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Abstract: A near-infrared to visible upconversion phosphor (β-NaYF4:Yb3+ (18%), Er3+ (2%)) has been applied at the back of a thin film hydrogenated amorphous silicon (a-Si:H) solar cell in combination with a white back reflector to investigate its response to sub-bandgap infrared irradiation. Current–voltage measurements were performed on the solar cells. A maximum current enhancement of 6.2μA was measured on illumination with a 980nm diode laser at 28mW. This corresponds to an external quantum efficiency (EQE) of 0.03% of the solar cell. A small part, 0.01%, was due to the direct absorption of sub-bandgap radiation but the larger part originates from upconversion. These experiments constitute a proof-of-principle for the utilization of photon upconversion in thin film solar cells. A close match between the non-linear behavior of the upconversion material and the EQE was found by varying the intensity of the laser light. [ABSTRACT FROM AUTHOR]

Using self-consistent computer modeling we find that the experimental current–voltage (J–V) and the spectral response (SR) characteristic curves of a-Si:H p–i–n solar cells can be fitted by either assuming a uniform density of dangling bonds (DB) in each device layer (UDM) or by relying on the defect pool model (DPM). Fittings within the DPM were achieved using the algorithms proposed by Powell and Deane and Schumm. One Si–H bond mediating in the creation of dangling bonds in the first expressions proposed by Powell and Deane and Schumm are appropriate for modeling solar cells in the initial state. The applicability of the algorithm proposed by Schumm for a-Si:H in the stabilized state is also discussed in solar cells. Using DPM we have explored the optimum doping and band gap profile in the intrinsic layer leading us to the maximum efficiency of a-Si:H p–i–n cells. [Copyright &y& Elsevier]

A diagnosis of the plasma process for high rate deposition of microcrystalline silicon by very high frequency plasma-enhanced chemical vapor deposition (VHF PECVD) is explored in this article. The Hα/Si* intensity ratio measured by optical emission spectroscopy is a fingerprint of the amorphous to crystalline transition. Irrespective of the mode of the plasma (α or γ type, different power, pressure and flow) the transition occurs at the same value of this ratio. Moreover, the depletion condition is not necessary for the crystalline regime of growth (at least for the VHF cases studied here). The intensity of hydrogen treatment per deposited Si species is the most dominant prerequisite for nucleation rather than radical/ionic species. An increase of the deposition rate (also Si* intensity) is achieved at high pressures; however, a balance between the increase in dissociation rate due to increased silane partial pressure and lowering of dissociation due to reduced electron temperature limits the optimum deposition rate. The crystallinity is reduced at high pressures. One-dimensional plasma modeling shows that the loss of atomic hydrogen concentration due to abstraction reactions (H + SiH4=SiH3 + H2) is negligible and thus does not explain the loss of crystallinity at high pressures (obtained from the modeling). A monotonous decrease of Hα/Si*, that explains the loss of crystallinity with increasing pressure is attributed to a decrease in electron temperature. [Copyright &y& Elsevier]

In this paper we investigate in a-Si:H-based devices the accuracy of approximating dangling bonds by pairs of donor-like and acceptor-like states. We discuss the impact of using this approximation in device modeling by studying the dark current–voltage, the illuminated current–voltage and the spectral response curves. We find that the relative error introduced by this approximation in these characteristic curves can be tolerated when the correlation energy is assumed to be positive and when the capture cross-section of neutral states adopted is much smaller than that of charged states. A wide range of intrinsic layer-thickness values, density of states and temperatures has been investigated. This approximation fails when the correlation energy adopted is negative, and is not accurate enough when the correlation energy is assumed to be positive but the capture cross-section of neutral states adopted is higher than that of charged states. [Copyright &y& Elsevier]

We deposited a silicon nitride (SiN x)-polymer hybrid multilayer moisture barrier in a hot wire chemical vapor deposition (HWCVD) process, entirely below 100 °C. The polymer, poly(glycidyl methacrylate) (PGMA), was deposited by initiated chemical vapour deposition and the SiN x in a dedicated HWCVD reactor. Line profile investigation of our barrier structures by cross-sectional scanning transmission electron microscopy and energy dispersive X-ray spectrometry reveals that, upon deposition of SiN x on top of our polymer layer, an intermediate layer of silicon oxide (SiO x) like material is formed. X-ray photoelectron spectroscopy measurements confirm the presence of this material and indicate the epoxy rings in the PGMA material open upon heating (to 100 °C) and exposure to atomic hydrogen and amine species in the HWCVD process. The oxygen atoms subsequently react with silicon and nitrogen containing radicals to form SiO xN y. The interlayer turns out to be highly beneficial for interlayer adhesion and this is considered to be one of the reasons for the excellent barrier properties of our multilayer. [ABSTRACT FROM AUTHOR]

The manuscript aims to elucidate the evolution of the bandgap and the semiconducting nature of quantum dot-sized SiGe alloy nanocrystals prepared at different plasma conditions in VHF PECVD. The technique of scanning tunneling spectroscopy (STS) is used for this, considering its highly localized nature, and to study the individual NCs. The STS analysis gives the density of states (DOS) spectrum of the SiGe alloy NCs. The present STS study reveals a dependence of the bandgap of SiGe alloy NCs on plasma parameters. The SiGe alloy NCs also exhibited a shallow p -type character despite the fact that no dopants were used during the deposition. Besides this, the midgap defect states are also observed in some of the samples. Further, a comparative bandgap study of the SiGe alloy NCs with its bulk indicates quantum confinement. These new findings from the QD-sized SiGe alloy NCs are important for the conceptual design and practical development of quantum dot-based third-generation solar cells. [ABSTRACT FROM AUTHOR]

We have extensively studied by multiple microscopic techniques the growth and crystallization of silicon nanoparticles in pulsed SiH 4 /Ar plasmas. We observe that the crystallinity of the particles can be tuned from amorphous to crystalline by altering the plasma ON time, t ON . Three phases can be identified as a function of t ON . Microscopic studies reveal that, in the initial gas phase (phase I) single particles of polycrystalline nature are formed which according to our hypothesis grow out of a single nucleus. The individual crystallites of the polycrystalline particles become bigger crystalline regions which marks the onset of cauliflower shaped particles (phase II). At longer t ON (phase III) distinct cauliflower particles are formed by the growth of these crystalline regions by local epitaxy. [ABSTRACT FROM AUTHOR]

To estimate the dust formation time scale in a silane-hydrogen plasma, optical and electrical plasma diagnostics are performed. We report a periodic fluctuation in emission intensity and electric current in a dusty plasma. The trends of the frequency of fluctuations with varying substrate temperatures and gas flows are studied. However, no such fluctuation is observed in the nondusty plasma. It is hypothesized that this fluctuation arises from the periodic formation and ejection of a dust cloud via the void formation when a critical dust size is reached. [ABSTRACT FROM AUTHOR]

Abstract: We report on the development of p-type μc-SiOx: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 E 04 of 2.56eV 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-SiOx: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. [Copyright &y& Elsevier]

Abstract: Hydrogen profiling, i.e., decreasing the H2 dilution during deposition, is a well-known technique to maintain a proper crystalline ratio of the nanocrystalline (nc-Si:H) absorber layers of plasma-enhanced chemical vapor-deposited (PECVD) thin film solar cells. With this technique a large increase in the energy conversion efficiency is obtained. Compared to PECVD, the unique characteristics of hot-wire CVD (HWCVD), such as the catalytic reactions, the absence of ion bombardment, the substrate heating by the filaments and filament aging effects, necessitate a different strategy for material and device optimization. We report in this paper the results of using a reverse H2 profiling technique, i.e., increasing the H2 dilution of silane instead of decreasing it, to improve the quality of HWCVD intrinsic nc-Si:H and the performance of this material in single junction n-i-p cells. Thus far, the efficiency of nc-Si:H n-i-p cells made on a stainless steel substrate with an Ag/ZnO textured back reflector has been improved to 8.5%, and the efficiency of triple junction solar cells with a structure of proto-Si:H(HWCVD) top cell/proto-SiGe:H (PECVD) middle cell/nc-Si:H (HWCVD, with reverse H2 profiling) bottom cell has reached 10.9%. These efficiency values show the viability of n-i-p cells comprising HWCVD nanocrystalline i-layers. [Copyright &y& Elsevier]

Abstract: The spectral response (SR) measured without bias light (dark SR) normally matches at each wavelength the lowest of the two sub cell responses under bias light (light SR), showing nearly a triangular shape. However, in some tandem structures a departure of this typical triangular shape has been observed, indicating the presence of an apparent current leakage. The dark SR shows some additional response coming from one of the sub cells and the open circuit voltage (V oc) of the tandem cell is observed to decrease. In this paper we investigate with computer modeling the causes behind the appearance of dark SR curves departing from the characteristic triangular shape by studying various amorphous and microcrystalline silicon based tandem structures. Our simulations indicate that the presence of thin defective regions perpendicular to the charge transport direction inside the intrinsic layers, especially near the p/i interface, can give rise to anomalous dark SR and to the deterioration of the tandem cell performance. Moreover, the dark SR is more sensitive to the presence of defects in the front region of the intrinsic layer of the bottom sub cell. [Copyright &y& Elsevier]

Abstract: Microcrystalline silicon (μc-Si:H) thin films prepared by plasma enhanced chemical vapour deposition (PECVD) at 37°C has been studied by cross-sectional TEM and ambient conductive AFM. We have succeeded in the combined measurement of topography and local conductivity under standard ambient conditions, overcoming the surface native oxide by more sensitive (pA range) current detection. We observed the columnar structure of the amorphous phase in the TEM micrograph and related it to the surface corrugation of the same size detected by AFM. [Copyright &y& Elsevier]

Abstract: Hot-wire chemical vapor deposition (HWCVD) has been shown to be promising for depositing thin film silicon materials at higher deposition rate than conventional plasma-enhanced CVD. Due to the higher atomic H density in the gas phase, it is not straightforward to utilize the full benefit of the higher deposition rate of the HWCVD in thin film solar cells in the p–i–n configuration on textured SnO2:F (superstrate configuration). The Transparent Conducting Oxide (TCO) is found to be very sensitive for high atomic hydrogen ambient. This paper presents our investigations on the implementation of ultrathin ZnO:Al protection layers, allowing higher temperatures during i-layer deposition, and thus, higher deposition rates. [Copyright &y& Elsevier]

In order to apply device quality polycrystalline silicon obtained by the hot-wire chemical vapour deposition technique in industrial processes a high deposition rate is desirable. Meanwhile it is important to maintain good thickness uniformity over large areas. To achieve this the catalytic area needs to be increased. The influence of increasing the catalytic area on the deposition rate and material properties was studied. Proportionally increasing the feedstock gas flows leads to an unexpected drop in deposition rate. The dissociation of the gases causes the filament temperature to decrease. Below 1850 K the dissociation process is limited by the silicon deposition on the filament. This leads to a drastic drop in deposition rate. By adjusting the process parameters to the increased catalytic area, device quality polycrystalline silicon is again obtained. The deposition rate of polycrystalline silicon has been increased from 6 to 21 A˚/s while maintaining the material properties. [Copyright &y& Elsevier]

Abstract: The technique of maintaining a proper crystalline ratio in microcrystalline silicon (μc-Si:H) layers along the thickness direction by decreasing the H2 dilution ratio during deposition (H2 profiling) was introduced by several laboratories while optimizing either n–i–p or p–i–n μc-Si:H cells made by PECVD. With this technique a great increase in the energy conversion efficiency was obtained. Compared to the PECVD technique, the unique characteristics of HWCVD, such as the catalytic reactions, the absence of ion bombardment, the substrate heating by the filaments and filament aging effects, necessitate a different strategy for device optimization. We report in this paper the result of our method of using a reverse H2 profiling technique, i.e. increasing the H2 dilution ratio instead of decreasing it, to improve the performance of μc-Si:H n–i–p cells with an i-layer made by HWCVD. The principle behind this technique is thought to be a compensation effect for the influence of progressing silicidation of the filaments during the growth of μc-Si:H, if the filament current is held constant during growth. The dependence of the material crystallinity on thickness with and without H2 profiling is discussed and solar cell J–V parameters are presented. Thus far, the best efficiency of μc-Si:H n–i–p cells made on a stainless steel substrate with an Ag/ZnO textured back reflector made in house has been improved to 8.5%, which is the highest known efficiency obtained for n–i–p cells with a hot-wire μc-Si:H i-layer. [Copyright &y& Elsevier]