Investigation of gap states in phosphorous-doped ultra-thin a-Si:H by near-UV photoelectron spectroscopy

A variant of photoelectron spectroscopy with near-UV light excitation was established and applied to an n-type doping series of ultra-thin a-Si:H layers (layer thickness ～10 nm). Using this technique, the position of the surface Fermi level E_Fs is obtained and the density of recombination active defect states in the a-Si:H band gap down to ～10¹⁵ states/cm³ can be detected. Defect densities are generally about one order of magnitude higher than in the bulk of thicker (several 100 nm) layers, and the minimum achievable distance of E_Fs from the conduction band is ～360 mV for doping with 10⁴ ppm PH₃. The optimum doping for the fabrication of solar cells is almost one order of magnitude lower. This discrepancy may be explained by enhanced recombination at the a-Si:H/c-Si interface at high doping levels, and in addition by an efficient recombination pathway where charge carriers tunnel from c-Si via a-Si:H band tail states into the a-Si:H and subsequently recombine at dangling bond states.     

Electronic states in a-Si:H/c-Si heterostructures

We have investigated PECVD-deposited ultrathin intrinsic a-Si:H layers on c-Si substrates using UV-excited photoemission spectroscopy (hν = 4–8 eV) and surface photovoltage measurements. For samples deposited at 230 °C, the Urbach energy is minimal, the Fermi level closest to midgap and the interface recombination velocity has a minimum. The a-Si:H/c-Si interface density of states is comparable to that of thermally oxidized silicon interfaces. However, the measured a-Si:H dangling bond densities are generally higher than in thick films and not correlated with the Urbach energy. This is ascribed to additional disorder induced by the proximity of the a-Si:H/c-Si interface and H-rich growth in the film/substrate interface region.     

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.     

Recombination and transport through localized states in hydrogenated amorphous and microcrystalline silicon

Electrical transport and recombination mechanisms in hydrogenated amorphous silicon, a-Si:H, are determined by localized band-tail states and deep defects. At low temperatures (T < 100 K) the photoluminescence originates from tunneling recombination between localized band-tail states and the photoconductivity arises from hopping in the band tail. This review describes the present understanding of transport and recombination mechanisms in this low-temperature regime with a focus on two aspects: (i) the kinetics of carrier recombination and the competition between geminate and non-geminate recombination, and (ii) the microscopic identification of recombination paths by magnetic resonance techniques and the proof of excitonic recombination. Inspite of its complex nanocrystalline morphology, hydrogenated microcrystalline silicon, μc-Si:H, behaves in many respects similarly to a-Si:H in that the low-temperature properties are also determined by disorder-induced localized band-tail states.     

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.     

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.     

Study of anomalous behavior of steady state photoconductivity in highly crystallized undoped microcrystalline Si films

The photoconductivity (σ_ph) of highly crystallized dense undoped hydrogenated microcrystalline silicon (μc-Si:H) films was measured as a function of light illumination over a wide temperature range (∼15–325 K). A thermal quenching behavior in σ_ph was observed at ∼240 K. The photoconductivity exponent (γ) was found to be sublinear with γ as low as 0.13. A density of states (DOS) profile having a steep conduction band tail, and valence band tail with two distinct distributions was found to be necessary to understand the electronic transport behavior in the inherently heterogeneous μc-Si:H films.     

Variation of the defect density in a-Si:H and μc-Si:H based solar cells with 2 MeV electron bombardment

Improvement of the performance of solar cells based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon requires understanding of the role of the deep defects – dangling bonds – in the bulk of the intrinsic a-Si:H or μc-Si:H absorber layers. A straightforward way to understand how these defects may affect the performance of the cells is to investigate changes in the device performance upon variation in the defect density.In the present work solar cells with a-Si:H and μc-Si:H absorber layers were exposed to 2 MeV electron bombardment. The performance of the cells after various bombardment doses and annealing steps was evaluated in view of the changes in the defect density of intrinsic layers, measured with ESR on nominally identical absorber layers irradiated in parallel with the cells.The defect density was varied over a range of 2 orders of magnitude. In the solar cells a strong degradation of performance is observed upon irradiation with the biggest effect on the short circuit current density J_SC for both types of absorber layers. In most cases both V_OC and J_SC recover after the final annealing step (at 160 °C) for both types of cells.     

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.     

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.     

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).     

Origin of the Voc enhancement with a p-doped nc-SiOx:H window layer in n-i-p solar cells

The introduction of a nc-SiOx:H material as window layer in single junction a-Si:H n-i-p solar cell leads to a V_oc enhancement of 80 mV compared to a μc-Si:H p-layer. According to numerical modeling of the V_oc, both the higher work function p-layer and the conduction band offset (CBO) at the i/p interface match well with the experimental V_oc increase with the oxygen content. Using the differential temperature method, the built-in voltage (V_bi) of the cells with the two different p-layers is measured to be similar, agreeing well with the CBO model. Thus we attribute the improvement of the V_oc to the reduction of recombination at the i/p interface, as a consequence of the CBO in this region.     

Amorphous silicon diamond based heterojunctions with high rectification ratio

We have fabricated and characterized diamond based heterojunctions composed of homoepitaxial diamond (B-doped film: p type) and hydrogenated amorphous silicon (a-Si:H film: n-type). All devices include an intrinsic amorphous silicon interface (i-a-Si:H). (J–V) characteristics of a-Si:H heterojunctions measured from 300 K to 460 K present a very high rectification ratio (in the range 10⁸–10⁹) and a current density of 10 mA/cm² under 2 V of forward bias. The reverse current up to ? 4 V is below the detection limit in the whole temperature range. The devices present two regimes of operation indicating that more than one mechanism governs the carrier transport. These characteristics are compared with a Schottky barrier diode (SBD) using a tungsten carbide metal on top of the p-type diamond as a Schottky contact. The SBD device exhibits J–V characteristic with an ideality factor n close to one and the heterojunction follows this trend for low bias voltages whereas for bias voltage above 1 V a second regime with larger ideality factors n ~ 3.6 is observed. These results point out the prominent role of transport mechanisms at heterointerface between the a-Si:H layers and the p-type doped diamond which degrades the current injection. The breakdown voltage reached ? 160 V indicating the good quality of the deposited layers.     

Light-induced annealing of hole trap states: A new aspect of light-induced changes in hydrogenated amorphous silicon

Details of light-induced annealing of hole trap state in undoped hydrogenated amorphous silicon (a-Si:H) have been studied; it has been found that prolonged illumination significantly reduces the density of hole trap states in the energy range deeper than 0.5 eV, and subsequent thermal annealing increases the density of hole trap states and restored the sample to the initial state before the illumination. We can speculate, from the experimental results and discussion in this work, that defect conversion processes are taking place during the long exposure to light; Si dangling bonds are generated from the precursors or latent sites which manifested as hole trap states located between 0.5 and 0.7 eV from the top of the valence band.     

Mechanisms controlling primary crystallisation of Ga20Te80 glasses

The kinetic study of the crystallisation process of Ga₂₀Te₈₀ glass from isothermal and continuous heating calorimetric data have been performed applying a recently developed procedure. The kinetic information was complemented with X-ray diffraction measurements. With this scope, three crystallisation patterns, with three-dimensional isotropic growth have been analysed: (i) site saturation and interface controlled growth. (ii) homogeneous nucleation with interface controlled growth and (iii) homogeneous nucleation with two simultaneous modes of crystal growth (interface- and diffusion-controlled). A complex model with two simultaneous modes of three-dimensional isotropic crystal growth with decreasing homogeneous nucleation and soft impingement has been applied for modelling primary crystallisation of the Ga₂₀Te₈₀ glass. The model goes beyond the isokinetic hypothesis when coupling isothermal and continuous heating kinetic data. The apparent activation energy E_a = (2.06 ± 0.03) eV/at obtained for the primary crystallisation of the phase Te is shown to correspond to an activation energy for nucleation E_I = (2.85 ± 0.03) eV/at and an interface controlled activation energy for growth E_u = (1.90 ± 0.03) eV/at at the crystallisation onset.     

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.     

Effects induced by 4.7 eV UV laser irradiation on pure silica core multimode optical fibers investigated by in situ optical absorption measurements

We investigated by in situ optical absorption measurements the effects induced by 4.7 eV UV laser irradiation on pure silica core optical fibers. Laser irradiation with 100 MW cm^? 2 laser intensity generates in the fiber E′ centers which partially decay after irradiation due to their reaction with diffusing H₂. An absorption band peaked at 5.3 eV is observed to grow in the post-irradiation stage with a kinetics anti-correlated to the decay of the 5.8 eV band of the E′ centers. The defect absorbing at 5.3 eV is proposed to be formed by trapping on pre-existing precursors of hydrogen atoms made available by breaking of H₂ on E′. We also show by repeated irradiation experiments that the 5.3 eV-absorbing center is photochemically destroyed by 4.7 eV laser light, and we estimate the cross section of this process. Possible structural models for this defect are discussed.     

Heat treatment of amorphous silicon p-i-n solar cells with high-pressure H2O vapor

We report improvement in characteristics of hydrogenated amorphous silicon (a-Si:H ) p-i-n structured solar cells by high-pressure H₂O vapor heat treatment. a-Si:H p-i-n solar cells were formed on glass substrates coated with textured SnO₂ layer. P-, i-, and n-type a-Si:H layers were subsequently formed by plasma enhanced chemical vapor deposition. Finally an indium-tin-oxide layer was coated on the n-type a-Si:H surface. Heat treatment at 210 °C with 2 × 10⁵ Pa H₂O vapor for 1 h was applied to the a-Si:H p-i-n solar cells. Electrical characteristics were measured when samples were kept in dark and illuminated with light of AM 1.5 at 100 mW/cm². The heat treatment with H₂O vapor increased fill factor (FF) and the conversion efficiency from 0.54 and 7.7% (initial) to 0.57 and 8.4%, respectively. Marked improvement in solar cell characteristics was also observed in the case of a poor a-Si:H p-i-n solar cell. FF and the conversion efficiency were increased from 0.29 and 3.2% (initial) to 0.56 and 7.7%, respectively.     

A novel structured plastic substrate for light confinement in thin film silicon solar cells by a geometric optical effect

We present a novel method to achieve light trapping in thin film silicon solar cells. Unlike the commonly used surface textures, such as Asahi U-type TCO, that rely on light scattering phenomena, we employ embossed periodically arranged micro-pyramidal structures with feature sizes much larger than the wavelength of visible light. Angular resolved transmission of light through these substrates indeed showed diffraction patterns, unlike in the case of Asahi U-type substrates, which show angular resolved scattering. Single junction amorphous silicon (a-Si) solar cells made at 125 °C on the embossed structured polycarbonate (PC) substrates showed an increase in current density by 24% compared to a similar solar cell on a flat substrate. The band gap and thickness of the i-layer made by VHF PECVD are 1.9 eV and 270 nm respectively. A double p-layer (nc-Si:H/a-Si:H) was used to make proper contact with ZnO:Al TCO.Numerical modeling, called DokterDEP was performed to fit the dark and light current–voltage parameters and understand the characteristics of the cell. The output parameters from the modeling suggest that the cells have excellent built-in potential (V_bi). However, a rather high recombination voltage, V_μ, affects the FF and short circuit current density (J_sc) for the cells on Asahi as well as for the cells on PC. A rather high parallel resistance ? 1 MΩ cm² (obtained from the modeling) infers that there is no significant shunt leakage, which is often observed for solar cells made at low temperatures on rough substrates. An efficiency of more than 6% for a cell on PC shows enormous potential of this type of light trapping structures.     

Radiation processes in oxygen-deficient silica glasses: Is ODC(I) a precursor of E′-center?

The accumulation of radiation-induced defects under non-destructive X-ray and destructive cathodoexcitation was studied in pure silica KS-4V glasses possessing an absorption band at 7.6 eV. The correspondence between the existence of this band and the creation of the E′-center by radiation was checked. Detection of induced defects was accomplished by measurement of the luminescence during irradiation, post irradiation afterglow or phosphorescence, induced optical absorption, and thermally stimulated luminescence. In all samples, these observed phenomena associated with charge trapping and recombination on the oxygen-deficient luminescence center. Others centers of luminescence were not significant contributors. In some samples, the intensity of the 7.6 eV absorption band was deliberately increased by treatment in hydrogen at 1200 C for 100 h. The intensity of luminescence in hydrogen-treated samples was smaller because of the known quenching effect of hydrogen on the luminescence of oxygen-deficient centers. The optical absorption method does not reveal an induced absorption band for the E′-center in the hydrogen-free samples with different levels of oxygen deficiency. Therefore, we did not detect the transformation of the defect responsible for the 7.6 eV absorption band or the ODC(I) defect into the E′-center. In the hydrogen-treated sample, the absorption of the E′-center was detected. The E′-centers creation in the hydrogen-treated sample was associated with precursors created by hydrogen treatment (≡Si–O–H and ≡Si–H) in the glass network. The destructive e-beam irradiation reveals an increase with dose of the ODC luminescence intensity in the sample exhibiting a small 7.6 eV band. That means that the corresponding luminescence centers are created. Optical absorption measurements in that case reveal the presence of E′-centers and a broad band at 7.6 eV. A compaction of the irradiated volume was detected. Therefore, we conclude that the E′-center is produced by heavy damage to the glass network or by the presence of precursors.