Although it has been clearly elucidated that the change in the overall bulk-heterojunction (BHJ) morphology of PM6:Y6 induced by the difference in solvents results in a variation in the performance of the entire solar cell, the charge dynamics during the charge separation process induced by morphological changes have not yet been deeply studied. Based on mobility and photocurrent-related studies, it has been inferred that holes are deeply involved in these charge generation and separation processes. The decrease in exciton dissociation probability observed in the PM6:Y6 solar cell fabricated using CB (PM6:Y6-CB) was found to be due to the change in the hole transfer state caused by morphological changes. For the PM6:Y6 solar cell fabricated using CF (PM6:Y6-CF), it was determined that the density of the state (DOS) of the effective hole transfer state (hECT) was small, as hECT and Y6 HOMO were almost degenerate, and the lower hECT, which can interfere with hole transfer, was also formed minimally. However, for PM6:Y6-CB, the overlapping region of hECT and the highest occupied molecular orbital (HOMO) of Y6 shifted towards the lower energy side (upper hECT), and a lower effective hECT that can act as a defect was significantly formed. These facts obtained by EL deconvolution were clearly confirmed in time-resolved photoinduced absorption spectroscopy. As a result, it is concluded that the decrease in fill factor (FF) and current density (JSC) in PM6:Y6-CB is due to the degradation of hole transfer from Y6 HOMO to PM6 HOMO. This analysis shows that the morphological changes in non-fullerene acceptor (NFA) solar cells affect the formation of hole transfer levels, which in turn affect the charge separation efficiency.
Conducting and semiconducting polymers are important materials in the development of printed, flexible, large area
electronics such as flat panel displays and photovoltaic cells. There has been rapid progress in developing conjugated
polymers with high transport mobility required for high performance field effect transistors (FETs), beginning with
mobilities around 10-5cm2/Vs to a recent report of 1cm2/Vs for poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno
[3,2-b]thiophene) (PBTTT). In this work, the electrical properties of PBTTT are studied at high charge densities both as the
semiconductor layer in FETs and in electrochemically doped films to determine the transport mechanism. We show that
data obtained using a wide range of parameters (temperature, gate-induced carrier density, source-drain voltage and
doping level) scale onto the universal curve predicted for transport in the Luttinger Liquid description of the onedimensional
"metal", where fermions along the 1D chain collectively behave as bosons, and where charge and spin are
decoupled.
Polymer field-effect transistors with a field-effect mobility of μ ≈0.3 cm2/V.s have been demonstrated using
regioregular poly(3-hexylthiophene) (rr-P3HT). Devices were fabricated by dip-coating the semiconducting polymer
followed by annealing at 150°C for 10 minutes. The heat annealed devices exhibit an increased field-effect mobility
compared with the as-prepared devices. Morphology studies and analysis of the channel resistance demonstrate that the
annealing process increases the crystallinity of rr-P3HT and improves the contact between the electrodes and the P3HT
films, thereby increasing the field effect mobility of the films. Based on the results obtained from unipolar FETs using rr-
P3HT, we have also applied postproduction heat treatment to ambipolar polymer FETs fabricated with rr-P3HT and C61-
butyric acid methyl ester (PCBM). Devices were fabricated using aluminum (Al) source and drain electrodes to achieve
an equivalent injection for the both holes and electrons. As the case of P3HT unipolar FETs, the thermal annealing
method also improves the film morphology, crystallinity, and the contact properties between Al and active layer, thereby
resulting in excellent ambipolar characteristics with the hole mobility of 1.7×10-3 cm2/V.s and the electron mobility of
2.0×10-3 cm2/V.s.
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