Experimental and theoretical radiation damage studies on crystalline silicon solar cells

An experimental facility was developed to asses in situ the degradation of crystalline silicon solar cells, fabricated by the Solar Energy Group of the National Atomic Energy Commission (CNEA), by measuring the current–voltage characteristic curve. The cells were irradiated with 10 MeV protons and fluences between 10⁸ and 10¹³ p/cm², using an external beam of the linear tandem accelerator TANDAR, at CAC-CNEA. Furthermore, theoretical simulations were performed to establish the relation between the variation of the electrical parameters and the degradation of the lifetime of minority carriers in the base. The damage constant for 10 MeV proton irradiated silicon solar cells of n⁺–p–p⁺ structure and 1 Ω cm base resistivity was determined. Finally, a proposal of a new model of radiation damage for silicon solar cells is discussed.     

2-Ethyl-1-hexanol based screen-printed titania thin films for dye-sensitized solar cells

Nanocrystalline titania thin films were prepared by screen printing in order to efficiently control and optimize the main step of the dye-sensitized solar cells (DSSCs) fabrication process. Different compositions of nanocrystalline titanium dioxide screen-printing pastes are described, based on 2-ethyl-1-hexanol solvent and commercial Degussa P25 TiO₂. The rheological properties of the prepared pastes are presented as the crucial parameter of the deposition procedure. The produced titania thin films are extensively characterized by means of spectroscopy (Raman, XRD) and microscopy (SEM, AFM). The performance (induced photon-to-current efficiency—IPCE% and overall energy conversion efficiency—η%) of the corresponding DSSCs is also reported.     

Zone melting recrystallization of silicon films for crystalline silicon thin-film solar cells

Coarse-grained silicon films for crystalline silicon thin-film solar cells have been prepared by zone melting recrystallization. A zone melting heater was modified to obtain better temperature homogeneity of the sample and higher reproducibility of the melt process. Various substrate materials of different purity and surface roughness have been tested concerning their suitability for, silicon deposition, zone melting and solar cell process. Solar cell efficiencies up to 10.5% could be achieved on silicon sheets from powder, capped by an intermediate layer. Silicon films on SiAlON ceramics were successfully processed to solar cells by a completely dry solar cell process.     

Doping CuSCN films for enhancement of conductivity: Application in dye-sensitized solid-state solar cells

Construction of dye-sensitized solid-state solar cells requires high band-gap (therefore, transparent) hole collectors which can be deposited on a dye-coated nanocrystalline semiconductor surface without denaturing the dye. Copper (I) thiocyanate (CuSCN) is an important p-type semiconductor satisfying the above requirements. However, the conductivity of this material, which depends on excess SCN, is not sufficiently high and polymerization of SCN prevents incorporation of sufficient amount of excess SCN during the process of synthesis of CuSCN. We have found that the conductivity of solid CuSCN can be increased by exposure to halogen gases which generate SCN or to a solution of (SCN)₂ in CCl₄. The latter method is suitable for doping of CuSCN films in dye-sensitized solid-state solar cells.     

Overview on SiN surface passivation of crystalline silicon solar cells

Silicon nitride (SiN) fabricated by plasma-enhanced chemical vapour deposition (PECVD) is increasingly used within the crystalline silicon (c-Si) photovoltaic industry as it offers the possibility to fabricate a surface and bulk passivating antireflection coating at low temperature (450°C). This article presents an overview on the present status of SiN for industrial as well as laboratory-type c-Si solar cells. Topics covered include the fundamentals of the PECVD technology, the present status of high-throughput PECVD machines for the deposition of SiN onto c-Si wafers, and a review of the fundamental properties of Si–SiN interfaces fabricated by PECVD.     

Quasi-solid-state dye-sensitized solar cells employing nanocrystalline TiO2 films made at low temperature

Quasi-solid-state dye-sensitized solar cells with enhanced performance were made by using nanocrystalline TiO₂ films without any template deposited on plastic or glass substrates at low temperature. A simple and benign procedure was developed to synthesize the low-temperature TiO₂ nanostructured films. According to this method, a small quantity of titanium isopropoxide (TTIP) was added in an ethanolic dispersion of TiO₂ powder consisting of nanoparticles at room temperature, which after alkoxide's hydrolysis helps to the connection between TiO₂ particles and to the formation of mechanically stable thick films on plastic or glass substrates. Pure TiO₂ films without any organic residuals consisting of nanoparticles were formed with surface area of 56 m²/g and pore volume of 0.383 cm³/g similar to that obtained for Degussa-P25 powder. The structural properties of the films were characterized by microscopy techniques, X-ray diffractometry, and porosimetry. Overall solar to electric energy conversion efficiencies of 5.3% and 3.2% (under 1sun) were achieved for quasi-solid-state dye-sensitized solar cells employing such TiO₂ films on F:SnO₂ glass and ITO plastic substrates, respectively. Thus, the quasi-solid-state device based on low-temperature TiO₂ attains a conversion efficiency which is very close to that obtained for cells consisting of TiO₂ nanoparticles sintered at high temperature.     

Electrodeposition of p–i–n type CuInSe2 multilayers for photovoltaic applications

Copper indium diselenide polycrystalline thin films of p-, i- and n-type electrical conductivity were grown using a one-step electrodeposition process in a single bath. The bulk structure and the stoichiometry of the layers were determined using X-ray diffraction and X-ray fluorescence. The material composition was correlated with the electrical conductivity type variation, detected by the photoelectrochemical cell. Atomic force microscopy analysis showed copper-rich films deposited at low cathodic potentials (0.6 V vs Ag/AgCl) are of spherical and granular morphology and the grain sizes were 0.3–0.5 μm, while stoichiometric CIS films deposited at 1.0 V vs Ag/AgCl have grain sizes of 0.1–0.4 μm. The initial studies of optoelectronic properties (V_oc, J_sc and FF) of the four-layer solar cell devices (glass/FTO/n-CdS/n-CIS/i-CIS/p-CIS/Au) are presented.     

Thickness effect of RF sputtered TiO2 passivating layer on the performance of dye-sensitized solar cells

We report on the characteristics of a TiO₂ passivating layer grown by radio frequency (RF) magnetron sputtering on F-doped SnO₂ (FTO) electrodes as a function of its thickness. The optical transparency, surface roughness and passivation properties of the TiO₂ layer passivating the FTO electrode depend on the thickness of the TiO₂ passivating layer. In addition, it was found that the power conversion efficiency of the dye-sensitized solar cells (DSSCs) is critically dependent on the thickness of RF sputtered TiO₂ layer inserted between FTO electrode and nanoporous TiO₂ layer. The DSSC fabricated on 50 nm thick TiO₂ passivating FTO electrode showed the maximum power conversion efficiency of 4.42% due to effective prevention of the electron transfer to electrolyte. This indicates that the thickness optimization of the TiO₂ passivating layer is one of the important parameter to obtain high performance DSSCs.     

Surface passivation of crystalline silicon wafer via hydrogen plasma pre-treatment for solar cells

The carrier lifetime of crystalline silicon wafers that were passivated with hydrogenated silicon nitride (SiN_x:H) films using plasma enhanced chemical vapor deposition was investigated in order to study the effects of hydrogen plasma pre-treatment on passivation. The decrease in the native oxide, the dangling bonds and the contamination on the silicon wafer led to an increase in the minority carrier lifetime. The silicon wafer was treated using a wet process, and the SiN_x:H film was deposited on the back surface. Hydrogen plasma was applied to the front surface of the wafer, and the SiN_x:H film was deposited on the hydrogen plasma treated surface using an in-situ process. The SiN_x:H film deposition was carried out at a low temperature (<350 °C) in a direct plasma reactor operated at 13.6 MHz. The surface recombination velocity measurement after the hydrogen plasma pre-treatment and the comparison with the ammonia plasma pre-treatment were made using Fourier transform infrared spectroscopy and secondary ion mass spectrometry measurements. The passivation qualities were measured using quasi-steady-state photoconductance. The hydrogen atom concentration increased at the SiN_x:H/Si interface, and the minority carrier lifetime increased from 36.6 to 75.2 μs. The carbon concentration decreased at the SiN_x:H/Si interfacial region after the hydrogen plasma pre-treatment.     

Novel iminocoumarin dyes as photosensitizers for dye-sensitized solar cell

Novel iminocoumarin dyes (2a-c and 3a-c) having carboxyl and hydroxyl anchoring groups onto the dyes skeletons have been designed and synthesized for the application of dye-sensitized nanocrystalline TiO₂ solar cells (DSSCs). The photophysical and electrochemical studies showed that these iminocoumarin dyes are suitable as light harvesting sensitizers in DSSC application. The dyes having carboxyl and hydroxyl anchoring groups (2a-c) showed better efficiency when compared to the dyes having carboxyl group (3a-c) alone. The cell consisted of dye 2a generated the highest solar-to-electricity conversion efficiency (η) of 0.767% (open circuit voltage (V_oc) = 0.491 V, short circuit photocurrent density (J_sc) = 2.461 mA cm⁻², fill factor (ff) = 0.635) under simulated AM 1.5 irradiation (1000 W m⁻²) with a total semiconductor area of 0.25 cm². The corresponding incident photon-to-current conversion efficiency (IPCE) of the above cell was 21.38%. The overall low efficiency of the dyes is ascribed to the lack of light harvesting ability at longer wavelength region.     

Evolution of space solar cells

The solar cells used in space for over 40 years are reviewed by discussing the semiconductor materials which have provided the best cells. Most emphasis was on high efficiency, combined with good tolerance to charged particle bombardment, and the steady increase in efficiency is discussed. The most important requirement is that the cells must be highly reliable, consistent in performance, and stable while operating in space. The need for highest reliability makes the costs less important. The progress to date has provided a good foundation for future applications for space cells.     

Radiation degradation of large fluence irradiated space silicon solar cells

In this paper, we present data on the electrical properties of 50 gm thick space silicon BSFR cells irradiated with 10 MeV protons with a fluence exceeding 1 x 10¹³ p/cm² and irradiated with 1 MeV electrons with a fluence exceeding 1 x 10¹⁶ e/cm², and discuss the anomalous degradation which was found in these large-fluence regions. These data show an increase of saturation current density and a decrease of diffusion voltage of the pn junction, and a decrease of majority carrier density and an increase of series resistance of the p-substrate as a result of the formation of a large amount of carrier traps by the large-fluence irradiation.     

On the photophysical and electrochemical studies of dye-sensitized solar cells with the new dye CYC-B1

In this study, the photoelectrochemical characteristics of a ruthenium photosensitizer with an alkyl bithiophene group, designated as CYC-B1, are studied. The effect of mesoporous TiO₂ film thickness on the photovoltaic performance of CYC-B1 and N3 dye-sensitized solar cells was investigated. The performance of the dye-sensitized nanocrystalline TiO₂ solar cells (DSSC) fabricated using CYC-B1 dye-anchored TiO₂ photoelectrode showed a convincing enhancement in cell efficiency when the TiO₂ film thickness was increased from 3 μm (eff.=5.41%) to 6 μm (eff.=7.19%). The efficiency of the CYC-B1-sensitized DSSC was maximum at 6 μm of the TiO₂ film thickness, reached its limiting value and remained constant up to 53 μm, although a similar trend was also observed for N3 dye-sensitized DSSC, however, the maximum efficiency achieved was only at 27 μm thickness (eff.=6.75%). As expected, the photocurrent density generated in the DSSC modified by CYC-B1 dye is larger than that from N3 dye. The effect of guanidinium thiocyanate (GuSCN) (additive) addition to the electrolyte on the photovoltaic performance of DSSCs based on CYC-B1 was also investigated. Furthermore, the electrochemical impedance spectroscopy (EIS) technique and photo-transient laser method have been employed to analyze the charge transfer resistances (R_ct) and the lifetime of the injected electrons on the TiO₂ containing different thicknesses.     

Improvement on surface texturing of single crystalline silicon for solar cells by saw-damage etching using an acidic solution

Texturing the surfaces of silicon wafer is one of the most important ways of increasing their efficiencies. The texturing process reduces the surface reflection loss through photon trapping, thereby increasing the short circuit current of the solar cell. The texturing of crystalline silicon was carried out using alkaline solutions. Such solutions resulted in anisotropic etching that leads to the formation of random pyramids. Before the texturing process was carried out, saw-damage etching was performed in order to remove the surface defects and damage caused by wire sawing. In general, potassium hydroxide (KOH) solution has been used for saw-damage etching. This etching results in a fairly flat surface. The results from this study showed that the outcome of the surface texturing is related to the original surface morphology of the silicon. It was found that saw-damage etching using an acidic solution improved the effects of the texturing. In this case, regular and small pyramids were formed on the surface of the silicon. This reduced the reflectance of the surface, thereby increased the short circuit current and the conversion efficiency of the solar cell.     

Study on hydrogenated silicon nitride for application of high efficiency crystalline silicon solar cells

The hydrogenated silicon nitride films (SiN_x:H) deposited by plasma enhanced chemical vapor deposition (PECVD) technique is commonly used as an antireflection coating as well as surface passivating layer of crystalline silicon solar cells. The refractive indices of SiN_x:H films could be changed by varying the growth gas ratio R(=NH₃/SiH₄+NH₃) and annealing temperature. For optimum SiN_x:H film, the optical and chemical characterization tools by varying the film deposition and annealing condition were employed in this study. Metal-insulator-semiconductor (MIS) devices were fabricated using SiN_x:H as an insulator layer and they were subjected to capacitance-voltage (C-V) and current-voltage (I-V) measurements for electrical characterization. The effect of rapid thermal annealing (RTA) on the surface passivation as well as antireflection properties of the SiN_x:H films deposited at various process conditions were also investigated for the fabrication of low cost and high efficiency silicon solar cells.     

Substrates for thin crystalline silicon solar cells

Choice of substrate for thin crystalline silicon solar cells requires a compromise between cost and quality. There are three generic substrate types, namely a transparent substrate (such as glass), an opaque substrate (such as a ceramic or metal) and low-cost multicrystalline silicon. Glass has the advantage of eliminating absorption within the substrate. However, the larger effective diffusion length, the improved surface passivation and the increased process flexibility obtainable with an opaque substrate, particularly low-cost multicrystalline silicon, may considerably outweigh the modest optical benefits of a transparent substrate. In this paper it is shown that the advantage in effective diffusion length that is required of a cell grown on an opaque substrate in order to offset the light-trapping advantages of a glass substrate is about a factor of two.     

Efficient hybrid carboxylated polythiophene/nanocrystalline TiO2 heterojunction solar cells

Efficient hybrid solar cells fabricated from TiO₂, novel carboxylated polythiophene poly (3-thiophenemalonic acid) P3TMA as sensitizer as well as hole conductor and poly (3-hexylthiophene) (P3HT) as hole transporter was described. UV-Vis absorption and morphology of the active layer were investigated. Device J/V characterizations with different P3HT layer thickness were measured and discussed. Efficiency improvements were observed in thinner P3HT layer thickness and with poly[3,4-(ethylenedioxy)-thiophene]:poly(styrene sulfonate) (PEDOT:PSS) as charge collection layer, and such device showed a short-circuit current density of 1.32 mA/cm², an open-circuit voltage of 0.44 V, a fill factor of 0.43, and a energy conversion efficiency of 0.25% at A.M. 1.5 solar illumination (100 mW/cm²).     

Hybrid inverted bulk heterojunction solar cells with nanoimprinted TiO2 nanopores

Photovoltaic devices with highly ordered nanoporous titanium dioxide (titania; TiO₂) were fabricated to improve the photovoltaic performances by increasing TiO₂ interface area. The nanoimprinting lithography technique with polymethyl methacrylate (PMMA) mold was used to form titania nanopores. The solar cell with poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) active layer on nanoporous titania showed higher power conversion efficiency (PCE) of 1.49% than on flat titania of 1.18%. The improved efficiency using nanoporous titania is interpreted with the enhanced-charge separation and collection by increasing the interface area between TiO₂ and active layer.     

Influences of water in bis-benzimidazole-derivative electrolyte additives to the degradation of the dye-sensitized solar cells

The chemical stability of dye-sensitized solar cells (DSSC) determines both the cell performance and the cell life-time. The presence of water in the solar cell from surrounding leakage through the imperfection packaging sealants causes the decrease in life-time of photogenerated electrons on the working electrodes, which induces the occurrence of the dark current to the electrolytes and thus leakage current significantly deteriorated the life-time of the DSSC. Reliable electrolyte additives diminishing the influences of water to the DSSCs degradation process becomes a critical issue in maintaining an acceptable cell life-time.In this work, the effects of four benzimidazole derivatives a-d as the electrolyte additives in the presence of water were comprehensively examined by time-dependent photovoltaic performance of the cells. As a result, open-circuit voltage (V_oc), short-circuit current (J_sc), efficiency (η), and fill factor (FF) collected from I-V curves were studied. In addition, electrochemical impedance spectroscopy (EIS) technique was implemented to evaluate the effects of the charge-transfer resistance (R_ct) at the interfaces between TiO₂/dye/electrolyte. Results showed that the bis-benzimidazole derivative c gives significant improvement in the long-term stability due to the effective protection of the ligands between dye and working electrodes from the attack by environmental water molecules.     

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).