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Electrical property modified hole transport layer (PEDOT:PSS) enhance the efficiency of perovskite solar cells: Hybrid co-solvent post-treatment
Journal Pre-proof Electrical property modified hole transport layer (PEDOT:PSS) enhance the efficiency of perovskite solar cells: hybrid co-solvent post-treatment
Ping Li, Mohamed Ibrahim Omer Mohamed, Cunyun Xu, Xizu Wang, Xiaohong Tang PII:
S1566-1199(19)30609-3
DOI:
https://doi.org/10.1016/j.orgel.2019.105582
Reference:
ORGELE 105582
To appear in:
Organic Electronics
Received Date:
05 September 2019
Accepted Date:
07 December 2019
Please cite this article as: Ping Li, Mohamed Ibrahim Omer Mohamed, Cunyun Xu, Xizu Wang, Xiaohong Tang, Electrical property modified hole transport layer (PEDOT:PSS) enhance the efficiency of perovskite solar cells: hybrid co-solvent post-treatment, Organic Electronics (2019), https://doi.org/10.1016/j.orgel.2019.105582
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Journal Pre-proof Graphical abstract
Variation of the conductivity of the PEDOT:PSS film treated without and with the various solvent by TAT and TBT method, respectively.
1|P a g e
Journal Pre-proof Electrical property modified hole transport layer (PEDOT:PSS) enhance the efficiency of perovskite solar cells: hybrid co-solvent post-treatment Ping Lia,b, Mohamed Ibrahim Omer Mohameda, Cunyun Xuc, Xizu Wang d, Xiaohong Tanga* a
School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798 b School of Physics and Electronic Science, Zunyi Normal university, Zunyi, China 563002 c Faculty of energy and materials, Southwest University, Chongqing, China, 400715 d Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research, #08-03, 2 Fusionopolis Way, Innovis, 138634, Singapore. *Corresponding Email: [email protected] Abstract Poly(3,4-ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS) layer as a hole transport layer (HTL) plays a key role in efficient hole extraction and transportation of the inverted planar perovskite solar cells (PSCs). However, the insulating nature of PSS lead to lower electrical property of the PEDOT:PSS film, which impedes transferring hole and results in a low photocurrent of the PSCs. In this work, dimethylformamide (DMF), methanol and their mixture solvents (co-solvent) were employed to treat the PEOT:PSS thin film after deposition with different methods to enhance the film’s electrical conductivity. Electrical conductivity of the PEDOT:PSS films was increased from 10-3 S.cm-1 to ~102 S.cm-1 after the co-solvents treatment. Using the highest conductivity co-solvent treated PEDOT:PSS thin film as the HTL in a perovskite solar cell, the power conversion efficiency (PCE) of the device has been measured improved by 17.5% as compared with that of the control device with the untreated PEDOT:PSS film as the HTL. This result shows that the hybrid co-solvent post-treatment is a simple and feasible way to modified the electrical properties of the PEDOT:PSS film as the HTL for high efficient PSCs and other thin film electronic and optoelectronic devices. Key words: Perovskite solar cells; PEDOT:PSS; Conductivity; Hole transporting layer; Solvent treatment 1. Introduction Conductive polymers have attracted much interest as the charge carriers’ transport layers in the solution-processed flexible electronic and optoelectronic devices, such as polymer based organic solar cells, organic light emitting diodes and the recently developed organic-inorganic lead halide perovskite solar cells (PSCs), etc [1-6]. Organic-inorganic lead halide perovskite solar cells have attracted much attention after experiencing a sky-rocketing power conversion efficiency since they were proposed in around 2009. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thin film with it’s high optical transparency in the visible light spectrum, easy aqueous solution processing, high work function facilitating the formation of an Ohmic contact with perovskite film, has been widely used as the hole transporting layer (HTL) to facilitate transporting holes to the anode in the perovskite solar cells [7-11]. However, the pristine 1
Journal Pre-proof PEDOT:PSS HTL films prepared from the PEDOT:PSS dispersion usually suffers from a very low electrical conductivity. This is because a moiety of insulating PSS existing on top of the PEDOT:PSS films when are formed from the aqueous dispersion [7,12 ]. Therefore, to fabricate high efficient PSCs, conductivity of the PEDOT:PSS film need to be improved. Need to be noticed that according to different volume ratio of PEDOT and PSS, there are different PEDOT:PSS films for different applications. When the volume ratio of the PEDOT and PSS is 1:2.5 (PH1000), the PEDOT:PSS films are normally used as the conductive electrode in organic photovoltaic (OPV) devices and organic light emitting diodes (OLED). The conductivity of the pure PH1000 film is usually less than 1 S.cm-1 [13-19]. With the solvent post-treatment, the PH1000 PEDOT:PSS films’ electrical conductivity can be increased to higher than 1000 S.cm-1 and even up to 3000 S.cm-1 [15,17]. High concentration of PSS in PEDOT:PSS is good for dispersing PEDOT into aqueous and enhances the work function of PEDOT:PSS. Therefore, PEDOT:PSS films with the PEDOT and PSS volume ratio of 1:6 (Al 4083) are widely used as the HTL in OPV, OLED and PSCs because of their high work function facilitating the formation of an Ohmic contact with the organic materials. But the Al4083 PEDOT:PSS films normally show very low conductivity due to the high volume ratio of PSS, which is not good for the charge transporting in the devices. Solvents posttreatment and doping are used to modify the electrical property of the insulating PEDOT:PSS (Al 4083) [20-25]. Chen et al. dipped the DMF on the PEDOT:PSS film with thermal annealing (TA), and then spin coated the DMF after 5 second [26]. With the post-treatment, they have increased the formed PEDOT:PSS film’s conductivity by 2 orders. In this work, a new DMF : Methanol cosolvent post-treatment methods of the Al4083 PEDOT:PSS film were proposed and investigated. Electrical conductivity of the PEDOT:PSS films was measured increased from 10-3 S.cm-1 to 101 S.cm-1 after the post-treatment. Using the co-solvent post treated PEDOT:PSS film as the hole transporting layer of the perovskite solar cell, the power conversion efficiency (PCE) of the device was obtained improved over 16.7% as compared with that of the device with the un-treated PEDOT:PSS as the HTL. 2. Experiment 2.1 Materials PEDOT:PSS aqueous solution (Clevios P VP Al 4083) used in this work was purchased from Heraeus. Methylammonium iodide (MAI), lead iodide (PbI2), lead chloride (PbCl2), dimethyl sulfoxide (DMSO), chlorobenzene (CB), methanol and dimethylformamide (DMF) were from Sigma-Aldrich. All materials were used as received. 2.2 Treatment of PEDOT:PSS and Fabrication of the PSCs The PEDOT:PSS films were prepared by spin coating their aqueous solution on pre-cleaned glass substrates of the size of 2.0×2.0 cm2. In the investigations, two methods were employed to treat the surface of the deposited PEOT:PSS films to enhance their conductivity. The first method is called as treatment before thermal annealing (TBT) of the PEDOT:PSS films. In the TBT treatment method, before thermal annealing the deposited PEDOT:PSS films, 80 μL of DMF, methanol and their mixed solvent (DMF:methanol cosolvent) was respectively dropped on the PEDOT:PSS film to cover the film surface for 10 seconds, and then, spin coated the sample for 30 s with 5000 rpm. After that, the samples were annealed at 130 C on hotplate for 20 min. The second method is called as treatment after thermal annealing (TAT) of the PEDOT:PSS films. 2
Journal Pre-proof In this treatment, after spin deposited the PEDOT:PSS films, the samples were thermal annealed at 130 C on the hotplate for 20 min. Then, 80 μL of DMF, methanol and their mixed solvent was respectively dropped on the samples to cover the PEDOT:PSS films’ surface for 10 seconds, and spin coating the solvent of the sample at 5000 rpm for 30 s. Then, the samples were annealed at 100 C on hotplate for 10 min. The ITO glass substrates were cleaned sequentially in deionized (DI) water, acetone, and isopropanol by sonication for 20 min. After N2 blow drying, the substrates were further treated with UV-ozone for 10 min. The PEDOT:PSS film with a thickness of 30 nm was deposited by spin coating the aqueous on the ITO substrates at 5000 rpm for 40 s, and subsequently treated with solvent by TAT or TBT method in air. For fabricating the perovskite solar cells, the ITO glass substrates coated with the PEDOT:PSS HTL were transferred into a glovebox filled with N2. MAI of 222.6 mg, PbCl2 of 38.9 mg and PbI2 of 580.86 mg were dissolved in l mL anhydrous DMF and DMSO co-solvent with volume ratio 9:1 as the perovskite precursor solution. The perovskite layer was formed by spin coating the precursor solution on the PEDOT:PSS coated ITO substrate at 4000 rpm for 30 seconds. After 10 seconds from started the spin coating the perovskite film, 200 ml anhydrous CB was added dropwise onto the spinning sample. The perovskite layer coated samples were annealed at 85 C for 30 min. The thickness of the perovskite films was around 450 nm. PCBM (20 mg.mL-1 in CB) solution without any additive was spun coated on top of the perovskite layer at 1000 rpm for 45 seconds to form the electron transporting layer. Then, BCP in IPA was spun coated on the samples at 1000 rpm for 30 seconds. Finally, metal silver (80 nm) electrode of the solar cell was thermal evaporated though a shadow mask. Each device had an area of 0.09 cm2. 2.3 Characterization of PEDOT:PSS film and PSCs The conductivity of the PEDOT:PSS films were measured by the four-point probe technique with a Keithley 2400 source/meter. UV absorption spectra of the films were measured by using a Varian Cary 5000 UV-vis-NIR spectrometer. The surface morphology of the films was measured by atomic force microscopic (AFM) acquired with a Veeco Nano-Scope IV Multi-Mode AFM in tapping mode. The contact angles (CA) of the PEDOT:PSS films with different solvent treatments were measured using a microscopic contact angle meter. The photovoltaic performance of the PSCs with the PEDOT:PSS films as the HTL was measured in glovebox with a computerprogrammed Keithley 2400 source/meter and a Newport’s solar simulator, which simulated the 1 sun AM1.5 sunlight with light density of 100 mW.cm-2. 3. Results and discussion Conductivity of the PEDOT:PSS films treated with different co-solvents by TAT and TBT treatment, respectively, (following denoted as PEDOT:PSS with solvent TAT or TBT) are compared in Figure 1. It shows that the conductivity of the pristine PEDOT:PSS film is 1.3×10-3 S.cm-1 which is consistent with previous reports [25,26]. A dramatic increase of the conductivity of the PEDOT:PSS films was received after these co-solvent treatments. Using the TAT treatment method, conductivity of the PEDOT:PSS film treated by pure of methanol and DMF achieved 1.02×101 S.cm-1 and 1.31×101 S.cm-1, respectively. Higher conductivity of the PEDOT:PSS films is achieved by treated the samples with the methanol:DMF co-solvents. The conductivity of the PEDOT:PSS film is depended on the volume ratio of the methanol:DMF co-solvent in the 3
Journal Pre-proof treatment. The highest conductivity of the PEDOT:PSS film reaches 5.13×101 S.cm-1 when it was treated by the TAT method with the methanol:DMF cosolvent with the volume ratio of 1:1. The conductivity enhancement of the PEDOT:PSS film with different volume ratio of the methanol:DMF co-solvent using the TBT treatment method was also investigated. The results show that the PEDOT:PSS film’s conductivity increased much after it was treated with the solvents TBT treatment. The highest conductivity of the film achieves 9.5×101 S.cm-1 when it was treated with the methanol:DMF co-solvent with the volume ratio 1:1. Compared with that of the PEDOT:PSS film without the treatment, conductivity of the PEDOT:PSS film was enhanced more than 70,000 times.
Fig.1 Variation of the conductivity of the PEDOT:PSS film treated without and with the various solvent by TAT (black) and TBT (red) method, respectively.
Fig. 2 Absorption of a untreated PEDOT:PSS and PEDOT:PSS films with methanol, DMF or methanol:DMF (1:1) co-solvent TAT or TBT. In order to reveal the effects of the treatments on the PEDOT:PSS films, some characterizations were carried out on the samples under the different solvent treatments. Fig. 2 shows the absorption spectrum of the PEDOT:PSS films with different treatments. All the samples show two absorption peaks which originated from the aromatic ring of PSS of the films [12, 16, 27]. The PEDOT:PSS film without the co-solvent treatment has the highest absorption peak. After gone through the treatments, the PSS absorption peaks of the film decreased significantly. The absorption peaks was 4
Journal Pre-proof the lowest when the film was treated with the methanol:DMF co-solvent using TBT treatment. This suggests that some PSS chains were removed from the PEDOT:PSS films during the solvent treatment. This result shows that the conductivity enhancement of the PEDOT:PSS film with the co-solvent treatment is because of the removal of the PSS from the PEDOT:PSS film surface during the solvent treatment [23, 24, 28-30]. To further prove the removal of PSS from the PEDOT:PSS film surface with the co-solvent treatment, water contact angle (CA) of the PEDOT:PSS films with different solvent treatment was measured. The measured contact angles of the PEDOT:PSS films after different solvent treatment by using TAT and TBT methods respectively are shown in Fig.3. The contact angle of the untreated PEDOT:PSS film is 10.8 degree. With the solvent treatment, the CA of the PEDOT:PSS film increases. This is ascribed to the hydrophobic PEDOT richer on the PEDOT:PSS film surface after the removal of hydrophilic PSS of the film during the co-solvent treatments. The CA of the films gone through the two treatment methods are compared and shown in Fig.3. It has been shown that the CA of the TBT treatment films are larger. The largest CA of the PEDOT:PSS film reaches 30 degree when it has been treated with Methanol:DMF co-solvent TBT. This result corresponds to the highest electrical conductivity of the films with the same treatment accordingly. All results are implied that the methanol:DMF co-solvent is an effective way to remove the insulating PSS parts from PEDOT:PSS film surface to improve it’s conductivity.
Fig.3 Variations of the contact angle of the PEDOT:PSS films with different solvent TAT or TBT treatment conditions. Surface morphology of the PEDOT:PSS film with the solvent treatment was also investigated. Fig. 4 shows the AFM surface images of the PEDOT:PSS films without and with the solvent treatments. It shows that for the untreated PEDOT:PSS film, it’s surface roughness was 1.22 nm which is consistent with previously reported result[31]. Surface roughness of the PEDOT:PSS films with the co-solvent TAT and TBT treatments was measured as 1.57 nm and 2.03 nm, respectively. The increased surface roughness of the films with the solvent treatment may be due to the increase of the grain size of the PEDOT:PSS film because of the PSS chains lost during the co-solvent treatment [12, 31].
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Fig. 4 AFM 3D images of a untreated PEDOT:PSS (a) and PEDOT:PSS with cosolvent TAT (b) or TBT (c), in which the co-solvent is formed of methanol and DMF with volume ratio of 1:1. All the images are with 5μm×5μm. It has been reported that in the fabrication of the perovskite solar cells, formation of the perovskite film is affected by the surface hydrophobic nature of the HTL underneath. A non-wetting HTL surface is beneficial to form large grain size perovskite films, which is good better performance of the PSC [32]. As described above， the contact angle (CA) of the PEDOT:PSS HTL with the solvent TBT or TAT treatment is larger than that of untreated PEDOT:PSS HTL film. As shown in Fig. 5 the SEM surface morphology images of the perovskite films deposited on the PEDOT:PSS HTL with different solvents TBT treatment are similar. This shows the solvent TBT treatment does not affect the formed perovskite film’s grain size.
Fig 5. SEM images of the perovskite film formed on PEDOT:PSS without (a) and with Methanol (b), Methanol:DMF cosolvent (c) and DMF (d) TBT treatment, respectively. Absorption spectrum and photoluminescence (PL) of the samples with the perovskite film deposited on the PEDOT:PSS HTL were measured. As shown in Figure 6(a), the absorption 6
Journal Pre-proof spectrum of the perovskite film formed on the PEDOT:PSS HTL film without and with the solvent treatment are almost same. But the PL intensity of the perovskite films formed on the HTL with different solvent treatment is much lower than the sample with the perovskite film deposited on the PEDOT:PSS HTL without the solvent treatment as show in Fig.6(b). The PL peak of all the samples locates at same wavelength 762 nm, which shows that the perovskite films formed on the PEDOR:PSS HTL without and with the solvent treatment have the same band gap [33]. But the PL intensity of the perovskite films formed on the PEDOT:PSS HTL with the solvent TBT treatment was much lower than that of the perovskite formed on the untreated PEDOT:PSS film. This is attributed to the carriers’ extraction at the interface between the perovskite film and PEDOT:PSS HTL. Faster charge transfer and extraction at the interface between the perovskite and the HTL after the co-solvent treatment [34-36] causes reduction of the PL intensity. The results show that the co-solvent treatment on the PEDOT:PSS HTL in the PSCs will not affect the optical absorption of the solar cells but enhance the carriers’ extraction of the device, which improve the PCE of PSCs.
Fig. 6 (a) Absorption and (b) the PL spectra of the perovskite film formed on the PEDOT:PSS HTL without and with a Methanol , Methanol:DMF (1:1) cosolvent and DMF TBT treatment. Applying the co-solvent TAT and TBT treated PEDOT:PSS as the HTLs, inverted planar PSCs devices with the structure of ITO/HTL/MAPbI3Cl/PCBM/BCP/Ag were fabricated and characterized using a solar simulator with 1sun AM 1.5G illumination (100 mW.cm-2). Fig. 7 shows the measured I-V characteristics of the PSCs based on the PEDOT:PSS HTLs with and without the solvent treatments, respectively. Fig.7(a) shows the I-V characteristics of the reference PSC and the PSCs with the solvents TBT treated PEDOT:PSS HTLs, while Fig.7(b) shows the IV characteristics of the reference PSC and the PSCs based on the solvents TAT treated PEDOT:PSS HTLs. Table 1 summarizes the measured performance parameters of the PSCs including their short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF) and PCE. All the value of the parameters are the average value measured from twelve devices of the same batch. The value of the parameter with parentheses is the best value of the parameter measured from the twelve devices. The reference device based on the untreated PEDOT:PSS HTL was measured the PEC of 13.56±0.65%, Jsc of 17.64±0.47 mA.cm-2, Voc of 1.00±0.01V, and FF of 76.85±1.01. All the devices with the solvent treated PEDOT:PSS as the HTL show the improvement in all device performance parameters. The best performance of the PSC was the device with the PEDOT:PSS 7
Journal Pre-proof HTL treated by methanol:DMF co-solvent using TBT treatment. The average PEC of the devices achieved 16.06±0.64%, with Jsc of 19.28±0.44 mA.cm-2, Voc of 1.02±0.01 V, and FF of 81.67±0.83. The best performance of the device was measured a PEC of 16.70%, Jsc of 19.72 mA.cm-2, Voc of 1.03 V, and FF of 82.20. Compared with the reference device, the PCE of the PSC with the cosolvent TBT treated PEDOT:PSS HTL has improved by 17.5%. It can be seen that the enhancement in PEC of the PSC is mainly due to the enhanced Jsc and FF of the devices with the solvent treated HTLs. As shown in Fig.7(a), for the device with the methanol:DMF co-solvent TBT treated PEDOT:PSS HTL, besides the Jsc and FF, the Voc of the device was also increased which may be because of the reduction of the charge recombination and improvement of charge transport in the device.
Fig 7. I-V characteristics of PSCs based on untreated PEDOT:PSS or PEDOT:PSS films with solvent TBT (a) and TAT (b) measured under AM 1.5G illumination. Table 1. Summary of photovoltaic performances of the PSCs based on the untreated PEDOT:PSS HTL and the PEDOT:PSS HTLs with Methanol, DMF or Methanol:DMF (1:1) cosolvent TBT and TAT. The value in parentheses is the best one of the devices measured device Untreat PEDOT:PSS Methanol TBT treatment Methanol:DMF TBT treatment DMF TBT treatment Methanol TAT treatment Methanol:DMF TAT treatment DMF TAT treatment aThe
Jsc (mA.cm-2)a
Voc (V)a
FF (%)a
PCE (%)a
17.64±0.47 (18.07) 18.47±0.51 (18.97) 19.28±0.44 (19.72) 18.99+0.64 (19.53) 18.49±0.40 (18.88) 18.91±0.59 (19.49) 18.79±0.54 (19.31)
1.00±0.01 (1.01) 0.99±0.02 (1.01) 1.02±0.01 (1.03) 1.00±0.01 (1.00) 1.01±0.01 (1.02) 1.01±0.01 (1.01) 1.01±0.01 (1.02)
76.85±1.01 (77.86) 77.81±1.15 (79.06) 81.67±0.83 (82.20) 80.01±1.19 (81.20) 78.81±1.37 (79.98) 81.00+1.01 (82.00) 79.53±0.55 (80.00)
13.56±0.65 (14.21) 14.29±0.86 (15.15) 16.06±0.64 (16.70) 15.19±0.66 (15.85) 14.72±0.68 (15.40) 15.47±0.67 (16.14) 15.09±0.67 (15.76)
averaged PCEs were calculated from twelve solar cells. 8
Journal Pre-proof 4. Conclusion In summary, PEDOT:PSS thin films were treated by different solvent post-treatments to improve it’s conductivity. After the treatment, electrical conductivity of the PEDOT:PSS films was enhanced from 10-3 S.cm-1 to ~102 S.cm-1. The PEDOT:PSS films with the solvent TBT treatment exhibited little higher conductivity than that of the PEDOT:PSS films went through the solvent TAT treatment using the same co-solvent. All the characterization, including optical absorption, contact angle and AFM, indicate that the enhancement in the conductivity of the PEDOT:PSS film is attributing to the removal of the PSS chains from the PEDOT:PSS film surface during the solvent treatment. Using the co-solvent TBT treated PEDOT:PSS films as the HTL of the perovskite solar cell, PEC of the PSC has been improved by 17.5% as compared with that of the PSC with the untreated PEDOT:PSS HTL of the same structure. The highest PCE of the PSC with the solvent treated PEDOT:PSS HTL reached 16.7%. The enhanced performance of the PSC is ascribed to the increased conductivity of the PEDOT:PSS HTL of the PSC by the solvent treatment, which mainly contributed the enhancement of Jsc and FF of the PSC. The investigations shows that the co-solvent treatment of the PEDOT:PSS HTL of the PSC facilitate the hole extraction and transport between the perovskite film and electrode so to enhanced the efficiency of the PSC.
Acknowledgements The author would like to thank MOE, Singapore, for providing the financial support: MOE AcRF Tier 1 RG176/16. This work was also supported by the National Natural Science Foundation of China (Grant No. 11704426), Scientific Research Foundation of Guizhou Province (ZSKH[2017]5, QKHJC[2016]1162) and Youth Science Foundation of Guizhou Province Education Ministry (QJHKY[2016]253).
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Journal Pre-proof enhanced performance and stability of inverted perovskite solar cells via a rational design of hole transport laye, Org. Electron. 73 (2019) 69–75.
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Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Journal Pre-proof Highlights 1. DMF and methanol co-solvent were employed to treat the PEOT:PSS thin film to enhance the film’s electrical conductivity. 2. Electrical conductivity of the PEDOT:PSS films was increased from 10-3 S.cm-1 to ~102 S.cm-1 after the co-solvents treatment. 3. The PCE of the device with co-solvents treat PEDOT:PSS film has been improved by 17.5% compared with that of the control device.
Ping Li, Mohamed Ibrahim Omer Mohamed, Cunyun Xu, Xizu Wang, Xiaohong Tang PII:
S1566-1199(19)30609-3
DOI:
https://doi.org/10.1016/j.orgel.2019.105582
Reference:
ORGELE 105582
To appear in:
Organic Electronics
Received Date:
05 September 2019
Accepted Date:
07 December 2019
Please cite this article as: Ping Li, Mohamed Ibrahim Omer Mohamed, Cunyun Xu, Xizu Wang, Xiaohong Tang, Electrical property modified hole transport layer (PEDOT:PSS) enhance the efficiency of perovskite solar cells: hybrid co-solvent post-treatment, Organic Electronics (2019), https://doi.org/10.1016/j.orgel.2019.105582
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Journal Pre-proof Graphical abstract
Variation of the conductivity of the PEDOT:PSS film treated without and with the various solvent by TAT and TBT method, respectively.
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Journal Pre-proof Electrical property modified hole transport layer (PEDOT:PSS) enhance the efficiency of perovskite solar cells: hybrid co-solvent post-treatment Ping Lia,b, Mohamed Ibrahim Omer Mohameda, Cunyun Xuc, Xizu Wang d, Xiaohong Tanga* a
School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798 b School of Physics and Electronic Science, Zunyi Normal university, Zunyi, China 563002 c Faculty of energy and materials, Southwest University, Chongqing, China, 400715 d Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research, #08-03, 2 Fusionopolis Way, Innovis, 138634, Singapore. *Corresponding Email: [email protected] Abstract Poly(3,4-ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS) layer as a hole transport layer (HTL) plays a key role in efficient hole extraction and transportation of the inverted planar perovskite solar cells (PSCs). However, the insulating nature of PSS lead to lower electrical property of the PEDOT:PSS film, which impedes transferring hole and results in a low photocurrent of the PSCs. In this work, dimethylformamide (DMF), methanol and their mixture solvents (co-solvent) were employed to treat the PEOT:PSS thin film after deposition with different methods to enhance the film’s electrical conductivity. Electrical conductivity of the PEDOT:PSS films was increased from 10-3 S.cm-1 to ~102 S.cm-1 after the co-solvents treatment. Using the highest conductivity co-solvent treated PEDOT:PSS thin film as the HTL in a perovskite solar cell, the power conversion efficiency (PCE) of the device has been measured improved by 17.5% as compared with that of the control device with the untreated PEDOT:PSS film as the HTL. This result shows that the hybrid co-solvent post-treatment is a simple and feasible way to modified the electrical properties of the PEDOT:PSS film as the HTL for high efficient PSCs and other thin film electronic and optoelectronic devices. Key words: Perovskite solar cells; PEDOT:PSS; Conductivity; Hole transporting layer; Solvent treatment 1. Introduction Conductive polymers have attracted much interest as the charge carriers’ transport layers in the solution-processed flexible electronic and optoelectronic devices, such as polymer based organic solar cells, organic light emitting diodes and the recently developed organic-inorganic lead halide perovskite solar cells (PSCs), etc [1-6]. Organic-inorganic lead halide perovskite solar cells have attracted much attention after experiencing a sky-rocketing power conversion efficiency since they were proposed in around 2009. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thin film with it’s high optical transparency in the visible light spectrum, easy aqueous solution processing, high work function facilitating the formation of an Ohmic contact with perovskite film, has been widely used as the hole transporting layer (HTL) to facilitate transporting holes to the anode in the perovskite solar cells [7-11]. However, the pristine 1
Journal Pre-proof PEDOT:PSS HTL films prepared from the PEDOT:PSS dispersion usually suffers from a very low electrical conductivity. This is because a moiety of insulating PSS existing on top of the PEDOT:PSS films when are formed from the aqueous dispersion [7,12 ]. Therefore, to fabricate high efficient PSCs, conductivity of the PEDOT:PSS film need to be improved. Need to be noticed that according to different volume ratio of PEDOT and PSS, there are different PEDOT:PSS films for different applications. When the volume ratio of the PEDOT and PSS is 1:2.5 (PH1000), the PEDOT:PSS films are normally used as the conductive electrode in organic photovoltaic (OPV) devices and organic light emitting diodes (OLED). The conductivity of the pure PH1000 film is usually less than 1 S.cm-1 [13-19]. With the solvent post-treatment, the PH1000 PEDOT:PSS films’ electrical conductivity can be increased to higher than 1000 S.cm-1 and even up to 3000 S.cm-1 [15,17]. High concentration of PSS in PEDOT:PSS is good for dispersing PEDOT into aqueous and enhances the work function of PEDOT:PSS. Therefore, PEDOT:PSS films with the PEDOT and PSS volume ratio of 1:6 (Al 4083) are widely used as the HTL in OPV, OLED and PSCs because of their high work function facilitating the formation of an Ohmic contact with the organic materials. But the Al4083 PEDOT:PSS films normally show very low conductivity due to the high volume ratio of PSS, which is not good for the charge transporting in the devices. Solvents posttreatment and doping are used to modify the electrical property of the insulating PEDOT:PSS (Al 4083) [20-25]. Chen et al. dipped the DMF on the PEDOT:PSS film with thermal annealing (TA), and then spin coated the DMF after 5 second [26]. With the post-treatment, they have increased the formed PEDOT:PSS film’s conductivity by 2 orders. In this work, a new DMF : Methanol cosolvent post-treatment methods of the Al4083 PEDOT:PSS film were proposed and investigated. Electrical conductivity of the PEDOT:PSS films was measured increased from 10-3 S.cm-1 to 101 S.cm-1 after the post-treatment. Using the co-solvent post treated PEDOT:PSS film as the hole transporting layer of the perovskite solar cell, the power conversion efficiency (PCE) of the device was obtained improved over 16.7% as compared with that of the device with the un-treated PEDOT:PSS as the HTL. 2. Experiment 2.1 Materials PEDOT:PSS aqueous solution (Clevios P VP Al 4083) used in this work was purchased from Heraeus. Methylammonium iodide (MAI), lead iodide (PbI2), lead chloride (PbCl2), dimethyl sulfoxide (DMSO), chlorobenzene (CB), methanol and dimethylformamide (DMF) were from Sigma-Aldrich. All materials were used as received. 2.2 Treatment of PEDOT:PSS and Fabrication of the PSCs The PEDOT:PSS films were prepared by spin coating their aqueous solution on pre-cleaned glass substrates of the size of 2.0×2.0 cm2. In the investigations, two methods were employed to treat the surface of the deposited PEOT:PSS films to enhance their conductivity. The first method is called as treatment before thermal annealing (TBT) of the PEDOT:PSS films. In the TBT treatment method, before thermal annealing the deposited PEDOT:PSS films, 80 μL of DMF, methanol and their mixed solvent (DMF:methanol cosolvent) was respectively dropped on the PEDOT:PSS film to cover the film surface for 10 seconds, and then, spin coated the sample for 30 s with 5000 rpm. After that, the samples were annealed at 130 C on hotplate for 20 min. The second method is called as treatment after thermal annealing (TAT) of the PEDOT:PSS films. 2
Journal Pre-proof In this treatment, after spin deposited the PEDOT:PSS films, the samples were thermal annealed at 130 C on the hotplate for 20 min. Then, 80 μL of DMF, methanol and their mixed solvent was respectively dropped on the samples to cover the PEDOT:PSS films’ surface for 10 seconds, and spin coating the solvent of the sample at 5000 rpm for 30 s. Then, the samples were annealed at 100 C on hotplate for 10 min. The ITO glass substrates were cleaned sequentially in deionized (DI) water, acetone, and isopropanol by sonication for 20 min. After N2 blow drying, the substrates were further treated with UV-ozone for 10 min. The PEDOT:PSS film with a thickness of 30 nm was deposited by spin coating the aqueous on the ITO substrates at 5000 rpm for 40 s, and subsequently treated with solvent by TAT or TBT method in air. For fabricating the perovskite solar cells, the ITO glass substrates coated with the PEDOT:PSS HTL were transferred into a glovebox filled with N2. MAI of 222.6 mg, PbCl2 of 38.9 mg and PbI2 of 580.86 mg were dissolved in l mL anhydrous DMF and DMSO co-solvent with volume ratio 9:1 as the perovskite precursor solution. The perovskite layer was formed by spin coating the precursor solution on the PEDOT:PSS coated ITO substrate at 4000 rpm for 30 seconds. After 10 seconds from started the spin coating the perovskite film, 200 ml anhydrous CB was added dropwise onto the spinning sample. The perovskite layer coated samples were annealed at 85 C for 30 min. The thickness of the perovskite films was around 450 nm. PCBM (20 mg.mL-1 in CB) solution without any additive was spun coated on top of the perovskite layer at 1000 rpm for 45 seconds to form the electron transporting layer. Then, BCP in IPA was spun coated on the samples at 1000 rpm for 30 seconds. Finally, metal silver (80 nm) electrode of the solar cell was thermal evaporated though a shadow mask. Each device had an area of 0.09 cm2. 2.3 Characterization of PEDOT:PSS film and PSCs The conductivity of the PEDOT:PSS films were measured by the four-point probe technique with a Keithley 2400 source/meter. UV absorption spectra of the films were measured by using a Varian Cary 5000 UV-vis-NIR spectrometer. The surface morphology of the films was measured by atomic force microscopic (AFM) acquired with a Veeco Nano-Scope IV Multi-Mode AFM in tapping mode. The contact angles (CA) of the PEDOT:PSS films with different solvent treatments were measured using a microscopic contact angle meter. The photovoltaic performance of the PSCs with the PEDOT:PSS films as the HTL was measured in glovebox with a computerprogrammed Keithley 2400 source/meter and a Newport’s solar simulator, which simulated the 1 sun AM1.5 sunlight with light density of 100 mW.cm-2. 3. Results and discussion Conductivity of the PEDOT:PSS films treated with different co-solvents by TAT and TBT treatment, respectively, (following denoted as PEDOT:PSS with solvent TAT or TBT) are compared in Figure 1. It shows that the conductivity of the pristine PEDOT:PSS film is 1.3×10-3 S.cm-1 which is consistent with previous reports [25,26]. A dramatic increase of the conductivity of the PEDOT:PSS films was received after these co-solvent treatments. Using the TAT treatment method, conductivity of the PEDOT:PSS film treated by pure of methanol and DMF achieved 1.02×101 S.cm-1 and 1.31×101 S.cm-1, respectively. Higher conductivity of the PEDOT:PSS films is achieved by treated the samples with the methanol:DMF co-solvents. The conductivity of the PEDOT:PSS film is depended on the volume ratio of the methanol:DMF co-solvent in the 3
Journal Pre-proof treatment. The highest conductivity of the PEDOT:PSS film reaches 5.13×101 S.cm-1 when it was treated by the TAT method with the methanol:DMF cosolvent with the volume ratio of 1:1. The conductivity enhancement of the PEDOT:PSS film with different volume ratio of the methanol:DMF co-solvent using the TBT treatment method was also investigated. The results show that the PEDOT:PSS film’s conductivity increased much after it was treated with the solvents TBT treatment. The highest conductivity of the film achieves 9.5×101 S.cm-1 when it was treated with the methanol:DMF co-solvent with the volume ratio 1:1. Compared with that of the PEDOT:PSS film without the treatment, conductivity of the PEDOT:PSS film was enhanced more than 70,000 times.
Fig.1 Variation of the conductivity of the PEDOT:PSS film treated without and with the various solvent by TAT (black) and TBT (red) method, respectively.
Fig. 2 Absorption of a untreated PEDOT:PSS and PEDOT:PSS films with methanol, DMF or methanol:DMF (1:1) co-solvent TAT or TBT. In order to reveal the effects of the treatments on the PEDOT:PSS films, some characterizations were carried out on the samples under the different solvent treatments. Fig. 2 shows the absorption spectrum of the PEDOT:PSS films with different treatments. All the samples show two absorption peaks which originated from the aromatic ring of PSS of the films [12, 16, 27]. The PEDOT:PSS film without the co-solvent treatment has the highest absorption peak. After gone through the treatments, the PSS absorption peaks of the film decreased significantly. The absorption peaks was 4
Journal Pre-proof the lowest when the film was treated with the methanol:DMF co-solvent using TBT treatment. This suggests that some PSS chains were removed from the PEDOT:PSS films during the solvent treatment. This result shows that the conductivity enhancement of the PEDOT:PSS film with the co-solvent treatment is because of the removal of the PSS from the PEDOT:PSS film surface during the solvent treatment [23, 24, 28-30]. To further prove the removal of PSS from the PEDOT:PSS film surface with the co-solvent treatment, water contact angle (CA) of the PEDOT:PSS films with different solvent treatment was measured. The measured contact angles of the PEDOT:PSS films after different solvent treatment by using TAT and TBT methods respectively are shown in Fig.3. The contact angle of the untreated PEDOT:PSS film is 10.8 degree. With the solvent treatment, the CA of the PEDOT:PSS film increases. This is ascribed to the hydrophobic PEDOT richer on the PEDOT:PSS film surface after the removal of hydrophilic PSS of the film during the co-solvent treatments. The CA of the films gone through the two treatment methods are compared and shown in Fig.3. It has been shown that the CA of the TBT treatment films are larger. The largest CA of the PEDOT:PSS film reaches 30 degree when it has been treated with Methanol:DMF co-solvent TBT. This result corresponds to the highest electrical conductivity of the films with the same treatment accordingly. All results are implied that the methanol:DMF co-solvent is an effective way to remove the insulating PSS parts from PEDOT:PSS film surface to improve it’s conductivity.
Fig.3 Variations of the contact angle of the PEDOT:PSS films with different solvent TAT or TBT treatment conditions. Surface morphology of the PEDOT:PSS film with the solvent treatment was also investigated. Fig. 4 shows the AFM surface images of the PEDOT:PSS films without and with the solvent treatments. It shows that for the untreated PEDOT:PSS film, it’s surface roughness was 1.22 nm which is consistent with previously reported result[31]. Surface roughness of the PEDOT:PSS films with the co-solvent TAT and TBT treatments was measured as 1.57 nm and 2.03 nm, respectively. The increased surface roughness of the films with the solvent treatment may be due to the increase of the grain size of the PEDOT:PSS film because of the PSS chains lost during the co-solvent treatment [12, 31].
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Fig. 4 AFM 3D images of a untreated PEDOT:PSS (a) and PEDOT:PSS with cosolvent TAT (b) or TBT (c), in which the co-solvent is formed of methanol and DMF with volume ratio of 1:1. All the images are with 5μm×5μm. It has been reported that in the fabrication of the perovskite solar cells, formation of the perovskite film is affected by the surface hydrophobic nature of the HTL underneath. A non-wetting HTL surface is beneficial to form large grain size perovskite films, which is good better performance of the PSC [32]. As described above， the contact angle (CA) of the PEDOT:PSS HTL with the solvent TBT or TAT treatment is larger than that of untreated PEDOT:PSS HTL film. As shown in Fig. 5 the SEM surface morphology images of the perovskite films deposited on the PEDOT:PSS HTL with different solvents TBT treatment are similar. This shows the solvent TBT treatment does not affect the formed perovskite film’s grain size.
Fig 5. SEM images of the perovskite film formed on PEDOT:PSS without (a) and with Methanol (b), Methanol:DMF cosolvent (c) and DMF (d) TBT treatment, respectively. Absorption spectrum and photoluminescence (PL) of the samples with the perovskite film deposited on the PEDOT:PSS HTL were measured. As shown in Figure 6(a), the absorption 6
Journal Pre-proof spectrum of the perovskite film formed on the PEDOT:PSS HTL film without and with the solvent treatment are almost same. But the PL intensity of the perovskite films formed on the HTL with different solvent treatment is much lower than the sample with the perovskite film deposited on the PEDOT:PSS HTL without the solvent treatment as show in Fig.6(b). The PL peak of all the samples locates at same wavelength 762 nm, which shows that the perovskite films formed on the PEDOR:PSS HTL without and with the solvent treatment have the same band gap [33]. But the PL intensity of the perovskite films formed on the PEDOT:PSS HTL with the solvent TBT treatment was much lower than that of the perovskite formed on the untreated PEDOT:PSS film. This is attributed to the carriers’ extraction at the interface between the perovskite film and PEDOT:PSS HTL. Faster charge transfer and extraction at the interface between the perovskite and the HTL after the co-solvent treatment [34-36] causes reduction of the PL intensity. The results show that the co-solvent treatment on the PEDOT:PSS HTL in the PSCs will not affect the optical absorption of the solar cells but enhance the carriers’ extraction of the device, which improve the PCE of PSCs.
Fig. 6 (a) Absorption and (b) the PL spectra of the perovskite film formed on the PEDOT:PSS HTL without and with a Methanol , Methanol:DMF (1:1) cosolvent and DMF TBT treatment. Applying the co-solvent TAT and TBT treated PEDOT:PSS as the HTLs, inverted planar PSCs devices with the structure of ITO/HTL/MAPbI3Cl/PCBM/BCP/Ag were fabricated and characterized using a solar simulator with 1sun AM 1.5G illumination (100 mW.cm-2). Fig. 7 shows the measured I-V characteristics of the PSCs based on the PEDOT:PSS HTLs with and without the solvent treatments, respectively. Fig.7(a) shows the I-V characteristics of the reference PSC and the PSCs with the solvents TBT treated PEDOT:PSS HTLs, while Fig.7(b) shows the IV characteristics of the reference PSC and the PSCs based on the solvents TAT treated PEDOT:PSS HTLs. Table 1 summarizes the measured performance parameters of the PSCs including their short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF) and PCE. All the value of the parameters are the average value measured from twelve devices of the same batch. The value of the parameter with parentheses is the best value of the parameter measured from the twelve devices. The reference device based on the untreated PEDOT:PSS HTL was measured the PEC of 13.56±0.65%, Jsc of 17.64±0.47 mA.cm-2, Voc of 1.00±0.01V, and FF of 76.85±1.01. All the devices with the solvent treated PEDOT:PSS as the HTL show the improvement in all device performance parameters. The best performance of the PSC was the device with the PEDOT:PSS 7
Journal Pre-proof HTL treated by methanol:DMF co-solvent using TBT treatment. The average PEC of the devices achieved 16.06±0.64%, with Jsc of 19.28±0.44 mA.cm-2, Voc of 1.02±0.01 V, and FF of 81.67±0.83. The best performance of the device was measured a PEC of 16.70%, Jsc of 19.72 mA.cm-2, Voc of 1.03 V, and FF of 82.20. Compared with the reference device, the PCE of the PSC with the cosolvent TBT treated PEDOT:PSS HTL has improved by 17.5%. It can be seen that the enhancement in PEC of the PSC is mainly due to the enhanced Jsc and FF of the devices with the solvent treated HTLs. As shown in Fig.7(a), for the device with the methanol:DMF co-solvent TBT treated PEDOT:PSS HTL, besides the Jsc and FF, the Voc of the device was also increased which may be because of the reduction of the charge recombination and improvement of charge transport in the device.
Fig 7. I-V characteristics of PSCs based on untreated PEDOT:PSS or PEDOT:PSS films with solvent TBT (a) and TAT (b) measured under AM 1.5G illumination. Table 1. Summary of photovoltaic performances of the PSCs based on the untreated PEDOT:PSS HTL and the PEDOT:PSS HTLs with Methanol, DMF or Methanol:DMF (1:1) cosolvent TBT and TAT. The value in parentheses is the best one of the devices measured device Untreat PEDOT:PSS Methanol TBT treatment Methanol:DMF TBT treatment DMF TBT treatment Methanol TAT treatment Methanol:DMF TAT treatment DMF TAT treatment aThe
Jsc (mA.cm-2)a
Voc (V)a
FF (%)a
PCE (%)a
17.64±0.47 (18.07) 18.47±0.51 (18.97) 19.28±0.44 (19.72) 18.99+0.64 (19.53) 18.49±0.40 (18.88) 18.91±0.59 (19.49) 18.79±0.54 (19.31)
1.00±0.01 (1.01) 0.99±0.02 (1.01) 1.02±0.01 (1.03) 1.00±0.01 (1.00) 1.01±0.01 (1.02) 1.01±0.01 (1.01) 1.01±0.01 (1.02)
76.85±1.01 (77.86) 77.81±1.15 (79.06) 81.67±0.83 (82.20) 80.01±1.19 (81.20) 78.81±1.37 (79.98) 81.00+1.01 (82.00) 79.53±0.55 (80.00)
13.56±0.65 (14.21) 14.29±0.86 (15.15) 16.06±0.64 (16.70) 15.19±0.66 (15.85) 14.72±0.68 (15.40) 15.47±0.67 (16.14) 15.09±0.67 (15.76)
averaged PCEs were calculated from twelve solar cells. 8
Journal Pre-proof 4. Conclusion In summary, PEDOT:PSS thin films were treated by different solvent post-treatments to improve it’s conductivity. After the treatment, electrical conductivity of the PEDOT:PSS films was enhanced from 10-3 S.cm-1 to ~102 S.cm-1. The PEDOT:PSS films with the solvent TBT treatment exhibited little higher conductivity than that of the PEDOT:PSS films went through the solvent TAT treatment using the same co-solvent. All the characterization, including optical absorption, contact angle and AFM, indicate that the enhancement in the conductivity of the PEDOT:PSS film is attributing to the removal of the PSS chains from the PEDOT:PSS film surface during the solvent treatment. Using the co-solvent TBT treated PEDOT:PSS films as the HTL of the perovskite solar cell, PEC of the PSC has been improved by 17.5% as compared with that of the PSC with the untreated PEDOT:PSS HTL of the same structure. The highest PCE of the PSC with the solvent treated PEDOT:PSS HTL reached 16.7%. The enhanced performance of the PSC is ascribed to the increased conductivity of the PEDOT:PSS HTL of the PSC by the solvent treatment, which mainly contributed the enhancement of Jsc and FF of the PSC. The investigations shows that the co-solvent treatment of the PEDOT:PSS HTL of the PSC facilitate the hole extraction and transport between the perovskite film and electrode so to enhanced the efficiency of the PSC.
Acknowledgements The author would like to thank MOE, Singapore, for providing the financial support: MOE AcRF Tier 1 RG176/16. This work was also supported by the National Natural Science Foundation of China (Grant No. 11704426), Scientific Research Foundation of Guizhou Province (ZSKH[2017]5, QKHJC[2016]1162) and Youth Science Foundation of Guizhou Province Education Ministry (QJHKY[2016]253).
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Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Journal Pre-proof Highlights 1. DMF and methanol co-solvent were employed to treat the PEOT:PSS thin film to enhance the film’s electrical conductivity. 2. Electrical conductivity of the PEDOT:PSS films was increased from 10-3 S.cm-1 to ~102 S.cm-1 after the co-solvents treatment. 3. The PCE of the device with co-solvents treat PEDOT:PSS film has been improved by 17.5% compared with that of the control device.