•Highly efficient polyfluorene copolymer PFOPV-based PLEDs were reported.•MEL study joint by time-resolved spectra were employed in measurements.•CT states between PFO and MEH-PPV components contribute largely to the EL emission.•Triplet CT states were harvested to generate singlets through RISC process.Recently, significant efforts have been made to develop highly efficient organic light-emitting diodes (OLEDs) employing reverse intersystem crossing (RISC) owing to its capability of harvesting non-emissive triplet states. Up to date, most of such emitters were small-molecular-weight materials that needed to be doped into a host material. Due to the advantages of ease device fabrication and excellent photoluminescent properties, polymer-based RISC emitters should be developed. In this study, we reported a polyfluorene-based copolymer of poly[{9,9-dioctyl-2,7-divinylene-fluorenylene-alt-co-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenyene}] (PFOPV), which is a solution-processable, non-doped, high-molecular-weight material exhibiting RISC characteristics. A systematical magneto-electroluminescence study joint by time-resolved spectra revealed that charge transfer (CT) states would be generated between PFO and MEH-PPV components of PFOPV, and triplet CT states can participate in RISC process and contribute significantly to the emission. The PFOPV device shows excellent EL performance with EL efficiency of 12.7 cd/A and EQE of 2.2%, which is about 10–15 times and 2–4 times higher than those of PFO (0.9 cd/A, 0.63%) and MEH-PPV (1.2 cd/A, 1.1%) homopolymer devices, respectively.

Energy transfer (ET) and charge injection (CI) in the hybrid organic/colloidal quantum dot light-emitting diodes (QD-LEDs) have been investigated by using magneto-electroluminescence (MEL) as an in situ tool. The feasibility and availability of MEL as an in situ tool were systematically demonstrated in the typical QD-LEDs based on CdSe–ZnS core–shell QDs. Our results suggest that the ET and CI processes can be well discerned by MEL measurements since these two processes exhibit distinct responses to the applied magnetic field. Through measurement of the MEL and current efficiency, we indicated that ET would be the main mechanism for light emission in the present hybrid QD-LEDs. This study strongly suggests that MEL could be a highly sensitive fingerprint for ET, which provides a facile and efficient method for the in situ investigation of fundamental processes in hybrid organic/colloidal QD-LEDs and other organic/inorganic composites.

•We realized a thin recombination zone for electrons and holes by using planar-heterojunction device structure.•TTA has been realized in our planar-heterojunction exciplex-based OLEDs by using a thin recombination zone to enhance the interfacial density of the triplet states.•TTA could even happens at room temperature with appropriate donor material of MeO-TPD that possess the favorable electron-donating ability.Triplet–triplet annihilation (TTA) for enhancement of luminous efficiency occurs with difficulty in exciplex-based organic light-emitting devices (OLEDs) because it is an interaction among several neighboring donor and acceptor molecules. However, TTA has been realized in our planar-heterojunction (PHJ) exciplex-based OLEDs by using a thin recombination zone to enhance the interfacial density of the triplet states. The TTA process, which is characterized by a high-field decrease (HFD) in the magneto-electroluminescence from the PHJ OLEDs, appears at approximately 150 K and becomes stronger with decreasing temperature. At a given temperature, the higher the injected current is, the stronger HFD is observed. Additionally, we find that TTA could even happens at room temperature with appropriate selection of the donor molecule, which may be attributed to the favorable electron-donating ability of the methoxy group (–OCH3) in the donor molecule and the matched overlaps of the intermolecular conformation of the donor and the acceptor.

•The low-field component (LFE) of MC was always larger than the high-field component in bipolar injection condition.•Hyperfine mixing is the dominant mechanism.•A positive correlation between the LFE intensity and the trap density.•Traps provide interaction sites for hyperfine mixing.The origin of magnetoconductance (MC) in organic light-emitting diodes under bipolar injection conditions was investigated using devices containing pristine Super-Yellow poly(phenylene vinylene) (SY-PPV) or SY-PPV:phenyl-C61-butyric acid methyl ester (PCBM) (x wt%) blends as the active layers. In pristine SY-PPV device, it was found that the low-field component of MC was always larger than the high-field component. Additionally, the low-field component increased and then saturated with increasing the electrical stressing time, whereas the high-field component remained unchanged. These behaviors were analyzed using empirical formula (containing a Lorentzian and a non-Lorentzian function), which suggested that the dominant mechanism in the MC response was hyperfine mixing between single and triplet polaron pairs that occurred on trap sites. The specific role of these traps, providing interaction sites for hyperfine mixing, was confirmed by controlling the lifetime of the trapped polaron-pairs states by doping the active layer with PCBM.

•We analyzed the coexistence of negative and positive MC responses in the rubrene-based devices.•Holes and electrons are dominant in different channels of triplet-charge interaction (TQI): holes via the dissociation channel while the electrons via scattering channel.•The appropriate design of devices can change the reaction probabilities of two channels of TQI.Although it is known that triplet excitons can be quenched via triplet-charge interaction (TQI), it is still unclear how this process occurs in rubrene-based devices. We found that magneto-conductance (MC) can be used to probe the detailed mechanism of TQI in rubrene-based organic light-emitting diodes and observed the coexistence of negative and positive MC responses in the high-field region when holes and electrons were the dominant charged species at different interfaces adjoining the rubrene layer, respectively. Further analysis suggests that the negative MC response was originated from the dissociation of triplet excitons by holes, while the positive MC response was due to electron scattering by triplet excitons. The MC responses of the devices were examined under different injection currents and temperatures to confirm our hypothesis. This work gives significant insight into mechanisms of TQI in organic semiconductors, which will allow for the design of new and improved devices.

Triplet fusion (TF) and singlet fission (SF) are two important spin-coupled exciton interactions that occur in rubrene-based organic light-emitting diodes (OLEDs). TF produces additional singlets, which increases fluorescence efficiency, while SF consumes singlets and lowers the fluorescence efficiency. In an effort to adjust the SF and TF processes in rubrene-based OLEDs, we changed the average molecular spacing (d) of rubrene by doping it at varying concentrations in the high triplet energy material 1,3-bis(9-carbazolyl)benzene (mCP). Using magneto-electroluminescence (MEL), we observed that TF increased, while SF decreased at ambient temperature as d was increased from 1.8 to 5.0 nm. This was further confirmed using MEL at different temperatures and current intensities. We found that the efficiency of rubrene-based OLEDs was improved by altering the value of d, with the highest efficiency being observed at d = 3.8 nm because of complete conversion of SF to TF (SF → TF). The SF → TF was explained using a model that describes Dexter- and Förster-energy transfer in SF and TF processes with functions that have a different dependence on d. This difference causes the rate constant of SF to decrease more rapidly than that of TF. The TF will be primary when d goes between the Dexter and Förster radii, leading to complete SF → TF at ambient temperature. This work presents a promising approach to improve the efficiency of rubrene-based OLEDs.

Negative magnetoconductance (MC) effects have been observed over a large temperature range from room temperature to 20 K in amorphous copper phthalocyanine (CuPc) thin film. It is found that the negative MC increases when the temperature decreases. The corresponding current density–voltage characteristics of the device at different temperatures reveal that this negative MC is related to the presence of traps in CuPc thin film. Moreover, the magnitude of negative MC scales with current density for nearly three orders. Based on these results, trap-assisted bipolaron formation, a developed mechanism based on bipolaron, has been proposed. We suggest that traps existing in CuPc thin film can assist the formation of bipolarons through lowering the formation energy. This model is further confirmed by the negative MC responses with light illumination.

In organic semiconductors, the triplet-charge annihilation (TCA) is one of the most common excitonic interactions influencing the opto-electronic power conversion efficiency of the devices. However, it is still unclear whether the TCA reaction goes through the “Scattering Channel” or the “Dissociation Channel”. In this work, by measuring the organic magneto-current (OMC) of the conjugated co-polymer poly[{9,9-dioctyl-2,7-divinylene-fluorenylene}-alt-co-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenyene}] (PFOPV)-based organic light-emitting diodes (OLEDs) containing both localized exciton (LE) and charge-transfer-complex (CT), it is found that 3LE and 3CT play a crucial role in the “Scattering Channel” and the “Dissociation Channel” of TCA, respectively. This argument was supported by the simulations of Lorentzian and non-Lorentzian functions used, respectively, for intersystem crossing (or reverse intersystem crossing, RISC) and TCA effects. Moreover, by inserting a tris(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB) layer between PFOPV and the cathode, we improved the electroluminescence efficiency of PFOPV-based OLEDs by suppressing the TCA when 3CT involves in RISC. Our results give insights into the spin-dependent TCA limiting the efficiency of hotly discussed CT-based OLEDs.

•A remarkable high-field (|B| > 50 mT) decay of magneto-electroluminescence (MEL) was observed at room temperatures in tris(8-hydroxyquinolinato) aluminum (Alq3)-based OLEDs.•B field can increase the extent of overlap between the electron–hole recombination zone and the organic/metal interface by suppressing electron mobility.•Intersystem crossing that is induced by spin–orbital coupling at the organic/metal interface increase with the extent of overlap leading to the observed high-field decay.We explored mechanisms for the high-field (|B| > 50 mT) decay of organic magneto-electroluminescence. The organic/metal interface in pristine tris (8-hydroxyquinolinato) aluminum-based organic light-emitting diodes was modified by changing the metal cathodes and their deposition methods. The metals investigated were Al, Au, and Cu and the methods used include molecular beam deposition, thermal resistive evaporation, and electron beam evaporation (EBE), respectively. Experimental results revealed that the high-field decay can be observed at room temperature when the cathode is: (i) Cu deposited by EBE or (ii) Au deposited by any of the three deposition methods. Furthermore, this decay is different from the previously reported high-field decay that originates from triplet–triplet annihilation, triplet-charge reaction processes or Δg mechanism. We suggest that the magnetic field can increase the extent of overlap between the electron–hole recombination zone and the organic/metal interface by suppressing electron mobility. The spin–orbital coupling at the organic/metal interface consequently induces intersystem crossing to increase with magnetic field leading to the observed high-field decay.

•In fission-sensitized photovoltaic devices, singlet excitons might undergo three decay pathways.•The competition between singlet fission, radiation, and dissociation was analyzed in detail.•In a rubrene-based photovoltaic device, the internal quantum efficiency can reach about 124%.In singlet fission sensitized photovoltaic devices, the photoexcited singlet excitons might undergo three different decay pathways including fission, radiation, and dissociation. For rubrene-doped amorphous films, we analyzed the competition between these three decay channels by using transient and steady-state measurements of photoluminescence and photocurrent, as well as their magnetic field effects. The experimental results and theoretical calculations show that the internal quantum efficiency of our device can reach about 124%. More importantly, the method developed in this work can also be applied to other fission materials for optimizing the design of fission sensitized photovoltaic devices.

•Changing the intermolecular distance can tune the intensity of singlet fission in doped rubrene.•The efficient singlet fission occurring in rubrene is proved to be a thermally activated process.•The fission probability of rubrene in amorphous solid is estimated to be as large as 67%.The magnetic field effects of photoluminescence (MPL) from rubrene doped organic films were recorded at different temperatures. The measured line shapes were attributed to the field modification on the rate constant of thermally activated singlet exciton fission which occurred between the doped rubrene molecules. And its amplitude exhibited a nonlinear dependence on the averaged intermolecular distance. Such an observation implies that the intermolecular coupling (IMC) which is modulated by changing the intermolecular distance is able to significantly affect the intensity of fission process. Therefore, investigating the variation of singlet fission with different strength of IMC could be an important means to study the dynamics of fission process. Our work reveals the importance of IMC factor which needs to be considered for the design of efficient singlet fission-sensitized organic photovoltaic devices.

•A large positive MEL (23.5%) was obtained at R.T.•The MELs changed their signs both at low-field and high-field components.•The singlet fission and triplet fusion are coexisted in rubrene.•Relative contribution of singlet fission and triplet fusion on MEL changes with decreasing temperature.Organic light emitting diodes (OLEDs) utilizing a singlet–triplet energy-resonant (ES ≈ 2ET) layer (rubrene) were fabricated to investigate the singlet fission and triplet fusion by the magneto-electroluminescence (MEL) of device from R.T. to 20 K. A large positive MEL (23.5%) was obtained at R.T. due to magnetic-field-suppressed singlet fission. With decreasing temperatures, the MELs changed their signs both at low-field and high-field components because of a gradual decrease in singlet fission simultaneously followed by an increasing triplet fusion, leading to a negative MEL around −7.5% at 20 K. Moreover, transient electroluminescence and MELs from the control devices were used to further confirm the exciton fission and fusion processes in rubrene-based OLEDs. Our findings of MEL may provide a useful pathway to study the microscopic dynamics of excited states in organic optoelectronic devices.

•The MEL can be significantly tuned from 19.3% to −1.0% at room temperature.•The tunable MEL was by the way of controlling the competition between SF and TF.•We doped different concentration of DCJTB into rubrene as an emissive layer.•Changing the working temperature is another method to tune the MEL.We report a tunable magneto-electroluminescence (MEL) in doped organic light-emitting diodes (OLEDs) with narrow energy gap molecule of 4-dicyanomethylene-2-t-butyl-6-1,1,7,7-tetramethy-ljulolidyl-9-4H-pyran (DCJTB) doped rubrene as active layers. It is noted that the MEL at 500 mT alters significantly from 19.3% to −1.0% by controlling the doping concentration of DCJTB in the active layer at room temperature. The results might be caused by the conversion from singlet fission to triplet fusion depending on the competition between energy transfer and charge trapping channels in doped layer. Moreover, the MEL can also be tuned by changing the working temperatures for the devices with certain doping concentration. Therefore, this work provides a feasible pathway to tune the MEL by controlling the competition between singlet fission and triplet fusion in OLEDs.

•The MC effects observed in three OLEDs can all be separated into two independent components which can be fitted with two non-Lorentzian functions.•The second MC component can be attributed to the triplet exciton-charge interactions according to its current and temperature dependences.•The sign of MC effect induced by the triplet exciton-charge interactions can be either positive or negative due to the different reaction channel.The magnetoconductance (MC) effects in three organic light-emitting diodes have been measured over a range of operating current and temperatures. The peculiar roles played by the hyperfine field and triplet excitons are outlined by fitting the MC traces using the sum of two non-Lorentzian functions. The MC response evident at large magnetic fields is confirmed to be resulted from the triplet exciton–charge interaction by investigating its variation with the triplet population. The remarkable agreement between the experimental data and fit lines demonstrates that the method used in this work could help to identify the correct models for understanding the fundamental mechanism behind the magnetic field effects universally observed in organic devices.Graphical abstract

•The ultra-small field MC effect can be observed in SY-PPV:PCBM blends.•The widths of these curves gradually broaden with increasing PCBM concentration.•The broadening is caused by an increase in the dissociation rate.•The interpretation is verified by an empirical formula fitting.The ultra-small field induced magnetoconductance (MC) responses in Supper Yellow Poly(phenylenevinylene) (SY-PPV):Phenyl-C61-butyric acid methyl ester (PCBM) blends have been investigated to clarify the role of competition ratio by changing the dissociation rate. It is found that the widths of the ultra-small field induced MC curves broaden from 0.4 mT (0 wt.%) to 1.9 mT (4 wt.%). The characteristics of electroluminescence-voltage suggest that the width broaden is assigned to an increase in the competition ratio induced by an increase in the dissociation rate when PCBM is blended. This conclusion is further verified by an empirical formula fitting.Graphical abstract

Organic magneto-electroluminescence (MEL) based on the charge-transfer (CT) states was investigated to clarify the electron–hole (e–h) pair mechanism for the organic magnetic field effects. The CT state is an ideal object because its emission is a direct intermolecular recombination process without forming intramolecular exciton. We found that the MEL of the CT states is not only greater than that of the exciton, but also exhibits almost no high-field decrease at low temperatures. Our results directly prove the e–h pair mechanism. Meanwhile, the transient electroluminescence measurements with and without magnetic fields confirm that magnetic field has no effect on the charge mobility but on the charge recombination process, implying the charge mobility-related mechanisms may be less dominant above the turn-on voltage.Graphical abstractHighlights► We report magneto-electroluminescence (MEL) of NPB:d(ppy)BF charge-transfer state. ► The MEL of NPB:d(ppy)BF CT state is larger than that of NPB exciton. ► The MEL of NPB:d(ppy)BF CT state does not change with injection current. ► The MEL of NPB:d(ppy)BF CT state exhibits no high-field decay at low temperature. ► Transient EL measurements revealed charge mobility was not changed by magnetic field.

We explore the magnetoconductance (MC) response of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine-based single-layer devices. The MC effect can be observed only if the device is irradiated with light, for measurement voltages |V| < 5 V. A positive MC with a non-Lorentzian line shape is obtained under forward bias. Under reverse bias, however, the MC shows both positive and negative components, forming a “W” shape with dips at about ±100 mT. The sign of the MC under reverse bias can be changed by controlling the carrier extraction from the anode/organic interface. We suggest that the positive MC is caused by triplet–polaron interaction and the negative MC by bipolaron formation.Graphical abstractHighlights► The MC effect can be observed only if the device is irradiated with light, for measurement voltages |V| < 5 V. ► The co-existence of positive and negative MC components with “W” shape was obtained under reverse bias. ► The sign of the MC can be changed by controlling the carrier extraction from the anode/organic interface.

Singlet exciton formation in working fluorescent devices was investigated by analyzing the magnetic field effects on electroluminescence intensity based on a rate model. Two magnetic field sensitive processes including charge recombination and triplet–triplet annihilation (TTA) are suggested to be involved in the generation of singlet excitons. It reveals that TTA process produces considerable extra singlets which account for as high as 19% of total singlets at 20 K in non-doped device, 34% at 20 K and 17% at room temperature in doped device, causing the total singlet generation yields exceeding the classical 0.25 spin statistics limit.Graphical abstractHighlights► Singlet exciton formation in working fluorescent devices was investigated by analyzing the magneto-electroluminescence. ► Charge recombination and triplet-triplet annihilation are suggested to be involved in the generation of singlet excitons. ► Triplet-triplet annihilation produces considerable extra singlets, causing total singlet generation yields exceeding 0.25.

Driving current and temperature dependences of magnetic-field modulated electroluminescence (EL) in tris(8-hydroxyquinoline) aluminum (Alq3)-based OLEDs have been thoroughly investigated. At low temperatures, the applied magnetic-field induces a sharp increase of the EL in low field regime (B ⩽ 35 mT) and a slow but apparent decrease at high fields (35 ⩽ B ⩽ 500 mT). The low-field increase in EL (LFE) survives at all working temperatures while the high-field decrease (HFE) gradually disappears as temperature is increased. At a given temperature, the higher the current level, the smaller LFE and stronger HFE are observed. To explain the observed MFEs a composite model based on magnetic-field dependent singlet-to-triplet conversion of electron-hole pairs and magnetic-field mediated triplet–triplet annihilation process is proposed in this paper.

The organic magnetoconductance (MC) effects in poly(3-hexylthiophene): [6,6]-phenyl-C61-butyricacid methylester based bulk heterojunction solar cells were studied in dark and under illumination. The correlations between the MC and current character were revealed in this study. Results show that the dark current always exhibits a negative MC whereas a sign change in MC under illumination occurs at the bias around the open circuit voltage Voc. We suggest that the positive MC in photocurrent is due to the field dependent conversion of singlet electron–hole pairs to triplet states and the negative MC is associated with space charge limited current with traps. Other possible mechanisms about the magnetoconductance effects are also discussed.

The effects of a magnetic field on the dissociation of triplet excitons by free charges (TCI) are well understood. However, the magneto-conductance (MC) characteristics of trapped triplet–polaron interactions (TtPI) and triplet–trapped polaron interactions (TPtI) within organic light emitting diodes (OLEDs) are not well understood. We have studied these interactions in an anthracene-based OLED. The electroluminescence spectra, current–voltage characteristics and magneto-electroluminescence indicated that the anthracene layer contained many defects that could trap either triplet excitons or polarons, which led to TPtI and TtPI. The MC curves at low temperature exhibited a complex line shape, which indicated that intersystem crossing, TPtI, TtPI, and TCI occurred simultaneously in the device. The individual MC characteristics of TPtI and TtPI were extracted from temperature dependant MC curves by fitting them to three empirical Lorentzian functions and one non-Lorentzian function. The MC of TPtI exhibited a negative sign, while that of TtPI exhibited a positive one, with characteristic magnetic fields (B0) of ∼10.5 and ∼15 mT, respectively. Both processes were prominent below 150 K and weakened with increasing temperature. TPtI was neglected above 200 K, while TtPI was observed even at ambient temperature. These results add significant insight into the magnetic field effects on triplet–polaron interactions.

In organic semiconductors, the triplet-charge annihilation (TCA) is one of the most common excitonic interactions influencing the opto-electronic power conversion efficiency of the devices. However, it is still unclear whether the TCA reaction goes through the “Scattering Channel” or the “Dissociation Channel”. In this work, by measuring the organic magneto-current (OMC) of the conjugated co-polymer poly[{9,9-dioctyl-2,7-divinylene-fluorenylene}-alt-co-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenyene}] (PFOPV)-based organic light-emitting diodes (OLEDs) containing both localized exciton (LE) and charge-transfer-complex (CT), it is found that 3LE and 3CT play a crucial role in the “Scattering Channel” and the “Dissociation Channel” of TCA, respectively. This argument was supported by the simulations of Lorentzian and non-Lorentzian functions used, respectively, for intersystem crossing (or reverse intersystem crossing, RISC) and TCA effects. Moreover, by inserting a tris(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB) layer between PFOPV and the cathode, we improved the electroluminescence efficiency of PFOPV-based OLEDs by suppressing the TCA when 3CT involves in RISC. Our results give insights into the spin-dependent TCA limiting the efficiency of hotly discussed CT-based OLEDs.

Energy transfer (ET) and charge injection (CI) in the hybrid organic/colloidal quantum dot light-emitting diodes (QD-LEDs) have been investigated by using magneto-electroluminescence (MEL) as an in situ tool. The feasibility and availability of MEL as an in situ tool were systematically demonstrated in the typical QD-LEDs based on CdSe–ZnS core–shell QDs. Our results suggest that the ET and CI processes can be well discerned by MEL measurements since these two processes exhibit distinct responses to the applied magnetic field. Through measurement of the MEL and current efficiency, we indicated that ET would be the main mechanism for light emission in the present hybrid QD-LEDs. This study strongly suggests that MEL could be a highly sensitive fingerprint for ET, which provides a facile and efficient method for the in situ investigation of fundamental processes in hybrid organic/colloidal QD-LEDs and other organic/inorganic composites.

Negative magnetoconductance (MC) effects have been observed over a large temperature range from room temperature to 20 K in amorphous copper phthalocyanine (CuPc) thin film. It is found that the negative MC increases when the temperature decreases. The corresponding current density–voltage characteristics of the device at different temperatures reveal that this negative MC is related to the presence of traps in CuPc thin film. Moreover, the magnitude of negative MC scales with current density for nearly three orders. Based on these results, trap-assisted bipolaron formation, a developed mechanism based on bipolaron, has been proposed. We suggest that traps existing in CuPc thin film can assist the formation of bipolarons through lowering the formation energy. This model is further confirmed by the negative MC responses with light illumination.