Light-harvesting semiconductor quantum dot nanocrystals integrated on photovoltaic radial junction nanopillars

In the last decades increased awareness for environmental problems, climate change, and limited energy sources has escalated research efforts around the globe to investigate photovoltaics as an alternative energy source. The sun offers an enormous potential for solar energy conversion, which makes photovoltaics quite attractive among green energy sources. However, there is still not an ultimate photovoltaic device which yet provides high efficiencies at low costs. Recently with the development of nanofabrication technologies, nanostructured solar cells have emerged at research labs (e.g., using nanowires, nanorods, nanotubes etc.). These nanostructured device architectures are intended to yield enhanced optical properties such as light trapping compared to existing planar devices. In certain architectures, however, there are some nanofabrication bottlenecks that adversely affect the overall device performance of such nanostructured photovoltaics. These technical challenges can possibly be overcome with maturing fabrication methods in time. But, there also exist limitations stemming from intrinsic properties of the active materials that cannot be easily fixed unless hybrid approaches are exploited or materials are replaced. For example, silicon, which is the most commonly used material in photovoltaic industry, suffers too strong absorption at short wavelengths (i.e., UV-blue), undesirably leading to poor performance in this spectral range due to high front surface recombination rates. However, UV-blue portion constitutes almost 10% of the sunlight, which is mostly unused by Si based photovoltaics. To address this problem, we propose and demonstrate light-trapping radial p-n junction Si nanopillar solar cells that are furnished with CdSe quantum dot nanocrystals to harvest short-wavelength radiation in addition to long-wavelengths. The basic operating principle of these light-harvesting nanocrystals integrated on nanopillars relies on the wavelength up-conversion idea of incident photon absorption in these nanocrystals at short wavelengths and subsequent photon emission at longer wavelengths, which is in turn reabsorbed by the furnished nanopillar diodes. Although such light-harvesters were previously studied on planar devices or those with elliptical holes in them, nanocrystal-decorated nanopillars have not been investigated to date. Here we demonstrate that this nanopillar architecture provides additional advantage of trapping photon emission from the light-harvesting nanocrystals for wavelength up-conversion, compared to the planar case.