Xianbi Xiang

Simulation of a-Si/a-SiGe thin film tandem junction solar cells

Amorphous silicon (a-Si) based thin film tandem junction solar cells are simulated based on a uniform field collection model. From the photovoltaic parameters of a single junction a-Si top cell and a few amorphous silicon–germanium (a-SiGe) bottom cells, the optimized a-Si/a-SiGe tandem cell can be predicted. The simulation results are in good agreement with the experiment. The highest efficiency a-Si/a-SiGe tandem cells are obtained with a combination of a-SiGe characteristics and a relatively large mismatch in the short circuit current between the top and bottom cells. A key reason for this behaviour is that the tandem cell may exhibit a larger fill factor than either one of the component cells under a certain current mismatch.

High-efficiency and highly stable a-Si:H solar cells deposited at high rate (8 Å/s) with disilane grading process

This paper presents our recent results on the high-rate deposition of high-efficiency and highly stable hydrogenated amorphous silicon (a-Si:H) solar cells with all layers deposited by 13.56 MHz radio frequency glow discharge. Using a linear disilane (Si2H6) grading process, high initial active-area efficiency of 11.42% has been obtained for the a-Si:H top cells with an effective i-layer deposition rate of 8 A/s. It is also found that the light-soaking stability of the a-Si:H top cells is much improved by the Si2H6 grading process with the best a-Si:H top cell exhibiting only 11.2% light-induced degradation after 1000 h of light-soaking. Integrating the high-rate deposited a-Si:H top cell in an amorphous silicon/amorphous silicon germanium (a-Si:H/a-SiGe:H) tandem cell, an initial active-area efficiency of 12.57% is achieved. After light soaking for 1008 h, the stable efficiency is still as high as 11.02%, corresponding to only a 12.31% degradation. To the best of our knowledge, this is the best performan...

Spectroscopic aspects of front transparent conductive films for a-Si thin film solar cells

This work demonstrates a method to optimize the indium tin oxide (ITO) thin films as front transparent electrode to maximize the efficiency of substrate type amorphous silicon (a-Si) based thin film solar cells. It shows that the total light intensity absorbed by the a-Si layer can be predicted by combining a multilayer optical simulation with the nonuniform solar spectrum and the spectroscopic response of the absorption coefficient of the a-Si film. Consequently, an optimized ITO film can be identified. The photovoltaic performances of experimentally obtained a-Si single junction solar cells confirm the simulation results, indicating an ITO film about 56 nm thick leads to the highest efficiency. Furthermore, it is shown that the ITO films should be deposited at relatively low temperature around 132 °C to avoid damage to the a-Si top p-layer and p-i-n junction. It is found that introducing a small fraction, ∼0.61% flow ratio, of O2 in the sputtering Ar gas reduces the sheet resistivity of the ITO film and...

Amorphous silicon germanium solar cells deposited on stainless steel at elevated pressure

This work reports our efforts on improving the deposition rate for amorphous silicon germanium (a-SiGe) absorber layer by increasing the PECVD process pressure. We demonstrate that at an elevated pressure of 1∼4 Torr, the deposition rate reaches 3.5∼4 Å/sec, which is about 4 times higher than previous low pressure processes (0.3∼0.6 Torr). Deposited at such a high rate, the single junction a- SiGe solar cells exhibit an efficiency comparable to that achieved at low pressure (12.5% initial, 10.4% stabilized). Furthermore, tandem junction cells using the high rate deposited a-SiGe as bottom cell show an initial efficiency as high as 13.27% and a stabilized efficiency of 11.50% after light soaking.

High efficiency amorphous silicon germanium solar cells

We report high-efficiency single-junction a-SiGe n-i-p solar cells deposited using rf PECVD on stainless steel (SS) substrates coated with metal/ZnO back-reflector (BR). The initial and stabilized active-area efficiencies have been improved to 12.5-13.0% and 10.4%, respectively, for 0.25 cm/sup 2/ a-SiGe cells. The achievement of single-junction cells with such high efficiencies, equivalent to those for the state-of-the-art triple-junction solar cells, are important since this would lead to significant cost reduction in manufacturing. The key factors leading to these high efficiencies include the use of: 1) an optimized GeH/sub 4/ to Si/sub 2/H/sub 6/ ratio leading to a Ge content ideal for high-efficiency single-junction a-SiGe cell, 2) an optimized level of hydrogen dilution for the i-layer, and, most importantly, 3) a hybrid p-layer with the sub-layer near a-SiGe i-layer deposited at high temperature (140 /spl deg/C) and the bulk of the p-layer deposited at low temperature (70 /spl deg/C) for better transparency.

Impacts of an intrinsic a-Si buffer layer between the p-type nc-Si layer and the intrinsic a-SiGe layer in single junction solar cells

This paper reports numerical modeling and experimental investigation for the impacts of an intrinsic a-Si buffer layer between the p-type nc-Si layer and the intrinsic a-SiGe layer with a narrow bandgap of 1.40–1.55eV on the performances of a-SiGe single junction solar cells. The effects of bandgap and thickness of the buffer layer were simulated by using Analysis of Microelectronic and Photonic Structures (AMPS) computer model developed at Penn State University. The results obtained by the simulation show that the intrinsic a-Si buffer layer can lead to an increase in the open circuit voltage (Voc), but cause a decrease of the fill factor (FF) and the conversion efficiency (Eff), depending on how large the band gap and thickness of the buffer layer are. Our experimental results are consistent with the simulations results; i.e., a thick and wide-bandgap buffer layer between i and p layers can cause a serious deterioration in FF. An optimal a-SiGe single junction solar cell without the a-Si buffer layer has achieved an efficiency of 9.41% with Voc=0.576V, Jsc=23.34 mA/cm2, and FF=70.0%.

Numerical simulation and experimental investigation of a-Si/a-SiGe tandem junction solar cells

This paper reports numerical modeling and experimental investigation of a-Si/a-SiGe tandem junction solar cells, based on a field-aided collection model. The performances of the composed a-Si/a-SiGe tandem cells could be predicted from the numerical modeling and experimental photovoltaic parameters of the component cells. An optimized choice for the tandem cells has been proposed by the aid of numerical simulation and verified by experimental results for the cases of a fixed a-Si top cell or a fixed a-SiGe bottom cell, respectively. It is found that 1) the highest efficiency (∼13%) of the tandem cells can be achieved with a certain mismatch in Jsc between the top and bottom cells; 2) a small amount of Ge can be incorporated into a-Si based top cells to obtain an optimized current match with the bottom cell; 3) the Jsc of the simulated tandem solar cells is always a little greater, rather than exactly equal to the limiting Jsc of the component cells.

High rate deposition of a-Si and a-SiGe solar cells near depletion condition

Amorphous silicon (a-Si) and amorphous silicon germanium (a-SiGe) absorber layers are deposited at high rates of 7∼8 Å/sec using RF plasma enhanced chemical vapor deposition. The single junction a-Si top and a-SiGe bottom cells deposited at such a high rate exhibit initial efficiencies of 10.06% and 9.96%, respectively, while the process is not yet fully optimized. A tandem junction cell made using the high rate deposited a-Si and a-SiGe shows an initial efficiency as high as 11.04%. A combination of proper RF power density, gas pressure, and H2 dilution enables the intrinsic layers being deposited near a depletion condition and is responsible for the promising performances.

Fine-grained nanocrystalline silicon p-layer for high open circuit voltage a-Si:H solar cells

Hydrogenated amorphous silicon (a-Si:H) single-junction solar cells with high open circuit voltage (V/sub oc/) are fabricated using a wide bandgap boron doped Si:H p-layer deposited at high hydrogen dilution, low substrate temperature and with H/sub 2/-plasma treatment that promotes nanocrystalline silicon (nc-Si:H) formation. This paper presents the structure of this p-type material characterized by Raman scattering spectroscopy and high resolution transmission electron microscope (HRTEM). It is found that the p-layer that leads to high V/sub oc/ a-Si:H solar cells is a mixed-phase material that contains fine-grained nc-Si:H embedded in a-Si:H matrix.

Impacts of n-type interface on the performances of a-Si based solar cells

This paper reports experimental results and numerical simulations for the impact of n-type interface on the performance of n-i-p type a-Si based solar cells deposited on stainless steel (SS) foil substrate. Here the n-type interface includes the interfaces of n-a-Si layer with i-a-Si layer and SS substrate. The obtained results show that 1) the extra interfacial layer between SS and the n-a-Si layer will cause a rollover behavior of the light I-V curve, and H2 plasma pretreatment on the SS surface may eliminate the extra layer and significantly improve the cell performance; 2) the contamination of n-dopants for the sequential growth of i-layer will seriously deteriorate the cell performances and cause a crossover of the dark and light I–V curves when the n-i-p a-Si solar cells were deposited in a single chamber mode; and 3) the i-a-Si buffer layer between i-a-SiGe and doped a-Si layers could improve the cell performance, but if the thickness of the buffer layer was beyond a critical value, it may cause a bending of the light I–V curves.