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Influence of Thiazole-modified Carbon Nitride Nanosheets with Feasible Electronic Properties on Inverted Perovskite Solar Cells Daniel Cruz, Jose Garcia Cerrillo, Baris Kumru, Ning Li, Jose Dario Perea, Bernhard V. K. J. Schmidt, Iver Lauermann, Christoph J. Brabec, and Markus Antonietti J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03639 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 16, 2019

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Journal of the American Chemical Society

Influence of Thiazole-modified Carbon Nitride Nanosheets with Feasible Electronic Properties on Inverted Perovskite Solar Cells Daniel Cruz,a,‡ Jose Garcia Cerrillo,b,‡ Baris Kumru,a Ning Li, b,,e Jose Dario Perea,b,f Bernhard V.K.J. Schmidt,a Iver Lauermann,d Christoph J. Brabec,b,c* and Markus Antonietti a,* a. Max-Planck-Institute of Colloids and Interfaces; Department of Colloid Chemistry, Am Mühlenberg 1, 14476 Potsdam, Germany b. Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University ErlangenNürnberg, Martensstraße 7, 91058 Erlangen, Germany c. Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy (HI-ErN), Immerwahrstraße 2, 91058 Erlangen, Germany d. Kompetenzzentrum Dünnschicht- und Nanotechnologie für Photovoltaik Berlin (PVcomB), Helmholtz-Zentrum Berlin für Materialien und Energie GmbH,–Schwarzschildstraße 3,D-12489 Berlin Germany e. National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, Zhengzhou, 450002, China f.Photovoltaic Research Laboratory, Massachusetts Institute of Technology (MIT), Cambridge, MA 02139 USA

KEYWORDS Inverted perovskite solar cells, carbon nitride, photovoltaic devices, Electron transporting layers (ETLs)

ABSTRACT: Effective, solution processable designs of interfacial electron transporting layers (ETLs) or hole-blocking layers are promising tools in modern electronic devices, e.g. to improve performance, cost and stability of perovskite-based solar cells. Herein, we introduce a facile synthetic route of thiazole-modified carbon nitride with 1.5 nm thick nanosheets which can be processed to a homogeneous, metal-free ETL for inverted perovskite solar cells. We show that thiazole-modified carbon nitride enables electronic interface enhancement via suppression of charge recombination, achieving 1.09 V in Voc and a rise to 20.17 mA/cm2 in Jsc. Hence, this report presents the successful implementation of a carbon nitride-based structure to boost charge extraction from the perovskite absorber towards the electron transport layer in p-i-n devices.

Introduction Interfacial solution-processed layers have played an essential role in modern electronic devices, for instance to enable further advances in the field of perovskite solar cells (PVSCs)1–5. This is possibly useful to diminish the recombination at the interfaces and increase photovoltaic performance6,7. At the same time, it is key to reduce manufacturing costs with alternative systems8–10. Hybrid organic-inorganic halide perovskite-based systems are some of the most promising photovoltaic technologies11–13, which is due to a whole number of superior physical key properties, such as low exciton binding energy14,15 and high charge carrier mobility16–18. These outstanding features have boosted the overall photovoltaic efficiency in PVSCs 19–22.

In this contribution, we employ PVSCs as an adequate platform to present our advances in the development and understanding of electron transporting layers (ETL)23–26. These structures are used to tackle significant recombination losses during the photovoltaic charge extraction 27–30, due to increased density of defects and energy level misalignment between the perovskite layer and transport contacts, creating both ohmic and capacitive resistance. A highly efficient ETL into a solar cell “just” diminishes such interface-related effects and remains a key aspect to be considered to achieve higher power conversion efficiencies (PCEs). Up to now, numerous interlayer structures were explored, e.g. mesoporous TiO2 structures, SnO2, fullerene C60 and phenyl-C61-butyric acid methyl ester (PCBM) 17,28,31–34. In

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Scheme 1: Schematic presentation of the exfoliation process and device fabrication. a)Thiazole-modified carbon nitride (CMB-vTA) was dispersed in Isopropanol:Acetonitrile (IPA:ACN) 1:1 solvent system, b) implementation of CMB-vTA as thin film by spin coating and c) fabricated inverted PVSC based on methylammonium lead iodide (MAPbI3), ITO/PTAA/MAPbI3/CMB-vTA/PC60BM/AZO/Ag and ITO/PTAA/MAPbI3/PC60BM/CMBvTA /AZO/Ag. [PTAA:Poly(tryarylamine)] architectures. principle many of those solutions have been demonstrated to work adequately for particular systems, however the synthesis and application of these materials are generally accompanied with disadvantages such as high temperature processing35, high density of boundary defects7,36, multiple and costprohibitive synthetic steps, harsh reaction environments and solvents, difficulties of purification and low stability issues37– 39, which hinders practical solar module processing. Therefore, novel materials which can tackle the aforementioned obstacles are always welcome, however not yet fully addressed. Recent research on metal-free semiconductor graphitic carbon nitride (g-CN) has undoubtedly extended the field of photocatalysis significantly40. g-CN ideally consists of 2D structures formed from repeating tri-s-triazine rings. In addition to being metal-free, g-CN can be synthesized from low-cost and abundant precursors (such as urea or melamine), while the choice of precursors affects the photophysical properties of g-CN, e.g. band gap values and photoluminescence41–45. As a heterogeneous colloidal material, g-CN has diverse applications, i.e. light induced water splitting, organic coupling reactions and polymer chemistry46–51 . Up to now the main drawback of g-CN materials for the discussed device applications was its processability, since coatings applied from organic solvents showed low quality and high roughness52–54, which prevented the application of g-CN in photovoltaic devices. Previous reports investigating the incorporation of g-CN in solar cells are either based on vacuum deposition techniques (Magnetron Sputtering or CVD) or non-homogenous films54– 61, which both led to non-satisfactory performance. Ordinary

g-CN features strong π - π interactions, which causes nondispersibility62, especially in organic solvents. Therefore, the film formation based on g-CN dispersion results in highdefect low-quality films that inhibit the implementation in solar cells as interface agents. Following the recent discovery by our group to modify g-CN via visible light-induced one-pot photografting63, the method was extended to 4-methyl-5vinylthiazole (vTA) modification to enhance its dispersibility in organic solvents, yielding exceptional organodispersions of g-CN-vTA with well-defined particle thickness and size, allowing transparent and homogenous film formation by spray-coating or inkjet printing64. The grafting of vTA on the g-CN rim led to spontaneous polarization and migration of negative charges on thiazole rims where g-CN nanosheets remain positive, which significantly influences the overall electron transport process and photophysical properties64. Herein we show that it is possible to utilize vTA-modified g-CN as a cheap, abundant and easily solution-processable interface modifier in inverted PVSCs. Providing an increases of short-circuit current and decreased recombination losses, cause an enhanced of electron extraction at the upper interface in p-i-n PVSCs. Results and Discussion The precursor choice for carbon nitride synthesis influences band gap properties of final g-CN structures. In the present case we have synthesized g-CN from cyanuric acid, melamine and barbituric acid (CMB), which reduce the LUMO level compared to traditional g-CNs 65. In order to promote organodispersibility and enhance its photophysical properties, CMB was subsequently modified with vinyl thiazole groups via photografting approach as described

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Journal of the American Chemical Society previously (CMB- vTA)64. Elemental analysis showed an increase in the mass content of sulfur in CMB- vTA compared to CMB (Table S1), which confirmed the succesful grafting. Additionally, CMB- vTA presented a reduced PL intensity compared to CMB, suggesting a decreased radiative recombination with efficient separation and delocalization of charge carriers due to the influence of thiazole groups as an internal heterojunction (Figure 1a). The effect of thiazole modification on the conductivity of CMB was then investigated (Figure S1). When comparing CMB and CMB-vTA thin films under otherwise similar conditions, an increase of current by about a factor of 2 arising from vTA modification under external voltage was observed. CMB- vTA retained its optical properties with a band gap of 2.2 eV (Figure 1b) and crystalline profile in XRD, showing typical orientation peaks at 15° and 27° without structural disordering (Figure 1c). X-ray photoelectron spectroscopy (XPS) survey spectra confirmed the typical core levels C1s, N1s peaks associated with the carbon nitride (Figure 1d), in contrast CMB post-modification showed a slight peak located at 161 eV assigned to S2p, thus confirming the presence of thiazole group.

Figure 1. a) Photoluminescence emission spectroscopy, b) UV-Vis diffuse reflectance spectra and Tauc-plot, c) X-ray diffraction patterns and d) XPS survey of CMB and CMB-vTA. HR-TEM was utilized for the morphology investigation of CMB and CMB-vTA particles, which shows ordered and sheetlike structure after vTA modification (Figure 2a-b). Importantly, CMBvTA featured excellent organodispersibility via electrostatic stabilization. A conventional isopropanol:acetonitrile mixture was chosen as dispersing medium for CMB- vTA where electrostatic forces play role for the exfoliation process (Table S2).The height profiles of CMB and CMB-vTA films (Figure 2b-c) were investigated by atomic force microscopy, showing that the CMB-vTA coating featured heights between 1-2 nm and lateral dimensions of around 100 nm. Unmodified CMB demonstrates higher thickness and heterogeneous particle distribution, which proves that vTA modification is indeed a

key step for excellent exfoliation of CN sheets and formation of transparent and uniform films.64

Figure 2. HR-TEM and AFM height profiles of a)-c) CMB and b)-d) CMB-vTA. In the next step, CMB and CMB-vTA dispersions was spin coated on two different layers of PVSCs (Scheme 1, experimental details in SI). With the aim to study the influence of CMB- vTA on the electronic properties of MAPbI3-based inverted perovskite solar cells, CMB-vTA was deposited on top of the MAPbI3 prior to or after the deposition of a PC60BM layer. Treatment of MAPbI3 with polar solvents such as ACN does not influence its degradation as evidenced by film transmittance as depicted in Figure S2. MAPbI3 and MAPbI3 treated with ACN presents similar pattern, indicating that perovskite retains its stability under polar solvent treatment during the spraying process. Furthermore, to gain insight into the charge carrier dynamics, we have analyzed the steady-state PL and time resolved photoluminescence (TRPL) for MAPbI3 deposited on glass with CMB-vTA and CMB thin films on top of perovskite layer. Figure S3 shows the main emission peak at 770 nm arising from the perovskite absorber, while the perovskite treated with CMB or CMB-vTA presents a decrease on peak intensity. Quenched PL intensity after CMB-vTA deposition implies a more effective charge extraction across the interface of MAPbI3-CMB-vTA.

Figure 3. a) Time resolved photoluminescence (TRPL) decay from the glass side, b) time resolved photoluminescence (TRPL) decay from the film side (the PL spectra of the film side are reported Figure S3).

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Table 1. Photovoltaic parameters for reference (MAPbI3) and CBM-vTA interface based devices. Device ITO/PTAA/MAPbI3/ PC60BM/AZO/Ag

Voc (V) 1.077

Jsc (mA cm-2) 18.66

FF (%) 75.13

PCE (%) 15.09

Rsh (Ω cm-2) 959

Rs (Ω cm-2) 7.71

ITO/PTAA/MAPbI3/CMB-vTA/PC60BM/AZO/Ag

1.095

19.57

77.50

16.61

1280

ITO/PTAA/MAPbI3/PC60BM/CMB-vTA/AZO/Ag

1.090

20.17

78.03

17.15

2010

6.829

ITO/PTAA/MAPbI3/ CMB-vTA/AZO/Ag

0.914

8.97

40.10

3.29

149

1.519

ITO/PTAA/MAPbI3/ PC60BM/CMB-vTA/Ag

0.9139

18.71

47.01

8.04

209

22.95

ITO/PTAA/MAPbI3/ PC60BM/CMB/Ag

1.017

17.79

49.88

9.03

93.7

2.768

ITO/PTAA/MAPbI3/ CMB/AZO/Ag

0.929

11.138

55.40

3.79

113

1.877

7.84

TRPL was applied to the previously described systems in order to observe the effect of CMB-vTA on the photodynamics of the interface (Figure 3). The PL decay was fitted via an exponential model which resulted in a lifetime (Table S3) of τ=139.9 ns for MAPbI3, τ =60.3 ns for MAPbI3/CMB and τ =15.6 ns for MAPbI3/CMB - vTA, hence demonstrating an efficient charge extraction processes at the interface. To study the role of CMB-vTA on the photovoltaic properties of devices with the basic structure ITO/PTAA/MAPbI3/PCBM/AZO/Ag, current density and voltage (J-V) scans were performed (Figure 4a), the results are summarized in table 1. The improvement of Jsc values was observed when we deposited CMB-vTA layers (Table 1, Figure 4a), pointing to increase the shunt resistance on the device. External quantum efficiency (EQE) spectra of the three devices are presented in Figure 4b, the integrated Jsc has increased to 19.823 mA cm-2 and 20.784 mA cm-2 over a reference device (19.184 mA cm-2) and lead to an increased carrier collection of the short wavelength light, when CMB-vTA was implemented, this is possibly due to the significantly improved electronic interactions and orbital overlaps with the covalent 2Dmaterial carbon nitride and the nature positive charges in the nanosheets and negative charges in the thiazole groups.64Therefore, charge delocalization and double layer interactions might be the reason for enhanced photovoltaic performance after thiazole modification.

Figure 4. a) Current density versus voltage characteristics of 0.104 cm2 MAPbI3 based solar cells with and without CMBvTA, b) external quantum efficiency spectra of MAPbI3 based solar cells with and without CMB-vTA. The role of CMB-vTA as electron transport layer or hole blocking layer is presented in figure S4-5 where PCBM or AZO layers were substituted by CMB-vTA. Utilizing CMB-vTA as a replacement of AZO results in better performance as indicated by the IV plots The improvement of Jsc is due to the decrease of number of recombination centers by improvement of the hole blocking layer interface.

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Journal of the American Chemical Society (without vTA modification) even inhibits solar cell performance, an explanation simply arises from the nonuniform film characteristics, i.e. a grainy and non-controlled layer thickness was formed, which was partly too thick and non-existing at all. Thanks to vTA modification, nano meterthick exfoliated CN-layers with spontaneous charge polarization (creating a donor-acceptor type structure) were formed, which promote electron transfer through all the analyzed interfaces, such as has been reported in our previous work64.

Figure 5. X-ray diffraction patterns for a) methyl ammonium lead iodide (MAPI) thin film and b) after CMB-vTA deposition on MAPI surface. XRD profiles of MAPbI3 and CMB-vTA deposited MAPbI3 were presented in Figure 5, which show the influence of carbon nitride colloids onto perovskite crystal. Typical peaks at 14.10°, 23.47°, 28.42°, 30.89° correspond to planes (110), (211), (220), (213) of tetragonal phase perovskite. During the deposition of CMB-vTA, peaks at 26.50°, 27.32° and 30.75° diminish, showing the positive influence on passivation of surface defects. A slight change on surface morphology and crystal packing was also observed via SEM images (Figure S6).

Ultraviolet photoelectron spectroscopy (UPS) was employed to give additional information on the energy band levels. The valence band maxima were shown to be -5.4 eV for MAPbI3 and -6.1 eV for CMB-vTA, whereas the conduction band minima of -3.7eV and -3.9 eV were determined by adding the band gap of the perovskite absorber. In comparison to PC60BM, the conduction band minimum of CMB-vTA is deeper, which acts as hole transport barrier. Moreover, the level of the conduction band minimum of CMB-vTA aligns with PC60BM such that it allows the efficient transfer of excited electrons from the PC60BM to the AZO (Figure 7). Correspondingly, the work function of CMB-vTA was estimated to be 4.5 eV using secondary electrons edge cut-off .

Furthermore, the functionality of the CMB-vTA as modification interfaces changes the hydrophobic nature of the surface of MAPbI3 (Figure S7), owing to increase the water contact angle from 75.6° to 88.0° post- CMB-vTA treatment.

Figure 7. Solar cell schematic architectures and diagram energy from ultraviolet photoemission spectroscopy (UPS).

Conclusion

Figure 6. Comparison of photovoltaic parameters for different types of p-i-n architectures with carbon nitride interlayers, a) power conversion efficiency (PCE), b) fill factor (FF), c) shortcircuit current (Jsc) and d) open circuit voltage (Voc). With the aim to analyze the statistical relevance of our devices, solar cells for each architecture included the nonmodified CMB (IV reported in Figure S4) were fabricated (Figure 6). The devices with CMB-vTA exhibited the best solar cell performance relate to another ones. Moreover, CMB itself

In conclusion, we presented the implementation of a novel interface layer based on carbon nitride materials. Successful modification of CMB with vTA groups promoted organodispersibility and improved its photophysical properties. This novel approach allowed the deposition of films with high homogeneity, reasonable electronic properties, and flexible adaption of work function. We found a systematic improvement of PCE up to 17.1% compared to the reference device. Likewise, the implementation of CMB-vTA as surface modifier offered and improvement on the interface between PCBM-AZO and MAPbI3-PC60BM. Besides, it enhanced the carrier collection due to the absorption in short wavelength light, leading to improved photovoltaic parameters. Overall, we have demonstrated that CMB-vTA constitutes an alternative interface layer that features metal-

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free, cheap and benign photophysical properties with high tunability. We strongly believe that the successful integration of this class of g-CN-based materials as new-generation interface modifiers would enhance the efficiency in a broader range of organic energy devices and bring them one step closer to daily life applications.

ASSOCIATED CONTENT Supporting Information. Experimental information, materials and characterization methods, elemental analysis, particle charge, PL, current density, SEM images and water contact angles.

AUTHOR INFORMATION Corresponding Author * Markus Antonietti,a Christoph J. Brabec b,c

Present Addresses a. Max-Planck-Institute of Colloids and Interfaces; Department of Colloid Chemistry, Am Mühlenberg 1, 14476 Potsdam, Germany b. Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg, Martensstraße 7, 91058 Erlangen, Germany c. Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy, Immerwahrstraße 2, 91058 Erlangen, Germany

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. / ‡These authors contributed equally.

Funding Sources Max Planck Society and Helmholtz association.

ACKNOWLEDGMENT This work was financially supported by Max Planck Society in collaboration with Helmholtz Association and I-MEET in Erlangen-Nürnberg. J. G. C. gratefully acknowledges the financial support from the Deutscher Akademischer Austausch Dienst (DAAD) through a doctoral scholarship. D.C. gratefully to Del-Lab-Campo for motivation and N.L. gratefully acknowledges the financial support from the DFG research grant: BR 4031/13-1. C.J.B. gratefully acknowledges the financial support through the “Aufbruch Bayern” initiative of the state of Bavaria (EnCN and SFF), the Bavarian Initiative “Solar Technologies go Hybrid” (SolTech) and the SFB 953 (DFG, project no. 182849149).

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