Review: Dye sensitized solar cells based on natural photosensitizers

The performance of dye sensitized solar cells is mainly based on the dye as a sensitizer. Natural dyes have become a viable alternative to expensive and rare organic sensitizers because of its low cost, easy attainability, abundance in supply of raw materials and no environment threat. Various components of a plant such as the flower petals, leaves and bark have been tested as sensitizers. The nature of these pigments together with other parameters has resulted in varying performance. This review briefly discusses the emergence, operation and components of dye sensitized solar cells together with the work done on natural dye based dye sensitized solar cells over the years.     

Dye sensitized solar cells incorporating obliquely deposited titanium oxide layers

Porous films of titanium oxide were deposited by oblique reactive electron beam evaporation. Both as-deposited and annealed samples of these films were structurally characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The as-deposited films were found to consist of amorphous columns with a very fine structure while the annealed films consisted of polycrystalline anatase columns whose fine structure showed evidence of conglomeration. These films were sensitized with a photoactive dye and implemented into a dye sensitized solar cell (DSSC) configuration as the electron collecting electrode. Solar cells incorporating annealed titanium oxide films fabricated at deposition angles between 60° and 75° produced higher short current densities than conventional DSSC based on colloidal TiO₂ film measured under the same conditions. The best performing solar cell incorporating annealed titanium oxide films was found to have a photoelectric conversion efficiency of 4.1%.     

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 substrates

The development of CdTe/CdS solar cells on flexible substrates is reviewed in this article. Photovoltaic structures on lightweight and flexible substrates have several advantages over the heavy glass based structures in both terrestrial and space applications. The cells mounted on flexible foil are not fragile, the requirements of the supporting structures are minimum and they can be wrapped onto any suitably oriented or curved structures. The specific power of the solar cells is an important factor in space applications and hence development of photovoltaic devices on light weight substrates is interesting. CdTe is one of the leading candidates for photovoltaic applications due to its optimum band gap for the efficient photo-conversion and robustness for industrial production with a variety of film preparation methods. Flexible solar cells with conversion efficiencies exceeding 11% have been developed on polyimide foils. The development of CdTe devices on metallic substrates is impeded due to the lack of a proper ohmic contact between CdTe and the substrate. The polymer substrate has the advantage that the devices can be prepared in both “superstrate” and “substrate” configurations.     

High-efficiency flexible CdTe solar cells on polymer substrates

Development of flexible and lightweight solar cells is interesting for terrestrial and space applications that require a very high specific power (kW/kg) and flexibility for curved shaping or rolling. Flexible CdTe/CdS solar cells of 11% efficiency in superstrate and 7.3% efficiency in substrate configurations have been developed with a “lift-off” approach. However, roll-to-roll manufacturing is desired in future.Therefore, flexible superstrate solar cells were directly grown on commercially available 10 μm thin polyimide (Upilex™) foils. A process for the deposition of ITO (front contact) has been developed to have a stable front contact on the Upilex™ foil. Post-deposition annealing treatments of the ITO/polyimide stacks bring a significant stability to the front contact, having almost the same sheet resistance at the beginning and at the end of the cell fabrication process. Solar cells with AM1.5 efficiency of 11.4% on Upilex™ foils (highest efficiency recorded for flexible CdTe cell) have been developed. A comparison of the cells prepared on different polyimides is presented.     

Preparation of 1-methyl-3-propylimidazolium acetate and its application in dye sensitized solar cells

In this paper, we reported the preparation of 1-methyl-3-propylimidazolium acetate (MPIAc), which proceeded via the metathesis of 1-methyl-3-propylimidazolium iodide (MPII) and lead acetate or potassium acetate. The apparent diffusion coefficients of triiodide and iodide in binary ionic liquids, MPIAc and MPII with various weight ratios, were demonstrated by cyclic voltammetry using a Pt ultramicroelectrode. It was found that the apparent diffusion coefficients of triiodide increased and those of iodide slightly increased with the weight ratio increase of MPIAc and MPII. The dye sensitized solar cells with the electrolyte, which was composed of 0.13 M I₂, 0.10 M LiI, 0.50 M 4-tert-butylpyrdine in the binary ionic liquid electrolyte of MPIAc (employing potassium acetate) and MPII (weight ratio 0.2), gave short circuit photocurrent density of 9.40 mA cm⁻², open circuit voltage of 0.62 V, and fill factor of 0.57, corresponding to the photoelectric conversion efficiency of 3.34% at the illumination (air mass 1.5, 100 mW cm⁻²).     

Polymer electrolyte based on polyethylene glycol for quasi-solid state dye sensitized solar cells

A gel polymer electrolyte containing I⁻/I₃⁻ redox couple was prepared using polyethylene glycol (PEG) as polymer matrix and propylene carbonate (PC) as organic solvent by sol-gel method. A dye sensitized solar cell (DSSC) employing the gel polymer electrolyte gives an open-circuit voltage of 0.7 V and a short-circuit current of 8.1 mA cm⁻² at an incident light intensity of 100 mW cm⁻². Fourier transform infrared spectroscopy was utilized to examine the chemical properties of produced gel electrolytes. Unlike the conventional covalent bond that bridges the different polymer segments, in this study, it was observed that hydrogen bonds bridged polyethylene glycol and propylene carbonate. Influences of different synthetic parameters such as reaction time and temperature were also investigated. Optimal DSSC performance was observed at electrolyte reaction temperature and time of 100 °C and 24 h, respectively, with a maximum photoelectric energy conversion efficiency of 2.2%.     

Solid-state D102 dye sensitized/poly(3-hexylthiophene) hybrid solar cells on flexible Ti substrate

Flexible solid-state dye sensitized solar cell is an important milestone for low-cost, large scale fabrication of dye-sensitized solar cells. Flexible solid-state dye-sensitized solar cell is fabricated for the first time on titanium substrates using D102 sensitizer and a sputtered platinum semi-transparent cathode. Devices are illuminated from the cathode side since titanium substrates are non-transparent. Due to rear-side illumination, significant proportion of radiation is absorbed and scattered by poly(3-hexylthiophene) and platinum, respectively. Limiting the amount of platinum and poly(3-hexylthiophene), up to a point, is found to enhance device efficiency. The amount of platinum and poly(3-hexylthiophene) is optimized on glass substrates before fabrication of flexible devices on titanium substrates. The rough surface of titanium substrates is smoothened until a mirror finish and the growth of a thin layer of native oxide enhanced the device efficiency. Under optimized conditions, an efficiency of 1.20% is obtained for devices fabricated on titanium foil substrates. The lower efficiency as compared to conventional devices is mainly due to light absorption/scattering from the poly(3-hexylthiophene) and platinum layers.     

Design issues in the fabrication of CdS–CdTe solar cells on molybdenum foil substrates

Investigations on CdTe–CdS solar cells on molybdenum foil substrates revealed that the depletion layer spans the entire CdS and CdTe film thickness and the cell should be conceived as a single junction device instead of the three separate junctions (Mo–CdTe, CdTe–CdS, CdS–TCO). Higher open circuit voltages were achieved when two CdS layers (separated by an air anneal) were used instead of a single CdS layer. The high series resistance of this solar cell continues to be the limiting factor in cell performance. Modeling and design issues for improving cell performance are presented.     

Study of hydrogen production in light assisted microbial electrolysis cell operated with dye sensitized solar cell

Hydrogen production with light as an additional energy source in a microbial electrolysis cell (MEC) is described. A ruthenium-dye (N719) sensitized solar cell with an open circuit potential (V_oc) of 602 mV was connected to the MEC. Hydrogen production was carried out by irradiating the DSSC connected across the MEC with a light intensity of 40 mW/cm² and also with natural sunlight. The DSSC was stable during various batch experiments. The acetate conversion efficiency and the coulombic efficiency based on the average of first two batches were 30.5 ± 2.5% and 40 ± 2% respectively. The cathodic recovery efficiency ranged from 72% to 86% during repeated batch experiments with an average of 78 ± 2.5%.     

Contribution of silicon substrates to the efficiencies of silicon thin layer solar cells

Although silicon solar cells based on layers less than 50 μm thick have become very popular, little attention has been paid to the role of the underlying silicon substrate. This treatment uses the device simulation program PC-1D and the ray tracing program SUNRAYS to examine the role of the substrate in contributing to the current and efficiency of textured and non-textured thin layer solar cells. For the case of a heavily doped silicon substrate, substrate contributions can be significant for cells with sufficiently thin base layers. For example, for the case of a silicon thin layer cell with a base layer thickness of 20 μm and a substrate doping of 6 × 10¹⁸ cm⁻³, the substrate contributes no more than 4% of the total short-circuit current. However, decreasing the base width to 5 μm results in an increase in this substrate contribution to 20%. Light trapping tends to alleviate the substrate contribution by increasing the effective path length in the base. Examination of the current components under forward bias reveals that for a thin layer cell with a high quality base and good front surface passivation, back diffusion of electrons into the substrate limits cell performance.     

Effect of acidic additives on the structure and performance of TiO2 films prepared by a commercial nanopowder for dye-sensitized solar cells

This work comprises an experimental study on the effect of various dispersing agents on the morphology of mesoporous TiO₂ films prepared by the doctor blade method and on the performance of the resulting dye-sensitized solar cells. TiO₂ films were prepared using a commercial nanopowder, Degussa P25, which was ground in a mortar, with different dispersing agents and under continuous grinding, in order to break the large agglomerates (>1 μm), present in the powder. These additives can be different acids or bases and must prevent also re-agglomeration of the nanoparticles, by forming a surface charge.Two strong acids (hydrochloric, HCl and nitric, HNO₃), a weak one (acetic acid, CH₃COOH) and a ketone (acetylacetone, C₅H₈O₂) were used in turn. The properties of the films were dependent on the concentration and the kind of the acid. With increasing concentration the coagulation of the nanoparticles also increases, thus affecting the efficiency of the devices. Of all the additives used, HNO₃ gave the best results and a 40% increment in efficiency was observed, compared to the standard dispersing agent that is acetylacetone. The improved homogeneity of the nanoparticle size (24 nm for anatase and 21.5 nm for rutile) and the better connectivity between them were responsible for the improvement in efficiency.     

Quasi solid state polymer electrolyte with binary iodide salts for photo-electrochemical solar cells

Quasi-solid-state polymer electrolytes can be used in dye sensitized solar cells (DSSCs) in order to overcome various problems associated with liquid electrolytes. Prior to fabricating commercially viable solar cells, the efficiency of quasi solid state DSSCs needs to be improved. Using electrolytes with a binary iodide mixture is a novel technique used to obtain such efficiency enhancement. In this work we report both conductivity and solar cell performance enhancements due to incorporation of a mixture containing LiI and tetrahexylammonium iodide in a quasi-solid-state electrolyte. The conductivity of the electrolyte increases with added amounts of LiI and thus the highest conductivity, 3.15 × 10⁻³ S cm⁻¹ at 25 °C, is obtained for the electrolyte 100 wt% LiI. The predominantly ionic behavior of the electrolytes was established from dc polarization measurements. The iodide ion conductivity, measured using iodine pellet electrodes decreased somewhat with increasing amount of LiI even though the overall conductivity increased. However, the highest efficiency was obtained for the DSSC containing a polymer electrolyte with Hex₄N⁺I^¯:LiI = 1:2 mass ratio. This cell had the largest short circuit current density of about 13 mA cm⁻² and more than 4% overall energy conversion efficiency. The results thus show that electrolytes with Hex₄N⁺I^¯/LiI mixed iodide system show better DSSC performance than single iodide systems.     

Synthesis of carbon nanomaterials for dye‐sensitized solar cells

In this paper, it was demonstrated that Na₂O can react with CO to produce carbon nanofibers at 500 °C and carbon nanosheets at 550 °C. Furthermore, the nanosheets exhibited excellent performance as a counter electrode for a dye‐sensitized solar cell (DSSC), leading to a high power conversion efficiency of 7.57%. The efficiency is larger than that (4.72%) of a DSSC with the carbon nanofiber counter electrode and even comparable with that of an expensive Pt‐based DSSC. Copyright © 2015 John Wiley & Sons, Ltd.     

A review on PEDOT‐based counter electrodes for dye‐sensitized solar cells

The dye‐sensitized solar cell (DSSC) is a promising alternative for the Si solar cell due to its low‐cost and easy fabrication. As a novel conductive polymer, poly(3,4‐ethylenedioxythiophene) (PEDOT) has attracted much attention for DSSCs. In this review article, the progress of PEDOT‐based counter electrodes for DSSCs is presented. First, the properties and structure of PEDOT are briefly described, and its feasibility as a DSSC counter electrode is demonstrated. Then, the effect of various treatments on the electrical conductivity and catalytic activity of PEDOT as well as its stability is examined. Furthermore, efficient and low‐cost composite counter electrodes consisting of PEDOT and other materials are deeply discussed. Finally, an outlook for PEDOT counter electrodes is provided. Copyright © 2014 John Wiley & Sons, Ltd.     

Flexible and light weight copper indium diselenide solar cells on polyimide substrates

Thin film flexible CuInSe₂ (CIS) solar cells have been fabricated for the first time on light-weight polymeric substrates. Evaporated Cu---In alloy precursors were selenized in H₂Se atmosphere at around 400°C to grow the CIS absorber layers. Low temperature techniques which are compatible with the polymeric substrates were used to deposit the window layers of CdS and ZnO. The demonstrated active area conversion efficiency of 9.3% makes this light-weight device very attractive for many terrestrial and space power generation applications where high specific power and mechanical flexibility are needed.     

3D graphene from CO2 and K as an excellent counter electrode for dye‐sensitized solar cells

3D graphene, which was synthesized directly from CO₂ via its exothermic reaction with liquid K, exhibited excellent performance as a counter electrode for a dye‐sensitized solar cell (DSSC). The DSSC has achieved a high power conversion efficiency of 8.25%, which is 10 times larger than that (0.74%) of a DSSC with a counter electrode of the regular graphene synthesized via chemical exfoliation of graphite. The efficiency is even higher than that (7.73%) of a dye‐sensitized solar cell with an expensive standard Pt counter electrode. This work provides a novel approach to utilize a greenhouse gas for DSSCs.     

CdS量子点敏化TiO2纳米线束阵列太阳能电池的研究

采用水热合成技术,以盐酸、去离子水和酞酸丁酯为反应前驱物,在透明导电玻璃衬底(FTO)上生长TiO2纳米线束阵列,以化学浴沉积技术制备CdS量子点敏化TiO2纳米线束阵列光阳极。研究了CdS量子点敏化的循环周期对太阳能电池的光伏性能、单色光光子-电子转换效率、静态和动态光电流的特性的影响规律。结果表明:CdS量子点的大小和密度随着敏化循环周期的增加而增加,当敏化循环的周期为15次时,单色光光子-电子转换效率最高,电池的短路电流密度为0.61 mA/cm2,开路电压为0.65 V,填充因子为0.50,光电转换效率为0.20%。通过强度调制的光电流谱分析,得到光生电子在光阳极中的扩散系数为3.2×10-6cm2/s,传输时间为2.1×10-2s。     

Thin-film solar cells on commercial ceramic tiles

Amorphous silicon solar cells were deposited on porcelain stoneware tiles in order to develop a fully integrated PV building element. In a previous work we demonstrated the feasibility of adopting porcelain stoneware tiles as thin-film solar cell substrates and we fabricated solar cells on “industrial-level” ceramic substrates showing more than 4% efficiency. In this study we focus our attention on larger area devices deposited on tiles. As the active area increases additional problems arise from the surface of the substrate. In particular we find that short-circuit paths originate from the unfavorable properties of the substrate tiles (roughness, porosity, etc.) having dramatic consequences on the performance of larger area devices. With the assumption that spot-like regions, in which the back layer and the front layer are in electrical contact (local short-circuits), are present all over the device, we propose a new device structure to overcome the substrate drawbacks. A new contacts arrangement was designed in order to minimize the shunting effect of short-circuit paths. As a result an initial efficiency of 2.5% has been obtained using the new device structure for devices deposited on a PV mini-module tile.     

A study of stainless steel-based dye-sensitized solar cells and modules

Stainless steel (StSt) has been applied as substrate material for efficient, flexible, nanoporous TiO₂ dye-sensitized solar cells (DSSCs) with the aim of improving the photochemical properties of current plastic-based flexible DSSCs. DSSCs with a StSt substrate show almost equivalent properties in efficiency and convenience to cells with a F-doped tin oxide (FTO) glass substrate. Specifically, the metal substrate allows application of high-temperature sintering processes and shows high conductance even after sintering. Cells fabricated with the StSt substrates have been investigated as individual cells and as modules. A comparison between conventional DSSCs with a FTO glass substrate and flexible DSSCs with a StSt substrate is presented. In addition, Pt-coated electrodes, which can serve as window electrodes for StSt-based DSSCs, are fabricated via two different methods, i.e., chemical reduction and annealing, and compared.