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Donor‐acceptor (D–A•) type luminescent organic radicals have received widespread attention as efficient doublet emitters. However, their generally low photoluminescence quantum efficiency (PLQE) and limited photostability restrict their various applications. Since unraveling the relationship between structure and properties of D–A• type luminescent radicals remains a challenge, here, a series of tri(2,4,6‐trichlorophenyl)methyl (TTM) radical derivatives, which differ by the location of their ring fusion sites and nature of their heteroatoms, is synthesized. The PLQE of isomers varies by ten times as a function of ring fusion sites. In particular, the PLQE of a radical undergoing ring fusion at the carbazole 3,4‐position is as high as 98.0%. Quantum‐chemical calculations show that in the case of overlapping holes and electrons, by increasing the effective distance between the D and A moieties, the radiative transition rates of the radicals increase. Also, decreasing the electronic coupling between the charge‐transfer and local‐excited states and avoiding large geometrical distortions between the ground state (D0)_and the first excited state (D1) can significantly reduce the nonradiative transition rates. This work offers a design strategy to obtain efficient and stable luminescent radicals by modifying the sites of ring fusion, which allows control of the radiative and nonradiative transition rates. [ABSTRACT FROM AUTHOR]

2D Ruddlesden–Popper metal‐halide perovskites exhibit structural diversity due to a variety of choices of organic ligands. Incorporating bifunctional ligands in such materials is particularly intriguing since it can result in novel electronic properties and functions. However, an in‐depth understanding of the effects of bifunctional ligands on perovskite structures and, consequently, their electronic and excitonic properties, is still lacking. Here, n = 1 2D perovskites built with organic ligands containing ─CN, ─OH, ─COOH, ─phenyl (Ph), and ─CH3 functional groups are investigated using ultraviolet and inverse photoemission spectroscopies, density functional theory calculations, and tight‐binding model analyses. The experimentally determined electronic gaps of the ─CN, ─COOH, ─Ph, and ─CH3 based perovskites exhibit a strong correlation with the in‐plane Pb─I─Pb bond angle, while the ─OH based perovskite deviates from the linear trend. Based on the band structure calculations, this anomaly is attributed to the out‐of‐plane dispersion, caused predominantly by significant interlayer electronic coupling that is present in ─OH based perovskites. These results highlight the complex and diverse impacts of organic ligands on electronic properties, especially in terms of the involvement of strong interlayer electronic coupling. The impact of the bifunctional ligands on the evolution of the exciton binding energy is also addressed. [ABSTRACT FROM AUTHOR]

Infrared (IR) thermal imaging is receiving a great deal of attention due to its wide range of applications. Given multiple issues (like cost and availability) with the inorganic materials currently exploited for IR imaging, there is nowadays a great push of developing organic imaging materials. Carbon‐based materials are known to have significant transparency in the visible and IR regions and some are used as transparent conductors. Here, whether π‐conjugated carbon‐based materials are suitable for long‐wave (LW) and mid‐wave (MW) IR imaging applications is computationally assessed. Using density functional theory calculations, the IR‐vibrational properties of molecules from acenes to coronenes and fullerenes, and of periodic systems like graphene and carbon nanotubes are characterized. Fullerenes, graphenes, and double‐walled carbon nanotubes are found to be very attractive as they are transparent in both the LWIR and MWIR regions, a feature resulting from the absence of hydrogen atoms. Also, it is found that replacing hydrogen atoms in a molecule with deuterium or sulfur atoms can be an efficient way to improve their LWIR or MWIR transparency, respectively. For fused‐ring systems having hydrogen atoms on the periphery, designing molecules with trio CH‐units is another way to enhance the transparency in the LWIR region. [ABSTRACT FROM AUTHOR]

2D Ruddlesden–Popper halide perovskites have attracted considerable attention due to their desirable optoelectronic properties, high chemical and structural tunability, and improved environmental stability. However, the understanding of their structure–properties relationships is still limited. In particular, the energy level positions and band alignments at interfaces involving these materials, which are important features to control in the context of any applications, are still under debate. Here, the electronic structure of high‐purity films of BA2MAn−1PbnI3n+1 for n = 1–5 (where BA stands for butylammonium and MA for methylammonium) is investigated, using optical absorption, ultraviolet, and inverse photoemission spectroscopies, and density functional theory calculations. This study determines the ionization energy and electron affinity of each compound and demonstrates a type‐I band alignment for the BA2MAn−1PbnI3n+1 series. This study further describes the evolution of the exciton binding energy as a function of the thickness of the inorganic layers. [ABSTRACT FROM AUTHOR]

Perovskite solar cells (PSCs) based on 2D/3D heterostructures show great potential to combine the advantages of the high efficiency of 3D perovskites and the high stability of 2D perovskites. However, an in‐depth understanding of the organic‐spacer effects on the 2D quantum well (QW) structures and electronic properties at the 2D/3D interfaces is yet to be fully achieved, especially in the case of 2D perovskites based on diammonium spacers/ligands. Here, a series of diammonium spacers is considered for the construct ion 2D/3D perovskite heterostructures. It is found that the chemical structure and concentration of the spacers can dramatically affect the characteristics of the 2D capping layers, including their phase purity and orientation. Density functional theory calculations indicate that the spacer modifications can induce shifts in the energy‐level alignments at the 2D/3D interfaces and therefore influence the charge‐transfer characteristics. The strong intermolecular interactions between the 2,2‐(ethylenedioxy)bis(ethylammonium) (EDBE) cations and inorganic [PbI6]4− slabs facilitate a controlled deposition of a phase‐pure QW structure (n = 1) with a horizontal orientation, which leads to better surface passivation and carrier extraction. These benefits endow the EDBE‐based 2D/3D devices with a high power conversion efficiency of 22.6% and remarkable environmental stability, highlighting the promise of spacer‐chemistry design for high‐performance 2D/3D PSCs. [ABSTRACT FROM AUTHOR]

The ability to passivate defects and modulate the interface energy‐level alignment (IEA) is key to boost the performance of perovskite solar cells (PSCs). Herein, we report a robust route that simultaneously allows defect passivation and reduced energy difference between perovskite and hole transport layer (HTL) via the judicious placement of polar chlorine‐terminated silane molecules at the interface. Density functional theory (DFT) points to effective passivation of the halide vacancies on perovskite surface by the silane chlorine atoms. An integrated experimental and DFT study demonstrates that the dipole layer formed by the silane molecules decreases the perovskite work function, imparting an Ohmic character to the perovskite/HTL contact. The corresponding PSCs manifest a nearly 20 % increase in power conversion efficiency over pristine devices and a markedly enhanced device stability. As such, the use of polar molecules to passivate defects and tailor the IEA in PSCs presents a promising platform to advance the performance of PSCs. [ABSTRACT FROM AUTHOR]

With the aim of achieving high‐performance thermally activated delayed fluorescence, a series of trioxoazatriangulene derivatives are systematically developed by modifying the donor substituents. The emission colors are shifted from green to greenish‐yellow and to yellow with rather broad spectral widths of 70–95 nm by introducing carbazole, triphenylamine, or diphenylamine donor units, indicating that each emission originates from a charge‐transfer transition. On the other hand, the trioxoazatriangulene modified with three diphenylamines shows orange emission with a narrow emission spectrum (45 nm), suggesting that the transition mainly originates from a multiple resonance effect. [ABSTRACT FROM AUTHOR]

In most instances, thermally activated delayed fluorescence (TADF) emitters are incorporated into a suitable host matrix at low doping concentration in order to reduce emission quenching and to improve organic light‐emitting diode (OLED) efficiency. Here, a combination of molecular dynamics simulations and density functional theory calculations is performed for thin films of 1) the neat 4CzIPN TADF emitter and 2) the (guest–host) 4CzIPN:mCBP system, in order to determine how guest–guest and guest–host interactions influence the morphological, electronic, and luminescence properties of the TADF emitters. The red‐shift in emission recently observed experimentally upon increasing the concentration in TADF emitters is attributed to the formation of guest–guest, i.e., dimer, intermolecular charge‐transfer states. It is found that the radiative and reverse intersystem crossing rates associated with these dimer states are similar to those of monomers. Thus, the contributions from both the dimer and monomer states need to be considered to describe TADF within the emissive layer. The exciton diffusion processes are also characterized; singlet excitons are calculated to be the main contributors to the diffusion length, in contrast to recently proposed models. [ABSTRACT FROM AUTHOR]

Hyperfluorescence is emerging as a powerful strategy to develop optoelectronic devices with high‐color purity and enhanced stability. It requires appropriate integration of a sensitizer displaying efficient thermally activated delayed fluorescence (TADF) and an emitter displaying strong, narrow‐band fluorescence. Here, through a joint computational and experimental approach, an unprecedented, end‐to‐end systems level description of the electronic and optical processes that take place in a hyperfluorescent emissive layer composed of a TADF sensitizer, 2,5‐bis(2,6‐di(9H‐carbazol‐9‐yl)phenyl)‐1,3,4‐oxadiazole (4CzDPO), and a fluorescent emitter, 2,5,8,11‐tetra‐tert‐butylperylene (TBPe) is provided. The photophysical properties measurement of the emissive layer is combined with the computational determination of the electronic properties, film morphology, and excitation transfer phenomena. The Förster resonance energy transfer rates from 4CzDPO to TBPe are on the order of 1011 s−1, considerably higher than the radiative and nonradiative recombination rates for 4CzDPO. These features ensure nearly complete energy transfer to TBPe, leading to a five‐fold increase in the photoluminescence quantum yields in the 4CzDPO:TBPe system in comparison to neat films of 4CzDPO. This approach highlights the factors that can provide efficient energy transfer from TADF molecules to fluorescent emitters, suppress energy transfer among TADF molecules, and avoid the need for a host material within the emissive layer. [ABSTRACT FROM AUTHOR]

Radical‐carrying organic molecules have received significant attention to bypass the issue related to harvesting triplet excitons in current light‐emitting materials. While the computational efforts conducted so far have treated these radical emitters as isolated entities, in actual devices, they are embedded in a host matrix and subject to emitter–host interactions. Here, by combining molecular dynamics simulations and density functional theory calculations, the impact of the host matrix on the optoelectronic performance of radical emitters is evaluated, taking as a representative example the (4‐ncarbazolyl‐2,6‐dichlorophenyl)bis(2,4,6‐trichlorophenyl)‐methyl (TTM‐3NCz) radical emitter dispersed in a 4,4‐bis(carbazol‐9‐yl)biphenyl (CBP) host. A morphological analysis shows that steric effects around the radical centers, carried by the TTM electron‐poor moieties of the emitters, disfavor π–π interactions with the host molecules, which leads to random intermolecular orientations around the TTM moieties. The 3NCz electron‐rich moieties of the emitters, however, have much lesser spatial hindrance for intermolecular π–π stacking, which modulates the structural and electronic properties of the emitters in the host matrix. The influence of dynamic and static disorders on the radiative and nonradiative recombination processes is also investigated and it is found that the rates of nonradiative recombination are small, which opens the way to 100% internal quantum efficiency for the doublet‐based emission process. [ABSTRACT FROM AUTHOR]

While adding a third component to a binary blend in the active layer of an organic solar cell is a promising approach to improve device performance, the control of active‐layer morphology also becomes more complex. Here, a combination of molecular dynamics simulations and long‐range corrected density functional theory calculations is used to examine the intermolecular packing and electronic properties in two polymer donor/small‐molecule acceptor binary blends, D/A1 (donor/acceptor1) and D/A2 (donor/acceptor2), in order to gain insight into the D/A1/A2 ternary blend. The focus is on the blends of the 4‐(3‐(2‐decyltetradecyl)‐5′‐(2,3‐difluoro‐4‐(5‐methylthiophen‐2‐yl)phenyl)‐[2,2′‐bithiophen]‐5‐yl)‐7‐(4‐(2‐decyltetradecyl)‐5‐methylthiophen‐2‐yl)‐5,6‐difluoro‐2‐propyl‐2H‐benzo[d][1,2,3]triazole (PTFB‐O) polymer donor with the 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(5‐hexylthienyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (ITIC‐Th) and 2,2′‐((2Z,2′Z)‐(((4,4,9,9‐tetrakis(5‐hexylthiophen‐2‐yl)‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(4‐octylthiophene‐5,2‐diyl))‐bis(methanylylidene))bis(3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))‐dimalononitrile (IEIC‐Th) acceptors. The intermolecular packings and extent of mixing between the polymer donor and the acceptor in both binary blends are found to be similar, which is consistent with the well‐mixed nature of the ITIC‐Th:IEIC‐Th phase and the quasi‐linear evolution of open‐circuit voltage as a function of ITIC‐Th concentration in the PTFB‐O:ITIC‐Th:IEIC‐Th ternary blend. The intermolecular packing patterns and electron‐transfer rates among the acceptors are explored to rationalize the higher electron mobility found in the PTFB‐O:ITIC‐Th blend. The energetic distribution of the charge‐transfer electronic states and non‐radiative recombination rates are also evaluated to understand the difference in voltage losses between the binary blends. [ABSTRACT FROM AUTHOR]

Reduced dimensionality forms of perovskites with alternating layers of organic ligands are a promising class of materials for achieving stable perovskite solar cells. Most work until now has focused on phases utilizing two ammonium terminated ligands per formula unit. However, phases utilizing a single diammonium ligand per formula unit are advantageous in that they can potentially have a thinner insulating organic layer between Pb‐halide layers, yet the structural effects on their optoelectronic properties are not yet well understood. In this study two organic ligands, butane 1,4‐diammonium (BDA) and N,N‐dimethylpropane diammonium (DMPD), are investigated as spacers in n = 1, 2D perovskites. Using ultraviolet and inverse photoelectron spectroscopies, BDAPbI4 is shown to have a larger transport gap by 350 meV and a larger exciton binding energy by 140 meV than DMPDPbI4. Through density functional theory calculations, the cause of this difference is traced to the out‐of‐plane tilting of the Pb‐halide octahedra provoked by the asymmetric ligand in DMPDPbI4. Parallel channels of nearly straight PbIPb bonds are formed in one direction, leading to enhanced electronic coupling and higher band dispersion in that direction. In BDAPbI4, no such channels exist, resulting in greater electronic confinement and a larger bandgap and exciton binding energy. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) emitters are typically embedded at low concentrations in a host matrix to suppress emission quenching. However, recent studies indicate that TADF compounds such as the oBFCzTrz emitter (5‐(2‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl)‐5H‐benzofuro[3,2‐c]carbazole) display insignificant concentration quenching dependence. To understand the origin of this beneficial behavior, the morphology, dynamics, electronic properties, and charge transport and energy transfer in a neat film of the oBFCzTrz emitter are characterized via molecular dynamic simulations combined with density functional theory calculations. The emissive layer shows glassy behavior at room temperature with the twisted configurations of the emitter molecules allowing for intramolecular donor–acceptor interactions, but disfavoring intermolecular π–π stacking, which suppresses the formation of intermolecular aggregate states. As a result, the electronic structure and luminescence of oBFCzTrz are not significantly altered by intermolecular interactions. The calculated diffusion lengths of the singlet and triplet excitons are small enough that there occurs no substantial concentration quenching effect. Overall, the design of new TADF emitters with structural motifs similar to those of oBFCzTrz offers potential to develop efficient organic light‐emitting diode devices in which the emissive layers are entirely composed of TADF molecules without the need for a host component. [ABSTRACT FROM AUTHOR]

Hematite (α-Fe2O3) is widely used as a catalytic electrode material in photoelectrochemical water oxidation, where its surface compositions and stabilities can strongly impact the redox reaction process. Here, its surface configurations in environmental or electrochemical conditions are assessed via density functional theory (DFT) calculations conducted at the Perdew, Burke, and Ernzerhof (PBE)+U level. The most energetically favorable surface domains of α-Fe2O3 (0001) and (1102) are predicted by constructing the surface phase diagrams in the framework of first-principle thermodynamics. The relative surface stabilities are investigated as a function of partial pressures of oxygen and water, temperature, solution pH, and electrode potential not only for perfect bulk terminations but also for defect-containing surfaces having various degrees of hydroxylation and hydration. In order to assess the impact on the redox reactions of the surface planes as well as of the extent of surface hydration/hydroxylation, the thermodynamics of the four-step oxygen evolution reaction (OER) mechanism are examined in detail for different models of the α-Fe2O3 (0001) and (1102) surfaces. Importantly, the results underline that the nature of the surface termination and the degree of near-surface hydroxylation give rise to significant variations in the OER overpotentials. [ABSTRACT FROM AUTHOR]

Abstract: Hematite (α‐Fe2O3) is widely used as a catalytic electrode material in photo‐electrochemical water oxidation, where its surface compositions and stabilities can strongly impact the redox reaction process. Here, its surface configurations in environmental or electrochemical conditions are assessed via density functional theory (DFT) calculations conducted at the Perdew, Burke, and Ernzerhof (PBE)+U level. The most energetically favorable surface domains of α‐Fe2O3 (0001) and (1 1 ¯02) are predicted by constructing the surface phase diagrams in the framework of first‐principle thermodynamics. The relative surface stabilities are investigated as a function of partial pressures of oxygen and water, temperature, solution pH, and electrode potential not only for perfect bulk terminations but also for defect‐containing surfaces having various degrees of hydroxylation and hydration. In order to assess the impact on the redox reactions of the surface planes as well as of the extent of surface hydration/hydroxylation, the thermodynamics of the four‐step oxygen evolution reaction (OER) mechanism are examined in detail for different models of the α‐Fe2O3 (0001) and (1 1 ¯02) surfaces. Importantly, the results underline that the nature of the surface termination and the degree of near‐surface hydroxylation give rise to significant variations in the OER overpotentials. [ABSTRACT FROM AUTHOR]

Abstract: This study presents a combined experimental and theoretical study of the electronic structure of two 2D metal halide perovskite films. Ultraviolet and inverse photoemission spectroscopies are performed on solution‐processed thin films of the n = 1 layered perovskite butylammonium lead iodide and bromide, BA2PbI4 and BA2PbBr4, characterized by optical absorption and X‐ray diffraction, to determine their valence and conduction band densities of states, transport gaps, and exciton binding energies. The electron spectroscopy results are compared with the densities of states determined by density functional theory calculations. The remarkable agreement between experiment and calculation enables a detailed identification and analysis of the organic and inorganic contributions to the valence and conduction bands of these two hybrid perovskites. The electron affinity and ionization energies are found to be 3.1 and 5.8 eV for BA2PbI4, and 3.1 and 6.5 eV for BA2PbBr4. The exciton binding energies are estimated to be 260 and 300 meV for the two materials, respectively. The 2D lead iodide and bromide perovskites exhibit significantly less band dispersion and a larger density of states at the band edges than the 3D analogs. The effects of using various organic ligands are also discussed. [ABSTRACT FROM AUTHOR]

Abstract: The morphology of the active layer of a bulk heterojunction solar cell, made of a blend of an electron‐donating polymer and an electron‐accepting fullerene derivative, is known to play a determining role in device performance. Here, a combination of molecular dynamics simulations and long‐range corrected density functional theory calculations is used to elucidate the molecular‐scale effects that even minor structural changes to the polymer backbone can have on the “local” morphology; this study focuses on the extent of polymer–fullerene mixing, on their packing, and on the characteristics of the fullerene–fullerene connecting network in the mixed regions, aspects that are difficult to access experimentally. Three representative polymer donors are investigated: (i) poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′″‐diyl)] (PffBT4T‐2OD); (ii) poly[(2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′″‐diyl)] (PBT4T‐2OD), where the fluorine atoms in the benzothiadiazole moieties of PffBT4T‐2OD are replaced with hydrogen atoms; and (iii) poly[(2,2′‐bithiophene)‐alt‐(4,7‐bis((2‐decyltetradecyl)thiophen‐2‐yl)‐5,6‐difluoro‐2‐propyl‐2H‐benzo[d][1,2,3]triazole)] (PT2‐FTAZ), where the sulfur atoms in the benzothiadiazole moieties of PffBT4T‐2OD are replaced with nitrogen atoms carrying a linear C3H7 side‐chain; these polymers are mixed with the phenyl‐C71‐butyric acid methyl ester (PC71BM) acceptor. This study also discusses the nature of the charge‐transfer electronic states appearing at the donor–acceptor interfaces, the electronic couplings relevant for the charge‐recombination process, and the electron‐transfer features between neighboring PC71BM molecules. [ABSTRACT FROM AUTHOR]

Abstract: The large voltage losses usually encountered in organic solar cells significantly limit the power conversion efficiencies (PCEs) of these devices, with the result that the current highest PCE values in single‐junction organic photovoltaic remain smaller than for other solar cell technologies, such as crystalline silicon or perovskite solar cells. In particular, the nonradiative recombinations to the electronic ground state from the lowest‐energy charge‐transfer (CT) states at the donor–acceptor interfaces in the active layer of organic devices, are responsible for a significant part of the voltage losses. Here, to better comprehend the nonradiative voltage loss mechanisms, a fully quantum‐mechanical rate formula is employed within the framework of time‐dependent perturbation theory, combined with density functional theory. The objective is to uncover the specific contributions of intramolecular vibrations to the CT‐state nonradiative recombinations in several model systems, which include small‐molecule and polymer donors as well as fullerene and nonfullerene acceptors. [ABSTRACT FROM AUTHOR]

Abstract: The uses of self‐assembled monolayers (SAMs) of dipolar molecules or of adsorbed molecular acceptors on electrode materials are common strategies to increase their work function, thereby facilitating hole injection into an organic semiconductor deposited on top. Here it is shown that a combination of both approaches can surpass the performance of the individual ones. By combined experimental and theoretical methods it is revealed that in a three‐component system, consisting of an indium‐tin‐oxide (ITO) electrode, a carbazole‐based phosphonic acid SAM, and a molecular acceptor layer on top of the SAM, charge transfer occurs from the ITO through the SAM to the acceptor layer, resulting in an electrostatic field drop over the charge‐neutral SAM. This result is in contrast to common expectations of either p‐doping the carbazole of the SAM or charge transfer complex formation between the carbazole and the acceptor molecules. A high work function of 5.7 eV is achieved with this combined system; even higher values may be accessible by exploiting the fundamental charge redistribution mechanisms identified here with other material combinations. [ABSTRACT FROM AUTHOR]

Two symmetrically substituted phenylenevinylene D-A-D′-A-D type siblings, (2Z,2′Z)-2,2′-(2,5-dimethoxy-1,4-phenylene)bis(3-(4-(dimethylamino)phenyl)acrylonitrile) (↑-dscn) and (2Z,2′Z)-3,3′-(2,5-dimethoxy-1,4-phenylene)bis(2-(4-(dimethylamino)phenyl)acrylonitrile) (↓-dscn), are prepared. We investigate the effect of different but symmetrical location of these cyano groups in vinylene bridges on the 1-photon and 2-photon absorption behaviors. We report that the closeness of CN group on the vinyl group to the central phenyl ring in ↑-dscn induces an intramolecular geometric distortion between the central phenyl ring and vinylene group, preventing the effective π-conjugation length in ground and excited states. Thus, the transition energy that is observed in 1-photon absorption and fluorescence is larger in ↑-dscn than in ↓-dscn. This leads to a different intramolecular charge distribution, as a result of which the linear and nonlinear optical properties strongly depend on the position of acceptors. These results are theoretically unraveled in terms of charge transfer pathways, charge distribution, and charge distribution differences on transition. [ABSTRACT FROM AUTHOR]

In the most efficient solar cells based on blends of a conjugated polymer (electron donor) and a fullerene derivative (electron acceptor),ultrafast formation of charge-transfer (CT) electronic states at the donor-acceptor interfaces and efficient separation of these CT states into free charges, lead to internal quantum efficiencies near 100%. However, there occur substantial energy losses due to the non-radiative recombinations of the charges, mediated by the loweset-energy (singlet and triplet) CT states; for example, such recombinations can lead to the formation of triplet excited electronic states on the polymer chains, which do not generate free charges. This issue remains a major factor limiting the power conversion efficiencies (PCE) of these devices. The recombination rates are, however, difficult to quantify experimentally. To shed light on these issues, here, an integrated multi-scale theoretical approach that combines molecular dynamics simulations with quantum chemistry calculations is employed in order to establish the relationships among chemical structures, molecular packing, and non-radiative recombination losses mediated by the lowest-energy charge-transfer states. [ABSTRACT FROM AUTHOR]

Two electronically delocalized molecules were designed as models to understand how molecular shape impacts the tradeoff between solubility and crystallization tendencies in molecular semiconductors. The more soluble compound TT contains a non-planar bithiophene central fragment, whereas CT has a planar cyclopentadithiophene unit. Calorimetry studies show that CT can crystallize more easily than TT. However, absorption spectroscopy shows that the initially amorphous TT film can eventually form crystals in which the molecular shape is significantly more planar. Two thermally reversible polymorphs for TT were observed by XRD and grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. These findings are relevant within the context of designing soft semiconductors that exhibit high solubility and a tendency to provide stable organized structures with desirable electronic properties. [ABSTRACT FROM AUTHOR]

In organic solar cells (OSCs), the energy of the charge-transfer (CT) complexes at the donor-acceptor interface, ECT, determines the maximum open-circuit voltage ( VOC). The coexistence of phases with different degrees of order in the donor or the acceptor, as in blends of semi-crystalline donors and fullerenes in bulk heterojunction layers, influences the distribution of CT states and the VOC enormously. Yet, the question of how structural heterogeneities alter CT states and the VOC is seldom addressed systematically. In this work, we combine experimental measurements of vacuum-deposited rubrene/C60 bilayer OSCs, with varying microstructure and texture, with density functional theory calculations to determine how relative molecular orientations and extents of structural order influence ECT and VOC. We find that varying the microstructure of rubrene gives rise to CT bands with varying energies. The CT band that originates from crystalline rubrene lies up to ≈0.4 eV lower in energy compared to the one that arises from amorphous rubrene. These low-lying CT states contribute strongly to VOC losses and result mainly from hole delocalization in aggregated rubrene. This work points to the importance of realizing interfacial structural control that prevents the formation of low ECT configurations and maximizes VOC. [ABSTRACT FROM AUTHOR]

Organic solar cells hold promise of providing low-cost, renewable power generation, with current devices providing up to 13% power conversion efficiency. The rational design of more performant systems requires an in-depth understanding of the interactions between the electron donating and electron accepting materials within the active layers of these devices. Here, we explore works that give insight into the intermolecular interactions between electron donors and electron acceptors, and the impact of molecular orientations and environment on these interactions. We highlight, from a theoretical standpoint, the effects of intermolecular interactions on the stability of charge carriers at the donor/acceptor interface and in the bulk and how these interactions influence the nature of the charge transfer states as well as the charge separation and charge transport processes. [ABSTRACT FROM AUTHOR]

The electrical properties of organic field-effect transistors (OFETs) are usually characterized by applying models initially developed for inorganic-based devices, which often implies the use of approximations that might be inappropriate for organic semiconductors. These approximations have brought limitations to the understanding of the device physics associated with organic materials. A strategy to overcome this issue is to establish straightforward connections between the macroscopic current characteristics and microscopic charge transport in OFETs. Here, a 3D kinetic Monte Carlo model is developed that goes beyond both the conventional assumption of zero channel thickness and the gradual channel approximation to simulate carrier transport and current. Using parallel computing and a new algorithm that significantly improves the evaluation of electric potential within the device, this methodology allows the simulation of micrometer-sized OFETs. The current characteristics of representative OFET devices are well reproduced, which provides insight into the validity of the gradual channel approximation in the case of OFETs, the impact of the channel thickness, and the nature of microscopic charge transport. [ABSTRACT FROM AUTHOR]

In organic solar cells, a major source of energy loss is attributed to nonradiative recombination from the interfacial charge transfer states to the ground state. By taking pentacene–C60 complexes as model donor–acceptor systems, a comprehensive theoretical understanding of how molecular packing and charge delocalization impact these nonradiative recombination rates at donor–acceptor interfaces is provided. [ABSTRACT FROM AUTHOR]

Branched-alkyl-substituted poly(thieno[3,4- c]pyrrole-4,6-dione- alt-3,4-difluorothiophene) (PTPD[2F]T) can be used as a polymer acceptor in bulk heterojunction (BHJ) solar cells with a low-band-gap polymer donor (PCE10) commonly used with fullerenes. The 'all-polymer' BHJ devices made with PTPD[2F]T achieve efficiencies of up to 4.4 %. While, to date, most efficient polymer acceptors are based on perylenediimide or naphthalenediimide motifs, our study of PTPD[2F]T polymers shows that linear, all-thiophene systems with adequately substituted main chains can also be conducive to efficient BHJ solar cells with polymer donors. [ABSTRACT FROM AUTHOR]

Polymer-fullerene packing in mixed regions of a bulk heterojunction solar cell is expected to play a major role in exciton-dissociation, charge-separation, and charge-recombination processes. Here, molecular dynamics simulations are combined with density functional theory calculations to examine the impact of nature and location of polymer side-chains on the polymer-fullerene packing in mixed regions. The focus is on poly-benzo[1,2-b:4,5-b′]dithiophene-thieno[3,4-c]pyrrole-4,6-dione (PBDTTPD) as electron-donating material and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as electron-accepting material. Three polymer side-chain patterns are considered: i) linear side-chains on both benzodithiophene (BDT) and thienopyrroledione (TPD) moieties; ii) two linear side-chains on BDT and a branched side-chain on TPD; and iii) two branched side-chains on BDT and a linear side-chain on TPD. Increasing the number of branched side-chains is found to decrease the polymer packing density and thereby to enhance PBDTTPD-PC61 BM mixing. The nature and location of side-chains are found to play a determining role in the probability of finding PC61BM molecules close to either BDT or TPD. The electronic couplings relevant for the exciton-dissociation and charge-recombination processes are also evaluated. Overall, the findings are consistent with the experimental evolution of the PBDTTPD-PC61BM solar-cell performance as a function of side-chain patterns. [ABSTRACT FROM AUTHOR]

Molecular doping is a key technique for flexible and low-cost organic complementary semiconductor technologies that requires both efficient and stable p- and n-type doping. However, in contrast to molecular p-dopants, highly efficient n-type dopants are commonly sensitive to rapid degradation in air due to their low ionization energies ( IEs) required for electron donation, e.g., IE = 2.4 eV for tetrakis(1,3,4,6,7,8-hexahydro- 2H-pyrimido[1,2- a]pyrimidinato)ditungsten(II) (W2(hpp)4). Here, the air stability of various host:W2(hpp)4 combinations is compared by conductivity measurements and photoemission spectroscopy. A partial passivation of the n-doping against degradation is found, with this effect identified to depend on the specific energy levels of the host material. Since host-W2(hpp)4 electronic wavefunction hybridization is unlikely due to confinement of the dopant highest occupied molecular orbital (HOMO) to its molecular center, this finding is explained via stabilization of the dopant by single-electron transfer to a host material whose energy levels are sufficiently low for avoiding further charge transfer to oxygen-water complexes. Our results show the feasibility of temporarily handling n-doped organic thin films in air, e.g., during structuring of organic field effect transistors (OFETs) by lithography. [ABSTRACT FROM AUTHOR]

Dimers of 2-substituted N,N′-dimethylbenzimidazoline radicals, (2-Y-DMBI)2 (Y=cyclohexyl (Cyc), ferrocenyl (Fc), ruthenocenyl (Rc)), have recently been reported as n-dopants for organic semiconductors. Here their structural and energetic characteristics are reported, along with the mechanisms by which they react with acceptors, A (PCBM, TIPS-pentacene), in solution. X-ray data and DFT calculations both indicate a longer CC bond for (2-Cyc-DMBI)2 than (2-Fc-DMBI)2, yet DFT and ESR data show that the latter dissociates more readily due to stabilization of the radical by Fc. Depending on the energetics of dimer (D2) dissociation and of D2-to-A electron transfer, D2 reacts with A to form D+ and A− by either of two mechanisms, differing in whether the first step is endergonic dissociation or endergonic electron transfer. However, the D+/0.5 D2 redox potentials-the effective reducing strengths of the dimers-vary little within the series (ca. −1.9 V vs. FeCp2+/0) (Cp=cyclopentadienyl) due to cancelation of trends in the D+/0 potential and D2 dissociation energy. The implications of these findings for use of these dimers as n-dopants, and for future dopant design, are discussed. [ABSTRACT FROM AUTHOR]

ABSTRACT Due to their inherent mechanical flexibility and stretchability, organic-based electronic devices have garnered a great deal of academic and industrial interest. Here, molecular-dynamics simulations are used to examine the molecular-scale details that govern the relationships among molecular weight, chain entanglement, persistence length, and the elastic characteristics of the widely studied π-conjugated polymer poly-(3-hexyl thiophene), P3HT. Oligomers containing at least 50 monomer units are required in the simulations to observe elastic behavior in P3HT, while much longer chains are required to ensure description of appropriate levels of entanglement: only when the molecular weight is greater than 50 kDa, that is, oligomers with approximately 400 monomer units, is truly entangled behavior observed. Interestingly, results from primitive path analysis of amorphous P3HT matches well with the observed onsets of inter-chain excitonic coherence with increased molecular weight. The simulations also indicate that the P3HT modulus saturates at 1.6 GPa for chain lengths of 50-100 monomers, a result that compares well with experimental results. This work highlights the care that needs to be taken to accurately model P3HT morphologies in relation to experimental measurements. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 934-942 [ABSTRACT FROM AUTHOR]

At metal/organic interfaces, the insertion of an organic monolayer can significantly modify the surface properties of the substrate, especially in terms of charge injection across the interface. Herein, we study the formation of an insulating monolayer of morpholine or amine-functionalized polystyrene on Al(111) and Au(111) surfaces and its impact on surface work-function and charge injection. First-principles calculations based on Density Functional Theory have been carried out and point to a significant decrease in the work-function of modified metal surfaces; this is in very good agreement with ultraviolet photoemission spectroscopy measurements performed on the Au(111) surface. In addition, a bilayer cathode, consisting of a thin film of highwork- function metal, such as Al and Au, and a layer of amine-functionalized polystyrene, was also fabricated and tested in organic light-emitting diodes. Such bilayer structures exhibit substantially enhanced efficiency when compared with controls without the functionalized polymers. Our combined theoretical and experimental investigation gives insight into how a thin layer of a commodity polymer can be used to transform rather high-work-function metals into high-performance cathodes to provide efficient electron injection. [ABSTRACT FROM AUTHOR]

The interfacial electronic structure between oxide thin films and organic semiconductors remains a key parameter for optimum functionality and performance of next-generation organic/hybrid electronics. By tailoring defect concentrations in transparent conductive ZnO films, we demonstrate the importance of controlling the electron transfer barrier at the interface with organic acceptor molecules such as C60. A combination of electron spectroscopy, density functional theory computations, and device characterization is used to determine band alignment and electron injection barriers. Extensive experimental and first principles calculations reveal the controllable formation of hybridized interface states and charge transfer between shallow donor defects in the oxide layer and the molecular adsorbate. Importantly, it is shown that removal of shallow donor intragap states causes a larger barrier for electron injection. Thus, hybrid interface states constitute an important gateway for nearly barrier-free charge carrier injection. These findings open new avenues to understand and tailor interfaces between organic semiconductors and transparent oxides, of critical importance for novel optoelectronic devices and applications in energy-conversion and sensor technologies. [ABSTRACT FROM AUTHOR]

The dimers of some Group 8 metal cyclopentadienyl/arene complexes and Group 9 metallocenes can be handled in air, yet are strongly reducing, making them useful n-dopants in organic electronics. In this work, the X-ray molecular structures are shown to resemble those of Group 8 metal cyclopentadienyl/pentadienyl or Group 9 metal cyclopentadienyl/diene model compounds. Compared to those of the model compounds, the DFT HOMOs of the dimers are significantly destabilized by interactions between the metal and the central CC σ-bonding orbital, accounting for the facile oxidation of the dimers. The lengths of these CC bonds (X-ray or DFT) do not correlate with DFT dissociation energies, the latter depending strongly on the monomer stabilities. Ru and Ir monomers are more reducing than their Fe and Rh analogues, but the corresponding dimers also exhibit much higher dissociation energies, so the estimated monomer cation/neutral dimer potentials are, with the exception of that of [RhCp2]2, rather similar (−1.97 to −2.15 V vs. FeCp2+/0 in THF). The consequences of the variations in bond strength and redox potentials for the reactivity of the dimers are discussed. [ABSTRACT FROM AUTHOR]

A combination of density functional theory and experimental measurements via ultraviolet and X-ray photoelectron spectroscopies is used to explore the nature of the interface between the stoichiometric molybdenum trioxide (MoO3) or its under-stoichiometric counterpart with oxygen vacancies, and an organic hole-transport layer represented by 4,4′- N, N′-dicarbazole-biphenyl (CBP). Upon adsorption of CBP, special attention is paid to i) the appearance of gap states and the reduction of the molybdenum oxide surface, and ii) the evolution of the work function. Very good agreement is found between theory and experiment. The near alignment of the CBP highest occupied molecular orbital with the Fermi level and the conduction band edge of molybdenum oxide points to facile hole collection or injection. [ABSTRACT FROM AUTHOR]

The ability to detail how molecules pack in the bulk and at the various materials interfaces in the active layer of an organic solar cell is important to further understanding overall device performance. Here, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), a preferred electron-acceptor material in organic solar cells, is studied through molecular dynamics (MD) simulations; the goal is to examine the effects of temperature and trace solvents on the packing and morphological features of bulk PCBM. Solubility (miscibility) parameters, melting and order-disorder transitions, surface energies, and orientational distributions as a function of different starting configurations are discussed. On the basis of the derived morphologies, electronic structure calculations and a kinetic Monte Carlo approach are combined to evaluate the parameters impacting electron mobility in crystalline and amorphous PCBM structures. [ABSTRACT FROM AUTHOR]

Organic solar cells based on the combination of squaraine dyes (as electron donors) and fullerenes (as electron acceptors) have recently garnered much attention. Here, molecular dynamics simulations are carried out to investigate the evolution of a squaraine-C60 bilayer interface as a function of the orientation and order of the underlying squaraine layer. Electronic couplings between the main electronic states involved in exciton dissociation and charge (polaron pair) recombination are derived for donor-acceptor complexes extracted from the simulations. The results of the combined molecular-dynamics−quantum-mechanics approach provide insight into how the degree of molecular order and the dynamics at the interface impact the key processes involved in the photovoltaic effect. [ABSTRACT FROM AUTHOR]

Gallium-doped zinc oxide (GZO) surfaces, both bare and modified with chemisorbed phosphonic acid (PA) molecules, are studied using a combination of density functional theory calculations and ultraviolet and X-ray photoelectron spectroscopy measurements. Excellent agreement between theory and experiment is obtained, which leads to an understanding of: i) the core-level binding energy shifts of the various oxygen atoms belonging to different surface sites and to the phosphonic acid molecules; ii) the GZO work-function change upon surface modification, and; iii) the energy level alignments of the frontier molecular orbitals of the PA molecules with respect to the valence band edge and Fermi level of the GZO surface. Importantly, both density of states calculations and experimental measurements of the valence band features demonstrate an increase in the density of states and changes in the characteristics of the valence band edge of GZO upon deposition of the phosphonic acid molecules. The new valence band features are associated with contributions from surface oxygen atoms near a defect site on the oxide surface and from the highest occupied molecular orbitals of the phosphonic acid molecules. [ABSTRACT FROM AUTHOR]

At metal/organic interfaces, the insertion of an organic monolayer can significantly modify the surface properties of the substrate, especially in terms of charge injection across the interface. Herein, we study the formation of an insulating monolayer of morpholine or amine-functionalized polystyrene on Al(111) and Au(111) surfaces and its impact on surface work-function and charge injection. First-principles calculations based on Density Functional Theory have been carried out and point to a significant decrease in the work-function of modified metal surfaces; this is in very good agreement with ultraviolet photoemission spectroscopy measurements performed on the Au(111) surface. In addition, a bilayer cathode, consisting of a thin film of high-work-function metal, such as Al and Au, and a layer of amine-functionalized polystyrene, was also fabricated and tested in organic light-emitting diodes. Such bilayer structures exhibit substantially enhanced efficiency when compared with controls without the functionalized polymers. Our combined theoretical and experimental investigation gives insight into how a thin layer of a commodity polymer can be used to transform rather high-work-function metals into high-performance cathodes to provide efficient electron injection. [ABSTRACT FROM AUTHOR]

Tri-isopropylsilylethynyl (TIPS)-functionalized polycyclic aromatic hydrocarbon (PAH) molecules incorporate structural components of graphene nanoribbons and represent a family of model molecules that form organic crystal semiconductors for electronic devices. Here, we report a series of TIPS-functionalized PAHs and discuss their electronic properties and crystal packing features. We observe that these soluble compounds easily form one-dimensional (1 D) packing arrangements and allow a direct evolution of the π stacking by varying the geometric shape. We find that the aspect ratio between length and width plays an important role on crystal packing. Our result indicates that when the PAH molecules have zigzag edges, these can provide enough volume for the molecules to rotate and reorient, alleviating the unfavorable electrostatic interactions found in perfectly cofacial π-π stacking. Density functional theory calculations were carried out to provide insights into how the molecular geometric shape influences the electronic structure and transport properties. The calculations indicate that, among the compounds studied here, 'brick-layer' stacks provide the highest hole mobility. [ABSTRACT FROM AUTHOR]