Influence of the volume of EGME-DMSO mixed co-solvent doping on the characteristics of PEDOT:PSS and their application in polymer solar cells

Abstract

Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) is one of the most widely used hole transport layer (HTL) in polymer solar cells (PSCs). However, the improving of the conductivity and transparency of PEDOT:PSS thin films is still needed. To solve this problem, we here introduce 2-methoxyethanol (EGME) and dimethyl sulfoxide (DMSO) mixed secondary solvent to PEDOT:PSS solution as a novel additive to the best of our knowledge. We determined that the EGME-DMSO doped PEDOT:PSS layer provides better energy level alignment, conductivity and morphology with the help of methods of UV-Vis spectroscopy, atomic force microscopy, etc. The addition of 15% (v/v.) volume of EGME-DMSO mixed co-solvent improves the efficiency from 2.8% of control device to 3.9%. The significant enhancement of the short-circuit current density (J_sc) of 13.7 to 16.5 mA cm⁻² is the main reason for this increase of performance due to better charge transport properties. This suggests that the EGME-DMSO mixed co-solvent doping into PEDOT:PSS solution is a simple approach to fabricate highly efficient PSCs.

Keywords

Polymer solar cells PEDOT:PSS mixed co-solvent doping performance enhancement

Introduction

In past decades, solar energy is having a great attention in the scientific world. Polymer solar cells (PSCs) have achieved a significant breakthrough in solar energy area due to their low-cost, high efficiency, light weight and processability on flexible surface. As known, the most commonly used PSCs architecture is bulk heterojunction (BHJ) architecture that consist the blend of P3HT:PCBM (poly(3-hexylthiopene): [6,6]-phenyl C61-butyric acid methyl ester) between a low work function (LWF) metal and an anode.¹ As silicon-based solar cells dominate photovoltaic industry today, the power conversion efficiency (PCE) of PSCs is still needed to improve. Numerous studies have been devoted to enhance the photovoltaic characteristics of PSCs, for instance, introducing new materials,^2,3 interfacial modifications,^4,5 doping the layers,⁶ and so on. Despite their low photovoltaic performance, PSCs have remarkable potential for commercial applications due to their applicability with solution-based fabrication methods.⁷

Solar cells produced by solution processing are a competitive candidate to supply low-cost solar power. Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) is among the most popular material utilized as hole transport layer (HTL) due to its high stability, solution-processed ability, high transparency, conductivity, and so on.⁸ In addition to that PEDOT:PSS could reduce the roughness of indium tin oxide (ITO), which is the most widely used transparent electrode with PEDOT:PSS, this HTL could enhance the selectivity of the anode. Even though, it offers many superior electrical and optical properties, the presence of PSS could cause a decrease in conductivity. Also, the acidic nature of PEDOT:PSS gives rise to corrosion of the electrode in contact with, which leads to a decreasing in the device performance.⁹ A large number of investigators have been researching to improve electrical and morphological characteristics of PEDOT:PSS thin film that effect photovoltaic performance of the solar cells fabricated. For example, WF of PEDOT:PSS can be simply altered by doping with different secondary solvent additives such as, dimethyl sulfoxide (DMSO),¹⁰ ethylene glycol (EG)¹¹ and methanol¹² or other additives (e.g. salts¹³ or ionic liquids¹⁴). Moreover, the addition of secondary solvent has been reported to develop the linear geometry of polymer chains in PEDOT:PSS solution leading to enhance the morphological and electrical properties of thin PEDOT:PSS layers.¹⁵ For instance, it was reported that ethylene glycol addition could easily increase the specific conductivity up to 200 S cm⁻¹ because of conformational changes.¹⁶ Pettersson et al. showed that PEDOT:PSS film with about 60 times higher conductivity than that of PEDOT:PSS film fabricated with as-bought PEDOT:PSS solution were obtained by adding sorbitol, followed by heat treatment.¹⁷ Crispin at el. found that the conductivity enhancement of PEDOT:PSS up to 10 S cm⁻¹ could be achieved by diethylene glycol addition into PEDOT:PSS solution, which leads to enhance the PEDOT:PSS phase separation.¹⁸ Most of the studies report the addition of only one secondary solvent into PEDOT:PSS. Additionally, doping of two different solvents at the same time can improve delocalization of charges in thin film obtained.¹⁹ Surprisingly, there are very few studies on the effect of using mixed co-solvent addition on morphological and electrical characteristics of PEDOT:PSS films obtained.

In this study, we fabricated PSCs that consist ITO (anode)/PEDOT:PSS added with mixed co-solvent of DMSO and 2-methoxyethanol (EGME)/P3HT:PCBM bulk heterojunction active layer/LiF and Al (cathode). We investigated the effect of EGME-DMSO mixed co-solvents with different volumes added into PEDOT:PSS solution on the electrical and morphological properties of thin films obtained. For the optimal addition concentration, electrical conductivity improved from 0.0159 to 0.11 S cm⁻¹. We optimized the additive volume of mixed co-solvents for enhancing charge transport to achieve higher PCE. It was found that the PCE improved from 2.8% to 3.9% with 15(v/v.)% mixed co-solvent doped PEDOT:PSS film. With this paper, we suggest a detailed and simple approach to enhance the performance of P3HT:PCBM based solar cells by doping of PEDOT:PSS with EGME-DMSO mixed co-solvents.

Experimental

Materials and device fabrication

The PSCs were produced with the configuration of ITO/PEDOT:PSS (doped with EGME-DMSO)/P3HT:PCBM/LiF/Al. Indium tin oxide (ITO) coated substrate was ultrasonically cleaned with Hellmanex, distilled water (DI), acetone, isopropyl alcohol (IPA) and DI sequentially for about 15 min. The cleaned substrate was dried by nitrogen gun. Prior to use, the cleaned and dried ITO substrate was treated under oxygen plasma for 5 min.

PEDOT:PSS (Clevious P VP AI 4083), dimethyl sulfoxide (DMSO, 99.9%) and 2-methoxyethanol (EGME, anhydrous, 99.8%) were purchased from Heraeus Clevios and Sigma-Aldrich, respectively. To prepare the pre-mixed stock solvent doping, 50 vol.% of DMSO and 50 vol.% of EGME were mixed and magnetically stirred for 2 h. 5, 6, 10, 15, and 30 vol.% of stock solvent doping were added into PEDOT:PSS solution (doped with 0.5 vol% Zonyl FS300) followed by stirring for 24 h. Final PEDOT:PSS solutions were filtered by a PTFE syringe filter with 0.45 µm pore size, and then spin coated at 1000 rpm for 1 min on a etched ITO substrate. The films obtained were annealed on a hot plate at 160°C for 10 min in ambient atmosphere. Absorber layer of P3HT:PCBM was spin coated on the top of PEDOT:PSS film at 2000 rpm for 1 min using a solution of P3HT:PCBM (25 mg:15 mg) in 1 mL of chlorobenzene: chloroform (1:1), followed by annealing at 100°C for 20 min. Finally, LiF (1 nm) and Al (100 nm) electrodes were physically deposited through a shadow mask to complete the device fabrication.

Film and device characterizations

Optical absorption spectra of PEDOT:PSS films were recorded using a Shimadzu UV Mini 1240 instrument. Morphology and grain size distribution of PEDOT:PSS films were observed using atomic force microscopy (AFM, NT-MDT INTEGRA Solaris). The conductivity of PEDOT:PSS films were determined by four-point probe instrument (ENTEK Elk. FPP 460 with Pt probes). Film thickness was determined using a surface profilometer (NanoMap-500LS 3D Stylus). Current density–voltage (J-V) characteristics of the solar cells were measured under nitrogen atmosphere in glove-box without encapsulation, using the simulated AM1.5 solar light source provided by ATLAS solar simulator.

Results and discussion

The possible conformational change of PEDOT:PSS thin films can be examined by AFM technique. Figure 1 (a-f) and Figure 2 (a-f) shows the height profile and their cross section of a 5 × 5 μm² EGME-DMSO doped PEDOT:PSS films. From non-doped PEDOT:PSS film to 30 (v/v.)% mixed co-solvent doped, the surface roughness significantly increased. On the cross section of doped films, the peaks could be attributed to rearrangement of polymer chains and, thereby, their aggregated grains, since non-doped PEDOT:PSS film is smoother. The aggregated PEDOT chains could improve the charge transport properties of PEDOT:PSS film.²⁰ It should be note that the surface roughness perceptibly increased after mixed co-solvent addition. The surface roughness could be used to have an idea about the crystallinity formation of a thin film. As known, the charge carrier mobility increases through HTL, when the better crystallinity is achieved. Therefore, it could be said that the conductivity increases with the increasing surface roughness.²¹

Figure 1.

AFM height profile of a 5 × 5 μm² PEDOT:PSS films (a) non-doped and doped with (b) 5 v/v.%, (c) 6 v/v.%, (d) 10 v/v.% (e) 15 v/v.% and (f) 30 v/v.% of EGME-DMSO mixed co-solvent.

Figure 2.

AFM cross section of a 5 × 5 μm² PEDOT:PSS films (a) non-doped and doped with (b) 5 v/v.%, (c) 6 v/v.%, (d) 10 v/v.% (e) 15 v/v.% and (f) 30 v/v.% of EGME-DMSO mixed co-solvent.

Figure 3 presents the film conductivities and surface roughness of mixed co-solvent doped PEDOT:PSS for different volumetric ratios. Clearly, it can be observed that the film conductivity shows a sharp increase from 0.0159 S cm⁻¹ to 0.998 S cm⁻¹ at a specific volume of EGME-DMSO (5% v/v.). The conductivity is about seven times higher at 15% (v/v.) volume of doping than that of PEDOT:PSS film without doping. This sharp increasing in conductivity after doping indicates the reorientation of polymer chains, which provides better mobility of charge carriers.²² This phenomenon can be explained by the better charge transport due to the interaction between polar organic solvent and positive charges on the PEDOT chains.²³ The surface roughness values shows that quite smooth PEDOT:PSS surfaces were obtained. Briefly, it can be observed that the mixed co-solvent doping could lead to increase in surface roughness. This is consistent with the AFM cross section results. To support the increase of highly conductive PEDOT grains, the statistical grain size distribution of films was analyzed by a software equipped with AFM technique. In Figure 4 (a), it can be concluded that the average grain size enlarged with the increasing volume of EGME-DMSO addition, which is consistent with the findings from AFM height profile. At a specific volume of EGME-DMSO (10% v/v.), the grain size was as high as 38 nm. However, when general trend was considered, it could be overlooked.

Figure 3.

The conductivity and average surface roughness values of PEDOT:PSS films.

Figure 4.

(a) Grain size distribution values and (b) UV-vis absorption spectra of PEDOT:PSS films.

The conducting PEDOT:PSS film in polymer solar cells is desired to be highly transparent. Optical absorption spectra of doped PEDOT:PSS films is given in Figure 4 (b). All doped PEDOT:PSS films displayed significantly good transparency. Especially, high transparency in the wavelength range of 450–800 nm leads to increase the absorption in the photoactive layer. Compared to the non-doped sample, the absorption spectra of doped PEDOT:PSS films showed a weak difference at 600–650 nm relating to the neutral state.²⁴ As known, the addition of dopants into PEDOT:PSS solution has a strong effect on optical properties of thin film obtained, thus, optical band gap values. The optical band gap values for thin films can be calculated from optical absorption data by using of the plot of αhu² versus hu. From Figure 5, it can be observed that the optical band gaps sharply increased after the mixed co-solvents addition. When the volume of mixed co-solvents addition increased, the optical band gaps of thin PEDOT:PSS films increased. This indicates a blue shift in absorption spectra of thin films and confirms the conformational change in the PEDOT:PSS structure. Since PEDOT:PSS layer is to increase the WF of ITO layer, it can be claimed that the addition of EGME-DMSO mixed co-solvents could tune the band structure of the PEDOT:PSS.²⁵

Figure 5.

Tauc plots according to the optical absorption data to calculate the band gap of the non-doped and doped PEDOT:PSS films.

To utilize EGME-DMSO doped PEDOT:PSS as a HTL for bulk heterojunction PSCs, we fabricated solar cells with the configuration of ITO/PEDOT:PSS (w or w/o EGME-DMSO)/P3HT:PCBM/LiF/Al. Figure 6 (a) and (b) shows the J-V results for the highest efficiency cells under light and dark conditions, respectively. Table 1 lists the device parameters extracted from J-V curves presented in Figure 6 (a). The reference device with non-doped PEDOT:PSS results a PCE 2.8%, with an open-circuit voltage (V_oc) of 0.616 V, a short-circuit current density (J_sc) of 13.7 mA cm⁻² and a fill factor (FF) of 32.7%. These are coherent results and close PCEs have been reported by Kaçuş et al. and Kırmacı et al. for devices with similar configuration.^26,27 As seen in Table 1, PCEs increases as the volume of EGME-DMSO mixed co-solvent increases from 5% (v/v.) to 15% (v/v.). Further increasing the volume of EGME-DMSO mixed co-solvent causes a decrease, even lower than PCE of reference device. The best efficiency of 3.9% is obtained when the PEDOT:PSS is doped at 15% (v/v.) volume of EGME-DMSO mixed co-solvent with a V_oc of 0.615 V, a J_sc of 16.5 mA cm⁻² and a FF of 38.0%. It should be noted that the improvement of PCE for devices fabricated with doped PEDOT:PSS solution is basically owing to the enhanced J_sc and FF values. Even though, there is no significant difference between the V_oc values of devices fabricated, the J_sc values show an increase ∼20% after co-solvent addition. The enhanced J_sc values could be related to better work function alignment, thereby, better ohmic contact between PEDOT:PSS and absorber layer. Additionally, an increase in the J_sc values could be a result of the improved charge transport properties and the improved hole extraction of doped PEDOT:PSS layer. The possible conformational change of the conductive PEDOT chains could create better charge hopping pathways through the PEDOT:PSS thin film. The enhanced FF values with the enhanced J_sc value could be due to better charge collection and lower charge transfer resistance.

Figure 6.

Current density–voltage characteristics of devices under (a) the light and (b) the dark conditions.

Table 1.

Summary of device parameters extracted from J-V curves presented in Figure 6.

Device	V_oc (V)	J_sc (mA/cm²)	FF (%)	Efficiency (%)
Non-modified	0.616	13.720	32.72	2.765
5%	0.622	16.300	33.10	3.356
6%	0.631	13.201	34.70	2.890
10%	0.625	15.668	36.45	3.569
15%	0.615	16.538	38.01	3.866
30%	0.620	10.560	32.63	2.136

Conclusion

To sum up briefly, this study presents a better understanding for the controlling of morphological and electrical properties of PEDOT:PSS layer via addition of EGME-DMSO mixed co-solvent. The doped and non-doped PEDOT:PSS films were spin coated on ITO coated surfaces for the fabrication of P3HT:PCBM based PSCs. After adding mixed co-solvent to improve the electrical properties of PEDOT:PSS layer, the grain size through the films become larger due to the well packing of conductive PEDOT chains. The PCE results showed that the J^sc and FF values improved by addition of EGME-DMSO mixed co-solvent into PEDOT:PSS solution thanks to improved charge transport properties and hole extraction. The photovoltaic characteristics presented that the addition of 15% (v/v.) EGME-DMSO mixed co-solvent is the optimal value. The control device showed a PCE of 2.8%, whereas the device with 15% (v/v.) of mixed co-solvent doped PEDOT:PSS showed a superior PCE of 3.9%. This supports that the EGME-DMSO mixed co-solvent addition into low conductivity PEDOT:PSS solution shows a significant effect to enhance the PCE of PSCs fabricated.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Çisem Kırbıyık Kurukavak

Selen Polat

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