Rethinking band bending at the P3HT-TiO(2) interface.

Area of Science:

• Materials Science
• Photovoltaics
• Organic Electronics

Background:

• Hybrid bulk heterojunction solar cells (HBHJs) utilize conjugated polymers interfaced with nanostructured TiO2.
• Efficient electron transfer from polymer to TiO2 is crucial for photovoltaic action.

Purpose of the Study:

• Investigate the impact of heterojunction formation on material energetics.
• Understand the mechanism of charge transfer at the poly(3-hexylthiophene) (P3HT) and TiO2 interface.
• Propose guidelines for enhancing HBHJ solar cell performance.

Main Methods:

• Spectroelectrochemistry to study charge transfer and energetics.
• X-ray photoelectron spectroscopy (XPS) to quantify vacuum level offset.

Main Results:

• Spontaneous charge transfer occurs at the P3HT/TiO2 heterojunction, forming deep trap states in TiO2 and hole polarons in P3HT.
• Interfacial dipoles and band bending result from charge transfer, inhibiting electron injection and reducing solar cell performance.
• A 0.8 eV vacuum level offset was quantified, with 0.1 eV in TiO2 and 0.7 eV in P3HT.

Conclusions:

• Current models neglect altered energetics upon heterojunction formation.
• Interfacial dipoles significantly impact HBHJ solar cell performance by hindering electron injection.
• Tuning heterojunction energetics to reverse interfacial dipole direction is proposed to enhance photoelectron injection and improve photocurrent.