Solution-Processed in Situ Growth of CuInS₂ Nanoparticle Films for Efficient Planar Heterojunction Solar Cells with a Dual Nature of Charge Generation

A facile solution-processing method featuring the applications of a stable molecular precursor solution and the repetition of spin-coating and annealing for film growth is reported for the in situ growth of CuInS2 nanoparticle films on TiO2/CdS film to provide TiO2/CdS/CuInS2 ternary superstrate heterojunction films for efficient solar cells. The novel and stable planar solar cells are fabricated using Spiro-OMeTAD as hole transporting material (HTM) and achieved a peak power conversion efficiency of 6.31% under AM 1.5 illumination at the optimized CuInS2 thickness of 200 nm and annealing temperature of 270 °C for CuInS2 formation. The efficiency is the highest among the CuInS2-based planar solar cells derived from molecular precursor methods; in particular, adopting HTM results in the open-circuit voltage up to 0.78 V, much higher than that of the similar devices without HTM. Our results demonstrate a dual nature of excitonic and nonexcitonic charge generation mechanisms in such solar cells. A depletion-field-assisted homojunction-interfacial charge transfer model is proposed to explain the generation and transfer of free charge carriers in the CuInS2 nanoparticle films, and the total charge generation in the TiO2/CdS/CuInS2 solar cells gets well elucidated from the model in combination with excitonic feature. Moreover, the CdS layer is found to act as an effective interfacial spacer to prevent the charge carriers generated upon CuInS2 absorption from recombination.