A Multifunctional Bis-Adduct Fullerene for Efficient Printable

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A Multifunctional Bis-Adduct Fullerene for Efficient Printable Mesoscopic Perovskite Solar Cells Chengbo Tian, Shujing Zhang, Anyi Mei, Yaoguang Rong, Yue Hu, Kai Du, Miao Duan, Yusong Sheng, Pei Jiang, Gengzhao Xu, and Hongwei Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18945 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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A Multifunctional Bis-Adduct Fullerene for Efficient Printable Mesoscopic Perovskite Solar Cells Chengbo Tian,[a]† Shujing Zhang,[a]† Anyi Mei,[a] Yaoguang Rong,[a] Yue Hu,[a] Kai Du,[b] Miao Duan,[a] Yusong Sheng,[a] Pei Jiang,[a] Gengzhao Xu,*[b] Hongwei Han*[a] a

Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for

Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, Hubei, P.R. China b

Suzhou Institute of Nano-Tech and Nano-Bionics, CAS, Suzhou 215123, Jiangsu P. R. China.

Keyword: perovskite solar cells, printable, fullerene derivative, electric, trap passivation

ABSTRACT

The printable mesoscopic perovskite solar cells (PMPSCs) have exhibited great attracting prospects in the energy conversion field due to their high stability and potential scalability. However, the thick perovskite film in the mesoporous layers challenge the charge transportation and increase grain boundary defects, limiting the performance of the PMPSCs. It is critical not only to improve the electric property of the perovskite film but also to passivate the charge traps to improve the device performance. Herein we synthesized a bis-adduct 2, 5-(Dimethyl ester) C60 fulleropyrrolidine (bis-DMEC60) via rational molecular design and incorporated it into the PMPSCs. The enhanced chemical interactions between perovskite and bis-DMEC60 improve conductivity of the perovskite film as well as elevate the passivation effect of bis-DMEC60 at the

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grain boundaries. As a result, the FF and PCE of the PMPSCs containing bis-DMEC60 reached 71% and 15.21%, respectively, significantly superior to analogous mono-adduct derivative (DMEC60) containing and control devices. This work suggests that fullerene derivatives with multifunctional groups are promising for achieving high performance PMPSCs.

1. Introduction Organic-inorganic hybrid perovskite materials (typically CH3NH3PbI3) with excellent properties, such as strong light absorption capacity,1,

2

tunable elementary composition and

bandgap,3 long-balanced carrier diffusion length,4-6 high charge carrier mobility,7 have attracted much attention from researchers for their application in the photovoltaic field. The efficiency of PSCs has rocketed from 3.9% to a certified 22.7% in just few years.8 This remarkable increase is a result of an extensive research effort towards the optimization of the perovskite film’s microstructure, through the application of new materials and evolution of device structures, making PSCs the most promising solar cells for the next-generation of photovoltaics.9-14 With efficiencies constantly rising, the stability and simple scalable processing of PSCs have become vital for their commercialization in the future.15-19 Specifically, Han and coworkers considered the properties of perovskite across the board, including the ambipolar transport property of the perovskite, as well as the scalability of the devices, and developed a holeconductor-free printable mesoscopic perovskite solar cell (PMPSC) using a triple layer of mesoporous TiO2, ZrO2 and a thick carbon electrode in 2013.20-22 Then, our group developed and employed a double cation perovskite material (5-AVA)x(MA)1-xPbI3 as a photoactive layer for the device, which showed an excellent stability in ambient condition under illumination and a certified efficiency exceeding 12%.12, 23-26

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However, the perovskite films filling in the thick nanocrystal layer and insulated mesoporous layer decrease the carrier transportation and increase the carrier recombination within the device, thus limiting further improvement of the PCE of PMPSCs. Therefore, it is vital for further optimizing the PMPSCs’ performance to enhance the conductivity of perovskites films and suppress the recombination at the grain boundaries. As a well-known material, fullerene derivative is an efficient additive for improving the performance of PSCs.27,

28

It has been

reported that the interactions between fullerene derivative and halide could facilitate the improvement of conductivity of fullerene derivative.29, 30 Meanwhile, the carrier traps at the grain boundaries of perovskite films can be passivated efficiently by fullerene derivatives.31,

32

According to previous reports, materials containing nitrogen and oxygen atoms with lone pairs of electrons are known to effectively passivate trap states in the perovskite film for improved device performance.33, 34 Thus, we propose a new fullerene derivative containing nitrogen and oxygen atoms in its adduct group, which could readily interact with the perovskite via lead-oxygen, hydrogen bonding, and fullerene-halide radical interactions. These interactions can elevate the passivation effect of the fullerene derivative on the perovskite film and also can improve the conductivity of the perovskite films.35, 36 Herein, we designed and synthesized a fullerene derivative, bis-DMEC60, with two functional groups containing nitrogen and oxygen atoms and incorporated it into PMPSCs. As depicted in Figure 1b, due to the effective interaction between the fullerene molecule and perovskite, bis-DMEC60 effectively passivated the grain boundary defects of the perovskite layer. Significantly, as verified by conductive AFM (c-AFM) technique, the conductivity of the perovskite films was improved obviously, which could elevate the efficiency of charge transportation in the thick perovskite film. Meanwhile, the high LUMO energy level of bis-

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DMEC60 can restrain the electron transportation from perovskite to fullerene derivative, thus facilitating the passivation effect of bis-DMEC60 for perovskite. Therefore, the efficiency of PMPSCs is boosted by the bis-DMEC60 from 13.01% to 15.21% with a significantly enhanced fill factor. The present work provides guidance for understanding the relationship between fullerene structure and device performance, and suggests that the high-LUMO-level fullerene derivatives with functional atoms in its substituent group are promising for high performance PMPSCs. 2. Results and discussion As presented in Scheme 1, bis-DMEC60 was synthesized following a previously reported procedure. 36 The oxygen and nitrogen atoms with lone pairs of electrons including in the adduct groups could facilitate the passivation effect of bis-DMEC60 for the perovskite film.33, 34 The two functional groups elevate the strength of the interactions between perovskite and bis-DMEC60. The interactions between fullerene derivative and iodide at the perovskite grain boundaries, facilitate the improvement of conductivity of perovskite films.29 In addition, the two adduct groups can efficiently increase the solubility of bis-DMEC60 in γ-butyrolactone, ensuring systematic exploration of the effective amount of fullerene in perovskite film.

Scheme 1. Synthesis of DMEC60 and bis-DMEC60.

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Figure 1. a) FTIR spectra of bis-DMEC60 and bis-DMEC60-PbI2. b) Schematic diagram of the interactions between bis-DMEC60 and perovskite. The interaction between bis-DMEC60 and PbI2 was detected using the Fourier Transform Infrared (FTIR) technology as displayed in Figure 1a. The C=O vibration appears at 1749 cm−1 in the pure bis-DMEC60, and shifted to 1740 cm−1 in the PbI2 and bis-DMEC60 mixture. The shift accounts for the decrease in bond strength for C=O upon the interaction between PbI2 and bisDMEC60. The stretching vibrations of the N-H bond appear at 3296 cm−1 for bis-DMEC60, and disappear upon combination of the compound with PbI2. The above results are similar with those that have been reported for DMEC60.36 The shift of the stretching C=O vibration peak and disappearance of the N-H vibration are reasonably attributed to the coordination effect between the oxygen and nitrogen atoms on bis-DMEC60 with the lead atoms of PbI2. From EDS mapping experiments (Figure S1), we infer that bis-DMEC60 and perovskite exhibit same distributions. Meanwhile, the XRD spectra of different perovskite films are in identical positions (Figure S7), indicating that bis-DMEC60 is located at the grain boundaries in the perovskite film, which is consistent with the previous report.34 Therefore, we propose that bis-DMEC60 can enhance the

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passivation for the carrier traps at the grain boundaries as well as improve the electric property of the perovskite films via fullerene-halide radical as depicted in Figure 1b.29 To study if bis-DMEC60 has suitable LUMO energy level for the perovskite, the electrochemical characteristics of bis-DMEC60 were analyzed using cyclic voltammetry (Figure S2). In agreement with previous literature and our photoluminescence (PL) studies (vide infra), bis-DMEC60 with a slight higher LUMO energy level can also extract electron from perovskite.27 The effect of bis-DMEC60 on the perovskite film were studied by the stead-state PL and TRPL technology.12 Note that since the highest device performance was yielded at Cbis-DMEC60 = 0.5 mg/ml when creasing Cbis-DMEC60 from 0 to 0.75 mg/ml (see Supporting Information), Cbis-DMEC60 = 0.5 mg/ml was selected for further studies. As exhibited in Figure 2a, the steady-state PL spectra of different perovskite films, which were fabricated on mesoporous ZrO2 layer coated glass substrates, were measured under the same conditions. The control perovskite film shows the highest PL intensity. In contrast, the DMEC60- and bis-DMEC60-containing perovskite films show obvious fluorescence quenching effect, which result from the charge transfer from the perovskite to DMEC60 and bis-DMEC60. Even though bis-DMEC60 has slight higher LUMO energy level than that of DMEC60, the bis-DMEC60-containing perovskite films displayed higher PL intensity than that of DMEC60 based perovskite films, suggesting the higher LUMO energy level of bis-DMEC60 might good for extending the carrier lifetime of perovskite films. On the other hand, the slight blue-shifting of steady-state PL spectra are observed in the bis-DMEC60containing and DMEC60-containing perovskite films, which can be attributed to the passivation effect.32

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Figure 2. a) steady-state PL spectra, and b) TRPL spectra of different perovskite films on the FTO/ZrO2 substrate. The TRPL curves of the different perovskite films were also obtained as depicted in Figure 2b. The control perovskite film prepared on the glass substrate exhibited a lifetime of about 44 ns. In contrast, the DMEC60- and bis-DMEC60-containing perovskite films exhibited lifetimes of 60 ns and 81 ns, respectively. These results demonstrate that the bis-DMEC60-containing perovskite film has less charge traps that suppress charge recombination and could result in a higher VOC and FF of the corresponding PMPSCs. To probe the effect of fullerene derivatives on the conductivity of the perovskite films, the electric properties of the perovskite films were studied by c-AFM under dark conditions. As shown in Figure 3a, b, c, the conductivity of both the control and fullerene derivative containing perovskite films is greatest at grain boundaries. It is noteworthy that the fullerene containing perovskite films show higher conductivity, consistent with the

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Figure 3. c-AFM images of the perovskite films a) control, b) DMEC60 and c) bis-DMEC60 on the FTO substrate recorded in the dark (positive bias voltage: 1 V). Corresponding AFM topographic images d) control, e) DMEC60 and f) bis-DMEC60. fullerene-iodide radical with high conductivity accumulating at grain boundaries.29 Meanwhile, probably due to the stronger interaction between bis-DMEC60 and perovskite materials, the bisDMEC60 containing perovskite film shows higher conductivity than that of DMEC60 based film. This results suggest that the highest conductivity of bis-DMEC60-containing perovskite films could significantly facilitate the charge transportation at the grain boundaries of perovskite film, improving the FF and efficiency of the devices. To rule out the effect of fullerene derivatives on the morphology of perovskite films, a series of experiments were conducted using AFM technology for the different perovskite films. As shown in the Figure 3d, e, f, there is no

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noticeable morphology change for the different perovskite films. The negligible morphological changes can also be observed from the cross-sectional SEM images in Figure S6. For comprehension of the effect of the bis-DMEC60 on the PMPSCs’ performance, triple mesoscopic

layer

structured

devices

were

prepared,

with

configuration

of

FTO/TiO2/ZrO2/Carbon/perovskite. The schematic structure of the device is presented in Figure 4a. The cross-section shown in Figure 4b allows clear observation of all the layers. For convenience, (5-AVA)0.03(MA)0.97PbI3 based devices was denoted as control devices, DMEC60 doped control devices as DMEC60-containing devices and bis-DMEC60 doped control devices as bis-DMEC60-containing devices in this paper. As shown in Figure S6, the cross-sectional SEM images of control, DMEC60- and bis-DMEC60-containing devices clearly show that the compactable and homogeneous grains of the perovskite film should provide a favorable performance and reproducibility. The XRD results, as shown in Figure S7, coincide with the above discussion that all the perovskite films show similar crystallinity along the (110) plane. As shown in Figure 4c and Table 1, the control devices produced a reasonable efficiency of 13.01%. The efficiency is comparable to the certified efficiency of the previous report.12 In contrast, the DMEC60-containing PMPSCs display a slight improvement in efficiency at 13.63%. We obtain the best efficiency of 15.21% for bis-DMEC60-containing device. Compared with control and DMEC60-containing devices, bis-DMEC60-containing PMPSCs show significant improvements in PCE and FF. The JSC and VOC also show slight improvement, which can be attributed to the reduced recombination of hole-electron and enhanced conductivity of the corresponding perovskite film. This results are consistent with the dark J-V curve of the PMPSCs (Figure S8).

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Figure 4. a) Schematic structure of the PMPSCs. b) Configuration of the PMPSCs from crosssectional SEM image c) J–V curves of PMPSCs with different perovskite components. d) steadystate output for bis-DMEC60 containing PMPSCs e) PCEs distribution of devices based on corresponding perovskite films. f) J-V curves of bis-DMEC60 containing devices with respect to forward and reverse scan. Table 1. The performance of PMPSCs with different perovskite components. Devices

VOC (V)

JSC (mA/cm2)

FF

PCE (%)

control

0.88

22.73

0.65

13.01

DMEC60

0.89

22.97

0.67

13.63

bis-DMEC60

0.92

23.30

0.71

15.21

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The JSC values of the control, DMEC60- and bis-DMEC60-containing devices were verified by the external quantum efficiency (EQE) spectrum shown in Figure S9. In order to check reproducibility, a statistical photovoltaic parameter of data of three device batches are shown in Figure 4d and Figure S10. The control, DMEC60- and bis-DMEC60-containing devices yield average efficiency of 12.6±0.5%, 13.3±0.3% and 14.9±0.3%, respectively. The maximal steadystate photocurrent outputs and their corresponding power outputs were applied to further confirm the efficiency of the devices. Bis-DMEC60-containing device yields a PCE of 15.1% with a photocurrent of 20.9 mA/cm2 at 0.72 V (Figure 4e). DMEC60-containing device yields a PCE of 13.5% with a photocurrent of 19.9 mA/cm2 at 0.68 V. Control device yields PCE of 12.9% with a photocurrent of 18.5 mA/cm2 at 0.70 V (Figure S11). All the parameters are consistent with the values determined from the J-V data. Therefore, the narrower distribution of the key photovoltaic parameters and coincident efficiencies illustrate that the device performance is reproducible and believable. A hysteresis was observed for the control device when testing the devices with increased or decreased bias (Figure S12 and Table S3). The hysteresis phenomenon results from the charge trapping and releasing processes in the devices.31 Due to the efficient interactions between bisDMEC60 (or DMEC60) and perovskite, the hysteresis was suppressed when DMEC60 or bisDMEC60 were present in the PMPSCs. The results suggest that DMEC60 and bis-DMEC60 can effectively passivate the presence of charge traps in the perovskite film. More specifically, as displayed in Figure 4f, the J-V curve of bis-DMEC60-containing devices show no clear differences, regardless of the scan direction, demonstrating the stronger interactions between bisDMEC60 and perovskite can efficiently improve the defect passivation. The conclusion is consistent with the faster photocurrent response speed of the DMEC60- and bis-DMEC60-

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containing devices. The fast progression of photocurrent of the DMEC60- and bis-DMEC60containing devices can be attributed to the passivation effect of the fullerene derivatives (Figure S13). In addition, the ambient stability tests have been performed on the devices in our lab as shown in Figure S14. The PMPSCs based on different perovskite film showed similar ambient stability, indicating DMEC60 and bis-DMEC60 doping did not affect the devices stability.

Figure 5. a) Nyquist plots of the PMPSCs with different perovskite components. (b) The plot of the charge recombination lifetime for PMPSCs based on different perovskite films. The above observations motivate further exploration of the charge transfer processes in PMPSCs. As shown in Figure 5a, electrical impedance spectroscopy (EIS) measurements were conducted for control, DMEC60- and bis-DMEC60-containing PMPSCs under illumination with a bias of 600 mV. After fitting the data to a simple equivalent circuit,37 we found that bisDMEC60-containing devices exhibited the smallest radius of the semicircle at the high frequency region, suggesting the lowest Rct is exsist in the bis-DMEC60 based devices. Meanwhile, the largest radius of the semicircle exists in the low frequency region, attributed to the largest recombination resistance in the bis-DMEC60-containing devices. The lowest Rct and the largest Rrec for the bis-DMEC60-containing devices could be the result of an improvement of the VOC and ACS Paragon Plus Environment

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FF. To further evaluate the role of bis-DMEC60, the charge recombination lifetimes of the devices were studied by intensity-modulated photovoltage spectroscopy (IMVS) as depicted in Figure 5b. The longest recombination lifetime for bis-DMEC60 containing devices demonstrated the slowest recombination rate of the corresponding devices, which contribute to an enhanced VOC.38 Therefore, all the results above are totally consistent with the improvement of performance of the bis-DMEC60-containing PMPSCs. 3. Conclusions In summary, we designed and synthesized bis-DMEC60 and incorporated it into PMPSCs. Benefiting from the enhanced conductivity of the perovskite films and the elevated passivation effect, the FF and PCE of PMPSCs containing bis-DMEC60 reached 71% and 15.21%, respectively, significantly superior to DMEC60-containing and control devices. Meanwhile, the bis-DMEC60-containing PMPSCs showed negligible hysteresis due to the suppressed trap filling process. The work described here first demonstrates the great usefulness of applying fullerene derivative to enhance the performance of PMPSCs. It facilitates our understanding of the relationship between the fullerene structure and device performance, and suggests that multiadduct fullerene derivatives with specific atoms are promising for high performance PMPSCs. It is highly anticipated that multifunctional fullerene derivatives will be altered for the development of efficient fullerene in the near future. 4. Experimental Section 4.1. Synthesis of bis-DMEC60. As presented in Scheme 1, bis-DMEC60 was synthesized following a previously reported procedure.36 After purification with a silica gel column, pure DMEC60 (14.5 mg, yield: 21%) and bis-DMEC60 (33.2 mg, yield: 32%) were obtained. The mole ratio of DMEC60 and bis-DMEC60 ACS Paragon Plus Environment

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is around 2:3. The compounds structure were determined by 1H NMR,

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13

C NMR, and the

atmospheric pressure chemical ionization sources coupled with a single quadruple mass spectrometer (APCI-MS) was used to determine the molecular mass (Figure S15-20). 4.2. Materials and perovskite precursor solution preparation. All the related chemical materials and agents were purchased from Acros organics and used without further purification. The perovskite precursor solution for the control devices was prepared with 368.8 mg PbI2, 123.4 mg MAI, 5.9 mg 5-AVAI and 1.0 ml γ-butyrolactone. The perovskite precursor solutions for DMEC60- and bis-DMEC60-containing devices were prepared in the same manner except for the varying amount of corresponding material in the solution. 4.3. Device Fabrication. The devices were prepared according to previous reports.12 Typically, fluorine-doped SnO2 (FTO) glass was patterned by a laser and then cleaned by sequential sonication with different solution or solvent. After the FTO glass substrates dried, a thin TiO2 compact layer was covered on the cleaned FTO substrates by spray pyrolysis technology. Then, the mesoporous TiO2 (800 nm), ZrO2 (1.5 µm) layer and carbon electrode (10 µm) were printed sequentially by screen printing technology with a pressure of 8 MPa on top of the compact TiO2 layer. To complete the devices, a 4 µL perovskite solution was dipped on the top of mesoporous layers of the device and annealed at 50 °C for 2 hours. 4.4. Characterization. J–V curves of the PMPSCs were obtained on a Keithley 2400 source measure unit under the air mass 1.5 illumination (AM1.5, 100 mW/cm2), and a standard Si solar cell was used for the light intensity calibration. The EQEs were tested by a 150 W xenon lamp (Oriel). The J-V and EQE tests were performed under ambient condition. The UV-Visible spectra of DMEC60 and bisDMEC60 were tested by a UV-3600 spectrophotometer. The interaction information between ACS Paragon Plus Environment

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PbI2 and fullerene derivative was recorded using a FTIR spectrometer from Bruker Company. The SEM images were provided by a field-emission SEM. The XRD curves were recorded using Cu Kα radiation (λ = 1.5418 Å). The PL spectra were provided by a LabRAM HR800 Raman Microscope with a laser beam at 532.16 nm. The TRPL experiments of the perovskite layer were conducted on a Delta Flex Fluorescence Lifetime System. EIS experiments were performed on an electrochemical workstation in the frequency range from 10 mHz to 4 MHz under illumination. The molecular mass of DMEC60 and bis-DMEC60 was provided by a Bruker Esquire HCT mass spectrometer with an atmospheric pressure chemical ionization (APCI) ion source.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publications website. EDS and cross-sectional SEM images of the devices with different perovskite components, XRD spectra of the perovskite films; J−V curves, key parameters distribution, IPCE curves, ambient stability, maximal steady-state photocurrent and power output of the PMPSCs with different perovskite components. Additional APCI-MS spectrum, 1H NMR spectrum, 13C NMR spectrum and CV curves for the DMEC60 and bis-DMEC60. AUTHOR INFORMATION Corresponding Author ACS Paragon Plus Environment

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[email protected] [email protected]

Author Contributions †

These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (91433203, 61474049, 51502141), the Ministry of Science and Technology of China (2015AA034601) and the 111 Project (No. B07038) for support of this work. The Analytical and Testing Center of HUST is also gratefully acknowledged for providing various characterization and measurements.

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REFERENCES (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E.; Gratzel, M.; Park, N. G. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591-597. (3) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764-1769. (4) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. (5) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344-347. (6) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-Hole Diffusion Lengths> 175 mm in Solution-Grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967-970.

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