Hiroaki Jinno2 1 Kenjiro Fukuda2 Xiaomin Xu2 Sungjun Park2 Yasuhito Suzuki2 Mari Koizumi1 Tomoyuki Yokota1 Itaru Osaka2 Kazuo Takimiya2 Takao Someya1 2

2, RIKEN, Tokyo, , Japan
1, Univ of Tokyo, Tokyo, , Japan

Energy harvesters with superior energy output are rapidly growing with development of novel organic material such as piezoelectric, thermoelectric and photovoltaic material. By combining these energy harvesters to electrical sensors, the sensor can extract power from environment which makes the sensors self-powered and applicable to the Internet of Things (IoTs). For continuous power source of wearable IoT sensors, ultraflexible organic photovoltaics (OPVs) are the most promising among all existing energy harvesters because of their flexibility, light-weight, and high power output (ref. 1). Also, ultraflexible OPVs are advantageous of textile-compatibility, thereby the area of the power source can be enlarged by applying the textile itself as the platform. Such textile-compatible power source must possess environmental stability in both air and water. Here we show ultraflexible, air/water stable and efficient OPVs based on inverted structure with an active layer of a D–A polymer with quaterthiophene and naphtho[1,2-c:5,6-c’]bis[1,2,5]thiadiazole (NTz) (PNTz4T) (ref. 2) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). The device shows high PCE of 7.9% on 1-μm-thick foil after peeled off from supporting glass and remains almost 50% of initial PCE even after 30 days storing in air. Additionally, the freestanding OPV was converted to double-side coated OPV by sandwiching the freestanding OPV with two pre-stretched elastomer, which results simultaneous achievement of stretchability and high stability against water immersion. First, we evaluated mechanical durability of double-side coated OPVs. Even the double-side coated OPV was compressed up to 52%, the device shows stable photovoltaic properties. Additionally, we cyclically compressed the OPVs with 52% compression. After 20 cycles of 52% compression, the PCEs for double-side coated OPV remained at 83%. Second, stability of OPVs against water immersion was examined. While the PCEs for the freestanding OPVs decreased by 20.8% after water immersion for 120 min, those for the double-side coated OPVs only decreased by 5.4%. Now we realize that highly efficient, stretchable, and air/water stable power sources with double-side elastomer coated OPVs. This double-side coated OPVs are promising textile-compatible power sources for future wearable applications (ref. 3).

[Reference 1] Kaltenbrunner, M. et al. Ultrathin and lightweight organic solar cells with high flexibility. Nat. Commun. 3, 770 (2012).
[Reference 2] Vohra, V. et al. Efficient inverted polymer solar cells employing favourable molecular orientation. Nat. Photonics 9, 403–408 (2015).
[Reference 3] Jinno, H. et al. Stretchable and waterproof elastomer-coated organic photovoltaics for washable electronic textile applications. Nat. Energy 2, 780–785 (2017).