Size-Dependent Photovoltaic Performance of CdSe Supraquantum Dot/Polymer Hybrid Solar Cells: “Goldilocks Problem” Resolved by Tuning the Band Alignment Using Surface Ligands

Abstract

This study explored the size dependence of colloidal CdSe nanocrystals (NCs) on the photovoltaic properties of CdSe NC/poly(3-hexylthiophene) (P3HT) hybrid bulk-heterojunction (BHJ) solar cell devices. The size-dependent photovoltaic performance was achieved by utilizing CdSe supraquantum dots (SQDs), which are three-dimensionally interconnected colloidal superstructures composed of hundreds of CdSe quantum dots (QDs). The average size of the SQDs can span tens of nanometers, which allow the formation of percolation networks in BHJ films. The open-circuit voltage of the devices was observed to be proportional to the size of the SQDs because of their ideal percolation networks. The photocurrents were determined by the competition between the charge separation and charge transport abilities controlled by the SQD sizes. Overall, the 46 nm-sized CdSe SQD-device demonstrated the highest power conversion efficiency (PCE) of 0.95%, which was 3.2 times higher than that of the control 4.3 nm-sized CdSe QD device. However, further increasing the SQD size resulted in a decrease in the PCE because of the inherent carrier recombination loss within the SQDs. To overcome this “Goldilocks problem,” we tuned the energy level at the surface region of the CdSe SQDs via electron-donating 4-methylthiophenol (MTP) ligand exchange. The MTP-treated CdSe SQDs further improved the device performance by enhancing the charge separation and increasing the energy level offset at the CdSe SQD/P3HT interface.