Low-cost and efficient perovskite solar cells using a surfactant-modified polyaniline:poly(styrenesulfonate) hole transport material

Accepted Manuscript Title: Low-cost and efficient perovskite solar cells using a surfactant-modified polyaniline:poly(styrenesulfonate) hole transport material Author: Kisu Lee Kyung Hee Cho Jaehoon Ryu Juyoung Yun Haejun Yu Jungsup Lee Wonjoo Na Jyongsik Jang PII: DOI: Reference:

S0013-4686(16)32666-4 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.103 EA 28575

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

31-10-2016 16-12-2016 19-12-2016

Please cite this article as: Kisu Lee, Kyung Hee Cho, Jaehoon Ryu, Juyoung Yun, Haejun Yu, Jungsup Lee, Wonjoo Na, Jyongsik Jang, Low-cost and efficient perovskite solar cells using a surfactant-modified polyaniline:poly(styrenesulfonate) hole transport material, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.103 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Low-cost and efficient perovskite solar cells using a surfactant-modified polyaniline:poly(styrenesulfonate) hole transport material

Kisu Lee,‡ Kyung Hee Cho,‡ Jaehoon Ryu, Juyoung Yun, Haejun Yu, Jungsup Lee, Wonjoo Na, and Jyongsik Jang*

School of Chemical and Biological Engineering, Seoul National University, 599 Gwanangro, Sillim-dong, Gwanak-gu, Seoul 151-742, Korea

*

Corresponding author. Fax: +82 2 888 1604. E-mail address: [email protected]

(J. Jang).

‡ These authors contributed equally to this work.

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Graphical abstract

Highlights 

PANI:PSS modified with a nonionic surfactant (Triton X–100) was successfully utilized as a hole transport material for perovskite solar cells.



Perovskite solar cells using PANI:PSS with the optimal Triton X–100 concentration of 1.0 wt% exhibited significantly improved photovoltaic parameters and average power conversion efficiency (from 6.59 to 10.90%), with a maximum efficiency of 11.67%.



Triton X–100 played a bifunctional role to enhance the hole extraction capability of PANI:PSS.

Abstract A facile synthesis and modification method of polyaniline:poly(styrenesulfonate) (PANI:PSS) is reported for an efficient hole transport material (HTM) in inverted planar perovskite solar cells (PSCs). PANI:PSS with Triton X–100, a nonionic surfactant, showed enhanced hole transport capability due to improved wetting on the substrate and a surface compositional change. The surfactant lowered the surface tension of a PANI:PSS aqueous solution, which enabled uniform casting of PANI:PSS films onto an

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indium tin oxide (ITO) substrate without any surface treatment. Additionally, selforganization of PSS nanogranules, induced by the surfactant, provided PANI-rich surfaces that were favorable for hole extraction. When surfactant-modified PANI:PSS (1.0 wt% Triton X–100) was used as an HTM in a PSC, the average short-circuit current density increased by 23% compared with pristine PANI:PSS (i.e., from 15.02 to 18.43 mA/cm2) and the fill factor also improved, eventually leading to a power conversion efficiency enhancement from 6.59 to 10.90%. Keywords: perovskite solar cells; conducting polymers; hole transport materials; polyaniline; surfactants.

1. Introduction Organic-inorganic hybrid perovskite solar cells (PSCs) offer the opportunity to realize low-cost, highly efficient next-generation photovoltaic technology. The first attempt to fabricate all-solid state PSCs used CH3NH3PbI3 nanodots; the power conversion efficiency (PCE) of the device was 9.7%.[1] PCEs have risen substantially within the past five years and now exceed 20%.[2,3] Along with the development of perovskite absorber materials and processing techniques, novel electron/hole transport materials for efficient charge separation and transfer of photo-generated excitons from the perovskite absorber layer to electrodes have been widely investigated.[4-8] Polymeric

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hole

transport

materials

(HTMs),

such

as

poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), poly(3-hexylthiophene2,5-diyl)

(P3HT),

poly(bis(4-phenyl)(2,5,6-trimentlyphenyl)amine)

(PTAA),

and

poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine) (poly-TPD), have been used as hole transport layers (HTLs) in PSCs due to their numerous advantages.[9-13] These include low-temperature solution processability, sufficiently high hole mobility from the conjugated structure, mechanical flexibility, and the ability to form thinner films than with small-molecule HTMs due to improved coverage.[14-17] However, despite these advantages, polymeric HTMs are relatively expensive and can increase the overall manufacturing cost of PSCs. Therefore, it is imperative to develop novel polymeric HTMs from low-cost conjugated polymers to achieve economical, efficient PSCs. Polyaniline (PANI), a representative p-type conducting polymer, has been widely used in electronic applications of organic light-emitting diodes (OLEDs), dye-sensitized solar cells (DSSCs), electrochromic devices, and sensor applications due to its ease of synthesis, good conductivity, high transmittance, and environmental stability.[18-22] Especially, the cost of aniline, which is the monomer of PANI, is much cheaper than the starting materials of other semiconducting polymers.[23] Thus, PANI has great potential as an alternative polymeric HTM in PSC applications. There have been a few studies on PANI-based HTMs, but the associated devices showed poor photovoltaic parameters

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and required complex synthetic routes.[24-26] Herein, we suggest a simple fabrication method of an solution-processable polymeric HTM for PSCs using a PANI:PSS blend and a novel strategy to improve the photovoltaic performance of PANI:PSS-based PSCs using a nonionic surfactant (Triton X–100). The PANI:PSS aqueous solution was prepared via oxidative chemical polymerization of aniline in the presence of commercial PSS; no further purification process was required. All processes associated with the fabrication of the PANI:PSSbased PSCs were carried out at a temperature below 125°C; thus, this method would be compatible with flexible substrates. Triton X–100 added to the PANI:PSS blend enhanced the hole extraction capability and significantly increased the PCE to 11.67% with a high short-circuit current density (JSC) exceeding 18 mA/cm2. In addition, the fabricated devices with surfactant-modified PANI:PSS exhibited smaller current density-voltage hysteresis with respect to the scan direction than those with pristine PANI:PSS. These improvements in photovoltaic performance were attributed to the improved wettability on the indium tin oxide (ITO) substrate and a change in the morphology of the PANI:PSS film surface into phase-separated PANI and PSS domains. The effects of the surfactant on device performance, reproducibility, and cell efficiency hysteresis were systematically investigated. 2. Materials and methods

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2.1. Materials. Unless otherwise stated, all chemicals were purchased from Sigma Aldrich and used as received. PCBM (Phenyl-C61-butyric acid methyl ester, > 99%) was purchased from Solenne BV. PbI2 (99.9985%) was obtained from Alfa Aesar.

2.2. Synthesis of surfactant modified-PANI:PSS solution. Polyaniline:poly(styrenesulfonate) (PANI:PSS) solution was synthesized by chemical oxidation polymerization of aniline in the presence of HCl and PSS. First, 17 mg of aniline monomer (0.18 mmol) was introduced drop-wise in 0.35 M aqueous HCl solution (11.66 mL) and was stirred for 2 h. Then, 0.4 g of commercially available PSS solution (Mw ~200,000, 30 wt% in water, Sigma Aldrich) was added to the mixed solution followed by vigorous stirring for 2 h. The polymerization was carried out by adding 52 mg of ammonium persulfate (0.23 mmol) as an oxidant over 12 h at 25 °C. As a result of polymerization, dark green solution of PANI:PSS was obtained. Afterwards, PANI:PSS solution was mixed with different weight percentage of Triton X–100 (0.5, 1.0, and 2.0 wt%), and used as a hole transport material of perovskite solar cells without further purification.

2.3. Fabrication of perovskite solar cells.

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The devices were fabricated on patterned indium tin oxide (ITO) glass with a sheet resistance of 15 Ω sq-1 (Pilkington). ITO substrates were cleaned sequentially in water, acetone, and 2-propanol for 30 min. A PANI:PSS layer was deposited by spincoating a prepared PANI:PSS solution at 2000 rpm, and annealed at 125 °C for 10 min in ambient air. Formamidinium iodide (FAI, CH(NH2)2I) was synthesized following a previous report.[27] Cs-doped FAPbI3 precursor solution was prepared by dissoving 0.159 g of FAI, 0.026 g of CsI, and 0.507 g of PbI2 in 0.75 mL of DMF/DMSO (v/v = 9/1) mixed solvent, and stirred at 70°C. The perovskite precursor solution was spun-cast on the pre-heated substrate at 1000 rpm for 10 s and 4000 rpm for 25 s under a N2 atmosphere. During the second spin coating step, 0.4 mL of diethyl ether (> 99.5 %) was dropped onto the substrate to promote fast crystallization of perovskite layer. After the whole spin coating process, the substrate was annealed at 125 °C for 10 min in ambient air to form Cs-doped FAPbI3 films. The electron transporting layer was spincoated from a PCBM solution (20 mg/mL in chlorobenzene) at 4000 rpm for 30 s. Finally, a 60-nm-thick Au top electrode was deposited by thermal evaporation. The active area of the fabricated device was 0.09 cm2. All solar cell fabrication procedures were carried out in ambient air (temperature= 25°C, relative humidity< 40%).

2.4. Measurement and Characterization.

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Scanning electron microscope (SEM) images were obtained with JSM-6701F microscopes (JEOL, Japan). X-ray diffraction (XRD) of FAPbI3 films was measured using Smartlab (Rigaku, Japan). Transmittance of PANI:PSS films and UV-Vis absorption spectra of the FAPbI3 films were measured with a double beam UV-Vis spectrophotometer of Lambda 35 (Perkin-Elmer, USA). Atomic force microscope (AFM) images of PANI:PSS films were acquired using an Innova SPM (Vecco, USA) and analyzed with SPMLabAnalysis software. The J–V characteristics of the PSCs were measured using a I-V tracer (MP-160, Eko Instruments) under standard AM 1.5G (100 mW/cm2) illumination from a 500-W xenon lamp, calibrated with a KG5-filtered Si reference cell (K801, McScience Inc.). The PSCs were masked with a customized solar cell jig with an aperture of 0.09 cm2.

External quantum efficiency (EQE) was

characterized from 350 nm to 850 nm using a K3100 IPCE Measurement System (McScience Inc., Korea; chopping frequency of 4 Hz, without bias light and 10-nm step wavelength). Time-resolved photoluminescence decay transients were recorded using a 520 nm pulse laser (PicoQuant). Electrochemical impedance spectra (EIS) of the PSCs were obtained by electrochemical workstation of Zive SP2 (Wonatech, Korea). Nyquist plots of EIS measurements were fitted using Zman software. 3. Results and discussion Figure 1a depicts the device structure of a PSC fabricated in this study and the

8

molecular structures of PANI, PSS, and Triton X–100. Digital photographs of spincoated surfactant-modified PANI:PSS (s-PANI:PSS) films on ITO and the fabricated PSC are shown in Figure S1. For the preparation of PANI:PSS, aniline monomer protonated by hydrochloric acid was mixed with commercially available PSS (Mw: ca. 200,000) in deionized water, and then ammonium persulfate was added to produce free radicals that initiated polymerization. As polymerization proceeded, the colorless reaction mixture slowly turned dark green. We chose an aniline:PSS feed ratio of 1:6 to simultaneously maintain the conductivity and dispersion stability. The synthesized polymer solution did not precipitate due to a strong electrostatic attraction between positively charged PANI and negatively charged PSS. Consequently, the PANI:PSS remained well-dispersed in an aqueous solution for months without agitation. After completion of the polymerization reaction, Triton X–100 nonionic surfactant was added and the solution was stirred for one hour. Triton X–100 has an amphiphilic structure with polyethylene oxide functional groups (Figure 1a) that causes phase separation of the conducting polymer and PSS.[28,29] To understand the effect of the surfactant on the hole transport ability of PANI:PSS, we prepared s-PANI:PSS with different weight percentages of Triton X–100 (0.5, 1.0, and 2.0 wt%) and used it as an HTL in PSCs. All component layers of the s-PANI:PSS-based PSC were spin-coated, except for the top Au electrode. Cesium-doped formamidinium lead iodide (FAPbI3)

9

was selected as the light absorber due to its excellent light absorbing capability and high photo- and moisture stability.[27] Figure 1b shows the energy diagram of the fabricated device. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels of the s-PANI:PSS film were determined using cyclic voltammetry (CV). The CV curves and the detailed procedure used to estimate the energy levels are provided in the Supporting Information (Figure S2). The HOMO level of s-PANI:PSS (−4.9 eV) is well-matched with the valence band of the perovskite layer (−5.4 eV), and hence facilitates efficient hole extraction. Inversely, the higher LUMO level of s-PANI:PSS (−3.7 eV) than the conduction band of the perovskite layer (−4.0 eV) prevents unwanted electron transfer from the perovskite layer to the ITO electrode. The current density–voltage (J–V) characteristics of the device using PANI:PSS with varied surfactant concentrations are displayed in Figure 2a, and the photovoltaic parameters are listed in Table 1. The photovoltaic performance of the PANI:PSS-based PSC was clearly influenced by the addition of Triton X–100. We observed noticeable improvements in the JSC, open-circuit voltage (VOC), and fill factor (FF), which led to a significant increase in the average PCE from 6.59 to 10.90%. The optimized amount of Triton X–100 was determined to be 1.0 wt% of the PANI:PSS solution; higher surfactant concentrations reduced the cell efficiency due to the insulating nature of

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Triton X–100. The optimal device yielded the maximum PCE of 11.67%, a JSC of 18.89 mA/cm2, an VOC of 0.87 V, and a FF of 0.71. The JSC values of the PSCs using sPANI:PSS were in good agreement with the external quantum efficiency (EQE) spectra shown in Figure 2b. In particular, the EQE spectrum of the device using s-PANI:PSS with 1.0 wt% Triton X–100 exhibited a broad plateau at ca. 73% over the range of 400– 800 nm. Figure 2c illustrates the efficiency distribution that was affected by the presence of surfactant in the PANI:PSS. Surfactant addition to the PANI:PSS not only increased the cell efficiency but also resulted in a narrower efficiency distribution, which indicated better reproducibility. Additionally, we optimized the thickness for the s-PANI:PSS (1.0 wt% Triton X–100) layer. The s-PANI:PSS solution was spin-coated at 2000, 3500, and 5000 rpm, resulting in 130-, 90-, and 50-nm-thick films, respectively (Figure S3). The fabricated device with a 130-nm-thick s-PANI:PSS film demonstrated the highest PCE of 11.37% and higher values for JSC and FF. To identify the influence of Triton X–100 in PANI:PSS on perovskite morphology, crystallinity, and light absorption, scanning electron microscopy (SEM), X-ray diffraction (XRD) and ultraviolet-visible (UV-Vis) absorption measurements were performed. In Figure S4, all perovskite films consisting of 200 nm-sized crystallites are thoroughly coated on the substrates and show no difference in their morphology. The XRD diffractograms (Figure S5a) show that all of the perovskite films on PANI:PSS

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made with the different surfactant concentrations had the same patterns with characteristic peaks corresponding to the black perovskite FAPbI3.[27] The UV-Vis absorption spectra of perovskite films (Figure S5b) also remained nearly unchanged by the addition of surfactant. These observations indicated that there was no significant change in the structural or optical properties of the perovskite film, and that the improved photovoltaic performance solely originated from efficient hole extraction of sPANI:PSS. One effect of Triton X–100 was to reduce the surface tension of an aqueous PANI:PSS solution; this improved the wetting of PANI:PSS on the ITO substrate. Figure 3a displays the surface tension of PANI:PSS solutions with different Triton X– 100 concentrations. The pristine PANI:PSS had a high surface tension of 69.2 mN/m, which is close to that of pure water (71.8 mN/m).[32] The addition of 0.5 wt% of Triton X–100 caused a steep drop in the surface tension to below half that of a pristine PANI:PSS solution (i.e., to 31.0 mN/m). Increasing the surfactant concentration to 1.0 and 2.0 wt% caused the surface tension to decrease slightly (i.e., to 29.8 and 29.5 mN/m, respectively). Figure 3b shows optical microscopy images of PANI:PSS and sPANI:PSS droplets on an ITO surface. The contact angles of the polymer solutions on ITO displayed the same trend as the surface tension measurements, indicating wettability enhancement. An s-PANI:PSS solution immediately spread on the ITO

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substrate and provided a continuous film during spin coating. Consequently, a PANI:PSS solution containing the surfactant could be used to form a more uniform HTL compared with pristine PANI:PSS (Figure 3c). No additional treatment such as plasma or ultraviolet–ozone (UVO) surface modifications was required. These factors could favorably influence photovoltaic performance and cell efficiency distribution.

In addition to improving the wetting properties of PANI:PSS, Triton X–100 induced a substantial change in the surface morphology and composition of PANI:PSS films and enabled better hole transport from the perovskite layer to the ITO substrate. Previous research reported that Triton X–100 can provoke segregation of PEDOT and PSS domains by phase-separation and cause nanofibril structures of both polymers to form with sharp interfaces.[28,33] However, the addition of Triton X–100 to PANI:PSS had a completely different effect on the surface morphology than in the PEDOT:PSS system. SEM images (Figure 4) show the surface morphologies of PANI:PSS and s-PANI:PSS films according to the surfactant concentration (0.5, 1.0, and 2.0 wt%). The pristine PANI:PSS film was composed of tiny (<10 nm) PANI–PSS complex nanoparticles. Adding 0.5 wt% Triton X–100 caused excess PSS to start building segregated domains on the film surface; self-organized nanogranules about 100-nm in length were fully formed in the presence of more than 1.0 wt% of surfactant. Atomic force microscopy

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(AFM) phase images (Figure 5), where dark and bright regions correspond to PANI-rich (hard segment) and PSS-rich (soft segment) regions, respectively, confirmed phase separation and the formation of PSS nanogranules on the film surface.[34,35] The small variation of brightness level (softness) for the pristine PANI:PSS film indicated a homogeneous distribution of PANI and PSS chains (Figure 5a). In strong contrast, Figure 5b shows separated domains of PSS nanogranules and PANI-rich regions in the s-PANI:PSS film. Self-organization of PSS nanogranules on the film surface was caused by Triton X–100, which revealed PANI-rich regions. Direct contact of these PANI networks with the perovskite layer was beneficial for hole extraction. A high transmittance of the HTL in the inverted PSC structure is essential for efficient light absorption by the perovskite layer. Despite the marked change in surface morphology, the s-PANI:PSS-coated ITO substrate maintained a high transmittance of over 80% in the visible and near-infrared regions. Thus, loss of light arising from the morphological change of PANI:PSS was expected to be negligible (Figure S7). To quantify the improved hole extraction capability of the s-PANI:PSS samples, time-resolved photoluminescence (PL) decay times were measured for the glass/(no HTM or PANI:PSS or s-PANI:PSS)/perovskite configuration (Figure 6a). PL decay lifetimes (τ1 and τ2) were determined by fitting the decay curves with a bi-exponential function; the results are summarized in Table S1. The fast decay (τ1) is considered to

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originate from charge carrier transport from the perovskite film to the contacting HTMs, while the slow decay (τ2) stems from radiative recombination of free charge carriers in the perovskite film.[36,37] All of the samples exhibited similar τ2 values, but the τ1 values of PANI:PSS and s-PANI:PSS were shortened to 18.19 and 11.19 ns, respectively, from that of the reference sample (39.85 ns). In particular, s-PANI:PSS displayed a more rapidly decaying curve than the pristine PANI:PSS, which implied that photo-excited holes were effectively transferred from perovskite to s-PANI:PSS.[31,38] Electrochemical impedance spectroscopy (EIS) was also conducted to characterize the charge transfer behavior of our devices. Figure 6b shows Nyquist plots measured at bias of 0.8 V under dark condition and the equivalent circuit model for fitting the spectra. The arcs are ascribed to the charge transfer resistance (RCT) and the plot of the device with sPANI:PSS has a smaller arc than that with pristine PANI:PSS.[39,40] It indicates a reduced contact resistance between s-PANI:PSS and perovskite layer due to the synergistic effect of the uniform coating with rapid hole extraction through PANI-rich regions of the s-PANI:PSS.[39,40] The time-resolved PL and EIS observations suggest that Triton X–100 strengthened the hole extraction capability of PANI:PSS, reduced contact resistance at the HTL or interfaces of HTL, and led to higher PSC photovoltaic parameter values. Hysteresis behavior between forward and reverse scan modes during J–V

15

characterization, the so-called J–V hysteresis, is an important indicator of cell efficiency and stability. Its origin is not fully understood, but it has been reported that the hysteresis behavior is highly dependent on electron/hole selective contact itself or the interface with the perovskite layer.[41,42] Figure 7 compares the J–V characteristics of PANI:PSS-based PSCs under forward and reverse scan directions; the related photovoltaic parameters are summarized in Table 2. The PCE of the PSC using pristine PANI:PSS decreased from 7.43 to 4.92% in the forward scan mode due to the considerably reduced VOC and FF. However, the J–V hysteresis of the cell was markedly relieved for s-PANI:PSS and exhibited a relatively small efficiency loss in the forward scan mode (i.e., from 11.17 to 10.85%) due to reduced contact resistance as discussed above. Our hysteresis analysis indicated that s-PANI:PSS improved the reliability of the cell efficiency in addition to the photovoltaic performance. Additionally, we have tested stability of PSCs based on s-PANI:PSS under ambient condition (temperature ~25°C, relative humidity ~30%) for 300 hours (Figure S8a). Figure S8b displays J-V curves of the device before-and-after the 300 hour stability test. The JSC was very stable but VOC and FF were decreased, resulting in efficiency degradation from 10.01 to 6.58%. The device maintained relative efficiency over 60 % of the initial value after 300 hours of exposure to the ambient condition due to high moisture-resistance of cesium-containing FAPbI3.[27]

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4. Summary PANI:PSS modified with Triton X–100 was successfully used as an efficient polymeric HTM for PSCs. Triton X–100, a nonionic surfactant, played a bifunctional role to enhance the hole extraction capability of PANI:PSS. First, the amphiphilic property of the surfactant lowered the surface tension of a PANI aqueous solution, which enabled the production of a flat, uniformly thick PANI:PSS film. Second, polyethylene oxide functional groups incorporated in the Triton X–100 induced a phase separation between PANI and PSS and, thus, produced conductive domains where the charge transporting species (PANI) were abundant. PSCs made with PANI:PSS with the optimal Triton X–100 concentration of 1.0 wt% exhibited a significantly increased average PCE (from 6.59 to 10.90%), with a maximum PCE of 11.67%. The procedures described herein provide an efficient and facile means of synthesizing a highperformance, reliable PANI:PSS HTM for low-cost PSCs. Acknowledgements This research was supported by Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea (2011–0031573). The authors thank Prof. Mansoo Choi for help in thermal evaporation deposition and Prof. Seong Keun Kim for photoluminescence measurement.

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Fig. 1. a) Schematic illustration of the perovskite solar cell (PSC) configuration, of which the hole transport layer is surfactant-modified polyaniline:poly(styrenesulfonate) (s-PANI:PSS). The molecular structures of PANI, PSS, and Triton X–100 are shown. b) Energy band diagram of the components in the PSC showing ideal electron and hole extractions. The energy levels of perovskite (formamidinium lead iodide) and phenylC61-butyric acid methyl ester (PCBM) were derived from references [30] and [31].

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Fig. 2. a) Current density–voltage (J–V) curves for the best-performing devices obtained with PANI:PSS and s-PANI:PSS hole transport layers, measured under standard AM 1.5G 100 mW/cm2 illumination. b) External quantum efficiency (EQE) of the PSCs with different PANI:PSS layers. c) Power conversion efficiency (PCE) distribution of PSCs with pristine PANI:PSS and s-PANI:PSS (1.0 wt% Triton X–100).

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Fig. 3. a) Surface tension of PANI:PSS solutions with different Triton X–100 concentrations. b) Optical microscopy images and contact angles of PANI:PSS solutions with different Triton X–100 concentrations on indium tin oxide (ITO) glass substrates. c) Cross-sectional scanning electron microscopy (SEM) images of PSCs made with PANI:PSS and s-PANI:PSS. The yellow arrows indicate the PANI:PSS and s-PANI:PSS layers.

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Fig. 4. SEM micrographs of spin-coated PANI:PSS and s-PANI:PSS for different Triton X–100 concentrations (0.5, 1.0, and 2.0 wt%).

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Fig. 5. Atomic force microscopy (AFM) phase images of spin-coated a) PANI:PSS and b) s-PANI:PSS films (1.0 wt% Triton X–100). Corresponding 2-and 3-dimensional topographic images are provided in the Supporting Information.

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Fig. 6. a) Time-resolved photoluminescence (PL) decay transients for perovskite films spin-coated on PANI:PSS and s-PANI:PSS. The samples were excited at 520 nm and PL decay transients were acquired at the peak emission wavelength of 810 nm. b) Nyquist plots for the PSCs made with PANI:PSS and s-PANI:PSS measured in the dark at V = 0.8 V. The frequency scan ranged from 0.5 Hz to 3 MHz. The inset is the equivalent circuit.

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Fig. 7. Hysteresis analysis of a PSC made with a) PANI:PSS and b) s-PANI:PSS hole transport layers.

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Table 1. Photovoltaic parameters of perovskite solar cells (PSCs) using PANI:PSS and surfactant-modified PANI:PSS (s-PANI:PSS) hole transport layers (HTLs).

HTL

Voca (V)

Jsca (mA/cm2)

FFa

ηavga (%)

ηmax (%)

PANI:PSS

0.77

15.02

0.57

6.59 (± 0.58)

7.46

s-PANI:PSS (0.5 wt%)

0.80

16.28

0.59

7.67 (± 0.51)

8.34

s-PANI:PSS (1.0 wt%)

0.85

18.43

0.69

10.90 (± 0.47)

11.67

9.63 (± 0.48) a Average values of the parameters were obtained for 25 devices.

s-PANI:PSS (2.0 wt%)

0.81

18.04

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0.66

10.01

Table 2. Photovoltaic parameters of PSCs made with PANI:PSS and s-PANI:PSS hole transport layers (HTLs) measured for different scan directions.

HTL

Scan direction

Voc (V)

Jsc (mA/cm2)

FF

η (%)

Forward a

0.66

15.70

0.47

4.92

Reverse a

0.79

15.69

0.60

7.43

Forward a

0.86

18.55

0.68

10.85

Reverse a

0.87

18.68

0.69

11.17

PANI:PSS

s-PANI:PSS a

The scan rate of the forward and reverse scan modes was 0.35 V/s.

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