Nano-heterojunctions for solar cells

Solar cell
solar cells tap sunlight and converts into electricity. Worldwide, the contribution of solar electricity is higher due to many installed solar modules. Both the potential and gap in solar energy utilization by solar  cells are enormous. Semiconductors serve as the light absorber to convert photons into electron-hole pairs, and the internal electric field. The fundamental processes in solar cells are light absorption and charge separation. Lifetime of minority carrier and carrier  mobility are  critical for high efficiency. The record  efficiencies of commercial-size cells range between 12% and 20%. The current best efficiency of inorganic single-junction solar cells is 20-25% and it is almost saturated during the last decade.
Inorganic solar cells
Solution-processed inorganic solar cells based on colloidal semiconducting quantum dots and nanocrystals show much promise because they can absorb light over a wide spectrum of wavelengths thanks to the fact that the bandgaps in quantum dots can be tuned over a large energy range. They are also comparatively cheap to produce. Inorganic solar cells are  made using quantum structures. Incorporation of MQW, SL and quantum dots into photovoltaic devices leads to a spectacular improvement of the theoretical maximum efficiency, compared to the conventional bulk-semiconductor-based solar cells. The nanorod-shaped donor-acceptor solar cells also exhibit the stable performance in air. There are challenges to reduce the gap between the ideal and real values of the conversion efficiency.
Nanorod-shaped heterojunction
Donor-acceptor solar cells are composed entirely of inorganic nanocrystals spin-cast from solution. The solar cells use the nanorod-shaped CdTe/CdSe nanocrystal heterojunction. Each ultrathin (~ 100 nm) nanocrystal is spin-cast from a filtered pyridine solution. This technology provides large area, flexible thin films of densely packed nanocrystal on virtually any substrate.
researchers in Spain have developed a new technique to prolong lifespan of charge carrier in colloidal nanocrystal solar cells by using nano-heterojunctions consisting of electron acceptor and donor nanomaterials. The technique allows for high quantum efficiencies even in photovoltaic materials with poor optoelectronic properties. Cadmium-based crystals were used by the researchers because charge carriers in these compounds last a fairly long time.
Prolonging lifespan
The researchers created a bulk nano-heterojunction in a solar-cell device by mixing delectron acceptor and donor materials in such a way that, when exposed to sunlight, photogenerated electron-hole pairs can separate at the nanoscale and travel along the device via two very different nano-paths, to reduce the chances of their recombination.
According to the published report the researchers claim that although the power conversion efficiency of their cells is still a bit lower than record efficiency devices based on PbS quantum dots and titania n-type electrodes, it does demonstrate the proof-of-principle, and unlike previous studies that relied on either sputtered oxide electron acceptors or high-temperature sintering at 500 °C, their technique works using fully solution-based process and at low temperatures of less than 100 °C  with non-negligible advantages for low-cost roll-to-roll manufacturing..


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