Xiesen Yang

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

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.

Origin of Optical Losses in Ag/ZnO Back-Reflectors for Thin Film Si Photovoltaics

Back-reflector (BR) fabrication for thin film Si solar cells has been studied using real time spectroscopic ellipsometry (SE) with a rotating compensator instrument. The structure explored here is relevant for the substrate/ BR/n-i-p configuration and consists of opaque Ag followed by ~1500 Aring of ZnO, both prepared by sputtering. The thickness of the initial roughness layer on the Ag has been varied from 6 to 55 Aring to investigate its effects on interface formation between Ag and ZnO in the specular regime. The dielectric functions epsiv=epsiv1+iepsiv2 of all layers including an observed Ag/ZnO interface layer have been determined, and the latter has been fit using (i) a free electron component, (ii) a plasmon component near 2.8 eV, and (iii) a bound electron component with an absorption onset near 3.7 eV. The measured reflectance spectra of the final specular BR structures with ~1500 Aring thick ZnO are consistent with those predicted from SE models and allow one to identify the physical origins of losses. A strong minimum in reflectance centered at 2.6 eV and an associated tail extending to 1.6 eV is found to be caused by interference-enhanced plasmon absorption. Simulations for thicker ZnO (db=3000 Aring) demonstrate that the dominant loss from 1.2 to 1.5 eV is due to intrinsic absorption in Ag, also enhanced by interference

Effects of hydrogen dilution grading in active layer on performance of nanocrystalline single junction bottom component and corresponding a-Si:H based triple junction solar cells

Using hydrogen dilution grading in nanocrystalline silicon (nc-Si:H) intrinsic layer, considerably high spectral response has been achieved in the longer wavelength (650nm-1000nm) of solar spectrum from single junction solar cells, to be used as bottom junction in monolithic triple junction a-Si:H based thin film solar cells. The intrinsic layers have been deposited with a high deposition rate of 8 Aring/s, using VHP PECVD deposition technique from Si2H6 diluted with H2. In the present work, using R (R=[H2]/[Si 2H6]) from an initial value 50 to different final values viz. 40, 37.5 and 35, single junction nc-Si:H cells have been fabricated and the same single junction components have been applied as bottom junction to fabricate a-Si:H/a-SiGe:H/nc-Si:H triple junction structure where the properties of top and middle junctions were kept fixed. The short circuit current of the triple junction cells is shown to be limited by the bottom junction and the fill factor has been improved in triple junction cells compared to the same obtained from the corresponding single junction cells used as nc-Si:H bottom junction. Comparison of the fill factors under red (>630nm) illumination and that under blue (<510nm) illumination for these single junction nc-Si:H cells indicated structural quality of p/i interface and bulk of the i-layer of nc-Si:H solar cells. A conversion efficiency of 11.2% has been achieved from triple junction solar cell. The light induced degradation studies are currently in process for triple junction cells under a 100 mW/cm2 white light illumination. Using spectroscopic ellipsometry, evaluation of crystalline volume fraction at different level of growth is undergoing, which will enable fine-tuning of grading in hydrogen dilution

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.

Effect of ZnO deposition condition for back reflector on the performance of nano-crystalline silicon solar cell

We have deposited ZnO at different substrate temperatures in a range of 120 ∼ 350 °C for back reflector (BR) with a structure of ZnO/Ag/Al/SS used in nano-crystalline silicon solar cells. AFM, XRD, SEM and optical transmittance have been employed to analyze the structure of ZnO films. It is found that the nano-crystalline solar cells on the back reflector with ZnO film deposited at 120 °C have a lower fill factor (FF) and a lower short-circuit current density (Jsc), but a higher open-circuit voltage (Voc). This may be caused by the high fraction of amorphous phase in the nano-crystalline silicon film, which grows on the low temperature deposited ZnO film. But when the ZnO deposition temperature is increased to 280 °C, the nano-crystalline silicon solar cells have a lower yield. The reason for this is the formation of a large amount of ZnO nano-crystalline rods on ZnO surface. The optimized BR has contributed to the development of high efficient a-Si:H/a-SiGe:H/nc-Si:H n-i-p triple solar cells with an efficiency of 12.5%.

Theoretical analysis and modeling of light trapping in a-Si based thin film solar cells

This paper analyses the light trapping effect and finds out that the light path enhancement factor of cells with an ideal Lambertian back reflector (BR) could be as high as n(n+1)2, instead of general quoted 4n2. The program PVOPTICS developed by NREL has been used to modeling the light trapping effect. It is found that 1) the texture angle is more critical than the texture height in determining the light trapping effect, but the latter of which was the only mentioned parameter in general light trapping designs; 2) Inserting a ZnO buffer layer can largely increase the Si absorption, while ZnO thickness does not affect the light trapping obviously, which were confirmed by experimental results; 3) Different impacts of texture starting from a-Si/ZnO or ZnO/metal interface on light trapping effects are studied. For ZnO/Al BR, the texture starting from the a-Si/ZnO interface will lead to a larger absorption in the Si layer; while for ZnO/Ag BR, the texture starting from the a-Si/ZnO interface or ZnO/Ag interface leads to similar results.

Fabrication of nc-Si:H n-i-p single-junction solar cells using VHF PECVD at a deposition rate of 10 Å/s

Efforts on the fabrication and optimization of hydrogenated nanocrystalline silicon (nc-Si:H) solar cells using VHP PECVD technique in our UT multi-chamber load-locked PECVD deposition system at a high deposition rate of 10 Aring/s are reported in this paper. nc-Si:H n-i-p single-junction solar cells with initial active-area (0.25 cm2) efficiencies of about 6.6% have been obtained with an i-layer deposition time of 25 min corresponding to a thickness of about 1500 nm and an short circuit current density Jsc of about 21 mA/cm2. The deposition conditions for n-layer and p-layer have also been optimized for the VHP nc-Si:H nip solar cells. With these optimizations, the best FF value of the VHP nc-Si:H single-junction solar cells was reached at 70.9% with an initial efficiency of eta=6.56%. The QE curve shows that the as-deposited nc-Si:H cell has a good spectral response in the long wavelength range and yields an AM1.5G integrated current density Jsc of 8.38 mA/cm2 over the wavelength region from 650 nm to 1000 nm, which is a good candidate as bottom component cell in a-Si:H/a-Si:H/nc-Si:H triple-junction solar cell