Xinhua Geng

Open circuit voltage improvement of high-deposition-rate microcrystalline silicon solar cells by hot wire interface layers

Significant improvement in open circuit voltage and fill factor was achieved for microcrystalline silicon (μc‐Si:H) solar cells deposited by plasma-enhanced chemical vapor deposition (PECVD) by the incorporation of an intrinsic μc‐Si:Hp∕i buffer layer fabricated by hot-wire (HW) CVD. The improved p∕i interface quality, likely due to the ion-free deposition on the p layers in the HWCVD process, was concluded from a considerably enhanced blue light response in such solar cells. Using this buffer layer concept allows the authors to apply high deposition rate PECVD processes for the μc‐Si:Hi layer material, yielding a high efficiency of 10.3% for a single junction μc‐Si:H solar cell.

Highly efficient organic photovoltaic devices using F-doped SnO2 anodes

Transparent F-doped SnO2 (FTO) is used as an anode material in organic photovoltaics based on poly(3-hexylthiophene) and [6, 6]-phenyl C61-butlyric acid methyl ester. Power conversion efficiency of 4.41% is achieved under 100 mW/cm2 simulated AM 1.5G solar illumination, which is comparable to that (4.25%) of the reference cells fabricated on indium tin oxide (ITO) glass substrates. Our results indicate that FTO anodes are a viable alternative to ITO for photovoltaic devices for cost effective fabrication of organic photovoltaic devices.

Effect of pretreatment on PET films and its application for flexible amorphous silicon solar cells

We proposed a low cost solution of flexible amorphous silicon solar cells on Polyethylene terephthalate(PET) polymer substrates deposited at low temperatures. PET films were firstly annealed both in the air and in vacuum at different temperatures, and the properties of PET films after annealing were evaluated. Then PET films were exposed to glow discharge Argon plasma in a standard PECVD system to improve their surface properties. The relationship between glow discharge parameters and the energy of Ar plasma were investigated by OES. After Ar plasma treatment, not only the surface morphology of PET but also the adhesion of the solar cell thin films to the PET substrates were improved. Finally, single junction a-Si solar cells with a p-i-n superstrate type were fabricated on PET/ITO substrates at low temperature of Ts=125°C, and an initial efficiency of 4.8% was obtained.

Hydrogenated Microcrystalline Silicon Single-Junction Nip Solar Cells

The microstructural properties of hydrogenated microcrystalline silicon solar cells are investigated using Raman spectroscopy and x-ray diffraction. It was found that the increase of grain size and crystalline volume fraction with thickness is the main reason for the deterioration of cell performance as using constant hydrogen dilution technique. In order to adjust grain size and crystalline volume fraction along the growth direction, gradient hydrogen dilution technique has been adopted to control the structural evolution. The experiment results demonstrated that the performance of solar cell can be much improved when there’s a higher crystallinity at n/i interface and a lower crystallinity at i/p interface. We have achieved an initial active-area efficiency of 5.7% (Voc=0.47V, Jsc=20.2mA/cm2, FF=60%) for the µc-Si:H single-junction n-i-p solar cells.

High-efficiency a-Si:H/μc-Si:H solar cells by optimizing A-Si:H and μc-Si:H sub-cells

The performance of a-Si:H/μc-Si:H tandem solar cell was improved by optimizing the a-Si:H top cell and μc-Si:H bottom cell, respectively. For the a-Si:H top cell, we focused on opto-electrical and structural properties of phosphorous-doped hydrogenated silicon (Si:H) films and their effect on the open circuit voltage (Voc). The experimental results indicated that when nanosized silicon crystalline grains existed in amorphous silicon matrix, the Voc of a-Si:H solar cells was much improved. An initial efficiency of 9.4% for a-Si:H solar cell was obtained. For the μc-Si:H bottom cell, we investigated the structural evolution along the growth direction of the intrinsic μc-Si:H layers. We introduced a high-quality initial seed layer at p/i interface to reduce the incubation layer thickness by lowering the silane concentration and very-high-frequency (VHF) power simultaneously. This initial seed layer acted as a seed layer for bulk μc-Si:H i-layer and the process reduced the ion bombardment on the p/i interface. We demonstrated a VHF power profiling technique by decreasing the VHF power step by step during the μc-Si:H deposition to control the structural evolution along the growth direction in the bulk i-layer. The advantage of this VHF power profiling technique was the reduced ion bombardments on growth surface because of the reduced VHF power. A high conversion efficiency of 9.36% was obtained for μc-Si:H p-i-n solar cell. Using a double n-layer (a-Si:H&μc-Si:H) in n/p tunnel recombination junction, we achieved the best conversion efficiency of 11.63% for a-Si:H/μc-Si:H tandem solar cells.

Research Progresses on High Efficiency Amorphous and Microcrystalline Silicon-Based Thin Film Solar Cells

This paper reviews our research progresses of hydrogenated amorphous silicon (a-Si:H) and microcrystalline (μc-Si:H) based thin film solar cells. It coves the three areas of high efficiency, low cost process, and large-area proto-type multi-chamber system design and solar module deposition. With an innovative VHF power profiling technique, we have effectively controlled the crystalline evolution and made uniform μc-Si:H materials along the growth direction, which was used as the intrinsic layers of pin solar cells. We attained a 9.36% efficiency with a μc-Si:H single-junction cell structure. We have successfully resolved the cross-contamination issue in a single-chamber system and demonstrated the feasibility of using single-chamber process for manufacturing. We designed and built a large-area multi-chamber VHF system, which is used for depositing a-Si:H/μc-Si:H micromorph tandem modules on 0.79-m 2 glass substrates. Preliminary module efficiency has exceeded 8%.

Controlling Structural Evolution by VHF Power Profiling Technique for High-efficiency Microcrystalline Silicon Solar Cells at High Deposition Rate

High rate deposition of hydrogenated microcrystalline silicon (μc-Si:H) films and solar cells were prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) process in a high power and high pressure regime. The experiment results demonstrate that in high-rate deposited μc-Si:H films, the structural evolution is much more dramatic than that in low-rate deposited μc-Si:H films. A novel VHF power profiling technique, which was designed by dynamically decreasing the VHF power step by step during the deposition of μc-Si:H intrinsic layers, has been developed to control the structural evolution along the growth direction. Another advantage of this VHF power profiling technique is the reduced ion bombardments on growth surface because of decreasing the VHF power. Using this method, a significant improvement in the solar cell performance has been achieved. A high conversion efficiency of 9.36% ( V oc =542mV, J sc =25.4mA/cm 2 , FF =68%) was obtained for a single junction μc-Si:H p - i - n solar cell with i -layer deposited at deposition rate over 10 �/s.

The research of N/I and I/P buffer layers in μC-Si:H N-I-P single-junction solar cells

Hydrogenated microcrystalline silicon (μc-Si:H) intrinsic films and solar cells with n-i-p configuration were prepared by plasma enhanced chemical vapor deposition (PECVD). The influence of n/i and i/p buffer layers on the μc-Si:H cell performance were studied in detail. The experimental results demonstrated that the efficiency can be much improved when theres a higher crystallinity at n/i interface and an optimized a-Si:H buffer layer at i/p interface. By combining above methods, the performance of μc-Si:H single-junction and a-Si:H/μc-Si:H tandem solar cells has been significantly improved.

Microcrystalline Silicon Materials and Solar Cells with High Deposition Rate

Although similar deposition rate (2.0nm/s) and defect absorption (α0.8eV =2.5cm-1) for intrinsic µc-Si:H films can be obtained at different total gas flow rate, the solar cell performance, which consists of the above intrinsic µc-Si:H as absorb layers, was obviously different. From the results of quantum efficiency (QE), dark J-V characteristic and Raman spectra, it was found that the amorphous silicon incubation layer is the main reason for the difference of the two solar cells. Increased the total gas flow rate can reduce the thickness of the amorphous silicon incubation layer, which can enhanced the QE response in the long wavelength and increase the short circuit current. These results demonstrate that the amorphous silicon incubation layer was a key factor for the fabrication of high efficiency microcrystalline silicon solar cell with high growth rate.

Controllable light utilization in silicon-based thin film solar cells

Controllable light utilization in silicon-based-thin-film solar cells with a DBR structure have been simulated. A experimental model cells constructed by a-Si pin/ZnO (~70 nm)/P+muc-Si(20 nm) were verified. The ratio of Isc increment can be reached to 14.1%.