Colloidal CIGS and CZTS nanocrystals: A precursor route to printed photovoltaics

This review article summarizes our research focused on Cu(In1–xGax)Se2 (CIGS) nanocrystals, including their synthesis and implementation as the active light absorbing material in photovoltaic devices (PVs). CIGS PV layers are typically made using a high temperature (>450 °C) process in which Cu, In and Ga are sequentially or co-evaporated and selenized. We have sought to use CIGS nanocrystals synthesized with the desired stoichiometry to deposit PV device layers without high temperature processing. This approach, using spray deposition of the CIGS light absorber layers, without high temperature selenization, has enabled up to 3.1% power conversion efficiency under AM 1.5 solar illumination. Although the device efficiency is too low for commercialization, these devices provide a proof-of-concept that solution-deposited CIGS nanocrystal films can function in PV devices, enabling unconventional device architectures and materials combinations, including the use of flexible, inexpensive and light-weight plastic substrates. - Graphical abstract: The semiconductor light-absorbing layers in photovoltaic devices can be deposited under ambient conditions using nanocrystal inks. Devices can be fabricated on glass or on mechanically flexible plastic substrates. Highlights: ► CIGS and CZTS nanocrystals are synthesized and formulated into inks. ► Nanocrystal films are spray deposited and used as light absorbing layers in photovoltaic devices. ► Photovoltaic devices were constructed from nanowire mats. ► Photovoltaic device efficiency is limited by electrical transport in the nanocrystal layers.