The role of alcoholic solvents in PEDOT:PSS modification as hole transport layers for polymer solar cells
Introduction
Currently, there are a number of environmental problems in the world due to the use of traditional energy sources. Additionally, the deposits of fossil fuel are expiring and in some regions they are completely depleted. All of these contributes to the need to develop systems of alternative energy sources [[1], [2], [3], [4]]. Renewable energy sources can reduce the demand of natural resources. They are abundant, free and environmentally friendly. These include solar radiation, wind, waves, geothermal heat to name a few [4,5]. However, the use renewable energy resources are still scare poor. The main challenge is to develop high-effective and cheap technologies and systems which can allow us to use nature gift energies.
For now solar energy is the most attractive source of alternative energy. It can be converted directly into both thermal and electrical energy [[6], [7], [8], [9]]. Photovoltaic technologies are growing rapidly and attract attention of scientists, researchers and engineers. One of the promising photovoltaic devices is solar cells based on polymeric materials due to their low cost and simple fabrication technology [[10], [11], [12], [13], [14]]. In short, polymer solar cells (PCS) has a sandwiched structure and consist from several functional layers [10,12,13]. The main layer is a photoactive layer, usually it is mixture of photoactive polymer and fullerene derivatives [12,15,16]. The photoactive polymer absorbs photons which leads to formation of excitons. Exciton is pair of electron and hole bonded by Coulomb force. Photoinduced excitons diffuse to the interface of polymer and fullerene where charge carriers can be separated. In order to deliver charge carriers to external electrodes, charge selective layers are needed to effectively extract charge carriers [12,17,18]. In the case of PSCs, hole selective layer is crucial, which extract holes from photoactive polymer [[19], [20], [21]]. One of the popular and promising hole selective and hole transport material is PEDOT:PSS.
One of the key components of PSC is hole transport material. The most popular hole transport material used in PSC is a conductive polymer (poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS) [[22], [23], [24]]. It has a number of advantages over other conductive polymers: relatively high transparency in the visible region, flexibility, thermal stability, and commercial availability as an aqueous solution [23,25,26]. However, the conductivity of PEDOT:PSS, when used as a stand-alone electrode, are low and barely reaches 1 S cm−1 [27,28]. The low conductivity of PEDOT:PSS is attributed with PSS, which is used as a counter ion and charge compensator for the polymerization of PEDOT. PSS is an insulator and it decreases electrical conductivity of the PEDOT:PSS films [[29], [30], [31]].
PEDOT:PSS layer usually is deposited on a substrate by spin-coating of PEDOT:PSS aqueous solution. Usually, coated PEDOT:PSS layer has agglomerates (Fig. 1), which increase roughness of the film [22,26,30,32]. In addition, according to literature agglomerates are PSS-rich region. For this reason, in order to fabricate PSCs with high performance, it is necessary to deposit PEDOT:PSS layers with optimal PSS distribution. As it was mentioned, PSS is isolator and the regions which have this PSS-rich agglomerates impede charge transfer [[33], [34], [35]]. In this work, we show easy method to reduce agglomerates and consequently PSS content by adding alcoholic solvents to PEDOT:PSS solution.
In our previous work [32], we studied the effect of modification of the structure of the PEDOT:PSS layer on the optical, electrical and PSC photovoltaic properties by adding 2-propnol to the PEDOT:PSS solution. That work showed that 2-proponal in PEDOT:PSS solution positively affect structure and morphology of PEDOT:PSS films. 2-proponal improves solubility of PSS content and prevents formation of PSS-rich agglomerates, which results in the formation of more uniform film composition and local conductivity. It was established that the hole extraction and transport rates, and the power conversion efficiency (PCE) of PSCs significantly depend on the structural features of PEDOT:PSS.
It worth to mention a few works, where authors also treated PEDOT:PSS with organic solvents in order to improve morphology and conductivity of PEDOT:PSS films serving as hole transport layer in polymer solar cells and perovskite based light emitting diodes [29,36,37]. D. Alemu et al. treated PEDOT:PSS films with methanol, ethanol and propanol and found that methanol has the most profound effect on PEDOT:PSS film morphology and conductivity. They explained the advantage of methanol over ethanol and propanol by its higher dielectric constant, which induce a stronger screening effect between counter ions and charge carriers, which reduces the Coulomb interaction between positively charged PEDOT and negatively charged PSS dopants. This will lead to a phase separation on the nanometer scale characterized by segregation of the excess PSS. The highly polar hydrophilic methanol can also easily dissolve the phase separated hydrophilic PSS and facilitate its removal from the film [36]. Y. Zheng and co-workers also investigated the effect of alcohol additives on PEDOT:PSS film conductivity [29]. They considered various alcohol additives including ethanol, glycerol, meso-erythritol, xylitol, and D-sorbitol. Y. Zheng and his colleagues also revealed that PEDOT:PSS film treatment with polar alcohol solvent lead to enhanced conductivity due to removing the PSS shell and achieving larger PEDOT grains. They also observed that work function of treated PEDOT:PSS films deeper than the pristine PEDOT:PSS, which is beneficial to the energy level matching with the photoactive polymer HOMO level. According their work, meso-erythritol and D-sorbitol shows larger potential for improving PEDOT:PSS properties. In the both mentioned works, authors assembled PSCs and observed enhanced efficiency with the treated PEDOT:PSS films, which is mostly associated with improved PEDOT:PSS conductivity and morphology. M. Wu et al. fabricated LEDs based on CH3NH3PbBr3 perovskite and tried to improve the hole injection by modifying PEDOT:PSS HTL properties by the solvent treatment including methanol, ethanol, and isopropanol. They also observed the superiority of methanol over ethanol and isopropanol and explained it by its higher polarity. A solvent with higher polarity induces stronger screening effect and can take away some insulator PSS from PEDOT:PSS during spin-coating process.
Nevertheless, it should be noted that studies on the influence of structural changes in the PEDOT:PSS film on its optical and electrical properties still remain relevant despite significant advances made in PSCs. In this work we chose ethanol and isopropanol as additives to aqueous PEDOT:PSS solution, and did not use methanol due to it is more toxic in comparison with ethanol and isopropanol. The difference of this work from the mentioned above is that we investigate the effect of PEDOT:PSS modification with organic solvents by impedance spectroscopy techniques, which allow to study the charge transfer process as both in bulk and the interface in more details than it was done in another works. We studied the effect of modification of PEDOT:PSS with ethanol, isopropanol and their mixture on its structural, optical, and electrical characteristics and show how these changes affect PSCs performance. The advantages of 2-propanol over ethanol are also discussed.
Section snippets
Experimental part
The preparation of the substrates was carried out according to the procedure described in other work [38]. To prepare PEDOT:PSS solution, we used commercial PEDOT:PSS aqueous solution (1%, Ossila Al4083), 2-proponl, and Ethanol (pure 99.9% Sigma Aldrich). PEDOT:PSS solution was filtered through a 0.45 μm filter before use. PEDOT:PSS films were obtained by spin-coating techniques. PEDOT:PSS solution was spin-coated on quartz and ITO substrates in order to form PEDOT:PSS thin films at a rotation
Results and discussions
3D AFM images of the surface topography of the PEDOT:PSS films spin-coated from the solution with various compositions are shown in Fig. 2. The PEDOT:PSS film obtained from the unmodified aqueous based solution (UM-PEDOT:PSS) has a more coarse surface compared with films obtained from solutions diluted with organic solvents implying the formation of PSS-rich agglomerates [32]. The surface roughness of UM-PEDOT:PSS film is around 1.03 nm. The PEDOT:PSS film obtained from the solution diluted
Conclusion
In this article, the influence of two types of alcohol, 2-propanol and ethanol, on PEDOT:PSS layer formation was studied. Our research showed that adding alcohol solvents to an aqueous solution of PEDOT:PSS positively affected the morphology of PEDOT:PSS layers coated by spin-coating technique. It was established that the alcohol solvent hinders the formation of PSS agglomerates and results in the formation of more homogeneous PEDOT:PSS layers. The reduction of the density and sizes of
CRediT authorship contribution statement
X.S. Rozhkova: Visualization, Formal analysis, Resources, Data curation. A.K. Aimukhanov: Methodology, Validation, Resources, Data curation, Writing – original draft, preparation, Writing – review & editing. B.R. Ilyassov: Conceptualization, Supervision, Project administration. A.K. Zeinidenov: Methodology, Validation, Resources, Data curation, Writing – review & editing.
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.
Acknowledgments
This research is funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (Grant No. AP08856176).
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