# Multicomponent organic blend systems: A review of quaternary organic photovoltaics

Kekeli N'Konou, Souk Kim, Nutifafa Doumon

<figure><img src="image_1.png" /></figure>

## To cite this version:

Kekeli N'Konou, Souk Kim, Nutifafa Doumon. Multicomponent organic blend systems: A review of quaternary organic photovoltaics. Carbon Energy, 2024, 10.1002/cey2.579. hal-04664973

HAL Id: hal-04664973

https://hal.science/hal-04664973v1

Submitted on 14 May 2025

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_3.png" /></figure>

# Multicomponent organic blend systems: A review of quaternary organic photovoltaics

Kekeli N'Konou<fnref n="1" /> ID | Souk Y. Kim<fnref n="2" /> ID | Nutifafa Y. Doumon<fnref n="2" />,<fnref n="3" />,<fnref n="4" />,<fnref n="5" /> ID

## Correspondence

Nutifafa Y. Doumon, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802 USA.

Email: nzd5349@psu.edu

## Abstract

Embedding a third and/or fourth component into a binary blend active layer of organic photovoltaics (OPVs) is a promising approach to achieve high-performance photovoltaic cells and modules. This multicomponent strategy favors absorption broadening via additional components. Quaternary OPV (QOPV) blends have four components in three possible configurations: (i) a donor and three acceptors, (ii) two donors and two acceptors, or (iii) three donors and an acceptor. Although quaternary systems have only been relatively recently studied compared to other systems in OPVs, leveraging the synergistic effects of the four components leads to record power conversion efficiencies, currently approaching 20%. QOPVs provide ample material choices for compatibility and channels for charge transfer mechanisms, possibly leading to optimized morphology and orientation. Reviewing recent progress in advancing QOPVs is essential for understanding their contribution to the OPV field. The review mainly discusses research progress in QOPVs with a keen interest in their various configurations, semitransparency, and outdoor and indoor applications. It describes the not-well-understood QOPV's general working mechanism. This review explores high-performance QOPVs based on the fourth component's contribution as a donor, acceptor, or dye molecule and beyond in photovoltaic applications. Finally, there is a discussion around QOPV's outlook and projected future research directions in this field. This review intends to provide an overview of the quaternary systems approach to OPVs and inform current and future researchers on investigating the full spectrum of OPVs.

## KEYWORDS

fullerene acceptors, multicomponent blend, nonfullerene acceptors, organic photovoltaics, quaternary organic photovoltaics, (semi)transparent and flexible devices

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2024 The Authors. *Carbon Energy* published by Wenzhou University and John Wiley & Sons Australia, Ltd.

<footnote n="1">¹CNRS, Centrale Lille, Université Polytechnique Hauts-de-France, Junia-ISEN, UMR 8520 -IEMN, Université de Lille, Lille, France</footnote>

<footnote n="2">²Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania, USA</footnote>

<footnote n="3">³Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania, USA</footnote>

<footnote n="4">⁴Institute of Energy and the Environment, The Pennsylvania State University, University Park, Pennsylvania, USA</footnote>

<footnote n="5">⁵Alliance for Education, Science, Engineering, and Design with Africa, The Pennsylvania State University, University Park, Pennsylvania, USA</footnote>
# 1 INTRODUCTION

Organic photovoltaics (OPVs) have garnered widespread attention over the past two decades owing to their desirable features, such as lightweight, flexibility, semitransparency, and low cost, simple fabrication process compatible with industrial large-scale roll-to-roll production.¹⁻⁶ Record OPVs are classically binary blend bulk heterojunctions (BHJs) composed of one donor (D) and one acceptor (A) material. The early-day fullerene-based photoactive materials of binary OPVs (BOPVs)⁷,⁸ inherently possessed narrow spectral absorption. Because the absorption range is limited, a single-junction device cannot fully cover the solar spectrum without compromising the open-circuit voltage (V<sub>oc</sub>). The performance of BOPVs remains hampered, even in the advent of nonfullerene acceptors (NFAs), compared to inorganic solar cells (e.g., silicon) or hybrid perovskite, owing to the short lifetime of the photoexcited carriers in the photoactive materials, low carrier mobility, and exciton transport in most organic semiconductors. Thus, considerable efforts are made to understand and improve the performance of BOPVs, focusing on key factors such as: (i) broadening of the absorption spectrum for ample capturing of photons, (ii) energy level alignment of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) at the D:A heterojunctions to provide an adequate driving force for exciton dissociation, and (iii) morphology optimization and nanoscale phase separation of the interpenetrating D:A mixture to facilitate exciton diffusion and charge separation. Besides energy level modulation of D and A materials and design and synthesis of new materials, alternative approaches have been revealed as successful ways to improve power conversion efficiencies (PCEs). They are possible morphology optimization by adding guest components,⁹⁻¹² cathodic and/or anodic interlayer engineering,¹³⁻¹⁷ solvent additive engineering,¹⁸⁻²¹ and so forth. For instance, introducing guest components into the active layers of BOPVs enables morphological optimization of multicomponent blends with various structural characteristics, such as control of crystallinity, domain purity, crystal orientation, and domain size, leading to better performance.²²⁻²⁴ The multicomponent OPV strategy of adding one or more donor and/or acceptor materials, namely, guest components, into the binary blend has been identified as a practical approach to improving OPV performance. This is because it can lead to broadening the absorption spectrum, maintaining optimized morphology, facilitating transfer kinetics, and reducing voltage loss in V<sub>oc</sub>.²⁵⁻²⁷ While multiple components OPVs have

complementary absorption spectra for improved photon harvesting in active layers, they also maintain a simple fabrication process.

The broader absorption wavelength and probable higher spectral response of quaternary blends are beneficial for light-harvesting in OPVs. Compared with their binary and ternary counterparts, the addition of the fourth component contributes efficiently to maximizing photon harvesting, leading to the optimization of the photovoltaic (PV) parameters of quaternary OPVs (QOPVs).²⁸,²⁹ The introduction of small molecules as the fourth component in QOPVs broadens absorption bandwidth, produces co-crystalline molecular conformations, and facilitates fast energy transfer processes within the quaternary architecture.²⁸ Considering that the quaternary blend has four materials with complementary absorption, its absorption can easily span the UV, visible, and near-infrared (NIR) regions (300–1000 nm) of the solar spectrum compared to their counterparts.³⁰

The relentless design of new PV materials and engineering device innovation has led to a PCE exceeding 19% for single-junction OPVs using a multicomponent strategy,³¹⁻³⁵ demonstrating their significant potential for commercial application. Multicomponent OPVs typically have more than two materials in the blend active layer. Ternary and quaternary blend OPVs are two of the simplest. They could maintain the simple processing of the binary blend devices and improve the absorption with multiple different bandgap materials.³⁶ Adding third and fourth components could also be pivotal in the frontier molecular orbital level tuning by modulating the electrical properties of the active layer, enhancing the film morphology, and forming homogeneous D:A phases.³⁶⁻³⁸ Ternary OPVs have shown steady progress with efficiencies of over 19.0%,³¹,³⁹⁻⁴² especially with NIR absorbing NFAs,⁴³,⁴⁴ providing opportunities for maximizing spectral absorption. Recent reviews have thoroughly addressed the basic mechanisms of ternary OPVs.⁴⁵⁻⁴⁹ Given that the application of these three materials can cover a broadband solar spectrum, researchers have adopted a quaternary strategy to improve the performance of solar cells.

Compared to OPVs, quaternary blend strategies are widely used in inorganic, dye, and quantum dotsensitized solar cells.⁵⁰⁻⁵⁵ In OPVs, such approaches insert a fourth component, either a donor or an acceptor, into the ternary blend (D:A₁:A₂ or D₁:D₂:A) or embed two components into a binary host blend (D:A), forming a quaternary (D:A₁:A₂:A₃, D₁:D₂:A₁:A₂, or D₁:D₂:D₃:A) active layer system. The fourth element can be a polymer donor (D), fullerene acceptor (FA) or its derivatives, NFA, or dye. Crucial requirements for QOPVs are processing, material cost, chemical structure
compatibility, and satisfying the physical properties of the quaternary blend. The polyvalent role of the fourth element provides flexibility in selecting the different materials. Though QOPV was introduced in early 2016 using P3HT:PC61BM:ZnO:TiO2,<sup>56</sup> it was only in late 2016 and early 2017 that it involved all organic or polymer molecules in the active layer.<sup>28,29</sup> For instance, the all-organic quaternary concept was implemented in OPVs in 2017, using two donors and two acceptors (PTB7: PCDTBT:PC71BM:PC61BM) as the active layer to achieve high-efficiency and long-term durable OPVs, effectively implementing the first D1:D2:A1:A2 blend.<sup>29</sup> The last 5 years have seen an evolution of OPV studies and increased effectiveness using the quaternary strategy, leading to a current record efficiency of over 19.0%.<sup>34,57-60</sup> Thus, with their performance reaching record efficiency with such little attention, as depicted in Figure 1, it is essential to be one of the first or very few reviews to highlight their progress and potential for different energy materials and technology applications. The acceptor, donor, and dye molecules discussed in this review with regards to multicomponent (e.g., quaternary) blends are shown in Figure 2, and the PV parameters of the corresponding OPVs based on multiple donor-acceptor pairs and dye molecules are listed in Table 1.

This review summarizes studies on QOPVs. We discuss strategies adopted in QOPVs focusing on the fourth component, as a donor polymer, FA, NFA, or a dye molecule in ternary host blends in small, large-area, and semitransparent quaternary blend devices for applications in outdoor and indoor technologies beyond PVs. First, the general working mechanisms and the optimization of the QOPV blend composition are briefly presented. We then discuss studies on

QOPVs, including those based on (i) DA1A2A3, (ii) D1D2A1A2, and (iii) D1D2D3A systems, and those with (iv) dye molecules. We also focus on their semi-transparent properties and outdoor and indoor applications. While high performance is a requirement for the commercialization of OPVs, other critical concerns, such as the need for a thick and large-area active layer<sup>57</sup> for mass production and stability of OPVs in air, along with other typical operational conditions, restrict their breakthrough to the market. Ultimately, we highlight possible challenges for QOPVs and provide an outlook to spur future research.

## 2 | MOVING FROM TERNARY TO QOPVS: GENERAL WORKING MECHANISMS

QOPV blends are made of a host blend of D:A combined with two guest components, either donors or acceptors or a donor and acceptor. It allows the selection of four materials, enabling more alternative mechanisms to improve the photon-electron conversion efficiency compared to ternary OPVs (TOPVs) and BOPVs. This strategy helps to compensate for deficiencies in electronic performance or compatibility. Donors or acceptors with similar molecular structures frequently display good compatibility with each other<sup>30,34,64</sup> and contribute to optimizing the quaternary blend morphology with improved crystallinity, resulting in better charge carrier transport in the devices. However, the more options there are for selecting materials, the more challenging it becomes to find four compatible materials among

<figure><img src="image_4.png" /><figcaption>FIGURE 1 (A) Progress of journal-article publications and efficiencies showing the trends of binary single-junction OPVs compared to ternary and quaternary single-junction OPVs searched from Web of Science from 2000 till date. One can deduce that since their inception in 2015, QOPVs received little attention, yet their progress is the most impressive, rivaling existing and well-studied binary and ternary systems. (B) Efficiencies of binary, ternary, and quaternary blend devices captured in this review since the advent of QOPVs.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>FIGURE 2 Chemical structures of semiconductor organic/polymer materials used in multicomponent blends, in this case, quaternary blends: (A) donor, (B) acceptor, and (C) dye molecules.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Y6</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>BTP</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>BTP-S11</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>BTP-eC9</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>ITIC</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>IEICO-4F</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>N3</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>BTP-BO-4F (Y12)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>BTP-S12</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>BTP-S16</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>ITCT</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>IDIC</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>O1-2F</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>L8-BO</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>BTP-S17</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>IT-T-IC</figcaption></figure>

(C) Dye molecules

<figure><img src="image_22.png" /><figcaption>ASSQ</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>DPSQ</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>SR197</figcaption></figure>

<caption>FIGURE 2 (Continued)</caption>
<table><caption>TABLE 1 Summary of the photovoltaic performances of selected binary, ternary (binary with a third component), and quaternary (ternary with a fourth component) OPVs.</caption><thead><tr><th rowspan="2">Binary blend</th><th colspan="2">Ratio (W<sub>1</sub>W<sub>2</sub>)</th><th colspan="2">J<sub>oc</sub> (mA/cm<sup>-2</sup>)</th><th colspan="2">Ratio (W<sub>1</sub>W<sub>2</sub>W<sub>3</sub>W<sub>4</sub>)</th><th colspan="2">J<sub>oc</sub> (mA/cm<sup>-2</sup>)</th><th colspan="2">Ratio (W<sub>1</sub>W<sub>2</sub>W<sub>3</sub>W<sub>4</sub>)</th><th colspan="2">J<sub>oc</sub> (mA/cm<sup>-2</sup>)</th><th rowspan="2">Year</th><th rowspan="2">References</th></tr><tr><th>V<sub>oc</sub> (V)</th><th>FF (%)</th><th>PCe (%)</th><th>TC</th><th>V<sub>oc</sub> (V)</th><th>FF (%)</th><th>PCe (%)</th><th>FC</th><th>V<sub>oc</sub> (V)</th><th>FF (%)</th><th>PCe (%)</th><th>FF (%)</th></tr></thead><tbody><tr><td>PM6:BTP-eC9</td><td>1:1.2</td><td>27.65</td><td>0.848</td><td>78.6</td><td>18.5</td><td>BTP-S16</td><td>1.072.0.48</td><td>27.73</td><td>0.863</td><td>80.64</td><td>19.3</td><td>BTP-S17</td><td>1.054.0.495.0.165</td><td>27.75</td><td>0.881</td><td>80.8</td><td>19.8</td><td>2023</td><td>[60]</td></tr><tr><td>PM6:BTP-S11</td><td>1:1.2</td><td>27.1</td><td>0.878</td><td>79.3</td><td>18.9</td><td>BTP-S17</td><td>1.072.0.48</td><td>27.59</td><td>0.873</td><td>79.55</td><td>19.2</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>D18-ClBTP-eC9</td><td>1:1.2</td><td>25.42</td><td>0.917</td><td>78.3</td><td>18.3</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>/PM6:L8-BO</td><td>1:1.2:1.1.2</td><td>27.02</td><td>0.898</td><td>80.8</td><td>19.6<sup>a</sup></td><td>2023</td><td>[59]</td></tr><tr><td>PM6:BTP-eC9</td><td>1:1.2</td><td>27.18</td><td>0.851</td><td>78.3</td><td>18.1</td><td>L8-BO</td><td>1:1.11.0.1</td><td>27.39</td><td>0.855</td><td>78.76</td><td>18.4</td><td>PY-IT</td><td>1:1.05.0.1.0.05</td><td>27.55</td><td>0.862</td><td>79.3</td><td>18.8</td><td>2023</td><td>[61]</td></tr><tr><td>PM6:BTP-eC9</td><td>1:1.2</td><td>27.18</td><td>0.851</td><td>78.3</td><td>18.1</td><td>L8-BO</td><td>1:1.11.0.1</td><td>27.39</td><td>0.855</td><td>78.76</td><td>18.4</td><td>PE2F</td><td>0.951.1.0.1.0.1.0.05</td><td>27.47</td><td>0.863</td><td>79.2</td><td>18.8</td><td>2023</td><td>[61]</td></tr><tr><td>PM6:BTP-eC9</td><td>1:1.2</td><td>27.59</td><td>0.845</td><td>78.5</td><td>18.3</td><td>L8-BO</td><td>1:0.69.0.6</td><td>27.04</td><td>0.864</td><td>78.81</td><td>18.4</td><td>BTP-S10</td><td>1.05.0.5.0.50.2</td><td>27.82</td><td>0.882</td><td>78.9</td><td>19.4</td><td>2022</td><td>[57]</td></tr><tr><td>PM6:BTP-BO-4F</td><td>1:1.2</td><td>24.4</td><td>0.812</td><td>66.1</td><td>13.1</td><td>BTP</td><td>1:1.2.0.15</td><td>25.2</td><td>0.843</td><td>71.2</td><td>15.1</td><td>Phi-Se</td><td>1:1.2.0.15.0.15</td><td>26.2</td><td>0.851</td><td>76.7</td><td>17.0</td><td>2022</td><td>[62]</td></tr><tr><td>PM6:Y6</td><td>1:1</td><td>26.52</td><td>0.845</td><td>75.9</td><td>17.0</td><td>Phi-Se</td><td>1:1.2.0.15</td><td>24.8</td><td>0.832</td><td>76.7</td><td>15.5</td><td>BTP</td><td>1:1.2.0.15.0.15</td><td>27.4</td><td>0.865</td><td>76.6</td><td>18.2</td><td>2022</td><td>[62]</td></tr><tr><td>PM6:Y6</td><td>1:1.2</td><td>25.92</td><td>0.822</td><td>74.6</td><td>15.9</td><td>L20</td><td>0.9:1.2.0.1</td><td>26.32</td><td>0.832</td><td>75.10</td><td>16.4</td><td>PC<sub>71</sub>BM</td><td>0.91.2.0.2.0.1</td><td>25.79</td><td>0.846</td><td>77.5</td><td>16.9</td><td>2022</td><td>[26]</td></tr><tr><td>PM6:Y6</td><td>1:1.4</td><td>24.48</td><td>0.844</td><td>74.9</td><td>15.5</td><td>PC<sub>71</sub>BM</td><td>1:1.2.0.2</td><td>25.44</td><td>0.844</td><td>75.85</td><td>16.3</td><td>ITIC</td><td>1.1.1.5.0.2.0.05</td><td>25.63</td><td>0.856</td><td>76.2</td><td>16.7</td><td>2021</td><td>[64]</td></tr><tr><td>PM6:Y6</td><td>1:1.2</td><td>25.12</td><td>0.830</td><td>74.5</td><td>15.6</td><td>SR197</td><td>1:1.11.0.1</td><td>26.87</td><td>0.834</td><td>75.08</td><td>16.8</td><td>PC<sub>71</sub>BM</td><td>1:1.1.0.1.0.1</td><td>27.11</td><td>0.841</td><td>76.6</td><td>17.5</td><td>2021</td><td>[65]</td></tr><tr><td>PM6:Y6</td><td>1:1.2</td><td>25.98</td><td>0.842</td><td>75.5</td><td>16.5</td><td>PM7</td><td>0.81.2.0.2</td><td>26.17</td><td>0.848</td><td>76.70</td><td>17.0</td><td>PC<sub>71</sub>BM</td><td>0.81.2.0.2.0.5</td><td>26.55</td><td>0.859</td><td>79.2</td><td>18.1</td><td>2021</td><td>[66]</td></tr><tr><td>PTQ10:N3</td><td>1:1.2</td><td>25.72</td><td>0.851</td><td>75.2</td><td>16.5</td><td>PM6</td><td>0.61.2.0.4</td><td>26.348</td><td>0.849</td><td>76.6</td><td>17.1</td><td>N3</td><td>0.61.2.0.4.0.2</td><td>26.78</td><td>0.852</td><td>77.7</td><td>17.7</td><td>2020</td><td>[67]</td></tr><tr><td>PM6:Y6</td><td>1:1.2</td><td>25.2</td><td>0.841</td><td>74.3</td><td>15.7</td><td>Bp-ITIC</td><td>1.1.14.0.0.6</td><td>25.2</td><td>0.847</td><td>74.7</td><td>15.9</td><td>PC<sub>71</sub>BM</td><td>1.1.08.0.1.2.0.12</td><td>25.8</td><td>0.853</td><td>76.4</td><td>16.8</td><td>2020</td><td>[68]</td></tr><tr><td>PM6:Y6</td><td>1:1</td><td>25.1</td><td>0.840</td><td>74.0</td><td>15.6</td><td>PC<sub>71</sub>BM</td><td>1.1.2.0.2</td><td>25.5</td><td>0.852</td><td>76.9</td><td>16.7</td><td>Phi-Se</td><td>1.1.2.0.2.0.15</td><td>26.3</td><td>0.851</td><td>76.8</td><td>17.2</td><td>2020</td><td>[69]</td></tr><tr><td>PM6:Y6</td><td>1:1</td><td>25.45</td><td>0.838</td><td>72.33</td><td>15.5</td><td>PC<sub>71</sub>BM</td><td>1:1.0.3</td><td>25.28</td><td>0.851</td><td>75.15</td><td>16.2</td><td>IDIC</td><td>1.1.0.0.2.0.1</td><td>26.19</td><td>0.866</td><td>75.3</td><td>17.1</td><td>2020</td><td>[70]</td></tr><tr><td>PBDTSF-TZNT-F8IC</td><td>1:1.4</td><td>22.61</td><td>0.77</td><td>65.5</td><td>11.4</td><td>F8OIC</td><td>1:1.0.4</td><td>24.63</td><td>0.80</td><td>72.6</td><td>14.3</td><td>PC<sub>71</sub>BM</td><td>1:1.0.4.0.2</td><td>25.28</td><td>0.80</td><td>74.8</td><td>15.1</td><td>2019</td><td>[71]</td></tr><tr><td>PTB7-Th.EICO-4F</td><td>1:1.5</td><td>22.06</td><td>0.75</td><td>58.24</td><td>9.7</td><td>PC<sub>71</sub>BM</td><td>1:1.3.0.2</td><td>23.17</td><td>0.75</td><td>65.03</td><td>11.3</td><td>PC<sub>61</sub>BM</td><td>1:1.1.0.2.0.2</td><td>24.37</td><td>0.74</td><td>69.3</td><td>12.5</td><td>2019</td><td>[72]</td></tr><tr><td>PDB7-TJTCT</td><td>1:1</td><td>17.88</td><td>0.850</td><td>72.3</td><td>11.3</td><td>JT-T-IC</td><td>1:1.0.1</td><td>18.00</td><td>0.863</td><td>71.5</td><td>11.5</td><td>PC<sub>71</sub>BM</td><td>1:1.0.1.0.2</td><td>19.15</td><td>0.875</td><td>72.7</td><td>12.5</td><td>2019</td><td>[73]</td></tr><tr><td>PTB7-Th.FOIC</td><td>1:1.5</td><td>23.18</td><td>0.74</td><td>65.62</td><td>11.3</td><td>PC<sub>71</sub>BM</td><td>1:1.2.0.3</td><td>24.34</td><td>0.74</td><td>67.49</td><td>12.2</td><td>DR3TBDTT</td><td>0.851.2.0.3.0.15</td><td>25.28</td><td>0.75</td><td>69.5</td><td>13.1</td><td>2019</td><td>[30]</td></tr><tr><td>PTB7-Th.FOIC</td><td>1:1.3</td><td>22.57</td><td>0.749</td><td>64.97</td><td>11.1</td><td>PDB-T</td><td>0.91.3.0.1</td><td>23.26</td><td>0.747</td><td>66.81</td><td>11.8</td><td>ITIC</td><td>0.9.1.17.0.13.0.1</td><td>23.89</td><td>0.746</td><td>67.3</td><td>12.5</td><td>2019</td><td>[74]</td></tr><tr><td>PDB-TJTIC</td><td>1:1</td><td>15.81</td><td>0.897</td><td>68.10</td><td>9.9</td><td>C<sub>70</sub>ICBA</td><td>1.07.0.4</td><td>16.67</td><td>0.930</td><td>69.5</td><td>11.1</td><td>C<sub>70</sub>PCBM</td><td>1.09.0.3.0.2</td><td>17.83</td><td>0.915</td><td>71.9</td><td>12.1</td><td>2018</td><td>[75]</td></tr><tr><td>C<sub>70</sub>ICBA</td><td>1:1.3</td><td>15.75</td><td>0.885</td><td>70.2</td><td>11.2</td><td>C<sub>70</sub>ICBA</td><td>1.07.0.4</td><td>16.59</td><td>0.930</td><td>69.5</td><td>11.0</td><td>C<sub>70</sub>PCBM</td><td>1.09.0.3.0.2</td><td>18.39</td><td>0.925</td><td>73.0</td><td>12.8</td><td>2018</td><td>[75]</td></tr><tr><td>C<sub>70</sub>PCBM</td><td>1:1.3</td><td>15.75</td><td>0.885</td><td>70.2</td><td>11.2</td><td>C<sub>70</sub>ICBA</td><td>1.07.0.4</td><td>16.59</td><td>0.930</td><td>69.5</td><td>11.0</td><td>C<sub>70</sub>ICBA</td><td>1.09.0.3.0.2</td><td>17.85</td><td>0.916</td><td>72.0</td><td>12.1</td><td>2018</td><td>[75]</td></tr><tr><td>C<sub>70</sub>PCBM</td><td>1:1.3</td><td>15.75</td><td>0.885</td><td>70.2</td><td>11.2</td><td>C<sub>70</sub>ICBA</td><td>1.07.0.4</td><td>16.59</td><td>0.930</td><td>69.5</td><td>11.0</td><td>C<sub>70</sub>ICBA</td><td>1.09.0.3.0.2</td><td>18.23</td><td>0.922</td><td>72.1</td><td>12.5</td><td>2018</td><td>[75]</td></tr></tbody></table>
<table><caption>TABLE 1 (Continued)</caption><thead><tr><th>Binary blend</th><th>Ratio (W<sub>1</sub>:W<sub>2</sub>)</th><th>J<sub>sc</sub> (mA/cm<sup>-2</sup>)</th><th>V<sub>oc</sub> (V)</th><th>FF (%)</th><th>PCE (%)</th><th>TC</th><th>Ratio (W<sub>1</sub>:W<sub>2</sub>:W<sub>3</sub>:W<sub>4</sub>)</th><th>J<sub>sc</sub> (mA/cm<sup>-2</sup>)</th><th>V<sub>oc</sub> (V)</th><th>FF (%)</th><th>PCE (%)</th><th>FC</th><th>Ratio (W<sub>1</sub>:W<sub>2</sub>:W<sub>3</sub>:W<sub>4</sub>:W<sub>5</sub>)</th><th>J<sub>sc</sub> (mA/cm<sup>-2</sup>)</th><th>V<sub>oc</sub> (V)</th><th>FF (%)</th><th>PCE (%)</th><th>Year</th><th>References</th></tr></thead><tbody><tr><td>PDBB-TJT-M</td><td>1:1</td><td>16.59</td><td>0.952</td><td>67.5</td><td>11.2</td><td>bis-PC<sub>71</sub>BM</td><td>1:1:0.2</td><td>16.79</td><td>0.959</td><td>72.6</td><td>12.0</td><td>1C<sub>60</sub>BA</td><td>1:1:0.2:0.2</td><td>18.21</td><td>0.968</td><td>73.1</td><td>13.3</td><td>2018</td><td>[72]</td></tr><tr><td>PDBB-TJT-M</td><td>1:1</td><td>16.59</td><td>0.952</td><td>67.5</td><td>11.2</td><td>bis-PC<sub>71</sub>BM</td><td>1:1:0.2</td><td>16.79</td><td>0.959</td><td>72.6</td><td>12.0</td><td>PC<sub>60</sub>BM</td><td>1:1:0.2:0.2</td><td>18.38</td><td>0.957</td><td>73.1</td><td>13.3</td><td>2018</td><td>[72]</td></tr><tr><td>PTB7-PC<sub>71</sub>BM</td><td>1:1.5</td><td>15.77</td><td>0.72</td><td>67.29</td><td>7.6</td><td>PCDTBT</td><td>0.9:1.50.1</td><td>16.14</td><td>0.73</td><td>69.26</td><td>8.2</td><td>PC<sub>60</sub>BM</td><td>0.9:1.2:0.1:0.3</td><td>16.31</td><td>0.74</td><td>70.3</td><td>8.4</td><td>2017</td><td>[29]</td></tr><tr><td>PTB7-Th:PC<sub>71</sub>BM</td><td>1:1.5</td><td>17.05</td><td>0.794</td><td>68.6</td><td>9.22</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>ASSQ:DFSQ</td><td>1:1:5.0:11.0.1</td><td>17.82</td><td>0.789</td><td>71.7</td><td>10.5</td><td>2016</td><td>[28]</td></tr><tr><td>P3HT:PC<sub>60</sub>BM</td><td>1:0.7</td><td>8.42</td><td>0.598</td><td>53.04</td><td>2.67</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>ZnO:TiO<sub>2</sub></td><td>1:0.550.075:0.075</td><td>9.21</td><td>0.604</td><td>55.6</td><td>3.1</td><td>2016</td><td>[56]</td></tr></tbody></table>

Abbreviations: ASSQ, assymetric squaraine; BHI, bulk heterojunction; FC, fourth component; FF, fill factor; OPV, organic photovoltaics; PCE, power conversion efficiencies; TC, third component.

<sup>a</sup>Double BHI quaternary OPVs.

themselves or in blocks of two or three vis-à-vis the others. Photon absorption, photogenerated excitons, excitons dissociation, and charge transport in QOPVs active layers are enhanced separately or simultaneously through successive blends of the appropriate third and fourth components. In QOPVs, the third and fourth components play various functions. They can operate independently to gradually optimize the OPV performance by maximizing the potential of each material. For instance, in 2019, Ma and co-workers designed an efficient QOPV combining a PTB7-Th:FOIC binary blend featuring PC<sub>71</sub>BM and DR3TBDTT for improved efficiency of over 13.5%. They dispersed a high-crystallinity DR3TBDTT into PTB7-Th to enhance domain purity, and the fourth component, PC<sub>71</sub>BM, is used as a phase modifier to facilitate beneficial FOIC packing.<sup>30</sup> Two years later, Jia et al. reported the successful fabrication of QOPV devices based on the PM6:Y6 blend by consecutively adding PC<sub>71</sub>BM for charge transport optimization and ITIC as the fourth component for maximized photon harvesting in the QOPV, leading to PCE of 16.7%.<sup>64</sup> Yet, while QOPVs' progress has been impressive, there is still a lot unknown and room for improvement, as the efficiency of single-junction BOPVs was also over 16% in 2019. The choice of the fourth component must consider the LUMO or HOMO levels of the different donors or acceptors to prevent the formation of deep charge trapping, thus leading to a better charge transfer.<sup>77,78</sup>

The working mechanisms of QOPVs are still a work in progress, and the theories behind them need to be better understood. It typically includes one or more of the four fundamental TOPV working mechanisms, shown in Figure 3: charge transfer (in Figure 3A), energy transfer (in Figure 3B), parallel-like (in Figure 3C), and the alloy model (in Figure 3D).<sup>48,79</sup> Building off a ternary dual-donor system, the operating mechanism of a multi-component system, for instance, a typical D<sub>1</sub>:D<sub>2</sub>:A<sub>1</sub>:A<sub>2</sub> quaternary blend enables double-cascaded energy level alignment in the active layer, allowing for efficient excitons separation and long carrier lifetimes. In this configuration, D<sub>2</sub> produces cascaded charge-hopping pathways that facilitate the manipulation of multiple charge transfer channels to minimize recombination and optimize carrier transport. A<sub>2</sub> enhances electron transport and extraction efficiency. The electronic structure and morphology of each of the four quaternary blend components are fine-tuned via molecular engineering to maximize light absorption, carrier extraction, and exciton splitting, potentially making QOPVs more efficient.<sup>66,80</sup> The absorption spectra of the third and fourth components must complement the absorption spectra of the D:A host blend for optimum and maximum sunlight harvesting to yield a higher J<sub>sc</sub>. However, the dynamic
<figure><img src="image_25.png" /><figcaption>FIGURE 3 Schematic of working mechanisms in ternary organic photovoltaic cells. (A) Charge transfer, (B) energy transfer, (C) parallel-like, and (D) alloy model. Reproduced with permission: Copyright 2021, The Royal Society of Chemistry.⁷⁹</figcaption></figure>

process of excitons and charge carriers in QOPVs is unusually complex, so selecting four appropriate functional, active layer components becomes arduous.

While details of the TOPV working mechanisms have been documented in the literature,⁴⁸,⁸¹⁻⁸³ it is not the case for quaternary and other multicomponent OPVs. In these systems, charge and energy transfer can occur concomitantly, but one working mechanism is prominent. There may be synchronous effects in quaternary blends, like simultaneous charge transfer/energy transfer⁸⁴ and parallel-like/alloy model effects.³⁰ Effective energy transfer necessitates a significant emission spectrum overlap of one component with another component and a spacing of <10 nm between them. A potential pathway to improve exciton use needs to be provided by energy transfer.⁸⁵,⁸⁶ Embedding more than two acceptors in OPVs is quite appealing to extend or combine the advantages of various strategies. The classical approach to introducing two acceptors into the active layer involves forming an alloy between them.

In 2019, Xu et al. introduced a concept for QOPV using three molecular acceptors connected in parallel series, using an analogy between a triple-acceptor QOPV and the well-established double-acceptor TOPV.⁷¹ The analogous working principles, either parallel or series connections, of the double acceptor-based TOPV active layer and three acceptors QOPV active layer are illustrated in Figure 4A,B.⁷¹ This particular structure was realized by opening an electron transfer pathway employing F8IC, a highly crystalline acceptor, and linking in series two poorly crystalline acceptors, F8OIC and PC₇₁BM, providing another parallel pathway. This proposed system combines the broad-spectrum coverage of multiple parallel connections with the effective charge transfer process of multiple series connections, yielding an improved *J*<sub>sc</sub> while maintaining a high *V*<sub>oc</sub> and fill factor (FF). The atomic force microscopy (AFM) images of PBDTSF-TZNT:F8IC,

PBDTSFTZNT:F8IC:F8OIC, and the quaternary mixtures show similar nanostructures in Figure 4C–H. The F8IC crystals are the primary features visible in all three AFM images; however, a reduction in roughness occurred as more components were embedded in the blend, yielding a better morphology for the triple-acceptor QOPV blend (Figure 4E,H) compared to single-acceptor BOPV (Figure 4D,G) and double-acceptor TOPV (Figure 4C,F) blends. In brief, the working principles of QOPVs are not well understood and are still under continual development. The OPV community will need to work collectively to gain an extensive and comprehensive awareness of the fundamental concepts of QOPVs.

A phenomenon recently described as the dilution effect has been proposed to gain insight into the basic working mechanism and material selection for optimal multicomponent OPVs such as QOPVs.⁸⁷ It involves material selection guidelines for high-performance multicomponent OPVs, with an emphasis on larger bandgap materials, strong miscibility, and a relatively greater LUMO offset between the acceptors of the D:A BHJ film. As a result, material selection based on the dilution effect may enhance the overall performance of the device. The dilution effect, typically observed when two compatible components are molecularly combined, leads to a range of optoelectronic property changes⁸⁷,⁸⁸ or developing OPV with strong mechanical deformability.⁸⁹ For instance, blending an NFA (dilute) with a highly miscible and larger bandgap NFA (diluent) results in observable changes like blue-shifted absorption, enhanced emission, and more distinct band tails alongside reduced electron-vibration interactions. It emphasizes selecting guest components, which helps form vertical gradient distribution and larger D:A interpenetrating regions, promoting exciton generation and dissociation.⁹⁰ Moreover, blending two compatible acceptor molecules can decrease *V*<sub>oc</sub> loss in the cell, attributed to reduced vibrational coupling and more defined band edges.
<figure><img src="image_26.png" /><figcaption>FIGURE 4 A schematic of the working principle of (A) double-acceptor devices showing alloy, parallel, and series connections and (B) triple-acceptor devices showing parallel, series, and parallel-series connections. (C–E) AFM height and (F–H) tapping phase images of single-acceptor, double-acceptor, and triple-acceptor devices containing F8IC. Reproduced with permission: Copyright 2019, The Royal Society of Chemistry.⁷¹</figcaption></figure>

3 | OPTIMIZATION OF THE QOPV BLEND COMPOSITION AND QOPV FABRICATION METHODS

Different approaches have been developed to optimize quaternary, and thus, multicomponent blend thin films for optimal PV performance. Ko and colleagues suggested an interesting approach for optimizing the composition of QOPVs.²⁹ It should be systematic, starting with the optimum binary D:A host blend ratio a:b by weight and varying first the ratio of the third (T) component in the blend such that D:A:T = (a–x):b:x or a:(b–x):x, where 0 < x < 1, and then gradually introducing the fourth (F) component. An optimized ternary to quaternary composition is more conveniently found by

varying the amounts of two additional components while maintaining the amount of the acceptor or donor in the host binary blend fixed. This offers options for engineering the physical properties of the active layer. After optimizing the TOPV blend composition by a trial-and-error approach, optimized quaternary blends are produced via similar steps by introducing a fourth component as an additional acceptor or donor. Furthermore, this approach facilitates control of the nanoscale morphology of the quaternary blends. Presumably, the additional donor and acceptor would interact with developing polymer donor crystal grains or aggregated domains of the FAs. As shown in Table 1, the choice of x between 0.1 and 0.2 likely yields the best V<sub>oc</sub> and FF values. According to the different ratios of quaternary
blends reported in Table 1, it can be assumed that most of the ratio of acceptors or donors in the blend varies relative to the host binary D:A ratio. For example, if the D:A ratio is a:b, the D:A:T:F quaternary ratio could be (a-x):x:(b-y):y, where x and y often range from 0.1 to 0.5.

Multicomponent OPV approaches, intended to circumvent the constraints of conventional single-junction devices, have proven to be a better way to overcome the limitation of BOPV blends in broadly absorbing the solar spectrum. However, controlling the molecular and morphological compatibility of all elements is challenging. The difficulty also involves developing optimization processes for blend solution parameters, such as solubility (types/choice of solvent and solvent additives if applicable), ratios between various components, film thickness, and any required pre- or post-treatment steps. They become significantly more relevant as the blend evolves from binary to ternary, quaternary, and so forth.

Moreover, PV performance is optimized if the benefits of the host and guest components are fully exploited through synergistic effects, indicating that incorporating an additional component may not necessarily guarantee optimal PV performance. Most QOPV blend thin films are fabricated by either (i) one deposition step (Figure 5A), (ii) two deposition steps (Figure 5B), or (iii) three deposition steps (Figure 5C).

In one deposition step, all component blend solution is deposited to form a BHJ thin film. Alternatively, a sequential three-deposition step is used by consecutively adding the third and fourth components in the host binary blend thin film that was previously coated (Figure 5C). However, these strategies face two challenges: compatibility of the four component materials and miscibility or orthogonality of the different solvents. As a result, an alternative fabrication method for addressing the shortcomings of the one-step multicomponent OPV approach, a layer-by-layer sequential double BHJ approach, is proposed (Figure 5B). This method usually involves two consecutive depositions of two different D:A binary BHJ blends. It has proven to be a simple, efficient way to fabricate high-efficiency QOPV and can result in a morphology more controllable and favorable than conventional QOPV BHJ morphology. Figure 5B,C shows two sequential deposition approaches identified as a simple alternative procedure for relatively optimizing QOPV performance. In Figure 5B, different QOPV BHJ configurations are formed as a quaternary blend active layer using simple sequential spin-coating of donor and acceptor materials. Figure 5C shows a layer-by-layer double-BHJ structure of the QOPV active layer using D₁:A₁ and D₂:A₂ binary blends separately.

<figure><img src="image_27.png" /><figcaption>FIGURE 5 Schematic diagram of (A) quaternary blend film prepared via conventional one-step deposition method of premixing four component materials; (B) sequential deposition process of second binary blend (D₂:A₂) atop the first binary blend (D₁:A₁) to form modified quaternary layer-by-layer double BHJs to form quaternary blend films; and (C) by subsequently adding the third component (A₂ or D₂ to form D₁:A₁:A₂ or D₁:D₂:A₁ ternary blend film) and the fourth component (A₃ or D₃ to form D₁:A₁:A₂:A₃ or D₁:D₃:A₁:A₂ or D₁:D₂:D₃:A₁ or D₁:D₂:A₁:A₃ quaternary blend film) in the host binary blend (D₁:A₁).</figcaption></figure>
4 | RECENT PROGRESS IN QOPVS AND BEYOND

This section presents some representative works in the field of QOPVs to achieve high efficiency based on (i) D:A₁:A₂:A₃ configurations, including D:NFA: FA:FA, D:NFA:NFA:FA, and D:NFA:NFA:NFA; (ii) D₁:D₂:A₁:A₂ configurations, including D₁:D₂:FA:FA; D₁:D₂:NFA:FA, and D₁:D₂:NFA:NFA; (iii) D₁:D₂:D₃:A configurations; (iv) the use of dye molecules; and for different alternative applications, including (v) indoor, (vi) semitransparency, and (vii) others such as light emitting diodes, photodetectors, field effect transistors, and so forth.

4.1 | QOPVs based on D:A₁:A₂:A₃ configuration

4.1.1 | D:NFA:FA:FA QOPVs

Small molecules usually have high crystallinity and, thus, possess good charge transport properties. Consequently, they are used as the fourth component in QOPVs for additional merits to broadening the absorption spectrum, such as binary film morphology tuning and host donor or host NFA crystallinity improvement.

In 2018, Yan et al. demonstrated an efficient QOPV by mixing a blend of small molecules bisPC<sub>71</sub>BM: PC<sub>71</sub>BM with a known PBDB-T:IT-M host system.<sup>76</sup> Grazing wide-angle X-ray scattering (GIWAXS) experiments on the host ternary PBDB-T:IT-M:bis-PC<sub>71</sub>BM and quaternary thin films with different PC<sub>71</sub>BM weight ratios reveal relevant information on the potential changes in molecular packing and orientation. The GIWAXS results indicated that PBDB-T out-of-plane scattering peak (100) occurs at 0.317 Å<sup>-1</sup> in broadband, and its peak (010) corresponding to the π-π stacking appears around 1.70 Å<sup>-1</sup> (dπ-π = 0.370 nm) in broadband form. After incorporating PC<sub>71</sub>BM into the PBDB-T:IT-M:bis-PC<sub>71</sub>BM ternary host system (in Figure 6A), some possible changes in the film morphologies were observed, as depicted in Figure 6B. PC<sub>71</sub>BM addition slightly enhances the polymer and IT-M phase coherence crystal length and their phase separation with a modest change in the root-mean-square surface roughness. The PV parameters revealed that the ternary device with PBDB-T:IT-M:bis-PC<sub>71</sub>BM as the active layer exhibited an average V<sub>oc</sub> of 0.959 V, J<sub>sc</sub> of 16.79 mA cm<sup>-2</sup>, and FF of 72.6%, with a maximum PCE of 12%. The PBDB-T:IT-M:PC<sub>71</sub>BM-based ternary cells showed comparable PCEs, with a larger J<sub>sc</sub> of 17.5 mA cm<sup>-2</sup> and a lower V<sub>oc</sub> of 0.938 V, consistent with the lower LUMO energy level of PC<sub>71</sub>BM versus bis-PC<sub>71</sub>BM. The

different V<sub>oc</sub> values of the bis-PC<sub>71</sub>BM and PC<sub>71</sub>BM ternary devices can be attributed to the formation of NFA:FA alloys.<sup>91</sup> Adding 0.1 PC<sub>71</sub>BM (w/w) can effectively lead to a significant increase in J<sub>sc</sub> (18.2 mA cm<sup>-2</sup> for quaternary vs. 16.8 mA cm<sup>-2</sup> for ternary and 16.6 mA cm<sup>-2</sup> for binary). This high J<sub>sc</sub> can be well conserved upon adding more PC<sub>71</sub>BM. Improving J<sub>sc</sub> and FF allows for achieving 13% PCE from the quaternary cell despite slightly reducing V<sub>oc</sub>.

Similarly, Shen et al. designed QOPVs by inserting bisPC<sub>71</sub>BM:PC<sub>61</sub>BM or bis-PC<sub>71</sub>BM:IC<sub>60</sub>BA into a PBDB-T:IT-M system.<sup>72</sup> A minor difference was found in the structures of the FAs, which reportedly led to significantly different device performance. The structure-matched IC<sub>60</sub>BA:PC<sub>61</sub>BM combination resulted in the best device performance. Morphology studies indicated clear, phase-separated light and dark domains in the bis-PC<sub>71</sub>BM-based ternary and quaternary blends, while both IC<sub>60</sub>BA-based ternary and quaternary blends exhibited larger domains. The one-dimensional GIXRD data indicated that the addition of IC<sub>60</sub>BA and PC<sub>61</sub>BM facilitated IT-M molecules to form compact π-π stacks in the out-of-plane direction, consistent with their increased electronic mobilities. The PV parameters of the devices revealed that the ternary devices based on PC<sub>61</sub>BM and IC<sub>60</sub>BA exhibited higher J<sub>sc</sub> than those based on bis-PC<sub>71</sub>BM. In contrast, the latter presented a higher FF, with all three ternary devices showing PCEs slightly above 12%. With a bis-PC<sub>71</sub>BM:PC<sub>61</sub>BM or bis-PC<sub>71</sub>BM:IC<sub>60</sub>BA blend as an additive, the QOPVs demonstrated an overall improvement in V<sub>oc</sub>, J<sub>sc</sub>, and FF, leading to an increased PCE of 13.3% as opposed to 11% for the binary and 12% for the ternary.

In 2018, Yao et al. developed a QOPV with two fullerene components (C<sub>60</sub>-/C<sub>70</sub>-PCBM and C<sub>60</sub>-/C<sub>70</sub>-ICBA) in a nonfullerene PBDB-T:ITIC binary system.<sup>75</sup> The two fullerenes in the blend lead to higher crystallinity in the ITIC and polymer donor phases. The PBDB-T:ITIC:PCBM:ICBA QOPV yielded an increased V<sub>oc</sub> and larger J<sub>sc</sub> and FF values than the host PBDB-T:ITIC binary blend device. The V<sub>oc</sub> of the quaternary device was between 0.915 and 0.925 V, falling between the extreme values of the ICBA and PCBM ternary devices. The favorable setting of V<sub>oc</sub> can be attributed to the formation of the acceptor alloy.<sup>75,92</sup> As a result, the QOPVs reached PCEs of up to 12.0% to 12.8% compared to the host binary device with 9.9% efficiency and ternary devices with 11.0% efficiency. The shape and size of C<sub>60</sub> and C<sub>70</sub> and the position of the second indene unit substituent on C<sub>60</sub>-/C<sub>70</sub>-ICBA affect the blend morphology upon adding the guest components. The increased crystallinity of the donor polymer is advantageous for rapid charge transport due to its regular molecular
<figure><img src="image_28.png" /><figcaption>FIGURE 6 Schematics of film morphology change upon introducing PC<sub>71</sub>BM into the PBDB-T:IT-M:bis-PC<sub>71</sub>BM ternary blend. (A) PBDB-T:IT-M:bisPC<sub>71</sub>BM ternary blend and (B) PBDB-T:IT-M:bisPC<sub>71</sub>BM:PC<sub>71</sub>BM quaternary blend. Reproduced with permission: Copyright 2018, American Chemical Society.<sup>76</sup></figcaption></figure>

arrangement. It promotes the formation of a bicontin-
uous interpenetrating lattice structure and could lead to
increased domain purity in most cases.⁹³

Liu et al. reported a 12.5% efficient QOPV in
2019,⁹⁴ where PC₇₁BM and PC₆₁BM fullerene alloys
were embedded into a PTB7-Th:IEICO-4F host system.
PTB7-Th is a typical narrow-band polymer donor. It is
compatible with fullerene derivatives, PC₆₁BM and
PC₇₁BM, and nonfullerene small-molecule, IEICO-4F,
leveraging PTB7-Th's favorable tolerance ratios to FAs
and the NIR absorption of the system. The results
revealed that PTB7-Th:IEICO-4F:PC₇₁BM:PC₆₁BM
QOPVs significantly outperformed the binary devices
without fullerene and ternary PTB7-Th:IEICO-
4F:fullerene devices with a significant increase in FF
(0.693 vs. 0.582 and 0.65) and Jₛₙ (24.37 vs. 22.06 and
23.17 mA cm⁻²). This led to a PCE of 12.5% for the
quaternary versus 9.7% for the binary and 11.3% for
the ternary. With the addition of PC₇₁BM:PC₆₁BM to
PTB7-Th:IEICO-4F blends, the resulting quaternary
devices successfully retained the high FF of PTB7-
Th:PC₇₁BM (or PTB7-Th:PC₆₁BM)-based devices and
high Jₛₙ of PTB7-Th:IEICO-4F-based devices. From the
external quantum efficiency (EQE) spectrum, PTB7-
Th:IEICO-4F-based devices show noticeable response
values above 70% from 700 to 900 nm and approxi-
mately 80% for the quaternary devices in the 600 to
900 nm range (Figure 7B). This appears to be the main
contributor to the improved Jₛₙ (Figure 7A). Both
PC₇₁BM and PC₆₁BM exhibited almost no absorp-
tion from ~750 to 900 nm but displayed favorable
electron mobility. Embedding them into the QOPVs,
they become highly favorable for improved exciton

separation and charge transport. Note that before
2020, QOPVs were usually made up of two fullerene
derivatives and one NFA. Thus, selecting FA deriva-
tives as the additional components in QOPVs is not
the best approach for obtaining high-efficiency
QOPVs (with efficiencies limited to lower than 14%
at the time).

4.1.2 | D:NFA:NFA:FA QOPVs

NFAs generally exhibit higher absorption than FAs. In
2020, Ma et al.⁶⁸ used a PM6:Y6:Br-ITIC:PC₇₁BM blend
with optimized morphology and distribution of photo-
generated excitons, and gained maximum benefit from
the complementary absorption of the components for
highly efficient QOPV. The PM6:Y6-based BOPVs
yielded a PCE of 15.7%, while the optimized PM6:Y6:
Br-ITIC- and PM6:Y6:PC₇₁BM-based TOPVs exhibited
PCEs of 16.4% and 16.2%, respectively. The optimized
QOPV yielded a PCE of 16.8% among the devices. This is
facilitated by the incorporation of Br-ITIC (NFA) and
PC₇₁BM (FA), respectively, resulting in a simultaneous
enhancement of Jsc at 25.8 mA cm⁻² (compared to
25.5 mA cm⁻² and 25.6 mA cm⁻² of TOPVs) and FF
reaching 76.4% (compared to 75.1% and 75.6% of TOPVs)
due to the optimized phase separation with PC₇₁BM. Jsc
and FF were significantly improved in the representative
TOPVs with fullerene derivatives as the third component.
This phenomenon has been routinely reported in the
literature.³⁹⁻⁴²,⁷⁹,⁸⁰,⁹⁵ The contribution of fullerene deri-
vatives to the performance improvement of TOPVs can
be attributed to higher photon harvesting at shorter
<figure><img src="image_29.png" /><figcaption>FIGURE 7 Characteristic curves of (A) J–V and (B) EQE comparison of PTB7-Th:PC₆₁BM-, PTB7-Th:PC₇₁BM-, and PTB7-Th:IEICO-4F-based OPVs to ternary- and quaternary-based OPVs under 100 mW cm⁻² AM 1.5 G irradiation. Reproduced with permission: Copyright 2020, Elsevier.⁹⁴</figcaption></figure>

wavelengths because FA acts as a photon collector and improved phase separation with a small amount of FA derivative incorporation since FA acts as a morphology regulator.²²,³⁸,⁴⁵,⁸¹,⁹⁶⁻⁹⁹ The photoluminescence results indicated that the emission intensity of Y6 in the blend films monotonically increased with increasing Br-ITIC content, suggesting efficient energy transfer from Br-ITIC to Y6. Also, the decrease in the EQE spectrum for the PM6:Y6-based BOPV in the 650–800 nm wavelength range can be progressively compensated by the steady addition of appropriate amounts of Br-ITIC and PC₇₁BM. The optimized phase separation in the quaternary active layer results in better exciton dissociation and charge transport, yielding improved EQE values in the long-wavelength range. A progressively balanced charge transfer could be achieved by adequately admixing Br-ITIC and PC71BM in the blended films, which is favorable for the stepwise improvement of the FF.

In the same year, Li et al. developed an improved QOPV with a PCE of 17.1% by adding crystalline IDIC and PC₇₁BM as the third and fourth components in an efficient PM6:Y6 binary system.⁷⁰ With the introduction of IDIC into the PM6:Y6 binary host, phase crystallinity has been improved, providing higher hole and electron mobilities. Adding PC₇₁BM as an acceptor guest to PM6:Y6 increased the phase purity¹⁰⁰ and reduced the nonradiative energy loss.¹⁰¹ A further increase in $J_{sc}$ is shown when IDIC and PC₇₁BM were added as acceptor guests to increase the hole and electron mobilities further. The weight ratio effects of the three acceptor components were explored, demonstrating that the increased hole and electron mobilities, accelerated hole transfer, and decreased monomolecular recombination are the factors that contribute to $J_{sc}$ and FF enhancement. The best quaternary device has a PCE of 17.1%

with a $V_{oc}$ of 0.866 V, $J_{sc}$ of 26.19 mA cm⁻², and FF of 75.3% compared to the PM6:Y6:PC₇₁BM ternary and PM6:Y6 binary (with $V_{oc}$ = 0.838 V, $J_{sc}$ = 25.45 mA cm⁻², and FF = 72.3%) devices. Efficiencies exceeding 16% are easily achieved with three acceptors with appropriate weight ratios, illustrating the high efficiency of this quaternary blend system due to the formation of nanoscale film morphologies for all quaternary mixtures. The nanoscale morphology can be attributed to the structural resemblance between Y6 and IDIC and the well-mixed nature of PC₇₁BM and NFAs, benefiting from forming the alloy-like acceptor phase. Also, IDIC loading tends to assist the donor polymer in connecting into a two-dimensional surface morphology, and the acceptor domains are surrounded independently.

Similarly, the enhancement in film morphology approach was utilized by Zhan et al. in 2019, designing QOPVs by including ITIC and PC₇₁BM in the PBDB-T:ITCT host blend. This led to improvement in $V_{oc}$ and $J_{sc}$.⁷³ The morphology studies indicated a slight enhancement of the donor phase crystallinity. The significant change in morphology originates from the observations of the 10–50 nm dark phases, consisting of donor and acceptor components, surrounded by linked donor and acceptor fibers, acting as new centers of light absorption, and charge separation and transport. Remarkably, the admixture of PC₇₁BMs drives the development of an unusual neuron-like morphology via their surrounding donor and acceptor fibers.

As previously mentioned, Xu et al. developed a successful technique relying on a parallel-series working mechanism in triple-acceptor devices.⁷¹ They designed triple-acceptor QOPVs with two parallel electron-transfer routes, one consisting of two series-connected acceptors. The donor polymer PBDTSF-TZNT and three small molecular acceptors,
F8IC:f8OIC:PC71BM, provided complementary absorption, yielding a high EQE response from 1.3 to 3.1 eV greater than 50%. The electrons were independently removed from the donor polymer by F8IC and f8OIC. F8IC forms a highly crystalline nanostructure that efficiently conducts electrons to the cathode. The f8OIC with low mobility and medium band gap also successfully transferred electrons to the cathode through the third acceptor PC71BM. Their unique parallel-series QOPV device structure offers advantages, such as high Jsc and FF, compared to their ternary counterparts. The PBDTSF-TZNT:F8IC:f8OIC: PC71BM QOPVs exhibited the highest PCE at 15.1%, compared to 14.3% of the PBDTSF-TZNT:F8IC:f8OIC ternary devices and 11.4% of the binary PBDTSF-TZNT:F8IC control devices.

This quaternary system highlights the potential of multicomponent devices to boost the functions and yields of OPV devices. It could pave the way for groundbreaking achievements regarding OPV efficiency and stability performance when multicomponent components compatibility is well understood. OPVs are an attractive technology for urban energy generation because they can be printed and integrated into building applications, such as outdoor wall surfaces or glass windows. However, the highly efficient laboratory-based photoactive area is small and usually below 0.1 cm². In addition, marketability is hampered by performance and reproducibility when the cell surface area is increased beyond a cm² scale to m². Sometimes, the D:NFA:NFA:FA configuration can deliver excellent PV performance for small- and large-area PV devices. In 2021, Jia et al. implemented a PM6:Y6:ITIC:PC71BM quaternary blend strategy in this way.⁶⁴ The PV performance of the small-area devices (0.063 cm²) exhibited a maximum PCE of 16.7% for the QOPVs, higher than 16.3% and 15.5%, respectively, for the PM6:Y6:PC71BM ternary and PM6:Y6 binary devices. Integrating PC71BM into the PM6:Y6 binary system significantly increased Jsc and FF. The significant enhancement in PV performance takes advantage of the more balanced charge transport caused mainly by the excellent electron mobility of fullerene.¹⁰² Consequently, for the large-area (19.34 cm²) QOPV modules with the optimal composition, the efficiency reached 13.3%, certified at 12.4%, representing the highest efficiencies among the different OPV modules. From the synergistic effects of these four components, QOPVs have great potential for realizing high-performance small- and large-area OPV devices. The fabricated large-area PV module was used to charge a cell phone under an indoor light-emitting diode (LED) lamp, demonstrating a high potential for commercialization.

### 4.1.3 | D:NFA:NFA:NFA QOPVs

Until recently, no QOPVs containing two or three NFAs were reported in the literature. While this section focuses on D:NFA:NFA:NFA blends, Section 4.2.3 discusses quaternary blends with two NFAs. NFAs generally exhibit better optoelectronic properties than FAs. Building on these advantages, in 2022, Zhan et al. studied QOPV with a single donor material with three different NFA blends, PM6:BTP-eC9:L8-BO:BTP-S10, as shown in Figure 8A, specifically to address thickness tolerance and device area upscaling issues.⁵⁷ Two acceptors, BTP-eC9 and L8-BO, are blended with PM6 donor polymer in a 1:1 (D:A₁ + A₂) ratio, adopted simply through contact angle measurement and wetting coefficient calculation. The wetting coefficient ($ω, ω_{A₂} = (γ_{A₁/A₂} - γ_{D/A₂})/γ_{D/A₁}$) of L8-BO for PM6(D):BTP-eC9(A₁):L8-BO(A₂) blend was calculated to be -0.70, where the $γ$ represents interfacial tensions. The wetting coefficient suggests that the third component (A₂) will percolate the phase of the host acceptor (A₁) when $ω < -1$, percolate the domain of the host donor when $ω > 1$, and place itself at the interface of D and A₁ when $-1 < ω < 1$. Since the calculated coefficient is between -1 and 1, it is inferred that L8-BO resides at the interfaces of PM6 and BTP-eC9, contributing to a parallel-like phase formation. This structure arrangement likely facilitates charge transport channels, thereby enhancing exciton separation and carrier transport. Similarly, the wetting coefficient of BTP-S10 was determined to be -1.31 for the PM6:BTP-eC9:BTP-S10 blend and -1.08 for the quaternary blend. Since values are less than -1, it confirms that BTP-S10 exists within the domain of BTP-eC9 in both ternary and quaternary, thus generating an alloy-like phase and possibly improving the phase purity of PM6 in the quaternary blend. Thus, for a 110-nm-thick small-area PM6:BTP-eC9:L8-BO:BTP-S10 QOPV, they obtained a PCE of 19.3% (certified 19.4%). For the 1 cm² (Figure 8B) and 72 cm² (Figure 8C) large-area QOPVs with the same thickness, the PCEs are around 18.3% and 12.2%, respectively. Finally, they reported an efficiency of 17.8% for a 305-nm-thick QOPV with an area of 0.047 cm².

Similarly, Chen et al. also recently achieved a PCE of 19.8% (certified at 19.4%), using a single donor and three distinct NFA materials.⁶⁰ To the best of our knowledge, this single-junction OPV has the highest efficiency reported for such devices to date. They synthesized and incorporated a combination of miscible NFAs, BTP-S17, with a comparatively wider bandgap, and BTP-S16, with a bandgap closer to that of BTP-eC9, into traditional PM6:BTP-eC9 blend. Figure 8D,E shows the device structure, highlighting the photoactive materials and normalized UV-vis spectra. The initial approach
<figure><img src="image_30.png" /><figcaption>FIGURE 8 (A) Structural schematic of PM6:BTP-eC9:L8-BO:BTP-S10 quaternary blend used on QOPV with J–V curves of the large-area devices with (B) 1.05 cm² and (C) 72.25 cm² device areas. Reproduced with permission: Copyright 2022, Wiley-VCH GmbH.⁵⁷ (D) Current record efficiency QOPV device structure showing PM6, BTP-eC9, BTP-S16, and BTP-S17 as active layer materials and (E) their normalized UV–vis spectra. Reproduced with permission: Copyright 2023, Wiley-VCH GmbH.⁶⁰</figcaption></figure>

involved a ternary system employing both NFAs sepa-
rately. While incorporating each NFA individually led to
a linear enhancement in the Voc of the devices, there was
a trade-off as Jsc decreased in these ternary blends.
Conversely, the FF of the quaternary devices increases
due to optimized carrier dynamics and morphology
alongside reduced recombination. Furthermore, the
quaternary device containing BTP-S16 and BTP-S17 (at
a 3:1 weight ratio) also exhibited a superior balance
between Voc and Jsc, credited to enhanced carrier
separation and mobility with an improved electron
packing in the quaternary system.

## 4.2 | QOPVs based on D₁:D₂:A₁:A₂

4.2.1 | Fullerene QOPVs: D₁:D₂:FA:FA

Nam et al. were the first to introduce a quaternary blend
in OPVs.²⁹ They used a D₁:D₂:A₁:A₂ system with PTB7
and PCDTBT as the donors and PC₇₁BM and PC₆₁BM as
acceptors. They achieved a QOPV with enhanced light
absorption, exciton dissociation rate, and charge trans-
port. The QOPV recorded a moderate PCE of 8.4%, with

a V<sub>oc</sub> of 0.74 V, J<sub>sc</sub> of 16.31 mA cm<sup>-2</sup>, and FF of 70.25%
compared to 8.2% for the PTB7:PCDTBT:PC<sub>71</sub>BM TOPV
and 7.6% for the reference PTB7:PC<sub>71</sub>BM BOPV. The
QOPV showed high J<sub>sc</sub> and FF owing to the broadened
absorption and improved internal photon conversion.
Notably, additional performance improvement can be
achieved through a more elaborate polymer:fullerene
ratio tuning (e.g., PTB7:PCDTBT:PC<sub>71</sub>BM:PC<sub>61</sub>BM with
ratio 0.9:0.1:1.6:0.4), exhibiting a PCE of 8.5%. As shown
in Figure 9A, the QOPV exhibited enhanced EQE
spectra compared to TOPVs and BOPVs. Figure 9B
displays the photocurrent density characteristics at the
effective voltage (J<sub>ph</sub>-V<sub>eff</sub>), which agrees with the EQE
results. Interestingly, the QOPV demonstrated en-
hanced thermal stability. Both experiments and simula-
tions revealed that the PTB7:PCDTBT:PC<sub>71</sub>BM:PC<sub>61</sub>BM
QOPV could maintain over 72% of its PCE after 365 days
at 65°C, as shown in Figure 9C. In contrast, the
reference binary OSC reportedly suffers from a consid-
erable decrease, retaining only 44.6% of initial PCE. The
stability improvement can be attributed to the hindered
domain growth and polymer:fullerene segregation in
the quaternary blend based on the early and long-term
performance decay mechanisms.
<figure><img src="image_31.png" /><figcaption>FIGURE 9 (A) EQE spectra, (B) $J_{ph}-V_{eff}$ characteristics, and (C) thermal stability at 65°C of PTB7:PC71BM binary, PTB7:PCDTBT:PC71BM ternary, and PTB7:PCDTBT:PC71BM:PC61BM quaternary OPVs. The EQE enhancement shows the ratio of ternary or quaternary devices to the PTB7-based binary device. Reproduced under terms of the CC-BY license (http://creativecommons.org/licenses/by/4.0/). Copyright 2017, The Authors, published by Springer Nature.²⁹</figcaption></figure>

### 4.2.2 | $D_1:D_2:NFA:FA$ QOPVs

NFAs are great alternatives to FAs due to the absorption limit of the latter. Thus, to benefit from the complementary absorption of the donors and acceptors, it is advisable to use NFAs, which generally have a stronger absorption than FA. In 2020, Ma's group developed a QOPV consisting of PTB7-Th:FOIC, with a highly crystalline $PC_{71}$BM, commonly used as the third component, and DR3TBDTT, a small molecule donor with a large bandgap as the fourth component.³⁰ This proposed design purposefully seeks to enhance the acceptor and donor phases in the thin film. The FOIC crystallites are isolated and suppressed in the PTB7-Th:FOIC blend. The ternary blend with $PC_{71}$BM showed reasonable phase separation due to the mixing between FOIC and $PC_{71}$BM. The FOIC refined “head-to-head” molecular packing upon adding $PC_{71}$BM along the backbone is due to their better intermolecular interactions. This morphological evolution best illustrates the unique gain of the guest modifier, $PC_{71}$BM. The donor’s crystallinity increased when DR3TBDTT was incorporated due to its high crystallinity nature and mixing compatibility with PTB7-Th. The modified morphology accelerated exciton dissociation and transfer of charge carriers with reduced recombination, leading to 16% PCE enhancement for QOPVs. Transient absorption spectroscopy measurements showed that incorporating $PC_{71}$BM and DR3TBDTT into the PTB7-Th:FOIC blend led to rapid polaron growth and a longer lifetime to increase charge transport efficiency by reducing recombination. The sequential improvement of the acceptor and donor nanostructures to realize better TOPVs and QOPVs yielded higher $J_{sc}$ (measured via EQE) and FF. The successful implementation of the PTB7-Th:FOIC:PC71BM:DR3TBDTT-based quaternary blend morphology for highly efficient OPVs allowed further morphology-oriented investigation into quaternary systems. The study also shows

a strategy for modified morphology at multiple scales using $PC_{71}$BM as a molecular adjuster for promoting better molecular packing (also achievable via end group substitution on molecules¹⁰³) and as a phase modifier. It also emphasizes how organic small molecules can further enhance the crystallinity of the polymer domains. Such a hierarchically modified morphology facilitates exciton splitting, controlled carrier transfer, increased carrier transport, and limited recombination.

Similarly, Arunagiri et al.⁶⁷ reported highly efficient QOPVs with a photoactive layer featuring two polymer donors, PM6 and PTQ10, and two acceptors, N3 and $PC_{71}$BM. The morphological investigation revealed that PTQ10 and $PC_{71}$BM act as separators between PM6 and N3 (Figure 10A), forming a “rivers and streams” type morphology. The multilength scale morphology helped to balance the previously unbalanced charge transport, implying enhanced internal and external quantum efficiencies and, thus, higher $J_{sc}$ (Figure 10B). The PM6:PTQ10:PC71BM:N3 QOPV attained an efficiency of 17.7%, significantly higher than that of the binary and ternary control systems. The QOPV showed significant $J_{sc}$ enhancement without compromising $V_{oc}$ and FF, demonstrating a synergistic interaction between the four functional components. The quaternary device demonstrated the longest charge-carrier lifetime of 0.377 ms (Figure 10C). Morphology analysis also revealed how $PC_{71}$BM is an essential component, facilitating functional multilength scale morphology. Figure 10D–G displays the GIWAXS measurements of the PM6:N3, PTQ10:N3, PM6:PTQ10:N3, and PM6:PTQ10:PC71BM:N3 blend films. Incorporating PM6 and PTQ10 into the ternary and quaternary systems showed little difference in the two-dimensional (2D) patterns, suggesting that the molecular orientation of the blended films remained unaffected. For the PM6:N3
<figure><img src="image_32.png" /><figcaption>FIGURE 10 (A) Energy level diagram with the proper spatial distribution of PTQ10 and PC<sub>71</sub>BM, the holes confined in PM6 and electrons in N3 cannot recombine. (B) *J*-*V* curves of the OPVs under AM 1.5 G illumination, 100 mW cm<sup>-2</sup>. (C) Transient photovoltage measurement. (D) GIWAXS 2D pattern of PM6:N3. (E) GIWAXS 2D pattern of PTQ10:N3. (F) GIWAXS 2D pattern of PM6:PTQ10:N3 (ternary). (G) GIWAXS 2D pattern of PM6:PTQ10:PC<sub>71</sub>BM:N3 (quaternary). Reproduced with permission: Copyright 2020, Elsevier Inc.⁶⁷</figcaption></figure>

binary blend, the corresponding coherence length ($L_c$) value for the (010) peak at ~1.72 Å<sup>-1</sup>, related to PM6, was 2 nm, and that for the $\pi$-$\pi$ stacking peak at ~1.79 Å<sup>-1</sup>, related to N3, was found at 5.7 nm. In the binary blend PTQ10:N3, the $L_c$ value for the (010) peak at ~1.80 Å<sup>-1</sup> is 3.7 nm. With the supplementary addition of PC<sub>71</sub>BM to form the quaternary system, the $L_c$ of the $\pi$-$\pi$ stacking peak was at ~1.72 Å<sup>-1</sup>, matching PM6, while that at ~1.79 Å<sup>-1</sup> is kept unaltered, implying similar molecular packing with the PM6:PTQ10:N3 ternary system.

In early 2021, Zhang et al.<sup>66</sup> conceived a monolayer QOPV with double cascade charge transport pathways with a PCE of 18% (certified to 17.35%), consisting of PM6:Y6 blend with additional PM7 donor and PC<sub>71</sub>BM acceptor components. The stepwise alignment of the energy levels is beneficial for cascade charge hopping pathways for high $J_{sc}$ and induces several charge transfer energies for improved $V_{oc}$. On the one hand, PM7 with a deeper HOMO<sup>104</sup> has a similar chemical structure to PM6 and is miscible. Their addition only improves cascaded energy level alignment without disturbing the established morphology. On the other hand, PC<sub>71</sub>BM, with a slightly higher LUMO than Y6, is compatible with PM6:Y6 and, as a blend, promotes cascaded energy level alignment as well. Having PC<sub>71</sub>BM in the acceptor mixture also helps improve $V_{oc}$. PM7 lowers the driving force of charge separation and favorably interacts with Y6, adjusting its crystallization to accommodate carrier transport better. The PM6:PM7:Y6:PC<sub>71</sub>BM QOPVs achieved

a maximum enhanced PCE of 18.07% due to the synergistic effect between components, with excellent stability (81% PCE after 1000 h of illumination; Figure 11A), about 5% higher than that of PM6:Y6 BOPVs. These findings highlight the electronic structure manipulation in thin films with concurrent morphology preservation, paving the way to high-performance OPVs. The donor and acceptor chemical structures provide a pathway to control the electronic structure and energy levels of the charge transfer state, allowing manipulation of hole transfer rates, nonradiative recombination losses, and carrier transport.

Recently, Kang et al.<sup>26</sup> reported high-performance QOPVs using two miscible donor molecules based on a novel synthesized polymer guest donor, L20 and PM6, and two acceptors, Y6 and PC<sub>71</sub>BM. The device architecture, enabling suppression of voltage loss in QOPV,<sup>26</sup> is ITO/PEDOT:PSS/active layer/PNDIT-F3N/Ag with L20:PM6:Y6:PC<sub>71</sub>BM blend as the active layer. The PV performance of the PM6:Y6 BOPV exhibited a PCE of 15.9%, while the PM6:L20:Y6 TOPV improved to 16.4%. The PM6:L20:Y6:PC<sub>71</sub>BM quaternary device yielded the best PCE at 16.9%, simultaneously increasing all device parameters ($V_{oc}$, $J_{sc}$, and FF). Morphological studies revealed that combining the L20 and PM6 donors with the optimal composition gave them a “network-like” morphology that favors hole transport. The co-presence of L20 and PC<sub>71</sub>BM is important to accomplish the desired hierarchical morphology. These virtues positively attenuate charge recombination and make
<figure><img src="image_33.png" /><figcaption>FIGURE 11 (A) Normalized J–V parameters of PM6:Y6 binary and PM6:PM7:Y6:PC71BM quaternary devices for 1000 h under illumination (1 sun). Reproduced under terms of the CC-BY license (http://creativecommons.org/licenses/by/4.0/). Copyright 2023, The Authors, published by Springer Nature.⁶⁶ (B) Normalized PCEs of PM6:BTP-eC9-based binary, PM6:BTP-eC9:L8-BO:PY-IT-based, and PM6:PB2F:BTP-eC9:L8-BO-based quaternary OPVs with different solvent treatments under illumination of a 100 mW cm⁻² white LED at 50–55°C. Reproduced with permission: Copyright 2023, The Royal Society of Chemistry.⁶¹</figcaption></figure>

charge collection easier in QOPVs, ascribed to improved FF in the device. Conveniently, the demonstrated PM6:L20: Y6:PC71BM QOPV has reduced voltage/energy loss relative to the corresponding binary and ternary devices based on decreased radiative and nonradiative recombination losses below the bandgap. Such properties are associated with the synergies of the modified energetics and film morphology of the quaternary blends.

Understanding how the guest components regulate the active layer structures is key to successful devices, even though a suitable selection of material systems remains challenging. Zhang et al.⁶⁹ synthesized two phthalimide-based ultrawide bandgap polymer donor guests, Phi-Se and Phi-Th, to be blended with PM6, Y6 NFA, and PC71BM FA. They fabricated parallel-type ternary and QOPVs with improved overall efficiencies. The presence of the individual Phi-Se phases improves light absorption and provides further paths for charge separation and hole transport. Also, after adding Phi-Se, they noticed a selective improvement in the electron mobility and structural order of the Y6 phase, resulting in better electron transport. The efficient charge transport, in combination with the HOMO levels

near the host and guest polymers, enables the PM6:Y6 and Phi-Se:Y6 subcells to operate synergistically by reducing the influences on the charge generation and transport of the host system, which subsequently contributes to boosting the current generation. A 17.2%-efficient Phi-Se:PM6:Y6:PC71BM quaternary device is obtained compared to a PCE of 16.7% and 15.6% for the PM6:Y6:PC71BM and Phi-Th:PM6:Y6 ternary devices, respectively. Lastly, they propose that phthalimide-based polymers may be promising candidates that offer simultaneously enhanced Voc and Jsc through fine-tuning of energy levels and the formation of additional pathways to improve current generation in parallel-type multicomponent OPVs.

These results indicated that with careful material selection and morphology control, quaternary blends with charge carriers inside a functional and unique hierarchical multilength scale morphology enable more balanced charge transport and minimized recombination, and, hence, better performance. Moreover, QOPV technology based on this type of “rivers and streams” morphology can provide unprecedented prospects for high-efficiency OPVs.
### 4.2.3 | Nonfullerene QOPVs: $D_1:D_2:NFA:NFA$

Many studies have proposed the ternary strategy to address the BHJ BOPV device limitations, leading to increased $V_{oc}$ values with improved FFs, typically by employing a higher LUMO acceptor guest.<sup>105-111</sup> However, the increase in $V_{oc}$ and $J_{sc}$ using NFA as an acceptor guest remained challenging. As shown in Table 1, the quaternary approach is identified as an alternative method to gain a simultaneous increase in $V_{oc}$ and $J_{sc}$, compared to the ternary approach. The fourth component in the active layer combines two components to synergistically modulate the complex photon-electron conversion mechanisms involved in OPVs. Until 2019, no QOPVs containing two NFA were reported in the literature. The dearth of QOPVs with two NFA may be related to (i) the late breakthrough of the NFAs as viable components in BOPV in 2015 and its slow rise into ternary or multicomponent OPV, and/or (ii) a lack of guidance concerning selecting materials to achieve for a good four-material blending configuration. For instance, in a $D_1:D_2:A_1:A_2$ QOPV system, if $A_1$ blends appropriately with a donor or phase separates because of poor compatibility with $A_2$, then the discontinuous charge transport pathways can drive severe nongerminated recombination toward a low FF. Hence, to obtain high FF and $J_{sc}$, the two donors must form phase-separated crystals with preferred front orientation, and the two acceptors must alloy to provide both effective charge separation and continuous charge transport channels.

Bi et al.<sup>74</sup> have developed a $D_1:D_2:NFA:NFA$ QOPV with an optimized PBDB-T:PTB7-Th:ITIC:FOIC blend morphology. It exhibited both parallel-like morphological configurations for the donor materials and alloy-like configurations for the acceptor materials, referred to as the parallel alloying QOPV model. The optimized QOPV blend boosts the advantage of a photoactive film with enhanced charge transport and separation features, leading to better FF and $J_{sc}$ over binary and ternary control devices. Adding PBDB-T to PTB7-Th:FOIC host blend forces a parallel bond between the donors and enhances the crystallinity of FOIC with an improvement in face-on orientation. Later, the addition of ITIC into the PBDB-T:PTB7-Th:FOIC ternary blend leads to domain size reduction and optimizes the discontinuous FOIC crystallite due to the creation of ITIC:FOIC alloy with a PCE of 12.5% (6% higher than that of the PBDB-T:PTB7-Th:FOIC QOPV and 12% higher than that of the PTB7-Th:FOIC BOPV).

In general, many OPV blends, including the ones discussed so far, use halogenated organic solvents, which are often toxic. The literature reported only a few works

with OPV blends processed from nonhalogenated or green solvents as they suffer from unbalanced charges mobilities and low $V_{oc}$, affecting their overall performance. However, Tan et al.<sup>62</sup> recently reported a nonhalogenated solvent-treated quaternary blend OPV with a PCE above 17.0%. They selected PhI-Se as a guest component of PM6:BTP-BO-4F to extend its potential to regulate the fluorinated phase of BTP-BO-4F selectively by sequencing and forming an individual donor phase different from PM6, thereby improving and balancing the charge mobilities. They carried out atomic force microscopy measurements for insights into the influence of PhI-Se and BTP addition on blend film morphology. The results indicated that the root-mean-square (rms) roughness value decreased progressively from 1.70 nm for the PM6:BTP-BO-4F binary blend to 1.26 nm for the quaternary blend (after adding both BTP and PhI-Se). A relatively smooth surface with rms of 1.29 and 1.28 nm was observed in the BTP and PhI-Se ternary mixtures, respectively. The smoother surface suppresses charge recombination and increases interfacial contact, leading to better $J_{sc}$ and FF. BTP and PhI-Se have been used as cooperative guest components of PM6:BTP-BO-4F to synergistically align the energy levels and modulate the film morphology by smoothing the surface. This combination provides a unique quaternary system consisting of (i) a polymer donor guest with a deeper HOMO higher than the host acceptor HOMO, (ii) an acceptor guest with an up-shifted HOMO with its level near to that of the host polymer donor, and (iii) a PM6:BTP-BO-4F binary host system. The host active layer treated with chlorobenzene showed an excellent device PCE of 18.2% with a $V_{oc}$ of 0.865 V, a $J_{sc}$ of 27.4 mA cm<sup>-2</sup>, and an FF of 76.6% compared to a toluene-treated quaternary device displaying a PCE of 13.1% with $V_{oc}$ = 0.812 V, $J_{sc}$ = 24.4 mA cm<sup>-2</sup>, and FF = 66.1%. An increase in $V_{oc}$ and FF can be observed in both ternary devices when adding BTP and PhI-Se as a third component. Comparatively, adding more BTP enhances the voltage, while PhI-Se further increases the FF. Eventually, a PCE higher than 15% is obtained. Inserting BTP and PhI-Se into the PM6:BTP-BO-4F binary system decreases recombination even more. Also, it increases hole and electron mobilities, which improves FF (to 76.7%) and $J_{sc}$ (to 26.1 mA cm<sup>-2</sup>) in combination with a boost of $V_{oc}$ (to 0.851 V), yielding a PCE of 17.0%. The guest donor polymer, PhI-Se, constitutes an individual phase besides the host donor PM6. With these characteristics in mind, upon adding BTP and PhI-Se to the host PM6:BTP-BO-4F blend of the BOPV, $J_{sc}$, $V_{oc}$, and FF are all boosted, thus improving the overall performance of the QOPV.

Early in 2023, Xu et al.<sup>59</sup> engineered a high-performance QOPV composed of two effective D:A pairs,
D18-Cl:BTP-eC9 and PM6:L8-BO, using a sequential deposition approach to form the double-BHJ D18-Cl:BTP-eC9/PM6:L8-BO and D18-Cl:PM6:L8-BO:BTP-eC9 blend OPVs. The double-BHJ D18-Cl:BTP-eC9/PM6:L8-BO OPV showed a dramatic enhancement of light absorption, as well as the preservation of optimal morphology without requiring the challenging morphological optimization in the traditional D18-Cl:PM6:L8-BO:BTP-eC9 quaternary blend. By logically matching each donor with an appropriate acceptor, they found that energy loss is reduced in the double BHJ devices, in contrast to the quaternary blend devices. As a result, the PCE is enhanced from 18.3% and 18.7% for D18-Cl:BTP-eC9 and PM6:L8-BO binary blend OPVs, respectively, to 19.6% for the double BHJ OPVs. However, the traditional D18-Cl:PM6:L8-BO:BTP-eC9 QOPV had a significantly reduced PCE of 15.8%. This finding highlights the inherent difficulty in achieving an optimized multicomponent blend in this quaternary for high-performance devices. This study has proven that multicomponent OPV effectiveness can be considerably boosted thanks to a double-BHJ of suitable binary blend composition processed via a relatively straightforward treatment procedure.

The double-BHJ quaternary blend strategy has also been proposed as an effective approach to suppress the nonradiative voltage loss (ΔV<sub>nr</sub>) and improve the charge transport in OPVs. Here, research by Li et al.<sup>63</sup> showed that an efficiency of 18.2% can be achieved in OPVs by applying the layer-by-layer method to produce a quasi-QOPV device. The quaternary blend is formed by sequentially depositing the PM6:PM7 blend as the donor layer and the Y6:O1-2F blend as the acceptor layer. Adding PM7 creates a cascade energy level alignment along with PM6, promoting effective charge transfer. O1-2F is a small molecule acceptor with low miscibility with donor materials and is inserted as the second acceptor, which can form an ideal vertical phase separation by impeding overmixing between the donor and acceptor in the transition layer, thus inhibiting charge recombination and enhancing charge transport and collection.<sup>63</sup> In addition, the acceptor alloys of O1-2F and Y6 optimize the active layer morphology thanks to the crystallization behavior observed in the acceptor alloy, forming an efficient charge separation and a continuous charge transport channel. Consequently, the PM6:Y6 binary layer device with a PCE of 17.0% and ΔV<sub>nr</sub> of 0.246 V underperformed compared to the optimized PM6:PM7/Y6:O1-2F QOPV with a maximum PCE of 18.2% and a reduced ΔV<sub>nr</sub> of 0.231 V.

Zhang et al.<sup>61</sup> also fabricated a nonfullerene high-
performance QOPV by employing two polymer donors,
PM6 and PB2F, two small-molecule NFAs, BTP-eC9 and

L8-BO, and one polymer NFA, PY-IT. PM6:BTP-eC9 OPVs exhibit exciton trapping during photothermal aging. Their research proposes using the QOPVs to suppress this effect and understand the basic correlation between diffusion and annihilation dynamics of an exciton and its coupling interactions with phonons. They demonstrated a “charge bridge” concept by adding PY-IT or PB2F as the fourth component in the PM6:BTP-eC9:L8-BO ternary blend. The idea is to dredge excitons and charge carriers, enabling exciton-phonon decoupling in QOPVs. Compared to ternary (18.4%) and binary (5.3%-18.1%) OPVs, both types of QOPVs fabricated with halogenated, chloroform (18.8%), and nonhalogenated, *ortho*-xylene (*o*-XY) (18.6%) solvents exhibited enhanced device efficiency and photothermal stability. These enhancements from 53.4% to 71.7% of initial PCE over 500 h, particularly for photothermal stability, as shown in Figure 11B, are primarily driven by the suppression of exciton and charge carrier trapping, ascribed to the generation of trap states and a slight increase in energy disorders. Thanks to the “charge bridge” approach, the influence of exciton-phonon coupling on exciton transport can be effectively attenuated, paving the way for improvements in the efficiency and stability of state-of-the-art nonfullerene multicomponent OPVs.

4.3 | D₁:D₂:D₃:A QOPVs

Studies on QOPV using multipolymer donors and fullerene or nonfullerene derivatives with a D₁:D₂:D₃:A configuration are still relatively rare in the literature to date. Indeed, the main challenge lies in assembling three polymer donors because such a configuration requires the following conditions to be satisfied: (i) choice of a solvent compatible with each polymer donor, (ii) compatibility of their physical properties (including crystallinity and molecular orientation), and (iii) similarity of their chemical properties, without omitting (iv) other aspects related to their impact on the optoelectronic properties or the blend morphology, which is uncommonly complicated to control.

However, in 2015, Yang et al.<sup>112</sup> reported the fabrication of high-performance multidonor BHJ PV cells. They investigated several dual and multidonor BHJ OPVs from a pool of materials with different absorption ranges and desired molecular structures. Their strategy for improving the performance of the multidonor systems involves optimizing the compatibility of the individual donor materials, enabling them to operate more as independent cells by considering both molecular compatibility and absorption characteristics. They found that additional absorption does not
necessarily yield better performance for PV devices. Only two successful TOPVs have been realized, and the best device achieved a PCE of 8.7% compared to the multipolymer:fullerene and host binary devices. Despite the poor performance exhibited by the multidonor systems, they have successfully demonstrated a correlation between device performance and the molecular structure of the donor materials. Their results conceptually indicate that blending two or even more donor materials in the active layer is feasible only if they have sufficient structural compatibility.

To bridge the FF reduction issue in multicomponent OPVs, Lee et al.<sup>113</sup> applied a new method based on matching HOMO energy levels of donor polymers to fabricate multicomponent OPVs (ternary and quaternary). The fabricated PTB7-Th:PBDT-DPPD:PC<sub>71</sub>BM TOPVs exhibited the highest PCE of 10.42% with a V<sub>oc</sub> of 0.80 V, J<sub>sc</sub> of 17.61 mA cm<sup>-2</sup>, and FF of 74%. The PCE improvement is primarily attributed to the effective charge transport because of similar HOMO energy levels of PTB7-Th and PBDT-DPPD (Figure 12A).

To further substantiate their findings, they implemented the same approach in designing a QOPV using three similar HOMO energy levels as donor polymers, PBDT-DPPD, YJ-55, and PBDTT-PDBT, and PC<sub>71</sub>BM as an acceptor, as shown in Figure 12A. The DPPD:YJ-55:PBDTPDBT:PC<sub>71</sub>BM QOPV provides the highest values for FF and PCE (Figure 12B), with a PCE of 6.04%, which is higher than that of PBDT-DPPD:PC<sub>71</sub>BM (~5.6%), PBDTT-PDBT:PC<sub>71</sub>BM (5.7%), and YJ-55:PC<sub>71</sub>BM (5.7%) BOPVs. The enhanced FF value in QOPV suggests efficient charge transport in the quaternary active layers. Their results strongly hint that efficient charge transport occurred in multicomponent photoactive layer OPVs with similar HOMO-level polymers. Furthermore, they successfully demonstrated

that matching HOMO values in ternary or quaternary blends can mitigate trap sites between two donor polymers interface, resulting in efficient charge transport without sacrificing the optimized morphology.

Three donor polymers and one acceptor QOPVs perform less than other QOPVs or ternary blend OPVs obtained from two donor polymers and one acceptor. This may be due to the unfavorable interactions between the polymers due to their different chemical and physical natures.

## 4.4 | QOPVs with dye molecules

Dye molecules are also extensively used as an additional component in OPVs to widen light absorption, thanks to the high absorption coefficient and tunable band gap, particularly in the NIR region. In mixtures with polymer:dye multidonor components, the dye molecules are mostly positioned at the primary donor and acceptor interface.<sup>65,114,115</sup> However, designing and mixing multiple donors successfully and minimizing unfavorable interactions related to morphological traps is challenging. Hence, using dye molecules as a third<sup>115</sup> or fourth component<sup>28,116,117</sup> typically does not disturb the original morphology of binary or ternary blends and forms a cascade structure for better charge transfer.

In 2016, Goh et al.<sup>28</sup> reported comprehensive studies of co-crystalline squaraines (SQs) blend QOPV with a PCE exceeding 10%. Their previous works<sup>118,119</sup> demonstrated the effectiveness of Förster resonance energy transfer (FRET) to reduce recombination and increase exciton harvesting in a ternary small molecule-polymer system, namely, SQ in combination with P3HT:PC<sub>61</sub>BM exhibiting a PCE of 4.51%. The FRET-enhanced ternary small molecule devices composed of asymmetric SQ

<figure><img src="image_34.png" /><figcaption>FIGURE 12 (A) Example of a triple-donor quaternary OPV blend energy-level diagram showing PBDTT-PDBT, YJ-55, and PC71BM. (B) Table showing the PV parameters of the QOPVs with an ITO/PEDOT:PSS/PBDT-DPPD:YJ-55:PBDTT-PDBT:PC<sub>71</sub>BM/Al structure. Reproduced with permission: Copyright 2018, Wiley-VCH GmbH.<sup>113</sup></figcaption></figure>
(ASSQ), SQ, and a fullerene, C₆₀, yielded a PCE of 6.2%. Next, adding small amounts of ASSQ and DPSQ (2,4-bis [4-(N,N-diphenylamino)-2,6 dihydroxyphenyl] squaraine) into the active layer of PTB7:PC₇₁BM, PTB7-Th:PC₇₁BM, and PT8:PC₆₁BM blends results in PCE increases of 33%, 19%, and 30%, respectively, with significant enhancements in both Jₛₙ and FF. For instance, the ASSQ:DPSQ:PTB7:PC₇₁BM QOPV reached a PCE of 10.7%, surpassing both the ternary and binary counterparts with PCEs of 8.9% and 7.17%, respectively. The PTB7:PC₇₁BM-based QOPVs also demonstrate an enhanced active layer morphology characterized by smaller domain sizes, denser π-π stacking, preferred orientation, and improved charge transport mobility. The observations from GIWAXS revealed that ASSQ and DPSQ form a co-crystalline phase in QOPV. Ultrafast transient absorption spectroscopy indicated that using multiple FRET pairs results in fast and successful energy transfer.

Ta and co-workers¹¹⁶ designed a novel PM6:Y6:SR197: PC₇₁BM QOPV using a hydrogen bonding strategy for the directional distribution of the photoactive layer materials. The SR197 N-H bonds interact with the F atom of Y6 to generate intermolecular hydrogen bonds, controlling the blend morphology and inhibiting bimolecular recombination. Therefore, quaternary devices fabricated by mixing PC₇₁BM and SR197 in the PM6:Y6 host blend resulted in a good performance with a maximum PCE of 17.5% compared to 16.8% and 16.7% for PM6:Y6:SR197 and PM6:Y6:PC₇₁BM ternary OPVs, respectively, and 15.6% for the binary devices. Y6 molecule contains F atoms with a high electron absorption capacity; thus, aggregation of the molecules is possible. The phenomenon of aggregation of the molecules was enhanced, with the formation of nanofibril structure when SR197 or PC₇₁BM was added. The material distribution in the binary and quaternary active layers can be simulated, as illustrated in Figure 13A,B. Introducing the SR197 and PC71BM mixture into the PM6:Y6 blend showed the best morphology for the quaternary co-blended film, thus favorable for

exciton dissociation and charge transport. The QOPVs showed the highest current value and FF. This is because the N-H group on SR197 can form intermolecular interactions with Y6 and PC₇₁BM, promoting better miscibility of acceptors and modulating the morphology film, forming a continuous pathway for charge transport.

Dye molecules can also be used as promising photosensitizers for collecting light in the NIR region. Ke et al.¹¹⁷ developed a new silicon naphthalocyanine (SiNC-1) photosensitizer in a P3HT:dye:PC₆₁BM TOPV. They also explored multicolor co-sensitized QOPVs, embedding a silicon phthalocyanine (SiPC-0 or SiPC-1) as a fourth material to complement SiNC-1. UV-vis and EQE measurements revealed that SiNC-1 offers significant photosensitivity in the NIR region (650-950 nm), yielding a 10% and 24% enhancement in Jₛₙ and PCE, respectively. The additional spectral absorption in the NIR of the EQE spectra shows that both dyes contributed to exciton generation. QOPVs with 7.5 wt% of SiPC and 5-7.5 wt% of SiNC-1 revealed better PCE than the TOPVs. AFM and transmission electron microscopy analyses indicated that even with a dye loading of 15-20 wt%, the morphology remained undisturbed for ternary and quaternary blends. Therefore, embedding dyes as a third and/or fourth material is a highly effective way to achieve high performance in OPVs. This versatile and facile approach could be implemented to enhance light harvesting efficiently, thus boosting the PCE and enabling easy access to already optimum morphology OPV systems.

## 4.5 | QOPVs for semitransparent and indoor applications

Semitransparent OPVs (ST-OPVs) are regarded as one of the most promising PV technologies.¹²⁰⁻¹²³ They play a critical role in IoT (Internet of Things) to the building- or vehicle-integrated solar power harvesting owing to their flexibility, lightweight, and transparency.¹²²,¹²⁴⁻¹²⁶ However, the

<figure><img src="image_35.png" /><figcaption>FIGURE 13 Schematic diagrams. Characteristic morphology of the blend films of (A) PM6:Y6-based BOPVs and (B) PM6:Y6:SR197:PC₇₁BM-based QOPVs. Reproduced with permission: Copyright 2021, The Royal Society of Chemistry and the Chinese Chemical Society.¹¹⁶</figcaption></figure>
<figure><img src="image_36.png" /><figcaption>FIGURE 14 Demonstration of 4T tandem PVs with the ST Q-OPV. (A) Schematic diagram of 4T tandem system based on the ST-QOPV top cell and NIR-responsive bottom cell. (B) ST-QOPV color-coded cover on NIR-sensitive module for commercial PVs. Reproduced with permission: Copyright 2019, Elsevier Ltd.¹²⁷ (C) Small electrical devices can be charged under dim lighting conditions using IOPVs. Reproduced with permission: Copyright 2019, Elsevier.¹²⁹ (D) Emission spectrum for the AM1.5 G and indoor light sources (fluorescent, LED, and halogen lamps). Reproduced with permission: Copyright 2019, Elsevier Ltd.¹³⁰</figcaption></figure>

trade-off between PCE and average visible transmission is a major issue for high-performance ST-OPVs. The efficiency of ST-OPVs is lower than that of opaque OPVs, and additional efforts to raise the efficiency while maintaining both transparency and color codability are needed for their commercial viability and marketability.

To this end, Ko et al.¹²⁷ designed inverted ST-QOPVs. They introduced an additional polymer donor, PBDB-T, and an acceptor, ITIC-Th, into the PTB7-Th:PC₇₁BM binary blend for an enhanced spectral response and a smoothened charge transport process. Interestingly, PBDB-T can contribute to additional photocurrent generation via complementary absorption at visible wavelengths and energy transfer to PTB7-Th.¹²⁸ At the same time, ITIC-Th provides efficient charge transport throughout the device by forming a cascade band alignment. ST-QOPVs offer high transparency and color tunability. For a realistic application, a four-terminal (4T) tandem PV was designed in which the color-coded ST-QOPVs covered an NIR PV cell. These sensitive OPVs (an add-on module for commercial PVs) allow for better photon harvesting and fulfill aesthetic features. The ST-QOPVs can cover all previously installed opaque PVs to form the 4T tandem configuration, as shown in

Figure 14A,B. The advantages of the quaternary blend platform include (i) successful operation under all irradiation circumstances (indoor and outdoor lighting), (ii) device color codability through adjustment of quaternary components, and (iii) the potential to extend ST-QOPVs to several practical applications (e.g., smart windows, building-integrated PVs, and portable electronics).

To ensure that ST-OPVs can be used proficiently, the design must consider versatile irradiation situations in addition to sunlight. It needs to supply electrical power with no sunlight or when operating indoors. Nam et al.¹²⁷ developed a new ST-QOPV based on the previous configuration, which is effective for both sunlight and artificial light irradiation, displaying the use of power generators in versatile irradiation situations.¹³¹ The key characteristics of the QOPV, such as extended spectral response and enhanced charge transport with reduced recombination losses, yielded PCEs of 9.5% under 1.5 G AM illumination and 16.3% under a 1000 lx LED. The new ST-QOPV would be adequate to operate low-power electronic devices independently. The quaternary strategy in OPVs is not limited to semitransparency but also indoor applications in general, which are discussed next.
As indoor electronic devices for the IoT are developed, researchers are driven to design PV devices that can efficiently convert indoor light into electricity. OPVs have demonstrated high potential for indoor applications due to outstanding properties, including low-cost solution process, low current leakage under low illumination, high absorption coefficient,¹³² and possibly better reliability and operational lifetimes.¹³³ The state-of-the-art indoor OPV (IOPV) illuminated with an LED lamp at an intensity in the range of 100–1700 lx yields a PCE of over 31%.¹³² Nevertheless, OPVs require additional improvements, such as improving their low light absorption and exploiting suitable bandgap materials, to leverage indoor lighting conditions fully. Based on these merits, IOPV is a preferential power generation or supplement source able to harvest energy from indoor light (Figure 14C), which is critical now and for future applications. The emission spectrum and irradiance intensities of indoor light sources are distinct from that of the AM1.5 G light source, as shown in Figure 14D. Again, multicomponent PV cells have demonstrated their promise for IOPV.

In 2019, Vincent et al.¹³⁴ addressed the optimization of QOPV design for indoor light harvesting and highlighted the key role of quaternary blends in indoor OPV. A finite difference time domain-based simulation has been employed to determine the number of photons absorbed in the active layer of the PCDTBT: PTB7:PC₆₁BM:PC₇₁BM QOPV under AM1.5 G and 500 lx white LED illumination. The optical simulations revealed that the absorption inside of the QOPVs depended greatly on the thickness of the active layer when illuminated under LED light because of the high oscillations in *J*<sub>sc,ideal</sub>. The maximum *J*<sub>sc,ideal</sub> occurs at 260 nm active layer thickness under AM1.5 G illumination and at 230 nm thickness under 500 lx indoor LED illumination. The simulation of the electric field intensity inside the QOPV showed that the electric field intensity is very sensitive to the active layer thickness compared to the reference cells. This occurred due to the interference between the incident light and the light reflected from the back electrode. Also, the indoor QOPVs achieved some characteristics of light absorption from their individual donors, thus leading to both cascade and parallel-type charge transfer between donors and acceptors, with PTB7 working mainly as a cascade energy level for enhanced hole extraction to the cathode. Similarly, Shin et al.¹³⁵ demonstrated panchromatic absorption in indoor OPVs using a quaternary system. The finite difference time domain simulation indicated that the PCDTBT:PTB7:PC₆₁BM:PC₇₁BM QOPV improved the PCE of indoor OPVs up to 10.6% under 500 lx LED illumination and broadened the OPV absorption window. The increase in PCE is driven by enhanced charge

transport properties, resulting in a better FF. The experimental *J*<sub>sc</sub> and the simulated *J*<sub>sc,ideal</sub> were very similar, resulting in high internal quantum efficiency close to approximately 100% and extremely high shunt resistances.

Nam et al.¹³⁶ recently investigated a quaternary BHJ active layer for high-performance indoor OPV applications via sequential deposition. The sequential method involves spin coating the first blend solution D<sub>1</sub>:A<sub>1</sub> (PTB7-Th:ITIC-Th) onto a ZnO-coated ITO substrate. Then, the second blend solution, D<sub>2</sub>:A<sub>2</sub> (PM6:PC<sub>71</sub>BM), was spin-coated on the first blend layer, as schematically illustrated in Figure 15A. The sequential QOPVs demonstrate efficient visible light absorption with no significant trade-off in component spectral response. Under 3000 K LED irradiation, the BOPV displayed a PCE of about 20% (Figure 15B). At the same time, the sequential QOPV took advantage of the simultaneously enhanced PV parameters, leading to a higher PCE of around 25% (Figure 15C). The enhancement in PCE was attributed to the tailored absorption and mitigated charge recombination loss. Again, the sequential deposition method has the potential to leverage the opportunities afforded by the multiple materials and their synergistic advantages and be a successful platform to facilitate the fabrication process of QOPVs for indoor and outdoor applications.

The outstanding features of QOPV, such as semi-transparency, color tunability, and blend thickness tolerance, coupled with high efficiency and thermal durability, could provide new possibilities for realistic and versatile applications that can be effectively used under multipurpose irradiation conditions even in the absence of sunlight or under indoor applications. However, the difficulty lies in the intricate complexity of the multicomponent system itself: (i) selection of the components, (ii) ensuring compatibility (structural, morphological, and energy level) among them, and (iii) with solvents and deposition methods, and (iv) the current lack of understanding of the full working mechanisms of multicomponent OPVs, the least being QOPVs.

## 4.6 | Quaternary organic blends: Applications beyond PVs

Beyond OPVs, multicomponent systems have been employed in other organic semiconductor technologies, including organic OLEDs,¹³⁷,¹³⁸ organic photodetectors (OPDs),¹³⁹⁻¹⁴¹ and organic field effect transistors (OFETs).¹⁴²,¹⁴³ Tanaka et al. investigated thermally activated delayed fluorescence (TADF) OLEDs. The
<figure><img src="image_37.png" /><figcaption>FIGURE 15 (A) Schematic diagram of the sequential deposition process. The D₂:A₂ (PM6:PC₇₁BM) blend solution was dropped and spin-coated onto the D₁:A₁ (PTB7-Th:ITIC-Th) blend layer, producing a D₁:D₂:A₁:A₂ quaternary BHJ photoactive layer. J–V characteristics of (B) binary and (C) sequential quaternary OPVs under 500 and 1000 lx 3000 K LEDs. Reproduced with permission: Copyright 2022, Elsevier B.V.¹³⁶</figcaption></figure>

ternary blend emission layer (EML) TADF-OLDEs could achieve close to 100% internal electroluminescence quantum efficiencies.¹⁴⁴ However, TADF-OLEDs typically degrade via exciton quenchers or carrier traps. A TADF-assisted fluorescence system was introduced to enhance operational stability by combining a fluorescent emitter and a TADF molecule with two host molecules (n-type and p-type) to fabricate TAF-OLEDs. The quaternary TAF-OLED EML blend exhibits an effective triplet-harvesting mechanism, leading to an ideal narrow-band electroluminescence from fluorescent molecules. It also inherently improves the stability of the device due to the rapid Förster energy transfer of singlet excitons from TADF to the fluorescent molecules, which reduces the triplet exciton density on TADF molecules. Figure 16A illustrates the EML concentration in TAF-OLED and the evaluated stability by the time it takes for the luminance to decrease by 5% (denoted as LT₉₅). Additionally, the study demonstrates how to control the recombination zone profile by using the graded carrier transport characteristics in the EML. Consequently, the optimized quaternary TAF-OLED achieves an LT₉₅ of 194 h, indicating a stability 17 times higher than that of its ternary TADF-OLED with a singular bipolar host.

Similarly, interest in quaternary blend OPDs is growing as they have an analogous structure to OPVs. Recently, Zhang et al. incorporated NFAs, ITIC, and IEICO-4F into P3HT:PC₆₁BM host blend in different ratios to improve the spectral range response and balance the response of OPDs (Figure 16B–E).¹⁴⁵ As shown in Figure 16D, the P3HT:PC₆₁BM:ITIC:IEICO-4F device with a blend ratio of 10:8:2:2 exhibits the highest responsivity (R) from 400 to 950 nm. This responsivity trend aligned with the photocurrent, *J*<sub>ph</sub>, as measured in their study. A crucial metric, the specific detection rate (*D*\*), is depicted in Figure 16E. It quantifies the capability of OPDs to detect weak light sources. OPDs should maintain a low dark current for optimal performance while ensuring high optical responsiveness. The optimized quaternary device configuration yields a notable value of *D*\* of 2.01 × 10¹² Jones at 630 nm. This is a significant enhancement in the *D*\* of the quaternary devices with increased photocurrent density *J*<sub>ph</sub> and reduced dark current *J*<sub>d</sub> in the 400–950 nm spectrum. This performance enhancement is attributed to well-balanced carrier transport, achieved by adjusting the ratios of the third and fourth components, ITIC and IEICO-4F. The detection rate throughout the
<figure><img src="image_38.png" /><figcaption>FIGURE 16 (A) Luminance decay curves of OLEDs with various EMLs. Reproduced with permission: Copyright 2020, American Chemical Society.¹⁴⁴ (B) P3HT:PC₆₁BM:ITIC:IEICO-4F quaternary photodetector device structure showing the multicomponent active layer materials with their (C) energy level diagram, (D) device responsiveness, and (E) specific detectivity of with different ratios. Reproduced with permission: Copyright 2023, IOP Publishing Ltd.¹⁴⁵ (F) Molecular structures of TIPS (6,13-bis(triisopropylsilylethynyl)pentacene), PFS, and PS and schematic illustrating the bar-assisted meniscus shearing (BAMS) method. Reproduced with permission: Copyright 2022, Wiley-VCH GmbH.¹⁴⁶</figcaption></figure>

absorption spectrum is also maximized by reducing bimolecular interactions.

OFETs are important organic semiconductor technology with applications in logic circuits and flexible electronics. However, they face the challenge of low stability. Thus, multicomponent strategy is propounded to be the way forward. As of today, and to the best of our knowledge, there is no quaternary blend of OFETs yet. However, in a related study, Tamayo et al. researched ternary blend OFETs to enhance the device stability.¹⁴⁶ A hydrophobic fluorinated polymer, poly(pentafluorostyrene) (PFS), is added to a blend of bis(triisopropylsilylethynyl)pentacene (PFS) with polystyrene (PS) to prevent water molecules from penetrating the organic layer, which is one of the main causes for device degradation. Figure 16F represents the molecular structure of the three materials and a schematic illustration of the bar-assisted meniscus shearing deposition method used in their study.

They optimized OFET with improved performance and shelf stability by tuning the ratio of PS:PFS. The device with an addition of 20% PFS demonstrated 1.5 years of stability, largely due to the hydrophobic nature of the PFS molecule. The PFS molecule serves as a barrier against humidity, thus reducing charge trapping and preventing the degradation of the crystalline film. Bias stability is also enhanced, as

evidenced by a reduced threshold voltage (V<sub>TH</sub>) shift. This improvement is also linked to reduced saturation mobility loss, typically associated with charge traps at interfaces, water and oxygen absorption, or alterations in the crystal structure induced by the field. Given the positive effects observed in OFETs with ternary systems, it is believed that quaternary systems hold significant potential for research to enhance the performance and stability of OFETs further.

This expansion of applications beyond PVs underscores the versatility and potential of quaternary systems in enhancing the efficiency, stability, and other crucial properties of organic electronics. As research continues to evolve, we believe these systems might pave the way for breakthroughs and innovations in organic electronics.

5 | PERSPECTIVE

While all these developments are promising, there are many challenges faced by QOPVs and by large OPVs, including stability, commercialization, end-of-life assessment, and toxicity. While all these developments are promising, there are many challenges faced by QOPVs and large OPVs, including stability, commercialization, end-of-life assessment, and toxicity. Addressing these challenges requires efforts that emphasize the fundamental role of circular economy and research into greener processing.
## 5.1 | Advantages and disadvantages of QOPVs

Before addressing the aforementioned challenges, let us summarize the advantages and disadvantages of multi-component OPVs, especially QOPVs, vis-à-vis BOPVs. Multicomponent blend OPVs boast many advantages, including the synergistic interaction between multiple, and in this case, four functional components, compared to binary blends.⁶⁷,¹⁴⁷ One such advantage is the facile processability of all the components into single-junction BHJ blends and devices compared to the complex structure of tandem, which also uses many materials. High-performance QOPVs offer various possibilities for selecting four donor and acceptor components and benefit from different mechanisms compared to ternary and BOPVs. For instance, QOPVs utilizing a mixed diluent approach can significantly reduce $V_{oc}$ loss due to suppressed nonradiative charge recombination. This approach balances charge generation and recombination in single-junction OPVs, resulting in a higher PCE of 19.76%.⁶⁰ Another advantage is component selection to have excellent or near-complete coverage of the absorption spectra, that is, absorption complementarity. This leads to enhanced photon energy harvesting using multiple organic materials, resulting in improved $J_{sc}$. Using a multicomponent blend can also help prolong the photogenerated exciton lifetime, resulting in subsequent enhanced exciton dissociation. This, coupled with the blend morphology optimization, is beneficial for charge dissociation and transport, leading to higher $J_{sc}$ and FF. Managing the molecular energy level alignment can positively affect charge and energy dynamics, suppressing the energy loss contributed by the charge generation and radiative/nonradiative recombination of the active layer, leading to improved $J_{sc}$ and $V_{oc}$. Multicomponent blend OPVs such as QOPVs can lead to enhanced stability in various ways by (i) morphology-locking, preventing thermally induced morphology changes, improving thermal and long-term stability; (ii) balancing the crystallinity and miscibility of the active layer and therefore enhancing the stability against burn-in, light, and mechanical stress; and (iii) suppressing the radical oxidation of the active layer using an antioxidant as one of the components to improve their photooxidation stability significantly.

With all these advantages, QOPVs still have not substantially outperformed binary and ternary OPVs, as depicted in Figure 17. Thus, the question is how can one improve their performance. There are a few ways to enhance QOPV performance. However, we believe using wide- or medium-bandgap donors in narrow-bandgap donors can extend light absorption in the quaternary

<figure><img src="image_39.png" /><figcaption>FIGURE 17 PCE evolution of binary, ternary, and quaternary OPVs for the last 7 years.</figcaption></figure>

photoactive layer, allowing the utilization of medium-infrared, long-infrared, and NIR photons. Moreover, due to their long wavelength and NIR absorption windows, these could be used as a second donor for NFA in QOPVs (D1:D2:A1:A2). We can then have the donor blends overlapping in the medium and long wavelength absorption regions (adjusting the $D_1:D_2$ ratio). Finally, the mixed diluent approach⁸⁷,⁸⁸,⁹⁰ can also be employed to enhance the QOPV efficiencies.

QOPVs are also faced with many disadvantages due to their complexity in relation to binary blends. It is time-consuming to select host D:A and the third and fourth components for complementarity absorption and find the appropriate ratio between the four components to build the optimal QOPVs requiring more than two or three different materials. Also, the issue of cost arises. It may be relatively costly to use four materials compared with TOPVs and BOPVs (purchasing only two and perhaps three materials). The unpredictability of morphology control could lead to morphological degradation in quaternary blends, impacting PCE and stability. Another difficulty that QOPV faces is the energy-level matching that must be considered for cascade energy-level alignment or energy-level shifting for the system to work. Though miscible or immiscible guest components can be selected deliberately to control the crystallinity, thermodynamic stability, and phase segregation of the blend, it is not a given that it will work because of the complex nature of multicomponent blends. For instance, morphology could be the biggest challenge. Although the third and fourth components in QOPVs prevent morphology changes by morphology-locking, understanding the process and replicating the blend morphology can be exceptionally challenging, given the complex composition involving various types of guest components. For this purpose, Wang et al. studied the effect of the location of guest components on PV performance in ternary systems.¹⁴⁸ The arrangement of guest components within
TOPVs played a crucial role in dictating their operational mechanisms. These mechanisms, including charge and energy transfers, are influenced by how the guest components interact with the donor and acceptor host materials. An optimal alignment of energy levels is key for facilitating a trap-free charge transfer. This phenomenon can obviously be observed in the quaternary blend, which becomes even more complex due to the additional guest component. A review on TOPVs written by Jin et al. covered the strategies for morphology regulation of active layer in TOPVs, including solvent engineering, additive engineering, and other regulation methods.<sup>149</sup>

## 5.2 | Machine learning for the design and discovery of high-performance materials

Machine Learning (ML) approach based on a fast-growing artificial intelligence has been recently used to accelerate the discovery of high-performance D:A pairs in NFA-based OPVs,<sup>150</sup> predicting the PCE,<sup>151</sup> screening acceptor and donor materials,<sup>152</sup> generating new OPV materials,<sup>153</sup> optimizing experimental parameters,<sup>154</sup> and so forth. The capability of ML to scan extensive data can be leveraged to provide an efficient prediction model and comprehensive data-based analysis for feature recognition and better inputs. This will help prevent time consumption and lead to high-throughput experiments. In addition, it is advisable to apply the active learning technique, which is a suitable ML approach for discovering the most promising D:A combinations in relatively fewer experiments, particularly for extrapolation tasks with small training data sets.<sup>155</sup>

## 5.3 | Green solvent processing

Green, eco-compatible solvents such as water, alcohol, or anisole are strongly advocated for solution-processed technologies such as OPVs.<sup>62,156,157</sup> Numerous high-efficiency OPVs have been fabricated with highly toxic halogenated solvents, such as chlorobenzene, chloroform, *ortho*-dichlorobenzene, and so forth. However, such solvents have unfortunately given rise to severe health, safety, and environmental issues, severely handicapping large-scale production and commercialization of OPVs. Over the past 5 years, efforts have been focused on the use of alternative solvents, such as nonhalogenated aromatic hydrocarbons and cyclic ethers, which, according to the United Nations' Globally Harmonized System (GHS) of classification and labeling

of chemicals, are not really “green.”<sup>158</sup> The GHS guidelines recommend water, alcohol, and anisole as solvents for safety, health, and low environmental impacts (Figure 18). Other environmentally friendly solvents, such as trimethylbenzene, o-XY, tetrahydrofuran (THF), and 2-methyltetrahydrofuran, have also been intensively studied as alternatives.<sup>160–162</sup> However, a significant drawback of these green solvents is their limited ability to dissolve photoactive materials, leading to comparatively lower device performance due to unfavorable morphology and inefficient charge transport. Anisole, a natural green, eco-compatible solvent widely available in the market at low cost, is a typical example that shows extremely poor compatibility due to its high polarity and the highly hydrophobic nature of the polymers. Thus, research efforts must continue in this direction toward new green or modified greener solvents. However, recent strategies have focused on modifying side chains of polymers/materials to facilitate the transition toward greener solvents. Notably, Hong et al. investigated modified Y6 and synthesized BTP-4F-12, which improved solubility in low-toxic solvents of o-XY, trimethylbenzene, and THF. This modification led to significantly higher PCEs. The PCEs achieved with the green solvents were comparable to or even exceeded those obtained with the conventional solvent CB.<sup>163</sup> Similarly, in the case of anisole, it was reported that incorporating a B-N covalent bond into the conjugated backbone of polymer donors can facilitate the fabrication of high-efficiency OPVs using anisole as the host solvent and limonene as the co-solvent for active layer deposition.<sup>159</sup> Thus, it emphasizes the need for chemical modification of either the solvents

<figure><img src="image_40.png" /><figcaption>**FIGURE 18** Ranking of common solvents investigated for OPV processing based on the overall greenness criterion. Reproduced with permission: Copyright 2023, The Royal Society of Chemistry.<sup>159</sup></figcaption></figure>
themselves or layer materials to achieve greener applications in organic PVs.

Recently, Corzo et al. investigated green terpene (natural compounds found in plants) solvents for organic electronics. They utilized Hansen solubility parameters, a quantitative model representing solubility as three-dimensional spheres in a complex space defined by dispersive, polar, and hydrogen-bonding intermolecular forces.¹⁶⁴ The study identified a group of terpene solvents (eucalyptol, D-limonene, β-pinene, and L-menthone) as high-performing green inks for various organic electronic devices. The devices fabricated with terpene solvents exhibit a maximum PCE of 15.7%.

Although research in green solvents for QOPVs is very scarce, we have highlighted two above.⁶¹,⁶² Also, Wang et al. demonstrated the promising potential of green solvents in QOPVs by utilizing toluene.¹⁶⁵ The QOPVs exhibited impressive PCEs of 17.4% for opaque and 13.1% for semitransparent devices. Interestingly, the PCE of the semitransparent device using toluene as the solvent was slightly higher than that achieved with a conventional solvent CF, suggesting that green solvent for QOPVs could be beneficial for the application of semitransparent devices. These findings would soon pave a promising way for developing high-efficiency OPVs via green processing, bringing us closer to relatively truer green energy.

## 5.4 | Stability

Despite the remarkable success in PCE enhancement, OPVs, including QOPVS, have limitations in their practical use due to their instability,¹⁶⁶⁻¹⁷⁴ leading to a modest operational lifetime. The origin of OPV instability can be assigned to various environmental conditions, such as heating, water, oxygen, irradiation, and other factors, including diffusion of electrode and buffer layer materials, mechanical stress, and metastable morphology. Investigating the degradation mechanism in OPVs and developing new strategies to improve their stability need to be promoted. Despite several efforts into different stability stressors, including the light-soaking stability of OPVs, more studies are still needed on the light stability of large-area modules above 100 cm². Meanwhile, developing light-soaked stable OPV submodules presents another new set of challenges since the large-area OPV submodule may present additional hurdles due to thin film uniformity, interconnections, and packaging. To remedy this stability concern, various approaches to improving stability can be foreseen and broadly categorized as follows:

- **Material design and selection:** the synthesis of novel low-cost material (D:A) systems capable of overcoming the abovementioned limitations (molecular compatibility, solvent, stability, etc.) in the devices with a quaternary blend. In addition, it is vital for industrial requirements to engineer inherently high-mobility new materials capable of forming nanomorphologies with suitable crystallinity and nanophase separation, driving reduced recombination such as the synthesis of block co-polymers to freeze or lock the nanomorphology.¹⁷⁵ Alongside the development of new materials, altering the properties of existing materials via the modification of (i) donor polymer side chains or NFAs to improve face-on stacking and material crystallinity or (ii) end groups of photoactive materials for efficient charge transport via the backbone, reorganization energy of NFAs, which plays a crucial role in photoelectric conversion processes; (iii) side-chain engineering to facilitate solubility, and so forth.

- **Device engineering:** Engineering OPVs with multicomponent photoactive layers can contribute to tackling the stability issues currently facing OPV technology. A recent demonstration is the highly thermally stable block copolymer QOPVs using a simple one-pot polymerization technique to eliminate the aggregation challenges of the blend films caused by heat and light exposure.¹⁷⁵ The findings proved that multicomponent photoactive layer OPVs offer excellent performance and outstanding device stability for over 1000 h (retains >80% initial efficiency).¹⁷⁵ In a recent article about hexanary blends, the similarities in electron affinities and structure of five acceptors were leveraged in a blend with a polymer donor to achieve thermal stability in multicomponent OPV beyond 20 days at 130°C.¹⁷⁶ This may open new avenues for the emergence of low-cost, long-term stable OPVs.

- **Processing techniques:** Developing new techniques for fabricating thick film OPVs may become crucial. They must be compatible with a large surface area to comply with industrial requirements more closely, enabling their commercialization. Multicomponent OPVs are an effective and simple alternative, offering the opportunity to try out different combinations and ratios of donor and acceptor materials, something not yet fully explored with accuracy in solution-processed devices. Moreover, a thorough study of morphology manipulation and structure-performance interrelationships is required to understand the stability concern in OPVs better. The development of new techniques compatible with these requirements would be key to the advancement of the technology.
Also, adapting existing techniques such as printing and encapsulating methods will significantly contribute to overcoming the stability challenges.

## 6 | CONCLUSION

This review gives a historical and recent account of works on multicomponent OPVs focusing on QOPVs. We explore the general working mechanism of QOPVs and discuss the various configurations and components, including dye molecules, polymer donors, small molecule donors, and acceptors in the host binary/ternary blend devices. We then highlight QOPV applications: outdoor, indoor, semitransparent PV applications, and beyond. The quaternary strategy with appropriate selection and incorporation of the fourth component has the merit of creating charge and/or energy transfer pathways, combined with cascade or parallel/series behaviors with improved morphology or without disturbing an earlier optimized host morphology. Additionally, it can enhance charge carrier transport and lower recombination, leading to increased PV performance. However, QOPVs face some challenges. Selecting multicomponent materials while ensuring their compatibility for an optimized morphology remains evasive and is still a mystery that has not been fully resolved. A better understanding of the full spectrum of the working mechanisms is needed. Thus, better insight into the distribution of the fourth component in the photoactive layer is required.

While QOPVs can lead to high efficiency, the recent record efficiency of over 19.0% for single junction BHJ BOPVs³¹,³²,¹⁷⁷ makes the case for QOPVs and multicomponent OPVs harder. It is only a matter of time before a new record of above 20% for single junction BOPVs is attained. This heartening result reflects a narrowing in PCE between the OPV and other PV devices, reinforcing our community's confidence in the OPV's rosy future. Hence, a deeper understanding of the structural-property relationship effect of multicomponent blends on OPVs can help boost their performance further. As highlighted above, the entire OPV community must work collectively to understand QOPV's working mechanisms thoroughly and explore the potential of OPVs further beyond the quaternary strategy.

## ACKNOWLEDGMENTS

K. N'Konou acknowledges the support of the research commission of the Catholic University of Lille and its Foundation and is grateful to the IEMN laboratory for the financial support. N. Y. Doumon acknowledges

support from the Materials Research Institute (MRI) and the Institute of Energy and the Environment (IEE) of the Pennsylvania State University.

## CONFLICT OF INTEREST STATEMENT

The authors declare that there are no conflicts of interests.

## ORCID

Kekeli N'Konou ID https://orcid.org/0000-0002-0510-6635

Souk Y. Kim ID https://orcid.org/0000-0002-9008-0731

Nutifafa Y. Doumon ID http://orcid.org/0000-0002-2625-1647

## REFERENCES

1. Kwon HC, Jeong W, Lee YS, et al. Overcoming the low-surface-energy-induced wettability problem of flexible and transparent electrodes for large-area organic photovoltaic modules over 500 cm². *Adv Energy Mater.* 2022; 12(21):2200023.

2. Adel R, Morse G, Silvestri F, et al. Understanding the blade coated to roll-to-roll coated performance gap in organic photovoltaics. *Sol Energy Mater Sol Cells.* 2022;245:111852.

3. Lv D, Jiang Q, Shang Y, Liu D. Highly efficient fiber-shaped organic solar cells toward wearable flexible electronics. *npj Flex lectron.* 2022;6(1):38.

4. Zhao F, Zhou J, He D, Wang C, Lin Y. Low-cost materials for organic solar cells. *J Mater Chem C.* 2021;9(43): 15395-15406.

5. Xie L, Zhang J, Song W, et al. Rational tuning of intermolecular and intramolecular interactions enabling high-efficiency indoor organic photovoltaics. *Nano Energy.* 2022;99:107414.

6. Fu H, Yao J, Zhang M, et al. Low-cost synthesis of small molecule acceptors makes polymer solar cells commercially viable. *Nat Commun.* 2022;13(1):3687.

7. Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. Polymer photovoltaic cells—enhanced efficiencies via a network of internal donor–acceptor heterojunctions. *Science.* 1995;270 (5243):1789-1791.

8. Shaheen SE, Brabec CJ, Sariciftci NS, Padinger F, Fromherz T, Hummelen JC. 2.5% efficient organic plastic solar cells. *Appl Phys Lett.* 2001;78(6):841-843.

9. Chen Z, Ge J, Guo Y, et al. Modification on the quinoxaline unit to achieve high open-circuit voltage and morphology optimization for organic solar cells. *ACS Energy Lett.* 2022; 7(10):3432-3438.

10. Chen S, He C, Li Y, et al. Improving the device performance of organic solar cells with immiscible solid additives. *J Mater Chem C.* 2022;10(7):2749-2756.

11. Yu K, Song W, Ge J, et al. 18.01% Efficiency organic solar cell and 2.53% light utilization efficiency semi-transparent organic solar cell enabled by optimizing PM6: Y6 active layer morphology. *Sci China Chem.* 2022;65(8): 1615-1622.

12. Chen Z, Song W, Yu K, et al. Small-molecular donor guest achieves rigid 18.5% and flexible 15.9% efficiency organic photovoltaic via fine-tuning microstructure morphology. *Joule.* 2021;5(9):2395-2407.
13. Chen Z, Li Q, Jiang Y, Lee H, Russell TP, Liu Y. Multi-site functional cathode interlayers for high-performance binary organic solar cells. *J Mater Chem A.* 2022;10(30):16163-16170.

14. Song H, Hu D, Lv J, Lu S, Haiyan C, Kan Z. Hybrid cathode interlayer enables 17.4% efficiency binary organic solar cells. *Adv Sci.* 2022;9(8):2105575.

15. Yao J, Chen Q, Zhang C, Zhang ZG, Li Y. Perylene-diimide-based cathode interlayer materials for high performance organic solar cells. *SusMat.* 2022;2(3):243-263.

16. Cao L, Du X, Li X, et al. Hydrogen bond-induced cathode engineering enables binary all-small-molecule organic solar cells with 15.88% efficiency and enhanced thermostability. *Solar RRL.* 2022;6(9):2200477.

17. Wang L, Chen Y, Tao W, et al. Polymerized naphthalimide derivatives as remarkable electron-transport layers for inverted organic solar cells. *Macromol Rapid Commun.* 2022;43(22):2200119.

18. Karuthedath S, Firdaus Y, Scaccabarozzi AD, et al. Trace solvent additives enhance charge generation in layer-by-layer coated organic solar cells. *Small Struct.* 2022;3(4):2100199.

19. Zhu L, Zhang QZ, Ding H, et al. Unraveling the effect of solvent additive and fullerene component on morphological control in organic solar cells. *Chin J Polym Sci.* 2022;40(12):1604-1612.

20. Qin J, Yang Q, Oh J, et al. Volatile solid additive-assisted sequential deposition enables 18.42% efficiency in organic solar cells. *Adv Sci.* 2022;9(9):2105347.

21. Alqahtani O, Lv J, Xu T, et al. High sensitivity of non-fullerene organic solar cells morphology and performance to a processing additive. *Small.* 2022;18(23):2202411.

22. Tan CAW, Wong BT. Unraveling the mystery of ternary organic solar cells: a review on the influence of third component on structure-morphology-performance relationships. *Solar RRL.* 2021;5(11):2100503.

23. Sha M, Zhu B, Wang Q, et al. Crystallinity and phase separation induced morphological modulation for efficient ternary all-polymer solar cells. *APL Energy.* 2023;1(1):016103.

24. Bi Z, Naveed HB, Wu H, et al. Tuning acceptor composition in ternary organic photovoltaics—impact of domain purity on non-radiative voltage losses. *Adv Energy Mater.* 2022;12(18):2103735.

25. Duan X, Song W, Qiao J, et al. Ternary strategy enabling high-efficiency rigid and flexible organic solar cells with reduced non-radiative voltage loss. *Energy Environ Sci.* 2022;15(4):1563-1572.

26. Kang H, Zheng B, Li Y, et al. Quaternary organic solar cells enable suppressed energy loss. *Solar RRL.* 2022;6(4):2101040.

27. Shi Y, Chang Y, Lu K, et al. Small reorganization energy acceptors enable low energy losses in non-fullerene organic solar cells. *Nat Commun.* 2022;13(1):3256.

28. Goh T, Huang JS, Yager KG, et al. Quaternary organic solar cells enhanced by cocrystalline squaraines with power conversion efficiencies >10%. *Adv Energy Mater.* 2016;6(21):1600660.

29. Nam M, Cha M, Lee HH, et al. Long-term efficient organic photovoltaics based on quaternary bulk heterojunctions. *Nat Commun.* 2017;8(1):14068.

30. Bi Z, Naveed HB, Sui X, et al. Individual nanostructure optimization in donor and acceptor phases to achieve efficient quaternary organic solar cells. *Nano Energy.* 2019;66:104176.

31. Zhu L, Zhang M, Xu J, et al. Single-junction organic solar cells with over 19% efficiency enabled by a refined double-fibril network morphology. *Nat Mater.* 2022;21(6):656-663.

32. Wei Y, Chen Z, Lu G, et al. Binary organic solar cells breaking 19% via manipulating the vertical component distribution. *Adv Mater.* 2022;34(33):2204718.

33. Sun R, Wu Y, Yang X, et al. Single-junction organic solar cells with 19.17% efficiency enabled by introducing one asymmetric guest acceptor. *Adv Mater.* 2022;34(26):2110147.

34. Zhan L, Li S, Li Y, et al. Manipulating charge transfer and transport via intermediary electron acceptor channels enables 19.3% efficiency organic photovoltaics. *Adv Energy Mater.* 2022;12(39):2201076.

35. Gao J, Yu N, Chen Z, et al. Over 19.2% efficiency of organic solar cells enabled by precisely tuning the charge transfer state via donor alloy strategy. *Adv Sci.* 2022;9(30):2203606.

36. Sharma GD, Dahiya H, Singh MK, Li P, Chayal G, Xu H. Efficient ternary polymer solar cells employing well matched medium band gap and narrow band gap nonfullerene acceptors. *ACS Appl Energy Mater.* 2022;5(6):7813-7821.

37. Yu T, He W, Jafari M, et al. 3D nanoscale morphology characterization of ternary organic solar cells. *Small Methods.* 2022;6(1):2100916.

38. Zhang Y, Lang Y, Li G. Recent advances of non-fullerene organic solar cells: from materials and morphology to devices and applications. *EcoMat.* 2023;5(1):e12281.

39. Peng W, Lin Y, Jeong SY, et al. Over 18% ternary polymer solar cells enabled by a terpolymer as the third component. *Nano Energy.* 2022;92:106681.

40. Guan M, Tao W, Xu L, et al. An asymmetric small-molecule donor enables over 18% efficiency in ternary organic solar cells. *J Mater Chem A.* 2022;10(17):9746-9752.

41. Gong Y, Yu R, Gao H, et al. Improving charge transport and reducing non-radiative energy loss via a nonacyclic carbazole-based third component for over 18% efficiency polymer solar cells. *J Mater Chem A.* 2022;10(13):7090-7098.

42. Zhang S, Ma X, Xu C, et al. Boosted efficiency over 18.1% of polymer solar cells by employing large extinction coefficients material as the third component. *Macromol Rapid Commun.* 2022;43(15):2200345.

43. Oh CM, Park S, Lee J, Park SH, Cho S, Hwang IW. Enhanced charge separation by interchain hole delocalization in nonfullerene acceptor-based bulk heterojunction materials. *Carbon Energy.* 2023;5(7):e302.

44. Li P, Meng X, Jin K, et al. Banana-shaped electron acceptors with an electron-rich core fragment and 3D packing capability. *Carbon Energy.* 2023;5(1):e250.

45. Xu X, Li Y, Peng Q. Ternary blend organic solar cells: understanding the morphology from recent progress. *Adv Mater.* 2022;34(46):2107476.

46. Zhang J, Peng R, Yu K, et al. Isogenous acceptor strategy enables highly efficient ternary organic solar cells via synergistic morphology regulation and charge recombination reduction. *Solar RRL.* 2022;6(10):2200508.

47. Zhao C, Wang J, Zhao X, Du Z, Yang R, Tang J. Recent advances, challenges and prospects in ternary organic solar cells. *Nanoscale.* 2021;13(4):2181-2208.
48. Doumon NY, Yang L, Rosei F. Ternary organic solar cells: a review of the role of the third element. *Nano Energy*. 2022;94:106915.

49. Günther M, Kazerouni N, Blätte D, Perea JD, Thompson BC, Ameri T. Models and mechanisms of ternary organic solar cells. *Nat Rev Mater*. 2023;8(7):456-471.

50. Hou M, Wang Y, Yang X, et al. Aryl quaternary ammonium modulation for perovskite solar cells with improved efficiency and stability. *Nano Energy*. 2022;94:106922.

51. Wang M, Li Y, Feng C, Zhao G, Wang ZS. Quaternary iron nickel cobalt selenide as an efficient electrocatalyst for both quasi-solid-state dye-sensitized solar cells and water splitting. *Chem Asian J*. 2019;14(7):1034-1041.

52. Oh WC, Cho KY, Jung CH, Areerob Y. Hybrid of graphene based on quaternary Cu₂ZnNiSe₄–WO₃ nanorods for counter electrode in dye-sensitized solar cell application. *Sci Rep*. 2020;10(1):4738.

53. Li F, Wei J, Liao G, et al. Quaternary quantum dots with gradient valence band for all-inorganic perovskite solar cells. *J Colloid Interface Sci*. 2019;549:33-41.

54. Boon-on P, Rajendran R, Yao YT, Lien SW, Chang TR, Lee MW. Band gap tunable quaternary PbₓCd₁₋ₓS₁₋ᵧSeᵧ quantum dot-sensitized solar cells with an efficiency of 9.24% under 1% sun. *Sustainable Energy Fuels*. 2022;6(11): 2783-2796.

55. Han D, Feng C, Du MH, et al. Design of high-performance lead-free quaternary antiperovskites for photovoltaics via ion type inversion and anion ordering. *J Am Chem Soc*. 2021; 143(31):12369-12379.

56. Ikram M, Murray R, Imran M, Ali S, Shah SI. Enhanced performance of P3HT/(PCBM:ZnO:TiO₂) blend based hybrid organic solar cells. *Mater Res Bull*. 2016;75:35-40.

57. Zhan L, Yin S, Li Y, et al. Multiphase morphology with enhanced carrier lifetime via quaternary strategy enables high-efficiency, thick-film, and large-area organic photovoltaics. *Adv Mater*. 2022;34(45):2206269.

58. Li S, He C, Chen T, et al. Refined molecular microstructure and optimized carrier management of multicomponent organic photovoltaics toward 19.3% certified efficiency. *Energy Environ Sci*. 2023;16(5):2262-2273.

59. Xu X, Jing W, Meng H, et al. Sequential deposition of multicomponent bulk heterojunctions increases efficiency of organic solar cells. *Adv Mater*. 2023;35(12):2208997.

60. Chen T, Li S, Li Y, et al. Compromising charge generation and recombination of organic photovoltaics with mixed diluent strategy for certified 19.4% efficiency. *Adv Mater*. 2023;35(21):2300400.

61. Zhang K, Jiang Z, Qiao J, et al. Dredging photocarrier trapping pathways via “charge bridge” driven exciton-phonon decoupling enables efficient and photothermal stable quaternary organic solar cells. *Energy Environ Sci*. 2023;16(8):3350-3362.

62. Tan H, Zhang W, Zhang P, et al. Green-solvent-processed 17% efficient polymer solar cell achieved synergistically by aligning energy levels and improving morphology with the quaternary strategy. *Solar RRL*. 2022;6(7):2200147.

63. Li S, Jia Z, Ma Q, et al. Highly efficient layer-by-layer processed quaternary organic solar cells with improved charge transport and reduced energy loss. *Solar RRL*. 2022;6(9):2200496.

64. Jia Z, Chen Z, Chen X, et al. 19.34 cm² large-area quaternary organic photovoltaic module with 12.36% certified efficiency. *Photonics Res*. 2021;9(3):324-330.

65. Gao X, Tao X, Xu Y, et al. A simple single-thiophene derivative assists efficient as-cast ternary organic solar cells through Förster resonance energy transfer. *New J Chem*. 2022;46(25):12177-12183.

66. Zhang M, Zhu L, Zhou G, et al. Single-layered organic photovoltaics with double cascading charge transport pathways: 18% efficiencies. *Nat Commun*. 2021;12(1):309.

67. Arunagiri L, Peng Z, Zou X, et al. Selective hole and electron transport in efficient quaternary blend organic solar cells. *Joule*. 2020;4(8):1790-1805.

68. Ma X, Wang J, An Q, et al. Highly efficient quaternary organic photovoltaics by optimizing photogenerated exciton distribution and active layer morphology. *Nano Energy*. 2020;70:104496.

69. Zhang W, Huang J, Xu J, et al. Phthalimide polymer donor guests enable over 17% efficient organic solar cells via parallel-like ternary and quaternary strategies. *Adv Energy Mater*. 2020;10(32):2001436.

70. Li K, Wu Y, Li X, Fu H, Zhan C. 17.1%-Efficiency organic photovoltaic cell enabled with two higher-LUMO-level acceptor guests as the quaternary strategy. *Sci China Chem*. 2020;63(4):490-496.

71. Xu X, Deng M, Lee YW, et al. Realizing high-efficiency multiple blend polymer solar cells via a unique parallel-series working mechanism. *J Mater Chem A*. 2019;7(43):24937-24946.

72. Shen F, Yan D, Li W, et al. Quaternary polymer solar cells with over 13% efficiency enabled by improving film-morphologies via binary mixed fullerene additive. *Mater Chem Front*. 2019;3(2):301-307.

73. Li W, Liu W, Zhang X, Yan D, Liu F, Zhan C. Quaternary solar cells with 12.5% efficiency enabled with non-fullerene and fullerene acceptor guests to improve open circuit voltage and film morphology. *Macromol Rapid Commun*. 2019;40(21):1900353.

74. Bi Z, Zhu Q, Xu X, et al. Efficient quaternary organic solar cells with parallel-alloy morphology. *Adv Funct Mater*. 2019;29(9):1806804.

75. Li W, Yan D, Liu F, Russell T, Zhan C, Yao J. High-efficiency quaternary polymer solar cells enabled with binary fullerene additives to reduce nonfullerene acceptor optical band gap and improve carriers transport. *Sci China Chem*. 2018;61(12): 1609-1618.

76. Yan D, Xin J, Li W, et al. 13%-Efficiency quaternary polymer solar cell with nonfullerene and fullerene as mixed electron acceptor materials. *ACS Appl Mater Interfaces*. 2019;11(1): 766-773.

77. Zhao Z, Wang J, Xu C, et al. Photomultiplication type broad response organic photodetectors with one absorber layer and one multiplication layer. *J Phys Chem Lett*. 2020;11(2): 366-373.

78. Yan C, Tang H, Ma R, et al. Synergy of liquid-crystalline small-molecule and polymeric donors delivers uncommon morphology evolution and 16.6% efficiency organic photovoltaics. *Adv Sci*. 2020;7(15):2000149.

79. Zhu L, Zhang M, Zhong W, et al. Progress and prospects of the morphology of non-fullerene acceptor based high-
efficiency organic solar cells. *Energy Environ Sci.* 2021;14(8): 4341-4357.

80. Yin Y, Zhan L, Liu M, et al. Boosting photovoltaic performance of ternary organic solar cells by integrating a multi-functional guest acceptor. *Nano Energy*. 2021;90: 106538.

81. Xu X, Li Y, Peng Q. Recent advances in morphology optimizations towards highly efficient ternary organic solar cells. *Nano Select*. 2020;1(1):30-58.

82. Ameri T, Khoram P, Min J, Brabec CJ. Organic ternary solar cells: a review. *Adv Mater*. 2013;25(31):4245-4266.

83. Gasparini N, Salleo A, McCulloch I, Baran D. The role of the third component in ternary organic solar cells. *Nat Rev Mater*. 2019;4(4):229-242.

84. Zhang T, Gasparini N. Ternary organic solar cells: insights into charge and energy transfer processes. *Appl Phys Lett.* 2022;120(25):250501.

85. Mohapatra AA, Kim V, Puttaraju B, et al. Förster resonance energy transfer drives higher efficiency in ternary blend organic solar cells. *ACS Appl Energy Mater.* 2018;1(9): 4874-4882.

86. Yang K, Wang J, Zhao Z, et al. Smart strategy: transparent Hole-Transporting polymer as a regulator to optimize photomultiplication-type polymer photodetectors. *ACS Appl Mater Interfaces*. 2021;13(18):21565-21572.

87. Zuo L, Jo SB, Li Y, et al. Dilution effect for highly efficient multiple-component organic solar cells. *Nat Nanotechnol.* 2022;17(1):53-60.

88. Fu Z, Wang T, Zhang WQ, et al. Structural-evolution dynamics and structural-defect suppression in high-efficiency all-polymer solar cells via dilution effect. *PRX Energy*. 2023;2(4):043012.

89. Zhu Q, Xue J, Zhao H, et al. Highly efficient organic solar cells with superior deformability enabled by diluting the small molecule acceptor content. *J Mater Chem A*. 2022;10(15):8293-8302.

90. Wen L, Mao H, Zhang L, et al. Achieving desired pseudo-planar heterojunction organic solar cells via binary-dilution strategy. *Adv Mater*. 2024;36(3):2308159.

91. Yan D, Liu W, Yao J, Zhan C. Fused-Ring nonfullerene acceptor forming interpenetrating J-architecture for fullerene-free polymer solar cells. *Adv Energy Mater.* 2018;8(31):1800204.

92. Castro-Chacón A, Castro-Carranza A, Amargós-Reyes O, et al. Relationship between the $V_{oc}$ tuning effect and the interface activation energy due to the third component concentration in ternary organic solar cells. *ACS Appl Energy Mater.* 2022;5(4):4288-4295.

93. Zhang L, Ma W. Morphology optimization in ternary organic solar cells. *Chin J Polym Sci.* 2017;35(2):184-197.

94. Liu L, Chen H, Chen W, He F. From binary to quaternary: high-tolerance of multi-acceptors enables development of efficient polymer solar cells. *J Mater Chem A*. 2019;7(13): 7815-7822.

95. Liu Z, Wang HE. Improving all ternary small-molecule organic solar cells by optimizing short wavelength photon harvesting and exciton dissociation based on a bisadduct analogue of [70]PCBM as a third component material. *Sustainable Energy Fuels*. 2022;6(3):744-755.

96. Khanam L, Srivastava SB, Do TT, Sonar P, Singh SP. Nonfullerene acceptor-based nanomorphology enhancement for efficient ternary organic solar cells. *Phys Status Solidi A*. 2022;219(18):2200143.

97. Li K, Wu Y, Tang Y, et al. Ternary blended fullerene-free polymer solar cells with 16.5% efficiency enabled with a higher-LUMO-level acceptor to improve film morphology. *Adv Energy Mater.* 2019;9(33):1901728.

98. Xia H, Zhang Y, Deng W, et al. Novel oligomer enables green solvent processed 17.5% ternary organic solar cells: synergistic energy loss reduction and morphology fine-tuning. *Adv Mater.* 2022;34(10):2107659.

99. Shan T, Ding K, Yu L, et al. Spatially orthogonal 2D sidechains optimize morphology in all-small-molecule organic solar cells. *Adv Funct Mater.* 2021;31(24):2100750.

100. Pan MA, Lau TK, Tang Y, et al. 16.7%-efficiency ternary blended organic photovoltaic cells with PCBM as the acceptor additive to increase the open-circuit voltage and phase purity. *J Mater Chem A*. 2019;7(36):20713-20722.

101. Liu X, Li Y, Ding K, Forrest S. Energy loss in organic photovoltaics: nonfullerene versus fullerene acceptors. *Phys Rev Appl.* 2019;11(2):024060.

102. Li S, Hamada F, Nishikubo R, Saeki A. Quantifying the optimal thickness in polymer:fullerene solar cells from the analysis of charge transport dynamics and photoabsorption. *Sustainable Energy Fuels*. 2022;6(3):756-765.

103. Li Y, Chen Z, Yan X, Ge Z. Molecular packing tuning via chlorinated end group enables efficient binary organic solar cells over 18.5%. *Carbon Energy*. 2024;6(3):e439.

104. Du X, Heumueller T, Gruber W, et al. Unraveling the microstructure-related device stability for polymer solar cells based on nonfullerene small-molecular acceptors. *Adv Mater.* 2020;32(16):1908305.

105. Zhang M, Zeng M, Deng X, et al. Simultaneously enhancing the $J_{sc}$ and $V_{oc}$ of ternary organic solar cells by incorporating a medium-band-gap acceptor. *ACS Appl Energy Mater.* 2021;4(4):3480-3486.

106. Wang Z, Nian Y, Jiang H, et al. High $V_{oc}$ ternary nonfullerene polymer solar cells with improved efficiency and good thermal stability. *Org Electron.* 2019;69:174-180.

107. Yan Y, Zhang Y, Liu Y, et al. Simultaneously decreasing the bandgap and $V_{oc}$ loss in efficient ternary organic solar cells. *Adv Energy Mater.* 2022;12(20):2200129.

108. Liu M, Wu J, Guo X, Wang Y, Yin Z, Zhang M. Fine-tuned morphology based on two well-miscible polymer donors enables higher open-circuit voltage and enhanced stability for highly efficient ternary all-polymer solar cells. *Macromol Rapid Commun.* 2022;43(22):2200411.

109. Liu Z, Wang N. Ternary organic solar cells with $NC_{70}$BA as a third component material exhibit high open-circuit voltage and small energy losses. *J Power Sources.* 2020; 448:227442.

110. Dayneko SV, Hendsbee AD, Cann JR, Cabanetos C, Welch GC. Ternary organic solar cells: using molecular donor or acceptor third components to increase open circuit voltage. *New J Chem.* 2019;43(26):10442-10448.

111. Zhou X, Li X, Yan Y, et al. Improved current density and fill factor of non-fullerene organic solar cells prepared under solvent vapor atmosphere. *Solar RRL.* 2022;6(9):2200424.
112. Yang Y, Chen W, Dou L, et al. High-performance multiple-donor bulk heterojunction solar cells. *Nat Photonics*. 2015;9(3):190-198.

113. Lee J, Tamilavan V, Rho KH, et al. Overcoming fill factor reduction in ternary polymer solar cells by matching the highest occupied molecular orbital energy levels of donor polymers. *Adv Energy Mater*. 2018;8(9):1702251.

114. Fan Z, Wang Y, Xu S, Hou S, Zhuang C, Wang B. Versatile third components in organic ternary solar cells. *Sol Energy*. 2022;231:732-757.

115. Wanwong S, Sangkhun W, Kumnorkaew P, Wootthikanokkhan J. Improved performance of ternary solar cells by using BODIPY triads. *Materials*. 2020;13(12):2723.

116. Li X, Zhou L, Lu X, et al. Hydrogen bond induced high-performance quaternary organic solar cells with efficiency up to 17.48% and superior thermal stability. *Mater Chem Front*. 2021;5(10):3850-3858.

117. Ke L, Gasparini N, Min J, et al. Panchromatic ternary/ quaternary polymer/fullerene BHJ solar cells based on novel silicon naphthalocyanine and silicon phthalocyanine dye sensitizers. *J Mater Chem A*. 2017;5(6):2550-2562.

118. Huang JS, Goh T, Li X, et al. Polymer bulk heterojunction solar cells employing Förster resonance energy transfer. *Nat Photonics*. 2013;7(6):479-485.

119. Goh T, Huang JS, Bielinski EA, et al. Coevaporated bisquaraine inverted solar cells: enhancement due to energy transfer and open circuit voltage control. *ACS Photonics*. 2015;2(1):86-95.

120. Wan J, Xia Y, Fang J, et al. Solution-processed transparent conducting electrodes for flexible organic solar cells with 16.61% efficiency. *Nano-Micro Lett*. 2021;13(1):44.

121. Li Y, Song W, Zhang J, Zhang X, Ge Z. High-performance and mechanically durable semi-transparent organic solar cells with highly transparent active layers. *Sci China Mater*. 2023;66(5):1719-1726.

122. Yang D, Zhang R, Shi Y, Guo X, Zhang M. High-performance semitransparent organic solar cells enabled by pseudo-planar heterojunction structures combined with optical engineering. *J Mater Chem C*. 2022;10(39):14597-14604.

123. Lee H, Jeong S, Kim JH, et al. Ultra-flexible semitransparent organic photovoltaics. *npj Flex Electron*. 2023;7(1):27.

124. Xu T, Luo Y, Wu S, et al. High-performance semitransparent organic solar cells: from competing indexes of transparency and efficiency perspectives. *Adv Sci*. 2022;9(26):2202150.

125. Milani EA, Piralaee M, Ahmadi S, Asgari A. The role of structural parameters on efficiency and transparency of semi-transparent non-fullerene organic solar cell. *Sci Rep*. 2022;12(1):14928.

126. Liu W, Sun S, Zhou L, et al. Design of near-infrared nonfullerene acceptor with ultralow nonradiative voltage loss for high-performance semitransparent ternary organic solar cells. *Angew Chem Int Ed.* 2022;61(19):e202116111.

127. Nam M, Noh HY, Kang JH, et al. Semi-transparent quaternary organic blends for advanced photovoltaic applications. *Nano Energy*. 2019;58:652-659.

128. Ma X, Mi Y, Zhang F, et al. Efficient ternary polymer solar cells with two well-compatible donors and one ultranarrow

bandgap nonfullerene acceptor. *Adv Energy Mater*. 2018; 8(11):1702854.

129. Alkhalayfeh MA, Abdul Aziz A, Pakhuruddin MZ, Katubi KMM, Ahmadi N. Recent development of indoor organic photovoltaics. *Phys Status Solidi A*. 2022;219(5): 2100639.

130. Shin SC, Koh CW, Vincent P, et al. Ultra-thick semi-crystalline photoactive donor polymer for efficient indoor organic photovoltaics. *Nano Energy*. 2019;58:466-475.

131. Nam M, Noh HY, Cho J, et al. All-day operating quaternary blend organic photovoltaics. *Adv Funct Mater*. 2019;29 (16):1900154.

132. Ma LK, Chen Y, Chow PCY, et al. High-efficiency indoor organic photovoltaics with a band-aligned interlayer. *Joule*. 2020;4(7):1486-1500.

133. Mathews I, Kantareddy SN, Buonassisi T, Peters IM. Technology and market perspective for indoor photovoltaic. *Joule*. 2019;3(6):1415-1426.

134. Vincent P, Shim JW, Jang J, et al. The crucial role of quaternary mixtures of active layer in organic indoor solar cells. *Energies*. 2019;12(10):1838.

135. Shin SC, Vincent P, Bae JH, et al. Quaternary indoor organic photovoltaic device demonstrating panchromatic absorption and power conversion efficiency of 10%. *Dyes Pigm*. 2019;163: 48-54.

136. Nam M, Lee C, Ko DH. Sequentially processed quaternary blends for high-performance indoor organic photovoltaic applications. *Chem Eng J*. 2022;438:135576.

137. Jeon SK, Jang HJ, Lee JY. Ternary exciplexes for high efficiency organic light-emitting diodes by self-energy transfer. *Adv Opt Mater*. 2019;7(5):1801462.

138. Zhao J, Zheng C, Zhou Y, et al. Novel small-molecule electron donor for solution-processed ternary exciplex with 24% external quantum efficiency in organic light-emitting diode. *Mater Horiz*. 2019;6(7):1425-1432.

139. Yang B, Wang Y, Li L, et al. High performance ternary organic phototransistors with photoresponse up to 2600 nm at room temperature. *Adv Funct Mater*. 2021;31(40):2103787.

140. An T, Pang J, Zhang R, Lu G. Wide spectrum quaternary organic photomultiplier detector based on double doping. *Opt Mater*. 2022;133:113001.

141. Kim J, Kang M, Lee S, So C, Chung DS. Interfacial electrostatic-interaction-enhanced photomultiplication for ultrahigh external quantum efficiency of organic photo-diodes. *Adv Mater*. 2021;33(52):2104689.

142. Panigrahi D, Hayakawa R, Wakayama Y. Antiambipolar transistor with double negative differential transconductances for organic quaternary logic circuits. *Adv Funct Mater*. 2023;33(20):2213899.

143. Pan Y, Yu G. Multicomponent blend systems used in organic field-effect transistors: charge transport properties, large-area preparation, and functional devices. *Chem Mater*. 2021;33(7): 2229-2257.

144. Tanaka M, Nagata R, Nakanotani H, Adachi C. Precise exciton management of quaternary emission layers for highly stable organic light-emitting diodes based on thermally
activated delayed fluorescence. *ACS Appl Mater Interfaces*. 2020;12(45):50668-50674.

145. Zhang R, Yang T, An T, Zhang S, Lu G. Wide band tri-receptor quaternary organic photodetector based on PC61BM, ITIC, and IICO-4F. *Semicond Sci Technol*. 2023;38(8):085001.

146. Tamayo A, Salzillo T, Mas-Torrent M. Organic field-effect transistors based on ternary blends including a fluorinated polymer for achieving enhanced device stability. *Adv Mater Interfaces*. 2022;9(6):2101679.

147. Jung S, Cho Y, Kang SH, Yoon SJ, Yang C. Effect of third component on efficiency and stability in ternary organic solar cells: more than a simple superposition. *Solar RRL*. 2022; 6(2):2100819.

148. Wang X, Wang J, Wang P, et al. Embedded host/guest alloy aggregations enable high-performance ternary organic photovoltaics. *Adv Mater*. 2023;35(51):2305652.

149. Jin J, Wang Q, Ma K, et al. Recent developments of polymer solar cells with photovoltaic performance over 17%. *Adv Funct Mater*. 2023;33(14):2213324.

150. Wu Y, Guo J, Sun R, Min J. Machine learning for accelerating the discovery of high-performance donor/acceptor pairs in non-fullerene organic solar cells. *npj Comput Mater*. 2020;6(1):120.

151. Lee MH. A machine learning-based design rule for improved open-circuit voltage in ternary organic solar cells. *Adv Intell Syst*. 2020;2(1):1900108.

152. Peng SP, Zhao Y. Convolutional neural networks for the design and analysis of non-fullerene acceptors. *J Chem Inf Model*. 2019;59(12):4993-5001.

153. Sun W, Zheng Y, Zhang Q, et al. Artificial intelligence designer for highly-efficient organic photovoltaic materials. *J Phys Chem Lett*. 2021;12(36):8847-8854.

154. Zhao ZW, Geng Y, Troisi A, Ma H. Performance prediction and experimental optimization assisted by machine learning for organic photovoltaics. *Adv Intell Syst*. 2022;4(6):2100261.

155. Malhotra P, Verduzco JC, Biswas S, Sharma GD. Active discovery of donor:acceptor combinations for efficient organic solar cells. *ACS Appl Mater Interfaces*. 2022;14(49): 54895-54906.

156. Shen YF, Zhang J, Tian C, Qiu D, Wei Z. Slot-die coated large-area flexible all-polymer solar cells by non-halogenated solvent. *Nano Res*. 2023;16(12):13008-13013.

157. Zhang W, Zhang G, Song H, et al. All-green solvent and additive combination enables efficient nonfullerene organic solar cells via sequential deposition strategy. *Solar RRL*. 2023;7(5):2201076.

158. Globally Harmonized System of Classification and Labelling of Chemicals (GHS). United Nations. Accessed November 13, 2023. https://unece.org/transport/standards/transport/dangerous-goods/ghs-rev9-202

159. Pang S, Chen Z, Li J, et al. High-efficiency organic solar cells processed from a real green solvent. *Mater Horiz*. 2023;10(2): 473-482.

160. Ye L, Xiong Y, Chen Z, et al. Sequential deposition of organic films with eco-compatible solvents improves performance and enables over 12%-efficiency nonfullerene solar cells. *Adv Mater*. 2019;31(17):1808153.

161. Ye L, Xiong Y, Zhang Q, et al. Surpassing 10% efficiency benchmark for nonfullerene organic solar cells by scalable coating in air from single nonhalogenated solvent. *Adv Mater*. 2018;30(8):1705485.

162. Zhao H, Naveed HB, Lin B, et al. Hot hydrocarbon-solvent slot-die coating enables high-efficiency organic solar cells with temperature-dependent aggregation behavior. *Adv Mater*. 2020;32(39):2002302.

163. Hong L, Yao H, Wu Z, et al. Eco-compatible solvent-processed organic photovoltaic cells with over 16% efficiency. *Adv Mater*. 2019;31(39):1903441.

164. Corzo D, Rosas-Villalva D, C A, et al. High-performing organic electronics using terpene green solvents from renewable feedstocks. *Nat Energy*. 2022;8(1):62-73.

165. Wang D, Liu H, Li Y, et al. High-performance and eco-friendly semitransparent organic solar cells for greenhouse applications. *Joule*. 2021;5(4):945-957.

166. Doumon NY, Wang G, Chiechi RC, Koster LJA. Relating polymer chemical structure to the stability of polymer:fullerene solar cells. *J Mater Chem C*. 2017;5(26):6611-6619.

167. Doumon NY, Dryzhov MV, Houard FV, et al. Photostability of fullerene and non-fullerene polymer solar cells: the role of the acceptor. *ACS Appl Mater Interfaces*. 2019;11(8): 8310-8318.

168. Doumon NY, Houard FV, Dong J, et al. Energy level modulation of ITIC derivatives: effects on the photodegradation of conventional and inverted organic solar cells. *Org Electron*. 2019;69:255-262.

169. Doumon NY, Koster LJA. Effects of the reduction and/or fluorination of the TT-units in BDT-TT polymers on the photostability of polymer:fullerene solar cells. *Solar RRL*. 2019;3(3):1800301.

170. Barreiro-Argüelles D, Ramos-Ortiz G, Maldonado JL, et al. Stability study in organic solar cells based on PTB7:PC71BM and the scaling effect of the active layer. *Sol Energy*. 2018;163: 510-518.

171. Dong Y, Zou Y, Yuan J, et al. Ternary polymer solar cells facilitating improved efficiency and stability. *Adv Mater*. 2019;31(52):1904601.

172. Doumon NY, Wang G, Qiu X, Minnaard AJ, Chiechi RC, Koster LJA. 1,8-Diiodooctane acts as a photo-acid in organic solar cells. *Sci Rep*. 2019;9(1):4350. doi:10.1038/s41598-019-40948-1

173. Doumon NY, Houard FV, Dong J, et al. Improved photo-stability in ternary blend organic solar cells: the role of [70] PCBM. *J Mater Chem C*. 2019;7(17):5104-5111.

174. Kim SY, Kumachang CCF, Doumon NY. Characterization tools to probe degradation mechanisms in organic and perovskite solar cells. *Solar RRL*. 2023;7(13):2300155.

175. Liu B, Sun H, Lee JW, et al. Efficient and stable organic solar cells enabled by multicomponent photoactive layer based on one-pot polymerization. *Nat Commun*. 2023;14(1):967.

176. Paleti SHK, Hultmark S, Han J, et al. Hexanary blends: a strategy towards thermally stable organic photovoltaics. *Nat Commun*. 2023;14(1):4608.

177. Wang J, Wang Y, Bi P, et al. Binary organic solar cells with 19.2% efficiency enabled by solid additive. *Adv Mater*. 2023;35(25):2301583.
## AUTHOR BIOGRAPHIES

<figure><img src="image_41.png" /></figure>

**Kekeli N'konou** received his Ph.D. degree in Micro-nanoelectronics from Aix-Marseille University in 2018. He briefly held a temporary research and teaching position at INSA-Toulouse, France, before joining JUNIA ISEN in

2019 as an assistant professor. His research interests focus on organic photovoltaic materials and devices.

<figure><img src="image_42.png" /></figure>

**Souk Y. Kim** is a Ph.D. student in the Department of Materials Science and Engineering at The Pennsylvania State University, USA. He earned his B.Eng. degree in 2020 and M.Eng. degree in 2022 from Pukyong National University (PKNU),

Republic of Korea. He worked as a student researcher at the Korea Institute of Materials Science (KIMS). His research covers the development and analysis of new photovoltaic materials, such as organic semiconductors and perovskites, with a keen interest in applications for flexible and large-area devices, focusing on their fundamental mechanisms and stability.

<figure><img src="image_43.png" /></figure>

**Nutifafa Y. Doumon** is an assistant professor at The Pennsylvania State University. He is a Virginia S. and Philip L. Walker Faculty Fellow in Materials Science and Engineering and the Fuels Science Program. He holds a

Ph.D. degree in Applied Physics and an M.Sc. degree in Nanoscience from the University of Groningen, the Netherlands. In 2011, he obtained an M.Sc. degree in Theoretical Physics from AUST-Abuja after a B.Sc. Physics in 2009 from the University of Ghana, Legon. His research interests are organic, polymer, and perovskite photovoltaic and optoelectronic device characterization, stability, and reliability testing.

**How to cite this article:** N'Konou K, Kim SY, Doumon NY. Multicomponent organic blend systems: a review of quaternary organic photovoltaics. *Carbon Energy*. 2024;6:e579.

doi:10.1002/cey2.579