Improved Hybrid Organic-Inorganic Photovoltaics via In Situ UV-Polymerization in Nanotube Arrays

Research Achievement Hybrid solar cells have been developed in the past decade as a promising alternative for traditional silicon-based solar cells. One approach for making inexpensive inorganic-organic hybrid photovoltaic (PV) cells is to fill nanostructured titania films with solid organic hole conductors such as conjugated polymers. These compounds can function as light-absorbing species and inject electrons into the conduction band of the n-type semiconductor, while at the same time they conduct the holes to the cathode. Nanotube films offer a distinct advantage over nanoparticle films in that they facilitate charge carrier transport. The infiltration of the polymer into the nanostructured metal oxide is of particular importance for optimizing the performance of these hybrid devices. Most of the reports on solar cells using conjugated polymers have employed wet processing deposition techniques. Since polymers suffer a loss of conformational entropy when they are confined in a channel whose radius is less than their radius of gyration, filling the pores with a polymer has been a challenge due to the possibility of the polymer chains clogging the pores of the nanotubular electrode. In this project we developed a new technique for deposition/infiltration of conjugated polymer into densely ordered TiO2 nanotube arrays. In particular, we report the effects of in situ ultraviolet (UV) polymerization of polythiophene vs. infiltration of pre-synthesized P3HT on the performance of solar cell devices. In our simple and efficient photochemical approach, a nanotubular TiO2 substrate is immersed in a 2,5-diiodothiophene monomer precursor solution and then irradiated with UV light in an argon environment. The selective UV photodissociation of the C–I bond (λ = 250–300 nm) in the precursor molecule produces monomer radicals with intact π-ring structure. Since the C–I bonds are present at the ends of the reaction coupling products, further photodissociation can take place forming oligomeric and polymer species that can couple to and self-assemble on the surface of TiO2 nanotubes (Figure [d]). The formation of polymeric semiconductor films by this method offers distinct advantages such as cost, uniformity, and scalability over other solution processing techniques. By polymerizing conjugated polymer inside an electron-accepting nanotube array, we have achieved significant improvements in optoelectronic device performance compared to those fabricated by infiltrating tubes with ex situ-synthesized polymer (Figure [c]). Our preparation leads to a strong coupling at the polymer-oxide interface, which is important for efficient exciton separation (Figure [b]), and fewer conformational defects (Figure [a]), which facilitates hole transport. The small molecule precursor also presumably achieves superior filling within the confined environment.