Pb-based versus Sn-based perovskite solar cells: Toxicity and environmental burden

Organometal halide perovskites have rapidly evolved into strong contenders to compete with silicon in the quest for low-cost photovoltaics, with their added value being solution-processability. Their primary drawback, however, is that so far the presence of lead (Pb) is required to obtain the highly favorable electro-optical properties of the most successful perovskite crystals such as CH3NH3PbI3. Together with their tendency to degrade under the influence of humidity, and the corresponding disintegration of the unit cell, this implies that Pb compounds can be released into the environment upon failure of a perovskite module. As already known from literature, Pb is a rather toxic element causing irreversible neurological, nephrotic and hepatic damage. Hence, finding a non-harmful alternative metal, exhibiting similar electro-optical characteristics in the resulting perovskite crystal, could be the solution to improve and ultimately commercialize perovskite-based solar cells. Tin (Sn), also being a group IV metal, has been deemed the most appropriate alternative. However, Sn is also enlisted as a harmful chemical. Animal and human volunteer studies have shown that toxicity symptoms like fever, nausea, nephropathy, etc. emerge upon excessive uptake, raising question marks regarding the suitability of Sn as a more environmentally friendly alternative to Pb in perovskite solar cells. This contribution aims to make a first step towards the assessment of the environmental burden of both Pb and Sn based solar cells in the form of a toxicity study. Well-established aquatic model organisms are exposed to the appropriate degradation products, according to well-defined guidelines of the Organization for Economic Co-operation and Development (OECD). This allows a systematic comparison of Sn and Pb-containing decayed compounds regarding their potentially harmful effects on the environment, and sheds light onto the applicability of both corresponding perovskite families in large-scale photovoltaic systems.