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NCCR MUST at Scientifica 2021Low driving forces in high-efficiency organic solar cells
February 11, 2020Organic photovoltaic (OPV) devices containing non-fullerene acceptors (NFAs) blended with conjugated polymers have impressive efficiencies close to 18%. One of the reasons is the strong light absorption by NFAs (absent in traditional fullerene acceptors), allowing better harvesting of the solar spectrum and opening up the hole transfer (HT) pathway for current generation. Another reason is the low driving force for the HT process, leading to high photovoltage. However, the low driving force is believed to slow down charge generation, causing a tradeoff between voltage and current.
The group of N. Banerji at the University of Bern has carried out a carefully designed transient absorption (TA) study on polymer:NFA systems, disentangling intrinsic charge-transfer rates from morphological aspects (determined using X-ray diffraction techniques) by comparing optimized blend morphologies with planar heterojunction systems and dilute blends containing a low NFA concentration. They show that both the electron transfer (ET) and HT times at the donor:acceptor interface remain ultrafast (< 1 ps) even if the driving force approaches zero.
The behavior at low HT driving force is consistent with the trend expected in the Marcus normal region for charge-transfer with moderate electronic coupling, whereby the high rates can be explained by a small reorganization energy, as supported by DFT calculations. HT is generally slower (< 1 ps) than ET (< 0.1 ps) at comparable driving force, likely related to higher electronic coupling for ET. The ultrafast ET rates can no longer by described within the non-adiabatic Marcus limit. Overall, the sub-picosecond times for both charge-transfer pathways over a large range of driving forces demonstrate that the energy offset at the heterojunction can be minimized without jeopardizing the charge-transfer rate and efficiency.
Figure 1. Driving force dependent sub-picosecond HT in polymer:NFA blends. a. Sensitive external quantum efficiency (sEQE) spectra for different polymer:m-ITIC 5:1 blends and neat m-ITIC, whereby m-ITIC is a typical NFA. The charge-transfer (CT) band indicates the driving force for the HT process, which is schematically illustrated for the different polymers at the bottom of the figure. b. m-ITIC exciton decay (top) and charge rise (bottom) dynamics for polymer:m-ITIC 5:1 BHJ samples, upon selective m-ITIC excitation at 730 nm, obtained from the analysis of the TA data. A sub-picosecond HT component is seen in all investigated blends.
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