Improved Performance and Reproducibility of Perovskite Solar Cells

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Improved performance and reproducibility of perovskite solar cells by well-soluble tris(pentafluorophenyl)borane as a p-type dopant Tengling Ye, Junhai Wang, Wenbo Chen, Yulin Yang, and Dongqing He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Applied Materials & Interfaces

Improved perovskite

performance solar

and

cells

reproducibility by

of

well-soluble

tris(pentafluorophenyl)borane as a p-type dopant

Tengling Ye, *,†,a Junhai Wang, †,a, Wenbo Chen, a Yulin Yang,*,a Dongqing Heb a

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and

Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China. E-mail: [email protected], [email protected] b

Institute of Advanced Technology , Heilongjiang Academy of Sciences, Harbin 150020, China.

+

Tengling Ye and Junhai Wang contributed equally to this work.

ACS Paragon Plus Environment

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Abstract: In this work, a well-soluble tris(pentafluorophenyl)borane (BCF) is introduced for the first time into spiro-OMeTAD as a p-dopant. The conductivity of spiro-OMeTAD films is dramatically enhanced. When the BCF-doped spiro-OMeTAD film is used as a hole transport layer (HTL) in perovskite solar cells (PSCs), nearly double increase in power conversion efficiency (PCE) is obtained compared with the PSCs based on pristine spiro-OMeTAD HTL. By the introduction of Li-TFSI and TBP into the BCF-doped spiro-OMeTAD film, the conductivity of spiro-OMeTAD film can be further enhanced and an optimum PCE of 14.65% is obtained. What’s more, the device average efficiency and the reproducibility of BCF-based PSCs are better than the ones of FK209-based PSCs. The working mechanism of the BCF doping effect on spiro-OMeTAD is detailed studied. The strong electron-accepting ability, excellent solubility in common organic solvents and the low cost make BCF a very attractive p-type dopant for spiro-OMeTAD.

Keywords: tris(pentafluorophenyl)borane, perovskite solar cells, spiro-OMeTAD, p-type dopant, conductivity

INTRODUCTION


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Recently, the rapid development of organic–inorganic halide perovskite solar cells (PSCs) has received much attention owing to the suitable bandgap of halide perovskite materials, high carrier mobility, long carrier diffusion length, etc1-2. The power conversion efficiency (PCE) of PSCs has boosted from the initial 3.8% in 20093 to 22% nowadays4. They are considered as a strong competitor of silicon based solar cells. Common device architectures of PSCs generally can be divided into two types, the mesoporous type and the planar type, as shown in Fig. 1. They are usually composed of electron transport layer (ETL), perovskite light absorption layer and hole transport layer (HTL). Among these functional layers, the HTL plays a very important role in determining the performance of PSCs. During the past several years, a number of organic, inorganic and polymeric HTLs have been developed, such as 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD),5-6 CuSCN,7-8 NiO,9-10 Poly(3hexylthiophene-2,5-diyl) (P3HT),11 polytertiary arylamine (PTAA)12-13 etc.. However, spiroOMeTAD remains the best choice of hole transport materials in PSCs when high efficiencies are required, due to its high solubility, high stability, well-matched energy level and amorphous nature.14-18 To our knowledge, the high efficiency over 20% up to now are mostly achieved using spiro-OMeTAD as the HTL.

Figure 1. The two types of device architectures of PSCs


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Nevertheless, the pristine spiro-OMeTAD suffers from low conductivity, which results that the spiro-OMeTAD must be doped to improve its conductivity when used in PSCs. One way to solve

this

problem

is

to

introduce

an

effective

additive

lithium

bis(trifluoromethanesulfonyl)imide(Li-TFSI) to oxidize spiro-OMeTAD. It is reported that LiTFSI can indirectly induce spiro-OMeTAD to be oxidized by oxygen when the light or thermal excitation is applied.19-21 In this case, the HTL will intensely influence the device reproducibility, stability and performance because the oxidation degree of the spiro-OMeTAD is uncontrollable by oxygen. What’s more, the oxidized process is in atmosphere environment and the condition such as light and moisture is harmful to the performance of PSCs.

22

Regarding to this, a much

better alternative solution to increase the conductivity of spiro-OMeTAD is the use of p-type dopants which can quantitatively increase the concentration of hole charge carriers and make the device performance more reliable. Researchers have explored some p-type dopants to boost the conductivity of spiro-OMeTAD, such as N(PhBr)3SbCl6,23 Co(III) complexes (FK102, FK209, FK269) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) 24-27. However, the above materials are usually limited by their high reactivity, low solubility in chlorobenzene (the common solvent of spiro-OMeTAD) or high cost. It is great urgent to develop an efficient, stable, highly soluble and low cost p-type dopant for spiro-OMeTAD. Tris(pentafluorophenyl)borane (BCF) is a kind of strong π-electron acceptors and is recently used as a novel and attractive p-type dopant for a wide range of lewis basic organic semiconductors containing elements of N, S, Se, etc.. The optical and charge transport properties of these organic semiconductors can be effectively tuned by BCF.28-31 There are two kinds of working mechanism to explain the doping effect of BCF. 32 One explanation is the supramolecular charge transfer complex is formed between BCF and organic semiconductor. 28 The B atom captures the lone


4

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electron pair of lewis basic organic semiconductors and the Lewis acid–base adduct is formed by a dative covalent bond N-B which makes the electron density on organic semiconductor redistributed and goes along with the partial charge transfer between the donor and acceptor species. The other explanation is that the ground-state integer charge transfer model. 31,33 An electron is completely transferred from organic semiconductor to the BCF acceptor and positive charge carrier on organic semiconductor and BCF anion are generated. The free positive charge carrier density is depending on the strength of the Coulomb binding between the positive charge carrier on organic semiconductor and BCF anion.

Figure 2. The molecular structures of spiro-OMeTAD and BCF

By far most of these BCF-doped materials have been used in organic light-emitting diodes and organic thin-film transistors, surprisingly little attention has been devoted to solar cells.29-30 In this paper, we introduced for the first time BCF into the well-known hole transport layer spiroOMeTAD as a p-type dopant and the doped spiro-OMeTAD was used as a HTL in PSCs. The strong electron-accepting ability, excellent solubility in common organic solvents and low price34 make BCF a very suitable p-type dopant for spiro-OMeTAD. The molecular structures between BCF and spiro-OMeTAD are shown in Figure 2. After carefully optimizing the BCF doping ratio to spiro-OMeTAD, we successfully proved BCF is a great substitute as a p-type dopant for spiroOMeTAD. The performance of PSCs based on BCF doped spiro-OMeTAD HTL is better than


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that of PSCs based on FK-209 doped one to a certain extent and the reproducibility of PSCs is increased. The working mechanism of the BCF doping effect on spiro-OMeTAD is detailed analyzed by UV-vis absorption spectra, photoluminescence (PL) spectra, electron spin resonance (ESR) spectroscopy, current-voltage (I-V) curves, cyclic voltammetry (CV) and atomic force microscope (AFM), etc. RESULTS AND DISCUSSION The doping effect of BCF on spiro-OMeTAD was first studied via UV-vis absorption spectra, PL spectra, ESR spectroscopy and the conductivity measurement. Figure 3(a) gives the absorption properties of pristine BCF, pristine spiro-OMeTAD and spiro-OMeTAD solution with different doping contents of BCF in chlorobenzene. It can be found that an evident new absorption peak appears around 526 nm when BCF is added into spiro-OMeTAD solution and the intensity of the peak increases as the BCF doping ratio increases. The peak around 526 nm could be attributed to the characteristic absorption of oxidized spiro-OMeTAD+, which has been proved by many other p-dopants

(FK102,

FK209,

or F4-TCNQ) 24,25,27 . What’s more, the intensity of the peak around 621 nm also increases as the BCF doping ratio increases and it can be ascribed to the increase of the concentration of BCF anion which behaves a broad characteristic absorption located around 2.0 eV. These results suggest that BCF has strong ability to oxidize spiro-OMeTAD and is able to accept electrons from spiroOMeTAD in the solid film to generate more free holes as an efficient p-type dopant. The ground state electron transfer from spiroOMeTAD to BCF can be directly observed by the immediate color change of the BCF-doped spiro-MeOTAD solutions from light green to dark red-brown as shown in the inset of figure 3(a). 33 In addition, we quantitatively evaluate the BCF doping ability on spiro-MeOTAD by analyzing the curve of the absorption intensity at 526 nm that can reflect


6

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the concentration of spiro-OMeTAD+ versus the doping ratio of BCF. The curve is shown in Figure S1. Initially, the intensity of the peak at 526 nm of spiro-OMeTAD+ increases superlinearly when the doping ratio of BCF increases to 2% and then it increases sub-linearly as the content of BCF improves from 2% to 20%. This observation further proves that electron can transfer from spiroOMeTAD to BCF effectively at low BCF doping concentration and the doping efficiency decreases when the doping ratio of BCF beyond 2% due to the increase of the recombination probability between BCF and spiro-MeOTAD+. Figure 3(b) presents PL spectra of the pristine spiro-OMeTAD film and spiro-OMeTAD films with different doping ratio of BCF. A decrease of the PL intensity with increasing the doing concentration of BCF can be obviously observed after exciting spiro-OMeTAD films at 367 nm. It indicates that BCF is a good electron acceptor that could quench the fluorescence of spiro-OMeTAD through the charge transfer from spiroOMeTAD to BCF, which supports the UV-vis absorption results. Another direct evidence indicating the efficient charge transfer from spiro-OMeTAD to BCF is to prove the existence of spiro-OMeTAD+ in solution. ESR spectroscopy is a sensitive, specific method for the identification and study of free radicals.35 As can be seen from Figure 3(c), no radical signal is detected for undoped spiro-OMeTAD solution in chlorobenzene. However, the BCF-doped spiroOMeTAD solution in chlorobenzene displays an obvious signal at the magnetic field of around 3455 G. It illustrates that the strong electron-accepting ability of BCF resulting in easy electron transfer from nitrogen atoms of spiro-OMeTAD onto the boron atom of BCF. Regarding the above results based on UV-vis absorption spectra, PL spectra and ESR spectroscopy, we believe that the main working mechanism of the p-doping effect of BCF on spiro-OMeTAD is the ground-state integer charge transfer. Ground state electron transfer from the HOMO level of spiroOMeTAD to the LUMO level of BCF can easily occur, resulting in the generation of oxidized spiro-OMeTAD+


7


(Figure 3(d)) which lead to an improvement of the conductivity of spiro-OMeTAD film. It is worth mention that we don’t observe any new peak for the BCF and spiro-OMeTAD adduct in UV-vis absorption spectra, which suggests that charge transfer complex is not the main working mechanism in this system.

1.5x104 BCF S S+2% BCF S+5% BCF S+10% BCF S+20% BCF

0.08

0.04

0.00

PL intensity (a.u.)

Absorption (a.u.)

0.12

500

600

700

800

S S+2% BCF S+5% BCF S+10% BCF S+20% BCF S+40% BCF

1.0x104

5.0x103

0.0 400

450

Wavelength (nm)

500

550

600

650

700

Wavelength (nm)

(b)

(a) 6.0x105 4.0x105

Intensity

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

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2.0x10

S S+5%BCF

5

0.0 -2.0x105 -4.0x105 -6.0x105 3400

3420

3440

3460

3480

3500

Magnetic Field

(c)

(d)

Figure 3. (a) Photographs and UV-vis absorption spectra of the BCF and a series of BCF-doped spiro-OMeTAD solutions. (b) PL intensity quenching on the increased ratios of BCF in spiroMeOTAD films. (c) ESR spectra of pure spiro-MeOTAD and BCF-doped spiro-MeOTAD. (d) working mechanism of the p-doping effect of BCF on spiro-OMeTAD.


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To confirm the p-doping induced enhanced conductivity by BCF, PSCs based on BCF-doped spiro-OMeTAD HTL were fabricated, the CV measurement and the conductivity of BCF-doped spiro-OMeTAD were carried out. Firstly, we fabricated PSCs with a structure of FTO substrate/block-TiO2/mesoporous-TiO2/MAPbI3/spiro-OMeTAD/Au.

BCF-doped

spiro-

OMeTAD films with different doping ratios (0%, 2%, 5% and 10%) were used as HTL. The crosssectional SEM image of the device is displayed in Figure 4(a). The corresponding photovoltaic parameters under simulated AM 1.5 G solar irradiation (100 mW/cm2) are summarized in Table 1. In comparison to the reference solar cells based on pristine spiro-OMeTAD HTL, spiroMeOTAD HTLs with increasing doped ratios of BCF lead to significant enhancements in shortcircuit current (Jsc), open-circuit voltage (Voc) and fill factor (FF). In particular, the device based on spiro-OMeTAD HTL with 5.0 mol % BCF gives the best performance with a PCE of 8.06% which is nearly an enhancement of twice as much as the PCE of the device based on pristine spiroOMeTAD HTL. Table 1. Photovoltaic parameters of devices using spiro-OMeTAD as hole conductor involving different mole ratios of BCF dopant. Voc (V)

Jsc (mA cm-2)

FF

PCE (%)

pristine S

0.72

10.46

0.60

4.54

S + 2% BCF

0.74

13.35

0.65

6.67

S + 5% BCF

0.79

14.96

0.68

8.06

S + 10% BCF

0.77

14.29

0.58

6.39

HTL compositions

The conductivity of spiro-OMeTAD films with BCF doped ratios from 0 to 10 mol % is characterized by current-voltage (I-V) curves. The corresponding I-V curves are presented in Figure 4(b). We can clearly found that all I-V curves show a good linear relationship and the


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prinstin spiro-OMeTAD film shows a smaller slope of the I-V curve than the doped spiroOMeTAD films does. With increasing doping ratios, the slope increased significantly, which illustrates the conductivity of the spiro-OMeTAD films indeed could be effectively enhanced by BCF. The conductivity values derived from the corresponding I-V curves of the BCF-doped spiroOMeTAD films are summarized in Table S1. Figure 4(c) shows the CV curves of pristine BCF, pristine spiro-OMeTAD and BCF-doped spiro-OMeTAD with different doping ratios in dichloromethane containing 0.1 M TBAPF6. It can be found that the highest occupied molecular orbital (HOMO) level keeps almost unchanged (For BCF doping ratios from 0 mol% to 20 mol%, the HOMO level decreases from 5.12 eV to 5.10 eV).36-37 Although the lower HOMO level of BCF-doped spiro-OMeTAD HTL is beneficial for hole extraction from MAPbI3 (5.43 eV) to Au (5.1 eV),38-39 the variation of HOMO level is negligible and the effect of HOMO levels on device performance is secondary. Therefore, we conclude that the enhancement of conductivity of HTL is the main reason that result in the enhancement of device performance. BCF can promote the generation of more holes and simultaneously the free holes increase the conductivity of the spiro-OMeTAD film. However, there is an optimum BCF doping ratio (5 mol%) for the performance of PSCs. At higher concentrations (10.0 mol%) of BCF, the PCE decreases compared with the PCE of 5 mol% BCF due to the obviously decreased FF value. This device performance decrease at higher BCF concentrations may because the excessive BCF dopant causes excessive organic anions BCF− distributed at the interface between MAPbI3 and spiro-MeOTAD HTL. The excessive organic anions become hole traps at the interface and increase the carrier recombination.


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5.0x10-9 4.0x10 Current (A)

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


S S+2%BCF S+5%BCF S+10%BCF S+20%BCF

S S+2% BCF S+5% BCF S+10% BCF

-9

3.0x10-9 2.0x10-9

-0.8 -0.6 -0.4 -0.2

0.0

0.2

0.4

0.6

0.8

BCF

1.0x10-9 0.0 0

1

2

3

4

Applied Bias (V)

(a)

-2.5 -2.0 -1.5 -1.0 -0.5 0.0

(b)

0.5

1.0

1.5

2.0

Potential (V vs. Fc+/Fc)

(c)

Figure 4. (a) Cross-sectional SEM image of our typical PSC. (b) The conductivity property of the spiro-OMeTAD films with different doping ratios of BCF.(c) Cyclic voltammetry of spiroOMeTAD doping with different mole ratio BCF and the pristine BCF. In general, BCF exhibits an excellent substitute as a p-dopant for spiro-OMeTAD hole transport material. Moreover, the good solubility in chlorobenzene and low price make BCF be a promising competitor with commercialized dopant such as FK209. To compare the p-dopant effect of BCF or FK209 on spiro-OMeTAD, three types of PSCs were fabricated: PSCs based on spiroOMeTAD HTL doped with Li-TFSI and 4-tert-butylpyridine (TBP) Device (I), with Li-TFSI, TBP and 10 mol% FK209 Device (II), with Li-TFSI, TBP and 5 mol% BCF Device (III). Here, the function of Li-TFSI and TBP has been reported by many groups.40 As is known, Li-TFSI can facilitate BCF/FK209 to oxidize spiro-OMeTAD and TBP is used to optimize the spiro-OMeTAD morphology. Therefore, both of them are helpful to further enhance the conductivity together with BCF or FK209.

20-21, 40

The best current density-voltage (J-V) curves of the three types of PSCs

are presented in Figure 5(a) and the corresponding photovoltaic parameters under simulated AM 1.5 G solar irradiation (100 mW/cm2) are summarized in Table 2. As can be seen, the device (III) based on the spiro-OMeTAD HTL doped with 5 mol% BCF, Li-TFSI and TBP shows the best photovoltaic performance among the three types of devices. The device (III) gives a Jsc of 20.3


11


mA·cm-2, a Voc of 1.02V, a FF of 0.70, leading to a highest PCE of 14.65%. Figure 5(b) shows the corresponding external quantum efficiency of the three types of PSCs, the highest integrated current density (20.01 mA·cm-2) was obtained from the BCF, Li salt and TBP-based device. The other two integrated current densities of device (I) and device (II) are 16.18 mA·cm-2 and 18.84 mA·cm-2, respectively. The integrated current density is basically consistent with the Jsc in J-V characteristic curves.

20

80 16 EQE (%)

Current density (mA/cm2)

100

12 8

S+Li+TBP S+Li+TBP+FK209 S+Li+TBP+BCF

0.2

0.4

60 S+Li+TBP S+Li+TBP+FK209 S+Li+TBP+BCF

40 20

4 0 0.0

0.6

0.8

0 300

1.0

400

500 600 Wavelength (nm)

Voltage (V)

(a)

700

800

(b)

8

2.5x10-8 S+Li+TBP S+Li+TBP+FK209 S+Li+TBP+BCF

4

2

0

S S+BCF S+FK209 S+BCF+Li+TBP S+FK209+Li+TBP

2.0x10-8 Current (A)

6 Count

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

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1.5x10-8 1.0x10-8 5.0x10-9 0.0

6

9

12

15

0

1

2

3

4

Applied Bias (V)

Power Conversion Efficiency (%)

(c)

(d)

Figure 5. (a,b) the best J-V characteristics and IPCE spectra of device I, II and III. (c) Statistical histogram of PCE of 20 devices for each of the three kinds of PSCs. (d) The conductivity of the spiro-OMeTAD films with different p-type dopants.


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Table 2. Photovoltaic parameters of the device I, II and III. HTL compositions

Voc (V)

Jsc (mA cm-2)

FF

PCEm (%)

PCEd (%)

S + Li-TFSI + TBP

0.79

16.93

0.64

8.48

6.25

S + Li-TFSI + TBP + 10%FK209

0.99

19.18

0.69

13.18

11.73

1.02

20.30

0.7

14.65

13.93

S + Li-TFSI + TBP + 5%BCF m

The values was the maximum efficiency. dThe values are average efficiencies.

The reproducibility of the performance was checked by preparing and measuring 20 devices for each type of PSCs. The statistical histogram of the PCE values is shown in Figure 5(c). The devices containing 5 mol % BCF, Li-TFSI and TBP exhibited an excellent reproducibility with an average PCE of 13.93% and the efficiency distribution is narrow. The other two types of PSCs show average PCEs of 6.25% (device I) and 11.73% (device II) and the efficiency distribution is wide. The higher average efficiency and reproducibility obtained for PSCs based on BCF-doped spiro-OMeTAD HTL could be mainly attributed to the high conductivity of the HTL. Figure 5(d) shows the I-V curves of the pristine spiro-OMeTAD film and spiro-OMeTAD films with 5 mol % BCF, 10 mol % FK209, 5 mol % BCF, Li-TFSI and TBP and 10 mol % FK209, Li-TFSI and TBP, respectively. The conductivity values derived from the corresponding I-V curves are summarized in Table 3. As expected, the conductivity of the spiro-OMeTAD films based on 5 mol % BCFdoped or 10 mol % FK209-doped is enhanced compared with the conductivity of pristine spiroOMeTAD film. After the introduction of the additives of Li-TFSI and TBP into the spiroOMeTAD films, the conductivity was further enhanced significantly. The conductivity of BCFdoped spiro-OMeTAD behaves the highest conductivity among the five films with 2.28×10-5 Scm-1. These results well explain the law of the device performance. The higher the conductivity is, the better the device performance is.


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Table 3. The conductivity values derived from the corresponding I-V curves of the spiroOMeTAD films with different p-type dopants in Figure 5(d). HTM

Conductivity

Spiro

1.33×10-7 Scm-1

Spiro+5%BCF

2.79×10-6 Scm-1

Spiro+10%FK209

4.49×10-7 Scm-1

Spiro+Li+TBP+5% BCF

2.28×10-5 Scm-1

Spiro+Li+TBP+10%FK209

1.14×10-5 Scm-1

In addition, the AFM measurement with 2 μm scale was performed to evaluate the morphology of the pristine spiro-OMeTAD film, the BCF-doped spiro-OMeTAD film and the FK209-doped spiro-OMeTAD film. The height and phase images are shown in Figure 6 (a-c) and Figure S2 (a-c), respectively. From the height images in Figure 6 a and b, we can find that the first two kinds of samples show a negligible difference of the root-mean-square roughness, and they are 0.748 nm, 0.700 nm respectively. However, the FK209-doped spiro-OMeTAD film exhibites a worse surface with the root-mean-square of 1.410 nm as shown in Figure 6 (c). We can observe an obvious island distribution in the optical microscope images of the FK209-doped spiroOMeTAD film as shown in Figure 6(f) and Figure S2 (f). The others are uniform shown in Figure 6(d, e) and Figure S2 (d, e). The rough surface and the island distribution of the FK209-doped spiro-OMeTAD film are attributed to its poor solubility in chlorobenzene, which drastically influence the morphology of the HTL and reduce the uniform of the oxidation process for spiroOMeTAD. In contrast, the excellent solubility of BCF in chlorobenzene makes BCF be an ideal


14

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p-type dopant to oxidize spiro-OMeTAD uniformly in the solution, which leads to higher conductivity and then better device performance.

(a)

(b)

(d)

(e)

(c)

(f)

Figure 6. AFM height images with 2μm scan area and 40 times optical microscope images. (a,d) pristine spiro-OMeTAD films using chlorobenzene as the solvent, (b,e) 5 mol% BCF-doped spiroOMeTAD films using chlorobenzene as the solvent, (c,f) 10 mol% FK209-doped spiro-OMeTAD films using the mixing solvent of chlorobenzene and acetonitrile. CONCLUSIONS In summary, the conductivity of the spiro-OMeTAD films can be enhanced by a well-soluble p-dopant BCF and the BCF-doped spiro-OMeTAD HTL was successfully applied in perovskite solar cells. The working mechanism of enhanced conductivity is attributed to ground-state integer charge transfer. BCF behaves very strong electron-accepting ability and electrons can easily


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transfer from the HOMO level of spiro-OMeTAD to the LUMO level of BCF, resulting in the generation of oxidized spiro-OMeTAD+. Going along with the generation of more holes and simultaneously result in the enhanced conductivity of the spiro-OMeTAD films. Compared with the PSCs based on pristine spiro-OMeTAD HTL, a nearly double increase in PCE is obtained for the PSCs based on 5 mol% BCF-doped spiro-OMeTAD HTL. By the introduction of Li-TFSI and TBP into the BCF-doped spiro-OMeTAD HTL, the conductivity of the spiro-OMeTAD films can be further enhanced. We obtained an optimum PCE with 14.65% for the PSC based on spiroOMeTAD HTL using BCF, Li-TFSI and TBP as dopant. The average efficiency and the reproducibility of BCF-based PSCs are better than the ones of FK209-based PSCs. The higher performance is owing to the higher conductivity and the excellent solubility of BCF in chlorobenzene. Together with the advantage of the low cost, BCF provides another facile way for p-type doping of spiro-OMeTAD. Experiments 1. Materials Methylammonium iodide(MAI) was purchased from Xi’an Polymer Light Technology Corp.,

Spiro-OMeTAD

was

bought

from

Luminescence

Technology

Corp.,

Tris(pentafluorophenyl)borane (BCF) was purchased from Energy Chemical and FK209 was purchased from Shanghai MaterWin New Materials Co., Ltd. All the other chemicals (Lead iodide, Titanium(IV) isopropoxide, N,N-Dimethylformamide(DMF), isopropanol (IPA) etc.) were purchased from Sigma-Aldrich and used without any purification. 2. Solar Cells Fabrication Fluorine-doped tin-oxide (FTO) coated glass substrates were etched with 2 M hydrochloric acid and zinc powder. Then the substrates were cleaned by ultrasonic in detergents, deionized


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water, acetone and ethanol and the remaining organic residues were cleaned for 10 mins by O2 plasma treatment. Initially, the compact TiO2 layer was deposited by spin-coating a mildy acid solution of titanium isopropoxide in IPA (2000 rpm,50 s) onto the surface of a pre-cleaned FTO substrates and annealed at 500℃ for 30 mins. Then the mesoporous layer TiO2 was deposited by spin-coating a ethanol diluted Dyesol paste (18NR-T) solution and annealed at 500℃ for 30mins. Finally, the substrates were immersed in 0.04 mM TiCl4 solution at 70℃ for 30mins and were then heat-treated at 500℃ for 30 mins. The CH3NH3PbI3 was prepared by subsequent spin-coating in the glovebox with N2 atmosphere. The PbI2 films were prepared by spin-coating process at 3000 rpm with a solution of 1 M PbI2 in DMF/DMSO (9/1 v/v) mixture. Then the films were annealed at 100℃ for 5 mins. After cooling down, the film was exposed to 0.05 M of MAI in IPA for 30 s, then the film was spun at 4000 rpm and annealed at 100℃ for 30 mins. After cooling to room temperature, the HTL solution was spin-coating at 4000 rpm for 30 s. The HTL solution was prepared in glovebox by adding

28.8

µL

4-tert-butylpyridine

and

17.5

µL

of

520

mg/ml

lithium

bis(trifluoromethylsulphonyl)imide to 72.3 mg spiro-OMeTAD in 1 mL anhydrous chlorobenzene. For the chemical dopant, the BCF was dissolved in chlorobenzene and mixed by desired BCF/spiro-OMeTAD mole ratios ranging from 2% to 10%. And 300 mg FK209 was dissolved in 1 mL acetonitrile and added to the spiro-MeOTAD + Li + TBP solution with 10% mole radio. All the oxidation process of spiro-OMeTAD was carried out in glovebox without the involvement of O2. After the fabrication of hole transport layers, they were immediately transferred into the vacuum chamber to evaporate 80nm Au back contact under a pressure of ca.10-6 Torr. 3. Characterization and Measurements.


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The absorption spectra of diluted spiro-OMeTAD solutions were measured by using a UVvis spectrometer(Japan Shimadzu model 2250). The photoluminescence emission intensity spectra were recorded by FLS 920 luminescence spectrometer. The electron spin resonance (ESR) spectra was measured using an X-band (9.69 GHz) Bruker ER083CS spectrometer at 300 K. The conductivity was measured according to a previous method.41 Indium tin oxide coated glass (ITO glass) was patterned into parallel row electrodes with width of 1 mm and length of 1 cm. Firstly, ITO substrates were carefully cleaned in detergents, deionized water, acetone and isopropanol. After O2 plasma treatment, the ITO substrates was dried in oven. Subsequently the HTL solutions were spin coated with the same concentrations as in the case of solar cells and the J-V characteristics were measured between two adjacent electrodes by a Keithley 236 source meter. The thickness of HTL was 180 nm measured by a Dektak 6M profilometer. The electrical conductivity (σ) was calculated by the following equation (1): σ=

W RLD

(1)

where L is the channel length with 10 mm, W is the channel width 1 mm, D is the thickness of HTL, and R is the film resistance calculated from the slope of the I-V curves. Field-emission scanning electron microscopy (FE-SEM) images were captured on S-4800 Hitachi (Tokyo, Japan). The atomic force microscopy (AFM) analysis were performed by using a Bruker Dimension ICON-PT with Co/Cr tips. Cyclic voltammetry was measured on CHI660D electrochemistry workstation (Shanghai Chenhua) with a glassy carbon working electrode, platinum counter electrode, and Ag/AgCl quasireference electrode using 0.1 M tetrabutylammonium hexafluorophosphate as supporting electrolyte in dichloromethane and a ferrocene/ferrocenium (Fc/Fc+) redox couple (0.386 V vs. the Ag/AgCl quasi-reference electrode) as an external standard. EHOMO = -(Eoxonset - Fc/Fc+ + 5.1) eV


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and ELUMO = -(Eredonset - Fc/Fc+ + 5.1) eV, respectively, where Eox on and E red on stand for the onset potentials for oxidation and reduction relative to the Ag/AgCl quasi-reference electrode, respectively.36,37 The J–V curves were characterized using a solar simulator (Newport, Oriel Class AAA, 94043A) with a source meter (Keithley 2400) under 100 mA/cm2 illumination AM 1.5G and a calibrated Si-reference cell certificated by NREL at a scan rate of 100 mV/s. Before the test, all the solar cells were placed under 100 mA/cm2 illumination AM 1.5G for 40 seconds exposure to be stable. Active area of the device was 0.06cm2 defined by a nonreflective metal aperture. The IPCE spectra were recorded with an IPCE measurement system (model 2931-C, Newport, U.S.A.) using a 300 W xenon lamp (model 66902, Newport, U.S.A.) with a 1/4 monochromator (model 74125 Oriel Cornerstone 260, Newport, U.S.A.). The light intensity was calibrated with a silicon detector (model 71675, Newport, U.S.A.). Both the J–V curves and the IPCE spectra were measured in air atmosphere.

ACKNOWLEDGEMENTS This work was supported by the National Science Foundation of China (Grant No. 51502058, 61504041, 21571042), the China Postdoctoral Science Foundation (Grant No. 2015M570284), the Postdoctoral Foundation of Heilongjiang Province (LBH-TZ0604), and the Fundamental Research Funds for the Central Universities (Grant No. HIT AUGA5710055014). SUPPORTING INFORMATION

The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed information on absorption intensity at 526 nm versus the doping ratio of BCF; The conductivity values derived from the corresponding I-V curves of the BCF-doped spiro-OMeTAD


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films in Figure 3(d); AFM phase images and optical microscope images of the pristine spiroOMeTAD films, 5 mol% BCF-doped spiro-OMeTAD films and 10 mol% FK209-doped spiroOMeTAD films on ITO glass; AFM height image and phase image of 10 mol% FK209-doped spiro-OMeTAD film with 2μm scan on the micron-sized island distribution area.

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Improved Performance and Reproducibility of Perovskite Solar Cells

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