Fabrication of Modules for Water Electrolysis Using Organic Solar Cells with Non-Fullerene Acceptors
Authors: Hayashi Juri , Aoki Atsushi 
Published: December 2024

Hydrogen energy is expected to contribute to a decarbonized society because it does not emit CO2 during combustion. In particular, CO2 emissions from hydrogen production can also be prevented by utilizing renewable energy such as solar light and wind power. As water electrolysis is one of major hydrogen production methods, combination of water electrolysis and solar cell can make hydrogen production without CO2 emission. However, single solar cells usually have not enough voltage to electrolyze water. Therefore, modularization of several solar cell units is needed to electrolyze water to hydrogen. We have fabricated organic thin-film solar cells using organic materials in the photoelectric conversion layer and have aimed to achieve highly efficient conversion of light energy to hydrogen energy through water electrolysis using these cells.

In this study, we fabricated solar cell modules for water electrolysis using two donor polymers, P3HT and PBDB-T and fullerene derivative, PC61BM and two non-fullerene acceptors, ITIC and IT-4F. The solar cell architecture consisted of indium tin oxide (ITO) cathode/ZnO/photoactive layer/MoO3/Ag anode. ZnO was deposited by sol-gel method, polymer donor:fullerene or non-fullerene acceptor by spin coating method, and MoO3 and Ag by vacuum deposition method.

First, three types of solar cells composed of A fullerene-based P3HT:PC61BM, non-fullerene-based PBDB-T:IT-4F and PBDB-T:ITIC were fabricated, and their performance was evaluated using a solar simulator. The short-circuit current density (Jsc) and open-circuit voltage (Voc) were increased by changing to the non-fullerene system, indicating that the non-fullerene system has longer wavelength absorption and larger energy gap between HOMO of donor polymer and LUMO of acceptor, respectively. In addition, the PBDB-T:ITIC system was expected to reduce the number of series solar cell units in the module for water electrolysis because Voc was higher than that of the PBDB-T:IT-4F system. The photovoltaic performance of the PBDB-T:ITIC system was then evaluated by changing film thickness in the PBDB-T:ITIC solar cell. Jsc reached its maximum at the film thickness indicating absorbance of 0.73 at 637 nm, while Voc did not depend on film thickness, and the fill factor (FF) decreased with increasing film thickness. As a result, the power conversion efficiency (PCE) was achieved to be 6.75%. Currently, the maximum output voltage (Vmax) is 0.63 V. Therefore, it was found that a module with three single solar cell units connected in series on the same substrate was required in order to achieve above the voltage of 1.23 V, which is thermodynamically necessary for water electrolysis.

3-cell series module with an area of 1.16 cm2 per cell were fabricated and their performance was evaluated. As a result, ideally, a 3-cell series module would have similar Jsc to that of a single solar cell, and Voc becomes about 3 times that of a single solar cell. However, the 3-cell series module has a particularly low FF of 0.40 compared to 0.48 for the single solar cell, resulting in decrease in PCE to 4.38%. To investigate the effect of cell area, single-cell solar cells with 1.16 and 0.126 cm2 were then fabricated and evaluated for performance. Jsc and Voc are comparable whereas FF is 0.57 for the small area, while it is lower at 0.48 for the large one. This indicates that an increase in area leads to an increase in series resistance and a decrease in FF. In addition, to investigate the effect of module structure, we evaluated the performance of 3 single solar cells with a cell area of 1.16 cm2 connected in series and parallel by crocodile clip. As a result, a large decrease in FF was observed for both connections, suggesting that crosstalk happened, caused by that ZnO, the electron transport layer was uniformly coated on the substrate. We will examine whether the performance of single solar cells can be maintained by changing the module structure.