How Do Surface Polar Molecules Contribute to High Open-Circuit Voltage in Perovskite Solar Cells?

To date, the improvement of open-circuit voltage (VOC ) offers a breakthrough for the performance of perovskite solar cells (PSCs) toward their theoretical limit. Surface modification through organic ammonium halide salts (e.g., phenethylammonium ions PEA+ and phenmethylammonium ions PMA+ ) is one of the most straightforward strategies to suppress defect density, thereby leading to improved VOC . However, the mechanism underlying the high voltage remains unclear. Here, polar molecular PMA+ is applied at the interface between perovskite and hole transporting layer and a remarkably high VOC of 1.175 V is obtained which corresponds to an increase of over 100 mV in comparison to the control device. It is revealed that the equivalent passivation effect of surface dipole effectively improves the splitting of the hole quasi-Fermi level. Ultimately the combined effect of defect suppression and surface dipole equivalent passivation effect leads to an overall increase in significantly enhanced VOC . The resulted PSCs device reaches an efficiency of up to 24.10%. Contributions are identified here by the surface polar molecules to the high VOC in PSCs. A fundamental mechanism is suggested by use of polar molecules which enables further high voltage, leading ways to highly efficient perovskite-based solar cells.

Pre-Buried Additive for Cross-Layer Modification in Flexible Perovskite Solar Cells with Efficiency Exceeding 22.

Halide perovskites have shown superior potentials in flexible photovoltaics due to their soft and high power-to-weight nature. However, interfacial residual stress and lattice mismatch due to the large deformation of flexible substrates have greatly limited the performance of flexible perovskite solar cells (F-PSCs). Here, ammonium formate (HCOONH4 ) is used as a pre-buried additive in electron transport layer (ETL) to realize a bottom-up infiltration process for an in situ, integral modification of ETL, perovskite layer, and their interface. The HCOONH4 treatment leads to an enhanced electron extraction in ETL, relaxed residual strain and micro-strain in perovskite film, along with reduced defect densities within these layers. As a result, a top power conversion efficiency of 22.37% and a certified 21.9% on F-PSCs are achieved, representing the highest performance reported so far. This work links the critical connection between multilayer mechanics/defect profiles of ETL-perovskite structure and device performance, thus providing meaningful scientific direction to further narrowing the efficiency gap between F-PSCs and rigid-substrate counterparts.