R.L. Stolk

Understanding light trapping by light scattering textured back electrodes in thin film n‐i‐p-type silicon solar cells

For substrate n‐i‐p-type cells rough reflecting back contacts are used in order to enhance the short-circuit currents. The roughness at the electrode∕silicon interfaces is considered to be the key to efficient light trapping. Root-mean-square (rms) roughness, angular resolved scattering intensity, and haze are normally used to indicate the amount of scattering, but they do not quantitatively correlate with the current enhancement. It is proposed that the lateral dimensions should also be taken into account. Based on fundamental considerations, we have analyzed by atomic force microscopy specific lateral dimensions that are considered to have a high scattering efficiency. Textured back reflectors with widely varying morphologies have been developed by the use of sputtered Ag and Ag:AlOx layers. For these layers we have weighted the rms roughness of the surface with the lateral dimensions of the effective scattering features. A clear correlation is found between the current generation under (infra)red light...

Mechanism of Shunting of Nanocrystalline Silicon Solar Cells Deposited on Rough Ag/ZnO Substrates

Using a textured substrate is a basic requirement for light trapping in a thin film solar cell. In this contribution, the structure of μc-Si:H n-i-p solar cells developed on a rough Ag/ZnO coated glass substrate is carefully studied, in order to understand the substrate surface morphology dependence of solar cell properties, especially of the yield of working cells. From cross-sectional transmission electron microscopy (TEM) images it is clear that cells developed on substrates with tilted large Ag crystal grains contain pinholes that result in short-circuiting of the entire device. The formation of these pinholes is due to the inability of conformal coverage of the sub-micron sized cavities that are created by these Ag grains. Controlling the Ag deposition temperature is found to be essential to have a well performing μc-Si:H n-i-p cell.

Triple Junction n-i-p Solar Cells with Hot-Wire Deposited Protocrystalline and Microcrystalline Silicon

We have implemented a number of methods to improve the performance of proto-Si/proto-SiGe/μc-Si:H triple junction n-i-p solar cells in which the top and bottom cell i-layers are deposited by Hot-Wire CVD. Firstly, a significant current enhancement is obtained by using textured Ag/ZnO back contacts developed in house instead of plain stainless steel. We studied the correlation between the integrated current density in the long wavelength range (650-1000 nm) with the back reflector surface roughness and clarified that the rms roughness from 2D AFM images correlates well with the long wavelength response of the cell when weighted with a Power Spectral Density function. For single junction 2-μm thick μc-Si:H n-i-p cells we improved the short circuit current density from the value of 15.2 mA/cm 2 for plain stainless steel to 23.4 mA/cm 2 for stainless steel coated with a textured Ag/ZnO back reflector. Secondly, we optimized the μc-Si:H n-type doped layer on this rough back reflector, the n/i interface, and in addition used a profiling scheme for the H 2 /SiH 4 ratio during i-layer deposition. The H 2 dilution during growth was stepwise increased in order to prevent a transition to amorphous growth. The efficiency that was reached for a single junction μc-Si:H n-i-p cell was 8.5%, which is the highest reported value for hot-wire deposited cells of this kind, whereas the deposition rate of 2.1 A/s is about twice as high as in record cells of this type so far. Moreover, these cells show to be totally stable under light-soaking tests. Combining the above techniques, a rather thin triple junction cell (total silicon thickness 2.5 μm) has been obtained with an efficiency of 10.9%. Preliminary light-soaking tests show that this type of triple cells degrades by less than 4%.

High Quality Hot-wire Microcrystalline Silicon for Efficient Single and Multijunction N-i-p Solar Cells

In this paper, the potential of hot-wire chemical vapor-deposited (HWCVD) microcrystalline silicon (µc-Si) for use in solar cells is explored. Incorporation of the material in the currentlimiting bottom cell of two tandem cells on plain stainless steel resulted in FF values as high as 0.77, which is much higher than the highest single junction FF. A combination of experiments, calculations and computer simulations was employed to identify causes for the observed high tandem cell FF values. Both the light intensity and the spectral composition of the bottom cell illumination in a tandem were found to contribute to an increase of the bottom cell FF. The fact that the operational voltage of a tandem cell is higher than that of the current-limiting subcell, was calculated to lead to a tandem FF that can be far higher than that of the limiting cell. Computer simulations with the AMPS computer code show that the current mismatch in a tandem cell reduces the recombination in the current-limiting cell, possibly by slightly enhancing the internal field of that cell. Use of a 1.5 µm µc-Si:H hot-wire deposited absorber layer in a single junction cell on a textured back reflector yielded a Voc, FF and Jsc of 0.543 V, 0.656 and 23.60 mA/cm 2 , respectively, which combine to an 8.4 % record efficiency for µc-Si single junction n-i-p cells with a hot-wire intrinsic layer.