Electrochemically induced charge injection in disordered organic conductive polymers

This paper deals with the electrochemically induced charge injection in the conductive polymer (CP), exemplified by well examined archetypal CP-poly(3-hexylthiophene-2,5-diyl). The polar solvent of acetonitrile with salt tetrabutylammonium hexafluorophosphate was used to transport electrons in the electrolyte. The decisive mechanism is the recombination current at the electrolyte/CP interface taking place at the Fermi energy of CP, whose energy position is determined by the externally applied voltage. The corresponding mechanism of the charge carrier transport in the polymer bulk is the space-charge limited current (SCLC) by holes or electrons (or more precisely positive and negative polarons) at the respective transport paths of HOMO and LUMO bands. The charge transport mechanisms and the occupation statistics are the basis of the energy-resolved electrochemical impedance spectroscopy for the mapping of the density of electronic states of conductive organic semiconductors [F. Schauer, V. Nadazdy, and K. Gmucova, J. Appl. Phys. 123, 161590 (2018)]. From the application point of view, the major message of the paper is that it is possible to pass high current densities of the order of 0.1 A cm(-2) via electrochemical systems with the CP, induced by means of doping processes of both CP surface and its bulk, leading to the charge injection and SCLC in CP. Published by AIP Publishing.Slovak Research and Development Agency [APVV-14-0891, APVV-0096-11]; Scientific Grant Agency VEGA [1/0501/15, 2/0163/17, 2/0081/18]; project Efficient controlling of the production and consumption of energy from renewable sources, ITMS [26240220028]; Research and Development Operational Programme - ERD

Thermally-Induced Degradation in PM6:Y6-Based Bulk Heterojunction Organic Solar Cells

Thermally induced degradation of organic photovoltaic devices hinders the commercialization of this emerging PV technology. Thus, a precise understanding of the origin of thermal device instability, as well as identifying strategies to circumvent degradation is of utmost importance. Here, it investigates thermally-induced degradation of state-of-the-art PBDB-T-2F (PM6):BTP (Y6) bulk heterojunction solar cells at different temperatures and reveal changes of their optical properties, photophysics, and morphology. The open-circuit voltage and fill factor of thermally degraded devices are limited by dissociation and charge collection efficiency differences, while the short-circuit current density is only slightly affected. Energy-resolved electrochemical impedance spectroscopy measurements reveal that thermally degraded samples exhibit a higher energy barrier for the charge-transfer state to charge-separated state conversion. Furthermore, the field dependence of charge generation, recombination, and extraction are studied by time-delayed collection field and transient photocurrent and photovoltage experiments, indicating significant bimolecular recombination limits device performance. Finally, coupled optical-electrical device simulations are conducted to fit the devices’ current-voltage characteristics, enabling us to find useful correlations between optical and electrical properties of the active layers and device performance parameters

Influence of thermal annealing on microstructure, energetic landscape and device performance of P3HT:PCBM-based organic solar cells

Thermal annealing alters the morphology of organic donor-acceptor bulk-heterojunction thin films used in organic solar cells. Here, we studied the influence of thermal annealing on blends of amorphous regio-random (RRa) and semi-crystalline regio-regular (RR) poly (3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C _60 -butyric acid methyl ester. Since the P3HT:PCBM blend is one of the most studied in the OPV community, the existing research provides a solid foundation for us to compare and benchmark our innovative characterization techniques that have been previously under-utilized to investigate bulk heterojunction organic thin films. Here, we combine advanced novel microscopies and spectroscopies, including polarized light microscopy, photo-deflection spectroscopy, hyperspectral photoluminescence imaging, and energy resolved-electrochemical impedance spectroscopy, with structural characterization techniques, including grazing-incidence wide-angle x-ray scattering, grazing-incidence x-ray diffraction, and Raman spectroscopy, in order to reveal the impact of thermal annealing on the microstructural crystallinity and morphology of the photoactive layer in organic solar cells. Coupled transfer matrix and drift-diffusion simulations were used to study the impact of the density of states on the solar cells’ device performance parameters, namely the short-circuit current ( J _SC ), open circuit voltage ( V _OC ), fill factor (FF), and power conversion efficiency (PCE)