Acceptor Heterojunction Research Articles

A Low‐Dimensional Donor‐Acceptor Perovskite for High‐Performance X‐Ray Detection

AbstractPerovskite materials hold significant promise for X‐ray detection due to their high X‐ray attenuation coefficient, efficient charge mobility, and tunable compositions and optoelectronic properties. Nonetheless, extracting the charges deep inside the thick semiconductors remains a challenge. Here, the 1D cytidine lead bromide perovskite (CytPbBr3) with a regularly arranged Donor (D)–Acceptor (A) heterojunction structure is reported. The organic interlayer contributes to the conduction band, while the inorganic framework constructs the valance band. Such a D–A heterojunction structure offers two separate channels for electrons and holes transport, suppressing the charge recombination rates with improved charge collection efficiency, and thus yields a high device sensitivity of 1.2 × 106 µC Gyair−1 cm−2 with a low noise equivalent dose (NED) of 44.6 pGyair.

Inside Cover Picture

Organic photovoltaic (OPV) devices show promising as the next‐generation renewable energy source. Currently, the non‐radiative charge recombination induced energy loss is considered to be a major reason for the low device efficiency. This can be further attributed to the use of donor:acceptor (D:A) heterojunction structures, which resolves the high exciton binding energy induced low charge generation issue, but introduces low emissive charge transfer states. In this work, we assess the necessity of D:A heterojunction, and demonstrate that the high‐performance D:A heterojunction‐free OPV device is possible when both photo‐induced spontaneous polaron generation and good ambipolar charge transport are realized. Moreover, the D:A heterojunction‐free structure exhibits much less non‐radiative charge recombination and more stable morphology for high performance, and probably becomes the next game‐changer in OPVs. More details are discussed in the article by Zuo et al. on page 252—258.image

Molecularly induced order promotes charge separation through delocalized charge-transfer states at donor-acceptor heterojunctions.

The energetic landscape at the interface between electron donating and accepting molecular materials favors efficient conversion of intermolecular charge-transfer (CT) states into free charge carriers (FCC) in high-performance organic solar cells. Here, we elucidate how interfacial energetics, charge generation and radiative recombination are affected by molecular arrangement. We experimentally determine the CT dissociation properties of a series of model, small molecule donor-acceptor blends, where the used acceptors (B2PYMPM, B3PYMPM and B4PYMPM) differ only in the nitrogen position of their lateral pyridine rings. We find that the formation of an ordered, face-on molecular packing in B4PYMPM is beneficial to efficient, field-independent charge separation, leading to fill factors above 70% in photovoltaic devices. This is rationalized by a comprehensive computational protocol showing that, compared to the more amorphous and isotropically oriented B2PYMPM, the higher structural order of B4PYMPM molecules leads to more delocalized CT states. Furthermore, we find no correlation between the quantum efficiency of FCC radiative recombination and the bound or unbound nature of the CT states. This work highlights the importance of structural ordering at donor-acceptor interfaces for efficient FCC generation and shows that less bound CT states do not preclude efficient radiative recombination.

Donor–π–acceptor heterojunctions constructed from the rGO network and redox-active covalent organic frameworks for high-performance supercapacitors

The orderly assembled Ni-bis(dithiolene) units with unique electronic structure, heterostructure formed between COFs and rGO, and efficient electron transfer contribute to the outstanding supercapacitor performance.

Machine learning study of D:A1:A2 ternary organic solar cells

Introducing third component into donor:acceptor heterojunction provide new dimension to improve power conversion efficiency of organic solar cells (OSCs). However, the combining different donors and acceptors, as well as optimizing fabricating procedures are exhaust and tedious by experiment for ternary OSCs. Herein, we constructed the database, including 280 groups of D:A1:A2 type ternary OSCs with non-fullerene acceptors. The descriptors include the mass ratios, the root-mean-square roughness of morphology, the frontier molecular orbital energies and molecular fingerprints. Four kinds of machine learning (ML) models, including random forest (RF), extra tree regression, gradient boosted regression tree and adaptive boosting, were trained and tested. The coefficient of determination R2, Pearson's correlation coefficient r, root mean square error and mean absolute error were used to score the prediction results of ML models. The score and the slope of regression lines indicate RF model is prevail to other ML models in power conversion efficiency (PCE) prediction. Combining donors and acceptors in the constructed dataset generates 429,688 groups of D:A1:A2 type new ternary OSCs. Their photovoltaic performances were predicted with the trained RF model and screened. There are 138 groups of new ternary OSCs with the predicted PCE greater than 18.50%, and the champion predicted PCE (18.83%) corresponds to D18:AQx-18:ZH2 ternary OSC. The results of this work provide the guidance for optimizing ternary OSCs and can accelerate the development of high performance ternary OSCs.

Charge-Transfer Dynamics in OLEDs with Coexisting Electroplex and Exciton States

The reverse intersystem-crossing (RISC) channels from triplet to singlet charge-transfer states (such as exciplex or electroplex states) frequently occur in donor:acceptor heterojunction organic light-emitting diodes (OLEDs) to achieve high external quantum efficiency. Although exciplexes have been extensively investigated, there are few reports on the physical microscopic processes for electroplex-based devices. Herein, three kinds of donor:acceptor heterojunction OLEDs are fabricated: two coexisting electroplex and exciton devices and one pure exciplex-based device. Amazingly, via the fingerprint magnetoelectroluminescence measurement at room temperature, we observe an abnormal current-dependent RISC, which is enhanced with increasing bias current (I) in the coexisting electroplex and exciton but electroplex-dominated device; the conversion from ISC to RISC in the coexisting electroplex and exciton but exciton-dominated device; and only the normal I-dependent ISC, which weakens with increasing I in the pure exciplex-based device. This is because low-energy-triplet electroplexes are well confined by the high triplet energies of electron donors and acceptors, promoting the occurrence of the RISC process in electroplexes, and Dexter energy transfer from triplet excitons to triplet electroplexes enhances the RISC process with increasing I, causing the abnormal I-dependent RISC and the conversion from ISC to RISC. Moreover, as the operational temperature rises from 10 to 300 K, these two coexisting electroplex and exciton devices present the conversion from ISC to RISC at different temperatures, but the pure exciplex-based device shows only the normal temperature-dependent ISC, which is enhanced with increasing temperature. This is because the RISC channel is an endothermic process and temperature-dependent electroluminescence measurements show that the electroplex emission weakens as the temperature is reduced, causing the weakened RISC process and even the presence of the ISC process at low temperature. These experimental results demonstrate that the electroplex emission plays an important role in the occurrence of strong RISC processes. Our work deepens our microscopic understanding of the charge-transfer states in donor:acceptor heterojunction devices but also paves the way for utilizing electroplex states to design highly efficient OLEDs.

Polariton enhanced free charge carrier generation in donor-acceptor cavity systems by a second-hybridization mechanism.

Cavity quantum electrodynamics has been studied as a potential approach to modify free charge carrier generation in donor-acceptor heterojunctions because of the delocalization and controllable energy level properties of hybridized light-matter states known as polaritons. However, in many experimental systems, cavity coupling decreases charge separation. Here, we theoretically study the quantum dynamics of a coherent and dissipative donor-acceptor cavity system, to investigate the dynamical mechanism and further discover the conditions under which polaritons may enhance free charge carrier generation. We use open quantum system methods based on single-pulse pumping to find that polaritons have the potential to connect excitonic states and charge separated states, further enhancing free charge generation on an ultrafast timescale of several hundred femtoseconds. The mechanism involves polaritons with optimal energy levels that allow the exciton to overcome the high Coulomb barrier induced by electron-hole attraction. Moreover, we propose that a second-hybridization between a polariton state and dark states with similar energy enables the formation of the hybrid charge separated states that are optically active. These two mechanisms lead to a maximum of 50% enhancement of free charge carrier generation on a short timescale. However, our simulation reveals that on the longer timescale of picoseconds, internal conversion and cavity loss dominate and suppress free charge carrier generation, reproducing the experimental results. Thus, our work shows that polaritons can affect the charge separation mechanism and promote free charge carrier generation efficiency, but predominantly on a short timescale after photoexcitation.

Elucidating the Role of Disorder in Charge-Carrier Photoseparation in Organic Solar Cells

Using high-precision simulation calculations and a semianalytical theory, we finally resolve the controversy over the role of energetic disorder in geminate electron–hole photoseparation in organic solids. We demonstrate that in efficient photovoltaic systems, where the charge-carrier separation probability is high, this probability decreases with increasing disorder, both in the absence and presence of a donor–acceptor heterojunction. This results from energy relaxation of the charged particles in the site energy distribution, which effectively deepens the electron–hole Coulomb well. In inefficient photovoltaic systems, the separation probability also initially decreases but turns into an increasing trend at large disorder. We also demonstrate that in the presence of disorder, the geminate charge-pair separation probability is affected by the mismatch between the electron and hole mobilities, even in homogeneous systems. We show how our theoretical results can be practically applied in an analysis of experimental data in organic photovoltaics.

High hysteresis and distinctive optoelectronic memory effect for ambipolar thin-film transistors using a conjugated polymer having donor–acceptor heterojunction

π-Conjugated p-type PBDB-T and n-type N2200 macromolecular units are alternatively bonded to generate ambipolar copolymer (i.e. P(BDBT-co-N2200)) for achieving donor-acceptor (D-A) heterojunction. From the laser confocal microscope photoluminescence (PL) spectra of the P(BDBT-co-N2200) copolymer, PL characteristic peaks of PBDB-T and N2200 are simultaneously observed at 695, 760, and 860 nm. The thin-film transistors (TFTs) using P(BDBT-co-N2200) copolymer show ambipolar transistor characteristics originating from the coexistence of p-type and n-type semiconducting macromolecular units. Interestingly, a high hysteresis is observed in the transfer and output characteristics of the TFTs because of the interface traps and near-interface bulk traps. Under light irradiation, distinctive photocurrents and hysteresis are observed, suggesting the optically mediated charge release and photogating effects caused by trap states. Charge trapping with a high hysteresis and photoconduction with the photogating effect of the P(BDBT-co-N2200)-based ambipolar TFTs induce stable and repeatable writing, reading, and erasing operations. The optoelectronic memory devices using the P(BDBT-co-N2200)-based ambipolar TFTs are realized with the merit of a long charge storage time of 40 s. The π-conjugated copolymer, P(BDBT-co-N2200) exhibiting D–A heterojunction, can be applied to multifunctional devices, such as photoresponsive ambipolar transistors and optoelectronic memory devices, for image sensing and data storage. Optoelectronic memory devices using the ambipolar thin-film transistors of π-conjugated P(BDBT-co-N2200) copolymer having donor-acceptor heterojunction are realized with the merit of a long charge storage time. • π-Conjugated p-type PBDB-T and n-type N2200 macromolecular units were alternatively bonded to generate ambipolar copolymer with donor-acceptor (D-A) heterojunction. • The P(BDBT-co-N2200)-based TFT showed photoresponsive ambipolar transistor characteristics. • The P(BDBT-co-N2200)-based ambipolar TFTs were successfully operated as optoelectronic memory devices with repeatable writing, reading, and erasing processes. • Readout current charges were constant regardless of the variation in waiting times and linearly increased with increasing light exposure time.

Molecular design for all-in-one self-assembled donor–acceptor organic solar cells

The stability of organic solar cells still remains one of the main challenges to make this technology suitable for large scale mass production. The stability of the morphology is one main degradation channel in currently used bulk heterojunction systems. We introduce three different all-in-one donor–acceptor amphiphilic triads and perform molecular dynamics simulations in order investigate the stacking of the donor and acceptor sub-molecules after performing a simulated annealing step. We show that the molecular volume of each sub-molecule of the triad plays an important role for the stability of the π -stacking within the bilayer sheet. Further, we found that TIPS-pentacene and TIPS-tetraazapentacene sub-molecules serving as donor and acceptor, respectively, keep their crystalline nature in the bilayer sheet. Thus, by such a system, charge carrier mobilities in the range of 1 cm 2 /Vs should be possible, which is three orders of magnitude higher compared to measured mobilities in existing highly efficient organic solar cells. Therefore, by this approach not only the stability can be increased, but also the fill factor and therefore the power conversion efficiency can be improved. • Design rules for an ideal donor–acceptor morphology based on bilayer sheets. • Molecular proposal for all-in-one donor–acceptor molecules. • Improve stability and power conversion efficiency by stable donor–acceptor heterojunctions.

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.

Persistent radical cation sp2 carbon-covalent organic framework for photocatalytic oxidative organic transformations

Persistent radical covalent organic frameworks (COFs) have exhibited great application potential in the fields of photocatalysis, magnetism, and biology. However, there is still a lack of a simple, green, and effective way to produce persistent radical COFs. Here, we synthesized two sp2 carbon-covalent organic frameworks (sp2C-COFs) with persistent triphenylamine radical cation (TPA+•), which can be generated by simple light irradiation since the effective donor–acceptor heterojunction is formed in ordered skeleton. The number of persistent TPA+• will increase with the intensity of light irradiation and the photogenerated electrons generated simultaneously can be consumed by O2 to form reactive oxygen species, which can be then used to carry out the oxidative organic transformations under visible light. In addition, femtosecond-transient absorption spectra prove that the difference in the structure of the two radical cation sp2C-COFs will significantly influence the charge recombination rate, so that the materials show different catalytic efficiency.

Synergy of intermolecular Donor-Acceptor and ultrathin structures in crystalline carbon nitride for efficient photocatalytic hydrogen evolution

Crystalline carbon nitride is regarded as the new generation of emerging metal-free photocatalysts as opposed to polymeric carbon nitride (g-C3N4) because of its high crystalline structure and ultrahigh photocatalytic water splitting performance. However, further advances in crystalline g-C3N4 are significantly restricted by the sluggish separation of charge carriers and limited active sites. In this study, we demonstrate the successful synthesis of heptazine-triazine donor–acceptor-based ultrathin crystalline g-C3N4 nanosheets (UCCN) using a combined hot air exfoliation and molten salt (NaCl/KCl) copolymerization approach. The synergy of the donor–acceptor heterojunction and the ultrathin structure greatly accelerated the separation of the charge carriers and enriched the active sites. Accordingly, the superior hydrogen evolution activity and an ultrahigh apparent quantum efficiency of 73.6% at 420 nm under a natural photosynthetic environment were achieved by UCCN, positioning this material at the top among reported conjugated g-C3N4 materials. This study provides a novel paradigm for the development of donor–acceptor-based ultrathin crystalline layered materials.

18.5% Efficiency Organic Solar Cells with a Hybrid Planar/Bulk Heterojunction.

The donor:acceptor heterojunction has proved as the most successful approach to split strongly bound excitons in organic solar cells (OSCs). Establishing an ideal architecture with selective carrier transport and suppressed recombination is of great importance to improve the photovoltaic efficiency while remains a challenge. Herein, via tailoring a hybrid planar/bulk structure, highly efficient OSCs with reduced energy losses (Eloss s) are fabricated. A p-type benzodithiophene-thiophene alternating polymer and an n-type naphthalene imide are inserted on both sides of a mixed donor:acceptor active layer to construct the hybrid heterojunction, respectively. The tailored structure with the donor near the anode and the acceptor near the cathode is beneficial for obtaining enhanced charge transport, extraction, and suppressed charge recombination. As a result, the photovoltaic characterizations suggest a reduced nonradiative Eloss by 25 meV, and the best OSC records a high efficiency of 18.5% (certified as 18.2%). This study highlights that precisely regulating the structure of donor:acceptor heterojunction has the potential to further improve the efficiencies of OSCs.

Long-Range Interfacial Charge Carrier Trapping in Halide Perovskite-C60 and Halide Perovskite-TiO2 Donor-Acceptor Films.

Interfacial electron transfer across perovskite-electron acceptor heterojunctions plays a significant role in the power-conversion efficiency of perovskite solar cells. Thus, electron donor-acceptor thin films of halide perovskite nanocrystals receive considerable attention. Nevertheless, understanding and optimizing distance- and thickness-dependent electron transfer in perovskite-electron acceptor heterojunctions are important. We reveal the distance-dependent and diffusion-controlled interfacial electron transfer across donor-acceptor heterojunction films formed by formamidinium or cesium lead bromide (FAPbBr3/CsPbBr3) perovskite nanocrystals with TiO2/C60. Self-assembled nanocrystal films prepared from FAPbBr3 show a longer photoluminescence lifetime than a solution, showing a long-range carrier migration. The acceptors quench the photoluminescence intensity but not the lifetime in a solution, revealing a static electron transfer. Conversely, the electron transfer in the films changes from dynamic to static by moving toward the donor-acceptor interface. While radiative recombination dominates the electron transfer at 800 μm or farther, the acceptors scavenge the photogenerated carriers within 100 μm. This research highlights the significance of interfacial electron transfer in perovskite films.

Sonochemically exfoliated polymer-carbon nanotube interface for high performance supercapacitors

Energy storage characteristics of organic molecules continue to attract attention for supercapacitor applications, as they offer simple processing and can be employed for flexible devices. The current study utilized the ultrasonically driven exfoliation to obtain poly diketo pyrrolopyrrole-thieno thiophene (PDPT) and multiwalled carbon nanotube (CNT) composite, subsequently fabricated a PDPT donor–π–acceptor heterojunction with CNT and investigated energy storage applications. The composite was characterized using series of standard analytical techniques. Morphology indicated well alighted CNT tubes on PDPT polymer nanosheets with an effective interface, providing efficient electrochemical regions, enabling fast charge transfer between PDPT and CNT. We also investigated the PDPT-CNT composite electrochemical behavior, achieving 319.2 and 105.7F.g−1 capacitances for PDPT-CNT and PDPT at 0.5 A.g−1 current density for three electrode configurations; and 126 and 42F.g−1 for symmetric structures, respectively. Experimental results confirmed that PDPT-CNT composite electrodes achieved two fold the capacitance compared with PDPT alone. The hypothesis and synthetic approach provide an excellent candidate for conjugated polymers with carbon nanotubes and energy related devices.

Voltage dependence of equivalent circuit parameters of bilayer organic photovoltaics

Despite the very different underlying physics of organic photovoltaics (OPVs), inorganic p-n junction’s Shockley’s diode equation is often applied to describe current density–voltage (JV) curves of OPVs. The model parameters, including the diode saturation current, diode ideality factor, series, and parallel resistances, are usually extracted and treated as constants in JV curve analyses. In this work, we develop a drift-diffusion bilayer interface (DD-BI) model for bilayer OPVs, which treats the donor–acceptor (D–A) heterojunction using the detailed balance between densities of polaron pairs, free electrons, and free holes. From the DD-BI model, we derive a diode equation, which is of Shockley’s equation form, but each parameter is explicitly written in terms of the D–A interface properties. We call this model the self-consistent diode (SCD) model as it is consistent with the DD-BI results provided that the key parameters are from the simulation data. By studying the effects of light intensity and carrier mobility, we find that the Shockley SCD parameters are voltage dependent because of space charge accumulation around the D–A heterojunction. Our models are successful in explaining the common discrepancies in OPV JV curve analyses, such as the validity of fitting for series resistance, deviation of ideality factor from the theoretical values, and different resistance values under light and dark conditions. The results provide a better understanding of OPVs with a D–A heterojunction and how we can capture its physics using the SCD equation.

Comparison of the mechanical properties of polymer blend and main-chain conjugated copolymer films with donor–acceptor heterojunctions

To implement a highly flexible polymer solar cell (PSC), the mechanical properties of the material constituting the active layer are considered crucial, having a significant influence on the stability of the device. In this study, the mechanical properties of a main-chain conjugated copolymer (PBDT2T-b-PNDI2T) containing donor- and acceptor-based macromolecular species in the polymer backbone was measured and compared with that of donor and acceptor polymer blend film (P(BDT2T):P(NDI2T)). It was observed that the PBDT2T-b-PNDI2T active layer exhibited a higher crack-onset-strain and toughness compared to P(BDT2T):P(NDI2T) active layer. Correspondingly, the YTRON/polyethylene naphthalate (PEN) based flexible PSC with a PBDT2T-b-PNDI2T active layer was used, which exhibited superior mechanical stability under multiple bending conditions. The results of this study indicate that flexible PSCs with excellent durability can be developed by replacing the polymer blend used for all-PSCs with PBDT2T-b-PNDI2T as the active layer material for highly efficient future applications.

Relationship between charge transfer state electroluminescence and the degradation of organic photovoltaics

The degradation of archetype organic photovoltaics comprising both vacuum and solution-deposited bulk heterojunction active regions is investigated and quantified using a theory based on detailed balance, which relates the open-circuit voltage to the efficiency of charge transfer state emission. To describe this relationship, we account for the difference between electroluminescent external quantum efficiency and the charge transfer emission efficiency. An empirical factor, m, is introduced to distinguish between nonradiative defect sites both within, m = 1, and outside, m >1, of the photoactive heterojunction. The m-factor is used to determine the primary sources of degradation for archetype solution- and vacuum-processed material systems. We conclude that degradation occurs primarily within the donor–acceptor heterojunction for the vacuum-processed devices (where m = 1.020 ± 0.002) and outside of the photoactive heterojunction for the solution-processed devices studied, both with and without an anode buffer layer (where m = 2.93 ± 0.09 and m = 1.90 ± 0.01, respectively).

Pentacene/non-fullerene acceptor heterojunction type phototransistors for broadened spectral photoresponsivity and ultralow level light detection

Pentacene/non-fullerene acceptor heterojunction type phototransistors were fabricated for broadened spectral photoresponsivity and ultralow level light detection.