Study On N-Methyl-Pyrrolidone Precursor Solution Processed CuIn（S,Se）₂ Thin-Film Solar Cells

Chalcopyrites copper indium sulfoselenide（CIS）and copper indium gallium sulfoselenide（CIGS）are ideal semiconducting materials for thin-film solar cells due to their high absorption coefficient,tunable band gap and good stability.However,their applications are seriously prevented because commercialized modules are all fabricated from vacuum-based technologies which are based on expensive equipment and complex fabrication process.To achieve the large-scale production of CIS and CIGS solar cells,a low-cost,high throughput and roll-to-roll fabrication technology is required.Thus,many non-vacuum approaches have been developed,in which processing CIS and CIGS solar cells via precursor solution is a promising one because of many advantages such as high materials utilization rate,simple fabrication process,and perfect industrial compatibility.For the application in the industrial production,several issues for precursor solution method should be addressed,including:（1）The currently used solvents have poor compatibility with the industrial production;（2）Most of the fabrication for precursor film is conducted in an inert environment and study on the processing in ambient air is very limited;（3）The grain growth mechanism of precursor solution method is missing,which cannot provide a feasible strategy for further improving device performance;（4）Study toward flexible solar cells is very limited.To address the above issues,this dissertation focuses on the following three topics:（1）Study on the effects of annealing atmosphere on the CIS films and devices.Highly industry-suitable solvent N-methyl-pyrrolidone（NMP）is for the first time used to prepare the precursor solution with simple compounds of Cu Cl,In Cl₃·4H₂O and thiourea,and the precursor solution is used to deposit precursor films in the air and inert environment（nitrogen）,followed with the fabrication of corresponding absorbers and devices.Characterizations show that air-annealing favors the decomposition of organic species in the precursor film,which results in the smooth absorber with large and dense grains.Compared to the deposition in nitrogen,air-deposited CIS absorber has lower Urbach energy（E_U）,potential（γ_g）and band gap fluctuation(σ_opt),and the corresponding device exhibits smaller ideality factor（A）,lower open circuit voltage deficit(V_oc,def)and better photovoltaic performance.（2）Study on the grain growth mechanism in precursor solution method.Taking the NMP-based precursor solution method as example,CIS precursor films are respectively prepared from with Cu-poor（Cu/In=0.9）,stoichiometric（Cu/In=1.0）and Cu-rich（Cu/In=1.1）compositions,and the development of phase,morphology,composition and photovoltaic property during the selenization for these films has been studied.The results indicate that the chalcopyrite structure already forms in the precursor film,and thus the precursor film takes a direct phase transformation grain growth mechanism,that is,the direct conversion from sulfide chalcopyrite to selenide chalcopyrite.The results also indicate that higher Cu/In ratio leads to the faster grain growth speed and earlier selenization time,and the corresponding absorber has better morphology and potentially higher photovoltaic performance.The direct phase transformation mechanism also exhibits high tolerance to Cu/In ratio,which enables fabrication of highly efficient CIS solar cell near stoichiometric composition and avoids detrimental Cu_2-xSe phase.After preliminary optimization,a 13.6%efficient CIS solar cell has been achieved in ambient air with Cu/In ratio of 0.93.（3）Study toward fabrication of flexible CIS solar cell.Two promising strategies for roll-to-roll production of flexible CIS solar cells based on molybdenum（Mo）foil and polyimide（PI）substrate are systematically studied.Considering that Mo foil easily leads to the excessively thick Mo Se₂ layer,it is optimized through annealing Mo foil in air and sputtering new Mo layer,and hence the Mo Se₂layer becomes much thinner after the selenization.An efficiency of 5.75%has been achieved in the optimized Mo foil based CIS solar cell,which is further improved to 6.47%by Na doping.For the poor temperature resistance of PI,structure engineering of precursor film is developed based on above studies on the grain growth mechanism in order to fabricate the CIS absorber at low selenization temperature（450°C）.Enough Cu_2-xSe is formed in the optimized precursor film during low temperature selenization,leading to fast grian growth,high-quility absorber and superior device.After structure engineering,the efficiency of low temperature selenized device raises from 3.83%to6.13%,indicating its feasibility for the future study of PI based CIS solar cell.