Hybrid photovoltaic cells are electronic devices designed for conversion of light into the noblest form of energy, electricity. They are cost effective alternatives to conventional silicon solar cells. Although their power conversion efficiency has been low so far, they can potentially surpass the conventional silicon cells not only in the production cost but also in performance because of their unique architectures combining organic and inorganic materials.

Figure 1 Hybrid solar cell: a) ZnO nanowires, b) flocky ZnO nanowires; c) architecture of a polymer infiltrated solar cell.

The project is engaged in development and investigation of novel hybrid photovoltaic cells that will form a base for cost effective devices with high power conversion. The design of organic/inorganic photocell structures with implementation of new fabrication approaches and study of nanomaterial interfaces is the essence of the project. Particularly ZnO nanostructures with different morphologies including vertical ZnO nanowires and flocky nanorods (Figure 1a and 1b) prepared by simple methods are investigated and used in the designed photovoltaic device structures (Figure 1c). The ZnO nanowires, inherently n-type semiconductors, are infiltrated by organic p-type conducting materials to provide large-area p-n heterojunctions. The problem of the size and functions of the heterojunction interface and the infiltration by organic semiconductors is specifically investigated for wettability, electronic interfacial structures, effective dissociation of photo-induced excitons, their diffusion length and interfacial field separation. It is foreseen that within the designed photovoltaic cells the charge recombination process will be suppressed considerably to provide a high conversion efficiency of light to electricity. Optimizing the electrode configuration and possible suppression of charge recombination in different parts of the devices by engineering the interfaces in hybrid photovoltaic devices will lead to considerable improvement of the device efficiency.

　

　

　

The project  focuses on fundamental engineering strategies for improving the conversion efficiency. The fact that only about a third of the excitons induced in the distance of the diffusion length (2-20nm) from the interface can reach the dissociative interface formulates the first research problem, i.e., optimization and control of the interspacing in nanowire arrays. The equally important task is providing a large electrically active interface between the p-type conjugated polymer and n-ZnO nanostructures. The large electrically active interface between the p-type conjugated polymer and n-ZnO nanostructures is especially important. This provision can hardly be met with technologies used so far, since i) large molecules of conjugated polymers can difficult to creep into small interspacing of the nanostructures, ii) adsorbed gases and gases present in cavities are enclosed at interface by infiltrated polymers and iii) poor wettability of the nanostructure to the conjugated polymers causing electrically inactive interfaces and interfacial separation. As a result the project introduces a novel technology to infiltrate ZnO nanostructures. The arrays of nanowires are outgassed at elevated temperature in vacuum and then infiltrated with solutions of conjugated polymers in a protective environment and under ultrasonic agitations. The proposed procedures of infiltration are anticipated to be crucial for final device performance. Since, wettability, dissociative electric field and open circuit voltage of final devices, which are closely related to the nature of interfaces, as well as functionalization of surface are investigated thoroughly too. 

Prof Igor BELLO
Department of Physics and Materials Science
City University of Hong Kong
apibello@cityu.edu.hk