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Copper iodide as inorganic hole conductor for perovskite solar cells with different thickness of mesoporous layer and hole transport layer
Accepted Manuscript Title: Copper iodide as inorganic hole conductor for perovskite solar cells with different thickness of mesoporous layer and hole transport layer Author: Minzan Huangfu Yue Shen Gongbo Zhu Kai Xu Meng Cao Feng Gu Linjun Wang PII: DOI: Reference:
S0169-4332(15)02346-6 http://dx.doi.org/doi:10.1016/j.apsusc.2015.09.215 APSUSC 31429
To appear in:
APSUSC
Received date: Revised date: Accepted date:
22-7-2015 25-9-2015 25-9-2015
Please cite this article as: M. Huangfu, Y. Shen, G. Zhu, K. Xu, M. Cao, F. Gu, L. Wang, Copper iodide as inorganic hole conductor for perovskite solar cells with different thickness of mesoporous layer and hole transport layer, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.09.215 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
1. Copper iodide (CuI) films were prepared by means of convenient airbrush process. 2. Perovskite solar cells with CuI layers reached a champion power conversion efficiency of 5.8%.
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3. The Voc remains low due to the high recombination in CuI devices.
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Graphical Abstract (for review)
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*Manuscript
Copper iodide as inorganic hole conductor for perovskite solar cells with different thickness of mesoporous layer and hole transport layer Minzan Huangfu, Yue Shen*, Gongbo Zhu, Kai Xu, Meng Cao*, Feng Gu, Linjun Wang
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School of Materials Science and Engineering, Shanghai University, China E-mail:[email protected]；[email protected]
ABSTRACT: This study is the first to report the preparation of Copper iodide (CuI)
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thick films by means of convenient airbrush process and their application as inorganic
hole transport layers (HTL) in organo-lead halide perovskite-based solar cells. CuI
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thick films exhibit high conductivity, wide-band-gap and solution-processable. Organo-lead halide perovskite solar cells with different thickness of mesoporous
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layers and CuI hole transport layers were fabricated. Performance of the cells were mainly controlled by the thickness of TiO2 mesoporous layers. Under optimized
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conditions, a power conversion efficiency of 5.8% has been achieved with short-circuit current density Jsc of 22.3 mA/cm2, open-circuit voltage Voc of 614 mV
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and fill factor of 42%. However, the Voc remains low in comparison with the state of the art perovskite-based solar cells, which is attributed to the high recombination in
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CuI devices as determined by impedance spectroscopy. Key words:Perovskite solar cell; Copper Iodide; Mesoporous Layer; Thickness
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1. Introduction
In recent years, organic-inorganic hybrid perovskite materials [1,2] have drawn tremendous attention applied into photovoltaic devices because of their low cost, ease to prepare, and high efficiency. Organic-inorganic hybrid perovskites possess intense light absorption, excellent ambipolar charge mobility and small exciton binding energy [3-5]. In late 2012, M.Gratzel et al. teamed up with N. G. Park used the solid hole conductor 2,2',7,7,'-tetrakis-(N,N-dimethoxyphenyl-amine)-9,9'-spirobiuorene (spiro-MeOTAD) on MAPbI3 layer, which accessed a PCE of 9.7% [6]. After that, perovskite solar cells have experienced a fast development adopting spiro-MeOTAD,
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skyrocketing from 14.1% [7,8] to an efficiency of 20.1% [9]. However, spiro-MeOTAD with low hole mobility, low conductivity [10] and ten times expensive than gold sometimes should impede the extensive use of the perovskite solar cells. On account of this, CuSCN and CuI become the most viable
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replacements for spiro-MeOTAD for its high conductivity, wide-band-gap and solution-processable, which have already applied in dye-sensitized solar cells and
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quantum dot-sensitized solar cells [11,12].Kamat et al. first employed CuI as the hole transport material (HTM) which was deposited by means of drop-casting obtaining a
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PCE of 6.0% [13]. Seigo Ito et al. made use of CuSCN as the HTM in MAPbI3 perovskite solar cells obtaining the PCE of 4.86% [14], then Peng Qin et al. prepared
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CuSCN layer by means of doctor blading at 65℃ receiving a PCE of 12.4% [15]. In this report, we employed CuI as the HTM deposited by spraying methods using
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an airbrush through the nitrogen. To some extent, the spray methods provide the possibility to fabricate large area organo-lead halide perovskite solar cells. The
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viability of using CuI as a HTL has been demonstrated in planar heterojunction-based FTO/TiO2/CH3NH3PbI3- xClx/CuI/Au solar cells with a champion PCE of 5.8%.
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Furthermore, the influences of the thickness of TiO2 mesoporous layer and CuI HTL on the performance of the organo-lead halide perovskite solar cells have been investigated. Current-voltage characteristics and impedance spectroscopy (IS) of the devices were measured under 100mW/cm2 AM 1.5G simulated sun irradiation. The
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high recombination is pointed out as the main limiting factor for the modest photovoltage.
2. Experimental section 2.1 Materials required. Fluorine-doped tin oxide coated glass (FTO, 13.5 ± 1.5 Ω sq-1, NSG), lead nitrate (Sinopharm Chemicl Reagent Co. , Ltd) , potassium iodide (SCRC) , methylamine hydrochloride (J&K Scientific Ltd) , chlorobenzene (J&K Scientific Ltd) , N,N-dimethylformamide (J&K Scientific Ltd) , propyl sulfide (J&K
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Scientific Ltd), copper iodide (SCRC) , TiO2 P25 (Degussa), ethyl cellulose (SCRC) , Alpha-Terpineol (SCRC) , ethanol (SCRC) , Pluronic P123 triblock copolymer (Aldrich) , acetylacetone (SCRC) , tetrabutyl titanate (SCRC) . 2.2 TiO2 Photo-anodes Preparation. Patterned FTO glass was heated at 500 oC
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for 2 hours to remove any organics and then was cleaned with acetone, ethanol and DI water, respectively. TiO2 blocking layers were prepared by spin coating method, with
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a mildly acidic TBT solution (5.1 mL) diluted in ethanol (40 mL). As-prepared TiO2
layers were annealed at 450 oC for 4 hours. TiO2 mesoporous layers were prepared by
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spin coating a TiO2 paste, which was prepared by molar weight ratios of 1.3 wt.% TiO2 P25, 0.7 wt.% ethyl cellulose, 46.4 wt.% ethanol and 51.6 wt.% Alpha-Terpineol
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(paste A) [16], and paste B contained TiO2 P25 two times as much as paste A by weight. The TiO2 mesoporous layers were then heated at 500 oC for 45 min. All the
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films were treated with a TiCl4 solution (40 mM in water) and sintered at 500 oC for 45 min [17].
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Varied types of TiO2 mesoporous layers were spin-coated as following, one layer of paste A (ml-250); one layer of paste B (ml-410); one layer of paste B and one layer
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paste A (ml-640); two layers of paste B (ml-900), as shown in Fig. 3. The TiO2 photo-anodes were distinguished by the TiO2 mesoporous layer mentioned above, and we also fabricated devices without mesoporous layer, only blocking layer, noted as
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ml-0.
2.3 Spin Coating of CH3NH3PbI3−xClx. The CH3NH3PbI3−xClx precursor solution was prepared by dissolving 1:1.5 molar amounts of lead iodide and methylamine hydrochloride in DMF (50 wt.%) at 70 oC. Lead iodide was prepared in advance by lead nitrate and potassium iodide. The solution was then dripped on mp-TiO2 layer and spun at 2000 rpm for 25 s. After the spin coating, the perovskite layer was heated on a hot plate to 90 oC for 30 min. 2.4 Spray Coating of Copper Iodide. 1.25 g copper iodide was dissolved in 1 ml
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propyl sulfide, and diluted to 6 ml with chlorobenzene. Then the solution was sprayed onto the perovskite layer by an airbrush through the nitrogen. Five different thickness of the CuI films were controlled by the amount of solution sprayed onto the perovskite layer, as shown in Fig. 5.
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2.5 Characterization. The morphology and structural properties of the CH3NH3PbI3−xClx film were analyzed by an FEI Sirion 200 scanning electron
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microscope (SEM) and a X-ray diffractometer (RigakuD/MAX-2200) with Cu Kα
radiation. UV-Vis absorption spectra and diffuse reflectance spectra of CH3NH3PbI3− were measured by Jasco UV-570 spectrophotometer. Incident photon to
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xClx samples
carrier efficiencies (IPCE) were measured using a Newport Oriel IQE 200
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measurement kit. Current density-voltage (J-V) characteristics and electrochemical impedance spectroscopy (EIS) were obtained on CHI660B electrochemical
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workstation (CHI660B, Shanghai Huachen Co.) under AM 1.5G illumination with an incident light intensity of 100 mW/cm2 (SDSB-09022, Newport Corporation). The
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active area of the devices were 0.25cm2.
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3. Results and Discussion
The prepared PbI2 powder and CH3NH3PbI3-xClx films were characterized by X-ray diffraction (XRD) and shown in Fig. 1A. It can be found that the patterns of PbI2 (in curve a) are in good accordance with the hexagonal structure (PDF card 07-0235) and
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peaks at 12.7° , 25.9° , and 39.5° can be attributed to the crystal planes (001) , (011) and (110) , respectively. Crystalline reflections of the CH3NH3PbI3-xClx (in curve b) indicate the formation of a tetragonal structure. The relatively strong intensity of the peaks located at 14.0° , 28.4° , 31.8° and 43.2° were observed corresponding to the planes of (110) , (220) , (310) and (330) [18,19]. CH3NH3PbI3-xClx films were prepared in the glove box, which were then annealed at 90 oC. As can be seen in Fig. 1B, TiO2 layer is covered by the cubic-like perovskite with dimension of about 1 µm, some of which exceeding 2 µm. The high temperature
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promotes perovskite grains growth. However there exsists some holes in the film
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which would give a bad impact on the performance of the devices.
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Figure 1. (A) X-ray diffraction patterns of CH3NH3PbI3-xClx annealed at 90 oC (B)Top-view SEM image of spin-coated CH3NH3PbI3-xClx
UV-Vis absorption spectra show a sharp lift at 758 nm and the absorption becomes
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stronger as the thickness of the mesoporous layers increases, revealing that more perovskite crystals are formed (see Fig. 2A). The band gap can be calculated from the
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extrapolation of the linear part of Kubelka-Munk spectrum, revealing a band gap Eg = 1.57 eV, which is pictured according to the expression [20], [(1-R)2hv/2R] P=A(hv - Eg) ,
(1)
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Where R is the percentage of light reflected, hv is the photon energy, A is a constant depending on transition probability and Eg is the optical band gap. P equals 2 for perovskite direct allowed transition according to the literature [21]. PL spectra of CH3NH3PbI3-xClx nanocrystals deposited on different thickness of mesoporous layers exhibit a single strong emission at 768nm (see Fig. 2B), which is attributed to the radiative recombinations of photogenerated carriers of organo-lead perovskite nanocrystals [22]. In general, samples with thicker TiO2 mesoporous layer present stronger PL emission, indicating which form a larger quantity of perovskite crystals.
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Figure 2. (A) UV-Vis absorption spectra of CH3NH3PbI3-xClx perovskite on different thickness of TiO2 mesoporous layers. Inset:the Kubelka-Munk spectrum for MAPbI3-xClx revealing a band gap Eg=1.59 eV and the absorbance spectrum of ml-0 layer infiltrated with CH3NH3PbI3-xClx perovskite. (B) Photoluminescence (PL) spectra with photoexcitation at 325 nm for different thickness of nanocrystalline TiO2/CH3NH3PbI3-xClx layers.
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In this work, five types of TiO2 photo-anodes with increasing thickness of mesoporous layers were prepared, corresponding to the thickness of approximate 0 nm, 250 nm, 410 nm, 640 nm, 900 nm separately, as showed in Fig. 3. A cross section
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of the TiO2 blocking layer with thickness of 200 nm displayed in Fig.3A exhibits a smooth and uniform film. The spin-coated mesoporous films are comprised with P25
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nanoparticles and exhibit a rough surface. Perovskite layers were spun coated onto the mesoporous layer and formed a film about 1 µm (Fig. 3F) . Then the CuI solution was sprayed onto the perovskite layer by an airbrush, forming a thickness of 17.5 ± 1.5
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µm CuI overlayer (Fig. 3G) . The cavitation in CuI layer, which can create paths that decrease the shunt resistance and increase the series resistance, is attributed to the low values of Voc and FF.
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ip t cr us an M ed ce pt Ac Figure 3. Cross sectional SEM images of (A) TiO2 blocking layer, (B) ml-250 layer, (C) ml-410 layer, (D) ml-640 layer and (E) ml-900 layer, (F) blocking layer and mesoporous layer (ml-640) coated with perovskite and (G) FTO glass/compact TiO2/mesoporous TiO2 infiltrated with CH3NH3PbI3-xClx/CuI. Inset: close-up view to show interfaces among each layer.
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J-V characteristics of devices with different thickness of mesoporous layers are displayed in Fig. 4A. The ml-640 device reached a champion power conversion efficiency of 4.9%. Five devices for different thickness of mesoporous layers were fabricated. The summary of device performance is exhibited in Table 1. Apparently the varied mesoporous layer thickness affects the performance of the devices. Five
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types devices all shows a low fill factor, which is due to the cavitation in CuI layer.
The average PCE exhibits: 1.4% for ml-0, 3.5% for ml-250, 4.2% for ml-410, 4.5%
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for ml-640 and 3.2% for ml-900. As the mesoporous layer thickness decreases from
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640 nm to 0 nm, the JSC and VOC of these devices drop from 21.0 to 9.4 mA/cm2 and from 646 to 516 mV, respectively. Due to that too thin mesoporous layer can not absorb enough thickness of perovskite, yielding a narrow depletion region which
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cannot separate electron-hole pairs efficiently [23]. With the thickness of mesoporous layer increasing from 640 nm to 900 nm, the average PCE drops to 3.2% (JSC = 18.6
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mA/cm2,VOC = 558 mV, FF =30%) , which is attributed to that a thicker mesoporous layer leads to a large quantity of recombination of the injected electron in TiO2
.
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conduction band with the hole in perovskite [24].
Figure 4. (A) PCE of five types of thickness of TiO2 mesoporous layers. (B) Effect of mesoporous layer thickness on PCE. Box represents the standard deviation, whiskers minimum and maximum values.
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1.4 ± 0.5
9.4 ± 1.7
516 ± 93
29 ± 5
ml-250
250
3.5 ± 0.4
19.3 ± 3.1
712 ± 43
26 ± 5
ml-410
410
4.2 ± 0.9
20.1 ± 1.5
633 ± 62
33 ± 2
ml-640
640
4.5 ± 0.6
21.0 ± 1.5
646 ± 46
33 ± 5
ml-900
900
3.2 ± 1.0
18.6 ± 3.0
558 ± 48
30 ± 5
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ml-0
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0
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Table 1. Photovoltaic Perfomance Parameters of Perovskite Solar Cells with different thickness of TiO2 mesoporous layers. _______________________________________________________________________________ Device ML Thickness (nm) PCE (%) JSC (mA/cm2) VOC (mV) FF (%)
To get a better understanding about impact of CuI thickness variation on devices
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performance, we fabricated four different thickness of CuI layer using ml-640 devices due to the highest PCE. Four different thickness of the CuI films were prepared by the
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controllable solution sprayed onto the perovskite layer, corresponding to the thickness of the whole devices without gold contact:17.5 ± 1.5 µm, 24.8 ± 2.6 µm, 28.8 ± 1.5 µm, 36.9 ± 2.8 µm, as shown in Fig. 5. We noted these devices as C1, C2, C3, and C4
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separately. From the SEM images, the morphology of the sufaces are rough and uneven, which will increase the series resistance, but the thickness of the CuI films is
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well be controlled (see Fig. 5E).
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Figure 5. Cross sectional SEM images of (A) C1 device, (B) C2 device, (C) C3 device, (D) C4 device and (E) the summary of four types of thickness of the whole devices without gold contact of 25 devices, scale bar: 20 µm.
The typical current density-voltage (J-V) curves of the devices are shown in Fig. 6A and the corresponding data are collected in Table 2. A champion PCE of 5.8% was achieved with the device thickness of 24.8 µm, the JSC, VOC and fill factor of which were determined as 22.3 mA/cm2, 614 mV and 42%. C2 devices gained the champion average PCE of 5.6 ± 0.2%, while C3 and C4 devices possessed higher Voc. The IPCE spectrum of the optimized performing C2 device showed in Fig. 6C exhibits a
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wide response in the range of 400-770 nm. The onset of 800 nm is in good accordance with the bandgap of the CH3NH3PbI3-xClx. The integrated photocurrent of the C2 device is 20.6 mA/cm2, which is consistent with the photocurrent density of 22.3 mA/cm2. As showed in Fig. 6D, the reflectance becomes stronger with the thickness
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ed
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of devices increases revealing that less light transmitted.
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Figure 6. (A) PCE of ml-640 devices with four types of thickness of CuI layer. (B) Effect of CuI layer's thickness on PCE. Box represents the standard deviation, whiskers minimum and maximum values. (C) IPCE spectrum of the optimized performing C2 device. (D) Diffuse reflectance spectra of devices with different thickness of CuI layers. Table 2. Photovoltaic Characters of Devices with different thickness of CuI layers. _______________________________________________________________________________ Device Device Thickness (µm) PCE (%) JSC (mA/cm2) VOC (mV) FF (%) _______________________________________________________________________________ C1 17.5 ± 1.5 4.5 ± 0.6 21.0 ± 1.5 646 ± 46 33 ± 5 C2
24.8 ± 2.6
5.6 ± 0.2
22.6 ± 0.8
640 ± 46
39 ± 4
C3
28.8 ± 1.5
5.0 ± 0.2
22.1 ± 1.4
698 ± 32
33 ± 6
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C4
36.9 ± 2.8
4.8 ± 0.2
22.7 ± 1.8
693 ± 46
31 ± 4
To further evaluate the origin of the Voc limitation, Impedance spectroscopy (IS) analysis was performed. Impedance spectra of C2 device (Fig. 7A) was recorded at applied voltage Vapp = 300 mV over the frequency range of 0.1 Hz to 100 kHz under
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100 mW/cm2 AM 1.5G illumination. And the equivalent circuit is present in the inset in Fig. 7A. The Nyquist plot presents two characteristic features [14]. In the equivalent circuit model, the high frequency arc in the Nyquist plot is assigned to the
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diffusion of holes through the hole transport material modeled by a HTM resistance,
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noted as RHTM. The lower frequency arc is attributed to a recombination resistance, Rrec. And at low frequencies a Gerischer pattern is identified that showed the diffusion
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length is shorter than the sample thickness. This fact illustrates that the Voc is limited by the low diffusion length [25,26].In Fig. 6B, Rrec is dominated by shunting at low
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potentials (200 mV) , revealing a flattening of the slope which also indicates that CuI
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has a large recombination. The low RHTM shows CuI has high conductivity [27,28].
Figure 7. (A) Nyquist plots of C2 device under 100 mW/cm2 AM 1.5G illumination. Inset in (A) is equivalent circuit model. (B) Summary of the recombination resistance, Rrec and the HTM resistance, RHTM.
4. Conclusion In summary, perovskite solar cells have been fabricated using CuI as the hole-transport materials with different thickness of TiO2 mesoporous layer. We found that the thickness of TiO2 mesoporous layer had great effects on photovoltaic
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performance of perovskite solar cells. The champion PCE value of 4.9% was achieved with the TiO2 mesoporous layer thickness of 640 nm. Through adjusting CuI thickness, a champion PCE of 5.8% was achieved with the device thickness of 24.8 µm. The characterization of the devices by means of impedance spectroscopy pointed out the short diffusion length as the main origin of the modest photovoltage (<700 mV). To
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fabricate large area organo-lead halide perovskite solar cells.
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some extent, the spray methods of depositing CuI layer provide the possibility to
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Acknowledgements
The work was supported by National Basic Research Program of China
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(2012CB934300), Shanghai city committee of Science and Technology (11530500200, 11ZR1412700), National Nature Science Foundation of China (61006089),
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Innovation program of Shanghai City (CXSJ-13-077) and Cooperation Fund of
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Shanghai Institute of Technical Physics.
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low-temperature-processed commercial carbon paste as cathode, ACS Appl.Mater.
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Interfaces 6 (2014) 16140-16146.
[25] V. Gonzalez-Pedro, E.J. Juarez-Perez, W.S. Arsyad, E.M. Barea, F.
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Fabregat-Santiago, I. Mora-Sero, J. Bisquert, General working principles of CH3NH3PbX3 perovskite solar cells, Nano Lett. 14 (2014) 888-893. [26] J. Bisquert, I. Mora-Sero, F. Fabregat-Santiago, Diffusion-recombination
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impedance model for solar cells with disorder and nonlinear recombination, ChemElectroChem. 1 (2014) 289-296. [27] M. Rusop, T. Soga, T. Jimbo, M. Umeno, Copper Iodide Thin films as a p-type electrical conductivity in dye-sensitized p-CuI/Dye/n-TiO2 heterojunction solid state solar cells, Surf. Rev. Lett. 11 (2004) 577−583. [28] 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) 643−647.
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ip t
Figure
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an
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Figure 1. (A) X-ray diffraction patterns of CH3NH3PbI3-xClx annealed at 90 oC (B)Top-view SEM image of spin-coated CH3NH3PbI3-xClx
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Figure 2. (A) UV-Vis absorption spectra of CH3NH3PbI3-xClx perovskite on different thickness of TiO2 mesoporous layers. Inset:the Kubelka-Munk spectrum for MAPbI3-xClx revealing a band gap Eg=1.59 eV and the absorbance spectrum of ml-0 layer infiltrated with CH3NH3PbI3-xClx perovskite. (B) Photoluminescence (PL) spectra with photoexcitation at 325 nm for different thickness of nanocrystalline TiO2/CH3NH3PbI3-xClx layers.
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ip t cr us an M ed ce pt Ac Figure 3. Cross sectional SEM images of (A) TiO2 blocking layer, (B) ml-250 layer, (C) ml-410 layer, (D) ml-640 layer and (E) ml-900 layer, (F) blocking layer and mesoporous layer (ml-640) coated with perovskite and (G) FTO glass/compact TiO2/mesoporous TiO2 infiltrated with CH3NH3PbI3-xClx/CuI. Inset: close-up view to show interfaces among each layer.
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.
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Figure 4. (A) PCE of five types of thickness of TiO2 mesoporous layers. (B) Effect of mesoporous layer thickness on PCE. Box represents the standard deviation, whiskers minimum and maximum values.
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Figure 5. Cross sectional SEM images of (A) C1 device, (B) C2 device, (C) C3 device, (D) C4 device and (E) the summary of four types of thickness of the whole devices without gold contact of 25 devices, scale bar: 20 µm.
Page 22 of 24
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Figure 6. (A) PCE of ml-640 devices with four types of thickness of CuI layer. (B) Effect of CuI layer's thickness on PCE. Box represents the standard deviation, whiskers minimum and maximum values. (C) IPCE spectrum of the optimized performing C2 device. (D) Diffuse reflectance spectra of devices with different thickness of CuI layers.
Figure 7. (A) Nyquist plots of C2 device under 100 mW/cm2 AM 1.5G illumination. Inset in (A) is equivalent circuit model. (B) Summary of the recombination resistance, Rrec and the HTM resistance, RHTM.
Page 23 of 24
Table
Table 1. Photovoltaic Perfomance Parameters of Perovskite Solar Cells with different thickness of TiO2 mesoporous layers. _______________________________________________________________________________ Device ML Thickness (nm) PCE (%) JSC (mA/cm2) VOC (mV) FF (%) 1.4 ± 0.5
9.4 ± 1.7
516 ± 93
29 ± 5
ml-250
250
3.5 ± 0.4
19.3 ± 3.1
712 ± 43
26 ± 5
ml-410
410
4.2 ± 0.9
20.1 ± 1.5
633 ± 62
33 ± 2
ml-640
640
4.5 ± 0.6
21.0 ± 1.5
646 ± 46
33 ± 5
ml-900
900
3.2 ± 1.0
18.6 ± 3.0
558 ± 48
30 ± 5
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ip t
0
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ml-0
24.8 ± 2.6
5.6 ± 0.2
640 ± 46
39 ± 4
C3
28.8 ± 1.5
5.0 ± 0.2
22.1 ± 1.4
698 ± 32
33 ± 6
C4
36.9 ± 2.8
22.7 ± 1.8
693 ± 46
31 ± 4
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22.6 ± 0.8
ed
C2
an
Table 2. Photovoltaic Characters of Devices with different thickness of CuI layers. _______________________________________________________________________________ Device Device Thickness (µm) PCE (%) JSC (mA/cm2) VOC (mV) FF (%) _______________________________________________________________________________ C1 17.5 ± 1.5 4.5 ± 0.6 21.0 ± 1.5 646 ± 46 33 ± 5
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4.8 ± 0.2
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S0169-4332(15)02346-6 http://dx.doi.org/doi:10.1016/j.apsusc.2015.09.215 APSUSC 31429
To appear in:
APSUSC
Received date: Revised date: Accepted date:
22-7-2015 25-9-2015 25-9-2015
Please cite this article as: M. Huangfu, Y. Shen, G. Zhu, K. Xu, M. Cao, F. Gu, L. Wang, Copper iodide as inorganic hole conductor for perovskite solar cells with different thickness of mesoporous layer and hole transport layer, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.09.215 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
1. Copper iodide (CuI) films were prepared by means of convenient airbrush process. 2. Perovskite solar cells with CuI layers reached a champion power conversion efficiency of 5.8%.
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3. The Voc remains low due to the high recombination in CuI devices.
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Graphical Abstract (for review)
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*Manuscript
Copper iodide as inorganic hole conductor for perovskite solar cells with different thickness of mesoporous layer and hole transport layer Minzan Huangfu, Yue Shen*, Gongbo Zhu, Kai Xu, Meng Cao*, Feng Gu, Linjun Wang
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School of Materials Science and Engineering, Shanghai University, China E-mail:[email protected]；[email protected]
ABSTRACT: This study is the first to report the preparation of Copper iodide (CuI)
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thick films by means of convenient airbrush process and their application as inorganic
hole transport layers (HTL) in organo-lead halide perovskite-based solar cells. CuI
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thick films exhibit high conductivity, wide-band-gap and solution-processable. Organo-lead halide perovskite solar cells with different thickness of mesoporous
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layers and CuI hole transport layers were fabricated. Performance of the cells were mainly controlled by the thickness of TiO2 mesoporous layers. Under optimized
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conditions, a power conversion efficiency of 5.8% has been achieved with short-circuit current density Jsc of 22.3 mA/cm2, open-circuit voltage Voc of 614 mV
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and fill factor of 42%. However, the Voc remains low in comparison with the state of the art perovskite-based solar cells, which is attributed to the high recombination in
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CuI devices as determined by impedance spectroscopy. Key words:Perovskite solar cell; Copper Iodide; Mesoporous Layer; Thickness
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1. Introduction
In recent years, organic-inorganic hybrid perovskite materials [1,2] have drawn tremendous attention applied into photovoltaic devices because of their low cost, ease to prepare, and high efficiency. Organic-inorganic hybrid perovskites possess intense light absorption, excellent ambipolar charge mobility and small exciton binding energy [3-5]. In late 2012, M.Gratzel et al. teamed up with N. G. Park used the solid hole conductor 2,2',7,7,'-tetrakis-(N,N-dimethoxyphenyl-amine)-9,9'-spirobiuorene (spiro-MeOTAD) on MAPbI3 layer, which accessed a PCE of 9.7% [6]. After that, perovskite solar cells have experienced a fast development adopting spiro-MeOTAD,
Page 3 of 24
skyrocketing from 14.1% [7,8] to an efficiency of 20.1% [9]. However, spiro-MeOTAD with low hole mobility, low conductivity [10] and ten times expensive than gold sometimes should impede the extensive use of the perovskite solar cells. On account of this, CuSCN and CuI become the most viable
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replacements for spiro-MeOTAD for its high conductivity, wide-band-gap and solution-processable, which have already applied in dye-sensitized solar cells and
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quantum dot-sensitized solar cells [11,12].Kamat et al. first employed CuI as the hole transport material (HTM) which was deposited by means of drop-casting obtaining a
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PCE of 6.0% [13]. Seigo Ito et al. made use of CuSCN as the HTM in MAPbI3 perovskite solar cells obtaining the PCE of 4.86% [14], then Peng Qin et al. prepared
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CuSCN layer by means of doctor blading at 65℃ receiving a PCE of 12.4% [15]. In this report, we employed CuI as the HTM deposited by spraying methods using
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an airbrush through the nitrogen. To some extent, the spray methods provide the possibility to fabricate large area organo-lead halide perovskite solar cells. The
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viability of using CuI as a HTL has been demonstrated in planar heterojunction-based FTO/TiO2/CH3NH3PbI3- xClx/CuI/Au solar cells with a champion PCE of 5.8%.
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Furthermore, the influences of the thickness of TiO2 mesoporous layer and CuI HTL on the performance of the organo-lead halide perovskite solar cells have been investigated. Current-voltage characteristics and impedance spectroscopy (IS) of the devices were measured under 100mW/cm2 AM 1.5G simulated sun irradiation. The
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high recombination is pointed out as the main limiting factor for the modest photovoltage.
2. Experimental section 2.1 Materials required. Fluorine-doped tin oxide coated glass (FTO, 13.5 ± 1.5 Ω sq-1, NSG), lead nitrate (Sinopharm Chemicl Reagent Co. , Ltd) , potassium iodide (SCRC) , methylamine hydrochloride (J&K Scientific Ltd) , chlorobenzene (J&K Scientific Ltd) , N,N-dimethylformamide (J&K Scientific Ltd) , propyl sulfide (J&K
Page 4 of 24
Scientific Ltd), copper iodide (SCRC) , TiO2 P25 (Degussa), ethyl cellulose (SCRC) , Alpha-Terpineol (SCRC) , ethanol (SCRC) , Pluronic P123 triblock copolymer (Aldrich) , acetylacetone (SCRC) , tetrabutyl titanate (SCRC) . 2.2 TiO2 Photo-anodes Preparation. Patterned FTO glass was heated at 500 oC
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for 2 hours to remove any organics and then was cleaned with acetone, ethanol and DI water, respectively. TiO2 blocking layers were prepared by spin coating method, with
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a mildly acidic TBT solution (5.1 mL) diluted in ethanol (40 mL). As-prepared TiO2
layers were annealed at 450 oC for 4 hours. TiO2 mesoporous layers were prepared by
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spin coating a TiO2 paste, which was prepared by molar weight ratios of 1.3 wt.% TiO2 P25, 0.7 wt.% ethyl cellulose, 46.4 wt.% ethanol and 51.6 wt.% Alpha-Terpineol
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(paste A) [16], and paste B contained TiO2 P25 two times as much as paste A by weight. The TiO2 mesoporous layers were then heated at 500 oC for 45 min. All the
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films were treated with a TiCl4 solution (40 mM in water) and sintered at 500 oC for 45 min [17].
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Varied types of TiO2 mesoporous layers were spin-coated as following, one layer of paste A (ml-250); one layer of paste B (ml-410); one layer of paste B and one layer
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paste A (ml-640); two layers of paste B (ml-900), as shown in Fig. 3. The TiO2 photo-anodes were distinguished by the TiO2 mesoporous layer mentioned above, and we also fabricated devices without mesoporous layer, only blocking layer, noted as
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ml-0.
2.3 Spin Coating of CH3NH3PbI3−xClx. The CH3NH3PbI3−xClx precursor solution was prepared by dissolving 1:1.5 molar amounts of lead iodide and methylamine hydrochloride in DMF (50 wt.%) at 70 oC. Lead iodide was prepared in advance by lead nitrate and potassium iodide. The solution was then dripped on mp-TiO2 layer and spun at 2000 rpm for 25 s. After the spin coating, the perovskite layer was heated on a hot plate to 90 oC for 30 min. 2.4 Spray Coating of Copper Iodide. 1.25 g copper iodide was dissolved in 1 ml
Page 5 of 24
propyl sulfide, and diluted to 6 ml with chlorobenzene. Then the solution was sprayed onto the perovskite layer by an airbrush through the nitrogen. Five different thickness of the CuI films were controlled by the amount of solution sprayed onto the perovskite layer, as shown in Fig. 5.
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2.5 Characterization. The morphology and structural properties of the CH3NH3PbI3−xClx film were analyzed by an FEI Sirion 200 scanning electron
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microscope (SEM) and a X-ray diffractometer (RigakuD/MAX-2200) with Cu Kα
radiation. UV-Vis absorption spectra and diffuse reflectance spectra of CH3NH3PbI3− were measured by Jasco UV-570 spectrophotometer. Incident photon to
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xClx samples
carrier efficiencies (IPCE) were measured using a Newport Oriel IQE 200
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measurement kit. Current density-voltage (J-V) characteristics and electrochemical impedance spectroscopy (EIS) were obtained on CHI660B electrochemical
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workstation (CHI660B, Shanghai Huachen Co.) under AM 1.5G illumination with an incident light intensity of 100 mW/cm2 (SDSB-09022, Newport Corporation). The
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active area of the devices were 0.25cm2.
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3. Results and Discussion
The prepared PbI2 powder and CH3NH3PbI3-xClx films were characterized by X-ray diffraction (XRD) and shown in Fig. 1A. It can be found that the patterns of PbI2 (in curve a) are in good accordance with the hexagonal structure (PDF card 07-0235) and
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peaks at 12.7° , 25.9° , and 39.5° can be attributed to the crystal planes (001) , (011) and (110) , respectively. Crystalline reflections of the CH3NH3PbI3-xClx (in curve b) indicate the formation of a tetragonal structure. The relatively strong intensity of the peaks located at 14.0° , 28.4° , 31.8° and 43.2° were observed corresponding to the planes of (110) , (220) , (310) and (330) [18,19]. CH3NH3PbI3-xClx films were prepared in the glove box, which were then annealed at 90 oC. As can be seen in Fig. 1B, TiO2 layer is covered by the cubic-like perovskite with dimension of about 1 µm, some of which exceeding 2 µm. The high temperature
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promotes perovskite grains growth. However there exsists some holes in the film
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which would give a bad impact on the performance of the devices.
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Figure 1. (A) X-ray diffraction patterns of CH3NH3PbI3-xClx annealed at 90 oC (B)Top-view SEM image of spin-coated CH3NH3PbI3-xClx
UV-Vis absorption spectra show a sharp lift at 758 nm and the absorption becomes
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stronger as the thickness of the mesoporous layers increases, revealing that more perovskite crystals are formed (see Fig. 2A). The band gap can be calculated from the
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extrapolation of the linear part of Kubelka-Munk spectrum, revealing a band gap Eg = 1.57 eV, which is pictured according to the expression [20], [(1-R)2hv/2R] P=A(hv - Eg) ,
(1)
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Where R is the percentage of light reflected, hv is the photon energy, A is a constant depending on transition probability and Eg is the optical band gap. P equals 2 for perovskite direct allowed transition according to the literature [21]. PL spectra of CH3NH3PbI3-xClx nanocrystals deposited on different thickness of mesoporous layers exhibit a single strong emission at 768nm (see Fig. 2B), which is attributed to the radiative recombinations of photogenerated carriers of organo-lead perovskite nanocrystals [22]. In general, samples with thicker TiO2 mesoporous layer present stronger PL emission, indicating which form a larger quantity of perovskite crystals.
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Figure 2. (A) UV-Vis absorption spectra of CH3NH3PbI3-xClx perovskite on different thickness of TiO2 mesoporous layers. Inset:the Kubelka-Munk spectrum for MAPbI3-xClx revealing a band gap Eg=1.59 eV and the absorbance spectrum of ml-0 layer infiltrated with CH3NH3PbI3-xClx perovskite. (B) Photoluminescence (PL) spectra with photoexcitation at 325 nm for different thickness of nanocrystalline TiO2/CH3NH3PbI3-xClx layers.
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In this work, five types of TiO2 photo-anodes with increasing thickness of mesoporous layers were prepared, corresponding to the thickness of approximate 0 nm, 250 nm, 410 nm, 640 nm, 900 nm separately, as showed in Fig. 3. A cross section
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of the TiO2 blocking layer with thickness of 200 nm displayed in Fig.3A exhibits a smooth and uniform film. The spin-coated mesoporous films are comprised with P25
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nanoparticles and exhibit a rough surface. Perovskite layers were spun coated onto the mesoporous layer and formed a film about 1 µm (Fig. 3F) . Then the CuI solution was sprayed onto the perovskite layer by an airbrush, forming a thickness of 17.5 ± 1.5
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µm CuI overlayer (Fig. 3G) . The cavitation in CuI layer, which can create paths that decrease the shunt resistance and increase the series resistance, is attributed to the low values of Voc and FF.
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ip t cr us an M ed ce pt Ac Figure 3. Cross sectional SEM images of (A) TiO2 blocking layer, (B) ml-250 layer, (C) ml-410 layer, (D) ml-640 layer and (E) ml-900 layer, (F) blocking layer and mesoporous layer (ml-640) coated with perovskite and (G) FTO glass/compact TiO2/mesoporous TiO2 infiltrated with CH3NH3PbI3-xClx/CuI. Inset: close-up view to show interfaces among each layer.
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J-V characteristics of devices with different thickness of mesoporous layers are displayed in Fig. 4A. The ml-640 device reached a champion power conversion efficiency of 4.9%. Five devices for different thickness of mesoporous layers were fabricated. The summary of device performance is exhibited in Table 1. Apparently the varied mesoporous layer thickness affects the performance of the devices. Five
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types devices all shows a low fill factor, which is due to the cavitation in CuI layer.
The average PCE exhibits: 1.4% for ml-0, 3.5% for ml-250, 4.2% for ml-410, 4.5%
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for ml-640 and 3.2% for ml-900. As the mesoporous layer thickness decreases from
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640 nm to 0 nm, the JSC and VOC of these devices drop from 21.0 to 9.4 mA/cm2 and from 646 to 516 mV, respectively. Due to that too thin mesoporous layer can not absorb enough thickness of perovskite, yielding a narrow depletion region which
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cannot separate electron-hole pairs efficiently [23]. With the thickness of mesoporous layer increasing from 640 nm to 900 nm, the average PCE drops to 3.2% (JSC = 18.6
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mA/cm2,VOC = 558 mV, FF =30%) , which is attributed to that a thicker mesoporous layer leads to a large quantity of recombination of the injected electron in TiO2
.
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conduction band with the hole in perovskite [24].
Figure 4. (A) PCE of five types of thickness of TiO2 mesoporous layers. (B) Effect of mesoporous layer thickness on PCE. Box represents the standard deviation, whiskers minimum and maximum values.
Page 10 of 24
1.4 ± 0.5
9.4 ± 1.7
516 ± 93
29 ± 5
ml-250
250
3.5 ± 0.4
19.3 ± 3.1
712 ± 43
26 ± 5
ml-410
410
4.2 ± 0.9
20.1 ± 1.5
633 ± 62
33 ± 2
ml-640
640
4.5 ± 0.6
21.0 ± 1.5
646 ± 46
33 ± 5
ml-900
900
3.2 ± 1.0
18.6 ± 3.0
558 ± 48
30 ± 5
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ml-0
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0
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Table 1. Photovoltaic Perfomance Parameters of Perovskite Solar Cells with different thickness of TiO2 mesoporous layers. _______________________________________________________________________________ Device ML Thickness (nm) PCE (%) JSC (mA/cm2) VOC (mV) FF (%)
To get a better understanding about impact of CuI thickness variation on devices
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performance, we fabricated four different thickness of CuI layer using ml-640 devices due to the highest PCE. Four different thickness of the CuI films were prepared by the
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controllable solution sprayed onto the perovskite layer, corresponding to the thickness of the whole devices without gold contact:17.5 ± 1.5 µm, 24.8 ± 2.6 µm, 28.8 ± 1.5 µm, 36.9 ± 2.8 µm, as shown in Fig. 5. We noted these devices as C1, C2, C3, and C4
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separately. From the SEM images, the morphology of the sufaces are rough and uneven, which will increase the series resistance, but the thickness of the CuI films is
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well be controlled (see Fig. 5E).
Page 11 of 24
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Figure 5. Cross sectional SEM images of (A) C1 device, (B) C2 device, (C) C3 device, (D) C4 device and (E) the summary of four types of thickness of the whole devices without gold contact of 25 devices, scale bar: 20 µm.
The typical current density-voltage (J-V) curves of the devices are shown in Fig. 6A and the corresponding data are collected in Table 2. A champion PCE of 5.8% was achieved with the device thickness of 24.8 µm, the JSC, VOC and fill factor of which were determined as 22.3 mA/cm2, 614 mV and 42%. C2 devices gained the champion average PCE of 5.6 ± 0.2%, while C3 and C4 devices possessed higher Voc. The IPCE spectrum of the optimized performing C2 device showed in Fig. 6C exhibits a
Page 12 of 24
wide response in the range of 400-770 nm. The onset of 800 nm is in good accordance with the bandgap of the CH3NH3PbI3-xClx. The integrated photocurrent of the C2 device is 20.6 mA/cm2, which is consistent with the photocurrent density of 22.3 mA/cm2. As showed in Fig. 6D, the reflectance becomes stronger with the thickness
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ed
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an
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of devices increases revealing that less light transmitted.
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Figure 6. (A) PCE of ml-640 devices with four types of thickness of CuI layer. (B) Effect of CuI layer's thickness on PCE. Box represents the standard deviation, whiskers minimum and maximum values. (C) IPCE spectrum of the optimized performing C2 device. (D) Diffuse reflectance spectra of devices with different thickness of CuI layers. Table 2. Photovoltaic Characters of Devices with different thickness of CuI layers. _______________________________________________________________________________ Device Device Thickness (µm) PCE (%) JSC (mA/cm2) VOC (mV) FF (%) _______________________________________________________________________________ C1 17.5 ± 1.5 4.5 ± 0.6 21.0 ± 1.5 646 ± 46 33 ± 5 C2
24.8 ± 2.6
5.6 ± 0.2
22.6 ± 0.8
640 ± 46
39 ± 4
C3
28.8 ± 1.5
5.0 ± 0.2
22.1 ± 1.4
698 ± 32
33 ± 6
Page 13 of 24
C4
36.9 ± 2.8
4.8 ± 0.2
22.7 ± 1.8
693 ± 46
31 ± 4
To further evaluate the origin of the Voc limitation, Impedance spectroscopy (IS) analysis was performed. Impedance spectra of C2 device (Fig. 7A) was recorded at applied voltage Vapp = 300 mV over the frequency range of 0.1 Hz to 100 kHz under
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100 mW/cm2 AM 1.5G illumination. And the equivalent circuit is present in the inset in Fig. 7A. The Nyquist plot presents two characteristic features [14]. In the equivalent circuit model, the high frequency arc in the Nyquist plot is assigned to the
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diffusion of holes through the hole transport material modeled by a HTM resistance,
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noted as RHTM. The lower frequency arc is attributed to a recombination resistance, Rrec. And at low frequencies a Gerischer pattern is identified that showed the diffusion
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length is shorter than the sample thickness. This fact illustrates that the Voc is limited by the low diffusion length [25,26].In Fig. 6B, Rrec is dominated by shunting at low
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potentials (200 mV) , revealing a flattening of the slope which also indicates that CuI
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ed
has a large recombination. The low RHTM shows CuI has high conductivity [27,28].
Figure 7. (A) Nyquist plots of C2 device under 100 mW/cm2 AM 1.5G illumination. Inset in (A) is equivalent circuit model. (B) Summary of the recombination resistance, Rrec and the HTM resistance, RHTM.
4. Conclusion In summary, perovskite solar cells have been fabricated using CuI as the hole-transport materials with different thickness of TiO2 mesoporous layer. We found that the thickness of TiO2 mesoporous layer had great effects on photovoltaic
Page 14 of 24
performance of perovskite solar cells. The champion PCE value of 4.9% was achieved with the TiO2 mesoporous layer thickness of 640 nm. Through adjusting CuI thickness, a champion PCE of 5.8% was achieved with the device thickness of 24.8 µm. The characterization of the devices by means of impedance spectroscopy pointed out the short diffusion length as the main origin of the modest photovoltage (<700 mV). To
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fabricate large area organo-lead halide perovskite solar cells.
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some extent, the spray methods of depositing CuI layer provide the possibility to
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Acknowledgements
The work was supported by National Basic Research Program of China
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(2012CB934300), Shanghai city committee of Science and Technology (11530500200, 11ZR1412700), National Nature Science Foundation of China (61006089),
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Innovation program of Shanghai City (CXSJ-13-077) and Cooperation Fund of
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Shanghai Institute of Technical Physics.
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Figure
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Figure 1. (A) X-ray diffraction patterns of CH3NH3PbI3-xClx annealed at 90 oC (B)Top-view SEM image of spin-coated CH3NH3PbI3-xClx
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Figure 2. (A) UV-Vis absorption spectra of CH3NH3PbI3-xClx perovskite on different thickness of TiO2 mesoporous layers. Inset:the Kubelka-Munk spectrum for MAPbI3-xClx revealing a band gap Eg=1.59 eV and the absorbance spectrum of ml-0 layer infiltrated with CH3NH3PbI3-xClx perovskite. (B) Photoluminescence (PL) spectra with photoexcitation at 325 nm for different thickness of nanocrystalline TiO2/CH3NH3PbI3-xClx layers.
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ip t cr us an M ed ce pt Ac Figure 3. Cross sectional SEM images of (A) TiO2 blocking layer, (B) ml-250 layer, (C) ml-410 layer, (D) ml-640 layer and (E) ml-900 layer, (F) blocking layer and mesoporous layer (ml-640) coated with perovskite and (G) FTO glass/compact TiO2/mesoporous TiO2 infiltrated with CH3NH3PbI3-xClx/CuI. Inset: close-up view to show interfaces among each layer.
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Figure 4. (A) PCE of five types of thickness of TiO2 mesoporous layers. (B) Effect of mesoporous layer thickness on PCE. Box represents the standard deviation, whiskers minimum and maximum values.
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Figure 5. Cross sectional SEM images of (A) C1 device, (B) C2 device, (C) C3 device, (D) C4 device and (E) the summary of four types of thickness of the whole devices without gold contact of 25 devices, scale bar: 20 µm.
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Figure 6. (A) PCE of ml-640 devices with four types of thickness of CuI layer. (B) Effect of CuI layer's thickness on PCE. Box represents the standard deviation, whiskers minimum and maximum values. (C) IPCE spectrum of the optimized performing C2 device. (D) Diffuse reflectance spectra of devices with different thickness of CuI layers.
Figure 7. (A) Nyquist plots of C2 device under 100 mW/cm2 AM 1.5G illumination. Inset in (A) is equivalent circuit model. (B) Summary of the recombination resistance, Rrec and the HTM resistance, RHTM.
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Table
Table 1. Photovoltaic Perfomance Parameters of Perovskite Solar Cells with different thickness of TiO2 mesoporous layers. _______________________________________________________________________________ Device ML Thickness (nm) PCE (%) JSC (mA/cm2) VOC (mV) FF (%) 1.4 ± 0.5
9.4 ± 1.7
516 ± 93
29 ± 5
ml-250
250
3.5 ± 0.4
19.3 ± 3.1
712 ± 43
26 ± 5
ml-410
410
4.2 ± 0.9
20.1 ± 1.5
633 ± 62
33 ± 2
ml-640
640
4.5 ± 0.6
21.0 ± 1.5
646 ± 46
33 ± 5
ml-900
900
3.2 ± 1.0
18.6 ± 3.0
558 ± 48
30 ± 5
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ml-0
24.8 ± 2.6
5.6 ± 0.2
640 ± 46
39 ± 4
C3
28.8 ± 1.5
5.0 ± 0.2
22.1 ± 1.4
698 ± 32
33 ± 6
C4
36.9 ± 2.8
22.7 ± 1.8
693 ± 46
31 ± 4
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22.6 ± 0.8
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C2
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Table 2. Photovoltaic Characters of Devices with different thickness of CuI layers. _______________________________________________________________________________ Device Device Thickness (µm) PCE (%) JSC (mA/cm2) VOC (mV) FF (%) _______________________________________________________________________________ C1 17.5 ± 1.5 4.5 ± 0.6 21.0 ± 1.5 646 ± 46 33 ± 5
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4.8 ± 0.2
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