Synthesis Of Cu₂FeSnS₄ Nanocrystals And Thin Films For Solar Cells

Quaternary Cu2FeSnS4(CFTS)semiconductor has been regarded as a promising absorber candidate for thin film solar cells due to its composition of nontoxic and earth-abundant elements,high absorption coefficient(>104 cm-1)and suitable band gap(1.28-1.5 eV).In this dissertation,we mainly investigate the solvothermal synthesis of CFTS nanocrystals and the in-situ growth of CFTS nanostructured and bulk films by solution-processing methods,and apply them to practical solar cells.The main contents and results are summarized as follows:1.The CFTS nanocrystals of 3-7 nm in size are controllably synthesized by solvothermal method.Using copper(?)acetate as a fixed primary reactant,the factors affecting the nanocrystal growth such as reaction temperature,reaction time,capping agent,and the sources of sulfur,iron and tin are investigated systematically.As a result,the optimal synthesis conditions are summarized as follows:the reaction temperature is 180 ?;the reaction time is at least 16 hours;the molar ratio of Cu:Fe:Sn:S is 2:1:1:5;thiourea,ferrous sulfate and tin tetrachloride are preferred to be used as sulfur source,iron source and tin source,respectively;Octadecylamine(refer to as ODA)is preferred to be used as capping agent,and the molar ratio of ODA:Cu is set as 10:1.2.Hybrid polymer-based solar cells are fabricated by using the films of polymer MEH-PPV and CFTS nanocrystal blends as photoactive layers.The data from absorption spectroscopy and cyclic voltammetric measurements show that as-prepared CFTS nanocrystals have an optical band gap(Eg)of ca.1.66 eV.Our data of the solar cells demonstrate that the CFTS nanocrystals are the effective electron acceptors with complementary absorption for the hybrid solar cells.Moreover,it is revealed that the content-related aggregation of CFTS nanocrystals in MEH-PPV matrix imposes a remarkable effect on the device Jsc and Voc,resulting in a peak efficiency of ?=0.29%for the MEH-PPV/CTFS weight ratio of 8/2.3.Compact nanocrystalline CFTS thin films with a controllable thickness(65-190 nm)are prepared by spin-coating CFTS precursor solution followed by a short thermal annealing process.The condensed thin films of CFTS nanoparticles(ca.10 nm)exhibit an absorption coefficient(?)of>104 cm-1 within 300-1100 nm and have a band gap of 1.53 eV.The inorganic thin film solar cells structured as FTO/CFTS/CdS/Al are fabricated and CFTS thicknesses have a remarkable effect on the device Jsc,Voc and FF.It is demonstrated that the highest efficiency of ?=0.08%for the optimized CFTS thickness of 125 nm.4.A high-quality CFTS bulk thin film with a thickness of 400 nm is prepared by the sulfurization of the CFTS nanoparticle film of 320 nm in thickness.We find that sulfurization temperature has a remarkable effect on the purity,morphology,and optical property of the resulting bulk film.It is revealed that,with increasing the sulfurization temperature,a phase transition from rhodostannite to stannite structure takes place in the processed CFTS film accompanied by the increase in grain size,and the grain size eventually becomes comparable to the CFTS bulk thin film(ca.400 nm)at 580 °C.The CFTS bulk thin film exhibits an absorption coefficient(a)of>104 cm-1 within 300-1100 nm and have a band gap of 1.42 eV.