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In operando morphology investigation of inverted bulk heterojunction organic solar cells by GISAXS

The correlation between morphology of the active layer and device performance of inverted polymer solar cells during continuous operation under illumination is revealed by in operando grazing incidence small angle X-ray scattering (GISAXS) and current−voltage (J-V) measurements. Consequently, the reason for enhanced stability is identified from the viewpoint of morphology.

Solar power as an alternative energy source to replace depleting fossil fuels has received great attention over last decades. To date, main photovoltaic devices are based on silicon, which possess very high power conversion efficiency. However, silicon solar panels are costly, material-consuming and inflexible. Thus, organic solar cells have emerged as one of the most promising alternatives to silicon based devices due to their unique properties, such as low-cost, light, flexible and easy to manufacture. Nevertheless, short lifetimes are a major obstacle for the commercial breakthrough. Therefore, stabilizing organic photovoltaic devices is an urgent demand.

Figure 1. (a) J-V curves of inverted P3HT:PCBM solar cells with different illumination time. They are recorded every 16 seconds until 240 min. Only the J-V curves are recorded at the same time as the GISAXS measurements are selected for illustration. (b) Time evolution of normalized photovoltaic parameters. The images are adapted from W. Wang et al., J. Mater. Chem. A 3, 8324 (2015) with permission from the Royal Society of Chemistry.

Figure 2. (a) Initial 2D GISAXS data of the P3HT:PCBM active layer in an inverted solar cell during operation under illumination. (b) Vertical line cuts and (c) Horizontal line cuts obtained from the 2D GISAXS data. Different line colors denote the illumination time of 0 min, 3 min, 10 min, 15 min, 30 min, 60 min, 120 min, 180 min and 240 min. In order to visualize the differences in the line cuts as a function of operation time, all the vertical or horizontal line cuts are plotted on top of each other. All the horizontal line cuts can be fitted with the same curve shown as a red line in (c). The images are adapted from W. Wang et al., J. Mater. Chem. A 3, 8324 (2015) with permission from the Royal Society of Chemistry.

In the present work it is found that poly(3-hexylthiophene-2,5-diyl) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells with inverted geometry are highly stable. In order to explore the correlation between the morphological evolution and the solar cell performance during continuous operation under illumination, in operando GISAXS measurements are carried out. J-V sweeps are recorded simultaneously to the GISAXS study as shown in Figure 1a. Surprisingly, the power conversion efficiency (PCE) of the inverted solar cell is preserved to 75% as compared to its maximum value after 240 min continuous operations under illumination. By taking into account all the photovoltaic parameters, it is noticed that the open circuit voltage (VOC) and fill factor (FF) show quite similar decay behaviors as in the standard solar cell, whereas the short circuit current density (JSC) exhibits a much higher stability (in Figure 1b). Therefore, the relative stable photovoltaic performance mainly originates from the higher stability of JSC.  

The in operando GISAXS measurements give the morphological evidence for this device stability. An example 2D GISAXS measurement is shown in Figure 2a. By analyzing the vertical and horizontal lines cuts of the in operando data, it is revealed that the P3HT:PCBM active layer in an inverted solar cell maintains a stable morphology, which does not alter during the device operation (Figure 2b and c). In contrast, in a standard solar cell P3HT domains increase in size along with illumination time, which is regarded as morphological degradation and which is responsible for the efficiency decay. Therefore, the stabilization of the P3HT:PCBM layer with an inverted geometry is directly correlated to the improved stability of the JSC and PCE as seen in our study for the first time

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In operando morphology investigation of inverted bulk heterojunction organic solar cells by GISAXS ;
W. Wang, C. J. Schaffer, L. Song, V. Körstgens, S. Pröller, E. Dwi Indari, T. Wang, A. Abdelsamie, S. Bernstorff, P. Müller-Buschbaum;
J. Mater. Chem. A 3, 8324 (2015). doi: 10.1039/c5ta01109d


Last Updated on Tuesday, 14 May 2019 17:05