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.

An investigation of design principles toward near infrared organic upconversion devices

Active layer and electrode thickness dependent performances of bottom contact photosensitive organic field-effect transistors

A numerical model for the current conduction in single-layer organic light-emitting devices is established under the basis of trapped charge-limited conduction with an exponential trap distribution. The dependences of the current density on the operation voltage, the thickness of the organic layer, and the trap properties are numerically studied. The current density decreases nearly exponentially with the thickness of the organic layer and the relative trap depth (l), and it is inversely proportional to the lth power of the total trap density. The results from simulations for the current–voltage characteristics agree very well with those from experiments.

Numerical study of the current conduction in single-layer organic light-emitting devices

The utility model discloses a Schottky contact organic photosensitive field effect tube structure. The Schottky contact organic photosensitive field effect tube structure is characterized in that: the Schottky contact is formed by a source electrode, a drain electrode and an organic photosensitive channel material. The structure can adopt bottom gate bottom contact, bottom gate top contact, top gate bottom contact or top gate top contact. The p-type organic photosensitive channel material selects a low work function metal (such as aluminum, magnesium, etc.) as the drain electrode and the source electrode, in order to form a hole Schottky contact. The n-type organic photosensitive channel material selects a high work function metal (such as platinum, gold, etc.) as the drain electrode and the source electrode to form an electronic Schottky contact. The source electrode, the drain electrode and the organic photosensitive channel form the Schottky contact, the impedance between the drain electrode and the source electrode is increased once without lighting, the dark current is greatly reduced, and the photocurrent is slightly reduced. The Schottky contact organic photosensitive field effect tube structure has advantages of small dark current, and large ratio of the photocurrent to the dark current.

Schottky contact organic photosensitive field effect tube

This paper presents a systematic numerical simulation of variation of blending ratio and cathode work function dependence of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-para-phenylene vinylene) (MDMO-PPV):phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) solar cells performance for MDMO-PPV concentration between 0 and 100 mol percentage and cathode work function interval 3.7–4.7 eV. From our studies it became evident that the open-circuit voltage (Voc), short-circuit current (Jsc) and fill factor (FF) decrease with the increasing cathode work function. It is shown that Jsc increases almost linearly with the increasing MDMO-PPV concentration from 0 to 60 mol%, and then decreases. With the increasing of MDMO-PPV concentration, FF augments gradually to 76.56% and then plunges to 19.53%. It is also demonstrated that when MDMO-PPV is larger than 45 mol%, the enhancing MDMO-PPV mol percentage is detrimental for the device performance.

Yingquan Peng

Papers

An investigation of design principles toward near infrared organic upconversion devices

Active layer and electrode thickness dependent performances of bottom contact photosensitive organic field-effect transistors

Numerical study of the current conduction in single-layer organic light-emitting devices

Schottky contact organic photosensitive field effect tube

Effect of mixture ratio on the performance of MDMO-PPV:PCBM bulk heterojunction solar cells: A numerical study