Improving the performance of lead-acetate-based perovskite solar cells using solvent controlled crystallization process
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 next-generation photovoltaics [[1], [2], [3], [4]]. Especially, the power conversion efficiency (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 fabrication 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], [10], [11]]. Basically, organolead trihalide perovskites can be prepared by simply spin-coating peroskite precursor solutions onto substrates [3,4,[9], [10], [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], [10], [11]]. However, 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 thermal annealing [15,[17], [18], [19], [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 very important roles in boosting the performance of PSCs [16]. In case of Pb(Ac)2 based PSCs, controling crystallization process, such as nucleation and the grain growth stages, is an effective way to improve the quality of perovskite [[17], [18], [19], [20]]. For example, hypophosphorous acid and PbCl2 incorporations can both slow down the crystallization rate of perovksite formation, enhancing the optoelectronic property of perovskite 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 perovskite, 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 processes. 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 evaporation 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.
Section snippets
Device fabrication
MAI, patterned indium-tin oxide (ITO) coated glass and PEN substrates 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(4-phenyl)(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
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], [18], [19], [20]].
As shown in Fig. S1 and Table S1, adding 5.0% (by volume) DMSO into GBL based perovskite precursor resulted
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 performance of PSCs. Compared with the use of DMF/DMSO, using GBL/DMSO could improve the quality of perovskite by retarding the crystallization 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,
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 & Technology of Jilin Province (Developmental Project of Science and Technology of Jilin Province, Funding No.: 20160414043GH), Jilin Province Department of Education (Grant No.: JJKH20180558KJ).
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