Lead selenide quantum dots and carbon dots amplify solar conversion capability of a TiO2/CdS photoanode

An integrated approach involving the use of visible light absorbing CdS quantum dots (QDs) and near infrared light harvesting PbSe QDs, along with highly conducting carbon dots (C-dots), resulting in impressive power conversion efficiency (PCE) is presented. A photoanode assembly composed of TiO2/PbSe/CdS/C-dots was prepared for the first time. The charge transfer processes in photoanodes of different compositions are explained using absorption studies, emission quenching and lifetime analysis. Upon illumination, photo-excited electrons are injected from the conduction bands of C-dots, CdS and PbSe QDs into TiO2; C-dots serve as excellent electron conduits, and promote electron transport from the photoactive assembly to the current collector and they also function as photosensitizers. Quantum dot solar cells (QDSCs) were fabricated with PbSe, CdS and C-dots based photoanodes, a S2− ion conducting hole transport layer, and multiwalled carbon nanotubes (MWCNTs) as the counter electrode. The cell with the TiO2/PbSe/CdS/C-dots photoanode delivered a remarkably high PCE of 4.97% for the champion cell, which was 35.8 and 48.1% greater than the PCEs produced by the cells with TiO2/PbSe/CdS and TiO2/CdS photoanodes. The increased efficiency, especially in comparison to the cell based on sole CdS, highlights the ability of (a) PbSe QDs, with a band gap of 0.98 eV to harvest red photons and convert them into electricity, and (b) C-dots to channelize the photogenerated electrons to the current collector, by virtue of their high electrical conductivity. This was clearly confirmed by the altered profiles of external and internal quantum efficiencies in the NIR region, for their magnitudes increased from zero to finite in the wavelength range of 550 to 1000 nm, only in the presence of PbSe QDs. Intensity modulated photocurrent/photovoltage data analysis showed fast electron transport and slow recombination for the TiO2/PbSe/CdS/C-dots photoanode. These studies furnish the rationalization for integrating NIR absorbing PbSe QDs and conductive C-dots with other visible light harvesting dyes or QDs to achieve increased photoconversion efficiencies