Rethinking Band Bending at the P3HT–TiO₂ Interface

Abstract

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.

Versatile Pendant Polymer for Selective Charge Carrier Transport via Controlling the Supramolecular Self‐Assembly

“…The large electron injection barrier formed at the interface of the ITO electrode reduces the dark current of the device under the reverse bias condition. Furthermore, band bending was observed upon introducing P0 and P4 interlayers in films of ITO/PEDOT : PSS, [21,24] owing to the formation of interfacial dipoles with P0 and P4. [25][26][27] The work function of P0 and P4 is increased by the removal of hydrocarbons through ozone treatment at the interface with ITO, [25,26] and the sulfur group of P4 forms a higher dipole and induces a relatively larger band bending at P4 than at P0.…”

Spray Deposition of Titania Films with Incorporated Crystalline Nanoparticles for All‐Solid‐State Dye‐Sensitized Solar Cells Using P3HT

“…This ability is in contrast to direct imaging techniques like SEM and atomic force microscopy (AFM), which only provide information of the surface morphology in a small observable area . However, the inner morphology of those films is of great interest for the performance of the DSSCs, as it is expected to directly influence the efficiency of charge carrier generation and transport . This is due to the dye molecules being adsorbed on the titania surface and due to the need of charge carriers to be transported along the corresponding p‐ and n‐type material phases to the respective electrodes .…”

Selective Release of Different Neurotransmitters Emulated by a p–i–n Junction Synaptic Transistor for Environment‐Responsive Action Control

“…Under positive V D and V G , the current is determined by the diffusion mechanism, and most of the carriers can travel through the contact surface to form a current. The current‐rectifying characteristics of the junction are generated, [ 26–29 ] so the current in the third quadrant ( V D < 0 and V G < 0) turns on slowly, and shows a weakening trend after the voltage increases to about 3.8 V. While a positive pulse is applied to the ionic gel, electrons collect in the P3HT/PEO layer to form an inversion layer, which increases carrier‐storage capacity and excellent long‐term plasticity in the TiO 2 thin film. [ 30,31 ] Finally, the two signals are collected and combined in the electrode to output the final signal; i.e., potentiation or inhibition.…”