Band Gap Inorganic Semiconductors Research Articles

Dependence of Exciton Binding Energy on Bandgap of Organic Semiconductors.

Strongly bound excitons crucially affect the operation of organic optoelectronic devices. Nevertheless, precise experimental data on the exciton binding energy of organic semiconductors are lacking. In this study, we determine the exciton binding energy as the difference between the optical and transport bandgaps with a precision of 0.1 eV. In particular, electron affinities with a precision higher than 0.05 eV determined by low-energy inverse photoelectron spectroscopy allow us to determine the transport gap and the exciton binding energies with such high precision. Through a systematic comparison of a wide range of organic semiconductors, including 42 organic solar cell materials (15 nonfullerene acceptors, 4 fullerene acceptors, 13 low-bandgap polymers, 7 organic light-emitting diode materials, and 3 crystalline materials), we found that the exciton binding energy is one-quarter of the transport gap regardless of the materials. We interpret this unexpected relation from a hydrogen atom-like model, i.e., the quantized energy levels in a Coulomb potential between the positive and the negative charges.

Insights into the Origins of Minority Carrier Traps in Solution‐Processed Organic Semiconductors and Their Effects on Transistor Photostability

AbstractMinority carrier traps in the bandgap of organic semiconductors (OSCs) are pervasive and of vital importance in determining the performance and stability of the optoelectronic device. Understanding their origins is one critical issue at both the fundamental and applied levels. However, relevant research has rarely been performed due to the lack of an effective strategy for quantitatively assessing the minority carrier traps buried in OSCs. Here, organic field‐effect transistors (OFETs) operated under strong and long‐term light illumination can be used as a model device to assess the amount of minority carrier traps in solution‐processed OSCs, are proposed. Based on the experimental results and theoretical calculations, first identified hydrated impurities (water and oxygen) in the residual organic solvents primarily contribute to formation of minority carrier traps within the OSC bandgap, giving rise to photo‐induced electrical instability of OFETs. To address this problem, a molecular additive strategy is developed to improve the OFET photostability by releasing trapped electrons from the levels of minority carrier traps. This work not only elucidates the significant role of water and oxygen in the residual organic solvents in forming minority carrier traps but also provides guidelines for improving OFET photostability for practical applications.

High-Detectivity and sensitive UVA photodetector of polycrystalline CH3NH3PbCl3 improved by α-Ga2O3 nanorod array

The detection and quantification of UVA radiation are indispensable for the reason that excess UVA radiation is harmful to skin, eyes, immune system and blood vessels of the human body. This study demonstrates a high-performance and accurate UVA (320–400 nm) detector based on polycrystalline CH3NH3PbCl3/α-Ga2O3 nanorod array hybrid structure, which was fabricated by using facile and low-cost solution processing method. The devices show the maximum responsivity of 8.02 mA/W under zero bias, which is two orders of magnitude larger than bare polycrystalline CH3NH3PbCl3 photodetector, as well as high speed photo-switching. Meanwhile, the detectivity of this self-powered device reaches up to 1.9 × 1011 Jones, which is even superior to most of the reported single-crystal CH3NH3PbCl3 photodetectors. This study reveals the potential in the combination of ultra-wide bandgap inorganic semiconductors with polycrystalline organic–inorganic hybrid perovskites in high performance of photodetector and other optoelectronic devices.

Elucidating the Role of Water‐Related Traps in the Operation of Polymer Field‐Effect Transistors

AbstractConjugated polymers have gained momentum as serious contenders for next‐generation flexible electronics, but their susceptibility to water represents a major problem. Atmospheric water is ubiquitous and its inadvertent diffusion into polymeric devices generates charge carrier traps, reducing their performance and stability. A good understanding of the physical processes associated with the presence of water is therefore necessary in order to be able to suppress the related trapping events and enable stable, high‐performance devices. Here, evidence is shown that water introduces traps in the bandgap of organic semiconductors and the impact of these traps on the electrical properties of polymer organic field‐effect transistors (OFETs) based on indacenodithiophene‐co‐benzothiadiazole (IDT‐BT) is investigated. Monitoring device parameters and the trap density of states (t‐DOS) during moisture extrication reveals the existence of two types of water‐related traps: shallow traps originating from water inhabiting the voids of the polymer film and deeper traps arising from chemisorbed water present at the dielectric/polymer interface. A trap passivation method based on flame‐annealing is introduced to eliminate the interfacial traps. As a result, stable OFETs, with threshold voltage shifts less than ΔVth = −0.3 V and constant mobilities (<10% variation) after three months of storage, are fabricated.

Narrowing the Band Gap: The Key to High-Performance Organic Photovoltaics.

ConspectusOrganic photovoltaics (OPVs) have attracted considerable attention in the last two decades to overcome the terawatt energy challenge and serious environmental problems. During their early development, only wide-band-gap organic semiconductors were synthesized and employed as the active layer, mainly utilizing photons in the UV-visible region and yielding power conversion efficiencies (PCEs) lower than 5%. Afterward, considerable efforts were made to narrow the polymer donor band gap in order to utilize the infrared photons, which led to the enhancement of the PCE from 5% to 12% in about a decade. Since 2017, the study of narrow-band-gap non-fullerene acceptors helped usher in a new era in OPV research and boosted the achievable the PCE to 17% in only 3 years. In essence, the history of OPV development in the last 15 years can be summarized as an attempt to narrow the band gap of organic semiconductors and better position the energy levels. There are multiple benefits of a narrower band gap: (1) considerable infrared photons can be utilized, and as a result, the short-circuit current density can increase significantly; (2) the energy offset of the lowest unoccupied molecular orbital energy levels or highest occupied molecular orbital energy levels between the donor and acceptor can be reduced, which will reduce the open-circuit voltage loss by minimizing the loss caused by the donor/acceptor charge transfer state; (3) because of the unique molecular orbitals of organic semiconductors, the red-shifted absorption will induce high transmittance in the visible region, which is ideal for the rear subcells in tandem-junction OPVs and transparent OPVs.In this Account, we first summarize our work beginning in 2008 on the design and synthesis of narrow-band-gap polymer donors/non-fullerene acceptors. Several strategies for constructing these materials, including enhancing the intramolecular charge transfer effect and steric hindrance/energy level engineering are discussed. In this part, in addition to systematic analyses of the design of narrow-band-gap polymer donors based on BDT/TT or BDT/DPP, donors/acceptors based on the new donor moieties DTP or BZPT are discussed as well. Especially, we highlight our work on the first report on the narrow-band-gap acceptor Y1 (based on the new donor moiety BZPT), which pioneered the future development and usage of acceptors belonging to the Y1 family (or series). Subsequently, we analyze several reported certified world record single-junction or tandem-junction OPVs that use these narrow-band-gap donors or acceptors. We share our experiences and insights from a device perspective in terms of donor/acceptor selection, energy level alignment management, film morphology control, current matching of subcells, interconnecting layer construction, interface engineering, and device geometry selection. In this part, the construction of high-performance ternary-blend OPVs and transparent OPVs based on these narrow-band-gap donors/acceptors is also discussed. Finally, in order to push the field into the 20-25% high-efficiency era in the next few years, some suggestions to further develop narrow-band-gap donors/acceptors and related device technologies are proposed.

Ultraviolet Photodetectors Based on CH3NH3PbCl3 Perovskite Quantum Dots‐Doped Poly(triarylamine)

Ultraviolet (UV) photodetectors have attracted significant attention because of their wide applications in environment monitoring, flame detecting, missile tracking, and so on. However, commercial UV photodetectors based on wide bandgap inorganic semiconductors normally require high‐vacuum and high‐temperature deposition techniques. To reduce the fabrication cost, solution‐processed organic or hybrid semiconductors are introduced as promising candidates for low‐cost UV detection. Particularly, due to the low cost, flexibility, and chemical variety, organic semiconductors have shown remarkable progress in the past two decades. However, UV photodetectors based on organic active layers are suffering from relatively low responsivity due to the poor charge transport. Therefore, the devices based on organic semiconductors commonly operate at high bias voltage to facilitate the charge transport, and the devices also exhibit large dark current. Herein, the organic UV active layer is doped with perovskite quantum dots (QDs) to improve the device performance. It is found that the responsivity and detectivity can effectively be enhanced by the doping strategy. Furthermore, the influence of doping effect on the charge carrier mobility and temporal response is also investigated, which shows that the mobility can be increased ≈6 times with the effective doping.

Tailoring tandem organics

Solar Cells Tandem solar cells can boost efficiency by using a wider range of the solar spectrum. The bandgap of organic semiconductors can be tuned over a wide range, but, for a two-terminal device that directly connects the cells, the currents produced must be nearly equal. Meng et al. used a semiempirical analysis to choose well-matched top- and bottom-cell active layers. They used solution processing to fabricate an inverted tandem device that has a power conversion efficiency as high as 17.4%. Science , this issue p. [1094][1] [1]: /lookup/volpage/361/1094?iss=6407

Organic and solution-processed tandem solar cells with 17.3% efficiency.

Although organic photovoltaic (OPV) cells have many advantages, their performance still lags far behind that of other photovoltaic platforms. A fundamental reason for their low performance is the low charge mobility of organic materials, leading to a limit on the active-layer thickness and efficient light absorption. In this work, guided by a semi-empirical model analysis and using the tandem cell strategy to overcome such issues, and taking advantage of the high diversity and easily tunable band structure of organic materials, a record and certified 17.29% power conversion efficiency for a two-terminal monolithic solution-processed tandem OPV is achieved.

A Lewis acid-base chemistry approach towards narrow bandgap dye molecules

Although it usually involves troublesome and time-consuming synthesis work, molecular structure tailoring has been the mainstream tool to adjust the optical absorption and bandgap of organic semiconductors. Herein, we provided a convenient and feasible method to prepare narrow bandgap small-molecule semiconductors by Lewis acid-base chemistry and the optical absorption was adjusted by the stoichiometry of Lewis acid-base reactions. In this work, three pyridine capped diketopyrrolopyrrole (DPP) molecules, i.e., DPPPy-Ph-F, DPPPy-Ph-3F, and DPPPy-Py-F, with medium bandgap were synthesized to coordinate with Lewis acid of B(C6F5)3 (BCF). By coordinating with BCF, as demonstrated by 1H, 19F, and 11B NMR spectra, the bandgaps of these DPP molecules were depressed largely. For DPPPy-Ph-F, with two pyridine units flanking symmetrically, 2 eq of BCF converted the two pyridine nitrogens stoichiometrically to Lewis acid adducts, along with absorption bands red-shifted and bandgap depressed (1.89 vs. 1.80 eV). For DPPPy-Ph-3F, with two pyridine units flanked and trifluorobenzene capped, adding 2 eq of BCF was unable to convert the two pyridine nitrogens to BCF adducts completely, due to the strong electron-drawing ability of trifluorobenzene substituent on pyridine, reducing its basicity. After adding excess BCF to coordinate with nitrogens, the bandgaps of DPPPy-Ph-3F also decreased from 1.92 eV to 1.84 eV. For DPPPy-Py-F, with four pyridine units flanked, 4 eq of BCF were required to bind the pyridine nitrogens. Accordingly, the red-shift of absorption bands and the depression of optical bandgaps (1.96 vs. 1.75 eV) for DPPPy-Py-F were most striking among the three molecules.

Probing the density of trap states in the middle of the bandgap using ambipolar organic field-effect transistors

The number of trap states in the band gap of organic semiconductors directly influences the charge transport as well as the threshold and turn-on voltage. Direct charge transport measurements have been used until now to probe the trap states rather close to the transport level, whereas their number in the middle of the band gap has been elusive. In this study, we use PDIF-CN2, a well known n-type semiconductor, together with vanadium pentoxide electrodes to build ambipolar field-effect transistors. Employing three different methods, we study the density of trap states in the band gap of the semiconductor. These methods give consistent results, and no pool of defect states was found. Additionally, we show first evidence that the number of trap states close to the transport level is correlated with the number of traps in the middle of the band-gap, meaning that a high number of trap states close to the transport level also implies a high number of trap states in the middle of the band gap. This points to a common origin of the trap states over a wide energy range.

Visible-to-Near-IR Wide-Range Light Harvesting by Interfacial Charge-Transfer Transitions between TiO2 and p-Aminophenol and Evidence of Direct Electron Injection to the Conduction Band of TiO2

Interfacial charge-transfer (ICT) transitions between wide band gap inorganic semiconductors such as titanium dioxide (TiO2) and organic compounds are characteristic electronic transitions which enable the absorption of visible light even using colorless low-molecular-weight organic compounds and also direct electron injection into TiO2. Because of these features, ICT transitions are expected to be a new mechanism of light absorption and photoinduced charge separation for solar energy conversions such as photovoltaics. However, in fact, almost all ICT transitions reported so far are limited in the visible region. Visible-to-near-IR wide-range light harvesting of solar energy by ICT transitions is an important subject for such applications. In addition, the conventionally used picture of the direct electron injection into the conduction band of TiO2 by ICT transitions still remains to be demonstrated clearly. In order to solve the two issues, in this paper we study ICT transitions in anatase TiO2 nanoparticles chemically adsorbed with o-, m-, and p-aminophenol, which possess an electron-donating amino group at different positions. We demonstrate that visible-to-near-IR wide-range light absorption due to ICT transitions up to ca. 1.1 μm is generated by using p-aminophenol. In addition, we reveal from the spectral analysis of the ICT band that the ICT transitions take place from the highest occupied molecular orbital (HOMO) of p-aminophenol adsorbates to the continuous conduction band of TiO2, not to discrete surface localized levels. This result offers a direct evidence of the direct electron injection into the conduction band of TiO2 by the ICT transitions. Our research provides novel knowledge on the light absorption and electron transfer properties of ICT transitions for their applications to solar energy conversions.

Interfacial Charge-Transfer Transitions in BaTiO3 Nanoparticles Adsorbed with Catechol

Interfacial charge-transfer (ICT) transitions between wide band gap inorganic semiconductors such as titanium dioxide (TiO2) and organic materials are characteristic electronic transitions that enable the absorption of lower energy photons than the band gap of inorganic semiconductors and the HOMO–LUMO gap of organic compounds and also direct photocarrier injection to inorganic semiconductors. Despite these features, inorganic semiconductors that show ICT transitions are quite limited. Here we report ICT transitions in barium titanate (BaTiO3) nanoparticles with a pseudocubic structure using 1,2-benzenediol (catechol) that is a representative aromatic bidentate ligand to induce ICT transitions. BaTiO3 nanoparticles are instantaneously colored orange by the addition of the colorless solution of catechol, showing a broad absorption band with an onset at ca. 620 nm. The observed FT-IR spectrum indicates that catechol molecules chemisorb on BaTiO3 surfaces by chelate coordination with a surface Ti atom more p...

Tuning the electronic band structure of PCBM by electron irradiation

Tuning the electronic band structures such as band-edge position and bandgap of organic semiconductors is crucial to maximize the performance of organic photovoltaic devices. We present a simple yet effective electron irradiation approach to tune the band structure of [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) that is the most widely used organic acceptor material. We have found that the lowest unoccupied molecular orbital (LUMO) level of PCBM up-shifts toward the vacuum energy level, while the highest occupied molecular orbital (HOMO) level down-shifts when PCBM is electron-irradiated. The shift of the HOMO and the LUMO levels increases as the irradiated electron fluence increases. Accordingly, the band-edge position and the bandgap of PCBM can be controlled by adjusting the electron fluence. Characterization of electron-irradiated PCBM reveals that the variation of the band structure is attributed to the molecular structural change of PCBM by electron irradiation.

Influence of transport-related material parameters on the I– V characteristic of inorganic–organic hybrid solar cells

The carrier transport in inorganic–organic hybrid cells based on small band gap inorganic semiconductors is theoretically studied. In such cells, photo-carriers are generated at the heterojunction (due to dissociation of the donor excitons) and in the bulk of the acceptor material due to direct generation in the small band gap inorganic material. By means of a two-dimensional drift–diffusion model, we demonstrate that material properties directly related to transport, i.e., (i) the carrier mobilities, (ii) the static dielectric constants, and (iii) transport level offsets give, essentially, rise to marked variations in the open circuit voltage, V oc , and the shape of the I– V curve. As the most striking consequence of the differing generation mechanisms, the role of the electron and hole level offsets entirely differ. A lack of a hole transport level offset represents the most serious loss factor for the power conversion efficiency (PCE) due to a pronounced sensitivity of the shape of the I– V curve towards the carrier mobilities and the dielectric constant. On the other hand, degenerate electron transport levels give rise to (i) appreciably large fill factors and (ii) values of V oc , j sc , and PCE that are insensitive towards the ratio of mobility and ϵ r values. However, a non-vanishing electron transport level offset is necessary to achieve a V oc beyond V built-in at all. Then, V oc is further (i) increased by an amount that corresponds to the smaller value of the transport level offsets and (ii) reduced due to a diffusion term that depends on the ratio of the mobility values in the organic and inorganic phase. The latter detrimental term is the smaller, the less the mobility ratio differs from unity, i.e., the smaller the mismatch in the mobility values becomes. Moreover, any mismatch in the carrier mobilities causes an effective mobility-induced diffusion of holes from the organic to the inorganic layer that gives rise to a s-shaped section in the I– V curve.

Conjugated Polymer-Based Organic Solar Cells

The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.

A photochargeable device composed of bilayer membranes of poly(3-methylthiophene) and prussian blue

A photochargeable device which can be charged under irradiation and discharged in the dark should open a new field of photoenergy conversion and storage. Such a device can be used also for information storage. Such a device can be used also for information storage. Some model cells for photoenergy conversion and storage have been constructed by using liquid-junction inorganic semiconductors in contact with a redox electrolyte solution [l-5]. Recently, organic semiconductors such as poly(acetylene) [6,7], poly(pyrrole) [&lo], poly(aniline) [ll], and poly(thienylene) [12,13] were reported to give photocurrents in the form of a Schottky type device. Although the light-to-electricity conversion efficiency of these organic semiconductor devices is inferior to that of inorganic semiconductor ones, they have merits in their easy fabrication, utilization as a film, and fairly high stability, except for poly(acetylene), in contrast to the corrosive narrow bandgap inorganic semiconductors. Prussian Blue (PB) is a polynuclear iron cyanide complex with the composition, Fe~‘[Fe”(CN),]~-. The electrochemical behavior of PB has been well characterized [14-161. Since PB is both oxidized and reduced reversibly, it can work as both the anode and cathode active material of a secondary battery [17-191. PB can be used as a fihn because of its high molecular weight structure. The photoelectrochemical reaction of a PB film coated on a TiO, liquid-junction photoanode was reported in ref. 20. In the present paper we will report the first example of an organic bilayer device composed of poly(3-methylthiophene) and PB which can be charged under irradiation and discharged in the dark reversibly.

Electrical and magnetic properties of single crystal [NEt 4] 2[Cu II(mnt) 2

We report electrical and magnetic studies of [NEt 4] 2[Cu II(mnt) 2]. This crystal is composed of chains of theplanar [Cu II(mnt) 2] −2 anions (space group P 1 and z = 1) which exhibit only weak magnetic interactions. The material behaves as a semiconductor; from 300–400°K the conductivity increases by six orders of magnitude and the resistivity values above 300°K are comparable to those of some of the better known wide band-gap inorganic semiconductors. In contrast with the behavior of other linear chain systems, at room temperature the conductivity along the chain (σ ∥) is less than that perpendicular to the chain (σ ⊥). As the temperature is increase, the anisotropy ratio, σ ∥/σ ⊥, becomes greater than unity and increases to 1.6 × 10 2 at 400°K.