Silicon Solar Cell Applications Research Articles

Microcrystalline B-doped window layers prepared near amorphous to microcrystalline transition by HWCVD and its application in amorphous silicon solar cells

The electronic and structural properties of p-type microcrystalline silicon films prepared near the microcrystalline to amorphous (μc-amorphous) transition by hot-wire chemical vapor deposition are studied. Silane is used as a source gas while H 2 as diluent and trimethylboron (TMB) and boron trifluoride (BF 3) as doping gases. Increasing TMB concentration from 0.01% to 5% favors the amorphous growth whereas for BF 3 the crystalline fraction remains constant. The dark conductivity ( σ d) of μc-Si:H p-layers remains approximately constant for TMB = 1–5% at constant crystalline fraction X c. This dark conductivity behavior is attributed to the decrease in doping efficiency with increasing TMB concentration. The best initial efficiency obtained for a 400 nm amorphous pin solar cell with optimized μc-Si:H p-layer is 7.7% ( V oc = 874 mV, J sc = 12.91 mA/cm 2, FF = 68%).

Use of Transparent Conductive Oxide Materials with Low Indices of Refraction in Amorphous Silicon-Based Solar Cell Technology

AbstractNew types of transparent conductive oxides with low indices of refraction have been developed for use in optical stacks for the amorphous silicon (a-Si) solar cell and other thin film applications. The alloys are ZnO based with Si and MgF added to reduce the index of the materials through the creation of SiO2 or MgF2, with n=1.3-1.4, or the addition of voids in the materials. Alloys with 12-14% Si or Mg have indices of refraction at λ=800nm between 1.6 and 1.7. These materials are presently being used in optical stacks to enhance light scattering by Al/multi-layer/ZnO back reflectors in a-Si based solar cells to increase light absorption in the semiconductor layers and increase open circuit currents and boost device efficiencies. In contrast to Ag/ZnO back reflectors which have long term stability issues due to electromigration of Ag, these Al based back reflectors should be stable and usable in manufactured PV products. In this manuscript, structural properties for the materials will be reported as well as the performance of solar cell devices made using these new types of materials.

Structure adjustment during high-deposition-rate growth of microcrystalline silicon solar cells

Preparation of microcrystalline silicon for solar cell applications is investigated under high-pressure, high-power conditions with plasma-enhanced chemical vapor deposition at 95MHz. It is found that the deposition rate depends mainly on the amount of silane in the reaction zone. Changes in the discharge power affect the deposition rate very little. This points to silane depletion under these process conditions. The amount of H radicals, on the other hand, increases with increasing discharge power and leads to structure changes of the material. Making use of this effect, optimum phase mixture material at the transition from highly crystalline to amorphous growth can be deposited at considerably higher deposition rates without loss in solar cell performance.

Effect of UV irradiations on the structural and optical features of porous silicon: application in silicon solar cells

The aim of this paper is to investigate the structural and optical stability of porous silicon layers (PSLs) planned to be used in silicon solar cells technology. The PSLs were prepared by a HNO3/HF vapor etching (VE) based method. Fourier transform infrared (FT-IR) spectroscopy shows that fresh VE-based PSLs contain NH and SiF bonds related to a ammonium hexafluorosilicate (NH4)2SiF6 minor phase, and conventional SiHx and SiOx bonds. Free air exposures of PSLs without and with UV irradiation lead to oxidation or photo-oxidation of the porous layer, respectively. FT-IR characterisation of the PSLs shows that UV irradiations modify the transformation kinetics replacing instable SiHx by SiOx or SiOH bonds. When fresh PSLs undergo free air oxidation within 7 days, the surface reflectivity decreases from 10 to about 8%, while it drops to about 4% when a 10min free air UV irradiation is applied. Long periods of free air oxidation do not ensure the reflectivity to be stable, whereas it becomes stable after only 10min of UV irradiation. This behaviour was explained taking into account the kinetic differences between oxidation with and without UV irradiation. Fresh VE-based PSLs were found to improve efficiently the photovoltaic (PV) characteristics of crystalline silicon solar cells. The passivating action of VE-based PSLs was discussed. An improvement of the PV performances was observed solely for stable oxidized porous silicon (PS) structures obtained from UV irradiations.

Silicontetrachloride based microcrystalline silicon for application in thin film silicon solar cells

The replacement of the process gas silane by silicontetrachloride for the deposition of microcrystalline silicon thin films is explored. The films were prepared by plasma enhanced chemical vapor deposition from mixtures of silicontetrachloride and hydrogen. Deposition conditions similar as employed for efficient silane-based microcrystalline silicon solar cells of high deposition rate were applied. Structural and electronic properties of the films are studied by infrared optical absorption and Raman spectroscopies and conductivity measurements. Increasing the flow ratio of silicontetrachloride to hydrogen, the transition from the crystalline to the amorphous state is found to occur in a similar range of flow ratio as in silane-based material. The lowest porosity is observed for a crystalline fraction near 80%. Porosity, void-related microstructure and chlorine content are found to decrease with increasing substrate temperature. For boron doped material deposited at a substrate temperature of 250 °C, conductivity values exceeding 50 (Ω cm) −1 were achieved. This latter material was incorporated as p-layer into microcrystalline pin silicon solar cells. Solar cell efficiencies similar as for solar cells with a silane-based p-layer were obtained.

High temperature processing of TiO2 thin films for application in silicon solar cells

Low temperature deposited coatings have the potential to reduce the production costs of silicon (Si) photovoltaic devices, such as the buried-contact (BC) solar cell. This work investigates the potential of titanium dioxide (TiO2) films to replace silicon dioxide (SiO2) or nitride coatings currently implemented in the BC fabrication sequence, which requires films to be stable at temperatures up to 1000 °C in a variety of gas atmospheres. The authors demonstrate that: (i) TiO2 films do not reduce the bulk minority carrier lifetime of the silicon wafer after lengthy high temperature processing, however chemical reduction of the TiO2 film can occur if samples are not loaded in an oxygen-containing ambient; (ii) a thin SiO2 passivation layer can be formed at the TiO2:Si interface by performing a brief oxidation after TiO2 film deposition; (iii) while TiO2 coatings function as a phosphorus diffusion barrier, reactions between TiO2 and the phosphorus source result in irreversible damage to the TiO2 film; (iv) phosphorus-doped TiO2 films can act as an n-type dopant source, however further reactions with phosphorus limit the usefulness of the film. Thus, TiO2 films are compatible with the high temperature processing required for BC solar cells, but not with any phosphorus diffusion steps.

Gettering by heat thermal processing: application in crystalline silicon solar cells

The aim of this work is to getter unwanted impurities from solar grade crystalline silicon (Si) wafers and then to enhance their electronic properties. This was done by forming a sacrificial porous silicon (PS) layer on both sides of the Si wafers and by performing infrared (IR) thermal annealing treatments (at around 950 °C) in a SiCl 4/N 2 controlled atmosphere. The process allows concentrating unwanted impurities in the PS layer and near the PS/silicon interface. These treatments reduce the resistivity by about two orders of magnitude at a depth of about 40 μm and improve the minority carrier diffusion length from 75 to 210 μm. This gettering method was also tested on silicon wafers where grooved fingers and back contacts were achieved using a chemical vapor etching (CVE) method. Front buried metallic contacts and small holes for local back surface field were then achieved after the gettering stage in order to realize silicon solar cells. It was shown that the photovoltaic parameters of gettered silicon solar cells were improved as regard to ungettered ones.

Modified technique of using conventional slider boat for liquid phase epitaxy of silicon for solar cell application

Epitaxial layers of silicon are grown on single crystal Si- substrate from a solution of silicon in indium using conventional graphite slider boat technique. The important problems of natural convection due to lower density of silicon compared to indium,poor wetting of substrate due to high angle of contact of indium solution on silicon substrate resulting in poor nucleation, melt removal from the growth substrate and saturation wafer associated with LPE in this technique are practically eliminated using sandwich method with simple modifications of the boat and the method of growth. Some experimental studies on the effect of different surface preparations of growth substrate are also reported. Growth results are shown and discussed. Further, improvization of slider boat to facilitate better study of growth parameters is suggested in the line of modification already carried out.

Dynamic etching of silicon for solar cell applications

If porous silicon (PSi) is going to be seriously considered for antireflection (AR) applications in the solar cell industry, it must not only offer superior AR properties, but must also prove minimally disruptive to the current manufacturing process. This limits its application to the final (backend) fabrication steps, after p–n junction formation. Since typical commercial junction depths are on the order of 350 nm, the PSi etching process must only consume a portion of this emitter layer if the electrical properties of these devices are to be maintained. In this study, we have focused on the development of a dynamic etching process to form thin (<120 nm) graded index PSi layers with excellent broadband AR properties.

Alternative phosphorus-doped amorphous silicon using trimethylphosphine diluted in hydrogen

The objective of our work was the development of an alternative doping method of amorphous and microcrystalline silicon for solar cell applications. n-type amorphous silicon films were prepared from gas mixtures containing the dopant vapor trimethylphosphine (TMP, P(CH 3) 3) diluted in hydrogen—instead of the well-known phosphorus source phosphine (PH 3)—silane and hydrogen. Different plasma sources were used—operating at 13.56, 27.12 MHz (parallel plate capacitive type system) and 163 MHz (VHF resonance plasma source). For all the investigated frequencies and plasma sources a dark conductivity maximum level (at room temperature) higher than 3×10 −3 Ω −1 cm −1 could be reached, with dopant levels (TMP/silane concentration) in the range of 10 −3–10 −2. The sharp decrease of the dark conductivity for higher dopant concentrations (which is characteristic for the hydrocarbon-based phosphorus sources due to the formation of silicon carbide within the silicon structure) was not detected up to dopant levels of 4×10 −2.

Effectiveness of anisotropic etching of silicon in aqueous alkaline solutions

Optical effectiveness of anisotropic etching of (1 0 0) silicon in inorganic alkaline solution has been studied from the view point of its application in commercial silicon solar cells. The damage caused by ID saw or wire saw during slicing of the wafer is required to be removed for fabrication of solar cells. The etch rates for removal of the surface damages for boron doped Czochralski wafers of 1–2 Ω cm resistivity in 20% NaOH solution at 80°C was measured and was found to be ∼1.4 μm/min. After the damage removal, texturisation was obtained in 2% NaOH solution buffered with isopropyl alcohol at 80°C. An optical effectiveness parameter f eff,λ was defined and its value was estimated from the study of reflectivity and topography of the wafers textured for different durations of time. The kinetics of anisotropic etching was studied which indicated that growth of pyramids begins at preferential sites which may arise due to crystalline defects or wetting. Silicon solar cells have been realized by standard process involving phosphorous diffusion and vacuum evaporated front and back contacts. The value of optical effectiveness parameter is found to have a direct correlation with the improvement in short circuit current density of the textured cells.

Surface recombination velocity of phosphorus-diffused silicon solar cell emitters passivated with plasma enhanced chemical vapor deposited silicon nitride and thermal silicon oxide

The emitter saturation current density (JOe) and surface recombination velocity (Sp) of various high quality passivation schemes on phosphorus-diffused solar cell emitters have been determined and compared. The passivation schemes investigated were (i) stoichiometric plasma enhanced chemical vapor deposited (PECVD) silicon nitride (SiN), (ii) forming gas annealed thermally grown silicon oxide, and (iii) aluminum annealed (alnealed) thermal silicon oxide. Emitters with sheet resistances ranging from 30 to 430 and 50 to 380 Ω/□ were investigated for planar and random-pyramid textured silicon surfaces, which covers both industrial and laboratory emitters. The electronic surface passivation quality provided by PECVD SiN films was found to be good, with Sp values ranging from 1400 to 25 000 cm/s for planar emitters. Thin thermal silicon oxides were found to provide superior passivation to PECVD SiN, with the best passivation provided by an alnealed thin oxide (Sp values between 250 and 21 000 cm/s). The optimized PECVD SiN films are, nevertheless, sufficiently good for most silicon solar cell applications.

A review of thin-film crystalline silicon for solar cell applications. Part 1: Native substrates

Approximately half the cost of a finished crystalline silicon solar module is due to the silicon itself. Combining this fact with a high-efficiency potential makes thin-film crystalline silicon solar cells a growing research area. This paper, written in two parts, aims to outline world-wide research on this topic. The subject has been divided into techniques which use native substrates and techniques which use foreign substrates. Light trapping, vapour- and liquid-phase deposition techniques, cell fabrication and some general considerations are also discussed with reference to thin-film cells.

A review of thin-film crystalline silicon for solar cell applications. Part 2: Foreign substrates

One of the most promising ways to reduce the cost of photovoltaics is thin-film crystalline silicon solar cells. This paper, together with part 1, reviews the current state of research in thin-film crystalline silicon solar cells. Deposition on silicon, novel techniques which use a high-quality, reusable silicon substrate and light trapping have been described in part 1 of this paper. This paper describes deposition on glass and ceramics and discusses cell designs for thin-film crystalline silicon solar cells.

Large-grained polycrystalline Si films obtained by selective nucleation and solid phase epitaxy

We investigated the formation of large-grain polycrystalline silicon films on glass substrates for application in low-cost thin film crystalline silicon solar cells. Since the use of glass substrates constrains process temperatures, our chosen approach to form large-grain polycrystalline silicon templates is selective nucleation and solid phase epitaxy (SNSPE). In this process, selective crystallization of an initially amorphous silicon film, at lithographically predetermined sites, enables grain sizes larger than those observed via random crystallization. Selective heterogeneous nucleation centers were created for both P-doped, B-doped and undoped, 100 nm thick amorphous silicon films, by masked implantation of In or Ni islands, followed by annealing at temperatures below 600°C. Seeded crystallization begins at the metal islands and continues via lateral solid phase epitaxy (SPE), thus obtaining crystallized regions of several tens of square microns. The maximum achievable grain size depends on the product of the SPE rate and the incubation time for the spontaneous nucleation. We have studied the dependence of the SPE rate and the incubation time on the type of metal (In and Ni) inducing the nucleation and on the electronic dopant (e.g. P and B) concentration in the 10 19–10 21 cm −3 range.

00/00366 Formation and characterization of porous silicon layers for application in multicrystalline silicon solar cells

Porous silicon (PS) layers are formed on multicrystalline silicon solar cells by chemically etching the emitter of these devices. Stain etched PS provides the simultaneous formation of a selective emitter and an antireflective layer. The optical behavior of the antireflective coating over the solar spectrum is determined, resulting in very low values of the reflectance. The variation of the topography, as well as the grain size of the PS and the metallic contacts before and after PS formation, is analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Furthermore, the electrical properties of both the emitter and the front metallic contacts are investigated. Finally, the effect of rapid thermal processing in nitrogen and oxygen atmospheres on the antireflective properties of the PS is studied.

Formation and characterization of porous silicon layers for application in multicrystalline silicon solar cells

The electrochemical formation of porous silicon (PS) layers in the n + emitter of silicon p–n + homojunctions for solar energy conversion has been investigated. During the electrochemical process under constant polarization, a variation of the current density occurs. This effect is explained by considering the doping impurity gradient in the emitter and by TEM characterization of the PS layer structure. Optical transmission measurements indicate that modifications of the refractive index and absorption coefficient of PS are mainly related to the porosity value. Reflectivity measurements, spectral response and I–V characteristics show that PS acts as an efficient antireflection coating layer. However, beyond a critical layer thickness, i.e. when PS reaches the p–n + interface, the junction properties are degraded.

Ni-Induced Selective Nucleation and Solid Phase Epitaxy of Large-Grained Poly-Si on Glass

AbstractWe investigated the formation of large-grain polycrystalline silicon films on glass substrates for application in low-cost thin film crystalline silicon solar cells. Since use of glass substrates constrains process temperatures, our approach to form large-grain polycrystalline silicon templates is selective nucleation and solid phase epitaxy (SNSPE). In this process, selective crystallization of an initially amorphous silicon film, at lithographically predetermined sites, enables grain sizes larger than those observed via random crystallization. Selective heterogeneous nucleation centers were created on undoped, 75 nm thick, amorphous silicon films, by masked implantation of Ni islands, followed by annealing at temperatures below 600 °. At this temperature, the Ni precipitates into NiSi2 particles that catalyze the transition from the amorphous to the crystalline Si phase. Seeded crystallization begins at the metal islands and continues via lateral solid phase epitaxy (SPE), thus obtaining crystallized regions of several tens of square microns in one hour. We have studied the dependence of the crystallization rate on the Ni-implanted dose in the seed, in the 5×1015/cm3 - 1016/cm3range. The large grained polycrystalline Si films were then used as a substrate for molecular beam epitaxy (MBE) depositions of 1 [.proportional]m thick Si layers. Transmission electron microscopy (TEM) analysis showed a strong correlation between the substrate morphology and the deposited layer. The layer presented a large grain morphology, with sizes of about 4 [.proportional]m.

Progress in Low-temperature Surface Passivation of Silicon Solar Cells using Remote-plasma Silicon Nitride

Using a remote-plasma technique as opposed to the conventional direct-plasma technique, significant progress has been obtained at ISFH in the area of low-temperature surface passivation of p-type crystalline silicon solar cells by means of silicon nitride (SiN) films fabricated at 350–400°C in a plasma-enhanced chemical vapour deposition system. If applied to the rear surface of the low-resistivity p-type substrates, the remote-plasma SiN films provide outstanding surface recombination velocities (SRVs) as low as 4 cm s−1, which is by a clear margin the lowest value ever obtained on a low-resistivity p-Si wafer passivated by a solid film, including highest quality thermal oxides. Compared to direct-plasma SiN films or thermally grown oxides, the remote-plasma films not only provide significantly better SRVs on low-resistivity p-silicon wafers, but also an enormously improved stability against ultraviolet (UV) light. The potential of these remote-plasma silicon nitride films for silicon solar cell applications is further increased by the fact that they provide a surface passivation on phosphorus-diffused emitters which is comparable to high-quality thermal oxides. Furthermore, if combined with a thermal oxide and a caesium treatment, the films induce a UV-stable inversion-layer emitter of outstanding electronic quality. Due to the low deposition temperature and the high refraction index, these remote-plasma SiN films act as highly efficient surface-passivating antireflection coatings. Application of these films to cost-effective silicon solar cell designs presently under development at ISFH turned out to be most successful, as demonstrated by diffused p-n junction cells with efficiencies above 19%, by bifacial p-n junction cells with front and rear efficiencies above 18%, by mask-free evaporated p-n junction cells with efficiencies above 18% and by MIS inversion-layer cells with a new record efficiency of above 17%. All cells are found to be stable during a UV test corresponding to more than 4 years of glass-encapsulated outdoor operation. © 1997 John Wiley & Sons, Ltd.

Recent progress of amorphous silicon solar cell applications and systems

Fifteen years have passed since the first industrial use of amorphous silicon (a-Si) solar cells for consumer products. At present, a-Si solar cells are entering a new age of use in power generating systems at private residences and other outdoor applications. This paper reviews recent advances in amorphous silicon (a-Si) solar cells and their applications. Technological developments in the field of a-Si solar cells are discussed. Various applications and systems that take advantage of the a-Si solar cell are then introduced. Finally, future prospects are discussed, including a new concept of GENESIS system for worldwide energy generation and transmission.