Impact of Ultrathin C60 on Perovskite Photovoltaic Devices - ACS

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Impact of Ultrathin C60 on Perovskite Photovoltaic Devices Dianyi Liu, Qiong Wang, Christopher J. Traverse, Chenchen Yang, Margaret Young, Padmanaban S. Kuttipillai, Sophia Y. Lunt, Thomas W. Hamann, and Richard R. Lunt ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08561 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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Impact of Ultrathin C60 on Perovskite Photovoltaic Devices Dianyi Liu†, Qiong Wang‡, Christopher J. Traverse†, Chenchen Yang†, Margaret Young†, Padmanaban S. Kuttipillai†, Sophia Y. Lunt†, §, Thomas W. Hamann‡, Richard R. Lunt†,

,

*

† Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan 48824, USA ‡

Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA

§

Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing,

Michigan 48824, USA ⁋

Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824,

USA KEYWORDS: ultrathin, fullerene, perovskite, photovoltaic, hysteresis.

ABSTRACT. Halide perovskite solar cells have seen dramatic progress in performance over the past several years. Certified efficiencies of inverted structure (p-i-n) devices have now exceeded 20%. In these p-i-n devices, fullerene compounds are the most popular electron transfer materials. However, the full function of fullerenes in perovskite solar cells is still under investigation, and the mechanism of photocurrent hysteresis suppression by fullerene remains unclear. In previous reports, thick fullerene layers (> 20 nm) were necessary to fully cover the perovskite film surface to make good contact with perovskite film and avoid large leakage currents. In addition, the solution-processed fullerene layer has ACS Paragon Plus Environment

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been broadly thought to infiltrate into the perovskite film to passivate traps on grain boundary surfaces, causing suppressed photocurrent hysteresis. In this work, we demonstrate an efficient perovskite photovoltaic device with only 1 nm C60 deposited by vapor deposition as the electron selective material. Utilizing a combination of fluorescence microscopy and impedance spectroscopy we show that the ultrathin C60 predominately acts to extract electrons from the perovskite film while concomitantly suppressing the photocurrent hysteresis by reducing space charge accumulation at the interface. This work ultimately helps to clarify the dominant role of fullerenes in perovskite solar cells while simplifying perovskite solar cell design to reduce manufacturing costs.

While the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has risen to 22.1% from 3.8%;1 the architecture has also been notably simplified from the original dye sensitized solar cell (DSSC) structure to planar structures.1, 3-5 Indeed, planar device structures now show a combination of many promising advantages, such as high PCE,6-9 low complexity,5, 10 versatile fabrication routes,4 and straightforward integration onto flexible substrates.10-11 Looking ahead, the ability to further simplify perovskite devices could aid in their eventual commercialization. Planar perovskite solar cell structures can be divided into regular (n-i-p) and inverted structures (p-in).12 The n- and p-type layers are responsible for selective electron and hole transport and extraction, respectively. The n-i-p planar structure device was first demonstrated in 2013.4-5 In these structures, organic molecules and polymers such as the 2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’bifluorene (spiro-OMeTAD) and Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) were broadly used as the hole transfer layer (HTL) to deliver a high efficiency over 20% in PSCs. However, most of the high efficiency perovskite n-i-p structures employ metal oxide compounds (e.g. TiO2 ， BaSnO3 and SnO2) as the electron transport layer (ETL),2, 6-8 which requires high sintering temperature to form a compact layer and has impeded the application of n-i-p structure device on flexible substrates. Moreover, the n-i-p structure devices usually suffer from considerable hysteresis during device

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measurement and operation.13-14 In contrast, the inverted planar perovskite structures usually utilize low temperature fabrication procedures to prevent damaging organic hole conduction layers.11, 15 To date, PCEs over 20% have been demonstrated for inverted planar structures, which are comparable with the best n-i-p device.9 Fullerenes have become a popular ETL in inverted structures since they can reduce or eliminate hysteresis and can be vacuum deposited or solution processed at low temperatures16. Fullerenes were first introduced in perovskite solar cells as the ETL in 2013, but early devices only showed an efficiency of 3.9%.17 Since then, the certified efficiency of the devices with fullerene-based ETL have been reported up to 20.59%.9 It is known that fullerenes can suppress hysteresis and work as excellent electron extraction materials.16-18 However, the full function of fullerenes in perovskite solar cells is still limited. Both Koster’s works and Huang’s work suggested that charge traps are mostly electron traps, which are close to the surface of the perovskite film.19-21 Huang’s research suggested that when a solution-processed fullerene layer is deposited on top of the perovskite film, a small amount of fullerene can penetrate into the perovskite film along the grain boundaries, and thus photocurrent hysteresis was suppressed via trap state passivation.16 Generally, fullerene layers with a thickness of 20 - 50 nm can form an efficient interface with the perovskite film to extract electrons.9, 22-23 When the thickness of the fullerene layer is reduced to less than 20 nm, the device performance has been shown to drop significantly.24-25 Previous work has also indicated that a sufficient thickness of fullerene layer is crucial for full surface coverage, which can help lead to high device performance and reduced hysteresis.16, 21, 26 In this work, we demonstrate that an ultrathin vapor deposited C60 (1 nm) selective layer works efficiently in an inverted structure PSC. The role of 1 nm C60 was also investigated. We find that the electron extraction efficiency is dramatically altered by such thin vapor deposited layers, even though the fullerene forms an incomplete monolayer over the perovskite film. The corresponding device presents the PCE of 18.2% under 1-sun illumination with negligible hysteresis of photocurrent. Because the fullerene is vapor deposited, fullerene molecules cannot readily diffuse into the perovskite film ACS Paragon Plus Environment

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along the grain boundaries. Thus, the function of ultrathin fullerene is studied by photoluminescence spectroscopy, microscopy, and impedance spectroscopy, which indicate that a nearly complete monolayer of C60 can switch on the perovskite device by primarily enabling efficient electron extraction. This demonstration could aid in the understanding of the true role of fullerenes while minimizing the necessary amount of materials for surrounding layers, reducing the impact of material variability without sacrificing performance, and enhancing the commercial scalability of PSCs. RESULTS AND DISCUSSION Figure 1a shows the scheme of the device structure used in this work. To study the role of ultrathin fullerene layers in devices, a 1 nm C60 device (C60-1 nm) and a 20 nm C60 device (C60-20 nm) were prepared, a fullerene-free (C60-0 nm) device is also prepared for comparison. The perovskite devices are fabricated on patterned ITO-coated glass substrates where the smooth ITO surface ensures each layer of the perovskite device is uniformly spread on the substrate to form smooth films. An ultrathin PEDOT layer is first spin-coated on the substrate by the diluted precursor solution. The thickness of PEDOT film is 1.6 ± 0.6 nm determined by ellipsometry. Next, a lead-based halide perovskite film (320nm) is deposited on top of the PEDOT layer by the vacuum-assistant method.24, 27-29 Subsequently, an ultrathin layer of C60 is deposited onto the perovskite film by thermal evaporation, with the nominal thickness controlled from 1 nm to 20 nm. All devices utilized a 7.5 nm of 2,9-dimethyl-4,7-diphenyl-1,10phenanthroline (BCP) as the buffer layer, to help form an Ohmic contact with top metal electrode and to protect the device from short or large current leak. The current–voltage (J–V) curves of three samples are given in Figure 1c. It shows that the presence of 1 nm C60 (even an incomplete monolayer) is vital to device performance; without the fullerene, the device barely functions as a photovoltaic cell. All the parameters including the short-circuit current density (JSC), open-circuit voltage (VOC) and the fill factor (FF) are very low, and the obtained PCE is only 0.6%. The FF is less than 0.25, and the voltage is modestly reduced to less than 0.9 V. This result ACS Paragon Plus Environment

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is similar to Chiang and Wu’s report of n-i-p structure perovskite device without the ETL.5 Surprisingly, when even an ultrathin C60 layer is introduced, the device exhibits excellent performance. All the device parameters reach high values with a JSC of 21.4 mA cm-2, a VOC of 1.04 V, and a FF of 69.5%, resulting in a PCE of 15.4%. Further increasing the C60 thickness to 20 nm can slightly improves FF, which helps to promote the device PCE to 16.3%. The external quantum efficiency (EQE) spectra of the devices are shown in Figure 1d. For the fullerene-free device, the EQE is limited to 10% over all absorption range of 300 nm to 800 nm. In comparison, the C60-1 nm device exhibits high EQE value with over 80% in the range of 420 nm to 720 nm wavelength, which is also identical to the spectral response of C60-20 nm device. To evaluate the effect of C60 presence and thickness in the perovskite device, the morphologies of the perovskite films with different C60 thickness deposition were first studied by SEM characterization (Figure 2a). The SEM images show the bare perovskite film has a smooth and clean surface with only a few pinholes. The large apparent grain size is close to 1 µm. These images indicate a good quality of perovskite film. After thermal evaporating 1 nm C60 onto the perovskite layer, the surface morphology clearly changes. Because of the small amount of C60 deposited, it cannot form a continuous film. For reference, a perfectly crystalline monolayer of C60 would be 1 nm thick (based on the lattice constant). The deposited C60 molecules aggregate to the separated spots and those spots are uniformly dispersed on the perovskite film surface. When increasing the deposition thickness to 20 nm, the aggregated C60 spots form a more continuous film. We infer that the partly-covered morphology of C60-1 nm device result in the slightly lower FF than the C60-20 nm device which has full coverage interface due to some interface resistance where the C60 is not present. Figure 2b shows the steady-state photoluminescence (PL) spectra of perovskite films deposited with different thickness of C60. Without C60, the PL is much stronger than with C60. For the C60-1 nm perovskite film, surprisingly, the PL is still efficiently quenched with a quenching efficiency of 88%.11 The high PL quenching efficiency implies high electron transfer efficiency from the perovskite film to ACS Paragon Plus Environment

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C60, preventing geminate carriers from recombining inside the perovskite layer. This indicates that only a very small amount C60 is needed to strongly and efficiently extract the electrons from perovskite absorber. Indeed, further increase in the deposition thickness to 20 nm does not result in improved PL quenching efficiency. This observation is consistent with the J-V and EQE results, since the JSC and EQE value of C60-1 nm device is much higher than the fullerene-free device, but comparable with the C60-20 nm device. To further gain insight into the effect of ultrathin C60 layer on the perovskite film, we performed fluorescence microscopy tests to simultaneously visualize the PL intensity of various thicknesses of C60 on perovskite films without (inset of Figure 2b) and with a non-ETL spacer (BCP) layer (Figure 2c). The 1 nm- and 20 nm-thickness of C60 were deposited on the perovskite film on the same substrate. The steady-state fluorescence microscopy image shows that the fluorescence of perovskite film (inset of Figure 2b) is uniform and strong. Adding 1 nm- and 20 nm-thick C60 layers can dramatically quench the fluorescence of the perovskite film, consistent with PL measurements. Considering that the higher PL quenching with 1 nm C60 could stem from either charge recombination caused by trap states or from enhanced carrier extraction, we performed additional timeresolved fluorescence microscopy with thin amorphous BCP spacer layers (1.5 nm and 7.5 nm) under the C60 layer to distinguish these mechanisms (Figure 3). We find that BCP alone does not significantly change the PL quenching efficiency (Figure 3a). In fact, the perovskite film becomes slightly brighter with only a neat BCP layer. However, when thin C60 layers are deposited over neat amorphous layers of BCP (1.5 nm), we see a similar increase in the PL quenching efficiency as without BCP (Figure 3b). These results indicate that the small molecules placed on the perovskite are not simply quenching defect states; rather, the C60 acts as an efficient electron receiver and efficiently pulls electrons out of the perovskite layer dark states via electron tunneling.31 This is confirmed by placing a larger BCP layer (7.5 nm) under the C60, where the PL quenching is significantly lessened but not entirely eliminated (Figure 3c). The time dependent evolution of this PL quenching is also indicates that the origin of the ACS Paragon Plus Environment

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perovskite luminescence is accumulated charge carriers. These experiments clearly suggest that the C60 is not needed for exciton dissociation (as with organic solar cells),32-37 rather just rapid electron extraction.Since the energetics could play a role in the charge transfer efficiency at the interface, we measure the work function and highest occupied molecular orbital levels of the devices with various C60 thicknesses by ultraviolet photoemission spectroscopy (UPS). The work function values are summarized in Table 1, where the work function of pristine perovskite is 4.7 eV, consistent with the previous reports.38-39 After depositing 1 nm C60, the work function shows a slight shift to 4.6 eV, which indicates there is no obvious band-bending at the interface of perovskite and C60 layer. Gao et al. systematically investigated the electronic structure of perovskite films with incrementally evaporated C60 on top of perovskite layers,38 and found a shift of about 0.2 eV to lower binding energy when 8 Å C60 was deposited, indicating electron transfer from the perovskite film to fullerene molecules. This is consistent with the results in this work but does not fully explain the sudden and dramatic turn-on of the device with the ultrathin C60. To further understand the working mechanism of the C60-1 nm device, we conducted impedance measurements for the devices with varying thicknesses of C60. The Nyquist plots of the devices measured at 0.6 V forward bias are given in Figure 4a & 4b. Two clearly separated semicircles are revealed from Figure 4. The equivalent circuit used to model the Nyquist plot is given in Figure 4c.40 Cg and R1 stand for the geometric capacitance and the transport resistance of the perovskite layer, and Cµ and R2 stand for the chemical capacitance and the recombination resistance. The first semicircle appearing at high frequency represents the bulk properties of the perovskite layer. The second semicircle appearing at the low frequency region represents the interface between perovskite and C60 layer. t2 can be calculated from R2Cµ, and represents the electron lifetime. The parameters extracted from the equivalent circuit are summarized in Table 1. The chemical capacitance Cµ of the 1 nm C60 device is reduced by about 25% of a device without the C60 layer under dark test condition. By further increasing the film thickness of C60 from 1 nm to 20 nm, ACS Paragon Plus Environment

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the charge accumulation at the interface of the perovskite and C60 is reduced by 80%. In addition, comparing the t2 of three devices shows that the electron lifetime is significantly reduced by an order of magnitude as the film thickness of C60 increases from 0 nm to 1 nm. The results measured under light condition show the same trend with the test under dark condition. Due to lack of an ETL, charge of C600 nm device accumulates at the surface of perovskite film, which leads to the two order of magnitude high capacitance than C60-1 nm device. The calculated lifetime of electrons of the C60-1 nm device is almost one thousand times lower compared to the C60-0 nm device. This indicates that electron extraction at the interface is indeed dramatically enhanced with the presence of even a monolayer of C60, and meanwhile the device is switch on with the PCE increasing from < 1% to > 18%. Further increases in the C60 thickness continues to decrease the electron lifetime, and results in the slightly enhanced FF. The effects of C60 layer in perovskite/C60 heterojunction are also investigated by understanding changes in the dark J-V characteristics. Figure 4d shows the dark J-V curves for the devices with varying thickness of C60. The dark curves are well-fit by the diode equation (1),

J=

  q (V − JRS )   V      J S  exp   − J SC  − 1 + RS + RP  nkT R P        Rp

(1)

where n is the diode ideality factor, Rp is the shunt resistance, Rs is the series resistance, and JS is the reverse saturation current. The fitting parameters are summarized in Table 2. As the control device, the C60-0 nm shows the largest current leakage, series resistance, and ideality factor. The shunt resistance C60-0 nm is about two orders of magnitude higher than C60-1 nm and C60-20 nm devices, and 2-3 times larger in series resistance compared to C60-1 nm and C60-20 nm devices. While the C60-1 nm device shows a slightly higher current leakage than the C60-20 nm device, the diode ideality factors are very close for those two devices. Based on the characterization optical and electrical characterization above, we find that the picture of efficient electron extraction sufficiently describes the data role of adding minute

levels

of

vapor-deposited

C60


onto

perovskite

devices.

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This demonstration shows the simplicity that is possible with highly efficient perovskite cells: since the perovskite provides a high degree of carrier dissociation and long-range ambipolar transport, very little is needed besides the thick perovskite layer itself and ultrathin carrier extraction layers. With such a simple structure, we demonstrate an average PCE of 16.2 ± 1.0% for 21 separated devices, and a champion PCE of 18.2%, with a JSC of 22.3 mA cm-2, a VOC of 1.08 V and a FF of 75.9% (Figure 5a). Notably, the device VOC is only slightly lower than the highest reported VOC for any inverted device structure (C60 has a slightly lower lowest unoccupied molecular orbital (LUMO) than PCBM) and has one of the highest VOCs reported for inverted PSCs utilizing only C60 on the n-side.41 We also note that the device photocurrent is absent of hysteresis when measuring the J-V curve under forward and reverse voltage sweeps (Figure 5b) for both C60 thicknesses but is significant without the C60, which reiterates that hysteresis in many cases can simply be tied to the build of space charge at the interface. In situations where processing conditions are not optimized, the traps in the bulk of perovskite films may also contribute to hysteresis effect. CONCLUSIONS In summary, we have demonstrated an inverted planar structure perovskite photovoltaic device with ultrathin hole/electron-selective layers. By systematically investigating the role of ultrathin vapordeposited fullerene layers, we find that the predominate role of the C60 layer is to aid in electron extraction and collection. This is consistently seen with a combination of spatially resolved fluorescence microscopy, impedance spectroscopy, and electrical characterization. Importantly, the device does not show the current hysteresis phenomenon, indicating that the hysteresis in PSC predominately originates from space charge accumulation at interfaces with poor charge-carrier extraction. Our results also indicate that very little C60 is needed to enhance carrier extraction. Utilizing this simple structure, we show that PCEs of over 18% are possible. This work ultimately helps to elucidate the role of fullerenes


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in perovskite solar cells and rethink the simplicity of perovskite solar cell design to reduce manufacturing costs. METHODS Materials and Precursor Preparation: Dimethylformamide (DMF, anhydrous, 99.8%, Aldrich.), dimethyl sulfoxide (DMSO, anhydrous, 99.9%, Aldrich.), PEDOT (Clevios PVP AI 4083, Heraeus. Diluted to 5% by water for use.), CH3NH3I (MAI, Lumtech.), PbI2 (99%, Aldrich.), PbCl2(98%, Aldrich), C60 (99.9%, MER Corporation.) 2,9-dimethyl-4,7-diphenyl1,10-phenanthroline (BCP, Lumtech.) were used as received. To prepare the perovskite precursor solution, MAI:PbI2:PbCl2 (191 mg, 461 mg and 27.8 mg, respectively) were added in a mixed solvent of DMF and DMSO (0.638 ml and 0.212 ml, respectively). The solutions were then stirred for 1 hour and filtered with 0.45 µm PTFE filters before use. Device Fabrication: The PEDOT solution was spin-coated onto pre-cleaned ITO substrates at 6000 rpm for 10 s and then annealed at 110 °C for 5 min. The perovskite precursor was spincoated on top of the PEDOT film at 6000 rpm for 12 s, and then moved into a homemade vacuum chamber, evacuated to ~ 10 mtorr, and left in the chamber for 3 min. The samples were then transferred to the hot plate and annealed at 80 °C for 10 min. The substrate was then moved into the evaporation chamber for deposition of C60 (0 nm, 1 nm or 20 nm) and BCP (7.5 nm). Finally, an 80 nm thick silver layer was deposited by thermal evaporation at a base pressure of 3 × 10−6 Torr through a shadow mask with a final measured device area of 4.85 mm2.


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Measurement and Characterization: The current density–voltage characteristics (J–V curves) were obtained using a Keithley 2420 sourcemeter under both dark and AM1.5 G solar simulation where the light intensity was measured using a NREL-calibrated Si reference cell with KG5 filter. Devices were scanned at a rate of 50 mV s-1. The EQE measurements were performed using a QTH lamp with a monochromator, chopper, lock-in amplifier, and calibrated Si detector to measure the intensity. Impedance measurements were conducted by µAutolabIII FRA2 Impedance Analyzer in the dark at 0.6 V forward bias. The frequency ranged from 105 Hz to 0.01Hz in equally spaced logarithmic steps. The J-V, EQE, PL and impedance were measured on un-encapsulated devices/samples in ambient air. A field-emission scanning electron microscopy (Carl Zeiss Auriga Dual Column FIB SEM) was used to acquire SEM images. Fluorescence microscopy images were collected using an inverted Leica DMi8 Fluorescence Microscope with an excitation wavelength of 635 nm and a 760nm long-pass dichroic block. AUTHOR INFORMATION Corresponding Author Prof. Richard R. Lunt *E-mail: ([email protected]) Department of Chemical Engineering and Materials Science, Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA ACKNOWLEDGMENT The authors acknowledge financial support from the Michigan State University Strategic Partnership Grant (SPG) (D. L.) and from the U.S. Department of Energy (DOE) Office of


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Science, Basic Energy Sciences (BES), under Award # DE-SC0010472 (structural and optical characterization) (P.S.K). FIGURE CAPTIONS Figure 1. (a) Device architecture. (b) Energy levels of the perovskite device.30 (c) Currentvoltage (J-V) curves of perovskite devices with various C60 thicknesses measured under 1-sun illumination, along with (d) the corresponding EQE spectra of the perovskite devices. Figure 2. (a) Surface morphology of perovskite films deposited with various thickness of C60. (left: 0 nm C60; middle: 1 nm C60; and right: 20 nm C60) The scale bar is 500 nm. (b) Steady-state photoluminescence spectra and steady-state fluorescence microscopy images (inset) of perovskite films deposited with various thicknesses of C60. (c) Fluorescence microscopy images of the perovskite film deposited with C60 or BCP between the C60 and perovskite (I: bare perovskite, II: perovskite/BCP-1.5 nm III: perovskite/BCP-1.5 nm/C60-20 nm, IV: perovskite /C60-20 nm).The scale bar is 100 µm for (b) and (c). The dash lines in figure (b) and (c) are used as a guide to the eye to indicate the interface positions. Figure 3. Time-resolved fluorescence microscopy images of perovskite film with (a) bare perovskite, (b) 1.5 nm amorphous BCP spacer with 20 nm C60, and (c) 7.5 nm BCP spacer with 20 nm C60. The BCP only coated perovskite film is provided in each panel for reference. The time scale is 0 second (left), 30 seconds (middle) and 60 seconds (right) for each figure - the brightening of the PL with time stems from space-charge accumulation that increases radiation recombination rates. The dash line is used as guide to the eye to indicate the interface position. The scale bar is 100 µm for all figures.


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Figure 4. Nyquist plots for the perovskite devices deposited with various thicknesses of C60, as measured under (a) dark condition and (b) 0.1-Sun illumination condition at 0.6 V bias. The inset is an expanded view of the Nyquist plot for the devices deposited with 1 nm and 20 nm C60. (c) Equivalent circuit employed to fit the Nyquist plots. (d) Dark current characteristics of the devices with various thicknesses of C60 (The solid line is the fitting curve for dark J-V curve, respectively). Figure 5. (a) J-V curves of the champion device with 1 nm C60 as ETL measured with reverse and forward scanning directions under 1-sun illumination and (b) J-V curves for the devices with various thicknesses of C60 measured under reverse and forward scans at a rate of 50 mV s-1. TABLES Test

Thickness of C60 Rs

R2

Cµ

t2

Work Function

(Ω)

(Ω)

(µF)

(s)

(eV)

0

48.97

3.67 × 108

0.026

9.45

4.7

1

35.77

6.22 × 106

0.019

0.12

4.6

20

38.25

3.63 × 106

0.0045

0.016

5.7

0

990.3

4.74 × 105

4.64

2.20

1

87.15

4.82 × 104

0.048

2.31 × 10-3

20

156.3

1.06 × 105

0.010

1.05 × 10-3

Condition (nm)

Dark

Light

Table 1. Fitting parameters for impedance data acquired under dark and 0.1-Sun illumination condition with 0.6 V bias and measured work functions by UPS. Thickness of C60

n

Rp (Ω cm2)

Rs (Ω cm2)

JS0 (A cm-2)

0 nm

1.80

90

0.64

8.80 × 10-11

1 nm

1.49

5.76 × 103

0.29

3.30 × 10-12

20 nm

1.51

1.82 × 104

0.16

6.90 × 10-12

Table 2. Extracted parameters for dark J-V curve fits using Equation (1).


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15. Malinkiewicz, O.; Yella, A.; Lee, Y. H.; Espallargas, G. M.; Graetzel, M.; Nazeeruddin, M. K.; Bolink, H. J., Perovskite Solar Cells Employing Organic Charge-Transport Layers. Nat. Photon. 2014, 8, 128-132. 16. Shao, Y.; Xiao, Z.; Bi, C.; Yuan, Y.; Huang, J., Origin and Elimination of Photocurrent Hysteresis by Fullerene Passivation in CH3NH3PbI3 Planar Heterojunction Solar Cells. Nat. Commun. 2014, 5, 5784. 17. Jeng, J.-Y.; Chiang, Y.-F.; Lee, M.-H.; Peng, S.-R.; Guo, T.-F.; Chen, P.; Wen, T.-C., CH3NH3PbI3 Perovskite/Fullerene Planar-Heterojunction Hybrid Solar Cells. Adv. Mater. 2013, 25, 3727-3732. 18. Chiang, C.-H.; Wu, C.-G., Bulk Heterojunction Perovskite–PCBM Solar cells With High Fill Factor. Nat. Photon. 2016, 10, 196-200. 19. Sherkar, T. S.; Momblona, C.; Gil-Escrig, L.; Bolink, H. J.; Koster, L. J. A., Improving Perovskite Solar Cells: Insights From a Validated Device Model. Adv. Energy Mater. 2017, 7, 1602432. 20. Sherkar, T. S.; Momblona, C.; Gil-Escrig, L.; Ávila, J.; Sessolo, M.; Bolink, H. J.; Koster, L. J. A., Recombination in Perovskite Solar Cells: Significance of Grain Boundaries, Interface Traps, and Defect Ions. ACS Energy Lett. 2017, 2, 1214-1222. 21. Fang, Y.; Bi, C.; Wang, D.; Huang, J., The Functions of Fullerenes in Hybrid Perovskite Solar Cells. ACS Energy Lett. 2017, 2, 782-794.


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Figure 1. (a) Device architecture. (b) Energy levels of the perovskite device.30 (c) Current-voltage (J-V) curves of perovskite devices with various C60 thicknesses measured under 1-sun illumination, along with (d) the corresponding EQE spectra of the perovskite devices. 200x159mm (300 x 300 DPI)


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Figure 2. (a) Surface morphology of perovskite films deposited with various thickness of C60. (left: 0 nm C60; middle: 1 nm C60; and right: 20 nm C60) The scale bar is 500 nm. (b) Steady-state photoluminescence spectra and steady-state fluorescence microscopy images (inset) of perovskite films deposited with various thicknesses of C60. (c) Fluorescence microscopy images of the perovskite film deposited with C60 or BCP between the C60 and perovskite (I: bare perovskite, II: perovskite/BCP-1.5 nm III: perovskite/BCP-1.5 nm/C60-20 nm, IV: perovskite /C60-20 nm).The scale bar is 100 µm for (b) and (c). The dash lines in figure (b) and (c) are used as a guide to the eye to indicate the interface positions. 221x157mm (300 x 300 DPI)


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Figure 3. Time-resolved fluorescence microscopy images of perovskite film with (a) bare perovskite, (b) 1.5 nm amorphous BCP spacer with 20 nm C60, and (c) 7.5 nm BCP spacer with 20 nm C60. The BCP only coated perovskite film is provided in each panel for reference. The time scale is 0 second (left), 30 seconds (middle) and 60 seconds (right) for each figure - the brightening of the PL with time stems from space-charge accumulation that increases radiation recombination rates. The dash line is used as guide to the eye to indicate the interface position. The scale bar is 100 µm for all figures. 254x72mm (300 x 300 DPI)


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Figure 4. Nyquist plots for the perovskite devices deposited with various thicknesses of C60, as measured under (a) dark condition and (b) 0.1-Sun illumination condition at 0.6 V bias. The inset is an expanded view of the Nyquist plot for the devices deposited with 1 nm and 20 nm C60. (c) Equivalent circuit employed to fit the Nyquist plots. (d) Dark current characteristics of the devices with various thicknesses of C60 (The solid line is the fitting curve for dark J-V curve, respectively). 210x161mm (300 x 300 DPI)


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Figure 5. (a) J-V curves of the champion device with 1 nm C60 as ETL measured with reverse and forward scanning directions under 1-sun illumination and (b) J-V curves for the devices with various thicknesses of C60 measured under reverse and forward scans at a rate of 50 mV s-1. 201x83mm (300 x 300 DPI)


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TOC figure 152x129mm (300 x 300 DPI)


Impact of Ultrathin C60 on Perovskite Photovoltaic Devices - ACS

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