P-type DSSC Research Articles

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.

“Iridium effect” in cyclometalated iridium complexes for p-type dye sensitized solar cells

Cyclometallated iridium complexes have shown great promises as photosensitizers for p-type dye sensitized solar cells (p-DSSC), in particular due to the occurrence of very long-lived interfacial charge separated states, leading to high open circuit potentials (Voc). To rationalize this experimental fact, we prepared a new series of iridium photosensitizers specially designed for NiO based p-DSSC, in order to assess the role of a panel of factors on the overall photovoltaic performances. We probed the electronic coupling between the iridium complex and the semi-conductor at the level of the anchor, the importance of the frontier orbitals distribution over the photosensitizers structures and the impact of iridium(III) vs. a lighter metal ion like ruthenium(II). The synthesis and full characterizations of these new photosensitizers are presented, and their performances within p-type DSSC are given and discussed in light of their structures. A combination of a high spin-orbit coupling and a smart design of the anchoring ligands explains the interesting performances of iridium complexes as photosensitizers for p-DSSC.

Investigation of the p-type dye-sensitized solar cell based on full Cu2O electrodes

The construction and testing of the p-type dye-sensitized solar cell based on full Cu2O electrodes using traditional P1 dye and the electrolyte based on the I3−/I− redox couple are reported. The counter electrode and the semiconductor material for the cathode based on Cu2O were prepared in single low-temperature hydrothermal technology using low cost precursor materials. The p-type dye-sensitized solar cell with full Cu2O electrodes has provided twice as much JSC and VOC as a DSSC using Pt as a counter electrode. Furthermore the power conversion efficiency is improved by about 60% by replacing conventional platinum with a Cu2O/Cu counter electrode. Based on these favorable characteristics this proof of principle "full-copper" p-type DSSC opens a fascinating opportunity towards environmentally friendly and large-scale production of 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.

P-Type dye-sensitized solar cells based on pseudorotaxane mediated charge-transfer.

The efficiency of p-type dye-sensitized solar cells (DSSCs) remains low compared to that of n-type congeners due to charge recombination events. We report a supramolecular approach to reduce recombination at the NiO-dye interface, realized by using the cyclophane cyclobis(paraquat-p-phenylene) ring (RING4+/RING3˙+) as a redox mediator and a dye (PN) functionalized with a 1,5-dioxynaphthalene (DNP) recognition site, promoting the supramolecular formation of a pseudorotaxane capable of directing charge transfer away from the NiO-dye interface. The binding affinity of RING4+ to PN is high (Kass = 3.4 × 104 M-1), with quenching of the photoexcited dye (PN*) ascribed to reduction of RING4+ to RING3˙+. The reduced RING3˙+ exhibits a lower binding affinity to PN, facilitating exchange with the excess RING4+ present in solution. This supramolecular phenomenon was implemented into p-type DSSCs by anchoring the PN dye on a NiO photocathode in conjunction with the RING4+/RING3˙+ redox couple, yielding a 10 fold enhancement in the short-circuit photocurrent (JSC) compared to control devices utilizing P1 dye or the methylviologen (MV2+/MV˙+) redox couple that cannot form pseudorotaxanes.

CrOOH as high-performance surface passivation material for dye-sensitized solar cell

α-CrOOH was firstly explored as a passivation material for p-type dye-sensitized solar cell based on 3R-CuCrO2 delafossite and its beneficial effect on photovoltaic performance of p-type DSSC was demonstrated. α-CrOOH nanomaterial was obtained by a simple hydrothermal method using strong acid solution. The n-type α-CrOOH passivation of the p-type 3R-CuCrO2 film has provided a doubling of JSC and a threefold increase in the VOC compared to the pure 3R-CuCrO2 photocathode in DSSC. This remarkable enhancement could be explained in that α-CrOOH layer acts as a barrier layer for charge recombination between the holes in the valence band and the electrolyte or dye. In addition, formed p-n junction would facilitate the charge separation. Therefore, the α-CrOOH is proposed as a promising material to modify the photocathode of p-type DSSC with excellent photovoltaic efficiency.

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.

Design of “push-pull” p-type quinoid-based organic dyes with near-IR absorption: A density function theory study

Based on an experimentally synthesized p-type dye QT-1, we screened and designed a series of quinoid-based “push-pull” dyes SPL401 - SPL407 by modifying the π-conjugated linker for ptype dye-sensitized solar cells (p-type DSSC). Further, we systematically calculated their electronic and optical properties by using the density functional theory (DFT) and the time-dependent DFT (TDDFT). The results indicate that compared to QT-1, SPL406 and SPL407, which have not only smaller energy gaps, higher oscillator strength f increased by 70% and 80%, and 141 nm and 110 nm redshifts, respectively, but also a broader absorption spectrum covering the entire visible range to the near-IR region of 1200 nm, may be the most promising candidates among SPL401 - SPL407 for p-type organic sensitizers.

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.

Influence of interface properties on charge density, band edge shifts and kinetics of the photoelectrochemical process in p-type NiO photocathodes

The surface structure of NiO is correlated to observed changes in the band energy, energetic distribution of the trap states density, charge interface transfer, charge transport, and as a result the p-type DSSC device performance.

The construction of tandem dye-sensitized solar cells from chemically-derived nanoporous photoelectrodes

A tandem dye-sensitized solar cell (tandem-DSSC) was synthesized on the basis of thin-film semiconductor electrodes. The nanoporous p-type NiO films were successfully obtained by simultaneous deposition of Al and Ni, followed by selective etching of Al and oxidation. Likewise, the n-type photoanode was made where Ag was etched in nitric acid after the initial formation of Ag/TiO2 nanocomposites. Such dye-sensitized photoelectrodes were combined to construct a tandem solar cell which exhibited an enhanced open-circuit voltage. Also, the tandem devices were subjected to various light fluxes to correlate the experimental cell parameters (open-circuit voltage, short-circuit current, fill factor, recombination shunt resistance, etc.) with the ideal one-diode model. Interestingly, impedance spectra of the tandem cell was well matched with the parameters from each of the n-type or p-type DSSC, indicative of successfully-designed tandem structure.

Characterization and control of the electronic properties of a NiO based dye sensitized photocathode

One compartment tandem DSSCs are based on two photoactive electrodes which are mediated by a redox electrolyte. Electron accumulation in the photoanode (n-type DSSC) alongside hole accumulation in the photocathode (p-type DSSC) should generate high photovoltage using different parts of the solar spectrum. While impressive efficiencies are reported for n-type DSSCs, the performance of the p-type analogue is very low due to insufficient understanding and a lack of materials. Electrochemical impedance spectroscopy of the p-type DSSC reveals that hole transport within the NiO mesoporous photocathode is the performance limiting factor. Modification of the NiO electrode with molecular dipoles significantly increases the cell photovoltage but has no significant effect on the photocurrent of the p-DSSC. Consequently, the development of better hole conducting materials in conjunction with surface dipole modification can lead to high photovoltage, high photocurrent p-DSSCs and thus to efficient tandem DSSCs.

Investigation of the photoinduced electron injection processes for p-type triphenylamine-sensitized solar cells

We have carried out theoretical investigation aiming at modelling the assessment of mechanisms of photoinduced processes in a recent p-type organic metal-free dye derived from the triphenylamine (P-1) structure. In the P-1 system, one uses the triphenylamine moiety as the electron donor, malononitrile as the electron acceptor, and a thiophene that plays the role of the conjugated chain. Basically, the difference between the P-1 dye and the common organic dyes used in the n-type DSSC field is the anchoring group located on the electron donor group. In a first step, DFT and TDDFT approaches have been exploited to calculate the key parameters controlling both the intramolecular charge transfer (ICT) and hole transfer rate constants in the Gurney-Gerischer-Marcus (GGM) formalism, for either a solvent-controlled adiabatic or a nonadiabatic electron transfer. These are: (i) the electronic coupling; (ii) the reorganization energies; and (iii) the variation of the Gibbs energy. The gathered results are in agreement with the experimental trends. (i) The vertical ICT excited states energy has been calculated at 2.67 eV, in perfect line with the experiment (2.65 eV). (ii) Two mechanisms can be conceived for the hole transfer and regeneration process. The first deals with the reduction of dye molecule at the excited state followed by an electron transfer from the reduced dye to the oxidized regenerator. The second implies a redox reaction between the excited dye and the oxidized regenerator, followed by an electron transfer from the cathode to the oxidized dye. (iii) Our theoretical investigation suggests that the first mechanism is dominant. Secondly, we propose structural modifications improving the TPA-based DSSCs hole transfer efficiency and we show that an additional –CN graft on the malononitrile unit combined to the functionalisation of the TPA moieties by –OMe groups (to give P-1b) should significantly improve the key parameters related to the electron injection. Indeed, for P-1b, we have noticed an increase of both the RLHE factor (0.907) and the injection driving force (−0.33 eV). This dye is therefore expected to be a very promising molecule in the p-type DSSC field.