Defect control and performance optimization in Li⁺/Na⁺ Co-doped HTL-free Cs₂AgBiBr₆ double perovskite solar cells: Toward an efficient HTL-free architecture

The Cs₂AgBiBr₆ double halide perovskite has catered for the problem related with lead toxicity and stability in perovskite solar cells (PSCs). Unfortunately, the unstable hole transport layer (HTL) such as spiro-OMeTAD, limited diffusion length of charge carriers, introduced interface defects, and inferior hole extraction capability results to poor performance. In this work, we presented, a simplified design and modeling of an improved stable HTL-free perovskite solar cells with Li⁺ and Na⁺ co-doped Cs₂AgBiBr₆-based absorber. Guided by solar capacitance simulation software (SCAPS-1D), this present work examined the performance of PSCs with both Cs₂AgBiBr₆ and Cs_1·96Li_0·01Na_0·03AgBiBr₆ absorbing material. The Cs₂AgBiBr₆-based device gave an open circuit voltage ( V _oc), short circuit current density ( J _sc), fill factor (FF) and power conversion efficiency (PCE) of 0.31V, 9.19 mA/cm², 68.15% and 1.97%, while the Cs_1·96Li_0·01Na_0·03AgBiBr₆ device presents 0.76 V, 10.53 mA/cm², 70.54% and 5.63% as V _oc, J _sc, FF and PCE. This shows that Cs_1·96Li_0·01Na_0·03AgBiBr₆-based device was more promising. The performance of the FTO/TiO₂/Cs_1·96Li_0·01Na_0·03AgBiBr₆/C device was optimized by varying the ETL thickness and doping concentration, perovskite thickness and doping concentration, perovskite defect density and band gap to obtain 0.02 μm, 10¹⁷ cm⁻³, 1.4 μm, 10¹⁸ cm⁻³, and 10¹² cm⁻², and 1.45eV optimal values. The values were used to obtain an optimized value with the following performance; PCE = 24.33%, supported with FF of 87.34%, J _sc of 25.83 mA/cm² and V _oc of 1.08 V. The effect of series resistance, shunt resistance, work function of back contact, temperature and light intensity were evaluated. At higher work function (say W _F = 5.9 eV for Se), a PCE of ∼30% was obtained with corresponding FF of ∼84 %, J _sc of ∼30 mA/cm² and V _oc of 1.2 V. By successfully controlling defect states and improving device performance through co-doping of the absorber layer, the results open the door for the design of an advanced, high-efficiency, and reasonably low PSC with HTL elimination.