High-density Cu-In intermetallic nanocrystal layers: towards high-efficiency printable CuInSe2 solar cells

This paper aims to demonstrate the importance of the precursor film density during the development of the microstructure and photovoltaic properties of CuInSe2 (CISe) thin films prepared from nanoparticulate precursors and to provide a simple but effective means to improve the density. In order to improve the packing density of a Cu-In alloy nanocrystalline layer deposited by a doctor-blade method, a mild, wet, ball-milling technique is applied to colloidal ink without the use of organic additives or mechanical pressing. The focused ion beam milled cross-sections clearly show that the relative density of the precursor layers increases with an increase in the milling duration. The CISe thin films obtained by annealing the intermetallic layers with a relative density of 0.78 exhibit extremely low porosity on the well-faceted surface, a thicker top layer with large grains in a typical double-layered structure, a uniform and planar surface morphology, and a thinner MoSe2 layer. The transmission electron microscopy analysis also reveals that the nanocrystals in the bottom layer exist in the chalcopyrite CISe phase, forming a well-connected mesoporous network. As a result, the PV performance of low-bandgap (1.0 eV) CISe solar cells is greatly improved, rendering a power conversion efficiency as high as 9.32%. Changes in the PV parameters induced by the increase in the relative density of the precursor layer are discussed in conjunction with the improved crystalline quality and the consequent improvements in diode characteristics, such as the series resistance, shunt conductance, and recombination current.