Letter pubs.acs.org/JPCL
Charge Injection at the Heterointerface in Perovskite CH3NH3PbI3 Solar Cells Studied by Simultaneous Microscopic Photoluminescence and Photocurrent Imaging Spectroscopy Daiki Yamashita,†,‡,∥ Taketo Handa,†,∥ Toshiyuki Ihara,† Hirokazu Tahara,† Ai Shimazaki,† Atsushi Wakamiya,† and Yoshihiko Kanemitsu*,† †
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Department of Physics and Electronics, Osaka Prefecture University, Sakai, Osaka 599-8570, Japan
‡
S Supporting Information *
ABSTRACT: Charge carrier dynamics in perovskite CH3NH3PbI3 solar cells were studied by means of microscopic photoluminescence (PL) and photocurrent (PC) imaging spectroscopy. The PL intensity, PL lifetime, and PC intensity varied spatially on the order of several tens of micrometers. Simultaneous PL and PC image measurements revealed a positive correlation between the PL intensity and PL lifetime, and a negative correlation between PL and PC intensities. These correlations were due to the competition between photocarrier injection from the CH3NH3PbI3 layer into the charge transport layer and photocarrier recombination within the CH3NH3PbI3 layer. Furthermore, we found that the decrease in the carrier injection efficiency under prolonged light illumination leads to a reduction in PC, resulting in light-induced degradation of solar cell devices. Our findings provide important insights for understanding carrier injection at the interface and lightinduced degradation in perovskite solar cells.
I
of inhomogeneous thin-film devices. Using microscopic photoluminescence (PL) measurements, the local optoelectronic properties of perovskite solar cells, including electron/ hole transport materials, have been studied.29 A combination of different optical techniques is more effective for a thorough understanding of their optical and electronic properties.11,30 In particular, photocurrent (PC) measurements can be used to directly evaluate charge carrier flow through the whole device under light illumination. Simultaneous PL and PC measurements clarify the recombination and transport processes of photogenerated carriers, and spatial image measurements are invaluable for spatial and statistical evaluation of the characteristics of solar cells. In this work, we performed microscopic imaging of PL and PC for CH3NH3PbI3 solar cells at room temperature. The PL intensity, PL lifetime, and PC intensity varied spatially for distances on the order of a few tens of microns. We found a positive correlation between PL intensity and PL lifetime, and a negative correlation between the PL and PC intensities. These correlations are determined by injection of the photocarriers from the perovskite absorption layer into the charge transport layers. Furthermore, prolonged light illumination causes a decrease in the carrier injection efficiency at the interface, leading to a reduction in the PC. We also discuss the
n recent years, solution-processed solar cells based on metal halide perovskite semiconductors, CH3NH3PbX3 (X = Cl, Br, I), have attracted much attention owing to their excellent photovoltaic properties.1−9 In particular, CH3NH3PbI3 thin films and devices have been intensively studied, and solar cells with power conversion efficiencies of over 20% have been reported so far.9 Many experimental and theoretical studies have clarified the intrinsic properties of CH3NH3PbI3 materials in thin film and bulk crystal forms. A high optical absorption coefficient resulting from the direct-gap band structure,10−12 the presence of free carriers at room temperature,13−17 a large carrier diffusion length,18−24 and photon recycling25,26 have been cited as reasons for the high power conversion efficiencies of CH3NH3PbI3-based solar cells. Although the intrinsic carrier behavior of CH3NH3PbI3 materials has been extensively studied, carrier recombination and transport processes during the operation of an actual diode device need to be clarified in order to further improve the power conversion efficiency and durability of the solar cells and light-emitting diodes. However, the carrier transport and recombination dynamics even in practically used singlecrystalline solar cells with heterojunctions are complicated.27,28 In addition, solution-processed polycrystalline thin-film devices exhibit spatial nonuniformity in their optical and electronic properties29−31 because of the nonflat microstructures of the perovskite grains, the mesoporous TiO2, and the lowtemperature spin-coating preparation method used. Microscopic spatially resolved optical spectroscopy is one of the most powerful methods for evaluating the optoelectronic properties © XXXX American Chemical Society
Received: June 6, 2016 Accepted: August 2, 2016
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The Journal of Physical Chemistry Letters mechanism underlying the light-induced degradation of solar cell devices. The CH3NH3PbI3 solar cells were fabricated by a two-step solution method for the perovskite layer, which contained mesoporous TiO2 structures and spiro-OMeTAD with an additive (LiTFSI salt) as the hole transporting layer (the details of the fabrication method and typical layer thickness are given in the Supporting Information).32,33 Figure 1a shows the
Figure 2. (a) PL decay curves for excitation at 688 nm. Red and blue curves were measured at points A and B of (b)−(d), respectively. The inset shows the PL spectrum from a perovskite solar cell. Spatial images of (b) the average PL lifetime (τave), (c) the time-integrated PL intensity (IPL), and (d) the PC intensity (IPC).
Figure 1. (a) I−V curves for AM 1.5G of a representative solar cell used in this work, recorded under forward (red) and reverse (black) scan. (b) PL image of the solar cell for excitation at 650 nm. (c) Image of the mesoporous TiO2/compact TiO2 on a glass substrate taken by an optical microscope.
microscopic PL spectrum obtained by photoexcitation with pulsed pumping at 688 nm. The spectral shape is approximately described by a Gaussian function with a center wavelength of 778 nm. PL from the solar devices is identical to that from the bare perovskite layer. The PL decay curves are nonexponential and are approximately described by a double exponential function (see Supporting Information); we obtained the average PL lifetime (τave) by double exponential fitting. The τave values calculated from the data for regions A and B in Figure 2a were 4.26 and 3.32 ns, respectively. These lifetimes are much shorter than the PL lifetime of the bare perovskite thin films, indicating the importance of carrier extraction from the perovskite layer in the multilayer structure.18,19 To clarify the reason for the shorter PL lifetimes in the photovoltaic devices, we conducted simultaneous PL and PC measurements for each point of the device and obtained spatial images. Figure 2b−d shows the spatial image of τave, the timeintegrated PL intensity (IPL), and PC intensity (IPC). We confirmed the high reproducibility of these measurements (see Figure S2 of Supporting Information for details). In all images, we observe a stripe structure on the order of a few tens of microns, which reflects the structural inhomogeneity of the perovskite solar cells, resulting from sample preparation by the spin-coating method. In Figure 3a,b, we found a positive correlation between IPL and τave, and a negative correlation between IPL and IPC, respectively. To evaluate these correlations in a more quantitative manner, we calculated the Pearson’s product-moment correlation coefficient, which is expressed by the equation35
current−voltage (I−V) curves of the prepared CH3NH3PbI3 solar cell device under AM 1.5G illumination. The red and black curves show the forward and reverse scans (scan rate: 0.05 V/s), respectively. No significant hysteresis behavior was observed in our devices. The short-circuit current (ISC), opencircuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE) were 20.7 mA/cm2, 0.98 V, 0.66, and 13.4%, respectively. Figure 1b shows a PL image of the solar cell for excitation at 650 nm. In all PL images, we observed a pattern radiating in all directions from the center of the solar cell. Figure 1c is the image of a sample consisting of mesoporous TiO2/compact TiO2 on a glass substrate taken by an optical microscope, which clearly exhibits the same stripe pattern as the PL image. Furthermore, Figure S1 in the Supporting Information confirms that the PL image of the CH3NH3PbI3 film on a quartz substrate exhibits no radial pattern. From these results, we conclude that the observed PL pattern in Figure 1b is a result of the inhomogeneous mesoporous TiO2 layer fabricated by spincoating. A similar stripe pattern is observed in the electroluminescence (EL) images.34 Since structural inhomogeneity exists in the perovskite solar cells, we performed microscopic PL and PC measurements (see Experimental Methods). For the PL lifetime measurements, a conventional time-correlated single photon counting method was used. By conducting simultaneous measurement of the PL intensity, lifetime, and PC intensity, the spatial distribution of the microscopic structure and the unique properties of each structure can be elucidated. Figure 2a shows the measured PL decay curves for the CH3NH3PbI3 layer of the solar cell device under short-circuit conditions. The red and blue PL decay curves were measured at points A and B indicated in Figure 2b−d, respectively. Points A and B are representative regions showing long and short PL lifetimes, respectively. The inset in Figure 2a shows the
n
r=
∑i = 1 (xi − x ̅ )(yi − y ̅ ) n
n
∑i = 1 (xi − x ̅ )2 ∑i = 1 (yi − y ̅ )2
(1)
where xi and yi represent the corresponding points of the two data sets, and x̅ and y ̅ indicate the averages of each data set. The 3187
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area with small IPC and large IPL, we consider the possibility of an interfacial barrier or a poor transport in the bulk transport layer. For a deeper understanding of the carrier injection and recombination behaviors in solar cells, we studied the spatial change in the PL and PC intensities after 1-sun AM1.5 global illumination for 1 h. Light illumination was carried out in air at room temperature under short-circuit condition. It is wellknown that the conversion efficiency of the CH3NH3PbI3 solar cell degrades upon light illumination in air under operating conditions.36,37 The advantage of using microscopic imaging is that it allows the statistical evaluation of solar cell characteristics in both the light-illuminated and nonilluminated areas of one sample, removing fluctuations arising from sample differences. We covered a part of the sample using a thin metallic cover (2 mm square hole) to clearly distinguish between the nonilluminated and light-illuminated areas. Figure 4a,b shows the spatial image of IPC and IPL. The left half of the figure represents the area illuminated by AM 1.5G
Figure 3. (a) Correlation diagram between IPL and τave. (b) Correlation diagram between IPL and IPC. The solid line represents the regression line, and r is the Pearson’s product-moment correlation coefficient. Cross-correlation coefficient images (c) between IPL and τave, and (d) between IPL and IPC.
Pearson’s product-moment correlation coefficients between IPL and τave and between IPL and IPC were calculated to be 0.88 and −0.81, respectively. We confirmed a strong positive correlation between IPL and τave and a strong negative correlation between IPL and IPC. In Figure 3a, the positive correlation between IPL and τave indicates that the positions with strong PL intensities show long PL lifetimes. Furthermore, in Figure 3b, it is shown that the simultaneous measured PC has a negative correlation with IPL, meaning less extraction at positions with strong PL. These results suggest that the PL intensity is determined by carrier extraction from the perovskite layer through the interface, rather than carrier recombination (including nonradiative recombination) within the perovskite layer. Under the excitation fluences used in this experiment, the PL intensity is proportional to the square of the carrier density in the perovskite layer.13 The negative correlation indicates that the measured photocurrent is inversely proportional to the recombination of carriers within the CH3NH3PbI3 layer. Additionally, the PL intensity in the CH3NH3PbI3 layer is reduced by charge injection from the CH3NH3PbI3 layer into the charge transport layers. From the above results, we conclude that the PL lifetime is short and the PC flows well in areas where hole or electron injection occurs efficiently at the interface. Our finding is clear evidence for the hypothesis that carrier injection at the interface determines the device performance. Furthermore, Figure 3c,d shows the spatial image of the cross-correlation coefficient ri, defined by ri =
Figure 4. Spatial images of (a) IPC and (b) IPL. Only the left-hand side area was irradiated by 1 sun AM 1.5G for 1 h. Histograms of (c) IPC, (d) the square root of IPL, (e) τave, and (f) the initial PL intensity at zero time delay (I0). The bars indicate the occurrence of each value and the solid curves show the data fitted with a Gaussian function.
(xi − x ̅ )(yi − y ̅ ) n ∑i = 1 (xi
n
− x ̅ )2 ∑i = 1 (yi − y ̅ )2
(2)
for 1 h, whereas the right half represents the nonilluminated area. In order to quantitatively compare the differences caused by prolonged light illumination, as seen in Figure 4a,b, we show the histograms of IPC and the square root of IPL in Figure 4c,d. The area enclosed by the black diagonal lines was excluded from the data in order to eliminate the effects of light scattering at the boundary. In the intensity histograms, the solid line
as an indicator of the relative correlation for each point in the image. In these images, the red (blue) points indicate a positive (negative) correlation. We clearly observe a stripe pattern in the correlations, reflecting the spatial inhomogeneity of charge carrier injection at the interface. With respect to the origin of the suppressed carrier injection rate, that is, the existence of an 3188
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shows the data fitted by a Gaussian function. The results of the experiment show a large decrease in the PC and a moderate decrease in the square root of PL intensities in the lightilluminated area. The half-width of the Gaussian fitting curve of the histogram was unchanged, indicating that light-induced degradation in the PC occurred uniformly across the plane, and that it was not due to the nonuniformity caused by the perovskite solar cell structures. This PC intensity decrease is due to suppressed charge carrier injection at the interfaces between the CH3NH3PbI3 and charge transport layers (TiO2 and/or spiro-OMeTAD) and the increase in the number of interfacial traps.38 Moreover, we note that the PC intensity change is significant when compared with the changes in the square root of PL intensity and PL lifetime. This means that the injection efficiency mainly determines the performance of the solar cell device. The PL intensity, PL lifetime, and PC intensity of the full devices are mainly determined by two parameters, that is, the carrier recombination rate in the CH3NH3PbI3 layer and the carrier injection rate at the interface. To discuss the lightinduced change in the carrier recombination in the perovskite layer, we plot the histograms of τave and the initial PL intensity at zero time delay (I0) before and after prolonged light illumination in Figure 4e,f. The decrease in PL lifetime suggests an increase in the nonradiative recombination rate in the perovskite layer. It has been reported that light illumination in air results in an increase in the number of nonradiative recombination traps in the CH3NH3PbI3 layer.39 However, the decrease in the PC intensity is more drastic compared to the decrease in the PL lifetime, that is, IPC decreased by 69%, while τave decreased by 31%. This implies that a more drastic change in the carrier injection rate occurs, compared to the change in the recombination rate in the CH3NH3PbI3 layer. The decrease in the carrier injection rate at the interface increases the survival time of electrons and holes in the CH3NH3PbI3 layer, resulting in enhancement of the radiative recombination of electrons and holes within the CH3NH3PbI3 layer just after photocarrier generation (see Figure 4f). We consider that the prolonged illumination under short-circuit condition can induce trapped carriers at the interface and also ion and/or ion vacancy migration.40−44 This results in the formation of a space charge region and consequently reduces the electric field for the carrier injection,40−43 leading to a suppressed carrier injection rate. In addition, the light-induced degradation of the transport layer may also lead to a decreased injection rate. The main reason for the light-induced degradation of the device appears to be a change in carrier injection at the interface. In conclusion, we investigated the charge carrier dynamics in perovskite CH3NH3PbI3 solar cells by using microscopic PL and PC imaging spectroscopy. The PL intensity, PL lifetime, and PC intensity varied spatially for distances on the order of few tens of microns. A positive correlation existed between IPL and τave, and a negative correlation existed between IPL and IPC. The carrier injection rate determined the carrier recombination and transport dynamics in CH3NH3PbI3 devices, whereas the reduction in the charge injection rate initiated the degradation of solar cell devices. These results indicate the importance of the development of a flat mesoporous TiO2 layer and excellent transport materials with high and stable charge injection for improved device stability.
EXPERIMENTAL METHODS Microscopic PL and PC Imaging Measurements. Figure 5 illustrates the experimental setup for spatially resolved PL
Figure 5. Schematic illustration of the microscopic PL and PC measurements.
and PC measurements. Spatial imaging of microscopic timeresolved PL measurements was conducted using a conventional time-correlated single photon counting (TCSPC) method. In addition, the PC flowing through the devices was measured simultaneously. The samples were moved 20 steps in each direction on the x and y axes, with each step spanning 20 μm, using an automatic three-axis micro stage. The excitation diode laser had a wavelength of 688 nm and a pulse frequency, set using a pulse picker (Advanced Laser Diode Systems, PiLas series), of 1 MHz. An objective lens (10×) was used for both excitation of the sample and detection of the PL. Images of the PL and excitation laser spot were obtained using an sCMOS camera (Hamamatsu, ORCAFlash4.0). From the sCMOS camera image, the diameter of the laser spot was estimated to be 7 μm. The excitation intensity was 0.8 μJ/cm2. Under this excitation condition, we confirmed that the origin of the PL was a radiative two-carrier recombination process involving electrons and holes. The excitation light was filtered using a long pass filter, and only the PL from the perovskite was detected using an avalanche photodiode (APD) with an active area of 50 μm (IDQ, ID100 Silicon Avalanche Photodiode). A TCSPC board (Becker & Hickl, SPC-130-EM) was used to record the signal of the APD and to measure the PL decay curves. The exposure time for the PL measurements at each position was 5 s. The PL data were acquired in the dark under ambient conditions at 25 °C with ∼30% humidity. The PL spectrum was measured under the condition shown in the inset of Figure 2a, using a monochromator and a liquid-nitrogen-cooled CCD camera. The PC flowing through the device was measured using an amperemeter (ADCMT, 6241A).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01231. Sample preparation, PL decay curves, origin of stripe pattern in photoluminescence and photocurrent images, reproducibility of microscopic photoluminescence and photocurrent images, and light-induced degradation: 1sun illumination. (PDF) 3189
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ∥
D.Y. and T.H. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Y. Yamada, T. Yamada, and D. M. Tex for participating in discussions. Part of this work was supported by JST CREST.
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DOI: 10.1021/acs.jpclett.6b01231 J. Phys. Chem. Lett. 2016, 7, 3186−3191
Letter
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DOI: 10.1021/acs.jpclett.6b01231 J. Phys. Chem. Lett. 2016, 7, 3186−3191
