Thin-film silicon solar cells with diffractive elements
Thin-film silicon p-i-n diodes based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon are among the most promising solar cell structures for future photovoltaic (PV) technology. These materials involve almost no ecological risk during manufacture, operation, or disposal. For single μc-Si:H diodes, and a-Si:H/μc-Si:H tandem cells, efficiencies above 10%1 and 12.5%, respectively, have been achieved.2 Since the absorber-layer thicknesses used are in the μm-range, low cell reflectivity and effective means of trapping light are required to make efficient use of the sun's spectrum.
As an alternative to the textured transparent conductive oxides (TCO) with a randomly-distributed surface morphologies commonly used for this purpose, we have been investigating periodic light-coupler gratings of aluminium-doped ZnO.3,4 The optical properties of the resulting solar cells were investigated both using the numerical-modelling-based finite-integration technique (FIT) and through experimental study. The simulated quantum efficiencies (QE) were found to agree well with our experimental results.
The transparent gratings were prepared using photolithographic and lift-off processes on a glass substrate. The period (P) and groove height (h) of the gratings were varied between 1–4μm and 100–600nm, respectively. Scanning-electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the morphology of the gratings. Their optical properties were studied using optical spectroscopy.
The ratio of diffracted to totally-transmitted light (haze) significantly increases with P. For gratings with periods of 3 and 4μm, the haze between 600 and 900nm exceeds 90%. This shows that effective light diffraction is taking place and, thus, high short-circuit currents should be possible. More details about the optical properties of the grating couplers are available elsewhere.3
On top of these rectangular-shaped couplers, μc-Si:H thin-film solar cells with an i-layer thickness of 1μm were deposited by plasma-enhanced chemical-vapor deposition.2 The multi-layer stack consists of a glass substrate, a periodically-textured front contact, a p-i-n diode, and a ZnO/Ag back contact. For these solar cells, the increasing haze of the substrates as a function of the grating period P is neither reflected in the spectral response data nor in the short-circuit current. This behavior can be explained by means of the diffraction angle: with increasing P, the diffraction angle decreases. In contrast with cells on smooth substrates, the spectral response in the long-wavelength region rises with elevating h, which is caused by the increasing diffraction efficiency of the grating.
The calculation of light propagation within a μc-Si:H solar cell was performed by a numerical simulation tool that rigorously solves Maxwell's equations in 3D. We wanted to intensively analyze the influence of the surface texture on light trapping so we varied the grating parameters P and h. Increasing h resulted in more efficient diffraction, and the absorption increased. The variation of P within the simulations showed that—for cells with an absorber-layer thickness of 1μm—the maximum red response is with a P of 0.7–1μm. The maximum current was obtained for h between 300 and 400 nm, and was nearly independent of P.5
Our experiments and simulations show no significant dependence on rectangular grating parameters for blue light, indicating that the light coupling is not enhanced for this part of the spectrum.
However, this situation can be improved by changing to blazed and triangular gratings. Here, the gain in the blue response is more pronounced for smaller P. Numerical and experimental studies of cells with diffractive elements show the simultaneous influence of optical and optoelectronic effects on the enhanced blue response of textured cells: improved coupling of light at the TCO/Si interface and improved extraction efficiency of photo-generated carriers in the vicinity of the p/i-interface.
The first 3D simulations of cells with a pyramidal structure have shown that a short-circuit current comparable to that of randomly-textured cells can be achieved. This suggests that diffractive elements have significant potential in solar applications.
