Abstract
The main purpose of this work is to show the thermal dependence of the refractive and extinction indices of conjugated polymer films used in optoelectronics devices. Herein, we present the results of optical investigations performed for the following polymers: poly(3-hexylthiophene) (P3HT), poly(3-octylthiophene) (P3OT), and their blends with [6,6]-phenyl C61 butyric acid methyl ester (PCBM). For our analysis, we chose well-known polythiophenes such P3HT and P3OT, often used in photovoltaic cells. Our addition of PCMB to the polythiophenes allows their conversion efficiency to be increased. This paper presents the results of our investigation determining the spectral dispersion of optical constants in a wavelength range of 190–1700 nm by using spectroscopic ellipsometry (SE). Furthermore, we show the temperature dependence of the refractive indices of polythiophene films for a heating and a cooling process in the temperature range 25–130 °C. Additionally, thermo-optic coefficients and an optical gap were established and are presented in the paper, followed by a discussion on the conditions of the thermal stability of polythiophene blends and reversibility issues in thermal processes. Our paper presents a new and fresh analysis of depolarization beams after their reflection from the studied films. The paper presents the results of thermo-optical studies of polymer blends which have not been included in previously published works.
Highlights
For many years, conjugated polymers have attracted the attention of researchers in the field of organic semiconductors
Optical properties in the visible range of the electromagnetic spectrum (VIS) region determined by π electrons and σ excitations are usually not taken into consideration [7]
We present the results of thermo-optical analysis done for P3HT/phenyl C61 butyric acid methyl ester (PCBM) and P3OT/PCBM blends in the temperature range of 25–130 ◦ C
Summary
For many years, conjugated polymers have attracted the attention of researchers in the field of organic semiconductors. The unique optoelectronic properties of π conjugated polymers are the result of π bonds in the electronic structure of the polymer [7]. Because π bonds are not strong, it is possible to excite electrons to excited states by the photons from the visible range of the electromagnetic spectrum (VIS). In most semiconductive polymers, σ bonds are much stronger than π bonds, and they can be excited only by photons from the ultraviolet range (UV). Optical properties in the VIS region determined by π electrons and σ excitations are usually not taken into consideration [7]. The other attractive properties of conjugated polymers are the following: their easy solubility, environmental stability, and solution processability [1,4]
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