Solution-processed Cu₂MSnS₄（M=Zn,Cd） And Cu₂SnS₃ Thin Films For Solar Cells

Conversion of sunlight into electricity by solar cells is an important way to use sustainable energy.Solution-processable inorganic semiconductor thin film materials are urgently desired in order to obtain low cost,stable and efficient solar cells.Multinary copper chalcogenides such as Cu2MSnS4(M=Zn,Cd)(CMTS)and Cu2SnS3(CTS)are promising light absorber materials for solar cells because of their earth-abundant elements,high stability to environment,suitable band gaps(Eg),and high light absorption coefficients(α).This dissertation aims to develop in-situ solution-processing methods to prepare CMTS and CTS thin films on transparent conducting substrates and to study the photovoltaic performance of their solar cells.The main research activities and conclusions in this dissertation are as follows:1)A repetition of spin-coating and annealing(RSCA)solution-processing approach,featuring the applications of a low annealing temperature(300℃),a short annealing time(5 min)and a stable molecular precursor solution,is developed to in-situ grow the compact thin films of Cu2ZnSnS4(CZTS)nanoparticles on ITO substrates.The as-prepared CZTS thin films have Eg=1.48 eV andα>104 cm-1.It is found that the inverted MAPbI3-basedplanar heterojunction perovskite solar cells with the CZTZ films as hole transporting materials(HTMs),structured as ITO/CZTS/MAPbI3/PCBM/BCP/Ag,exhibit the device performance remarkably dependent on CZTS film thickness and the CZTS film with a thickness<100 nm is an effective HTM for the perovskite solar cells;moreover,the efficiency and stability of the perovskite solar cells are much higher than the counterpart devices with PEDOT:PSS film as HTM2)A new RSCA solution-processing method featuring the applications of a low annealing temperature(250-350℃),a short annealing time(5 min),and a new and stable molecular precursor solution,is used to in-situ grow porously nanostructured thin films of Cu2SnS3(CTS)with Eg=1.35-1.5 eV and α>104cm-1,and prepare the novel planar heterojunction solar cells with a structure of FTO/TiO2/CdS/CTS/P3HT/MoO3/Ag.It is found that the solar cells display the device performance greatly dependent on the annealing temperature and the CTS film thickness;the solar cell with the extremely CTS thin film(ca.60 nm in thickness)of nanoparticles prepared at 300℃ achieve the best power conversion efficiency of 2.03%under AM 1.5G(100 mW/cm2),thanks to the shortest transit time and the longest lifetime of photogenerated electrons wherein.3)A solid-state Cd-diffusion strategy is developed to prepare bulk Cu2CdSnS4(CCTS)thin film of monolayered large grains by sulfurizing the nanostructured CdS(50 nm thick)/CTS(230-470 nm in thickness)planar heterojunction films at 550℃.It is found that,after sulfurization,a phase transformation from CTS through a mixture of Cd-doped CTS and CCTS to pure CCTS at the fixed CdS thickness takes place as the CTS film thickness decreases due to the different Cd content in the films rather than the Cd diffusion length wherein;the thickness of resulting bulk CCTS film is a bit smaller than that of the initial CTS film.It is also found that pure CCTS always formed in the tested sulfurization temperature range of 520-580℃ at the CdS(50 nm)and CTS film(230 nm)thickness.The prepared bulk CCTS films have Eg=1.30-1.36 eV and α>105 cm-1 within 300-750 nm.The planar heterojunction solar cells with a structure of FTO/TiO2/CCTS/P3HT/MoO3/Ag displays a performance tightly dependent on the sulfurization temperature due to the resulting microstructure in the CCTS film,and the bulk CCTS film of 208 nm in thickness prepared under the condition of 230-nm-thick CTS film,50-nm-thick CdS film and sulfurization temperature of 550℃ achieves the best power conversion efficiency of 3.81%under AM 1.5G(100 mW/cm2).4)Bulk CZTS films of monolayered large grains with sizes extending the film thickness(515 nm)on FTO and FTO/TiO2 substrates were prepared by the sulfurizing the CZTS nanoparticles at 500-580℃.It is found that the sulfurization temperature>550℃ is necessary to obtain the bulk CZTS films,and the usage of TiO2 interlayer on FTO substrate is capable to prevent the FTO layer from damage during sulfurization process and improve the adhesion of the CZTS film to the FTO substrate.Preliminarily,the planar heterojunction solar cell structured as FTO/TiO2/CZTS/P3HT/MoO3/Ag achieves the power conversion efficiency of 2.11%under AM 1.5G(100 mW/cm2).