A fluorinated polythiophene hole-transport material for efficient and stable perovskite solar cells

Accepted Manuscript A fluorinated polythiophene hole-transport material for efficient and stable perovskite solar cells Inyoung Jeong, Jea Woong Jo, Seunghwan Bae, Hae Jung Son, Min Jae Ko PII:

S0143-7208(18)32588-9

DOI:

https://doi.org/10.1016/j.dyepig.2019.01.002

Reference:

DYPI 7275

To appear in:

Dyes and Pigments

Received Date: 25 November 2018 Revised Date:

31 December 2018

Accepted Date: 3 January 2019

Please cite this article as: Jeong I, Jo JW, Bae S, Son HJ, Ko MJ, A fluorinated polythiophene holetransport material for efficient and stable perovskite solar cells, Dyes and Pigments (2019), doi: https:// doi.org/10.1016/j.dyepig.2019.01.002. 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.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

A

fluorinated

polythiophene

hole-transport

material for efficient and stable perovskite solar

RI PT

cells

Photovoltaics Laboratory, Korea Institute of Energy Research (KIER), Daejeon 34129,

Republic of Korea. b

Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 04620,

Republic of Korea. c

Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology,

TE D

Seoul 02792, Republic of Korea. d

M AN U

a

SC

Inyoung Jeong a,1, Jea Woong Jo b,1, Seunghwan Bae c,1, Hae Jung Son c,*, and Min Jae Ko d,**

Department of Chemical Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-

EP

gu, Seoul 04763, South Korea.

* Corresponding author.

AC C

** Corresponding author.

E-mail: [email protected] (M. J. Ko); [email protected] (H. J. Son), 1

These authors equally contributed to this work.

KEYWORDS: hole-transport layer, polythiophene, hydrophobicity, stability, perovskite solar cell

ACCEPTED MANUSCRIPT Abstract: Charge-transport materials for use in highly efficient and stable perovskite solar cells (PSCs) must exhibit energy levels appropriate for high charge selectivity, sufficiently high charge-transport ability for efficient charge collection, and high

RI PT

humidity resistance for long-term device stability. Polythiophenes are a promising class of hole-transport layer (HTL) materials that could satisfy these requirements. However, PSCs fabricated using conventional poly(3-hexylthiophene) (P3HT) HTLs show limited

SC

efficiencies of < 16% owing to the shallow highest occupied molecular orbital (HOMO) energy level and poor charge extraction ability of P3HT. Herein, we demonstrate that

M AN U

the fluorinated polythiophene derivative FEH is a suitable replacement for P3HT and a promising HTL material for perovskite solar cells. The FEH was found to have a deeper HOMO and exhibit more efficient charge-extraction ability at the perovskite/HTL interface than P3HT. This is attributed to the electron-withdrawing nature of the

TE D

fluorine atoms in FEH and its ability to form more uniform films on the perovskite layer. Thus, when FEH was employed as the HTL, the corresponding PSC showed an improved efficiency of 18.0% and an enhancement of all device parameters compared

EP

with control devices fabricated using P3HT (10.8%) and Spiro-OMeTAD (17.0%) HTLs. Moreover, fluorination on the conjugated backbone of the polymer increases its

AC C

hydrophobicity, and the resulting hydrophobic surface of the FEH HTL prevents the ingress of water, resulting in an improvement of the long-term stability of the corresponding PSCs under air exposure.

ACCEPTED MANUSCRIPT 1. Introduction In the last decade, organometal trihalide perovskites (OTPs) have garnered worldwide research interest as promising candidate materials for next-generation photovoltaic devices

RI PT

owing to their unique optoelectronic properties [1–6]. Enormous efforts have been devoted to improving the device performances of perovskite solar cells (PSCs) in terms of OTP design, control of processing conditions, device structure optimization, and interface engineering. As

SC

a result, the power conversion efficiencies (PCEs) of PSCs have been improved from 3.9% to 23.7% in a relatively short period of time [7–15].

M AN U

Typical PSCs are fabricated with a sandwich configuration comprising a parallel cathode and anode upon which an electron-transport layer (ETL) and a hole-transport layer (HTL) are deposited, respectively. Importantly, the properties of these interfacial layers (i.e., the ETL and HTL) significantly affect not only the charge extraction from the perovskite absorber to

TE D

the electrodes but also the device stability in air. To realize highly efficient and stable PSCs, the interface layer materials should satisfy several requirements. These are: 1) Suitable energy levels that minimize potential energy loss at the perovskite/electrode interface and

EP

increase charge selectivity at the corresponding electrodes; 2) high hole or electron mobility for efficient charge transport and charge collection; and 3) high resistance to humidity and

AC C

photodegradation for long-term device stability [16–20]. A wide variety of inorganic and organic materials, including TiO2, ZnO, SnO2, phenyl-C61butyric acid methyl ester (PC61BM), 2,2′,7,7′-tetrakis(N,N-p-dimethoxyphenylamino)-9,9′spirobifluorene

(Spiro-OMeTAD),

NiOx,

and,

poly(3,4-ethylenedioxythiophene):poly

(styrenesulfonate) (PEDOT:PSS), have been investigated as a means to developing suitable perovskite/electrode interfaces [21–25]. Poly(3-hexylthiophene) (P3HT) is an attractive HTL candidate for PSCs owing to its high hole-transporting ability (hole mobility (µh) ≈ 10−3 cm2 V−1 s−1) and cost-effectiveness. However, despite these advantages, PSCs fabricated with

ACCEPTED MANUSCRIPT P3HT HTLs exhibit limited device performances and low PCEs (10–16%) owing to the relatively shallow highest occupied molecular orbital (HOMO) energy level of P3HT (4.6−5.0 eV), which limits the built-in potential of the corresponding PSC devices [26–30].

RI PT

Various polythiophene derivatives have been designed and synthesized as alternatives to P3HT for organic field-effect transistors and organic photovoltaics [31–34]. Among them, FEH, which consists of difluoro-bithiophene and 3,4-dialkylthiophene building blocks, has a

SC

deeper HOMO energy level than that of P3HT owing to the strong electron-withdrawing nature of the fluorine atoms on its conjugated backbone [35,36]. Moreover, because

M AN U

fluorination of an organic compound could contribute to an increases its hydrophobicity, FEH is expected to improve stability under highly humid conditions [37–41]. Accordingly, in this work, we evaluated the optoelectronic properties of FEH as an HTL. PSC devices were prepared using Spiro-OMeTAD, P3HT, or FEH as the HTL, and the potential utility of FEH

2. Experimental section

EP

2.1. Device fabrication

TE D

was assessed by comparing the device performances and stabilities of the resultant PSCs.

AC C

A fluorine-doped tin oxide glass (FTO, TEC8, Pilkington) substrate was washed by sonication in ethanol, acetone, and isopropanol for 20 min. After UV-ozone treatment for 20 min, a compact (c)-TiO2 layer was deposited on the FTO substrate by spin coating a diluted titanium diisopropoxide bis(acetylacetonate) solution (75% in isopropanol, Aldrich) with 1butanol at 2000 rpm for 40 s followed by heating at 500 °C for 30 min. The solution for the mesoporous (mp)-TiO2 layer was prepared by diluting a lab-made TiO2 paste [24] with anhydrous ethanol (1:3.5 weight ratio). The mp-TiO2 layer was coated onto the c-TiO2 layer by spin coating the diluted TiO2 paste solution at a spin rate of 3500 rpm for 40s and then

ACCEPTED MANUSCRIPT annealing at 500 °C for 30 min. A MAPbI3 perovskite solution was prepared by dissolving 1.4 M PbI2 (99.9985%, Alfa Aesar) and 1.4 M CH3NH3I (Greatcell Solar) in 1 mL of N,Ndimethylformamide (DMF, 99.8%, Alfa Aesar) and dimethyl sulfoxide (DMSO, 99.9%,

RI PT

Aldrich) mixed solvent (9:1, v/v). The MAPbI3 layer was prepared by spin-coating the perovskite solution at 1000 rpm for 5 s and then 4000 rpm for 15 s onto the ETLs. During spin-coating, diethyl ether was dropped onto the rotating film 7 s before the end of the

SC

coating process and the film was heated on a hot plate at 65 °C for 1 min and then at 100 °C for 10 min. The Spiro-OMeTAD solution was prepared by dissolving 56 mg of Spiro-

M AN U

OMeTAD (Merck), 30 mg of 4-tert-butylpyridine (tBP, 96%, Aldrich), and 6 mg of lithiumbis(trifluoromethanesulfonyl) imide (LiTFSI, 99.95%, Aldrich) in 1 mL of chlorobenzene (99.8%, Aldrich). The Spiro-OMeTAD solution was then spin-coated at 2500 rpm for 20 s. For the polymeric HTLs, an FEH or P3HT solution was prepared by dissolving 15 mg of the polymer in 1 mL chlorobenzene. Then, 5 µL of a LiTFSI stock solution (520 mg in 1 mL

TE D

acetonitrile) and 15.2 µL of tBP were added to the solution. The polymer solution was coated onto the perovskite layer at a spin-rate of 3000 rpm for 30 s. Finally, an 80 nm-thick Au

EP

electrode was deposited on top of the device by thermal evaporation.

AC C

2.2. Characterization

The number of average molecular weights (Mn) and polydispersity index (PDI) of polymers were obtained by measuring gel permeation chromatography (Waters, refractive index detector Waters 2414) eluted with chlorobenzene as an eluent (P3HT: Mn = 52 kDa, PDI = 2.8; FEH: Mn = 14 kDa, PDI = 2.13). The surface morphology of the HTLs and cross-sectional images of the devices were obtained by atomic force microscopy (AFM, XE-100, Park Systems) and field-emission scanning electron microscopy (FESEM, Inspect F, FEI). Ultraviolet photoelectron spectroscopy (UPS) measurements were performed with a scanning

ACCEPTED MANUSCRIPT XPS microprobe (PHI 5000 VersaProbe, Ulvac-PHI) using HeI (21.2 eV). The UV-Vis absorbance spectra of the HTLs were obtained using a UV-Vis spectrometer (Lambda 35, PerkinElmer). The space charge limited current (SCLC) current-voltage (J–V) curves were

RI PT

measured using hole-only devices (ITO/PEDOT:PSS/HTL material/Au) under dark conditions. The SCLC hole mobilities of the HTL materials were obtained using the MottGurney square law, J = (9/8) ε0 εr µ (V2 / L3), where ε0 is vacuum permittivity, εr is the

SC

dielectric constant of the HTL material, µ is the charge carrier mobility, V is the effective applied voltage, and L is the thickness of the film. Steady-state photoluminescence (PL)

M AN U

spectra were measured using a Fluorolog3 PL spectrometer system with a monochromator (iHR320, HORIBA Scientific) excited at 530 nm. Fluorescence-lifetime imaging microscopy (FLIM) was performed using an inverted-type scanning confocal microscope (MicroTime200, Picoquant, Germany). As an excitation source, a single-mode 470-nm pulsed diode laser

TE D

with a pulse width of ≈ 100 ps and an average power of < 0.1 µW was used. A dichroic mirror (490 DCXR, AHF), a long-pass filter (HQ500lp, AHF), and a single photon avalanche diode (PDM series, MPD) were used to measure emissions from the samples, and time-correlated

EP

single-photon counting (TCSPC) was used to count the fluorescence photons. Current density-voltage curves were recorded using a Keithley 2400 source measurement unit and a

AC C

solar simulator equipped with a 180 W xenon lamp (Yamashida Denso, YSS-50A). A National Renewable Energy Laboratory-calibrated Si solar cell equipped with a KG-3 filter was used to adjust a light intensity to solar conditions (AM 1.5G and 100 mW cm-2). The active area of each cell was measured using an optical microscope camera (Moticam 1000). External quantum efficiencies (EQE) were obtained using a K3100 spectral EQE measurement system (McScience, Inc.).

ACCEPTED MANUSCRIPT 3. Results and discussion Fig. 1a shows the chemical structures of Spiro-OMeTAD, P3HT, and FEH. The FEH polymer composed of 3,3′-difluoro-2,2′-bithiophene and 3,4-di(2-ethylhexyl)thiophene was

RI PT

synthesized via a Stille coupling reaction with Pd(PPh3)4 as the catalyst [35]. Due to the bulky 2-ethylhexyl side chains, FEH shows good solubility in various solvents, including toluene, chloroform, and chlorobenzene, and thereby was able to be deposited onto the

SC

perovskite layer using a solution-based spin-coating process without the decomposition of the

M AN U

perovskite crystals [42].

The energy levels of the HTLs were obtained using ultraviolet photoelectron spectroscopy (UPS) (Fig. S1a). As shown by the energy level diagrams in Fig. 1b, the HOMO energy level of FEH (–5.1 eV) is deeper than those of Spiro-OMeTAD (–4.9 eV) and P3HT (–4.6 eV), which is attributed to the strong electron-withdrawing characteristics of

TE D

fluorine atoms on the conjugated backbone of FEH. From the previous studies, it is well known that a HOMO level close to the valence band maximum (VBM) of the perovskite

EP

leads to minimization of energy loss and a higher open circuit voltage (VOC) [18,36,43–45]. Considering the VBM of perovskite (−5.4 eV), the deeper HOMO energy level of FEH forms

AC C

a favorable energy level alignment with the perovskite and would be beneficial for obtaining PSCs with higher VOC values than those of devices based on P3HT and Spiro-OMeTAD. The LUMO energy levels of the HTL materials were calculated by adding the optical band gap to the HOMO energy level. Compared with the conduction band minimum of perovskite (−3.8 eV), all the HTLs showed higher LUMO levels, indicating that unwanted electron transfer from the perovskite to the HTLs is effectively blocked.

ACCEPTED MANUSCRIPT The hole mobilities of the HTL materials were investigated from dark J−V curves using the space charge limited current (SCLC) model (Fig. S1b). As summarized in Table 1, P3HT and FEH polymers show hole mobilities of 7.8 × 10−3 and 2.1 × 10−3 cm2 V−1 s−1, respectively.

RI PT

The small-molecule HTL material Spiro-OMeTAD shows a much lower hole mobility of 8.1 × 10−5 cm2 V−1 s−1 compared to those of the polymeric HTLs. This is because thiophenebased polymers exhibit strong intermolecular interactions between the polymer backbones

SC

and thus form a regular packing structure in the corresponding film, which is verified by the vibrational peaks in the UV-Vis absorption spectra of P3HT and FEH at approximately 598

M AN U

nm (Fig. S2). The strong vibronic shoulder peak of polythiophenes indicates enhanced interchain interactions between the polymer backbones [35]. In PSCs, p-type doping is wellknown to enhance the hole-transport properties of organic HTLs. Thus, we carried out p-type doping of HTL materials by adding the widely used LiTFSI and tBP to the HTL solution

TE D

[12,29,36]. When the electrical conductivities of the doped-HTL materials were calculated from the slopes of current-voltage plots by using Ohm’s law (Fig. S1c and Table 1), P3HT and FEH exhibit higher conductivities than that of Spiro-OMeTAD, indicating that the

EP

charge-transport properties of the polymeric HTLs are better than those of Spiro-OMeTAD.

AC C

To study the charge-extraction/-collection from the perovskite absorber to the HTLs, steady-state photoluminescence (PL) spectra were obtained. As shown in Fig. 2a, perovskite films in contact with FEH and Spiro-OMeTAD HTLs exhibit more efficient PL quenching compared with those with P3HT, indicating the more efficient hole-extraction of FEH and Spiro-OMeTAD than that of P3HT. To further understand the charge-extraction abilities of HTLs, time-resolved PL (TRPL) decay traces for perovskite/HTLs were obtained (Fig. 2b) and the decay curves were fitted using a tri-exponential decay function to extract the PL lifetimes. As summarized in Table S1, the pristine perovskite exhibits the long PL lifetime of

ACCEPTED MANUSCRIPT 90.8 ns, and upon the deposition of P3HT the PL lifetime decreases to 67.4 ns. The PL lifetimes for FEH- and Spiro-OMeTAD-coated perovskites are significantly decreased to 10.3 ns and 11.7 ns, respectively. The much faster PL decay for FEH compared to that of

RI PT

P3HT indicates that hole extraction from perovskite to FEH is more efficient than that from perovskite to P3HT.

More importantly, uniform and consistent PL quenching is observed in the confocal

SC

images of fluorescence-lifetime imaging microscopy (FLIM) for perovskite films coated with FEH and Spiro-OMeTAD, whereas inhomogeneous PL quenching behavior is observed for

M AN U

perovskite coated with P3HT (Fig. 2c). These different quenching homogeneities may originate from the different morphologies of the HTLs. Therefore, we performed atomic force microscopy (AFM) measurements to compare the morphologies of the HTLs deposited on the perovskite. As shown in Figs. S3 and S4, the perovskite layer prepared by the anti-solvent-

TE D

based method exhibits a very uniform and densely-packed surface with a root-mean-square (RMS) roughness of 5.5 nm. The P3HT film coated on the MAPbI3 layer shows the formation of a large fibrillar morphology and a rough surface with the high RMS roughness

EP

of 15.3 nm. In contrast, FEH forms finer fibrils than P3HT and has a smooth surface with a

AC C

low RMS roughness of 4.6 nm. The different morphologies of the two thiophene-based polymeric HTLs are thought result from the different fibril formations; the FEH polymer forms narrower fibrils than those formed by the P3HT polymer (Figure S3), which may be associated with the different alkyl side chains of the two thiophene-based polymers [35]. The smooth surface of FEH allows better contact at the perovskite/HTL interface and thus promotes more efficient and homogeneous charge extraction by the FEH HTL. Additionally, Fig. S3b showed that the Spiro-OMeTAD film contains nanoscale pinholes, which could be detrimental to the photovoltaic performance and stability of the corresponding PSCs, as

ACCEPTED MANUSCRIPT reported previously. [46,47] Meanwhile, pinholes are rarely observed in the FEH film owing to its excellent film-forming ability. The improved contact at the perovskite/HTL interface could lead to less charge recombination and, consequently, enhancement of Voc.

RI PT

The photovoltaic performances of PSCs fabricated with different HTLs were investigated using a conventional n-i-p device configuration of FTO/c-TiO2/mp-TiO2/CH3NH3PbI3 perovskite/HTL/Au (Figs. 3a and b). The typical thickness of Spiro-OMeTAD layers is in the

SC

hundreds-of-nanometers range, which is required for efficient charge separation and extraction by small-molecule-based HTLs. In contrast, thicknesses in the tens-of-nanometers

M AN U

range are sufficient for polymeric HTLs due to their superior film coverage. As shown in Fig. S5, the optimized thicknesses of the polymeric HTL and Spiro-OMeTAD films are ≈ 50 nm and ≈ 150 nm, respectively. Fig. 3c shows representative J–V curves of the PSCs measured by reverse scanning direction with a delay time of 200 ms for each voltage step and the

TE D

individual device parameters are summarized in Table 2. The P3HT-based device exhibits a low short-circuit current density (Jsc) of 17.7 mA cm−2, a fill factor (FF) of 66.9%, and a VOC of 0.91 V. The poor performances, especially the lower VOC compared to those of the other

EP

HTLs (Spiro-OMeTAD and FEH), originate from the large energy offset between the HOMO

AC C

level of P3HT and the VBM of perovskite as well as inefficient hole extraction. As shown in Fig. 3d, the PSC fabricated using P3HT exhibits a lower external quantum efficiency (EQE) over the whole visible range owing to severe charge recombination. The best-performing

-

device, i.e., that based on FEH, exhibits a PCE of 18.0% with a VOC of 1.06 V, a JSC of 22.8 mA cm–2, and an FF of 74.5%, while the Spiro-OMeTAD-based device exhibits a PCE of 17.0% with a VOC of 1.05 V, a JSC of 22.7 mA cm–2, and an FF of 71.2%. Compared to the Spiro-OMeTAD-based device, the PSC fabricated using FEH exhibits a better spectral response, especially at long wavelengths. The statistical distributions of photovoltaic

ACCEPTED MANUSCRIPT parameters for PSCs made with different HTLs are compared in Fig. S6. The superior photovoltaic performances and EQE of the FEH-based devices indicate that hole collection at the perovskite/HTL interface and hole transport in the HTL are more efficient for the FEH, as

RI PT

evidenced by the superior electrical properties and fast PL quenching at the perovskite/FEH interface. To corroborate the photovoltaic performances under real working condition, we measured and compared the stabilized photocurrent density and power output of the PSCs

SC

based on different HTLs at each maximum power point voltage under illumination (Fig. S7). The stabilized PCE values of P3HT, FEH, and Spiro-OMeTAD-based devices were 10.54%,

reverse scanned J-V curves.

M AN U

17.96%, and 16.7%, respectively, which are similar to the PCE values extracted from the

The long-term stabilities of the PSCs prepared using different HTLs were evaluated. The PSCs were stored without encapsulation in the dark under ambient atmosphere (50−60%

TE D

relative humidity at 25 °C) and photovoltaic performances of the devices were measured under ambient condition. As show in Fig. 4a, the PSC fabricated with FEH exhibits slow PCE degradation and retains more than 80% of its initial PCE over 500 h of exposure.

EP

However, compared with the FEH PSC, the devices fabricated with Spiro-OMeTAD and

AC C

P3HT have poorer device stabilities and their PCEs when exposed to ambient conditions decreased more rapidly. The water contact angles for each HTL were measured in order to characterize their hydrophobicities (Fig. 4b). All HTLs were doped with additives (LiTFSI and tBP). FEH film shows a larger contact angle (104°) of than those of Spiro-OMeTAD (74°) and P3HT (97°), as demonstrated in images of water droplets on the real devices (Fig. S8). This indicates that FEH forms a more hydrophobic surface than the other HTL materials, which may be attributed to the fluorination of the conjugated backbone in the FEH polymer. The hydrophobic surface of FEH has higher resistance against moisture penetration which

ACCEPTED MANUSCRIPT causes decomposition of the perovskite layer and could result in improved stability of PSCs.[48–50].

RI PT

4. Conclusions We employed the fluorinated polythiophene derivative FEH as an HTL for PSCs and investigated the optoelectronic and photovoltaic properties of the resultant device by

SC

comparing them with those of devices prepared using conventional P3HT and SpiroOMeTAD HTL materials. FEH has a deeper HOMO energy level than that of P3HT, which

M AN U

originates from the strong electron-withdrawing nature of the fluorine atoms on the conjugated backbone. The deeper HOMO level of the FEH HTL leads to favorable energy level alignment with perovskite, resulting in a reduced energy level offset at the holeextraction interface and thus a higher VOC compared to that achieved with a P3HT HTL. In addition, FEH forms a uniform film while the P3HT film has a rougher surface, resulting in

TE D

differences in the homogeneity of PL quenching behavior. Compared to Spiro-OMeTAD, the FEH HTL exhibits higher hole mobility and electrical conductivity, satisfying the

EP

requirements for both good hole extraction and transport capability, resulting in superior photovoltaic performances of FEH-based PSCs compared to those of Spiro-OMeTAD-based

AC C

PSCs. Accordingly, when FEH was employed as the HTL, a PCE of 18.0% was achieved, which is higher than the values obtained for control devices fabricated using Spiro-OMeTAD (17.0%) and P3HT (10.8%) and is one of the highest PCEs reported for PSCs containing polythiophene-based polymers as HTLs. More importantly, because of the hydrophobic characteristics of the fluorinated FEH polymer, PSCs fabricated using FEH show improved stability under air exposure compared to P3HT- and Spiro-OMeTAD-based devices. Thus, it may be concluded that, in order to realize highly efficient PSCs containing polymeric HTLs, the chemical structure of the polymeric HTL should be rationally modified considering its

ACCEPTED MANUSCRIPT electronic and morphological properties. We believe that this study will help to understand the development of HTL materials for highly efficient and highly stable PSCs, which potentially contributes to the realization of cost-effective PSC manufacturing based on

RI PT

encapsulation-free architecture and large area roll-to-roll production in atmosphere.

Acknowledgments

SC

This work was conducted under the framework of the Global Frontier R&D Program on Center for Multiscale Energy System Research, the Technology Development Program to Climate

Changes

(2017M1A2A2087353),

and

M AN U

Solve

Research

Program

(2018R1A2B2006708) funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Republic of Korea. This work is also supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of

TE D

Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20173010013200 and No. 2018201010636A), KIST institutional program (2E28300) and Research and Development

EP

Program of the Korea Institute of Energy Research (KIER) (B8-2425).

Appendix A. Supplementary data

AC C

Supplementary data related to this article can be found at http:// dx.doi.org/.

References [1]

Dong Q, Fang Y, Shao Y, Mulligan P, Qiu J, Cao L, et al. Electron-hole diffusion lengths > 175 µm in solution-grown CH3NH3PbI3 single crystals. Science 2015; 347: 967–70.

ACCEPTED MANUSCRIPT [2]

Xing G, Mathews N, Sun S, Lim SS, Lam YM, Grätzel M, et al. Long-range balanced electron- and hole-transport lengths in organic-inorganic CH3NH3PbI3. Science 2013; 342: 344–7. Stranks SD, Eperon GE, Grancini G, Menelaou C, Alcocer MJP, Leijtens T, et al.

RI PT

[3]

Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 2013; 342: 341–4.

Lin Q, Armin A, Nagiri RCR, Burn PL, Meredith P. Electro-optics of perovskite solar cells. Nat Photon 2015; 9: 106–12.

Miyata A, Mitioglu A, Plochocka P, Portugall O, Wang JTW, Stranks SD, et al. Direct

M AN U

[5]

SC

[4]

measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites. Nat Phys 2015; 11: 582–7. [6]

Brenner TM, Egger DA, Kronik L, Hodes G, Cahen D. Hybrid organic—inorganic

TE D

perovskites: low-cost semiconductors with intriguing charge-transport properties. Nat Rev Mater 2016; 1: 15007. [7]

Kojima A, Teshima K, Shirai Y, Miyasaka T. Organometal halide perovskites as

[8]

EP

visible-light sensitizers for photovoltaic cells. J Am Chem Soc 2009; 131: 6050–1. Im JH, Lee CR, Lee JW, Park SW, Park NG. 6.5% efficient perovskite quantum-dot-

[9]

AC C

sensitized solar cell. Nanoscale 2011; 3: 4088–93. Lee MM, Teuscher J, Miyasaka T, Murakami TN, Snaith HJ. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 2012; 338: 643–7.

[10] Kim HS, Lee CR, Im JH, Lee KB, Moehl T, Marchioro A, et al. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci Rep 2012; 2: 591.

ACCEPTED MANUSCRIPT [11] Jeong I, Jung H, Park M, Park JS, Son HJ, Joo J, et al. A tailored TiO2 electron selective layer for high-performance flexible perovskite solar cells via low temperature UV process. Nano Energy 2016; 28: 380–9.

RI PT

[12] Jeon NJ, Noh JH, Yang WS, Kim YC, Ryu S, Seo J, et al. Compositional engineering of perovskite materials for high-performance solar cells. Nature 2015; 517: 476–80. [13] Park YH, Jeong I, Bae S, Son HJ, Lee P, Lee J, et al. Inorganic rubidium cation as an

SC

enhancer for photovoltaic performance and moisture stability of HC(NH2)2PbI3 perovskite solar cells. Adv Funct Mater 2017; 27: 1605988.

M AN U

[14] Bi D, Tress W, Dar MI, Gao P, Luo J, Renevier C, et al. Efficient luminescent solar cells based on tailored mixed-cation perovskites. Sci Advances 2016; 2: e1501170. [15] National Renewable Energy Laboratory, Best Research-Cell Efficiencies chart; www.nrel.gov/ncpv/images/efficiency_chart.jpg.

TE D

[16] Schulz P, Edri E, Kirmayer S, Hodes G, Cahen D, Kahn A. Interface energetics in organo-metal halide perovskite-based photovoltaic cells. Energy Environ Sci 2014; 7: 1377–81.

EP

[17] Lim KG, Ahn S., Kim YH, Qi Y, Lee TW. Universal energy level tailoring of selforganized hole extraction layers in organic solar cells and organic–inorganic hybrid

AC C

perovskite solar cells. Energy Environ Sci 2016; 9: 932–9. [18] Jeon NJ, Na H, Jung EH, Yang TY, Lee YG, Kim G, et al. A fluorene-terminated holetransporting material for highly efficient and stable perovskite solar cells. Nature Energy 2018; 3: 682–9. [19] Bakr ZH, Wali Q, Fakharuddin A, Schmidt-Mende L, Brown TM, Jose R. Advances in hole transport materials engineering for stable and efficient perovskite solar cells. Nano Energy 2017; 34: 271–305.

ACCEPTED MANUSCRIPT [20] Calio L, Kazim S, Grätzel M, Ahmad S. Hole-transport materials for perovskite solar cells. Angew Chem Int Ed 2016; 55: 14522–45. [21] Bai Y, Meng X, Yang S. Interface engineering for highly efficient and stable planar p‐

RI PT

i‐n perovskite solar cells. Adv Energy Mater 2018; 8: 1701883. [22] Jo JW, Seo MS, Park M, Kim JY, Park JS, Han IK, et al. Improving performance and stability of flexible planar‐heterojunction perovskite solar cells using polymeric hole‐

SC

transport material. Adv Funct Mater 2016; 26: 4464–71.

[23] Wang Y, Yue Y, Yang X, Han L. Toward long‐term stable and highly efficient

M AN U

perovskite solar cells via effective charge transporting materials. Adv Energy Mater 2018; 8: 1800249.

[24] Jeong I, Park YH, Bae S, Park M, Jeong H, Lee P, et al. Solution-processed ultrathin TiO2 compact layer hybridized with mesoporous TiO2 for high-performance perovskite

TE D

solar cells. ACS Appl Mater Interfaces 2017; 9: 36865–74.

[25] Sun S, Buonassisi T, Correa-Baena JP, State‐of‐the‐art electron‐selective contacts in perovskite solar cells. Adv Mater Interfaces 2018; 1800408.

EP

[26] Giacomo FD, Razza S, Matteocci F, D’Epifanio A, Licoccia S, Brown TM, et al. High efficiency CH3NH3PbI(3−x)Clx perovskite solar cells with poly(3-hexylthiophene) hole

AC C

transport layer. J Power Sources 2014; 251: 152–6. [27] Nia NY, Matteocci F, Cina L, Carlo AD. High‐efficiency perovskite solar cell based on poly(3‐hexylthiophene): influence of molecular weight and mesoscopic scaffold layer. ChemSusChem 2017; 10: 3854–60. [28] Wang JY, Hsu FC, Huang JY, Wang L, Chen YF. Bifunctional polymer nanocomposites as hole-transport layers for efficient light harvesting: application to perovskite solar cells. ACS Appl Mater Interfaces 2015; 7: 27676–84.

ACCEPTED MANUSCRIPT [29] Heo JH, Im SH. CH3NH3PbI3/poly‐3‐hexylthiophen perovskite mesoscopic solar cells: performance enhancement by Li‐assisted hole conduction. Phys Status Solidi RRL 2014; 8: 816–21.

RI PT

[30] Guo Y, Liu C, Inoue K, Harano K, Tanaka H, Nakamura E. Enhancement in the efficiency of an organic–inorganic hybrid solar cell with a doped P3HT holetransporting layer on a void-free perovskite active layer. J Mater Chem A 2014; 2:

SC

13827–30.

[31] Ko S, Verploegen E, Hong S, Mondal R, Hoke ET, Toney MF, et al. 3,4-Disubstituted

Chem Soc 2011; 133: 16722–5.

M AN U

polyalkylthiophenes for high-performance thin-film transistors and photovoltaics. J Am

[32] Zhang M, Guo X, Ma W, Ade H, Hou J. A polythiophene derivative with superior properties for practical application in polymer solar cells. Adv Mater 2014; 26: 5880–5.

TE D

[33] Son SY, Kim Y, Lee J, Lee GY, Park WT, Noh YY, et al. High-field-fffect mobility of low-crystallinity conjugated polymers with localized aggregates. J Am Chem Soc 2016; 138: 8096–103.

EP

[34] Fei Z, Boufflet P, Wood S, Wade J, Moriarty J, Gann E, et al. Influence of backbone fluorination in regioregular poly(3-alkyl-4-fluoro)thiophenes. J Am Chem Soc 2015;

AC C

137: 6866–6879.

[35] Jo JW, Jung JW, Wang HW, Kim P, Russell TP, Jo WH. Fluorination of polythiophene derivatives for high performance organic photovoltaics. Chem Mater 2014; 26: 4214– 20.

[36] Kim Y, Jung EH, Kim G, Kim D, Kim BJ, Seo J. Sequentially fluorinated PTAA polymers for enhancing VOC of high‐performance perovskite solar cells. Adv Energy Mater 2018; 8: 1801668.

ACCEPTED MANUSCRIPT [37] Hwang I, Jeong I, Lee J, Ko MJ, Yong K. Enhancing stability of perovskite solar cells to moisture by the facile hydrophobic passivation. ACS Appl Mater Interfaces 2015; 7: 17330–6.

RI PT

[38] Kim G, Jang H, Yoon YJ, Jeong J, Park SI, Walker B, et al. Fluorine functionalized graphene nano platelets for highly stable inverted perovskite solar cells. Nano Lett 2017; 17: 6385–90.

SC

[39] Liu K, Dai S, Meng F, Shi J, Li Y, Wu J, et al. Fluorinated fused nonacyclic interfacial materials for efficient and stable perovskite solar cells. J Mater Chem A 2017; 5:

M AN U

21414–21.

[40] Bella F, Griffini G, Correa-Baena JP, Saracco G, Grätzel M, Hagfeldt A, et al. Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers. Science 2016; 354: 203–6.

TE D

[41] Bi D, Gao P, Scopelliti R, Oveisi E, Luo J, Grätzel M, et al. High‐performance perovskite solar cells with enhanced environmental stability based on amphiphile‐ modified CH3NH3PbI3. Adv Mater 2016; 28: 2910–5.

EP

[42] Lee KM, Lin CJ, Liou BY, Yu SM, Hsu CC, Suryanarayanan V, et al. Selection of antisolvent and optimization of dropping volume for the preparation of large area sub-

AC C

module perovskite solar cells. Solar Energy Mater Solar Cells 2017; 172: 368–75. [43] Edri E, Kirmayer S, Cahen D, Hodes G. High open-circuit voltage solar cells based on organic–inorganic lead bromide perovskite. J Phys Chem Lett 2013; 4: 897–902. [44] Azmi R, Lee UH, Wibowo FTA, Eom SH, Yoon SC, Jang SY, et al. Performance Improvement in low-temperature-processed perovskite solar cells by molecular engineering of porphyrin-based hole transport materials. ACS Appl Mater Interfaces 2018; 10: 35404–10.

ACCEPTED MANUSCRIPT [45] Kim H, Lim KG, Lee TW. Planar heterojunction organometal halide perovskite solar cells: roles of interfacial layers. Energy Environ Sci 2016; 9: 12–30. [46] Arora N, Dar MI, Hinderhofer A, Pellet N, Schreiber F, Zakeeruddin SM, et al.

greater than 20%. Science 2017; 358: 768–71.

RI PT

Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies

[47] Hawash Z, Ono L. K, Raga S. R, Lee M. V, Qi Y. Air-exposure induced dopant

SC

redistribution and energy level shifts in spin-coated Spiro-MeOTAD films. Chem. Mater 2015; 27: 562-9.

M AN U

[48] Reddy SS, Gunasekar K, Heo JH, Im SH, Kim CS, Kim D-H, et al. Highly efficient organic hole transporting materials for perovskite and organic solar cells with long‐ term stability. Adv Mater 2016; 28: 686–93.

[49] Zhang J, Xu B, Yang L, Ruan C, Wang L, Liu P, et al. The importance of pendant

TE D

groups on triphenylamine‐based hole transport materials for obtaining perovskite solar cells with over 20% efficiency. Adv Energy Mater 2017; 8: 1701209. [50] Kwon YS, Lim J, Yun H-J, Kim Y-H, Park T. A diketopyrrolopyrrole-containing hole

EP

transporting conjugated polymer for use in efficient stable organic–inorganic hybrid

AC C

solar cells based on a perovskite. Energy Environ Sci 2014; 7: 1454–60.

ACCEPTED MANUSCRIPT Table 1 Optical and electronic properties of the HTLs. HOMO [eV]

LUMO [eV]

Hole mobilitya [cm2 V−1 s−1]

Electrical conductivityb [S cm−1]

P3HT

1.9

−4.6

−2.7

7.8 × 10−3

1.2 × 10−3

FEH

1.9

−5.1

−3.2

2.1 × 10−3

9.7 × 10−4

Spiro-OMeTAD

2.9

−4.9

−2.0

8.1 × 10−5

a

RI PT

Eg [eV]

3.1 × 10−4

M AN U

SC

Obtained from dark J−V curve of HTLs without doping by using the space charge limited current (SCLC) model. b Calculated from the slopes of current–voltage plots of doped HTLs by using the Ohm’s law.

Table 2 Photovoltaic parameters of PSCs prepared with different HTLs. JSC [mAcm−2]

VOC [V]

FF [%]

PCE [%]

P3HT

18.7 ± 0.94 (17.7)

0.88 ± 0.03 (0.91)

60.6 ± 6.35 (66.9)

9.90 ± 0.86 (10.8)

FEH

21.7 ± 0.82 (22.8)

1.05 ± 0.01 (1.06)

74.5 ± 2.95 (74.5)

16.9 ± 0.89 (18.0)

1.04 ± 0.02 71.0 ± 2.24 21.3 ± 0.93a b (22.7) (1.05) (71.2) a The average values with a standard deviation calculated from 20 cells. b The values of the best devices.

15.8 ± 0.60 (17.0)

AC C

EP

Spiro-OMeTAD

TE D

HTL

RI PT

ACCEPTED MANUSCRIPT

SC

Fig. 1. (a) Chemical structures and (b) energy level diagrams of the HTLs, i.e., SpiroOMeTAD, P3HT, and FEH (energy levels for the other components of PSCs are also shown

AC C

EP

TE D

M AN U

for comparison).

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 2. (a) Steady-state PL spectra, (b) TRPL spectra, and (c) FLIM images of a bare perovskite film and perovskite films coated with different HTLs. Symbols and solid lines in

AC C

EP

TE D

Fig. 2b represent raw data and fitted data, respectively.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 3. (a) Schematic illustration and (b) a cross-sectional SEM image of a device with a typical mesoscopic n-i-p configuration. (c) J−V curves and (d) EQE spectra for PSCs

AC C

EP

TE D

prepared using different HTLs.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 4. (a) PCEs of PSCs prepared with three different HTLs as a function of exposure time to ambient atmosphere (50−60% relative humidity at 25 °C). (b) Water droplet contact angles

AC C

EP

TE D

for HTL films doped with additives.

ACCEPTED MANUSCRIPT

Supporting Information for

fluorinated

polythiophene

hole-transport

RI PT

A

material for efficient and stable perovskite solar

SC

cells

a

M AN U

Inyoung Jeong a,1, Jea Woong Jo b,1, Seunghwan Bae c,1, Hae Jung Son c,*, and Min Jae Ko d,** Photovoltaics Laboratory, Korea Institute of Energy Research (KIER), Daejeon 34129,

Republic of Korea. b

Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 04620,

c

TE D

Republic of Korea.

Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology,

Seoul 02792, Republic of Korea.

Department of Chemical Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-

EP

d

AC C

gu, Seoul 04763, South Korea.

* Corresponding author.

** Corresponding author.

E-mail: [email protected] (M. J. Ko), [email protected] (H. J. Son), 1

These authors equally contributed to this work.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

TE D

Fig. S1. (a) UPS spectra of Spiro-OMeTAD, P3HT, and FEH films. (b) Dark J−V characteristics of Spiro-OMeTAD, P3HT, and FEH with hole-transport layer only device. (c) Current–voltage plots of the HTL films (thickness = 200 nm) doped with additives (LiTFSI

AC C

EP

and tBP).

Fig. S2. UV−Vis Spectra of Spiro-OMeTAD, P3HT, and FEH films.

ACCEPTED MANUSCRIPT

Table S1 Time-resolved PL parameters fitted using tri-exponential decay function.a A1 τ1 A2 τ2 A3 τ3 Samples [%] [ns] [%] [ns] [%] [ns] 0.92

0.23

26

0.31

Perov/P3HT

0.54

9.0

0.38

34

0.08

134

67.4

Perov/FEH

0.5

1.21

0.28

5.88

0.22

14.3

10.3

Perov/Spiro

0.34

0.57

0.34

4.11

0.32

14.5

11.7

,

: base-line offset,

.

)/(

SC

Fitting function =

90.8

RI PT

0.46

: decay amplitude, τi: decay time.

AC C

EP

TE D

b

Glass/Perov

M AN U

a

104

τavgb [ns]

Fig. S3. AFM topography of (a) bare perovskite layer, (b) Spiro-OMeTAD, (c) P3HT, and (d) FEH coated on the perovskite layer, respectively. Red circles in Fig. S3b represent pinholes of Spiro-OMeTAD film.

SC

RI PT

ACCEPTED MANUSCRIPT

M AN U

Fig. S4. Top-view SEM images of perovskite layers covered with polymeric HTLs; (a) P3HT

AC C

EP

TE D

(uncovered part shows surface of perovskite layer) and (b) FEH, respectively.

Fig. S5. Cross-sectional SEM images of PSCs prepared using (a) FEH and (b) SpiroOMeTAD without Au electrode.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. S6. Statistical distribution of photovoltaic parameters for the PSCs based on different

AC C

EP

TE D

HTLs.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. S7. Stabilized photocurrent density and power output of (a) P3HT, (b) FEH, and (c) Spiro-OMeTAD based devices measured at each maximum power point voltage under continuous 1 sun illumination.

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

Fig. S8. Photographs of the real devices with a water droplet.

ACCEPTED MANUSCRIPT Highlights Fluorinated polythiophene derivative was used as hole-transport layer (HTL) of perovskite solar cells (PSCs)

RI PT

The fluorinated polymeric HTL shows efficient charge extraction and hydrophobic surface property. The PSCs based on the HTL exhibit a best efficiency of 18.0% and better stability compared to

AC C

EP

TE D

M AN U

SC

conventional Spiro-OMeTAD HTL.