Photorefractive performance of poly(triarylamine)-Based polymer composites: An approach from the photoconductive properties

Photorefractive Response Enhancement in Poly(triarylamine)-Based Polymer Composites by a Second Electron Trap Chromophore

Photorefractive (PR) performances are affected by the components of the photoconductor, sensitizer, nonlinear optical dye, and plasticizer. A photoconductor with high hole mobility promises a faster response time, whereas it induces higher photoconductivity, which leads to easy dielectric breakdown. Adding a second electron trap is effective in controlling photoconductivity. In this study, the role of a second electron trap 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) was investigated in a PR composite consisting of a photoconductor of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] with a high hole mobility, a nonlinear optical chromophore of piperidinodicyanostyrene, a plasticizer of (2,4,6-trimethylphenyl)diphenylamine, and a sensitizer of [6,6]-phenyl C₆₁ butyric acid-methyl ester. The minimum time response with the maximum optical diffraction efficiency and sensitivity was measured at a 1 wt % content of TmPyPB. These results were consistent with the number of charge carriers trapped per unit volume and per unit time N _c (cm^-3 s^-1), which is defined as the ratio between the initial trap density T _i (cm^-3) and response time τ (s), at a 1 wt % content of TmPyPB. A faster response time of 149 μs, optical diffraction of 24.1% (external diffraction of 4.8%), and a sensitivity of 2746 cm² J^-1 were measured at 50 V μm^-1 for the sample with 1 wt % TmPyPB. High loading of 5 wt % TmPyPB led to a large decrease in photoconductivity and effectively suppressed the dielectric breakdown under a stronger electric field, whereas a slower response time with lower diffraction efficiency was observed for optical diffraction.

Light Intensity Analysis of Photovoltaic Parameters for Perovskite Solar Cells

The number of publications on perovskite solar cells (PSCs) continues to grow exponentially. Although the efficiency of PSCs has exceeded 25.5%, not every research laboratory can reproduce this result or even pass the border of 20%. Unfortunately, it is not always clear which dominating mechanism is responsible for the performance drop. Here, a simple method of light intensity analysis of the JV parameters is developed, allowing an understanding of what the mechanisms are that appear in the solar cell and limit device performance. The developed method is supported by the drift-diffusion model and is aimed at helping in the explanation of parasitic losses from the interface or bulk recombination, series resistance, or shunt resistance in the perovskite solar cell. This method can help not only point toward the dominating of bulk or interface recombination in the devices but also determine which interface is more defective. A detailed and stepwise guidance for such a type of light intensity analysis of JV parameters is provided. The proposed method and the conclusions of this study are supported by a series of case studies, showing the effectiveness of the proposed method on real examples.

Enhancement of the response time in organic photorefractive composites using alkoxy-substituted PDCST as a nonlinear optical chromophore

The ease of the molecular orientation of a chromophore has an important effect on the electro-optical (EO) properties of polymeric photorefractive (PR) composites. A derivative of 4-piperidinobenzylidene-malononitrile (PDCST) with an alkoxy group added as a side branch was synthesized to improve the molecular orientation characteristics. Electrophoresis was performed on the polymeric PR composite to which the improved PDCST had been added. The optical properties and response times were examined to evaluate the effects of the substitution of the alkoxy group. PDCST substituted with the alkoxy group showed enhanced EO properties and a PR grating formation rate.

Optimal composition of the poly(triarylamine)-based polymer composite to maximize photorefractive performance

A holographic display system requires the external diffraction efficiency to be greater than 10% and four orders of magnitude of sensitivity for practical usage. To achieve such requirements, the photorefractive (PR) performance of PR composite based on poly[bis(2,4,6-trimethylpheneyl)amine] (PTAA) has been investigated. In the present report, the change of the content of PTAA as a photoconductive polymer, (2,4,6-trimethylphenyl)diphenylamine (TAA) as a photoconductive plasticizer, and second trap agent bathophenanthroline (BPhen) reasonably optimized the PR response time and external diffraction efficiency. High sensitivity of 1851 cm² J^-1 with response time of 494 μs and external diffraction efficiency of 23.9% were achieved at 532 nm and 60 V μm^-1 by reducing the content of PTAA and increasing the contents of TAA and BPhen. Decreasing the amount of PTAA and increasing the contents of TAA and BPhen lowered the absorption coefficient, resulting in the high external diffraction efficiency. The narrower distribution of the electronic density of states (DOS) for PTAA/TAA (43.5/20 and 33.5/30) also contributed to the shorter PR response time of hundreds of microseconds.

Organic Optoelectronic Materials: Mechanisms and Applications

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.

Sub-Millisecond Response Time in a Photorefractive Composite Operating under CW Conditions

Extensive study of photorefractive polymeric composites photosensitized with semiconductor nanocrystals has yielded data indicating that the inclusion of such nanocrystals enhances the charge-carrier mobility, and subsequently leads to a reduction in the photorefractive response time. Unfortunately, the included nanocrystals may also act as a source of deep traps, resulting in diminished diffraction efficiencies as well as reduced two beam coupling gain coefficients. Nonetheless, previous studies indicate that this problem is mitigated through the inclusion of semiconductor nanocrystals possessing a relatively narrow band-gap. Here, we fully exploit this property by doping PbS nanocrystals into a newly formulated photorefractive composite based on molecular triphenyldiamine photosensitized with C₆₀. Through this approach, response times of 399 μs are observed, opening the door for video and other high-speed applications. It is further demonstrated that this improvement in response time occurs with little sacrifice in photorefractive efficiency, with internal diffraction efficiencies of 72% and two-beam-coupling gain coefficients of 500 cm⁻¹ being measured. A thorough analysis of the experimental data is presented, supporting the hypothesized mechanism of enhanced charge mobility without the accompaniment of superfluous traps. It is anticipated that this approach can play a significant role in the eventual commercialization of this class of materials.

Photorefractive dynamics in poly(triarylamine)-based polymer composites

The photorefractive (PR) response and dynamics are investigated in a methyl-substituted poly(triarylamine) (PTAA)-based PR composite. The charge transfer complex between PTAA and an added small amount of second acceptor, (tris(8-hydroxyquinolinato)aluminium) Alq(3), effectively suppresses the photoconductivity, and thus the sample is able to withstand the dielectric breakdown at a high electric field. The resulting PR response is enhanced at a higher electric field. Sub-millisecond PR response times were observed for both optical diffraction and optical amplification: i.e., 350 μs for optical amplification and 860 μs for optical diffraction observed under 532 nm illumination (0.534 W cm(-2)) at 60 V μm(-1). The response time of optical amplification followed the photocurrent response time of 367 μs.