Abstract
Solution-processed inverted bulk heterojunction (BHJ) organic solar cells (OSCs) are expected to play a significant role in the future of large-area flexible devices and printed electronics. In order to catch the potential of this inverted BHJ technology for use in devices, a solar cell typically requires low-resistance ohmic contact between the photoactive layers and metal electrodes, since it not only boosts performance but also protects the unstable conducting polymer-based active layer from degradation in the working environment. Interfacial engineering delivers a powerful approach to enhance the efficiency and stability of OSCs. In this study, we demonstrated the surface passivation of the ZnO electron transport layer (ETL) by an ultrathin layer of tetracyanoethylene (TCNE). We show that the TCNE film could provide a uniform and intimate interfacial contact between the ZnO and photo-active layer, simultaneously reducing the recombination of electron and holes and series resistance at the contact interface. After successful insertion of TCNE between the ZnO film and the active layer, the parameters, such as short circuit current density (Jsc) and fill factor (FF), greatly improved, and also a high-power conversion efficiency (PCE) of ∼8.59% was achieved, which is ∼15% more than that of the reference devices without a TCNE layer. The devices fabricated were based on a poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b : 4,5-b′]dithiophene-2,6-diyl]-[3-fluoro-2[(2-ethylhexyl)-carbonyl]-thieno[3,4-b]thiophenediyl]] (PTB7):(6,6)-phenyl C71 butyric acid methyl ester (PC71BM) blend system. These results suggest that this surface modification strategy could be readily extended in developing large-scale roll-to-roll fabrication of OSCs.
Highlights
Bulk-heterojunction (BHJ) organic solar cells (OSCs) have attracted signi cant consideration in the past few years due to their promising potential to provide environmentally safe, exible, lightweight, and cost-effective solar cells
The most widely followed passivation methodologies are the insertion of suitable interfacial modi ers, such as conjugated polyelectrolytes (CPEs),[27] alcohol/water-soluble conjugated polymers,[28] self-assembled monolayers (SAMs),[29] small molecules,[30] and ionic liquids (ILs),[27] at the interface which improved the electronic coupling between Zinc Oxide (ZnO)/photo-active blend resulting enhanced device characteristics
We propose that the solution-processed organic small molecule TCNE can effectively passivate the surface defects of ZnO based electron transport layer (ETL)
Summary
Bulk-heterojunction (BHJ) organic solar cells (OSCs) have attracted signi cant consideration in the past few years due to their promising potential to provide environmentally safe, exible, lightweight, and cost-effective solar cells. ZnO is a more attractive material owing to its excellent optical transparency, relatively high electron mobility, environment-friendly nature, and ease of fabrication.[19,20] The energy levels of zinc oxide are about 4.3 eV (conduction band minimum) and 7.8 eV (valence band maximum) This energy band position enables ZnO to play a signi cant role in electron collection and hole blocking.[21] A variety of low-temperature and solution-based fabrication methods have been demonstrated to deposit ZnO as ETL.[22] Despite the advantages of ZnO as ETL, sur cial defects on ZnO thin- lm can behave as recombination centers for photogenerated charge carriers, causing signi cant harm in both photovoltage and photocurrent, worsening the performance of devices.[23,24] they create adsorption sites for environmental oxygen and water molecules, which are highly detrimental to device stability.[25,26] the surface defect passivation strategy of ZnO has become vital concurrently in enhancing the PCE and the sustainability of inverted OSCs. The most widely followed passivation methodologies are the insertion of suitable interfacial modi ers, such as conjugated polyelectrolytes (CPEs),[27] alcohol/water-soluble conjugated polymers,[28] self-assembled monolayers (SAMs),[29] small molecules,[30] and ionic liquids (ILs),[27] at the interface which improved the electronic coupling between ZnO/photo-active blend resulting enhanced device characteristics. It could be a promising alternative for other optoelectronic devices, which is bene cial for large-scale production
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