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.