Molecular engineering of sensitizers for dye‐sensitized solar cell applications

Dye‐sensitized solar cells (DSSCs) have attracted considerable attention in recent years as they offer the possibility of low‐cost conversion of photovoltaic energy. This account focuses on recent advances in molecular design and technological aspects of sensitizers based on metal complexes, metal‐free organics and tetrapyrrolic compounds which include porphyrins, phthalocyanines as well as corroles. Special attention has been paid to the design principles of these dyes, and co‐sensitization, an emerging technique to extend the absorption range, is also discussed as a way to improve the performance of the device. This account also focuses on recent advances of efficient ruthenium sensitizers as well as other metal complexes and their applications in DSSCs. Recent developments in the area of metal‐free organic and tetrapyrrolic sensitizers are also discussed. DOI 10.1002/tcr.201100044     

Synthesis of organic sulfobetaine‐based polymer gel electrolyte for dye‐sensitized solar cell application

We have synthesized eco‐friendly, economic, and equally efficient polysulfobetaine‐based gel electrolyte to the alternative of liquid electrolyte in the fabrication of dye‐sensitized solar cells (DSSCs) for the first time. This nitrogen‐rich and highly conductive polysulfobetaine was synthesized by an easy and facile method without the use of any catalyst and explored for its DSSC application. The synthesized polymer gel electrolyte exhibited good ionic conductivity about 6.8 × 10^?3 Scm^?1 at ambient temperatures. DSSCs were fabricated based on this polysulfobetaine gel electrolyte and studied for their performance based on photovoltaic parameters. The DSSC photovoltaic results were appreciable and are Voc = 0.82 V, Jsc = 11.49 mA/cm², FF = 66%, and PCE = 6.26% at 1 sun intensity. These values are slightly lower than conventional liquid electrolyte‐based DSSC shown as Voc = 0.78 V, Jsc = 12.90 mA/cm², FF = 69%, and PCE = 7.07%, both at 100 mWcm^?2. Conductivity and photovoltaic parameters of the device reveals that as prepared polysulfobetaine‐based polymer gel electrolyte may be useful in the fabrication of DSSC and other electrochemical devices. Copyright © 2017 John Wiley & Sons, Ltd.     

Molecular design of organic dyes with diketopyrrolopyrrole for dye‐sensitized solar cell: A theoretical approach

Three designed metal‐free dyes based on 3‐(10‐butyl‐8‐(methylthio)‐10H‐phenothiazin‐3‐yl)‐2‐cyanoacrylic acid (V5) are investigated by density functional theory (DFT) and time‐dependent DFT to improve the efficiency of V5‐based solar cell devices. We have studied the geometrical structures, excitations, electronic structures, and conduction band shift caused by dye adsorption. The results indicate that the designed dyes have several merits compared with V5 including: (i) smaller energy band gaps and the LUMO closer to conduction band of TiO₂; (ii) wider absorption spectra and higher oscillator strength; (iii) larger dipole moment that lead to higher V_oc value. Our work suggests that the modification of π‐bridge with diketopyrrolopyrrole unit is very effective for designing novel metal‐free dyes with improved performance for dye‐sensitized solar cells (DSSCs). These findings are expected to provide a bright way to design new efficient metal‐free organic DSSCs. © 2014 Wiley Periodicals, Inc.     

Room temperature polymerization of poly(3,4‐ethylenedioxythiophene) as transparent counter electrodes for dye‐sensitized solar cells

Poly(3,4‐ethylenedioxythiophene) (PEDOT) counter electrode is prepared with in situ polymerization of 3,4‐ethylenedioxythiophene on a fluorine‐doped tin oxide over‐layer glass at room temperature. The cyclic voltammetry, electrochemical impedance spectroscopy, and Tafel polarization are measured to evaluate the catalytic activity of PEDOT counter electrode for I₃^?/I^? redox couple. Comparing the data with that of traditional thermal decomposed Pt counter electrode, it is found that PEDOT has higher catalytic activity than that of Pt counterpart. Power conversion efficiency of the dye‐sensitized solar cell (DSC) with PEDOT counter electrode can attain to 7.713%, a little higher than that of the cell with Pt counter electrode (7.300%). Taking the advantage of high transparency of PEDOT counter electrode, an Ag mirror is put on the back side of PEDOT counter electrode of the DSC to reflect light back for power conversion. Power conversion efficiency of the DSC with this special structure can be further enhanced to 8.359%, which mainly stems from the improved short‐circuit current density by the increased irradiated light intensity. Copyright © 2014 John Wiley & Sons, Ltd.     

X‐ray photoelectron spectroscopy analysis of the stability of platinized catalytic electrodes in dye‐sensitized solar cells

The stability of platinized catalytic electrodes prepared by thermal decomposition of hexachloroplatinic acid was investigated. The platinum on the electrode did not dissolve in the presence of the electrolyte containing an iodide/triiodide redox couple, even under anodic bias. The electrocatalytic activity of platinized catalytic electrodes sealed in a cell with oxygen‐free electrolyte did not decrease within 23 weeks. However, the charge transfer resistance value of platinized catalytic electrodes increased tenfold when the electrodes were heated at 150° for 15 min in air during the sealing process and doubled when the electrodes were reused. The XPS analysis results showed that part of the platinum catalyst on the surface of the electrode was transformed to Pt[II] and Pt[IV] during the thermal sealing process, which led to the decrease of catalytic activity of the platinized catalytic electrodes for the reduction of triiodide. A large amount of inactive iodine absorbed on the surface of the reused electrode, which was confirmed by XPS, also decreased the electrocatalytic activity of the electrodes. The electrocatalytic activity of reused electrodes can be recovered by heating again at 390 °C or removing the platinum oxide and inactive iodine by the electrochemical method. Copyright © 2004 John Wiley & Sons, Ltd.     

Synergistical assembly of multiwalled carbon nanotubes/polyaniline network for dye‐sensitized solar cells

In this study, an aniline (ANI) solution containing well‐dispersed multiwall carbon nanotubes (CNTs) has been prepared. With an aim of improving the dispersability of CNTs in ANI monomer, we synthesize CNTs/ANI complexes using a reflux technique which can be electrochemically polymerized to form well‐dispersed CNTs/polyaniline (PANI) films. The refluxed CNTs/ANI solution can be used to prepare high porous CNTs/PANI network via an electrochemical polymerization for applying as counter electrodes in dye‐sensitized solar cells. Compared with the pristine PANI, the multiwalled CNTs/PANI network shows more porous morphology and higher electrocatalytic activity, resulting in the acceleration of the reaction (triiodide (I^3?) to iodide (I^?)) of the redox electrolyte. The enhancement of the electrocatalytic activity is attributed to the interactions between multiwalled CNTs and PANI, promoting the quinoid ring structure and thus enhancing the conductivity of the polymer chains. The device, assembled with multiwalled CNTs/PANI network as counter electrodes, delivers 7.67% power conversion efficiency, which is comparable to 7.43% of that with Pt. Copyright © 2014 John Wiley & Sons, Ltd.     

Morphology and permeability of exfoliated PVAc‐MMT nanocomposite films cast from soap‐free emulsion‐polymerized latices

In this study, we exclusively found that the exfoliated poly(vinyl acetate)‐montmorillonite (PVAc‐MMT) nanocomposite latices could be straightforwardly fabricated by soap‐free emulsion polymerization and cast into a film. The as‐fabricated films were transparent with the exfoliated MMT nanoplatelets in flat form uniformly dispersed in the PVAc matrix. Certain bonding of PVAc matrix to the exfoliated MMT nanoplatlets refrained it from removal by acetone in Soxhlet extraction. Exfoliated MMT nanoplatlets (10 wt %) in the film was able to reduce the water vapor permeability coefficient to only 9% that of the neat PVAc. According to the generalized Nielsen's permeability model for the composites containing impermeable fillers in sheet form, the average aspect ratio of exfoliated MMT platelets was calculated as 327, similar to those directly estimated by atomic force microscopy and transmission electron microscopy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5583–5589, 2007     

A study of novel macrocyclic copper complex/graphene‐based composite materials for counter electrodes of dye‐sensitized solar cells

In this study, a newly synthesized macrocyclic copper complex, [Cu(C₁₀H₂₀N₈)(C₄H₈N₄)](BF₄)₂, was used for a reaction with graphene oxide. Macrocyclic copper complex/graphene‐based composite materials were prepared and applied to the counter electrodes (CEs) of dye‐sensitized solar cells (DSSCs). As the level of the macrocyclic copper complex increased, the catalytic sites on the surface of the CE increased. The results showed that the device efficiency of the composite GO/Cu (1:10) CE was 7.61%, which was better than that of the Platinum (Pt) CE (7.04%). The device efficiency of the DSSC was enhanced effectively because the electrocatalytic activity of the CE was enhanced, and the interface impedance of the device was reduced. Therefore, the macrocyclic copper complex/graphene‐based composite materials may have the potential to replace traditional Pt to increase efficiency and reduce the fabrication cost of DSSCs.     

Laser desorption/ionization mass spectrometry of dye‐sensitized solar cells: identification of the dye‐electrolyte interaction

Dye‐sensitized solar cells (DSCs) have great potential to provide sustainable electricity from sunlight. The photoanode in DSCs consists of a dye‐sensitized metal oxide film deposited on a conductive substrate. This configuration makes the photoanode a perfect sample for laser desorption/ionization mass spectrometry (LDI‐MS). We applied LDI‐MS for the study of molecular interactions between a dye and electrolyte on the surface of a TiO₂ photoanode. We found that a dye containing polyoxyethylene groups forms complexes with alkali metal cations from the electrolyte, while a dye substituted with alkoxy groups does not. Guanidinium ion forms adducts with neither of the two dyes. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis and characterization of cross‐linkable ruthenium complex dye and its application on dye‐sensitized solar cells

A new crosslinkable light sensitizer, Ru(2,2′‐bipyridine‐4,4′‐bicarboxylic acid)(4,4′‐bis(11‐dodecenyl)‐2,2′‐bipyridine)(NCS)₂, denoted as Ru‐C for titanium oxide nanocrystalline‐based solar cells was synthesized with its crosslinking properties invesitigated by Fourier‐transform infrared and UV‐vis absorption spectroscopies. After crosslinking by itself or copolymerizing with methyacrylic acid, their sensitized solar cells with poly(methylacrylate)‐gelled electrolyte system not only attained more than 5% of power conversion efficiency at AM 1.5 illumination (100 mW/cm²), but also gave rise to long storage life. To the best our knowledge, this is the first crosslinkable dye ever applied to the DSSC in the literature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 366–372, 2010     

Design of high‐performance chlorine type dyes for dye‐sensitized solar cells

Intrinsic defect of electronic structure for the chlorine‐type porphyrin 1, which was synthesized for use in dye‐sensitized solar cell (DSSC), is found by theoretical calculation including density functional method (DFT), time‐dependent DFT, and C+/C? function. It is believed that the limited cell performance obtained by using dye 1 as the sensitizer is due to the existence of this electronic defect. To avoid this defect, a series of novel molecules with electron deficient π bridge were designed. The subsequent theoretical calculation indicated that the electron deficient π bridge in the newly designed molecule is quite effective in offsetting the electronic defect observed for dye 1. The parameters for the designed molecules closely associated with open‐circuit voltage and short‐circuit current density including dipole moment of dye vertical to the surface of semiconductor and light‐harvesting efficiency were then evaluated. By comparing these parameters of designed dyes with those of dye 1, we can predict that the DSSC based on dye 4 (2, 6‐cyan benzoic acid as anchoring group) should possess enhanced performance, which would be a valuable theoretical guidance for the practical work. © 2013 Wiley Periodicals, Inc.     

Efficient gel‐type electrolyte with bismaleimide via in situ low temperature polymerization in dye‐sensitized solar cells

This study reports the characteristics of gel‐type dye‐sensitized solar cells (DSSCs), fabricated with gel‐type electrolyte containing poly‐1,1′‐(methylenedi‐4,1‐phenylene)bismaleimide (PBMI), or poly‐1,1′‐(3,3′‐dimethyl‐1,1′‐biphenyl‐4,4′‐diyl)bismaleimide (PDBBMI), or poly‐N,N′‐(4‐methyl‐1,3‐phenylene)bismaleimide (PMPBMI), prepared by in situ polymerization of the corresponding monomer without an initiator at 30 °C. Incorporating 0.3 wt % content of exfoliated alkyl‐modified nanomica (EAMNM) into PBMI‐gelled electrolyte leads to higher short‐circuit current density (J_sc = 17.14 mA cm^?2) and efficiency (η = 7.02%) than that of neat PBMI‐gel electrolyte (J_sc = 15.32 mA cm^?2, η = 6.41%). Incorporating 0.3 wt % EAMNM into PBMI‐gelled electrolyte results in remarkably stable device performance under continuous light soaking under one sun (100 mW cm^?2) at 55 °C. The efficiency of DSSCs based on PBMI/0.3 wt % EAMNM‐gelled electrolyte drops by only 1.7% (η = 6.93%) after 500 h of continuous light soaking. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Graphene oxide sheet–polyaniline nanohybrids for enhanced photovoltaic performance of dye‐sensitized solar cells

In this study, photovoltaic (PV) properties of dye‐sensitized solar cells (DSSCs) incorporated with graphene oxide nanosheet‐polyaniline (GOS‐PANI) nanohybrid/poly(ethylene oxide) (PEO) blend gel electrolytes were investigated. Chemical structure and composition of GOS‐PANI nanohybrids were characterized by Raman spectroscopy and X‐ray photoelectron spectroscopy. The images of transmission electron microscopy revealed that PANI nanorods were anchored to the single‐layered GOS for the GOS‐PANI nanohybrids. Ionic conductivities of the GOS‐PANI/PEO–based gel electrolytes were measured using a conductivity meter. The electrochemical catalytic activities of the GOS‐PANI nanohybrids were determined through cyclic voltammetry. These GOS‐PANI nanohybrids were served as the extended electron transfer materials and catalyst for the electrochemical reduction of I₃^?. Due to the enhancement of the ionic conductivity and electrochemical catalytic activity of the gel electrolyte, better PV performance was observed for the DSSCs based on the GOS‐PANI containing electrolytes as compared to the pristine PEO electrolyte‐based DSSC sample. Moreover, PV performances of the GOS‐PANI/PEO–based DSSCs were closely related to the PANI content of GOS‐PANI nanohybrids. The highest photo‐energy conversion efficiency (5.63%) was obtained for an optimized GOS‐PANI/PEO (5:95, w/w) blend gel electrolyte‐based DSSC sample. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 321–332     

Template‐free synthesis of polyaniline nanobelts as a catalytic counter electrode in dye‐sensitized solar cells

Polyaniline (PAn) nanobelts were synthesized by simply mixing aniline and hydrochloric acid aqueous solution with ammonium peroxydisulfate and hydrochloric acid aqueous solution at room temperature without any templates. The PAn nanobelt paste with polytetrafluoroethylene as binder is proper for low‐cost and efficient casting processes such as doctor blade method, screen printing, spin‐casting, and even roll‐to‐roll painting. The porous network structure of the PAn nanobelt counter electrode was obtained after drying the paste on the conductive glass. The as‐prepared PAn nanobelt counter electrode showed nearly equivalent electrochemical catalytic activity as that of thermal decomposed Pt counter electrode, owing to the well‐connected conductive network and high active surface area. The power conversion efficiency of dye‐sensitized solar cell with PAn nanobelt counter electrode attained 90% of the value of the cell with Pt counter electrode. Copyright © 2014 John Wiley & Sons, Ltd.     

Quasi‐solid‐state dye‐sensitized solar cells containing P (MMA‐co‐AN)‐based polymeric gel electrolyte

Quasi‐solid state dye‐sensitized solar cells (QS‐DSSC) containing poly (methyl methacrylate‐co‐acrylonitrile) [P (MMA‐co‐AN)] gel electrolytes were fabricated. By tuning AN molar percentage in P (MMA‐co‐AN), the optimized polymeric gel electrolyte for fabricating QS‐DSSC can be obtained. QS‐DSSC containing polymeric gel electrolyte with 45 mol.% AN in P(MMA‐co‐AN) shows higher energy conversion efficiency than that of QS‐DSSCs containing polymeric gel electrolytes with either pure PMMA or PAN. So it presents an effective way to improve the photovoltaic performance of QS‐DSSC by tuning the components of polymeric gelling agent. Copyright © 2010 John Wiley & Sons, Ltd.     

Relationship between dye–iodine binding and cell voltage in dye‐sensitized solar cells: A quantum‐mechanical look

It has been proposed that iodine binding to dyes may actually decrease the cell efficiency of a dye‐sensitized solar cell. A previous experimental study showed that a two‐atom change from oxygen to sulfur increased recombination of iodine with injected electrons by a factor of approximately 2. Here, it is shown that iodine binding is a plausible explanation for this effect. The steric and conjugation effects are quantified separately using a set of model compounds. Quantum‐chemical calculations show that elongation of the hydrocarbon chain has only an insignificant effect on the iodine and bromine binding to the chalcogen atoms (O, S, Se). The conjugation, however, significantly disfavors the iodine and bromine interaction. Iodine and bromine binding to the dye and model compounds containing sulfur is significantly more favorable than to their oxygen containing counterparts. Bromine binding to dyes is shown to be stronger than that of iodine. Accordingly, bromine binding to dyes may contribute significantly to the observed lower efficiencies in cells using Br/Br^? as the redox couple. © 2012 Wiley Periodicals, Inc.     

Theoretical study of metal‐free organic dyes based on different configurations for efficient dye‐sensitized solar cells

Considering different solar dyes configuration, four novel metal‐free organic dyes based on phenoxazine as electron donor, thiophene and cyanovinylene linkers as the ‐conjugation bridge and cyanoacrylic acid as electron acceptor were designed to optimize open circuit voltage and short circuit current parameters and theoretically inspected. Density functional theory and time‐dependent density functional theory calculations were used to study frontier molecular orbital energy states of the dyes and their optical absorption spectra. The results indicated that D2‐4 dyes can be suitable candidates as sensitizers for application in dye sensitized solar cells and among these three dyes, D3 showed a broader and more bathochromically shifted absorption band compared to the others. The dye also showed the highest molar extinction coefficient. This work suggests optimizing the configuration of metal‐free organic dyes based on simple D‐ ‐A configuration containing alkyl chain as substitution, starburst conformation, and symmetric double D‐ ‐A chains would produce good photovoltaic properties.     

Optimization of electrospun TiO2 nanofibers photoanode film for dye‐sensitized solar cells through interfacial pre‐treatment,controllable calcination,and surface post‐treatment

Dye‐sensitized solar cells (DSSCs) are generally viewed as next generation photovoltaic devices. Electrospun TiO₂ nanofibers (NFs) film can be used to construct photoanode for DSSCs. A systematic strategy to optimize such a novel photoanode material of DSSCs was elaborated in this paper. A main drawback of NFs photoanode is the poor adhesion of ceramic NFs film to its conductive glass substrate. This problem can be well solved by sandwiching a transition layer between the overlaid NFs film and the underlaid glass substrate through an interfacial spin‐coating pre‐treatment. After electrospinning, a controllable calcination is also indispensable for obtaining an ideal nanofibrous mat with good morphology and adhesion. The choice of calcination parameters including temperature, holding time, and heating rate was discussed in detail. In addition, a surface TiCl₄ post‐treatment can further improve adhesion as well as strength for the NFs photoanode film. And the performance of the resulting DSSCs will benefit from the TiCl₄ post‐treatment. Copyright © 2013 John Wiley & Sons, Ltd.     

Quantitative investigation of local electric field through absorption spectrum in dye‐sensitized solar cells: Atomistic simulations

The local transient electric field in dye‐sensitized solar cells (DSSCs), created by heterointerfacial charge transfer on illumination, exerts an important role in the internal photoelectronic processes occurring at dye/semiconductor heterointerface in the time‐resolved measurement, for example, transient absorption spectroscopy. Herein, on the basis of theoretical simulations, we proposed a general strategy to quantitatively examine the nature of local electric field (microscopic nature) through absorption spectra of adsorbed dye molecule (macroscopic observation). A controllably modulated external electric field is imposed on the sensitized system to couple to local electric field, resulting in the absorption spectrum shift of ground‐state dye molecules. By fitting absorption spectrum under applied external electric field with Gauss–Stark equation, the quantitative property of local electric field would be obtained. The Gauss–Stark equation reflects the quantitative relationship between the local electric field and absorption spectrum shift of adsorbed ground‐state dye molecule. Our study not only provides origin and magnitude of Stark effect in DSSCs but also offers a promising route to explore the elementary photoelectronic processes experimentally and theoretically.     

Electrochemical impedance characterization and photovoltaic performance of N719 dye‐sensitized solar cells using quaternized ammonium iodide containing polyfluorene electrolyte solutions

This study develops a series of titanium oxide electrode‐based N719 dye‐sensitized solar cells (DSSCs) using quaternized ammonium iodide containing main‐chain and star‐shaped polyfluorene (MPF‐E and SPF‐E) electrolyte solutions. The electrochemical impedance and photovoltaic properties of the polyfluorene electrolyte‐based DSSCs were studied and compared to those of the poly(ethylene oxide) (PEO) electrolyte‐based DSSCs. As with the PEO electrolyte‐based DSSCs, the recombination impedance increased with increase in the polymer content for the MPF‐E electrolyte‐based DSSCs, whereas the photovoltaic performance did otherwise. Nevertheless, the reduction in the photovoltaic properties was not significant for the SPF‐E electrolyte‐based DSSCs. The electrochemical impedance and photovoltaic properties of the different polymer‐based DSSCs are also discussed as a function of the polymer concentration. Copyright © 2010 John Wiley & Sons, Ltd.