Strategies for high current densities in non-fullerene acceptors based organic solar cells

Strategies for high current densities in non-fullerene acceptors based organic solar cells

Xin Song, Joel Troughton, Nicola Gasparini, Derya Baran. "Strategies for high current densities in non-fullerene acceptors based organic solar cells"

Xin Song, Joel Troughton, Nicola Gasparini, Derya Baran
NonFullererne, organic solar cells, current densities, photovoltaics
2018
High photocurrent density (Jsc) is a crucial parameter for champion efficiencies in organic solar cells (OSCs). In the bulk heterojunction (BHJ) configuration, consisting of donor and fullerene acceptor materials intimately mixed together, achieving high Jsc (over 20 mA/cm2) was challenging. In fact, the low absorption coefficient as well as the limited absorption window of the conjugated polymer donor were the limiting factors for high efficiency solar devices. In order to overcome such limitation, scientists firstly considered to introduce a strong electron push-pull building block to extend the absorption range from the ultraviolet and visible to the near-infrared region (700 nm-1000 nm). For example, Yan and coworkers reported a series of polythiophene based polymers with a strong absorption form 300 nm to 800 nm. The corresponding devices yielded the current density over 20 mA/cm2. 1 State-of-the-art materials are the diketopyrrolopyrrole (DPP) based polymers, which can get the absorption range approximately to 900 nm. Based on the device engineering, 23 mA/cm2 has been reported in the DPP system. 2,3 However, the inherent drawbacks (low absorption coefficient and absorption range) of the fullerene derivate acceptors limited the improvement room for boosting the current density. Fortunately, the non-fullerene acceptors (NFA) has been put forward and made a great success recently, which obtained efficiencies over 13% together with a current density of 20 mA/cm2 in the literature.4In comparison with the fullerene acceptors, the success of NFA can be attributed to several reasons:

(i) By the strong electron push-pull interaction between the electron withdrawing groups to the electron donating core, it is possible to achieve a narrow optical bandgap and enhanced π-π coherence length.

(ii) By the incorporation of the strong rigid building block, the molecules would possess a small steric hindrance and a large overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the NFA, which leads to a greater oscillator strength and therefore a stronger absorption of light.

(iii) By coupling with fluorine or chlorine atoms in the end group, due to the high electronegativity and strong intramolecular effect of these atoms, the electron cloud will be much more delocalized in the small molecule, which would cause the significant redshift in comparison with the small molecule without the F or Cl incorporation.