Solution-Processed Transparent Conducting Electrodes for Flexible Organic Solar Cells with 16.61% Efficiency

The PEDOT:PSS flexible electrodes with a unique CF3SO3H treatment exhibited high electrical characteristics and stability. An energy level tuning effect was induced to create a suitable work function. Flexible organic solar cells yielded a record-high efficiency of 16.61%, a high flexibility, and a good thermal stability. Electronic supplementary material The online version of this article (10.1007/s40820-020-00566-3) contains supplementary material, which is available to authorized users.

Trifluoromethanesulfonic acid (CF 3 SO 3 H) is a super acid with ultrahigh acidity (pKa = −15) over sulfuric acid (H 2 SO 4 , pKa = −3.0) and methanesulfonic acid (CH 3 SO 3 H, pKa = −1.9); thereby it provides a strong protonation of hydrogen ions (H + ) to insulating PSS for raising the film conductivity. Moreover, owing to a polarization of carbon-fluorine (C-F) polar covalent bonds of CF 3 SO 3 H with uncoupling charge centers, CF 3 SO 3 H is capable of polarizing the PEDOTs and raising the work function of the PEDOT:PSS films. It potentially minimizes the energy level mismatch among the PEDOT:PSS electrodes, PEDOT:PSS (Clevios P VP AI4083) buffer layers, and electron-donors of active blends for an effective charge transport. It should be noted that, although 98 wt% CF 3 SO 3 H treatment was mentioned before, the harsh treatment made the PEDOT:PSS films unsuitable for application into flexible optoelctronics, because underlying plastic substrates were destroyed in the requisite strong acid process (Fig. S1). Besides, a combined pre-treatment using CF 3 SO 3 H and ionic liquids was developed for making transistor fabrications [12]. However, acid pre-treatments caused large aggregates in PEDOT:PSS aqueous solutions, which were unfavorable for the smooth and uniform anodes required by flexible OSCs. Thus, it is necessary and urgent to develop a unique CF 3 SO 3 H treatment for the development of the solution-processed PEDOT:PSS electrodes for flexible OSCs.
In this work, we proposed a low-temperature and lowconcentration CF 3 SO 3 H post-treatment and demonstrated an efficient solution-processed flexible OSC based on the CH 3 SO 3 H-doped PEDOT:PSS films as transparent anodes and PM6:Y6 as active layers. The 0.8 M CF 3 SO 3 H post-treatment at 50 °C induced a series of merits, including low sheet resistances, high work functions, superior hydrophilicities and a little acid residue, for the PEDOT:PSS anodes. The optimized flexible OSCs exhibited an efficiency of 16.41% with the maximum value of 16.61%. To the best of our knowledge, 16.61% is the highest PCE for single-junction flexible OSCs reported so far. Furthermore, the solution-processed devices maintained a high flexibility and a good thermal stability. This work demonstrates the advance of the unique CH 3 SO 3 H post-treatment, and it provides a simple route to enable a flexible PEDOT:PSS anode with high conductivity, high work function and good stability for the realization of efficient and stable solution-processed flexible OSCs.

Device Fabrication
For the rigid ITO-based solar cells, glass substrates (size: 2.0 × 2.0 cm 2 ) were cleaned through using ultrasonic treatments in deionized (DI) water, acetone, and isopropyl alcohol (IPA), and then were processed in UVozone chambers for 10 min. For the flexible solar cells, the polyethylene terephthalate (PET) plastic substrates were cleaned by IPA followed by baking at 100 °C for 10 min. Next, PEDOT:PSS aqueous solutions (PH1000) were filtered through 0.45 µm syringe filter. The PH1000 solutions were spin-coated on the as-employed substrates at 3500 rpm followed by the drying at 80 °C for 15 min in the air atmosphere. The spin-coating of PH1000 and the drying treatment were conducted again. Subsequently, the super acid treatment was conducted via dipping the CF 3 SO 3 H solutions on the PEDOT:PSS surfaces with a controlled temperature. Upon the CF 3 SO 3 H treatments for 5 min, the acid residues were washed off by DI-water and IPA followed by a baking at 80 °C for 15 min. Then, a PEDOT:PSS hole-transporting layer (Clevios P VP AI4083) was spin-coated on the PEDOT:PSS electrodes at 3000 rpm for 30 s, and the hole-transporting layer was dried at 100 °C for 10 min. After that, the active layer blends of PM6:Y6 were dissolved in chloroform with 1-chloronaphthalene (0.5%, v/v) and were spin-coated at 3000 rpm for 30 s onto the PEDOT:PSS (P VP AI4083) films. Then, perylene-diimide (PDINN) in methanol (1.0 mg mL -1 ) was spin-coated on the surfaces of active layers at 2000 rpm to obtain the electron transporting layers. Finally, the Al cathodes were thermally evaporated under a pressure at 10 -4 Pa. The active area is 6.0 mm 2 . Note that metal probes were contacted with the Al top cathodes and the PEDOT:PSS bottom anodes coated with Ag pastes for the current density-voltage (J-V) characteristics.

Characterizations
Sheet resistance was measured through using van der Pauw four-point probe method. Film thickness was conducted by a surface profile-meter (Talysurf Series II). The carrier concentration of the PEDOT:PSS electrodes was measured by the Hall measurement system (Lake Shore, 7704A) with the van der Pauw four-point probe method. UV-Vis spectra were taken on GS54T spectrophotometers (Shanghai Lengguang Technology Co., China). Film morphology was conducted through using a scanning probe microscope (SPM, VEECO Dimension 3100V) and transmission electron microscopy (TEM, TECNAI G20, FEI). The molecule structures were conducted through using Fourier transform infrared (FTIR) spectroscopy (NICO-LET 6700, THermo, USA) and Raman spectroscopy (Renishaw inVia Reflex). Element compositions of the PEDOT:PSS films were conducted by X-ray photoelectron spectroscopy (XPS, XSAM800). Ultraviolet photoelectron spectra (UPS) measurements were carried out using a Kratos AXIS ULTRA DALD UPS system. J-V characteristics were measured in N 2 -filled glove-boxes using a Keithley 2400 sourcemeter under the illumination of AM 1.5G, with a AAA solar simulator (Newport, model 94023A). The lamp was calibrated by a 2 × 2 cm 2 monocrystalline silicon reference cell (KG5 filter) provided by Newport Corporation. The light intensity was calibrated with a standard silicon detector. External quantum efficiency was conducted through the Newport quantum efficiency measurement system (ORIEL IQE 200TM) in the ambient atmosphere. The light intensity was calibrated with a standard Si/Ge solar cell. To evaluate the device flexibility, the solar cells were bent with radii of 1.5 mm and underwent the 1000 cycle harsh bending. To evaluate the thermal stability, the solar cells were placed in a hot plate at 85 °C in the glove-boxes filled with N 2 . Figure 1a, b plots the main sheet resistance (R sh ) and the optical transparency (T%) at λ = 550 nm of the PEDOT:PSS electrodes with the CF 3  The PEDOT:PSS electrodes showed a high optical transparency that was raised with the increase in the doping treatment temperatures. For example, after the 0.8 M CF 3 SO 3 H doping treatment at 180 °C, T% was increased from 87.3 to 91.7% at λ = 550 nm, which was accompanied with a morphology evolution and a reduced content of insulating PSS. With the lower concentration (0.1-0.8 M) CF 3 SO 3 H doping treatments at no higher than 50 °C, the PEDOT:PSS electrodes exhibited high optical transparencies over 87% in the visible regions of 400-550 nm (Fig. S2), which might be attributed to a large removal of hydrophilic PSS components from the PEDOT:PSS matrices as well as the formation of orderly stacked PEDOT molecules. The films with the 0.8 M CF 3 SO 3 H treatment at 50 °C having high optical and electrical characteristics should be suitable to be used as transparent electrodes for flexible OSCs. We measured the carrier concentration (n) of the doped PEDOT:PSS films using the Hall effect. The carrier mobility (µ) of the PEDOT:PSS films was calculated from the relationship between the electrical conductivity (σ) and the carrier concentration: σ = neμ. Figure 1c, d plots the n and µ of the PEDOT:PSS films doped with 0.8 M CF 3 SO 3 H. With the increase in the doping treatment temperatures, n was increased from 4.77 to 8.62 cm −3 , mostly attributed to the high protonation of the lower concentration CF 3 SO 3 H to PSS (H + ionized from CF 3 SO 3 H) and polarization of C-F polar covalent bonds. A strong polarization induced weak Coulomb interactions between PEDOT and PSS, thereby leading to a favorable phase-separated morphology.

Optical and Electrical Characteristics of PEDOT:PSS Electrodes
The n values (6.52-8.62 cm −3 ) of the PEDOT:PSS films with 0.8 M CF 3 SO 3 H treatments are comparable to the best value (≈ 6.2 cm −3 ) of the optimal PEDOT:PSS films with 98 wt% sulfuric acid treatments [25]. Notably, the lower concentration CF 3 SO 3 H treatments avoided destroying underlying plastic substrates (Fig. S1), and it would reduce substantially strong acid residues on electrode surfaces. We also found that the µ values (3.93 and 3.44 cm 2 V −1 s −1 ) of the PEDOT:PSS films doped by CF 3 SO 3 H at low temperatures (≤ 50 °C) were higher than that of the PEDOT:PSS films doped by CF 3 SO 3 H at high temperatures (≥ 100 °C). The higher µ values were related to a morphology evolution of these doped films. The results demonstrated a high electrical characteristic achieved by the PEDOT:PSS films with the CF 3 SO 3 H treatments; and the low-temperature and low-concentration CF 3 SO 3 H treatments are compatible to thermoplastic substrates for flexible device integration.

Morphological and Structural Characteristics of PEDOT:PSS Electrodes
The high electrical characteristics are mostly attributed to an evolution in phase-segregated morphology and a reduced content of insulating PSS components. Figure 2 presents the morphologies of the PEDOT:PSS films (PH1000) through using SPM and TEM. As-cast films exhibited an inferior phase segregation with a low root-mean-square (RMS) roughness of 1.51 nm (Fig. 2a, g). Via the 0.8 M CF 3 SO 3 H doping at RT, the PEDOT:PSS films exhibited the physically continuous networks with a higher roughness of 2.63 nm. The morphology differs from the cluster morphology of the conventional PEDOT:PSS films that were doped with 6 vol% dimethylsulfoxide (DMSO) (Fig. S3). With the increase in the doping treatment temperatures to 50 °C, it induced a physically continuous network that consisted of spherical/ elliptical-like nanoparticles and nanofibrils (Fig. 2c, h). The results demonstrated a favorable phase-segregated morphology, thanks to a full penetration of H+ into the PEDOT:PSS matrix followed by the reaction (H+ + PSS− → PSSH). The morphology was favorable for charge-carrier transport and collection by the PEDOT:PSS anodes in solar cells. With the CF 3 SO 3 H treatments at the higher temperatures of 100 and 140 °C, the PEDOT:PSS films showed a morphology evolution from linear/expended nanofibril-like networks to coiled small aggregated nanoparticles (Fig. 2d, e, g). The major sizes of the small nanoparticles were 20-40 nm. By increasing the doping treatment temperatures to 180 °C, the smallest nanoparticles with major sizes of 10-25 nm appeared (Fig. 2f, i), suggesting a weak Columbic attraction between the PEDOT molecules and the PSS chains. The PEDOT:PSS (PH1000) films are then characterized by the FTIR spectroscopy. As shown in Fig. S4a, the FTIR bands located between 1000 and 1100 cm −1 originate from the stretching vibrations of the PSS-H bonds [41,42]. After the CF 3 SO 3 H treatments, the valleys blue-shifted and the valley's intensity became stronger, indicating the PSSH formations in the PEDOT:PSS matrices. Figure S4b shows the Raman spectra of the PEDOT:PSS (PH1000) films including the as-cast ones, and the as-doped ones with the 0.8 M CF 3 SO 3 H treatments at 50 and 140 °C. The peak at 756 cm −1 corresponds to the stretching mode of CF 3 SO 3 − [43]. The peak at 756 cm −1 wasn't observed in the films, suggesting little CF 3 SO 3 H residuals on the surfaces of the CF 3 SO 3 H-doped films. The strongest band between 1400 and 1500 cm −1 originated from the C α =C β stretching vibration of the PEDOTs. The C α =C β vibration peaks in Raman spectroscopy red-shifted and were narrower in width in the as-doped PEDOT:PSS films as compared to that of the ascast films. The results indicate an evolution of the PEDOTs from benzoid structures to quinoid structures, leading to a more planar backbone. This planarity is probably attributed to more efficient charge delocalization and a higher packing order [44]. The peak at 700 cm −1 corresponds to the sym C-S-C vibrations in PEDOTs [45]. The peak had a weak intensity and became wider in width, probably attributed to the formation of the quinoid structures of the PEDOTs.

Components and Work Functions of PEDOT:PSS Electrodes
To elucidate the effect of the CF 3 SO 3 H doping treatments on the components of the PEDOT:PSS films, we showed the XPS full spectra (Fig. S5) and the fitted curves (Fig. 3a-c). For as-cast films, the ratio of sulfur (S) atoms in sulfonate moieties of PSS to S atoms in thiophene rings of PEDOT (called S PSS :S PEDOT ) is 1.26:1 (Fig. 3a), whereas for the PEDOT:PSS films with the 0.8 M CF 3 SO 3 H treatment at 50 °C, the ratio is sharply decreased to 0.37:1 (Fig. 3b), indicating a large removal of insulating PSS from the PEDOT:PSS matrices. Note that, to accurately calculate the ratio, sulfonate moieties originated from CF 3 SO 3 H were considered as well. For the PEDOT:PSS films with the 0.8 M CF 3 SO 3 H treatment at 140 °C, the ratio is 0.82:1 (Fig. 3c). The less PSS components contributed to the reduced sheet resistances. Besides, the content of the fluorine atoms is 3.75 and 7.40 atom% for the PEDOT:PSS films doped at 50 and 140 °C, respectively ( Fig. 3d). The electron-withdrawing fluorine groups led to the interfacial dipoles pointing toward the active layers of the OSCs. We also found that the CF 3 SO 3 H contents on the PEDOT:PSS film surfaces were much lower than that (14.08 atom%) of the PEDOT:PSS films with 40 wt% viscous phosphoric acid treatments [15], due to the use of the lower concentration CF 3 SO 3 H solutions with a low viscosity. It is favored to make a smooth anode and a highquality coating of buffer layers on tops. Furthermore, we compared the binding energies of the S 2p 3/2 emission for PEDOT to evaluate the impact of the doping treatments on the oxidation states of sulfurs of PEDOT. Binding energy of S 2p 3/2 of PEDOT in as-cast films is 164.0 eV [39]. Both PEDOT:PSS films doped at 50 and 140 °C showed a visible shift of ≈ 0.34 eV toward higher binding energies of PEDOT emission, as compared to as-cast ones. The results demonstrated an evolution in the electronic environment of the sulfur atoms and a raised oxidation level of the thiophene sulfur of PEDOT molecules. We probed the energy levels of the PEDOT:PSS films (PH1000) through UPS. Figure 3e, f shows the low kinetic energy cutoff (E cutoff ) and the Fermi levels (E Fermi ) of the PEDOT:PSS films including as-cast ones and 0.8 M CF 3 SO 3 H-doped ones. According to the relation below: Work function (Φ) is obtained. The CF 3 SO 3 H-doped films exhibited a high Φ of ≈ 5.0 eV (4.99 eV), which was independent of the doping treatment temperatures. It allowed a formation of Ohmic contacts, and it was favored for hole transport from active layers of OSCs to PEDOT:PSS anodes. Besides, the small offset between the Φ of PEDOT:PSS and the highest occupied molecular orbital (HOMO) of electron donors of active layers could maximize the charge extraction and minimize the recombination losses, thereby resulting in a raised built-in field for a high open-circuit voltage (V OC ) of OSCs. These results encouraged us to apply the highly conductive PEDOT:PSS anodes with a high work function into efficient solution-processed flexible OSCs.

Device Efficiency and Flexibility
We fabricated the ITO-free flexible OSCs based on the PEDOT:PSS anodes with the 0.8 M CF 3 Fig. 4b. We also plotted the work functions of the PH1000 films including as-cast ones, the conventional ones [26,37] [15,26,37], the PEDOT:PSS anodes were prepared via using 99.5 wt% CH 3 SO 3 H treatments at room temperature [15,37], and 8.0 M CH 3 SO 3 H treatments at 140 °C [26], respectively. As shown in Fig. 4e, Table 1 summarizes the performance data of the OSC devices for reference.
The efficiency is significantly promoted through using the optimized PEDOT:PSS (PH1000) anodes that affected the hole mobility (μ h ) of devices. The μ h values of three-kinds of hole-only devices with a structure of PET/PEDOT:PSS anodes/PEDOT:PSS (P VP AI4083)/PM6:Y6/Au were calculated by the space charge limited current (SCLC) model using Theott-Gurney square law [47]: where ε r is the dielectric constant of active layer materials, ε 0 is the permittivity of free space, L is the distance between the polymeric anode and the metal cathode (Au), which is equivalent to the thickness of active layers, and J D is the dark current density. Figures  The flexible OSCs have the advantage of high mechanical flexibility over the devices fabricated on ITO (180 nm)/ PET substrates, as shown in Fig. 6a. With 1,000 cyclic bending at r of 1.5 mm, the flexible devices based on the CF 3 SO 3 H-doped PEDOT:PSS anodes maintained 94.4% of the initial efficiency, whereas the devices with the ITO (≈ 180 nm)/PET substrates showed a large drop in PCE down to 11.0% of the initial value at r of 1.5 mm, that meant, active regions of the ITO electrodes had been scrapped to restrain charge-carrier collections. We found that, for the small bending at the radius of 1.5 mm, the PEDOT:PSS anodes coated on the PET substrates (2 × 2 cm 2 ; two edges were coated with Ag pastes) had no visible increase in resistance, while the resistance of the ITO/PET was increased to 3 orders of magnitude (from 1 to over 2,800), as shown in Fig. S8; this implies a structure damage of the ITO films with crack generations.

Device Stability
Furthermore, we investigated the thermal stability of the PEDOT:PSS anodes and the as-integrated flexible OSCs. The PEDOT:PSS anodes were thermal annealed at 85 °C in the air atmosphere (relative humidity: 85%). As shown in Fig. 6b, the sheet resistance of the PEDOT:PSS anodes was increased by ≈ 3.6% for the 168 h testing and by ≈ 6.9% for 500 h testing. The PEDOT:PSS anodes had a slight increase in R sh from 35.0 to 37.4 Ω sq −1 in the long-time thermal process for 500 h. A high thermal stability of the PEDOT:PSS anodes is energetically favored to raise the performance stability of the flexible OSCs. Figure 6c-f shows the photovoltaic parameters (V OC , FF, J SC , and PCE) of the flexible OSCs based on the optimized PEDOT:PSS anodes as a function of thermal processing time, respectively. Notably, the flexible OSCs were thermally processed at 85 °C for 200 h in an inert glove-box filled with nitrogen (N 2 ). Obviously, V OC is almost independent of the thermal processing times. The FF of the flexible devices changed a little, and it was of a high value (higher than 73.0%) for the flexible OSCs with the thermal treatment for 25 h. The FF of the flexible OSCs with the thermal treatment for 200 h is 71.6%, which was a little lower than that of the flexible OSCs in a shorter thermal process. However, the J SC of the flexible OSCs changed significantly in the thermal process. As mentioned above, the flexible OSCs with the optimized PEDOT:PSS anodes showed the initial J SC of 25.78 mA cm −2 . Increasing the processing time to 150 and 200 h led to a lower J SC of 24.51 and 24.23 mA cm −1 , respectively. Thus, the PCE was decreased from the initial value of 16.41% to 14.92% in the thermal stability test. The decrease in PCE might be attributed to the insulating PSS components of the PEDOT:PSS anodes and P VPAI4083 buffer layers that absorbed moisture (e.g., alcohols, H 2 O), leading to a volume expansion. Besides, the rigid main-chain structures and irregular arrangement of PDINN molecules potentially caused a contact issue at interfaces and an electrical stability concern in the thermal process. The performance stability of solution-processed flexible devices would be promoted through removing PSS components from the PEDOT:PSS matrices, and employing stable buffer layers for an interface shield in the future.

Conclusions
We demonstrated the flexible OSCs based on the 0. anodes showed a favorable phase-separated morphology with physically continuous networks; and the films had a high charge concentration (6.54 × 10 21 cm −3 ) and mobility (3.44 cm 2 V −1 s −1 ), accounting for a low sheet resistance. The work function was raised by the electron-withdrawing fluorine groups of CF 3 SO 3 H that reduced the charge recombination loss at interfaces, substantially accounting for the improvement in V OC . Besides, the CF 3 SO 3 H-doped PEDOT:PSS anodes exhibited a better wettability, resulting in an intimate contact with the hole-transporting layers (P VP AI4083). The PCE of the solution-processed flexible OSCs was maintained well in the bending tests, in which 94.4% of the initial value was retained after 1000 cyclic bending at r of 1.5 mm. The solution-processed flexible OSCs also exhibited a good thermal stability in the 200 h thermal process at 85 °C, i.e., a decrease by 9.1% in PCE in the stability test. This work demonstrates a solution-processed flexible OSC with a high efficiency, a high flexibility and a good thermal stability. It widens the adaptation of the flexible PEDOT:PSS anodes into high-performance flexible optoelectronics.