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.     

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.     

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.     

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.     

Charge collection in amorphous silicon solar cells: Cell analysis and simulation of high-efficiency pin devices

The drift length L_drift = μτE within the i layer of a-Si:H solar cells is a crucial parameter for charge collection and efficiency. It is strongly reduced not only by light-induced reduction of μτ, but also by electric field deformation ΔE by charges near the p–i and i–n interfaces. Here, a simple model is presented to estimate contributions of free carriers, charges trapped in band tails and charged dangling bonds to ΔE. It is shown that the model reproduces correctly trends observed experimentally and by ASA simulations: charged dangling bonds contribute most to ΔE of meta-stable cells. Electrons trapped in the conduction band tail near the i–n interface lead to the strongest field deformation in the initial state, while positively charged dangling bonds near the p–i interface get more important with degradation under AM1.5g spectrum. The measurable parameter V_coll is proposed as an indirect parameter to estimate the electric field, and an experimental technique is presented that could enable the distinction of defects near the p–i and the i–n interfaces.     

Correlation between micro-roughness,surface chemistry,and performance of crystalline Si/amorphous Si:H:Cl hetero-junction solar cells

The correlation between micro-roughness, surface chemistry, and performance of crystalline Si/amorphous Si:H:Cl hetero-junction solar cells is discussed through a deposition study of amorphous Si:H:Cl (a-Si:H:Cl) films by rf plasma-enhanced chemical vapor deposition using a SiH₂Cl₂–H₂ mixture. The degree of H- and Cl-termination on the growing surface determined the degree of micro-roughness at the p-type a-Si:H:Cl/intrinsic a-Si:H:Cl interface and solar cell performance. A higher degree of Cl-termination compared to H-termination was effective to suppress the micro-roughness at the growing surface and oxygen incorporation into the film, as well as chemical reduction of the intrinsic a-Si:H:Cl layer during the underneath p-layer formation. The study showed that a-Si:H:Cl deposited from SiH₂Cl₂ is a potential material for c-Si hetero-junction solar cells with an intrinsic a-Si:H:Cl thin layer.     

Enhanced luminescence from Si quantum dots/SiO2 multilayers by hydrogen annealing

Si quantum dots/SiO₂ multilayers were prepared by annealing a-Si:H/SiO₂ stacked structures at 1100 °C . Photo- and electro-luminescence band around 750 nm can be observed from Si QDs/SiO₂ multilayers due to the recombination of electron-hole pairs in Si QDs/SiO₂ interfaces. The electro-luminescence intensity was obviously enhanced after post hydrogen annealing at 400 °C. Electron spin resonance measurements were used to characterize the change of the defect states after hydrogen annealing. It is found that there exists a-centers (g value = 2.006), which is related to the Si dangling bonds in Si QDs in our samples. Hydrogen annealing can significantly reduce non-luminescent a-centers and enhance the electro-luminescence intensity consequently.     

Distribution of hydrogen in low temperature passivated amorphous silicon (a-Si:H) films from neutron reflectivity

Hydrogen plays a critical role in the passivation of dangling bonds in hydrogenated amorphous silicon (a-Si:H) to enable acceptable semiconducting characteristics during operation in devices. Low temperature processing enables fabrication of high performance transistors on flexible substrates such as plastic or stainless steel foils, but also leads to a decrease in the stability of the electronic performance. Generation of defects at the a-Si:H/insulator (hydrogenated silicon nitride, SiN:H) during electrical use due to localized heating will lead to decreased performance unless the dangling bonds are passivated in-situ by residual hydrogen. For this reason, the distribution of hydrogen within a-Si:H may be critical to understanding their aging phenomena. Here the distribution of hydrogen within both a-Si:H and SiN:H layers is probed with sub-nanometer resolution using neutron reflectivity. The hydrogen concentration within the bulk of the a-Si:H (11 ± 2 at.%) and SiN:H (18 ± 3 at.%) agree well with previous reports, but the increased resolution of the neutron measurement is able to identify an approximate three fold increase in the concentration within 2 nm of the semiconductor-insulator interface. This enhanced hydrogen content may act in the short-term as a sink to passivate any dangling bonds formed during operation.     

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.     

Characterization of HF-PECVD a-Si:H thin film solar cells by using OES studies

Hydrogenated amorphous silicon (a-Si:H) films show considerable potential for the fabrication of thin film solar cells. In this study, the a-Si:H thin films have been deposited in a parallel-plate radio frequency (RF) plasma reactor fed with pure SiH₄. The plasma diagnostics were performed simultaneously during the a-Si:H solar cell deposition process using an optical emission spectrometer (OES) in order to study their correlations with growth rate and microstructure of the films. During the deposition, the emitting species (SiH*, Si*, H*) was analyzed. The effect of RF power on the emission intensities of excited SiH, Si and H on the film growth rate has been investigated. The OES analysis revealed a chemisorption-based deposition model of the growth mechanism. Finally, the a-Si:H thin film solar cell with an efficiency of 7.6% has been obtained.     

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%.     

Structure of the ESR spectra of thin film silicon after electron bombardment

Defect creation by MeV electron bombardment of a-Si:H and μc-Si:H thin films is used to explore hidden features of the electron spin resonance spectra. Different dynamics of creation and annealing for different paramagnetic states is expected and found. In a-Si:H the g-value of the db resonance does not change after irradiation, but a pair of satellites is observed on its wings. In the spectra of μc-Si:H three additional lines can be extracted after irradiation, overlapping with the central resonance. Careful analysis of the spectra shows also modification of the dangling bond resonance in μc-Si:H that is compatible with variations of two components of the spectra and supports the model of two dominant defect states in μc-Si:H.     

Numerical simulation of a low- and a high-electric-field photocurrent decay in a-Si:H

We report a numerical simulation of the photocurrent decay (PCD), from the steady state, in two different structure configurations based on the a-Si:H. The standard DOS of the a-Si:H is used. The high-electric-field PCD is considered in a structure configuration based on a metal/a-Si:H junction. Poisson’s and the two continuity equations are numerically solved in a one-dimensional space to calculate the current density. Two different boundary conditions of the a-Si:H film are considered. The low electric field PCD, which may occurs in a coplanar configuration, is calculated from the solution of a system of two non linear coupled rate equations which govern the free carriers concentrations and the different charges on the localized states in the gap. The calculated PCD versus time curves, for the two configurations, show a shoulder around 1 μs which separate two main regions. We can see that the initial current decay is dominated by the electron emission from the conduction-band tail and the recombination via the dangling bonds states. The second current decay is mainly due to the electrons emission from the dangling bonds. We show also that the PCD curve tends towards the PCD of the coplanar configuration when the electric field decreases.     

The dangling-bond defect in amorphous silicon: Statistical random versus kinetically driven defect geometries

Amorphous and micro-crystalline silicon (a-Si:H, μc-Si) are key materials for resource-saving thin-film solar cells. However, the efficiency of such devices is severely limited by light-induced Si dangling-bond defects, which can be detected by electron paramagnetic resonance (EPR). We report density-functional theory calculations on a set of random dangling bonds created in supercell models of a-Si:H and compare calculated hyperfine and g-tensor distributions to the ones obtained from a recent multi-frequency EPR spectral analysis. Our results show that the g-tensor does not exhibit axial symmetry as has been previously assumed, but is clearly rhombic. The hyperfine coupling to the undercoordinated Si atom, on the other hand, is almost perfectly axial. This apparent discrepancy in the symmetry properties is shown to be a consequence of the underlying coupling mechanisms and how these are influenced by structural disorder.However, the hyperfine distribution calculated from our random models underestimates the experimentally observed 30% red-shift when going from c-Si to a-Si:H. We suggest that only a subset of possible dangling-bond configurations is observed in experiment. We discuss plausible mechanisms that would give rise to such a selection, and new experiments to test these hypotheses.     

Doping dependence of crystallization growth rate in a-Si CVD films

Crystallization growth rates (V_g) on boron and phosphorus doped a-Si C VD films are obtained using conductivity measurements during isothermal annealings at temperatures 510<T_A<650°C. For boron doping, the associated activation energy of V_g is equal to 2.9 eV in the whole doping range (up to 2 × 10^?3 B₂H₆), whereas V_g increases by a factor 4 in the range 0 ? 7 × 10^?6 and remains almost constant for higher doping. In this low range, the neutral dangling bonds became positively charged and non paramagnetic by electronic compensation with the acceptors, and the E.S.R. signal decreases from 10¹⁹ to 10¹⁷ cm^?3. These results indicate clearly that dangling bonds and their charge state play an important role in the growth rate process.     

Thirty years trajectory of amorphous and nanocrystalline silicon materials and their optoelectronic devices

A review is given on research trajectory of hydrogenated amorphous and nanocrystalline silicon (a-Si:H and nc-Si) materials with their device applications ongoing since the period of 1970. A brief explanation on the motivation to start amorphous semiconductors research is given to produce a new kind of synthetic semiconductor having continuous energy gap controllability with valency electron controllability. Due to the result of some basic research on the film quality improvement of a-Si:H and nc-Si, some innovative devices had been developed since middle of 1980s in R&D phase such as a-SiC/a-Si heterojunction solar cells, a-Si/a-SiGe and also a-Si/nc-Si tandem type solar cells. Finally, the state of the art on the industrialization of the new devices is introduced and discussed.     

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.     

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.     

Advanced deposition phase diagrams for guiding Si:H-based multijunction solar cells

Phase diagrams have been established to describe very high frequency (vhf) plasma-enhanced chemical vapor deposition (PECVD) of intrinsic hydrogenated silicon (Si:H) and silicon–germanium alloy (Si_1?xGe_x:H) thin films on crystalline Si substrates that have been over-deposited with n-type amorphous Si:H (a-Si:H). The Si:H and Si_1?xGe_x:H films are prepared under conditions used for the top and middle i-layers of high efficiency triple-junction a-Si:H-based n–i–p solar cells. Identical n/i cell structures were co-deposited in this study on textured (stainless steel)/Ag/ZnO which serve as substrate/back-reflectors in order to relate the phase diagrams to the performance parameters of single-junction solar cells. This study has reaffirmed that the highest efficiencies for a-Si:H and a-Si_1?xGe_x:H solar cells are obtained when the i-layers are prepared under previously-described maximal H₂ dilution conditions.