Studies On Cu₂SnS₃ Related Thin Films And Photovoltaic Devices

Copper tin sulfide?Cu₂SnS₃,CTS?has attracted extensive attention for its high absorption coefficients(>10⁴ cm^-1)and the optimal band gap?0.8-1.8 eV?which can be used as alternative PV materials for absorbers of thin-film solar cells.At present,most CTS thin-film solar cells with high efficiency can be obtained by the vacuum method,the process is complicated and the record efficiency still has a great gap compared with CIGS and CZTS thin-film solar cells.The basic properties and synthetic method need to be further explored.Based on the low-cost sol-gel technique,the synthesis of CTS thin films has been systematically investigated including regulating its composition,optimizing annealing process to improve the quality of the films and understand the fundamental physical properties of CTS.A microstructure change induced by Na/Bi addition has also been explored.High efficiency CTS thin-film solar cell has been finally achieved.1.Based on sol-gel reaction,metal salts were added to non-toxic or low toxicity organic solvents,the precursors were synthesized by spin-coating combining with pre-annealing process.The precursors were post-annealed and the optimum sulfurization temperature has been investigated.Monoclinic CTS thin films have been synthesized with improved crystallinity.It is demonstrated that the element composition of precursors prepared with dimethyl sulfoxide is close to the stoichiometric ratio.It also shows dense surface with nanoparticles and a band gap of 0.88 eV.Precursors were sulfurized with a post-annealed process.The grain size of the film prepared at 540?reaches the micron level,the film surface is homogeneous,indicating that the monoclinic CTS can be successfully prepared by sol-gel method.2.The effects of composition modulation on the physical properties of CTS films have been investigated systematically.The modulation of Cu content has been achieved by changing the ratio of Cu/Sn in the sol.The content of thiourea in the sol has important influence on the microstructure and phase of the films as well as the performance of solar cell whether when thiourea acts as the only sulfur source or used combining with the powder of sulfur.Modulating the content of Cu in the sol has been demonstrated an effective method to adjust the Cu content in CTS thin films.CTS thin film has improved crystallnity with a ratio of 1.85 of Cu/Sn in the sol.The content of thiourea plays an important role and can be demonstrated even in the precursors.A compact and dense surface can be obtained when the molar ratio of thiourea/metal is 2-3.It is found that Cu₄SnS₄ formed when the molar ratio of thiourea/metal is 1 which can be inhibited by regulating the content of thiourea.The stability of CTS phase can be improved by a treatment with the powder of sulfur combining with thiourea.Based on the suitable content of thiourea and the powder of sulfur,CTS thin film with good crystallinity has been synthesized and CTS solar cell with efficiency of 1.51%has been achieved.3.CTS photovoltaic device was optimized by layer.For absorber,duration time and heating rate need to be further optimized.PL spectra is used to study CTS thin film with good quality,it is concluded that PL peaks conforms to DAP recombination luminescence.Finally,CTS solar cells have been fabricated with a conversion efficiency of 3.18%.The grain size of CTS thin film is improved under slow heating rate with more loss of Sn.The volatilization of SnS promotes the grain growth.XRD and Raman analysis confirm that CTS films prepared under slow heating rate have better crystallinity.The corresponding performance of the device has been also improved.As the holding time increases,the grain size of CTS thin films increases.The duration time has been optimized about 20 min.CTS phase is unstable with long duration time.Based on the analysis of the parameters of CTS solar cell,it is found that the improvement of the quality of absorber significantly improves the efficiency to 3.18%.CTS thin film with good crystallinity was selected for PL spectra analysis,Cu vacancy defect and Sn-Cu disorder defect are considered as main defects of CTS thin films.4.The influence of metal element?Bi,Na?doping on the properties of CTS thin film as well as the devices have been explored.Proper amount of Na element doping can improve the quality of CTS film effectively and promote the grain growth.The conversion efficiency CTS thin film solar cell with Na doping achievies 4.49%which is the highest efficiency of CTS solar cell synthesized by chemical method.The results indicate that Bi doping has negative effect on the grain growth of CTS thin film and induces Cu-S as secondary phase,leaving holes on surface.In a word,it should be careful to introduce Bi to CTS thin films.CTS thin film prepared with the sol in which Na/metal ratio equal to 1 has better crystallinity with denser surface and larger grain size.The conversion efficiency of the solar cell fabricated by the CTS film with1%Na doping reaches 4.49%.EQE analysis shows that the improvement of conversion efficiency is not related to buffer layer and window layers,which is attributed to the improvement of absorber.Absorber with great quality promotes the transport of photon carriers,reducing the recombination of photon carriers at the interface.By the time of writing the paper,it is the highest efficiency synthesized by chemical method and is significant to the development of CTS thin film solar cells.