Researchers from the University of Tokyo, Riken , Waseda University, University of California - Santa Barbara, Toray Industries, and Japan Science and Technology Agency have published research on the development of an "ultra-thin organic solar cell" featuring both high heat resistance and high energy conversion efficiency.
The cell can be directly attached to clothes by using a "hot melt method," which is a bonding process using an adhesive that is melted by heat.
The thickness of the newly-developed ultra-thin organic solar cell (including everything from a substrate to sealing film) is only 3μm. But its maximum energy conversion efficiency reaches 10%, and its element deterioration is small enough to be ignored even at a temperature of 100°C (high thermal resistance). Also, after being stored for 80 days in the atmosphere, its performance deteriorates by less than 20%.
For this development the research group developed "PBDTTT-OFT," a new semiconductor polymer featuring both a high energy conversion efficiency and high thermal resistance. Its structure is similar to that of "PBDTTT-EFT (or PTB7-Th)," which has been widely used as a material for organic solar cells, except that it has a linear side chain. The group found that the chain enables to form a film with a high crystallinity and that the deterioration of conductivity caused by heat is smaller than in the case of conventional materials.
The structural formulas of the "PBDTTT-OFT" newly-developed semiconductor polymer and "PBDTTT-EFT" conventional material. The high crystallinity and high heat resistance were realized by the side chain boxed in red. (source: Riken)
Thermally stable, highly efficient, ultraflexible organic photovoltaics
Xiaomin Xu | Kenjiro Fukuda | Akchheta Karki | Sungjun Park | Hiroki Kimura | Hiroaki Jinno | Nobuhiro Watanabe | Shuhei Yamamoto | Satoru Shimomura | Daisuke Kitazawa | Tomoyuki Yokota | Shinjiro Umezu | Thuc-Quyen Nguyen | Takao Someya
PNAS April 16, 2018. 201801187 | published ahead of print April 16, 2018 | https://doi.org/10.1073/pnas.1801187115
Significance
We have developed an ultraflexible organic photovoltaic (OPV) that achieves sufficient thermal stability of up to 120 °C and a high power conversion efficiency of 10% with a total thickness of 3 μm. By combining an inherently stable donor:acceptor blend as the active layer and ultrathin substrate and barriers with excellent thermal capability, we were able to overcome the trade-offs between efficiency, stability, and device thickness. The ultraflexible and thermally stable OPV can be easily integrated into textiles through the commercially available hot-melt process without causing performance degradation, thereby presenting great potential as a ubiquitous and wearable power source in daily life.
Abstract
Flexible photovoltaics with extreme mechanical compliance present appealing possibilities to power Internet of Things (IoT) sensors and wearable electronic devices. Although improvement in thermal stability is essential, simultaneous achievement of high power conversion efficiency (PCE) and thermal stability in flexible organic photovoltaics (OPVs) remains challenging due to the difficulties in maintaining an optimal microstructure of the active layer under thermal stress. The insufficient thermal capability of a plastic substrate and the environmental influences cannot be fully expelled by ultrathin barrier coatings. Here, we have successfully fabricated ultraflexible OPVs with initial efficiencies of up to 10% that can endure temperatures of over 100 °C, maintaining 80% of the initial efficiency under accelerated testing conditions for over 500 hours in air. Particularly, we introduce a low-bandgap poly(benzodithiophene-cothieno[3,4-b]thiophene) (PBDTTT) donor polymer that forms a sturdy microstructure when blended with a fullerene acceptor. We demonstrate a feasible way to adhere ultraflexible OPVs onto textiles through a hot-melt process without causing severe performance degradation.