Novel Organic Superbase Dopants for Ultraefficient N-Doping of Organic Semiconductors

Strong Bases and beyond: The Prominent Contribution of Neutral Push-Pull Organic Molecules towards Superbases in the Gas Phase

In this review, the principles of gas-phase proton basicity measurements and theoretical calculations are recalled as a reminder of how the basicity PA/GB scale, based on Brønsted-Lowry theory, was constructed in the gas-phase (PA-proton affinity and/or GB-gas-phase basicity in the enthalpy and Gibbs energy scale, respectively). The origins of exceptionally strong gas-phase basicity of some organic nitrogen bases containing N-sp3 (amines), N-sp2 (imines, amidines, guanidines, polyguanides, phosphazenes), and N-sp (nitriles) are rationalized. In particular, the role of push-pull nitrogen bases in the development of the gas-phase basicity in the superbasicity region is emphasized. Some reasons for the difficulties in measurements for poly-functional nitrogen bases are highlighted. Various structural phenomena being in relation with gas-phase acid-base equilibria that should be considered in quantum-chemical calculations of PA/GB parameters are discussed. The preparation methods for strong organic push-pull bases containing a N-sp2 site of protonation are briefly reviewed. Finally, recent trends in research on neutral organic superbases, leaning toward catalytic and other remarkable applications, are underlined.

Dopant-induced Morphology of Organic Semiconductors Resulting in High Doping Performance

Doping plays a crucial role in modulating and enhancing the performance of organic semiconductor (OSC) devices. In this study, the critical role of dopants is underscored in shaping the morphology and structure of OSC films, which in turn profoundly influences their properties. Two dopants, trityl tetrakis(pentafluorophenyl) (TrTPFB) and N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DMA-TPFB), are examined for their doping effects on poly(3-hexylthiophene) (P3HT) and PBBT-2T host OSCs. It is found that although TrTPFB exhibits higher doping efficiency, OSCs doped with DMA-TPFB achieve comparable or even enhanced electrical conductivity. Indeed, the electrical conductivity of DMA-TPFB-doped P3HT reaches over 67 S cm-1, which is a record-high value for mixed-solution-doped P3HT. This can be attributed to DMA-TPFB inducing a higher degree of crystallinity and reduced structural disorder. Moreover, the beneficial impact of DMA-TPFB on the OSC films' morphology and structure results in superior thermoelectric performance in the doped OSCs. These findings highlight the significance of dopant-induced morphological and structural considerations in enhancing the film characteristics of OSCs, opening up a new avenue for optimization of dopant performance.

Reductive Doping Inhibits the Formation of Isomerization-Derived Structural Defects in N-doped Poly(benzodifurandione) (n-PBDF)

Recently, solution-processable n-doped poly(benzodifurandione) (n-PBDF) has been made through in-situ oxidative polymerization and reductive doping, which exhibited exceptionally high electrical conductivities and optical transparency. The discovery of n-PBDF is considered a breakthrough in the field of organic semiconductors. In the initial report, the possibility of structural defect formation in n-PBDF was proposed, based on the observation of structural isomerization from (E)-2H,2'H-[3,3'-bibenzofuranylidene]-2,2'-dione (isoxindigo) to chromeno[4,3-c]chromene-5,11-dione (dibenzonaphthyrone) in the dimer model reactions. In this study, we present clear evidence that structural isomerization is inhibited during polymerization. We reveal that the dimer (BFD1) and the trimer (BFD2) can be reductively doped by several mechanisms, including hydride transfer, forming charge transfer complexes (CTC) or undergoing an integer charge transfer (ICT) with reactants available during polymerization. Once the hydride transfer adducts, the CTC, or the ICT product forms, structural isomerization can be effectively prevented even at elevated temperatures. Our findings provide a mechanistic understanding of why isomerization-derived structural defects are absent in n-PBDF backbone. It lays a solid foundation for the future development of n-PBDF as a benchmark polymer for organic electronics and beyond.

Charge transfer in superbase n-type doping of PCBM induced by deprotonation

N-type electronic doping of organic semiconductors (OSCs) by using superbase compounds shows high doping efficiency (H. Wei, Z. Cheng, T. Wu, Y. Liu, J. Guo, P.-A. Chen, J. Xia, H. Xie, X. Qiu, T. Liu, B. Zhang, J. Hui, Z. Zeng, Y. Bai and Y. Hu, Adv. Mater. 2023, 35, 2300084). While a deprotonation reaction is believed to trigger the doping process, the detailed mechanism therein is not yet fully understood. In the present work we theoretically study the electronic structure of the deprotonated Phenyl-C61-butyric acid methyl ester (PCBM) molecule, as well as the charge transfer (CT) between PCBM and its deprotonated species. We find that deprotonated PCBM without formation of a new bond between the deprotonated side chain and fullerene induces electronic structure with broken spin symmetry, where an in-gap state is singly occupied by an unpaired electron. A second scenario that we find to be possible is the formation of a new bond between the deprotonated side chain and a fullerene. This leads to a spin symmetric electronic structure with partially localized in-gap state, which is expected to contribute less to the effective doping. These results show that the deprotonated PCBM species without new bond formation predominantly accounts for the effective n-type doping of PCBM, an insight that will be useful for optimization of this recently discovered doping method.