Cuprous thiocyanate as an inorganic hole transport material for carbon-based flexible perovskite solar cells

Flexible perovskite solar cells (F-PSCs) are highly promising for harvesting solar energy in various environments, both indoors and outdoors. Nonetheless, one of the main hurdles to the widespread commercial use of F-PSCs is the thermal evaporation of the metal top electrode, a time-consuming process that substantially increases the cost related to both raw materials and fabrication equipment. Consequently, developing effective alternatives is essential for harnessing the full potential of this technology. One promising approach is to replace the top metal electrode with carbon-based materials, which can effectively serve as both the hole transport layer (HTL) and back electrode. These materials are low cost and compatible with inexpensive, simple, and scalable deposition techniques, such as blade coating. However, HTL-free carbon-based PSCs (C-PSCs) currently suffer from power conversion efficiency (PCE) lower than their metal counterparts, due to inefficient charge transfer and collection, associated with an ineffective perovskite (PVK) and carbon electrode interface. By utilizing a suitable HTL between the PVK and the carbon electrode, the charge extraction can be effectively improved and the interfacial recombination reduced. Throughout this work, a screening of suitable hole transport materials (HTMs) was carried out to select the most promising candidate to improve the performance of C-PSCs on flexible substrates. Copper(i) thiocyanate (CuSCN) was employed as the HTL with a wide band gap (3.5-3.8 eV). At the optimized concentration of 10 mg ml-1, a PCE of 9.4% was achieved on 1 cm2 flexible devices. The results obtained were compared with the performance of F-PSCs with gold top electrodes using organic PTAA as the HTL as state-of-the-art reference. The optimization of the HTL allowed for the demonstration of a significant improvement in the performance of the device, which could pave the way for the large-scale commercialization of PSCs with low environmental impact and promising cost-effectiveness.