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Annealing effects on high-performance CH3NH3PbI3 perovskite solar cells prepared by solution-process
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 122 (2015) 1047–1051 www.elsevier.com/locate/solener
Annealing effects on high-performance CH3NH3PbI3 perovskite solar cells prepared by solution-process Lung-Chien Chen a,⇑, Cheng-Chiang Chen b, Jhih-Chyi Chen a, Chun-Guey Wu b a
Department of Electro-Optical Engineering, National Taipei University of Technology, 1, Section 3, Chung-Hsiao E. Road, Taipei 106, Taiwan b Research Center for New Generation Photovolatics, National Central University, Taoyuan 32001, Taiwan Received 30 July 2015; received in revised form 6 October 2015; accepted 9 October 2015
Communicated by: Associate Editor Takhir Razykov
Abstract This work studies annealing effects of the CH3NH3PbI3 perovskite film for solar cell applications by time-resolved photoluminescence (TR-PL) spectra. The different thermal annealing temperatures process influences the quality and exciton lifetime of the CH3NH3PbI3 perovskite film and importantly the device performance. The CH3NH3PbI3 perovskite film of the optimal thermal annealing temperatures (100 °C) would significantly extended exciton lifetime and yield a higher open-circuit voltage, with short-circuit current density (JSC) = 21.58 mA/cm2, open-circuit voltage (VOC) = 0.84 V, fill factor (FF) = 61.4%, and g = 11.12%, respectively. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Perovskite; Quenching effect; Annealing; Exciton lifetime
1. Introduction Perovskite solar cell has attracted great attention over the past two years because of its unique properties and the perovskite solar cell using a continuous coating technique on flexible substrates. It was known as a group of material with interesting magnetic and electrical properties. Solution-processed the hybrid organic/inorganic lead halide perovskite (e.g. CH3NH3PbX3, X = I, Br, Cl) materials, perovskite has a high average power conversion efficiency (PCE) values of 16% using the phenyl-C61butyric acid methyl ester (PCBM)-based planar structure (Kojima et al., 2009; Kim et al., 2012; Sun et al., 2014; Docampo et al., 2013; Lee et al., 2015; Zhang et al., 2015; You et al., 2014; Seo et al., 2014; Liang et al., 2014; Lim ⇑ Corresponding author.
E-mail address: [email protected] (L.-C. Chen). http://dx.doi.org/10.1016/j.solener.2015.10.019 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
et al., 2014; Wang et al., 2014; Xiao et al., 2014a,b; Chiang et al., 2014; Subbiah et al., 2014; Zhu et al., 2014; Wu et al., 2014). The perovskite and its derivatives have been achieved in various types of solar cell architectures including perovskite-sensitized solar cells, mesoporous TiO2 scaffold material and planar p–i–n heterojunction solar cells. Compared with the perovskite solar cells structure with perovskite-sensitized mesoporous TiO2 scaffold, the planar heterojunction perovskite solar cells could be fabricated with much simpler process. The perovskite light-absorber was deposited on a mesoporous TiO2 layer, which also required high temperature to sinter or crystallize (Jeon et al., 2014, 2015; Ball et al., 2013; Liu and Kelly, 2014). However, CH3NH3PbI3 perovskite films can be prepared by vapor-assisted solution (Liu et al., 2013), sequential deposition process (Liu and Kelly, 2014), onestep spin-coating procedures (Eperon et al., 2014) and two-step spin-coating procedures (Im et al., 2014) for
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L.-C. Chen et al. / Solar Energy 122 (2015) 1047–1051
CH3NH3PbI3 formation (Burschka et al., 2013; Zhou et al., 2014), which has many advantages such as low cost, lowtemperature, and the potential of the perovskite cells using a continuous coating technique on flexible substrates (Jeng et al., 2013). In this work, we report the solution-process fabrication of perovskite solar cells comprising a architecture CH3 NH3PbI3 perovskites formed by a one-step spin-coating process. This study investigated the optical, structural, and surface properties of an perovskite film that is grown on PEDOT:PSS-coated ITO substrate by one-step spincoating process as functions of different thermal annealing temperatures in high-performance perovskite solar cells. 2. Experimental In this study, the patterned ITO substrate was cleaned with ultrasonic treatment in deionized water, acetone and isopropanol for 20 min, followed by an UV-ozone for 15 min. A PEDOT:PSS (1.3–1.7 wt%, from H.C. Stark Baytron P AI 4083) film was spin-coated on a patterned ITO substrate at 5000 rpm for 30 s and then annealed at 140 °C for 10 min under air. The perovskite precursor solution was deposited by the solvent-engineering technology of 1.25 M Pbl2 and 1.25 M methylammonium iodide (MAI) in a cosolvent of dimethyl sulfoxide (DMSO) and c-butyrolactone (GBL) (vol.ratio = 1:1) in a glove box filled with highly pure nitrogen. The perovskite solutions were then coated onto the PEDOT:PSS-coated ITO substrate by two consecutive spin-coating steps, at 1000 rpm and 6000 rpm for 40 s and 20 s, respectively. At 6000 rpm for 20 s, the wet spinning film was quenched by dropping 100 ll of anhydrous toluene. After spin coating, the film was then annealed in an oven at different thermal annealing temperatures ranging from 80 to 140 °C for 5 min. A solution of PC61BM was spin-coated on the perovskite/ PEDOT:PSS/ITO substrate at 1250 rpm for 30 s. Finally,
an Ag electrode was completed by thermal deposition with a thickness of 100 nm. Fig. 1 schematically depicts the complete structure. A field emission scanning electron microscope (FESEM) (LEO 1530) was used to observe the cross-section and surface morphology of the cells. The crystal structure of perovskite film was investigated with an X-ray diffraction (XRD) equipped with a Cu Ka source operated at 10 kV and 50 mA, an 1° divergence slit, we carried out the acquisition in the range of 10–45° in h–2h mode. The photoluminescence (PL) spectra and time-resolved photoluminescence (TR-PL) of a spincoated perovskite layer was performed using a spectrofluorometer, which was measured at 767 nm. Moreover, the current density–voltage (J–V) characteristics were measured using a Keithley 2420 programmable source meter under irradiation by a 1000 W xenon lamp. Finally, the irradiation power density on the surface of the sample was calibrated as 1000 W/m2. 3. Results and discussion The crystal structure and morphology evolution of the CH3NH3PbI3 perovskite films along different thermal annealing temperatures (from 80 to 140 °C) is further investigated by a XRD and FESEM measurement. Fig. 2 reveals evidence for the perovskite films with different thermal annealing temperatures ranging from 80 to 140 °C of 2h of 14.2 (1 1 0), 28.5 (2 2 0), and 31.8° (3 1 0), the small peak at 12.7° that belongs to the PbI2 phase (Liu and Kelly, 2014; Chen et al., 2014). We believe that CH3NH3PbI3 decompose upon heating at 120 °C, where CH3NH3I species escaped from the perovskite film to form the PbI2 phase. Fig. 3 presents the surface FESEM images of the perovskite film on glass substrate with different thermal annealing temperatures ranging from 80 to 140 °C for 5 min. The states of CH3NH3PbI3 perovskite films showed
Ag PCBM CH3NH3PbI3
INTENSITY (arb.units)
(110)
(220) (310)
0
(PbI2)
Perovskite anneal 140 C
(PbI2)
Perovskite anneal 120 C
(PbI2)
Perovskite anneal 100 C
0
0
0
Perovskite anneal 80 C
PEDOT:PSS ITO Glass Fig. 1. Schematic of the perovskite device configuration consisting of a structure of Ag/PC61BM/perovskite/PEDOT:PSS/ITO substrate.
10
20
30
40
DEGREE (2θ) Fig. 2. XRD diffractograms of the CH3NH3PbI3 perovskite film after different thermal annealing temperatures.
L.-C. Chen et al. / Solar Energy 122 (2015) 1047–1051
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Fig. 3. FESEM morphological image of perovskite film. (a) Thermal annealing to 80 °C. (b) Thermal annealing to 100 °C. (c) Thermal annealing to 120° C. (b) Thermal annealing to 140 °C.
different morphologies owing to their different thermal annealing temperatures, the average CH3NH3PbI3 crystal size from about 100 nm to about 300 nm. The enhanced grain size may also serve to increase the carrier mobility within the CH3NH3PbI3 perovskite film and reduce recombination from PbI2 to perovskite by the intercalation of CH3NH3I. Further increase the thermal annealing temperatures to 140 °C, the grain size of the CH3NH3PbI3 thin film increases, more grains with light contrast were observed in the grain boundaries, which is consistent with the enhanced intensity of PbI2 phase in the XRD pattern (Chen et al., 2014). The PL and TR-PL of a spincoated perovskite film was performed using a spectrofluorometer, as shown in Fig. 4. The different thermal annealing temperatures experimental conditions, the PL quantum yield of the perovskite film with heat treatment at temperature of 140 °C is greatly reduced (Fig. 4(a)). Therefore, it was found that the perovskite film annealed at 140 °C demonstrated more strikingly quenching effect than the perovskite film annealed at 100 °C. The Columbic interaction between the electron and hole can be used to estimate the exciton binding energy (Franceschetti and Zunger, 1997; Zhen et al., 2009). The enhanced exciton lifetime indicates the reduced recombination in the CH3NH3PbI3 perovskite film, and it is associated with the appearance of PbI2 phases in the
CH3NH3PbI3 perovskite film. The exciton lifetime indicative of the recombination behavior of the CH3NH3PbI3 perovskite film affects the device performance, as shown in Table 1. As shown in Fig. 4(b), the exciton lifetime (5.15 ns) of the CH3NH3PbI3 perovskite film annealed at 140 °C is lower than that of the film annealed at 100 °C (15.69 ns). Fig. 5 shows the absorbance spectra of the CH3NH3PbI3 perovskite films treated with different annealing temperatures ranging wavelength from 600 to 800 nm. The absorption of the CH3NH3PbI3 perovskite film treated with low annealing temperatures, such as 80 and 100 °C, is lower than that of the film treated with high annealing temperature, such as 120 and 140 °C. This indicates the reaction between precursors was not completed after drying. Therefore, the CH3NH3PbI3 perovskite films need a thermal annealing process to drive the interdiffusion between the precursors. In other words, more sunlight can be absorbed to generate photocurrent in the perovskite films treated with high annealing temperature. Fig. 6 plots photocurrent J–V curves of the perovskite solar cell obtained under standard 1 sun AM 1.5 simulated solar irradiation. The cell has an active area of 5 2 mm2 and no antireflective coating. As shown in Fig. 5, the optimum device exhibits outstanding performance (see Table 2), with short-circuit current density (JSC) = 21.58 mA/cm2,
L.-C. Chen et al. / Solar Energy 122 (2015) 1047–1051
o
PL INTENSITY (arn.units)
80 C o 100 C o 120 C o 140 C
1.1
(a)
o
80 C o 100 C o 120 C o 140 C
1.0
ABSORBANCE (arb.units)
1050
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2
700
720
740
760
780
800
820
600
840
650
750
800
Fig. 5. Absorbance spectra of the CH3NH3PbI3 perovskite films treated with different annealing temperatures. o
80 C o 100 C o 120 C o 140 C
1.0 0.8
(b)
0.6 0.4 0.2 0.0
0
5
10
15
20
25
CURRENT DENSITY (mA/cm 2 )
NORMALIZED PL INTENSITY (arb.units)
700
WAVELENGTH (nm)
WAVELENGTH (nm)
0 o
80 C o 100 C o 120 C o 140 C
-5 -10 -15 -20
TIME (ps) Fig. 4. Photoluminescence decay curves of perovskite films prepared by postannealing the CH3NH3PbI3 perovskite film in different thermal annealing temperatures; (a) photoluminescence spectra of perovskite films on glass; (b) nanosecond time-resolved photoluminescence of perovskite films on glass.
Table 1 Fitting decay times of perovskite films prepared by postannealing the CH3NH3PbI3 perovskite film in different thermal annealing temperatures. Thermal annealing temperatures (°C)
Time constant (s) (ns)
80 100 120 140
12.74 15.69 13.69 5.15
open-circuit voltage (VOC) = 0.84 V, fill factor (FF) = 61.4%, and g = 11.12%. The exciton lifetime indicative of the recombination behavior of the CH3NH3PbI3 perovskite film affects the device performance. However, further increase the thermal annealing temperatures to 140 °C resulted in decrease of g = 9.60% value (VOC = 0.78 V), a CH3NH3PbI3 perovskite film of optimal the thermal annealing temperatures (100 °C) would significantly extended exciton lifetime and yield a higher open-circuit voltage.
-25
0.0
0.2
0.4
0.6
0.8
1.0
VOLTAGE (V) Fig. 6. Current–voltage (J V) characteristics of perovskite solar cell constructed using the Ag/PC61BM/perovskite/PEDOT:PSS/ITO substrate under a simulated illumination with a light intensity of 100 mW/cm2 (AM 1.5).
Table 2 Parameters of the CH3NH3PbI3 perovskite film with different thermal annealing temperatures. Thermal annealing temperatures (°C)
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
80 100 120 140
19.52 21.58 21.85 22.59
0.82 0.84 0.82 0.78
0.58 0.61 0.59 0.54
9.29 11.11 10.45 9.60
4. Conclusions To summarize, we have been demonstrated the characteristics of the CH3NH3PbI3 perovskite with thermal annealing at temperatures ranging from 80 to 140 °C for 5 min. In optimal condition, the thermal annealing temperatures (100 °C) would significantly extended exciton lifetime and yield a higher open-circuit voltage, with relative low temperature to form the CH3NH3PbI3 perovskite in
L.-C. Chen et al. / Solar Energy 122 (2015) 1047–1051
a solvent-engineering technology. We believe that CH3NH3PbI3 decompose upon heating at 120 °C, where CH3NH3I species escaped from the perovskite film to form the PbI2 phase. The states of CH3NH3PbI3 perovskite films showed different morphologies owing to their different thermal annealing temperatures, the average CH3NH3PbI3 crystal size from about 100 nm to about 300 nm. The optimum device exhibits outstanding performance, with JSC = 21.58 mA/cm2, VOC = 0.84 V, FF = 61.4%, and g = 11.12%, respectively. Acknowledgments Financial support of this paper was provided by the Ministry of Science and Technology of the Republic of China under Contract No. MOST 103-2221-E-027-029MY2 References Ball, J.M., Lee, M.M., Hey, A., Snaith, H.J., 2013. Low-temperature processed meso-superstructured to thin-film perovskite solar cells. Energy Environ. Sci. 6, 1739–1743. Burschka, J., Pellet, N., Moon, S.J., Baker, R.H., Gao, P., Nazeeruddin, M.K., Gra¨tzel, M., 2013. Sequential deposition as a route to highperformance perovskite-sensitized solar cells. Nature 499, 316–319. Chen, Q., Zhou, H., Song, T.-B., Luo, S., Hong, Z., Duan, H.-S., Dou, L., Liu, Y., Yang, Y., 2014. Controllable self-induced passivation of hybrid lead iodide perovskites toward high performance solar cells. Nano Lett. 14, 4158–4163. Chiang, C.-H., Tseng, Z.-L., Wu, C.-G., 2014. Planar heterojunction perovskite/PC71BM solar cells with enhanced open-circuit voltage via a (2/1)-step spin-coating process. J. Mater. Chem. A 2, 15897–15903. Docampo, P., Ball, J.M., Darwich, M., Eperon, G.E., Snaith, H.J., 2013. Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates. Nat. Commun. 4, 2761. Eperon, G.E., Burlakov, V.M., Docampo, P., Goriely, A., Snaith, H.J., 2014. Morphological control for high performance, solution-processed planar heterojunction perovskite solar cells. Adv. Funct. Mater. 24, 151–157. Franceschetti, A., Zunger, A., 1997. Direct pseudopotential calculation of exciton coulomb and exchange energies in semiconductor quantum dots. Phys. ReV. Lett. 78, 915–918. Im, J.H., Jang, I.H., Pellet, N., Gr¨atzel, M., Park, N.G., 2014. Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells. Nat. Nanotechnol. 9, 927–932. Jeng, J.-Y., Chiang, Y.-F., Lee, M.-H., Peng, S.-R., Guo, T.-F., Chen, P., Wen, T.-C., 2013. CH3NH3PbI3 perovskite/fullerene planar-heterojunction hybrid solar cells. Adv. Mater. 25, 3727–3732. Jeon, N.J., Noh, J.H., Kim, Y.C., Yang, W.S., Ryu, S., Seok, S.I., 2014. Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells. Nat. Mater. 13, 897–903. Jeon, N.J., Noh, J.H., Yang, W.S., Kim, Y.C., Ryu, S., Seo, J., Seok, S.I., 2015. Compositional engineering of perovskite materials for highperformance solar cells. Nature 517, 476–480. 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., Gra¨tzel, M., Park, N.-G., 2012. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2, 591.
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Kojima, A., Teshima, K., Shirai, Y., Miyasaka, T., 2009. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 6050–6051. Lee, K.M., Chang, S.H., Wang, K.H., Chang, C.M., Cheng, H.M., Kei, C.C., Tseng, Z.L., Wu, C.G., 2015. Thickness effects of ZnO thin film on the performance of tri-iodide perovskite absorber based photovoltaics. Sol. Energy 120, 117-112. Liang, P.W., Liao, C.Y., Chueh, C.C., Zuo, F., Williams, S.T., Xin, X.K., Lin, J., Jen, A.K., 2014. Additive enhanced crystallization of solutionprocessed perovskite for highly efficient planar-heterojunction solar cells. Adv. Mater. 26, 3748–3754. Lim, K.-G., Kim, H.-B., Jeong, J., Kim, H., Kim, J.Y., Lee, T.-W., 2014. Boosting the power conversion efficiency of perovskite solar cells using self-organized polymeric hole extraction layers with high work function. Adv. Mater. 26, 6461–6466. Liu, D., Kelly, T.L., 2014. Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nat. Photon. 8, 133–138. Liu, M., Johnston, M.B., Snaith, H.J., 2013. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 501, 395–398. Seo, J., Park, S., Chan Kim, Y., Jeon, N.J., Noh, J.H., Yoon, S.C., Seok, S.I., 2014. Benefits of very thin PCBM and LiF layers for solutionprocessed p–i–n perovskite solar cell. Energy Environ. Sci. 7, 2642. Subbiah, A.S., Halder, A., Ghosh, S., Mahuli, N., Hodes, G., Sarkar, S. K., 2014. Inorganic hole conducting layers for perovskite-based solar cells. J. Phys. Chem. Lett. 5, 1748–1753. Sun, S., Salim, T., Mathews, N., Duchamp, M., Boothroyd, C., Xing, G., Sum, T.C., Lam, Y.M., 2014. The origin of high efficiency in lowtemperature solution-processable bilayer organometal halide hybrid solar cells. Energy Environ. Sci. 7, 339–407. Wang, Q., Shao, Y.C., Dong, Q.F., Xiao, Z.G., Yuan, Y.B., Huang, J.S., 2014. Large fill-factor bilayer iodine perovskite solar cells fabricated by a low-temperature solution-process. Energy Environ. Sci. 7, 2359– 2365. Wu, Z., Bai, S., Xiang, J., Yuan, Z., Yang, Y., Cui, W., Gao, X., Liu, Z., Jin, Y., Sun, B., 2014. Efficient planar heterojunction perovskite solar cells employing graphene oxide as hole conductor. Nanoscale 6, 10505–10510. Xiao, Z., Bi, C., Shao, Y., Dong, Q., Wang, Q., Yuan, Y., Wang, C., Gao, Y., Huang, J., 2014a. Efficient, high yield perovskite photovoltaic devices grown by interdiffusion of solution-processed precursor stacking layers. Energy Environ. Sci. 7, 2619–2623. Xiao, Z., Dong, Q., Bi, C., Shao, Y., Yuan, Y., Huang, J., 2014b. Solvent annealing of perovskite-induced crystal growth for photovoltaic-device efficiency enhancement. Adv. Mater. 26, 6503–6509. You, J., Hong, Z., Yang, Y.M., Chen, Q., Cai, M., Song, T.B., Chen, C. C., Lu, S., Liu, Y., Zhou, H., Yang, Y., 2014. Low-temperature solution-processed perovskite solar cells with high efficiency and flexibility. ACS Nano 8, 1674–1680. Zhang, Z., Wei, D., Xie, B., Yue, X., Li, M., Song, D., Li, Y., 2015. High reproducibility of perovskite solar cells via a complete spin-coating sequential solution deposition process. Sol. Energy 122 (4619), 97–103. Zhen, C.-G., Becker, U., Kieffer, J., 2009. Tuning electronic properties of functionalized polyhedral oligomeric silsesquioxanes: a DFT and TDDFT study. J. Phys. Chem. A 113, 9707–9714. Zhou, H., Chen, Q., Li, G., Luo, S., Song, T., Duan, H.-S., Hong, Z., You, J., Liu, Y., Yang, Y., 2014. Interface engineering of highly efficient perovskite solar cells. Science 345, 542–546. Zhu, Z., Bai, Y., Zhang, T., Liu, Z., Long, X., Wei, Z., Wang, Z., Zhang, L., Wang, J., Yan, F., Yang, S., 2014. High-performance holeextraction layer of sol–gel-processed NiO nanocrystals for inverted planar perovskite solar cells. Angew. Chem., Int. Ed. 53, 12571–12575.
ScienceDirect Solar Energy 122 (2015) 1047–1051 www.elsevier.com/locate/solener
Annealing effects on high-performance CH3NH3PbI3 perovskite solar cells prepared by solution-process Lung-Chien Chen a,⇑, Cheng-Chiang Chen b, Jhih-Chyi Chen a, Chun-Guey Wu b a
Department of Electro-Optical Engineering, National Taipei University of Technology, 1, Section 3, Chung-Hsiao E. Road, Taipei 106, Taiwan b Research Center for New Generation Photovolatics, National Central University, Taoyuan 32001, Taiwan Received 30 July 2015; received in revised form 6 October 2015; accepted 9 October 2015
Communicated by: Associate Editor Takhir Razykov
Abstract This work studies annealing effects of the CH3NH3PbI3 perovskite film for solar cell applications by time-resolved photoluminescence (TR-PL) spectra. The different thermal annealing temperatures process influences the quality and exciton lifetime of the CH3NH3PbI3 perovskite film and importantly the device performance. The CH3NH3PbI3 perovskite film of the optimal thermal annealing temperatures (100 °C) would significantly extended exciton lifetime and yield a higher open-circuit voltage, with short-circuit current density (JSC) = 21.58 mA/cm2, open-circuit voltage (VOC) = 0.84 V, fill factor (FF) = 61.4%, and g = 11.12%, respectively. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Perovskite; Quenching effect; Annealing; Exciton lifetime
1. Introduction Perovskite solar cell has attracted great attention over the past two years because of its unique properties and the perovskite solar cell using a continuous coating technique on flexible substrates. It was known as a group of material with interesting magnetic and electrical properties. Solution-processed the hybrid organic/inorganic lead halide perovskite (e.g. CH3NH3PbX3, X = I, Br, Cl) materials, perovskite has a high average power conversion efficiency (PCE) values of 16% using the phenyl-C61butyric acid methyl ester (PCBM)-based planar structure (Kojima et al., 2009; Kim et al., 2012; Sun et al., 2014; Docampo et al., 2013; Lee et al., 2015; Zhang et al., 2015; You et al., 2014; Seo et al., 2014; Liang et al., 2014; Lim ⇑ Corresponding author.
E-mail address: [email protected] (L.-C. Chen). http://dx.doi.org/10.1016/j.solener.2015.10.019 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
et al., 2014; Wang et al., 2014; Xiao et al., 2014a,b; Chiang et al., 2014; Subbiah et al., 2014; Zhu et al., 2014; Wu et al., 2014). The perovskite and its derivatives have been achieved in various types of solar cell architectures including perovskite-sensitized solar cells, mesoporous TiO2 scaffold material and planar p–i–n heterojunction solar cells. Compared with the perovskite solar cells structure with perovskite-sensitized mesoporous TiO2 scaffold, the planar heterojunction perovskite solar cells could be fabricated with much simpler process. The perovskite light-absorber was deposited on a mesoporous TiO2 layer, which also required high temperature to sinter or crystallize (Jeon et al., 2014, 2015; Ball et al., 2013; Liu and Kelly, 2014). However, CH3NH3PbI3 perovskite films can be prepared by vapor-assisted solution (Liu et al., 2013), sequential deposition process (Liu and Kelly, 2014), onestep spin-coating procedures (Eperon et al., 2014) and two-step spin-coating procedures (Im et al., 2014) for
1048
L.-C. Chen et al. / Solar Energy 122 (2015) 1047–1051
CH3NH3PbI3 formation (Burschka et al., 2013; Zhou et al., 2014), which has many advantages such as low cost, lowtemperature, and the potential of the perovskite cells using a continuous coating technique on flexible substrates (Jeng et al., 2013). In this work, we report the solution-process fabrication of perovskite solar cells comprising a architecture CH3 NH3PbI3 perovskites formed by a one-step spin-coating process. This study investigated the optical, structural, and surface properties of an perovskite film that is grown on PEDOT:PSS-coated ITO substrate by one-step spincoating process as functions of different thermal annealing temperatures in high-performance perovskite solar cells. 2. Experimental In this study, the patterned ITO substrate was cleaned with ultrasonic treatment in deionized water, acetone and isopropanol for 20 min, followed by an UV-ozone for 15 min. A PEDOT:PSS (1.3–1.7 wt%, from H.C. Stark Baytron P AI 4083) film was spin-coated on a patterned ITO substrate at 5000 rpm for 30 s and then annealed at 140 °C for 10 min under air. The perovskite precursor solution was deposited by the solvent-engineering technology of 1.25 M Pbl2 and 1.25 M methylammonium iodide (MAI) in a cosolvent of dimethyl sulfoxide (DMSO) and c-butyrolactone (GBL) (vol.ratio = 1:1) in a glove box filled with highly pure nitrogen. The perovskite solutions were then coated onto the PEDOT:PSS-coated ITO substrate by two consecutive spin-coating steps, at 1000 rpm and 6000 rpm for 40 s and 20 s, respectively. At 6000 rpm for 20 s, the wet spinning film was quenched by dropping 100 ll of anhydrous toluene. After spin coating, the film was then annealed in an oven at different thermal annealing temperatures ranging from 80 to 140 °C for 5 min. A solution of PC61BM was spin-coated on the perovskite/ PEDOT:PSS/ITO substrate at 1250 rpm for 30 s. Finally,
an Ag electrode was completed by thermal deposition with a thickness of 100 nm. Fig. 1 schematically depicts the complete structure. A field emission scanning electron microscope (FESEM) (LEO 1530) was used to observe the cross-section and surface morphology of the cells. The crystal structure of perovskite film was investigated with an X-ray diffraction (XRD) equipped with a Cu Ka source operated at 10 kV and 50 mA, an 1° divergence slit, we carried out the acquisition in the range of 10–45° in h–2h mode. The photoluminescence (PL) spectra and time-resolved photoluminescence (TR-PL) of a spincoated perovskite layer was performed using a spectrofluorometer, which was measured at 767 nm. Moreover, the current density–voltage (J–V) characteristics were measured using a Keithley 2420 programmable source meter under irradiation by a 1000 W xenon lamp. Finally, the irradiation power density on the surface of the sample was calibrated as 1000 W/m2. 3. Results and discussion The crystal structure and morphology evolution of the CH3NH3PbI3 perovskite films along different thermal annealing temperatures (from 80 to 140 °C) is further investigated by a XRD and FESEM measurement. Fig. 2 reveals evidence for the perovskite films with different thermal annealing temperatures ranging from 80 to 140 °C of 2h of 14.2 (1 1 0), 28.5 (2 2 0), and 31.8° (3 1 0), the small peak at 12.7° that belongs to the PbI2 phase (Liu and Kelly, 2014; Chen et al., 2014). We believe that CH3NH3PbI3 decompose upon heating at 120 °C, where CH3NH3I species escaped from the perovskite film to form the PbI2 phase. Fig. 3 presents the surface FESEM images of the perovskite film on glass substrate with different thermal annealing temperatures ranging from 80 to 140 °C for 5 min. The states of CH3NH3PbI3 perovskite films showed
Ag PCBM CH3NH3PbI3
INTENSITY (arb.units)
(110)
(220) (310)
0
(PbI2)
Perovskite anneal 140 C
(PbI2)
Perovskite anneal 120 C
(PbI2)
Perovskite anneal 100 C
0
0
0
Perovskite anneal 80 C
PEDOT:PSS ITO Glass Fig. 1. Schematic of the perovskite device configuration consisting of a structure of Ag/PC61BM/perovskite/PEDOT:PSS/ITO substrate.
10
20
30
40
DEGREE (2θ) Fig. 2. XRD diffractograms of the CH3NH3PbI3 perovskite film after different thermal annealing temperatures.
L.-C. Chen et al. / Solar Energy 122 (2015) 1047–1051
1049
Fig. 3. FESEM morphological image of perovskite film. (a) Thermal annealing to 80 °C. (b) Thermal annealing to 100 °C. (c) Thermal annealing to 120° C. (b) Thermal annealing to 140 °C.
different morphologies owing to their different thermal annealing temperatures, the average CH3NH3PbI3 crystal size from about 100 nm to about 300 nm. The enhanced grain size may also serve to increase the carrier mobility within the CH3NH3PbI3 perovskite film and reduce recombination from PbI2 to perovskite by the intercalation of CH3NH3I. Further increase the thermal annealing temperatures to 140 °C, the grain size of the CH3NH3PbI3 thin film increases, more grains with light contrast were observed in the grain boundaries, which is consistent with the enhanced intensity of PbI2 phase in the XRD pattern (Chen et al., 2014). The PL and TR-PL of a spincoated perovskite film was performed using a spectrofluorometer, as shown in Fig. 4. The different thermal annealing temperatures experimental conditions, the PL quantum yield of the perovskite film with heat treatment at temperature of 140 °C is greatly reduced (Fig. 4(a)). Therefore, it was found that the perovskite film annealed at 140 °C demonstrated more strikingly quenching effect than the perovskite film annealed at 100 °C. The Columbic interaction between the electron and hole can be used to estimate the exciton binding energy (Franceschetti and Zunger, 1997; Zhen et al., 2009). The enhanced exciton lifetime indicates the reduced recombination in the CH3NH3PbI3 perovskite film, and it is associated with the appearance of PbI2 phases in the
CH3NH3PbI3 perovskite film. The exciton lifetime indicative of the recombination behavior of the CH3NH3PbI3 perovskite film affects the device performance, as shown in Table 1. As shown in Fig. 4(b), the exciton lifetime (5.15 ns) of the CH3NH3PbI3 perovskite film annealed at 140 °C is lower than that of the film annealed at 100 °C (15.69 ns). Fig. 5 shows the absorbance spectra of the CH3NH3PbI3 perovskite films treated with different annealing temperatures ranging wavelength from 600 to 800 nm. The absorption of the CH3NH3PbI3 perovskite film treated with low annealing temperatures, such as 80 and 100 °C, is lower than that of the film treated with high annealing temperature, such as 120 and 140 °C. This indicates the reaction between precursors was not completed after drying. Therefore, the CH3NH3PbI3 perovskite films need a thermal annealing process to drive the interdiffusion between the precursors. In other words, more sunlight can be absorbed to generate photocurrent in the perovskite films treated with high annealing temperature. Fig. 6 plots photocurrent J–V curves of the perovskite solar cell obtained under standard 1 sun AM 1.5 simulated solar irradiation. The cell has an active area of 5 2 mm2 and no antireflective coating. As shown in Fig. 5, the optimum device exhibits outstanding performance (see Table 2), with short-circuit current density (JSC) = 21.58 mA/cm2,
L.-C. Chen et al. / Solar Energy 122 (2015) 1047–1051
o
PL INTENSITY (arn.units)
80 C o 100 C o 120 C o 140 C
1.1
(a)
o
80 C o 100 C o 120 C o 140 C
1.0
ABSORBANCE (arb.units)
1050
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2
700
720
740
760
780
800
820
600
840
650
750
800
Fig. 5. Absorbance spectra of the CH3NH3PbI3 perovskite films treated with different annealing temperatures. o
80 C o 100 C o 120 C o 140 C
1.0 0.8
(b)
0.6 0.4 0.2 0.0
0
5
10
15
20
25
CURRENT DENSITY (mA/cm 2 )
NORMALIZED PL INTENSITY (arb.units)
700
WAVELENGTH (nm)
WAVELENGTH (nm)
0 o
80 C o 100 C o 120 C o 140 C
-5 -10 -15 -20
TIME (ps) Fig. 4. Photoluminescence decay curves of perovskite films prepared by postannealing the CH3NH3PbI3 perovskite film in different thermal annealing temperatures; (a) photoluminescence spectra of perovskite films on glass; (b) nanosecond time-resolved photoluminescence of perovskite films on glass.
Table 1 Fitting decay times of perovskite films prepared by postannealing the CH3NH3PbI3 perovskite film in different thermal annealing temperatures. Thermal annealing temperatures (°C)
Time constant (s) (ns)
80 100 120 140
12.74 15.69 13.69 5.15
open-circuit voltage (VOC) = 0.84 V, fill factor (FF) = 61.4%, and g = 11.12%. The exciton lifetime indicative of the recombination behavior of the CH3NH3PbI3 perovskite film affects the device performance. However, further increase the thermal annealing temperatures to 140 °C resulted in decrease of g = 9.60% value (VOC = 0.78 V), a CH3NH3PbI3 perovskite film of optimal the thermal annealing temperatures (100 °C) would significantly extended exciton lifetime and yield a higher open-circuit voltage.
-25
0.0
0.2
0.4
0.6
0.8
1.0
VOLTAGE (V) Fig. 6. Current–voltage (J V) characteristics of perovskite solar cell constructed using the Ag/PC61BM/perovskite/PEDOT:PSS/ITO substrate under a simulated illumination with a light intensity of 100 mW/cm2 (AM 1.5).
Table 2 Parameters of the CH3NH3PbI3 perovskite film with different thermal annealing temperatures. Thermal annealing temperatures (°C)
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
80 100 120 140
19.52 21.58 21.85 22.59
0.82 0.84 0.82 0.78
0.58 0.61 0.59 0.54
9.29 11.11 10.45 9.60
4. Conclusions To summarize, we have been demonstrated the characteristics of the CH3NH3PbI3 perovskite with thermal annealing at temperatures ranging from 80 to 140 °C for 5 min. In optimal condition, the thermal annealing temperatures (100 °C) would significantly extended exciton lifetime and yield a higher open-circuit voltage, with relative low temperature to form the CH3NH3PbI3 perovskite in
L.-C. Chen et al. / Solar Energy 122 (2015) 1047–1051
a solvent-engineering technology. We believe that CH3NH3PbI3 decompose upon heating at 120 °C, where CH3NH3I species escaped from the perovskite film to form the PbI2 phase. The states of CH3NH3PbI3 perovskite films showed different morphologies owing to their different thermal annealing temperatures, the average CH3NH3PbI3 crystal size from about 100 nm to about 300 nm. The optimum device exhibits outstanding performance, with JSC = 21.58 mA/cm2, VOC = 0.84 V, FF = 61.4%, and g = 11.12%, respectively. Acknowledgments Financial support of this paper was provided by the Ministry of Science and Technology of the Republic of China under Contract No. MOST 103-2221-E-027-029MY2 References Ball, J.M., Lee, M.M., Hey, A., Snaith, H.J., 2013. Low-temperature processed meso-superstructured to thin-film perovskite solar cells. Energy Environ. Sci. 6, 1739–1743. Burschka, J., Pellet, N., Moon, S.J., Baker, R.H., Gao, P., Nazeeruddin, M.K., Gra¨tzel, M., 2013. Sequential deposition as a route to highperformance perovskite-sensitized solar cells. Nature 499, 316–319. Chen, Q., Zhou, H., Song, T.-B., Luo, S., Hong, Z., Duan, H.-S., Dou, L., Liu, Y., Yang, Y., 2014. Controllable self-induced passivation of hybrid lead iodide perovskites toward high performance solar cells. Nano Lett. 14, 4158–4163. Chiang, C.-H., Tseng, Z.-L., Wu, C.-G., 2014. Planar heterojunction perovskite/PC71BM solar cells with enhanced open-circuit voltage via a (2/1)-step spin-coating process. J. Mater. Chem. A 2, 15897–15903. Docampo, P., Ball, J.M., Darwich, M., Eperon, G.E., Snaith, H.J., 2013. Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates. Nat. Commun. 4, 2761. Eperon, G.E., Burlakov, V.M., Docampo, P., Goriely, A., Snaith, H.J., 2014. Morphological control for high performance, solution-processed planar heterojunction perovskite solar cells. Adv. Funct. Mater. 24, 151–157. Franceschetti, A., Zunger, A., 1997. Direct pseudopotential calculation of exciton coulomb and exchange energies in semiconductor quantum dots. Phys. ReV. Lett. 78, 915–918. Im, J.H., Jang, I.H., Pellet, N., Gr¨atzel, M., Park, N.G., 2014. Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells. Nat. Nanotechnol. 9, 927–932. Jeng, J.-Y., Chiang, Y.-F., Lee, M.-H., Peng, S.-R., Guo, T.-F., Chen, P., Wen, T.-C., 2013. CH3NH3PbI3 perovskite/fullerene planar-heterojunction hybrid solar cells. Adv. Mater. 25, 3727–3732. Jeon, N.J., Noh, J.H., Kim, Y.C., Yang, W.S., Ryu, S., Seok, S.I., 2014. Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells. Nat. Mater. 13, 897–903. Jeon, N.J., Noh, J.H., Yang, W.S., Kim, Y.C., Ryu, S., Seo, J., Seok, S.I., 2015. Compositional engineering of perovskite materials for highperformance solar cells. Nature 517, 476–480. 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., Gra¨tzel, M., Park, N.-G., 2012. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2, 591.
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