Crystal-Domain Orientation and Boundary in Highly Ordered Organic Semiconductor Thin Film

Abstract

Conduction of electric charges is often done in polycrystalline materials. Unavoidably, the crystallite size, orientation and domain boundaries (DBs) affect the transport of the charge carriers. It is particularly so for organic semiconductors known to be highly anisotropic and strongly dependent on DBs. Understanding those effects will have a strong impact on improving the performance of organic electronic and optoelectronic devices. Herein, we report our investigation on the crystal-domain orientation and boundary on the charge transport of operating device with copper phthalocyanine (CuPc) thin films grown on parasexiphenyl (p-6P) by kelvin probe force microscopy. In CuPc intra-domains, the voltage drop increases as the angle increases between the domain orientation and the source-drain electric field. In DBs, the potential wells and steep voltage drops were observed. The increase of the DBs width and the angle between the orientations of neighboring domains results in the raise of voltage drop across the DBs, which restrict the charge transport in DBs simultaneously.The mobility of CuPc thin films increases with the domain size, resulting from the reduction of the mismatched orientation degree and the number of DBs.

Uniform electroactive fibre-like micelle nanowires for organic electronics

“…4b), with mobility values ranging over two orders of magnitude, from 2.4 × 10 −5 to 4.4 × 10 −3  cm 2  V −1  s −1 . A similar relationship has been reported between the charge-carrier mobility and (crystal) grain sizes for OFETs based on pentacene52, copper phthalocyanine,53 oligo(thiophene)s54 and P3HT crystals55. In all of these previously reported cases, the increase in mobility was attributed to a decreased density of grain boundaries.…”

High‐Performance Organic Heterojunction Phototransistors Based on Highly Ordered Copper Phthalocyanine/para‐Sexiphenyl Thin Films

“…The mobility of CuPc/p-6P thin films have been reported that a hole field-effect mobility as high as 0.18 cm 2 V −1 s −1 could be obtained, which was about 2 orders of magnitude higher than that of CuPcbased field-effect transistors (FETs) (≈2.1 × 10 −3 cm 2 V −1 s −1 ). [19,20] The results in Figure 2b,c show that, under the dark condition, the off-current of CuPc/p-6P and CuPc phototransistors is about 1.6 × 10 −11 and 5.2 × 10 −11 A, respectively, while the on-current of two devices is about 1.4 × 10 −5 and 2.3 × 10 −6 A, respectively. Because the highly ordered CuPc thin films with large-size grain can greatly improve the charge transport, and the electrical properties of CuPc/p-6P phototransistors are much better than that of CuPc FETs.…”

“…[17,18] Based on disordered CuPc and highly ordered CuPc/p-6P thin films, the corresponding phototransistors were fabricated, and the structure schematic of CuPc/p-6P heterojunction photo transistors is shown in Figure 2a. [19,20] The results in Figure 2b,c show that, under the dark condition, the off-current of CuPc/p-6P and CuPc phototransistors is about 1.6 × 10 −11 and 5.2 × 10 −11 A, respectively, while the on-current of two devices is about 1.4 × 10 −5 and 2.3 × 10 −6 A, respectively. The mobility of CuPc/p-6P thin films have been reported that a hole field-effect mobility as high as 0.18 cm 2 V −1 s −1 could be obtained, which was about 2 orders of magnitude higher than that of CuPcbased field-effect transistors (FETs) (≈2.1 × 10 −3 cm 2 V −1 s −1 ).…”

Energy barriers at grain boundaries dominate charge carrier transport in an electron-conductive organic semiconductor

“…1c ). The strong dependence of µ on the density of grain boundaries is expected and has been experimentally verified multiple times, This is typically explained by the presence of shallow or deep valleys trapping charge carriers at the grain boundary 40 , 44 , 46 – 48 .…”

“…In practice, this means that if the grain boundary density is high, all free charge can be trapped at the grain boundary leading to a depletion of the grain itself 15 , 46 . Finally, in the literature it is typically assumed, that the main mechanism that limits µ upon transport through a grain boundary are energy barriers due to trapped majority carriers 14 , 15 , 45 , 46 , 48 . Again, up to now no attention has been paid to the role of energy barriers at grain boundaries that do not originate from filled valleys, even though one could imagine them to have a dramatic impact on charge transport in the case that they are located at a grain boundary that percolates across the entire channel.…”