Energy Level Alignment in Hybrid Bulk Heterojunctions and New Redox Mediators for Quantum Dot Solar Cells

The advancement of quantum dot sensitized solar cell (QDSSC) technology depends on optimizing directional charge transfer between light absorbing quantum dots, TiO2, and a redox mediator. Kinetically, reduction of oxidized quantum dots by the redox mediator should be rapid and faster than the back electron transfer between TiO2 and oxidized quantum dots to maintain photocurrent. Thermodynamically, the reduction potential of the redox mediator should be sufficiently positive to provide high photovoltages. To satisfy both criteria and enhance power conversion efficiencies, we introduced charge transfer spin-crossover MnII/III complexes as promising redox mediator alternatives in QDSSCs. High photovoltages ~ 1 V were achieved by a series of Mn poly(pyrazolyl)borates, with reduction potentials ~0.51 V vs Ag/AgCl. Back electron transfer rates were slower than Co(bpy)3, where bpy = 2,2'-bipyridine. This is indicative of a large barrier to recombination imposed by spin-crossover in these complexes. By capitalizing on these characteristics, efficient MnII/III-based QDSSCs can be achieved with more soluble Mn-complexes.

In hybrid bulk heterojunction solar cells (HBHJs), light-absorbing conjugated polymers are interfaced with films of nanostructured TiO2. Photovoltaic action requires photoelectrons in the polymer to transfer into the TiO2, and therefore, polymers are designed with lowest unoccupied molecular orbital levels higher in energy than the conduction band of TiO2 for thermodynamically favorable electron transfer. Currently, the energy level values used to guide solar cell design are referenced from the separated materials, neglecting the fact that upon heterojunction formation material energetics are altered. With spectroelectrochemistry, we discovered that spontaneous charge transfer occurs upon heterojunction formation between poly(3-hexylthiophene) (P3HT) and TiO2. It was determined that deep trap states in TiO2 accept electrons from P3HT and form hole polarons in the polymer. This equilibrium charge separation alters energetics through the formation of interfacial dipoles and results in band bending that inhibits desired photoelectron injection into TiO2, limiting HBHJ solar cell performance. New guidelines for improved photocurrent are proposed by tuning the energetics of the heterojunction to reverse the direction of the interfacial dipole, enhancing photoelectron injection.