Rationally Controlled Synthesis of CdSe(x)Te(1−x) Alloy Nanocrystals and Their Application in Efficient Graded Bandgap Solar Cells

CdSe(x)Te(1−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 CdSe(x)Te(1−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 CdSe(x)Te(1−x) active thin film is nonlinearly correlated with the different compositions, which is perceived as optical bowing. The solar cell devices based on CdSe(x)Te(1−x) NCs with the structure of ITO/ZnO/CdSe/CdSe(x)Te(1−x)/MoO(x)/Au and the graded bandgap ITO/ZnO/CdSe(w/o)/CdSe(x)Te(1−x)/CdTe/MoO(x)/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/CdSe(0.2)Te(0.8)/MoO(x)/Au. From another perspective, in terms of the graded bandgap structure of ITO/ZnO/CdSe/CdSe(x)Te(1−x)/CdTe/MoO(x)/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/CdSe(0.8)Te(0.2)/CdSe(0.2)Te(0.8)/CdTe/MoO(x)/Au indicate power conversion efficiency (PCE) of 6.37%, a value among the highest for solution-processed inversely-structured CdSe(x)Te(1−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.