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Dopant-Free Tetrakis-triphenylamine Hole Transporting Material for Efficient Tin-Based Perovskite Solar Cells Weijun Ke, Pragya Priyanka, Sureshraju Vegiraju, Constantinos C. Stoumpos, Ioannis Spanopoulos, Chan Myae Myae Soe, Tobin J. Marks, Ming-Chou Chen, and M. G. Kanatzidis J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10898 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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Journal of the American Chemical Society

Dopant-Free Tetrakis-triphenylamine Hole Transporting Material for Efficient Tin-Based Perovskite Solar Cells Weijun Ke,†# Pragya Priyanka,‡# Sureshraju Vegiraju,‡ Constantinos C. Stoumpos,† Ioannis Spanopoulos,† Chan Myae Myae Soe,† Tobin J. Marks,*,† Ming-Chou Chen,*,‡ and Mercouri G. Kanatzidis*,† † ‡

Department of Chemistry, Northwestern University, Evanston, IL 60208, USA Department of Chemistry, National Central University, Taoyuan, Taiwan

ABSTRACT Developing dopant-free hole transporting layers (HTLs) are critical in achieving high-performance and robust state-of-the-art perovskite photovoltaics, especially for the air-sensitive tin-based perovskite systems. The commonly used HTLs require hygroscopic dopants and additives for optimal performance that adds extra cost to manufacturing and limits long-term device stability. Here we demonstrate the use of a novel tetrakis-triphenylamine (TPE) small molecule prepared by a facile synthetic route as a superior dopant-free hole HTL for lead-free tin-based perovskite solar cells. The best-performing tin iodide perovskite cells employing the novel mixed-cation ethylenediammonium/formamidinium with the dopant-free TPE HTL achieve a power conversion efficiency as high as 7.23%, ascribed to the HTL’s suitable band alignment and excellent hole extraction/collection properties. This efficiency is one of the highest reported so far for tin halide perovskite systems, highlighting potential application of TPE HTL material in low-cost high-performance tin-based perovskite solar cells.

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INTRODUCTION Organic-inorganic halide perovskites have attracted widespread attention in the optoelectronics science and technology community because of their suitable band gaps, long carrier diffusion lengths, and high absorption coefficients.1-5 The power conversion efficiency (PCE) of lead-based perovskite solar cells has skyrocketed to ~22.1% in few years.6-13 Tin-based perovskites have also attracted great attention due to their similar optical and electric properties, and especially lower toxicity compared with lead-based perovskites.3,14-20 However, the tin-based perovskites are more air-sensitive and require special handling to control the oxidation from Sn2+ to Sn4+, which results in uncontrollable p-type doping, rendering the tin-based perovskites too conductive, and compromising device performance.3 High-performance perovskite solar cells typically have a sandwich structure, consisting of a perovskite absorber between an electron transporting layer (ETL) and a hole transporting layer (HTL).21 These charge injection/extraction layers play key roles in promoting the transport and extraction of the photogenerated charge carriers as well as suppressing charge recombination at the interfaces, thus enhancing the device performance.22-25 While metal oxides such as TiO2 and SnO2 are typically used as ETLs,26 the most effective organic

HTLs

have

been

limited

to

triarylamine

derivatives,

such

as

2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA),27 for the perovskite solar cells with regular structures. However, these organic HTLs require complicated multistep synthesis and are typically expensive.28 Additionally, 2


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they suffer from low hole mobility and low conductivity,29 and for optimal device performance, additives and dopants such as tert-butylpyridine (t-BP), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, and cobalt complexes must be added to the HTL to increase their hole conductivity.29 These additives add extra cost to the manufacturing, and more importantly, their deliquescent behavior and the necessary pre-oxidization step of the HTL with atmospheric O2 can accelerate the degradation of perovskite films, especially for tin-based perovskites.29 Therefore, developing new dopant-free and low-cost HTLs for tin-based perovskite solar cells represents a challenging and compelling issue. An excellent HTL candidate should effectively transport holes and block electrons and exhibit a high hole mobility, with an energetically suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for efficient charge transfer.27,30 Many efforts have been devoted to finding a HTL substitute for the expensive spiro-OMeTAD and PTAA.27-39 New HTLs should ideally be synthesized by simple methods, be solution processable, and achieve high device performance in perovskite solar cells. Here, we report that a new type of branched small molecule organic, comprised of one tetraphenylethene core with four end-capped triphenylamine units (TPE), serves as an excellent HTL for tin-based perovskite

solar

cells.

The

best-performing

mixed-cation

ethylenediammonium/formamidinium tin iodide ({en}FASnI3) hollow perovskite solar cell employing the dopant-free tetrakis-triphenylamine (TPE) HTL achieves a champion PCE of 7.23% with an open-circuit voltage (Voc) of 459.91 mV, a 3



short-circuit current density (Jsc) of 22.54 mA cm-2, and a fill factor (FF) of 69.74%, comparable with those of the devices using the doped PTAA HTLs and superior to those of the devices using the doped spiro-based HTLs. The simplicity of the synthetic procedure, the intrinsic high HTL hole mobility, no need for dopants, and the possibility of excellent device performance make this new HTL very appealing for fabricating low-cost, efficient, and large-scale tin-based perovskite solar cells. EXPERIMENTAL DETAILS Synthesis of TPE (1)

Scheme 1. Synthetic route to TPE (1). Compound TPE (1) To a stirred solution of compound 2 (1.0 g, 0.65 mmol, see supporting information for detailed synthesis of 2) and Zn powder (0.17 g, 2.59 mmol) in anhydrous THF (40 mL) under a nitrogen atmosphere, TiCl4 (0.1 g, 0.52 mmol) was slowly added via syringe at 0°C. The reaction mixture was allowed to warm to room temperature and 4


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then heated to reflux for 12 h. The reaction mixture was quenched with saturated K2CO3 solution and extracted with CH2Cl2 (3 × 20 mL). The combined organic layers were dried over NaSO4, filtered, and concentrated. The crude material was purified by column chromatography (DCM/Hexanes) to obtain pure product 1 as yellow solid. (0.65 g, 67%). 1H NMR (500 MHz, DMSO-d6): δ 7.53 (d, J = 5.0 Hz, 2 H), 7.41 (d, J = 10 Hz, 6 H), 7.37-7.34 (m, 10 H), 7.02-6.99 (m, 22 H), 6.90-6.88 (m, 16 H), 6.75 (d, J = 5.0 Hz, 6 H), 6.73 (d, J = 10 Hz, 2 H), 3.72 (s, 24 H).

13

C NMR (125 MHz,

DMSO-d6): δ 156.24, 148.27, 142.08, 140.39, 138.13, 131.91, 131.22, 127.45, 127.15, 125.55, 119.89, 115.42, 55.69, 40.52. HRMS (m/z, FAB+) calcd for C106H88N4O8: 1544.6602, found 1545.6690. Device Fabrication The {en}FASnI3 precursor solution was prepared by adding 172 mg of FAI (Dyesol), 24 mg of SnF2 (99%, Sigma-Aldrich), 428 mg of home-made SnI2,3 and 6.4 µL of en ( ≥99%, Sigma-Aldrich) in a mixed solvent of 632.8 µL of N,N-dimethylformamide and 70.8 µL of dimethyl sulfoxide.40 The pristine spiro-OMeTAD (99.0%, Shenzhen Feiming Science and Technology Co., Ltd) and PTAA (99%, Sigma-Aldrich) solutions were prepared by adding 72.3 mg of spiro-OMeTAD and 20 mg of PTAA in 1 mL of chlorobenzene (CB), respectively. The doped spiro-OMeTAD solution was added extra 17.5 µL of Li-TFSI salt solution (500 mg of Li-TFSI dissolved in 1 mL of acetonitrile) and 28.8 µL of t-BP, whereas the doped PTAA solution was added extra 2.25 mg of TPFB (TCI America). The TPE solution was prepared by adding 5-30 mg of TPE in a mixed solvent of 0.5 mL of CB and 0.5 mL of chloroform (CF). 5



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The process of preparing compact and mesoporous TiO2 layers on FTO substrate followed the procedures in our previous work.41 The perovskite precursor solution was spin-coated on the mesoporous TiO2 coated substrate at 1500 rpm for 60 s by a one-step method without anti-solvent. After spin-coating, the film was annealed at 70°C for 5 min and at 120°C for 10 min in a sequential manner.40 The spiro-OMeTAD, PTAA, and TPE solutions were spin-coated on the perovskite films at 2000 rpm for 60 s, 1500 rpm for 30 s, and 2000 rpm for 30 s, respectively. To complete the devices, an 80-nm-thick Au electrode was thermally evaporated on top of the hole transporting layers through a metal mask. The active area of the solar cells was 0.09 or 0.039 cm2. All the devices were unencapsulated and measured in ambient air with a humidity of 10-40% and a temperature of 20-25°C. RESULTS AND DISCUSSION The synthetic route for the preparation of the tetrakis-triphenylamine TPE (1) is shown in Scheme 1, which involves commercially available low-cost raw materials in a two-step reaction. The TPE HTL molecule consisting one tetraphenylethene core connected to four end-capped triphenylamine units (Figure 1a). Triphenylamine functionalized conjugated systems have been reported with excellent hole transport capacity.36,42,43 The TPE molecule 1 was synthesized by the McMurray coupling of di-triphenylamine ketone 2 in presence of TiCl4 and Zn. The successful synthesis and purity of the TPE molecule was confirmed by the 1H NMR,

13

C NMR, and HMRS

spectroscopy, as shown in Figures S1-3. Note that ketone 2 was synthesized by Stille cross coupling of dibromobenzophenone 3 with stannylated dimethoxytriphenylamine 6


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4, as confirmed by 1H NMR, and 13C NMR spectroscopy in Figures S4-5. The overall yield and cost of TPE are around 34% and $30/g, respectively. In comparison, the traditional spiro-OMeTAD and PTAA HTLs normally require multistep synthetic processes and are expensive.27,30 The newly derived TPE compound dissolves easily in common organic solvents such as toluene, CB, CF, and tetrahydrofuran, allowing the formation of uniform thin films. The ultraviolet-visible (UV-vis) absorption spectrum of the TPE in o-C6H4Cl2 has a maximum absorption peak at ~350 nm and an optical gap of 2.77 eV, as shown in Figure 1b. To estimate the HOMO level of TPE, differential

pulse

voltammetry

(DPV)

measurements

were

employed

in

dichlorobenzene at 25°C, as shown in Figure 1c. The DPV of TPE exhibits an oxidation peak at +0.79 V (using ferrocene/ferrocenium as the internal standard at +0.6 V). The HOMO level of TPE is therefore estimated at −4.99 eV using the equation of EHOMO = −(4.20 + Eox) and assuming ferrocene/ferrocenium oxidation at +4.8 eV.44 Together with the optical energy gap, the calculated LUMO of TPE is estimated at –2.22 eV. The HOMO of TPE is slightly shallower than those of spiro-OMeTAD and PTAA (~-5.2-5.3 eV). It is well known that the valence band of tin-based perovskites is usually shallower than that of lead-based perovskites.41,45-47 Therefore, electron and hole transporting layers with shallower HOMO and LUMO levels will be more suitable for tin-based perovskite solar cells.41,47 Additionally, it is critical to evaluate thermal stability of TPE to ensure its compatibility with environmentally stable solar cells. Based on the thermogravimetric analysis (TGA) measurements (Figure S6a), TPE exhibits a high decomposition temperature of 7



~425 °C, confirming its good thermal stability. Differential scanning calorimetry (DSC) analysis (Figure S6b) illustrates that the compound is of amorphous nature and exhibits a glass phase transition temperature at 159 °C, which is higher than that of spiro-OMeTAD (125 °C).31 Based on the above attractive properties of TPE, we next investigated it as the HTL for the tin-based perovskite solar cells. Figure 1d shows the device structure, including a bottom FTO substrate, TiO2 ETL, a {en}FASnI3 perovskite absorber, and a top HTL covered with a gold electrode. Just as ETLs and HTLs, the perovskite absorber film quality is pivotal to ensure delivery of good device performance. Here, we used mixed-cation {en}FASnI3 hollow-perovskite films as the solar cell absorber due to its excellent optoelectronic properties and film-forming ability, as demonstrated in our previous work.40,48 The {en}FASnI3 films are crystalline and exhibit a strong absorption in the 300-880 nm wavelength range (Figure 2a, b). Figure 2c shows a top-view SEM image of the {en}FASnI3 perovskite film, prepared by a simple one-step spin-casting method and with 15% SnF2 addition.40 It can be seen that the resulting perovskite film is of high-quality with desirable smoothness and a pin-hole less morphology, both essential for preventing device shorting and reducing the charge recombination. The SEM images of the {en}FASnI3 perovskite films coated with spiro-OMeTAD, PTAA, and TPE films are also shown in Figure S7. Each HTL-coated film exhibits a different surface morphology, likely due to differences in film thickness and crystallinity arising from the application of different optimal HTL solution concentrations. 8


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To investigate the effect of different HTLs on the device performance, we fabricated solar cells using various HTLs. Here spiro-OMeTAD and PTAA HTLs are used as references to evaluate the TPE material for HTL application. The photocurrent density-voltage (J-V) plots of the solar cells using different HTLs are shown in Figure 3a. Due to the low conductivity and mobility of the spiro-OMeTAD and PTAA HTLs, dopants are required to partially oxidize the materials to increase their hole transporting properties. Using pristine spiro-OMeTAD and PTAA HTLs without any dopants and additives yields very poor device performance, producing low PCEs of 1.77% and 3.51%, respectively. These PCEs are only slightly greater than those of HTL-free devices which produce a PCE of 1.56% (Figure S8). Note that the device with the dopant-free spiro-OMeTAD HTL even gives a lower Jsc than that of the HTL-free device, suggesting the inferior performance of these common HTLs without dopants. To improve the HTL film conductivity and mobility, spiro-OMeTAD is doped with t-BP and Li-TFSI, which are widely used for Pb-based perovskite solar cells,7,11,49 while PTAA is doped with 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (TPFB) instead of t-BP and Li-TFSI salt as shown in our previous work.40,41,47 As a result, a significant increase in device performance is observed for both doped spiro-OMeTAD and PTAA HTLs, yielding PCEs of 5.20% and 6.67%, respectively. Despite these PCE enhancements, note that the Sn-based perovskite film is more sensitive when exposed to the t-BP and lutidine dopants and acetonitrile solvent of the Li-TFSI salt than the corresponding Pb perovskite films. Degradation of the Sn-based perovskite film during the spin-coating of the doped 9



spiro-OMeTAD solution with t-BP and acetonitrile is unavoidable even via very fast processing. In addition, the requirement of the oxidation process of the film for optimal performance of the doped spiro-OMeTAD in air cannot be carried out for the air-sensitive tin-based perovskite films. We find that devices using the new TPE HTL material can achieve outstanding performance even without any additives and dopants. As shown in Figure 3a, a high PCE of 7.02% with a Voc of 460.88 mV, a Jsc of 22.39 mA cm-2, and a FF of 68.01%, are obtained, which are comparable to those of the doped PTAA HTL devices and superior to the doped spiro-OMeTAD HTL devices. The photovoltaic parameters of the present devices using various HTLs are summarized in Table 1. Finding the optimal HTL thickness is also imperative for good device performance. If the HTL film is too thin, it cannot effectively block the electrons and the metal electrode will contact the perovskite film directly. If too thick, the high resistance of the HTL film will reduce the device FF and Jsc. To optimize the TPE HTL thickness, devices using various concentrations of TPE were fabricated. Figure S9 shows the J-V plots of the solar cells using the HTLs prepared with 5, 10, 15, 20, 30 mg/mL of TPE in a mixed solvent of CF and CB (1:1 in volume ratio), measured by reverse voltage scan. The associated photovoltaic parameters of the devices are summarized in Table S1. Upon increasing the TPE concentration, the device performance initially shows an increase and then a drop in PCE. The best performance is obtained from the device using the TPE with a concentration of 15 mg/mL whose thickness is around 80 nm. Additionally, we measured the EQE spectra of the devices using different HTLs. As 10


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shown in Figure 3b, the device using the dopant-free TPE HTL shows a higher average value than those using the doped spiro-OMeTAD and PTAA HTLs. The Jsc integrated from the EQE plots of the doped spiro-OMeTAD, doped PTAA, and undoped TPE devices are 20.0, 21.1, 21.7 mA cm-2, respectively, in good agreement with the trend of the Jsc obtained from the J-V plots. To understand the superior performance of the TPE devices, we compared the series resistance (Rs) of the devices using different HLTs, according to the diode eq. 1,23,50 -dV/dJ =AKBT(JSC-J)-1e-1 +RS

(1)

where T is the absolute temperature, KB is Boltzmann constant, and A is ideality factor. According to eq. (1), the Rs of the doped spiro-OMeTAD, doped PTAA, and undoped TPE devices are estimated to be 1.36, 0.65, 0.54 Ω cm2, respectively (Figure 3c). The device using the doped spiro-OMeTAD HTL has a higher Rs and lower Rsh consistent with the lower conductivity and mobility even after doping, resulting in a lower device FF and Jsc. The device using the dopant-free TPE HTL exhibits the lowest Rs and highest Rsh consistent with the superior extraction/collection properties, confirmed by hole mobility measurements. We evaluated the hole transporting properties of different HTLs using space-charge-limited-current methods.51,52 Figure 3d shows the dark I-V plots of the hole-only devices consist of FTO/Au/HTLs/Au, which can be used to estimate the hole mobility of the materials, according to the modified Mott– Gurney eq.2,51 J = 9ε0εrµh(e)V2/8L3 11


(2)


where L is film thickness, µh(e) is hole mobility, ε0 is the permittivity of free space, and εr is the permittivity of the polymer. The thicknesses of Spiro-OMeTAD, PTAA, and TPE films are around 250, 250, 220 nm, respectively. According to eq. (2), the hole mobilities of the doped spiro-OMeTAD, doped PTAA, and undoped TPE materials are estimated at 1.3 ± 0.1×10-3, 1.6 ± 0.2×10-3, 1.4 ± 0.1×10-3 cm2 V-1 s-1, respectively. Each value is averaged from three hole-only devices. Thus, it can be seen that the mobility of undoped TPE is quite comparable to those of the doped spiro-OMeTAD and PTAA. Photoluminescence (PL) spectra confirm that the perovskite film coated with TPE has an efficient charge transfer. As shown in Figure S10, the perovskite film coated with TPE has a good quenching effect, similar to those films coated with spiro-OMeTAD and PTAA. Therefore, the good hole transport ability of TPE enables the high Jsc and FF of the solar cells. To additionally confirm the reproducibility of the device performance, a number of devices were fabricated. The 36 cells using the undoped TPE HTLs achieved the highest average PCE of 6.26 ± 0.46% with an average Jsc = 22.50 ± 1.15 mA cm-2, Voc = 431.57 ± 26.31 mV, and FF = 64.61 ± 3.66%. In comparison, the average Jsc, Voc, FF, and PCE for the 30 cells using the doped spiro-OMeTAD HTLs are only 17.38 ± 4.11 mA cm-2, 358.85 ± 74.85 mV, 38.24 ± 12.05%, and 2.50 ± 1.31 %, respectively. The average Jsc, Voc, FF, and PCE for the 36 cells using the doped PTAA HTLs are 21.35 ± 1.25 mA cm-2, 435.55 ± 17.81 mV, 64.88 ± 4.15%, and 6.03 ± 0.59%, respectively. The statistics of all the device photovoltaic parameters including Jsc, Voc, FF, and PCE are shown in Figures S11a-c and Figure 4a, 12


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respectively. Note that the solar cells using the doped spiro-OMeTAD HTLs have a larger deviation than those devices using the doped PTAA and undoped TPE HTLs, presumably due to etching53 of the perovskite films by t-BP and acetonitrile in the doped spiro-OMeTAD solution. Furthermore, the dopant of Li-TFSI with a highly deliquescent behavior can accelerate the degradation of perovskite films, especially for tin-based perovskites. As shown in Figure S12, the unencapsulated device using the spiro-OMeTAD HTL doped with Li-TFSI and t-BP has the worst air stability. On the contrary, the device with the dopant-free TPE HTL shows the best stability under the same conditions. Figure 4b shows the J-V plots of the champion solar cell using the undoped TPE HTL measured under different voltage scan directions. This solar cell achieves a PCE of 6.85% with a Voc of 453.17 mV, a Jsc of 22.60 mA cm-2, and a FF of 66.92% when measured under forward voltage scan and a slight higher PCE of 7.23% with a Voc of 459.91 mV, a Jsc of 22.54 mA cm-2, and an FF of 69.74% when measured under reverse voltage scan, indicating a small hysteresis. Figure 4c shows the measured EQE spectrum of the solar cell using the undoped TPE HTL, revealing an integrated Jsc of 22.15 mA cm-2, which is close to the Jsc obtained from the J-V measurements. Figure 4d shows the stabilized power output of the dopant-free TPE HTL-based device. The device achieves a high initial PCE of 7.05% and then stabilizes at a PCE of 6.85% after applying a maximum power voltage of 0.358 V for 100 s. We also fabricated a {en}FASnI3 solar cell using the undoped TPE HTL with a larger active area of 0.39 cm2. Figure S13 shows the J-V plot of this TPE HTL-based device 13



yielding Jsc = 21.87 mA cm-2, Voc = 447.53 mV, FF = 60.12%, and a high PCE = 5.88%, showing the potential utility of dopant-free TPE materials for low-cost and large-scale device fabrication.

CONCLUSIONS The small molecule tetrakis-triphenylamine-based ethylene HTL can be synthesized in few steps and in high yield. We showed that it can be successfully implemented in high-performance tin-based perovskite solar cells as an excellent HTL material without the need of additional dopants or additives. The champion solar cell devices employing our previously developed high-quality {en}FASnI3 hollow-perovskite light absorber and the dopant-free TPE HTL achieve a high PCE of 7.23%, which is comparable with those of the devices using the doped PTAA HTLs and superior to those of the devices using the doped spiro-based HTLs. The outstanding performance of the TPE HTLs can be attributed to the effective charge transfer and the favorable energy-matched band alignment. These results illustrate the potential of small molecule TPE as a superior HTL material towards the development of large-scale economical and high-performance lead-free and other types of perovskite solar cells. ASSOCIATED CONTENT Supporting Information Synthesis details of compound 2; 1H NMR, 13C NMR, HRMS spectra of compound 1 and 2; TGA and DSC plots of TPE; SEM images of different HTL films; J-V plots of a HTL-free device, a device with larger active area, and the devices with various concentrations of TPE HTLs; PL spectra of the perovskite films covered with various 14


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HTLs; air-stability test of devices; statistics of Voc, Jsc and FF of the devices using various HTLs, including Figures S1-S13 and Table S1 (PDF). AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] *[email protected]

Author Contributions # W.K and P.P. contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported in part by the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award DE-SC0001059. This work made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center, and the Nanoscale Science and Engineering Center (EEC-0118025/003), both programs of the National Science Foundation; the State of Illinois; and Northwestern University. The support at National Central University was received from the Ministry of Science and Technology of Taiwan (MOST).

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21



Figure 1. (a) Structure, (b) optical absorption spectrum, and (c) DPV plot of TPE HTL material. (d) Device structure of the {en}FASnI3 perovskite solar cells.

22


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Figure 2. (a) UV-vis absorption spectrum, (b) XRD patterns, and (c) Top view SEM image of a {en}FASnI3 perovskite film coated on a mesoporous TiO2 film. (d) Cross-sectional SEM image of a completed device using a TPE HTL.

23



Current density (mA cm-2)

(a)25

(b)100

20 EQE (%)

80

15 10 5 0 0.0

spiro doped spiro PTAA doped PTAA TPE

0.1

60 40

doped spiro doped PTAA TPE 400 600 800 Wavelength (nm)

20

0.2 0.3 Voltage (V)

0

0.4

1000

(d)

(c)

0.01

4 Current (A)

-dV/dJ (Ω 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

Page 24 of 27

1E-3

2

0 0.00

doped spiro doped PTAA TPE

doped spiro doped PTAA TPE

1E-4 1E-5

0.03 0.06 0.09 (Jsc-J)-1(mA-1 cm2)

0

1 Voltage (V)

2

Figure 3. (a) J-V plots measured under reverse voltage scan, (b) EQE plots, and (c) plots of dV/dJ versus (Jsc-J)-1 of the {en}FASnI3 solar cells using various HTLs. (d) Dark I–V plots of the hole-only devices using various HTLs.

24


(b) 25

4 2 0

80

20

60

15

40

10

20

5

0

400 600 800 Wavelength (nm)

0 1000

-2

25

Integrated Jsc (mA cm

(c)100

)

Doped sprio Doped PTAA TPE

20

Forward Reverse

15 10 5

Voc Jsc FF [mV] [mA cm-2] [%] reverse 459.91 22.54 69.74 forward 453.17 22.60 66.92

0 0.0

0.1

(d) 25

0.2 0.3 Voltage (V)

PCE [%] 7.23 6.85

0.4

0.5

PCE~6.85%

)

6

20

8 6

J~19.13 mA cm

15

-2

4

10 2

5 0

0

25

50 75 Time (s)

0 100

Figure 4. (a) PCE statistics for the {en}FASnI3 solar cells using various HTLs. (b) J-V plots of the best-performing {en}FASnI3 solar cell using the TPE HTL measured under reverse and forward voltage scans. (c) EQE and integrated Jsc plots of a {en}FASnI3 solar cell using a TPE HTL. (d) Steady-state efficiency of a {en}FASnI3 solar cell using a TPE HTL at a constant bias voltage of 0.358 V.

25


PCE (%)

Current density (mA cm

-2

)

8

Current density (mA cm

PCE (%)

(a)

EQE (%)

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


-2

Page 25 of 27


Page 26 of 27

Table 1. Summary of the photovoltaic parameters of {en}FASnI3 solar cells using the indicated HTLs.

Voc

Jsc

FF

PCE

Rsh

[mV]

[mA cm-2]

[%]

[%]

[Ω cm2]

spiro

428.85

13.38

30.79

doped spiro

436.89

20.76

57.35

5.20

228.02

PTAA

384.49

21.12

43.22

3.51

119.65

doped PTAA

446.37

21.85

68.37

6.67

389.30

TPE

460.88

22.39

68.01

7.02

397.58

26


1.77

55.93

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TOC

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