Enhanced Efficiency of Planar Heterojunction Perovskite Solar Cells

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Enhanced efficiency of planar heterojunction perovskite solar cells by a light soaking treatment on BCF-doped PTAA solution Tengling Ye, Wenbo Chen, Shan Jin, Sue Hao, Xiaochen Zhang, Hongcheng Liu, and Dongqing He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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ACS Applied Materials & Interfaces

Enhanced

efficiency

of

planar

heterojunction

perovskite solar cells by a light soaking treatment on BCF-doped PTAA solution Tengling Ye*†, Wenbo Chen†, Shan Jin†, Sue Hao†, Xiaochen Zhang‡, Hongcheng Liu‡, Dongqing He*‡ † 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. ‡Institute of Advanced Technology, Heilongjiang Academy of Sciences, Harbin 150020, P. R. China. KEYWORDS: perovskite solar cells, PTAA, light soaking, BCF, frustrated Lewis pairs

ACS Paragon Plus Environment

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ABSTRACT

This research used Lewis acid tris(pentafluorophenyl)borane (BCF) as a p-type dopant and a light soaking (LS) treatment to improve the conductivity of polytriarylamine (PTAA). Specifically, the conductivity of PTAA films was improved by two orders of magnitude using BCF as a p dopant, and the conductivity of BCF-doped PTAA films could be further improved by using the LS treatment on its solution. The working mechanism of the formation of frustrated Lewis pairs between BCF and PTAA was proposed to explain the BCF doping and LS treatment effect on the hole transport property of PTAA. When 5 min LS-PTAA films with 8 wt.% BCF were used as the hole transport layer in p-i-n planar heterojunction perovskite solar cells, a maximum power conversion efficiency of 17.12% was achieved. This work provides a deep understanding of the enhancement of the conductivity of PTAA by BCF doping and LS treatment. In addition, a convenient and quick light soaking method was explored to improve the conductivity of the PTAA hole transport material. Our findings may help in improving the hole transport properties of other organic photoelectric materials and devices.


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Recently, great progress has been achieved in the efficiency enhancement of perovskite solar cells (PVSCs) with power conversion efficiency (PCE) values rising from 3.8% to 22.7% only in 10 years.1,2 The device structure can be divided into mesoporous or planar heterojunction PVSCs. The planar heterojunction (PHJ) PVSCs is more attractive due to their less hysteresis effects, low-temperature solution processability, and adaptable mechanical flexibility.3,4 A typical device structure of p-i-n PHJ-PVSCs includes the stacking of a positive electrode, a holetransport layer (HTL), a perovskite light-absorption layer, an electron-transport layer, and a negative electrode (ITO/HTL/Perovskite/ETL/Al or Ag). Among these functional layers, the HTL maintains the extraction of photogenerarated holes and ensures holes to be efficiently transported to the positive electrode.5 A broad range of HTLs has been reported, including CuSCN, CuI, NiO, poly(N,N ′ -bis(4-butylphenyl)-N,N ′ -bis(phenyl)-benzidine) (Poly-TPD), poly(3,4-ethylenedioxythiophene)

polystyrene

sulfonate

(PEDOT:PSS),

and

poly[bis(4-

phenyl)(2,4,6- trimethylphenyl)amine] (PTAA). Inorganic materials usually suffer from poor solubility, leaky film, or high-temperature calcination.5,6 As a result, polymeric hole transport materials (HTMs) are adopted more frequently for p-i-n PHJ-PVSCs. Among these polymeric HTMs, PTAA is the most efficient HTM due to its good film forming characteristic at low temperature, matched highest occupied molecular orbital (HOMO) level, and non-wetting surface.7–9 However, it still suffers from low conductivity, and p-type additives are needed to obtain efficient PHJ-PVSCs.10,11 Tris(pentafluorophenyl)borane (BCF), a strong Lewis acid with excellent solubility in common solvents and low cost, has been used as an efficient p-type dopant in a wide range of Lewis basic organic semiconductors containing element of N, S, or Se to improve their hole transport properties.12–14 In our previous work, we introduced BCF as a p-type dopant into the well-known HTM spiro-OMeTAD in PVSCs for the first time, and the


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corresponding PCE of the mesoporous type PVSCs had been dramatically improved.15 So it is expected that BCF should be an efficient p-type dopant for PTAA. The light soaking (LS) effect is a common phenomenon in typical n-i-p PVSCs or inverted polymer solar cells. A kink shape J-V curve with a low PCE was usually observed for the devices incorporating metal oxide electron transport layers (e.g., TiOx, ZnO, and ITO); However, the kinked shape disappeared and an improved device performance was observed after the light exposure to the device.16,17 The LS effect of metal oxides probably originates from the electron traps that can be filled by LS treatment in the metal oxides.18 However, the LS phenomenon is usually observed in the side of the electron transport layer and it is rarely reported in the side of the hole transport layer. Combining the two strategies of doping and LS, this work presents a methodology of increasing the conductivity of PTAA HTM step by step simply by using the BCF dopant and LS treatment. By the introduction of the BCF dopant into PTAA, the PCE of PHJ-PVSCs could be improved as expected, and after the LS treatment, the PCE of PHJ-PVSCs could be further increased. A maximum power conversion efficiency of 17.03% was achieved. An interaction mechanism of the formation of frustrated Lewis pairs between BCF and PTAA is proposed and then the BCF doping and the LS treatment effects on the performance of PHJ-PVSCs were studied.

Figure 1. (a) Molecular structures of PTAA and BCF, (b) photograph and UV-vis absorption of BCF, PTAA, and BCF-doped PTAA solutions with 2%, 8%, 14% weight ratios.


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The molecular structures of PTAA and BCF are shown in Figure 1a. The pristine BCF solution in chlorobenzene is colorless and transparent, the pristine PTAA solution is yellowgreen with fluorescence, and the color of PTAA solutions with different doped ratios of BCF gradually deepens from yellow-green to pink. UV–vis absorption spectra of PTAA solutions with different BCF concentration and pristine BCF solution in chlorobenzene are shown in Figure 1b, and there is nearly no absorbance from 450 nm to 600 nm for pristine PTAA and BCF solutions. In contrast, the solutions of BCF-doped PTAA exhibited an obvious peak at 522 nm, and the intensity increases with increased BCF dopant concentration. Considering the molecular structures of the strong Lewis base triphenylamine donor unit in PTAA and the strong Lewis acid triarylboron acceptor unit in BCF, new absorption peaks of BCF-doped PTAA solutions should be generated because of the interaction between the PTAA host and the BCF dopant.14,19 To demonstrate the interaction mechanism between BCF and PTAA, proton nuclear magnetic resonance (1H NMR and 11B NMR) and Fourier transform infrared (FTIR) spectroscopy were performed. 1H NMR and 11B NMR can provide information about the interaction between various molecules by changing the chemical shift of hydrogen and boron atoms. The change of chemical shift is determined by the inductive effect, and the strong electron accepting ability of the BCF dopant can reduce the electron density of PTAA, resulting in the chemical shift movement of hydrogen atoms to the lower external magnetic field.20 Figure 2a shows the 1H NMR spectra of PTAA with and without BCF in chloroform-d solution. Different chemical shifts of hydrogen atoms on PTAA are distinguished as a, b, c, d, and e. As expected, the chemical shifts of b, c, d, and e all moved to a low external magnetic field, and the spin-spin splitting of a became overlapped with the introduction of BCF. These results indicate that the electron cloud density around the hydrogen atoms can be reduced by the very electron-


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deficient triarylborane group on BCF. Due to the strong steric hindrance of PTAA and BCF, the direct interaction to form classical Lewis pairs between B and N atoms is difficult. In this case, a frustrated Lewis pair (FLP) is expected to form, and it is a compound or mixture containing a Lewis acid and a Lewis base that cannot combine to form a classical adduct because of steric hindrance.21 As shown in Figure 2b, a working mechanism of the formation of BCF-doped PTAA FLP was proposed, and a BCF radical anion and a triphenylamine radical cation were first formed owing to the strong electron donating ability of triphenylamine, named FLP ion pair Ⅰ. The triarylborane group on the BCF radical anion is not stable and can attack the phenyl group on PTAA. Then, the tetra-coordinated BCF anion and triphenylamine cation could be produced, comprising FLP ion pair Ⅱ. FLP ion pair Ⅰ can reversibly turn to FLP ion pair Ⅱ. Both the triphenylamine radical cation and the triphenylamine cation are in the planar structure and can serve as the free charge carrier of mobile holes which determine the hole transport of PTAA.22 The presence of a tetra-coordinated BCF anion was confirmed by an 11B NMR resonance at δ= 11.3 ppm, which was shown in Figure S1.23–26 Similar FLPs have also been observed in many studies.21,27,28 The mechanism of the formation of FLP Ⅱ can be further supported by the FTIR spectra of pristine BCF powder and films based on PTAA with and without BCF, as shown in Figure 2c. FTIR spectroscopy is an established tool for structural characterization, and different electron cloud densities around the functional group should lead to different bond polarities on the functional group. Also, stronger bond polarities of the functional group correlate to higher absorption band wavenumbers. The IR peak of pristine BCF at 973.9 cm−1 is ascribed to the vibration absorbance of B-C bond, and no peak around 973.9 cm−1 for pristine PTAA was observed; The peak shifts to 977.7 cm−1 for the film of BCF-doped PTAA, indicating that the electron cloud density of B-C bond increased owing to the strong electron accepting ability of


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the triarylborane group. Correspondingly, the absorbance peaks of C=C bonds shifts from 1683.5 cm−1 to 1679.7 cm−1 for the pristine PTAA and BCF-doped PTAA films. The electron cloud density of C=C bonds on PTAA molecules decreased for the film of BCF-doped-PTAA due to the interaction between the triarylborane group and the phenyl group.20,26

Figure 2. (a) 1H NMR spectra of PTAA with and without BCF in chloroform-d solution, (b) the working mechanism of the BCF-doped PTAA HTL, (c) FTIR spectra of pristine BCF power and films based on PTAA with and without BCF. Since the BCF doping can increase the concentration of amine cations that can become the free mobile holes, the conductivity of PTAA film should be increased by BCF doping. The conductivity of PTAA film with different BCF-doped concentration can be derived from the IV curves (shown in Figure S2), and the results are summarized in Table S1. The results show that the conductivity of PTAA was improved by two orders of magnitude by BCF doping. So we can expect that the application of BCF-doped PTAA as an HTM in PHJ-PVSCs will improve device


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performance. A typical device structure of the inverted p-i-n PHJ-PVSCs (ITO/PTAA (or BCFdoped PTAA)/Perovskite/PC61BM/Bphen/Ag) was used, as shown in Figure S3. The average short-circuits current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and PCE of PVSCs with different BCF doping ratios are given in Figure S4(a-d), and the photovoltaic properties are summarized in Table 1. As the introduction of BCF to PTAA HTM, the Jsc improves from 18.74 mA/cm2 to 19.77 mA/cm2 and the FF and the Voc also increase with the introduction of the BCF dopant. As a result, the average PCE gives an optimal value of 15.49% at 8% BCF dopant concentration, and the PCE decreases when the concentration of BCF is further increased. The excessive BCF dopant may behave as recombination centers at the interface between FA0.5MA0.5PbI2.7Br0.3 and the PTAA HTL, which results in the decreased device performace. Table 1. Average device performance parameters of PVSCs based on PTAA HTL with different BCF-doped ratios. HTM ( w t . % )

Jsc(mA/cm2)

Voc(V)

FF (%)

Rs(ohm)

Rsh(Kohm)

PCE (%)

PTAA

18.74(±0.50)

1.00(±0.01).

72.29(±1.93)

1.14(±0.45)

3.32(±1.66)

13.60 (±0.63)

2% BCF-PTAA

19.52 (±0.31)

1.03(±0.01)

73.02 (±3.65)

0.62(±0.18)

6.24(±3.76)

14.73 (±0.68)

8% BCF-PTAA

19.77 (±0.15)

1.04(±0.00)

75.35 (±1.42)

0.60(±0.33)

3.49(±3.15)

15.49 (±0.26)

14% BCF-PTAA

19.60(±0.31)

1.02 (±0.02)

75.26(±2.29)

1.03(±0.32)

5.75(±2.98)

15.06(±0.90)

20% BCF-PTAA

19.43(±0.52)

1.03 (±0.01)

74.63(±1.84)

1.35(±0.62)

7.81(±4.06)

14.97(±0.17)

The results in Table1 show that 8 wt.% BCF is the optimal dopant concentration for PTAA and that excess BCF leads to increased carrier recombination. Is there any other way to increase the conductivity of PTAA without introducing excess BCF dopant? According to the forming process of the BCF-doped PTAA FLP Ⅰ, amine free radical can improve the content of amine


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cations. Also, light irradiation is a convenient method to strengthen the formation of amine free radicals on PTAA chains by exciting one electron on the N atoms to its excited state, the electrons of excited state are more active and can be got by BCF more easily.29 Thus, from the mechanism in Figure 2b, light irradiation can produce more amine free radicals and more free mobile holes on PTAA chains. The solutions of PTAA were exposed with 8 wt.% BCF under AM1.5G sunlight for 0-60 min, and the pink solution turned from light pink to dark red, as shown in the inset of Figure 3a. The corresponding UV–vis absorption spectra are consistent with the color change in Figure 3a. The UV-vis absorption peak at 522 nm gradually increases as the duration of LS treatment increases, and the intensity of the peak starts to decrease after the duration of LS treatment is over 30 min. Thus, the amount of amine cations can be improved dramatically by LS treatment with the same BCF dopant concentration. To show which wave band is the effective source for the light soaking, we treated the PTAA+8% BCF solution under solar simulator (1 sun) with different optical filter. From the absorption spectra in Figure 3b, it can seen that both ultraviolet (Filter ZWB3, transmission range 300nm-400nm, peak@357nm) and visible light (Filter QB22, transmission range 350nm-600nm peak@480nm) are effective for the light soaking, so we can control the light by simply adjusting the time of light soaking. The increase of amine free radicals on PTAA chains can be confirmed by electron spin resonance (ESR) spectroscopy, which is a sensitive method for the recognition of free radicals. In Figure 3c, there is nearly no radical signal for pristine PTAA solution, while a small peak is observed in the spectrum of the PTAA solution with 8 wt.% BCF. In comparison, the free radical characteristic peak for the PTAA solution with 8 wt.% BCF under LS treatment becomes obvious. The existence of free radicals can support the formation of FLP Ⅰ and prove that LS treatment can produce a higher free radical density; thus, more amine cations and a higher hole


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transport property of PTAA are expected. The conductivities of pure PTAA film, the PTAA film with 8 wt.% BCF, and the PTAA film with 8 wt.% BCF under LS treatment for 3 min were then compared. As shown in Figure 3d, PTAA film gives the lowest I-V curve, and the I-V curve becomes steep when BCF is introduced. The PTAA film with 8 wt.% BCF under LS treatment shows the highest I-V curve. The conductivity values can be derived from the I-V curves in Figure 3d, and the values are summarized in Table S1. The conductivity increases from 5.98×107

S·cm-1 for pure PTAA film to 4.55×10-5 S·cm-1 for the PTAA film with 8 wt.% BCF under LS

treatment for 3 min. These results show that the doping efficiency can be further improved by LS treatment at the level of 8 wt.% BCF. A new, convenient and quick route to generating more free mobile holes based on BCF doped PTAA films is provided by LS treatment to their solutions.

Figure 3. (a) Photograph and UV-vis absorption spectra of 8 wt.% BCF-doped PTAA solutions without or with LS treatment for different duration. (b) The absorption spectra of 10 min LS-8 wt.% BCF-doped PTAA solution using different optical filters. (c) ESR spectra of PTAA solution, 8 wt.% BCF-doped PTAA solution and 3 min LS-8 wt.% BCF-doped PTAA solution;


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(d) I-V curves of PTAA film, 8 wt.% BCF-doped PTAA film, and 3 min LS-8 wt.% BCF-doped PTAA film on 50 μm patterned ITO glass. To determine the LS effect on the device performance of p-i-n PHJ-PVSCs, we fabricated PVSCs based on 8 wt.% BCF-doped PTAA HTL by optimizing the duration of LS treatment. The corresponding I-V curves are given in Figure S5(a), and the photovoltaic properties are summarized in Table 2. The results prove the device performance can be further improved by LS treatment, and the optimal device performance is obtained when the duration of LS treatment is 5 min. The Jsc and FF can be further increased without additional BCF dopant because of the increase of the conductivity of BCF-doped PTAA HTL. The PVSC with 8 wt.% BCF-doped PTAA HTL under LS treatment for 5 min exhibits the best PCE of 17.12%. The J-V and the EQE curves are shown in Figure 4a and b. Figure 4c shows photoluminescence (PL) spectra of films on quartz glass (MAPbBrxI3-x, MAPbBrxI3-x/PTAA, and MAPbBrxI3-x/8 wt.% BCF-doped PTAA with and without LS treatment). Perovskite was excited at 470 nm with a 450 nm filter to exclude the PTAA luminescence. The pristine perovskite film shows a much higher PL intensity at 765 nm than the other films, and the intense fluorescence of the MAPbBrxI3-x film has been quenched efficiently after the introduction of HTLs. Greater fluorescence quenching implies more efficient hole extraction from perovskite to HTL. These fluorescence quenching results indicate an efficient hole extraction when PTAA is doped by BCF, and LS treatment can further strengthen the hole extraction ability of BCF-doped PTAA. These results also agree well with the results of the conductivity measurements. Thus, BCF is an effective dopant to promote the hole transport of PTAA, and LS treatment makes the hole transport ability of PTAA better. The corresponding devices display higher Jsc and FF compared with the devices without LS. In addition, the Voc also increases a bit from 1 V (pristine PTAA) to 1.04 V (doped PTAA). We


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have measured the Peakforce KPFM to illustrate the change of energy level, as shown in Figure 4 d. It was found that the work function decreased from -5.15 eV (PTAA) to -5.20 eV (8% BCF doped PTAA) and -5.21 eV (LS 3min-8% BCF doped PTAA). The decreased work function lead to the increase of Voc. Together with the improvement of the conductivity of PTAA, the device performance is improved. When the duration of LS treatment is over 5 min, the device performance gradually decreases. Although hole carrier concentration is increasing until 30min, the device performance didn’t increase further. It is probably because higher hole carriers concentration (more FLPs) results in the increasing density of traps and scattering centers in the HTL, which plays the detrimental effects on the charge transport. In other words, the increased hole carrier concentration (FLP) does not necessarily mean an increase in the charge carrier mobility, which lead to the slightly lower Jsc and FF values at longer LS treatment.30 Table 2. Optimized device performance parameters of PVSCs based on PTAA and 8% BCFdoped PTAA HTLs with different durations of LS treatment. HTM ( w t . % )

Jsc(mA/cm2)

Voc(V)

FF (%)

PTAA

19.63

1

74.47

14.62

8% BCF+PTAA

20.07

1.04

75.52

15.76

1 min LS-8% BCF+PTAA

20.20

1.04

74.05

15.55


20.73

1.04

79.40

17.12


19.93

1.04

77.10

15.98


20.19

1.04

76.24

16.01

P C E

( % )


12

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19.45

1.04

76.10

15.39

The stability of the PSCs with LS-8% BCF+PTAA as HTLs was also studied. To illustrate the stability of the LS-8 wt.% BCF-doped PTAA film, we measured the absorption spectra of 3min LS-PTAA+8% BCF Film, the results are shown in Figure S5(b). It can be seen that the absorption spectra of 3minLS-PTAA+8% BCF film didn’t changed a lot from 450nm to 700nm. It means that the radical is relatively stable in film and then the film conductivity is sable. It may be because that the steric hindrance increases the stability of the radical. Figure S5(c) gives the stability test of the device based on 10min LS-8% BCF+PTAA HTL and the performance can maintain 70% after 100h. We also measured the device hysteresis as shown in Figure S5(d), the devices based on 10min LS-PTAA+8%BCF HTL exhibit small hysteresis J–V characteristics with forward and reverse scanning directions.

Figure 4. (a) The optimal J-V curve and (b) EQE of PVSC with 5 min LS-PTAA+8% BCF HTL, (c) Steady-state PL spectra of the perovskite films on different HTMs (d) Peakforce KPFM and the energy diagram.


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In conclusion, BCF as a p-type dopant was introduced to PTAA HTM, and the conductivity of PTAA was improved by two orders of magnitude. By using the BCF dopant and treating with LS, the conductivity of PTAA can be further improved. We proposed the working mechanism of the formation of FLPs between PTAA and BCF to explain the effects of the BCF dopant and the LS treatment on the hole transport property. The results showed that both the BCF dopant and LS treatment can improve the conductivity of PTAA by the formation of FLP. The intrinsic difference between BCF and LS treatment is that LS treatment improves the doping efficiency without increasing the concentration of BCF. After introducing the BCF dopant, the device efficiency increased from 13.60% to 15.49%, and the device efficiency could be further increased by LS treatment. A maximum PCE of 17.12% was achieved with a Jsc of 20.73 mA/cm2, a Voc of 1.04 V and a large FF 79.40%, and the optimal duration of LS treatment is 5 min. This work provides a simple and quick route to improve the hole transport property of ptype organic semiconductors and may be extended to other organic photoelectric fields. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: Materials, device fabrication, characterization and measurements, B NMR, conductivity results, device structure diagram and transmission spectra of the two optical filters, box charts of Jsc Voc, FF and PCE of PVSC devices with different BCF doped ratios, I-V curvers of PVSC devices listed in table 2, Nyquist plots, the stability of the LS-8 wt.% BCF-doped PTAA film, the stability test of the device based on LS-8% BCF-doped PTAA HTL and the device hysteresis. AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] *E-mail:[email protected] ORCID Tengling Ye: 0000-0002-6979-3315 Dongqing He: 0000-0003-3950-2588 Funding Sources This work was supported by the National Science Foundation of China (Grant No. 51502058, 61504041), the China Postdoctoral Science Foundation (Grant No. 2015M570284), the Postdoctoral Foundation of Heilongjiang Province (Grant No. LBH-TZ0604), the Special Fund of Technological Innovation Talents in Harbin City (Grant No. 2017RAQXJ085), and the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (Grant No. HIT.NSRIF2019042). REFERENCES (1)

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