Xianbo Liao

Amorphous silicon and silicon germanium materials for high-efficiency triple-junction solar cells

In this paper, we report our recent progress in the amorphous silicon (a-Si)-based photovoltaic research program at The University of Toledo (UT). We have achieved the fabrication of (1) wide bandgap a-Si solar cells with an open-circuit voltage of 0.981 and a fill factor of 0.728 using high hydrogen dilution for the i-layer deposition, (2) mid bandgap a-SiGe solar cells having an open-circuit voltage of 0.815 and a fill factor of 0.65, (3) narrow bandgap a-SiGe solar cells with 9.17% initial efficiency, and (4) triple-junction, spectrum-splitting a-Si/a-SiGe/a-SiGe solar cells with 10.6% initial efficiency.

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.

AMPS modeling of nanocrystalline Si p-layer in a-Si nip solar cells

. This paper reports numerical simulations for the impact of a wide bandgap p-type hydrogenated nanocrystalline silicon (nc-Si:H) on the performances of a-Si based component solar cells, using Analysis of Microelectronic and Photonic Structures (AMPS) computer model developed at Penn State University. The effects of band offset and potential barrier formed at the interfaces of player with i-layer and ITO front contact were also investigated. The simulated results show that 1) with increasing bandgap of p-nc-Si:H (E/sub gp/), the V/sub oc/ increases beyond 1 V, then decreases, due to the band offset at the p/i interface, which also leads to an anomalous illuminated I-V characteristics with a bending close to the open circuit point; and 2) the front contact barrier plays a similar role to hinder the hole collection and may cause the illuminated I-V curve to bend seriously.

Damage mechanisms in thin film solar cells during sputtering deposition of transparent conductive coatings

Amorphous silicon (a-Si) based thin film solar cell grown on flexible stainless steel substrate is one of the most promising energy conversion devices in the future. This type of solar cell uses a transparent conductive oxide (TCO) film as top electrode. It has been a widely accepted opinion that the radio frequency sputtering deposition of the TCO film produces a higher yield than direct current sputtering, and the reason is not clear. Here we show that the damage to the solar cell during the sputtering process is caused by a reverse bias applied to the n-i-p junction. This reverse bias is related to the characteristics of plasma discharge. The mechanism we reveal may significantly affect the solar cell process.

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.

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.

Raman and IR study of narrow bandgap a-SiGe and /spl mu/c-SiGe films deposited using different hydrogen dilution

Hydrogenated amorphous silicon-germanium (a-SiGe:H) films and n-i-p solar cells near the threshold of microcrystalline formation have been prepared by plasma enhanced chemical vapor deposition (PECVD) with a fixed germane to disilane ratio of 0.72 and a wide range of hydrogen dilution R/sub H/=(H/sub 2/ flow)/(GeH/sub 4/+Si/sub 2/H/sub 6/ flow) values of 1.7, 10, 30, 50, 120, 180 and 240. The effects of RH on the structural properties of the films were investigated using Raman scattering and Fourier transform infrared (FTIR) absorption spectroscopy. It is found that H dilution causes the H content, especially that in SiH/sub 2/ configuration, in a-SiGe:H films to decrease and finally leads the films through amorphous to microcrystalline transition. The onset of the phase transition occurs at RH about 180, and the crystalline formation begins first in the Si-rich region. Light soaking tests on the solar cells demonstrate that the devices with higher RH exhibit higher stabilized efficiency after 1000 hours of 1 sun light soaking.