Identifying an Optimum Perovskite Solar Cell Structure by Kinetic

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Identifying an optimum perovskite solar cell structure by kinetic analysis: planar, mesoporous based or extremely thin absorber structure Maning Liu, Masaru Endo, Ai Shimazaki, Atsushi Wakamiya, and Yasuhiro Tachibana ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00515 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Applied Energy Materials

Identifying an Optimum Perovskite Solar Cell Structure by Kinetic Analysis: Planar, Mesoporous Based or Extremely Thin Absorber Structure Maning Liu,† Masaru Endo,§ Ai Shimazaki,§ Atsushi Wakamiya§,# and Yasuhiro Tachibana*,†,‡,# †

School of Engineering, RMIT University, Bundoora, VIC 3083, Australia; §Institute for

Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan; #Japan Science and Technology Agency (JST), PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan; ‡

Office for Industry-University Co-Creation, Osaka University, 2-1 Yamada-oka, Suita, Osaka

565-0871, Japan

KEYWORDS: Planar structure, Mesoporous, Extremely thin absorber, Charge transfer, Interfacial charge recombination, Solar cells, Perovskite, Charge transfer yield

ACS Paragon Plus Environment

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Abstract

Perovskite solar cells have rapidly been developed over the last several years. Choice of the most suitable solar cell structure is crucial to improve the performance further. Here, we attempt to determine an optimum cell structure for methylammonium lead iodide (MAPbI3) perovskite sandwiched by TiO2 and spiro-OMeTAD layers, among planar heterojunction, mesoporous structure and extremely thin absorber structure, by identifying and comparing charge carrier diffusion coefficients of the perovskite layer, interfacial charge transfer and recombination rates using transient emission and absorption spectroscopies. An interfacial electron transfer from MAPbI3 to compact TiO2 occurs with a time constant of 160 ns, slower than the perovskite photoluminescence (PL) lifetime (34 ns). In contrast, fast non-exponential electron injection to mesoporous TiO2 was observed with at least two different electron injection processes over different time scales; one (60~70 %) occurs within an instrument response time of 1.2 ns and the other (30~40 %) on nanosecond time scale, while most of hole injection (85 %) completes in 1.2 ns. Analysis of the slow charge injection data revealed an electron diffusion coefficient of 0.016 ± 0.004 cm2 s-1 and a hole diffusion coefficient of 0.2 ± 0.02 cm2 s-1 inside MAPbI3. To achieve an incident photon-to-current conversion efficiency (IPCE) of >80%, a minimum charge carrier diffusion coefficient of 0.08 cm2 s-1 was evaluated. An interfacial charge recombination lifetime was increased from 0.5 to 40 ms by increasing a perovskite layer thickness, suggesting that the perovskite layer suppresses charge recombination reactions. Assessments of charge injection and interfacial charge recombination processes indicate that the optimum solar cell structure for the MAPbI3 perovskite is a mesoporous TiO2 based structure. This comparison of kinetics has been applied to several different types of photoactive semiconductors such as perovskite, CdTe and GaAs, and the most appropriate solar cell structure was identified.


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Introduction Perovskite solar cells have been rapidly developed over the last several years,1-7 since pioneering studies were reported for perovskite sensitized solar cells8 and solid state perovskite solar cells.910

The solar energy conversion efficiency of single perovskite solar cell has exceeded 22 %.1

Although a typical perovskite solar cell employs an organic-inorganic lead halide perovskite layer, recently several different types of metal based halide perovskite such as tin, bismuth, germanium and antimony have been employed to replace lead.11-16 As a solar cell structure, a perovskite layer is sandwiched by p-type semiconductor (such as spiro-OMeTAD,7 PEDOT17 or NiO18) and n-type semiconductor (such as TiO2,7 ZnO18 or PCBM17) layers. Such cell structure arrangement is significantly important, since the performance of perovskite solar cells, particularly photocurrent generation, is controlled by charge transfer dynamics at the material interface. Also, morphology, homogeneity and quality of semiconductor layers and cell operation conditions such as irradiation of light wavelength and intensity, potentially influence charge separation and recombination dynamics, and thus the solar cell performance. For example, we recently reported that increase in the methylammonium lead iodide (MAPbI3) perovskite excitation intensity accelerates electron-hole recombination rates within the perovskite layer, competing with the electron and hole injection processes, thereby resulting in decrease of the electron injection and hole injection yields.19 To optimize perovskite solar cell performance, choice of a solar cell structure is crucial. Scheme 1 shows solar cell structures reported so far, (a) planar heterojunction, (b) mesoporous structure and (c) sensitized type or extremely thin absorber (ETA) structure, respectively. Although the planar structure shows the high efficiency,2 the highest performance was achieved by the mesoporous structure with organic-inorganic lead iodide perovskites.1, 4 For any structure,


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the charge injection yield increases if the charge injection is faster than the perovskite PL decay or the electron-hole recombination inside the perovskite layer, following the exciton dissociation.19 The charge collection yield at the electrode increases if the recombination of the separated charges (electron and hole), i.e. interfacial charge recombination, is sufficiently slow. Several previous studies have assessed charge carrier diffusions by employing a one dimensional diffusion model3, 20 or by obtaining charge carrier mobility,21-22 and interfacial charge injection rates using a rate law9, 23-26 to correlate with the solar cell performance. The former assumes extremely fast interfacial charge transfer rates. The latter includes both charge carrier diffusions and charge transfer rates, however they have not been distinguished from each other. Interfacial charge recombination studies were rather limited, but conducted for specific cases.19,

27-28

Thorough systematic investigations to study charge carrier diffusion, interfacial charge transfer and recombination dynamics have rarely been conducted, although their direct measurements and evaluations of identical samples can provide a guide to design the most appropriate cell structure.

Scheme 1. Schematic diagrams of metal halide perovskite solar cell structures. (a) planar heterojunction structure, (b) mesoporous structure, and (c) sensitized type or extremely thin absorber (ETA) solar cell structure.


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Here we attempt to determine the optimum cell structure among the structures shown in Scheme 1 by identifying and comparing charge carrier diffusion coefficients inside a perovskite layer, interfacial charge transfer and recombination rates using transient emission and absorption spectroscopies (TES and TAS). These systematic optical studies using an identical sample are extremely important. For example, even if a fast charge transfer process is achieved for a specific sample, but if the charge recombination rate is also high, the photocurrent generation will be limited. TAS can directly monitor the number of photo-generated carriers and their interfacial recombination dynamics. Therefore, the film structure influencing interfacial charge recombination lifetime can be determined. TES under different excitation intensity is particularly useful to identify charge diffusion coefficient and interfacial charge transfer dynamics individually. For this study, we employ MAPbI3 as a perovskite layer, since this type of perovskite has achieved one of the highest efficiency.7 By comparing these dynamics, we will propose the method to determine the optimized solar cell structure.

Experimental Methods Sample preparation. A MAPbI3 layer was deposited on a various different substrate. The substrate includes a slide glass (glass/MAPbI3), a fluorine doped tin oxide (FTO, 15 mm × 25 mm) glass (FTO/MAPbI3), a nanocrystalline Al2O3 film prepared on a slide glass (Al2O3/MAPbI3), a compact TiO2 layer (c-TiO2) prepared on FTO (FTO/c-TiO2/MAPbI3), and a mesoporous TiO2 film (m-TiO2) with (FTO/c-TiO2/m-TiO2/MAPbI3/OMeTAD) or without (FTO/c-TiO2/m-TiO2/MAPbI3) spiro-OMeTAD. A c-TiO2 layer was deposited on a FTO substrate by spray pyrolysis of 0.05 M titanium di-isopropoxide bis(acetylacetonate) solution in ethanol at 450 ºC.29


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Al2O3 nanocrystalline films, m-Al2O3, (thickness: 3.0 µm) were prepared by the screen printer with the different printing mask, using a home-made Al2O3 paste, following the previously reported method.19, 30-31 The printed Al2O3 films were calcined at 500 °C for 1 h in an air flow oven. A transparent m-TiO2 film was prepared on the as-prepared c-TiO2 layer or a slide glass. The film thickness was controlled by depositing with two different methods, spin coating and screen printing methods. A thin transparent m-TiO2 (thickness: 150 nm) was deposited by spin coating at 5000 rpm for 30 s using an ethanol suspension of TiO2 paste (TiO2 paste:ethanol = 1:3.5 wt. ratio). A thick transparent m-TiO2 (thickness: 2.8 µm) was prepared by the screen printing method with the TiO2 paste. The film, after printing, was levelled for 15 min, heated up to 500 °C at 15.8 °C/min, and calcined at 500 °C for 1 h in an air flow oven. The film thickness was measured by a surface profiler (KLA-Tencor P-16+), and confirmed by observing a crosssection image using scanning electron microscope (JEOL JEM-6500F) with an operating voltage of 10.0 kV. A MAPbI3 perovskite layer was deposited on a various different film or substrate, as discussed above, inside a glove box, following the reported method.19, 32-33 A PbI2 solution with various concentration (0.2 ~ 1.1 M) in dehydrated DMF was spin-coated at 70 °C. After drying at 70 °C for 1 h, the film was dipped for 40 s in a 0.06 M MAI solution in dehydrated 2-propanol, forming into a perovskite layer. To remove excess amount of MAI, the films were quickly rinsed with 2propanol and dried at 70 °C for 30 min. For some films, a hole-transporting layer was deposited on top of TiO2-MAPbI3 and Al2O3-MAPbI3 by spin-coating a solution of spiro-OMeTAD in dehydrated chlorobenzene (0.058 M). The film was fixed in a vacuum chamber with optical windows, and was kept under vacuum (~10-3 Torr) during steady state or transient optical measurements. For all optical experiments, the films were excited from a glass substrate side.


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Characterization. The details of film characterization are described in the Supporting Information. Analysis. Charge injection and interfacial charge recombination reaction. Electron and hole injection reaction from a perovskite layer into an electron and hole acceptor materials, typically n-type and p-type semiconductor, respectively, is generally characterized by transient emission spectroscopy. Since the perovskite layer absorbs the entire visible wavelengths, unless a thin perovskite layer is deposited, we could not employ ns-TAS to assess charge injection reactions owing to lack of the probe light intensity. Charge injection processes are described in Scheme 2. Following light excitation, an electron and a hole are immediately dissociated from the exciton, and diffused to their respective n-type and p-type semiconductor interface.34 At the interface, charge transfer reaction occurs. Therefore, the total charge injection time, τinj, is described by the addition of the charge transport time (inside the perovskite layer), L2/D, and the interfacial charge transfer time, τCT, where D is the charge carrier diffusion coefficient, and L is the perovskite layer thickness. One dimensional diffusion model has been employed to assess electron and hole diffusion coefficients in a planar heterojunction structure.3, 20 However, this diffusion model assumes that the interfacial electron or hole transfer occurs at an infinite rate (kCT = 1/τCT = ∞), i.e. the entire charge injection process is therefore assumed to occur with charge carrier diffusion limit (τinj ≈ L2/D).3, 20 This model is inappropriate for a case that the interfacial charge transfer process is relatively slow. Here, we consider three different cases to analyse transient emission data, (a) kCT = (1/τCT) >> D/L2, (b) kCT ≈ D/L2, and (c) kCT > D/L2. (b) τCT, is similar to L2/D, i.e. kCT ≈ D/L2. (c) τCT, is longer than L2/D, i.e. kCT 0.05 ± 0.01 cm2 s-1. In contrast, slight decay acceleration was observed for FTO/cTiO2/MAPbI3. In this case, the total electron injection process must be interfacial charge transfer limited rather than charge carrier diffusion limited. The estimated time of interfacial electron transfer from the MAPbI3 conduction band to the c-TiO2 conduction band, τET, in Scheme 2a is 160 ns, using Equation S16. The observation of this slow electron transfer is plausible owing to the small Gibbs free energy difference (∆G = -0.05 eV according to Scheme 2) between the


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perovskite and c-TiO2 conduction band edges. Such slow electron transfer at the c-TiO2/MAPbI3 interface was also observed previously.35 We note that the current c-TiO2 layer is not suitable as an electron acceptor layer if the interfacial electron transfer is the rate limiting process in the perovskite solar cells, and suggest that doping the c-TiO2 layer is required to control the conduction band edge, and thus to increase an interfacial electron transfer rate.36 Figure 1c compares PL decays of glass/MAPbI3, FTO/c-TiO2/m-TiO2/MAPbI3 and FTO/cTiO2/m-TiO2/MAPbI3/OMeTAD. Clear acceleration of PL decay was observed for FTO/cTiO2/m-TiO2/MAPbI3, indicating that electron transfer from the perovskite to the m-TiO2 is significantly faster than the electron transfer from the perovskite to the c-TiO2. The decay appears to be non-exponential with at least two different electron injection processes over different time scales; one (60 %) occurs within an instrument response time of 1.2 ns and the other (40 %) on nanosecond time scale. The time scale of this slower component will be discussed in the next section. This observation clarifies that the electron injection processes can be monitored over the different time scales (from sub-picoseconds to nanoseconds), as reported previously.9, 23-25 As most of excited light at 405 nm is absorbed by the perovskite inside the mesoporous TiO2 structure and the generated electron inside the perovskite can reach the TiO2 particle surface without long diffusion distance (pore size: 20 nm on average37), the reaction can be interfacial electron transfer limited. The slow component decay was fitted well with Equation S16. Further acceleration was observed for FTO/c-TiO2/m-TiO2/MAPbI3/OMeTAD, similar to the decay observed for FTO/MAPbI3 (Figure 1b), suggesting that the hole transfer from the perovskite to the spiro-OMeTAD is fast. The majority of the decay completes within an instrument response time of 1.2 ns (85 %), and the residual is seen on nanosecond time scale (15 %). The slow component is clearly hole diffusion limited, and therefore was fitted using


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Equation S6, resulting in the hole diffusion coefficient, Dh, of 0.2 ± 0.02 cm2 s-1. The hole injection reaction was also assessed with acceleration of PL decay for glass/mAl2O3/MAPbI3/OMeTAD, as shown in Figure S1a, requiring at least Dh of 0.2 ± 0.02 cm2 s-1. The evaluated hole diffusion coefficient for MAPbI3 is significantly larger than those reported previously.3, 20 Our preparation method of the MAPbI3 perovskite structure clearly shows longer bulk PL lifetime (34 ns compared to 9.6 ns3 with similar excitation intensity), and thus may sufficiently passivate hole trap states. Charge injection yields to the electron acceptors and donor (c-TiO2, m-TiO2 and spiroOMeTAD) were estimated as a function of excitation intensity using Equation S22 or S24, and presented in Figure 1d. The hole injection yield is higher than the electron injection (to m-TiO2) yield, while the electron injection yields to the c-TiO2 are low (> D/L2 or (b) kCT ≈ D/L2 (see the Supporting Information for more detail).


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(b)

(a) 2

De = 0.012 cm s Se = 1100 cm s-1

-1

glass/MAPbI3

De = 0.005 cm2 s-1 1.1 M 1.0 M 0.5 M

De = 0.01 cm2 s-1

0.2 M

(c) 1.1 M

hole

electron

1.0 M 0.5 M 0.2 M (d)

Figure 3. (a) Transient emission decays (slower electron injection process) at 770 nm, obtained for an m-TiO2 film with MAPbI3 thickness of 230 nm (PbI2 concentration of 1.1 M) with 625 nm excitation and excitation intensity of 0.1 µJ cm-2. The green broken lines show results of fitting using Equation S4. The red solid line shows results of fitting using Equation S9. (b) Transient emission decays at 770 nm, obtained for an m-TiO2 film with various MAPbI3 thickness with 625 nm excitation and excitation intensity of 0.1 µJ cm-2. The number indicates PbI2


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concentration used to prepare the perovskite layer. PL decay obtained for glass/MAPbI3 (prepared with 1.1 M PbI2) under the same excitation condition is also shown as reference. (c) Transient absorption decays monitored at 1,600 nm, for an m-TiO2 film with various MAPbI3 thickness and with spiro-OMeTAD with 625 nm excitation and excitation intensity of 120 µJ cm2

. The black solid line indicates results from the stretched exponential fitting. (d) Effective

perovskite thickness dependence of charge injection yield (electron and hole) and interfacial charge recombination lifetimes. The dotted line for each data set is shown as a guide.

For the film prepared with 1.1 M PbI2 solution, the faster decay process completes within an instrument response time of 1.2 ns, and is clearly interfacial charge transfer limited. In contrast, the slower decay data in Figure 3b indicates that the slow electron injection process is limited by either diffusion of electrons to reach the m-TiO2/MAPbI3 interface or the interfacial electron transfer rate at the m-TiO2/MAPbI3 interface. The data was fitted with the above two different cases ((a) kCT = (1/τCT) >> D/L2 and (b) kCT ≈ D/L2), and the fitted results are shown in Figure 3a. It is clear that fitting with the 2nd model, kCT ≈ D/L2, with De of 0.012 cm2 s-1 and Se of 1,100 cm s-1, resulting in the total injection time, τe-inj, of 39 ns, is more appropriate compared to the 1st model, kCT = (1/τCT) >> D/L2. Global fitting with other decay data except the data for the film prepared with 0.2 M PbI2 solution results in De of 0.016 ± 0.004 cm2 s-1 and Se of 1,300 ± 200 cm s-1. This surface electron transfer velocity is larger than the recently obtained surface recombination velocity, 100 cm s-1,21, 41 and thus the decay is limited by both electron diffusion and interfacial electron transfer into the m-TiO2. The fitted electron diffusion coefficient is in agreement with that evaluated previously.3 For the film prepared with 0.2 M PbI2 solution, the slow PL decay cannot be comfortably fitted with these two diffusion models, (a) kCT = (1/τCT) >>


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D/L2 and (b) kCT ≈ D/L2. The film shows a typical ETA structure, shown in Scheme 1c, i.e. the perovskite was available only inside the m-TiO2, and that the photo-generated electrons travel within 10 nm (average pore size: 20 nm37) before reaching the TiO2 surface. Therefore, the reaction is limited by the interfacial electron transfer process, as shown in Scheme 2c. The nanosecond PL decay data was fitted with (c) kCT 90 %, = > 1, > = 1 −

! "

% $ > 90 %, < 0.1 $

(2)

(3)

where LHE(λ) is the light harvesting efficiency, ϕinj is the charge injection yield, η is the charge collection yield, A is the absorbance, α is the absorption coefficient of a perovskite film, L is the perovskite film thickness, τinj is the total charge injection time, and τb is the bulk PL lifetime.


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If the interfacial change transfer is extremely fast, the total charge injection process will be limited by the charge diffusion time in the perovskite layer.

≈

./ 0

< 0.1 $ , 1 >

!2 ./ 3

>

!2 " / 3

(4)

This relation provides the minimum required charge carrier diffusion coefficient, Dm = 10/(α2τb), if the charge injection is limited by the charge diffusion in the perovskite, but not by the charge drift.21 This relation further suggests that the identified minimum diffusion coefficients of electrons and holes determine an optimum solar cell structure from the three different configurations, (a) planar structure, (b) mesoporous based structure and (c) ETA solar cell structure. In the discussion below, we consider requirement of charge injection reactions as well as interfacial charge recombination reactions. Planar structure. In a planar structure (Scheme 1a), the charge injection reaction should be limited by charge carrier diffusion. Assuming that the interfacial charge transfer is extremely fast, if the electron and hole diffusion coefficients, De and Dh, of a perovskite layer satisfies Equation 4, then any solar cell structure would be suitable for efficient charge injection processes. In contrast, for the interfacial charge recombination process, the charge separated state lifetime is the longest with the thickest perovskite layer (see Figure 3d). Therefore, the planar structure would be the optimum solar cell structure. Mesoporous based structure. If one of charge carrier diffusion coefficients (De or Dh) does not satisfy Equation 4, a mesoporous based solar cell structure as shown in Scheme 1b is the most suitable. For example, if the electron diffusion coefficient does not satisfy Equation 4, a mesoporous n-type semiconductor film such as an m-TiO2 structure should be employed in the


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solar cell to accept electrons with short diffusion length inside the porous structure. In this case, the perovskite film thickness can be sufficiently large, and thus relatively long charge separated state lifetime is expected to maximize solar cell performance. ETA solar cell structure. If both De and Dh do not satisfy Equation 4, light generated charge carriers in a perovskite layer should be collected sufficiently with short diffusion length, before they recombine. Therefore, a thin layer of perovskite has to be bound by a porous n and p type semiconductor. Dye sensitized solar cell or ETA solar cell structure as shown in Scheme 1c is the most appropriate. However, in this case, the perovskite layer is thin, compared to the planar or mesoporous based structure, and thus, the interfacial charge recombination is relatively faster. This type of solar cell structure is the least suitable owing to relatively faster interfacial charge recombination reactions. Role of m-TiO2 for MAPbI3 perovskite. In the present study, we have investigated influence of the MAPbI3 perovskite layer and m-TiO2 film thickness on the charge injection and recombination dynamics. For FTO/c-TiO2/MAPbI3, the estimated electron injection time constant is 160 ns, and thus, the current c-TiO2 layer is not suitable as an electron acceptor layer. Assuming that the minimum absorption coefficient of a perovskite layer is 2.5 ×104 cm-1, the perovskite layer thickness of 400 nm is required to absorb 90% of incident light. To ensure at least 90 % charge injection efficiency, the charge injection reaction has to complete within 20 ns, since the typical bulk PL decay rate is 200 ns. Assuming that the interfacial charge transfer occurs significantly faster than the charge diffusion time, following Equation 4, Dm becomes 0.08 cm2 s-1, required to support a planar structure. The present study has identified that MAPbI3 has De and Dh of 0.016 ± 0.004 and 0.2 ± 0.02 cm2 s-1, respectively. Since De is far smaller than 0.08 cm2 s-1, the planar structure is not suitable, and m-TiO2 should be utilised in the cell


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(Scheme 1b), while no mesoporous hole acceptor is required. The similar conclusion has been reached by Cahen et al. by identifying the smaller electron diffusion coefficient using the electron beam-induced current (EBIC) technique.47 Here we found that the electron diffusion distance should be shorten to design the solar cells. This suggestion agrees with the previous studies that the solar cell efficiency of >20 % has been achieved by employing an m-TiO2 structure for MAPbI3.1 In contrast, the hole injection yield remains high (>95 %) with the perovskite layer thickness of up to 2.8 µm. The charge injection efficiency can be maximized (close to 100 %) by employing FTO/cTiO2/m-TiO2/MAPbI3/OMeTAD/Au solar cell structure. This in turn indicates that the cell performance is essentially controlled by the interfacial charge recombination process, i.e. longer charge separated state lifetime potentially improves solar cell performance. Employing an mTiO2 film also influences interfacial charge recombination rates. Figure S5 clearly shows extended charge separated state lifetimes for m-TiO2/MAPbI3/OMeTAD compared to mAl2O3/MAPbI3/OMeTAD. We therefore conclude that employing MAPbI3, m-TiO2 is required to separate charges efficiently and retard interfacial charge recombination reactions, and thus to optimize solar cell performance. Comparison with other studies. The above method selecting an appropriate solar cell structure can be in principle applicable to any perovskite film or semiconductor light absorber. Here we discuss other perovskite types and semiconductors reported so far to apply this structure selection method. The required condition is to obtain IPCE of >80 %, assuming that the total charge injection is limited by the charge carrier diffusion. Examples of the results are summarized in Table 2.


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Table 2. Summary of semiconductor types, absorption coefficient (α), required film thickness (Lm), bulk PL lifetime (τb), electron diffusion coefficient (De), hole diffusion coefficient (Dh), required minimum diffusion coefficient (Dm), and recommended cell structure, assuming infinite interfacial

charge transfer rates.

Type

α / cm-1

Lm /

MAPbI3

2.5 ×104 at 750 nm

MAPbI3xClx

τb / ns

De / cm2 s-1

Dh / cm2 s-1

Dm / cm2 s-1

Recommended structure

Ref.

400

200

0.016

0.2

0.08

n-type meso

This 20 study,

2.7 ×104 at 750 nm

385

273

0.042

0.054

0.05

n-type meso

3

MAPbBr3

6.0 ×104 at 470 nm

167

51

0.22

0.23

0.054

Planar

48

CsPbI3

2.0 ×104 at 640 nm

500

50

0.025

1.8 × 10-3

0.5

ETA

49

MASnI3

1.0 ×104 at 720 nm

1,000

3.7

1.28

0.59

27

ETA

50

CsSnI3

1.0 ×105 at 880 nm

100

6.6

1.3

>1.3

0.15

Planar

51-52

(CH3NH3) 3Bi2I9

1.0 ×105 at 550 nm

100

5.6

0.064

0.023

0.18

ETA

12

CdTe

5.0 ×104 at 780 nm

200

0.83~180

17

>17

0.022~ 4.8

Planar

53-55

GaAs

7,000 at 870 nm

1,430

38~1,300

5~200

5.5~16 5

0.16~5 .4

Planar

56-58

nm

MAPbI3 and MAPbI3-xClx show relatively smaller electron diffusion coefficients compared to hole diffusion coefficients and the required minimum coefficients. Therefore, this class of organic lead iodide perovskite requires an m-TiO2 film for efficient charge injection reactions and to retard charge recombination reactions with sufficient perovskite layer thickness. The combination of this type of perovskite with a m-TiO2 film has been successful to exceed a solar


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energy conversion efficiency of 22 %.1 The planar structure is appropriate for the semiconductors with high absorption coefficients or large electron and hole diffusion coefficients. It is interesting to note that CsSnI3 has high absorption coefficients as well as large charge diffusion coefficients. In contrast, the perovskite requiring ETA structure generally have small electron and hole diffusion coefficients compared to the required minimum coefficients or small absorption coefficients. Passivating surface recombination sites of those perovskites is extremely important to develop a new type of perovskite films.

Conclusion We have employed transient emission and absorption spectroscopies to identify charge carrier diffusion coefficients inside a perovskite layer, interfacial charge injection and recombination rates for a MAPbI3 perovskite layer sandwiched by TiO2 and spiro-OMeTAD films. For the planar heterojunction structure, the electron injection from the perovskite to c-TiO2 is extremely slow (injection time constant: 160 ns), compared to the PL decay (34 ns) and thus is the interfacial electron transfer limited rather than the electron diffusion limited, suggesting that doping the c-TiO2 layer is required. With the mesoporous structure, the electron injection processes appear to be non-exponential with at least two different electron injection processes over different time scales; one (60~70 %) occurs within an instrument response time of 1.2 ns and the other (30~40 %) on nanosecond time scale. For the slower process, the interfacial electron transfer rate was found to be similar to the electron diffusion time (De: 0.016 ± 0.004 cm2 s-1) in the perovskite layer, and thus the slower electron injection process is both interfacial electron transfer and electron diffusion limited. In contrast, minimum Dh of 0.2 ± 0.02 cm2 s-1 was obtained. The hole injection yield remains high even if thick m-TiO2 is employed. By


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increasing a perovskite layer thickness on top of m-TiO2, the interfacial charge recombination is significantly retarded (by two orders of magnitude, up to 40 ms), suggesting that the increased coverage with the perovskite layer extends the charge separated state lifetime. By obtaining absorption coefficient and bulk PL decay rate of the MAPbI3 perovskite, the minimum diffusion coefficient, Dm, required to support a planar structure, was evaluated to be 0.08 cm2 s-1. This value is larger than De (0.016 ± 0.004 cm2 s-1), but smaller than Dh (0.2 ± 0.02 cm2 s-1). Thus, we conclude that to maximize the charge injection yield for the MAPbI3 perovskite, an m-TiO2 film is required. Employing an m-TiO2 film also retards interfacial charge recombination reactions, and thus optimizes solar cell performance. The method to work out the required minimum diffusion coefficient was applied to several different semiconductors, and the most appropriate solar cell structure was identified.

ASSOCIATED CONTENT Supporting Information. Experimental (Characterization and Analysis) details; Spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources Y. T. received funds from Japan Science and Technology agency, PRESTO program (Photoenergy Conversion Systems and Materials for the Next Generation Solar Cells), the Collaborative Research Program of Institute for Chemical Research, Kyoto University (grant number 2017-75), ARC LIEF grant (LE170100235), Australia, and JSPS KAKENHI Grant Number 16K05885, Japan.

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Number 16K05885, and partly supported by the Collaborative Research Program of Institute for Chemical Research, Kyoto University (grant number 2017-75), and the JST PRESTO program (Photoenergy Conversion Systems and Materials for the Next Generation Solar Cells), Japan. We also acknowledge supports from an ARC LIEF grant (LE170100235), Australia, and Office for Industry-University Co-Creation at Osaka University.


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Table of Contents Graphic


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Identifying an Optimum Perovskite Solar Cell Structure by Kinetic

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