Hole Tunneling Injection Research Articles

Suppressing the Dark Current Under Forward Bias for Dual‐Mode Organic Photodiodes

AbstractTremendous research efforts are developed to suppress the reverse dark current (Jd) and enhance the responsivity of organic photodiodes (OPDs). The functional layers of traditional OPDs usually follow the principle of energy level alignment to make unobstructed photo‐carriers transport under reverse bias, but this inevitably leads to a large forward Jd. Herein, a universal strategy is proposed to manipulate the carrier dynamics and effectively suppress the forward Jd of OPDs, that is, tuning the energy level and electron traps of the anode interface layers (AILs). The bandgap and electron traps of typical organometallic chelate AIL (PEIE‐Co) can be well controlled by adjusting the component ratio of PEIE and metal ions. The wide bandgap increases the carrier injection barrier under reverse and forward bias, endowing OPD with a much lower Jd; the electron traps induce hole tunneling injection by capturing photo‐generated electrons under forward bias, thereby enabling the photomultiplication effect. The obtained OPD exhibits photoconductive/photomultiplication working mode at reverse/forward bias and the specific detectivity approaches ≈1013/1012 Jones, showing promise for adaptively detecting faint and strong light. This study presents an intelligent strategy to achieve dual‐mode OPDs, paving the way for the multifunctional development of photodetectors.

Boosting the UV-vis-NIR Photodetection Performance of MoS2 through the Cavity Enhancement Effect and Bulk Heterojunction Strategy.

Navigating more effective methods to enhance the photon utilization of photodetectors poses a significant challenge. This study initially investigates the impact of morphological alterations in 2H-MoS2 on photodetector (PD) performance. The results reveal that compared to layered MoS2 (MoS2 NLs), MoS2 nanotubes (MoS2 NTs) impart a cavity enhancement effect through multiple light reflections. This structural feature significantly enhances the photodetection performance of the MoS2-based PDs. We further employ the heterojunction strategy to construct Y-TiOPc NPs:MoS2 NTs, utilizing Y-TiOPc NPs (Y-type titanylphthalocyanine) as the vis-NIR photosensitizer and MoS2 NTs as the photon absorption enhancer. This approach not only addresses the weak absorption of MoS2 NTs in the near-infrared region but also enhances carrier generation, separation, and transport efficiency. Additionally, the band bending phenomenon induced by trapped-electrons at the interface between ITO and the photoactive layer significantly enhances the hole tunneling injection capability from the external circuit. By leveraging the synergistic effects of the aforementioned strategies, the PD based on Y-TiOPc NPs:MoS2 NTs (Y:MT-PD) exhibits superior photodetection performance in the wavelength range of 365-940 nm compared to MoS2 NLs-based PD and MoS2 NTs-based PD. Particularly noteworthy are the peak values of key metrics for Y:MT-PD, such as EQE, R, and D* that are 4947.6%, 20588 mA/W, and 1.94 × 1012 Jones, respectively. The multiperiod time-resolved photocurrent response curves of Y:MT-PD also surpass those of the other two PDs, displaying rapid, stable, and reproducible responses across all wavelengths. This study provides valuable insights for the further development of photoactive materials with a high photon utilization efficiency.

High‐Power AlGaN‐Based Ultrathin Tunneling Junction Deep Ultraviolet Light‐Emitting Diodes

AbstractTunnel junctions (TJs) offer a unique approach to utilizing nonequilibrium tunneling injection of holes and have demonstrated potential applications in ultraviolet (UV) emitters. However, high operating voltage caused by the wide bandgap of the III‐nitrides has impeded the further promotion of TJ. Here, 275 nm n+‐Al0.45Ga0.55N/p+‐Al0.5Ga0.5N ultrathin tunnel junction (UTJ) deep‐UV light‐emitting diodes (LEDs) are developed for minimizing the electrical losses and achieve, to the current knowledge, the lowest operating voltage (5.7 V) at 300 mA reported to date. This study discovers that Mg‐Si co‐doping occurs in the n+‐Al0.45Ga0.55N layer during the growth of n+‐Al0.45Ga0.55N/p+‐Al0.5Ga0.5N UTJ due to the memory and diffusion effect of Mg, which leads to the formation of Ohmic contact between high‐work‐function Ni/Au and Mg‐Si co‐doped n+‐ Al0.45Ga0.55N layer in UTJ. The Zener diode, fluorine resin, and optimally designed glass lens are incorporated into the chip encapsulation to enhance the reliability and optical performance of the device. Furthermore, a high‐efficiency sterilization deep‐UV light source is developed integrated with 120 UTJ deep‐UV LED chips. Complete surface inactivation at a long irradiation distance (20 cm) is achieved within only seconds using the high‐power sterilization deep‐UV light source. These results indicate that UTJ is promising in developing deep UV LEDs and their integrated light sources.

Revisiting Hole Injection in Quantum Dot Light‐Emitting Diodes

AbstractInjecting holes from the hole transport layer (HTL) into the quantum dot (QD) emitting layer in quantum dot light‐emitting diodes (QLEDs) is considered challenging due to the presence of a relatively high hole injection barrier at the HTL/QD interface. However, QLEDs with exceptional brightness and efficiency are achieved, prompting a reevaluation of the traditional hole injection mechanisms. This study examines the hole injection mechanism in QLEDs using a combination of experiments and simulations. The results demonstrate that the applied bias significantly reduces the barrier height between the highest occupied molecular orbital level of the HTL and the valence band (VB) of the QDs, facilitating hole injection. The bending of the lowest unoccupied molecular orbital energy level of the HTL at the HTL/QD interface confines electrons within the QD, effectively minimizing leakage current. Additionally, the triangle‐shaped potential barrier arising from the bending of the VB energy level of the QDs creates favorable conditions for hole–tunneling injection. Moreover, both simulations and experiments consistently demonstrate that the predominant pathway for hole injection from the HTL to the QDs in the QLED device involved thermally assisted tunneling. This study is important to understand the hole injection mechanism in QLEDs.

Filter‐Free Narrowband Photomultiplication‐Type Planar Heterojunction Organic Photodetectors

AbstractFilter‐free narrowband photomultiplication‐type planar heterojunction (PHJ) organic photodetectors (PM‐PHOPDs) are first realized by employing a thick front donor layer and an ultrathin PC71BM layer. The thick front donor layer is employed as an optical field adjusting (OFA) layer. The sequentially coated PC71BM will diffuse slightly into OFA layer, which works as interfacial electron traps to capture photogenerated electrons for assisting hole tunneling injection. The P3HT/PC71BM‐based PM‐PHOPDs exhibit narrowband response with full‐width of half‐maximum of 32 nm and external quantum efficiency (EQE) of 1700% at 650 nm under −20 V bias. Due to the enhanced hole transport and reduced charge recombination in PHJ compared to those in bulk heterojunction (BHJ), the EQE of P3HT/PC71BM‐based narrowband PM‐PHOPDs is twice as P3HT:PC71BM BHJ‐based narrowband PM‐OPDs under the same bias. The response peak of PM‐PHOPDs is adjusted from 650 to 695 or 745 nm by incorporating SMPV1 or DRCN5T in OFA layers due to the red‐shifted absorption edge. The EQEs of 3600% at 695 nm and 870% at 745 nm are obtained for P3HT:SMPV1 and P3HT:DRCN5T‐based PM‐PHOPDs under −20 V bias, respectively. This work provides a smart strategy to achieve narrowband PM‐OPDs by designing different OFA layers.

Panchromatic photomultiplication-type organic photodetectors with planar/bulk heterojunction structure

Broadband organic photodetectors (OPDs) can be integrated into various wearable devices and show great application potential in health monitoring and other fields. Here, we demonstrated high-performance broadband photomultiplication-type OPDs (PM-OPDs) based on MoO3 trapping electron-assisted hole tunneling injection mechanism with small-molecule lead (II) phthalocyanine (PbPc) and C70 fullerene as the donor and acceptor, respectively. In order to control the crystal phase structure formed by PbPc molecules, we prepared PbPc:C70 bulk heterojunction (BHJ) and PbPc/C70 planar heterojunction (PHJ) devices. It can be seen that the PHJ device exhibits stronger near-infrared (NIR) absorption characteristics, which is more conducive to the formation of the triclinic phase structure of the PbPc molecule. We further prepared flat panchromatic PM-OPDs in the range of 300–1000 nm by using materials (PbPc and SubPc) with absorption complementary characteristic as the active layers and PHJ/BHJ hybrid heterojunction structure. The external quantum efficiency of the resulting panchromatic PM-OPDs exceeds 1000% in the whole spectral response range under −8 V reverse bias. Finally, we prepared flexible panchromatic PM-OPDs on polyethylene terephthalate substrate and successfully realized the detection of the human pulse signal. This work provides a new strategy for obtaining high-performance panchromatic PM-OPDs by designing device structures and selecting appropriate materials.

Photomultiplication type organic photodetectors with different response characteristics under forward or reverse bias

Photomultiplication type organic photodetectors (PM-OPDs) with a sandwich structure of ITO/PNDIT-F3N/P3HT:[71]PCBM(100:1, wt/wt)/Al were successfully developed, exhibiting different response characteristics under forward or reverse bias. The rather less [71]PCBM can act as isolated electron traps to capture photogenerated electrons. The trapped electrons near ITO (Al) electrode can make interfacial band bending for hole tunneling injection from external circuit under forward (reverse) bias. The injected holes can be efficiently transported along the channel provided by P3HT and then collected by the corresponding electrode under bias. The external quantum efficiency (EQE) spectral shape of PM-OPDs is primarily determined by the trapped electron distributions near ITO or Al electrode under forward or reverse bias. The PM-OPDs exhibit flat EQE spectra under forward bias and ‘M-shaped’ EQE spectra under reverse bias, which is consistent with the curves of trapped electron distribution near ITO or Al electrode. The champion EQE values of PM-OPDs arrive to 2550% at 425 nm under 10 V bias and 2360% at 365 nm under −10 V bias. The PM-OPDs exhibit distinct response speed under forward or reverse bias, which is primarily attributed to the different trapped electron density near ITO or Al electrode under incident light from ITO side. • The PNDIT-F3N interfacial layer makes PM-OPDs work well under forward or reverse bias. • The response speed of PM-OPDs was investigated under distinct polarity bias, resulting from trapped electrons near electrode. • The champion EQE values of PM-OPDs arrive to 2550% at 425 nm under 10 V bias and 2360% at 365 nm under −10 V bias.

Highly sensitive photomultiplication type polymer photodetectors by manipulating interfacial trapped electron density

Photomultiplication type polymer photodetectors (PM-PPDs) are successfully achieved via interfacial trap-assisted hole tunneling injection from external circuit. The external quantum efficiency (EQE) of PM-PPDs is mainly determined by the number of injected holes from external circuit and flowing across the devices. Although the trapped electron density near Al electrode can be increased for efficient hole tunneling injection by decreasing the thickness of active layers, which also makes the PM-PPDs vulnerable to breakdown. A smart strategy is firstly proposed to obtain large EQE values by decreasing the thickness of active layers and keep excellent withstanding large bias characteristic of PM-PPDs by using wide bandgap polymer poly-TPD as buffer layer. The optimal double layered ternary PM-PPDs were obtained with 40 nm poly-TPD as buffer layer and 90 nm P3HT:PBDB-T:PC61BM (80:20:2, wt/wt/wt) as active layers, exhibiting a rather large EQE value of 21400% and specific detectivity of 1.0 × 1013 Jones at 430 nm under −8 V bias. This work indicates that inserting wide bandgap and high hole mobility material as buffer layer should be an efficient strategy to prepare highly sensitive PM-PPDs, benefiting from the increased hole tunneling injection from external circuit and efficient hole flowing across the devices.

Employing liquid crystal material as regulator to enhance performance of photomultiplication type polymer photodetectors

Photomultiplication type polymer photodetectors (PM-PPDs) were achieved with PBDB-T:BTPV-4F (98:2, wt/wt) as blend active layer based on a sandwich structure. Isolated electron traps are formed due to the rather less BTPV-4F surrounded by PBDB-T in blend active layers. Trapped electrons in BTPV-4F near Al electrode will induce interfacial band bending for hole tunneling injection into blend active layers. The tunneling injected holes will be transported along the channels formed by PBDB-T in blend active layers, photomultiplication phenomenon will be obtained when the number of holes crossing through blend active layers is larger than that of incident photons per unit time. Liquid crystal material BTR was selected as regulator to adjust PBDB-T molecular arrangement for improving hole mobility in blend active layers, leading to optimized performance of PM-PPDs. Optimal PM-PPDs were obtained with PBDB-T:BTR:BTPV-4F (89:9:2, wt/wt/wt) as blend active layer. The external quantum efficiency (EQE) values of optimal PM-PPDs are 1.5 times that of PM-PPDs based on PBDB-T:BTPV-4F (98:2, wt/wt) in the whole spectral response range under −10 V applied bias. The EQE of optimal PM-PPDs can be markedly improved by increasing applied bias, which is improved from 600% to 19,300% at 660 nm with applied bias increased from −10 to −20 V. The responsivity of optimal PM-PPDs arrives to 102 A W−1 at 660 nm under −20 V applied bias.

Highly Sensitive Narrowband Photomultiplication‐Type Organic Photodetectors Prepared by Transfer‐Printed Technology

AbstractNarrowband photomultiplication‐type organic photodetectors (PMOPDs) are realized with poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) as the optical field adjusting (OFA) layer and transfer‐printed P3HT: [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) (50:1, w/w) as the photomultiplication (PM) layer. The thickness of the OFA layers is adjusted to optimize interfacial trapped electron distribution and density, which determines the external quantum efficiency (EQE) and spectral response range of PMOPDs. Narrowband PMOPDs with 2.5 µm thick P3HT as the OFA layer exhibit two narrow response peaks at 350 and 660 nm, and the corresponding EQE values at 350 and 660 nm are 180% and 760% under an applied bias of −20 V. A wide bandgap polymer poly[N,N′‐bis(4‐butylphenyl)‐N,N′‐bis(phenyl)benzidine] (P‐TPD) is deliberately incorporated into OFA layer for adjusting interfacial trapped electron distribution near Al electrode. Narrowband PMOPDs exhibit only one response peak at 660 nm with the enhanced EQE value of 1120% under the same bias. The enhanced EQE of PMOPDs with P‐TPD is primarily attributed to the increased hole tunneling injection and transport, which can be ascribed to the enhanced trapped electron density near the Al electrode and the improved hole mobility, respectively. Clearly resolved images can be obtained from the imaging system with the narrowband PMOPDs as sensing pixel without any current preamplifier, indicating the promising potential of PMOPDs in imaging sense.

Smart Strategy: Transparent Hole-Transporting Polymer as a Regulator to Optimize Photomultiplication-type Polymer Photodetectors.

Photomultiplication-type polymer photodetectors (PM-PPDs) were fabricated with hole-only transport active layers containing polymer(s): [6,6]-phenylC61-butyric acid methyl ester (PC61BM) with a weight ratio of 100:2. The rather less PC61BM content in active layers prefers to generate a large amount of isolated electron traps surrounded by polymers. Photogenerated electrons prefer to be trapped by the isolated PC61BM due to the lack of continuous electron-transport channels. The trapped electrons by the isolated PC61BM close to the Al electrode would like to seduce hole tunneling injection. The transparent polymer poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine] (poly-TPD) was incorporated as a regulator to improve hole mobility (μh) and adjust the trapped-electron distribution in active layers, leading to the enhanced performance of PM-PPDs. The optimal PM-PPDs were achieved using poly(3-hexylthiophene) (P3HT):poly-TPD:PC61BM (80:20:2, wt/wt/wt) as active layers. External quantum efficiency (EQE) values at 620 nm are 3900 and 1250% for PM-PPDs based on P3HT:poly-TPD:PC61BM (80:20:2, wt/wt/wt) and P3HT:PC61BM (100:2, wt/wt) under -10 V applied voltage, respectively. The EQE at 620 nm of optimal PM-PPDs is improved from 650 to 63,000% along with the applied voltage increase from -5 to -20 V. This work provides a new strategy of using transparent polymer with large μh as a regulator for EQE and response speed improvement, as well as the flattened EQE spectral shape of PM-PPDs.

Broadband organic color photodetectors with high gain based on C60-doped tri-phase bulk heterojunction

We experimentally investigate the spectrum broadening of P3HT:PC61BM:PBDT-TT-F tri-phase bulk heterojunction active layer, and the influence of material mixing degree on the optoelectronic performance of detectors is also discussed. In addition, the mechanism of gain for C60-doped P3HT:PC61BM:PBDT-TT-F detectors is studied. On the basis, we obtain a broadband with responsive spectra from 350 ~ 750 nm and photomultiplication (PM) detector. The resulting detector exhibits responsivity of 2.1, 3.95, 5.05 A/W and the external quantum efficiency of 415%, 924%, and 1361%, respectively, under the illumination of three primary light at − 4 V bias. Moreover, the specific detectivity approaches or exceeds 1012 Jones. The results indicates that the absorption spectrum of active layer can be used to broaden by adding small amount of spectrum broadening material while keeping the original morphology of binary bulk heterojunction. A small weight of C60 dispersed in tri-phase bulk heterojunction active layer can be regarded as electron trap. Combining the advantage of high exciton dissociation efficiency of bulk heterojunction, PM-type organic photodetectors working at low bias can be achieved by triggering hole tunneling injection from cathode.

Photomultiplication type all-polymer photodetectors with single carrier transport property

Photomultiplication (PM) type all-polymer photodetectors (all-PPDs) are first demonstrated with the sandwich structure of ITO/PEDOT:PSS/PBDB-T:PZ1 (100: x , wt/wt)/Al. The optimal PM type all-PPDs with PBDB-T:PZ1 (100:3, wt/wt) as active layers exhibit external quantum efficiency (EQE) of >100% in the spectral range from 310 to 790 nm. Under 675 nm light illumination, the champion EQE value arrives to 1,470% at −20 V bias and the specific detectivity approaches 1× 1012 Jones at −10 V bias. The PM phenomenon in all-PPDs results from hole tunneling injection assisted by interfacial band bending induced by trapped electrons in PZ1 near Al electrode. The EQE values of optimal PM type all-PPDs still remained over 90% of the original value after 60 d of the storage in a high-purity nitrogen-filled glove box.

Realizing photomultiplication-type organic photodetectors based on C60-doped bulk heterojunction structure at low bias**Project supported by the National Natural Science Foundation of China (Grant No. 61106043), the Natural Science Foundation of Shaanxi Province, China (Grant No. 2015JM6267), and Program of Xi’an University of Technology, China (Grant No. 103-4515013).

Photomultiplication (PM) structure has been widely employed to improve the optoelectronic performance of organic photodetectors (OPDs). However, most PM-type OPDs require a high negative operating voltage or complex fabrication. For obtaining high-efficiency OPDs with low voltage and easy process, here the bulk heterojunction (BHJ) structure of high exciton dissociation efficiency combined with the method of trap-assisted PM are applied to the OPDs. In this paper, we investigate the operating mechanism of OPDs based on poly(3-hexylthiophene) (P3HT): (phenyl-C61-butyric-acid-methyl-ester) (PC61BM), and poly-{[4,8-bis[(2-ethylhexyl)oxy]-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[3-fluore-2-(octyloxy)carbonyl-thieno[3,4-b]thiophene-4,6-diyl]} (PBDT-TT-F):PC61BM doped with C60 as active layer. Furthermore, the influence of C60 concentration on the optoelectronic performances is also discussed. With 1.6 wt.% C60 added, the P3HT:PC61BM:C60 OPD exhibits a 327.5% external quantum efficiency, a 1.21 A·W−1 responsivity, and a 4.22 × 1012 Jones normalized detectivity at −1 V under 460 nm (0.21 mW·cm−2) illumination. The experimental results show that the effective electron traps can be formed by doping a small weight of C60 into BHJ active layer. Thus the PM-type OPDs can be realized, which benefits from the cathode hole tunneling injection assisted by the trapped electrons in C60 near the Al side. The efficiency of PM is related to the C60 concentration. The present study provides theoretical basis and method for the design of highly sensitive OPDs with low operating voltage and facile fabrication.

Photoelectronic multiplication organic photodetectors with facile fabrication and controllable operating voltage

Abstract Organic photodetectors (OPDs) possessing high responsivity are promising building blocks for exploring good performance integrated optoelectronics products. Herein, photoelectronic multiplication (PM) type OPDs are fabricated by doping small ratio C60 as electron traps into PBDT-TT-F:PC61BM bulk heterojunction (BHJ). The mechanism of PM assisted by C60 is analyzed systematically, especially for the influence of C60 concentration on the device performance. For obtaining an ideal operating voltage of required OPDs, we further change the PC61BM concentration to adjust the voltage happening tunneling injection. On the basis, the PM type OPD with low bias and facile fabrication is achieved where the optimized weight ratio of PBDT-TT-F:PC61BM:C60 is 6:6:0.2. The EQE spectra of the resulting OPD shows broadband response and >100% EQE values in visible region under reverse bias. At −3.0 V bias, they arrive to 629.1%, 655.7%, and 739.8% under the illumination of three selected wavelengths of 460, 530, and 630 nm, respectively. The experimental results indicate that the effective electron traps can be formed by introducing a small weight of C60 into the active layer. Moreover, the observed >100% EQE values are attributed to the cathode hole tunneling injection assisted by the trapped electrons in C60 near the Al cathode. The present study provides theoretical basis and method to obtain the desirable operating voltage of PM type OPDs with facile fabrication.

Photomultiplication Type Organic Photodetectors with Broadband and Narrowband Response Ability

AbstractIt is a great challenge to achieve highly sensitive organic photodetectors (OPDs) with broadband and narrowband response ability. The OPDs with an extraordinary structure of ITO/PFN‐OX/P3HT:PTB7‐Th:PC61BM (100‐x:x:1, wt/wt/wt)/Al are elaborately fabricated, exhibiting tunable spectral response dependent on the bias polarity. The optimized ternary OPDs with 3 µm P3HT:PTB7‐Th:PC61BM (40:60:1) as active layers exhibit broadband spectral response from 350 to 800 nm under forward bias and narrowband spectral response from 750 to 850 nm with a full width half maximum (FWHM) of ≈40 nm under reverse bias. In particular, the FWHM of narrowband response windows can be kept well under different reverse biases. Moreover, the optimized ternary OPDs also exhibit photomultiplication (PM) phenomenon under forward and reverse bias, resulting from the interfacial trap‐assisted hole tunneling injection from external circuit. This should be the first demonstration of PM type OPDs with broadband and narrowband response ability, which would possess wide potential applications, especially in integrated optoelectronic products.

Organic color photodetectors based on tri-phase bulk heterojunction with wide sectrum and photoelectronic mltiplication

In order to obtain highly sensitive broadband organic photodetectors (OPDs) used for image sensors with the stable ability to detect three primary colors (RGB), in this paper, the spectral broadening of organic active layer based on tri-phase bulk heterojunction formed by P3HT:PCBM doped with narrow band material PBDT-TT-F which absorbs red light is investigated. The influences of PBDT-TT-F doping ratio on the morphology of active layer film and detector photoelectric properties are further analyzed. Finally, the operating mechanism of trap-assisted photoelectronic multiplication is discussed. On this basis, the detector with 350-750 nm wide spectrum is obtained where the optimum mixing ratio of P3HT:PCBM:PBDT-TT-F is 12:8:3. At a small reverse bias of 1 V, the values of responsivity and external quantum efficiency of the photodetector can reach 470, 381, 450 mA/W and 93%, 89%, 121% respectively under the illumination of three primary colors and its normalized detectivity to the RGB is close to 1012 Jones. Additionally, the maximum relative difference between each parameter and its average value is lower than 20%; the bandwidths are 5, 8, and 8 kHz respectively, which reach the imaging requirements for image sensors. The experimental results show that not only the absorption spectra of the active layer can be broadened but also the carriers collection efficiency of respective electrodes can be well maintained by adding a small quantity of spectral broadening material while keeping the microstructure of the original binary bulk heterojunction. Utilizing the reasonable combination of materials to form electron traps, photoelectronic multiplication can be realized by trap-assisted hole tunneling injection from the Al cathode into active layer, and thus improving the normalized detectivity. Moreover, in order to detect different light intensities, the hole injection barrier width should be controlled by the corresponding light intensity. The resulting OPD shows a good liner response to all three primary colors when light intensity increases from 0.1 to 10 mW/cm2. By adjusting the mixing ratio of the tri-phase materials, the stable ability to detect the primary color can be achieved. The present study paves the way for high responsivity broadband OPDs based on tri-phase bulk heterojunction.

Tunneling Injection and Exciton Diffusion of White Organic Light-Emitting Diodes with Composed Buffer Layers

Four configurations of buffer layers were inserted into the structure of a white organic light emitting diode, and their impacts on the hole tunneling-injection and exciton diffusion processes were investigated. The insertion of a single buffer layer of 4,4′-bis(carbazol-9-yl)biphenyl (CBP) resulted in a balanced carrier concentration and excellent color stability with insignificant chromaticity coordinate variations of Δx < 0.023 and Δy < 0.023. A device with a 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) buffer layer was beneficial for hole tunneling to the emission layer, resulting in a 1.45-fold increase in current density. The tunneling of holes and the diffusion of excitons were confirmed by the preparation of a dual buffer layer of CBP:tris-(phenylpyridine)-iridine (Ir(ppy)3)/BCP. A maximum current efficiency of 12.61 cd/A with a luminance of 13,850 cd/m2 was obtained at 8 V when a device with a dual-buffer layer of CBP:6 wt.% Ir(ppy)3/BCP was prepared.

Highly Narrowband Photomultiplication Type Organic Photodetectors.

Filterless narrowband response organic photodetectors (OPDs) present a great challenge due to the broad absorption range of organic semiconducting materials. The reported narrowband response OPDs also suffer from low external quantum efficiency (EQE) in the desired response window and low rejection ratio. Here, we report highly narrowband photomultiplication (PM) type OPDs based on P3HT:PC71BM (100:1, wt/wt) as active layer without an optical filter. The full width at half-maximum (fwhm) of the PM-type OPDs can be well retained less than 30 nm under different biases. Meanwhile, the champion EQE and rejection ratio approach 53 500% and 2020 at -60 V bias, respectively. The small fwhm should be attributed to the sharp absorption edge of active layer with small amount of PC71BM. The PM phenomenon is attributed to hole tunneling injection from the external circuit assisted by trapped electron in PC71BM near the Al electrode under light illumination. These highly narrowband PM-type OPDs should have great potential applications in sensitively detecting specific wavelength light and be blind to light outside of the desired response window.

Stability in organic solar cells with Ag/pentacene anode buffer under repetitive illumination

The degradation phenomenon in small-molecule organic solar cells (OSCs) with a structure of indium-tin oxide (ITO, anode)/anode buffer/copper phthalocyanine (CuPc, donor)/fullerene (acceptor)/bathocuproine (cathode buffer)/Ag (cathode) was experimentally investigated. Two kinds of anode buffer layers, a 2-nm-thick pentacene layer and a 0.5-nm-thick Ag layer, were examined. The devices were subjected to repetitive illumination stress through the transparent ITO electrode in air. The ultra-thin Ag layer enhanced the ability of the devices to withstand the illumination stress. A thinner Ag layer resulted in better device performance. Resonant light absorption in the nanometer-scale Ag clusters probably accumulates trapped electrons near the ITO/CuPc interface, shrinks the depletion layer, enhances tunneling injection of holes, and suppresses the S-shaped kink during the illumination. The thin pentacene layer provides good ohmic contact between the anode and the CuPc layer, resulting in a larger short-circuit current. OSCs fabricated with both Ag and pentacene anode buffer layers exhibited only a 10% decrease in power conversion efficiency at 60 repetitions, whereas OSCs without an anode buffer layer exhibited a decrease of approximately 45%. The large decrease in efficiency was dominated by the decrease in the fill factor.