Wenhui Du

Impact of hydrogen dilution on microstructure and optoelectronic properties of silicon films deposited using trisilane

We explored the deposition of hydrogenated amorphous silicon (a-Si: H) using trisilane (Si3H8) as a gas precursor in a radiofrequency plasma enhanced chemical vapour deposition process and studied the suitability of this material for photovoltaic applications. The impact of hydrogen dilution on the deposition rate and microstructure of the films is systematically examined. Materials deposited using trisilane are compared with that using disilane (Si2H6). It is found that when using Si3H8 as the gas precursor the deposition rate increases by a factor of similar to 1.5 for the same hydrogen dilution (R = [H-2]/[Si3H8] or [H-2]/[Si2H6])- Moreover, the structural transition from amorphous to nanocrystalline occurs at a higher hydrogen dilution level for Si3H8 and the transition is more gradual as compared with Si2H6 deposited films. Single-junction n-i-p a-Si: H solar cells were prepared with intrinsic layers deposited using Si3H8 or Si2H6. The dependence of open circuit voltage (V-oc) on hydrogen dilution was investigated. V-oc greater than 1 V can be obtained when the i-layers are deposited at a hydrogen dilution of 180 and 100 using Si3H8 and Si2H6, respectively.

Fabrication and Characterization of Triple-junction Amorphous Silicon Based Solar Cell with Nanocrystalline Silicon Bottom Cell

Recent research activities and results on the fabrication and characterization of high-efficiency triple-junction hydrogenated amorphous silicon (a-Si:H) based solar cells with hydrogenated nanocrystalline silicon (nc-Si:H) bottom cells at the University of Toledo (UT) are briefly summarized and reported in this paper. Using VHF PECVD technique, new deposition regimes have been developed in UT multi-chamber load-locked PECVD deposition system for the preparation of high quality a-Si:H, a-SiGe:H and nc-Si:H i-layers at deposition rates in the range of 2-15 A/s. Incorporating various improvements in device fabrication and characterization, 7.8% initial and 7.4% stable active-area (0.25 cm2) cell efficiencies have been achieved for VHF nc-Si n-i-p single-junction solar cells. Initial efficiency of 11.0% for a-Si/nc-Si tandem-junction was obtained. 12.4% initial and 11.0% stable cell efficiencies for a-Si/a-SiGe/nc-Si triple-junction solar cells have also been achieved. We also report 7.2% initial efficiency for single-junction nc-Si:H cells having nc-Si:H i-layer deposited at high rate using RF PECVD at a high pressure of 8 Torr

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.

Film adhesion in triple junction a-Si solar cells on polyimide substrates

A major issue encountered during fabrication of triple junction a-Si solar cells on polyimide substrates is the adhesion of the solar cell thin films to the substrates. Here, we present our study of film adhesion in amorphous silicon solar cells made on different polyimide substrates (Kapton VN, Upilex-S and Gouldflex), and the effect of tie coats on film adhesion.

Triple-junction a-Si solar cells with heavily doped thin interface layers at the tunnel junctions

Triple-junction a-Si based solar cells, having a structure of SS/Ag/ZnO/n/sup +//n/b/a-SiGe-i/b/p/p/sup +//n/sup +//n/b/a-SiGe-i/b/p/p/sup +//n/sup +//n/a-Si-i/p /p/sup +//ITO, are fabricated at the University of Toledo using a multi-chamber, load-locked PECVD system. We studied the effect of heavily doped p/sup +/ and n/sup +/ layers deposited at the tunnel junction interfaces between the top and middle component cells and between the middle and bottom component cells on the efficiency of triple-junction solar cells. Preliminary results show that thin, /spl sim/1nm, interface p/sup +//n/sup +/ layers improve the solar cell efficiency while thicker interface layers, /spl sim/4nm thick, cause the efficiency to decrease. Incorporating the improved interface layers at the tunnel junctions, as well as earlier improvements in the intrinsic layers, the p-i interface in terms of reducing the band-edge offset, and the a-SiGe component cells using bandgap-graded buffer layers, we fabricated triple-junction solar cells with 12.71% efficiency in the initial state and 10.7% stable efficiency after 1000 hours of 1-sun light soaking. Samples sent to NREL for independent measurements show 11.8% total-area (or 12.5% active-area) initial efficiency.

Modeling of triple junction a-Si solar cells using ASA: analysis of device performance under various failure scenarios

Triple junction a-Si solar cells have been modeled and simulated using the Advanced Semiconductor Analysis (ASA). The device performance is analyzed with numerically simulated I-V characteristics. We have studied several failure scenarios such as variations in the thickness of different layers of the multilayered triple-junction structure. Distinctive features of the I-V characteristics and solar parameters have been found which have been correlated with experimentally obtained results.

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.