P-type DSSCs Research Articles

New Carbazole-Based Sensitizers for p-Type DSSCs: Impact of the Position of Acceptor Units on Device Performance

We report the design, synthesis, and characterization of two new carbazole-based organic dyes PC1–2 as potential sensitizers for NiO-based p-type dye-sensitized solar cells (p-DSSCs). The D-A-π-A’ configured PC1 dye comprises a thienyl unit as a π-spacer and a malononitrile as an end-capping acceptor unit, whereas in PC2, the cyanovinylene group serves as an acceptor unit and a thienyl group acts as a donor unit in a D–A–D configuration. These molecules achieved excellent solubility due to their long-branched alkyl chains. The current work encompasses their structural, photophysical, thermal, electrochemical, theoretical, and photoelectrochemical studies, establishing structure–property relationships. PC1–2 exhibit λabs and λemi in the range of 389–404 and 448–515 nm, respectively, with a band gap in the range of 2.88–2.92 eV. Electrochemical studies confirm the feasibility of electron injection, regeneration, and recombination. The introduction of an additional electron-withdrawing group (cyanovinylene group) on the dye PC1 skeleton endows it with a higher dye loading capacity, high hole injection, and a strengthened intramolecular charge transfer (ICT) effect, resulting in a redshifted ICT absorption with a higher molar extinction coefficient. Among the two new dyes, the device based on PC1 achieved the highest power conversion efficiency (PCE) of 0.027% with a short-circuit current density (JSC) of 1.29 mA·cm–2, open-circuit voltage (VOC) of 67 mV, and fill factor (FF) of 31%, whereas the device with dye PC2 performed less efficiently (PCE: 0.018%, JSC: 0.92 mA·cm–2, VOC: 68 mV, and FF: 30%). Conclusively, the study provides insights into the intricacies involved in the structural modification of carbazole-based p-type dyads for the development of highly efficient DSSCs.

Sensitizers designed toward efficient intramolecular charge separation for p-type dye-sensitized solar cells

Effective charge separation is demonstrated as one of the rational design strategies for organic sensitizers of both n-type and p-type dye-sensitized solar cells. Herein, we report the design and synthesis of novel D-A dye cTPAPh-NO2 and D-A1-A2 dye cTPABT-PyCN2 using triphenylamine as the donor, di-carboxylic acid group as anchoring group. Photophysical and electrochemical properties of both dyes were investigated to discuss the effect of anchoring group and molecular structure on the photophysical processes and charge separation feature of the sensitizers. For D-A dyes, the attachment of carboxyl groups reduces both the light harvesting and intramolecular charge separation ability of cTPAPh-NO2. In comparison, cTPABT-PyCN2 exhibits weak fluorescence and efficient intramolecular charge separation characteristics. Exceeding 80% of cTPABT-PyCN2 singlets decays via intramolecular electron transfer from the first acceptor to the dicyanovinyl pyrimidine. Finally, these synthesis dyes were used as sensitizers in p-type DSSCs and cTPABT-PyCN2 showed the overall power conversion efficiency of 0.103%, which is comparable to that of reported dyes. The results demonstrate that D-A1-A2 structure can be a promising strategy to design p-type sensitizers with effective charge separation characteristics.

Photoelectrochemical properties of dyads composed of porphyrin/ruthenium catalyst grafted on metal oxide semiconductors

In the current work, we present the use of two free-base and two zinc-metallated porphyrin-ruthenium(II) polypyridine dyads, along with two reference porphyrin derivatives, as sensitizers in both n- and p-type DSSCs and DSPECs. Two of the dyads contain the well-known Ru(bpy)3 unit (HOOC-DMP-Ru(bpy)3 and HOOC-(Zn)DMP-Ru(bpy)3), while in the other two terpyridine-Ru(Cl)-bypiridine was used (HOOC-DMP-tpy-Ru and HOOC-(Zn)DMP-tpy-Ru). In all systems, the amide-bonding motif was utilized for the connection of the counterparts comprising each dyad. Photophysical investigation of the reported systems indicated sufficient electronic interactions for the dyads in their excited states (emission measurements). The photovoltaic measurements revealed that the presence of the ruthenium complex improves the overall performance of the dyads with the most efficient dyad being HOOC-(Zn)DMP-tpy-Ru in both n- and p-type DSSCs. Consequently, HOOC-(Zn)DMP-tpy-Ru was used to fabricate n- and p-DSPECs towards the oxidation of methoxy-benzyl alcohol and the reduction of CO2, respectively.

Theoretical Study of Effects of Anchoring Groups on Photovoltaic Properties of a Triarylamine-Based p-Type Sensitizer.

The effects of anchoring groups on triarylamine-based p-type dyes were studied by substituting the strong electron-withdrawing carboxyl group with the weak electron-withdrawing pyridyl and the electron-rich catechol groups. Judged by the index t, the charge separation would be improved greatly when the carboxyl group of P4 is replaced by the pyridyl or catechol groups. Although carboxyl as an anchoring group lowers the HOMO energy and facilitates the hole injection in comparison with pyridyl and catechol groups, the weak electron-withdrawing pyridyl and the electron-rich catechol groups facilitate the charge separation. Eg becomes narrow as the electron-withdrawing abilities of the anchoring groups decrease or as the conjugation extends. Both the extended π-spacers and the substitution of carboxyl with pyridyl and catechol groups promote the redshifts of adsorption wavelengths. The oscillator strengths for all dyes are over 2.00, indicating that all the dyes are able to harvest the sunlight strongly. The ΔGCR values of P4, DF4, and DZ4 are smaller than those of the other dyes. Also, these dyes have larger adsorption over infrared visible light, indicating that these dyes may be good candidates for p-type DSSCs.

New pyran-based molecules as both n- and p-type sensitizers in semi-transparent Dye Sensitized Solar Cells

We report here on the synthesis of four novel pyran-based DSSC photosensitizers. The dyes are constituted of a pyran core functionalized with different electron acceptor groups and with peripheral cyano-acrylic groups. The molecules were carefully characterized for what concerns optical and electrochemical properties and a clear dependence from the strength of the electron-acceptor group functionalizing the pyran core was observed. A detailed DFT analysis on the dyes gave a better insight of the electronic properties and showed a good accord with the experimental properties. The dyes were used as photosensitizers both in n-type and p-type DSSCs. In the first case, the stabilization of the LUMO achieved partially prevented a fast kinetics of the electron injection. To overcome this, we used an I-based electrolyte containing an excess of LiI: the photoconversion efficiency significantly increases for all the dye-sensitized devices, up to a maximum of 2.1%. The dyes were successfully employed the as photosensitizers in p-type DSSC, obtaining PCEs approaching 0.1%, a promising result in the field on NiO-based devices. More remarkably, as far as we are aware, this is the first paper in which a single series of dyes is effectively employed as sensitizers in both n and p-type DSSCs.

Ab initio Study of Anchoring Groups for CuGaO2 Delafossite-Based p-Type Dye Sensitized Solar Cells.

Here we report the first theoretical characterization of the interface between the CuGaO2 delafossite oxide and the carboxylic (–COOH) and phosphonic acid (–PO3H2) anchoring groups. The promising use of delafossites as effective alternative to nickel oxide in p-type DSSC is still limited by practical difficulties in sensitizing the delafossite surface. Thus, this work provides atomistic insights on the structure and energetics of all the possible interactions between the anchoring functional groups and the CuGaO2 surface species, including the effects of the Mg doping and of the solvent medium. Our results highlight the presence of a strong selectivity toward the monodentate binding mode on surface Ga atoms for both the carboxylic and phosphonic acid groups. Since the binding modes have a strong influence on the hole injection thermodynamics, these findings have direct implications for further development of delafossite based p-type DSSCs.

Improving charge injection and charge transport in CuO-based p-type DSSCs – a quick and simple precipitation method for small CuO nanoparticles

Novel CuO photocathodes based on synthesized nanoparticles achieve superior charge injection and transport properties in p-DSSCs.

Exploring the application of new carbazole based dyes as effective p-type photosensitizers in dye-sensitized solar cells

Herein, we report the design and the synthesis of three new D-A type metal-free carbazole based dyes (C1–3) as effective photosensitizers for p-type DSSCs. In this new design, the electron rich carboxy substituted carbazole unit has been attached to three different electron withdrawing species, viz. N,N-dimethyl barbituric acid, N,N-diethyl thiobarbituric acid and N-ethyl rhodanine. They were well-characterized by spectral, photophysical and electrochemical analyses. Further, their optical and electrochemical parameters along with molecular geometries, optimized from DFT have been employed to apprehend the effect of structures of C1–3 on their photovoltaic performances. Further, the photovoltaic performance of C1–3 was determined along with the standard dye P1 and their PCE values were found to be in the order of P1 (0.047%)>C2 (0.040%)>C1 (0.016%)>C3 (0.001%). Interestingly, the NiO based p-type DSSC fabricated with C2 carrying electron withdrawing N,N-diethyl thiobarbituric acid displayed VOC as 59±4mV and FF as 29±1%, which are higher than that of benchmark reference P1. This is attributed to the highest light harvesting ability, the greatest regeneration driving force and the lowest interfacial charge recombination of C2 among the tested dyes. Conclusively, the results showcase the potential of carbazole based D-A type sensitizers in the development of efficient p-type DSSCs.

Photovoltaic Performance of Co-doped CuCrO2 for p-type Dye-sensitized Solar Cells Application

For the first time, Co-doped CuCrO2+δ non-stoichiometric nanocrystals with δ≈0.1 aresynthesized bylow-temperature hydrothermal method. The synthesis mechanism proposed and the structural characterizations have confirmed that the excess oxygen from delafossite structure is given by the specific hydrothermal conditions (temperature, pressure and molarity of basic solution). The decreasing of the observed electrical resistivity in the non-stoichiometric nanocrystals is enhanced by Co-subtitution. Thus, hydrothermal synthesis has allowed to obtain doped delafossite nanocrystalline compound with excess oxygen and mixed valancies of both sites (A and B sites) and could open the study of its cumulative effect on physical properties. The performance of p-type DSSCs based on Co-doped CuCrO2+δnanocrystals photocathodeswas made using two type of dye: Coumarin C343 and Ruthenium N719. The short-circuit density (Jsc) is notably increased by approximately 60% when N719 is used as dye.

New cyclometalated iridium(III) dye chromophore complexes for p-type dye-sensitised solar cells

The synthesis of seven iridium complexes [Ir(CˆN)2(NˆN)][PF6] (AS9-15) designed as dyes for p-type DSSC is described. These complexes comprise a 4-(pyrid-2-yl)benzoic acid as the cyclometalating/anchoring ligand with different diimine ligands acting as electron accepting ancillary ligands. DFT analysis together with photophysical investigations reveal how using different π-systems on the ancillary ligand it is possible to tune the absorption spectra of these complexes and to enhance the spatial separation between the HOMO and LUMO. Computational studies demonstrate an ideal HOMO to LUMO charge transfer directionality for the presented [Ir(CˆN)2(NˆN)]+ frameworks, promoting a favourable hole transfer into the NiO cathode valence band upon photoexcitation in p-type DSSC devices. Preliminary tests on NiO-based p-type DSSCs have been carried out confirming the potential use of complexes AS9-15 as a basis for continued development as DSSC chromophores.

Increasing p-type dye sensitised solar cell photovoltages using polyoxometalates.

Lindqvist polyoxometalate (POM) additives increase VOC in p-type DSSCs by up to 140%, yielding substantial efficiency gains for poorly matched dyes and redox mediators. For better dye/electrolyte combinations, these gains are typically outweighed by losses in JSC. Charge lifetime and transient IR measurements show that this is due to retardation of both recombination and electron transfer to the mediator, and a positive shift in the NiO valence band edge. The POMs also show their own, limited sensitizing effect.

Effect of polymorphism on photovoltaic performance of CuAlO2 delafossite nanomaterials for p-type dye-sensitized solar cells application

In this paper, we report the successful hydrothermal synthesis of CuAlO2 nanocrystals, 3R or mixture of 2H and 3R polytypes. According to the literature, the potential of CuAlO2 with 2H and 3R polytype as cathode elements in DSSCs has not been fully explored by either theory or experiment. We have made a comparative study of nanocrystalline CuAlO2 polytypes prepared by hydrothermal method in subcritical conditions. The stability diagram for Cu-Al-H2O system allows preparing of nanocrystalline CuAlO2, either 3R pure phase or mixture of 2H and 3R. Together with temperature and autogenous pressure, the time reaction seems to be a key factor in the competition between both polytypes, 3R and 2H. The nanocrystalline CuAlO2 polytypes have been characterized from the structural and optical point of views prior to be used for DSSC preparation and characterization. We have notably found that even in small concentration, the presence of 2H polytype in CuAlO2 compound leads to an increase of 49% of the energy conversion efficiency compared to pure 3R- CuAlO2 polytype.The future work will be focused on the hydrothermal synthesis of pure nanocrystalline 2H-CuAlO2 polytype and the study of its effect on the performance of p-type DSSCs.

Investigation of a new bis(carboxylate)triazole-based anchoring ligand for dye solar cell chromophore complexes.

A novel anchoring ligand for dye-sensitised solar cell chromophoric complexes, 1-(2,2'-bipyrid-4-yl)-1,2,3-triazole-4,5-dicarboxylic acid (dctzbpy), is described. The new dye complexes [Ru(bpy)2(dctzbpy)][PF6]2 (AS16), [Ir(ppy)2(dctzbpy)][PF6] (AS17) and [Re(dctzbpy)(CO)3Cl] (AS18) were prepared in a two stage procedure with intermediate isolation of their diester analogues, AS16-Et2, AS17-Et2 and AS18-Et2 respectively. Electrochemical analysis of AS16-Et2, AS17-Et2 and AS18-Et2 reveal reduction potentials in the range -1.50 to -1.59 V (vs. Fc+/Fc) which are cathodically shifted with respect to that of the model complex [Ru(bpy)2(dcbH2)]2+ (1) (Ered = -1.34 V, dcbH2 = 2,2'-bipyridyl-4,4'-dicarboxylic acid). This therefore demonstrates that the LUMO of the complex is correctly positioned for favourable electron transfer into the TiO2 conduction band upon photoexcitation. The higher energy LUMOs for AS16 to AS18 and a larger HOMO-LUMO gap result in blue-shifted absorption spectra and hence reduced light harvesting efficiency relative to their dcbH2 analogues. Preliminary tests on TiO2 n-type and NiO p-type DSSCs have been carried out. In the cases of the Ir(iii) and Re(i) based dyes AS17 and AS18 these show inferior performance to their dcbH2 analogues. However, the Ru(ii) dye AS16 (η = 0.61%) exhibits significantly greater efficiency than 1 (η = 0.1%). In a p-type cell AS16 shows the highest photovoltaic efficiency (η = 0.028%), almost three times that of cells incorporating the benchmark dye coumarin C343.

KuQuinones as sensitizers for NiO based p-type dye-sensitized solar cells

KuQuinones have been used for the first time as dyes in NiO-based p-type DSSCs.

Comparison of p-type sensitizers with different electron-induced effects in dye-sensitized solar cells: A theoretical investigation

Two classical p-type photosensitizers with modified conjugation spacers by the electron-rich unit (thiophene) and electron-deficient unit (pyrimidine) are calculated and investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). Creatively, regeneration reactions between reduced sensitizers and I3− as well as I2− are explored theoretically, reappearing the regeneration process that electrons are transferred from reduced dyes to I3− or I2− in p-type dye-sensitized solar cells (p-type DSSCs) intuitively, and uncovering the difference of π-spacers with different electron-induced effects in the process of dye regeneration. Besides, calculation also indicates that thiophene-based dye (PS) has better light capture efficiency but unfavorable performance of charge transfer when compared with the pyrimidine-based dye (PN). From another perspective, both sensitizers have efficient driving forces of injection. In addition, dyes absorbing on the (NiO)9 semiconductor are also considered. Our comparison discussion of two sensitizers with different π-spacers provides great assistance for deeply investigating p-type photosensitizes with higher efficiencies and their regeneration processes, contributing to the potential tandem DSSCs.

Optimizing CuO p-type dye-sensitized solar cells by using a comprehensive electrochemical impedance spectroscopic study.

We introduce a novel and comprehensive approach for the evaluation and interpretation of electrochemical impedance spectroscopy (EIS) measurements in p-type DSSCs. In detail, we correlate both the device performance and EIS figures-of-merit of a series of devices in which, the calcination temperature, film thickness, and electrolyte concentration have been systematically modified. This new approach enables the separation of the different processes across the dye/semiconductor/electrolyte interface, namely the unfavorable charge recombination and the favorable electron injection/regeneration processes. In addition, studies on non-sensitized CuO and NiO electrodes provide insights into their affinity towards a reaction with the electrolyte - CuO is far less reactive towards the polyiodide species. Overall, this work underlines the superior features of CuO with respect to NiO for p-DSSCs and demonstrates a comprehensive optimization of the CuO-based DSSCs with respect to the device architecture by the aid of EIS analysis.

Determining the most promising anchors for CuSCN: ab initio insights towards p-type DSSCs

We investigate a large number of potential monodentate and bidentate anchors in the framework of p-type DSSCs based on copper thiocyanate.

Valence Band Edge Shifts and Charge-transfer Dynamics in Li-Doped NiO Based p-type DSSCs

ABSTRACT The objective of this work is to gain deeper insight into behavior of p-type DSSCs, paying special attention to photoelectric properties of Li-doped NiO photoelectrode, and studying their effect on interfacial photoelectrochemical processes under illumination and device performance. Here we find that Li-doped NiO photoelectrodes show higher charge carrier densities, down-shift of valence band and narrowing trap energy distribution. Furthermore, the performance enhancement of p-type DSSCs is observed for the Li-doped NiO photoelectrodes with respect to the unmodified ones. These improvement is found to arise from more rapid hole transport combined with reduced recombination, contributing to improved hole collection. With regard to interfacial carriers transfer process, electrochemical impedance spectroscopy (EIS), intensity-modulated photovoltage spectroscopy (IMPS) and ultra-violet photoemission spectra (UPS) provide evidence that the decreased recombination of Li-doped NiO DSSCs is attributed to the downward shift of valence band, which increases energy barrier of interface recombination. Another reason results in narrowing trap energy distribution of Li-doped NiO photoelectrode, which can weaken energy overlap with the LUMO level of dye, and subsequently the recombination path of hole back transfer to dye is hindered. These findings suggest that it will be important to carefully optimize band edge alignment and trap states distribution of p-type oxide semiconductors film to further improve the efficiency of p-DSSCs.

A double-acceptor as a superior organic dye design for p-type DSSCs: high photocurrents and the observed light soaking effect.

Herein, we report three novel single donor double acceptor dyes, BH2, 4, and 6, for use in p-type dye sensitized solar cells (DSSCs). BH4 yields one of the highest photocurrents, 7.4 mA cm(-2), to date. The high performance is achieved via a shorter synthetic route and no exotic materials or cell-building techniques. We suggest a structural principle when building dyes whereby one adopts a double acceptor/single anchor when a triphenylamine moiety is incorporated into a dye for p-type DSSCs. This strategy increases the molar extinction coefficient while simultaneously reducing the number of synthetic steps. The molar extinction coefficients (99 980 M(-1) cm(-1)) reported herein are among the highest reported. Finally, we report the first-ever-observed light soaking effect in p-type DSSCs.

A Rational Design for Dye Sensitizer: Density Functional Theory Study on the Electronic Absorption Spectra of Organoimido-Substituted Hexamolybdates

The novel dyes of organoimido-substituted hexamolybdates for positive type dye-sensitized solar cells (p-type DSSCs) have been studied on the basis of time-dependent density functional theory (TDDF...