Polycrystalline Silicon Thin-film Solar Cells Research Articles

Crystallization behavior of electron-beam-evaporated amorphous silicon films on textured glass substrates by flash lamp annealing

Flash lamp annealing (FLA) is one of the short-duration annealing methods suitable for the crystallization of micrometer-order-thick amorphous silicon (a-Si) films for thin-film polycrystalline silicon (poly-Si) solar cells. The crystallization of electron-beam- (EB-) evaporated a-Si films formed on flat glass substrates by FLA is known to take place through explosive crystallization (EC), by which laterally stretched large crystal grains are formed. In this study, we experimentally investigate the behavior of the FLA-induced crystallization of EB-evaporated a-Si films on glass substrates with intentionally roughed surfaces. When a-Si films on the textured glass substrates receive a FL pulse light consisting of sub-pulses with variable emission frequency, macroscopic stripe patterns with a constant width are observed on the poly-Si surface. This indicates the emergence of EC in EB-evaporated a-Si films also on textured glass substrates, similar to the case of a-Si films on flat substrates. Poly-Si films formed on textured glass substrates have smaller Si crystal grains in the vicinity of the Si/glass interface than the grains near the surface. Moreover, the lateral crystallization velocity of a-Si films on textured glass substrates is smaller compared to that on flat glass substrates. These indicate that the behavior of the EC of EB-evaporated a-Si films is affected by the surface morphology of glass substrates.

Comparing n- and p-type polycrystalline silicon absorbers in thin-film solar cells

We have investigated fine grained polycrystalline silicon thin films grown by direct chemical vapor deposition on oxidized silicon substrates. More specifically, we analyze the influence of the doping type on the properties of this model polycrystalline silicon material. This includes an investigation of defect passivation and benchmarking of minority carrier properties. In our investigation, we use a variety of characterization techniques to probe the properties of the investigated polycrystalline silicon thin films, including Fourier Transform Photoelectron Spectroscopy, Electron Spin Resonance, Conductivity Activation, and Suns-Voc measurements. Amphoteric silicon dangling bond defects are identified as the most prominent defect type present in these layers. They are the primary recombination center in the relatively lowly doped polysilicon thin films at the heart of the current investigation. In contrast with the case of solar cells based on Czochralski silicon or multicrystalline silicon wafers, we conclude that no benefit is found to be associated with the use of n-type dopants over p-type dopants in the active absorber of the investigated polycrystalline silicon thin-film solar cells.

Micro-structural defects in polycrystalline silicon thin-film solar cells on glass by solid-phase crystallisation and laser-induced liquid-phase crystallisation

The microstructural properties of polycrystalline silicon films obtained by either solid-phase crystallisation (SPC) or laser-induced liquid-phase crystallisation (LPC) were investigated by transmission electron microscopy (TEM). In SPC films, the most common intra-grain defects are dislocations with the density as high as 1E10cm−2 determined from cross-sectional weak-beam dark-field images. The highest dislocation density in LPC film is at least two orders of magnitude lower than the SPC film, 1E8cm−2 and typically it is below 1E6cm−2. The most common defect type in LPC films is twin boundaries and other junctions of different coincidence site lattice (CSL) boundaries. Such differences in the material structural properties result in far superior electrical performance of solar cells made of LPC films, such as mobility up to 400cm2V−1s−1, similar to c-Si wafers, and the higher open-circuit voltage up to 585mV.

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.

Periodic nanostructures imprinted on high-temperature stable sol–gel films by ultraviolet-based nanoimprint lithography for photovoltaic and photonic applications

Nanostructured sol–gel films with high-temperature stability are used in the area of electronics, photonics or biomimetic materials as light-trapping architectures in solar cells, displays, waveguides or as superhydrophobic surfaces with a lotus effect. In this work, high-temperature stable 2-μm nanostructured surfaces were prepared by ultraviolet-based nanoimprint lithography using an alkoxysilane binder incorporating modified silica nanoparticles. Material densification during thermal curing and microstructural evolution which are destined for a high structural fidelity of nanostructured films were investigated in relation to precursor chemistry, particle morphology and particle content of the imprint resist. The mechanism for densification and shrinkage of the films was clarified and correlated with the structural fidelity to explain the influence of the geometrical design on the optical properties. A high internal coherence of the microstructure of the nanostructured films results in a critical film thickness of >5μm. The structured glassy layers with high inorganic content show thermal stability up to 800°C and have a high structural fidelity >90% with an axial shrinkage of 16% and a horizontal shrinkage of 1%.This material allows the realization of highly effective light-trapping architectures for polycrystalline silicon thin-film solar cells on glass but also for the preparation of 2D photonic crystals for telecommunication wavelengths.

Polycrystalline silicon thin-film solar cells prepared by layered laser crystallization with 540 mV open circuit voltage

Polycrystalline silicon thin film solar cells on a glass substrate are investigated. The solar cell layer structure was generated by a two-step process in which first a 100–600nm thin seed layer is formed by diode laser crystallization of electron beam evaporated amorphous silicon. In a second step this layer is epitaxially thickened to 2–3.5μm by layered laser crystallization. In this process further amorphous silicon is deposited and in situ repeatedly is irradiated by excimer laser pulses. The polycrystalline layer consists of grains several hundreds of microns long and several tens of microns wide and it contains a p+–p–n+ doping profile. After deposition a rapid thermal annealing and hydrogen passivation steps follow. The back and front contacts are prepared after mesa structuring. The influence of the seed layer thickness on the solar cell performance was investigated. In addition, the absorber contamination due to the background pressure during absorber deposition and its influence on the short circuit current density was investigated. The best parameters reached for various solar cells are 540mV open circuit voltage, 20.3mA/cm2 short circuit current density (without light trapping), 75% fill factor, and 5.2% efficiency.

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.

Efficiency and stability enhancement of laser-crystallized polycrystalline silicon thin-film solar cells by laser firing of the absorber contacts

Polycrystalline silicon thin-film solar cells produced by continuous-wave diode-laser crystallization at the University of New South Wales were recently reported to have reached a conversion efficiency above 10%. One drawback of these cells, however, was that they exhibited efficiency degradation within several hours after the cell fabrication was completed. In this work we show that by applying laser firing to the rear point contacts of the solar cells, it is possible to stabilize and even to enhance the performance of these devices. Our investigation indicates that it is the poor quality of the contact between the aluminum and the silicon absorber that causes the cell degradation and offers an elegant and industrial-compatible process to improve the cell performance. This is the first time that the laser firing process, initially developed for alloying an aluminum layer through a dielectric layer on crystalline silicon wafer solar cells, is being applied to polycrystalline silicon thin-film solar cells.

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.

Technological status of polycrystalline silicon thin-film solar cells on glass

Abstract This short review article gives an overview on the current status of polycrystalline silicon thin-film solar cells on glass and discusses the future perspectives of this technology.

Aluminum-induced crystallization for thin-film polycrystalline silicon solar cells: Achievements and perspective

Thin-film polycrystalline silicon (pc-Si) solar cell technology tries to combine the cost benefit of thin-film technology and the quality potential of crystalline Si technology. For this type of solar cells the challenge is to fabricate high quality coarse-grained (grain size of 1–100 µm) layers on non-silicon substrates in combination with superb light management. One possible material fabrication approach is metal induced crystallization (MIC). Many metals lower the crystallization temperature of a-Si but for photovoltaic (PV) solar cell applications aluminum seems to be the most interesting. Approximately a decade after the suggestion of Nast et al. to use aluminum induced layer exchange for thin-film solar cells, this paper will discuss the aluminum induced crystallization process itself as well as the implementation into workable solar cells. The record aluminum induced crystallization (AIC) solar cell today features an energy conversion efficiency of 8.5%, too low to be competitive with other PV technologies. The main problems and limitations as encountered today will be described. We believe that an energy conversion efficiency of 8.5% is not the limit of AIC solar cells. Different possible solutions to improve the AIC process as well as the resulting solar cells will be discussed together with our vision on the direction and future of AIC solar cell research.

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.

Highest Efficiency Plasmonic Polycrystalline Silicon Thin-Film Solar Cells by Optimization of Plasmonic Nanoparticle Fabrication

Excitation of surface plasmons in metallic nanoparticles is a promising method for increasing the light absorption in solar cells and hence the cell photocurrent. Comprehensive optimization of a nanoparticle fabrication process for enhanced performance of polycrystalline silicon thin-film solar cells is presented. Three factors were studied: the Ag precursor film thickness, annealing temperature and time. The thickness of the precursor film was 10, 14 and 20 nm; annealing temperature was 190, 200, 230 and 260 °C; and annealing time was varied between 20 and 95 min. Performance enhancement due to light-scattering by nanoparticles was calculated by comparing absorption, short-circuit current density and energy conversion efficiency in solar cells with and without nanoparticles formed under different process conditions. Nanoparticles formed from 14-nm-thick Ag precursor film annealed at 230 °C for 53 min result in the highest absorption enhancement in the 700–1,100 nm wavelength range, in the highest enhancement of total short-circuit current density. The highest photocurrent enhancement was 33.5 %, which was achieved by the cell with the highest absorption enhancement in the 700–1,100 nm range. The plasmonic cell efficiency of 5.32 % was achieved without a back reflector and 5.95 % with the back reflector; which is the highest reported efficiency for plasmonic 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.

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.

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.

Compatibility of glass textures with E-beam evaporated polycrystalline silicon thin-film solar cells

For polycrystalline silicon thin films on glass, E-beam evaporation capable of high-rate deposition of amorphous silicon (a-Si) film precursor up to 1 μm/minute is a potentially low-cost solution to replace the main stream a-Si deposition method—plasma enhanced chemical vapour deposition (PECVD). Due to weak absorption of near infrared light and a target of 2 μm Si absorber thickness, glass substrate texturing as a general way of light trapping is vital to make E-beam evaporation commercially viable. As a result, the compatibility of e-beam evaporation with glass textures becomes essential. In this paper, glass textures with feature size ranging from ∼200 nm to ∼1.5 micron and root-mean-square roughness (Rms) ranging from ∼10 nm to 200 nm are prepared and their compatibility with e-beam evaporation is investigated. This work indicates that e-beam evaporation is only compatible with small smooth submicron sized textures, which enhances Jsc by 21 % without degrading Voc of the cells. Such textures improve absorption-based Jsc up to 45 % with only 90 nm SiNx as the antireflection and barrier layer; however, the enhancement degrades to ∼10 % with 100 nm SiOx+90 nm SiNx as the barrier layer. The absorption-based Jsc is abbreviated by Jsc(A), which is deduced by integrating the multiplication product of the measured absorption and the AM1.5G spectrum in the wavelength range 300–1050 nm assuming unity internal quantum efficiency at each wavelength.

Static Large-Area Hydrogenation of Polycrystalline Silicon Thin-Film Solar Cells on Glass Using a Linear Microwave Plasma Source

Hydrogenation of polycrystalline silicon thin-film solar cells on glass is performed to improve the open-circuit voltage Voc of the devices. The hydrogenation process is performed using linear microwave plasma sources that are capable of generating a uniform hydrogen-argon plasma over a large area. The substrate is fixed (i.e., does not move) during the hydrogenation process. The optical emission intensities from the hydrogen-argon plasma are recorded at two wavelengths using an optical emission spectroscopy system and are used to study the impact of several process parameters. The impact of these process parameters on the device's Voc is also studied. We demonstrate that this plasma reactor is able to hydrogenate samples with a size of up to 400 cm2. The uniformity of the hydrogenation process is evaluated by measuring the 1-sun Voc with a suns-Voc tester, giving voltage variations of less than ±3%. The highest average Voc achieved in this study is 465 mV on a 10 cm × 10 cm textured sample and 428 mV on a 20 cm × 20 cm textured sample. In addition, secondary ion mass spectroscopy is used to measure the hydrogen concentration in the poly-Si films, giving an average concentration of about 6 × 1019 cm-3.

Influence of deep defects on device performance of thin-film polycrystalline silicon solar cells

Employing quantitative electron-paramagnetic resonance analysis and numerical simulations, we investigate the performance of thin-film polycrystalline silicon solar cells as a function of defect density. We find that the open-circuit voltage is correlated to the density of defects, which we assign to coordination defects at grain boundaries and in dislocation cores. Numerical device simulations confirm the observed correlation and indicate that the device performance is limited by deep defects in the absorber bulk. Analyzing the defect density as a function of grain size indicates a high concentration of intra-grain defects. For large grains (>2 μm), we find that intra-grain defects dominate over grain boundary defects and limit the solar cell performance.