Purification of Colloidal Nanocrystals Along the Road to Highly Efficient Photovoltaic Devices

Colloidal semiconducting nanocrystals, also called colloidal quantum dots (QDs) afford efficient photoconversion from visible to infrared wavelength owing to their size-dependent optoelectronic properties. To manufacture the highly performing devices utilizing colloidal NCs, however, unreacted impurities should be removed following synthesis. As the scale of NCs synthesis increases, especially in industry, the need is heightened for large-scale purification methods that can retain the desirable optoelectronic characteristics as of as-synthesized samples. Particularly, for the use of colloidal quantum dots (QD) films for photovoltaic active layers, control over the surface during the purification needs keen attention because residual impurities or trap states introduced by inappropriate treatments during the purification are detrimental to the carrier collection efficiency of the device. In this article, we review several approaches to the purification of QDs and their successful implications for formation of the efficient photovoltaic devices. We group the purification methods according to the key property by which the separation is achieved, and discuss the scalability of each method specifically focusing on the possibility of implementing a continuous process flow that is compatible with continuous synthesis processes developed for large scale production of QDs. Finally, we present recent efforts for the highly efficient photovoltaic QD devices and discuss the importance of purification in terms of device performance.