Abstract
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.
Highlights
Owing to their large optical nonlinearities, low permittivity and low cost, organic photorefractive (PR) composites are potentially useful in a variety of real-time optical applications[1,2,3,4,5]
In addition to the charge-transporting molecular TPD, the composite described contains the non-linear optical (NLO) dye 4-homopiperidinobenzylidenemalononitrile (7-DCST) providing the electro-optic activity required for the PR effect25. 7-DCST has been used in conjunction with PVK as well as poly(acrylic tetraphenyldiaminobiphenyl) (PATPD) and is a common choice of NLO dye for PR composites in which the optimization of τ is of interest[24,25,30,31,32,33]
Since the primary goal of this study was to study the performance of a PR composite in which the charge-carrier μ is enhanced through the inclusion of narrow band-gap Q-dots, there was no need for the Q-dots to perform as the photosensitizer as in most of the previous studies involving the inclusion of Q-dots in an otherwise all-organic PR composite
Summary
Owing to their large optical nonlinearities, low permittivity and low cost, organic photorefractive (PR) composites are potentially useful in a variety of real-time optical applications[1,2,3,4,5]. In addition to acting as a photosensitizer, a significant amount of experimental data, from time-of-flight characterizations, indicate the included Q-dots increase the charge-carrier mobility, μ, within the PR composite[10] This enhancement in μ is primarily attributed to the ability of the free charge-carriers to enter into, and subsequently be transported through the included Q-dots, where they experience a faster μ relative to that attributed to the organic charge-transporting species. In a separate study it was established that by exchanging the relatively wide band-gap QCdSe with QCdTe, which possesses a comparatively narrower band-gap, an improvement in the τ was realized[17] These studies provided strong evidence that in addition to the photosensitization of PR composites, Q-dots may alternatively be used for enhancing the μ and thereby substantially reducing the τ of a PR composite. This significant decrease in τ is especially exciting due to the ability to avoid the detrimental effect over PR efficiency, with ηext decreasing by only 4%, to 59%, as a result of introducing the QPbS into the composite
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