While regulation of the nanoscale microstructure of the active layers in organic bulk heterojunction (BHJ) solar cells, particularly for conjugated polymer–fullerene blend systems, has been shown to be highly important when maximizing power conversion efficiency, little is known about the role of disordered polymer chains in the photovoltaic (PV) behaviors and electrochemical potential drops of polymer–fullerene interfaces. In this study, the microstructural-dependent PV properties of a series of poly(3-hexylthiophene) (P3HT):fullerene (i.e., [6,6]-phenyl-C61-butyric acid methyl ester, or PCBM) blending films with different compositions have been investigated using several experiments (i.e., absorption spectroscopy, Raman spectroscopy, X-ray diffraction, and atomic force microscopy) and theoretical methods (i.e., spectroscopic simulation and quantum mechanical calculations). A strong correlation exists between amorphous P3HT chain properties, characterized by degree of conjugation (L
eff), and PV parameters. The impact of L
eff of amorphous P3HT on exciton dissociation is addressed, thus providing an ideal structural model for organic BHJ solar cells. Although bigger P3HT and PCBM domains favor carrier transport, the control of disordered P3HT segments and PCBM contact is crucial to exciton dissociation, which can consequently optimize PV performance.
The
blending of organic semiconductor and insulating polymer as
an active layer of organic transistors has been assessed by several
studies and may open new possibilities for advantageous functions.
However, studies on the use of insulating polymer doping in organic
solar cells are rarely conducted. In this research, a blending film
comprising regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was doped with a soft insulating
poly(methyl methacrylate) (PMMA) to act as an active layer of a solar
cell. The fill factor (FF) and open-circuit voltage (V
OC) of P3HT:PCBM solar cells were improved after PMMA
doping. The microstructure-dependent photovoltaic properties of devices
with different fractions of PMMA were investigated using spectroscopy
and quantum chemical calculations. We highlight that PMMA doping leads
to a more homogeneous conformation of P3HT chains, fewer vacancies,
and fewer leakage pathways in blending films. Consequently, charges
can be efficiently transported to the electrodes of devices, resulting
in enhanced FF and V
OC. The results of
theoretical calculations at the microscopic molecular level also confirmed
the enhanced electrical performance of devices from PMMA doping.
This study focuses on the microstructural modifications of regioregular poly(3-hexylthiophene) (rr-P3HT) in the small active channel of thin-film transistors (TFTs) during operations. Polarized absorption and micro-Raman spectroscopy analyses allow us to probe directly the conformation transitions of rr-P3HT chains parallel or perpendicular to the channel by means of exciton bandwidth, interchain electronic coupling, and effective conjugation length. The results of absorption spectra and a joint experimental-theoretical study of Raman spectra show that an external source-to-drain electric field can align rr-P3HT chains parallel to the channel, improving electrical performance after long-term operations, especially charge transport properties. In comparison, the applied external gate field induced an increase in amorphous fraction of the rr-P3HT films. After the analysis, we propose a chain rearrangement model driven by an external electric field to interpret the changes of the effective conjugation length of rr-P3HT, rather than thermal annealing. Our observations provide a thorough explanation for the previously unknown relationships of structure-electronic properties under the extended operations of polymer TFT devices.
The role of residual solvents and vacancies within poly(3-hexylthiophene) (P3HT) active layers, which are made from different boiling point (bp) solvents, on the electrical hysteresis characteristics of P3HT-based transistors was investigated. The improved electrical performance and reduced hysteresis of P3HT films, which are spin coated by high bp solvents, can be interpreted by superior crystalline quality and homogeneity and low vacancies. The hysteresis is dominated by the vacancy-related charge traps in the semiconductor created during film solidification and subsequence solvent evaporation. Furthermore, residual solvents, which initially occupied the vacancies, can contribute to conductivity of regioregular P3HT, thus altering electrical properties and smaller hysteresis.
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