Thermal Annealing Reduces Geminate Recombination in TQ1:N2200 All-Polymer Solar Cells
Khan, Jafar Iqbal
KAUST DepartmentPhysical Sciences and Engineering (PSE) Division
KAUST Solar Center (KSC)
Materials Science and Engineering Program
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AbstractA combination of steady-state and time-resolved spectroscopic measurements is used to investigate the photophysics of the all-polymer bulk heterojunction system TQ1:N2200. Upon thermal annealing a doubling of the external quantum efficiency and an improved fill factor (FF) is observed, resulting in an increase in the power conversion efficiency. Carrier extraction is similar for both blends, as demonstrated by time-resolved electric-field-induced second harmonic generation experiments in conjunction with transient photocurrent studies, spanning the ps-µs time range. Complementary transient absorption spectroscopy measurements reveal that the different quantum efficiencies originate from differences in charge carrier separation and recombination at the polymer-polymer interface: in as-spun samples ~35 % of the charges are bound in interfacial charge-transfer states and recombine geminately, while this pool is reduced to ~7 % in thermally-annealed sample, resulting in higher short-circuit currents. Time-delayed collection field experiments demonstrate a field-dependent charge generation process in as-spun samples, which reduces the FF. In contrast, field-dependence of charge generation is weak in annealed films. While both devices exhibit significant non-geminate recombination competing with charge extraction, causing low FFs, our results demonstrate that the donor/acceptor interface in all-polymer solar cells can be favourably altered to enhance charge separation, without compromising charge transport and extraction.
CitationKaruthedath S, Melianas A, Kan Z, Pranculis V, Wohlfahrt M, et al. (2018) Thermal Annealing Reduces Geminate Recombination in TQ1:N2200 All-Polymer Solar Cells. Journal of Materials Chemistry A. Available: http://dx.doi.org/10.1039/C8TA01692E.
SponsorsThe research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST). M. Wohlfahrt acknowledges a VSRP internship from KAUST. Research in Linköping University was funded by the Knut and Alice Wallenberg foundation and the Swedish Science council VR.
PublisherRoyal Society of Chemistry (RSC)
JournalJournal of Materials Chemistry A