Charge Injection Mechanism at Heterointerfaces in CH3NH3PbI3

Subscriber access provided by University of Colorado Boulder

Letter 3

3

3

Charge Injection Mechanism at Heterointerfaces in CHNHPbI Perovskite Solar Cells Revealed by Simultaneous TimeResolved Photoluminescence and Photocurrent Measurements Taketo Handa, David M. Tex, Ai Shimazaki, Atsushi Wakamiya, and Yoshihiko Kanemitsu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02847 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

Charge Injection Mechanism at Heterointerfaces in CH3NH3PbI3 Perovskite Solar Cells Revealed by Simultaneous Time-Resolved Photoluminescence and Photocurrent Measurements Taketo Handa, David M. Tex, Ai Shimazaki, Atsushi Wakamiya, and Yoshihiko Kanemitsu* Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

AUTHOR INFORMATION Corresponding Author *[email protected]

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

ABSTRACT: Organic-inorganic hybrid perovskite solar cells are attracting much attention due to their excellent photovoltaic properties. In these multi-layered structures, the device performance is determined by complicated carrier dynamics. Here, we studied photocarrier recombination and injection dynamics in CH3NH3PbI3 perovskite solar cells using time-resolved photoluminescence (PL) and photocurrent (PC) measurements. It is found that a peculiar slowdown in the PL decay time constants of the perovskite layer occurs for higher excitation powers, followed by a decrease of the external quantum efficiency for PC. This indicates that a carrier-injection bottleneck exists at the heterojunction interfaces, which limits the photovoltaic performance of the device in concentrator applications. We conclude that the carrier-injection rate is sensitive to the photogenerated carrier density and the carrier-injection bottleneck strongly enhances recombination losses of photocarriers in the perovskite layer at high excitation conditions. The physical origin of the bottleneck is discussed based on the result of numerical simulations.

TOC GRAPHICS


2

Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Organic-inorganic hybrid perovskites provide excellent photovoltaic and optoelectronic properties even when prepared by low-temperature solution-processes.1–10 So far, comprehensive studies on the perovskite thin films and single crystals have revealed their intrinsic superior optoelectronic properties. One of the best-studied perovskites, CH3NH3PbI3 (MAPbI3), provides high absorption coefficients with a very sharp onset at the band gap due to extremely low density of sub-band gap states,11–13 and photogenerated carries which behave as free carriers at room temperature rather than excitons.14–17 The latter property is the origin of the observed long diffusion lengths.18–22 Furthermore, recent studies demonstrated that strong photon recycling occurs in this class of materials.21–25 These superior material properties are the reason for high conversion efficiencies in perovskite-based solar cells. For further improvement of the conversion efficiency and the device durability, it is important to understand and control carrier dynamics in an actual device. Perovskite solar cells are usually composed of a perovskite thin-film absorber layer sandwiched between carrier transport layers, which selectively collect and transport oppositely charged carriers.1–8 The carrier injection at the heterointerface from the absorber layer into the transport layer, and the transport in the latter play a crucial role for the device performance, but investigations for a general model are still ongoing.3,6,26–28 Unravelling the physics governing the injection mechanism enables us to optimize the device architecture. Time-resolved photoluminescence (PL) is a powerful technique for investigating carrier recombination processes in solar cell materials and devices.29–31 In perovskite thin films and single crystals, various intrinsic physical processes have been studied using this method.14,21,22.32– 34

However, the perovskite solar cell devices have a heterojunction design and their photocarrier

dynamics are complicated, compared to the case of GaAs- or Si-based solar cells. Thus, for a


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

quantitative evaluation and a thorough understanding of the carrier dynamics in the multi-layered heterojunction device, a combination of different techniques is more effective.11,23,28 In particular, photocurrent (PC) directly reflects the net charge-carrier flow through the whole device,28,35 and therefore a combination of PL and PC enables us to investigate the details of the carrier injection and recombination. In this Letter, we studied the excitation fluence dependence of time-resolved PL and PC for MAPbI3 solar cells, and discuss the carrier injection mechanism at the heterointerface. Three types of samples were prepared; a MAPbI3 thin film, MAPbI3 thin film on TiO2, and solar cell device. We found that the PL lifetime in the device becomes longer with higher excitation intensity (up to 100 nJ/cm2), which is completely different from the trend observed for thin film samples. Simultaneously performed PC measurements revealed that the external quantum efficiency (EQE) of PC maintains a constant value of 80% below excitation intensities of 100 nJ/cm2, but drops significantly for higher excitation powers. These trends indicate that the carrier separation and injection at the heterointerface become slower under higher excitation, which limits the photovoltaic performance of the device. In addition, based on a simple rate equation taking account for carrier-density-dependent injection rate, we discuss how the carrier injection, the nonradiative and radiative recombination affect the performance of perovskite solar cells. To evaluate the impact of the carrier injection from the perovskite absorber layer to the transport layer on the photovoltaic performance, three different samples were prepared: (i) MAPbI3 thin film on quartz substrate (MAPbI3/SiO2), (ii) glass/FTO/compact TiO2/mesoporous TiO2/MAPbI3 (MAPbI3/TiO2), and (iii) glass/FTO/compact TiO2/mesoporous TiO2/MAPbI3/Spiro-OMeTAD/Au (solar cell device). The detailed preparation procedure is given in the Supporting Information. Under simulated AM1.5G 1 sun illumination the solar cell


4

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


device achieved a high power-conversion efficiency of 16.6% owing to the fabrication of a perovskite layer with good quality. Current-voltage curves and the extracted photovoltaic parameters verify this (Fig. S1 in the Supporting Information). We performed simultaneous PL and PC measurements under nitrogen atmosphere, and confirmed that the conversion efficiency showed no essential degradation after the measurements (Fig. S1b). As previously reported,32,34,36–38 our solar cell device also showed a slow transient photo- and electricalresponse on the order of seconds under laser irradiation (Fig. S2). We discuss the time region where the sample has settled down, by time-averaging PL and PC values between 60 and 120 s laser irradiation time.

Figure 1. Excitation fluence dependence of PL decay curves for (a) MAPbI3/SiO2, (b) MAPbI3/TiO2, and (c) the solar cell device. The PL from the device was measured under shortcircuit condition. The inset shows the normalized PL decay curves. The excitation intensity was varied between 0.8 and 800 nJ/cm2 for (a) and (c), and 0.89 to 890 nJ/cm2 for (b). Dotted lines represent the fitting results.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

Figure 1 shows the excitation fluence dependence of the PL decay curves for the samples consisting of MAPbI3/SiO2, MAPbI3/TiO2, and the solar cell device. For excitation of the perovskite layer, a pulsed picosecond laser with wavelength of 650 nm was used, which was incident from the transparent glass side for all samples. In the Supporting Information, we show further details about the experimental setup and the PL spectra of the three samples (Fig. S3). The PL curves of MAPbI3/SiO2 in Fig. 1a are single-exponential under weak excitation, but show a non-exponential fast decay in the high excitation regime. This trend has been also observed in previous reports14,32, and was explained by a dominant radiative bimolecular (electron-hole) recombination process under high photocarrier densities. Figure 1b represents the decay curves for MAPbI3/TiO2, showing a very fast decay component with time constant of about 1 ns, and a slow component with 90 ns for weak excitation. The inset provides the PL curves normalized at their maximum values, and we confirm that the trend is very different compared to that observed for MAPbI3/SiO2. Since the MAPbI3/SiO2 sample has no transport layers, the carrier density at the interface is almost the same as inside the sample due to negligible surface recombination. The introduction of an electron transport layer leads to a strong drop in the electron carrier density at the interface, because diffusion causes the transport of photogenerated electrons from the interface into the transport layer, i.e., electron injection occurs. We consider that for the first several nanoseconds efficient electron-injection into TiO2 occurs, resulting in a fast PL decay. However, the electron mobility in TiO239,40 is smaller by magnitudes than that in MAPbI3.41,42 A smaller mobility means a large series resistance and thus an additional voltage drop in the TiO2 layer. Therefore, the injected electrons in the TiO2 layer proceed slowly to the contact and accumulate right after


6

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the perovskite/TiO2 heterointerface. We should expect a drastic state filling effect immediately after the interface. As a result, further electron-injection from MAPbI3 to TiO2 is suppressed, which causes a slowdown of the effective injection time, as seen in Fig. 1b. Obviously, the fast component seems to disappear under strong excitation. This is considered to be a result of an instantaneous injection of a large amount of carriers just after the excitation and thus slowdown of injection occurs immediately. The PL decay curves of the device are shown in Fig. 1c. In contrary to the other samples the device PL shows a fast decay for weak excitation (a slow component like for MAPbI3/TiO2 is no longer observed), whereas the PL decay becomes longer with increasing excitation power. Two mechanisms can explain this trend. First, the device has a Spiro-OMeTAD layer, and thus hole injection can occur, resulting in shorter PL lifetimes under weak excitation, but suppression of the injection occurs at high excitation due to unbalanced mobilities.39–43 Second, the PL curves in Fig. 1c were obtained under short-circuit condition, resulting in an internal electric field across the perovskite layer,44 which accelerates the charge-carrier separation. If the carrier mobilities are high enough, as in the case for GaAs,45 the charge carriers can reach the heterointerfaces almost immediately, and carrier-injection would occur. However, since the mobilities in MAPbI3 are not so high (µe, µh~ 10-20 cm2V-1s-1),41,42 compared to typical inorganic semiconductors (GaAs: µe~ 8000 cm2V-1s-1, µh~ 400 cm2V-1s-1, Si: µe~ 1450 cm2V-1s-1, µh~ 500 cm2V-1s-1),45 the photogenerated carriers cannot reach the interfaces fast enough. Therefore, the distribution of the electrons and holes reduces the internal field for separation. For increased photocarrier densities, the internal field weakens, and a slower PL decay is observed, as shown in Fig. 1c.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

Figure 2. (a) Excitation intensity dependence of (a) PL lifetime, and time-integrated PL intensity for (b) MAPbI3/SiO2, (c) MAPbI3/TiO2, and (d) the solar cell device. The blue and red solid lines represent the linear and quadratic dependence, respectively. The PL signal from the solar cell was recorded under short-circuit condition.

To discuss the carrier dynamics in the samples, the effective PL lifetimes are plotted in Fig. 2a. The PL decays in Figs. 1a,c were fitted with a stretched-exponential function,46 while those in Fig. 1b were fitted with a sum of single- and stretched-exponential. The fitting procedure and calculation of effective lifetimes are explained in the Supporting Information. For MAPbI3/SiO2, a PL lifetime of 230 ns (Fig.2a black squares) was observed under weak excitation, which is comparable to that reported for high quality samples.14,32 The shorter lifetimes under high excitation are due to dominant radiative electron-hole recombination.14,32


8

Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The PL lifetimes for MAPbI3/TiO2, shown with the gray dots in Fig. 2a, are averages of the fast and slow components (see eq. S3 in the Supporting Information), initially fast and slowing down for power densities up to 100 nJ/cm2 due to the disappearance of the fast component. This trend is even stronger in the device, which shows a one order increase in the PL lifetime between 1 and 100 nJ/cm2 (Fig. 2a, yellow dots). The results in Fig. 2a suggest that the injection rates are lowered under high carrier densities, as a result of a reduced electric field and charge accumulation at the interfaces as explained above. Later we show that other models based on traps, such as different trapping rates or depopulation of traps, can be ruled out, based on our simultaneously obtained PC results. Next, we show that the analysis of the time-integrated PL intensity enables us to study the carrier-injection mechanism. Figures 2b-d show the excitation power dependence of the integrated PL intensities for all samples excluding background counts. We start the discussion with the simple bare film case (MAPbI3/SiO2). Below the excitation intensity of 100 nJ/cm2, the integrated PL intensity (Fig. 2b) shows an approximately quadratic dependence (red dotted line in the figure), while for the very weak excitation condition (~1 nJ/cm2) as well for high excitation (above 100 nJ/cm2) the power dependence is closer to linear. This behavior can be well described with a simple bulk rate equation as shown in the following. For the bare film samples, nonradiative and radiative recombination govern the PL dynamics,14

dn(t ) = − Anonrad n(t ) − Brad n(t ) 2 dt

(1)

where n(t) represents the photoexcited carrier density. Here we assume that electrons and holes behave similar, because photoexcitation generates equal number of electrons and holes. Anonrad represents the single-carrier trapping and/or recombination rate (Shockley-Read-Hall process),


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

whereas Brad stands for the radiative recombination constant. With regard to the former process, we consider trapping to deep levels (energy difference ≫ ), which can be treated as nonradiative recombination. Note that the excitation intensity 1000 nJ/cm2 corresponds to a photocarrier density of 1.4 × 10 cm just after the pulse excitation, calculated using an absorption coefficient of 4 × 10 cm at 1.9 eV (650 nm).47 Considering the reported kinetic recombination coefficients in MAPbI3 thin films (bimolecular recombination constant: 10-10 cm3/s,14,48–50 three body Auger recombination constant: 10-28 cm6/s48–50), we can ignore the contribution of Auger recombination. In addition, the exciton formation and dissociation are considered to be negligible due to the small exciton binding energy of the material15–17 and the present experimental conditions (298K and the photocarrier density of 10 cm < < 10 cm). The exact solution of eq 1 is given as:

n(t ) =

n0 exp(− Anonrad t ) B n 1 + rad 0 (1 − exp(− Anonrad t )) Anonrad

(2)

where represents the carrier density generated by the pump pulse at zero delay time, which is proportional to the excitation intensity of the pump laser. The PL intensity is determined by the band-to-band radiative bimolecular recombination: I PL (t ) ∝ Brad n(t ) 2 + Brad n(t ) N 0 , N0 is the density of unintentionally doped carriers, for example the density of shallow traps (< 3 ).14 Accordingly, the time-integrated PL intensity is given by: () = + #$ −

≅ &

,)*+ '(')*+

&'(')*+ ,)*+

,

- ln #1 + & )*+

($ +

01

'(')*+

01 3 4

)

-

( ≪

&'(')*+ ,)*+

(3) )


10

Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


≅

( ≫

&'(')*+ ,)*+

)

Along with the exact expression, the limits under low ( ≪ 6nonrad⁄;rad ) and high excitations ( ≫ 6nonrad⁄;rad) are also given. Note that the typical value of N0 is 1015 cm-3,14 while that of 6=>=?@A ⁄;?@A is 1016-1017 cm-3,14,48–50 meaning 6=>=?@A ⁄;?@A ≫ N0. These expressions predict that the power dependence of the time-integrated PL intensity follows (i) a linear dependence for high excitation (initial photocarrier density >1016 cm-3) due to the dominant electron-hole recombination, (ii) a quadratic dependence for (relatively) weak excitation (1015 cm-3< 0 100 nJ/cm2).

Figure 3. (a) PL decay curves and (b) corresponding integrated PL intensities for eq. 2 and 3. (c) PL decay curves and (d) integrated PL intensities for eq. 4 and 5. Parameters are: 6=>=?@A = ACS Paragon Plus Environment

12

Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3 × 10 s , ;?@A = 1.7 × 10 cm s , F = 10 s , F4 (0) = 10G s , H = 2 × 10G cm s , $ = 2.5 × 10 cm . The blue and red dotted lines represent the linear and quadratic power dependence.

So far, we showed experimental evidence for the excitation dependent carrier separation and injection. Characteristic results in the device were that the PL decay showed a nonexponential behavior and the lifetime became longer with increasing excitation intensity. Additionally, the time-integrated PL intensity showed a power law of larger than two in the high excitation regime. By solving a simple rate equation system, we show that these results can be explained consistently with saturation of the separation and injection efficiency at the heterointerface, in other words, an intrinsic bottleneck of the carrier injection. First, we plot the theoretical PL decay curves and the time-integrated PL intensity for bare films in Figs. 3a,b, based on eq 2 and 3. The used recombination constants are: Anonrad =3 × 10K s , Brad = 1.7 × 10 cm s , and N0 = 2.5 × 10 cm , which are values for highquality MAPbI3 thin films.14,48–50 It is confirmed that the calculated PL trends agree well with the experimental results (Figs. 1a,2b). In contrast to the case of bare film samples, the solar cell device involves carrier separation and injection from the MAPbI3 layer into the transport layers. For such a situation, carrier dynamics in the perovskite layer are described with

dn(t ) = − Anonrad n(t ) − Brad n(t ) 2 − Cinj n(t ) dt

(4)

where Cinj represents the total carrier-injection rate at the heterointerfaces. The injection term Cinjn(t) is assumed to be linear in n(t), because we consider that the injection dynamics are


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

governed by the continuity equation. The PL decay curves in Fig. 1b indicated that the electroninjection occurs efficiently just after the photoexcitation, and shows a slow effective injection time afterwards. We suggest that the injection rate should consist of a time-independent term (C1) and a time-dependent term (C2(t)), because Cinj should be proportional to the electric field, which is in turn proportional to the bias (time-independent) and inverse proportional to the electric field induced by the electrons and holes which were separated and accumulated at the TiO2 and Spiro-OMeTAD interfaces, respectively (time-dependent). Hereafter, we consider the time dependence of the latter. The internal field for the carrier separation is reduced when a large amount of electrons and holes was accelerated to opposite sides within the absorber layer. In addition, a serious suppression of the carrier injection would occur when the amount of the instantaneously injected carriers is large, because the carriers accumulate in the low-mobility transport layers. Thus, we can approximately assume that the suppression of both the separation and injection are directly proportional to n(t):

Cinj = C1 + C2 (t )

(5a)

dC2 (t ) = −αC 2 (t )n(t ) dt

(5b)

where H is a coefficient relating the instantaneous carrier density with the severity of the bottleneck. To compare the theoretical model with experimental results, we numerically solved this equation system by assuming: F = 10 s , F4 (0) = 10G s , H = 2 × 10G cm s , and plotted the obtained PL decay curves and integrated PL intensities in Figs. 3c,d. The calculated PL curves in Fig. 3c show a slower decay with increasing excitation intensity, reproducing the experimentally obtained trend of the device (Fig. 1c). In addition, the


14

Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


simulated time-integrated PL intensities show a power law larger than two for strong excitation, reproducing the characteristic trend in Fig. 2d. From these results, we conclude that the slower injection time under strong excitation should be a result of a bottleneck effect due to the weakened internal field and the accumulated carriers in the transport layer. Such a bottleneck effect can be lifted by eliminating the difference of the electron (hole) mobility between the absorber layer41,42 and the electron (hole) transport layer,39,40,43 namely by enhancing the electron (hole) mobility in the transport layer and by reducing the imbalance between electron- and holeextraction rates at the heterointerfaces. To provide possible solutions further studies are necessary.

Figure 4. (a) Excitation dependent external quantum efficiency (EQE) of the device. The inset shows a linear plot of the photocurrent (PC) intensity. The dotted lines represent a constant EQE of 80%. (b) Theoretical evaluation of the injection efficiency (LM=N ,orange), radiative (Irad, red), and nonradiative (Inonrad, blue) losses, including transmission loss (Itrans, black) as a function of initial photocarrier density, calculated using eq.4 and 5.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

The PL measurements and calculations revealed that a carrier-injection bottleneck exists in the high excitation regime. It is important to understand how this phenomenon affects the EQE of the PC. To clarify this point, we performed PC measurements in parallel with the PL measurements discussed above. The excitation dependence of the EQE is plotted in Fig. 4a, calculated from the PC intensity in the inset and the incident photon energy of 1.9 eV (650nm). The corresponding carrier densities just after the photoexcitation are also plotted on the top axis of Fig. 4a. From Fig. 4a, it is found that the EQE maintains a constant value of 80% below 100 nJ/cm2, but drops above. The region above corresponds to the case of slow carrier-injection, as inferred from the PL decays. We thus clarified that a large photocarrier density results in a drop of the steady-state current generation rate of the device. It has been reported that the steady-state photocarrier density under 1 sun illumination should be around 1012 cm-3 if the carrier lifetime in the solar cell is 1ns (Fig. 2a).52,53 However, the direct comparison of the pulsed excitation and continuous excitation is difficult because in the device structure the carrier lifetime varies greatly with the photocarrier density (Fig. 2a). At least the rough estimate given above indicates that the photocarrier density where the EQE starts to drop off should be much higher than that for 1 sun condition, and thus the bottleneck effect should be important for concentrator applications. Here, we would like to emphasize the importance of the simultaneous PL and PC measurements. For example, if we would only consider the EQE results, we cannot exclude the possibility of degradation of the MAPbI3 layer (increase of the nonradiative recombination centers, such as deep trap states). However, we confirmed longer PL lifetimes (Fig. 2a) and large PL intensities (Fig. 2d) for strong excitation, and thus this possibility can be rejected. Conversely, by only considering the PL results, we cannot exclude the possibility of carrier-trapping to subband gap states,54 but the constant EQE under low fluences indicates that the trapping is not


16

Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dominant in the device. The simultaneous PL and PC measurements enable us to eliminate various complicated possibilities and discuss the details of the carrier-injection. Using the proposed simple rate equation system, we can numerically estimate the injection efficiency. In Fig. 4b, the injection efficiency (LM=N ), the radiative (Irad), and nonradiative (Inonrad) recombination losses are plotted as a function of the initial photocarrier density, by solving eq. 4 and 5 with the same parameters used for Figs. 3c,d. Each value is obtained via the time integration of the corresponding term in eq. 4, considering the transmission loss (Itrans) of the glass/FTO/compact TiO2/mesoporous TiO2 substrate (transmittance is 0.87 at 650nm, Fig. S3b). The details of the calculation procedure are provided in the Supporting Information. If there are no further losses, the injection efficiency would correspond exactly to the EQE which we obtained from the PC extracted from the device. The estimated injection efficiency shows a drop above 1016 cm-3, which reproduces the experimental EQE data very well. Fig. 4b indicates that the recombination losses become larger due to the slow injection rate under strong excitation conditions. Furthermore, around the excitation intensity of 1017 cm-3, the radiative recombination loss (the red area in Fig. 4b) is greatly enhanced, resulting in a decreased injection efficiency. In the Supporting Information, we show that the loss due to Auger recombination process is negligible below 1017 cm-3 and is not responsible for the observed EQE drop. We successfully explained the observed PL and PC tendency using eq. 4 and 5, and thus conclude that the carrier-injection bottleneck limits the device performance in perovskite solar cells under photoexcitation conditions that are presumably higher than the 1 sun equivalent. In addition, our results evidence that the injection rate can drop for high excitation intensities, and therefore a careful evaluation of time constants is essential for determining the carrier-injection rate and efficiency.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

In conclusion, we investigated the excitation fluence dependence of time-resolved PL and PC for MAPbI3 solar cells, and discuss the carrier-injection mechanism at the heterointerface. We found that the PL lifetime in the device shows a peculiar slowdown for increasing excitation intensities up to 100 nJ/cm2. Simultaneously performed PC measurements revealed that the EQE maintains a constant value below excitation intensities of 100 nJ/cm2, but drops above. These results indicate that the charge injection at the heterointerface becomes slower under higher excitation, limiting the photovoltaic performance of the device in concentrator applications. In addition, we proposed a simple rate equation accounting for the carrier-injection bottleneck, and the calculated PL and PC trends were in good agreement with the experimental data. Thus we conclude that the carrier-injection rate is sensitive to the photocarrier density and the injection bottleneck effect strongly enhances recombination losses of photocarriers in the perovskite layer at high excitation conditions. Our work provides an important insight for evaluating the carrierinjection process in actual solar cell devices.

AUTHOR INFORMATION Corresponding Author. *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


18

Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The authors thank H. Tahara, T. Yamada, D. Yamashita, T. Ihara, and Y. Yamada for fruitful discussions and experimental help, and acknowledge K. Matsuda for the SEM measurements. Part of this work was supported by CREST, JST. ASSOCIATED CONTENT Supporting Information. Sample preparation, photoluminescence and photocurrent measurements, transient photoelectrical response, absorption and photoluminescence spectra, fitting of PL decay curves, calculation of the carrier injection efficiency and recombination losses, negligible contribution of the Auger recombination. (PDF)

REFERENCES (1) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643–647. (3) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance PerovskiteSensitized Solar Cells. Nature 2013, 499, 316–319. (4) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395–398.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

(5) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-B.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542–546. (6) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent Engineering for High-Performance Inorganic-Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897–903. (7) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476–480. (8) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. HighPerformance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 1234–1237. (9) Tan, Z. -K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; et al. Bright Light-Emitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687–692. (10)

Dou, L.; Yang, Y. M.; You, J.; Hong, Z.; Chang, W. H.; Li, G.; Yang, Y.

Solution-Processed Hybrid Perovskite Photodetectors with High Detectivity. Nat. Commun. 2014, 5, 5404. (11)

Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. Near-

Band-Edge Optical Responses of Solution-Processed Organic-Inorganic Hybrid Perovskite CH3NH3PbI3 on Mesoporous TiO2 Electrodes. Appl. Phys. Express 2014, 7, 032302. (12)

De Wolf, S.; Holovsky, J.; Moon, S.-J.; Löper, P.; Niesen, B.; Ledinsky, M.;


20

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Haug, F.-J.; Yum, J.-H.; Ballif, C. Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance. J. Phys. Chem. Lett. 2014, 5, 1035–1039. (13)

Shirayama, M.; Kadowaki, H.; Miyadera, T.; Sugita, T.; Tamakoshi, M.; Kato,

M.; Fujiseki, T.; Murata, D.; Hara, S.; Murakami, T. N.; et al. Optical Transitions in Hybrid Perovskite Solar Cells: Ellipsometry, Density Functional Theory, and Quantum Efficiency Analyses for CH3NH3PbI3. Phys. Rev. Applied 2016, 5, 0140121. (14)

Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.; Kanemitsu, Y.

Photocarrier Recombination Dynamics in Perovskite CH3NH3PbI3 for Solar Cell Applications. J. Am. Chem. Soc. 2014, 136, 11610–11613. (15)

Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.; Kanemitsu, Y.

Photoelectronic Responses in Solution-Processed Perovskite CH3NH3PbI3 Solar Cells Studied by Photoluminescence and Photoabsorption Spectroscopy. IEEE J. Photovolt. 2015, 5, 401–405. (16)

Miyata, A.; Mitioglu, A.; Plochocka, P.; Portugall, O.; Wang, J. T. W.; Stranks,

S. D.; Snaith, H. J.; Nicholas, R. J. Direct Measurement of the Exciton Binding Energy and Effective Masses for Charge Carriers in Organic–Inorganic Tri-Halide Perovskites. Nat. Phys. 2015, 11, 582–587. (17)

Galkowski, K.; Mitioglu, A.; Miyata, A.; Plochocka, P.; Portugall, O.; Eperson,

G. E.; Wang, J. T.; Stergiopoulos, T.; Stranks, S. D.; Snaith, H. J. et al. Determination of the Exciton Binding Energy and Effective Masses for Methylammonium and Formamidinium Lead Tri-halide Perovskite Semiconductors. Energy Environ. Sci. 2016, 9, 962–970.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(18)

Page 22 of 31

Stranks, S.; Eperon, G.; Grancini, G.; Menelaou, C.; Alcocer, M.; Leijtens, T.;

Herz, L.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341– 344. (19)

Xing, G.; Mathews, N.; Sun, S.; Lim, S.; Lam, Y.; Grätzel, M.; Mhaisalkar, S.;

Sum, T. Long-Range Balanced Electronand Hole-Transport Lengths in OrganicInorganic CH3NH3PbI3. Science 2013, 342, 344−347. (20)

Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-

Hole Diffusion Lengths > 175 µm in Solution-Grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970. (21)

Yamada, Y.; Yamada, T.; Phuong, L.; Maruyama, N.; Nishimura, H.;

Wakamiya, A.; Murata, Y.; Kanemitsu, Y. Dynamic Optical Properties of CH3NH3PbI3 Single Crystals As Revealed by One- and Two-Photon Excited Photoluminescence Measurements. J. Am. Chem. Soc. 2015, 137, 10456–10459. (22)

Yamada, T.; Yamada, Y.; Nishimura, H.; Nakaike, Y.; Wakamiya, A.; Murata,

Y.; Kanemitsu, Y. Fast Free-Carrier Diffusion in CH3NH3PbBr3 Single Crystals Revealed by Time-Resolved One- and Two-Photon Excitation Photoluminescence Spectroscopy. Adv. Electron. Mater. 2016, 2, 1500290. (23)

Pazos-Outón, L. M.; Szumilo, M.; Lamboll, R.; Richter, J. M.; Crespo-

Quesada, M.; Abdi-Jalebi, M.; Beeson, H. J.; Vrućinić, M.; Alsari, M.; Snaith, H. J.; et al. Photon Recycling in Lead Iodide Perovskite Solar Cells. Science 2016, 351 1430– 1433. (24)

Staub, F.; Hempel, H.; Hebig, J. C.; Mock, J.; Paetzold, U. W.; Rau, U.;


22

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Kirchartz, T. Beyond Bulk Lifetimes: Insights into Lead Halide Perovskite Films from Time-Resolved Photoluminescence. Phys. Rev. Applied 2016, 6, 044017. (25)

Yamada, T; Yamada, Y; Nakaike, Y.; Wakamiya, A.; Kanemitsu, Y. Photon

Emission and Reabsorption Processes in CH3NH3PbBr3 Single Crystals Revealed by Time-Resolved Two-Photon-Excitation Photoluminescence Microscopy. Phys. Rev. Applied 2017, 7, 014001. (26)

Marchioro, A.; Teuscher, J.; Friedrich, D.; Kunst, M.; Van De Krol, R.; Moehl,

T.; Grätzel, M.; Moser, J.-E. Unravelling the Mechanism of Photoinduced Charge Transfer Processes in Lead Iodide Perovskite Solar Cells. Nat. Photonics 2014, 8, 250– 255. (27)

Zhu, Z.; Ma, J.; Wang, Z.; Mu, C.; Fan, Z.; Du, L.; Bai, Y.; Fan, L.; Yan, H.;

Phillips, D. L. et al. Efficiency Enhancement of Perovskite Solar Cells through Fast Electron Extraction: The Role of Graphene Quantum Dots. J. Am Chem. Soc. 2014, 136, 3760–3763. (28)

Yamashita, D.; Handa, T.; Ihara, T.; Tahara, H.; Shimazaki, A.; Wakamiya, A.;

Kanemitsu, Y. Charge Injection at the Heterointerface in Perovskite CH3NH3PbI3 Solar Cells Studied by Simultaneous Microscopic Photoluminescence and Photocurrent Imaging Spectroscopy. J. Phys. Chem. Lett. 2016, 7, 3186–3191. (29)

Tex, D. M.; Ihara, T.; Akiyama, H.; Imaizumi, M.; Kanemitsu, Y. Time-

Resolved Photoluminescence Measurements for Determining Voltage-Dependent Charge-Separation Efficiencies of Subcells in Triple-Junction Solar Cells. Appl. Phys. Lett. 2015, 106, 013905. (30)

Tex, D. M.; Imaizumi, M.; Kanemitsu, Y. Charge Separation in Subcells of


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

Triple-Junction Solar Cells Revealed by Time-Resolved Photoluminescence Spectroscopy. Opt. Express 2015, 23, A1687–A1692. (31)

Tex, D. M.; Imaizumi, M.; Kanemitsu, Y. Analyzing the Electrical Performance

of a Solar Cell with Time-Resolved Photoluminescence: Methodology for Fast Optical Screening. Phys. Rev. Applied 2017, 7, 014019. (32)

Stranks, S. D.; Burlakov, V. M.; Leijtens, T.; Ball, J. M.; Goriely, A.; Snaith, H.

J. Recombination Kinetics in Organic-Inorganic Perovskites: Excitons, Free Charge, and Subgap States. Phys. Rev. Applied 2014, 2, 034007. (33)

Leijtens, T.; Srimath Kandada, A. R.; Eperon, G. E.; Grancini, G.;

D’Innocenzo, V.; Ball, J. M.; Stranks, S. D.; Snaith, H. J.; Petrozza, A. Modulating the Electron−Hole Interaction in a Hybrid Lead Halide Perovskite with an Electric Field. J. Am. Chem. Soc. 2015, 137, 15451–15459. (34)

deQuilettes, D. W.; Zhang, W.; Burlakov, V. M.; Graham, D. J.; Leijtens, T.;

Osherov, A.; Bulović, V.; Snaith, H. J.; Ginger, D. S.; Stranks, S. D. Photo-Induced Halide Redistribution in Organic–Inorganic Perovskite Films. Nat. Commun. 2016, 7, 11683. (35)

Tex, D. M.; Ihara, T.; Kamiya, I.; Kanemitsu, Y. Microscopic

Photoluminescence and Photocurrent Imaging Spectroscopy of InAs Nanostructures: Identification of Photocarrier Generation Sites for Intermediate-Band Solar Cells. Phys. Rev. B 2014, 89, 125301. (36)

Sanchez, R. S.; Gonzalez-Pedro, V.; Lee, J. W.; Park, N. G.; Kang, Y. S.; Mora-

Sero, I.; Bisquert, J. Slow Dynamic Processes in Lead Halide Perovskite Solar Cells. Characteristic Times and Hysteresis. J. Phys. Chem. Lett. 2014, 5, 2357–2363.


24

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37)

Kim, H. S.; Park, N. G. Parameters Affecting I−V Hysteresis of CH3NH3PbI3

Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer. J. Phys. Chem. Lett. 2014, 5, 2927−2934. (38)

Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.;

Grätzel, M. Understanding the Rate-Dependent J–V Hysteresis, Slow Time Component, and Aging in CH3NH3PbI3 Perovskite Solar Cells: The Role of a Compensated Electric Field. Energy Environ. Sci. 2015, 8, 995−1004. (39)

Crossland, E. J. W.; Noel, N.; Sivaram, V.; Leijtens, T.; Alexander-Webber, J.

A.; Snaith, H. J. Mesoporous TiO2 Single Crystals Delivering Enhanced Mobility and Optoelectronic Device Performance, Nature 2013, 495, 215–219. (40)

Leijtens, T.; Lim, J.; Teuscher, J.; Park, T.; Snaith, H. J. Charge Density

Dependent Mobility of Organic Hole-Transporters and Mesoporous TiO2 Determined by Transient Mobility Spectroscopy: Implications to Dye-Sensitized and Organic Solar Cells. Adv. Mater. 2013, 25, 3227–3233. (41)

Leijtens, T.; Stranks, S. D.; Eperon, G. E.; Lindblad, R.; Johansson, E. M.;

McPherson, I. J.; Rensmo, H.; Ball, J. M.; Lee, M. M.; Snaith, H. J. Electronic Properties of Meso-Superstructured and Planar Organometal Halide Perovskite Films: Charge Trapping, Photodoping, and Carrier Mobility. ACS Nano 2014, 8, 7147–7155. (42)

Oga, H.; Saeki, A.; Ogomi, Y.; Hayase, S.; Seki, S. Improved Understanding of

the Electronic and Energetic Landscapes of Perovskite Solar Cells: High Local Charge Carrier Mobility, Reduced Recombination, and Extremely Shallow Traps. J. Am. Chem. Soc. 2014, 136, 13818–13825. (43)

Nishimura, H.; Ishida, N.; Shimazaki, A.; Wakamiya, A.; Saeki, A.; Scott, L. T.;


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

Murata, Y. Hole-Transporting Materials with a Two-Dimensionally Expanded O‑ System around an Azulene Core for Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137, 15656–15659. (44)

Edri, E.; Kirmayer, S.; Henning, A.; Mukhopadhyay, S.; Gartsman, K.;

Rosenwaks, Y.; Hodes, G.; Cahen, D. Why Lead Methylammonium Tri-Iodide Perovskite-Based Solar Cells Require a Mesoporous Electron Transporting Scaffold (but Not Necessarily a Hole Conductor) Nano Lett. 2014, 14, 1000–1004. (45)

Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, 3rd ed.; Wiley-

Interscience: Hoboken, NJ, 2007. (46)

Lindsey, C. P.; Patterson, G. D. Detailed Comparison of the Williams–Watts

and Cole–Davidson Functions. J. Chem. Phys. 1980, 73, 3348. (47)

Absorption coefficients are given in the Supporting Information in Löper, P.;

Stuckelberger, M.; Niesen, B.; Werner, J.; Filipič, M.; Moon, S. J.; Yum, J.-H.; Topič, M.; De Wolf, S.; Ballif, C. Complex Refractive Index Spectra of CH3NH3PbI3 Perovskite Thin Films Determined by Spectroscopic Ellipsometry and Spectrophotometry. J. Phys. Chem. Lett. 2015, 6, 66–71. (48)

Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H.J.; Herz, L. M.

High Charge Carrier Mobilities and Lifetimes in Organolead Trihalide Perovskites. Adv. Mater. 2014, 26, 1584–1589. (49)

Milot, R. L.; Eperon, G. E.; Snaith, H. J.; Johnston, M. B.; Herz, L. M.

Temperature-Dependent Charge-Carrier Dynamics in CH3NH3PbI3 Perovskite Thin Films. Adv. Funct. Mater. 2015, 25, 6218–6227. (50)

Yang, Y.; Yang, M.; Li, Z.; Crisp, R.; Zhu, K.; Beard, M. C. Comparison of


26

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Recombination Dynamics in CH3NH3PbBr3 and CH3NH3PbI3 Perovskite Films: Influence of Exciton Binding Energy. J. Phys. Chem. Lett. 2015, 6 4688–4692. (51)

Saba, M.; Cadelano, M.; Marongiu, D.; Chen, F.; Sarritzu, V.; Sestu, N.; Figus,

C.; Aresti, M.; Piras, R.; Lehmann, A. G. et al. Correlated Electron–Hole Plasma in Organometal Perovskites. Nat. Commun. 2014, 5, 5049. (52)

Yamada, Y.; Yamada, T.; Shimazaki, A.; Wakamiya, A.; Kanemitsu, Y.

Interfacial Charge-Carrier Trapping in CH3NH3PbI3-Based Heterolayered Structures Revealed by Time-Resolved Photoluminescence Spectroscopy. J. Phys. Chem. Lett. 2016, 7, 1972–1977. (53)

Johnston. M. B.; Herz. L. M. Hybrid Perovskites for Photovoltaics: Charge-

Carrier Recombination, Diffusion, and Radiative Efficiencies. Acc. Chem. Res. 2016, 49, 146–154. (54)

Wen, X.; Feng, Y.; Huang, S.; Huang, F.; Cheng, Y. B.; Green, M.; Ho-Baillie,

A. Defect Trapping States and Charge Carrier Recombination in Organic–Inorganic Halide Perovskites. J. Mater. Chem. C 2016, 4, 793−800.


27

PL intensity (counts/s)

10

6

(c) (b) The Journal of Physical Chemistry Letters Page 28 of 31

(a)

5

1 4 10 2 3 3 10 4 102 5 1 6 10 7 100 8 -1 10 9 -2 1010 0 11 12

0

20 (ns)

10 20 (ns)

800 nJ/cm2

80

89

16

18

4

3.6

16

0.8

0.89

4


100 150 200 0 Time (ns)

0

890 nJ/cm2

800 nJ/cm2

50

10

PL (norm.)

10

7

PL (norm.)

10

50

100 150 200 0

Time (ns)

80

0.8

50

100 150 200

Time (ns)

Integrated PL int. (a.u.) Integrated PL int. (a.u.)

3

PL lifetime (ns)

8 10The29 Page of 31 of Physical Chemistry Letters 10 (b) (a)Journal

10

2

SiO2

TiO2

10

6

Integrated PL int. (a.u.)

1 1 4 10 10 2 Device MAPbI3/SiO2 3 0 2 10 10 4 1 10 100 1000 1 10 100 1000 8 5 9 10 (d) 10 (c) 6 6 7107 10 8 5 4 910 10 10 3 MAPbI 3/TiO 2 Device 2 ACS Paragon Plus Environment 10 10 11 10 100 1000 1 10 100 1000 12 1 -2 -2 Excitation intensity (nJ cm ) Excitation intensity (nJ cm ) 13

10

7

10

7

(a) (c) Page The Journal of Physical Chemistry Letters 30 of 31 6 6

PL intensity (arb. units)

10 10

10

5

10

5

Integrated PL int. (a.u.)

1 4 4 10 2 10 3 3 103 10 4 2 2 10 5 10 1 6 101 10 7 0 0 10 8 10 -1 9 10-1 10 0 50 100 150 200 0 50 100 150 200 10 Time (ns) Time (ns) 11 12 7 13108 (b) 10 (d) 14 6 5 10 1510 3 16104 10 17 1 2 ACS Paragon Plus 10 Environment 1810 14 15 16 17 19 1014 1015 1016 1017 10 10 10 10 -3 20 Photocarrier density (cm-3) Photocarrier density (cm )

-3

Photocarrier density (cm )

100 80

10

(a)

10

10

10

EQE , Irad , Inonrad,Itrans (%)

EQE at 650 nm (%)

Page The 31Journal of1531 of Chemistry Letters 14 16 Physical 17 100 80

(b)

PC (µA)

40 1 30 60 60 2 20 40 3 40 100 1000 0 500 20 20 -2 4 cm ACSnJ Paragon Plus Environment 0 0 5 14 15 16 1 10 100 1000 10 10 10 6 -2 Photocarrier density 7 Excitation intensity (nJ cm )

10

17

-3

(cm )

Charge Injection Mechanism at Heterointerfaces in CH3NH3PbI3

Recommend Documents