- Email: [email protected]
NaCl doped electrochemical PEDOT:PSS layers for inverted perovskite solar cells with enhanced stability
Synthetic Metals 257 (2019) 116178
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
NaCl doped electrochemical PEDOT:PSS layers for inverted perovskite solar cells with enhanced stability Eider A. Erazoa,1, Daniel Castillo-Bendeckb,1, Pablo Ortizb, María T. Cortésa, a b
T
⁎
Department of Chemistry, Universidad de los Andes, Bogotá D.C. 111711, Colombia Department of Chemical Engineering, Universidad de los Andes, Bogotá D.C. 111711, Colombia
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite solar cell Inverted architecture Electrodeposited PEDOT:PSS Stability Redox state tuning
Perovskite solar cells are experiencing an unprecedented growth and might soon replace the conventional solar technologies. For inverted architecture cells, spin coated (SC) PEDOT:PSS has been widely implemented as hole transport material, but it has still unresolved issues regarding stability and scalability. In this work we deposited PEDOT:PSS layers by an alternative electrochemical (EC) route that offers precise synthesis control, scale-up potential and enhanced cell stability. The EC-PEDOT:PSS layers were deposited on ITO substrates from an aqueous solution of EDOT and NaPSS by cyclic voltammetry. Additionally, NaCl in different concentrations was added to the synthesis solution to tune the redox state of the polymer, as confirmed by UV-vis measurements. Photoluminescence emission spectra of MAPbI3 perovskite deposited on EC- and SC-PEDOT:PSS layers showed that both had a similar charge collection efficiency. Furthermore, SEM images demonstrated that MAPbI3 grew similarly on both films. Finally, inverted perovskite solar cells were fabricated using these layers. The results showed that 0.1 M NaCl doped EC-PEDOT:PSS performed similarly as the SC-PEDOT:PSS, achieving efficiencies as high as 11% and improved fill factors exceeding 80%. Most importantly, the EC-PEDOT:PSS significantly improved the stability of the cells, allowing the devices to maintain 90% of their average efficiency after 15 days.
1. Introduction Since their introduction in 2009, Perovskite Solar Cells (PVSC) have experienced an unprecedented growth compared to previous solar technologies. In only 6 years, their record lab-efficiency raised from 4% to 21% and the current published record stands at 25.2% [1], a value comparable to the best lab-efficiencies of c-Si cells. The good performance of these devices has been explained through the excellent optoelectronic properties of the hybrid metalorganic perovskite family, which include a high visible-light absorption efficiency, a tunable direct band-gap, a low exciton binding energy, and relatively high electron and hole diffusion lengths [2–4]. Additionally, the perovskite films can be deposited through ambient temperature solvent methods, which makes them suitable for low energy, cost effective, high throughput mass production [5]. There are two types of architecture in PVSC. The direct architecture commonly uses TiO2 and spiro-OMeTAD as electron transport material (ETM) and hole transport material (HTM) respectively. However the use of spiro-OMeTAD and its dopants is highly expensive and results in instability issues [6,7]. Although important advances have been made
to replace spiro-OMeTAD [8], alternatives with inverted architecture have gained a lot of interest due to advantages such as compatibility with flexible substrates because of low temperature fabrication processes (< 120 °C) [9], a balanced hole- and electron-transport resulting in hysteresis-less photovoltaic behavior [10,11], and the possibility of using them in multijunction solar cells [12]. The conductive polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) has been extensively used as the HTM for the inverted architecture, achieving efficiencies as high as 20.1% [13]. The most common way to deposit this material for labscale cells is by spin coating it from an aqueous suspension. Its main advantages include low temperature solution processability, relatively high visible transparency and conductivity, good mechanical flexibility, and low costs [14]. Nonetheless, it has also been demonstrated that PEDOT:PSS is directly associated with serious long-term device instability, one of the still unresolved issues for PVSC and the main hindrance for their industrial scale-up [15]. The sulfonate group in PSS is acidic in nature and highly hygroscopic. This has been shown to be detrimental when deposited on the frequently used indium-tin-oxide (ITO) electrode layer, since acidic
⁎
Corresponding author. E-mail address: [email protected] (M.T. Cortés). 1 Contributed equally to this work. https://doi.org/10.1016/j.synthmet.2019.116178 Received 14 August 2019; Received in revised form 6 September 2019; Accepted 16 September 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
solution was ultrasonicated for 2 minutes to disperse all the EDOT and then bubbled during 5 min with N2. During the synthesis, a N2 atmosphere was maintained on top of the solution, and PEDOT was deposited on the ITO substrate by cyclic voltammetry with an Autolab PGSTAT302N potentiostat-galvanostat between −0.1 and 1.2 V at 0.1 V/s. The thickness of the films was kept constant by fixing the electrosynthesis charge at 4 mC. Then, the films of oxidized PEDOT were electrochemically reduced in an aqueous solution of 10 mM NaCl by running 5 cycles of pulses at −0.4 V for 5 s and 0 V for 2 s. Finally, the films were annealed inside a N2 filled glove box on a hot plate at 110 °C.
solutions etch it and liberate indium atoms that contaminate the PEDOT:PSS and perovskite layers [16]. Moreover, the stability of the PVSC is severely affected by the presence of hygroscopic films, since the perovskite degrades easily with water molecules [17]. Despite these undesired effects, PSS is widely used because it stabilizes the aqueous suspensions of PEDOT and increases its work function, making it more compatible with the valence band of the perovskite [18]. Furthermore, it reduces the presence of pinholes in the perovskite layer resulting in higher fill factors [19] and its acidic nature can be decreased by the using basic additives like urea [20]. PEDOT:PSS layers can also be obtained from the electropolymerization of EDOT in the presence of NaPSS, which is a simple and scalable process. Since PEDOT:PSS electrodeposition combines polymerization and deposition in a single step, the problem of solubility is reduced and, with it, the need for large amounts of detrimental PSS surfactant [21]. Furthermore, the electrodeposition has other advantages such as great reproducibility and tunable electronic and surface properties, offering an attractive alternative to the conventional spin coated PEDOT:PSS layers. Moreover, the electropolymerization process does not generate significant material waste and can be used in large area, curved substrates, making this technology compatible with large-scale applications [22]. The use of electrochemically deposited PEDOT:PSS has shown good results in organic solar cells and dye sensitized solar cells [23,24,21,25]. Already in 2002, Frohne et al. tuned the work function of electrodeposited PEDOT:PSS layers by an electrochemical reduction to optimize the interfacial contact in organic solar cells [23]. In PVSC the electrodeposition of PEDOT:PSS has been explored little and only in the direct architecture. Xiao et al. electrochemically deposited a PEDOT layer on top of a fluorine-doped-tin-oxide (FTO) substrate and used it to sandwich the perovskite layer deposited on another substrate to obtain efficiencies as high as 12.33% [26]. Furthermore, in a novel approach, Samu et al. electrochemically deposited the PEDOT:PSS layer directly on top of the perovskite layer achieving efficiencies close to 6% [27]. Regarding the inverted architecture, some groups have started to study the electrochemical deposition of HTMs, for example polythiopene [28] or copper thyocianate [29]. However, to the best of our knowledge, no one has yet published results with electrochemically deposited PEDOT:PSS in inverted PVSC. In this work, we electrochemically deposited PEDOT:PSS layers on ITO substrates to fabricate inverted architecture perovskite solar cells with efficiencies around 11% and fill factors exceeding 80%. This architecture has the advantage that the PEDOT:PSS layer is deposited before the perovskite layer, such that aqueous suspensions of PEDOT:PSS may be used without harming the devices. Moreover, the ITO substrate offers a homogeneous, well conducting working electrode for the deposition.
2.2. Spin coating of PEDOT:PSS This process was conducted inside a dry air (< 30% RH) glovebox. A PEDOT:PSS Clevios Al 4083 solution (Ossila, 1.3–1.7% wt.) was used as received for the process. The cleaned ITO substrates were preheated at 140 °C on a heating plate, as suggested in [30] and then the hot substrate was spun at 6000 RPM. After 10 s, 35 μL of the PEDOT:PSS suspension were dynamically added and the substrate was left to rotate for another 30 s. Finally, the films were annealed at 140 °C. 2.3. Solar cell fabrication The solar cells were manufactured inside a glove box with constant N2 flow. The perovskite precursor solution consisted of a 38% wt. stoichiometric mixture of MAI (Great Cell Solar) and PbI2 (Aldrich, 99%) in DMF:DMSO, with 10:1 volume ratio (Panreac 99.8 and 99.5% respectively) [31]. 30 μL of this solution were spread on the PEDOT:PSS layer and the substrate was spun at 1000 rpm for 10 s and at 6000 rpm for 30 s. 200 μL of chlorobenzene were quickly added 6 s after the start of the second stage. Then, the films were annealed for 3 min at 100 °C on a hot plate. PC61BM (1-Material, > 99%) was spin coated on top of the perovskite from a 20 mg/mL solution in chlorobenzene at 4000 rpm. The BCP layer was similarly deposited from a 0.5 mg/mL solution in methanol at 4000 rpm. Finally, the silver contacts were thermally evaporated at a rate of 0.2 Å/s during the first 5 nm and at 1 Å/s until 100 nm. 2.4. Characterization methods The UV-vis spectra were taken in a Thermo Scientific Genesys 10S UV-vis spectrophotometer and the PL measurements in a Cary Varian Eclipse Fluorescence Spectrophotometer with an excitation wavelength of 405 nm. The estimation of the HOMO level by cyclic voltammetry was made in anhydrous acetonitrile with 0.1 M TBAPF6 (Aldrich, 98%) at 100 mV/s, using 1 mM ferrocene (Alfa Aesar, 99%) as the electrochemical reference. The potential was measured against an Ag/AgCl reference electrode using a double-junction chamber filled with 3 M NaCl. Raman spectra were obtained by an Xplora Horiba Scientific Raman microscope. The AFM images were taken with an Asylum Research microscope model MFP-3D-BIO in tapping mode and KPFM measurements were carried out on the same microscope using the KPFM mode. The SEM images were taken with a Tescan Lyra 3 microscope. Finally, the JV curves of the solar cells were measured under simulated AM 1.5G light (100 mW/cm2) from a solar simulator Abet Technologies model 10500 sweeping from −0.1 to 1.0 V at 50 mV/s with an Autolab AUT84194 potentiostat. The light intensity was calibrated with a Hamamatsu S1133 photodiode and the photoactive area was estimated as the intersection between the ITO and Ag electrodes with a value of 0.067 cm2.Contact angles were measured using an Attension model theta contact angle meter.
2. Experimental 2.1. Electrodeposition of PEDOT:PSS Prepatterned ITO glass substrates (KINTEC, 15 Ω/□) were sequentially ultrasonicated in a neutral detergent solution and in deionized water. After drying the substrates with N2, they were plasma-cleaned for 15 min. The PEDOT:PSS electrodeposition was carried out in a three-electrode cell, where the cleaned ITO substrates were used as working electrodes and a platinum wire was used as counter electrode. Additionally, the potential was measured vs an Ag/AgCl (3 M NaCl) reference electrode. The EDOT monomer was vacuum distilled to avoid the presence of oligomers. The synthesis solution consisted of an aqueous medium with 4.5 g/L NaPSS (Aldrich, avg. Mw 70.000) and 0.02% v/v EDOT. These conditions were adapted from a procedure reported elswhere [26]. Optionally, NaCl (Panreac ACS) at 0.001, 0.01 or 0.1 M concentration was added to tune the redox state of the polymer. This
3. Results and discussion The voltammogram corresponding to the electrochemical (EC) 2
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
Fig. 1. (a) Voltammogram for the electrochemical synthesis of PEDOT:PSS by cyclic voltammetry at 100 mV/s. The number in parenthesis denotes the cycle number. In black, the aqueous synthesis solution contained only EDOT and NaPSS. In red, 0.1 M NaCl was added. (b) UV-vis absorption spectra of the PEDOT:PSS films synthesized at different concentrations of NaCl and subsequently electrochemically reduced in 10 mM NaCl. In the absence of NaCl denoted by ”ECPEDOT:PSS”. the spectra are relative to an ITO substrate.
through Raman spectroscopy and the results are displayed in Fig. S2. It was observed that indeed the electrochemically deposited films present the characteristic PEDOT peaks at 1570, 1545, 1372, 863, 707, 586 and 440 cm−1 [35]. The peak centered at 1437 cm−1 is assigned to the Cα=Cβ symmetric stretching vibration; the bands at 1511 cm−1 and 1570 cm−1 to the Cα=Cβ asymmetric stretching vibrations [36]; the 1372 cm−1 peak to C-C stretching deformations; and the 1255 cm−1 to C-C in-plane symmetric stretching. The form of the Raman band associated with Cα=Cβ at 1400–1500 cm−1 is used to distinguish between benzenoid and quinoid structures and gives information about doping [37]. Finally, the main vibrational modes of PSS are located at 999 cm−1 and 1112 cm−1[38]. Fig. 2 (a) shows the normalized Raman spectra between 1350 and 1550 cm−1. Here it can be noted that the Cα = Cβ symmetric stretching vibration of the PEDOT five-member ring presents an evident blue shift for the electrochemical variations, especially for the Cl− doped one. This indicates that the ratio of quinoid to benzenoid structures is higher for the spin coated films than for the electrochemically deposited ones [38]. Thus, the SC-PEDOT:PSS had a higher doping level and charge carrier mobility followed by the EC-PEDOT:PSS and finally the ECPEDOT:PSS-Cl. The incorporation of Cl− anions to the polymer lattice destabilizes the multiple positive charges (polarons and bipolarons) on the PEDOT chains reducing their doping level and forcing a configuration change from a quinoid to a benzoid structure. This will result in a suppressed conductivity since the benzenoid structure hinders the intra-chain conductivity and the charge transport relies on hopping between polymer chains [39–41]. The effect of the presence of Cl− during the electropolymerization of PEDOT:PSS is contrary to the effect of adding Cl− salts to the spin coated PEDOT:PSS suspension. In the latter, an increase in conductivity has been reported due to the interaction of cations such as Na+ with the PSS. This allows the removal of excess PSS and an increase in the conductivity because the structure of PEDOT changes from coiled to linear [42,43]. A steady state photoluminescence (PL) study was conducted on the EC-PEDOT:PSS-Cl and SC-PEDOT:PSS layers to assess their quality in terms of charge collection. After annealing both the EC and the SC films, MAPbI3 perovskite was spin coated through a one-step anti-solvent method on top of them and the PL emission spectra of the samples were compared to the PL spectrum of the same perovskite deposited on glass. The results, including two independent replicates for each variation, are presented in Fig. 2(b). Here, the PL spectra of both MAPbI3/ SC-PEDOT:PSS/ITO and MAPbI3/EC-PEDOT:PSS-Cl/ITO samples overlap and are significantly below the spectrum of the perovskite on glass sample. This confirmed that the electrochemical synthesis produced reproducible films with a similar hole extraction capacity as the SC-PEDOT:PSS. To study possible effects of annealing temperature, three different MAPbI3/EC-PEDOT:PSS-Cl/ITO samples were prepared by annealing the polymeric films at 110, 130 and 150 °C. Their (PL) emission spectra were measured (see supp. Fig. S3) and the results showed a similar
synthesis of PEDOT:PSS in the presence of EDOT and NaPSS (without NaCl) can be seen in black in Fig. 1(a). The corresponding UV-vis absorption spectra for the subsequently reduced layer is presented in black in Fig. 1(b) (EC-PEDOT:PSS). Despite the reduction process, the absorption around 900 nm was strong and coincided with that of an oxidized PEDOT:PSS film with a high presence of polarons [32,33]. This was initially attributed to the fact that the PSS is a bulky dopant, which might hinder its removal from the polymeric lattice during the reduction and would leave the PEDOT polymer still highly doped. For the material to act as a good HTM it must have a well-defined band gap, a HOMO level compatible with the valence band of the perovskite and a LUMO level as high as possible to block leakage electron currents [34]. In this sense, more reduced films were preferred, since they don’t have mid-gap energy levels introduced by the presence of polarons and bipolarons in the polymer chain. An attempt was made to further reduce the films by applying a stronger reduction potential of -0.5 V, but the films were damaged and detached from the ITO at this voltage. Therefore, we added the less voluminous Cl− secondary counterion during the synthesis process to facilitate the reduction process. This was performed by adding NaCl to the PEDOT:PSS synthesis solution in 0.001, 0.01 and 0.1 M concentrations. In Fig. 1(a) the voltammogram for the synthesis with an addition of 0.1 M NaCl is shown in red. Here, an increase in the peak current can be noticed when adding the NaCl, which may be due to the decreased solution resistance. The polymerization potential also increased, probably because the additional Na+ cations induce a destabilization that cannot be compensated by the Cl− anions. The latter, since these are not as good as the PSS− at stabilizing the polymerization intermediary radicals [21]. The UV-vis spectra of the films prepared with NaCl are shown in Fig. 1(b), where it is clear that there is a gradual decrease in the absorption between 800 and 1000 nm as the NaCl content of the synthesis solution is increased. This evidences a more reduced state of the films and a lower density of polarons due to the presence of the Cl− anions [32,33].To further investigate this, UV-vis spectra of films synthesized with and without 0.1 M NaCl were measured before and after the reduction to investigate whether this change in shape was due to the synthesis or the reduction processes (see supp. Fig. S1). The results show that the main absorption characteristics of the films were determined by the synthesis conditions; the reduction only caused minor changes in the spectra. Thus, it can be concluded that the lower density of polarons in the NaCl sample is mainly due to the nature of the dopants (Cl− vs PSS−) rather than a removal of these species during the reduction. This confirms what was found by Nasybulin et al. that the number of free carriers in the PEDOT depends to a large extent on the nature of the dopants [21]. Since only the most reduced state was desired, we kept working only with the 0.1 M NaCl variation, which will be henceforth denoted by EC-PEDOT:PSS-Cl. The electrochemical PEDOT:PSS films (EC-PEDOT:PSS and ECPEDOT: PSS-Cl) were compared with spin coated films (SC-PEDOT:PSS) 3
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
Fig. 2. (a) Normalized Raman spectra of a spin coated PEDOT:PSS film (SC-PEDOT:PSS), an electrochemically deposited PEDOT:PSS film (EC-PEDOT:PSS) and an electrochemically deposited PEDOT:PSS film in the presence of 0.1 M NaCl (EC-PEDOT:PSS-Cl). The laser excitation wavelength was 532 nm. (b) Photoluminescence emission spectra of perovskite layers excited at 405 nm deposited on a glass substrate (black), a spin coated PEDOT:PSS (green) and an electrochemically deposited PEDOT:PSS in the presence of 0.1 M NaCl (red). In all cases the excitation beam reaches the perovskite from the ITO/glass side. (c) Cyclic voltammetry of ECPEDOT:PSS-Cl films in acetonitrile with 0.1 M TBAPF6 at 100 mV/s, Vox = 0.70 V. (d-f) AFM topographic images of (d) SC-PEDOT:PSS (RMSR = 1.1 nm), (e) ECPEDOT:PSS (RMSR = 7.3 nm) and (f) EC-PEDOT:PSS-Cl (RMSR = 1.7 nm). RMSR stands for root mean square roughness.
quench for all temperatures, so the lowest was chosen to favor low energy consumption. The electrochemical films were then characterized through cyclic voltammetry to estimate their HOMO levels and ferrocene was used as the electrochemical reference for the vacuum level (see supp. Fig. S4). The voltammogram for EC-PEDOT:PSS-Cl is shown in Fig. 2(c) and the corresponding figure for EC-PEDOT:PSS is presented in supp. Fig. S4. Both yield an estimated HOMO value of −5.1 eV that is similar to the value reported in other studies [44] and that is compatible with the MAPbI3 perovskite valence band level (−5.4 eV). This suggests that the addition of NaCl to the synthesis solution does not affect the HOMO level. In addition, the Kelvin probe force microscopy (KPFM) technique was used to estimate the work functions of the EC-PEDOT:PSS and ECPEDOT:PSS-Cl in comparison to the SC-PEDOT:PSS value. By fixing the work function of the latter at 5.2 eV [20,45,46], the values for ECPEDOT:PSS and EC-PEDOT:PSS-Cl were 5.02 and 5.08 eV respectively. The KPFM images are shown in Fig. S5. AFM topographic images of the SC-PEDOT:PSS, EC-PEDOT:PSS and EC-PEDOT:PSS-Cl films are presented in Fig. 2(d)-(f). Whereas the ECPEDOT:PSS film has a root mean square roughness (RMSR) of 7.3 nm, both SC-PEDOT:PSS and EC-PEDOT:PSS-Cl films are smoother, having an RMSR of 1.1 and 1.7 nm respectively. There is a slight morphological difference between both samples though. The SC one is more homogeneous, whereas the EC layer seems to aggregate. To check if this difference had any impact on perovskite growth, top-view and crosssection SEM images of perovskite deposited on these layers were taken, as shown in supp. Fig. S6 and Fig. 3 respectively. The top-view images suggest that there is no clear difference between the perovskites grown on SC-PEDOT:PSS-Cl and EC-PEDOT:PSS, both having similar crystal size distribution. The cross-section image shows a perovskite thickness and a vertical distribution of grains similar in both cases. An important observation is that the perovskite films do not consist of single vertical crystals, but instead of multiple vertical layers. Finally, the cross-section SEM images evidence that the EC-PEDOT:PSS is much thinner than its spin coated counterpart, almost imperceptible at the displayed resolution. This was confirmed by an AFM step profile measurement which demonstrated that the film was ultra-thin (Fig. S7).
Fig. 3. Cross section SEM images of solar cells made with: (a) SC-PEDOT:PSS and (b) EC-PEDOT:PSS-Cl.
Inverted perovskite solar cells were then fabricated with ECPEDOT:PSS and EC-PEDOT:PSS-Cl to test their functionality and compare them to cells fabricated with SC-PEDOT:PSS. Fig. 3 shows the SEM cross-section images of the cells, both with the SC-PEDOT:PSS and the EC-PEDOT:PSS-Cl layers. Fig. 4 presents the JV-curves of the champion solar cell for each variation. The first observation is that the EC route can produce cells with similar performances to the cells with SC-PEDOT:PSS, having champion efficiencies of 11.2% and 11.4% respectively. Secondly, the results suggest that the presence of NaCl during the PEDOT:PSS synthesis yields cells with better currents and fill factors, as will be statistically discussed below. The similar results between SCPEDOT:PSS and EC-PEDOT:PSS-Cl were expected, given the same PL quenching behavior and the same perovskite growth. In all three cases, the short circuit currents are relatively low in comparison with other literature reported values. This could be explained by the multiple rows of perovskite crystals observed in the vertical direction in Fig. 3. Since the charges must travel through many grain boundaries to reach the adjacent layers, they are more likely to 4
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
work functions of the EC-PEDOT:PSS-Cl and EC-PEDOT:PSS variations were estimated to be very close (5.08 and 5.02 eV), the difference in Voc must have been mainly due to recombination and not a difference in band alignment. The lower recombination in a more reduced state could also explain why the EC-PEDOT:PSS-Cl with an intermediate work function (5.08 eV) has an average Voc higher than the SC-PEDOT:PSS that has the highest work function (5.20 eV). Regarding the difference in Jsc, the stronger absorption of light of EC-PEDOT:PSS in comparison to EC-PEDOT:PSS-Cl in the range of 400–800 nm might have caused it (see Fig. 1(b)). There is still room for improvement, though, in terms of optimizing the thickness and reduction level of the EC-PEDOT:PSS-Cl, such that the recombination due to mid-gap states is overcome and a similar or better Jsc is obtained in comparison to the spin coated films. Interestingly, the EC-PEDOT:PSS variation presents a significantly higher efficiency dispersion in Fig. 5(a). This could be attributed to undesired effects that occur during the reduction process for this variation. As mentioned above, the EC-PEDOT:PSS films detached from the ITO at a reduction potential of −0.5 V and, most likely, there were still unnoticeable damages at lower voltages. Similar problems have been reported for chemical and electrochemical reduction of PEDOT layers [47]. The addition of NaCl increased the film resilience so that it could withstand −0.5 and even −0.7 V without detaching. Fig. S8 shows the UV-Vis spectra of EC-PEDOT:PSS-Cl films reduced at −0.5V and −0.7V and it is clear that there is no fundamental difference between them. We speculate that the improved resilience of the EC-PEDOT:PSS-Cl is related to conformational rearrangements of the polymeric chains. During the electrochemical reduction, the small and mobile Cl− anions are expelled from the film as the positive charges in the polymer chains are neutralized, resulting in a compact polymer lattice. In contrast, for immobile polyanions like PSS− the reduction involves the entrance of cations, hence there is an increase in volume [48]. In some cases, the volume change could create enough mechanical stress to cause a local polymer delamination and thus cause the greater efficiency dispersion. Finally, a stability study was conducted on cells fabricated with SCPEDOT:PSS and EC-PEDOT:PSS-Cl. Fig. 6 shows the temporal evolution of the photovoltaic parameters of the unencapsulated cells. The parameters were measured at ambient conditions with a relative humidity higher than 60%, but the cells were stored in a dark nitrogen atmosphere in-between. The significantly better stability for the EC-PEDOT:PSS-Cl variation is striking. On the one hand, the average efficiency of the SC-PEDOT:PSS cells decreases rapidly over time, dropping to less than 60% of the initial value in only 8 days. The main cause of this drop is a drastic reduction in short circuit current, while the voltage drops slower. The same observation has been established already in the literature and is attributed to the degradation problems associated with PSS [20,14,49]. In contrast, the EC-PEDOT:PSS–Cl cells maintain on average a little more than 80% of their original efficiency after 25 days.
Fig. 4. JV curves of the best performing cells for three PEDOT:PSS deposition variations: spin coated (SC-PEDOT:PSS), electrochemically deposited (ECPEDOT:PSS) and electrochemically deposited in the presence of 0.1 M NaCl (EC-PEDOT:PSS-Cl).
recombine, thus reducing the measured current. A statistical study was conducted to establish the effects of the different PEDOT:PSS variations on efficiency, open circuit voltage (Voc), short circuit current (Jsc) and fill factor (FF). The box plots of the results are presented in Fig. 5. Even though, on average, the SC-PEDOT:PSS cells are more efficient, the EC-PEDOT:PSS-Cl yields comparable cell efficiencies, with the champion devices exceeding 11% in both cases. The reason why the SC-PEDOT:PSS cells are a bit more efficient is because they have higher Jsc values, but in terms of Voc and FF the ECPEDOT:PSS-Cl cells have better and less dispersed data. In particular, the FF values are remarkable, some even exceeding 80%. This originates from the fact that the series resistance of the EC-PEDOT:PSS-Cl layers was smaller, as evidenced by the steeper slope at open circuit conditions in Fig. 4. Even though the Raman results in Fig. 2(a) indicated a lower hole mobility for the electrochemically deposited layer, its ultra-thin thickness compensates this and yields a smaller series resistance [38,40]. In all three cases, the Voc values are relatively small in comparison to other PVSC reported in the literature, most likely because of the ∼0.3 V misalignment between the PEDOT:PSS HOMO level and the perovskite valence band maximum. From Fig. 5 it is also clear that the EC-PEDOT:PSS-Cl yields cells significantly more efficient than the EC-PEDOT:PSS, with better Voc and Jsc values. This can be rationalized in the following way. The ECPEDOT:PSS-Cl is in a more reduced state than the EC-PEDOT:PSS, as evidenced by the UV-vis spectra in Fig. 1(b) and the Raman spectra in Fig. 2 (a). Hence, it has a more defined energy gap between the HOMO and LUMO levels such that there are less mid-gap energy levels available that could provide recombination pathways. Given the fact that the
Fig. 5. Box plot of (a) efficiency, (b) Voc, (c) Jsc and (d) FF for 13 devices with SC-PEDOT:PSS, 26 devices with EC-PEDOT:PSS-Cl and 13 devices with EC-PEDOT:PSS. Due to space reasons, the word PEDOT has been removed from all labels. 5
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
Fig. 6. Temporal evolution of efficiency (a), Voc (b), Jsc (c) and FF (d) of unencapsulated inverted perovskite solar cells manufactured with spin coated PEDOT:PSS (SC), and electrochemically deposited PEDOT:PSS in the presence of 0.1 M NaCl (EC). The number of measured devices was 4 and 8 respectively. The dots represent the mean value and the bars show the standard deviation. The data was normalized to the day 0 mean values.
SEM images, water contact angle measurement, and a photograph comparing the stability of cells can be found in the electronic supplementary material.
These results are supported by visually comparing the cells fabricated using both methods, as shown in Fig. S9, where the perovskite in the cell with SC-PEDOT:PSS displays a notorious yellow degradation, while the EC-PEDOT:PSS-Cl cell remains quite intact. The improved stability is probably related to the lower acidic PSS content in the EC films, since the SC-PEDOT:PSS needs a greater amount of PSS, not only to dope the polymer, but to keep it in a stable suspension before the deposition. This hypothesis is supported by the static water contact angle measurements shown in Fig. S10, where a lower contact angle denotes a higher hydrophilic PSS content. Whereas the spin coated PEDOT:PSS film is highly hydrophilic with a contact angle of 9°, the electrodeposited films present a more hydrophobic nature with contact angles close to 47°. Moreover, a higher content of hydrophilic PSS in the spin coated film is also in agreement with its measured higher work function, since the work function of PEDOT:PSS increases with the content of PSS [18].
Acknowledgement The authors thank the Universidad de los Andes (Chemistry Department and Chemical Engineering Department) for providing funding. María T. Cortés acknowledges support from the Science Faculty (Proyecto INV-2017-51-1456). Eider A. Erazo acknowledges support from the Science Faculty (Proyecto INV-2018-33-1308) and CEIBA foundation. Appendix A. Supplementary Data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.synthmet.2019.116178.
4. Conclusions References In conclusion, it was demonstrated that the PEDOT:PSS obtained by electropolymerization in the presence of 0.1 M NaCl yielded a functional HTM for inverted perovskite solar cells with efficiencies around 11%, fill factors exceeding 80% and a similar performance to devices fabricated with spin coated PEDOT:PSS. To achieve these values, the redox level of the polymer was tuned to a more reduced state by the addition of NaCl to the synthesis solution, and by applying a voltage reduction signal to the layer. Most importantly, the electrochemical polymerization of PEDOT:PSS resulted in a less hydrophilic coating which improved the stability of the devices in a significant manner, probably because of a lower PSS content. Future work will be done to further optimize several parameters such as the thickness of the layers, the NaCl content and the polymer oxidation level to attain better short circuit currents and open circuit voltages. The great reproducibility of electropolymerization, the ease of tuning of the properties of the films, the promising photovoltaic results and the enhanced stability of the cells demonstrate that the electropolymerization of PEDOT:PSS should be further studied in pursuit of the large-scale implementation of PVSC technology.
[1] NREL, Best Research-Cell Efficiencies, (2019) https://www.nrel.gov/pv/cell-efficiency.html. [2] N.-G. Park, Perovskite solar cells: an emerging photovoltaic technology, Mater. Today 18 (2015) 65–72. [3] J.M. Ball, M.M. Lee, A. Hey, H.J. Snaith, Low-temperature processed meso-superstructured to thin-film perovskite solar cells, Energy Environ. Sci. 6 (2013) 1739–1743. [4] J. Bouclé, N. Herlin-Boime, The benefits of graphene for hybrid perovskite solar cells, Synth. Metals 222 (2016) 3–16. [5] F.X. Xie, D. Zhang, H. Su, X. Ren, K.S. Wong, M. Gratzel, W.C. Choy, Vacuumassisted thermal annealing of CH3NH3PbI3 for highly stable and efficient perovskite solar cells, ACS Nano 9 (2015) 639–646. [6] A. Dubey, N. Adhikari, S. Venkatesan, S. Gu, D. Khatiwada, Q. Wang, L. Mohammad, M. Kumar, Q. Qiao, Solution processed pristine pdpp3t polymer as hole transport layer for efficient perovskite solar cells with slower degradation, Solar Energy Mater. Solar Cells 145 (2016) 193–199. [7] E.A. Gaml, A. Dubey, K.M. Reza, M.N. Hasan, N. Adhikari, H. Elbohy, B. Bahrami, H. Zeyada, S. Yang, Q. Qiao, Alternative benzodithiophene (bdt) based polymeric hole transport layer for efficient perovskite solar cells, Solar Energy Mater. Solar Cells 168 (2017) 8–13. [8] S. Mabrouk, M. Zhang, Z. Wang, M. Liang, B. Bahrami, Y. Wu, J. Wu, Q. Qiao, S. Yang, Dithieno [3,2-b: 2’, 3’-d] pyrrole-based hole, J. Mater. Chem. A 6 (2018) 7950–7958. [9] J. You, Z. Hong, Y. Yang, Q. Chen, M. Cai, T.-B. Song, C.-C. Chen, S. Lu, Y. Liu, H. Zhou, et al., Low-temperature solution-processed perovskite solar cells with high efficiency and flexibility, ACS Nano 8 (2014) 1674–1680. [10] J.H. Heo, H.J. Han, D. Kim, T.K. Ahn, S.H. Im, Hysteresis-less inverted CH3NH3PbI3 planar, Energy Environ. Sci. 8 (2015) 1602–1608. [11] F. Wu, R. Pathak, K. Chen, G. Wang, B. Bahrami, W.-H. Zhang, Q. Qiao, Inverted current-voltage hysteresis in perovskite solar cells, ACS Energy Lett. 3 (2018) 2457–2460. [12] K.A. Bush, A.F. Palmstrom, J.Y. Zhengshan, M. Boccard, R. Cheacharoen, J.P. Mailoa, D.P. McMeekin, R.L. Hoye, C.D. Bailie, T. Leijtens, et al., solar cells with improved stability, Nature Energy 2 (2017) 17009. [13] C.-H. Chiang, M. Khaja Nazeeruddin, M. Gr“atzel, C.-G. Wu, The synergistic
Declarations of interest There are no conflicts to declare. Supplementary Material Additional UV-vis, Raman and PL figures, HOMO level estimation voltammograms and calculations, KPFM images, top-view perovskite 6
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
controlling cell growth, ACS Appl. Mater. Interfaces 7 (2015) 17993–18003. [33] N. Massonnet, A. Carella, O. Jaudouin, P. Rannou, G. Laval, C. Celle, J.-P. Simonato, Improvement of the seebeck coefficient of PEDOT:PSS by chemical reduction combined with a novel method for its transfer using free-standing thin films, J. Mater. Chem. C 2 (2014) 1278–1283. [34] A. Isakova, P.D. Topham, Polymer strategies in perovskite solar cells, J. Polym. Sci. Part B: Polym. Phys. 55 (2017) 549–568. [35] X. Hu, L. Chen, L. Tan, T. Ji, Y. Zhang, L. Zhang, D. Zhang, Y. Chen, In situ polymerization of ethylenedioxythiophene from sulfonated carbon nanotube templates: toward high efficiency ito-free solar cells, J. Mater. Chem. A 4 (2016) 6645–6652. [36] B. Xu, S.-A. Gopalan, A.-I. Gopalan, N. Muthuchamy, K.-P. Lee, J.-S. Lee, Y. Jiang, S.-W. Lee, S.-W. Kim, J.-S. Kim, et al., Corrigendum: Functional solid additive modified pedot: Pss as an anode buffer layer for enhanced photovoltaic performance and stability in polymer solar cells, Scientific Reports 7 (2017) 46779. [37] S.S. Kalagi, P.S. Patil, Secondary electrochemical doping level effects on polaron and bipolaron bands evolution and interband transition energy from absorbance spectra of PEDOT:PSS thin films, Synth. Metals 220 (2016) 661–666. [38] S.H. Chang, K.-F. Lin, K.Y. Chiu, C.-L. Tsai, H.-M. Cheng, S.-C. Yeh, W.-T. Wu, W.N. Chen, C.-T. Chen, S.-H. Chen, et al., Improving the efficiency of CH3NH3PbI3 based photovoltaics by tuning the work function of the PEDOT:PSS hole transport layer, Solar Energy 122 (2015) 892–899. [39] R. Kroon, D.A. Mengistie, D. Kiefer, J. Hynynen, J.D. Ryan, L. Yu, C. M”uller, Thermoelectric plastics: from design to synthesis, processing and structure-property relationships, Chem. Soc. Rev. 45 (2016) 6147–6164. [40] B. Vaagensmith, K.M. Reza, M.N. Hasan, H. Elbohy, N. Adhikari, A. Dubey, N. Kantack, E. Gaml, Q. Qiao, Environmentally friendly plasma-treated pedot:pss as electrodes for ito-free perovskite solar cells, ACS Appl. Mater. Interfaces 9 (2017) 35861–35870. [41] Y. Kim, W. Cho, Y. Kim, H. Cho, J.H. Kim, Electrical characteristics of heterogeneous polymer layers in PEDOT:PSS films, J. Mater. Chem. C 6 (2018) 8906–8913. [42] B. Kadem, W. Cranton, A. Hassan, Metal salt modified pedot: Pss as anode buffer layer and its effect on power conversion efficiency of organic solar cells, Org. Electron. 24 (2015) 73–79. [43] L. Hu, K. Sun, M. Wang, W. Chen, B. Yang, J. Fu, Z. Xiong, X. Li, X. Tang, Z. Zang, et al., Inverted planar perovskite solar cells with a high fill factor and negligible hysteresis by the dual effect of NaCl-doped PEDOT:PSS, ACS Appl. Mater. Interfaces 9 (2017) 43902–43909. [44] S. Ma, W. Qiao, T. Cheng, B. Zhang, J. Yao, A. Alsaedi, T. Hayat, Y. Ding, Z. Tan, S. Dai, Optical-electrical-chemical engineering of pedot:pss by incorporation of hydrophobic nafion for efficient and stable perovskite solar cells, ACS Appl. Mater. Interfaces 10 (2018) 3902–3911. [45] Y.-H. Seo, J.-S. Yeo, N. Myoung, S.-Y. Yim, M. Kang, D.-Y. Kim, S.-I. Na, Blending of n-type semiconducting polymer and pc61bm for an efficient electron-selective material to boost the performance of the planar perovskite solar cell, ACS Appl. Mater. Interfaces 8 (2016) 12822–12829. [46] A. Al Mamun, T.T. Ava, K. Zhang, H. Baumgart, G. Namkoong, New pcbm/carbon based electron transport layer for perovskite solar cells, Phys. Chem. Chem. Phys. 19 (2017) 17960–17966. [47] T. Lindfors, A. Österholm, J. Kauppila, M. Pesonen, Electrochemical reduction of graphene oxide in electrically conducting poly (3,4-ethylenedioxythiophene) composite films, Electrochim. Acta 110 (2013) 428–436. [48] T.F. Otero, H.-J. Grande, J. Rodríguez, Reinterpretation of polypyrrole electrochemistry after consideration of conformational relaxation processes, J. Phys. Chem. B 101 (1997) 3688–3697. [49] K.M. Reza, S. Mabrouk, Q. Qiao, A review on tailoring pedot: Pss layer for improved performance of perovskite solar cells, Proc. Nat. Res. Soc. 2 (2018) 02004.
effect of H2O and dmf towards, Energy Environ. Sci. 10 (2017) 808–817. [14] Q. Wang, C.-C. Chueh, M. Eslamian, A.K.-Y. Jen, Modulation of pedot:pss ph for efficient inverted perovskite solar cells with reduced potential loss and enhanced stability, ACS Appl. Mater. Interfaces 8 (2016) 32068–32076. [15] Q. Fu, X. Tang, B. Huang, T. Hu, L. Tan, L. Chen, Y. Chen, Recent progress on the long-term stability of perovskite solar cells, Adv. Sci. 5 (2018) 1700387. [16] M. De Jong, L. Van Ijzendoorn, M. De Voigt, Stability of the interface between indium-tin-oxide and poly (3,4-ethylenedioxythiophene)/poly (styrenesulfonate) in polymer light-emitting diodes, Appl. Phys. Lett. 77 (2000) 2255–2257. [17] J. Yang, Z. Yuan, X. Liu, S. Braun, Y. Li, J. Tang, F. Gao, C. Duan, M. Fahlman, Q. Bao, Oxygen-and water-induced energetics degradation in organometal halide perovskites, ACS Appl. Mater. Interfaces 10 (2018) 16225–16230. [18] W. Kim, S. Kim, S.U. Chai, M.S. Jung, J.K. Nam, J.-H. Kim, J.H. Park, Thermodynamically self-organized hole transport layers for high-efficiency inverted-planar perovskite solar cells, Nanoscale 9 (2017) 12677–12683. [19] H. Liu, X. Li, L. Zhang, Q. Hong, J. Tang, A. Zhang, C.-Q. Ma, Influence of the surface treatment of pedot:pss layer with high boiling point solvent on the performance of inverted planar perovskite solar cells, Organic Electron. 47 (2017) 220–227. [20] H. Elbohy, B. Bahrami, S. Mabrouk, K.M. Reza, A. Gurung, R. Pathak, M. Liang, Q. Qiao, K. Zhu, Tuning hole transport layer using urea for high-performance perovskite solar cells, Adv. Functional Mater. (2018) 1806740. [21] E. Nasybulin, S. Wei, I. Kymissis, K. Levon, Effect of solubilizing agent on properties of poly (3,4-ethylenedioxythiophene)(pedot) electrodeposited from aqueous solution, Electrochim. Acta 78 (2012) 638–643. [22] H. Ellis, N. Vlachopoulos, L. H”aggman, C. Perruchot, M. Jouini, G. Boschloo, A. Hagfeldt, Pedot counter electrodes for dye-sensitized solar cells prepared by aqueous micellar electrodeposition, Electrochim. Acta 107 (2013) 45–51. [23] H. Frohne, S.E. Shaheen, C.J. Brabec, D.C. M“uller, N.S. Sariciftci, K. Meerholz, Influence of the anodic work function on the performance of organic solar cells, ChemPhysChem 3 (2002) 795–799. [24] J. Yan, C. Sun, F. Tan, X. Hu, P. Chen, S. Qu, S. Zhou, J. Xu, Electropolymerized poly (3,4-ethylenedioxythiophene): poly (styrene sulfonate)(PEDOT:PSS) film on ito glass and its application in photovoltaic device, Solar Energy Mater. Solar Cells 94 (2010) 390–394. [25] T.-L. Zhang, H.-Y. Chen, C.-Y. Su, D.-B. Kuang, A novel tco-and pt-free counter electrode for high efficiency dye-sensitized solar cells, J. Mater. Chem. A 1 (2013) 1724–1730. [26] Y. Xiao, G. Han, J. Wu, J.-Y. Lin, Efficient bifacial perovskite solar cell based on a highly transparent poly (3,4-ethylenedioxythiophene) as the p-type hole-transporting material, J. Power Sources 306 (2016) 171–177. [27] G.F. Samu, R. Scheidt, G. Zaiats, P.V. Kamat, C. Janáky, Electrodeposition of holetransport layer on methylammonium lead iodide film: A new strategy to assemble perovskite solar cells, Chem. Mater. (2018). [28] W. Yan, Y. Li, Y. Li, S. Ye, Z. Liu, S. Wang, Z. Bian, C. Huang, Stable high-performance hybrid perovskite solar cells with ultrathin polythiophene as hole-transporting layer, Nano Res. 8 (2015) 2474–2480. [29] S. Ye, W. Sun, Y. Li, W. Yan, H. Peng, Z. Bian, Z. Liu, C. Huang, Cuscn-based inverted planar perovskite solar cell, Nano Lett. 15 (2015) 3723–3728. [30] C.-H. Chiang, Z.-L. Tseng, C.-G. Wu, Planar heterojunction perovskite/PC71BM solar cells with enhanced open-circuit voltage via a (2/1)-step spin-coating process, J. Mater. Chem. A 2 (2014) 15897–15903. [31] N. Ahn, D.-Y. Son, I.-H. Jang, S.M. Kang, M. Choi, N.-G. Park, Highly reproducible perovskite solar cells with lewis base adduct of lead (ii) iodide, J. Am. Chem. Soc. 137 (2015) 8696–8699. [32] M. Marzocchi, I. Gualandi, M. Calienni, I. Zironi, E. Scavetta, G. Castellani, B. Fraboni, Physical and electrochemical properties of PEDOT:PSS as a tool for
7
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
NaCl doped electrochemical PEDOT:PSS layers for inverted perovskite solar cells with enhanced stability Eider A. Erazoa,1, Daniel Castillo-Bendeckb,1, Pablo Ortizb, María T. Cortésa, a b
T
⁎
Department of Chemistry, Universidad de los Andes, Bogotá D.C. 111711, Colombia Department of Chemical Engineering, Universidad de los Andes, Bogotá D.C. 111711, Colombia
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite solar cell Inverted architecture Electrodeposited PEDOT:PSS Stability Redox state tuning
Perovskite solar cells are experiencing an unprecedented growth and might soon replace the conventional solar technologies. For inverted architecture cells, spin coated (SC) PEDOT:PSS has been widely implemented as hole transport material, but it has still unresolved issues regarding stability and scalability. In this work we deposited PEDOT:PSS layers by an alternative electrochemical (EC) route that offers precise synthesis control, scale-up potential and enhanced cell stability. The EC-PEDOT:PSS layers were deposited on ITO substrates from an aqueous solution of EDOT and NaPSS by cyclic voltammetry. Additionally, NaCl in different concentrations was added to the synthesis solution to tune the redox state of the polymer, as confirmed by UV-vis measurements. Photoluminescence emission spectra of MAPbI3 perovskite deposited on EC- and SC-PEDOT:PSS layers showed that both had a similar charge collection efficiency. Furthermore, SEM images demonstrated that MAPbI3 grew similarly on both films. Finally, inverted perovskite solar cells were fabricated using these layers. The results showed that 0.1 M NaCl doped EC-PEDOT:PSS performed similarly as the SC-PEDOT:PSS, achieving efficiencies as high as 11% and improved fill factors exceeding 80%. Most importantly, the EC-PEDOT:PSS significantly improved the stability of the cells, allowing the devices to maintain 90% of their average efficiency after 15 days.
1. Introduction Since their introduction in 2009, Perovskite Solar Cells (PVSC) have experienced an unprecedented growth compared to previous solar technologies. In only 6 years, their record lab-efficiency raised from 4% to 21% and the current published record stands at 25.2% [1], a value comparable to the best lab-efficiencies of c-Si cells. The good performance of these devices has been explained through the excellent optoelectronic properties of the hybrid metalorganic perovskite family, which include a high visible-light absorption efficiency, a tunable direct band-gap, a low exciton binding energy, and relatively high electron and hole diffusion lengths [2–4]. Additionally, the perovskite films can be deposited through ambient temperature solvent methods, which makes them suitable for low energy, cost effective, high throughput mass production [5]. There are two types of architecture in PVSC. The direct architecture commonly uses TiO2 and spiro-OMeTAD as electron transport material (ETM) and hole transport material (HTM) respectively. However the use of spiro-OMeTAD and its dopants is highly expensive and results in instability issues [6,7]. Although important advances have been made
to replace spiro-OMeTAD [8], alternatives with inverted architecture have gained a lot of interest due to advantages such as compatibility with flexible substrates because of low temperature fabrication processes (< 120 °C) [9], a balanced hole- and electron-transport resulting in hysteresis-less photovoltaic behavior [10,11], and the possibility of using them in multijunction solar cells [12]. The conductive polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) has been extensively used as the HTM for the inverted architecture, achieving efficiencies as high as 20.1% [13]. The most common way to deposit this material for labscale cells is by spin coating it from an aqueous suspension. Its main advantages include low temperature solution processability, relatively high visible transparency and conductivity, good mechanical flexibility, and low costs [14]. Nonetheless, it has also been demonstrated that PEDOT:PSS is directly associated with serious long-term device instability, one of the still unresolved issues for PVSC and the main hindrance for their industrial scale-up [15]. The sulfonate group in PSS is acidic in nature and highly hygroscopic. This has been shown to be detrimental when deposited on the frequently used indium-tin-oxide (ITO) electrode layer, since acidic
⁎
Corresponding author. E-mail address: [email protected] (M.T. Cortés). 1 Contributed equally to this work. https://doi.org/10.1016/j.synthmet.2019.116178 Received 14 August 2019; Received in revised form 6 September 2019; Accepted 16 September 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
solution was ultrasonicated for 2 minutes to disperse all the EDOT and then bubbled during 5 min with N2. During the synthesis, a N2 atmosphere was maintained on top of the solution, and PEDOT was deposited on the ITO substrate by cyclic voltammetry with an Autolab PGSTAT302N potentiostat-galvanostat between −0.1 and 1.2 V at 0.1 V/s. The thickness of the films was kept constant by fixing the electrosynthesis charge at 4 mC. Then, the films of oxidized PEDOT were electrochemically reduced in an aqueous solution of 10 mM NaCl by running 5 cycles of pulses at −0.4 V for 5 s and 0 V for 2 s. Finally, the films were annealed inside a N2 filled glove box on a hot plate at 110 °C.
solutions etch it and liberate indium atoms that contaminate the PEDOT:PSS and perovskite layers [16]. Moreover, the stability of the PVSC is severely affected by the presence of hygroscopic films, since the perovskite degrades easily with water molecules [17]. Despite these undesired effects, PSS is widely used because it stabilizes the aqueous suspensions of PEDOT and increases its work function, making it more compatible with the valence band of the perovskite [18]. Furthermore, it reduces the presence of pinholes in the perovskite layer resulting in higher fill factors [19] and its acidic nature can be decreased by the using basic additives like urea [20]. PEDOT:PSS layers can also be obtained from the electropolymerization of EDOT in the presence of NaPSS, which is a simple and scalable process. Since PEDOT:PSS electrodeposition combines polymerization and deposition in a single step, the problem of solubility is reduced and, with it, the need for large amounts of detrimental PSS surfactant [21]. Furthermore, the electrodeposition has other advantages such as great reproducibility and tunable electronic and surface properties, offering an attractive alternative to the conventional spin coated PEDOT:PSS layers. Moreover, the electropolymerization process does not generate significant material waste and can be used in large area, curved substrates, making this technology compatible with large-scale applications [22]. The use of electrochemically deposited PEDOT:PSS has shown good results in organic solar cells and dye sensitized solar cells [23,24,21,25]. Already in 2002, Frohne et al. tuned the work function of electrodeposited PEDOT:PSS layers by an electrochemical reduction to optimize the interfacial contact in organic solar cells [23]. In PVSC the electrodeposition of PEDOT:PSS has been explored little and only in the direct architecture. Xiao et al. electrochemically deposited a PEDOT layer on top of a fluorine-doped-tin-oxide (FTO) substrate and used it to sandwich the perovskite layer deposited on another substrate to obtain efficiencies as high as 12.33% [26]. Furthermore, in a novel approach, Samu et al. electrochemically deposited the PEDOT:PSS layer directly on top of the perovskite layer achieving efficiencies close to 6% [27]. Regarding the inverted architecture, some groups have started to study the electrochemical deposition of HTMs, for example polythiopene [28] or copper thyocianate [29]. However, to the best of our knowledge, no one has yet published results with electrochemically deposited PEDOT:PSS in inverted PVSC. In this work, we electrochemically deposited PEDOT:PSS layers on ITO substrates to fabricate inverted architecture perovskite solar cells with efficiencies around 11% and fill factors exceeding 80%. This architecture has the advantage that the PEDOT:PSS layer is deposited before the perovskite layer, such that aqueous suspensions of PEDOT:PSS may be used without harming the devices. Moreover, the ITO substrate offers a homogeneous, well conducting working electrode for the deposition.
2.2. Spin coating of PEDOT:PSS This process was conducted inside a dry air (< 30% RH) glovebox. A PEDOT:PSS Clevios Al 4083 solution (Ossila, 1.3–1.7% wt.) was used as received for the process. The cleaned ITO substrates were preheated at 140 °C on a heating plate, as suggested in [30] and then the hot substrate was spun at 6000 RPM. After 10 s, 35 μL of the PEDOT:PSS suspension were dynamically added and the substrate was left to rotate for another 30 s. Finally, the films were annealed at 140 °C. 2.3. Solar cell fabrication The solar cells were manufactured inside a glove box with constant N2 flow. The perovskite precursor solution consisted of a 38% wt. stoichiometric mixture of MAI (Great Cell Solar) and PbI2 (Aldrich, 99%) in DMF:DMSO, with 10:1 volume ratio (Panreac 99.8 and 99.5% respectively) [31]. 30 μL of this solution were spread on the PEDOT:PSS layer and the substrate was spun at 1000 rpm for 10 s and at 6000 rpm for 30 s. 200 μL of chlorobenzene were quickly added 6 s after the start of the second stage. Then, the films were annealed for 3 min at 100 °C on a hot plate. PC61BM (1-Material, > 99%) was spin coated on top of the perovskite from a 20 mg/mL solution in chlorobenzene at 4000 rpm. The BCP layer was similarly deposited from a 0.5 mg/mL solution in methanol at 4000 rpm. Finally, the silver contacts were thermally evaporated at a rate of 0.2 Å/s during the first 5 nm and at 1 Å/s until 100 nm. 2.4. Characterization methods The UV-vis spectra were taken in a Thermo Scientific Genesys 10S UV-vis spectrophotometer and the PL measurements in a Cary Varian Eclipse Fluorescence Spectrophotometer with an excitation wavelength of 405 nm. The estimation of the HOMO level by cyclic voltammetry was made in anhydrous acetonitrile with 0.1 M TBAPF6 (Aldrich, 98%) at 100 mV/s, using 1 mM ferrocene (Alfa Aesar, 99%) as the electrochemical reference. The potential was measured against an Ag/AgCl reference electrode using a double-junction chamber filled with 3 M NaCl. Raman spectra were obtained by an Xplora Horiba Scientific Raman microscope. The AFM images were taken with an Asylum Research microscope model MFP-3D-BIO in tapping mode and KPFM measurements were carried out on the same microscope using the KPFM mode. The SEM images were taken with a Tescan Lyra 3 microscope. Finally, the JV curves of the solar cells were measured under simulated AM 1.5G light (100 mW/cm2) from a solar simulator Abet Technologies model 10500 sweeping from −0.1 to 1.0 V at 50 mV/s with an Autolab AUT84194 potentiostat. The light intensity was calibrated with a Hamamatsu S1133 photodiode and the photoactive area was estimated as the intersection between the ITO and Ag electrodes with a value of 0.067 cm2.Contact angles were measured using an Attension model theta contact angle meter.
2. Experimental 2.1. Electrodeposition of PEDOT:PSS Prepatterned ITO glass substrates (KINTEC, 15 Ω/□) were sequentially ultrasonicated in a neutral detergent solution and in deionized water. After drying the substrates with N2, they were plasma-cleaned for 15 min. The PEDOT:PSS electrodeposition was carried out in a three-electrode cell, where the cleaned ITO substrates were used as working electrodes and a platinum wire was used as counter electrode. Additionally, the potential was measured vs an Ag/AgCl (3 M NaCl) reference electrode. The EDOT monomer was vacuum distilled to avoid the presence of oligomers. The synthesis solution consisted of an aqueous medium with 4.5 g/L NaPSS (Aldrich, avg. Mw 70.000) and 0.02% v/v EDOT. These conditions were adapted from a procedure reported elswhere [26]. Optionally, NaCl (Panreac ACS) at 0.001, 0.01 or 0.1 M concentration was added to tune the redox state of the polymer. This
3. Results and discussion The voltammogram corresponding to the electrochemical (EC) 2
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
Fig. 1. (a) Voltammogram for the electrochemical synthesis of PEDOT:PSS by cyclic voltammetry at 100 mV/s. The number in parenthesis denotes the cycle number. In black, the aqueous synthesis solution contained only EDOT and NaPSS. In red, 0.1 M NaCl was added. (b) UV-vis absorption spectra of the PEDOT:PSS films synthesized at different concentrations of NaCl and subsequently electrochemically reduced in 10 mM NaCl. In the absence of NaCl denoted by ”ECPEDOT:PSS”. the spectra are relative to an ITO substrate.
through Raman spectroscopy and the results are displayed in Fig. S2. It was observed that indeed the electrochemically deposited films present the characteristic PEDOT peaks at 1570, 1545, 1372, 863, 707, 586 and 440 cm−1 [35]. The peak centered at 1437 cm−1 is assigned to the Cα=Cβ symmetric stretching vibration; the bands at 1511 cm−1 and 1570 cm−1 to the Cα=Cβ asymmetric stretching vibrations [36]; the 1372 cm−1 peak to C-C stretching deformations; and the 1255 cm−1 to C-C in-plane symmetric stretching. The form of the Raman band associated with Cα=Cβ at 1400–1500 cm−1 is used to distinguish between benzenoid and quinoid structures and gives information about doping [37]. Finally, the main vibrational modes of PSS are located at 999 cm−1 and 1112 cm−1[38]. Fig. 2 (a) shows the normalized Raman spectra between 1350 and 1550 cm−1. Here it can be noted that the Cα = Cβ symmetric stretching vibration of the PEDOT five-member ring presents an evident blue shift for the electrochemical variations, especially for the Cl− doped one. This indicates that the ratio of quinoid to benzenoid structures is higher for the spin coated films than for the electrochemically deposited ones [38]. Thus, the SC-PEDOT:PSS had a higher doping level and charge carrier mobility followed by the EC-PEDOT:PSS and finally the ECPEDOT:PSS-Cl. The incorporation of Cl− anions to the polymer lattice destabilizes the multiple positive charges (polarons and bipolarons) on the PEDOT chains reducing their doping level and forcing a configuration change from a quinoid to a benzoid structure. This will result in a suppressed conductivity since the benzenoid structure hinders the intra-chain conductivity and the charge transport relies on hopping between polymer chains [39–41]. The effect of the presence of Cl− during the electropolymerization of PEDOT:PSS is contrary to the effect of adding Cl− salts to the spin coated PEDOT:PSS suspension. In the latter, an increase in conductivity has been reported due to the interaction of cations such as Na+ with the PSS. This allows the removal of excess PSS and an increase in the conductivity because the structure of PEDOT changes from coiled to linear [42,43]. A steady state photoluminescence (PL) study was conducted on the EC-PEDOT:PSS-Cl and SC-PEDOT:PSS layers to assess their quality in terms of charge collection. After annealing both the EC and the SC films, MAPbI3 perovskite was spin coated through a one-step anti-solvent method on top of them and the PL emission spectra of the samples were compared to the PL spectrum of the same perovskite deposited on glass. The results, including two independent replicates for each variation, are presented in Fig. 2(b). Here, the PL spectra of both MAPbI3/ SC-PEDOT:PSS/ITO and MAPbI3/EC-PEDOT:PSS-Cl/ITO samples overlap and are significantly below the spectrum of the perovskite on glass sample. This confirmed that the electrochemical synthesis produced reproducible films with a similar hole extraction capacity as the SC-PEDOT:PSS. To study possible effects of annealing temperature, three different MAPbI3/EC-PEDOT:PSS-Cl/ITO samples were prepared by annealing the polymeric films at 110, 130 and 150 °C. Their (PL) emission spectra were measured (see supp. Fig. S3) and the results showed a similar
synthesis of PEDOT:PSS in the presence of EDOT and NaPSS (without NaCl) can be seen in black in Fig. 1(a). The corresponding UV-vis absorption spectra for the subsequently reduced layer is presented in black in Fig. 1(b) (EC-PEDOT:PSS). Despite the reduction process, the absorption around 900 nm was strong and coincided with that of an oxidized PEDOT:PSS film with a high presence of polarons [32,33]. This was initially attributed to the fact that the PSS is a bulky dopant, which might hinder its removal from the polymeric lattice during the reduction and would leave the PEDOT polymer still highly doped. For the material to act as a good HTM it must have a well-defined band gap, a HOMO level compatible with the valence band of the perovskite and a LUMO level as high as possible to block leakage electron currents [34]. In this sense, more reduced films were preferred, since they don’t have mid-gap energy levels introduced by the presence of polarons and bipolarons in the polymer chain. An attempt was made to further reduce the films by applying a stronger reduction potential of -0.5 V, but the films were damaged and detached from the ITO at this voltage. Therefore, we added the less voluminous Cl− secondary counterion during the synthesis process to facilitate the reduction process. This was performed by adding NaCl to the PEDOT:PSS synthesis solution in 0.001, 0.01 and 0.1 M concentrations. In Fig. 1(a) the voltammogram for the synthesis with an addition of 0.1 M NaCl is shown in red. Here, an increase in the peak current can be noticed when adding the NaCl, which may be due to the decreased solution resistance. The polymerization potential also increased, probably because the additional Na+ cations induce a destabilization that cannot be compensated by the Cl− anions. The latter, since these are not as good as the PSS− at stabilizing the polymerization intermediary radicals [21]. The UV-vis spectra of the films prepared with NaCl are shown in Fig. 1(b), where it is clear that there is a gradual decrease in the absorption between 800 and 1000 nm as the NaCl content of the synthesis solution is increased. This evidences a more reduced state of the films and a lower density of polarons due to the presence of the Cl− anions [32,33].To further investigate this, UV-vis spectra of films synthesized with and without 0.1 M NaCl were measured before and after the reduction to investigate whether this change in shape was due to the synthesis or the reduction processes (see supp. Fig. S1). The results show that the main absorption characteristics of the films were determined by the synthesis conditions; the reduction only caused minor changes in the spectra. Thus, it can be concluded that the lower density of polarons in the NaCl sample is mainly due to the nature of the dopants (Cl− vs PSS−) rather than a removal of these species during the reduction. This confirms what was found by Nasybulin et al. that the number of free carriers in the PEDOT depends to a large extent on the nature of the dopants [21]. Since only the most reduced state was desired, we kept working only with the 0.1 M NaCl variation, which will be henceforth denoted by EC-PEDOT:PSS-Cl. The electrochemical PEDOT:PSS films (EC-PEDOT:PSS and ECPEDOT: PSS-Cl) were compared with spin coated films (SC-PEDOT:PSS) 3
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
Fig. 2. (a) Normalized Raman spectra of a spin coated PEDOT:PSS film (SC-PEDOT:PSS), an electrochemically deposited PEDOT:PSS film (EC-PEDOT:PSS) and an electrochemically deposited PEDOT:PSS film in the presence of 0.1 M NaCl (EC-PEDOT:PSS-Cl). The laser excitation wavelength was 532 nm. (b) Photoluminescence emission spectra of perovskite layers excited at 405 nm deposited on a glass substrate (black), a spin coated PEDOT:PSS (green) and an electrochemically deposited PEDOT:PSS in the presence of 0.1 M NaCl (red). In all cases the excitation beam reaches the perovskite from the ITO/glass side. (c) Cyclic voltammetry of ECPEDOT:PSS-Cl films in acetonitrile with 0.1 M TBAPF6 at 100 mV/s, Vox = 0.70 V. (d-f) AFM topographic images of (d) SC-PEDOT:PSS (RMSR = 1.1 nm), (e) ECPEDOT:PSS (RMSR = 7.3 nm) and (f) EC-PEDOT:PSS-Cl (RMSR = 1.7 nm). RMSR stands for root mean square roughness.
quench for all temperatures, so the lowest was chosen to favor low energy consumption. The electrochemical films were then characterized through cyclic voltammetry to estimate their HOMO levels and ferrocene was used as the electrochemical reference for the vacuum level (see supp. Fig. S4). The voltammogram for EC-PEDOT:PSS-Cl is shown in Fig. 2(c) and the corresponding figure for EC-PEDOT:PSS is presented in supp. Fig. S4. Both yield an estimated HOMO value of −5.1 eV that is similar to the value reported in other studies [44] and that is compatible with the MAPbI3 perovskite valence band level (−5.4 eV). This suggests that the addition of NaCl to the synthesis solution does not affect the HOMO level. In addition, the Kelvin probe force microscopy (KPFM) technique was used to estimate the work functions of the EC-PEDOT:PSS and ECPEDOT:PSS-Cl in comparison to the SC-PEDOT:PSS value. By fixing the work function of the latter at 5.2 eV [20,45,46], the values for ECPEDOT:PSS and EC-PEDOT:PSS-Cl were 5.02 and 5.08 eV respectively. The KPFM images are shown in Fig. S5. AFM topographic images of the SC-PEDOT:PSS, EC-PEDOT:PSS and EC-PEDOT:PSS-Cl films are presented in Fig. 2(d)-(f). Whereas the ECPEDOT:PSS film has a root mean square roughness (RMSR) of 7.3 nm, both SC-PEDOT:PSS and EC-PEDOT:PSS-Cl films are smoother, having an RMSR of 1.1 and 1.7 nm respectively. There is a slight morphological difference between both samples though. The SC one is more homogeneous, whereas the EC layer seems to aggregate. To check if this difference had any impact on perovskite growth, top-view and crosssection SEM images of perovskite deposited on these layers were taken, as shown in supp. Fig. S6 and Fig. 3 respectively. The top-view images suggest that there is no clear difference between the perovskites grown on SC-PEDOT:PSS-Cl and EC-PEDOT:PSS, both having similar crystal size distribution. The cross-section image shows a perovskite thickness and a vertical distribution of grains similar in both cases. An important observation is that the perovskite films do not consist of single vertical crystals, but instead of multiple vertical layers. Finally, the cross-section SEM images evidence that the EC-PEDOT:PSS is much thinner than its spin coated counterpart, almost imperceptible at the displayed resolution. This was confirmed by an AFM step profile measurement which demonstrated that the film was ultra-thin (Fig. S7).
Fig. 3. Cross section SEM images of solar cells made with: (a) SC-PEDOT:PSS and (b) EC-PEDOT:PSS-Cl.
Inverted perovskite solar cells were then fabricated with ECPEDOT:PSS and EC-PEDOT:PSS-Cl to test their functionality and compare them to cells fabricated with SC-PEDOT:PSS. Fig. 3 shows the SEM cross-section images of the cells, both with the SC-PEDOT:PSS and the EC-PEDOT:PSS-Cl layers. Fig. 4 presents the JV-curves of the champion solar cell for each variation. The first observation is that the EC route can produce cells with similar performances to the cells with SC-PEDOT:PSS, having champion efficiencies of 11.2% and 11.4% respectively. Secondly, the results suggest that the presence of NaCl during the PEDOT:PSS synthesis yields cells with better currents and fill factors, as will be statistically discussed below. The similar results between SCPEDOT:PSS and EC-PEDOT:PSS-Cl were expected, given the same PL quenching behavior and the same perovskite growth. In all three cases, the short circuit currents are relatively low in comparison with other literature reported values. This could be explained by the multiple rows of perovskite crystals observed in the vertical direction in Fig. 3. Since the charges must travel through many grain boundaries to reach the adjacent layers, they are more likely to 4
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
work functions of the EC-PEDOT:PSS-Cl and EC-PEDOT:PSS variations were estimated to be very close (5.08 and 5.02 eV), the difference in Voc must have been mainly due to recombination and not a difference in band alignment. The lower recombination in a more reduced state could also explain why the EC-PEDOT:PSS-Cl with an intermediate work function (5.08 eV) has an average Voc higher than the SC-PEDOT:PSS that has the highest work function (5.20 eV). Regarding the difference in Jsc, the stronger absorption of light of EC-PEDOT:PSS in comparison to EC-PEDOT:PSS-Cl in the range of 400–800 nm might have caused it (see Fig. 1(b)). There is still room for improvement, though, in terms of optimizing the thickness and reduction level of the EC-PEDOT:PSS-Cl, such that the recombination due to mid-gap states is overcome and a similar or better Jsc is obtained in comparison to the spin coated films. Interestingly, the EC-PEDOT:PSS variation presents a significantly higher efficiency dispersion in Fig. 5(a). This could be attributed to undesired effects that occur during the reduction process for this variation. As mentioned above, the EC-PEDOT:PSS films detached from the ITO at a reduction potential of −0.5 V and, most likely, there were still unnoticeable damages at lower voltages. Similar problems have been reported for chemical and electrochemical reduction of PEDOT layers [47]. The addition of NaCl increased the film resilience so that it could withstand −0.5 and even −0.7 V without detaching. Fig. S8 shows the UV-Vis spectra of EC-PEDOT:PSS-Cl films reduced at −0.5V and −0.7V and it is clear that there is no fundamental difference between them. We speculate that the improved resilience of the EC-PEDOT:PSS-Cl is related to conformational rearrangements of the polymeric chains. During the electrochemical reduction, the small and mobile Cl− anions are expelled from the film as the positive charges in the polymer chains are neutralized, resulting in a compact polymer lattice. In contrast, for immobile polyanions like PSS− the reduction involves the entrance of cations, hence there is an increase in volume [48]. In some cases, the volume change could create enough mechanical stress to cause a local polymer delamination and thus cause the greater efficiency dispersion. Finally, a stability study was conducted on cells fabricated with SCPEDOT:PSS and EC-PEDOT:PSS-Cl. Fig. 6 shows the temporal evolution of the photovoltaic parameters of the unencapsulated cells. The parameters were measured at ambient conditions with a relative humidity higher than 60%, but the cells were stored in a dark nitrogen atmosphere in-between. The significantly better stability for the EC-PEDOT:PSS-Cl variation is striking. On the one hand, the average efficiency of the SC-PEDOT:PSS cells decreases rapidly over time, dropping to less than 60% of the initial value in only 8 days. The main cause of this drop is a drastic reduction in short circuit current, while the voltage drops slower. The same observation has been established already in the literature and is attributed to the degradation problems associated with PSS [20,14,49]. In contrast, the EC-PEDOT:PSS–Cl cells maintain on average a little more than 80% of their original efficiency after 25 days.
Fig. 4. JV curves of the best performing cells for three PEDOT:PSS deposition variations: spin coated (SC-PEDOT:PSS), electrochemically deposited (ECPEDOT:PSS) and electrochemically deposited in the presence of 0.1 M NaCl (EC-PEDOT:PSS-Cl).
recombine, thus reducing the measured current. A statistical study was conducted to establish the effects of the different PEDOT:PSS variations on efficiency, open circuit voltage (Voc), short circuit current (Jsc) and fill factor (FF). The box plots of the results are presented in Fig. 5. Even though, on average, the SC-PEDOT:PSS cells are more efficient, the EC-PEDOT:PSS-Cl yields comparable cell efficiencies, with the champion devices exceeding 11% in both cases. The reason why the SC-PEDOT:PSS cells are a bit more efficient is because they have higher Jsc values, but in terms of Voc and FF the ECPEDOT:PSS-Cl cells have better and less dispersed data. In particular, the FF values are remarkable, some even exceeding 80%. This originates from the fact that the series resistance of the EC-PEDOT:PSS-Cl layers was smaller, as evidenced by the steeper slope at open circuit conditions in Fig. 4. Even though the Raman results in Fig. 2(a) indicated a lower hole mobility for the electrochemically deposited layer, its ultra-thin thickness compensates this and yields a smaller series resistance [38,40]. In all three cases, the Voc values are relatively small in comparison to other PVSC reported in the literature, most likely because of the ∼0.3 V misalignment between the PEDOT:PSS HOMO level and the perovskite valence band maximum. From Fig. 5 it is also clear that the EC-PEDOT:PSS-Cl yields cells significantly more efficient than the EC-PEDOT:PSS, with better Voc and Jsc values. This can be rationalized in the following way. The ECPEDOT:PSS-Cl is in a more reduced state than the EC-PEDOT:PSS, as evidenced by the UV-vis spectra in Fig. 1(b) and the Raman spectra in Fig. 2 (a). Hence, it has a more defined energy gap between the HOMO and LUMO levels such that there are less mid-gap energy levels available that could provide recombination pathways. Given the fact that the
Fig. 5. Box plot of (a) efficiency, (b) Voc, (c) Jsc and (d) FF for 13 devices with SC-PEDOT:PSS, 26 devices with EC-PEDOT:PSS-Cl and 13 devices with EC-PEDOT:PSS. Due to space reasons, the word PEDOT has been removed from all labels. 5
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
Fig. 6. Temporal evolution of efficiency (a), Voc (b), Jsc (c) and FF (d) of unencapsulated inverted perovskite solar cells manufactured with spin coated PEDOT:PSS (SC), and electrochemically deposited PEDOT:PSS in the presence of 0.1 M NaCl (EC). The number of measured devices was 4 and 8 respectively. The dots represent the mean value and the bars show the standard deviation. The data was normalized to the day 0 mean values.
SEM images, water contact angle measurement, and a photograph comparing the stability of cells can be found in the electronic supplementary material.
These results are supported by visually comparing the cells fabricated using both methods, as shown in Fig. S9, where the perovskite in the cell with SC-PEDOT:PSS displays a notorious yellow degradation, while the EC-PEDOT:PSS-Cl cell remains quite intact. The improved stability is probably related to the lower acidic PSS content in the EC films, since the SC-PEDOT:PSS needs a greater amount of PSS, not only to dope the polymer, but to keep it in a stable suspension before the deposition. This hypothesis is supported by the static water contact angle measurements shown in Fig. S10, where a lower contact angle denotes a higher hydrophilic PSS content. Whereas the spin coated PEDOT:PSS film is highly hydrophilic with a contact angle of 9°, the electrodeposited films present a more hydrophobic nature with contact angles close to 47°. Moreover, a higher content of hydrophilic PSS in the spin coated film is also in agreement with its measured higher work function, since the work function of PEDOT:PSS increases with the content of PSS [18].
Acknowledgement The authors thank the Universidad de los Andes (Chemistry Department and Chemical Engineering Department) for providing funding. María T. Cortés acknowledges support from the Science Faculty (Proyecto INV-2017-51-1456). Eider A. Erazo acknowledges support from the Science Faculty (Proyecto INV-2018-33-1308) and CEIBA foundation. Appendix A. Supplementary Data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.synthmet.2019.116178.
4. Conclusions References In conclusion, it was demonstrated that the PEDOT:PSS obtained by electropolymerization in the presence of 0.1 M NaCl yielded a functional HTM for inverted perovskite solar cells with efficiencies around 11%, fill factors exceeding 80% and a similar performance to devices fabricated with spin coated PEDOT:PSS. To achieve these values, the redox level of the polymer was tuned to a more reduced state by the addition of NaCl to the synthesis solution, and by applying a voltage reduction signal to the layer. Most importantly, the electrochemical polymerization of PEDOT:PSS resulted in a less hydrophilic coating which improved the stability of the devices in a significant manner, probably because of a lower PSS content. Future work will be done to further optimize several parameters such as the thickness of the layers, the NaCl content and the polymer oxidation level to attain better short circuit currents and open circuit voltages. The great reproducibility of electropolymerization, the ease of tuning of the properties of the films, the promising photovoltaic results and the enhanced stability of the cells demonstrate that the electropolymerization of PEDOT:PSS should be further studied in pursuit of the large-scale implementation of PVSC technology.
[1] NREL, Best Research-Cell Efficiencies, (2019) https://www.nrel.gov/pv/cell-efficiency.html. [2] N.-G. Park, Perovskite solar cells: an emerging photovoltaic technology, Mater. Today 18 (2015) 65–72. [3] J.M. Ball, M.M. Lee, A. Hey, H.J. Snaith, Low-temperature processed meso-superstructured to thin-film perovskite solar cells, Energy Environ. Sci. 6 (2013) 1739–1743. [4] J. Bouclé, N. Herlin-Boime, The benefits of graphene for hybrid perovskite solar cells, Synth. Metals 222 (2016) 3–16. [5] F.X. Xie, D. Zhang, H. Su, X. Ren, K.S. Wong, M. Gratzel, W.C. Choy, Vacuumassisted thermal annealing of CH3NH3PbI3 for highly stable and efficient perovskite solar cells, ACS Nano 9 (2015) 639–646. [6] A. Dubey, N. Adhikari, S. Venkatesan, S. Gu, D. Khatiwada, Q. Wang, L. Mohammad, M. Kumar, Q. Qiao, Solution processed pristine pdpp3t polymer as hole transport layer for efficient perovskite solar cells with slower degradation, Solar Energy Mater. Solar Cells 145 (2016) 193–199. [7] E.A. Gaml, A. Dubey, K.M. Reza, M.N. Hasan, N. Adhikari, H. Elbohy, B. Bahrami, H. Zeyada, S. Yang, Q. Qiao, Alternative benzodithiophene (bdt) based polymeric hole transport layer for efficient perovskite solar cells, Solar Energy Mater. Solar Cells 168 (2017) 8–13. [8] S. Mabrouk, M. Zhang, Z. Wang, M. Liang, B. Bahrami, Y. Wu, J. Wu, Q. Qiao, S. Yang, Dithieno [3,2-b: 2’, 3’-d] pyrrole-based hole, J. Mater. Chem. A 6 (2018) 7950–7958. [9] J. You, Z. Hong, Y. Yang, Q. Chen, M. Cai, T.-B. Song, C.-C. Chen, S. Lu, Y. Liu, H. Zhou, et al., Low-temperature solution-processed perovskite solar cells with high efficiency and flexibility, ACS Nano 8 (2014) 1674–1680. [10] J.H. Heo, H.J. Han, D. Kim, T.K. Ahn, S.H. Im, Hysteresis-less inverted CH3NH3PbI3 planar, Energy Environ. Sci. 8 (2015) 1602–1608. [11] F. Wu, R. Pathak, K. Chen, G. Wang, B. Bahrami, W.-H. Zhang, Q. Qiao, Inverted current-voltage hysteresis in perovskite solar cells, ACS Energy Lett. 3 (2018) 2457–2460. [12] K.A. Bush, A.F. Palmstrom, J.Y. Zhengshan, M. Boccard, R. Cheacharoen, J.P. Mailoa, D.P. McMeekin, R.L. Hoye, C.D. Bailie, T. Leijtens, et al., solar cells with improved stability, Nature Energy 2 (2017) 17009. [13] C.-H. Chiang, M. Khaja Nazeeruddin, M. Gr“atzel, C.-G. Wu, The synergistic
Declarations of interest There are no conflicts to declare. Supplementary Material Additional UV-vis, Raman and PL figures, HOMO level estimation voltammograms and calculations, KPFM images, top-view perovskite 6
Synthetic Metals 257 (2019) 116178
E.A. Erazo, et al.
controlling cell growth, ACS Appl. Mater. Interfaces 7 (2015) 17993–18003. [33] N. Massonnet, A. Carella, O. Jaudouin, P. Rannou, G. Laval, C. Celle, J.-P. Simonato, Improvement of the seebeck coefficient of PEDOT:PSS by chemical reduction combined with a novel method for its transfer using free-standing thin films, J. Mater. Chem. C 2 (2014) 1278–1283. [34] A. Isakova, P.D. Topham, Polymer strategies in perovskite solar cells, J. Polym. Sci. Part B: Polym. Phys. 55 (2017) 549–568. [35] X. Hu, L. Chen, L. Tan, T. Ji, Y. Zhang, L. Zhang, D. Zhang, Y. Chen, In situ polymerization of ethylenedioxythiophene from sulfonated carbon nanotube templates: toward high efficiency ito-free solar cells, J. Mater. Chem. A 4 (2016) 6645–6652. [36] B. Xu, S.-A. Gopalan, A.-I. Gopalan, N. Muthuchamy, K.-P. Lee, J.-S. Lee, Y. Jiang, S.-W. Lee, S.-W. Kim, J.-S. Kim, et al., Corrigendum: Functional solid additive modified pedot: Pss as an anode buffer layer for enhanced photovoltaic performance and stability in polymer solar cells, Scientific Reports 7 (2017) 46779. [37] S.S. Kalagi, P.S. Patil, Secondary electrochemical doping level effects on polaron and bipolaron bands evolution and interband transition energy from absorbance spectra of PEDOT:PSS thin films, Synth. Metals 220 (2016) 661–666. [38] S.H. Chang, K.-F. Lin, K.Y. Chiu, C.-L. Tsai, H.-M. Cheng, S.-C. Yeh, W.-T. Wu, W.N. Chen, C.-T. Chen, S.-H. Chen, et al., Improving the efficiency of CH3NH3PbI3 based photovoltaics by tuning the work function of the PEDOT:PSS hole transport layer, Solar Energy 122 (2015) 892–899. [39] R. Kroon, D.A. Mengistie, D. Kiefer, J. Hynynen, J.D. Ryan, L. Yu, C. M”uller, Thermoelectric plastics: from design to synthesis, processing and structure-property relationships, Chem. Soc. Rev. 45 (2016) 6147–6164. [40] B. Vaagensmith, K.M. Reza, M.N. Hasan, H. Elbohy, N. Adhikari, A. Dubey, N. Kantack, E. Gaml, Q. Qiao, Environmentally friendly plasma-treated pedot:pss as electrodes for ito-free perovskite solar cells, ACS Appl. Mater. Interfaces 9 (2017) 35861–35870. [41] Y. Kim, W. Cho, Y. Kim, H. Cho, J.H. Kim, Electrical characteristics of heterogeneous polymer layers in PEDOT:PSS films, J. Mater. Chem. C 6 (2018) 8906–8913. [42] B. Kadem, W. Cranton, A. Hassan, Metal salt modified pedot: Pss as anode buffer layer and its effect on power conversion efficiency of organic solar cells, Org. Electron. 24 (2015) 73–79. [43] L. Hu, K. Sun, M. Wang, W. Chen, B. Yang, J. Fu, Z. Xiong, X. Li, X. Tang, Z. Zang, et al., Inverted planar perovskite solar cells with a high fill factor and negligible hysteresis by the dual effect of NaCl-doped PEDOT:PSS, ACS Appl. Mater. Interfaces 9 (2017) 43902–43909. [44] S. Ma, W. Qiao, T. Cheng, B. Zhang, J. Yao, A. Alsaedi, T. Hayat, Y. Ding, Z. Tan, S. Dai, Optical-electrical-chemical engineering of pedot:pss by incorporation of hydrophobic nafion for efficient and stable perovskite solar cells, ACS Appl. Mater. Interfaces 10 (2018) 3902–3911. [45] Y.-H. Seo, J.-S. Yeo, N. Myoung, S.-Y. Yim, M. Kang, D.-Y. Kim, S.-I. Na, Blending of n-type semiconducting polymer and pc61bm for an efficient electron-selective material to boost the performance of the planar perovskite solar cell, ACS Appl. Mater. Interfaces 8 (2016) 12822–12829. [46] A. Al Mamun, T.T. Ava, K. Zhang, H. Baumgart, G. Namkoong, New pcbm/carbon based electron transport layer for perovskite solar cells, Phys. Chem. Chem. Phys. 19 (2017) 17960–17966. [47] T. Lindfors, A. Österholm, J. Kauppila, M. Pesonen, Electrochemical reduction of graphene oxide in electrically conducting poly (3,4-ethylenedioxythiophene) composite films, Electrochim. Acta 110 (2013) 428–436. [48] T.F. Otero, H.-J. Grande, J. Rodríguez, Reinterpretation of polypyrrole electrochemistry after consideration of conformational relaxation processes, J. Phys. Chem. B 101 (1997) 3688–3697. [49] K.M. Reza, S. Mabrouk, Q. Qiao, A review on tailoring pedot: Pss layer for improved performance of perovskite solar cells, Proc. Nat. Res. Soc. 2 (2018) 02004.
effect of H2O and dmf towards, Energy Environ. Sci. 10 (2017) 808–817. [14] Q. Wang, C.-C. Chueh, M. Eslamian, A.K.-Y. Jen, Modulation of pedot:pss ph for efficient inverted perovskite solar cells with reduced potential loss and enhanced stability, ACS Appl. Mater. Interfaces 8 (2016) 32068–32076. [15] Q. Fu, X. Tang, B. Huang, T. Hu, L. Tan, L. Chen, Y. Chen, Recent progress on the long-term stability of perovskite solar cells, Adv. Sci. 5 (2018) 1700387. [16] M. De Jong, L. Van Ijzendoorn, M. De Voigt, Stability of the interface between indium-tin-oxide and poly (3,4-ethylenedioxythiophene)/poly (styrenesulfonate) in polymer light-emitting diodes, Appl. Phys. Lett. 77 (2000) 2255–2257. [17] J. Yang, Z. Yuan, X. Liu, S. Braun, Y. Li, J. Tang, F. Gao, C. Duan, M. Fahlman, Q. Bao, Oxygen-and water-induced energetics degradation in organometal halide perovskites, ACS Appl. Mater. Interfaces 10 (2018) 16225–16230. [18] W. Kim, S. Kim, S.U. Chai, M.S. Jung, J.K. Nam, J.-H. Kim, J.H. Park, Thermodynamically self-organized hole transport layers for high-efficiency inverted-planar perovskite solar cells, Nanoscale 9 (2017) 12677–12683. [19] H. Liu, X. Li, L. Zhang, Q. Hong, J. Tang, A. Zhang, C.-Q. Ma, Influence of the surface treatment of pedot:pss layer with high boiling point solvent on the performance of inverted planar perovskite solar cells, Organic Electron. 47 (2017) 220–227. [20] H. Elbohy, B. Bahrami, S. Mabrouk, K.M. Reza, A. Gurung, R. Pathak, M. Liang, Q. Qiao, K. Zhu, Tuning hole transport layer using urea for high-performance perovskite solar cells, Adv. Functional Mater. (2018) 1806740. [21] E. Nasybulin, S. Wei, I. Kymissis, K. Levon, Effect of solubilizing agent on properties of poly (3,4-ethylenedioxythiophene)(pedot) electrodeposited from aqueous solution, Electrochim. Acta 78 (2012) 638–643. [22] H. Ellis, N. Vlachopoulos, L. H”aggman, C. Perruchot, M. Jouini, G. Boschloo, A. Hagfeldt, Pedot counter electrodes for dye-sensitized solar cells prepared by aqueous micellar electrodeposition, Electrochim. Acta 107 (2013) 45–51. [23] H. Frohne, S.E. Shaheen, C.J. Brabec, D.C. M“uller, N.S. Sariciftci, K. Meerholz, Influence of the anodic work function on the performance of organic solar cells, ChemPhysChem 3 (2002) 795–799. [24] J. Yan, C. Sun, F. Tan, X. Hu, P. Chen, S. Qu, S. Zhou, J. Xu, Electropolymerized poly (3,4-ethylenedioxythiophene): poly (styrene sulfonate)(PEDOT:PSS) film on ito glass and its application in photovoltaic device, Solar Energy Mater. Solar Cells 94 (2010) 390–394. [25] T.-L. Zhang, H.-Y. Chen, C.-Y. Su, D.-B. Kuang, A novel tco-and pt-free counter electrode for high efficiency dye-sensitized solar cells, J. Mater. Chem. A 1 (2013) 1724–1730. [26] Y. Xiao, G. Han, J. Wu, J.-Y. Lin, Efficient bifacial perovskite solar cell based on a highly transparent poly (3,4-ethylenedioxythiophene) as the p-type hole-transporting material, J. Power Sources 306 (2016) 171–177. [27] G.F. Samu, R. Scheidt, G. Zaiats, P.V. Kamat, C. Janáky, Electrodeposition of holetransport layer on methylammonium lead iodide film: A new strategy to assemble perovskite solar cells, Chem. Mater. (2018). [28] W. Yan, Y. Li, Y. Li, S. Ye, Z. Liu, S. Wang, Z. Bian, C. Huang, Stable high-performance hybrid perovskite solar cells with ultrathin polythiophene as hole-transporting layer, Nano Res. 8 (2015) 2474–2480. [29] S. Ye, W. Sun, Y. Li, W. Yan, H. Peng, Z. Bian, Z. Liu, C. Huang, Cuscn-based inverted planar perovskite solar cell, Nano Lett. 15 (2015) 3723–3728. [30] C.-H. Chiang, Z.-L. Tseng, C.-G. Wu, Planar heterojunction perovskite/PC71BM solar cells with enhanced open-circuit voltage via a (2/1)-step spin-coating process, J. Mater. Chem. A 2 (2014) 15897–15903. [31] N. Ahn, D.-Y. Son, I.-H. Jang, S.M. Kang, M. Choi, N.-G. Park, Highly reproducible perovskite solar cells with lewis base adduct of lead (ii) iodide, J. Am. Chem. Soc. 137 (2015) 8696–8699. [32] M. Marzocchi, I. Gualandi, M. Calienni, I. Zironi, E. Scavetta, G. Castellani, B. Fraboni, Physical and electrochemical properties of PEDOT:PSS as a tool for
7