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Improving the performance of lead-acetate-based perovskite solar cells using solvent controlled crystallization process
Organic Electronics 78 (2020) 105552
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Improving the performance of lead-acetate-based perovskite solar cells using solvent controlled crystallization process Zhihai Liu a, *, Lei Wang b, Jiqu Han a, Fanming Zeng c, Guanchen Liu c, d, Xiaoyin Xie d, ** a
School of Opto-Electronic Information Science and Technology, Yantai University, Yantai, 264005, China Department of Packaging Engineering, Beijing Technology and Business University, Beijing, 100048, China c School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun, 130022, China d Department of Chemical Technology, Jilin Institute of Chemical Technology, Jilin, 132022, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Mix-solvent Crystallization Lead acetate Perovskite solar cells Power conversion efficiency
Lead accetate is an important lead source for preparation of organolead trihalide perovskites. In this work, we improved the performance of lead accetate based perovskite solar cells (PSCs) using a mix-solvent of γ-butyr olactone/dimethyl-sulfoxide (GBL/DMSO). Compared with the use of N,N-dimethylformamide (DMF)/DMSO, using GBL/DMSO efficiently retarded the crystallization of perovskite, which improved the crystallinity of the perovskite films. Consequently, the perovskite films processed from GBL/DMSO showed higher light harvesting, longer carrier lifetime and suppressed charge recombination properties in the PSCs. Power conversion efficiency (PCE) of the PSCs was significantly improved from 16.6 to 17.7% with simultaneous improvement in open-circuit voltage, short-circuit current density and fill factor. The best PSC showed a champion PCE of 18.1%, with a stable power output and negligible hysteresis. Moreover, the flexible PSCs (based on polyethylene naphthalate sub strates) with perovskite processed from GBL/DMSO exhibited a high PCE of 13.2%. Our results indicate that using GBL/DMSO as a mix-solvent is a simple and effective way for performance improvement of lead-acetatebased PSCs.
1. Introduction Owing to the advantages of high performance, light weight, low cost, and simple fabrication process, organolead trihalide perovskite solar cells (PSCs) have emerged as one the of the most promising nextgeneration photovoltaics [1–4]. Especially, the power conversion effi ciency (PCE) of the PSCs has significantly been improved from 3.9 to 25.2% within only a few years, indicating a high potential for future commercialization of PSCs [5]. In standard structured PSCs, high temperature processed TiO2 (ca. 500 � C) is typically employed as the electron transport layer (ETL) [3,4, 6]. Although high PCEs can be achieved for this kind of PSCs, the complicated TiO2 sintering operation limits its application in large-scale industrial production [7,8]. As a result, inverted planar structured PSCs have been intensively investigated because of their convenient fabrica tion process, in which organic ETL (such as phenyl-C61-butyric acid methyl ester (PCBM)) can be deposited using a solution method at low temperatures [9–11]. Basically, organolead trihalide perovskites can be
prepared by simply spin-coating peroskite precursor solutions onto substrates [3,4,9–11]. Perovskite precursors are usually prepared by dissolving lead halide (lead iodide (PbI2) or lead chloride (PbCl2)) and methylammonium iodide (MAI) into organic solvents [2,9–11]. How ever, in order to obtain high quality perovskite (CH3NH3PbI3) films, using PbI2 as the lead source usually require an anti-solvent treatment, which is not easy for experimentally controlling [12]. Although the anti-solvent process can be skipped by replacing PbI2 into PbCl2, the necessary long time thermal annealing (ca. 100 � C for 2 h) procedure would limit its application in the large-scale future commercialization [13,14]. Recently, lead accetate (Pb(Ac)2) has been widely used as the lead source for perovskite preparations. Compared with PbI2 and PbCl2, using Pb(Ac)2 is more convenient, because the CH3NH3PbI3 perovskite films can be realized without anti-solvent treatment or long time ther mal annealing [15,17–20]. Moreover, using Pb(Ac)2 could induce better CH3NH3PbI3 perovskite film with enhanced crystallinity, resulting in a higher PCE of the PSCs than those of PbI2 and PbCl2 based ones [15]. It is well-known that controling the morphology of perovskite layers plays
* Corresponding author. Yantai University, School of Opto-Electronic Information Science and Technology, Yantai University, Yantai, 264005, China. ** Corresponding author. Jilin Institute of Chemical Technology, Department of Chemical Technology, Jilin, 130022, China. E-mail addresses: [email protected] (Z. Liu), [email protected] (X. Xie). https://doi.org/10.1016/j.orgel.2019.105552 Received 16 September 2019; Received in revised form 2 November 2019; Accepted 4 November 2019 Available online 1 December 2019 1566-1199/© 2019 Published by Elsevier B.V.
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very important roles in boosting the performance of PSCs [16]. In case of Pb(Ac)2 based PSCs, controling crystallization process, such as nucle ation and the grain growth stages, is an effective way to improve the quality of perovskite [17–20]. For example, hypophosphorous acid and PbCl2 incorporations can both slow down the crystallization rate of perovksite formation, enhancing the optoelectronic property of perov skite films [17,18]. A thermal annealing free process was developed by Tan et al., resulting in slow growth of perovskite fim and PCE improvement of the relevant PSCs [19]. In our previous work, dimethyl-sulfoxide (DMSO) was demonstrated to be an effective co-solvent to retard the crystallization process of Pb(Ac)2 based perov skite, which lead to a significant PCE improvement from 12.88 to 16.59% [20]. In that work, N,N-dimethylformamide (DMF) was used as the host solvent to prepare the perovskite precursor solution. However, γ-butyrolactone (GBL) is another frequently used host solvent for fabricating PbI2 based PSCs [3,4,21]. Considering the evaporation property of GBL (with high boiling point and low vapor pressure), using GBL for Pb(Ac)2 based perovskite preparation could further retard the crystallization rate during spin-coating and thermal annealing pro cesses. Thus, it is very important to investigate the effect of using GBL on the performance of Pb(Ac)2 based PSCs. In this work, we improved the performance of Pb(Ac)2 based PSCs using a mix-solvent of GBL/DMSO for dissolving Pb(Ac)2 and MAI. Compared with the use of DMF/DMSO, using GBL/DMSO could further retard the crystallization process of perovskite due to the lower evapo ration property of GBL. Quality of the Pb(Ac)2 based perovskite was improved with enhanced crystallinity and enlarged perovskite grain size, which are beneficial for charge generation and dissociation in PSCs. When using GBL/DMSO, PCE of the PSCs was significantly improved from 16.6 to 17.7% with simultaneous improvement in open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF). The best PSC showed a champion PCE of 18.1%, with a stable power output and negligible hysteresis. Based on this technique, we also fabricated flexible PSCs on polyethylene naphthalate (PEN) substrates, which exhibited a high PCE of 13.2%. We provide a simple and effective way to control the growth of Pb(Ac)2 based perovskite and improve the performance of Pb(Ac)2 based PSCs.
PCBM/BCP/Ag architecture. First, the PTAA solution (in CB, 10 mg mL 1) was spin coated onto the pre-washed ITO coated glass and PEN substrates at 3500 rpm for 40 s. Then the samples were thermally annealed at 80 � C for 10 min to fully remove the solvent. The Pb(Ac)2 based perovskite precursor solution was prepared by dissolving Pb (Ac)2⋅3H2O and MAI (molar ratio ¼ 1:3) in different organic solvents (GBL, GBL/DMSO or DMF/DMSO) at a total concentration of 45 wt%. The perovskite precursor solutions were spin coated onto the PTAA layers at 4000 rpm for 45 s in a N2 gas filled glove box. Then the CH3NH3PbI3 perovskite films were formed after being thermally annealed at 100 � C for 5 min on a hot plate. For ETL deposition, the PCBM solution (20 mg mL 1 in CB) was spin coated onto the perovskite films at 1000 rpm for 45 s. Then the BCP solution (in IPA at 0.5 mg mL 1) was spin coated onto the PCBM ETLs at 4500 rpm for 45 s. The fabrication of the PSCs was finished by thermally evaporating a Ag cathode (100 nm thick) onto the samples. The effective working area of the Pb(Ac)2 based PSCs was 0.06 cm2, which was defined by a shadow mask. 2.2. Characterization The cross-sectional image of the PSC and the top view images of the perovskite surfaces were measured using an SU8020 scanning electron microscope (SEM, Hitachi, Japan), operated at an acceleration voltage of 8 kV. The ultraviolet–visible (UV–vis) absorption spectrum was characterized by using a Perkin Elmer Lambda 750 (USA). The X-ray diffraction (XRD) was performed using an X-ray diffractometer (Pan alytical, Netherlands). The photoluminescence (PL) and time resolved photoluminescence (TRPL) spectra were measured by a spectrometer (FLS920, Edinburgh Instruments, UK). The current density–voltage (J–V) characteristics of the PSCs were measured for an irradiation in tensity of 100 mW cm 2 (AM1.5). The incident photon-to-current effi ciency (IPCE) was measured using a Solar Cell IPCE measurement system (Solar Cell Scan 100, Zolix, China). 3. Results and discussion The cross-sectional SEM image in Fig. 1(b) shows a well-condensed layer-by-layer structure of the inverted structured PSCs, which is in consistent with the schematic in Fig. 1(a). The SEM image also indicates that the thicknesses of perovskite and PCBM layers are about 350 and 55 nm, respectively, which are typical thicknesses for high performance inverted PSCs [15,17–20]. As shown in Fig. S1 and Table S1, adding 5.0% (by volume) DMSO into GBL based perovskite precursor resulted in the highest PCE of the PSCs, which is similar with DMF based case [20]. This is because DMSO can form adduct with Pb(Ac)2, which could effectively retard the crys tallization of perovskite [20]. The J–V characteristic results of the PSCs are presented in Fig. 2(a), with the average device parameters summa rized in Table 1. By replacing DMF/DMSO into GBL/DMSO for perov skite preparation, PCE of the PSCs was further improved from 16.6 to
2. Experimental section 2.1. Device fabrication MAI, patterned indium-tin oxide (ITO) coated glass and PEN sub strates were purchased from Ying Kou You Xuan Trade Co., Ltd (China). Lead acetate trihydrate (Pb(Ac)2⋅3H2O), DMF, GBL, DMSO, isopropanol (IPA), and chlorobenzene (CB) were purchased from Sigma-Aldrich (USA). PCBM was purchased from Nano-C Inc. (USA). Poly(bis(4phenyl)(2,4,6-trimethylphenyl)amine) (PTAA) and bathocuproine (BCP) were purchased from EM Index (Korea) and Xi’an Polymer Light Technology Corp. (China), respectively. As shown in Fig. 1(a), our PSCs were fabricated using a structure of substrate/ITO/PTAA/perovskite/
Fig. 1. (a) Schematic structure of the PSCs fabricated in this work; (b) Cross-sectional SEM image of a PSC fabricated in this work. 2
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Organic Electronics 78 (2020) 105552
Fig. 2. (a) J–V characteristics (under forward scan) of the PSCs with perovskite layers processed from DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5); (b) IPCE spectra of the PSCs with perovskite layers processed from DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5); (c) Forward and reverse J–V characteristics of the highest performing PSC with perovskite layer processed from GBL/DMSO (volume ratio ¼ 95:5); (d) Current density and PCE as a function of time for the best PSC under a forward bias of 0.88 V.
17.7%, with simultaneous improvements in Voc (from 1.03 to 1.05 V), Jsc (from 21.7 to 22.2 mA cm 2) and FF (from 0.74 to 0.76). The average PCE values with standard deviations of the 2 kinds of PSCs are shown in Fig. S2, indicating the high reproducibility of our devices. As shown in Fig. 2(b), the IPCE of the PSCs was improved over a wide wavelength range between 350 and 750 nm. Usually the improved IPCE can be induced by enhancing light absorption and/or improving charge disso ciation of PSCs, which will be discussed later. The integrated Jsc values from IPCE spectra are 21.2 and 21.8 mA cm 2 for PSCs with perovskite layers processed from DMF/DMSO and GBL/DMSO, respectively. The small difference between Jsc values obtained from IPCE and J–V per formance indicates the high accuracy of our J–V characteristic mea surements. The best device from GBL/DMSO processed group exhibited a champion PCE of 18.1%, which is an excellent value for Pb(Ac)2 based inverted PSCs [15,17–20]. As shown in Fig. 2(c), under reverse scan, a similar J–V performance can be observed for the best device, indicating a negligible hysteresis of the PSCs. The steady-state current density and PCE with respect to time for the best PSC are shown in Fig. 2(d). The current density stabilized at 20.1 mA cm 2 for 300 s, yielding a stabi lized PCE of 17.7%, which indicates a stable output of the GBL/DMSO processed PSC. From Figs. S3(a) and (b), the best sample with perovskite layer processed from DMF/DMSO also shows a negligible hysteresis and
stable power output. To analyze the performance improvement of the PSCs, the quality of the Pb(Ac)2 based perovskite films was characterized using SEM, UV–vis absorption and XRD measurements. As shown in Fig. 3(a) and (b), both the perovskite films showed a full surface coverage without significant pinholes, indicating the well perovskite depositions. Compared with DMF/DMSO, using GBL/DMSO resulted in a more condensed surface with enlarged domain size, indicating the improved crystallization of the perovskite films [19,20]. Fig. 3(c) and (d) showed that, by using GBL/DMSO as solvent, light absorption was enhanced over a broad wavelength range, which is in good agreement with the improvement in IPCE spectra (see Fig. 2(b)). Thus the improved light absorption is an important aspect for improving Jsc. As shown from Fig. 3(d), the absorbance onset edge showed a slightly red shift, which might be caused by the improved crystallization of the perovskite film. The slightly decreased optical bandgap of the perovskite films could effi ciently facilitate electron excitation under irradiation [20,22]. More over, the XRD patterns of the perovskite films is shown in Fig. 4, which confirmed the improved crystallization of perovskite films by using GBL/DMSO as the solvent. From Fig. S4, the intensities of the perovskite (110) and (220) characteristic peaks at 14.1� and 28.4� were enhanced. The intensity of the peak at 12.5� , which indicates the (001) lattice planes of hexagonal (2H polytype) PbI2, was dramatically reduced, indicating the promoted crystallization of perovskite. Moreover, the crystallite sizes of DMF/DMSO and GBL/DMSO processed perovskite films were calculated to be 78 and 85 nm, respectively, indicating the improved crystallinity. The sizes of crystallites are similar with those of perovskite grains in the SEM images (Fig. 3(a) and (b)), which indicates the lower nuclei density of the GBL/DMSO processed perovskite film. The lower nuclei density would further lead to fewer boundaries and less
Table 1 Average device parameters for PSCs with perovskite layers processed from DMF/ DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5). Solvent
Voc (V)
Jsc (mA cm 2)
FF
PCE (%)
DMF/DMSO GBL/DMSO
1.03 � 0.01 1.05 � 0.01
21.7 � 0.4 22.2 � 0.4
0.74 � 0.02 0.76 � 0.02
16.6 � 0.4 17.7 � 0.3
3
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Fig. 3. SEM images of the perovskite films processed from (a) DMF/DMSO (volume ratio ¼ 95:5) and (b) GBL/DMSO (volume ratio ¼ 95:5); Wide (c) and narrow (d) wavelength range UV–vis spectra of perovskite films processed from DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5). Table 2 Solvent parameters of DMF and GBL [26,27]. Solvent
Boiling point (� C)
Vapor pressure (mm Hg)
DMF GBL
153 204
2.7 1.5
perovskite films. Figs. S5(a) and (b) represents thermal annealing pro cess of the samples spin coated from precursors using DMF/DMSO and GBL/DMSO. After spin coating, an orange color can be observed for the sample processed DMF/DMSO, whereas the GBL/DMSO processed sample looks quite transparent. When thermal annealed at 100 � C, the DMF/DMSO processed sample exhibited dark black color very quickly (within 30 s) indicating fast phase transition of the perovskite (from α phase to δ phase) [23,24]. However, it takes longer time (40 s) for phase transition of the GBL/DMSO processed sample, indicating a slow perovskite formation procedure [17,20,23]. Usually, the perovskite film formation process can be described as initial nucleation and later grain growth. The difference in evaporation property of DMF and GBL could influence the initial nucleation by changing the supersaturated state and activation energy, which acts as the driving force in perovskite forma tion [25]. The retarded crystallization would further facilitate the perovskite growth during spin coating and thermal annealing operations [17,20], which is in good agreement with the SEM, UV–vis absorption and XRD analysis. For deeper understanding the quality of perovskite on the perfor mance of PSCs, we also measured PL and TRPL spectra of the perovskite layers processed from DMF/DMSO and GBL/DMSO. As shown from the steady state PL performance in Fig. 5(a), the PL intensity of GBL/DMSO processed perovskite is about 30% higher than that of DMF/DMSO processed perovskite, indicating improved optoelectronic property of the perovskite [20,28,29]. An about 5 nm red shift of the PL peak can be observed from the spectra, which is in good agreement with the result of
Fig. 4. Schematic of the formation of perovskite layers processed from pre cursors using solvents of DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5).
defects, which resulted in lower resistance of the perovskite film [20, 23]. The lower resistance of the GBL/DMSO processed perovskite film is beneficial for improving Jsc and FF of the PSCs, which further contribute to PCE improvement [20]. The different intensity ratios of characteristic peaks ((110) to (310)) indicates the preferential growth of (110) perovskite plane by using GBL/DMSO [23], which is in consistent with the SEM images. The analyses of the XRD patterns show clear evidences for the improved crystallinity in the perovskite film processed from GBL/DMSO [15,17–20]. Table 2 summarizes the solvent parameters of DMF and GBL. Compare to DMF, the higher boiling point and lower vapor pressure of GBL indicate the slower evaporation property of the solvent. As sche matically shown in Fig. 4, during the perovskite formation process (spin coating and thermal annealing), GBL/DMSO would be slowly evapo rated from the samples, resulting in a slow growth of the perovskite. This hypothesis can be demonstrated by observing the color variation of the samples over time, which clearly shows the formation process of the 4
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Fig. 5. PL (a) and TRPL (b) spectra of the Pb(Ac)2 based perovskite layers (on glass) processed from DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5).
light absorption measurement in Fig. 3(c) and (d). From the TRPL spectra in Fig. 5(b), a significantly improved PL lifetime can be observed by using GBL/DMSO. After bi-exponential fitting to the spectra, the PL lifetime was enhanced from 202 to 325 ns when using GBL/DMSO for perovskite preparation. The TRPL results indicate the longer surviving time of excitons, which is beneficial for improving PCE of the PSCs [28, 29]. The PL and TRPL results demonstrate a lower defect concentration and superior electronic quality of the perovskite film processed from GBL/DMSO, which is consistent with performance improvement of the PSCs (shown in Fig. 2(a) and Table 1). We also analyzed the charge recombination properties of the PSCs by measuring the current density and voltage on incident light intensity (from 0.1 to 1 sun). According to previous studies, Jsc follows a powerlaw dependence on light intensity (Jsc ¼ Pαlight) [14,29–31]. When α value equals to 0.75, the device is space charge limited because of the interfacial barrier or carrier imbalance. When α value equals to 1, no space charge exists for the related device [29,30]. As shown in Fig. 6(a), the calculated slopes from the dependence of Jsc and light intensity of the PSCs with DMF/DMSO and GBL/DMSO processed perovskite layers are 0.96 and 0.99, respectively. This indicates a slightly suppressed bimolecular recombination in the PSCs based on GBL/DMSO processed perovskite film [30,31]. As shown in Fig. 3(a) and (b), the GBL/DMSO processed perovskite showed a smoother surface morphology than DMF/DMSO based perovskite, which is beneficial for improving the contact with PCBM. A better interfacial contact between perovskite and PCBM can more efficiently facilitate charge to electrode, improving Jsc
and FF [29–31]. On the other hand, from the dependence of Voc and light intensity in Fig. 6(b), the PSCs without PCBM layer showed a high slope of 1.49 kT/q, where k is the Boltzmann constant, T is the absolute temperature, and q is the elementary charge. For the PSCs with GBL/DMSO processed perovskite, the slope was significantly decreased to 1.32 kT/q, which indicated the reduced trap-assisted recombination of the PSCs [30–32]. This can be explained by the improved quality of perovskite film using different solvent as analyzed from the SEM, UV–vis absorption and XRD results. A high-quality perovskite layer (processed from GBL/DMSO) with good crystallinity shows less defects, which would reduce the carrier traps in the PSCs [33,34]. Moreover, the slightly improved Voc can be described by the Shockley equation [13,29, 35]: Voc �
nkT Jsc lnð Þ q J0
(1)
where J0 is the reverse saturation current density, and n is the ideality factor. The decreased charge recombination from the light intensity dependence analysis indicates the reduced J0. As a result, the suppressed charge recombination (resulting in decreased J0) and enhanced Jsc can explain the improvement in Voc (from 1.03 to 1.05 V). In addition, we also fabricated the flexible PSCs based on GBL/DMSO processed perovskite layer on PEN substrates. The inset of Fig. 7(a) showed the real image of the flexible PSC, which has excellent flexi bility. From the J–V characteristic in Fig. 7(a), the flexible PSCs showed a PCE of 13.2%, with Voc, Jsc, and FF of 1.02 V, 19.9 mA cm 2, and 0.65,
Fig. 6. Light intensity dependence of (a) Jsc and (b) Voc for PSCs with perovskite layers processed from DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5). 5
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Fig. 7. J–V characteristic of the flexible PSC (based on GBL/DMSO processed perovskite) on PEN substrate using PCBM as the electron transport layer. The inset shows the flexible PSC with the effective working area of 0.06 cm2.
respectively. The decreased PCE was mainly induced by the reductions in Jsc and FF, which probably induced by the rougher PEN substrate and higher series resistance of the flexible PSCs [7,36–38]. 4. Conclusions In conclusion, we investigated the use of GBL/DMSO as a mix-solvent to prepare Pb(Ac)2 based perovskite precursor and improved the per formance of PSCs. Compared with the use of DMF/DMSO, using GBL/ DMSO could improve the quality of perovskite by retarding the crys tallization process, which is caused by the lower evaporation property of GBL. The SEM, UV–vis absorption and XRD results indicate the enhanced crystallinity and enlarged grain size for perovskite layers upon using GBL/DMSO. The PL, TRPL and light intensity dependent analysis demontrate the improved optoeletronic property of GBL/DMSO pro cessed perovskite, which are beneficial for charge generation and dissociation in PSCs. As a result, PCE of the GBL/DMSO processed PSCs was significantly improved from 16.6 to 17.7% with simultaneous improvement in Voc, Jsc and FF. The best PSC showed a champion PCE of 18.1%, with a stable power output and negligible hysteresis. Based on this technique, we also fabricated flexible PSCs on PEN substrates, which exhibited a high PCE of 13.2%. We provide a simple and effective way to control the growth of Pb(Ac)2 based perovskite and improve the per formance of Pb(Ac)2 based PSCs. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Improving the performance of lead-acetate-based perovskite solar cells using solvent controlled crystallization process”. Acknowledgments This work was supported by the Department of Science & Technol ogy of Jilin Province (Developmental Project of Science and Technology of Jilin Province, Funding No.: 20160414043GH), Jilin Province
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Organic Electronics journal homepage: http://www.elsevier.com/locate/orgel
Improving the performance of lead-acetate-based perovskite solar cells using solvent controlled crystallization process Zhihai Liu a, *, Lei Wang b, Jiqu Han a, Fanming Zeng c, Guanchen Liu c, d, Xiaoyin Xie d, ** a
School of Opto-Electronic Information Science and Technology, Yantai University, Yantai, 264005, China Department of Packaging Engineering, Beijing Technology and Business University, Beijing, 100048, China c School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun, 130022, China d Department of Chemical Technology, Jilin Institute of Chemical Technology, Jilin, 132022, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Mix-solvent Crystallization Lead acetate Perovskite solar cells Power conversion efficiency
Lead accetate is an important lead source for preparation of organolead trihalide perovskites. In this work, we improved the performance of lead accetate based perovskite solar cells (PSCs) using a mix-solvent of γ-butyr olactone/dimethyl-sulfoxide (GBL/DMSO). Compared with the use of N,N-dimethylformamide (DMF)/DMSO, using GBL/DMSO efficiently retarded the crystallization of perovskite, which improved the crystallinity of the perovskite films. Consequently, the perovskite films processed from GBL/DMSO showed higher light harvesting, longer carrier lifetime and suppressed charge recombination properties in the PSCs. Power conversion efficiency (PCE) of the PSCs was significantly improved from 16.6 to 17.7% with simultaneous improvement in open-circuit voltage, short-circuit current density and fill factor. The best PSC showed a champion PCE of 18.1%, with a stable power output and negligible hysteresis. Moreover, the flexible PSCs (based on polyethylene naphthalate sub strates) with perovskite processed from GBL/DMSO exhibited a high PCE of 13.2%. Our results indicate that using GBL/DMSO as a mix-solvent is a simple and effective way for performance improvement of lead-acetatebased PSCs.
1. Introduction Owing to the advantages of high performance, light weight, low cost, and simple fabrication process, organolead trihalide perovskite solar cells (PSCs) have emerged as one the of the most promising nextgeneration photovoltaics [1–4]. Especially, the power conversion effi ciency (PCE) of the PSCs has significantly been improved from 3.9 to 25.2% within only a few years, indicating a high potential for future commercialization of PSCs [5]. In standard structured PSCs, high temperature processed TiO2 (ca. 500 � C) is typically employed as the electron transport layer (ETL) [3,4, 6]. Although high PCEs can be achieved for this kind of PSCs, the complicated TiO2 sintering operation limits its application in large-scale industrial production [7,8]. As a result, inverted planar structured PSCs have been intensively investigated because of their convenient fabrica tion process, in which organic ETL (such as phenyl-C61-butyric acid methyl ester (PCBM)) can be deposited using a solution method at low temperatures [9–11]. Basically, organolead trihalide perovskites can be
prepared by simply spin-coating peroskite precursor solutions onto substrates [3,4,9–11]. Perovskite precursors are usually prepared by dissolving lead halide (lead iodide (PbI2) or lead chloride (PbCl2)) and methylammonium iodide (MAI) into organic solvents [2,9–11]. How ever, in order to obtain high quality perovskite (CH3NH3PbI3) films, using PbI2 as the lead source usually require an anti-solvent treatment, which is not easy for experimentally controlling [12]. Although the anti-solvent process can be skipped by replacing PbI2 into PbCl2, the necessary long time thermal annealing (ca. 100 � C for 2 h) procedure would limit its application in the large-scale future commercialization [13,14]. Recently, lead accetate (Pb(Ac)2) has been widely used as the lead source for perovskite preparations. Compared with PbI2 and PbCl2, using Pb(Ac)2 is more convenient, because the CH3NH3PbI3 perovskite films can be realized without anti-solvent treatment or long time ther mal annealing [15,17–20]. Moreover, using Pb(Ac)2 could induce better CH3NH3PbI3 perovskite film with enhanced crystallinity, resulting in a higher PCE of the PSCs than those of PbI2 and PbCl2 based ones [15]. It is well-known that controling the morphology of perovskite layers plays
* Corresponding author. Yantai University, School of Opto-Electronic Information Science and Technology, Yantai University, Yantai, 264005, China. ** Corresponding author. Jilin Institute of Chemical Technology, Department of Chemical Technology, Jilin, 130022, China. E-mail addresses: [email protected] (Z. Liu), [email protected] (X. Xie). https://doi.org/10.1016/j.orgel.2019.105552 Received 16 September 2019; Received in revised form 2 November 2019; Accepted 4 November 2019 Available online 1 December 2019 1566-1199/© 2019 Published by Elsevier B.V.
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very important roles in boosting the performance of PSCs [16]. In case of Pb(Ac)2 based PSCs, controling crystallization process, such as nucle ation and the grain growth stages, is an effective way to improve the quality of perovskite [17–20]. For example, hypophosphorous acid and PbCl2 incorporations can both slow down the crystallization rate of perovksite formation, enhancing the optoelectronic property of perov skite films [17,18]. A thermal annealing free process was developed by Tan et al., resulting in slow growth of perovskite fim and PCE improvement of the relevant PSCs [19]. In our previous work, dimethyl-sulfoxide (DMSO) was demonstrated to be an effective co-solvent to retard the crystallization process of Pb(Ac)2 based perov skite, which lead to a significant PCE improvement from 12.88 to 16.59% [20]. In that work, N,N-dimethylformamide (DMF) was used as the host solvent to prepare the perovskite precursor solution. However, γ-butyrolactone (GBL) is another frequently used host solvent for fabricating PbI2 based PSCs [3,4,21]. Considering the evaporation property of GBL (with high boiling point and low vapor pressure), using GBL for Pb(Ac)2 based perovskite preparation could further retard the crystallization rate during spin-coating and thermal annealing pro cesses. Thus, it is very important to investigate the effect of using GBL on the performance of Pb(Ac)2 based PSCs. In this work, we improved the performance of Pb(Ac)2 based PSCs using a mix-solvent of GBL/DMSO for dissolving Pb(Ac)2 and MAI. Compared with the use of DMF/DMSO, using GBL/DMSO could further retard the crystallization process of perovskite due to the lower evapo ration property of GBL. Quality of the Pb(Ac)2 based perovskite was improved with enhanced crystallinity and enlarged perovskite grain size, which are beneficial for charge generation and dissociation in PSCs. When using GBL/DMSO, PCE of the PSCs was significantly improved from 16.6 to 17.7% with simultaneous improvement in open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF). The best PSC showed a champion PCE of 18.1%, with a stable power output and negligible hysteresis. Based on this technique, we also fabricated flexible PSCs on polyethylene naphthalate (PEN) substrates, which exhibited a high PCE of 13.2%. We provide a simple and effective way to control the growth of Pb(Ac)2 based perovskite and improve the performance of Pb(Ac)2 based PSCs.
PCBM/BCP/Ag architecture. First, the PTAA solution (in CB, 10 mg mL 1) was spin coated onto the pre-washed ITO coated glass and PEN substrates at 3500 rpm for 40 s. Then the samples were thermally annealed at 80 � C for 10 min to fully remove the solvent. The Pb(Ac)2 based perovskite precursor solution was prepared by dissolving Pb (Ac)2⋅3H2O and MAI (molar ratio ¼ 1:3) in different organic solvents (GBL, GBL/DMSO or DMF/DMSO) at a total concentration of 45 wt%. The perovskite precursor solutions were spin coated onto the PTAA layers at 4000 rpm for 45 s in a N2 gas filled glove box. Then the CH3NH3PbI3 perovskite films were formed after being thermally annealed at 100 � C for 5 min on a hot plate. For ETL deposition, the PCBM solution (20 mg mL 1 in CB) was spin coated onto the perovskite films at 1000 rpm for 45 s. Then the BCP solution (in IPA at 0.5 mg mL 1) was spin coated onto the PCBM ETLs at 4500 rpm for 45 s. The fabrication of the PSCs was finished by thermally evaporating a Ag cathode (100 nm thick) onto the samples. The effective working area of the Pb(Ac)2 based PSCs was 0.06 cm2, which was defined by a shadow mask. 2.2. Characterization The cross-sectional image of the PSC and the top view images of the perovskite surfaces were measured using an SU8020 scanning electron microscope (SEM, Hitachi, Japan), operated at an acceleration voltage of 8 kV. The ultraviolet–visible (UV–vis) absorption spectrum was characterized by using a Perkin Elmer Lambda 750 (USA). The X-ray diffraction (XRD) was performed using an X-ray diffractometer (Pan alytical, Netherlands). The photoluminescence (PL) and time resolved photoluminescence (TRPL) spectra were measured by a spectrometer (FLS920, Edinburgh Instruments, UK). The current density–voltage (J–V) characteristics of the PSCs were measured for an irradiation in tensity of 100 mW cm 2 (AM1.5). The incident photon-to-current effi ciency (IPCE) was measured using a Solar Cell IPCE measurement system (Solar Cell Scan 100, Zolix, China). 3. Results and discussion The cross-sectional SEM image in Fig. 1(b) shows a well-condensed layer-by-layer structure of the inverted structured PSCs, which is in consistent with the schematic in Fig. 1(a). The SEM image also indicates that the thicknesses of perovskite and PCBM layers are about 350 and 55 nm, respectively, which are typical thicknesses for high performance inverted PSCs [15,17–20]. As shown in Fig. S1 and Table S1, adding 5.0% (by volume) DMSO into GBL based perovskite precursor resulted in the highest PCE of the PSCs, which is similar with DMF based case [20]. This is because DMSO can form adduct with Pb(Ac)2, which could effectively retard the crys tallization of perovskite [20]. The J–V characteristic results of the PSCs are presented in Fig. 2(a), with the average device parameters summa rized in Table 1. By replacing DMF/DMSO into GBL/DMSO for perov skite preparation, PCE of the PSCs was further improved from 16.6 to
2. Experimental section 2.1. Device fabrication MAI, patterned indium-tin oxide (ITO) coated glass and PEN sub strates were purchased from Ying Kou You Xuan Trade Co., Ltd (China). Lead acetate trihydrate (Pb(Ac)2⋅3H2O), DMF, GBL, DMSO, isopropanol (IPA), and chlorobenzene (CB) were purchased from Sigma-Aldrich (USA). PCBM was purchased from Nano-C Inc. (USA). Poly(bis(4phenyl)(2,4,6-trimethylphenyl)amine) (PTAA) and bathocuproine (BCP) were purchased from EM Index (Korea) and Xi’an Polymer Light Technology Corp. (China), respectively. As shown in Fig. 1(a), our PSCs were fabricated using a structure of substrate/ITO/PTAA/perovskite/
Fig. 1. (a) Schematic structure of the PSCs fabricated in this work; (b) Cross-sectional SEM image of a PSC fabricated in this work. 2
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Fig. 2. (a) J–V characteristics (under forward scan) of the PSCs with perovskite layers processed from DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5); (b) IPCE spectra of the PSCs with perovskite layers processed from DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5); (c) Forward and reverse J–V characteristics of the highest performing PSC with perovskite layer processed from GBL/DMSO (volume ratio ¼ 95:5); (d) Current density and PCE as a function of time for the best PSC under a forward bias of 0.88 V.
17.7%, with simultaneous improvements in Voc (from 1.03 to 1.05 V), Jsc (from 21.7 to 22.2 mA cm 2) and FF (from 0.74 to 0.76). The average PCE values with standard deviations of the 2 kinds of PSCs are shown in Fig. S2, indicating the high reproducibility of our devices. As shown in Fig. 2(b), the IPCE of the PSCs was improved over a wide wavelength range between 350 and 750 nm. Usually the improved IPCE can be induced by enhancing light absorption and/or improving charge disso ciation of PSCs, which will be discussed later. The integrated Jsc values from IPCE spectra are 21.2 and 21.8 mA cm 2 for PSCs with perovskite layers processed from DMF/DMSO and GBL/DMSO, respectively. The small difference between Jsc values obtained from IPCE and J–V per formance indicates the high accuracy of our J–V characteristic mea surements. The best device from GBL/DMSO processed group exhibited a champion PCE of 18.1%, which is an excellent value for Pb(Ac)2 based inverted PSCs [15,17–20]. As shown in Fig. 2(c), under reverse scan, a similar J–V performance can be observed for the best device, indicating a negligible hysteresis of the PSCs. The steady-state current density and PCE with respect to time for the best PSC are shown in Fig. 2(d). The current density stabilized at 20.1 mA cm 2 for 300 s, yielding a stabi lized PCE of 17.7%, which indicates a stable output of the GBL/DMSO processed PSC. From Figs. S3(a) and (b), the best sample with perovskite layer processed from DMF/DMSO also shows a negligible hysteresis and
stable power output. To analyze the performance improvement of the PSCs, the quality of the Pb(Ac)2 based perovskite films was characterized using SEM, UV–vis absorption and XRD measurements. As shown in Fig. 3(a) and (b), both the perovskite films showed a full surface coverage without significant pinholes, indicating the well perovskite depositions. Compared with DMF/DMSO, using GBL/DMSO resulted in a more condensed surface with enlarged domain size, indicating the improved crystallization of the perovskite films [19,20]. Fig. 3(c) and (d) showed that, by using GBL/DMSO as solvent, light absorption was enhanced over a broad wavelength range, which is in good agreement with the improvement in IPCE spectra (see Fig. 2(b)). Thus the improved light absorption is an important aspect for improving Jsc. As shown from Fig. 3(d), the absorbance onset edge showed a slightly red shift, which might be caused by the improved crystallization of the perovskite film. The slightly decreased optical bandgap of the perovskite films could effi ciently facilitate electron excitation under irradiation [20,22]. More over, the XRD patterns of the perovskite films is shown in Fig. 4, which confirmed the improved crystallization of perovskite films by using GBL/DMSO as the solvent. From Fig. S4, the intensities of the perovskite (110) and (220) characteristic peaks at 14.1� and 28.4� were enhanced. The intensity of the peak at 12.5� , which indicates the (001) lattice planes of hexagonal (2H polytype) PbI2, was dramatically reduced, indicating the promoted crystallization of perovskite. Moreover, the crystallite sizes of DMF/DMSO and GBL/DMSO processed perovskite films were calculated to be 78 and 85 nm, respectively, indicating the improved crystallinity. The sizes of crystallites are similar with those of perovskite grains in the SEM images (Fig. 3(a) and (b)), which indicates the lower nuclei density of the GBL/DMSO processed perovskite film. The lower nuclei density would further lead to fewer boundaries and less
Table 1 Average device parameters for PSCs with perovskite layers processed from DMF/ DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5). Solvent
Voc (V)
Jsc (mA cm 2)
FF
PCE (%)
DMF/DMSO GBL/DMSO
1.03 � 0.01 1.05 � 0.01
21.7 � 0.4 22.2 � 0.4
0.74 � 0.02 0.76 � 0.02
16.6 � 0.4 17.7 � 0.3
3
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Fig. 3. SEM images of the perovskite films processed from (a) DMF/DMSO (volume ratio ¼ 95:5) and (b) GBL/DMSO (volume ratio ¼ 95:5); Wide (c) and narrow (d) wavelength range UV–vis spectra of perovskite films processed from DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5). Table 2 Solvent parameters of DMF and GBL [26,27]. Solvent
Boiling point (� C)
Vapor pressure (mm Hg)
DMF GBL
153 204
2.7 1.5
perovskite films. Figs. S5(a) and (b) represents thermal annealing pro cess of the samples spin coated from precursors using DMF/DMSO and GBL/DMSO. After spin coating, an orange color can be observed for the sample processed DMF/DMSO, whereas the GBL/DMSO processed sample looks quite transparent. When thermal annealed at 100 � C, the DMF/DMSO processed sample exhibited dark black color very quickly (within 30 s) indicating fast phase transition of the perovskite (from α phase to δ phase) [23,24]. However, it takes longer time (40 s) for phase transition of the GBL/DMSO processed sample, indicating a slow perovskite formation procedure [17,20,23]. Usually, the perovskite film formation process can be described as initial nucleation and later grain growth. The difference in evaporation property of DMF and GBL could influence the initial nucleation by changing the supersaturated state and activation energy, which acts as the driving force in perovskite forma tion [25]. The retarded crystallization would further facilitate the perovskite growth during spin coating and thermal annealing operations [17,20], which is in good agreement with the SEM, UV–vis absorption and XRD analysis. For deeper understanding the quality of perovskite on the perfor mance of PSCs, we also measured PL and TRPL spectra of the perovskite layers processed from DMF/DMSO and GBL/DMSO. As shown from the steady state PL performance in Fig. 5(a), the PL intensity of GBL/DMSO processed perovskite is about 30% higher than that of DMF/DMSO processed perovskite, indicating improved optoelectronic property of the perovskite [20,28,29]. An about 5 nm red shift of the PL peak can be observed from the spectra, which is in good agreement with the result of
Fig. 4. Schematic of the formation of perovskite layers processed from pre cursors using solvents of DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5).
defects, which resulted in lower resistance of the perovskite film [20, 23]. The lower resistance of the GBL/DMSO processed perovskite film is beneficial for improving Jsc and FF of the PSCs, which further contribute to PCE improvement [20]. The different intensity ratios of characteristic peaks ((110) to (310)) indicates the preferential growth of (110) perovskite plane by using GBL/DMSO [23], which is in consistent with the SEM images. The analyses of the XRD patterns show clear evidences for the improved crystallinity in the perovskite film processed from GBL/DMSO [15,17–20]. Table 2 summarizes the solvent parameters of DMF and GBL. Compare to DMF, the higher boiling point and lower vapor pressure of GBL indicate the slower evaporation property of the solvent. As sche matically shown in Fig. 4, during the perovskite formation process (spin coating and thermal annealing), GBL/DMSO would be slowly evapo rated from the samples, resulting in a slow growth of the perovskite. This hypothesis can be demonstrated by observing the color variation of the samples over time, which clearly shows the formation process of the 4
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Fig. 5. PL (a) and TRPL (b) spectra of the Pb(Ac)2 based perovskite layers (on glass) processed from DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5).
light absorption measurement in Fig. 3(c) and (d). From the TRPL spectra in Fig. 5(b), a significantly improved PL lifetime can be observed by using GBL/DMSO. After bi-exponential fitting to the spectra, the PL lifetime was enhanced from 202 to 325 ns when using GBL/DMSO for perovskite preparation. The TRPL results indicate the longer surviving time of excitons, which is beneficial for improving PCE of the PSCs [28, 29]. The PL and TRPL results demonstrate a lower defect concentration and superior electronic quality of the perovskite film processed from GBL/DMSO, which is consistent with performance improvement of the PSCs (shown in Fig. 2(a) and Table 1). We also analyzed the charge recombination properties of the PSCs by measuring the current density and voltage on incident light intensity (from 0.1 to 1 sun). According to previous studies, Jsc follows a powerlaw dependence on light intensity (Jsc ¼ Pαlight) [14,29–31]. When α value equals to 0.75, the device is space charge limited because of the interfacial barrier or carrier imbalance. When α value equals to 1, no space charge exists for the related device [29,30]. As shown in Fig. 6(a), the calculated slopes from the dependence of Jsc and light intensity of the PSCs with DMF/DMSO and GBL/DMSO processed perovskite layers are 0.96 and 0.99, respectively. This indicates a slightly suppressed bimolecular recombination in the PSCs based on GBL/DMSO processed perovskite film [30,31]. As shown in Fig. 3(a) and (b), the GBL/DMSO processed perovskite showed a smoother surface morphology than DMF/DMSO based perovskite, which is beneficial for improving the contact with PCBM. A better interfacial contact between perovskite and PCBM can more efficiently facilitate charge to electrode, improving Jsc
and FF [29–31]. On the other hand, from the dependence of Voc and light intensity in Fig. 6(b), the PSCs without PCBM layer showed a high slope of 1.49 kT/q, where k is the Boltzmann constant, T is the absolute temperature, and q is the elementary charge. For the PSCs with GBL/DMSO processed perovskite, the slope was significantly decreased to 1.32 kT/q, which indicated the reduced trap-assisted recombination of the PSCs [30–32]. This can be explained by the improved quality of perovskite film using different solvent as analyzed from the SEM, UV–vis absorption and XRD results. A high-quality perovskite layer (processed from GBL/DMSO) with good crystallinity shows less defects, which would reduce the carrier traps in the PSCs [33,34]. Moreover, the slightly improved Voc can be described by the Shockley equation [13,29, 35]: Voc �
nkT Jsc lnð Þ q J0
(1)
where J0 is the reverse saturation current density, and n is the ideality factor. The decreased charge recombination from the light intensity dependence analysis indicates the reduced J0. As a result, the suppressed charge recombination (resulting in decreased J0) and enhanced Jsc can explain the improvement in Voc (from 1.03 to 1.05 V). In addition, we also fabricated the flexible PSCs based on GBL/DMSO processed perovskite layer on PEN substrates. The inset of Fig. 7(a) showed the real image of the flexible PSC, which has excellent flexi bility. From the J–V characteristic in Fig. 7(a), the flexible PSCs showed a PCE of 13.2%, with Voc, Jsc, and FF of 1.02 V, 19.9 mA cm 2, and 0.65,
Fig. 6. Light intensity dependence of (a) Jsc and (b) Voc for PSCs with perovskite layers processed from DMF/DMSO (volume ratio ¼ 95:5) and GBL/DMSO (volume ratio ¼ 95:5). 5
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Organic Electronics 78 (2020) 105552
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Fig. 7. J–V characteristic of the flexible PSC (based on GBL/DMSO processed perovskite) on PEN substrate using PCBM as the electron transport layer. The inset shows the flexible PSC with the effective working area of 0.06 cm2.
respectively. The decreased PCE was mainly induced by the reductions in Jsc and FF, which probably induced by the rougher PEN substrate and higher series resistance of the flexible PSCs [7,36–38]. 4. Conclusions In conclusion, we investigated the use of GBL/DMSO as a mix-solvent to prepare Pb(Ac)2 based perovskite precursor and improved the per formance of PSCs. Compared with the use of DMF/DMSO, using GBL/ DMSO could improve the quality of perovskite by retarding the crys tallization process, which is caused by the lower evaporation property of GBL. The SEM, UV–vis absorption and XRD results indicate the enhanced crystallinity and enlarged grain size for perovskite layers upon using GBL/DMSO. The PL, TRPL and light intensity dependent analysis demontrate the improved optoeletronic property of GBL/DMSO pro cessed perovskite, which are beneficial for charge generation and dissociation in PSCs. As a result, PCE of the GBL/DMSO processed PSCs was significantly improved from 16.6 to 17.7% with simultaneous improvement in Voc, Jsc and FF. The best PSC showed a champion PCE of 18.1%, with a stable power output and negligible hysteresis. Based on this technique, we also fabricated flexible PSCs on PEN substrates, which exhibited a high PCE of 13.2%. We provide a simple and effective way to control the growth of Pb(Ac)2 based perovskite and improve the per formance of Pb(Ac)2 based PSCs. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Improving the performance of lead-acetate-based perovskite solar cells using solvent controlled crystallization process”. Acknowledgments This work was supported by the Department of Science & Technol ogy of Jilin Province (Developmental Project of Science and Technology of Jilin Province, Funding No.: 20160414043GH), Jilin Province
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