Elsevier

Nano Energy

Volume 82, April 2021, 105712
Nano Energy

Review
Polymer strategies for high-efficiency and stable perovskite solar cells

https://doi.org/10.1016/j.nanoen.2020.105712Get rights and content

Highlights

  • The review is focused on polymer strategies for improving PSCs performance and stability.

  • Polymer strategies including polymer additives, polymer CTL, polymer interface, and polymer encapsulation are summarized.

  • The functions of the incorporated polymers in improving performance and related influencing mechanisms are elucidated.

  • Main challenges and the future prospects are outlooked.

Abstract

Polymer strategy has been widely adopted for efficient, stable, and hysteresis-reduced perovskite solar cells (PSCs). Herein, a comprehensive review of polymer strategy is provided, by categorizing the polymers as additives in the perovskite active layer and charge transport layer, as an interfacial layer, and as an encapsulation layer. In addition, the various polymers adopted in polymer strategy and the diverse ways to introduce them into PSCs are summarized. Moreover, the functions of polymers in each layer, such as morphology modulation, energy level alignment adjustment, non-radiative recombination suppression, stability and flexibility enhancement, are emphasized, and the related underlying mechanisms are discussed. Furthermore, future research directions to optimize polymer strategy for further improving the efficiency and stability of PSCs is proposed.

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

References (227)

  • J. Yuan et al.

    Band-aligned polymeric hole transport materials for extremely low energy loss α-CsPbI3 perovskite nanocrystal solar cells

    Joule

    (2018)
  • W. Chen et al.

    N-type conjugated polymer as efficient electron transport layer for planar inverted perovskite solar cells with power conversion efficiency of 20.86%

    Nano Energy

    (2020)
  • D. Li et al.

    Amino-functionalized conjugated polymer electron transport layers enhance the UV-photostability of planar heterojunction perovskite solar cells

    Chem. Sci.

    (2017)
  • G.E. Eperon et al.

    Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells

    Energy Environ. Sci.

    (2014)
  • L. Protesescu et al.

    Nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut

    Nano Lett.

    (2015)
  • S. De Wolf et al.

    Organometallic halide perovskites: sharp optical absorption edge and its relation to photovoltaic performance

    J. Phys. Chem. Lett.

    (2014)
  • Q. Dong et al.

    Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals

    Science

    (2015)
  • S.D. Stranks et al.

    Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber

    Science

    (2013)
  • G. Xing et al.

    Long-range balanced electron- and hole-transport lengths in organic-inorganic CH3NH3PbI3

    Science

    (2013)
  • C. Wehrenfennig et al.

    High charge carrier mobilities and lifetimes in organolead trihalide perovskites

    Adv. Mater.

    (2014)
  • J. Kang et al.

    High defect tolerance in lead halide perovskite CsPbBr3

    J. Phys. Chem. Lett.

    (2017)
  • K.X. Steirer et al.

    Defect tolerance in methylammonium lead triiodide perovskite

    ACS Energy Lett.

    (2016)
  • A. Miyata et al.

    Direct measurement of the exciton binding energy and effective masses for charge carriers in organic-inorganic tri-halide perovskites

    Nat. Phys.

    (2015)
  • J.W. Lee et al.

    Two-step deposition method for high-efficiency perovskite solar cells

    MRS Bull.

    (2015)
  • H.S. Ko et al.

    15.76% efficiency perovskite solar cells prepared under high relative humidity: importance of PbI2 morphology in two-step deposition of CH3NH3PbI3

    J. Mater. Chem. A

    (2015)
  • J.J. Shi et al.

    Modified two-step deposition method for high-efficiency TiO2/CH3NH3PbI3 heterojunction solar cells

    ACS Appl. Mater. Interfaces

    (2014)
  • C. Dong et al.

    A green anti-solvent process for high performance carbon-based CsPbI2Br all-inorganic perovskite solar cell

    Sol. RRL

    (2018)
  • M. Jung et al.

    Perovskite precursor solution chemistry: from fundamentals to photovoltaic applications

    Chem. Soc. Rev.

    (2019)
  • R.Z. Liu et al.

    Solvent engineering for perovskite solar cells: a review

    Micro Nano Lett.

    (2020)
  • K.T. Cho et al.

    Highly efficient perovskite solar cells with a compositionally engineered perovskite/hole transporting material interface

    Energy Environ. Sci.

    (2017)
  • J. Seo et al.

    Rational strategies for efficient perovskite solar cells

    Acc. Chem. Res.

    (2016)
  • A. Kojima et al.

    Organometal halide perovskites as visible-light sensitizers for photovoltaic cells

    J. Am. Chem. Soc.

    (2009)
  • ...
  • T. Baikie et al.

    Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3) PbI3 for solid-state sensitised solar cell applications

    J. Mater. Chem. A

    (2013)
  • Y.G. Rong et al.

    Beyond efficiency: the challenge of stability in mesoscopic perovskite solar cells

    Adv. Energy Mater.

    (2015)
  • X.X. Liu et al.

    Highly stable new organic-inorganic hybrid 3D perovskite CH3NH3PdI3 and 2D perovskite (CH3NH3)3Pd2I7: DFT analysis, synthesis, structure, transition behavior, and physical properties

    J. Phys. Chem. Lett.

    (2018)
  • D. Bryant et al.

    Light and oxygen induced degradation limits the operational stability of methylammonium lead triiodide perovskite solar cells

    Energy Environ. Sci.

    (2016)
  • D. Li et al.

    Recent progress on stability issues of organic-inorganic hybrid lead perovskite-based solar cells

    RSC Adv.

    (2016)
  • X. Zhao et al.

    Stability issues on perovskite solar cells

    Photonics

    (2015)
  • J.A. Christians et al.

    Transformation of the excited state and photovoltaic efficiency of CH3NH3PbI3 perovskite upon controlled exposure to humidified air

    J. Am. Chem. Soc.

    (2015)
  • C.C. Boyd et al.

    Understanding degradation mechanisms and improving stability of perovskite photovoltaics

    Chem. Rev.

    (2019)
  • H. Huang et al.

    Solar-driven metal halide perovskite photocatalysis: design, stability, and performance

    ACS Energy Lett.

    (2020)
  • S. Zhang, Z. Liu, W. Zhang, Z. Jiang, W. Chen, R. Chen, Y. Huang, Z. Yang, Y. Zhang, L. Han, W. Chen, Barrier Designs...
  • W. Zhu et al.

    Recycling of FTO/TiO2 substrates: route toward simultaneously high-performance and cost-efficient carbon-based, all-inorganic CsPbIBr2 solar cells

    ACS Appl. Mater. Interfaces

    (2020)
  • S. Wieghold et al.

    Understanding the effect of light and temperature on the optical properties and stability of mixed-ion halide perovskites

    J. Mater. Chem. C

    (2020)
  • F. Zhang et al.

    Polymeric, cost-effective, dopant-free hole transport materials for efficient and stable perovskite solar cells

    J. Am. Chem. Soc.

    (2019)
  • X. Kong et al.

    Dopant-free F-substituted benzodithiophene copolymer hole-transporting materials for efficient and stable perovskite solar cells

    J. Mater. Chem. A

    (2020)
  • H.D. Pham et al.

    Development of dopant-free organic hole transporting materials for perovskite solar cells

    Adv. Energy Mater.

    (2020)
  • G.D. Niu et al.

    Progress of interface engineering in perovskite solar cells

    Sci. China Mater.

    (2016)
  • J. Ali et al.

    Interfacial and structural modifications in perovskite solar cells

    Nanoscale

    (2020)
  • Cited by (63)

    View all citing articles on Scopus

    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.

    1

    Both the authors contributed equally to this work.

    View full text