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

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

Novel iminocoumarin dyes as photosensitizers for dye-sensitized solar cell

Novel iminocoumarin dyes (2a-c and 3a-c) having carboxyl and hydroxyl anchoring groups onto the dyes skeletons have been designed and synthesized for the application of dye-sensitized nanocrystalline TiO₂ solar cells (DSSCs). The photophysical and electrochemical studies showed that these iminocoumarin dyes are suitable as light harvesting sensitizers in DSSC application. The dyes having carboxyl and hydroxyl anchoring groups (2a-c) showed better efficiency when compared to the dyes having carboxyl group (3a-c) alone. The cell consisted of dye 2a generated the highest solar-to-electricity conversion efficiency (η) of 0.767% (open circuit voltage (V_oc) = 0.491 V, short circuit photocurrent density (J_sc) = 2.461 mA cm⁻², fill factor (ff) = 0.635) under simulated AM 1.5 irradiation (1000 W m⁻²) with a total semiconductor area of 0.25 cm². The corresponding incident photon-to-current conversion efficiency (IPCE) of the above cell was 21.38%. The overall low efficiency of the dyes is ascribed to the lack of light harvesting ability at longer wavelength region.     

Influence of n-type chemical doping layer on the performance of organic photovoltaic solar cells

We report the performance improvement of organic solar cell by addition of an n-type chemical doping layer in organic bulk heterojunction device. The power conversion efficiency (PCE) of P3HT and PCBM-71 based polymer solar cells increases by adding a mixture of TCNQ (7,7,8,8-tetracyanoquinodimethane) and LCV (Leucocrystal violet) between active layer and cathode electrode. The PCE of the cell increases by 14% compared to the control cell with Al-only cathode electrode. The device with an organic n-doped layer shows the J_SC of 8.88 mA/cm², V_OC of 0.51 V, FF of 60.1%, and thus the PCE of 2.72% under AM1.5 illumination of 100 mW/cm².     

Development of DA-type polymers with phthalimide derivatives as electron withdrawing units and a promising strategy for the enhancement of photovoltaic properties

We report on two push-pull type polymer semiconductors involving phthalimide derivatives as electron withdrawing units. The solubility and energy level of phthalimide could be easily controlled by introducing various functional groups in its nitrogen site. Additionally, the V_OC value of polymer semiconductor materials with phthalimide as an electron withdrawing unit could be efficiently enhanced because of the low HOMO energy level of phthalimide. Nevertheless, there are just a few of studies regarding the use of phthalimide in OPVs. In this study, we synthesized two photovoltaic polymer materials based on phthalimide with high V_OC value, PFTPT and PCTPT. Between the two polymers, PCTPT/PC₇₁BM-based photovoltaic cell afforded the best PCE value of 1.4% (V_OC=0.94 V, J_SC=4.41 mA/cm², FF=0.33) under 100 mW/cm² irradiation. In addition, a promising strategy for the development of high performance photovoltaic polymers with phthalimide derivatives as electron withdrawing units was investigated.     

Synthesis and characterization of trivalent metal porphyrin with NCS ligand for application in dye-sensitized solar cells

Two novel trivalent metal porphyrin dyes, P_Mn-HT-SCN and P_Ga-HT-SCN, were designed, synthesized and firstly applied in dye-sensitized solar cells (DSSCs). These two dyes possess porphyrin donor modified with manganese (III) and gallium (III) as coordination metal and NCS as the second ligand, cyanoacrylic acid as electron-accepting moiety and 4-hexylthiophene as π-spacers. Each of the porphyrin showed different adsorption behavior and saturated coverage on the TiO₂ surface. Between the two dyes, the P_Mn-HT-SCN-based DSSCs afforded the best photovoltaic performance: a short-circuit photocurrent density (J_sc) of 4.32 mA/cm², an open-circuit photovoltage (V_oc) of 0.61 V and a fill factor (FF) of 0.58, corresponding to a solar-to-electricity conversion efficiency of 1.53% under 100 mW/cm² irradiation.     

Fabrication of multilayer TiO2 thin films for dye-sensitized solar cells with high conversion efficiency by electrophoresis deposition

This research coats a commercial TiO₂ nanoparticle Degussa P25 with good roundness and size uniformity on an indium tin oxide (ITO) glass substrate and to be photoelectrical electrode by electrophoresis deposition. It combined with dye N719, electrolyte I^-/ and counter-electrode of Pt layer to produce dye-sensitized solar cells (DSSCs). Through the electrophoretic technique, a multilayer film of an appropriate thickness is deposited in the suspension containing TiO₂ nanoparticles and isopropanol. In this process, electric current, voltage, and the number of deposition cycles are well controlled to obtain a single TiO₂ film of around 3.3 μm thick. Stacking is then performed to obtain a multilayer-typed TiO₂ film of around 12 μm thick. As the sintering temperature reaches 400 °C, the prepared multilayer TiO₂ film with a good compactness can increase the dye adsorption capability of the thin film and enhance its adsorption percentage. In addition, the heat treatment will transfer a portion of the rutile crystalline into the anatase crystalline, resulting in better material properties for DSSCs application. DSSCs produced are exposed to metal halide lamp and their energy conversion efficiency is measured. The I-V curve of the produced DSSCs shows that it has an excellent energy conversion efficiency of 6.9%.     

Dimethyl-2H-benzimidazole based small molecules as donor materials for organic photovoltaics

Utilization of 2,2-dimethyl-2H-benzimidazole has received strong attention as the electron-deficient unit for the generation of electron donor material for organic photovoltaic cells (OPVs). This paper reports the first small organic molecules based on dimethyl-2H-benzimidazole, which can produce intramolecular charge transfer. Two soluble small organic molecules, MMM and OMO, with dimethyl-2H-benzimidazole unit were synthesized by Suzuki coupling reaction with Pd(0)-catalyst. The spectra of MMM and OMO in the solid thin films show absorption bands with maximum peaks at 374, 598 and 373, 588 nm, and the absorption onsets at 678 and 673 nm, corresponding to band gaps of 1.83 and 1.84 eV, respectively. The devices comprising MMM with PC61BM (1:3) showed a V_OC of 0.66 V, a J_SC of 2.03 mA/cm², and a fill factor (FF) of 0.27, giving a power-conversion efficiency of 0.37%.     

Synthesis of triarylamines with secondary electron-donating groups for dye-sensitized solar cells

Three organic dyes XS24–26 containing N,N-dimethylaniline and butoxybenzene have been designed, synthesized and applied in the dye-sensitized solar cells (DSSCs). The influence of secondary electron-donating groups on the performance of DSSCs is discussed. The dimethylaniline is beneficial to extend absorption spectrum, whereas butoxybenzene is useful to suppress electron recombination. XS26 containing butoxybenzene and thiophene unit gives the highest power efficiency η of 5.67%, with a J_SC of 12.36 mA cm^?2, V_OC of 680 mV, and ff of 0.67.     

Dye-sensitized solar cells with a micro-porous TiO2 electrode and gel polymer electrolytes prepared by in situ cross-link reaction

The ionic conductivities and performances of dye-sensitized solar cells (DSSCs) of gel polymer electrolytes (GPEs) prepared by in situ cross-link reaction with different cross-linkers were investigated. The poly(imidazole-co-butylmethacrylate)-based GPE containing the 1,2,4,5-tetrakis(bromomethyl)benzene (B4Br) cross-linker showed a higher ionic conductivity than that containing cross-linkers with a linear structure, due to the formation of micro-phase separation that resulted in an increase in ion transport paths in the GPE. Moreover, the co-adsorbent ((4-pyridylthio) acetic acid, PAA) co-adsorbed with N3 dye on the TiO₂ electrode not only reduced dye aggregation, but also reacted with the cross-linkers in the GPE at the TiO₂/GPE interface. A decrease in the charge transport resistance at the TiO₂/GPE interface was noted after forming the gel; thus the value of J_SC significantly increased from 7.72 to 10.00 mA cm⁻². In addition, in order to reduce the ionic diffusion resistance within the TiO₂ electrode, incorporation of monodispersed PMMA in the TiO₂ paste was considered. With the optimal weight ratio of PMMA/TiO₂ (w/w=3.75), the TiO₂ electrode exhibited larger pores (ca. 350 nm) uniformly distributed after sintering at 500 °C, and the ionic diffusion resistance within the TiO₂ film could significantly be reduced. The cell conversion efficiency increased from 3.61% to 5.81% under illumination of 100 mW cm⁻², an improvement of ca. 55%.     

St. Lucie cherry,yellow jasmine,and madder berries as novel natural sensitizers for dye‐sensitized solar cells

Natural dyes extracted from fruits, vegetables, flowers, and leaves are considered as promising alternative sensitizers to replace synthetic dyes for dye‐sensitized solar cells (DSSCs). Generally, solar activity of natural dyes stem from anthocyanin pigment. Carbonyl, carboxyl, and hydroxyl groups present in the anthocyanin molecule improve the adsorption ability of dye on TiO₂ and therefore facilitate charge transfer. Here, for the first time, novel natural dyes extracted from St. Lucie cherry, yellow jasmine, and madder berries are reported to act as sensitizer in DSSCs. These novel natural dye extracts are prepared by dissolving related fruits in ethanol. The ingredient of the dyes is identified by FT‐IR spectroscopy. Accordingly, FT‐IR spectrum reveals that novel natural dye extracts exhibit all the characteristic peaks of anthocyanin pigment. Specifically, St. Lucie cherry consists of more distinct carbonyl group than other sources. Also, photoanodes composed of three TiO₂ layers are prepared by using a spin‐coating method. Then, they are immersed into natural dyes and analyzed by conducting UV‐Vis spectroscopy. Compared with bare TiO₂, natural dye–loaded photoanodes demonstrate far higher absorption ability in the visible region. After fabrication of devices with different novel natural dye sensitizers, current‐voltage characteristics and electrochemical impedance spectroscopy measurements are performed. The best power conversion efficiency (PCE) of 0.19% is obtained by sensitization of St. Lucie cherry with an open‐circuit voltage (V_oc) of 0.56 V, short‐circuit current density (J_sc) of 181 μA cm^?2, and fill factor (FF) of 0.55. Furthermore, St. Lucie cherry–sensitized devices show the lowest charge transfer and highest recombination resistances. This result can be attributed to the obvious carbonyl group exhibited by St. Lucie cherry.     

Synthesis of TiO<Subscript>2</Subscript> nanoparticle using sol–gel route and testing its photovoltaic performance in dye-sensitized solar cell

In the present work, sol–gel method is used to synthesize TiO₂ nanoparticle. The characterization of the prepared TiO₂ powder is done using Powder X-ray diffraction (powder XRD), Scanning Electron Microscope (SEM), Energy-Dispersive X-Ray Spectroscopy (EDS) and Ultraviolet-Visible Spectrophotometry (UV-Vis). The XRD pattern reveals formation of anatase phase TiO₂. The SEM images reveal agglomeration of nanoparticles. The absorbance spectrum of TiO₂ nanoparticles was observed with excitonic peaks at 327 nm and the band gap came out to be ~3.2 eV. This prepared TiO₂ was tested for photovoltaic performance by using it in the Dye sensitized solar cell (FTO/TiO₂/N719/KI-I₂/Pt). Conversion of solar light energy to electricity was successfully done using this TiO₂. The fabricated cell showed an open-circuit voltage (V _OC) of 587 mV and short-circuit current density (J _SC) of 5.06 mA/cm². Maximum power (P _max) generated was 1.912 mW/cm² with a fill factor (FF) of 0.644 and a conversion efficiency of 1.91%.     

Visible light-operated saccharide–O2 biofuel cell based on the photosensitization of chlorophyll derivative on TiO2 film

The visible light-operated saccharide–O₂ biofuel cell consisting of zinc chlorin-e₆ (ZnChl-e₆) adsorbed on nanocrystalline TiO₂ layer coated onto optical transparent conductive glass electrode (OTE) as an anode, platinum-coated OTE as a cathode, and the fuel solution containing sucrose as a saccharide, invertase, glucose dehydrogenase (GDH) and NAD⁺ is studied as a new type biofuel cell. The short-circuit photocurrent (I_SC) and the open-circuit photovoltage (V_OC) of this cell are 9.0 μA cm⁻² and 415 mV, respectively. The peaks in the photocurrent action spectrum of this cell are observed at 400 and 800 nm and the incident photon-to-current efficiency (IPCE) values at 400 and 800 nm are estimated to be ca. 17.3% and 10.6%. Thus, a new type of visible light-operated saccharide–O₂ biofuel cell with the visible and near IR photosensitization of ZnChl-e₆ molecules on nanocrystalline TiO₂ film electrode is accomplished.     

Incorporating carbon nanotube in a low-temperature fabrication process for dye-sensitized TiO2 solar cells

Dye-sensitized solar cells (DSSCs) incorporating TiO₂ porous films, prepared at a low temperature (150 °C), along with multi-wall carbon nanotubes (MWCNTs) were studied using two different electrolytes, namely LiI and THI. Electrochemical impedance spectroscopy (EIS) was employed to quantify the charge transport resistance and electron lifetime (τ_e) under different levels (wt%) of MWCNTs and electrolytes. The charge transport resistance at the TiO₂/dye/electrolyte interface (R_ct2) increased as a function of the MWCNT concentration, which ranged 0.1-0.5 wt%, due to a decrease in the surface area and decreased dye adsorption. The characteristic peak shifted to a lower frequency at 0.1 wt% of MWCNT, indicating a longer electron lifetime. The DSSC with the TiO₂ electrode containing 0.1 wt% of MWCNT resulted in a higher short-circuited current density (J_SC) of 9.08 mA/cm², an open-circuit voltage (V_OC) of 0.781 V, and a cell conversion efficiency of 5.02%. EIS was also conducted under dark conditions. The large value at a middle frequency represented electron transport at the TiO₂/dye/electrolyte interface (R_rec). The R_rec for 0.1 wt% MWCNT/TiO₂ was found to be 114 Ω, and for those with 0.3 and 0.5 wt% were 35 and 30 Ω, respectively. The significantly higher value of R_rec suggested that the charge recombination between injected electrons and electron acceptors in the redox electrolyte, I₃⁻, was remarkably retarded. Finally, electrolytes with LiI and THI were used to compare the cell conversion performance under the same conditions. It was found that more electrons were injected in the TiO₂ electrode and the electron recombination reaction was faster in the DSSC with THI than that with LiI.     

Low band gap dyes based on 2-styryl-5-phenylazo-pyrrole: Synthesis and application for efficient dye-sensitized solar cells

A new series of low band gap dyes, C1, C2 and S, based on 2-styryl-5-phenylazo-pyrrole was synthesized. These dyes contain one carboxy, two carboxy and one sulfonic acid anchoring groups, respectively. They were soluble in common organic solvents, showed long-wavelength absorption maximum at ∼620 nm and optical band gap of 1.66-1.68 eV. The photophysical and electrochemical properties of these dyes were investigated and found to be suitable as photosensitizers for dye sensitized solar cells (DSSCs). The quasi solid state DSSCs with dye S showed a maximum monochromatic incident photon to current efficiency (IPCE) of 78% and an overall power conversion efficiency (PCE) of 4.17% under illumination intensity of 100 mW cm⁻² (1.5 AM), which is higher than the other dyes (3.26% for C2 and 2.59% for C1). Even though dye S contains one sulfonic acid anchoring group, the higher PCE for the DSSCs based on this dye has been attributed to the higher dye loading at the TiO₂ surface and enhanced electron lifetime in the device, as indicated by absorption spectra and electrochemical impedance spectra measurements. Finally, by increasing the molecular weight of poly(ethylene oxide) (PEO) in electrolyte, the PCE also increases up to 4.8% for the electrolyte with PEO molecular weight of 2.0 × 10⁶. This improvement has been attributed to the enhancement in iodide ions diffusion due to the increase in free volume of polymer gel electrolyte.     

Characterization of some metal-free organic dyes as photosensitizer for nanocrystalline ZnO-based dye sensitized solar cells

We report the photoelectrochemical characteristics of some biologically used dye: Bromophenol, Ponceau S, Sudan IV, Giemsa, and Acridine Orange as sensitizers. The J_SC from 2.5 to 0.47 mA cm⁻² with the order Bromophenol > Ponceau S > Sudan IV > Giemsa > Acridine orange, the V_OC from 642 to 384 mV, the fill factor (FF) from 0.61 to 0.40, and P_max from 855 to 84 μW cm⁻² were obtained from the DSSCs sensitized with these metal free organic dyes. Among these dyes, Bromophenol gave the best performance as sensitizer with maximum current, which is due to the better interaction between the hydroxyl groups of the dye on the surface of ZnO porous film. Incident photon- to-current conversion efficiency (IPCE) achieved with the use of these dyes follows the order Ponceau S > Sudan IV > Bromophenol > Giemsa > Acridine orange.     

Syntheses and photovoltaic properties of polymeric sensitizers using thiophene-based copolymer derivatives for dye-sensitized solar cells

We synthesized the thiophene-based copolymers (P(3TAF-co-3TAa)-A-n and P(3TAF-co-3TAa)-B-n) using two different kinds of thiophene monomers, (N-(3-thienylmethylene)-2-aminofluorene and 3-thiophene acetic acid), as sensitizers on the DSSCs. P(3TAF-co-3TAa)-A-n (n=1, 2, 3) was synthesized with different molar ratios (3TAF:3TAa=1:5, 1:10, 1:20) of monomers at room temperature, respectively. Also, P(3TAF-co-3TAa)-B-n (n=1, 2, 3) was synthesized with above molar ratios of monomers at 0 °C, respectively. The DSSCs devices were fabricated using the thiophene-based copolymers as sensitizers and their photovoltaic performances were measured by using a solar simulator under AM 1.5. In the DSSCs devices using polymeric sensitizers, V_oc is 0.53-0.60 V, J_sc is 1.9-4.5 mA/cm², FF is 0.51-0.63 and the power conversion efficiency is 0.63-1.53%, respectively.     

Cubic titanium dioxide photoanode for dye-sensitized solar cells

Following from the recently evolved concept of significantly improving the photovoltaic efficiency in dye-sensitized solar cells (DSSCs) by reducing the loss of electrons on the spherical surface of titanium dioxide, this study examines the synthesis of cubic TiO₂ with a special morphology to overcome this electron loss and investigates its application to DSSCs. Cubic TiO₂ is synthesized by an advanced rapid hydrothermal method, with the addition of an amine species additive. Transmission electron microscopy (TEM) images confirm the cubic shape of the TiO₂ particles with a diameter less than 5-10 nm. Using N719 dye under illumination with 100 mW cm⁻² simulated sunlight, the application of cubic TiO₂ to DSSCs affords an energy conversion efficiency of approximately 9.77% (4.0-μm thick TiO₂ film), which is considerably enhanced compared with that achieved using a commercial, spherical TiO₂. Electrostatic force microscopy (EFM) and impedance analyses reveal that the electrons are transferred more rapidly to the surface of a cubic TiO₂ film than on a spherical TiO₂ film.     

To make polymer: Quantum dot hybrid solar cells NIR-active by increasing diameter of PbSnanoparticles

We fabricate NIR-active solar cells based on PbS quantum dots and a conventional conjugated polymer. These devices act as solar cells under exclusively NIR wavelengths above 650 nm. Here PbS nanoparticles absorb photons in the NIR range that in turn generate excitons. We show that with an assistance from a strong electron-acceptor (TiO₂), these excitons can be dissociated to electrons and holes to yield a photocurrent in the external circuit. We then aim to extend the spectral window of the solar cells to higher wavelength region by increasing the diameter of PbS nanoparticles to make the cells further NIR-active. We observe that the short-circuit current (J_SC) shows a peak when the diameter of PbS nanoparticles increases. Here, the spectral window can be extended till conduction band-edge of PbS quantum dots falls below that of TiO₂ nanostructures cutting off the electron-transfer pathway. The NIR-active photovoltaic solar cells yield a short-circuit current (J_SC) of 1.0 mA/cm², open-circuit voltage (V_OC) of 0.42 V, and power conversion efficiency (η) of 0.16% and remain operative till 1200 nm.     

Long-term stable dye-sensitized solar cells based on UV photo-crosslinkable poly(ethylene glycol) and poly(ethylene glycol) diacrylate based electrolytes

UV photo-crosslinkable polymer electrolytes based on poly(ethylene glycol) (PEG) and poly(ethylene glycol) diacrylate (PEGDA) were used in dye-sensitized solar cells (DSSCs). PEG and bifunctional PEGDA formed a crosslinked structure upon UV light illumination, confirmed by the solubility test and FTIR spectroscopy. The polymeric electrolyte was prepared by photo-polymerization after injecting the monomer electrolyte solution into the porous film in order to make close contact with the TiO₂ porous film. Under AM 1.5 (100 mW/cm²) light irradiation for up to 20 min, a maximum 62% increase in the photo-conversion efficiency (η%) was observed. The DSSCs with the crosslinkable PEG/PEGDA based polymer electrolyte showed improved long-term stability in comparison to those with electrolytes containing only PEG. Also, the effects of solvent on stability of the DSSCs were investigated.     

Red Sicilian orange and purple eggplant fruits as natural sensitizers for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) were assembled by using red Sicilian orange juice (Citrus Sinensis) and the purple extract of eggplant peels (Solanum melongena, L.) as natural sensitizers of TiO₂ films. Conversion of solar light into electricity was successfully accomplished with both fruit-based solar cells. The best solar energy conversion efficiency (η=0.66%) was obtained by red orange juice dye that, under AM 1.5 illumination, achieved up to J_sc=3.84 mA/cm², V_oc=0.340 V and fill factor=0.50. In the case of the extract of eggplant peels, the values determined were up to J_sc=3.40 mA/cm², V_oc=0.350 V and fill factor=0.40. Cyanidine-3-glucoside (cyanine) and delphinidin 3-[4-(p-coumaroyl)-l-rhamnosyl(1-6)-glucopyranoside]-5-glucopyranoside (nasunin) are the main pigments of cocktail dyes for red orange and eggplant, respectively. Actually, their application is far below the industrial requirements. Nevertheless, their study is an interesting multidisciplinary exercise useful for dissemination of knowledge and to educate people on renewable energy sources. Here, we report and discuss the role of the structure, the absorption spectra and the sensitization activity of the mentioned compounds.