Open Atmosphere Processed Stable Perovskite Solar Cells Using

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C: Energy Conversion and Storage; Energy and Charge Transport

Open Atmosphere Processed Stable Perovskite Solar Cells Using Molecular Engineered, Dopant-Free, Highly Hydrophobic Polymeric Hole Transporting Materials: Influence of Thiophene and Alkyl Chain on Power Conversion Efficiency Prem Jyoti Singh Rana, Rajendra Kumar Gunasekaran, Sung Heum Park, Vellaiappillai Tamilavan, Senthil Karuppanan, Hee-Je Kim, and Kandasamy Prabakar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11898 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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The Journal of Physical Chemistry

Open Atmosphere Processed Stable Perovskite Solar Cells Using Molecular Engineered, Dopant-Free, Highly Hydrophobic Polymeric Hole Transporting Materials: Influence of Thiophene and Alkyl Chain on Power Conversion Efficiency Prem Jyoti Singh Rana, †[a] Rajendra Kumar Gunasekaran, †[a] Sung Heum Park, [b] Vellaiappillai Tamilavan, [b] Senthil Karuppanan, [c] Hee-Je Kim, [a] Kandasamy Prabakar[a]

aDepartment

of Electrical Engineering, Pusan National University, 2 Busandaehak-ro 63beon-gil,

Geumjeong-gu, Busan-46241, Republic of Korea. bDepartment

of Physics, Pukyong National University, Busan 608-737, Republic of Korea.

cDepartment

of Physics, Bannari Amman Institute of Technology, Sathyamangalam 638 401,

Tamil Nadu, India. †These authors contribute equally to this work. Corresponding Author: [email protected]

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ABSTRACT:

Developing an efficient and stable perovskite solar cells (PSCs) in open atmosphere desperately require a robust hole transporting material (HTMs) with high hole conductivity and rich hydrophobicity. Here, we present two dopant-free, highly hydrophobic, donor-π-acceptor conducting polymeric HTMs by interconnecting three monomer units of 4,8‐bis(5‐(2‐ethylhexyl) thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene (BDTT), pyrrolo[3,4-c]pyrrole-1,3-dione (DPPD) and [3-ﬂuoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (TT) named as R1 and two monomer unit of [4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] (BDT) and TT named as R2. These two R1 and R2 HTMs integrated in PSCs exhibit an excellent photovoltaic performance of ~15.8% and ~13.5% at open atmospheric conditions respectively. This distinguished photovoltaic performance is strongly correlated with their hole mobility, solubility and energetic alignment with perovskite valence band. Briefly, the excess thiophene rings with extended alkyl chains in R1 brought significant impact on photovoltaic performance due to (i) Sbased heterocyclic thiophene strengthen the interaction with perovskite/HTM interface and increase its conductivity, (ii) retard recombination rate, (iii) high solubility helps to obtain uniform film coverage over perovskite, (iv) the HOMO level is well aligned with perovskite energy band and (v) increased hydrophobicity. Except the top gold metal contact, the complete PSCs device was fabricated in an open atmosphere from low-temperature solution process and these dopantfree HTMs paves the way for attaining stable and efficient PSCs towards potential applications


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Introduction Recently, the hybrid perovskite solar cells (PSCs) achieved the noticeable PCE over 23.3% in inert conditions, however, still unsatisfactory for practical applications due to the stability issues.1 Thus, making an effective and stable PSCs in open atmosphere would be the only promising way to advance the lab scale technology to potential applications in near future.2–4 However, the global perovskite photovoltaics researches are concentrating on improving the power conversion efficiency (PCE) of the devices made inside the glove box and hence the urgent concern is to fabricate the air stable solar cells without compromising the efficiency in order to make it viable for commercial applications.5–7 The open atmospheric fabrication of PSCs is still a challenging task and the moisture infiltration is the major issue that rapidly degrade the device performance. Generally, the PSCs composed of an electron transporting material (ETM),8,9 a perovskite absorber,10–12 a hole transporting material (HTM)13–15 and a metal contact.16–18 The photogenerated electrons and holes in perovskite absorber are separated by ETM and HTM layers based on its energy level positions, thus the current flow occurs in PSCs.19,20 In conventional architecture, the HTM serves as an active barrier to prevent the moisture ingression into the perovskite absorber and extract the positive charge carriers effectively to the external circuit.21,22 At

present,

the

2,2′,7,7′-tetrakis(N,N-di-p-ethoxyphenylamine)-9,9′-spirobifluorene

(spiro-OMeTAD) is mainly used as HTM which shows the PCE above 20%.23,24 However, the poor hole mobility (∼10−5 cm2V−1s−1) of spiro-OMeTAD require additives such as tertbutylpyridine (tBP), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to increase the hole mobility but high hygroscopic nature of these ionic dopants rapidly decompose the perovskite film.25–27 Moreover, the low doping of LiTFSI leads to the generation of deep Coulombic traps and immobile acceptor units that further reduce the spiro-OMeTAD hole mobility.28,29 The gold


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migration is also observed into the perovskite layer via spiro-MeOTAD HTMs which form AuI2 defects at the perovskite-HTM interface.30,31 Thus, it is urgently needed to develop a dopant free HTMs with high hole mobility and hydrophobicity by a simple synthetic route for efficient and stable PSCs fabrication in open atmosphere. So far, varieties of HTMs were developed and integrated in PSCs, which composed of organic,32–34 inorganic35,36 and polymer37,38 HTMs as an alternative to spiro-MeOTAD. Each HTMs are having few merits and demerits, thus, it is essential to find the relevant HTMs for governing the stable and efficient PSCs. Most of the conducting polymers having a shallow and mismatch energy states with the perovskite valence levels, resulting in the limited choice of polymers as HTM in PSCs. Therefore, in present the scientific society is mainly focused on the development of dopant free polymers with suitable over potential and intriguing properties.39,40 To date, the -bridge engineering is the unique strategy to make a conjugated polymer with strong interchain interactions, have offered a high hydrophobic property, easy processability and tunable energy levels. Based on this strategy, the molecular engineered Donor--Acceptor (D--A) type conjugated derivatives can be synthesized for tuning the polymer energy levels, conductivity and surface energy.41 Recently, D--A type derivatives were utilized as HTM in perovskite photovoltaics based on diketopyrrolopyrrole (DPP)--PDPPDBTE and achieved PCE of 9.2% in inert conditions.42 Further, the PSCs based on TTB–TTQ HTM achieved an inferior device efficiency of 6.2% and exhibited 14.1% with additives (LiTFSI and tBP) at inert atmosphere. Few reports are already available on D–π–A type polymeric HTMs, however, all of them were processed in glove box conditions.43–46 Herein, we present two D–π–A polymeric HTMs by crosslinking

different

monomeric

units

such

as

4,8‐bis(5‐(2‐ethylhexyl)

thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene (BDTT), [4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-


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b:4,5-b′]dithiophene-2,6-diyl] (BDT) as a donor moiety and pyrrolo[3,4-c]pyrrole-1,3-dione (DPPD) and [3-ﬂuoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (TT) as acceptor unit. The R1 polymer was obtained by interconnecting BDTT, DPPD, and TT units whereas the R2 by BDT and TT units. These derivatives employed as dopant free HTMs in PSCs yielded the PCE of 15.8 % and 13.5% for R1 and R2 respectively in open atmospheric conditions.

Experimental Section Device Fabrication The complete device fabrication was done in open atmosphere at room temperature 25C with relative humidity of 50%. The synthesis of two polymers were reported in elsewhere9. The complete device fabrication is given as follows: The UV/Ozone treated pre-patterned fluorinedoped tin oxide (FTO) glass substrates (Sigma-Aldrich, 10Ω/sq.) were used as a transparent electrode for device fabrication. First, the precursor solution of 15 mM titanium diisopropoxide bis(acetylacetonate) (Sigma-Aldrich, 75 wt% in isopropanol) in 1-butanol (Sigma-Aldrich, 99.8%) was drop casted on FTO at 2000 rpm for 20 sec to form a uniform TiO2 compact layer, followed by annealing at 500 C for 30 min in air. Mesoporous TiO2 paste (18NR-T) was diluted in absolute ethanol (1:3.5, w/w), and spin coated on the compact TiO2 layer at 2000 rpm for 40 sec, followed by heating at 500 C for 60 min. To improve the conductivity and cover the pin-holes, the substrates were further treated with 20 mM aqueous TiCl4 (Sigma-Aldrich, > 98%) solution at 90 C

for 10 min, cleaned with deionized water and then annealed at 500oC for 30 min. The perovskite

adduct solution was prepared by mixing CH3NH3I with stoichiometric amount (1:1 molar ratio) of PbI2 (Sigma-Aldrich, 99%) and DMSO (Sigma-Aldrich, 99.5%) in 0.6315 ml of DMF (SigmaAldrich, 99.8%) at room temperature. The well dissolved solution was filtered, and spin coated at 4000 rpm for 25 sec and the diethyl ether (500 µL) was slowly dripped onto the rotating substrate


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precisely at 10 sec. The obtained transparent film was heated at 65 C for 2 min and 100 C for 10 min to get uniform and shiny black perovskite film. Then, the as-synthesized HTMs were dissolved in chlorobenzene (20 mg/ml) and subsequently drop casted on the perovskite film at 2000 rpm for 40 sec, followed by annealing at 70 C for 20 min. Finally, Au electrode (100 nm) was coated by thermal evaporation under high vacuum at the rate of 0.03 nm/s.

Characterization Structural and Morphological Analysis The CH3NH3PbI3 films structure was analyzed by X-ray diffraction (XRD; Bruker D8-Advance) with Cu Kα radiation (λ = 1.540Å) source operated at 40 kV and 30 mA in the range 10−50. The surface morphology and cross-sectional image of the films was obtained from a field-emission scanning electron microscopy (FE-SEM) (Hitachi, model S-4200) operated at 15 kV,150 W. Surface and Chemical State Analysis X-ray photoelectron spectroscopy (XPS) was done using a Thermo Fisher Scientific (U.K) ESCALAB 250 system with monochromatic Al K radiation of 1486.6 eV and with an electron take-off angle of 45. The pressure of the chamber was kept at 10−10 Torr during measurement and to avoid the reconfiguration of the bonds the measurement was conducted without ion etching. The survey spectrum was scanned in the binding energy (BE) range 100 - 1200 eV in steps scan of 1 eV with spot size 500 µm. All the observed binding energies were compensated with the core level peak of adventitious carbon (C 1s) at 284 eV, kept as a reference. Peak fitting and quantitative analysis were performed using Casa XPS program (Casa Software Ltd) and the identified components BE was assigned from NIST X-ray Photoelectron Spectroscopy Database (NIST Standard Reference Database 20, Version 4.1).


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Optical Studies UV-visible spectroscopy was performed using an Optizen 3220 UV spectrophotometer. Timeresolved and steady-state photoluminescence measurements were conducted in the prepared samples: glass/perovskite or glass/perovskite/HTMs. Time-resolved PL (TRPL) study was performed using an inverted-type scanning confocal microscope (MicroTime-200, Picoquant, Germany). The measurements were performed at the Korea Basic Science Institute (KBSI), Daegu Center. A single-mode pulsed diode laser (470 nm with a ~30 ps pulse width and a ~0.1 μW average power) was used as an excitation source. A dichroic mirror (490 DCXR, AHF), a longpass filter (HQ500lp, AHF), a 700 nm long-pass filter, and a single photon avalanche diode (PDM series, MPD) were used to collect emissions from the samples. Time-correlated single-photon counting (TCSPC) technique was used to count emission photons. The TRPL images consisted of 200×200 pixels were recorded using the time-tagged time-resolved (TTTR) data acquisition method. Exponential fittings for the measured PL decays were performed using the Symphotime64 software (Ver. 2.2) by an exponential decay model;

𝐼(𝑡) =

∑

―𝑡

𝐴𝑖𝑒

𝜏

where I(t) is the time-dependent PL intensity, A is the amplitude, and τ is the PL lifetime. Ultraviolet Photoelectron Spectroscopy (UPS) analysis UPS test was performed by an advanced in-situ Surface Analysis System in Korea Basic Science Institute in Jeonju, Korea. Micro X-ray/UV spectrometer were equipped with a He I (21.2 eV) discharge lamp as the excitation source. The spectra from sputter-etched gold (Au) was used for reference to eliminate the possible instrumental bias during data acquisition. The spectrometer


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chamber kept at the base pressure of typically 8×10-11 torr while the evaporation chamber maintained at 1×10-6 torr. Photovoltaic measurements The current−voltage characteristics of the PSCs was performed under 1 sun illumination (AM 1.5G 100 mW cm−2) using ABET Technologies solar stimulator having the irradiance uniformity of ±3%. Contact angle measurements Water contact angles were measured by contact angle 101 measuring system (Plasma systems and materials at South Korea).

Results and discussion Figure 1 shows the molecular structure of the R1 and R2 polymer HTMs based on D–π–A architecture. The synthesized polymer shows highly solubility in organic solvent (chlorobenzene). Here, we have used three different monomer units to synthesize R1 polymer namely BDTT, DPPD and TT in 2:1:1 ratio. Polymer R2 consists two monomer units BDT and TT in 1:1 ratio. The detailed synthesis procedure of these polymers, and its thermal analysis (TGA), differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) are given elsewhere.47,48 The absorption spectra of the R1 and R2 polymer recorded in chloroform show the absorption maximum at 546 nm and 629 nm respectively as in shown in Figure 2 (a) and the relevant data is given in Table 1. The absorption in the visible region is mainly attributed to π–π* intramolecular electronic transition in both polymers.49,50 The thin film deposited on glass substrates show the absorption maximum at 552 nm and 690 nm for R1 and R2 polymer respectively due to interchain


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interaction and π–π stacking in the polymer. The optical band gap for R1 and R2 was calculated to be 1.60 eV and 1.60 eV from thin film absorption spectra respectively, shown Figure S1. Recombination losses are mainly attributed due to the imperfect energy band alignment between the perovskite absorber and HTM.51,52 Therefore, the two D–π–A conducting polymer HTMs were synthesized to optimize the minimal energy offset between the highest occupied molecular orbital (HOMO) of HTM with perovskite valence band. The HOMO and lowest unoccupied molecular orbital (LUMO) energy levels of R1 and R2 HTMs were determined from the valence band offset (Ei) and cutoff (Ecutoff) energy regions in the UPS measurements shown in Figure 2 (b). The full spectra and the energy level of polymer HTM are shown in Figure S2 and S3. Based on the equation φ = 21.2−(Ecutoff - Ef), the HOMO energy level was derived to be -5.42 eV and -5.37 eV for R1 and R2 respectively. Both HTMs have a deep and suitable HOMO level with perovskite valence band (-5.43 eV), might lead to the efficient hole transport from perovskite absorber to the metal contact. The LUMO levels were calculated to be -3.82 and -3.70 eV for R1 and R2 respectively, estimated by subtracting the HOMO level with the optical bandgap. The LUMO levels are significantly higher than conduction band energy level of perovskite (-3.93 eV) and hence could effectively prevent the back electron transfer from absorber to the metal electrode. The state-of-the-art Spiro-OMeTAD HTMs suffer with huge energy mismatch (-0.23 eV)26 results in high over potential between perovskite absorber and HTM, thus these HTMs are suitable alternative for effective charge extraction in PSCs. The X-ray diffraction spectrum of the perovskite show (110) and (220) peaks at 14.16 and 28.51 respectively for the solution processed perovskite film, indicating tetragonal phase with highly uniform crystal structure displayed in Figure 3 (a). No signature of PbI2 peaks found in our experiment confirmed the formation of stable perovskite by solution process in open atmosphere.


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Further, the high-resolution X-ray photoelectron spectroscopy (HRXPS) was studied to identify the surface binding states and chemical composition of the perovskite film processed in open atmosphere. Figure 3 (b) shows the HRXPS spectra (I 3d and Pb 4f doublets) fitted with an equal FWHM for each oxidation states and fixed intensity ratio of 2:3 (d3/2:d5/2) and 4:3 (f7/2: f5/2) respectively.53 A single oxidation state is fitted for I (3d5/2 and 3d3/2) and Pb 4f (4f7/2 and 4f5/2) that are separated by the spin-orbit splitting of 4.8 and 11.5 eV respectively. The I 3d and Pb 4f spectra depicts the presence of I3- and Pb2+ chemical states corresponding to their binding energy positions, indicating there are no impurity states in the perovskite film surface.30 The C 1s spectra for the perovskite film are shown in Figure S4. The morphology and cross-sectional FESEM images of the PSCs are shown in Figure 4 (a-c), and the device configuration FTO/compact-TiO2/mesoporous-TiO2/CH3NH3PbI3/R1 or R2/Au is shown in Figure 4 (d). For comparison, the devices were made without HTMs in open atmosphere with similar fabrication process. The solution processed perovskite film comprises a uniform and well-defined grains across the full surface without pinholes, which are favorable for efficient photovoltaic performance. The energy level of HTM with respect to perovskite is represented in Figure 4 (e). Both polymers have a rich solubility in chlorobenzene which are beneficial for complete coverage of polymer films over perovskite surface. The cross-sectional image indicates that the 345 nm dense perovskite layer deposited on mesoporous TiO2 layer (280 nm) and the polymer film thickness was found to be 95, 94 nm for R1 and R2 with same solution concentration (20 mg/ml) and spin speed (3000 rpm) respectively. Figure 5 (a) represents the absorption spectra of perovskite, Perovskite/R1 and Perovskite/R2 recorded by UV–Vis–NIR spectroscopy. The perovskite film shows the broad absorption up to near IR region with clear absorption onset at 796 nm. There is no significant change in the absorption onset of perovskite/HTM film was found, but


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the absorption intensity is higher than the pristine perovskite. To better understand the charge transport/recombination mechanism and the interface properties of perovskite/HTM, we carried out the photoluminescence (PL) and time-correlated single-photon counting (TCSPC) spectroscopy measurements. Figure 5 (b) shows the pristine perovskite film’s emission maximum at 780 nm due to the interband transition between electrons in the CB and the holes in the VB.54,55 The PL spectra of perovskite/R1 or R2 film was strongly quenched compared to pristine perovskite due to fast extraction of the holes across at perovskite/HTM interface. The PL of perovskite/R1 polymer was extensively quenched compared to R2 due to high hole mobility of R1, which enhance the hole transfer rate. Further, the recombination dynamics and charge transport kinetics of perovskite and perovskite/HTM interface was quantitatively estimated by TCSPC measurement with a 460 nm pulsed laser excitation. The PL lifetime decay of pristine perovskite and perovskite/HTM are shown in Figure 5 (c). The TCSPC data was fitted bi-exponentially indicating two different relaxation pathways (fast decay and slow decay) in perovskite. The fast decay is related to quenching of free carrier in perovskite film through effective holes extraction via polymer HTMs, whereas the slow decay is attributed to radiative relaxation of electron from the CB of the perovskite to the ground state.56–58 We observed the significant quenching in lifetime decay of perovskite/HTM samples in initial 2 seconds. Pristine perovskite film decay shows two components: long component of 14.51 ns was assigned to free carrier recombination in the radiative channel and slow components of 3.01 ns is related to bimolecular recombination in the perovskite layer. The average PL decay lifetime of perovskite/R1 and perovskite/R2 was decreased to 0.80 ns and 0.85 ns compared to pristine perovskite (10.27 ns) respectively. The fast decay life time (τ1) was decreased to 0.46 ns, and 0.48 ns for R1 and R2 respectively from 3.01 ns (HTM free) due to fast charge transfer by


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HTM.59 Life time decay data of Perovskite and Perovskite/HTM are summarized in Table S2. Notably, the R1 HTM extract the holes much faster than R2 due to presence of excess number of thiophene rings which make the strong interfacial interactions (Pb-S ions) between the perovskite/HTM interface. The fast lifetime decay is accomplished by the high hole conductivity and the near-Ohmic contact between perovskite/HTMs interface. Time-Resolved Confocal Fluorescence (TRCF) image elucidates the uniformity of perovskite film with different HTMs (R1 and R2) shown in Figure 5 (d-f). Pristine perovskite film shows the intense fluorescence and exhibit high lifetime decay whereas perovskite/R1 and R2 film, the lifetime and fluorescence were extensively reduced, supported by PL study. The photovoltaic performance of the PSCs with HTM films (R1 and R2) were studied by the current density-voltage (J-V) characteristics measured at 100 mW cm-2 illumination (AM 1.5G) in open atmosphere are displayed in Figure 6 (a). As a reference, the PSCs fabricated without HTM under open atmospheric conditions also investigated and their corresponding photovoltaic parameters are summarized in Table 2. The HTM free devices exhibited low PCE of 7.64%, with short-circuit current (Jsc) of 14.98 mA/cm2, open-circuit voltage (Voc) of 0.793 V and fill factor (FF) of 0.64 and showed a small hysteresis behavior in reverse bias. Conversely, the devices integrated with dopant free R1 and R2 HTMs showed substantial improvement in the photovoltaic performance, yielded a highest PCE of 15.82% and 13.35% in forward and 15.88% and 13.57% in reverse bias, respectively. Notably, the R1 device produced a higher PCE than R2 which is directly correlated with the hole conductivity, solubility and energy offset of the respective HTMs. Briefly, the R1 possess higher Voc (0.98 V) attributed to the suitable and deep HOMO energy level (-5.42 eV) with perovskite valence band which minimize the voltage loss. In R2 device, the small number of thiophene rings create a shallow HOMO levels (-5.37 eV) cause large energy offset


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with perovskite valence band, ensuing the photovoltage loss at the perovskite/HTM interface. Eventually, the Voc of R2 device is reduced to 0.87 V due to its increased overpotential. Furthermore, the devices with HTMs demonstrated very low hysteresis in comparison with HTM free device, suggesting that the rich hole conductivity of R1 and R2 reduce the charge accumulation at the interface, thus minimizing the hysteresis. However, the R1 device achieved the highest Jsc of 23.48 mA/cm2 compared to R2 due to the following reasons: (i). The suitable over potential of R1 with perovskite valence band facilitates high driving force for effective hole transport from perovskite to the metal contact. Hence, the recombination losses related to the energy band offset between absorber and their respective HTM is negligible. (ii). The long alkyl chains with S-based hectocycles in R1 enhance the Pb-S interaction which is collectively increasing the hole mobility and charge extraction ability. (iii). From PL and TRPL measurements, it is confirmed that the R1 is effectively quenching the charge carriers, thus exhibited a very short decay time. The fast quenching is directly correlated with the high hole transport ability of R1 than R2, leading to the difference in Jsc value. (iv) The minimum grain boundaries with larger grain size of perovskite might also help to reduce the surface recombination, leading to high Voc and Jsc. Therefore, we conclude that the remarkably high photovoltaic performance of R1 devices is mainly associated by its preferential charge extraction and energetic alignment which is strongly correlated to its high conductivity. Our dopant free polymer HTM devices exhibit much better performances compared to doped HTM’s used perovskite devices fabricated in open atmosphere and a comparison is given in the Table S3. However, a fair comparison is valid only for those devices made with same active area.


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Electrochemical impedance spectroscopy (EIS) measurements were performed to study the charge transport properties and the recombination process at perovskite/HTM interface.60,61 Figure 6 (b) shows the Nyquist plots of the perovskite device with and without HTMs under illumination. The obtained spectra were fitted with a single RC equivalent circuit (inset in Figure 6 (b)) model to estimate the recombination resistance (Rrec) and series resistance (Rs) summarized in Table S1. The Rs of all the devices is found to be less than 25 Ω, indicating the low defect density and high crystallinity of the perovskite film. Each device exhibits distinguished charge recombination lifetime (τrec) according to the HTM conductivity and energy level alignment with perovskite. The τrec is determined from the equation (τrec = RrecCrec), calculated to be 9.305 µs, 7.299 µs and 5.238 µs for R1, R2 and HTM free devices respectively. Amongst, the R1 device exhibit much longer lifetime due to high hole conductivity and minimal deep trap states. Further, we carried out the water contact angle measurements to identify the hydrophobicity of these HTM in perovskite devices.62 The water contact angle of perovskite and perovskite with HTM (R1 and R2) films were found to be 63.8, 103.1 and 99.5 respectively shown in Figure 6(c). The high hydrophobic nature of the HTMs prevent the moisture penetration into the hydrophilic perovskite layer, thus resulting prolonged stability in open atmosphere. The change in water contact angle in the time scale of 1, 2 and 5 minutes for the perovskite containing R1 and R2 HTMs at ambient atmosphere is shown in Figure S5. The contact angle of perovskite was decreased from 103.1 to 93.8 for R1 and 99.5 to 76 for R2. The R1 HTM exhibits a minor change in the contact angle with steady shape of water droplet up to 5 minutes, whereas a drastic change in the contact angle is observed in R2 HTM. Still, both HTMs possess good stability against moisture by holding hydrophobic nature in open atmospheric conditions. Overall, the R1 HTM endow superior moisture stability to protect the PSCs.


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We also performed the stability test of the PSCs by storing in open atmosphere with a humidity of 50 RH% without encapsulation. Figure 7 shows that the R1 devices exhibited superior photovoltaic performance with negligible changes up to the aging time of 30000 minutes due to its high hydrophobic nature. In contrast, the device with R2 HTM showed partial decrease in its original performance after 30000 minutes, while the HTM free device highly decomposed leading to an uneven device performance. Thus, we can conclude that the R1 and R2 device exhibit longterm stability due to its high-water repellent nature while the HTM free devices rapidly decomposed due to the ingression of water molecules into it. The thermal stability data of the PSCs was measured at a constant temperature of 70C with different time interval as shown in Figure S6. The PCE was found to be increased initially for 10 minutes for both R1 and R2 based devices may be due to the improvement in HTMs crystallinity, whereas the drastic PCE drop was observed in HTM free device. However, the device performance was gradually degraded with prolonged heating time at 70C. Though, the HTM free devices were degraded completely within two hours of heating at 70C, the R1 and R2 HTM protected PSCs exhibited the efficiency over 10 %. These results demonstrate that these polymer HTMs films serve as an excellent moisture barrier with high hole conductivity, resulting for the stable and efficient perovskite devices in open atmosphere. Figure 8 shows the device statistics (open circuit voltage, short circuit current, fill factor and PCE) of 20 control cells for HTM free, dopant free R1 and R2 polymers. We have also provided the 20 devices power conversion efficiency data of HTM free, dopant free R1 and R2 polymer device in open atmosphere in Table S4, S5 and S6 in ESI. The average power conversion efficiency of HTM free, dopant free R1 and R2 polymer device in open atmosphere are 6.62%, 15.07% and 12.87 % respectively. In conclusion, the cost-effective synthesis and outstanding


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photovoltaic performance with excellent stability would make these HTMs as an effective alternative to the conventional HTMs for making stable devices in open atmosphere.

Conclusions We have successfully fabricated the PSCs in open atmosphere by the support of dopant-free polymeric HTMs (R1 and R2) in low temperature solution process. The extended conjugations of excess alkyl chains and thiophene rings of these HTMs facilitate a substantial impact on device performance due to (a) High solubility provides an excellent and uniform film coverage over the perovskite (b) Perfect energy band alignment increase the charge extraction ability and affords high open circuit voltage. (c) long alkyl chains with S-based hectocycles strengthen the Pb-S interaction at perovskite/HTM interface and retard recombination rate (d) High water contact angle enhances the device lifetime in open atmosphere, (e) High hole mobility and good ohmic contact between perovskite/HTMs interface supports the efficient extraction of charge carriers. The excellent PCEs of 15.82% and 13.35% was obtained for PSCs using dopant-free HTMs showed minimal hysteresis without any encapsulation in open atmosphere. The present results demonstrate that these dopant free and hydrophobic HTMs would be the promising group of materials to develop an efficient and stable PSCs in open atmosphere. Supplementary material Thin film absorption studies of HTMs, Full UPS spectra and its energy level diagram, core-level spectra of C 1s, changing trend of water contact angles, thermal stability data of PSCs and Efficiency comparison of the perovskite devices fabricated in open atmosphere.


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Conflicts of interest There are no conflicts to declare. Acknowledgements This research was supported by Basic Research Laboratory through the National Research Foundations of Korea funded by the Ministry of Science, ICT and Future Planning (NRF2015R1A4A1041584). The authors would like to thank Mr. Jin-Soo Bak and Mr. In-Ho Cho for assisting to access the characterizations.

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(R2)

(R1) Figure 1. Molecular structures of polymer HTMs


26

(a)

Wavelength (nm) Intensity (Counts/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Normalized Absorbance

Page 27 of 35

(b) 1.03

R1

2.63 R2

Binding Energy (eV) Figure 2. (a) Absorbance of R1 and R2 Polymer in chloroform solution (b) Evalence and cut-off regions in UPS spectra of polymer HTMs (R1 and R2).


27


(a)

Intensity (a.u.)

XRD

2

HRXPS

(b)

Intensity (Counts/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

Binding Energy (eV) Figure 3. (a) Diffraction spectra of polycrystalline perovskite film (b) HRXPS spectra of I 3d and Pb4f chemical states with spinorbital splitting of 4.8 and 11.5 eV.


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(a)

(d)

Au

Au Polymer HTM m-TiO2/Perovskite

Perovskite (bl/mp TiO2) FTO Glass

C-TiO2

(b)

Glass+FTO

(e)

(c)

Au HTM

Perovskite (bl/mp TiO2) FTO Glass

Figure 4. SEM morphological and cross-sectional images of (a) pristine perovskite, (b) R1, (c) R2. (d, e) Pictorial representation of the fabricated device and its corresponding energy level diagram. The energy level values were extracted from UPS analysis.


29


(a)

(c)

(b)

Wavelength (nm)

Glass/Perovskite

(d)

Counts

Intensity

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

Wavelength (nm)

Glass/Perovskite/R1 (e)

Time (ns)

Glass/Perovskite/R2 (f)

Figure 5. Steady state (a) Absorption (b) Photoluminescence spectra of pristine perovskite with different HTMs. (c) Life-time decay kinetics of perovskite with different HTMs, whereas solid lines show the biexponential fitting of decay curve (d, e, f) timeresolved confocal fluorescence image of different HTMs over perovskite film.


30

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(a)

(b)

Rrec Rs

Crec

-Z”(Ω)

Current Density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Z’(Ω)

Voltage (V)

(c)

HTM Free (d)

63.8

103.1

R1 (e)

99.5 

R2

Figure 6. (a) Current density versus voltage (J–V) characteristics of perovskite solar cells incorporated with different HTMs scanned in forward and reverse direction under 1.5 AM G (100 mW cm−2). (b) Nyquist plots (C) Water contact angle for perovskite, with different polymer HTM.


31


(b)

Voc (V)

Jsc (mA/cm2)

(a)

(c)

(d) PCE (%)

Fill Factor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

Time (minutes)

Time (minutes)

Figure 7. Stability test of the fabricated device with two HTMs (R1 and R2) and HTM free devices was considered as reference.


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Figure 8. Statistics of 20 devices (HTM free, R1 and R2) at open atmospheric conditions (RH 50% and 25C).


33


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Table 1. Summary of photophysical and energy level parameters of different HTMs (R1 and R2)

(nm)b

Eg (eV)c

Hole Mobility (cm2V-1S-1)d

Evalence e

High B.E (eV)f

Work Function (eV)g

HOMO (eV)h

LUMO (eV)i

546

552

1.60

3.7×10-3

1.03

16.80

4.39

-5.42

-3.82

629

690

1.60

5.8×10-4

2.63

18. 53

2.67

-5.37

-3.70

HTMs

 𝒂𝒃𝒔 𝒎𝒂𝒙soln.

 𝒂𝒃𝒔 𝒎𝒂𝒙fil.

(nm)a

R1 R2

aAbsorption

maxima of the HTMs in chloroform solution bThin film absorption maxima of the HTMs cBandgap

obtained from the onset wavelength of the optical absorption in a thin film dmobility ederived from valence band, fderived

from UPS spectra energy cutoff gWork function was calculated by the equation φ = 21.2−(Ecutoff - Ef), hHOMO

level derived from equation : HOMO = Evalence + WF IThe LUMO level of the polymer was estimated via following equation: LUMO = HOMO – Eg.

Table 2. Summary of PSC device parameters incorporated with different HTMs measured under AM 1.5 G (100 mW cm-2).

Polymer name

Bias

Voc

Jsc

FF

Efficiency ( )

Reverse

0.79

14.40

0.670

7.69

Forward

0.79

14.98

0.643

7.64

Reverse

0.98

23.70

0.683

15.88

Forward

0.98

23.48

0.687

15.82

Reverse

0.88

23.10

0.662

13.57

Forward

0.87

23.47

0.648

13.35

HTM Free

R1

R2


34

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R2

Au Polymer HTM m-TiO2/Perovskite C-TiO2 Glass+FTO

R1


35

Open Atmosphere Processed Stable Perovskite Solar Cells Using

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