Scientific Column
Do you know the amino group doping IT-4F of PFN leads to unfavorable charge accumulation?
Highlights
- It was discovered for the first time that the amino group doping IT-4F of PFN leads to unfavorable charge accumulation, which results in the formation of a densely charged negative molecular layer-4F due to the poor electron transport ability of non-fullerene acceptor IT.
- The negatively charged molecular layer can prevent the transfer of electrons from the active layer to the interlayer and cause series charge recombination at the active layer/cathode interface. This mechanism can be verified by ESR measurement and pure electronic equipment.
- By replacing PFN with PFN-Br, the over-doping effect between the cathode interlayer and IT-4F is eliminated, thereby greatly improving the charge transfer and collection.
- Conclusion, a high PCE of 13.5% is achieved in PSC without fullerene.
Summary
In August 2019, the Journal of Energy Chemistry published that Xu Bowei, an associate researcher of the Institute of Chemistry, Chinese Academy of Sciences, has a significant influence of doping effect on photovoltaic performance of efficient fullerene-free polymer solar cells. The modification mechanism of the water/alcohol cathode interlayer is one of the most complex issues in the field of organic photovoltaics, and it has not been clearly clarified; this greatly limits the further enhancement of polymer solar cells PCE. Here, we clarified the different functions of PFN and its derivatives, namely poly[(9,9-bis(3-((N,N-dimethyl)-N-ethylammonium)-propyl)-2 ,7-fluorene)-alt -2,7-(9,9-dioctylfluorene)] (PFN-Br) is used to modify fullerene-free PSC.
Background
Polymer solar cells (PSC) use conjugated polymers as electron donors and fullerene derivatives (FD) or non-fullerene (NF)-based molecules as electron acceptors. In the past few decades, it has achieved Great progress [1-4]. The power conversion efficiency (PCE) of single-junction PSC has reached more than 15%, showing a bright prospect for the practical application of organic photovoltaic technology [5,6]. A typical PSC device has a multilayer stack structure, including a bulk heterojunction (BHJ) active layer, an anode and cathode, and an interlayer between the BHJ and the electrode.
At present, the active layer materials are developing rapidly, and the related photoelectric conversion mechanism has been deeply studied and understood [7-10]. Compared with the active layer materials, the development of the intermediate layer is relatively lagging behind. In particular, due to the complex energy level structure at the interface of the interlayer/active layer, the understanding of the charge collection process is still vague [11-13]. In fact, interface engineering has proven to be an effective way to improve the PCE of PSC.
Key Results
Figure 2.
(a) PSC devices F-PFN-Br, F-PFN, NF-PFN-Br and NF-PFN (a) current density-voltage (J-V) characteristics and (b) EQE. (c) Nyquist plots of devices F-PFN-Br, F-PFN, NF-PFN-Br and NF-PFN. (d) Equivalent circuit model for EIS fitting of equipment.
Table 1.
Photovoltaic parameters of conventional structural devices based on F-PFN-Br, F-PFN, NF-PFN-Br and NF-PFN under AM 1.5 G and 100 mW cm -2 illumination.
Table 2.
Fitting results of the Nyquist plot of the device.
Figure 4.
(a) ESR spectra of pure IT-4F, PFN-Br and PFN films. (b) ESR spectrum of IT-4F:PFN-Br and IT-4F:PFN mixture. (c) Schematic diagram of the electron transmission process in PBDB-T-2F:IT-4F/PFN and (d) PBDB-T-2F:IT-4F/PFN-Br.
Figure 5.
(a) The chemical structures of F-N and F-Br. (b) J-V and (c) EQE curves of PSC devices modified with small molecule models F-N and F-Br. (d) ESR spectrum of IT-4F:F-Br and IT-4F:F-N mixture.
Figure 6. Semi-logarithmic current density and voltage characteristics of pure electronic devices
(a) A: ITO/ZnO/PC 71 BM/PFN/Al and B: ITO/ZnO/PC 71 BM/PFN-Br/ Al (b) C : ITO/ZnO/IT-4F/PFN/Al and D: ITO/ZnO/IT-4F/PFN-Br/Al (c) E: ITO/ZnO/IT-4F/FN/Al and F: ITO/ZnO /IT-4F/F-Br/Al.
Conclusion
This article proved the different effects of PFN and PFN-Br on modified PSC, and deeply studied the working mechanism of PFN and PFN-Br. Although PFN and PFN-Br interlayers work well in fullerene-based PSCs, significant contrasts between the use of PFN and PFN-Br interlayers to modify fullerene-free devices have been observed. Due to the poor ability to accept excess electrons, the doping on the NF acceptor IT-4F leads to the accumulation of unpaired electrons near the IT-4F/PFN interface and forms a dense layer of negatively charged molecules, thereby preventing electrons from moving from the active molecules. Transfer. Layer to interlayer; this reduces the charge collection efficiency of the device.
The pure electronic device further validates our inference that the excessive doping between PFN and IT-4F hinders the collection of electrons in the PSC device. By replacing PFN with PFN-Br, the charge collection efficiency is greatly improved, and a high PCE of 13.5% is achieved. The results of this work clarify the different effects of doping on the modification of fullerene-based and fullerene-free PSCs, which provides new inspiration for selecting suitable interface materials to construct high-efficiency PSCs.
Article Information
Significant influence of doping effect on photovoltaic performance of efficient fullerene-free polymer solar cells
Qian Kang, Qi Wang, Cunbin An, Chang He, Bowei Xu, Jianhui Hou
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