Elsevier

Organic Electronics

Volume 99, December 2021, 106305
Organic Electronics

High-performance hole transport layer based on WS2 doped PEDOT:PSS for organic solar cells

https://doi.org/10.1016/j.orgel.2021.106305Get rights and content

Highlights

  • WS2 NS doped PEDOT:PSS (PEDOT:PSS:WS2 NS) was used as hole transport layers in OSCs.

  • The PEDOT:PSS:WS2 NS interlayer showed higher WF and increased hole mobility compared with pure PEDOT:PSS interlayer.

  • The device based on PEDOT:PSS:WS2 NS with PM6:Y6 as the active layer obtained PCE of 15.67%.

Abstract

In this work, we developed WS2 nanosheets (WS2 NS) doped into PEDOT:PSS (PEDOT:PSS/WS2 NS) as an efficient hole transport layer (HTL) for organic solar cells (OSCs). WS2 NS is obtained by a simple and cost-effective liquid-phase exfoliation (LPE) method. The results show that the PEDOT:PSS:WS2 NS anode interlayer has higher work function, reduced roughness and improved hole mobility. The devices based on PEDOT:PSS:WS2 NS interlayer show enhanced extraction and transportation of holes and improved fill factor and short-circuit current density. Based on PM6:Y6, the power conversion efficiency (PCE) increased from 14.35% to 15.67% after WS2 NS doping.

Introduction

In the last decade, organic solar cells (OSCs) have developed rapidly because of their simple fabrication, flexibility, large-scale production, wide material sources and low cost. Recently, the power conversion efficiency (PCE) of OSCs has exceeded 18% [[1], [2], [3]], surpassing the threshold of industrial production. The rapid enhancement for the OSCs performance was attributed to the designing of new photo-active materials, optimizing the morphology of blended film and interfacial engineering [[4], [5], [6], [7], [8]]. Interlayer is crucial for improving the overall PCE of OSCs due to its ability of reducing the energy barrier in the charge extraction from photoactive layer to electrodes [[9], [10], [11], [12], [13], [14]].

The conjugated polymer poly (3,4-ethylenedioxythiophene):poly (4-styrenesulfonic) (PEDOT:PSS) is the most widely used hole transport layer (HTL) in OSCs for decades, owing to the excellent water-solubility and high conductivity [15]. However, the hygroscopicity and acidity of PEDOT:PSS could corrode ITO substrates and decrease the stability of the device [16]. In addition, the relatively low work function 5.1eV of ITO/PEDOT:PSS is more suitable for early donor materials with relatively high highest occupied molecular orbital (HOMO), such as P3HT, but not suitable for the current efficient donor materials (most of them with HOMO deeper than 5.3eV) [8,17]. In order to overcome these shortcomings, a variety of PEDOT:PSS optimization methods have been proposed, such as doping with nanoparticles or graphene [15,[18], [19], [20], [21], [22], [23], [24]].

Two-dimensional transition metal sulfides, MX2 (M represents transition metal, X represents S Se), has attracted research attention in recent years due to their adjustable band gap and high charge carrier mobility [25,26]. Recently, WS2 nanosheets (WS2 NS) was reported to be used as a hole transport layer in OSCs, and does not require complicated post-processing [27]. Because of the special structure of the single-layer WS2 NS, the lone-pair electrons of the S atom can carry out ballistic transport, thereby, increasing the carrier mobility [28]. The suitable energy level structure and high hole mobility of WS2 NS inspired us to use it to dope PEDOT:PSS, hoping to tune the WF of PEDOT:PSS more matched with the HOMO of donor.

In this work, we demonstrate a high-performance hole transport layer for OSCs by dopingWS2 nanosheets (WS2 NS) into PEDOT:PSS. WS2 NS is obtained by a simple and cost-effective liquid-phase exfoliation method. After doping by WS2 NS, the PEDOT:PSS:WS2 NS interlayers show increased WF and smoother surface. In OSCs, the devices based on WS2 NS doped interlayers show increased hole mobility, increased hole extraction efficiency and reduced charge carrier recombination loss. As a result, the PCE of WS2 NS doped PEDOT:PSS based device is 15.67% while that for pure PEDOT:PSS based device is only 14.35%.

Section snippets

Results and discussion

The preparation of WS2 NS relies on the method of liquid-phase exfoliation. Firstly, weigh 10 mg WS2 powder accurately and dissolve it in 10 mL of a solvent, which is a mixture of ethanol and water with equal volume ratio. Then, the exfoliation was carried out for 5.5 h with sonic probe (150W) in a water bath at 5 °C. The purpose of the low temperature water bath is to prevent the suspension from being heated. After that, the WS2 NS suspension was processed at a centrifugal speed of 6000 rpm

Conclusions

In summary, we doped WS2 NS into the PEDOT:PSS as a effective HTL for OSCs. The WS2 NS was obtained by cost-effective liquid-phase exfoliation method. By doping WS2 NS into PEDOT:PSS, the interlayer showed increased WF, forming Ohmic contact between photoactive layer and anode. The devices based on WS2 NS doped interlayers showed increased hole mobility and reduced charge carrier recombination. As a result, the PCE of WS2 NS doped PEDOT:PSS based device increased 9.2% compared to that for pure

Author contributions

N. Li and L. Nian conceived the idea and designed the experiment. N. Li, D. Yuan and L. Nian directed the project. Device fabrication and characterization were carried out by Y. Wang. All the authors contributed to the device measurement, discussion of results and analysis of data. All the authors discussed the results and contributed to the writing of the manuscript.

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.

Acknowledgment

This work was supported by Natural Science Foundation of China (51803063), Science and Technology Program of Guangzhou (No. 2019050001), Guangdong Basic and Applied Basic Research Foundation (2019B151502060), and Guangdong Natural Science Foundation (2018A030313257).

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      On the other hand, p-type organic semiconducting polymers (Alkhalayfeh et al., 2021; Ma et al., 2021) and inorganic metal oxides (Liu, Y. et al., 2019; Sacramento et al., 2021) have been commonly used as HTLs in BHJ-OSCs. Among these, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a p-type organic semiconducting polymer, has been widely used due to its highly visible region transparency for efficient photon transmission to the active layer, and high work function for selective hole extraction and transportation (Li, J. et al., 2020; Wang, Y. et al., 2021a). PEDOT:PSS also helps smooth out the rough surface of the indium tin oxide (ITO) anode, reducing short-circuits and the trap density; hence, minimizing leakage current and electron-hole recombination (Muchuweni et al., 2020a).

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