CdSe Quantum Dot Sensitized Molecular Photon Upconversion Solar Cells

Incorporating photon upconversion, via triplet–triplet annihilation (TTA-UC), directly into a solar cell is an intriguing strategy for harnessing sub-band gap photons and surpassing the Shockley–Queisser limit. A majority of TTA-UC solar cells to date rely on difficult to synthesize and expensive platinum and/or palladium porphyrin sensitizers. Here, we present, as far as we know, the first TTA-UC solar cell that integrates quantum dot (QD) sensitizers directly into the photocurrent generation mechanism. The photoanodes are composed of a nanocrystalline TiO2 substrate, 4,4′-(anthracene-9,10-diyl)bis(4,1-phenylene)diphosphonic acid (A) as the annihilator molecule, and CdSe QDs as the sensitizer in an inorganic–organic-inorganic layered architecture (TiO2-A-QD). The TiO2-A-QD devices generate a photocurrent that is more than 1.4 times the sum of its parts and does so via a TTA-UC mechanism as demonstrated by intensity dependence, IPCE, and spectroscopic measurements. The maximum efficiency onset threshold (i.e., the Ith value) of 0.9 mW cm–2 (1.9 × 1015 ex s–1 cm–2) is below solar flux and on par with some of the lowest Ith values reported to date. However, the Jsc for the QD sensitized device (29 μA cm–2) is still lower than comparable molecular sensitized devices (185 μA cm–2) due in part to lower sensitizer surface loadings, less than unity energy transfer yields (∼40–80%), slow regeneration kinetics, and competitive QD* quenching by the CoII/III(phen)3 redox mediator. Nonetheless these results demonstrate that multilayer assemblies containing QD sensitizers is an effective strategy to harness UC in a TTA-UC solar cell.