Journal
NANO ENERGY
Volume 75, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.nanoen.2020.105032
Keywords
Fill factor; HOMO energy offset; Voltage loss; Charge-separated state lifetime
Categories
Funding
- National Natural Science Foundation of China [21875012, 21674006]
- National Natural Science Foundation of China (NSFC) [21773045]
- National Key Research and Development Program of China [2017YFA0206600]
- Chinese Academy of Science (100 Top Young Scientists Program)
- Knut and Alice Wallenberg Foundation [2016.0059]
- Swedish Government Research Area in Materials Science on Functional Materials at Link_oping University (Faculty Grant SFO-Mat-LiU) [200900971]
- China Scholarship Council (CSC) [201708370115]
- program of Academic Excellence Foundation of BUAA for PhD Students
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Fill factor (FF) is an important parameter governing the power conversion efficiency (PCE) in non-fullerene organic solar cells (NF-OSCs), which however is less studied than the other two parameters (short-circuit current J(sc) and open-circuit voltage V-oc). To understand how energy offsets, exciton and charge carrier dynamics impact the FF, four groups of bulk heterojunctions (BHJs) NF-OSCs are investigated with FFs varying from 0.61 to 0.78 under progressive changes of HOMO-HOMO offsets (Delta(HOMOs), from 0.09 to 0.24 eV). By pump-probe optical spectroscopy, we find that the FF exhibits a positive dependence on Delta(HOMO) and charge-separated state lifetime (tau(CS)) in the blends, a result of inhibited back charge transfers and recombination at the donor-acceptor interface under higher Delta(HOMO)s. Moreover, we observe a fast charge extraction with decreased sensitivity to internal electric-fields in high-FF devices. Despite these merits, the gains of FF are at the expense of increasing the voltage loss to non-radiative recombination in our studied systems. The combined results suggest that remaining appropriate energetic offsets is essential for controlling the carrier dynamics with longer-lived CS-states, restraining charge back transfer and reducing charge recombination toward high FFs and photovoltaic efficiencies.
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