Abstract
High performance flexible inverted organic solar cells (OSCs) employing the low temperature cathode buffer bilayer combining the aqueous solution-processed ZnO and polyethylenimine ethoxylated (PEIE) are investigated based on Poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butryric acid methyl ester (P3HT:PC61BM) and Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl}):[6,6]-phenyl-C71-butyric acid methyl ester (PTB-7:PC71BM) material systems. It is found that, compared with pure ZnO or PEIE cathode buffer layer (CBL), the proper combination of low-temperature processed ZnO and PEIE as the CBL enhanced the short circuit current density (JSC), resulting in better device performance. The increased JSC results from the enhanced electron collection ability from the active layer to the cathode. By using the ZnO/PEIE CBL, a power conversion efficiency (PCE) as high as 4.04% for the P3HT:PC61BM flexible device and a PCE as high as 8.12% for the PTB-7:PC71BM flexible device are achieved, which are higher than the control devices with the pure ZnO CBL or pure PEIE CBL. The flexible inverted OSC also shows a superior mechanical property and it can keep 92.9% of its initial performance after 1000 bending cycles with a radius of 0.8 cm. These results suggest that the combination of the low temperature aqueous solution processed ZnO and PEIE can be a promising cathode buffer bilayer for flexible inverted OSCs.
Highlights
Due to the potential of low-cost, flexibility, light weight and compatibility with roll-to-roll fabrication, organic solar cells (OSCs) have attracted much research attention [1,2,3,4]
The flexible inverted OSCs showed a superior mechanical property. These results suggest that the combination of the low temperature aqueous solution processed zinc oxide (ZnO) and polyethylenimine ethoxylated (PEIE) can be a promising cathode buffer bilayer for flexible inverted OSCs
Jph is obtained by subtracting the current density in the dark from the current with the above OSCs, the device with ZnO/PEIE cathode buffer layer (CBL) exhibits the best performance as shown in Figure 2c,d, which obtains a power conversion efficiency (PCE) as high as 4.04%, with a JSC of 9.81 mA/cm2, a V OC of 0.65 V, and a fill factor (FF) of 63.83%
Summary
Due to the potential of low-cost, flexibility, light weight and compatibility with roll-to-roll fabrication, organic solar cells (OSCs) have attracted much research attention [1,2,3,4]. Compared with the conventional structure, the inverted structure have the opposite electrode polarities, where the modified ITO acts as the cathode and a high-work-function metal such as silver (Ag) or gold (Au) acts as the top anode, so that both the commonly used acidic PEDOT:PSS and low-work-function metal top cathode can be avoided and morphology of the active layer become more stable This structure has been considered as an efficient approach for improving the cell stability [15,16,17,18,19]. Inverted OSCs showed a superior mechanical property
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