Vitamin C treatment improves stability of inverted organic solar cells



A team of researchers from the University of Southern Denmark (SDU) sought to match the advances being made in power conversion efficiencies for organic solar cells (OPV) made with non-fullerene acceptor (NFA)  materials with stability improvements.

The team selected ascorbic acid, commonly known as vitamin C, and used it as a passivation layer between a zinc oxide (ZnO) electron transport layer (ETL) and the photoactive layer of in NFA OPV cells fabricated with an inverted device layer stack and a semiconducting polymer (PBDB-T:IT-4F).

The scientists built the cell with an indium tin oxide (ITO) layer, the ZnO ETL, the vitamin C layer, the PBDB-T:IT-4F absorber, a molybdenum oxide (MoOx) carrier-selective layer, and a silver (Ag) metal contact.

The group found that the ascorbic acid produces a  photostabilizing effect, reporting that antioxidant activity alleviates the degradative processes arising from exposure to oxygen, light and heat. Tests, such as ultraviolet-visible absorption, impedance spectroscopy, light-dependent voltage and current measurements, also revealed that vitamin C reduces the photobleaching of NFA molecules and suppresses the charge recombination, noted the research.

Their analysis showed that, after 96 h of continuous photodegradation under 1 Sun, the encapsulated devices containing the vitamin C interlayer retained 62% of their original value, with the reference devices retaining only 36%.

The results also showed that the stability gains did not come at a cost of efficiency. The champion device achieved a power conversion efficiency of 9.97 %, an open-circuit voltage of 0.69 V, a short-circuit current density of 21.57 mA/cm2, and a fill factor of 66%. The reference devices containing no vitamin C, exhibited 9.85 % efficiency, an open-circuit voltage of 0.68V, a short-circuit current of 21.02 mA/cm2, and a fill factor of  68%.

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When asked about commercialization potential and scalability, Vida Engmann who heads a group at the Center for Advanced Photovoltaics and Thin-Film Energy Devices (SDU CAPE), told pv magazine, “Our devices in this experiment were 2.8 mm2 and 6.6 mm2, but can be scaled up in our roll-to-roll lab at SDU CAPE where we regularly fabricate OPV modules too.”

She emphasized that the manufacturing method can be scaled, pointing out that the interfacial layer is an “inexpensive compound that is soluble in usual solvents, so it can be used in a roll-to-roll coating process like the rest of the layers” in an OPV cell.

Engmann sees potential for additives beyond OPV in other third-generation cell technologies, such as perovskite solar cells and dye-sensitized solar cells (DSSC). “Other organic/hybrid semiconductor-based technologies, such as DSSC and perovskite solar cells, have similar stability issues as organic solar cells, so there is a good chance they can contribute to solving stability problems in these technologies as well,” she stated.

The cell was presented in the paper “Vitamin C for Photo-Stable Non-fullerene-acceptor-Based Organic Solar Cells,” published in ACS Applied Material Interfaces. The first author of the paper is SDU CAPE's Sambathkumar Balasubramanian. The team included researchers from SDU and Rey Juan Carlos University.

Looking ahead the team has plans for further research into stabilization approaches using naturally occurring antioxidants. “In the future, we are going to continue investigating in this direction,” said Engmann referring to promising research on a new class of antioxidants.

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