Microcrystalline silicon–germanium alloys for solar cell application: Growth and material properties

Microcrystalline silicon–germanium (μc-Si_1−xGe_x:H) alloy films have been grown by 100-MHz glow-discharge of a SiH₄/GeH₄/H₂ gas mixture. Alloys over a full range of compositions were prepared to gain a comprehensive understanding of their growth and material properties. With increasing GeH₄ concentration in the gas-phase, we observed a preferential Ge incorporation behavior in the solid. Growth rate studies revealed that the Ge incorporation efficiency from source gas to solid is about five times greater than for Si at growth temperature of 200 °C, which accounts for the variation of alloy composition. With increasing Ge incorporation in the solid, on the other hand, we find a monotonic decrease in photoconductivity, followed by an electrical transition from weak n-type to strong p-type conduction at x > 0.7. At x ≈ 0.4, however, we obtained relatively high photoconductivity gains by a factor of 20 and strong infrared response in the solar cell structure. The Ge incorporation behavior and its effect on charge carrier transport are discussed.     

Cross-contamination in single-chamber processes for thin-film silicon solar cells

Boron (B) and phosphorus (P) cross-contamination for single-chamber deposited a-Si:H, μc-Si:H, and a-Si:H/μc-Si:H tandem solar cells has been investigated by studying their impact on the different layers of solar cells. To reduce the B and P cross-contamination into the i-layer and p-layer, respectively, to a tolerable level, for a-Si:H and μc-Si:H cells a 15' evacuation cycle prior to the i-layer deposition is applied. The effect of P cross-contamination into the i-layer is strongly reduced by the p-layer deposition and a 15’ evacuation cycle prior to the i-layer deposition. The p-layer is assumed to cover up or to fix (in form of P-B complexes) most of the P at the chamber walls. This leads to high quality μc-Si:H cells and a-Si:H cells with only slightly reduced performance. Here, a soft-start of the a-Si:H i-layer led to high quality cells, presumably due to reduced P recycling. Further, there is no need to clean the process chamber with, e.g. NF₃, after each p-layer, as applied in many industrial processes. Instead, many cells are deposited without cleaning the process chamber. We established a single-chamber tandem cell process with 15' evacuation cycles prior to the μc-Si:H p-layer and to each i-layer with a cell efficiency of ~ 11.1%.     

Micro-Raman mapping on layers for crystalline silicon thin-film solar cells

Micro-Raman mappings have been used for characterization of our layers system developed for thin-film silicon solar cells. For the cubic SiC barrier layer a preferential orientation of the grains in 〈1 1 1〉 direction normal to the substrate was revealed. A high density of stacking faults resulted in the splitting of transversal optical (TO)-phonon modes, usually only observed in several non-cubic SiC polytypes. Within the silicon layers, which were obtained by zone melting recrystallization (ZMR) and subsequent epitaxial growth, a high residual stress of about 625 MPa was measured near the boundary towards the SiC layer. Outside of this boundary no residual stress could be detected, in spite of commonly found twin boundaries. Thus the main origin of residual stress in the silicon layers is due to the different expansion coefficients of the respective layers, while grain boundaries have no dominant effect.     

Drift-mobility characterization of silicon thin-film solar cells using photocapacitance

We have applied the photocapacitance method to the measurements of hole drift-mobilities in silicon solar cells. We found a simple analysis that yields drift-mobilities even in the presence of anomalously dispersive transport. On one thick sample we measured the hole drift-mobility using both the photocapacitance and the time-of-flight methods; the two methods gave results that were consistent with each other and with the established bandtail multiple-trapping model. We then applied the method to thinner samples that are more characteristic of the conditions in solar modules, but are not generally usable for the time-of-flight method. These samples showed much smaller hole drift-mobilities than expected from the bandtail trapping model. We speculate that the hole drift-mobility has smaller values in regions close to the substrate during deposition than has been reported for thicker samples.     

Influence of crystalline volume fraction on the performance of high mobility microcrystalline silicon thin-film transistors

The influence of the crystalline volume fraction of hydrogenated microcrystalline silicon on the device performance of thin-film transistors fabricated at temperatures below 200 °C was investigated. Transistors employing microcrystalline silicon channel material prepared close to the transition to amorphous growth regime exhibit the highest charge carrier mobilities exceeding 50 cm²/V s. The device parameters like the charge carrier mobility, the threshold voltage and the subthreshold slope will be discussed with respect to the crystalline volume fraction of the intrinsic microcrystalline silicon material.     

Relation between substrate surface morphology and microcrystalline silicon solar cell performance

In the present paper, the structural and electrical performances of microcrystalline silicon (μc-Si:H) single junction solar cells co-deposited on a series of substrates having different surface morphologies varying from V-shaped to U-shaped valleys, are analyzed. Transmission electron microscopy (TEM) is used to quantify the density of cracks within the cells deposited on the various substrates. Standard 1 sun, variable illumination measurements (VIM) and Dark J(V) measurements are performed to evaluate the electrical performances of the devices. A marked increase of the reverse saturation current density (J₀) is observed for increasing crack densities. By introducing a novel equivalent circuit taking into account such cracks as non-linear shunts, the authors are able to relate the magnitude of the decrease of V_oc and FF to the increasing density of cracks.     

Properties of thin-film silicon solar cells at very high irradiance

The focussed beam of a low-power helium–neon laser is used to study accelerated light-induced degradation (Staebler–Wronski effect) and high steady-state photocarrier generation rates in amorphous and microcrystalline silicon thin-film solar cells, at up to 13 MW m^? 2 irradiance. Even at these high power densities, COMSOL® simulations indicate that heat diffusion into the substrate, aided by spreading conduction via the Ag back-contact, restricts the temperature rise to less than 14 °C. Short-circuit current may be measured directly, and the I–V characteristic estimated by taking into account shunting by the inactive part of the cell. The improved resistance to degradation of microcrystalline silicon cells is shown to persist to high irradiance. Computer simulations of an amorphous silicon solar cell are presented that are consistent with measured un-degraded and degraded properties, and offer insight into prevailing defect creation processes and carrier recombination mechanisms.     

Enhanced spectral response by silicon nitride index matching layer in amorphous silicon thin-film solar cells

We employed the low temperature hydrogenated amorphous silicon nitride (a-SiN_x:H) prepared by plasma-enhanced chemical vapor deposition as a refractive index (n) matching layers in a silicon-based thin-film solar cell between glass (n = 1.5) and the transparent conducting oxide (n = 2). By varying the stoichiometry, refractive index and thickness of the a-SiN_x:H layers, we enhanced the spectral response and efficiency of the hydrogenated amorphous silicon thin-film solar cells. The refractive index of a-SiN_x:H was reduced from 2.32 to 1.78. Optimizing the a-SiN_x:H thickness to 80 nm increased the J_SC from 8.3 to 9.8 mA/cm² and the corresponding cell efficiency increased from 4.5 to 5.3%, as compared to the cell without the a-SiN_x:H index-matching layer on planar substrate. The a-SiN_x:H layers with graded refractive indices were effective for enhancing the cell performance.     

Inline deposited thin-film silicon solar cells on imprinted foil using linear PECVD sources

The standard way to improve the light management of thin film solar cells is to introduce a light scattering structure, either on the front window or at the back reflector. Usually, growth conditions of TCO layers are adjusted to get random surface roughness on the front window. In this paper we present an alternative method, which can be applied both on the front window and at the back reflector. It involves imprinting a UV curable coating layer allowing full control on the texture (random or periodic) to fully optimise the light trapping. Light trapping is even more important for microcrystalline Si solar cells. We have fabricated thin film nip Si solar cells with sputtered Ag/ZnO back contacts on embossed barrier layers on steel foil. We show that the UV curable coating is well-suited as imprintable barrier layer between the steel foil and the active layers. For nip a-Si cells we can obtain light trapping, as measured by the short-circuit current, that is almost as good as that of nip a-Si cells made on Asahi U-type glass, covered with a Ag/ZnO back reflector. Furthermore, we show that dynamically processed a-Si nip cells on foil realised efficiencies of over 7%, which are only slightly less than for cells made in a UHV lab-scale cluster tool in static processing. Finally, a-Si/a-Si tandems and μc-Si/a-Si tandems have been fabricated. Initial efficiencies of around 8% on textured barrier layer on steel foil have been achieved.     

Design and application of dielectric distributed Bragg back reflector in thin-film silicon solar cells

A dielectric distributed Bragg reflector (DBR) formed by four pairs of hydrogenated amorphous silicon/silicon nitride layers is used as the back reflector in thin-film silicon solar cells. The DBR was designed to perform in a broad wavelength range with the peak reflectance at 600 nm. The DBR was fabricated at low substrate temperature (172 °C) and applied at the rear side of flat and textured amorphous silicon single-junction solar cells in both superstrate (pin) and substrate (nip) configurations. The spectral response and electrical I–V characteristics were measured. Solar cells with optimized DBR exhibit an enhanced external quantum efficiency in the long wavelength range and the electrical performance is comparable to solar cells having conventional Ag back reflector.     

Improved light absorption in thin-film silicon solar cells by integration of silver nanoparticles

Silver nanoparticles, produced by thermal evaporation and a subsequent annealing treatment, were integrated at the back side of thin-film silicon solar cells. Metallic nanoparticles can lead to (i) a strong enhancement of the electric field in their surrounding when they are irradiated by light and (ii) significant scattering of the light when they have the proper diameter (>100 nm). In this study, we investigated the optical properties of two types of substrates, one with large and well separated ellipsoidal silver nanoparticles (with average lateral size of 300 nm), and the other with silver nanostructures connected to each other. Furthermore, these substrates were used as back reflectors in microcrystalline silicon solar cells in substrate (n–i–p) configuration.     

Comparison of amorphous silicon solar cell performance following light and high-energy electron-beam induced degradation

In this article a comparison is reported between amorphous silicon (a-Si:H) solar cells that have been degraded using light soaking and 1 MeV electron-beam irradiation. Solar cells were degraded in open- and short-circuit condition, with the aim to change the recombination profile in the cell. For light-soaked solar cells a clear difference is found between open- and short-circuit conditions. Under open-circuit condition the solar cells degrade much more, which is explained by a much higher recombination rate under illumination in this case. These recombination events are believed to initiate defect formation. The performance of thin solar cells degrades less, as expected. For solar cells degraded under electron-beam irradiation no difference is found between open- and short-circuit conditions. Therefore we think that during electron-beam irradiation defect creation is not initiated by recombination events, but by energy transfer during collisions. The fill factor of thin solar cells degrades more after electron-beam irradiation. This effect is ascribed to a significant increase of the activation energy of the doped layers after irradiation.     

Influence of textured c-Si surface morphology on the interfacial properties of heterojunction silicon solar cells

For the HIT solar cells, the properties of interface between intrinsic thin film and c-Si are critical for the resulting device. The interfacial properties mainly depend on the surface passivation quality of c-Si, which is found to be affected by the morphology of textured surfaces. In this study, four kinds of textured c-Si substrates are fabricated: large pyramids without chemical polished (CP), large pyramids with CP, small pyramids without CP and small pyramids with CP. We investigated the effects of textured-surface morphology on the passivation of c-Si, the thin layer coverage and the interfacial properties of heterojunction prepared by HWCVD. Minority carrier lifetime measurements show that the wafer with small pyramids leads to better surface passivation than the one with large pyramids. The good coverage and contact between the thin film and the substrate can be achieved and no epitaxial growth occurs on the wafer with small pyramids through the study of TEM. Dark I-V measurements reveal that the heterojunction on wafer with small pyramids and CP has low recombination at the a-Si:H/c-Si interface. Our results indicate that the surface with small pyramids and low surface roughness is beneficial to the performance of HIT solar cells.     

Microstructures of high-growth-rate (up to 8.3 nm/s) microcrystalline silicon photovoltaic layers and their influence on the photovoltaic performance of thin-film solar cells

Microstructures of microcrystalline silicon (μc-Si) deposited at a high-growth-rate have been investigated in order to apply to the photovoltaic i-layer. μc-Si films were prepared by very-high-frequency (100 MHz) plasma-enhanced chemical vapor deposition at 180 °C. High growth rates of 3.3–8.3 nm/s have been achieved utilizing high deposition pressures up to 24 Torr and large input powers. Applying μc-Si to n–i–p junction solar cells, as the optimum result in this experimental series, a conversion efficiency of 6.30% (J_SC: 22.1 mA/cm², V_OC: 0.470 V, and FF: 60.7%) has been achieved employing the i-layer deposited at 8.1 nm/s. Raman scattering and X-ray diffraction measurements revealed the crystalline volume fraction of around 50% with the (2 2 0) crystallographic preferential orientation, respectively. The cross-sectional transmission electron microscope image shows densely columnar structure grown directly on the underlying n-layer. These structural features are basically in good agreement those of low-growth-rate μc-Si used for a high efficiency solar cell as previously reported, implying advantages of the use of high pressures with regard to providing the photovoltaic i-layers. Finally, the implication is discussed from the photovoltaic performance as a function of the crystalline volume fraction of i-layer, and current problems in improving the photovoltaic performance are extracted.     

Two-dimensional modelling and simulation of hydrogenated amorphous silicon p-n-n solar cell

Two-dimensional device modelling for hydrogenated amorphous silicon p⁺-n-n⁺ solar cell has been carried out by using MEDICI^™ device simulator and the influence of absorber layer thickness, doping concentration, and dangling bond density of states in absorber layer on photo parameters are investigated. A strong correlation between n-type doping and dangling bond density in the absorber layer relative to the stability of the a-Si:H solar cell is observed. An increased stabilized efficiency is obtained when n-type dopant concentration in the absorber layer is higher than optimum value for higher initial efficiency. The window layer (p⁺ layer) of the device is designed with a three layered structure of graded doping for higher device performance. This window layer structure in the a-Si:H p⁺-n-n⁺ cell resulted in higher open circuit voltage and fill factor and hence higher efficiency of the cell. The efficiency of the modified amorphous silicon solar cell structure is found to be 12.85%.     

Effect of ex-situ hydrogen passivation on the structural properties of e-beam evaporated amorphous silicon thin film for solar cell applications

Thin-film photovoltaics greatly reduce the semiconductor material content in the finished product, using 150–200 times less material as compared with conventional Si wafer based cells. Electron beam evaporation (e-beam), a non-ultra-high vacuum technique has the potential for being inexpensive, and simpler process for a-Si deposition. It offers specific advantages such as high Si deposition rate (up to 1 μm/min), excellent Si source material usage, avoidance of toxic gases, and simple sample preparation conditions. In this work, we report the growth of a-Si films using e-beam at a growth rate exceeding 30 Å/s (1–5 Å/s for conventional PECVD process). We report the effect of hydrogen passivation on amorphous silicon network and on silicon-bonded hydrogen configuration under ex-situ hydrogenation in hydrogen plasma. The hydrogen concentration and silicon-hydrogen bonding configuration was evaluated using nuclear reaction analysis (NRA) and Fourier transform infrared spectroscopy (FTIR). Hydrogen plasma treatment shows an increase in the monohydride bond concentration with substrate temperature, and is corroborated by our FTIR investigation, in addition to reducing clustered monohydride bonds or polyhydride bonds in a-Si:H film. Raman analysis indicates reduction in silicon bond angle as well as the bond distance, both leading to significant structural improvement in short-range and medium range order in the amorphous phase. Thus, ex-situ hydrogenation clearly demonstrates the possibility of comparable hydrogen passivation in e‐beam evaporated a-Si films with high growth rate. One can easily extrapolate this result to microcrystalline film growth, assuming the structural improvement of the silicon network preceding the microcrystalline nucleation, where ex-situ passivation is most effective. Thus ex-situ hydrogenation opens up new possibilities in minutely tailoring the a:Si film properties especially for solar cell applications.     

Toward the fast deposition of highly crystallized microcrystalline silicon films with low defect density for Si thin-film solar cells

In this study, employing a high-density, low-temperature SiH₄–H₂ mixture microwave plasma, we investigate the influence of source gas supply configuration on deposition rate and structural properties of microcrystalline silicon (μc-Si) films, and demonstrate the plasma parameters for fast deposition of highly crystallized μc-Si films with low defect density. A fast deposition rate of 65 Å/s has been achieved for a SiH₄ concentration of 67% diluted in H₂ with a high Raman crystallinity of X_c > 65% and a low defect density of (1–2) × 10¹⁶ cm⁻³ by adjusting source gas supply configuration and plasma conditions. A sufficient supply of deposition precursors, such as SiH₃, as well as atomic hydrogen H on film growing surface is effective for the high-rate synthesis of highly crystallized μc-Si films, for the reduction in defect density, and for the improvement in film homogeneity and compactability. A preliminary result of p–i–n structure μc-Si thin-film solar cells using the resulting μc-Si films as an intrinsic absorption layer is presented.     

Ellipsometry characterization of a-Si:H layers for thin-film solar cells

In order to determine microscopic structures of hydrogenated amorphous silicon (a-Si:H) layers incorporated in a-Si:H-based thin-film solar cells, the spectroscopic ellipsometry (SE) analysis of a-Si:H layers prepared by plasma-enhanced chemical vapor deposition has been performed. In particular, we have characterized the a-Si:H layers by applying a new dielectric function model that allows the evaluation of the SiH₂ bond densities in a-Si:H networks. This model is based on our finding that the a-Si:H dielectric functions in the visible/ultraviolet region vary systematically with the formation of SiH₂-clustered microvoids. We have applied this model to estimate the SiH₂ content in a-Si:H layers fabricated on glass substrates, on which the characterization of the SiH₂ bonding is generally difficult. The validity of the SE analysis has been confirmed from the direct characterization of the SiH_n local structures using infrared ellipsometry.     

In-situ Raman spectroscopy used to study and control the initial growth phase of microcrystalline absorber layers for thin-film silicon solar cells

The continuous deposition of microcrystalline silicon has been monitored with in-situ Raman spectroscopy. The process and measurement settings were chosen such that one spectrum was taken during approximately 9 nm of layer growth. This allows observing the crystallinity in the initial growth phase of microcrystalline silicon absorber layers. The influence of different p-doped seed layers has been studied. Under constant deposition conditions, an initial decrease in crystallinity was observed over the first tens of nanometers. By profiling the process gas flows during the initial phase it was possible to reduce the amount of amorphous material that was detected during the initial phase of deposition.     

Improving the performance of amorphous and crystalline silicon heterojunction solar cells by monitoring surface passivation

The influence of thermal annealing on the crystalline silicon surface passivating properties of selected amorphous silicon containing layer stacks (including intrinsic and doped films), as well as the correlation with silicon heterojunction solar cell performance has been investigated. All samples have been isochronally annealed for 1 h in an N₂ ambient at temperatures between 150 °C and 300 °C in incremental steps of 15 °C. For intrinsic films and intrinsic/n-type stacks, an improvement in passivation quality is observed up to 255 °C and 270 °C, respectively, and a deterioration at higher temperatures. For intrinsic/n-type a-Si:H layer stacks, a maximum minority carrier lifetime of 13.3 ms at an injection level of 10¹⁵ cm^? 3 has been measured. In contrast, for intrinsic/p-type a-Si:H layer stacks, a deterioration in passivation is observed upon annealing over the whole temperature range. Comparing the lifetime values and trends for the different layer stacks to the performance of the corresponding cells, it is inferred that the intrinsic/p-layer stack is limiting device performance. Furthermore, thermal annealing of p-type layers should be avoided entirely. We therefore propose an adapted processing sequence, leading to a substantial improvement in efficiency to 16.7%, well above the efficiency of 15.8% obtained with the ‘standard’ processing sequence.