Influence of flexible substrates on inverted organic solar cells using sputtered ZnO as cathode interfacial layer
Graphical abstract
Introduction
Because of their high mechanical flexibility, simple fabrication process, low cost and light weight, flexible organic solar cells (OSCs) give an attractive alternative to Si based flexible thin film solar cells [1]. Flexibility means that the solar cells can be easily mass-produced using roll-to-roll processing. However, in order to achieve high device efficiencies on flexible substrates, further studies on the appropriate materials and processes used in the OSCs are still needed. This is especially true for inverse OSCs, which have attracted a growing attention since their initial development [2]. With respect to conventional structures, inverted devices have many advantages, such as the absence of the indium tin oxide/poly(3,4-ethylene dioxythiophene):poly(styrenesulfonaten) (ITO/PEDOT:PSS) interface [3], and the possibility to use air-stable metal as anode for collecting holes (e.g. gold or silver) [4]. It was also reported that the phenomenon of vertical phase segregation between poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) makes the inverted structure a better choice for OSCs [5]. Several studies have shown that inverted devices can exhibit high power conversion efficiencies (PCEs) using P3HT:PCBM or newly developed donors [6]. However, up to now, research has been mainly focused on efficiency and stability, while the effects of the flexible substrate on OSC performances have not been considered. These effects are something far more important so that recent progress in OCSs has been achieved using semiconducting metal oxides as interfacial layers [7]. As a consequence of this, further restrictive processes that can affect flexible devices are required and have to be investigated. Among interface materials investigated as cathode interfacial layers, zinc oxide (ZnO) is considered as particularly promising, mainly due to its chemical stability, optical properties (near-UV emission and transparency) and high electric conductivity [8]. Many fabrication techniques have been used to grow ZnO as cathode interfacial layer. Most of them are relying on wet processing and have been proved to be compatible with flexible substrate using ZnO nanoparticles [9]. However, ZnO nanoparticles are also known to be unstable in solution and their properties are strongly depending on the particles size, which impedes a precise control of the layer thickness. On the other hand, the sol–gel method is considered as a more stable and controlled wet processing technique and has been extensively investigated to grow ZnO films. Usually, in order to promote crystallization and remove impurities from the organic solution, a high annealing temperature, much over 200 °C, is usually required, which is not compatible with flexible substrates [10]. Recent works reported that uniform sol–gel-derived ZnO films can be obtained at relatively low annealing temperatures (130, 150 and 200 °C) and used for inverse OSCs with high conversion performances [11]. Nevertheless, these progresses in wet processing cannot exclude the emergence of dry deposition technologies used in production to grow interfacial layers for flexible OSCs. High quality ZnO thin films can be prepared on large-area ITO-based flexible substrates for OSCs, using, among other techniques, plasma-enhanced chemical vapor deposition (PECVD) [12], microwaves plasma chemical vapor deposition (MPCVD) [13], atomic layer deposition (ALD) [14], and sputtering [15]. These processes can operate at relatively low-temperature to develop dense and reproducible films, which can be a determining factor in future industrial production of OSCs. Among them, the sputtering technique allows high deposition rates and is compatible with roll-to-roll coating processes [16]. Furthermore, sputtered ZnO films can be doped with metallic ions (such as Al) and thus lead to highly conductive layers which may substitute the more commonly used but expensive ITO electrode [17]. As known from recent studies, the properties of metal oxides grown by sputtering are determined by the crystalline quality and stoichiometry of the resulting film [18]. In addition, a thermal annealing is needed to improve the crystalline quality. However, the use of flexible substrates limits the window of annealing temperatures in OSC devices. For instance, a polymer substrate such as poly(ethylene glycol)2,6-naphthalate (PEN) is not supposed to be heated above 220 °C [19]. Indeed, higher temperatures can modify the surface properties of the polymer or/and increase the thermal strain at the interface that can significantly reduce the integrity of the ZnO layer and therefore the solar device performances. For a better understanding of these effects, a detailed investigation on the influence of the annealing process on the crystalline quality of the sputtered ZnO thin film on flexible substrate for OSCs is necessary. To our knowledge, such a study is still missing in literature. In this work, rf magnetron sputtering is employed to integrate ZnO interfacial films, at room temperature, between a flexible/ITO cathode and the organic photoactive layer in inverted OSC structures. As flexible substrate, we choose a commercially available PEN foil coated with ITO. Different annealing treatments are carried out on the PEN/ITO-coated ZnO films and their influence on the photovoltaic performances of the final cells is investigated. From the analysis of the surface and structural properties of the interfacial ZnO layer using atomic force microscopy (AFM) and X-ray diffraction (XRD), we show that high quality ZnO films can be obtained by appropriate thermal annealing. However, by varying the substrate surface properties, the crystalline quality of ZnO and P3HT can be modified with a positive impact on the photovoltaic characteristics of the device. Thus, we demonstrate that PEN photovoltaic cells reached similar performances to those of the corresponding cells prepared on glass with ZnO films annealed between 160 and 180 °C (see Supporting information). Interestingly, the best results for flexible cells are obtained for ZnO layers annealed at 160 °C when the S-shaped kinks observed in the J–V curves completely disappear. These particular features are analyzed and interpreted in the present study.
Section snippets
Experimental section
The device architecture, integrating flexible substrate, anodic and organic active layers elaborated in our study, is schematically shown in Fig. 1. The ZnO film is integrated between an ITO-coated PEN substrate and a P3HT:PCBM active layer as bulk heterojunction blend of the OSC. The top metallic anode of the structure is made of silver (Ag) and has been deposited onto an interfacial layer based on molybdenum trioxide (MoO3).
Surface morphology
Fig. 2 shows AFM images of the as-prepared and annealed (140, 160 and 180 °C) ZnO thin films grown by sputtering technique on PEN/ITO substrate. From AFM images, it is shown that except for the as-prepared sample, which exhibits a relatively large roughness (rms = 6.63 nm), the other samples annealed at 140, 160 and 180 °C present smoother surfaces with rms values of about 3.87, 3.80 and 3.24 nm, respectively (Table 1). Note that, unlike glass, polymer substrates cannot be polished and the surface
Evolution of P3HT crystallites size
As observed, by increasing the annealing temperature, the quality of ZnO is continuously improved up to 160 °C and the performances of inverted OSCs significantly enhanced. However, higher annealing temperatures lead to a decrease of the device performances, especially Jsc and PCE. No modifications in the PEN/ITO/ZnO quality have been observed, which can suggest little changes of the ZnO properties following the annealing treatment between 140 and 180 °C. Consequently, we can attribute this
Conclusion
In conclusion, the performances of inverted polymer solar cells integrating interfacial ZnO layers grown by low-temperature sputtering process on flexible substrates (PEN/ITO) and annealed at different temperatures were investigated. Using AFM and XRD techniques, we revealed that the thermal annealing of the ZnO films grown on flexible substrates, processed in the range of compatible temperatures, leads to modification that induces the enhancement of the ZnO and P3HT crystallites sizes. These
Acknowledgement
This work is supported by the French Ministry of Research and Education. The authors are very grateful to N. Zimmermana, J. Bartringera and B. Heinrichb for technical assistance.
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