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Improved External Quantum Efficiency from Solution-Processed (CHNH)PbI Perovskite/PC BM Planar Heterojunction for High Efficiency Hybrid Solar Cells 71

Sanghyun Paek, Nara Cho, Hyeju Choi, Hanbin Jeong, Jin Sung Lim, Jun Yeon Hwang, Jae Kwan Lee, and Jaejung Ko J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508162p • Publication Date (Web): 22 Oct 2014 Downloaded from http://pubs.acs.org on October 26, 2014

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The Journal of Physical Chemistry

Improved External Quantum Efficiency from Solution-Processed (CH3NH3)PbI3 Perovskite/PC71BM Planar Heterojunction for High Efficiency Hybrid Solar Cells Sanghyun Paek,† Nara Cho,† Hyeju Choi,† Hanbin Jeong,‡ Jin Sung Lim,‡ Jun-Yeon Hwang,§ Jae Kwan Lee,‡,* and Jaejung Ko†,* †

Department of New Material Chemistry, Korea University, Chungnam, 330-700, Republic of

Korea. ‡Department of Carbon Materials and Department of Chemistry Education, Chosun University, Kwangju, 501-759, Republic of Korea. §Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Jeonbuk 565-902, Republic of Korea.

ABSTRACT. Well-organized (CH3NH3)PbI3 perovskite films fabricated from various solution processing conditions were characterized and used in hybrid solar cells with PC71BM planar heterojuncion films, exhibiting a high power-conversion efficiency of 12.2% with the better photocurrent and fill factor compared to those with PC61BM due to a better spectral response in the visible region and a better planar junction with Ag electrode than the PC61BM.

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KEYWORDS. Perovskite, Nanocrystal, Planar heterojunction, Hybrid, Solar cell.

INTRODUCTION Over the past few years, organometal halide perovskite solar cells have received considerable attention in the scientific community due to their promising breakthrough of over 15% power conversion efficiency (PCE) and readily available using efficient solution-processed techniques.1-5 The alkylammonium lead halides, (RNH3)PbX3 (R = alkyl, X = Cl, Br, I), are direct band gap materials with hybrid organic-inorganic perovskite structures, which could be synthesized through chemical deposition or spin-casting with precursor solutions.6-7 These were initially used as photosensitizer in liquid dye-sensitized solar cells (DSSCs) because they exhibit strong light harvesting ability across the visible solar spectrum. Since that, improving the performance of solar cell using perovskite materials have been extensively investigated in recent years.8-10 In particular, the solid-state perovskite solar cells with hole-transporting material such as 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-MeOTAD) were achieved up to high PCE of 15%, as reported by Gratzel and Snaith research groups, where the (RNH3)PbX3 perovskite stuctures effectively performed as electron conductors as well as photosensitizers.11,12 Very recently, Jeon et al. reported that (CH3NH3)Pb(I1-xBrx)3 perovskite materials fabricated from a mixed solvent of γ-butyrolactone (GBL) and dimethylsulphoxide (DMSO) followed by toluene drop-casting led to significantly improved PCE of 16.2% in device structure with TiO2 mesoporous and compact layer.13 Moreover, these perovskite materials also exhibited the outstanding performances as superior hole conductor14-16 and photosensitizer in the planar heterojunction (PHJ) hybrid devices with and [6,6]-phenyl-C61-butyric acid methyl ester


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(PC61BM) like a conventional bilayer structure of organic solar cells.17 Interestingly, both electron and hole diffusion length were determined as long-ranged scales of ~100 nm and above 1 µm in triiodide (CH3NH3)PbI3 (MAPbI3)and mixed halide (CH3NH3)PbI3-xClx perovskite materials, respectively.18,19 Recently Sun et al. reported a PCE of 7.4% from simple PHJ solar cell comprising of MAPbI3/PC61BM bilayer configuration.20 And Liang et al. reported that the use of additive such as 1,8-diiodooctane (DIO) has afforded these device structure with high PECs of ~12% from enhanced crystallization of perovskite film.21 Therefore, it has been challenging to find efficient fabrication methods of solution-processed (RNH3)PbX3 perovskite films in PHJ hybrid solar cells. Very recently, the highest efficiency of 14.1% in MAPbI3/PC61BM PHJ solar cell was reported by Seo et al.22 via using LiF buffer layer and very thin PC61BM on MAPbI3 perovskite materials fabricated from a mixed DMSO-GBL followed by toluene dripping. Thusly inspired, we have attempted to improve the performances of PHJ hybrid solar cells using MAPbI3 perovskite layer and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) layer, which often exhibited a better external quantum efficiency in photocurrent than PC61BM due to a better spectral response in the visible region23, and used the Ag electrode as electron collecting material due to better practical in all solution-processed system than Al electrode. Herein, we report the high-performance PHJ hybrid solar cells comprising of MAPbI3 (DMSO/GBL-Tol) perovskite, which was fabricated from solvent-engineering techniques reported by Seo et al.22, and PC71BM PHJ films, exhibiting high PCEs of 12.2%. These outstanding performances were achieved from PHJ hybrid devices configuration of ITO/PEDOT:PSS/MAPbI3 (DMSO/GBL-Tol)/PC71BM/Ag without insertion of Bis-C60 surfactant, bathocuproine, or LiF buffer layers between PCBM film and metal electrode20-22, which were compared with the PHJ hybrid solar cells fabricated with


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various counterparts such as MAPbI3 (DMF) and MAPbI3 (GBL) perovskite films, which was obtained from dimethylformamide(DMF) and GBL solution, respectively, or PC61BM layer. Scheme 1 shows description of (a) PHJ solar cell device structure, (b) energy level diagram, and (c) transmittance spectra of MAPbI3/PC71BM (or PC61BM) used in this work.

EXPERIMENTAL SECTION Measurements and Instruments : J-V measurements were made under simulated 100 mW/cm2 AM 1.5G irradiation from a 1000 W Xe arc lamp (Oriel 91193). The light intensity was adjusted with a Si solar cell that was double-checked with a National Renewable Energy Laboratory (NREL)-calibrated Si solar cell (PV measurement Inc.). The applied potential and cell currents were measured using a Keithley model 2400 digital source meter. The J-V curves were measured at the voltage settling time of 100 ms. The IPCE spectra for the cells were recorded on an IPCE measuring system (PV measurements) with 5 seconds per one point (60 points) in 300-900 nm at applied bias voltage of 0.65 V. Device Fabrication Method : PHJ films were prepared under optimized conditions according to reported protocol.22 Indium tin oxide (ITO)-coated glass substrates were cleaned with detergent, ultrasonicated in acetone and isopropyl alcohol, and subsequently dried overnight in an oven. PEDOT:PSS (Heraeus, Clevios P VP.AI 4083) in aqueous solution was spin-cast on the ITO substrates to form a film ~35 nm thick. The substrate was dried for 10 min at 140 °C in air, then transferred into a glove box before spin-casting on the photoactive layer. PbI2 was purchased from Aldrich and CH3NH3I was prepared similarly to method reported previously.8 PbI2 and CH3NH3I were added to a 250 mL flame-dried 2-neck round-bottom flask with GBL. The reaction mixture was heated to 100oC for 30 min. The reaction mixture was then cooled to


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solidify followed by filtration using Büchner funnel. Then, the MAPbI3 was washed with hexane to obtain the purified form. This approach may minimize the PbI2 or CH3NH3I structures and facilitate the inter-networking to extend MAPbI3 crystalline domains. MAPbI3 solution was stirred in DMF, GBL, or mixture of GBL and (DMSO) (7:3 v/v) at 60 oC for 12 h. The MAPbI3 solution was spin-cast on the PEDOT:PSS/ITO substrate with consecutive two step at 2000 and 5000 rpm for 30s and 20s, respectively. The toluene of 1mL was introduced drop-wise onto the substrate during the second step of spin coating according to reported protocol.18 Then the substrate was dried on a hot plate at 100 °C for 10 min. The PCBM (12 mg/mL) solutions were spin-cast on top of the MAPbI3 layer, and the substrate was dried for 1 h at room temperature in air. Finally, a metal electrode such as Ag, Al, or Au was deposited with thickness of ~100 nm on top of the MAPbI3/PCBM PHJ film under reduced pressure (lower than 10–6 Torr).

RESULTS AND DISCUSSION Figure 1 shows field emission scanning electron microscope (FESEM) surface images and X-ray diffraction (XRD) patterns of MAPbI3 (DMF) (a), MAPbI3 (GBL) (b), and MAPbI3 (DMSO/GBL-Tol) (c)

perovskite films spin-cast on the ITO/PEDOT:PSS substrate with

precursor solutions prepared in DMF, GBL, and a mixed DMSO-GBL followed by toluene dripping, respectively. In this work, we employed the spin-casting method with consecutive two rpms to make the better morphologies of MAPbI3 perovskite films. The toluene treatment during casting of precursor solution did not affect the morphology of films cast from DMF or GBL solution. (see the Figure S1 in supporting information). As shown in Figure 1, the MAPbI3 (DMF) and MAPbI3 (GBL) perovskite films showed overally longitudinal- and omnidirectional constructive molphologies, respectively having larger grain sizes of ~ 500 nm than


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that (~200 nm) of MAPbI3 (DMSO/GBL-Tol), but sparse surface coverage on the ITO/PEDOT:PSS substrate. Meanwhile, the MAPbI3 (DMSO/GBL-Tol) perovskite film revealed completly homogeneous surface coverage on ITO/PEDOT:PSS, which was consistent with those reported previously.22 Note that these surface coverage on substrate should be one of the most important factors for affecting the high performance of PHJ device. Figure 1d shows the XRD patterns of these perovskite films. The MAPbI3 (DMSO/GBL-Tol) film presented the well-formed perovskite crystallinity of MAPbI3, but the MAPbI3 (DMF) and MAPbI3 (GBL) films exhibited heterogeneous crystallinity of with PbI2 partially.11 Subsequently the PC71BM in chlorobenzene was spin-cast on these MAPbI3 perovskite films, and the PC61BM was also compared from spin-casting on the MAPbI3 (DMSO/GBL-Tol) film. Figure 2 shows the atomic force microscope (AFM) surface morphologies of PC71BM films cast on MAPbI3 (DMF) (a), MAPbI3 (GBL) (b), and MAPbI3 (DMSO/GBL-Tol) (c) perovskite film substrates, which compared with that (d) of PC61BM films cast on MAPbI3 (DMSO/GBLTol) perovskite film substrates. Interestingly, the surface morphologies of PCBM films were closely correlated with the morphologies of each perovskite films observed in SEM images of Figure 1. As shown in Figure 2, the spin-cast PC71BM could not make up the large empty spaces in sparse surface coverage of MAPbI3 (DMF) and MAPbI3 (GBL) perovskite films. while both spin-cast PC71BM and PC61BM provided uniformed and fully covered layers on MAPbI3 (DMSO/GBL-Tol) perovskite film. The PC71BM on perovskite films showed more smooth surface morphology with root-mean-square (rms) value of 8.4 nm than that (12.1 nm) of the PC61BM, indicating the difference of interfacial attraction between PC61BM and PC71BM with perovskite films.


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Although these surface morphologies was mostly investigated in PHJ solar cell device with MAPbI3 perovskite film, there are a little survey in the vertically formed morphology and interfacial heterojunction of MAPbI3 perovskite film with ITO/PEDOT:PSS or PCBM. Regarding this, we prepared the thin section ("slice") of ~100 nm in PHJ solar cell device with MAPbI3 (GBL/DMSO-Tol)/PC71BM using focused ion beam (FIB) technique. Figure 3 shows (a) the description for cross-sectioning the PHJ solar cell device with MAPbI3 (GBL/DMSOTol)/PC71BM using FIB technique and (b and c) cross-sectional morphologies of this "slice" observed using high resolution transmission electron microscopy (HRTEM). As shown in figure 3b, the PHJs between each layers were well-organized without interfacial 'dead space'. These could afford to facilitate the exciton dissociation in heterojunction. We also observed polycrystalline MAPbI3 throughout thick film of ~350 nm as shown in Figure 3c similarly with that reported by Sun et al.20 Next, PHJ solar cell devices fabricated the aforementioned MAPbI3 (DMF), MAPbI3 (GBL), and MAPbI3 (DMSO/GBL-Tol) perovskite films with PC71BM or PC61BM were employed. Comparison of more than 250 solar cell devices revealed to optimize the photovoltaic performances. Various metal electrodes such as Al, Ag, and Au were also investigated in this work. Although these metal electrodes could be successfully applied in PHJ perovskite solar cells,20-22 we found the Ag electrode exhibited the better performances than Al and Au in our devices during the optimization process. (see the Figure S2 in Supporting Information) The MAPbI3 films spin-cast with consecutive two step at 2000 and 5000 rpm exhibited the most efficient morphologies with thickness of ~320 ± 30nm. The optimum thickness of PC71BM film cast on MAPbI3 perovskite film was ~150 ± 10 nm. Figure 4 shows the J-V curves under AM 1.5 irradiation (100 mW·cm-2) and the incident photon-to-current efficiency (IPCE) spectra for PHJ


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solar cell devices fabricated under optimized processing conditions with these perovskite films and PCBM. The corresponding values are summarized in Table 1. As shown in Figure 4a and Table 1, the PHJ devices fabricated with MAPbI3 (DMF)/PC71BM and MAPbI3 (GBL)/PC71BM presented some dismal photovoltaic performances. These might be caused by their poor morphologies shown in Figure 1 and 2. Meanwhile, most of devices fabricated using MAPbI3 (DMSO/GBL-Tol)/PC71BM or PC61BM showed significantly improved photovoltaic performances compared to those of MAPbI3 (DMF)/PC71BM and MAPbI3 (GBL)/PC71BM due to superior morphology of MAPbI3 (DMSO/GBL-Tol) perovskite film. The PHJ devices fabricated with MAPbI3 (DMSO/GBL-Tol)/PC61BM exhibited high PCEs (maximum/average) of 8.56/6.98% with Jsc = 17.15 mA·cm-2, Voc = 0.90 V, and F.F = 0.56, which was comparable with those on thickness of PC61BM layer reported previously.22 Since the PC71BM often exhibited a better spectral response in the visible region than PC61BM. The best PCEs (maximum/average) of 12.22/9.75% with a Jsc of 18.66 mA·cm-2, Voc of 0.87 V, and a F.F of 0.75 was obtained in PHJ devices fabricated with MAPbI3 (DMSO/GBL-Tol)/PC71BM, showing the better Jsc and F.F compared to those with PC61BM without LiF buffer layer. The IPCE spectra of these devices (Figure 4b) overlap well with their optical absorptions shown in Scheme 1c, resulting in a close correlation with the photocurrents in the J-V curves. The superior F.F values in PHJ devices fabricated with MAPbI3 (DMSO/GBL-Tol)/PC71BM might be originated by more smooth surface morphology of PC71BM film with than PC61BM film cast on MAPbI3 (DMSO/GBL-Tol) perovskite film as shown in Figure 2, reducing series resistance from better planar junction between PC71BM and Ag electrode compared to that of PC61BM film. The outstanding photovoltaic performances in PHJ devices fabricated with MAPbI3 (DMSO/GBL-Tol)/PC71BM were mostly affected by morphology and thickness of MAPbI3


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(DMSO/GBL-Tol) perovskite films varied under processing conditions and showed the following large variations; PCEs of 7~12% with short-circuit current densities (Jsc) of 13~22 mA·cm-2, open-circuit voltages (Voc) of 0.76~0.94 V, and fill factors (F.F) of 0.44~0.76. (see the Figure S3 in Supporting Information). Finally, Figure S4 shows the hysteretic characteristics of J-V curve of solar cell devices fabricated with MAPbI3 (DMSO/GBL-Tol)/PC71BM (or PC61BM) PHJ films from forward (short circuit to open circuit) and reverse (open circuit to short circuit) scans at the voltage settling time of 100 ms under AM 1.5 irradiation. The perovskite solar cells often present some unambiguous efficiency values on current-voltage scan direction.24,25 The observed J-V curve behavior for reverse and forward scan directions in device with MAPbI3 (DMSO/GBL-Tol)/PC71BM exhibited a little different efficiency of 11.4% and 10.65%, respectively. Meanwhile, the device with MAPbI3 (DMSO/GBL-Tol)/PC61BM showed a negligible hysteretic J-V curve behavior even in reverse and forward scan directions showing the PCEs of 8.56% and 8.96%, respectively.

CONCLUSION We have demonstrated the high-performance PHJ hybrid solar cells comprising of wellorganized MAPbI3 (DMSO/GBL-Tol) perovskite and PC71BM films, exhibiting high PCEs without insertion of Bis-C60 surfactant, bathocuproine, or LiF buffer layers between PCBM film and metal electrode. We have also characterized the MAPbI3 perovskite films fabricated from various processing conditions using DMF, GBL, and DMSO-GBL solution, and used them in PHJ hybrid solar cells with PCBM. Moreover, the PHJ devices fabricated with MAPbI3 perovskite/PC71BM exhibited the better photovoltaic performances with higher Jsc and F.F


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compared to those with PC61BM owing to a better spectral response in the visible region and a better planar junction with Ag electrode than the PC61BM. We believe that the findings of this study introduce a new direction for the development of low-cost high efficiency next-generation solar cells. Further studies on the interfactial photophysics, distribution and orientation between materials in PHJ hybrid solar cells are ongoing.

Acknowledgment. This work was supported by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning, Korea (2013K000203), the International Science and Business Belt Program through the Ministry of Education, Science and Technology (no. 2012K001573), the ERC (the Korean government (MEST)) program (no. 2013004800), and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by NRF-2013R1A1A4A01005961.

Author Information. *Corresponding authors: Fax : 82-62-232-8122 (J.K.L), 82-44-860-1331 (J.K) ; 82-62-2307319 (J.K.L), Tel : 82-44-860-1337 (J.K) ; E-mail : [email protected] (J.K.L), [email protected] (J.K)

Supporting Information Available: FESEM images, variation of photovotaic performances, hysteretic J-V behavior. This information is available free of charge via the Internet at http://pubs.acs.org


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Table 1. Photovoltaic performances of devices fabricated with MAPbI3 perovskite/PCBM PHJa) Jsc (mAcm-2)

MAPbI3

PCBM

MAPbI3 (DMF) b)

PC71BM

-

-

-

-

MAPbI3 (GBL)

PC71BM

1.78

0.52

0.46

0.42/0.23

MAPbI3 (DMSO/GBL-Tol)

PC71BM

18.66

0.87

0.75

12.22/9.75

MAPbI3 (DMSO/GBL-Tol)

PC61BM

17.15

0.90

0.56

8.56/6.98

a)

Voc (V)

F.F (%)

η max./average

The performances are determined under simulated 100 mW/cm2 AM 1.5G illumination. The

light intensity using calibrated standard silicon solar cells with a proactive window made from KG5 filter glass traced to the National Renewable Energy Laboratory (NREL). The masked active area of device is 4 mm2; b)the performances were not observed.


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Scheme Captions Scheme 1. Description of (a) PHJ solar cell device structure, (b) energy level diagram, and (c) transmittance spectra of MAPbI3/PC71BM (or PC61BM) used in this work.

Figure Captions Figure 1. FESEM surface images of (a) MAPbI3 (DMF), (b) MAPbI3 (GBL), and (c) MAPbI3 (DMSO/GBL-Tol) perovskite films spin-cast on the ITO/PEDOT:PSS substrate and their (d) XRD patterns Figure 2. AFM surface morphologies of PC71BM films cast on (a) MAPbI3 (DMF), (b) MAPbI3 (GBL), and (c) MAPbI3 (DMSO/GBL-Tol) films, which were compared with (d) PC61BM films cast on MAPbI3 (DMSO/GBL-Tol) films Figure 3. (a) Description and (b) TEM image of thin section in PHJ solar cell device with MAPbI3 (GBL/DMSO-Tol)/PC71BM using FIB technique and (c) crystalline stucture of MAPbI3 (DMSO/GBL-Tol) films. Figure 4. (a) J-V curves under AM 1.5 irradiation (100 mW·cm-2) of devices fabricated with MAPbI3 (GBL)/PC71BM, and MAPbI3 (DMSO/GBL-Tol)/PC71BM (or PC61BM) PHJ and (b) IPCE spectra for devices fabricated with MAPbI3 (DMSO/GBL-Tol)/PC71BM (or PC61BM) PHJ


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Scheme 1. Paek et. al.


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Figure 1. Paek et. al.


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