Back reflectors and substrate texturing are used to improve the power output of most thin film solar cells. Figure 41.6a shows the “quantum efficiencies” measured for a 250-nm-thick a-Si:H solar cell with varying texturing and back reflectors (Hegedus et al. 1996), along with absorptances calculated from the optical absorption coefficient spectrum for a-Si:H. The dashed absorptance curve labeled “no light trapping”
Fig. 41.6 (a) Absorption spectra calculated for a 250 nm a-Si:H, and quantum efficiency spectra measured for a series of a-Si:H nip solar cells with a 250 nm thick i-layer and varying substrates and back reflectors (Hegedus et al. 1996). E: (untextured, none); V: (untextured, untext.); ▲ untextured, textured), (textured, none); ■ (textured, untextured). (b) Calculated absorption spectra and quantum efficiency measured for a nc-Si:H solar cell (Yan et al. 2010) corresponds to A(k) = 1 – exp(-a(X)d), and is the fraction of light that is absorbed by a 250 nm layer of a-Si:H assuming normal incidence and neglecting reflection from both the front and rear interfaces. This absorptance falls with increasing wavelength, which corresponds to the absorption coefficient spectrum of Fig. 41.2 and the thickness of 250 nm.
The quantum efficiency (QE) of the solar cell is defined as the ratio, at a specific wavelength, of the photocurrent density j (A/cm2) to the incident photon flux f (cm-2s-1):
QE(k) = j(k)/[ef(k) ]. (41.3)
Electrons generated in a p-layer are more likely to be captured by a dopant or a dangling bond than to escape to the i-layer, and similarly for holes generated in an n-layer. Figure 41.6 shows, that the introduction of a back reflector doubles the QE for the longer wavelengths, which are weakly absorbed. The other curves show that “texturing” of the substrate and of the back reflector further increases the QE from the cells, by allowing light to traverse more. The substrate texturing also leads to a modest improvement of the QE in the blue spectral region (beyond 2.5 eV), which is due to a reduction in the front-surface reflectance of the cell. For a-Si:H, the best texturing increases the short-circuit current by about 25 % (Hegedus et al. 1996; Lechner et al. 2000). The texturing and back reflectors and of a front antireflection coating, vary substantially between superstrate and substrate cells. Superstate cells usually have a textured, transparent conductive oxide (TCO) coating on the transparent substrate (usually glass).The back reflector deposited on top of the semiconductor layers is often a thin TCO layer, followed by the reflective metal (typically
Fig. 41.7 Multi-junction solar cells: (a) a-Si/a-Si superstrate tandem; (b) a-Si/nc-Si “micro – morph” superstrate tandem; (c) a-Si/a-SiGe/a-SiGe substrate triple; and (d) a-Si/nc-Si/nc-Si substrate triple. The light enters from the top. The black region is the metal contact. TCO as ITO or ZnO. The differences of the i – layer thickness for the different cell types is indicated
Ag, for best reflectivity, or Al, for improved production yield). Substrate cells are deposited onto the back reflector, a textured silver or aluminum metallization and then a textured TCO (Banerjee and Guha 1991).