Showing papers by "Baojie Yan published in 2012"

Amorphous and nanocrystalline silicon thin film photovoltaic technology on flexible substrates

Abstract: This paper reviews our thin film silicon-based photovoltaic (PV) technology, including material and device studies as well as roll-to-roll manufacturing on a flexible substrate. Our current thin film silicon PV products are made with hydrogenated amorphous silicon (a-Si:H) and amorphous silicon germanium (a-SiGe:H) alloys. The advantages of a-Si:H-based technology are low cost, capability of large scale manufacturing, abundance of raw materials, and no environmental concerns. One disadvantage of a-Si:H PV technology is lower energy conversion efficiency than solar panels made of crystalline and polycrystalline silicon and compound crystal thin film semiconductors. Significant efforts have been made to improve efficiency. First, a-Si:H and a-SiGe:H material quality has been improved by optimizing deposition conditions, especially using high hydrogen dilution to deposit the amorphous materials close to the amorphous/nanocrystalline transition. Second, cell efficiency has been improved by engineering the dev...

Advances in Light Trapping for Hydrogenated Nanocrystalline Silicon Solar Cells

Abstract: We optimized Ag/ZnO back reflectors (BR) for hydrogenated nanocrystalline silicon (nc-Si:H) solar cells by independently changing the textures of the Ag and ZnO layers. We found that Ag/ZnO with textured Ag and thin ZnO provides the highest nc-Si:H solar cell efficiency. Optimized Ag texture with an rms = 40 nm effectively scatters light without seriously degrading the nc-Si:H material quality. Using this type of BR and nc-Si:H cells with ∼1-µm-thick intrinsic layer, we obtained a short-circuit current density J sc = 24.6 mA/cm2 and conversion efficiency E ff = 9.47%. By increasing the nc-Si:H layer to∼3.1 µm, we attained a J sc >30 mA/cm2. In order to increase the J sc further, we increased the texture of the ZnO layer. With highly textured Ag/ZnO BRs, the J sc was increased. However, the high textures caused poor fill factors, and hence, relatively low efficiency. By using nanocrystalline silicon-oxide (nc-SiO x :H) to replace both the n-layer and dielectric layer, the texture-induced deterioration of nc-Si:H material quality was suppressed and the cell structure was simplified by removing the ZnO, conventional n-layer, n/i buffer layer, and the seed layer. A high J sc over 27 mA/cm2 and high-cell efficiency of 8.8% were attained using a 2.5-µm-thick nc-Si:H cell with an nc-SiO x :H n-layer.

Hydrogenated Nanocrystalline Silicon based Solar Cell with 13.6% Stable Efficiency

Abstract: Multi-junction solar cells incorporating hydrogenated nanocrystalline silicon (nc-Si:H) exhibit a high current capability and low light-induced degradation. In this paper, we report our recent progress in developing nc-Si:H solar cells using a modified very-high-frequency glow discharge technique. We achieved a short-circuit current density >30 mA/cm 2 and 10.6% conversion efficiency from single-junction solar cells. Using the improved nc-Si:H cells in an a-Si:H/nc-Si:H/nc-Si:H triple-junction structure, we attained initial and stabilized efficiencies of 13.9% and 13.6%, respectively. Issues related to improving material properties and device structures are addressed. Besides using the conventional techniques, such as hydrogen dilution profiling, optimized Ag/ZnO back reflector, and buffer layers, we found that compensation from Boron and Oxygen micro-doping is also critical in obtaining the above achievements.

High efficiency amorphous and nanocrystalline silicon thin film solar cells on flexible substrates

Abstract: We review the progresses and issues towards manufacturing hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) based thin film multi-junction solar cells using a roll-to-roll process on flexible substrates. United Solar has been heavily involved in the research and development of high efficiency a-Si:H and nc-Si:H multi-junction solar cells since 2001. We have resolved several critical issues limiting nc-Si:H solar cell performance, such as nanocrystalline evolution, impurities, and porosity/ambient degradation. We have developed new cell designs, including the proper optimization of the seeding layer for nc-Si:H growth and proper n/i and i/p buffers for improvement of cell efficiency. We have optimized Ag/ZnO back reflectors for nc-Si:H cell performance. Combining all of the efforts in the improvement of material quality and optimization of device structure, we have advanced thin film silicon solar cell efficiency. We reported 16.3% initial active-area efficiency using an a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cell. Furthermore, we attained 12.5% stable total area (0.27 cm2) efficiency using a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells and 11.3% stable aperture area (800 cm2) efficiency using the same cell structure, where the efficiencies were measured by NREL and were the records for thin film silicon photovoltaic technology.

Drift-mobility characterization of silicon thin-film solar cells using photocapacitance

Abstract: We have applied the photocapacitance method to the measurements of hole drift-mobilities in silicon solar cells. We found a simple analysis that yields drift-mobilities even in the presence of anomalously dispersive transport. On one thick sample we measured the hole drift-mobility using both the photocapacitance and the time-of-flight methods; the two methods gave results that were consistent with each other and with the established bandtail multiple-trapping model. We then applied the method to thinner samples that are more characteristic of the conditions in solar modules, but are not generally usable for the time-of-flight method. These samples showed much smaller hole drift-mobilities than expected from the bandtail trapping model. We speculate that the hole drift-mobility has smaller values in regions close to the substrate during deposition than has been reported for thicker samples.

Microscopic Measurements of Electrical Potential in Hydrogenated Nanocrystalline Silicon Solar Cells

Abstract: We report on a direct measurement of electrical potential and field profiles across the n-i-p junction of hydrogenated nanocrystalline silicon (nc-Si:H) solar cells, using the nanometer-resolution potential imaging technique of scanning Kelvin probe force microscopy (SKPFM). It was observed that the electric field is nonuniform across the i layer. It is much higher in the p/i region than in the middle and the n/i region, illustrating that the i layer is actually slightly n-type. A measurement on a nc-Si:H cell with a higher oxygen impurity concentration shows that the nonuniformity of the electric field is much more pronounced than in samples having a lower O impurity, indicating that O is an electron donor in nc-Si:H materials. This nonuniform distribution of electric field implies a mixture of diffusion and drift of carrier transport in the nc-Si:H solar cells. The composition and structure of these nc-Si:H cells were further investigated by using secondary-ion mass spectrometry and Raman spectroscopy, respectively. The effects of impurity and structural properties on the electrical potential distribution and solar cell performance are discussed.

Nanostructured Silicon Oxide Dual-Function Layer in Amorphous Silicon Based Solar Cells

Abstract: We report the results of using n-type hydrogenated nanocrystalline silicon oxide alloy (nc-SiOx:H) in hydrogenated nanocrystalline silicon (nc-Si:H) and amorphous silicon germanium alloy (a-SiGe:H) single-junction solar cells. We used VHF glow discharge to deposit nc-SiOx:H layers on various substrates for material characterizations. We also used VHF glow discharge to deposit the intrinsic layer in nc-Si:H solar cells. RF glow discharge was used for the deposition of the doped layers and the intrinsic layer in a-SiGe:H solar cells. Various substrates such as stainless steel (SS), Ag coated SS, and ZnO/Ag coated SS were used for different cell structures. We found that by using nc-SiOx:H to replace the ZnO and the a-Si:H n-layer in nc-Si:H solar cells, the cell structure is greatly simplified, while the cell performances remain nearly identical to those made using the conventional n-i-p structure on standard ZnO/Ag BR’s. Solar cells with nc-SiOx:H as the n layer directly deposited on textured Ag show similar quantum efficiency (QE) as the n-i-p cells on ZnO/Ag BRs. In both cases, QE is higher than that in the n-i-p cells made directly on Ag coated SS. This effect is probably caused by the shift of surface plasmon-polariton resonance frequency due to the difference in index of refraction of ZnO, nc-SiOx:H, and Si.

Light Trapping in Hydrogenated Amorphous and Nanocrystalline Silicon Based Thin-Film Solar Cells With Ag/ZnO Back Reflectors

Abstract: We report on our systematic study of light trapping effects using Ag/ZnO BRs for nc-Si:H solar cells. The texture of Ag and ZnO was optimized to achieve enhancement in photocurrent. The light trapping effect on photocurrent enhancement in solar cells was carefully investigated. Comparing to single-junction solar cells deposited on flat stainless steel substrates, the gain in Jsc by using Ag/ZnO BRs is 57% for nc-Si:H solar cells. This gain in Jsc is much higher than what has been achieved by advanced light trapping approaches using photonic structures or plasmonic light trapping reported in the literature. We achieved a Jsc of 29-30 mA/cm2 in a nc-Si:H single-junction solar cell with an intrinsic layer thickness of ∼2.5 μm. We compared the quantum efficiency of single-junction cells to the classical limit of fully randomized scattering and found that there is a 6-7 mA/cm2 difference between the measured Jsc and the classical limit, in which 3-4 mA/cm2 is in the long wavelength region. However, by taking into consideration the losses from reflection of the top contact, absorption in the doped layers, and imperfect reflection in the BRs, the difference disappears. This implies we have reached the practical limit if the scattering from randomly textured substrates is the only mechanism of light trapping. Therefore, we believe future research for improving photocurrent should be directed toward reducing (i) reflection loss by the top contact, the absorption in ZnO and at the Ag/ZnO interface, and (ii) p layer absorption.

Improved back reflector with nanocrystalline photovoltaic device

Abstract: A photovoltaic device and processes of manufacture are provided that employ particularly configured, textured back reflector structures that maintain a smooth, non-textured surface at the interface between the lowermost doped layer of semiconductor material and the intrinsic, light absorbing layer of nanocrystalline semiconductor material. The back reflector structure provides exhibit both superior short circuit current and a superior fill factor to a photovoltaic device such as those using nanocrystalline semiconductor materials.