Electrostatically Self‐Assembled Nonconjugated Polyelectrolytes as an Ideal Interfacial Layer for Inverted Polymer Solar Cells

Abstract

Polymer solar cells (PSCs) based on the bulk heterojunction (BHJ) blend of a conjugated polymer donor and a fullerene acceptor have gained considerable attention as a cost-effi cient, fl exible, and portable energy source. [1][2][3] Considering the practical aspects of its commercialization, the inverted device structure of PSCs (I-PSCs), consisting of a BHJ photoactive layer between indium tin oxide (ITO) as a bottom cathode and a high work function (WF) metal (Ag or Au) as a top anode, is an advantageous approach due to its superior long-term stability and printability. [4][5][6] In this type of inverted device structure, an additional interfacial layer between the active layer and ITO electrode must be introduced to establish the device concept. The high WF of the bottom ITO cathode (approximately 4.8 eV) hampers the formation of an ohmic contact with the lowest unoccupied molecular orbital (LUMO) level of fullerene and makes the creation of a high built-in fi eld used to break the electrical symmetry of the device diffi cult. Thus, many researchers explored a variety of interfacial materials to shift and modify the energy level of the ITO cathode. [4][5][6][7][8][9][10][11][12][13][14][15][16] Inorganic metal oxides (MOs), such as titanium oxide (TiO x ) or zinc oxide (ZnO), have been widely used as interfacial materials due to their solution processability (via a sol-gel precursor and nano-particle solutions) and their electron selective properties, which originate from their conduction band edges (typically at approximately 4 eV) that correspond well with the LUMO level of fullerene and the deep valence bands at approximately 8 eV. [8][9][10][11][12] However, I-PSCs using MOs typically require a high annealing temperature (over 200 ° C) to achieve a crystallinity of the MO that is suffi ciently high to yield a high charge carrier mobility, [ 8 , 9 ] and these devices require a crucial post-UV treatment to enhance the device performance (typically referred to as a 'light-soaking' problem). [10][11][12] Because the high-temperature annealing process is incompatible with typical printing processes that use fl exible plastic substrates and the post-UV treatment causes a harmful photo-oxidation in the conjugated polymers, these processes should be eliminated to promote the practical use of I-PSCs.To circumvent the inherent weaknesses of the MO interlayers, polymer-based interfacial materials, such as conjugated polyelectrolytes (CPEs) and nonconjugated polyethylene oxide (PEO), have also been employed. These polymer-based interlayers alter the WF of ITO by forming extremely thin interfacial dipoles (typically 1-2 nm) and facilitate charge transport into the ITO electrode. [17][18][19][20] Becuase cationic CPEs have charged ionic pendant groups in their structures, they can induce signifi cantly stronger dipole moments than the neutral PEO, resulting in a higher device performance than that of the I-PSC using PEO. [ 19 ] However, because CPEs typically require a delicate and complicated synthesis procedure, nonc...

Interfacial Materials for Organic Solar Cells: Recent Advances and Perspectives

“…As an alternative to conjugated polymer CILs, non‐conjugated polyelectrolytes (NCPEs) with charged ionic groups in their structures can also be employed to tune WFs of cathodes so as to reduce interfacial energy barriers and increase the built‐in potential of inverted devices 122, 123. Representative NCPE interlayer materials such as polyethyleneimine (PEI), polyallylamine (PAA), and ethoxylated polyethyleneimine (PEIE) have been directly used as CILs for OSCs 122, 123, 124.…”

“…Representative NCPE interlayer materials such as polyethyleneimine (PEI), polyallylamine (PAA), and ethoxylated polyethyleneimine (PEIE) have been directly used as CILs for OSCs 122, 123, 124. The use of solution‐processed PEI layer can drastically reduce the WF of ITO from 4.8 to 4.0 eV, which is originated from the strong electrostatic self‐assembled dipoles created by the presence of protonated amines within the ITO/PEI cathode.…”

Performance Study of Water/Alcohol Soluble Polymer Interface Materials in Polymer Optoelectronic Devices

“…与此同时, 非共轭的水/醇溶聚合物界面材料也在 有机光电器件上得到了成功的应用, 实现了高效的聚合 物发光器件 [24,25] , 聚合物太阳电池 [12,26,27] 等. 目前的研 究表明, 共轭及非共轭水/醇溶界面材料中含有的极性 基团, 在它们的独特界面修饰作用中起了关键作用, 可 以与金属或金属氧化物电极发生相互作用形成界面偶 极而降低电极功函数, 增强电极电子的注入与收集.…”

Influences of surface capping with electrostatically self-assembled PEI on the photoresponse of a TiO2 thin film

“…13,14 Solution-processed organic interfacial materials, which can alter the work function (WF) on the surface of semiconductor materials by forming extremely thin interfacial dipoles (typically 30 1-2 nm), have been demonstrated good substitute for inorganic counterparts in inverted polymer solar cells and light-emitting devices, [15][16][17][18][19] especially the nonconjugated polyelectrolytes (NPEs) due to their high stability, easier synthesis procedure than conjugated polyelectrolytes, and unique film formation 35 characteristics of ionic self-assembly onto oppositely charged surfaces. 20,21 However, the application of NPEs in UVPDs has been seldom reported. As the Schottky barrier between metal electrodes and wide band-gap semiconductors can be tuned with the variation of WF on the surface of semiconductors, NPEs can 40 be expected to be ideal interfacial materials for photodetectors.…”

“…1(a). In aqueous 55 solution, the functional amines of PEI can be partially protonated by accepting protons (H + ) dissociated from the water due to their strong basicity, 21 which makes PEI exhibit cationic characteristics. When PEI is deposited on the hydroxylated surface of TiO 2 from its aqueous solution, the electrostatic self-60 assembly of PEI occurs, as shown in Fig.…”