Solution-processing of ultra-thin CdTe/ZnO nanocrystal solar cells

• We study the growth of nanocrystalline CdTe thin films from colloidal nanocrystals.
• We examine the CdTe growth profiles as a function of surface chemistry.
• We show that nanocrystalline CdTe is susceptible to oxidation under air annealing.
• We show how this oxidation influences performance in CdTe/ZnO solar cells.
• We demonstrate CdTe/ZnO solar cells with an efficiency of 7.7% fabricated in air.

We have carried out a detailed study into how modifications of the physical, chemical and optical properties of solution-processed, nanocrystalline CdTe layers influence the photovoltaic performance of sintered CdTe/ZnO nanocrystal solar cells. Such solar cells are fabricated through layer-by-layer assembly, which is enabled through an inter layer chemical and thermal treatment cycle. In this manner we are able to fabricate working solar cells with sintered CdTe layers as low as 90 nm, provided that grain size is precisely controlled. We show that the extent of grain growth achieved during the CdTe sintering process is strongly dependent on nanocrystal surface chemistry and chemical environment, with the removal of the organic capping ligands and the introduction of CdCl2 prior to annealing leading to greatly enhanced growth. Due to the air processing involved and the nanocrystalline nature of the CdTe, the overall performance of these solar cells is shown to be strongly dependent on both annealing temperature and time, with optimal results requiring a balance between crystal growth and degradation due to oxidation. Using this simple bi-layer device structure, optimized treatment conditions result in power conversion efficiencies of up to 7.7% and peak internal quantum efficiencies in excess of 95%.