page_title: Organic photorefractive materials
section_title: Organic Photorefractive Materials
hierarchical_section_title: Organic photorefractive materials / Organic Photorefractive Materials
caption_attribution_description: English: Electronic states of PATPO
context_page_description: Organic photorefractive materials are materials that exhibit a temporary change in refractive index when exposed to light. The changing refractive index causes light to change speed throughout the material and produce light and dark regions in the crystal. The buildup can be controlled to produce holographic images for use in biomedical scans and optical computing. The ease with which the chemical composition can be changed in organic materials makes the photorefractive effect more controllable.
context_section_description: In general, photorefractive materials can be classified into the following categories, the border between categories may not be sharp in each case
  Inorganic crystal and compound semiconductor
  Multiple quantum well structures
  Organic crystalline materials
  Polymer dispersed Liquid crystalline materials (PDLC)
  Organic amorphous materials
  In the field of this research, initial investigations were mainly carried out with inorganic semiconductors. There have been huge varieties of inorganic crystals such as BaTiO₃, KNbO₃, LiNbO₃ and inorganic compound semiconductors such as GaAs, InP, CdTe are reported in literature.
  First photorefractive (PR) effect in organic materials was reported in 1991 and then, research of organic photorefractive materials has drawn major attention in recent years compare to inorganic PR semiconductors. This is due to mainly cost effectiveness, relatively easy synthetic procedure, and tunable properties through modifications of chemical or compositional changes.
  Polymer or polymer composite materials have shown excellent photorefractive properties of 100% diffraction efficiency. Most recently, amorphous composites of low glass transition temperature have emerged as highly efficient PR materials. These two classes of organic PR materials are also mostly investigated field.
  These composite materials have four components -conducting materials, sensitizer, chromophore, and other dopant molecules to be discussed in terms of PR effect. According to the literature, design strategy of hole conductors is mainly p-type based  and the issues on the sensitizing are accentuated on n-type electron accepting materials, which are usually of very low content in the blends and thus do not provide a complementary path for electron conduction.
  In recent publications on organic PR materials, it is common to incorporate a polymeric material with charge transport units in its main or side chain. In this way, the polymer also serves as a host matrix to provide the resultant composite material with a sufficient viscosity for reasons of processing. Most guest-host composites demonstrated in the literature so far are based on hole conducting polymeric materials.
  The vast majority of the polymers are based on carbazole containing polymers like poly-(N-vinyl carbazole) (PVK) and polysiloxanes (PSX). PVK is the well studied system for huge varieties of applications.
  In polymers, charge is transported through the HOMO and the mobility is influenced by the nature of the dopant mixed into the polymer, also it depends on the amount of dopant which may exceed 50 weight percent of the composite for guest-host materials.
  The mobility decreases as the concentration of charge-transport moieties decreases, and the dopant's polarity and concentration increases.
  Besides the mobility, the ionization potential of the polymer and the respective dopant has also significant importance. The relative position of the polymer HOMO with respect to the ionization potential of the other components of the blends determines the extent of extrinsic hole traps in the material.
  TPD (tetraphenyldiaminophenyl) based materials are known to exhibit higher charge carrier mobilities and lower ionization potentials compare to carbazole based (PVK) materials. The low ionization potentials of the TPD based materials greatly enhance the photoconductivity of the materials.  This is partly due to the enhanced complexation of the hole conductor, which is an electron donor, with the sensitizing agents, which is an electron acceptor.
  It was reported a dramatic increase of the photogeneration efficiency from 0.3% to 100% by lowering the ionization potential from 5.90 eV (PVK) to 5.39 eV ( TPD derivative PATPD). This is schematically explained in the diagram using the electronic states of PVK and PATPD.
caption: Electronic states of PATPD (a) and PVK (b)-based composite with DBDC and 7-DCST as chromophores and C60 as sensitizer
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