Bulk Heterojunction Photodetectors Research Articles

Enhancing the performance of PbS:CsPbBr3 bulk-heterojunction photodetectors by treating with imidazolium-based ionic liquids.

All-inorganic lead halide perovskites and quantum dots (QDs) have gained significant attention since their emergence, owing to their immense potential for applications in optoelectronic devices. Here, enhanced-performance broadband photodetectors based on the bulk-heterostructure of a CsPbBr3 perovskite and PbS colloidal quantum dots (CQDs) are presented, and 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4) ionic liquids as a dual-purpose additive were introduced in the blended film to regulate the surface of QDs by facilitating surface passivation, adjusting energy levels, and coupling with longer alkyl chains as compared to iodide ions (I-). As a result, a superior-quality bulk-heterostructure based photodetector with long-term stability was obtained, showing outstanding performance in photodetection across the visible to near-infrared wavelength range, demonstrating a high photoresponsivity of 22.4 A W-1 with a response time of 16.2 ms and a specific detectivity of 1.58 × 1014 Jones under 405 nm illumination. Thus, this work provides a novel modification strategy for PbS:CsPbBr3 as a promising material for novel optoelectronics.

Solution-processed Self-driven Bulk-heterojunction Photodetectors by Passivating with Polymers for Ultrasensitive Upconverters

Solution-processed Self-driven Bulk-heterojunction Photodetectors by Passivating with Polymers for Ultrasensitive Upconverters

High-stability lead-free tin(II)-perovskites by A-site cation engineering and surface-passivating engineering for high-performance hybrid bulk-heterojunction photodetectors

Tin-based perovskites have attracted more and more attention in recent years due to their non-toxic properties. However, the poor stability of Tin-based perovskites severely limits its further applications. In order to obtain more stable Tin-based perovskites, in this work, Cs ions are introduced as a precursor besides SnF2 during the synthesis of Tin-based perovskites. Further, poly(N-vinylcarbazole) (PVK) is introduced into the above perovskites to improve the separation rate of photogenerated carriers and to suppress both O2 and H2O diffusion into Tin-perovskites. Enhanced-performance Tin-based perovskite photodetectors with hybrid bulk-heterojunctions were obtained, and our experimental data showed that the responsivity and the specific detectivity of the photodetectors ITO/ZnO/FA0.9Cs0.1SnBr3:PVK/MoO3/Ag reached to 409.8 A/W and 1.23 × 1014 Jones, respectively, under 25.3 μW/cm2 405 nm illumination at − 0.2 V. In addition, the devices also show great stability, whose photocurrent decreases only slightly after its deposing in glove box for 14 days. Thus, such a kind of combing A-site cation engineering with surface-passivating engineering on hybrid bulk-heterojunction provides a facile method for high-performance Tin-perovskite photodetectors with high-stability.

Water-soluble Ag2S QD modified CsPbI2Br heterojunction photodetector with ultra-low dark current and ultra-fast response speed

Mixed halide inorganic perovskite CsPbI3-xBrx has attracted attention in the field of optoelectronics. As a desirable photoactive material, the application of CsPbI2Br in the field of photodetector (PD) remains to be explored. The synergistic passivation strategy based on Ag2S quantum dot (QD) aqueous solution improves the morphology of the CsPbI2Br perovskite film. At the same time, Ag2S/CsPbI2Br bulk heterojunction optimizes the energy level arrangement between perovskite and Au electrode, improves the carrier transmission and collection capacity, and thus improves the performance of PD. Ag2S/CsPbI2Br bulk heterojunction PD has an ultra-low dark current of 7.74 pA, an ultra-fast response speed of 2.95/3.07 μs, and a high switching ratio of 7.67 × 105. This work provides a method for the preparation of high-quality mixed halide perovskite materials and a strategy for the development of high-performance perovskite PDs based on bulk heterojunction.

Lead-free tin-based perovskites nanocrystals for high-performance self-driven bulk-heterojunction photodetectors

Currently, photodetectors based on metal-halide perovskites are the fastest-emergent photovoltaics technology in terms of research and development. Lead-based perovskite semiconductors exhibit unusual structural and electrical properties due to the chemical instability of Pb+2 content. All-inorganic lead-free perovskites have become an alternative perovskite material for optoelectronic devices. However, high trap state density and the surface non-uniformity of the active layer make them inappropriate for high-performance photodetectors. Therefore, in this work, a feasible method for surface-passivation of all-inorganic lead-free perovskites with poly (3-hexylthiophene) (P3HT) as the photoactive layer for high-performance photodetectors is presented, and the underlain mechanisms to enhance device performance are investigated by experiments and simulation. As the result, the photoresponsivity of 1.56 A/W with a specific detectivity of 1.40 × 1014 Jones was obtained from photodetectors ITO/ZnO/CsSnBr3:P3HT/CuSCN/Au under 0.1 mW/cm2 532 nm illumination in self-driven mode. Our experiments and simulation show that the enhanced-performance origins from both the enhanced transportation of the photogenerated charge carriers and the suppressed dark current through the device.

Skin‐Mimicking, Stretchable Photodetector for Skin‐Customized Ultraviolet Dosimetry

AbstractDespite extensive research on photoelectric sensors that typically output signals to indicate the transient ultraviolet (UV) intensity, developing wearable, electrometric photodetectors that can reveal the cumulative UV exposure remains challenging. Here, an organic–inorganic hybrid bulk heterojunction (BHJ) is used to construct a skin‐mimicking, wearable photodetector, based on conductive polymer sensitized with semiconducting nanoparticles. Owing to the UV‐induced reduction reactions, the BHJ photodetectors (PDs) can long remember the UV radiation previously exposed. Moreover, based on the solution processability of bulk heterojunction blends and rich surface functional groups of polydopamine, BHJ PDs can fully inherit the ultraflexibility from electrospun fiber‐networked structure. Thus, this fiber‐configured photodetector is working well even at the strains of up to 60%. Without requiring any complicated analytical tool, its memory effect and simple‐to‐process output facilitate the working mechanism of power‐free UV monitor, while catering to the individual need of different skin types. This work combines the bottom materials design to the top device application, pointing out a new direction in wearable UV sensor technologies.

Platinum(II)‐Acetylide Conjugated Polymer Containing Aza‐BODIPY Moieties for Panchromatic Photodetectors

AbstractBroadband photodetectors have driven the development of optical communication, image sensing, and health monitoring technologies, yet confronting a challenge in acquiring high performance in the whole spectral range. Herein, a novel low‐bandgap platinum(II)‐acetylide conjugated polymer is designed and synthesized by introducing near‐infrared‐absorbing aza‐BODIPY moieties. Benefiting from the good solubility and extended conjugated systems, it can serve as the donor materials for photodetector. By blending with (6,6)‐phenyl‐C61‐butyric acid methyl ester, organic bulk heterojunction photodetectors are successfully constructed, exhibiting a high detectivity over 1011 Jones in a broad band of 330–930 nm. Especially, the photodetector exhibits external quantum efficiency mostly beyond 15% in the NIR region. This work opens a new possibility for high‐performance organic broadband photodetector application.

Lateral organic-inorganic hybrid Vis-NIR photodetectors based on GaN nanowires promoting photogenerated carriers transfer

The narrow bandgap of the low-energy near-infrared (NIR) polymer would lead to overlap between adjacent energy levels, which is a major barrier to the preparation of Vis-NIR polymer bulk heterojunction (BHJ) photodetectors with small responsivity and photocurrent. In this study, a high-performance lateral inorganic-organic hybrid photodetector was constructed to eliminate this barrier by combining GaN nanowires (GaN-NWs) with PDPP3T:PC61BM-based BHJ. In stage one, high-quality GaN-NWs were synthesized by the catalyst-free CVD method. The mechanism for controlling GaN-NWs morphology by adjusting the NH3 flow rate was revealed. In stage two, the GaN-NWs with large electron mobility were used to accelerate the transfer of photogenerated carriers in the BHJ layer. Finally, compared with the BHJ device, the BHJ/GaN device demonstrated obvious improvements in responsivity and photocurrent at the wavelength between 400 and 1000 nm. The responsivity and photocurrent increased over 20-fold at the NIR band of 800–900 nm. Besides, owing to the energy level gradient effect, the BHJ/GaN device has a response speed of 7.8/<5.0 ms, which increases over three orders of magnitude than that of the GaN-NWs-based device (tr/tf: 7.1/10.9 s). Therefore, the novel device structure proposed in this work holds great potential for preparing high-performance Vis-NIR photodetectors.

Effect of oligothiophene π-bridge length in D-π-A star-shaped small molecules on properties and photovoltaic performance in single-component and bulk heterojunction organic solar cells and photodetectors

Donor-acceptor molecules with thiophene fragments as the π-bridge represent a promising class of materials for organic photovoltaics especially in single-component (SC) organic solar cells (OSCs) and other related applications. However, the effect of the oligothiophene π-bridge length on physicochemical properties, photophysics, charge transport, and photovoltaic performance of these materials has not been thoroughly addressed. Here, we report on the synthesis and comprehensive investigation of the series of star-shaped donor-acceptor molecules (0T–4T) with triphenylamine as a donor core linked through an oligothiophene π-bridge of variable length to the terminal hexyl-dicyanovinyl electron-withdrawing groups. We found that variation of the π-bridge length from 0 to 4 thiophene units strongly impacts their properties such as the solubility, highest occupied molecular orbital energy, optical absorption and photophysics, film morphology, phase behavior, and molecular packing as well as the charge carrier mobility. The performance of the SC and bulk heterojunction OSCs and photodetectors is comprehensively studied and compared. The results obtained provide insight into how to fine-tune and predict properties and photovoltaic performance of small molecules for organic solar cells and photodetectors.

Responsivity Spectrum Tailoring of Pentacene:ZnO Multi-Nano Film-Based Bulk Heterojunction Photodetector

A Al/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /Pentacene:ZnO/ PEDOT:PSS/ITO photodetector structure is proposed and analyzed. The pentacene:ZnO blend film used as an active layer of the device is studied briefly for its structural and optical properties. The proposed photodetector is shown to be tailored for operation in UV and visible region by changing pentacene:ZnO (P:Z) blend film ratio as 1:1, 1:2 and 2:1. For the proposed photodetector optimum performance in the UV region is obtained for P:Z ratio of 1:2. The obtained responsivity is 9.26 A/W, detectivity is 1.74 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> Jones and external quantum efficiency (EQE) is 3.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> %, at wavelength 302 nm. Further, for the visible region, responsivity is 0.71 A/W, detectivity is 0.15 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> Jones and external quantum efficiency (EQE) is 140%, at wavelength 628 nm for P:Z ratio of 2:1. Thus, responsivity tailoring by changing the blend ratio of the active layer facilitates the route for the fabrication of suitable optoelectronic devices for a wide variety of applications.

Organic bulk heterojunction photodetectors based on polymer blends with unbalanced mobilities

We investigate bulk heterojunction organic photodetectors with unbalanced charge carrier mobilities in the active layer. We present an experimental example of an organic photodiode with very unbalanced mobilities but achieving state-of-the-art parameters in terms of external quantum efficiency exceeding 75% (at illumination 638 nm), and of bandwidth exceeding 1 MHz. The active layer of the photodiode is a 50 nm thick bulk heterojunction made of a blend of PCBM and copolymer of 5,6-difluoro2,1,3-benzothiadiazole, with mobility of slower charge carriers of order of 5 × 10−7 , estimated using space-charge-limited photocurrent technique. To understand why such a low mobility does not have deteriorating effects on the photodetecting performance of the photodiode, we performed drift-diffusion simulations of bulk heterojunction photodiodes with Langevin recombination. According to the simulation, the bandwidth of the photodiode is approximately independent of the mobility of slower charge carriers in the blend, and the negative effect of low mobility on the responsivity can be compensated by increasing the reverse bias. Our study shows that well-performing organic photodetectors can be fabricated using organic semiconductors having too low mobility for photovoltaic applications.

Color-Selective Printed Organic Photodiodes for Filterless Multichannel Visible Light Communication.

Future lightweight, flexible, and wearable electronics will employ visible-light-communication schemes to interact within indoor environments. Organic photodiodes are particularly well suited for such technologies as they enable chemically tailored optoelectronic performance and fabrication by printing techniques on thin and flexible substrates. However, previous methods have failed to address versatile functionality regarding wavelength selectivity without increasing fabrication complexity. This work introduces a general solution for printing wavelength-selective bulk-heterojunction photodetectors through engineering of the ink formulation. Nonfullerene acceptors are incorporated in a transparent polymer donor matrix to narrow and tune the response in the visible range without optical filters or light-management techniques. This approach effectively decouples the optical response from the viscoelastic ink properties, simplifying process development. A thorough morphological and spectroscopic investigation finds excellent charge-carrier dynamics enabling state-of-the-art responsivities >102 mA W-1 and cutoff frequencies >1.5 MHz. Finally, the color selectivity and high performance are demonstrated in a filterless visible-light-communication system capable of demultiplexing intermixed optical signals.

Large-scale roll-to-roll printed, flexible and stable organic bulk heterojunction photodetector

A flexible and stable photodetector shows great potential applications in intelligent wearable devices, health monitoring, and biological sensing. The high-output fabrication of flexible and stable photodetector via the large-scale printing process would accelerate its commercialization. Herein, a high performance, flexible organic bulk heterojunction (BHJ) photodetector with good stability is designed and fabricated via a large-scale roll-to-roll (R2R) micro-gravure printing technique on polyethylene terephthalate (PET) or paper substrate, in which the organic BHJ active layer is structured with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and a donor–acceptor copolymer, i.e., employing 4,8-bis(2-ethylhexylthiophene) benzo[1,2-b;3,4-b′] dithiophene (BDTT) as the donor unit and 5,8-bis(5-thiophen-2-yl)-6,7-difluoro-2,3-bis(4-ethylhexyloxy-1-mata-luorophenyl) quinoxaline (ffQx) as the acceptor unit (PBDTT-ffQx). The PBDTT-ffQx/PCBM BHJ photodetector shows a broad photoresponse in ultraviolet and visible light, a high detectivity (D*) value up to 6.19 × 1011 Jones, and an excellent Iphoto/Idark as high as 5.6 × 102. It exhibits excellent flexibility and stability. Its performance parameters could maintain over 80% of original values after bending 10,000 cycles or exposing in ambient condition (humidity ~50%, temperature ~30 °C) for 50 days without any encapsulation. More importantly, the R2R micro-gravure printed PBDTT-ffQx/PCBM BHJ active layer is great homogeneous, and the responsivity (R) values of photodetector arrays show a very narrow distribution. The research results show that a high-performance PBDTT-ffQx/PCBM BHJ photodetector with well reliability and reproducibility can be fabricated via the R2R micro-gravure printing technique, which provides an available strategy for fabricating large-area and flexible electronic and optoelectronic devices.

Effect of compositions of acceptor polymers on dark current and photocurrent of all-polymer bulk-heterojunction photodetectors

A series of electron-acceptor polymers, copolymers and blends were used in all-polymer BHJ photodetectors and the effect of acceptor compositions on the key device parameters of dark current density and photocurrent was investigated. Compared with acceptor polymers and polymer blends, the devices based on acceptor copolymer showed lowered dark current and higher photocurrent, due to optimal molecular stacking and morphology of the BHJ active layer. The acceptor blends tend to cause a large phase separation and rough surface of the active layer, thus leading to a low detectivity of the device. Among all the acceptor compositions studied in this work, the all-polymer BHJ photodetector based on a donor polymer (PolyD) and an acceptor copolymer (PolyAA′50) exhibited the highest specific detectivity of over 1012 Jones in the spectral region of 320–980 nm under −0.1 V bias.

Enhancement of photodetector performance by tuning donor-acceptor ratios in diketopyrrolopyrrole- and thiophene-based polymers

In order to investigate the effect of the ratio of donor (D) and acceptor (A) in conjugated D-A polymers on the photovoltaic properties and photodetector performance, a series of polymers containing a weak electron donor of thiophene (T) and a strong electron acceptor of pyrrolo [3,4-c]pyrrole-1,4-dione (DPP) were designed and synthesized. Five polymers P1–P5 were obtained with different D/A (T/DPP) ratios of 3.0:1, 2.7:1, 2.5:1, 2.3:1 and 2.0:1, respectively. With increase of the DPP content, the polymers exhibited a red shift in maximal absorption and a gradual decrease of the LUMO energy level. At the D/A ratio of 2.7:1 for P2, its film morphology was found to be ideal for the bulk-heterojunction photodetector and the device based on P2 exhibited the highest specific detectivity of over 1012 Jones in the spectral region of 330–920 nm under −0.1 V bias. These results manifest the feasibility of improving the photovoltaic property simply by tuning the D/A ratio in conjugated D-A polymers.

Space charge region effects in bidirectional illuminated P3HT:PCBM bulk heterojunction photodetectors

Organic semiconductors are widely investigated for their application in photovoltaics and photodetectors. We show that the efficiency of these devices is strongly influenced by the position of the space charge region, due to unintentional doping, and wavelength-dependent absorption properties in bulk heterojunctions. Spray-coated P3HT:PCBM bulk heterojunction photodiodes with thicknesses up to 4.2μm and semitransparent top contact enable the characterization of exciton generation and separation in both irradiation directions. A large difference in external quantum efficiency (EQE) is observed for top and bottom illuminated configurations and is explained by a bias dependent arrangement of the space charge region at the two contact electrodes. Numerical drift–diffusion simulations allow to get insight into first order mechanisms behind the spectral features of EQE data in highly-doped organic photodiodes.

Breakdown mechanisms and reverse current-voltage characteristics of organic bulk heterojunction solar cells and photodetectors

We investigate the reverse current-voltage characteristics and breakdown mechanisms of organic bulk heterojunction solar cells and photodetectors. Dark current and photo current measurements at different temperatures indicate that tunneling is the dominant mechanism at high reverse voltage. A band-to-band tunneling model that accommodates either Gaussian or exponential-parabolic density of states distributions is developed and used for simulations. At high reverse bias, the model explains the observed breakdown, which differs from the bias independent dark current behavior predicted by the Onsager-Braun model. At low reverse voltage, the incorporation of shunt resistance in the model provides good agreement between the measured reverse bias characteristics and simulations.

Solution-processed pentathiophene dendrimer based photodetectors for digital cameras

The photosensing ability of two blue-absorbing dendrimers is presented. Bulk heterojunction photodetectors were fabricated by spin-coating a blend solution of quinquethiophene (pentathiophene) cored conjugated dendrimers and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The photoresponse characterized under monochromatic illumination at 442nm increases linearly with a dynamic range of ∼105. The response approaches that required for spectral selectivity for camera applications, and the external quantum efficiency of the photodetectors reached up to 22.5% at 435nm.

MEH-PPV/Alq3-based bulk heterojunction photodetector

In this paper, we present the effect of varied illumination levels on the electrical properties of the organic blend bulk heterojuction (BHJ) photodiode. To prepare the BHJ blend, poly(2-methoxy-5(2′-ethylhexyloxy) phenylenevinylene (MEH-PPV) and aluminum-tris-(8-hydroxyquinoline) (Alq3) are used as donor and acceptor materials, respectively. In order to fabricate the photodiode, a 40-nm thick film of poly(3, 4-ethylendioxythiophene):poly(styrensulfonate) (PEDOT:PSS) is primarily deposited on a cleaned ITO coated glass substrate by spin coating technique. The organic photosensitive blend is later spun coated on the PEDOT:PSS layer, followed by the lithium fluoride (LiF) and aluminium (Al) thin films deposition by thermal evaporation. The optical properties of the MEH-PPV:Alq3 blend thin films are investigated using photoluminescence (PL) and UV-Vis spectroscopy. The photodiode shows good photo-current response as a function of variable illumination levels. The responsivity value ∼ 8 mA/W at 3 V is found and the ratio of photo-current to dark current (IPh/IDark) is found to be 1.24.

Visible Light Communication System Using an Organic Bulk Heterojunction Photodetector

A visible light communication (VLC) system using an organic bulk heterojunction photodetector (OPD) is presented. The system has been successfully proven indoors with an audio signal. The emitter consists of three commercial high-power white LEDs connected in parallel. The receiver is based on an organic photodetector having as active layer a blend of poly(3-hexylthiophene) (P3HT) and phenyl C61-butyric acid methyl ester (PCBM). The OPD is opto-electrically characterized, showing a responsivity of 0.18 A/W and a modulation response of 790 kHz at −6 V.