Jessica M. Owens

Optimization of back reflector for high efficiency hydrogenated nanocrystalline silicon solar cells

We have studied the effect of texture in Ag/ZnO back reflectors (BRs) on the performance of hydrogenated nanocrystalline silicon (nc-Si:H) solar cells. While a larger texture provides superior light trapping, it also deteriorates the nc-Si:H quality. We have used total and diffused reflection and atomic force microscopy to evaluate the BR texture. A BR with textured Ag and thin ZnO layers has been found to give the best cell performance. Using the optimized BR, we have achieved an initial active-area efficiency of 10.2% in a nc-Si:H single-junction cell and a stable total-area efficiency of 12.5% in a hydrogenated amorphous silicon/nc-Si:H/nc-Si:H triple-junction cell.

Light-induced metastability in hydrogenated nanocrystalline silicon solar cells

Light-induced metastability in hydrogenated nanocrystalline silicon (nc-Si:H) single-junction solar cells has been studied under different light spectra. The nc-Si:H studied contains a certain fraction of hydrogenated amorphous silicon (a-Si:H). We observe no light-induced degradation when the photon energy used is lower than the bandgap of a-Si:H, while degradation occurs when the photon energy is higher than the bandgap. We conclude that the light-induced defect generation occurs mainly in the amorphous phase. Light soaking experiments on a-Si:H∕a-SiGe:H∕nc-Si:H triple-junction solar cells show no light-induced degradation in the bottom cell, because the a-Si:H top and a a-SiGe:H middle cells absorb most of the high-energy photons.

Hydrogen structures and the optoelectronic properties in transition films from amorphous to microcrystalline silicon prepared by hot-wire chemical vapor deposition

Transition films from amorphous (a-) to microcrystalline (μc-) silicon were prepared by hot-wire chemical vapor deposition using silane decomposition with either varied hydrogen-to-silane ratio, R, or with fixed R=3 but a varied substrate temperature, Ts. Raman results indicate that there is a threshold for the structural transition from a- to μc-Si:H in both cases. The onset of the structural transition is found to be R≈2 at Ts=250 °C and Ts≈200 °C at R=3. The properties of the material were studied by infrared absorption, optical absorption, photoluminescence (PL), and conductivity temperature dependence. We observed that the peak frequency of the SiH wag mode remains at 630−640 cm−1 for all the films, but the hydrogen content shows two regimes of fast and slow decreases separated by the onset of microcrystallinity. When microcrystallinity increased, we observed that (a) the SiO vibration absorption at 750 cm−1 and 1050−1200 cm−1 appeared, (b) the relative intensity of the 2090 cm−1 absorption increased...

Over 15% Efficient Hydrogenated Amorphous Silicon Based Triple-Junction Solar Cells Incorporating Nanocrystalline Silicon

We report our recent progress in the optimization of a-Si:H, a-SiGe:H, and nc-Si:H materials for high efficiency triple-junction solar cells. The main technique for the optimization of a-Si:H and a-SiGe:H materials is using hydrogen dilution to approach the amorphous to nanocrystalline transition. The nc-Si:H intrinsic layers are deposited using a modified VHF glow discharge technique at high rates. The material quality and cell performance are improved by using hydrogen dilution profiling for controlling the nanocrystalline evolution. Two triple-junction structures have been studied. We achieve an initial active-area efficiency of 15.1% using an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure, and 14.1% using an a-Si:H/nc-Si:H/nc-Si:H configuration. After prolonged light soaking, both structures stabilized at 13.3%, which is higher than the 13.0% stable efficiency previously reported for an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure

Optical Enhancement by Textured Back Reflector in Amorphous and Nanocrystalline Silicon Based Solar Cells

We present the results of systematic studies of optical enhancements by textured Ag/ZnO back reflectors in a-SiGe:H and nc-Si:H solar cells. First, the back reflector materials were characterized by AFM, XRD, and TEM. The results showed that the ZnO layers deposited by sputtering exhibit an increased surface texture with the deposition temperature and film thickness. The material structure showed a strong (002) preferential orientation. The large columnar crystal structure determines the surface texture. Second, the solar cell performance was correlated to the back reflector structure. We found that with a thin ZnO layer, a textured Ag layer results in more light scattering than a flat Ag layer. However, when a thick ZnO layer is used, a flat Ag layer can produce similar or more light scattering than a textured Ag layer. Third, we developed a method to estimate the optical enhancement for a-SiGe:H and nc-Si:H solar cells on various structures of Ag/ZnO back reflectors. Comparing the quantum efficiency data from solar cells made using the same recipe but one on a flat stainless steel substrate and another on a textured Ag/ZnO BR substrate revealed that the optical enhancement for the long wavelength light can be as high as 20 to 30. Compared to the theoretical value of 4 n 2 , there is still scope for further improvement

Light trapping effect from randomized textures of Ag/ZnO back reflector on hydrogenated amorphous and nanocrystalline silicon based solar cells

We report our progress in the optimization of Ag/ZnO back reflectors (BR) for a-Si:H and nc-Si:H solar cells. Theoretically, a BR with a smooth metal surface and a textured dielectric surface would be more desirable. A smooth metal/dielectric interface reduces the plasmonic resonance loss and parasitic losses due to light trapped in sharp angles; a textured dielectric/semiconductor interface provides scattering for light trapping. In order to obtain sufficient light scattering at the ZnO/silicon interface, a highly textured ZnO layer is normally used. However, a highly textured ZnO surface causes deterioration of nc-Si:H material quality. In addition, to make a highly textured ZnO surface, a thick ZnO layer is needed, which could introduce additional absorption in the bulk ZnO layer and reduce the photocurrent density. Therefore, Ag/ZnO BR structures for nc-Si:H solar cells needs to be optimized experimentally. In this study, we found that an optimized Ag/ZnO BR for nc-Si:H solar cells is constructed with textured Ag and thin ZnO layers. Although a textured Ag layer might cause certain losses resulting from plasmonic absorption, the enhanced light scattering by a moderately textured Ag layer makes it possible to use a thin ZnO layer, where the absorption in the ZnO layer is low. With such a BR, we achieved a short-circuit current density of over 29 mA/cm2 from a nc-Si:H single-junction solar cell. Using the high performance nc-Si:H cell in an a-Si:H/nc- Si:H/nc-Si:H triple-junction structure, we achieved an initial active-area efficiency of 14.5% with a total current density exceeding 30 mA/cm2.

Improvement of a-Si:H and nc-Si:H multi-junction solar cells by optimization of textured back reflectors

The effect of the texture of Ag/ZnO back reflector (BR) on nc-Si:H single-junction solar cell performance has been investigated systematically. Using a high textured BR, a 74% gain in short circuit current density (Jsc) was obtained over a cell made using the same recipe on specular stainless steel. However, the texture reduced the fill factor (FF) from 0.73 to 0.54. Dark current versus voltage measurements showed a significant increase in reverse saturate current when the texture is increased, indicating a poor nc-Si:H material quality in the nc-Si:H cells deposited on highly textured Ag/ZnO BR. In order to maintain both high Jsc and FF, we have optimized the BR texture. With the improved BR, we have achieved initial efficiencies of 9.5% in a nc-Si:H single-junction and 13.4% in an a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells made at 10 Å/s.

Effects of Hydrogen Dilution on a-Si:H and its Solar Cells Studied by Raman and Photoluminescence Spectroscopy

a-Si:H films and their n-i-p solar cells were prepared using plasma-enhanced CVD. The samples were prepared with no-, low-, standard, and high-H dilution. Raman and photoluminescence (PL) were used to characterize the i-layer. The main results are (a) Raman shows typical a-Si:H mode except for a c-Si peak in the 450 nm-thick film with high-H dilution, and (b) PL shows two regimes. (I) Below the onset of microcrystallinity characterized by x-ray diffraction, a blue-shift of the 1.4 eV PL peak energy and a decrease of the band width occur. (II) Above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4-5, and the PL peak energy is red-shifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells.

Distribution of Local Open-Circuit Voltage on Amorphous and Nanocrystalline Mixed-Phase Si:H and SiGe:H Solar Cells

Local open-circuit voltage (Voc) distributions on amorphous and nanocrystalline mixed-phase silicon solar cells were measured using a scanning Kelvin probe microscope (SKPM) on the p layer of an n-i-p structure without the top ITO contact. During the measurement, the sample was illuminated with a laser beam that was used for the atomic force microscopy (AFM). Therefore, the surface potential measured by SKPM is the sum of the local Voc and the difference in workfunction between the p layer and the AFM tip. Comparing the SKPM and AFM images, we find that nanocrystallites aggregate in the amorphous matrix with an aggregation size of ~0.5 mum in diameter, where many nanometer-size grains are clustered. The Voc distribution shows valleys in the nanocrystalline aggregation area. The transition from low to high Voc regions is a gradual change within a distance of about 1 mum. The minimum V oc value in the nanocrystalline clusters in the mixed-phase region is larger than the Voc of a nc-Si:H single-phase solar cell. These results could be due to lateral photo-charge redistribution between the two phases. We have also carried out local Voc measurements on mixed-phase SiGe:H alloy solar cells. The magnitudes of Voc in the amorphous and nanocrystalline regions are consistent with the J-V measurements

Microcrystalline Silicon Solar Cell Deposited Using Modified Very-High-Frequency Glow Discharge and Its Application in Multi-junction Structures

We have used the modified very-high-frequency glow discharge technique to deposit hydrogenated microcrystalline silicon (m c-Si:H) solar cells at high rates for use as the bottom cell in a multi-junction structure. We have investigated c-Si:H single-junction, a-Si:H/ c-Si:H double-junction, and a-Si:H/a-SiGe:H/m c-Si:H triple-junction solar cells and achieved initial active area efficiencies of 7.7%, 12.5%, and 12.4%, respectively. Issues related to improving material properties and device structures are addressed. By taking advantage of a lower degradation in m c-Si:H than a-Si:H and a-SiGe:H alloys, we have minimized the light induced effect in multi-junction structures by designing a bottom-cell-limited current mismatching. As a result, we have obtained a stable active-area cell efficiency of 11.2% with an a-Si:H/a-SiGe:H/μ c-Si:H triple-junction structure.