Research Team

First Author: Qian Kang 

Corresponding Author: Bowei Xu

DOI: 10.1016/j.jechem.2019.08.005

Key Highlights

1. Doping Effect Discovery: First discovery that PFN’s amine doping of IT-4F leads to unfavorable charge accumulation, forming a dense negatively charged molecular layer due to the poor electron transport capability of non-fullerene acceptor IT-4F.
2. Mechanism Verification: The negatively charged molecular layer can block electron transfer from the active layer to the interlayer and cause series charge recombination at the active layer/cathode interface. This mechanism can be verified through ESR measurements and pure electronic devices.
3. Performance Enhancement: Significant improvement in charge transport and collection achieved by replacing PFN with PFN-Br, eliminating excessive doping effects between the cathode interlayer and IT-4F.
4. Significant Breakthrough: Finally achieved high power conversion efficiency of 15% in fullerene-free polymer solar cells.

Introduction

In August 2019, the Journal of Energy Chemistry published research by Associate Researcher Bowei Xu from the Institute of Chemistry, Chinese Academy of Sciences, regarding the significant influence of doping effects on photovoltaic performance in high-efficiency fullerene-free polymer solar cells. The modification mechanism of water/alcohol cathode interlayers remains one of the most complex issues in the organic solar cell field and has not yet been clearly elucidated; this greatly limits further improvements in polymer solar cell power conversion efficiency. Here, this paper elucidates the different roles of PFN and its derivative, poly[(9,9-bis(3-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN-Br) used for modifying fullerene-free polymer solar cells.

Background

Polymer Solar Cells (PSCs) using conjugated polymers as electron donors and fullerene derivatives or non-fullerene-based molecules as electron acceptors have made tremendous progress over the past decades [1-4]. Single-junction polymer solar cells have achieved power conversion efficiencies exceeding 15%, demonstrating bright prospects for practical applications of organic solar cell technology [5,6]. Typical polymer solar cell devices have a multi-layer stack structure, including a bulk heterojunction (BHJ) active layer, anode and cathode, and interlayers between the BHJ and electrodes.

Currently, active layer materials are developing rapidly, and related photoelectric conversion mechanisms have been deeply studied and understood [7-10]. Compared to active layer materials, interlayer development has relatively lagged behind. In particular, understanding of charge collection processes remains unclear due to complex energy level structures at the interlayer/active layer interface [11-13]. In fact, interface engineering has proven to be an effective method for improving polymer solar cell power conversion efficiency.

Figure Analysis

Figure 2

• Electrical Measurements: (a) Current density-voltage (J-V) characteristics of polymer solar cell devices F-PFN-Br, F-PFN, NF-PFN-Br, and NF-PFN
• Quantum Efficiency: (b) External Quantum Efficiency (EQE) analysis
• Impedance Analysis: (c) Nyquist plots of devices F-PFN-Br, F-PFN, NF-PFN-Br, and NF-PFN
• Equivalent Circuit: (d) Equivalent circuit model used for fitting electrochemical impedance spectroscopy (EIS) of devices

Table 1

Photovoltaic parameters of conventional structure devices based on F-PFN-Br, F-PFN, NF-PFN-Br, and NF-PFN under AM 1.5 G, 100 mW cm^-2 illumination.

Table 2

Fitting results of the Nyquist plots of the devices.

Figure 4

(a) Electron spin resonance (ESR) spectra of neat IT-4F, PFN-Br, and PFN films
(b) ESR spectra of IT-4F:PFN-Br and IT-4F:PFN blends
(c) Schematic diagram of electron transport process in PBDB-T-2F:IT-4F/PFN and
(d) PBDB-T-2F:IT-4F/PFN-Br

Figure 5

(a) Chemical structures of F-N and F-Br
(b) J-V curves
(c) EQE curves of polymer solar cell devices modified with small molecule models F-N and F-Br
(d) ESR spectra of IT-4F:F-Br and IT-4F:F-N blends

Figure 6 Semi-logarithmic current density-voltage characteristics of pure electronic devices

• Structures A and B: (a) A: ITO/ZnO/PC₇₁BM/PFN/Al and B: ITO/ZnO/PC₇₁BM/PFN-Br/Al
• Structures C and D: (b) C: ITO/ZnO/IT-4F/PFN/Al and D: ITO/ZnO/IT-4F/PFN-Br/Al
• Structures E and F: (c) E: ITO/ZnO/IT-4F/FN/Al and F: ITO/ZnO/IT-4F/F-Br/Al

Conclusions

This work demonstrates the different effects of PFN and PFN-Br in modifying polymer solar cells and deeply investigates their working mechanisms. Although PFN and PFN-Br interlayers work well in fullerene-based polymer solar cells, significant differences are observed when using PFN and PFN-Br interlayers to modify non-fullerene devices. Due to poor acceptance capability of excess electrons, doping on non-fullerene acceptor IT-4F leads to unpaired electron accumulation near the IT-4F/PFN interface and forms a dense layer of negative molecules, which blocks electron transfer from the active layer to the interlayer, reducing device charge collection efficiency.

Pure electronic devices further verify our inference that excessive doping between PFN and IT-4F hinders electron collection in polymer solar cell devices. By replacing PFN with PFN-Br, charge collection efficiency was significantly improved, achieving a high power conversion efficiency of 13.5%. The results of this work elucidate the different effects of doping on the modification of fullerene-based and fullerene-free polymer solar cells, providing new inspiration for selecting appropriate interface materials to construct efficient polymer solar cells.

Publication 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

DOI: 10.1016/j.jechem.2019.08.005