Optimization and characterization of TiO2-based solar cell design using diverse plant pigments

The design of electrochemical solar cells (SCs), including those composed of biological pigments is an actively developing direction of obtaining alternative energy. SCs were studied under different temperatures, light intensities and spectral conditions. Furthermore, to understand processes occurring in the SCs, investigations characterizing the efficiency and stability with regard to environmental factors are also required. For this aim, novel instrumentation for the investigation of environmental effects on photocurrent generated by SCs has been designed and constructed. The system can be a model, which reflects conditions required for effective and stable functioning of the solar cells. Preliminary results are shown for two types of solar cells with two photosensitizers: thylakoid membrane preparations and anthocyanin-enriched raspberry extracts. It was shown that electrogenic activity decreased by a half at 40 °C and returned back to the initial value under gradual cooling. Maximum current obtained from the thylakoid-based SC was 0.46 μA, while maximum current generated by the anthocyanin-based SC was 1.75 μA. The goal of this investigation is to find new ways to increase efficiency and stability of bio-based SCs. In future, this measuring system can be used for investigation of solar cells based on long-wave forms of chlorophylls (Chls d and f) and components of the photosynthetic apparatus comprising these chlorophylls.     

Comparative study of high concentrating photovoltaics integrated with phase‐change liquid film cooling system

In high concentrating photovoltaic systems, thermal regulation is of great importance to the conversion efficiency and the safety of solar cells. Direct‐contact liquid film cooling technique is an effective way of thermal regulation with low initial investment. Tilt of solar cells is common in concentrating solar systems. An evaluation of direct‐contact liquid film cooling technique behind tilted high concentration photovoltaics was performed using both experimental and computational approaches. In the experiment, deionized water was used as the coolant at the back of simulated solar cells. Solar cell inclination of 0° to 75° with inlet water flow rate of 100–300 L/hour and inlet temperature of 30°C to 75°C were experimentally investigated. A two‐dimensional model was developed using computational fluid dynamics technique and validated by experimental results. The effects of inclination on average temperature, temperature uniformity, and heat transfer coefficient were discovered in this paper. The results indicated that 20° is the optimum angle for liquid film cooling. In addition, optimum inlet width, temperature, and velocity for inclination over 30° are 0.75 mm, 75°C, and 0.855 m/s, respectively.     

Bridging TiO2 nanoparticles using graphene for use in dye‐sensitized solar cells

The use of graphene to bridge TiO₂ particles in the photoanode of dye‐sensitized solar cell for reduced electrical resistance has been investigated. The difficulty in dispersing graphene in TiO₂ paste was overcome by first dispersing graphene oxide (GO) into the TiO₂ paste. The GO was then reduced to graphene after the sintering of TiO₂. This is shown through transmission electron microscopy and X‐ray photoelectron spectroscopy analysis. Cell performance was evaluated using a solar simulator, incident photon to electron conversion efficiency, intensity modulated photocurrent/photovoltage spectroscopy under blue light, and electrochemical impedance spectroscopy. Depending on the amount of graphene in the photoanode, the cell performance was enhanced to different degrees. A maximum increase of 11.4% in the cell efficiency has been obtained. In particular, the inclusion of graphene has reduced the electron diffusion time by as much as 23.4%, i.e. from 4.74 to 3.63 ms and increased the electron lifetime by as much as 42.3%, i.e. from 19.58 to 27.85 ms. Copyright © 2014 John Wiley & Sons, Ltd.     

Omnidirectional light‐harvesting enhancement of dye‐sensitized solar cells with ZnO nanorods

In this work, the chemical solution method was used to prepare one‐dimensional (1‐D) ZnO nanorod (NR) photoelectrodes, which were subsequently used in dye‐sensitized solar cells (DSSCs). The effects of ZnO NRs on the omnidirectional light‐harvesting performance of DSSCs were investigated by growing ZnO NRs with varying lengths as photoelectrodes. On the basis of field‐emission scanning electron microscopy and ultraviolet (UV)–vis‐near infrared (NIR) spectroscopy measurements on the ZnO NR photoelectrodes of varying lengths, it was observed that the dye adsorption and light‐scattering properties of NRs are affected by their length. In addition, DSSCs were prepared using ZnO NRs of varying lengths. These DSSCs were examined via electrochemical impedance spectroscopy, monochromatic incident photon‐to‐electron conversion efficiency measurements, and solar simulations to measure their photovoltaic efficiencies, carrier lifetimes, and device characteristics in omnidirectional antireflection measurements. The highest photovoltaic efficiency between these DSSCs was 0.33%. Omnidirectional antireflection measurements were performed on DSSCs with different ZnO NR lengths, and it was observed that the smallest change in efficiency between angles of incidence of 0° and 60° was 23%. Therefore, the light‐scattering properties of ZnO NR photoelectrodes improve the omnidirectional antireflection light capture characteristics of DSSCs.     

Use of single‐wall carbon nanohorns as counter electrodes in dye‐sensitized solar cells

The catalytic activity of single‐wall carbon nanohorns (SWNH) as counter electrodes (CE) of dye‐sensitized solar cells (DSC) was studied for the iodide/triiodide redox reaction. The catalytic activities of SWNH and high surface SWNH (HS‐SWNH) obtained by partial oxidation of SWNH were assessed based on charge‐transfer resistances (R_ct) and current–voltage curves. A half‐cell configuration was used, and CE performances were compared to CEs made of carbon black (CB) and Pt. A CE assembled with HS‐SWNH and mixed with 10 wt.% of hydroxyethyl cellulose (HEC) ‐ HS‐SWNH/HEC was found to have the highest electrocatalytic activity (lowest R_ct) among all the carbon‐based CEs tested when annealed at 180 °C (R_ct = 141 Ω cm²); however, a very thick film (several tens of µm) would be required in order to perform comparably to a Pt CE. The annealing of such CE at higher temperatures (above 400 °C) did not improve its catalytic activity, contrary to the other studied carbonaceous CEs. The redox catalytic activity of SWNH and HS‐SWNH decorated with Pt was also studied on a half‐cell configuration and compared to that of Pt/CB and pristine Pt. The Pt/SWNH/HEC CE showed the highest electrocatalytic activity per mass of Pt, needing just 50% of Pt load to yield the same electrocatalytic activity of a DSC equipped with a Pt CE, but having half of its transparency. Additionally, applications in temperature‐sensitive substrates are envisioned for the Pt/SWNH/HEC CE due to the use of lower annealing temperatures. Copyright © 2012 John Wiley & Sons, Ltd.     

TiO2/ZnO/TiO2 sandwich multi‐layer films as a hole‐blocking layer for efficient perovskite solar cells

A continuous and compact hole‐blocking layer is crucial to prevent photocurrent recombination at the photoanode/electrode interface of high‐performance mesostructure perovskite‐based solar cells. Novel TiO₂/ZnO/TiO₂ sandwich multi‐layer compact film prepared as hole‐blocking layer for perovskite solar cell. Herein, TiO₂, ZnO, and TiO₂ layers were successfully deposited by spin‐coating onto FTO glass substrate in sequence. The fill factor and power conversion efficiency of the perovskite solar cell are remarkably improved by the employment of a TiO₂/ZnO/TiO₂ sandwich compact layer. Perovskite solar cell based on TiO₂/ZnO/TiO₂ sandwich film has been observed to exhibit maximum incident‐photon‐to‐current conversion efficiency in the visible region (400–780 nm) and reach a power conversion efficiency of 12.8% under AM1.5G illumination. Copyright © 2016 John Wiley & Sons, Ltd.     

Reduced graphene oxide–titania nanocomposite‐modified photoanode for efficient dye‐sensitized solar cells

We report the successful application of reduced graphene oxide–titania (rGO–TiO₂) nanocomposite as an efficient photoanode for dye‐sensitized solar cell (DSSC). The DSSC assembled with the rGO–TiO₂‐modified photoanode demonstrated an enhanced solar to electrical energy conversion efficiency of 4.74% compared with the photoanode of DSSC composed with unmodified TiO₂ (2.19%) under full sunlight illumination (100 mW/cm², AM 1.5G) as a result of the better charge collection efficiency of rGO, which reduced the back electron transfer process. Influence of the rGO content on the overall efficiency was also investigated, and the optimal rGO content for TiO₂ was 0.5 mg. Further, the modification of rGO–TiO₂ on the compact layer TiO₂ surface led to an increase in efficiency to 5.83%. The superior charge collection and enhanced solar energy conversion efficiency of the rGO–TiO₂ nanocomposite makes it to be used as a promising alternative to conventional photoanode‐based DSSCs. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

Performance evaluation of a hot‐box reflector solar cooker using a microcontroller‐based measurement system

The performance of a low‐cost compound box‐reflector solar cooker designed and constructed by the department of Mechanical Engineering, at the University of Zimbabwe, was investigated and evaluated using a microcontroller‐based measurement system over a period of 3 months. Solar radiation and temperature measurements are sent directly to the computer for monitoring and subsequent analysis using a spreadsheet program. The system is connected to the computer through the RS232 port. Temperature was measured by LM335 temperature sensors, whereas solar radiation was measured by a Kipp & Zonen CM3 thermopile‐based pyranometer that was initially calibrated against the Eppley Precision Spectral Pyranometer. Peak temperatures of about 90°C for the food can be attained in about 5 h on a clear day in Bindura, Zimbabwe (18°S, 31°E). A standardized cooking power of 11 W and an overall efficiency of 15% were found for this cooker. Copyright © 2008 John Wiley & Sons, Ltd.     

Numerical study of heat transfer enhancement and fluid flow with inline common‐flow‐down vortex generators in a plate‐fin heat exchanger

Three‐dimensional numerical simulations are performed on a plate‐fin heat exchanger (with triangular fins as inserts between the plates) to evaluate the laminar heat transfer and fluid flow characteristics with longitudinal vortex generators (LVGs). The effect with an inline rectangular winglet pair (RWP) with a common‐flow‐down (CFD) configuration is studied. The numerical results indicate that the application of inline LVGs effectively enhances the heat transfer of the channel. The heat transfer further increases with the increase in the Reynolds number from 200 to 500 and angle of attack from β = 15° to 22.5°. The computations are also performed to find the best location for the second RWP. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20414     

Multilayered large‐area WO3 films on sheet and mesh‐type stainless steel substrates for photoelectrochemical hydrogen generation

The objective of this study is to demonstrate the significant improvement in the photoelectrochemical (PEC) hydrogen generation by a photoanode owing to the increased surface area of the substrate. In this work, multilayered tungsten oxide (WO₃) films have been successfully synthesized onto the large‐area sheet (9 × 9cm²) and mesh (1 × 20cm²) ‐type stainless steel (SS) substrates using screen printing and brush painting methods, respectively. All the WO₃ films are porous and nanocrystalline (30–80 nm) in nature with a monoclinic crystal structure as revealed from X‐ray diffraction and scanning electron microscopy studies. The PEC water splitting study is performed under simulated 1 SUN illumination (AM1.5 G) in a typical two‐electrode cell configuration with WO₃ photoanode and Pt wire immersed in 0.5 M H₂SO₄ electrolyte. The photocurrent as well as hydrogen generation rate for WO₃ photoanodes coated on the plane SS sheet substrate is relatively low and showed minimal change with increasing film thickness. On the other hand, the photocurrent as well as the hydrogen generation is enhanced by a 3–4 fold degree for the WO₃ photoanodes coated on SS mesh. We attribute such efficient water splitting to the increment in the filling factor of the WO₃ material due to the large effective surface area of the SS mesh as compared to the SS sheet substrate. Copyright © 2011 John Wiley & Sons, Ltd.     

Lauric and myristic acids eutectic mixture as phase change material for low‐temperature heating applications

Lauric acid (m.p.: 42.6°C) and myristic acid (m.p.: 52.2°C) are phase change materials (PCM) having quite high melting points which can limit their use in low‐temperature solar applications such as solar space heating and greenhouse heating. However, their melting temperatures can be tailored to appropriate value by preparing a eutectic mixture of lauric acid (LA) and myristic acid (MA). In the present study, the thermal analysis based on differential scanning calorimetry (DSC) technique shows that the mixture of 66.0 wt% LA forms a eutectic mixture having melting temperature of 34.2°C and the latent heat of fusion of 166.8 J g^?1. This study also considers the experimental establishment of thermal characteristics of the eutectic PCM in a vertical concentric pipe‐in‐pipe heat storage system. Thermal performance of the PCM was evaluated with respect to the effect of inlet temperature and mass flow rate of the heat transfer fluid on those characteristics during the heat charging and discharging processes. The DSC thermal analysis and the experimental results indicate that the LA–MA eutectic PCM can be potential material for low‐temperature solar energy storage applications in terms of its thermo‐physical and thermal characteristics. Copyright © 2005 John Wiley & Sons, Ltd.     

A solar ejector air‐conditioning system using environment‐friendly working fluids

In this paper, the performance of the solar‐driven ejector air conditioning with several environment‐friendly working fluids is studied. The effect of the fluid nature and operating conditions on the ejector performance is examined. This performance is calculated using an empirical correlation. Thermodynamic properties of functioning fluids are obtained with a package REFPROP7. It appears that the refrigerant R717 offers the highest coefficient of performance (COP). For generator temperature T_B = 90°C, condenser temperature T_C = 35°C and evaporator temperature T_E = 15°C and with R717, the COP of ejector air‐conditioning system is 0.408. Using a meteorological data for the city of Tunis, the system performance is computed for three collector types. The air‐conditioning season and period were taken for six months from April to September. The daily period is between 8 and 17 h. For the solar air‐conditioning application, the COP of the overall system varied from 0.21 to 0.28 and the exergy efficiency varied from 0.14 to 0.19 with the same working conditions and total solar radiation (351–875 Wm^?2) in July. Copyright © 2008 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.     

Photovoltaic cells energy performance enhancement with down‐converting photoluminescence phosphors

Phosphors, synthesized by the urea homo‐precipitation method, were examined as ultraviolet‐spectral down conversion materials for improving the light absorption and electrical characteristics of commercial single‐junction silicon solar cells. The photovoltaic (PV) cells were coated with erbium and terbium doped gadolinium oxysulfide phosphors encapsulated in ethyl vinyl‐acetate binder using blade screen printing technique, and the optimum concentration of phosphor in the composite resulted in the largest light conversion, and superior electrical output and energy transfer efficiency. Moreover, the results demonstrated that the composition of dispersed phosphors has a strong influence on the amount of ultraviolet‐light converted and electron transition capacity of PV cells. The experimental results showed in an optimized PV cell, an enhancement of 0.54% (from 12.11% to 12.65%) in the energy conversion of a Si‐based PV cell was achieved. Copyright © 2015 John Wiley & Sons, Ltd.     

Innovative multi‐processed N‐doped carbon and Fe3O4 cathode for enhanced bioelectro‐Fenton microbial fuel cell performance

A novel bioelectro‐Fenton microbial fuel cell (BEF‐MFC) cathode has been fabricated by modification of electrode using multi‐processing of nitrogen‐doped carbon (NDC)/nano‐Fe₃O₄ method with the aims of cost‐effectiveness, high oxygen reduction reaction (ORR) efficiency, and power performance enhancement. In this study, BEF‐MFC with carbon cloth (CC) cathode pyrolyzed with NDC‐M100/Fe₃O₄ at 700°C achieved higher ORR activity compared with the commercial Pt/C under same operational conditions. It also exhibited excellent crystalline structure according to high‐resolution transmission electron microscope (HRTEM) analysis. Moreover, using NDCN/Fe₃O₄ can facilitate further Fenton‐like reaction for the treatment of wastewater. Chemical oxygen demand (COD) removal efficiency of the reactor was 78% with maximum power density of 1.57 W/m³ in 216 hours. Thus, an innovative multi‐processing method with feasibility for enhanced wastewater treatment and improved power performance of the MFC was investigated. This can be effectively applied in related alternative energy production techniques and bio‐electrochemical systems in the future.     

Theoretical and experimental study on the application of diffuse‐reflection concentrators in PV/T solar system

In order to improve the practicability of PV/T solar system, we proposed the theory and method on the application of diffuse‐reflection concentrator in the PV/T solar system and analyzed the concentration characteristics of this proposed application. In addition, we designed experimental prototype of PV/T solar system and conducted test and analysis of the thermal and electrical characteristics of the PV/T solar system with or without a concentrator, respectively. The results showed that for the PV/T solar system with diffuse‐reflection concentrator, the amount of incident irradiance was increased by an average of 26% during test period, and the 200‐L water in the system was heated to 58 °C, which was 12 °C higher than that of PV/T solar system without diffuse‐reflection concentrator; moreover, the max output power was increased by 11%. Therefore, it is a feasible way to improve the practicability of PT/V solar system by integrating a diffuse‐reflection concentrator. Copyright © 2016 John Wiley & Sons, Ltd.     

Reduced graphene oxide–coated anode gas diffusion layer for performance enhancement of proton‐exchange membrane fuel cell

In this study, we produced reduced graphene oxide (RGO) by reduction of graphene oxide (GO) in Teflon‐lined autoclave, maintained at 100°C for 12 hours, and coated on the anode gas diffusion layer (GDL) of a proton‐exchange membrane fuel cell (PEMFC) to improve the cell performance. Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy analysis showed the presence of residual oxygen‐containing functional groups in RGO. Field‐emission scanning electron microscopy images revealed the uniform and adequate coating of the GDLs with RGO. The wettability of RGO‐coated GDL was determined by the sessile drop method and has optimum contact angle 117°. The power densities for the membrane electrode assembly (MEA) with RGO coated on the anode GDL were 30.92%, 41%, and 36.20% higher than those for the MEA without the RGO coating at anode gas humidified temperatures of 25°C, 45°C, and 65°C, respectively.     

Electrospray‐deposited nickel ferrite thin film electrode for hydrogen production in PV‐assisted water electrolysis system

Nanocrystalline Ni ferrite thin film was prepared by electrospray deposition technique and characterized by different analytical techniques at different annealing temperatures. All these films were studied by photovoltaic‐assisted water electrolysis system for solar to hydrogen production efficiency measurement. Highly dense and uniform surface morphology was observed in as‐deposited film, which changed into agglomerated nanocrystalline grains of irregular size and shape with change in annealing temperature. The X‐ray photoelectron spectroscopy study showed that the as‐deposited film was a mixture of an oxyhydroxide form of iron and an Ni₂O₃ form of nickel, whereas it changed into ferrite phase with change in annealing temperature. The as‐deposited film was observed to be of amorphous phase, which changed to crystalline cubic spinel structure with change in annealing temperature. The solar to hydrogen production efficiency was found to increase in a film with an increase in annealing temperature. The film annealed at 500°C showed a high solar to hydrogen production efficiency (8.29%) with constant performance of up to initial 500 h. Thereafter, the performance slowly declined by 11% when up to 1000 h. Copyright © 2011 John Wiley & Sons, Ltd.     

Three-layer organic solar cell with high-power conversion efficiency of 3.5%

To prepare organic solar cells with practical level of energy-conversion efficiency, the following strategies were adopted. By using HD as the photosensitizer, which is heterodimer consisting of a weak electron-donating 5, 10, 15, 20-tetra(2, 5-dimethoxyphenyl)porphyrinatozinc and a weak electron-accepting 5, 10, 15-triphenyl-20-(3-pyridyl)porphyrin, intramolecular photoinduced electron transfer is promoted resulting in effective charge separation. To create an energetically well-arranged system, the HD was placed between an electron-acceptor layer of PV (perylene-3, 4, 9, 10-tetracarboxyl-bis-benzimidazole) and strong electron-donor layer of MC (3-carboxymethyl-5-[(3-ethyl-2(3H)-benzothiazolylidine)ethylidene]-2-thioxo-4-thiazolidinone), where photoinduced intermolecular electron transfer from HD to PV and rapid injection of electrons from MC to HD suppress back electron transfer in the charge-separated HD. As a result of this, the three-layer solar cell Al/PV/HD/MC/Au showed fairly good photovoltaic properties, short-circuit photocurrent quantum yield of 49.2%, open-circuit photovoltage of 0.39 V, fill factor of 0.51, and energy conversion yield of 3.51% when irradiated with 445 nm monochromatic light of 12 μW cm⁻² intensity transmitted through the Al/PV interface. Since the photocurrent hardly decreased with age, the photocurrent observed here is really from energy conversion and not from photocorrosion of Al electrode being occasionally responsible for the photocurrent.