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Solar & Alternative Energy

Thin-film silicon solar cells with diffractive elements

Nano-textured substrates can be used to trap sunlight and couple it into thin-film silicon solar cells.
12 April 2006, SPIE Newsroom. DOI: 10.1117/2.1200602.0150

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.


Figure 1. AFM and SEM images of grating structures prepared by photolithographic and lift-off processes.
 

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.


Figure 2. The QE calculation compared to experimental results for P=1.2μm and h=300nm. By integrating the grating structure, there is an increase of ∼3mA/cm2 as compared to a flat cell.
 

Figure 3. Simulation of the 3D power-loss profile for a pyramidal structure integrated into a thin-film silicon solar cell.
 

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.


Authors
Christian Haase and Helmut Stiebig
Institute of Photovoltaics, Forschungszentrum Juelich
Juelich, Germany 
Christian Haase is a PhD student investigating the optical properties of thin-film silicon solar cells with diffractive elements by means of optical simulations and optical measurements. He previously worked on GaN-based optical memory devices and thin-film silicon solar cells made with high-temperature CVD.
Helmut Stiebig received his PhD in electrical engineering from the University of Technology, Aachen. He is head of the research group on device analysis and sensor technology. He investigates the optoelectronic properties of amorphous and microcrystalline silicon and related materials, the development of innovative device structures for solar-cell applications, device analysis and numerical modelling, and the development of optical sensors based on thin-film technology.

References:
1. Y. Mai, S. Klein, R. Carius, H. Stiebig, X. Geng, F. Finger,
Applied Physics Letters,
Vol: 87, 2005.
2. B. Rech, T. Roschek, T. Repmann, J. Müller, R. Schmitz, W. Appenzeller,
Thin Solid Films,
Vol: 427, pp. 157, 2003.
3. H. Stiebig, N. Senoussaoui, C. Zahren, C. Haase, J. Müller,
Progress in Photovoltaics,
Vol: 14, no. 1, pp. 13-24, 2005.
4. R. H. Morf,
Condensed Matter Theory Group, Paul-Scherrer-Institut, Villigen, Switzerland
Ch. 5232, web-report 2002..
5. C. Haase, H. Stiebig,
Progress in Photovoltaics,
(in press).