# Record High Efficiency Achievement under LED Light in Low Bandgap Donor-Based Organic Solar Cell through Optimal Design

Authors: Yongju Lee; Swarup Biswas; Hyojeong Choi; Hyeok Kim
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Recently, a low bandgap donor named poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4b]thiophene-)-2-carboxylate-2-6-diyl)]- (PTB7-Th-) based organic photovoltaic (OPV) devices has exhibited interesting behavior when tested under indoor light. Theoretically, a PTB7-Th : [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) active layer-based OPV can show >23% power conversion efficiency (PCE) under light-emitting diode (LED) light. However, to date, the experimentally achieved PCE (~11.63%) is significantly lower than the theoretical one. Therefore, we design an indoor OPV having PTB7-Th : PC70BM active layer and low-acidic and cheaper polypyrrole : polystyrene sulfonate (PPY : PSS) as the hole transport layer (HTL), by optimizing active layer thickness and processing conditions (spin coating speed and doping concentration) of the HTL via optical simulations and experiments. The results show that the device having 100 nm thick active layer and a PPY : PSS-based HTL (PPY : PSS; weight ratio between PPY and PSS 1 : 2) coated at 5000 rpm can exhibit a record high PCE value (16.35%) during its operation under 1000 lx LED lamp. In comparison, a commercially available PEDOT : PSS-based OPV can achieve maximum 14.21% PCE under the same conditions.


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## Body

## 1. Introduction

Nowadays, the use of Internet of Things (IoT) network has grown rapidly facilitating a swift and automated technology-based living. It is believed that by 2025, a major part of our daily life will be mediated through ~75 billion low-powered IoT devices such as sensors, actuators, and health care units. Interestingly, most of these IoT devices are expected to be employed as indoor devices [1, 2]. According to their applications, these devices need to be autonomous (i.e., the device operation should be independent of batteries or a grid connection) [3, 4]. Therefore, it is essential to develop a suitable energy source that can provide local power to the IoT devices. Indoor photovoltaic (IPV) cells are one of the finest solutions for powering such devices through the absorption of indoor light energy [5, 6]. The irradiance power of the indoor light is significantly lower than that of the outdoor light (i.e., 1-sun condition) and lies in the visible range (300–700 nm) [7]. It has been observed that the commercially available inorganic PV cells are not efficient as indoor light energy harvesters, owing to their large thicknesses and mismatch between the irradiance and absorption spectra exhibited by their active layers [8, 9]. Although the organic PV (OPV) cells exhibit lower power conversion efficiencies (PCEs) than the inorganic PV cells for operating under 1-sun condition, they exhibit a relatively higher performance in indoor because of their higher photoabsorption characteristic within the visible range [10]. Furthermore, these cells are light weight, super flexible, and ultrathin, rendering them suitable for use as the power sources for different small-sized micropowered IoT devices [10]. Despite these advantages, organic IPVs have not been commercialized to date because of various limitations such as no universal applicability for different indoor light sources, high production costs, short shelf lifetime, and poor stabilities [11]. To overcome these shortcomings, various strategies like the development of suitable donor-acceptor materials [12], optimization of the device fabrication process and architecture [13, 14], and development of different charge transport interlayers (such as the hole transport layer (HTL) [15, 16] and electron transport layer [17]) have been developed to date. Recently, a lower optical bandgap (~1.6 eV) polymer donor, namely, poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) with a thieno[3,4-b]thiophene/benzodithiophene backbone and fullerene acceptor ([6,6]-phenyl-C71-butyric acid methyl ester (PC70BM)) based organic PV cell, has reportedly demonstrated a significantly higher PCE under indoor light [18, 19]. Mori et al. [19] have theoretically calculated that the PTB7-Th : PC70BM active layer-based OPV can exhibit a maximum PCE of 23.1% under a light-emitting diode (LED) lamp. However, experimentally, a maximum PCE of only 11.63%, under a LED lamp, was obtained for the OPV. Besides this, there are various light sources such as LED, fluorescent lamp, halogen, and incandescent lamp which are generally used to illuminate indoor environment. Each and every source’s irradiance spectrum has a different wavelength range [6]. Therefore, the optimal energy bandgap values (according to Shockley-Queisser limit) of donor material for harvesting the light energy from those light sources most efficiently are not same [6]. For example, high energy bandgap donors (~2.0 eV) are ideal for light-emitting diode and fluorescent lamp, whereas low energy bandgap donor materials (~1.0 eV) are ideal for harvesting the light generated from halogen and incandescent lamp. So higher energy bandgap donor material-based OPV will have very poor performance under halogen/incandescent lamp, and very lower energy bandgap donor material-based OPV will have very poor performance under light-emitting diode and fluorescent lamp due to the large voltage loss. PTB7-Th has the optical energy bandgap value ~1.6 eV which is not very high and also not very low. So, due to its specific optical energy value, PTB7-Th may be an appropriate donor for developing an OPV that can operate under all artificial light sources with a reasonable PCE value, but to the best of our knowledge, no significant effort has been devoted to improve the PCE of PTB7-Th : PC70BM active material-based OPV for operating under the indoor light. Andreoli et al. [20] used an electrochemically deposited polypyrrole : polystyrene sulfonate- (PPY:PSS-) based HTL in an OPV, which showed maximum 2.50% PCE for 1-sun conditions. Although this device has a little lower PCE than a spin-coated PEDOT : PSS HTL-based OPV device, the environmental robustness (i.e., self-lifetime) of the device has been greatly boosted due to the less etching of indium tin oxide (ITO) anode by acidic PPY : PSS. Furthermore, although being an essential parameter that can regulate carrier transport characteristic of the material, the doping concentration in PPY : PSS has not been evaluated in this reported work. Moreover, the active layer thickness and processing conditions of the HTL in an OPV critically influence the device performance. However, these factors were not considered judiciously in the previously reported studies on the PTB7-Th : PC70BM active material-based indoor OPVs [21, 22].Therefore, here, we developed an indoor OPV with PTB7-Th : PC70BM (i.e., a low bandgap donor+fullerene acceptor) as the active material, through the optimal design and a judicious utilization (i.e., optimization of the processing conditions such as the doping concentration and spin coating speed) of the low-cost, easy-to-process, and low-acidic HTL named PPY : PSS. The device exhibited a maximum 16.35% PCE under the 1000 lx LED lamp. By contrast, the device with a PEDOT : PSS HTL showed a maximum 14.21% PCE, even after using an active layer (PTB7-Th : PC70BM) with an optimized thickness (100 nm).


## 2. Experimental

### 2.1. Materials

PTB7-Th and PC70BM (>99% pure) were obtained from 1-Material Inc. Anhydrous chlorobenzene (CB) (>99.8% pure) and isopropanol (70% in water) were supplied by Sigma-Aldrich. Pyrrole (PY) (>98% pure) (purified twice through vacuum distillation) and PSS (MW 70000) were supplied by Alfa Aesar. Samchun Chemicals supplied ammonium peroxodisulfate (APS) (>98% pure). Clevios Al 4083 PEDOT : PSS (1.3-1.7 wt.% (in water)) was supplied by Heraeus Epurio. Deionized (DI) water (18 MΩ) was prepared using a pure reverse osmosis system (RO 15). Patterned ITO-coated glass was supplied by All for LAB.


### 2.2. Solution Preparation

The PTB7-Th : PC70BM-blended solutions were prepared by the mixing of PTB7-Th and PC70BM into CB through constant stirring (50°C) in a glovebox (GB) for 12 h. PTB7-Th and PC70BM have a fixed weight ratio of 1.5 : 1. The solution’s concentration was set at 20 mg/mL.


### 2.3. Synthesis of HTL

In this study, we synthesized the PPY : PSS HTL through an easy chemical route. First, we prepared five batches of PY solution in water by dissolving 100μL of PY into 15 mL DI water and stirring at 1000 rpm for 1 h. After that, a fixed PSS concentration (50, 100, 150, 200, and 250 mg) was mixed with various batches and rapidly stirred for 5 h at 1000 rpm. On the other side, we prepared five APS solutions by mixing 335 mg of APS in 7.5 mL DI water and vigorously stirring for 1 h and stored at 0-5°C for 3 hours. Next, we combined these APS solutions with the five previously made PY : PSS solutions and stirred at 1000 rpm for 12 h to complete the polymerization and create a black-colored water-stable polymer. The final materials were named as PY1, PY2, PY3, PY4, and PY5 corresponding to the PSS concentrations of 50, 100, 150, 200, and 250 mg in PPY : PSS, respectively.


### 2.4. Device Fabrication

To fabricate the OPV devices, initially, the ITO (patterned) coated glass substrates were washed through ultrasonication in detergent and subsequently rinsed by using DI water. After that, we ultrasonicated those substrates in DI water, acetone, and 2-propanol, respectively, and blown by N2-blowing gun. Next, we treated (15 min) those substrates with O2-plasma. Next, we formed HTLs through the spin coating of PPY : PSS and PEDOT : PSS (reference) on the substrates. For optimizing the rotation speed, the fabricated OPVs were coated with PPY : PSS at 3000, 5000, and 6000 rpm to form the HTLs. Following that, the substrates were heated for 30 min at 120°C to eliminate water from the HTLs. Next, we transferred the HTL-coated substrates into an N2-filled GB, and the PTB7-Th : PC70BM solution was spin-coated (3000 rpm for 30 s) on HTL. Again, we heated the substrates at 130°C (30 min) in the GB. Next, we deposited (deposition rate: 0.01–0.02 nm/s; pressure ~1 μPa) an ultrathin (0.5 nm) layer of lithium fluoride (LiF), by using shadow mask, on the PTB7-Th : PC70BM layer in a thermal evaporation system (vacuum). Here, we used LiF interlayer to reduce the electrode work function for more effective electron extraction. Moreover, during the deposition of top electrode through heat evaporation, it acts as a buffer layer and blocks the migration of metal into the active layer. Finally, we completed the device fabrication procedure through the deposition (deposition rate: 0.5–0.6 nm/s; pressure ~1 μPa) of a 100 nm thick Al layer onto the previously deposited LiF. The OPV active area was 0.0225 cm2.


### 2.5. Characterization

First, the PSS-doped PPY synthesis was validated using high-resolution Fourier transform infrared (FT-IR) spectroscopy of PPY : PSS films in the 4000 cm-1-400 cm-1 range. The FTIR spectra were recorded by a PerkinElmer FTIR spectrometer. The transmittance and absorbance spectra (300–900 nm) of the samples were recorded by a Shimadzu, UV-2401PC UV-vis spectrophotometer. Furthermore, the samples were analyzed using ultraviolet photoelectron spectroscopy (UPS) with an AXIS Ultra DLD system (He I (21.2 eV) source). The scanning electron microscopy (SEM) images of the HTLs were recorded by a JEOL, JSM 7610F SEM. Surface profiles of various movies made using PSS-doped PPY and PEDOT : PSS were examined using a PSIA XE-100 atomic force microscope (AFM). Variations of current density (J) with voltage (V) for the fabricated OPVs were recorded by a Keithley SMU 2401, Cleveland, OH, source meter (programmed). A solar simulator (McScience) equipped with a 1000 lx (280 μW/cm) LED lamp (white linear COB LED, McScience) was used as the light source. We estimated the active area of the fabricated OPVs using an optical microscope (Olympus). The wavelength-dependent incident-photon-to-current efficiency (IPCE) values of several OPV devices were measured using an ORIEL IQE 200 IPCE measurement equipment.


### 2.6. Optical Simulation

We utilized the Lumerical finite-difference time-domain (FDTD) solution software to perform a two-dimensional optical simulation-based study of the indoor OPV devices. We included the wavelength-dependent refractive index (n) and extinction coefficient (k) values of the various layers to specify the characteristics (optical) of those layers. The length of the device was considered as 1000 nm, and the breadth was considered 1000 nm. The thickness of ITO, HTL, LiF, and Al layer was considered as 150, 40, 0.5, and 100 nm, respectively, and the active layer thickness was varied from 10 to 250 nm at fixed intervals of 5 nm. To reduce its impact on the simulation results, the device structure was meshes at the optimal mesh density (mesh accuracy: 3, min mesh step: 0.25 nm, dt stability factor: 0.99, override x, y, and z mesh: 0.001). Then, we placed two perfectly matched layers (PML), with the solar cell axis limiting the simulation zone (underneath the bottom electrode and above the top electrode). The optical simulation was used to evaluate how an electric field (photo generated) caused by photon absorption in a photoactive layer would be distributed. By assuming a 100% internal quantum efficiency, this was then utilized to compute the photogenerated ideal short circuit current density (JSC,Ideal) of the indoor OPVs [23, 24]. Moreover, we considered that the light was travelling through the indoor OPV device at an angle of 90° with respect to its surface. The system’s impulse response was assessed using a continuous-wave normalized source (a 1000 lx LED light), and throughout the simulation, the impulse response was multiplied by the corresponding source spectrum to provide a user-defined power spectrum.


## 2.1. Materials

PTB7-Th and PC70BM (>99% pure) were obtained from 1-Material Inc. Anhydrous chlorobenzene (CB) (>99.8% pure) and isopropanol (70% in water) were supplied by Sigma-Aldrich. Pyrrole (PY) (>98% pure) (purified twice through vacuum distillation) and PSS (MW 70000) were supplied by Alfa Aesar. Samchun Chemicals supplied ammonium peroxodisulfate (APS) (>98% pure). Clevios Al 4083 PEDOT : PSS (1.3-1.7 wt.% (in water)) was supplied by Heraeus Epurio. Deionized (DI) water (18 MΩ) was prepared using a pure reverse osmosis system (RO 15). Patterned ITO-coated glass was supplied by All for LAB.


## 2.2. Solution Preparation

The PTB7-Th : PC70BM-blended solutions were prepared by the mixing of PTB7-Th and PC70BM into CB through constant stirring (50°C) in a glovebox (GB) for 12 h. PTB7-Th and PC70BM have a fixed weight ratio of 1.5 : 1. The solution’s concentration was set at 20 mg/mL.


## 2.3. Synthesis of HTL

In this study, we synthesized the PPY : PSS HTL through an easy chemical route. First, we prepared five batches of PY solution in water by dissolving 100μL of PY into 15 mL DI water and stirring at 1000 rpm for 1 h. After that, a fixed PSS concentration (50, 100, 150, 200, and 250 mg) was mixed with various batches and rapidly stirred for 5 h at 1000 rpm. On the other side, we prepared five APS solutions by mixing 335 mg of APS in 7.5 mL DI water and vigorously stirring for 1 h and stored at 0-5°C for 3 hours. Next, we combined these APS solutions with the five previously made PY : PSS solutions and stirred at 1000 rpm for 12 h to complete the polymerization and create a black-colored water-stable polymer. The final materials were named as PY1, PY2, PY3, PY4, and PY5 corresponding to the PSS concentrations of 50, 100, 150, 200, and 250 mg in PPY : PSS, respectively.


## 2.4. Device Fabrication

To fabricate the OPV devices, initially, the ITO (patterned) coated glass substrates were washed through ultrasonication in detergent and subsequently rinsed by using DI water. After that, we ultrasonicated those substrates in DI water, acetone, and 2-propanol, respectively, and blown by N2-blowing gun. Next, we treated (15 min) those substrates with O2-plasma. Next, we formed HTLs through the spin coating of PPY : PSS and PEDOT : PSS (reference) on the substrates. For optimizing the rotation speed, the fabricated OPVs were coated with PPY : PSS at 3000, 5000, and 6000 rpm to form the HTLs. Following that, the substrates were heated for 30 min at 120°C to eliminate water from the HTLs. Next, we transferred the HTL-coated substrates into an N2-filled GB, and the PTB7-Th : PC70BM solution was spin-coated (3000 rpm for 30 s) on HTL. Again, we heated the substrates at 130°C (30 min) in the GB. Next, we deposited (deposition rate: 0.01–0.02 nm/s; pressure ~1 μPa) an ultrathin (0.5 nm) layer of lithium fluoride (LiF), by using shadow mask, on the PTB7-Th : PC70BM layer in a thermal evaporation system (vacuum). Here, we used LiF interlayer to reduce the electrode work function for more effective electron extraction. Moreover, during the deposition of top electrode through heat evaporation, it acts as a buffer layer and blocks the migration of metal into the active layer. Finally, we completed the device fabrication procedure through the deposition (deposition rate: 0.5–0.6 nm/s; pressure ~1 μPa) of a 100 nm thick Al layer onto the previously deposited LiF. The OPV active area was 0.0225 cm2.


## 2.5. Characterization

First, the PSS-doped PPY synthesis was validated using high-resolution Fourier transform infrared (FT-IR) spectroscopy of PPY : PSS films in the 4000 cm-1-400 cm-1 range. The FTIR spectra were recorded by a PerkinElmer FTIR spectrometer. The transmittance and absorbance spectra (300–900 nm) of the samples were recorded by a Shimadzu, UV-2401PC UV-vis spectrophotometer. Furthermore, the samples were analyzed using ultraviolet photoelectron spectroscopy (UPS) with an AXIS Ultra DLD system (He I (21.2 eV) source). The scanning electron microscopy (SEM) images of the HTLs were recorded by a JEOL, JSM 7610F SEM. Surface profiles of various movies made using PSS-doped PPY and PEDOT : PSS were examined using a PSIA XE-100 atomic force microscope (AFM). Variations of current density (J) with voltage (V) for the fabricated OPVs were recorded by a Keithley SMU 2401, Cleveland, OH, source meter (programmed). A solar simulator (McScience) equipped with a 1000 lx (280 μW/cm) LED lamp (white linear COB LED, McScience) was used as the light source. We estimated the active area of the fabricated OPVs using an optical microscope (Olympus). The wavelength-dependent incident-photon-to-current efficiency (IPCE) values of several OPV devices were measured using an ORIEL IQE 200 IPCE measurement equipment.


## 2.6. Optical Simulation

We utilized the Lumerical finite-difference time-domain (FDTD) solution software to perform a two-dimensional optical simulation-based study of the indoor OPV devices. We included the wavelength-dependent refractive index (n) and extinction coefficient (k) values of the various layers to specify the characteristics (optical) of those layers. The length of the device was considered as 1000 nm, and the breadth was considered 1000 nm. The thickness of ITO, HTL, LiF, and Al layer was considered as 150, 40, 0.5, and 100 nm, respectively, and the active layer thickness was varied from 10 to 250 nm at fixed intervals of 5 nm. To reduce its impact on the simulation results, the device structure was meshes at the optimal mesh density (mesh accuracy: 3, min mesh step: 0.25 nm, dt stability factor: 0.99, override x, y, and z mesh: 0.001). Then, we placed two perfectly matched layers (PML), with the solar cell axis limiting the simulation zone (underneath the bottom electrode and above the top electrode). The optical simulation was used to evaluate how an electric field (photo generated) caused by photon absorption in a photoactive layer would be distributed. By assuming a 100% internal quantum efficiency, this was then utilized to compute the photogenerated ideal short circuit current density (JSC,Ideal) of the indoor OPVs [23, 24]. Moreover, we considered that the light was travelling through the indoor OPV device at an angle of 90° with respect to its surface. The system’s impulse response was assessed using a continuous-wave normalized source (a 1000 lx LED light), and throughout the simulation, the impulse response was multiplied by the corresponding source spectrum to provide a user-defined power spectrum.


## 3. Results and Discussion

Figure1(a) presents the FTIR spectra of the PY2-based film, showing two sharp deeps at ~1553 and~1461 cm-1 that represent the stretching of the PY ring and that of the conjugated C–N bond, respectively [25, 26],. It confirms that the synthesized sample is of PPY. Additionally, the noticeable deeps at ~1176 ((SO3H) group stretching), ~1130 (benzene ring vibration), ~1039 (stretching of sulfur dioxide (SO2) group), ~1002 (benzene ring bending vibration), and ~680 cm-1 (C–S stretching) confirm the presence of PSS within PPY [26]. The deep at 925 cm-1 indicates the doping status of PPY, which shows the inclusion of dopant ions into the produced polymer. Figure 1(b) represents the transmittance spectra of the different HTL samples, viz., PY1, PY2, PY3, PY4, PY5, and PEDOT : PSS, within the wavelength range of 370–800 nm. Clearly, all of the samples had transmittance values greater than 80%, and the light transmission performance of PPY : PSS is equivalent to that of commercially available PEDOT : PSS (the average transmittance (AVT) within the visible range (370-740 nm) of PY1-, PY2-, PY3-, PY4-, PY5-, and PEDOT : PSS-based films is 83.23%, 84.21%, 85.12%, 93.06%, 88.04%, and 90.29%, respectively) [27]. Furthermore, PY4 exhibits a higher transmittance than does the PEDOT : PSS. Therefore, it can be expected that a large number of photons can transmit through an OPV if we use PY4 as its HTL. It is also noteworthy to note that AVT value of PPY : PSS-based film first rises with the PSS amount up to a certain point before falling off with an increase in PSS concentration. The transmittance of these types of material-based film is highly affected by two important factors: film surface morphology and light absorption (400-500 nm range) capacity (owing to the π-π∗ transition). They both have different impacts. In general, the absorption of PPY : PSS should increase with the enhancement of its doping concentration due to the enhancement of π-π∗ transition. If so, the AVT should decrease along with this increase and be at its highest level at the lowest doping concentration. However, at lower doping concentrations, the film morphology is very poor, and many pinholes and defects typically form as a result of poor polymer chain binding. Additionally, significant photon energy loss may occur as a result of irregular light scattering. Because of this, the PPY : PSS film has a lower AVT value at lower doping concentrations. PPY’s processability is improved by PSS in addition to doping. The quality of the film improves as the amount of PSS within PPY : PSS-based film rises, and owing to this, an improvement in AVT anticipated. So, in our experience, the PY4-based HTL has the highest AVT at the optimal doping level and thereafter shows a decreasing trend in AVT when the doping level is raised owing a progressive increase in absorption ability. Because the acidity of an HTL is an important factor that can affect the lifetime of an OPV, we measured the acidity of the PY1, PY2, PY3, PY4, PY5, and PEDOT : PSS solutions. Figure 1(c) shows the pH value of the different HTL samples, indicating that the pH values of the PY1, PY2, PY3, PY4, PY5, and PEDOT : PSS samples are approximately 2.4, 2.2, 2.1, 2.0, 1.9, and 1.6, respectively. It implies that PEDOT : PSS is highly acidic, which is not favorable for fabricating an OPV with a longer lifetime. However, the acidity of PPY : PSS is rather low, making it suitable for use as the HTL of an OPV. Figure 1(d) depicts PPY : PSS resistivity variations with increasing concentrations of PSS. Notably, the resistivity of PPY : PSS decreases with increasing PSS doping concentration, up to 200 mg (i.e., PY4), and then increases. PSS is an insulating organic polymeric acid that plays two opposite roles in PPY : PSS. Due to its acidic nature, its doping in PPY through protonation enhances the conductivity of PPY. In contrast, its insulating behavior can hinder the charge transport mechanism in PPY : PSS [22]. Therefore, at the optimized concentration of PSS, PPY : PSS exhibits the lowest resistivity, and beyond this PSS concentration, PPY : PSS shows a higher resistivity value [4]. Notably, the resistivity value of PY4 is approximately same as that of PEDOT : PSS, rendering it suitable for application as an HTL. Figure 2 depicts the AFM images of the various films formed using the PY1, PY2, PY3, PY4, PY5, and PEDOT : PSS HTL solutions. The root mean square values of the surface roughness of the films prepared using the PY1, PY2, PY3, PY4, and PY5 HTL samples are approximately 7.34, 18.4, 23.3, 28.5, and 27.5 nm, respectively, in stark contrast to that of the film prepared using PEDOT:PSS (~1.05 nm only) as the HTL. The surface roughness of the PPY : PSS-based film is much greater than that of the PEDOT : PSS-based film. This might be related to the development of larger PPY : PSS nanoparticles than PEDOT : PSS nanoparticles. This trend corresponds to the change of HTL conductivity and AVT value. According to reports, the performance of an OPV device is strongly reliant on the thickness of its active layer [13]. Therefore, before fabricating the real device, we optimized the active layer thickness (Figure 3(a)) through optical simulations using Lumerical FDTD solution for reducing the experimental cost and time. The optical properties of different layers were defined by the incorporation of their frequency-dependent n and k values into the simulation (Figure S1), as mentioned before in detail. Figure 3(b) reveals the relation between the estimated JSC,ideal and the active layer thickness of the OPV device for operation under a 1000 lx LED lamp. Evidently, the JSC,Ideal value of the OPV shows an oscillating behavior with variations in its active layer thickness. Initially, it increases with the increasing active layer thickness and reaches a maximum value at a thickness of 100 nm. After that, it decreases up to a certain limit with the increasing active layer thickness and then increases again. The oscillating behavior of JSC,Ideal is caused by the interference of incident and reflected light at the device’s active layer [28]. This result can be used to explain the dissimilarities in the distribution of normalized electric field intensity (E2) within the devices containing active layers with different thicknesses (Figure 3(c), Figure S2A, and Figure S2B). Figure 3(c) shows the distribution of wavelength-dependent E2 within the device containing an active layer with optimized thickness (100 nm). We evaluated the E2 distribution at two different wavelengths, viz., 450 and 650 nm, because at these wavelengths, the active layer exhibits the strongest absorption (Figure S2C). From Figure 3(c), it is worthy to note that the peak value of the E2 distribution is centered at the active layer; however, for the 50 (Figure S2B) and 150 nm thick (Figure S2C) active layers, E2 (at active layer) does not show a similar trend and is much lower than that observed in the 100 nm thick active layer. These results confirm that the optimum thickness of active layer integrated into our fabricated device was approximately 100 nm, which was used in the subsequent investigations. The energy levels of the different layers of an OPV are also crucial factors that influence its performance. As a result, the energy levels of different layers should be well matched. For estimating the energy associated with the highest occupied molecular orbital (HOMO) and that corresponding to the lowest unoccupied molecular orbital (LUMO) of different HTLs, we utilized UPS (Figure 4(a)) and ultraviolet-visible (UV-vis) absorption spectra (Figure 4(b)) of those HTLs. The energy value of the HOMO level (EHOMO) can be calculated through the analysis of the UPS spectra using the following equation [29–31].
(1)EHOMO=hν−W,where W is the peak width in the UPS spectra and hν=21.22eV. Conversely, the LUMO level (ELUMO) energy value can be estimated by the following equation [29].
(2)ELUMO=EHOMO−Eg,where Eg is the energy bandgap. From the analysis of the UPS spectra of PY4 and PEDOT : PSS, it was found that the EHOMO values of PY4 and PEDOT : PSS were -4.83 and -5.16 eV, respectively. The values of Eg of these samples were calculated through the analysis of the UV-vis absorption (Figure 4(c)) curves using the Tauc relation (Equation (3)) as [29]
(3)αhν=Ahν−Egn,where α, ν, h, and A represent the absorption coefficient, frequency, Plank’s constant, and a constant, respectively. Furthermore, we considered the value of n as 1/2 due to direct transitions. From this analysis, it was observed that the optical energy bandgaps of PY and PEDOT : PSS are 3.12 eV and 3.42 eV, respectively. By contrast, theELUMO of PY4 and PEDOT : PSS film is -1.71 eV and -1.74 eV, respectively. Using these HOMO and LUMO energy level values of PY4 and PEDOT : PSS and extracting the energy values of the other layers (i.e., ITO, PTB7-Th, PC70BM, LiF, and Al) of the OPVs from other reports [32], we draw energy level diagram of the OPV device, as shown in Figure 4(d). Notably, the energy value of the HOMO level of our as-synthesized PPY : PSS is similar to the HOMO level of the donor. This energy matching can contribute in accelerating the hole movement from the active layer to the anode of the OPV. Moreover, the broad energy bandgap (~3.12 eV) of the PPY : PSS results in a sufficiently low energy (LUMO) level (-1.71 eV) of PPY : PSS. The huge difference between the LUMO levels of PPY : PSS and donor PTB7-Th (3.74 eV) is favorable for blocking the movement of electron towards opposite side. Thus, it is expected that PPY : PSS can be an efficient candidate for developing the HTL of an OPV. Based on the aforementioned results, we fabricated the OPV devices by optimizing the processing condition of PPY : PSS (i.e., the spin coating speed). Figure 5(a) reveals the J-V characteristic curves of the three different OPVs (operating under a 1000 lx LED lamp) fabricated using the PY3 (HTL) solution coated at 3000, 5000, and 6000 rpm by fixing the processing conditions of the other layers. The device performance parameters, estimated from Figure 5(a), are summarized in Table 1. Evidently, the OPV device with the PY3 HTL coated at 5000 rpm exhibits the highest PCE of 13.45%, possibly due to its relatively smooth surface (Figure S3). Next, we fabricated and tested six OPVs, with HTLs constructed from the PY1, PY2, PY3, PY4, PY5, and PEDOT : PSS HTL, under a 1000 lx LED. Figure 5(b) shows the J-V characteristics of these devices under 1000 lx LED, and Table 2 presents the summary of the device performance parameters. From Table 2, it can be ascertained that the OPV device performance is strongly dependent on the doping concentration of its HTL (i.e., PPY : PSS). Initially, its PCE value (which was calculated by using the relation PCE=VOC×JSC×FF/Pin, where VOC, JSC, FF, and Pin are the open circuit voltage, short circuit current density, fill factor, and power intensity of light, respectively) is gradually increasing with increasing doping concentration and reaches a maximum of 16.35% (which is record value (Table S1) for this particular category (low bandgap donor-based OPV for indoor application)) at a concentration of 200 mg (i.e., PY4). Subsequently, it decreases with further increase in the doping concentration (Figure 5(c)). Moreover, the OPV device with optimally doped PPY : PSS (PY4) shows a higher PCE than does the device with commercially available PEDOT : PSS (14.21%) as the HTL. This might be attributed to PY4’s lowest resistivity, which results in the device’s highest JSC, shunt resistance (RSH), and lowest series resistance (RS) (the equation (Equation S1) used to estimate RSH and RS value is presented in Supplementary Information), and highest AVT value. Figure 5(d) depicts the wavelength-dependent IPCE of the OPVs having PY4 and PEDOT : PSS as the HTLs. Notably, both the devices exhibit approximately the same IPCE values within the wavelength range of 300–900 nm, except from 400 to 500 nm. In the range of 400–500 nm, the PY4 HTL-based OPV shows higher IPCE values than does the PEDOT : PSS HTL-based OPV. This may be due to the higher transparency (Figure 1(b)) of the PY4 film than the PEDOT : PSS film within this wavelength range. This might be another reason behind the better performance of the PY4 HTL-based OPV than of the PDEOT : PSS HTL-based OPV, even though both exhibit similar resistivity values.Figure 1
(a) FTIR spectra of PY2, (b) transmittance spectra, (c) pH value, and (d) resistivity of the films formed by PY1, PY2, PY3, PY4, PY5, and PEDOT : PSS.
(a)(b)(c)(d)Figure 2
AFM images of the films formed by (a) PEDOT : PSS, (b) PY1, (c) PY2, (d) PY3, (e) PY4, and (f) PY5.
(a)(b)(c)(d)(e)(f)Figure 3
(a) Schematic diagram of the OPV device. (b) Variation of the simulatedJSC,Ideal with increasing active layer thickness. (c) Distribution of the normalized E2 within the OPV device with a 100 nm thick active layer (PTB7-Th : PC70BM).
(a)(b)(c)Figure 4
(a) UPS spectra, (b) UV-vis absorption spectra, (c) Tauc plot obtained from the UV-vis absorption spectra of PEDOT : PSS and PY4, and (d) energy band diagram of the OPV device.
(a)(b)(c)(d)Figure 5
(a) J-V characteristic curves of various OPVs coated with PY3 HTL at different rotation speeds. (b) J-V characteristic curves of the different OPVs with the PY1, PY2, PY3, PY4, PY5, and PDEOT : PSS HTLs. (c) Variation of PCE with doping concentration of the HTL (PPY : PSS). (d) IPCE spectra of the OPVs with the PY4 and PEDOT : PSS-based HTLs.
(a)(b)(c)(d)Table 1
Summary of the device performance parameters (standard deviation for 8 devices) of the PTB7-Th : PC70BM active layer-based OPVs, with the PY3 HTL, deposited at different spin coating speeds.
Spin coating speed (rpm)VOC (V)JSC (μA/cm2)FF (%)PCEmax (PCEavg) (%)30000.44±0.0192.3±7.2065.10±1.7211.40 (10.55±0.85)50000.65±0.0096.10±4.2055.43±1.0013.45 (13.20±0.25)60000.62±0.0097.40±1.8364.07±1.5012.58 (12.03±0.55)Table 2
Summary of the device performance parameters (standard deviations for 8 devices) of the PTB7-Th : PC70BM active layer-based OPVs with different HTLs.
HTLVOC (V)JSC (μA/cm2)FF (%)PCEmax (PCEavg) (%)RS (Ω × cm2)RSH (Ω × cm2)PY10.65±0.0319.6±1.1023.16±3.201.28 (0.96±0.32)27.18±0.51108890±10213PY20.64±0.0097.1±3.9053.27±3.5510.23 (10.08±0.15)15.43±0.16146152±14615PY30.65±0.01100.3±5.3556.43±0.9414.09 (13.45±0.64)10.20±0.0975223±6411PY40.64±0.00107.1±3.4373.60±0.3416.35 (15.76±0.59)8.13±0.21105664±8035PY50.63±0.00105.2±1.7073.3±0.3215.24 (15.11±0.13)9.06±0.8195590±8621PEDOT : PSS0.64±0.00100.5±1.8569.76±0.4414.21 (13.98±0.23)9.82±1.0166481±5855


## 4. Conclusion

Here, we developed a PTB7-Th : PC70BM (low bandgap donor) active material-based indoor OPV through the optimization of its active layer thickness and the judicious utilization (optimization of the processing conditions such as doping concentration and spin coating speed) of the low-cost, easy-to-process, and low-acidic PPY : PSS-based HTLs. This fabricated OPV was able to achieve a record high (for PTB7-Th) PCE value (16.35%) under LED lamp (1000 lx). First, we optimized the active layer thickness through optical simulations, which indicated that the 100 nm thick PTB7-Th : PC70BM layer would show the maximum absorption of the incident photon energy. Next, we extracted the optimal processing conditions such as the spin coating speed (5000 rpm) and doping concentration (200 mg; PPY:PSS=1:2) for our newly synthesized low-cost and low-acidic (pH=1.9) PPY : PSS HTLs to develop a highly efficient indoor OPV with PTB7-Th : PC70BM as the active layer.



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*Source: 3823460-2023-02-03.xml*