Film Polycrystalline Silicon Solar Cells Research Articles

Thin Film Polycrystalline Silicon Solar Cells Studied by Transient Terahertz Probe Spectroscopy

Abstract Thin film polycrystalline silicon (poly-Si) solar cells were annealed in hydrogen plasma or in water vapour to suppress electrical activity of defects and impurities (passivation). The hydrogen plasma passivation procedure was optimized and a modification of several hydrogenation parameters was suggested, e.g. higher hydrogen pressure of 300 Pa and keeping bias voltage Vbias constant during the whole passivation process until the plasma was switched off. Since the plasma hydrogenation process was run as a closed system (without hydrogen flow) and the achieved passivation results (up to 497 mV) were close to the limit for poly-Si (500 mV), the generally accepted necessity to operate the process with a hydrogen flow was called into question. Samples passivated in a hydrogen plasma at different processing conditions were analyzed by transient terahertz probe spectroscopy and Suns-VOC method to measure a solar cell's charge carrier lifetime and its open-circuit voltage VOC, respectively. A correlation of these measured parameters was observed even though both these techniques characterize solar cells in completely different states. The transient terahertz probe spectroscopy appears to be a useful tool for a contactless investigation of materials where ultrafast processes of ns-long lifetimes play a crucial role, e.g. nanostructured objects and materials.

ANNEALING OF POLYCRYSTALLINE THIN FILM SILICON SOLAR CELLS IN WATER VAPOUR AT SUB-ATMOSPHERIC PRESSURES

Thin film polycrystalline silicon (poly-Si) solar cells were annealed in water vapour at pressures below atmospheric pressure. PN junction of the sample was contacted by measuring probes directly in the pressure chamber filled with steam during passivation. Suns-VOC method and a Lock-in detector were used to monitor an effect of water vapour to VOC of the solar cell during whole passivation process (in-situ). Tested temperature of the sample (55°C – 110°C) was constant during the procedure. Open-circuit voltage of a solar cell at these temperatures is lower than at room temperature. Nevertheless, voltage response of the solar cell to the light flash used during Suns-VOC measurements was good observable. Temperature dependences for multicrystalline wafer-based and polycrystalline thin film solar cells were measured and compared. While no significant improvement of thin film poly-Si solar cell parameters by annealing in water vapour at under-atmospheric pressures was observed up to now, in-situ observation proved required sensitivity to changing VOC at elevated temperatures during the process.

Advanced microhole arrays for light trapping in thin film silicon solar cells

We present a comprehensive theoretical analysis accompanying our experimental development of micro-structured thin film polycrystalline silicon solar cell absorbers. Our focus is on 2D-periodic arrays of holes which do not act as a superficial scattering grating but pierce the entire silicon absorber layer. We deviate from the commonly used model of vertical hole sidewalls by employing conical frustum shaped holes with a range of opening angles. Additionally, an experimentally motivated front texture is applied to the simulated solar cell absorbers and varied in texture aspect. Based on this parametric absorber model, for a single domain period of 2μm, we discuss light trapping and optical losses in thin film cells with absorptive front and back contacts and a perfectly transparent glass encapsulation layer. Implications from this study are that light trapping in square periodic arrays with periods larger than the wavelength of light can be enhanced by microholes but that it cannot replace a front texture which provides both antireflection and scattering.

PREPARATION AND CHARACTERIZATION OF P3HT-PCBM ORGANIC SOLAR CELLS

Organic solar cells using P3HT: PCBM as an active layer on ITO coated glass substrates were fabricated and characterized. Different air annealing procedures and cathode materials were tried and the characteristics were compared with that of a standard thin film polycrystalline silicon solar cell. It was found that the sample prepared with post-deposition air annealing at 130 oC improves the open circuit voltage (Voc) considerably. Besides, short circuit current (Isc) and the efficiency (η) were highest for the sample with a non annealed active layer. Series resistance (Rs) for this sample was lowest, but 103 times higher than that of the silicon solar cell, which in turn may have reduced the efficiency value for the organic cell compared to silicon.

Laser annealing of thin film polycrystalline silicon solar cell

Performances of thin film polycrystalline silicon solar cell grown on glass substrate, using solid phase crystallization of amorphous silicon can be limited by low dopant activation and high density of defects. Here, we investigate line shaped laser induced thermal annealing to passivate some of these defects in the sub-melt regime. Effect of laser power and scan speed on the open circuit voltage of the polysilicon solar cells is reported. The processing temperature was measured by thermal imaging camera. Enhancement of the open circuit voltage as high as 210% is achieved using this method. The results are discussed.

On the diffusion length and grain size homogeneity requirements for efficient thin-film polycrystalline silicon solar cells

We examine the influence of intragrain defects and grain boundaries on the macroscopic performance of a thin film polycrystalline silicon solar cell. In addition, we evaluate the effect of grain size inhomogeneity on the cell performance via circuit simulations. From an analytical study of charge transport in individual grains and homogeneous grain systems, we obtain the grain size and intragrain diffusion length requirements for a desired efficiency. We identify the conditions under which the grain size and the intragrain diffusion length dominate the cell characteristics. In devices with intragrain effective diffusion length Lmono ⩽ 100 µm and grain boundary recombination velocity SGB ⩽ 104 cm s−1, achieving a larger grain size beyond several µm is not crucial. The inhomogeneous distribution circuit simulations show that grain size inhomogeneity is not the main limiting factor in polycrystalline silicon solar cells. This is so even in thin polycrystalline silicon films with a broad grain size distribution such as those made with aluminum-induced crystallization at low annealing temperature. The main reason is that the optimum bias point for grains of different sizes only differ by about ∼50 mV over a fairly wide grain diameter range 0.5–50 µm even when Lmono = 100 µm and SGB = 105 cm s−1.

Light confinement in e‐beam evaporated thin film polycrystalline silicon solar cells

AbstractPolycrystalline Si thin‐film (poly‐Si) solar cells need effective light‐trapping to compensate for the moderate absorption. Recent developments in the poly‐Si cell technology focus on film fabrication by e‐beam evaporation due to its manufacturing benefits, such as a very high deposition rate above 15 nm/s and inline compatible process. However, evaporated poly‐Si cells exhibit limited compatibility with textured glass. In this Letter, the coupling of the light in the absorber layer is enhanced by introducing a rough interface at the back of the solar cells. This increases the conversion efficiency from 6% to 7.1% for, respectively, planar and back textured interface with current density of 26.6 mA/cm2 for only 3.6 μm absorber thickness. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of inhomogeneous grain size distribution in polycrystalline silicon solar cells

Abstract We investigate the effects of grain size inhomogeneity on the performance of thin film polycrystalline silicon solar cells with columnar grains. We focus on cells with an exponential distribution of the grain diameter, which approximates the grain size inhomogeneity in polycrystalline silicon films made via aluminum induced crystallization (AIC) with low temperature annealing. Through equivalent circuit simulation, we demonstrate that the size inhomogeneity will only play a role when intra grain defects, which is common in AIC cells, can be avoided. We further demonstrate that the inhomogeneity itself is not a dominant factor which limits the efficiency of these cells.

Quantifying the effectiveness of SiO2/Au light trapping nanoshells for thin film poly-Si solar cells

In order to enhance light absorption of thin film poly-crystalline silicon (TF poly-Si) solar cells over a broad spectral range, and quantify the effectiveness of nanoshell light trapping structure over the full solar spectrum in theory, the effective photon trapping flux (EPTF) and effective photon trapping efficiency (EPTE) were firstly proposed by considering both the external quantum efficiency of TF poly-Si solar cell and scattering properties of light trapping structures. The EPTF, EPTE and scattering spectrum exhibit different behaviors depending on the geometric size and density of nanoshells that form the light trapping layer. With an optimum size and density of SiO2/Au nanoshell light trapping layer, the EPTE could reach up to 40% due to the enhancement of light trapping over a broad spectral range, especially from 500 to 800 nm.

Nickel-induced crystallization of amorphous silicon

The nickel-induced crystallization of hydrogenated amorphous silicon (a-Si:H) is used to obtain large grained polycrystalline silicon thin films on glass substrates. a-Si:H is deposited by plasma enhanced chemical vapour deposition at 200 °C, preparing intrinsic and slightly p-doped samples. Each sample was divided in several pieces, over which increasing Ni concentrations were sputtered. Two crystallization methods are compared, conventional furnace annealing (CFA) and rapid thermal annealing (RTA). The crystallization was followed by optical microscopy and scanning electron microscopy observations, X-ray diffraction, and reflectance measurements in the UV region. The large grain sizes obtained – larger than 100 μm for the samples crystallized by CFA – are very encouraging for the preparation of low-cost thin film polycrystalline silicon solar cells.

Efficiency potential of thin film polycrystalline silicon solar cells by silane-gas-free process using aluminum-induced-crystallization

Abstract Analyzing the performance of thin film polycrystalline silicon solar cells fabricated by silane-gas-free process including the aluminum-induced-crystallization technique by using the device simulation program “PC1D”, we have estimated the efficiency of them. In addition, we have discussed the issues to make the silane-gas-free process practical. In the cell fabrication by silane-gas-free process, segregation of impurity atoms at the grain boundaries of the Si film is one of the serious problems. By suppressing the impurity inclusion and optimizing the cell parameters, the simulated efficiency is to be about 13% in single-junction cells.

Thin film polycrystalline silicon solar cell on ceramics with a seeding layer formed via aluminium-induced crystallisation of amorphous silicon

Thin film polycrystalline silicon solar cells on foreign substrates are viewed as one of the most promising approaches to cost reduction in photovoltaics. To enhance the quality of the film, the use of ‘seeding layers’ prior to deposition of active material is being investigated. It has been shown that a phenomenon suitable to create such a seeding layer is the aluminium-induced crystallisation of amorphous silicon. Previous work mainly considered glass as the substrate of choice, thereby introducing limitations on the deposition temperature. Results concerning the application of such a technique to ceramic substrates (allowing the use of high-temperature CVD) are described. Also, the first reported results of a solar cell made in silicon deposited on these seeding layers are presented.

Influence of substrate texture on microstructure and photovoltaic performances of thin film polycrystalline silicon solar cells

Thin film polycrystalline silicon (poly-Si) solar cells have been fabricated by plasma enhanced chemical vapor deposition on differently textured ZnO/Ag/SnO 2/glass substrates. Using a textured substrate with the root mean square (RMS) roughness ( σ) of 38 nm, the conversion efficiency of 8.22% has been achieved due to improvement of long wavelength responses. Whereas using textured substrates with σ>38 nm , (2 2 0) preferential growth is deteriorated, yielding decreases in both short circuit current and open circuit voltage. The field-dependent carrier collection behaviors reveal that the carrier diffusion length in the poly-Si layer on textured substrates with σ>38 nm decreases due to the change in poly-Si microstructure.

Analysis of Device Performance by Quasi Three-Dimensional Simulation for Thin Film Polycrystalline Silicon Solar Cells with Columnar Structure

In order to realize high efficiency thin film polycrystalline silicon (poly-Si) solar cells, a novel method of quasi three-dimensional simulation using a cylindrical coordinate system was proposed. Optimum design of cell configuration and analysis of device performance have been demonstrated. In this simulation we used realistic physical values for parameters, such as interface recombination velocity at grain boundaries and diffusion length of minority carriers. Interface recombination velocity at grain boundaries (which has a strong effect on cell performance) should be less than 103 cm/s to obtain high cell performance. Even at a relatively short diffusion length of 50 µm, high efficiency of more than 16–18% can be expected at a film thickness of 5–20 µm when grain size is 10 µm, which is appropriate for the utilization of solar grade Si of reasonable material quality.

16.0% Efficiency of large area (10 cm×10 cm) thin film polycrystalline silicon solar cell

High efficient large area thin film polycrystalline Si solar cell based on a silicon on insulator (SOI) structure prepared by zone-melting recrystallization (ZMR) is reported. Fabrication process of the via-hole etching for the separation of thin films (VEST) is newly developed. It is found that phosphorus treatment and back surface field (BSF) are quite effective for the VEST structure and the ZMR thin film polycrystalline silicon. The conversion efficiency as high as 16.0% for a practical size (10 cm×10 cm) is achieved. This is the highest for large area thin film polycrystalline Si solar cells ever reported.

High efficiency polycrystalline silicon solar cells using low temperature PECVD process

Conventionally directionally solidified (DS) and silicon film (SF) polycrystalline silicon solar cells are fabricated using gettering and low temperature plasma enhanced chemical vapor deposition (PECVD) passivation. Thin layer (/spl sim/10 nm) of PECVD SiO/sub 2/ is used to passivate the emitter of the solar cell, while direct hydrogen rf plasma and PECVD silicon nitride (Si/sub 3/N/sub 4/) are implemented to provide emitter and bulk passivation. It is found in this work that hydrogen rf plasma can significantly improve the solar cell blue and long wavelength responses when it is performed through a thin layer of PECVD Si/sub 3/N/sub 4/. High efficiency DS and SF polycrystalline silicon solar cells have been achieved using a simple solar cell process with uniform emitter, Al/POCl/sub 3/ gettering, hydrogen rf plasma/PECVD Si/sub 3/N/sub 4/ and PECVD SiO/sub 2/ passivation. On the other hand, a comprehensive experimental study of the characteristics of the PECVD Si/sub 3/N/sub 4/ layer and its role in improving the efficiency of polycrystalline silicon solar cells is carried out in this paper. For the polycrystalline silicon used in this investigation, it is found that the PECVD Si/sub 3/N/sub 4/ layer doesn't provide a sufficient cap for the out diffusion of hydrogen at temperatures higher than 500/spl deg/C. Low temperature (/spl les/400/spl deg/C) annealing of the PECVD Si/sub 3/N/sub 4/ provides efficient hydrogen bulk passivation, while higher temperature annealing relaxes the deposition induced stress and improves mainly the short wavelength (blue) response of the solar cells.

Effect of grain size and dislocation density on the performance of thin film polycrystalline silicon solar cells

Three kinds of important properties of the solar cell were calculated: short-circuit current density, open-circuit voltage, and conversion efficiency. Two equations which show the relationship between the minority-carrier diffusion length and the grain size or the etch pit density were used for the calculation. The dependence of the properties on the cell thickness were estimated as a function of grain size and etch-pit density. The effect of the internal reflectance with varying minority-carrier diffusion length was also examined. The results show that thin film polycrystalline silicon solar cells have the potential to attain an efficiency of 17% even at a film thickness of 2 μm if the grain size is bigger than 10 μm and the etch-pit density of less than 1×106 cm−2. The principal requirement is to achieve efficient light trapping.

Anomalously high collection probability in thin film polycrystalline silicon solar cells from numerical modeling

The impact of grain size on the average collection probability of fine grained (<10 μm) polycrystalline silicon has been investigated using a numerical device simulator. This model predicts that the collection probability can improve dramatically once the depletion regions around adjacent grain boundaries overlap. Thus, contrary to most analytical models, the collection probability is not monotonic with grain size. The collection probability has a local maximum at a grain size determined by the grain boundary trap state density. The magnitude of the improvement is dependent upon the grain boundary recombination velocity.

Thin Film Polycrystalline Silicon Solar Cells Prepared by Plasma CVD

The use of RF plasma-assisted chemical vapour deposition (PACVD) for the growth of thin film polycrystalline silicon solar cells on carbon substrates is reported. Efficiencies of upto 5.3% (air mass 1.5, 100 mW/cm 2 irradiation) have been attained in structures employing 10 to 15 μm thick silicon layers deposited at 800 °C, with growth rates upto 10 μm/h. The cells exhibited excellent stabilities under continuous light soaking over a period of 3600 h which was the limit of the test. Fill factor and open circuit voltage were the main factors limiting cell efficiency, the former due to a high series resistance from which all the cells suffered. Short circuit currents were promising at upto 23 mA/cm 2 without the use of AR coatings or light trapping. In view of the non-optimised nature of the cells (e.g. series resistance, doping levels, emitter thicknesses, optical losses, etc.) and the small grain size (≤1 μm) of the layers, the results suggest that further improvements in performance should be possible with optimisation.

High-efficient operation of large-area (100 cm2) thin film polycrystalline silicon solar cell based on SOI structure

High-efficient operation of a large-area thin film polycrystalline Si solar cell with a novel structure based on a silicon on insulator (SOI) structure prepared by zone-melting recrystallization (ZMR) is reported. The (100) crystal orientation area over 90% has successfully been obtained by controlling the ZMR conditions, which allowed to form a uniform random pyramidal structure at the cell surface. The effect of hydrogen passivation has also been investigated for further improvement of the cell characteristics. By employing a light trapping structure (textured surface) and hydrogen passivation, an efficiency of 14.22% was obtained for a practical 100 cm 2 size.