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Forming Intermediate Phase on the Surface of PbI2 Precursor Films by Short-time DMSO Treatment for High-efficiency Planar Perovskite Solar Cells via Vapor-assisted Solution Process Haibin Chen, Xihong Ding, Pan Xu, Tasawar Hayat, Ahmed Alsaedi, Jianxi Yao, Yong Ding, and Songyuan Dai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17781 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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ACS Applied Materials & Interfaces

Forming Intermediate Phase on the Surface of PbI2 Precursor Films by Short-Time DMSO Treatment for High-Efficiency Planar Perovskite Solar Cells via Vapor-Assisted Solution Process Haibin Chena, Xihong Dinga, Pan Xub,*, Tasawar Hayatc,d, Ahmed Alsaedid, Jianxi Yaoa, Yong Dinga,* and Songyuan Daia,* a

Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing, 102206,

P. R. China. b

Key Laboratory of Photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences, Hefei, Anhui,

230088, P. R. China. c

Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan

d

NAAM Research Group, Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah

21589, Saudi Arabia.

* Corresponding author.

E-mail: [email protected]. Phone: +86 55165593222

E-mail: [email protected]

E-mail: [email protected]. Phone: +86 1061772268

ABSTRACT Morphology regulation is vital to obtain high-performance perovskite films. Vapor-assisted deposition provides a simple approach to prepare perovskite films with controlled vapor-solid reaction. However, dense PbI2 precursor films with large crystal grains make it difficult for organic molecules to diffuse and interact with inner PbI2 frame. Here, a surface modification process is developed to optimize the surface layer morphology of PbI2 precursor films and lower 1

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the resistance of induced period in crystallization. The vapor optimization time is shortened to several-second level and the intermediate phase forms on the surface layer of PbI2 films. We achieve porous PbI2 surface with smaller grains through dimethyl sulfoxide (DMSO) vapor treatment, which promotes migration and reaction rate between CH3NH3I vapor and PbI2 layer. The PbI2 precursor films emerge dramatic morphological evolution, due to the formed intermediate phase on PbI2 surface layer. Taking advantage of the proposed surface modification process, we achieve high quality uniform perovskite films with larger crystal grains and without residual PbI2. The repeatable perovskite solar cells (PSCs) with modified films exhibit power conversion efficiency (PCE) up to 18.43% for planar structure. Moreover, the devices show less hysteresis because of improved films quality and reduced films density states. Our work expands the application of morphology control through forming intermediate phase and demonstrates an effective way to enhance the performance of the PSCs. KEYWORDS: PbI2 precursor films, surface layer, morphology, modification, intermediate phase

1. INTRODUCTION Recently, hybrid organic-inorganic perovskite solar cells (PSCs) have drawn enormous attention owing to its low cost fabrication process and excellent photovoltaic performance.1-5 The power conversion efficiency (PCE) of PSCs has been increased from 3.8%1 to 22.7%,6 taking advantages of perovskite materials with high absorption coefficient,7 long carrier lifetime8 and high carrier mobility.9 Previous studies suggested that perovskite films could be obtained by various approaches, such as one-step deposition, two-step sequential deposition, vapor-assisted solution process (VASP) and so on.3, 10-22 Compared to other methods, the VASP approach could retard nucleation and enable motivated reorganization for perovskite growth.7, 23-24 The perovskite films 2


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fabricated with VASP have complete surface coverage and uniform morphology.13, 25 Yang et al first reported the perovskite solar cells with VASP, achieving a PCE of 12.1%.7 It is well accepted that the steps of VASP including inorganic framework (PbI2) deposition and subsequently in-situ reaction with desired organic vapor (CH3NH3I, MAI).24 Generally, this reaction occurs on the surface of PbI2 films. While dense PbI2 films impede the reaction with MAI vapor, resulting in the residual of PbI2. The PbI2 residual does have negative effect on the PCEs, especially in VASP.7, 23, 26-27 Thus it takes a rather long time for PbI2 to completely convert to the final perovskite. It is noted that the extended heating time would destroy the lattices and lower the crystallinity of perovskite. Therefore, it is essential to modify the surface morphology of PbI2 for high performance devices.13, 28 Various methods were adopted to optimize the morphology and decrease the residual PbI2 in perovskite films such as solvent engineering,25, 29 solvent annealing optimization30-34 and the formation of intermediate phases,25, 35-36 while most of them are utilized in solution method or acquired for a long time processing.37 Huang et al introduced N, N-dimethylformamide (DMF) vapor during the inter-diffusion of PbI2 and MAI to enhance the crystallinity and grain sizes of perovskite.30 The solvent annealing process suppress the defects of perovskite films and thus enhance the performance of PSCs (PCE 15.6%). However, the DMF vapor annealing process sustains at 100ºC for 1 h. Liu et al reported the alcohol-vapor solvent annealing treatment on PbI2 films aiming to improve the crystal growth and increase the grain sizes of MAPbI3 crystal.34 While the alcohol-vapor thermal annealing process lasts for 2 h because of the lower polarities of the alcohols. Li et al prepared high crystallinity perovskite films with DMF/CB mixed solvents vapor annealing at room temperature and the PCE of PSCs reached 16.4%.33 The process is reduced to 6 min which is still too long for mass production. Chen and his 3



co-workers annealed PbI2 films in diverse solvent atmospheres including DMF, DMSO, acetone and isopropanol.31 They reported that the employing of DMSO solvent annealing process led to porous PbI2, which promote the conversion of PbI2 to perovskite. The annealing time is shortened to 5 min, while it is still too long compared to our method. Herein, the vapor handling time is shortened to several-second level to form the intermediate phase on the surface of PbI2 films. The surface morphology of PbI2 are finely controlled via second-level DMSO vapor treatment and highly reproducible perovskite films are fabricated via VASP. The surface modification process (SMP) reduces the obstruction of induction period, promoting the completely reaction between PbI2 and MAI. The reconstructed perovskite films without residual PbI2 have exhibited excellent photovoltaic performance. Repeatable devices based on perovskite films prepared via SMP exhibit PCE up to 18.43% with less hysteresis. The substantial reduced treatment time is beneficial to actual production of PSCs. Meanwhile, our results present that it is not essential to reconstruct all the PbI2 films to the intermediate phase in order to obtain high quality perovskite films. The outstanding devices performance can be ascribed to the high quality and decreased defects of perovskite films.

2. EXPERIMENTAL 2.1. Materials PbI2 (98%), MAI (99.99%) were purchased from TCI and Xi’an Polymer Light Technology Corp., respectively. DMF (99.5%), DMSO (99.8%), 1, 2-dichlorobenzene (99%), C60 (99.0%), 4-tert-butylpyridine (TBP, 98%) and lithium bis (trifluoromethylsulphonyl) imide (Li-TFSI, 98%) were purchased from Aldrich. The spiro-MeOTAD (99.8%) was obtained from Borun New Material Technology Co. Ltd. All chemicals were directly used without further purification. 4


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2.2. Device Fabrication The devices are prepared as previous reports.7, 23 FTO glass substrates were etched with HCl (2M) and Zn powder, followed by dipping in alkali solution for 15 min. After that the glass substrates were sequentially cleaned with detergent, deionized (DI) water and ethanol for 30 min each, followed by drying with an air flow. Finally, the FTO substrates were annealed at 510ºC for 30 min in a plate furnace. TiO2 compact layer was deposited via spray pyrolysis at 465ºC for 40 min from precursor solution of titanium diisopropoxide bis (acetylacetonate) in isopropanol (volume ration 1:7). MAPbI3 was prepared via VASP method and the following spin-coated processes were carried out in a glove box with cold and dry air (humidity≤20%, temperature≤20ºC). The C60 scaffold was spin-coated at 3000 rpm for 20 s and then annealed at 60ºC for 5 min, in which pristine powder were diluted in 1, 2-dichlorobenzene (1 M) with stirring for 8 h. Next, PbI2/DMSO (1:1, molar ratio) solutions in DMF (1.4 M) were spin-coated on the FTO substrates at 1000 rpm and 3000 rpm for 10 s and 30 s, respectively. The formed PbI2 films are annealed at 105ºC for 5 min, subsequently treated by DMSO saturated steam for 0 s, 5 s, 10 s, 15 s and 20 s at 105ºC. After that, the modified PbI2 films reacted with MAI to synthesize perovskite as previous report.7 The spiro-MeOTAD solution was prepared by previous method29 and spin-coated on the perovskite films at 3000 rpm for 20 s. Finally, 60 nm of Au contact was deposited on top of the device by thermally evaporating. 2.3. Characterization The crystal structure and intermediate products of the films were measured by X-ray diffraction (XRD, X’Pert Pro, Netherlands) Cu Kα beam (λ=1.54 Å). We further confirmed the intermediate products with Fourier transform infrared spectroscopy (Thermo Fisher IS50R, USA). 5



The films morphologies were characterized by a field-emission scanning electron microscope (FE-SEM, sirion 200, FEI Corp., Holland). The films morphologies were characterized by polarizing microscope (Leica, DM 2500 P). UV/Vis absorption spectra were obtained on a UV/Vis spectrophotometer (U-3900 H, Hitachi, Japan). Electrochemical impedance spectra were measured on an electrochemical workstation in the dark with a 0.9 V forward bias in a frequency range of 10−2 ～ 106 Hz. The steady state photoluminescence measurements are carried out on a Fluorescence Detector (QM 400 and Laser Strobe, Photo Technology International, USA) and they are carried out on a standard 450 W xenon CW lamp and pulsed nitrogen laser, respectively. Transient absorption (LKS 80, England) measurements were performed to characterize the charge recombination dynamics. The pump light wavelength and probe light wavelength for the samples were 500 nm. The transient absorption measurements 5 Hz repetition rate and 150 J cm−2 laser energy. The current−voltage characteristics (J−V curves) were measured by a solar simulator (solar AAA simulator, Oriel USA) with a source meter (Keithley Instruments, Inc., OH) at 100 mW cm−2, AM 1.5 Gillumination. The active area for each device was 0.09 cm2 and ensured by masking a black mask on the device. With a 50 mV s−1 of scan rate, the applied bias voltages for the reverse scan and forward scan were from 1.2 to −0.1 V and from −0.1 to 1.2 V. The monochromatic IPCE spectra were conducted using a QE/IPCE measurement kit (Newport Corporation, USA).

3. RESULTS AND DISCUSSION A schematic illustration of the proposed SMP is shown in Figure 1. First, we spin coat the PbI2 precursor solution on fluorine-doped tin oxide/crystalline TiO2 (FTO/c-TiO2) substrate to form uniform PbI2 framework films. Then, the PbI2 films get combined with DMSO molecules within 6


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several seconds, forming a buff PbI2-DMSO compound on the surface layer of PbI2 films. Finally, the modified PbI2 films are heated in MAI vapor to obtain polycrystalline perovskite films. Compared to traditional VASP, attractive changes of the films are observed with SMP. With DMSO vapor optimization, the PbI2 precursor films gradually get faded and become hazy. Finally the films color change to dark brown after in suit reaction with MAI vapor.

Figure 1 The schematic illustration of perovskite films formation through SMP. 3.1. Phase Compositions Figure 2 displays the XRD patterns of PbI2 and perovskite films prepared with SMP method. In Figure 2a, the XRD pattern of pure PbI2 (SMP-0, ICSD 68819) exhibits sharp diffraction peak at 12.7º and PbI2-DMSO intermediate peaks appear at 9.8º and 9.3º with DMSO vapor treatment (SMP-5 for 5 s, SMP-10 for 10 s and SMP-15 for 15 s), similar with previous reports.30, 38-39 The SMP time could be reduced to several seconds due to the rapid reaction between DMSO vapor and PbI2 films. With the increasing of handling time, the peak intensity of PbI2 decreases gradually. On the contrary, the PbI2-DMSO intermediate peaks intensity get increased. It is well known that PbI2 is a semiconductor material with layer structure and the interlayer spacing can be inserted with various guest molecules due to the weak bonding within the two planes by van der Waals-type interactions, bringing about the expansion of interlayer distance along c axis.29, 40 The main peak 7



diffraction degree would become smaller along with the increasing of interplanar distance. The appearance of small angle diffraction peaks suggest that DMSO molecules are absorbed in PbI2 films and then insert into the crystal lattice of PbI2 forming PbI2-DMSO intermediate phase as previous reports.29, 41-42 Further increasing the disposal time, the peak intensity of PbI2 gradually disappear and the low angle peaks intensity get increased stepwise, indicating the content of the intermediate phase increases with the increasing of DMSO dosage. The diffraction pattern for perovskite films (Figure 2b) have strong peaks at 14.1º, 28.4º, 31.8º, corresponding to (110), (220) and (310) faces of the MAPbI3 crystal (ICSD 194994) as previous reports.12, 43-44 For the controlled sample (SMP-0), there is a tiny peak at 12.7º, assigned to the characteristic diffraction peak of PbI2. The residual PbI2 as electrons and holes recombination centers have adversely influence on devices performance.23, 45-46 The intensity of (220) peak at 28.48º get stronger than (310) peak at 31.86º with SMP-10 and SMP-15 process, signifying a little preferred orientation along the [110] axis. Attractively, we find PbI2 films with DMSO vapor modification exhibit a faster conversion to MAPbI3 during the in-suit reaction with MAI vapor and the signature peak of PbI2 disappeared. The SMP would lower the surface energy of PbI2 films, reducing the reaction obstruction of induced period.47-48 The optimized PbI2 precursor films would influence the diffusion kinetic of MAI vapor molecules inserting into PbI2, as well as the reaction rate and diffusion distance.24, 28 According to the main peak intensity, the optimum time of DMSO modification is 10 s (SMP-10), which has the higher crystallinity of perovskite crystal without any PbI2 residual. The results of XRD indicates that it is not necessary to completely convert the PbI2 film into intermediate phase for high crystallinity perovskite grains.

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Figure 2 XRD patterns of PbI2 (a) and perovskite (b) films prepared with SMP. SMP-0 to SMP-15 represent PbI2 precursor films modified by DMSO saturated steam with 0 s, 5 s, 10 s and 15 s, respectively. Fourier transform infrared spectroscopy (FTIR) is implemented to verify the composition of intermediate phase, as shown in Figure 3. The stretching vibration of S=O (ν (S=O)) and bending vibration of -CH3 (δ (-CH3)) appear for bare DMSO. Vibration frequency is proportional to square root of force constant according to harmonic motion for diatomic model.25, 49 For the PbI2 films with DMSO modification, the ν (S=O) absorption peaks of pure DMSO decreased from 1032 cm-1 to 1022 cm-1, due to the decreasing bond strength between sulphur and oxygen as the consequence of intermediate phases formation.25,

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The detection of ν (S=O) deformation modes in

intermediate phase guarantees the inclusion of DMSO into the interlayer space of PbI2. The ν (S=O) absorption peak intensity get stronger as the increase of DMSO modification time. This discovery confirms the existence of DMSO in intermediate phases, consistent with previous reports.23, 25, 28

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Figure 3 (a) Fourier transform infrared spectra of DMSO and PbI2 precursor complex films with DMSO vapor treatment, (b) expanded finger print region for the S=O vibrations. SMP-0 to SMP-15 represent PbI2 precursor films modified by DMSO saturated steam with 0 s, 5 s, 10 s and 15 s, respectively. 3.2. Film Morphology Vapor atmosphere has strong impact on the morphology of PbI2 precursor films as well as perovskite grain growth process. By decorating PbI2 precursor films with DMSO vapor, the films colors and surface morphology are significantly changed due to the formed intermediate phase (PbI2-DMSO). The DMSO vapor molecules prefer being absorbed at the grain boundaries of PbI2 films, as the adsorption formation energy in the grain boundaries is lower than other positions.47, 50 The adjacent PbI2 grains are consumed by absorbed DMSO molecules, accompanying with the formation of intermediate phase. Figure 4a show the polarized microscope images of PbI2 films with SMP method. Numerous small spots emerges on the film surface with SMP-5 and then cracks appear on the surface with SMP-10, breaking the integral film into many isolated parts. Further increasing the handling time, the PbI2 surface appears collapses. The SEM images of PbI2 films with SMP are shown in Figure 4b. There are apparent differences on the appearance of PbI2 films with small amount of DMSO treatment and the morphologies are obviously changed. With 10


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the increasing of handling time, the color of PbI2 films gradually get faded and become hazy. The morphology of PbI2 surface passes through branching, granulation and restructuring process. The controlled PbI2 film (SMP-0) has well-distributed crystalline grains with size about 250 nm, meanwhile some voids scattered among adjacent grains. The morphology of PbI2 film with SMP-5, namely branching process exhibits cross-linked porous grains with lots of voids. The porous grains are caused by the reaction between PbI2 and DMSO vapor, which could erode the dense arranged inorganic particles. Interestingly, the sizes of grains get smaller and some rod-like crystals appear on the image, when DMSO treating time increases to 10 s. The grains broke at the weaker connection of the cross-linked and we get the isolated quasi-spherical particles, which is classified as granulation process. At the restructuring process with the increasing of DMSO treatment time, the sticks get larger with different directions and some smaller spherical grains districting around the sticks. When handling time is longer than 20 s, the films' macroscopic appearance and micro morphology would get worse. The sectional view of PbI2 films with SMP are shown in Figure 5. The formation of intermediate phase is accompanied with the changes of morphology and thickness of PbI2 films. The thickness of PbI2 films for SMP-0 to SMP-15 are 220, 303, 448 and 351 nm, respectively. The increase in thickness indicates that DMSO vapor molecules insert into the crystal lattice of PbI2, bringing about the expansion of PbI2 interlayer distance. The enlarged space could reduce the obstruction of MAI entering into the inner PbI2 framework. The PbI2-DMSO intermediate phase and PbI2 crystal are coexisted in the PbI2 precursor films combined with the XRD data of PbI2 films. We get the optimum thickness with SMP-10 and the PbI2 framework would get collapsed when further increase the vapor handling time. 11



Figure 4 The morphology of PbI2 precursor films treated with SMP. (a) The polarized microscope images, (b) surface SEM images. The inserted images are the appearance of PbI2 films with SMP. SMP-0 to SMP-15 represent PbI2 precursor films modified by DMSO saturated steam with 0 s, 5 s, 10 s and 15 s, respectively.

Figure 5 The sectional view of PbI2 films with SMP. SMP-0 to SMP-15 represent PbI2 precursor films modified by DMSO saturated steam with 0 s, 5 s, 10 s and 15 s respectively. PbI2 with porous surface morphology and a smaller crystal grains can promote atomic motion during vapor-solid reaction with MAI and thereby increase the grain size of perovskite films.23, 26 Perovskite films prepared by traditional VASP exhibit grain size about 500 nm (Figure 6a SMP-0). However, the grain sizes extend to 600 nm and 1 µm for SMP-5 and SMP-10, respectively. When the time of DMSO modification increases to 15 s, perovskite films get course with uneven surface which have adversely effects on device performance. Compared to conventional VASP films, much larger crystal grains are prepared with porous PbI2 via SMP method. The formed intermediate phase is more reactive than PbI2 itself and could reduce the reaction obstruction of 12


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induced period,48, 51-52 finally facilitating perovskite crystallization process.28 There are no residual PbI2 diffraction peaks (Figure 2b) for SMP method-based perovskite films, indicating the advantage of crystallization with porous structure through modified reaction interface. The larger crystals of the films with SMP-10 are closely aligned on the surface of FTO/c-TiO2 substrates with well-defined grains across the film thickness, as confirmed by the cross-sectional SEM images (Figure 6a). Figure 6b and 6c exhibit the grains distribution and average size of perovskite films, where the films with 10 s vapor optimization has the narrow distribution range and the larger average size, indicating a high quality perovskite films. The SMP-10 has the better result compared with others, revealing the advantages of the coexistence of intermediate phase (surface layer) and PbI2 (substratum).

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Figure 6 (a) The morphology of perovskite films prepared with PbI2 precursor films modified by DMSO vapor. (b) and (c) exhibit the grains distribution and average size of perovskite films. SMP-0 to SMP-15 represent PbI2 precursor films modified by DMSO saturated steam with 0 s, 5 s, 10 s and 15 s, respectively. 3.3. Mechanism Perovskite crystallization processes include vapor-solid reaction and rearrangement process via in-situ annealing.7 Generally, vapor-solid reaction occurs on the surface layer of PbI2 films thus the surface morphology has vital influence on the crystallization process. A porous structure with smaller PbI2 grains has larger number of nucleation sites which would facilitate the diffusion 14


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and reaction of MAI. We speculate that DMSO vapor molecules are absorbed on the surface of PbI2 films and then insert into the crystal lattice breaking the lamellar structure of PbI2 grains as shown in Figure 7.28, 40 The interlayer spacing of PbI2 is expanded by DMSO vapor modification, due to the strong interaction between DMSO molecules and PbI2. The PbI2-DMSO intermediate phase just forms on the surface layer of PbI2, which greatly reduce the obstruction of induce period.47-48 The modified surface has smaller work function compared with the flat surface.23 Smaller PbI2 grains with porous structure could promote completely conversion from PbI2 to MAPbI3, which could reduce the crystallization time, improving the crystallinity and morphology of perovskite films. The SMP method increases the numbers of hole on PbI2 precursor film, which decreases the obstruction of MAI entering to PbI2 framework and provides sufficient space for perovskite crystals growth. Simultaneously, in-situ annealing further accelerates the rearrangement of perovskite grains. Through SMP method, we get high quality perovskite films with larger grains uniformly arranged in both horizontal and vertical direction. We would further explore the effect of various solution polarity on PbI2 films.

Figure 7 The Schematic diagram of film transformation process. 3.4. Device Performance The devices adopt planar structure with C60 and spiro-MeOTAD as electron and hole transport layers contacting with FTO/c-TiO2 and Au electrode, respectively, which is shown in 15



Figure 8a. We study the effects of DMSO vapor treatment on devices performance, as shown in Figure 8b, S1 and Table 1. The appropriate DMSO play a vital role in enhancing the quality of perovskite films with larger grains and fewer grain boundaries, which lead to improve the JSC and PCE apparently. The devices with traditional VASP (SMP-0) exhibit a best PCE of 15.15%, with VOC of 1.04V, JSC of 19.12 mA cm-2, and FF of 0.75, while after PbI2 films treated by DMSO vapor with 5 s, the devices performance have unobvious improvement as the similar grain sizes with SMP-5. With DMSO optimization time raised to 10 s, the PSCs performance get obvious enhanced to 18.43%, with VOC of 1.06V, JSC of 22.40 mA cm-2 and FF of 0.77. However, with 15 s DMSO treatment, the device performance drops owing to poor morphology with uneven size of perovskite particles. The reproducibility of the device performance and the stabilized power output of the corresponding devices with different SMPs are shown in Figure S1. It is obvious that the devices with SMP-10 have the highest PCEs. Uniform films with larger crystalline grains as well as increased crystallinity are beneficial to reduce grain-boundary traps, thus the PSCs performance get enhanced. The integrated JSC of SMP-10 and SMP-0 are 20.00 and 17.33 mA cm-2 in IPCE spectra (Figure 8c), which agree with the measured values in J-V curves. The increasing in IPCE efficiency suggests that large crystals with well-defined grains may strengthen light absorption, boost charge transport and lower charge recombination. Figure 8d show the ultraviolet visible light absorption spectra (UV-vis) of perovskite films with SMP-0 and SMP-10 process. We can see the intensity of absorption between 450 nm to 800 nm wavelength getting obviously improved and the absorption edge appears a little red shift with the optimization of SMP-10. The SMP method could improve the repeatability of PSCs performance, as shown in Figure 8e. The average and best PCEs of SMP-10 are substantially improved compared to PSCs with traditional VASP. After 30 days 16


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stored in the air atmosphere (humidity≥50%, temperature≥25ºC), the devices of SMP-10 and SMP-0 retained 90% and 70% of their initial performance, respectively, indicating the superior stability of the SMP based cells. The decreasing hysteresis is owing to the smooth and pinhole-free perovskite film. Hysteresis effect of J-V curves for devices with traditional VASP method and the SMP-10 method are shown in Figure 8f, 8g. For the reference sample (Figure 8f), strong hysteresis between forward and reverse scan is observed and the forward scan exhibits PCE with 13.83%, which indicates over 8% deviation from that of reverse scan (15.15%). For the device with 10 s DMSO treatment (Figure 8f), the forward scan shows a PCE of 17.57%, within 5% reduction compared with reverse scan (18.42%). Hence, hysteresis is suppressed by surface modification and the crystallization of perovskite is enhanced as confirmed by XRD and SEM patterns. The modified morphology with smooth and pinhole-free of perovskite is also in favor of suppressing the charge trapping and accumulation, leading to better device performance with lower hysteresis.53

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1.2

0.2

0.4

0.6

0.8

1.0

1.2

Voltage (V)

Voltage (V)

Figure 8 Device structure and performance of PSCs with SMP. (a) Device architecture of perovskite solar cells, (b) J-V characteristic of perovskite solar cells with SMP, (c) IPCE spectra of PSCs with SMP, (d) UV-vis spectra of perovskite films with SMP-0 and SMP-10, (e) PCE histograms for 20 devices fabricated with SMP-0 and SMP-10 respectively, (f) Hysteresis J-V curves of PSCs with SMP-0, (g) Hysteresis J-V curves of PSCs with SMP-10. SMP-0, SMP-5, SMP-10, SMP-15 represent PbI2 precursor films treated by DMSO saturated steam with 0 s, 5 s, 10 s and 15 s, respectively. 18


Page 19 of 32 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 1. Photovoltaic performances of devices with SMP. Samples

VOC (V)

JSC (mA

FF

PCE (%)

cm-2) SMP-0

SMP-5

SMP-10

SMP-15

Average

Error

PCE (%)

Reverse

1.05

19.12

0.75

15.15

14.32

±0.83

Forward

0.99

19.28

0.70

13.83

13.04

±0.86

Reverse

1.07

19.20

0.75

15.56

15.03

±0.57

Forward

1.06

18.77

0.71

14.23

13.56

±0.68

Reverse

1.06

22.41

0.77

18.43

17.95

±0.48

Forward

1.06

21.21

0.76

17.57

17.09

±0.51

Reverse

0.98

20.04

0.71

13.75

13.22

±0.84

Forward

0.96

19.19

0.67

12.56

11.98

±0.75

3.5. Carrier Transport Performance We perform steady-state photoluminescence measurements (PL) to analyze the carrier recombination dynamics. The PL for perovskite films deposited on glass substrate are shown in Figure 9a. The increasing peak intensity is due to the decrease of spontaneous radiative recombination.54 With the increasing of DMSO vapor handling time, the PL intensity of perovskite films get strengthen effectively and the perovskite film with SMP-10 is 2.5 times as high as that of the control film. Further increasing the treating time of DMSO vapor, the PL intensity of perovskite film get decrease. Electrochemical Impedance spectroscopy (EIS) is performed to investigate the interfacial charge transfer processes in the PSCs. Figure 9b shows the Nyquist plots of PSCs measured at 0.9 V under dark condition. The equivalent circuit can be used to analyze EIS spectra. The lower 19



frequency arc is attributed to a charge recombination resistance (Rrec), in parallel with a chemical capacitance (Crec).55 The Rrec is the most important parameter that affects the VOC of solar cells and it could be estimated from the EIS spectra.56-57 In EIS spectra, the first arc is related to the carrier transport behavior at cathode/HTM. The arc in the lower frequency results from the recombination between HTM and TiO2 which is the focus of the study.58-59 It is noted that the Rrec of SMP-10 optimizing devices is higher than that of the others, indicating the recombination in the devices with SMP-10 optimizing is slower than that in the controlled samples. Thus, the solar cells with SMP-10 optimizing exhibit a higher VOC due to a slower recombination.

(a)

(b) SMP-0

700

250

Rrec

RHTM

Crec

CHTM

SMP-0

RS

SMP-5

SMP-5

PL Intensity (a.u.)

SMP-15

SMP-10

200

SMP-10

-Z″″ (Ω)

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

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SMP-15 150 100 50

750

800

0

850

0

100

Wavelength / nm

200

300

400

500

Z′′ (Ω)

Figure 9 (a) PL and (b) EIS curves for perovskite films with SMP-0 to SMP-15. Charge recombination dynamics of the MAPbI3 prepared via SMP is also investigated by transient absorption (TA). The TA characterizations are conducted on MAPbI3/glass, to prove the destinies of electrons and holes after separating in perovskite films. In our study, the pump light wavelength is 500 nm to excite the sample and the probe wavelength is 760 nm as previous reports.51 A signal with slow decay is observed for the reference sample (Figure 10a) and the TA signal could be well fitted to mono-exponential decay ( y = A0 + Ae

−∆t /τ

) with a time constant

( ∆t ) of 53.55 ns. Since there is no electron injection from perovskite to the glass, the slow decay process could be ascribed to the slow recombination of electron and hole in perovskite film. The 20


Page 21 of 32

lifetimes extend to 55.42 ns and 85.05 ns with SMP-5 and SMP-10 respectively, which means the perovskite film quality get improved by vapor optimization. However, when the time of DMSO modification increase to 15 s, two processes in TA decay are observed and the TA signal could be well fitted to bi-exponential function ( y=A0 + A1e

−∆t1 /τ1

+ A2 e −∆t2 /τ 2 ) as previous reports. The

time constants ∆t1 and ∆t 2 of the bi-exponential function are calculated 17.02 ns (occupied 95.89%) and 101.05 ns (occupied 4.11%), respectively. The faster decay process ( ∆t1 ) could be attributed to electron and hole non-radiative recombination through defects or trap states in perovskite or at MAPbI3/glass interface. The slower decay process ( ∆t 2 ) could be attributed to the recombination of free electrons and holes in MAPbI3.60 This result suggests that the photo-excited charge carriers are most recombined by trap states. Both PL, EIS and TA studies illustrate the faster carrier transportation and slower charge recombination of perovskite films with SMP-10.

(b)

(a)

SMP-5 Fitting

0.00

△A

△A

0.00

SMS-1 Fitting

-0.05

-0.05 -0.10

-0.10

100

200

300

-0.15

400

100

200

300

400

Time (ns)

Time (ns)

(c)

(d) 0.0 0.0 2

SMP-10 Fitting

SMP-15 Fitting

0.00

△A

-0.1

△A

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


-0.05

-0.2 -0.10 -0.3 100

200

300

400

100

200

Time (ns)

Time (ns) 21


300

400


Figure 10 TA characterizations of perovskite conducted on CH3NH3PbI3/glass. (a)-(d) represent the SMP-0 to SMP-15 respectively. SMP-0, SMP-5, SMP-10, SMP-15 represent PbI2 precursor films treated by DMSO saturated steam with 0 s, 5 s, 10 s and 15 s, respectively.

4. CONCLUSION In summary, we demonstrate a simple DMSO surface treatment on PbI2 layer. The morphology of PbI2 surface passes through branching, granulation and restructuring process with the increase of handling time. The optimal treating time is 10 s and the intermediate phase is formed on the surface layer of PbI2 films. The SMP could lower the resistance of induced period and promote MAI vapor diffusing into PbI2 framework. Thus we obtain perovskite films with larger crystal grains and reduced defect states. Consequentially, a champion device with PCE of 18.43% with less hysteresis is obtained. The SMP provides a simple approach to prepare high quality perovskite films and paves the way for high reproducibility of perovskite films.

ACKNOWLEDGEMENTS This work was supported by National Basic Research Program of China under Grant No. 2016YFA0202400, The 111 Project under Grant NO. B16016, Science and Technology Commission of Beijing Municipality, China under Grant No. Z141100003314003, Project of Scientific and Technological Support Program in Jiang Su Province under Grant No. BE2014147-X, The National High Technology Research and Development Program of China (863 Program) under Grant No. 2015AA050602, The National Key Basic Research Program of China (973 Program) under Grant No. 2015CB932201, The National Natural Science Foundation of China under Grant No ： 51572080, 51372083, 51772095, 5573042, 21303049, 51303052, 51702096 and the Fundamental Research Funds for the Central Universities under Grant No: 22


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2014ZZD07, 2015ZZD06, 2014MS35.

ASSOCIATED CONTENT Supporting Information Error bar graph of PCEs versus different SMPs. Stabilized power output of the corresponding devices with different SMPs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +86 1061772268 Notes The authors declare no competing financial interest.

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