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%.     

Boron-doped hydrogenated microcrystalline silicon oxide (μc-SiOx:H) for application in thin-film silicon solar cells

We report on the development of p-type μc-SiO_x:H material, in particular the relationship between the deposition parameters and the material properties like band gap, electrical conductivity, and crystalline volume fraction. The material was deposited from gas mixtures of silane, carbon dioxide and hydrogen by RF-PECVD. The gas flows were varied systematically to evaluate their influence on the material properties. An increase of the oxygen content in the material disturbs the crystalline growth. This can be counteracted by appropriate hydrogen dilutions. Materials with a combination of reasonably high conductivity of 4 × 10^? 6 S/cm at a high optical band gap E₀₄ of 2.56 eV and a refractive index of 1.95 are obtained. Applied in single junction μc-Si:H pin solar cells the improved properties of the μc-SiO_x:H p-layers are reflected in higher quantum efficiency in the short wavelength range by 10% compare to cells without adding CO₂ during p-layer deposition.     

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.     

Deposition of phosphorus doped a-Si:H and μc-Si:H using a novel linear RF source

A roll-to-roll PECVD system for thin film silicon solar cells on steel foil has been developed by ECN in collaboration with Roth and Rau AG. It combines MW–PECVD for fast deposition of intrinsic Si and novel linear RF sources, which apply very mild deposition conditions, for the growth of doped Si layers. The RF and MW sources can be easily scaled up to deposition widths of up to 150 cm. Here, we report on n-type doping, achieved by RF–PECVD from a H₂/SiH₄/PH₃ mixture in the reaction chamber. The best n-type a-Si:H layers showed E_act = 0.27 eV and σ_d = 2.7 × 10^?3 S/cm. Also thin layers down to 20 nm were of device quality and were deposited at a rate of 0.4 Å/s. Furthermore, n-type μc-Si:H layers with thicknesses of 150 nm, with E_act = 0.034 eV and σ_d = 2 S/cm were grown. Good quality n-type μc-Si:H layers can be made for layer thicknesses down to 50 nm at a rate of 0.15 Å/s. To conclude, the novel RF source is well-suited for the growth of n-doped a-Si:H and μc-Si:H layers for roll-to-roll solar cell production.     

Fabrication and optimization of single-junction nc-Si:H n–i–p solar cells using Si:H phase diagram concepts developed by real time spectroscopic ellipsometry

In this study, we have developed and applied deposition phase diagrams in the plane of the bulk layer thickness d_b and the H₂-dilution ratio R = [H₂]/[Si₂H₆] for Si:H materials deposited by 70 MHz VHF PECVD from [H₂] + [Si₂H₆] mixed gases on c-Si/(native-oxide)/n-layer substrates. To establish the phase diagrams, series of Si:H depositions having different R values over the range of 60–150 were measured in real time using a rotating-compensator multichannel ellipsometer. Using phase diagram concepts for guidance, we have fabricated high efficiency single-junction nc-Si:H n–i–p solar cells with ～3 Å/s intrinsic layers using the VHF PECVD process. We have found that the nc-Si:H solar cells with the best performance are obtained by incorporating i-layers deposited in the single-phase nanocrystalline silicon regime near the transition boundary to mixed-phase (a + nc)-Si:H. Applying insights from real time spectroscopic ellipsometry moreover, we have investigated in detail the effects of the phase of the underlying n-layer on the phase evolution of the overdeposited Si:H i-layer and on the overall device performance. With the strategy developed here, a stabilized efficiency of η = 9.46% (V_oc = 0.516 V, J_sc = 24.65 mA/cm², FF = 0.744) has been achieved for nc-Si:H solar cells (0.25 cm² in active area) fabricated with an i-layer deposition rate of ～2.2 Å/s.     

Dynamics of low temperature PECVD growth of microcrystalline silicon thin films: Impact of substrate surface treatments

Microcrystalline silicon (μc-Si) films have been deposited on polyimide, Corning glass and c-Si(0 0 1) by rf plasma-enhanced chemical vapour deposition (PECVD) using both SiF₄–H₂ and SiH₄–H₂ plasmas. The effect of substrate pre-treatment using SiF₄–He and H₂ plasmas on the nucleation of crystallites is investigated. Real-time laser reflectance interferometry monitoring (LRI) revealed the existence of a ‘crystalline seeding time’ that strongly impacts on the crystallite nucleation, on the structural quality of the substrate/μc-Si interface and on film microstructure. It is found that SiF₄–He pre-treatment of substrates is effective in suppressing porous and amorphous interface layer at the early nucleation stage of crystallites, resulting in direct deposition of μc-Si films also on polyimide at the temperature of 120 °C.     

Large grain μc-Si:H films deposited at low temperature: Growth process and electronic properties

We have studied structural and electronic properties of μc-Si:H films deposited from SiH₄ + H₂ and SiH₄ + H₂ + Ar gas mixtures. The use of Ar containing gas mixtures for depositions allows us to increase deposition rate by a factor of two and to obtain films with an important fraction of large grains in comparison with SiH₄ + H₂ gas mixtures. Electronic properties of fully crystallized films become more intrinsic with the increase of large grain fraction. Deposition of highly p- and n-doped μc-Si:H layers from the dopant/SiH₄ + H₂ gas mixture at a temperature of 175 °C is possible without any remarkable changes in crystallinity in comparison with undoped films deposited with the same discharge conditions.     

Impurities in thin-film silicon: Influence on material properties and solar cell performance

The influence of oxygen and nitrogen impurities on the material properties of a-Si:H and μc-Si:H films and on the corresponding solar cell performances was studied. For intentional contamination of the i-layer the impurities were inserted by two contamination sources: (i) directly into the plasma through a leak at the chamber wall or (ii) into the gas supply line. The critical oxygen and nitrogen concentrations for silicon solar cells were determined as the lowest concentration of these impurities in the i-layer causing a deterioration of the cell performance. Similar critical concentrations for a-Si:H and μc-Si:H cells in the range of 4–6 × 10¹⁸ cm^? 3 for nitrogen and 1–5 × 10¹⁹ cm^? 3 for oxygen by applying a chamber leak are observed. Similar increase of conductivity with increasing impurity concentration in the a-Si:H and μc-Si:H films is found. A more detailed study shows that the critical oxygen concentration depends on the contamination source and the deposition parameters. For a-Si:H cells, the application of the gas pipe leak leads to an increased critical oxygen concentration to 2 × 10²⁰ cm^? 3. Such an effect was not observed for nitrogen. For μc-Si:H, a new deposition regime with reduced discharge power was found where the application of the gas pipe leak can also result in an increase of the oxygen concentration to 1 × 10²⁰ cm^? 3.     

Argon-plasma-induced growth of crystalline grains in microcrystalline silicon: Formation mechanism of grains

A thick (∼300 nm) microcrystalline silicon (μc-Si:H) film with a low crystalline volume fraction (∼24%) and a columnar grain size of about 100 nm was exposed to an argon plasma at a substrate temperature of 220 °C after deposition. It is shown that argon plasma treatment significantly enhances film-crystallinity throughout the μc-Si:H layer: over a factor of 2 in crystalline fraction and by a factor of 3 in columnar grain size after a 90-min argon treatment. Based on these experimental results, it is proposed that crystallization of μc-Si:H is likely mediated by the energy transferred from energetic argon atoms.     

Fractional composition of large crystallite grains: A unique microstructural parameter to explain conduction behavior in single phase undoped microcrystalline silicon

We have studied the dark conductivity of a broad microstructural range of plasma deposited single phase undoped microcrystalline silicon (μc-Si:H) films in a wide temperature range (15–450 K) to identify the possible transport mechanisms and the interrelationship between film microstructure and electrical transport behavior. Different conduction behaviors seen in films with different microstructures are explained in the context of underlying transport mechanisms and microstructural features, for above and below room temperature measurements. Our microstructural studies have shown that different ranges of the percentage volume fraction of the constituent large crystallite grains (F_cl) of the μc-Si:H films correspond to characteristically different and specific microstructures, irrespective of deposition conditions and thicknesses. Our electrical transport studies demonstrate that each type of μc-Si:H material having a different range of F_cl shows different electrical transport behaviors.     

Light-induced changes in hydrogenated amorphous silicon solar cells deposited at the edge of crystallinity

A series of hydrogenated amorphous silicon (a-Si:H) films were deposited in the transition region from amorphous to nanocrystalline phases by changing hydrogen dilution ratio R, deposition gas pressure, and RF power. Single junction a-Si:H solar cells were made using these materials as the intrinsic layers in the structure of n–i–p type on ZnO/Ag/stainless steel substrates. Light-induced degradations in the photovoltaic parameters were characterized on these cells after 1 Sun solar illumination for 150 h. The stabilized efficiencies were compared in conjunction with the structures in the intrinsic layers, which were revealed by high resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrometry (FTIR). It was found that the solar cells incorporated protocrystalline intrinsic layer as the i-layer give a better initial efficiency, while solar cells made from nanostructured i-layers have a better stability of ～7% degradation against light soaking, as a result, both have nearly the same final stabilized efficiency. The best device stabilized efficiency reaches ～10.2% (0.25 cm², AM1.5G) for the intrinsic layer deposited at a high pressure of 2 Torr.     

Low concentrator hetero-junction microcrystalline silicon solar cells

Multi-junction silicon-based thin-film solar cells are attractive materials for further cost-reduction and high efficiency. Meanwhile, it is also well considered that a concentrator solar cell is another alternative approach to enhance the conversion efficiency. In concentrator solar cells, the photocurrent linearly increases with the concentration ratio of incident light. At the same time, the open-circuit voltage (V_oc) of solar cells increases logarithmically with the photocurrent. This leads to an increase in efficiency with increasing sunlight intensity.We proposed a novel hetero-junction structure microcrystalline silicon (μc-Si:H) solar cell structure using wide-gap microcrystalline silicon oxide (μc-Si_1 ? xO_x:H) as p-layer and it has some advantages over conventional homo-junction μc-Si:H solar cells under low concentrations. It was observed that wide-gap doped layers can reduce carrier recombination rate especially in p-layer and at the p/i interface and V_oc enhancement with increasing light intensity improves as the band gap of p-layer is increased. Our best solar cell has efficiencies of 9.2% at 1 sun and 10.4% at 11.8 suns. We also investigated the degradation behavior of hetero-junction μc-Si:H solar cells. The degradation in efficiency for this type of solar cell was less than 6%. Therefore, hetero-junction μc-Si:H solar cell is the promising alternative for low-light concentration.     

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.     

Comparison of growth methods for Si/SiO2 nanostructures as nanodot hetero-emitters for photovoltaic applications

Two different growth mechanisms are compared for the fabrication of Si/SiO₂ nanostructures on crystalline silicon (c-Si) to be used as hetero-emitter in high-efficiency solar cells: (1) The decomposition of substoichiometric amorphous SiO_x (a-SiO_x) films with 0 < x < 1.3 and (2) the dewetting of thin amorphous silicon (a-Si) layers.The grown layers are investigated with regard to their structural properties, their passivation quality for c-Si wafer substrates and their electrical properties in order to evaluate their suitability as a nanodot hetero-emitter. While by layer decomposition, no passivating nanodots could be formed, the dewetting process allows fabricating nanodot passivation layers at temperatures as low as 600 °C. The series resistance through Ag/[Si-nanodots in SiO₂]/c-Si/Al structures for dewetting is similar to nanostructured silicon rich SiO_x films. Still, a nanodot hetero-emitter which exhibits both a satisfying passivation of the substrate and induces a high band bending by doping at the same time could not be fabricated yet.     

Properties of n-type hydrogenated nanocrystalline cubic silicon carbide films deposited by VHF-PECVD at a low substrate temperature

n-Type hydrogenated nanocrystalline cubic silicon carbide (nc-3C–SiC:H) films have been deposited by very high-frequency plasma-enhanced chemical vapor deposition at a low substrate temperature of about 360 °C to apply this material to the window layer of heterojunction crystalline silicon (HJ-c-Si) solar cells. We investigated the effect of in situ doping on deposition rate, crystalline volume fraction and dark conductivity to optimize properties of the material. We also fabricated HJ-c-Si solar cells with a n-type nc-3C–SiC:H window layer. The solar cells shows high internal quantum efficiency of 0.90 at a wavelength of 400 nm, indicating that n-type nc-3C–SiC:H deposited by VHF-PECVD is a promising candidate of the window layer of HJ-c-Si solar cells.     

Nanocrystalline silicon TFT process using silane diluted in argon–hydrogen mixtures

We have investigated the effect of Ar dilution on the deposition process of intrinsic nc-Si:H (hydrogenated nanocrystalline silicon) thin films used as active layers of top-gate TFTs, in order to improve the TFTs performances. The nc-Si:H films were deposited by plasma enhanced chemical vapor deposition (PECVD) at low temperature (165 °C) and the related TFTs were fabricated with a maximum process temperature of 200 °C. During the nc-Si:H films deposition, the SiH₄ fraction and the total flow of the diluting gases Ar + H₂ mixture was kept constant, H₂ being substituted by Ar. We have pointed out the active role played by the metastable states of excited Ar atoms in both the dissociation of SiH₄ and H₂ by quenching reactions in the plasma. The role of the atomic hydrogen during the film deposition seems to be promoted by the addition of argon into the discharge, leading to an increase of the deposition rate by a factor of about three and an enhancement of the crystalline quality of the nc-Si:H films. This effect is maximized when the Ar fraction in the Ar + H₂ gases mixture reaches 50%. The evolution with Ar addition of the carriers mobility of the related TFTs is closely connected to the evolution of the crystalline fraction of the intrinsic nc-Si:H film. Mobilities values as high as 8 cm² V^?1 s^?1 are obtained at the Ar fraction of 50%. For higher Ar fractions, the fall of the mobility comes with a degradation of the I_D–V_G transfer characteristics of the processed TFTs due to a degradation of the nc-Si:H films quality. OES measurements show that the evolution of the H_α intensity is closely connected to the evolution of the deposition rate, intrinsic films crystalline fraction and TFTs mobility, providing an interesting tool to monitor the TFTs performances.     

Thin film silicon modules on plastic superstrates

The aim of this research is to fabricate high efficiency a-Si/μc-Si tandem solar cell modules on flexible (polymer) superstrates using the Helianthos concept. As a first step we began by depositing the top cell which contains an amorphous silicon (a-Si:H) i-layer of ～350 nm made by VHF PECVD at 50 MHz in a high vacuum multichamber system called ASTER, with hydrogen to silane gas flow ratio of 1:1. Such amorphous cells on-foil showed an initial active area (0.912 cm²) efficiency of 7.69% (V_oc = 0.834 V, FF = 0.71). These cells were light soaked with white light at a controlled temperature of 50 °C. The efficiency degradation was predominantly due to degradation of FF that amounted to only 11% after 1000 h of light soaking. The cell-on-foil data prove that thin film silicon modules of high stability on cheap plastics can be made at a reasonable efficiency within 30 min of deposition time. A minimodule of 8 × 7.5 cm² area (consisting of 8 cells interconnected in series) with the same single junction a-Si:H p–i–n structure had an initial efficiency of 6.7% (V_oc = 6.32 V, FF = 0.65).     

A narrow process window for the preparation of polytypes of microcrystalline silicon carbide thin films by hot-wire CVD method

Effects of deposition conditions on the structure of microcrystalline silicon carbide (μc-SiC) films prepared by hot-wire chemical vapor deposition (hot-wire CVD) method have been investigated. It is found from X-ray diffraction patterns of the film that a diffraction peak from crystallites from hexagonal polytypes of SiC is observed in addition to those of 3 C-SiC crystallites. This result is obtained in the film under a narrow deposition conditions of SiH₃CH₃ gas pressure of 8 Pa, the H₂ gas pressure of 80–300 Pa and the total gas pressure of 40–300 Pa under fixed substrate and filament temperatures employed in this study. Furthermore, the grain size of hexagonal crystallites (about 20 nm) on c-Si substrates becomes larger than that of 3 C-SiC crystallites (about 10 nm) for the films deposited under the total gas pressure of 36–88 Pa. The fact that microcrystalline hexagonal SiC can be deposited under limited deposition conditions could be interpreted in the context of a result for c-SiC polytypes prepared by thermal CVD method.     

Effect of tellurium deposition rate on the properties of Cu–In–Te based thin films and solar cells

To investigate the effects of tellurium (Te) deposition rate on the properties of Cu–In–Te based thin films (Cu/In=0.30–0.31), the films were grown on both bare and Mo-coated soda-lime glass substrates at 200 °C by co-evaporation using a molecular beam epitaxy system. The microstructural properties were examined by means of scanning electron microscopy and X-ray diffraction. The crystalline quality of the films was improved with increase in the deposition rate of Te, and exhibited a single CuIn₃Te₅ phase with a highly preferred (1 1 2) orientation. Te-deficient film (Te/(Cu+In)=1.07) grown with a low Te deposition rate showed a narrow bandgap of 0.99 eV at room temperature. The solar cell performance was affected by the deposition rate of Te. The best solar cell fabricated using CuIn₃Te₅ thin films grown with the highest deposition rate of Te (2.6 nm/s) yielded a total area (0.50 cm²) efficiency of 4.4% (V_oc=309 mV, J_sc=28.0 mA/cm², and FF=0.509) without light soaking.     

Effect of hot-wire passivation on the properties of hydrogenated microcrystalline silicon films to reduce post-deposition oxidation

Hydrogenated microcrystalline silicon (μc-Si:H) films have a large number of grain boundaries that oxidize after deposition, leading to deterioration of device performance. In this study, post-treatment of μc-Si:H thin films was carried out with methane-related radicals generated by a hot wire. The effect of the hot-wire passivation on the properties of the μc-Si:H thin films was investigated using Fourier-transform infrared (FT-IR) transmission spectroscopy. Through post-treatment, hydrogen on the silicon-crystallite surface was substituted with hydrocarbon. Further, an increase in filament temperature (T_ft) was found to enhance passivation. For films treated at T_ft above 1700 °C, post-oxidation and nitridation hardly occurred, whereas films treated at T_ft below 1400 °C were oxidized and nitrided even after post-treatment.