poly-Si Thin-film Solar Cells Research Articles

A glass thinning and texturing method for light incoupling in thin-film polycrystalline silicon solar cells application

For polycrystalline silicon (poly-Si) thin-film solar cells on ∼3 mm borosilicate glass, glass thinning reduces the glass absorption and light leaking to neighbouring cells; the glass texturing of the sun-facing side suppresses reflection. In this Letter, a labour-free wet etching method is developed to texture and thin the glass at the same time in contrast to conventionally separated labour-intensive glass thinning and texturing processes. For 2 cm2 size poly-Si thin-film solar cells on glass superstrate, this wet etching successfully thins down the glass from 3 mm to 0.5 mm to check the ultimate benefit of the process and introduces a large micron texture on the sun-facing glass surface. The process enhances Jsc by 6.3% on average, with the optimal Jsc enhancement of 8%, better than the value of 4.6% found in the literature. This process also reduces the loss in external quantum efficiency (EQE loss), which is due to light leaking to neighbouring cells, dramatically. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical Investigation of Light Trapping in Polycrystalline Silicon Thin-Film Solar Cells

In this paper we present a theoretical study of light trapping in polycrystalline silicon (poly-Si) thin-film solar cells with scattering surfaces, using the commercial software Advanced Semiconductor Analysis (ASA). Light scattering in ASA is modelled by haze parameters and angular distribution functions. The effects of these functions on the calculated absorption in poly-Si thin-film solar cells are investigated. An expected result of this investigation is that the optical absorption increases with an increasing fraction of light being scattered. This increased absorption results in a higher photocurrent generation and, thus, in an improved solar cell efficiency. For poor material quality, however, a higher haze value can also result in a decrease in the short-circuit current due to increased recombination losses. Additionally, our results show that, for poor material quality, a front surface texture is to be preferred over a rear surface texture.

Liquid-phase explosive crystallization of electron-beam-evaporated a-Si films induced by flash lamp annealing

We succeed in the formation of micrometer-order-thick polycrystalline silicon (poly-Si) films through the flash-lamp-induced liquid-phase explosive crystallization (EC) of precursor a-Si films prepared by electron-beam (EB) evaporation. The velocity of the explosive crystallization (vEC) is estimated to be ∼14m/s, which is close to the velocity of the liquid-phase epitaxy (LPE) of Si at a temperature around the melting point of a-Si of 1418K. Poly-Si films formed have micrometer-order-long grains stretched along a lateral crystallization direction, and X-ray diffraction (XRD) and electron diffraction pattern measurements reveal that grains in poly-Si films tend to have a particular orientation. These features are significantly different from our previous results: the formation of poly-Si films containing randomly-oriented 10-nm-sized fine grains formed from a-Si films prepared by catalytic chemical vapor deposition (Cat-CVD) or sputtering. One possible reason for the emergence of a different EC mode in EB-evaporated a-Si films is the suppression of solid-phase nucleation (SPN) during Flash Lamp Annealing (FLA) due to tensile stress which precursor a-Si films originally hold. Poly-Si films formed from EB-evaporated a-Si films would contribute to the realization of high-efficiency thin-film poly-Si solar cells because of large and oriented grains.

Characterization of poly-Si thin-film solar cell functions and parameters with IR optical interaction techniques

Three different optical interaction techniques have been employed to characterise the electrical and material parameters of polycrystalline silicon (poly-Si) thin-film solar cells with an interdigitated mesa structure. First, Light Beam Induced Current (LBIC) in the infrared range was used to locally analyse the light collection properties. Second, electroluminescence in forward bias (EL) yielding information on band to band recombination was performed. Third, electroluminescence in reverse bias (ELR) was utilized to gain information on the intraband relaxation. The EL and ELR measurements were performed using cooled Si-CCD (Silicon-based Charge Coupled Device) and InGaAs (Indium Gallium Arsenide) detectors. The high resolution IR-LBIC measurement equipped with a 1,064 nm wavelength laser has been applied to investigate the grain boundary characteristics in the absorber layer. Additionally, the local electrical characteristics of the absorber layer (diffusion length, doping concentration and built-in potential) have been extracted by performing a bias-dependent IR-LBIC measurement based on a simple theoretical model with the assumption of relatively small diffusion length compared to the absorber layer thickness. The local/spatial distribution of the diffusion length in the absorber layer of the thin-film solar cell has been extracted. Furthermore, the temperature dependence of the photocurrent of thin-film solar cells in a temperature range of −25 to +70 °C has been locally investigated using IR-LBIC. Additionally, the temperature dependence of the reverse bias characteristics of the poly-Si thin-film solar cell is analysed and compared with that of monocrystalline Si solar cell. For the EL and ELR measurements a spectral analysis of the emitted light has been performed. From the EL results material properties like diffusion length and process induced defects have been deduced and insights on the quality of production processes like metallization and etching were gained. The complementary information from the ELR measurements provides access to additional types of defects resulting from generation centres, such as lattice disorder, crystal defects and charged coulomb centers.

Contact resistivity measurements of the buried Si–ZnO:Al interface of polycrystalline silicon thin-film solar cells on ZnO:Al

An experimental method is developed for contact resistivity measurements of a buried interface in polycrystalline silicon (poly-Si) thin-film solar cell devices on aluminum doped zinc oxide (ZnO:Al) layers. The solar cell concept comprises a glass substrate covered with a temperature-stable ZnO:Al film as transparent front contact layer, a poly-Si n +/p −/p + cell, as well as a metal back contact. Glass/ZnO:Al/poly-Si/metal test stripe structures are fabricated by photolithographic techniques with the ZnO:Al stripes locally bared by laser ablation. The high-temperature treatments during poly-Si fabrication, e.g. a several hours lasting high-temperature step at 600 °C, are found to have no detrimental impact on the ZnO:Al/Si interface contact resistivity. All measured ρ C values range well below 0.4 Ω cm 2 corresponding to a relative power loss ΔP below 3% for a solar cell with 500 mV open circuit voltage and 30 mA/cm 2 short circuit current density. By inclusion of a silicon nitride (SiN x) diffusion barrier between ZnO:Al and poly-Si the electrical material quality of the poly-Si absorber can be significantly enhanced. Even in this case, the contact resistivity remains below 0.4 Ω cm 2 if the diffusion barrier has a thickness smaller than 10 nm.

Growth of Large Grain Polycrystalline Silicon Thin Film on Soda-lime Glass at Low Temperature for Solar Cell Applications

ABSTRACTHigh quality polycrystalline silicon (poly-Si) thin film solar cell was successfully fabricated on soda-lime glass substrates by electron beam (Ebeam) evaporation at low processing temperature. The initial poly-Si seed layer (p+-type 0.5 μm thick) was grown via the aluminum induced crystallization (AIC) method at 450 °C. Prominent interdiffusion and Si crystallization have been observed. X-ray diffraction (XRD) shows that (111) is the dominating crystalline orientation. Post annealing at 450 °C for six hours has produced densely packed Si grains with dimension of more than 10 μm in the plane of the film. Non-destructive Raman spectroscopy reveals the remarkable crystalline improvement for samples after thermal treatment. After removing the top diffused Al by chemical means, an absorber layer (p-type) of 0.9 μm thick was subsequently deposited onto the seed layer by Ebeam evaporation at 500 °C. Transmission electron microscopy (TEM) confirmed good homo-epitaxial growth. Without breaking the high vacuum, an n-type amorphous Si (a-Si) layer (0.7 μm thick) was coated onto the absorber layer to form p-n junction. The corresponding I-V characteristics suggest that our low temperature processing technique is applicable for production of poly-Si thin film solar cell on low cost substrates.

Polycrystalline silicon thin films by high-rate electronbeam evaporation for photovoltaic applications – Influence of substrate texture and temperature

Abstract Polycrystalline silicon (poly-Si) thin-films have the potential to overcome the limits of today's silicon thin-film solar cell technology because of the chance to grow high-quality material by low-cost deposition techniques. As poly-Si thin-film fabrication is independent of rather slow and costly PECVD processes, it is possible to switch to physical high-rate deposition methods such as electron-beam (e-beam) evaporation exhibiting a strong cost-saving potential. In this contribution, the challenges and opportunities of two different poly-Si solar cell preparation techniques using e-beam evaporated silicon are investigated: Thermal solid phase crystallization (SPC) of initially amorphous silicon layers, and direct growth of poly-Si films at elevated temperatures >500 °C. For both approaches, attention is given to the interplay between substrate texture, structure of the grown silicon and the final solar cell performance. Poly-Si thin film solar cells with 7.8% conversion efficiency were prepared on a smooth substrate in cooperation with CSG solar, demonstrating the equality of e-beam evaporation as deposition technique compared to PECVD. However since e-beam evaporation leads to a non-conformal deposition of rough surfaces, the implementation of a textured substrate for light trapping is highly desired but still challenging. A different behavior is observed for poly-Si thin films directly grown at higher temperatures. For the best photovoltaic performance, a certain substrate microstructure is even necessary and can be optimized by the use of glass coated by differently textured ZnO films as substrate.

Extraction of local thin-film solar cell parameters by bias-dependent IR-LBIC

A bias-dependent Infrared Light Beam Induced Current (IR-LBIC) analysis was performed on polycrystalline silicon (poly-Si) thin-film solar cells on glass. The local electrical characteristics of the absorber layer can be extracted from a simple theoretical model based on the assumption of the diffusion length smaller than the absorber layer thickness. The depletion depth and spatially resolved doping concentration were obtained from the results. Furthermore, the diffusion length in the absorber layer of the thin-film solar cell can also be extracted spatially. This is possible due to the low absorption coefficient of poly-Si in the IR range.

The influence of base doping density on the performance of evaporated poly-Si thin-film solar cells by solid-phase epitaxy

The phosphorous base doping dependence of evaporated poly-Si thin-film solar cell by aluminium-induced crystallization solid-phase epitaxy (ALICE) has been investigated. It is found that the open-circuit voltage ( V oc ) of the the poly-Si thin-film solar cell increases with the decrease of base doping density due to the defect-rich nature of poly-Si thin-film material and effectiveness of the back surface field. Meanwhile, the short-circuit current ( J sc ) also increases with the decrease of the base doping density as a result of the reduced doping-induced defects. Therefore, the maximum V oc and J sc are simultaneously achieved when the lowest phosphorous base doping density (~ 5.5 × 10 15 cm − 3 ) is applied.

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.

Influences of oxygen contamination on evaporated poly-Si thin-film solar cells by solid-phase epitaxy

The influences of the oxygen contaminations on the crystal quality and performances of the evaporated polycrystalline silicon (poly-Si) thin-film solar cells prepared by solid-phase epitaxy were investigated by applying different deposition rates and base pressures. The experimental results show that although the evaporated poly-Si thin-film solar cell obtained at high base pressures (9.33 × 10 − 5 Pa) and high deposition rate (300 nm/min) has small amount of SiO 2 precipitations, it still shows the similar good material quality and performances as the cell prepared at low base pressure (1.33 × 10 − 6 Pa) and high deposition rate (300 nm/min) with oxygen interstitials. On the other hand, the poly-Si thin-film solar cell deposited at low base pressure (1.33 × 10 − 6 Pa) and low deposition rate (50 nm/min) has large amount of SiO 2 precipitations and resulting worse material quality and hence cell performances. Therefore, the high deposition rate is desirable to maximize the solar cell performance, as well as the throughput. It is a more influential factor than the base pressure.

Growth of polycrystalline silicon on glass for thin-film solar cells

Polycrystalline Si (poly-Si) thin-film solar cells on glass feature the potential to reach high single-junction efficiencies at low costs. However, the preparation is challenging because the process temperatures are limited by the glass to about 600 °C. There are several methods to prepare poly-Si films on glass. So far, the best poly-Si thin-film solar cells on glass have been prepared by solid phase crystallization (SPC) of amorphous Si (a-Si). In this paper, we give an overview on the formation of poly-Si films by both SPC and the aluminum-induced layer exchange (ALILE) process (which is based on aluminum-induced crystallization (AIC) of a-Si). Due to the fact that the utilization of ZnO:Al-coated glass is an attractive option for future poly-Si thin-film solar cells, we discuss the influence of an additional ZnO:Al layer on the formation of poly-Si films. Such an additional ZnO:Al layer leads to enhanced nucleation. Thus, the time necessary to form the poly-Si film (process time) and the resulting grain size of the poly-Si film are reduced.

Large-Grain Polycrystalline Silicon Solar Cell on Epitaxial Thickening of AIC Seed Layer by Hot Wire CVD

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Large-grain polycrystalline silicon (poly-Si) films were prepared on foreign substrates by the epitaxial thickening of seed layers. The seed layers were formed by aluminum-induced crystallization (AIC). Large-grain n-i-p poly-Si solar cells were deposited on epitaxial seeds by hot-wire chemical vapor deposition (HWCVD). Highly (93%) crystalline fractions with a lateral grain size of 5 <formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> and an intrinsic layer were grown without incubation. These techniques were employed to prepare large-grain poly-Si thin-film solar cells. An ITO/n-i-p (HWCVD)/p<formula formulatype="inline"><tex Notation="TeX">$+$</tex></formula> (AIC)/Ti/glass-structured poly-Si thin-film solar cell with an initial efficiency of 5.6% was obtained. </para>

Relationship between average slope of textured substrate and poly-Si thin film solar cells performance

This paper presents a nanoscopic characterisation of a series of textured substrates with a structure of Ag-grid/ZnO/p-i-n/ZnO/Ag/SnO2/glass for poly-Si thin film solar cells performed by atomic force microscopy, in order to find the optimum texture substrate and realise the light trapping. The relationship between average slopes (tanθ) and photovoltaic performance are studied based on the experimental results. The experimental results show that the textured ZnO/Ag/SnO2/glass with tanθ of 0·13 yields the maximum conversion efficiency for Jsc and Voc. Although the poly-Si deposited on highly textured substrate (σ>37 nm) shows excellent light trapping properties, however, in its microstructure shows a crystal deterioration and exhibits n-type characteristic, yielding poor photovoltaic performances.

Analysis of poly-Si thin filmp+-n-n+homojunction solar cell and heterojunction solar cell with and without a thinμc-Si layer at the interface ofa-Si and poly-Si layers

In this study, new possibilities for higher efficiency poly-Si thin film solar cells are investigated using MEDICI TM device simulator. The poly-Si p + - n - n + thin film solar cell with a thin a -Si p + layer is found to have higher efficiency than the homojunction p + - n - n + cell. Further improvement in efficiency of the heterojunction p + - n - n + cell is achieved by introducing a thin μ c-Si layer at the interface of a -Si emitter and poly-Si absorber layers. The μ c-Si layer at the interface is found to reduce the recombination losses at the interface and improved the fill factor and efficiency of the cell. The photovoltaic parameters of the cell and the absorber layer thickness for optimum efficiency are highly influenced by grain size and passivation at the grain boundary.

Polycrystalline silicon thin-film solar cells on glass

Poly-Si thin-film solar cells on glass feature the potential to reach single-junction efficiencies of 15% or even higher at low costs. In this paper innovative approaches are discussed, which could lead to substantial efficiency improvements and significant cost reductions: (i) preparation of large-grained poly-Si films using the ‘seed layer concept’ targeting at high material quality, (ii) utilization of ZnO:Al-coated glass enabling simple contacting and light-trapping schemes, (iii) utilization of high-rate electron-beam evaporation for the absorber deposition offering a high potential for cost reduction.

Solid-phase crystallization of amorphous silicon on ZnO:Al for thin-film solar cells

The suitability of ZnO:Al thin films for polycrystalline silicon (poly-Si) thin-film solar cell fabrication was investigated. The electrical and optical properties of 700 -nm-thick ZnO:Al films on glass were analyzed after typical annealing steps occurring during poly-Si film preparation. If the ZnO:Al layer is covered by a 30 nm thin silicon film, the initial sheet resistance of ZnO:Al drops from 4.2 to 2.2 Ω after 22 h annealing at 600 °C and only slightly increases for a 200 s heat treatment at 900 °C. A thin-film solar cell concept consisting of poly-Si films on ZnO:Al coated glass is introduced. First solar cell results will be presented using absorber layers either prepared by solid-phase crystallization (SPC) or by direct deposition at 600 °C.

Formation of Several-Micrometer-Thick Polycrystalline Silicon Films on Soda Lime Glass by Flash Lamp Annealing

We have succeeded in forming polycrystalline silicon (poly-Si) films with thicknesses of over 4 µm on soda lime glass by flash lamp annealing (FLA) of precursor amorphous Si (a-Si) films deposited by catalytic chemical vapor deposition (Cat-CVD). The insertion of Cr thin films between glass substrates and a-Si significantly improves the adhesion of Si films to the glass substrates, resulting in uniform crystallization of a-Si in 20×20 mm2 area. Several types of substrate, such as quartz substrates, are also used instead of soda lime glass to elucidate the effects of the properties of glass substrates on formation of the poly-Si films. a-Si films tend to be crystallized under lower irradiance than those on quartz glass substrates, which can be described by the lower thermal conductivity and the thermal diffusion length of soda lime glass. Raman spectra of the poly-Si films on soda lime glass show high crystallinity close to unity. The utilization of soda lime glass with poor thermal resistivity is of great importance for the cost-effective mass production of thin-film poly-Si solar cells.

Shunting problems due to sub‐micron pinholes in evaporated solid‐phase crystallised poly‐Si thin‐film solar cells on glass

AbstractRecent progress in the metallisation of poly‐silicon thin‐film solar cells on glass, created by solid phase crystallisation (SPC) of evaporated amorphous silicon (EVA), revealed that shunting through sub‐micron holes (density 100–200 mm−2) in the films causes severe shunting problems when the air‐side metal contact is deposited onto these diodes, by creating effective shunting paths between the two highly doped layers of EVA cells. We present evidence of these pinholes by optical transmission and focussed ion beam (FIB) microscopic images and confirm the point‐like pinhole shunts using lock‐in thermographic images. The latter revealed that the Al rear electrode induces strong ohmic shunts below the grid lines and a high density of weak non‐linear shunts away from the grid lines. Two distinctly different approaches are shown to reduce the shunting problem to a negligible level: (i) to contact only a small fraction of the rear Si surface via a point contacting scheme, whereby the metal layer needs to be thin (&lt;1 µm) and the fractional area coverage small (&lt;5%), and (ii) to deposit line contacts in a bifacial interdigitated scheme, whereby a thick layer of metal is deposited followed by a wet‐chemical etching step that effectively reduces shunting by preferentially etching away the shunting paths. Test devices with an area of 1 cm2 achieve pseudo fill factors (pFF) of above 75% and diode ideality factors of below 1·3, demonstrating that the proposed methods are well suited for the metallisation of the rear surface of EVA solar cells. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Development of a rapid thermal annealing process for polycrystalline silicon thin-film solar cells on glass

In this report, we discuss the influence of rapid thermal annealing (RTA) on the performance of polycrystalline Si (poly-Si) thin-film solar cells on glass where the poly-Si layers are differently prepared. The first part presents a comprehensive study of RTA treatments on poly-Si thin-films made by solid phase crystallization (SPC) (standard material of CSG Solar AG, Thalheim). By varying both plateau temperature (up to 1050 °C) and duration (up to 1000 s) of the annealing profile, we determined the parameters for a maximum open-circuit voltage ( V OC). In addition, we applied our standard plasma hydrogenation treatment in order to passivate the remaining intra-grain defects and grain boundaries by atomic hydrogen resulting in a further increase of V OC. We found, that the preceding RTA treatment increases the effect of hydrogenation already at comparable low RTA temperatures. The effect on hydrogenation increases significantly with RTA temperature. In a second step we investigated the effect of the RTA and hydrogenation on large-grained poly-Si films based on the epitaxial thickening of poly-Si seed layers.