Mechanism of integer charge transfer (ICT) mediated doping and Polaron-Assisted Near-Infrared Photodetection (IMAGE)

Light Publishing Center, Changchun Institute of Optics, Fine Mechanics And Physics, CAS

Mechanism of integer charge transfer (ICT) mediated doping and Polaron-Assisted Near-Infrared Photodetection

Caption

Mechanism of integer charge transfer (ICT) mediated doping and Polaron-Assisted Near-Infrared Photodetection a, Flat energy band diagrams before and after doping. The doping process involves the C-14PBTTT polymer acting as a Donor (D), while the TCNQF4 dopant molecule functions as an acceptor (A). In systems where the Ionization Energy of a conjugated polymer D is lower than the Electron Affinity of A, the doping mechanism follows the ICT model. In this model, upon doping, an electron is transferred entirely from the D to the A, resulting in the creation of  D+A- on the polymer chain. This localized structural distortion of the chain, coupled with D+ formation, defines a positive polaron. This polaron formation is accompanied by the creation of a singly occupied energy level above the valence band edge and an unoccupied energy level below the conduction band edge. b, Distinction between Frenkel exciton（Fex）and polaron excitations (P1, P2). These newly formed sub-gaps result in two strong subgap absorptions in the IR region. These absorptions indicate the transition from the valence band edge to the half-filled subgap level (P1, 0.95-2.5 µm) and the transition from the lower to the higher subgap level (P2, 0.7-0.95 µm). In contrast, the transition from the valence band edge to the conduction band in the UV-Vis region (0.2-0.7 µm) is attributed to the transition of neutral molecules (Fex transition). c, This ICT-based type II bulk heterojunction comprises a solitary layer of radical and neutral TCNQF4/C14-PBTTT molecules, serving dual roles as the channel and the photoactive layer. 1) This process involves the excitation of electrons to the subgap level, 2) followed by polarons (hole) transfer, and 3) subsequent liberation of bound polarons into free charge carriers under an external electric field, enhancing sensitivity to IR light. The peak EQE of 107 % at 1.0 μm demonstrates the effective intramolecular migration of P1-excited polarons, transitioning into free polarons.

Credit

by Muhammad Ahsan Iqbal et al.

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