Donor engineering of a benzothiadiazole-based D-A-D-type molecular semiconductor for perovskite solar cells: a theoretical study.

Due to the easy formation of compact molecular packing arrangements and the favorable photophysical and electrochemical properties, donor-acceptor-donor (D-A-D)-type small molecule hole-transporting materials (HTMs) have been widely synthesized and researched to improve the efficiency and stability of perovskite solar cells (PSCs). The main approach in recent experiments has been to seek good acceptors, whereas the influence of the electron-donating units has been less reported. In this work, six new benzothiadiazole-based D-A-D-type HTMs are tailored by employing the ethyl-substituted phenoxazine (POZ), phenothiazine (PTZ) and carbazole (CZ) as the donors. To obtain an elementary understanding of new HTMs, the electronic, optical, hole-transporting and interfacial properties are simulated with quantum chemistry methods. The results indicate that all tailored HTMs exhibit suitable energy alignment compared with the band structures of the perovskite, and the continuous highest occupied molecular orbital (HOMO) levels will be helpful for interfacial energy regulation. In comparison with the YN1, the maximum absorption wavelengths of the newly designed HTMs are red-shifted due to the decreased excitation energies from the ground-state to the first singlet excited-state. Importantly, the hole mobilities of all designed HTMs are distinctly higher than the referenced YN1, which is contributed by the better planarity of the molecular skeleton and the easier orbital overlapping between adjacent molecules. The interfacial simulations manifest that the FAPbI3/SM37 system displays a more stable adsorption configuration and greater charge redistributions at the interface compared to YN1, which further promotes the separation of photogenerated electron-hole pairs. Moreover, larger Stokes shifts and better solubility are also acquired for the new HTMs. In summary, our calculations not only propose several potential highly efficient HTMs, but also provide useful insights at the atomic level for the experimental synthesis of new D-A-D-type HTMs.

Buried Interface Regulation by Bio-Functional Molecules for Efficient and Stable Planar Perovskite Solar Cells.

Among the factors that lead to the reduction of the efficiency of perovskite solar cells (PSCs) the difficulty involved in realizing a high-quality film and the efficient charge transfer that takes place at the interface between electron-transport layer (ETL) and perovskite is worth mentioning. Here, a strategy for planar-type devices by natural bio-functional interfaces that uses a buried electron-transport layer made of cobalamin complexed tin oxide (SnO2 @B12 ) is demonstrated. Having systematically investigated the effects of SnO2 @B12 interfacial layer in perovskite solar cells, it can be concluded that cobalamin can chemically link the SnO2 layer and the perovskite layer, resulting in improved perovskite film quality and interfacial defect passivation. Utilizing SnO2 @B12 improves the efficiency of planar-type PSCs by 20.60 %. Furthermore, after 250 h of exposure to an ambient atmosphere, unsealed PSCs containing SnO2 @B12 degrade by 10 %. This research provides a viable method for developing bio-functional molecules that will increase the effectiveness and durability of planar-perovskite solar cells.

Tailoring of the core structure towards promising small molecule hole-transporting materials for perovskite solar cells: a theoretical study.

The design of new molecules with theoretical chemistry methods and further obtaining a fundamental understanding of the structure-property relationship are important for the development of high-efficiency hole-transporting materials (HTMs). Herein, the effect of semi-locked and fully-locked cores was systematically investigated based on two conformation-tunable tetrathienylethene (TTE) and tetraphenylethylene (TPE) units. Our results show that the highest occupied molecular orbital (HOMO) levels of the locked TTE-2 and TTE-3 are clearly down-shifted compared with that of the unlocked TTE-1, which is due to the decreased electronic conjugation between the locked cores and the triphenylamine (TPA) arms, whereas the same situation is not found for TPE-3 due to the twisted core configuration. Compared with the TTE-series, the TPE-series exhibits less optical absorption in the visible light region and enhanced stability. Meanwhile, the hole mobility of the designed HTMs displays an increased trend from the unlocked core to the semi-locked and fully-locked cores due to the gradually increasing hole transfer integral with enhanced molecular planarity. In addition, we also found that the reorganization energy of the locked TTE cores is obviously lowered compared to that of the unlocked one, which plays an important role in increasing the hole mobility. In summary, this work can provide some useful clues for designing high-efficiency two-dimensional HTMs, and two potential promising candidates (TTE-3 and TPE-3) are proposed.

Boosting the hole transport of conductive polymers by regulating the ion ratio in ionic liquid additives.

Poly(3,4-ethylenedioxythiophene) (PEDOT) has aroused great interest in organic electrics because of its high electrical conductivity and mechanical flexibility. To improve the charge transport, it can act as an ionic liquid (IL) additive due to its ion characteristics and high electrical conductivity. Herein, we investigated the hole-transport performance of PEDOT treated by ILs featuring specific ion ratios (4 : 1, 3 : 1, 2 : 1, 1 : 1, 1 : 2, 1 : 3, and 1 : 4) of the cation and anion through classical dynamics simulations and quantum mechanics computations. The hole mobility of the amorphous PEDOT, constituting nine EDOT monomers, could be improved to 16.81, 18.03, and 10.14 cm2 V-1 s-1 when synergistically regulating the ion ratio to 2 : 1, 3 : 1, and 4 : 1. Consequently, these ratios potentially achieved nearly a 100-fold improvement in the electrical conductivity with respect to the pristine system. The improvements mainly stemmed from the fact that decreasing the amount of anions in ILs and prolonging the chain length of PEDOT yielded an ordered face-to-face π-π stacking. The electronic coupling and charge excitation further confirmed that the anions play an active role in tunneling the hole transport in ILs/heterogeneous PEDOT, and the highest occupied molecular orbital (HOMO) energy level of PEDOT was up-shifted significantly after treatment by the ratios of 2 : 1, 3 : 1, and 4 : 1, which favored the electron-donating ability and was in line with the extraordinary enhancement of the hole mobility. Our results imply that regulating the ion ratio in ILs is a novel strategy for modulating the electronic properties and π-stacked morphology of PEDOT.

Understanding the effects of the co-sensitizing ratio on the surface potential, electron injection efficiency, and Förster resonance energy transfer.

Multiple absorbers that function in different absorption regions (near infra-red (NIR) and UV-Visible (UV-Vis)) have been widely used in solar cell applications to enhance the light-harvesting. Herein, two special co-sensitizing Models 1 and 2, which feature either saturated dye IQ21 or saturated co-sensitizer S2, have been added to a TiO2 surface to explore the effect of the altered sensitizing sequence, namely the co-sensitizing ratio of IQ21/S2 and S2/IQ21 on the electrostatic potential variation (ΔV), electron injection efficiency (ηinj'), and Förster resonance energy transfer (FRET), using density functional theory and first-principle molecular dynamics simulations. The ΔV related to the open-circuit voltage (Voc) is insensitive in both Models 1 and 2. However, the absorption (λabs) and ηinj' associated with the short-circuit density (Jsc) display a significant deviation (the λabs for 1 is red-shifted compared to that of 2, and the ηinj' for 1 is improved by 56%). Meanwhile, Model 1 manifests a suppressed FRET and potentially favors co-sensitizer S2 functioning as the electron-injector and not the energy-donor. Another two possible Models 3 and 4 that feature a reduced adsorption of IQ21 and S2 relative to 1 and 2 were considered further, and the result mirrors the main trend in 1 and 2, except for the ηinj'. Overall, it implies that sensitizing a larger absorber with NIR features to saturate it first, then introducing a smaller absorber with UV-Vis features, can potentially improve the electron injection and diminish electron-hole recombination considerably. Our results provide a comprehensive analysis of the active role of an optimized sensitizing sequence to improve the conversion efficiency.

Probing effects of molecular conformation on the electronic and charge transport properties in two- and three-dimensional small molecule hole-transporting materials: a theoretical investigation.

Thiophene/benzene-fused π-conjugated systems are normally employed as the core units of two- and three-dimensionally expanded small molecule hole-transporting materials (HTMs) to improve their electronic and charge transport properties, whereas comparison studies between two-dimensional and three-dimensional core conformations are less reported. To further find useful clues for the design of highly-efficient small molecule HTMs and to find new core units, in this work, four HTM molecules are designed by employing triphenylene, benzotrithiophene, triptycene, and thiophenetriptycene as the core units, and simulated with density functional theory combined with the Marcus hopping model. Our results show that all the considered HTMs display appropriate molecular energy levels, less optical absorption in the visible light region and large Stokes shifts, and high hole mobilities (9.80 × 10-2 cm2 V-1 s-1). Compared with the two-dimensional core structures, the three-dimensional cores exhibit evident superiorities with the same chemical components. Meanwhile, we also find that the quasi-degenerate HOMO energy levels will be helpful to enlarge the transfer integrals between adjacent molecules, and further to promote the hole transport in HTMs. By considering the various elements simultaneously, these investigated HTMs (S-1-S-4) with thiophene- and benzene-fused cores can be expected as potential promising candidates to help create more efficient solar cells.

How to design more efficient hole-transporting materials for perovskite solar cells? Rational tailoring of the triphenylamine-based electron donor.

Designed with a symmetrical naphthatetrathiophene (NTT) core and triphenylamine (TPA)-based side arms, a series of novel organic small molecule hole-transporting materials are simulated for perovskite solar cells (PSCs) using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. As a fundamental understanding, the energy level alignments and the charge transport behavior are explored for their potential applications. Our results show that, adding an oxygen-bridge between the neighboring phenyl groups of TPA side arms makes the highest occupied molecular orbital (HOMO) levels up-shift, whereas the carbon-carbon single bond stabilizes the HOMOs by about 0.3-0.4 eV. By structural tailoring of the TPA side arms, the HOMO levels of newly designed molecules range from -5.08 eV to -5.61 eV, which provides more possibilities for the interfacial energy regulation. Meanwhile, our results also indicate that the quasi-planar molecular architecture and the delocalized frontier molecular orbitals can effectively enhance the electronic coupling between adjacent molecules. In addition, the reorganization energies are distinctly lowered in the cases of the mixed carbon-carbon bond and oxygen-bridge, and the double oxygen-bridge models. As a result, these molecules with the additional carbon-carbon bond and oxygen-bridge exhibit high hole mobilities. Several promising candidates are proposed toward more efficient PSCs, and more importantly, this work offers some new insights for the design of organic small molecule materials.

The electron injection rate in CdSe quantum dot sensitized solar cells: from a bifunctional linker and zinc oxide morphology.

Herein, we have investigated the effect of both the bifunctional linker (L1, L2, L3, and L4) and ZnO morphology (porous nanoparticles (NPs), nanowires (NWs), and nanotubes (NTs-A and NTs-Z)) on the electron injection in CdSe QD sensitized solar cells by first-principles simulation. Via calculating the partitioned interfaces formed by different components (linker/QDs and ZnO/linker), we found that the electronic states of QDs and every ZnO substrate are insensitive to any linker, while the frontier orbitals of L1-L4 (with increased delocalization) manifest a systematical negative-shift. Because of the lowest unoccupied molecular orbital (LUMO) of L1 compared to its counterparts aligned in the region of the virtual states of QDs or the substrate with a high density of states, it always yields a stronger electronic coupling with QDs and varied substrates. After characterization of the complete ZnO/linker/QD system, we found that the electron injection time (τ) vastly depends on both the linker and substrate. On the one hand, L1 bridged QDs and every substrate always achieve the shortest τ compared to their counterpart associated cases. On the other hand, NW supported systems always yield the shortest τ no matter what the linker is. Overall, the NW/L1/QD system achieves the fastest injection by ∼160 fs. This essentially stems from the shortest molecular length of L1 decreasing the distance between QDs and the substrate, subsequently improving the interfacial coupling. Meanwhile, the NW supported cases generate the less sensitive virtual states for both the QDs and NWs, ensuring a less variable interfacial coupling. These facts combined can provide understanding of the effects contributed from the linker and the oxide semiconductor morphology on charge transfer with the aim of choosing an appropriate component with fast directional electron injection.

Theoretical investigation on structural and electronic properties of organic dye C258 on TiO2(101) surface in dye-sensitized solar cells.

The structural and electronic properties of an organic dye C258 before and after being adsorbed onto a TiO2(101) surface by two adsorption modes, monodentate (Mha) and bidentate bridging (BBH), have been investigated in detail. The combination of density functional tight-binding (DFTB), density functional theory (DFT), and time-dependent DFT (TDDFT) approaches have been employed. DFT calculations show that C258 has remarkable charge-transfer characteristics, which favors fast electron injection from the excited dye to the conduction band of TiO2. A detailed analysis of the adsorbate contributions of the dye molecule to band states of TiO2 shows a strong coupling of the adsorbate orbitals with the substrate orbitals. Significant electronic transfer characteristics across the interface reveal a direct electron injection mechanism arising from the electronic excitation of the anchoring group of C258 to the conduction bands of TiO2. The adsorption energy and the electron density distribution demonstrate that the BBH structure is more stable and has a stronger coupling with TiO2 than the Mha pattern, which is able to better promote the electron injection to increase the efficiency of dye-sensitized solar cells (DSSCs).

Iodinated Al(III)-based phthalocyanines are promising sensitizers for dye-sensitized solar cells; a theoretical comparison between Zn(II), Mg(II), and Al(III)-based phthalocyanine sensitizers.

To design efficient dyes for dye-sensitized solar cells (DSSCs), using a Zn-coordinated phthalocyanine (TT7) as the prototype, a series of phthalocyanine dyes (Pcs) with different metal ions and peripheral/axial groups have been investigated by means of density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Computational results show that the iodinated Al-based dye with a peripheral amino group (Al-I-NH2-Pc) exhibits the largest redshift in the maximum absorbance (λ(max)). In addition, Al-based dyes have appropriate energy-level arrangements of frontier orbitals to keep excellent balance between electron injection and regeneration of oxidized dyes. Further, it has been found that the intermolecular π-staking interaction in Al-I-Pc molecules is weaker than the other metal-based Pcs, which may effectively reduce dye aggregation on the semi-conductor surface. All these results suggest iodinated Al-based Pcs (Al-I-Pcs) to be potentially promising sensitizers in DSSCs.