Hole transporting materials for perovskite solar cells and a simple approach for determining the performance limiting factors†
The synthesis and characterization of three novel HTMs with different highest occupied molecular orbital (HOMO) energy levels and their performances in MAPbI3-based devices in comparison with Spiro-OMeTAD is reported. Without systematic optimization, the HTMs performed well. The devices delivered fill factors comparable to the one with Spiro-OMeTAD but suffered from short-circuit current (JSC). Interestingly, despite the significant differences in HOMO energy levels, all three HTMs generated the same open-circuit voltage (VOC). We explored the performance limiting factors of the HTMs by simple transient photovoltage/photocurrent (TPV/TPC) measurements along with drift-diffusion simulations. We found no correlation between HOMO energy levels of the HTMs and VOC of the devices. Performances of the devices are limited by high trap density as well as low carrier mobility of the HTMs, and by the shunts present in the devices. Furthermore, the high trap density and low carrier mobility of the HTM are found to induce ion migration effect in the device causing slow decaying components in TPV/TPC. Nevertheless, it is confirmed that JSC and VOC, measured at steady state, are not influenced by the ion migration effect. These HTMs can be improved further by optimizing their conductivity, trap density, morphology, and can be used as alternatives to Spiro-OMeTAD or other expensive, synthetically challenging HTMs. The simple and inexpensive approach presented in this work can also be applied for effectively evaluating charge transporting materials for perovskite solar cells.