A Universal Strategy to Utilize Polymeric ...

FULL PAPER Perovskite Solar Cells

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A Universal Strategy to Utilize Polymeric Semiconductors for Perovskite Solar Cells with Enhanced Efficiency and Longevity Fangchao Li, Jianyu Yuan,* Xufeng Ling, Yannan Zhang, Yingguo Yang, Sin Hang Cheung, Carr Hoi Yi Ho, Xingyu Gao, and Wanli Ma* increased rapidly from ≈3.0% to 22.1%.[1,5–13] In general, the PSCs with the best performance usually employ a sandwich-type configuration, composed of a layer of electron transport material (ETM) that is infiltrated by an intrinsic organometal halide ABX3 perovskite (normally CH3NH3PbX3, X = halogen) light absorber, followed by a layer of hole transport material (HTM), and a metal back contact.[6–8] Organometal halide perovskite-based materials exhibit advantages such as high optical absorption coefficients, long diffusion lengths, and high mobility of charge carriers, which are thought to be responsible for the rapid and significant progress of PSCs.[4,14] Despite these advances and processing advantages, stability problem of PSCs is thought to be one of the most crucial issues hindering their commercialization. Thus, it has been recognized that the stability of organic–inorganic hybrid perovskite materials with regard to humidity, heat, light, and oxygen should be carefully considered.[15–17] Therefore, various efforts have been made to improve both efficiency and stability such as composition substitution,[18–20] modification of film fabrication processing[21–23] and interface engineering.[24–30] Perovskite materials suffer most from a rapid degradation upon exposure to humidity, and this process also appears to be accelerated by heat. First, Cation (A) and anion (X) substitutions have been proved to be efficient in improving the chemical stability of the conventional MAPbI3 (MA = CH3NH3) organometal halide perovskite, where the iodine in MAPbI3 can be replaced by either chlorine (Cl) or bromine (Br). Meanwhile, the MA can be successfully exchanged by the cation ions FA (CH(NH2)2+), or inorganic cation Cs+.[31] Second, different processing methods have been developed to improve the morphology and stability of perovskite films, such as gas blowing,[32] solvent vapor annealing,[33,34] vacuum treatment,[35] and antisolvent treatment.[23,36] Among these, the antisolvent treatment is critical for achieving high power conversion efficiency and has been widely used in the fabrication of PSCs. Finally, rational interfacial design is also beneficial for improving the device efficiency and stability. The interface engineering at either electron transporting layer (ETL)/perovskite or perovskite/ hole transporting layer (HTL) interface is important for charge extraction, transportation, and recombination.[36–38] However, despite of these recent achievements, it is still a challenge to

In this contribution, a facile and universal method is successfully reported to fabricate perovskite solar cells (PSCs) with enhanced efficiency and stability. Through dissolving functional conjugated polymers in antisolvent chlorobenzene to treat the spinning CH3NH3PbI3 perovskite film, the resultant devices exhibit significantly enhanced efficiency and longevity simultaneously. In-depth characterizations demonstrate that thin polymer layer well covers the top surface of perovskite film, resulting in certain surface passivation and morphology modification. More importantly, it is shown that through rational chemical modification, namely molecular fluorination, the air stability and photostability of the perovskite solar cells are remarkably enhanced. Considering the vast selection of conjugated polymer materials and easy functional design, promi sing new results are expected in further enhancement of device performance. It is believed that the findings provide exciting insights into the role of conjugated polymer in improving the current perovskite-based solar cells.

1. Introduction Solution-processed thin-film solar cells based on organic–inorganic halide perovskite are emerging as a promising photovoltaic technology.[1–4] During the past 7 years, perovskite solar cells (PSCs) have attracted tremendous scientific and industrial interest, and the power conversion efficiencies (PCEs) have F. Li, Prof. J. Yuan, X. Ling, Y. Zhang, Prof. W. Ma Institute of Functional Nano and Soft Materials (FUNSOM) Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices Joint International Research Laboratory of Carbon-Based Functional Materials and Devices Soochow University 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou Jiangsu 215123, P. R. China E-mail: [email protected]; [email protected] Dr. Y. Yang, Prof. X. Gao Shanghai Synchrotron Radiation Facility Shanghai Institute of Applied Physics Chinese Academy of Sciences Shanghai 201204, P. R. China Dr. S. H. Cheung, Dr. C. H. Y. Ho Department of Physics and Institute of Advanced Materials Hong Kong Baptist University Kowloon Tong, 999077 Hong Kong, P. R. China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201706377.

DOI: 10.1002/adfm.201706377

Adv. Funct. Mater. 2018, 1706377

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explore simple and universal processing protocols to improve the efficiency and stability of PSCs simultaneously. Conjugated polymer has also been proved to be advantageous in cost-effective synthesis and high-quality large-scale film preparation.[39–41] The molecular structure and property of semiconducting conjugated polymers can be highly tunable; as a result, polymer-based organic solar cells have shown impressive progress in the past decade. Quite recently, several attempts have been made to switch the antisolvent process to antisolution treatment, through dissolving compounds such as poly(methyl methacrylate)[42] and [6,6]-phenyl C61 butyric acid methyl ester (PCBM)[43] in antisolvents. These methods aim to control nucleation and crystal growth of perovskite film, achieving enhanced efficiency and stability simultaneously. However, in these reports, the criteria of materials selection are not clear and the reported materials have almost fixed structures, leading to limited opportunity for further improvement. Therefore, it is now urgent to expand the polymer materials used in antisolution treatment, which should have tunable structures and desired functions to further help control the perovskite film morphology and improve device efficiency and stability. In our research, four conjugated polymers, including p-type polymer with or without (w/wo) molecular fluorination (PF-0, PF-1) and n-type polymer w/wo molecular fluorination (N2200, F-N2200) were chosen in antisolution treatment for MAPbI3 perovskite films. Our results showed that the functional conjugated polymer leads to: (a) good coverage of perovskite film without affecting the perovskite crystal grain size and energetic disorder, (b) improved perovskite surface morphology and surface passivation of the trap sites at the grain boundary, and

(c) better protection of the perovskite film from humidity by using fluorinated conjugated polymers. As a result, PSCs using fluorinated p-type (PF-1) and n-type (F-N2200) polymers obtained an improved PCE of 18.7% and 18.4%, respectively, compared to the best PCE of 17.7% for the control device. More importantly, PSCs utilizing conjugated polymer, especially fluorinated polymer, exhibit significantly enhanced stability without encapsulation under ambient environment and with illumination.

2. Results and Discussion 2.1. The Impact of Conjugated Polymer on Solar Cell Performance The conventional planar-heterojunction perovskite solar cells were fabricated with the device structure shown in Figure 1a, where TiO2 processed from hydrothermal method[44] is used as the ETM, the organic–inorganic MAPbI3 perovskite as the light harvester and Spiro-OMeTAD as the HTM. The entire conjugated polymers studied in this work were reported in our previous work of organic polymer solar cells,[45,46] and the molecular structures of polymers are shown in Figure 1b. The detailed materials information (absorption, energy levels, and molecular packing) are shown in Figures S1–S4 in the Supporting Information. Figure 1c shows a schematic illustration of the conjugated polymer-assisted growth process for the perovskite-polymer layer. PbI2 and methylammonium iodide (MAI) were dissolved in dimethyl sulfoxide (DMSO): γ-butyrolactone. The perovskite solution was spin coated at 2000 rpm for 10 s

Figure 1. a) Schematic illustration of the device structure of perovskite solar cell, b) molecular structural formula of the conjugated polymers, and c) spin-coating process for fabricating perovskite films with antisolvent process.


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Figure 2. Current–voltage curves of optimized perovskite solar cells w/wo n-type polymer N2200, F-N2200 a) and p-type polymer PF-O and PF-1 b), measured under AM1.5 simulated sun light. The inset pictures show the PCE distribution of PSCs based on corresponding 20 devices.

The control devices with pure chlorobenzene treatment exhibit a best PCE of 17.7%, with a short-circuit current (Jsc) of 22.5 mA cm−2, an open-circuit voltage (Voc) of 1.05 V, a fill factor (FF) of 0.75, which are comparable with the previous reports.[47] Perovskite solar cells with N2200 treatment present similar performance with a highest PCE of 17.9%, Jsc of 22.4 mA cm−2, Voc of 1.05 V, and FF of 0.76, while the fluorinated polymer F-N2200 contained solar cells show slightly improved performance with an optimal PCE of 18.4%, Jsc of 22.7 mA cm−2, Voc of 1.06 V, and FF of 0.76. Quite interestingly, perovskite solar cells containing p-type polymer PF-0 and PF-1 also exhibit comprehensive improvements in Jsc, Voc, and FF. As shown in Figure 2b, similar trend can be found when treating perovskite film with p-type conjugated polymer. In comparison with the control device, PF-0 contained perovskite solar cells exhibit slightly improved performance with a highest PCE of 18.1%, a Jsc of 22.8 mA cm−2, a Voc of 1.07 V, and an FF of 0.75, and the fluorinated polymer PF-1 contained perovskite solar cells present further improved performance with a maximum PCE of 18.7%, Jsc of 22.8 mA cm−2, Voc of 1.08 V, and FF of 0.76. Figure S8 in the Supporting Information presents the corresponding external quantum efficiencies (EQEs) and the integrated products of the EQEs with AM1.5 G photon flux for the champion device. The integrated Jsc values are 21.96 mA cm−2 for the optimized cells, in good agreement with the J–V measurement results. We also recorded the photocurrent of this cell held at a forward bias as a function of time to gain some understanding of the stabilized power output under working conditions (Figure S9, Supporting Information). The photocurrent stabilizes instantly at ≈22.4 mA cm−2, yielding a stabilized power conversion efficiency of 18.3%. This J–V scans provide a more direct representation of the steady power output of our photovoltaic devices. In order to confirm the device performance, we fabricated 20 devices for each condition, and the distribution of PCE is shown in the insert of Figure 2. From the efficiency statistics, both p-type and n-type polymer contained solar cells exhibit improved performance compared to the control devices. And the use of fluorinated polymer can further boost the improvement. As previously reported, adding PCBM as templating agent plays a critical role in improving the quality of the active layer, leading to large grains and fewer grain boundaries; the improvement in Jsc was ascribed to improved photoelectron transport. However, although the p-type and n-type polymers used in our work have opposite nature of charge transport properties and totally different molecular structure, they still result in similar device enhancement. Thus it is intriguing to

followed by 4000 rpm for 30 s. During the last 20 s of the second spin coating step, 100 µL chlorobenzene w/wo conjugated polymer was dropped onto the perovskite. As shown in Figure S5 in the Supporting Information, the UV–vis absorption curve of both film w/wo using polymer exhibit similar features of MAPbI3 perovskite. The only slight difference between 400 and 500 nm may be attributed to the absorbance of the incorporated conjugated polymers. The Table 1. Summary of photovoltaic parameters derived from J−V measurements of perovskite device optimization includes fine adjustment based devices with and without additives. of polymer concentration in chlorobenzene, which exhibits significantly impact on the FF PCEa) [%] Voc [V] Jsc [mA cm−2] relevant PSCs performance (Figures S6–S7 MAPbI3 1.05 (1.03 ± 0.02) 22.5 (22.2 ± 0.3) 0.75 (0.73 ± 0.02) 17.7 (17.5 ± 0.3) and Tables S1 and S2, Supporting Informa1.05 (1.02 ± 0.01) 22.4 (22.1 ± 0.3) 0.76 (0.74 ± 0.02) 17.9 (17.7 ± 0.2) tion). The current–voltage (J–V) characteris- MAPbI3+N2200 1.06 (1.03 ± 0.02) 22.7 (22.4 ± 0.2) 0.76 (0.73 ± 0.02) 18.4 (18.0 ± 0.3) tics of the optimized devices adopting p-type MAPbI3+F-N2200 and n-type polymers under AM 1.5 G illumi- MAPbI3+PF-0 1.07 (1.05 ± 0.03) 22.8 (22.4 ± 0.3) 0.75 (0.73 ± 0.03) 18.1 (17.8 ± 0.2) nation with light intensity of 100 mW cm−2 MAPbI +PF-1 1.08 (1.05 ± 0.01) 22.8 (22.5 ± 0.2) 0.76 (0.74 ± 0.02) 18.7 (18.3 ± 0.3) 3 are shown in Figure 2a,b, respectively, with a)Average results based on 20 devices on each condition. the detailed parameters listed in Table 1.


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carry out further investigation to reveal the mechanisms for the improvement. Consequently, we may then further boost the device performance by rationally tailor the molecular structure and property of semiconducting conjugated polymers.

2.2. Photothermal Deflection Spectroscopy (PDS) Measurements The working mechanism of an antisolvent is reported to speed up heterogeneous nucleation via the creation of an instantaneous local supersaturation on the spinning substrate.[8] During this process, complicated interactions are happening simultaneously. The impurities (catalyst, low molecular weight portion, etc.) in the conjugated polymer may dope the perovskite and change the physicochemical properties.[48,49] To gain insight into the electrical properties of MAPbI3 perovskite w/wo conjugated polymer, we employ a highly sensitive technique for subgap absorption measurement, known as PDS. The PDS spectra of the perovskite MAPbI3 w/wo conjugated polymer are shown in Figure 3. For clarity, we only show PDS spectra for neat MAPbI3, MAPbI3+F-N2200, MAPbI3+PF-1 film prepared on quartz. First, all PDS spectra exhibit sharp optical absorption edge with similar energy gap (Eg) of 1.59 eV. Second all films exhibit similar Urbach energy (Eu) of ≈20 meV, which is a low value and consistent with previous report.[50] The Eu characterizes the exponential decay of band edge and is generally decided by inherent structural and energetics disorder of the material.[51] Therefore, the comparable value measured for the neat MAPbI3 and MAPbI3 with either p-type or n-type polymer suggests similar structural and energetics disorder information in these films, which seem not affected by the impurities in the applied polymers.

2.3. Impact of Conjugated Polymer on Morphology of Perovskite Crystals Lots of previous works have demonstrated that the efficiency and stability of PSCs are strongly affected by the grain size of

Figure 3. PDS spectra of MAPbI3 perovskite thin films of the glass substrate, all measured at room temperature. The inset table is the calculated results of the Eg and Eu of films w/wo polymers.


perovskite crystals. Therefore, a better understanding of the impact of conjugated polymer on the perovskite morphology is crucial to understand the working mechanism of this treatment and to further improve device performance. Herein, the morphology of MAPbI3 perovskite films w/wo conjugated polymer was examined by using scanning electron microscopy (SEM) (Figure 4) and 2D grazing incidence wide-angle X-ray scattering (GIWAXS). SEM images of MAPbI3 perovskite film treated with pure antisolvent, F-N2200 and PF-1 solutions at a concentration of 0.2 mg mL−1 were taken using a Carl Zeiss Supra 55 and are shown in Figure 4 with corresponding size histograms. Pristine MAPbI3 films show a smooth morphology with apparent grain boundary, which is similar to the previous reports. Interestingly, by incorporating conjugated polymer into antisolvent process, we find the conjugated polymers can form a thin and continuous layer on the surface of perovskite film, which may be due to the lower surface energy of conjugated polymer compared to that of MAPbI3 perovskite. In addition, the morphology of some polymer layer is distinctly different, which may be determined by the crystallinity and structure of the adopted polymers. It is reasonable that semicrystalline polymer N2200 and F-N2200 form more ordered thin film on the perovskite surface, whereas less crystalline polymer PF-0 and PF-1 show amorphous film morphology (see Figure S11, Supporting Information). Due to the extremely low content of conjugated polymer (0.2 mg mL−1), it is not surprising to find that the difference of grain size between pristine and polymer treated perovskite film is not significant (Figure 4). Meanwhile, the grain boundary becomes less clear under the well coverage of polymer thin film which may help alleviate the notorious issue of boundary and surface trap. To further verify the influence of conjugated polymer on the crystallization of MAPbI3, GIWAXS is used to probe the crystal features in the pristine and polymer-treated perovskite films. Figure 5a–e shows the 2D GIWAXS patterns of perovskite films of MAPbI3 and MAPbI3 with different conjugated polymers. Due to the strong crystalline nature of perovskite films and the low polymer contents, we do not observe any diffraction peak from conjugated polymers, even for highly crystalline polymer N2200 and F-N2200. All the GIWAXS patterns exhibited typical scattering features of MAPbI3 perovskite, with a stronger peak in the out-of-plane direction at q = 10 nm−1 for all the films, which corresponds to the (110) peak of MAPbI3 perovskite. In addition, the (110) diffraction peak of MAPbI3 perovskite in all 2D GIWAXS patterns is similar under the same intensity scale, indicating similar degree of crystallization for all perovskite films. Similar trend can be also found in the line-cuts of 2D GIWAXS profiles, as shown in Figure 5f, the (110) peak in the out-of-plane direction exhibit similar full width at half maximum (FWHM) values. According to Scherrer equation, the crystal size is closely related to the FWHM of diffraction peaks. Therefore, dissolving relatively low content of conjugated polymer in the antisolvent will not significantly change the grain size of perovskite crystals, which further confirms the results of the SEM measurement. As shown in Figure 5g, we finally investigate the crystal orientation by azimuthally integrating scattering intensity of GIWAXS patterns along the ring at q = 10 nm−1,

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Figure 4. a) SEM of pristine perovskite films, b) MAPbI3+F-N2200, and c) MAPbI3+PF-1, the corresponding column graphs below are the grain size statistics.

which is assigned to the (110) peak of corresponding perovskite structures. Along with the sharp peaks at the azimuth angle of 90°, preferential orientations with peaks distributed at the azimuth angles between 0°–90° and 90°–180° are observed for perovskite film treated with conjugated polymers. These results indicate that multiple-order orientations of perovskite crystal are formed by polymer treatment, which would result in efficient charge transport along multiple directions.[52] Thus, we speculate that it is one of the main factors for the improvement of device performance.

2.4. Steady and Time Resolved Photoluminescence (TRPL) Measurements To further understand the improved device performance by the treatment of conjugated polymers, steady photoluminescence (PL) measurement was introduced to investigate the MAPbI3 perovskite devices w/wo conjugated polymers. Figure 6a,b shows the PL spectra of these samples. We observe apparent PL quenching effect in perovskite films covered with conjugated polymer, suggesting more efficient charge carrier extraction by using polymer treatment. Meanwhile, we confirmed the change of photoluminescence properties of corresponding MAPbI3 film on a quartz substrate (see Figure S12, Supporting Information). The introduction of polymer N2200 onto the perovskite layer quenches and blueshifts the PL peak. Using fluorinated polymer F-N2200 further enhances this effect. This blueshifting of PL peak is attributed to a decrease of spontaneous radiative recombination induced by trap states.[53] Then, TRPL decay measurement was further performed to gain information of charge carrier lifetime. As shown in Figure 6c,d and


stretched exponential decay lifetimes are obtained by fitting the data with a biexponential decay law  t   t  Y = y 0 + A1 exp −  + A2 exp −  (1)  τ1   τ2  where A1 and A2 are the relative amplitudes and τ1 and τ2 are the lifetimes for the fast and slow recombination, respectively. As shown in Table 2, the calculated lifetime reveals a slightly longer PL decay for the polymer treated films than the pristine films. The polymer-containing perovskite films exhibit similar fast decay lifetime of ≈2 ns compared to that in the pristine films, and a longer slow decay lifetime of 52.6 ns (F-N2200), 57.4 ns (N2200), 53.8 ns (PF-1) and 49.8 ns (PF-0), respectively. In contrast, the pristine perovskite film only shows a smaller value of 43.0 ns. In summary, the polymer treatment can passivate the perovskite surface and reduce the defects concentration.

2.5. The Impact of Conjugated Polymer on Device Stability To push the emerging photovoltaic technology based on perovskite materials from academic studies into real life applications, the lifetime is now becoming one of the most important issues need to be addressed. It is known to all that the decomposition of perovskite active layer via corrosion by moisture that intrudes into the active layer through the degraded top electrode is mainly associated with degradation of solar cell devices.[31] Here, we further studied the solar cell stability under illumination in ambient environment, with the facility setup shown in Figures S13 and S14 in the Supporting Information. Figure 7 shows the results of stability tests of the corresponding perovskite solar cells: in an

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Figure 5. 2D GIWAXS images of a) pristine MAPbI3, b) MAPbI3+N2200, c) MAPbI3+F-N2200, d) MAPbI3+PF-0, and e) MAPbI3+PF-1, insets: closeup of the lamellar (110), diffraction region; f) out-of-plane line-cuts of (110) diffraction and g) pole figures extracted from the (110) for corresponding films.

ambient environment with 30–40% relative humidity at 25 °C without encapsulation; under continuous one-sun illumination in ambient environment with 30–40% relative humidity at 25 °C without encapsulation. It is clear from these stability results that although devices containing either n-type polymer N2200 or p-type polymer PF-0 give better stability than that of the control perovskite device. Devices utilizing their fluorinated analog present further substantial improvement in stability. For example, Figure 7a shows the F-N2200-containing perovskite-based devices maintain 85.1% of its initial PCE after 30 d without encapsulation, whereas under the same conditions the measured PCEs decreased to 68.4% and 75.6% of their initial values in the pristine perovskite and N2200-containing perovskite based solar cells, respectively. Similar trend can be found in device adopting p-type conjugated polymer PF-0 and PF-1,


with the results shown in Figure 7b. Moreover, a preliminary test of stability under illumination in Figure 7c,d shows that the perovskite device with fluorinated polymer treatment is more resistant to light stress than the device with nonfluorinated polymer and control device. Figure 7c,d indicates that there is only ≈20% efficiency drop after 50 h continuous one-sun illumination for PSCs containing either F-N2200 or PF-1, whereas under the same conditions the measured PCEs decreased to 20% of their initial values for the pristine perovskite devices. Apparently, devices w/wo polymer treatment exhibit decreased stability under continuous one-sun illumination compared to those under ambient environment. According to recent report,[54] we believe the heating from one-sun illumination will result in the increased sample temperature, which will affect the solar cell device stability.

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Figure 6. Steady PL and time resolved PL measurements of MAPbI3, MAPbI3+N2200, and MAPbI3+F-N2200 a,c) and of MAPbI3, MAPbI3+PF-0, and MAPbI3+PF-1 c,d).

Degradation of perovskite solar cells is mainly associated with the decomposition of the active layer via corrosion by moisture. Here, the device stability under either ambient environment without encapsulation or light stress is significantly improved through incorporating fluorinated conjugated polymers. According to the SEM characterization, the polymer can form continues thin layer mainly on the surface of perovskite film, resulting in smoother surface morphology with less pinholes and grain boundaries. We also investigated the surface properties of perovskite film w/wo polymer. Figure 8a–e shows the contact-angle measurements of using deionized water droplet of pristine, N2200, F-N2200, PF-0, and PF-1-containing perovskite films. With the coverage of conjugated polymers, the derived contact angles increase from 61.1° to 80.4° and 89.1° with nonfluorinated polymer N2200 and PF-0, respectively. The contact angles further increase to 82.3° and 91.8° with fluorinated polymer F-N2200 and PF-1, respectively. These results evidently demonstrate a change of perovskite films from hydrophilic to hydrophobic through introducing conjugated polymers, especially for the fluorinated ones. Therefore, we assumed it to be the main factor for improvement of device stability against moisture. As shown in Figure 8f, we present the Table 2. Summary of charge carrier lifetime calculated from TRPL measurements of perovskite w/wo polymers. Pristine [ns] N2200 [ns] F-N2200 [ns] PF-0 [ns] PF-1 [ns] Fast phase lifetime

2.2

2.9

2.1

2.1

2.0

Slow phase lifetime

43.0

52.6

57.4

53.8

49.8

Average phase lifetime

4.37

4.64

7.12

8.44

7.69


images of the corresponding perovskite films w/wo conjugated polymer after 30 d under the atmosphere environment with ≈30% relative humidity at 25 °C. The pristine perovskite film decomposed heavily and turned to yellow after 30 d keeping in the ambient environment. However, under same conditions, the N2200 and PF-0-containing perovskite films exhibit enhanced stability. More interestingly, the fluorinated polymers F-N2200 and PF-1-containing perovskite films maintain the same color like the fresh made perovskite films. Therefore, we ascribe the improved stability of polymer-containing perovskite mainly to: favorable hydrophobic surface to protect perovskite from moisture, improved interface with less pinholes and grain boundaries. As a result, the antisolvent process adopting conjugated polymers could be an efficient and universal strategy to improve device performance of PSCs, especially the long-term stability.

3. Conclusion In summary, by dissolving functional conjugated polymer in antisolvent to treat the spinning perovskite film, we have demonstrated an efficient and universal protocol for fabricating perovskite solar cells with simultaneously enhanced efficiency and longevity. The conjugated polymers exhibit less effect on the grain size of perovskite crystal but are beneficial for achieving improved charge transport along multiple-direction perovskite crystals. The presence of thin conjugated polymer layer can be observed evidently on the top surface of perovskite film through SEM measurements, which plays an important role in optimizing the morphology, surface passivation, and turning the

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Figure 7. The stability of corresponding perovskite solar cells without encapsulation in an ambient environment with 30–40% relative humidity at 25 °C, a) MAPbI3 w/wo n-type polymer, b) MAPbI3 w/wo p-type polymer, under illumination c) MAPbI3 w/wo n-type polymer, d) MAPbI3 w/wo p-type polymer.

surface to hydrophobic one. All these aspects govern the device efficiency and long-term stability. More importantly, we have shown that through molecular fluorination, the polymers can further strengthen these effects. Through optimizing the concentration of conjugated polymer in antisolvent, an improved PCE of 18.4% and 18.7% was achieved for n-type polymer PF-1

and n-type F-N2200, respectively, in comparison with PCE of 17.7% for the control device. Meanwhile, the remarkable enhancement of the device stability is more attractive for the future commercialization of perovskite solar cells. We believe that the core function of conjugated polymer is a thin protection layer regardless of energy levels, absorption, crystallinity,

Figure 8. The contact angles between perovskite films and the water droplet: a) pristine, b) MAPbI3+N2200, c) MAPbI3+F-N2200, d) MAPbI3+PF-0, e) MAPbI3+PF-1, and f) images of the corresponding perovskite films after 30 d under the ambient environment with a 30% relative humidity and a room temperature.


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and p-type/n-type properties. Considering the vast selection of conjugated polymer materials and easy functional design, our work may provide a facile, versatile, and universal way to fabricate efficient and stable perovskite solar cells.

4. Experimental Section Materials: P-type polymer PF-0 and PF-1, n-type polymer N2200 and F-N2200 were prepared according to the previous report.[45,46] Methylammonium iodide (CH3NH3I), PbI2, and 2,2’,7,7’-tetrakis(N,Ndip-methoxyphenylamine)-9,9’-spirobifluorene(spiro-OMeTAD) were purchased from Xi’an Polymer Light Technology Corp. Anhydrous DMSO and γ-butyrolactone were purchased from Sigma-Aldrich. The planar TiO2 layer was obtained by the hydrothermal deposition which has been reported in previous work. Instruments: General: UV–vis–NIR spectra were recorded on a Perkin Elmer model Lambda 750. A field emission scanning electron microscope (Carl Zeiss Supra 55) was used to perform the SEM. The steady-state and time-resolved PL spectra were recorded with a FluoroMax-4 spectrofluorometer (HORIBA Scientific). The contact angle measurement was conducted Dataphysics Dataphysics equipment with 3 µL deionized water dropping on the surface of perovskite films. Photothermal Deflection Spectroscopy: PDS measurements were carried out with a 1 kW Xe arc lamp and a 1/4 m grating monochromator (Oriel) as the tunable light source. The pump beam was modulated at 13 Hz by a mechanical chopper before irradiating on the sample. Perfluorohexane was used as the deflection fluid. A Uniphase HeNe laser was directed parallel to sample surface as the probe laser. A quadrant cell (United Detector Technology) was used as the position sensor for monitoring the photothermal deflection signal of the probe beam. The output of the detector was fed into a lock-in amplifier (Stanford Research, Model SR830) for phase-sensitive measurements. All PDS spectra were normalized to the incident power of the pump beam. Detailed experimental setup can be found in ref. [55]. Grazing-Incidence Wide Angle X-Ray Scattering: GIWAXS measurements were performed at the Shanghai Synchrotron Radiation Facility Laboratory on Beamline BL14B1 using X-ray with a wavelength of λ = ≈1.24 Å. 2D GIWAXS patterns were acquired by a MarCCD mounted vertically at a distance ≈194 mm from the sample with a grazing incidence angle of 0.4° and an exposure time of 50 s. The 2D GIWAXS patterns were analyzed using the FIT2D software and displayed in scattering vector q coordinates with q = 4πsinθ/λ, where θ is half of the diffraction angle and λ is the wavelength of incident X-ray. Fabrication and Characterization of Perovskite Solar Cells: PSCs were fabricated with a structure of fluorine-doped tin oxide (FTO)/TiO2/MAPbI3/ spiro-OMeTAD/Au. The FTO glass substrates was ultrasonically cleaned and treated with UV−ozone for 20 min. The TiO2 films were deposited onto the substrates by the hydrothermal method. Before the deposition of the perovskite films, a 10 min UV–ozone was carried. The MAPbI3 perovskite films were deposited through a one-step spin-coating process, with antisolvent dripping. CH3NH3I and PbI2 was dissolved in dimethylsulfoxide (DMSO) (99.9%, Alfa Aesar) and γ-butyrolactone (99.9%, Alfa Aesar) (3:7, v/v) at 1.2 m and stirring in room temperature for 4 h. The MAPbI3 precursor solution was coated onto the TiO2 substrate and spun at 1000 and 4000 rpm for 10 and 40 s, respectively. The antisolvent process was utilized to make the perovskite films with a better morphology and crystallization, in which 120 µL of chlorobenzene w/wo conjugated polymer was swiftly dripped onto the substrate 20 s preliminary to the end of the program during the high-speed spin-coating step. Conjugated polymers with different concentration were dissolved in the chlorobenzene and stirred for 4 h at 45 °C before antisolvent process. The substrate was then dried at 100 °C for 10 min. The solution of spiro-OMeTAD was spin coated on the MAPbI3 films at 5000 rpm for 30 s before a 100 nm of Au was deposited by thermal evaporation under vacuum of 2 × 10−6 mbar. The active area of the cell was defined as 7.25 mm−2 through a shadow mask.


The current density–voltage (J−V) curves of the devices were obtained with a Keithley 2400 digital source meter under simulated AM 1.5G spectrum at 100 mW cm−2, with a solar simulator (Class AAA, 94023A-U; Newport Corporation). The light intensity was calibrated by a National Renewable Energy Laboratory-certified Oriel reference cell (91150 V). The incident photon-to-electron conversion efficiency (IPCE) spectra were recorded through certified IPCE equipment (Solar Cell Scan 100; Zolix Instruments Co. Ltd.).

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the National Key Research Projects (Grant No. 2016YFA0202402), the Natural Science Foundation of Jiangsu Province of China (BK20170337), the National Natural Science Foundation of China (Grant No. 51761145013 and 61674111), and “111” projects. The authors thank the Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University. The authors also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Conflict of Interest The authors declare no conflict of interest.

Keywords conjugated polymers, fluorination, perovskite solar cells, stability Received: November 2, 2017 Revised: December 27, 2017 Published online:

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Supporting Information for Adv. Funct. Mater., DOI: 10.1002/adfm.201706377

A Universal Strategy to Utilize Polymeric Semiconductors for Perovskite Solar Cells with Enhanced Efficiency and Longevity Fangchao Li, Jianyu Yuan,* Xufeng Ling, Yannan Zhang, Yingguo Yang, Sin Hang Cheung, Carr Hoi Yi Ho, Xingyu Gao, and Wanli Ma*

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2016.

Supporting Information A Universal Strategy to Utilize Polymeric Semiconductor for Perovskite Solar Cells with Enhanced Efficiency and Longevity Fangchao Li, Jianyu Yuan,* Xufeng Ling, Yannan Zhang, Yingguo Yang, Sin Hang Cheung, Carr Hoi Yi Ho, Xingyu Gao, Wanli Ma*

Content

1. UV-vis absorption 2. Optimization of perovskite solar cells 3. EQE and steady output 4. Scanning electron microscopy 5. Steady and time resolved photoluminescence measurement 6. Stability measurement

1

Figure S1. Photograph and molecular structure of different additives in chlorobenzene (0.4 mg/mL).

Figure S2. Thin film absorption of PF-0, PF-1, N2200 and F-N2200 cast from chloroform solutions.

Figure S3. Cyclic voltammograms of PF-0, PF-1, N2200 and F-N2200. 2

Figure S4. Line-cuts of 2d GIWAXS patterns extracted from PF-0, PF-1, N2200 and F-N2200.

Figure S5. UV-vis absorption spectra of w/wo additives in perovskite films on the quartz substrate.

Figure S6. J–V characteristics of PSCs with different concentration of N2200 in the antisolvent.

3

Table S1 Photovoltaic parameters of planar heterojunction PSCs with different concentration of N2200 in the anti-solvent. N2200

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

0 mg/mL

1.04

22.4

0.73

17.1

0.1 mg/mL

1.05

21.8

0.76

17.3

0.2 mg/mL

1.07

21.4

0.77

17.6

0.4 mg/mL

1.06

21.5

0.71

16.2

0.8 mg/mL

1.03

21.5

0.70

15.4

2

Current Density (mA/cm )

25

20

15 0 mg/mL 0.1 mg/mL 0.2 mg/mL 0.4 mg/mL 0.8 mg/mL

10

5

0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage (V) Figure S7. J–V characteristics of PSCs with different concentration of PF-0 in the anti-solvent. Table S2 Photovoltaic parameters of planar heterojunction PSCs with different concentration of PF-0 in the anti-solvent. PF-0

Voc (V)

Jsc 2 (mA/cm )

FF

PCE (%)

0 mg/mL

1.04

22.4

0.73

17.1

0.1 mg/mL

1.07

22.8

0.72

17.7

0.2 mg/mL

1.08

22.9

0.73

17.9

0.4 mg/mL

1.05

22.3

0.74

17.3

0.8 mg/mL

1.04

22.3

0.70

16.3

4

20

60

15

40

10

20

5

0 300

400

500

600

700

800

2

80

Integrated Jsc (mA/cm )

25

EQE (%)

100

0

Wavelength (nm)

25

25

20

20

15

15

2

Current density (mA/cm )

Figure S8. EQE and integrated Jsc spectra of the champion device.

10

Efficiency (%)

10 Power maximum at 0.91 V 5

0

20

40

60

80

5 100

Decay Time (s)

Figure S9. Steady output of photocurrent density and power conversion efficiency measured under maximum power at 0.91 V. The cell was illuminated under 1 sun AM1.5G prior to the start of the measurement.

5

Figure S10. J-V curves of the champion devices w/wo polymer treatment under different scanning directions.

Figure S11. SEM of (a) MAPbI3+N2200 and (b) MAPbI3+PF-0

Figure S12. Steady-state PL spectra of corresponding films on quartz substrates. 6

Figure S13. Image of equipment for stability measurements in darkness, the temperature is around 25 oC and the relatively humidity is 30-40 %.

Figure S14. Image of equipment for stability measurements under illumination, the relatively humidity is 30 %.

7