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Studies solar cells with wafers subjected to an extra atomic hydrogen prior to deposition of the amorphous layer. Fabrication of pn and pin a-SiC:H/c-Si heterojunction solar cells. Hydrogen passivation; Heterojunction structure.

Presents information on a study that investigated the influence of deposition parameters on the silicon film growth and properties. Use of hot-wire chemical vapor deposition; Experimental details; Influence of substrate temperature and pressure; Conclusions.

Abstract The leader of todays solar energy revolution is undoubtedly the silicon photovoltaic (PV) module. However, despite the immense progress in efficiency and the phenomenal drop of manufacturing and installation costs the dark blue flat panels have not found widespread use in the modern urban environment. The scarcity of available rooftop space, the high cost of land and the irregular metropolitan skyline have not allowed conventional solar technologies to supply cities with clean energy. Thus, new concepts are being investigated to integrate solar generators into new and existing buildings in the form of facades or windows. Luminescent Solar Concentrators (LSCs) offer a novel approach for the utilization of solar irradiation in the form of transparent glazing systems that have the potential to become functional elements of the building envelope. This paper highlights and compares the most recent technological advances in the field of LSC technology and the contribution of colloidal chemistry with reabsorption-free emitters offering broadband absorption and enhanced stability. Combined with a critical study of the newly emerged LSC applications in various fields this study will also attempt to give a possible glimpse of the near future of transparent solar harvesting devices. Graphical abstract Image 1 Highlights • State-of-the-art review in LSCs based on nanoparticles. • Reabsorption in LSCs can be minimized. • Applications in large scale LSCs are presented. [ABSTRACT FROM AUTHOR]

The consequences of implementing a Hot Wire Chemical Vapor Deposition (HWCVD) chamber into an existing in-line or roll-to-roll reactor are described. The hardware and operation of the HWCVD production reactor is compared to that of existing roll-to-roll reactors based on Plasma Enhanced Chemical Vapor Deposition. The most important consequences are the technical consequences and the economic consequences, which are both discussed. The technical consequences are adaptations needed to the hardware and to the processing sequences due to the different interaction of the HWCVD process with the substrate and already deposited layers. The economic consequences are the reduced investments in radio frequency (RF) supplies and RF components. This is partially offset by investments that have to be made in higher capacity pumping systems. The most mature applications of HWCVD are moisture barrier coatings for thin film flexible devices such as Organic Light Emitting Diodes and Organic Photovoltaics, and passivation layers for multicrystalline Si solar cells, high mobility field effect transistors, and silicon heterojunction cells (also known as heterojunction cells with intrinsic thin film layers). Another example is the use of Si in thin film photovoltaics. The cost perspective per unit of thin film photovoltaic product using HWCVD is estimated at 0.07 €/Wp for the Si thin film component. [ABSTRACT FROM AUTHOR]

Abstract: Tungsten- and titanium-doped indium oxide (IWO and ITiO) films were deposited at room temperature by radio frequency (RF) magnetron sputtering, and vacuum post-annealing was used to improve the electron mobility. With increasing deposition power, the as deposited films showed an increasingly crystalline nature. Compared with ITiO films, IWO films showed crystallinity at lower RF power. IWO films are partially crystallized at 10W deposition power and become nearly fully crystalline at 20W. ITiO films are fully crystalline only at 75W. For this reason, film thickness has a greater impact on the electrical properties of IWO films than ITiO films. Vacuum post-annealing is more effective in improving electron mobility for amorphous than for (partially) crystalline IWO and ITiO films. Changes in the electrical properties of ITiO films can be better controlled as a function of annealing temperature than those of IWO films. Finally, post annealed 308nm-thick IWO and 325nm-thick ITiO films have approximately 80% transmittance in visible and near infrared wavelengths (up to 1100nm), while their sheet resistances decrease to 9.3 and 10Ω/□, and their electron mobilities are 51cm2V−1 s−1 and 50cm2V−1 s−1, respectively, making them suitable for use as Transparent Conductive Oxide layers of low bandgap solar cells. [Copyright &y& Elsevier]

Abstract: A considerable improvement in performance has been achieved for multibandgap proto-Si/proto-SiGe/nc-Si:H triple junction n–i–p solar cells in which hot-wire chemical vapor deposition (HWCVD) is used to obtain the absorber layers of the bottom and the top cell. To achieve this, optimized Ag/ZnO layers are prepared in house with respect to rms roughness and morphology. Apart from optimizing the light scattering from this back reflector, we found that it is important to control the morphology of surfaces to avoid cavities and shunting paths. A further drawback of such rough metallic surfaces is the parasitic absorption due to surface particle plasmons. Nevertheless, optimized back reflectors lead to an enhancement of the photocurrent of as much as 50%. The stable efficiency for a single junction n–i–p cell with optimized back reflector reached 8.6%, which is the highest reported value for n–i–p cells with HWCVD nc-Si:H i-layer. The triple junction cells of 2.5-μm thickness, using silicon germanium (1.5eV) in the middle cell, have an efficiency of 11%. These cells are stable within 3.5% relative. By keeping all component cells very thin, these triple cells are almost insensitive to light-induced defects. The difference between the best single junction and triple junction n–i–p cells obtained so far in our laboratory and the reported best cells with plasma enhanced chemical vapor deposition (PECVD) i-layers can be mainly attributed to the differences in the rough substrates and to the use of rather thin i-layers. [Copyright &y& Elsevier]

Abstract: The research effort in the field of Hot Wire CVD (also called Catalytic CVD or initiated CVD) has increased considerably over the last 10 years. An increasing variety of thin film materials can be obtained with this method, with good feedstock utilization and high deposition rate. The properties of the deposited films are notably different from those of films made with conventional methods. A number of applications, such as diamond deposition, functional polymer deposition, and passivating silicon nitride deposition, have already found its way to commercial manufacturing. In this paper we discuss some of the modern research issues in the realm of Hot Wire CVD (Cat-CVD) and i-CVD. [Copyright &y& Elsevier]

Abstract: The technology of Hot Wire Chemical Vapor Deposition (HWCVD) or Catalytic Chemical Vapor Deposition (Cat-CVD) has made great progress during the last couple of years. This review discusses examples of significant progress. Specifically, silicon nitride deposition by HWCVD (HW-SiN x ) is highlighted, as well as thin film silicon single junction and multijunction junction solar cells. The application of HW-SiN x at a deposition rate of 3 nm/s to polycrystalline Si wafer solar cells has led to cells with 15.7% efficiency and preliminary tests of our transparent and dense material obtained at record high deposition rates of 7.3 nm/s yielded 14.9% efficiency. We also present recent progress on Hot-Wire deposited thin film solar cells. The cell efficiency reached for (nanocrystalline) nc-Si:H n-i-p solar cells on textured Ag/ZnO presently is 8.6%. Such cells, used in triple junction cells together with Hot-Wire deposited proto-Si:H and plasma-deposited SiGe:H, have reached 10.9% efficiency. Further, in our research on utilizing the HWCVD technology for roll-to-roll production of flexible thin film solar cells we recently achieved experimental laboratory scale tandem modules with HWCVD active layers with initial efficiencies of 7.4% at an aperture area of 25 cm2. [Copyright &y& Elsevier]

Abstract: Nanocrystalline (or microcrystalline) silicon (nc-Si:H or μc-Si:H) is an absorber material that is crucial for obtaining thin film silicon tandem solar cells with high efficiency. This material is conventionally produced by direct plasma-enhanced chemical vapor deposition (PECVD), which is based on the dissociation of silane (SiH4) and hydrogen (H2) in a radiofrequency (rf) plasma. During the last few decades, the plasma deposition parameter regime has been explored intensively, in particular to improve the quality of nc-Si:H and to increase its deposition rate. The desired formation of nanocrystallites occurs under growth conditions close to the transition regime. A technique that is not plasma based is hot-wire CVD (HWCVD), which is based on catalytic decomposition of SiH4 and dissociation of H2 gases on a hot filament. Because source gases are catalytically decomposed, the method is often referred to as catalytic CVD (Cat-CVD). The HWCVD technique has been shown to be a viable method for the deposition of silicon-based thin films and solar cells. We discuss the progress at our laboratory. Specifically, recent single junction and multijunction n–i–p-type solar cells are highlighted. Improvements have been achieved by addressing, among others, the morphology of the light-scattering rough surfaces, to avoid cavities and shunting paths, and by studying the structural evolution of the nanocrystalline phase during growth. The obtained solar cells are highly stable against light soaking. [Copyright &y& Elsevier]

Abstract: By implementing nanostructure in multiband-gap proto-Si/proto-SiGe/nc-Si:H triple junction n–i–p solar cells, a considerable improvement in performance has been achieved. The unalloyed active layers in the top and bottom cell of these triple junction cells are deposited by Hot-Wire CVD. A significant current enhancement is obtained by using textured Ag/ZnO back contacts instead of plain stainless steel. We studied the correlation between the integrated current density in the long-wavelength range (650–1000 nm) with the back reflector surface roughness and clarified that the rms roughness from 2D AFM images correlates well with the long-wavelength response of the cell when weighted with a Power Spectral Density function. For single junction 2-μm thick nc-Si:H n–i–p cells we improved the J sc from 15.2 mA/cm2 for plain stainless steel to 23.4 mA/cm2 using rough back reflector. We introduced profiling of the H2 dilution during growth of the nc-Si:H layer to prevent a transition to amorphous growth. The efficiency for a single junction n–i–p cell reached 8.5%, the highest reported value for HWCVD cells of this kind. Moreover, these cells show to be totally stable under light-soaking tests. Combining the above techniques, an efficiency of 10.9% has been obtained for triple junction cells (J sc =8.35 mA/cm2, V oc =1.98 V, FF=0.661). By using effective light trapping techniques and three different band-gap materials, the required thickness could be kept small (∼2.5 μm total). [Copyright &y& Elsevier]

Abstract: The dielectric functions of thin film silicon materials on glass were measured by spectroscopic ellipsometry (SE) and simulated by using the Tauc-Lorentz (TL) dispersion law, which provided information on disorder (C) and density (A). A VHF plasma-deposited sample made at 100°C with an optimum hydrogen dilution shows density (relative packing density of Si–Si bonds as estimated by SE) and structural disorder that are comparable to samples made at 200°C. HWCVD materials made at 100°C at lower hydrogen dilution conditions have a less dense structure and higher roughness compared to the plasma-deposited samples. This can be attributed to the absence of ion bombardment on the growing film. Out of all samples investigated, the HWCVD sample made at a hydrogen to silane flow ratio value of 20 showed a remarkably low structural disorder (C =1.67) even though the deposition temperature was only 100°C. A small bond angle variation of ∼6.4° as determined from its Raman spectrum, the presence of small (1–1.5nm) dispersed crystalline-like islands in the silicon matrix, and sharp rings in the selective area diffraction pattern point towards a special ordered structure. The photoresponse of this material is >105. [Copyright &y& Elsevier]

Abstract: Using the hot wire (HW) chemical vapor deposition (CVD) method for the deposition of silicon nitride (SiN x ) and amorphous silicon (a-Si:H) thin films we have achieved high deposition rates for device quality materials up to 7.3nm/s and 3.5nm/s, respectively. For thin films of SiN1.3, deposited at 3nm/s, the mass-density of the material reached a very high value of 3.0g/cm3. The silane utilization rate for this fast process is 77%. The high mass-density was consistent with the low 16BHF etch rate of 7nm/min. We tested this SiN1.3 in “all hot wire” thin film transistors (TFTs), along with a-Si:H material in the protocrystalline regime at 1nm/s. Analysis shows that these “all hot wire” TFTs have a V th =1.7–2.4V, an on/off ratio of 106, and a mobility of 0.4cm2/Vs after a forming gas anneal. We therefore conclude that the HWCVD provides SiN x materials with dielectric properties at least as good as PECVD does, though at a much higher deposition rate and better gas utilization rates. [Copyright &y& Elsevier]

Abstract: We present recent progress on hot-wire deposited thin film solar cells and applications of silicon nitride. The cell efficiency reached for μc-Si:H n–i–p solar cells on textured Ag/ZnO presently is 8.5%, in line with the state-of-the-art level for μc-Si:H n–i–p''s for any method of deposition. Such cells, used in triple junction cells together with hot-wire deposited proto-Si:H and plasma-deposited SiGe:H, have reached 10.5% efficiency. The single junction μc-Si:H n–i–p cell is entirely stable under prolonged light soaking. The triple junction cell, including protocrystalline i-layers, is within 3% stable, due to the limited thicknesses of the two top cells. The application of SiN x :H at a deposition rate of 3 nm/s to polycrystalline Si wafer solar cells has led to cells with 15.7% efficiency. We have also achieved record high deposition rates of 7.3 nm/s for transparent and dense SiN x ;H. Hot-wire SiN x :H is likely to be the first large commercial application of the Hot Wire CVD (Cat-CVD) technology. [Copyright &y& Elsevier]

Hot-wire chemical vapor deposition is a rapidly developing CVD technique for the deposition of silicon thin films and silicon alloys and may become a competitor of the plasma-enhanced (PE) CVD method due to significant advantages such as high deposition rate, efficient source gas utilization, lack of ion bombardment, and low equipment cost. Little is known, however, about the mechanisms for catalytic decomposition of the source gases, gas phase reactions at commonly used pressures, and the growth reactions. In this article, the differences in the reactions at various filament materials are discussed and it is shown that the subsequent reactions in the gas phase and reactions contributing to film growth can be substantially different from those in PE-CVD, due to the lack of energetic electrons and ions. Further work is necessary to identify the role of each precursor for the deposition of amorphous and microcrystalline films. [Copyright &y& Elsevier]

We investigate the impact of new growth techniques on the mobility and stability of amorphous silicon (a-Si:H) thin film transistors (TFTs). It was suggested that the key parameter controlling the field-effect mobility and stability is the intrinsic mechanical stress in the a-Si:H layer. We study a series of bottom-gate TFTs incorporating a-Si:H deposited by VHF PECVD and hot-wire CVD. All TFTs exhibit good characteristics with mobilities of 0.6–0.7 cm2/V s. The mean activation energy EA and the slope of the barrier-height distribution kBT0 for defect creation in the a-Si:H are determined. EA correlates to the intrinsic stress. [Copyright &y& Elsevier]

This paper addresses the current trends in research and development for: (1) Thin film transistors (TFTs) on plastic substrates, (2) low-temperature poly-silicon (LTPS) for the pixel TFTs and for row and column drivers on glass, (3) addressing of organic light emitting diodes by silicon TFTs. For these advanced applications of TFTs the relevant issues are: (i) higher electron mobility, (ii) stability, and (iii) defect free, uniform deposition of thin silicon films and gate dielectrics at a high deposition rate (reduced cost). At Utrecht University, we are investigating hot wire (catalytic) chemical vapor deposition (CVD) as a deposition technique for novel TFTs that have a high potential to meet the above mentioned requirements. Bottom gate, inverted staggered TFTs with hot wire CVD (HWCVD) silicon films have been made with an electron mobility of 1.5 cm2/V s, and with field effect characteristics that are completely stable under operating conditions. Top gate, coplanar TFTs with polycrystalline silicon (poly-Si) films have been made, which showed a mobility of 4.7 cm2/V s. This has been obtained without any post treatment, and the hot wire technology can thus avoid expensive, time-consuming steps such as laser recrystallization as currently used in the production of the latest poly-Si lap top displays. HWCVD is also suitable for the deposition of SiNx:H gate dielectrics. TFTs with a hot wire silicon nitride gate dielectric have been deposited. [Copyright &y& Elsevier]

Describes the electron transport in undoped heterogeneous silicon deposited by hot-wire chemical vapor deposition found to exhibit conventional and pronounced inverse Meyer-Neldel behavior. Heterogeneous nature of the semiconductor in the channel region near to the gate insulator allowing the Fermi level to be pushed deeply into the conduction-band tail states.

Investigates the stability of amorphous-silicon thin-film transistors. Information on the incorporation of hot-wire chemical vapor deposition technique; Optimization of hot-wire layer electronic properties; Implication for interface between gate dielectric and hot-wire layer.

Examines the link between the strength of three vibrational modes to interpret the silicon-hydrogen stretching doublet in hydrogenated amorphous silicon species deposited by radio frequency glow discharge. Effect of the deposition temperature on the mode strength; Evidence for a straightforward correlation between the modes; Consideration of the vibrating dipole.

We demonstrate single junction and tandem solar cells using an ultrathin (35 nm) hydrogenated amorphous silicon germanium absorber which is deposited by hot wire chemical vapor deposition within only 90 s. We take advantage of the extremely small thickness of the absorber layer and the capability of hot wire chemical deposition to create conformal layers, by fabricating solar cells on morphologies consisting of self-assembled zinc oxide nanorods. We created solar cells in tandem configuration using the hydrogenated amorphous silicon germanium absorber layer in the bottom cell. Our cells include a cell structure that is “vertical”, which has great potential for high photocurrent generation. We show that the nanorod morphology particularly improves the generation of current in the bottom cell. [ABSTRACT FROM AUTHOR]

Hot wire CVD (HWCVD) and initiated CVD (iCVD) are very well suited deposition techniques for the fabrication of transparent thin film gas barriers. Single inorganic or organic layers, however, face challenges, which are hard to overcome: unavoidable defects and low intrinsic barrier function. We demonstrate that by combining inorganic HWCVD films and organic iCVD films, a water vapor transmission rate a low as 5 ∗ 10 − 6 g/m 2 /day at 60 °C and 90% RH for a simple pinhole free three layer structure is obtained even with non-optimized individual layers. Given the 100 °C deposition temperature, the layer stacks can be deposited on any sensitive electronic device. [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 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 1.9eV and 270nm 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 (Vbi). However, a rather high recombination voltage, Vμ, affects the FF and short circuit current density (Jsc) for the cells on Asahi as well as for the cells on PC. A rather high parallel resistance≫1MΩcm2 (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. [Copyright &y& Elsevier]

Abstract: To be able to use the simple technique of optical emission spectroscopy (OES) for the prediction of the transition of growth from a-Si to nc-Si via the Hα/Si⁎ emission ratio, a regime-dependent correction factor is required to relate the measured Hα/Si⁎ emission ratio to the true flux (to the substrate) ratio of atomic hydrogen to deposited silicon radicals. Through an in-depth study in a very high frequency plasma enhanced chemical vapor deposition process, we obtained that the flux ratio of atomic hydrogen and deposited silicon radicals to the growing surface,Γ H/Γ Si, is related to the emission ratio of Hα and Si⁎, I rad H α /I rad Si*, by the relation, , where the parameters p (pressure), d (inter-electrode distance) and T gas (gas temperature) are experimentally obtained quantities and R rad is the ratio of the rate coefficients for radiation of Si⁎ and Hα. We obtained the calibration parameters a and b to be 1.9·10−21 ±2·10−22 Pam−1 and 5.5±1.9 respectively which is valid in a broad range of power and pressure settings. With these parameters, it is easy to estimate the flux ratio of atomic hydrogen and silicon species at any deposition condition using the OES data and this will allow accurate prediction of the phase transition. According to simulations in the linear low-pressure regime, the amorphous to nanocrystalline phase transformation occurs at the flux ratio Γ H/Γ Si =12, which translates, using the factors a and b, to the required emission ratio. [Copyright &y& Elsevier]

Abstract: Titanium-doped indium oxide (ITiO) films were deposited on Corning glass 2000 substrates at room temperature by radio frequency magnetron sputtering followed by vacuum post-annealing. With increasing deposition power, the as-deposited films showed an increasingly crystalline nature. As-deposited amorphous ITiO films obtained at 20W began to crystallize at the annealing temperature of 155°C. Although there was no significant change in the crystalline structure of the films, electron mobility improved gradually with further increase in the annealing temperature. After post-annealing at 580°C, the highest electron mobility of 50cm2 V−1 s−1 was obtained. Compared with the amorphous ITiO films, the ITiO films with a certain degree of crystallinity obtained at high deposition power were less affected by the vacuum annealing. Their electron mobility also improved due to post-annealing, but the increase was insignificant. After post-annealing, the optical transmission of the 325 nm-thick ITiO films showed approximately 80% at wavelengths ranging from 530 to 1100nm, while the sheet resistance decreased to 10Ω/sq. This makes them suitable for use as transparent conductive oxide layers of low bandgap solar cells. [Copyright &y& Elsevier]

Abstract: The applicability of the very high frequency (VHF) plasma-enhanced chemical vapor deposition (PECVD) technique to the fabrication of solar cells in an n–i–p configuration at 100°C substrate temperature is being investigated. Amorphous and microcrystalline silicon cells are made with the absorber layers grown in conditions close to the amorphous-to-microcrystalline transition, which proved to give the best quality layers. It was observed that post-deposition annealing at 100°C resulted in a relative increase of the efficiency of up to 50% for both amorphous and microcrystalline cells. For an amorphous solar cell deposited on stainless steel foil with a non-textured back reflector, an efficiency of 5.3% was achieved. A too rough substrate (textured back reflector), with an rms roughness higher than 80nm, was found to give rise to shunting paths. [Copyright &y& Elsevier]

Abstract: Amorphous silicon films have been made by HWCVD at a very low substrate temperature of ≤100 °C (in a dynamic substrate heating mode) without artificial substrate cooling, through a substantial increase of the filament–substrate distance (∼80 mm) and using one straight tantalum filament. The material is made at a reasonable deposition rate of 0.11 nm/s. Optimized films made this way have device quality, as confirmed by the photosensitivity of >105. Furthermore, they possess a low structural disorder, manifested by the small Γ/2 value (half width at half maximum) of the transverse optic (TO) Si–Si vibration peak (at 480 cm−1) in the Raman spectrum of ∼30.4 cm−1, which translates into a bond angle variation of only ∼6.4°. The evidence gathered from the studies on the structure of the HWCVD grown film by three different techniques, Raman spectroscopy, spectroscopic ellipsometry and transmission electron microscopy, indicate that we have been able to make a photosensitive material with a structural disorder that is smaller than that expected at such a low deposition temperature. Tested in a p–i–n solar cell on Asahi SnO2:F coated glass (without ZnO at the back reflector), this i-layer gave an efficiency of 3.4%. To our knowledge, this is the first report of a HWCVD thin film silicon solar cell made at such a low temperature. [Copyright &y& Elsevier]

Abstract: From TEM, XRD and Raman measurements, we compare the crystallization kinetics when HWCVD and PECVD a-Si:H films, containing different initial film hydrogen contents (C H), are crystallized by annealing at 600 °C. For the HWCVD films, the nucleation rate increases, and the incubation time and the full width at half maximum (FWHM) of the XRD (111) peak decrease with decreasing film C H. However, the crystallization kinetics of HWCVD and PECVD films of similar initial film C H are quite different, suggesting that other factors beside the initial film hydrogen content affect the crystallization process. Even though the bonded hydrogen evolves very early from the film during annealing, we suggest that the initial spatial distribution of hydrogen plays a critical role in the crystallization kinetics, and we propose a preliminary model to describe this process. [Copyright &y& Elsevier]

Abstract: Micromorph silicon tandem solar cells on Asahi U-type SnO2:F coated glass substrates have been fabricated by very high frequency plasma enhanced chemical vapour deposition (VHF-PECVD) in an ultra high vacuum multichamber system called ASTER. The hydrogenated microcrystalline silicon (μc-Si:H) intrinsic layer (i-layer) was deposited using a capacitively coupled reactor with a shower head cathode at high pressure depletion (HPD) conditions. We made bottom cell current limited tandem cells on Asahi U-type substrates, and they showed an initial efficiency of 10.2% (V oc =1.34 V, FF=0.69). To develop the fabrication process of such tandem cells as flexible solar cells, we began as a first step the deposition of hydrogenated amorphous silicon (a-Si:H) p-i-n single junction cells on aluminium foil (provide by Helianthos b.v.) which was then transferred to plastic substrate at Helianthos b.v. Such a-Si:H cells showed an initial active area efficiency of 7.69% (V oc =0.834 V, FF=0.70) and FF degraded by only 11% after 1000 h of light soaking, showing a high stability of these single junction cells. A minimodule (consisting of 8 cells) on foil delivered an initial aperture area efficiency of 6.7% (V oc =6.32 V, FF=0.65). [Copyright &y& Elsevier]

Abstract: Transient grating and photon echo measurements of Ge-H stretch modes in hydrogenated amorphous germanium are reported. These experiments reveal that the Ge-H vibrational population decays into two bending modes at 570cm−1, with the excess energy bridged by Ge–Ge vibrations of the amorphous host. The vibrational dephasing has an excitation density dependence (as has been observed for Si-H stretch modes in amorphous silicon) which we attribute to additional dephasing contributions from non-equilibrium phonons. At temperatures above 30K, the dephasing rate follows an exponentially activated trend due to elastic phonon scattering processes. [Copyright &y& Elsevier]

Abstract: A p++ μc-Si:H layer for the use as deposited back surface field (BSF) has to be highly doped in order to achieve a truly functioning BSF. We optimized a p++ μc-Si:H deposited on glass in terms of activation energy by varying the deposition parameters. The activation energy of the 30nm thick μc p++ layer on glass reaches an optimum at E a =0.15±0.01eV at a flow ratio TMB/SiH4 of 0.016. To obtain a measure of the value of the activation energy of the p++ μc-Si:H layer on wafers, the layers were deposited on highly crystalline intrinsic μc-Si:H layers due to which the incubation layer of the μc p-layer can be avoided. The optimum TMB flow is higher on the microcrystalline i-layer, simulating the wafer, and the activation energy is greatly reduced to E a =0.08±0.01eV. The best bifacial silicon heterojunction cell (SHJ) using such a low activation energy layer as the deposited BSF has an active area efficiency of 14.87%. This is high compared to the efficiency of 11.86% for the cell without BSF, due to a 22.4mV higher V oc and a 4.6mA/cm2 higher J sc, demonstrating that the deposited BSF performs successfully. [Copyright &y& Elsevier]

Abstract: We have used a free electron laser to study the vibrational relaxation of the Ge–H symmetric stretching mode in a-Ge:H thin films. The overall dynamics of the Ge–H stretch mode are remarkably similar to those measured for the Si–H stretch mode in a-Si:H. Transient grating measurements of the vibrational population decay exhibits a non-exponential shape due to the involvement of the Ge–H bending modes in the decay processes. Photon echo experiments reveal a temperature dependent pure dephasing rate dominated by elastic scattering of TO-like Ge–Ge vibrations with an excitation dependent component due to non-equilibrium host vibrations. [Copyright &y& Elsevier]

Abstract: Planar converters containing quantum dots as wavelength-shifting moieties on top of a multi-crystalline silicon and an amorphous silicon solar cell were studied. The highly efficient quantum dots are to shift the wavelengths where the spectral response of the solar cell is low to wavelengths where the spectral response is high, in order to improve the conversion efficiency of the solar cell. It was calculated that quantum dots with an emission at 603nm increase the multi-crystalline solar cell short-circuit current by nearly 10%. Simulation results for planar converters on hydrogenated amorphous silicon solar cells show no beneficial effects, due to the high spectral response at low wavelength. [Copyright &y& Elsevier]

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]

The hot-wire chemical vapor deposition technique was used for the deposition of amorphous and microcrystalline silicon. The best microcrystalline silicon solar cells were deposited at the edge of the transition from the microcrystalline to the amorphous regime. Under conditions that are close to this edge, the crystalline ratio decreased with accumulated film thickness, due to changes in the filament properties during deposition. Microcrystalline silicon made at slightly higher hydrogen dilution was homogeneously structured. Multibandgap a-Si:H/μc-Si:H solar cells were made, and high open-circuit voltages were obtained for both n–i–p/n–i–p and p–i–n/p–i–n structures. The tunnel-recombination junction of n–i–p/n–i–p cells requires careful optimization in order to prevent current leakage under low light intensity conditions, such as during spectral response measurements. [Copyright &y& Elsevier]

High growth rate poly- and microcrystalline silicon materials such as those made by hot-wire chemical vapor deposition (HWCVD) require doped layers that are not only resistant to high temperatures, but also resistant to high atomic hydrogen fluxes. This has been achieved in our case by Layer-by-Layer (LbL) deposition at high temperatures. Layer-by-Layer deposition was achieved by alternating either boron (p-type) or phosphorous (n-type) doped amorphous silicon and hydrogen plasma treatments by very high frequency chemical vapor deposition (VHF PECVD). The experiments revealed the following observations. (1) An optimum thickness per deposition cycle (total thickness/deposition cycle) of 1.4 nm/cycle is needed for crystallization (irrespective of dopants and deposition temperature). We found that for continuous wave deposition (CW) increased boron doping leads the growth regime to amorphous nature, whereas at the same doping condition we obtain microcrystalline films by LbL deposition if the aforesaid thickness per cycle is followed. (2) The etching rate during the hydrogen treatment decreases monotonously at increasing substrate temperature. The observation that films grown at 400 °C (where etching is negligible) are microcrystalline, implies that etching does not play an important role in nucleation. (3) A minimum thickness of the first layer is needed for sustaining growth in the LbL process. From the above studies we propose a hydrogen mediated nucleation process, which is not affected by dopants at the growing surface as in the case of continuous growth. The doping efficiencies in our LbL deposited layers are orders of magnitude higher than those in CW deposition (for p layers a doping efficiency of 39% in case of LbL, compared to 1% for CW). The best high-temperature doped layers with a small thickness have properties as follows: LbL p-type μc-Si:H (Ts=350 °C, 29 nm): activation energy=0.11 eV and dark conductivity=0.1 Ω-1 cm-1; LbL n-type μc-Si:H (Ts=400 °C, 31 nm): activation energy = 0.056 eV and dark conductivity=2.7 Ω-1 cm-1. A test cell using an HWCVD deposited μc-Si:H i-layer on top of the high temperature LbL μc-Si:H n-layer in an n-i-p cell configuration on a stainless steel substrate without a back reflector showed a high open circuit voltage of 0.65 V and a fill factor of 0.68, proving the high doping efficiency and crystallinity of the n-layer as well as its resistance against high-temperature conditions. [Copyright &y& Elsevier]

In the past few years hot-wire chemical vapor deposition (HWCVD) has become a popular technique for the deposition of silicon-based thin-film transistors (TFTs). Several groups have been using hot-wire deposited amorphous and microcrystalline silicon as the active layers in TFTs. In such devices either thermal SiO2 or plasma-deposited silicon nitride was the gate insulator. Recently ‘All-Hot-Wire TFTs’ have been realized, with also the silicon nitride deposited by HWCVD. This paper reviews the characteristics of hot-wire TFTs with amorphous and microcrystalline silicon using plasma- or hot-wire deposited silicon nitride as the gate insulator. It has been shown that hot-wire TFTs have a higher stability upon gate-bias stress as compared to their plasma-deposited counterparts. We present an overview of the stability of hot-wire TFTs deposited at a range of substrate temperatures. The higher stability of hot-wire TFTs that have been deposited at temperatures of 400–500 °C is ascribed to an enhanced structural order, i.e. a higher degree of medium-range order of the silicon network. [Copyright &y& Elsevier]

Using two tungsten (W) filaments and a filament–substrate spacing of 3.2 cm, we have explored the deposition of microcrystalline silicon (μc-Si) solar cells, with the i-layer deposited at high deposition rates (Rd), by the hot-wire CVD (HWCVD) technique. These cells were deposited in the n–i–p configuration on textured stainless steel (SS) substrates, and all layers were deposited by HWCVD. Thin, highly crystalline seed layers were used to facilitate crystallite formation at the n–i interface. Companion devices were also fabricated on flat SS substrates, enabling structural measurements (by XRD) to be performed on i-layers used in actual device structures. Using a filament temperature of 1750 °C, device performance was explored as a function of i-layer deposition conditions, including variations in i-layer substrate temperature (Tsub) using constant H2 dilution, and also variations in H2 dilution during i-layer deposition. The intent of the latter is to affect crystallinity at the top surface of the i-layer (i–p interface). We report device performance resulting from these studies, with all i-layers deposited at Rd>5 A˚/s, and correlate them with i-layer structural studies. The highest device efficiency reported is 6.57%, which is a record efficiency for an all-hot-wire solar cell. [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]

The passivating quality of hydrogenated amorphous silicon thin films is essential for achieving high open-circuit voltages in silicon heterojunction solar cells. This work reports our progress towards the use of hot wire chemical vapor deposition for fabricating these passivation layers. We have achieved 19.4% energy conversion efficiency for a solar cell with a conventional alkali-etched pyramidal texture. Next, we tested radial heterojunction devices that consist of up to 20-μm high micropillar arrays. Proper passivation of these devices is challenging, since the structures feature an enlarged surface area and vertical walls. We have made use of the ability of hot wire chemical vapor deposition to deposit conformal thin films at high deposition rate. The micropillar solar cells show only limited reduction of open-circuit voltage in comparison to a flat reference solar cell and have an improved overall performance. We created additional stacked multijunction silicon-based junctions on top of the micropillars to increase the open-circuit voltage of the solar cells for future applications in which a higher potential is required such as water splitting devices. [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]

We report on the gas phase synthesis of highly crystalline and homogeneously alloyed Si 1−x Ge x nanocrystals in continuous and pulsed plasmas. Agglomerated nanocrystals have been produced with remarkable control over their composition by altering the precursor GeH 4 gas flow in a continuous plasma. We specially highlight that in the pulsed plasma mode, we obtain quantum-sized free standing alloy nanocrystals with a mean size of 7.3 nm. The presence of Si 1−x Ge x alloy particles is confirmed with multiple techniques, i.e. Raman spectroscopy, XRD (Xray diffraction) and HRTEM (high resolution transmission electron microscopy) studies, with each of these methods consistently yielding the same composition. The nanocrystals synthesized here have potential applications in band-gap engineering for multijunction solar cells. [ABSTRACT FROM AUTHOR]

An effective light trapping method for substrate-type hydrogenated amorphous silicon (a-Si:H) thin film solar cells is the use of a back reflector (BR) of high roughness, e.g., ‘hot silver’, which is deposited at temperatures higher than 450 °C. In this work, textured silver-aluminum (Ag–Al) BR films were fabricated by depositing Ag on Al film at Ag-deposition temperatures ( T Ag ) ranging from 25 to 350 °C. The surface morphology and roughness of Ag–Al films were strongly affected by T Ag . The Al and Ag films were formed entirely of Ag 2 Al alloy at T Ag of 330 °C or higher, while the Ag–Al films maintained a double-layered structure at 290 °C or below. Although the films did not undergo alloying at T Ag of 290 °C, the Ag–Al films have a well-developed surface structure with high diffuse-reflectance, compared to Ag films deposited at the same temperature. The conversion efficiency of an a-Si:H thin film solar cell on a flexible stainless steel substrate increased from 7.63% to 8.44% as T Ag was increased from 25 to 290 °C, as a result of more effective light scattering by Ag–Al BRs, producing increased short-circuit current. However, at higher T Ag , Ag 2 Al alloy films with sharp crystallite edges were formed, and were not appropriate as BRs. The present work clearly shows that double-layered Ag–Al films fabricated at temperatures as low as 290 °C could be useful back reflectors for substrate-type thin film solar cells. [ABSTRACT FROM AUTHOR]

This work studies hydrogenated amorphous silicon germanium films, deposited by hot wire chemical vapor deposition, to be used as low band gap absorber material in thin film solar cells. Material properties, such as the bonding configurations, the ambipolar diffusion length and the optical band gap, were examined as a function of the substrate temperature and germanium content. Our best materials were incorporated in single junction solar cells with high long-wavelength response and a tandem solar cell with an efficiency of 10.42%. [ABSTRACT FROM AUTHOR]

We present the synthesis of three-dimensional tungsten oxide (WO 3 − x ) nanostructures, called nanocacti, using hot-wire chemical vapor deposition. The growth of the nanocacti is controlled through a succession of oxidation, reduction and re-oxidation processes. By using only a resistively heated W filament, a flow of ambient air and hydrogen at subatmospheric pressure, and a substrate heated to about 700 °C, branched nanostructures are deposited. We report three varieties of simple synthesis approaches to obtain hierarchical homo- and heterogeneous nanocacti. Furthermore, by using catalyst nanoparticles site-selection for the growth is demonstrated. The atomic, morphological and crystallographic compositions of the nanocacti are determined using a combination of electron microscopy techniques, energy-dispersive X-ray spectroscopy and electron diffraction. [ABSTRACT FROM AUTHOR]

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]

Microcrystalline silicon oxide (µc-SiO x :H) alloys prepared by plasma enhanced chemical vapor deposition (PECVD) represent a versatile material class for opto-electronic applications especially for thin-film and wafer based silicon solar cells. The material is a phase mixture of microcrystalline silicon (µc-Si:H) and amorphous silicon oxide (a-SiO x :H). The possibility to enhance the optical band gap energy and to adjust the refractive index over a considerable range, together with the possibility to dope the material p-type as well as n-type, makes μc-SiO x :H an ideal material for the application as window layer, as intermediate reflector (IR), and as back reflector in thin-film silicon solar cells. Analogously, μc-SiO x :H is a suitable material for p- and n-type contact layers in silicon hetero junction (SHJ) solar cells. The present paper gives an overview on the range of physical parameters (refractive index, optical band gap, conductivity) which can be covered by this material by variation of the deposition conditions. The paper focuses on the interdependence between these material properties and optical improvements for amorphous silicon/microcrystalline silicon (a-Si:H/µc-Si:H) tandem solar cells prepared on different substrates, such as Asahi (VU) and sputtered ZnO:Al. It gives a guideline on possible optical gains when using doped µc-SiO x :H in silicon based solar cells. As intermediate reflector in a-Si:H/µc-Si:H tandem cells µc-SiO x :H leads to an effective transfer of short circuit current generation from the bottom cell to the top cell resulting in a possible thickness reduction of the top cell by 40%. Within another series of solar cells shown in this paper a short circuit current density of 14.1mA/cm2 for an a-Si:H/µc-Si:H tandem solar cell with a µc-SiO x :H intermediate reflector is demonstrated. A SHJ solar cell on a flat (non-textured) wafer using p- and n-type doped µc-SiO x :H contact layers with an effective area efficiency of 19.0% is also presented. [ABSTRACT FROM AUTHOR]

Abstract: We deposited a silicon nitride/polymer hybrid multilayer moisture barrier for flexible electronics in a hot wire chemical vapor deposition process, entirely below 100°C. We were able to reach a water vapor transmission rate (WVTR) as low as 5×10−6 g/m2/day at a temperature of 60°C and a relative humidity of 90% for a simple three-layer structure consisting of two low-temperature silicon nitride (SiNx) layers and a polymer layer in between. This WVTR is low enough for organic and polymer devices. In a second experiment it is investigated how the yield of our samples increases with the number of SiNx layers, while keeping the total SiNx thickness constant. Cross sectional scanning electron microscopy images of degraded samples reveal a high structural robustness of our multilayers. [Copyright &y& Elsevier]

Abstract: 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 1h 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 13.3ms 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. [Copyright &y& Elsevier]