Stable and Tunable Chemical Doping for Organic Semiconductors in Aqueous Solution under Ambient Conditions

Doping levels in chemical doping of organic semiconductors are determined by semiconductor and dopant energy levels. While precise control of doping is required to fabricate advanced devices, as seen in silicon semiconductors, control in doping levels at a scale of thermal energy (25 meV) has been challenging for organic semiconductors. Most of dopants are unstable under ambient conditions owing to the reactivity with water or oxygen in air, which further decreases precision in doping levels. Low precision in doping levels and need for inert processing environment have been bottlenecks for developing and manufacturing advanced organic semiconductor devices employing chemical doping.<br/>In this study, doping levels of organic semiconductors were controlled reproducibly and precisely in aqueous dopant solution under ambient conditions [1]. In our method, chemical doping is based on proton-coupled electron transfer (PCET) reactions that are widely observed in biochemistry, unlike conventional methods that employ single electron transfer reactions. Our aqueous dopant solutions contain dopant salts, pH adjusting agents and PCET-based redox couples. As a PCET-based redox agent, for instance, Benzoquinone/Hydroquinone (BQ/HQ) couple was employed. Through redox reactions, BQ gets transformed into HQ by accepting two electrons and two protons, where its redox potential depends on proton activity, pH, of the aqueous solution following the Nernst equation. A well-known p-type semiconducting polymer, poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT), was doped by BQ/HQ doping solutions. PBTTT got oxidized when the redox potential of BQ/HQ exceeded the Fermi level of PBTTT in a low pH environment. The positive charge introduced into PBTTT was compensated by intercalation of anions in aqueous solution similarly to anion-exchange doping in organic solvents [2]. Bis(trifluoromethanesulfonyl)imide (TFSI) or bis(nonafluorobutanesulfonyl)imide (NFSI) was employed as dopant anion introduced into PBTTT, which would realize stable doped state owing to their closed-shell structures and hydrophobicity [3].<br/>Doping level of PBTTT was controlled by the pH value of dopant aqueous solution, where the redox potential of BQ/HQ showed pH dependence of 59 mV/pH. According to the optical absorption and conductivity measurement, the doping level of PBTTT increases with decreasing solution pH. Tuning of doping levels were also confirmed by structural analysis based on X-ray diffraction and elemental analysis based on X-ray photoelectron spectroscopy measurements. Doping levels of one PBTTT film were modulated repeatedly by cycles of increase and decrease in solution pH, which demonstrates that both of oxidation and reduction of organic semiconductors take place in our system that develops chemical equilibrium. It was verified that one can control the Fermi level of organic semiconductors reproducibly with a high degree of precision, <i>ca.</i> thermal energy of 25 meV at RT, over a few hundred meV around the band edge. This is a critical range to control electronic properties of semiconductors.<br/>Our PCET-based system provides a facile, scalable, precisely controlled, reproducible and reliable chemical doping method, which would lead to ambient processed organic electronics and bioelectronics.<br/>[1] M. Ishii, Y. Yamashita, S. Watanabe, K. Ariga, J. Takeya, <i>under review</i>, preprint DOI: 10.21203/rs.3.rs-2011544/v1.<br/>[2] Y. Yamashita, J. Takeya, S. Watanabe et al., <i>Nature</i> <b>572</b>, 634–638 (2019).<br/>[3] Y. Yamashita, J. Takeya, S. Watanabe et al., <i>Commun. Mater.</i> <b>2</b>, 45 (2021).