Rationally Controlled Synthesis of CdSexTe1−x Alloy Nanocrystals and Their Application in Efficient Graded Bandgap Solar Cells

Abstract

CdSe_xTe_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_xTe_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_xTe_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_xTe_1-x NCs with the structure of ITO/ZnO/CdSe/CdSe_xTe_1-x/MoO_x/Au and the graded bandgap ITO/ZnO/CdSe(<i>w</i>/<i>o</i>)/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_0.2Te_0.8/MoO_x/Au. From another perspective, in terms of the graded bandgap structure of ITO/ZnO/CdSe/CdSe_xTe_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_0.8Te_0.2/CdSe_0.2Te_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_xTe_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.