Physicochemical Phenomena and Application in Solar Cells of

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Physico-Chemical Phenomena and Application in Solar Cells of Perovskite:Fullerene Films Jorge Pascual, Juan Luis Delgado, and Ramon Tena-Zaera J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00968 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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The Journal of Physical Chemistry Letters

Physico-Chemical Phenomena and Application in Solar Cells of Perovskite:Fullerene Films Jorge Pascual,[a,b] Juan Luis Delgado,*[a,c] Ramón Tena-Zaera*[b] [a] J. Pascual, Prof. J. L. Delgado, POLYMAT, University of the Basque Country UPV/EHU, Avenida de Tolosa 72, Donostia-San Sebastián 20018 (Spain), E-mail: [email protected] [b] J. Pascual, Dr. R. Tena-Zaera, CIDETEC, Parque Tecnológico de San Sebastián, Paseo Miramón 196, Donostia-San Sebastián 20014 (Spain), E-mail: [email protected] [c] Prof. J. L. Delgado, Ikerbasque, Basque Foundation for Science, Bilbao 48013 (Spain) AUTHOR INFORMATION Corresponding Author * Ramón Tena-Zaera, e-mail address: [email protected] ORCID: 0000-0002-1525-7760 * Juan Luis Delgado, e-mail address: [email protected] ORCID: 0000-0002-6948-8062 ABSTRACT

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Beyond the use of fullerenes as electron-transporting layers in perovskite solar cells, their introduction in the perovskite active layer has been ascertained as a very promising strategy for device improvement. In this sense, this perspective summarizes the studies in which perovskite:fullerene systems were employed, covering the different methodologies for introducing fullerenes inside the perovskite layer. In particular, fill factor was the most benefited parameter, what was ascribed to minimized pinhole density and fullerene passivating role. We discussed the importance of their ability to passivate trap states and, in this regard, focused on the affinity of fullerenes towards these sites. Additionally, the different nature of the fullerene and its environment in the active layer were found as determining for the proper distribution of fullerene so that they could passivate the defects along grain boundaries. Understanding this mechanism would provide further insights for new methodologies and fullerene derivatives with enhanced trap-passivating ability.

TOC GRAPHICS

KEYWORDS

Perovskite:fullerene

films,

Solar

cells,

ETL-free,

Perovskite-fullerene

interactions, Fullerene derivatives


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Jorge Pascual obtained his Degree in Chemistry and carried out his Master thesis at the University of the Basque Country in Leioa in 2015. He is currently working as a PhD candidate at the Basque Center for Macromolecular Design and Engineering (POLYMAT) and CIDETEC in Donostia-San Sebastian under the supervision of Juan Luis Delgado and Ramón Tena-Zaera. His current research focuses on the introduction of fullerenes and new organic molecules in perovskite solar cells.

Juan Luis Delgado obtained his PhD in Chemistry at UCLM (Spain) in 2004 under the supervision of Prof. F. Langa and Prof. P. de la Cruz. Afterwards he joined the group of Prof. J.-F. Nierengarten, at the ECPMCNRS (Strasbourg, France) where he worked as a postdoctoral fellow. He then joined the group of Prof. N. Martín at University Complutense of Madrid as a “Juan de la Cierva” postdoctoral fellow. From 2008-2014 he was a Senior “Ramón & Cajal” Researcher at IMDEA-Nanoscience (Madrid, Spain). In 2013 he was awarded with the” Young researcher award” of RSEQ-Lilly Spain and the “Emerging Investigator award” of RSEQ-Sigma Aldrich Spain. In 2014 he was awarded with an “Ikerbasque Research Fellowship” and became the leader of the Hybrid Materials for Photovoltaics group at POLYMAT (DonostiaSan Sebastián, Spain). His research interest is mainly focused on the synthesis of organic materials suitable for their utilization in Organic Photovoltaics, Dye Sensitized Solar Cells, Light Harvesting, Artificial Photosynthetic Systems and Perovskite-Based Solar Devices.


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Ramon Tena-Zaera received his Ph. D. degree in Applied Physics at University of Valencia in 2004. After 4 years at the Institute de Chimie et des Matériaux Paris Est (ICMPE-CNRS), he was awarded as Ramon y Cajal Felow and then joined CIDETEC. He is now leading the Nanosurfaces Unit of CIDETEC. His research interest is mainly focused on the wet-chemistry synthesis and thin film deposition approaches to develop functional (nano) materials and coatings for application in different fields such as energy and

emerging photovoltaic

technologies in particular. He is co-author of above 95 scientific papers (h-index: 33) in international peer-reviewed journals and co-inventor of 7 patents.

Perovskite solar cells (PSC) have recently revolutionized the field of photovoltaic (PV) technology1-3 by eclipsing in just a few years other emerging technologies such organic (OSC)4-6 and Dye-sensitized Solar Cells (DSC)7-9 in terms of power conversion efficiency (PCE). This is the result of the perovskite outstanding optoelectronic properties,10-13 as well as the high versatility in its processing. Most of the research work has been carried on the optimization of composition and deposition engineering of perovskite layer,14,15 but also on the development of optimized charge transporting materials.16-18 In this sense, carbon allotropes and, more in particular, fullerenes have been widely employed in PSCs as electron transporting layers (ETLs).19-30 Apart from this classical application, which have been recently reviewed,31-36 fullerenes show especially appealing properties for being


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beneficial in perovskite:fullerene films. In fact, the passivation of surface trap states of perovskite, which was claimed to happen at the perovskite/ETL interface when fullerene and derivatives were used as ETL,24-29 could also occur on the perovskite grain boundaries when fullerene is dispersed in the perovskite film having stronger positive effect on the solar cell performance. Moreover, when fullerenes are integrated in the perovskite-based photoactive films, their electron acceptor ability may open wide possibilities for advanced strategies to act on the electron distribution and charge balance in the solar cell. Furthermore, the versatile chemistry of fullerenes,37,38 offers many possibilities to prepare modified fullerenes “a la carte”. Thus, a suitable fullerene functionalization would allow among others: enhancing solubility in the solvent used for the perovskite film processing, tailoring the electronic density and energy of electronic levels as well as interactions between the fullerene and the perovskite. Fullerenes, in fact, represent a very special family of compounds, whose use in PSCs cannot be compared to the one of other carbon nanostructures. In this sense, they have unique electronic properties and reactivity. If compared to graphene and derivatives, fullerenes have a different hybridization (some pyramidal character sp2-sp3 vs sp2 from graphene). Furthermore, fullerene derivatives may be specifically functionalized to make them dispersible or soluble in DMF or others. This perspective presents a critical overview of the major findings about perovskite:fullerene films and their application in solar cells. Their main benefits in cell production and performance will be specified, providing an extensive discussion on their physicochemical origin by focusing on the particularities of perovskite:fullerene interactions. An outlook, including actions to address the existing challenges and make able to fully exploit the many possibilities that perovskite:fullerene blends offer, is also provided.


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One of the main advantages of the perovskite:fullerene system is the wide range of processing ways in which it can be done. This, in turn, allowed integrating fullerenes in several solar cell architectures, including simplified ones such as ETL-free.39 The first reported inclusion of fullerene in the perovskite layer can be found in the work by Shao and co-workers.40 Inspired by the works from the groups of Jeng and Malinkiewicz,41,42 they deposited PCBM (Figure 1c) on top of the perovskite layer as ETL in p-i-n (sometimes called “inverted”)25,43,44 architecture (Figure 1a). The authors suggested that the applied thermal annealing allowed fullerene to permeate along perovskite grain boundaries. Therefore, depositing a PCBM layer over the perovskite and promoting interpenetration of fullerene along active layer grains signified the first type of integration of fullerene in the perovskite film. The other main way for entering fullerene into the perovskite layer was based on a much more direct, controllable approaches, by doping the perovskite layer in a certain step of its processing, forming a perovskite:fullerene film. PCBM was introduced in p-i-n devices by Wu et al.45 by dissolving this fullerene in the toluene for the dripping step, leading to the interpenetration of the fullerene in the perovskite layer (Figure 1a). During the dripping process, a thin film of PCBM is also formed, thick enough to separate the active layer from the back metal electrode. By an alternative processing way based on adding PCBM to the perovskite precursors solution (i.e. formulating an “all-in-one” processing solution), Xu and co-workers46 deposited a perovskite:fullerene film on top of the electron-transporting TiO2 layer and prepared a n-i-p (sometimes called “regular”27,47,48 architecture, as shown in Figure 1b. Moreover, dissolving a fullerene derivative in the perovskite solution (i.e. using “all-in-one” processing solutions)39,46,4954

allowed introducing easily a modified fullerene into the blend films without the use of toxic

processing solvents like chlorobenzene or dichlorobenzene, generally used for the processing of


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fullerene as ETL. Due to the uncertainty of knowing about the actual distribution of fullerene within the perovskite matrix, the authors used a thiophene-containing derivative, [60]ThCBM ([6,6]-(2-Thienyl)-C61-butyric acid methyl ester) to use sulfur as the tracer element and effectively detect by secondary ion mass spectrometry (SIMS) the uniform concentration of fullerene throughout the thickness of the perovskite:fullerene film. In fact, large aggregates of PCBM were clearly detected at grain boundaries in SEM micrographs of films processed from solutions with too high concentrations of this fullerene. Therefore, PCBM was suggested to be preferentially located at the perovskite grain boundaries. Furthermore, Pascual et al.39 demonstrated clearly the advantages of using perovskite:C70 (Figure 1c) films, also processing from “all-in-one” blended solutions, for achieving efficient ETL-free perovskite solar cells. This finding opens wide possibilities to reduce the number of layers and processing steps (i.e. solvent and/or energy saving) in the perovskite photovoltaics manufacturing. Apart from above main two processing ways, it is worth to mention that without an initial intention of introducing fullerenes into the perovskite film, Collavini and co-workers27 demonstrated the partial dissolution of the fullerene ETL in n-i-p devices when the perovskite film was processed by spinning DMF-based solutions. This finding, in agreement with previous limitations found out by Wojciechowski et al.,55 may open some room for perovskite:fullerene films in n-i-p solar cells in which initially were not considered. Therefore, three different ways to obtain perovskite:fullerene blend films have been reported. However, the distribution of the fullerene may depend significantly on the processing way. Briefly, fullerene is expected to be only at the upper perovskite grain boundaries when added as an upper film in the solar cell fabrication. However, distribution in whole blend film is possible when added by using “all-in-one” processing solution (Figure 1b) although further knowledge on


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the preferential sites for fullerenes and tailoring ways seem to be needed. Similar scenario may occur when the integration of the fullerene on the perovskite films occurs due to the partial dissolution of the fullerene ETL during the deposition of perovskite in n-i-p solar cells, although larger fullerene concentration at the bottom of the perovskite film might be expected.


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Figure 1. (a) Fullerene-doped PSC with p-i-n architecture, where “fullerene” means fullerene and derivatives. (Adapted with permission from Ref. 40. Copyright 2014 Macmillan Publishers Limited). (b) Fullerene-doped PSC with n-i-p architecture, where “fullerene” means fullerene and derivatives. (c) Structure of fullerenes and derivatives introduced in perovskite films through interpenetration from ETL (top-left square) and by “all-in-one” methodology (bottom-right square). PCBM was used in both strategies. Independently of the PSC architecture, the use of perovskite:fullerene layers has been clearly demonstrated to result in a direct and positive impact in the power conversion efficiency. In particular, notable improvements were seen in JSC and VOC values for the use of fullerene derivatives49,50 and PCBM (Figure 2a),45,46 which for JSC were ascribed to the improved IPCE as shown in the inset, suggesting better electron extraction from the perovskite. The elimination of the photocurrent hysteresis was another of the benefits of using these systems. Previous studies suggest that this phenomenon is originating from charge traps, ferroelectric properties of perovskites and ion electromigration in perovskite layers.56,57 A complete elimination could be found through the use of perovskite:PCBM blends as shown in Figure 2b.46 It is worth to note, that the elimination of the hysteresis phenomenon was also detected when blend films including fullerene derivatives A10C60,49 and PCBSD50 (Figure 1c), were used. A reduction of hysteresis was also found for solar cells based on perovskite:fluoroalkyl-substituted fullerene DF-C60 (Figure 1c) films, and increased the stability of p-i-n devices as well.51 Additionally, a big improvement in the photostability was detected in ETL-free devices based on perovskite:C70 films when illuminated under continuous illumination at 1 sun (Figure 2c). The same result can be found for solar cells not only based on perovskite:PCBM films,45 but


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also when they were obtained by permeation of the PCBM along the perovskite grain boundaries by thermal annealing.40 An increase in stability was also seen for a blend with pure isomer α-bisPCBM (Figure 1c), achieving an excellent PCE value of 20.8%.52 Although the beneficial effect of fullerene is clear, it is worth noting that the record value of 20.8% is also due to the high PCE of the control device (18.8%). Nevertheless, as it can be seen in Table 1, the main contributor to power conversion efficiency enhancement in solar cells based on perovskite:fullerene films was the increase in FF value. The highest increase for this parameter was achieved by Wang and co-workers49 with a 10carboxylic-acid-substituted fullerene A10C60 in a perovskite blend BHJ, obtaining a FF of 86.7%, the highest value obtained so far for PSCs. Another derivative, the cross-linking fullerene PCBSD, also led to an improvement in FF in a perovskite blend by Li and co-workers.50 This beneficial effect can also be seen for perovskite:PCBM blend-containing n-i-p devices, increasing FF from 66% to 74% as shown in Figure 2b.46 Perovskite:C70 blends, in particular, allowed achieving acceptable FF values (i.e. 74%) in n-i-p ETL-free devices that generally suffer very significant FF limitations.58,59 Moreover, Pascual et al.60 showed recently that ETL-free solar cells based on perovskite:C70 films processed using o-xylene as cosolvent (i.e. 1:4 ratio to DMF) exhibited enhanced FF value up to 77%, which is the highest one reported for ETL-free perovskite solar cells.


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Figure 2. (a) J-V curves and IPCE spectra in inset of fullerene-free (black) and interpenetrated PCBM-containing (red) PSCs. (Adapted with permission from Ref. 45. Copyright 2016 Macmillan Publishers Limited). (b) Direct (black) and direct (colored) J-V curves of fullerenefree (blue) and PCBM-containing in “all-in-one” methodology (red) PSCs. (Adapted with permission from Ref. 46. Copyright 2015 Macmillan Publishers Limited). (c) Photostability of fullerene-free PSCs with TiO2 as ETL (black) and without ETL (blue), and ETL-free C70containing in “all-in-one” methodology (red) PSCs. (Adapted with permission from Ref. 39. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim). Table 1. PV parameters improvement for PSCs based on perovskite:fullerene blends.


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Fullerene

JSC (mA cm-2)

VOC (V)

FF (%)

PCE (%)

Ref

14.2

0.94

61

8.1

C6027,a

15.1

1.00

69

10.4

Ref

14.3

0.96

61

8.4

C7027,a

15.4

0.99

75

11.4

Ref

14.9

1.02

70

10.6

C7039

17.4

1.06

74

13.6

Ref

17.9

0.93

68

11.3

PCBM45

22.0

1.08

79

18.8

Ref

14.4

0.98

65

8.1

PCBM46,b

18.0

1.09

75

14.4

Ref

17.0

1.04

70

10.5

PCBM54,c

14.5

1.06

72

11.2

IS-154,c

16.7

1.03

69

11.8

IS-254,c

17.3

1.08

78

14.3

PI-254,c

16.5

1.02

69

11.7

DPM-654,c

16.1

1.04

69

11.6

Ref

17.3

0.86

77

11.5

A10C6049

19.4

0.88

82

14.0

Ref

18.7

0.95

71

12.6

C-PCBSD50

22.8

0.98

77

17.2

Ref

23.3

1.09

71

18.8

PCBM52

23.7

1.11

73

19.9

α-bis-PCBM52

24.0

1.13

74

20.8

Ref

19.4

1.07

75

15.7

DF-C6051

21.1

1.09

79

18.1

Ref

21.2

1.10

79

18.4


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F153

20.7

1.09

73

16.4

F253

17.8

1.06

76

15.1

F353

18.5

1.07

78

16.4

Refd

21.5

0.95

75

15.2

PCBEH61,b

22.1

0.95

78

16.3

a

The ETL was made out of the same fullerene. b Fullerene interpenetrated in perovskite layer from the ETL in p-i-n architecture. c ETL-free, n-i-p architecture device. d Reference device made with PCBM. Overall, the main effect of perovskite:fullerene films is the increase of device FF. In this sense, an answer over the role of the fullerene in the perovskite and its effect on this parameter is required, so as to understand these systems and exploit better perovskite:fullerene mixtures. The use of “all-in-one” processing solutions may have a direct impact on the morphology of the deposited layer. As a clear example, Pascual et al.39 demonstrated that using C70-containing “all-in-one” processing solution was a straightforward approach to improve the pinhole issue in perovskite films directly processed on FTO surfaces, which was previously identified as a highly challenging scenario.58,59 It is also worth noting that Li and co-workers50 found that the addition of cross-linking fullerene derivative PCBSD to the processing solution assisted in the consecution of perovskite:fullerene films with a decreased density of pinholes on PEDOT:PSS surfaces too as it can be seen in FESEM micrographs (Figure 3a). The electrochemical impedance spectroscopy (EIS) analysis showed indeed an increased Rsh in devices based on perovskite:fullerene films, benefitting from the decreased pinhole density in these films. The EIS characterization also revealed decreased RS values, which were attributed to the larger grains (i.e. improved electron conductivity) in perovskite:fullerene films (Figure 3b). The improvement in resistance values was said to contribute to FF increase for devices based on the perovskite:fullerene films.


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EIS was also used by Wang et al.49 for explaining the high FF value, finding that devices based on perovskite:A10C60 films show smaller charge-transfer resistance (RCT) in comparison to those based on perovskite films (Figure 3c). These results were addressed to the larger interfacial area between perovskite and fullerene. However, fullerene percolation was assumed and, according to the TEM micrographs of perovskite:C70 films,39 this does not necessarily occur in perovskite:fullerene films. Therefore, other phenomena affecting the charge transfer should not be ruled out. Indeed, Wang et al.49 also found a large increase in the recombination resistance for devices based on perovskite:fullerene films, suggesting other significant roles for fullerene too. In this sense, Wu and co-workers45 proposed the assistance of PCBM fullerene in the crystallization of perovskite, improving its structural quality and crystal size. Apart from the morphology, trap states in perovskite may affect significantly FF of the solar cells. These are charge recombination sites that decrease the recombination resistance, leading to lower FF values,62 apart from being a cause for perovskite degradation63 and hysteresis.64 The origin of trap states has been ascribed to the existence of sites with a higher concentration of uncoordinated Pb atoms,65-67 as well as Pb-I antisites where the trimer PbI3- is formed, due to the occupation of a Pb site by I anion, as stated by Xu et al.46 Furthermore, these authors clarify by density functional theory (DFT) calculation of density states (DOS) how trap states go shallower when PCBM binds defect sites, resulting in devices with larger FFs. In the same line, Shao and co-workers40 found in an experimental work that perovskites present a large density of defect states, from 1017 to 1019 m-3 without fullerene passivation (Figure 3d). After depositing PCBM, the tDOS over 0.40 eV of energy experienced a considerable decrease of almost two orders of magnitude, which was in consistency with the decrease in the photocurrent hysteresis. Thus, after interpenetration of PCBM along perovskite grain boundaries by thermal annealing, they found a


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reduction in the tDOS for trap states in the 0.35-0.40 eV range, pointing out that deeper (> 0.4 eV) trap states on perovskite surface can be passivated by PCBM without thermal annealing. However thermal annealing is required to passivate inner shallow (0.35-0.40 eV) trap states located along grain boundaries, thus reaching maximized FFs. This passivation turns out to be particularly relevant for efficient device performance, since electron extraction at cathode is highly affected by electron recombination on the perovskite surface. In fact, the smaller series resistance that was found for the devices in which PCBM was said to participate in passivation might have led to the increase in JSC and FF. Therefore, these experimental results showed how PCBM is thermodynamically favored to bind these excess-halide-containing sites, avoiding the formation of traps. Reports on other fullerene derivatives, such as α-bis-PCBM52 and DF-C60,51 also revealed their role in defect passivation and charge transport. In this sense, the FF improvement in perovskite:fullerene blends might be associated to the passivating role of fullerene. Hence, a deeper understanding of the fullerene-trap state system is required, so that the passivating nature of fullerenes can be maximized.


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Figure 3. (a) Top-view micrograph by FE-SEM means of the elimination of pinhole presence by the introduction of C70 in the perovskite film by “all-in-one” strategy. (Adapted with permission from Ref. 39. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim). (b) SEM micrographs of the removal of large voids and pinholes by the use of perovskite:PCBSD “all-inone” active layers. (Adapted from Ref. 50 with permission from The Royal Society of Chemistry). (c) Nyquist plots of fullerene-free (blue) and A10C60-containing (red) PSCs measured in the dark and at an applied voltage close to the open-circuit voltage of the PSC. (Adapted from Ref. 49 with permission from The Royal Society of Chemistry). (d) Trap density of states (tDOS) obtained by thermal admittance spectroscopy for devices without PCBM


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(orange), with PCBM but no thermal annealing (red), with 15 min thermal annealing PCBM (green), 45 min thermal annealing PCBM (blue). (Adapted with permission from Ref. 40. Copyright 2014 Macmillan Publishers Limited). The affinity of fullerenes to these trap states needs to be understood through the potential interactions that might be involved between fullerenes and perovskite components. In this sense, perovskite/fullerene-based ETL bilayer systems may be a simplified scenario to provide further insights into the interaction between perovskite and fullerenes in blend films. Tian and coworkers68 prepared indeed some p-i-n devices in which the interactions between perovskite and different fullerene derivatives (i.e. in the ETLs) were analyzed. These were PCBM and DMEC60, a fullerene with two pyrrolidine esters, as well as their C70 analogues. The characterization of fullerene layers by Fourier-transform infrared spectroscopy (FTIR) means showed a shift of the stretching C=O vibration (1720-1750 cm-1) to lower energies when they were deposited on perovskite, meaning a decrease in the strength of this bond (Figure 4a). Moreover, as the same figure points out, the stretching N-H vibrations (~3300 cm-1) that are observed in DMEC60 and DMEC70 layers could not be seen when deposited over perovskite. The authors propose that the shift or elimination of their vibrations in FTIR spectra is happening through complexation in the interface of C=O and N-H moieties with Pb atoms from perovskite, which is the responsible of the reduction of charge recombination and increase in the electron extraction in the fullerene-perovskite interfaces. These results were in agreement with the computational calculations by Taufique and co-workers69 of the attachment of PCBM to different perovskite surfaces. They showed how the carbonyl moiety from fullerene was the closest moiety to Pb and H atoms from perovskite surface, while phenyl carbon had less affinity


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to perovskite. Radial distribution functions revealed the preference of carbonyl group to coordinate the perovskite. Additionally, Sun and co-workers70 pointed out the preference of fullerenes for iodine through π interactions. These were actually found critical in n-doping and transporting electrons to fullerenes at perovskite/fullerene interface in p-i-n PSCs. Iodine-fullerene π interactions helped in charge transport and hysteresis suppression in these PSCs with PCBAN fullerene as HTM. In the same line, Quarti and colleagues71 explained in their computational calculations the impact of the fullerene-perovskite interaction in the energy levels of the perovskite in relation to electrodes and HTMs. DFT simulations revealed how the conduction band edge (CBE) of the perovskite lied above the LUMO of C60 only for MAI-terminated perovskite surface. C60 could interact with perovskite surface with this termination, decreasing the energy difference between the CBE of perovskite and the LUMO of C60. Therefore, perovskite with preferential MAI-termination is thermodynamically more stable than PbI-terminated one, which, in addition, has a more favorable energy level diagram for electron extraction to ETLs based on fullerenes. These π interactions might have been determining in the work by Pascual and colleagues,60 where C70 was used in “all-in-one” solutions for the preparation of efficient ETL-free devices. In this work, the STEM-EELS characterization of the perovskite:C70 films revealed significant differences in the C70 distribution as a function of the processing solvents (Figure 4b). Briefly, when DMF was used as a solvent C70 was not clearly identified in the STEM-EELS carbon map micrographs although higher C-K signal was clearly detected in the EELS spectra (inset Figure 4b), suggesting fullerene may be integrated inside the crystalline structure of perovskite. However, when o-xylene was used as cosolvent, STEM-EELS carbon maps showed a different distribution for C70, leaving it visible, probably laying along the grain boundaries (Figure 4b).


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These results were in accordance with the micrographies obtained by AFM means (Figure 4c), for which a different distribution of the fullerene in the perovskite layer was observed. While the film processed with pure DMF showed a homogeneous phase, the use of an aromatic cosolvent led to island-like regions of few nanometers distributed along the grains. The improvement in PV parameters might be derived from the new distribution of fullerene along the boundaries, a preferred situation for trap passivation, as was also found in a previous work when Cu(thiourea)I was used as additive.72 This probably happened since C70 integrated in the perovskite structure when it was dissolved with it in the precursor solution, a result that does not happen when an aromatic cosolvent was added, which could establish π-stacking interactions with the fullerene, weakening its affinity for perovskite and therefore distributing itself along the grain boundaries. The variation in affinity of fullerene for different nature cosolvents was proven by UV-vis spectra of solutions of C70 in the different cosolvents (Figure 4d). Apart from the higher absorption for higher energy bands that can be seen for aromatic cosolvents, the bathochromic effect that they experience was pointing out the presence of charge-transfer interactions. C70 shows a clear preference to be surrounded by aromatic cosolvent molecules, as the wavelength value of the main absorption band of the fullerene is determined mainly just by this cosolvent instead of DMF. Hence, the interactions between fullerene and perovskite are of particular relevance when determining its distribution throughout the active layer, being particularly interesting that methodology (processing, fullerene modification…) that allows dispersing these carbon nanostructures along perovskite grain boundaries, where they can passivate defects or participate in charge-transfer processes.


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Figure 4. (a) FTIR spectra of the films with different components with key N-H and C=O vibrations highlighted in green. (Adapted with permission from Ref. 68. Copyright 2016 American Chemical Society). (b) HAADF STEM micrographs and composition maps obtained by EELS of C K and I M edges of cross sections of solar cells based on perovskite:C70 films processed with and without o-xylene, while the inset in the center shows a comparison of EELS


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spectra from perovskite:C70 blend films with (red) and without o-xylene (black). (Adapted with permission from Ref. 39. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim). (c) AFM results showing differences in high magnification phase images of perovskite:C70 layers processed with and without o-xylene. (Adapted with permission from Ref. 60. Copyright 2018 American Chemical Society). (d) UV-vis absorption spectra for C70 dissolved in different aromatic and aliphatic co-solvents. (Adapted with permission from Ref. 60. Copyright 2018 American Chemical Society). Therefore, perovskite:fullerene interactions have been proven as essential for affording an efficient electron transfer and defect passivation in perovskite. The different affinity between perovskite and fullerenes might actually depend on the nature of the latter ones. In this sense, the preparation and application of fullerene derivatives might be a key point for optimizing the affinity for perovskite and, in the same line, maximizing their passivating character through their controlled distribution along grain boundaries.35 In general, fullerene derivatives have assisted the processing of perovskite layers with enhanced morphology quality, due to their passivating character, leading to enhanced PV parameters and stability. This is the case of the 10-carboxylic acid-containing fullerene A10C60 by Wang and co-workers,49 which, even for the high degree of functionalization, had very similar LUMO and HOMO energy levels compared to PCBM, since the substituents did not change any optoelectronic property from C60 (Figure 1c). However, it was successfully modified for a significant increase in the resistance to moisture. Recently, based on previous studies about the stability to moisture through the employment of hygroscopic PEG in PSCs,73 some fullerenes were derivatized (i.e. F1, F2 and F3) with these polymeric moieties by Collavini and co-workers (Figure 1c).53 Although a little decrease in PCE was experienced for them, the fullerenes were


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still successful in the enhancement of stability to moisture, as expected from their PEG functionalization. By dissolving them in the perovskite precursor solution, the authors achieved a huge increase in the resistance to ageing and moisture. Fullerene PCBSD also assisted in the resistance to moisture thanks to its cross-linking ability,50 and fluorinated fullerene DF-C60 increased the ambient stability of the device.51 Moreover, it helped in the passivation of pinholes in perovskite layer. In a different way, Tian and co-workers61 used a fullerene derivative with a long alkyl chain with enhanced solubility as ETL in p-i-n architectures. The better solubility was claimed to allow fullerene to permeate deeper in the perovskite and distribute more efficiently along grain boundaries, passivating trap states and improving film morphology. Considering other aspects, in a recent paper by Sandoval-Torrientes et al.54 a series of LUMO-tailored fullerene derivatives were prepared. Based on the enhancement of the electron-accepting character of fullerene, the authors

synthesized

a

pair

of

isoxazolinofullerenes

(IS-1,

IS-2,

Figure

1c)

and

pyrazolinofullerenes (PI-1, PI-2, Figure 1c) that showed, in ETL-free n-i-p PSCs, a trend between LUMO energy of the fullerene derivative and the resulting VOC in the device. The use of o-xylene in this architecture in combination with the optimized IS-2, allowed overcoming the previous highest PCE with a value of 14.3%. Fullerenes are a family of compounds with a wide range of chemical possibilities. They allow plenty of modifications by which we can tune their properties, adapting them to a specific application. Therefore, they are presented as one of the most promising groups of compounds, not only due to their benefits and versatility in PSCs, but also to the opportunities for optimization that they offer.


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Summary and Future Outlook. The use of fullerenes in many diverse ways has contributed significantly for the enormously fast development of PSCs. More in particular, as extensively discussed in this perspective, their introduction in the perovskite layer has led to an exceptional enhancement of the PV parameters, being FF the most affected one. Additionally, hysteresis was largely decreased and durability of the devices increased in comparison to fullerene-free standard PSCs. This is mainly due to the increase in the morphological quality (e.g. decreased pinhole density and larger grains) of the perovskite films through the inclusion of fullerene in it, leading to significant increase and decrease of the shunt and series resistances, respectively, of the resulting solar cells. Furthermore, we have pointed out the trap sites-passivating role of fullerenes as the source of the beneficial effects on the optoelectronic properties of the perovskite:fullerene films. In fact, fullerenes, and more in particular the ones with Lewis basecharacter, have shown a particular preference for these trap sites, as they have been claimed to coordinate with the different elements present in these defects, such as triiodide moieties or uncoordinated Pb. However, further studies on the fullerene-perovskite interactions in defects are required so as to gain further insights into their role in these systems. We have also discussed the potential importance of the location of fullerenes in the perovskite layer, since different results were obtained when fullerene was incorporated into the perovskite crystals or placed at the grain boundaries, suggesting that passivation of the relevant traps can be favored when fullerene is distributed along the perovskite grain boundaries. Owing to the different compatibility that differently functionalized fullerenes could have with perovskite, many fullerene derivatives have been introduced in this layer. We have enumerated them and indicated the relevance of their functionalization in their beneficial character. Considering these aspects, focused investigations are needed to figure out the interactions that govern perovskite:fullerene systems, so that the


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factors that cause the different fullerene distribution and affinity for perovskite can be identified. It is desirable that the particular interactions between fullerene and perovskite moieties are studied, in order that preferred functionalizations of fullerenes can be classified. Future strategically modified fullerenes may be consciously designed for enhanced trap site-passivation by distributing them preferentially along regions with major density of relevant traps (e.g. grain boundaries). All in all, there is still much work to do in the characterization of the perovskite:fullerene systems, to completely understand the role of these carbon nanostructures in these devices. Unraveling this issue will lead to a new generation of molecules capable of enhancing PV parameters, as well as other benefits as hysteresis elimination or stability enhancement, through easy “all-in-one” procedures. Furthermore, from practical point of view, up-scaling of ETL-free solar cells based on “all-in-one” processed perovskite:fullerene films is also necessary to confirm the real potential of this simplified architecture to the industrialization of the perovskite photovoltaics.

Funding Sources Basque Government (Grant number PC2015-1-03 (16-79). MINECO of Spain (Grant numbers MAT2013-47192-C3-2-R, CTQ2015-70921 and ENE201679282-C5-4-R, IEDI-2015-00666). Iberdrola Foundation Notes

The authors declare no competing financial interests.


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ACKNOWLEDGMENT This work was partially supported by the MINECO of Spain (Grant Number CTQ2015-70921, ENE2016-79282-C5-4-R). J.P. acknowledge the Basque Government for a PhD research grant. J.P. and J.L.D. acknowledge Iberdrola Foundation for financial support. We thank Prof. Andrey Chuvilin from CIC nanoGUNE Consolider for the STEM and EELS studies.

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