Self-Powered Organic Photodetectors with High Detectivity for Near Infrared Light Detection Enabled by Dark Current Reduction

Organic photodetectors (OPDs) for near infrared (NIR) light detection represents cutting-edge technology for optical communication, environmental monitoring, biomedical imaging, and sensing. Herein, a series of self-powered OPDs with high detectivity are constructed by the sequential deposition (SD) method. The dark currents (J_d) of SD devices are effectively reduced in comparison to blend casting (BC) ones due to the vertical phase segregation structure. Impressively, the J_d values of SD devices based on D18 and Y6 system is reduced to be 2.1 × 10⁻¹¹ A cm⁻² at 0 V, which is two orders of magnitude lower than those of the BC devices. The D* value of the SD device is superior to that of BC device under different bias voltages (0, −0.5, −1.0, and −2.0 V) due to the reduction of dark current, which originates from the fine vertical phase separation structure of the SD device. The mechanism studies shows that the vertical phase segregation structure can effectively suppress the unfavorable charge injection, thus reducing the dark current. Also, the surface energy is proven to play a key role in the photocurrent stability. In addition, the flexible OPDs demonstrate excellent performance in photoplethysmography test.     

Low dark current inverted organic photodetectors employing MoOx:Al cathode interlayer

We report low dark current small molecule organic photodetectors (OPDs) with an inverted geometry for image sensor applications. Adopting a very thin MoO_x:Al cathode interlayer (CIL) in the inverted OPD with a reflective top electrode results in a remarkably low dark current density (J_d) of 5.6 nA/cm² at reverse bias of 3 V, while maintaining high external quantum efficiency (EQE) of 56.1% at visible wavelengths. The effectiveness of the CIL on the diode performance has been further identified by application to inverted OPDs with a semi-transparent top electrode, leading to a significantly low J_d of 0.25 nA/cm², moderately high EQE_{540 nm} of 25.8%, and subsequently high detectivity of 8.95 × 10¹² Jones at reverse bias of 3 V. Possible origins of reduced dark currents in the OPD by using the MoO_x:Al CIL are further described in terms of the change of interfacial energy barrier and surface morphology.     

Toward Faster Organic Photodiodes: Tuning of Blend Composition Ratio

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.     

Light Detection in Open‐Circuit Voltage Mode of Organic Photodetectors

Organic photodetectors (OPDs) are promising candidates for next‐generation light sensors as they combine unique material properties with high‐level performance in converting photons into electrical signals. However, low‐level light detection with OPD is often limited by device dark current. Here, the open‐circuit voltage (V_oc ) regime of OPDs is shown to be efficient for detecting low light signals (<100 µW cm^?2). It is established that the light‐dependence of V_oc exhibits two distinct regimes as function of irradiance: linear and logarithmic. Whereas the observed logarithmic regime is well understood in organic photovoltaic cells (OPVs), it is shown experimentally and theoretically that the linear regime is due to the non‐infinite shunt resistance of the OPD device. Overall, OPDs composed of rubrene and fullerene show photovoltage light sensitivity across nine orders of magnitude with a detection limit as low as 400 pW cm^?2. A photovoltage responsivity of 1.75 V m² W^?1 demonstrates highly efficient performance without the necessity to supress high dark current. This approach opens up new possibilities for resolving low light signals and provides simplified design rules for OPDs.     

X-ray Sensitive hybrid organic photodetectors with embedded CsPbBr3 perovskite quantum dots

X-ray detection is an important technology for medical diagnosis as well as industrial and security inspections. While today's commercial X-ray detectors are bulky, photodetectors based on organic semiconductors have attracted increasing attention owing to their low temperature processing capabilities, flexibility and low cost. Nonetheless, the low X-ray attenuation coefficient of organic semiconductors still hinders their practical application. Herein, a new organic-inorganic hybrid strategy is proposed to improve the X-ray sensitivity of organic photodetectors (OPDs). A solution-processed X-ray sensitive hybrid OPD is fabricated by embedding CsPbBr₃ quantum dots (QDs) into a P3HT:PC₆₁BM bulk heterojunction photodiode. The QDs, acting as embedded scintillators in the organic active layer, maintain a high radioluminescence. The proposed hybrid structure enables indirect X-ray detection in a comprehensive manner. These hybrid photodetectors exhibit suppressed dark current densities in the range of tens of picoamperes per square centimeters for different weight ratios of blended QDs. The best OPD achieves a sensitivity of 229.6 e nGy⁻¹ mm⁻² (3.67 μC Gy⁻¹ cm⁻²) and a dark current of 23.3 pA cm⁻² at a low operating voltage (−3 V) for 20–80 kV “soft” X-rays, thus representing great potential for the development of next generation low cost, portable, and highly sensitive X-ray detectors.     

An Organic Electrochemical Transistor Integrated Photodetector for High Quality Photoplethysmogram Signal Acquisition

The organic photodiode (OPD) is a promising building block for solution-processable, flexible, lightweight, and miniaturized photodetectors, ideal for wearable applications. Despite the advances in materials used in OPDs, their photocurrent and light responsivity are limited, and alternative methods are required to boost the signal response. Herein, a miniaturized organic electrochemical transistor (OECT) is integrated with an OPD module to unlock the potential of OPDs to acquire physiological signals. In this integrated photodetector (IPD) system, the light intensity regulates the OPD voltage output that modulates the OECT channel current. The high transconductance of the OECT provides efficient voltage-to-current conversion, enhancing the signal-to-noise ratio on the sensing site. A microscale, p-type enhancement-mode OECT with high g_m and fast switching speed performs better in this application than depletion-mode OECT of the same geometry. The IPD achieves a photocurrent and responsivity 318 and 140 times higher than the standalone OPD, respectively. It is shown that with the IPD, the amplitude of the photoplethysmogram signals detected by the OPD is enhanced by a factor of 2.9 × 10³, highlighting its potential as a wearable biosensor and to detect weak, often uncaptured, light-based signals from living systems.     

Systematic optimization of low bandgap polymer/[6,6]-phenyl C70 butyric acid methyl ester blend photodiode via structural engineering

Synthetic approaches for optimizing polymer-based organic photodiodes (OPDs) by systematically analyzing the effects of the hole-blocking layer, the electron-blocking layer, and the thickness and morphology of the active layer with respect to the dark current and detectivity have been reported. PBDTT-DPP with a repeating alkylthienylbenzodithiophene (BDTT) and diketopyrrolopyrrole (DPP) units is used as a p-type polymer for achieving both broadband absorption and a high absorption coefficient in conjunction with n-type [6,6]-phenyl C₇₀ butyric acid methyl ester (PC₇₀BM) for constructing photoactive layers. Through systematic investigations of various interfacial layers, we found that the thickness of the active layer and the energy level of the hole/electron blocking layer were critical for minimizing the dark current of OPDs. By changing the deposition method of the PBDTT-DPP/PC₇₀BM blend and using post treatment, we discovered that the morphology of the active layer was directly related to the photocurrent of OPDs. Furthermore, we conducted a comparative study between a bulk heterojunction and a planar heterojunction (PHJ) to demonstrate the effectiveness of the PHJ for suppressing the dark current. Consequently, we realized a high detectivity of 5.3 × 10¹² Jones with an optimized device architecture and morphology. This work shows the importance of a synthetic approach for optimizing OPDs that requires both a high photocurrent and a low dark current in the reverse saturation regime.     

Factors governing photo- and dark currents in lateral organic photo-detectors

We study the factors governing the dark and photo currents in lateral organic photodetectors (OPD). The dark current is found to be strongly limited by space charge limited conduction (SCLC) across a highly depleted gap and arises mostly from transient capacitive currents due to the charge accumulation in the organic layers near the contacts. Similarly, the photocurrent is found to be strongly limited by the collection of photogenerated carriers at the contacts, which limits the sensitivity of lateral OPDs. Furthermore, evidence of the contribution of some photons falling outside of the gap area are reported and have to be taken into account when one wants to conduct accurate external quantum efficiency estimates of lateral OPDs. Finally, it is found that the dark current is significantly increased after the device is exposed to light, likely to be due to the filling of charge traps by photogenerated carriers, while the photocurrent remains unchanged, leading to a decrease in the overall sensitivity of the device upon repeated exposure to light. The results shed the light on the performance limitations in lateral OPDs.     

Organic Bidirectional Phototransistors Based on Diketopyrrolopyrrole and Fullerene

It is shown that simple bilayer devices consisting of the diketopyrrolopyrrole (DPP) monomer Ph‐TDPP‐Ph as donor and C₆₀ as acceptor feature J–V‐characteristics of a bidirectional organic phototransistor where illumination intensity plays the role of the gate voltage as compared to a conventional field‐effect transistor. The output current may therefore be controlled both electrically and optically. The underlying mechanism is based on the good charge transport in Ph‐TDPP‐Ph and C₆₀, the intrinsic dissociation properties of C₆₀, and the presence of an injection barrier for holes. In addition to this, it is demonstrated that the observed behavior of the DPP/C₆₀ system allows the realization of basic logic elements like NOT‐, AND‐, and OR‐Gates, which may provide the basis for advanced analog and digital applications.     

Transparent photodetectors with ultra-low dark current and high photoresponse for near-infrared detection

A remarkable progress in research works regarding flexibility and transparency of organic optoelectronic devices has been observed in the past decade compared to their inorganic counterparts. However, few studies have been devoted to the advancement of a transparent organic photodetector. In this study, we have used a wavelength-selective bulk-heterojunction of ClAlPc:C₆₀ as active layer and Cu:Ag/WO₃ metal alloy as electrode to realize a see-through organic photodetector (OPD) with an average visible transmission of 76.92%. The optimized transparent OPDs show an average dark current density of 0.36 nA cm⁻² and a rise/fall time of <5 μs under a bias voltage of −2 V, which could be potentially applied in a home security system based on invisible near-infrared detection.     

Wavelength-Selective Organic Photodetectors

Spectroscopic sensing combined with optical imaging is crucial with respect to today's ever-growing demand for instant analytical techniques to be incorporated in various handheld and wearable devices. Further miniaturization and integration of such types of sensors is critical and wavelength-selective organic photodetectors (OPDs) may provide the required technology. In this progress report, some early OPD applications and their potential are presented. Crucial device parameters such as the specific detectivity, external quantum efficiency, and dark current density of visible and near-infrared wavelength-selective photodetectors are compared and assayed to theoretical and semi-empirical limits. The different organic detector approaches include the use of inherently narrow-band absorbers as well as internally filtered and microcavity devices. Each of these strategies comes with its own specific material and device design criteria, around which material development and selection should be centered to move beyond the current state of the art. As OPD technology matures, device stability becomes important and is hence also briefly discussed. Via this perspective, it is aimed to provide the reader with critical insights into the device physics and chemistry of wavelength-selective OPDs, hereby providing leverage for new ideas to bring this technology to the market.     

Efficient Charge Carrier Injection and Balance Achieved by Low Electrochemical Doping in Solution‐Processed Polymer Light‐Emitting Diodes

Charge carrier injection and transport in polymer light‐emitting diodes (PLEDs) is strongly limited by the energy level offset at organic/(in)organic interfaces and the mismatch in electron and hole mobilities. Herein, these limitations are overcome via electrochemical doping of a light‐emitting polymer. Less than 1 wt% of doping agent is enough to effectively tune charge injection and balance and hence significantly improve PLED performance. For thick single‐layer (1.2 µm) PLEDs, dramatic reductions in current and luminance turn‐on voltages (V_J = 11.6 V from 20.0 V and V_L = 12.7 V from 19.8 V with/without doping) accompanied by reduced efficiency roll‐off are observed. For thinner (<100 nm) PLEDs, electrochemical doping removes a thickness dependence on V_J and V_L, enabling homogeneous electroluminescence emission in large‐area doped devices. Such efficient charge injection and balance properties achieved in doped PLEDs are attributed to a strong electrochemical interaction between the polymer and the doping agents, which is probed by in situ electric‐field‐dependent Raman spectroscopy combined with further electrical and energetic analysis. This approach to control charge injection and balance in solution‐processed PLEDs by low electrochemical doping provides a simple yet feasible strategy for developing high‐quality and efficient lighting applications that are fully compatible with printing technologies.     

Effect of Cyano Substitution on Non-Fullerene Acceptor for Near-Infrared Organic Photodetectors above 1000 nm

Near-infrared organic photodetectors (NIR OPDs) comprising ultra-narrow bandgap non-fullerene acceptors (NFA, over 1000 nm) typically exhibit high dark current density under applied reverse bias. Therefore, suppression of dark current density is crucial to achieve high-performance of such NIR OPDs. Herein, cyano (CN) with a strong electron-withdrawing property is introduced into alkoxy thiophene as a π-bridge to adjust its optoelectronic characteristics, and the correlation between dark current density and charge injection barrier is investigated. Compared with their motivated NFA (COTH), the novel CN-substituted NFAs, COTCN and COTCN2, exhibited deeper-lying highest occupied molecular orbital energy levels and narrower optical bandgap (<1.10 eV), owing to the strong inductive and resonance effect of CN. The dark current and total noise currents are minimized as the number of substituted CN increases because of the larger hole injection barrier. Consequently, PTB7-Th:COTCN2 exhibited the best shot-noise limited detectivity (D^*_sh, 1.18 × 10¹² Jones) and total noise detectivity (D^*_n, 1.33 × 10¹¹ Jones) compared with those of PTB7-Th:COTH (D^*_sh, 2.47 × 10¹¹ Jones and D^*_n, 1.96 × 10¹⁰ Jones).     

Effects of Photo‐oxidation on the Performance of Poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene]:[6,6]‐Phenyl C61‐Butyric Acid Methyl Ester Solar Cells

A study of the photo‐oxidation of films of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV) blended with [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM), and solar cells based thereon, is presented. Solar‐cell performance is degraded primarily through loss in short‐circuit current density, J_SC. The effect of the same photodegradation treatment on the optical‐absorption, charge‐recombination, and charge‐transport properties of the active layer is studied. It is concluded that the loss in J_SC is primarily due to a reduction in charge‐carrier mobility, owing to the creation of more deep traps in the polymer during photo‐oxidation. Recombination is slowed down by the degradation and cannot therefore explain the loss in photocurrent. Optical absorption is reduced by photo‐bleaching, but the size of this effect alone is insufficient to explain the loss in device photocurrent.     

A Cross‐Linkable Donor Polymer as the Underlying Layer to Tune the Active Layer Morphology of Polymer Solar Cells

For polymer solar cells (PSCs) with conventional configuration, the vertical composition profile of donor:acceptor in active layer is detrimental for charge carrier transporting/collection and leads to decreased device performance. A cross‐linkable donor polymer as the underlying morphology‐inducing layer (MIL) to tune the vertical composition distribution of donor:acceptor in the active layer for improved PSC device performance is reported. With poly(thieno[3,4‐b]‐thiophene/benzodithiophene):[6,6]‐phenyl C₇₁‐butyric acid methyl ester (PTB7:PC₇₁BM) as the active layer, the MIL material, PTB7‐TV , is developed by attaching cross‐linkable vinyl groups to the side chain of PTB7. PSC device with PTB7‐TV layer exhibits a power conversion efficiency (PCE) of 8.55% and short‐circuit current density (J_SC) of 15.75 mA cm^?2, in comparison to PCE of 7.41% and J_SC of 13.73 mA cm^?2 of the controlled device. The enhanced device performance is ascribed to the much improved vertical composition profile and reduced phase separation domain size in the active layer. These results demonstrate that cross‐linked MIL is an effective strategy to improve photovoltaic performance of conventional PSC devices.     

On the determination of the emitter saturation current density from lifetime measurements of silicon devices

Contactless photoconductance measurements are commonly used to extract the emitter saturation current density (J_oe) for crystalline silicon samples containing an emitter on the surface. We review the physics behind the analysis of J_oe and compare the commonly used approximations with more generalised solutions using two‐dimensional device simulations. We quantify errors present in such approximations for different test conditions involving varying illumination conditions and surface properties in samples with the same emitter on both sides. The simulated J_oe obtained from the dark hole current from the emitter into the bulk is nearly the same as the simulated J_oe determined by photoconductance measurements of the rear diffusion. The simulated J_oe at the front emitter is equivalent to that at the rear emitter only when the sample is subject to a nearly constant and flat generation profile. For illumination conditions including visible light, the simulated J_oe at the front emitter is smaller than the simulated J_oe at the rear emitter. Both J_oe at the rear emitter and from the dark hole current in the emitter remain nearly constant over a wide range of base doping densities. The approximations used for the determination of J_oe from photoconductance measurements make J_oe dependent on the excess minority carrier density. Lifetime measurements demonstrate that, even in high‐quality silicon, J_oe should be determined from the analytical solution as a function of excess minority carrier density including Shockley‐Read‐Hall recombination. Copyright © 2012 John Wiley & Sons, Ltd.     

Towards Efficient Integrated Perovskite/Organic Bulk Heterojunction Solar Cells: Interfacial Energetic Requirement to Reduce Charge Carrier Recombination Losses

Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (J_sc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in J_sc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices.     

Influence of band gap of p-type hydrogenated nanocrystalline silicon layer on the short-circuit current density in thin-film silicon solar cells

The impact of the optical band gap (E_g) of a p-type hydrogenated nanocrystalline silicon layer on the short-circuit current density (J_sc) of a thin-film silicon solar cell is assessed. We have found that the J_sc reaches maximum when the E_g reaches optimum. The reason for the J_sc on E_g needs to be clarified. Our results exhibit that maximum J_sc is the balance between dark current and photocurrent. We show here that this dark current results from the density of defects in the p-layer and the barrier at the interface between p-and i-layers. An optimum cell can be designed by optimizing the p-layer via reducing the density of defects in the p-layer and the barrier at the p/i interface. Finally, a 6.6% increase in J_sc was obtained at optimum E_g for n-i-p solar cells.     

Understanding Temperature‐Dependent Charge Extraction and Trapping in Perovskite Solar Cells

Understanding the factors that limit the performance of perovskite solar cells (PSCs) can be enriched by detailed temperature (T)‐dependent studies. Based on p‐i‐n type PSCs with prototype methylammonium lead triiodide (MAPbI₃) perovskite absorbers, T‐dependent photovoltaic properties are explored and negative T‐coefficients for the three device parameters (V_OC, J_SC, and FF) are observed within a wide low T‐range, leading to a maximum power conversion efficiency (PCE) of 21.4% with an impressive fill factor (FF) approaching 82% at 220 K. These T‐behaviors are explained by the enhanced interfacial charge transfer, reduced charge trapping with suppressed nonradiative recombination and narrowed optical bandgap at lower T. By comparing the T‐dependent device behaviors based on MAPbI₃ devices containing a PASP passivation layer, enhanced PCE at room temperature is observed but different tendencies showing attenuating T‐dependencies of J_SC and FF, which eventually leads to nearly T‐invariable PCEs. These results indicate that charge extraction with the utilized all‐organic charge transporting layers is not a limiting factor for low‐T device operation, meanwhile the trap passivation layer of choice can play a role in the T‐dependent photovoltaic properties and thus needs to be considered for PSCs operating in a temperature‐variable environment.     

Interfacial‐Tunneling‐Effect‐Enhanced CsPbBr3 Photodetectors Featuring High Detectivity and Stability

Inorganic halide perovskite (HP)‐based photodetectors (PDs) have exhibited fast response speed and high responsivity, but with low detectivity due to the high dark current of devices. Additionally, the intrinsic instability of HPs and interface deterioration originating from ion migration inhibit their practical applications severely. A tunneling organic layer is introduced to solve both these problems. Light‐induced charge carriers can flow across the interfacial (poly(methyl methacrylate), PMMA) layer with appropriate thickness via the Fowler–Nordheim tunneling effect. Due to the effective control of dark current, the photo‐/dark‐current ratio reaches a giant value of 2.13 × 10⁸, and the peak detectivity is as high as 1.24 × 10¹³ Jones. With such superiority, light signal as weak as 244 pW is accurately imaged by a PD array. Additionally, the hydrophobic organic layer inhibits the destruction of HPs caused by moisture and ion migration induced interface reaction, and negligible response attenuation is observed during continuous work in a humid environment for 48 h. This heterojunction structure design provides a new strategy to enhance the performance and stability of perovskite‐based photoelectric and photovoltaic devices.