Red, White & Blue: Identifying Recombination Loss Mechanisms in Perovskite Solar Cells

Photovoltaics play a key role in the renewable energy transition, where perovskite solar cells stand out as one of the most promising with high-power conversion efficiencies, desirable characteristics, and great versatility. The efficiency of these cells is mainly limited by nonradiative recombination, whether at one of the interfaces between the perovskite layer and transport layers or within the bulk of the perovskite itself. However, it is difficult to probe which of these dominates the losses during operation, which complicates further improvements. Here we introduce and demonstrate a simple, inexpensive, and easy-to-integrate method for identification of the limiting aspect of perovskite solar cells under operating conditions in terms of recombination losses. We illuminate a device with red, blue, or white light, each absorbed differently depending on the position in the device. We show that, in perovskite solar cells, the diffusion length in the bulk is of secondary importance and rather the interfaces between layers dominate the charge carrier distributions, and thus light of different wavelengths will result in changing performance. Using the fill factor from the device characteristics for each case, we are able to identify whether the front or back interface of the cell limits the performance with nearly 97% accuracy. Finally, we apply this principle experimentally using a co-evaporated perovskite solar cell, where we show a decrease of nearly 5% in fill factor depending on the wavelength of light used.