image: 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.
Organic infrared (IR) photodetectors encounter significant obstacles in achieving high external quantum efficiencies (EQE) due to inadequate Frenkel exciton dissociation (high exciton binding energy) within organic IR materials, exacerbated by pronounced non-radiative recombination at narrow bandgaps. Current approaches, such as integrating IR-absorbing organic materials with high-mobility materials like graphene, have resulted in reduced device sensitivity. This highlights the critical need for novel IR organic materials and innovative strategies to achieve both high EQE as well as ultra-high sensitivity without relying on hybrid structures.
In a new paper published in Light: Science & Applications, a team of scientists led by Professor Xueqian Fang and Dr. Muhammad Ahsan Iqbal from the School of Environment and Civil Engineering and the Guangdong Provincial Key Laboratory of Intelligent Disaster Prevention and Emergency Technologies for Urban Lifeline Engineering, Dongguan University of Technology, China, in collaboration with Professor Yu-Jia Zeng from the Key Laboratory of Optoelectronic Devices and Systems of the Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, China, and co-workers, has proposed a novel strategy utilizing ICT between D-A molecules. ICT forms neutral and radical D-A blended molecules, introducing a high degree of structural and energetic disorder that broadens the density of states (low-energy IR states). This localized structural distortion of the donor polymer chain, along with the formation of D+, defines a positive polaron. Although most of these polarons are typically Coulombically bound by the anion, polaron excitation (low-energy IR states) in these radical and neutral D-A blended molecules enables bound charges to overcome Coulombic attraction to their counterions. This can lead to an elevated EQE in the polaron absorption region compared to Frenkel excitons. Moreover, strong low-energy subgap absorptions in the IR region offer a pathway to high performance in next-generation IR photodetectors without hybrid structures.
These scientists claim outstanding results of their infrared photodetector:
“By blending radical and neutral Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (C-14PBTTT) donor polymer with Tetrafluorotetracyanoquinodimethane (TCNQF4) acceptor molecules forms a bulk type-II heterojunction, leading to remarkable femtowatt optoelectronic performance with high EQE of ~1.57 × 107 %, and femtowatt sensitivity (NEP) of ~0.12 fW/√Hz at 1 µm, outperforming traditional inorganic detectors.”
“The efficiency stems from the rapid intramolecular migration of IR-excited polarons, transitioning into free polarons. The performance divergence between visible and IR regions highlights the superiority of polaron excitons over Frenkel excitons in IR photodetection.” They added.
These discoveries not only underscore the capacity of organic materials to outperform conventional inorganic alternatives but also lay the groundwork for future advancements in IR sensing technology.” the scientists forecast.
Regarding the significance of this work, these scientists' approach is innovative not only in its utilization of polaron mechanisms to enhance IR photodetection compared to Frenkel excitons but also in its simplicity of device fabrication and structure. Their work introduces a novel experimental framework/strategy for leveraging polarons to enhance charge separation and transport in narrow bandgap materials, minimizing non-radiative recombination losses. Their work demonstrates that significant performance improvements can be achieved with straightforward methods and commercially available materials, offering competitive results in the field of NIR photodetection.
Journal
Light Science & Applications
Article Title
Unlocking High-Performance Near-Infrared Photodetection: Polaron-Assisted Organic Integer Charge Transfer Hybrids