Spatial Fourier‐Transform Analysis of the Morphology of Bulk Heterojunction Materials Used in “Plastic” Solar Cells

Abstract

Polymer solar cells based on poly(3-hexylthiophene) (P3HT):fullerene bulk heterojunction (BHJ) materials have demonstrated promising (and continuously improving) performance and remarkable thermal stability. Extensive studies of the morphology have revealed that polymer solar cell performance depends strongly on the nanoscale phase separation of the P3HT and fullerene components within the charge-separating-and charge-transporting film. [1][2][3] Thermal annealing has proven to be the most effective method to control and enhance the phase separation and, thereby, improve the device performance. When annealed at elevated temperatures, the P3HT and fullerene components demix and simultaneously develop crystallinity while maintaining the phase-separated networks. Upon changing the annealing time and/or temperature, the length scale of phase separation evolves and "ripens" in a manner reminiscent of that observed in two-phase systems during spinodal decomposition.[4]We have carried out a focused study of the morphology utilizing an approach not previously applied to BHJ network systems: spatial Fourier-transform analysis of images obtained using transmission electron microscopy (TEM). TEM was used to investigate the phase separation morphology; the digital TEM images were then analyzed by carrying out a spatial Fourier transform of the image intensity to obtain the power spectral density (PSD). PSD has been widely used on digital atomic force microscopy (AFM) images to achieve surface roughness information. [5][6][7] In this report, we introduce spatial Fourier transform and PSD as a method for quantitative analysis of TEM images of the polymer of the BHJ materials used in the fabrication of "plastic" solar cells.Polymer solar-cell devices were fabricated under optimum conditions which have been reported previously.[3] In order to relate device performance to morphology change we made TEM specimens using the same polymer film applied to device fabrication. As shown in Figure 1A and B, TEM images of the films were captured before and after annealing at 150°C for 10 min. The TEM images clearly show the changes in the phase separation morphology before and after annealing. The film shown in Figure 1A was cast from solution and dried at room temperature; the phase separated microstructure can be seen, but it is poorly defined. After annealing the same film at 150°C for 10 minutes, the TEM image shows well-defined phase segregation with a quasi-periodic appearance, as shown in Figure 1B. One can see interconnected dark and bright regions which form an interpenetrating network throughout the entire area of the image. The bright regions are attributed to P3HT-rich domains because the electron scattering density of [6,6]-Phenyl-C 61 butyric acid methyl ester (PCBM) is higher than that of poly(3-hexylthiophene) (P3HT). [8,9] Upon annealing the blend film above the glass transition temperature (T g ) of P3HT (for P3HT, T g ≈ 50°C), the two components demix and the length scale of the phase separated structure evolves with time...

Quantifying organic solar cell morphology: a computational study of three-dimensional maps

“…37, but a brief description is included in the Supplementary Information. The previous inability to accurately represent morphology limited quantification to averaged quantities like domain size 38,39 or was based on incomplete binary maps of the active layer, as in our previous work. 40,41 But by quantifying these directly measured three-phase morphology maps, not only can the effects of three-phases be determined, but features which lead to high performance devices can be extracted, leading to a much more complete understanding of morphology.…”

Probing the nanoscale phase separation in binary photovoltaic blends of poly(3-hexylthiophene) and methanofullerene by energy transfer

“…The 9 nm domain size can be compared with the results obtained using the transmission electron microscopy (TEM), which showed a peak of the spatial period of two phases at around 16 nm in a blend with a similar ratio of P3HT and PCBM. 5 As the spatial period corresponds to the |P3HT|PCBM| domain size, the dominant PCBM domain size from TEM is about 8 nm. The length scale of <10 nm is at the resolution limit of TEM, whilst energy transfer can be used to probe even smaller length scales.…”

Comparative study: the effect of annealing conditions on the properties of P3HT:PCBM blends

“…The PCE of the solar cells based on the P3HT/fullerene system depends strongly on the processing conditions [12] and can be improved by increasing the crystalline content of the P3HT. Different approaches have been suggested for the optimization of the morphology, such as thermal annealing [5,6,[13][14][15], solvent annealing [16], electric field treatment [17], the use of various solvent additives, e.g., alkanedithiols [18], 1-chloronaphthalene [19], 1,8-diiodooctane [20], and nitrobenzene [21], or the use of different solvents [22,23] to lead to a molecular 2 rearrangement of the spin-coated film. Thermal annealing has been shown to be a critical step in the fabrication of P3HT/PCBM solar cells [24].…”

“…The PCE of the solar cells based on the P3HT/fullerene system depends strongly on the processing conditions [12] and can be improved by increasing the crystalline content of the P3HT. Different approaches have been suggested for the optimization of the morphology, such as thermal annealing [5,6,[13][14][15] …”