Carbon-Based CsPbBr3 Perovskite Solar Cells: All ... - ACS Publications

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Carbon-Based CsPbBr3 Perovskite Solar Cells: All-Ambient Processes and High Thermal Stability Xiaowen Chang,† Weiping Li,† Liqun Zhu,† Huicong Liu,† Huifang Geng,† Sisi Xiang,† Jiaming Liu,† and Haining Chen*,† †

School of Materials Science and Engineering, Beihang University, No. 37 Xueyuan Road, Haidian District, Beijing 100191, People’s Republic of China S Supporting Information *

ABSTRACT: The device instability has been an important issue for hybrid organic−inorganic halide perovskite solar cells (PSCs). This work intends to address this issue by exploiting inorganic perovskite (CsPbBr3) as light absorber, accompanied by replacing organic hole transport materials (HTM) and the metal electrode with a carbon electrode. All the fabrication processes (including those for CsPbBr3 and the carbon electrode) in the PSCs are conducted in ambient atmosphere. Through a systematical optimization on the fabrication processes of CsPbBr3 film, carbon-based PSCs (C-PSCs) obtained the highest power conversion efficiency (PCE) of about 5.0%, a relatively high value for inorganic perovskite-based PSCs. More importantly, after storage for 250 h at 80 °C, only 11.7% loss in PCE is observed for CsPbBr3 C-PSCs, significantly lower than that for popular CH3NH3PbI3 C-PSCs (59.0%) and other reported PSCs, which indicated a promising thermal stability of CsPbBr3 C-PSCs. KEYWORDS: perovskite solar cells, CsPbBr3, hole transport materials-free, carbon electrode, thermal stability

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INTRODUCTION Hybrid organic−inorganic halide perovskite solar cells (PSCs) have exhibited booming development in power conversion efficiency (PCE), from 3.8% in 2009 to 22.1% in 2016,1−9 which is mainly attributed to the extraordinary photoelectric properties of perovskite materials, such as high carrier mobility, long carrier diffusion, small band gaps, and large absorption coefficient. However, owing to the utilization of unstable organic hole transport materials (HTMs, e.g., PTAA, spiro-OMeTAD) and the instability of hybrid organic−inorganic halide perovskite materials, the stability, especially for thermal stability, of the traditional PSCs is poor, which significantly restricts their practical application.10−12 In order to avoid the use of unstable organic HTMs, some solutions have been reported. On the one hand, the stable inorganic HTMs (e.g., NiOx, MoOx, and VOx) have been used to replace the organic HTMs.13−15 On the other hand, HTM-free PSCs was discovered to be possible due to the unique ambipolar property of perovskite that can serve as both a light harvester and a hole conductor.16−21 Hole carriers in perovskite could be effectively extracted by the Au or carbon electrodes that are directly deposited on perovskite layers, which could remove the unstable organic HTM layer to afford PSCs with improved stability. Compared with the HTM-free PSCs with Au electrode, the HTM-free PSCs with carbon electrode (C-PSCs) have exhibited more impressive promise not only because of low-cost materials, simple process, and lower equipment requirement, but also the high water resistance property and the inner property to © 2016 American Chemical Society

halide ions in perovskite, which help to enhance device stability.22−36 Though HTM-free PSCs, especially for C-PSCs, have significantly improved the device stability, the instability of organic−inorganic halide perovskite materials still constrains the application of HTM-free PSCs. Owing to the higher stability, inorganic perovskite (e.g., CsPbI3, CsPbBr3) has attracted extensive attention,37−40 and many works on HTM-based PSCs with inorganic perovskite have been reported. For example, Luo et al. fabricated the CsPbI3 HTM-based PSCs under open-air conditions, which achieved a PCE of 4.13%.41 Kulbak et al. reported the HTM-based PSCs with CsPbBr3 as absorber, which achieved a PCE of 6.2%.42 All these HTM-based PSCs using inorganic perovskite have demonstrated a relatively enhanced stability. However, the presence of organic HTM still limited the device stability. To avoid so, Ma et al. fabricated a HTM-free PSC by using Au as hole extraction electrode for CsPbIBr2 perovskite, which obtained a PCE of 4.7%.43 However, the metal (such Au, Ag, etc.) electrode has been proved to be easy to react with halide ions in perovskite,44,45 which will also limit the stability of Aubased HTM-free PSCs with inorganic perovskite. HTM-free PSCs using inorganic perovskite seems to be promising to achieve high stability, but no successful case has been demonstrated. Received: September 8, 2016 Accepted: November 22, 2016 Published: November 22, 2016 33649

DOI: 10.1021/acsami.6b11393 ACS Appl. Mater. Interfaces 2016, 8, 33649−33655

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ACS Applied Materials & Interfaces

Figure 1. Characterizations of Pb−Br precursor film. (a) XRD pattern, (b) SEM image, and (c, d) AFM images.

Figure 2. Preparation and evaluation of CsPbBr3 film and CsPbBr3 C-PSCs. (a) XRD pattern, (b) SEM image, and (c) UV−vis absorption spectrum of the CsPbBr3 film obtained by immersing Pb−Br precursor into 17 mg/mL CsBr methyl alcohol solution at 50 °C for 10 min, followed by annealing at 250 °C for 10 min. (d) Schematic illustration of deposition process of carbon electrode and cross-sectional structure, (e) working principle, (f) J−V curve, and (g) IPCE spectrum of CsPbBr3 C-PSCs.

film using PbBr2-solvent adduct as precursor, while carbon electrode was deposited by directly painting carbon paste on the CsPbBr3 film. After optimizing the reaction time and temper-

Herein, the above issues will be addressed by using carbon electrode in the HTM-free PSCs based on inorganic perovskite (CsPbBr3). A two-step method was used to prepare the CsPbBr3 33650


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temperature fixed at 50 °C. XRD patterns in Figure 3a show obviously strong diffraction peaks of PbBr2 without any obvious

ature during the deposition processes of CsPbBr3, a highest PCE of 5.0% was obtained, a relatively high PCE for the PSCs based on inorganic perovskite. Furthermore, after storage for 250 h at 80 °C, only 11.7% loss in PCE was observed for the CsPbBr3 CPSCs, significantly lower than that for the popular CH3NH3PbI3 C-PSCs (59.0%) and other reported PSCs, demonstrating a promising thermal stability of CsPbBr3 C-PSCs.

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RESULTS AND DISCUSSION CsPbBr3 perovskite film was prepared by a two-step method, in which a Pb−Br precursor film was first deposited, followed by the conversion in CsBr solution to CsPbBr3. A 1.4 M PbBr2 solution in a mixed solvent of dimethylformamide (DMF) and dimethyl sulfoxide (DMSO; 9:1 volume ratio) was used as precursor that was spin-coated on a TiO2 porous scaffold serving as Pb−Br precursor film. As observed, the Pb−Br precursor film was transparent and smooth. The XRD pattern in Figure 1a indicates that, in addition to the diffraction peaks of FTO glass and TiO2, other diffraction peaks at 11.7°, 12.4°, 16.8°, 21.4°, 23.9°, and 24.7° are observed for the Pb−Br precursor film, which could not be indexed to any compound in PDF card or reported works. Since the DMSO molecule has strong coordination with Pb2+,46−50 PbBr2(DMSO)x would form in the PbBr2 DMF/ DMSO solution and in Pb−Br precursor film, which results in the appearance of several nonreported diffraction peaks. The SEM image in Figure 1b shows a relatively smooth surface with fibrous crystals. This morphology is confirmed by AFM images (Figure 1c,d), which obtained a roughness of only 5.20 nm. The Pb−Br precursor was then converted to CsPbBr3 by immersing into 17 mg/mL CsBr methyl alcohol solution at 50 °C for 10 min, followed by annealing at 250 °C for 10 min. The XRD pattern in Figure 2a shows obvious diffraction peaks at 15.8°, 22.2°, 24.7°, and 31.2°, which are consistent with the planes of (100), (110), (111), and (200) of CsPbBr3, respectively. However, the presence of the diffraction peaks of PbBr2 at 12.3° and 19.4° indicates an incomplete conversion of the Pb−Br precursor at the present conditions. The SEM image in Figure 2b presents a nonuniform but smooth film consisting of varisized crystal grains with the sizes ranging from 0.17 to 0.68 μm. The UV−vis absorption spectrum in Figure 2c presents an obvious absorption onset at around 540 nm, which affords a calculated bandgap of 1.83 eV from the (F(R)hv)2-hv plot, as shown in the inset of Figure 2c. To evaluate the device performance, C-PSC was fabricated based on the above CsPbBr3 by directly painting carbon paste, as illustrated in Figure 2d. In this device, photogenerated electrons and holes on the conduction band (CB) and the valence band (VB) of CsPbBr3 will be injected to the CB of TiO2 and carbon electrode, respectively (Figure 2e). Current density−voltage (J− V) curves of C-PSCs are displayed in Figure 2f, with a shortcircuit current (JSC) of 2.6 mA/cm2, open-circuit voltage (VOC) of 0.99 V, fill factor (FF) of 0.49, and PCE of 1.3%. Such low performance should be attributed to the presence of a large amount of PbBr2 residual in CsPbBr3, which would suppress effective charge injection and transport. The IPCE spectrum in Figure 2g exhibits a similar onset at around 540 nm, which is consistent with the absorption range (Figure 2c). However, the IPCE values are all below 40% in the wavelength range of 300− 550 nm, leading to the low JSC. In order to promote the conversion of Pb−Br precursor to CsPbBr3 and hence enhance the device performance, reaction time and temperature in the conversion process were further optimized. First, reaction time was varied with the reaction

Figure 3. Effects of growth conditions on the compositions and morphologies of CsPbBr3. (a) XRD patterns and (c1−c3) SEM images of CsPbBr3 prepared by immersing Pb−Br precursor into CsBr methyl alcohol solution at 50 °C for different reaction times (10, 40, and 90 min); (b) XRD patterns and (d1−d3) SEM images of CsPbBr3 prepared by immersing Pb−Br precursor into CsBr methyl alcohol solution at different reaction temperatures (room temperature (RT), 50, and 70 °C) for 40 min.

peaks of CsPbBr3 for 10 min reaction (10 min−50 °C film), indicating a negligible conversion for such a short duration. After prolonging the reaction time to 40 min (40 min−50 °C film), the diffraction peaks of PbBr2 become obviously weak with the appearance of several strong diffraction peaks of CsPbBr3, suggesting a considerable conversion of PbBr2. For longer reaction time (90 min, 90 min−50 °C film), the diffraction peaks of PbBr2 are barely visible in the XRD pattern, but the diffraction peaks of CsPbBr3 become stronger, demontrating that most of Pb−Br precursor has been converted to CsPbBr3. The composition change with reaction time has leaded to different film morphologies of CsPbBr3. As shown in Figure 3c1, almost pure PbBr2 in 10 min−50 °C film exhibits a smooth surface with large grain size (from several hundreds of nanometers to about 2 μm). For 40 min−50 °C film, grain size decreases slightly with some cubic-shape crystals (about hundreds of nanometers in diameter) dispersed on the surface (Figure 3c2). For 90 min−50 °C film, the surface becomes more rough and more crystals are dispersed on the surface with larger grain size (Figure 3c3), partially indicating an Ostwald rippening effect for longer reaction time.51 In addition to reaction time, the reaction temperature of the conversion process was also investigated with the reaction time fixed at 40 min. As indicated in Figure 3b, compared with 40 min−50 °C film, room-temperature (RT) conversion results in the obvious appearance of the diffraction peaks of PbBr2 for 40 min−RT film, indicating the incomplete conversion of Pb−Br precursor due to slow conversion rate at lower temperature. For increasing reaction temperature to 70 °C (40 min−70 °C film), conversion process is obviously accelerated, as confirmed by the considerablely weaken diffraction peaks of PbBr2 and the more 33651


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afford the highest JSC. For different reaction temperatures, the CPSCs based on the CsPbBr3 film fabricated at RT (40 min−RT device) get a very low performance with a PCE of 1.4%, VOC of 1.05 V, JSC of 3.0 mA/cm2, and FF of 0.44, which were all improved for the reaction temperature of 50 °C (40 min−50 °C device), as shown above. With temperature increasing to 70 °C, the perforamnce of 40 min−70 °C device declines again with a PCE of 2.4%, VOC of 1.14 V, JSC of 4.1 mA/cm2, and FF of 0.51. The change in IPCE spectra (Figure 4d) are also consistent with the JSC in J−V curves. As reported in previous literature, both the perovskite composition and morphology greatly influenced the performance of C-PSCs. In our present case, the presence of PbBr2 would hinder the electron injection from CsPbBr3 into TiO2, which could explain the low performance of 10 min−50 °C and 40 min−RT devices. Besides, the more rough surface of 90 min−50 °C and 40 min−70 °C CsPbBr3 films would induce a poor contact between CsPbBr3 film and carbon electrode, which would suppress hole extraction at this interface and hence result in poor performance (Figure 4e). Due to the presence of a small amount of PbBr2 residual and the relatively even surface for 40 min−50 °C CsPbBr3 film, C-PSCs based on this film obtains the optimal performance. After preliminary optimization, the best CsPbBr3 C-PSC yields a PCE of 5.0% with a VOC of 1.29 V, JSC of 5.7 mA/cm2, and a FF of 0.68 (Figure 5a). IPCE spectrum in Figure 5b indicates that the IPCE values are over 70% in the wavelength range from 400 to 500 nm, which obtains an integrated current density of about 5.9 mA/cm2, consistent with the JSC. This PCE is among the highest reported PCE values for CsPbBr3-based PSCs without or with HTMs.38,42 The J−V curves under forward and reverse scannings have been recorded and shown in Figure S1, which demonstrates a small hysteresis for the CsPbBr3 C-PSCs. Thermal stability of CsPbBr3 C-PSCs was further evaluated in comparison with the most famous CH3NH3PbI3 C-PSCs. The devices were exposed in an ambient environment with a relative humidity (RH) of 30−50% and a temperature of 80 °C, with the PCEs being recorded after periods of exposion. As displayed in Figure 5c, after 250 h, better thermal stability is obviously found for CsPbBr3 C-PSCs with a PCE loss of only 11.7%, while CH3NH3PbI3 C-PSCs shows an evident decay, with a PCE loss of 59.0%. As is well-known, the organic CH3NH3+ cation has higher volatility and hydrophilicity than the inorganic Cs+ cation, which as a result, tends to deviate from CH3NH3PbI3 crystalline structure under the environment of relatively high RH and temperature, leading to the degradation of CH3NH3PbI3 and further the decay of device performance. Therefore, the carbonbased HTM-free PSCs using inorganic perovskite not only exhbits a decent PCE, but also obtains higher device stability.

intense diffraction peaks of CsPbBr3 in XRD pattern. SEM image in Figure 3d1 presents a similar surface morphology of 40 min− RT film with that of 40 min−50 °C film (Figure 3d2), with slightly larger grain size and less dispersed crystals on the surface, while for 40 min−70 °C film (Figure 3d3), lots of small grains (from tens of nanometers to hundreds of nanometers in size) and their clusters are distributed randomly to construct a very rough surface. Owing to the low boiling point of methyl alcohol (64.7 °C), small bubbles could generate on the film surface, which to some extent influenced the surface morphology. To determine the optimal CsPbBr3 film for C-PSCs, the CsPbBr3 films fabricated with different conditions were used to construct C-PSCs for photovoltaic performance evaluation, whose J−V curves, IPCE spectra, and photovoltaic parameters are presented in Figure 4 and Table 1. For different reaction

Figure 4. Performance dependence of C-PSCs on the CsPbBr3 films fabricated under different conditions. (a) J−V curves and (b) IPCE spectra of the C-PSCs with the CsPbBr3 films fabricated under different reaction times, (c) J−V curves and (d) IPCE spectra of the C-PSCs with the CsPbBr3 films fabricated under different reaction temperatures. (e) Schematic illustration of the performance dependence caused by PbBr2 residual and poor interface contact.

Table 1. Photovoltaic Parameters Calculated from the J−V Curves in Figure 4 devices

JSC (mA/cm2)

VOC (V)

FF

PCE (%)

10 min−50 °C 90 min−50 °C 40 min−50 °C 40 min−70 °C 40 min−RT

2.8 3.4 5.3 4.1 3.0

0.99 1.14 1.20 1.14 1.05

0.49 0.60 0.64 0.51 0.44

1.3 2.3 3.9 2.4 1.4

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CONCLUSION We have demonstrated a new kind of PSCs using carbon electrode as hole extraction electrode without HTM accompanied by the application of inorganic CsPbBr3 perovskite as light absorber. Two-step preparation processes of the CsPbBr3 film were conducted in ambient atmosphere, which made the fabrication of C-PSCs processable in ambient atmosphere. Through a systematical optimization on reaction time and temperature for CsPbBr3 film fabrication, the optimal conditions are found to be 50 °C for 40 min. The champion CsPbBr3 CPSCs reached a PCE of 5.0% (VOC = 1.29 V), a relatively high value for the PSCs based on inorganic perovskite. In stability test, after storage for 250 h at 80 °C, only 11.7% loss in PCE was

times, 10 min reaction only gets a very low performance (10 min−50 °C device) with a PCE of 1.3%, VOC of 0.99 V, JSC of 2.8 mA/cm2 and FF of 0.49, while performance is largely improved as reaction time increases to 40 min (40 min-50 °C device) with a PCE of 3.9%, VOC of 1.20 V, JSC of 5.3 mA/cm2 and FF of 0.64. However, performance is lowered for the reaction time prolonging to 90 min (90 min−50 °C device) with a PCE of 2.3%, VOC of 1.14 V, JSC of 3.4 mA/cm2, and FF of 0.60. IPCE spectrum of the 40 min−50 °C device in Figure 4b also presents the highest IPCE values in almost the whole wavelength range to 33652


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Figure 5. Performance of champion device and device stability evaluation. (a) J−V curve and (b) IPCE spectrum of the champion CsPbBr3 C-PSCs. (c) Thermal stability analysis of CsPbBr3 and CH3NH3PbI3 PSCs. tometer. Surface morphologies and roughness were characterized by a Bruker Dimension ICON atomic force microscopy (AFM). The J−V curves were recorded on a CHI660E electrochemical workstation (Shanghai Chen Hua Instruments Inc., China) under the illumination of solar simulator (CHF-XM-500W, 100 mW cm−2, AM 1.5). The active cell area was all fixed at about 6.25 mm2. IPCE spectra were recorded using IPCE kit (E0201, Institute of Physics, Chinese Academy of Sciences) in AC mode.

observed for CsPbBr3 C-PSCs, significantly lower than that for the popular CH3NH3PbI3 C-PSCs (59.0%) and other reported PSCs, indicating a promising thermal stability of CsPbBr3 CPSCs. Therefore, our present works have well demonstrated the possibility to fabricate decent carbon-based HTM-free PSCs using inorganic perovskite, which could help to improve the device stability.

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EXPERIMENTAL SECTION

ASSOCIATED CONTENT

S Supporting Information *

Preparation of TiO2 Blocking Layer and Porous Scaffold. First, a TiO2 blocking layer was deposited on cleaned FTO glass (15 mm × 15 mm) by spin-coating 0.15 M titanium diisopropoxide bis(acetylacetonate) in 1-butanol solution at 2000 rpm for 20 s, followed by heating at 125 °C for 5 min. Then, a TiO2 porous scaffold was spincoated on the blocking layer at 5000 rpm for 30 s using a commercial TiO2 paste (Dyesol 30 NRD, Dyesol) that was diluted by ethanol with a weight ratio of 1:2.5. After drying at 100 °C for 5 min, the TiO2 scaffold was gradually heated to 550 °C, baked at this temperature for 2 h, and then cooled to room temperature. Preparation of CH3NH3I. CH3NH3I was synthesized by reacting methylamine and hydroiodic acid with a molar ratio of 1.6:1 under N2 atmosphere in an ice bath for 2 h with stirring. After rotary evaporation at 50 °C for 1 h, the obtained precipitate was washed by normal hexane for three times, and then dried at 60 °C in vacuum for 24 h to acquire CH3NH3I. Deposition of PbBr2 and PbI2 Film on TiO2 Scaffolds. PbBr2 and PbI2 precursor solutions were prepared by dissolving 1.4 M PbBr2 or PbI2 into DMSO/DMF (VDMSO/VDMF = 1:9) mixed solvent at 80 °C. PbBr2 or PbI2 films were deposited by spin-coating precursor solutions on TiO2 scaffolds at 2000 rpm for 20 s (FTO glass/TiO2 blocking layer/ TiO2 scaffolds/PbBr2 (PbI2)). Conversion of PbBr2 and PbI2 Film to Perovskite. For CsPbBr3 films, the PbBr2 films on substrates (FTO glass/TiO2 blocking layer/ TiO2 scaffold) were immersed into 17 mg/mL CsBr methyl alcohol solution for different reaction times (10, 40, and 90 min) at different temperatures (RT, 50, and 70 °C). After being washed with isopropanol and heated at 250 °C for 10 min, CsPbBr3 films were obtained. For CH3NH3PbI3 films, the PbI2 films deposited on substrates (FTO glass/TiO2 blocking layer/TiO2 scaffold) were immersed into 1 mg/mL CH3NH3I IPA/cyclohexane (CYHEX) (VIPA/VCYHEX = 1:9) solution for 12 h. The film was taken out and heated at 100 °C for 15 min to form CH3NH3PbI3 films. Deposition of Carbon Counter Electrode. A commercial carbon paste produced by Guangzhou Seaside Technology Co., Ltd. was used for carbon electrode, which was composed of carbon black and graphite. The carbon paste was painted on perovskite layer under ambient condition followed by heating at 100 °C for 30 min. Characterizations. Surface morphologies were observed by a JSM7500F field emission scanning electron microscopy (FESEM). X-ray diffraction (XRD) patterns were recorded by a Rigaku D/MAX-2500 Xray diffractometer with Cu Kα radiation. Absorption spectra were measured by a UV-3000 ultraviolet and visible (UV−vis) spectropho-

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11393. More results and analysis of J−V curves about CsPbBr3, MAPbBr3, and MAPbI3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Haining Chen: 0000-0002-7543-3674 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Young Talent of “Zhuoyue” Program of Beihang University and the National Natural Science Foundation of China (Nos. 51371020 and 21603010).

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