Efficiency-Enhanced Planar Perovskite Solar Cells via an Isopropanol

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Efficiency-Enhanced Planar Perovskite Solar Cells via Isopropanol/Ethanol Mixed Solvent Process Peng Mao, Qing Zhou, Zhiwen Jin, Hui Li, and Jizheng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08863 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on August 24, 2016

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ACS Applied Materials & Interfaces

Efficiency-Enhanced Planar Perovskite Solar Cells via Isopropanol/Ethanol Mixed Solvent Process Peng Mao†‡, Qing Zhou†‡, Zhiwen Jin†‡, Hui Li† and Jizheng Wang†‡*

†Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ‡University of Chinese Academy of Science, 19 A Yuquan Rd, Shijingshan District, Beijing 100049, China ABSTRACT: Solution processable perovskite solar cells (PSCs) traditionally employed isopropanol as solvent of CH3NH3I in two-step method. One of the largest issues of this technique is the uncontrollable morphology of the perovskite film. In this study, homogeneous and dense PbI2 film was prepared by introducing DMSO as additive into DMF, then reacted with CH3NH3I dissolved in isopropanol/ethanol solvent to fabricate high quality perovskite films. Results revealed that ethanol played a crucial role on morphology and components of perovskite films. By optimizing the ratio of isopropanol/ethanol, power conversion efficiency (PCE) of 15.76% was achieved, which was averagely ~50% higher than PSCs without DMSO and ethanol processing. KEYWORDS: perovskite, solar cell, mixed solvent, morphology, component

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INTRODUCTION Over the last few years, Organic–inorganic halide perovskite solar cells (PSCs) have attracted considerable attention due to their superb photovoltaic performance. The merits of large absorption coefficient1, small exciton binding energy2-3, long charge diffusion length4 (>100 nm) and good solution processability, have promoted the power conversion efficiency (PCE) up to more than 20%5 rapidly. Numerous techniques like vapour deposition6, inkjet printing7, sequential deposition8 have been carried out to fabricate high-performance PSCs, among which the wet processes, which can be divided into one-step method and two-step method, were the most significant representative for their simple preparation. At early stage most studies were based on facile one-step method, as perovskite layer could be prepared easily by coating the precursor solution (PbX2 (X = Cl, I) and methylamine iodide (MAI) dissolved in appropriate solvent) on top of the metal oxide substrate9-11. Many reports have demonstrated high-quality perovskite film could be obtained with the introduction of additives (e.g. PEG, MEH-PPV, HI, DIO)12-15. However, forming a homogeneous pin-hole free perovskite layer by one-step coating process without additive is still difficult. Poor coverage of perovskite layer leads to weak light absorption and severe charge recombination because of the occurrence of electrical shorting which significantly lower the device efficiency16-17. In this context, burgeoning studies have focused on two-step method within the last two years5,

18-19

, for which PbI2 solution was

deposited, followed by a dip in a solution of MAI in isopropanol. The two-step perovskite with full coverage is more compact than one-step perovskite, showing enhanced recombination kinetics due to its well established layer with free-void enabling to prevent the HTM infiltration and thus decrease the recombination probability.20 In addition, a two-step spin-coating procedure


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enabled to control the size of CH3NH3PbI3 cuboids by altering concentration of MAI solution, which significantly affected light-harvesting efficiency and charge-carrier extraction21. Film quality of the perovskite active layer is the most essential for PSCs because charge carrier generation, charge transfer and trap-assisted recombination are largely depend on it. At present, one of the main issues encountered in device fabrication processes is how to finely control the film morphology during the crystallization and growth of the perovskite layer. For two-step method, two problem is found in fabrication process: one is the incomplete conversion of PbI2 owing to the big size of PbI2 crystal, the other is the uncontrollable morphology of perovskite film for the rapid reaction rate between precursors. For these reasons, to fabricate a pin-hole free and completely converted perovskite film by two step method still remains a challenge, especially on the top a organic compound (e.g. PEDOT:PSS). Hence, much effort toward improving perovskite film quality (morphology, crystallization size and speed) has been devoted. One of the most effective strategy is the introduction of solvent additives. For instance, Leung et al. incorporated hydrochloric acid (HCl) into PbI2 precursor solution, found that HCl additive inhibited the rod-shape PbI2 crystallization and promoted homogeneous nucleation as well as crystal growth, therefore improving the uniformity and coverage of the perovskite film.22 Durstock’s group demonstrated that uniform thin films with micron size perovskite grains can be prepared during low-temperature thin-film growth by adding a controlled amount of sodium ions into the precursor solution.23 Wang et al. retarded the crystallization of PbI2 in dimethylformamide (DMF) by solvent additive of dimethyl sulfoxide (DMSO), forming PbI2(DMSO)x complexes. An intramolecular exchange of DMSO with MAI enabled the complexes to convert to an ultraflat and dense thin CH3NH3PbI3 film.24 It is noteworthy that all of these methods are introduction of additives into PbI2 precursor solution, while few attention


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have been paid to the solvent of MAI precursor. Considering the solvents of both precursor solutions impact the growth and morphology of perovskite film in two-step method, an appropriate solvent of MAI enhancing film quality is also desirable to improve device performance. In this work, we focus on the need for high-quality perovskite film and demonstrate an efficiency-enhanced solution-processed planar CH3NH3PbI3 perovskite solar cell via facile isopropanol (IPA)/ethyl alcohol (EA) solvent engineering. To form a uniform and pin-hole free PbI2 film, PbI2 was dissolved in DMF with DMSO additive, and the solution was spin-coated on top of PEDOT:PSS to form the first deposition layer. Afterwards IPA/EA mixed solvent was used to control the rate of reaction between PbI2 and MAI precursors. By optimizing the ratio of IPA/EA, homogeneous and flat perovskite films were obtained and a maximum PCE as high as 15.76% was achieved. To the best of our knowledge, IPA/EA mixed solvent was used to fabricate PSCs for the first time and no better performance of ethanol-based PSCs have been reported. RESULTS AND DISCUSSION Previous reports have confirmed that a high quality and full coverage PbI2 film on PEDOT:PSS greatly facilitates the formation of a smooth, highly crystalline, adhesive and continuous perovskite film, therefore enhancing the device performance. 15, 21-24 In spite of the excellent nature of PEDOT:PSS in hole transportation, the crystallization of ionic compound (e.g. perovskite) on a smooth organic surface via a facile solution process is too fast to form a homogeneous thin film owing to the poor affinity between PEDOT:PSS and ionic reagents. Compared with PbI2, DMSO is more compatible with PEDOT:PSS for their similar elemental composition, PbI2 solution with DMSO additive can therefore form a more continuous and more


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Figure 1. SEM images of PbI2 films processed by adding (a) 0%, (b) 5%, (c) 8% and (d) 10% DMSO in DMF (volume ratio), cross-sectional images of the facture of (e) glass/ ITO/PEDOT:PSS/PbI2 and (f) glass/ITO/PEDOT:PSS/PbI2(8%DMSO). Layer 1, 2, 3 and 4 refer to glass, ITO, PEDOT:PSS and PbI2 layer, respectively.

homogeneous film on the substrate than PbI2 solution. We explored the morphology of PbI2(x%DMSO) films (PbI2(x%DMSO) represents the film processed from PbI2/DMF solution with x v% of DMSO) prepared from PbI2 solutions containing various amounts of DMSO additive by SEM (Fig. 1). Results showed that the content of DMSO in the precursor solution has a critical impact on the morphology of PbI2(x%DMSO) films. A pinhole-free, flat, uniform film could be obtained only when 8% DMSO is added into PbI2 solution (Fig. 1c). A lower DMSO content gives rise to a film covered with small particles (Fig. 1b), while a higher DMSO content results in a coarse and discrete film with plenty of voids (Fig. 1d). Particularly, a non-continuous and very rough film could be observed when the precursor solution contains no DMSO (Fig. 1a). This observation indicates PbI2 solution


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containing DMSO combine with PEDOT:PSS more readily than that without DMSO. The SEM cross section images of ITO/PEDOT:PSS/ PbI2(x%DMSO) film are shown in Fig. 1e-f, which identify that the inner morphology of the film prepared from PbI2(8%DMSO) solution is also more uniform and compact than that from PbI2 solution. No pin-hole is observed in PbI2(8%DMSO) film and it is composed of small dense grains (Fig. 1f), which contributes to the homogeneity of reaction between precursors, thus improving the reproducibility of PSCs. It has previously reported that PbI2 and DMSO could form PbI2(DMSO) and PbI2(DMSO)2 complexes readily. To understand the crystallinity of PbI2(x%DMSO) films, X-ray diffraction (XRD) was employed to characterize the films. Fig. S1 shows the XRD patterns of PbI2 films, it illustrates that the content of DMSO in precursor solution also impacts the crystallinity of the asprepared film significantly. The diffraction peaks at 2θ of 12.8°(001), 25.6°(002), 38.8°(003) and 52.5°(004) demonstrate PbI2(0%DMSO) film and PbI2(2.5%DMSO) film are crystalline with the same d-spacing. With the increase of DMSO content in precursor solution, the crystallinity of PbI2(x%DMSO) film varies distinctly. Spike peaks indicate good crystallinity of PbI2 when the amount of additive in precursor solution are less than 5%. However, when DMSO is 5% or more, no evident peaks can be observed. Considering the measurement was carried out just after the films were prepared, PbI2(x%DMSO) films based on precursor solution containing abundant DMSO likely incorporate residual DMSO which doesn’t coordinate with Pb2+. This result is in agreement with the observation of literature, which stated amorphous PbI2 film could efficiently generate perovskite crystals with a small distribution of particle sizes25. Unlike the results reported by Seok et. al., no characteristic peaks of PbI2(DMSO) (~9.6°)5 and PbI2(DMSO)2 (~10.5°)26 complexes are observed in our experiment, we ascribe this observation to the weak XRD intensity of complexes compared with PbI2 crystal (leading to unconspicuous signals) and


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the short time of solvent evaporation with regard to the high boiling point of DMSO (leading to amorphous form). Based on above, PbI2 precursor solution containing 8% DMSO was used to fabricate photovoltaic devices. The fabrication process for the solar cells with a structure of ITO/PEDOT:PSS/perovskite/PCBM/Ca/Al was a modified two-step method. Compare with conventional two-step method, MAI solution was dropped on top of spinning PbI2 films rather than coated on the stationary film in this study. Table S1 displays the photovoltaic performance of devices fabricated from precursor solution with and without 8% DMSO. The parameters clearly reveal that DMSO additive greatly affects the photovoltaic performance of PSCs. With the addition of 8% DMSO, Voc and Jsc of devices are enhanced from 972 mV to 1013 mV and 14.09 mA/cm2 to 16.90 mA/cm2, respectively. Nevertheless, an obvious decrease in FF (from 71.69% to 63.86%) results in a small improvement (from 10.99% to 12.50%) in PCE. Noted that the color of the as-deposited perovskite film obtained from PbI2 solution is dark-brown (Fig. S2a) while that from PbI2(8%DMSO) solution is light-brown (Fig. S2b), it is assumed that the reaction of perovskite formation is retarded by DMSO, which is consistent with the previous report25. One of the merits of this reaction rate-retarded method is that it overcomes the problem of uncontrolled morphology of perovskite without mesoporous scaffolds. However, the significant reduction of reaction rate probably causes incomplete conversion of the deep layer of PbI2. To validate this speculation, cross section SEM was performed (Fig. 2a-b). Expectedly, cross section SEM images show a layer of unconverted PbI2 between perovskite and PEDOT:PSS when the precursor solution contain 8% DMSO (Fig. 2b), while this phenomenon is not found when no DMSO is added into solvent (Fig. 2a). This phenomenon may explain the frustrating FF of PSCs prepared from precursor solution with 8% DMSO.


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Figure 2. cross-sectional images of the facture of CH3NH3PbI3 perovskite deposited on glass/ITO/PEDOT:PSS substrates. (a) PbI2 (in DMF) and MAI (in IPA) as precursor solutions. PbI2 dissolved in DMF with 8% DMSO and MAI dissolved in (b) IPA, (c) 25% EA, (d) 50% EA, (e) 75% EA and (f) pure EA as precursor solutions. Layer 1, 2, 3 and 4 refer to glass, ITO, PEDOT:PSS and perovskite layer, respectively. Layer 5 in panel (b) refers to unconverted PbI2.

In light of the great impact of reaction rate on perovskite morphology and its components, a proposal of mixed solvent for MAI was implemented to enhance reaction rate. Ethanol alcohol, similar to IPA, could dissolve MAI readily while hardly dissolve PbI2. Furthermore EA can dissolve a small amount of strong-polar CH3NH3PbI3 due to the strong polarity of EA. In previous report, strong-polar ethanol solution rather than less-polar isopropanol solution was applied to realize smooth surface morphology of CH3NH3PbI3 perovskite thin films, and a corresponding PCE of 11.45% was achieved27. Unfortunately, low Voc (0.88 V) of the photovoltaic performance limited the advance of pure ethanol solvent in PSCs. Herein, IPA and


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EA were mixed at different ratios to use as the solvent of MAI to adjust and control the reaction rate of precursors and morphology of perovskite films. In order to understand the underlying solvent effect on the photovoltaic performance of the PSCs, SEM was employed to probe the morphology of perovskite films. Surface images are shown in Fig. S3, and cross-sectional SEM images are shown in Fig. 2. Only when the mixed solvent contains 25% (Fig. 2c) could we obtain a homogeneous, flat and pin-hole free perovskite film covering the entire surface area of the PEDOT:PSS. Pure IPA solvent lead to a inhomogeneous film with many grain boundaries (Fig. 2b), and uneven films, which are filled with pin-holes and defects, are observed when the EA content are 50% or more (Fig. 2d-f). Considering the fact that full surface coverage is crucial to the overall PCE, these observation is consistent with the photovoltaic performance of PSCs. Fig. S2 shows photographs of the as-deposited perovskite films based on various mixed solvents without annealing. As can been seen that with the increase of EA content in mixed solvents, the color of the as-deposited perovskite film becomes darker. This observation could be explained by the factor that perovskite is slightly soluble in EA while it is completely insoluble in IPA. When MAI is dissolved in IPA, due to the rapid reaction between PbI2 and MAI, the quick formation of the upper perovskite film retards the inner PbI2 to react with CH3NH3I, leading to the remnant PbI2 exists between PEDOT:PSS and MAPbI3. On the contrast, when EA is added into solvent, MAI can penetrate into inner PbI2 through the channels formed by dissolution of perovskite, thereby controlling the conversion of precursors. Current density (J)–voltage (V) characteristics of the devices based on different solvents of MAI were recorded, the Voc/Jsc/FF/PCE parameters are illustrated in Fig. 3 and summarized in Table 1. As expected, the EA content in solvent significantly affects all the device parameters.


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Figure 3. (a) Photovoltaic parameter statistics of PSCs fabricated by IPA , 25% EA，50% EA, 75% EA, and pure EA solvents (based on 20 devices for each). (b) J-V characteristics of best performing devices with different sweep directions (sacn rate 200mV/s).

With the increase of EA content in solvent, Voc of the corresponding device changes slightly. However, when pure EA is used as the solvent, Voc decreases dramatically to as low as 0.883 V, which is in agreement with other reports27-28. For Jsc, it increases with EA content and reaches a maximum of 19.19 mA/cm2 when the EA/IPA volume ratio is 1:3, further increase results in a decreased Jsc. FF exhibits the similar trend to that of Jsc and Voc. In consequence, a promising PCE of 15.76%with a Voc of 1.044 V, Jsc of 19.52 mA/cm2 and FF of 77.30% was obtained. The average PCE (14.82%) is ~35% higher than that of IPA based PSCs (10.95%). Compared with other spin-dropping (dropping when the substrate is spinning) methods which are far sensitive to dropping time after spinning29-30, our method barely depends on dropping time. No distinct


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Table 1. Summary of the photovoltaic properties of PSCs fabricated from IPA , 25% EA， 50% EA, 75% EA, and pure EA solvents EA content

Voc(V)

Jsc(mA/cm2)

FF(%)

PCE(%)

Best PCE

0%

1.013(0.024)

16.90(0.86)

63.86(2.80)

10.95(0.90)

12.50

25%

1.026(0.015)

19.19(0.51)

75.22(1.20)

14.82(0.46)

15.76

50%

1.019(0.022)

16.46(0.92)

69.61(2.52)

11.68(0.82)

12.86

75%

0.961(0.022)

16.38(0.68)

70.23(3.39)

11.06(0.80)

11.96

100%

0.883(0.022)

2.94(0.43)

63.28(2.89)

1.65(0.27)

2.18

The values in the bracket are standard deviations.

differences of film morphology and device performance are observed with regard to different dropping time in our work. Fig. 4 exhibits the XRD patterns of perovskite films prepared from various mixed solvents. The characteristic peaks at 2θ of 14.2°, 28.5°, 32.0° and 42.7°confirmed the tetragonal perovskite structure of the films. As expected, weak peaks at 12.8° and 25.6° indicates that PbI2 impurity exists in the perovskite film based on pure IPA solvent, which is in agreement with SEM cross section observation. Interestingly, it’s can been found that the 25% and 50% EA based films probably contain more PbI2 species than that from pure IPA, owing to their more evident XRD peaks of PbI2. As far as we know, EA could speed up the reaction rate of formation of CH3NH3PbI3, more EA in solvent signifies a faster interaction of PbI2 and MAI. A probable reason for the existence of more unreacted PbI2 contents in pero(25%EA) (pero(x%EA) represents the perovskite film prepared from MAI dissolved in IPA/EA mixed solvent containing x v% EA) and pero(50%EA) films is the lower boiling point of EA, which evaporate much faster


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Figure 4. XRD patterns of pure IPA, 25% EA, 50% EA, 75% EA and pure EA based CH3NH3PbI3 perovskite films. ★ and ▲ represent the characteristic peaks of PbI2 and CH3NH3PbI3 perovskite, respectively.

than IPA during spinning, leading to a reduction of reaction time due to lack of solvent. When EA content in solvent is increased to 75% or 100%, almost no PbI2 peak is observed in the XRD patterns, this may be explained by the highly improved reaction rate stemmed from a mass of EA in solvent. The role of residual PbI2 in overall efficiencies of PSCs is a controversial issue. On the one hand, the bottom PbI2 blocks the hole injection from perovskite to PEDOT, as the valence band of PbI2 is located lower than that of the perovskite, therefore degrading the device performance. This is the reason why most literature about PSCs demonstrates a complete conversion of PbI2 is prerequisite to high-efficiency device31-32. On the other hand, the remnant


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PbI2 acts as a blocking layer between the charge transfer layer and the perovskite, reducing the probability of charge recombination. In this case, trace amounts PbI2 contributes to the improvement of device performance. For these reasons, the real effect of PbI2 on PCE basically depends on the distribution and amount of PbI2 in the film. A consecutive and relative thick PbI2 layer in perovskite film will definitely impede charge transfer because of the low conductivity of PbI2, yielding a reduced PCE. However, if PbI2 is presented in perovskite grain boundaries and at the relevant interfaces or the thickness of PbI2 layer is appropriate, it turns to be beneficial for the device performance for its passivation effect33-38.

In our case, trace amount PbI2 is not

detrimental to the device performance with regard to the enhanced Jsc of pero(25%EA) and pero(50%EA) based devices. The light absorption of the glass/perovskite was studied by UV-vis spectroscopy (Fig. S4). All the perovskite thin films have strong absorption from UV down to the near-infrared range. Except the one from pure EA solvent, an absorption onset at 780–790 nm could be observed which is in good agreement with the optical band gap of CH3NH3PbI3 perovskite (~1.58 eV). The slightly higher absorption of pero(75%EA) films may be ascribed to its special surface for light absorption. The exceptional UV-vis spectroscopy of pero(100%) film may be attributed to its poor crystallinity which can be concluded from its XRD pattern (Fig. 4). To test the solvent effect on perovskite/PCBM interface, time-resolved photoluminescence (TR-PL) of glass/perovskite/PCBM was carried out (Fig. S5). Except for pero(100%) film, the similar lifetimes of different solvents based perovskite reveals that the introduction of EA has little influence on perovskite/PCBM interface in terms of charge transfer. Then the effect of solvent was examined by external quantum efficiency (EQE) measurements. It is well known that the EQE spectrum could provide photoelectric information about the absorber converting


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Figure 5. EQE curve of PSCs based on IPA/EA mixed solvent with different EA contents.

absorbed photons to the collected electrons and holes. As shown in Fig. 5 , the EQE spectra shows the fundamental reason for the best photovoltaic performance of pero(25%EA) was its highest EQE among all the devices. The pero(25%EA) based device exhibits highest EQE value up to 83% within the spectra range of 480~560 nm. The performance in EQE suggests that EA significantly impacts the photoelectric properties of the corresponding device. To further investigate charge transfer properties of PSCs, impedance spectroscopy (IS) of different solvents based PSCs was carried out (Fig. 6). The IS was measured in dark with an applied voltage of VOC, under which the recombination resistance (Rrec) is the lowest and much lower than charge transfer resistance (RCT). Thus the equivalent circuit can be simplified to the


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Figure 6. Fitted Nyquist plots of PSCs measured in dark at an applied voltage of open-circuit voltage. The inset in top right corner is the magnified version of low impedance zone. The inset in centre is the equivalent circuit of PSCs

circuit model shown in the inset of Fig. 6, where RS is the sheet resistance of the conductive electrode.39 Considering the identical electrode of PSCs in our study, the RS is reasonably assumed to be the same. Therefore, the major difference of IS is RCT which can be extracted from the semicircle of Nyquist plots. As can be seen, the RCT of PSCs based on pero(25%), pero(0%), pero(50%), pero(75%) and pero(100%) gradually increases from 3.2 x104 Ω to 2.0 x106 Ω. A much larger RCT for the pero(100%) based PSCs originates from the inferior perovskite crystal morphology and a mass of defects. In the contrast, the minimum RCT is obtained from pero(25%) based PSCs, indicating its more orientated crystal and fewer defectassisted traps. Compare with pure IPA based PSCs, the improved quality of pero(25%)


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demonstrates the charge carrier transfer process can greatly facilitated by adding a certain amount of EA into IPA solvent, leading to enhanced Jsc and FF. CONCLUSIONS In this work, we introduced DMSO as an additive into DMF to smoothen PbI2 film, employed EA/IPA mixed solvent to accelerate reaction rate and control the morphology CH3NH3PbI3 perovskite film. The best volume percentage of DMSO and the optimized volume ratio of EA/IPA were determined to be 8% and 1:3, respectively. By this method, ultra-smooth PbI2 films and homogeneous, pin-hole free perovskite film were obtained. The existence of trace amount of PbI2 in perovskite film is found to be not detrimental to our devices. Compared with pure IPA based devices, the optimized PSCs based on mixed solvent exhibit significant enhancement on Jsc and FF, mainly due to its improved morphology and charge transfer efficiency. As a result, a maximum PCE of 15.76% was achieved which demonstrated a remarkable improvement compared to the counterpart PSCs without DMSO and ethanol processing. We anticipate this mixed solvent based technique to be a promising method for high performance PSCs in the future. EXPERIMENTAL SECTION Materials. All chemicals were purchased from commercial suppliers and used without further purification. MAI was synthesized by reacting 10 mL hydroiodic acid (57% in water, SigmaAldrich), 14 mL methylamine (33% in absolute ethanol, Sigma-Aldrich) in 50 mL ethanol in a 250 mL round-bottomed flask at 0 °C for 2 h with stirring. After 30 min rotary evaporating, a white precipitate of MAI was obtained and the product was then dissolved in ethanol. Diethyl ether was droplet added into the resulting solution to recrystallize MAI, pure MAI was obtained after dried at 60 °C under vacuum for 24 h. The obtained pure MAI was dissolved in anhydrous


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isopropanol (Aldrich)/ethanol alcohol (Aldrich) mixed solvent with different ratios at a concentration of 40 mg/mL. Lead iodide (PbI2 , 99.9995%, Sigma-Aldrich) was dissolved in anhydrous DMF (Aldrich) with different DMSO (Aldrich) contents to form 1 M PbI2(x%DMSO) solutions. The solutions was filtered through a 0.4-µm-pore PTFE filter after stirring at 70 °C for 4 h and stored under a dry nitrogen atmosphere. Devices fabrication. The patterned ITO (15 ohm/sq) glass substrates were washed sequentially with detergent and deionized water, acetone, and isopropanol with ultrasonication for 10 min each, and then were dried and treated by O2 plasma. An aqueous dispersion of PEDOT:PSS (levios, Al 4083, filtered through a 0.45 µm nylon filter) was spin-coated on the ITO substrate at 3000 rpm for 50 s and then annealed at 140 °C for 10 min in air. In order to avoid the influence of moisture on fabrication of perovskite thin-film, the substrates were transferred into a simply equipped glove-box, where the humidity was controlled to be lower than 30% by silica gel. PbI2(x%DMSO) precursor solution was spin-coated on top of PEDOT:PSS at 2000 rpm for 60 s. Afterward MAI solution with different IPA/EA ratios were added on the center of the asprepared PbI2(x% DMSO) thin-film immediately when the spin speed reached at 2000 rpm. After spinning for 30 s, the devices were annealed on a hot plate (Stuart SD160, UK) at 100 °C for 10 min to form perovskite films. The acceptor layer was prepared by spin coating (1000 rpm, 40 s) 2 wt% PCBM (99.5%, Sigma-Aldrich ) in chlorobenzene (Aldrich ) onto the surface of the perovskite film. The PCBM film was solvent annealed by adding 3 µL chlorobenzene into a Petri dish on a hot plate and covering the film with another Petri dish for 6 min at 80 °C. Finally, the PEDOT:PSS/perovskite/PCBM film was transferred to a vacuum chamber for coating the 20 nm thick Ca and 80 nm thick Al as electrode under less than 2x10-4 Pa.


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Characterization. The X-ray diffraction (XRD) spectra of the PbI2(x%)DMSO films and the perovskite films were measured using Rigaku-2500 X-ray diffractometer with an X-ray tube (Cu Kα , λ = 1.5406 Å). The morphology of the films was recorded using a scanning electron microscope (SEM, HITACH2100). External quantum efficiency (EQE) was recorded by a Newport Oriel IQE-200 by a power source (Newport 300 W Xenon lamp, 66920) with a monochromatic (Newport Cornerstone 260). The wavelength interval we used for measuring the EQE curve is 20 nm. All J-V curves were measured using a source meter (Keithley 2420, USA) under AM 1.5 sunlight at an irradiance of 100 mW/cm2 provided by a solar simulator (Newport, Oriel Sol3A Class AAA, 94043A). Light intensity was calibrated using a monocrystalline silicon reference cell with KG5 window (Newport, Oriel 91150). The J-V curves were measured by reverse (1.2 V→−0.7 V)) or forward (−0.7 V→1.2 V) scan with a rate of 200 mV/s. The device area was 0.168 or 0.044 cm2, determined by the overlap of the cathode and anode. The UV-vis absorption spectra were measured on a U-3010 spectrophotometer (Hitachi) instrument. In order to make a parallel comparison and avoid the influence of ITO, the perovskite films were deposited on glass substrates. Time-resolved PL decay spectra were recorded by using a timecorrelated single-photon counting system (Edinburgh Instruments FL980), and the wavelength of laser was 460 nm, and the signal was monitored at 775 nm. To avoid the effect of solvent evaporation and moisture, all the films and devices were measured within 30min after prepared. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxxxx.


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Additional data including XRD patterns of PbI2(x%DMSO) films, SEM images, photograph of the as-prepared perovskite films, UV-vis absorption spectra, time-resolved PL decay spectra of mixed solvent based perovskite films, and photovoltaic parameters of the PSCs prepared from PbI2 solution. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions P.M. performed the experimental work, the data analysis and the experimental planning; The project was conceived, planned and supervised by P.M. and J.W.; Q.Z. and Z.J. performed SEM and EQE characterization. The manuscript was written and reviewed by P.M. and J.W. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge the financial support by 973 Program (Grant No. 2014CB643600, 2014CB643503), National Natural Science Foundation of China (61405208), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12030200) and the CAS/SAFEA International Partnership Program for Creative Research Teams. REFERENCES 1. De Wolf, S.; Holovsky, J.; Moon, S. J.; Loper, P.; Niesen, B.; Ledinsky, M.; Haug, F. J.; Yum, J. H.; Ballif, C.,


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Efficiency-Enhanced Planar Perovskite Solar Cells via an Isopropanol

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