Investigation of charge collection layers for thin film rhenium sulfide solar cells

Rhenium sulfide (ReS2) is an exciting two-dimensional (2D) material employed in various optoelectronic applications due to panchromatic visible absorption properties. However, unlike other transition-metal dichalcogenides (TMDs) such as Mo(S/Se)2 and W(S/Se)2, ReS2 has not been explored for solar energy conversion applications. In this work, ReS2 nanolayers are employed as photoabsorber in both liquid and solid-state dye-sensitized type solar cells, in combination with TiO2 and SnO2 as electron transporting layers (ETLs). Various devices with compact/mesoporous TiO2 and mesoporous SnO2 ETLs were tested for photovoltaic performance with spiro-OMeTAD hole conductor as well as with different redox mediators. While no photovoltaic response was seen for mesoporous TiO2 (both liquid and solid-state), a clear photovoltaic performance was observed for mesoporous SnO2 solid-state devices. This was attributed to the energy level alignments unsuitable for charge carrier injection from ReS2 to ETL in the former and suitable in the latter cases. In the case of only compact layer TiO2 ETL, devices in both solid and liquid-state configurations exhibited photovoltaic response with latter showing higher photocurrent than the former, due to additional role played by the energy level of redox mediator. While impedance measurements revealed limitations in conductivity of ReS2 photoabsorber, presence of photovoltaic response in them indicate the significant role played by the energy level alignment of individual components. The presence of photovoltaic effect for compact layer TiO2 devices, in spite of unsuitable energy level alignment seen for TiO2, is attributed to the charge carrier injection assisted by the trap states.