In situ Al2O3 incorporation enhances the efficiency of CuIn(S,Se)2 solar cells prepared from molecular-ink solutions

Abstract

We report an efficiency enhancement of solution-processed CuIn(S,Se)₂ (CISSe) thin film solar cells via in situ incorporation of Al₂O₃. These films were produced using inks containing CuCl, InCl₃, AlNO₃ (Al/Al + In: 0.1) and thiourea dissolved in methanol. After spin coating of these solutions in air, samples were subjected to a selenization process. Auger electron spectroscopy depth-profiling analysis showed that Al is evenly distributed throughout the bulk of the film. Transmission electron microscopy revealed that AlNO₃ precursor reacted with oxygen to form nanosized amorphous Al₂O₃ grains located within the bulk and grain boundaries of CISSe, as well as at both the top and bottom interfaces. Power conversion efficiency (PCE) as high as 11.6% (J_SC: 35.8 mA cm⁻², V_OC: 518 mV, FF: 62.2%, no anti-reflection coating) was achieved with Al–CISSe solar cell devices integrated with CdS (chemical bath deposition, thickness: 80 nm) and ZnO/ITO bilayers (sputtered, thickness: 300 nm). The average PCE (10.1%, 〈J_SC〉: 34.5 mA cm⁻², 〈V_OC〉: 491 mV, 〈FF〉: 59.8%) was nearly 4% (absolute) higher than that measured on CISSe baseline cells fabricated from solutions without Al (〈PCE〉 = 6.4%, 〈J_SC〉: 32.8 mA cm⁻², 〈V_OC〉: 410 mV, 〈FF〉: 47.3%). This in situ Al₂O₃ incorporation is speculated to play a role in the enhancement of the V_OC and FF of the devices through passivation of defects in CISSe reducing interface and bulk recombination, as evidenced by a reduced defect density and an increased activation energy of the dominant recombination mechanism from capacitance and temperature-dependent V_OC measurements, respectively.