Reviving "Fool's Gold" as an Environmentally Sustainable Solar Cell Material
Eray S. Aydil
Pyrite FeS2, also known as fool’s gold, has long been considered an ideal semiconductor for low-cost, sustainable solar cells because it is composed of earth-abundant, non-toxic, inexpensive elements and it absorbs light so strongly that a film one thousandth of the thickness of a human hair (100 nm) can absorb >90 % of incoming sunlight. Moreover, minority charge carrier diffusion lengths (100-1000 nm) exceed the film thickness required for complete light absorption, a significant advantage for efficient collection of photogenerated carriers to produce current. For these reasons, pyrite was pursued vigorously in the 1980’s as a potential absorber in thin film solar cells. All attempts failed, however, producing solar cells with disappointing efficiencies, less than 3 % . With the rise of other thin film solar cell materials, such as cadmium telluride (CdTe) and copper indium gallium diselenide (CuInGaSe2 or CIGS), with higher efficiencies, enthusiasm for pyrite vanished. Fool’s gold was dead as a solar cell material. Interest in FeS2 reemerged around 2009, however, motivated in part by the sustainability, cost, and toxicity concerns with commercialized solar cells based on CdTe and CIGS. This time, however, a few groups, including ours, are pursuing the fundamental origins of the disappointing performance of FeS2 rather than attempting to produce efficient cells via the trial-and-error approach that previously failed. Three issues have been identified: the possible presence of secondary phases and their deleterious effects, anomalous surface conduction, and a lack of understanding and control of doping. In this talk, we address the third, and most important issue, defects and doping in pyrite FeS2. For three decades, electronic transport data from thin FeS2 films were interpreted as p-type, while single crystals have been unambiguously established as n-type. This unexpected difference came to be known as the “Doping Puzzle”. Recently, we resolved this puzzle and showed that rigorously phase-pure pyrite FeS2 films are not p-type but in fact n-type. In thin films with low mobility (1 cm2 V-1s-1) hopping conduction artificially inverts Hall coefficient, incorrectly indicating p-type conduction. In single crystals and high quality films with mobilities over 1 cm2 V-1s-1, these artifacts caused by hopping conduction are absent and electronic transport measurements reveal that pyrite films are in fact n-type. For three decades, FeS2 based solar cells might in fact have been designed based on a mistaken presumption. Most recently, we have amassed the strongest evidence to date that the common dopant that makes FeS2 n-type is the sulfur vacancy. Significantly, we achieved control over sulfur vacancy concentrations and electron densities by varying the sulfur vapor pressure during crystal growth via chemical vapor transport.
- 10:30 Refreshments
- 10:45–12:00 Talk