poly-Si Thin-film Solar Cells Research Articles

Investigation of Interdigitated Metallization Patterns for Polycrystalline Silicon Thin-Film Solar Cells on Glass

In this study, we evaluate the impact of interdigitated metallization patterns on the 1-sun performance of poly-Si thin-film solar cells on glass. We implement a model of the solar cell with an interdigitated metallization pattern in the simulation software Silvaco Atlas. Simulation and experimental results show consistently that the dominant factor for the performance of the metallization pattern is its impact on the current generation capability of the solar cell. For this reason, we study the effect of a variation of the emitter finger pitch, rear contact size, and carrier lifetime on current generation. For a pitch between 500 and 600 μm, the solar cell efficiency varies by less than 0.2% absolute. We also find that the optimum emitter finger pitch does not depend strongly on the charge carrier lifetime in the absorber layer.

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.

Integration of a 2-D Periodic Nanopattern Into Thin-Film Polycrystalline Silicon Solar Cells by Nanoimprint Lithography

The integration of two-dimensional (2D) periodic nanopattern defined by nanoimprint lithography and dry etching into aluminum induced crystallization (AIC) based polycrystalline silicon (Poly-Si) thin film solar cells is investigated experimentally. Compared to the unpatterned cell an increase of 6% in the light absorption has been achieved thanks to the nanopattern which, in turn, increased the short circuit current from 20.6 mA/cm2 to 23.8 mA/cm2. The efficiency, on the other hand, has limitedly increased from 6.4% to 6.7%. We show using the transfer length method (TLM) that the surface topography modification caused by the nanopattern has increased the sheet resistance of the antireflection coating (ARC) layer as well as the contact resistance between the ARC layer and the emitter front contacts. This, in turn, resulted in increased series resistance of the nanopatterned cell which has translated into a decreased fill factor, explaining the limited increase in efficiency.

Epitaxially grown polycrystalline silicon thin-film solar cells on solid-phase crystallised seed layers

This paper presents the fabrication of poly-Si thin film solar cells on glass substrates using seed layer approach. The solid-phase crystallised P-doped seed layer is not only used as the crystalline template for the epitaxial growth but also as the emitter for the solar cell structure. This paper investigates two important factors, surface cleaning and intragrain defects elimination for the seed layer, which can greatly influence the epitaxial grown solar cell performance. Shorter incubation and crystallisation time is observed using a simplified RCA cleaning than the other two wet chemical cleaning methods, indicating a cleaner seed layer surface is achieved. Cross sectional transmission microscope images confirm a crystallographic transferal of information from the simplified RCA cleaned seed layer into the epi-layer. RTA for the SPC seed layer can effectively eliminate the intragrain defects in the seed layer and improve structural quality of both of the seed layer and the epi-layer. Consequently, epitaxial grown poly-Si solar cell on the RTA treated seed layer shows better solar cell efficiency, Voc and Jsc than the one on the seed layer without RTA treatment.

Modeling the Performance of Biaxially-Textured Silicon Solar Cells

ABSTRACTGrain boundaries (GBs) in polycrystalline silicon (poly-Si) thin film solar cells are frequently found to be detrimental for device performance. Biaxiallytextured silicon with grains that are well-aligned in-plane and out-of-plane can possess fewer GB defects. In this work, we use TCAD Sentaurus device simulator and known experimental work to investigate and quantify the potential performance gains of biaxially-textured silicon. Simulation shows there can be performance gain from well-aligned grains when GB defects dominate carrier recombination or when grains are small. On the other hand, when intra-grain defects dominate recombination and grains are large, well-aligned grains do not lead to much performance gain. Another important result from our simulation is when intra-grain and GB defects are few, Jsc is almost independent of grain size while Voc drops with decreasing grain size.

Enhanced light trapping in polycrystalline silicon thin-film solar cells using plasma-etched submicron textures

In this work a highly scattering rear Si surface texture (RST) is realized by plasma etching of polycrystalline silicon (poly-Si) thin-film solar cells on glass. The resulting RST shows reflection haze values of more than 95% at the Si–air interface. The average feature size of the texture is around 200nm. We use a model based on the scalar scattering theory to calculate the scattering properties of the textured surface. We also use a commercial thin-film solar cell simulator to evaluate the light trapping and current enhancement induced by the texture. Combining this submicron RST with a micrometer-scale glass texture can produce a multi-scale rear Si surface texture. Assuming a 1900nm thick poly-Si solar cell on glass with a high-quality back surface reflector (silicon dioxide/silver stack), the calculated photon density absorbed in the poly-Si solar cell with the multi-scale rear Si surface texture corresponds to a 1-sun short-circuit current density (jsc) of 31.1mA/cm2, which is 1mA/cm2 more than the calculated jsc of a poly-Si solar cell with the same thickness on textured glass but without RST. The calculated current densities do not fu lly take current loss due to parasitic absorption into consideration, hence are slightly overestimated.

Impact of the n+ emitter layer on the structural and electrical properties of p-type polycrystalline silicon thin-film solar cells

The effect of the phosphine (PH3) flow rate on the doping profile, in particular the peak doping concentration of the n+ emitter layer, of solid phase crystallised polycrystalline silicon thin-film solar cells on glass is investigated by electrochemical capacitance-voltage profiling. The peak n+ layer doping is found to increase with increasing PH3 gas flow, resulting in a shift of the p-n junction location towards the centre of the diode. The impact of the PH3 flow rate on the crystal quality of the poly-Si films is analysed using ultraviolet (UV) reflectance and UV/visible Raman spectroscopy. The impact of the PH3 flow rate on the efficiency of poly-Si thin-film solar cells is investigated using electrical measurements. An improvement in the efficiency by 46% and a pseudo energy conversion efficiency of 5% was obtained through precise control of the flow rate at an intermediate n+ emitter layer doping concentration of 1.0 × 1019 cm−3. The best fabricated poly-Si thin-film solar cell is also found to have the highest crystal quality factor, based on both Raman and UV reflectance measurements.

A novel silicon nanostructure with effective light trapping for polycrystalline silicon thin film solar cells by means of metal-assisted wet chemical etching

Effective light trapping is required for poly-Si thin film solar cells to compensate for the moderate light absorption. Recent developments of light trapping in the poly-Si cell technology focus on random light scattering in the absorber layer by three means: glass substrate texture, silicon film etch-back texture, and plasmonic nanoparticles. Although silicon nanostructures like porous silicon, silicon nanowires, and silicon nanoholes demonstrate good optical properties, they are seldom considered for poly-Si thin film solar cells due to generally poor electronic properties and difficulty of fabrication. In this paper, the first poly-Si thin film solar cells with a novel silicon nanostructure are fabricated. The Si nanostructure is fabricated by means of metal-assisted wet chemical etching. Silver nanoparticles created by thermal annealing of evaporated silver thin film are used as etching catalyst. Light absorption for both glass-side and air-side illumination are significantly improved. The short-circuit current is enhanced by 21.0% as measured from the glass side and 53.5% from the air side. The open-circuit voltage is improved by 22 mV. It is first demonstration of working poly-Si thin film cells made of such material.

Polycrystalline silicon thin-film solar cells: Status and perspectives

The present article gives a summary of recent technological and scientific developments in the field of polycrystalline silicon (poly-Si) thin-film solar cells on foreign substrates. Cost-effective fabrication methods and cheap substrate materials make poly-Si thin-film solar cells promising candidates for photovoltaics. However, it is still the challenge for research and development to achieve the necessary high electrical material quality known from crystalline Si wafers on glass as a prerequisite to harvest the advantages of thin-film technologies. A wide variety of poly-Si thin-film solar cell approaches has been investigated in the past years, such as thermal solid phase crystallization – the only technology that had already been matured to industrial production so far – the seed layer concept where a large-grained seed layer is epitaxially thickened, direct growth of fine grained material, and liquid phase crystallization methods by laser or electron beam. In the first part of this paper, the status of these four different poly-Si thin-film solar cell concepts is summarized, by comparing the technological fabrication methods, as well as the structural and electrical properties and solar cell performances of the respective materials. In the second part, three promising technologies are described in more detail due to their highly auspicious properties regarding material quality and throughput aspects during fabrication: (1) High-rate electron–beam evaporation of silicon for the low-cost deposition of high-quality material, (2) large-area periodic nano- and micro-structuring of poly-Si by the use of imprinted substrates providing a large absorption enhancement by a factor of six at a wavelength of 900nm, (3) liquid-phase crystallization of silicon thin-film solar cells by electron–beam, yielding an excellent poly-Si material quality reflected by an open-circuit voltage of 582mV which has been achieved only very recently. A successful combination of these three complementary technologies is envisaged to be the basis for a prospective low-cost and highly efficient poly-Si solar cell device.

Simulation of a GaP/ Si Heterojunction Thin Film Solar Cell on Glass Substrate

This work presents a 1D simulation of light J-V characteristics of a GaP/ Si heterojunction thin film solar cell on glass substrate. The device is composed of a GaP/ Si n-p heterojunction, where the p-type Si layer serves as the absorber. A heavily doped p-type Si layer is used between the absorber and the substrate as a Back Surface Field (BSF) layer. The obtained results show slight improvement in shortcircuit current density (Jsc) and efficiency, compared to the present thin film poly-Si solar cells fabricated on glass substrate. At 1 sun, under AM1.5G, the open-circuit voltage (Voc) and the short-circuit current density (Jsc) were obtained as 0.5582 V and 28.42 mA/cm 2 , respectively. With a fill factor of 0.8274, the efficiency was calculated as 13.83%. Afterwards, a number of thin film cell designs were proposed, with corresponding simulation outcomes. Besides this, saturation in short-circuit current density (Jsc) and open-circuit voltage (Voc) with increasing absorber layer thickness was illustrated, in light of relevant simulation results.

Polycrystalline silicon heterojunction thin-film solar cells on glass exhibiting 582 mV open-circuit voltage

In this paper we present silicon heterojunction solar cells based on polycrystalline silicon (poly-Si) prepared by electron-beam induced liquid phase crystallisation. A single sided contact system has been developed to tap the full potential of the heterojunction concept. Open-circuit voltages as high as 582mV demonstrate the high potential of poly-Si absorber material. This is supported by a high pseudo fill factor of 80.5% as determined by Suns-VOCmeasurements. The still moderate best efficiency of 5.7% can be attributed to ohmic losses due to a broken front contact grid and to short circuit current densities not exceeding 16mAcm−2. The latter are largely explained by the missing light-trapping scheme and by recombination losses in the absorber or at the burried SiC/Si interface. The results demonstrate that open-circuit voltages above 600mV are in reach for poly-Si thin-film solar cells on glass, opening up exciting perspectives towards high efficiencies for a new type of potentially low cost silicon based thin-film solar cells.

Impact of rapid thermal annealing temperature on non-metallised polycrystalline silicon thin-film diodes on glass

A rapid thermal annealing (RTA) process is performed to activate the dopants and anneal point defects in planar polycrystalline silicon (poly-Si) thin-film solar cell diodes on glass. The impact of the RTA peak temperature on the open-circuit voltage (Voc), the sheet resistance and the doping profiles of the non-metallised samples was investigated. An annealing temperature of about 1000°C was found to give the highest Voc and the lowest sheet resistance. The doping concentration was measured using the electrochemical capacitance voltage (ECV) method. The doping concentration was found to increase with increasing RTA temperature, before it decreased at high temperature. The junction depth was found to move away from the glass-side with increasing annealing temperature. The sheet resistances were calculated based on the ECV doping profiles and were found to have a trend similar to the experimental values obtained by the four-point probe method. The highest average Voc obtained in this work is 471mV and it corresponds to the lowest sheet resistance.

Chemical speciation at buried interfaces in high-temperature processed polycrystalline silicon thin-film solar cells on ZnO:Al

The combination of polycrystalline silicon (poly-Si) thin films with aluminum doped zinc oxide layers (ZnO:Al) as transparent conductive oxide enables the design of appealing optoelectronic devices at low costs, namely in the field of photovoltaics. The fabrication of both thin-film materials requires high-temperature treatments, which are highly desired for obtaining a high electrical material quality. Annealing procedures are typically applied during crystallization and defect-healing processes for silicon and can boost the carrier mobility and conductivity of ZnO:Al layers. In a combined poly-Si/ZnO:Al layer system, an in-depth knowledge of the interaction of both layers and the control of interface reactions upon thermal treatments is crucial. Therefore, we analyze the influence of rapid thermal treatments up to 1050 °C on solid phase crystallized poly-Si thin-film solar cells on ZnO:Al-coated glass, focusing on chemical interface reactions and modifications of the poly-Si absorber material quality. The presence of a ZnO:Al layer in the solar cell stack was found to limit the poly-Si solar cell performance with open circuit voltages only below 390 mV (compared to 435 mV without ZnO film), even if a silicon nitride (SiN) diffusion barrier was included. A considerable amount of diffused zinc inside the silicon was observed. By grazing-incidence X-ray fluorescence spectrometry, a depth-resolving analysis of the elemental composition close to the poly-Si/(SiN)/ZnO:Al interface was carried out. Temperatures above 1000 °C were found to promote the formation of new chemical compounds within about 10 nm of interface, such as zinc silicates (Zn2SiO4) and aluminium oxide (AlxOy). These results give valuable insights about the temperature-limitations of Si/ZnO thin-film solar cell fabrication and the formation of high-mobility ZnO-layers by thermal anneal.

Optimisation of the Back Surface Reflector for Textured Polycrystalline Si Thin Film Solar Cells

The back surface reflector of a solar cell not only enhances back reflection, but also may contribute to further randomising light. In this paper, three different types of back surface reflectors (dielectric/metal bi-layer, pigmented diffuse reflector and Ag nanoparticles) are investigated and compared with one another to determine the optimal for polycrystalline Si (poly-Si) thin film solar cells grown on textured glass superstrates. The optimal dielectric layer material and thickness of an interposed dielectric layer between Si and metal contact is optimised via WVASE simulation for ∼2μm random textured poly-Si thin film solar cells on glass. Experimentally, the Si thin films are deposited on Aluminium Induced Textured (AIT) glass and those with optical absorption close to Lambertian limited absorption are selected for investigation. The experiment confirms the simulation results and finds the best back reflector configuration— 500 nm MgF2+Ag. Theoretically Ag nanoparticles on the rear surface of a cell should induce surface plasmon effects, scattering light obliquely into the Si film. However, this work indicates that Ag nanoparticles degrade Jsc of textured poly-Si thin film solar cells. The reason why Ag nanoparticles do not work on textured poly-Si thin film solar cells needs further investigation.

Influence of Surface Morphology of Textured Substrate against Poly-Si Thin Film Solar Cells Performance

Since poly-Si is an indirect band gap material and has low optical absorption coefficient in the visible-infrared region, the light trapping in thin film poly-Si layer by using textured substrate is one of the important technical issue for achievement of high short current. Surface texture of a transparent conductive oxide (TCO) layer on a glass substrate as well as SnO2 with a large grain are usually utilized for the light-trapping technique, i.e., path lengths of the incident light in the poly-Si layer are effectively enhanced by the light-scattering at the textured surface. In this paper, a systematic investigation has been carried out concerning the relationship between poly-Si thin film solar cells performance and surface morphology of substrate texture as a function of root mean square roughness of substrate surface, in order to find the optimum textured substrate and realize the light trapping in the poly-Si solar cells. Furthermore, the influence of textured substrate on optical reflectance, poly-Si microstructure and photovoltaic performance are also discussed.

2D modelling of polycrystalline silicon thin film solar cells

The influence of grain boundary (GB) properties on device parameters of polycrystalline silicon (poly-Si) thin film solar cells is investigated by two-dimensional device simulation. A realistic poly-Si thin film model cell composed of antireflection layer, (n + )-type emitter, thick p -type absorber, and (p + )-type back surface field was created. The absorber consists of a low-defect crystalline Si grain with an adjacent highly defective grain boundary layer. The performances of a reference cell without GB, one with n -type and one with p -type GB, respectively, are compared. The doping concentration and defect density at the GB are varied. It is shown that the impact of the grain boundary on the poly-Si cell is twofold: a local potential barrier is created at the GB, and a part of the photogenerated current flows within the GB. Regarding the cell performance, a highly doped n -type GB is less critical in terms of the cell’s short circuit current than a highly doped p -type GB, but more detrimental in terms of the cell’s open circuit voltage and fill factor.

Interdigitated Metallisation Pattern Design based on Lumped Series Resistance Calculations

An interdigitated metallisation scheme was developed and optimised for polycrystalline silicon thin-film solar cells on glass. In the investigated metallisation approach, polycrystalline silicon is partially and selectively removed from the glass substrate in order to contact the emitter of the cell, which is located at the glass-side surface of the solar cell. In this metallisation approach, the design of the metal pattern is of key importance. On the one hand, if the number of fingers on the glass side is too small, a significant part of the photogenerated charge carriers will be lost due to recombination. On the other hand, if active material is removed, the area covered with absorbing material decreases and the number of absorbed photons is reduced. To optimise the metal pattern, a model based on the lumped series resistance of the metal pattern is used to provide analytical solutions for the different resistive losses. The impact from the size of the glass-side contacting area and the number of glass-side fingers on the electrical properties of the polycrystalline silicon thin-film solar cells is investigated based on this model. It is found that for a poly-Si thin-film solar cell with current density of 19 mA/cm2 and pseudo fill factor of 74%, a metal pattern with 500 μm pitch and 15 μm emitter finger width results in the minimum power loss due to metallisation.

A novel silver nanoparticle assisted texture as broadband antireflection coating for solar cell applications

To maximise the degree of light incoupling, therefore improving the short circuit current and efficiency of solar cells, a novel subwavelength (SW) structured ‘moth-eye’ antireflection coating (ARC) is formed on the sun-facing side of the glass superstrate. This moth-eye structure in theory can suppress reflection entirely in the 300–1200nm wavelength range. It is produced by fabricating a mask, followed by reactive ion etch. In this paper, we present a novel etch mask, a random Ag nanoparticle array on glass with feature size ranging from ∼50nm to ∼350nm. The subwavelength features can be adjusted by varying the size, shape and distribution of the Ag nanoparticle array, which in turn is tuned by varying as-deposited Ag thickness. The optimal structures reduce reflection loss by a current equivalent of 1.25mA/cm2 for a 3.3mm borosilicate glass (BSG) superstrate, by 1.39mA/cm2 for 1.1mm BSG and enhances Jsc by 3.4% on average for 2.4μm poly-Si thin-film solar cells on 3.3mm BSG.On Si film side of the glass surface, a preliminary modulated texture investigation, consisting of growing SW structure on micron sized aluminium induced textured (AIT) glass reduced reflection loss by ∼7% compared to original AIT samples.

Shrinking and Growing: Grain Boundary Density Reduction for Efficient Polysilicon Thin-Film Solar Cells

Polycrystalline Si (poly-Si) thin-film, due to its low Si consumption, low substrate cost, and good stability, is an attractive candidate for cost-effective solar cells, but the as-deposited poly-Si typically has a columnar structure with grain boundaries in between, severely limiting the efficiency of the poly-Si. Here, we report a micropillar poly-Si solar cell that utilizes the columnar structure of the as-deposited poly-Si grains. We first formed submicrometer diameter poly-Si pillars, smaller than the initial grain sizes, and used these pillars as the seeds for the subsequent epitaxial growth of Si, which effectively reduces grain boundary density in the final poly-Si crystal. In addition, the vertically aligned micropillar arrays form radial p-n junctions that further mitigate the grain boundary recombination losses by improving the light absorption and charge-carrier collection efficiencies. Consequently, the maximum efficiency of micropillar poly-Si thin-film solar cells is 6.4%, that is, ∼1.5 times higher than that of the planar cells.

Defect annealing processes for polycrystalline silicon thin-film solar cells

► Defect annealing processes were applied to polycrystalline silicon thin films. ► Conventional rapid thermal annealing was compared to novel annealing processes using a laser system and a zone-melting recrystallization setup. ► The open circuit voltages could be enhanced from below 170 mV up to 482 mV. ► Increase in Sun's- V OC values with decrease in FWHM of the TO Raman phonon of crystalline silicon. ► Solar cells were fabricated for I – V -measurements: Best solar cell efficiency of 6.7%. A variety of defect healing methods was analyzed for optimization of polycrystalline silicon (poly-Si) thin-film solar cells on glass. The films were fabricated by solid phase crystallization of amorphous silicon deposited either by plasma enhanced chemical vapor deposition (PECVD) or by electron-beam evaporation (EBE). Three different rapid thermal processing (RTP) set-ups were compared: A conventional rapid thermal annealing oven, a dual wavelength laser annealing system and a movable two sided halogen lamp oven. The two latter processes utilize focused energy input for reducing the thermal load introduced into the glass substrates and thus lead to less deformation and impurity diffusion. Analysis of the structural and electrical properties of the poly-Si thin films was performed by Suns- V OC measurements and Raman spectroscopy. 1 cm 2 cells were prepared for a selection of samples and characterized by I – V -measurements. The poly-Si material quality could be extremely enhanced, resulting in increase of the open circuit voltages from about 100 mV (EBE) and 170 mV (PECVD) in the untreated case up to 480 mV after processing.