Abstract
Despite significant development recently, improving the power conversion efficiency of organic photovoltaics (OPVs) is still an ongoing challenge to overcome. One of the prerequisites to achieving this goal is to enable efficient charge separation and small voltage losses at the same time. In this work, a facile synthetic strategy is reported, where optoelectronic properties are delicately tuned by the introduction of electron-deficient-core-based fused structure into non-fullerene acceptors. Both devices exhibited a low voltage loss of 0.57 V and high short-circuit current density of 22.0 mA cm−2, resulting in high power conversion efficiencies of over 13.4%. These unconventional electron-deficient-core-based non-fullerene acceptors with near-infrared absorption lead to low non-radiative recombination losses in the resulting organic photovoltaics, contributing to a certified high power conversion efficiency of 12.6%.
Highlights
From the aspect of molecular design, synthesis of donor and acceptor materials with complementary absorption profiles intended to maximize the coverage of the solar spectrum has been one of the prerequisites for achieving a high photocurrent
To meet the rules mentioned above, our molecule design rational consists of tuning the HOMO levels relatively close to that of the donor and, lowering the LUMO levels to make the absorption spectra of donor material and acceptor materials complementary: (1) Introducing the nitrogen atoms in the center core unit serve as heteroatomic bridges for covalent planarization[36]: this provides stronger electron-donating character and allow for charge carrier mobility in contrast to cyclopentadiene[37,38], increasing the HOMO energy levels; (2) Adding a weak electron-withdrawing moiety of 2-ethylhexyl-benzo[d]- [1,2,3]-triazoles segment[39] in the middle of the central core to form a fused deficient-core-based fused structure (DAD) structure: this helps to improve the efficient radiative recombination pathway and enhance the electroluminescence yield of the single components
In contrast to previous designs, it does not need to synthesize spiro-like structures, and this is a good example of non-spiro-like molecule that show excellent performance as an non-fullerene acceptors (NFAs)
Summary
From the aspect of molecular design, synthesis of donor and acceptor materials with complementary absorption profiles intended to maximize the coverage of the solar spectrum has been one of the prerequisites for achieving a high photocurrent. A commercial donor polymer PBDB-T35 is selected because of its negligible band offsets with Y1 and Y2 In spite of this small energetic offset, which results in relatively high EQEEL (approximately 0.5 × 10−4) and small voltage losses, charge separation is still efficient and offers decent short-circuit current density (over 22 mA cm−2), resulting in PCEs of over 13.4% (13.42% for Y1 and 13.40% for Y2). These devices have been certified at the photovoltaic Lab of Newport Corporation, showing a 12.6% efficiency, which is the high efficiency of current single-junction organic cells meeting the ISO 17025 Standards reported so far.
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