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Letter

Analysis of Photo-Induced Carrier Recombination Kinetics in Flat and Mesoporous Lead Perovskite Solar Cells Nuria F. Montcada, José Manuel Marin-Beloqui, Werther Cambarau, Jesus Jimenez-Lopez, Lydia Cabau, Kyung Taek Cho, Mohammad Khaja Nazeeruddin, and Emilio Palomares ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00600 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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ACS Energy Letters

Analysis of Photo-Induced Carrier Recombination Kinetics in Flat and Mesoporous Lead Perovskite Solar Cells Núria F. Montcada1, José Manuel Marín-Beloqui1, Werther Cambarau1, Jesús Jiménez-López1, Lydia Cabau1, Kyung Taek Cho3, Mohammad Khaja Nazeeruddin3 and Emilio Palomares1,2* 1

Institute of Chemical Research of Catalonia (ICIQ). The Barcelona Institute of Science and

Technology. Avinguda del Països Catalans 16, 43007 Tarragona, Spain. 2

Catalan Institution for Research and Advanced Studies (ICREA), Passeig de Lluis Companys

23, 08010 Barcelona, Spain. 3

Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and

Engineering, Ecole Polytechnique Fédérale de Lausanne, CH-1951 Sion, Switzerland.

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ABSTRACT:

In this work, we analyze the carrier recombination kinetics and the associated charge

carrier density in methyl ammonium lead iodide perovskite (MAPI) solar cells that use mesoporous TiO2 as selective contact (m-MAPI) and flat solar cells (without the mesoporous TiO2, f-MAPI), which are the most common device architectures for perovskite solar cells. The use of PIT-PV (Photo-induced Transient Photo-Voltage) and L-TAS (Laser Transient Absorption Spectroscopy) showed that for devices that cannot reach efficiencies close to 19% there is a slow component of the photovoltage decay that corresponds to a charge recombination pathway for carrier losses responsible for the lower device efficiency. Moreover, we do have also identified a primary interfacial charge recombination pathway for carrier losses that is common in all devices studied, independently of their efficiency or their device structure, which we have associated with the recombination reaction between electrons in the perovskite and holes in the organic semiconductor material used as the selective contact.

TOC GRAPHICS


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Carrier recombination processes govern the solar cell efficiency.1 In solar cells with materials that have high dielectric constant (compared to most of organics) such as silicon, CdTe, and MAPI perovskite, upon light irradiation electrons and holes (carriers) are directly generated and, later transported to the selective contacts. However, electrons and holes may recombine during transport or in the contacts, decreasing the number of collected charges (carriers) at the electrodes and, thus, limiting the overall solar cell efficiency. Thus, understanding and minimizing the existent pathways for electron-hole recombination is of utmost importance to increase the solar cell light-to-energy conversion efficiency. In MAPI solar cells literature there are a myriad of manuscripts studying different ways to fabricate the MAPI film2-5, describing the synthesis and characterization of novel hole transporting materials (HTM)2,

6-10

and employing different device structures and device

fabrication methods.11-16 Yet, there are only a handful of studies about the charge transfer dynamics in complete devices under solar cell working (illumination) conditions17-19. For instance, initial studies by our group showed that PIT-PV is a convenient technique to measure decay transients20. The decay transient is generated, while illuminating the solar cell with a constant white light source that generates an open-circuit voltage (VOC), with a fast (nanoseconds) light pulse. Moreover, the decay lifetime can be associated with the variation of the initial VOC and, hence, to the process involved in restoring the initial value. For example, in Grätzel solar cells and organic solar cells, the decay was directly related to the carrier recombination process of the excess of carriers promoted by the fast light pulse and, hence, it was a direct measurement of the carrier lifetime at a given voltage (or illumination intensity).21-22 Furthermore, in a recent work, O’Regan-Palomares-Barnes et al.23, have shown that in MAPI solar cells the PIT-PV decays were unexpectedly bi-exponential, at light intensities close to 1 sun


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conditions, with a fast component (τ = 1-2 µs) and a slower component of hundreds of microseconds. In fact, additional measurements of the solar cell carrier density using PIDC (Photo-Induced Differential Charging) made possible to reproduce the solar cell photocurrent by combining the fast decay component and the measured charge23. However, it was not possible to establish a clear correlation between the voltage decay components and the carrier recombination lifetime because the voltage decay may be also due to different physical processes as, for example, changes in the MAPI dipole moment after the light pulse exposure, which upon dipole rearrangement may lead to a change in voltage.24-28 Thus, still remain to determine the processes that lead to a bi-exponential transient decay in the PIT-PV measurement in MAPI solar cells. The novelty of this work is, therefore, that we have characterized for the first time the kinetics for the carrier losses in MAPI solar cells using mesoporous TiO2 as scaffold and electron selective contact. We compare different MAPI solar cells fabricated under identical conditions that differ only in the presence of the mesoporous TiO2 layer (described in detail in the SI) using the above-mentioned techniques (PIT-PV and PIDC). As shown below, the absence of the mesoporous TiO2 layer has a clear effect on the decay components of the PIT-PV, clarifying the nature of the fast component of the voltage decay. As discussed later on this work, the biexponential nature of the carrier recombination decay is correlated with the m-TiO2 coverage by the perovskite. Furthermore, the analysis of the L-TAS decay under identical illumination conditions used during PIT-PV measurements, allowed us to assign the slow component of the PIT-PV unequivocally. In fact, we demonstrate that both decay components result to be the consequence of two different carrier recombination processes in MAPI solar cells under operating conditions.


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MAPI solar cell characterization. Figure 1 illustrates the J-V curves measured at 1 sun simulated conditions (AM 1.5G, 100 mW/cm2) of a f-MAPI and a m-MAPI solar cells. In both cases the MAPI thickness was intended to be similar to allow a fair comparison, as can be seen in the SEM (Scanning Electron Microscopy) images (Figure S1a-b). We could observe, in those images, that the MAPI coverage over the TiO2 layer is quite different depending on the use of mesoporous or flat structure.

Figure 1. J-V curves for f-MAPI and m-MAPI solar cells measured in reverse scan (forward scan curves are shown in the SI. At a first sight, it is remarkable the difference in solar cell VOC (Table 1, Figure S2a in the SI) and the fact that the short-circuit current (JSC) is almost identical (∼20 mA/cm2) in both devices which implies that charge collection in both devices is comparable. It is also noticeable that these values are in the order of the measured photocurrents of some of the best-reported devices (∼2024 mA/cm2)2-3, 12-13. Moreover, the f-MAPI solar cell presents more hysteresis (see Table 1 and Figure S2b) in comparison with the m-MAPI device in good agreement with previous reports.29


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The origin of the hysteresis in MAPI solar cells and the fact that f-MAPI solar cells show more severely this characteristic is still under scientific debate.24, 28, 30-32 Table 1. Photovoltaic parameters of devices used in this study under standard (AM 1.5) 1 sun illumination. Cell*

Measure

JSC (mA·cm-2)

VOC (mV)

FF (%)

PCE (%)

m-MAPI

Forward

20.9

862

57.0

10.3 (9.1±0.2)†

Reverse

20.3

873

71.0

12.6 (10.2±0.2)

Forward

21.2

867

44.8

8.31 (6.4±0.2)

Reverse

21.3

941

62.8

12.6 (11.2±0.2)

f-MAPI

* Each value corresponds to the champion cell of the set. †The value in brackets is the average of 6 solar cells.

Photo-induced charge extraction (PICE). The PICE method has been already detailed in previous manuscripts for MAPI solar cells23, as well as, for Grätzel33 and organic solar cells34-35. In brief, it allows the measurement of the carrier density at a given light bias (solar cell voltage produced by continuous white illumination) (see details in the SI). From our past experience, in m-MAPI solar cells the PICE decay was much slower (milliseconds) than the PIT-PV (shown later on this work). As can be seen in Figure 2a, the PICE decay for m-MAPI solar cells is in the millisecond time scale as expected while, in contrast, the f-MAPI is in the microsecond time scale, even though the PICE decay was slower than PIT-PV they both belongs to the same timescale.


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a)

b)

c)

d)

Figure 2. VOC decays for m-MAPI (a) and f-MAPI (b) devices from PICE setup and m-MAPI (c) and f-MAPI (d) from PIT-PV setup. All decays were measured at 1sun conditions and for PIT-PV a perturbation of λex = 580 nm (67 µJ/cm2) was applied. See section S3 of SI for further details.

This result already remarks that the presence of the mesoporous scaffold slows down the charge extraction dynamics. Upon irradiation, in m-MAPI solar cells, charges (electrons) are transferred from the perovskite conduction band (CB) to the TiO2 CB. These charges are then transported through the nanocrystalline TiO2 nanoparticles to the contact that requires a trapping/de-trapping process that is inexistent in f-MAPI solar cells. However, it is feasible that other kind of charges (ions for example) can also move to compensate the electron transport but with slower kinetics that require more time to be extracted by PICE27. These facts may lead to a


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incorrect estimation of total electric charge extracted and forced us the use of an alternative method to obtain a representative charge density such the PIDC described above. Photo-induced differential charging (PIDC). Unlike the PICE, the PIDC has shown to be a useful alternative tool to measure the carrier density at different light bias. As detailed by Maurano et al.36, PIDC is a method that combines the data obtained from PIT-PV and PIT-PC (Photo-Induced Transient Photo-Current, see Figures S3f-g in the SI). In brief, the PIDC allows the calculation of the charge density from the differential capacitance of the device defined as C = (dQ/dt)/(dV/dt).23 This method assumes that the baseline of the resulting trend of the capacitance with voltage corresponds to the device geometrical capacitance. Thus, after subtracting this value and integrating over voltage, the calculation leads to a good estimation of the photo-generated charge density. The charge distribution at different light bias obtained by PIDC is depicted in Figure 3 for both f-MAPI and m-MAPI devices.

Figure 3. Charge distribution obtained using PIDC method for three f-MAPI devices (green triangles) and three m-MAPI devices (red circles).


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Photo-induced transient photo-voltage (PIT-PV).The PIT-PV measurement has been widely employed by our group28,29,33 and others37-38 to study VOC transients that can be directly correlated to carrier recombination dynamics. Figure 2c-d shows typical PIT-PV transients for fMAPI and m-MAPI solar cells. Unlike other PIT-PV measurements carried out in Grätzel and organic solar cells and conforming to our previous analysis in MAPI solar cells, the measured decays show a biexponential trend (See Figure S3d), indicating two different recombination processes with different kinetics. For f-MAPI solar cells the decay can be fitted to a bi-exponential function with time constants of τ1 = 0.9 µs and τ2 = 3.8 µs (although a mono-exponential fit would not generate a notable error), while for m-MAPI the values are τ1 = 1 µs and τ2 = 78 µs, where τ2 is 20 times larger than the one obtained for f-MAPI solar cell. It is important also to notice that in both transients the fastest decay is almost identical and therefore we could correlate the difference of the second (slower) decay to the presence of the mesoporous TiO2 layer. In our previous work we already quoted this fast component as “recombination” lifetime associated with a specific carrier density,23 unfortunately this could not be assigned to any specific carrier recombination process. Now, based on these last PIT-PV measurements and the L-TAS measurements we could assign this fastest component, for the first time, to the carrier recombination between electrons in the MAPI and holes in the HTM. In fact, we exclude it refers to the recombination within the MAPI material, since it occurs in the nanoseconds range as demonstrated by other groups.39,40 All of that supports our initial assumption, correlating the slow decay component to a charge recombination process between electrons in the m-TiO2 layer and holes in the HTM. This makes


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us hypothesize an alternative pathway for the charge carriers, most probably due to a direct contact between the layers of TiO2 and the HTM. FIB-SEM (Focus Ion Beam-Scanning Electron Microscopy) images were taken to corroborate this hypothesis (see SI) confirming that both flat and mesoporous devices present relatively large uncovered zones in the perovskite layer that are subsequently filled with the Spiro-OMeTAD (HTM) and, thus the interface HTM/TiO2 layer is created. Such percolation pathways for carrier recombination are more evident in the mesoporous TiO2 than in the TiO2 flat devices. For the sake of comparison, we analyzed an outstanding (external) device, previously published,41 fabricated using the same structure as m-MAPI described in this manuscript in order to ensure a full-coverage of the m-TiO2 by the perovskite (the characterization of this device is shown in detail in the SI). The PIT-PV analysis of this fully-covered device (Figure S3e) shows a completely monoexponential decay with a time constant of τ = 1.6 µs at 1 sun light intensity, which is in the same time scale as the fast component of both decays for m-MAPI and f-MAPI devices examined in this work. Laser Transient Absorption Spectroscopy (L-TAS). The L-TAS technique (See SI) allows to determine unambiguously the species formed upon light excitation of the solar cells and their lifetime. In this case, we monitored the presence of positive polarons in the HTM (SpiroOMeTAD+) upon photo-excitation at, which correspond to the maximum absorbance of the Spiro-OMeTAD+, and measure the recombination kinetics of this Spiro-OMeTAD+ species with electrons stored in the TiO2 as illustrated in Figure 4. On the other hand, upon irradiating the mMAPI solar cells with 1 sun equivalent background white light we measured the PIT-PV decay and monitor the L-TAS decay at λ = 1400 nm. Furthermore, the L-TAS decay also fits with the


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ACS Energy Letters

slower PIT-PV decay measured for the solar cell at a light bias equivalent to 1 sun. Thus, we conclude that the process that origins the slow component of the PIT-PV is the charge recombination between the electrons in the m-TiO2 and the holes in the Spiro-OMeTAD.

Figure 4. PIT-PV transient decay at 1 sun light irradiation of an f-MAPI (green) and m-MAPI (red) solar cell directly compared to an L-TAS decay (black) of a MAPI/Spiro-OMeTAD film grown over the mesoporous TiO2 layer with 1 sun background (λex = 500 nm and λpr = 1400 nm). Charge recombination lifetime vs carrier density. After identifying the origin of the slow component in the PIT-PV decays and then discarding it as any recombination process that involve the MAPI layer, we focus the attention on the fast component, which seems a common feature in PIT-PV decays for solar cells with moderate or high efficiency. Figure 5 shows a comparison between the carrier lifetime vs charge density, for all the devices studied herein.


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Figure 5. Fast PIT-PV decay components for f-MAPI (green), m-MAPI (red) and m-MAPI with full perovskite coverage (blue) vs carrier density from PIDC. As can be seen, the carrier lifetimes expand from hundreds of microseconds to a few microseconds at high charge density (higher illumination intensities). We believe that the process related to these measured kinetics is the interfacial charge recombination between the electrons in the perovskite and the holes in the HTM. In conclusion, two different MAPI solar devices using Spiro-OMeTAD as the HTM have been fabricated, using the two most common architectures reported in bibliography, flat and mesoporous structures, incorporating in this last case the mesoporous TiO2 layer as an additional layer for the selective contact electrode. PICE was performed lead to great differences in the measured charge densities and some limitations of the technique were detected, thus, as an alternantive, PIDC was measured to calculate the carriers stored in the device. The PIT-PV measurements were carried out obtaining two very different transient decays. Both transients could be fitted to a bi-exponential decay: for m-MAPI devices the slow component was found to be 20 times slower than in f-MAPI and, on the contrary, the fast component is almost equal in both cases.


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In fact, we have demonstrated that the slowest component of the carrier lifetime in m-MAPI corresponds to the carrier recombination between electrons in the mesoporous TiO2 and holes in the HTM due to a non-homogeneous perovskite capping layer as supported by the SEM characterization of the films. Furthermore, L-TAS measurements carried out under same conditions as for the PIT-PV experiments registered an identical profile for the slow component of the decay pointing out again a charge transfer process between TiO2 and HTM. Finally, comparing the PIT-PV measurements for both the architectures presented in this work and supported by a “fully-covered” device characterization, we can conclude that the fast component of the PIT-PV decay is related to the carrier recombination process between the electrons in the MAPI and the holes in the HTM, a primary interfacial pathway for carrier losses in hybrid perovskite solar cells.

ASSOCIATED CONTENT Supporting Information. The characterization of the devices such as the experimental section, the FIB-SEM images, J-V characterization, Photo-induced time-resolved characterization, XRD diffractogram of the MAPI, and all the characterization of the fully covered m-MAPI device are compiled in this section. AUTHOR INFORMATION Corresponding Author * (E. Palomares) E-mail: [email protected]; Fax: +34 977 920 823; Tel: +34 977 920 200 Author Contributions


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The manuscript was written through contributions of N. F. Montcada, J. M. Marín-Beloqui, W. Cambarau, L. Cabau and E. Palomares. Some of the data to complete this work was provided by J. Jiménez-López, K. T. Cho and M. K. Nazeeruddin. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors would like to thank MINECO for projects CTQ2013-47183 and the Severo Ochoa Excellence Accreditation 2014-2018 (SEV-2013-0319). E. Palomares also thanks AGAUR for the SGR project 2014 SGR 763 and FICIQ and ICREA for economical support. MN and KC thank the Swiss National Science foundation under NRP 70, grant N°: 407040_154056. The authors would like to thank Dr. Joaquim (Quim) Puigdollers for the FIB-SEM measurements.

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