Interface In Dye-sensitized Solar Cells Research Articles

Computational Study on the Interaction of Iodide Electrolyte/Dye/TiO2 Interface in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) represent an emerging and promising candidate as a future renewable energy resource because of their potential low-cost fabrication cost, environmentally friendly, stability, and efficient solar energy conversion1. However, the intrinsic dye/iodide interaction with the dye on the semiconductor surface in understanding the DSSCs mechanism remains unclear2.In this work, three metal-free organic dyes based on D–π–A architecture were designed and computationally investigated for understanding the DSSCs mechanism and improving its efficiency2 - 4. The different donor groups were employed for the comparative study noted as D1Y, D2Y, and D3Y. This computational study mainly aims at determining the effect of the interaction of the iodide/triiodide with the dye on the semiconductor surface using the first-principles calculations (see scheme.1). We investigated the structural and electronic properties of these dyes when standing alone and being adsorbed on the oxide surface with the iodide electrolyte. Our studies revealed that the incorporation of a strong donor unit in D1Y and D 2 Y dyes in combination with iodide electrolytes on the TiO2 surface provides better adsorption and electronic properties in comparison to those from the dye alone on the TiO2 surface. Analysis of density of states (DOS) revealed that the dye with a strong electron-donating group, mainly D1Y and D2Y with an iodide electrolyte on the surface remarkably upshifts the Fermi energy, thereby improving the efficiency of the DSSCs by increasing the open-circuit voltage (Voc). These current findings provide a valuable hint for attaining a deeper understanding of the intrinsic interaction taking place at the electrolyte/dye/TiO2 interface. References O’Regan, B.; Grätzel, M.A Low-Cost High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737– 740.Hailu, M. and J.-C. Jiang Phys. Chem. Chem. Phys., 2020, 22, 26410-26418.Hailu, W.-R. Shie, S. Nachimuthu and J.-C. Jiang, ACS Sustain. Chem. Eng., 2017, 5, 8619-8629.Hailu, M. T. Nguyen and J.-C. Jiang, Phys. Chem. Chem. Phys., 2018, 20, 23564-23577. Figure 1

Characterizing Interfacial Structures of Dye-Sensitized Solar Cell Working Electrodes.

In this feature article, we discuss the fundamental use of materials-characterization methods that directly determine structural information on the dye···TiO2 interface in dye-sensitized solar cells (DSCs). This interface is usually buried within the DSC and submerged in solvent and electrolyte, which renders such metrological work nontrivial. We will show how ex-situ X-ray reflectometry (XRR), atomic-force microscopy (AFM), grazing-incidence X-ray scattering (GIXS), pair-distribution-function analysis of X-ray diffraction data (gaPDF), and in-situ neutron reflectometry (NR) can be used to deliver specific structural information on the dye···TiO2 interface regarding dye anchoring, dye aggregation, molecular dye orientation, intermolecular spacing between dye molecules, interactions between the dye molecules and the TiO2 surface, and interactions between the dye molecules and the electrolyte components and precursors. Some of these materials-characterization techniques have been developed specifically for this purpose. We will demonstrate how the direct acquisition of such information from materials-characterization experiments is crucial for assembling a holistic structural picture of this interface, which in turn can be used to develop DSC design guidelines. Moreover, we will show how these methodologies can be used in the experimental-validation process of "design-to-device" pipelines for big-data- and machine-learning-based materials discovery. We conclude with an outlook on further developments of this design-to-device approach as well as the materials characterization of more dye···TiO2 interfacial structures that involve known DSC dyes using the methods described herein. In addition, we propose to combine these formally disparate metrologies so that their complementary merits can be exploited simultaneously. New metrologies of this kind could serve as a "one-stop-shop" for the materials characterization of surfaces, interfaces, and bulk structures in DSCs and other devices with layered architectures.

Ni2+ enriched carbon nanotubes nanohybrids based non-platinum counter electrodes for dye sensitized solar cells

The development of competent substitutes for Pt counter electrode (CE), possessing excellent electrocatalytic activity, stability, and ready availability, is a key issue to materialize the long-term stability and economical large-scale industrial production of future generation dye sensitized solar cells (DSSCs). In the present work, a cost effective and solution processable hydrothermal technique has been utilized to synthesize Ni2+ enriched nanohybrids of nickel oxide (NiO) and nickel sulfide (Ni3S2) nanoparticles with multi-walled carbon nanotubes (MWCNTs) as CE in DSSCs. NiO/MWCNTs nanohybrids showed superior electrocatalytic behaviour among MWCNTs, NiO, Ni3S2, nanohybrids, and Pt. The nanohybrids containing aligned network MWCNTs exhibited good electrical conductivity, high specific surface area and better stability. These nanohybrids have been further employed as CE in DSSCs and observed higher Power Conversion Efficiency (PCE) for NiO/MWCNTs nanohybrids CE (3.80%) among others and is almost comparable to that of Pt based DSSC (3.90%). It has been explained on the basis of Ni2+content present in the nanohybrids. The higher content of Ni2+species provides a greater number of active sites representing higher electrocatalytic activity that would further productively catalyze the reactions occurring at CE/electrolyte interface in DSSCs.

Probing Interfacial Halogen-Bonding Effects with Halogenated Organic Dyes and a Lewis Base-Decorated Transition Metal-Based Redox Shuttle at a Metal Oxide Interface in Dye-Sensitized Solar Cells

Metal-free bis-cyclopentadithiophene-based molecular sensitizers with varying halogens (Cl, Br, and I) installed at a terminal position in conjugation with the dye frontier molecular orbital π-system are studied in combination with two cobalt redox shuttles (RSs): with and without a nonmetal-coordinated nitrogen atom (pyrazine or pyridine) accessible to the halide-decorated dyes. This systematic study employs UV–vis absorption, cyclic voltammetry, density functional theory calculations, and nanosecond transient absorption spectroscopy to probe the influence of possible halogen-bonding between the dyes and the pyrazine-based RS on electron-transfer reactions and effects on dye-sensitized solar cell performances. The results of this study imply a possible halogen-bonding event occurring between the halogenated dyes and the halogen binding RS with substantial effects on electron-transfer reaction rates.

Current status and future prospects of metal-organic frameworks at the interface of dye-sensitized solar cells.

The implementation of metal-organic frameworks (MOFs) as an integral part of dye-sensitized solar cells has received increasing attention over the past decade. Much effort has been devoted to improving the performance of these cells by optimizing the photosensitizer, photoanode, and counter electrode. This Frontier Article provides a snapshot of the recent advances in each of these three major directions achieved via MOF implementation.

Selective Deposition of Al₂O₃ on the Upper Side-Photoelectrode to Improve Dye-Sensitized Solar Cell Efficiency.

Charge recombination at the photoelectrode/dye/electrolyte interface decreases the energy conversion efficiency of dye-sensitized solar cells (DSSCs). To suppress charge recombination at this interface in DSSCs, an aluminum oxide (Al₂O₃) film can be deposited as an insulating metal oxide layer on the photoelectrode to form an energy barrier. However, the Al₂O₃ energy barrier can also disturb the transport of injected electrons to the working electrode through the titanium dioxide (TiO₂) photoelectrode. In this study, Al₂O₃ was selectively deposited as an insulating metal oxide layer on the upper side of a TiO₂ photoelectrode, which has a high probability of charge recombination, using plasma-enhanced atomic layer deposition. Deposition of the Al₂O₃ layer by this method helped to minimize the transport rate deterioration of injected electrons. This resulted in an increase of the efficiency of DSSCs containing the Al₂O₃ layer by 42.3% compared with that of a reference DSSC without the insulating metal oxide layer.

Solution-Based Synthesis of Ultrasmall Nb2O5 Nanoparticles for Functional Thin Films in Dye-Sensitized and Perovskite Solar Cells

A facile and new solution-based synthesis for the preparation of Nb2O5 nanoparticles (NPs) is firstly introduced. Chain-like particles of 30 to 50nm are synthesized through a hydrolysis reaction using an ethanol-based NbCl5 precursor and H2O2 as a strong oxidizer by UV treatment. A uniform dispersion of ultrasmall Nb2O5 NPs with a quantum size of 2 to 5nm is further obtained by modifying the procedure with acetic acid as a stabilizer. The resultant Nb2O5 NPs are controllably coated on fluorine-doped tin oxide (FTO) conductive glass through 2-volt electrophoresis. The Nb2O5-coated thin film functioned as an effective blocking layer between a mesoporous TiO2 and FTO; it thereby reduced the recombination at the TiO2/FTO/electrolyte interface in dye-sensitized solar cells. Furthermore, it serves as an ultrathin and compact hole blocking layer in perovskite solar cells, resulting in moderate hysteresis effect and improved photovoltaic efficiency from high-transmittance, low capacitance and low series resistance.

Multilayer Dye Aggregation at Dye/TiO2 Interface via π…π Stacking and Hydrogen Bond and Its Impact on Solar Cell Performance: A DFT Analysis

Multilayer dye aggregation at the dye/TiO2 interface of dye-sensitized solar cells is probed via first principles calculations, using p-methyl red azo dye as an example. Our calculations suggest that the multilayer dye aggregates at the TiO2 surface can be stabilized by π…π stacking and hydrogen bond interactions. Compared with previous two-dimensional monolayer dye/TiO2 model, the multilayer dye aggregation model proposed in this study constructs a three-dimensional multilayer dye/TiO2 interfacial structure, and provides a better agreement between experimental and computational results in dye coverage and dye adsorption energy. In particular, a dimer forms by π…π stacking interactions between two neighboring azo molecules, while one of them chemisorbs on the TiO2 surface; a trimer may form by introducing one additional azo molecule on the dimer through a hydrogen bond between two carboxylic acid groups. Different forms of multilayer dye aggregates, either stabilized by π…π stacking or hydrogen bond, exhibit varied optical absorption spectra and electronic properties. Such variations could have a critical impact on the performance of dye sensitized solar cells.

The interface and its role in carrier transfer/recombination dynamics for the planar perovskite solar cells prepared under fully open air conditions

The planar heterojunction perovskite solar cells, where the perovskite film is deposited directly onto a flat hole blocking layer, have recently attracted a great deal of attention owing to their high performance and ease of processing. However, the interface and its underlying role in carrier transport/recombination kinetics for such perovskite devices prepared under ambient air is still obscure. Herein, we addressed this issue by a dynamic intensity modulated photovoltage spectroscopy (IMVS) model using a continuity equation. The interface and its role in charge-carrier transport/recombination kinetics have been explored and discussed as an approach to understand the origin of the photovoltaic properties for the devices prepared under ambient air. The experimental IMVS responses were measured and satisfactorily fitted to the analytical results. Compared to the typical IMVS model based on dye-sensitized solar cells (DSSCs), the better IMVS fitting results presented in this study indicated that there was a discrepancy between the planar perovskite devices and those of DSSCs in electron transport/recombination properties, because carrier transfer across the TiO2/liquid electrolyte interface in DSSCs has been modified. That is, the Schottky interface in DSSCs needs to be replaced by the semiconductor heterojunction interface in perovskite solar cells (PSCs). Besides, the interface exhibits a more significant role in determining the carrier transport/recombination process by influencing the boundary conditions in a continuity equation. Furthermore, the intensity modulated photocurrent/photovoltage spectroscopy responses demonstrated that the carrier recombination characteristic is ultimately related with the surface and defect density in the interface. Interfacial modification, such as air-annealing, resulting in crystallographic changes, oxygen passivation, and variation in grain domain size, could suppress carrier recombination and prolong charge lifetime, which can yield more photo-generated electrons to be collected by anode, subsequently resulting in strikingly improving photovoltaic performance of the devices. In short, the dynamic IMVS model would help in elucidating the role of interface and the importance of interfacial modification or even interface design in order to obtain a highly efficient solar cell. The study can not only pave the way to construct the current−voltage curve using a continuity equation model, but also provide new insights into the performance-improving steps for the PSCs prepared under fully open air conditions, which is of great importance for their future commercialization.

Facile realization of efficient blocking from ZnO/TiO2 mismatch interface in dye-sensitized solar cells and precise microscopic modeling adapted by circuit analysis

In the present research, the effect of -based blocking layers on the operational features of -based dye-sensitized solar cells is investigated. A facile solution-based coating method is applied to prepare an interfacial highly transparent compact blocking layer (CBL) to enhance the efficiency of dye-sensitized solar cells. Different precursor molar concentration were tested to find the optimum concentration. Optical and electrical measurements were carried out to confirm the operation of the CBLs. Morphological characterizations were performed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) to investigate the structure of the compact layers. We have also developed a set of modeling procedures to extract the effective electrical parameters including the parasitic resistances and charged carrier profiles to investigate the effect of CBLs on the dye-sensitized solar cell (DSSC) performance. The adopted modeling approach should establish a versatile framework for diagnosis of DSSCs and facilitates the exploration of critical factors influencing device performance.

Highly stable dye-sensitized solar cells with quasi-solid-state electrolyte based on Flemion

A polymer gel electrolyte containing Flemion® (a perfluorocarboxylated polymer) as polymer matrix, lithium iodide, iodine, 1,2 dimethyl-3-propylimidazolium iodide (DMPII) and 4-tert-butylpyridine (TBP) was developed for quasi-solid-state dye-sensitized solar cells (DSSCs). The Flemion-based electrolyte was gelled by heating the Flemion solution to evaporate the ethanol solvent. Scanning electron microscopy, energy-dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy suggested three regions in the Flemion-based gel electrolyte: the fluorocarbon polymer frame, the ion cluster, and their interface. The open-circuit voltage, the short-circuit current density, and the fill factor were markedly enhanced by adding TBP and DMPII, respectively, to the Flemion-based gel electrolyte. When irradiated with light at 100mWcm−2, the DSSC containing the Flemion-based gel electrolyte with 0.6M DMPII and 0.5M TBP exhibited an energy conversion efficiency (η) of 4.1%, while η of the acetonitrile-based DSSC was 5.1%. According to electrochemical impedance spectra, gelation greatly increased the charge transfer resistance at the TiO2/electrolyte interface in DSSCs containing the Flemion-based electrolyte without DMPII. However, this effect was negated by adding 0.6M DMPII to the electrolyte. The η of Flemion-gel-based DSSCs stored for up to 4392h decreased by less than 10%. In contrast, the η of Flemion-liquid-based and acetonitrile-based DSSCs decreased by 95% during 480h storage, and by 93% during 115h storage, respectively. This result demonstrates that Flemion-based gel electrolytes solve the leakage problem that degrades the performance of DSSCs.

Acetonitrile Solution Effect on Ru N749 Dye Adsorption and Excitation at TiO2 Anatase Interface

We investigated stable structures and photoexcitation character of Ru N749 dye (black dye (BD)) adsorption to TiO2 anatase (101) interface immersed in bulk acetonitrile (AN) solution, a most representative electrode interface in dye-sensitized solar cells (DSCs). Density-functional-theory-based molecular dynamics (DFT-MD) with explicit solvent molecules was used to take into account the fluctuations of solvation shells and adsorbed molecules. We demonstrated that BD adsorption via deprotonated carboxylate two anchors (d2) is the most stable at the interface, while the one protonated carboxyl anchor (p1) has the average energy only slightly higher than the d2. This indicates that the p1 state can still coexist with the d2 under equilibrium. It is in contrast with the calculated large stability of the p1 in vacuo. Inhomogeneous charge distribution and anchor fluctuation enhanced by AN solution causes this d2 stabilization. The calculated projected densities of states and the photoabsorption spectra clearly ...

Rediscovering a Key Interface in Dye-Sensitized Solar Cells: Guanidinium and Iodine Competition for Binding Sites at the Dye/Electrolyte Surface

We propose a new mechanism by which the common electrolyte additive guanidinium thiocyanate (GdmSCN) improves efficiency in dye-sensitized solar cells (DSSCs). We demonstrate that binding of Gdm(+) to TiO2 is weak and does not passivate recombination sites on the TiO2 surface as has been previously claimed. Instead, we show that Gdm(+) binds strongly to the N719 and D131 dyes and probably to many similar compounds. The binding of Gdm(+) competes with iodine binding to the same molecule, reducing the surface concentration of dye-I2 complexes. This in turn reduces the electron/iodine recombination rate constant, which increases the collection efficiency and thus the photocurrent. We further observe that GdmNO3 can increase efficiency more than the current Gdm(+) source, GdmSCN, at least in some DSSCs. Overall, the results point to an improved paradigm for DSSC operation and development. The TiO2/electrolyte surface has long been held to be the key interface in DSSCs. We now assert that the dye layer/electrolyte interaction is at least, and probably more, important.

Theoretical investigation of the donor group related electronic structure properties in push-pull organic sensitizers

The judicious selection of donors is one common tactic to elaborately tailor metal-free chromophores, which can harness visible as well as near-infrared solar photons and modulate the charge transfer kinetics at titania/dye/electrolyte interface in dye-sensitized solar cells. In this paper, to gain insight into the effect of the donor on spectroscopic properties, electronic structures, excited state lifetimes and reorganization energies which are related to light-harvesting capacity and charge transfer reaction, we carried out a computational investigation on a variety of ordinary arylamine electron donors, by use of three cyclopentadithiophene dyes (C246, C218 and K201) with the respective N-hexyl-carbazole (HCZ), dihexyloxy-substituted triphenylamine (HTPA) and bisfluorenylaniline (HFA) electron donors. Amongst these electron donors, N-hexyl-carbazole is found to display the weakest electron-donating capacity, accounting for the corresponding C246 dye with evidently blue-shifted light absorption in comparison with the other two congeners, and endowing the hole mainly localizing on the conjugated segment, which will slow down the regeneration reaction. The HFA electron donor group with large and rigid structure not only presents longer excited state lifetime, which could be beneficial to the photoinduced electron injection, but also features smaller reorganization energy for the regeneration process.

Application of Pt sputter-deposited counter electrodes based on micro-patterned ITO glass to quasi-solid state dye-sensitized solar cells

The effects of the Pt catalyst sputter deposited on the patterned ITO glass (SD-Pt/pITO) on the photovoltaic properties and charge-transfer characteristics at the Pt/electrolyte interface of dye-sensitized solar cells (DSSCs) are studied and compared with those of a conventional Pt counter electrode deposited by thermal reduction on fluorine-doped tin oxide (FTO) glass (TD-Pt/FTO). The DSSC with the SD-Pt/pITO (sample cell) shows a lower charge-transfer resistance than that of the DSSC with the TD-Pt/FTO (reference cell), which leads to an improvement of its fill factor by about 7.6%. The long-term durability test performed for 1000 h at room temperature reveals that the sample cell retains up to 99% of its energy conversion efficiency, while that of the reference cell is degraded by about 7.2%.

Probing Dye-Correlated Interplay of Energetics and Kinetics in Mesoscopic Titania Solar Cells with 4-tert-Butylpyridine

The adsorption of 4-tert-butylpyridine (TBP) on the surface of oxide semiconductors can tune their interband state distributions as well as the conduction band edges, offering a straightforward tactic to modulate the energy alignments at the titania/dye/electrolyte interface in dye-sensitized solar cells. In this work, by stepwise augmenting the TBP concentration in electrolytes, we investigate the energetic and kinetic interplays in mesoscopic titania solar cells based upon two push–pull organic dyes C218 and D205. TBP at a higher concentration, while better retarding triiodide interception of photoinjected electrons in C218 cells, has strikingly induced an acceleration of interfacial charge recombination with triiodide in devices employing the D205 dye, accounting for a much smaller photovoltage enhancement with respect to the C218 counterpart. Transient emission measurements disclose that the exciton dissociation at the titania/C218 dye interface does not noticeably vary despite a TBP-correlated conduc...

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.

Insights into Working Principles of Ruthenium Polypyridyl Dye-Sensitized Solar Cells from First Principles Modeling

With the aim to better describe the phenomena taking place at the dye/semiconductor interface in dye-sensitized solar cells and to understand the interfacial electron transfer from the excited dye to the semiconductor, the cis-[Ru(4,4′-COOH-2,2′-bpy)2(NCS)2]/TiO2 system has been investigated using density functional theory (DFT) in conjunction with a periodic approach and a hybrid functional. At this level of theory, the interplay of the electronic and geometrical coupling between the semiconductor and the dye has been analyzed, and the feasibility of the interfacial electronic transfer has been discussed. Our results show that the electronic transfer is highly favorable from a thermodynamic point of view, the LUMO of the dye being significantly higher in energy than the conduction band of the semiconductor. In addition, the theoretical injection time, computed using the Newns−Anderson model, is in fair agreement with the observed value.

Anion Effects in Organic Dye-Sensitized Mesoscopic Solar Cells with Ionic Liquid Electrolytes: Tetracyanoborate vs Dicyanamide

We compare the impacts of tetracyanoborate and dicyanamide anions in solvent-free ionic liquid electrolytes upon the optoelectronic features of dye-sensitized solar cells based on an organic push−pull chromophore. With respect to dicyanamide, tetracyanoborate confers a bathochromic and higher photocurrent response upon a dye-coated mesoporous titania film. Numerical simulations on electrical impedance spectroscopies disclose that tetracyanoborate anions evoke a downward displacement of the conduction-band edge of an electrolyte-sinking nanocrystalline film in comparison with dicyanamide, generating a more favorable energy-offset at the titania/dye interface and therefore a better exciton dissociation yield, as proved via the transient emission measurements. An effective recombination reaction rate constant U0k is used to depict the charge-transfer behavior at the titania/electrolyte interface in dye-sensitized solar cells. The negative impact of an about 4 times larger U0k of the dicyanamide cell is overwhelmed by the noticeably higher conduction band edge, leading to the experimentally observed anion-dependent open-circuit photovoltage.

Effects of 1,3-dialkylimidazolium cations with different lengths of alkyl chains on the Pt electrode/electrolyte interface in dye-sensitized solar cells

The electrochemical behaviors of the tri-iodide (I 3 −)/iodide (I −) redox couple of symmetric cells were investigated by cyclic voltammetry, steady-state polarization, chronocoulometry, and electrochemical impedance spectroscopy. 1,3-Dialkylimidazolium cations affected the characteristics of the Pt electrode/electrolyte interface by adsorbing on the Pt electrode, as a result of electrostatic interactions, and further affected the reduction of I 3 − on the Pt electrode. Capacitance of the double layers of the Pt electrode/electrolyte interface was chiefly determined by capacitance of the compact layer according to the Helmoholtz theory.