Ultrathin and Efficient Organic Photovoltaics with Enhanced Air Stability by Suppression of Zinc Element Diffusion

Abstract Ultrathin (thickness less than 10 µm) organic photovoltaics (OPVs) can be applied to power soft robotics and wearable electronics. In addition to high power conversion efficiency, stability under various environmental stresses is crucial for the application of ultrathin OPVs. In this study, the authors realize highly air‐stable and ultrathin (≈3 µm) OPVs that possess high efficiency (15.8%) and an outstanding power‐per‐weight ratio of 33.8 W g−1. Dynamic secondary‐ion mass spectrometry is used to identify Zn diffusion from the electron transport layer zinc oxide (ZnO) to the interface of photoactive layer; this diffusion results in the degradation of the ultrathin OPVs in air. The suppression of the Zn diffusion by a chelating strategy results in stable ultrathin OPVs that maintain 89.6% of their initial efficiency after storage for 1574 h in air at room temperature under dark conditions and 92.4% of their initial efficiency after annealing for 172 h at 85 °C in air under dark conditions. The lightweight and stable OPVs also possess excellent deformability with 87.3% retention of the initial performance after 5000 cycles of a compressing–stretching test with 33% compression.


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sequentially deposited onto the ITO electrode as contact pads. The tPI substrate with the ITO electrode was sequentially rinsed with acetone and 2-propanol. Subsequently, the substrates were blown dry using nitrogen flow. Before the deposition of ZnO, the substrate was treated with oxygen plasma at 300 W for 1 min (PC-300, SAMCO). The ZnO precursor solution was prepared by dissolving 109.8 mg zinc acetate dehydrate and 31.2 μL ethanolamine in 1 mL 2-methoxyethanol. The ZnO precursor solution was spin-coated on the tPI/ITO substrate at 3500 rpm for 45 s, followed by a thermal annealing at 180 °C for 30 min in air. The PEI-Zn precursor solution was prepared by dissolving 70 mg zinc acetate dehydrate in 1 wt% PEIE 2-methoxyethanol. To form the PEI-Zn film, the precursor solution was spin-coated at 3500 rpm for 45 s, and then thermally annealed at 180 °C for 30 min in air. PM6:Y6 (7 mg:9 mg) was dissolved in 1 mL chloroform:1-chloronaphthalene (1-CN) mixed solvent (99.5:0.5, volume ratio).
The active layer solution was spin-coated at 3500 rpm for 45 s, and then annealed at 110 °C for 10 min in a glovebox. Then, a MoO x hole-transporting layer (thickness of 7.5 nm) and an Ag electrode (thickness of 100 nm) were sequentially deposited ULVAC). The effective area of the solar cells was 4 mm 2 . To connect the ultrathin OPVs, an external wiring on the polyimide substrates with Cr (3-nm-thick)/Au (100-nm-thick) was attached to the cathode and anode of the OPVs by electrically conductive adhesive tape (ECATT 9703, 3M Company). Finally, the devices were encapsulated by 1-μm-thick parylene (diX-SR, Daisan Kasei) evaporated by chemical vapor deposition (PDS 2010, KISCO Company).

Characterization of devices:
The current density-voltage (J-V) characteristics of S5 ultrathin OPVs were measured using a Keithley 2400 Source Meter under an illumination of 1 sun using a solar simulator (AM 1.5 global spectrum calibrated using a silicon reference diode). For the stability test, all devices were stored in ambient air. For the maximum point power tracking (MPPT) test, the voltages at the maximum power point to the OSCs were continuously applied to the devices during the test. The voltage at the maximum power point was updated every 30 min based on the latest J-V characteristics measurement. The external quantum efficiency (EQE) measurements were performed with monochromatic light (SM-250F, Bunkoukeiki) calibrated using a silicon reference diode.
Characterization of the film: The absorbance of the films was characterized using an ultraviolet/visible/near infrared (UV-vis-NIR) spectrophotometer JASCO) and the thicknesses were determined using a surface profiler (DEKTAK 6M, Bruker).
Atomic force microscopy (AFM) was performed using a scanning probe microscope (SPM-9700HT, Shimadzu) in tapping mode. The dynamic secondary-ion mass spectrometry (D-SIMS) test (ADEPT-1010, ULVAC-PHI, Inc.) was performed by the TORAY Research Center, Inc. A photoelectron spectroscopy system (PHI5000 Versa Probe II, ULVAC-PHI Inc.) was used for the X-ray photoelectron spectroscopy (XPS) measurements.
Compression Test: The freestanding ultrathin OPVs were laminated on a pre-stretched elastomer (VHB Y-4905J, 3M). To control the amount of compression and stretching, a screw machine controlled by a program with pre-designed parameters was developed and manufactured in-house. The J-V curve was measured with a source S6 meter (2400 series, Keithley).
Calculation of the power-per-weight ratio: For the accurate calculation of the power-per-weight ratio of the fabricated ultrathin OPVs, the ITO, PEI-Zn, active layer, and MoO x /Ag were completely formed on the transparent polyimide substrates (24 × 24 mm 2 ) without a pattern. To eliminate the nonuniformity at the edge, we cut the 24 × 24 mm 2 substrate to a size of 10 × 10 mm 2 . The total weight of the three ultrathin OPVs (300 mm 2 ) was 1.4 mg (ATX224, Shimadzu).
Calculation of the power-per-weight ratio: (c) The power-per-weight ratio of the ultrathin OPVs is given by the value of p/.
The power-per-weight ratio was 33.8 W g −1 .
S7 Figure S1 (a) Chemical structure of the PM6 donor polymer and Y6 acceptor. (b) Absorption curves of the PM6 and Y6 films.