Role of graphene and transition metal dichalcogenides as hole transport layer and counter electrode in solar cells

Photovoltaic (PV) technology got much attention in the past few decades in developing advanced and environment friendly solar cells (SCs). However, high cost, unstable nature, and low efficiency are major limitations towards commercialization of SCs. To overcome the issues, two-dimensional materials (2DMs) have been exploited due to low cost, high catalytic activity, fast charge separation, and better electrochemical performance. The review emphasis on (a) the electrochemical performance of graphene and transition metal dichalcogenides (TMDCs) as a hole transport layer (HTL) in SCs and (b) to explore low-cost and effective counter electrode (CE) based on graphene and TMDCs for dye-sensitized solar cell (DSSC). The review presents a comparative analysis of 2DMs as HTL and CE to attain highly efficient and low-cost PV devices. Multiple combinations of the material with graphene, graphene oxide (GO), reduced graphene oxide (rGO), tungsten disulfide (WS₂), molybdenum disulfide (MoS₂) as HTL, and CE material in PV cells are discussed and comparatively analyzed. Numerous strategies are briefly discussed to enhance the efficiency of SCs by utilizing graphene and TMDCs based HTL and CEs. The review focuses on the recent progress in developing low-cost and highly efficient PV devices by using 2DMs. Our study reveals that GO/PEDOT:PSS demonstrate a maximum power conversion efficiency (PCE) of 13.1% when fabricated at different revolutions. Moreover, our statistical analysis unveils that efficiency of the cell can be enhanced by optimizing the layer thickness, which provide a route to develop highly efficient and better performance SCs that can be exploited for future commercial applications.     

VC@C composites derived from polyoxovanadate assisted by dicyandiamide as low-cost platinum-free counter electrode electrocatalyst for dye-sensitized solar cells

Vanadium-based carbides have been applied as Pt-free counter electrodes (CEs) electro-catalysts for dye-sensitized solar cells (DSSCs) due to the advantages of earth-abundant reserves, diverse composition, ease modification, and low cost. Herein, the polyoxovanadate (NH₄)₂V₆O₁₆ as V source assisted by dicyandiamide (C₂H₄N₄) as C source via simply physical mixing by ball-milling to assemble VC@C precursors. And then, five different VC@C composites derived from precursors with mass ratios of dicyandiamide to polyoxovanadate of 5:1, 10:1, 15:1, 20:1 and 25:1 at 900 °C, and further achieved power conversion efficiencies (PCEs) of 5.4%, 5.6%, 6.6%, 6.2% and 5.1% as CEs for regenerate traditional I₃⁻/I⁻ couple in the encapsulated DSSCs, respectively. The effects of different mass ratio of dicyandiamide on the catalytic performances of VC@C composite CEs were also assessed using cyclic voltammetry, electrochemical impedance spectroscopy and Tafel polarization experiments. The photocurrent-photovoltage (J-V) results indicated that VC@C composites CEs had high conductivity and rich number of active sites, which indicated that VC@C composites could be a cost-effective and high-performance alternative Pt-based CEs catalyst for DSSCs.     

Influence of doped PEDOT:PSS on the performance of polymer solar cells

The influence of anode buffer layers of doped poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) on the performance of solar cells made from blends of poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁-buytyric acid methyl ester has been investigated. Different concentration of ethylene glycol were added into the PEDOT:PSS solution to increase its conductivity. The surface roughness of the doped PEDOT:PSS film was changed, which was examined by atomic force microscopy. The best doped device with a power conversion efficiency of 4.39% as compared to 3.41% for the pristine device has been achieved. The enhanced PEDOT:PSS conductivity improved the short circuit current and fill factor of the doped device. The almost constant open circuit voltage indicated the well-established ohmic contact between the anode and active layer irrespective of the doping of the PEDOT:PSS. The changed surface roughness of the doped PEDOT:PSS film did not correlate with the morphology of the consequent active layer and the resultant device performance.     

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 effects of inclusion of iodine in CdTe thin films on material properties and solar cell performance

CdTe thin films were potentiostatically electrodeposited from a non-aqueous electrolytic bath containing ethylene glycol. In order to dope the CdTe using an n-type dopant according to a proposed new model, varying concentrations of iodine were added into the electrolytic bath. The resulting materials were studied for structural, morphological, optical and electrical properties. Structural analysis indicated the formation of CdTe layers without other possible phases at a concentration of 0.05 M of iodine in the bath. Optical absorption measurements have yielded a direct band gap value of 1.42±0.03 eV without showing any noticeable changes of the energy gap. Inclusion of iodine in CdTe layers have increased the electrical conductivity by a factor of 5, indicating positive n-type doping effects. The diodes of FTO/CdS/CdTe/Au structures showed improved current–voltage characteristics indicating the presence of a high potential barrier of 1.20 eV with low ideality factors around 1.40. These results demonstrate a considerable reduction of active recombination and generation centres from the structure. Although the fill factors observed are low, for the studied batches in this project, remarkable improvement of short-circuit current densities over 40 mA cm⁻² were observed together with open circuit voltage values in the range 500–700 mV. Capacitance–voltage measurements indicate a formation of a fully depleted device, desirable for photovoltaic conversion.     

Heteroatom‐doped graphene and its application as a counter electrode in dye‐sensitized solar cells

The most frequently used counter electrode (CE) in dye‐sensitized solar cells (DSSCs) is platinum on fluorine‐doped tin oxide glass. This electrode has excellent electrical conductivity, chemical stability, and high electrocatalytic affinity for the reduction of triiodide. However, the high cost of metallic platinum and the poor electrochemical stability pose a major drawback in the commercial production. This has necessitated a search for a non‐precious metal and metal‐free electrocatalyst that demonstrates better catalytic activity and longer electrochemical stability for practical use in DSSCs. Graphene has been at the centre of attention due to its excellent optoelectronic properties. However, a defect‐free graphene sheet is not suitable as a CE in DSSCs, because of its neutral polarity which often restricts efficient charge transfer at the graphene/liquid interface, irrespective of the high in‐plane charge mobility. Hence, heteroatom‐doped graphene‐based CEs are being developed with the aim to balance electrical conductivity for efficient charge transfer and charge polarization for enhanced reduction activity of redox couples simultaneously. The elements commonly used in chemical doping of graphene are nitrogen, oxygen, boron, sulfur, and phosphorus. Halogens have also recently shown great promise. It has been demonstrated that edge‐selective heteroatom‐doping of graphene imparts both efficient in‐plane charge transfers and polarity, thereby enhancing electrocatalytic activity. Thus, heteroatom‐doped graphene serves as a good material to replace conventional electrodes and enhance power conversion efficiency in DSSCs. The focus is to reduce the cost of DSSCs. This review explores the performance of DSSCs, factors that influence the power conversion efficiency, and various physicochemical properties of graphene. It further outlines current progress on the synthetic approaches for chemical doping (substitutional and surface transfer doping) of graphene and graphene oxide with different heteroatoms in order to fine‐tune the electronic properties. The use of heteroatom‐doped graphene as a CE in DSSCs and how it improves the photovoltaic performance of cells is discussed.     

Effects of graphene counter electrode and CdSe quantum dots in TiO2 and ZnO on dye‐sensitized solar cell performance

Fabrication and performance study of dye‐sensitized solar cells using different counter electrodes and photoanodes is reported. Spin coated, E‐beam coated platinum, and graphene electrodes were used as counter electrodes. Different combinations of TiO₂ nanoparticle and ZnO nanorods (NRs) with CdSe quantum dots were prepared and used as photoanodes. The photoanodes comprising of both ZnO NRs and TiO₂ nanoparticles have shown improved performances in short‐circuit current density and open‐circuit voltage comparing the devices fabricated using only ZnO NR or TiO₂ nanoparticles. The inclusion of CdSe quantum dots has been found to increase the performance of dye‐sensitized solar cell for all the photoanodes. In case of counter electrodes, the cells fabricated with graphene showed improved performance. Copyright © 2014 John Wiley & Sons, Ltd.     

Recent advances of high-efficiency single crystalline silicon solar cells in processing technologies and substrate materials

Single crystalline silicon solar cells have demonstrated high-energy conversion efficiencies up to 24.7% in a laboratory environment. One of the recent trends in high-efficiency silicon solar cells is to fabricate these cells on different silicon substrates. Some silicon wafer suppliers are also involved in such development. Another recent trend is the increased production of high-efficiency silicon cells, some of them with low-cost structures. This paper will discuss the progress at the University of New South Wales, and these trends in other organisations.     

Enhancement of the photoelectric performance of dye-sensitized solar cells by using a CaCO3-coated TiO2 nanoparticle film as an electrode

Core-shell-type nanoparticles with TiO₂ cores and CaCO₃ shells were applied as the electrode of dye-sensitized solar cells. The performance of the cell was significantly improved (as high as 26.7%) compared to the case when un-coated TiO₂ particle film was used as electrode. The improved energy conversion efficiency has been ascribed to (i) enhanced dye adsorption due to the high isoelectric point of the overlayer, and (ii) the prevention of the back electron transfer by the insulating nature of the overlayer.     

Application of micro-porous polycarbonate membranes in dye-sensitized solar cells: Cell performance and long-term stability

This research investigates the cell performance and long-term stability of dye-sensitized solar cells (DSSCs) containing micro-porous polycarbonate (PC) film as the frame work material to stabilize the electrolyte solution. The track-etched PC film has cylindrical pore geometry, which is beneficial for ion transport in the electrolyte trapped inside the PC film. The photovoltaic efficiency of the DSSC with 0.2-μm PC membrane is 5.75 ± 0.73% under irradiation of 100 mW cm⁻², which is slightly lower than that (6.34 ± 0.44%) of cells without PC film. The differences in fill factor and open-circuit voltage between the DSSCs with and without PC film are not statistically significant. The long-term cell performance is carried out at continuous illumination of 100 mW cm⁻² (1 sun) and in darkness at 60 °C for up to 1000 h. There is no significant efficiency difference between the cells with and without PC film in light soaking (4.33% vs. 4.52%) for 960 h. In darkness, however, the cells with PC film demonstrate much higher efficiency (at 2.37%) than cells without PC (0.85%) after 1000 h. The improved long-term efficiency data and the higher percentage of working cells confirm the superior lifetime and performance using the micro-porous PC film.     

Investigation of thermal performance of solar air heater having roughness elements as a combination of inclined and transverse ribs on the absorber plate

An experimental investigation has been carried out to study the heat transfer and friction characteristics by using a combination of inclined as well as transverse ribs on the absorber plate of a solar air heater. The experimental investigation encompassed the Reynolds number (Re) ranges from 2000 to 14 000, relative roughness pitch (p/e) 3–8 and relative roughness height (e/D_h) 0.030. The effect of these parameters on the heat transfer coefficient and friction factor has been discussed in the present paper and correlations for Nusselt number and friction factor has been developed within the reasonable limits. A procedure to compute the thermal efficiency based on heat transfer processes in the system is also given and the effect of these parameters on thermal efficiency has been discussed.     

Improving the performance of textured silicon solar cells through the field-effect passivation of aluminum oxide layers and up-conversion via multiple coatings with Er/Yb-doped phosphors

This paper examines the influence of field-effect passivation (from a coating of aluminum oxide) in conjunction with up-conversion (from multiple coatings containing Er/Yb-doped phosphors) on the performance of silicon solar cells. Note that the phosphors were applied to the rear surface of the cells. The surface morphology of the coatings was characterized by scanning electron microscopy and the chemical composition of Er/Yb-doped phosphors coating was examined using energy-dispersive X-ray spectroscopy. The fluorescence emissions of the coatings were examined using photoluminescence and optical image measurements. We examined the influence of field-effect passivation on dark current-voltage as well as photo-current density and external quantum efficiency (EQE). Improvements in photovoltaic performance after applying coatings containing Er/Yb-doped phosphors were estimated in terms of EQE and conversion efficiency. The field-effect passivation of Al₂O₃ and up-conversion provided by Er/Yb-doped phosphors resulted in EQE enhancements over a wavelength range of 600 to 1050 nm. Field-effect passivation was shown to enhance the conversion efficiency by 1.77% (from 16.91% to 17.21%), up-conversion enhanced conversion efficiency by 2.9% (from 17.21% to 17.71%), and a combination of field-effect passivation and up-conversion enhanced conversion efficiency by 4.73% (from 16.91% to 17.71%).     

The use of nickel oxide as a hole transport material in perovskite solar cell configuration: Achieving a high performance and stable device

Solar cells incorporated with organic-inorganic lead or tin halide-based perovskite materials as active light-absorber surfaces are referred to as perovskite solar cells (PSCs). This fast advancing solar technology has recorded an increase in its efficiency from 3.8% in 2009 to above 25% in recent years. The technology creates room for diverse device architectures, which enhances further development of thin-film solar cells and photovoltaics. This article reviews the use of nanocrystalline nickel oxide (NiO) film as a hole transport material in PSCs. The literature on pure nickel oxide and doped nickel oxide films has been discussed. The principle of operation, charge separation of PSCs and the various parameters that affect the efficient hole transport mechanisms, power conversion efficiency, growth mechanism, and stability of PSCs have also been discussed. Possible electron-blocking applications and future perspective of nickel oxide films have also been discussed.     

Comparison of minority carrier diffusion length measurements in silicon solar cells by the photo-induced open-circuit voltage decay (OCVD) with different excitation sources

The photoinduced open-circuit voltage decay technique was used to investigate the minority carrier lifetime in crystalline and polycrystalline silicon solar cells. This convenient investigation technique allows a fast determination of the diffusion length of minority carriers in semiconductor materials and is an important technique to predict the solar cell performance. The decay curves were obtained with different excitation sources, a xenon stroboscope lamp and a Nd:YAG laser, and the results were compared.     

Experimental studies and limitations of the light trapping and optical losses in microcrystalline silicon solar cells

This study addresses the potential of different approaches to improve the generated current in silicon thin-film solar cells and modules. Decreasing the carrier concentration in the front contact has proven to increase the quantum efficiency and the cell-current density significantly. Additionally, an optically improved ZnO/Ag back reflector and the optimized light incoupling by anti-reflection layers were studied. In this contribution, we show the potential of the different optical components and discuss combinations thereof in order to obtain a maximized cell-current density in silicon thin-film solar cells. Limitations of the cell-current density are discussed with respect to theoretical calculations.     

High performance membrane electrode assemblies by optimization of coating process and catalyst layer structure in direct methanol fuel cells

High performance membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs) are developed by changing the coating process, optimizing the structure of the catalyst layer, adding a pore forming agent to the cathode catalyst layer, and adjusting the hot-pressing conditions, such as pressure and temperature. The effects of these MEA fabrication methods on the DMFC performance are examined using a range of physicochemical and electrochemical analysis tools, such as FE-SEM, electrochemical impedance spectroscopy (EIS), polarization curves, and differential scanning calorimetry (DSC) of the membrane. EIS and polarization curve analysis show that an increase in the thickness and porosity of the cathode catalyst layer plays a key role in improving the cell performance with reduced cathode reaction resistance, whereas the MEA preparation methods have no significant effects on the anode impedance. In addition, the addition of magnesium sulfate as a pore former reduces the cathode reaction transfer resistance by approximately 30 wt%, resulting in improved cell performance.     

Study the performance of photogalvanic cells for solar energy conversion and storage: Rose Bengal-d-Xylose-NaLS system

The Rose Bengal is used as photosensitizer with d-Xylose as reductant and sodium lauryl sulphate (NaLS) as surfactant for the enhancement of the conversion efficiency and storage capacity of photogalvanic cell for its commercial viability. The observed value of the photogeneration of photopotential was 885.0 mV and photocurrent was 460.0 μA whereas maximum power of the cell was 407.10 μW. The observed power at power point was 158.72 μW and the conversion efficiency was 1.52%. The fill factor 0.3151 was experimentally determined at the power point of the cell. The rate of initial generation of photocurrent was 63.88 μA min⁻¹. The photogalvanic cell so developed can work for 145.0 min in dark on irradiation for 165.0 min, i.e. the storage capacity of the photogalvanic cell is 87.87%. A simple mechanism for the photogeneration of photocurrent has also been proposed.     

Theoretical variations of the thermal performance of different solar collectors and solar combi systems as function of the varying yearly weather conditions in Denmark

The thermal performances of solar collectors and solar combi systems with different solar fractions are studied under the influence of the Danish design reference year, DRY data file, and measured weather data from a solar radiation measurement station situated at the Technical University of Denmark in Kgs. Lyngby. The data from DRY data file are used for any location in Denmark. The thermal performances of the solar heating systems are calculated by means of validated computer models. The measured yearly solar radiation varies by approximately 23% in the period from 1990 until 2002, and the investigations show that it is not possible to predict the yearly solar radiation on a tilted surface based on the yearly global radiation.The annual thermal performance of solar combi systems cannot with reasonable approximation be fitted to a linear function of the annual total radiation on the solar collector or the annual global radiation. Solar combi systems with high efficient solar collectors are more influenced by weather variations from one year to another than systems with low efficient solar collectors.The annual thermal performance of solar collectors cannot be predicted from the global radiation, but both the annual thermal performance and the annual utilized solar energy can with a reasonable approximation be fitted to a linear function of the yearly solar radiation on the collector for both flat plate and evacuated tubular solar collectors. Also evacuated tubular solar collectors utilize less sunny years with large parts of diffuse radiation relatively better than flat plate collectors.     

Study the performance of photogalvanic cells for solar energy conversion and storage: Toluidine blue–D‐Xylose–NaLS system

Toluidine blue has been used as the photosensitizer with D ‐Xylose as the reductant and s odium lauryl sulphate as the surfactant for the enhancement of the conversion efficiency and storage capacity along with reduction in cost of the photogalvanic cell for its commercial viability. The observed value of open circuit voltage is 1110.0 mV and the photogeneration of photopotential is 945.0 mV, whereas the maximum photocurrent is 510.0 µA and the short circuit current or photocurrent at equilibrium is 430.0 µA. The rate of initial generation of photocurrent is 56.66 µA min^?1. The determined maximum power and power at power point are 406.35 µW and 148.96 µW, respectively. The observed conversion efficiency is 1.4323%. The fill factor 0.3120 has been experimentally determined at the power point of the cell, where the absolute value is 1.0. The photogalvanic cell so developed can work for 130.0 min in dark if it is irradiated for 145.0 min, i.e. storage capacity of the photogalvanic cell is 89.65%. A mechanism has also been proposed for the photogeneration of the photocurrent. Copyright © 2010 John Wiley & Sons, Ltd.     

Structural defects caused by a rough substrate and their influence on the performance of hydrogenated nano-crystalline silicon n-i-p solar cells

We present a cross-sectional transmission electron microscopy study of a set of hydrogenated nano-crystalline silicon n-i-p solar cells deposited by hot-wire chemical vapour deposition on Corning glass substrates coated with ZnO-covered Ag layers with various surface roughnesses. Strip-like structural defects (voids and low-density areas) are observed in the silicon layers originating from micro-valleys of Ag grains. A correlation between the opening angles of the textured surface and the appearance of these strips was found. We propose that in order to grow high-quality hydrogenated nano-crystalline silicon absorber layers for solar cell applications, the morphology of the Ag surface is a critical property, and the micro-valleys at the ZnO surface with an opening angle smaller than around 110° should be avoided.