The case against hole injection through SAMs in perovskite solar cells

Abstract

Self-assembled molecules (SAMs) are widely used as hole-selective contacts in perovskite solar cells (PSCs). They are traditionally designed to facilitate charge injection by aligning their highest occupied molecular orbital (HOMO) with the perovskite's valence band. However, interfacial energy barriers may not necessarily hinder performance, and in some cases, can boost the devices' open-circuit voltage, thereby improving efficiency. This raises an important question: is injection through the SAM, to promote charge extraction, a necessary or even desirable criterion? To investigate this, we compare two Spiro-OMeTAD derivatives: Spiro-A, which is directly attached to the indium-doped tin oxide (ITO) anode by a carboxylic acid moiety, forcing the HOMO level to be in close proximity to the ITO, and Spiro-B, which incorporates a spacer group to separate the HOMO from ITO spatially. Contrary to expectations, Spiro-B achieves a higher open-circuit voltage (V_OC) and power conversion efficiency (PCE) than Spiro-A despite having a lower built-in potential (V_BI). Stabilise and pulse (SaP) measurements confirm that Spiro-B promotes charge accumulation by reducing interfacial recombination, thus increasing quasi-Fermi level splitting (QFLS). Furthermore, the carbazole-based reference SAM (Me-4PACz) achieves the highest V_OC, demonstrating that direct charge injection is not always beneficial. These results challenge conventional molecular design strategies, emphasising the importance of controlling interfacial recombination over maximising charge injection. This work provides new insights for optimising SAMs in PSCs, offering a pathway toward higher efficiency through tailored energy barriers and charge accumulation dynamics.