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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: 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]

Abstract: Thin film hydrogenated nanocrystalline silicon (nc-Si:H) solar cells were deposited at a high growth rate of 5 nm/s by Very High Frequency Plasma Enhanced Chemical Vapor Deposition (VHF PECVD). A single deposition yielded 30 working cells out of 30 test cells. After characterization of J-V characteristics under AM1.5 conditions, spectral response and Fourier Transform Photocurrent Spectroscopy (FTPS) for defect density, the cells were subjected to a beam of 1 MeV protons, with different irradiation times for a series of cells, up to a maximum fluence of 1015 protons per cm2, to investigate the feasibility of using this type of solar cells in space. After degradation, the cells were characterized again. The highest fluence reduced the conversion efficiency of the solar cells by a factor of 10. These values are compared to a similar experiment in literature for thin film amorphous silicon cells. The spectral response shows that the quantum efficiency for low energy photons is drastically reduced, suggesting that the damage is mainly inflicted to the bulk of the absorber material. This is strongly supported by the FTPS results, which show a clear trend of increasing defect density with increasing fluence. The optical absorption coefficient at 0.8 eV increased by a factor of 20. After characterization, the cells were isochronally annealed, up to the deposition temperatures, to investigate if the original performance of the cell could be restored. The mid-gap absorption decreased by a factor of 5 after annealing. [ABSTRACT FROM AUTHOR]

Abstract: In order to increase industrial viability and to find niche markets, high deposition rate and low temperature depositions compared to standard deposition conditions are two recent trends in research areas concerning thin film silicon. In situ diagnostic tools to monitor gas phase conditions are useful in quick optimization processes of deposition parameters without going into time consuming material characterizations. Optical emission spectroscopy is an efficient technique to monitor/predict growth rate and phase of the material (amorphous or nanocrystalline). However, at high growth rate conditions which are generally achieved at high chamber pressures (p), the simple correlation breaks down. We see that at high pressure condition a higher Hα/Si* is needed for the onset of crystallinity than that is found at lower pressure conditions. Additional methods such as estimating the silane depletion from the experiment and the flux ratio of atomic hydrogen to deposited silicon atoms from simulations can be used for fine-tuning the amorphous to nanocrystalline transition regime. On the other hand, intensity of Si* line loses its character as a monitoring tool for deposition rate. Moreover, the plasma changes its character when the pressure is varied, even when the pd product is kept constant. In situ diagnosis of the ion energy distribution function by a retarding field ion energy analyzer has thrown new lights on the role of hydrogen dilution for depositions at low substrate temperature conditions, namely to compensate the loss in ion energy due to lower gas temperature. [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: 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 ∼350nm made by VHF PECVD at 50MHz 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 (0.912cm2) efficiency of 7.69% (V oc =0.834V, 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 1000h 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 30min of deposition time. A minimodule of 8×7.5cm2 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 =6.32V, FF=0.65). [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]

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]

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]

Abstract: In order to deposit thin film silicon solar cells on plastics and papers, the deposition process needs to be adapted for low deposition temperatures. In a very high frequency plasma-enhanced chemical vapor deposition (VHF PECVD) process, both the gas phase and the surface processes are affected by low process temperature. Using an electrostatic ion energy analyzer the effect of deposition temperature on the energies of ions reaching the substrate was measured. The ion energy decreases with decreasing temperature, but this can be compensated by diluting the silane source gas by hydrogen. [Copyright &y& Elsevier]

Abstract: Microcrystalline silicon based single junction solar cells have been deposited on Asahi U-type SnO2:F and texture-etched ZnO by means of very high-frequency plasma-enhanced chemical vapor deposition (VHF PECVD) using a showerhead cathode at high pressures in depletion conditions. The defect densities in the intrinsic layers (in the solar cell configuration) have been estimated by means of Fourier-transform photocurrent spectroscopy. An Urbach tail parameter of 35meV and a defect density of smaller than 1015 cm−3 show the high quality of the material. It was found that the cell performance of our cells is dominated by the bulk defect density of the i-layers: the fill factor and open circuit voltage of solar cells deposited at various conditions show a consistent decrease with increasing defect density. It was found that the bulk defect density increases one order of magnitude upon an increase in deposition rate from 0.45 to 4.5nm/s. We show that this increase in defect density can be reduced when the ion bombardment energy is reduced by artificially lowering the plasma potential in the deposition process using an external DC bias. On texture-etched ZnO:Al, at a deposition rate of 0.45nm/s, a stabilized conversion efficiency of 10.0% (4mm×4mm cell area) is obtained for a single junction solar cell with a μc-Si i-layer of 1.5μm thick. This belongs to the highest efficiencies reported for microcrystalline silicon solar cells in the superstrate (p–i–n) configuration. Upon increasing the deposition rate to 4.5nm/s, an efficiency of 6.4% (initial) was obtained. It is observed that the performance of solar cells that are deposited at high deposition rates improves under light soaking conditions at 50°C, even though these cells are deposited at the amorphous to crystalline transition regime. [Copyright &y& Elsevier]

Abstract: Microcrystalline silicon solar cells with intrinsic layer thicknesses between 500 and 3000 nm deposited using the hot-wire CVD technique are investigated, combining experimental characterisation with computer simulations. Fitting of the solar cell characteristic curves shows that this material has a density of dangling bonds and drift mobility that are comparable to that of amorphous silicon, whereas its mobility band gap is closer to the value of crystalline silicon. These fittings can be done assuming homogeneous electrical parameters in the intrinsic layers. A maximum in solar cell performance was seen for i-layer thickness of 3000 nm. [Copyright &y& Elsevier]

We have investigated the material properties and n–i–p solar cell quality of hot-wire deposited amorphous and microcrystalline silicon. Although it is possible to make high quality amorphous silicon solar cells, occasionally many cells show shunting behavior. Therefore, better control over the variation in cell performance is needed. We prove that this behavior is correlated with the filament age and different methods for improving the reproducibility of the cell performance are presented. Furthermore, the influence of different deposition parameters of microcrystalline silicon layers on the material and solar cell properties was studied. Although some of these microcrystalline layers are porous and oxidize in air, an initial efficiency of 4.8% is obtained for an n–i–p cell on untextured stainless steel. [Copyright &y& Elsevier]

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]

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]

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 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: 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: We present an overview of the scientific challenges and achievements during the development of thin film silicon based single and multijunction solar cells with hot-wire chemical vapor deposition (HWCVD) of the active silicon layers. The highlights discussed include the development of Ag/ZnO coatings with a proper roughness and morphology for optimal light trapping in single and multijunction thin film silicon solar cells, studies of the structural defects created by a rough substrate surface and their influence on the performance of nc-Si:H n–i–p single junction solar cells, and studies of the phase change during the growth of nc-Si:H by HWCVD and the use of a ‘reverse’ H2 profiling technique to achieve nc-Si:H single junction n–i–p cells with high performance. Thus far, the best AM1.5 efficiency reached for n–i–p cells on stainless-steel with HWCVD i-layers is 8.6% for single junction nc-Si:H solar cells and 10.9% for triple junction solar cells. The opportunities for further improvement of cell efficiency are also discussed. We conclude that the uniqueness of HWCVD and of the i-layers deposited with this technique require some adjustments in the strategy for optimization of single or multijunction solar cells, such as using a reverse H2 profiling technique for the deposition of nc-Si:H i-layers. However, the output performance of solar cells with HWCVD deposited i-layers is close to those with i-layers deposited by other techniques. The difference between the best nc-Si:H n–i–p cells obtained so far in our lab and the reported best n–i–p cells with 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 bonding rearrangement upon thermal annealing of amorphous silicon nitride (a-SiN x :H) films deposited by hot-wire chemical vapor deposition was studied. A wide range of N/Si atom ratio between 0.5 and 1.6 was obtained for the a-SiN x :H sample series by varying the source gases ratio only. Evolutions of Si–N, Si–H and N–H bonds upon annealing were found to depend strongly on the N/Si atom ratio of the films. According to the above observations, we propose possible reaction pathways for bonding rearrangement in a-SiN x :H with different N/Si ratios. [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: 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]

Abstract: This paper examines the potential of μc-Si deposited by hot-wire chemical vapor deposition (HWCVD) at a constant hydrogen dilution of 0.95, as absorber layer in n–i–p thin film solar cells. For a series of single junction cells onto plain stainless steel, the highest open circuit voltage (V oc) and Fill Factor (FF) were 0.54V and 0.72. From J–V measurements at reduced light intensities, the change of the AM1.5-normalized FF values with light intensity was found to depend on the μc-Si i-layer thickness. This is attributed to the thickness evolution of the i-layer crystallinity, which influences the cell properties, particularly for i-layers thinner than 2.0μm. For thicker layers the measurements clearly show drift-driven charge carrier collection. Tandem cells showed FF values as high as 0.77, which indicates the very high optoelectronic quality of the μc-Si material. A 2.0μm cell on a textured back reflector shows an efficiency of 7.8%. [Copyright &y& Elsevier]

Abstract: In developing n–i–p structured narrow band gap protocrystalline SiGe:H thin film solar cells, an S-shape was encountered in the cells’ current–voltage characteristics, which was not present in the curves of p–i–n cells that incorporated the same i-layer material. With the help of a carefully designed series of annealing experiments, the S-shape was found to consist of two barriers, one of which is located at the p/i interface while the other one is at the ITO/p front contact. Further investigations using thin layer Raman spectroscopy and activation energy measurements pointed out that the barriers are mainly due to a drifting of the μc-Si p-layer deposition from optimized conditions. By optimizing the p-layer deposition and with the use of post-deposition annealing of the entire cell structure, an efficiency of 8.7% was achieved for a 1.55eV band gap protocrystalline SiGe:H n–i–p cell on an Asahi U-type substrate coated with a Ag/ZnO back reflector. [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 vapour deposition (HWCVD) with two tantalum filaments has been employed to deposit highly crystalline microcrystalline silicon (μc-Si:H) for use in the bottom cell of multijunction solar cells. The material was compared to previously developed poly2 material, which is deposited under slightly different conditions. The present material has smaller crystallites, which show a less preferential (220) orientation, and has a lower crystalline volume fraction (80%). A higher oxygen content, suggesting a less compact structure, and well-passivated grain boundaries were observed. A single junction nip-configured solar cell, in which a 1.1-μm-thick layer of the material was incorporated as absorber layer, showed a higher V oc and FF (0.412 V and 0.664) and a lower J sc (16.08 mA/cm2) than the best poly2-containing cell. The lower J sc is a result of poorer light-trapping due to the reduced (220) orientation and size of the materials crystallites. [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]

Abstract: A new regime of high rate deposition of near-stoichiometric silicon nitride by Hot Wire Chemical Vapor Deposition was investigated. The layers were tested on multicrystalline silicon solar cells in order to assess their antireflection and passivation properties. The present design of the filament arrangement and the showerhead gas supply system allows for virtually unlimited scale-up and deposition rates of around 3 nm/s were obtained. By varying the SiH4 flow, the refractive index at 630 nm (n) could be controlled from 1.90±0.05 to 2.12±0.05, and the extinction coefficient at 400 nm (k) was <0.007 for all films of interest. The cells had state-of-the-art values for all photovoltaic parameters, similar to cells with a conventional SiN x antireflection coating. An efficiency of 14.3% was reached using HWCVD SiN x for multicrystalline Si solar cells with an industrial emitter. [Copyright &y& Elsevier]

Abstract: For amorphous and ‘micromorph’ silicon multi-junction solar cells, we have developed tunnel recombination junctions consisting of two microcrystalline doped layers with a defect-rich interface. While the solar cells performed reasonably well under AM 1.5 light, we found in spectral response measurements that the first deposited cell of tandem structures in nip and pin configuration was apparently leaking under low light conditions. Insertion of a thin protection layer of n-type amorphous silicon solved this issue, and led to an increase in open-circuit voltage. Voltages as high as 1.76V have been obtained for a-Si/a-Si pinpin tandem cells. [Copyright &y& Elsevier]

Innovative multibandgap a-Si:H/poly-Si tandem solar cells have been developed, where the two absorbing layers have been deposited by hot-wire CVD. These n–i–p/n–i–p cells have been deposited on a flexible stainless steel substrate, where the microcrystalline doped layers are made by PECVD. No enhanced back reflector was applied. Although the bottom cell shows a shunting problem under low-light conditions, the best tandem cell has an efficiency of 8.1% under AM-1.5 illumination, a fill factor of 0.60, an open-circuit voltage of 1.18 V, and a short-circuit current density of 11.4 mA/cm2. The total thickness of the tandem structure is only 1.1 μm. [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]