Stable polymer solar cells using conjugated polymer as solvent barrier for organic electron transport layer
Graphical abstract
Introduction
Wearable technology has been considered as one of the advanced technologies for next-generation smart living. Devices made with strechability and flexibility are indepensible for wearable electronics. Generally, a power supply is needed to drive the passive device components. To be an effective power supply for flexible and wearable electronic circuits, it is essential to develop stretchable and flexible power sources such as polymer solar cells (PSCs) for instance. Typically, a PSC is composed of a polymer blend as the photoactive layer sandwiched between electron and hole transport layers (ETL & HTL) before contacting the anode and cathode. Metal-oxide such as ZnO, TiO2, etc. [[1], [2], [3]] of good electron mobility and conductivity are candidates for ETLs. Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), MoO3, and so on [[4], [5], [6]] are commonly used HTL materials. Lately, PSCs fabricated on rigid glass substrates with power conversion efficiency (PCE) exceeding 16% have been achieved [[7], [8], [9]]. Meanwhile, a few groups [[10], [11], [12], [13], [14], [15]] have also successfully demonstrated flexible/stretchable PSCs with rational design. Among those reports, metal-oxide film is the most popular ETL. However, the brittle nature of metal-oxide film can limit mechanical performance of devices. Organic materials carry superior inherent mechanical properties to their inorganic counterparts and can be good candidates for ETL materials to develop PSCs with good stretching/bending performance.
Thanks for the progressive development in PSCs recently, a few n-type semiconducting molecules other than fullerene and its derivatives have been synthesized. Poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (N2200) is a newly developed n-type semiconductor with good electron mobility [16], which is a substance suitable for ETL. Generally, most organic materials are dissolvable in organic solvents and the organic solvents can highly possible destory the underneath organic layer during processing. Therefore, it is difficult to stack organic films having similar solvent solubility together. To solve this problem, using orthogonal solvents to deposit the second organic layer can be a solution. However, it may not be efficient because the solubility of organic materials in orthogonal solvents might be limited and the morphology as well as the electrical properties of the resulting films can be greatly affected. Alternatively, it might be possible to introduce a solvent barrier between two consecutive organic layers to overcome this problem. Water-/alcohol-soluble poly[9,9-bis(6’-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis-(3-ethyl(oxetane-3-ethyloxy)-hexyl)-fluorene] (PFN-OX) has shown its good electrical properties as an interlayer in high performance inverted PSCs [17]. PFN-OX can be thermally cross-linked and becomes insoluble in most organic solvents. With rational design, one can incorporate the solvent-resistance feature of PFN-OX in fabricating flexible/stretchable PSCs to fully explore their mechanical property.
In this study, we introduce thermally cross-linkable polymer PFN-OX as solvent barrier predeposited on ETL for the fabrication of inverted PSCs. The most prominent polymer blend, P3HT:PCBM, is used as the photoactive material. We expect that the cross-linked property of PFN-OX can effectively prevent the subsecquent processing solvent from penetrating into the ETL and causing damage. The approach of introducing solvent barrier is facile, doable, and straightforward for stacking organic layers having similar solvent solubility together. To demonstrate our idea, we prepare devices on rigid glass substrates and their performance are compared with conventional inverted cells using ZnO as ETL (control devices). Devices using N2200/PFN-OX bilayer (N2200/PFN-OX devices) show much improved performance and stability. Similar behavior is also obtained when the pohotoactive material changes to PBDBT:N2200 blend. Further, by replacing N2200 with small molecule ITIC, PSCs can also be successfully fabricated and possess better performance than those with ZnO as ETL. Thus, the proposed approach is general. The advantages of using organic-ETL/PFN-OX are better surface compability, less interface recombination as well as excellent mechanical flexibility and stretchability over sol-gel derived ZnO film. Our organic-ETL/PFN-OX design idea not only offers a simple yet highly effective approach to fabricate all organic layer devices, but also can improve device performance, mechanical stability, and device stability as PSCs shown in this work.
Section snippets
Material preparation
ZnO sol-gel solution was prepared according to Ref. [18]. In brief, the ZnO precursor was prepared by dissolving 100 mg of zinc acetate dihydrate (Aldrich, 99.9%) and 27 μL ethanolamine (Aldrich, 99.5%) in 1 mL 2-methoxyethanol (Aldrich, 99.8%) under vigorous stirring at 60 °C for 12 h to the hydrolysis reaction in air. P3HT, PCBM, and PFN-OX were purchased commercially from Lumtech. Inc. and PBDBT, N2200, and ITIC were obtained from Solarmer. All materials were used as received without
Results and discussion
Fig. 1 (a) shows the fabricated inverted PSC device of layered structure of ITO/N2200/PFN-OX/P3HT:PCBM/MoO3/Ag along with the chemical structures of N2200 and PFN-OX materials. N2200 is the selected material for ETL and PFN-OX is introduced as the solvent barrier layer. We also fabricate PSCs using conventional ZnO as ETL for comparison. Fig. 1(b) displays the absorption spectrum of P3HT:PCBM and the transmitance of ITO/ETL(/PFN-OX). It clearly shows that ITO/ZnO possesses over 80%
Conclusion
In conclusion, we have successfully demonstrated to stack two organic layers of similar solvent solubility together through introducing a solvent barrier layer in the fabrication of inverted PSCs. The approach of introducing a solvent barrier layer to prevent an organic film from re-dissolution by the subsequent processing organic solvent is simple, doable, and straightforward. As demonstrated, the polymer blend P3HT:PCBM (or PBDBT:N2200) can be successfully deposited on organic-ETLs, including
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by Ministry of Science and Technology, Taiwan (Project Nos. MOST 108-2112-M-239-002 & MOST 109-2112-M-239-001).
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