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Owing to the exceptional photovoltaic and optoelectronic properties of metal halide perovskites, they have sparked an intensive interest in the research community. CsPbBr3perovskite nanocrystals (NCs) have come into sight due to their versatile properties that can be achieved through structural modification. While the prior research on perovskite nanocrystals has focused mainly on tuning optical and electronic properties, the understanding of their electronic-ionic kinetics still remains a significant research gap. In this work, we explore how the size of CsPbBr3NCs impacts their electronic-ionic properties by using electrochemical impedance spectroscopy (EIS). We systematically tune NC size and investigate the resulting dielectric properties, conductivity, and capacitance. Notably, larger NCs exhibit anomalous behavior similar to that of perovskite polycrystalline thin films in the range of 0.4–0.6 V, indicating strong electronic-ionic coupling. Conversely, smaller NCs display weak electronic-ionic coupling due to ion localization. Additionally, this study sheds light on the electronic-ionic behavior of NCs approaching quantum confinement with a size reduction, suggesting opportunities for defect engineering. Ultimately, this work will pave the way for developing advanced electronic devices utilizing perovskite nanocrystals.

Lead halide perovskites have a range of spectacular properties and interesting phenomena and are a serious candidate for the next generation of photovoltaics with high efficiencies and low fabrication costs. An interesting phenomenon is the anomalous hysteresis often seen in current–voltage scans, which complicates accurate performance measurements but has also been explored to obtain a more comprehensive understanding of the device physics. Herein, we demonstrate a wavelength and illumination intensity dependency of the hysteresis in state-of-the-art perovskite solar cells with 18% power conversion efficiency (PCE), which gives new insights into ion migration. The perovskite devices show lower hysteresis under illumination with near band edge (red) wavelengths compared to more energetic (blue) excitation. This can be rationalized with thermalization-assisted ion movement or thermalization-assisted vacancy generation. These explanations are supported by the dependency of the photovoltage decay with illumination time and excitation wavelength, as well as by impedance spectroscopy. The suggested mechanism is that high-energy photons create hot charge carriers that either through thermalization can create additional vacancies or by release of more energetic phonons play a role in overcoming the activation energy for ion movement. The excitation wavelength dependency of the hysteresis presented here gives valuable insights into the photophysics of the lead halide perovskite solar cells.

Lead bromide perovskites have a larger band gap and are significantly more stable than their iodine counterparts, offering the perspective for higher voltage, tandem photovoltaics exceeding the Shockley–Queisser limit, and shorter time to deployment of photovoltaics. However, their efficiencies still need to be rivaling the iodine ones. Herein, the photophysics of FAPbBr3and the ones behind electron transfer from FAPbBr3to SnO2, one of the most effective electron transporting materials (ETMs), are reported. Time- and energy-resolved photoluminescence studies revealed the existence of two emitting states in the perovskite, which were assigned to bounded excitons and free carriers. SnO2extracted electrons from excitons and free carriers, with a selectivity related to the SnO2surface treatment. This new insight helps explain SnO2’s unique qualities as an ETM to produce photovoltaics with reduced voltage losses. Furthermore, this study illustrates the importance of performing time- and energy-resolved photoluminescence to capture the intricacies of the photophysical process.

Cesium and methylammonium bismuth iodides (Cs3Bi2I9and MA3Bi2I9) are new low-toxic and air stable compounds in the perovskite solar cell family with promising characteristics. Here, the electronic structure and the nature of their optical transitions, dielectric constant, and charge carrier properties are assessed for photovoltaic applications with density functional theory (DFT) calculations and experiments. The calculated direct and indirect band gap values for Cs3Bi2I9(2.17 and 2.0 eV) and MA3Bi2I9(2.17 and 1.97 eV) are found to be in good agreement with the experimental optical band gaps (2.2, 2.0 eV and 2.4, 2.1 eV for Cs3Bi2I9and MA3Bi2I9, respectively) estimated for solution-processed films. There is an error cancelation in the DFT calculated band gap similar to that for lead perovskites. However, fully relativistic DFT calculations indicate that the size of the spin orbit coupling (SOC) error cancelation for bismuth perovskite (0.5 eV) is less than for lead perovskite (1 eV), and other factors are therefore also important. Band structure calculations show high effective masses of the charge carriers along the c-axis but on the other hand lower electron effective mass in the a–bplanes, revealing the interesting possibility for a directional charge transport. Calculations of dielectric constants, absorption coefficients, carrier effective masses, and exciton binding energies emphasize the fundamental differences between the lead and bismuth iodide perovskites and clarify the reasons behind the lower power conversion efficiency of bismuth iodide perovskite solar cells. Also the calculations show that the orientational disorder of the MA dipoles in the lattice has meaningful impacts on the near valence and conduction band edge of the electronic structure.

The effects of coadsorbing the amphiphilic ruthenium-based dye Z907 (cis-bis(isothiocyanato)(2,20-bipyridyl-4,40-dicarboxylato)(4,40-dinonyl-20-bipyridyl)ruthenium(II)) with the coadsorbent DPA (n-decylphosphonic acid) and with the organic dye D35 ((E)-3-(5-(4-(bis(2′,4′-dibutoxybiphenyl-4-yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid) on mesoporous TiO2were investigated using photoelectron spectroscopy (PES). Z907 is expected to adsorb to the TiO2surface via the carboxylic acid groups. However, Z907 also shows signs of interacting with the TiO2via the sulfur of the thiocyanate groups, and this interaction is affected by both the addition of DPA and D35. DPA, when added, exchanges with Z907 at the TiO2surface, and each Z907 is replaced by six DPA molecules, but it does not affect the energy level alignment between Z907 and TiO2substantially. Adding D35 to Z907 induces changes in the adsorption configuration of Z907 by the means of suppressing the interaction of the thiocyanate ligands and the TiO2surface. The HOMO level of Z907 is shifted by the addition of D35. Coadsorbing Z907 with D35 thus gives changes at a molecular level, meaning that this is an example of collaborative sensitization.

We applied organic donor-π-acceptor (D-π-A) sensitizers for photoelectrochemical polymerization (PEP) because of their appropriate energy levels and high light absorption. The polymerized conducting polymer PEDOT was used as hole conductor in all-solid-state dye-sensitized solar cells (ssDSCs). By combination of the D-π-A sensitizers and the generated PEDOT from PEP of bis-EDOT in acetonitrile, the resulting device showed an average power conversion efficiency of 5.6%. Furthermore, the PEP in aqueous micellar electrolytic medium was also employed because of the ability to decrease oxidation potential of the precursor, thereby making the polymerization process easier. The latter method is a cost-effective and environmentally friendly approach. Using as hole conductor the so-obtained PEDOT from PEP of bis-EDOT in aqueous electrolyte, the devices exhibited impressive power conversion efficiency of 5.2%. To compare the properties of the generated polymer from bis-EDOT in these two PEP methods, electron lifetime, photoinduced absorption (PIA) spectra, and UV–vis–NIR spectra were measured. The results showed that PEDOT from organic PEP exhibits a delocalized conformation with high conductivity and a smooth and compact morphology; a rough morphology with high porosity and polymer structure of relatively shorter chains was assumed to be obtained from aqueous PEP. Therefore, better dye regeneration but faster charge recombination was observed in the device based on PEDOT from aqueous PEP of bis-EDOT. Subsequently, to extend the aqueous PEP approach in consideration of the ability to decrease the oxidation potential of the precursor, the easily available precursor EDOT was for the first time used for PEP in aqueous medium in a variant of the aforementioned procedure, and the device based on the so-obtained PEDOT shows a more than 70-fold increase in efficiency, 3.0%, over that based on the polymer generated from EDOT by PEP in organic media. It was demonstrated that aqueous micellar PEP with EDOT as monomer is an efficient strategy for generation of conducting polymer hole-transporting materials.

We investigate dye-sensitized solar cells (DSSCs) based on PEDOT as hole conductor and prepared by photoelectrochemical polymer deposition at different light intensities. We specifically investigate the effect of light intensity on the PEDOT polymer and in turn the efficiency of the solar cells. We find that the PEDOT prepared by this method is largely oxidized and contains significant amounts of polarons and bipolarons and only a small fraction of neutral PEDOT. Photoelectrochemical polymer deposition under low light intensity leads to a particularly low fraction of neutral PEDOT and a high fraction of bipolarons as measured in the UV–vis spectra. The solar cells based on PEDOT as a hole conductor prepared under these conditions are the most efficient with a higher power conversion efficiency, which can be explained by a longer electron lifetime, faster charge transport, and higher transparency of the PEDOT. Interestingly, we conclude that in this type of solid-state DSSCs the mechanism of dye regeneration occurs from PEDOT polarons that then form bipolarons, which is different from the mechanism of dye regeneration proposed in standard solid-state DSSCs.

Postsynthesis thermal treatments are key to promote crystallinity and reduce the defect density in solution-processed nanomaterials. In particular, the annealing atmosphere strongly influences the functional properties of ZnO nanowires (NWs) and specifically their performance as photoanodes in dye-sensitized solar cells (DSCs). We prepared vertically aligned ZnO NWs by a low-cost, high-yield, and up-scalable hydrothermal method and studied the effect of the postannealing atmosphere on their optoelectronic properties and on their performance as electrodes in DSCs. When annealing ZnO NWs under argon, instead of air, significantly higher photoluminescence (PL) UV emission and relatively lower defects-related visible PL emission were obtained. At the same time, Ar-annealing rendered ZnO NWs with higher electrical conductivities, as observed from single NW measurements using conductive-atomic force microscopy. Furthermore, DSCs based on ZnO NWs annealed in argon were characterized by 50% higher photocurrents than those obtained from air-annealed ZnO. As a result 30% efficiency increases were systematically obtained when using argon as the annealing atmosphere. These results are discussed within the framework of a multiple trapping model for transport and charge transfer, taking into account differences in the defect concentration introduced during the annealing.

Hard X-ray photoelectron spectroscopy (HAXPES) has been used to study the effects of adding Li-TFSI to hole conducting molecular thin films of 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD). The work shows that a procedure of mixing a Li-TFSI solution into a spiro-OMeTAD solution, and subsequent spin-coating this mixture into a solid thin film causes the Fermi level of the molecular film to move closer to the HOMO level. Hence, adding the Li-TFSI gives similar effects to spiro-OMeTAD as a p-dopant. Specific effects from doping on the valence levels were also characterized. Absorbance measurements also showed that the spiro-OMeTAD film was partially oxidized when Li-TFSI was added before spin-coating. By varying the photon energy in the photoelectron spectroscopy measurements, the probe depth varies between being surface sensitive (<1 nm) and bulk sensitive (inelastic mean free path ≥10 nm). This property was used to follow differences in the composition at different depth of the spiro-OMeTAD/Li-TFSI film. It could be concluded that there was a concentration gradient in the molecular film and that the concentration of Li-TFSI was dominating at the interface between the spiro-OMeTAD/Li-TFSI film and vacuum.

Interfacial charge separation in hybrid solar cells depends on both the energetic alignment and electronic coupling between the inorganic and organic semiconducting materials at the hetero interface. In the present work, bilayer solar cells comprising the small molecular semiconducting dye TDCV-TPA (tris-(thienylene-vinylene)-triphenylamine) and dense titanium dioxide (TiO2) films were investigated. The internal quantum efficiency and degree of photoluminescence quenching were found to be excitation energy dependent. The molecular interaction and interfacial energy level alignment was investigated using a combination of UV–vis and photoelectron spectroscopy (PES). Stationary and time-dependent density functional theory calculations were used to assign and distinguish between different experimentally determined molecular energy levels (PES) and electronic transitions (UV–vis). Photoelectron spectroscopy results suggest surface induced interactions of TDCV-TPA with TiO2involving the peripheral CN-groups of the molecule which would imply a favorable electronic coupling for photoinduced interfacial charge transfer. In an energy level diagram distinguishing between the different electronic transitions in the molecule, the differences in the thermodynamic driving force for electron injection from the excited states were found small. Therefore, it is suggested that the observed higher internal quantum efficiency at shorter wavelength can be rationalized by a more favorable driving force for the regeneration of holes created at the hetero interface at higher excitation energy.

In dye-sensitized solar cells (DSC) an efficient transfer of holes from the oxidized dye to the contact is necessary, which in solid-state DSC is performed by hole-conductor molecules. In this report we use photoinduced absorption and transient absorption spectroscopy to show that a small hole-conducting molecule, tris(p-anisyl)amine, regenerates dye molecules in the pores of the dye-sensitized TiO2nanoparticle electrode efficiently even for thick (>5 μm) electrodes. For similar thicknesses we observe incomplete regeneration using a larger polymer hole-conductor. However, the performance of the solar cells with the small hole-conductor molecules is poor due to that inefficient hole conduction in these small molecules may limit the collection of the charges at the contacts. Polymer hole-conductors, which may have a good hole conductivity, also have a high molecular weight, which makes these polymers difficult to infiltrate into the smallest pores in the electrode. We show that a conducting polymer, P3HT, may be added to the small molecule hole-conductor, to enable better transport of the charges to the contact and to reduce recombination and therefore increase the photocurrent. This new device construction with a small molecule efficiently regenerating the dye molecules, and a polymer conducting the holes to the contact is therefore a promising pathway for solid-state dye-sensitized solar cells.

ZnO based dye-sensitized solar cells have been studied using N719 and Z-907 as sensitizing dyes, with and without including water to the dye solution. The solar cells have been characterized with photoelectric measurements and the interface between the dye and the ZnO surface has been studied using photoelectron spectroscopy. It was shown that water in the dye solution greatly reduces surface dye aggregation and thereby enhances the solar cell performance for N719. For Z-907 where no sign of dye aggregation could be found, the presence of water had minor effect on the surface structure and solar cell performance.

The influence of water on the surface electronic and molecular properties of three Ru-dyes adsorbed at nanostructured TiO2was investigated using photoelectron spectroscopy (PES). The dyes investigated were the Ru(dcbpy)2(NCS)2in its acid (N3) and in its 2-fold deprotonated form (N719) as well as a similar dye (Z-907) containing the hydrophobic ligand 4,4′-dinonyl-2,2′-bipyridine. Trends in surface structures depending on water exposure were followed for the three dyes. The results showed that the hydrophobic chains of the Z-907 dye effectively inhibit surface reorganization while large changes in surface electronic and molecular structure were observed for the N3 and N719 molecular layers. Specifically, large effects involving the thiocyanate ligands were detected, and the S2p and N1s core level spectra indicate that the changes involve mixing of only two dominating surface configurations. Moreover, the PES results also showed water-induced changes in the energy level matching between the dye and the TiO2, and water induced desorption of the TBA+counterion. Basic photoelectrochemical trends depending on water exposure to dye-sensitized solar cell systems were also verified.

We recently reported on a perylene sensitizer, ID176, which performs much better in solid state dye-sensitized solar cells than in those using liquid electrolytes with iodide/tri-iodide as the redox couple (J. Phys. Chem. C2009, 113, 14595−14597). Here, we present a characterization of the sensitizer and of the TiO2/dye interface by UV−visible absorption and fluorescence spectroscopy, spectroelectrochemistry, photoelectron spectroscopy, electroabsorption spectroscopy, photoinduced absorption spectroscopy, and femtosecond transient absorption measurements. We report that the absorption spectrum of the sensitizer is red-shifted by addition of lithium ions to the surface due to a downward shift of the excited state level of the sensitizer, which is of the same order of magnitude as the downward shift of the titanium dioxide conduction band edge. Results from photoelectron spectroscopy and electrochemistry suggest that the excited state is largely located below the conduction band edge of TiO2but that there are states in the band gap of TiO2which might be available for photoinduced electron injection. The sensitizer was able to efficiently inject into TiO2, when a lithium salt was present on the surface, while injection was much less effective in the absence of lithium ions or in the presence of solvent. In the presence of the hole conductor 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-MeOTAD) and LiTFSI, charge separation was monitored by the emergence of a Stark shift of the dye in transient absorption spectra, and both injection and regeneration appear to be completed within 1 ps. Regeneration by spiro-MeOTAD is therefore several orders of magnitude faster than regeneration by iodide, and ID176 can even be photoreduced by spiro-MeOTAD.

Different molecular layers on TiO2were prepared by using the p-dimethylaniline triphenylamine based organic dye, D29, together with the coadsorbents decylphosphonic acid (DPA), dineohexyl bis(3,3-dimethylbutyl)phosphinic acid (DINHOP), and chenodeoxycholic acid (CDCA). The surface molecular structure of dye and coadsorbent layers on TiO2was investigated by photoelectron spectroscopy (PES). A focus was to determine the surface molecular concentrations using characteristic photoelectron core levels. Dye-sensitized solar cells (DSCs) were prepared from the same substrate and were further characterized by photoelectrochemical methods. Together the investigation gives information on the arrangement of the mixed molecular layer and a first insight to the extent to which the coadsorbents exchange with dye molecules on the TiO2surface for the examined conditions.

Photoinduced energy conversion from multilayers of organic dye on dense TiO2films was investigated in bilayer hybrid solar cells. Dye layers of varying thicknesses were prepared by spin-casting the star-shaped dye [tris(dicyano-vinyl-2-thienyl)phenyl]amine (1) from solutions onto dense TiO2on conducting glass substrates. A spin-cast layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and graphite powder was used for contacting the devices. Excitons generated in the dye multilayer contribute to the power conversion efficiency, reaching a maximum of ca. 0.3% at a dye layer thickness of ca. 8 nm for the devices described herein. For dye layers exceeding 5 nm, the cell performance becomes limited by the exciton diffusion length LEDand the hole mobility in the organic layer. Using dye multilayers is a viable way to increase light harvesting in solid-state dye-sensitized solar cells.

One-electron outer-sphere redox couples, such as ferrocene/ferrocenium, are an interesting alternative to the iodide/triiodide redox couple that is normally employed in dye-sensitized solar cells (DSCs) because they should reduce the driving force needed to regenerate the dye. Unfortunately, one-electron redox couples also show enhanced recombination with photoinjected electrons, and methods to inhibit this recombination are needed for functioning DSCs. In this study, dye-sensitized titanium dioxide surfaces were passivated by a trichloromethylsilane reaction in order to decrease the fast recombination rates when using the ferrocene redox couple. The formation and binding of poly(methylsiloxane) on the dye-sensitized TiO2surface was verified with infrared spectroscopy and photoelectron spectroscopy. Photoelectrochemical characterization of the silanization method showed that the treatment decreased the recombination rate of photoinjected electrons with ferrocenium and thereby improved the efficiency of the DSC. Transient absorption spectroscopy revealed, however, that the poly(methylsiloxane) coatings slowed down the regeneration of the oxidized dye by the ferrocene and prevented the regeneration of some of the dye molecules.

The purpose of the present paper is to experimentally map the energy levels governing the trends observed in oxidation potentials and absorption spectra of M(bpy)32+complexes (bpy = 2,2′-bipyridine, M = Fe(II), Ru(II), and Os(II)). Molecular films of the transition metal complexes were investigated with element specific methods using photoelectron spectroscopy (PES) at high kinetic energy using hard X-rays and by X-ray absorption spectroscopy (XAS). The results were compared to electronic structure calculations on the complexes and the ligand. The approach allows us to experimentally measure and interpret the energy levels in terms of spin−orbit coupling and metal−ligand interactions. Specifically, it was verified that the anomaly in the trend in oxidation potentials could be explained by a large spin−orbit coupling for the Os(bpy)32+. The influence of the different metal ions on the state formed upon light absorption was also investigated by N 1s X-ray absorption, and from the spectra we could determine the relative position of the levels originating from d−σ and π contributions. The results for the occupied and unoccupied electronic levels explain the lower energy of the MLCT transition of the Os(bpy)32+in comparison to the Ru(bpy)32+.

Photoconversion properties are demonstrated for a device based on a small dye molecule, absorbing light in the near-IR region, mixed with two organic charge transport materials and together forming a dye-sensitized organic bulk heterojunction. The organic dye molecule, phthalocyanine (1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine), mixed with a blend of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61(PCBM), shows a photoconversion spectrum extended more than 150 nm toward longer wavelengths, as compared to a device without such dye sensitization. In the dye-sensitized region of the photoconversion spectrum the maximum internal quantum efficiency was estimated to 40%. With higher dye concentrations the internal quantum efficiency decreases. Transient laser spectroscopy measurements show that after excitation of the dye there is an electron transfer from the dye to PCBM and a subsequent hole transfer from the dye to P3HT, which results in a long-lived (P3HT+/dye/PCBM−) charge-separated state.