Multiple Roles of Cobalt Pyrazol-Pyridine ... - ACS Publications

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Multiple Roles of Cobalt Pyrazol-Pyridine Complexes in High Performing Perovskite Solar Cells Jianfeng Lu, Andrew D Scully, Jingsong Sun, Boer Tan, Anthony Sidney Richard Chesman, Sonia Ruiz Raga, Liangcong Jiang, Xiongfeng Lin, Narendra Pai, Wenchao Huang, Yi-Bing Cheng, Udo Bach, and Alexandr N. Simonov J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01783 • Publication Date (Web): 21 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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The Journal of Physical Chemistry Letters

Multiple Roles of Cobalt Pyrazol-Pyridine Complexes in High Performing Perovskite Solar Cells Jianfeng Lu,†,‡ Andrew D. Scully,§ Jingsong Sun,‡,‖ Boer Tan,†,‡ Anthony S. R. Chesman,§ Sonia R. Raga,†,‡ Liangcong Jiang,‡,‖ Xiongfeng Lin,†,‡ Narendra Pai,┴ Wenchao Huang,‖ Yi-Bing Cheng,# Udo Bach,*,†,‡,§ and Alexandr N. Simonov*,┴,∇ †

Department of Chemical Engineering, Monash University, Victoria 3800, Australia

‡

ARC Centre of Excellence for Exciton Science, Monash University, Victoria, 3800, Australia

§

Commonwealth Scientific and Industrial Research Organisation, Clayton, Victoria 3168,

Australia ‖

Department of Materials Science and Engineering, Monash University, Victoria, 3800, Australia

┴

School of Chemistry, Monash University, Victoria, 3800, Australia,

#

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan

University of Technology, Wuhan 430070, P. R. China ∇

ARC Centre of Excellence for Electromaterials Science, Monash University, Victoria, 3800,

Australia

AUTHOR INFORMATION Corresponding Author *[email protected] (UB), [email protected] (ANS).

ACS Paragon Plus Environment

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Chemical doping is a ubiquitously applied strategy to improve the charge-transfer and conductivity characteristics of spiro-OMeTAD, a hole-transporting material (HTM) used widely in solutionprocessed perovskite solar cells (PSCs). Cobalt(III) complexes are among the commonly employed HTM dopants, which major role is to oxidize spiro-OMeTAD, i.e. provide p-doping for better conductivity. The present work discloses additional, previously unknown important functions of cobalt complexes in the HTM films that influence the photovoltaic performance. Specifically, it is demonstrated that commercial p-dopant FK269 (bis(2,6-di(1H-pyrazol-1yl)pyridine)

cobalt(III)

tris(bis(trifluoromethylsulfonyl)imide))

reduces

the

interfacial

recombination and alleviates the decomposition and corrosion of perovskite active material under the action of tert-butylpyridine and lithium bis(trifluoromethanesulfonyl)imide. These effects are demonstrated for 1-cm2 sized PSCs that achieve a stabilized PCE of 19 % under 1 sun irradiation.


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High-performance perovskite solar cells (PSCs) require an efficient hole-transport material (HTM) in direct contact with the perovskite active layer,1-2 to extract photo-generated holes from the lightabsorber and transport these holes to the electrode.3 An efficient charge-transfer suppresses recombination and reduces losses in the solar cell photovoltage.4 Thus, an ideal HTM should exhibit very high conductivity and affinity for holes.5 Spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-dip-methoxyphenylamine)-9,9′-spirobifluorene; Scheme 1)6 has been used as a HTM in numerous high-performance PSCs displaying power conversion efficiencies (PCE) exceeding 23%.7-8 In its pristine form, spiro-OMeTAD has a very low conductivity, only moderate hole-mobility, and cannot support the charge-transfer necessary for solar cells with PCE >12%.9-10 However, conversion of just a few mol% of [spiro-OMeTAD]0 to its single-electron oxidized form, [spiroOMeTAD]+, provides additional charge carriers, which substantially improves the holeconducting capacity, making spiro-OMeTAD one of the best-performing HTMs.11 This so-called p-doping can be achieved using lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to promote the oxidation of spiro-OMeTAD by dioxygen, typically occurring upon exposure to air.12-13 Addition of 4-tert-butylpyridine (tBP) along with LiTFSI is known to control the HTM morphology, although the exact mechanism of the promoting effect of tBP remains the subject of debate.14-16 A drawback of this method is the difficulty in achieving satisfactory reproducibility, as ambient conditions can be variable even within the same laboratory.17 It is also well known that exposure to air promotes perovskite degradation.18-19


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Scheme 1. Molecular structures of spiro-OMeTAD, CoII/III(pztbpy)3 and CoII/III(dpzpyr)2 (TFSIcounter-ions not shown).

A more reproducible strategy is to generate [spiro-OMeTAD]+ by introducing chemical oxidants into the precursor solution. By following this approach, small-area (ca 0.1 cm2) PSCs having PCEs in excess of 23% have been produced.8 Metal complexes, such as bis(2,6-di(1H-pyrazol-1yl)pyridine) cobalt(III) tris(bis(trifluoromethylsulfonyl)-imide) (Scheme 1; CoIII(dpzpyr)2) and tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine) cobalt(III) tris-(bis(trifluoromethylsulfonyl)imide)) (Scheme 1; CoIII(pztbpy)3), are amongst the most commonly used p-dopants.20-21 Previous studies focused on the oxidation ability of these compounds,22-23 but the role of complexes in operating devices has not received significant attention. However, the latter cannot be ignored as both chemically reduced and unreacted dopants remain in the HTM layer and are likely to affect the cell operation. The present work aims to unveil the additional, previously not described functions


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of cobalt complexes in spiro-OMeTAD-based PSCs by systematically investigating the effects of CoII and CoIII-complexes on the conductivity of HTM, morphology of the perovskite layer and recombination in the centimeter-sized solar cells. To examine the effect of the cobalt complexes in Scheme 1 on the device performance, n–i–p PSCs with

a

generic

structure

FTO|c-TiO2|m-TiO2+Cs0.05FA0.79MA0.16PbI2.49Br0.51|spiro-

OMeTAD+additives|Au (c-TiO2 and m-TiO2 are compact and mesoporous titania layers, respectively) were fabricated. The quality of the perovskite film was confirmed by X-ray diffraction (Figure S1), showing a cubic Cs0.05FA0.79MA0.16PbI2.49Br0.51 phase with a mean crystallite size of 137 ± 4 nm. Low-intensity peaks at 11.1º and 12.6º are associated with minor admixtures of yellow hexagonal δH phase and PbI2.24 The open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF) and PCE were extracted from the photocurrent-voltage (J-V) curves (Figure 1a-b, Figure S2) for 35 devices from 7 independent batches of each type and used to construct diagrams in Figure 1c. The JSC values obtained from J-V measurements and the integrated current densities derived from the incident photon-to-current conversion efficiency spectra (Figure S3) agreed closely for all PSCs examined (Table S1). Additionally, PCEs were more reliably determined by steady-state measurements at the voltage corresponding to the maximal power point in the J-V data and were confirmed to follow the same trend as those measured in the potentiodynamic mode (Figure 1a-b, Figure S2, Table S1).


5

S

(c) 19

1.14

q-SPO: 17.8%

1.08

18

q-SPO: 19.0%

14 12

0

0

0.0

0.2

50

100 150 Time (s)

0.4 0.6 0.8 Voltage (V)

21

200

1.2

72 70

20

1.0

74

S+CoII(pztbpy)3

q-SPO

20

22

76

S+CoIII(pztbpy)3

S+CoIII(dpzpyr)2

16

16 23

1.2

S+CoII(dpzpyr)2

1.0

20

10

17

200

S+CoIII(dpzpyr)2

0.4 0.6 0.8 Voltage (V)

1.11

Fill Factor (%)

0.2

100 150 Time (s)

S+CoII(pztbpy)3

0.0

50

S+CoIII(pztbpy)3

0

0

S+CoII(dpzpyr)2

12

S+CoIII(dpzpyr)2

14

18

S

16

JSC (mA cm-2)

q-SPO

10

18

VOC (V)

PCE (%)

20 20

Page 6 of 25

S

(a)

(b) Current density (mA cm-2)

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

Current density (mA cm-2)


Figure 1. (a-b) Typical photocurrent-voltage characteristics (sweep rate 0.100 V s-1) with corresponding quasi-stabilized power output data measured at the voltage corresponding to the maximal power output (insets), and (c) summary of the major photovoltaic metrics (PCE, VOC, JSC and fill factor) for FTO|c-TiO2|m-TiO2+Cs0.05FA0.79MA0.16PbI2.49Br0.51|spiro-OMeTAD + additives|Au solar cells based on spiro-OMeTAD (containing LiTFSI + tBP) without and with cobalt complexes added. Data in panel (c) were derived from J-V curves (0.100 V s-1; forwardbias to short-circuit) measured under simulated 1-sun illumination (100 mW cm-2, AM 1.5G) with a metal aperture providing an irradiated area of 1.00 cm2; the same conditions were used to record data in panels (a-b).

Control devices were prepared with spiro-OMeTAD containing LiTFSI and tBP that was exposed to air for 1 h; hereinafter, this type of HTM is referred to S. Modification of spiro-OMeTAD with 3 mol% (hereinafter, with respect to spiro-OMeTAD) of each of the examined CoII and CoIII complexes induced changes in all photovoltaic parameters (cobalt-containing HTMs also contained LiTFSI + tBP and were subject to oxidation by oxygen air for 1 h). The average PCE of the devices followed the trend: S+CoIII(dpzpyr)2 > S+CoII(dpzpyr)2 ≈ S+CoIII(pztbpy)3 >


6

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S+CoII(pztbpy)3 ≈ S. Thus, for cobalt complexes of the same structure, better performance was found when the oxidized (CoIII) form was used. More interestingly, the non-oxidizing CoII(dpzpyr)2 complex and oxidant CoIII(pztbpy)3 provided very similar performance improvements. Modification with CoII(dpzpyr)2 and CoIII(dpzpyr)2 increased FF and VOC (Figure 1), which is the major reason for the higher PCE for devices based on S+CoIII(dpzpyr)2 and S+CoII(dpzpyr)2. The superior performance of S+CoIII(dpzpyr)2 as opposed to S+CoII(dpzpyr)2 was mainly due to higher VOC, which might be explained at this stage by the presence of additional [spiro-OMeTAD]+ providing better conductivity of the CoIII(dpzpyr)2-modified HTM. However, the use of more than 3 mol% of CoIII(dpzpyr)2 to oxidize spiro-OMeTAD did not improve the photovoltaic performance further (Figure S4, Table S2), despite the higher degree of p-doping provided. Increasing the amount of CoII(dpzpyr)2 did not produce any additional positive effects either (Figure S5, Table S2). Examination of the surface morphology of the spiro-OMeTAD layer by visible light microscopy revealed the formation of large heterogeneous grains upon increase in the amount of the introduced dopants to 6 mol% and beyond (Figure S6). This contrasts the almost perfectly flat, pinhole-free HTM surface produced with no and with 3 mol% cobalt complexes added, as visualized by scanning electron microscopic (SEM) (Figure S7) and atomic force microscopic analysis (Figure S8). To understand the differences in the photovoltaic parameters of devices based on different HTMs, comprehensive characterization of key components of PSCs was undertaken. Photoelectron spectroscopy in air was employed to determine the ionization potentials (EI) for spiro-OMeTAD films produced under different conditions (Figure S9).25 EI values measured in this manner can be considered to represent a virtual valence band edge (EVB) of a p-type semiconductor resulting from p-doping of a spiro-OMeTAD film.26 The ionization potential measured for spiro-OMeTAD


7


Page 8 of 25

containing no additives was -5.00 eV (hereinafter, on a vacuum scale). As compared to pure spiroOMeTAD, EI shifts down to -5.15 eV upon 1 h oxidation in air in the presence of LiTFSI and tBP. The ionization potentials for S+CoIII(dpzpyr)2 and S+CoIII(pztbpy)3 were more negative by 0.10.2 eV than that for S samples attesting to the increased p-character due the partial oxidation of [spiro-OMeTAD]0 to [spiro-OMeTAD]+ by CoIII species, while no changes were induced to EI by the modification with CoII complexes, as expected (Figure S9). Importantly, EI for all HTM layers examined were significantly more positive than that of the valence band edge for the Cs0.05FA0.79MA0.16PbI2.49Br0.51 perovskite (EVB = -5.56 eV)25 employed herein, meaning that differences in ionization potentials could hardly explain different photovoltaic performance. Another factor that can affect the efficiency of PSCs is the conductivity of the HTM layer, which in turn depends on the level of p-doping. Hence, the amount of [spiro-OMeTAD]+ in different films was quantified by UV-Vis absorption spectrophotometry (Figures S10, Table S3). The [spiro-OMeTAD]+ content in the freshly-deposited spiro-OMeTAD films (containing LiTFSI and tBP, but no cobalt complexes) that had not been exposed to air was not higher than 0.3 mol%. In contrast, S control samples that were exposed to air for ca 1 h contained around 2 mol% [spiroOMeTAD]+. Comparable [spiro-OMeTAD]+ amounts were found in the films modified with 3 mol% of the reduced (CoII) forms of both complexes. Addition of 3 mol% CoIII-based complexes produced samples containing higher [spiro-OMeTAD]+ concentration of 4.5 ± 0.1 mol%, corresponding to an oxidation yield of 90 ± 3 % (Table S3). Taking into account all experimental uncertainties, this value is in a good agreement with the yield of 82 ± 1 % found for the reaction of spiro-OMeTAD and CoIII(dpzpyr)2 or CoIII(pztbpy)3 in the solution (Figure S11). The conductivity (σ) of HTM films was quantified using interdigitated electrodes.27-28 Variation in the duration of the exposure to air of spiro-OMeTAD films (with LiTFSI and tBP) allowed for the


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monitoring of changes in σ as a function of [spiro-OMeTAD]+ concentration, which was found to be essentially linear within the examined range (Figure S12a). The kinetics of [spiro-OMeTAD]0 oxidation by ambient O2 was comparatively slow and could be approximated as a pseudo firstorder reaction with an effective rate constant of ca 0.12 h-1 (Figure S12b). As such, the [spiroOMeTAD]+ content, and hence σ, increased notably during the initial 5 h of the oxidation by ambient oxygen only. The conductivities of S+CoII(dpzpyr)2 and S+CoII(pztbpy)3 were slightly higher than that of S, while σ of S+CoIII(dpzpyr)2 and S+CoIII(pztbpy)3 was significantly better (Table S3).29 Therefore, lower series resistance (RS) in the devices based on S+CoIII(dpzpyr)2 and S+CoIII(pztbpy)3 was expected. Numerous studies have demonstrated that the morphology of the photoactive layer and the quality of the perovskite|HTM interface significantly affect the performance of PSCs. However, impact of LiTFSI, tBP and cobalt complexes that are routinely added to HTMs and the surface morphology of the underlying perovskite layer were rarely investigated. Only recently, the degradation of perovskite under the action of tBP has been reported.16 Herein, these effects were further examined for the Cs0.05FA0.79MA0.16PbI2.49Br0.51 perovskite using SEM. Samples were prepared following the standard procedure for the deposition of a HTM layer, but with no spiroOMeTAD present. The as-deposited unmodified perovskite surface was compact and uniform, with a grain size of 400-600 nm (Figure 2a). Modification with the LiTFSI+tBP solution deteriorated the perovskite surface morphology by generating small voids and SEM-bright, i.e. poorly conducting, species (Figure 2b). Very similar effects were produced upon treatment with the LiTFSI+tBP+CoII/III(pztbpy)3 mixtures (Figure 2c, Figure S13a), but not when applying LiTFSI+tBP along with CoIII(dpzpyr)2 (Figure 2d) or CoII(dpzpyr)2 (Figure S13b). The presence of either CoIII(dpzpyr)2 or CoII(dpzpyr)2 preserved highly uniform and compact surface of the


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Page 10 of 25

perovskite. Possibly, this protection might be provided by the coordination of Co-dpzpyr complexes with the pyridine ring of tBP,30 although it is noted that the latter was present in a significant excess with respect to CoII/III(dpzpyr)2 in the deposition solutions. The exact mechanism of the protective effect is yet to be established.

Figure 2. Scanning electron micrographs of FTO|c-TiO2|m-TiO2 + Cs0.05FA0.79MA0.16PbI2.49Br0.51 perovskite films treated with (a) chlorobenzene and (b-c) chlorobenzene solutions of (b) LiTFSI + tBP, (c) CoIII(pztbpy)3 + LiTFSI + tBP, or (d) CoIII(dpzpyr)2 + LiTFSI + tBP. Treatment conditions were similar to those used when depositing spiro-OMeTAD HTM layer to produce solar cells but with

no

spiro-OMeTAD.

(e)

Cross-section

image

of

a

FTO|c-TiO2|m-TiO2

+

Cs0.05FA0.79MA0.16PbI2.49Br0.51|S + CoIII(dpzpyr)2|Au solar cell. All scale bars correspond to 1 μm.


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It was also found that non-encapsulated solar cells based on S+CoIII(dpzpyr)2 and S+CoII(dpzpyr)2 demonstrate slightly better stability during long-term storage (air, dark, 22 °C, relative humidity

Multiple Roles of Cobalt Pyrazol-Pyridine ... - ACS Publications

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