Electrical and optical characterization of crystalline silicon/porous silicon heterojunctions

We have investigated the photovoltage and photocurrent spectra of crystalline silicon/porous silicon heterojunctions. The porous silicon layers were prepared using anodic etching of p-type crystalline silicon at a current density of 25 mA/cm². From the spectral dependence of the photovoltage and photocurrent, we suggest that the photovoltaic properties of the junction are dominated by absorption in crystalline silicon only. We have also studied the effect of increase in the thickness of porous silicon layers on these spectra. We find that the open-circuit voltage of the devices increases, but the short-circuit current decreases with an increase in the thickness of the porous silicon layers. We propose a qualitative explanation for this trend, based on the increase in the series and the shunt resistance of these devices. The effect of hydrogen passivation on the junction properties by exposing the devices to hydrogen plasma is also reported.     

Electrical properties of Cl incorporated hydrogenated amorphous silicon

The electrical properties of chlorine incorporated hydrogenated amorphous silicon films were studied. The samples were deposited with dichlosilane/silane mixtures. The conductivity of these chlorine-incorporated hydrogenated amorphous silicon films decreases with increasing dichlosilane to silane ratio. The Fermi level shifts toward the valence band with increasing chlorine content, resulting in a corresponding conductivity activation energy and room temperature conductivity decrease. However, the defect density and Urbach energy are noot significantly changed in films containing chlorine.     

Surface contributions to the effective optical properties of porous silicon

Porous silicon (PS) presents efficient photoluminescence and electroluminescence with potential applications in the optoelectronic industry, in particular in photovoltaic devices. It is now generally accepted that the interesting optical properties of PS are due to two combined aspects: on the one hand, the quantum confinement and on the other, the surface states. Although there has been a great effort to study PS, its transport properties are still not well understood. Due to the complex structure of PS an averaging theory to describe its effective properties is justified. In this work the effective dielectric function, effective absorption coefficient and effective refractive index are calculated using the volume averaging method for a model of periodic columns with different surface coatings simulating porous silicon. This approach allows analytical results within certain approximations and the analysis of surface contributions. The method uses parameters to characterize the bulk and the surface. We choose for the bulk c-Si, and cover it with three different possible surfaces: siloxane, a-Si : H and SiO₂. The results are compared with experimental data and other theoretical approaches for silicon wires. We obtain good agreement with some experimental results showing the important role of the surface in the effective response of porous silicon.     

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.     

Electrical and photovoltaic properties of Cr/Si Schottky diodes

The electrical properties of the Cr/p-Si(111) and Cr/n-Si(100) junctions were investigated through capacitance–voltage and current–voltage measurements, performed under dark and light conditions at room temperature. Diode parameters of Cr/Si Schottky diode like ideality factor and barrier height were obtained and variations of them were monitored as a function of temperatures. Also, an attempt to explore the governing current flow mechanism was tried. The reverse biased I–V measurement under illumination exhibited anomalous behavior as well as high photosensitivity. The former was explained in terms of minority carrier injection phenomenon. The photovoltaic parameters, such as open circuit voltage and short circuit current were obtained as 370 mV and I_sc = 44.5 μA, respectively.     

Structural and electrical properties of Cr-doped SrTiO3 porous materials

Series of single-phase materials with assumed formula SrTi_1?xCr_xO₃ (where x = 0, 1, 4, 6 mol.%) were obtained by sol-gel method. The structure and microstructure of materials were characterised by X-ray diffraction and scanning electron microscopy methods. Moreover, the study of electrical properties and evaluation of chemical stability in CO₂/H₂O atmosphere was performed by electrochemical impedance spectroscopy and thermogravimery methods, respectively. The possibility of participation of Cr-doped strontium titanate in oxidation–reduction processes was analysed by temperature-programed reduction (TPR) and temperature-programed oxidation (TPOx) measurements. The changes of lattice parameters together with XPS analysis, the Seebeck coefficient measurements results and TPR profiles obtained for SrTi_1?xCr_xO₃ materials prove the presence of chromium on +3 and +6 oxidation stages. Thus, chromium can be treated as both acceptor- and donor-type dopant in the SrTiO₃ structure. The Cr³⁺/Cr⁶⁺ ratio strongly affects the electrical properties, as the change of conduction mechanism was observed. The results of performed stability test clearly indicate that incorporation of chromium into SrTiO₃ structure results with decrease of chemical stability in CO₂ atmosphere.     

Correlation between reflectivity and photoluminescent properties of porous silicon films

Porous silicon (PS) layers were formed on p-type, 〈1 0 0〉 oriented, 1-5 Ω cm resistivity Cz silicon wafers by electrochemical etching in an HF:C₂H₅OH (1:2 by volume) electrolyte at room temperature at a constant current density 20 mA/cm². The etching duration was varied to achieve PS layers of different morphologies and thicknesses. Both the photoluminescence (PL) and the total diffused reflectivity spectra of the PS layers were measured. It was found that for the PS layers grown for etching durations of less than 90 s the PL emission is insignificant and reflectivity is quite low. Such PS layers can be used as antireflection coatings (ARC) on solar cells. The PS layers formed for etching durations greater than 90 s show a significant PL emission in 500-800 nm range with peak lying in 630-660 nm wavelength range. When etching duration increases from 90 s to 8 min the PL intensity increases and the PL peak shows a blue shift. With further increase in etching duration the PL intensity decreases and PL peak shows a red shift. The reflectivity of the photoluminescent layers increases with etching duration showing a highest value for a sample grown for 8 min. Further increase in etching duration up to 20 min the reflectivity decreases and then increases. Striking observation is that both the PL emission intensity and reflectivity in the wavelength range of 550-800 nm are maximum for the PS layer grown for the etching duration of 8 min.     

Multicrystalline silicon solar cells with porous silicon emitter

A review of the application of porous silicon (PS) in multicrystalline silicon solar cell processes is given. The different PS formation processes, structural and optical properties of PS are discussed from the viewpoint of photovoltaics. Special attention is given to the use of PS as an antireflection coating in simplified processing schemes and for simple selective emitter processes as well as to its light trapping and surface passivating capabilities. The optimization of a PS selective emitter formation results in a 14.1% efficiency mc-Si cell processed without texturization, surface passivation or additional ARC deposition. The implementation of a PS selective emitter into an industrially compatible screenprinted solar cell process is made by both the chemical and electrochemical method of PS formation. Different kinds of multicrystalline silicon materials and solar cell processes are used. An efficiency of 13.2% is achieved on a 25 cm² mc-Si solar cell using the electrochemical technique while the efficiencies in between 12% and 13% are reached for very large (100–164 cm²) commercial mc-Si cells with a PS emitter formed by chemical method.     

The influence of porous silicon on junction formation in silicon solar cells

In this work the results of a structural investigation by SEM of porous silicon (PS) before and after diffusion processes are reported. The formation of PS n⁺/p structures were carried out on PS p/p silicon wafers with two methods: from POCl₃ in a conventional furnace and from a phosphorous doped paste in an infrared furnace. Sheet resistance was found to be a strong function of PS structure. Further details on sheet resistance distribution are reported. The electrical contacts in prepared solar cells were obtained by screen printing process, with a Du Ponte photovoltaic silver paste for front contacts and home-prepared silver with 3% aluminium paste for the back ones. Metallization was done in the infrared furnace. Solar cell current–voltage characteristics were measured under an AM 1.5 global spectrum sun simulator. The average results for multi-crystalline silicon solar cells without antireflection coating are: I_sc=720 (mA), V_oc=560 (mV), FF=69%, Eff=10.6% (area 25 cm²).     

Optimization of porous silicon reflectance for silicon photovoltaic cells

The antireflection properties of electrochemically formed porous silicon (PS) layers in the 0.3 μm thick n⁺ emitter of Si p–n⁺ junctions, have been optimized for application to commercial silicon photovoltaic cells. The porosity and thickness of the PS layers are easily adjusted by controlling the electrochemical formation conditions (current density and anodization time). The appropriate PS formation conditions were determined by carrying out a two steps experiment. A first set of samples allowed to determine the optimal porosity and a second one to adjust the thickness of the PS layers, by evaluating the interference features of the reflectance produced by the layers. A PS layer with optimal antireflection coating (ARC) characteristics was obtained in 30% HF in only 3.5 s. The effective reflectance is reduced to 7.3% between 400 and 1150 nm which leads to a gain of up to 33% in the theoretical short circuit current of a p–n⁺ shallow junction compared to a reference junction without a PS layer. The effective reflectance with optimized PS layers is significantly less than that obtained with a classical TiO₂ ARC on a NaOH pretextured multicrystalline surface (11%).     

Enhancement of photovoltaic properties of multicrystalline silicon solar cells by combination of buried metallic contacts and thin porous silicon

Photovoltaic properties of buried metallic contacts (BMCs) with and without application of a front porous silicon (PS) layer on multicrystalline silicon (mc-Si) solar cells were investigated. A Chemical Vapor Etching (CVE) method was used to perform front PS layer and BMCs of mc-Si solar cells. Good electrical performance for the mc-Si solar cells was observed after combination of BMCs and thin PS films. As a result the current-voltage (I-V) characteristics and the internal quantum efficiency (IQE) were improved, and the effective minority carrier diffusion length (Ln) increases from 75 to 110 μm after BMCs achievement. The reflectivity was reduced to 8% in the 450-950 nm wavelength range. This simple and low cost technology induces a 12% conversion efficiency (surface area = 3.2 cm²). The obtained results indicate that the BMCs improve charge carrier collection while the PS layer passivates the front surface.     

Study of electrical properties of oxidized porous silicon for back surface passivation of silicon solar cells

Back surface passivation becomes a key issue for the silicon solar cells made with thin wafers. The high surface recombination due to the metal contacts can be lowered by reducing the back contact area and forming local back surface field (LBSF) in conjunction with the passivation with dielectric layer. About 3×10^-7 m thick porous silicon (PS) layer with pore diameter mostly of 1×10^-8–5×10^-8 m was formed by chemical etching of silicon using the acidic solution containing hydrofluoric acid (HF), nitric acid (HNO₃) and De-ionized water in the volume ratio 1:3:5 at 298 K for which etching time was kept constant for 360 s. Electrical properties of oxidized PS was studied through the current–voltage (I–V) and capacitance–voltage (C–V) characteristics of the metal–insulator–semiconductor (MIS) device in which the oxidized PS was used as an insulating layer and the results were further analyzed. The C–V curves of all the studies MIS devices showed the negative flatband voltage varying from -2 to , confirming that the oxidized layer of PS has fixed positive charge.     

Efficiency improvement by porous silicon coating of multicrystalline solar cells

Shallow junction multicrystalline Si solar cells have been processed by an anodical etching technique. More than 25% improvement in short-circuit current and photovoltaic energy conversion efficiency was demonstrated. It was shown that improved performance was caused by antireflection action of the porous silicon layer as well as by the cell surface and grain boundary passivation.     

Fabrication of buried contact silicon solar cells using porous silicon

We report the fabrication of buried contact solar cells using porous silicon as sacrificial layer to create well-defined channels (for buried contacts) in silicon. In this paper, the salient features of the technology have been presented. No detrimental effect was found in the performance of buried contact solar cell with partially filled contact area compared to the solar cells having conventional planar contacts. However, a marked difference in the short circuit current density was seen when channel was fully filled with metal by screen printing, without degradation in the open-circuit voltage. It is expected that improved processing in combination with optimized buried metallic contact parameters may yield higher efficiencies that may result in substantial decrease in solar cell cost.     

Analysis of the interband transitions in porous silicon

Porous silicon (PS) presents interesting phenomena such as efficient luminescence and a peculiar transport of carriers. Due to its possible optoelectronic applications, it is important to calculate the dielectric function from interband optical transitions in PS to include quantum effects. In this work, we apply a supercell model for PS within an sp³s^* tight-binding technique, to analyze the effects of pores on the above-mentioned transitions. The polarized light absorption is studied by observing the oscillator strength behavior within two different schemes, which are applied and compared. We have found a significant enlargement of the optically active zone in the k-space, due to the localization of the wave function. The calculated dielectric functions for crystalline silicon and PS are compared with experimental results, giving the correct energy range and shape.     

Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N⁺/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N⁺/P, 78.5 cm²) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I_sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I–V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.     

Reactive wave propagation in energetic porous silicon composites

A parametric study of reactive wave propagations in porous silicon (PS) – oxidizer composites is presented. This study investigates the effects of the composite equivalence ratio and the oxidizer, and also the nanoscale and microscale structure, and the effect of dopant atoms, which are specific to this nanostructured composite material. The reactive wave speed and structure for energetic PS composites formed by depositing sodium, magnesium, or calcium perchlorates within the nanoscale pores were analyzed with high speed video recordings and spectroscopic temperature measurements. The findings indicate that heavily doped samples that do not yield a microscale structure result in slow propagation speeds, and low doped substrates with randomly formed micro-crack patterns during the electrochemical dissolution result in high speed propagations. A systematic study of the mixture composition revealed very wide flammability limits and flame speed and temperature measurements independent of the equivalence ratio, consistent with thermochemical equilibrium calculations. Also, while all the composites considered in this study are fuel rich with equivalence ratios greater than 1.60, the composites with equivalence ratios closer to unity exhibited lower temperatures and propagation speeds than more fuel rich composites. This unusual behavior of the composites is attributed to the inhomogeneity of the system even though the reactants are mixed at the nanometer scale. This was illustrated by developing a phenomenological model describing the interaction of silicon and the oxidizer within a single nanometer scale pore, which revealed that the reactive wave propagation is more strongly controlled by the specific surface area than the global equivalence ratio, due to the diffusion length scales involved.     

Hybrid copper–indium disulfide/polypyrrole photovoltaic structures prepared by electrodeposition

Photovoltaic structures on the basis of photoactive polycrystalline CuInS₂ (CIS) in combination with electrically conductive polymer polypyrrole (PPy) were prepared using the electrodeposition technique. In order to prepare stable PPy films with a good adherence to the surface of CIS, the appropriate concentrations of reagents, current densities and electrodeposition potentials were selected experimentally. Electrochemical polymerization of pyrrole to PPy on the CIS surface is faster under white light irradiation, and the polymerization starts at a lower potential than in the dark. Significant photovoltage and photocurrent of the fabricated CIS/PPy structures have been observed under standard white light illumination. The best structure showed an open-circuit voltage of 509 mV, a short-circuit current density of 6.45 mA/cm², fill-factor 0.47 and photoconversion efficiency of 1.53% under white light illumination of 100 mW/cm².     

Electrical/electronic effects of titanium and iron impurities in EFG and FZ solar cell silicon: SPV/EBIC analysis

In this paper results on surface photovoltage (SPV) and electron beam induced conductivity (EBIC) studies of edge-defined film-fed growth (EFG) and floating zone (FZ) silicon solar cell materials (both p-type) are presented. A systematic comparison based on minority carrier diffusion length and carrier recombination is made between: (i) samples contaminated with Ti and/or Fe, (ii) samples gettered by phosphorous diffusion, and (iii) as-received samples. Deep level transient spectroscopy (DLTS) measurements, together with the iron-boron (FeB) pairing kinetics [1] have successfully been used to detect the presence of Fe in the samples. Even though this process is effective in revealing Fe impurities in p-type FZ silicon it is evidently not suitable for Fe identification in p-type EFG silicon. Ti, like Fe, is found to be a prominent lifetime-limiting metallic impurity in both EFG and FZ silicon. Phosphorous diffusion is proven to be an effective external gettering technique for fast-diffusing impurities such as Fe, but not for Ti.     

Gettering impurities from crystalline silicon by phosphorus diffusion using a porous silicon layer

The possible benefits of phosphorus-based gettering applied to crystalline silicon wafers have been evaluated. The gettering process is achieved by forming porous silicon (PS) layers on both sides of the Si wafers. The PS layers were formed by the stain-etching technique, and phosphorus diffusion using liquid POCl₃-based source was done on both sides of the Si wafer. The realized phosphorus/PS/Si/PS/phosphorus structure undergoes a heat treatment in an infrared furnace under an O₂/N₂ controlled atmosphere. This heat treatment allows phosphorus to diffuse throughout the PS layer and to getter eventual metal impurities towards the phosphorus doped PS layer. The gettering effect was evaluated using four probe points, Hall effect measurements and the light beam induced current (LBIC) technique. These techniques enable to measure the density and the mobility of the majority carrier and the minority carrier diffusion length (L_d) of the Si substrate. We noticed that the best gettering is achieved at 900 °C for 90 min of heat treatment. After gettering impurities, we found an apparent enhancement of the mobility and the minority carrier diffusion length as compared to the reference substrate.