Quantitative Comparison of Organic Photovoltaic Bulk Heterojunction Photostability Under Laser Illumination

Abstract

The photostability of bulk heterojunction organic photovoltaic films containing a polymer donor and a fullerene-derivative acceptor was examined using resonance Raman spectroscopy and controlled laser power densities. The polymer donors were poly(3-hexylthiophene-2,5-diyl) (P3HT), poly [[9-(1-octylnonylFour sample preparation methods were studied: (i) thin or (ii) thick films with fast solvent evaporation under nitrogen, (iii) thick films with slow solvent evaporation under nitrogen, and (iv) thin films dried under nitrogen followed by thermal annealing. Polymer order was assessed by monitoring a Raman peak's full width at half-maximum and location as a function of illumination time and laser power densities from 2.5 × 10 3 to 2.5 × 10 5 W cm −2. Resonance Raman spectroscopy measurements show that before prolonged illumination, PCDTBT and PTB7 have the same initial order for all preparation conditions, while P3HT order improves with slow solvent drying or thermal annealing. All films exhibited changes to bulk heterojunction structure with 2.5 × 10 5 Wcm −2 laser illumination as measured by resonance Raman spectroscopy, and atomic force microscopy images show evidence of sample heating that affects the polymer over an area greater than the illumination profile. Photostability data are important for proper characterization by techniques involving illumination and the development of devices suitable for real-world applications.

Characterizing Electric Field Exposed P3HT Thin Films Using Polarized‐Light Spectroscopies

“…An XY translation stage (ProScan, Prior Scientific, Rockland, MA) controlled by a lab‐developed LabVIEW program (2010 version, National Instruments, Austin, TX) was used to move 1 mm to each new location. The spectra were collected for 10 s with two accumulations and illuminated with 1.09 × 10 4 W cm −2 , which is a low enough power density to avoid film degradation . Two accumulations were used in order to remove cosmic rays.…”

“…The spectra were collected for 10 s with two accumulations and illuminated with 1.09 × 10 4 W cm −2 , which is a low enough power density to avoid fi lm degradation. [ 47 ] Two accumulations were used in order to remove cosmic rays.…”

“…Raman spectroscopy can be used to characterize polymerbased organic photovoltaic device morphology by measuring changes in the FWHM and the resulting polarization of the scattered light. [ 47,48,[58][59][60][61] Extensive work on P3HT aggregation has been reported by Grey and co-workers. [ 62,63 ] They classify P3HT fi lms with a ratio of the peak intensities for aggregated and unaggregated chains ( I 1450 cm −1 / I 1470 cm −1 ) greater than 1.5 as highly aggregated chains with correlated planarity and an average intrastack chain-to-chain spacing of ≈3.8 Å.…”

Optical-based spectroscopic methods for measuring chemical, optical, and physical properties of thin polymer waveguide films

“…47,48,[57][58][59][60] Extensive work on P3HT aggregation has been reported by Grey et al 61,62 They classify P3HT films with a ratio of the peak intensities for aggregated and unaggregated chains (I1450 cm -1 /I1470 cm -1 ) greater than 1.5 as highly aggregated chains with correlated planarity and an average intrastack chain-to-chain spacing of approximately 3.8 Å. Table III presents measured parameters for the P3HT carbon-carbon double bond peak at 1450 cm -1 for films prepared in the absence of an external electric field, or with an electric field oriented in the noted directions.…”

“…SPR requires a metal film, capable of generating a surface plasmon, 46 which is susceptible to changes in the adjacent layer's refractive index or thickness. 11,47 Due to the inherent limitation of the technique only refractive index or thickness is measured in a traditional SPR experiment, but there are advanced SPR-based techniques that work around this limitation. 45,48,49 On the other hand, ellipsometry is capable of simultaneously providing a thin polymer film's refractive index and thickness.…”