Cooperative effect of carbon black and dimethyl sulfoxide on PEDOT:PSS hole transport layer for inverted planar perovskite solar cells
Introduction
Recent emergence of perovskite solar cells (PSCs) has been attracting a great deal of attention on account of impressive photovoltaic performances and easy device manufacturing (Timothy et al., 2016). Until now, PSCs have reached the highest power conversion efficiency (PCE) of over 22.1%, owing to tremendous efforts made on the crystal optimization and new structure development of perovskite light harvest layer, the structure design and preparation technology innovation of electron transport layer (ETL), the doping modification and new materials application of hole transport layer (HTL), and the interface modification and band gap regulation (Li et al., 2016, Nie et al., 2015, Zhou et al., 2014). In general, the cell structure of PSCs can be classified into two types: porous and planar architectures. Among them, the planar PSCs have distinctive merits, such as flexible, solution-processable and cost-effective, showing greater potential for large-scale applications (Wei et al., 2014, Zhou et al., 2015).
The role of the HTL is vital to efficiently collect and transport the holes extracted from the perovskite light harvest layer (Chen et al., 2013). P-type conjugated polymer, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), is one of the most popular HTL materials used to construct the inverted planar PSCs with p–i–n junction (HTL/perovskite/ETL) in vision of its simple water solution-processability and high transparency (Jo et al., 2016, Zuo and Ding, 2017, Wu et al., 2014). However, many reports have found that the pristine PEDOT: PSS HTL is flawed in many ways. For example, inefficient charge transport within the PEDOT:PSS HTL because of low conductivity is an major drawback, which mainly results in charge accumulation at the interface between HTL and perovskite layer that in turn increases the leakage current in the device (Huang et al., 2017). Furthermore, the challenging control of the high-covered perovskite layer with large-sized grains deposited onto a smooth polymeric surface is another major drawback (Giuri et al., 2016). Thus, altering the surface topography and characteristics of the PEDOT:PSS HTL is of paramount importance to induce the heterogeneous nucleation and decrease the nucleation energy barrier, facilitating the growth of perovskite crystal. To address the issues mentioned above, an impactful attempt to modify the PEDOT:PSS HTL has been performed by introducing the polar organic additives like dimethyl sulfoxide (DMSO) (Huang et al., 2017), polyethylene oxide (PEO) (Huang et al., 2016a, Huang et al., 2016b), methanesulfonic acid (MSA) (Sun et al., 2015) and Zonyl (Adam et al., 2016), and the inorganic carbonaceous additives like graphene oxide (GO) (Wu et al., 2014) and carbon nanotubes (CNTs) (Rincón et al., 2015). However, single additive for modifying the PEDOT:PSS HTL is somewhat difficult to perfectly take on the dual responsibilities of enhancing the conductivity and improving the surface topography and characteristics.
In this work we prepared a nanocomposite film based on PEDOT:PSS modified with dual additives of carbon black (CB) and DMSO used in inverted planar PSCs as a HTL for the first time. It has reported that the charge transport capability of the PEDOT:PSS can be enhanced by employing DMSO additive due to the reduction of the interaction between PEDOT and PSS chains and the formation of the PEDOT-rich domain (Huang et al., 2017, Lee et al., 2015, Ma et al., 2016). Findings by Zhang et al. (2016) have indicated that CNTs embedded in the perovskite light harvest layer can assist its crystal growth, and it is reasonable to speculate that the CB nanoparticles have a similar function to that of the CNTs. Moreover, as a widely available and cost-effective material, CB is typically used as conductive filler in polymers by forming a continuous network polymer matrix leading to high conductivity (Elimat, 2006). In the current study, we indeed observe a cooperative effect of CB and DMSO derived from their unique and complementary properties in the co-modified film of CB-DMSO-PEDOT:PSS where the dispersed CB nanoparticles bulged on the PEDOT:PSS matrix chiefly boost the growth of the perovskite crystal with micron-sized grains on the co-modified film while the DMSO mainly strengthens the charge transport capability of the co-modified film. As expected, when the co-modified film is implemented in PSCs as a HTL, the device possesses a higher PCE of 5.11%, which shows 293%, 152%, and 38% improvements in contrast to the values of the devices based on pristine PEDOT:PSS, DMSO-PEDOT:PSS, and CB-PEDOT:PSS films, respectively.
Section snippets
Preparation of co-modified CB-DMSO-PEDOT:PSS films
PEDOT:PSS aqueous solution with a concentration of 1.3 wt.% was purchased from Sigma-Aldrich and used without purification. The ratio of PSS to PEDOT is 1 to 0.625. DMSO (Alfa, 99.9%) was added into PEDOT:PSS aqueous solution with a weight ratio of 1:11. After stirring for 6 h, 0.5 wt.% of CB (Alfa, surface area: 75 m2 g−1, 99.9+%) was dispersed in the obtained DMSO-PEDOT:PSS solution and sonicated for 2 h. The dispersion was subsequently centrifuged at 8000 rpm for 30 min to remove large CB
Results and discussion
In inverted planar PSCs, the transmittance of HTL can influence the device performance since the incident light has to pass through the HTL before being illuminated to the perovskite light harvest layer. Fig. 1 demonstrates the UV–Vis spectra and images of the pristine and modified PEDOT:PSS films. It can be seen that the pristine PEDOT:PSS film shows excellent transparency in the visible range (400–700 nm) with a transmittance of 90.6% at 550 nm. Compared with the pristine PEDOT:PSS film, the
Conclusion
A nanocomposite film based on PEDOT:PSS modified with dual additives of CB and DMSO used in inverted planar PSCs as a HTL was synthesized for the first time. Benefited from the cooperative effect of CB and DMSO, the co-modified film of CB-DMSO-PEDOT:PSS exhibits superior capabilities to collect and transport the charge induced by decreasing sheet resistance and to assist the growth of the perovskite light harvest layer with enlarged grains in micron scale on its surface in comparison with the
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (No. 51462035), Training Programme Foundation for Young Scientist of Jiangxi (No. 20133BCB23035), Natural Science Foundation of Jiangxi (No. 20161BAB206106) and Educational Commission of Jiangxi (No. KJLD13100).
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