Energy Of Charge Transfer State Research Articles

Investigation on voltage loss in organic triplet photovoltaic devices based on Ir complexes

A higher Voc is achieved in Ir(FOtbpa)3-based devices despite a lower energy charge transfer state compared to Ir(Ftbpa)3-based devices, which is attributed to the reduced radiative and non-radiative recombination.

Relating free energy and open-circuit voltage to disorder in organic photovoltaic systems.

Efficient exciton dissociation into mobile charge carries a crucial factor underscoring the performance of organic polymer-based bulk-heterojunction photovoltaic devices. In this paper, we compute the energies of charge-transfer (CT) states of the model donor-acceptor lattice system with varying degrees of structural disorder to investigate how fluctuations in the material properties affect electron-hole separation. We also demonstrate how proper statistical treatment of the CT energies recovers the experimentally observed "hot" and "cold" exciton dissociation pathways. Using a quantum mechanical model for a model heterojunction interface, we recover experimental values for the open-circuit voltage at 50 and 100 meV of site-energy disorder. We find that energetic and conformational disorder generally facilitates charge transfer; however, due to excess energy supplied by photoexcitation, highly energetic electron-hole pairs can dissociate in unfavorable directions, potentially never contributing to the photocurrent while "cold" excitons follow the free energy curve defined at the operating temperature of the device.

High lying energy of charge-transfer states and small energetic offsets enabled by fluorinated quinoxaline-based alternating polymer and alkyl-thienyl side-chain modified non-fullerene acceptor

Abstract Significant driving forces are the prerequisite to achieve fast and efficient charge separation in fullerene derivatives-based polymer solar cells to achieve high power conversion efficiency (PCE). However, the large driving forces both in photo-induced hole transfer (PHT) and in photo-induced electron transfer (PET) processes lead to significant energy losses, resulting in low open-circuit voltage in the devices. Recent studies indicate the driving forces in non-fullerene acceptors-based devices can be reduced to very low values but still with high PCE and low energy losses. Herein, we report a new donor:acceptor system with high lying energy of charge-transfer excitons (ECT) of 1.50 eV and very small driving forces (PHT of 0.28 eV and PET of 0.11 eV), in which a fluorinated quinoxaline-based alternating polymer (FTQ) and an alkyl-thienyl side-chain modified small molecule (ITIC-Th) are taken as the donor material and non-fullerene acceptor material, respectively. A high power conversion efficiency (PCE) of 8.19% with maximal external quantum efficiency of 71% are achieved successfully in FTQ:ITIC-Th-based device after appropriate thermal annealing treatment, indicating FTQ can be further applied as donor materials with other highly efficient NF-acceptors to achieve enhanced performances and low energy losses.

Alkyl Branching Position in Diketopyrrolopyrrole Polymers: Interplay between Fibrillar Morphology and Crystallinity and Their Effect on Photogeneration and Recombination in Bulk-Heterojunction Solar Cells

Diketopyrrolopyrrole (DPP)-based donor–acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP-based donor–acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (ECT) of the blend films lies exceptionally c...

Correct determination of charge transfer state energy from luminescence spectra in organic solar cells

Generation and recombination of electrons and holes in organic solar cells occurs via charge transfer states located at the donor/acceptor interface. The energy of these charge transfer states is a crucial factor for the attainable open-circuit voltage and its correct determination is thus of utmost importance for a detailed understanding of such devices. This work reports on drastic changes of electroluminescence spectra of bulk heterojunction organic solar cells upon variation of the absorber layer thickness. It is shown that optical thin-film effects have a large impact on optical out-coupling of luminescence radiation for devices made from different photoactive materials, in configurations with and without indium tin oxide. A scattering matrix approach is presented which accurately reproduces the observed effects and thus delivers the radiative recombination spectra corrected for the wavelength-dependent out-coupling. This approach is proven to enable the correct determination of charge transfer state energies.

High efficiency non-fullerene organic solar cells without electron transporting layers enabled by Lewis base anion doping

Interfacial charge extraction and recombination play a critical role in photovoltaic characteristics of organic bulk heterojunction solar cells. Here we propose a methodology based on Lewis base n-doping with tetrabutylammonium acetate (TBAA) salts to boost charge extraction and resultant power conversion efficiencies (PCE) in OPVs with non-fullerene acceptors (NFA). Based on the PBDB-T:IT-M model system, we show that the TBAA doping via interfacial electron transfer is advantageous in maintaining the preferential intermolecular stacking and built-in potential necessitated for device operation compared to conventional bulk doping. Benefitting from the melioration of interfacial charge extraction and recombination, gains of photocurrent and fill factor are obtained in PBDB-T:IT-M solar cells. We found that the energy of charge transfer states after doping becomes slightly raised, which promotes the mitigation on radiative losses for open-circuit voltage. Furthermore, the described TBAA doping is found with the generality in modifying the device characteristics in non-fullerene based organic solar cells with different LUMO energies. Based on the PBDBT-2F:IT-4F BHJ blends, the solar cell doped with TBAA yields a PCE exceeding 13% without electron-transporting layer (ETL). This strategy allows for high efficiency ETL-free organic solar cells with potentials of reduced fabrication cost.

Theoretical estimation of the dissociation energy of CT states at the acenes/C60 interfaces using fragmental-based ALMO method

The dissociated energy (ECTdiss) of charge-transfer state at the DA interaction is a crucial parameter to determine the efficiency of organic solar cells. The ΔSCF-liked absolutely-localized molecular method (ALMO) combining with DFT is applied to investigate ECTdiss of the lowest CT with prototype oligoacenes/C60 models. The lowest CT energies obtained from ALMO coupled with various exchange-correlation (XC) functionals behave a correct 1/d asymptotic feature. Although CT energies show XC functional dependent feature, ECTdiss behaves XC functional independence and correct 1/d asymptotic character. These mean that ALMO can well describe the electrostatic interactions between positive donor and negative acceptor. ALMO also provide an XC-independent method to evaluate ECTdiss. The condensed-phase ECTdiss also are simply studied by ALMO coupled with polarizable continuum model. The results of ECTdiss demonstrate that increasing the conjugation-length, high-dielectric constant, and adjusting molecular packing to control the electrostatic polarization are effective methods to dissociate the lowest CT into free electron and hole carriers.

Triptycene as a Supramolecular Additive in PTB7:PCBM Blends and Its Influence on Photovoltaic Properties.

Additives play an important role in modifying the morphology and phase separation of donor and acceptor molecules in bulk heterojunction (BHJ) solar cells. Here, we report triptycene (TPC) as a small-molecule additive for supramolecular control of phase separation and concomitant improvement of the power conversion efficiency (PCE) of PTB7 donor and fullerene acceptor-based BHJ polymer solar cells. An overall 60% improvement in PCE is observed for both PTB7:PC61BM and PTB7:PC71BM blends. The improved photovoltaic (PV) performance can be attributed to three factors: (a) TPC-induced supramolecular interactions with donor:acceptor components in the blends to realize a nanoscale phase-separated morphology; (b) an increase in the charge transfer state energy that lowers the driving force for electron transfer from donor to acceptor molecules; and (c) an increase in the charge carrier mobility. An improvement in efficiency using TPC as a supramolecular additive has also been demonstrated for other BHJ blends such as PBDB-T:PC71BM and P3HT:PCBM, implying the wide applicability of this new additive molecule. A comparison of the photostability of TPC as an additive for PTB7:PCBM solar cells to that of the widely used 1,8-diiodooctane additive shows ∼30% higher retention of PV performance for the TPC-added solar cells after 34 h of AM 1.5G illumination. The results obtained suggest that the approach of using additives that can promote supramolecular interactions to modify the length scale of phase separation between donor and acceptor is very promising and can lead to the development of highly efficient and stable organic photovoltaics.

Variable charge transfer state energies at nanostructured pentacene/C60 interfaces

While it has recently been recognized that organic donor-acceptor charge transfer (CT) state energies can vary substantially under different interfacial morphologies, this behavior is under-appreciated in the context of organic singlet fission solar cells where a specific alignment between the triplet state of the fission material and the CT state of the donor-acceptor interface is necessary to the function of the device. In this work, we demonstrate that the CT state energy of a prototypical pentacene-C60 singlet fission system is around 1 eV in most systems, but can vary over 300 meV depending on the composition and morphology of the interface. Moreover, we show that the inclusion of a poly(3-hexylthiophene-2,5-diyl) underlayer, which commonly serves as a triplet blocker and hole collector in pentacene/C60 solar cells, helps promote active layer morphologies with stabilized, low energy CT states. These trends in the interfacial energetics are correlated with structural characterization of the films by atomic force microscopy and x-ray diffraction.

Understanding the free energy barrier and multiple timescale dynamics of charge separation in organic photovoltaic cells.

By employing several lattice model systems, we investigate the free energy barrier and real-time dynamics of charge separation in organic photovoltaic (OPV) cells. It is found that the combined effects of the external electric field, entropy, and charge delocalization reduce the free energy barrier significantly. The dynamic disorder reduces charge carrier delocalization and results in the increased charge separation barrier, while the effect of static disorder is more complicated. Simulation of the real-time dynamics indicates that the free charge generation process involves multiple time scales, including an ultrafast component within hundreds of femtoseconds, an intermediate component related to the relaxation of the hot charge transfer (CT) state, and a slow component on the time scale of tens of picoseconds from the thermally equilibrated CT state. Effects of hot exciton dissociation as well as its dependence on the energy offset between the Frenkel exciton and the CT state are also analyzed. The current results indicate that only a small energy offset between the band gap and the lowest energy CT state is needed to achieve efficient free charge generation in OPV devices, which agrees with recent experimental findings.

Effect of Halogenation on the Energetics of Pure and Mixed Phases in Model Organic Semiconductors Composed of Anthradithiophene Derivatives and C60

Halogenation, particularly fluorination, is commonly used to manipulate the energetics, stability, and morphology of organic semiconductors. In the case of organic photovoltaics (OPVs), fluorination of electron donor molecules or polymers at appropriate positions can lead to improved performance. In this contribution, we use ultraviolet photoelectron spectroscopy, external quantum efficiency measurements of charge-transfer (CT) states, and density functional theory calculations to systematically investigate the effects of halogenation on the bulk solid-state energetics of model anthradithiophene (ADT) materials, their interfacial energetics with C60, and the energetics of various ADT:C60 blend compositions. In agreement with previous work, nonhalogenated ADT molecules show higher energy CT states in blends with C60 and lower energy CT states in the ADT/C60 bilayers. However, this trend is reversed in the halogenated ADT/C60 systems, wherein the CT state energies of ADT:C60 blends are lower than those in t...

Effect of trap-assisted recombination on open-circuit voltage loss in phthalocyanine/fullerene solar cells

Abstract Large energy losses in open-circuit voltage (VOC) are still an issue for the photo-conversion efficiency of organic solar cells (OSCs). We clarify the relationship between charge recombination and VOC loss for phthalocyanine/fullerene planar heterojunction (PHJ) OSCs. We quantify the VOC loss relative to the charge-transfer state energy by the temperature dependence of VOC. The charge recombination order obtained from impedance measurements indicates the presence of trap-assisted recombination. Our results suggest that the VOC losses are caused by the broad distribution of the tail state near the donor/acceptor (D/A) interface in the PHJ device. Thus, reducing the number of trap states near the D/A interface could lead to an increase in VOC.

The Janus-faced chromophore: a donor-acceptor dyad with dual performance in photon up-conversion.

An electron donor-acceptor dyad based on BODIPY (acceptor) and anthracene (donor) plays either the role of sensitizer or emitter in triplet-triplet annihilation photon up-conversion (TTA-UC). This Janus-like behavior was achieved via altering the relative ordering of charge-transfer and local excited state energies in the dyad through the polarity of TTA-UC media.

Relating open-circuit voltage losses to the active layer morphology and contact selectivity in organic solar cells

By manipulating the active-layer morphologies in OSCs, we achieve different open-circuit-voltages without affecting the energy of charge-transfer state.

Effect of Infrared Pulse Excitation on the Bound Charge-Transfer State of Photovoltaic Interfaces.

The nature and dynamics of the bound charge-transfer (CT) state in the exciton dissociation process in organic solar cells are of critical importance for the understanding of these devices. It was recently demonstrated that this state can be probed by a new experiment in which an infrared (IR) push-pulse is used to dissociate charges from the bound excited state. Here we proposed a simple quantum dynamics model to simulate the excitation of the IR pulse on the bound CT state with model parameters extracted from quantum chemical calculations. We show that the pulse dissociates the CT state following two different mechanisms: one, fairly expected, is the direct excitation of higher energy CT states leading to charge separation; the other, proposed here for the first time, is a rebound mechanism in which the negative charge is transferred in the opposite direction to form the neutral Frenkel exciton state from where it dissociates.

Partial atomic multipoles for internally consistent microelectrostatic calculations

An extension of the extant microelectrostatic methodologies, based on the concept of distributed generalized polarizability matrix derived from the Coupled Perturbed Hartree-Fock (CPHF) equations, is proposed for self-consistent calculation of charge carrier and charge-transfer (CT) state electrostatic energies in molecular solids, including the doped, defected and disordered ones. The CPHF equations are solved only once and the generalized molecular polarizability they yield enables low cost iterations that mutually adjust the molecular electronic distributions and the local electric field in which the molecules are immersed. The approach offers a precise picture of molecular charge densities, accounting for atomic partial multipoles up to order 2, which allows one to reproduce the recently reported large charge-quadrupole contributions to CT state energies in low-symmetry local environments. It is particularly well suited for repetitive calculations for large clusters (up to 300,000 atoms), and may potentially be useful for describing electrostatic solvent effects. © 2017 Wiley Periodicals, Inc.

Prediction of the lowest charge-transfer excited-state energy at the donor-acceptor interface in a condensed phase using ground-state DFT calculations with generalized Kohn-Sham functionals.

Knowledge of the charge (electron) transfer process at the donor-acceptor interface is required to understand the working mechanisms of different organic photovoltaic materials. Investigating the lowest charge-transfer state in organic donor-acceptor solar cells is important as it allows the destruction/formation of excitons and polarons to be studied, and is directly related to the open circuit voltage. By performing low-cost and feasible calculations of ground-state electronic structures using the Mulliken rule as well as the optimally tuned range-separated hybrid (OTRSH) density functional and a regular long-range corrected functional, the lowest charge-transfer (CT) state energies of a series of dimers containing functionalized anthracene (the donor) and tetracyanoethylene (the acceptor) were obtained. The jumping distances of excited electrons during CT were calculated. The polarizable continuum model was applied to account for the effects of the solvent methylene chloride (CH2Cl2) on the lowest CT state energies obtained from gas-phase calculations. The calculated lowest CT state energies of the dimers were close to the corresponding experimental results, with a root mean square deviation (RMSD) of 0.22eV.

Ternary Organic Solar Cells with Minimum Voltage Losses

AbstractA new strategy for designing ternary solar cells is reported in this paper. A low‐bandgap polymer named PTB7‐Th and a high‐bandgap polymer named PBDTTS‐FTAZ sharing the same bulk ionization potential and interface positive integer charge transfer energy while featuring complementary absorption spectra are selected. They are used to fabricate efficient ternary solar cells, where the hole can be transported freely between the two donor polymers and collected by the electrode as in one broadband low bandgap polymer. Furthermore, the fullerene acceptor is chosen so that the energy of the positive integer charge transfer state of the two donor polymers is equal to the energy of negative integer charge transfer state of the fullerene, enabling enhanced dissociation of all polymer donor and fullerene acceptor excitons and suppressed bimolecular and trap assistant recombination. The two donor polymers feature good miscibility and energy transfer from high‐bandgap polymer of PBDTTS‐FTAZ to low‐bandgap polymer of PTB7‐Th, which contribute to enhanced performance of the ternary solar cell.

Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency

A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics—however, the results have important implications on the operation of all optoelectronic devices with donor/acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.

Donor-fullerene dyads for energy cascade organic solar cells

Organic bulk heterojunction (BHJ) solar cells require broad absorption of the incident solar spectrum by the donor and acceptor in the blend. Fullerene derivatives with covalently linked dyes, referred to as dyads, were designed to have strong optical absorption and appropriate electronic levels for electron transfer with common donor polymers in BHJs. Dyads with dyes based on diketopyrrolopyrrole and benzothiadiazole were synthesized using either a methanofullerene or a fulleropyrrolidine linkage. The performance of these dyad acceptors in BHJ solar cells with poly(3-hexylthiophene) (P3HT) and a low optical gap co-polymer of thiophene and diketopyrrolopyrrole (PDPP2FT) were examined. Although the solar power conversion efficiencies were low, charge generation from the molecular dye in the dyad could be observed in BHJs with PDPP2FT. The low power conversion efficiency was attributed to the morphology of the BHJs based on grazing incidence wide angle X-ray scattering patterns. Density functional theory was used to examine the charge transfer states between the donor and the dyad. The lowest energy charge-transfer state was found to be a transition from the donor polymer to the fullerene portion of the dyad.