By applying the specific fabrication conditions summarized in the Experimental section and post‐production annealing at 150 °C, polymer solar cells with power‐conversion efficiency approaching 5 % are demonstrated. These devices exhibit remarkable thermal stability. We attribute the improved performance to changes in the bulk heterojunction material induced by thermal annealing. The improved nanoscale morphology, the increased crystallinity of the semiconducting polymer, and the improved contact to the electron‐collecting electrode facilitate charge generation, charge transport to, and charge collection at the electrodes, thereby enhancing the device efficiency by lowering the series resistance of the polymer solar cells.

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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...

The operational lifetime of polymer light-emitting diodes was studied at several temperatures in the range from 25 to 85 °C. When operated in constant current mode, at luminances greater than 100 cd/m 2, lifetimes of around 20 000 h were noted. Two significant changes in performance were found during continuous operation: the luminance of the devices varied in a nonmonotonic fashion, and the operating voltage increased in a linear fashion. Both of these changes were thermally activated, with the changes accelerated at higher temperatures. These changes are also accelerated at higher current densities. We discuss possible mechanisms for these degradation processes.