Photoactive Core–Shell Nanorods as Bifunctional Electrodes for Boosting the Performance of Quantum Dot Sensitized Solar Cells and Photoelectrochemical Cells

The performance of solar energy conversion and storage devices is contingent to the appropriate design of photosensitizers that make the electrodes. In quantum dot sensitized solar cells (QDSSCs) the power conversion efficiency (PCE) is mainly limited by the impediments of electron–hole pair generation in the trap state abundant QDs at the photoanode and below expected catalytic activity of the counter electrode (CE). Employing photoactive CEs, that too with precisely tuned core/shell nanorods (NRs) array is thereby a new approach in addressing these limitations. Herein CdS/Cu2S core/shell p-n junction NRs increase photon harvesting over a wider solar spectrum as well as demonstrate enhanced catalytic activity to reduce the oxidized polysulfide electrolyte. The NRs were also employed as photoanodes in photoelectrochemical (PEC) water splitting. The NRs with varying shell thickness were synthesized by meticulously controlled cation exchange and their vertical c-axis orientation not only offers high surface area to react with the electrolyte but also generates low sheet resistance. With a graded interface for better charge flow, electron tunnelling in the CE increases the photocurrent. In QDSSCs, when the Cu2S shell is thicker, Cu-d and S-p hybridization facilitates the tunnelling of holes from Cu2S shell to thinner CdS core increasing the PCE of QDSSC device, well validated by density functional theory (DFT) calculations. In PEC water splitting, the thin shell CdS/Cu2S NR photoanode however provides the highest photon-to-current conversion efficiency since thicker Cu2S shell creates larger depletion region increasing the recombination of charge carriers.