纳米硅薄膜及其太阳电池研究进展

纳米硅薄膜具有新颖的结构特征和一系列独特的物理性质,可望应用于新型光电子器件、量子功能器件、集成电路等领域.本文综述了纳米硅薄膜的研究现状及其优良的光电性能和纳米硅薄膜太阳电池的研究进展,指出在生产制备与性能方面纳米硅薄膜太阳电池所具有的优势,具有良好的发展前景.     

微晶硅同质结薄膜太阳电池的数值模拟分析

采用太阳电池电容模拟软件(简称SCAPS)对p-i-n结构的微晶硅同质结薄膜太阳电池进行了数值模拟。研究了本征层的厚度和缺陷态浓度及窗口层的厚度等参数对电池性能的影响。得到的主要结论如下:(1)随着本征层缺陷态浓度Nt的增加,电池的各性能参数均单调下降。(2)随着本征层厚度的增加,长波段的光谱响应逐渐改善,但该层过厚则导致中波段的光谱响应急剧下降,在Nt=1.0×1016/cm3的条件下,本征层厚度在1.5～2.0μm范围内电池效率均可达到7.0%以上。(3)p型窗口层的厚度对短波段的光谱响应及短路电流密度JSC有较大影响。     

Applications of microcrystalline hydrogenated cubic silicon carbide for amorphous silicon thin film solar cells

We demonstrated the fabrication of n-i-p type amorphous silicon (a-Si:H) thin film solar cells using phosphorus doped microcrystalline cubic silicon carbide (μc-3C-SiC:H) films as a window layer. The Hot-wire CVD method and a covering technique of titanium dioxide TiO₂ on TCO was utilized for the cell fabrication. The cell configuration is TCO/TiO₂/n-type μc-3C-SiC:H/intrinsic a-Si:H/p-type μc- SiC_x (a-SiC_x:H including μc-Si:H phase)/Al. Approximately 4.5% efficiency with a V_oc of 0.953 V was obtained for AM-1.5 light irradiation. We also prepared a cell with the undoped a-Si_1−xC_x:H film as a buffer layer to improve the n/i interface. A maximum V_oc of 0.966 V was obtained.     

Advanced APCVD-processes for high-temperature grown crystalline silicon thin film solar cells

Crystalline silicon thin film (cSiTF) solar cells based on the epitaxial wafer-equivalent (EpiWE) concept combine advantages of wafer-based and thin film silicon solar cells. In this paper two processes beyond the standard process sequence for cSiTF cell fabrication are described. The first provides an alternative to wet chemical saw damage removal by chemical vapor etching (CVE) with hydrogen chloride in-situ prior to epitaxial deposition. This application decreases the number of process and handling steps. Solar cells fabricated with different etching processes achieved efficiencies up to 14.7%. 1300 degrees C etching temperature led to better cell results than 1200 degrees C. The second investigated process aims for an improvement of cell efficiency by implementation of a reflecting interlayer between substrate and active solar cell. Some characteristics of epitaxial lateral overgrowth (ELO) of a patterned silicon dioxide film in a lab-type reactor constructed at Fraunhofer ISE are described and first solar cell results are presented.     

Recent advances in very high frequency plasma enhanced CVD process for the fabrication of thin film silicon solar cells

We have deposited amorphous silicon (a-Si) and nanocrystalline silicon (nc-Si) materials and the total p-i-n configurations for solar cells in a high vacuum multichamber system ASTER using very high frequency plasma enhanced chemical vapour deposition (VHF PECVD) process. The deposition process is monitored and controlled by in-situ diagnostic tools to maintain reproducibility of the material quality. In this paper we show our recent results on single junction (amorphous silicon) and tandem (a-Si/nc-Si) cells on plastic foil using the Helianthos concept. The tandem cell efficiency on Asahi U-type SnO₂:F coated glass is ~ 12% and this is achieved by employing nc-Si deposited at high pressure (p) conditions of 5 mbar and a small inter-electrode distance (d) of 5 mm. The deposition scheme of this cell on glass was adapted for the SnO₂:F coated Al foil substrates from Helianthos b.v., especially taking into account the expansion of the foil during deposition. The inter-electrode distance d was one of the variables for this optimisation process. Depositions at four inter-electrode distances of 6 mm, 8 mm, 10 mm and 12 mm (keeping the pressure-distance product constant) revealed that the deposition rate increases at higher distances, reaching 0.6 nm/s at a d of 10 mm and pressure p of 3.0 mbar. The Raman crystalline ratio showed a monotonic increase with the combination of higher d and lower p. Tandem cells with an area of 2.5 cm² on plastic foil fabricated by the Helianthos concept and employing the above mentioned nc-Si made at 0.6 nm/s in the bottom cell and a-Si in the top cell, showed an efficiency of 8.12%, with a short circuit current density of 10 mA/cm². The combined deposition time of the photoactive silicon layers of the top and bottom cells amounted to only 85 min.     

Vacuum-deposited polycrystalline silicon solar cells on foreign substrates

The problem of fabricating low cost solar cells on foreign substrates has been addressed through the use of vacuum-deposited polycrystalline silicon films. The experimental cells consisted of the following layers on an Al₂O₃ substrate: TiB₂ bottom electrode/p-type polycrystalline silicon film/n-type silicon region/Ti---Ag electrode. The formation and properties of each layer are described. Interfacial reactions and purity were examined by secondary ion mass spectrometry, X-ray diffraction and scanning electron microscopy. A reactionbettween silicon and TiB₂ resulting in large silicon crystallite growth has been identified. The n-type region was formed by standard phosphorus diffusion techniques. Typical photovoltaic responses without an antireflection coating were V_oc = 0.28 V, J_sc = 18 mA cm^-2, an efficiency of 2.7% and a fill factor of 0.55. The factors limiting the cell efficiency were primarily the grain size and the purity of the p-type silicon layer.     

Doping-free fabrication of silicon thin films for schottky solar cell

Thin film Schottky solar cells were fabricated without doping processes, which may provide an alternative approach to the conventional thin film solar cells in the n-i-p configuration. A thin Co layer was coated on a substrate, which worked as a back contact metal and then Si film was grown above it. Deposition condition may modulate the Si film structure to be a fully amorphous Si (a-Si) or a mixing of microcrystalline Si (mc-Si) and a-Si. A thin Au layer was deposited above the grown Si films, which formed a Schottky junction. Two types of Schottky solar cells were prepared on a fully a-Si film and a mixing of mc-Si and a-Si film. Under one sun illumination, the mixing of mc-Si and a-Si device provided 35% and 68.4% enhancement in the open circuit voltage and fill factor compared to that of the amorphous device.     

Improving low pressure chemical vapor deposited zinc oxide contacts for thin film silicon solar cells by using rough glass substrates

Compared to zinc oxide grown (ZnO) on flat glass, rough etched glass substrates decrease the sheet resistance (R_sq) of zinc oxide layers grown on it. We explain this R_sq reduction from a higher thickness and an improved electron mobility for ZnO layers deposited on rough etched glass substrates. When using this etched glass substrate, we also obtain a large variety of surface texture by changing the thickness of the ZnO layer grown on it. This new combination of etched glass and ZnO layer shows improved light trapping potential compared to ZnO films grown on flat glass. With this new approach, Micromorph thin film silicon tandem solar cells with high total current densities (sum of the top and bottom cell current density) of up to 26.8 mA cm^− 2 were fabricated.     

The influence of Na on low temperature growth of CIGS thin film solar cells on polyimide substrates

The objective of this work is to study the influence of Na on the properties of Cu(In,Ga)Se₂ (CIGS) absorber layers and finished solar cell devices on polyimide substrates. For this study Na is added to 3-stage grown CIGS thin films by evaporation of a NaF precursor layer prior to the absorber deposition. The precursor layer modifies the CIGS growth kinetics. A stronger Ga-gradient and a decrease of grain size are observed when the Na content increases. An increase in V_oc for a higher Na concentration at a nominal growth temperature of T_sub,max = 500 °C during CIGS deposition is explained by a higher carrier density, as obtained by DLCP measurements. The higher carrier concentration for the higher Na content could be attributed to the reduction of a compensating donor. However, a low J_sc does not allow for an enhanced efficiency possibly due to a shorter depletion region, as observed by admittance spectroscopy, and effective diffusion length.     

Development of CZTS-based thin film solar cells

The low cost, environmental harmless Cu₂ZnSnS₄ (CZTS)-based thin film solar cells are fabricated by using abundant materials. The CZTS film possesses promising characteristic optical properties; band-gap energy of about 1.5 eV and large absorption coefficient in the order of 10⁴ cm^− 1. All constituents of this CZTS film, which are abundant in the crust of the earth, are non-toxic. Therefore, if we can use CZTS film practically as the absorber of solar cells, we will be free from both of the resource saving problem and the environmental pollution.In our CZTS project, CZTS absorber films were prepared by two independent techniques. One is three rf sources co-sputtering followed by annealing in sulfurized atmosphere. The latest conversion efficiency of over 6.7% was achieved by this technique. The other is co-evaporation technique. CZTS films were grown on Si (100) by vacuum co-evaporation using elemental Cu, Sn, S and binary ZnS as sources. XRD patterns indicated that the polycrystalline growth was suppressed and the orientational growth was relatively induced in a film grown at higher temperatures.In this presentation, the development of CZTS-based thin film solar cells will be surveyed.     

Plasma deposition of n-SiOx nanocrystalline thin film for enhancing the performance of silicon thin film solar cells

In this paper, we firstly optimized the properties of n-SiO_x nanocrystalline thin film through tuning deposition parameters by plasma enhanced chemical vapor deposition, so that we can actively control the properties of materials obtained. Secondly, we proposed using n-SiO_x/Al as back reflector for amorphous silicon (a-Si:H) solar cells. Compared to Al single-layer as back reflector, adding an n-SiO_x layer into the back reflector could improve the solar cell performance, which not only enhances the short circuit current density by an improvement of spectral response in the wavelength range of 550-750 nm, but also improves the open circuit voltage. With an optimized n-SiO_x/Al back reflector, a-Si:H solar cells with an intrinsic layer thickness of 270 nm show 13.1% enhancement in efficiency. In addition, a-Si:H/μc-Si:H tandem solar cells with n-SiO_x as intermediate reflector were also researched. As a result, it evidently balanced the current matching between top and bottom cell.     

Intrinsic microcrystalline silicon prepared by hot-wire chemical vapour deposition for thin film solar cells

Microcrystalline silicon (μc-Si:H) prepared by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature T_S and low deposition pressure exhibits excellent material quality and performance in solar cells. Prepared at T_S below 250 °C, μc-Si:H has very low spin densities, low optical absorption below the band gap, high photosensitivities, high hydrogen content and a compact structure, as evidenced by the low oxygen content and the weak 2100 cm⁻¹ IR absorption mode. Similar to PECVD material, solar cells prepared with HWCVD i-layers show increasing open circuit voltages V_oc with increasing silane concentration. The best performance is achieved near the transition to amorphous growth, and such solar cells exhibit very high V_oc up to 600 mV. The structural analysis by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows considerable amorphous volume fractions in the cells with high V_oc. Raman spectra show a continuously increasing amorphous peak with increasing V_oc. Crystalline fractions X_C ranging from 50% for the highest V_oc to 95% for the lowest V_oc were obtained by XRD. XRD-measurements with different incident beam angles, TEM images and electron diffraction patterns indicate a homogeneous distribution of the amorphous material across the i-layer. Nearly no light induced degradation was observed in the cell with the highest X_C, but solar cells with high amorphous volume fractions exhibit up to 10% degradation of the cell efficiency.     

Towards high efficiency thin film solar cells

As an alternative to single crystal silicon photovoltaics, thin film solar cells have been extensively explored for miniaturized cost-effective photovoltaic systems. Though the fight to gain efficiency has been severely engaged over the years, the battle is not yet over. In this review, we comb the fields to elucidate the strategies towards high efficiency thin films solar cells and provide pointers for further development. Starting from the photoelectron generation, we look into the fundamental issues in photoelectric conversion processes, including light harvesting and charge handling (separations, transportations and collections). The emerging organic-inorganic halide perovskite systems, as well as the rapidly developed polycrystalline inorganic systems, organic photovoltaics and amorphous silicon cells are discussed in details. The biggest bottleneck for the cost-effective polycrystalline inorganic cells is the composition sensitivity and deep defects; for amorphous silicon cells, it is the quantum of the dangling bonds; for organic cells, it is the low charge carrier mobility and high exciton binding energy; and for perovskite cells, it is the environmental degradation and the controversial mechanisms of generation of I-V hysteresis. Strategies of light harvesting and charge handling as well as directions to break the bottlenecks are pointed out.     

The design and fabrication of high efficiency thin film CdS/Cu2S solar cells

Thin film photovoltaic cells of CdS/Cu₂S which exhibit conversion efficiencies in excess of 9% have been designed and fabricated. Specific cell designs are prepared from an analysis of optical and electronic loss mechanisms operative in the cell. Material and engineering modifications to the fabrication process are then made to minimize specific energy conversion losses. The present cell design consists of five thin film layers which are sequentially prepared on a copper substrate 35 μm thick. In addition to the material control required for each component layer, the electrical, chemical, mechanical and topological compatibilities at the interfaces between each adjoining layer must be assured to achieve the desired cell performance. Our present analysis shows that a fully optimized solar cell based on a CdS/Cu₂S junction will have a practical conversion efficiency limit of about 11%. It is anticipated that practical conversion efficiencies of 14–15% can be achieved utilizing a (CdZn)S/Cu₂S junction designed to produce the maximum open-circuit voltage possible using Cu₂S as the absorbing layer. Present cell results which incorporate this design are presented.     

Novel aspects in thin film silicon solar cells-amorphous, microcrystalline and nanocrystalline silicon

The improvement of photodegradation of a-Si:H has been studied on the basis of controlling the subsurface reaction and gaseous phase reaction. We found that higher deposition temperature, hydrogen dilution and triode method are effective to reduce the SiH₂ density in the film and to suppress the photodegradation of solar cells. These results are explained in terms of the hydrogen elimination reaction in the subsurface region and the contribution of the higher silane radicals to the film growth. The high-rate deposition of μc-Si:H was obtained by means of a high-pressure method and further improvement in deposition rate and the film quality was achieved in combination with the locally high-density plasma, which enables effective dissociation of source gases without thermal damage. It was also found that the deposition pressure is crucial to improve the film quality for device. This technique was successfully applied to the solar cells and an efficiency of 7.9% was obtained at a deposition rate of 3.1 nm/s. The potential application of nanocrystalline silicon is also discussed.     

Textured surface boron-doped ZnO transparent conductive oxides on polyethylene terephthalate substrates for Si-based thin film solar cells

Textured surface boron-doped zinc oxide (ZnO:B) thin films were directly grown via low pressure metal organic chemical vapor deposition (LP-MOCVD) on polyethylene terephthalate (PET) flexible substrates at low temperatures and high-efficiency flexible polymer silicon (Si) based thin film solar cells were obtained. High purity diethylzinc and water vapors were used as source materials, and diborane was used as an n-type dopant gas. P-i-n silicon layers were fabricated at ~ 398 K by plasma enhanced chemical vapor deposition. These textured surface ZnO:B thin films on PET substrates (PET/ZnO:B) exhibit rough pyramid-like morphology with high transparencies (T ~ 80%) and excellent electrical properties (Rs ~ 10 Ω at d ~ 1500 nm). Finally, the PET/ZnO:B thin films were applied in flexible p-i-n type silicon thin film solar cells (device structure: PET/ZnO:B/p-i-n a-Si:H/Al) with a high conversion efficiency of 6.32% (short-circuit current density J_SC = 10.62 mA/cm², open-circuit voltage V_OC = 0.93 V and fill factor = 64%).     

Low temperature (< 100 °C) fabrication of thin film silicon solar cells by HWCVD

Amorphous silicon films have been made by HWCVD at a very low substrate temperature of ≤ 100 °C (in a dynamic substrate heating mode) without artificial substrate cooling, through a substantial increase of the filament-substrate distance (∼ 80 mm) and using one straight tantalum filament. The material is made at a reasonable deposition rate of 0.11 nm/s. Optimized films made this way have device quality, as confirmed by the photosensitivity of > 10⁵. Furthermore, they possess a low structural disorder, manifested by the small Γ/2 value (half width at half maximum) of the transverse optic (TO) Si-Si vibration peak (at 480 cm^− 1) in the Raman spectrum of ∼ 30.4 cm^− 1, which translates into a bond angle variation of only ∼ 6.4°. The evidence gathered from the studies on the structure of the HWCVD grown film by three different techniques, Raman spectroscopy, spectroscopic ellipsometry and transmission electron microscopy, indicate that we have been able to make a photosensitive material with a structural disorder that is smaller than that expected at such a low deposition temperature.Tested in a p-i-n solar cell on Asahi SnO₂:F coated glass (without ZnO at the back reflector), this i-layer gave an efficiency of 3.4%. To our knowledge, this is the first report of a HWCVD thin film silicon solar cell made at such a low temperature.