How cation migration across a 2D/3D interface dictates perovskite solar cell efficiency

Perovskite solar cells with 2D/3D architecture are claimed to exhibit better stability compared to pristine 3D films at room temperature. However, under illumination and/or heat, cation migration causes the exchange of the bulky spacer cations (2D phase) with the smaller A-site cations (3D phase), creating a gradient heterostructure at the 2D/3D interface. We have evaluated the performance of BA2MAPb2I7/MAPbI3 (2D/3D) and MAPbI3 (3D) solar cells at different temperatures, while simultaneously probing the absorption changes of the 2D/3D perovskite layer and the photovoltaic performance of solar cell devices. The 2D/3D solar cells were more stable at room temperature but exhibited deterioration of photovoltaic performance at high temperatures. By employing in situ measurements of operating solar cells to track both the photoconversion efficiency and absorption changes at different temperatures, we show that the cation exchange at the 2D/3D interface contributes to the efficiency losses.