Investigation of alkali elemental doping effects on solution processed Cu2ZnSn(S,Se)4 thin films for photovoltaics
Date of Issue2016
School of Materials Science and Engineering
Energetics Research Institute
Due to the great potential for large scale and low cost production in photovoltaic industry, kesterite Cu2ZnSn(S,Se)4 (CZTSSe) based materials have been deemed as one of the most promising absorber materials for photovoltaic applications. Composed of relatively low toxic and earth abundant elements, this kesterite material could be used to avoid the limitation of elemental scarcity and relieve the environmental issues which are troubling the CdTe and Cu(In,Ga)(S,Se)2 (CIGS) based thin film technologies. Besides, CZTSSe also shows excellent electronic and optical properties, like high optical absorption coefficient (>104 cm-1) and direct tunable bandgap from 1.0 to 1.5 eV, making it good alternative to CIGS. Similar to CIGS, both solution and vacuum based approaches could be adopted to fabricate CZTSSe thin film absorber. In order to reduce the manufacturing cost, there is urgent demand towards the usage of non-vacuum solution based approach. So in this thesis, a solution based sol-gel method for the synthesis of CZTS/CZTSSe thin film absorbers was presented. Generally, CZTS precursor films were successfully synthesized by a simple spin-coating process, followed by a preheating process at 280 °C. Further high temperature annealing processes were required to enhance the grain growth and get high quality dense films for photovoltaic application. Therefore, the influence of annealing conditions such as annealing atmosphere and selenization temperature on both thin films and device performance has been investigated. Alkali elemental doping such as sodium and potassium doping was employed into CZTSSe thin film solar cells by solution based method to improve the grain size and further enhance the device performance. It was found that the open circuit voltage (Voc) and fill factor (FF) were improved due to the alkali elemental doping, thus leading to the improvement of solar cell power conversion efficiency (PCE). By tuning the doping concentration and selenization condition, the CZTSSe thin film solar cell with 1.5 mol% K doping under selenization at 560 °C for 30 min exhibited the best efficiency of 7.78%. The alkali elemental doping effects on the microstructure, chemical composition, electronic properties of CZTSSe thin films and solar cell performance were analyzed in details. It was revealed that the improvement of solar cell performance with K doping could be mainly attributed to the increase of carrier concentration. However, excess K together with non-ideal annealing condition may induce deep level states, high series resistance and low carrier mobility, thus quenching the performance. In order to compare the individual role of each alkali element (Li, Na, K), the CZTSSe absorbers were also deposited onto the substrates with alkali diffusion barrier (SLG/Al2O3/Mo). As a result, similar effects of increasing the carrier concentration and reducing depletion width were found. But the degree of improvement may be a little different, possibly due to different incorporation amount of each alkali element. In summary, our solution based alkali elemental doping in CZTSSe offers some new insights into the improvement of thin film solar cell performance by alkali elemental doping. Our work also suggests a potential application of solution based alkali elemental doping in flexible solar cell devices.