Exploring charge generation and separation in tandem organic light-emitting diodes based on magneto-electroluminescence.

5,6,11,12-tetraphenylnaphthacene (rubrene) exhibits resonant energy properties (ES1,rub≈ 2ET1,rub), resulting in rubrene-based organic light-emitting diode (OLED) devices that undergo the singlet fission (STT) process at room temperature. This unique process gives rise to a distinct magneto-electroluminescence (MEL) profile, differing significantly from the typical intersystem crossing (ISC) process. Therefore, in this paper, we investigate charge generation and separation in the interconnector, and the mechanism of charge transport in tandem OLEDs at room temperature using MEL tools. We fabricate tandem OLEDs comprising green (Alq3) and yellow (Alq3:rubrene) electroluminescence (EL) units using different interconnectors. The results demonstrate that all devices exhibited significant rubrene emission. However, the MEL did not exhibit an STT process with an increasing magnetic field, but rather a triplet-triplet annihilation (TTA) process. This occurrence is attributed to direct carrier trapping within doped EL units, which hinders the transport of rubrene trapped charges, consequently prolonging the lifetime of triplet excitons (T1,rub). Thus, the increased T1,rubconcentration causes TTA to occur at room temperature, causing the rapid decrease of MEL in all devices under high magnetic fields. In devices where only the TTA process occurs, the TTA increases with the increasing current. Consequently, the high magnetic field of devices A-C is only related to TTA. Notably, there exists a high magnetic field TTA of device D in the Alq3/1,4,5,8,9,11-Hexaazatriphenylene-hexacarbonitrile interconnector regardless of the current. This occurs because both EL units in the device emit simultaneously, resulting in the triplet-charge annihilation process of Alq3in the high magnetic field of the MEL. Moreover, the rapid increase in MEL at low magnetic field across all devices is attributed to the ISC between Alq3polaron pairs. This entire process involves Förster and Dexter energy transfer. This article not only provides novel insights into charge generation and separation in the interconnector but also enhances our understanding of the microscopic mechanisms in tandem OLED devices.

Realizing high efficiency and luminance in green, yellow and blue organic light emitting diode by deoxyribonucleic acid.

Although the effect of the electron blocking layer (EBL) material, deoxyribonucleic acid (DNA), on the electroluminescence (EL) performance of organic light-emitting diodes (OLEDs) has been studied, the process of DNA regulation of exciton recombination region within the device is still unclear. Herein, devices with and without EBL were fabricated using different DNA spin-coating speeds, and the photoelectric performance of device were measured. By using DNA compounded with cetyltrimethyl ammonium (CTMA) as the EBL and hole buffer layer, so-called BioLEDs. The DNA-based green Alq3BioLEDs achieve higher luminance (39 000 cd m-2) and higher current efficiency (8.4 cd A-1), which are increased by 49% and 54%, respectively, compared to the reference OLEDs without the addition of DNA. Similarly, the maximum luminance and efficiency of yellow Rubrene BioLEDs is increased by 64% (from 12 120 to 19 820 cd m-2) and 74% (from 1.36 to 2.36 cd A-1), the luminance and efficiency of blue TCTA BioLEDs is increased by 101% and 245%. Specifically, we found that as the thickness of DNA-CTMA increases, the exciton recombination region moves towards the interface between the emitting layer (EML) and the hole transport layer (HTL). By better confining excitons within the EML, the current efficiency of the BioLEDs is effectively improved. Accordingly, we provide a possible idea for achieve high performance DNA-based BioLEDs by adding DNA-CTMA EBL and hole buffer layers. Meanwhile, as the DNA thickness increases, the exciton recombination region moves towards the EML/HTL interface, thereby enhancing the efficiency of the DNA-based BioLEDs.

Investigations on exciton recombination and annihilation in TmPyPB-ETL OLEDs using magnetic field effects.

Although the effect of the electron transport layer (ETL) material TmPyPb on the electroluminescence performance of organic light-emitting diodes (OLEDs) has been extensively studied, the process of TmPyPb regulating exciton recombination and annihilation within the device is still unclear. Here, we fabricated devices of various TmPyPb thicknesses with and without ETL. Subsequently, we measured the magneto-electroluminescence (MEL) of these devices. Specifically, at the same luminance, the triplet-charge annihilation (TQA) process is more likely to occur as the thickness of TmPyPb increases, resulting in a decrease in the maximum luminance of devices. Due to electron leakage and exciton recombination region moving towards the cathode, leading to a decrease in luminance efficiency at first and then an enhancement with an increase in the thickness of TmPyPb. Furthermore, at room temperature, the application of a large bias voltage suppresses singlet fission (SF) processes by modulating the dissociation of singlet polaron pairs (PPS) and the concentration of triplet exciton (T1). This leads to the conversion of SF to the TQA process. At low temperatures, the bias voltage and temperature can regulate the concentration and lifetime of PPS and T1. Therefore, as the temperature decreases, the transition of SF → TQA → triplet-triplet annihilation (TTA) and TQA coexistence → TTA process occurs. Moreover, MEL responses of the TmPyPb-ETL device show a W-linear pattern owing to the combined effect of the hyperfine interaction (HFI) and Zeeman splitting at 145 K. Accordingly, we explored the electroluminescence (EL) performance of TmPyPB-ETL OLEDs and investigated the evolution of SF, TQA, and TTA processes using MEL. Our study revealed the effect of exciton recombination and annihilation in OLEDs with varying thicknesses of TmPyPb.

Effect of Conductive Polymers PEDOT:PSS on Exciton Recombination and Conversion in Doped-Type BioLEDs.

Although the effect of the conductive polymers PEDOT:PSS on the electroluminescence performance of doped-type organic light-emitting diodes (OLEDs) has been studied, the process of PEDOT:PSS regulation of exciton recombination region and concentration within the deoxyribonucleic acid (DNA)-based doped-type BioLEDs is still obscure. In this study, we fabricated Bio-devices with and without PEDOT:PSS using varying spin-coating speeds of PEDOT:PSS. The Alq3:Rubrene-based BioLEDs achieve higher luminance (44,010 cd/m2) and higher luminance efficiency (8.1 cd/A), which are increased by 186% and 478%, respectively, compared to the reference BioLEDs without PEDOT:PSS. Similarly, the maximum luminance and efficiency of blue TCTA:TPBi exciplex-type BioLEDs are increased by 224% and 464%. In particular, our findings reveal that with an increasing thickness of PEDOT:PSS, the region of exciton recombination shifts towards the interface between the emitting layer (EML) and the hole transport layer (HTL). Meanwhile, the concentration of singlet exciton (S1,Rub) and triplet exciton (T1,Rub) increases, and the triplet-triplet annihilation (TTA) process is enhanced, resulting in the enhanced luminescence and efficiency of the devices. Accordingly, we provide a possible idea for achieving high performance doped-type BioLEDs by adding conductive polymers PEDOT:PSS, and revealing the effect of exciton recombination and conversion in BioLEDs given different PEDOT:PSS thicknesses.

Mediation of exciton concentration on magnetic field effects in NPB : Alq3-based heterojunction OLEDs.

Organic light-emitting diodes (OLEDs) are considered one of the most promising new display technologies owing to their advantages, such as all-solid-state, high color gamut, and wide viewing angle. However, in terms of special fields, the brightness, lifetime, and stability of the devices need further improvement. Therefore, heterojunction devices with different concentrations were prepared to regulate device brightness. The brightness of the bulk heterojunction device is enhanced by 9740 cd m-2, with a growth rate of about 26.8%. The impact of various temperatures and various exciton concentrations on the device magneto-conductance (MC) and magneto-electroluminescence (MEL) was investigated. Experimental results demonstrate that the exciton concentration inside the device can be tuned to improve optoelectronic properties and organic magnetic effects. The complex spin mixing process inside the bulk heterojunction device is deeply investigated, which provides a reliable basis for the design of bulk heterojunction devices.

An Inverted Perovskite Solar Cell with Good Comprehensive Performance Realized by Reducing the Concentration of Precursors.

Inverted perovskite solar cells (PSCs) exhibit great potential for industrial application thanks to their low complexity and low fabrication temperature. Aiming at commercial applications, it is necessary to comprehensively consider the material consumption and its corresponding electrical performance. Here, a simple strategy has been proposed to obtain inverted PSCs with comprehensive performance, that is, reaching an acceptable electrical performance by reducing the usage of perovskite. More precisely, the inverted PSCs, whose perovskite film is prepared by 1.0 M precursor, yields a power conversion efficiency (PCE) of 15.50%, fulfilling the requirement for real commercial application. In addition, the thickness of the electron transport layer (C60 in this work) in the above inverted PSCs was further optimized by comparing the simulated absorption spectrum, J-V characteristics and impedance with three different thicknesses of C60 layer. More excitingly, the optimized device displays high storage stability which maintains more than 90% of its initial PCE for 28 days. Therefore, our work provides a simple and cost-effective method to reach good comprehensive performance of inverted PSCs for commercial applications.

Enhancing the Luminance Efficiency of Formamidinium-Based Dion-Jacobson Perovskite Light-Emitting Diodes via Compositional Engineering.

In this paper, the phase formation mechanism of formamidinium (FA)-based Dion-Jacobson (DJ) perovskites is uncovered for the first time, which includes the formation of the n1 domain (n = 1 perovskite phase) and a trace amount of the large n domain in the spin-coating process and the growth of the n2 domain and the large n domain during the thermal annealing stage. The phase formation mechanism clearly reveals the different phase distributions between FA-based Ruddlesden-Popper (RP) and FA-based DJ perovskite films at different stages of film preparation due to the larger formation energy of the DJ perovskites. According to this phase formation mechanism, we put forward an effective strategy of the small A-site cation compositional engineering. Then, excess perovskitizer cations (FA+) are introduced to increase the crystallinity, modulate the phase distribution and passivate defects without disturbing the structure of DJ perovskites simultaneously. The final green-light DJ PeLED devices show a maximum luminance of 41 520 cd/m2 and a maximum current efficiency of 31.1 cd/A (EQE: 8.5%), which are the record values so far. The final DJ PeLEDs show an improved operational lifetime of ca. 15 min at an initial luminance of ca. 800 cd/m2. Our results suggest that DJ perovskites can be promising for light-emitting applications.

Smart Ternary Strategy in Promoting the Performance of Polymer Solar Cells Based on Bulk-Heterojunction or Layer-By-Layer Structure.

Although the rapid development of polymer solar cells (PSCs) has been achieved, it is still a great challenge to explore efficient ways for improving power conversion efficiency (PCE) of PSCs from materials and device engineering. Ternary strategy has been confirmed as an efficient way to improve PCE of PSCs by employing three kinds of materials. In this work, one polymer donor PM6, and two non-fullerene materials N3 and MF1 are selected to prepare ternary PSCs with layer-by-layer (LbL) or bulk-heterojunction (BHJ) structure. The LbL and BHJ-PSCs exhibit PCEs of 16.75% and 16.76% with 15 wt% MF1 content in acceptors, corresponding to over 5% or 4% PCE improvement compared with N3-based binary PSCs with LbL or BHJ structure. The PCE improvement is mainly attributed to the fill factor enhancement from 73.29% to 76.95% for LbL-PSCs or from 74.13% to 77.51% for BHJ-PSCs by employing the ternary strategy. This work indicates that ternary strategy has great potential in preparing highly efficient LbL-PSCs via simultaneously optimizing molecular arrangement and the thickness of each layer.

Influence of Temperature on Exciton Dynamic Processes in CuPc/C60 Based Solar Cells.

Although the effect of high temperature on the performance of organic solar cells has been widely investigated, it is inevitably influenced by the associated annealing effect (which usually leads to film morphology change and variation in electrical properties), which makes the discussion more sophisticated. In this study, we simplified the issue and investigated the influence of low temperatures (from room temperature to 77 K) on the photocurrent and internal/external quantum efficiency of a CuPc/C60 based solar cell. We found that besides the charge dynamic process (charge transport), one or more of the exciton dynamic processes, such as exciton diffusion and exciton dissociation, also play a significant role in affecting the photocurrent of organic solar cells at different temperatures. Additionally, the results showed that the temperature had negligible influence on the absorption of the CuPc film as well as the exciton generation process, but obviously influenced the other two exciton dynamic processes (exciton diffusion and exciton dissociation).

Ultra-Narrow-Band NIR Photomultiplication Organic Photodetectors Based on Charge Injection Narrowing.

Ultra-narrow-band NIR photomultiplication organic photodetectors (PM-OPDs) were realized in ITO/PEDOT:PSS/active layers/Al based on an interfacial-trap-induced charge injection narrowing (CIN) concept. The rather less Bod Ethex-Hex (BEH) is imbedded in a polymer donor matrix to form large amounts of isolated electron traps. Trapped electrons in BEH close to an Al electrode will enforce hole-tunneling injection induced by interfacial band bending, resulting in a photomultiplication phenomenon. PM-OPDs with P3HT:BEH as the active layer exhibit a narrow response peak at 850 nm with a full-width at half-maximum (fwhm) of 27 nm as well as a rather weak response from 650 to 800 nm. The EQE of 29 700% at 850 nm was achieved in PM-OPDs by incorporating 0.02 wt % of F6TCNNQ under -13 V of applied voltage. The rejection ratio (RR) of the optimized PM-OPDs with F6TCNNQ is 11 for EQE850 nm/EQE700 nm and 10 for EQE850 nm/EQE750 nm, respectively. An EQE of 15 300% at 850 nm was achieved in the ternary PM-OPDs under -13 V of applied voltage, with markedly enhanced RRs of 44 for EQE850 nm/EQE700 nm and 30 for EQE850 nm/EQE750 nm.

Coordinated Optical Matching of a Texture Interface Made from Demixing Blended Polymers for High-Performance Inverted Perovskite Solar Cells.

The continuing increase of the efficiency of perovskite solar cells has pushed the internal quantum efficiency approaching 100%, which means the light-to-carrier and then the following carrier transportation and extraction are no longer limiting factors in photoelectric conversion efficiency of perovskite solar cells. However, the optimal efficiency is still far lower than the Shockley-Queisser efficiency limit, especially for those inverted perovskite solar cells, indicating that a significant fraction of light does not transmit into the active perovskite layer to be absorbed there. Here, a planar inverted perovskite solar cell (ITO/PTAA/perovskite/PC61BM/bathocuproine (BCP)/Ag) is chosen as an example, and we show that its external quantum efficiency (EQE) can be significantly improved by simply texturing the poly[bis (4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) layer. By washing the film prepared from a mixed polymer solution of PTAA and polystyrene (PS), a textured PTAA/perovskite interface is introduced on the light-input side of perovskite to inhibit internal optical reflection. The reduction of optical loss by this simple texture method increases the EQE and then the photocurrent of the ITO/PTAA/perovskite/PC61BM/BCP/Ag device with the magnitude of about 10%. At the same time, this textured PTAA benefits the band edge absorption in this planar solar cell. The large increase of the short-circuit current together with the increase of fill factor pushes the efficiency of this inverted perovskite solar cell from 18.3% up to an efficiency over 20.8%. By using an antireflection coating on glass to let more light into the device, the efficiency is further improved to 21.6%, further demonstrating the importance of light management in perovskite solar cells.

Nuclei position-control and crystal growth-guidance on frozen substrates for high-performance perovskite solar cells.

Nucleation and crystal growth are key stages for high-quality perovskite films that dominate the performance of perovskite solar cells. However, the position of nuclei in the films and the orientation of the crystal growth have not yet been intendedly controlled during their fabrication. In this study, we developed a method of spin-coating perovskite films on frozen substrates to control the position of the nuclei and the direction of the crystal growth at the same time. In this way, the position of the crystal nuclei and the growth orientation of the perovskite crystals in the perovskite films can be simultaneously controlled. A high-quality perovskite film with grains spanning vertically the entire film thickness has been obtained using this new method. And an efficient inverted planar solar cell (ITO/PEDOT:PSS/CH3NH3PbI3/PC61BM/BCP/Ag) with the highest power conversion efficiency of 17.14% and open-circuit voltage of 1.14 V has been achieved by using this technique.

High performance planar p-i-n perovskite solar cells based on a thin Alq3 cathode buffer layer.

As a thin cathode buffer layer (CBL) tris-(8-hydroxyquinoline), aluminum (Alq3) is successfully introduced into the planar p-i-n perovskite solar cells (PSC) between the PCBM layer and cathode with a device structure of ITO/PEDOT:PSS/CH3NH3PbI3(Cl)/PCBM/Alq3/Ag. Due to the as-introduced thin Alq3 CBL, a high performance planar PSC has been achieved with a fill factor (FF) of 72% and maximum power conversion efficiency (PCE) of 14.22%. The PCE value is approximately 29% higher than that of the reference device without Alq3 CBL. Concerning the results of AC impedance spectra and transient photocurrent measurements, such a remarkable improvement of PCE is mainly attributed to the Alq3-caused better charge-extraction at the cathode, which is induced by reducing charge accumulation between PCBM and Ag.

[Influence of MnO3 on Photoelectric Performance in Organic Light Emitting Diodes].

Organic Light Emitting Diodes (OLEDs) has been a promising new research point that has received much attention recently. Emission in a conventional OLED originates from the recombination of carriers (electrons and holes) that are injected from external electrodes. In the device, Electrons, on the other hand, are injected from the Al cathode to an electron-transporting layer and travel to the same emissive zone. Holes are injected from the transparent ITO anode to a hole-transporting layer and holes reach an emitting zone through the holetransporting layer. Electrons and holes recombine at the emissive film to formsinglet excited states, followed by emissive light. It is because OLED is basically an optical device and its structure consists of organic or inorganic layers of sub-wavelength thickness with different refractive indices. When the electron and holes are injected through the electrodes, they combine in the emission zone emitting the photons. These photons will have the reflection and transmission at each interface and the interference will determine the intensity profile. The emissive light reflected at the interfaces or the metallic electrode returns to the emissive layer and affects the radiation current efficiency. Microcavity OLED can produce saturated colors and narrow the emission spetrum as a new kind of technique. In the paper, we fabricate microcavity OLED using glass substrate. Ag film acts as the anode reflector mirror; NPB serves as the hole-transporting material; Alq3 is electron-transporting material and organic emissive material; Ag film acts as cathode reflector mirror. The microcavity OLED structures named as A, B, C and D are glass/Ag(15 nm)/MoO3 (x nm)/NPB(50 nm)/Alq3 (60 nm)/A1(100 nm). Here, A, x = 4 nm; B, x = 7 nm; C, x = 10 nm; D, x = 13 nm. The characteristic voltage, brightness and current of these devices are investigated in the electric field. The luminance from the Devices A, B, C and D reaches the luminance of 928, 1 369, 2 550 and 2 035 cd x m(-2), respectively at 13 V. At 60 mA x cm(-2), the current efficiency of the microcavity OLEDs using MnO3 are about 2.2, 2.6, 3.1 and 2.6 cd x A(-2) respectively. It is found that electrons are majority carriers and holes are minority carriers in this microcavity OLEDs. MnO3 film can improve hole injection ability from 4 to 10 nm. In addition, hole injection ability is increased with the increasing thickness of the MnO3 film.

The triplet-charge annihilation in copolymer-based organic light emitting diodes: through the "Scattering Channel" or the "Dissociation Channel"?

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 (3)LE and (3)CT 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 (3)CT involves in RISC. Our results give insights into the spin-dependent TCA limiting the efficiency of hotly discussed CT-based OLEDs.

[Fabrication and spectral analysis of a novel panel full color device].

It is demonstrated that a new solid-state cathodoluminescence device based on accelerated electron in SiO2 can be fabricated using organic light emitting materials as a fluorescent film. This device (the device of sandwiched structure) is composed of a glass substrate, an ITO layer, a double SiO2 films, and an organic fluorescent film. When a bias voltage is applied to the devices, they can uniformly emit red, green, and blue light. To some extent, when the voltage is low, the red (green) emission is obtained; and when the voltage is high, the blue emission is acquired. These emissions belong to the solid-state cathodoluminescence, they originate from the accelerated electrons in SiO2, which bombard the organic layers. Their devices have achieved full color (RGB) lighting and improved blue emitting.