ABSTRACT
Nanophotonics is essential for increasing the absorption in thin-film solar cells. Two particularly promising examples are plasmonics and grating structures. In this chapter, we discuss the physical concepts behind light trapping using these nanophotonic structures and the state of the art in this area. 3.1 Introduction
The worldwide installed capacity of solar photovoltaic (PV) power has escalated from 1.3 GW in 2001 to 15.2 GW in 2008 and 22.9 GW in 2009 and has had an average growth rate of 40% over the last five years. Given that the earth receives more energy from the sun in one day (1021 J) (International Energy Agency, 2010) than is used by the world population in one year, PV contribution to the world energy has vast potential. PV can be used to generate electricity as well as
fuels like hydrogen. Another advantage of PV, apart from the fact that it is a renewable energy source, is that the world’s energy can be produced in a decentralized manner. Setting up small-scale solar power generators in a decentralized manner eliminates the need to transport electricity over long distances, which is inherently lossy. Of the electricity generated from conventional sources, 30% is actually lost in resistive heating while being transported. Decentralized solar electricity generation also eliminates the need for large-capacity transmission lines and transformer stations. Decentralized solar power generators are also more resilient to natural disasters as there will be no single points of failure that can bring down the entire grid. Currently, 80-90% of the PV market is based on crystalline Si (c-Si) solar cells (Solarbuzz, https://www.solarbuzz.com/). Si is the third-most abundant element on the earth and has near-ideal bandgap energy for maximizing the efficiency of a single-junction solar cell. c-Si solar cells have now exceeded an efficiency of 25% in the laboratory, and silicon modules have reached an efficiency of over 22% (Green, Emery, Hishikawa, and Warta, 2010). High-purity Si used in the fabrication of conventional c-Si solar cells requires expensive and energy-intensive refining of the Si feedstock. Material costs account for ~40% of the total cost of a typical c-Si PV module. Only 25% of the total costs are spent on actual cell fabrication. The rest is the module fabrication cost. An effective approach to reducing the cost per watt of PV-generated electricity to a level comparable with that generated from conventional fossil fuels would be to design and fabricate high-efficiency solar cells based on thin active layers. Conventional c-Si solar cells are fabricated from 180-300 μm thick Si wafers. Fabricating thin-film solar cells with an active layer thickness of hundreds of nanometers to a few microns would reduce the material usage by a factor of 100. In addition to reduced materials usage, thin-film solar cells also have the advantage of reduced carrier collection lengths. The photogenerated carriers in the cell should reach the external contacts before they recombine in order to generate electric current. The distance travelled by the carriers before recombination is called carrier diffusion length. For efficient collection of photogenerated carriers, the carrier diffusion length in the active material should be a few times larger than the thickness of the active layer. Reduced carrier collection lengths facilitate the use of lower-quality active material (material with
lower carrier diffusion lengths) for cell fabrication, further reducing the material and deposition costs. This approach also opens up opportunities to use alternative, relatively cheap semiconductors based on earth-abundant materials like Cu, Zn, and Sn for solar cells. These semiconductors are usually of poor quality and can be useful for PVs only in thin-film configuration. However, a thin active layer compromises the optical absorption in the solar cell. Figure 3.1 shows the spectral irradiance (in W/m2/nm) on the earth’s surface for the AM1.5g solar spectrum and the irradiance absorbed by a 2 μm thick Si layer, neglecting reflection losses (i.e., assuming a perfect antireflection coating [ARC] on the front surface). As can be seen from Fig. 3.1, for wavelengths greater than 500 nm, not all of the incident photons are absorbed in the Si layer. Part of the incident energy is lost because of transmission of light through the Si layer. These transmission losses are more significant in the long-wavelength region (700-1180 nm), closer to the band edge of Si. Transmission losses can be reduced by “folding” light multiple times into the absorbing region of the solar cell, thereby increasing the optical path length of light and hence the probability of its absorption inside the solar cell. This process is known as light trapping. By employing light trapping in a solar cell, the “optical thickness” of the active layer is increased several times, keeping its physical thickness unaltered. The ratio of the optical thickness to the physical thickness, that is, the ratio of the path length travelled by photons inside the cell in the presence of light trapping to that in the absence of light trapping is known as path length enhancement. This is an important parameter that enables quantitative comparison of different light-trapping techniques. Conventionally, light trapping is achieved by modifying the surface of the solar cell to enhance the probability of total internal reflection. By doing so, light gets reflected back into the active volume several times. Theoretically, the most widely studied light-trapping scheme employs the “Lambertian surface.” A Lambertian surface is an isotropic scattering surface. For a solar cell with a front Lambertian surface and a perfect rear reflector, the average path length becomes 4n2w, or the average path length enhancement with respect to a planar semiconductor of thickness w is 4n2, where n is the refractive index of the active material (Patrick and Martin, 1987; Goetzberger, 1981). This estimation is valid only in a weakly absorbing limit, that is, when there is negligible absorption inside the structure or
on the surface of the structure. Another important assumption is that the optical mode density in the structure is continuous and is unaffected by wave optical effects. These conditions are satisfied if the optical thickness of the cell is much greater than λ/2n, where λ is the wavelength of the incident light, n is the refractive index of the material, and the surface texture is random or has a period much larger than λ.
