Unraveling the role of energy band alignment and mobile ions on interfacial recombination in perovskite solar cells

A key limitation in perovskite solar cell (PSC) performance is suboptimal electronic properties at the perovskite–transport layer (TL) interfaces, which result in parasitic nonradiative recombination. Interface recombination depends on the concentration of recombination-active defects, but as a recombination event requires both an electron and a hole, the magnitude and sign of charge accumulation at the perovskite–TL heterojunctions are also critical. Here, we employ a well-established numerical ion-electron drift-diffusion model of PSCs to illustrate how the work function of the transport layer is an important factor in determining the total recombination activity at the perovskite interfaces. We show that the equilibrium electrostatics of the perovskite–TL heterojunctions, which are determined by the work function difference between the two materials, can result in increased recombination rates for any given concentration of interface defects. As a case study, we compare PSCs incorporating a NiO_X hole transport layer with those with a spiro-OMeTAD. We show that the work function of NiO_X can induce greater electron accumulation at the perovskite–NiO_X interface, which leads to increased interface recombination. Finally, a higher ion concentration is found to be beneficial to overall device performance by displacing accumulated electrons or holes at the TL interfaces and thus reducing recombination rates.