Molecular photon upconversion via triplet-triplet annihilation (TTA-UC), combining two or more low energy photons to generate a higher energy excited state, is an intriguing strategy to surpass the maximum efficiency for a single junction solar cell (<34%). Here, we introduce self-assembled bilayers on metal oxide surfaces as a strategy to facilitate TTA-UC emission and demonstrate direct charge separation of the upconverted state. A 3-fold enhancement in transient photocurrent is achieved at light intensities as low as two equivalent suns. This strategy is simple, modular and offers unprecedented geometric and spatial control of the donor-acceptor interactions at an interface. These results are a key stepping stone toward the realization of an efficient TTA-UC solar cell that can circumvent the Shockley-Queisser limit.

Self-assembled bilayers offer a promising strategy to directly harness photon upconversion via triplet-triplet annihilation (TTA-UC) and increase maximum theoretical solar cell efficiencies from 33% to >43%. Here we demonstrate that the choice of redox mediator in these solar cells has a profound influence on both the light harvesting and TTA-UC efficiency. Devices with Co(phen) as the redox mediator produced the highest photocurrent yet generated from TTA-UC (0.158 mA cm) under 1 sun. Despite generating less photocurrent, Co(pz-py-pz) devices achieved maximum TTA-UC efficiency at excitation intensities well below solar irradiance (0.8 mW cm), which is on par with the lowest value yet reported for any TTA-UC system. The large variation in performance with respect to mediator is attributed to triplet excited-state quenching via (1) energy transfer or paramagnetic quenching by the Co species and (2) excited-state electron transfer to Co species.

Molecular photon upconversion, by way of triplet−triplet annihilation (TTA-UC), is an intriguing strategy to increase solar cell efficiencies beyond the Shockley−Queisser limit. Here we introduce self-assembled bilayers of acceptor and sensitizer molecules on high surface area electrodes as a means of generating an integrated TTA-UC dye-sensitized solar cell. Intensity dependence and IPCE measurements indicate that bilayer films effectively generate photocurrent by two different mechanisms: (1) direct excitation and electron injection from the acceptor molecule and (2) low-energy light absorption by the sensitizer molecule followed by TTA-UC and electron injection from the upconverted state. The power conversion efficiency from the upconverted photons is the highest yet reported for an integrated TTA-UC solar cell. Energy transfer and photocurrent generation efficiency of the bilayer device is also directly compared to the previously reported heterogeneous UC scheme.

An intramolecular oxidative C(sp)-H amination from unprotected anilines and C(sp)-H bonds readily occurs under mild conditions using t-BuOK, molecular oxygen and N,N-dimethylformamide (DMF). Success of this process, which requires mildly acidic N-H bonds and an activated C(sp)-H bond (BDE < 85 kcal/mol), stems from synergy between basic, radical, and oxidizing species working together to promote a coordinated sequence of deprotonation: H atom transfer and oxidation that forges a new C-N bond. This process is applicable for the synthesis of a wide variety of N-heterocycles, ranging from small molecules to extended aromatics without the need for transition metals or strong oxidants. Computational results reveal the mechanistic details and energy landscape for the sequence of individual steps that comprise this reaction cascade. The importance of base in this process stems from the much greater acidity of transition state and product for the 2c,3e C-N bond formation relative to the reactant. In this scenario, selective deprotonation provides the driving force for the process.

Self-assembled trilayers on metal oxide surfaces are used to increase absorption cross section and photocurrent generation efficiency via triplet–triplet annihilation.