We utilize herein ultrafast mid-infrared probe laser pulses to explore the mechanism for the charge recombination/trapping process of photogenerated charges within the band gap of TiO2 and across interfaces. Low-energy photons solely probe the free electrons present in the conduction band of TiO2 and those captured in shallow-trap states. We found that >70% of the photogenerated charges disappear from the conduction band in the first few nanoseconds due to electron trapping followed by charge recombination at longer time scales. Moreover, the behavior of the dynamics of the free electrons within the band gap of TiO2 and electrons generated at the interface of adsorbed organic dyes was investigated and compared. This comparison shows that the main driving force for the efficient charge trapping of photogenerated charges within the picosecond time scale is the presence of photogenerated holes, within the band gap of TiO2, or close to the interface of TiO2. If the hole is far from the TiO2 surface, the electron trapping process is hindered, and almost 100% of photogenerated charges can survive up to nanoseconds. This work offers a deeper understanding of the charge trapping and charge recombination processes, by knowing the spatial hole effect, in TiO2 and similar semiconductors upon utilization in photonic devices and photocatalysis.

Adequate hole mobility is the prerequisite for dopant-free polymeric hole-transport materials (HTMs). Constraining the configurational variation of polymer chains to afford a rigid and planar backbone can reduce unfavorable reorganization energy and improve hole mobility. Herein, a noncovalent conformational locking via S–O secondary interaction is exploited in a phenanthrocarbazole (PC) based polymeric HTM, PC6, to fix the molecular geometry and significantly reduce reorganization energy. Systematic studies on structurally explicit repeats to targeted polymers reveals that the broad and planar backbone of PC remarkably enhances π–π stacking of adjacent polymers, facilitating intermolecular charge transfer greatly. The inserted “Lewis soft” oxygen atoms passivate the trap sites efficiently at the perovskite/HTM interface and further suppress interfacial recombination. Consequently, a PSC employing PC6 as a dopant-free HTM offers an excellent power conversion efficiency of 22.2 % and significantly improved longevity, rendering it as one of the best PSCs based on dopant-free HTMs. 

In the ambition to improve the power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs), it will be essential to understand the mechanisms and rates of dye regeneration. Although the mechanism of dye regeneration has been studied by static density functional theory (DFT) and classical molecular dynamics (CMD) simulations, ab initio molecular dynamics simulation (aiMD) has the potential to combine the insights from both methods for a deeper understanding. In this work, a series of aiMD simulations has been performed to study the interaction between an oxidized organic model dye, LEG4, and an electrolyte containing iodide ions as reducing agents. Dynamic Mulliken and natural spin population analyses show that two iodide ions, I-center dot center dot center dot I-, are required for dye regeneration. It was found that a distance between I-center dot center dot center dot I(-)of less than 6.5 angstrom at site 1 benefits from the electrostatic environment of the triphenylamine group of the LEG4 dye, and a corresponding distance of 4.8 angstrom at site 2 is essential for the dye regeneration process to take place. The rate constants of the LEG4 regeneration by two iodine ions range from 10(5) to 10(12) s(-1), spanning a window in which results from both experimental and static theoretical calculations fall. It is also verified that the probability of electron transfer from a radical I-2(-) to the oxidized LEG4 dye is extremely low due to the rapid electron back transfer. However, it has been found that the addition of an additional iodide ion at a distance of 5 angstrom with respect to the radical I-2(-) opens the pathway for the reduction of the oxidized LEG4 dye with an associated formation of I-3(-). The current results highlight the necessity for a dynamical approach for a full understanding of the regeneration process.

To gain a deeper understanding of the underlying charge processes in dye sensitized photocathodes, lateral electron hopping across dye-sensitized NiO photocathodes was investigated. For dye-sensitized systems, hole hopping across photoanodes has been studied extensively in the literature but no expansive studies on electron hopping in sensitized photocathodes exist today. Therefore, an organic p-type dye (TIP) with donor-linker-acceptor design, showing high stability and electrochemical reversibility, was used to study the electron transfer dynamics (electron-hopping) between dyes with temperature dependent spectroelectrochemistry and computational simulations. Besides intermolecular electron-hopping across the surface with a rate constant in the order of 10(5) s(-1), our results show a second electron hopping pathway between NiO surface states with a rate constant in the order of 10(7) s(-1), which precedes the electron hopping between the dyes. Upon application of a potential step negative enough to reduce both the dye and NiO surface states, the majority of NiO surface states need to be reduced before intermolecular electron transfer can take place. The results indicate that, in contrast to sensitized photoanodes where intermolecular charge transfer is known to influence recombination kinetics, intermolecular charge transport processes in TIP dye sensitized NiO photocathodes is less relevant because the fast electron transport between NiO surface states likely dominates recombination kinetics.

Ab initio molecular dynamics simulations were employed to investigate the regeneration of the oxidized organic dye LEG4+by the reducing agents Fc0and Co[(bpy)3]2+. Dynamical Mulliken spin population analyses suggest that the oxidized LEG4+may be regenerated by Fc0and Co[(bpy)3]2+directly in specific configurations providing that the Fe2+and Co2+are in a low-spin state. An exponential coupling relation was found between the distance between the dye and the redox mediators. The rate of the LEG4+regeneration by Fc0and Co[(bpy)3]2+ranges between 5.41 μs-1∼3.80 ps-1and 0.58 μs-1∼0.04 ps-1, respectively, which spans the window of all experimentally reported rates.

A new class of polymeric hole-transport materials (HTMs) are explored by inserting a two-dimensionally conjugated fluoro-substituted pyrene into thiophene and selenophene polymeric chains. The broad conjugated plane of pyrene and “Lewis soft” selenium atoms not only enhance the π–π stacking of HTM molecules greatly but also render a strong interaction with the perovskite surface, leading to an efficient charge transport/transfer in both the HTM layer and the perovskite/HTM interface. Note that fluorine substitution adjacent to pyrene boosts the stacking of HTMs towards a more favorable face-on orientation, further facilitating the efficient charge transport. As a result, perovskite solar cells (PSCs) employing PE10 as dopant-free HTM afford an excellent efficiency of 22.3 % and the dramatically enhanced device longevity, qualifying it among the best PSCs based on dopant-free HTMs. 

A new crosslinked polymer, called P65, with appropriate photo-electrochemical, opto-electronic, and thermal properties, has been designed and synthesized as an efficient, dopant-free, hole-transport material (HTM) for n-i-p type planar perovskite solar cells (PSCs). P65 is obtained from a low-cost and easily synthesized spiro[fluorene-9,9′-xanthene]-3′,6′-diol (SFX-OH)-based monomer X65 through a free-radical polymerization reaction. The combination of a three-dimensional (3D) SFX core unit, hole-transport methoxydiphenylamine group, and crosslinked polyvinyl network provides P65 with good solubility and excellent film-forming properties. By employing P65 as a dopant-free hole-transport layer in conventional n-i-p type PSCs, a power conversion efficiency (PCE) of up to 17.7% is achieved. To the best of our knowledge, this is the first time a 3D, crosslinked, polymeric dopant-free HTM has been reported for use in conventional n-i-p type PSCs. This study provides a new strategy for the future development of a 3D crosslinked polymeric dopant-free HTM with a simple synthetic route and low-cost for commercial, large-scale applications in future PSCs.

The charge-transport dynamics at the dye-TiO2interface plays a vital role for the resulting power conversion efficiency (PCE) of dye sensitized solar cells (DSSCs). In this work, we have investigated the charge-exchange dynamics for a series of organic dyes, of different complexity, and a small model of the semiconductor substrate TiO2. The dyes studied involve L1, D35 and LEG4, all well-known organic dyes commonly used in DSSCs. The computational studies have been based onab initiomolecular dynamics (aiMD) simulations, from which structural snapshots have been collected. Estimates of the charge-transfer rate constants of the central exchange processes in the systems have been computed. All dyes show similar properties, and differences are mainly of quantitative character. The processes studied were the electron injection from the photoexcited dye, the hole transfer from TiO2to the dye and the recombination loss from TiO2to the dye. It is notable that the electronic coupling/transfer rates differ significantly between the snapshot configurations harvested from the aiMD simulations. The differences are significant and indicate that a single geometrically optimized conformation normally obtained from static quantum-chemistry calculations may provide arbitrary results. Both protonated and deprotonated dye systems were studied. The differences mainly appear in the rate constant of recombination loss between the protonated and the deprotonated dyes, where recombination losses take place at significantly higher rates. The inclusion of lithium ions close to the deprotonated dye carboxylate anchoring group mitigates recombination in a similar way as when protons are retained at the carboxylate group. This may give insight into the performance-enchancing effects of added salts of polarizing cations to the DSSC electrolyte. In addition, solvent effects can retard charge recombination by about two orders of magnitude, which demonstrates that the presence of a solvent will increase the lifetime of injected electrons and thus contribute to a higher PCE of DSSCs. It is also notable that no simple correlation can be identified between high/low transfer rate constants and specific structural arrangements in terms of atom-atom distances, angles or dihedral arrangements of dye sub-units.

Conjugated polymers are regarded as promising candidates for dopant-free hole-transport materials (HTMs) in efficient and stable perovskite solar cells (PSCs). Thus far, the vast majority of polymeric HTMs feature structurally complicated benzo[1,2-b:4,5-b']dithiophene (BDT) analogs and electron-withdrawing heterocycles, forming a strong donor-acceptor (D-A) structure. Herein, a new class of phenanthrocarbazole (PC)-based polymeric HTMs (PC1, PC2, and PC3) has been synthesized by inserting a PC unit into a polymeric thiophene or selenophene chain with the aim of enhancing the pi-pi stacking of adjacent polymer chains and also to efficiently interact with the perovskite surface through the broad and planar conjugated backbone of the PC. Suitable energy levels, excellent thermostability, and humidity resistivity together with remarkable photoelectric properties are obtained via meticulously tuning the conformation and elemental composition of the polymers. As a result, PSCs containing PC3 as dopant-free HTM show a stabilized power conversion efficiency (PCE) of 20.8% and significantly enhanced longevity, rendering one of the best types of PSCs based on dopant-free HTMs. Subsequent experimental and theoretical studies reveal that the planar conformation of the polymers contributes to an ordered and face-on stacking of the polymer chains. Furthermore, introduction of the "Lewis soft" selenium atom can passivate surface trap sites of perovskite films by Pb-Se interaction and facilitate the interfacial charge separation significantly. This work reveals the guiding principles for rational design of dopant-free polymeric HTMs and also inspires rational exploration of small molecular HTMs.

The ways to overcome surface charge recombination and poor interface contact are still the central challenges for the development of inorganic-organic hybrid halide perovskite solar cells (PSCs). [6,6]-Phenyl C-61 butyric acid methyl ester (PCBM) is commonly employed in PSCs, but it has some disadvantages including high charge recombination and poor surface coverage. Therefore, the addition of an interfacial engineering layer showing efficient surface passivation, electron extraction, and excellent interface contact can solve the above problems. Furthermore, by employing interface engineering with a spike structure of the energy levels, the reduced energy losses are beneficial to elevating the open-circuit voltage (V-oc) in PSCs. Herein, the linear naphthalene imide dimer containing an indacenodithiophene unit (IDTT2NPI) has been developed as an excellent interface engineering material to strengthen the perovskite performance. The introduction of a spike interface on the top of a methylammonium lead triiodide (MAPbI(3)) film resulted in a high V-oc of 1.12 V with the optimal efficiency reaching 20.2%. The efficiency enhancement can be traced to the efficient surface passivation and enhanced interface contact. The mechanism of IDTT2NPI as the interface engineering layer was investigated by both experiments and theoretical calculations. This work provides a promising naphthalene imide-based interfacial material for high-efficiency and stable PSCs.