Guofu Hou

An efficient light trapping scheme based on textured conductive photonic crystal back reflector for performance improvement of amorphous silicon solar cells

An efficient light trapping scheme named as textured conductive photonic crystal (TCPC) has been proposed and then applied as a back-reflector (BR) in n-i-p hydrogenated amorphous silicon (a-Si:H) solar cell. This TCPC BR combined a flat one-dimensional photonic crystal and a randomly textured surface of chemically etched ZnO:Al. Total efficiency enhancement was obtained thanks to the sufficient conductivity, high reflectivity and strong light scattering of the TCPC BR. Unwanted intrinsic losses of surface plasmon modes are avoided. An initial efficiency of 9.66% for a-Si:H solar cell was obtained with short-circuit current density of 14.74 mA/cm2, fill factor of 70.3%, and open-circuit voltage of 0.932 V.

Optimal design of one-dimensional photonic crystal back reflectors for thin-film silicon solar cells

For thin-film silicon solar cells (TFSC), a one-dimensional photonic crystal (1D PC) is a good back reflector (BR) because it increases the total internal reflection at the back surface. We used the plane-wave expansion method and the finite difference time domain (FDTD) algorithm to simulate and analyze the photonic bandgap (PBG), the reflection and the absorption properties of a 1D PC and to further explore the optimal 1D PC design for use in hydrogenated amorphous silicon (a-Si:H) solar cells. With identified refractive index contrast and period thickness, we found that the PBG and the reflection of a 1D PC are strongly influenced by the contrast in bilayer thickness. Additionally, light coupled to the top three periods of the 1D PC and was absorbed if one of the bilayers was absorptive. By decreasing the thickness contrast of the absorptive layer relative to the non-absorptive layer, an average reflectivity of 96.7% was achieved for a 1D PC alternatively stacked with a-Si:H and SiO2 in five periods. T...

Entire band absorption enhancement in double-side textured ultrathin solar cells by nanoparticle imprinting

Entire band light management is crucial for amorphous silicon (a-Si) solar cells, especially when the absorbing layer becomes ultrathin. Here, we propose and demonstrate a double-side texture strategy to effectively manage light in ultrathin solar cells via a simple and scalable nanoparticle imprinting technique. SiO2 nanoparticles are half embedded into the top surface of the solar cells to introduce the double-side texture. Using a solar cell with a 150 nm thick a-Si layer as an example, we observe significant enhancement over the entire absorption band of a-Si both theoretically and experimentally. A maximum short circuit current density enhancement as high as 43.9% has been achieved experimentally compared with a flat solar cell.

Mechanism insight into the effect of I/P buffer layer on the performance of NIP-type hydrogenated microcrystalline silicon solar cells

A simulation and experimental study on the effect of the buffer layer at the I/P interface on the performance of NIP-type hydrogenated microcrystalline silicon (μc-Si:H) single-junction solar cells is presented. Device-quality hydrogenated amorphous silicon (a-Si:H) material as a buffer layer at the I/P interface obviously improves the performance of NIP-type μc-Si:H single-junction solar cells. In addition to the well-known mechanism that an a-Si:H I/P buffer layer can reduce the recombination current density at I/P interfaces, the optically and electrically calibrated simulations and supporting experimental results in this study illustrate that the performance improvement also originates from the mitigation of the electric screening effect due to the reduced defect density at the I/P interfaces, which reinforces the bulk electric field. Integrating an optimized hydrogen profiling strategy and adding a-Si:H I/P buffer layer yielded an initial efficiency of 9.20% for μc-Si:H single-junction solar cells wi...

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.

Application of metal nanowire networks on hydrogenated amorphous silicon thin film solar cells.

We demonstrate the application of metal nanowire (NW) networks as a transparent electrode on hydrogenated amorphous Si (a-Si:H) solar cells. We first systematically investigate the optical performances of the metal NW networks on a-Si:H solar cells in different electrode configurations through numerical simulations to fully understand the mechanisms to guide the experiments. The theoretically optimized configuration is discovered to be metal NWs sandwiched between a 40 nm indium tin oxide (ITO) layer and a 20 nm ITO layer. The overall performances of the solar cells integrated with the metal NW networks are experimentally studied. It has been found the experimentally best performing NW integrated solar cell deviates from the theoretically predicated design due to the performance degradation induced by the fabrication complicity. A 6.7% efficiency enhancement was achieved for the solar cell with metal NW network integrated on top of a 60 nm thick ITO layer compared to the cell with only the ITO layer due to enhanced electrical conductivity by the metal NW network.

Triple-functional n-type microcrystalline silicon oxide layers in hydrogenated amorphous silicon/microcrystalline silicon tandem solar cells

A novel tunnel recombination junction (TRJ) consisted of n type hydrogenated microcrystalline silicon oxide (n-μc-SiO_x:H) layer and p type hydrogenated nanocrystalline silicon oxide (p-nc-SiOx:H) layer was proposed in hydrogenated amorphous silicon/microcrystalline silicon (a-Si:H/μc-Si:H) tandem solar cell. The absence of n-μc-Si:H compared to conventional n-μc-SiO_x:H/n-μc-Si:H/p-nc-SiO_x:H TRJ reduced parasitic absorption. Meanwhile, the new TRJ indicated an ohmic contact, which is suitable for the tandem solar cell. The application of the new TRJ significantly improved the short-circuit current (J_sc) of bottom cell. Moreover, n-μc-SiOx:H layer functioned as intermediate reflector layer to ensure high J_sc of top cell. Initial conversion efficiency of optimized a-Si:H/μc-Si:H tandem solar cell with novel TRJ based on as-grown MOCVDZnO: B (BZO) substrate reached up to 12.99%.

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.