非晶硅太阳电池的原理

非晶硅太阳电池是２０世纪７０年代中期发展起来的一种新型薄膜太阳电池，与其他太阳电池相比，非晶硅电池具有以下突出特点：（１）制作工艺简单，在制备非晶硅薄膜的同时就能制作pin结构。（２）可连续、大面积、自动化批量生产。（３）非晶硅太阳电池的衬底材料可以是玻璃、不锈钢等，因而成本小。（４）可以设计成各种形式，利用集成型结构，可获得更高的输出电压和光电转换效率。（５）薄膜材料是用硅烷（SiH４）等的辉光放电分解得到的，原材料价格低。１非晶硅太阳电池的结构、原理及制备方法非晶硅太阳电池是以玻璃、不锈钢及特种…     

Numerical approach for high-efficiency a-Si solar cells

We have developed the first precise numerical simulator for thin-film solar cells with two-dimensional structures, such as a submicron textured a-Si solar cell. Conventional simulators for thin-film solar cells were all one-dimensional, which made precise simulation of the behavior of light and carrier transport in the cell impossible. Using the 2D simulator, guidelines for cell design, including textured structures, were obtained. One proposal to increase the conversion efficiency of the textured a-Si single-junction solar cell is to make the texture period longer than the film thickness.     

Fabrication technology for large-area a-Si solar cells

The fabrication process technology for large-area a-Si photovoltaic (PV) modules and their performance are reviewed. Our present technology enables us to provide 10% efficient large-area submodules with a stabilized efficiency of 8.5%. To study the practicability of the a-Si solar panels, we carried out an outdoor test for our a-Si modules. The results show that the a-Si solar PV modules generate power very efficiently in outdoor systems. The advantage of the a-Si modules under outdoor uses is presented and discussed.     

Modeling of a-Si/poly-Si and a-Si/poly-Si/poly-Si stacked solar cells

A computer model for the poly-Si thin film-related solar cells is established, with which the solar cells with the structure of single junction poly-Si cell, a-Si/poly-Si tandem cell and a-Si/poly-Si/poly-Si triple cell are simulated. The results indicate that the practical structure for poly-Si-related solar cell is a-Si/poly-Si/poly-Si triple cell with the best matched thickness of 0.23/0.95/3 μm and with optical confinement structure, which has the highest simulated efficiency of 22.74%.     

a-Si alloy three-stacked solar cells with high stabilized-efficiency

a-Si alloy three-stacked solar cells have been studied to improve the stabilized efficiency of a-Si: H based solar cells. Based on the analysis by the individual characterization method of the component cells in stacked type cells, the a-Si :H middle cell was replaced with an a-SiGe :H cell. Furthermore, the optical confinement technology was improved to obtain a high-output current with thin i-layer thickness in the a-SiGe :H bottom cell. By this device design, the initial conversion efficiency was improved up to 12.4% and more than a 10% stabilized efficiency was obtained in a-SiC :H/a-SiGe :H/a-SiGe :H three-stacked cells. These cell characteristics were confirmed by measurements at the JQA Organization (the former JMI Institute).     

Amorphous silicon oxide and its application to metal/n-i-p/ITO type a-Si solar cells

P-type and n-type amorphous silicon oxide (a-SiO) films with a microcrystalline Si phase were deposited by plasma CVD using a gas mixture of SiH₄---CO₂---H₂---B₂H₆ and SiH₄---CO₂---H₂---PH₃, respectively. These films had lowe absorption coefficients than conventional a-SiO due to larger oxygen contents and microcrystalline phase, but their conductivities were still high since they contained some microcrystalline Si phase. Furthermore, it was found that it is easier to make microcrystalline a-SiO than a-SiC films at low substrate temperature. By applying these films to the p-layer of metal/nip/ITO type cells, higher performance was obtained, compared to the cells with conventional microcrystalline Si p-layer deposited at lower temperature and conventional a-SiO p-layer. From these results, we consideredhat these novel a-SiO films with microcrystalline Si phase are a promising material for the window layer of a-Si solar cells.     

Limiting carrier effect in a-Si:H solar cells

The limiting carrier effect in a-Si:H p–i–n and n–i–p solar cells has been investigated computationally, by adjusting the values of the carrier band mobilities. Using a realistic optical generation rate profile, it was found that the effect is significant in both types of cell, with the electron identified as the limiting carrier in the p–i–n cell, and the hole in the n–i–p cell. However, using a uniform generation rate profile in the simulation indicated that the limiting carrier effect was much less significant, and that device performance in both cells seemed to be slightly more sensitive to the hole transport properties than to the electron transport properties.     

Optically induced degradation in a-Si:H solar cells

Difference spectra of optically induced changes in the surface photo-voltage response of a-Si:H solar cells show that illumination generates two sets of near midgap states which have energy levels consistent with the two energy states for dangling bond defects in this material. Recombination of photo-generated carriers in these states leads to significant degradation of cell performance for photon energies larger than the bandgap. At low temperature (−160°C), the difference spectrum indicates that although the defect states are still present, such degradation does not occur.     

Single-junction a-Si solar cells with over 13% efficiency

Single junction hydrogenated amorphous silicon solar cells having a high conversion efficiency of 13.2% were developed by combining three approaches. First, a new type of p-layer, such as (a-Si/a-C)_n multilayers, was investigated. The high open-circuit voltage was obtained without lowering the short-circuit current and the fill factor. Second, alternately repeating deposition and hydrogen plasma treatment method was applied to the fabrication of an a-SiC or wide gap a-Si : H films for p/i interface layer. High photoconductive and wide bandgap materials were obtained applicable to the p/i buffer layers. Third, the relationships between defect density of films or fill factors of solar cells and hydrogen radical in plasma were investigated. It was suggested that the H^*/SiH^* ratio was an effective parameter to improve the defect and fill factor, and the excess hydrogen radical deteriorated quality of films and cells.     

Measuring and modelling of a-Si, Ge:H solar cells

We have measured and modelled a-Si,Ge:H pin cells with the purpose of refining the existing modelling parameters, so that the model can be used for designing cells. In this work, we have employed the numerical device model AMPS. We have found that we can satisfactorily model the performance of as-deposited alloy cells by suitably changing the band gap, mobility gap, optical absorption spectrum, and electron affinity.     

Advanced fabrication technologies of integrated-type structure for a-Si solar cells

This paper proposes a new advanced fabrication technology for a low-cost integrated-type a-Si solar cell. Integrated-type cells provide many advantages and have been industrialized with a laser patterning method. However, a higher throughput and more efficient patterning method was required for applying a-Si solar cells to a power generating system. Plasma CVM (Chemical Vaporization Machining) was first applied to advanced patterning because of its advantages of high speed and selectivity. In this method, a plasma generated under high pressure localizes near the wire electrode and concentrates reactive radicals. As a result, we achieved an etching rate of more than 1 μm/s and selective patterning of a 200 μm-wide a-Si layer in 1 s multiline patterning was also developed for large-area modules.     

Development of an ultralight, flexible a-Si solar cell submodule

An ultralight, flexible a-Si solar cell fabricated on a polyimide film substrate has been developed. It was found that prebaking the polyimide film substrate improved the output performance of the cell by reducing the amount of H₂0 and polymers released from the substrate. Also, using an i-layer fabricated at high temperature (250°C) and a p-layer fabricated at low temperature (80°C) improved the cell's output performance to a level of over 10%. A maximum output of 782.4 mW and a power-to-weight ratio of 340 mW/g were obtained for an integrated-type flexible a-Si solar cell submodule with a size of 113 mm × 120 mm.     

Optical confinement in high-efficiency a-Si solar cells with textured surfaces

The experimental spectral response and reflectance of high-efficiency a-Si solar cells are systematically investigated by using an optical simulation based on realistic optical properties of the transparent conducting oxide (TCO), a-Si, and metal electrode, in order to improve the spectral response. It is shown that a practically important optical loss results from absorption by the TCO, which is enhanced by the optical confinement effect. This suggests that improvement in the spectral response is possible by suppressing the optical confinement in the TCO.     

Incorporation and critical concentration of oxygen in a-Si:H solar cells

For different process conditions, series of hydrogenated amorphous silicon p-i-n solar cells with various oxygen concentrations in the intrinsic absorber layer were fabricated by plasma-enhanced chemical vapor deposition at 13.56 MHz using process gas mixtures of SiH₄ and H₂. Oxygen was introduced into the gas phase during the deposition process by a controllable leak in the chamber wall and the amount of oxygen supply is characterized by the oxygen base pressure p_b. It is found that for a certain deposition regime defined by silane and H₂ flows, deposition pressure and substrate temperature the oxygen incorporation follows an expected dependence on the ratio p_b/r_d with r_d the deposition rate. This relation is not valid for the comparison of different deposition regimes. A high hydrogen flow is found to reduce the oxygen incorporation strongly. The photovoltaic parameters of the solar cells were measured in the initial state as well as after 1000 h of light-soaking. The critical oxygen concentration (i.e. the upper limit of incorporated oxygen not leading to a decay of the solar cell performance) was determined for each regime in the initial and light-soaked state. For all deposition regimes, the results show no difference in these critical oxygen concentrations for the initial and light-soaked state. The critical oxygen concentration, is found to differ for the different process regimes and turns out to be the highest (approximately 1×10²⁰ cm⁻³) for the deposition regime with the highest hydrogen flow rate, which interestingly is the regime with the lowest oxygen incorporation at a given p_b/r_d ratio. This combination makes the regime of high hydrogen gas flow suitable for depositing high-efficiency solar cells at high base pressure.     

Optimization of ZnO for front and rear contacts in a-Si solar cells

The enhancement of the reflection from the rear contact of p-i-n a-Si solar cells using ZnO combined with metals (Ag/Al) as a back reflector was demonstrated theoretically and experimentally. Futhermore, the incorporation of unreacted H₂O as source gas in the ZnO films was clearly observed through the thermal evolution measurement, suggesting the need for employing the pre-annealing technique for ZnO films before using them as a front contact in p-i-n a-Si solar cells. By using these approaches, the a-Si solar cells with glass/annealed-ZnO/delta-doped p/buffer/i/n/ZnO/metals(Ag/Al) structure were successfully fabricated and a conversion efficiency of 12.1% (AM-1.5, area 3×3 mm²) was obtained. Moreover, the solar cells with a structure of AR coated glass/SnO₂/delta-doped/p/buffer/i/n/ZnO/metals(Ag/Al) were also fabricated and by optimizing the use of the ZnO layer at the rear contact, a conversion efficiency of 12.6% was obtained. To make the ZnO films more appropriate for solar cells application, the growth rate of the ZnO films was increased by increasing the flow rate of diethylzinc used as a source gas.     

Phase engineering of a-Si:H solar cells for optimized performance

Until recently, the advances in hydrogenated amorphous silicon (a-Si:H) solar cell performance and stability have been achieved materials prepared with hydrogen dilution following primarily empirical approaches. This paper discusses the recently obtained insights into the growth, microstructure and nature of these materials. Such protocrystalline Si:H materials are more ordered than the a-Si:H obtained without dilution and evolve with thickness from an amorphous phase into first a mixed amorphous–microcrystalline and subsequently into a single microcrystalline phase. The development of deposition phase diagrams, characterize their microstructural evolution during growth which can be used to guide the fabrication of solar cell structures in a controlled way. Examples are presented and discussed of their application in solar cell fabrication to obtain a fundamental understanding of the properties of the phase transitions as well as the systematic optimization of cell performance.     

Experimental model and long-term prediction of photovoltaic conversion efficiency of a-Si solar cells

Degradation and recovery tests have been conducted on recent single-junction a-Si solar cells under various light intensities and temperature conditions to predict long-term stability. Saturation phenomena of the degradation of efficiency have been shown experimentally. The degradation characteristics are expressed as the second extreme distribution function of the largest value with saturation introduced. The dependence of saturation on the cell temperature and the light intensity are investigated. The worst efficiency of seasonal changes over a long term can be predicted from the saturation characteristics.     

Photovoltaic water electrolysis using the sputter-deposited a-Si/c-Si solar cells

The solar cell which is employed for photovoltaic water electrolysis was fabricated by sputter-depositing a-Si:H on Si wafer. The electrolysis was conducted by connecting the series-connected solar cell externally to the hydrogen and the oxygen evolution electrodes. The conversion efficiency of 3.0% from the solar to hydrogen chemical energy was obtained and can be expected to be improved further by optimizing the system.     

Potential of VHF-plasmas for low-cost production of a-Si: H solar cells

Compared to the use of the standard glow discharge technique the production of amorphous silicon solar cells by the very high frequency glow discharge (VHF-GD) bears yet additional cost reduction potentials:Using VHF-GD at excitation frequencies higher than 13.56 MHz, a more efficient dissociation of silane gas is obtained; thus, higher deposition rates are achieved; this reduces considerably the deposition time for intrinsic amorphous and microcrystalline silicon layers.Furthermore, by itself and even more so, in combination with argon dilution, VHF-GD technique improves silane utilisation and leads, thus, to further cost reduction.Finally, by combining the VHF-GD technique and the “micromorph” concept “real” tandem cells (i.e. a superposition of two cells with distinctly different band gaps) can be deposited at low temperatures without the use of expensive germane gas.     

非晶硅薄膜太阳电池应用分析

简要介绍了当前几种薄膜太阳电池,并分析了它们的优缺点;详细介绍了非晶硅薄膜太阳电池,分析了其光致衰退效应与影响光电性能的各种因素,总结并展望了优化非晶硅太阳电池的各种技术。