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perovskite interface linker for highly efficient perovskite solar cells
Accepted Manuscript Bifunctional electron transporting layer/perovskite interface linker for highly efficient perovskite solar cells Fei Han, Guimin Hao, Zhongquan Wan, Junsheng Luo, Jianxing Xia, Chunyang Jia PII:
S0013-4686(18)32370-3
DOI:
https://doi.org/10.1016/j.electacta.2018.10.130
Reference:
EA 32929
To appear in:
Electrochimica Acta
Received Date: 21 September 2018 Revised Date:
14 October 2018
Accepted Date: 20 October 2018
Please cite this article as: F. Han, G. Hao, Z. Wan, J. Luo, J. Xia, C. Jia, Bifunctional electron transporting layer/perovskite interface linker for highly efficient perovskite solar cells, Electrochimica Acta (2018), doi: https://doi.org/10.1016/j.electacta.2018.10.130. 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.
ACCEPTED MANUSCRIPT Bifunctional Electron Transporting Layer/Perovskite Interface Linker for Highly Efficient Perovskite Solar Cells Fei Hana, Guimin Haob, Zhongquan Wana, Junsheng Luoa, Jianxing Xiaa, Chunyang Jiaa,*
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
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a
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China.
Langfang Yanjing Polytechnic College, Langfang 065200, P. R. China.
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Graphical abstract
ACCEPTED MANUSCRIPT Bifunctional Electron Transporting Layer/Perovskite Interface Linker for Highly Efficient Perovskite Solar Cells Fei Hana, Guimin Haob, Zhongquan Wana, Junsheng Luoa, Jianxing Xiaa, Chunyang Jiaa,*
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
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a
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China.
Langfang Yanjing Polytechnic College, Langfang 065200, P. R. China.
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Abstract: The electron transporting layer/perovskite interface is crucial to the power conversion efficiency of perovskite solar cell. In this work, we study the effect of bifunctional 4-picolinic acid self-assembled monolayer as electron transporting
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layer/perovskite interface linker on perovskite crystallinity and electron transporting layer/perovskite interface transfer property for high performance perovskite solar cell. Our result shows that the 4-picolinic acid self-assembled monolayer modified device
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exhibits the highest efficiency of 18.90% and negligible hysteresis index of 0.03
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under illumination of AM 1.5G (100 mW cm-2), compared to that (14.65%) of the unmodified device. It can be concluded that the 4-picolinic acid self-assembled monolayer modified device shows significantly improved device performance and negligible hysteresis are due to the bifunctional self-assembled monolayer can be grappled with the TiO2, increase the grain sizes of MAPbI3, effectively passivate TiO2/perovskite interface, and preferably balance electrons and holes transfer. Our results also indicate bifunctional self-assembled monolayer is a preferable pathway 1
ACCEPTED MANUSCRIPT for fabricating high performance and low hysteresis perovskite solar cells.
Keywords: perovskite solar cell, self-assembled monolayer, TiO2/perovskite interface
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linker, hysteresis
---------------------------------------------------* Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia) 2
ACCEPTED MANUSCRIPT 1. Introduction Since the first report of perovskite solar cell (PSC) by Miyasaka et al. [1], hybrid organometal trihalide perovskites (MAPbX3, X = Cl, Br or I) have attracted
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extensive interests due to their excellent optoelectronic properties, such as optimal band gaps, high absorption coefficients, long charge carriers lifetime and exciton diffusion length, small exciton binding energies and simple solution approaches
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[2-7]. Particularly, PSCs using MAPbX3 as light harvesters have developed rapidly.
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So far, the highest power conversion efficiency (PCE) has been as high as 23.3% (certifed) in a typical n-i-p device architecture [8].
As the heart of the PSC, the quality of perovskite film and the electron transporting layer (ETL)/perovskite interface play a critical role in determining the
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device performance [9-16]. Incomplete surface coverage of perovskite film on the TiO2 and plenty of defect-assisted traps in the perovskite film can greatly reduce the device performances.
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To achieve better control over the morphology of perovskite film, a few of
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amphiphilic organic coupling agents have been employed to guide the formation of perovskite film [17-19]. According to previous reports, self-assembled monolayer (SAM) often forms strong electron coupling between the ETL and perovskite layer to facilitate charge carrier transport and collection, and suppress interfacial charge recombination [20-24]. For example, Snaith et al. modified the n-type contact with a fullerene SAM to “switch on” the electron transfer and found that the best PCE of fullerene-modified device achieves up to 17.3% with 3
ACCEPTED MANUSCRIPT significantly reduced hysteresis, and stabilized power output reaches 15.7% under simulated AM 1.5 sunlight [20]. Furthermore, SAM could affect the energy level, passivate ETL surface states and control the growth processes of organic
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semiconductors, which could dramatically change the electron density and energy level structure of ETL. However, the effect of bifunctional SAM on forming uniform perovskite film and ETL/perovskite interface linker has seldom been
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studied.
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Herein, a bifunctional 4-picolinic acid (4-PA) SAM as ETL/perovskite interface linker is chosen to study the effect of bifunctional SAM on perovskite crystallinity and ETL/perovskite interface transfer property for highly efficient PSCs. Before this, a series of studies have investigated the structural and
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electronic properties of mono- and multi-layers of 4-PA on anatase and rutile TiO2 in dye-sensitized solar cells [25, 26]. Fig. 1 shows the schematic diagram of the device structure and the 4-PA interface linker between the TiO2 and perovskite
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layers, and the photographs of semitransparent FTO/TiO2/4-PA/perovskite films,
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respectively. In this paper, we used the first-principal calculations (Fig. S1) and fourier transform infrared (FTIR) spectra (Fig. S2) to verify our suppositions about TiO2/4-PA and 4-PA/perovskite interactions. The hydroxyl groups on the TiO2 surface can chemically react and form bonds with the -COOH of 4-PA. The
strong peaks of the FTIR spectra at around 1623 and 1414 cm−1 for N=C and N-C bonds vibrations in 4-PA shifts to a lower frequency of around 1619 and 1408 cm−1, respectively, demonstrating a bonding interaction of the Pb atom with the N 4
ACCEPTED MANUSCRIPT atom [13]. Similar results have been reported by Xu’ [27], Dai’s [28], Wang’s [29], and Ma’s [30] papers. tBP has a tertiary butyl group on one end, in the meantime, the strong coordination between the nitrogen atom on the other end of tBP and the
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Pb2+ can result in numerous pyridyl groups to be anchored on the surface of PbI2. Hence, 4-PA is bifunctional. Our work further illuminates the effect of bifunctional SAM as ETL/perovskite interface linker on fabricating high
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performance and low hysteresis PSCs. Further, we prospect that the bifunctional
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4-PA interface linker could effectively: i) passivate TiO2/perovskite interface and facilitate interfacial charge transfer, ii) suppress charge recombination, balance electrons and holes transfer, and reduce the hysteresis, iii) improve the
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short-circuit current density (Jsc), and iv) enhance the open-circuit voltage (Voc).
Fig. 1 (a) Schematic diagram of the device structure and the 4-PA interface linker between TiO2 and perovskite layers, respectively. (b-c) The photographs of semitransparent FTO/TiO2/4-PA/perovskite films.
2. Results and discussion 2.1 Characterizations of the bare and 4-PA SAM-modified TiO2 films Initially, the presence and distribution of 4-PA SAM on the TiO2 film was 5
ACCEPTED MANUSCRIPT confirmed by energy dispersive X-ray (EDX) spectroscopy. Relevant EDX elemental mapping of bare and 4-PA SAM-modified TiO2 substrates were shown in Fig. 2. The characteristic element of 4-PA is nitrogen. There is no nitrogen in the FTO/TiO2
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sample. In contrast, the nitrogen (Fig. 2b) was found to be uniformly dispersed on the 4-PA SAM-modified TiO2 film. Based on the previous literature [22], the surface energy of TiO2 can be adjusted by the insert of 4-PA, thence, contact angle test was
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used to examine the surface wettability. The presence of 4-PA SAM on the TiO2 film
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was also verified by H2O contact angle test. A small droplet of water (~5 µL) formed a relatively hydrophilic contact with the TiO2 substrates, whose contact angles (Fig. 3) are 15.8 and 11.0° for bare and 4-PA SAM-modified TiO2 substrates, respectively. Moreover, the contact angles of DMF (Fig. S3) are 11.7 and 9.2° for bare and 4-PA
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SAM-modified TiO2 substrates, respectively. Based on the experimental results, 4-PA SAM-modified TiO2 shows better hydrophilic ability and smaller wetting angle. The surface energy of TiO2 can be adjusted by the insert of 4-PA can be used to explain
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smaller wetting angle for 4-PA SAM-modified TiO2 substrate. Similar results have
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been reported by Huang’s paper [22]. The smaller wetting angle ensures that the perovskite precursor solution can more easily infiltrate into the mesoporous-TiO2 (mp-TiO2). Detailed H2O and DMF contact angle information for bare and 4-PA SAM-modified TiO2 substrates is shown in Table S1 and S2, respectively.
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Fig. 2 The EDX elemental mapping of (a) bare and (b) 4-PA SAM-modified TiO2
substrates, respectively. The images that indicate the atomic symbols correspond to
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the EDX.
Fig. 3 The H2O contact angles of (a) bare and (b) 4-PA SAM-modified TiO2 substrates,
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respectively.
Scanning electron microscopy (SEM) measurement was used to study the surface
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morphologies of the MAPbI3 perovskite films on bare and 4-PA SAM-modified TiO2
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substrates. After 4-PA SAM modification, the grain sizes of MAPbI3 film on FTO/bl-TiO2/mp-TiO2 substrate are increased from the SEM result (Fig. 4b), which indicates that the 4-PA SAM can effectively improve the grain sizes of MAPbI3 film due to the reduced nucleation centers of the perovskite layer [24, 31]. The strong coordination between the nitrogen atom on the other end of 4-PA and the Pb2+, which can markedly influence the nucleation of the perovskite layer to produce highly dense films with increased grain sizes. Fig. 4c and 4d show the cross-sectional SEM and
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photograph of PSC based on 4-PA SAM-modified TiO2 substrate, respectively.
Fig. 4 SEM of MAPbI3 films on (a) bare and (b) 4-PA SAM-modified TiO2
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substrates, respectively. (c) The cross-sectional SEM of PSC based on 4-PA SAM-modified TiO2 substrate. (d) The photograph of PSC based on 4-PA
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SAM-modified TiO2 substrate.
The absorption spectra of as-prepared films cover the broad wavelength range from 400 up to 800 nm, which are typical for MAPbI3 material (Fig. 5a). The energy gap (Eg, Fig. 5b) are 3.76 and 3.77 eV for bare and 4-PA SAM-modified TiO2
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substrates, respectively. The absorption spectrum of MAPbI3 material on 4-PA SAM-modified TiO2 substrate shows a slight increase from ~430 to ~760 nm than that
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of MAPbI3 material on bare TiO2 substrate. The increased absorption results from highly dense films with increased grain sizes, which may be conducive to promoting
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the device performance, for example, Jsc. X-ray diffraction (XRD) patterns of MAPbI3 perovskite films based on bare and 4-PA SAMs-modified TiO2 substrates were measured to demonstrate the structural variations, as were shown in Fig. 5c (XRD patterns of the 4-PA powder are shown in Fig. S4). The result shows that a tetragonal crystal structure of perovskite was obtained [14-16, 32, 33]. Strong diffraction peaks at 2θ = 14.1, 23.6, 28.3, 34.9, 40.5 and 43.2° in Fig. 5c corresponding to the (110), (211), (220), (310), (224) and (314) places of MAPbI3 are observed and other three 8
ACCEPTED MANUSCRIPT diffraction peaks at 2θ = 26.5, 37.7 and 51.5° are diffraction peaks of FTO and TiO2. The XRD trace of the film processed from the standard solution reveals the reflection at 23.6° that is consistent with the (211) reflection of the tetragonal phase of MAPbI3
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[19]. The perovskite film exhibits similar XRD peak positions and slightly enhanced intensities after 4-PA SAM modification. Therefore, the changes in device performances could mainly be attributed to the variations in grain sizes of perovskite
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film and interfacial optoelectronic properties of PSC [19, 34].
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The Femi level (EF) of the interfacial layer is critical for achieving perfect contact at interfaces of PSCs. Efforts were made to utilize organic or inorganic materials for perfect contact, such as PFN [35] and CdS [36], etc. to improve the device performance. These interface engineering approaches are demonstrated to
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achieve the optimized interface energy level alignment, resulting in perfect contact at interfaces of PSCs. Hence, ultraviolet photoelectron spectroscopy (UPS) with HeI radiation (hv = 21.22 eV) was used to estimate the EF and maximum valance band
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energy (EVB) level of bare and 4-PA SAM-modified TiO2 substrates. According to our
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previous researches [14, 37], the energy difference between the EVB and EF was derived from the low binding energy tails (in the range below 4 eV, Fig. 5d). As shown in Fig. 5d, the surface EF are ~3.94 (bare TiO2) and ~3.97 (4-PA SAM-modified TiO2) eV, respectively. Conduction band edge (ECB) values were obtained by adding the Eg (Fig. 5b) to EVB. The Eg of bare and 4-PA SAM-modified TiO2 are calculated on the basis of Tauc relation (1) [38, 39]: ℎ = (ℎ − ) 9
(1)
ACCEPTED MANUSCRIPT where α is the absorption coefficient obtained by the absorption spectra, B (proportionality constant) represents the slope of tangent, and hv is the energy of absorbed light. Based on the Tauc relation, the Eg can be obtained by plotting (αhv)2
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vs. hv and extrapolating the linear portion of (αhv)2 to zero. The ECB (3.72 eV) of 4-PA SAM-modified TiO2 shows a negative deviation than that (3.70 eV) of bare TiO2. Thus, the 4-PA SAM modification forms a perfect contact at the ETL/perovskite
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interface, which will lead to a larger device Voc result from negative shifting of the
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ECB of TiO2.
Reference 4-PA(SAM)
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Absorbance / a. u.
(a)
500
600 Wavelength / nm
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400
10
700
800
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(b)
(ahυ )
2
Reference 4-PA(SAM)
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Eg = 3.76 eV Eg = 3.77 eV
(c)
3.4
3.5 3.6 3.7 3.8 Photenergy hυ / eV
3.9
4.0
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FTO/TiO2/4-PA/MAPbI3 FTO/TiO2/MAPbI3
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Relative Intensity
FTO/TiO2
(110)
(211)
20
30
(314) (224)
40
2Theta / degree
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(220) (310)
11
50
60
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(d)
EF = 3.94 eV EF = 3.97 eV 17.4
17.2
17.0
16.8
16.6
EVB-EF=3.52 eV
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10
15
20
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Intensity / a. u.
Reference 4-PA(SAM)
Bonding Energy / eV
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Fig. 5 (a) The absorption spectra of MAPbI3 films on bare and 4-PA SAM-modified TiO2 substrates, respectively. (b) Plot of (αhv)2 vs. hv on bare and 4-PA SAM-modified TiO2 substrates, respectively. (c) XRD patterns of perovskite films based bare and 4-PA SAM-modified TiO2 substrates, respectively. (d) UPS of bare
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2.2 Device performances
Extensive experiments including J-V curves (different scanning speeds) of 4-PA
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SAM-modified PSC, and the stability measurements of best PSC devices based on
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4-PA SAM-unmodified and 4-PA SAM-modified TiO2 substrates were used to study the photovoltaic (PV) performances of PSCs, whose results were shown in Fig. S5, Fig. S6 and Table S3. From these experiment results, it is not difficult to find that 4-PA SAM was conducive to enhancing PV performance of PSC and inproving the device stability. The better stability of 4-PA SAM-modified PSC can attributed to the stabilizing interactions of TiO2/4-PA (The -COOH group of 4-PA can be chemically grappled with mp-TiO2) and 4-PA/perovskite (The nitrogen atom of 4-PA and Pb2+ 12
ACCEPTED MANUSCRIPT form strong coordination) interfaces. The J-V curves of the 4-PA SAM-modified PSC are shown in Fig. 6a, it is quite clear that the 4-PA SAM-modified device exhibits better PCE and the corresponding hysteresis is negligible (Table 1). The best PCE of
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4-PA SAM-modified PSC is 18.08% (Jsc = 22.96 mA cm-2, Voc = 1.05 V and FF = 0.75) under forward scanning (FS), while 18.90% (Jsc = 23.15 mA cm-2, Voc = 1.06 V and FF = 0.77) under reverse scanning (RS). From Fig. 6a, we can obtain that the PCE of
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SAM-unmodified PSC is 11.74% (Jsc = 20.30 mA cm-2, Voc = 0.98 V and FF = 0.59)
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under FS, while 14.65% (Jsc = 20.55 mA cm-2, Voc = 0.99 V and FF = 0.72) under RS. Incident photo-to-current conversion efficiency (IPCE) of the SAM-unmodified and 4-PA SAM-modified devices were performed to further verify the accuracies of PCEs. The integrated current densities (Fig. 6b) are 18.69 and 21.23 mA cm-2, which are in
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good agreement with the values from J-V curves. The steady-state J-t and PCE curves of the SAM-unmodified and 4-PA SAM-modified devices at the maximum power point (MPP) were measured to verify the accuracies of efficiencies. As shown in Fig.
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6c, the steady-state photocurrent densities are 18.53 (VMPP = 0.78 V) and 21.71 (VMPP
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=0.84 V) mA cm-2, which are also close to those from the J-V curves under FS/RS. While the steady-state PCEs are 13.34 and 16.72%, which are close to those calculated by PV parameters. In the experimental process, full sunlight irradiation and temperature increase during the measurement should contribute jointly to the stabilized Jsc obtained in J-t curves and the Jsc calculated by J-V curves, which are higher than the integrated Jsc estimated from IPCE spectra [6, 14]. We tried to better understand the improved IPCE, which could be given by the following Eq. (2) [29]: 13
ACCEPTED MANUSCRIPT IPCE(λ) = LHE(λ)φinjηcol
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where LHE(λ) is the light-harvesting efficiency by the active layer, φinj is the electron injection efficiency from the perovskite layer to the electron collection layer,
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and ηcol is the charge collection efficiency at the electrodes. The φinj is positively related to the ETL's electron extraction ability from the perovskite layer. The ηcol cumulatively takes into account the transport and recombination dynamics and could
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be defined by the following equation: ηcol = transport rate/(transport rate +
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recombination rate). The 4-PA facilitates the electrons transport from the perovskite layer to the electron collection layer and simultaneously reduces the back flow of electrons from the SAM to the perovskite layer. Thus, the φinj and ηcol are enhanced. The 4-PA SAM-modified PSCs are of high stability and reproducibility. All the
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PSCs were prepared and tested under the same conditions. The stability measurements of best PSC devices based on 4-PA SAM-unmodified and 4-PA SAM-modified TiO2 substrates are shown in Fig. S6. Fig. 6d shows the statistic parameters (Jsc, Voc, FF and
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PCE) of the SAM-unmodified (Reference) and 4-PA SAM-modified PSCs.
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Reference(RS) Reference(FS) 4-PA(SAM, RS) 4-PA(SAM, FS)
0.2
0.4
0.6
0.8
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Voltage / V
(b)100
60 40
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IPCE / %
80
20
1.0
400
500
Reference 4-PA(SAM)
600
Wavelength / nm
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15
1.2
700
25 2
0 0.0
20 15 10 5 0 800
Integrated JSC / mA/cm
10
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Current density / mA/cm
2
(a)
25 20 15
Reference 4-PA(SAM)
10 5 0 0
Reference 4-PA(SAM)
5
PCE / %
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(c)
Current density / mA/cm2
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10 15
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100
200
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400
1.16
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(d)
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500
Jsc (mA/cm )
Voc (V)
1.12
24
1.08 1.04
21
1.00 0.96
18 Reference 0.85
FF 0.80
0.70
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0.65
4-PA
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0.75
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0.92
Reference
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4-PA
PCE (%)
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4-PA
Reference
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Fig. 6 (a) The J-V curves of SAM-unmodified (Reference) and 4-PA SAM-modified PSCs. (b) IPCE of SAM-unmodified (Reference) and 4-PA SAM-modified PSCs. (c) The steady-state J-t and PCE curves of SAM-unmodified (Reference) and 4-PA SAM-modified PSCs. (d) The statistic parameters (Jsc, Voc, FF and PCE) of SAM-unmodified (Reference) and 4-PA SAM-modified PSCs.
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ACCEPTED MANUSCRIPT Table 1 The photovoltaic parameters and the corresponding hysteresis index of the SAM-unmodified (Reference) and 4-PA SAM-modified PSCs. Samples
Scanning
Jsc
Voc
Direction
(mA/cm2)
(V)
FF
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a
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Index (HI)
Hysteresis
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Forward
20.30
0.98
0.59
Reverse
20.55
0.99
0.72
Forward
22.96
1.05
0.75
Reverse
23.15
1.06
0.77
a
11.74
0.31
14.65
18.08
0.03
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[40]
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HI = [JRS(0.8Voc)-JFS(0.8Voc)]/JRS(0.8Voc) [40]; JRS(0.8Voc) and JFS(0.8Voc) represent photocurrent density at 80% of Voc for the RS and FS, respectively.
It is particularly interesting to understand why the 4-PA SAM-modified PSC obtained better PCE of 18.90% with low HI of 0.03, comparing with the PCE of 14.65% (Reference) and HI of 0.31 (Reference) in Table 1. Our results indicate the 4-PA SAM
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is a preferable pathway for fabricating high performance and low hysteresis PSCs. The 4-PA SAM-modified PSC exhibits a higher PCE with negligible hysteresis is due to that both TiO2 and perovskite can bond with 4-PA. The mechanism can be
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explained by two aspects: i) The -COOH group of 4-PA can be chemically grappled
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with mp-TiO2, which facilitates electron transport and reduces the charge accumulation at the interface. ii) The strong coordination between the nitrogen atom on the other end of 4-PA and the Pb2+, which passivates perovskite surface close to TiO2. The strong N-Pb coordination can markedly influence the nucleation of the perovskite layer to produce highly dense films with increased grain sizes. Finally, the device PCE is improved and the hysteresis is suppressive. 2.3 Charge transfer and extraction mechanisms Charge extraction kinetics at the TiO2/perovskite interface is crucial to the device 17
ACCEPTED MANUSCRIPT performance [41-43]. To insight into the effect of 4-PA SAM modifying on electron transfer and extraction at TiO2/perovskite interface, steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements are presented in
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Fig. 7a and 7b, respectively. From Fig. 7a, we can know that the PL quenching for the 4-PA SAM-modified perovskite sample is more efficient, suggesting enhanced electron extraction ability. The TRPL curves are fitted with a biexponential decay
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function: = + / + / . The fitted average PL lifetime (τPL) of
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the glass/perovskite sample is 52.46 ns. The fitted average τPL are 45.54 and 26.10 ns for the SAM-unmodified and 4-PA SAM-modified perovskite samples, respectively. The TRPL results shown in Fig. 7b indicate faster photo-induced electron transfer from the 4-PA SAM-modified MAPbI3 to TiO2, which are because the -COOH group
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is chemically grappled with mp-TiO2. Furthermore, pyridine group of 4-PA can passivate perovskite surface close to TiO2, which facilitates interfacial charge transfer and increases MAPbI3 grain sizes. Ultimately, the Voc of 4-PA SAM-modified PSC is
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enhanced. The 4-PA SAM modification causes a larger Voc by negative shifting of the
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ECB of TiO2, whose result is consistent with that according to Table 1.
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(a)
700
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PL intensity / a. u.
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Glass/perovskite Reference 4-PA (SAM) Glass/perovskite (Fit) Reference (Fit) 4-PA (SAM, Fit)
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900
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Glass/perovskite Reference 4-PA (SAM)
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Fig. 7 (a) The steady-state PL of perovskite films based on the glass, SAM-unmodified (Reference) and 4-PA SAM-modified TiO2 substrates, respectively. (b) The time-resolved PL of perovskite films based on the glass, SAM-unmodified (Reference) and 4-PA SAM-modified TiO2 substrates, respectively.
3. Conclusions In this paper, we study the effect of bifunctional 4-picolinic acid (4-PA) 19
ACCEPTED MANUSCRIPT self-assembled monolayer (SAM) as electron transporting layer (ETL)/perovskite interface linker on perovskite crystallinity and ETL/perovskite interface transfer property for high performance perovskite solar cell (PSC). Our results show that the
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4-PA SAM-modified PSC exhibits the highest power conversion efficiency (PCE) of 18.90% and negligible hysteresis index (HI) of 0.03 under illumination of AM 1.5G (100 mW cm-2), compared to PCE = 14.65% and HI = 0.31 of the SAM-unmodified
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PSC. It can be concluded that the 4-PA SAM-modified PSC shows improved
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performance and negligible hysteresis are due to the bifunctional 4-PA SAM can be grappled with the TiO2, increase the grain sizes of MAPbI3, effectively passivate TiO2/perovskite interface, and preferably balance electrons and holes transfer. Our results also indicate the bifunctional SAM is a preferable pathway for further
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fabricating high performance and low hysteresis PSCs.
4.1 Materials
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4. Experimental section
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PTAA (Mol. Weight (Mw)：17500 by GPC), MAI (>99.5%), PbI2 (>99.99%) and TiO2 paste (Dyesol, 18NR-T) were purchased from Xi'an Polymer Light Technology Corp. Lithium bis(trifluoromethylsulphonyl)imide (LiTFSI, >95%), acetonitrile (>99.8%), titanium diisopropoxidebis (acetylacetonate) (75 wt% in isopropanol), 1-butanol (>99.8%), 4-PA (>99%), chlorobenzene (CB, >99.8%), anhydrous N, N-dimethylformamide (DMF, >99.8%), toluene (TL, >99.8%) and tBP (>96%) were purchased from Sigma Aldrich. Unless stated otherwise, all materials were used 20
ACCEPTED MANUSCRIPT without further purification. FTO glasses (Tec 7, Pilkington) were patterned by etching with Zn powder and 2 M HCl. To prepare 1.2 M MAPbI3 precursor solution, 553.2 mg PbI2 and 190.8 mg
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MAI were dissloved in 1.0 mL DMF with stirring. The PTAA solution was prepared by dissolving 15 mg PTAA, 7.5 µL tBP/acetonitrile (v:v, 1:1) and 7.5 µL of a stock solution of 170.0 mg mL-1 LiTFSI in acetonitrile in 1.0 mL TL.
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4.2 Device fabrication
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4.2.1 Substrate preparation
The etched FTO substrates were cleaned with alkaline aqueous washing solution, deionized water, isopropanol, ethanol and finally treated under O3/ultraviolet for 15 min. The deposition of TiO2 followed our previous work [15]. Most SAMs can be
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dissolved in methanol/H2O at a concentration of ~0.5 mg/ml. As everyone knows, the 4-PA is slightly soluble in cold water. The actual solvent of 4-PA SAM is methanol not water. In our manuscript, the mp-TiO2 substrates were immersed in a 0.5 mg/mL
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solution of 4-PA in methanol (Here, 0.3 vol% water is added to slow down the
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evaporation of methanol) for several hours to induce a 4-PA SAM on the surface of mp-TiO2. After that, the mp-TiO2 substrates were rinsed with methanol and ethanol to remove excess water, and then dried under a flow of nitrogen. 4.2.2 Perovskite solar cells fabrication The synthesis of MAPbI3 was carried out by a one-step deposition technique, which was deposited onto bare and 4-PA SAM-modified mp-TiO2 films by spin-coating with 5000 r.p.m. for 30 s. At the remaining spin time of ~23 s, 500.0 µL 21
ACCEPTED MANUSCRIPT CB was quickly added onto the MAPbI3 film. After spin-coating, the substrate was annealed at 100 °C for 10 min. The PTAA solution was deposited on MAPbI3 film by spin-coating at 3000 r.p.m. for 30 s. Devices were then left overnight in a dry box full
~10−6 Torr, at a rate of ~3 Å s-1, to complete the devices. 4.2.3 Device measurement and characterization
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of oxygen. Finally, 60~80 nm of gold was thermally evaporated under vacuum of
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EDX spectroscopy was performed to determine the chemical composition using
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an Oxford X-max LN2-free Silicon Drif Detector, with 80 mm2 of sensor active area and 129 eV of energy resolution at 5.9 k eV (MnKα). SEM was performed using a JEOL JSM-7600F device. The contact angle tests were performed with a standard DSA25 (KRUSS, Germany) Contact Angle Measurement Instrument. The XRD
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analysis of the perovskite films was characterized by a D1 Evolation (JORDAN VALLEY, USA) using Cu-Ka radiation at a coincident scan rate of 6° min-1. Absorption spectra of the MAPbI3 perovskite films on FTO/TiO2 substrates were
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determined with SHIMADZU (model UV1700) ultraviolet to visible (UV-vis)
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spectrophotometer. The UPS were performed with EscaLab 250Xi. The light source of UPS is a HeI discharge lamp (hv = 21.22 eV). PL lifetime measurement was performed on a lifetime spectrometer (FLS 920, Edinburgh Instrument). The monitored wavelength is at about 770 nm. The steady-state PL spectra were measured using an integrated Raman system (Horiba JY HR800) with an Olympus 15X NUV lens (NA=0.32). A 325 nm helium-cadmium (He-Cd) laser was used as the excitation source. The laser power density on the film surface was about 2 W cm-2 and the 22
ACCEPTED MANUSCRIPT wavelength scale was from 500 to 900 nm. The J-V characteristics of the PSCs were measured with a fixed active area of 0.09 cm2 using a CH Instruments 660D electrochemical workstation (Shanghai CH Instruments Co., China) under
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illumination of a simulated sunlight (AM 1.5G, 100 mW cm-2) provided by a short-arc xenon lamp (CHF-XW-500W, Trusttech Co. Ltd., Beijing, China). The scan rate is 0.20 V/s, which is a frequently-used parameter. The measurement of the IPCE was
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obtained by a commercial setup (Qtest Station 2000 IPCE Measurement System,
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CROWNTECH, USA). FTIR spectra were performed on a Nexus 670 FTIR spectrometer in the range 400-4000 cm-1. The first-principal calculations within dispersion-corrected density functional theory1 were performed using the Vienna ab initio simulation package with a projector-augmented-wave2 method. The wave
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function was expanded by plane wave with kinetic cutoff of 400 eV. The generalized gradient approximation with the spin-polarized Perdew-Burke-Ernzerh of function
[19, 47]:
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was used for all calculations [44-46]. The bonding energy was calculated using Eq. (3)
! (3)
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= − −
where Etot is the total energy of the system studied, Esur is the energy of a clean
surface, and Emol is the energy of free 4-PA. The negative bonding energy signifies that the chemisorption structure is stable.
Acknowledgments We thank the National Natural Science Foundation of China (Grant Nos. 23
ACCEPTED MANUSCRIPT 21572030 and 51773027) for financial support.
References
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ACCEPTED MANUSCRIPT Bifunctional Electron Transporting Layer/Perovskite Interface Linker for Highly Efficient Perovskite Solar Cells Fei Hana, Guimin Haob, Zhongquan Wana, Junsheng Luoa, Jianxing Xiaa, Chunyang Jiaa,*
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
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a
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China.
Langfang Yanjing Polytechnic College, Langfang 065200, P. R. China.
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Highlights
1. Bifunctional 4-picolinic acid interface linker was studied.
2. The 4-picolinic acid optimizing device exhibits the highest PCE of 18.90%. 3. The 4-picolinic acid optimized PSC exhibits negligible hysteresis.
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4. Bifunctional interface linker is a preferable pathway for highly efficient PSCs.
S0013-4686(18)32370-3
DOI:
https://doi.org/10.1016/j.electacta.2018.10.130
Reference:
EA 32929
To appear in:
Electrochimica Acta
Received Date: 21 September 2018 Revised Date:
14 October 2018
Accepted Date: 20 October 2018
Please cite this article as: F. Han, G. Hao, Z. Wan, J. Luo, J. Xia, C. Jia, Bifunctional electron transporting layer/perovskite interface linker for highly efficient perovskite solar cells, Electrochimica Acta (2018), doi: https://doi.org/10.1016/j.electacta.2018.10.130. 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.
ACCEPTED MANUSCRIPT Bifunctional Electron Transporting Layer/Perovskite Interface Linker for Highly Efficient Perovskite Solar Cells Fei Hana, Guimin Haob, Zhongquan Wana, Junsheng Luoa, Jianxing Xiaa, Chunyang Jiaa,*
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
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a
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China.
Langfang Yanjing Polytechnic College, Langfang 065200, P. R. China.
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Graphical abstract
ACCEPTED MANUSCRIPT Bifunctional Electron Transporting Layer/Perovskite Interface Linker for Highly Efficient Perovskite Solar Cells Fei Hana, Guimin Haob, Zhongquan Wana, Junsheng Luoa, Jianxing Xiaa, Chunyang Jiaa,*
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
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a
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China.
Langfang Yanjing Polytechnic College, Langfang 065200, P. R. China.
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Abstract: The electron transporting layer/perovskite interface is crucial to the power conversion efficiency of perovskite solar cell. In this work, we study the effect of bifunctional 4-picolinic acid self-assembled monolayer as electron transporting
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layer/perovskite interface linker on perovskite crystallinity and electron transporting layer/perovskite interface transfer property for high performance perovskite solar cell. Our result shows that the 4-picolinic acid self-assembled monolayer modified device
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exhibits the highest efficiency of 18.90% and negligible hysteresis index of 0.03
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under illumination of AM 1.5G (100 mW cm-2), compared to that (14.65%) of the unmodified device. It can be concluded that the 4-picolinic acid self-assembled monolayer modified device shows significantly improved device performance and negligible hysteresis are due to the bifunctional self-assembled monolayer can be grappled with the TiO2, increase the grain sizes of MAPbI3, effectively passivate TiO2/perovskite interface, and preferably balance electrons and holes transfer. Our results also indicate bifunctional self-assembled monolayer is a preferable pathway 1
ACCEPTED MANUSCRIPT for fabricating high performance and low hysteresis perovskite solar cells.
Keywords: perovskite solar cell, self-assembled monolayer, TiO2/perovskite interface
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linker, hysteresis
---------------------------------------------------* Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia) 2
ACCEPTED MANUSCRIPT 1. Introduction Since the first report of perovskite solar cell (PSC) by Miyasaka et al. [1], hybrid organometal trihalide perovskites (MAPbX3, X = Cl, Br or I) have attracted
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extensive interests due to their excellent optoelectronic properties, such as optimal band gaps, high absorption coefficients, long charge carriers lifetime and exciton diffusion length, small exciton binding energies and simple solution approaches
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[2-7]. Particularly, PSCs using MAPbX3 as light harvesters have developed rapidly.
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So far, the highest power conversion efficiency (PCE) has been as high as 23.3% (certifed) in a typical n-i-p device architecture [8].
As the heart of the PSC, the quality of perovskite film and the electron transporting layer (ETL)/perovskite interface play a critical role in determining the
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device performance [9-16]. Incomplete surface coverage of perovskite film on the TiO2 and plenty of defect-assisted traps in the perovskite film can greatly reduce the device performances.
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To achieve better control over the morphology of perovskite film, a few of
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amphiphilic organic coupling agents have been employed to guide the formation of perovskite film [17-19]. According to previous reports, self-assembled monolayer (SAM) often forms strong electron coupling between the ETL and perovskite layer to facilitate charge carrier transport and collection, and suppress interfacial charge recombination [20-24]. For example, Snaith et al. modified the n-type contact with a fullerene SAM to “switch on” the electron transfer and found that the best PCE of fullerene-modified device achieves up to 17.3% with 3
ACCEPTED MANUSCRIPT significantly reduced hysteresis, and stabilized power output reaches 15.7% under simulated AM 1.5 sunlight [20]. Furthermore, SAM could affect the energy level, passivate ETL surface states and control the growth processes of organic
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semiconductors, which could dramatically change the electron density and energy level structure of ETL. However, the effect of bifunctional SAM on forming uniform perovskite film and ETL/perovskite interface linker has seldom been
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studied.
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Herein, a bifunctional 4-picolinic acid (4-PA) SAM as ETL/perovskite interface linker is chosen to study the effect of bifunctional SAM on perovskite crystallinity and ETL/perovskite interface transfer property for highly efficient PSCs. Before this, a series of studies have investigated the structural and
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electronic properties of mono- and multi-layers of 4-PA on anatase and rutile TiO2 in dye-sensitized solar cells [25, 26]. Fig. 1 shows the schematic diagram of the device structure and the 4-PA interface linker between the TiO2 and perovskite
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layers, and the photographs of semitransparent FTO/TiO2/4-PA/perovskite films,
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respectively. In this paper, we used the first-principal calculations (Fig. S1) and fourier transform infrared (FTIR) spectra (Fig. S2) to verify our suppositions about TiO2/4-PA and 4-PA/perovskite interactions. The hydroxyl groups on the TiO2 surface can chemically react and form bonds with the -COOH of 4-PA. The
strong peaks of the FTIR spectra at around 1623 and 1414 cm−1 for N=C and N-C bonds vibrations in 4-PA shifts to a lower frequency of around 1619 and 1408 cm−1, respectively, demonstrating a bonding interaction of the Pb atom with the N 4
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Pb2+ can result in numerous pyridyl groups to be anchored on the surface of PbI2. Hence, 4-PA is bifunctional. Our work further illuminates the effect of bifunctional SAM as ETL/perovskite interface linker on fabricating high
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performance and low hysteresis PSCs. Further, we prospect that the bifunctional
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4-PA interface linker could effectively: i) passivate TiO2/perovskite interface and facilitate interfacial charge transfer, ii) suppress charge recombination, balance electrons and holes transfer, and reduce the hysteresis, iii) improve the
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short-circuit current density (Jsc), and iv) enhance the open-circuit voltage (Voc).
Fig. 1 (a) Schematic diagram of the device structure and the 4-PA interface linker between TiO2 and perovskite layers, respectively. (b-c) The photographs of semitransparent FTO/TiO2/4-PA/perovskite films.
2. Results and discussion 2.1 Characterizations of the bare and 4-PA SAM-modified TiO2 films Initially, the presence and distribution of 4-PA SAM on the TiO2 film was 5
ACCEPTED MANUSCRIPT confirmed by energy dispersive X-ray (EDX) spectroscopy. Relevant EDX elemental mapping of bare and 4-PA SAM-modified TiO2 substrates were shown in Fig. 2. The characteristic element of 4-PA is nitrogen. There is no nitrogen in the FTO/TiO2
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sample. In contrast, the nitrogen (Fig. 2b) was found to be uniformly dispersed on the 4-PA SAM-modified TiO2 film. Based on the previous literature [22], the surface energy of TiO2 can be adjusted by the insert of 4-PA, thence, contact angle test was
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used to examine the surface wettability. The presence of 4-PA SAM on the TiO2 film
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was also verified by H2O contact angle test. A small droplet of water (~5 µL) formed a relatively hydrophilic contact with the TiO2 substrates, whose contact angles (Fig. 3) are 15.8 and 11.0° for bare and 4-PA SAM-modified TiO2 substrates, respectively. Moreover, the contact angles of DMF (Fig. S3) are 11.7 and 9.2° for bare and 4-PA
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SAM-modified TiO2 substrates, respectively. Based on the experimental results, 4-PA SAM-modified TiO2 shows better hydrophilic ability and smaller wetting angle. The surface energy of TiO2 can be adjusted by the insert of 4-PA can be used to explain
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smaller wetting angle for 4-PA SAM-modified TiO2 substrate. Similar results have
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been reported by Huang’s paper [22]. The smaller wetting angle ensures that the perovskite precursor solution can more easily infiltrate into the mesoporous-TiO2 (mp-TiO2). Detailed H2O and DMF contact angle information for bare and 4-PA SAM-modified TiO2 substrates is shown in Table S1 and S2, respectively.
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Fig. 2 The EDX elemental mapping of (a) bare and (b) 4-PA SAM-modified TiO2
substrates, respectively. The images that indicate the atomic symbols correspond to
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the EDX.
Fig. 3 The H2O contact angles of (a) bare and (b) 4-PA SAM-modified TiO2 substrates,
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respectively.
Scanning electron microscopy (SEM) measurement was used to study the surface
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morphologies of the MAPbI3 perovskite films on bare and 4-PA SAM-modified TiO2
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substrates. After 4-PA SAM modification, the grain sizes of MAPbI3 film on FTO/bl-TiO2/mp-TiO2 substrate are increased from the SEM result (Fig. 4b), which indicates that the 4-PA SAM can effectively improve the grain sizes of MAPbI3 film due to the reduced nucleation centers of the perovskite layer [24, 31]. The strong coordination between the nitrogen atom on the other end of 4-PA and the Pb2+, which can markedly influence the nucleation of the perovskite layer to produce highly dense films with increased grain sizes. Fig. 4c and 4d show the cross-sectional SEM and
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photograph of PSC based on 4-PA SAM-modified TiO2 substrate, respectively.
Fig. 4 SEM of MAPbI3 films on (a) bare and (b) 4-PA SAM-modified TiO2
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substrates, respectively. (c) The cross-sectional SEM of PSC based on 4-PA SAM-modified TiO2 substrate. (d) The photograph of PSC based on 4-PA
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SAM-modified TiO2 substrate.
The absorption spectra of as-prepared films cover the broad wavelength range from 400 up to 800 nm, which are typical for MAPbI3 material (Fig. 5a). The energy gap (Eg, Fig. 5b) are 3.76 and 3.77 eV for bare and 4-PA SAM-modified TiO2
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substrates, respectively. The absorption spectrum of MAPbI3 material on 4-PA SAM-modified TiO2 substrate shows a slight increase from ~430 to ~760 nm than that
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of MAPbI3 material on bare TiO2 substrate. The increased absorption results from highly dense films with increased grain sizes, which may be conducive to promoting
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the device performance, for example, Jsc. X-ray diffraction (XRD) patterns of MAPbI3 perovskite films based on bare and 4-PA SAMs-modified TiO2 substrates were measured to demonstrate the structural variations, as were shown in Fig. 5c (XRD patterns of the 4-PA powder are shown in Fig. S4). The result shows that a tetragonal crystal structure of perovskite was obtained [14-16, 32, 33]. Strong diffraction peaks at 2θ = 14.1, 23.6, 28.3, 34.9, 40.5 and 43.2° in Fig. 5c corresponding to the (110), (211), (220), (310), (224) and (314) places of MAPbI3 are observed and other three 8
ACCEPTED MANUSCRIPT diffraction peaks at 2θ = 26.5, 37.7 and 51.5° are diffraction peaks of FTO and TiO2. The XRD trace of the film processed from the standard solution reveals the reflection at 23.6° that is consistent with the (211) reflection of the tetragonal phase of MAPbI3
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[19]. The perovskite film exhibits similar XRD peak positions and slightly enhanced intensities after 4-PA SAM modification. Therefore, the changes in device performances could mainly be attributed to the variations in grain sizes of perovskite
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film and interfacial optoelectronic properties of PSC [19, 34].
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The Femi level (EF) of the interfacial layer is critical for achieving perfect contact at interfaces of PSCs. Efforts were made to utilize organic or inorganic materials for perfect contact, such as PFN [35] and CdS [36], etc. to improve the device performance. These interface engineering approaches are demonstrated to
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achieve the optimized interface energy level alignment, resulting in perfect contact at interfaces of PSCs. Hence, ultraviolet photoelectron spectroscopy (UPS) with HeI radiation (hv = 21.22 eV) was used to estimate the EF and maximum valance band
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energy (EVB) level of bare and 4-PA SAM-modified TiO2 substrates. According to our
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previous researches [14, 37], the energy difference between the EVB and EF was derived from the low binding energy tails (in the range below 4 eV, Fig. 5d). As shown in Fig. 5d, the surface EF are ~3.94 (bare TiO2) and ~3.97 (4-PA SAM-modified TiO2) eV, respectively. Conduction band edge (ECB) values were obtained by adding the Eg (Fig. 5b) to EVB. The Eg of bare and 4-PA SAM-modified TiO2 are calculated on the basis of Tauc relation (1) [38, 39]: ℎ = (ℎ − ) 9
(1)
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vs. hv and extrapolating the linear portion of (αhv)2 to zero. The ECB (3.72 eV) of 4-PA SAM-modified TiO2 shows a negative deviation than that (3.70 eV) of bare TiO2. Thus, the 4-PA SAM modification forms a perfect contact at the ETL/perovskite
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interface, which will lead to a larger device Voc result from negative shifting of the
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Reference 4-PA(SAM)
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Absorbance / a. u.
(a)
500
600 Wavelength / nm
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10
700
800
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(b)
(ahυ )
2
Reference 4-PA(SAM)
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Eg = 3.76 eV Eg = 3.77 eV
(c)
3.4
3.5 3.6 3.7 3.8 Photenergy hυ / eV
3.9
4.0
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3.3
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3.2
FTO/TiO2/4-PA/MAPbI3 FTO/TiO2/MAPbI3
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Relative Intensity
FTO/TiO2
(110)
(211)
20
30
(314) (224)
40
2Theta / degree
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10
(220) (310)
11
50
60
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(d)
EF = 3.94 eV EF = 3.97 eV 17.4
17.2
17.0
16.8
16.6
EVB-EF=3.52 eV
5
10
15
20
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0
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Intensity / a. u.
Reference 4-PA(SAM)
Bonding Energy / eV
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Fig. 5 (a) The absorption spectra of MAPbI3 films on bare and 4-PA SAM-modified TiO2 substrates, respectively. (b) Plot of (αhv)2 vs. hv on bare and 4-PA SAM-modified TiO2 substrates, respectively. (c) XRD patterns of perovskite films based bare and 4-PA SAM-modified TiO2 substrates, respectively. (d) UPS of bare
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and 4-PA SAM-modified TiO2 substrates, respectively.
2.2 Device performances
Extensive experiments including J-V curves (different scanning speeds) of 4-PA
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SAM-modified PSC, and the stability measurements of best PSC devices based on
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4-PA SAM-unmodified and 4-PA SAM-modified TiO2 substrates were used to study the photovoltaic (PV) performances of PSCs, whose results were shown in Fig. S5, Fig. S6 and Table S3. From these experiment results, it is not difficult to find that 4-PA SAM was conducive to enhancing PV performance of PSC and inproving the device stability. The better stability of 4-PA SAM-modified PSC can attributed to the stabilizing interactions of TiO2/4-PA (The -COOH group of 4-PA can be chemically grappled with mp-TiO2) and 4-PA/perovskite (The nitrogen atom of 4-PA and Pb2+ 12
ACCEPTED MANUSCRIPT form strong coordination) interfaces. The J-V curves of the 4-PA SAM-modified PSC are shown in Fig. 6a, it is quite clear that the 4-PA SAM-modified device exhibits better PCE and the corresponding hysteresis is negligible (Table 1). The best PCE of
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4-PA SAM-modified PSC is 18.08% (Jsc = 22.96 mA cm-2, Voc = 1.05 V and FF = 0.75) under forward scanning (FS), while 18.90% (Jsc = 23.15 mA cm-2, Voc = 1.06 V and FF = 0.77) under reverse scanning (RS). From Fig. 6a, we can obtain that the PCE of
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SAM-unmodified PSC is 11.74% (Jsc = 20.30 mA cm-2, Voc = 0.98 V and FF = 0.59)
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under FS, while 14.65% (Jsc = 20.55 mA cm-2, Voc = 0.99 V and FF = 0.72) under RS. Incident photo-to-current conversion efficiency (IPCE) of the SAM-unmodified and 4-PA SAM-modified devices were performed to further verify the accuracies of PCEs. The integrated current densities (Fig. 6b) are 18.69 and 21.23 mA cm-2, which are in
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good agreement with the values from J-V curves. The steady-state J-t and PCE curves of the SAM-unmodified and 4-PA SAM-modified devices at the maximum power point (MPP) were measured to verify the accuracies of efficiencies. As shown in Fig.
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6c, the steady-state photocurrent densities are 18.53 (VMPP = 0.78 V) and 21.71 (VMPP
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=0.84 V) mA cm-2, which are also close to those from the J-V curves under FS/RS. While the steady-state PCEs are 13.34 and 16.72%, which are close to those calculated by PV parameters. In the experimental process, full sunlight irradiation and temperature increase during the measurement should contribute jointly to the stabilized Jsc obtained in J-t curves and the Jsc calculated by J-V curves, which are higher than the integrated Jsc estimated from IPCE spectra [6, 14]. We tried to better understand the improved IPCE, which could be given by the following Eq. (2) [29]: 13
ACCEPTED MANUSCRIPT IPCE(λ) = LHE(λ)φinjηcol
(2)
where LHE(λ) is the light-harvesting efficiency by the active layer, φinj is the electron injection efficiency from the perovskite layer to the electron collection layer,
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and ηcol is the charge collection efficiency at the electrodes. The φinj is positively related to the ETL's electron extraction ability from the perovskite layer. The ηcol cumulatively takes into account the transport and recombination dynamics and could
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be defined by the following equation: ηcol = transport rate/(transport rate +
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recombination rate). The 4-PA facilitates the electrons transport from the perovskite layer to the electron collection layer and simultaneously reduces the back flow of electrons from the SAM to the perovskite layer. Thus, the φinj and ηcol are enhanced. The 4-PA SAM-modified PSCs are of high stability and reproducibility. All the
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PSCs were prepared and tested under the same conditions. The stability measurements of best PSC devices based on 4-PA SAM-unmodified and 4-PA SAM-modified TiO2 substrates are shown in Fig. S6. Fig. 6d shows the statistic parameters (Jsc, Voc, FF and
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PCE) of the SAM-unmodified (Reference) and 4-PA SAM-modified PSCs.
14
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Reference(RS) Reference(FS) 4-PA(SAM, RS) 4-PA(SAM, FS)
0.2
0.4
0.6
0.8
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Voltage / V
(b)100
60 40
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IPCE / %
80
20
1.0
400
500
Reference 4-PA(SAM)
600
Wavelength / nm
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0 300
15
1.2
700
25 2
0 0.0
20 15 10 5 0 800
Integrated JSC / mA/cm
10
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20
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Current density / mA/cm
2
(a)
25 20 15
Reference 4-PA(SAM)
10 5 0 0
Reference 4-PA(SAM)
5
PCE / %
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(c)
Current density / mA/cm2
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10 15
0
100
200
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20 300
400
1.16
2
(d)
600
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Time / s
500
Jsc (mA/cm )
Voc (V)
1.12
24
1.08 1.04
21
1.00 0.96
18 Reference 0.85
FF 0.80
0.70
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0.65
4-PA
EP
0.75
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0.92
Reference
Reference 20
4-PA
PCE (%)
16
12
4-PA
Reference
4-PA
Fig. 6 (a) The J-V curves of SAM-unmodified (Reference) and 4-PA SAM-modified PSCs. (b) IPCE of SAM-unmodified (Reference) and 4-PA SAM-modified PSCs. (c) The steady-state J-t and PCE curves of SAM-unmodified (Reference) and 4-PA SAM-modified PSCs. (d) The statistic parameters (Jsc, Voc, FF and PCE) of SAM-unmodified (Reference) and 4-PA SAM-modified PSCs.
16
ACCEPTED MANUSCRIPT Table 1 The photovoltaic parameters and the corresponding hysteresis index of the SAM-unmodified (Reference) and 4-PA SAM-modified PSCs. Samples
Scanning
Jsc
Voc
Direction
(mA/cm2)
(V)
FF
PCE
a
(%)
Index (HI)
Hysteresis
4-PA
Forward
20.30
0.98
0.59
Reverse
20.55
0.99
0.72
Forward
22.96
1.05
0.75
Reverse
23.15
1.06
0.77
a
11.74
0.31
14.65
18.08
0.03
18.90
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Reference
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[40]
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HI = [JRS(0.8Voc)-JFS(0.8Voc)]/JRS(0.8Voc) [40]; JRS(0.8Voc) and JFS(0.8Voc) represent photocurrent density at 80% of Voc for the RS and FS, respectively.
It is particularly interesting to understand why the 4-PA SAM-modified PSC obtained better PCE of 18.90% with low HI of 0.03, comparing with the PCE of 14.65% (Reference) and HI of 0.31 (Reference) in Table 1. Our results indicate the 4-PA SAM
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is a preferable pathway for fabricating high performance and low hysteresis PSCs. The 4-PA SAM-modified PSC exhibits a higher PCE with negligible hysteresis is due to that both TiO2 and perovskite can bond with 4-PA. The mechanism can be
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explained by two aspects: i) The -COOH group of 4-PA can be chemically grappled
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with mp-TiO2, which facilitates electron transport and reduces the charge accumulation at the interface. ii) The strong coordination between the nitrogen atom on the other end of 4-PA and the Pb2+, which passivates perovskite surface close to TiO2. The strong N-Pb coordination can markedly influence the nucleation of the perovskite layer to produce highly dense films with increased grain sizes. Finally, the device PCE is improved and the hysteresis is suppressive. 2.3 Charge transfer and extraction mechanisms Charge extraction kinetics at the TiO2/perovskite interface is crucial to the device 17
ACCEPTED MANUSCRIPT performance [41-43]. To insight into the effect of 4-PA SAM modifying on electron transfer and extraction at TiO2/perovskite interface, steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements are presented in
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Fig. 7a and 7b, respectively. From Fig. 7a, we can know that the PL quenching for the 4-PA SAM-modified perovskite sample is more efficient, suggesting enhanced electron extraction ability. The TRPL curves are fitted with a biexponential decay
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function: = + / + / . The fitted average PL lifetime (τPL) of
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the glass/perovskite sample is 52.46 ns. The fitted average τPL are 45.54 and 26.10 ns for the SAM-unmodified and 4-PA SAM-modified perovskite samples, respectively. The TRPL results shown in Fig. 7b indicate faster photo-induced electron transfer from the 4-PA SAM-modified MAPbI3 to TiO2, which are because the -COOH group
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is chemically grappled with mp-TiO2. Furthermore, pyridine group of 4-PA can passivate perovskite surface close to TiO2, which facilitates interfacial charge transfer and increases MAPbI3 grain sizes. Ultimately, the Voc of 4-PA SAM-modified PSC is
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enhanced. The 4-PA SAM modification causes a larger Voc by negative shifting of the
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ECB of TiO2, whose result is consistent with that according to Table 1.
18
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(a)
700
800
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Wavelength / nm
PL intensity / a. u.
(b) 10000
Glass/perovskite Reference 4-PA (SAM) Glass/perovskite (Fit) Reference (Fit) 4-PA (SAM, Fit)
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5000
0
900
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600
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Relative intensity
Glass/perovskite Reference 4-PA (SAM)
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50
100
150
Time / ns
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Fig. 7 (a) The steady-state PL of perovskite films based on the glass, SAM-unmodified (Reference) and 4-PA SAM-modified TiO2 substrates, respectively. (b) The time-resolved PL of perovskite films based on the glass, SAM-unmodified (Reference) and 4-PA SAM-modified TiO2 substrates, respectively.
3. Conclusions In this paper, we study the effect of bifunctional 4-picolinic acid (4-PA) 19
ACCEPTED MANUSCRIPT self-assembled monolayer (SAM) as electron transporting layer (ETL)/perovskite interface linker on perovskite crystallinity and ETL/perovskite interface transfer property for high performance perovskite solar cell (PSC). Our results show that the
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4-PA SAM-modified PSC exhibits the highest power conversion efficiency (PCE) of 18.90% and negligible hysteresis index (HI) of 0.03 under illumination of AM 1.5G (100 mW cm-2), compared to PCE = 14.65% and HI = 0.31 of the SAM-unmodified
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PSC. It can be concluded that the 4-PA SAM-modified PSC shows improved
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performance and negligible hysteresis are due to the bifunctional 4-PA SAM can be grappled with the TiO2, increase the grain sizes of MAPbI3, effectively passivate TiO2/perovskite interface, and preferably balance electrons and holes transfer. Our results also indicate the bifunctional SAM is a preferable pathway for further
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fabricating high performance and low hysteresis PSCs.
4.1 Materials
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4. Experimental section
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PTAA (Mol. Weight (Mw)：17500 by GPC), MAI (>99.5%), PbI2 (>99.99%) and TiO2 paste (Dyesol, 18NR-T) were purchased from Xi'an Polymer Light Technology Corp. Lithium bis(trifluoromethylsulphonyl)imide (LiTFSI, >95%), acetonitrile (>99.8%), titanium diisopropoxidebis (acetylacetonate) (75 wt% in isopropanol), 1-butanol (>99.8%), 4-PA (>99%), chlorobenzene (CB, >99.8%), anhydrous N, N-dimethylformamide (DMF, >99.8%), toluene (TL, >99.8%) and tBP (>96%) were purchased from Sigma Aldrich. Unless stated otherwise, all materials were used 20
ACCEPTED MANUSCRIPT without further purification. FTO glasses (Tec 7, Pilkington) were patterned by etching with Zn powder and 2 M HCl. To prepare 1.2 M MAPbI3 precursor solution, 553.2 mg PbI2 and 190.8 mg
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MAI were dissloved in 1.0 mL DMF with stirring. The PTAA solution was prepared by dissolving 15 mg PTAA, 7.5 µL tBP/acetonitrile (v:v, 1:1) and 7.5 µL of a stock solution of 170.0 mg mL-1 LiTFSI in acetonitrile in 1.0 mL TL.
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4.2 Device fabrication
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4.2.1 Substrate preparation
The etched FTO substrates were cleaned with alkaline aqueous washing solution, deionized water, isopropanol, ethanol and finally treated under O3/ultraviolet for 15 min. The deposition of TiO2 followed our previous work [15]. Most SAMs can be
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dissolved in methanol/H2O at a concentration of ~0.5 mg/ml. As everyone knows, the 4-PA is slightly soluble in cold water. The actual solvent of 4-PA SAM is methanol not water. In our manuscript, the mp-TiO2 substrates were immersed in a 0.5 mg/mL
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solution of 4-PA in methanol (Here, 0.3 vol% water is added to slow down the
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evaporation of methanol) for several hours to induce a 4-PA SAM on the surface of mp-TiO2. After that, the mp-TiO2 substrates were rinsed with methanol and ethanol to remove excess water, and then dried under a flow of nitrogen. 4.2.2 Perovskite solar cells fabrication The synthesis of MAPbI3 was carried out by a one-step deposition technique, which was deposited onto bare and 4-PA SAM-modified mp-TiO2 films by spin-coating with 5000 r.p.m. for 30 s. At the remaining spin time of ~23 s, 500.0 µL 21
ACCEPTED MANUSCRIPT CB was quickly added onto the MAPbI3 film. After spin-coating, the substrate was annealed at 100 °C for 10 min. The PTAA solution was deposited on MAPbI3 film by spin-coating at 3000 r.p.m. for 30 s. Devices were then left overnight in a dry box full
~10−6 Torr, at a rate of ~3 Å s-1, to complete the devices. 4.2.3 Device measurement and characterization
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of oxygen. Finally, 60~80 nm of gold was thermally evaporated under vacuum of
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EDX spectroscopy was performed to determine the chemical composition using
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an Oxford X-max LN2-free Silicon Drif Detector, with 80 mm2 of sensor active area and 129 eV of energy resolution at 5.9 k eV (MnKα). SEM was performed using a JEOL JSM-7600F device. The contact angle tests were performed with a standard DSA25 (KRUSS, Germany) Contact Angle Measurement Instrument. The XRD
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analysis of the perovskite films was characterized by a D1 Evolation (JORDAN VALLEY, USA) using Cu-Ka radiation at a coincident scan rate of 6° min-1. Absorption spectra of the MAPbI3 perovskite films on FTO/TiO2 substrates were
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determined with SHIMADZU (model UV1700) ultraviolet to visible (UV-vis)
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spectrophotometer. The UPS were performed with EscaLab 250Xi. The light source of UPS is a HeI discharge lamp (hv = 21.22 eV). PL lifetime measurement was performed on a lifetime spectrometer (FLS 920, Edinburgh Instrument). The monitored wavelength is at about 770 nm. The steady-state PL spectra were measured using an integrated Raman system (Horiba JY HR800) with an Olympus 15X NUV lens (NA=0.32). A 325 nm helium-cadmium (He-Cd) laser was used as the excitation source. The laser power density on the film surface was about 2 W cm-2 and the 22
ACCEPTED MANUSCRIPT wavelength scale was from 500 to 900 nm. The J-V characteristics of the PSCs were measured with a fixed active area of 0.09 cm2 using a CH Instruments 660D electrochemical workstation (Shanghai CH Instruments Co., China) under
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illumination of a simulated sunlight (AM 1.5G, 100 mW cm-2) provided by a short-arc xenon lamp (CHF-XW-500W, Trusttech Co. Ltd., Beijing, China). The scan rate is 0.20 V/s, which is a frequently-used parameter. The measurement of the IPCE was
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obtained by a commercial setup (Qtest Station 2000 IPCE Measurement System,
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CROWNTECH, USA). FTIR spectra were performed on a Nexus 670 FTIR spectrometer in the range 400-4000 cm-1. The first-principal calculations within dispersion-corrected density functional theory1 were performed using the Vienna ab initio simulation package with a projector-augmented-wave2 method. The wave
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function was expanded by plane wave with kinetic cutoff of 400 eV. The generalized gradient approximation with the spin-polarized Perdew-Burke-Ernzerh of function
[19, 47]:
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was used for all calculations [44-46]. The bonding energy was calculated using Eq. (3)
! (3)
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= − −
where Etot is the total energy of the system studied, Esur is the energy of a clean
surface, and Emol is the energy of free 4-PA. The negative bonding energy signifies that the chemisorption structure is stable.
Acknowledgments We thank the National Natural Science Foundation of China (Grant Nos. 23
ACCEPTED MANUSCRIPT 21572030 and 51773027) for financial support.
References
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25
ACCEPTED MANUSCRIPT Bifunctional Electron Transporting Layer/Perovskite Interface Linker for Highly Efficient Perovskite Solar Cells Fei Hana, Guimin Haob, Zhongquan Wana, Junsheng Luoa, Jianxing Xiaa, Chunyang Jiaa,*
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
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a
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China.
Langfang Yanjing Polytechnic College, Langfang 065200, P. R. China.
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b
Highlights
1. Bifunctional 4-picolinic acid interface linker was studied.
2. The 4-picolinic acid optimizing device exhibits the highest PCE of 18.90%. 3. The 4-picolinic acid optimized PSC exhibits negligible hysteresis.
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4. Bifunctional interface linker is a preferable pathway for highly efficient PSCs.