We introduce an ultrathin donor-acceptor solar cell comprised entirely of inorganic nanocrystals spin-cast from solution. These devices are stable in air, and postfabrication processing allows for power conversion efficiencies approaching 3% in initial tests. This demonstration elucidates a new class of photovoltaic device with potential for stable, low cost power generation.Small molecule and polymer organic solar cells are promising candidates for affordable wide-scale solar energy conversion, but while organic materials offer strong potential for cost reduction vis-à-vis conventional solar cells, their spectrally limited absorption and low carrier mobilities impose limitations to achieving commercially viable device efficiencies (1). Colloidal inorganic nanocrystals share all of the primary advantages of organics -scalable and controlled synthesis, an ability to be processed in solution, and a decreased sensitivity to substitutional doping -while retaining the broadband absorption and superior transport properties of traditional photovoltaic semiconductors (2-4). Accordingly, utilizing inorganic nanocrystals for electron transport has been found to

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