Performance enhancement of bulk heterojunction solar cells with direct growth of CdS-cluster-decorated graphene nanosheets

Two-dimensional graphene-CdS (G-CdS) semiconductor hybrid nanosheets were synthesized in situ by graphene oxide (GO) quantum wells and a metal-xanthate precursor through a one-step growth process. Incorporation of G-CdS nanosheets into a photoactive film consisting of poly[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[2-(2-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl] (PBDTTT-C-T) and [6,6]-phenyl C70 butyric acid methyl ester (PC70 BM) effectively decreases the exciton lifetime to accelerate exciton dissociation. More importantly, the decreasing energy levels of PBDTTT-C-T, PC70 BM, and G-CdS produces versatile heterojunction interfaces of PBDTTT-C-T:PC70 BM, PBDTTT-C-T:G-CdS, and PBDTTT-C-T:PC70 BM:G-CdS; this offers multi-charge-transfer channels for more efficient charge separation and transfer. The charge transfer in the blend film also depends on the G-CdS nanosheet loadings. In addition, G-CdS nanosheets improve light utilization and charge mobility in the photoactive layer. As a result, by incorporation of G-CdS nanosheets into the active layer, the power-conversion efficiency of inverted solar cells based on PBDTTT-C-T and PC71 BM is improved from 6.0 % for a reference device without G-CdS nanosheets to 7.5 % for the device with 1.5wt % G-CdS nanosheets, due to the dramatically enhanced short-circuit current. Combined with the advantageous mechanical properties of the PBDTTT-C-T:PC70 BM:G-CdS active layer, the novel CdS-cluster-decorated graphene hybrid nanomaterials provide a promising approach to improve the device performance.