Henry S. Povolny

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.

Study of a-SiGe:H films and n-i-p devices used in high efficiency triple junction solar cells

We report our systematic studies on a-SiGe:H thin films and n-i-p solar cells for GeH 4 /Si 2 H 6 ratio varying from 1.43 to 0. This results in a variation of band gap from 1.37 to 1.84 eV. The FTIR studies show that the total hydrogen content in these films decreases as Ge content increases. For Ge rich films, the hydrogen also goes in to Ge-H mode. I V measurements on n-i-p solar cells with i-layer having different Ge content show that as Ge content increase, Short circuit current (J sc ) increases, whereas open circuit voltage (V oc ). fill factor (FF) and conversion efficiency (η) decrease. For Ge rich films, J sc does not significantly increase after GeH 4 /Si 2 H 6 ratio increases beyond 0.72; however V oc , FF and η decrease drastically. The quantum efficiency (QE) measurements in the subgap absorption range show that band gap and Urbach slope of the i-layer can very well be estimated in the devices.

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.

Selected Lifetime and Oscillator Strength Measurements in Si II

We have remeasured the lifetimes of the 3s24s and 3s25s2S1/2 levels in Si II using beam foil spectroscopic techniques. Measured values for the lifetimes and oscillator strengths derived from them are presented and compared with previous measurements and theoretical calculations. Agreement with recent theoretical calculations is now quite good: for 3s24s it is excellent and for 3s25s it is satisfactory, although the theoretical uncertainties in that calculation are still somewhat larger than desired.

Comparison Study of a-SiGe Solar Cells and Materials Deposited Using Different Hydrogen Dilution

A-SiGe n-i-p solar cells with i-layer deposited via plasma enhanced chemical vapor deposition (PECVD) with a germane to disilane ratio of 0.72 and hydrogen dilution R=(H2 flow)/(GeH4+Si2H6 flow) values of 1.7, 10, 30, 50, 120, 180 and 240 were deposited on stainless steel substrates. This germane to disilane ratio is what we typically use for the i-layer in the bottom cell of our standard triple-junction solar cells. Solar cell current-voltage curves (J-V) and quantum efficiency (QE) were measured for these devices. Light soaking tests were performed for these devices under 1 sun light intensity at 50 o C. While device with R=30 showed the highest initial efficiency, the device with R=120 exhibit higher stabilized efficiency after 1000 hours of light soaking. Single-layer a-SiGe films (~500 nm thick) were deposited under the same conditions as the i-layer of these devices on a variety of substrates including 7059 glass, crystalline silicon, and stainless steel for visible-IR transmission spectroscopy, FTIR, and hydrogen effusion studies. It is interesting to note 1) the H content in the film decreased with increasing R based on both the IR and H effusion measurements, and 2) while the H content changes significantly with different R, the change in Eg is relatively small. This is most likely due to a change in Ge content in the film for different R.

High-rate deposition of amorphous silicon films using hot-wire CVD with a coil-shaped filament

To reduce the manufacturing cost of amorphous silicon (a-Si:H)-based photovoltaic devices, it is important to deposit highquality a-Si:H and related materials at a high deposition rate. To this end, we designed and constructed a hot-wire deposition chamber with a coiled filament design and with multiple gas inlets. The process gas could be directed into the chamber through the filament coil and have maximum exposure to the high-temperature filament surface. Using such a chamber design, we deposited a-Si:H films at high deposition rates up to 800 A s and dense, low-void a-Si:H at rates up to 240 A s . y1 y1 ˚˚ 2003 Elsevier Science B.V. All rights reserved.

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.

Hot-wire deposition of amorphous and microcrystalline silicon using different gas excitations by a coiled filament

Abstract Microcrystalline silicon (μc-Si:H) and amorphous silicon (a-Si:H) films were deposited using a hot-wire CVD (HWCVD) system that employs a coiled filament. Process gasses, H2 and Si2H6, could be directed into the deposition chamber via different gas inlets, either through a coiled filament for efficient dissociation or into the chamber away from the filament, but near the substrates. We found that at low deposition pressure (e.g. 20 mTorr) the structure of the films depends on the way gases are introduced into the hot-wire chamber. However, at higher pressure (e.g. 50 mTorr), Raman measurement shows similar results for films deposited with different gas inlets.

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.