What Limits the V_OC of Br-based Perovskite Solar Devices?

Thanks to the efforts of many research groups worldwide in halide perovskite (HaP) research, >22 % efficient small photovoltaic (PV) devices have been reported [1]. Such devices, based on HaPs with mostly iodide as halide, can show a remarkably low voltage loss (E_G-V_OC) of ~ 0.4 V. Unfortunately, up to now, for the higher bandgap HaPs, where the halide is only Br, the highest reported V_OC still leaves (E_G-V_OC) ~ 0.75 V [2]. This large loss severely detracts from the potential of HaPs that have enough Br substitution for I, for optimal use in the higher E_G-based cell for tandem configurations or spectral splitting systems. The origin of this increased loss remains an open question.

There are 3 main issues to be checked to understand what limits the V_OC from getting closer to the theoretical maximum value for pure APbBr₃-based PV cells (~2.0 V).

–1- what is the absorber’s in-gap density of states and what range of energies have these states in the material’s band gap;

-2- as -1-, for interfacial in-gap states @ the HaP-hole/-electron transport layer interface;

-3- what is the mismatch in energy level alignment of the cell’s layers, to see if this can explain the significant energetic losses?

Here we used (FM_0.85MA_0.1Cs_0.05)PbBr₃ (from hereon mixed-cation) as the photo-absorber. Previously we showed that this mixed-cation HaP has relatively long carrier diffusion lengths and low carrier density compared to single cation Br-based HaPs, more resembling the I-based HaPs, with which the small (E_G-V_OC) cells can be made.[1]

By using optoelectronic measurements such as Contact Potential Difference, Surface Photovoltage, Electroluminescence and Capacitance Voltage on mixed cation Br-based HaP stand-alone films, as well in different device configurations, we can now provide insights into the origins of the large (E_G-V_OC) for the BR-based HaP-based PV cells and suggest how these losses may be minimized.