Research ArticleEffect of different terminal groups of phenyl boronic acid self-assembled monolayers on the photovoltaic performance of organic solar cells
Graphical abstract
Introduction
Thin film solar cells fabricated with organic semiconductor materials are considered to be good alternative to conventional silicon based solar cells, since they offer excellent advantages including solution processability, flexibility, low-cost production, light weight and easy integration to different technologies [1]. Among other thin film organic solar cells, bulk heterojunction (BHJ) structured organic solar cells (OSCs) has presented a remarkable performance improvement over a power conversion efficiency (PCE) of 18% during the last two decades [2]. In a BHJ OSCs, photoactive layer is a bicontinuous interpenetrating network blended of donor and acceptor materials and it is mainly placed between anode and low work function (WF) cathode [3]. In most widely studied OSCs structure, conjugated polymers and pigments or oligomers are used as donor materials, while fullerene derivatives are typically are used as acceptor materials [4]. Among other alternative photoactive blends, numerous effort have been employed on poly(3-hexylthiophene) [6,6]: phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend [5]. However, further approaches still need to be developed to improve the level of photovoltaic performance of OSCs with P3HT:PCBM as photoactive layer. Up to now, many approaches including developing of new efficient materials, designing of device structure or interfacial modification have been utilized [[6], [7], [8], [9]].
The interfacial modification is common and easy way to increase the performance of solar cells, since it could not only enhance the charge transport properties but also provide better upper layer morphology. For example, Lee et al. introduced zinc phthalocyanine between P3HT and PCBM in OSCs. It was reported that the device efficiency improved from 3.15% to 3.56% [10]. In other study, phosphonic acid‐anchored fullerene self‐assembled monolayers (SAMs) was used as interfacial modifiers between aluminium‐doped zinc oxide and P3HT:PCBM layer in OSCs by Stubhan et al. [11]. They increased the PCE from 2.9% to 3.3% due to the unique properties offered by SAMs.
SAMs are considered to be the key element in nanotechnology studies. Since the SAMs are small size molecules (from the sub-nm to a few nm) allowing to provide ultra-thin interfacial layer, they are promising to build organic electronic devices [12]. SAMs are important to enhance organic electronic device performance, not only for leading better interfacial contact between organic and inorganic layer but also for improving the charge transfer and morphological properties [13]. With these, SAMs could be used to decrease back charge recombination, which is one of the most significant factor to obtained improved solar cell performance [14]. In addition, work function of surfaces could be easily modulated by SAMs [15]. Despite all these advantages, the insufficient surface coverage of SAMs could lead to form delocalized defect on this modification layer. This issue is basically related to structure of SAMs molecules, especially to the anchor group. The reactive anchor groups of SAMs selectively interact with surfaces, which could lead to sufficient coverage [16]. There have been many reports on SAMs with different anchor groups including silane, thiol or phosphoric acid on different surfaces [17]. Surprisingly, there are very limited reports on the organic electronic device modified through SAMs with boronic acid anchor group. According to studies reported, this new SAMs class seems to be able to enhance the solar cells efficiencies by attaching to metal or metal oxide surfaces [18,19]. In this framework, we suspected that phenyl boronic acid SAMs could be applied as a successful interfacial modification layer on OSCs.
With the large diversity of the using of boronic acid derivatives in medical applications, catalysis, material science and so on, a series of phenyl boronic acid derivative SAMs with various functional groups terminated were introduced as an interfacial modification layer to OSCs structure and their effect on the photovoltaic performance was investigated. The SAMs used in this study are illustrated in Fig. 1. The conventional OSCs comprising these phenyl boronic acid SAMs showed improved efficiencies. It was determined that the efficiencies of OSCs are strongly depend on the variety of functional groups terminated, since these functional groups affect the surface charge states and packing pattern of SAMs [20]. Additionally, it is worth to note that these SAMs can be effectively used to control the growth of upper layer, thereby morphology. Our study provides a development of easy and new approach to enhance the interfacial contact and charge transport properties between organic and inorganic layer in OSCs.
Section snippets
Materials and device fabrication
1.5 cm × 1.5 cm indium tin oxide (ITO) coated glass substrates (Lumtec Corp., with a resistance of 15 Ω/square with a thickness of 1.1 mm) were successively sonicated with diluted Hellmanex detergent solution at the ratio of 1:50 (Sigma Aldrich, Hellmanex III), deionized (DI) water, acetone and isopropanol for 10 min, each. After being dried by N2, ITO surfaces were treated by oxygen plasma for 5 min. To obtain SAM modified ITO surfaces, all SAM solutions at a concentration of 1 mM were
Surface analyses
To determine the surface properties including wettability and surface energy, water contact angle measurements were performed by sessile drop method onto ITO surfaces non-modified and modified with SAMs and calculated from the values at three different points. The images of sessile water drop, and surface energy values calculated were given in Fig. 2 and Table 1, respectively. It should be note that all SAMs modified ITO surfaces produced an important decrease in contact angles in comparison to
Conclusion
As summary, an effective strategy was proposed to fabricate highly efficient OSCs by novel phenyl boronic acid SAMs as interfacial modification layer. With this approach, we observed the improved Jsc and FF values, thereby PCEs. The highest efficiency of 3.17% for NO2-PBA SAM modified OSC was determined with a Voc of 0.583 V, a Jsc of 9.84 mA cm−2 and a FF of 44.3%. The champion device presented over 45.7% enhancement than that of control device without SAM layer (a PCE of 2.18%). The improved
CRediT authorship contribution statement
Çisem Kırbıyık Kurukavak: Conceptualization, Formal analysis, Writing - original draft, Conception and design of study, Analysis and/or interpretation of data, Drafting the manuscript, Revising the manuscript critically for important intellectual content. Tuğbahan Yılmaz: Conceptualization, Formal analysis, Writing - original draft, Conception and design of study, Analysis and/or interpretation of data, Drafting the manuscript, Revising the manuscript critically for important intellectual
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.
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