| Work Detail |
Researchers at the University of Surrey in the UK built the cell with a hole transport layer (HTL) based on PEDOT:PSS and apparently managed to reduce losses caused by this compound by using a thiocyanate additive. The laboratory-sized champion device achieved an open circuit voltage of 0.875 V, a short circuit current density of 31.84 mA cm-2 and a fill factor of 83.23%. An international research team led by the University of Surrey with Imperial College London has identified a strategy to improve both the performance and stability of lead-tin perovskite solar cells, achieving a champion device with a power conversion efficiency of 23.2%, which it says is one of “the best results” achieved with this material and “importantly, a design strategy that improves the lifetime of these devices by 66%.” The researchers built the cell with a hole transport layer (HTL) based on PEDOT:PSS, a polymer known for its low-cost and easy-to-prepare properties, and said this material is widely used for mixed lead-tin narrow bandgap perovskites that are employed in all perovskite tandem solar cells and also in multi-junction solar cells. “This subcell is expected to replace silicon in current perovskite-based tandem and multijunction cells,” Imalka Jayawardena, co-author of the study, told pv magazine . The team investigated the performance loss and mechanisms of decreased stability of PEDOT:PSS-based perovskite optoelectronics. They observed that amine-containing organic cations degrade PEDOT:PSS. It can be “partially recovered by thiocyanate additives,” but the improvement comes at the expense of device stability due to cyanogen formation from the thiocyanate-iodine interaction, which the team says is accelerated in the presence of moisture. “Our work demonstrates that organic cations can diffuse into PEDOT:PSS, leading to a loss of efficiency,” explains Jayawardena. “We also show that this diffusion process can be alleviated by a thiocyanate additive. However, thiocyanate can also accelerate the degradation of the perovskite solar cell in the any-humidity process for which we have demonstrated a solution.” The biggest challenge was isolating the factors that could mask the real mechanism of diffusion and degradation, according to Jayawardena. The team observed that in the presence of moisture, thiocyanates form cyanogens, which accelerate perovskite degradation, regardless of the hole transport layer used. They also explained that the device efficiency and stability of lead-tin perovskites under ambient conditions could be improved with iodine reduction inside the bulk as a key strategy. “To mitigate this degradation pathway, we incorporated an iodine reducer into the lead-tin PSCs. The resulting devices exhibit an improved power conversion efficiency of 23.2%, one of the highest reported for lead-tin PSCs,” the team says, adding that this translated into a 66% improvement in TS80 lifetime under ambient and maximum power point tracking conditions. Additionally, the champion lab devices had an open-circuit voltage of 0.875 V, a short-circuit current density of 31.84 mA cm-2, and a fill factor of 83.23%. “In comparison, a control device showed a lower efficiency of 21.86% with a short-circuit current density of 31.53 mA cm-2, an open-circuit voltage of 0.852 V, and a fill factor of 81.41%,” the researchers explain. Looking ahead, the researchers aim to identify additives that improve the stability of perovskite absorbers, alternatives to PEDOT:PSS, and develop minimodules using environmentally friendly solvent systems. “At Surrey we are carrying out accelerated stress testing under more demanding conditions,” explains Jayawardena, who adds that high humidity and temperature testing are planned, as well as an outdoor test bed to evaluate the team’s minimodules. The novel cell concept was presented in the study “ 23.2% efficient low band gap perovskite solar cells with cyanogen management,” published in Energy Environmental Science . The research team consisted of scientists from Sichuan University, the UK National Physical Laboratory, the University of Cambridge, London South Bank University, University College London, the US Department of Energy’s National Renewable Energy Laboratory (NREL), and Fluxim AG, |
