Compared with conventional silicon solar cells, organic solar cells have the advantages of light weight, softness and low cost, and can be widely used in wearable devices and integrated photovoltaic generation. A key measure of flexible organic solar cells is their energy efficiency and mechanical stability. In recent years, with the rapid development of non-fullerene receptor materials, the energy efficiency of glass-based organic solar cells has been increased to more than 16% . However, the conversion efficiency of solar cells fabricated on flexible substrates is always lower than that of conventional rigid solar cells. Therefore, how to further improve the conversion efficiency and mechanical stability of flexible organic solar cells is an urgent problem to be solved in its commercialization.

Recently, a team from RIKEN in Tokyo, Japan, successfully fabricated an organic solar cell 3 microns thick on an ultra thin Parylene substrate The stretchability is achieved by transferring the ultra-flexible device onto a pre-stretched elastomer. The crystallization of the non-fullerene Electron acceptor is regulated by introducing a third component, which improves both the Exciton dissociation efficiency and the ductility of the active layer film A super flexible organic solar cell with energy conversion efficiency of 13% (JET efficiency of 12.3%) was successfully prepared.

After 1000 cycles of compression-tension, the performance of the solar cell decreased by only 11% , and after 1000 cycles of bending (5 mm Bending Radius) , the performance of the solar cell decreased by only 3% . The research, published in the Cell Press Journal Joule, is titled: "Efficient and Mechanicallyrobust Ultraflexible Organic Solar Cells Based on Mixed Acceptors. " The research provides a new way to improve both the energy efficiency and the mechanical properties of the devices, and has important implications for the further development of flexible or stretchable organic solar cells. Dr Wen Chao Huang and Jiang Zhi are co First Authors, Kenjiro Fukuda and Takao Someya are co corresponding authors.


Figure 1: 3 micron ultra-flexible organic solar cell. a) molecular structure formula; b) energy level diagram; c) ultraviolet and visible absorption spectrum; d) device structure diagram; e) optical photograph; f) current density-voltage (J-V) characteristic curve.

In this study, 13% of the ultra-flexible organic solar cells were successfully fabricated on 1 Micron Parylene substrate using PBDTTT-OFT: IEICO-4F: PC71BM ternary active layer The device is then packaged in a 1 Micron Parylene, which is less than 3 microns thick. The researchers found that introducing a third component, PC71BM, into the binary system of broad band polymer donor PBDTTT-OFT and narrow band gap polymer acceptor IEICO-4F could enhance the exciton separation and charge transfer efficiency in the polymer donor PBDTTT-OFT Reduced carrier recombination (Fig. 3). Therefore, based on PBDTTT-OFT: IEICO-4F: PC71BM three-component system of ultra-flexible solar energy devices, compared with PBDTTT-OFT: IEICO-4F two-component system devices, has higher JSC, VOC, FF and energy efficiency.


Figure 2: Solar Cell Device Physics. A) external quantum efficiency curve EQE; b) short circuit current under different light intensity; c) open circuit voltage under different light intensity; d) transient photocurrent TPC; e) transient photovoltage TPC.

The effect of the third component PC71BM on the morphology of PBDTTT-OFT: IEICO-4F films was further investigated (Fig. 3) . From the AFM results, it can be concluded that the ternary blend film has smaller phase separation and Surface roughness. Giwaxs results show that both the polymer donor PBDTTT-OFT and the non-fullerene acceptor IEICO-4F exhibit face-on Orientation, which is beneficial to the effective carrier transport in the active layer. After the introduction of the third component PC71BM, the crystallization of the non-fullerene acceptor ICO-4F was inhibited, and the mixing degree between acceptors was increased, which resulted in higher exciton separation efficiency. At the same time, the formation of more amorphous IEICO-4F will help to improve the ductility of active layer film, which will lead to better mechanical properties of super-flexible solar cells. Although the decrease of the crystallinity of the non-fullerene acceptor may affect the electron transport properties, the newly introduced third-component fullerene acceptor has a high Electron mobility, which makes up for this deficiency. Through the SCLC test found that the three-component devices have a higher Electron mobility.



Figure 3: Characterization of active layers in solar cells. a）PBDTTT-OFT:IEICO-4F and b）PBDTTT-OFT:IEICO-4F:PC71BMAFM images of thin films；c）PBDTTT-OFT:IEICO-4F and d）PBDTTT-OFT:IEICO-4F:PC71BMTwo-dimensional GIWAXS images of thin films；e）PBDTTT-OFT:IEICO-4F and g）PBDTTT-OFT:IEICO-4F:PC71BMOne-dimensional X-ray diffraction pattern of GIWAXS films in the out-of-plane Direction；f）PBDTTT-OFT:IEICO-4F和and h）PBDTTT-OFT:IEICO-4F:PC71BMOne-dimensional in-plane X-ray diffraction pattern of GIWAXS films；i）PBDTTT-OFT:IEICO-4F and j）PBDTTT-OFT:IEICO-4F:PC71BMNear-edge x-ray absorption fine structure on the upper surface of thin films.

The mechanical stability of the super-flexible organic solar cell was studied. The stretchability of the ultra-flexible solar cell was realized by prestretching transfer as well as prestressing release as bending pleat. The VOC and FF of the battery do not change much when the battery is compressed from 0% to 45% , and the decrease in energy efficiency is mainly due to the reduction in effective light area. After 1,000 compression-tensile cycles, the battery retains 89% of its initial efficiency. At the same time, the flexure of the super-flexible solar cell was tested. After 1000 times of continuous flexure (bending radius is 5mm) , the initial efficiency of the solar cell was 97% . The researchers also successfully fabricated a large area super flexible organic solar cell with an effective area of 1 CM2, energy efficiency 11.6% .


Figure 4: Mechanical Stability of ultra-flexible Organic Solar Cell Devices. a）Optical photographs of ultra-flexible organic solar energy during compression-stretching。b）Current density-voltage (J-V) characteristic curve of the device during compression；c）Efficiency Degradation of small area (0.04 CM2) super flexible organic solar cells during 1,000 compression-tensile cycles; D) Bend Test Schematic and optical photographs；e）Efficiency Degradation of small area (0.04 CM2) ultra-flexible organic solar cells during 1000 bending cycles；f）Current density-voltage (J-V) characteristic curve of large area (1cm2) super flexible organic solar cells；g）Efficiency Degradation of small area (1cm2) ultra-flexible organic solar cells during 1000 compression-tensile cycles.


Relevant papers published online：
https://www.sciencedirect.com/science/article/abs/pii/S2542435119305239