Solution-Processed Chalcopyrite Solar Cells: the Grain Growth Mechanism and the Effects of Cu/In Mole Ratio

Solution-processed Cu(In,Ga)(S,Se) 2 solar cells have reached 18% efficiency but still remain much lower compared to state-of-the-art vacuum based solar cells. In comparison to vacuum deposited precursor films, which mostly consist of stacked metal and/or metal chalcogenide layers and takes a liquid Cu 2− x Se assisted grain growth mechanism, solution-processed precursor films normally have a chalcopyrite structure that is already developed. Understanding the grain growth mechanism of solution-processed absorbers is crucial to control the electronic properties and further improve the device photovoltaic performance. Here, the grain growth mechanism of a N -methyl-pyrrolidone solution processed precursor film with composition from Cu-poor to Cu-rich is systematically investigated. Characterizations show that the chalcopyrite structured CuInS 2 precursor film takes a direct phase transformation grain growth mechanism and forms the CuIn(S,Se) 2 (CISSe) absorber without the presence of a detrimental Cu 2− x Se phase with Cu/In ratio up to unit. Beyond the stoichiometric composition, the coexistence of Cu 2− x Se facilitates grain growth but deteriorates device performance. The direct phase transformation mechanism not only avoids detrimental Cu 2− x Se but also enables fabrication of a highly efficient CISSe device near stoichiometric composition with high tolerance to the Cu/In ratio (from 0.90 to 1.05). By preliminary optimization, a CISSe solar cell with an efficiency of 13.6% is achieved in ambient air with a Cu/In ratio of 0.93.