H. B. T. Li

Light trapping in ultrathin plasmonic solar cells

We report on the design, fabrication, and measurement of ultrathin film a-Si:H solar cells with nanostructured plasmonic back contacts, which demonstrate enhanced short circuit current densities compared to cells having flat or randomly textured back contacts. The primary photocurrent enhancement occurs in the spectral range from 550 nm to 800 nm. We use angle-resolved photocurrent spectroscopy to confirm that the enhanced absorption is due to coupling to guided modes supported by the cell. Full-field electromagnetic simulation of the absorption in the active a-Si:H layer agrees well with the experimental results. Furthermore, the nanopatterns were fabricated via an inexpensive, scalable, and precise nanopatterning method. These results should guide design of optimized, non-random nanostructured back reflectors for thin film solar cells.

Improved red-response in thin film a-Si:H solar cells with soft-imprinted plasmonic back reflectors

The impact of controlled nanopatterning on the Ag back contact of an n-i-p a-Si:H solar cell was investigated experimentally and through electromagnetic simulation. Compared to a similar reference cell with a flat back contact, we demonstrate an efficiency increase from 4.5% to 6.2%, with a 26% increase in short circuit current density. Spectral response measurements show the majority of the improvement between 600 and 800 nm, with no reduction in photocurrent at wavelengths shorter than 600 nm. Optimization of the pattern aspect ratio using electromagnetic simulation predicts absorption enhancements over 50% at 660 nm.

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.

Exploring dark current voltage characteristics of micromorph silicon tandem cells with computer simulations

The transport mechanisms controlling the forward dark current-voltage characteristic of the silicon micromorph tandem solar cell were investigated with numerical modeling techniques. The dark current-voltage characteristics of the micromorph tandem structure at forward voltages show three regions: two with an exponential dependence and a third where the current grows more slowly with the applied voltage. In the first exponential region the current is entirely controlled by recombination through gap states of the top cell. In the second exponential region the current is controlled by the mixture of recombination through gap states of both the top and bottom cells and by free carrier diffusion along the a-Si:H intrinsic layer. In the third region the onset of the electron space charge limited current on the a-Si:H top cell can be observed along with some other mechanisms that are discussed in the paper. The high forward dark J-V curve of the tandem cell can be used as diagnosis tool to quickly inspect the e...

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.

Plasmonic light trapping for thin film A-SI:H solar cells

Here we discuss the design, fabrication, and simulation of ultrathin film n-i-p a-Si:H solar cells incorporating light trapping plasmonic back reflectors which exceed the performance of n-i-p cells on randomly textured Asahi substrates. The periodic patterns are made via an inexpensive and scalable nanoimprint method, and are structured directly into the metallic back contact. Compared to reference cells with randomly textured back contacts and flat back contacts, the patterned cells exhibit higher short-circuit current densities and improved overall efficiencies than either reference case. Angle-resolved photocurrent measurements confirm that the enhanced photocurrents are due to coupling to waveguide modes of the cell. Electromagnetic modeling is shown to agree well with measurements, and used to understand further details of the device.

Photogating effect as a defect probe in hydrogenated nanocrystalline silicon solar cells

The measurement of the spectrally resolved collection efficiency is of great importance in solar cell characterization. Under standard conditions the bias light is a solar simulator or a light source with a similar broadband irradiation spectrum. When a colored blue or red bias light is used instead, an enhanced collection efficiency effect, in the literature known as the photogating effect, can be observed under certain conditions. While most of the published reports on such effect were on solar cells with amorphous silicon based absorber layers, we have shown that the enhanced collection efficiency could be also present in thin film silicon solar cells where hydrogenated nanocrystalline silicon (nc-Si:H) is used as the absorber layer. In this article we present detailed experimental results and simulations aiming at a better understanding of this phenomenon. We show that the collection efficiency is strongly dependent on the intensity of bias light and the intensity of the monochromatic light. These exp...

Hot wire CVD for epitaxial thickening of polycrystalline silicon seed layers made by AIC on glass

Large-grained polycrystalline silicon thin films on glass are attractive as the absorber layers for highly efficient, inexpensive, and stable solar cells that could replace the dominant silicon wafer-based technology. As an alternative to crystallization treatments of deposited amorphous silicon precursor layers, we are studying the approach of epitaxial thickening by hot wire CVD (at Utrecht University; UU) of large-grained polycrystalline seed layers produced by aluminium induced crystallization (AIC; at UNSW) at <500 /spl deg/C on glass. We found that after the transfer of the AIC layers from Sydney to Utrecht, it is important to thoroughly clean them in order to achieve epitaxial thickening. At UU, the AIC layers were thickened by 150-300 nm, where poly2 conditions led to 60% crystalline ratio and epi conditions to 70% crystalline ratio. In both cases the optical (lateral) feature size was -5 /spl mu/m, and epitaxial growth was observed to have taken place locally both in cross-sectional TEM and in optical micrographs. We report on preliminary solar cell structures made with AIC and HWCVD technology.