Thermally Stable Mesoporous Perovskite Solar Cells Incorporating

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Thermally Stable Mesoporous Perovskite Solar Cells Incorporating Low-Temperature Processed Graphene/Polymer Electron Transporting Layer Shi Wun Tong,† Janardhan Balapanuru, Deyi Fu,† and Kian Ping Loh*,†,‡ †

Department of Chemistry and Centre for Advanced 2D Materials, National University of Singapore, 3 Science Drive 3, Singapore 119260 ‡ SinBeRISE CREATE, National Research Foundation, CREATE Tower, 1 Create Way, 138602, Singapore S Supporting Information *

ABSTRACT: In the short time since its discovery, perovskite solar cells (PSCs) have attained high power conversion efficiency but their lack of thermal stability remains a barrier to commercialization. Among the experimentally accessible parameter spaces for optimizing performance, identifying an electron transport layer (ETL) that forms a thermally stable interface with perovskite and which is solution-processable at low-temperature will certainly be advantageous. Herein, we developed a mesoporous graphene/polymer composite with these advantages when used as ETL in CH3NH3PbI3 PSCs, and a high efficiency of 13.8% under AM 1.5G solar illumination could be obtained. Due to the high heat transmission coefficient and low isoelectric point of mesoporous graphene-based ETL, the PSC device enjoys good chemical and thermal stability. Our work demonstrates that the mesoporous graphene-based scaffold is a promising ETL candidate for high performance and thermally stable PSCs. KEYWORDS: perovskite solar cells, electron transport layer, mesoporous, graphene, polymer, low temperature

1. INTRODUCTION Recently, a new-type of solar cell based on organic−inorganic halide perovskite has witnessed rapid development. Mesoporous TiO2-based PSCs have already surpassed a power conversion efficiency (PCE) of 20% since its first development in 2009.1 Although the electron injection rate from the perovskite absorber to TiO2 electron transporting layer (ETL) is very fast, this is offset by the high electron recombination rate due to the low electron mobility of TiO2.2,3 The relatively high density of electronic trap states below the conduction band of TiO2 degrade both efficiency and stability of the PSCs.4 Another drawback is that TiO2-based PSCs require high temperature sintering (500 °C) to generate the electrically conducting anatase phase, thus limiting its application in flexible electronics. Contrary to TiO2, ZnO is known to have a higher electron mobility and lower processing temperature (∼120 °C). Electrodeposited or spin coated ZnObased ETLs have been utilized in PSCs, showing PCEs of 15− 16%.5 However, ZnO tends to decompose at a relative low temperature, hindering its long-term device operation.5,6 Hence, there is an ongoing drive to search for a lowtemperature processed and thermally stable ETL. Graphene and its derivatives like reduced graphene oxide (rGO), processed in the form of 2D flakes, had been applied as additives in ETL.7,8 The attractiveness of graphene comes from its high transparency, high electrical conductivity, and high heat transfer coefficient.9 Previously, ETL made of rGO and © 2016 American Chemical Society

mesoporous TiO2 showed higher PCE compared with the standalone TiO2 devices in PSCs.7 Lithium functionalized graphene oxide (Li−GO) can show good energy alignment to the TiO2/perovskite interface, affording in a high PCE of 11.8%.8 In addition, Li−GO can passivate oxygen vacancies on TiO2 and also protect it from moisture attack, thus improving stability. However, a high temperature sintering step is still necessary for the TiO2 constituent in ETLs. Hence, development of a new ETL that can be prepared at low temperature is useful. The low thermal conductive property of perovskite CH3NH3PbI3 has been reported by Pisoni et al.10 Heat generated inside the operating devices cannot be dissipated quickly and induce mechanical stress that limits the lifetime of PSCs. Placing a heat spreader adjacent to the perovskite will help to decrease the heat accumulation in PSCs. The high thermal conductivity of rGO allows it to act as a heat spreader.11 In view of the low-temperature processing requirement and long-term stability limits in PSCs,12,13 we report the first application of a low-temperature processed mesoporous graphene/polymer (mp−GP) ETL for high performance PSCs in this paper. The network structure of highly conducting graphene and granular-like polyaniline contains well-defined Received: August 16, 2016 Accepted: October 12, 2016 Published: October 12, 2016 29496

DOI: 10.1021/acsami.6b10278 ACS Appl. Mater. Interfaces 2016, 8, 29496−29503

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ACS Applied Materials & Interfaces

Figure 1. SEM images show the topography of the thin films consisted of (a) rGO, (b) polyaniline and (c) mp−GP. Topographic images of perovskite grown on thin films shown in (a−c) are indicated from AFM images in (d−f), respectively. 15 min each. Clean ITO substrates were oxygen-plasma treated using 50W plasma power, with an oxygen flow rate of 50 sccm for 10 min. One wt % of rGO was mixed with 0.5 M aniline in 10 mL 1 M HCl acid. Another 10 mL 1 M HCl acid mixed with 0.12 M potassium persulfate was prepared. After tuning the pH of both solutions to 1.8, we mixed both solutions together immediately, followed by 3 min ultrasonication in a 5 °C water bath. Plasma treated ITO was immersed into the solution at once and kept at 5 °C for 1.5, 3, 5, or 8 min in order to get mp−GP thickness of 180, 265, 380, and 650 nm. The uniform mp−GP films were deposited on ITO substrates and washed by DI water and IPA before usage. Reference sample made of 4 nm graphene ETL was prepared by spin coating rGO solution (10 mg/mL) from DMF at 3000 rpm for 40 s on clean ITO substrate, followed by heating at 100 °C for 30 min. 2.3. Materials Characterizations. SEM images were captured by using FEI Verios 460L field-emission scanning electron microscope, operated at 2 kV. XRD patterns were obtained using an X ray diffractometer (Paanalytical X’Pert Pro) equipped with Cu−Kα radiation (λ = 1.540 50 Å). SEM and XRD images measured on mp−GP films with different rGO loading: 0.2, 0.5, and 1 wt % of rGO was mixed with 0.5 M aniline in 10 mL 1 M HCl acid. Preparation of mp−GP film with larger than 1 wt % of rGO loading is neglected as undesirable aggregation starts with heavy rGO loading in aniline solution. AFM images were captured by using Bruker atomic force microscope in scanning size of 5 × 5 μm2. UV−vis absorption measurements were recorded with an Agilent 8453 UV−vis Spectroscopy System at room temperature. The steady-state photoluminescence spectra of perovskite with different interfaces (mp−GP and rGO) were measured under excitation at λ of 600 nm. PL quenching efficiency (%) is equal to 1 − (IPL_ETL/IPL_perovskite) × 100%14 where IPL_ETL and IPL_perovskite are PL intensity of perovskite after and before coating with electron transport layer (ETL). UPS was performed using He I UV lamp (VG Microtech, UK) with a concentric electron analyzer operated at a pass energy of 1 eV. The sample was biased at −3 V to overcome the analyzer function in order to observe the low energy cutoff. All samples were prepared on ITO substrates. Thermal stability of perovskite coated mp−GP film was investigated by heating the film at 150 °C in air for 30 min.

pores to serve as fast electronic channels. The mesoporous mp−GP ETL affords penetrable microvoid volume for the infiltration of active layers, resulting in a complete surface coverage of highly crystalline perovskite formation. In contrast to the 2D form of graphene, mp−GP ETL not only shows an efficiency enhancement in PSC and also offers a stable 3-D scaffold to protect perovskite layer due to its chemical inertness and its ability to provide encapsulation of the perovskite crystals within, thus affording protection from moisture and reactive interface formation during high temperature operation. The effects of mp−GP ETL on the optical absorption, surface morphology, and crystallinity of the perovskite film as well as the energy level alignment at mp−GP/perovskite interface were investigated to explain why there is a remarkable PCE improvement (∼48%) in the mp−GP based device, from 9.3% to 13.8%, compared with the reference graphene based device.

2. EXPERIMENTAL SECTION 2.1. Materials PreparationSynthesis of Reduced Graphene Oxide (rGO). Under constant stirring, 1 g of expanded graphite and 5.6 g of KMnO4 was added to a 9:1 mixture of conc. H2SO4/H3PO4 (90:10 mL). The reaction mixture was stirred at room temperature for 24 h and 30% H2O2 (5 mL) was slowly added to the reaction mixture followed by the addition of 100 mL DI water. After separating the acid mixtures via filtering/centrifugation, the crude GO was then digested with 1 M HCl solution. GO solution was purified by further washing with DI water via centrifugation at 10 000 rpm until the pH reaches 4−5, followed by 3-day dialysis for purification. For the chemical reduction of GO, 0.5 mL of GO solution (3 mg/mL) was mixed with 4.5 mL of DMF (volume ratio of water and DMF is 1:9). One μL of hydrazine monohydrate was added and stirred at 60 °C for 12 h. Finally the rGO was washed with DMF and filtered dry. 2.2. Preparation of Mesoporous Graphene/Polymer (mp− GP) Film with 1 wt % rGO. ITO substrates were ultrasonically cleaned with detergent, deionized water, acetone, and isopropanol for 29497


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ACS Applied Materials & Interfaces 2.4. Solar Cell Fabrication. All ITO substrates were ultrasonically cleaned with detergent, deionized water, acetone, and isopropanol for 15 min each. Clean ITO substrates were oxygen plasma-treated using 50 W power and an oxygen flow rate of 50 sccm for 10 min. 265 nm thick mp−GP film was deposited on ITO via electrostatic attraction in solution mixed with aniline and rGO as mentioned in the section Preparation of Mesoporous Graphene/Polymer (mp−GP) Film with 1 wt % rGO. For the ZnO device, 25 nm ZnO film was deposited by spin coating ZnO nanoparticles solution on ITO at 3000 r.p.m. for 30 s, followed by heating at 120 °C for 5 min. This procedure was repeated three times. Cs2CO3 was deposited by spin coating (3000 rpm for 30 s) 0.5 wt % solution of Cs2CO3 from 2-ethoxyethanol and dried at 150 °C for 5 min. For all devices, the deposition of perovskite layer, poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2octyldodecyl)-2,2′;5′,2″;5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T2OD) and Ag were prepared inside glovebox (O2 and H2O level 2 μm.25,26 After optimizing various thickness, we found that 265 nm thick mp−GP scaffold supports the overgrowth of homogeneous, large sized crystallite, thus this is chosen as the optimal thickness for the PV cell. Figure 4a presents the device structure with mp−GP ETL. The energy levels of the materials used for fabrication of mp− GP based PSC are plotted in Figure 4c. To achieve highperformance PSCs, matching energy level alignment at the interface is essential. In conventional device structure, the perovskite is sandwiched between an electron transporting layer and a hole transporting layer with low LUMO and high HOMO level, respectively. In this study, we applied poly[(5,6difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldo29499


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ACS Applied Materials & Interfaces decyl)-2,2′;5′,2″;5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T2OD) as the hole transporting layer in the device. Compared to conventional hole conductor spiro-OMeTAD, PffBT4T2OD has a deeper HOMO27 and hence a larger open-circuit voltage (Voc) (Figure S5) can be generated in PSC.28 The large LUMO energy offset at PffBT4T-2OD/perovskite interface is favorable for blocking the electrons being transported from the perovskite layer to the Ag anode. As shown from UPS results in Figure S6, the work function of mp−GP film (4.4 eV) may induce a relatively large electron extraction barrier height of 0.47 eV at the interface of mp−GP/perovskite. Suitable interfacial engineering with low work function (2.3 eV)29 Cs2CO3 has been applied on ETL in high performance photovoltaic cell.30,31 Here, the insertion of ultrathin Cs2CO3 coating at mp−GP/perovskite interface can effectively reduce the work function of mp−GP film from 4.4 to 4.13 eV (Figure S6). Efficient electron extraction at mp−GP_Cs2CO3/perovskite interface is thus induced without affecting the morphology and optical properties of perovskite (Figure S7 and S8). The tunable energy level of mp−GP composite with Cs2CO3 is advantageous for reducing interfacial energy barrier in PSCs. The higher electron mobility of the mp−GP (1.7 × 10 −2 cm2/ (V s) as shown in Figure S9) over conventional ETL such as TiO2 (1.7 × 10 −4 cm2/(V s))32 further suggests that mp−GP can be a good alternative electron transporting scaffold in PSCs. The good electron transport ability of mp−GP film is endowed by rGO, rather than from polymer PANI. Four-point probe measurement indicated that the conductivity of rGO (31 S cm−1) is much higher than that of PANI (Figure S10 and Table S1). Since rGO have intrinsically high electrical conductivity with its large sp2 hybridized surface area, a high electron mobility of mp−GP is generated after coupling rGO with PANI. Figure 4b shows the PV characteristics of PSCs using different ETLs: mp−GP, rGO, mp−GP_Cs 2 CO 3 , or rGO_Cs2CO3. All ETLs were prepared under mild (8.7).39 The higher basicity of ZnO contributes to the instability of perovskite/ZnO system, producing PbI2 crystallites as the decomposition products. The low boiling point of CH3NH2 (−6 °C)39 and HI (−35.36 °C)40 will drive the decomposition (eq 1) forward continuously until the whole layer of CH3NH3PbI3 degrades. The strong HI acid also potentially reacts with ZnO and generates ZnI2 and H2O, the latter can cause further autocatalytic decomposition (eq 2). ZnO + 2HI ⇔ ZnI 2 + H 2O

ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.P.L.). Notes

(2)

The authors declare no competing financial interest.

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H2O acts as a Lewis base and triggers a new decomposition cycle by extracting one proton from ammonium, leading to further decomposition into CH3NH2, HI, and PbI2.37 Contrary to ZnO, the lower basicity of mp−GP film prevents the occurrence of reactions described in (eq 1) and (eq 2) and thus it enjoys a thermally and chemically stable interface with perovskite.

ACKNOWLEDGMENTS K.P.L. thanks A-star-DST program 1425203139 “Flexible and high performance based solar cells on graphene” and Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) for funding support. 29501


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Thermally Stable Mesoporous Perovskite Solar Cells Incorporating

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