Simultaneous Top and Bottom Perovskite Interface Engineering by

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Simultaneous top and bottom perovskite interface engineering by fullerene surface modification of titanium dioxide as electron transport layer John Ciro, Santiago Mesa, Juan Felipe Montoya, Jose Ignacio Uribe, Rafael Betancur, and Franklin Jaramillo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06343 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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ACS Applied Materials & Interfaces

Simultaneous top and bottom perovskite interface engineering by fullerene surface modification of titanium dioxide as electron transport layer John Ciro1, Santiago Mesa1, Juan Felipe Montoya1, José Ignacio Uribe1,2, Rafael Betancur1 and Franklin Jaramillo1* 1

Centro de Investigación, Innovación y Desarrollo de Materiales – CIDEMAT, Facultad de Ingeniería, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Colombia.

2

Grupo de Estado Sólido, Instituto de Física, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Colombia

ABSTRACT Optimization of the interface between the electron transport layer (ETL) and hybrid perovskite is crucial to achieve high performance perovskite solar cell (PSC) devices. Fullerene-based compounds have attracted attention as modifiers on the surface properties of TiO2, the archetypal ETL in regular n-i-p PSCs. However, the partial solubility of fullerenes in the aprotic solvents used for perovskite deposition hinders its application to low-temperature solution-processed PSCs. In this work, we introduce a new method for fullerene modification of TiO2 layers derived from nanoparticles (NPs) inks. Atomic force microscopy characterization reveals that the resulting ETL is a network of TiO2-NPs interconnected by fullerenes. Interestingly, this surface modification enhances the bottom interface of the perovskite by improving the charge transfer as well as the top perovskite interface by reducing surface trap states enhancing the contact with the p-type buffer layer. As a result, rigid PSCs reached a 17.2% power conversion efficiency (PCE) while flexible PSCs exhibited a remarkable stabilized PCE of 12.2% demonstrating the potential application of this approach for further scale-up of PSC devices.

KEYWORDS: electron transport layer; interface engineering; TiO2 modification; n-i-p planar perovskite solar cell; fullerene.

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INTRODUCTION Most efficiency breakthroughs in perovskite solar cells (PSC) have been reached implementing the regular n-i-p device structure1 with TiO2 representing the archetypal electron transport layer (ETL) due to its low cost, phase stability and effective carrier dynamics control when employed as scaffold.2–6 However, the advancement towards planar n-i-p devices has not been easy making apparently due to the need to accomplish a precise control of the TiO2/perovskite interface. This objective is transversal to all the perovskite photovoltaic research since Yang et al.7 recently demonstrated that recombination in the top and bottom surfaces of the perovskite is more important than even the recombination intraand inter- perovskite grains. Two main strategies have been explored for effectively controlling the TiO2/perovskite interfaces: growing the TiO2 using a more controlled deposition method or incorporating specific interlayers. In the first approach, TiO2 growing methods such as sputtering,5 e-beam2 or atomic layer deposition8 led to a good device performance. In fact, promising power conversion efficiency (PCE) ranged from 12.2% to 17.6% but were essentially limited by the relatively complex deposition methodology. The second strategy, based on optimizing the interface between TiO2 and the perovskite absorber,2,4,5,9–15 has been more successful. Very recently, the designing of chlorine-capped TiO2 nanocrystals demonstrated an unprecedented 20.9% stabilized PCE.13 A slightly simpler strategy consists in incorporating interlayers such as ionic-liquids5,16 or fullerene derivatives.3,4,9–11,17–19 Although this latter case has been specially studied, it is hard to preserve the integrity of the fullerene layer because of its partial solubility in the solvents used for perovskite deposition.18 As a result, several alternatives including depositing the perovskite by thermal evaporation,3,10,11 chemically modifying the fullerene4,17,18 or depositing additional protective layer atop the fullerene9 have been studied. A very complete explanation of the fullerene effect on the perovskite p-i-n structure was recently reported.20 Fullerene derivatives can act in different ways, firstly suppressing hysteresis, slightly increase the Voc and improve stability in such configuration. On the other hand, when considering flexible n-i-p PSC, the polymeric substrate imposes a sub 150 °C restriction on the processing temperature. In that context, the best reported flexible devices relied on a sputtered TiO2 layer.21,22 Strategies employing solution-processed low temperature TiO2 have reached around 10% PCE.23–25 In both rigid and flexible n-i-p 2 ACS Paragon Plus Environment

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devices, the surface engineering of the TiO2 is intended for passivating surface defects and suppressing ion migration, which is ultimately critical to achieve highly efficient and hysteresis-free n-i-p devices.13

In this work, we introduce a new method for engineering the surface of nanoparticle-based solution-processed TiO2 layer (TiO2-NP) leading to a unique simultaneous passivation of the top and bottom interfaces of the perovskite n-i-p device structure. Through atomic force microscopy related techniques, we show that this specific modification simultaneously improves the charge transfer from the bottom perovskite contact to the TiO2 as well as reduces surface trap states enhancing the top perovskite contact with the p-type buffer layer. The developed TiO2-NP:PC70BM layer is a network of TiO2 nanoparticles interconnected by fullerene clusters. Due to the particular topography of this layer, fullerene is not dissolved by the perovskite precursor solution. At the end, we obtained a 18.4% for low-temperature solution-processed n-i-p PSC in rigid substrates and a remarkable 12.2% stabilized PCE in flexible substrates.

RESULTS AND DISCUSION The effective surface modification of a TiO2-NP layer with the fullerene derivative PC70BM (referred in the following as TiO2-NP:PC70BM) configures a novel ETL enabling a champion 18.4% PCE in a planar n-i-p PSC (Figure 1A). Since non-sintered TiO2-NPs were implemented, the layer on top is expected to infiltrate. Accordingly, when depositing perovskite directly onto the TiO2-NP, the perovskite layer is likely to have a direct contact with the ITO semitransparent electrode in detriment of the device photovoltaic parameters. Such is the case for devices without PC70BM as shown in Figure 1B and Table 1. Since the TiO2-NP-based device exhibited a strong hysteresis, it was not possible determine its stabilized power output (SPO). This behavior has been already reported for devices with severe hysteresis 26,27. Incorporating PC70BM into the TiO2-NP layer strongly enhanced the device performance with a slight dependence on the concentration of the PC70BM solution (Figure S1 and Table S1). PC70BM provided compactness to the TiO2-NP layer, which is analyzed thoroughly later, enabling devices with high external quantum efficiency (EQE) and SPO reaching 17.2% as shown in Figures 1C and 1D, respectively. Moreover, the 3 ACS Paragon Plus Environment


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obtained PCE values are slightly dependent on the scan rate used for J-V curves as shown in Table S2 and Figure S2. Therefore, the TiO2-NP:PC70BM layer enabled a significant reduction of hysteresis in the J-V curve of the optimized devices. Previously, the origin of the hysteresis in planar n-i-p solar cells based on TiO2 was attributed to a contact resistance for electron transfer between perovskite and TiO2

26

. Here, we observed the same

phenomena for solar cells based on TiO2-NPs. However, modification of TiO2 with PC70BM leaded to solar cells with reduced hysteresis. Although out of the scope of this work, we fabricated a device based on a compact TiO2 layer (referred as C-TiO2) grown from a typical spray pyrolysis process as comparison. Such device reached a 13.36% champion PCE demonstrating the correlation between compactness of the ETL and the resulting device performance.

Figure 1. Enhanced planar perovskite solar cell based on a TiO2-NP electron transporting layer surface modified with PC70BM reaching a champion 18.4% efficiency. (a) Planar n-ip device structure. (b) Statistical dispersion of the obtained photovoltaic parameters. (c)


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External quantum efficiency and integrated Jsc. (d) Resulting 17.2% stabilized power output.

Table 1. Photovoltaic parameters of the optimized solar cell devices with TiO2-NP an TiO2-NP:PC70BM ETLs. Best values achieved are presented in parenthesis. J-V curves were recorded from forward bias (FB) to short circuit (SC) condition and vice-versa. ETL

Scan

Jsc

direction

(mA/cm2)

Voc (V)

FF (%)

FB to SC

21.82 ± 1.58 1.05 ± 0.02

72.98 ± 4.36

SC to FB

20.22 ± 2.20 1.05 ± 0.02

69.21 ± 5.94

FB to SC

13.78 ± 8.27 0.80 ± 0.33

SC to FB

5.79 ± 1.65

0.38 ± 0.07

26.59 ± 1.14

FB to SC

18.65 ± 1.01 1.01 ± 0.02

65.69 ± 4.15

SC to FB

17.92 ± 0.96 0.99 ± 0.01

42.79 ± 5.30

TiO2-NP: PC70BM

PCE (%) 16.69 ± 1.21 (18.39) 15.08 ± 1.25 (15.27)

44.10 ±

6.23 ± 6.47

17.42

(16.45)

TiO2-NP

C-TiO2

0.52 ± 0.16 (0.76) 12.49 ± 1.29 (13.36) 7.66 ± 0.96 (8.59)

TiO2-NP surface properties were deeply modified by the incorporation of PC70BM. While bare TiO2-NP showed an almost flat surface potential averaging 79 mV, the TiO2NP:PC70BM exhibited several fluctuations from 150 to 240 mV as shown in Figures 2A and 2B, respectively. Those fluctuations behave accordingly to the topography variations (Figure S3). In that sense, the PC70BM did not form a uniform layer, but induced a surface modification creating a network of interconnected fullerenes all over the TiO2 layer as observed in Figures 2B and S3. The uniformity of the PC70BM layer fully depended on the concentration of the PC70BM precursor. In fact, using a precursor more concentrated than 5 mg/mL, led to continuous layers as shown in Figure S3C. The optimal performance of the device using a low concentrated PC70BM (3 mg/mL) indicates a successful surface 5 ACS Paragon Plus Environment


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modification of the TiO2-NP layer without using PC70BM in excess. Conversely, PC70BM was able to cover possible pinholes in the bare TiO2-NP surface, which results in an improved interface with the perovskite. This result is consistent with the report by Conings et al., who state that fullerene is able to infiltrate a nanoporous SnO2 layer leading to passivation of the metal oxide and the blockage of possible shunt paths.28 In addition, the incorporation of PC70BM into the TiO2-NP was confirmed by evaluating the optical absorption presented in Figure S3B. We found that the PC70BM needed to interact with the TiO2-NP during several hours to reach a proper adsorption preventing its dissolution by the perovskite precursor solvent. At the end, the TiO2-NP surface showed a low 10° contact angle to the perovskite precursor (Figure 2C) which quickly reduced presumably due to infiltration into the nanoporous TiO2-NP layer. On the other hand, contact angle on the TiO2-NP:PC70BM was stable around 20°. Previous research demonstrated a close correlation between higher contact angles and improved optoelectronic quality of the grown perovskite films29 which indeed took place in this case as discussed in the following.

Figure 2. Surface potential maps of (a) TiO2-NP layer on ITO and (b) TiO2-NP:PC70BM layer on ITO. Corresponding contact angle of the perovskite precursor with the (c) TiO2-NP surface and the (d) TiO2-NP:PC70BM surface. Besides modifying the surface properties of the TiO2-NP layer, the PC70BM implementation also enhances the electronic properties of the perovskite grown atop. In 6 ACS Paragon Plus Environment

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fact, the charge extraction at the ETL/perovskite interface was enhanced by the TiO2NP:PC70BM layer while simultaneously the resulting grown perovskite exhibited surface donor states on its top interface. As a result, there is a subsequent accumulation of holes at the perovskite surface facilitating charge transfer towards the atop hole transporting layer (HTL). In first place, photocurrent analysis of the ITO/ETL/perovskite multilayer demonstrated a more uniform photocurrent collection when employing TiO2-NP:PC70BM as shown in Figure 3A. Enhancement in photocurrent evidences a better charge transfer across the perovskite/ETL interface.30 In opposite, the ITO/TiO2-NP/perovskite multilayer presents several grains with null photocurrent (Figure 3B). These results are in agreement with steady-state photo-luminescence (PL) measurements (Figure S4) showing higher quenching of perovskite PL signal when it is grown over the TiO2-NP:PC70BM layer. Accordingly, the PC70BM modification of the TiO2-NP surface contributes to maintaining photocurrent uniformity all over the perovskite interface. This result is consistent with results by Wojciechowski et. al., who found improved electron extraction from the perovskite as a result of fullerene modification of TiO2.4 On the other hand, surface photovoltage (SPV) analysis revealed the changes of the band bending at the surface of the perovskite. Variation of the SPV with the light intensity is shown in Figure 3C. As described elsewhere,31,32 the SPV sign is related to the type of surface states while its magnitude relates to the density of states. A positive SPV corresponds to donor states (surface downward band bending) which accumulate positively charge surface donors at the semiconductor surface, while a negative SPV corresponds to acceptor states (surface upward band bending) that accumulate electrons, as shown schematically in Figure 3D. In the case of the perovskite grown on TiO2-NP:PC70BM, positively charged surface donors accumulated at the top perovskite surface which is convenient for the subsequent hole transfer towards the Spiro-MeOTAD HTL because electrons are transferred from the HTL valence band to the surface donor states and from there to the valence band of the perovskite. An opposite behavior was found for the perovskite grown on bare TiO2-NP where electrons are accumulated in acceptor surface states. Consequently, effective hole transfer at the Spiro-MeOTAD/perovskite interface is hampered by the accumulation of negative charge on the perovskite surface. These changes in the perovskite SPV can be explained by the morphological and superficial differences of the underlying layer. As 7 ACS Paragon Plus Environment


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shown in Figure 3C, perovskite grown on bare TiO2-NP and ITO has negative SPV values, indicating presumably that pinholes on the TiO2-NP layer allows the contact of perovskite with the ITO electrode. In contrast, an almost flat SPV response was found for the perovskite grown on the TiO2-NP:PC70BM layer which also happened for the perovskite grown on the pin-hole free C-TiO2 layer confirming the influence of getting compact ETLs. Hence, the TiO2-NP:PC70BM layer effectively blocks the contact of perovskite with ITO and change the direction of the surface band bending of the perovskite. Moreover, the lower magnitude of the SPV for the perovskite grown on TiO2-NP:PC70BM indicated a reduction of surface trap density within the perovskite absorber. This flattening of the perovskite bands reduce the electrostatic barrier for hole transfer across the perovskite/HTL interface.4 These features have been pointed out as key factors to achieve highly efficient and hysteresis-free PSC.5 In order to provide insight into the physical mechanism behind the modifications in the electronic properties of the perovskite grown on TiO2-NP:PC70BM, the perovskite morphology and grain boundaries were evaluated. In first place, the resulting perovskite facets and crystalline structure depended on the underlying ETL, as shown in Figure S5. Since facets are closely related to the photovoltage mapping,33 modifications in the perovskite electronic behavior are clearly anticipated. On the other hand we verified that each grain boundary has its own SPV behavior. As shown in Figure S6, irrespective of the ETL employed each grain boundary presents a different SPV response referred to the average of the whole SPV map. Accordingly, it is not possible to assign a particular charge accumulation at the grain boundaries. From the resulting photovoltaic performance, we corroborate the influence of the TiO2-NP:PC70BM interface in determining a perovskite with higher optoelectronic quality. In summary, the PC70BM incorporation engineered both interfaces of the perovskite towards enhancing simultaneously the electron and hole extraction, which explains the achieved good performance of the device.


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Figure 3. Effect of the ETL on the superficial electronic properties of the perovskite. Photocurrent of the perovskite deposited on (a) TiO2-NP:PC70BM and (b) TiO2-NP layers, respectively. c) Dependence of SPV with light intensity for perovskite films grown on different ETLs. d) Schematic of the band bending at the surface space charge region (SCR) of the perovskite depending on the underlying ETL.

Finally, the potential of the studied low-temperature processed TiO2-NP:PC70BM layer was demonstrated in the fabrication of bendable solar cells. Figure 4 and Table 2 summarize the obtained photovoltaic parameters. Interestingly, a 12.2% SPO was achieved which is a record for TiO2-NP as ETL on a flexible substrate and comparable to the performance of flexible n-i-p devices fabricated using more complex deposition techniques such as atomic layer deposition.8 Accordingly, the presented methodology holds a great potential for scaling up n-i-p devices.



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Figure 4. Bendable solar cells. (a) Measured J–V curve for the flexible device PET/ITO/TiO2-NP:PC70BM/CH3NH3PbI3/spiro-MeOTAD/Ag. (b) Corresponding EQE spectrum and integrated JSC. (c) Stabilized current density and power output measured close to the maximum power point (∼0.8 V).

Table 2. Photovoltaic parameters of the optimized flexible solar cell devices based on the on the TiO2-NP:PC70BM ETL. Best values achieved are presented in parenthesis. Scan

Jsc

direction

(mA/cm2)

FB to SC

16.18 ± 1.16

1.03 ± 0.02

SC to FB

14.15 ± 0.89

1.02 ± 0.02

Voc (V)

FF (%)

PCE (%)

66.23 ±

11.02 ± 1.12

3.70

(12.21)

64.62 ±

9.32 ± 0.83

2.24

(10.86)

CONCLUSIONS In summary, we have developed a new method for the deposition of ETLs with enhanced optoelectronic properties. Our approach is based on the surface modification of a TiO2 layer derived from an ink of dispersed NPs and processed at low temperature (150 oC). Atomic Force microscopy (AFM) measurements show that fullerene interconnect the TiO2 NPs in the resulting TiO2-NP:PC70BM composite layer. Since fullerene results physisorbed into a network of TiO2 NPs, the perovskite precursor solvent does not remove it. Planar PSC devices based on the TiO2-NP:PC70BM layer reached PCEs up to 18.4% and SPO of 17.23%. To understand the origin of this high PCE, surface photovoltage (SPV) analysis of the perovskite surface was determinant. We demonstrate that surface modification of TiO2 10 ACS Paragon Plus Environment

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not only influence the properties at the ETL/perovskite interface but determines a proper hole transport layer (HTL)/perovskite interface. It was found that PC70BM surface modification change the direction of the surface band bending within the perovskite semiconductor and reduce its density of trap-states at the interface with the HTL layer. In consequence, TiO2 modification by fullerenes enables better charge transfer towards the ntype and p-type buffer layers. This study provides new insight into the mechanism of enhancement of n-i-p PSC by fullerene surface modification. Moreover, the TiO2NP:PC70BM was implemented in bendable n-i-p PHJ-PSCs devices achieving 12.2% SPO. The studied TiO2-NP:PC70BM layer opens a promising route for further perovskite photovoltaics scale-up and production of flexible PSCs.

EXPERIMENTAL METHODS Device fabrication Devices were fabricated on ITO coated glass (Naranjo). The substrates were washed with neutral soap (Inmunodet neutro) and sequentially sonicated in DI water, acetone and isopropanol. Then, they were treated with ultraviolet ozone for 5 min. The electron transporting layer was fabricated by spin-coating an aqueous 25 mg/mL nanoparticles dispersion (Anatase TiO2, average diameter of 5nm purchased from NanoAmor Inc.) at 3000 rpm for 30 s with a 3 s ramp, dried at 100 °C for 20 minutes. For the devices based on fullerene modified TiO2 film, a PC70BM solution in chlorobenzene (up to 20 mg/mL) was deposited onto the TiO2 layer by spin-coating at 2000 rpm for 30 s. The resulting TiO2-NP: PC70BM layers were left stand in the glove box for 12 hours in order to reach adsorption equilibrium of PC70BM on TiO2. For the devices based on a compact TiO2 layer, a C-TiO2 layer was deposited by spray pyrolysis of a mildly acidic solution of titanium isopropoxide in anhydrous ethanol (350 µL in 5 mL of ethanol with 0.016M HCl) sintered at 500 °C. The perovskite layer was then deposited by spin-coating a precursor solution of methylammonium iodide (Dyesol), lead iodide and dimethyl sulfoxide (DMSO) (1:1:1 molar ratio; 51% wt.) in N,N-dimethylformamide (DMF) at 4000 rpm for 25 s. During spinning, 500 µL of Diethyleter (Aldrich) were dripped on the substrate after 10 seconds. Under these conditions, the perovskite layer has a thickness of 400 ± 10 nm. The films were then annealed at 65 oC for 1 min plus 100 oC for 30 min. The 40 µL of spiro-MeOTAD 11 ACS Paragon Plus Environment


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solution, consisting of 72.3 mg spiro-MeOTAD (1-Material), 28.8 µL of 4-tert-butyl pyridine and 17.5 µl of lithium bis(trifluoromethanesulfonyl) imide (Li-TFSI) solution (520 mg Li-TSFI in 1 ml acetonitrile (Sigma–Aldrich, 99.8%)) in 1 ml of chlorobenzene, was spin-coated on the perovskite layer at 4000 rpm for 25 s. Finally, to complete the devices, 110 nm silver electrodes were thermally evaporated under vacuum (≈10 −6 mbar) at a deposition rate around ≈ 0.1 nm s−1.

Characterization The electrical characterization of the devices was performed using a 4200SCS Keithley system at a voltage swept speed around 400 mV/s in combination with an Oriel sol3A sun simulator, which was calibrated to AM1.5G standard conditions using an oriel 91150 V reference cell. The J-V curves were recorded from forward bias (FB) to the short circuit (SC) condition and vice-versa. Devices were masked with a metal aperture of 9 mm2 to define the active area. No device pre-conditioning was applied before starting the measurement. An Oriel IQE 200 was used to determine the external quantum efficiency. Surface roughness and thickness of the different layers was obtained using a Bruker DektakXT profilometer.

Atomic force microscopy analysis Surface potential and surface photovoltage (SPV) measurements where performed employing a MFP-3D AFM (Asylum Research) using dual-pass technique. Topography is mapped using tapping mode AFM during the first pass, which is then traced at a set lift height above the surface performing the surface potential measurement. During the second pass of Kelvin probe force microscopy (KPFM), the mechanical drive to the cantilever is disabled and an AC bias voltage (VAC = 1 V) is applied to the probe at the mechanical resonance of the cantilever. The VAC causes the cantilever to oscillate due to the attractive and repulsive electrostatic interaction between the probe and the sample.34 A feedback loop monitors and maintain constant the amplitude of the cantilever oscillations by applying a compensating VDC to the probe to cancel the probe-sample electrostatic forces. The tips used were Silicon Ti-Ir coated (Asyelec-01) with nominal spring constant of 2.89 N/m and resonance frequency of 71.7 KHz. The scan size was 5 µm at a scan rate of 0.5 Hz. 12 ACS Paragon Plus Environment

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Photoconductive AFM technique was carried out employing white LED illumination, in contact mode using a solid platinum probe (12Pt300B) with nominal spring constant of 0.81 N/m. The scan rate was 0.5 Hz. The contact potential difference Vc, between the tip and the perovskite sample surface, was measured in air conditions with controlled humidity. The contact potential Vc, between the AFM tip and the perovskite surface, was determined at different light intensities. From there, the induced surface photovoltage (SPV) defined as the difference between Vc-light and Vc-dark was calculated.

ASSOCIATED CONTENT Supporting Information Available: Photovoltaic performance of devices with different concentration of PCBM solutions, AFM images and UV-vis spectra of TiO2-NP:PC70BM layers; AFM, XRD and SPV characterization of perovskite grown on different ETL layers.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Note: The authors declare the no competing financial interests.

ACKNOWLEDGMENTS We thank the Colombian “Departamento Nacional de Planeación”, SGR collaborative project 2013000100184 between Empresas Públicas de Medellin, Andercol S.A., Sumicol S.A.S., and Universidad de Antioquia, for supporting this work. Authors thanks to Daniel Ramirez for the fabrication and characterization of PSC devices based on C-TiO2.

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Simultaneous Top and Bottom Perovskite Interface Engineering by

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