Bio-inspired multi-scale structures in dye-sensitized solar cell

Dye-sensitized solar cells (DSSCs) provide a technique and economic alternative concept to present p–n junction photovoltaic devices. For a DSSC, light is absorbed by a sensitizer, which is anchored to the surface of a wide band semiconductor. Charge separation takes place at the interface via photo-induced electron injection from the dye into the conduction band of the semiconductor. Nanocrystalline oxide semiconductor photo-anode films play an important role in photo-electrical conversion efficiency of DSSCs. In this review, we summarize the recent advances of multi-scale structures of DSSCs in the view of bio-inspired materials and analyze the influence factors of a variety of multi-scale structures on photo-electrical conversion in DSSCs, which will provide a strategy for structure design on the novel solar cell.     

Influence of a TiCl4 post-treatment on nanocrystalline TiO2 films in dye-sensitized solar cells

In this study, the influence of the TiCl(4) post-treatment on nanocrystalline TiO(2) films as electrodes in dye-sensitized solar cells is investigated and compared to nontreated films. As a result of this post-treatment cell efficiencies are improved, due to higher photocurrents. On a microscopic scale TiO(2) particle growth on the order of 1 nm is observed. Despite a corresponding decrease of BET surface area, more dye is adsorbed onto the oxide surface. Although it seems trivial to match this finding with the improved photocurrent, this performance improvement cannot be attributed to higher dye adsorption only. This follows from comparison between incident photon to current conversion efficiency (IPCE) and light absorption characteristics. Since the charge transport properties of the TiO(2) films are already more than sufficient without treatment, the increase in short circuit current density J(SC) cannot be related to improvements in charge transport either. Transient photocurrent measurements indicate a shift in the conduction band edge of the TiO(2) upon TiCl(4) treatment. It is concluded that the main contribution to enhanced current originates from this shift in conduction band edge, resulting in improved charge injection into the TiO(2).     

Photovoltaic performance of injection solar cells and other applications of nanocrystalline oxide layers

The direct conversion of sunlight to electricity via photoelectrochemical solar cells is an attractive option that has been pursued for nearly two decades in several laboratories. In this paper, we review the principles and performance features of very efficient solar cells that are being developed in our laboratories. These are based on the concept of dye-sensitization of wide bandgap semiconductors used in the form of mesoporous nanocrystalline membrane-type films. The key feature is charge injection from the excited state of an anchored dye to the conduction band of an oxide semiconductor such as TiO₂. In the use of the semiconductor in the form of high surface area, highly porous film offers several unique advantages: monomeric distribution of a large quantity of the dye in a compact (few micron thick) film, efficient charge collection and drastic inhibition of charge recombination (‘capture of charge carriers by oxidized dye’). Near quantitative efficiency for charge collection for monochromatic light excitation gives rise to sunlight conversion efficiency in the range of 8–10% This has led to fruitful collaboration with several industrial partners. Possible applications and commercialization of these solar cells and also other practical applications of nanosized films are briefly outlined.     

Nanocrystalline ceramic films for efficient conversion of light into electricity

Transparent nanocrystalline films of oxide semiconductors such as TiO₂ and Fe₂O₃ have been prepared on a conducting glass support employing a sol-gel procedure. The films are composed of nanometer-sized particles sintered together to allow for percolative charge carrier transport. The internal surface of these films is very high, roughness factors of the order of 1000 being readily obtained. Electric polarization was applied for forward and reverse biasing of the films and the resulting optical changes have been analyzed to derive their flat band potential. Band gap excitation of such nanocrystalline semiconductors produces electron-hole pairs which migrate through the film to be collected as electric current. Steady state photolysis and time resolved laser techniques have been applied to scrutinize the mechanism of light induced charge separation within the nanostructure. When derivatized with a suitable chromophore, TiO₂ films give extraordinary efficiencies for the conversion of incident photons into electric current, exceeding 90% for certain transition metal complexes within the wavelength range of their absorption band. The underlying physical principles of these astonishing findings will be discussed. Exploiting this discovery, we have developed a new type of photovoltaic device whose overall light to electric energy conversion yield is 10% under simulated AM 1.5 solar radiation.     

Femtosecond to millisecond studies of electron transfer processes in a donor-(π-spacer)-acceptor series of organic dyes for solar cells interacting with titania nanoparticles and ordered nanotube array films

Time-resolved emission and absorption spectroscopy are used to study the photoinduced dynamics of forward and back electron transfer processes taking place between a recently synthesized series of donor-(π-spacer)-acceptor organic dyes and semiconductor films. Results are obtained for vertically oriented titania nanotube arrays (inner diameters 36 nm and 70 nm), standard titania nanoparticles (25 nm diameter) and, as a reference, alumina nanoparticle (13 nm diameter) films. The studied dyes contain a triphenylamine group as an electron donor, cyanoacrylic acid part as an electron acceptor, and differ by the substituents in a spacer group that causes a shift of its absorption spectra. Despite a red-shift of the dye absorption band resulting in an improved response to the solar spectrum, smaller electron injection rates and smaller extinction coefficients result in reduced dye sensitized solar cell (DSSC) conversion efficiencies. For the most efficient dye, TPC1, electron injection from the hot locally excited state to titania on a time scale of about 100 fs is suggested, while from the relaxed charge transfer state it proceeds in a non-exponential way with time constants from 1 ps to 50 ps. Our results imply that the latter process involves the trap states below the conduction band edge (or the sub-bandgap tail of the acceptor states), localized close to the dye radical cation, and is accompanied by fast electron recombination to the parent dye's ground state. This process should limit the efficiency of DSSCs made using these types of organic dyes. The residual, slower recombination can be described by a stretched exponential decay with a characteristic time of 0.5 μs and a dispersion parameter of 0.33. Both the electron injection and back electron transfer dynamics are similar in titania nanoparticles and nanotubes. Variations between the two film types are only found in the time resolved emission transients, which are explained in terms of the difference in local electric fields affecting the position of the emission bands.     

Enhanced photovoltaic characterization and charge transport of TIO2 nanoparticles/nanotubes composite photoanode based on indigo carmine dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) have established themselves as an alternative to conventional solar cells owing to their remarkably high power conversion efficiency, longtime stability and low-cost production. DSSCs composed of a dyed oxide semiconductor photoanode, a redox electrolyte and a counter electrode. In these devices, conversion efficiency is achieved by ultra-fast injection of an electron from a photo excited dye into the conduction band of metal oxide followed by subsequent dye regeneration and holes transportation to the counter electrode. The energy conversion efficiency of DSSC is to be dependent on the morphology and structure of the dye adsorbed metal oxide photoanode. Worldwide considerable efforts of DSSCs have been invested in morphology control of photoanode film, synthesis of stable optical sensitizers and improved ionic conductivity electrolytes. In the present investigation, a new composite nano structured photoanodes were prepared using TiO₂ nano tubes (TNTs) with TiO₂ nano particles (TNPs). TNPs were synthesized by sol–gel method and TNTs were prepared through an alkali hydrothermal transformation. Working photoanodes were prepared using five pastes of TNTs concentrations of 0, 10, 50, 90, and 100 % with TNPs. The DSSCs were fabricated using Indigo carmine dye as photo sensitizer and PMII (1-propyl-3-methylimmidazolium iodide) ionic liquid as electrolyte. The counter electrode was prepared using Copper sulfide. The structure and morphology of TNPs and TNTs were characterized by X-ray diffraction and electron microscopes (TEM and SEM). The photocurrent efficiency is measured using a solar simulator (100 mW/cm²). The prepared composite TNTs/TNPs photoanode could significantly improve the efficiency of dye-sensitized solar cells owing to its synergic effects, i.e. effective dye adsorption mainly originated from TiO₂ nanoparticles and rapid electron transport in one-dimensional TiO₂ nanotubes. The results of the present investigation suggested that the DSSC based on 10 % TNTs/TNPs showed better photovoltaic performance than cell made pure TiO₂ nanoparticles. The highest energy-conversion efficiency of 2.80 % is achieved by composite TNTs (10 %)/TNPs film, which is 68 % higher than that pure TNPs film and far larger than that formed by bare TNTs film (94 %). The charge transport and charge recombination behaviors of DSSCs were investigated by electrochemical impedance spectra and the results showed that composite TNTs/TNPs film-based cell possessed the lowest transfer resistances and the longest electron lifetime. Hence, it could be concluded that the composite TNTs/TNPs photoanodes facilitate the charge transport and enhancing the efficiencies of DSSCs.     

Photoinduced charge carrier dynamics of Zn-porphyrin-TiO2 electrodes: the key role of charge recombination for solar cell performance

Time resolved absorption spectroscopy has been used to study photoinduced electron injection and charge recombination in Zn-porphyrin sensitized nanostructured TiO(2) electrodes. The electron transfer dynamics is correlated to the performance of dye sensitized solar cells based on the same electrodes. We find that the dye/semiconductor binding can be described with a heterogeneous geometry where the Zn-porphyrin molecules are attached to the TiO(2) surface with a distribution of tilt angles. The binding angle determines the porphyrin-semiconductor electron transfer distance and charge transfer occurs through space, rather than through the bridge connecting the porphyrin to the surface. For short sensitization times (1 h), there is a direct correlation between solar cell efficiency and amplitude of the kinetic component due to long-lived conduction band electrons, once variations in light harvesting (surface coverage) have been taken into account. Long sensitization time (12 h) results in decreased solar cell efficiency because of decreased efficiency of electron injection.     

Mesoporous oxide junctions and nanostructured solar cells

Mesoporous films of wide-band gap semiconductor oxides are an important new class of electronic materials. They are constituted by a network of nanocrystalline particles of oxides, such as titania, niobia or zinc oxide, sintered together to allow for charge carrier transport to take place. The pores between the nanoparticles are filled with an electrolyte or a solid state organic hole conductor forming an interpenetrating heterojunction of very large contact area. These junctions exhibit extraordinary opto-electronic properties due to their large surface area to volume ratio leading to applications in different domains, such as photovoltaics, intercalation batteries, electrochromic and electroluminescent displays, photocatalysis and chemical sensors. Of particular interest are dye-sensitized heterojunctions, where photo-induced charge separation occurs at the interface between the mesoporous oxide and the hole conductor or the electrolyte. Photovoltaic cells based on this concept form a viable alternative to conventional silicon cells. Solar to electric power conversion efficiencies exceeding 10% have been reached with mesoporous titania films derivatized with molecular charge transfer sensitizers and used in conjunction with organic iodide/triiodide-based redox electrolytes. Long-term accelerated light-soaking tests have shown the system to be intrinsically stable. This article summarized recent developments in this field including a discussion of solid state dye-sensitized heterojunctions employing spirobifluorene-connected arylamines as hole transport materials.     

Lithium-modulated conduction band edge shifts and charge-transfer dynamics in dye-sensitized solar cells based on a dicyanamide ionic liquid

Lithium ions are known for their potent function in modulating the energy alignment at the oxide semiconductor/dye/electrolyte interface in dye-sensitized solar cells (DSCs), offering the opportunity to control the associated multichannel charge-transfer dynamics. Herein, by optimizing the lithium iodide content in 1-ethyl-3-methylimidazolium dicyanamide-based ionic liquid electrolytes, we present a solvent-free DSC displaying an impressive 8.4% efficiency at 100 mW cm(-2) AM1.5G conditions. We further scrutinize the origins of evident impacts of lithium ions upon current density-voltage characteristics as well as photocurrent action spectra of DSCs based thereon. It is found that, along with a gradual increase of the lithium content in ionic liquid electrolytes, a consecutive diminishment of the open-circuit photovoltage arises, primarily owing to a noticeable downward movement of the titania conduction band edge. The conduction band edge displacement away from vacuum also assists the formation of a more favorable energy offset at the titania/dye interface, and thereby leads to a faster electron injection rate and a higher exciton dissociation yield as implied by transient emission measurements. We also notice that the adverse influence of the titania conduction band edge downward shift arising from lithium addition upon photovoltage is partly compensated by a concomitant suppression of the triiodide involving interfacial charge recombination.     

苯基磷酸联吡啶钌络合物敏化纳晶多孔TiO2薄膜电极光电性能研究

用有机光敏染料敏化半导体,通过染料分子的吸附功能基团与半导体相互作用,使染料分子与半导体表面之间建立电性耦合,进行有效的电荷转移,可以形成有机-半导体复合新型光电功能材料。联吡啶钌络合物有较强的可见光吸收、氧化还原性能可逆、氧化态稳定性高,是一类性能优越的有机光敏染料。近来许多研究发现,羧酸联吡啶钌的强吸附与TiO₂纳晶薄膜的大比表面相结合,导致光生电荷快速注入TiO₂导带达到有效的电荷分离,得到了接近100%的单色光光电流效率^[1]。为研究联吡啶钌分子的不同吸附功能基团与TiO₂纳晶薄膜表面的相互作用对提高光电性能的影响,本文报道苯基磷酸取代的联吡啶钌络合物敏化纳晶多孔TiO₂薄膜的光电性能。     

Significant efficiency improvement of the black dye-sensitized solar cell through protonation of TiO2 films

This paper describes the influence of acid pretreatment ofTiO2 mesoporous films prior to dye sensitization on the performance of dye-sensitized solar cells based on [(C4H9)4N]3[Ru(Htcterpy)(NCS)3] (tcterpy = 4,4',4"-tricarboxy- 2,2',2"-terpyridine), the so-called black dye. The HCl pretreatment caused an increase in overall efficiency by 8%, with a major contribution from photocurrent improvement. It is speculated, from the analysis of incident photon-to-electron conversion efficiency, UV-vis absorption spectra, redox properties of the dye and TiO2, and the impedance spectra of the dye-sensitized solar cells, that photocurrent enhancement is attributed to the increases in electron injection and/or charge collection efficiency besides the improvement of light harvesting efficiency upon HCl pretreatment. Open-circuit photovoltage (V(oc)) remained almost unchanged in the case of significant positive shift of flat band potential for TiO2 upon HCl pretreatment. The suppression of electron transfer from conduction band electrons to the I3- ions in the electrolyte upon HCl pretreatment, reflected by the increased resistance at the TiO2/dye/electrolyte interface and reduced dark current, resulted in a V(oc) gain, which compensated the V(oc) loss due to the positive shift of the flat band. Using the HCl pretreatment approach, 10.5% of overall efficiency with the black dye was obtained under illumination of simulated AM 1.5 solar light (100 mW cm(-2)) using an antireflection film on the cell surface.     

Photoinduced Oxygen Reduction Reaction Boosts the Output Voltage of a Zinc–Air Battery

Utilization of solar energy is of great interest for a sustainable society, and its conversion into electricity in a compact battery is challenging. Herein, a zinc–air battery with the polymer semiconductor polytrithiophene (pTTh) as the cathode is reported for direct conversion of photoenergy into electric energy. Upon irradiation, photoelectrons are generated in the conduction band (CB) of pTTh and then injected into the π_2p* orbitals of O₂ for its reduction to HO₂^?, which is disproportionated to OH^? and drives the oxidation of Zn to ZnO at the anode. The discharge voltage was significantly increased to 1.78 V without decay during discharge–charge cycles over 64 h, which corresponds to an energy density increase of 29.0 % as compared to 1.38 V for a zinc–air battery with state‐of‐the‐art Pt/C. The zinc–air battery with an intrinsically different reaction scheme for simultaneous conversion of chemical and photoenergy into electric energy opens a new pathway for utilization of solar energy.     

Molecular approaches to solar energy conversion with coordination compounds anchored to semiconductor surfaces

Strategies toward the realization of molecular control of interfacial charge transfer at nanocrystalline semiconductor interfaces are described. Light excitation of coordination compounds, based on (dpi)6 transition metals, anchored to wide band-gap semiconductors, such as TiO2, can initiate electron-transfer processes that ultimately reduce the semiconductor. Such photoinduced charge-separation processes are a key step for solar energy conversion. The thermodynamics and kinetic rate constants for three different interfacial charge separation mechanisms are discussed. Tuning the energetic position of the semiconductor conduction band relative to the molecular sensitizer has provided new insights into interfacial charge transfer. Supramolecular compounds that efficiently absorb light, promote interfacial electron transfer, and feature additional functions such as intramolecular electron transfer when bound to semiconductor surfaces have also been studied. New approaches for enhancing charge-separation lifetimes for solar energy conversion are presented.     

Interparticle Charge Transfer in Dye-Sensitized Films Composed of Two Kinds of Semiconductor Crystallites

Interparticle charge transfer between different types of semiconductor crystallites in contact on band gap excitation or dye-sensitization is documented. The general consensus had been that electrons always transfer from particles of higher conduction band position to those with lower conduction band position. Observation on dye-sensitizated photoelectrochemical cells made from SnO(2)/ZnO films sensitized with different dyes suggests that the electron transfer could occur in either direction, that is from semiconductor of high band position to the semiconductor of the low band position or vice versa, depending on which surface adsorbs the dye more strongly. Copyright 2001 Academic Press.     

Charge separation versus recombination in dye-sensitized nanocrystalline solar cells: the minimization of kinetic redundancy

In this paper we focus upon the electron injection dynamics in complete dye-sensitized nanocrystalline metal oxide solar cells (DSSCs). Electron injection dynamics are studied by transient absorption and emission studies of DSSCs and correlated with device photovoltaic performance and charge recombination dynamics. We find that the electron injection dynamics are dependent upon the composition of the redox electrolyte employed in the device. In a device with an electrolyte composition yielding optimum photovoltaic device efficiency, electron injection kinetics exhibit a half time of 150 ps. This half time is 20 times slower than that for control dye-sensitized films covered in inert organic liquids. This retardation is shown to result from the influence of the electrolyte upon the conduction band energetics of the TiO2 electrode. We conclude that optimum DSSC device performance is obtained when the charge separation kinetics are just fast enough to compete successfully with the dye excited-state decay. These conditions allow a high injection yield while minimizing interfacial charge recombination losses, thereby minimizing "kinetic redundancy" in the device. We show furthermore that the nonexponential nature of the injection dynamics can be simulated by a simple inhomogeneous disorder model and discuss the relevance of our findings to the optimization of both dye-sensitized and polymer based photovoltaic devices.     

Electronic structures and absorption properties of three kinds of ruthenium dye sensitizers containing bipyridine-pyrazolate for solar cells

The geometries, electronic structures and the electronic absorption spectra of three kinds of ruthenium complexes, which contain tridentate bipyridine-pyrazolate ancillary ligands, were studied using density functional theory (DFT) and time-dependent DFT. The calculated results indicate that: (1) the strong conjugated effects are formed across the pyrazoalte-bipyridine groups; (2) the interfacial electron transfer between electrode and the dye sensitizers is an electron injection processes from the excited dyes to the conduction band of TiO2; (3) the absorption bands in visible region have a mixed character of metal-to-ligand charge transfer and ligand-to-ligand charge transfer, but the main character of absorption bands near UV region ascribe to π→π* transitions; (4) introducing pyrazolate and -NCS groups are favorable for intra-molecular charge transfer, and they are main chromophores that contribute to the sensitization of photon-to-current conversion processes, but introducing -Cl and the terminal group -CF3 are unfavorable to improve the dye performance in dye sensitized solar cells.     

Charge transport versus recombination in dye-sensitized solar cells employing nanocrystalline TiO2 and SnO2 films

We report a comparison of charge transport and recombination dynamics in dye-sensitized solar cells (DSSCs) employing nanocrystalline TiO(2) and SnO(2) films and address the impact of these dynamics upon photovoltaic device efficiency. Transient photovoltage studies of electron transport in the metal oxide film are correlated with transient absorption studies of electron recombination with both oxidized sensitizer dyes and the redox couple. For all three processes, the dynamics are observed to be 2-3 orders of magnitude faster for the SnO(2) electrode. The origins of these faster dynamics are addressed by studies correlating the electron recombination dynamics to dye cations with chronoamperometric studies of film electron density. These studies indicate that the faster recombination dynamics for the SnO(2) electrodes result both from a 100-fold higher electron diffusion constant at matched electron densities, consistent with a lower trap density for this metal oxide relative to TiO(2), and from a 300 mV positive shift of the SnO(2) conduction band/trap states density of states relative to TiO(2). The faster recombination to the redox couple results in an increased dark current for DSSCs employing SnO(2) films, limiting the device open-circuit voltage. The faster recombination dynamics to the dye cation result in a significant reduction in the efficiency of regeneration of the dye ground state by the redox couple, as confirmed by transient absorption studies of this reaction, and in a loss of device short-circuit current and fill factor. The importance of this loss pathway was confirmed by nonideal diode equation analyses of device current-voltage data. The addition of MgO blocking layers is shown to be effective at reducing recombination losses to the redox electrolyte but is found to be unable to retard recombination dynamics to the dye cation sufficiently to allow efficient dye regeneration without resulting in concomitant losses of electron injection efficiency. We conclude that such a large acceleration of electron dynamics within the metal oxide films of DSSCs may in general be detrimental to device efficiency due to the limited rate of dye regeneration by the redox couple and discuss the implications of this conclusion for strategies to optimize device performance.     

Comparing spiro-OMeTAD and P3HT hole conductors in efficient solid state dye-sensitized solar cells

Two hole conductor materials, spiro-OMeTAD and P3HT, were compared in solid-state dye-sensitized solar cells. Two organic dyes containing one anchor unit (D35) or two anchor units (M3) were used in the comparison. Absorbed photon to current conversion efficiency close to unity was obtained for the devices with spiro-OMeTAD. Energy conversion efficiencies of 4.7% and 4.9% were measured for the devices with spiro-OMeTAD and the dyes D35 and M3, respectively. For the devices using the P3HT hole conductor the results were rather different comparing the two dye molecules, with energy conversion efficiencies of 3.2% and 0.5% for D35 and M3, respectively. Photo-induced absorption measurements suggest that the regeneration of the dyes, and the polymer infiltration, is not complete using P3HT, while spiro-OMeTAD regenerates the dyes efficiently. However, the TiO(2)/D35/P3HT system shows rather high energy conversion efficiency and electrochemical oxidation of the dyes on TiO(2) indicates that D35 have a more efficient dye to dye hole conduction than M3, which thereby might explain the higher performance. The dye hole conduction may therefore be of significant importance for optimizing the energy conversion in such hybrid TiO(2)/dye/polymer systems.     

A novel organic chromophore for dye-sensitized nanostructured solar cells

A novel and efficient polyene-diphenylaniline dye for dye-sensitized solar cells has been synthesized. The dye has a short synthesis route and is readily adsorbed on TiO2 under a variety of dye-bath conditions. The overall solar-to-energy conversion efficiency is over 5% in the preliminary tests, in comparison with the conventional N719 dye which gives 6% under the same conditions. The dye is designed for future use also in solid state devices, with triarylamine based hole conductors.     

Photoacoustic measurement of electron injection efficiencies and energies from excited sensitizer dyes into nanocrystalline TiO2 films

Time-resolved photoacoustic calorimetry is used to measure the energy released upon injection of an electron from an electronically excited dye adsorbed to nanocrystalline TiO2 into the conduction band of this material. More energy is released when the environment of the dye is made less polar, because the energy of the dye-oxidized state has a more pronounced solvent dependence than the edge of the conduction band of the TiO2 semiconductor. Such energy dependences should be considered in the design of more efficient dye-sensitized solar cells.