Chemical Doping of the Organic Semiconductor C8‐BTBT‐C8 Using an Aqueous Iodine Solution for Device Mobility Enhancement

Abstract

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....

Synergistic Effect of Solvent Vapor Annealing and Chemical Doping for Achieving High-Performance Organic Field-Effect Transistors with Ideal Electrical Characteristics

“…The comparison of the UV–vis–NIR spectra of the pristine and treated films did not show any significant difference (Figure S5). The absence of a charge transfer absorption band is in agreement with the fact that no charge transfer process occurs between the C8-BTBT-C8 and iodine . UV resonance Raman spectra on pristine C8-BTBT-C8:PS film and on films doped with I 2 /water and I 2 /CH 3 CN vapors were performed.…”

“…Thin films of blends of 2,7-dioctyl[1]­benzothieno­[3,2- b ]­[1]­benzothiophene (C8-BTBT-C8) and polystyrene (PS) in a mass ratio of 4:1 were prepared on SiO 2 substrates employing the bar-assisted meniscus shearing technique (BAMS, Figure a), as previously reported. , The deposition of these blends by BAMS results in a vertical phase separation of the two materials, where a crystalline layer of the C8-BTBT-C8 semiconductor sits on top of a PS layer that is in contact with the SiO 2 substrate. ,, Gold source-drain top contacts were thermally evaporated through a shadow mask consisting of device motifs with identical channel widths ( W ) but with different channel lengths ( L ) (see Experimental Section).…”

High throughput processing of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) organic semiconductors

“…Recently, it was reported that the treatment of OSC films with aqueous iodine solution resulted in a reduction in R C , improving the effective device mobility. 52 Accordingly, we doped the S-DNTT-10 films following the same methodology (see Experimental section) and we explored again the performance of the OFETs of different L . In Fig.…”

“…Recently, it was reported that the treatment of OSC films with an aqueous iodine solution resulted in a reduction in R C , improving the effective device mobility. 52 Accordingly, we doped the S-DNTT-10 films following the same methodology (see Experimental Section) and we explored again the performance of the OFETs of different L. In Figure S5 representative transfer characteristics of devices before and after doping are shown. It can be clearly observed that after doping the source-drain current increases and the transfer curves are shifted towards positive voltage values.…”

Dopant Diffusion Inhibition in Organic Field-Effect Transistors Using Organic Semiconductor/High-Molecular-Weight Polymer Blends

“…In bottom-contact devices, the metal contacts can be modified with molecular self-assembled monolayers to tune their work function . In addition, chemical doping of the OSC represents also a promising approach that can improve the OFETs contact resistance. , However, controlling the level of OSC doping can be challenging and it can result in over doped devices that exhibit high off-currents. Additionally, doping processes can also impact on the OSC crystallinity affecting the material transport properties.…”