By directly converting sun light to electricity, photovoltaic technology is of great importance as part of a sustainable energy solution. Currently the challenge for utility scale photovoltaic power generation is the cost – efficiency dilemma. For high efficiency cells, made of high quality crystalline semiconductor, their price is high, and for low cost cells, such as organic PV, their efficiency is not sufficient. One way of reducing the cost is to replace expensive semiconductor material with relatively cheaper optics, a solar concentrator, and reduce the size of the active photovoltaic cells. The bulkiness of a conventional solar concentrator becomes pricy and impractical for certain applications. A micro solar cell array module was thus developed to overcome these limitations and provide a cheap and compact solution. By miniaturizing the solar cells to produce micro-scale solar cells, several advantages can be achieved.The micro solar cell enables a distributed micro lens array to deliver the same benefit of cost reduction as a large concentrator system but with a sleeker form factor that allows for easier deployment and integration. In this work a micro lens array solar concentrator for micro solar cells is demonstrated and achieves a concentration factor of 2.5 while removing the need for solar tracking in a single dimension.After exploiting an imaging based micro solar cell concentrator, it is desirable to achieve light concentration without the need to accurately track the sun, as is needed for a lens based concentrator.The specific method explored was the use of a Luminescent solar concentrator (LSC). Traditionally, luminescent concentrators suffer from loss of emitted light due to poor wave guiding and self-absorption of emitted light by the fluorophore.Together these effects have limited the possible collecting efficiency in LSC structures.Using a distributed array of micro-solar cells, the wave guiding loss and self-absorption loss can be greatly reduced. Part of this work is focused on distributed luminescent solar concentrator for micro solar cell array. Organic dye doped polymer LSC was studied and it was shown that the distributed system was an effective solar concentrator achieving a maximum concentration ratio of 3.2 ×. This system is still limited in the concentration ratio that could be achieved as the flux gain saturates at dimensions on the order of a couple millimeters.To understand the fundamental limitations of the system, a rigorous 3D ray tracing model was developed to understand the LSC system and provide design guidelines, and predict 3D structure performance as well. Key parameters affecting LSC performance were identified and each investigated for their contribution. It is found that the matrix absorption loss, including the dye self-absorption and matrix host absorption determines the ultimate concentration ratio, and the dye self-absorption plays a dominant role. By way of geometric ray tracing, it is concluded that to achieve higher optical flux gain a large Stokes shift is required, agreeing with the previously developed theory according to thermodynamics.As called for by theory, modeling and shown from organic dye doped LSC, a semiconductor nanocrystal doped LSC was demonstrated. The careful design of these quantum materials by our collaborators enabled a large Stokes shift. The results show that the photon flux gain increases to large geometric gain, well beyond the situation limited by dye self-absorption, and keeps increasing until the matrix absorption limits the LSC performance. The drawback is the narrow spectral range the nanocrystals absorb, as it only covers 10% of the solar spectrum. This points out to future direction of development of other type of such nanocrystals that can be utilized to make a multilayered tandem LSC that covers the entire solar spectrum.
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Ultra thin micro solar cells with concentration optics: fabrication, modeling and characterization
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