Small grains as recombination hot spots in perovskite solar cells

Abstract

•Role of grain boundaries in perovskite solar cells is systematically investigated•Small grains lead to high recombination losses, limiting the open-circuit voltage•The complete distribution of grain sizes must be considered for future optimization Unlike interfacial recombination, much remains unknown about bulk recombination in polycrystalline metal halide perovskite materials. In this work, we systematically investigate how changing the grain size of perovskite films influences their properties and their photovoltaic device performance. We demonstrate that small grains introduce significant recombination losses and propose an intuitive heuristic model that explains this observation by taking into account the geometry of the grain and the dynamics of charge diffusion, which dominates transport near and at open-circuit voltage conditions. Our results allow us to simulate the effects of grain size distribution on the device performance, highlighting the need to improve the microstructure of perovskite thin films by reducing the prevalence of small grains that act as recombination hot spots due to their very high density of defects. Non-radiative recombination in the perovskite bulk and at its interfaces prohibits the photovoltaic performance from reaching the Shockley-Queisser limit. While interfacial recombination has been widely discussed and demonstrated, bulk recombination and especially the influence of grain boundaries remain under debate. Most studies explore the role of grain boundaries on perovskite films rather than devices, making it difficult to link the film properties with those of the devices. Here, we systematically investigate the effects of grain boundaries on the performance of perovskite solar cells by two different methods. By combining experimental characterization with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and hence is inversely proportional to the grain area to the power of 3/2. Consequently, the prevalence of small grains—which act as recombination hot spots—across the perovskite active layer dictates the photovoltaic performance of the perovskite solar cells. Non-radiative recombination in the perovskite bulk and at its interfaces prohibits the photovoltaic performance from reaching the Shockley-Queisser limit. While interfacial recombination has been widely discussed and demonstrated, bulk recombination and especially the influence of grain boundaries remain under debate. Most studies explore the role of grain boundaries on perovskite films rather than devices, making it difficult to link the film properties with those of the devices. Here, we systematically investigate the effects of grain boundaries on the performance of perovskite solar cells by two different methods. By combining experimental characterization with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and hence is inversely proportional to the grain area to the power of 3/2. Consequently, the prevalence of small grains—which act as recombination hot spots—across the perovskite active layer dictates the photovoltaic performance of the perovskite solar cells. Starting from their first exploration in mesostructured solar cells, perovskite semiconductors have shown a steady and continuous increase in their power-conversion efficiency (PCE) from just above 10% to over 25% in less than a decade.1Lee M.M. Teuscher J. Miyasaka T. Murakami T.N. Snaith H.J. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites.Science. 2012; 338: 643-647Crossref PubMed Scopus (8070) Google Scholar,2NREL Best research-cell efficiency chart.https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200104.pdf.Google Scholar By means of perovskite composition optimization, perovskite/transport layer interface engineering, and crystallographic modification, the short-circuit current (JSC) of perovskite solar cells is approaching the Shockley-Queisser limit (∼27 mA cm−2) for a band gap of 1.55 eV. However, the open-circuit voltage (VOC) and fill factor (FF) are far behind their expected limits, restricting the maximum possible PCE.3Saliba M. Matsui T. Seo J.-Y. Domanski K. Correa-Baena J.-P. Nazeeruddin M.K. Zakeeruddin S.M. Tress W. Abate A. Hagfeldt A. et al.Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency.Energy Environ. Sci. 2016; 9: 1989-1997Crossref PubMed Google Scholar, 4Turren-Cruz S.H. Hagfeldt A. Saliba M. Methylammonium-free, high-performance, and stable perovskite solar cells on a planar architecture.Science. 2018; 362: 449-453Crossref PubMed Scopus (566) Google Scholar, 5Jiang Q. Zhao Y. Zhang X. Yang X. Chen Y. Chu Z. Ye Q. Li X. Yin Z. You J. Surface passivation of perovskite film for efficient solar cells.Nat. Photon. 2019; 13: 460-466Crossref Scopus (2295) Google Scholar, 6Kim M. Kim G.-H. Lee T.K. Choi I.W. Choi H.W. Jo Y. Yoon Y.J. Kim J.W. Lee J. Huh D. et al.Methylammonium chloride induces intermediate phase stabilization for efficient perovskite solar cells.Joule. 2019; 3: 2179-2192Abstract Full Text Full Text PDF Scopus (765) Google Scholar, 7Min H. Kim M. Lee S.U. Kim H. Kim G. Choi K. Lee J.H. Seok S. Il Efficient, stable solar cells by using inherent bandgap of a-phase formamidinium lead iodide.Science. 2019; 366: 749-753Crossref PubMed Scopus (572) Google Scholar, 8Guillemoles J.F. Kirchartz T. Cahen D. Rau U. Guide for the perplexed to the Shockley-Queisser model for solar cells.Nat. Photon. 2019; 13: 501-505Crossref Scopus (79) Google Scholar, 9Zheng X. Hou Y. Bao C. Yin J. Yuan F. Huang Z. Song K. Liu J. Troughton J. Gasparini N. et al.Managing grains and interfaces via ligand anchoring enables 22.3%-efficiency inverted perovskite solar cells.Nat. Energy. 2020; 5: 131-140Crossref Scopus (513) Google Scholar It is well established that the VOC and FF are strongly related to the non-radiative recombination in the perovskite bulk and at the interfaces.10Chen J. Park N.G. Causes and solutions of recombination in perovskite solar cells.Adv. Mater. 2019; 31: 1803019Crossref PubMed Scopus (230) Google Scholar,11Tress W. Perovskite solar cells on the way to their radiative efficiency limit - insights into a success story of high open-circuit voltage and low recombination.Adv. Energy Mater. 2017; 7: 1602358Crossref Scopus (269) Google Scholar The main loss process is trap-assisted Shockley-Read-Hall (SRH) recombination, which takes place through defect states that may form in the bulk of the perovskite grains or, more likely, at its interfaces with either the charge extraction layers or other adjacent grains (i.e., grain boundaries).12Wetzelaer G.-J.A.H. Scheepers M. Sempere A.M. Momblona C. Ávila J. Bolink H.J. Trap-assisted non-radiative recombination in organic-inorganic perovskite solar cells.Adv. Mater. 2015; 27: 1837-1841Crossref PubMed Scopus (542) Google Scholar, 13Leijtens T. Eperon G.E. Barker A.J. Grancini G. Zhang W. Ball J.M. Kandada A.R.S. Snaith H.J. Petrozza A. Carrier trapping and recombination: the role of defect physics in enhancing the open circuit voltage of metal halide perovskite solar cells.Energy Environ. Sci. 2016; 9: 3472-3481Crossref Google Scholar, 14Sha W.E.I. Zhang H. Wang Z.S. Zhu H.L. Ren X. Lin F. Jen A.K.-Y. Choy W.C.H. Quantifying efficiency loss of perovskite solar cells by a modified detailed balance model.Adv. Energy Mater. 2018; 8: 1701586Crossref Scopus (63) Google Scholar, 15Vogel M. Doka S. Breyer C. Lux-Steiner M.C. Fostiropoulos K. On the function of a bathocuproine buffer layer in organic photovoltaic cells.Appl. Phys. Lett. 2006; 89: 163501Crossref Scopus (169) Google Scholar Therefore, to further improve device efficiencies and reach the Shockley-Queisser limit, it is important to understand the recombination processes in the bulk and interfaces of polycrystalline perovskite solar cells. Interfacial non-radiative recombination has been explored at the interface of both perovskite/charge transport layers (CTLs) and CTLs/electrode. For example, it was reported that surface defects at perovskite/CTLs assist SRH recombination, thus reducing the photovoltaic performance.5Jiang Q. Zhao Y. Zhang X. Yang X. Chen Y. Chu Z. Ye Q. Li X. Yin Z. You J. Surface passivation of perovskite film for efficient solar cells.Nat. Photon. 2019; 13: 460-466Crossref Scopus (2295) Google Scholar,16Peng J. Wu Y. Ye W. Jacobs D.A. Shen H. Fu X. Wan Y. Duong T. Wu N. Barugkin C. et al.Interface passivation using ultrathin polymer-fullerene films for high-efficiency perovskite solar cells with negligible hysteresis.Energy Environ. Sci. 2017; 10: 1792-1800Crossref Google Scholar, 17An Q. Fassl P. Hofstetter Y.J. Becker-Koch D. Bausch A. Hopkinson P.E. Vaynzof Y. High performance planar perovskite solar cells by ZnO electron transport layer engineering.Nano Energy. 2017; 39: 400-408Crossref Scopus (96) Google Scholar, 18Wang R. Xue J. Wang K.L. Wang Z.K. Luo Y. Fenning D. Xu G. Nuryyeva S. Huang T. Zhao Y. et al.Constructive molecular configurations for surface-defect passivation of perovskite photovoltaics.Science. 2019; 366: 1509-1513Crossref PubMed Scopus (425) Google Scholar, 19Yoo J.J. Wieghold S. Sponseller M.C. Chua M.R. Bertram S.N. Hartono N.T.P. Tresback J.S. Hansen E.C. Correa-Baena J.-P. Bulović V. et al.An interface stabilized perovskite solar cell with high stabilized efficiency and low voltage loss.Energy Environ. Sci. 2019; 12: 2192-2199Crossref Google Scholar Photoluminescence (PL) measurements revealed that the energetic misalignment between the perovskite layer and the CTL lowers the quasi-Fermi level splitting, i.e., enhancing non-radiative recombination.20Wolff C.M. Zu F. Paulke A. Toro L.P. Koch N. Neher D. Reduced interface-mediated recombination for high open-circuit voltages in CH3NH3PbI3 solar cells.Adv. Mater. 2017; 29: 1700159Crossref Scopus (156) Google Scholar, 21Stolterfoht M. Wolff C.M. Márquez J.A. Zhang S. Hages C.J. Rothhardt D. Albrecht S. Burn P.L. Meredith P. Unold T. et al.Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells.Nat. Energy. 2018; 3: 847-854Crossref Scopus (463) Google Scholar, 22Stolterfoht M. Caprioglio P. Wolff C.M. Márquez J.A. Nordmann J. Zhang S. Rothhardt D. Hörmann U. Amir Y. Redinger A. et al.The impact of energy alignment and interfacial recombination on the internal and external open-circuit voltage of perovskite solar cells.Energy Environ. Sci. 2019; 12: 2778-2788Crossref Google Scholar, 23Wolff C.M. Caprioglio P. Stolterfoht M. Neher D. Nonradiative recombination in perovskite solar cells: the role of interfaces.Adv. Mater. 2019; 31: 1902762Crossref Scopus (212) Google Scholar Additionally, by inserting a dipole moment at the CTLs/electrode interface, the non-radiative recombination of minority carriers is suppressed by reinforcing the built-in electric field.24Lee J.-H. Kim J. Kim G. Shin D. Jeong S.Y. Lee J. Hong S. Choi J.W. Lee C.-L. Kim H. et al.Introducing paired electric dipole layers for efficient and reproducible perovskite solar cells.Energy Environ. Sci. 2018; 11: 1742-1751Crossref Google Scholar, 25An Q. Sun Q. Weu A. Becker-Koch D. Paulus F. Arndt S. Stuck F. Hashmi A.S.K. Tessler N. Vaynzof Y. Enhancing the open-circuit voltage of perovskite solar cells by up to 120 mV using π-extended phosphoniumfluorene electrolytes as hole blocking layers.Adv. Energy Mater. 2019; 9: 1901257Crossref Scopus (22) Google Scholar, 26Butscher J.F. Intorp S. Kress J. An Q. Hofstetter Y.J. Hippchen N. Paulus F. Bunz U.H.F. Tessler N. Vaynzof Y. Enhancing the open-circuit voltage of perovskite solar cells by embedding molecular dipoles within their hole-blocking layer.ACS Appl. Mater. Interfaces. 2020; 12: 3572-3579Crossref PubMed Scopus (17) Google Scholar On the other hand, in the bulk of polycrystalline perovskite film, grain boundaries provide a large area at which defects may form due to the discontinuity, but also due to the naturally varying orientation of the crystal lattices.27Seager C.H. Grain boundaries in polycrystalline silicon.Annu. Rev. Mater. Sci. 1985; 15: 271-302Crossref Google Scholar Although these defects could serve as recombination centers, the precise role of grain boundaries in perovskite photovoltaics is still under debate. In this context, one also has to consider the rich microstructure of perovskite films. Not all grains necessarily have the same crystallographic orientation, which may influence the type of grain boundaries that are formed. Indeed, it has been shown that the rate of ionic transport is influenced by the relative orientation of the grains,28Fassl P. Ternes S. Lami V. Zakharko Y. Heimfarth D. Hopkinson P.E. Paulus F. Taylor A.D. Zaumseil J. Vaynzof Y. Effect of crystal grain orientation on the rate of ionic transport in perovskite polycrystalline thin films.ACS Appl. Mater. Interfaces. 2019; 11: 2490-2499Crossref PubMed Scopus (18) Google Scholar suggesting that grain orientation may also influence other properties. Grain boundaries may also exhibit other phenomena, such as complexation,29Cantwell P.R. Tang M. Dillon S.J. Luo J. Rohrer G.S. Harmer M.P. Grain boundary complexions.Acta Mater. 2014; 62: 1-48Crossref Scopus (485) Google Scholar chemical excess of either PbI230Yang J. Xiao A. Xie L. Liao K. Deng X. Li C. Wang A. Xiang Y. Li T. Hao F. Precise control of PbI2 excess into grain boundary for efficacious charge extraction in off-stoichiometric perovskite solar cells.Electrochim. Acta. 2020; 338: 135697Crossref Scopus (14) Google Scholaror organic halides,31Son D.Y. Lee J.W. Choi Y.J. Jang I.H. Lee S. Yoo P.J. Shin H. Ahn N. Choi M. Kim D. et al.Self-formed grain boundary healing layer for highly efficient CH3NH3PbI3 perovskite solar cells.Nat. Energy. 2016; 1: 16081Crossref Scopus (751) Google Scholaror the presence of amorphous phases,32Adhyaksa G.W.P. Brittman S. Āboliņš H. Lof A. Li X. Keelor J.D. Luo Y. Duevski T. Heeren R.M.A. Ellis S.R. et al.Understanding detrimental and beneficial grain boundary effects in halide perovskites.Adv. Mater. 2018; 30: 1804792Crossref PubMed Scopus (76) Google Scholar especially in the case of non-stoichiometric compositions; however, recent work by Rothmann et al. has shown that grain boundaries in high-quality polycrystalline perovskite films are atomically clean.33Rothmann M.U. Kim J.S. Borchert J. Lohmann K.B. O’Leary C.M. Sheader A.A. Clark L. Snaith H.J. Johnston M.B. Nellist P.D. et al.Atomic-scale microstructure of metal halide perovskite.Science. 2020; 370: eabb5940Crossref PubMed Scopus (14) Google Scholar The situation may be further complicated by the fact that not all crystallographic grains are resolved by commonly used microscopic methods such as scanning electron microscopy (SEM) or atomic force microscopy (AFM). The identification of “grains” via these methods is based on the visible grooves that separate them, thus providing contrast. It has been shown that some SEM/AFM-derived grains actually contain several crystallographic grains and hence should be considered as “domains,” and several recent works have demonstrated the existence of so-called domains that encompass several grains. For example, Li et al. have reported on the presence of subgrain boundaries that cannot be imaged via AFM or SEM, but are visible via PL microscopy.34Li W. Yadavalli S.K. Lizarazo-Ferro D. Chen M. Zhou Y. Padture N.P. Zia R. Subgrain special boundaries in halide perovskite thin films restrict carrier diffusion.ACS Energy Lett. 2018; 3: 2669-2670Crossref Scopus (49) Google Scholar Jariwala et al. employed electron backscatter diffraction (EBSD), which is capable of directly visualizing grains in perovskite thin films. By comparing the grains as identified by standard SEM with grains measured by EBSD, the authors showed that in certain cases a domain may consist of several grains.35Jariwala S. Sun H. Adhyaksa G.W. Lof A. Muscarella L.A. Ehrler B. Garnett E.C. Ginger D.S. Local crystal misorientation influences non-radiative recombination in halide perovskites.Joule. 2019; 3: 3048-3060Abstract Full Text Full Text PDF Scopus (91) Google Scholar More recently, Song et al. utilized tomographic AFM to demonstrate the presence of grain boundaries that do not result in the formation of a groove and thus cannot be easily identified in standard AFM measurements.36Song J. Zhou Y. Padture N.P. Huey B.D. Anomalous 3D nanoscale photoconduction in hybrid perovskite semiconductors revealed by tomographic atomic force microscopy.Nat. Commun. 2020; 11: 3308Crossref PubMed Scopus (27) Google Scholar Importantly, the authors concluded that such “hidden grain boundaries” are insipid and represent at most 5% of all grain boundaries, thus suggesting that while “domain” may be a more general description for AFM/SEM data, the term “grain” still describes the results rather well. Some previous studies show that grain boundaries are benign37Yin W.J. Shi T. Yan Y. Unique properties of halide perovskites as possible origins of the superior solar cell performance.Adv. Mater. 2014; 26: 4653-4658Crossref PubMed Scopus (1349) Google Scholar and present no gap states38Yin W.-J. Shi T. Yan Y. Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber.Appl. Phys. Lett. 2014; 104: 63903Crossref Scopus (1783) Google Scholar,39Yun J.S. Ho-Baillie A. Huang S. Woo S.H. Heo Y. Seidel J. Huang F. Cheng Y.B. Green M.A. Benefit of grain boundaries in organic-inorganic halide planar perovskite solar cells.J. Phys. Chem. Lett. 2015; 6: 875-880Crossref PubMed Scopus (351) Google Scholar or that grain boundaries are not recombination active but rather dominate the ion migration in the film.40Correa-Baena J.-P. Anaya M. Lozano G. Tress W. Domanski K. Saliba M. Matsui T. Jacobsson T.J. Calvo M.E. Abate A. et al.Unbroken perovskite: interplay of morphology, electro-optical properties, and ionic movement.Adv. Mater. 2016; 28: 5031-5037Crossref PubMed Scopus (211) Google Scholar, 41Shao Y. Fang Y. Li T. Wang Q. Dong Q. Deng Y. Yuan Y. Wei H. Wang M. Gruverman A. et al.Grain boundary dominated ion migration in polycrystalline organic-inorganic halide perovskite films.Energy Environ. Sci. 2016; 9: 1752-1759Crossref Google Scholar, 42Yang M. Zeng Y. Li Z. Kim D.H. Jiang C.-S. Lagemaat J.van de Zhu K. Do grain boundaries dominate non-radiative recombination in CH3NH3PbI3 perovskite thin films?.Phys. Chem. Chem. Phys. 2017; 19: 5043-5050Crossref PubMed Google Scholar Other reports, however, suggest that grain boundaries are detrimental and act as recombination centers following the SRH recombination mechanism.43Nie W. Tsai H. Asadpour R. Blancon J.C. Neukirch A.J. Gupta G. Crochet J.J. Chhowalla M. Tretiak S. Alam M.A. et al.High-efficiency solution-processed perovskite solar cells with millimeter-scale grains.Science. 2015; 347: 522-525Crossref PubMed Scopus (2605) Google Scholar, 44Ren X. Yang Z. Yang D. Zhang X. Cui D. Liu Y. Wei Q. Fan H. Liu S. Frank ) Modulating crystal grain size and optoelectronic properties of perovskite films for solar cells by reaction temperature.Nanoscale. 2016; 8: 3816-3822Crossref PubMed Google Scholar, 45Xu W. Lei G. Tao C. Zhang J. Liu X. Xu X. Lai W.-Y. Gao F. Huang W. Precisely controlling the grain sizes with an ammonium hypophosphite additive for high-performance perovskite solar cells.Adv. Funct. Mater. 2018; 28: 1802320Crossref Scopus (49) Google Scholar Garnett and co-workers reported that grain boundaries in perovskite films might lead to both detrimental and advantageous effects.32Adhyaksa G.W.P. Brittman S. Āboliņš H. Lof A. Li X. Keelor J.D. Luo Y. Duevski T. Heeren R.M.A. Ellis S.R. et al.Understanding detrimental and beneficial grain boundary effects in halide perovskites.Adv. Mater. 2018; 30: 1804792Crossref PubMed Scopus (76) Google Scholar Several reports linked increased grain size with improved photovoltaic performance, although a careful examination reveals conflicting origins for this improvement. For example, Kim et al. showed that increasing the grain size from ∼150 nm to ∼5 μm leads to no change in the VOC of the device but a significant improvement in JSC.46Kim M.K. Jeon T. Park H. Il Lee J.M. Nam S.A. Kim S.O. Effective control of crystal grain size in CH3NH3PbI3 perovskite solar cells with a pseudohalide Pb(SCN)2 additive.CrystEngComm. 2016; 18: 6090-6095Crossref Google Scholar Patil et al. used thiourea as an additive in double cation perovskites, resulting in a doubling of the average grain size from 1 μm to 2 μm.47Patil J.V. Mali S.S. Hong C.K. A thiourea additive-based quadruple cation lead halide perovskite with an ultra-large grain size for efficient perovskite solar cells.Nanoscale. 2019; 11: 21824-21833Crossref PubMed Google Scholar This led to a reduction in the VOC and an increase in the JSC. Kim and co-workers, on the other hand, showed that increased grain size leads to a significant increase in the VOC,48Kim H. Do Ohkita H. Benten H. Ito S. Photovoltaic performance of perovskite solar cells with different grain sizes.Adv. Mater. 2016; 28: 917-922Crossref PubMed Scopus (248) Google Scholar with similar results reported more recently.6Kim M. Kim G.-H. Lee T.K. Choi I.W. Choi H.W. Jo Y. Yoon Y.J. Kim J.W. Lee J. Huh D. et al.Methylammonium chloride induces intermediate phase stabilization for efficient perovskite solar cells.Joule. 2019; 3: 2179-2192Abstract Full Text Full Text PDF Scopus (765) Google Scholar Several reports demonstrated a more complex relationship between the grain size and the VOC. For example, Xu et al. reported that increasing the grain size leads to an increase in VOC up to a point, followed by a decrease for the samples with the largest grains.45Xu W. Lei G. Tao C. Zhang J. Liu X. Xu X. Lai W.-Y. Gao F. Huang W. Precisely controlling the grain sizes with an ammonium hypophosphite additive for high-performance perovskite solar cells.Adv. Funct. Mater. 2018; 28: 1802320Crossref Scopus (49) Google Scholar Similar results were also reported by Im et al., whose samples with medium-size grains resulted in the largest VOC while small and large grain sizes led to lower voltages.49Im J.H. Jang I.H. Pellet N. Grätzel M. Park N.G. Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells.Nat. Nanotechnol. 2014; 9: 927-932Crossref PubMed Scopus (1458) Google Scholar In another work, the same group reported that devices with the smallest grain size led to the highest VOC.50Kim H.S. Park N.G. Parameters affecting I-V hysteresis of CH3NH3PbI3 perovskite solar cells: effects of perovskite crystal size and mesoporous TiO2 layer.J. Phys. Chem. Lett. 2014; 5: 2927-2934Crossref PubMed Scopus (902) Google Scholar Several studies reported that no relationship exists between the grain size and VOC. Correa-Baena et al. reported that so long as the device is of a certain thickness, increasing the grain size does not lead to a change in the VOC.40Correa-Baena J.-P. Anaya M. Lozano G. Tress W. Domanski K. Saliba M. Matsui T. Jacobsson T.J. Calvo M.E. Abate A. et al.Unbroken perovskite: interplay of morphology, electro-optical properties, and ionic movement.Adv. Mater. 2016; 28: 5031-5037Crossref PubMed Scopus (211) Google Scholar,51Correa-Baena J.P. Tress W. Domanski K. Anaraki E.H. Turren-Cruz S.H. Roose B. Boix P.P. Grätzel M. Saliba M. Abate A. et al.Identifying and suppressing interfacial recombination to achieve high open-circuit voltage in perovskite solar cells.Energy Environ. Sci. 2017; 10: 1207-1212Crossref Google Scholar Despite the conflicting literature results, the authors concluded that grain boundaries do not dramatically influence the recombination and VOC in perovskite solar cells.52Castro-Méndez A. Hidalgo J. Correa-Baena J. The role of grain boundaries in perovskite solar cells.Adv. Energy Mater. 2019; 9: 1901489Crossref Scopus (103) Google Scholar A range of microscopic techniques has been used to visualize the grains and grain boundaries and identify their impact. Cathodoluminescence (CL) microscopy probes the radiative decay of charges generated by exposure to an electron beam and has been utilized in many studies on perovskite materials.53Guthrey H. Moseley J. A review and perspective on cathodoluminescence analysis of halide perovskites.Adv. Energy Mater. 2020; 10: 1903840Crossref Scopus (12) Google Scholar While the effect of tuning the grain size has not been systematically investigated using CL microscopy, conflicting results have been observed regarding the luminescence intensity of grains and grain boundaries. For example, Bischak et al. observed a reduced CL intensity at grain boundaries and a large variation of grain-to-grain luminescence, with many large grains—surprisingly—appearing dark.54Bischak C.G. Sanehira E.M. Precht J.T. Luther J.M. Ginsberg N.S. Heterogeneous charge carrier dynamics in organic-inorganic hybrid materials: nanoscale lateral and depth-dependent variation of recombination rates in methylammonium lead halide perovskite thin films.Nano Lett. 2015; 15: 4799-4807Crossref PubMed Scopus (120) Google Scholar Bi et al. observed that the CL intensity is strongest at the grain boundaries with no clear relationship between grain size and emission intensity.55Bi D. Li X. Milić J.V. Kubicki D.J. Pellet N. Luo J. LaGrange T. Mettraux P. Emsley L. Zakeeruddin S.M. et al.Multifunctional molecular modulators for perovskite solar cells with over 20% efficiency and high operational stability.Nat. Commun. 2018; 9: 4482Crossref PubMed Scopus (192) Google Scholar PL microscopy has also often been employed to investigate the radiative emission properties of perovskite films.56Goetz K.P. Taylor A.D. Paulus F. Vaynzof Y. Shining light on the photoluminescence properties of metal halide perovskites.Adv. Funct. Mater. 2020; 30: 1910004Crossref Scopus (53) Google Scholar Early work by de Quilettes et al. reported that PL intensity at the grain boundaries is 65% lower than in the interior of the grains accompanied by a significant heterogeneity of emission intensity between different grains.57de Quilettes D.W. Vorpahl S.M. Stranks S.D. Nagaoka H. Eperon G.E. Ziffer M.E. Snaith H.J. Ginger D.S. Impact of microstructure on local carrier lifetime in perovskite solar cells.Science. 2015; 348: 683-686Crossref PubMed Scopus (1516) Google Scholar This heterogeneity does not seem to correlate with grain size and is dependent on the exact processing conditions of the films.58Fassl P. Zakharko Y. Falk L.M. Goetz K.P. Paulus F. Taylor A.D. Zaumseil J. Vaynzof Y. Effect of density of surface defects on photoluminescence properties in MAPbI3 perovskite films.J. Mater. Chem. C. 2019; 7: 5285-5292Crossref Google Scholar For example, while some studies reported strong differences in PL for grains of different sizes,59D’Innocenzo V. Srimath Kandada A.R. De Bastiani M. Gandini M. Petrozza A. Tuning the light emission properties by band gap engineering in hybrid lead halide perovskite.J. Am. Chem. Soc. 2014; 136: 17730-17733Crossref PubMed Scopus (472) Google Scholar,60Srimath Kandada A.R. Petrozza A. Photophysics of hybrid lead halide perovskites: the role of microstructure.Acc. Chem. Res. 2016; 49: 536-544Crossref PubMed Scopus (95) Google Scholar other, more recent studies found no such correlation.61Wen Y. Tang Y.G. Yan G.Q. Large grain size CH3NH3PbI3 film for perovskite solar cells with hydroic acid additive.AIP Adv. 2018; 8: 095226Crossref Scopus (18) Google Scholar,62Muscarella L.A. Hutter E.M. Sanchez S. Dieleman C.D. Savenije T.J. Hagfeldt A. Saliba M. Ehrler B. Crystal orientation and grain size: do they determine optoelectronic properties of MAPbI3 perovskite?.J. Phys. Chem. Lett. 2019; 10: 6010-6018Crossref PubMed Scopus (49) Google Scholar Jariwala et al. attributed differences in the PL intensity of certain grains to the degree of misorientation of crystallographic grains within them.35Jariwala S. Sun H. Adhyaksa G.W. Lof A. Muscarella L.A. Ehrler B. Garnett E.C. Ginger D.S. Local crystal misorientation influences non-radiative recombination in halide perovskites.Joule. 2019; 3: 3048-3060Abstract Full Text Full Text PDF Scopus (91) Google Scholar It is noteworthy that PL microscopy has a limited resolution, making it difficult to image and resolve small grains. Moreover, using three-dimensional PL tomography, Stranks and co-workers recently revealed that grains that appear bright at the surface might be dark in the bulk of the film and vice versa, placing previous conclusions obtained via surface PL microscopy under question.62Muscarella L.A. Hutter E.M. Sanchez S. Dieleman C.D. Savenije T.J. Hagfeldt A. Saliba M. Ehrler B. Crystal orientation and grain size: do they determine optoelectronic properties of MAPbI3 perovskite?.J. Phys. Chem. Lett. 2019; 10: 6010-6018Crossref PubMed Scopus (49) Google Scholar Kelvin probe microscopy has also been exploited for the characterization of perovskite films, in particular for the study of ion migration and environmental degradation, albeit without conclusive results about the role of grain boundaries as recombination centers.63Kang Z. Si H. Shi M. Xu C. Fan W. Ma S. Kausar A. Liao Q. Zhang Z. Zhang Y. Kelvin probe force microscopy for perovskite solar cells.Sci. China Mater. 2019; 62: 776-789Crossref Scopus (50) Google Scholar One of the reasons for such inconclusive experimental observations is that some of the results are recorded on perovskite morphologies with uncontrollable amorphous grain boundaries in both vertical and horizontal directions, making it difficult to pinpoint and separate their effects. Another possible reason is that some of the research objects are chosen to be different sizes of grains within the same film, which can originate from local inhomogeneities in the films and thus represent a special case. Similarly, previous studies that employe