Modelling and optimization of a lead and tin free Cs₂TiI₆ and La₂NiMnO₆ bilayer photovoltaic cell

Abstract Bilayer perovskite solar cells (PSCs) represent an emerging strategy for enhancing photovoltaic (PV) performance. In this work, we present a lead-free bilayer PSC architecture utilizing Cs₂TiI₆ as the primary absorber and La₂NiMnO₆ as the secondary absorber, chosen for their favourable band alignment and environmental stability. Their complementary absorption profiles efficiently harvest different portions of the solar spectrum maximizing the photon utilization. This design also avoids the use of Sn²⁺-based materials, thereby preventing common issues related to oxidation-induced degradation. WS₂ is employed as the electron transport layer (ETL), while PEDOT: PSS serves as the hole transport layer (HTL). Starting from a single-layer Cs₂TiI₆ solar cell that achieved a power conversion efficiency (PCE) of 24.3%, this bilayer architecture offered enhanced device performance, resulting in a PCE of 28.5%, with a fill factor (FF) of 75.5%, a short-circuit current density (J SC ) of 29.98 mA/cm 2 and an open-circuit voltage (V OC ) of 1.26 V under standard AM 1.5G illumination at 300 K. This architecture thus offers a blueprint for scalable, environmentally favourable bilayer photovoltaics outperforming single-absorber PSCs.