Simultaneous reduction and sulfonation of graphene oxide for efficient hole selectivity in polymer solar cells

https://doi.org/10.1016/j.cap.2018.02.016Get rights and content

Highlights

  • Development of S-RGO through acid treatment of GO in ambient.

  • Tuning S-RGO properties via control of water content of the reaction mixture.

  • Employing the S-RGOs as hole extraction layer materials in P3HT:PC61BM solar cells.

  • Performance interpretation in terms of series & shunt resistances (RS & RSh).

  • RS & RSh elaboration in terms of energy level diagram and the nature of the S-RGO.

Abstract

We developed sulfonated, reduced graphene oxide (S-RGO) through fuming/concentrated sulfuric acid treatment of graphene oxide (GO) in ambient conditions. It was demonstrated that the optical band gap and electrical conductivity of S-RGO are easily tunable, and depend on the level of reduction and sulfonation of GO. Whereas, reduction and sulfonation were found dependent on SO3 content, acid strength, and gas tightness of the reaction mixture. It's actually the water content of oleum that determines the nature of the final product. The easily adjustable band gap and electrical conductivity suggest that S-RGO can be employed as a potential hole extraction layer (HEL) material for several donor-acceptor systems. For P3HT:PC61BM based inverted polymer solar cells, it was observed that the shape of the J–V curve is tailorable with the choice of HEL. Compared to a 2.75% power conversion efficiency (PCE) attained with PEDOT:PSS, a PCE of 2.80% was achieved with tuned S-RGO. Our results imply that an S-RGO of sufficiently high band gap and conductivity can replace some of the state of the art HEL materials for a host of device applications.

Introduction

The nature of the interface between the organic blend layer and the electrode is crucial to determining the overall performance of a bulk-heterojunction (BHJ) polymer solar cell (PSC) [1]. For non-ohmic contacts with some energy barrier height at the organic active layer/electrode interface, charge accumulation results in recombination loss at the interface. Ohmic contacts with the least energy barrier height at these interfaces are thus required for efficient charge extraction [2]. To overcome this and other associated problems, some modification to the interfaces is critical to realize devices with higher efficiency and more stability. One solution is to use dedicated charge extraction layers (CELs), sandwiched between organic blend layer and electrode, to facilitate selective extraction of photogenerated carriers to respective electrodes. Recently, the CELs are widely debated in the scientific community as sites affecting a BHJ-PSC's performance [3,4].

CELs are meant to efficiently extract the photogenerated charge carriers from the BHJ layer, and then transport these carriers to the respective electrodes with least resistance in the transport path. CELs need to be charge selective, allowing only one type of charge carrier to be extracted, while blocking the other as well as excitons. CELs develop ohmic contacts for selective carriers' extraction and transport by minimizing the energy barrier height at the active layer/electrode interfaces. The ohmic contacts, as a consequence, help reduce charge accumulation and thus minimize charge recombination at the interfaces. Moreover, an interlayer may serve as a buffer layer to help prevent chemical reactions that may take place, in case of direct contact between the photoactive layer and the electrode metal/transparent conducting oxide (TCO). Furthermore, it may avoid the diffusion of metal ions from either of the electrodes into the photoactive layer. If suitably thick and transparent, it may also serve as an optical spacer, which helps distribute the incident light in BHJ-PSCs in a way that maximum incident light is harvested by the photoactive layer at its center. The overall effect is an increase in short circuit current density and improved stability of BHJ-PSCs [[2], [3], [4], [5], [6]].

CELs can be grouped into electron extraction layers (EELs), and hole extraction layers (HELs). The proper choice of both the EEL and HEL is critical for efficient device performance. For a host of donor-acceptor systems, low work function metal oxides such as zinc oxide (ZnO) and titanium oxide (TiO2) are popular EEL materials, as they fulfil most of the chemical and physical requirements of an EEL material. On the other hand, the quest for an efficient and easily processable HEL material for PSCs is on the ascendant these days. Sufficiently high work function [7], precisely tuned band structure [8,9], and good p-type electrical conductivity remain the key criteria for good hole selectivity and transport by HEL materials. Low contact resistance with the neighboring layers is another critical performance indicator of a HEL. Low temperature synthesis and solution processibility are other attributes of a versatile and cost-effective hole extraction system, whose importance becomes even more pronounced in inverted BHJ-PSCs. Typical HEL materials which have been in common use are conductive polymers, self-assembled organic molecules, and vacuum or solution deposited metal oxide layer (e.g., MoO3, WO3, V2O5, NiO) [4,10,11]. The state of the art HEL material is poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), used widely in organic photovoltaics. However, its acidity, tendency to absorb water and inability to block electrons make it less than ideal [12]. PEDOT:PSS films often show inhomogeneous electrical and morphological properties [13,14], resulting in poor long-term stability of PSCs. Thus, compelling material scientists to search for alternative HEL materials.

Tuned graphene materials are potential candidates for replacing PEDOT:PSS as HEL in many solar cell technologies. Graphene materials are currently being intensively studied for applications in electronics [10,[15], [16], [17]], bio medics [18,19], structural reinforcement [20,21], capacitors [22,23], batteries [24,25], GO sponges [26], and sensors [27,28]. However, use of pristine graphene in electronics is limited by its zero band gap and inertness to reaction. Often some functionalization to its honeycomb structure is desired to tune its properties for specific applications. Furthermore, functionalization makes it solution processable, which in turn facilitates low cost fabrication [29]. Functionalization opens innumerable ways to research and apply graphene materials for a host of electronic applications. It remains a challenge however, to design graphene materials with controlled electronic properties as HEL for high performance PSCs [11].

As a general rule, surface modification of materials by introducing surface dipoles onto their surfaces through chemisorption, modifies the band structure of materials [30]. In general, the electron withdrawing power of a chemisorbate can be used to determine the possible kind of shift in work function. More electrophilic chemisorbates relative to the substrates, drag electrons away from the surface, thus creating an excess negative charge outside, while an excess positive charge inside the surface. This is responsible for an increase in work function. On the contrary, a decrease in work function takes place with nucleophilic chemisorption. Thus, the more the electron withdrawing power of the electrophile and larger the dipole content on the surface, the larger the electron density redistribution and shift in work function [30,31].

In case of graphene, its electronic structure, conductivity, and solubility can be widely modified through surface functionalization [32]. For graphene based HEL development, the idea should be to introduce the least number of electrophilic groups, that would sparsely disrupt the conjugation and conductivity, yet sufficiently increase the work function and band gap. In this regard, a wide array of electrophiles is available, however, only strong electrophiles that could be easily and cost effectively attached to the graphene structure would be considered for the study. Sulfonic acid and bisulfate groups (–SO3H and –OSO3H, respectively) are two preferred choices for this study. These are strong electron withdrawing groups. The strong electrophilic nature of these groups arises from the strong electronegative atoms (O and S) that constitute the group [33]. Since these groups can be easily and cost effectively attached to the graphene structure using wet chemical approaches, hence are considered for this study.

Surface functionalized graphene materials such as graphene oxide (GO) and sulfated GO (prepared by treating GO with oleum in inert) have already been tried as HEL materials for normal geometry PSCs [10,11,34]. Sulfonated, reduced graphene oxide (S-RGO) synthesis by a lyophilization-assisted method from a mixture of GO and (NH4)2SO4 with a subsequent thermal treatment in inert, has been reported for fuel cell applications [35]. Herein, the development of S-RGO through acid treatment of GO in ambient conditions is reported. We modify its electronic and optical properties through modification of process conditions, and apply the resulting S-RGOs as HEL in inverted BHJ-PSCs.

This work is structured as follows: An account of the experimentation and the materials involved is presented. Here, we elaborate the procedures involved in the development of S-RGO and its variants through treating GO with acids of different strengths in different atmospheres. It is followed by the procedures adopted for zinc oxide sol preparation. Processing conditions for device fabrication along with the dimensional specifications of each layer are highlighted next. Different instruments and characterization techniques employed are mentioned in the next section. Then we start discussing the results obtained. EDS spectra for elemental analysis, XRD patterns for crystal structure analysis, FTIR for chemical bond analysis, Kubelka-Munk treatment of diffuse reflectance spectra (DRS) for band gap analysis, and hall effect measurements for electrical and electronic characterization are discussed in sufficient detail. Based on the results obtained, an interpretation of the simultaneous reduction and sulfonation in terms of the underlying chemistry is suggested. In an attempt to demonstrate S-RGO as a potential HEL material, the S-RGOs are tried as hole selective contacts for one of the most widely studied polymer donor-acceptor system, that is, the poly(3-hexylthiophene)-regio-regular (RR-P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) donor-acceptor system. Performance in terms of current density–voltage (J–V) characteristics is compared to that of the PEDOT:PSS device. The change in performance is explained in terms of the series and shunt resistances (RS and RSh respectively) as well as energy level diagram, and is correlated to the nature of each S-RGO employed.

Section snippets

S-RGO development

Scheme 1 describes the basic synthesis route to the development of S-RGO. S-RGO was prepared from GO, by treating it with fuming/concentrated sulfuric acid. The precursor GO was prepared according to a modified Hummer's method reported elsewhere [36]. Three variants of S-RGO with varying sulfur (S) content were prepared. One of the samples, which we call S-RGO1, was prepared by reacting a mass of 100 mg of GO with 30 ml of 2% fuming sulfuric acid. The reaction mixture was stirred for 2.5 days

Results and discussion

It is well documented that the carbon basal plane of GO is mostly occupied by hydroxyl and epoxide groups, whereas, at the periphery, GO sheets mostly terminate with hydroxyl and carboxyl groups. Presence of these groups disrupts the π electron conjugation in GO [36,37]. With this understanding about GO, lets further analyze GO and its derivatives, S-RGO1, S-RGO2, and S-RGO3.

The EDS spectra of Fig. 1 has two distinct sets of peaks – the red ones and the green ones (labelled). The red peaks mark

Conclusion

With a considerable attention to exploit the potential of S-RGO, its electro-optical properties can be engineered to meet a variety of device requirements. S-RGO, in this work, was prepared through fuming/concentrated sulfuric acid treatment of GO in ambient conditions. Further, three variants of S-RGO, varying in C/S and C/O were prepared by simply varying the reaction parameters/conditions such as SO3 content, H2SO4 strength, and gas tightness of the reaction mixture. Relation between the

Acknowledgements

This work was financially supported by the Research and PGP Directorates of National University of Sciences and Technology (NUST), Islamabad, Pakistan. We are thankful to School of Chemical and Materials Engineering, NUST, Islamabad, Pakistan and Ira A. Fulton Schools of Engineering, ASU, Arizona, USA, for granting access to their processing and characterization facilities.

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