Impact of Organic Hole Transporting Material and Doping on the

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C: Energy Conversion and Storage; Energy and Charge Transport

Impact of Organic Hole Transporting Material and Doping on the Electrical Response of Perovskite Solar Cells Maria Ulfa, Thierry Pauporte, Thanh-Tuân Bui, and Fabrice Goubard J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02141 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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The Journal of Physical Chemistry

Revised version

Impact of Organic Hole Transporting Material and Doping on the Electrical Response of Perovskite Solar Cells Maria Ulfa,a Thierry Pauporté,a* Thanh-Tuân Bui,b Fabrice Goubardb, a

Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris (IRCP), 11 rue P. et M. Curie, F-75005 Paris, France b

Laboratoire de Physicochimie des Polymères et des Interfaces, Université de CergyPontoise, 5 mail Gay Lussac, 95031 Neuville-sur-Oise, France. * Corresponding author: [email protected]

Abstract The hole transport material (HTM) layer is a key component of the perovskite solar cells (PSCs) that must be optimized to reach high efficiency. The development of new HTMs alternative to Spiro-OMeTAD and the understanding of the role of doping agents on these layers are important research axes in the field. It requires the use of appropriate characterization tools enabling to discriminate the bulk and interface effects. In the present paper, we fully analyze the effect of HTM doping and of the material on the impedance response of PSCs. The approach has been implemented on two different molecular HTMs, Spiro-OMeTAD and a new molecular carbazole HTM, called B186, and with various doping levels. We show that limitations by poor doping are characterized by an extra high frequency impedance loop which capacitance and resistance analysis gives the dielectric constant and conductivity of the material, respectively. On the other hand, the low frequency part of the spectra provides important information on the charge accumulation/outflow and on the recombination levels. More generally, the presented approach is of high practical interest for the development of new organic HTMs and for the optimization of the layer doping.

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1. Introduction In recent years, hybrid organolead perovskites (HPs) have emerged as one of the most promising family of materials for the development of high-performance optoelectronic devices1-6 and have revolutionized the field of photovoltaic solar cells.7-19 The record efficiency for solar radiation power conversion of these systems has increased rapidly to achieve a current certified world record of 22.7%.20 In most cases, the general architecture used for PSC consists in placing the HP absorber layer between two selective contacts (Figure S1a, Supporting Information). On one side, perovskite is contacted by a wide bandgap n-type oxide. This oxide is typically anatase TiO2. This layer blocks the hole transfer and ensures the electron transport.21 On the other side, an electron blocking layer is used which is commonly made of an organic semiconductor. This layer ensures the transport of holes between the absorber and the back contact. The hole transporting material (HTM) is a key component of the device to reach high efficiency. It must have a high conductivity and a good affinity for the holes transferred from the perovskite. Moreover, it must show thermal, morphological and photochemical stability, and must have good hydrophobic properties to protect the perovskite materials against moisture. Three different compound families have demonstrated good performances for the application, namely small molecules,17,21 polymers,22 and carbon

23

based materials. Small

molecule HTMs are especially attractive due to their benefits in terms of synthetic reproducibility, well-definition in molecular weight and structure. 2,2′,7,7′-tetrakis-(N,N-di-4methoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD) is the most commonly used HTM for PSCs and has allowed to reach record efficiencies above 22%.20 Pristine SpiroOMeTAD has a relatively low conductivity and moderate hole-mobility which limit its performances in PSC devices. P-doping is required to increase Spiro-OMeTAD conductivity by several orders of magnitude. The charge-carrier density in the material is increased by oxidation.24-26 Chemical oxidant additives typically used in the literature include lithium salts such as Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI),27-29 cobalt(III) salts,30-32 ionic liquids33,34 and SnCl4.35 For instance, Li-TFSI is a chemical mediator which promotes the reaction between Spiro-OMeTAD and O2.29 On the other hand, Co(III) salts dope SpiroOMeTAD through a direct and rapid oxidation reaction.31 More details are given in the Supporting Information, Section A. Similar strategies have been also implemented to increase the performances of many molecular HTMs alternative to the expensive Spiro-OMeTAD. Because HTM doping is a requirement in many cases and must be optimized to get efficient 2 ACS Paragon Plus Environment

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PSCs, new tools must be developed in order to evaluate and optimize its effect on the device performances and electrical responses. These tools must permit to discriminate the effects related to layer conductivity/ dielectric relaxation and to the interface with the perovskites (hole injection and charge recombinations), especially in the case of new HTMs developed for the application. In most papers dedicated to HTMs for perovskite solar cells (PSCs), the cells are mainly electrically characterized by the measurement of their J-V curves under forward and reverse bias scan directions. However, deeper electrical characterizations are required to better understand and study the solar cells, especially the behavior of new HTMs and the effect of doping under operating conditions. They will also help their optimization as well. Impedance spectroscopy (IS) has emerged recently as a powerful technique for the investigation of PSCs.13,14,19,21,36-38 In this technique, the device electrical response is linearized by superimposing a small ac perturbation varied over a large frequency range to the dc polarization (Vappl). The spectra of PSCs show various features produced by physical processes occurring with distinct relaxation times. These features evolve with Vappl. The technique has permitted the fine analysis for instance of the effects of the hole blocking layer,21 of the absorber composition,14,19 of the scaffold layer37… in PSCs. The scope of the present paper is to study the effect of organic HTM and doping on the overall cell performances and impedance spectra. We focus on molecular HTMs by comparing the benchmark Spiro-OMeTAD (Figure 1a) and a new carbazole-based molecular compound, named B186, with a core dendritic structure (Figure 1a), for which doping is found to induce a dramatic change in the cell PCE and device IS response.

2. Experimental The F-doped tin oxide coated glass (TEC7, Pilkington) was cut, and etched patterned using HCl 10% and Zn powder. It was then cleaned by using a concentrated 2.5 mol.L-1 NaOH ethanolic solution, rinsed with water, clean with a detergent, rinsed with MilliQ water and dried with compressed air. The substrates were then annealed 30 min at 500°C. The TiO2 blocking layer was prepared by an aerosol spray pyrolysis technique as described in Ref.[21]. A precursor solution was prepared by mixing 0.6 mL of titanium isopropoxide (TTIP), 0.4 mL of acetyl acetone in 7 mL of isopropanol. The substrate was placed on a hotplate at 455°C for 20 min prior to start the spraying. The deposited layer was then annealed at 455°C for 40 min 3 ACS Paragon Plus Environment


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before to be let to cool down. The mesoporous TiO2 layer was prepared by diluting the NR30D paste from Dyesol with ethanol (1:8 mass ratio). 40 µL of the solution was deposited on the blocking layer and spin-coated 20s at 5000 rpm. The layer was then dried on a hotplate at 100°C for 10 min and subsequently annealed at 500°C for 30 min. The CH3NH3PbI3 (MAPI) precursor solution was prepared by mixing 553 mg of PbI2 and 190 mg of MAI (Dyesol) in 1 mL of DMSO solvent. The solution was heated at 70°C until full dissolution. 50 µL of this solution was deposited on the substrate, and spun at 6000 rpm for 35s. The layer was dripped with 100 µL chlorobenzene after 25s. The perovskite layer was finally annealed at 105°C for 1h on a hotplate. The samples were then removed from the hotplate

and

let

to

cool

down

to

room

temperature

during

10

min.

For

FA0.87MA0.13Pb(I0.87Br0.17)3 (FAMA) preparation, the precursor solution contained 1M FAI, 0.2M MABr, 1.1M PbI2 and 0.22 M PbBr2 dissolved in a 4:1 volume ratio mixture of DMF and DMSO. 45µL of the perovskite precursor solution was spin-coated on FTO/ TiO2 (BL) /meso-TiO2 at 1000 rpm for 20 s (acceleration 200 rpm/s) and then at 6000 rpm for 30 s (acceleration 3000 rpm/s). During the second step, after 20s, 100 µL Chlorobenzene was dripped on the sample. The perovskite layer was finally annealed at 100 ºC for 1 h. For the doped-HTM cells, a solution was prepared by dissolving 72 mg of HTM (SpiroOMeTAD

or

B186)

in

1

mL

chlorobenzene.

Then,

17.5µl

of

bis(trifluoromethylsulfonyl)imide lithium salt solution (LiTFSI) solution (520 mg in 1 mL ACN), 28 µL of tBP (tert-butylpyridine) and 6 µL of

tris(2-(1H-pyrazol-1-yl)-4-tert-

butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] (FK209 from Dyesol) (300 mg in 1 mL ACN) were added to this solution. The solutions of undoped Spiro-OMeTAD or undoped B186 were the same without the chemical additives. In the case of the oxidized HTM, this precursor solution was stirred for two days in dry air before use. 35 µL of these HTM solutions was spin-coated at 4000 rpm for 20 s. Finally, the device was completed by thermally evaporating a 70-80 nm thick gold back contact on the Spiro-OMeTAD layer. The solar cell surface area delimited by the back contact was about 0.24 cm2. A schematic of the solar cell configuration is shown in Figure S1a. The J-V curves were recorded by a Keithley 2410 digital sourcemeter, using a 0.10 -1

V.s voltage scan rate. The solar cells were illuminated with a solar simulator (Abet Technology Sun 2000) filtered to mimic AM 1.5G conditions.39 The illuminated surface was delimited by a black mask with an aperture diameter of 3 mm. The power 4 ACS Paragon Plus Environment

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density was calibrated to 100 mW.cm-2 by the use of a reference silicon solar cell.40 The impedance spectra were measured at room temperature, between 600 kHz and 30 mHz, using a PGSTAT 20 apparatus from Autolab. All the measured cells had the same contact geometries. The AC signal was 20 mV. All the impedance spectra were measured at room temperature, over a large applied voltage range under a ~1 sun light power provided by a halogen Schott lamp. These spectra were analyzed using the Zview software from National Instrument.

3. Results and Discussion The molecular structures of the two investigated molecules are presented in Figure 1a. In both HTMs, the N-alkylated core group (three carbazoles in the case of B186) are end-capped by di(4-methoxyphenyl)aminyl function in 3,6 positions. These groups are electron rich and modulate the optoelectrochemical and electrical properties of the compounds. The methoxy groups have a good interaction with perovskite surface notably with Pb(II) ion. Additionally, their presence in para positions of phenyl rings of the diphenylamino moiety favors the charge transport properties and reduces charge recombination. B186 has a glass transition temperature (Tg) at 146°C which is higher than Tg of Spiro-OMeTAD (125°C) (Figure 1b). It is attributed to the higher molecular weight of B186 (MW: 1979.4 g.mol-1) compared to Spiro-OMeTAD (MW: 1225.4 g.mol-1). Interestingly, both molecules are wide bandgap semiconductors. The bandgap energy is 3.0 eV for Spiro-OMeTAD and 2.8 eV for B186. Moreover the energies of the top of their valence band are similar at -4.70 eV and -4.62 eV vs vacuum for Spiro-OMeTAD and B186, respectively (Figure 1b).41,42 These energies are higher than -5.4 eV vs vacuum, the energy of the top of the conduction band of CH3NH3PbI3 (MAPI).7



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Figure 1. (a) Molecular structure of the HTMs. (b) Thermal (Tg), bandgap (Eg) and band energy (Ev and Ec) characteristics of Spiro-OMeTAD and B186 (a after Ref.[41]; b after Ref.[42]. (c) J-V curves measured in the reverse scan bias direction.

The two molecular HTMs have been implemented in PSCs at different doping and oxidation levels. For the cells with the doped HTMs, the dopant composition and concentrations giving the best device performances were retained. In that case, LiTFSI and tris(2-1H-pyrazol-1-yl)-4-tert-butylpyridine)-

cobalt(III)-

ris(bis(trifluoromethylsulfonyl)

imide, two oxidizing compounds, were used. tert-buylpyridine (tBP) was also added to the HTM. The corresponding cells are noted Doped-Spiro and Doped-B186 in the following. The J-V curves of the record cells are disclosed in Figure 1c and the curve parameters are gathered in Table 1. The average values and standard deviations are gathered in Table S1 (Supporting Information). The best Doped-Spiro cell delivered a PCE of 17.7%. The best PCE of a Doped-B186 cell was 14.6% and these cells presented a large hysteresis. We have also tested two different undoped Spiro-OMeTAD cells, namely with and without a partial oxidation of the HTM precursor solution by stirring for two days in dry air. The cell J-V parameters are given in Table 1 and Table S1 (Supporting Information). The absence of chemical doping decreases dramatically the device performances. Best results are found after oxidation with a PCE maximum at 6.6% versus 5.1% without oxidation. Similar trends were found for the average values (Table S1, Supporting Information). Undoping the B186 HTM results in very 6 ACS Paragon Plus Environment

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low cell performances (PCE at 0.2%) due to very low Jsc and FF (Table 1). The J-V curve is typical of a highly resistive device and problems of charge injection and transport occur. For both HTMs, without doping, the device suffers from a low FF that can be attributed to the low HTM conductivity and therefore high charge transport resistances. Table 1: Photovoltaic J-V curve parameters and power conversion efficiencies of the best MAPI cells with Spiro-OMeTAD and B186 HTMs (AM1.5 solar spectrum irradiated at 100 mW.cm-2) HTM

Doping

Cell Name

Doped

DopedSpiro

Undoped Oxidized

Ox-Spiro

Undoped

Spiro

Doped

DopedB186

Undoped

B186

SpiroOMeTAD

B186

Scan Direction

Voc / V

Jsc/ mA.cm-2

_

σ / x107 S.cm-1 _

5.6

6.3

2.4

3.0

_

_

3.3

1.0

εr FF/ %

PCE/ %

Rev

1.01

22.15

78.22

17.68

For

1.00

22.72

63.02

14.45

Rev For Rev For Rev For

0.93 0.92 0.94 0.94 0.98 0.92

16.41 15.35 16.67 16.44 20.11 20.27

42.61 26.80 32.62 26.55 73.27 33.76

6.55 3.79 5.13 4.10 14.59 6.33

Rev For

0.74 0.74

1.52 1.44

16.89 16.64

0.19 0.17

In order to acquire a deep understanding of the effects of doping and material on the electric behavior of the PSCs, the photovoltaic devices have been comprehensively studied by impedance spectroscopy. The cells measured were the record devices or devices having close record J-V parameters. Figure 2 shows the Nyquist plots (imaginary versus real part of the impedance) of doped, oxidized and undoped Spiro-OMeTAD cells measured at 0.0V (Figure 2a,a’) and 0.6V (Figure 2b,b’) applied potentials. The spectra of Doped-Spiro were characterized by two main arcs of circle. The low frequency relaxation is noted 4 and the high frequency relaxation is noted 2 in Figure 2. At the foot of the second loop, there is another impedance feature (arc of circle) which gives the Z3 element in the general equivalent electrical circuit (EEC) in Figure 2d. For the Spiro and the Ox-Spiro cells, an additional relaxation at very high frequency is found which is noted 1 in Figures 2a’ and 2b’. This feature is due to the HTM layer. More dramatic changes are observed between the doped and undoped B186 cells. The electrical behavior of the former is close to that of the Doped-Spiro. In the absence of dopant, the spectra are dominated by a large loop with a large impedance which size increases with the applied potential. This loop is flattened and has been deconvoluted into two relaxations, named 1 and 1’ in Figure 2c. The spectra have been analyzed using the general EEC shown in Figure 2d. Simplified circuits have been employed depending on the doping and HTM. The EECs actually used for the analysis are detailed and 7 ACS Paragon Plus Environment


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explained in the section B- General electrical equivalent circuit and its simplications (Supporting Information). Because the arcs of circle are not semi-circles, they have been fitted using R//CPE circuits. The CPE impedance is defined by ZCPE =

Q where ω is the angular ( jω) p

frequency related to the frequency as f = ω , j is the square root of -1 and p is a number lower 2π

than 1. The equivalent capacitances have been then calculated from these CPE parameters as described in our previous works.19,21


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(d)

Figure 2. Nyquist plots of the impedance spectra of (a-b) doped, oxidized and undoped SpiroOMeTAD based PSCs measured at 0.0V (a, a’) and 0.6V (b, b’). Nyquist plots of the impedance spectra (c, c’) of doped and undoped B186 based PCSs. The letter prime figures are zoom views. (d) General equivalent electrical circuit used to fit the impedance spectra.

C1 of undoped cells have been determined and are plotted in Figure 3a. C1 does not change with the applied voltage and shows no charge accumulation effect. This parameter is measured at about 1.4 10-8 F.cm-2 and 1.5 10-8 F.cm-2 for pristine Spiro and pristine B186 cells, respectively. The oxidation in air of Spiro-OMeTAD leads to an increase of C1 and an average 3.3 10-8 F.cm-2 value is found. C1 is assigned to the bulk dielectric capacitance of the HTM layer. This capacitance is expressed by C/S=εrε0/d=C1, with ε0 the vacuum permittivity 9 ACS Paragon Plus Environment


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(8.85×10−12 F.m-1), εr the relative permittivity, S the cell surface area and d the HTM layer thickness. The HTM layer thicknesses were measured on cross sections at about 150 nm for Spiro-OMeTAD and 200 nm for B186 (Figure S1b and S1c, Supporting Information). εspiro is found at 2.4 and εB186 at 3.3 whereas, after oxidation, εspiro is determined at 5.6 (Table 1). The relaxation 1 is then assigned to the HTM layer and R1 is the resistance of the HTM layers. Its values at various applied voltage are displayed in Figure 3b. They do not change significantly with the applied voltage. A higher resistance is found for B186 compared to Spiro. Moreover, R1 decreases with the air oxidation due to the increase of the material conductivity. From R1, the conductivity, σHTM, is calculated according to σHTM =d/R1. It is determined at 3.0 10-7 S.cm-1 for Spiro and, as expected, increases with air oxidation at 6.3 10-7 S.cm-1. σ186 of B186 is measured at 1.0 10-7 S.cm-1 (Table 1). In the literature, σ of Spiro-OMeTAD varies in a large extent due to various compound origins and to their uncontrolled oxidation state. However, these very low values can be compared for instance to 0.7-1.10-7 S.cm-1, 0.3.10-7 S.cm-1 or 52 10-7 S.cm-1 reported by Abate et al.,29 Nguyen et al.

26

and Koh et al.,32

respectively. With LiTFSI and Co(III) complex, the conductivity of the HTM increases by several orders of magnitude due to the strong oxidation.26,29 Consequently, the relaxation time of the HTM layer decreases dramatically and this feature is not observed in the impedance spectra measured below 600 kHz of Doped-Spiro and in Doped-B186 cells in Figure 2.


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(a)

(b)

(c)

Figure 3: Effect of HTM on C1 (a) and R1 (b) of undoped PSCs measured at various applied potentials. (c) C2 versus the applied voltage of PSCs prepared with Spiro-OMeTAD and B186 at various doping levels and with two different hybrid perovskite absorbers, MAPI and FAMA.

From relaxation 2, C2 was calculated and is reported in Figure 3c. C2 did not vary significantly with Vappl up to 0.6V. The average values ranged between 1.1 and 1.4 10-8 F.cm-2 and was not influenced by the HTM and its doping. The C2 capacitance is analyzed as the bulk dielectric capacitance of the organolead perovskite layer. The slight increase in C2 above 0.6V is assigned to some charge accumulations. The assignment was confirmed by preparing B186 and Spiro-OMeTAD cells with another HP absorber, mixing both cations and anion and with composition FA0.87MA0.13Pb(I0.87Br0.17)3 (FAMA).19 The J-V curve parameters of these cells are disclosed in Table S3 (Supporting Information). Figure 3c shows that C2 changed with the HP but not with the HTM. So there is no contribution of the HTM layer and HTM



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interface to this parameter. The fact that the bulk capacitance C2 was lower for FAMA compared to MAPI suggests a lower εr for this former compound compared to MAPI.

(a)

(b)

(c)

Figure 4. (a) Effect of Spiro-OMeTAD doping on the variation of C4 with the applied potential. (b) Effect of B186 doping on C4 and comparison with Doped-Spiro. (c) Effect of Spiro-OMeTAD doping on the variation of R4 with the applied potential.

The C3 and R3 parameters were found and determined for doped-HTM cells. However, in the case of the Doped-B186 cells, C3 was somehow difficult to extract and the points are scattered. These parameters are plotted as a function of Vappl in Figure S3 (Supporting Information). R3 for both HTM are of the same order of magnitude. It slightly decreases over a large Vappl before to go down at high potential. C3 of Doped-Spiro cells continuously increases with Vappl. In the literature, these parameters are usually coupled with an inductor element to fit the intermediate frequency inductive loop found in many PSCs under light shining. However, these parameters are poorly discussed in the literature and their assignment 12 ACS Paragon Plus Environment

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remains unclear. For Guillén et al., R3 is related with recombination resistance.43 In other papers, it is related with interfacial processes since its presence and magnitude depend strongly on the electron selective material used in the PSC preparation.36,37,44 With respect to C3, this parameter has been identified as a surface state capacitance.36,37 The low frequency loop was analyzed as a R4//CPE4 circuit. The effect of Spiro-OMeTAD oxidization and doping on C4 is shown in Figure 4a. C4 is assigned to charge accumulation at the interfaces. Figure 4 shows a contribution of the HTM/perovskite interface on this parameter since the curve is different for the pristine Spiro cell. In that case, the C4 curve has a S-shape and increases more rapidly than for the Ox-Spiro and the Doped-Spiro cells. The two latter curves are quite close and reveal that oxidizing the HTM is required to improve the interface between HP and HTM. The formation of a defective interface is likely at the origin of the small amplitude and not well-defined low frequency loop in the case of the undoped B186 solar cells (Figure 2c). In Figure 4b the variation of C4 with Vappl for various doped HTMs and HPs are compared. The curves are rather similar and in the reproducibility uncertainty. R4 is related to the recombination phenomena at the interfaces and the higher R4, the lower the recombinations. Figure 4c shows the effect of oxidizing and doping on this resistance. Oxidizing by air increases this resistance. As we have shown, oxidation increases the HTM conductivity, favor the charge flow and avoid their accumulation near the interface. A better interface is also expected after oxidation. When the dopants are added, R4 increases more. Moreover, in the optimized cells tBP is added. This compound is supposed to adsorb at the perovskite surface, reduces the defects and favor the hole transfer.45 Also the shape of the R4 curve is important. To get a high FF, a flat curve up to high voltage before a steep decrease near the Voc is expected and is the signature of a high FF. Such a curve is found for the Doped-Spiro cell whereas the unoxidized Spiro cell gives exhibits a continuous decrease of R4 in agreement with the low FF and low Voc measured in this case (Table 1 and Table S1, Supporting Information). The R4 curve of the Ox-Spiro cell is localized above and is more curved than the Spiro cell curve, in agreement with the recorded intermediate FF and Voc. Figure S4 (Supporting Information) compares the R4 versus Vappl curves for Spiro-OMeTAD and B186 devices. Their order of magnitude is the same. Hovewer, the curve decreases more rapidly for the B186 cells in agreement with their lower FF and Voc (Table 1 and Table S1, Supporting information). We have noted above that the Doped-B186 cells presented a large hysteresis compared to the Doped-Spiro ones (Table 1) and it is interesting to control that the 13 ACS Paragon Plus Environment


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impedance measurements are in agreement with this electrical feature. Actually, the scan rate for the J-V curve measurement corresponds to an impedance frequency in the hertz order. In Figure S5 (Supporting Information), the imaginary part of Z (Z”) is plotted as a function of the frequency for the doped cells. As expected larger Z” is found for B186 compared to SpiroOMeTAD and it exhibits a more capacitive electric behavior.

4. Conclusions In conclusion, we have developed the impedance spectroscopy analysis of perovskite solar cells in relation with the HTM optimization. We have shown that the technique permits to discriminate the effects related to layer conductivity and to the interface with the perovskites (hole injection and charge recombinations), especially in the case of new HTMs developed for the application. We have shown that the presence of a loop at very high frequency is the signature of an unoptimized layer with a low conductivity. The analysis of this loop has allowed us to determine the εr and σ of undoped Spiro-OMeTAD and of undoped B186. The low frequency part is the impedance response of the interfaces. In the case of undoped B186, a reduced noisy loop was found as a signature of a defective interface. We show that oxidation and doping improve dramatically the quality of the perovskite/HTM interface and then the charge transfer. The recombination phenomena are reduced and it allows the achievement of high PCE. Based on these conclusions, the present work paves the way to the use of impedance spectroscopy for the fine study and optimization of new HTMs for PSCs.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc. Existing literature on the oxidation mechanisms of Spiro-OMeTAD; Solar cell exploded view; HTM SEM cross-sectional views; Best, averaged, standard deviation of the J-V solar cell parameters and PCEs; General electrical equivalent circuit and its simplifications; R3 and C3 of Doped-HTM cells; Effect of HTM and doping on the R4 parameter; Imaginary part of the impedance versus the frequency.


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Ackncknowledgements M.U. acknowledges the Indonesia Endowment Fund for Education (LPDP) scholarship for funding.

References (1) Byun, J.; Cho, H.; Wolf, C.; Jang, M.; Sadhanala, A.; Friend, R.H.; Yang, H.; Lee, T.W. Efficient Visible Quasi-2D Perovskite Light-Emitting Diodes. Adv. Mater. 2016, 28, 75157520. (2) Veldhuis, S.A.; Boix, P.P.; Yantara, N.; Li, M.J.; Sum, T.C.; Mathews, N.; Mhaisalkar, S.G. Perovskite Materials for Light-Emitting Diodes and Lasers. Adv. Mater. 2016, 28, 68046834. (3) Zhou, J.; Chu, Y.; Huang, J. Photodetectors Based on Two-Dimensional Layer-Structured Hybrid Lead Iodide Perovskite Semiconductors. ACS Appl. Mater. Interfaces 2016, 8, 2566025666. (4) Chen, S.; Shi G. Two-Dimensional Materials for Halide Perovskite-Based Optoelectronic Devices. Adv. Mater. 2017, 29, 1605448. (5) Deng, W.; Huang, L.; Xu, X.; Zhang, X.; Jin, X.; Lee, S.T.; Jie, J. Ultrahigh-Responsivity Photodetectors from Perovskite Nanowire Arrays for Sequentially Tunable Spectral Measurement. NanoLett., 2017, 17, 2482. (6) Xue, J.; Gu, Y.; Q. Shan, Y. Constructing Mie-Scattering Porous Interface-Fused Perovskite Films to Synergistically Boost Light Harvesting and Carrier Transport. Angew. Chem., Int. Ed. 2017, 56, 5232-5236. (7) Kim, H.S.; Lee, C.R.; Im J.H.; Lee, K.H.; Moehl, T.; Marchioro, A.; Moon, S.J.; Humphry-Baker, R.; Yum, J.H.; Moser, J.E. et al. M. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (8) Lee, M.M.; Teuscher, J.; Miyasaka, T.; Murakami, T.N.; Snaith, H.J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organo-Metal Halide Perovskite. Science 2012, 338, 643-647.



Page 16 of 20

(9) Liu, M.; Johnston, M.B.; Snaith, H.J. Efficient Planar Heterojunction Perovskite Solar Cells By Vapour Deposition. Nature 2013, 501, 395-398. (10) Burschka, J.; Pellet, N.; Moon, S.J.; Humphy-Baker, R.; Gao, P., Nazeeruddin, M.K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. (11) Zhang, J.; P. Barboux, P.; T. Pauporté, T. Electrochemical Design of Nanostructured ZnO Charge Carrier Layers for Efficient Solid-State Perovskite-Sensitized Solar Cells. Adv. Energy Mater. 2014, 4, 1400932. (12) 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. High-Efficiency Solution-Processed Perovskite Solar Cells with Millimeter-Scale Grains. Science 2015, 347, 522-525. (13) Zhang, J.; Juárez-Pérez, E. J.; Mora-Seró, I.; Viana, B.; Pauporté, T. Fast and Low Temperature Growth of Electron Transport Layers for Efficient Perovskite Solar Cells. J. Mater. Chem. A. 2015, 3 4909–4915. (14) Zhang, J.; Pauporté, T. Effect of Oxide Contact Layer on the Preparation and Properties of CH3NH3PbI3 for Perovskite Solar Cell Application. J. Phys. Chem. C 2015, 119, 14919−14928. (15) Zhang J.; Pauporté, T. One-Dimensional Free-Standing TiO2 Nanotube Arrays Designed for Perovskite Solar Cells Application. ChemPhysChem, 2015, 16, 2836–2841. (16) You, J.B.; Meng, L.; Song, T.B.; Guo, T.F.; Yang, Y.; Chang, W.H.; Hong, Z.R.; Chen, H.J.; Zhou, H.P.; Chen, Q. Improved Air Stability of Perovskite Solar Cells via Solution-Processed Metal Oxide Transport Layers. Nat. Nanotechnol. 2016, 11, 75-81. (17) Yang, W.S.; Park, B.W.; Jung, E.H.; Jeon, N.J.; Kim, Y.C.; Lee, D.U.; Shin, S.S.; Seo, J.; Kim, E.K.; Noh, J.H.; et al. Iodide Management in Formamidinium-Lead-Halide–Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376–1379. (18) Leblanc, A.; Mercier, N.; Allain, M.; Dittmer, J.; Fernandez, V.; Pauporté, T. Lead and Iodide Deficient MAPI, d-MAPI: the Bridge Between 2D and 3D Hybrid Perovskites. Angew. Chem., Int. Ed., 2017, 56, 16067 –16072 (19) Wang, P.; Ulfa, M.; Pauporté, T. Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells. J. Phys. Chem. C 2018, 122 1973−1981. (20) NREL chart (2018) https://www.nrel.gov/pv/assets/images/efficiency-chart.png.


Page 17 of 20 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


(21) Wang, P.; Shao, Z.; Ulfa, M.; Pauporté, T. Insights into the Hole Blocking Layer Effect on the Perovskite Solar Cell Performance and Impedance Response. J. Phys. Chem. C, 2017, 121, 9131–9141. (22) Yang, W.S.; Park, B.W.; Jung, E.H.; Jeon, N.J.; Kim, Y.C.; Lee, D.U.; Shin, S.S.; Seo, J.; Kim, E.K.; Noh, J.H. et al. Iodide Management in Formamidinium-Lead-Halide–Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376–1379. (23) Hu, Y.; Zhang, Z.; Mei, A.; Jiang, Y.; Hou, X.; Wang, Q.; Du, K.; Rong, Y.; Zhou, Y.; Xu, G. et al. Improved Performance of Printable Perovskite Solar Cells with Bifunctional Conjugated Organic Molecule. Adv. Mater. 2018, 1705786. (24) Fang, Y.; Wang, X.; Wang, Q.; Huang, J.; Wu, T. Impact of Annealing on SpiroOMeTAD and Corresponding Solid-State Dye Sensitized Solar Cells. Phys. Status Solidi A 2014, 211, 2809–2816. (25) Wang, S.; Yuan, W.; Meng, Y.S. Spectrum-Dependent Spiro-OMeTAD Oxidization Mechanism in Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 24791−24798. (26) Nguyen, W.H.; Bailie, C.D.; Unger, E.L.; and McGehee, M.D. Enhancing the HoleConductivity of Spiro-OMeTAD without Oxygen or Lithium Salts by Using Spiro(TFSI)2 in Perovskite and Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2014, 136, 10996−11001. (27) Li, Z.; Tinkham, J.; Schulz, P.; Yang, M.; Kim, D.H; Berry, J.; Sellinger, A.; Zhu K. Acid Additives Enhancing the Conductivity of Spiro-OMeTAD Toward High-Efficiency and Hysteresis-Less Planar Perovskite Solar Cells. Adv. Energy Mater. 2017, 7, 1601451. (28) Bach, U.; Lupo, D.; Comte, P.; Moser, J.E.; Weissortel, F.; Salbeck, J.; Spreitzer, H.; Grätzel, M. Solid-State Dye-Sensitized Mesoporous TiO2 Solar Cells with High Photon-toElectron Conversion Efficiencies. Nature 1998, 395, 583. (29) Abate, A.; Leijtens, T.; Pathak, S.; Teuscher, J.; Avolio, R.; Errico, M.E.; Kirkpatrik, J.; Ball, J.M.; Docampo, P.; McPherson, I. et al. Lithium Salts as "Redox Active" p-Type Dopants for Organic Semiconductors and their Impact in Solid-State Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 2572-2579. (30) Burschka, J.; Dualeh, A.; Kessler, F.; Baranoff, E.; Cevey-Ha, C., Yi, Y.; Nzeeruddin, Gratzel, M. Tris(2-(1H-pyrazol-1-yl)pyridine)cobalt(III) as p-Type Dopant for Organic Semiconductors and Its Application in Highly Efficient Solid-State Dye-Sensitized Solar Cells. J. Am. Chem. Soc., 2011, 133, 18042-18045. (31) Burschka, J.; Kessler, F.; Nazeeruddin, M.K.; Grätzel, M. Co(III) Complexes as pDopants in Solid-State Dye-Sensitized Solar Cells. Chem. Mater. 2013, 25, 2986- 2990.



Page 18 of 20

(32) Koh, T.M.; Dharani, S.; Li, H.; Prabhakar, R.R.; Mathews, N.; Grimsdale, A.C.; Mhaisalkar, S.G. Cobalt Dopant with Deep Redox Potential for Organometal Halide Hybrid Solar Cells. ChemSusChem 2014, 7, 1909-1914. (33) Abate, AA.; Hollman, D.J.; Teuscher, J.; Pathak, S.; Avolio, R.; D’Errico, G.; Vitiello, G.; Fantacci, S.; Snaith, H.J. Protic Ionic Liquids as p-Dopant for Organic Hole Transporting Materials and Their Application in High Efficiency Hybrid Solar Cells. J. Am. Chem. Soc. 2013, 135, 13538-13548. (34) Zhang, H.; Shi, Y.; Yan, F.; Wang, L.; Wang, K.; Xing, Y.; Donga, Q.; Ma, T. A dual Functional Additive for the HTM Layer in Perovskite Solar Cells. Chem. Commun. 2014, 50, 5020-5022. (35) Xu, M. ; Rong, Y. ; Ku, Z. ; Mei, A.; Li, X. ; Han, H. Improvement in Solid-State Dye Sensitized Solar Cells by p-Type Doping with Lewis Acid SnCl4. J. Phys. Chem. C. 2013, 117, 22492-22496. (36) Guerrero, A.; Garcia-Belmonte, G.; Mora-Sero, I.; Bisquert, J.; Kang, Y.S.; Jacobsson, T.J.; Correa-Baena, J.P.; Hagfeldt, A. Properties of Contact and Bulk Impedances in Hybrid Lead Halide Perovskite Solar Cells Including Inductive Loop Elements. J. Phys. Chem. C 2016, 120, 8023−8032. (37) Anaya, M.; Zhang, W.; Hames, B.C.; Li, Y.L.; Fabregat-Santiago, F.; Calvo, M.E.; Snaith, H.J.; Miguez, H.; Mora-Sero, I. Electron Injection and Scaffold Effects in Perovskite Solar Cells. J. Phys. Chem. C. 2017, 5, 634-644. (38) Pitarch-Tena, D.; Ngo, T.T.; Vallés-Pelarda, M.; Pauporté, T.; Mora-Seró, I. Impedance Spectroscopy Measurements in Perovskite Solar Cells. Device Stability During the Measurement and Noise Reduction. ACS Energy Lett. 2018, 3, 1044−1048. (39) Guérin, V.M.; J. Elias, J. ; T.T. Nguyen, T.T. ; L. Philippe, L. ; T. Pauporté ; T. Ordered Networks of ZnO-Nanowire Hierarchical Urchin-like Structures for Improved Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2012, 14, 12948–12955. (40) Magne, C.; Dufour, F. ; Labat, F. ; Lancel, G. ; Durupthy, O. ; Cassaignon, S. ; Pauporté, T. Effects of TiO2 Nanoparticle Polymorphism on Dye-Sensitized Solar Cell Photovoltaic Properties. J. Photochem. Photobiol. A 2012, 232, 22-31. (41) Bui, T.T.; Shah, S.K.; Abbas, M.; Sallenave, X.; Sini, G.; Hirsch, L.; Goubard, F. Carbazole-Based Molecular Glasses as Hole-Transporting Materials in Solid State DyeSensitized Solar Cells. ChemNanoMat, 2015, 1, 203–210.


Page 19 of 20 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


(42) Bui, T.T.; Ulfa, M.; Maschietto, F.; Ottochian, A.; Nghiem, M.P.; Ciofini, I.; Goubard, F. Pauporté, T. Design of Dendritic Core Carbazole-Based Hole Transporting Materials for Efficient and Stable Hybrid Solar Cells. Org. Electron. Submitted. (43) Guillén, E.; Ramos, F.J.; Anta, J.A.; Ahmad, S. Elucidating Transport-Recombination Mechanisms in Perovskite Solar Cells by Small-Perturbation Techniques. J. Phys. Chem. C 2014, 118, 22913–22922. (44) Juarez-Perez, E.J.; Wubler, M.; Fabregat-Santiago, F.; Lakus-Wollny, K.; Mankel, E.; Mayer, T.; Jaegermann, W.; Mora-Sero, I. Role of the Selective Contacts in the Performance of Lead Halide Perovskite Solar Cells. J. Phys. Lett. 2014, 5, 680–685. (45) Wu,Y.H; Shi, X.Q.; Ding, X.H.; Ren, Y.K.; Hayat, T.; Alsaedi, A.; Ding, Y.; Xu, P.; Dai, S.Y. Incorporating 4-tert-Butylpyridine in an Antisolvent: A Facile Approach to Obtain Highly Efficient and Stable Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2018, 10, 3602-3608.



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TOC Graphic


Impact of Organic Hole Transporting Material and Doping on the

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