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Organic-inorganic lead halide perovskites containing a mixture of iodide and bromide anions consistently perform better in donor-acceptor heterojunction solar cells than the standard methylammonium lead triiodide material. This observation is counterintuitive, as it is generally expected for photovoltaic materials that heterogeneities and compositional disorder cause reduced carrier diffusion length and conversion efficiency. Here, we combine ultrafast photoinduced electroabsorption and broadband fluorescence up-conversion spectroscopy measurements to scrutinize the carrier dynamics in mixed-cations, mixed-halide lead perovskite thin films. Our results evidence the formation of charge transfer excitons (CTE) across the boundaries of domains of various halide compositions. A global analysis of photoinduced transient Stark signals shows that CTE evolve gradually from Br-rich to I-rich domains over tens to hundreds of picoseconds. Rather than constituting recombination centres, boundaries between domains of various halide compositions appear then to favour charge carrier separation by driving photogenerated holes along channels of decreasing bromide content., Comment: 10 pages, 9 figures

Lead halide perovskites have over the past few years attracted considerable interest as photo absorbers in PV applications with record efficiencies now reaching 22%. It has recently been found that not only the composition but also the precise stoichiometry is important for the device performance. Recent reports have, for example, demonstrated small amount of PbI2 in the perovskite films to be beneficial for the overall performance of both the standard perovskite, CH3NH3PbI3, as well as for the mixed perovskites (CH3NH3)x(CH(NH2)2)(1–x)PbBryI(3–y). In this work a broad range of characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photo electron spectroscopy (PES), transient absorption spectroscopy (TAS), UV–vis, electroluminescence (EL), photoluminescence (PL), and confocal PL mapping have been used to further understand the importance of remnant PbI2 in perovskite solar cells. Our best devices were over 18% efficient, and had in line with previous results a small amount of excess PbI2. For the PbI2-deficient samples, the photocurrent dropped, which could be attributed to accumulation of organic species at the grain boundaries, low charge carrier mobility, and decreased electron injection into the TiO2. The PbI2-deficient compositions did, however, also have advantages. The record Voc was as high as 1.20 V and was found in PbI2-deficient samples. This was correlated with high crystal quality, longer charge carrier lifetimes, and high PL yields and was rationalized as a consequence of the dynamics of the perovskite formation. We further found the ion migration to be obstructed in the PbI2-deficient samples, which decreased the JV hysteresis and increased the photostability. PbI2-deficient synthesis conditions can thus be used to deposit perovskites with excellent crystal quality but with the downside of grain boundaries enriched in organic species, which act as a barrier toward current transport. Exploring ways to tune the synthesis conditions to give the high crystal quality obtained under PbI2-poor condition while maintaining the favorable grain boundary characteristics obtained under PbI2-rich conditions would thus be a strategy toward more efficiency devices.

PBEsol-optimized structures of AVA2FAn–1PbnI3n+1, n = 1, 2, and 3 PBEsol-optimized structure of alpha-FAPbI3 Thermal distributions at T = 300 K from 6 ps first-principles MD simulations corresponding to Fig. 6 Histogram of intermolecular O···H bond distances between COOH groups of adjacent spacers of opposite layers Distribution of vertical penetration distances of NH3+ into the inorganic slab Distribution of polar, θ, and azimuthal, φ, angles of AVA in the spacer layers.

A series of new pyrido[3,4-b]pyrazine-based organic sensitizers (PP-I and APP-I–IV) containing different donors and p-spacers have been synthesized and employed in dye-sensitized solar cells (DSSCs). The absorption spectra properties of dyes are analysed by density functional theory (DFT). The calculated results in combination with the experiments suggest that the absorption characteristics and excited state features will mainly be dominated by charge transfer transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and to higher LUMO orbitals. Furthermore, attaching the octyloxy groups significantly extends the π-conjugation of the donor in APP- IV, which raises the HOMO energy and facilitates its oxidation. As a consequence, APP-IV exhibits the lowest HOMO–LUMO energy gap among all dyes, which, in turn, corresponds well with the red shift of the absorption spectra. Transient photovoltage and photocurrent decay experiments as well as electrochemical impedance spectroscopy indicate that the electron lifetime and charge recombination resistance are increased due to the introduction of octyloxy chains on the donor unit, resulting in the high photovoltage based on APP-IV. It was found that APP-IV based DSSCs with liquid electrolyte display the highest power conversion efficiency (PCE) of 7.12%. Importantly, a PCE of 6.20% has been achieved for APP-IV based DSSCs with ionic-liquid electrolytes and retained 97% of the initial value after continuous light soaking for 1000 h at 60 C. This renders these pyrido[3,4-b]pyrazine-based sensitizers quite promising candidates for highly efficient and stable DSSCs.

Dye-sensitized solar cells (DSSCs) based on Cu-II/I bipyridyl or phenanthroline complexes as redox shuttles have achieved very high open-circuit voltages (V-OC, more than 1 V). However, their short-circuit photocurrent density (J(SC)) has remained modest. Increasing the J(SC) is expected to extend the spectral response of sensitizers to the red or NIR region while maintaining efficient electron injection in the mesoscopic TiO2 film and fast regeneration by the Cu-I complex. Herein, we report two new D-A-pi-A-featured sensitizers termed HY63 and HY64, which employ benzothiadiazole (BT) or phenanthrene-fused-quinoxaline (PFQ), respectively, as the auxiliary electron-withdrawing acceptor moiety. Despite their very similar energy levels and absorption onsets, HY64-based DSSCs outperform their HY63 counterparts, achieving a power conversion efficiency (PCE) of 12.5 %. PFQ is superior to BT in reducing charge recombination resulting in the near-quantitative collection of photogenerated charge carriers.

Cyclometalated ruthenium sensitizers have been synthesized that differ with number of thiophene units on the auxiliary ligands. Sensitizers possessing four (SA25, SA246, and SA285) or none (SA282) sulfur atoms in their structures, were tested in solar cell devices employing I3−/I− redox mediator, enabling an estimation of the influence of sulfur−iodine/iodide interactions on dye-sensitized solar cell (DSC) performance. Power conversion efficiencies over 6% under simulated AM 1.5 illumination (1 Sun) were achieved with all the sensitizers. Consistently higher opencircuit voltage (VOC) and fill factor (FF) values were measured using SA282. Scrutinizing the DSCs with these dyes by transient absorption spectroscopy (TAS) and electrochemical impedance spectroscopy (EIS) indicate that sulfur atom induced recombination cancels favorable increased regeneration resulting in decreased power conversion efficiencies (PCEs). The data indicate that, to reduce charge recombination channels, the use of sulfur-containing aromatic rings should be avoided if possible in the dye structure when I3−/I− redox mediator is used.

The outstanding photovoltaic performances of lead halide perovskite (LHP) thin film solar cells are due in particular to the large diffusion length of photocarriers. The mechanism behind this property and its dependence on the various anions and cations in the LHP material composition are still an active topic of debate. Here, we apply ultrabroadband terahertz spectroscopy with a time resolution < 200 fs to probe the early carrier mobility dynamics of photoexcited perovskite samples of different chemical compositions. An increase in the carrier mobility with time constants in the range of 0.3 to 0.7 ps is observed for different LHPs, in which the signal amplitude is larger at shorter excitation wavelengths. This feature is assigned to the relaxation of hot carriers from low-curvature regions of the band structure to the bottom of the valley, where their effective mass accounts for the maximum mobility. The dependence of the mobility dynamics on the initial photogenerated carrier population shows that carrier cooling competes with a dynamic screening effect associated with polaron formation and relaxation. A kinetic model that considers all competing processes is proposed. Simulations based on this model allow us to fit the experimental data well and evidence a composition dependence of the dynamic screening lifetime., Main text: 17 pages, 3 figures, 2 schemes, 2 tables. Attached supplementary information: 6 pages, 4 figures

We present a study of the kinetics of electron injection in ruthenium(II) cis-(2,2‘-bipyridyl-4,4‘-dicarboxylate)2(NCS)2-sensitized nanocrystalline TiO2 films as a function of electrical potential applied to the TiO2 film and as a function of the composition of the electrolyte in which the film is immersed. At moderate applied potentials (−0.2 V vs Ag/AgCl), and in the presence of potential determining ions (0.1 M Li+) in the electrolyte, the electron injection kinetics were found to be multiphasic, with a half time for electron injection of 500 fs. These injection kinetics were retarded by either the omission of potential determining ions or the application of more negative potentials. Omission of Li+ ions from the electrolyte resulted in a 7-fold retardation of the injections kinetics. The application of −0.7 V to the TiO2 electrode resulted in a 25-fold retardation of the injection kinetics. These observations are discussed in terms of nonadiabatic interfacial electron transfer theory. The retardation of the injection kinetics in the absence of potential determining ions is attributed to the influence of these ions upon the electronic density of states of the TiO2 electrode. The retardation of the injection kinetics at negative applied potentials is attributed to the increased occupancy of this density of states. Fits to the potential dependence of the injection kinetics following nonadiabatic theory yield a reorganizational energy for the electron injection process of 0.25 ± 0.05 eV.

The photophysical properties of the triple-stranded dimetallic helicates [Ln2(LC−2H)3]·H2O (Ln = Nd, Sm, Dy, Yb) are determined in water and D2O solutions, and energy transfer processes are modeled for SmIII. The luminescence of NdIII, SmIII, and YbIII is sensitized by (LC−2H)2-, but the energy transfer from the ligand to the LnIII ions is not complete, resulting in residual ligand emission. The luminescence of the NdIII helicate is very weak due to nonradiative de-excitation processes. On the other hand, the YbIII and SmIII helicates exhibit fair quantum yields, 1.8% and 1.1% in deuterated water, respectively. The energy transfer rates between (LC−2H)2- and SmIII levels are calculated by direct and exchange Coulomb interaction models. This theoretical modeling coupled to numerical solutions of the rate equations leads to an estimate of the emission quantum yields in H2O and D2O, which compares favorably with experimental data. The main component of the ligand-to-metal energy transfer (97.5%) goes through a 3ππ* → 5G5/2(1) path, and the operative mechanism is of the exchange type. For the YbIII helicate, minor effects of oxygen on the sensitization of YbIII and nanosecond time-resolved spectroscopy point to the energy transfer mechanism being consistent with a recently proposed pathway involving fast electron transfer and YbII. No up-conversion process could be identified. Ligand-field splitting of the 2F5/2 (3E1/2 + E3/2) and 2F7/2 (2E1/2 + E3/2) levels of YbIII is consistent with D3 symmetry.

Copper indium sulfide quantum dots (QDs) have attracted substantial attention in recent years due to environmental issues and diverse applications. We report the synthesis and characterization of copper-zinc-indium-sulfide (CZIS) QDs and CZIS treated with excess Zn2+ at different temperatures, denoted here as CZIS/ZnS 100 and CZIS/ZnS 200. Zn2+ can diffuse into the lattice by an exchange-cation reaction, replacing Cu+ and In3+. We employed transient absorption (TA) spectroscopy to study the role of Zn2+ in the lattice. The data were treated by global analysis, which yielded the decay-associated spectra (DAS). Through DAS and second-order derivative absorption spectra, we could for the first time isolate the excitonic contribution from the metal–ligand conduction band charge transfer (MLCBCT) absorption in the TA spectrum, finding the hole localization lifetime by its spectral and dynamical features. The hole localization lifetime increases from 0.3 to 1.7 ps by increasing the Zn2+ concentration. We also measured the electron-trapping constants to be dozens of picoseconds and nonradiative recombination larger than 1 ns. Finally, we concluded that suppression of the electron-trapping rate is not the only process responsible for increasing the photoluminescence quantum yield (PLQY); however, suppression of this process is important since it is the first step of the nonradiative recombination pathway. The detailed mechanism was proposed, and our results suggest that the introduction of Zn2+ in the lattice of copper indium sulfide (CIS) QDs could be beneficial for charge extraction.

The high conversion efficiency has made metal halide perovskite solar cells a real breakthrough in thin film photovoltaic technology in recent years. Here, we introduce a straightforward strategy to reduce the level of electronic defects present at the interface between the perovskite film and the hole transport layer by treating the perovskite surface with different types of ammonium salts, namely ethylammonium, imidazolium and guanidinium iodide. We use a triple cation perovskite formulation containing primarily formamidinium and small amounts of cesium and methylammonium. We find that this treatment boosts the power conversion efficiency from 20.5% for the control to 22.3%, 22.1%, and 21.0% for the devices treated with ethylammonium, imidazolium and guanidinium iodide, respectively. Best performing devices showed a loss in efficiency of only 5% under full sunlight intensity with maximum power tracking for 550 h. We apply 2D- solid-state NMR to unravel the atomic-level mechanism of this passivation effect.

Solar cells that operate efficiently under indoor lighting are of great practical interest as they can serve as electric power sources for portable electronics and devices for wireless sensor networks or the Internet of Things. Here, we demonstrate a dye-sensitized solar cell (DSC) that achieves very high power-conversion efficiencies (PCEs) under ambient light conditions. Our photosystem combines two judiciously designed sensitizers, coded D35 and XY1, with the copper complex Cu(II/I)(tmby) as a redox shuttle (tmby, 4,4′,6,6′-tetramethyl-2,2′-bipyridine), and features a high open-circuit photovoltage of 1.1 V. The DSC achieves an external quantum efficiency for photocurrent generation that exceeds 90% across the whole visible domain from 400 to 650 nm, and achieves power outputs of 15.6 and 88.5 mW cm–2 at 200 and 1,000 lux, respectively, under illumination from a model Osram 930 warm-white fluorescent light tube. This translates into a PCE of 28.9%.

Lead halide perovskites have emerged as promising materials for light-emitting devices. Here, we report the preparation of colloidal CsPbBr3 nanoplatelets (3 × 4 × 23 nm3) experiencing a strong quasi-one-dimensional quantum confine- ment. Ultrafast transient absorption and broadband fluorescence up-conversion spectroscopies were employed to scrutinize the carrier and quasiparticle dynamics and to obtain a full description of the spectroscopic properties of the material. An exciton binding energy of 350 meV, an absorption cross section at 3.2 eV of 5.0 ± 0.3 × 10−15 cm−2, an efficient biexciton Auger recombination lifetime of 9 ± 1 ps, and a biexciton binding energy of 74 ± 4 meV were determined. Moreover, a short-lived emission from hot excitons was observed, which is related to the formation of band-edge excitons. The time constant of both processes is 300 ± 50 fs. These results show that CsPbBr3 nanoplatelets are indeed quite promising for light-emitting technological applications.

The mechanism of photoinduced symmetry-breaking charge separation in solid cyanine salts at the base of organic photovoltaic and optoelectronic devices is still debated. Here, we employ femtosecond transient absorption spectroscopy (TAS) to monitor the charge transfer processes occurring in thin films of pristine pentamethine cyanine (Cy5). Oxidized dye species are observed in Cy5-hexafluorophosphate salts upon photoexcitation, resulting from electron transfer from monomer excited states to H-aggregates. The charge separation proceeds with a quantum yield of 86%, providing the first direct proof of high efficiency intrinsic charge generation in organic salt semiconductors. The impact of the size of weakly coordinating anions on charge separation and transport is studied using TAS alongside electroabsorption spectroscopy and time-of-flight techniques. The degree of H-aggregation decreases with increasing anion size, resulting in reduced charge transfer. However, there is little change in carrier mobility, as despite the interchromophore distance increasing, the decrease in energetic disorder helps to alleviate the trapping of charges by H-aggregates.

One of the most salient features of hybrid lead halide perovskites is the extended lifetime of their photogenerated charge carriers. This property has now been shown experimentally to originate from a slow, thermally activated recombination process.

In the recent years, the heterojunction solar cells based on quantum dots (QDs) have attracted attention due to strong light absorbing characteristics and the size effect on the bandgap tuning. This paper reports on the kinetics of interfacial charge separation of PbS QDs/(001) TiO2 nanosheets heterojunction solar cells. PbS QDs are deposited using a bifunctional linker molecule on two different TiO2 films, i.e., TiO2 nanosheets (with 001 dominant exposed facet) and TiO2 nanoparticles (with 101 dominant exposed facet). Upon bandgap excitation, electrons are transferred from the PbS QDs conduction band to the lower lying conduction band of TiO2. Based on the ultrafast pump-probe laser spectroscopy technique, the kinetics of charge separation is scrutinized at the PbS/TiO2 interface. The interfacial charge separation at PbS/TiO2 nanosheets films made of (001) dominant exposed facets is five times faster than that on (101) dominant exposed facets TiO2 nanoparticles. The quantum yields for charge injection are higher for the (001) TiO2 nanosheets than the (101) TiO2 nanoparticles due to enhanced interfacial interaction with (001) surface compared to the (101) nanoparticles. The superior interfacial charge separation at PbS/(001) nanosheets respect to PbS/(101) nanoparticles is consistent with the higher photocurrent and enhanced power conversion efficiency in the PbS QDs/(001) TiO2 heterojunction solar cell. The use of (001) TiO2 nanosheets can be a better alternative to conventional mesoporous TiO2 films in QD heterojunction solar cells and perovskites-based heterojunction solar cells.

The relatively large voltage loss (Vloss) in excitonic type solar cells severely limits their power conversion efficiencies (PCEs). Here, we report a comprehensive control of Vloss through efficacious engineering of the sensitizer and redox mediator, making a breakthrough in the PCE of dye-sensitized solar cells (DSSCs). The targeted down-regulation of Vloss is successfully realized by three valid channels: (i) reducing the driving force of electron injection through dye molecular engineering, (ii) decreasing the dye regeneration overpotential through redox mediator engineering, and (iii) suppressing interfacial electron recombination. Significantly, the ‘‘trade-off’’ effect between the dye optical band gap and the open-circuit voltage (VOC) is minimized to a great extent, achieving a distinct enhancement in photovoltaic performance (PCE 4 11.5% with VOC up to 1.1 V) for liquid junction cells. The solidification of the best-performing device leads to a PCE of 11.7%, which is so far the highest efficiency obtained for solid-state DSSCs. Our work inspires further development in highly efficient excitonic solar cells by comprehensive control of Vloss.

Nanostructured TiO2 hollow fibers have been prepared using natural cellulose fibers as a template. This cheap and easily processed material was used to produce highly porous photoanodes incorporated in dye-sensitized solar cells and exhibited remarkably enhanced electron transport properties compared to mesoscopic films made of spherical nanoparticles. Photoinjected electron lifetime, in particular, was multiplied by 3-4 in the fiber morphology, while the electron transport rate within the fibrous photoanaode was doubled. A nearly quantitative absorbed photon-to-electrical current conversion yield exceeding 95% was achieved upon excitation at 550 nm and a photovoltaic power conversion efficiency of 7.2% reached under simulated AM 1.5 (100 mW cm(-2)) solar illumination.

The photoelectrochemical properties of TiO2 anatase nanoparticles were studied at the water|1,2-dichloroethane (DCE) interface under potentiostatic control. The interfacial concentration of the electrostatically stabilized particles can be effectively tuned by the Galvani potential difference and the pH of the aqueous phase. At pH values lower than that corresponding to the point of zero zeta potential, the particles assemble at the liquid|liquid boundary region upon positive polarization with respect to the Galvani potential in the organic phase. Upon band-gap illumination, photocurrent responses are observed in the presence of ferrocene in the organic phase. The potential and wavelength dependencies of the photocurrent unambiguously reveal that the photoresponses arises from the transfer of valence-band holes to the redox couple in DCE. The hole transfer is mediated via the generation of OHs• at the particle surface. On the other hand, photocurrents of the opposite sign were observed in the presence of TCNQ at pH's higher than 10. In this pH range, the particles are negatively charged, and the formation of the interfacial assembly takes place upon negative polarization with respect to the organic phase. The effect of the pH and the Galvani potential difference on the photocurrent suggests that heterogeneous charge transfer is in effective competition with recombination via OHs• as well as oxygen evolution at the particle surface. These studies open new possibilities for contactless photoelectrochemical and spectroscopic characterization of nanoparticles in solution in the presence of an electric field.

PBEsol-optimized structures of AVA2FAn–1PbnI3n+1,n= 1, 2, and 3 PBEsol-optimized structure of alpha-FAPbI3 Thermal distributions atT= 300 K from 6 psfirst-principles MD simulations corresponding to Fig. 6 Histogram of intermolecular O···Hbond distances between COOH groups of adjacent spacers of opposite layers Distribution of vertical penetration distances of NH3+into the inorganic slab Distribution of polar,θ, and azimuthal,φ, angles of AVA in the spacer layers. &nbsp

Strongly confined, fully inorganic cesium lead halide perovskite nanocrystals are of great promise for light-emitting devices in the blue spectral range owing to their high photoluminescence quantum yields. A combination of broadband fluorescence up-conversion and transient absorption spectroscopies was used to study early exciton dynamics in quasi-1D CsPbBr3 nanoplatelets (3 × 4 × 23 nm3, NPls). This allowed to reveal emitting band-edge states in the NPls that form instantaneously upon photoexcitation and then relax within the first picosecond to lower energy confined hole states (CHSs). The influence of the pump excitation intensity on the latter process was further scrutinized. The band-edge population lifetime was found to increase with the rise of the photon fluence due to CHS filling. When the concentration of NPls in solution becomes very high, nanoparticles overlap, resulting in a decrease in their external absorption cross-section and an increase in the emission Stokes shift due to photon reabsorption.

Identifying and characterizing systems that generate well-defined states with large electron spin polarization is of high interest for applications in molecular spintronics, high-energy physics, and magnetic resonance spectroscopy. The generation of electron spin polarization on free-radical substituents tethered to pentacene derivatives has recently gained a great deal of interest for its applications in molecular electronics. After photoexcitation of the chromophore, pentacene-radical derivatives can rapidly form spin-polarized triplet excited states through enhanced intersystem crossing. Under the right conditions, the triplet spin polarization, arising from m(s)-selective intersystem crossing rates, can be transferred to the tethered stable radical. The efficiency of this spin polarization transfer depends on many factors: local magnetic and electric fields, excited-state energetics, molecular geometry, and spin-spin coupling. Here, we present transient electron paramagnetic resonance (EPR) measurements on three pentacene derivatives tethered to Finland trityl, BDPA, or TEMPO radicals to explore the influence of the nature of the radical on the spin polarization transfer. We observe efficient polarization transfer between the pentacene excited triplet and the trityl radical but do not observe the same for the BDPA and TEMPO derivatives. The polarization transfer behavior in the pentacene-trityl system is also investigated in different glassy matrices and is found to depend markedly on the solvent used. The EPR results are rationalized with the help of femtosecond and nanosecond transient absorption measurements, yielding complementary information on the excited-state dynamics of the three pentacene derivatives. Notably, we observe a 2 orders of magnitude difference in the time scale of triplet formation between the pentacene-trityl system and the pentacene systems tethered with the BDPA and TEMPO radicals.

Dye-sensitized solar cells (DSCs) based on copper(II/I) redox mediators have recently achieved notable performance progress through tailoring the ligand structure of the copper complexes, exploiting cosensitization, and introducing an advanced device structure. In order to further improve the power conversion efficiency (PCE), it is imperative to develop wide spectral-response photo- sensitizers compatible with the copper(II/I) redox mediators while attaining a high Voc. Herein, a blue photosensitizer coded R7 is reported, which is purposely designed for highly efficient DSCs with copper-based electrolytes. R7 features a strong electron-donating segment of 9,19-dihydrobenzo[1′,10′ ] phenanthro[3′,4′:4,5]thieno[3,2-b]benzo[1,10]phenanthro[3,4-d]thiophene conjugated with a bulky auxiliary donor N-(2,4′-bis(hexyloxy)-[1,1′-biphenyl]-4- yl)-2,4′-bis(hexyloxy)-N-methyl-[1,1′-biphenyl]-4-amine and the electron acceptor 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid. The blue dye R7 with bulkier auxiliary donor moiety in the DSC largely outperforms the reference dye R6, which has a smaller bis(4-(hexyloxy)phenyl)amine donor unit. Transient absorption spectroscopy and electrochemical impedance spectroscopy meas- urements show that the R7 based DSC has a higher charge separation yield, a higher charge collection efficiency, and lower charge recombination with the copper electrolyte than the R6 based counterpart. A cosensitized system of R7 and Y123 for the DSC presents an outstanding PCE of 12.7% and retains 90% of its initial value after 1000 h of light soaking under 1 sun at 45 °C.

Post-treatment of mesoporous titanium dioxide films by TiCl4 solutions is commonly applied during the fabrication of solid-state dye-sensitized solar cells (ssDSCs), as this operation markedly improves the performance of the photovoltaic device. The effect of the post-treatement upon the charge carrier dynamics was scrutinized in ssDSC aiming at unraveling its mechanism. Kinetic studies carried out using femtosecond and nanosecond transient absorption spectroscopy, showed that a biphasic electron injection from the dye excited state is observed, for both treated and non-treated films, which kinetics is not significantly affected by the surface modification step. However, hole injection in the hole transport material (HTM) spiro-OMeTAD and charge recombination were found to be markedly slower in TiCl4-treated films. These findings are rationalized by a model describing the interaction at the interface between TiO2, the dye-sensitizer and spiro-OMeTAD. Rather than resulting from a modification of the energetics of the conduction band of the oxide, the effect of the TiCl4 post-treatment appears to be associated with a subtle change of the film morphology. Results emphasize the importance of controlling the contact at the heterojunction between the HTM and the sensitized semiconductor oxide network.

The high conversion efficiency has made metal halide perovskite solar cells a real breakthrough in thin film photovoltaic technology in recent years. Here, we introduce a straightforward strategy to reduce the level of electronic defects present at the interface between the perovskite film and the hole transport layer by treating the perovskite surface with different types of ammonium salts, namely ethylammonium, imidazolium and guanidinium iodide. We use a triple cation perovskite formulation containing primarily formamidinium and small amounts of cesium and methylammonium. We find that this treatment boosts the power conversion efficiency from 20.5% for the control to 22.3%, 22.1%, and 21.0% for the devices treated with ethylammonium, imidazolium and guanidinium iodide, respectively. Best performing devices showed a loss in efficiency of only 5% under full sunlight intensity with maximum power tracking for 550 h. We apply 2D- solid-state NMR to unravel the atomic-level mechanism of this passivation effect., Various approaches have been developed to push higher the efficiency of halide perovskite solar cells. Here Alharbi et al. show that ammonium salts treatment can reduce the defect density at the perovskite surface and understand the passivation mechanism with 2D-solid state NMR.

Redox mediators play a major role determining the photocurrent and the photovoltage in dye-sensitized solar cells (DSCs). To maintain the photocurrent, the reduction of oxidized dye by the redox mediator should be significantly faster than the electron back transfer between TiO2 and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of our new copper complexes as promising redox mediators in DSCs both criteria are satisfied to enhance power conversion efficiencies. In this study, two copper bipyridyl complexes, Cu((II/I))(dmby)2TFSI2/1 (0.97 V vs SHE, dmby = 6,6'-dimethyl-2,2'-bipyridine) and Cu((II/I))(tmby)2TFSI2/1 (0.87 V vs SHE, tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), are presented as new redox couples for DSCs. They are compared to previously reported Cu((II/I))(dmp)2TFSI2/1 (0.93 V vs SHE, dmp = bis(2,9-dimethyl-1,10-phenanthroline). Due to the small reorganization energy between Cu(I) and Cu(II) species, these copper complexes can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1 V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite the small driving forces for dye regeneration, fast and efficient dye regeneration (2-3 μs) was observed for both complexes. As another advantage, the electron back transfer (recombination) rates were slower with Cu((II/I))(tmby)2TFSI2/1 as evidenced by longer lifetimes. The solar-to-electrical power conversion efficiencies for [Cu(tmby)2](2+/1+), [Cu(dmby)2](2+/1+), and [Cu(dmp)2](2+/1+) based electrolytes were 10.3%, 10.0%, and 10.3%, respectively, using the organic Y123 dye under 1000 W m(-2) AM1.5G illumination. The high photovoltaic performance of Cu-based redox mediators underlines the significant potential of the new redox mediators and points to a new research and development direction for DSCs.