Can Al2O3 Tunnelling Layer Revolutionize Lead Free Germanium Based Perovskite Solar Cells?

The urgent need for renewable energy solutions has made perovskite solar cells pivotal in combating climate change. Among lead-free alternatives, germanium-based perovskites, particularly CsGeI3 with a 1.6 eV bandgap, show great promise for single-junction solar cells. However, their full potential remains untapped due to significant open-circuit voltage (Voc) losses arising from non-radiative recombination and low fill factor (FF), which limit device efficiency. In this work, we present for the first time the simulation of a p-i-n solar cell with the structure ITO/NiOx/CsGeI3/Al2O3/PCBM/C60/Ag, introducing Al2O3 as a tunnel layer to optimize charge transport and suppress recombination at the perovskite/PCBM interface. An optimal 4 nm Al2O3 layer not only enhances electron tunnelling efficiency but also prevents holes from the perovskite layer from recombining with electrons at the ETL, thereby significantly increasing recombination resistance, as revealed by Nyquist plot analysis. These optimizations result in a notable Voc of 1.26 V and FF of 81%, boosting single-junction efficiency by 4%. Physics based understanding of this improvement have been discussed through Capacitance-Voltage (CV) and Impedance-Spectroscopy (IS) study. Furthermore, thickness optimization across the device enables the construction of a tandem cell, pairing the CsGeI3-based top cell with a Silicon H-I-T bottom cell in a 2T configuration. The tandem cell achieves a remarkable efficiency of 31.39%, with Voc of 1.97 V, Jsc of 19.40 mA/cm², and FF of 82.17%. This study shows the transformative potential of Al2O3 tunnel layers in advancing high-performance lead-free perovskite-silicon tandem solar cells, offering a critical pathway for sustainable and efficient photovoltaic solutions.