Perovskite‐Based Phototransistors and Hybrid Photodetectors

In the past several years, organic–inorganic hybrid perovskites and all inorganic perovskites have attracted enormous research interest in a variety of optoelectronic applications including solar cells, light‐emitting diodes, semiconductor lasers, and photodetectors for their plenty of appealing electrical and optoelectrical properties. Benefiting from the inherent amplification function of transistors and the pronounced photogating effect, perovskite‐based phototransistors and hybrid photodetectors can provide very high photoresponsivity and gain, rendering them highly promising for some specific applications especially ultrasensitive light detection. A review on the recent progress of phototransistors and hybrid photodetectors using perovskites as light‐sensitive materials is presented. The efforts and development in 3D and 2D perovskite‐based phototransistors, and perovskite/functional material (e.g., graphene, 2D semiconductors, organic semiconductors, and other semiconductors) heterojunction‐based hybrid photodetectors are introduced and discussed systematically. Some processing techniques for optimizing device performance are also addressed. In the final section, a conclusion of the research achievements is presented and possible challenges as well as outlook are provided to guide future activity in this research field.     

Electrical‐Polarization‐Induced Ultrahigh Responsivity Photodetectors Based on Graphene and Graphene Quantum Dots

Hybrid quantum dot–graphene photodetectors have recently attracted substantial interest because of their remarkable performance and low power consumption. However, the performance of the device greatly depends on the interfacial states and photogenerated screening field. As a consequence, the sensitivity is limited and the response time is relatively slow. In order to circumvent these challenges, herein, a composite graphene and graphene quantum dot (GQD) photodetector on lead zirconate titanate (Pb(Zr_0.2Ti_0.8)O₃) (PZT) substrates has been designed to form an ultrasensitive photodetector over a wide range of illumination power. Under 325 nm UV light illumination, the device shows sensitivity as high as 4.06 × 10⁹ A W^?1, which is 120 times higher than reported sensitivity of the same class of devices. Plant derived GQD has a broad range of absorptivity and is an excellent candidate for harvesting photons generating electron–hole pairs. Intrinsic electric field from PZT substrate separates photogenerated electron–hole pairs as well as provides the built‐in electric field that causes the holes to transfer to the underlying graphene channel. The composite structure of graphene and GQD on PZT substrate therefore produces a simple, stable, and highly sensitive photodetector over a wide range of power with short response time, which shows a way to obtain high‐performance optoelectronic devices.     

Novel p–p Heterojunctions Self‐Powered Broadband Photodetectors with Ultrafast Speed and High Responsivity

Novel inorganic/organic self‐powered UV–vis photodetectors based on single Se microtube and conducting polymers—polyaniline (PANI), polypyrrole (PPy), and poly(3,4‐ethylenedioxythiophene) (PEDOT)—are fabricated. The conducting polymers are directly coated on the surface of a single Se microtube via a facile and low‐cost in situ polymerization method. The integrated Se/PANI photodetector with 45‐nm‐thick PANI layer shows excellent self‐powered behavior under UV–vis light illumination. In particular, it exhibits high on/off ratio of 1.1 × 10³, responsivity (120 mA W^?1), large detectivity (3.78 × 10¹¹ Jones), and ultrafast response speed (rise time of 4.5 µs and fall time of 2.84 ms) at zero bias at 610 nm (0.434 mW cm^?2)‐light illumination. Moreover, the individual Se/PPy and Se/PEDOT self‐powered photodetectors also exhibit fast and stable responses, including responsivity of 70 and 5.5 mA W^?1, rise time of 0.35 and 1.00 ms, fall time of 16.97 and 9.78 ms, respectively. Given the simple device architecture and low cost fabrication process, this work provides a promising way to fabricate inorganic/organic, high‐performance, self‐powered photodetectors.     

Ultrahigh Responsivity of Ternary Sb–Bi–Se Nanowire Photodetectors

High‐quality single‐crystalline ternary (Sb_1‐xBi_x)₂Se₃ nanowires (NWs) (x = 0–0.88) are synthesized by chemical vapor deposition. Nanowires with x from 0 to 0.75 are indexed as an orthorhombic structure. With increasing Bi incorporation ratio, (Sb_1‐xBi_x)₂Se₃ NWs exhibit remarkable photoresponsivities, which originate from growing surface Se vacancies and augmented oxygen chemisorptions. Notably, spectra responsivity and external quantum efficiency of an (Sb_0.44Bi_0.56)₂Se₃ NW photodetector reach as high as ≈8261.4 A/W and ≈1.6 × 10⁶ %, respectively. Those excellent performances unambiguously demonstrate that Sb–Bi–Se NWs are promising for the utilizations of high‐sensitivity and high‐speed photodetectors and photoelectronic switches.     

High Mobility WS2 Transistors Realized by Multilayer Graphene Electrodes and Application to High Responsivity Flexible Photodetectors

The electrical contact is one of the main issues preventing semiconducting 2D materials to fulfill their potential in electronic and optoelectronic devices. To overcome this problem, a new approach is developed here that uses chemical vapor deposition grown multilayer graphene (MLG) sheets as flexible electrodes for WS₂ field‐effect transistors. The gate‐tunable Fermi level, van der Waals interaction with the WS₂, and the high electrical conductivity of MLG significantly improve the overall performance of the devices. The carrier mobility of single‐layer WS₂ increases about a tenfold (50 cm² V^?1 s^?1 at room temperature) by replacing conventional Ti/Au metal electrodes (5 cm² V^?1 s^?1) with the MLG electrodes. Further, by replacing the conventional SiO₂ substrate with a thin (1 µm) parylene‐C flexible film as insulator, flexible WS₂ photodetectors that are able to sustain multiple bending stress tests without significant performance degradation are realized. The flexible photodetectors exhibited extraordinarily high gate‐tunable photoresponsivities, reaching values of 4500 A W^?1, and with very short (<2 ms) response time. The work of the heterostacked structure combining WS₂, graphene, and the very thin polymer film will find applications in various flexible electronics, such as wearable high‐performance optoelectronics devices.     

Polarity‐Controlled Attachment of Cytochrome C for High‐Performance Cytochrome C/Graphene van der Waals Heterojunction Photodetectors

Biomolecule/graphene van der Waals heterojunction provides a generic platform for designing high‐performance, flexible, and scalable optoelectronics. A key challenge is, in controllable attachment, the biomolecules to form a desired interfacial electronic structure for a high‐efficiency optoelectronic process of photoabsorption, exciton dissociation into photocarriers, carrier transfer, and transport. Here, it is shown that a polarity‐controlled attachment of the Cytochrome c (Cyt c) biomolecules can be achieved on the channel of graphene field effect transistors (GFET). High‐efficiency charge transfer across the formed Cyt c/graphene interface is demonstrated when Cyt c attaches with positively charged side to GFET as predicted by molecular dynamics simulation and confirmed experimentally. This Cyt c/GFET van der Waals heterojunction nanohybrid photodetector exhibits a spectral photoresponsivity resembling the absorption spectrum of the Cyt c, confirming the role of Cty c as the photosensitizer in the device. The high visible photoresponsivity up to 7.57 × 10⁴ A W^?1 can be attributed to the high photoconductive gain in exceeding 10⁵ facilitated by the high carrier mobility in graphene. This result therefore demonstrates a viable approach in synthesis of the biomolecule/graphene van der Waals heterojunction optoelectronics using polarity‐controlled biomolecule attachment, which can be expanded for on‐chip printing of high‐performance molecular optoelectronics.     

Inch‐Size Single Crystal of a Lead‐Free Organic–Inorganic Hybrid Perovskite for High‐Performance Photodetector

Large‐size crystals of organic–inorganic hybrid perovskites (e.g., CH₃NH₃PbX₃, X = Cl, Br, I) have gained wide attention since their spectacular progress on optoelectronic technologies. Although presenting brilliant semiconducting properties, a serious concern of the toxicity in these lead‐based hybrids has become a stumbling block that limits their wide‐scale applications. Exploring lead‐free hybrid perovskite is thus highly urgent for high‐performance optoelectronic devices. Here, a new lead‐free perovskite hybrid (TMHD)BiBr₅ (TMHD = N,N,N,N‐tetramethyl‐1,6‐hexanediammonium) is prepared from facile solution process. Emphatically, inch‐size high‐quality single crystals are successfully grown, the dimensions of which reach up to 32 × 24 × 12 mm³. Furthermore, the planar arrays of photodetectors based on bulk lead‐free (TMHD)BiBr₅ single crystals are first fabricated, which shows sizeable on/off current ratios (≈10³) and rapid response speed (τ_rise = 8.9 ms and τ_decay = 10.2 ms). The prominent device performance of (TMHD)BiBr₅ strongly underscores the lead‐free hybrid perovskite single crystals as promising material candidates for optoelectronic applications.     

Spectroscopic Evaluation of Mixing and Crystallinity of Fullerenes in Bulk Heterojunctions

The microstructure of blend films of conjugated polymer and fullerene, especially the degree of mixing and crystallization, impacts the performance of organic photovoltaic devices considerably. Mixing and crystallization affect device performance in different ways. These phenomena are not easy to screen using traditional methods such as imaging. In this paper, the amorphous regiorandom poly(3‐hexylthiophene) is blended with the potentially crystalline fullerene [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester PCBM and the amorphous bis‐adduct. First, the degree of mixing of polymer: fullerene blends is evaluated using UV–Vis absorption, steady‐state and ultra‐fast photoluminescence spectroscopy. The blue‐shift of the polymer emission and absorption onset are used in combination with the saturation of the polymer emission decay time upon fullerene addition in order to infer the onset of aggregation of the blends. Second, the crystallinity of the fullerene is probed using variable angle spectroscopic ellipsometry (VASE), electroluminescence and photoluminescence spectroscopy. It is shown that the red‐shift of charge transfer emission in the case of PCBM based blends cannot be explained solely by a variation of optical dielectric constant as probed by VASE. A combination of optical spectroscopy techniques, therefore, allows to probe the degree of mixing and can also distinguish between aggregation and crystallization of fullerenes.     

Organic Photodetectors and their Application in Large Area and Flexible Image Sensors: The Role of Dark Current

Organic photodetectors (OPDs) have gained increasing interest as they offer cost‐effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible‐light OPDs are proposed for use in indirect‐conversion X‐ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near‐infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (J_d) is crucial to improve important figures of merits such as the signal‐to‐noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to J_d in OPDs. J_d reduction is typically addressed by fine‐tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental J_d values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.     

Direct Growth of Nanocrystalline Graphene/Graphite Transparent Electrodes on Si/SiO2 for Metal‐Free Schottky Junction Photodetectors

Conventional methods to produce graphene/silicon Schottky junctions inevitably involve graphene transfer and metal deposition, which leads to the techniques being complicated, high‐cost, and environmentally unfriendly. It is possible to directly grow hybrid nanocrystalline graphene/graphite transparent electrodes from photoresist on quartz without any catalyst. Due to the source material being photoresist, nanographene/graphite patterns can easily be made on Si/SiO₂ structures to form nanographene/silicon Schottky junctions via commercial photolithography and silicon techniques. The obtained Schottky junctions exhibit excellent properties with respect to photodetection, with photovoltage responsivity of 300 V W^‐1 at a light power of 0.2 μW and photovoltage response time of less than 0.5 s. The devices also exhibit an excellent reliability with the photovoltage deviating less than 1% when cycled over 200 times.     

High‐Performance Organic‐Inorganic Hybrid Photodetectors Based on P3HT:CdSe Nanowire Heterojunctions on Rigid and Flexible Substrates

Organic‐inorganic hybrid photoelectric devices draw considerable attention because of their unique features by combining the relatively low ionization potential of the organic molecules and the high electron affinity of inorganic semiconductors. Hybrid organic‐inorganic poly(3‐hexylthiophene) (P3HT):CdSe nanowire heterojunction photodetectors are first demonstrated on silicon substrates, exhibiting a greatly enhanced photocurrent, a fast response, and a recovery time shorter than 0.1 s. Flexible hybrid photodetectors with excellent mechanical flexibility and stability are also fabricated on both poly(ethylene terephthalate) (PET) substrates and printing paper. The flexible devices are successfully operated under bending up to almost 180° and show an extremely high on/off switching ratio (larger than 500), a fast time response (about 10 ms), and excellent wavelength‐dependence, which are very desirable properties for its practical application in high‐frequency or high‐speed flexible electronic devices.     

Structural Design and Pyroelectric Property of SnS/CdS Heterojunctions Contrived for Low‐Temperature Visible Photodetectors

The traditional photodetectors based on photoelectric effect exhibit inferior response or even out of operation with the decrease of temperature. However, cryogenic visible light detection is increasingly demanded in deep space and polar exploration. Herein, a self‐powered visible photodetector coupling pyroelectricity and photoelectricity to optimize the cryogenic detecting performance is designed in which hydrothermally grown CdS nanorod array is covered by SnS nanoflakes. The choice of SnS allows the detector with strong visible light absorption and great photoelectric conversion efficiency, while the CdS nanorod structure with pyroelectricity can effectively modulate the behavior of photogenerated carriers at low temperatures. It is found that the response characteristics of the photodetector are dominated by the combination of pyroelectric and photoelectric effects, which becomes more significant with the reduced temperature. Specifically, at 130 K temperature, the photoresponse current under 650 nm light is improved by 7.5 times as that at room temperature, while the ratio of pyroelectric current to photocurrent can be increased to 400%. Meanwhile, the responsivity and detectivity are as high as 10.4 mA W^?1 and 3.56 × 10¹¹ Jones, respectively. This work provides a promising approach to develop high‐performance self‐powered visible photodetectors with low‐temperature operating capability.     

Working Principles of Perovskite Photodetectors: Analyzing the Interplay Between Photoconductivity and Voltage‐Driven Energy‐Level Alignment

Organic–inorganic lead halide perovskites have recently received significant attention as active materials for high‐performance photovoltaics and photodetectors. However, the understanding of their operation mechanism remains limited. High‐gain, low‐voltage CH₃NH₃PbI₃ photodetectors in various architectures are demonstrated herein. Photomultiplication in all structures with direct contact of fluorine‐doped tin oxide (FTO) and perovskite with the highest responsivity 208 A W^?1 corresponding to an incident photon‐to‐current efficiency of 47 000% is observed. Studying the dynamics and temperature dependence, a slow process with an activation energy of 420 ± 90 meV in the time scale of seconds is found, which is essential to photocurrent multiplication. A model based on ion migration to explain the observed transients and the photomultiplication is developed. The accumulation of negative ionic charge at the FTO/perovskite interface under reverse bias lowers the FTO work function allowing for direct hole injection into the perovskite valence band. Under illumination, the conductivity of perovskite is increased and the device behaves similar to a photoconductor.     

Highly Sensitive Low‐Bandgap Perovskite Photodetectors with Response from Ultraviolet to the Near‐Infrared Region

It is a great challenge to obtain broadband response perovskite photodetectors (PPDs) due to the relatively large bandgaps of the most used methylammonium lead halide perovskites. The response range of the reported PPDs is limited in the ultraviolet–visible range. Here, highly sensitive PPDs are successfully fabricated with low bandgap (≈1.25 eV) (FASnI₃)_0.6(MAPbI₃)_0.4 perovskite as active layers, exhibiting a broadband response from 300 to 1000 nm. The performance of the PPDs can be optimized by adjusting the thicknesses of the perovskite and C₆₀ layers. The optimized PPDs with 1000 nm thick perovskite layer and 70 nm thick C₆₀ layer exhibit an almost flat external quantum efficiency (EQE) spectrum from 350 to 900 nm with EQE larger than 65% under ?0.2 V bias. Meanwhile, the optimized PPDs also exhibit suppressed dark current of 3.9 nA, high responsivity (R ) of over 0.4 A W^?1, high specific detectivity (D* ) of over 10¹² Jones in the near‐infrared region under ?0.2 V. Such highly sensitive broadband response PPDs, which can work well as self‐powered conditions, offer great potential applications in multicolor light detection.     

Tunable Nanochannels along Graphene Oxide/Polymer Core–Shell Nanosheets to Enhance Proton Conductivity

Simultaneous manipulation of topological and chemical structures to induce ionic nanochannel formation within solid electrolytes is a crucial but challenging task for the rational design of high‐performance electrochemical devices including proton exchange membrane fuel cell. Herein, a novel generic approach is presented for the construction of tunable ion‐conducting nanochannels via direct assembly of graphene oxide (GO)/poly(phosphonic acid) core–shell nanosheets prepared by surface‐initiated precipitation polymerization. Using this simple and rapid approach to engineer GO/polymer nanosheets at the molecular‐level, ordered and continuous nanochannels with interconnected hydrogen‐bonded networks having a favorable water environment can be created. The resulting membranes exhibit proton conductivities up to 32 mS cm^?1 at 51% relative humidity, surpassing state‐of‐the‐art Nafion membrane and all previously reported GO‐based materials.     

Formation of Bulk Heterojunctions by Alternative Thermal Deposition and Its Structure Analysis for High Efficiency Small Molecular Organic Photovoltaics

We propose a new method to form small‐molecule based bulk heterojunctions (SM‐BHJs) through alternative thermal deposition (ATD), which is a simple modification of conventional thermal evaporation. By ATD, the thicknesses of alternative donor and acceptor layers are precisely controlled down to 0.1 nm, which is critical to form BHJs. The formation of a BHJ in copper(II) phthalocyanine (CuPc) and fullerene (C₆₀) systems is confirmed by atomic force microscopy (AFM), grazing incidence X‐ray small angle scattering (GISAXS), and absorption measurements. From analysis of the data, we find that the CuPc|C₆₀ films fabricated by ATD are composed of nanometer sized disk‐shaped CuPc nano grains and aggregated C₆₀, which explains the phase separation of CuPc and C₆₀. On the other hand, the co‐deposited CuPc:C₆₀ films do not show the existence of separated CuPc nano grains in the CuPc:C₆₀ matrix. The OPV cells fabricated using the ATD method show significantly enhanced power conversion efficiency compared to the co‐deposited OPV cells with the same composition.     

Bulk Heterojunction Solar Cells: Impact of Minor Structural Modifications to the Polymer Backbone on the Polymer–Fullerene Mixing and Packing and on the Fullerene–Fullerene Connecting Network

The morphology of the active layer of a bulk heterojunction solar cell, made of a blend of an electron‐donating polymer and an electron‐accepting fullerene derivative, is known to play a determining role in device performance. Here, a combination of molecular dynamics simulations and long‐range corrected density functional theory calculations is used to elucidate the molecular‐scale effects that even minor structural changes to the polymer backbone can have on the “local” morphology; this study focuses on the extent of polymer–fullerene mixing, on their packing, and on the characteristics of the fullerene–fullerene connecting network in the mixed regions, aspects that are difficult to access experimentally. Three representative polymer donors are investigated: (i) poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′″‐diyl)] (PffBT4T‐2OD); (ii) poly[(2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′″‐diyl)] (PBT4T‐2OD), where the fluorine atoms in the benzothiadiazole moieties of PffBT4T‐2OD are replaced with hydrogen atoms; and (iii) poly[(2,2′‐bithiophene)‐alt‐(4,7‐bis((2‐decyltetradecyl)thiophen‐2‐yl)‐5,6‐difluoro‐2‐propyl‐2H‐benzo[d][1,2,3]triazole)] (PT2‐FTAZ), where the sulfur atoms in the benzothiadiazole moieties of PffBT4T‐2OD are replaced with nitrogen atoms carrying a linear C₃H₇ side‐chain; these polymers are mixed with the phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) acceptor. This study also discusses the nature of the charge‐transfer electronic states appearing at the donor–acceptor interfaces, the electronic couplings relevant for the charge‐recombination process, and the electron‐transfer features between neighboring PC₇₁BM molecules.     

Efficiency and Stability Enhancement in Perovskite Solar Cells by Inserting Lithium‐Neutralized Graphene Oxide as Electron Transporting Layer

This work proposes a new perovskite solar cell structure by including lithium‐neutralized graphene oxide (GO‐Li) as the electron transporting layer (ETL) on top of the mesoporous TiO₂ (m‐TiO₂) substrate. The modified work‐function of GO after the intercalation of Li atoms (4.3 eV) exhibits a good energy matching with the TiO₂ conduction band, leading to a significant enhancement of the electron injection from the perovskite to the m‐TiO₂. The resulting devices exhibit an improved short circuit current and fill factor and a reduced hysteresis. Furthermore, the GO‐Li ETL partially passivates the oxygen vacancies/defects of m‐TiO₂ by resulting in an enhanced stability under prolonged 1 SUN irradiation.     

Multilayer Hybrid Films Consisting of Alternating Graphene and Titania Nanosheets with Ultrafast Electron Transfer and Photoconversion Properties

Alternating graphene (G) and titania (Ti_0.91O₂) multilayered nanosheets are fabricated using layer‐by‐layer electrostatic deposition followed by UV irradiation. Successful assemblies of graphene oxide (GO) and titania nanosheets in sequence with polyethylenimine as a linker is confirmed by UV–vis absorption and X‐ray diffraction. Photocatalytic reduction of GO into G can be achieved upon UV irradiation. Ultrafast photocatalytic electron transfer between the titania and graphene is demonstrated using femtosecond transient absorption spectroscopy. Efficient exciton dissociation at the interfaces coupled with cross‐surface charge percolation allows efficient photocurrent conversion in the multilayered Ti_0.91O₂/G films.     

Development of a Seedless Floating Growth Process in Solution for Synthesis of Crystalline ZnO Micro/Nanowire Arrays on Graphene: Towards High‐Performance Nanohybrid Ultraviolet Photodetectors

A seedless solution process is developed for controllable growth of crystalline ZnO micro/nanowire arrays directly on single‐layer graphene sheets made in chemical vapor deposition (CVD). In particular, the alignment of the ZnO micro/nanowires correlates well with the density of the wires, which is determined by both the sample configuration in solution and the graphene surface cleaning. With increasing wire density, the ZnO micro/nanowire array alignment may be varied from horizontal to vertical by increasing the physical confinement. Ultraviolet photodetectors based on the vertically aligned ZnO micro/nanowires on graphene show high responsivity of 1.62 A W^?1 per volt, a 500% improvement over epitxial ZnO sensors, a 300% improvement over ZnO nanoparticle sensors, and a 40% improvement over the previous best results for nanowire/graphene hybrid sensors. This seedless, floating growth process could be scaled up for large scale growth of oriented ZnO micro/nanowires on graphene at low costs.