Enhancing the performance of dye-sensitized solar cells by incorporating nanosilicate platelets in gel electrolyte

Two kinds of gel-type dye-sensitized solar cells (DSSCs), composed of two types of electrolytes, were constructed and the respective cell performance was evaluated in this study. One electrolyte, TEOS-Triton X-100 gel, was based on a hybrid organic/inorganic gel electrolyte made by the sol–gel method and the other was based on poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP) copolymer. TEOS-Triton X-100 gel was based on the reticulate structure of silica, formed by hydrolysis, and condensation of tetraethoxysilane (TEOS), while its organic subphase was a mixture of surfactant (Triton X-100) and ionic liquid electrolytes. Both DSSC gel-type electrolytes were composed of iodine, 1-propy-3-methyl-imidazolium iodide, and 3-methoxypropionitrile to create the redox couple of I₃⁻/I⁻. Based on the results obtained from the I–V characteristics, it was found that the optimal iodine concentrations for the TEOS-Triton X-100 gel electrolyte and PVDF-HFP gel electrolyte are 0.05 M and 0.1 M, respectively. Although the increase in the iodine concentration could enhance the short-circuit current density (J_SC), a further increase in the iodine concentration would reduce the J_SC due to increased dark current. Therefore, the concentration of I₂ is a significant factor in determining the performance of DSSCs.In order to enhance cell performance, the addition of nanosilicate platelets (NSPs) in the above-mentioned gel electrolytes was investigated. By incorporating NSP-Triton X-100 into the electrolytes, the J_SC of the cells increased due to the decrease of diffusion resistance, while the open circuit voltage (V_OC) remained almost the same. As the loading of the NSP-Triton X-100 in the TEOS-Triton X-100 gel electrolyte increased to 0.5 wt%, the J_SC and the conversion efficiency increased from 8.5 to 12 mA/cm² and from 3.6% to 4.7%, respectively. However, the J_SC decreased as the loading of NSP-Triton X-100 exceeded 0.5 wt%. At higher NSP-Triton X-100 loading, NSPs acted as a barrier interface between the electrolyte and the dye molecules, hindering electron transfer, hence, reducing the cell's photocurrent density. The same behavior was also observed in the PVDF-HFP gel electrolyte DSSC system.     

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).     

The infrared processing in multicrystalline silicon solar cell low-cost technology

New directions in photovoltaics depend very often on financial possibilities and new equipment. In this paper, we present the modification of a standard screen-printing technology by using an infrared (IR) furnace for forming a n⁺/p structure with phosphorus-doped silica paste on 100 cm² multicrystalline silicon wafers. The solar cells were fabricated on 300 μm thick 1 Ω cm p-type multicrystalline Bayer silicon. The average results for 100 cm² multicrystalline silicon solar cells are: I_sc=2589 mA, V_oc=599 mV, FF=0.74, E_ff=11.5%. The cross-sections of the contacts metallized in the IR furnace, as determined by scanning electron microscopy, and the phosphorus profile measured by an electrochemical profiler are shown. IR processing offers many advantages, such as a small overall thermal budget, low power and time consumption, in terms of a cost-effective technology for the continuous preparation of solar cells.     

A comparative study of gel polymer electrolytes based on PVDF-HFP and liquid electrolytes, containing imidazolinium ionic liquids of different carbon chain lengths in DSSCs

The photoelectrochemical characteristics of titanium dioxide (TiO₂)-based dye-sensitized solar cells (DSSCs) containing gel polymer electrolyte (GPE) and organic liquid electrolyte (OLE) were studied in detail. GPE was prepared by adding poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP) to imidazolinium ionic liquids (IILs) of the type, 1-methyl-3-alkyl imidazolinium iodides (alkyl is C_nH_2n+1, where n=3–10) in methoxy propionitrile (MPN) and the OLE contained the above molten salt in MPN. The IILs were synthesized in the laboratory and characterized by ¹H nuclear magnetic resonance spectroscopy (NMR). The conductivities (σ) of both GPE and OLE decrease with increase in chain length (n) of the alkyl group of IILs; however, the effect is more drastic in the former case. The performance of the DSSCs containing OLE increases with the increase in alkyl chain length of IIL from C3 to C7, whereas, there is a linear decrease in the efficiency of the DSSCs incorporated with GPE containing IIL of alkyl chain length from C3 to C10. The change in short circuit current density (J_SC) determines the cell efficiency as the V_OC of the DSSCs remains almost the same with increase of alkyl chain length of IILs for both the electrolytes. The change in J_SC values and the consistency of the V_OC of the DSSCs for both the electrolytes may be explained on the basis of increase in viscosity of IILs from C3 to C10 and the dominating role of the 4-tertiary butyl pyridine (TBP), respectively, on the phenomenon of charge recombination.     

Synthesis and characterization of naphthyridine and acridinedione ligands coordinated ruthenium (II) complexes and their applications in dye-sensitized solar cells

Naphthyridine and acridinedione coordinated ruthenium (II) complexes were synthesized and characterized. Their applications in dye-sensitized solar cells were demonstrated. From the I–V curves, the short-circuit photocurrent (I_SC) and the open-circuit photovoltage (V_OC) were measured. A maximum current conversion efficiency (η) of about 7.7% was obtained for 5-amino-4-phenyl-2-(4-methylphenyl)-7-(pyrrolidin-1-yl)-1,6-naphthyridine-8-carbonitrile (pmpn) coordinated ruthenium (II) complex.     

5–20 MeV proton irradiation effects on GaAs/Ge solar cells for space use

This paper reports the high-energy proton irradiation effects on GaAs/Ge space solar cells. The solar cells were irradiated by protons with energy of 5–20 MeV at a fluence ranging from 1×10⁹ to 7×10¹³ cm⁻², and then their electric parameters were measured at AM0. It was shown that the I_sc, V_oc and P_max degrade as the fluence increases, respectively, but the degradation rates of I_sc, V_oc and P_max decrease as the proton energy increases, and the degradation is relative to proton irradiation-induced defect Ec−0.41 eV in irradiated GaAs/Ge cells.     

Determination of front surface recombination velocity of silicon solar cells using the short-wavelength spectral response

A method of determination of recombination velocity S_f of minority carriers at the front surface of an n⁺–p–p⁺(p⁺–n–n⁺) silicon solar cell in which the n⁺(p⁺) front emitter is made by diffusion of dopant impurity in the p(n) region is presented. This method uses the short-wavelength spectral response of the cell to determine S_f and is applicable if the front emitter of the cell has a linearly varying built-in field. It was applied to a p⁺–n–n⁺ solar cell that had a Gaussian distribution of the dopant impurity in the p⁺ front emitter up to a depth of 0.078 μm from the surface. Using the spectral response data of cell in 380 nm<λ< 500 nm range S_f was found to have a nearly constant value 6×10⁵ cm s⁻¹ in 400 nm<λ<460 nm range. Below and above this wavelength range the value of S_f was found to be slightly smaller. For comparison the value of S_f was also determined assuming the p⁺ region to be uniformly doped, and this value was found to be significantly smaller than based on the diffused emitter model. The analysis showed that for a diffused junction cell, the assumption that the front emitter is uniformly doped, ignores the presence of the built-in field in the emitter region and leads to overestimation of minority carrier recombination in the emitter. Consequently for a given contribution of the front emitter region to the spectral response of the cell, this assumption underestimates the front surface recombination and determines a smaller value of S_f. On the other hand, the present method can be expected to determine a realistic value of S_f independent of λ for most diffused junction silicon solar cells using the spectral response data in a suitable short-wavelength range since each such cell indeed has a built-in electric field in the emitter region.     

Quantum dot integration in heterostructure solar cells

InAs self-assembled quantum dots (SA-QDs) were incorporated into GaAlAs/GaAs heterostructure for solar cell applications. The structure was fabricated by molecular beam epitaxy on p-GaAs substrate. After the growth of GaAs buffer layer, multi-stacked InAs QDs were grown by self-assembly with a slow growth rate of 0.01 ML/s, which provides high dot quality and large dot size. Then, the structure was capped with n-GaAs and wide band gap n-GaAlAs was introduced. One, two or three stacks of QDs were sandwiched in the p–n heterojunction. The contribution of QDs in solar cell hetero-structure is the quantized nature and a high density of quantized states. I–V characterization was conducted in the dark and under AM1 illumination with 100 mW/cm² light power density to confirm the solar cell performance. Photocurrent from the QDs was confirmed by spectral response measurement using a filtered light source (1.1-μm wavelength) and a tungsten halogen lamp with monochromator with standard lock-in technique. These experimental results indicate that QDs could be an effective part of solar cell heterostructure. A typical I–V characteristic of this yet-to-be-optimized solar cell, with an active area of 7.25 mm², shows an open circuit voltage V_oc of 0.7 V, a short circuit current I_sc of 3.7 mA, and a fill factor FF of 0.69, leading to an efficiency η of 24.6% (active area).     

Effects of illumination and Co γ-ray irradiation on the electrical characteristics of porous silicon solar cells

A new approach for hybrid metal–insulator–semiconductor (MIS) Si solar cells is adopted by Institute of Fundamental Problems for High Technology, Ukrainian Academy of Sciences. In order to interpret the effect of illumination and ⁶⁰Co γ-ray radiation dose on the electrical characteristics of solar cells are studied at room temperature. Before the solar cells are subjected to stressed irradiation six different illumination levels of forward and reverse bias I–V measurements are carried out at room temperature. The solar cells are irradiated with ⁶⁰Co γ-ray source irradiation, with a dose rate of 2.12 kGy/h and an over dose range from 0 to 500 kGy. Experimental results shows that both the values of capacitance and conductance increase with increasing illumination levels and give the peaks at high illumination levels. γ-ray irradiation induces an increase in the barrier heights Φ_b(C–V) which are obtained from reverse-bias C–V measurements, whereas barrier heights Φ_b(I–V) which are deducted from forward-bias I–V measurements remain essentially constant. This negligible change of Φ_b(I–V) is attributed to the low barrier height (BH) in regions associated with the surface termination of dislocations. Both the I–V and C–V characteristics indicate that the total-dose radiation hardness of the Si solar cells cannot be neglected according to illumination levels.     

Heteropolyacid-impregnated PVDF as a solid polymer electrolyte for dye-sensitized solar cells

Heteropolyacid (HPA)-impregnated polyvinylidene fluoride (PVDF) with iodine/iodide was prepared as a new polymer electrolyte for bio-mimicking natural photosynthesis. With this new polymer electrolyte, dye-sensitized solar cell was fabricated using N3 dye-adsorbed over TiO₂ nanoparticles (photoanode) and conducting carbon cement coated on conducting glass (photocathode). The fabricated cell generates high open circuit voltage (V_OC 426 mV) and short circuit current (I_SC 3.90 mA) upon illumination with visible light. It is also demonstrated that the polymer electrolytes prevent the back-electron transfer reactions taking place in dye-sensitized hetero-junctions and are highly promising for solar energy conversion to electricity.     

Wet-etch texturing of ZnO:Ga back layer on superstrate-type microcrystalline silicon solar cells

Surface wet etching is applied to the ZnO:Ga (GZO) back contact in μc-Si thin film solar cells. GZO transparency increases with increasing deposition substrate temperature. Texturing enhances reflective scattering, with etching around 5-6 s producing the best scattering, whereas etching around 5 s produces the best fabricated solar cells. Etching beyond these times produces suboptimal performance related to excessive erosion of the GZO. The best μc-Si solar cell achieves FF=68%, V_OC=471 mV and J_SC=21.48 mA/cm² (η=6.88%). Improvement is attributed to enhanced texture-induced scattering of light reflected back into the solar cell, increasing the efficiency of our lab-made single μc-Si solar cells from 6.54% to 6.88%. Improved external quantum efficiency is seen primarily in the longer wavelengths, i.e. 600-1100 nm. However, variation of the fabrication conditions offers opportunity for significant tuning of the optical absorption spectrum.     

Polycrystalline silicon solar cells on low cost foreign substrates

The use of polycrystalline silicon layers on low-cost substrates is a promising approach for the fabrication of low-cost solar cells. Using low-carbon steel and graphite as substrates, solar cell structures have been deposited by the thermal decomposition of silane and appropriate dopants.Steel was selected as a substrate on the sole basis of its low cost. However, steel and silicon are not compatible in their properties, and an interlayer of a diffusion barrier, such as borosilicate, must be used to minimize the diffusion of iron from the substrate into the deposit. The deposited silicon on borosilicate/steel substrates is polycrystalline with a grain size of 1–5 μm, depending on deposition conditions. P-n junction solar cells were found to have low open-circuit voltages and poor current-voltage characteristics, and Schottky-barrier solar cells were found to show negligible photovoltages.Graphite is more compatible with silicon in properties than steel, and silicon deposited on graphite substrates shows considerably better microstructures. A number of solar cells, 2·5×2·5 cm in area, have been fabricated from n⁺-silicon/p-silicon/p⁺-silicon/graphite structures. The best cell to date had a V_oc of 0·35 V and an AMO efficiency of 1·5% (no antireflection coating). This type of solar cell is very promising because of the simplicity in fabrication.     

Density of interface states, excess capacitance and series resistance in the metal–insulator–semiconductor (MIS) solar cells

Dark and illuminatied current–voltage (I–V) characteristics of Al/SiO_x/p-Si metal–insulator–semiconductor (MIS) solar cells were measured at room temperature. In addition to capacitance–voltage (C–V) and conductance–voltage (G–V), characteristics are studied at a wide frequency range of 1 kHz–10 MHz. The dark I–V characteristics showed non-ideal behavior with an ideal factor of 3.2. The density of interface states distribution profiles as a function of (E_ss−E_v) deduced from the I–V measurements at room temperature for the MIS solar cells on the order of 10¹³ cm⁻² eV⁻¹. These interface states were responsible for the non-ideal behavior of I–V, C–V and G–V characteristics. Frequency dispersion in capacitance for MIS solar cells can be interpreted only in terms of interface states. The interface states can follow the a.c. signal and yield an excess capacitance, which depends on the relaxation time of interface states and the frequency of the a.c. signal. It was observed that the excess capacitance C_o caused by an interface state decreases with an increase of frequency. The capacitances characteristics of MIS solar cells are affected not only in interface states but also series resistance. Analysis of this data indicated that the high interface states and series resistance leads to lower values of open-circuit voltage, short-circuit current density, and fill factor. Experimental results show that the location of interface states and series resistance have a significant effect on I–V, C–V and G–V characteristics.     

Three cascade solar cells with graded band-gap layer on the base of GaAs–AlGaAs heterosystem

A new structure of three cascade solar cells with graded band-gap layer on the base of GaAs–AlGaAs heterosystem is designed to decrease the thermal losses, arising as a result of absorption of short-wavelength radiation. These solar cells were created by combining liquid-phase epitaxy with gas-phase zinc diffusion technologies. In these structures multilayer cascade elements reduce losses, caused by generation of “hot” carriers and the upper graded band-gap layer improves the conversion of short-wavelength radiation. The cell structure exhibited high short-wavelength sensitivity and the following parameters (under 1-sun conditions): open-circuit voltage (V_oc=1.03 V); short-circuit current (I_sc=24.9 mA/cm²); fill factor (FF=0.74) and efficiency (η=22.3%). Therefore it can be successfully utilized as power supplies in high-located areas, and as sensors of ultra-violet radiation.     

Characterization of high-photovoltage CuPc-based solar cell structures

Organic solar cells of the configuration ITO/PEDOT:PSS/CuPc/PTCBI/Al (Indium Tin Oxide/poly(3,4-ethylenedioxythiophene): polystyrene sulfonic acid/Copper phthalocyanine/3,4,9,10-perylenetetracarboxylic bisbenzimidazole/Aluminum ) were investigated. A high open circuit voltage (V_OC) of 1.15 V was obtained when the PTCBI layer was 7 nm thick. Lower V_oc values were observed for the same structure with silver, copper and gold electrodes instead of aluminum. However, short-circuit current density (J_SC) with these electrodes was much higher (4 mA/cm²) than in the case of aluminum (0.12 mA/cm²). Incorporating a 10 nm thick CdS interlayer between PTCBI and aluminum resulted in an increase in current density to 0.3 mA/cm². Results were interpreted in terms of a modified CuPc/Al Schottky diode for the thin PTCBI case and a CuPc/PTCBI heterojunction for the thick PTCBI case. Also, the formation of a thin, protective aluminum oxide layer under the aluminum electrode was postulated. For devices with silver, copper and gold electrodes, absence of this protective layer was thought to be the cause of a relatively lower V_oc and higher J_SC.     

A technique to compare polythiophene solid-state dye sensitized TiO2 solar cells to liquid junction devices

In this communication, we report on a technique to fabricate solid-state polythiophene-based dye sensitized solar cells (DSSCs) that can be directly compared to analogous liquid junction devices. The device configuration is based on non-porous TiO₂ thin films and one of the three undoped polythiophene hole conductors: poly[3-(11 diethylphosphorylundecyl) thiophene], P3PUT, poly(4-undecyl-2,2′-bithiophene), P4UBT, or poly(3-undecyl-2,2′-bithiophene), P3UBT. These polymers were spin coated and cast from organic solutions onto the TiO₂ films. The dense TiO₂ thin films (ca. 30 nm) were deposited on conductive glass via facile spray pyrolysis and sol–gel techniques. After that, cis-(SCN)₂ Bis(2,2′ bipyridyl-4,4′-dicarboxylate) ruthenium(II) (a.k.a. Ru N3 dye) was adsorbed on the TiO₂ surface, and the polythiophenes were utilized as hole conductors in a simplified solar cell geometry. The results were compared to the control DSSC device made with dense TiO₂ and a liquid electrolyte, or 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (a.k.a. Spiro-MeOTAD). The polythiophenes exhibited bandgaps in the range 1.9–2.0 eV, and HOMO energy levels of approximately 5 eV (vs. vacuum). The P3PUT DSSC device exhibited an AM1.5 V_OC=0.8 V, a J_SC=0.1 mA/cm², as well as an IPCE=0.5–1%. The AM1.5 short-circuit photocurrents and quantum efficiencies for DSSCs made with the polythiophenes, the Spiro-MeOTAD and the standard liquid electrolyte (I⁻/I₃⁻) were found to be identical within the limits of experimental uncertainty and reproducibility. Our results indicate that a solid-state replacement to the liquid junction is not necessarily limited by the fundamental aspect of hole transfer, one of the three fundamental aspects that must be met for an efficient DSSC. Rather than suggest that P3UBT or P4UBT could be used to create efficient “organic solar cells” with the exclusion of the Ru dye, we suggest that transparent thiophene compounds could be attractive candidates for high-surface area solid-state DSSCs, and that the technique presented can be applied to other hole conductors. It can allow a verification of one of the things necessary for the DSSC, so that parallel studies using high-surface area materials can proceed with confidence.     

CdTe/CdS Solar cells on flexible molybdenum substrates

Development of CdTe/CdS solar cells on flexible metallic substrates is highly interesting due to the light weight and flexible nature of the solar modules. We have deposited CdTe films onto flexible molybdenum substrates using close-spaced sublimation technique and the CdTe/CdS junction was developed by depositing a thin layer of CdS onto the CdTe substrate from a chemical bath. The devices were characterized by Current–voltage (I–V) and photocurrent spectroscopy techniques. Prior to the deposition of the transparent conducting layer, the devices were annealed in air at different temperatures and found that the devices annealed at 400°C have better photovoltaic parameters. The efficiency of a typical device under 60 mW cm⁻² illumination was estimated as 3.5%.     

Mechanism for the anomalous degradation of proton- or electron-irradiated silicon solar cells

A mechanism of the anomalous increase of the short-circuit current of n⁺–p–p⁺ silicon space solar cells under high fluence of the high-energy 10 MeV protons or 1 Mev electrons is proposed. In distinction to other models this mechanism takes place as a result of the conversion of conductivity type and increased minority carrier lifetime with respect to that of majority carriers. This mechanism occurs in solar cells with deep centers, whose energy level is close to the middle of the band gap.     

Study of silicon solar cell at different intensities of illumination and wavelengths using impedance spectroscopy

The electrical properties of an n⁺–p–p⁺ structure-based single-crystalline silicon solar cell were studied by impedance spectroscopy, I–V and spectral response. The impedance spectrum is measured in dark, under different intensities (14, 43, 57, 71, 86, 100 mW/cm²) of illumination and wavelengths (400–1050 nm) of light. Under dark and at low intensities of illumination (<50 mW/cm²) the impedance spectra show perfect semicircles but at high intensities the semicircles are distorted at low frequencies. It is found that illumination provides an additional virtual R₁C₁ network parallel to the initial bulk R_pC_p network observed under dark conditions. The value of virtual resistance R₁ depends on the illumination wavelength and shows an inverse relationship with the spectral response of the device.     

The effect of temperature on the performance of dye-sensitized solar cells based on a propyl-methyl-imidazolium iodide electrolyte

The performance of dye-sensitized solar cells (DSSC), based on an ionic liquid (propyl-methyl-imidazolium iodide) electrolyte, has been evaluated at varying iodine concentrations and cell temperatures (5–50 °C) for two irradiancies (0.1 and 1 sun). At 1 sun and at lower temperatures, the short-circuit current (J_SC) is limited by the diffusion of tri-iodide, while at higher temperatures, the J_SC decreases due to more pronounced recombinations. Also, the conversion efficiency of a DSSC resembles the J_SC behaviour. At 0.1 sun irradiance, the efficiency monotonically decreases with increasing temperature, while at 1 sun, a five-fold increase in efficiency is observed.