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Non-reduction-active hole-transporting layers enhancing opencircuit voltage and efficiency of planar perovskite solar cells Tiefeng Liu, Fangyuan Jiang, Fei Qin, Wei Meng, Youyu Jiang, Sixing Xiong, Jinhui Tong, Zaifang Li, Yun Liu, and Yinhua Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13324 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 19, 2016

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ACS Applied Materials & Interfaces

Non-reduction-active hole-transporting layers enhancing open-circuit voltage and efficiency of planar perovskite solar cells Tiefeng Liu,a,b Fangyuan Jiang,a,b Fei Qin,a Wei Meng,a Youyu Jiang,a,b Sixing Xiong,a Jinhui Tong,a Zaifang Li,a,b Yun Liu,a and Yinhua Zhoua,b*

a:Wuhan National Laboratory for Optoelectronics, and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China b:Research Institute of Huazhong University of Science and Technology in Shenzhen, Shenzhen 518057, China E-mail: [email protected]

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Abstract Inverted planar perovskite solar cells using poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) as the hole-transporting layer (HTL) are very attractive because of their low-temperature and easy processing. However, the planar cells with the PEDOT:PSS HTL typically display lower open-circuit voltage (VOC) (about 0.90 V) than that of devices with TiO2-based conventional structure (1.0 - 1.1 V). The underlying reasons are still not clear. In this work, we report the PEDOT:PSS that is intrinsically p-doped can be chemically reduced by methylamine iodide (MAI) and MAPbI3. The reaction reduces the work function (WF) of PEDOT:PSS, which suppresses the efficient hole collection and yields lower VOC. To overcome this issue, we

adopt

undoped

semiconducting

polymers

that

are

intrinsically

non-reduction-active (NRA) as the HTL for inverted planar perovskite solar cells. The cells display enhanced VOC from 0.88 ± 0.04 V (PEDOT:PSS HTL, reference cells) to 1.02 ± 0.03 V (P3HT HTL) and 1.04 ± 0.03 V (PTB7 and PTB-Th HTL). The power conversion efficiency (PCE) of the devices with these NRA HTL reaches about 17%.

Keywords: perovskite solar cells, hole-transporting layers, open-circuit voltage, non-reduction-active, PEDOT:PSS.

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1. Introduction The organic-inorganic hybrid perovskite solar cells have attracted enormous research interest in past few years due to the rapid increase of their PCEs and prospects for low-cost solution-processing fabrication.1-5 Among the structure of perovskite solar cells, mesoscopic TiO2-based structure is one of the most widely used. This structure delivers the record-high PCE for perovskite cells so far.4 A drawback of this structure is that the fabrication of the mesoscopic TiO2 layer requires high-temperature (about 500 ºC) sintering that is energy-consuming and not compatible with flexible plastic substrates. Inverted planar perovskite solar cells are another type of attractive structure because of their low-temperature fabrication and compatibility with flexible devices. The polarity of the inverted structure is h-AL-e (direction: from substrate to top electrode, where h- and e- denote hole- and electron-transporting layer.) The e-AL-h and h-AL-e structures are generally called conventional structure and inverted structure, respectively. Among the HTLs of inverted planar cells, PEDOT:PSS is the most popular one because of its low cost, easy processing, and also yielding a good device performance.6-11 However, there is deficiency for PEDOT:PSS-based devices, i. e, the cells display lower VOC of about 0.90 V comparing with the typical TiO2-based devices12, 13, or other e-AL-h cells with PCBM as the ETL14, or other inverted planar perovskite cells with NiOX HTL15-17. As shown in Figure 1a, the VOC comparison of four structures is presented (obtained from 20 devices). The underlying reasons are still not clear. Some reports ascribe it to the mismatched energy levels between WF of -3-



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PEDOT:PSS (−5.1 eV) and the valence band of MAPbI3 (−5.4 eV).18, 19 However, some

HTLs

[such

as:

4,4,4,4-(ethene-1,1,2,2-tetrayl)tetrakis(N,N-bis(4-methoxyphenyl)aniline): -4.90 eV; PTPAFSONa:

-5.16

eV;

2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene: -5.11 eV] have similar highest occupied molecular orbital (HOMO) energy levels and can also reach a higher VOC above 1.0 V.19-21 The VOC can be influenced by many factors, such as grain size, trap density, chemical composition, and charge-collecting interfaces.22-26 As for PEDOT:PSS, the PEDOT:PSS are synthesized via oxidation polymerization and PEDOT chain is highly p-doped via oxidation (the doping ratio is 1/3).27 The highly

doped

property

indicates

the

PEDOT:PSS

films

are

intrinsically

chemical-reduction-active. Indeed, PEDOT:PSS could be chemically reduced by amine

groups

such

as

polyethylenimine28,

29

,

methylamine30

and

tetrakis(dimethylamine)ethylene31. The chemical reduction of PEDOT:PSS leads to the decrease of the WF as well as conductivity, which will yield poor hole collection in solar cells.30 In this work, we report that MAI and MAPbI3 perovskite solution could chemically reduce PEDOT:PSS HTL and reduce its WF, which suppresses the efficient hole collection and yields a relatively lower VOC (about 0.9 V). To overcome this issue, we select undoped semiconducting polymers as HTLs, including poly(3-hexylthiophene)

(P3HT),

poly[(ethylhexyloxy)-benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7), and -4-


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poly[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)thienothiophene] (PTB7-Th). These polymers usually act as electron donors in organic photovoltaics. HOMO levels of these polymers are close to the WF of PEDOT:PSS. Different from the oxidation state of PEDOT:PSS, these polymers are in neutral states and intrinsically non-reduction-active (NRA). The devices with NRA HTLs display enhanced VOC of 1.0 - 1.1V that is 0.1 - 0.2 V higher than that of the cells based on PEDOT:PSS HTL. The cells with the NRA HTLs exhibit a PCE of about 17%.

2. Results and discussions 2.1 Chemical reduction of PEDOT:PSS hole-transporting layer by MAI and MAPbI3 perovskite solution The absorbance spectra of PEDOT:PSS films with different treatment was depicted in Figure 1b. With the solvent [isopropanol (IPA) or mixture of γ-butyrolactone (γ-GBL) and dimethylsulfoxide (DMSO), 7:3 (v/v)] treatment, there is nearly no difference in the absorbance spectrum comparing to the pristine film. However, after the MAPbI3 perovskite film deposited on PEDOT:PSS layer and was washed by solvent, there appeared a small shoulder at around 1000 nm. Moreover, when PEDOT:PSS film was treated by MAI, the absorbance at around 1000 nm becomes stronger and absorbance at longer wavelength decreases (>1400 nm). This is a typical characteristic of the reduction of PEDOT:PSS,29-31 indicating that PEDOT:PSS film could be reduced by the MAPbI3 precursor solution during the film -5-



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coating. The PEDOT:PSS HTL is synthesized by oxidative polymerization and its aqueous dispersion is stabilized in oxidized state (rather than neutral state). Therefore, the PEDOT:PSS is reduction-active. It can be reduced by strong reduction reagent. The reduction will result in the decrease of WF.30, 31 The iodide in MAI is Lewis base and is a potential reduction reagent,32,

33

which contribute to the reduction of

PEDOT:PSS and lead to the absorance change. To further test if the interaction exists between the MAI and PEDOT:PSS, a 5 wt.% of MAI was added into the PEDOT:PSS dispersion. As shown in Figure 1c, the PEDOT:PSS dispersion became to gel after the addition of 5 wt.% MAI which indicates the interaction between PEDOT:PSS and MAI. The MAPbI3 precursor solution consists of the MAI and PbI2 dissolved in solvents. The precursor could play the similar role as the MAI only in solvent and reduce the WF of PEDOT:PSS HTL film. The WF of pristine and solvent-treated PEDOT:PSS is nearly the same around 5.14 eV, as shown in Figure 1d. After depositing of MAPbI3 perovskite on PEDOT:PSS layer from its precursor solution and washed away by solvent, the WF of the PEDOT:PSS film decrease from 5.14 eV to 4.82 eV. The MAI-treatment decreases the WF from 5.14 eV to 4.75 eV. The change tendency of WF is consistent with the results of absorbance spectra. Both WF and absorbance spectra indicate that the PEDOT:PSS film suffers chemical reduction during the perovskite formation. The reduced WF of PEDOT:PSS increases the energy-level mismatch between the WF of PEDOT:PSS HTL and the HOMO level of perovskite layer, which yields lower VOC, -6-


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as shown in Figure 1e.

2.2 Non-reduction-active hole-transporting layer for perovskite solar cells To avoid the reduction during the perovskite formation, we choose three intrinsically NRA polymers as the HTL for perovskite solar cells: P3HT, PTB7, and PTB7-Th. These three polymers are widely used as electron-donors in organic solar cells and they have high hole motilities.34, 35 HOMO levels of the three polymers are 5.00 eV (P3HT), 5.17 eV (PTB7), and 5.22 eV (PTB7-Th) respectively,36 which are close to the WF of PEDOT:PSS. The device structure, HOMO levels and molecular formula of HTLs are shown in Figure 2.

2.2.1 Wetting and film morphology on the semiconducting HTL

Different from PEDOT:PSS, the surface of NRA polymer films are more hydrophobic with low surface energy. The contact angle images of perovskite precursor solutions on the NRA polymer films are shown in Figure 3. Compared with PEDOT:PSS with a contact angle of 3.51°, the perovskite precursor solutions on the surfaces of NRA HTLs display much larger contact angles: 71.9° for P3HT, 63.3° for PTB7, and 63.8° for PTB7-Th. The perovskite can hardly form a full coverage film on these NRA polymers. In previous studies, Zhao et al.37 and Bi et al.38 have prepared perovskite films on the surfaces of poly-TPD and PTAA (that are also hydrophobic) -7-



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through depositing hot lead iodide solution and MAI in sequence. Here, to fulfill the simple one-step solution deposition requirement, we treated NRA polymer with air-plasma to reduce the contact angle. After air-plasma treatment for 30 s, the contact angle of the MAPbI3 perovskite precursor solution on NRA polymers decreases significantly (P3HT: from 71.9° to 27.9°, PTB7: from 63.3° to 6.8°, and PTB7-Th: from 63.8° to 9.0°) that could allow the deposition of high-quality perovskite films. The morphology of MAPbI3 perovskite films deposited on different HTLs was characterized using scanning electron microscopy (SEM). According to the SEM images, all the perovskite could form compact and pinhole-free films the grain size is about 200 nm regardless of the HTLs (Figure 4a-d). The X-ray diffraction (XRD) spectroscopy was used to examine the perovskite structures deposited on different HTLs. From the XRD spectra (Figure 4e), the main peaks at 13.98o, 28.32o and 43.06o, corresponding to the (110), (220), and (310) diffraction planes of the perovskite respectively, were in accordance with the previous studies.30

2.2.2 PL lifetime and quenching

We tested the steady-state photoluminescence (PL) and time-resolved PL decay of the perovskite films to investigate the hole-extraction ability of the HTLs. The samples with configurations of glass/MAPbI3 and glass/HTL/MAPbI3 were measured. As shown in Figure S1, compared with the PEDOT:PSS/MAPbI3 sample, the NRA polymer/MAPbI3 samples show the similar PL quenching and decay time, indicating that all the HTLs can collect and transport hole efficiently. The related parameters of -8-


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time-resolved PL decays are summarized in Table S1.

2.2.3 Solar cells performance

The statistical data of photovoltaic performance of 35 devices are summarized in Table 1 and the champion current density-voltage (J-V) curves are presented in Figure 5a. The cells with PEDOT:PSS HTL displayed an champion PCE of 13.37% with open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF) of 0.88 V, 20.25 mA cm−2, and 0.75, respectively, which is in accordance with the previous report.39,

40

By substitution of the PEDOT:PSS with P3HT, PTB7 and

PTB7-Th, the best PCEs of devices are 16.80%, 17.02% and 17.08% respectively, which are higher than the PEDOT:PSS-based devices. The PCE distributions of different HTLs were presented in Figure 5d. Particularly, the major parameter that contributed to the PCEs improvement in the NRA polymer-based devices is VOC. All the NRA polymer-based devices have a VOC above 1.0 V while the VOC of PEDOT:PSS-based devices is around 0.88 V. Moreover, the devices with PEDOT:PSS HTL presented a wider distribution of VOC, compared with the NRA polymer-based devices, as shown in Figure 5c. This may be related to the varied reduction degree of PEDOT:PSS during each perovskite forming processing, leading the variable WF of PEDOT:PSS.41 On the contrary, the neutral NRA polymers are not affected significantly during the coating of the perovskite precursor solution. Besides VOC, the increasing JSC also leads to the PCE improvement, and the increase of JSC is in agreement with the external quantum efficiency (EQE) spectrum (Figure 5b). FF of -9-



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these devices is similar around 0.74. We also measured the hysteresis behavior under different scan directions (Figure S2) and scan rates (Figure S3). Notably, all devices displayed insignificant hysteresis (< 1% in PCEs). The insignificant hysteresis of inverted devices should be due to the fullerene passivation effect, which is in agreement with the previous study.42 Because of the hydrophobic of the NRA polymers, the stability of devices should be improved as compared with PEDOT:PSS.38 We monitored devices in dark and under an ambient condition of 25 °C and 80 % relative humidity without any encapsulation. As shown in Figure 6, after 200 hours, the NRA polymer-based devices maintained more that 60 % of their initial PCEs while the PEDOT:PSS-based devices only maintained 40 % of their initial PCEs. The main loss of the PCE was contributed to JSC. As mentioned above, the poorer stability might attribute to the acidity and hygroscopicity of PEDOT:PSS that influence the moisture-sensitive MAPbI3 perovskite films.41, 43

Conclusions In summary, we report the PEDOT:PSS that is intrinsically p-doped can be chemically reduced by MAI and MAPbI3 during the perovskite formation processing. The chemical reaction reduces the WF of PEDOT:PSS, which suppresses the efficient hole collection and yields lower VOC. By simply replacing PEDOT:PSS with NRA polymer, we achieved perovskite solar cells with high Voc (> 1.0 V) and high PCE (~ - 10 -


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17%). In addition, owing to the hydrophobicity of NRA, the stability of perovskite solar cells also was improved. The work reveals a reason of the lower VOC in PEDOT:PSS-based cells and provided a new route to enhance VOC and efficiency of perovskite solar cells.

Acknowledgements The work is supported by the Recruitment Program of Global Youth Experts, the National High-tech R&D Program of China (863 Program, No. 2015AA034601), the National Natural Science Foundation of China (Grant No. 21474035, 51403071), the Fundamental Research Funds for the Central Universities, HUST (Grant No. 2016JCTD111),

Science

and

Technology

Program

of

Shenzhen

(JCYJ20160429182443609) and China Postdoctoral Science Foundation funded projects (2015T80794).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Experiment section; Steady-state PL spectra and time-resolved PL; J-V characteristics of perovskite cells with different hole-transporting layers at different scan directions and rates.

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Author contributions T.F.L. and Y.H.Z. conceived the idea. T.F.L., F.Y.J, F.Q., Y.Y.J., S.X.X., J.H.T., Z.F.L., and Y.L, performed the solar cell fabrication, characterization and optimization. W.M. optimized and characterized the hole-transporting layer. Y.H.Z. directed this work. T.F.L. wrote the first draft of the manuscript. All the authors revised and approved the manuscript.

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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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Table 1. The photovoltaic parameters of perovskite solar cells (averaged over 35 devices) with different HTLs under 100 mW cm−2 AM 1.5 G illumination. (a: best means the highest efficiency.) Hole-transporting

V (V) oc

materials PEDOT:PSS

P3HT

PTB7

PTB7-Th

2

J (mA/cm )

FF

PCE (%)

sc

average

0.88±0.04

18.08±0.91

0.77±0.02

12.24±0.91

besta

0.88

20.25

0.75

13.37

average

1.02±0.03

19.21±1.39

0.77±0.03

15.02±1.02

besta

1.02

21.87

0.75

16.80

average

1.04±0.02

18.77±1.07

0.74±0.03

14.49±1.18

besta

1.07

20.37

0.78

17.02

average

1.04 ±0.02

19.49±0.97

0.74±0.04

15.01±1.20

besta

1.05

20.79

0.78

17.08

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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


Figure 1. (a) The VOC comparison of different device structures, 1 and 2 are in conventional structure with PEPOT:PSS and NiOX as the HTL respectively; 3 and 4 are in inverted structure with TiO2 and doped-PCBM as the ETL respectively. (b) Absorbance spectra of PEDOT:PSS film with different treatment. (Insert is the molecular structure of PEDOT:PSS) (c) Pictures of PEDOT:PSS before and after adding MAI. (d) Work function of PEDOT:PSS with different treatment. (e) Schematic energy diagrams of the PEDOT:PSS-based perovskite solar cells.

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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

Figure 2. (a) Schematic device structure of the inverted planar perovskite solar cells. (b) Energy diagrams of the cells using different hole-transporting materials. (c) The molecular structure of non-active-reduction polymer.

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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


Figure 3. The contact angle images of MAPbI3 perovskite precursor solutions on different HTLs: (a) PEDOT:PSS; (b) air-plasma treated PEDOT:PSS; (c) P3HT; (d) air-plasma treated P3HT; (e) PTB7; (f) air-plasma treated PTB7; (g) PTB7-Th; (h) air-plasma treated PTB7-Th.

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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

Figure 4. SEM images of MAPbI3 perovskite films on different HTLs: (a) P3HT; (b) PTB7; (c) PTB7-Th; (d) PEDOT:PSS. (e) XRD patterns of these perovskite films.

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Figure 5. (a) J–V characteristics; (b) EQE spectra; histogram of (c) VOC; (d) PCE of perovskite solar cells with different HTLs.

2

(b) 100

0 -5

80

PEDOT:PSS P3HT PTB7 PTB7-Th

-10

EQE (%)

-15 -20

60

PEDOT:PSS P3HT PTB7 PTB7-Th

40

20

-25

0.0

0.2

0.4

0.6

0.8

1.0

700

800

PEDOT:PSS

8

4

4

0

0

8

P3HT

4 0 16 12 8 4 0 16 12 8 4 0 0.80

600

Wavelength (nm)

PEDOT:PSS

8

500

(d)

Voltage (V)

(c)

0 400

Number of devices

Current density (mA/cm )

(a)

Number of devices

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


PTB7

8

P3HT

4 0 PTB7

8 4 0

PTB7-Th

8

PTB7-Th

4 0.84

0.88

0.92

0.96

1.00

1.04

0

1.08

VOC (V)

9

10

11

12

13

14

PCE (%)

- 21 -


15

16

17

18


Figure 6. Normalized (a) VOC, (b) JSC, (c) FF and (d) PCE of perovskite solar cells based on different HTLs as a function of storage time in dark and under an ambient condition of 25 °C and 80 % relative humidity without any encapsulation.

(a)

(b)

1.2

1.0 1.0

0.6

HTL: PEDOT:PSS P3HT PTB7 PTB7-Th

0.4 0.2 0.0

(c)

Norm. JSC Jsc Normalization

Norm. Voc

0.8

0

50

100

150

200

Time (h)

0.8

0.4 0.2 0.0

1.0

1.0

0.8

0.8

0.6 0.4 0.2 0.0

PEDOT P3HT PTB7 PTB7-TH

0.6

(d)

Norm. PCE

Norm. FF

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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0

50

100

150

200

150

200

Time (h)

0.6 0.4 0.2

0

50

100

150

200

0.0

0

50

Time (h)

100 Time (h)

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Graphical abstract

0.20

2

0.15

0.10

PEDOT:PSS PEDOT:PSS+DMSO/GBL PEDOT:PSS+IPA

0.05

0.00

600

800

1000

1200

1400

Current density (mA/cm )

0 PEDOT:PSS+perovskite+DMSO/GBL PEDOT:PSS+MAI

Absorbance (a.u.)

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


1600

Ag PEI PCBM

-5

CH3NH3PbI3

-10

HTL -15

ITO Glass

-20 P3HT PTB7 PTB7-Th

PEDOT:PSS

-25 0.0

0.2

Wavelength (nm)


0.4

0.6

Voltage (V)

0.8

1.0

1.2

Nonreduction-Active Hole-Transporting Layers ... - ACS Publications

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