Journal Pre-proof Vapor-assisted deposition of CsPbIBr2 films for highly efficient and stable carbonbased planar perovskite solar cells with superior Voc Xingyue Liu, Junjie Li, Zhiyong Liu, Xianhua Tan, Bo Sun, Shuang Xi, Tielin Shi, Zirong Tang, Guanglan Liao PII:
S0013-4686(19)32138-3
DOI:
https://doi.org/10.1016/j.electacta.2019.135266
Reference:
EA 135266
To appear in:
Electrochimica Acta
Received Date: 23 July 2019 Revised Date:
4 November 2019
Accepted Date: 9 November 2019
Please cite this article as: X. Liu, J. Li, Z. Liu, X. Tan, B. Sun, S. Xi, T. Shi, Z. Tang, G. Liao, Vaporassisted deposition of CsPbIBr2 films for highly efficient and stable carbon-based planar perovskite solar cells with superior Voc, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135266. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Vapor-assisted deposition of CsPbIBr2 films for highly efficient and stable carbon-based planar perovskite solar cells with superior Voc are demonstrated. A champion PCE of 8.76% with an superior Voc of 1.289 V as well as excellent moisture and thermal stabilities are achieved.
Vapor-assisted deposition of CsPbIBr2 films for highly efficient and stable carbon-based planar perovskite solar cells with superior Voc Xingyue Liua, Junjie Lia, Zhiyong Liua, Xianhua Tana, Bo Suna, Shuang Xib, Tielin Shia, Zirong Tanga and Guanglan Liaoa,c* a
State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong
University of Science and Technology, Wuhan 430074, China b
School of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing
210037, China c
Shenzhen Huazhong University of Science and Technology Research Institute, Shenzhen 518057,
China * Address correspondence to (G. Liao)
[email protected].
Abstract: CsPbIBr2 perovskite, as a promising light harvester, possesses the most balanced bandgap and stability characters among all-inorganic perovskite materials. However, the poor quality of the traditionally one-step solution-processed CsPbIBr2 film always leads to a severe recombination loss and thus a low output potential difference (Voc). Herein, we demonstrate a novel vapor-assisted deposition strategy to construct high-quality CsPbIBr2 films for the first time, in which the crystallization kinetics of the CsPbIBr2 is more easily controllable than that of the one-step spin-coated one. The PbBr2 film acting as the template in the CsPbIBr2 crystal growth is firstly prepared via an antisolvent-washing technique and CsI is then vacuum evaporated onto the PbBr2 layer. By precisely tuning the thickness of the CsI film, highly phase-pure and crystallized CsPbIBr2 crystals are successfully obtained. The optimized CsPbIBr2 film also exhibits a
homogeneous morphology and full coverage over the substrate with large grain sizes up to microscale and ultrahigh light absorption capability. The corresponding carbon-based CsPbIBr2 solar cells achieve a champion power conversion efficiency of 8.76% with a superior Voc of 1.289 V. The large-area (1 cm2) devices also deliver an efficiency of 6.78% with an impressive Voc of 1.336 V. Moreover, under the protection of the highly hydrophobic and chemically stable CuPc layer and carbon counter electrode, the unencapsulated devices present excellent moisture and thermal stabilities. Our work provides a new approach for the preparation of cost-effective, highly efficient and robust CsPbIBr2 photovoltaics. Keywords: vapor-assisted; CsPbIBr2; perovskite solar cell; high Voc; highly efficient and stable
1. Introduction Perovskite solar cells (PSC) are considered as promising candidates for the next-generation photovoltaics due to the merits of high efficiency, cost-effectiveness and easy-to-preparation, etc [1-3]. The certified power conversion efficiency (PCE) of PSCs has already reached to as high as 25.2%, surpassing that of the commercial multicrystalline silicon (22.3%), CdTe (22.1%) and CIGS (23.3%) thin-film solar cells [4]. However, the stability issue of the organic-inorganic hybrid perovskites, owing to the existence of volatile and hygroscopic organic moiety (such as FA+, MA+), hinders the commercialization of PSCs [5, 6]. In this case, all-inorganic perovskites with much higher stability (especially thermal stability) than the hybrid counterparts are now becoming a research hotspot in photovoltaic filed. CsPbI3 and CsPbI2Br with high thermal stability and suitable bandgaps of 1.73 and 1.92 eV, respectively, are the most studied materials among all the inorganic perovskites so far. The PCEs of the CsPbI3- and CsPbI2Br-based PSCs have been raised to 17.06% and 16.2%, respectively [7, 8]. It’s discouraging that CsPbI3 and CsPbI2Br would convert into non-perovskite phase quickly when exposed to ambient air [9, 10], hampering their practical applications. CsPbBr3 perovskite has proved to possess the best moisture and thermal stabilities among all the inorganic perovskite materials [11]. But the large bandgap (2.32 eV) of CsPbBr3 impedes its light absorption beyond 535 nm, leading to a low short-circuit current density (Jsc) of the PSCs [12]. CsPbIBr2 with a relatively suitable bandgap of 2.05 eV can stay stable up to 460 oC under ambient condition, exhibiting the most balanced bandgap and stability features among all the inorganic perovskites [10]. Thus, it seems a desirable alternative to organic-inorganic hybrid perovskite light absorbers. Till now, most of the CsPbIBr2 light-harvesting layers in CsPbIBr2 PSCs are deposited by
conventional one-step spin-coating route. There are always large number of pinholes, grain boundaries as well as compositional defects existing in the spin-coated CsPbIBr2 films [10, 13], limiting the improvement of the device performance, especially the Voc. Zhang, et al. created a light-processing technique to ameliorate the crystallization process of the solution-processed CsPbIBr2 [14]. This method could effectively increase the continuity, phase purity and crystallinity of the CsPbIBr2 perovskites. The as-treated devices achieved a champion PCE of 8.6% with a high Voc of 1.283 V. Hao’s group further boosted the efficiency of CsPbIBr2 PSCs to 9.16% by a simple intermolecular exchange strategy [10]. The grain sizes and coverage of the as-fabricated CsPbIBr2 was greatly improved, provoking a high Voc of 1.245 V. Element doping and interface modification were also found favorable for promoting the device performance. Liang, et al. employed Mn doping to enhance both the electronic and morphological properties of the CsPbIBr2 perovskites [15]. The best-performing device with the optimized CsPb0.095Mn0.005I1.01Br1.99 light absorber gained a PCE of 7.36%, showing an increase of 19.9% in PCE compared to the none-doped counterparts. Sn2+ has also been introduced to partially substitute the B-site Pb2+ to promote the efficiency of the CsPbIBr2 devices [16, 17]. Nevertheless, this enhancement was largely originated from the widened light response range, rather than the decreased grain boundaries and defects in the CsPbIBr2 film actually [10]. Let alone that Sn-containing perovskite always suffers from terrific stability when exposed to oxygen and moisture [18]. CsBr and SmBr3 were used by several groups to modify the interface of electron transport layer (ETL)/CsPbIBr2, contributing to the formation of a gradient energy band [19, 20]. The charge transport ability of the devices was pronouncedly enhanced and an excellent PCE of 10.88% was achieved for the SmBr3-modified devices, which was the highest efficiency
for CsPbIBr2 PSCs reported yet. Unfortunately, the SmBr3-treated CsPbIBr2 perovskite was even composed of smaller crystals with more grain boundaries. There still existed a serious energy loss (Eloss) related to many defect states in the CsPbIBr2 film and only a low Voc of 1.17 V was obtained. The expensive Spiro-OMeTAD hole transport layers (HTLs) and noble metal Au electrodes employed in their devices substantially increased the whole production costs. Very recently, Ma’s group adopted SnCl2 solution to adjust the energy band alignment and suppress the recombination process at the SnO2 ETL/CsPbIBr2 interface [21]. Although a high Voc of 1.31 V was announced, the best-performing device merely delivered a PCE of 7.0% with a low Voc of 1.23 V. Compared to the solution-processing route, vapor and spray deposition methods are supposed to be more suitable for the fabrication of homogeneous films in large area [22]. Ho-Baillie’s group have tried preparing CsPbIBr2 films by a co-evaporation process and investigated the impact of the annealing temperature on the film quality [23]. The device only obtained a PCE of 4.7% with a low open-circuit output potential difference (Voc) of 0.959 V due to the unsatisfying quality of the CsPbIBr2 film. The same group then proposed a spray-assisted strategy to construct CsPbIBr2 film, in which the CsI was sprayed onto the spin-coated PbBr2 film [24]. The discontinuous film finally resulted in a poor PCE and Voc of 6.3% and 1.127 V, respectively. Vapor-assisted deposition method, with the advantages of easily controllable and reproducible, has been attempted for the fabrication of high-quality organic-inorganic hybrid perovskite films in the previous studies [25-27]. Yang’s group first employed a vapor-assisted deposition method to fabricate MAPbI3 perovskite [25], where the PbI2 film was firstly spin-coated onto the substrate and the MAI vapor was then reacted with the pre-deposited PbI2. A
highly phase-pure, low-roughness perovskite was obtained and the PSCs got a PCE of 12.1%. Chen. et al, used the same method to prepare (FA)x(MA)1-xPbI3 with high coverage and phase-stability [26]. The corresponding PSCs achieved a high PCE of 16.48%. Liu’s group developed a fast gas-solid reaction technique to construct CH3NH3PbI3(Cl) perovskite, in which the chlorine-incorporated hydrogen lead triiodide was reacted with methylamine gas [28]. Over 1 µm-thick and large-area perovskite film with smooth surface was obtained. The PSC delivered a champion PCE of 20.0% in small area and a PCE of 15.3% for 5×5 cm solar modules. However, this technique has never been introduced to the all-inorganic perovskite system. In this study, we develop a highly controllable and reproducible vapor-assisted deposition strategy to fabricate high-quality CsPbIBr2 perovskite films for the first time. The PbBr2 film acting as the template in the CsPbIBr2 crystal growth is firstly deposited via an antisolvent-washing technique. It is found that the chlorobenzene (CB) washing can effectively increase the coverage of the PbBr2 seed layer. The CsI precursor layer is then thermally evaporated onto the PbBr2 film. By precisely tuning the thickness of the CsI film, highly phase-pure and crystallized CsPbIBr2 crystals are obtained. The as-prepared CsPbIBr2 film presents
a
homogeneous
morphology
and
full
coverage
over
the
substrate
with
vertically-orientated grains. The film also possesses larger grain sizes and much higher light absorption capability than the traditional one-step solution-processed one. The corresponding carbon-based CsPbIBr2 devices achieve a champion PCE of 8.76%. More importantly, the introduction of cost-effective and chemically stable CuPc HTL and carbon counter electrode (CE) can not only decrease the whole costs but also enhanced the device stability. The unencapsulated devices show superior stability when exposed to ambient air and under thermal attack of 60 oC
for one month. Our work provides a feasible and easily controllable approach for the fabrication of high-quality CsPbIBr2 perovskites, which show great application prospects in the photovoltaic and photodetector fields.
2. Experimental section 2.1. Materials Lead (II) bromide (PbBr2, ≥99.99%) and cesium iodide (CsI, ≥99.99%) powders are bought from Xi’an p-OLED. N, N-Dimethylformamide (DMF, ≥99.9%), titanium tetrachloride (TiCl4, ≥99.5%) and copper(II) phthalocyanine (CuPc) are purchased from Aladdin. Nickel(II) nitrate hexahydrate (NiCl2·6H2O, ≥98.0%) powder as Ni source is from Sinopharm Chemical Reagent Co., Ltd. The commercial carbon paste is from Shenzhen Dongdalai Chemical Co., Ltd. All the chemicals and reagents are directly used without any further purification.
2.2. Device fabrication The transparent fluorine-doped tin oxide (FTO) glass (NSG-10) substrates were etched by laser. Then they were sequentially cleaned with detergent, acetone, anhydrous alcohol and deionized water in an ultrasonic bath each for 15 min, followed by an O3/ultraviolet treatment for 20 min. The Ni-doped compact TiO2 (c-TiO2) ETLs were deposited through the hydrolysis of the TiCl4 aqueous solution according to our previous report [12]. All the preparation process were carried out under a low temperature of no more than 200 oC. The CsPbIBr2 perovskite films were fabricated by a facile vapor-assisted deposition method. 1.0 M PbBr2 in DMF solution was heated at 75 oC until it got fully dissolved. The substrates and PbBr2 solution were pre-heated at 75 oC before the PbBr2 film spin-coating process. Then, the PbBr2 precursor was spin-coated on the TiO2 ETLs at a speed of 2000 rpm for 30 s and 120 µL chlorobenzene was quickly dropped onto the substrates at a delay time of about 5 s from the start of the spin-coating process. The PbBr2 films were heated at 90 oC for 1 h after the spin-coating process. The CsI films with different
thickness of 200, 230, 260 nm were vacuum evaporated onto the pre-deposited PbBr2 films under a base pressure of ~9×10-4 Pa at a speed of ~2 Å/s determined by a quartz crystal monitor. The substrates were then annealed at 280 oC for 10 min in N2 atmosphere to get highly crystalline CsPbIBr2 films. The one-step solution-processed CsPbIBr2 films were fabricated according to our previous report [29]. 369 mg PbBr2 and 260 mg CsI were dissolved in 1 ml DMSO and heated at 55 oC until dissolved. Then, the CsPbIBr2 precursor solution was spin-coated onto the substrates at a speed of 1500 rpm for 20 s and 5000 rpm for 60 s, followed by being annealed at 280 oC for 10 min in N2 atmosphere. Thin CuPc hole transport layers (HTLs) with a thickness of ~25 nm were thermally evaporated onto the CsPbIBr2 films at a low speed of ~0.5 Å/s under a pressure of ~1×10-3 Pa. Finally, the carbon counter electrodes were constructed by a doctor-blade technique, in which the commercial carbon paste was bladed on the CuPc HTL, and dried at 100 oC for 20 min.
2.3. Device characterizations The film morphologies and element mapping scan analysis of the PbBr2 and CsPbIBr2 films as well as the cross-sectional images of the CsPbIBr2 perovskites and whole devices were obtained by the scanning electron microscopy (SEM, Sirion 200, FEI, Heland and GeminiSEM 300, Carl Zeiss, German). The X-ray diffraction (XRD) patterns of the CsPbIBr2 perovskites with varied amounts of CsI precursors were measured by an x'pert3 powder X-ray diffractometer (PANalytical, Netherland) and the data were collected with a 0.013° step size (2θ). The absorbance spectra of the CsPbIBr2 films with varied thickness of CsI were gained by a UV-visible spectrophotometer (UV 2600, Shimadzu). The atomic force microscopy (AFM)
images and the corresponding root-mean-square roughness of the CsPbIBr2 perovskites were recorded by an Innova SPM 9700 (Shimadzu, Japan) in tapping mode. The X-ray photoelectron spectroscopy (XPS) measurements of the CsPbIBr2 films were carried out on a photoelectron spectrometer (AXIS-ULTRA DLD-600W, Kratos, Shimadzu, Japan). The steady-state photoluminescence (PL) and time-resolved photoluminescence (TR-PL) decay curves of the CsPbIBr2 samples with varied thickness of CsI were obtained via FluoTime300 (PicoQuant, German) at an excitation wavelength of 485 nm. The current density-applied potential difference (J-V) curves of the CsPbIBr2 cells were recorded by a electrochemical station (Autolab PGSTA302N, Netherlands) at a scan rate of 0.02 V/s under reverse scan direction (if not specially mentioned) and a simulated AM 1.5G illumination (100 mW/cm2) generated by a solar simulator (Oriel 94043A, Newport Corporation, Irvine, CA, USA). The light intensity of the solar simulator was calibrated by a NREL-traceable KG5 filtered silicon reference cell. The active area of the CsPbIBr2 cells is set as 0.071 cm2 in the measurements. The electrochemical impedance spectroscopy (EIS) measurements are performed from 2 MHz to 0.01 Hz at an applied potential difference of 0.8 V under light illumination and the capacitance-applied potential difference measurements were performed under dark condition on the same electrochemical station. The EIS spectra were fitted via a Nova software, from which the resistance of the device can be assessed.
3. Results and Discussion Fig. 1a shows the schematic illustration of the vapor-assisted deposition strategy for the preparation of high-quality CsPbIBr2 perovskites. The PbBr2 seed layer is firstly spin-coated on the low-temperature processed c-TiO2 ETL. The CsI precursor film is subsequently vacuum evaporated onto the PbBr2 layer. Different from the conventional one-step solution-processing route, the vapor-assisted deposition approach avoids a high reaction rate of the precursors and fast crystal growth rate, which usually provokes a nonuniform film morphology. In our two-step method, the pre-deposited PbBr2 layer provides the kinetically favorable nucleation centers and acts as the template for the growth of the CsPbIBr2 crystals [25]. Therefore, the quality of the PbBr2 film has a great impact on the resultant CsPbIBr2 film. During the PbBr2 deposition process, an antisolvent-washing technique is introduced [30], which can greatly increase the coverage of the PbBr2 and the resultant CsPbIBr2 film (Fig. S1). This can be mainly ascribed to the increase of supersaturation of the PbBr2 precursor caused by the fast removal of the DMF solution once the antisolvent is dropped during the spin-coating process [31]. After the CsI is evaporated onto the PbBr2 layer, the intercalation reaction takes place between the two precursors and highly crystallized CsPbIBr2 perovskite is obtained after a post-annealing process. The step-by-step manipulation strategy is much more controllable than the traditional one-step solution-processing route.
Fig. 1. (a) Schematic illustration of the vapor-assisted deposition process of the CsPbIBr2 films in which the PbBr2 layer is firstly spin-coated via an anti-solvent technique and the CsI film is subsequently vacuum evaporated onto the PbBr2 layer. (b) XRD patterns, (c) UV-vis absorption spectra and (d) the plots of (Ahv)2 versus the photon energy (hv) of the as-prepared CsPbIBr2 perovskites with varied thickness of CsI. The black dash line represents the CsPbIBr2 film deposited via traditional one-step spin-coating method.
Apart from the deposition of the PbBr2 layer, another key point of this work is the precise control of the CsI amount. The XRD patterns of the as-fabricated CsPbIBr2 films deposited on FTO substrates with varied thickness of CsI are depicted in Fig. 1b. All the as-formed perovskite films exhibit diffraction peaks at 15.05o, 21.35o, 23.44o, 24.93o, 30.25o, 37.49o, and 43.43o, well corresponding to the (100), (110), (121), (112), (200), (141) and (240) crystal facets of the CsPbIBr2 phase (PDF#32-0223) [10], respectively. For the 200 nm CsI-based CsPbIBr2 perovskites, there also exists apparent peaks at 11.6 o, 17.42o, 27.44o and 28.93o. These peaks can be attributed to the existence of the untreated PbBr2 (PDF#46-0992) [14], suggesting an insufficiency of CsI. The large excess amount of PbBr2 will accelerate the formation of grain boundaries and set barriers for the charge transport [32]. When the thickness of CsI comes to 230
nm, the peaks of CsPbIBr2 get stronger and sharper while the PbBr2 peaks disappear. It signifies the much enhanced phase-purity and crystallinity of the 230 nm CsI-based film. The peak positions of the 230 nm CsI-based CsPbIBr2 film is consistent with the one-step spin-coated counterpart (Fig. S2). Differently, the former film exhibits a more preferential orientation along the (100), (110) and (200) directions, while the later film mainly orientate along the (100) and (200) directions. Further increasing the amount of CsI, the intensity of the CsPbIBr2 peaks get a decrease with the peaks at 12.34o, 28.16o and 29.42o occurring, indicating the generation of δ-CsPbI2Br and α-CsPbI2Br phase (PDF#22-1074). Since the δ-CsPbIBr2 phase has bad electronic and optical properties and α-CsPbIBr2 phase is not stable in air [33], the excess amount of CsI are harmful for the performance and stability of the devices. The UV-vis absorption spectra of the CsPbIBr2 films with varied thickness of CsI are given in Fig. 1c. It appears that the 230 nm CsI-based CsPbIBr2 possesses the highest light absorption coefficient among all the films fabricated by the vapor-assisted deposition process. This is mainly due to the higher crystallinity and lower impurity level of the film [12]. The 230 nm CsI-based CsPbIBr2 film also exhibits a much higher absorption coefficient than the conventionally one-step spin-coated CsPbIBr2 film (represented by the black dash line). Notably, the higher absorption ability of the light absorbers is favorable for the achievement of a higher Jsc and Voc for PSCs [14]. The absorption edge of the 230 nm CsI-based CsPbIBr2 is at approximately 604 nm, corresponding to a bandgap of 2.05 eV (Fig. 1d). This coincides well with the previous report in which the CsPbIBr2 were prepared both by co-evaporation and one-step solution-processing route [10, 14, 23]. The X-ray photoelectron spectroscopy (XPS) survey is performed to evaluate the actual
compositional structure of the 230 nm-based CsPbIBr2 film (Fig. 2a) as calibrated on the basis of C 1s (285.0 eV). The peaks at the binding energies of 738.5 and 724.6 eV in the high-resolution XPS spectrum correspond to Cs 3d3/2 and Cs 3d5/2 (Fig. 2b) while the peaks at 142.6 and 137.8 eV can be well assigned to Pb 4f5/2 and Pb 4f7/2 (Fig. 2c), respectively. The peaks at 630.7 and 619.1 eV belong to I 3d3/2 and I 3d5/2 (Fig. 2d) whilst the fitting peaks at lower binding energies of 69.5 and 68.4 eV relate to Br 3d3/2 and Br 3d5/2 (Fig. 2e). All the peaks are in good agreement with the previously reported binding energy values of the component elements of CsPbIBr2 perovskites [10, 14]. The relative atomic ratio of the constituent elements Cs, Pb, I and Br is 14.08%: 14.46%: 13.76%: 27.93%, approximating to 1:1:1:2. This suggests the fine manipulation of the film components for the vapor-assisted deposition technique and phase-pure CsPbIBr2 are fabricated as expected. The distributions of the constituent elements (Cs, Pb, I, Br) of the 230 nm-CsI based CsPbIBr2 film are investigated by EDS elemental mapping-scan analysis (Fig. 2f-j). The relative atomic ratio of the elements Cs, Pb, I and Br obtained from the EDS analysis is about 15.71 %: 15.24%: 15.68%: 30.88%, also close to 1:1:1:2. Besides, all the elements are homogeneously distributed in the scan window, unveiling the adequate reaction of the PbBr2 and CsI precursors and high uniformity of the resultant CsPbIBr2 film.
Fig. 2. (a-e) XPS survey of the 230 nm CsI-based CsPbIBr2 film and the corresponding high-resolution XPS spectra of the Cs 3d, Pb 4f, I 3d and Br 3d peaks. (f-j) The components (Cs, Pb, I, Br) distribution of the 230
nm CsI-based CsPbIBr2 film obtained by the EDS mapping-scan analysis.
The morphology evolutions of the CsPbIBr2 films with different thickness of evaporated CsI are studied by scanning electron microscopy (SEM) as presented in Fig. 3a-f. Apparently, all the films prepared by our method exhibit a high coverage over the substrates. For the 200 nm CsI-based CsPbIBr2 film, the majority grain sizes of the perovskite are less than 1 µm (Fig. 3a). There also lies many tiny pinholes and grain boundaries in the film. Note that the pinholes in the perovskite always serve as the current-shunting pathways, leading to a severe current leakage and recombination loss [34, 35]. The cross-sectional SEM image of the film (Fig. 3d) further reveals the existence of many small crystals within the bulk perovskite, yielding massive internal grain boundaries. The 230 nm CsI-based phase-pure CsPbIBr2 film is composed of many larger crystals with grain sizes over 1 µm. The vertically-orientated grains (Fig. 3e) enables the photogenerated charge carriers to transfer out of the CsPbIBr2 perovskite without passing through any grain boundaries [36]. By contrast, the grain sizes of the one-step spin-coated CsPbIBr2 perovskite are much smaller with numerous pinholes and grain boundaries (Fig. S3). It has been reported that there are many surface impurities and trap states lying in the grain boundaries, serving as the recombination sites [37, 38]. Thus a more serious charge carrier recombination loss may occur in the one-step spin-coated films. The grain boundaries-induced shallow states near the valence band edge will also hamper the hole diffusion [9], unbeneficial for the charge extraction process. When 260 nm CsI is evaporated onto the PbBr2 layer, the film morphology get much worse. It may be due to the generation of the unstable δ-CsPbI2Br and α-CsPbI2Br phase caused by the excess amount of CsI precursor. This phenomenon is also witnessed in the CsPbBr3 film with excessive CsBr [12].
Atomic force microscopy (AFM) images displayed in Fig. 3g-i shows that the 200 nm CsI-based CsPbIBr2 film exhibits a smooth surface with a low root-mean-square roughness (RMS) of 43.65 nm. The slightly higher RMS of the 230 nm CsI-based CsPbIBr2 may be caused by the larger grain sizes of the perovskite crystals. The 260 nm CsI-based film gets the roughest surface with a high RMS of 71.36 nm, which is attributed to the formation of many irregular-arranged small crystals and unfavorable for the formation of a close contact between the HTL and CsPbIBr2.
Fig. 3. (a-c) Top-view SEM images, (d-f) cross-sectional SEM images and (g-i) AFM images of the as-prepared CsPbIBr2 films based on 200, 230 and 260 nm CsI, respectively.
Furthermore, the photoluminescence (PL) decay measurements are performed to get an
insight into the charge transport dynamics and carrier lifetimes of the CsPbIBr2 perovskites [39]. During the examinations, the photoinduced carriers cannot be extracted out immediately because no charge transport layer is inserted between the CsPbIBr2 samples and FTO substrates. Thus the PL quenching is considered to be closely related to the trap-assisted radiative recombination behavior. A faster PL quenching is usually linked to more trap states in the perovskites [40, 41]. The steady-state PL characteristics (Fig. 4a) shows that the 230 nm CsI-based CsPbIBr2 film exhibits the weakest PL quenching, indicating the lowest defect density. This can be mainly ascribed to the lower impurity level and less grain boundaries of the 230 nm CsI-based sample than that of the other two counterparts. The emission peak position of the film lies at ~614 nm, in good line with the previous report [10]. The 260 nm CsI-based film gets the strongest PL quenching caused by the fastest recombination rates. This may originate from the poor film quality with massive grain boundaries as observed from the SEM image (Fig. 3c). The corresponding time-resolved PL (TR-PL) spectra of the same samples are given in Fig. 4b. The defects at the grain boundaries can greatly influence the intralayer charge transfer process within the bulk perovskites [42]. A longer carrier lifetime represents a slower intralayer carrier recombination rate. The PL curves can be suitably fitted by a bi-exponential decay function [43]: f(t)= A1exp(-t/τ1) + A2exp(-t/τ1)+B
(1)
where τ1 and τ2 are the slow and fast decay time constants, A1 and A2 are the corresponding fractional amplitude of τ1 and τ2 while B is a constant for the base-line offset. The slow decay process corresponds to the trap-assisted radiative recombination in the bulk CsPbIBr2 phase whilst the fast decay process is supposed to be relevant to the quenching process of the free carriers at the interface, respectively [36, 44]. The lifetime parameters derived from the fitted
TR-PL curves of these samples are summarized in Table 1. The 230 nm CsI-based CsPbIBr2 perovskite possesses the longest carrier lifetime of 16.58 ns with a slow decay time τ1 and fast decay time τ2 of about 18.35 and 6.25 ns, respectively. This indicates a more suppressed recombination rate within the film compared to the other two samples, consistent with the weakest PL quenching observed in Fig. 4a. The much elongated carrier lifetime and decreased recombination loss contribute to a higher Jsc and Voc for the 230 nm CsI-based devices. The shortest average carrier lifetime of the 260 nm-CsI-based CsPbIBr2 samples (3.91 ns) may result from the most surface and bulk defects existing in the grain boundaries. Table 1 Lifetime parameters derived from the TR-PL spectroscopy of the CsPbIBr2 perovskites with different thickness of CsI. Thickness of CsI (nm)
τave [ns]
τ1 [ns]
A1
τ2 [ns]
A2
200 230 260
9.57 16.58 3.91
15.15 18.35 8.12
26.7% 85.4% 25.8%
7.54 6.25 2.44
73.3% 14.6% 74.2%
Fig. 4. (a) Steady-state PL and (b) TR-PL spectra of the CsPbIBr2 films with different thickness of CsI deposited on FTO substrates. (c) The cross-sectional SEM image and (d) energy diagram of the whole device with a structure of FTO/c-TiO2/CsPbIBr2/CuPc/Carbon.
Fig. 4c depicts the cross-sectional SEM image of the complete carbon-based planar PSC with a simple configuration of FTO/c-TiO2/CsPbIBr2/CuPc/carbon. The Ni-doped c-TiO2 ETL is fabricated via the hydrolysis process of the TiCl4 aqueous solution at low temperature according to our previous report [34]. The thickness of the TiO2 ETL and CsPbIBr2 perovskite layer is around 100 nm and 470 nm, whilst the CuPc HTL is too thin to be observed. It is noteworthy that the thickness of the CsPbIBr2 prepared by the vapor-assisted deposition method is higher than that of the traditionally one-step solution-processed one [21]. Since polycrystalline perovskites have a long carrier diffusion length of over 1 µm [45], the relatively thicker light absorber (within the carrier diffusion length) allows for a higher light absorption ability and thus a higher Jsc to
some extent. This is verified by the results of the UV-vis absorption measurements (Fig. 1c). The band alignment diagram of the whole device is drawn in Fig. 4d. The well matched energy levels of the components enable the effective transport of the photocarriers generated in the CsPbIBr2 absorber to the ETL and HTL, respectively. Table 2 Key J-V parameters of the PSCs based on varied thickness of CsI. Device 200 nm 230 nm 260 nm
average champion average champion average champion
Jsc (mA cm-2)
Voc (V)
FF
PCE (%)
9.36±0.54 9.65 10.03±0.47 10.40 8.72±0.61 9.24
1.202±0.034 1.236 1.258±0.030 1.289 1.162±0.036 1.193
0.609±0.027 0.631 0.638±0.024 0.653 0.573±0.035 0.602
6.86±0.65 7.53 8.06±0.056 8.76 5.81±0.072 6.63
The current density-applied potential difference (J-V) parameters of the as-prepared PSCs recorded under AM 1.5G (100 mW/cm2) is listed in Table 2. These data are extracted from 25 samples for each kind of devices with 200, 230, 260 nm CsI, respectively. The 230 nm CsI-based devices exhibit a much higher average PCE of 8.06% than that of the 200 nm CsI-based (6.86%) and 260 nm CsI-based (5.81%) ones. Notably, the average Voc of the 230 nm CsI-based devices reaches to a very high level of 1.258 V. To assess the reproducibility more intuitively, the box charts of the Jsc, Voc, fill factor (FF) and PCE of these PSCs are plotted in Fig. 5a-d. It is clear that all the key photovoltaic parameters of the 230 nm CsI-based device are higher than those of the other two counterparts. This can be mainly attributed to the augmented light absorption and phase-purity as well as decreased current-shunting pathways and trap-assisted recombination loss in the 230 nm CsI-based CsPbIBr2 film as discussed above. Besides, the narrow PCE distribution of the PSCs manifests the high reproducibility of our vapor-assisted deposition technique. Fig. 5e displays the J-V characteristics of the best-performing devices with varied thickness of CsI. The 230 nm CsI-based device outperforms the PbBr2-rich and CsBr-rich counterparts,
demonstrating a high PCE of 8.76% with a Jsc of 10.40 mA cm-2, a Voc of 1.289 V, and a FF of 0.653. To the best of knowledge, 1.289 V is one of the best Voc achieved by CsPbIBr2 solar cells according to the summary in recent work [21]. As a comparison, the device based on one-step spin-coated CsPbIBr2 film only gets a champion PCE of 7.27%, with a Jsc of 9.27 mA cm-2, a Voc of 1.133 V, and a FF of 0.692 (Fig. S4a). The much declined Jsc and Voc value mainly originates from the poor light absorption capability and large Eloss caused by the poor quality of the spin-coated CsPbIBr2 film. The hysteresis behaviors of the PSCs based on the one-step spin-coated and vapor-assisted CsPbIBr2 films are also studied as show in Fig. S4. The degree of hysteresis can be assessed by the hysteresis index (HI) [36]: HI =
ୖܬୗ ሺ0.8ܸ୭ୡ ሻ − ܬୗ ሺ0.8ܸ୭ୡ ሻ ୖܬୗ ሺ0.8ܸ୭ୡ ሻ
(2)
in which JRS(0.8Voc) and JFS(0.8Voc) represent the current density at an applied potential difference of 80% Voc for the reverse and forward scan, respectively. The calculated HI for the PSCs based on the spin-coated and 230 nm CsI-based CsPbIBr2 films is about 0.112 and 0.097. The smaller HI of the 230 nm CsI-based devices indicates a more balanced charge transport. This further embodies the superiority of our vapor-assisted deposition technique compared to the traditional one. The steady-state photocurrent and PCE outputs recorded under the maximum power (MMP) point of the devices with different thickness of CsI are given in Fig. 5f. The PbBr2-rich cell obtains a steady-state current output of 6.82 mA cm-2 under an applied potential difference of 0.98 V, yielding a PCE output of 6.68%. The best-performing 230 nm CsI-based device delivers a steady-state current of 7.96 mA cm-2 in parallel with a PCE of 8.12%, which is close to the result of the J-V measurement. The steady-state performance of the device shows no degradation at
MMP point for over 400 s, exhibiting a good operational stability. The CsI-rich (260 nm) device gains the lowest steady-state current and PCE output of 6.46 mA cm-2 and 5.81%, respectively, under a low applied potential difference of 0.9 V. Since the evaporation techniques are suitable for preparing homogeneous films in large area (Fig. S5), devices with an active area of 1cm2 are also constructed. The large-area device demonstrates a champion PCE of 6.78% with a Jsc of 8.38 mA cm-2, a very impressive Voc of 1.336 V, and a FF of 0.606 (Fig. 5g). As far as we know, this is the first report about large-area CsPbIBr2 solar cell and the PCE of 6.78% is comparable to or even better than that of many CsPbI2Br devices with active areas of no more than 0.1 cm2 [13, 15, 21, 23, 24, 46]. The stability of PSCs in working condition is still a widely concerned issue before their practical applications. The durability tests of the optimized 230 nm CsI-based devices without encapsulation are conducted both in ambient air and upon thermal attack of 60 oC for one month. The device retains about 98.8% of its initial PCE when exposed to ambient condition with a humidity of ~40% for one month (Fig. 5h). The excellent air stability can mainly ascribed to the protection effect of the highly hydrophobic and chemically stable CuPc HTL and carbon CE, which prevent the CsPbIBr2 perovskite from damage caused by moisture and oxygen in air. Compared to the widely used organic small-molecule or polymeric hole conductor (Spiro-OMeTAD, PTAA, P3HT, etc), there is no any deliquescent and volatile dopants (such as Li-TFSI) adopted in the CuPc HTL [47, 48]. After being heated at 60 oC in air for one month, our device still maintains ~94.5% of its initial PCE. It has been reported that CsPbIBr2 itself can stay stable up to its melting point over 460 oC under ambient atmosphere [10, 16], comparable to CsPbBr3 perovskites [12]. Besides, the halogen ions migrated from the perovskites, which can
react with noble metals (Au, Ag), won’t react with the carbon electrode. These all contribute to a high thermal stability for our devices. Moreover, the CuPc and commercial carbon materials used in the device have great price advantages over the expensive polymeric hole conductor and noble metal materials, which is favorable for lowing down the whole production costs.
Fig. 5. Box-charts of the (a) Jsc, (b) Voc, (c) FF and (d) PCE of the devices with varied thickness of CsI. (e) J-V characteristics and (f) steady-state photocurrent and PCE output of the best-performing PSCs based on 200, 230 and 260 nm CsI, respectively. (g) J-V characteristics of the best-performing large-area (1cm2) device based on
230 nm CsI. (h) PCE variations of the 230 nm CsI-based device when stored in ambient air with a relative humidity of ~40% and under persistent thermal attack at 60 oC for one month.
More characterizations are further carried out to shed light on the impact of the compositional structure on the device performance. Fig. 6a illustrates the dark J-V characteristics of the PSCs with varied thickness of CsI. It appears that the pure CsPbIBr2 (with 230 nm CsI)-based device possesses a smaller leakage current than the PbBr2-rich and CsI-rich devices. This can be mainly ascribed to the less pinholes and grain boundaries serving as shunting pathways in the 230 nm CsI-based CsPbIBr2 films. The lower dark current conduces to a suppressed carrier recombination and thus a higher Jsc and FF [19]. According to the intercept of the linear portion of the dark J-V curve to the x-axis, the Voc of a PSC can be approximately estimated [39]. The Voc values evaluated from the dark J-V curves (Fig. S6) of the 200, 230, and 260 nm CsI-based devices are 1.24, 1.30 and 1.20 V, consistent to the results of the J-V measurements taken under one sunlight illumination. This manifests the 230 nm CsI-based PSC possesses a higher intrinsic output potential difference. Fig. 6b presents the electrochemical impedance spectra (EIS) of the PSCs based on different thickness of CsI, from which the interfacial charge transport and recombination dynamics of them can be investigated. These measurements are performed from 2 MHz to 0.01 Hz at an applied potential difference of 0.8 V under light illumination. Obviously, there are two semicircles in each Nyquist plot. The small semicircle at high frequency is closely related to the hole transport process at the CuPc/CsPbIBr2 interface, reflecting a charge transfer resistance (Rct) in parallel with a HTM capacitance (CPE1). The CPE1 is usually considered as a space charge capacitance or ionic capacitance in HTM [49]. The large semicircle at low frequency reveals a recombination process at the TiO2 ETL/CsPbIBr2 interface, which can be expressed by a recombination
resistance (Rrec) in parallel with a chemical capacitance (CPE2) [49]. The simplified equivalent circuit is drawn in the inset of Fig. 6b. The 230 nm CsI-based device exhibits a much smaller Rct of 46.3 Ω than the 200 nm CsI-based (70.6 Ω) and 260 nm CsI-based (88.5 Ω) counterparts. The decreased Rct reveals a more effective charge extraction and collection process in the 230 nm CsI-based device, leading to a higher Jsc. The Rrec of the 230 nm CsI-based PSC is around 344 Ω, higher than that of the PbBr2-rich (291 Ω) and CsI-rich (212 Ω) devices, contributing to a lower recombination rate and higher Voc. The series resistance (Rs) derived from the starting point in the real part of the Nyquist plot is 42.5, 38.8 and 49.7 Ω for the PSCs with 200, 230 and 260 nm CsI, respectively. The lowest Rs of the 230 nm CsI-based device may originates from the declined leakage current and is beneficial to the achievement of a higher FF. In addition, the smaller CPE1 value of the 230 nm CsI-based PSC (44.2 nF/cm2) than that of the PbBr2-rich (67.4 nF/cm2) and CsI-rich (84.4 nF/cm2) devices certifies a suppressed interfacial charge accumulation behavior in the phase-pure CsPbIBr2-based device.
Fig. 6. (a) Dark J-V characteristics of the PSCs with varied thickness of CsI. (b) Nyquist plots of the PSCs based on varied thickness of CsI measured under light illumination, with the simplified equivalent circuit shown in the inset. (c) Voc variations of the PSCs based on 200, 230 and 260 nm CsI recorded under changed light intensities. (d) Mott-Schottky plots under varied applied potential difference extracted from the impedance analysis of the cells based on different thickness of CsI.
The recombination process of the photogenerated carriers in PSCs can also be revealed by the dependence of the open-circuit output potential difference Vλ on the intensity of the incident light Pλ [50]. At Vλ, the current output is 0, suggesting no flow of current. Thus all the photocarriers are considered to recombine in the perovskite film. When the Pλ is larger than 0.1 sun (10 mW/cm2), a reduction in Vλ is related to the interfacial recombination loss. The Voc of PSC usually decreases rapidly when Pλ is less than 0.1 sun, due to the current shunting and trap-assisted recombination [51]. The relationship between the Vλ and Pλ can be described by the following equation [52]:
Vλ= Voc +
ా ்
In
ഊ
(3)
౩
where Voc is the open-circuit output potential difference recorded under one sunlight illumination, n is the ideality factor, kB and T are the Bolzman constant and absolute temperature, q is the elementary charge whilst Ps is the intensity of one sunlight (100 mW/cm2). The interfacial charge recombination process in the PSC can be assessed by n, which can be calculated by the following equation according to eqn. 3: ݊=
ݍ dܸ ݇ ܶ dሺInܲఒ ሻ
(4)
The as-calculated ideality factor of the 200, 230 and 260 nm CsI-based device is 1.42, 1.25 and 1.53 (Fig. 6c). In general, a higher n value means a more severe trap-assisted recombination behavior in the device. When the n is approaching to 1, a free carrier recombination is dominant in the device [52]. The smaller ideality factor of the 230 nm CsI-based PSC than that of the PbBr2-rich and CsI-rich counterparts manifests a lower trap-assisted recombination loss in the device. Furthermore, capacitance-applied potential difference measurements are conducted for the same devices under dark conditions and the results are shown in Fig. 6d. The relationship between the capacitance and applied potential difference can be summarized by the Mott-Schottky equation [20]: ଵ మ
ଶ
= ఌఌ
బ ேీ
ሺܸୠ୧ − V −
் ୯
)
(5)
where C is the capacitance of the space charge region, ε and ε0 represent the dielectric constant of the perovskite and the vacuum permittivity, V and Vbi equal to the applied potential difference and the built-in potential, respectively. Thus, the Vbi can be extracted from the intercept of the linear portion of the Mott-Schottky plots with the x-axis. The built-in potential of the 230 nm CsI-based device is located at 1.34 V, much higher than that of the other two counterparts (1.21 and 1.25 V).
This can be mainly attributed to the decreased defects in the bulk CsPbIBr2 perovskite. The larger built-in potential can not only facilitate the dissociation of the photogenerated carriers by providing a stronger driven force but also form an extended depletion region to suppress the recombination efficiently [53]. Therefore, the phase-pure CsPbIBr2-based devices are supposed to deliver a higher output potential difference.
4. Conclusions In summary, we demonstrate a novel and facile vapor-assisted deposition strategy to fabricate high-quality CsPbIBr2 films. A highly-covered PbBr2 film acting as the template in the CsPbIBr2 crystal growth is firstly deposited via an antisolvent-washing technique and the CsI precursor layer is then thermally evaporated onto the PbBr2 film. The step-by-step manipulation strategy is much more controllable and reproducible than the conventional one-step solution-processing route. Tuning the thickness of the CsI precursor to an optimized value of 230 nm, highly phase-pure and crystallized CsPbIBr2 crystals are obtained. The as-prepared CsPbIBr2 film presents a homogeneous morphology and full coverage over the substrate with vertically-orientated grains. It also possesses larger grain sizes (over 1 µm) and much higher light absorption capability than the traditional one-step solution-processed CsPbIBr2 film. The corresponding carbon-based CsPbIBr2 devices achieve a champion PCE up to 8.76% with a Jsc of 10.40 mA cm-2, a superior Voc of 1.289 V, and a FF of 0.653. The devices with a large active area of 1 cm2 also deliver a PCE of 6.78% with a prominent Voc of 1.336 V. More importantly, under the protection of the highly hydrophobic and chemically stable CuPc HTL and CE, the unencapsulated devices show excellent stability when exposed to ambient air and under thermal attack of 60 oC for one month. Our work provides a feasible and easily controllable approach for the fabrication of cost-effective, highly efficient and stable CsPbIBr2 photovoltaics.
Conflicts of interest There are no conflicts of interest to declare.
Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant nos. 51675210, 51675209 and 51805195), the China Postdoctoral Science Foundation (Grant no. 2018M640691) and Fund from Science, Technology and Innovation Commission of Shenzhen Municipality (Grant no. JCYJ20170818165724025). We also appreciate Flexible Electronics Research Center and the Analytical and Testing Center of Huazhong University of Science and Technology for the SEM, AFM, XRD and XPS measurements.
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Highlights (1)
A novel and facile vapor-assisted deposition method is demonstrated for the fabrication of high-quality CsPbIBr2 perovskites for the first time.
(2)
Highly phase-purity and crystallized CsPbIBr2 film with large grain sizes and ultrahigh light absorption capability is obtained.
(3)
A high PCE of 8.76% with a superior Voc of 1.289 V is achieved.
(4)
A PCE of 6.78% with an impressive Voc of 1.336 V is gained for the large-area (1 cm2) devices.
(5)
Excellent moisture and thermal stabilities are obtained for our devices.
Declaration of Interest Statement There are no conflicts of interest to declare.