Photoelectron spectroscopic study on band alignment of poly(3-hexylthiophene-2,5-diyl)/polar-ZnO heterointerface

• Band alignment at P3HT/ZnO interface was studied.
• Polarity dependence of band alignment at P3HT/ZnO interface was observed.
• ZnO surface has a short depletion layer.
• Band of P3HT bends downward from surface to interface.

We used hard X-ray photoelectron spectroscopy (HX-PES) to investigate the polarity effect of ZnO, which has Zn terminated (+ polar) and O terminated (− polar) surfaces, on poly(3-hexylthiophene-2,5-diyl) (P3HT)/ZnO photovoltaic structures. HX-PES, which has a longer inelastic mean free path than conventional X-ray photoelectron spectroscopy, revealed that the open circuit voltage (VOC), estimated from the gap between the highest occupied molecular orbital of P3HT and the conduction band minimum of Zn-polar ZnO, was larger than that of the O-polar ZnO. Although polarity dependence was observed for the electrical structure at the P3HT/ZnO interface, the VOC of the photovoltaic property of Zn-polar ZnO was almost the same as that of O-polar ZnO (~ 0.4 V) due to electron–hole recombination, which degrades the photovoltaic properties at the interface. The HX-PES results also suggested that the recombination could be attributed to the short depletion length of the ZnO surface.