Rethinking Band Bending at the P3HT–TiO₂ Interface

The advancement of solar cell technology necessitates a detailed understanding of material heterojunctions and their interfacial properties. In hybrid bulk heterojunction solar cells (HBHJs), light-absorbing conjugated polymers are often interfaced with films of nanostructured TiO₂ as a cheaper alternative to conventional inorganic solar cells. The mechanism of photovoltaic action requires photoelectrons in the polymer to transfer into the TiO₂, and therefore, polymers are designed with lowest unoccupied molecular orbital (LUMO) 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 nanocrystalline TiO₂. It was determined that deep trap states (0.5 eV below the conduction band of 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. X-ray photoelectron spectroscopic studies quantified the resultant vacuum level offset to be 0.8 eV. Further spectroelectrochemical studies indicate that 0.1 eV of this offset occurs in TiO₂, whereas the balance occurs in P3HT. 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

Mn^II/III Complexes as Promising Redox Mediators in Quantum-Dot-Sensitized 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. The nature of the redox mediator plays a pivotal role in determining the photocurrent and photovoltage from the solar cell. 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 Mn^II/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 (recombination) rates were slower than Co­(bpy)₃, where bpy = 2,2′-bipyridine, evidenced by electron lifetimes up to 4 orders of magnitude longer. This is indicative of a large barrier to electron transport imposed by spin-crossover in these complexes. Low solubility prevented the redox mediators from sustaining high photocurrent due to mass transport limits. However, with high fill factors (∼0.6) and photovoltages, they demonstrate competitive efficiencies with Co­(bpy)₃ redox mediator at the same concentration. More positive reduction potentials and slower recombination rates compared to current redox mediators establish the viability of Mn poly­(pyrazolyl)­borates as promising redox mediators. By capitalizing on these characteristics, efficient Mn^II/III-based QDSSCs can be achieved with more soluble Mn-complexes