Role of fullerene electron transport layer on the morphology and optoelectronic properties of perovskite solar cells

Accepted Manuscript Role of fullerene electron transport layer on the morphology and optoelectronic properties of perovskite solar cells Mushfika Baishakhi Upama, Naveen Kumar Elumalai, Md Arafat Mahmud, Dian Wang, Faiazul Haque, Vinicius R. Gonçales, J. Justin Gooding, Matthew Wright, Cheng Xu, Ashraf Uddin PII:

S1566-1199(17)30388-9

DOI:

10.1016/j.orgel.2017.08.001

Reference:

ORGELE 4248

To appear in:

Organic Electronics

Received Date: 28 March 2017 Revised Date:

13 July 2017

Accepted Date: 2 August 2017

Please cite this article as: M.B. Upama, N.K. Elumalai, M.A. Mahmud, D. Wang, F. Haque, V.R. Gonçales, J.J. Gooding, M. Wright, C. Xu, A. Uddin, Role of fullerene electron transport layer on the morphology and optoelectronic properties of perovskite solar cells, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.08.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

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Influence of fullerene ETL on the perovskite grain growth and photovoltaic performance

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Role of fullerene electron transport layer on the morphology and optoelectronic properties of perovskite solar cells Mushfika Baishakhi Upama*,a, Naveen Kumar Elumalai*,a, Md Arafat Mahmuda, Dian Wanga, Faiazul Haquea, Vinicius R. Gonçalesb, J. Justin Goodingb, Matthew Wrighta, Cheng Xua, Ashraf Uddina a

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School of Photovoltaic and Renewable Energy Engineering, The University of New South Wales, 2052, Sydney, Australia.

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School of Chemistry, Australian Centre for NanoMedicine and ARC Centre of Excellence for Convergent Bio-Nano Science, The University of New South Wales, Sydney, NSW 2052, Australia

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Corresponding authors * E-mail: [email protected] * E-mail: [email protected]

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High performance, hysteresis-free, low temperature n-i-p perovskite solar cells are successfully fabricated by solution processing using fullerene electron transport layer (ETL). PC71BM fullerene, with broader absorption spectrum and lower HOMO level, when incorporated in the perovskite solar cell yielded average power conversion efficiency (PCE) of 13.9%. This is the highest reported PCE in n-i-p perovskite solar cells with PC71BM ETL. The devices exhibited negligible hysteresis and high open-circuit voltage (Voc). On the contrary, devices with PC61BM, a common fullerene ETL in perovskite solar cell, exhibited large hysteresis and lower Voc. The underlying mechanisms of superior performance of devices with PC71BM ETL were found to be correlated with fullerene surface wettability and perovskite grain size. The influence of fullerene ETL on the perovskite grain growth and subsequent photovoltaic performance was investigated by contact angle measurement, morphological characterization of the surface topography and electrochemical impedance analysis.

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Keywords- perovskite solar cells, fullerene ETL, n-i-p structure, hysteresis, morphology

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1. Introduction

Since the first successful implementation of organic/inorganic metal halide perovskite solar cell in 2009 [1], it has become a promising photoactive material. Within the past few years, perovskite solar cells have accomplished impressive power conversion efficiency (PCE), exceeding 22% in 2016 [2]. Such high efficiencies have been possible due to the material’s excellent tunable optical band gap [3], high absorption coefficient [4], balanced electron/hole mobility [5], small exciton binding energy [6-8], slow recombination of free charges [9, 10], and good carrier transport [11, 12] and longer exciton diffusion length compared to organic semiconductors [13]. As such, perovskite solar cells are a promising candidate to serve as a high performance energy source at low cost, which is the ultimate target of the photovoltaics industry [14]. One obstacle on the pathway to the successful commercialization of perovskite solar cell is the high temperature processing (>450 °C [15-19]) of metal oxide layers, such as mesoporous (mp)- TiO2, used as electron transport layer in n-i-p configuration. Although 1

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conventional mesoporous n-i-p architecture is still at top in producing higher efficiency cells compared to its p-i-n counterpart [20, 21], the high temperature processing is an impediment to roll-to-roll processing using flexible plastic substrate, for commercial production [21-25]. At the same time, research is continuing on methods to improve the efficiency of p-i-n structure. For example, using advanced material preparation methods, efficiencies as high as 18.9% have been achieved [26]. Such structure typically employs PEDOT: PSS as the hole transport layer (HTL) and PC61BM fullerene as the electron transport layer. n-type fullerene interfacial layer, such as PC61BM, not only acts as an efficient electron acceptor in fullerene/perovskite devices but also helps to increase photocurrent by broadening the overall device absorption window and contributing to efficient photon harvesting, thus providing excellent photovoltaic performance [27]. In contrast, PEDOT: PSS is acidic and hygroscopic in nature, which is detrimental to long-term device stability [28, 29]. Under such circumstance, it is imperative to search for an alternative solution-processed, low-temperature perovskite solar cell structure which can replace PEDOT: PSS by a more stable HTL. Studies have been conducted to achieve this goal by applying variety of electron transport layers (ETLs) such as ZnO and TiO2 nano-structures [30, 31], and low temperature solutionprocessed ZnO (~150 °C).[32-34] However, these interfacial layers possess high density of trap states [29], rigorous synthesis procedure of nanostructures [35-37] and material deposition techniques less suitable for roll-to-roll processing [5], simultaneously affecting the device performance and mass production compatibility.

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One way to overcome the challenges mentioned above is to reverse the position of PC61BM fullerene ETL in p-i-n structure and substitute non-hygroscopic material for the PEDOT: PSS. Ryu et al. [28] were the first to report a n-i-p structure with fullerene (PC61BM) as ETL, the structure also included methyl ammonium lead iodide (CH3NH3PbI3) perovskite as photoabsorbing layer and poly[bis(4-phenyl)(2,5,6-trimentlyphenyl)amine poly(triarylamine) (PTAA) as the hole transport layer (HTL). They achieved 14.4% PCE, with a low processing temperature (≤100 °C). The fabricated device required sophisticated solvent engineering technique and suffered from hysteresis effects. The study did however, pave way to low temperature and solution processed n-i-p perovskite solar cell. Since this first report, very few studies have focussed on the immense potential offered by this structure. Even fewer studies have been done to explore the applicability of other solution processed fullerenes such as PC71BM, which is widely used as acceptor material in organic solar cell [29, 38-41]. Typically PC71BM is used instead of PC61BM in organic solar cells due to its broader absorption spectrum that leads to higher efficiency [42-44]. However, the device efficiency does not solely depend on higher light absorption of PC71BM. Other factors also need to be considered. For example, molecular ordering and electron donor-fullerene interaction [45]. The donor-fullerene interface heavily depends on the type of fullerene derivative used [45]. Similarly, the fullerene/perovskite interface is also expected to vary depending on the type of fullerene ETL used (PC71BM or PC61BM). Hence the significance of fullerene on perovskite morphology needs to be investigated. Chang et al. [46] briefly mentioned the use of PC71BM fullerene as ETL in a n-i-p structure or so-called “normal structure” perovskite solar cell. However, deeper investigations are required into the role of PC71BM fullerene as an electron transport layer and the impact on the morphology evolution of the adjacent perovskite layer, 2

ACCEPTED MANUSCRIPT which is fundamental to enhance the photovoltaic performance of perovskite solar cells [47]. Such thorough investigations will provide insight into the relationship between the underlying fullerene ETLs and top perovskite layer such that the charge carrier dynamics in these devices can be better understood. The present work aims to close this knowledge gap. The attained knowledge can also be applied to tandem solar cell application by providing versatility in interfacial layer selection.

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In this work, we have fabricated low temperature processed (≤100 °C) CH3NH3PbI3 perovskite solar cells with 2 different fullerene ETLs (PC61BM and PC71BM) with n-i-p structure. The effect of underlying fullerene layer on the morphology evolution of adjacent light absorbing perovskite layer is investigated. The concomitant influence on photovoltaic performance and the optoelectronic properties of the devices are also analysed. The PC61BM devices showed reduced hysteresis compared with previous report on related structures [28]. Furthermore, hysteresis-free perovskite cells were fabricated using PC71BM ETL. Both types of devices showed high PCE with best efficiencies of 13.9% and 12.7% for devices with PC71BM and PC61BM, respectively. It is the highest efficiency reported for n-i-p perovskite solar cell with PC71BM ETL. We have compared the surface wettability of the fullerenes with perovskite precursor solution to find the link between interface quality and perovskite grain size. In addition, we have conducted an in-depth study of fullerene/perovskite devices by means of electrochemical impedance spectroscopy (EIS) to gather information about significant physical phenomena such as interfacial charge transfer and carrier recombination, which has direct impact on the device performance. We have also conducted capacitancevoltage measurement to understand the hysteresis phenomenon inside the device. MottSchottky analysis is employed to analyse the improvement of open circuit voltage (Voc) in devices with PC71BM ETL.

2. Experimental section

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2.1 Device Preparation: Fig. 1a displays the architecture of the fabricated perovskite devices: ITO/PC71BM or PC61BM/CH3NH3PbI3 perovskite/Spiro-OMeTAD/MoO3/Ag. Patterned ITO glass substrates (12 mm × 12 mm, Lumtec) were cleaned sequentially via ultrasonication in soapy DI water, DI water, acetone and isopropanol, each for 10 min. [6,6]phenyl C71 butyric acid methyl ester (PC71BM) and phenyl-C61-butyric acid methyl ester (PC61BM) (1- Material, Inc., concentration: 20 mg/mL) were dissolved in chlorobenzene and spin-coated on top of the ITO coated glasses at 1000 rpm for 60 s. 460 mg lead iodide (PbI2) and 159 mg methyl ammonium iodide (CH3NH3I or MAI) were dissolved in 1 mL DMF (N, N–dimethylformamide) at 70 °C overnight in a N2 filled glovebox. The MAPbI3 solution was spin-cast on the PC71BM and PC61BM layers at 2500 rpm. Later, the substrates were annealed at 100 °C for 10 min. For HTL, 72.3 mg/mL Spiro-OMeTAD (2,2',7,7'-Tetrakis (N,N-di-pmethoxyphenylamino)-9,9'-spirobifluorene) was doped with 17.5 µl Li-TFSI (520 mg/mL in acetronitrile) and 28.8 µL 4-TBP in chlorobenzene. The Spiro-OMeTAD layer was spin 3

ACCEPTED MANUSCRIPT coated dynamically on the perovskite layer with 2500 rpm for 40 s in order to prevent the fullerene layer from damage since both layers can dissolve in chlorobenzene. To ensure better hole transport, an additional layer of thin MoO3 film (6.5 nm) was deposited on top of the Spiro-OMeTAD layer via thermal evaporation. Finally, 100 nm Ag layer was deposited on the Spiro-OMeTAD/MoO3 HTL layer by thermal evaporation with an evaporation rate of 2.5 Å/s under a vacuum condition of 1 × 10 mBar.

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2.2 Characterization: The current density–voltage (J–V) measurements were performed using a solar cell I–V testing system from PV Measurements, Inc. (using a Keithley 2400 source meter) under illumination power of 100 mW/cm2 by an AM 1.5G solar simulator. The device area was fixed to be 0.12 cm2 with the use of a metal shadow mask. UV–VIS-NIR spectrometer (Perkin Elmer – Lambda 950) was used to measure the transmission and absorption of various layers. The contact angles of PC71BM and PC61BM layers were measured by a Ramé-Hart 200-F1 goniometer employing 8 µL of DMF solvent. The contact angles were registered by a NET-GmbH 1354 digital camera and measured using the DROPimage standard software (version 2.1.3). Perovskite surface topology image was captured by FEI Nova NanoSEM 230 FE-SEM and surface roughness was extracted from atomic force microscopy (AFM) images by Bruker Dimension ICON SPM. X-ray diffraction (XRD) with Cu Kα radiation was performed at an angle ranging from 20° to 60° by stepscanning with a step size of 0.02°. The impedance and Mott-Schottky measurements were performed with an Autolab PGSTAT-30 equipped with a frequency analyser module in the frequency range of 106-1 Hz. AC oscillating amplitude was as low as 20 mV (rms) to maintain the linearity of the response.

3. Results and discussion

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In the present work, n-i-p fullerene-MAPbI3 heterojunction perovskite solar cell is fabricated using fullerene derivatives, PC71BM and PC61BM, as the hole blocking layer. Both fullerenes had similar concentration (20 mg/mL). The perovskite layers are grown on both of the fullerenes in an identical one step fabrication process. Fig. 1b shows the energy band diagram of the fabricated device, Spiro-OMeTAD/MoO3 assists the hole transport and electron blocking at the anode. Before characterizing the device, optical and morphological characterizations of the ETL and perovskite layer were conducted. Fig. 2a and 2b displays the transmission patterns of fullerene layers and absorbance of the MAPbI3 deposited on top of different ETLs. As can be seen from the figures, in the short wavelength region (<375 nm), PC71BM transmits slightly higher than PC61BM. However, PC61BM transmission rapidly exceeds that of PC71BM in the mid- and long wavelength region (375-720 nm). After 720 nm, the transmission patterns of both materials become similar. The patterns observed in Fig. 2a reflect on the absorption spectra of perovskite layer grown on top of the different ETL layers. In short wavelength region, perovskite on PC61BM absorbs lower than that on PC71BM. In contrast, the absorption increases within 375-720 nm (Fig. 2b) owing to the high transmission of PC61BM layer in this region. Fig. 2c and d illustrate the contact angles between the perovskite solvent (DMF) and the ETLs. In the images, droplets of DMF solvent 4

ACCEPTED MANUSCRIPT can be seen on top of PC71BM and PC61BM. The contact angle of PC61BM layer was measured to be 19.6°+0.5°, which is slightly lower than the measured contact angle of PC71BM layer (25.3°+0.4°), implying higher wettability of PC61BM to perovskite precursor, compared to PC71BM layer.

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Fig. 3a and 3b presents the scanning electron microscopy (SEM) images of perovskite layers on top of PC71BM and PC61BM, respectively, to provide information on the morphology of the perovskite film. From the figures, it is apparent that the grain size of MAPbI3 perovskite layer grown on top of PC71BM is larger than that grown on top of PC61BM. It is well-known that the growth of the perovskite is influenced by the structure of previously formed layers [33, 48, 49]. The fact that, identically fabricated perovskite layer (MAPbI3 in both cases) can grow differently on non-identical underlying layers, is supported by previous reports [33, 48, 49]. The grain size difference of perovskite layer on top of different organic ETLs can be connected to the wettability of the ETLs to perovskite precursor [48, 50]. Earlier literature suggests that non-wetting interfacial layers, such as ETL and HTL, underlying the perovskite layer, can suppress heterogeneous nucleation process of perovskite, resulting in less dense nuclei and finally, larger grain size [51]. Large grain size is reported to curb both bimolecular recombination [33, 52] and trap-assisted recombination [51] in perovskite solar cell. Grain boundaries in perovskite films have been reported to cause increasing charge recombination due to the presence of large density of charge traps [53, 54]. Perovskite film with a larger grain size can reduce the charge trap density since such film possesses less grain boundaries. The proposed mechanisms of perovskite morphology evolution on PC61BM and PC71BM films are illustrated in Fig. 4. From contact angle measurements shown in Fig. 2c and 2d, PC71BM has less wettability compared to PC61BM. Due to the slightly higher wettability of PC61BM substrates for perovskite solvent, we hypothesize that the surface tension dragging force (STDF) of such substrates reduce the grain boundary mobility [51]. As a result, dense nuclei are expected to form and finally grow into smaller grains. On the other hand, for less wetting surface of PC71BM substrate presenting higher contact angles, the STDF decreases which assists the growth of larger grains of perovskite on PC71BM. The hypothesis is supported by the SEM images where the average grain size of perovskite layer on top of PC71BM (~200 nm) is approximately twice as large as that on top of PC61BM (~90 nm).

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Fig. 5a and 5b depicts 2D and 3D AFM images of MAPbI3 film on top of PC71BM and PC61BM, respectively. The AFM images also reflect larger grain size of perovskite on lesswettable PC71BM layer. The difference in the surface roughness of perovskite on PC71BM and PC61BM is negligible. From statistical quantitative analysis of the images, the RMS surface roughness of the perovskite films was found to be 10.9 and 8.1 nm on PC71BM and PC61BM, respectively (Table S1). In both cases, the surface is smooth enough to produce a perovskite photoactive layer which assists minimal leakage current and efficient charge extraction [55]. Fig. S1 shows the XRD patterns of the perovskite films on PC71BM and PC61BM. The presence of distinct (110), (220), (310), (224) and (314) characteristic diffraction peaks is evident in both perovskite layers. Both XRD patterns are identical, nevertheless the peaks confirm the construction of tetragonal perovskite structure in both cases [33]. 5

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In order to assess the device performance of n-i-p MAPbI3 perovskite solar cell including low temperature fullerene ETLs (PC71BM and PC61BM), we fabricated the following device: ITO/PC71BM or PC61BM/CH3NH3PbI3 perovskite/Spiro-OMeTAD/MoO3/Ag (Fig. 1a). Henceforth, the PC71BM and PC61BM ETL based devices are referred to as PC71BM and PC61BM devices. Table 1 contains the photovoltaic performance of the devices, as determined from the I–V measurement. The average values are presented with the corresponding standard deviation values of six samples from a random batch. The good reproducibility of the fabricated devices can be comprehended from the statistical box-charts presented in Fig. S4 (for both PC61BM and PC71BM devices), showing the range of variation in photovoltaic parameters in them. In terms of efficiency, PC71BM devices show better performance than PC61BM devices. The best performing PC71BM device exhibits a PCE of 13.9% with short circuit current density (Jsc), open circuit voltage (Voc) and fill-factor (FF) of 20.5 mA/cm2, 1.08V and 62.7%, respectively. The average values of PCE, Jsc, Voc and FF for these devices are 13.7%, 22.1 mA/cm2, 1.02V and 60.7%, respectively. The average PCE reported in the current study is the highest ever achieved for n-i-p perovskite devices with PC71BM ETL. In contrast, PC61BM devices demonstrate comparatively lower average PCE of 11.6% with the average Jsc, Voc and FF values of 23.8 mA/cm2, 0.814V and 59.9%, respectively. Even the best performing device has relatively lower PCE (12.7%) compared to the average PCE of PC71BM devices. Fig. 6a and 6b show the light and dark J-V characteristics graphs of the best performing PC71BM and PC61BM devices.

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From Table 1, significant differences in Jsc and Voc of PC71BM and PC61BM devices are clearly visible. These differences appear to play a role in attaining 15.3% enhancement in the efficiency of PC71BM devices. By comparison, the short-circuit current density of PC61BM devices is higher than that of PC71BM devices. PC61BM devices have an average Jsc of 23.8 mA/cm2, whereas it is 22.1 mA/cm2 for PC71BM devices. The ~8% increase in average Jsc is expected to be caused by the enhanced absorption inside the photo-absorbing perovskite layer (Fig. 2b) due to the higher transmission of light through the PC61BM ETL (Fig. 2a). The variation in Jsc is further confirmed by the external quantum efficiency (EQE) spectra, plotted in Fig. S5. The figure shows higher photon-to-electron conversion efficiency in the PC61BM devices in 375-720 nm wavelength region, in which the maximum EQE peak reaches over 87% (the photocurrent value estimated from the EQE spectrum is 20 mA/cm2, which is 84% of the value obtained from J-V curve). In PC71BM devices, the photocurrent value estimated from the EQE spectrum is ~17 mA/cm2, which is 85% of the value obtained from J-V curve of the best device. The Jsc extracted from I–V scans, which is within 20% of the value calculated from EQE measurements, indicates reasonable correlation as the phenomenon is inextricably tied to the specifics of the device being investigated and arises due to nonlinearities in the recombination mechanisms caused by a wide variety of processes [56]. The differences between the Jsc values from these two characterizations are also consistent with the reports from previous perovskite literatures [57-59]. The variation in the Jsc values obtained from integrated EQE spectrum and J-V characteristics curve could originate from multiple sources, such as light soaking effect [57] or photodoping [60-62], slow transient response [57] and/or different characterization time spans [63]. The EQE spectra, combined

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Nonetheless, it is the striking boost of 25.6% in the Voc of PC71BM devices that finally yields their enhanced PCE. There is also a slight increment in fill factor (FF) of the PC71BM devices. However the influence is not as dominant as that of Voc. Three possible mechanisms can play a key role in the enhancement of the Voc in PC71BM devices, compared to that in PC61BM devices. First, PC71BM passivates the electron trap states near the MAPbI3/PC71BM interface. Due to low temperature stability of perovskite, large density of defects or charge trap sites are created at its surfaces and grain boundaries [64, 65]. It is a well-known fact that fullerene can passivate the trap states both inside bulk perovskite[66] and surface [52, 64, 67, 68]. The surface passivation can effectively reduce radiative recombination pathways near MAPbI3/fullerene interface [64]. An example of radiative recombination is the direct recombination of the electrons in the conduction band with the holes in the valence band, also known as bimolecular recombination. Recent studies suggest that bimolecular recombination is the dominant recombination in perovskite layer [12, 54, 69]. According to Yang et al. [63], the reduction in this bimolecular recombination is the primary source of high Voc in good quality, well-grown, high crystalline perovskite solar cell. The effect of bimolecular recombination on device Voc can be understood by the following equation [63]:

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Here,  is the material bandgap,  is a material-dependent prefactor that accounts for the reduction in bimolecular recombination rate,  is Langevin’s recombination coefficient,  ,  are the density-of-states (DOS) of conduction band and valence band, and  is the charge generation rate. When fullerene passivates the perovskite surface, bimolecular recombination rate goes down, which eventually reduces the value of . With the decrease in , Voc increases significantly and the loss in Voc reduces notably. We speculate that PC71BM acts as a more superior passivator than PC61BM by reducing the surface charge trap states more effectively, which contributes to the higher Voc of PC71BM devices.

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Table 1: Average and best performance of devices with ITO/PC71BM or PC61BM/MAPbI3 perovskite/SpiroOMeTAD or (Spiro-OMeTAD/MoO3)/Ag device structures. J-V parameters of best performing devices are presented at both FB-SC (forward bias to short-circuit) and SC-FB (short-circuit to forward bias) scan directions. All the J-V curves are measured at the same scan rate (10 V/s).

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Best Value

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Current Density, Jsc (mA/cm2)

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Open

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Factor, FF (%)

Efficiency (%)

Series Resistance, RS (Ω.cm2)

Shunt Resistance, Rsh (Ω.cm2)

Average

22.1+1.01

1.022+0.05

60.7+2.62

13.7+0.92

16.2+2.73

1217.5+321

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20.5

1.080

62.7

13.9

21.7

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Best (SC-FB)

20.4

1.082

63.1

13.9

17.4

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Average

23.8+0.44

0.814+0.03

59.9+4.00

11.6+0.79

14.3+0.42

499.8+47.5

Best (FB-SC)

23.9

0.841

62.9

12.7

7.0

621.6

Best (SC-FB)

24.2

0.800

55.6

10.8

7.5

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Second, the grain size of perovskite crystal is larger when grown on top of PC71BM ETL, as can be seen from the SEM and AFM images (Fig. 3 and Fig. 5). The distance between ionized acceptor and donor pair in large grain size perovskite is longer than that in a smaller grain size perovskite crystal [33, 52]. This distance is inversely related to the photon energy of DAP (donor–acceptor pair) recombination by following equation [33]: (2)

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Here, EA, ED, -, -. and R correspond to acceptor and donor binding energy, free space dielectric constant, relative dielectric constant and the distance between ionized donor– acceptor pair, respectively. In PC71BM devices, the value of R is higher than that in PC61BM devices due to larger grain sizes of perovskite. From equation (2), the significance of higher R value is lower DAP recombination energy, which means lower bimolecular recombination in the perovskite active layer. As stated in previous paragraph, reduced bimolecular recombination leads to high Voc. Larger grain size can also lead to improved quality of MAPbI3/PC71BM interface with less surface defects, which can contribute to the reduction in trap density-of-states (tDOS) inside the forbidden energy gap [51]. These deep trap energy levels can assist trap-assisted non-radiative recombination, resulting in a reduction in Voc [63]. We hypothesize that PC71BM is passivating the bottom surface of the perovskite layer better than PC61BM, such that density of deep trap levels are reduced and the Voc is improved in PC71BM devices. Hence, although the hydrophobicity of PC71BM is higher, it is good for the large grain growth of perovskite and the subsequent improvements in electronic parameters. It is worth mentioning that, both ETL and HTL in our device structures are soluble in similar solvents. It is imperative to ensure complete surface coverage of a thick perovskite layer such that the underneath interfacial layer is not washed away.

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Third, from the energy-level alignment of the fullerene ETLs and perovskite in Fig. 1b, the HOMO level of PC71BM is situated at a lower level (-6.1 eV [70]) than that of PC61BM (-5.9 eV [71]). Hence, PC71BM can act as a better hole blocking layer compared to PC61BM by ensuring better charge selectivity at cathode. The relationship between charge selectivity of electrical contacts and Voc can be explained from following equation [33]:  =

 

/

ln  /0  , +

(3)

1. denotes the charge recombination of a diode in the dark, it quantifies the leakage of minority carriers due to poorly selective electrical contacts [57]. PC71BM devices have low 1. which is apparent from low leakage current density presented in the dark J-V characteristic curve in Fig. 6b and very high shunt resistance, Rsh (Table 1), compared to PC61BM devices.

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ACCEPTED MANUSCRIPT The reduced saturated current density results in the augmentation in average Voc value of PC71BM devices.

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During the optimization stage, we also investigated the photovoltaic performance of devices without the MoO3 layer. Table S2 presents the comparative solar cell parameters of the fabricated devices. Since the Spiro-OMeTAD and fullerene ETLs are soluble in the same solvent (chlorobenzene), a thin layer of Spiro-OMeTAD was deposited on the perovskite film via dynamic spin coating at a very high spin rate (3500 rpm), to prevent the fullerene ETL from dissolving by the Spiro-OMeTAD solvent. This thin Spiro-OMeTAD layer is insufficient for efficient hole transport and electron blockage, which can be seen from the diminished fill-factor of the devices with only Spiro-OMeTAD HTL (Table S2). For PC71BM devices, the fill-factor falls from 60.7% to 50.4% if the MoO3 layer is removed from the HTL. The parameter goes down from 59.9% to 45.3%, in a similar fashion, in PC61BM devices. As a result, on average, devices with Spiro-OMeTAD HTL showed less efficiency than devices with Spiro-OMeTAD/MoO3 HTL. For Spiro-OMeTAD HTL, the PCE decreases by 30.7% and 21.6% in devices with PC71BM and PC61BM, respectively.

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In order to confirm the speculated mechanisms behind the enhanced Voc of PC71BM devices, EIS was conducted on both type of devices (with PC71BM and PC61BM). EIS is an effective tool to shed light upon the charge recombination and transfer phenomena inside an electronic device [29, 32-34]. All the Nyquist plots, extracted from EIS under dark and different biasing conditions, were fitted with an equivalent electrical circuit presented in Fig. 7a. The equivalent circuit model consists of a series resistance, Rs, in conjunction with three parallel R-CPE (constant phase element) components. The symbol Rs corresponds to the resistance originating from the wire connection and metal contact. The Nyquist plots of the devices at 0.85V bias are illustrated in Fig. 7b. Both plots consist of three arcs: a) the high frequency arc refers to the charge transfer at the cathode, b) the mid-frequency arc is attributed to the charge recombination phenomenon at the fullerene/perovskite interface, and c) the lowfrequency region corresponds to the slow ion motion in perovskite. The circuit in Fig. 7a reflects the corresponding parallel resistance and non-ideal capacitance branches ((a) RtCPEt, (b) Rrec-CPErec, (c) Rion-CPEion), connected in series. Liu et al.[64] observed similar arc patterns in CH3NH3PbI3 perovskite solar cells, however in their case, the ETL was kept constant (TiO2), and the hole collectors were varied between NiO nano-sheets and NiO nanoparticles. In this study, the hole collector was MoO3, and the ETL were either PC71BM or PC61BM. Table 2 lists the fitted values of different electronic parameters extracted from the Nyquist plots in Fig. 7b. It can be seen from Table 2 that the PC71BM devices have smaller charge transfer resistance (0.017 kΩ.cm2) than that of PC61BM devices (0.155 kΩ.cm2), which suggests better charge collection at the PC71BM/perovskite interface [29, 32-34]. PC71BM devices also exhibit larger recombination resistance (0.152 kΩ.cm2) over PC61BM devices (0.028 kΩ.cm2). The related charge recombination lifetime, τrec (=Rrec.Crec), is also longer in these devices, which implies prolonged electron lifetime such that they have better chance to get collected at the cathode before being recombined [64]. A similar trend is observed for all the biases. The Rrec and τrec at different biases are shown in Fig. 7c and 7d. Corresponding Nyquist plots and capacitance values (Crec) are displayed in Fig. S6, S7 and 9

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S8a. In all cases, the recombination resistance is higher and the recombination lifetime is longer in PC71BM devices, which implies our conjecture made earlier- PC71BM devices have lower probability of carrier recombination due to larger grain size of perovskite grown on top of PC71BM layer and lesser surface defects at PC71BM/perovskite interface. In addition, PC71BM devices demonstrate higher resistance against ion motion inside perovskite (Rion (PC71BM)= 0.625 kΩ.cm2, Rion (PC61BM)= 0.089 kΩ.cm2) which can be related to the mitigation of interface defects, larger grain size of perovskite, reduced interface and bulk charge recombination, and a longer charge carrier lifetime, as we have already stated. Fig. 7e and 7f show the Rion and τion values at different biases. Corresponding capacitance values (Cion) are illustrated in Fig. S8b. Slow moving ions are a root cause of hysteresis in perovskite solar cell,[64-66] high resistance against these species suggest lower hysteresis in the perovskite devices. The motion related time constant, τion, is also smaller in PC71BM devices (Table 2), this observation explains the absence of hysteresis in the J-V characteristics of PC71BM devices displayed in Fig. 6a, whereas PC61BM devices showed a moderate amount of hysteresis.

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Table 2: Fitted values of different electronic parameters from Nyquist plot of MAPbI3 Perovskite solar cells (with PC71BM and PC61BM as ETL) at 0.85 V under dark condition. Rt

Type

(kΩ.cm2)

Rrec (kΩ.cm2)

Rion (kΩ.cm2)

Crec (nF/cm2)

Cion (nF/cm2)

τrec (sec)

τion (sec)

PC71BM

0.017

0.152

0.625

80.7

0.101

1.23x10-2

6.00x10-5

PC61BM

0.155

0.028

0.089

0.197

28.67

5.50x10-6

2.56x10-3

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Another evidence of reduced recombination in PC71BM device can be found from the (capacitance)-2 vs voltage curve in Fig. 7g, extracted from the Mott-Schottky analysis, by means of impedance spectroscopy. The data were extracted by imposing a small ac signal at 10 kHz frequency and sweeping a DC voltage from 0-1.4V under dark. The x-axis intercept of the extrapolated linear section of the Mott–Schottky curve provides the flat band potential (Vfb) of respective device [33]. Table S3 contains the flat band potential for the fabricated devices; the values are 1.1V and 0.95V for PC71BM and PC61BM devices, respectively. Vfb can be defined as the potential that can counterbalance the energetic difference between the quasi Fermi levels of perovskite and electron selective contact, in our case fullerene/ITO contact [33]. The relationship can be expressed in following equation: 234 = 36 − 7 ,

(4)

Here, 36 is the quasi Fermi level of fullerene/ITO contact and 7 is the quasi Fermi level of the perovskite film. Value of Vfb of PC71BM devices is 150 mV higher than that of PC61BM devices. The difference in the values can arise from both an increase in 36 and a decrease in 7 . In PC71BM devices, large perovskite grain size and fullerene passivation can lead to reduced bimolecular recombination; hence free charge carrier density in the active layer will be higher. Thus, the quasi fermi level, 7 , will go down to be closer to the LUMO level of the perovskite film. Furthermore, due to fullerene passivation and larger grain size, both surface 10

ACCEPTED MANUSCRIPT and bulk defect states will be passivated, resulting in lower trap-mediated monomolecular recombination. A parameter to measure the p-doping density of electrical defects is to find the doping density in the bulk active layer from the slope of the straight line in (capacitance)-2 vs voltage curve [58]. The extracted acceptor densities are listed in Table S3. The defect density in almost twice in PC61BM devices (2.43 x 1016 cm-3) than in PC71BM devices (1.23 x 1016 cm-3). Since PC71BM ETL support less defect density, more electrons are collected at the cathode and the quasi Fermi level of fullerene/ITO contact (36 ) will have an upward shift.

2 = 36 − 38 ,

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The increase in 36 also explains the 208 mV increment in the average Voc of the PC71BM devices compared to that of the PC61BM devices. The device Voc can be defined as the energetic offset between 36 and the quasi Fermi level of HTL/ anode contact, 38 . It can be expressed as,[33]

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(5)

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Since, the HTL layer (Spiro-OMeTAD/MoO3) is similar for both device structures, the improvement in Voc of PC71BM devices are majorly coming from the upward shift of the quasi Fermi level of PC71BM/ITO contact (36 ).

9: =

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Apart from high Voc, PC71BM devices also exhibited significant suppression of hysteresis. Hysteresis prevents the accurate measurement of efficiencies in perovskite solar cells [64]. Fig. 8a and 8b display the J-V characteristics curves of devices with PC71BM and PC61BM ETLs, respectively, at different scan rates (ranging from 1-100 V/s) and scan directions (FBSC and SC-FB). It is clear from the figures that PC71BM devices show negligible hysteresis compared to PC61BM devices. This is not unexpected since fullerene passivation can eliminate the photocurrent hysteresis in perovskite solar cells [64]. Recently, Li et al.[67] have demonstrated that densely packed and uniformly distributed large crystal grains in perovskite films can lead to weak hysteresis behaviour. In a similar fashion, we observed negligible hysteresis in PC71BM devices due to the larger grain growth of perovskite films on top of PC71BM layer. The PC71BM passivation is more effective than PC61BM passivation owing to the reduction in unwanted charge trapping and charge recombination in both bulk and at the interface of PC71BM devices, as congruent with surface morphology characterization (Fig. 3 and Fig. 5). To compare the hysteresis phenomenon, the hysteresis index (HI) of the devices is calculated using following equation [72]: @ @ /;<=>?  A B/>?=;<  A  '

@ /;<=>?  A 

where 1CDBEF 

'

,

(6)

'

GA H

 represents the photocurrent at voltage bias of

(from Voc to short circuit) and 1EFBCD 

GA H

GA H

for the negative scan

 represents the photocurrent for the forward scan.

PC71BM devices have ~65% lower HI compared to PC61BM devices, as can be seen from Table 3.

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ETL Type

L

(V)

JFB-SC 

IJK L



IJK L

(V)

JSC-FB 

IJK L



HI

PC71BM

0.539

19.9

0.541

19.7

0.011

PC61BM

0.420

22.8

0.399

22.1

0.031

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Finally, Fig. 8c, illustrating the frequency-dependent capacitive response, can also elucidate the variation in the hysteresis phenomenon. Hysteresis in perovskite solar cell can originate from slow ionic motion and/or charge accumulation at the interfacial trap states at perovskite/ETL or perovskite/HTL interface, giving rise to electrode polarization [33]. We have already shown from EIS that the PC71BM devices possess higher resistance against ion motion inside the perovskite layer (Fig. 7e and Fig. 7f), thus lowering the hysteresis. On the other hand, electrode polarization by interfacial trap states in perovskite device can be demonstrated by the excess capacitance in the capacitance vs. frequency curve, particularly at low frequency range [59]. The excess capacitance in the low frequency region can alter the required time period to reach steady state condition, thus giving rise to the hysteresis phenomena [33]. From Fig. 8c, the interfacial capacitance of PC71BM devices at low frequency region is 4.2 µF/cm2, which is ~54 times higher than that of PC61BM devices (C = 226.4 µF/cm2). Smaller low frequency capacitance of PC71BM devices explains the absence of hysteresis phenomenon in the devices, due to large grain-assisted and fullerene passivated reduction in charge trapping in both bulk and at the interface of the devices.

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Incorporation of an additional interfacial layer, primarily a work function modifier, such as FPI-PEIE [68] or PEI [28], employed between the electrodes (ITO or FTO) and the fullerene ETL layer would enable more efficient extraction of charge carriers across the interface, consequently improving the PCE of the devices further. Such modification will be a focus of study in our future work.

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4. Conclusion

In conclusion, metal oxide free, low temperature, solution-processed n-i-p perovskite solar cells were fabricated by using the ITO/PC71BM or PC61BM/CH3NH3PbI3/SpiroOMeTAD/MoO3/Ag configuration. Dynamic spin-coating at a high spin rate was used to deposit thin layer of Spiro-OMeTAD HTL such that the HTL solvent cannot damage underneath fullerene ETL layer. Both device structures performed well with an average PCE of 13.9% and 12.7% for devices with PC71BM and PC61BM ETLs, respectively, under standard AM1.5G 100 mW.cm-2 illumination. PC71BM devices showed 25.6% higher Voc compared to that of PC61BM devices. Moreover, PC71BM devices exhibited significantly higher suppression of hysteresis, which is highly desirable in perovskite solar cells for greater accuracy and stability. The mechanisms underpinning the improvements were analysed by means of contact angle measurement, microscopic images of surface morphology and electrochemical impedance analysis. The perovskite morphology evolution was found to be 12

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dependent on the fullerene/perovskite interface. PC71BM fullerene ETL had lower wettability with perovskite precursor solution, assisting the growth of large size perovskite grain. As a result, the PC71BM/perovskite devices had lower bulk and interface defects leading to better device performance. This study provides a simple method for achieving a low-temperature, hysteresis-free, solution processed device which has the potential of low-cost, large-scale application of perovskite solar cell. In addition, high Voc PC71BM devices can be used as a useful sub-cell in tandem cells, rendering flexibility in the selection of interfacial layer and higher device performance.

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Acknowledgements

References

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The authors would like to thank the Australian Centre for Advanced Photovoltaics, UNSW staff and technicians for their support. We acknowledge Future Solar Technologies for providing funding. We also thank the staffs of the Photovoltaic and Renewable Energy Engineering School (SPREE), the Electron Microscope Unit (EMU), and the Solid State and Elemental Analysis Unit (SSEAU) of University of New South Wales (UNSW). J.J.G. acknowledges the ARC for the ARC Australian Laureate Fellowship (FL150100060).

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List of Figures Role of fullerene electron transport layer on the morphology and optoelectronic properties of perovskite solar cells

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Mushfika Baishakhi Upama*,a, Naveen Kumar Elumalai*,a, Md Arafat Mahmuda, Dian Wanga, Faiazul Haquea, Vinicius R. Gonçalesb, J. Justin Goodingb, Matthew Wrighta, Cheng Xua, Ashraf Uddina a

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School of Photovoltaic and Renewable Energy Engineering, The University of New South Wales, 2052, Sydney, Australia. b School of Chemistry, Australian Centre for NanoMedicine and ARC Centre of Excellence for Convergent BioNano Science, The University of New South Wales, Sydney, NSW 2052, Australia

Fig. 1. (a) Schematic representation of fabricated MAPbI3 perovskite solar cells with PC71BM or PC61BM as ETL. (b) The energy levels of the device related materials in eV.

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Fig. 2. (a) Transmittance patterns of PC71BM and PC61BM films on ITO/glass substrates. (b) Absorption curve

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of MAPbI3 perovskite films on PC71BM or PC61BM/ITO/glass substrates. (c)-(d) Contact angle measurement

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with DMF solution (MAPbI3 solvent) on PC61BM and PC71BM film, respectively.

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Fig. 3. Top view scanning electron microscopy (SEM) images of MAPbI3 perovskite film, (a) on top of

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PC71BM/ITO/glass substrate, and (b) on top of PC61BM/ITO/glass substrate.

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Fig. 4. Mechanism of perovskite grain growth on PC71BM and PC61BM layers with different wettability.

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Fig. 5. Two and three dimensional atomic force microscopy (AFM) images of MAPbI3 perovskite film, (a) on

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top of PC71BM/ITO/glass substrate, and (b) on top of PC61BM/ITO/glass substrate.

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Fig. 6. (a) J–V characteristics curve of the best performing solar cells with ITO/PC71BM or PC61BM/MAPbI3 perovskite/Spiro-OMeTAD/MoO3/Ag device structures at FB-SC (forward bias to short circuit) and SC-FB (short-circuit to forward bias) scan directions with a scan rate of 10 V/s at room temperature under AM1.5G illumination at 100 mW/cm2. (b) Dark J-V characteristics of the solar cells with ITO/PC71BM or PC61BM/MAPbI3 perovskite/Spiro-OMeTAD/MoO3/Ag device structures.

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Fig. 7. (a) Equivalent circuit model used to fit the experimental data from Nyquist plot. (b) Nyquist plot of MAPbI3 perovskite devices at a bias of 0.85V under dark condition. (c) Plot of charge recombination resistance (Rrec), (d) charge recombination lifetime (τrec), (e) low frequency resistance (Rion), (f) lifetime corresponding to ion motion in perovskite (τion) at the ETL/perovskite interface obtained from impedance measurements under dark at the given bias (bias range: 0-0.85V). (g) Mott Schottky curve of MAPbI3 perovskite devices at 10 KHz frequency under dark. (PC71BM ETL: red square and PC61BM ETL: blue circle).

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Fig. 8. Photocurrent hysteresis phenomena in fabricated MAPbI3 perovskite solar cells with (a) PC71BM and (b) PC61BM as ETL. The J-V curves for both type of devices are shown at FB-SC (forward bias to short-circuit) and SC-FB (short-circuit to forward bias) scan directions with scan rate of 1 V/s, 10 V/s and 100 V/s. (c) Device capacitance at zero bias, as a function of frequency for MAPbI3 Perovskite devices with PC71BM and PC61BM ETL.

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Supplementary Information Role of fullerene electron transport layer on the morphology and optoelectronic properties of perovskite solar cells

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Mushfika Baishakhi Upama*,a, Naveen Kumar Elumalai*,a, Md Arafat Mahmuda, Dian Wanga, Faiazul Haquea, Vinicius R. Gonçalesb, J. Justin Goodingb, Matthew Wrighta, Cheng Xua, Ashraf Uddina a

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School of Photovoltaic and Renewable Energy Engineering, The University of New South Wales, 2052, Sydney, Australia. b School of Chemistry, Australian Centre for NanoMedicine and ARC Centre of Excellence for Convergent BioNano Science, The University of New South Wales, Sydney, NSW 2052, Australia

Fig. S1 XRD patterns of MAPbI3 perovskite on PC71BM and PC61BM layer, showing the major diffraction peaks and the corresponding crystal orientation (The * signs denote the peaks for ITO).

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Fig. S2 Two dimensional atomic force microscopy (AFM) images: (a)-(b) Spiro OMeTAD and MoO3/Spiro OMeTAD on top of MAPbI3 perovskite/PC71BM/ITO/glass substrate and Spiro

OMeTAD

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(c)-(d)

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Fig. S3 Three dimensional atomic force microscopy (AFM) images: (a)-(b) Spiro OMeTAD and MoO3/Spiro OMeTAD on top of MAPbI3 perovskite/PC71BM/ITO/glass substrate and Spiro

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Table S1: RMS surface roughness of different films (in nm), extracted from AFM. RMS Surface

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Film Description

Roughness (nm)

MAPbI3 on PC71BM

10.9

MAPbI3 on PC61BM

8.1

ITO/PC71BM/MAPbI3/Spiro-OMeTAD

1.7

ITO/PC61BM/MAPbI3/Spiro-OMeTAD

2.9

ITO/PC71BM/MAPbI3/Spiro-OMeTAD/MoO3

2.1

ITO/PC61BM/MAPbI3/Spiro-OMeTAD/MoO3

3.0

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Short Open

HTL

Current

Circuit

Description

Density,

Voltage,

JSC

Voc (V)

Fill Factor, FF (%)

(mA/cm2)

OMeTAD/MoO3 Spiro-OMeTAD

22.1

1.022

21.1

0.895

60.7

(%)

Series

Shunt

Resistance,

Resistance

RS (Ω.cm2)

RSh (Ω.cm2)

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MAPbI3 Perovskite Devices with PC71BM ETL

50.4

9.5

16.2

1217.5

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848.4

Series

Shunt

MAPbI3 Perovskite Devices with PC61BM ETL Short Open

HTL

Current

Circuit

Description

Density,

Voltage,

JSC

Fill

Factor,

FF (%)

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Voc (V)

Efficiency (%)

Resistance, 2

Resistance

RS (Ω.cm )

RSh (Ω.cm2)

(mA/cm2)

OMeTAD/MoO3

0.814

24.1

0.825

59.9

11.6

14.3

499.8

45.3

9.1

16.9

164.6

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Fig. S4 Statistical box-chart showing the range of variation in (a) Jsc (mA/cm2), (b) FF (%), (c) Voc (V) and (d) PCE (%) in FB-SC direction obtained from six identically fabricated ITO/PC61BM or PC71BM/MAPbI3 perovskite /Spiro-OMeTAD/MoO3/Ag devices in a single, random batch at a scan rate of 10 V/s.

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Fig. S5 External quantum efficiency (EQE) curves for PC61BM and PC71BM devices.

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Fig. S6 Nyquist plot of MAPbI3 perovskite devices with PC71BM ETL at different biases

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Fig. S7 Nyquist plot of MAPbI3 perovskite devices with PC61BM ETL at different biases

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Fig. S8 Plot of capacitance corresponding to (a) charge recombination and (b) ionic motion in

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perovskite under dark at the given bias (bias range: 0-0.85 V).

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Acceptor Density (cm-3)

Flat-band Potential (V)

16

1.23 x 10

PC61BM

2.43 x 1016

1.10 0.95

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Lower wettability of PC71BM fullerene assists large size perovskite grain growth

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PC71BM/Perovskite devices exhibit negligible hysteresis and high Voc

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