| Abstract Scope |
Structure-property relations have guided the development and processing of organic electronic materials and devices over the past two decades, leading to films with field effect mobilities around 1cm<sup>2</sup>/Vs. Such studies have focused on relative crystallinity, grain boundaries, film cracking, as well as molecular packing and orientation, in attempt to understand device-scale charge transport and performance. Grain size or grain boundary density is often cited as a reason for improved or degraded device performance. However, even the determination of grain size can be difficult: grain dimensions are often extracted from top-surface scanning probe techniques, rough estimation from X-ray peak widths or optical microscopy of crystalline domains with similar polarization extinction characteristics; the latter technique is especially common (and sometimes misleading) in small molecule films. Furthermore, the details of intra-grain order are important when considering charge transport within a grain. Deviations from ideal crystal packing will affect the crystalline transport and may shed light on the origins of intrinsic charge trapping in semi/polycrystalline organic materials. The study of disorder within crystalline grains is challenging due to the instability of organic materials under the prolonged electron beam exposure necessary for transmission electron microscopy. The small grain size, often on the order of tens of nanometers, further complicates characterization. One promising analysis technique is order-dependent Fourier X-ray peak shape analysis, developed by Warren and Averbach in the early 1950’s. The Warren-Averbach (WA) analysis allows one to simultaneously extract grain size, as well as parameters relating to non-uniform strain and statistical disorder (paracrystallinity) within the grain, both of which are the primary causes of higher-order peak broadening.
To this end, we present a series of datasets with crystalline order both out-of-plane and in-plane and discuss the implementation of WA analysis as a tool to extract disorder parameters. Diffraction data is collected from a number of solution processable, high performance transistor and solar cell materials, including poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-<i>b</i>]thiophenes) (PBTTT), poly{[N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6,-diyl]-alt-5,5’-(2,2’-bithiophene)}, P(NDI2OD-T2), and triisopropylsilane (TIPS) pentacene. We discuss how parameters extracted from WA analysis may influence charge transport. Experiments are designed to elucidate the effects of annealing, alignment and blending on grain size, inter-grain disorder and strain for a number of crystallographic directions. In general, we find that polymer grains exhibit paracrystalline disorder on the order of 2-6%, and small molecules <1%. We propose that this numerical representation of deviations from ideal crystalline order along with crystalline mobility (determined from device data fitting) and simulations is important for understanding crystalline transport and trapping in organic semiconductor thin films. |