Chemical Dopants Engineering in Hole Transport Layer for Efficient

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Chemical Dopants Engineering in Hole Transport Layer for Efficient Perovskite Solar Cells: An Insight into the Interfacial Recombination Jinbao Zhang, Quentin Daniel, Tian Zhang, Xiaoming Wen, Bo Xu, Licheng Sun, Udo Bach, and Yi-Bing Cheng ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06062 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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Chemical Dopants Engineering in Hole Transport Layer for Efficient Perovskite Solar Cells: An Insight into the Interfacial Recombination Jinbao Zhang, a Quentin Daniel,b Tian Zhang,a Xiaoming Wen,c Bo Xu,d Licheng Sun,b,e Udo Bach,f, g, h Yi-Bing Chenga,i*

a

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

Australia. b

Organic Chemistry, Centre of Molecular Devices, Department of Chemistry, Chemical

Science and Engineering, KTH Royal Institute of Technology, SE-10044, Stockholm, Sweden c

Centre for Micro-Photonics, Swinburne University of Technology, Melbourne, Victoria,

3122, Australia d

Physical Chemistry, Department of Chemistry-Ångström Laboratory, Uppsala University,

Box 523, 751 20 Uppsala, Sweden e

State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center

on Molecular Devices, Dalian University of Technology, 116012 Dalian, China f

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

g

CSIRO Manufacturing, Clayton, VIC, 3168, Australia

h

Melbourne Centre for Nanofabrication, Clayton, VIC, 3800, Australia

h

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of

Technology, Wuhan, Hubei, 430070 China

1 ACS Paragon Plus Environment

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TOC

1.15

∆V: ∼ +100 mV

1.10 1.05

Voc / V

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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1.00

Chemical dopants 1

0.95

N Cu

0.90

N

0.85

N 2+

PF6PF6-

N

TiO2

Perovskite S Spiro-OMeTAD

CuII(dpm)2(PF6)2

0

5

10

S Cu2+

S S S+ S

S S

HTM layer Au S+ Spiro-OMeTAD+

Electron recombination Hole injection

JQ-1

0.80

+ + + +

15

20

25

30

35

40

45

Time / s


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ABSTRACT Chemical doping of organic semiconductors has been recognized as an effective way to enhance the electrical conductivity. In perovskite solar cells (PSCs) various types of dopants have been developed for organic hole transport materials (HTMs), however, the knowledge on the basic requirements for being efficient dopants as well as the comprehensive roles of the dopants in PSCs has not been clearly revealed. Here, three copper-based complexes with controlled redox activities are applied as dopants in PSCs, and it is found that the oxidative reactivity of dopants presents substantial impacts on conductivity, charge dynamics and solar cell performance. A significant improvement of open-circuit voltage (Voc) by more than 100 mV and an increase of power conversion efficiency from 13.2% to 19.3% have been achieved by tuning the doping level of the HTM. The observed large variation of Voc for three dopants reveals their different recombination kinetics at the perovskite/HTM interfaces and suggests a model of interfacial recombination mechanism. We also suggest that the dopants in HTMs can also affect the charge recombination kinetics as well as the solar cell performance. Based on these findings, a strategy is proposed to physically passivate the electron-hole recombination by inserting an ultrathin Al2O3 insulating layer between the perovskite and the HTM. This strategy contributes a significant enhancement of the power conversion efficiency and environmental stability, indicating that dopant engineering is one crucial way to further improve the performance of PSCs. Key words: chemical dopants; hole transport materials; perovskite solar cells; interfacial recombination; passivation; Al2O3.


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Organic-inorganic hybrid perovskites ABX3 (A: cations; B: Pb or Sn; X: halides) exhibit excellent photovoltaic properties as light harvesting materials in solar cells owing to their high absorption coefficient, high defect tolerance, long charge carrier lifetime and long carriers diffusion length.1-4 Comprehensive optimization of the perovskite composition as well as the device architecture lead to a rapid increase in power conversion efficiency (PCE) of perovskite solar cells (PSCs) from 3.6% to over 22% during the past few years.5,6 Furthermore, the hole selective contacts, work function alignment, carrier mobility as well as their interfaces play crucial roles in charge extraction and transportation in the device,7 which thus significantly determine the PCE, current-voltage hysteresis and stability of the PSCs.7,8 Organic molecules have shown great promise as HTMs owing to the ease of tuning optical gap through structural variability and their high potential for cheap and large-scale processing. However, organic small molecules generally have poor charge transport property. In this respect, great efforts have been made to design efficient hole transport materials (HTMs) or to modify HTM composition in order to achieve high conductivity, due to the fact that typically pristine organic molecules (e. g. for the state-of-the-art HTM spiro-OMeTAD: ~9×10-7 S·cm-1) have low conductivity that significantly limits the photovoltaic performance of PSCs.9-13 Therefore, chemical doping has been widely recognized as one of the most valuable strategies to improve the electronic conductivity of the HTM and thereby to enhance the PCE of the PSCs.14-16 So far various types of dopants have been developed to increase spiro-OMeTAD conductivity,

such

as

tetracyanoquinodimethane,

cobalt-based 18

complexes,17

2,3,5,6-

tetrafluoro-7,7,8,8-

SnCl4,19 silver bis-(trifluoro-methane-sulfonyl)-imide,20

1,1,2,2-tetrachloroethane,21 protic ionic liquid,22 copper-based complexes,23 iridiumbased complexes,24 phosphoric acid,25 and O2 in the presence of LiTFSI.26 Therein,


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metal complexes as dopants have been commonly applied in PSCs due to their easy synthesis, tunable redox potentials by changing ligands as well as good charge transport property.23,27 The key function of dopants is to partially oxidize spiroOMeTAD so as to introduce hole concentration by spiro-OMeTAD+ formation, thereby to accelerate the hole hopping process in the HTM. Although different types of dopants have been used in PSCs, there is limited understanding of the basic requirements for being an efficient dopant for PSCs. Furthermore, there are hardly any studies on the comprehensive roles of dopants on the charge dynamics (such as interfacial charge transfer, transport and recombination) as well as on the photovoltaic parameters in PSCs. Hence, it is desirable to have a further understanding of the effects of the dopants in HTM and to establish a design strategy of dopants for organic molecules in efficient PSCs.


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(a) N + Cu 2 N N

N + Cu 2 N N

N

PF6PF6-

CuII(dpm)2(PF6)2

JQ3

2.5x104

0.3

2.0x104

0.2

0.1

1.5x10

4

1.0x10

4

5.0x103 0.0

JQ2

N

JQ-2

JQ-3

JQ1

JQ3

JQ2

0.002

0.0

0.000 -0.002

Wavelength / nm

0.07 V

-0.004

0.17 V

0.33 V

-0.006 -0.008 -0.010

250 300 350 400 450 500

-0.4 -0.2 0.0 0.2 0.4 0.6-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Voltage / V vs Fc/Fc+

(d)


0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 110% 130% 150% 170% 190% 210%

JQ3

1.5

PF6PF6-

2+

CuII(dmbp)2(PF6)2

0.004

Current / A

JQ1

Cu N

(c) 0.006

3.0x104 Extinction coefficient / M-1cm-1

Absorbance

0.4

N

CuII(dpe)2(PF6)2

JQ-1

(b)

N

PF6PF6-

spiro-OMeTAD+ 523 nm 1.0

0.5


(e) 50 45

mol% of spiroOMeTAD+

N

Absorbance

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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40

FK209

35 30

JQ3

25 20 15

JQ2

10

JQ1

5 0

0.0 300

400

500

600

700

800

0

Wavelength / nm

5

10 15 20 25 30 35 40 45 50

mol % of dopants

Figure 1. (a) Chemical structure of the dopants JQ1, JQ2, and JQ3. (b) UV-vis absorption spectra and extinction coefficient of JQ1, JQ2 and JQ3 in acetonitrile with a concentration of 0.015 mM. (c) Cyclic voltammetry curves of JQ1, JQ2 and JQ3 in acetonitrile; working electrode: glassy carbon, reference electrode: Ag/AgCl, counter electrode: platinum wire, and electrolyte: 1 mM JQ1, JQ2 or JQ3 in 0.1 M LiTFSI. (d) UV-vis absorption of 0.015 mM spiro-OMeTAD in chlorobenzene when gradually adding JQ3 (3 mg/mL in acetonitrile) with increasing molar ratios. (e) Dependence of the generation of the spiro-OMeTAD+ on the concentration of JQ1, JQ2, JQ3 and FK209.


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In this article, we present three copper-based complexes (JQ1, JQ2 and JQ3) as dopants in HTM, and systematically reveal the effects of dopants relating to hole conductivity, charge recombination dynamics as well as device performance. Firstly, the redox potential of these copper complexes significantly influences the doping level and thus the conductivity of spiro-OMeTAD. In addition, we found that a higher doping level in spiro-OMeTAD than the optimal condition will produce detrimental effects on the interface recombination. Balancing the charge transport and interface recombination yields a high Voc increase by ~100 mV in PSCs. The optimized PSC with dopant JQ1 delivers a high PCE of 19.3%, which is much higher than that with the state-of-the-art cobalt dopant FK209 (18%) and also the one without dopant (13.2%). Secondly, by keeping same level of spiro-OMeTAD+ with different dopants, it is obvious to see a variation of Voc and different recombination kinetics, which suggest that dopants can also act as recombination sites in PSCs. Based on these findings we have established an interfacial recombination mechanism at the perovskite/HTM interface, and also revealed that the redox activity of the dopant is one of the most important selection criteria for efficient p-dopants. Lastly, a strategy of interfacial passivation by an insulator Al2O3 layer has been successfully developed to suppress the aforementioned recombination pathways, leading to a significant enhancement of PCE from 15.6% to 18% for the strong dopant JQ3.


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RESULTS/DISCUSSION Figure 1a shows the chemical structures of the copper-based complexes, namely JQ1, JQ2 and JQ3, respectively. All three complexes have hexafluorophosphate (PF6) as the counter ions, given that PF6 is inert, highly soluble in organic solvent and therefore suitable for the organometallic synthesis. It has been reported that the raw materials for the synthesis of such copper-based complexes are less expensive compared to the conventional cobalt-based complex FK209.23 In addition, the cobalt (III) complex FK209 is prepared via the oxidation of its reduced state cobalt (II) complex; while in the case of copper (II) complexes, Cu(ClO4)2 can directly react with the desired ligand to produce the final product without the need of any oxidation reaction.23 The UV-vis absorption spectra and extinction coefficient of the three dopants are presented in Figure 1b. JQ3 shows broader absorption than the other two (JQ1 and JQ2), which could be mainly due to the more extended conjugation in the structure. The electrochemical redox potentials of these three complexes were determined as 0.07, 0.17, 0.33 V/Fc/Fc+ (Fc/Fc+: ferrocene/ferrocene+) (see Table 1) via cyclic voltammetry as shown in Figure 1c. After converting those values to vacuum scale (E/vacuum=Eox (vs Fc/Fc+) -5.1 eV, see Table 1),23 E/vacuum values for three dopants are more negative than that of spiro-OMeTAD (-5.08 eV), which demonstrates that all three copper complexes can be potentially applied as chemical dopants of spiroOMeTAD. The potential difference (∆V, see Table 1) between the dopant and spiroOMeTAD can be seen as a driving force of the following doping process: + = + . A higher ∆V promotes the oxidation process and induces a higher doping level of spiro-OMeTAD.

The

difference of ∆V for the different dopants (JQ1: ∆V=90 mV; JQ2: ∆V=190 mV; JQ3: ∆V=360 mV; FK209: ∆V=370 mV) allows us to systematically investigate the impact


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of the dopants’ redox potentials on their reactivity, hole conductivity as well as solar cell performance.

Table 1. Detailed molecular parameters on the optical, electrochemical and photoelectrical properties of the complexes JQ1, JQ2, JQ3, FK209 and spiroOMeTAD (spiro). Dopants

JQ1

λmax

Eredox

E/ eV +

∆V/

ƞdoping

σ

nm

(mV vs Fc/Fс ) (vs. vacuum)

mV

%

Scm-1

259

70

90

8

1.3×10-5 (3%);

-5.17

1.4×10-4 (7%) 5.8×10-4 (9%); 6.6×10-4 (10%) JQ2

260

170

-5.27

190

25

3.3×10-4 (3%)

JQ3

310

330

-5.44

360

80

6.0×10-4 (3%)

FK209

-

3405

-5.45

370

85

6.1×10-4 (3%)

Spiro (pristine) -

-

-5.08 23

-

-

9.1×10-7 23

Spiro+LiTFSI +TBP

-

-

-

-

3.0×10-6 20

-

The spiro-OMeTAD doping efficiencies for the three copper complexes were investigated from UV-vis absorption spectra of solutions containing spiro-OMeTAD and dopants, as shown in Figure 1d and Figure S1. By gradually adding more JQ3 in the spiro-OMeTAD solution, a decrease of the absorption band at 390 nm, dedicated to neutral spiro-OMeTAD, as well as an increase of absorption bands at 523 nm and 688 nm, assigned to the singly and doubly oxidized spiro-OMeTAD molecules, were observed. 23,28 Tracing the evolution of the absorption intensity at 523 nm allows us to estimate the doping efficiency of copper complexes,29 as exhibited in Figure 1e. According to the linear relationship between the concentration of spiro-OMeTAD+ and the concentration of dopants, as shown in Figure 1e, the neutral-to-oxidized spiro-


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OMeTAD conversion efficiency by different dopants is estimated to be around 8%, 25%, and 80% for JQ1, JQ2 and JQ3, respectively (see Table 1), as compared to ~85% for the reference FK209. The doping efficiency by three complexes follows the trend of their electrochemical potentials. Therefore, we conclude that a more positive electrochemical potential than spiro-OMeTAD is crucial for p-dopants to accomplish the doping process; and the difference in their potentials ∆V as a driving force can determine the doping efficiency (ƞ) accordingly. And the quantity of spiro+ in the HTM can be estimated by: C(spiro+) = C(dopant) × ƞ(doping).


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Figure 2. (a) Device configuration of perovskite solar cells. (b) Cross-section image of the complete perovskite solar cells. (c) Current-voltage characteristics of the perovskite solar cell based on spiro-OMeTAD doped by JQ1, JQ2, and JQ3 with a doping ratio of 3%. (d) Current-voltage characteristics of the perovskite solar cells based on spiro-OMeTAD doped by JQ1, JQ2, and JQ3 with a ratio of 10%. (e) Voc evolution over time for the devices with spiro-OMeTAD doped by different dopants. (f) Current-voltage characteristics of the perovskite solar cells based on the same doping level of spiro-OMeTAD by 10% JQ1, 3% JQ2 and 1% JQ3. (g) Currentvoltage characteristics of the perovskite solar cells based on spiro-OMeTAD by different ratios of JQ1.

Table 2. The average photovoltaic performance of the PSCs (for each condition at least two devices in the same batch) based on dopants JQ1, JQ2 and JQ3 with different ratios in comparison of the standard device with FK209. Metal-complex dopant

mol%

No JQ1

JQ2 JQ3

FK209

Voc

Jsc

FF -2

Ƞ

(mV)

(mAcm )

(%)

0%

995±5

20.8±1.1

0.64±0.03

13.0±0.4

3%

1070±20

22.0±0.1

0.65±0.01

15.4±0.1

7%

1105±2

22.5±0.1

0.73±0.01

18.0±0.3

8%

1116±2

22.8±0.2

0.73±0.01

18.8±0.1

9%

1120±6

22.8±0.3

0.75±0.01

19.3±0.2

10%

1105±5

22.7±0.2

0.74±0.01

19.0±0.3

11%

1090±10

22.6±0.1

0.74±0.01

18.4±0.3

3%

1070±10

22.5±0.3

0.64±0.01

15.7±0.1

10%

1040±10

21.8±0.3

0.68±0.02

15.9±0.3

1%

1090±10

21.9±0.3

0.72±0.02

17.5±0.2

3%

1095±5

21.9±0.2

0.73±0.01

18.0±0.1

10%

1020±5

21.8±0.3

0.66±0.03

15.5±0.5

3%

1090±10

22.0±0.5

0.74±0.02

18.0±0.3

10%

1025±5

20.4±0.5

0.70±0.03

15.2±0.5


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In order to investigate the effects of three dopants on solar cell performance, PSCs have been fabricated with a regular device configuration (see Figure 2a): mesoporous TiO2 as ETM, Rb0.05Cs0.05FA0.8MA0.1Pb(I0.85Br0.15)3 (FA: formamidinium; MA: methylammonium) as light absorber and doped spiro-OMeTAD as HTM. The crosssection image of the complete device is shown in Figure 2b. Three dopants with the same concentration of 3% (this concentration is the optimal condition for FK209) were firstly added in HTMs and the J-V curves of the devices are presented in Figure 2c. The detailed photovoltaic parameters can be found in Table 2. Specifically, JQ3-based PSC shows a much higher PCE of 18.0% (Voc=1101 mV, Jsc=22.0 mAcm-2, FF=0.75) than the PSCs with JQ1 (15.5%) and JQ2 (15.7%). The reference device based on 3% FK209 exhibits a high PCE of 18.2% (see Figure S2). It is noted that the main difference for their photovoltaic parameters is the FF. This can be due to the much higher doping efficiency for JQ3 (see Figure 1e) than the others leading to a higher conductivity. The conductivity measurements of the doped spiro-OMeTAD thin film by three dopants are presented in Figure S3, showing that JQ3 doped spiro-OMeTAD has a higher conductivity (6.0×10-4 S cm-1) than JQ1 (1.3×10-5 S cm-1) and JQ2 (3.3×10-4 S cm-1), as listed in Table 1. Therefore, we suggest that the higher FF obtained for JQ3-based device can be primarily attributed to the higher carrier conductivity in the device. This indicates that stronger dopants with higher doping efficiency are favorable to obtain higher conductivity, as similarly reported in previous reports.17,23 Additionally, the dependence of the HTM conductivity on the JQ1 concentration has been added in Figure S4 and the conductivity results are included in Table 1. It can be seen that the conductivity increases (from 3×10-6 Scm-1 to 6.6×10-4 Scm-1) with the addition of JQ1 (doping ratios: 0% to 10%).


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However, solely increasing the doping level for higher HTM conductivity does not promise higher solar cell efficiency. To investigate the influence of high doping level of spiro-OMeTAD on the solar cell performance, we further increased the concentration of three dopants up to 10%. The J-V curves of the corresponding devices are shown in Figure 2d and Figure S2. Contrary to the previous case, JQ1-based device delivers a high PCE of 19.1% with a Voc of 1109 mV, Jsc of 22.8 mAcm-2 and FF of 0.75; while stronger dopants JQ2, JQ3 and FK209-based devices exhibited lower PCEs of 16.2%, 15.6% and 15.4%, respectively, in spite of their expected higher hole conductivities. The higher PCE for JQ1 than the other two can be mainly attributed to the enhanced Voc and FF. Figure 2e depicts the evolution of the Voc with time for different devices. It is noted that JQ1-based device produced a significant Voc enhancement by around ~80 mV compared to those for JQ2 and JQ3 when 10% doping ratio was applied. The obtained high Jsc and FF for the device based on JQ1 with relatively lower redox potential suggest that conductivity is not the limiting factor for the device performance. Based on previous findings, we believe that the higher concentration of spiro-OMeTAD+ in HTM generated from strong dopants JQ2 and JQ3 may induce more charge recombination at the perovskite/HTM interface, resulting in lower Voc.30 In order to take account of the effect of oxygen in the environment on the doped devices, we have fabricated the encapsulated devices with 10% dopants. The J-V curves and photovoltaic parameters are shown in Figure S5. JQ1 based PSCs exhibit higher efficiency of 19.2% as compared to 16.1% for JQ2 and 15.7% for JQ3. The encapsulated devices showed a similar trend in photovoltaic performance as the un-encapsulated devices (Figure 2d). Therefore, we believe that the O2 in the environment has a similar influence on the devices with the three dopants.


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To gain more insights into the interfacial recombination kinetics, we have controlled the same concentration of spiro-OMeTAD+ in HTM by adding different concentration of dopants in spiro-OMeTAD solution according to their different oxidative reactivity (Figure 1e): 10% of JQ1, 3% JQ2 and 1% JQ3, which is confirmed by the similar UV-vis absorption spectra of those solutions as shown in Figure S7. The J-V curves of such PSCs are shown in Figure 2f. The device with JQ2 showed a PCE of 15.7%, which is much lower than that for JQ3 (17.4%) and JQ1 (19.1%). It can be noted that JQ2-based devices have significantly lower Voc and FF values compared to the others. In solar cells, the FF and Voc can be affected by different factors, such as conductivities of the charge transport layers and the interfacial electron recombination rates. Given the facts that (1) the HTM layers based on three dopants are expected to have very similar conductivities due to the same concentration of spiro-OMeTAD+; and (2) the devices have same architectures and materials except the HTMs, thus, we believe that the different charge dynamics at interfaces are responsible for the different Voc and FF values observed in the devices as shown in Figure 2f, which will be discussed below. In order to gain an overview about how the doping level influences the solar cell performance, the devices with different concentrations of JQ1 were fabricated, and the J-V curves are exhibited in Figure 2g. The inset diagram in Figure 2g shows the dependence of PCEs on doping ratios of JQ1. The device without dopants presents the lowest PCE of 13.2% due to the low conductivity of the HTM. With gradually increases in the doping ratio, the devices present a significant PCE improvement to a highest efficiency of 19.3%, and the Voc increased from 1090 mV at 3% JQ1 to 1126 mV at 9% JQ1, indicating that enhanced conductivity of HTM favors higher Voc as low transport resistance of HTM produces a low Voc drop, as similarly observed previously.31 The increase of PCE for JQ1 based 14 ACS Paragon Plus Environment

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devices can be partially attributed to the increased conductivity (Figure S4), and the optimal conductivity for the highest performance device is about 5.8×10-4 Scm-1, which is similar to the previously reported values (~5×10-4 Scm-1) for other types of dopants.20,21,23 The J-V hysteresis of the devices with JQ1 has been measured, as shown in Figure S6, indicating that the efficiency presents a very small difference between the forward and reverse scans. The incident photon-to-current conversion efficiency (IPCE) spectra for the high-efficiency devices with dopants are shown in Figure S8. The integrated current density for devices with JQ1 (10%), JQ2 (10%) and JQ3 (3%) are 22.1, 22.0 and 21.97 mAcm-2, which are very consistent with the Jsc values for the same devices obtained from J-V measurements (see Table 2). It is noteworthy that a high Voc enhancement by more than 100 mV has been achieved via dopant engineering in HTM, as shown in Figure 2e. Moreover, both the PCE and Voc decreased when further increasing the concentration of JQ1 beyond 9%, which suggests that high interfacial recombination due to the high quantity of charge carriers (spiro-OMeTAD+) is the dominating factor to influence both Voc and PCE. Additionally, the device efficiencies as a function of the concentration of three dopants and the formed spiro+ are shown in Figure S9. Figure S9a shows that the weaker dopant JQ1 has an optimal doping ratio of about 9%, as compared to the stronger dopants JQ2 (~4%) and JQ3 (~2%), indicating that higher quantity is required for weaker dopants in order to obtain best solar cell performance. Figure S9b demonstrates the device efficiencies as a function of the estimated amount of spiro+, showing that a rather similar quantity of spiro+ (molar ratio of spiro+/spiro: ~1%) in HTM is required for best performing devices with different dopants. This estimated optimal concentration of 1% for spiro+ in the HTM layer will provide important


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guidance to optimize the doping level of organic HTMs for obtaining high-efficiency PSCs. Therefore, we believe that optimizing the redox potential of the dopants in order to balance the charge transport and interfacial charge recombination is crucial to maximize the photovoltaic performance of PSCs.

Figure 3. (a) Scanning electronic microscopy (SEM) images of perovskite on mesoporous TiO2/FTO glass substrate and spiro-OMeTAD films doped by 10% of different dopants on the substrates of FTO/mesoporous TiO2/perovskite. (b) Energy diagram and charge transfer 16 ACS Paragon Plus Environment

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and transport processes in PSCs. (c) Work function of the doped spiro-OMeTAD by 10% dopants determined by PESA (photon electron spectroscopy in air) with substrates of FTO/TiO2/doped spiro-OMeTAD. (d) Electron lifetime of the PSCs based on 10% dopants determined by IMVS (intensity modulated voltage spectroscopy). (e) Transient open-circuit voltage decay (OCVD) measurements for the PSCs with the same doping level of spiroOMeTAD (b: 10% JQ1; 3% JQ2; 1% JQ3). (f) Steady state photoluminescence (PL) and (g) transient PL decay of perovskite films coated with and without doped spiro-OMeTAD by different dopants (10% JQ1; 3% JQ2; 1% JQ3). To reveal the origins for the large difference of Voc obtained for the devices with 10% dopants (see Figure 2c), we have investigated their properties on morphology, energy levels and charge recombination. The top view images of the perovskite film as well as the top view of HTM films with different dopants are displayed in Figure 3a. The perovskite film shows large grains with an average size of ~500 nm. In addition, all the HTM layers with three different dopants exhibit uniform and complete coverage on perovskite, meaning that the dopant is unlikely to have a direct impact on the HTM morphology. From the device point of view (see Figure 3b), their Voc values greatly depends on the bulk and interface recombination, as well as the energy levels of the selective contacts. In this respect, the work function (WF) of the doped spiroOMeTAD by three dopants was measured by photon electron spectroscopy in air (PESA), as shown in Figure 3c and Figure S10. The WF for doped HTM can be estimated to be 5.25 eV, 5.27 eV and 5.29 eV for the cases of JQ1, JQ2 and JQ3, respectively, which is consistent with their doping efficiency. However, the higher WF for JQ3-doped spiro-OMeTAD does not correlate to the lower Voc measured for JQ3based device, suggesting that the WF is not responsible for the variation of Voc of the devices. The work functions of the HTM layer with JQ1 at different ratios have been 17 ACS Paragon Plus Environment

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measured and shown in Figure S11. It is shown that the work function of the HTM becomes higher when increasing the doping ratios of the JQ1. And the work function of the HTM for the best performing device is located at around 5.25 eV. Therefore, the dopants play important roles in determining the work function of the HTM as well as the interfacial recombination; these two factors together significantly influence the Voc of the devices. Furthermore, charge recombination in the device has been studied by intensity modulated voltage spectroscopy (IMVS) in order to estimate the recombination lifetime in solar cells.24,30 The extracted electron lifetime for different dopants is shown in Figure 3d. It clearly demonstrates that JQ3-based devices have a much shorter electron lifetime than those doped with JQ1 and JQ2, leading to a lower Voc in the device. It indicates that the higher concentration of spiro-OMeTAD+ generated from the stronger dopant JQ3 greatly accelerates the interfacial recombination: + = . Hence, it confirms that the interfacial recombination rates scale with the level of HTM doping, and the optimal doping ratio for different dopants is greatly influenced by their redox potentials. As interestingly observed in Figure 2f, the devices present variations in Voc, FF and PCE even when comparing devices with similar concentrations of spiroOMeTAD+. To gain further insights into the effects of the different dopants on interface carrier dynamics, transient open-circuit voltage decay (OCVD) and photoluminescence (PL) measurements have been performed, as shown in Figure 3e3g. JQ1-based device showed a much slower Voc decay as compared to the other two dopants, indicating that the different recombination lifetime results in a variation of Voc for three types of devices. In order to further study the interfacial recombination, we performed steady state and transient photoluminescence (PL) with a device 18 ACS Paragon Plus Environment

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structure of glass/perovskite/HTM. When illuminated from the HTM side, the excited electrons and holes are dominantly generated near the interface of perovskite/HTM where holes can be extracted into the HTM side preferably.32 In this case, the relaxation rate of the accumulated holes in HTM critically determine the hole transfer rate from perovskite to HTM, because the state fitting effect (the occupancy of the accept states) can significantly influence the charge transfer rate.33-37 At an opencircuit condition, the holes in HTM essentially have two channels of relaxation: (1) intrinsic relaxation by trapping into the bulk defects (vacancy) and finally depleting the energy by phonon emission; (2) surface recombination between holes and the excited electrons at the perovskite/HTM interface. The bulk defects are not expected to change with the addition of dopants in the HTM layers. The PL intensity and lifetime for the perovskite/HTM substrates can be significantly influenced by both (a) the hole transfer yield from perovskite to the HTM, and (b) non-radiative recombination at the interface, such as the electrons in perovskite recombination with the holes in the HTMs. The overall PL intensity or lifetime depends on the dominating process of these two. Therefore, the relatively higher PL intensity for high-efficiency JQ1-based device (see Figure 3f) indicates that (a) the interfacial recombination, rather than the hole transfer, is the dominating process that influences the PL intensity; (b) much less surface recombination has occurred at or near the perovskite/HTM-JQ1 interface as compared to those for JQ2 and JQ3, as discussed previously.38 The observed significant differences of steady-state PL intensities (JQ1˃JQ3˃JQ2) can be further supported by time-resolved photoluminescence (TRPL) decay measurements (Figure 3g). A bi-exponential function was applied to fit the PL decay and the extracted average lifetimes were shown in Figure 3g. JQ1-based device presents a much longer PL lifetime (τ=73.4 ns), as compared to JQ2 (τ=5.2 ns) and JQ3 (τ=13.9 ns). Given


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that the dopants are the only difference in the HTMs, this result may imply that dopants can also influence the charge recombination kinetics at perovskite/HTM interface, dramatically affecting the Voc and FF of the PSCs, as found in Figure 2f. This agrees well with the OCVD measurements (see Figure 3e), demonstrating that JQ1-based devices with less surface recombination contribute to a longer electron lifetime and thus higher performance in PSCs. At this point, the faster recombination for JQ2 and JQ3 may result from their higher reactivity with spiro-OMeTAD (see Figure 1e), even though in this case the amount of JQ2 (3%) and JQ3 (1%) are lower than that of JQ1 (10%). This assumption on the recombination kinetics is reinforced by the observation of lower Voc and FF for JQ2 and JQ3-based devices (see Figure 2f). Therefore, we conclude that both the quantity of dopants and redox reactivity of the dopants play important roles in the interface recombination. The high recombination from dopants may explain the low performance of the PSCs based on metal complexes as HTMs.39 It is interesting to discover that the dopants in the HTMs influence the charge recombination kinetics as well as the solar cell performance. Figure

4a summarizes the possible recombination pathways across the

perovskite/spiro-OMeTAD interface. Under VOC condition holes will be transfered from the perovskite to the spiro-OMeTAD layer until the fermi levels of the two layers are equilibrated. This process will result in a charging of the interface with excess negative charge (electrons) accumulating in the perovskite near the interface. Based on the above findings, two recombination mechanisms are proposed: (a) electron + spiroOMeTAD+ = spiro-OMeTAD and (b) electron + Cu2+ = Cu1+. In addition, JQ2-based device presents the fastest recombination (see Figure 3g) compared to JQ3 (strongest dopant) and JQ1 (highest concentration), suggesting that both the reactivity (redox potentials) and quantities of the copper complexes play important roles in the 20 ACS Paragon Plus Environment

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interfacial recombination kinetics. In order to suppress this interfacial recombination, it is possible to do interface passivation by atomic layer deposition of an ultrathin layer of Al2O3 between the perovskite and HTM layers in order to block the aforementioned recombination pathways.

Figure 4. (a) Scheme of the interfacial recombination mechanism at the perovskite/HTM interface; S represents spiro-OMeTAD; S+ represents oxidized spiro-OMeTAD; Cu2+ 21 ACS Paragon Plus Environment

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represents dopants. The processes include: 1. electron transport in perovskite; 2. hole transfer from perovskite to spiro-OMeTAD; 3. electron recombination with dopants cu2+ complexes; 4. electron recombination with oxidized spiro-OMeTAD). (b) Scheme of perovskite solar cell structure with Al2O3 passivation layer. (c) Principle of the interface passivation by high bandgap Al2O3. Photovoltaic parameters of the PSCs based on Al2O3: (d) efficiency, (e) Voc and (f) FF. (g) J-V curves at 1 sun light intensity and (h) dark J-V curves of the devices without and with Al2O3. (i) Electron lifetime of the devices with and without Al2O3. To illustrate this effect, we used the strongest dopant JQ3 (10%) based device as a reference. For comparison, devices with Al2O3 (thickness: 1.0 nm and 1.5 nm, respectively) have been fabricated with a device structure as displayed in Figure 4b. Owing to the large bandgap, Al2O3 is expected to efficiently block electron transfer from perovskite to HTM (see Figure 4c), hence to reduce the possibility of recombination through the dopant induced recombination center at Al2O3/HTM interface. Meanwhile, the Al2O3 layer needs to be thin enough to allow efficient hole tunneling from the perovskite to the hole conductor layer.40 Figure 4d-f shows the photovoltaic parameters of the devices comprising an Al2O3 interface layer. As expected, the PCE is significantly increased from 15.6% for Al2O3-free reference device to a PCE of 18% with both increased Voc and FF when 1 nm Al2O3 is applied (see Figure 4g). While when further increasing the thickness of Al2O3 to 1.5 nm, the PCE drops down to 13.2%, which is mainly due to the decreased FF despite a higher Voc. We believe that the low FF for 1.5 nm Al2O3 is related to its high series resistance, as demonstrated by the decreased dark current (Figure 4h). The same passivation approach has been applied to the PSCs with the more efficient dopant JQ1 (10%) and the J-V curves of the devices are shown in Figure S12. It is noted that the devices with a 1 nm Al2O3 passivation layer present a slightly lower efficiency 22 ACS Paragon Plus Environment

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(18.8%) as compared to 19.1% for the reference devices (no Al2O3 at the interface). However, a slightly higher Voc is obtained, indicating that the Al2O3 passivation is beneficial to reduce interface recombination. A further decrease of efficiency and FF is observed when a 1.5 nm Al2O3 layer was used at the interface, suggesting that the thick Al2O3 (1.5 nm) layer produces a high resistance for the hole transfer, causing low FF in PSCs. Therefore, we believe that two factors must be considered for the insertion of Al2O3 at the perovskite/HTM interfaces, namely the interfacial recombination and the hole transfer resistance. And the thickness of the passivation layer has to be optimized for different passivation materials and different types of dopants. To quickly demonstrate the successful suppression of interfacial recombination by Al2O3, IMVS is adopted to directly study the electron lifetime in the devices, as exhibited in Figure 4I. Clearly, longer lifetimes are observed for devices comprising an Al2O3 layer, suggesting that the interface recombination across the HTM/perovskite interface is a crucial limiting factor for the Voc. Accordingly we believe that interface engineering by high bandgap semiconductor tunneling junction is an efficient strategy to further improve the Voc. In order to investigate the influences of the Al2O3 layer on the environmental stability of the PSCs, the unsealed devices (10% JQ3) with (1nm and 1.5 nm) and without Al2O3 have been stored in the ambient condition with an average humidity of 50% in dark and 25 oC. Figure S13 shows the efficiency change during the aging time for one week. It is shown that the devices with the Al2O3 passivation layers present more stable performance as compared to the reference devices without Al2O3. This indicates that the passivation layer Al2O3 can not only reduce the interface recombination, but also enhance the long term stability of the PSCs, possibly by providing a barrier layer for preventing the water penetration to the 23 ACS Paragon Plus Environment

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perovskite layer. Therefore, we believe that effective interface engineering is one of the most critical ways to optimize the efficiency and stability of PSCs.

CONCLUSION In conclusion, we have introduced three copper-based complexes as chemical dopants in HTMs for perovskite solar cells. We found that the redox potentials of the dopants are crucial for the doping efficiency and conductivity of HTM as well as device performance. By optimizing the doping level and dopant categories, an efficiency improvement (from 13.2% to 19.3%) as well as a significant Voc increase by ~100 mV has been achieved. In addition, we observed that one of the main limiting factors for the Voc of PSCs is the interfacial charge recombination across perovskite/hole conductor interface. Detailed characterization by electron lifetime and transient photoluminescence measurement suggested that both dopants and the oxidized spiro-OMeTAD (spiro-OMeTAD+) play important roles in the recombination kinetics. By controlling the concentration and redox potential of the dopants, the interfacial recombination kinetics as well as the device performance can be significantly optimized. This work demonstrates that the dopants in the HTMs participate in the interfacial recombination and significantly influence the Voc of the devices. Insights into the interface recombination mechanisms in PSCs have been established on the basis of in-depth investigations. Finally, a strategy of interface passivation has been successfully applied to suppress such interfacial recombination by physically inserting an ultrathin Al2O3 insulating layer between perovskite and the HTM, and a significant PCE enhancement from 15.6% to 18.0% has been achieved for stronger dopant JQ3-based device. This work provides an important guide to control 24 ACS Paragon Plus Environment

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the effects of dopants in PSCs. High-performance dopants for HTM should balance the charge transport resistance as well as the interfacial recombination in order to further enhance the performance of PSCs. In addition, the passivation layer can be beneficial for the environmental stability of PSCs. This work clearly illustrates the comprehensive roles of the dopants in PSCs, which will provide an important guidance for the future development of efficient dopants of organic semiconductors for solar cell applications. And optimizing the interface between the perovskite and HTM is one of the most critical ways to improve both efficiency and stability of PSCs.

METHODS/EXPERIMENTAL Synthesis of copper complexes All chemicals and solvents were purchased from Sigma-Aldrich, if not stated otherwise, and were used without further purification. Di(pyridin-2-yl)methane (dpm) was purchased from AK Scientific, Inc. Cu(I/II)(dpm)2(PF6)2(JQ1): one equivalent of Cu(ClO4)2 was mixed with 2.2 equivalents of dpm in ethanol, under nitrogen atmosphere, at room temperature for 2 hours. Cu(I/II)(dpm)2(ClO4)2 was collected by filtration and washed with water and diethyl ether. Then, Cu(I/II)(dpm)2(ClO4)2 was dissolved in a 1:2 ethanol/water mixture. To this solution, 20 equivalents of ammonium hexafluorophosphate were added. The solution was stirred for 3 hours at room temperature under nitrogen atmosphere. Cu(I/II)(dpm)2(PF6)2 was collected by filtration and washed with water and diethyl ether. The total yield was 85 % (mol).


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Cu(I/II)(dpe)2(PF6)2(JQ2): one equivalent of Cu(ClO4)2 was mixed with 2.2 equivalents of dpe in ethanol, under nitrogen atmosphere, at room temperature for 2 hours. Cu(I/II)(dpe)2(ClO4)2 was collected by filtration and washed with water and diethyl ether. Then, Cu(I/II)(dpe)2(ClO4)2 was dissolved in a 1:2 ethanol/water mixture. To this solution, 20 equivalents of ammonium hexafluorophosphate were added. The solution was stirred for 3 hours at room temperature under nitrogen atmosphere. Cu(I/II)(dpe)2(PF6)2 was collected by filtration and washed with water and diethyl ether. The total yield was 90 % (mol). Cu(I/II)(dmbp)2(PF6)2(JQ3): one equivalent of Cu(ClO4)2 was mixed with 2.2 equivalents of dmbp in ethanol, under nitrogen atmosphere, at room temperature for 2 hours. Cu(I/II)(dmbp)2(ClO4)2 was collected by filtration and washed with water and diethyl ether. Then, Cu(I/II)(dmbp)2(ClO4)2 was dissolved in a 1:2 ethanol/water mixture. To this solution, 20 equivalents of ammonium hexafluorophosphate were added. The solution was stirred for 3 hours at room temperature under nitrogen atmosphere. Cu(I/II)(dmbp)2(PF6)2 was collected by filtration and washed with water and diethyl ether. The total yield was 70 % (mol). Optical and electrochemical and conductivity characterization The UV-Vis absorption spectra of three dopants were measured in acetonitrile solution. Cyclic voltammetry measurements were carried out by three-electrode setup, with glassy carbon (d=3mm) as working electrode, Ag/AgCl (3M KCl) as reference electrode, and platinum wire as counter electrode. The dopants are dissolved in acetonitrile with 0.1 M of tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) as electrolyte. The scan rates were 50 mV/s. The reference electrode is calibrated by ferrocene. The conductivity of the doped spiro-OMeTAD by different dopants was 26 ACS Paragon Plus Environment

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performed by two-contact electrical conductivity setup. The details about this measurement can be found in previous work.41,42 Specifically, the device structure for conductivity measurement is glass/mesoporous TiO2/HTM/Au. Mesoporous TiO2 layer was prepared by spin-coating with a diluted TiO2 paste (Dyesol DSL 18NR-T) with terpineol (1:3, mass ratio). The thickness of the film is around 500 nm. Followed the sintering of TiO2 on a hotplate at 500 °C for 30 min, a layer of HTM solution with same composition as used in the solar cell was spin coated on TiO2. Finally, a 200 nm thick Ag was deposited on top of the HTM layer by thermal evaporation. Perovskite Solar Cell Device Fabrication A

regular

device

structure

of

FTO

glass/TiO2

underlayer/mesoporous

TiO2/perovskite/HTM/Au was applied for perovskite solar cell, as described in our previous work.43 In brief, after cleaning of FTO glass, a thin TiO2 compact layer was coated on FTO by spray pyrolysis with a precursor solution bis(isopropoxide)-bis(acetylacetonate) titanium(IV)

in

19

mL

containing 1 mL isopropanol. A

mesoporous TiO2 layer was prepared by spin coating of a diluted TiO2 paste (1g 30nm-TiO2 original paste in 6g ethanol) gel on the substrate with a speed of 4000 rpm for 30 s with a ramp of 2000 rpm·s-1, followed by a 30 mins sintering on hotplate (500oC). After that, a perovskite solution was spin coated on top of TiO2 with the following spin program: first at 2000 rpm for 10 s with a ramp of 200 rpm·s-1; second at 4000 rpm for 30 s with a ramp of 2000 rpm·s-1. During the spinning process, 110 µL chlorobenzene was dropped on the substrate during the second spin coating step 15 s before the end of the procedure. The perovskite precursor solution was prepared in the mixed solvent of DMF (dimethylformamide) and DMSO (dimethyl sulfoxide) with a volume ratio of 4:1. The details can be found in our previous work.44 The perovskite


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film was then sintered at 100°C for 90 min on a hotplate. Then HTM solutions (72 mg spiro-OMeTAD with the addition of 17.5 uL LiTFSI (from a stock solution in acetonitrile with concentration of 520 mg/ mL), 28.8 uL tert-butyl pyridine and 8 uL FK209 (Dyenamo) with a concentration of 1.5M or other dopants with required concentration (from a stock solution in acetonitrile with concentration of 0.5 M)) was spin coated on the top of the perovskite layer with a speed of 3000 rpm for 30 s. For the reference cells, the spiro-OMeTAD solution is prepared with additives LiTFSI and TBP, but without metal complex dopants. Finally, a 80 nm gold layer was deposited by thermally evaporated on the HTM layer. Device Characterization Current-voltage characterization was measured using a Keithley 2400 source meter and an Oriel solar simulator equipped with a xenon lamp (100 mW cm-2) and an AM 1.5G filter. Non-reflective metal apertures of 0.16 cm2 was used to define the irradiation area. The J-V scans are performed from 1.2 V to 0 V with a scan rate of 0.05 V/s. The devices are kept in dark dry box at room temperature before J-V measurements. The PL was measured by using glass /Perovskite/HTM. Time-resolved PL was measured using time correlation single photon counting (TCSPC) technique. The sample was excited by a 405 nm pulse laser with repetition rate of 2MHz. each PL !

decay can be well fitted by two-exponential function = ∑ exp − and the "#

average lifetime was calculated by $ = ∑ $ / ∑ $ . The instruments used are FEI Nova NanoSEM 450 FEGSEM and Zeiss MERLIN Field Emission SEM, Germany.


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CONFLICS OF INTERESTS There are no conflicts of interests.

ACKNOWLEDGEMENTS The authors acknowledge the Australian Centre for Advanced Photovoltaics (ACAP), the Australia Research Council Centre of Excellence in Exciton Science (project number CE 170100026), the Swedish Energy Agency and the Swedish Foundation for Strategic Research for their ﬁnancial support. Electron microscopy access provided by the Monash University Centre for Electron Microscopy (MCEM) is greatly acknowledged. The authors are also grateful to J. Sun for the PESA measurement, and L. Jiang for SEM measurements, respectively. Supporting Information Available: UV-vis spectra of doped spiro-OMeTAD; Conductivity of HTMs with different dopants; J-V curves, IPCE and hysteresis of devices; PESA; Stability. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES 1. Xing, G. C.; Mathews, N.; Sun, S. Y.; Lim, S. S.; Lam, Y. M.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C., LongRange Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3 . Science 2013, 342, 344-347. 2. Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. 3. Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M. J.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, y.; Zhang, X.; Dowben, P.; Mohanmmed O.; Sargent, E.; Bakr, O., Low Trap-state Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519-522. 4. Nie, W. Y.; Tsai, H. H.; Asadpour, R.; Blancon, J. C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.; Wang, H.; Mohite, A. D., High-efficiency Solution-processed Perovskite Solar Cells with Millimeter-scale Grains. Science 2015, 347, 522-525. 5. Yang, W. S.; Park, B.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S., Iodide Management in Formamidinium-lead-halide–based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376-1379 6. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050. 7. Shi, J.; Xu, X.; Li, D.; Meng, Q., Interfaces in Perovskite Solar Cells. Small 2015, 11, 2472-86. 8. Fakharuddin, A.; Schmidt-Mende, L.; Garcia-Belmonte, G.; Jose, R.; Mora-Sero, I., Interfaces in Perovskite Solar Cells. Adv. Energy Mater. 2017, 1700623.


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