Preparation of microcrystalline silicon solar cells on microcrystalline silicon carbide window layers grown with HWCVD at low temperature

N-type microcrystalline silicon carbide layers prepared by hot-wire chemical vapor deposition were used as window layers for microcrystalline silicon n–i–p solar cells. The microcrystalline silicon intrinsic and p-layers of the solar cells were prepared with plasma-enhanced chemical vapor deposition at a very high frequency. Amorphous silicon incubation layers were observed at the initial stages of the growth of the microcrystalline silicon intrinsic layer under conditions close to the transition from microcrystalline to amorphous silicon growth. ‘Seed layers’ were developed to improve the nucleation and growth of microcrystalline silicon on the microcrystalline silicon carbide layers. Raman scattering measurement demonstrates that an incorporation of a ‘seed layer’ can drastically increase the crystalline volume fraction of the total absorber layer. Accordingly, the solar cell performance is improved. The correlation between the cell performance and the structural property of the absorber layer is discussed. By optimizing the deposition process, a high short-circuit current density of 26.7 mA/cm² was achieved with an absorber layer thickness of 1 μm, which led to a cell efficiency of 9.2%.     

Electron and hole transport in microcrystalline silicon solar cells studied by time-of-flight photocurrent spectroscopy

A photocurrent time-of-flight study of carrier transport in microcrystalline silicon pin diodes prepared over a range of crystallinities is presented. Electron and hole drift mobilities at a crystalline volume fraction >0.35 are typically 3.8 and 1.3 cm²/(V s) respectively at 300 K and a thickness to electric field ratio of 1.8 × 10⁻⁷ cm²/V. A factor of five enhancement in hole mobility over amorphous silicon persists at a crystalline volume fraction as low as 0.1. Current decays are dispersive and mobilities are thermally activated, although detailed field-dependence is still under investigation. Evidence for a sharp fall in the density of states at 0.13 eV above the valence band edge is presented. Similarities in behaviour with certain amorphous and polymorphous silicon samples are identified.     

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.     

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.     

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.     

Alternative designs for nanocrystalline silicon solar cells

We discuss the use of two alternative design techniques for enhancing the performance of nanocrystalline Si solar cells. The first technique involves the use of alternating layers of nanocrystalline and amorphous silicon, where the amorphous silicon layer is used to effectively passivate the grain boundaries in nano Si. We show that the use of amorphous Si layer increases photon absorption and leads to higher quantum efficiency in infrared wavelength. The second design involves enhancing the grains in nano Si by growing at higher temperatures, followed by anneal in a hydrogen plasma to preserve grain boundary passivation. This technique results in significant improvement in infrared quantum efficiency of solar cells while preserving good electronic properties.     

Photocarrier diffusion lengths of high-growth-rate microcrystalline silicon

We report on photocarrier transport of high-growth-rate microcrystalline Si (μc-Si) in conjunction with the lateral size, σ_L, of crystallites’ conglomerate (grain) determined from the atomic force microscope (AFM) topographic images on the basis of fractal concepts. μc-Si films were prepared using very-high-frequency plasma-enhanced chemical vapor deposition at a high deposition rate of 6.8 ± 0.5 nm/s. μc-Si thicknesses, d, were varied from 0.53 μm to 5.6 μm. With an increase in d, σ_L increased from 70 nm to 590 nm. At the same time, the ambipolar diffusion lengths, L_amb, of photocarriers, observed using the steady-state photocarrier grating (SSPG) technique, increased from 50 nm to 420 nm. Log–log plots of L_amb versus d and σ_L versus d were both expressed as a power law with an exponent of 0.9, yielding a simple linear relation between L_amb and σ_L. Moreover, their ratio, L_amb/σ_L, was below unity, implying the intra-grain carrier diffusion. From these results, the role of the grain (column) boundaries for photocarrier diffusion in μc-Si is discussed.     

Density-of-states in microcrystalline silicon from thermally-stimulated conductivity

The technique of thermally-stimulated currents has been applied to extract the density-of-states profile in microcrystalline silicon. Exploiting the experimental parameter space a consistent density-of-states profile emerges with an exponential conduction band tail and a broader deeper distribution. Calibrating the absolute density-of-states profile from other techniques like modulated photoconductivity, steady-state photocarrier grating technique and intensity-dependent photoconductivity allows a determination of the capture coefficient of the probed localized states.     

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.     

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%.     

Influence of boron doping on roughness microcrystalline silicon

We have studied roughness of boron-doped microcrystalline Si (μc-Si) surfaces with an emphasis on the influence of heavy doping. μc-Si films were prepared using plasma-enhanced chemical vapor deposition (PECVD) with different boron concentrations in gas phase from 0% to 2%. Growth-induced roughening of μc-Si surfaces was monitored ex situ using an atomic force microscope (AFM). With an increase in the deposition time, the surface width (rms roughness), w, of undoped μc-Si surface exhibited usual behaviors; first, (a) w increased, (b) slightly dropped, (c) rose again, and then (d) gradually increased. In the case of B-doped μc-Si, w differently behaved; (a) w increased very soon, (b) slightly dropped, (a′) rose again, (b′) slightly dropped again, (c) rose, and finally (d) gradually increased. The quick increase in w indicates that boron doping promotes the nucleation, and the repeated nucleation is responsible for the behavior (a′)–(b′). Additionally, the nucleation density, that was derived using the lateral correlation length of surface heights, monotonically increased with an increase in the boron concentration. The effects of boron doping are discussed with the catalytic effects and the formation of the surface-covering layer.     

Metastable dark and photoconductive properties of microcrystalline silicon

Metastable changes in the dark conductivity of microcrystalline silicon upon heat treatment at different temperatures obey the Meyer–Neldel rule. Dark conductivity variations are accompanied by changes in the photoconductivity or the majority-carrier mobility-lifetime product. The minority-carrier mobility-lifetime product is not affected. The observations can be related to Fermi-level induced changes of the excess carrier lifetimes.     

Scaling properties of growing surfaces of microcrystalline silicon

Growth-induced roughening of microcrystalline Si (μc-Si) surfaces has been studied from the viewpoint of self-similar and fractal structures in conjunction with crystallographic preferential orientations of μc-Si films. Typically, μc-Si films are prepared by plasma enhanced chemical vapor deposition (PECVD) with various preparation conditions including excitation frequency. Irrespective of preparation conditions, self-similarity of the μc-Si surface roughness derived by an atomic force microscope is well characterized in terms of its scaling exponents. Furthermore, the scaling exponents revealed that growth-induced roughness shows different behaviors in accordance with the crystallographic preferential orientations of the μc-Si films. Experimental results of scaling exponents are discussed regarding the origins of surface roughening in comparison to analytical results and numerical simulation results.     

Improvement of amorphous silicon n-i-p solar cells by incorporating double-layer hydrogenated nanocrystalline silicon structure

We develop a double-layer p-type hydrogenated nanocrystalline silicon (p-nc-Si:H) structure consisting of a low hydrogen diluted i/p buffer layer and a high hydrogen diluted p-layer to improve the hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells. The electrical, optical and structural properties of p-nc-Si:H films with different hydrogen dilution ratio (R_H) are investigated. High conductivity, low activation energy and wide band gap are achieved for the thin films. Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the thin films contain nanocrystallites with grain size around 3-5 nm embedded in the amorphous silicon matrix. By inserting a p-nc-Si:H buffer layer at the i/p interface, the overall performance of the solar cell is improved significantly compared to the bufferless cell. The improvement is correlated with the reduction of the density of defect states at the i/p interface.     

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.     

Hole mobilities and the physics of amorphous silicon solar cells

The effects of low hole mobilities in the intrinsic layer of pin solar cells are illustrated using general computer modeling; in these models electron mobilities are assumed to be much larger than hole values. The models reveal that a low hole mobility can be the most important photocarrier transport parameter in determining the output power of the cell, and that the effects of recombination parameters are much weaker. Recent hole drift-mobility measurements in a-Si:H are compared. While hole drift mobilities in intrinsic a-Si:H are now up to tenfold larger than two decades ago, even with recent materials a-Si:H cells are low-mobility cells. Computer modeling of solar cells with parameters that are consistent with drift-mobility measurements give a good account for the published initial power output of cells from United Solar Ovonic Corp.; deep levels (dangling bonds) in the intrinsic layer were not included in this calculation. Light-soaking creates a sufficient density of dangling bonds to lower the power from cells below the mobility limit, but in contemporary a-Si:H solar cells degradation is not large. We discuss the speculation that light-soaking is ‘self-limiting’ in such cells.     

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.     

Recombination and transport in microcrystalline pin solar cells studied with pulsed electrically detected magnetic resonance

An analysis of spin-dependent processes in microcrystalline silicon (μc-Si:H) pin solar cells is presented using pulsed electrically detected magnetic resonance (pEDMR). In this first study it is shown that by modulating the morphology of the n-type contact layer from amorphous to microcrystalline, pronounced changes in the pEDMR spectra may be observed. Due to the fact that pEDMR allows a deconvolution of the spin-dependent signals in time as well as in magnetic field domain, we were able to significantly reduce the complexity of the spectra compared to conventional EDMR. In the samples containing amorphous n-type contact layers we found signals from shallow localized conduction band tail states and phosphorous donor states. Upon replacement of this layer by its microcrystalline counterpart both signals disappeared. Possible spin-dependent transport mechanisms involving paramagnetic states in the various layers are discussed in view of sign and time evolution of the associated pEDMR signals.     

Characterization of microcrystalline silicon by Hall and photo-Hall measurements

Transport properties of microcrystalline silicon are studied by Hall and photo-Hall measurements. The temperature dependence of the mobility shows that there exist potential barriers for majority carrirs to jump over. Illumination increases the majority carrier mobility while the minority carrier mobility turns out to be very small (<0.1 cm²/Vs) compared with the majority carrier mobility (5–30 cm²/Vs).     

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.