Preparation Of CuInS₂-based Inorganic Heterojunction Films For Solar Cells

Photovoltaic conversion of renewable solar energy into electricity for sustainable energy production requires efficient,stable,and low-cost solar cells.Developing solution-processed inorganic heterojunction solar cells is a practical scenario in virtue of the high charge mobility and good stability of inorganic semiconductors.Copper indium sulfide(CuInS2),an environmentally-friendly and direct band gap material,is considered to be a promising light-harvesting candidate for solar cells,due to its suitable band gap(Eg)of 1.5-1.6 eV matching the solar spectrum well,large absorption coefficient(??105 cm-1)in the visible spectral range,and high stability.In this dissertation,high-quality CuInS2 nanoparticle films are in situ solution-processed,novel CuInS2-based planar and bulk heterojunction solar cells are fabricated,and the correlation between the device performance and fabrication conditions and the optoelectric conversion mechanism involved are studied.The main research activities and conclusions are summarized as follows:(1)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 established 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 and achieve a peak power conversion efficiency of 6.31%under AM 1.5 illumination(100 mW/cm2)at the optimized CuInS2 thickness of 200 nm and annealing temperature of 270? for CuInS2 formation.The efficiency is the highest amongst 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 from femtosecond transient absorption spectroscopy demonstrate a dual nature of excitonic and non-excitonic 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 film,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.(2)A solution-processed all-inorganic planar heterojunction solar cell based on the CuInS2 nanoparticle film is prepared by using an antimony trisulfide(Sb2S3)nanoparticle film as interfacial layer between the CuInS2 photon harvesting layer and cathode.All of the component layers in the solar cell are in a superstrate architecture and sequentially in situ grown on transparent conducting glass by solution-processing methods.The dependences of device performance on the thickness of Sb2S3 film and the reduction of hole-trapping centers in the Sb2S3 film by thioacetamide treatment are investigated.The optimized all-inorganic device exhibits a best power conversion efficiency of 4.85%under AM 1.5G illumination and an excellent thermal stability.It is found that the Sb2S3 interfacial layer sandwiched between CuInS2 photon harvesting layer and counter electrode has dual functions,that is,to provide the complementary absorption after CuInS2 attenuation and to act as an effective hole transporting layer to selectively extract photogenerated holes for effective charge collection efficiency.(3)Indium sulfide(In2S3)is a promising non-toxic electron-transfer/buffer material for thin film solar cells,due to its ideal band gap(2.5-3.0 eV).A facile molecular precursor solution method is developed for preparing In2S3 nanosheet arrays,the In2S3 nanosheet array films are in situ grown on TiO2 films,and the In2S3/CuInS2 inorganic bulk heterojunction(BHJ)films are obtained by in situ growth of CuInS2 nanoparticles into the In2S3 nanosheet arrays.With Spiro-OMeTAD film as hole transporting layer on the BHJ film,the novel In2S3/CuInS2 bulk heterojunction solar cells are fabricated.The optimized all-inorganic device exhibits a peak power conversion efficiency of 2.58%under AM 1.5 illumination(100 mW/cm2)depending on the In2S3 nanosheet array height,suggesting that the In2S3 nanosheet arrays are effective electron acceptors.