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We present a general framework that enables quantification with atomic resolution of the overall London dispersion energy, which can be readily integrated with currently available energy decomposition schemes. This approach can be used to determine the contribution of individual atoms and functional groups to molecular recognition, conformational preferences, molecular stability, and reactivity. Its efficacy across diverse realms of molecular chemistry and biology is demonstrated with application to molecular balances in solution, asymmetric organocatalytic transformations, and a subcomplex of the F1FO ATP synthase.

Intense research activities have been exploring the possibility of harnessing solar energy via photovoltaic and photocatalytic applications of ferroelectric materials using the built-in electric field for an efficient separation of photoexcited charge carriers. However, one of the most important bottlenecks in these efforts is to find ferroelectricity in suitably low-band-gap materials for harvesting a sizable part of the solar spectrum, with most of the known ferroelectric materials having band gaps larger than 2.5 eV. In the present work, we show that the known chiral and polar compound, (R-/S-MBA)2CuCl4, with MBA = α-methyl benzylamine, is also ferroelectric and its ligand-to-metal charge transfer (LMCT) band gap (∼2.53 eV) can be systematically decreased via substitution of Cl–with Br–forming the solid solution (R-/S-MBA)2CuCl4–xBrx. These compounds retain their chiral ferroelectric state until x= 2 and reach a significantly low band gap of ∼2.09 eV for (R-/S-MBA)2CuCl2Br2, which is the smallest band gap reported so far among layered hybrid ferroelectric materials. We elucidate the origin of the band gap reduction and other changes in the electronic structure with the help of state-of-the-art electronic structure calculations. Chiral ferroelectrics constitute an interesting class of materials, with ferroelectricity being able to discriminate between electron and hole charge transports, while chirality may have the ability to discriminate between up- and down-spin transports.

Herein, we show that [Cp*Co(2-ampy)I]I (2-ampy = 2-aminomethyl-pyridine) is an extremely active catalyst for HER, exhibiting a TOF of 109 000 s−1in phosphate buffered water solution (pH 7). The key to this remarkable activity stems from the establishment of a network of weak interactions in the second coordination sphere. As a matter of fact, both experimental and theoretical studies strongly suggest that the –NH2functionality of the 2-ampy ligand acts as an anchoring and orienting group for H2PO4−through the establishment of an intermolecular hydrogen bonding with it that, in turn, intermolecularly donates a proton to Co–H liberating H2.

The compositional tunability of 2D metal halide perovskites enables exploration of diverse semiconducting materials with different structural features. However, rationally tuning the 2D perovskite structures to target physical properties for specific applications remains challenging, especially for lead-free perovskites. Here, we study the effect of the interplay of the B-site (Ge, Sn, and Pb), A-site (cesium, methylammonium, and formamidinium), and spacer cations on the structure and optical properties of a new series of 2D Ruddlesden–Popper perovskites using the previously unreported spacer cation 4-bromo-2-fluorobenzylammonium (4Br2FBZ). We report eight new crystal structures and study the consequence of varying the B-site (Pb, Sn, Ge) and dimension (n= 1, 2, vs 3D). Dimension strongly influences local distortion and structural symmetry, and the increased octahedral tilting and lone pair effects in Ge perovskites lead to a polar n= 2 perovskite that exhibits second harmonic generation, (4Br2FBZ)2(Cs)Ge2I7. In contrast, the analogous Sn and Pb perovskites remain centrosymmetric, but the B-site metal influences the photoluminescence properties. The Pb perovskites exhibit broad, defect-mediated emission at low temperature, whereas the Sn perovskites show purely excitonic emission over the entire temperature range, but the carrier recombination dynamics depend on dimensionality and dark excitonic states. Wholistic understanding of these differences that arise based on cations and dimensionality can guide the rational materials design of 2D perovskites for targeting physical properties for optoelectronic applications based on the interplay of cations and the connectivity of the inorganic framework.

In the search for lead-free perovskites, silver pnictohalides recently gained attention as novel perovskite-inspired materials for photovoltaics due to their high stability, low toxicity, and promising early efficiencies, especially for indoor applications. Recent research on such “rudorffites” mainly addresses silver bismuth iodides (Ag–Bi–I), while their antimony analogues are hardly investigated due to intrinsic challenges in the synthesis of Sb-based thin films. Here, we establish a synthetic route to prepare Ag–Sb–I thin films by employing thiourea as a Lewis-base additive. Thin film morphologies were further optimized by alloying them with Cu, resulting in solar cells with an improved power conversion efficiency of 0.7% by reducing undesired side phases. Density functional theory calculations and optical characterization methods support the incorporation of Cu into a Cu1–xAgxSbI4phase, keeping the overall stoichiometry and band gap virtually unchanged upon alloying. Our results further reveal the detrimental role of Ag point defects representing trap states in the band gap, being responsible for low open-circuit voltages and subgap absorption and emission features. Moreover, additional minor amounts of Bi are shown to boost the efficiency and stabilize the performance over a wider compositional range. Despite the remaining challenges regarding device performance, we demonstrate a strong increase in external quantum efficiency when reducing the light intensity, highlighting the potential of Ag–Sb–I rudorffites for indoor photovoltaics.

Finding stable analogues of three-dimensional (3D) lead halide perovskites has motivated the exploration of an ever-expanding repertoire of two-dimensional (2D) counterparts. However, the bandgap and exciton binding energy in these 2D systems are generally considerably higher than those in 3D analogues due to size and dielectric confinement. Such quantum confinements are most prominently manifested in the extreme 2D realization in (A)mPbI4(m= 1 or 2) series of compounds with a single inorganic layer repeat unit. Here, we explore a new A-site cation, 4,4′-azopyridine (APD), whose size and hydrogen bonding properties endow the corresponding (APD)PbI42D compound with the lowest bandgap and exciton binding energy of all such compounds, 2.19 eV and 48 meV, respectively. (APD)PbI4presents the first example of the ideal Pb–I–Pb bond angle of 180°, maximizing the valence and conduction bandwidths and minimizing the electron and hole effective masses. These effects coupled with a significant increase in the dielectric constant provide an explanation for the unique bandgap and exciton binding energies in this system. Our theoretical results further reveal that the requirement of optimizing the hydrogen bonding interactions between the organic and the inorganic units provides the driving force for achieving the structural uniqueness and the associated optoelectronic properties in this system. Our preliminary investigations in characterizing photovoltaic solar cells in the presence of APD show encouraging improvements in performances and stability.

Heteroatom alloying of lead-free perovskite derivatives is a highly promising route to tailor their optoelectronic properties and stability for multiple applications. Here, we demonstrate the facile solution-based synthesis of Sn-alloyed layered MA3Sb2I9thin films by precursor engineering, combining acetate and halide salts. An increasing concentration of tin halides in different oxidation states leads to a strong boost in absorption over the whole visible spectrum. We demonstrate phase-pure synthesis and elucidate the heterovalent incorporation of Sn into the MA3Sb2I9lattice, proving the formation of additional electronic states in the bandgap by theoretical calculations. On this basis, we dissect the strong absorption increase into three components that we attribute to intervalence and heteroatom-induced interband absorption. Finally, we show the charge-stabilizing effect of the system through robustness toward precursors in mixed oxidation states and trace the improved ambient stability of this material back to this feature.

The application of self-assembled molecules (SAMs) as a charge selective layer in perovskite solar cells has gained tremendous attention. As a result, highly efficient and stable devices have been released with stand-alone SAMs binding ITO substrates. However, further structural understanding of the effect of SAM in perovskite solar cells (PSCs) is required. Herein, three triphenylamine-based molecules with differently positioned methoxy substituents have been synthesized that can self-assemble onto the metal oxide layers that selectively extract holes. They have been effectively employed in p-i-n PSCs with a power conversion efficiency of up to 20%. We found that the perovskite deposited onto SAMs made by para- and ortho-substituted hole selective contacts provides large grain thin film formation increasing the power conversion efficiencies. Density functional theory predicts that para- and ortho-substituted position SAMs might form a well-ordered structure by improving the SAM’s arrangement and in consequence enhancing its stability on the metal oxide surface. We believe this result will be a benchmark for the design of further SAMs.

ABX3metal halide perovskites revolutionized the research and development of new optoelectronics, including solar cells and light-emitting diodes. Processing polycrystalline thin films from precursor solutions is one of the core advantages of these materials since it enables versatile and cost-effective manufacturing. The perovskite film morphology, that is, continuous substrate coverage and low surface roughness, is of paramount importance for highly efficient solar cells and optoelectronic devices in general. Controlling the chemistry of precursor solutions is one of the most effective strategies to manage the perovskite film morphology. Herein, we show the fundamental influence of the A-site cation composition on the perovskite precursor arrangement and the consequent film formation. Extended X-ray absorption fine structure spectroscopy and small-angle X-ray scattering give unprecedented insights into the complex structural chemistry of the perovskite precursors and, in particular, their repulsive interactions as a crucial parameter for colloidal stability. Combining these techniques with in situgrazing incidence wide-angle X-ray scattering during thin-film formation allows us to identify the mechanism for using alkali metals as a decisive criterion to control the colloidal stability of the perovskite precursor and thus the thin-film morphology. We illustrate the fundamental principle behind the systematic use of alkali metals regardless of whether they are incorporated in the lattice or not. Hence, this work provides tools to selectively control the morphology and crystal growth in present and future systems.

Tin halide perovskites have recently emerged as promising materials for low band gap solar cells. Much effort has been invested on controlling the limiting factors responsible for poor device efficiencies, namely self-p-doping and tin oxidation. Both phenomena are related to the presence of defects; however, full understanding of their implications in the optoelectronic properties of the material is still missing. We provide a comprehensive picture of the competing radiative and non-radiative recombination processes in tin-based perovskite thin films to establish the interplay between doping and trapping by combining photoluminescence measurements with trapped-carrier dynamic simulations and first-principles calculations. We show that pristine Sn perovskites, i.e.sample processed with commercially available SnI2used as received, exhibit extremely high radiative efficiency due to electronic doping which boosts the radiative band-to-band recombination. Contrarily, thin films where Sn4+species are intentionally introduced show drastically reduced radiative lifetime and efficiency due to a dominance of Auger recombination at all excitation densities when the material is highly doped. The introduction of SnF2reduces the doping and passivates Sn4+trap states but conversely introduces additional non-radiative decay channels in the bulk that fundamentally limit the radiative efficiency. Overall, we provide a qualitative model that takes into account different types of traps present in tin-perovskite thin films and show how doping and defects can affect the optoelectronic properties.

Despite the remarkable progress in power conversion efficiency of perovskite solar cells, going from individual small-size devices into large-area modules while preserving their commercial competitiveness compared with other thin-film solar cells remains a challenge. Major obstacles include reduction of both the resistive losses and intrinsic defects in the electron transport layers and the reliable fabrication of high-quality large-area perovskite films. Here we report a facile solvothermal method to synthesize single-crystalline TiO2rhombohedral nanoparticles with exposed (001) facets. Owing to their low lattice mismatch and high affinity with the perovskite absorber, their high electron mobility and their lower density of defects, single-crystalline TiO2nanoparticle-based small-size devices achieve an efficiency of 24.05% and a fill factor of 84.7%. The devices maintain about 90% of their initial performance after continuous operation for 1,400 h. We have fabricated large-area modules and obtained a certified efficiency of 22.72% with an active area of nearly 24 cm2, which represents the highest-efficiency modules with the lowest loss in efficiency when scaling up.

Tin is a valuable candidate for replacing lead in perovskites solar cells. Tin oxidation, however, strongly limits the efficiency and the long-term stability of tin halide perovskites (THP) in devices. To mitigate this issue, capping 3D THP with analogue 2D perovskites through the addition of large cations is an emerging strategy, leading to increased performance and stability. In this study, based on the state of the art density functional theory calculations, the possible beneficial effects of large cations (BA, PEA, and AVA) on the tin stability at the surface is investigated. Our results show that large cation dipoles of the 2D perovskites modulate tin oxidation potential at the surface of 3D perovskites by hindering the formation of tin vacancies and the degradation of the material. This study confirms that stabilization of tin at the surface is key for the full exploitation of THP in long-term stable devices.

Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high colour purity1,2. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colours, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent—approaching those of commercial organic light-emitting diodes—have been demonstrated in both the infrared and the green emission channels1,3,4. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and colour-stable red electroluminescence from mixed-halide perovskites has not yet been realized5,6. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate colour-stable, red emission centred at 620 nanometres, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to ‘clean’ the nanocrystal surface through the removal of lead atoms. Density functional theory calculations reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)crystalline surface and presents a route through which to control the formation and migration of surface defects. This is critical to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications—such as photovoltaics—for which bandgap stability is required.

Water is generally considered an enemy of metal halide perovskites, being responsible for their rapid degradation and, consequently, undermining the long-term stability of perovskite-based solar cells. However, beneficial effects of liquid water have been surprisingly observed, and synthetic routes including water treatments have shown to improve the quality of perovskite films. This suggests that the interactions of water with perovskites and their precursors are far from being completely understood, as water appears to play a puzzling dual role in perovskite precursor solutions. In this context, studying the basic interactions between perovskite precursors in the aqueous environment can provide a deeper comprehension of this conundrum. In this context, it is fundamental to understand how water impacts the chemistry of iodoplumbate perovskite precursor species, PbIx2–x. Here, we investigate the chemistry of these complexes using a combined experimental and theoretical strategy to unveil their peculiar structural and optical properties and eventually to assign the species present in the solution. Our study indicates that iodide-rich iodoplumbates, which are generally key to the formation of lead halide perovskites, are not easily formed in aqueous solutions because of the competition between iodide and solvent molecules in coordinating Pb2+ions, explaining the difficulty of depositing lead iodide perovskites from aqueous solutions. We postulate that the beneficial effect of water when used as an additive is then motivated by its behavior being similar to high coordinative polar aprotic solvents usually employed as additives in one-step perovskite depositions.

By combining time-correlated single photon counting (TCSPC) measurements, density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations, we herein investigate the role of protons, in solutions and on semiconductor surfaces, for the emission quenching of indoline dyes. We show that the rhodanine acceptor moieties, and in particular the carbonyl oxygens, undergo protonation, leading to nonradiative excited-state deactivation. The presence of the carboxylic acid anchoring group, close to the rhodanine moiety, further facilitates the emission quenching, by establishing stable H-bond complexes with carboxylic acid quenchers, with high association constants, in both ground and excited states. This complexation favors the proton transfer process, at a low quencher concentration, in two ways: bringing close to the rhodanine unit the quencher and assisting the proton release from the acid by a partial-concerted proton donation from the close-by carboxylic group to the deprotonated acid. Esterification of the carboxylic group, indeed, inhibits the ground-state complex formation with carboxylic acids and thus the quenching at a low quencher concentration. However, the rhodanine moiety in the ester form can still be the source of emission quenching through dynamic quenching mechanism with higher concentrations of protic solvents or carboxylic acids. Investigating this quenching process on mesoporous ZrO2, for solar cell applications, also reveals the sensitivity of the adsorbed excited rhodanine dyes toward adsorbed protons on surfaces. This has been confirmed by using an organic base to remove surface protons and utilizing cynao-acrylic dye as a reference dye. Our study highlights the impact of selecting such acceptor group in the structural design of organic dyes for solar cell applications and the overlooked role of protons to quench the excited state for such chemical structures.

Self-trapping of excitons or of free charges associated with the formation of color centers is typical of conventional halides. By analogy, lead halide perovskites could in principle show self-trapping of photogenerated charge carriers, possibly leading to defect formation and long-term material instability. Here we investigate the energetics of hole self-trapping in methylammonium lead iodide (MAPbI3) by performing first-principles electronic structure calculations. The thermodynamics and kinetics for the formation of bridging I2–dimers and iodine vacancy/I3–trimer Frenkel defects, originated by self-trapping of one and two holes, respectively, are investigated both in the bulk and at selected surfaces, in both pristine and defective systems. Our results indicate that hole self-trapping is unlikely to occur in the bulk, being thermodynamically unfavorable with associated high-energy barriers. Self-trapping remains unfavorable at surfaces, though it is significantly stabilized compared to the bulk. The inclusion of typical hole-trapping defects, such as the lead vacancy and the interstitial iodine, further stabilizes the formation of color centers, which eventually become stable for the PbI2-terminated MAPbI3surface. Overall, our results clearly indicate that surfaces and grain boundaries are the main instability sources in lead iodide perovskites and that tailoring surface passivation is crucial for improving the performance and long-term stability of devices based on lead halide perovskites.

Halide perovskites are a strong candidate for the next generation of photovoltaics. Chemical doping of halide perovskites is an established strategy to prepare the highest efficiency and most stable perovskite-based solar cells. In this study, we unveil the doping mechanism of halide perovskites using a series of alkaline earth metals. We find that low doping levels enable the incorporation of the dopant within the perovskite lattice, whereas high doping concentrations induce surface segregation. The threshold from low to high doping regime correlates to the size of the doping element. We show that the low doping regime results in a more n-type material, while the high doping regime induces a less n-type doping character. Our work provides a comprehensive picture of the unique doping mechanism of halide perovskites, which differs from classical semiconductors. We proved the effectiveness of the low doping regime for the first time, demonstrating highly efficient methylammonium lead iodide based solar cells in both n-i-p and p-i-n architectures.

Three-dimensional (3D)/two-dimensional (2D) mixed dimensional perovskites have potentially overcome the stability issues of conventional 3D perovskites without significantly compromising the solar cell performance. 3D/2D perovskite heterostructures rely on the alignment of energy levels across the 3D/2D interface, which determines carrier confinement and possibly charge separation. Using state-of-the-art first-principles calculations, we show that the confined dipoles of spacer cations and the perovskite surface termination have a crucial role in determining interfacial energy alignment. Considering the experimentally employed BA2PbI4(BA = n-butylammonium) and AVA2PbI4(AVA = protonated aminovaleric acid) 2D perovskite interfaces with the prototypical MAPbI33D perovskite, we systematically investigate the band alignment at the interfaces of 3D and 2D quantum wells considering different types of surface terminations. Our study shows that BA2PbI4has favorable alignment of band edges with MAPbI3irrespective of the surface termination of MAPbI3, however, with the alteration of the band edge positions between 2D and 3D parts depending on surface termination. On the other hand, the alignment of AVA2PbI4with MAPbI3is very much sensitive to surface termination, having favorable alignment by virtue of confined interface dipoles of the −COOH groups. This study provides a global picture of the interface engineering in 3D/2D perovskites, setting the required background to further explore the anchoring of hole/electron transport materials at the preferential side of the film for improved carrier extraction.

Dye-sensitized solar cells (DSC) represent a valuable, efficient, and low-cost alternative to conventional semiconductor photovoltaic devices. A deeper understanding of the interactions at the dye/semiconductor heterointerface is fundamental for future progress in DSC technology. Here we present an interesting heteroleptic ruthenium(II) sensitizer that shifts the conduction band of TiO2by 80 mV resulting in high open circuit potential for improved device efficiency.

Hybrid halide perovskites represent one of the most promising solutions toward the fabrication of all solid nanostructured solar cells, with improved efficiency and long-term stability. This article aims at investigating the structural properties of iodide/chloride mixed-halide perovskites and correlating them with their photovoltaic performances. We found out that, independent of the components ratio in the precursor solution, Cl incorporation in an iodide-based structure, is possible only at relatively low concentration levels (below 3–4%). However, even if the material band gap remains substantially unchanged, the Cl doping dramatically improves the charge transport within the perovskite layer, explaining the outstanding performances of meso-superstructured solar cells based on this material.

We report a computational method for evaluating the dynamical Rashba interaction coefficient of tetragonal and cubic methylammonium lead iodide (MAPbI3) perovskite, at various size scales, through Car–Parrinello molecular dynamics trajectories. This strategy involves the calculation of a time-dependent band structure of the target systems using periodic boundary conditions and the evaluation of the amplitude of band-splitting due to the spin–orbit coupling (SOC) with the computation of a local electric field. This model, physically motivated by a rewriting of the SOC Hamiltonian according to the heterogeneity of three-dimensional systems and our choice of k-space sampling, involves directly the methylammonium configuration space sampling. Originally applied to tetragonal and cubic unit cells with static point-charges, this model is further ameliorated in order to take into account the replication of this unit cell through space and to account for the dynamical nature of charge distribution. Once our protocol has been calibrated based on a toy model, it is exploited for investigating MAPbI3systems in both tetragonal and cubic phases.

A near solvent-free synthetic route for Ce-UiO-66 metal–organic frameworks (MOFs) is presented. The MOFs are obtained by energetically grinding the reagents, cerium ammonium nitrate (CAN) and the carboxylic linkers, in a mortar for a few minutes with the addition of a small amount of acetic acid (AcOH) as a modulator (8.75 equiv, 0.5 mL). The slurry is then transferred into a 2 mL vial and heated at 120 °C for 1 day. The MOFs have been characterized for their composition, crystallinity, and porosity and employed as heterogeneous catalysts for the photo-oxidation reaction of substituted benzylic alcohols to benzaldaldehydes under near-ultraviolet light irradiation. The catalytic performances, such as selectivity, conversion, and kinetics, exceed those of similar systems studied by chemical oxidation using similar Ce-MOFs as a catalyst. Moreover, the MOFs were found to be reusable up to three cycles without loss of activity. Density functional theory (DFT) calculations were used to fully describe the electronic structure of the best performing MOFs and to provide useful information on the catalytic activity experimentally observed.

Two-dimensional (2D) and quasi-2D perovskite materials have enabled advances in device performance and stability relevant to a number of optoelectronic applications. However, the alignment among the bands of these variably quantum confined materials remains a controversial topic: there exist multiple experimental reports supporting type-I, and also others supporting type-II, band alignment among the reduced-dimensional grains. Here we report a combined computational and experimental study showing that variable ligand concentration on grain surfaces modulates the surface charge density among neighboring quantum wells. Density functional theory calculations and ultraviolet photoelectron spectroscopy reveal that the effective work function of a given quantum well can be varied by modulating the density of ligands at the interface. These induce type-II interfaces in otherwise type-I aligned materials. By treating 2D perovskite films, we find that the effective work function can indeed be shifted down by up to 1 eV. We corroborate the model via a suite of pump–probe transient absorption experiments: these manifest charge transfer consistent with a modulation in band alignment of at least 200 meV among neighboring grains. The findings shed light on perovskite 2D band alignment and explain contrasting behavior of quasi-2D materials in light-emitting diodes (LEDs) and photovoltaics (PV) in the literature, where materials can exhibit either type-I or type-II interfaces depending on the ligand concentration at neighboring surfaces.

Halide ion mobility in metal halide perovskites remains an intriguing phenomenon, influencing their optical and photovoltaic properties. Selective injection of holes through electrochemical anodic bias has allowed us to probe the effect of hole trapping at iodide (0.9 V) and bromide (1.15 V) in mixed halide perovskite (CH3NH3PbBr1.5I1.5) films. Upon trapping holes at the iodide site, the iodide gradually gets expelled from the mixed halide film (as iodine and/or triiodide ion), leaving behind re-formed CH3NH3PbBr3domains. The weakening of the Pb–I bond following the hole trapping (oxidation of the iodide site) and its expulsion from the lattice in the form of iodine provided further insight into the photoinduced segregation of halide ions in mixed halide perovskite films. Transient absorption spectroscopy revealed that the iodide expulsion process leaves a defect-rich perovskite lattice behind as charge carrier recombination in the re-formed lattice is greatly accelerated. The selective mobility of iodide species provides insight into the photoinduced phase segregation and its implication in the stable operation of perovskite solar cells.

Dye-sensitized solar cells (DSCs) have gained great attention in recent years due to their low-cost fabrication, flexibility, and high power conversion efficiency. In a DSC, due to interfaces between the dye and the charge-transport materials, the interface electrostatics becomes a key factor determining the overall performance of the cell. Liquid-electrolyte-based DSCs suffer from low stability, electrolyte leakage, and, in some cases, electrode corrosion. Replacing liquid electrolyte with a solid semiconducting material leads to poor interfacial contacts, hence the interface electrostatics becomes one of the limiting factors. In this work, we present a drift-diffusion and density functional theory (DFT) study of solid-state DSCs to investigate the electrostatics at the TiO2/organic dye/Spiro-OMeTAD interface and its impact on the adsorbed dye energy levels, its absorption spectrum, and the related charge injection. In our three-dimensional drift-diffusion model, we solve a set of drift-diffusion equations coupled to Poisson equation for electrons, holes, doping impurities, and interface traps simultaneously. After that, we use first-principles DFT modeling of dye-sensitized interfaces in the presence of the calculated electric fields. We find that interface traps located below the conduction band edge of mesoporous TiO2influence the accumulation of photogenerated holes and built-in electric field near the interface. The built-in electric field leads to change in the energetics at the dye/TiO2interface, leading to poor charge injection from excited dye into TiO2. The simulations were carried out for different electronic trap densities in TiO2and different doping levels in the Spiro-OMeTAD hole-transport layer. This study helps to a better understanding of the interface electrostatics and its role in the charge injection mechanism of solid-state DSCs.

In the present work, the substitution of cesium (Cs+) with rubidium (Rb+) in fully inorganic tin bromide perovskites Cs1–xRbxSnBr3, has been experimentally demonstrated by synthesizing pure single-phase samples in the CsSnBr3–Cs0.70Rb0.30SnBr3compositional range. The substitution of Cs with Rb is responsible for structural modification from cubic to orthorhombic symmetry, which has been correlated with optical properties, as the band gap varies from 1.719 to 1.817 eV from CsSnBr3to Cs0.70Rb0.30SnBr3sample. Notably, all of the rubidium-embedding alloys present good air stability. All of these results are very straightforward and open the possibility to exploit the electrical and optical capabilities of this very promising family of lead-free materials.

Doping organic metal-halide perovskites with cesium could be the best solution to stabilize highly-efficient perovskite solar cells. The understanding of the respective roles of the organic molecule, on one hand, and the inorganic lattice, on the other hand, is thus crucial to be able to optimize the physical properties of the mixed cation structures. In particular, the study of the recombination mechanisms is thought to be one of the key challenges toward full comprehension of their working principles. Using molecular dynamics and frozen phonons, we evidence subpicosecond anharmonic fluctuations in the fully inorganic CsPbI3perovskite. We reveal the effect of these fluctuations, combined with spin–orbit coupling, on the electronic band structure, evidencing a dynamical Rashba effect. Our study shows that under certain conditions space disorder can quench the Rashba effect. As for time disorder, we evidence a dynamical Rashba effect which is similar to what was found for MAPbI3and which is still sizable despite temperature disorder, the large investigated supercell, and the absence of the organic cations’ motion. We show that the spin texture associated with the Rashba splitting cannot be deemed responsible for a consistent reduction of recombination rates, although the spin mismatch between the valence and conduction bands increases with the ferroelectric distortion causing the Rashba splitting.

Heterovalent doping in lead halide perovskites was only marginally explored. Particular attention was focused on Bi3+dopant, which was found to increase the α-phase stability for CsPbI3, leading to high efficiency of fully inorganic perovskite solar cells. It was recently demonstrated that the absorption onset red-shift of the Bi-doped perovskite is due to the increased number of defect states and a significant increase in the sub-band-gap density of states. Here we computationally simulated the electronic properties of the Bi-doped MAPbI3(MA = CH3NH3+) perovskite to gain insight into the electronic structure modifications occurring upon heterovalent doping. Our results confirm the presence of deep trap states induced by the Bi dopant, with the Bi3+acting as deep electron trap. The absorption onset red-shift observed upon Bi-doping of MAPbI3is mainly related to transitions to the Bi defect states, while the perovskite band gap is essentially unaltered.

The role of chlorine doping in CH3NH3PbI3represents an important open issue in the use of hybrid perovskites for photovoltaic applications. In particular, even if a positive role of chlorine doping on perovskite film formation and on material morphology has been demonstrated, an inherent positive effect on the electronic and photovoltaic properties cannot be excluded. Here we carried out periodic density functional theory and Car–Parrinello molecular dynamics simulations, going down to ∼1% doping, to investigate the effect of chlorine on CH3NH3PbI3. We found that such a small doping has important effects on the dynamics of the crystalline structure, both with respect to the inorganic framework and with respect to the cation libration motion. Together, we observe a dynamic spatial localization of the valence and conduction states in separated spatial material regions, which takes place in the 10–1ps time scale and which could be the key to ease of exciton dissociation and, likely, to small charge recombination in hybrid perovskites. Moreover, such localization is enhanced by chlorine doping, demonstrating an inherent positive role of chlorine doping on the electronic properties of this class of materials.

Mixed-cation metal halide perovskites have shown remarkable progress in photovoltaic applications with high power conversion efficiencies. However, to achieve large-scale deployment of this technology, efficiencies must be complemented by long-term durability. The latter is limited by external factors, such as exposure to humidity and air, which lead to the rapid degradation of the perovskite materials and devices. In this work, we study the mechanisms causing Cs and formamidinium (FA)-based halide perovskite phase transformations and stabilization during moisture and air exposure. We use in situX-ray scattering, X-ray photoelectron spectroscopy, and first-principles calculations to study these chemical interactions and their effects on structure. We unravel a surface reaction pathway involving the dissolution of FAI by water and iodide oxidation by oxygen, driving the Cs/FA ratio into thermodynamically unstable regions, leading to undesirable phase transformations. This work demonstrates the interplay of bulk phase transformations with surface chemical reactions, providing a detailed understanding of the degradation mechanism and strategies for designing durable and efficient perovskite materials.

Over the past 20 years, ruthenium(II)-based dyes have played a pivotal role in turning dye-sensitized solar cells (DSCs) into a mature technology for the third generation of photovoltaics. However, the classic I3–/I–redox couple limits the performance and application of this technique. Simply replacing the iodine-based redox couple by new types like cobalt(3+/2+) complexes was not successful because of the poor compatibility between the ruthenium(II) sensitizer and the cobalt redox species. To address this problem and achieve higher power conversion efficiencies (PCEs), we introduce here six new cyclometalated ruthenium(II)-based dyes developed through ligand engineering. We tested DSCs employing these ruthenium(II) complexes and achieved PCEs of up to 9.4% using cobalt(3+/2+)-based electrolytes, which is the record efficiency to date featuring a ruthenium-based dye. In view of the complicated liquid DSC system, the disagreement found between different characterizations enlightens us about the importance of the sensitizer loading on TiO2, which is a subtle but equally important factor in the electronic properties of the sensitizers.

A coherent account of adsorption modes, dynamics, self-aggregation, and spectroscopic properties of an indoline organic dye adsorbed on TiO2anatase (101) substrates is reported. The study is performed by combining reactive molecular dynamics (reaxFF) simulations with time-dependent density functional theory calculations, and the reliability of the results is assessed through comparison with theoretical and experimental data available in the literature. The use of a theoretical multilevel approach has proven to be crucial to gain a deep understanding, at an atomistic level, of the morphology and electronic properties of dye-sensitized heterogeneous interfaces. A realistic description of the functionalized anatase (101) interface, where a variety of binding modes are present, has been achieved by means of extensive molecular dynamics simulations of the adsorption of dye clusters made of different molecular units on medium/large size TiO2anatase slabs. Our results disclose that the main driving forces toward formation of ordered surface aggregates are π stacking and T-shaped interactions between the aromatic rings of the donor moiety of the molecules, as well as the tendency to maximize the anchoring points with the surface. The dye aggregates were found to be organized in domains, characterized by a different orientation of the packing units, and, in the high coverage limit, presenting a certain degree of short-to-medium range order.

The rational design of ligand molecules has earned lots of attention as an elegant means to tailor the electronic and optical properties of semiconductor quantum dots (QDs). Aromatic dithiocarbamate molecules, in particular, are known to greatly influence the optoelectronic properties of CdSe QDs, red-shifting the absorption features and enhancing the photoluminescence. Here, we present an integrated computational study, which combines ab initiomolecular dynamics and excited state calculations including thousands of excitations, aimed at understanding the impact of this kind of surface ligand on the optoelectronic properties of CdSe QDs. We demonstrate that the valence electronic states of the dithiocarbamate molecules, mostly localized in the anchoring moiety, are responsible for the red-shift of the absorption features of capped CdSe QDs. Ligands develop interfacial electronic states close to the band edges of the CdSe, which enhance the absorption features of the QD and might open new channels for the radiative decay from the excited state, improving optical emission. Hybridized QD/ligand states could also funnel interfacial charge transfer between the inorganic core and surface molecules, a process that lies at the heart of many photovoltaic and photocatalytic devices. This work may pave the way toward the design of new capping ligands that, adsorbed on the QD surface, could provide control over the optoelectronic properties of the semiconductor core.

We report on the density functional theory (DFT) modeling of core/shell quantum dot (QD) sensitized solar cells (QDSSCs), a device architecture that holds great potential in photovoltaics but has not been fully exploited so far. To understand the working mechanisms of this kind of solar cells, we have investigated ZnSe- and ZnSe/CdS-sensitized TiO2models. Both the core-only and the core/shell QDs are predicted to strongly adsorb on the oxide surface, driven by the electrostatic interaction between the metal atoms on the QD surface and the O atoms exposed by the oxide substrate. Accordingly, the QD conduction states are strongly mixed with the TiO2acceptor states, giving rise to bridge states that should funnel the interfacial electron transfer. Accordingly, quite fast electron injection processes are predicted, with computed rates of 135 and 163 fs. The back-electron transfer is much slower for ZnSe/CdS, due to the weak coupling between the newly injected charge and the holes trapped in the sensitizer core. Therefore, the core/shell QDs deliver much better efficiencies. Moreover, the interfacial dipole established between the TiO2-injected electrons and the holes confined in the QD are found to shift the conduction band edge of the oxide, which further improves the performance of the device in terms of the open circuit voltage (VOC). We believe that this work sets the ground for future computational works in the field, which could in turn guide the fabrication of new device architectures with improved efficiencies.

We present an innovative methodology for the computer simulation of mesoporous nanoparticulated oxide films based on the random packing of faceted nanopolyhedra in the form of bifrustums, reproducing the experimentally observed TiO2anatase bifrustum shape. A computer simulation employing nanospheres as packing objects was preliminary considered to verify the validity of the developed procedure. The pore size distribution and other fundamental characteristics of the simulated films (porosity, radial distribution function, coordination number, contact number) are computed for nanospheres and nanopolyhedra simulated films. Our results show that the use of faceted bifrustums, while involving a computationally more demanding procedure, is essential to attain a reliable description of the morphology and local three-dimensional structure of mesoporous TiO2nanoparticles films.

We have performed density functional theory calculations to describe the changes in the electronic structure of oligothiophene derivatives equipped with carboxylic acid anchoring groups upon chemical grafting on TiO2clusters. The adsorption promotes a partial pinning effect for the lowest unoccupied molecular orbital of the dye, i.e., a LUMO energy level alignment with respect to the TiO2conduction band edge (CBE) irrespective of the oligothiophene length, which we ascribe to a strong hybridization between the dye discrete (LUMO) level and the cluster conduction band (CB). This is borne out by the fact that no pinning is observed, when decoupling the conjugated segment from the metal oxide substrate, e.g., by introducing a phenylene (PTn-Ph) or vinylene phenylene (PTn-Vi-Ph) moiety in a meta configuration or when there is a large energy mismatch between the dye frontier levels and the oxide conduction band, as is the case for oligothiophene-S,S-dioxides (with LUMO levels deep below the oxide CBE). Implications for electron injection into the titania and the optical absorption properties of the oxide–dye hybrids are discussed.

The structure and energetics of excitons and individual electron and hole polarons in a model anatase TiO2nanoparticle (NP) are investigated by means of Density Functional Theory (DFT) and Time Dependent (TD)-DFT calculations. The effect of the Hartree–Fock exchange (HF-exc) contribution in the description of TiO2NPs with unpaired electrons is examined by comparing the results from semilocal and hybrid DFT functionals with different HF-exc percentages, including a long-range corrected hybrid functional. The performances of TD-DFT and ground state (SCF) DFT approaches in the description of the photoexcited polaron states in TiO2NPs are also analyzed. Our results confirm that the HF-exc contribution is essential to properly describe the self-trapping of the charge carriers. They also suggest that long-range corrected functionals are needed to properly describe excited state relaxation in TiO2NPs. TD-DFT geometry optimization of the lowest excited singlet and triplet states deliver photoluminescence values in close agreement with the experimental data.

Making use of the recently developed excited state charge displacement analysis [E. Ronca et al., J. Chem. Phys. 140, 054110 (2014)], suited to quantitatively characterize the charge fluxes coming along an electronic excitation, we investigate the role of the density relaxation effects in the overall description of electronically excited states of different nature, namely, valence, ionic, and charge transfer (CT), considering a large set of prototypical small and medium-sized molecular systems. By comparing the response densities provided by time-dependent density functional theory (TDDFT) and the corresponding relaxed densities obtained by applying the Z-vector postlinear-response approach [N. C. Handy and H. F. Schaefer, J. Chem. Phys. 81, 5031 (1984)] with those obtained by highly correlated state-of-the-art wave function calculations, we show that the inclusion of the relaxation effects is imperative to get an accurate description of the considered excited states. We also examine what happens at the quality of the response function when an increasing amount of Hartree–Fock (HF) exchange is included in the functional, showing that the usually improved excitation energies in the case of CT states are not always the consequence of an improved description of their overall properties. Remarkably, we find that the relaxation of the response densities is always able to reproduce, independently of the extent of HF exchange in the functional, the benchmark wave function densities. Finally, we propose a novel and computationally convenient strategy, based on the use of the natural orbitals derived from the relaxed TDDFT density to build zero-order wave function for multireference perturbation theory calculations. For a significant set of different excited states, the proposed approach provided accurate excitation energies, comparable to those obtained by computationally demanding ab initio calculations.

We report on the relevance of spin–orbit coupling on the optical properties of Ru(II)- and Os(II)-polypyridyl dyes effectively employed in dye-sensitized solar cells (DSCs). We include relativistic effects on time-dependent density functional theory calculations of selected complexes by using different levels of calculations, i.e., the scalar zero-order regular approximation (ZORA) and the fully relativistic ZORA including spin–orbit coupling, in such a way so as to disentangle and evaluate the spin–orbit effect. The widely investigated [M(bpy)3]2+(M = Ru(II) and Os(II)) have been selected as benchmark complexes in our calculations; this is followed by investigation of “realistic” dyes used in DSCs, such as the prototypical N3 dye, its Os-based analogue, and a panchromatic Os(II) dye. We find that in Ru(II) complexes, spin–orbit coupling leads to a slight correction of the spectral shape, whereas only when we include the spin–orbit coupling we are able to reproduce the low-energy absorption bands characteristic of the Os(II) complexes. This study allows us to find a quantitative correlation between the strength of spin–orbit coupling and the metal center, highlighting the secondary effect of the different ligands experienced by the metal center.

We report a joint theoretical and experimental investigation on the effect of TiO2protonation on the interfacial electronic coupling and injection rates in organic dye-sensitized solar cells (DSCs). We model the electronic structure of different organic dye-sensitized TiO2cluster models at different degrees of surface protonation and experimentally show the enhancement in the photocurrent generation upon the acidic treatment of the substrate. By merging theory and experiments, we elucidate the role of TiO2protonation on the relative alignment and electronic coupling (injection rates) between the dye’s lowest unoccupied molecular orbital and the semiconductor conduction band states, also in relation to the different electronic structure of the anchored dye (length of conjugation, conjugated vs not conjugated anchoring group). The photocurrent enhancement observed with TiO2protonation is attributed to a combined effect of both red-shifted absorption of the protonated TiO2films and to an overall improvement in the interfacial charge generation

Three novel dyes based on ZnIIporphyrinates combined, in β-pyrrolic position, with the π unit dithienylethylene (DTE) have been synthesized and investigated for application in DSSCs. The panchromatic effect due to elongation of the π-delocalized system through a bridge between the porphyrinic ring and the DTE unit such as the 4-ethynylstyryl (1), ethynyl (2), and ethenyl (3) bonds have been investigated by computational, electrochemical, and photoelectrochemical methods. For all three dyes the π conjugated substituents in the β position produced the expected panchromatic effect with broadened electronic absorption spectra over a wide range of wavelengths and IPCE spectra featuring a broad plateau in the region 430–650 nm. In addition both DFT computational and electrochemical data have shown a smaller HOMO–LUMO energy gap for dye 3,when compared to dye 2suggesting a slightly more facile conjugation between the porphyrinic core and the DTE unit through the ethenylic bond. Conversely the photoelectrochemical investigation showed improved DSSC performances from 3to 1. These results have been rationalized by an in-depth DFT computational study of dyes 2and 3interacting with a cluster of 82 TiO2units. The small energetic overlap between the LUMO and the TiO2conduction band characterizing the more structurally distorted dye 3would suggest low quantum yield of electron injection, while dye 2shows a greater interaction between the LUMO of the dye and the semiconductor. Consequently the increased linearity and planarity of the structure of dye 1seems to be the origin of its best performance in DSSC. Therefore it appears that the nature of the bridge between the DTE unit and the porphyrinic ring is quite relevant for the efficiency of these dyes for DSSC, due to distortion from the planarity and linearity of the structure of the dye and the consequent changes on the dye π conjugation.

We carry out an accurate computational analysis on the nature and distribution of electronic trap states in shape-tailored anatase TiO2structures, investigating the effect of the morphology on the electronic structure. Linear nanocrystal models up to 6 nm in length with various morphologies, reproducing both flattened and elongated rod-shaped TiO2nanocrystals, have been investigated by DFT calculations, to clarify the effect of the crystal facet percentage on the nanocrystal electronic structure, with particular reference to the energetics and distribution of trap states. The calculated densities of states below the conduction band edge have been very well fitted assuming an exponential distribution of energies and have been correlated with experimental capacitance data. In good agreement with the experimental phenomenology our calculations show that elongated rod-shaped nanocrystals with higher values of the ratio between (100) and (101) facets exhibit a relatively deeper distribution of trap states. Our results point at the crucial role of the nanocrystal morphology on the trap state density, highlighting the importance of a balance between the low-energy (101) and high-energy (100)/(001) surface facets in individual TiO2nanocrystals.

Quantum mechanical calculations on the structural and optoelectronic features of two realistic wurtzite-like ZnS quantum dot (QD) models, namely, (ZnS)33and (ZnS)116, are presented both in vacuo and in an explicit water solution environment. Car–Parrinello molecular dynamics (CPMD) simulation and excited-state, Time-Dependent Density Functional Theory (DFT/TDDFT) calculations on extended models are combined to unravel hitherto inaccessible atomistic features of the investigated systems. Ultrasmall QDs are predicted to exhibit strong dynamical fluctuations. Accordingly, the bare (ZnS)33model undergoes a drastic structural rearrangement and evolves from the starting bulk-like structure to an amorphous phase. The geometrical changes occurring over the time are reflected on the opto-electronic properties. The band-edge states and the optical absorption onset both sizably vary along the CPMD trajectory. Eventually, the optical gap decreases due to the emergence of high-lying occupied orbitals. These midgap states are mainly localized in undercoordinated S sites and could act as trap states for the photogenerated holes. Water molecules are predicted to form strong Zn–OH2bonds with the surface Zn atoms. Hydration seems to lower the surface energy, stabilize the wurtzite polymorph, hinder the Zn–S bond breaking, and largely prevent the appearance of trap states. Besides, adsorbed water molecules produce a notable blue-shift of the optical gap. The electrostatic field induced by the solvent shell and the electron-donor properties of the water molecules are supposed to be responsible for the opening of the gap. Moreover, capping the QDs with water molecules increases the intensity of the lowest-lying electronic excitations. This study sheds light on the important opto-electronic modifications occurring for realistic QD in water solution and offers at the same time the methodological framework to investigate photocatalytic reactions mediated by ZnS.

We report a joint experimental and computational work on new organic donor–acceptor dye sensitizers in which a carbazole (CZ) and a phenothiazine (PTZ) units are linked together by an alkyl C6H13, while two different anchoring groups are employed: the cyanoacrylic acid (CS1A, CSORG1) and the rhodanine-3-acetic acid (CS4A, CSORG4). The CZ moiety has multiple roles of (i) acting as an extra-electron donor portion, providing more electron density on the PTZ; (ii) suppressing the back-electron transfer from TiO2to the electrolyte by forming a compact insulating dye layer; (iii) modulating dye aggregation on the semiconductor surface; and (iv) acting as an antenna, collecting photons and, through long-range energy transfer, redirecting the captured energy to the dye sensitizer. We show that the introduction of the CZ donor remarkably enhances the photovoltaic performances of the rhodanine-based dye, compared to the corresponding simple PTZ dye, with more than a two-fold increase in the overall efficiencies, while it does not bring beneficial effects in the case of the cyanoacrylic-based sensitizer. Based on quantum mechanical calculations and experimental measurements, we show that, in addition to a favored long-range energy transfer, which increases the light absorption in the blue region of the spectrum, the presence of the CZ unit in the CSORG4 dye effectively induces a beneficial aggregation pattern on the semiconductor surface, yielding a broadened and red-shifted light absorption, accounting for the two-fold increase in the generated photocurrent.

We report a computational modeling investigation, based on DFT and TDDFT calculations, on the structural, electronic, and optical properties of three prototypical donor–acceptor polymers based on the isoindigo unit acceptor moiety, namely, PTI-1, PBDT-I, and PBDT-TIT, in order to calibrate a computational protocol to screen new candidate polymers and to get a better understanding of the properties of the investigated series. Starting from the monomeric units and by using a growing-up approach, we were able to reproduce the experimental electrochemical and optical properties and to estimate the effective conjugation length of these polymers. This study can support the choice of suitable donor and acceptor building blocks and provides the computational framework for an in silico screening of new target photoactive polymeric systems.

We computationally investigate organometal CH3NH3PbX3and mixed halide CH3NH3PbI2X perovskites (X = Cl, Br, I), which are key materials for high efficiency solid-state solar cells. CH3NH3PbX3perovskites exhibited the expected absorption blue shift along the I → Br → Cl series. The mixed halide systems surprisingly showed the CH3NH3PbI3and the CH3NH3PbI2Cl (or CH3NH3PbI3–xClx) perovskites to have similar absorption onset at ∼800 nm wavelength, whereas CH3NH3PbI2Br absorbs light below ∼700 nm. To provide insight into the structural and electronic properties of these materials, in light of their application as solar cell active layers, we perform periodic DFT calculations on the CH3NH3PbX3and CH3NH3PbI2X perovskites. We find a good agreement between the calculated band structures and the experimental trend of optical band gaps. For the mixed halide perovskites our calculations show the existence of two different types of structures with different electronic properties, whose relative stability varies by varying the X group. For these systems, the calculated formation energies decrease in the order I > Br > Cl, in line with the observed miscibility of CH3NH3PbI3and CH3NH3PbBr3compounds, while suggesting a comparatively smaller chlorine incorporation into CH3NH3Pb(I1–xClx)3compounds. We also show that Cl atoms preferentially occupy the apical positions in the PbI4X2octahedra, while Br atoms may occupy both apical and equatorial positions, consistent with reported lattice parameters. The interplay of the organic and inorganic components of the perovskites, possibly mediated by hydrogen bonding between the ammonium groups and the halides, seems to be the key to the observed structural variability.

Photoinduced electron transfer at low-band-gap ruthenium-based dye/TiO2has been investigated by means of ultrafast transient absorption and DFT/TDDFT calculations. We demonstrate that although the charge generation mechanism is triplet mediated upon band gap excitation, as already proven in high band gap dyes such as the well-known N3 and N719, when excess energy is provided which allows to reach high energy singlet states still in the visible spectral range, ultrafast electron transfer takes place. No intersystem crossing process is observed and charge generation happens only from the singlet excited state.

Combining computational modeling and experimental optical analyses, we investigate two prototypical phenothiazine-based organic solar cell sensitizers with the aim to understand the individual effects of solvation and aggregation on the dyes optical properties. Dye solvation and aggregation play a crucial role in determining the photoelectrochemical properties of these systems and the interplay of these two factors can lead to a misinterpretation of the underlying phenomenology due to their similar spectroscopic signals. In particular, upon adsorption of the dye onto the metal oxide surface, the dye UV–vis absorption spectrum may attain either a blue or a red shift compared to the dye in solution, which can either be originated from aggregation of surface-adsorbed dye and solvatochromism in the initial dye solution. Understanding the origin of these spectral changes along with their possible effect on charge-transfer properties is important for the further improvement of dye-sensitized solar cells. Based on our results, we show that the optical properties of phenothiazine-based dyes are much more sensitive to the type of explicit interactions with the solvent than to aggregation on the TiO2surface. Therefore, this study gets new insights into the understanding of these properties and may assist the molecular engineering of new and more efficient dyes sensitizers.

Theoretical and computational modeling is a powerful tool to investigate and characterize the structural, electronic, and optical properties of the main components of dye-sensitized solar cells (DSCs). In this article we focus on the description of the ground and excited state properties of both standalone and TiO2-adsorbed metallorganic and fully organic dyes, relevant to modeling the dye→semiconductor electron injection process, which is the primary charge generation step in DSCs. By reviewing previous data from our laboratory, integrated with new calculations, we wish to critically address the potential and limitations of current DFT and TDDFT computational methods to model DSCs. While ruthenium dyes are accurately described by standard DFT approaches, for highly conjugated organic dyes, characterized by strong charge transfer excited states, specifically tailored exchange-correlation functionals are needed. For ruthenium dye/semiconductor interfaces, a strategy is presented, which accurately describes the electronic and optical properties andthe alignment of ground and excited state levels at the same time, allowing us to discuss the coupling and the energetics of the excited states underlying the ultrafast electron injection. For donor–acceptor organic dyes, a simple picture based on the dye lowest unoccupied molecular orbital (LUMO) broadening accounts for the different interfacial electronic coupling exhibited by dyes with different anchoring groups. The explored DFT/TDDFT methods, however, are not capable to deliver at the same time a balanced description of the dye@TiO2excited states and of the alignment of the dye excited states with the semiconductor manifold of unoccupied states. This represents a challenge which should be addressed by next generation DFT or post-DFT methods.