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. 2020 Sep 1;5(36):23334–23342. doi: 10.1021/acsomega.0c03184

N-Substituted Phenothiazines as Environmentally Friendly Hole-Transporting Materials for Low-Cost and Highly Stable Halide Perovskite Solar Cells

Jagadish Salunke †,*, Xing Guo ‡, Maning Liu †, Zhenhua Lin ‡, Nuno R Candeias †,§, Arri Priimagi †, Jingjing Chang ‡,*, Paola Vivo †,*
PMCID: PMC7496006  PMID: 32954184

Abstract

graphic file with name ao0c03184_0007.jpg

Most of the high-performing halide perovskite solar cells (PSCs) leverage toxic chlorinated solvents (e.g., o-dichlorobenzene or chlorobenzene) for the hole-transporting material (HTM) processing and/or antisolvents in the perovskite film fabrication. To minimize the environmental and health-related hazards, it is highly desirable, yet at the same time demanding, to develop HTMs and perovskite deposition processes relying on nonhalogenated solvents. In this work, we designed two small molecules, AZO-III and AZO-IV, and synthesized them via simple and environmentally friendly Schiff base chemistry, by condensation of electron-donating triarylamine and phenothiazine moieties connected through an azomethine bridge. The molecules are implemented as HTMs in PSCs upon processing in a nonchlorinated (toluene) solvent, rendering their synthesis and film preparation eco-friendly. The enhancement in the power conversion efficiency (PCE) was achieved when switching from AZO-III (9.77%) to AZO-IV (11.62%), in which the thioethyl group is introduced in the 2-position of the phenothiazine ring. Additionally, unencapsulated PSCs based on AZO-III displayed excellent stabilities (75% of the initial PCEs is retained after 6 months of air exposure for AZO-III to be compared with a 48% decrease of the initial PCE for Spiro-OMeTAD-based devices). The outstanding stability and the extremely low production cost (AZO-III = 9.23 $/g and AZO-IV = 9.03 $/g), together with the environmentally friendly synthesis, purification, and processing, make these materials attractive candidates as HTMs for cost-effective, stable, and eco-friendly PSCs.

1. Introduction

Over the last decade, organic small molecules have received tremendous attention as hole-transporting materials (HTMs) in halide perovskite solar cells (PSCs) because of their reproducible synthesis, low-temperature solution processability, and large freedom of functionalization1−6 To date, 2,2,7,7-tetrakis(N,N-dipmethoxyphenylamine) 9,9-spirobifluorene (Spiro-OMeTAD) is by far the most commonly used small-molecular HTM in conventional n–i–p PSC structures. Spiro-OMeTAD-based devices exhibit impressive power conversion efficiencies (PCEs) in the range of 20–22%7−10 close to the existing record of 25.2%11 reached by PSCs. However, the multistep synthesis, difficult purification, high-cost (91.67 $/g, raw material cost), and poor hole-mobility of Spiro-OMeTAD limit its large-scale production.12,13 Furthermore, all the Spiro-OMeTAD-based high-performing devices require the addition of external dopants for enhancing the mediocre hole-mobility of pristine Spiro-OMeTAD. Nevertheless, because of their hygroscopic nature, these dopants cause moisture-induced degradation pathways in PSCs, thus reducing their long-term stability.14−17 Therefore, to overcome the limitations of Spiro-OMeTAD, various types of organic small molecular HTMs based on fluorene,2,18 carbazole,12,19 pyrene,20,21 and several others functional groups have been widely investigated.1,6

However, all the above-mentioned high-performing HTMs were either synthesized by using toxic and expensive palladium catalysts via multistep synthesis or were processed with hazardous halogenated solvents with severe acute toxicity, which can not only harm the environment but also hamper the large-scale production and commercialization of PSCs.3,22−26 Hence, when designing new HTMs, one should ideally account for a good solubility of the materials in nonhalogenated solvents, in addition to the other obvious characteristics such as well-aligned energy levels with the perovskite conduction/valence band (VB), high hole-mobility, and cost-effective facile synthesis with high overall conversion yields. There are very few reports regarding HTMs processed in nonhalogenated solvents.24−28 Nevertheless, in all the reported examples, the HTMs were mostly synthesized in modest overall yields via onerous multistep syntheses employing costly and toxic palladium catalysts and were purified via stringent and long procedures that limit their scalability.24−28 Thus, HTMs that combine green solvents processability with cost-effective and eco-friendly synthesis and purification are still lacking.

Because of their easy availability, chemical stability, low-cost, good solubility in common organic solvents, large freedom of functionalization, and excellent hole-mobility,29 phenothiazine-based small molecules have been extensively studied in organic electronics.30−32 Recently, they have also been considered as low-cost and effective HTMs for PSCs.33−37 In our previous work,37 we designed two phenothiazine-core, azomethine-based HTMs (AZO-I and AZO-II) via an environmentally friendly Schiff base chemistry approach, starting from low-cost precursors. Even though AZO-I and AZO-II provided a high-performing, stable, and low-cost alternative to Spiro-OMeTAD, their insolubility in nonhalogenated solvents limited their eco-friendliness as regarding their processing in the device fabrication.37

Here, we report on two new azomethine-based phenothiazines, AZO-III and AZO-IV, whose synthesis routes are presented in Scheme 1 and details are mentioned in Section 2 and in Supporting Information. Both were synthesized by using low-cost precursors and simple Schiff base chemistry with only water as a byproduct. Upon the utilization of AZO-III and AZO-IV in mixed perovskite (CsFAMA)-based devices, the thioethyl-substituted AZO-IV exhibited an improved PCE of 11.62% compared to that of AZO-III (9.77%). Additionally, the AZO-III-based PSCs provided stable power output after the unencapsulated devices were kept in ambient conditions (relative humidity, RH = 30%, at 25 °C) for 6 months. AZO-III and AZO-IV are the cheapest phenothiazine-core HTMs processed in nonhalogenated solvents (AZO-III = 9.23 $/g and AZO-IV = 9.03 $/g; in comparison, the raw material cost of conventional Spiro-OMeTAD is 91.67 $/g. See Table S1 in Supporting Information).

Scheme 1. Molecular Design and Synthetic Route for AZO-III and AZO-IV; Previously Reported AZO-I and AZO-II Molecules Are Also Added for Comparison; Scheme Adapted with Permission from ACS Appl. Energy Mater.2019,2, 3021–3027; Copyright 2019 American Chemical Society.

Scheme 1

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2. Results and Discussion

To ensure the solubility in nonhalogenated solvents while guaranteeing the maximum HTM loading capacity on the perovskite surface and at the same time minimizing the synthesis steps, we modified our previously reported structures (AZO-I, AZO-II, Scheme 1)37 by (i) making them smaller in size, (ii) introducing an aryl spacer between the phenothiazine and the azomethine bridge (AZO-III), and (iii) further adding a thioethyl (AZO-IV) group in the 2-position of the phenothiazine unit to enhance the solubility, and possibly also the HTM/perovskite interactions.2

Furthermore, the methoxy-substituted triarylamine group was introduced to the phenothiazine core to ensure good alignment between the highest occupied molecular orbital (HOMO) levels of the HTMs and the VB of the perovskite. These design principles combined good solubility (20 mg/mL) in toluene, reduction in cost/synthesis steps (in comparison, the cost of AZO-I and AZO-II was 9 and 12 $/g, respectively), and excellent synthesis yields of 92% (AZO-III) and 87% (AZO-IV). Further details on the cost analysis of HTMs calculated based on previously reported cost models34,38 and on the chemical characterization of the synthesized products are provided in Table S1 and Figures S1–S4 in Supporting Information.

The UV–vis absorption spectra of AZO-III and AZO-IV were measured in chloroform (1 × 10–5 M) and in thin films and presented in Table 1 and Figure S5 in Supporting Information. The absorption peaks (λmax) are at 409 and 412 nm for AZO-III and at 412 and 417 nm for AZO-IV in solution and film phases, respectively. These bands correspond to the intramolecular charge transfer from the electron-donating methoxy-substituted triarylamine to the azomethine unit(s). The slight increase in the λmax of AZO-IV compared to AZO-III is ascribed to the presence of the additional electron-rich thioethyl group in AZO-IV.39 The minor red-shift in the absorption maxima in the case of thin films for both compounds is attributed to intermolecular interactions in the solid state.40 The absorption onsets of AZO-III and AZO-IV in films are at 484 and 488 nm, and the corresponding optical bandgaps (Eg) are 2.56 eV and 2.54 eV, respectively. The steady-state photoluminescence (PL) spectra of AZO-III and AZO-IV show the peak positions at 565 nm and 567 nm for AZO-III and at 565 and 576 nm for AZO-IV in chloroform (1 × 10–5 M) and in thin films, respectively (Figure S5, Table 1).

Table 1. Optical and Electrochemical Characterization of AZO-III and AZO-IV.

  λmax Abs/Emi (nm)
        
HTMs DCM film HOMO (eV) LUMO (eV) bEg (eV) dEg (eV)
AZO-III 409/565 412/567 –4.97a, −4.93b –2.41c, −1.64b 3.29b 2.56d
AZO-IV 412/565 417/576 –4.99a, −4.91b –2.45c, −1.64b 3.27b 2.54d
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a

HOMO level determined by DPV.

b

Energy levels and bEg determined by NBO analysis.

c

LUMO level by using optical band gap.

d

Optical bandgap.

To determine the energy levels of the two HTMs, as well as that of the reference HTM Spiro-OMeTAD, differential pulse voltammograms have been recorded (see Figure S6) with the key results summarized in Table 1. Because the HOMO levels correspond to the ionization potentials of the materials, both AZO-III and AZO-IV display similar electrochemical properties, with their HOMO levels lying at −4.97 and −4.99 eV, respectively. The HOMO level of AZO-III is similar to that of the previously reported AZO-I because it possesses a similar core with the only difference being the position of the electron-donating methoxy-substituted triarylamine units.37 The HOMO level of Spiro-OMeTAD, as determined by differential pulse voltammetry (DPV) in identical conditions, is −4.82 eV. Because of the small difference between the HOMO levels of AZO-III (0.15 eV) or AZO-IV (0.17 eV) with respect to Spiro-OMeTAD, both new HTMs show high compatibility with the triple-cation halide perovskite (e.g., Cs0.05MA1–yFAyPbI3–xClx) employed in this work, whose VB lies at −5.9 eV.37 The corresponding energy level diagram, comprising all the materials used in our solar cell structures, is depicted in Figure 1a. Because no clear reduction peaks are detected in DPV curves, the lowest unoccupied molecular orbital (LUMO) levels are derived from the optical Eg and the corresponding values for AZO-III and AZO-IV are −2.41 eV and −2.45 eV, respectively (in Table 1).

Figure 1.

Figure 1

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(a) Energy level diagram for AZO-III, AZO-IV, and Spiro-OMeTAD, based on DPV experiments, and the reported values for ITO, SnO2, and Au [37]. (b) Optimized geometries, selected molecular orbitals, and frontier molecular orbitals energies of AZO-III and AZO-IV, calculated with DFT at PBE1PBE/6-31G** level of theory in DCM.

AZO-III and AZO-IV were further studied with density functional theory (DFT), at the PBE1PBE/6-31G** level, followed by time-dependent calculations (Figure 1b, Tables S2 and S3 in Supporting Information). For the computational analysis, the ethyl substituent in the thioether group of AZO-IV has been replaced by a methyl for simplicity, and the further details are described in Section 4.4. A comparison of the unoccupied frontier orbital energies calculated by DFT (−1.64 eV) and determined by using the optical band gap (−2.41 and −2.45 eV) clearly shows that the computational method used overestimates the energy of the orbitals. The absolute excitation energies are overestimated by the method by ca 0.6–0.9 eV, although the simulated energies of the occupied frontier orbitals (−4.91 to −4.93 eV) are close to the values determined experimentally (−4.97 to −4.99 eV) differing less than 0.1 eV. The electron density in the case of HOMO energy levels of both the compounds is located on the electron-donating triarylamine units, while the LUMO energy levels are spread over the electron-withdrawing azomethine linkage and the aryl spacer unit between phenothiazine and azomethine(s).41,42 Simulations of the vertical excitations of AZO-III and AZO-IV indicate that the first transition refers to the HOMO–LUMO transitions (Tables S2 and S3) through intramolecular charge transfer.

To investigate the hole extraction process at the perovskite|HTM interface, we carried out PL quenching experiments, as presented in Figure 2a. Upon deposition of AZO-III, AZO-IV, and Spiro-OMeTAD films on top of the perovskite layer, the steady-state PL spectra of the pristine perovskite layer on glass quench significantly, indicating that the holes can be effectively injected from the VB of the perovskite layer to the HOMO level of the HTMs. By comparing the amplitude of the PL spectra before and after HTM deposition, the quenching efficiency, that is, hole injection yield, can be calculated as 92.2, 94.6, and 95.8% for Spiro-OMeTAD, AZO-III, and AZO-IV, respectively. The improved hole injection yields for AZO-III and AZO-IV compared to the reference Spiro-OMeTAD can be attributed to the better energy level alignment because the HOMO levels of both AZO-III and AZO-IV are effectively deepened toward the VB of the perovskite, which favors an efficient hole extraction’.43 We also assign even the higher PL quenching efficiency of AZO-IV to the presence of additional sulfur atoms as compared to AZO-III, which has been previously suggested to enhance the adhesion and the interaction between the HTM and the perovskite surface.2,44

Figure 2.

Figure 2

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(a) Steady-state and (b) time-resolved PL spectra of Spiro-OMeTAD, AZO-III, and AZO-IV deposited on the perovskite surface, excited at 510 nm with an excitation intensity of 5.8 W cm–2.

The hole transfer dynamics have been further investigated with the perovskite excited state and the hole injection from the VB of the perovskite to the HOMO level of HTMs have been investigated with time-resolved PL (TR-PL) measurements (Figure 2b). All PL decays can be fitted well with a single-exponential function with two components, and the fitting results are summarized in Table S4 (Supporting Information). We assign the fast component to the trap-assisted recombination of charge carriers/excitons and to the hole injection dynamics from perovskite to HTMs and the slow component to the radiative recombination of free charge carriers/excitons in the bulk, respectively.45 The contribution factors (A1) for the fast components of all the HTMs are more than 95%, indicating that the fast component indeed dominates the charge recombination dynamics (Table S4 in Supporting Information).

To simplify the comparison of lifetimes, we here define an effective lifetime, that is, t1/e,46 expressed as I(t1/e) = I(0)/e, where I(t1/e) is the PL intensity at time t1/e. From the effective lifetimes for the PL decays of the perovskite films without (t1/e(perov)) and with HTMs (t1/e(HTM)), we can work out the hole injection yield (Φh-inj) using the following equation45,47

2. 1

Accordingly, the hole injection yields for spiro-OMeTAD, AZO-III, and AZO-IV are 86.0, 96.0, and 99.3%, respectively, which is consistent with the trend of quenching efficiency variations shown in Figure 2a. The remarkably accelerated PL decays for both AZO-III and AZO-IV compared to spiro-OMeTAD clearly demonstrate their improved hole-injection dynamics for the newHTMs.

To investigate the performance of AZO-III and AZO-IV in PSCs, we fabricated devices using mixed halide perovskite as light-harvester (complete structure: ITO/SnO2/Cs0.05MA1–yFAyPbI3–xClx/HTM/Au, see device fabrication details in Section 4.3). The AZO-III and AZO-IV films were first cast from toluene with a concentration of 60, 40, and 20 mg/mL. However, the corresponding PSCs displayed a very low JSC, probably due to the low conductivity of the HTMs. Hence, we focused on the optimized concentration of 10 mg/mL. In addition, both HTMs possessed good solubility in tetrahydrofuran (THF), but because of the poor film quality, we did not test the THF-casted films in solar cells.

Initially, all the HTMs were used to fabricate dopant-free PSCs, but because of poor PCEs (Table S5), conventional dopants (see details in Section 2.2) were added to the HTMs to improve the PCEs The reference Spiro-OMeTAD films were deposited from a chlorobenzene solution with a much higher concentration of 72.5 mg/mL (corresponding to the known optimized concentration for HTM),37 because its solubility in toluene is modest.2 It has been shown that the use of different solvents for Spiro-OMeTAD films has a minimal impact on the corresponding device performance.48 Importantly, the use of toluene in less concentrated (10 mg/mL) AZO-III and AZO-IV solutions, compared to that of the chlorobenzene (72.5 mg/mL) solution of Spiro-OMeTAD with higher concentration, removes the necessity of utilization of toxic-halogenated solvent and at the same time reduces the overall device fabrication costs.28

The current density–voltage (J–V) curves of each HTM’s champion PSCs under 1 sun simulated illumination (AM 1.5G, 100 mW/cm2) are presented in Figure 3a. The photovoltaic parameters extracted from the J–V characteristics are summarized in Table 2. In Table 2, both the results of the champion devices and the averaged photovoltaic parameters obtained from 20 independent devices are reported. The very low standard deviations demonstrate the high reproducibility of these experiments.

Figure 3.

Figure 3

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(a) Current density–voltage (J–V) curves measured with a scan rate of 10 mV/s in a backward scan, under AM 1.5 simulated solar light illumination, (b) EQE spectra, and (c) long-term stability of PSCs employing Spiro-OMeTAD, AZO-III, and AZO-IV as HTM.

Table 2. Photovoltaic Parameters of the Champion Devices Obtained from J–V Curves Based on Different HTMsa.

HTMs   JSC (mA/cm2) VOC (V) FF (%) PCE (%) R` (Ω·cm2) RSh (kΩ·cm2)
Spiro-OMeTAD champion 22.69 1.08 0.76 18.63 3.95 4.42
  average 22.78 ± 0.34 1.06 ± 0.01 0.76 ± 0.01 18.38 ± 0.14 3.98 ± 0.03 4.37 ± 0.04
AZO-III champion 19.75 1.01 0.49 9.77 15.63 0.68
  average 18.98 ± 0.81 1.01 ± 0.01 0.46 ± 0.02 8.75 ± 0.72 15.75 ± 0.12 0.65 ± 0.08
AZO-IV champion 19.83 1.01 0.58 11.62 5.23 0.74
  average 18.09 ± 1.62 1.01 ± 0.01 0.55 ± 0.04 10.04 ± 0.87 5.32 ± 0.09 0.69 ± 0.11
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a

The averaged parameters are also reported together with the standard deviations.

The performance gap for AZO-III/IV devices versus Spiro-OMeTAD cells is mainly caused by the lower short-circuit current (JSC) and fill factor (FF). Because both new HTMs show higher hole injection yields than that of spiro-OMeTAD, the decrease in JSC can be attributed to the poor charge transfer within AZO-III or AZO-IV, probably in relation to the low hole mobility, which has been reported for similar phenothiazine core azomethine compounds (e.g., AZO-I) in earlier studies.37,49−52 To clarify the reasons for the lower FF, we have extracted the series (RS) and shunt (RSh) resistance of the devices, according to the 1-diode model,53 from the J–V curves in a backward scan (Table 2). The control Spiro-OMeTAD devices show the lowest RS and highest RSh, thus leading to higher FF than those of AZO-III- and AZO-IV-based PSCs. It is interesting to observe a nearly three times larger RS of the AZO-III device compared to that of the AZO-IV device, indicating that the charge recombination can be effectively hindered in the case of AZO-IV. We further carried out the external quantum efficiency (EQE) measurements for all the champion devices (Figure 3b). The reference device shows the expected highest EQE value of 95.6% at around 520 nm, while the AZO-III and AZO-IV devices display the highest EQE values of 74.5 and 85.0%, respectively, which are consistent with their demonstrated lower JSC. This indicates that the internal quantum efficiency (IQE) of the Spiro-OMeTAD-based devices is much higher than those of AZO-based devices because the absorbance of all the cells at 520 nm is nearly identical, close to 1. This results in a much higher charge collection efficiency in the reference devices compared to those of AZO-based devices based on the equation of IQE = charge separation efficiency × charge collection efficiency.54 The higher charge collection efficiency in the reference devices further confirms that the bulk hole mobility for the doped Spiro-OMeTAD is higher than that of doped AZO-III or AZO-IV, mainly responsible for the poorer performance of the new HTM-based solar cells.

This again supports the enhancement of charge separation/collection upon the additional thioethyl attachment in the case of AZO-IV. Finally, we have monitored the stability of unencapsulated devices employing AZO-III, AZO-IV, and Spiro-OMeTAD (20 cells) upon storing them in air with relative humidity (RH) ∼ 30% over six months (Figure 3c). The AZO-III devices exhibited high stability and maintained 75% of their initial performance over 180 days (six-months) period, while AZO-IV and Spiro-OMeTAD cells retained 56 and 48% of the initial PCEs, respectively.

If we focus on the performance after the first two months of storage, AZO-III (PCE unchanged or, in other words, 100% of its initial PCE is retained) also outperforms the previously reported AZO-I and AZO-II, which retained 68 and 91% of the initial PCEs, respectively, under identical conditions and storage time.37 To understand the reason behind the superior stability of AZO-III and AZO-IV devices compared to that of Spiro-OMeTAD cells, we measured water contact angle (CA) of HTMs shown in Figure 4 to study their hydrophobicity because this plays an important role in protecting the PSC layer against moisture. Both AZO-III (86.6°) and AZO-IV (92.0°) films exhibited higher CAs compared to Spiro-OMeTAD (77.3°), showing a highly hydrophobic nature. Such enhanced hydrophobicity for both AZO-III and AZO-IV arises from the reduced number of hydrophilic methoxy groups compared to Spiro-OMeTAD (three methoxy groups in AZOs vs eight methoxy groups in Spiro-OMeTAD).54

Figure 4.

Figure 4

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Water CAs of (a) Spiro-OMeTAD_CAaverage = 77.3°, (b) AZO-III_CAaverage = 86.6°, and (c) AZO-IV_CAaverage = 92.0°.

Even though AZO-IV exhibited higher CA (92°) than AZO-III (86.2°), the lower stability of AZO-IV can be explained by its rapid oxidative degradation in the presence of p-type dopants caused by the strong charge localization in the radical cation state of AZO-IV (compared to AZO-III) because of additional thioalkyl substitution in 2-position, as shown in earlier reports.34,55

3. Conclusions

In summary, we have designed and synthesized two low-cost azomethine-functionalized phenothiazine-core HTMs, AZO-III and AZO-IV, via simple Schiff base chemistry with excellent synthesis yields. Upon the processing of these HTMs in nonhalogenated solvents, the champion AZO-IV-based corresponding device yielded a PCE of 11.62% with an inherently high hydrophobicity (water CA = 91.96°) and an extremely low production cost of 9.03 $/g. On the other hand, AZO-III HTM displayed excellent stability, retaining 75% of its initial performance after 6 months shelf-storage in 30% RH environment. Hence, despite the lower performance of AZO-III and AZO-IV (9.77 and 11.62%, respectively) cells compared to Spiro-OMeTAD-based reference device (18.63%), the significantly lower cost, good stability, and the environmentally friendly synthesis and processing make them rather competitive HTMs. Further optimization of these eco-friendly HTM designs may include the introduction of alkyl chains, as well as electron-rich units, in the phenothiazine core to ensure high hole-transportability, solution processability, and tunable energy levels.

4. Experimental Section

4.1. Synthesis

All the chemical precursors, namely, 4-iodoanisole, 4-nitroaniline, Cu powder, K2CO3, 18-Crown-6, Pd/C (10%), KOH, 1,10-phenanthroline, CuI, and phenothiazine, as well as the solvents, were purchased form Sigma-Aldrich and used without further purification. Compound 3 and 4 were synthesized according to previously reported methods.56,57 All the reactions were carried out under an inert environment in Schleck tube. 1H and 13C NMR spectra were recorded with 500 MHz JEOL spectrometer in CDCl3 against tetramethylsilane as reference. Mass spectroscopy was carried out using a high-resolution ESI-TOF LCT Premier XE mass spectrometer (Waters Corp.). The analyte was dissolved in chloroform/methanol (c ≈ 0.01 mg/mL). More details on the synthesis are in the Supporting Information.

4.2. Characterization

The steady-state absorption spectra were recorded with a Shimadzu UV-2501PC spectrophotometer both in solution and in thin films. The solid-state films of AZO-III and AZO-IV were deposited by spin-coating (WS-400B-6NPP/LITE, Laurell Technologies) from CHCl3 solution (1000 rpm, 1 min) onto clean glass substrates. Steady PL and TR-PL were measured by using the Pico Quant FluoTime 300 with a 510 nm picosecond pulsed laser. DPV measurements of the target HTMs and the reference Spiro-OMeTAD were performed by employing a potentiostat (Compact-Stat, Ivium Technologies) and a three-electrode cell configuration. Dry tetrabutyl ammonium tetrafluoroborate in dichloromethane (DCM) (0.1 M) was the supporting electrolyte, glass platinum was the electrode the working electrode, Pt wire was the the counter-electrode, Ag/AgCl wire was the the pseudo-reference electrode, and ferrocene/ferrocenium (Fc/Fc+) couple the internal standard reference to scale the measured potentials against the vacuum level.

4.3. Device Fabrication and Stability Test Measurements

All the devices were prepared by using mixed perovskite with ITO/SnO2/Cs0.05MA1–yFAyPbI3–xClx/HTM/Au (or Ag) device structure. The substrates were cleaned by detergent, deionized water, acetone, and isopropyl alcohol, successively, and then, the substrates were further cleaned by UV ozone treatment for 15 min. The SnO2 film as an ETM was prepared by spin-coating the SnO2 aqueous colloidal dispersion at 4000 rpm for 30 s followed by thermal annealing at 150 °C for 30 min. The Cs0.05MA1–yFAyPbI3–xClx film was deposited by a two-step method according to a previous report.58 1.36 M PbI2, 0.24 M PbCl2, and 0.08 M CsI were dissolved in the dimethylformamide (DMF) and stirred for 3 h at 70 °C. 70 mg of MAI and 30 mg of FAI were dissolved in 1 mL of IPA with 10 μL of DMF added. After that, around 70 μL of PbX2 precursor solution was spin-coated onto SnO2 or SnO2/LiF substrates at 3000 rpm for 45 s. Then, 200 μL of MAI/FAI solution was spin-coated onto the PbX2 at 3000 rpm for 45 s. Then, the samples were thermally annealed on a hot plate at 100 °C for 10 min. The spiro-OMeTAD film as an HTM was prepared by spin-coating 72.5 mg/mL solution on the perovskite layer at 4000 rpm for 45 s. AZO-III and AZO-IV were prepared by dissolving the powders in toluene or chlorobenzene with concentrations 20, 15, 10, and 5 mg/mL. Both the HTMs along with Spiro-OMeTAD were first used without dopants, but because of the poor performance of the corresponding PSCs, doped devices were fabricated by doping the HTMs with 17.5 μL of Li-TFSI (520 mg/mL in acetonitrile), 28.8 μL of FK209 (300 mg/mL in acetonitrile), and 28.8 μL of tbp. Furthermore, the films of AZO-III and AZO-IV were deposited on perovskite layers by spin-coating at 3000 rpm for 45 s. The devices were finished by thermally evaporated 100 nm Ag or Au. All the devices had an effective area of 7.5 mm2. All current density–voltage (J–V) curves were recorded using a Kethley 2400 source meter unit under simulated AM 1.5G illumination at an intensity of 100 mW/cm2 with an XES-70S1 solar simulator. The system was calibrated using a NREL-certified monocrystal Si photodiode detector before device testing. Steady PL and TR-PL were measured by using the Pico Quant FluoTime 300 with a 510 nm picosecond pulsed laser. The stability test is averaged based on 20 devices, and for both the HTMs, the chosen concentration was 10 mg/mL.

4.4. Computational Details

All calculations were carried out using the Gaussian 0959 software package without symmetry constraints. DFT60 and time-dependent DFT61 were used for computation of the ground state and vertical excitations (considering the six lower-lying transitions) of AZO-III and AZO-IV. Solvent effects (DCM) were considered in every calculation using the polarizable continuum model initially devised by Casida62 and further studied by Tomasi and co-workers63−65 as implemented on Gaussian 16, with radii and non-electrostatic terms for Truhlar and co-workers’ SMD solvation model.65 All calculations have been performed using the PBE1PBE functional and 6-31G(d, p)66−70 basis set. This functional uses a hybrid generalized gradient approximation, including 25% mixture of Hartree–Fock71 exchange with DFT exchange–correlation, given by Perdew, Burke, and Ernzerhof functional (PBE).72,73 Frequency calculations were performed to confirm the nature of the stationary points, yielding nonimaginary frequency for the optimized geometries. Natural population analysis74,75 was performed as implemented on Gaussian 09 to study the electronic structure of the optimized species.76−81

Acknowledgments

J.S. is grateful to the Fortum Foundation (201800260) and Finnish Foundation for Technology Promotion. A.P. is grateful to the Academy of Finland (Decision number 311142). P.V. acknowledges Jane & Aatos Erkko foundation (project ASPIRE) for financial support. This work is part of the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN, Decision number 320165).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03184.

  • Cost analysis of the newly designed materials and details on their synthesis and characterization (optical, electrochemical, and computational) (PDF)

Author Contributions

J.S. and X.G. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao0c03184_si_001.pdf (1.2MB, pdf)

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