The lack of long-term stability in thin films of organic semiconductors can often be caused by the low structural stability of metastable phases that are frequently formed upon deposition on a substrate surface. Here, thin films of 2,7-dioctyloxy[1]benzothieno[3,2-b]benzothiophene (C 8 O-BTBT-OC 8) and blends of this material with polystyrene by solution shearing are fabricated. Both types of films exhibit the metastable surface-induced herringbone phase (SIP) in all the tested coating conditions. The blended films reveal a higher device performance with a field-effect mobility close to 1 cm 2 V −1 s −1 , a threshold voltage close to 0 V, and an on/off current ratio above 10 7. In situ lattice phonon Raman microscopy is used to study the stability of the SIP polymorph. It is found that films based on only C 8 O-BTBT-OC 8 slowly evolve to the Bulk cofacial phase, significantly impacting device electrical performance. In contrast, the blended films stabilize the SIP phase, leading to devices that maintain a high performance over 1.5 years. This work demonstrates that blending small-molecule organic semiconductors with insulating binding polymers can trap metastable polymorphs, which can lead to devices with both improved performance and long-term stability.

The present work assesses improved carrier injection in organic field-effect transistors by contact doping and provides fundamental insight into the multiple impacts that the dopant/semiconductor interface details have on the long-term and thermal stability of devices. We investigate donor [1]benzothieno-[3,2-b]-[1]benzothiophene (BTBT) derivatives with one and two octyl side chains attached to the core, therefore constituting asymmetric (BTBT-C8) and symmetric (C8-BTBT-C8) molecules, respectively. Our results reveal that films formed out of the asymmetric BTBT-C8 expose the same alkyl-terminated surface as the C8-BTBT-C8 films do. In both cases, the consequence of depositing fluorinated fullerene (C 60 F 48 ) as a molecular p-dopant is the formation of C 60 F 48 crystalline islands decorating the step edges of the underlying semiconductor film surface. We demonstrate that local work function changes along with a peculiar nanomorphology lead to the double beneficial effect of lowering the contact resistance and providing long-term and enhanced thermal stability of the devices.

Despite these encouraging results, the performance of OFETs is still severely limited by factors such as contact resistance (R c ) [9] and charge trapping. [10] When the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO) of a p-type or n-type OSC, respectively, and the electrodes work-function are not aligned, charge injection is significantly hindered by the high energy barrier and high contact resistance values are extracted. [11] In order to confront these issues, different electrode engineering approaches have been proposed. For example, the work-function of the metal electrodes can be modified to match the OSC energy level by using self-assembled molecular monolayers (SAMs) in bottom-contact devices or by inserting a charge injection layer in top-contact OFETs. [12][13][14] Another source that prevents OSCs from realizing their instrinsic charge carrier mobilities is charge trapping. In the energy gap of OSCs, electronic states can appear due to the presence of chemical impurities or defects that trap mobile charge carriers, thus causing OFETs to deviate from the ideal behavior. [15] It has been previously reported that passivation of the dielectric layer or the use of OSC:insulating polymer blends are appealing routes to decrease the dielectric interfacial trap density. [16] Nonetheless, traps are also present at the metal-OSC interface, grain boundaries and thin film structural inhomogeneities. [15] Chemical doping is a suitable way to modify the electronic properties of OSCs, which consists in adding a small percentage of species able to donate (n-doping) or accept (p-doping) an electron to or from the OSC, respectively. Doping of semiconductors is a well-established strategy in inorganic transistors with great success also in organic optoelectronics, especially in OLEDs and solar cells. [17][18][19][20][21][22] Regarding OFET devices, doping has mainly been exploited to increase device mobility, adjust the threshold voltage, fill up trap states, or to improve charge injection by contact doping. [23] Although recent works have demonstrated that doping can be a key enabler for high performing OFETs, the progress of organic semiconductors doping is stillThe performance of organic field-effect transistors is still severely limited by factors such as contact resistance and charge trapping. Chemical doping is considered to be a promising key enabler for improving device performance, although there is a limited number of established doping protocols as well as a lack of understanding of the doping mechanisms. Here, a very simple doping methodo logy based on exposing an organic semiconductor thin film to an aqueous iodine solution is reported. The doped devices exhibit enhanced device mobility, which becomes channel-length independent, a decreased threshold voltage and a reduction in the density of interfacial traps. The device OFF current is not altered, which is in agreement with the spectroscopic data that points out that no charge transfer processes are occurring....

Establishing the rather complex correlation between structure and charge transfer in organicorganic heterostructures is of utmost importance for organic electronics and requires spatially resolved structural, chemical and electronic details. Insight in this issue is provided here by combining atomic force microscopy, Kelvin probe force microscopy, photoemission electron microscopy and low-energy electron microscopy for investigating a case study. We select the interface formed by pentacene (PEN), benchmark among the donor organic semiconductors, and a p-type dopant from the family of fluorinated fullerenes. As for Buckminsterfullerene (C60), the 2 growth of its fluorinated derivative C60F48 is influenced by thickness and crystallinity of the PEN buffer layer, but the behaviour is markedly different. We provide a microscopic description of the C60F48/PEN interface formation and analyse the consequences in the electronic properties of the final heterostructure. For just one single layer of PEN, a laterally complete but non-compact C60F48/PEN interface is created, importantly affecting the surface work function. Nonetheless, from the very beginning of the second layer formation, the presence of epitaxial and non-epitaxial PEN domains dramatically influences the growth dynamics and extremely well packed twodimensional C60F48 islands develop. Insightful element maps of the C60F48/PEN surface spatially resolve the non-uniform distribution of the dopant molecules, which leads to a heterogeneous work function landscape.

Two derivatives of [1]benzothieno[3,2-b][1]benzothiophene (BTBT), namely, 2,7-dioctyl-BTBT (C8-BTBT) and 2,7-diphenyl-BTBT (DPh-BTBT), belonging to one of the best performing organic semiconductor (OSC) families, have been employed to investigate the influence of the substitutional side groups on the properties of the interface created when they are in contact with dopant molecules. As a molecular p-dopant, the fluorinated fullerene C 60 F 48 is used because of its adequate electronic levels and its bulky molecular structure. Despite the dissimilarity introduced by the OSC film termination, dopant thin films grown on top adopt the same (111)-oriented FCC crystalline structure in the two cases. However, the early stage distribution of the dopant on each OSC film surface is dramatically influenced by the group side, leading to distinct host−dopant interfacial morphologies that strongly affect the nanoscale local work function. In this context, Kelvin probe force microscopy and photoelectron emission spectroscopy provide a comprehensive picture of the interfacial electronic properties. The extent of charge transfer and energy level alignment between OSCs and dopant are debated in light of the differences in the ionization potential of the OSC in the films, the interface nanomorphology, and the electronic coupling with the substrate.