Charge Injection, Carriers Recombination and HOMO Energy Level Relationship in Perovskite Solar Cells

TLDR

The authors compare polymeric hole‑transport materials with spiro‑OMeTAD in perovskite solar cells to assess their impact on device performance. They employ advanced photoinduced spectroscopy and photoinduced differential charging to quantify recombination currents and correlate them with short‑circuit current measurements under standard illumination. The results show no correlation between HTM HOMO levels and open‑circuit voltage; spiro‑OMeTAD and P3HT share a HOMO of ~5.2 eV yet differ by ~200 mV in VOC, a shift that aligns with charge‑voltage measurements, and polymeric HTMs exhibit less efficient hole transfer while their recombination currents agree with the measured JSC.

Abstract

We present a comparative study between a series of well-known semiconductor polymers, used in efficient organic solar cells as hole transport materials (HTM), and the state-of-the art material used as hole transport material in perovskite solar cells: the spiro-OMeTAD. The observed differences in solar cell efficiencies are studied in depth using advanced photoinduced spectroscopic techniques under working illumination conditions. We have observed that there is no correlation between the highest occupied molecular orbital (HOMO) energy levels of the organic semiconductors and the measured open-circuit voltage (VOC). For instance, spiro-OMeTAD and P3HT have a comparable HOMO level of ~5.2 eV vs vacuum even though a difference in VOC of around 200 mV is recorded. This difference is in good agreement with the shift observed for the charge vs voltage measurements. Moreover, hole transfer from the perovskite to the HTM, estimated qualitatively from fluorescence quenching and emission lifetime, seems less efficient for the polymeric HTMs. Finally, the recombination currents from all devices were estimated by using the measured charge (calculated using photoinduced differential charging) and the carriers' lifetime and their value resulted in accordance with the registered short-circuit currents (JSC) at 1 sun.