Journal of Power Sources 344 (2017) 160e169
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Readily synthesized dopant-free hole transport materials with phenol core for stabilized mixed perovskite solar cells Yuyuan Xue, Ying Wu, Yuan Li* School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, 510641 Guangzhou, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A PCE of 16.97% was obtained with a dopant-free HTM in PVSC. A very low hole mobility and electron blocking will also ensure high PCE. Phenol and its derivatives show great potential as building block for HTMs. Device stability is enhanced with extremely cheap HTMs.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2016 Received in revised form 13 January 2017 Accepted 29 January 2017
With the dramatic development of the power conversion efficiency (PCE) of perovskite solar cells (PVSCs), device lifetime has become one of the extensive research interests and concerns. To enhance the device durability, developing high performance dopant-free hole transport materials (HTMs) is a promising strategy. Herein, two new C3-symmetric HTMs with phenol core, TCP-OH and TCP-OC8 are readily prepared and show ultra-wide energy band-gap and excellent film-formation property. PCEs of 16.97% and 15.28% are achieved with pristine TCP-OH and TCP-OC8 film as HTMs, respectively, even though their hole mobilities are as low as 106 cm2 V1 s1. Phenol acts as hole trap in traditional concept, however, TCP-OH shows higher hole mobility than that of TCP-OC8. Moreover, TCP-OH shows higher glass transition temperature and better matching band alignment than those of TCP-OC8. Phenol shows great potential as building block for HTMs as it is beneficial to enhance hole mobility of HTMs. Moreover, our study demonstrates an interesting viewpoint to design HTMs with the balance of hole mobility and electron blocking effect. © 2017 Elsevier B.V. All rights reserved.
Keywords: Perovskite solar cells Dopant-free Hole transport materials Ultra-wide bandgap Phenol
1. Introduction Perovskite solar cells (PVSCs) has captured tremendous attention because of the special optical and electrical properties of
* Corresponding author. E-mail address:
[email protected] (Y. Li). http://dx.doi.org/10.1016/j.jpowsour.2017.01.121 0378-7753/© 2017 Elsevier B.V. All rights reserved.
perovskite material [1e3]. As a great light harvester, perovskite shows a wide light absorption over the whole visible solar emission spectrum [2h]. Moreover, perovskite materials also behave as efficient semiconductors with high charge mobility and a long carrier diffusion length. For example, methylammonium lead iodide, CH3NH3PbI3 exhibits high carrier mobilities ranging from 7.5 cm2 V1 s1 for electrons to 12.5 cm2 V1 s1 - 66 cm2 V1 s1 for holes, and the carrier diffusion length ranged between 100 nm
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and 1000 nm [2a]. Since the original research by Miyasaka in 2009 [1a], power conversion efficiency (PCE) of the record PVSCs has been boosted from 3.8% to over 20% in the last 7 years [1a,3f], making them as good candidates for the next generation solar cells. Considering the future practical application of PVSCs, low fabrication cost, high PCE and lifetime are critically important aspects that need to be considered. Among the reported PVSC device structures, mesoporous PVSCs showed extremely high performance [2b]. Mesoporous PVSCs are composed of mesoporous layer of TiO2, perovskite material and hole transport materials (HTMs). To achieve high power conversion efficiency and device stability of PVSCs, HTMs should be further developed [2,3]. HTMs play a central role to transport and extract holes at the anode interface [3e5]. The highest occupied molecular orbital (HOMO) energy level of HTMs must be well matched with the perovskite layer, meanwhile, the hole mobility in the HTM should be reasonably high, which are favorable to achieve high PCE. In general, chemical doping is indispensable to improve the hole mobility of HTMs due to the low hole mobility in their pristine films. As the most successful HTMs, 2,20 ,7,70 -tetrakis(N,N-dis(pmethoxy-phenyl)amine)-9,90 -spirobifluorene (Spiro-OMeTAD) and polytriarylamine (PTAA) showed extremely high PCEs in PVSCs, after chemical doping with lithium bis(trifluoromethane-sulfonyl) imide (LiTFSI) and tert-butylpyridine (TBP) [4]. However, in spite of the dramatical enhancement of the PCE with doped HTMs, chemical dopants suffered a series of problems including fabrication cost and device stability [4b]. Recently, developing dopant-free HTM with high hole mobility has attracted wide interest of many researchers [5]. Nevertheless, progress on HTM is not smooth and the PCEs in PVSC devices are sparse over 16% based on dopant -free HTM. Meanwhile, most of them show a relatively high preparation cost owing to their complex synthesis and purification steps considering their large scale €tzel and co-workers designed a novel industrial production. Gra dopant-free HTM with an impressive PCE up to 16.3% based on a butterfly-shaped structure following a hole mobility of 8.49 104 cm2 V1 s1 [5b]. Yang et al. reported a donor-acceptor (D-A) conjugation dopant-free HTM with a hole mobility of 1.0 104 cm2 V1 s1 and it exhibited a PCE of 16.2% in PVSC [5c]. Park et al. presented a polymeric dopant free HTM with a PCE up to 17.3% and its hole mobility is 3.09 103 cm2 V1 s1 [5d]. Chen et al. reported a remarkable PCE of 18.6% in an inverted-type PVSC based on a dopant-free HTM, which exhibited a hole mobility of 3.6 103 cm2 V1 s1 [5e]. Despite these HTMs show an efficient performance in PVSCs, some of these HTMs show relatively weak electron blocking property due to the narrow energy band gap (Eg) [5c,5d,5h]. The relatively weak electron blocking capability might increase the chance of charge recombination. Hence, weak electron blocking capability of these HTMs are remedied by their high hole mobility. Therefore, the balance between hole mobility and band alignment is an important factor to be considered for the design of highly efficient dopant-free HTMs for PVSCs. Will a low hole mobility HTM with excellent band alignment get high performance in perovskite cells? To our best knowledge, there was rare report based on this hypothesis, to date. In this work, two novel phenol derivatives with the wide energy band-gap (TCP-OH and TCP-OC8), have been readily synthesized. In spite of their hole mobility are as low as 106 cm2 V1 s1, the best PCEs of these two HTMs, which are employed in PVSC by dopantfree process, are 16.97% and 15.28%, respectively. These two HTMs were carefully characterized to reveal their physical/optoelectronic properties and performance in PVSCs. The mechanism was investigated and discussed in detail.
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2. Results and discussion 2.1. Structural design, preparation and characterization of TCP-OH and TCP-OC8 2.1.1. Structural design of TCP-OH and TCP-OC8 The structural design of TCP-OH and TCP-OC8 was motivated by the traditional HTM in organic light-emitting diodes. 4,40 ,400 -Tri(Ncarbazolyl)-triphenylamine (TCTA) is a traditional HTM and has a wide energy band-gap (3.2 eV) with a hole mobility of 2 105 cm2 V1 s1 [6]. However, the HOMO energy level of TCTA (5.9 eV) is too deep to match the valence band of perovskite materials. To enhance the HOMO energy level of TCTA, phenol core is chosen to replace the nitrogen atom in the triphenylamine structure of TCTA, which is inspired by the recent work and concept on phenol-based organic electronic (PBOE) in our group [7d]. Our recent works show that non-conjugated phenol-based polymers and even weakly conjugated lignin were potential p-type transport materials [7]. 2.1.2. Preparation of TCP-OH and TCP-OC8 Two HTMs named 2,4,6-tris(9-phenyl-9H-carbazol-3-yl)phenol (TCP-OH) and 3,30 ,300 -(2-(octyloxy)-benzene-1,3,5-triyl)tris(9phenyl-9H-carbazole) (TCP-OC8) were prepared (Scheme 1). The star-shaped and twisted structure is constructed by introducing 9phenyl-9H-carbazole (CP) into the o-/p- positions of phenol via one-step Suzuki-Miyaura coupling reaction (Scheme 1). (9-phenyl9H-carbazol-3-yl)boronic acid (PCBA) was purchased from Soochiral Chemical Science &Technology Co.,Ltd, China, and the price of this reagent is even lower than 1.5 $/g. 1,3,5-tribromo-2-(octyloxy)benzene was synthesized according to a previously reported procedure [8]. All the other materials were purchased from commercial sources and used as received. With the loading of excessive PCBA, the yields of TCP-OH and TCP-OC8 were as high as 87% and 93%, respectively, after the purification of silicon gel column chromatography. The simple synthesis of TCP-OH and TCP-OC8 based on commercially available and cheap starting materials, make these two HTMs very promising for their potential industrial production in the future. A detailed cost accounting was presented in supporting information (Fig. S17 and Tables S3eS5 in SI). TCP-OH and TCP-OC8 showed obvious advantages over the other high performance dopant-free HTMs applied in PVSCs. 2.1.3. Characterization of TCP-OH and TCP-OC8 The chemical structures of HTMs were characterized by 1H/13C NMR spectroscopy and high-resolution mass spectrometry (Figs. S1-S6). The two materials have good solubility in common organic solvents, including chloroform, toluene, chlorobenzene (>300 mg/mL), DMF and DMSO. It is noteworthy that TCP-OH
Scheme 1. The synthetic route of TCP-OH and TCP-OC8.
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exhibits excellent solubility in low polar solvents even though phenol and phenol-based derivatives are usually soluble in high polar solvents. The normalized UVevis absorptions of TCP-OH and TCP-OC8 were shown in Fig. 1a. The corresponding data of TCP-OH and TCPOC8 (labs, lonset, optical bandgap, and energy levels) are listed in Table 1. The absorptions of TCP-OH and TCP-OC8 were centered at 294 and 298 nm, respectively (Fig. 1a). The two compounds have the same adsorption onsets at 359 nm, which indicates they have the same optical energy band-gap (Eopt g ) of 3.45 eV. Cyclic voltammetry (CV) was used to analyze the energy level of the HTMs (Fig. S7). The CV curves were depicted in Fig. 1b and the corresponding energy level was illustrated in Fig. 2a. The HOMO energy levels of TCP-OH and TCP-OC8 were calculated to be 5.47 eV and 5.56 eV, respectively (Fig. 1b). The HOMO energy levels of TCP-OH and TCP-OC8 were much lower than that of Spiro-OMeTAD (5.09 eV). To gain further understanding on the geometrical configuration and electronic structure, main contribution frontier molecular orbitals of TCP-OH and TCP-OC8 were calculated using density functional theory (DFT) calculation at B3LYP/6-31G* level by Gaussian 09 program [4h]. As depicted in Fig. 1c, the HOMO electron density of the HTMs was relatively delocalized within the whole molecule and the LUMO electron density was primarily localized on one group of 9-phenyl-9H-carbazole. In addition, the HOMO/LUMO energy levels were calculated to be 4.95/0.93 eV and 5.11/0.66 eV for TCP-OH and TCP-OC8, respectively. The HOMO of TCP-OH is higher than that of TCP-OC8, which showed the same trend to the experimentally determined value. A relatively hydrophobic surface will be beneficial to improve the device performance, especially for device stability, by expelling moisture away from perovskite film. TCP-OH and TCP-OC8
exhibited contact angles of 79.6 and 85.0 (Figs. S8aeS8b), respectively, which showed the films of TCP-OH and TCP-OC8 were more hydrophobic than that of pristine/doped Spiro-OMeTAD (Figs. S8ceS8d). It was proposed that the hydrophobic property of TCP-OH and TCP-OC8 was ascribed to the whole branched and twist molecule structure based on the twist 9-phenyl-9H-carbazole substituent group, despite phenol-containing material is thought of commonly more hydrophilic. The large steric hindrance group of two 9-phenyl-9H-carbazoles around phenolic hydroxyl group would reduce its hydrophilic effect and there was only one phenolic hydroxyl group in the molecular structure of TCP-OH. Moreover, both of TCP-OC8 and TCP-OH have an extremely good solubility in chlorobenzene (>300 mg/mL). It indicated that TCP-OH showed a very strong hydrophobic property, which was comparable with that of TCP-OC8. The thermal properties of the materials were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements (Figs. S10eS12). The temperature was ramped from 40 to 800 C with rate of 10 C/min in nitrogen atmosphere. The thermal stability of HTMs was determined by the 5% weight loss temperature. TCP-OC8 showed a main decomposition at 408 C. The first decomposition of TCP-OC8 occurred when the side chains began to cleave. TCP-OH showed excellent thermal stability with a high thermal decomposition temperature (Td) of 538 C. About 5% mass loss at 100 C was ascribed to the bound water due to intermolecular hydrogen bonding in TCP-OH. It is noteworthy that the 5% weight loss temperature of TCP-OH was calculated after the bound water was removed. The high thermal stability of amorphous films is very important for its application in device. The glass transition temperature (Tg) of TCP-OH and TCPOC8 was 158 C and 86 C, respectively. DSC revealed that both of TCP-OH and TCP-OC8 were amorphous, owning to their twisted
Fig. 1. (a) UVevis absorption spectra of TCP-OH, TCP-OC8 and Spiro-OMeTAD in dichloromethane. (b) Cyclic voltammograms of TCP-OH, TCP-OC8 and Spiro-OMeTAD in dichloromethane solution. (c) Calculated frontier molecular orbitals of TCP-OH and TCP-OC8.
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Table 1 Optical, electrochemical properties and energy level of TCP-OH, TCP-OC8 and Spiro-OMeTAD. Samples
labs (nm)a
lonset (nm)a
b Eopt g (eV)
Eonset (V vs Ag/AgCl)c ox
HOMO (eV)d
LUMO (eV)e
m (cm2 V1 s1)f
TCP-OH TCP-OC8 Spiro-OMeTAD
294 298 386
359 359 418
3.45 3.45 2.97
0.77 0.86 0.39
5.47 5.56 5.09
2.02 2.11 2.12
5.85 106 2.56 107 8.65 105
a
Measured in CH2Cl2 solution. onset Eopt is the onset potential of the first oxidative wave. g : optical energy gap derived from the lowest energy absorption onset in the absorption spectra, where Eox Measured in CH2Cl2 solution with 0.1 M tetrabutylammoniun hexafluorophosphate as the supporting electrolyte at a scan rate of 50 mV/s, a Ag/AgCl electrode as the reference electrode, a carbon-glass electrode as the working electrode, a Pt electrode as the counter electrode and ferrocene/ferrocenium (Fc/Fcþ) as an internal reference. d The conversion E(Fc/Fcþ) ¼ 0.63 V vs NHE was used. The oxidation potential of Fc/Fcþ was 0.43 V vs Ag/AgCl electrode. HOMO ¼ (4.7 þ Eonset ox ). e LUMO ¼ (Eopt g þ HOMO). f The hole mobility are in their pristine forms. b c
perovskite to HTM. Well-matched HOMO energy level is an essential prerequisite for highly efficient HTMs. The HOMO energy level of TCP-OH and TCP-OC8 are about 5.47 eV and 5.56 eV, respectively, which are too low to match the valence band of CH3NH3PbI3 (5.40 eV). Seok et al. reported an excellent work on a double-mixed perovskite (DMPV) layer by incorporating methylammonium lead bromide (MAPbBr3) into formamidinium lead iodide (FAPbI3) and the PCE was improved comparing with the single component [2d]. The valence band of DMPV ((FAI)0.81(PbI2)0.85(MAPbBr3)0.15) was as low as 5.65 eV, which was a favorable valence band for TCP-OH and TCP-OC8 [10]. Compared with the valence band level of DMPV, TCP-OH and TCP-OC8 will reduce the energy level barrier and favor the hole transport from perovskite layer to the Au anode. Moreover, the LUMO energy level of TCP-OH and TCP-OC8 were 2.02 eV and 2.11 eV, which were estimated according to their HOMO and Eg, respectively. Meanwhile, the large energy band (1.81e1.93 eV) between the LUMO of perovskite and HTMs will favor the electron blocking effect and thus largely reduce the recombination chance. Consequently, TCP-OH and TCP-OC8 acted as not only an HTM, but also an electron blocking material.
Fig. 2. (a) Energy level diagram for each layer in PVSC device. (b) Cross-sectional structure of a representative device.
structure [8,9]. Atomic force microscopy (AFM) measurement of TCP-OH and TCP-OC8 was conducted and the results indicated that they showed extremely smooth surface with root-mean-square (RMS) value of only about 0.4 nm in 5 5 mm2 (Fig. S9). It is exciting that our HTMs have comparable film-formation capability to that of SpiroOMeTAD. Such a good hydrophobicity and high quality film will be beneficial to achieve high performance in PVSCs. The high filmformation capability during the spin-coating process might be attributed to their amorphous properties [9].
2.2. Performance of TCP-OH and TCP-OC8 in double-mixture perovskite solar cells 2.2.1. The choice of the double-mixed perovskite material The HOMO energy level of the HTM must be higher than the valence band of perovskite, facilitating the transfer of holes from
2.2.2. Performance of TCP-OH and TCP-OC8 in double-mixture perovskite solar cells The hole extract properties of HTMs were demonstrated by fabricating the DMPV-based solar cells with a structure of FTO/ TiO2/(FAI)0.81(PbI2)0.85 (MAPbBr3)0.15/HTMs/Au (Fig. 2 and Fig. S13) [2b]. The details of device preparation were provided in SI. The current density-voltage (J-V) and incident photon-tocurrent efficiency (IPCE) spectra are shown in Fig. 3. The J-V curves were collected by scanning the applied voltage at 125 mV s1 from forward and reverse scans. The photovoltaic parameters of these devices are listed in Table 2, Tables S1 and S2. TCP-OH and TCP-OC8 were employed as HTMs in PVSC with dopant-free process. The best TCP-OH based PVSC device exhibited a PCE of 16.97%, with an open circuit voltage (Voc) of 1.07 V, a short current density (Jsc) of 23.15 mA cm2 and a fill factor (FF) of 66.7% under reverse scan conditions. The device with TCP-OC8 as HTM also showed a good performance, yielding the highest PCE of 15.28% in the reverse scan (Voc ¼ 1.09 V, Jsc ¼ 22.38 mA cm2, FF ¼ 61.4%). The PCE of TCP-OH based device is better than that of TCP-OC8 based device, resulting from the improved Jsc and FF. The small difference in FF between the two HTMs based devices is attributed to their slight difference of series resistance (Rs) [4e]. The lower Jsc of TCP-OC8 based devices might be induced by its lower hole mobility, comparing with those of TCP-OH based devices, because the difference in morphology is negligible (Fig. 3). Moreover, the Voc of the TCP-OH based device is slightly lower than that of TCP-OC8 device, which is consistent with their HOMO energy levels of the two HTMs. Hysteresis behavior is not favorable in PVSCs. The PCE
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790 nm, and the photocurrents obtained from the IPCE data were in relatively close agreement with those of current-voltage measurements and showed similar trends. Phenol acts as hole trap in traditional concept, however, TCP-OH shows a better performance in PVSC and higher hole mobility than those of TCP-OC8. Phenol shows great potential as building block for HTMs as it is beneficial to enhance hole mobility of HTMs [7], and this result is similar with our previous concept on PBOE [7d]. The reproducibility of PVSC devices based on TCP-OH and TCP-OC8 as HTMs, was demonstrated in Tables S1 and S2. PVSC devices with TCP-OH and TCP-OC8 exhibited relatively narrow distributions in the ten individual devices, respectively. The PCEs of TCP-OH-based devices ranged from 14.77 to 16.97% and the PCEs for TCP-OC8based devices were between 12.10 and 15.28%. The variation of PCEs might be related with the dripping process in the doublemixture perovskite film fabrication [2i]. As the whole, these results revealed that TCP-OH and TCP-OC8 showed a relatively good reproducibility in PVSC and the performance of TCP-OH based device is better than that of TCP-OC8 based device. 2.3. The hole mobility of TCP-OH and TCP-OC8
Fig. 3. (a) J-V curves of the best cells recorded with reverse (from Voc to Jsc) and forward (from Jsc to Voc) scanning directions. (b) The corresponding IPCE spectra taken with chopped monochromatic light under a white light bias corresponding to 5% solar intensity.
differences between the forward and reverse scanning were 5.7% and 7% for TCP-OH and TCP-OC8 based devices, respectively. To further evaluate the performance of TCP-OH, the photocurrent density and power output were measured as a function of time at the maximum power point (0.81 V) (Fig. S16). The stabilized power output of TCP-OH approached 16.22% with the photocurrent density of 20.10 mA cm2 over the measurement window, which was in consistent with the PCE result in I-V measurement [6d]. Fig. 3b showed the IPCE spectra for the PVSC cells using TCP-OH and TCP-OC8 as HTMs. The generation of photocurrent began at
The hole mobility of TCP-OH, TCP-OC8 and Spiro-OMeTAD was evaluated with the hole-only devices fabricated with device structure: indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene): polystyrene (PEDOT: PSS)/TCP-OH, TCP-OC8 or (Spiro-OMeTAD)/MoO3/Al (Fig. 4). The hole mobilities were calculated by fitting the data using the space charge limited current (SCLC) model (Fig. 4a and Fig. S15). The hole mobilities of pristine TCP-OH and TCP-OC8 were 5.85 106 and 5.56 107 cm2 V1 s1, respectively. TCP-OH showed a higher hole mobility than that of TCP-OC8, owning to the reactively good molecular rigidity of TCP-OH. Meanwhile, the substituent of -OC8H17 is insulated and the flexible alkyl chain results in the disordered molecular arrangement in film, which might have negative effects on the mobility [11c]. The hole mobility of SpiroOMeTAD was calculated to be 8.65 105 cm2 V1 s1 and the measurement result was in good agreement with the previous reports [4b,5d]. The hole mobility of doped Spiro-OMeTAD was about 3.1 104 cm2 V1 s1 in the reported work [5g]. The hole mobility of TCP-OH and TCP-OC8 are much lower than that of doped SpiroOMeTAD or even that of pristine Spiro-OMeTAD. The low hole mobilities of TCP-OH and TCP-OC8 are reasonable due to their branched chemical structure and amorphous property. In traditional concept, the mobility with 106 cm2 V1 s1 was unacceptable in PVSC. 2.4. Performance of pristine and doped Spiro-OMeTAD in doublemixture perovskite solar cells To understand the high performance of TCP-OH and TCP-OC8 in PVSCs, the devices based on Spiro-OMeTAD were also studied (Table 2 and Fig. S14). The photovoltaic parameters are listed in
Table 2 Solar Cell Performance Parameters of dopant-free TCP-OH, TCP-OC8 and pristine/doped Spiro-OMeTAD.a Sample
Scan direction
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
Rs (U)
TCP-OH
Reverse Forward Reverse Forward Reverse Forward Reverse Forward
23.15 23.09 22.38 22.35 21.39 21.40 23.10 23.14
1.07 1.06 1.09 1.09 0.96 0.96 1.11 1.09
66.7 63.1 61.4 57.5 63.1 56.0 72.6 62.4
16.97 15.68 15.28 14.22 13.26 11.87 18.85 15.99
35.76
TCP-OC8 Pristine Spiro-OMeTAD Doped Spiro-OMeTAD a
The best-performing devices of TCP-OH, TCP-OC8 and pristine/doped Spiro-OMeTAD at a scanning rate of 125 mV/s under AM 1.5100 mW cm2.
46.52 36.01 13.44
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OC8, it confirmed that our two HTMs showed much better matching energy level alignment with the valence band of DMPV layer than that of pristine Spiro-OMeTAD.
2.6. The mechanism of efficient performance in PVSC with TCP-OH and TCP-OC8 as HTMs As a whole, we concluded that expect for hole mobility, band alignment also was an important factor which should be considered to ensure the remarkable efficiency in double-mixed perovskite solar cell. We proposed the following three points to understand our high performance. 1) Comparing with the narrow band gap HTMs: The HTMs with high hole mobility and narrow band gap could also show a good device performance [5c,5d,5h]. However, the LUMO energy level of these HTMs is about 3.7 eV, which is close to the conduction band of perovskite (3.9 eV), and most of them showed an ambipolar transport property [13]. The relatively weak electron blocking capability might increase the chance of charge recombination. Our HTMs displayed an excellent electron blocking capability, which would decrease the chance of charge recombination (Fig. 6). 2) Comparing with pristine Spiro-OMeTAD:
Fig. 4. (a) JV curves of hole-only devices with SCLC fitting for TCP-OH, TCP-OC8 and Spiro-OMeTAD. (b) Energy level diagram for each layer in SCLC device.
Table 2. Despite the hole mobility of pristine Spiro-OMeTAD was higher than that of TCP-OH and TCP-OC8, a low PCE was observed in the pristine Spiro-OMeTAD-based device, yielding the highest PCE of 13.26% (Voc ¼ 0.96 V, Jsc ¼ 21.39 mA cm2, FF ¼ 0.631) with a PCE difference of 10.5% between the forward and reverse scanning. Interestingly, this PCE based on pristine Spiro-OMeTAD as HTM, was one of the highest efficiency, which was a new finding reported for the first time. Doped Spiro-OMeTAD exhibited a high PCE up to 18.85% with a Voc of 1.11 V, a Jsc of 23.10 mA cm2 and a FF of 0.726. After the chemical doping, all of the three photovoltaic parameters were obviously improved. However, no significant increase was observed in Voc with the doping process in the CH3NH3PbI3 layer solar cells [5d,5g]. In general, chemical doping would improve the hole mobility of HTMs, meanwhile, it would also decrease the HOMO energy level of HTMs (5.6 eV of doped Spiro-OMeTAD film, see Fig. 5). Comparing the performance of Spiro-OMeTAD in different perovskite materials before and after chemical doping, it indicates that band alignment plays a very important role in DMPV solar cell. 2.5. The HOMO energy levels of TCP-OH, pristine and doped SpiroOMeTAD films The HOMO energy levels of TCP-OH, pristine and doped SpiroOMeTAD films were further measured using an ultraviolet photoelectron spectrometer (UPS) (Fig. 5). We failed to measure the HOMO energy level of TCP-OC8 on ITO substrate by UPS due to its low conductivity. The resulted HOMO energy levels for TCP-OH, pristine and doped Spiro-OMeTAD films were 5.52 eV, 5.08 eV and 5.60 eV, respectively. Considering the similar Voc of doped Spiro-OMeTAD HTM based PVSC with those of TCP-OH and TCP-
Spiro-OMeTAD showed a good selective contact for extracting holes and blocking electrons, however, a lower PCE was observed in the pristine Spiro-OMeTAD-based device, comparing with the PCEs of TCP-OH and TCP-OC8 based devices in this work. The parameters of Voc and Jsc were lower than these of TCP-OH and TCP-OC8 based devices. The well matched HOMO energy level between DMPV layer and our HTMs resulted in the high Voc and Jsc. 3) Contribution derived from double-mixed perovskite layer (DMPV): For the in-depth understanding of underlying mechanism for the high PCE of our HTMs, the perovskite layer structure is another important aspect which should be considered. DMPV was chosen in our study and the valance band of DMPV would well match the HOMO energy level of our HTMs, comparing with 5.4 eV of CH3NH3PbI3 (Fig. 6a and b). Moreover, it has been demonstrated that perovskite exhibited as an ambipolar semiconductor resulting from the high carrier mobilities for electrons and holes [1b,2h,11a,11b], and the recombination in perovskite occurs until the microsecond time scale [2h]. As reported, FAPbI3 and MAPbI3 display as a p-type and n-type properties, respectively [2h,3b]. Comparing with MAPbI3, FAPbI3 has a larger hole-diffusion length than the electron-diffusion length [12]. This indicates that many more holes than electrons should travel a long distance in the DMPV layer, comparing with the single component perovskite material of MAPbI3. Balanced electron and hole mobilities remain highly up to the microsecond time scale. As a result, under the promise of electron blocking effect, the relatively low hole mobility of our HTMs is acceptable and enough to achieve efficient hole extract. This phenomenon was further confirmed by the device performance of pristine and doped Spiro-OMeTAD, which showed significantly different device parameters between CH3NH3PbI3 and DMPV solar cells. Therefore, we can conclude that a high PCE would be produced in double-mixed perovskite solar cell with the HTM which has a relatively low hole mobility and excellent band alignment.
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Fig. 5. Ultraviolet photoelectron spectroscopy (UPS) measurements of TCP-OH, pristine and doped Spiro-OMeTAD films spin-coated on ITO. A sample bias of 4.8 V was applied to clear the surface vacuum level.
Fig. 6. (a) The HOMO energy level of HTM (HOMO: 5.5 eV) is too deep for MAPbI3. (b) Well matching energy level between double-mixed perovskite and HTM (HOMO: 5.5 eV). (c) Dopant-free HTM with high hole mobility and narrow bandgap in MAPbI3 PVSC. (d) The HTM with high hole mobility and ultra-wide bandgap in double-mixed PVSC.
2.7. The long-term stability of PVSC devices Finally, the long-term stability of PVSC is a crucial factor to evaluate the performance of perovskite cells. The stability of our HTMs and doped Spiro-OMeTAD-based PVSC was investigated by subjecting continuously to a 10 mW cm2 UV-filtered simulated
sun light at 45 C in ambient air and maintained at the open circuit condition with encapsulation (Fig. 7) [5b]. The devices based on TCP-OH and TCP-OC8, presented higher stability than that based on doped Spiro-OMeTAD after 720 h. The devices based on TCP-OH and doped Spiro-OMeTAD exhibited a slight increase of the PCE before 200 h. After that, a moderate decrease in the photovoltaic
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Fig. 7. Time-course variation of the photovoltaic parameters of TCP-OH, TCP-OC8 and doped spiro-OMeTAD-based perovskite solar cells under 10 mW cm2 UV-filtered simulated sun light at 45 C in ambient air and maintained at the open circuit condition. a) Normalized PCE. b) Normalized Voc. c) Normalized Jsc. d) Normalized FF.
performance was observed in the cells with TCP-OH and TCP-OC8 as HTMs. The PCE for PVSC based on TCP-OH droped 3.0%, 6%, and 13% after 372, 456, and 720 h light soaking, respectively. However, in case for doped Spiro-OMeTAD, there was an obvious 40% drop in the PCE after 720 h under the same condition. The decrease of doped Spiro-OMeTAD-based photovoltaic performance might be attributed to the decomposition of perovskite layer, due to a slight decrease in Jsc and Voc during prolonged light soaking. In the absent of dopant, TCP-OH and TCP-OC8 based devices showed a good performance of long-term stability. 3. Conclusion In summary, two readily prepared and low cost dopant-free HTMs, TCP-OH and TCP-OC8, for highly efficient and stable perovskite solar cells, were reported. In general, HTMs with high mobility are favorable and exhibit promising performance. In contrast, our study demonstrated that excellent electron blocking effect, matching energy level and good film-formation property could also ensure the high performance of PVSCs, even though the hole mobility is as low as 106 cm2 V1 s1. We forecast that high hole mobility and good electron blocking will produce higher performance HTM than TCP-OH in PVSCs. This encouraging result will be a starting point to explore high performance HTMs based on phenol and its derivatives in future. Acknowledgment The authors would like to acknowledge the financial support of
the National Natural Science Foundation of China (21402054, 21436004) and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.01.121. References [1] a) A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050e6051; b) M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (2012) 643e647; c) M.Z. Liu, M.B. Johnston, H.J. Snaith, Efficient planar heterojunction perovskite solar cells by vapour deposition, Nature 501 (2013) 395e398; d) J.H. Im, C.R. Lee, J.W. Lee, S.W. Park, N.G. Park, 6.5% efficient perovskite quantum-dot-sensitized solar cell, Nanoscale 3 (2011) 4088e4093; e) H.S. Kim, C.R. Lee, J.H. Im, K.B. Lee, T. Moehl, A. Marchioro, S.J. Moon, R. Humphry-Baker, J.H. Yum, J.E. Moser, M. Gratzel, N.G. Park, Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%, Sci. Rep. 2 (2012) 591; f) J. Burschka, N. Pellet, S.J. Moon, R. Humphry-Baker, P. Gao, €tzel, Sequential deposition as a route to highM.K. Nazeeruddin, M. Gra performance perovskite-sensitized solar cells, Nature 499 (2013) 316e319; €tzel, S. Ahmad, Perovskite as light g) S. Kazim, M.K. Nazeeruddin, M. Gra harvester: a game changer in photovoltaics, Angew. Chem. Int. Ed. 53 (2014) 2812e2824; h) N. Pellet, P. Gao, G. Gregori, T.Y. Yang, M.K. Nazeeruddin, J. Maier, M. Gr€ atzel, Mixed-organic-cation perovskite photovoltaics for enhanced solarlight harvesting, Angew. Chem. Int. Ed. 53 (2014) 3151e3157;
168
[2]
[3]
[4]
[5]
Y. Xue et al. / Journal of Power Sources 344 (2017) 160e169 i) M.D. Xiao, F.Z. Huang, W.C. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. GrayWeale, U. Bach, Y.B. Cheng, L. Spiccia, A fast deposition-crystallization procedure for highly efficient lead iodide perovskite thin-film solar cells, Angew. Chem. Int. Ed. 126 (2014) 10056e10061; j) A.Y. Mei, X. Li, L.F. Liu, Z.L. Ku, T.F. Liu, Y.G. Rong, M. Xu, M. Hu, J.Z. Chen, Y. Yang, M. Gr€ atzel, H.W. Han, A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability, Science 345 (2014) 295e298. a) M. Gr€ atzel, The light and shade of perovskite solar cells, Nat. Mater 13 (2014) 838e842; b) D.Q. Bi, W.G. Tress, M.I. Dar, P. Gao, J.S. Luo, C. Renevier, K. Schenk, A. Abate, F. Giordano, J.P.C. Baena, J.D. Decoppet, S.M. Zakeeruddin, M.K. Nazeeruddin, €tzel, A. Hagfeldt, Efficient luminescent solar cells based on tailored M. Gra mixed-cation perovskites, Sci. Adv. 2 (2016) e1501170; c) X. Li, M.I. Dar, C.Y. Yi, J.S. Luo, M. Tschumi, S.M. Zakeeruddin, M.K. Nazeeruddin, H.W. Han, M. Gr€ atzel, Improved performance and stability of perovskite solar cells by crystal crosslinking with alkylphosphonic acid uammonium chlorides, Nat. Chem. 7 (2015) 703e711; d) N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Compositional engineering of perovskite materials for high-performance solar cells, Nature 517 (2015) 476e480; e) L. Meng, J.B. You, T.F. Guo, Y. Yang, Recent advances in the inverted planar structure of perovskite solar cells, Acc. Chem. Res. 49 (2015) 155e165; f) S. Ameen, M.A. Rub, S.A. Kosa, K.A. Alamry, M.S. Akhtar, H.S. Shin, H.K. Seo, A.M. Asiri, M.K. Nazeeruddin, Perovskite solar cells: influence of hole transporting materials on power conversion efficiency, ChemSusChem 9 (2016) 10e27; g) M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photonics 8 (2014) 506e514; h) C.C. Stoumpos, C.D. Malliakas, M.G. Kanatzidis, Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties, Inorg. Chem. 52 (2013) 9019e9038; i) H. Kim, H. Jeong, J.K. Lee, Highly efficient, reproducible, uniform (CH3NH3) PbI3 layer by processing additive dripping for solution-processed planar heterojunction perovskite solar cells, Chem. Asian J. 11 (2016) 2399e2405. a) H.P. Zhou, Q. Chen, G. Li, S. Luo, T.B. Song, H.S. Duan, Z.R. Hong, J.B. You, Y.S. Liu, Y. Yang, Interface engineering of highly efficient perovskite solar cells, Science 345 (2014) 542e546; b) W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (2015) 1234e1237; c) H.C. Liao, T.L.D. Tam, P.J. Guo, Y.L. Wu, E.F. Manley, W. Huang, N.J. Zhou, C.M.M. Soe, B.H. Wang, M.R. Wasielewski, L.X. Chen, M.G. Kanatzidis, A. Facchetti, R.P.H. Chang, T.J. Marks, Dopant-free hole transporting polymers for high efficiency, environmentally stable perovskite solar cells, Adv. Energy Mater 6 (2016) 1600502; coppet, J.S. Luo, S.M. Zakeeruddin, A. Hagfeldt, d) X. Li, D.Q. Bi, C.Y. Yi, J.D. De €tzel, A vacuum flash-assisted solution process for high-efficiency largeM. Gra area perovskite solar cells, Science 353 (2016) 58e62; e) B. Xu, D. Bi, Y. Hua, P. Liu, M. Cheng, M. Gr€ atzel, L. Kloo, A. Hagfeldt, L. Sun, A low-cost spiro[fluorene-9,9’xanthene]-based hole transport material for highly efficient solid-state dye-sensitized solar cells and perovskite solar cells, Energ. Environ. Sci. 9 (2016) 873e877; f) NREL chart, www.nrel.gov/ncpv/images/efficiency_chart.jpg. €tzel, Temperature depena) A. Dualeh, T. Moehl, M.K. Nazeeruddin, M. Gra dence of transport properties of spiro-meotad as a hole transport material in solid-state dye-sensitized solar cells, ACS Nano 7 (2013) 2292e2301; b) Y.K. Wang, Z.C. Yuan, G.Z. Shi, Y.X. Li, Q. Li, F. Hui, B.Q. Sun, Z.Q. Jiang, L.S. Liao, Dopant-free spiro-triphenylamine/fluorene as hole-transporting material for perovskite solar cells with enhanced efficiency and stability, Adv. Funct. Mater 26 (2016) 1375e1381; c) P. Ganesan, K. Fu, P. Gao, I. Raabe, K. Schenk, R. Scopelliti, J. Luo, L.H. Wong, M.K. Nazeeruddin, M. Gr€ atzel, A simple spiro-type hole transporting material for efficient perovskite solar cells, Energ. Environ. Sci. 8 (2015) 1986e1991; d) S. Ma, H. Zhang, N. Zhao, Y. Cheng, M. Wang, Y. Shen, G. Tu, Spiro-thiophene derivatives as hole-transport materials for perovskite solar cells, J. Mater. Chem. A 3 (2015) 12139e12144; e) N.J. Jeon, H.G. Lee, Y.C. Kim, J. Seo, J.H. Noh, J. Lee, S.I. Seok, o-methoxy substituents in spiro-ometad for efficient inorganic-organic hybrid perovskite solar cells, J. Am. Chem. Soc. 136 (2014) 7837e7840; f) P. Gratia, A. Magomedov, T. Malinauskas, M. Daskeviciene, A. Abate, €tzel, S. Ahmad, M. Gra M. Getautis, M.K. Nazeeruddin, A methoxydiphenylamine-substituted carbazole twin derivative: an efficient hole-transporting material for perovskite solar cells, Angew. Chem. Int. Ed. 54 (2015) 11409e11413; g) J.H. Heo, S.H. Im, J.H. Noh, T.N. Mandal, C.S. Lim, J.A. Chang, Y.H. Lee, H.J. Kim, A. Sarkar, M.K. Nazeeruddin, M. Gr€ atzel, S.I. Seok, Efficient inorganicorganic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors, Nat. Photonics 7 (2013) 486e491; h) S.S. Reddy, K. Gunasekar, J.H. Heo, S.H. Im, C.S. Kim, D.H. Kim, J.H. Moon, Y.L. Jin, M. Song, S.H. Jin, Highly efficient organic hole transporting materials for perovskite and organic solar cells with long-term stability, Adv. Mater 28 (2016) 686e693. a) Y.K. Song, S.T. Lv, X.C. Liu, X.G. Li, S.R. Wang, H.Y. Wei, D.M. Li, Y. Xiao, Q.B. Meng, Energy level tuning of TPB-based hole-transporting materials for
[6]
[7]
[8]
[9]
[10]
highly efficient perovskite solar cells, Chem. Commun. 50 (2014) 15239e15242; b) F. Zhang, C.Y. Yi, P. Wei, X.D. Bi, J.S. Luo, G. Jacopin, S.R. Wang, X.G. Li, S.M. Zakeeruddin, M. Gr€ atzel, A novel dopant-free triphenylamine based molecular “butterfly” hole-transport material for highly efficient and stable perovskite solar cells, Adv. Energy Mater 6 (2016) 1600401; c) Y.S. Liu, Z.R. Hong, Q. Chen, H.J. Chen, W.H. Chang, Y. Yang (Michael), T.B. Song, Y. Yang, Perovskite solar cells employing dopant-free organic hole transport materials with tunable energy levels, Adv. Mater 28 (2016) 440e446; d) G.W. Kim, G. Kang, J. Kim, G.Y. Lee, H.I. Kim, L. Pyeon, J. Lee, T. Park, Dopantfree polymeric hole transport materials for highly efficient and stable perovskite solar cells, Energy Environ. Sci. 9 (2016) 2326e2333; e) C.Y. Huang, W.F. Fu, C.Z. Li, Z.Q. Zhang, W.M. Qiu, M.M. Shi, P. Heremans, A.K.Y. Jen, H.Z. Chen, Dopant-free hole-transporting material with a C3h symmetrical truxene core for highly efficient perovskite solar cells, J. Am. Chem. Soc. 138 (2016) 2528e2531; f) C. Chen, M. Cheng, P. Liu, J.J. Gao, L. Kloo, L.C. Sun, Application of benzodithiophene based A-D-A structured materials in efficient perovskite solar cells and organic solar cells, Nano Energy 23 (2016) 40e49; g) M. Franckevi cius, A. Mishra, F. Kreuzer, J.S. Luo, S.M. Zakeeruddin, M. Gr€ atzel, A dopant-free spirobi[cyclopenta[2,1-B:3,4-B’]dithiophene] based hole-transport material for efficient perovskite solar cells, Mater. Horiz. 2 (2015) 613e618; h) Y.S. Liu, Q. Chen, H.S. Duan, H.P. Zhou, Y. Yang (Michael), H.J. Chen, S. Luo, T.B. Song, L.T. Dou, Z.R. Hong, Y. Yang, A dopant-free organic hole transport material for efficient planar heterojunction perovskite solar cells, J. Mater. Chem. A 3 (2015) 11940e11947; i) X.M. Zhao, F. Zhang, C.Y. Yi, D.Q. Bi, X.D. Bi, P. Wei, J.S. Luo, X.C. Liu, €tzel, A novel one-step synthesized S.R. Wang, X.G. Li, S.M. Zakeeruddin, M. Gra and dopant-free hole transport material for efficient and stable perovskite solar cells, J. Mater. Chem. A 4 (2016) 16330e16334; j) F. Zhang, X.C. Liu, C.Y. Yi, D.Q. Bi, J.S. Luo, S.R. Wang, X.G. Li, Y. Xiao, €tzel, Dopant-free donor (D)-p-D-p-D conjugated S.M. Zakeeruddin, M. Gra hole-transport materials for efficient and stable perovskite solar cells, ChemSusChem 9 (2016) 2578e2585; k) J.H. Yun, S. Park, J.H. Heo, H.S. Lee, S. Yoon, J. Kang, S.H. lm, H. Kim, W. Lee, B. Kim, M.J. Ko, D.S. Chung, H.J. Son, Enhancement of charge transport properties of small molecule semiconductors by controlling fluorine substitution and effects on photovoltaic properties of organic solar cells and perovskite solar cells, Chem. Sci. 7 (2016) 6649e6661. a) D.H. Lee, Y.P. Liu, K.H. Lee, H. Chae, S.M. Cho, Effect of hole transporting materials in phosphorescent white polymer light-emitting diodes, Org. Electron 11 (2010) 427e433; b) K. Walzer, B. Maennig, M. Pfeiffer, K. Leo, Highly efficient organic devices based on electrically doped transport layers, Chem. Rev. 107 (2007) 1233e1271; c) Y. Kuwabara, H. Ogawa, H. Inada, N. Noma, Y. Shirota, Thermally stable multilared organic electroluminescent devices using novel starburst molecules, 4,4’,4”-tri(N-carbazolyl) triphenylamine (TCTA) and 4,4’,4”-Tris(3methylphenylphenylamino)triphenylamine(m-MTDATA), as hole-transport materials, Adv. Mater 6 (1994) 677e679; d) Q.F. Xue, G.T. Chen, M.Y. Liu, J.Y. Xiao, Z.M. Chen, Z.C. Hu, X.F. Jiang, B. Zhang, F. Huang, W. Yang, H.L. Yip, Y. Cao, Improving film formation and photovoltage of highly efficient inverted-type perovskite solar cells through the incorporation of new polymeric hole selective layers, Adv. Energy Mater 6 (2016) 1502021. a) Y. Li, Y.Y. Xue, L.P. Xia, L.T. Hou, X.Q. Qiu, 1,3,5-triazine crosslinked 2,5dibromohydroquinone as new hole-transport material in polymer lightemitting diodes, Phys. Status. Solidi. A 213 (2015) 429e435; b) L.P. Xia, Y.Y. Xue, K. Xiong, C.S. Cai, Z.S. Peng, Y. Wu, Y. Li, J.S. Miao, D.C. Chen, Z.H. Hu, J.B. Wang, X.B. Peng, Y.Q. Mo, L.T. Hou, Highly improved efficiency of deep-blue fluorescent polymer light-emitting device based on a novel hole interface modifier with 1,3,5-triazine core, ACS Appl. Mater. Inter 7 (2015) 26405e26413; c) Y. Li, N.L. Hong, An efficient hole transport material based on PEDOT dispersed with lignosulfonate: preparation, characterization and performance in polymer solar cells, J. Mater. Chem. A 3 (2015) 21537e21544; d) Y. Wu, J.Y. Wang, X.Q. Qiu, R.Q. Yang, H.M. Lou, X.C. Bao, Y. Li, Highly efficient inverted perovskite solar cells with sulfonated lignin doped PEDOT as hole extract layer, ACS Appl. Mater. Inter 8 (2016) 12377e12383; e) Y. Xue, P. Guo, H.L. Yip, Y. Li, Y. Cao, General design of self-doped small molecules as efficient hole extraction materials for polymer solar cells, J. Mater. Chem. A (2017) 12377e12383, http://dx.doi.org/10.1039/ C6TA09925D. Y. Li, A.Y. Li, B.X. Li, J. Huang, L. Zhao, B.Z. Wang, J.W. Li, X.H. Zhu, J. Peng, Y. Cao, D.G. Ma, J. Roncali, Asymmetrically 4,7-disubstituted benzothiadiazoles as efficient non-doped solution-processable green fluorescent emitters, Org. Lett. 11 (2009) 5318e5321. Y. Shirota, Photo-and electroactive amorphous molecular materials-molecular design, syntheses, reactions, properties, and applications, J. Mater. Chem. 15 (2005) 75e93. K. Rakstys, A. Abate, M.I. Dar, P. Gao, V. Jankauskas, G. Jacopin, E. Kamarauskas, €tzel, M.K. Nazeeruddin, Triazatruxene-based hole S. Kazim, S. Ahmad, M. Gra transporting materials for highly efficient perovskite solar cells, J. Am. Chem.
Y. Xue et al. / Journal of Power Sources 344 (2017) 160e169 Soc. 137 (2015) 16172e16178. [11] a) D.B. Mitzi, Templating and structural engineering in organic-inorganic perovskites, Dalton Trans. 1 (2001) 1e12; b) C.S. Ponseca Jr., T.J. Savenije, M. Abdellah, K.B. Zheng, A. Yartsev, T. Pascher, € m, T. Harlang, P. Chabera, T. Pullerits, A. Stepanov, J.P. Wolf, V. Sundstro Organometal halide perovskite solar cell materials rationalized: ultrafast charge generation, high and microsecond-long balanced mobilities, and slow recombination, J. Am. Chem. Soc. 136 (2014) 5189e5192; c) S.Y. Chen, B. Sun, W. Hong, Z.Q. Yan, H. Aziz, Y.Z. Meng, J. Hollinger, D.S. Seferos, Y.N. Li, Impact of N-substitution of a carbazole unit on molecular packing and charge transport of DPP-carbazole copolymers, J. Mater. Chem. C
169
2 (2014) 1683e1690. [12] a) G.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith, Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells, Energ. Environ. Sci. 7 (2014) 982e988; b) G.C. Xing, N. Mathews, S.Y. Sun, S.S. Lim, Y.M. Lam, M. Gr€ atzel, S. Mhaisalkar, T.C. Sum, Long-range balanced electron-and hole-transport lengths in organic-inorganic CH3NH3PbI3, Science 342 (2013) 344e347. [13] L.T. Dou, J.B. You, Z.R. Hong, Z. Xu, G. Li, R.A. Street, Y. Yang, 25th Anniversary article: a decade of organic/polymeric photovoltaic research, Adv. Mater 25 (2013) 6642e6671.