Mixed-solvent-vapor annealing of perovskite for photovoltaic device efficiency enhancement
Graphical abstract
Introduction
Organic−inorganic hybrid methylammonium lead trihalide perovskites (CH3NH3PbX3, where X is halogen I, Br or Cl) have gradually become a spotlight for low-cost, high efficient solar cells due to their obvious advantages such as high absorption coefficient, broad light absorption range from visible to the near-infrared region, bandgap tunability by varying different components, long charge carrier diffusion length, bipolar transport properties, and solution processing approach [1], [2], [3], [4], [5], [6], [7], [8]. In 2009, Miyasaka and co-workers employed the organometal trihalide perovskite as the sensitizer in the dye-sensitized solar cell (DSSC) with liquid electrolyte, demonstrating the first perovskite solar cell with a modest PCE of 3.8% [1]. The power conversion efficiency of pervoskite solar cells (PSCs) has increased from 3.8% to over 20% just over the past six years [1], [2], [3], [4], [5], [6], [7], [8], [9]. Many methods such as interface engineering [6], compositional engineering [7], new materials [9] and device structures [10] have been proved to be effective for improving the PSC performance, including the device efficiency and stability. Among these methods, however, there is no doubt that improving the crystal quality of perovskite materials is the key to improve the PSC performance. Huang et al. demonstrated an interdiffusion approach to form high quality CH3NH3PbI3 films, which could be considered as the modified two-step process in which perovskite films were formed by interdiffusion of CH3NH3I into PbI2 [8]. Seok et al. employed solvent engineering approach to get high quality perovskite films in a simple way with good reproducibility and negligible hysteresis [9]. This method has been proved to be effective to optimize the perovskite quality by forming a complex MAI-DMSO-PbI2 immediate phase. Chang et al. achieved higher film crystallinity by controlling the precursor purity [11] and precursor compositions [12]. Although the water could cause great damage to the perovskite film, You et al. discovered that moisture could improve the perovskite quality during the perovskite precursor film annealing [13]. Sang et al. used HI as an additive in MAPbI3/DMSO solution and MAPbI3/DMF solution, which could passivate the defects, and the corresponding devices showed lower current density – voltage (J-V) hysteresis [14]. Besides above findings, recently solvent vapor annealing was also demonstrated to be one of the most effective and facile ways to improve the crystal quality of perovskites.
Huang et al. found that the introduction of DMF solvent vapor during the crystal growth of CH3NH3PbI3 by interdiffusion of CH3NH3I into PbI2 could effectively enhance the film crystallinity and grain size, passivate the defects, and thus improve the device performance [15]. The reason is that there is a good solubility of PbI2 and CH3NH3I in DMF which can retard the perovskite crystal formation and help to form a preferable state of the CH3NH3PbI3 grains. However, if the DMF vapor was introduced into the PbI2-CH3NH3I system and the recrystallization rate was not precisely controlled during the perovskite reaction, DMF solvent could compromise the pre-deposited dense and compact PbI2 and generate pinholes, which will lead to the inferior perovskite morphology and low device performance. To overcome this problem, Liu et al. proposed to introduce a PbI2 anti-solvent environment such as alcohol vapors instead of DMF vapor in the annealing process, aiming to improve the crystal growth and increase the grain size of CH3NH3PbI3 crystal [16]. In alcohols, CH3NH3I rather than PbI2 possesses a good solubility, so that the alcohol solvent vapors in the annealing process could selectively control the dynamic transitions of CH3NH3I, leaving the PbI2 layer as a dense film to ensure a pinhole-free perovskite layer. By using this method, higher crystallinity and pinhole-free CH3NH3PbI3 thin films are realized, and the improved PSCs along with high reproducibility and eliminated photocurrent hysteresis behavior are obtained. Although the above studies have shown that the solvent vapor annealing can effectively improve the device performance, both of them are based on the perovskite produced by the two-step method. Compared with the two-step method, it is widely accepted that the one-step method is more preferred because it is simpler, cheaper and more compatible with the roll-to-roll process. Otherwise, although the standalone solvent annealing or anti-solvent annealing have been proved to be efficient for the perovskite quality improvement, there is still no reports about whether the mixed solvent and anti-solvent annealing is efficient for the device performance improvement or not. Thus, the mixed solvent annealing in the one-step method produced perovskite needs to be investigated further, especially for the perovskite fabricated by the solvent engineering method since there is PbI2-MAI-DMSO intermediate phase [17] before annealing.
In this work, we report that when the perovskite precursor film (PbI2-MAI-DMSO intermediate phase) deposited by the solvent engineering one-step method is annealed in anhydrous isopropanol vapor environment, in which CH3NH3PbI3 possesses a poor solubility, a dense uniform and pinhole-free perovskite film can be obtained and the performance of corresponding PSCs is significantly improved. When a little polar aprotic DMF vapor is introduced in the isopropanol vapor, a positive effect could be observed. By using this mixed-solvent vapor treatment in the annealing process, it is found that the average grain size of CH3NH3PbI3 crystals can be further increased, resulting in enhanced short-circuit current density (JSC), suppressed reverse dark current, and reduced recombination loss in PSCs. Mixed-solvent annealing can greatly improve the electronic properties and stability of PSCs, and the best-performing device with planar heterojunction structure reaches the efficiency of over 15% under air mass global (AM 1.5 G) spectrum illumination, which is over 23% enhancement than the reference device along with high reproducibility.
To further improve the performance of planar heterojunction solar cell, CH3NH3PbI3−xClx perovskite precursor and interface engineering have been employed besides using the mixed-solvent-vapor annealing and PCEs achieved around 19%. Introducing chlorine ions into organolead triiodide perovskite can increase the electron-hole diffusion length [4] and retard the crystallization of perovskite to improve the surface morphology and improve the crystalline quality [18]. BCP possesses deep HOMO energy level (−7.0 eV) and will act as the hole blocking layer in the inverted PSCs. This will help to reduce the charge recombination at the interface. By employing BCP layer between PC61BM and Ag cathode and incorporation of chlorine ions into organolead triiodide perovskite, the performance of the perovskite solar cell has been significantly improved.
Section snippets
Materials
All the materials, Methylammonium iodide (CH3NH3I, 99.8%, Dyesol), Lead iodide (PbI2, 99.999%, Sigma), lead chloride (PbCl2, 99.999%, Sigma), phenyl-C61-butyric acid methyl ester (PCBM, 98%, nano-c), Bathocuproine (BCP, 96%, Sigma), poly(3,4-ethy-lenedioxythiophene) poly(styrenesulfonate) (PEDOT: PSS, Clevios PVP Al 4083), Dimethyl sulphoxide (99.8%, Sigma), N,N′-Dimethylformamide (99.8%, Aladdin), Chlorobenzene (99.8%, Sigma), Toluene (99.8%, Sigma-Aldrich), γ-Butyrolactone (99.8%, Aladdin),
Results and discussion
The CH3NH3PbI3 PSCs in this study were structured as ITO/PEDOT: PSS/CH3NH3PbI3/PCBM/Ag as shown in Fig. 1a, where the PEDOT: PSS and PCBM act as hole and electron transport layers, respectively. Here an inverted planar heterojunction structure is used instead of the conventional structure due to some disadvantages for the conventional structure. For example, mesoporous device structures employ an n-type TiO2 layer as the bottom electron transport layer. However, it usually needs a high
Conclusion
In conclusion, the mixed solvent vapor annealing for CH3NH3PbI3 perovskite thin films is an effective method to improve the electronic property of material and photovoltaic performance of the devices. Compared with the IPA solvent annealing, the introduction of a little polar aprotic solvent such as DMF can further improve the device performance. The XRD and SEM analysis demonstrates that the average grain size of perovskite film via IPA/DMF mixed-solvent (100:1, v/v) vapor annealing has been
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