Hybrid organic-on-inorganic photovoltaic devices

Hybrid organic-on-inorganic structures may represent an interesting alternative to all-inorganic and all-organic photovoltaic devices, allowing association of the stability and conversion efficiency of n-type inorganic semiconductors with the tunability of band level energies of organic p-type semiconductors. Efficient heterojunctions can be realized by this approach, as demonstrated by the example described: n-type GaAs associated with an organic p-type polythiophene derivative. A thin film (25 nm) of polythiophene has been electrochemically grown onto an n-GaAs substrate, and the cell has been completed by a thin layer of Au providing a front ohmic contact to the polymer layer. This device has been illuminated under a polychromatic 100 mW cm(-2) tungsten lamp, through the Au layer. The photovoltaic characteristics of this hybrid cell have been directly compared to those of a classical Schottky junction, n-GaAs/Au. Photocurrent action spectra confirm that the light is principally absorbed in the n-GaAs. The power characteristics of these cells show an increase of the conversion efficiency on passing from the Schottky cell (14%) to the hybrid cell (17.5%), which is due to the large increase of the open-circuit voltage. The parameters of the junction have been analysed by dark-current-voltage measurements, and interpreted following Anderson's model, which allows an energy band diagram to be proposed for this heterojunction. The improvement of V-oc is attributed to the creation of a high built-in potential of 0.93 eV, together with the reduction of the dark direct current.