Polycrystalline silicon on glass for thin-film solar cells

Although most solar cell modules to date have been based on crystalline or polycrystalline wafers, these may be too material intensive and hence always too expensive to reach the very low costs required for large-scale impact of photovoltaics on the energy scene. Polycrystalline silicon on glass (CSG) solar cell technology was developed to address this difficulty as well as perceived fundamental difficulties with other thin-film technologies. The aim was to combine the advantages of standard silicon wafer-based technology, namely ruggedness, durability, good electronic properties and environmental soundness with the advantages of thin-films, specifically low material use, large monolithic construction and a desirable glass superstrate configuration. The challenge has been to match the different preferred processing temperatures of silicon and glass and to obtain strong solar absorption in notoriously weakly-absorbing silicon of only 1–2 micron thickness. A rugged, durable silicon thin-film technology has been developed with amongst the lowest manufacturing cost of these contenders and confirmed efficiency for small pilot line modules already in the 10–11% energy conversion efficiency range, on the path to 12–13%.     

对硅薄膜型太阳电池的一些思考

在当前迅速发展的绿色环保能源中,硅太阳电池一直占据着首要地位.然而晶体硅太阳电池(单晶硅和多晶硅)由于价格昂贵和材料短缺已不能满足绿色能源快速发展的需要.因此,薄膜型太阳电池已经被视为今后发展的主要方向.非晶硅薄膜太阳电池虽然在性能上还具有不少缺点,但随着薄膜沉积技术的改进以及膜本身质量的不断提高,它在太阳电池领域中仍占有一席之地.多晶硅薄膜太阳电池集晶体硅与非晶硅电池的优点为一体,也受到人们的关注.然而,后起之秀纳米硅薄膜太阳电池,依靠其本身的优越性以及当前纳米技术的进展,将会成为一个新的亮点.     

High-temperature processing of crystalline silicon thin-film solar cells

The crystalline silicon thin-film solar cell combines, in principle, the advantages of crystalline silicon wafer-based solar cells and of thin-film solar cell technologies. Its efficiency potential is the highest of all thin-film cells. In the “high-temperature approach” thin silicon layers are deposited on substrates that withstand processing temperatures higher than 1000 °C. The basic features of the high-temperature crystalline silicon thin-film cell technology are described and some important results are discussed. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Fabrication of large-grain polycrystalline silicon for solar cells

The rapid recrystallization of a-Si films utilizing excimer laser crystallization (ELC) is investigated. The melt duration of liquid silicon (liquid Si) is measured by in-situ time-resolved optical measurements during ELC. The grain size of crystallized poly-Si films are characterized by FE-SEM. The substrate temperature as a function of the longest melt duration of liquid Si for different Si film thicknesses are presented. In-situ timeresolved optical measurements reveal that the longest melt duration of 400-nm-thick a-Si thin films at a substrate temperature of 500 °C is 1714 ns. The diameter of the disk grain as large as 4.7 μm is produced by ELC, which can be applied to solar cells with a high conversion efficiency and reliability.     

Highly crystallized sputtered silicon with textured morphology for thin-film solar cells

A light-trapping structure with textured morphology for thin-film solar cell is demonstrated in this paper. It is fabricated through Al evaporation, and has a root-mean-roughness (Rms) of about 120 nm and lateral width of about 1 μm for single bulge. A Mo layer is introduced to be a barrier layer. Subsequently sputtered amorphous silicon film is 100% crystallized by Cu induced crystallization. Reflectivity of samples with different silicon thickness is studied to reveal the light-trapping efficiency and the reflectivity as low as 10% is obtained with only 840 nm thick silicon film. This is a low-cost structure promising for future thin-film solar cells with high efficiency.     

Application of the chemical vapor-etching in polycrystalline silicon solar cells

This paper reports a study of the application of chemical vapor-etching (CVE) for the rear surface and in the emitter of polycrystalline silicon (pc-Si) solar cells. The CVE technique consists of exposing pc-Si wafers to a mixture of HF/HNO₃. This technique is used to groove the rear surface of the pc-Si wafers for acid vapors rich in HNO₃ (HNO₃/HF > 1/4), in order to realize rear-buried metallic contacts (RBMC) and the formation of a porous silicon (PS) layer on the frontal surface of the cell for volume ratio of HNO₃/HF = 1/7. A significant increase of the spectral response in the long wavelength range was observed when a RBMC is formed. This increase was attributed to the reduction of the effective thickness of the base of the cells and grain boundary Al gettering. The achievement of a PS layer on the emitter of the pc-Si cells passivates the surface and reduces the reflectivity. The dark I-V characteristics of pc-Si cells with emitter-based PS show an important reduction of the reverse current together with an improvement of the rectifying behaviour. The I-V characteristic under AM1.5 illumination shows an enhancement of both short circuit current density and fill factor. The internal quantum efficiency is improved, particularly in the short wavelengths region.     

High-temperature laser annealing for thin film polycrystalline silicon solar cell on glass substrate

Thin film polycrystalline silicon films grown on glass substrate were irradiated with an infrared continuous wave laser for defects annealing and/or dopants activation. The samples were uniformly scanned using an attachment with the laser system. Substrate temperature, scan speed and laser power were varied to find suitable laser annealing conditions. The Raman spectroscopy and Suns-V _oc analysis were carried out to qualify the films quality after laser annealing. A maximum enhancement of the open circuit voltage V _oc of about 100?mV is obtained after laser annealing of as-grown polysilicon structures. A strong correlation was found between the full width half maximum of the Si crystalline peak and V _oc. It is interpreted as due to defects annealing as well as to dopants activation in the absorbing silicon layer. The maximum V _oc reached is 485?mV after laser treatment and plasma hydrogenation, thanks to defects passivation.     

Analysis of the laser ablation processes for thin-film silicon solar cells

A detailed analysis of the monolithical series connection of thin-film silicon modules with ZnO/Ag back contact is presented. In this study, pulsed lasers with wavelengths of 1064 nm and 532 nm were used. The influence of various laser parameters like laser power, pulse overlap, etc., on the different patterning steps is discussed. The focus of this study was on the back contact patterning process. Here (i) the flake formation process during the ablation and (ii) the influence of a NIR-laser source as an alternative approach to the green laser were investigated in detail. The latter would reduce system costs if only one NIR-laser source could be used for all patterning steps.     

Amorphous thin-film solar cells

This report gives an overview of the present status of thin-film solar cells made from hydrogenated amorphous semiconductors (a-Si:H, a-Ge:H) together with new results emphasizing the physics of amorphous materials and devices. Preparation techniques, quality and performances of a-Si:H and a-Ge:H films as well as solar cells with pin structures are reviewed. Dark and light current-voltage I(V) characteristics and spectral response measurements give information about photovoltaic diodes and allow further insights into the physics of these kinds of materials and solar cells. Simulation calculations and device modelling of such solar cells have increased our understanding of amorphous semiconductors and their devices. The introduction of pin/pin stacked and/or tandem structures has improved the long-term stability and conversion efficiency of amorphous solar cells.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

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 J_sc by 6.3% on average, with the optimal J_sc 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)     

锥形二维光子晶体太阳电池数值模拟

采用时域有限差分方法,模拟研究在本征吸收层引入锥形二维光子晶体(2D PC)后,其结构参数变化对单结微晶硅电池各膜层吸收的影响规律.研究表明,2D PC的纵横比(高度与周期之比)对电池本征吸收具有决定性影响.周期小于1μm时,本征吸收随着纵横比的增大先上升后下降,纵横比为1时达到最大值;周期大于1μm时,本征吸收达到最大值的纵横比小于1,且周期越大,实现本征吸收最大化的纵横比越小.当周期为0.5μm,纵横比为1时,锥形2D PC电池的本征吸收达到峰值,短路电流密度为27.8 mA/cm2;与平面结构相比,短路电流密度提升5.8 mA/cm~2,相对增加27%.该研究突破了以往认为绒面陷光效果主要取决于绒面形貌横向特征尺寸的观点,对实验获取最佳的周期或随机绒面陷光结构具有指导意义.     

Investigation on antireflection coatings for Al:ZnO in silicon thin-film solar cells

We numerically investigate the role of antireflection (AR) coatings, composed of SiO₂ and/or ZnO, in suppression of interfacial reflections in the presence of the transparent conducting oxide, Al-doped ZnO (AZO). Three structures are simulated: (a) AR coatings in organic light-emitting diodes (OLEDs) and flat panel displays, (b) AR coating located between the glass and the AZO, and (c) same as the case b except the involvement of another AR coating between the AZO and the amorphous silicon layers. The weighted average transmittance according to the AM1.5 solar spectrum, the photovoltaic transmittance (T_pv), suggests that there is no evident difference between the structures a and b, especially when the layer number of AR coatings is less than three. An effective way to improve T_pv is presented in the structure c, where T_pv is increased from 73.54% to 76.32% with a three-layered AR coating located between AZO and a-Si. It implies that the suppression of interfacial reflections, resulting from the considerable mismatch of refractive indices at the interface of AZO and a-Si, would benefit the efficiency improvement of silicon thin-film solar cells.     

Hysteresis suppression improvement of polycrystalline silicon thin-film transistors by two-step hydrogenation

The effect of two-step hydrogenation, consisting of plasma hydrogenation and annealing in hydrogen, on the hysteresis phenomenon of metal-induced unilaterally crystallized silicon thin-film transistors (MIUC-Si TFTs) was investigated. The large hysteresis level of the conventional MIUC-Si TFTs caused a wide variation of the drain current with the previous gate voltage. As the plasma exposure time increased, the plasma hydrogenation commonly used for stability in poly-Si TFTs was found to increase the hysteresis level of MIUC-Si TFTs after a minimum point. This is because plasma-induced damages correlated with unique defects of MIUC-Si such as metal-related weak bonds, are accompanied by passivation. The following annealing repaired the damages. Consequently the hysteresis level was lower, which resulted in a narrower variation of the drain current.     

n-Type microcrystalline silicon oxide layer and its application to high-performance back reflectors in thin-film silicon solar cells

Light trapping is a key issue in improving the efficiency of thin-film Si solar cells, and using a back reflector material plays a critical role in improving a cell's light-trapping efficiency. In this study, we developed n-type microcrystalline silicon oxide (n-μc-SiO_x) films that are suitable for use as back reflectors in thin-film silicon solar cells. They exhibit a lower refractive index and lower absorption spectra, especially at long wavelengths of >700 nm, than conventional ZnO:Al materials, which are beneficial for this application. The n-μc-SiO_x films were prepared by the PECVD (plasma-enhanced chemical vapor deposition) method and applied to the fabrication of back reflectors in μc-Si:H solar cells. We also characterized the changes in cell performance with respect to the refractive index, conductivity, and thickness of the n-μc-SiO_x back reflectors. The novel back reflector boosts the total current density by up to 3.0% with the help of the enhanced long-wavelength response. It also improves open circuit voltage (V_oc) and fill factor (FF), which may be attributed to the reduced shunt current caused by the anisotropic electrical characteristics of the n-μc-SiO_x layer. Finally, we could achieve a conversion efficiency for the hydrogenated microcrystalline silicon (μc-Si:H) solar cells of up to 9.3% (V_oc: 0.501 V, J_sc: 27.4 mA/cm², FF: 0.68) using the n-μc-SiO_x back reflector.     

New method to measure whole-wavelength transmittance of TCO substrates for thin-film silicon solar cells

High transmittance of transparent conductive oxide (TCO) substrates is one of the most important factors for achieving high efficiency in thin-film silicon solar cells. Immersion (IM) method with CH₂I₂ liquid is widely used for the evaluation of optical properties (transmittance, reflectance and absorption) for TCO substrates with textured surface in order to reduce the scattering at the TCO surface. However, in order to measure transmittance accurately, three problems have been found. (1) CH₂I₂ liquid itself absorbs the light in short wavelength region. (2) The transmittance around the absorption edge of CH₂I₂ liquids is very sensitive to its amount. (3) Scattering cannot be suppressed when the scattering surfaces are more than 2 surfaces (for example, TCO on reactive ion etching (RIE) processed glass). To overcome these problems, we proposed a new setup to measure optical properties of TCO substrates by holding the samples inside the integral sphere. As the results, we have confirmed that their absorption in all wavelength could be measured accurately and the transmittance measured by the new method was well consistent with the external quantum efficiency (EQE) of the fabricated cell while the transmittance measured with conventional IM method showed differently. Therefore, this new method could be a useful tool to evaluate TCO substrates for thin-film silicon solar cells.     

Microstructured design of metallic diffraction gratings for light trapping in thin-film silicon solar cells

In thin-film silicon solar cells, a flexible optical design for light collection is developed that can enhance the optical path length in absorber layer. In this paper, we demonstrate the rectangular, trapezoidal and triangular metallic diffraction gratings as back reflectors. The potential of the structures in the solar cells is investigated by means of numerical simulations, i.e., the Rigorous Coupled Wave Analysis enhanced by the Modal Transmission Line theory. The results show that the 0-order reflection can be suppressed and the energy can be transferred to high diffraction orders by metallic diffraction gratings, especially the triangular metallic diffraction gratings.     

Light trapping by chemically micro-textured glass for crystalline silicon solar cells

Results of the studies of optical properties of anti-reflective glasses with various texturization patterns, which were used as a coating for crystalline silicon solar cells, are presented. It was found that glass samples sorted by their optical transmittance demonstrated the same order as when sorted by their solar-cell short-circuit current enhancement parameter. The value of the latter depended on the parameters of texturization, such as the surface density of inclusions and their profile, and the depth of etching pits. A 2% relative increase of the solar cell efficiency was obtained for the best glass sample for null degree angle of incidence, proving enhanced light trapping properties of the studied glass.     

Preparation of silver-coated glass frit and its application in silicon solar cells

A simple electroless plating process was employed to prepare silver-coated glass frits for solar cells. The surface of the glass frits was modified with polyvinyl-pyrrolidone(PVP) before the electroless plating process. Infrared(IR) spectroscopy,field emission scanning electron microscopy(FESEM), and x-ray diffraction(XRD) were used to characterize the PVP modified glass frits and investigate the mechanism of the modification process. It was found that the PVP molecules adsorbed on the glass frit surface and reduced the silver ions to the silver nanoparticles. Through epitaxial growth, these nanoparticles were uniformly deposited onto the surface of the glass frit. Silicon solar cells with this novel silver coating exhibited a photoelectric conversion efficiency increase of 0.33%. Compared with the electroless plating processes, this method provides a simple route to prepare silver-coated glass frits without introducing impurity ions.