Packaging of organic photovoltaic (OPV) devices is an important issue which has been rarely addressed in the past. With the recent reports of high efficiency organic photovoltaics (6%), the need to produce materials which can effectively protect the device from degradation due to exposure to oxygen, moisture and radiation is pressing. We report a novel Saran (a co-polymer of vinylidene chloride and acrylonitrile) based polymer nanotube composite, which shows high transparency in the visible region, good barrier properties and thermal stability, for use as an encapsulant for OPV devices. Different loadings of Saran and boron nitride nanotubes were taken and the composites were prepared to optimize the composition of the composite. UV-visible spectroscopy, infra-red spectroscopy and thermal analysis were used to characterize the composite. The barrier properties of the composite were tested on poly(3-hexylthiophene), which is used in high efficiency OPV devices.
Purpose:
To analytically estimate first‐order x‐ray scatter for kV cone beam x‐ray imaging with high computational efficiency.
Methods:
In calculating first‐order scatter using the Klein‐Nishina formula, we found that by integrating the point‐to‐point scatter along an interaction line, a “pencil‐beam” scatter kernel (BSK) can be approximated to a quartic expression when the imaging field is small. This BSK model for monoenergetic, 100keV x‐rays has been verified on homogeneous cube and cylinder water phantoms by comparing with the exact implementation of KN formula. For heterogeneous medium, the water‐equivalent length of a BSK was acquired with an improved Siddon's ray‐tracing algorithm, which was also used in calculating pre‐ and post‐ scattering attenuation. To include the electron binding effect for scattering of low‐kV photons, the mean corresponding scattering angle is determined from the effective point of scattered photons of a BSK. The behavior of polyenergetic x‐rays was also investigated for 120kV x‐rays incident to a sandwiched infinite heterogeneous slab phantom, with the electron binding effect incorporated. Exact computation and Monte Carlo simulations were performed for comparisons, using the EGSnrc code package.
Results:
By reducing the 3D volumetric target (o(n3)) to 2D pencil‐beams (o(n2)), the computation expense can be generally lowered by n times, which our experience verifies. The scatter distribution on a flat detector shows high agreement between the analytic BSK model and exact calculations. The pixel‐to‐pixel differences are within (‐2%, 2%) for the homogeneous cube and cylinder phantoms and within (0, 6%) for the heterogeneous slab phantom. However, the Monte Carlo simulation shows increased deviation of the BSK model toward detector periphery.
Conclusion:
The proposed BSK model, accommodating polyenergetic x‐rays and electron binding effect at low kV, shows great potential in efficiently estimating the first‐order scatter from small imaging fields. We are investigating more thoroughly to improve performance and explore applications.
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