We have developed the colloidal synthesis of I-III-VI2 semiconductor nanocrystals of Cu(In,Ga)Se2 (CIGS) and CuIn(SeS)2 (CISS). These nanocrystals can be spray-deposited into photovoltaic devices (PVs) with power conversion efficiencies of up to 3% without the use of high temperature annealing. Mechanically flexible PVs on plastic substrates can also be made using these materials. The gallium content can be varied systematically in the nanocrystals as well to tune the bandgap energy. By creating a library of CIGS nanocrystal inks with varying Ga composition, PVs with an absorber layer with a Ga composition gradient can be fabricated, yielding devices with graded bandgaps. The stability of the nanocrystal dispersions is important for creating uniform films of nanocrystals and functioning PVs. Typically, oleylamine is used as a capping ligand, but nanocrystals of relatively large diameter (> 15 nm) are usually obtained. By manipulating the ligand chemistry, nanocrystals with much smaller diameter, below the size needed for quantum confinement, can be synthesized. These CISS nanocrystals emit light in the red/near-IR region of the spectrum and are being examined as luminescent contrast agents for medical imaging.
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