The ability of a light‐sensor to detect fast variation in incident light intensity is a vital feature required in imaging and data transmission applications. Solution‐processed bulk heterojunction (BHJ) type organic photodiodes (OPDs) have gone through key developments, including dark current mitigation and longer linear dynamic range. In contrast, there has been less focus on increasing OPD response speed (f–3dB). Here, bulk heterojunction OPDs based on electron‐donating polymer poly[thiophene‐2,5‐diyl‐alt‐5,10‐bis((2‐hexyldecyl)oxy)dithieno[3,2‐c:3′,2′‐h][1,5]naphthyridine‐2,7‐diyl] (or PTNT) and electron‐accepting phenyl‐C71‐butyric acid methyl ester (or PC71BM) are reported. The intrinsic charge transport characteristics required for fast speed OPDs are discussed, and an analytical model for the same is developed. The OPDs present 0.8 MHz f–3dB under no applied voltage bias for a typical blend ratio of 1:1 by weight. It is shown that balanced electron and hole mobility is a critical criterion for faster speed OPDs, which can be realized by tuning the composition ratio of the bulk heterojunction. By tuning PTNT and PC71BM blend ratio, the f–3dB was successfully raised by more than quadruple to 4.5 MHz. The findings provide a tool to set device architecture for faster next‐generation light sensors.

Over the years, achieving efficient electroluminescence (EL) while simultaneously having low light amplification thresholds under optical excitation has been the key to progression toward the long‐thought objective of electrically pumped organic lasers. While significant progress in this regard has been made for organic semiconductors emitting in the blue–green region of the visible spectrum, organic laser dyes with low‐energy emission (>600 nm) still suffer from high amplified spontaneous emission (ASE) thresholds and low external quantum efficiencies (EQEs) in devices. Herein, low ASE thresholds and efficient EL are reported from a solution‐processable organic laser dye dithiophenyl diketopyrrolopyrrole (DT‐DPP). The ASE threshold of 4 µJ cm−2 at the wavelength of 620 nm is obtained while making constructive use of triplet excitons by doping DT‐DPP in a green‐emitting host matrix, which exhibits thermally activated delayed fluorescence (TADF). The organic light‐emitting diode fabricated from this system gives a high EQE of 7.9% due to the efficient utilization of triplet excitons. Transient EL studies further show that a high reverse intersystem crossing rate is crucial in achieving lasing under electrical pumping from such TADF‐assisted fluorescent systems.

Polaron-induced exciton quenching in thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) can lead to external quantum efficiency (EQE) roll-off and device degradation. In this study, singlet-polaron annihilation (SPA) and triplet-polaron annihilation (TPA) were investigated under steady-state conditions and their relative contributions to EQE roll-off were quantified, using experimentally obtained parameters. It is observed that both TPA and SPA can lead to efficiency roll-off in 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) doped OLEDs. Charge imbalance and singlet-triplet annihilation (STA) were found to be the main contributing factors, whereas the device degradation process is mainly dominated by TPA. It is also shown that the impact of electric field-induced exciton dissociation is negligible under the DC operation regime (electric field < 0.5 MV cm−1). Through theoretical simulation, it is demonstrated that improvement to the charge recombination rate may reduce the effect of polaron-induced quenching, and thus significantly decrease the EQE roll-off.

tion. [1] Recently, they have gained significant interest primarily due to their cost-effective approach to fabrication, higher throughput additive manufacturing on mechanically flexible substrates, and spectral tunability comparable to that of conventional inorganic photodiodes. [1][2][3] Along with these exciting features, OPDs have been reported for their potential in a number of applications, including pulse oximeters, [4] biometric analysis systems, [5] retinal prosthesis, [6] wearable electronics, [7] and image sensing. [8] As a result, it has recently gained interest amongst commercial manufacturers as well. [9][10][11] An OPD has similar architecture to an organic photovoltaic device, but operates in reverse bias. While photovoltaic solar cells rely strongly on matching to the incident solar spectrum in order to maximize power conversion efficiencies, the OPD characteristics on the other hand are tailored toward lower dark current (i.e., current measured in the device under null illumination), higher detectivity, and longer linear dynamic range (LDR). The active materials of OPDs commonly include evaporated molecules, solution-processable molecules, and polymers as electron-donating materials. In particular, the semiconducting The field of organic photodiodes (OPDs) has witnessed continuous development in the last decade. Although a considerable portion of electron-donating materials are polymers, there has been an existential gap in deciphering the influence of the polymer's molecular-weight on the photodiode performance. We take up OPDs based on 5,5′-[(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(2,1,3benzothiadiazole-7,4diylmethylidyne)]bis[3-ethyl-2-thioxo-4-thiazolidinone] (FBR) acceptor material blended with three different molecular-weights of defect-free form of a well-known donor polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) are taken up, and their optoelectronic performance along with morphological characteristics are studied. Disparity of up to a decade in key photodetecting characteristics is observed. Further, the tools of near-edge X-ray absorption fine-structure spectroscopy, resonant soft X-ray scattering spectroscopy, atomic force microscopy, and time-delayed collection-field measurements are employed to decipher the difference in the fundamental photo-physical processes and the operating mechanisms of the OPDs. It is concluded that the molecular weight and the resulting morphology of the active layer strongly influence photodiode performance, in particular, dark current, linear dynamic range, and specific-detectivity.

In an attempt to overcome the limitations of the presently prevailing transparent conducting electrode (TCE) -indium tin oxide (ITO) -many materials have been considered for replacing ITO. Recently, a novel method has been reported for the synthesis of Au-Ag nanowire (NW) mesh, and tested successfully for organic-light-emitting-diodes (OLEDs). It employs UV-induced reduction of gold-and silver-precursors to form Au-Ag NW mesh. In this report, Au-Ag NW mesh thinfilms are synthesized on glass substrates with an objective for use as facing-electrode for Organic Photovoltaics. Various issues and factors affecting the fabrication-process have been improved, and are also discussed here. The electrode showed good transmitivity, of around 95% (excluding that of glass substrate). The advantage of the technique is its simple processing method and cost-effectiveness.