There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

While the field of perovskite-based optoelectronics has mostly been dominated by photovoltaics, light-emitting diodes, and transistors, semiconducting properties peculiar to perovskites make them interesting candidates for innovative and disruptive applications in light signal detection. Perovskites combine effective light absorption in the broadband range with good photo-generation yield and high charge carrier mobility, a combination that provides promising potential for exploiting sensitive and fast photodetectors that are targeted for image sensing, optical communication, environmental monitoring or chemical/biological detection. Currently, organic–inorganic hybrid and all-inorganic halide perovskites with controlled morphologies of polycrystalline thin films, nano-particles/wires/sheets, and bulk single crystals have shown key figure-of-merit features in terms of their responsivity, detectivity, noise equivalent power, linear dynamic range, and response speed. The sensing region has been covered from ultraviolet-visible-near infrared (UV-Vis-NIR) to gamma photons based on two- or three-terminal device architectures. Diverse photoactive materials and devices with superior optoelectronic performances have stimulated attention from researchers in multidisciplinary areas. In this review, we provide a comprehensive overview of the recent progress of perovskite-based photodetectors focusing on versatile compositions, structures, and morphologies of constituent materials, and diverse device architectures toward the superior performance metrics. Combining the advantages of both organic semiconductors (facile solution processability) and inorganic semiconductors (high charge carrier mobility), perovskites are expected to replace commercial silicon for future photodetection applications.

Perovskite-based photodetectors: materials and devices

The electronic structure of the perovskite LaCoO$_3$ for different spin states of Co ions was calculated in the LDA+U approach. The ground state was found to be a nonmagnetic insulator with Co ions in a low-spin state. Somewhat higher in energy we found two intermediate-spin states followed by a high-spin state at significantly higher energy. The calculation results show that Co 3$d$ states of $t_{2g}$ symmetry form narrow bands which could easily localize whilst $e_g$ orbitals, due to their strong hybridization with the oxygen 2$p$ states, form a broad $\sigma^*$ band. With the increase of temperature which is simulated by the corresponding increase of the lattice parameter, the transition from the low- to intermediate-spin states occurs. This intermediate-spin (occupation $t_{2g}^5e_g^1$) can develop an orbital ordering which can account for the nonmetallic nature of LaCoO$_3$ at 90 K$<$T$<$500 K. Possible explanations of the magnetic behavior and gradual insulating-metal transition are suggested.

https://arxiv.org/pdf/cond-mat/9709060.pdf

Intermediate-spin state and properties of LaCoO_3

Perovskite photodetectors (PPDs), which combine the advantages of perovskite semiconductor materials with superior optical and electronic properties and solution-processed manufacturing, have emerged as a new class of revolutionary optoelectronic devices with potential for various practical applications. Encouraged by the development of various solution-synthesis and film-deposition techniques for controlling the morphology and composition of perovskite materials with interesting optoelectronic properties, increasing research attention is focused on the development of high performance PPDs. In this review, the recent progress on emerging PPDs is comprehensively summarized from the perspective of device physics and materials science. The strategies for extending the spectral response range of PPDs and improving the performance of devices are investigated. Furthermore, the methods for realizing narrowband photodetectors are also discussed, where filter-free and self-filter narrowband PPDs are achieved based on the concept of charge collection narrowing. Meanwhile, the promising future directions in this research field are proposed and discussed, including multifunctional PPDs, perovskite–organic hybrid photodetectors, flexible and transparent PPDs, self-powered PPDs, and photodetector systems and arrays. This review provides valuable insights into the current status of highly sensitive PPDs and will spur the design of new structures and devices to further enhance their photo-detection performances and meet the need of versatility in practical application.

Recent progress on highly sensitive perovskite photodetectors

Multi-functional organic field-effect transistors (OFETs), an emerging focus of organic optoelectronic devices, hold great potential for a variety of applications. This report introduces recent progress on multi-functional OFETs including OFETs based sensors, phototransistors, light-emitting transistors, memory cells, and magnetic field-effect OFETs. Key strategies towards multi- functional integration of OFETs, which involves the exploration of functional materials, interfaces modifications, modulation of condensed structures, optimization of device geometry, and device integration, are summarized. Furthermore, remaining challenges and perspectives are discussed, giving a comprehensive overview of multi-functional OFETs.

Multi-functional integration of organic field-effect transistors (OFETs): advances and perspectives.

Photogenerated carriers in active double-layer 3,4,9,10-perylenetetracarboxylic dianhydride/copper phthalocyanine (PTCDA/CuPc) organic field-effect transistor (OFET) has been investigated. The PTCDA and CuPc layers have n- and p-type conductivities, respectively; thus, PTCDA can be assumed as an acceptor and CuPc as a donor in the meaning of photovoltaic device. It has been found that the charges are generated in the bulk of the organic material rather than close to the organic-organic interface.

http://ieeexplore.ieee.org/iel5/6410088/6418225/06418514.pdf

Photocurrents in double-layer organic field-effect transistors

In this paper, we report on the mixed programming software of VB and Fortran to determine the charge transport characteristics of organic semiconductors. Exponentially distributed trap and Frenkel effect space charge limited current (SCLC) model was studied respectively. In order to verify the effectiveness of the simulation software, we fabricated and characterized single-hole devices based on organic semiconductor NPB (N,N′-Bis (naphthalen-1-yl)-N,N′-bis (phenyl) benzidine). The simulation current–voltage characteristic curves of the device were consistent with the experimental ones.

Simulation Software Development for Charge Transport Characteristics in Organic Semiconductors Based on VB and Fortran Mixed Programming

Employing a photosensitive donor/acceptor planar heterojunction (DA-PHJ) with complementary optical absorption as the active layer is one of the key strategies for realizing broad spectral organic photodiodes (BS-OPDs). To achieve superior optoelectronic performance, it is vital to optimize the thickness ratio of the donor layer to acceptor layer (the DA thickness ratio) in addition to the optoelectronic properties of the DA-PHJ materials. In this study, we realized a BS-OPD exploiting tin(II) phthalocyanine (SnPc)/3,4,9,10-perylenete-acarboxylic dianhydride (PTCDA) as the active layer and investigated the effect of the DA thickness ratio on the device performance. The results showed that the DA thickness ratio has a significant impact on the device performance, and an optimized DA thickness ratio of 30:20 was found. Upon the optimization of the DA thickness ratio, improvements of 187% in photoresponsivity and 144% in specific detectivity were achieved on average. Trap-free space-charge-limited photocarrier transport and balanced optical absorption over the wavelength range can be ascribed to the improved performance at the optimized DA thickness ratio. These results establish a solid photophysical foundation for improving the performance of BS-OPDs via thickness ratio optimization.

Thickness-ratio-dependent performances of broadband organic photodiodes based on a tin phthalocyanine/PTCDA heterojunction.

The influence of the cathode work function, carriers mobilities and temperature on the short-circuit current of single layer organic solar cells with Schottkey contacts was numerically studied, and the quantitative dependences of the short-circuit current on these quantities were obtained. The results provide the theoretical foundation for experimental study of single layer organic solar cells with Schottkey contacts.

Numerical study on the short-circuit current in single layer organic solar cells with Schottkey contacts

Organic photodiode array has recently attracted a lot of attention for large-area light detection applications and imaging. In this work, Organic photodiode 3 × 2 (Sample A), 4 × 3 (Sample B), 5 × 4 (Sample C), and 6 × 5 (Sample D) arrays based on copper phthalocyanine (CuPc)/C60 planar heterojunction in which CuPc used as optical sensitive material were fabricated and characterized. It is found that the dark current density of the device increases as the array increases through experimental data. A circuit model was constructed to analyze the cause of this phenomenon, and an analytical expression between dark current density and array size was obtained.

Yingquan Peng

Papers

Photocurrents in double-layer organic field-effect transistors

Simulation Software Development for Charge Transport Characteristics in Organic Semiconductors Based on VB and Fortran Mixed Programming

Thickness-ratio-dependent performances of broadband organic photodiodes based on a tin phthalocyanine/PTCDA heterojunction.

Numerical study on the short-circuit current in single layer organic solar cells with Schottkey contacts

Size Dependent Performances of Organic Photodiode Arrays