Rationally Controlled Synthesis of CdSe x Te 1-x Alloy Nanocrystals and Their Application in Efficient Graded Bandgap Solar Cells.

CdSex Te1-x semiconductor nanocrystals (NCs), being rod-shaped/irregular dot-shaped in morphology, have been fabricated via a simple hot-injection method. The NCs composition is well controlled through varying molar ratios of Se to Te precursors. Through changing the composition of the CdSex Te1-x NCs, the spectral absorption of the NC thin film between 570-800 nm is proved to be tunable. It is shown that the bandgap of homogeneously alloyed CdSex Te1-x active thin film is nonlinearly correlated with the different compositions, which is perceived as optical bowing. The solar cell devices based on CdSex Te1-x NCs with the structure of ITO/ZnO/CdSe/CdSex Te1-x /MoOx /Au and the graded bandgap ITO/ZnO/CdSe( w / o )/CdSex Te 1-x /CdTe/MoOx /Au are systematically evaluated. It was found that the performance of solar cells degrades almost linearly with the increase of alloy NC film thickness with respect to ITO/ZnO/CdSe/CdSe0.2 Te0.8 /MoOx /Au. From another perspective, in terms of the graded bandgap structure of ITO/ZnO/CdSe/CdSex Te1-x /CdTe/MoOx /Au, the performance is improved in contrast with its single-junction analogues. The graded bandgap structure is proved to be efficient when absorbing spectrum and the solar cells fabricated under the structure of ITO/ZnO/CdSe0.8 Te0.2 /CdSe0.2 Te0.8 /CdTe/MoOx /Au indicate power conversion efficiency (PCE) of 6.37%, a value among the highest for solution-processed inversely-structured CdSex Te1-x NC solar cells. As the NC solar cells are solution-processed under environmental conditions, they are promising for fabricating solar cells at low cost, roll by roll and in large area.