ReviewPolymer strategies for high-efficiency and stable perovskite solar cells
Graphical Abstract
Introduction
Halide perovskites have exceptional photoelectric properties including tunable composition and bandgap [1], [2], a high absorption coefficient [3], long charge carrier diffusion length [4], [5], [6], high mobility [7], high defect tolerance [8], [9], and low exciton binding energy [10]. Optimized solution processing methods, such as sequence deposition [11], [12], [13], anti-solvent engineering [14], [15], additive engineering [16], and composition engineering [17], [18], enable researchers to prepare perovskite films with ever-increasing quality. Well-established solar cells such as dye-sensitized solar cells (DSSCs) and organic photovoltaic devices provide rich experiences on the optimization of the perovskite solar cells (PSC) device structure. Benefiting from these aspects, the power conversion efficiency (PCE) of PSCs has been enhanced significantly from a mere 3.8% [19] to a certified record of 25.2%, which is comparable to that of single crystal silicon solar cells [20]. It can be said that efficiency is no longer the bottleneck restricting the commercialization of PSCs.
At present, the long-term instability of PSCs under harsh environments, e.g., heat [21], [22], [23], light [24], and moisture [25], [26], [27], has been the most challenging issue. Among them, moisture is shown to be the main instability inducement of PSCs. In addition, the presence of moisture will accelerate the degradation of perovskite induced by heat and light [28]. To mitigate the instability issue of PSCs, many technologies including the engineering of a perovskite active layer (AL) [29], [30], [31], [32], [33], [34], modifying or replacing charge transport layer (CTL) materials [35], [36], [37], [38], [39], inserting moisture barriers [40], [41], and even encapsulating whole devices [42], [43], [44], [45], [46], [47] have been developed. Among these technologies, polymer strategy is widely used, e.g., using polymer additives in perovskite AL, adopting polymeric CTL, employing polymer dopants in CTL (i.e. hole transport layer (HTL) and electron transport layer (ETL)), inserting polymeric interfacial layers (IL), and capping polymer encapsulation layers (EL). For example, Zhao et al. reported poly(ethylene glycol)(PEG)-based perovskite films with strong moisture resistance and self-healing behavior [48]. The resulting unencapsulated devices retained high output for up to 300 h in a highly humid environment (i.e. 70% relative humidity, or RH). Zhang et al. replaced a traditional 2,2′,7,7′‐tetrakis‐(N,N‐di‐4–methoxyphenylamino)‐9,9′‐spirobifluorene (spiro-MeOTAD) HTL with fluorinated polymeric HTL (P3). The hydrophobic features of the polymers conferred the PSCs with traits of elongated durability and morphology stability, enabling the PSCs to maintain over 96% of their original performance after 40 days of the stability test [49]. Zuo et al. developed a homogeneous bulk-mixed ETL by blending n-type poly[(9,9-dioctyluorene)−2,7-diyl-alt-(4,7-bis(3-hexylthien-5-yl)−2,1,3-benzothiadiazole)−2′,2″-diyl] (F8TBT) with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) [50]. The PSCs with such ETLs retained 80% of the initial performance after being exposed to the ambient air in the dark for 45 days. Wang et al. inserted a thin poly(methyl methacrylate) (PMMA) layer between perovskite AL and spiro-MeOTAD HTL to improve the stability [51]. The PSCs containing a polymeric IL exhibited a slight decrease of only approximately 5% from the original value during exposure to ambient moisture over 20 days. Bella et al. encapsulated PSCs with fluorinated photopolymer coatings synthesized by an in-situ rapid light-induced free-radical polymerization method [52]. In that case, the coated PSCs reproducibly retained their full functional performance during prolonged operation, even for more than six months after a series of severe aging tests was carried out. These results indicate that polymer strategy is promising for obtaining stable PSCs.
In addition to enhancing device stability, polymer strategy also shows remarkable performance in improving device performance. For example, Bi et al. used PMMA as a template to control nucleation and crystal growth [53]. As a result, except for the improved stability, the devices exhibited a PCE of up to 21.6% with negligible J-V hysteresis behavior. Wu et al. and Peng et al. inserted polystyrene (PS) and PMMA into both perovskite/HTL and perovskite/ETL interfaces in their PSCs, respectively [54], [55]. Consequently, the resulting PSCs showed impressive PCE values of 21.89% and 20.8% with negligible J-V hysteresis behavior, respectively.
Polymers adopted for PSCs contain various types, such as insulating polymers [48], [53], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], p-type semiconductor polymers [49], [74], [75], [76], [77], [78], [79], [80], [81], and n-type semiconductor polymers [50], [74], [75], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91]. Among insulating polymers, some are amorphous [62], [63], [64], [65], some are crystalline [48], [66], [67], [68], [69], [70], and some are otherwise unique such as liquid crystalline polymers, elastic polymers, and ferroelectric polymers [59], [71], [72], [73]. On the one hand, the diversity of available polymers results from the good coordination ability of lead cations and iodide anions as well as the hydrogen bonding ability of organic ammonium cations in halide perovskites. On the other hand, such diversity also results due to the rich chemistry of polymers.
To further improve PSC stability towards commercial application and improve the efficiency towards their theoretical Shockley–Queisser (S–Q) efficiency limit of more than 30% [92], adopting polymer strategy is one of the most promising methods, which deserves more attention and trial application. Herein, we review polymer strategies used for efficient, stable, and hysteresis-reduced PSCs. We first briefly describe the methods to introduce polymers into PSCs. Then, we classify and summarize various polymers used in PSCs according to their positions in devices (e.g., AL, CTL, IL, and EL). Furthermore, we also discuss in detail their functions of modulating morphology, adjusting energy level alignment, suppressing non-radiative recombination, reducing hysteresis, and enhancing the stability and flexibility of PSCs.
Section snippets
Ex-situ polymerization
Ex-situ polymerization is the most widely used approach to fabricate polymer-contained PSCs, due to its simplicity, convenience, and flexibility of operation as well as wide universality. Typically, pre-synthesized or commercially acquired polymers are directly introduced into PSCs by mixing with perovskite precursor solutions, or charge transport material solutions, or using their pure solutions. In view of the great importance of the morphology of perovskite film to the device performance, we
Polymer strategies applied to perovskite AL
The quality of perovskite AL determines the photovoltaic performance of PSCs to a large extent, as the quality of perovskite AL including the crystallinity, homogeneity, surface morphology, coverage of film, and density of defects affects many photo-physical properties, such as light harvesting, charge carrier transport, diffusion length, and charge recombination [111], [112], [113], [114], [115]. In particular, the defects produced within the grains, at the grain boundaries, and on the surface
Polymer strategies in CTLs
Charge transport layers play significant roles in the photovoltaic performance for PSCs, as their energy level, surface properties, chemical and physical properties, transparency, and transport properties affect many photo-physical properties, such as selective charge transfer, nonradiative recombination, light harvesting, and charge carrier transport [153]. For conventional PSCs, the issues related to defects and instability not only exist in the perovskites or at the perovskite interfaces,
Polymer strategies employed in the IL
Except for optimizing the basic three functional layers, i.e., AL, ETL, and CTL, interface engineering has proven to be very efficient in obtaining better device performance and stability. In this section, we summarize polymers used as an IL between perovskite films and CTLs as well as between CTLs and electrodes, and the summary of polymers used as ILs is also presented in Table 3.
Polymeric EL
The performance of PSC devices is known to be highly susceptible to deterioration upon exposure to ambient atmospheric conditions, due to the susceptibility of the perovskite to decompose when in contact with moisture. Therfore, encapsulation is an important component of photovoltaic devices, as this protects it from damaging effects of oxygen and moisture. Device encapsulation is expected to play a role in the commercialization of PSCs. Compared to traditional glass encapsulation, where the
Summary and outlook
Tremendous attempts have been made to obtain efficient, stable, and hysteresis-free PSCs. Polymer seems to be the best partner for an inherently humidity-sensitive perovskite. Polymer strategy, that involves applying polymers into PSCs, shows great potential for achieving this goal due to the unique chemical and physical properties of polymers. We first review several prevalent methods to introduce polymers into PSCs and the various polymers widely adopted in the polymer strategy. Then we
CRediT authorship contribution statement
Sisi Wang: Conceptualization and Writing - original draft. Zhipeng Zhang: Writing - review & editing. Zikang Tang: Project administration, Chenliang Su: Writing - review & editing; Wei Huang: Project administration; Ying Li: Co-supervision; Guichuan Xing: Funding acquisition and Co-supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge financial support from Macau Science and Technology Development Fund, China (FDCT-0044/2020/A1, FDCT-091/2017/A2, FDCT-014/2017/AMJ), University of Macau Research Grant, China (MYRG2018-00148-IAPME, MYRG2018-00142-IAPME) from University of Macau, the Natural Science Foundation of China, China (91733302, 61935017), and Natural Science Foundation of Guangdong Province, China (2019A1515012186).
Sisi Wang received her B.Sc degree in Chemistry from Hubei University in 2010, and then received her Ph.D. degree in polymer chemistry and physics from in Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2016. She worked as a co-supervised postdoctor in the Shenzhen university and Macau university since 2018. Her current research interests include fabrication of photovoltaic devices, interfacial materials, morphology control and characterization, and condensed matter
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Sisi Wang received her B.Sc degree in Chemistry from Hubei University in 2010, and then received her Ph.D. degree in polymer chemistry and physics from in Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2016. She worked as a co-supervised postdoctor in the Shenzhen university and Macau university since 2018. Her current research interests include fabrication of photovoltaic devices, interfacial materials, morphology control and characterization, and condensed matter physics.
Zhipeng Zhang received his bachelor’s degree in 2015 from Nanjing Tech University (NanjingTech). He is currently a Ph.D. candidate in the Institute of Applied Physics and Materials Engineering at University of Macau, supervised by Professor Guichuan Xing. His current research interest focuses on the synthesis of metal halide perovskite nanocomposites and their applications in optoelectronics.
Zikang Tang is chair professor and director in the Institute of Applied Physics and Materials Engineering at University of Macau, China. He obtained his Ph.D. in Condensed Matter Physics from Tohoku University, Japan in 1992. Following he worked at Japanese Science & Technology Agency as research fellow (1992–1994). He then joined the faculty at Hong Kong University of Science & Technology (1994–2015) and joined the University of Macau from 2016. His research focuses on nano-structured electronic materials, 2D materials, ZnO crystal thin films, wide-gap semiconductors and their photo-electronic devices.
Chenliang Su received his B.S. degree (2005) and Ph.D. degree (2010) from the Department of Chemistry at the Zhejiang University of China. After that he worked as a research fellow at the Advanced 2D Materials and Graphene Research Centre at the National University of Singapore (2010–2015). He is now a full-professor at the International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology (ICL-2D MOST), Shenzhen University and a Principal Investigator of the ICL-2D MOST in materials science. His current interests include the study of nanostructured materials for heterogeneous catalysis and energy applications.
Wei Huang received his B.Sc, M.Sc, and Ph.D. degrees in Chemistry from Peking University in 1983, 1988, and 1992, respectively. He worked as a postdoctor in the Chemistry Department at the National University of Singapore in 1993, and then as a chair professor at Fudan University in 2001. He was appointed as the Deputy President of Nanjing University of Posts and Telecommunications in 2006, as the President of Nanjing Tech University in 2012, as the Provost of Northwestern Polytechnical University in 2017. His research interests include organic optoelectronics, nanomaterials, polymer chemistry, plastic electronics, and bioelectronics.
Ying Li is now a Full Professor at the International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology (ICL-2D MOST) of Institute of Microscale Optoelectronics, Shenzhen University, China. She received her Ph.D. degree in optical engineering from Fudan University in 2010. Her current research interest mainly focuses on the study of nonlinear optics and all-optical devices including the broadband optical nonlinearities of low dimensional materials and their applications in photonics. She has authored more than 50 publications in international journals.
Guichuan Xing is an Associate Professor in the Institute of Applied Physics and Materials Engineering at University of Macau, China. He received his Ph.D. in physics from National University of Singapore, Singapore, in 2011 and then worked as a research assistant, research fellow, and senior research fellow in the Division of Physics & Applied Physics at Nanyang Technological University, Singapore from 2009 to 2016. His research interests lie in ultrafast laser spectroscopy, nano optoelectronics, perovskites for light harvesting and light emission, nonlinear optical properties, and ultrafast carrier dynamics in novel optoelectronic materials and devices.
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Both the authors contributed equally to this work.