The influence of monovalent cation halide additives on the optical, excitonic, and electrical properties of CH₃NH₃PbI₃ perovskite is reported. Monovalent cation halide with similar ionic radii to Pb²⁺, including Cu⁺, Na⁺, and Ag⁺, have been added to explore the possibility of doping. Significant reduction of sub-bandgap optical absorption and lower energetic disorder along with a shift in the Fermi level of the perovskite in the presence of these cations has been observed. The bulk hole mobility of the additive-based perovskites as estimated using the space charge limited current method exhibits an increase of up to an order of magnitude compared to the pristine perovskites with a significant decrease in the activation energy. Consequentially, enhancement in the photovoltaic parameters of additive-based solar cells is achieved. An increase in open circuit voltage for AgI (≈1.02 vs 0.95 V for the pristine) and photocurrent density for NaI- and CuBr-based solar cells (≈23 vs 21 mA cm⁻² for the pristine) has been observed. This enhanced photovoltaic performance can be attributed to the formation of uniform and continuous perovskite film, better conversion, and loading of perovskite, as well as the enhancement in the bulk charge transport along with a minimization of disorder, pointing towards possible surface passivation.

Ionic liquid electrolytes are prepared using sulfolane as a plasticizer for eutectic melts to realize highly stable and efficiently performing dye-sensitized solar cells (DSCs) in hot climate conditions. Variations in the viscosity of the formulations with sulfolane content are measured and performance in DSCs is investigated using the ruthenium dye C106 as a sensitizer. A power conversion efficiency (PCE) of 8.2% is achieved under standard reporting conditions. Apart from lowering the viscosity, the addition of sulfolane induces a negative shift of the TiO₂ conduction band edge. Strikingly the device performance increases to 8.4% at 50 °C due to higher short circuit photocurrent and fill factor, over-compensating the loss in open circuit voltage with increasing temperature. The PCE increases also upon decreasing the light intensity of the solar simulator, reaching up to 9% at 50 mW cm⁻². Devices based on these new electrolyte formulations show excellent stability during light soaking for 2320 h under full sunlight at 60 °C and also during a 1065 h long heat stress at 80 °C in the dark. A detailed investigation provides important information about the factors affecting the principal photovoltaic parameters during the aging process and the first results from a series of outdoor measurements are reported.

Overcoming ionic diffusion limitations is essential for the development of high-efficiency dye-sensitized solar cells based on cobalt redox mediators. Here, improved mass transport is reported for photoanodes composed of mesoporous TiO₂ beads of varying pore sizes and porosities in combination with the high extinction YD2-o-C8 porphyrin dye. Compared to a photoanode made of 20 nm-sized TiO₂ particles, electrolyte diffusion through these films is greatly improved due to the large interstitial pores between the TiO₂ beads, resulting in up to 70% increase in diffusion-limited current. Simultaneously, transient photocurrent measurements reveal no mass transport limitations for films of up to 10 μm thickness. In contrast, standard photoanodes made of 20 nm-sized TiO₂ particles show non-linear behavior in photocurrent under 1 sun illumination for a film thickness as low as 7 μm. By including a transparent thin mesoporous TiO₂ underlayer in order to reduce optical losses at the fluorine-doped tin oxide (FTO)-TiO₂ interface, an efficiency of 11.4% under AM1.5G 1 sun illumination is achieved. The combination of high surface area, strong scattering behavior, and high porosity makes these mesoporous TiO₂ beads particularly suitable for dye-sensitized solar cells using bulky redox couples and/or viscous electrolytes.

Electron recombination is one of the major loss factors in dye-sensitized solar cells (DSC), especially, with single electron outer sphere redox shuttle electrolyte. Insulating sub-nanometer oxide tunneling layers deposited by atomic layer deposition (ALD) are known to block the electron recombination, thereby leading to an increase in the open-circuit potential and the collection efficiency of the solar cell. A general perception in the DSC community is that any insulating oxide layer can block the recombination. However, in this work, it is unraveled that the insulating property of oxides alone is not sufficient. In addition, the properties such as the conduction band position and the oxidation state of the insulating oxide, the electronic structural modification induced to the underlying TiO₂ mesoporous film, modification of surface charges (isoelectric point) and charge of the electrolyte species have to be considered. A complete photovoltaic study is done by depositing different cycles (by ALD) of four different insulating oxides (Ga₂O₃, ZrO₂, Nb₂O₅, and Ta₂O₅) and their recombination characteristics, surface electronic properties, transport rate, and injection dynamics are investigated with a standard organic dye and Co²⁺/Co³⁺ redox mediator. A comparison is made with the conventional iodide/triiodide electrolyte.

Organic–inorganic hybrid perovskites have attracted attention as successful light harvesting materials for mesoscopic solid-state solar cells and led to record breaking efficiencies. The photovoltaic performance of these devices is greatly dependent on the film morphology, which in turn is dependent on the deposition techniques and subsequent treatments employed. In this work the perovskite film is deposited by spin-coating a precursor solution of PbCl₂ and CH₃NH₃I (1 to 3 molar ratio) in dimethylformamide. Here, the role of the temperature used in the annealing process required to convert the as deposited solution into the perovskite material is investigated. It is found that the conversion requires sufficiently high temperatures to ensure the vaporization of solvent and the crystallization of the perovskite material. However, increasing the annealing temperature too high leads to the additional formation of PbI₂, which is detrimental to the photovoltaic performance. Furthermore, the effect of the annealing temperature on the film formation, morphology, and composition is examined and correlated with the photovoltaic performance and device working mechanisms.

Hybrid organic–inorganic lead halide perovskite APbX₃ pigments, such as methylammonium lead iodide, have recently emerged as excellent light harvesters in solid-state mesoscopic solar cells. An important target for the further improvement of the performance of perovskite-based photovoltaics is to extend their optical-absorption onset further into the red to enhance solar-light harvesting. Herein, we show that this goal can be reached by using a mixture of formamidinium (HN=CHNH₃⁺, FA) and methylammonium (CH₃NH₃⁺, MA) cations in the A position of the APbI₃ perovskite structure. This combination leads to an enhanced short-circuit current and thus superior devices to those based on only CH₃NH₃⁺. This concept has not been applied previously in perovskite-based solar cells. It shows great potential as a versatile tool to tune the structural, electrical, and optoelectronic properties of the light-harvesting materials.

Porphyrins have drawn much attention as sensitizers owing to the large absorption coefficients of their Soret and Q bands in the visible region. In a donor and acceptor zinc porphyrin we applied a new strategy of introducing 2,1,3-benzothiadiazole (BTD) as a π-conjugated linker between the anchoring group and the porphyrin chromophore to broaden the absorption spectra to fill the valley between the Soret and Q bands. With this novel approach, we observed 12.75 % power-conversion efficiency under simulated one-sun illumination (AM1.5G, 100 mW cm⁻²). In this study, we showed the importance of introducing the phenyl group as a spacer between the BTD and the zinc porphyrin in achieving high power-conversion efficiencies. Time-resolved fluorescence, transient-photocurrent-decay, and transient-photovoltage-decay measurements were employed to determine the electron-injection dynamics and the lifetime of the photogenerated charge carriers.

A new phthalocyanine (Pc) bearing bulky peripheral substituents and a carboxylic anchoring group directly attached to the macrocycle has been prepared and used as a sensitizer in DSSCs, reaching 5.57 % power conversion efficiency. In addition, an enhanced performance for the TT40 dye, previously reported by us, was achieved in optimized devices, obtaining a new record efficiency with Pc-sensitized cells.

We designed and synthesized two new zinc porphyrin dyes for dye-sensitized solar cells (DSCs). Subtle molecular structural variation in the dyes significantly influenced the performance of the DSC devices. By utilizing these dyes in combination with a cobalt-based redox electrolyte using a photoanode made of mesoporous TiO₂, we achieved a power conversion efficiency (PCE) of up to 12.0 % under AM 1.5 G (100 mW cm⁻²) simulated solar light. Moreover, we obtained a high PCE of 6.4 % for solid-state dye-sensitized solar cells by using 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene as a hole-transporting material.

Hybrid organic–inorganic lead halide perovskite APbX₃ pigments, such as methylammonium lead iodide, have recently emerged as excellent light harvesters in solid-state mesoscopic solar cells. An important target for the further improvement of the performance of perovskite-based photovoltaics is to extend their optical-absorption onset further into the red to enhance solar-light harvesting. Herein, we show that this goal can be reached by using a mixture of formamidinium (HN=CHNH₃⁺, FA) and methylammonium (CH₃NH₃⁺, MA) cations in the A position of the APbI₃ perovskite structure. This combination leads to an enhanced short-circuit current and thus superior devices to those based on only CH₃NH₃⁺. This concept has not been applied previously in perovskite-based solar cells. It shows great potential as a versatile tool to tune the structural, electrical, and optoelectronic properties of the light-harvesting materials.

Bridged triphenylamine was chosen as the donor unit for metal-free organic sensitizers in dye-sensitized solar cells (DSSCs). The easily constructed alkene linkage between the donor and the spacer improves the molecule's delocalization and causes a large red shift in its absorption peaks. Planarization of the donor and the use of alkene linkage have proven powerful in extending the red light response of the sensitizer, leading to a significant enhancement in photocurrent density of the device. As a result, devices sensitized with target sensitizers LC-2 (η = 7.51%) and LC-3 (η = 8.00%) exhibited more than 70% efficiency increase over devices sensitized with the reference sensitizer LC-1 (η = 4.44%).

A new class of thiocyanate-free Ru(II) sensitizers with 4,4′-dicarboxyvinyl-2,2′-bipyridine anchor and two trans-oriented pyrid-2-yl pyrazolate (or triazolate) functional chromophores is synthesized, characterized, and evaluated in dye-sensitized solar cells (DSCs). Despite their enhanced red response and absorptivity when compared to the parent sensitizer TFRS-2 that possesses standard 4,4′-dicarboxyl-2,2′-bipyridine anchor and shows the best conversion efficiency of η = 9.82%, the newly synthesized carboxyvinyl-pyrazolate sensitizers, TFRS-11–TFRS-13, exhibit inferior performance characteristics in terms of short-circuit current density (J_SC), open-circuit voltage (V_OC), and power conversion efficiency (η), the latter being recorded to be in the range 5.60–7.62%. The reduction in device efficiencies is attributed to a combination of poor packing of these sensitizers on the TiO₂ surface and less positive ground-state oxidation potentials, which, respectively, increase charge recombination with I₃⁻ in electrolytes and impede the regeneration of sensitizers by I⁻ anions. The latter obstacle can be circumvented in part by the replacement of the pyrazolates with triazolates, forming the TFRS-14 sensitizer, which exhibits an improved J_SC, V_OC, and η of 16.4 mAcm⁻², 0.77 V, and 9.02%, respectively.

In this paper, a way of utilizing thin and conformal overlayer of titanium dioxide on an insulating mesoporous template as a photoanode for dye-sensitized solar cells is presented. Different thicknesses of TiO₂ ranging from 1 to 15 nm are deposited on the surface of the template by atomic layer deposition. This systematic study helps unraveling the minimum critical thickness of the TiO₂ overlayer required to transport the photogenerated electrons efficiently. A merely 6-nm-thick TiO₂ film on a 3-μm mesoporous insulating substrate is shown to transport 8 mA/cm² of photocurrent density along with ≈900 mV of open-circuit potential when using our standard donor-π-acceptor sensitizer and Co(bipyridine) redox mediator.

A series of squaraine-based sensitizers with various π bridges and anchors were prepared and examined in dye-sensitized solar cells. The carboxylic anchor group was attached onto a squaraine dye through π bridges with and without an ethynyl spacer. DFT studies indicate that the LUMO is delocalized throughout the dyes, whilst the HOMO resides on the squaraine core. The dye that incorporates a 4,4-di-n-hexyl-cyclopentadithiophene group that is directly attached onto the π bridge, JD10, exhibits the highest power conversion efficiency in a DSC; this result is attributed, in part, to the deaggregative properties that are associated with the gem-di-n-hexyl substituents, which extend above and below the π-conjugated dye plane. Dye JD10 demonstrates a power-conversion efficiency of 7.3 % for liquid-electrolyte dye-sensitized solar cells and 7.9 % for cells that are co-sensitized by another metal-free dye, D35, which substantially exceed the performance of any previously tested squaraine sensitizer. A panchromatic incident-photon-to-current-conversion efficiency curve is realized for this dye with an excellent short-circuit current of 18.0 mA cm⁻². This current is higher than that seen for other squaraine dyes, partially owing to a high molar absorptivity of >5 000 M⁻¹ cm⁻¹ from 400 nm to the long-wavelength onset of 724 nm for dye JD10.

Two donor-π-acceptor (D-π-A) dyes are synthesized for application in dye-sensitized solar cells (DSSC). These D-π-A sensitizers use triphenylamine as donor, oligothiophene as both donor and π-bridge, and benzothiadiazole (BTDA)/cyanoacrylic acid as acceptor that can be anchored to the TiO₂ surface. Tuning of the optical and electrochemical properties is observed by the insertion of a phenyl ring between the BTDA and cyanoacrylic acid acceptor units. Density functional theory (DFT) calculations of these sensitizers provide further insight into the molecular geometry and the impact of the additional phenyl group on the photophysical and photovoltaic performance. These dyes are investigated as sensitizers in liquid-electrolyte-based dye-sensitized solar cells. The insertion of an additional phenyl ring shows significant influence on the solar cells' performance leading to an over 6.5 times higher efficiency (η = 8.21%) in DSSCs compared to the sensitizer without phenyl unit (η = 1.24%). Photophysical investigations reveal that the insertion of the phenyl ring blocks the back electron transfer of the charge separated state, thus slowing down recombination processes by over 5 times, while maintaining efficient electron injection from the excited dye into the TiO₂-photoanode.

Two donor–acceptor molecular tweezers incorporating the 10-(1,3-dithiol-2-ylidene)anthracene unit as donor group and two cyanoacrylic units as accepting/anchoring groups are reported as metal-free sensitizers for dye-sensitized solar cells. By changing the phenyl spacer with 3,4-ethylenedioxythiophene (EDOT) units, the absorption spectrum of the sensitizer is red-shifted with a corresponding increase in the molar absorptivity. Density functional calculations confirmed the intramolecular charge-transfer nature of the lowest-energy absorption bands. The new dyes are highly distorted from planarity and are bound to the TiO₂ surface through the two anchoring groups in a unidentate binding form. A power-conversion efficiency of 3.7 % was obtained with a volatile CH₃CN-based electrolyte, under air mass 1.5 global sunlight. Photovoltage decay transients and ATR-FTIR measurements allowed us to understand the photovoltaic performance, as well as the surface binding, of these new sensitizers.

Dye-sensitized solar cells based on electrolytes containing cobalt complexes as redox shuttles typically suffer a major limitation in terms of slow diffusion of those couples through the mesoporous TiO₂ film. This results in a drop of the photocurrent density, particularly at high incident light intensities, reducing the overall cell performance. This work illustrates how tuning the four characteristic parameters of the mesoporous TiO₂ layer, namely film thickness, particle size, pore size and porosity, by simply optimizing the TiCl₄ post-treatment, completely eliminates diffusion problems of cobalt(III/II) tris(2,2′-bipyridine) and at the same time maximizes the short-circuit photocurrent density. As a result, a power conversion efficiency of 10.0 % at AM 1.5 G 100 mW cm⁻² was reached in conjunction with an organic sensitizer.

The use of mixed self-assembled monolayers, combining hydrophobic co-adsorbents with the sensitizer, has been demonstrated to enhance the efficiency of dye-sensitized solar cells (DSCs). Herein, the influence of the anchoring groups of the co-adsorbents on the performance of the DSCs is carefully examined by selecting two model molecules: neohexyl phosphonic acid (NHOOP) and bis-(3,3-dimethyl-butyl)-phosphinic acid (DINHOP). The effect of these co-adsorbents on the photovoltaic performance (J–V curves, incident photon-to-electron conversion efficiency) is investigated. Photoelectron spectroscopy and electrochemical impedance spectroscopy are performed to assess the spatial configuration of adsorbed dye and co-adsorbent molecules. The photoelectron spectroscopy studies indicate that the ligands of the ruthenium complex, containing thiophene groups, point out away from the surface of TiO₂ in comparison with the NCS group.

A novel heteroleptic Ru^II complex (BTC-2) employing 5,5′-(2,2′-bipyridine-4,4′-diyl)-bis(thiophene-2-carboxylic acid) (BTC) as the anchoring group and 4,4′- dinonyl-2,2′-bipiridyl and two thiocyanates as ligands is prepared. The photovoltaic performance and device stability achieved with this sensitizer are compared to those of the Z-907 dye, which lacks the thiophene moieties. For thin mesoporous TiO₂ films, the devices with BTC-2 achieve higher power conversion efficiencies than those of Z-907 but with a double-layer thicker film the device performance is similar. Using a volatile electrolyte and a double layer 7 + 5 μm mesoporous TiO₂ film, BTC-2 achieves a solar-to-electricity conversion efficiency of 9.1% under standard global AM 1.5 sunlight. Using this sensitizer in combination with a low volatile electrolyte, a photovoltaic efficiency of 8.3% is obtained under standard global AM 1.5 sunlight. These devices show excellent stability when subjected to light soaking at 60 °C for 1000 h. Electrochemical impedance spectroscopy and transient photovoltage decay measurements are performed to help understand the changes in the photovoltaic parameters during the aging process. In solid state dye-sensitized solar cells (DSSCs) using an organic hole-transporting material (spiro-MeOTAD, 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene), the BTC-2 sensitizer exhibits an overall power conversion efficiency of 3.6% under AM 1.5 solar (100 mW cm⁻²) irradiation.

Panchromatic response is essential to increase the light-harvesting efficiency in solar conversion systems. Herein we show increased light harvesting from using multiple energy relay dyes inside dye-sensitized solar cells. Additional photoresponse from 400–590 nm matching the optical window of the zinc phthalocyanine sensitizer was observed due to Förster resonance energy transfer (FRET) from the two energy relay dyes to the sensitizing dye. The complementary absorption spectra of the energy relay dyes and high excitation transfer efficiencies result in a 35 % increase in photovoltaic performance.

A promising route to increase the performance of hematite (α-Fe₂O₃) photoelectrodes for solar hydrogen production through water-splitting is to use an extremely thin layer of this visible light absorber on a nanostructured scaffold. However, the typically poor performance of ultrathin (ca. 20 nm) films of hematite has been the limiting factor in implementing this approach. Here, the surprising effect of a substrate pretreatment using tetraethoxysilicate (TEOS) is reported; it results in drastic improvements in the photoperformance of 12.5 nm thick films of hematite. These films exhibit a water oxidation photocurrent onset potential at 1.1 V versus the reversible hydrogen electrode (vs. RHE) and a plateau current of 0.63 mA cm⁻² at 1.5 V vs. RHE under standard illumination conditions, representing the highest reported performance for ultrathin hematite films. In contrast, almost no photoactivity is observed for the photoanode with the same amount of hematite on an untreated substrate. A detailed study of the effects of the TEOS treatment shows that a monolayer of SiO_x is formed, which acts to change the hematite nucleation and growth mechanism, increases its crystallinity, reduces the concentration of carrier trapping states of the ultrathin films, and suggests its further application to quantum-dot and extremely-thin-absorber (ETA)-type solar cells.

A ruthenium sensitizer (coded C101, NaRu (4,4′-bis(5-hexylthiophen-2-yl)-2,2′-bipyridine) (4-carboxylic acid-4′-caboxylate-2,2′-bipyridine) (NCS)₂) containing a hexylthiophene-conjugated bipyridyl group as an ancillary ligand is presented for use in solid-state dye-sensitized solar cells (SSDSCs). The high molar-extinction coefficient of this dye is advantageous compared to the widely used Z907 dye, (NaRu (4-carboxylic acid-4′-carboxylate) (4,4′-dinonyl-2,2′-bipyridine) (NCS)₂). In combination with an organic hole-transporting material (spiro-MeOTAD, 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine) 9, 9′-spirobifluorene), the C101 sensitizer exhibits an excellent power-conversion efficiency of 4.5% under AM 1.5 solar (100 mW cm⁻²) irradiation in a SSDSC. From electronic-absorption, transient-photovoltage-decay, and impedance measurements it is inferred that extending the π-conjugation of spectator ligands induces an enhanced light harvesting and retards the charge recombination, thus favoring the photovoltaic performance of a SSDSC.

Nanoporous layers of poly(3,4-propylenedioxythiophene) (PProDOT) were fabricated by electrical-field-assisted growth using hydrophobic ionic liquids as the growing medium. A series of PProDoT layers was prepared with three different ionic liquids to control the microstructure and electrochemical properties of the resulting dye-sensitized solar cells, which were highly efficient and showed a power conversion efficiency of >9 % under different sunlight intensities. The current–voltage characteristics of the counter electrodes varied depending on the ionic liquids used in the synthesis of PProDOT. The most hydrophobic ionic liquids exhibited high catalytic properties, thus resulting in high power conversion efficiency and allowing the fabrication of platinum-free, stable, flexible, and cost-effective dye-sensitized solar cells.

In solid-state dye sensitized solar cells (SSDSCs) charge recombination at the dye-hole transporting material interface plays a critical role in the cell efficiency. For the first time we report on the influence of dipolar co-adsorbents on the photovoltaic performance of sensitized hetero-junction solar cells. In the present study, we investigated the effect of two zwitterionic butyric acid derivatives differing only in the polar moiety attached to their common 4 carbon-chain acid, i.e., 4-guanidinobutyric acid (GBA) and 4-aminobutyric acid (ABA). These two molecules were implemented as co-adsorbents in conjunction with Z907Na dye on the SSDSC. It was found that a Z907Na/GBA dye/co-adsorbent combination increases both the open circuit voltage (V_oc) and short-circuit current density (J_sc) as compared to using Z907Na dye alone. The Z907Na/ABA dye/co-adsorbent combination increases the J_sc. Impedance and transient photovoltage investigations elucidate the cause of these remarkable observations.

Ionic liquids have been identified as a new class of solvent that offers opportunities to move away from the traditional solvents. The physical-chemical properties of ionic liquids can be tuned and controlled by tailoring their structures. The typical properties of ionic liquids, such as non-volatility, electrochemical stability and high conductivity, render them attractive as electrolytes for dye-sensitized solar cells. However, the high viscosity of ionic liquids leads to mass transport limitations on the photocurrents in the solar cells at full sunlight intensity, but the contribution of a Grotthous-type exchange mechanism in these viscous electrolytes helps to alleviate these diffusion problems. This article discusses recent developments in the field of high-performance dye-sensitized solar cells with ionic liquid-based electrolytes and their characterization by electrochemical impedance analysis.

Lead sulfide (PbS) and cadmium sulfide (CdS) quantum dots (QDs) are prepared over mesoporous TiO₂ films by a successive ionic layer adsorption and reaction (SILAR) process. These QDs are exploited as a sensitizer in solid-state solar cells with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) as a hole conductor. High-resolution transmission electron microscopy (TEM) images reveal that PbS QDs of around 3 nm in size are distributed homogeneously over the TiO₂ surface and are well separated from each other if prepared under common SILAR deposition conditions. The pore size of the TiO₂ films and the deposition medium are found to be very critical in determining the overall performance of the solid-state QD cells. By incorporating promising inorganic QDs (PbS) and an organic hole conductor spiro-OMeTAD into the solid-state cells, it is possible to attain an efficiency of over 1% for PbS-sensitized solid-state cells after some optimizations. The optimized deposition cycle of the SILAR process for PbS QDs has also been confirmed by transient spectroscopic studies on the hole generation of spiro-OMeTAD. In addition, it is established that the PbS QD layer plays a role in mediating the interfacial recombination between the spiro-OMeTAD⁺ cation and the TiO₂ conduction band electron, and that the lifetime of these species can change by around 2 orders of magnitude by varying the number of SILAR cycles used. When a near infrared (NIR)-absorbing zinc carboxyphthalocyanine dye (TT1) is added on top of the PbS-sensitized electrode to obtain a panchromatic response, two signals from each component are observed, which results in an improved efficiency. In particular, when a CdS-sensitized electrode is first prepared, and then co-sensitized with a squarine dye (SQ1), the resulting color change is clearly an addition of each component and the overall efficiencies are also added in a more synergistic way than those in PbS/TT1-modified cells because of favorable charge-transfer energetics.

We report a new type of dendritic terthiophene attached to a 2,2’-bipyridine ligand for ruthenium-based dye-sensitized solar cells. As a result of the incorporation of this electron-rich terthiophene donor unit into the heteroleptic ruthenium sensitizer [Ru(dcbpy)(3Tbpy)(NCS)₂] (3T), the lowest-energy metal-to-ligand charge transfer (MLCT) transition is red-shifted and the molar extinction coefficient is increased compared to the analogous standard Z907Na sensitizer. A preliminary investigation of the 3T dye in combination with an iodine/iodide-based electrolyte shows an interesting photovoltaic performance, with a maximum power conversion efficiency of 7.4 % measured under air mass 1.5 global sunlight irradiation. Accelerated testing of these devices at 60 °C under full sunlight soaking shows a remarkable stability over 1000 h.

Electrochemical impedance spectroscopy (EIS) and transient voltage decay measurements are applied to compare the performance of dye sensitized solar cells (DSCs) using organic electrolytes, ionic liquids and organic-hole conductors as hole transport materials (HTM). Nano-crystalline titania films sensitized by the same heteroleptic ruthenium complex NaRu(4-carboxylic acid-4′-carboxylate) (4,4′-dinonyl-2,2′-bipyridyl)(NCS)₂ , coded Z-907Na are employed as working electrodes. The influence of the nature of the HTM on the photovoltaic figures of merit, that is, the open circuit voltage, short circuit photocurrent and fill factor is evaluated. In order to derive the electron lifetime, as well as the electron diffusion coefficient and charge collection efficiency, EIS measurements are performed in the dark and under illumination corresponding to realistic photovoltaic operating conditions of these mesoscopic solar cells. A theoretical model is established to interpret the frequency response off the impedance under open circuit conditions, which is conceptually similar to photovoltage transient decay measurements. Important information on factors that govern the dynamics of electron transport within the nanocrystalline TiO₂ film and charge recombination across the dye sensitized heterojunction is obtained.

Dye-sensitized solar cells based on nanocrystalline TiO₂ have been fabricated with an amphiphilic ruthenium sensitizer NaRu(4-carboxylic acid-4′-carboxylate)(4,4′-dinonyl-2,2′-bipyridine)(NCS)2, coded as Z-907Na, and a series of non-volatile 3-methoxyproprionitrile (MPN)-based electrolytes with different concentration of 1-methyl-3-propylimidazolium iodide (PMII). The short-circuit photocurrent density increases with increasing iodide concentration until at 1.5 M practically quantitative dye regeneration is achieved as proved by time-resolved laser experiments. Devices containing 1.0 M PMII electrolyte show excellent stability during long-time thermal aging at 80 °C and under light soaking at 60 °C.

A promising redox system, offering an alternative to the widely used iodide/triiodide couple, based on the stable organic radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) has been employed in dye-sensitized solar cells. The photovoltaic performance of this new redox couple has been evaluated by employing nanocrystalline TiO₂ films with different thickness. Judicious selections of a 5.0 μm photoanode made from TiO₂ mesoscopic particles and an organic sensitizer with a high molar extinction coefficient yield an overall solar-to-electric power conversion efficiency of 5.4 % under AM 1.5 illumination at 100 mW cm^–2, which is unprecedented for an iodine-free mediator system.