(Invited) Interface Engineering for Organic and Perovskite Photovoltaics
Authors: Ohkita Hideo 
Published: February 2024

Organic and perovskite solar cells have been intensively studied because they are considered as one of the most promising photovoltaic devices. Indeed, power conversion efficiencies (PCEs) have recently exceeded 19% for organic solar cells and 25% for perovskite solar cells. In both devices, interface engineering is highly important for efficient photovoltaic performance. In particular, it has great impact on open-circuit voltage (V
OC) because charge carriers would recombine bimolecularly at the interface. In this talk, I will discuss how interface impact on V
OC in polymer solar cells and perovskite solar cells.
First, I will discuss how interfacial charge transfer (CT) states impact on V
OC in polymer solar cells. More specifically, we employed two crystalline polymers (PTzBT-BOHD and PTzBT-12OD) and a fullerene derivative (PCBM). PTzBT-BOHD and PTzBT-12OD consist of the same backbone with different side chains: PTzBT-BOHD has two branched side chains of butyloctyl (BO) and hexyldecyl (HD) groups and PTzBT-12OD has a linear side chain of dodecyl (12) group and a branched side chain of octadecyl (OD) group. Interestingly, these PTzBT-BOHD/PCBM and PTzBT-12OD/PCBM solar cells exhibited different V
OCs, although the two crystalline polymer neat films exhibit the same ionization energy because of the same backbone structure. In order to address the origin of the different V
OCs, we carefully analyzed the interfacial CT state in the blend films. As a result, we found the interfacial CT state energy is higher in PTzBT-BOHD/PCBM than in PTzBT-12OD/PCBM blend films. This is because HOMO level of PTzBT-BOHD is deeper than that of PTzBT-12OD in disordered mixed phase in blend films, which results from twisted backbone of PTzBT-BOHD with branched side chains [1].
Next, I will discuss how aging and passivation impact on interfacial energy matching and hence V
OC in perovskite solar cells. As reported previously, V
OC is often improved by ambient storage, which is called aging effect, and also improved by addition of passivation layers, which is called passivation effect. Such improvements in V
OC suggest recombination would be suppressed effectively in these devices. Interestingly, V
OC was gradually improved during ambient storage even for the passivated device where surface recombination is effectively suppressed by the passivation layer. By analyzing electronic properties of each layer, we found that no change was observed for the perovskite layer but the HOMO level deepening was observed for a hole-transporting material of spiro-OMeTAD during the ambient storage. As a result, energy matching between perovskite and spiro-OMeTAD layers was significantly improved and hence surface recombination was further effectively suppressed, resulting in the improvement in V
OC [2,3].
These findings clearly show that the electronic state at the interface critically impacts on V
OC not only in polymer solar cells but also in perovskite solar cells, and therefore suggest that device design based on interface engineering is of particular importance for further improvement in V
OC.

References

[1] T. Fukuhara, K. Yamazaki, T. Hidani, M. Saito, Y. Tamai, I. Osaka, H. Ohkita, “Molecular Understanding of How the Interfacial Structure Impacts the Open-Circuit Voltage of Highly Crystalline Polymer Solar Cells”, ACS Appl. Mater. Interfaces, 2021, 13, 34357–34366.
[2] Y. Cho, H. D. Kim, J. Zheng, J. Bing, Y. Li, M. Zhang, M. A. Green, A. Wakamiya, S. Huang, H. Ohkita, A. W. Y. Ho-Baillie, “Elucidating Mechanisms behind Ambient Storage-Induced Efficiency Improvements in Perovskite Solar Cells”, ACS Energy Lett., 2021, 6, 925–933.
[3] Y. Cho, H. D. Kim, Y. Li, J. Zheng, J. Bing, S. Tang, M. A. Green, A. Wakamiya, S. Huang, H. Ohkita, A. W. Y. Ho-Baillie, “Immediate and Temporal Enhancement of Power Conversion Efficiency in Surface-Passivated Perovskite Solar Cells”, ACS Appl. Mater. Interfaces, 2021, 13, 39178–39185.

Acknowle
d
gments

This study was partly supported by a green innovation fund project commissioned by the New Energy and Industrial Technology Development Organization (NEDO), and by JST MIRAI projects Grant Number JPMJMI20E2 and JPMJMI22E2, Japan.