Photoelectrochemical Solar Cells Research Articles

Control of Interfacial Properties between Polymer Gel Electrolytes and Counter Electrodes for Efficient Quasi-Solid Photoelectrochemical Solar Cells

There has been much attention towards dye-sensitized solar cells (DSSCs) for the past decades due to their attractive features such as reasonable energy conversion efficiency and low energy production cost. Recently, cobalt complexes have been suggested as one of the most promising candidates for replacing conventional iodine redox shuttles owing to their high voltage characteristics. Although high energy efficiency exceeding 12% was reported by employing liquid electrolyte that consists of cobalt complex redox mediator and volatile solvent, the use of volatile liquid solvent cannot ensure the long-term stability of DSSCs. In this work, therefore, novel polymer gel electrolytes (PGEs) including cobalt complexes as redox couples have been developed for efficient and long-term stable DSSCs. The physicochemical properties of PGEs were adjusted by selecting proper solvent and functional additives (for improving cobalt complex solubility and cation transference number etc.). Polyethers with polar groups (e.g. PEO and PMO) and polymers without polar groups (e.g. PVdF-HFP) were used as the polymer additives for the gelation of the liquid electrolytes with various blending ratios. Especially, various organic solvents with relatively high boiling points were tested to prepare long-term stable polymer gel electrolytes containing cobalt complex redox shuttles. Solvents with high donor number induced significant dissolution of dyes attached on TiO2 and therefore the low volatile solvents with moderate donor number and dielectric constant were utilized in this work. DSSCs employing PGEs with cobalt redox couples also showed better long-term stability compared with those of DSSCs filled with liquid electrolytes at 50 oC for 700 h. In addition, several types of conjugated polymer based counter electrodes have been prepared and used for the fabrication of DSSCs to decrease the high charge transport resistance occurred between the PGE and counter electrode. The photovoltaic and electrochemical performances of the DSSCs have been systematically analyzed via the J-V curve measurement and various electrochemical characterizations such as EIS and CV analyses. Acknowledgments: This work was supported by a grant (No.2017000140002/ RE201702218) from the Environmental Industry Advancement Technology Development Project of Korea Environmental Industry & Technology (KEITI) funded by Korea Ministry of Environment (MOE).

In situ formation of porous TiO2 nanotube array with MgTiO3 nanoparticles for enhanced photocatalytic activity

Heterostructured semiconductor photocatalysts are desirable for applications such as purification of pollutants and water splitting due to the enhanced charge separation. In this work, porous TiO2 nanotube arrays loaded with MgTiO3 nanoparticles (MgTiO3/P-TiO2 NTAs) are prepared hydrothermally by anodization of TiO2 NTAs in the magnesium acetate (Mg(AC)2) solution. The photocatalytic (PC) activity of MgTiO3/P-TiO2 NTAs evaluated by monitoring the degradation of methyl blue (MB) reveals that the effectiveness is related to the thickness of MgTiO3 layer on TiO2. The best PC activity is observed from the MgTiO3/P-TiO2 NTAs hydrothermally treated at 200 °C for 1 h, the degradation rate is twice that of pristine anatase TiO2 NTAs and the transient photocurrent is three times higher. The heterostructured MgTiO3/P-TiO2 NTAs with good PC characteristics have large potential in photoelectrochemical (PEC) water decomposition and solar cell applications.

Role of Photogenerated Iodine on the Energy-Conversion Properties of MoSe2 Nanoflake Liquid Junction Photovoltaics.

Transition metal dichalcogenides (TMDs) such as MoSe2 and WSe2 are efficient materials for converting solar energy to electrical energy in photoelectrochemical photovoltaic cells. One limiting factor of these liquid junction solar cells is that photogenerated oxidation products accumulate on the electrode surface and decrease the photocurrent efficiency. However, it is unclear where the reaction products accumulate on the electrode surface and how they impact the local photoelectrochemical response. This open question is especially important for the structurally heterogeneous TMD nanoflake thin-film electrodes that are promising for large-area solar energy conversion applications. Here, we use a single-nanoflake photoelectrochemical and Raman microscopy approach to probe how the photogenerated I2/I3- products impact the photocurrent collection efficiency and the onset potential in MoSe2-nanoflake|I-/I2|Pt photoelectrochemical solar cells. We observed localized I2/I3- deposition on all types of MoSe2 nanoflake surface motifs, including basal planes, perimeter edges, and interior step edges. Illuminated nanoflake spots with the highest photocurrent collection efficiency are the first to be limited by I2/I3- formation under high-intensity illumination. Interestingly, I2/I3- formation occurs on illuminated surface spots that have the lowest photocurrent onset potential for iodide oxidation, corresponding to the highest open circuit voltage ( VOC). The VOC shifts could be attributed to variations in the surface reaction kinetics or doping density across the nanoflake. Our results highlight important limiting factors of nanoflake thin-film TMD liquid junction photovoltaics under concentrated solar illumination intensities.

Effect of films morphology on the performance of Cu2O PEC solar cells

Copper oxide, a p-type semiconductor, offers multiple device applications due to the lower band gap, non-toxic nature and perfect band alignment necessary for solar water splitting. We report the synthesis of copper oxide thin films on ITO glass substrates via electrodeposition technique. The results of this study manifest variation in particle size, crystallite orientation, shape and crystallinity of the films as a function of current density. We further conclude that significant variations in the efficiency of Photoelectrochemical (PEC) solar cell are attributed to the particle size and crystals faces exposed at the junction interface. The optimized parameters relevant to the efficient PEC solar cell are experimental identified as; open circuit voltage (VOC) = 0.370 V, short circuit current density(ISC) = 1.48 mA/cm2, fill factor (FF) = 0.50 and η = 0.28%, with high shunt resistance and lower series resistance corresponding to a current density of −1.0 mA/cm2. Mott–Schottky plots illustrate that the flat-band potential and charge carrier concentration increase with the particle size, crystallinity, and orientation of crystal faces.

ZnS passivated PbSe sensitized TiO2 nanorod arrays to suppress photocorrosion in photoelectrochemical solar cells

Harmonized effect of TiO2 and PbSe on the photoconversion efficiency of semiconductor sensitized solar cells (SSSCs) is studied. Rutile TiO2 nanorods (NRs) grown by hydrothermal method provides remarkably high surface area, superior injection and charge collection efficiency. TiO2 NRs are sensitized by PbSe using chemical bath deposition (CBD) and ZnS layer where ZnS acts as a surface passivation layer grown by successive ionic layer adsorption and reaction (SILAR) method. Photoelectrodes made up of sensitization can absorb the ultraviolet (UV) as well as the visible region of the solar spectrum efficiently. Synthesized photoelectrodes were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), optical and solar cell performances. FESEM images of the photoelectrodes illustrate uniform coverage of PbSe over TiO2 NRs increases with increasing deposition time. The ZnS layer avoids the recombination of charge carriers. The photoelectrochemical (PEC) study is carried out using an optimized device structure i.e. Glass/FTO/TiO2/PbSe/ZnS/Polysulphide/Pt/FTO/Glass. Under AM 1.5 G illumination, the T18-PbSe2-ZnS3 photoelectrode shows 0.34% photoconversion efficiency demonstrate lead chalcogenides have potential applications in solar cells.

A robust and self-assembled route to synthesis of CdZn(Se1−xTex)2 photoanodes as light harvesters for photoelectrochemical solar cells

This work reports on the development of CdZn(Se1−xTex)2 thin films utilized as the photoanode for photoelectrochemical cells (PECs). It was found that the incorporation of tellurium plays an important role in determining the optostructural, morphological, compositional and PEC performance of thin films. XRD measurements showed that the deposited thin films are in the mixed phases with a nanocrystalline nature. SEM images indicated that the surface morphology is favourable for effective light absorption in the solar spectrum. The EDS spectrum confirmed that thin film deposition occured in a stoichiometric manner. A detailed quantitative study was also executed using XPS and revealed the presence of Cd2+, Zn2+, Se2− and Te2− elements in the deposited thin film. Finally, the deposited thin films were tested for their photoelectrochemical (PEC) performance. The PEC study illustrated that CdZn(Se1−xTex)2 thin film showed the highest power conversion efficiency (η) of 1.13% among reported values.

All spray pyrolysis-coated CdTe\u2013TiO2 heterogeneous films for photo-electrochemical solar cells

Cadmium telluride (CdTe) thin films of different thicknesses deposited onto titanium dioxide (TiO2) nanoparticle layer by spray pyrolysis deposition (SPD) are demonstrated as major photo-active semiconductor in photo-electrochemical solar cell configuration using iodide/triiodide (I−/I3−) redox couple as a hole transport layer. The CdTe–TiO2 heterogeneous films were characterized by X-ray photoelectron spectroscopy which identified doublet split of Cd 3d and Ti 2p which confirms CdTe and TiO2. Optical absorbance and transmittance of CdTe and TiO2 films which were examined by UV–Vis spectroscopy confirm that the optical bandgap of CdTe is 1.5 eV with a dominant photo-absorption in the spectral window of 350–800 nm, while TiO2 showed a bandgap of 3.1 eV and is optically transparent in the visible spectral window. The present work examined photo-anodes comprising 1, 3, 5, and 10 SPD cycles of CdTe coated on TiO2 nanoparticle layer. The solar cell with 5 SPD cycles of CdTe resulting in 0.4% efficiency. Results can be articulated to the CdTe deposited by 5 SPD cycles provided an optimum surface coverage in the bulk of TiO2, while the higher SPD cycles leads to agglomeration which blocks the porosity of the heterogeneous films.

Highly efficient tandem photoelectrochemical solar cells using coumarin6 dye-sensitized CuCrO2 delafossite oxide as photocathode

In this study, we introduce a new concept for the highly efficient tandem p-n photoelectrochemical cell consisting of coumarin6 organic dye-anchored p-type CuCrO2 delafossite semiconductor as photocathodes coupling with traditional n-type TiO2 based photoanodes. Also, we have systematically studied the photovoltaic performance of tandem cells as a function of post-annealing of CuCrO2 films before sensitization. Hydrothermally synthesized CuCrO2 nanocrystals exhibited a high surface area with small crystallite size of 12 nm, phase-pure and well-crystalline after the optimized post-annealing conditions. The experimental results indicated that the optimal post-annealing temperature was 450 °C for 1 h due to the larger active surface area, lower Rct, and higher Jsc and Voc values. This tandem cell, fabricated by employing the CuCrO2 photocathodes, iodide-based redox mediator and a coumarin6 organic dye, afforded an impressive efficiency of 2.33% with Voc of 813 mV, Jsc of 4.83 mA cm−2, and fill factor of 0.59. The obtained parameters are acceptably high in comparison to NiO photocathode-based tandem cells previously reported in literature under similar experimental conditions. Therefore, this work opened the way for developing highly-efficient tandem photoelectrochemical solar cells.

Photoelectrochemical Analysis of Tin Selenide (SnSex) Thin Films Formed Using Electrochemical Atomic Layer Deposition (E-ALD)

Tin selenide is an attractive material for photovoltaic devices as an absorber layer.1 There are two derivatives: SnSe and SnSe2. SnSe is a p-type material that exhibits a band gap of 1.0 eV. SnSe2 is a n-type material with a band gap of 1.6 eV.1 In this study, tin selenide thin films have been electrochemically deposited using a process known as electrochemical atomic layer deposition (E-ALD). E-ALD uses the phenomenon known as underpotential deposition to deposit an atomic monolayer of each element separately. This process is known as a cycle and is repeated until the desired thickness is achieved.2 These films need further optimization for the selection of one species over the other. Initial photolectrochemical studies have shown the current thin film is p-type and is an option for a photocathode for hydrogen production in a photoelectrochemical cell. Further studies will include improving the deposition cycle used to increase the photoresponse of the material and to gain control over the appearance of SnSe or SnSe2. Subramanian, B; Snajeeviraja, C.; Jayaychandran, M. Review on materials properties Sn(S,Se) compound semiconductors useful for photoelectrochemical solar cells. Electrochem. 2002, 18 (8), 349-366.Stickney, J. L.; Villegas, I.; Suggs, D. W.; Gregory, B. W., Electrochemical Atomic Layer Epitaxy (ECALE). Abstr Pap Am Chem S 1991, 201, 289-Coll.

(Invited) Designing Efficient Photoelectrochemical Solar Energy Conversion Devices and Their Integration with Redox Flow Battery Devices

Due to the intermittent nature of sunlight, practical solar energy utilization systems demand both efficient solar energy conversion and inexpensive large scale energy storage. We will first discuss the rational design and demonstration of efficient photoelectrochemical hydrogen generation systems using efficient semiconductors and earth-abundant catalyst materials. We have further developed novel hybrid solar-charged storage devices that integrate regenerative photoelectrochemical solar cells and redox flow batteries (RFBs) that share the same pair of redox couples. In these integrated solar flow batteries (SFBs), solar energy is absorbed by semiconductor electrodes and photoexcited caries are collected at the semiconductor-liquid electrolyte interface and used to convert the redox couples in the RFB to fully charge up the battery. When electricity is needed, the charged up redox couples will be discharged on carbon electrodes to generate the electricity as in a RFB. We have demonstrated that such SFB devices can be charged under solar light without external electric bias and deliver a high discharge capacity comparable with state-of-the-art RFBs over many cycles. After developing silicon solar cells and high performance solar cells, carefully matching them with various organic or inorganic redox couples, and optimizing several generations of SFB device designs, we have recently achieved integrated SFB device with an overall direct solar-to-output electricity efficiency (SOEE) of 14%. These high performance SFBs can serve as distributed and standalone solar energy conversion and storage systems in remote locations and enable practical off-gird electrification.

Photovoltaic Properties of Aluminum Doped Zinc Oxide Electrodes Based on Variation of Aluminum Impurities in the Semiconductor

Photoelectrochemical (PEC) solar cell studies of Al doped zinc oxide (AZO) thin film electrodes has been carried out by photocurrent-voltage (I-V) characteristic. The concentration of Al in ZnO was varied between 1-5 at.% in order to study the effect of the variation on the photovoltaic performance of the electrodes. The current-voltage (I-V) characteristics measured in the dark and under 80 W simulated illumination revealed enhanced PV performance for the AZO electrodes in contrast to the undoped electrode. The best response was achieved for AZO electrode with 2 at.% Al concentration thus, recording higher conversion efficiency and fill factor. This is a clear indication that AZO electrodes are superior to undoped ZnO in PV applications. The remarkable enhanced properties of Al doping on ZnO were carefully studied by means of thickness measurement, X-ray diffraction (XRD), scanning electron microscope (SEM) and UV-Vis spectroscopy, and the results were in good agreement, and confirmed that AZO thin film electrodes are better for PV applications than undoped ZnO semiconductor in sodium sulphate (Na2SO4) electrolyte.

Effect of electron beam irradiation on chemically synthesized nanoflake-like CdS electrodes for photoelectrochemical applications.

In this paper, we chemically synthesized interconnected nanoflake-like CdS thin films for photoelectrochemical solar cell applications and subsequently irradiated them with electron beam irradiation at various doses of irradiation. The as-synthesized and irradiated samples were characterized by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and electrochemical measurements. XRD and XPS results confirmed the formation of CdS with a hexagonal crystal structure. FE-SEM and HR-TEM studies confirmed the photoelectrochemical performance, which was dependent on the surface morphology. The calculated values for efficiency demonstrated an outstanding photoelectrochemical performance with a fill factor of 0.38 and efficiency of 3.06% at 30 kGy. The high photoelectrochemical performance may be due to the interconnected nanoflake-like nanostructure and higher active surface area of the CdS samples. These results show that the electron beam irradiation is capable as an electrode for photoelectrochemical solar cells.

Graphitic carbon nitride (g-C3N4) electrodes for energy conversion and storage: a review on photoelectrochemical water splitting, solar cells and supercapacitors

Application of g-C3N4in energy conversion and storage such as solar to fuel conversion, solar cells and supercapacitors.

Low toxicity environmentally friendly single component aqueous organic ionic conductors for high efficiency photoelectrochemical solar cells

Low-toxic, environment friendly single component organic ionic conductors are synthesized by facile methods and used as electrolytes in Dye Sensitized Solar Cells (DSSCs).

Accuracy of dielectric-dependent hybrid functionals in the prediction of optoelectronic properties of metal oxide semiconductors: a comprehensive comparison with many-body GW and experiments

Understanding the electronic structure of metal oxide semiconductors is crucial to their numerous technological applications, such as photoelectrochemical water splitting and solar cells. The needed experimental and theoretical knowledge goes beyond that of pristine bulk crystals, and must include the effects of surfaces and interfaces, as well as those due to the presence of intrinsic defects (e.g. oxygen vacancies), or dopants for band engineering. In this review, we present an account of the recent efforts in predicting and understanding the optoelectronic properties of oxides using ab initio theoretical methods. In particular, we discuss the performance of recently developed dielectric-dependent hybrid functionals, providing a comparison against the results of many-body GW calculations, including G0W0 as well as more refined approaches, such as quasiparticle self-consistent GW. We summarize results in the recent literature for the band gap, the band level alignment at surfaces, and optical transition energies in defective oxides, including wide gap oxide semiconductors and transition metal oxides. Correlated transition metal oxides are also discussed. For each method, we describe successes and drawbacks, emphasizing the challenges faced by the development of improved theoretical approaches. The theoretical section is preceded by a critical overview of the main experimental techniques needed to characterize the optoelectronic properties of semiconductors, including absorption and reflection spectroscopy, photoemission, and scanning tunneling spectroscopy (STS).

A promising hybrid graphite counter electrode doped with fumed silica nano-spacers for efficient quasi-solid state dye sensitized solar cells

Preparation of low-cost electrocatalytic counter electrode (CE) at low temperature is desired in the fabrication of photo-electrochemical quasi-solid state dye-sensitized solar cell (Qs-DSSC). We present facile doping of fumed silica (SiO2) as mesoporous nano-spacers in cationically functionalized graphite network and its synthesis into an electrically conductive hybrid paste for Qs-DSSC CEs. Polyethylene oxide (3%) based electrolyte solution is used as a quasi-solid gel electrolyte. With an optimized quantity of fumed silica nano-spacers (10%), an increase of 53.6% is achieved in power conversion efficiency (PCE) compared to bare cationised graphite coated CE. Our suggested Si@G 10% CE demonstrates commendable power conversion efficiency of 6.42% compared to 7.32% with expensive platinum (Pt) based Qs-DSSC. It exhibits a low charge transfer resistance (RCT) of 0.61 Ω cm2, owing to the improved interconnection, formation of the porous structure, and enhanced mobility of electrolyte ions. The cyclic voltammetry (CV) shows excellent electro-catalytic activity towards the I−/I3− redox couple and it is stable up to 50 cycles at a scan rate of 10 mV s−1. The facile synthesis route, excellent electro-catalytic activity, appreciable stability, and considerable photovoltaic performance makes Si@G 10% based CE a promising candidate for Qs-DSSC fabrication.

Surfactant mediated synthesis of bismuth selenide thin films for photoelectrochemical solar cell applications

In the present report, nanostructured bismuth selenide (Bi2Se3) thin films have been successfully deposited by using arrested precipitation technique (APT) at room temperature. The effect of three different surfactants on the optostructural, morphological, compositional and photoelectrochemical properties of Bi2Se3 thin films were investigated. Optical absorption data indicates direct and allowed transition with a band gap energy varied from 1.4 eV to 1.8 eV. The X-ray diffraction pattern (XRD) revealed that Bi2Se3 thin films are crystalline in nature and confirmed rhombohedral crystal structure. SEM micrographs shows morphological transition from interconnected mesh to nanospheres like and finally granular morphology. Surface topography of Bi2Se3 thin films was determined by AFM. Compositional analysis of all samples was carried out by energy dispersive X-ray spectroscopy (EDS). Finally, all Bi2Se3 thin films shows good PEC performance with highest photoconversion efficiency 1.47%. In order to study the stability of Bi2Se3 thin films four cycles are repeated after gap of one week each. Further PEC performance of all Bi2Se3 thin films are also supported by electrochemical impedance (EIS) measurement study.

Optimizing the photochemical conversion of UV–vis light of silver-nanoparticles decorated TiO2 nanotubes based photoanodes

Homogeneous decoration of TiO2 nanotubes (NTs) by Ag metallic nanoparticles (NPs) was carried out by a relatively simple photoreduction process. This Ag-NPs decoration was found to improve the photoconversion efficiency of the TiO2-NTs based photoanodes. The x-ray photoelectron spectroscopy and x-ray diffraction analyses confirmed that all the Ag-NPs are metallic and the underlying TiO2-NTs crystallize in the anatase phase after their annealing at 400 °C, respectively. Transmission electron microscopy observations have confirmed the effective decoration of the TiO2-NTs’ surface by Ag-NPs, and allowed to measure the average Ag-NPs size, which was found to increase linearly from (4 ± 2) nm to (16 ± 4) nm when the photoreduction time is increased from 5 to 20 min. The diffuse reflectivity of the Ag-NPs decorated TiO2-NTs was found to decrease significantly as compared to the undecorated TiO2-NTs. Interestingly, the Ag-NPs decorated TiO2-NTs exhibited a significantly enhanced photochemical response, under visible radiation, with regards to the undecorated NTs. This enhancement was found to reach its maximum for the TiO2-NTs decorated with Ag-NPs having the optimal average diameter of ∼8.5 nm. The maximum photoconversion efficiency of Ag-NPs decorated TiO2-NTs was about two times greater than for the undecorated ones. This improved photo-electro-chemical response is believed to be associated with the additional absorption of visible light of Ag-NPs through the localized surface plasmon resonance phenomenon. This interpretation is supported by the fact that the photoluminescence intensity of the Ag-NPs decorated TiO2-NTs was found to decrease significantly as compared to undecorated NTs, due to charge carriers trapping in the Ag NPs. This demonstrates that Ag-NPs decoration promotes photogenerated charges separation in the TiO2-NTs, increasing thereby their capacity for current photogeneration. The surface decoration of TiO2 NTs by noble metals NPs is expected to impact positively the use of TiO2-NTs based photoanodes in some energetic applications such as hydrogen generation and photo-electrochemical solar cells.

Photoelectrocatalytic degradation of sugarcane factory wastewater using WO3/ZnO thin films

In the present work, layered WO3/ZnO thin films have been prepared by simple chemical spray pyrolysis method. As prepared films are characterized by photoelectrochemical (PEC) solar cell, X-ray diffraction (XRD), Raman spectroscopy, Field emission scanning electron microscopy (FE-SEM), Brunauer–Emmer–Teller (BET) and energy dispersive X-ray spectroscopy techniques. XRD analysis reveals the formation of hexagonal and monoclinic phases of ZnO and WO3 respectively. Raman analysis confirms the formation of layered WO3/ZnO thin films. FE-SEM images demonstrate that the surface morphology of layered WO3/ZnO consists of nano balls like morphology. The specific surface area of the layered WO3/ZnO thin film is found to be 65.12 m2 g−1. The photoelectrocatalytic degradation properties of layered WO3/ZnO thin films were investigated by studying degradation of sugarcane factory wastewater. The end result shows that the degradation percentage of sugarcane factory wastewater using layered WO3/ZnO photo electrode has reached 94.44% after 100 min. under sunlight illumination.

Effect of annealing on microstructural, compositional, optical, and photoelectrochemical properties of CdSe thin films

Thin films of CdSe have been prepared on indium-doped tin oxide-coated conducting glass substrates from an aqueous electrolytic bath containing CdSO4 and SeO2. The process of annealing has been carried out to improve microstructural, optical, and photoelectrochemical properties of the prepared films. X-ray diffraction analysis revealed that as-deposited and annealed films exhibit hexagonal structure with preferential orientation along (002) plane. Nelson-Riley plot analysis has been carried out to determine exact value of lattice constants for as-deposited and annealed films. Optical absorption analysis has been captured to determine the value of optical parameters such as band gap, refractive index, and extinction coefficient of the deposited films. Photoelectrochemical solar cell analysis has been carried out and power output characteristics of the prepared films are investigated.