Obviating the requirement for oxygen inSnO2-based solid-state dye-sensitized solar cells

Abstract

Organic semiconductors employed in solar cells are perfectly stable to solar irradiationprovided oxygen content can be kept below 1 ppm. Paradoxically, the state-of-the-artmolecular hole-transporter-based solid-state dye-sensitized solar cells only operateefficiently if measured in an atmosphere containing oxygen. Without oxygen, these devicesrapidly lose photovoltage and photocurrent and are rendered useless. Clearly this peculiarrequirement has detrimental implications to the long term stability of these devices.Through characterizing the solar cells in air and in oxygen-free atmospheres, andconsidering the device architecture, we identify that direct contact between the metalliccathode and the mesoporous metal oxide photo-anode is responsible for a shunting paththrough the device. This metal–metal oxide contact forms a Schottky barrier underambient conditions and the barrier is suitably high so as to prevent significant shunting ofthe solar cells. However, under light absorption in an anaerobic atmosphere the barrierreduces significantly, opening a low resistance shunting path which dominatesthe current–voltage characteristics in the solar cell. By incorporating an extrainterlayer of insulating mesoporous aluminum oxide, on top of the mesoporoussemiconducting metal oxide electrode, we successfully block this shunting pathand subsequently the devices operate efficiently in an oxygen-free atmosphere,enabling the possibility of long term stability of solid-state dye-sensitized solar cells.