Light trapping in layer-transferred quasi-monocrystalline porous silicon solar cell

Quasi-monocrystalline porous silicon (QMPS) layers have a top surface like crystalline silicon with small voids in the body. Such layers are reported to have significantly higher absorption coefficient compared to crystalline silicon at the wavelength of interest for solar cells. A model has been developed to account for higher absorption coefficient of QMPS layer. The model conforms to the experimental results. The model is then extended to predict absorption coefficient of QMPS layer for different thickness, porosity and void size. Interesting results are obtained, particularly regarding the dependence of absorption coefficient on thickness and void diameter of QMPS layers. Computed values of absorption coefficient and some experimental results relating to electronic properties of QMPS layers are used to investigate the solar cell potential of QMPS layers. Short circuit current density of about 31 mA/cm² is predicted for a QMPS layer of thickness 4 μm having average void radius of about 15 nm assuming effective diffusion length to be 5 μm.     

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%).     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells

We have investigated the photovoltaic (PV) characteristics of both glow discharge deposited hydrogenated amorphous silicon (a-Si:H) on crystalline silicon (c-Si) in a n⁺ a-Si:H/undoped a-Si:H/p c-Si type structure, and DC magnetron sputtered a-Si:H in a n-type a-Si:H/p c-Si type solar cell structure. It was found that the PV properties of the solar cells were influenced very strongly by the a-Si/c-Si interface. Properties of strongly interface limited devices were found to be independent of a-Si thickness and c-Si resistivity. A hydrofluoric acid passivation prior to RF glow discharge deposition of a-Si:H increases the short circuit current density from 2.57 to 25.00 mA/cm² under 1 sun conditions.DC magnetron sputtering of a-Si:H in a Ar/H₂ ambient was found to be a controlled way of depositing n type a-Si:H layers on c-Si for solar cells and also a tool to study the PV response with a-Si/c-Si interface variations. 300 Å a-Si sputtered onto 1–10 ω cm p-type c-Si resulted in 10.6% efficient solar cells, without an A/R coating, with an open circuit voltage of 0.55 V and a short circuit current density of 30 mA/cm² over a 0.3 cm² area. High frequency capacitance-voltage measurements indicate good junction characteristics with zero bias depletion width in c-Si of 0.65 μm. The properties of the devices have been investigated over a wide range of variables like substrate resistivity, a-Si thickness, and sputtering power. The processing has focused on identifying and studying the conditions that result in an improved a-Si/c-Si interface that leads to better PV properties.     

The influence of porous silicon coating on silicon solar cells with different emitter thicknesses

The influence of the emitter thickness on the photovoltaic properties of monocrystalline silicon solar cells with porous silicon was investigated. The measurements were carried out on n⁺p silicon junction whose emitter depth was varied between 0.5 and 2.2 μm. A thin porous silicon layer (PSL), less than 100 nm, was formed on the n⁺ emitter. The electrical properties of the samples with PS were improved with decrease of the n⁺p junction depth. Our results demonstrate short-circuit current values of about 35–37 mA/cm² using n⁺ region with 0.5 μm depth. The observed increase of the short-circuit current for samples with PS and thin emitter could be explained not only by the reduction of the reflection loss and surface recombination but also by the additional photogenerated carriers within the PSL. This assumption was confirmed by numerical modeling. The spectral response measurements were performed at a wavelength range of 0.4–1.1 μm. The relative spectral response showed a significant increase in the quantum efficiency of shorter wavelengths of 400–500 nm as a result of the PS coating. The obtained results point out that it would be possible to prepare a solar cell with 19–20% efficiency by the proposed simple technology.     

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.     

Thickness determination of thin (20 nm) microcrystalline silicon layers

For the development of thin, doped microcrystalline silicon (μc-Si) layers, it is necessary to have an accurate tool to determine the thickness and material properties of layers around 20 nm. Here, we report on the interpretation of UV-VIS-NIR spectroscopy (reflection/transmission) measurements using the O’Leary, Johnson, Lim (OJL) model in which we add extra information to compensate for the loss of density information due to the lack of fringes. Moreover, using this method we extract information that can be correlated to the crystalline ratio of μc-Si:H thin layers. We correlate thicknesses and material properties obtained from the optical method to the results obtained from various other techniques: Raman spectroscopy, Rutherford back scattering (RBS) and cross-sectional transmission electron microscopy (X-TEM).By analyzing the data of thin μc-Si:H layers (20 nm) as well as of thicker layers (100 nm) and comparing the results to thicknesses measured with X-TEM, we conclude that as long as the density of thin layers is identical to the thicker layers, with the optical method a good approximation of thickness of microcrystalline silicon layers is possible at a layer thickness down to 20 nm.     

Release of hydrogen during transformation from porous silicon to silicon oxide at normal temperature

The use of porous silicon as an energy carrier is investigated. NaOH and solid Mg alloy are used to introduce OH⁻ in water to react with the porous silicon and the porous silicon treated with Mg alloy in water is converted to transparent silicon oxide hydride. The amount and release rate of hydrogen from the reaction between porous silicon and water are determined and the efficiency is also studied. The total amount of released hydrogen does not vary much with the pH value but the release rate is sensitive to the pH value. The average amount of hydrogen produced form porous silicon can reach 63.2 mmol per gram of porous silicon. A moderate rate of about 1.77 mol of H₂ per mol of porous silicon can be obtained per day with the aid of the Mg alloy. This technique is potential useful in supplying hydrogen to fuel cells at normal temperature.     

Interface effects on the carrier transport and photovoltaic properties of hydrogenated amorphous silicon/crystalline silicon solar cells

Current-voltage-temperature (I-V-T) characteristics evaluated near 150K and 300K were used to study the photovoltaic property variations in hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) solar cells. The possible carrier transport mechanisms in such devices were examined from the I-V-T data which indicated a significant influence of the amorphous /crystalline interface on the short-circuit current density (J_sc) and open-circuit voltage (V_oc) of the solar cells. Carrier transport near 300K for forward biases was by a multi-tunneling mechanism and became space charge limited with increasing bias. For devices having low J_sc and V_oc an additional region was seen in both forward and reverse biases, at low temperatures, where the current simply varied linearly with the applied bias. This characteristic manifested in both high and low temperatures region for devices with still lower photovoltaic properties, which has been reasoned to be due to a higher interface density. Passivating the c-Si surface with HF just prior to the amorphous layer deposition resulted in a large improvement in the properties. The most significant effect was on the J_sc which improved by an order of magnitude. The treatment also affected the lower temperature I-V-T data in that the current fell to very low levels. The spectral response of the treated solar cells showed enhanced blue/violet response compared with the unpassivated devices. The interface passivation plus reducing a-Si thickness has improved the solar cell efficiency from 0.39% to 9.5%.     

Formation of a low reflective surface on crystalline silicon solar cells by chemical treatment using Ag electrodes as the catalyst

A new method was developed for making a porous silicon layer as an anti-reflective coating on the top of crystalline silicon solar cells. The porous silicon layer was formed in a mixed solution of H₂O₂ and HF by using screen-printed Ag front electrodes as the catalyst. With the help of the catalytic effect, porous silicon layers were formed by treatment in a solution chemically milder than conventional solutions. The multi-crystalline silicon solar cell covered with the porous silicon layer showed a surface reflectance below 15% in a wavelength region of 400–800 nm.     

Process development of amorphous silicon/crystalline silicon solar cells

We have already investigated some crucial limiting process steps of the amorphous silicon (a-Si)/crystalline silicon (c-Si) solar cell technology and some specific characterization tools of the ultrathin amorphous material used in devices. In this work, we focus our attention particularlyon the technology of the ITO front contact fabrication, that also is used as an antireflective coating. It is pointed out that this layer acts as a barrier layer against the diffusion of metal during the annealing treatments of the front contact grid. The criteria of the selection of the metal to be used to obtain good performance of the grid and the deposition methods best suited to the purpose are shown. We were able to fabricate low temperature heterojunction solar cells based p-type Czochralski silicon, and a conversion efficiency of 14.7% on 3.8 cm² area was obtained without back surface field and texturization.     

Investigation of porous silicon thin films for electrochemical hydrogen storage

In this study, nanoporous silicon (PS) layers have been elaborated and used for hydrogen storage. The effect of the thickness, porosity and specific surface area of porous silicon on the amount of hydrogen chemically bound to the nanoporous silicon structures is studied by Infrared spectroscopy (FTIR), cyclic voltammetry (CV), contact angle and capacitance –voltage measurements. The electrochemical characterization and hydrogen storage were carried out in a three-electrode cell, using sulfuric acid 3 M H₂SO₄ as electrolyte by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge. The results indicate the presence of two oxidation peaks at 0.2 V and 0.4 V on the anodic side corresponding to hydrogen desorption and a reduction peak at −0.2 V on the cathodic side corresponding to the sorption of hydrogen. Moreover, the EIS studies performed on PS electrode in 3 M H₂SO₄ show that the hodograph contains a semicircle at high frequency region and a line in the lower frequency zone. An equivalent circuit has been proposed; the values of the equivalent circuit elements corresponding to the experimental impedance spectra have been determined and discussed. Finally, the highest hydrogen storage in PS was 86 mAh/g. This storage capacity decreases by only 7% of the initial capacity value, after 40 cycles.     

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.     

Amorphous silicon solar cells

The perfectioning of the deposition techniques of amorphous silicon over large areas, in particular film homogeneity and the reproducibility of the electro-optical characteristics, has allowed a more accurate study of the most intriguing bane of this material: the degradation under sun-light illumination. Optical band-gap and film thickness engineering have enabled device efficiency to stabilize with only a 10–15% loss in the as-deposited device efficiency. More sophisticated computer simulations of the device have also strongly contributed to achieve the highest stable efficiencies in the case of multijunction devices. Novel use of nanocrystalline thin films offers new possibilities of high efficiency and stability. Short term goals of great economical impact can be achieved by the amorphous silicon/crystalline silicon heterojunction. A review is made of the most innovative achievements in amorphous silicon solar cell design and material engineering.     

Deposition of hydrogenated amorphous silicon (a-Si:H) films by hot-wire chemical vapor deposition (HW-CVD) method: Role of substrate temperature

Hydrogenated amorphous silicon (a-Si:H) thin films were deposited from pure silane (SiH₄) using hot-wire chemical vapor deposition (HW-CVD) method. We have investigated the effect of substrate temperature on the structural, optical and electrical properties of these films. Deposition rates up to 15 Å s⁻¹ and photosensitivity 10⁶ were achieved for device quality material. Raman spectroscopic analysis showed the increase of Rayleigh scattering in the films with increase in substrate temperature. The full width at half maximum of TO peak (Γ_TO) and deviation in bond angle (Δθ) are found smaller than those obtained for P-CVD deposited a-Si:H films. The hydrogen content in the films was found <1 at% over the range of substrate temperature studied. However, the Tauc's optical band gap remains as high as 1.70 eV or much higher. The presence of microvoids in the films may be responsible for high value of band gap at low hydrogen content. A correlation between electrical and structural properties has been found. Finally, the photoconductivity degradation of optimized a-Si:H film under intense sunlight was also studied.     

CF4/O2 dry etching of textured crystalline silicon surface in a-Si:H/c-Si heterojunction for photovoltaic applications

We investigated a dry cleaning procedure of the crystalline substrate, both mono- and multi crystalline silicon, to leave an uncontaminated surface using an etching process involving CF₄/O₂ mixture. A detailed investigation was performed to find compatibility and optimisation of amorphous layer depositions both on flat and textured silicon by changing the plasma process parameters. We found evidence that plasma etching acts by removing the native oxide and the damages of textured silicon and by leaving an active layer on silicon surface suitable for the emitter deposition. SEM analysis confirmed that it is possible to find plasma process conditions where no appreciable damages and change in surface morphology are induced. By using this process we achieved on amorphous crystalline heterostructure a photovoltaic conversion efficiency of 13% on 51 cm² and 14.5% on 1.26 cm² active area. We also investigated compatibility of the process with industrial production of large area devices.     

A critical appraisal of the factors affecting energy production from amorphous silicon photovoltaic arrays in a maritime climate

Contradictory reports exist in the literature regarding the energy production from amorphous silicon photovoltaic arrays. The majority claims high-energy output compared to crystalline silicon arrays of the same power rating (i.e. high kW h/kW_p), but some reports point to less favourable comparisons. The reasons for these conflicting reports are investigated using long-term measurements of the I–V characteristics of a number of amorphous silicon devices, in conjunction with in situ measurements of the solar spectrum and other relevant environmental parameters. It is shown that the variation in the performance of devices produced by different manufacturers is so significant that one cannot speak of the performance of amorphous silicon devices in general; one has to investigate each type of amorphous silicon panel separately.

The causes of differences in energy production are investigated in detail. The major factor impacting on the seasonal performance in the UK is identified to be variations in the solar spectrum. Single junction devices exhibit some seasonal thermal annealing but multi-junctions do not show this effect at a significant level.

Scope for further improvement is identified, largely in the photon absorption. The response to different spectra can be modified to some extent, which would bridge the gap between the best and the worst performers in the field. It is also shown that in the case of multi-junction devices an optimised current matching might bring a 5% increase in energy production for this location. Differences in the magnitude of the fill factor have been identified to be the second most significant cause for performance variation between the different samples in the test, suggesting additional scope for improvement.     

Investigations of solar cells with porous silicon as antireflection layer

The possibility of using porous silicon layers as antireflection coating instead of the antireflection coatings in common silicon solar cells was investigated. A technology for the manufacture of porous silicon antireflection layers was developed. The comparison of the photovoltaic and optical characteristics of investigated samples of solar cells with ZnS antireflection coating and with porous silicon antireflection coating is presented. It is shown that the formation of the porous layer under optimal technological regimes leads to significant improvement of the main photovoltaic parameters–short-circuit current and open-circuit voltage.     

Effect of gas flow rates on PECVD-deposited nanocrystalline silicon thin film and solar cell properties

Nanocrystalline silicon films have been deposited at a plasma excitation frequency of 54.24 MHz by varying the flow rates of SiH₄+H₂ mixture in the reaction chamber. It has been found that with increase in gas flow rate from 100 to 300 sccm the defect density, microstructural defect fraction and the crystalline volume fraction in the film decrease. Films deposited at optimum total gas flow rate of 200 sccm with comparable crystalline volume fraction have shown better structural and optoelectronic properties compared to the films deposited at 100 sccm total gas flow rate for application in solar cell. Solar cells have been fabricated using these layers as absorber layers and the maximum cell efficiency obtained is 6.2% (AM1.5, 28 °C) at 200 sccm total gas flow rate. It has been found that material prepared using higher total gas flow rate of 200 sccm together with higher hydrogen dilution is better suited for solar cell application.     

Crystalline silicon surface passivation by amorphous silicon carbide films

This article reviews the surface passivation of n- and p-type crystalline silicon by hydrogenated amorphous silicon carbide films, which provide surface recombination velocities in the range of 10 cm s⁻¹. Films are deposited by plasma-enhanced chemical vapor deposition from a silane/methane plasma. We determine the passivation quality measuring the injection level (Δn)-dependent lifetime (τ_eff(Δn)) by the quasi-steady-state photoconductance technique. We analyze the experimental τ_eff(Δn)-curves using a physical model based on an insulator/semiconductor structure and an automatic fitting routine to calculate physical parameters like the fundamental recombination velocities of electrons and holes and the fixed charge created in the film. In this way, we get a deeper insight into the effect of the deposition temperature, the gas flow ratio, the doping density of the substrate and the film thickness on surface passivation quality.