A Simple Approach to Fabricate an Efficient Inverted Polymer Solar Cell with a Novel Small Molecular Electrolyte as the Cathode Buffer Layer

Abstract

A novel small-molecule electrolyte, 1,1'-bis(4-hydroxypropyl)-[4,4'-bipyridine]-1,1'-diium bromide (V-OH), containing a mixture of PTB7:PCBM has been designed and synthesized as a cathode buffer layer for inverted polymer solar cells (iPSCs). The molecular structure of this new compound comprises a viologen skeleton with hydroxyl group terminals. While the viologen unit is responsible for generating a favorable interface dipole, the two terminal hydroxyl groups of V-OH may generate a synergy effect in the magnitude of the interface dipole. Consequently, the devices containing the V-OH interlayer exhibited a power conversion efficiency (PCE) of 9.13% (short circuit current = 17.13 mA/cm, open circuit voltage = 0.75 V, fill factor = 71.1%). The PCE of the devices with V-OH exhibited better long-term stability compared to that of the devices without V-OH. Thus, we found that it is possible to enhance the efficiency of PSCs by a simple approach without the need for complicated methods of device fabrication.

Polymer Solar Cells—Interfacial Processes Related to Performance Issues

“…The formation of the interface dipoles leaves a significant influence on the device performance which has been explained through various studies carried out on wide variety of organic materials (Collins et al, 1994; Kaspar et al, 1994; Schmidt et al, 1995a,b; Van Slyke et al, 1996; Katz, 1997; Hiller et al, 1998; Lee et al, 1998, 1999; Chen et al, 2013; Liu C. et al, 2015; Kim et al, 2016a,b; Liu et al, 2016; Zhang et al, 2016; Yang et al, 2017; Yu et al, 2017). It has been reported in these studies that interface dipoles strongly influence the process of charge carrier injection/extraction across the electrode interfaces, which in certain cases can lead to the generation of the S-kink in the current-voltage characteristics of the solar cells.…”

“…It has been reported in these studies that interface dipoles strongly influence the process of charge carrier injection/extraction across the electrode interfaces, which in certain cases can lead to the generation of the S-kink in the current-voltage characteristics of the solar cells. On the other hand, the formation of the favorable interface dipoles may improve the charge extraction efficiency by minimization of the interfacial energy barriers, therefore increasing the built in potential, and suppressing charge carrier recombination at the interfaces (Chen et al, 2013; Liu C. et al, 2015; Kim et al, 2016a,b; Liu et al, 2016; Zhang et al, 2016; Yang et al, 2017; Yu et al, 2017). Furthermore, the formation of such surface dipoles is also important in organic heterojunctions, which play important role in photovoltage formation and exciton dissociation occurring at this heterojunction interface (Schmidt et al, 1995a,b; Katz, 1997; Hiller et al, 1998).…”

“…The study has reported that the spontaneous orientation of liquid-crystal polar groups can induce a dipole moment at the interface which leads to better energy-level alignment (Liu C. et al, 2015). A small-molecule electrolyte, 1,1′-bis(4-hydroxypropyl)-[4,4′-bipyridine]-1,1′-diium bromide (V-OH), used as an interlayer between ZnO and active layer of PTB7:PC 71 BM has been used as a cathode buffer layer for inverted polymer solar cells (Kim et al, 2016b). The molecular structure of the electrolyte has been reported to be responsible for generating a favorable interface dipole which leads to an improvement in the device efficiency from 7.41 to 9.13% (Kim et al, 2016b).…”

“…A small-molecule electrolyte, 1,1′-bis(4-hydroxypropyl)-[4,4′-bipyridine]-1,1′-diium bromide (V-OH), used as an interlayer between ZnO and active layer of PTB7:PC 71 BM has been used as a cathode buffer layer for inverted polymer solar cells (Kim et al, 2016b). The molecular structure of the electrolyte has been reported to be responsible for generating a favorable interface dipole which leads to an improvement in the device efficiency from 7.41 to 9.13% (Kim et al, 2016b). Other types of small-molecule electrolytes have also been shown to improve the device efficiency from 7.35% up to 9.20% (Kim et al, 2016a).…”

Organic Electrolytes Doped ZnO Layer as the Electron Transport Layer for Bulk Heterojunction Polymer Solar Cells

“…Reducing the energy gap at the interface and passivation of ZnO surface are important to improve the performances of the devices. Many approaches have been tried to modify the surface of ZnO such as deposition of thin layer of conjugated or non‐conjugated polyelectrolytes (i.e., interlayer), metal chelates, and self‐assembled monolayer (SAM) treatments on the top of the ZnO surface to improve electron collection at the ZnO surface. In case of the former method, the performance of the device is very sensitive to the thickness of the interlayer, which is normally in the range of 5–10 nm and should be optimized.…”