Energy Of Charge Transfer State Research Articles

On the role of asymmetric molecular geometry in high-performance organic solar cells

Although asymmetric molecular design has been widely demonstrated effective for organic photovoltaics (OPVs), the correlation between asymmetric molecular geometry and their optoelectronic properties is still unclear. To access this issue, we have designed and synthesized several symmetric-asymmetric non-fullerene acceptors (NFAs) pairs with identical physical and optoelectronic properties. Interestingly, we found that the asymmetric NFAs universally exhibited increased open-circuit voltage compared to their symmetric counterparts, due to the reduced non-radiative charge recombination. From our molecular-dynamic simulations, the asymmetric NFA naturally exhibits more diverse molecular interaction patterns at the donor (D):acceptor (A) interface as compared to the symmetric ones, as well as higher D:A interfacial charge-transfer state energy. Moreover, it is observed that the asymmetric structure can effectively suppress triplet state formation. These advantages enable a best efficiency of 18.80%, which is one of the champion results among binary OPVs. Therefore, this work unambiguously demonstrates the unique advantage of asymmetric molecular geometry, unveils the underlying mechanism, and highlights the manipulation of D:A interface as an important consideration for future molecular design.

(Invited) Interface Engineering for Organic and Perovskite Photovoltaics

Organic and perovskite solar cells have been intensively studied because they are considered as one of the most promising photovoltaic devices. Indeed, power conversion efficiencies (PCEs) have recently exceeded 19% for organic solar cells and 25% for perovskite solar cells. In both devices, interface engineering is highly important for efficient photovoltaic performance. In particular, it has great impact on open-circuit voltage (V OC) because charge carriers would recombine bimolecularly at the interface. In this talk, I will discuss how interface impact on V OC in polymer solar cells and perovskite solar cells.First, I will discuss how interfacial charge transfer (CT) states impact on V OC in polymer solar cells. More specifically, we employed two crystalline polymers (PTzBT-BOHD and PTzBT-12OD) and a fullerene derivative (PCBM). PTzBT-BOHD and PTzBT-12OD consist of the same backbone with different side chains: PTzBT-BOHD has two branched side chains of butyloctyl (BO) and hexyldecyl (HD) groups and PTzBT-12OD has a linear side chain of dodecyl (12) group and a branched side chain of octadecyl (OD) group. Interestingly, these PTzBT-BOHD/PCBM and PTzBT-12OD/PCBM solar cells exhibited different V OCs, although the two crystalline polymer neat films exhibit the same ionization energy because of the same backbone structure. In order to address the origin of the different V OCs, we carefully analyzed the interfacial CT state in the blend films. As a result, we found the interfacial CT state energy is higher in PTzBT-BOHD/PCBM than in PTzBT-12OD/PCBM blend films. This is because HOMO level of PTzBT-BOHD is deeper than that of PTzBT-12OD in disordered mixed phase in blend films, which results from twisted backbone of PTzBT-BOHD with branched side chains [1].Next, I will discuss how aging and passivation impact on interfacial energy matching and hence V OC in perovskite solar cells. As reported previously, V OC is often improved by ambient storage, which is called aging effect, and also improved by addition of passivation layers, which is called passivation effect. Such improvements in V OC suggest recombination would be suppressed effectively in these devices. Interestingly, V OC was gradually improved during ambient storage even for the passivated device where surface recombination is effectively suppressed by the passivation layer. By analyzing electronic properties of each layer, we found that no change was observed for the perovskite layer but the HOMO level deepening was observed for a hole-transporting material of spiro-OMeTAD during the ambient storage. As a result, energy matching between perovskite and spiro-OMeTAD layers was significantly improved and hence surface recombination was further effectively suppressed, resulting in the improvement in V OC [2,3].These findings clearly show that the electronic state at the interface critically impacts on V OC not only in polymer solar cells but also in perovskite solar cells, and therefore suggest that device design based on interface engineering is of particular importance for further improvement in V OC.

Development of high-performance organic photodetectors by understanding origin of dark current density with synthesis of photoconductive polymers

Photoconductive polymers (PCPs) are key materials that determine the static and dynamic performance of p-n bulk heterojunction organic photodetectors (OPDs). Herein, we developed benzo[1,2-d:4,5-d']bis(oxazole)-based novel PCPs with π-spacer molecular engineering to promote charge-transport while maintaining low dark-current densities (Jd) in OPDs. The origin of Jd was elucidated by measuring hole charge-transfer (CT) state energy. We found that the degree of hole accumulation at the CT state rather than the CT state energy was crucial to maintaining a low Jd and promoting a fast signal response in the devices. The optimized OPDs showed highly promising responsivity (R) of 0.43 A/W and specific detectivity (D*) of 6.85 × 1012 Jones at −1 V under 532 nm LED (5.0 mW cm−2) illumination, which are superior to those of the control device (R = 0.29 A/W and D* = 5.46 × 1011 Jones). In addition, the dynamic properties were significantly improved by the introduction of π-spacers on the PCPs. −3 dB cut-off frequency was 200.4 kHz, and the rising and falling response times was ultrafast as 2.3 and 2.8 μs, respectively. Understanding the origin of Jd through the molecular engineering of PCPs is expected to contribute greatly toward expanding the knowledge required to develop high-performance OPDs.

Increasing the Ionization Energy Offset to Increase the Quantum Efficiency in Non‐Fullerene Acceptor‐Based Organic Solar Cells: How Far Can We Go?

AbstractMolecular engineering of organic semiconductors provides a virtually unlimited number of possible structures, yet only a handful of combinations lead to state‐of‐the‐art efficiencies in photovoltaic applications. Thus, design rules that guide material development are needed. One such design principle is that in a bulk heterojunction consisting of an electron donor and lower bandgap acceptor an offset (ΔIE) of at least 0.45 eV is required between both materials ionization energies to overcome energy level bending at the donor–acceptor interface, in turn maximizing the charge separation yield and the cell's internal quantum efficiency. The present work studies energy losses associated with ΔIE and, based on 24 blends, finds that losses are minimal up to a ΔIE of 0.6 eV. Electroluminescence spectroscopy shows that low energy losses are achieved when the charge transfer state energy (ECT) is similar to the acceptor's optical bandgap (EgA). Further ΔIE increase lowers ECT with respect to EgA, thus decreasing VOC. Within that 0.45–0.6 eV ΔIE sweet range, the fill factor FF, hence the power conversion efficiency, increases only marginally as the FF is often already close to maximal for ΔIE = 0.45 eV. The results are extended to 76 binary and ternary blends.

Direct Observation of Ultrafast Relaxation Dynamics of a Mixed Excimer State in Perylene Monoimide Dimer by Femtosecond Transient Absorption.

A J-type dimer PMI-2, two perylene monoimides linked by butadiynylene bridger was prepared, and its excited-state dynamics was studied using ultrafast femtosecond transient absorption spectroscopy, along with steady-state spectroscopy and quantum chemical calculations. It is evidently demonstrated that the symmetry-breaking charge separation (SB-CS) process in PMI-2 is positively mediated by an excimer, which is mixed by localized Frenkel excitation (LE) and an interunit charge transfer (CT) state. Kinetic studies show that, with the polarity increasing of the solvent, the transformation of excimer from a mixture to the CT state (SB-CS) is accelerated, and the recombination time of the CT state is reduced obviously. Theoretical calculations indicate that these are due to PMI-2 obtaining more negative free energy (ΔGcs) and lower CT state energy levels in highly polar solvents. Our work suggests that the mixed excimer can be formed in a J-type dimer with suitable structure, in which the charge separation the process is sensitive to the solvent environment.

Quantifying the effect of energetic disorder on organic solar cell energy loss

•Correlation between non-radiative energy loss and CT-state energetic disorder •D-A interface optimization strategy for reduced non-radiative energy loss •Universal energy loss metric incorporating disorder Understanding the factors affecting energy loss in organic photovoltaics (OPVs) is imperative to achieve further improvements in their efficiency and to establish design rules for the development of new materials. Here, we provide direct experimental evidence supporting correlation between charge-transfer (CT) state static disorder and energy loss. Specifically, upon studying several planar and bulk heterojunction solar cells, we demonstrate that the non-radiative energy loss component quadratically increases with increasing Gaussian CT-state disorder. We also show that by defining the total energy loss in terms of the peak of the CT-state distribution, obtained from temperature-dependent external quantum efficiency measurements, the effect of disorder on OPV performance can be unambiguously identified, offering a universal metric for quantifying energy loss across various devices. Understanding the factors affecting energy loss in organic photovoltaics (OPVs) is imperative to achieve further improvements in their efficiency and to establish design rules for the development of new materials. Here, we provide direct experimental evidence supporting correlation between charge-transfer (CT) state static disorder and energy loss. Specifically, upon studying several planar and bulk heterojunction solar cells, we demonstrate that the non-radiative energy loss component quadratically increases with increasing Gaussian CT-state disorder. We also show that by defining the total energy loss in terms of the peak of the CT-state distribution, obtained from temperature-dependent external quantum efficiency measurements, the effect of disorder on OPV performance can be unambiguously identified, offering a universal metric for quantifying energy loss across various devices.

Open-circuit voltage of organic solar cells: interfacial roughness makes the difference

Organic photovoltaics (PV) is an energy-harvesting technology that offers many advantages, such as flexibility, low weight and cost, as well as environmentally benign materials and manufacturing techniques. Despite growth of power conversion efficiencies to around 19 % in the last years, organic PVs still lag behind inorganic PV technologies, mainly due to high losses in open-circuit voltage. Understanding and improving open circuit voltage in organic solar cells is challenging, as it is controlled by the properties of a donor-acceptor interface where the optical excitations are separated into charge carriers. Here, we provide an electrostatic model of a rough donor-acceptor interface and test it experimentally on small molecule PV materials systems. The model provides concise relationships between the open-circuit voltage, photovoltaic gap, charge-transfer state energy, and interfacial morphology. In particular, we show that the electrostatic bias generated across the interface reduces the photovoltaic gap. This negative influence on open-circuit voltage can, however, be circumvented by adjusting the morphology of the donor-acceptor interface.

Cyclo[18]carbon‐A new class of electron acceptor for organic solar cells applications

The all-carboatomic ring, cyclo[18]carbon (C18), has the potential to act as an electron acceptor due to promising electronic and optical properties. In this study, we first illustrated the geometrical, electronic, and excited-state properties of C18 using various hybrid and long-range corrected density functional theory (DFT) methods. Further, we studied the nature of intermolecular interactions between dimers of C18 to gain insights into packing configurations of cyclo[18] in dimer and trimer configurations. Also, using the state-of-the-art DFT methods, we have reported the thorough characterization of the lowest excited-state (i.e. charge-transfer state) in various donor-acceptor model complexes based on pentacene and C18. We established an interplay between the molecular packing of C18 and pentacene molecules on the energy of charge transfer state. All these results could help in the designing of more efficient organic solar cells.

Over 19.2% Efficiency of Organic Solar Cells Enabled by Precisely Tuning the Charge Transfer State Via Donor Alloy Strategy.

The large energy loss (E loss) is one of the main obstacles to further improve the photovoltaic performance of organic solar cells (OSCs), which is closely related to the charge transfer (CT) state. Herein, ternary donor alloy strategy is used to precisely tune the energy of CT state (E CT) and thus the E loss for boosting the efficiency of OSCs. The elevated E CT in the ternary OSCs reduce the energy loss for charge generation (ΔE CT), and promote the hybridization between localized excitation state and CT state to reduce the nonradiative energy loss (ΔE nonrad). Together with the optimal morphology, the ternary OSCs afford an impressive power conversion efficiency of 19.22% with a significantly improved open‐circuit voltage (V oc) of 0.910 V without sacrificing short‐cicuit density (J sc) and fill factor (FF) in comparison to the binary ones. This contribution reveals that precisely tuning the E CT via donor alloy strategy is an efficient way to minimize E loss and improve the photovoltaic performance of OSCs.

Spiro rhodamine-coumarin compact electron donor-acceptor dyads: synthesis and spin-orbit charge transfer intersystem crossing.

We prepared spiro rhodamine (RB)-coumarin (Cou) compact electron donor-acceptor dyads (RB-Cou-CF3 and RB-Cou-CN), to study the charge transfer (CT) and spin-orbit CT intersystem crossing (SOCT-ISC). The π-conjugation planes of the rhodamine and coumarin units in both dyads are in nearly orthogonal geometry (dihedral angle: 86.3°). CT state emission was observed for RB-Cou-CF3 (at 550nm) and RB-Cou-CN (at 595nm). Although the fluorescence of the pristine coumarin units (fluorescence quantum yields ΦF = 59%) was quenched in the dyads (ΦF = 0.5 ~ 1.1% in n-hexane), the triplet state quantum yields of the dyads are also low (singlet oxygen quantum yield, ΦΔ = 2.3-7.5% in n-hexane). Nanosecond transient absorption spectra show that the 3Cou* state was formed, which shows a triplet state lifetime of 11-15.6μs. The proposed photophysical path for the dyads is as follows: RB-1Cou* → RB+•-Cou-• → RB-3Cou*. The low SOCT-ISC yield is attributed to the slightly lower charge-transfer state energy (1.94eV in toluene) as compared to the 3Cou* state energy (2.23eV) and the shallow potential energy curve (PEC) at energy minima of the dyads. This work indicates that orthogonal conformation of donor-acceptor units is inadequate for achieving efficient SOCT-ISC. These results are useful for studying charge separation and intersystem crossing of electron donor/acceptor dyads.

Delocalization suppresses nonradiative charge recombination in polymer solar cells

Suppressing nonradiative deactivation of charge transfer (CT) states is pivotal to realizing further improvements in the power conversion efficiencies of polymer solar cells (PSCs). According to the energy gap law, the nonradiative decay rate constant knr scales exponentially with decreasing CT state energy ECT; thereby, as long as knr is governed by the energy gap law, a decrease in ECT will inevitably increase nonradiative deactivation of CT states and hence decrease the power conversion efficiency. Here, we report the nonradiative decay dynamics of CT states generated in various nonfullerene-acceptor-based PSCs by using transient absorption spectroscopy. The absence of a strong correlation between knr and ECT indicates that the energy gap law is not valid for these PSCs and that parameters other than ECT contribute significantly to knr. We found that knr decreased with an increase in materials’ crystallinities, indicating that increasing crystallinity leads to CT state delocalization, which in turn mitigates the nonradiative deactivation of CT states. We report the nonradiative decay dynamics of charge transfer (CT) states generated in nonfullerene-acceptor-based polymer solar cells. The nonradiative decay rate constants knr decreased with an increase in the efficiency for dissociation of CT states into free carriers, indicating that nonradiative decay of CT states can be mitigated by increasing the delocalization of the CT state wave function.

Effects of additional π-bridges on a terpolymer based on the second acceptor unit of DTBT and the performance of organic solar cells

Fused electron acceptor units play a significant role in donor materials for organic solar cells (OSCs). This paper introduces the fused dithieno[3′,2':3,4;2″,3'':5,6]benzo[1,2-c][1,2,5]thiadiazole (DTBT) units into the benzo[1,2-b:4,5-b']dithiophene-benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDT-BDD) backbones to synthesize terpolymers. The photovoltaic performance of the terpolymers is investigated by adding the π-bridges on both sides of the DTBT. For Y6-based OSCs, the devices based on DTBT terpolymers without additional π-bridges can obtain high open-circuit voltage (V OC ) but low fill factor (FF). On the contrary, DTBT terpolymers with 3-octylthiophene π-bridges significantly enhance the FF but decrease the V OC in devices. By investigating terpolymers' photovoltaic and quantum chemistry properties, we find that the presence of π-bridge structures on both sides of the DTBT unit can effectively promote carrier collection and reduce the bimolecular recombination in the devices. However, its reduced charge-transfer state energy level results in high voltage losses. Therefore, the design of DTBT terpolymers for how to balance the voltage loss due to π-bridges brought to the optimization of polymer conformation and charge excitation and conversion will become an important subject. • A series of terpolymers based on dithienobenzothiadiazole are synthesized. • Additional π-bridges play non-negligible roles in terpolymers properties. • Balancing voltage loss and fill factor is vital in DTBT terpolymer-based OSC by π-bridges.

Exploring optimal multimode vibronic pathways in singlet fission of azaborine analogues of perylene.

The development of new singlet fission chromophores is a vibrant area of research to explore the possibility of efficient photovoltaic devices. Using high-level ab-initio density matrix renormalization group calculations, we present a systematic analysis of BN-doped perylenes for their potential application as singlet fission candidates. Four singlet fission chromophores are identified considering the monomer-based properties and their excitonic characters are further analyzed in a dimer configuration optimized in a six-dimensional space for local maxima of fission rates. Furthermore, a multistate multimode vibronic Hamiltonian is employed to identify intra- and interstate vibrational pathways for excitation energy modulation. Several photophysical properties such as Davydov splitting, activation energy and vibronic admixture of multiexcitonic and charge-transfer states are calculated for physically accessible dimers. The optimal dimer packing results in appropriate vibrational relaxation of singlet fission states and promotes significant population transfer which would be more attenuated without such couplings. This work not only identifies potential singlet fission systems with favorable electronic properties but also highlights the sensitivity of dimer packings with respect to the substitution patterns in singlet fission chromophores.

Optimized photoelectric conversion properties of PbS x Se1−x -QD/MoS2-NT 0D–1D mixed-dimensional van der Waals heterostructures

Zero-dimensional (0D)–one-dimensional (1D) mixed-dimensional van der Waals (MvdW) heterostructures have shown great potential in electronic/optoelectronic applications. However, addressing the interface barrier modulation and charge-transfer mechanisms remain challenging. Here, we develop an analytic model to illustrate the open-circuit voltage and charge-transfer state energy in PbS x Se1−x -quantum dots (QDs)/MoS2-nanotube (NT) 0D–1D MvdW heterostructures based on atomic-bond-relaxation approach, Marcus theory and modified-detailed balance principle. We find that the band alignment of PbS x Se1−x -QDs/MoS2-NT heterostructures undergoes a transition from type II to type I, and the threshold of size is around 5.6 nm for x = 1, which makes the system suitable for various devices including photocatalytic device, light-emission device and solar cell under different sizes. Our results not only clarify the underlying mechanism of interfacial charge-transfer in the heterostructures, but also provide unique insight and new strategy for designing multifunctional and high-performance 0D–1D MvdW heterostructure devices.

Mapping the energy level alignment at donor/acceptor interfaces in non-fullerene organic solar cells

Energy level alignment (ELA) at donor (D) -acceptor (A) heterojunctions is essential for understanding the charge generation and recombination process in organic photovoltaic devices. However, the ELA at the D-A interfaces is largely underdetermined, resulting in debates on the fundamental operating mechanisms of high-efficiency non-fullerene organic solar cells. Here, we systematically investigate ELA and its depth-dependent variation of a range of donor/non-fullerene-acceptor interfaces by fabricating and characterizing D-A quasi bilayers and planar bilayers. In contrast to previous assumptions, we observe significant vacuum level (VL) shifts existing at the D-A interfaces, which are demonstrated to be abrupt, extending over only 1–2 layers at the heterojunctions, and are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of donor - non-fullerene-acceptor systems.

Inner mechanism of Pr3+ luminescence thermometers based on the intervalence charge transfer state

Owing to their ultra-high temperature sensitivity, luminescence thermometers based on the intervalence charge transfer state of Pr3+ have drawn more and more attention. But the inner mechanism of Pr3+ luminescence thermometers, especially which parameters could affect the value of relative sensitivity, is still not fully revealed. In this paper, the luminescence thermometer properties of Ba1-xCaxTiO3:1%Pr3+ (x=0-0.7) with variation temperature have been systematically studied. The process of electrons in the 3P0 state passed through the intervalence charge-transfer state to the 1D2 state has been found to be sensitive to temperature variation. Through the intervalence charge transfer state engineering, we successfully realized the effective regulation of the intervalence charge transfer state energy position by replacing Ba2+ with Ca2+ and find that the effective energy gap between the 3P0 state and the intervalence charge transfer state is the key parameter that affects the relative sensitivity of luminescence thermometers. Moreover, the maximum relative sensitivity of Pr3+-doped BaTiO3, evaluated to be 2.26% K−1 at 413K under 450 nm excitation, is the one of the best performances among the Pr3+-based luminescence thermometers. This research could lead to the novel development of an efficient and high-performance optical thermometer.

Ultrafast charge transfer in a nonfullerene all-small-molecule organic solar cell: a nonadiabatic dynamics simulation with optimally tuned range-separated functional.

Herein, we have employed linear-response time dependent density functional theory (LR-TDDFT)-based nonadiabatic dynamics simulations to investigate the ultrafast charge transfer in a nonfullerene all-small-molecule donor-acceptor (D-A) system formed by a porphyrin small-molecule donor ZnP and a recently developed nonfullerene small-molecule acceptor 6TIC, during which the optimally tuned range-separated hybrid (OT-RSH) functional was adopted. In combination with static electronic structure calculations, several important conclusions were drawn. Firstly, the ZnP and 6TIC are more likely combined together non-covalently in parallel rather than in perpendicular to form ZnP-6TIC due to the much larger adsorption energies, i.e. -44.6 kcal mol-1vs. -25.2 kcal mol-1. Secondly, the excited state properties obtained by OT-RSH functionals seem more consistent with the experimental results compared to their untuned versions. Specifically, the energy of the lowest charge transfer (CT) state was predicted to be smaller than the lowest lying local excitation (LE) states using the OT-RSH functional-based LR-TDDFT calculations, which is beneficial for the charge transfer process that might be crucial for the high power conversion efficiency (PCE) achieved experimentally. In contrast, the untuned RS functionals all predict higher CT state energies, which is contradictory to the high PCE obtained in the experiment. Moreover, strong hybridization upon excitation between these states was revealed, which might be one of the reasons responsible for the high PCE observed in the experiment. Finally, ultrafast excited state relaxation can be completed within 500 fs due to the small energy gaps and the strong nonadiabatic couplings between these states, which is accompanied by ultrafast photoinduced electron transfer from ZnP to 6TIC and photoinduced hole transfer the other way around. The efficient charge transfer processes and the involvement of two charge generation channels might be another cause resulting in the excellent photovoltaic performance of ZnP-6TIC based OSCs. Our present work not only provides solid evidence for elucidating the underlying mechanism observed in previous experiments, but also suggests that the combination of OT-RSH functionals and LR-TDDFT-based nonadiabatic dynamics simulations might be a powerful tool for investigating the excited state dynamics of organic D-A systems, which is crucial for the theoretical design of novel OSCs with better performances in the future.

Tuning Electronic and Morphological Properties for High‐Performance Wavelength‐Selective Organic Near‐Infrared Cavity Photodetectors

AbstractIncorporation of compact spectroscopic near‐infrared (NIR) light detectors into various wearable and handheld devices opens up new applications, such as on‐the‐spot medical diagnostics. To extend beyond the detection window of silicon, i.e., past 1000 nm, organic semiconductors are highly attractive because of their tunable absorption. In particular, organic NIR wavelength‐selective detectors have been realized by incorporating donor:acceptor thin films, exhibiting weak intermolecular charge‐transfer (CT) absorption, into an optical microcavity architecture. In this work, the alkyl side chains of the well‐known PBTTT donor polymer are replaced by alkoxy substituents, hereby redshifting the CT absorption of the polymer:PC61BM blend. It is shown that the unique fullerene intercalation features of the PBTTT polymer are retained when half of the side chains are altered, hereby maximizing the polymer:fullerene interfacial area and thus the CT absorption strength. This is exploited to extend the detection range of organic narrow‐band photodetectors with a full‐width‐at‐half‐maximum of 30–38 nm to wavelengths between 840 and 1340 nm, yielding detectivities in the range of 5 × 1011 to 1.75 × 1010 Jones, despite the low CT state energy of 0.98 eV. The broad wavelength tuning range achieved using a single polymer:fullerene blend renders this system an ideal candidate for miniature NIR spectrophotometers.

Direct Observation of the Charge Transfer States from a Non-Fullerene Organic Solar Cell with a Small Driving Force.

For organic solar cells (OSCs), the charge generation mechanism and the recombination loss are heavily linked with charge transfer states (CTS). Measuring the energy of CTS (ECT) by the most widely used technique, however, has become challenging for the non-fullerene-based OSCs with a small driving force, resulting in difficulty in the understanding of OSC physics. Herein, we present a study of the PM6:Y6 bulk heterojunction. It is demonstrated that electro-absorption can not only reveal the dipolar nature of Y6 but also resolve the morphology-dependent absorption signal of CTS in the sub-bandgap region. The device with the optimum blending weight ratio shows an ECT of 1.27 eV, which is confirmed by independent measurements. Because of the charge transfer characteristics of Y6, the charge generation at PM6:Y6 interfaces occurs efficiently under a small but non-negligible driving force of 0.14 eV, and the total recombination loss is as low as 0.43 eV.

Three-Dimensional Symmetry Decides the Description of the Energy of Charge-Transfer State in Organic Solar Cells

Three-Dimensional Symmetry Decides the Description of the Energy of Charge-Transfer State in Organic Solar Cells