Elucidation of Charge Recombination and Accumulation Mechanism

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Elucidation of Charge Recombination and Accumulation Mechanism in Mixed Perovskite Solar Cells Pankaj Yadav, Silver Hamill Turren Cruz, Daniel Prochowicz, Mohammad Mahdi Tavakoli, Kavita Pandey, Shaik Mohammed Zakeeruddin, Michael Grätzel, Anders Hagfeldt, and Michael Saliba J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03948 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Journal of Physical Chemistry

Elucidation of Charge Recombination and Accumulation Mechanism in Mixed Perovskite Solar Cells Pankaj Yadav,*1,3 Silver-Hamill Turren-Cruz,2 Daniel Prochowicz,3,4 Mohammad Mahdi Tavakoli,3,5,6 Kavita Pandey,7 Shaik M. Zakeeruddin,3 Michael Grätzel,3 Anders Hagfeldt2 and Michael Saliba*7 1

Department of Solar Energy, Pandit Deendayal Petroleum University, Gandhinagar 382007, Gujarat, India. E-mail: [email protected]

2

Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland. 3

Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.

4

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland.

5

Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA02139,USA. 6

Department of Materials Science and Engineering, Sharif University of Technology, 14588 Tehran, Iran.

7

Adolphe Merkle Institute, Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland. E-mail:

[email protected]

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ABSTRACT Organic-inorganic perovskite solar cells (PSCs) have gained considerable attention owing to their impressive photovoltaic properties and simple device manufacturing. In general, PSC employs a perovskite absorber material sandwiched between an electron and hole selective transport layer optimized with respect to optimal band alignment, efficient charge collection, and low interfacial recombination. The interfaces between the perovskite absorber and respective selective contacts play a crucial role in determining photovoltaic performance and stability of PSCs. However, a fundamental understanding is lacking and there is poor understanding in controlling the physical processes at the interfaces. Herein, we investigate the interfacial characteristics of PSCs with both planar and mesoporous architecture that provide a deeper insight into the charge recombination and accumulation mechanism, and the origin of open-circuit voltage (Voc). The effect of electron- and hole-selective contacts in the final cell performance of PSCs has been analyzed by impedance spectroscopy and Capacitance-Frequency analysis. This study demonstrates that the excess of charges accumulation under illumination in planar based devices is responsible for the origin of Voc and hysteresis phenomena.

INTRODUCTION In the last two decades various types of solar cells such as organic heterojunction solar cells and dye sensitized solar cells have been widely studied. However, the low power conversion efficiency and comparably poor stability prevented broad-scale commercialization. Recently, hybrid organic-inorganic perovskite solar cells (PSCs) have received significant research attention due to their impressive power conversion efficiency (PCE) with certified values of 22.1%.1 Major improvements were made in as well as interface engineering which provided excellent photovoltaic parameters. However, many fundamental physical and electrical processes during device operation remain poorly understood.1-5 Particular attention has been 2 ACS Paragon Plus Environment

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paid to those processes occurring at interfaces between the perovskite absorber layer and the carrier transport layers. This is because the interfacial phenomena at the perovskite/electron selective contact interface can lead to hysteresis that is responsible for the difference in opencircuit voltage (Voc) between forward and reverse scans in the current-voltage (J-V) curve. The origin of J-V hysteresis has been attributed to the charge accumulation at the interface of the perovskite thin film, ion migration, and/or ferroelectric polarization.6-11 The hysteresis behaviour not only depends on the intrinsic characteristic of perovskite absorber material, but also on the selective contacts (for example mesoporous or planar configuration), sweep rate, and poling conditions.12-14 However, recent work by Juan et al. shows that the extend of charge accumulation at electron transporting layer (ETL) and perovskite interface can also govern the Voc in parts along with the main contribution from ETL-perovskite junction.15-16 In particular, the charge accumulation process is connected with the changes in interfacial energetics that ultimately defines the Voc of devices. Notably, the processes that lead to differences in Voc between the main kinds of PSCs architectures i.e. planar and mesoporous, have been rarely reported in literature. In this context, it is crucial to discuss and understand the specific interfacial electronic properties between selective contacts and the absorber layer. Herein, we investigate the interfacial characteristics of various types of PSCs that provide a deeper insight into the recombination and charge accumulation mechanism as well as the origin of Voc. PSCs with planar and mesoporous architecture containing different electron and hole transporting layers were fabricated to systematically study the interfacial characteristics. Moreover, the capacitance-frequency analyses under dark and illumination conditions were used to elucidate the evolution of Voc and interfacial charge accumulations. Our results demonstrate that the excess of charge accumulation after illumination in both mesoporous and planar based devices is responsible for the origin of Voc and hysteresis phenomena. RESULTS AND DISCUSSIONS 3 ACS Paragon Plus Environment


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Figure 1a and b show schematic representations of the mesoporous and planar architecture of PSCs, respectively. In this study, we used mesoporous device architecture of FTO/bl-TiO2/ms-TiO2/perovskite layer/hole transporting material/Au. We employed the mixed-halide and mixed-cation formulation Cs5(MA0.17FA0.83)95Pb(I0.83Br0.17)3

as a

perovskite absorber layer and two different hole transporting materials (HTMs) i.e. spiroOMeTAD or PTAA.17 For the remainder of this work, mesoporous titania devices with spiroOMeTAD and PTAA were referred to as Meso_S and Meso_P, respectively. To determine the role of electron transporting layer, we have also prepared TiO2- and SnO2-based planar PSCs denoted as planar_TiO2 and planar_SnO2, respectively. In these devices, only spiro-OMeTAD was used as a HTM. For more details of the device fabrication see the Supporting Information (SI).

Figure 1. a) Schematic representation of mesoporous devices with Spiro-OMeTAD and PTAA as the hole transporting layer. b) Schematic representation of planar devices with TiO2 and SnO2 as the electron transporting layer. c) J–V curves from reverse scan for the devices 4 ACS Paragon Plus Environment

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recorded under standard illumination of AM 1.5 G and d) the corresponding dark J–V characteristics.

Film characterizations of perovskite absorber layers deposited on mesoporous and planar ETLs are shown in Figure S1. To fabricate perovskite film on top of planar and mesoporous TiO2 ETLs, we have employed anti-solvent method. Top-view SEM images of perovskite for both case show similar morphology with almost same grain size. The optical and structural property of synthesized perovskite absorber layer on planar and mesoporous TiO2 ETLs were also studied by using UV-Vis, PL and XRD measurements. We found that deposition of perovskite on mesoporous or on planar substrates has no significant influence on the optical and structural properties. From the detailed optical and structural characterization techniques we observed that the deposition of perovskite on either planar or mesoporous TiO2 does not alter the properties of the perovskite layer. However, the crystallization of perovskite within the pores of the meso-TiO2 is very different to that of planar TiO2 interface which may be one of the source to observe difference in these solar cells. The current-voltage (J-V) characteristics of devices under 1 sun illumination (AM 1.5G) are shown in Figure 1c (Table S1 shows all corresponding performance parameters). The efficiencies of these devices are comparable with a spread of ± 2%. We found that the devices with planar configuration exhibit a higher Voc and a lower fill factor (FF) when compared to PSCs with a mesoporous configuration. Moreover, a slight difference in Voc is observed in both planar PSCs with different ETL and mesoporous PSCs with different HTMs. We note that the change in Voc was only observed when ETL characteristics were modified or altered. Focusing on the major difference in Voc between planar and mesoporous PSCs, statistics of 38 devices are summarized in Figure S2. This signifies that major contribution for interfacial recombination stems from the perovskite/ETL junction.18-22 A recent study by Wang has shown that accumulation of charge at perovskite/HTM interface is also take place if there is valance band offset. Authors have shown that HTM doped with cobalt exhibits minimum charge 5 ACS Paragon Plus Environment


accumulation which is consistent with present study also.23 In the present study all the samples are made in a same batch under identical condition with the only difference in ETL layers. Therefore observed difference in device photovoltaic characteristics is mainly from ETL/perovskite interface, this claim is further supported by capacitance-frequency (C-F) measurements discuss in next section. To investigate the recombination mechanism, we measured the dark J-V characteristics at a sweep rate of 50 mV/s from 0 to VOC for all devices. The ideality factor (n) was obtained by using the expression:

This formula is referred to as (Exp. 1) where the equation is only valid for the exponential region of log-log (J-V) curve (for reference see inset of Figure 1c). The dark J-V curve of PSCs with planar structure shows a lower dark current and a higher knee voltage compared to PSCs with mesoporous structure. This was attributed to lower recombination and distinct interfacial energetics. From the exponential region (See Figure 1d inset and Figure S2) of J-V characteristics and based on Exp. 1, we calculated an approx. value of n as 1.8, 1.9 for Planar_SnO2, Planar_TiO2, and 2.2, 2.1 for Meso_P and Meso_S devices, respectively. In general, n of the solar cell refers to the recombination phenomena. The observed differences in the n-value signifies that the recombination in devices is mainly dependent on the nature of the selective contacts. The reverse saturation current (J0) of 7.21e−13, 4.89e−12, 2.81e−10 and 9.83e−11 A/cm2 were obtained for Planar_SnO2, Planar_TiO2, Meso_P and Meso_S, respectively. The J0 current or leakage current in solar cell defines the interface quality and are in consistent with the findings of n. To get further insight into recombination mechanism in Meso and Planar devices by means of n, Voc of devices as a function of illumination is measured and shown in Figure S3. A lower value of n in planar devices illustrates a lower recombination in these devices.


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Figure 2. Logarithmic plot of capacitance versus frequency for the investigated devices measured under 1 sun illumination at zero applied potential.

Capacitance-frequency (C-F) measurements are commonly used to determine the processes at interfaces between the perovskite absorber and the selective contacts as well as at the perovskite bulk.24-25 Figure 2 shows the C-F plot measured under 1 Sun illumination at zero bias voltage. In general, the low frequency capacitance was attributed to interfacial charge accumulation due to the external electric field or illumination conditions. However, the mid frequency and high frequency capacitance corresponds to chemical and dielectric polarization of the perovskite absorber. As shown in Figure 2, Meso_P and Meso_S PSCs having the same ETL but with different HTL, exhibits almost the same capacitance value and frequency dependence indicating that the low frequency response is independent of HTL/perovskite interface. Recent studies by Gottesman et al. demonstrated that the interfacial charge accumulation, predominantly occurring at the perovskite/electron selective contact interface, affects the space charge layer and/or charge collection efficiency.21 The higher values of low frequency capacitances were observed for PSCs with planar structure compared to PSCs with mesoporous structure. We note that under dark condition, the interfacial capacitance is mainly 7 ACS Paragon Plus Environment


dominated by a Helmholtz layer and the excess of photogenerated charge carriers are negligible.24 Therefore the low frequency capacitance should be lower than the corresponding capacitance measured under illumination. We found that the low frequency capacitances measured under dark condition are comparable for PSCs with planar and mesoporous structure (see Figure S4). This indicates that under illumination the excess of photogenerated charge carriers are more prone to accumulation at interfaces for devices with planar structure. The role of these accumulated charge carriers on device characteristics is discussed in further section. The accumulation of charge carriers can also be extracted from the slope of C-V plot in a semi-log scale. Figure S5 shows the C-V plot measured under illumination for all fabricated PSCs. Indeed, we observed a high value of capacitance for PSCs with planar structure suggesting that the perovskite/ETL interface has a tendency to accumulate higher charge carriers. The exact phenomena of charge carriers accumulation in relation with nature of selective contact needs further investigations. Further evidence for higher charge carrier accumulation in planar devices is evident from the observed higher hysteresis effect because accumulated charge carriers create a surface dipole at the interface affecting the charge dynamics (see Figure S6). This illustrates that the accumulated charge carriers at perovskite/ETL interface under illumination can be responsible for the hysteresis behaviour in planar devices, but not due to the perovskite absorber alone.


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Figure 3. Nyquist spectra at zero bias under 1 sun illumination for fabricated a) mesoporous and b) planar based devices. c) Equivalent circuit to fit the Nyquist spectra and d) variation of recombination resistance as a function of applied voltage for planar_TiO2, planar_SnO2, Meso_S and Meso_P devices.

Figure 3 shows the Nyquist spectra of fabricated devices at zero bias and under 1 sun illumination (for a detailed bias dependent Nyquist spectra under illumination and dark conditions see Figure S7 and S8). A broad distinguishable characteristic of Nyquist spectra between both types of PSCs is observed. In particular, the PSCs exhibit a semicircle at high frequency region and second incomplete arc at low frequency region. The observed lower high frequency resistance and higher low frequency capacitance for PSCs with planar structure is related to interfacial contact area between perovskite and ETL.26-30 The devices measured under dark condition exhibit a semicircle at high frequency spectra region followed by a straight line in the low frequency spectra region, which undergoes a major change after 9 ACS Paragon Plus Environment


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illumination (see Figure S7 and S8). As the resulting PSCs show similar characteristics measured under dark condition and to have a better comparison, we only consider the data measured under illumination. The J-V characteristics of PSCs were measured before and after impedance spectroscopy measurements to confirm the device stability (Figure S9). By keeping the devices overnight under dark condition in a dry box, we could restore the initial performance. In our recent work, we have shown the various favourable and non-favourable conditions necessary for device restoration.31-32 Next, we fit the obtained impedance spectra by using electrical equivalent circuit shown in Figure 3c and example of fittings are shown in Figure S10. The electrical equivalent circuit consists of series resistance RS due to contacts, resistance RHF due to transport phenomena, RLF (Rrec) due to recombination resistance, CHF represent geometrical or bulk capacitance and CLF defines accumulation capacitance. From the high frequency semicircle arc, lower values of real impedance for the Meso_P and Planar_SnO2 devices suggest a better charge transport at perovskite/ETL interfaces. The smaller semi-circle appeared at high frequency in planar cells can also be refers to the high density of charges in the bulk of perovskite film. Consequently, one of the roles of selective contact is to improve the charge transport across the device. Recently the work by Zarazúa et al. shows

that high- and low-frequency

resistance are correlated with the same voltage dependence and corelated to recombination meachanism.33 However, this discussed phenomena generally takes place at high forward bias. Where in low forward bias a voltage independent characteristic of RHF is observed (See Figure S11) and correlated to the transport characteristics of device. An in-depth discussion on the high frequency resistance is done by Zhu et.al.

34-35

To further confirm that the high

frequency arc is associated with conductivity of device, mesoporous PSCs with MAPbI3 were investigated through IS by using Spiro-OMeTAD as HTL; with and without additives. Figure S12 clearly shows that Spiro-OMeTAD with additive improves the conductivity and reduces the real magnitude of the high frequency arc. Moreover, apart from improved conductivity of 10 ACS Paragon Plus Environment

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HTM, in the present study the Planar_SnO2 device also shows a lower magnitude of the real part compared to the Planar_TiO2 device. This signifies that detailed analysis of the high frequency spectra allows determining the device conduction or transport characteristics. However, it is challenging to get separate IS features corresponding to HTL or ETL conductivities. Another effective tool for characterization and analysis of the ETL/perovskite interface is photoluminescence and time resolved photoluminescence (TRPL). As shown in Figure S13, planar SnO2-based device has a lower PL emission intensity compared to planar TiO2-based device. This is attributed to a lower recombination at SnO2/perovskite junction. Moreover, as shown in Figure S1, SnO2 based device shows shorter lifetime than its TiO2 counterpart, resulting in a more effective quenching effect at interface. Interestingly, during the transition from the low to high frequency region, an inductive loop appeared at mid frequency in the case of planar devices. A recent studies attributes this to good charge extraction at the contact, which is consistent for the planar devices that do exhibit better charge separation (or extraction) compared to mesoporous PSCs. Figure 3d shows the recombination resistance (Rrec) from the fitting of low frequency IS or Nyquist spectra as a function of applied bias for all PSCs. The planar PSCs exhibit higher Rrec values close to open-circuit voltage, which corresponds to higher Voc, whereas at low forward bias, a lower Rrec value in these devices can limit FF.19, 26 Therefore, in the present case of having the same perovskite absorber layer, but different type of selective contacts, the observed difference in Rrec demonstrates the role of the selective contact on the solar cell performance by affecting Voc and FF. This demonstrated that absolute difference in Rrec for planar and mesoporous based devices should explain the obtained VOC. IS measurements at open-circuit voltage and at different light intensities were done for Meso and Planar devices and are shown in Figure S14. A higher value of Rrec is obatained for Planar devices



illustrates the lower recombination in these devices.These observations are in consistent with the finding of J-V characteristics measured under dark.

CONCLUSIONS In conclusion, we have studied the charge recombination and accumulation mechanisms in mixed PSCs consist of mesoporous and planar configurations. To elucidate the charge recombination and accumulation in the resulted PSCs J-V, Impedance spectroscopy and capacitance-frequency measurements under dark and illumination are carried out. The J-V measurements under dark condition shows that the recombination in devices is mainly depend on the nature of selective contacts. The capacitance-frequency measurements under dark condition exhibits comparable low frequency capacitances for planar and mesoporous PSCs. Whereas, under illumination the excess of photogenerated charge carriers are more prone to accumulation at interfaces for devices with planar structure. We show that these accumulated charge carriers at perovskite/ETL interface under illumination can be responsible for hysteresis behaviour in planar devices, but not due to the perovskite absorber alone. We also found that these accumulated charges at ETL/perovskite interface in planar based devices are found to be responsible phenomena for the observed higher Voc. These results provide a way to deep understanding and analysis of PSCs as well as highlight the importance of interfaces in PSCs. Further works on the modification of ETL in planar PSCs to suppress the current leakage paths leading higher FF and PCE are in progress.

SUPPORTING INFORMATION Experimental section on fabrication and characterization of perovskite solar cells, SEM, UVVisible spectra and PL spectra of perovskite, semi-log plot of dark J-V characteristics, Logarithmic plot of capacitance versus frequency, Capacitance versus voltage plot, Forward


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and reverse J–V characteristics of perovskite solar cells, Nyquist spectra under different conditions

ACKNOWLEDGEMENTS D. P. acknowledges support from the funded Marie Skłodowska Curie fellowship, H2020 Grant agreement no. 707168. M. S. acknowledges support from the co-funded Marie Skłodowska Curie fellowship, H2020 Grant agreement no. 665667. K. P. acknowledges the funding from SERB Overseas Postdoctoral fellowship, Govt. of India.

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Elucidation of Charge Recombination and Accumulation Mechanism

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