# Investigation of the Impact of Active Layer and Charge Transfer Layer Materials on the Performance of Polymer Solar Cells through Simulation

Authors: Fekadu Gochole Aga; Fetene Fufa Bakare; Solomon Tiruneh Dibaba; Demissie Jobir Gelmecha; Chernet Amente
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

This article reports polymer solar cell performance evaluated using general-purpose photovoltaic device model (GPVDM) software for various device structures. The essential parameters of the cell such as short circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE) are evaluated for these structures. The simulation result shows the performance of the cells could be highly affected by both interfacial and active layers. Among the demonstrated device structures, better performance is observed for device structure ITO/V2O5/PTB7 : PC70BM/TiOx/Al with 10.4 mA/cm2 short circuit current density, 0.89 V open-circuit voltage, 59.5% fill factor, and 6.1% power conversion efficiency. The study is useful for a better understanding of the powerful effect of the materials in the device structure on the performance of the cell and thereby determines the proper device structure and materials for improving the performance of the cell.


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## Body

## 1. Introduction

Nowadays, the serious problems observed in the world are global warming and the enhanced greenhouse effect as the result of fossil fuel-based energy consumption [1, 2]. The solar energy that is sustainable and inexhaustible is one of the renewable sources of energy converted sunlight into electrical energy using solar cell devices which are dubbed as the best solution for the aforementioned problems. The conversion of solar radiation into electricity using solar cells has no pollutants, and the energy obtained is proportional to the quantity of absorbed solar radiations [3, 4]. Based on the materials used to fabricate the cell, there are various kinds of solar cells; however, the challenges facing solar cells are efficiency, lifetime, cost, and the availability of the materials for fabrication [5–9]. Among various kinds of solar cells, polymer-based organic solar cells have many interesting features and research progress on polymer solar cells, and polymer-based organic solar cells are a promising renewable source of energy for the coming generation [10–13]. The techniques, low fabrication cost, flexibility, and portability make polymer solar cells advantageous over silicon-based solar cells [14–17]. The power conversation efficiency of nonfullerene acceptor-based polymer solar cells is maintained over 16% through various techniques [18–20]; however, fullerene-based polymer solar cells are still low with respect to nonfullerene. This work focuses on the method of optimizing the performance of the bulk heterojunction conjugated polymer and electron acceptor fullerene-based polymer solar cells that can be easily synthesized by a simple process based on device structure using materials-based laboratory general-purpose photovoltaic device model (GPVDM) software. GPVDM is the freely available software designed to simulate optoelectronics devices, such as OPV, OLED, and OFTE [21].The basic device structure of polymer solar cells comprises indium tin oxide (ITO) coated glass used as a front electrode which is considered as an anode, light-absorbing layer, and back electrode for photon-created electrons collector which acts as a cathode [22] as shown in Figure 1. Few experimental investigations of the cell performance based on device structure have been carried out; however, the detailed analysis of device architecture impact on the cell performance is still an infant and not yet evaluated using GPVDM software. The most important light-absorbing layer was a blend of the donor and acceptor organic materials that play a vital role in the creation of excitons (electron-hole pairs) and is also used to reduce the recombination of electron-hole that minimize the performance of the cell [23, 24]. In addition to the active layer, the electron and hole transport layers between the active layer and electrode are used to reduce the recombination of the free charge carriers and also prevent the interpenetration of the active layer into the electrode during device fabrication [25]. Thus, an appropriate selection of materials for device structure is very important to optimize cell performance.Figure 1
The basic device structure of polymer solar cells.In this study, we investigate the impact of device architecture on the performance of the cell for different structures. In the simulation, the various light-absorbing materials along with different thin films as hole and electron transport layers are conducted in order to optimize the performance of the cell. Figures2(a) and 2(b) show the molecular structures of PTB7 and PC70BM, respectively, that give better performance among used active layers blend, and the list of parameters used for simulation purposes are given in Table 1.Figure 2
Molecular structure of PTB7 (a) and PC70BM (b) (colors: white represents hydrogen, red represents oxygen, yellow represents sulfur, green represents fluorine, and dark grey represents carbon).
(a)(b)Table 1
Electrical parameters used for simulation [26–28].
ParametersValueBand gap of P3HT (eV)1.9Band gap of PTB7 (eV)1.84Band gap of PCBM (eV)2.4Band gap of PC70BM (eV)1.8Electron mobility (cm2/V.s)1.0 × 10−3Hole mobility (cm2/V.s)2.0 × 10−4Donor concentration (m−3)2.8 × 1027Acceptor concentration (m−3)2.8 × 1025


## 2. Result and Discussion

The exciton generation by a photon in a blend of donor and acceptor and subsequent separation of exciton into electron and hole to generate power are discussed in detail. Two blends of donor and acceptor along with various electron and hole transport layers at different thicknesses were used to investigate the performance of the cells with the help of GPVDM software that consists of full information of the electrical and optical properties of the materials used in the device structure. GPVDM software for solar cell simulation describes electron and hole transport based on bipolar drift diffusion in 1D as that can be given by equations (1) and (2) [29]. Figure 3 illustrates the electrical simulation window of GPVDM used to analyze the cell performance.(1)Jn=qμen∂ELUMO∂x+qDn∂n∂x,(2)Jp=qμpp∂EHOMO∂x−qμp∂p∂x.Figure 3
Electrical simulation window for the architecture of the studied solar cell.Through the simulation work, dependencies of device performance on the structure were clarified by changing donor/acceptor materials and electron transfer layers at different thicknesses. The comparison of the simulation results for different configurations demonstrated that the architecture of the device is critically important to achieving performance improvement of the cell. For those various active and different interface layers, the current density versus voltage is shown in Figure4. Figure 4(a) shows the current density versus voltage for the active layer of P3HT : PCBM and Figure 4(b) for the active layer of PTB7 : PC70BM. We also investigate the performance of the cells for TiOx and ZnO as electron transfer layers at the same thickness. The comparison of the simulation results demonstrated that the cell with structure ITO/V2O5/PTB7 : PC70BM/TiOx/Al with 20 nm TiOx as electron transfer layer together with 5 nm vanadium pentoxide (V2O5) hole transfer layer shows better performance according to our investigation. As shown in the simulation results, the thickness and materials used for the interface layers also play a major role in cell performance. This is due to the ohmic contact of low work function metal oxide with electron acceptor that makes low series resistance, selectively transfers electron, and improves parallel resistance [30]. For all device structures, the current density (J) versus voltage (V) is evaluated under solar irradiance of 100 mW/cm2 at constant room temperature.Figure 4
Current density vs. voltage for (a) P3HT : PCBM and (b) PTB7 : PC70BM active layers with different interfaces used as hole and electron transfer layer with various thicknesses.
(a)(b)Figure5 illustrates the photon absorber density in each component of the device structure ITO/V2O5/PTB7 : PC70BM/TiOx/Al that indicates the wide absorption spectrum edges which match in the range of visible photon wavelength spectrum. As indicated in the figure, the high photon absorbed density observed in the active layer could play enhancing role in the creation of more excitons to optimize the performance of the cell.Figure 5
Absorbed photon density in different components of the device architecture of ITO/V2O5/PTB7 : PC70BM/TiOx/Al.The evaluated short circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE) of the various cell structures are given in Table2. Better performances are obtained for 20 nm TiOx thick electron transfer layer for the device structure ITO/V2O5/PTB7 : PC70BM/TiOx/Al with 10 mA/cm2 short circuit current density, 0.89 V open-circuit voltage, 59.5% fill factor, and 6.1% power conversion efficiency. As a result of photon absorption by the active layer, excitons are created with sequential charge carrier separation into hole and electron at the interface of donor and acceptor. Electron and hole generation rate as the position of device components have been demonstrated and shown in Figures 6(a) and 6(b), respectively. Accordingly, both charge carriers’ generation rates are high in the bulk heterojunction blend of donor and acceptor. Thus, the bulk heterojunction used as an active layer plays a major role in polymer solar cell efficiency optimization.Table 2
Evaluated parameters of the cells for various active and interface layers.
Active layerThickness of interface layerJSC (mA/cm2)VOC (V)FF (%)PCE (%)PTB7 : PC70BMTiOx/20 nm10.40.8959.56.1ZnO/20 nm10.20.8957.75.6P3HT : PCBMTiOx/20 nm10.10.60168.94.4ZnO/40 nm10.40.60268.84.7Figure 6
Generation rate of (a) electron and (b) hole for ITO/V2O5/PTB7 : PC70BM/TiOx/Al device architecture.
(a)(b)The absorbance of PTB7 : PC70BM and external quantum efficiency (EQE) of the device structure of ITO/V2O5/PTB7 : PC70BM/TiOx/Al were demonstrated and are shown in Figures7(a) and 7(b), respectively. As shown in Figure 7(a), the device shows better absorption in visible spectra, and in Figure 7(b), EQE follows the absorption in the visible spectrum and shows opposite to the absorption in UV [27, 31].Figure 7
(a) Absorbance of PTB7 : PC70BM and (b) EQE spectra of ITO/V2O5/PTB7 : PC70BM/TiOx/Al.
(a)(b)


## 3. Conclusion

The work widely shows the importance of device structure to optimize the performance of polymer solar cells and helps to understand the challenge in developing the active layer and the interfacial influence on the cell performance optimization. The simulation result with 5 nm V2O5 thickness as hole transport layer and 20 nm TiOx as electron transport layer with ITO/V2O5/PTB7 : PC70BM/TiOx/Al device structure shows 10.4 mA/cm2 short circuit current density, 0.89 V open-circuit voltage, 59.5% fill factor, and 6.1% power conversion efficiency. The study gives guidance for a better understanding of the powerful impact of the structure of materials on the performance of the cell and helps to find the solution in the progress of improving the efficiency of polymer solar cells.



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