Narrow-bandgap pi-conjugated copolymers attract great interest due to their perspective utilization in novel molecular devices [1]. Outstanding properties of such devices can be explained by their bilayer "column-like" morphology and ambipolarity. This causes, e.g., ultrafast charge separation before localization of the primary excitations to form a bound excitons in bulk heterojunctions of their fullerene composites [2]. The type of the initial excitons should also determine functionality and efficiency of electronic devices. Milichko et al. showed [3], that irradiation of a bilayer ordered inorganic composite initiates intralayer excitons inside each layer and interlayer excitons, whose electrons and holes are formed on neighboring layers. Because some organic narrow-bandgap copolymers consist of the donor and acceptor subgroups (ambipolarity), one may expect the formation of the analogous excitons also in their bulk heterojunctions. So, the morphology and ambipolarity of such systems should predetermine their exciton state and electronic properties.

Both charge carriers possess spin. The lifetime of the intralayer excitations and the resultant spin pairs is shorter than that of the less mobile interlayer excitons. This allows one to obtain by the direct light-induced EPR method (LEPR) [4] the correlations of electronic parameters of a composite with the photoinitiated exciton type and the main spin parameters of resulting charge carriers which can be used in organic electronics, photovoltaics and spintronics. This method was used for the study of magnetic resonance, relaxation and dynamic parameters of polarons and methanfullerene radical anions, initiated by the near IR-Vis-UV photons in interfaces formed by narrow-bandgap [(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] (F8T2), poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT) and poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) copolymers with [6,6]-phenyl- C₆₁-butyric acid methyl ester (PC61BM) globules. The formation of spin traps in the copolymer matrix and the exchange interaction between different spin packets cause the extreme sensitivity of these parameters to the number and energy of the initiating photons. Compared to F8T2 and PFO-DBT, the structure of the PCDTBT composite was shown to be more organized, resulting in a decrease in the number of spin traps and a change in the order of the charge recombination process. The decay of free charge carriers is interpreted in terms of the trapping-detrapping spin diffusion in bulk heterojunctions. The recombination of both charge carriers in the F8T2:PC₆₁BM and PFO-DBT:P C₆₁BM can be described in terms of a bimolecular process of the second order, whereas charge decay in the PCDTBT:P C₆₁BM interfaces follows the first order. This evidences the formation of different types of excitons in the compounds under study which controls their magnetic, relaxation and electronic properties.

1. Huang, F.; Yip, H.-L.; Cao, Y., Polymer Photovoltaics: Materials, Physics, and Device Engineering. The Royal Society of Chemistry: Cambridge, 2015.

2. Banerji, N.; Cowan, S.; Leclerc, M.; Vauthey, E.; Heeger, A. J., Exciton Formation, Relaxation, and Decay in PCDTBT. J. Am. Chem. Soc. 2010, 132, 17459-17470.

3. Milichko, V. A.; Makarov, S. V.; Alexey V. Yulin; Vinogradov, A. V.; Krasilin, A. A.; Ushakova, E.; Dzyuba, V. P.; Hey-Hawkins, E.; Pidko, E. A.; Belov, P. A., van der Waals Metal-Organic Framework as an Excitonic Material for Advanced Photonics. Adv. Mater. 2017, 29, 1606034/01 - 1606034/09

4. Lupton, J. M.; McCamey, D. R.; Boehme, C., Coherent Spin Manipulation in Molecular Semiconductors: Getting a Handle on Organic Spintronics. ChemPhysChem. 2010, 11, 3040-3058; Krinichnyi, V. I. EPR Spectroscopy of Polymer:Fullerene Nanocomposites. In Spectroscopy of Polymer Nanocomposites, 1 ed.; Thomas, S.; Rouxel, D.; Ponnamma, D., Eds.; Elsevier: Amsterdam, 2016; pp 202-275; Naveed, K.-u.-R.; Wang, L.; Yu, H.; Ullah, R. S.; Haroon, M.; Fahad, S.; Li, J.; Elshaarani, T.; Khan, R. U.; Nazir, A., Recent Progress in the Electron Paramagnetic Resonance Study of Polymers. Pol. Chem. 2018, 9, 3306-3335.