Within four to seven years, electricity generated from solar cells will cost less than grid electricity, making it the cleanest, cheapest, and most abundant energy source on the planet. The rise of solar energy, however, could come to an untimely end if current solar cell technologies fail to meet the staggering manufacturing volumes needed to sustain current growth rates. Nanostructured donor/acceptor photovoltaics utilizing small molecule organics or conjugated polymers offer processing advantages that might enable high-throughput, large-area production. However, power conversion efficiencies of these structures have remained low, due in large part to low open-circuit voltages (VOC). Using printing methods, we deposit a layer of colloidal cadmium selenide (CdSe) quantum dots (QDs) onto a wide band-gap organic hole-transporting thin film of N,N'-bis(3-methylphenyl)-N,N'-bis-(phenyl)-9,9-spirobiuorene (spiro-TPD) in order to form a unique planar heterojunction photovoltaic device. This structure is found to produce much higher VOC than previously predicted for donor/acceptor heterojunction photovoltaics. Absorption and charge generation occur primarily in the QD layer and indium tin oxide (ITO) provides the top contact, allowing for exceptional device stability and full transparency below the QD bandgap of 2.0 eV. Overall power conversion efficiencies remain low at 0.03% because only a small percentage of the incident light is absorbed (4% at the rst QD excitonic peak of 2.1 eV) and ll factors are near 0.4, yet VOC is 1.3V.
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