A low-cost, nonvacuum fabrication route for CuInSe 2 and CuInS 2 thin films is presented. To produce these films, binder-free colloidal precursors were prepared using Cu−In intermetallic nanoparticles that were synthesized via a chemical reduction method. The Cu−In alloy precursor films were transformed to CuInSe 2 and CuInS 2 by reactive annealing in chalcogen-containing atmospheres at atmospheric pressure. The as-synthesized nanoparticles and the annealed films were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectrometry, electron probe X-ray microanalysis, Raman spectroscopy, and Auger electron spectroscopy depth profile measurements to elucidate the phase evolution pathway and the densification mechanism of the Cu−In−Se−S system. Solar cell devices made with CuInSe 2 and CuInS 2 absorbing layers exhibited power conversion efficiencies of 3.92% and 2.28%, respectively. A comparison of the devices suggested that the microstructure of the absorbing layer had a greater influence on the overall photovoltaic performance than the band gap energy. A diode analysis on the solar cell devices revealed that the high saturation current density and diode ideality factor caused lower open-circuit voltages than would be expected from the band gap energies. However, the diode analysis combined with the microstructural and compositional analysis offered guidance about how to improve the photovoltaic performance of these devices.

We demonstrate here that an improvement in the green density leads to a great enhancement in the photovoltaic performance of CuInSe2 (CISe) solar cells fabricated with Cu-In nanoparticle precursor films via colloidal solution deposition. Cold-isostatic pressing (CIP) increases the precursor film density by ca. 20%, which results in an appreciable improvement in the microstructural features of the sintered CISe film in terms of a lower porosity, a more uniform surface morphology, and a thinner MoSe2 layer. The low-band-gap (1.0 eV) CISe solar cells with the CIP-treated films exhibit greatly enhanced open-circuit voltage (V(OC), typically from 0.265 to 0.413 V) and fill factor (FF, typically from 0.34 to 0.55), compared to the control devices. As a consequence, an almost 3-fold increase in the average efficiency, from 3.0 to 8.2% (with the highest value of 9.02%), is realized. Diode analysis reveals that the enhanced V(OC) and FF are essentially attributed to the reduced reverse saturation current density and diode ideality factor. This is associated with suppressed recombination, likely due to the reduction in recombination sites at grain/air surfaces, intergranular interfaces, and defective CISe/CdS junctions. From the temperature dependences of V(OC), it is revealed that CIP-treated devices suffer less from interface recombination.