Coherent light trapping in thin-film photovoltaics

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Introduction Thin-film photovoltaic (PV) technologies are attractive because they offer a lower overall module cost due to savings in both materials and processing. The lower module cost offers the potential for a lower cost kilowatt-hour produced by PV installation. The use of thin films also allows material of relatively poor quality to be used, since charge carriers only have to diffuse over a distance of the order of the film thickness. An inherent characteristic of both wafer-based and thin-film PV cells is incomplete light absorption in the spectral region near the bandgap of the photoactive material, which means that light-trapping approaches must be used to achieve optimal cell efficiencies.1–4 In thin-film PV cells, light trapping is particularly important because of the desire to produce cells with the thinnest possible active layers, but it remains relatively poorly understood and technologically challenging. A common way to increase the optical absorption of a slab of material is to use an anti-reflective coating (ARC)5,6 (Figure 1a). An ARC reduces the reflectivity from air to the photoactive region, but also prevents the optical energy, once inside the photoactive region, from making multiple round trips inside the photoactive region. In contrast, the goal of a light-trapping approach is to enhance the optical path length (i.e., the distance a photon travels before escaping) inside the photoactive region, as shown in Figure 1b. ARCs and light-trapping approaches are used in

tandem to maximize the number of photons absorbed in the photoactive region. Traditionally, geometric scattering1 at one or both surfaces that bound the active layer has been used for trapping incident light (Figure 1b). Light absorption is maximized by randomizing the occupation of the photon density-of-states (PDOS)2,4 in the active layer, thereby ensuring that, once trapped in the photoactive layer, the exit probability of a photon is reduced to a minimum possible value of 1/n2, where n is the index of refraction of the active material. Since this approach is based on the principles of statistical ray optics, the limit strictly applies only to PV cells, whose active layer thickness is much larger than the wavelength. It is used to maximize light absorption in both wafer-based and thin-film PV cells, but is much less effective in thin-film PV cells where the wavelength of the incident light is comparable to or larger than the film thickness. The main reasons for the reduced effectiveness are that subwavelength structures that effectively scatter light across a wide spectral range are challenging to design and fabricate,8 and thin films have a lower local PDOS compared to the bulk. Because of the shortcomings of geometric light-trapping schemes for thin-film PV cells, light harvesting approaches based on wave optics have attracted significant interest9 (Figure 1c). Light trapping based on wave optics or coherent light-trapping approaches works in the regime where light needs to be treated

Shrestha Basu Mallick, Stanford University; sbasumal@sta

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