The development of suitable colloidal nanocrystal inks are a key step in the development of low-cost solar cells since they enable the use of fast and inexpensive coating processes such as spray coating and roll coating to form a thin film photoabsorbing layer. Chalcopyrite structure copper indium diselenide (CuInSe2) is a key photoabsorbing material for existing thin film solar cells due to its serendipitous defect chemistry and material properties. Here, we report the first solution-phase synthesis of chalcopyrite structured CuInSe2 nanocrystals. The synthesis proceeds rapidly from elemental and halide reagents via a simple batch reaction without “hot injection” in a single component coordinating solvent. This facile synthesis is environmentally benign and amenable to large-scale production. The nanocrystals are easily precipitated and may be redispersed in a variety of organic solvents to form an ink that is used to coat molybdenum/soda-lime glass substrates by simple drop casting to form 1-2 micron thick layers of the nanocrystals. The structure of the CuInSe2 nanocrystals after synthesis may be controlled via the synthesis conditions to yield sphalerite, chalcopyrite, or ordered defect compounds. Specific cation orderings have been identified by powder x-ray diffraction simulations using DiffaX and from electron diffraction and dark field transmission electron microscopy.

The use of these nanocrystals for solar cells has been demonstrated by fabricating Mo/CuInSe2/CdS/i-ZnO/ITO/Ni/Al devices. The first cells had an efficiency of 2.0% under standard AM1.5 illumination after fabrication. The cells are robust and increase in efficiency with time exhibiting similar serendipitous defect chemistry as layers formed by vacuum co-evaporation. After 5 months in normal laboratory conditions the very first solar cells have an efficiency of 2.8% with an open circuit voltage 280 mV and a short circuit current of 25.8 mA/cm2. External quantum efficiencies of these devices are 70 % in the 500 nm – 900 nm range. In addition, J-V measurements under varying incident light intensities and temperatures give insight into the nature of the limiting recombination mechanisms.