Ambient Engineering for High-Performance OrganicâInorganic

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Ambient Engineering for High Performance Organic-Inorganic Perovskite Hybrid Solar Cells Jiabin Huang, Xuegong Yu, Jiangsheng Xie, Dikai Xu, Zeguo Tang, Can Cui, and Deren Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06682 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Ambient Engineering for High Performance Organic-Inorganic Perovskite Hybrid Solar Cells Jiabin Huang, Xuegong Yu, *,† Jiangsheng Xie, † Dikai Xu, † Zeguo Tang, *,‡ Can Cui, § †

and Deren Yang† †

State Key Laboratory of Silicon Materials and School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China. ‡ Ritsumeikan Global Innovation Research Organization, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan § Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China

ABSTRACT Considering the evaporation of solvents during fabrication of perovskite films, the organic ambience will present a significant influence on the morphologies and properties of perovskite films. To clarify this issue, various ambiences of N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and chlorobenzene (CBZ) are introduced during fabrication of perovskite films by two-step sequential deposition method. The results reveal that CBZ ambience is favorable to control the nucleation and growth of CH3NH3PbI3 grains while the others present negative effect. The statistical results show that the average efficiencies of perovskite solar cells processed under CBZ ambience can be significantly improved by a relatively average value of 35% comparison with those processed under air. The efficiency of the best perovskite solar cells can be improved from 10.65% to 14.55% by introducing this ambience engineering technology. The CH3NH3PbI3 film with large-size grains produced in CBZ ambience can effectively reduce the density of grain boundaries, and then the recombination centers for photo-induced carriers. Therefore, a higher short-circuit current density is achieved, which makes main contribution to the efficiency improvement. These results provide vital progress towards the understanding of the role of ambience in the realization of highly efficient perovskite solar cells.

KEYWORDS: perovskite solar cells, ambience, morphology, grain growth, reproducibility 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 20

1. INTRODUCTION Organic-inorganic hybrid perovskite solar cells based on CH3NH3PbI3 have drew enormous attention since the perovskite layer has advantages of high light absorption coefficient,

1, 2

excellent bipolar carrier transport properties,

fabrication process and low non-radiative carrier recombination rates.

5

3, 4

facile

The first

report on the perovskite solar cell has demonstrated an efficiency of 3.8% in 2009, which utilized the organic lead halides as sensitizers in liquid electrolyte. 6 Afterwards, many efforts have been paid on the efficiency improvement of perovskite solar cells, and notable progress has been achieved in the last few years. By nowadays, the highest certified power conversion efficiency (PCE) of perovskite solar cells has already reached to more than 20% by Seok’s group. 7 This achievement shed light on the practical application of perovskite solar cells, which could compete with the position of silicon solar cells in photovoltaic industry. For the fabrication of high-performance perovskite solar cells, the formation of a uniform and pinhole-free perovskite layer is one of the key issues.

8

Various

approaches have been developed to control the properties of perovskite layers and consequent device performances. 9-14 Among them, the solution process based on a simple spin-coating technique has received intensive interest due to its low cost, which is classified into one-step and two-step sequential deposition. One-step spin-coating technique using the mixed solution of lead halides (PbCl2 or PbI2) and CH3NH3I could prepare a highly oriented and smooth perovskite film with large size domains, achieving an efficiency of more than 19% for the solar cells. 15-17 Meanwhile, two-step sequential deposition technique, which is based on the reaction of CH3NH3I with the spin-coated PbI2 layer to form a CH3NH3PbI3 film, has been explored as an effective approach to achieve high efficiency solar cells. 10, 18-22 By applying some strategies, such as pre-heating the substrate, 21 retarding the crystallization of PbI2, 22 applying additive into precursor solution, 23-24 the photovoltaic performance can be effectively increased. Consequently, high efficiency of 20.1% for the perovskite solar cells has been obtained by the direct intramolecular exchange of DMSO molecules 2


Page 3 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intercalated in PbI2 with formamidinium iodide. 7 However, ether one-step or two-step sequential deposition, the reproducibility is one of challenging questions. Especially, the cell performance exhibits significant fluctuation of solar cells prepared at different batches even all the processes are conducted in the same glove box.

15

Many

literatures have mentioned that the atmosphere presented considerable influence on the formation of perovskite films and the eventual efficiency, 25-27 but there lack of systematic study on the influence of atmosphere upon perovskite films and related performance of devices. Rira Kang et al. stated that the varied evaporation rate of organic solvents is one of the reasons accounting for the poor performance.28 Considering the evaporation of organic solvents during fabrication processes of solar cells, the ambience in the glove box will be influenced by the organic solvents, and then the quality of perovskite films as well as the photovoltaic performance. In this study, we concentrate on the influence of ambience on the properties of perovskite films and photovoltaic performance based on a two-step sequential deposition method. The perovskite solar cells are prepared in various ambiences such as air (humidity around 35% ± 5%), CBZ, DMF and DMSO. When the whole process of solar cell fabrication is performed in the CBZ ambience, the uniform perovskite films with larger grain size of crystals are achieved compared to those in the other ambiences. The growth processes of perovskite grains in CBZ are discussed and a reasonable explanation is proposed. This ambience engineering technology enables a fully solution-processed perovskite solar cell with an efficiency of 14.55%.

2. EXPERIMENTAL DETAILS 2.1 Materials synthesis. CH3NH3I was synthesized via the reaction of 30 ml CH3NH2 (40 wt% in methanol, TCI) with 32.3 ml HI (57 wt% in water, Aldrich) in a round bottom flask at a rotation speed of 300 rpm for 2 hours in an ice bath. The residual solvents of water and methanol were evaporated using a rotary evaporator at 50 ℃. After washing and recrystallization by mixed solvents of diethyl ether and ethanol, the precipitated CH3NH3I was collected. Finally, the CH3NH3I crystals were dried in a vacuum oven at 60 ℃ for 24 hours. 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 20

2.2 Solar cell fabrication. Solar cells were fabricated under indoor humidity (about 35%±5%). FTO glass (NSG, 8 ohm/square ) was cleaned with deionized water in ultrasonic cleaner for 10 min, followed by washing with detergent solution, acetone and ethanol for 10 min, respectively. The FTO substrates were treated in a UV-Ozone cleaner for 30 min. About 50 nm thick compact TiO2 was formed on a cleaned FTO glass by spray pyrolysis at 550 ℃ using 0.25M Ti (IV) bis (ethylacetoacetal)-diisopropoxide in ethanol solution. After cooling to room temperature, the TiO2 paste was spin-coated on the compact layer at 5000 r.p.m. for 30 s, where the pristine paste was diluted in ethanol (with mass ratio of 1:4). After drying at 130 ℃ for 5 min, the film was annealed at 550 ℃ for 15 min. CH3NH3PbI3 was formed by two-step sequential method. PbI2 solution was prepared by dissolving 553 mg PbI2 (98.5%, Alfa-Aesar) in 1 ml DMF (99.8%, Alfa-Aesar) under stirring at 70 ℃. PbI2 solution (60 µl) was spin-coated on the mesoporous TiO2 film at 3,000 r.p.m. for 5 s and then 6,000 r.p.m. for 5 s. After spinning, the film was heated at 70 ℃ for 5 min and then 100 ℃ for 5 min, and after cooling to room temperature, 200 µl of 0.038 M (6 mg.ml-1) CH3NH3I solution in 2-propanol (99.9%, Sigma-Aldrich) was loaded on the PbI2-coated substrate with desired reaction time after introducing corresponding organic gases (DMF, DMSO and CBZ) into the chamber of spin coater, which was spun at 4,000 r.p.m. for 30 s and annealed at 100 ℃ for 5 min. The Chamber is cylinder-like, with a diameter of 25 cm and height of 6 cm. The pressure of DMF, DMSO and CBZ are all equal to one atmosphere (P=101.325 kPa). For each sample, the consumption volumes of DMF, DMSO and CBZ are about 1.7, 1.4 and 1.0 ml, respectively. Afterwards, a volume of 40 µl of Spiro-OMeTAD solution was spin-coated on the CH3NH3PbI3 perovskite layer at 4000 r.p.m. for 30 s. A Spiro-OMeTAD solution was prepared by dissolving 72.25 mg of Spiro-OMeTAD in 1 mL of CBZ, to which 28.75 µl of 4-tert-butyl pyridine and 17.5 µl of lithium bis (trifluoromethanesulfonyl) imide (Li-TFSI) solution (520 mg Li-TSFI in 1 m acetonitrile, Sigma-Aldrich, 99.8%) were added.

Finally,

100

nm

of

gold

was

thermally

4


evaporated

on

the

Page 5 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Spiro-OMeTAD-coated film.

2.3 Device characterization. The photocurrent density and voltage curves were recorded using a Keithley 2400 source meter under AM 1.5 G one-sun illumination (100 mW/cm2) provided by a solar simulator (94022A, Newport®). The light intensity was calibrated using a standard Si solar cell (PVM937, Newport®). While measuring current and voltage, the cell was covered with a black mask with an area of 0.1256 cm2, and the step widths was fixed at 20 mV with delay time of 40 ms. The external quantum efficiency (EQE) of solar cells was measured by an EQE measurement system (QEX10, PV Measurements, Inc.) across a wavelength range of 300-800 nm. Optical absorbance spectra of perovskite films on glass were measured using a UV-vis-NIR spectrophotometer (U-4100, Hitachi, Japan) in the wavelength range of 300-850 nm. The crystal structures of the PbI2 and CH3NH3PbI3 formed on the mesoporous-TiO2 films were investigated using an X-ray diffraction measurement (PANalytical, Utrecht, The Netherlands) with Cu Kα radiation (λ=1.5406 Å) at 40 kV and 40 mA. Morphologies were investigated using a field emission scanning electron microscope (FE-SEM HITACH S4800). Photoluminescence (PL) spectra were measured on glass using a Fluorescence Spectrophotometer (FLS920, Edinburgh Instruments), where the excited wavelength was 405 nm and detected in the wavelength range of 700 to 850 nm with 10 nm increment. Time-resolved PL measurements were photoexcited using a 405 nm laser head.

3. RESULTS AND DISCUSSION Figure 1 presents a schematic diagram of the cell architecture and deposition of CH3NH3PbI3 perovskite materials by the ambience engineering technology using spin-coating.

The

conventional

mesoporous

solar cells

with structure

of

FTO/block-TiO2/mesoporous-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au are prepared, as shown in Figure 1a. On the top of the mesoporous TiO2 layer is a perovskite active layer with a thickness of ~350 nm. Spiro-OMeTAD and Au metal were subsequently deposited on the active layer as a hole-transporting material and electrode, respectively. For high efficiency solar cells, the formation of a homogeneous 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 20

perovskite films with large grain size is extremely important, and we develop an ambience engineering technology as an effective approach to prepare such a layer. The processes involve four stages, as illustrated in Figure 1b. First, PbI2 films are fabricated by spin coating on the mesoporous TiO2 substrate, followed by a thermal annealing treatment at 70 ℃ for 5 min and 100 ℃ for 5 min. Second, CH3NH3I solution in 2-proponal is dripped onto the dried PbI2 film in a sealed spin coater with full of organic solvents gases (CBZ, DMF and DMSO) for a desired reaction time. Third, all constituents were frozen to a uniform layer on the removal of the residual 2-proponal by spin-coating. Finally, the highly quality perovskite crystalline of CH3NH3PbI3 were formed by annealing at 100 ℃ for 5 min. This organic solvents ambience can significantly affect the morphologies of the perovskite thin films, compared with that prepared in indoor atmosphere (humidity around 35%±5%). The morphologies of perovskite films prepared under relevant atmospheres are investigated via the measurement of scanning electron microscope (SEM) image. Figure 2 shows the corresponding surface SEM images for perovskite films prepared under different atmosphere (air, CBZ, DMF and DMSO) with reaction time of 60 s. Evidently, the morphologies of the perovskite thin layers are markedly affected by organic solvents ambience engineering process, as compared with the perovskite films fabricated under air atmosphere. Figure 2a demonstrates that cuboid CH3NH3PbI3 crystals are closely packed and the average grain size of cuboids is about ~600 nm. Figure 2b shows the morphology of CH3NH3PbI3 film fabricated under CBZ ambience, the CH3NH3PbI3 crystals are also cuboids, similar with that prepared in air, but the grain size can up to ~2 µm. Nam-Gyu Park et al.

18

reported that the size of

CH3NH3PbI3 cuboids in the capping layer was key to obtain high light harvesting and charge carrier extraction, and the change in size of CH3NH3PbI3 was likely to affect charge extraction behavior, which eventually determined the photocurrent. It is interesting that the morphologies of perovskite films prepared under DMF and DMSO ambiences exhibit significant difference compared to that obtained under air atmosphere and CBZ ambience. Dendrites are formed in the case of DMF ambient (Figure 2c). This phenomenon is associated with the dissolution of PbI2 and MAIPbI3 6


Page 7 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


crystals by the DMF vapor during the formation of perovskite film, as previously reported on one-step perovskite filming process

29-31

. In the case of DMSO ambient,

the perovskite film is composed of fine grains with high density (Figure 2d). The best power conversion efficiencies (PCEs) and average PCEs of perovskite solar cells based on these films shown in Table 1 reveal that the CBZ ambience is favorable to form larger CH3NH3PbI3 cuboids while DMF and DMSO present negative effect. Figure S1 shows the statistical photovoltaic parameters of CH3NH3PbI3 perovskite solar cells fabricated under different ambience (Air, CBZ, DMF and DMSO). We can see that short current density (Jsc) and fill factor (FF) of perovskite solar cells fabricated in CBZ is much higher than that in air, DMF and DMSO. This result gives us a hint that the evaporation of DMF and DMSO during fabrication of perovskite films responsible for the poor performance and reproducibility. The introduction of CBZ ambience is an effective approach to enhance the efficiency and reproducibility. To further scrutinize the effect of CBZ ambience on surface morphology and crystal growth behavior, we traced the evolution in surface morphology as a function of the reaction time of the CH3NH3I solution with the PbI2 precursor film. Figure 3 shows the SEM images at different reaction times (20 s, 40 s and 60 s) in air (a, b, c) and CBZ ambience (d, e, f), respectively. From Figures 3a and 3d, we can see that the cuboid CH3NH3PbI3 crystals haven’t completely covered the substrate in both the cases due to the short reaction time of 20 s. Notably, the density of crystal seeds under CBZ ambience is less than that under air. This indicates that the CBZ can suppress the perovskite grain nucleation. In fact, the nucleation rate of perovskite is determined by the reaction rate between PbI2 and CH3NH3I for the two-step perovskite fabrication process. It is suspected that the CBZ ambient can reduce the reaction constant between PbI2 and CH3NH3I and therefore their reaction rate. With increasing the reaction time to 40 s, CH3NH3PbI3 crystals almost cover the surface in the case of air atmosphere (Figure 3b) while bare parties are still observed in the case of CBZ (Figure 3e). Further increasing the reaction time to 60 s, the crystals grow bigger and the size of grains in CBZ case is larger than that in air. Figure 3g shows the histogram of CH3NH3PbI3 cuboid size under CBZ ambience and air, respectively. The average 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

grain size in CBZ ambience and in air are about ～1 µm and ～0.5 µm, respectively. The average grain size is 2 times larger in CBZ ambience than that in air, which should greatly enhance the charge extraction efficiency. The CBZ ambient would be helpful for the reduction of the surface energy of perovskite grains and therefore enhance the grain growing-up. As a result, a larger-grain-sized perovskite film can be easily obtained under the CBZ ambient. We also carried out the ultraviolet-visible (UV-vis) absorption spectra of the resulting perovskite films (Figure S2) with different reaction time prepared under air and CBZ ambiences, respectively. It is found that absorption edges increase with the reaction time in both the cases, and reaches to the saturation after the reaction time of 60 s. The absorption edge increases faster with the increasing reaction time in the case of air atmosphere than that in CBZ ambience, which demonstrates that the reaction rate is suppressed by CBZ ambience. Furthermore, the solar cells based on the corresponding perovskite films are fabricated, and the power conversion efficiency achieves a maximum at the reaction time of 60 s in both the cases (Figure S3). This result is consistent with the SEM and absorption results. From above discussion, we think that the CBZ atmosphere can suppress the formation of seed density of CH3NH3PbI3, thereby achieving larger CH3NH3PbI3 grains. Figure 4a shows the characteristic peaks of PbI2 and CH3NH3PbI3 existed in the X-ray diffraction (XRD) pattern of the films fabricated under air and CBZ ambiences, which indicates that the PbI2 hasn’t completely converted into CH3NH3PbI3 in both the cases. The PbI2 film displays the strong characteristic XRD peaks at about 12.57o, 38.69o and 52.39o while CH3NH3PbI3 shows the characteristic peaks at about 14.03o, 28.38o and 31.81o, which is assigned to the (110), (220) and (310) planes of the tetragonal perovskite structure, respectively

32

. The intensity of the CH3NH3PbI3

diffraction peaks fabricated under CBZ atmosphere is stronger than that prepared under air, suggesting the crystallinity of CH3NH3PbI3 in CBZ atmosphere is better than that in air. Meanwhile, the positions of peaks originated from PbI2 and CH3NH3PbI3 phases are identified under CBZ and air, which means that the ambient-engineering process affects the grain size rather than crystal structure. 8


Page 8 of 20

Page 9 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Further investigation based on steady-state and time-resolved photoluminescence (PL) measurements are carried out to figure out the role of CBZ atmosphere in the crystallization process of the perovskites films. The steady-state PL spectra of pristine perovskite films fabricated under CBZ and air on glass with the reaction time of 60 s are shown in Figure 4b. As a result, the enhanced PL indicates that the non-radiative carrier recombination is significantly suppressed in the perovskite films prepared under CBZ ambience. Here, it should be mentioned that PL samples were fabricated by using a two-step spin-coating process on glass substrate. This two-step process is exactly the same as that for the device fabrication. The intensity of PL signals related to intrinsic perovskite film would be largely reduced if the TiO2 substrate was utilized. The reason is that the built-in electric field formed between perovskite and TiO2 can promote the carrier extraction. The glass substrate, as an insulator, is unable to extract the photo-induced carriers and therefore will not affect the PL signal of intrinsic perovskite film on it. Meanwhile, our experimental results have clarified that the morphology of peroskite films prepared on the glass substrate have no obvious difference from those on mesoporous TiO2, as shown in Figure S4. All these issues lead us to choose the glass substrate for PL measurements. Furthermore, we present the time-resolved PL decays with the same samples, measuring the peak emission at ~777 nm for the perovskite absorbers that fabricated under CBZ and air ambience, respectively. Obviously, the perovskite film fabricated under CBZ atmosphere gives an improved PL lifetime of ~319 ns, a much larger value compared with the film prepared under air atmosphere of ~206 ns. The findings of steady-state and time-resolved PL measurements reveal that the non-radiative recombination centers in the perovskite films fabricated under CBZ atmosphere could be inhibited to some extent. This result suggests that the bulk defects in the CH3NH3PbI3 perovskite films can be reduced by introducing CBZ atmosphere during the fabrication process. Leone Spiccia et al. reported that the large crystalline grains with the size up to microns of CH3NH3PbI3 perovskite films could be achieved by exposure the films into CBZ solvent.

20

Introduction of the CBZ solution can control the nucleation and grain

growth of CH3NH3PbI3, and then achieves reproducible fabrication of high-quality 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

perovskite thin films. In present study, introducing CBZ gas during fabricated process increases the grain size of CH3NH3PbI3, and then improves the quality of perovskite films. Noticeably, CBZ gas is more uniform than CBZ solution that is dropped on CH3NH3PbI3 films. This feature will significantly increase the reproducibility (Figure S1). The CBZ ambience suppresses the formation of seed crystal of CH3NH3PbI3, and thus results in the formation of larger crystals as shown in Figure 3. Eventually, the influence of CBZ ambience upon photovoltaic performance of perovskite solar cells is investigated based on the conventional mesoscopic structure solar cells of FTO/block-TiO2/mesoporous-TiO2/CH3NH3PbI3/spiro-OMeTAD/Au. Figure S5 shows the cross-sectional SEM image of a complete device based on (a) air and (b) CBZ ambience. We can find from the Figure S4 that the cross-sectional SEM image of perovskite solar cells fabricated under air and CBZ ambience have no significant differences, and the thickness of block-TiO2, mp-TiO2 and spiro-OMeTAD layer are about ～50 nm, ～300 nm and ～100 nm, respectively. But the thickness of MAPbI3 based on CBZ ambience is thicker than that in air due to the bigger grain size. Figure 5a shows the current density-voltage (J-V) characteristics of CH3NH3PbI3 perovskite solar cells fabricated under CBZ and air ambiences recorded under AM 1.5 G condition (100 mW/cm2), respectively. The reference device fabricated under air exhibits a PCE of 10.65% with the open circuit voltage (Voc) of 1.04 V, the short-circuit current density (Jsc) of 16.5 mA/cm2, and the fill factor (FF) of 62.2%. In the case of CBZ ambience, the device shows a significantly enhanced PCE of 14.55% with a Voc of 1.05 V, Jsc of 19.0 mA/cm2 and FF of 72.6%. Voc of the cells fabricated under air and CBZ ambiences are almost identical, but the Jsc and FF of the cells fabricated under CBZ atmosphere is improved ~15% and ~17%, respectively. The best device fabricated by our ambient-engineering process also shows a very broad EQE plateau of over 70% between 400 nm and 700 nm, as shown in Figure 5b. The Jsc value integrated from the EQE are 18.9 mA/cm2 and 16.3 mA/cm2 in CBZ and air, respectively, which is agreed well with that measured by I-V. Figure 5c shows the stabilized PCE of the devices fabricated under air and CBZ 10


Page 10 of 20

Page 11 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


atmosphere at a constant bias of 0.730 V and 0.771 V, respectively. The stabilized PCEs are 14.2% and 10.3% for the CBZ and air based solar cells, respectively, which are close to the J–V result. It indicates that our devices contain less density of traps for electrons or holes

33

, and meanwhile the CBZ ambient processing can be more

efficient for device fabrication. In Figure 5d, a histogram of the average PCEs for all of the independently fabricated cells is presented. Around 75% of the cells made using our ambient-engineering process exhibited an overall efficiency exceeding of 12% under one sun condition. At the end, we have achieved average power conversion efficiency of 12.7% and the maximum PCE of 14.55% for our solar cells fabricated under CBZ ambience. Effects of ambience on photovoltaic parameters, such as Voc, Jsc, FF and PCE are also illustrated in Figure S1. It is found that the CBZ ambience improves the PCE by increasing Jsc and FF, which is attributed to the enhanced grain size. The hysteresis effects is also evaluated by changing the scan directions during I-V measurement with step of 20 mV and delay time of 40 ms, as shown in Figure S6 and Table S1. Hysteresis is observed in both cases. It can be seen that an efficiency of 14.2% is obtained with reverse scan direction and 10.93% with forward scan direction for the device obtained under CBZ ambient. The corresponding values for the cell prepared in air are 10.65% and 7.61%, respectively.

4. CONCLUSIONS In summary, we systematically investigated the influence of ambience on the properties of perovskite films and photovoltaic performance. The findings revealed that CBZ ambience was favorable to the fabrication of high-quality perovskite films with larger grain size while DMF and DMSO ambiences exhibited negative effect. The evaporation of DMF and DMSO solvents during the fabrication of perovskite films was the possible reason accounting for the poor performance and reproducibility. The introduction of CBZ ambience was critical for achieving high-quality perovskite films. We proposed a plausible growth processes for the formation of uniform and larger grain size perovskite crystals under the CBZ ambience. The formation of seed 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

crystals was suppressed by introducing CBZ ambience, which caused the larger grain. The perovskite films with larger grain demonstrated a lower density of grain boundaries and a higher carrier lifetime, an indicator of less non-radiative recombination center in the films. Finally, the ambient-processed perovskite solar cell with a PCE of 14.55% was achieved. These results provided an effective strategy for fabrication of perovskite layer with larger grain size and enhanced reproducibility.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Statistics for photovoltaic parameters of the perovskite solar cells, absorption spectra, J-V curves, cross sectional SEM images, hysteresis. AUTHOR INFORMATION Corresponding Author: Email: [email protected] (Xuegong Yu), [email protected] (Zeguo Tang) Present Address †

State Key Laboratory of Silicon Materials and School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China. ‡ Ritsumeikan Global Innovation Research Organization, Ritsumeikan University, Nojihigashi, Kusatsu, Shiga 525-8577, Japan Author Contributions Jiabin Huang, Jiangsheng Xie and Dikai Xu do the experiments. Jiabin Huang wrote the draft. All authors discussed and revised the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (No. 61422404), Central basic scientific research in colleges and universities operating expenses, Program for Innovative Research Team in University of Ministry of 12


Page 12 of 20

Page 13 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Education of China (IRT13R54), and State Key Laboratory of Optoelectronic Materials and Technologies (Sun Yatsen University).

REFERENCES (1) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (2) Yin, W.-J.; Shi, T. T.; Yan, Y. F. Unique Properties of Halide Perovskites as Possible Origins of the Superior Solar Cell Performance. Adv. Mater. 2014, 26, 4653-4658. (3) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science. 2013, 342, 341-344. (4) Xing, G. C.; Mathews, N.; Sun, S. Y.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S. Sum, T. C. Long-Range Balanced Electron and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science. 2013, 342, 344-347. (5) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics. 2014, 8, 506-514. (6) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am.Chem. Soc. 2009, 131, 6050-6051. (7) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. II. High-Performance Photovoltaic Perovskite Layers Fabricated Trough Intramolecular Exchange. Science. 2015, 348, 1234-1237. (8) Zhao, Y. X.; Zhu, K. Solution Chemistry Engineering toward High-Efficiency Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 4175-4186. (9) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science. 2012, 338, 643-647. (10) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature. 2013, 499, 316-319. (11) Jeng, J.-Y.; Chiang, Y.-F.; Lee, M.-H.; Peng, S.-R.; Guo, T.-F.; Chen, P.; Wen, T.-C. CH3NH3PbI3 Perovskite/Fullerene Planar-Heterojunction Hybrid Solar Cells. Adv. Mater. 2013, 25, 3727-3732. (12) Liu, D.; Kelly, T. L. Perovskite Solar Cells with a Planar Heterojunction Structure Prepared Using Room-Temperature Solution Processing Techniques. Nat. Photonics. 2014, 8, 133-138. (13) Liu, M. Z.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature. 2013, 501, 395-398. 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(14) Chen, Q; Zhou, H. P.; Hong, Z. R.; Luo, S.; Duan, H.-S.; Wang, H.-H.; Liu, Y. S.; Li, G.; Yang, Y. Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process. J. Am. Chem. Soc. 2014, 136, 622-625. (15) Zhou, H. P.; Chen, Q.; Li, G.; Luo, S.; Song, T.-B.; Duan, H.-S.; Hong, Z. R.; You, J. B.; Liu, Y. S.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science. 2014, 345, 542-546. (16) Ahn, N.; Son, D.; Jang, I.; Kang, S. M.; Choi, M.; Park, N. G. Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead(II) Iodide. J. Am. Chem. Soc. 2015, 137, 8696-8699. (17) Kim, H. D.; Ohkita, H.; Benten, H.; Ito, S. Photovoltaic Performance of Perovskite Solar Cells with Different Grain Sizes. Adv. Mater. 2016, 28, 917-922. (18) Im, J.-H.; Jang, I.-H.; Pellet, N.; Gratzel, M.; Park, N.-G. Growth of CH3NH3PbI3 Cuboids with Controlled Size for High-Efficiency Perovskite Solar Cells. Nat. Nanotechnol. 2014, 9, 927-932. (19) Zhu, L. F.; Shi, J. J.; Lv, S. T.; Yang, Y. Y.; Xu, X.; Xu, Y. Z.; Xiao, J. Y.; Wu, H. J.; Luo, Y. H.; Li, D. M.; Meng, Q. B. Temperature-Assisted Controlling Morphology and Charge Transport Property for Highly Efficient Perovskite Solar Cells. Nano Energy. 2015, 15, 540-548. (20) Xiao, M. D.; Huang, F. Z.; Huang, W. C.; Dkhissi, Y.; Zhu, Y.; Etheridge, J.; Gray-Weale, A.; Bach, U.; Cheng, Y.-B.; Spiccia, L. A Fast Deposition-Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells. Angew. Chem. 2014, 126, 10056-10061. (21) Ito, S.; Tanaka, S.; Nishino, H. Substrate-Preheating Effects on PbI2 Spin Coating for Perovskite Solar Cells via Sequential Deposition. Chem. Lett. 2015, 44, 849-851. (22) Wu, Y. Z.; Islam, A.; Yang, X. D.; Qin, C. J.; Liu, J.; Zhang, K.; Peng, W. Q.; Han, L. Y. Retarding the Crystallization of PbI2 for Highly Reproducible Planar-Structured Perovskite Solar Cells via Sequential Deposition. Energy Environ. Sci. 2014, 7, 2934-2938. (23) Li, W. Z.; Fan, J. D.; Li, J. W.; Mai, Y. H.; Wang, L. D. Controllable Grain Morphology of Perovskite Absorber Film by Molecular Self-Assembly toward Efficient Solar Cell Exceeding 17%. J. Am. Chem. Soc. 2015, 137, 10399-10405. (24) Zhang, H.; Mao, J.; He, H.; Zhang, D.; Zhu, H. L.; Xie, F.; Wong, K. S.; Grätzel, M.; Choy, W. C. H. A Smooth CH3NH3PbI3 Film via a New Approach for Forming the PbI2 Nanostructure Together with Strategically High CH3NH3I Concentration for High Efficient Planar-Heterojunction Solar Cells. Adv. Energy Mater. 2015, 23, 1501354. (25) Pathak, S.; Sepe, A.; Sadhanala, A.; Deschler, F.; Haghighirad, A.; Sakai, N.; Goedel, K. C.; Stranks, S. D.; Noel, N.; Price, M.; Huttner, S.; Hawkins, N. A.; Friend, R. H.; Steiner, U.; Snaith, H. J. Atmospheric Influence upon Crystallization and Electronic Disorder and Its Impact on the Photophysical Properties of OrganicInorganic Perovskite Solar Cells. ACS Nano. 2015, 9, 2311-2320. (26) Sheikh, A. D.; Bera, A.; Haque, M. A.; Rakhi, R. B.; Gobbo, S. D.; Alshareef, H. N.; Wu, T. Atmospheric Effects on the Photovoltaic Performance of Hybrid Perovskite 14


Page 14 of 20

Page 15 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Solar Cells. Solar Energy Materials & Solar Cells. 2015, 137, 6-14. (27) Xiao, Z. G.; Dong, Q. F.; Bi, C.; Shao, Y. C.; Yuan, Y. B.; Huang, J. S. Solvent Annealing of Perovskite-Induced Crystal Growth for Photovoltaic-Device Efficiency Enhancement. Adv. Mater. 2014, 26, 6503-6509. (28) Jeon, Y.-J.; Lee, S.; Kang, R.; Kim, J.-E.; Yeo, J.-S.; Lee, S.-H.; Kim, S.-S.; Yun, J.-M.; Kim, D.-Y. Planar Heterojunction Perovskite Solar Cells with Superior Reproducibility. Sci. Reports. 2014, 4, 6953. (29) Eperon, G. E.; Burlakov, V. M.; Docampo, P.; Goriely, A.; Snaith, H. J. Morphological Control for High Performance, Solution-Processed Planar Heterojunction Perovskite Solar Cells. Adv. Funct. Mater. 2014, 24, 151–157. (30) Sharenko, A.; Toney, M. F. Relationships between Lead Halide Perovskite

Thin-Film Fabrication, Morphology, and Performance in Solar Cells. J. Am. Chem. Soc. 2016, 138, 463−470. (31) Huang, F.; Dkhissi, Y.; Huang, W.; Xiao, M.; Benesperi, I.; Rubanov, S.; Zhu, Y.; Lin, X.; Jiang, L.; Zhou, Y.; Gray-Weale, A.; Etheridge, J.; McNeill, C. R.; Caruso, R. A.; Bach, U.; Spiccia, L.; Cheng, Y.-B. Gas-Assisted Preparation of Lead Iodide Perovskite Films Consisting of A Monolayer of Single Crystalline Grains for High Efficiency Planar Solar Cells. Nano Energy. 2014, 10, 10–18. (32) Tang, Z.; Tanaka, S.; Ito, S.; Ikeda, S.; Taguchi, K.; Minemoto, T. Investigating Relation of Photovoltaic Factors with Properties of Perovskite Films Based on Various Solvents. Nano Energy. 2016, 21, 51–61. (33) Yin, W.-J.; Shi, T.; Yan, Y. Unusual Defect Physics in CH3NH3PbI3 Perovskite Solar Cell Absorber. Appl. Phys. Lett. 2014, 104, 063903.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Graphic

16


Page 16 of 20

Page 17 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1．．Device architecture, scheme of solvent engineering process. (a), Device architecture of the perovskite solar cell (glass/FTO/bl-TiO2/mp-TiO2/perovskite nanocomposite layer/spiro-OMeTAD/Au) (b), ambient engineering procedure for preparing the uniform and large grain size perovskite film.

Figure 2. Surface scanning electron microscope (SEM) images of CH3NH3PbI3 perovskite films fabricated under different ambient (a) air, (b) CBZ, (c) DMF and (d) DMSO.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 20

Table 1. Photovoltaic parameters of best CH3NH3PbI3 perovskite solar cells and average photovoltaic parameters provided by 40 devices (show in parenthesis) fabricated under different ambient (Air, CBZ, DMF and DMSO). 2

Voc(V)

Jsc(mA/cm )

FF

PCE(%)

Air

1.04 (0.99)

16.47 (16.48)

62.2 (57.4)

10.65 (9.27)

CBZ

1.05 (1.04)

19.01 (19.45)

72.6 (62.5)

14.55 (12.71)

DMF

0.96 (0.98)

18.49 (14.76)

46.5 (47.8)

8.19 (6.93)

DMSO

0.99 (1.00)

13.2 (11.97)

51.2 (45.2)

6.74 (5.30)

Figure 3. CH3NH3PbI3 nucleation and crystal growth in air and CBZ ambient, respectively. Surface SEM images for the reaction times of 20 s, 40 s and 60 s in air (a, b, c) and CBZ ambient ( d, e, f), respectively. (g) Histogram of the grain size for the perovskite films prepared under air and CBZ ambient. 18


Page 19 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) XRD patterns and (b) Steady-state PL spectra and (c) Time-resolved PL kinetics of CH3NH3PbI3 films grown via the typical two-step sequential solution deposition using pure PbI2 precursor film with CH3NH3I solution under air and chlorobenzene ambient, respectively.

Figure 5. (a) J-V curves recorded under AM 1.5 simulated sun light (100 mW/cm2) for the best devices, in which the perovskite layer was fabricated under CBZ ambient or air. The PCE(%), Jsc (mA/cm2),Voc (V) and FF are shown in the inset. (b) EQE and integrated Jsc of the best devices fabricated under air and CBZ ambient, respectively. (c) Stabilized PCE of the devices under air and CBZ ambient at a 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

constant bias of 0.730 V and 0.771 V. (d) Histogram of the average PCEs for all of the independently fabricated cells.

20


Page 20 of 20

Ambient Engineering for High-Performance OrganicâInorganic

Recommend Documents

Ambient Engineering for High-Performance OrganicâInorganic