Quantifying Hole Transfer Yield from Perovskite to Polymer Layer

Article pubs.acs.org/journal/apchd5

Quantifying Hole Transfer Yield from Perovskite to Polymer Layer: Statistical Correlation of Solar Cell Outputs with Kinetic and Energetic Properties Naoki Ishida,† Atsushi Wakamiya,‡,§ and Akinori Saeki*,†,§ †

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ‡

S Supporting Information *

ABSTRACT: Organic−inorganic hybrid perovskites provide not only an exceptionally rich area of research but also remarkable power conversion efficiency relevant to commercial use. However, developing efficient organic hole transport layers remains challenging, due partly to the subtle electronic behavior of perovskite and complications introduced by the use of reactive dopants. Here we show, through time-resolved microwave conductivity, the quantification of a hole transfer process from methylammonium lead triiodide perovskite to eight kinds of conjugated polymers with and without a Li dopant. The time evolution of hole transfer yield is characterized by kinetic parameters, which are further examined in conjunction with solar cell performance, energetics, and temporal profiles triggered by exposure to air at the minute scale. Using statistics and LASSO (least absolute shrinkage and selection operator) analysis, we identify an accurate descriptor that correlates with device output. This work explores the design of organic hole transport materials, and the presented evaluation technique may be employed as a facile screening method. KEYWORDS: organometal halide perovskite solar cell, conjugated polymer, time-resolved microwave conductivity, hole transfer process, LASSO analysis

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(trifluoromethylsulfonyl) imide salt (CoTFSI) together with 4-tert-butylpyridine (TBP) significantly outperform nondoped materials;21 however, the control and stabilization of doped HTLs are still problematic issues.22,23 Conjugated polymers, such as regioregular poly(3-hexylthiophene) P3HT24 and diketopyrrolopyrrole-based copolymers,25 which have been used as organic photovoltaic active layers, have been widely investigated as HTL materials. The PCEs were inferior to those obtained with doped spiroOMeTAD because of the rough surface morphology of perovskite crystallites, necessitating a relatively thick HTL (>100 nm) without pin-holes.26 However, state-of-the-art wetprocessing of perovskite has enabled the formation of smooth surfaces by solvent engineering, allowing the fabrication of efficient perovskite solar cells with thin HTLs (ca. 50 nm) of poly(triarylamine) (PTAA).13 Chemical doping of the polymer is still necessary to achieve high performance, but the doping process can be detrimental to the practical application of the material in view of the instability and poor reproducibility of the oxidized form of the HTL. Thus, HTLs comprising small molecules that work without doping, or inorganic HTLs, are

ver the last 5 years, impressive progress in nextgeneration solar cells has been made with organic− inorganic perovskite materials.1−3 Their potentially costeffective fabrication from inexpensive organic cation, metal cation, and trihalide sources,4−8 and their power conversion efficiencies (PCEs) of over 20%,9,10 have made this class of material a promising candidate for the harvesting of abundant solar energy. The constant improvements in PCE have been driven mainly by developments in process engineering that allow the fabrication of large, high-quality, densely packed perovskite layers via two-step methods,11,12 poor-solvent treatment,13 vapor treatment,14 thermal treatment,15 and intermediate crystal engineering.16 Currently, the predominant hole transport layer (HTL) material is 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene (spiro-OMeTAD), which has been the milestone small molecule in solid-state dye-sensitized solar cells since its introduction in 1998.17 This is because spherically shaped spiro-OMeTAD exhibits excellent compatibility with mesoporous superstructured solar cells, most likely owing to its amorphous features with high pore-filling ability18,19 and moderate hole mobility.20 HTL materials doped with lithium bis-(trifluoromethylsulfonyl) imide salt (LiTFSI) and tris(2(1H-pyrazol-1-yl)-4-tertbutylpyridine) cobalt(III) bis© XXXX American Chemical Society

Received: May 12, 2016

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DOI: 10.1021/acsphotonics.6b00331 ACS Photonics XXXX, XXX, XXX−XXX

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attractive alternatives.27−29 However, their PCEs are not as good as those of doped PTAA and spiro-OMeTAD. The hole extraction process underlying device output remains relatively unexplored as the debate has so far been largely focused on device characterization, which is a consequence of several complex issues. In addition to the high susceptibility of these devices to extrinsic factors like humidity,30 heat,31 and impurities,32 anomalous hysteresis33−35 and fluctuation of device output have hindered the acquisition of fundamental insight into hole transport and the role of doping. In this report, we present the direct observation of the hole transfer process from a prototype perovskite layer of methylammonium lead triiodide (MAPbI3) to conjugated polymer HTLs with different highest occupied molecular orbital (HOMO) levels (Figure 1 and Table 1. The

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RESULTS AND DISCUSSION Local Hole and Electron Mobilities in MAPbI3. The MAPbI3 layers were prepared by a one-step method from a dimethyl sulfoxide (DMSO) solution of CH3NH3I and high purity PbI2 (1:1 molar fraction)32,38 followed by poor-solvent (toluene) treatment during spin-coating.13 After thermal annealing at 100 °C for 10 min, an opaque cocrystal of MAPbI3 and DMSO was converted to 3D perovskite, followed by the HTL with or without dopants being deposited by spincoating. Figure 2a shows the TRMC transient of pristine perovskite on bare quartz without a mesoporous titanium dioxide (mpTiO2) scaffold excited at 630 nm, exhibiting a ϕΣμ (ϕ: charge carrier generation yield, Σμ: sum of local hole and electron mobilities) maximum as high as 73 cm2 V−1 s−1. The ϕ at a low excitation density (I0 ca. 1011 photons cm−2 pulse−1) is assumed to be close to unity, and thereby, ϕΣμmax is regarded as the minimum charge carrier mobility, as supported by the flat dependence of maxima and long-lived decay in the low I0 region (Supporting Figure S4). It is noteworthy that the mobility is almost 3 times higher than those in materials prepared by previous one-step or two-step methods (10−20 cm2 V−1 s−1) as evaluated by TRMC39 and terahertz timedomain spectroscopy.40,41 The improved Σμ by the new onestep method indicates its higher electronic quality, which is related to the large and densely packed crystals that induce reduced charge recombination and enhanced mobility. It should be noted that the observed local charge carrier mobility is consistent with the reported Hall effect hole mobilities of MAPbI3 crystals (66 cm2 V−1 s−1)42 and single crystal (34−105 cm2 V−1 s−1),43,44 but one order smaller than that of single crystal evaluated by THz spectroscopy in subpicosecond time domain (∼800 cm2 V−1 s−1).45 Indeed, the improved Σμ of perovskite via the present one-step method affords an improved average PCE of 17.1% (Jsc = 22.6 mA cm−2, Voc = 1.01 V, FF = 0.749, Figure 2b and Table 1), much higher than our previous PCE (9.6%) afforded by the two-step method (Jsc = 19.2 mA cm−2, Voc = 0.90 V, FF = 0.56).39 The slight hysteresis (forward scan = 16.7%, backward scan = 17.4%) and fluctuation among the cells (15.5 ± 1.3%, Supporting Figure S5) should be noted along with the high external quantum efficiency (the integrated Jsc = 22.5 mA cm−2, Supporting Figure S6). The scanning electron microscopy (SEM) and atomic force microscopy (AFM) images in Figure 2c visualize the large, dense-packed grains relevant to a high quality perovskite as reported previously.13 The cross sectional SEM and the stability of PCE (∼30 min, without encapsulation) are appended in Supporting Figure S7. Top-coating of phenyl-C61-butyric acid methyl ester (PCBM) onto the perovskite layer effected the suppression of ϕΣμmax concomitant with decelerated transient decay (Figure 2a). PCBM has been frequently utilized as the electron transport layer (ETL),46 in particular, of low temperatureprocessed planar perovskite solar cell (e.g., with poly(3,4ethylenedioxythiophene) polystyrenesulfonate or another conducting polymer as the HTL), with PCEs of up to 18% being reported.47−49 Hence, electrons generated in the perovskite layer are effectively transferred to the PCBM capping layer, as is evident from the pronounced photoluminescence quenching of perovskite.50 Provided that electrons are captured by the PCBM layer at the end-of-pulse and the TRMC electron mobility of PCBM (ca. 0.1 cm2 V−1 s−1)51 is much lower than that of perovskite, the remaining signal is readily attributed to

Figure 1. HTL conjugated polymers. (a) Chemical structures. (b) Energetics of MAPbI3 perovskite (black bar, the deep energy: VB maximum, the shallow energy: conduction band minimum) and HTLs (colored bars, the deep energy: HOMO, the shallow energy: the lowest unoccupied molecular orbital, LUMO) evaluated by PYS of the films and their optical bandgaps.

photoelectron yield and photoabsorption spectra are appended in Supporting Figures S1−S3). The transient photoconductivities of these composite films were evaluated using flashphotolysis time-resolved microwave conductivity (TRMC).36,37 From the kinetic comparison of pristine perovskite and the doped/nondoped HTL-capped perovskite, we explored the correlation between TRMC transients and short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and PCE. We further identified the key role of doping in hole transfer at the delayed time scale of several microseconds, as well as the oxidation in air effect at the tens-of-minutes scale. Along with electrochemical information, our study highlights the importance of fine-tuning HTL HOMO levels, and facilitates a deeper understanding of dopant effects. This has wide-ranging implications for the design of efficient and stable HTLs for perovskite solar cells. B


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Table 1. Summary of Energetic, Kinetic, and Device Parameters hole transfer kineticsa

HTL material P3HT PTAA TQ1d PPV FT37 FT55 F8T2e FT73

HOMO (doped)/eV −4.92 −5.17 −5.18 −5.21 −5.36 −5.48 −5.52 −5.59

(−4.74) (−5.30) (−5.17) (−5.21) (−5.64) (−5.70) (−5.68) (−5.93)

c

η0

ηsat

0.47 0.76 0.16 0.32 0.34 0.60 0.02 0.01

0.96 0.99 0.85 0.97 0.95 0.67 0.71 0.11

6

k (10 )/s 9.2 8.7 1.5 4.9 5.7 0.65 2.1 0.38

−1

MAPbI3 solar cellb β 1.00 0.86 0.85 1.00 0.99 1.00 0.73 1.00

−2

Jsc/mA cm 21.1 22.6 17.7 19.3 21.5 20.2 12.2 6.08

Voc/V

FF

PCE/%

0.906 1.010 0.943 0.904 0.953 0.818 0.839 0.789

0.721 0.749 0.561 0.562 0.656 0.462 0.286 0.313

13.8 17.1 9.39 9.81 13.4 7.65 2.92 1.50

a Evaluated from TRMC transients of doped HTL on MAPbI3/mpTiO2. The transients were measured at 5 min after the samples were taken out from a glovebox. bFTO/compact TiO2/mpTiO2/MAPbI3/HTL(LiTFSI/TBP)/Au. Each parameter is the averaged value between forward and backward scans. cDetermined by PYS for the film on ITO/glass. The HTLs are sorted in descending order of neutral HOMO levels. The VB of MAPbI3 was −5.50 eV. dPoly[[2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl] ePoly[[2,2′-bithiophene]-5,5′-diyl(9,9-dioctyl-9Hfluorene-2,7-diyl)]

comparable to that of mpTiO2/perovskite without PCBM (33 cm2 V−1 s−1), and their decay speeds are quite similar. Hole Transfer Yield. To gain direct access to the hole transfer process, we conducted TRMC evaluation of MAPbI3 on mpTiO2 with and without an HTL for which photoconductivity is dominated by holes of MAPbI3. Here, we introduce the hole transfer yield, ηHT(t), defined by ηHT(t ) =

ϕΣμ(t ) − ϕΣμHTL (t ) ϕΣμ(t )

(1)

where ϕΣμ(t) and ϕΣμHTL(t) are time-dependent ϕΣμ without and with an HTL, respectively. The unity of ηHT represents complete hole transfer from the perovskite to the HTL. Figure 3a shows the ϕΣμ(t) and ϕΣμHTL(t) transients using an HTL of PTAA or FT55 with and without LiTFSI and TBP dopants. The prompt decay of MAPbI3/PTAA, which is much faster than that observed for MAPbI3/FT55, is directly linked to the larger energetic offset between the valence band (VB) maximum of MAPbI3 (−5.50 eV) and the HOMO level of PTAA (−5.17 eV) than that of FT55 (−5.48 eV). Notably, doping suppresses the ϕΣμ maxima in both the HTLs, indicative of improved hole transfer yield. The ηHT values of PTAA and FT55 were evaluated according to eq 1 as shown in Figure 3b, where we define ηHT(t) at 10− 40 ns (ca. the time resolution of TRMC) and ca. 3 μs as η0 and ηsat, respectively. The doped PTAA exhibits a higher η0 (0.76) than that of nondoped PTAA (0.41). In addition, their ηHT(t) values increase rapidly and reach an ηsat of 0.98−0.99. Conversely, the doped and nondoped FT55 exhibit low η0 of 0.60 and 0.17, respectively, followed by small increases to ηsat of 0.67 and 0.23, respectively. The normalized kinetics, that is, (ηsat − ηHT(t))/(ηsat − η0), were analyzed by the least-squaresmean fit of a stretched exponential function, exp(−(kt)β), and the results are shown in Figure 3c. The delayed hole transfer rates k are 8.7 × 106 and 6.5 × 105 s−1 for doped PTAA and FT55, respectively, intuitively demonstrating the more efficient hole extraction by PTAA than that of FT55. The resultant PCE of the doped FT55 devices are as low as 7.7% (Jsc = 20.2 mA cm−2, Voc = 0.818 V, FF = 0.462), less than half that of the doped PTAA device (Table 1). This clearly indicates that the hole transfer yield limits the device performance, in particular, Jsc and FF (detailed correlations are discussed later). In the course of our evaluations, we noticed that ηHT(t) is sensitive to exposure to air after the films are removed from a N2 glovebox. We therefore examined the dependence of ηHT(t)

Figure 2. Photoconductivity transients of layered perovskite films and solar cell outputs. (a) ϕΣμ transients measured by TRMC (λex = 630 nm, 1.6 × 1011 photons cm−2 pulse−1) for each layered structure. (b) J−V curves of MAPbI3 solar cell with different HTLs (doped) under AM 1.5G pseudosunlight (100 mW cm−2). The solid and dotted lines represent the forward and backward scans, respectively. (c) SEM and AFM images of MAPbI3 layer.

the hole mobility of perovskite. The spatial separation of the electrons in PCBM and holes in the perovskite layer inhibits their recombination, leading to the deceleration of TRMC decay. Importantly, the hole and electron mobilities are found to be 33 and 40 cm2 V−1 s−1, respectively (i.e., much more balanced than those exhibited by the previous two-step perovskite).39 This is relevant to the balanced and lightweight effective masses of charges (0.1−0.3 me) obtained from calculations52,53 and magnetic field spectroscopy.54 A perovskite layer on mpTiO2 affords decreased ϕΣμmax and slow decay, similar to the perovskite/PCBM film; however, its ϕΣμmax is 1.5 times higher than the latter (Figure 2a). This is explained by incomplete electron transfer to the mpTiO2, which agrees well with the slightly faster decay than that in the perovskite/PCBM. The presence of both underlying mpTiO2 and top-coated PCBM completely quenches the electron contribution, whereas the ϕΣμmax (36 cm2 V−1 s−1) is C


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on the air-exposure time from 1 to 30 min. The time evolution of η0, ηsat, and k obtained for PTAA, P3HT, FT37, and FT55 are shown in Figure 3d−f (ηHT(t) and fitting curves are provided in Supporting Figures S8 and S9). For all the doped HTLs, the kinetic parameters (η0, ηsat, and k) undergo a rapid change from 1 to 5 min and remain largely unchanged after 10 min. The temporal profile of the hole concentration of doped PTAA was evaluated from the steady-state photoabsorption at 530 nm, which exhibits a rapid increase at 5 min and gradual increase from 5−30 min (Supporting Figures S10). This corresponds approximately to the growth of η0 and k of the doped HTLs (Figure 3d,f). In contrast, nondoped HTLs exhibit slow but large changes of ηHT(t) up to 30 min. For instance, the η0 of nondoped PTAA increases from ca. 0.0 at 1 min to 0.83 at 30 min in an exponential manner (τ = 4.9 min), which is slower than that of doped PTAA (τ < 1.8 min). The electrical conductivity of an HTL increases in intensity by oxidation through indirect electron transfer to Li+, as has also been reported for spiro-OMeTAD.55 HTL + O2 ↔ (HTL·+ + O2−)

(2)

(HTL·+ + n/4O2−) + (Li − TFSI) → (HTL·+ + TFSI−) + 1/2Li 2On

(n = 1 or 2)

(3)

Thus, exposure to air is essential for doping. This is confirmed by observation of the progressive decrease in series resistance at the beginning of device characterization, in particular for low-performing devices. Schölin et al. have reported the shift of Fermi energy level upon doping and its saturation at a specific value (0.8 eV), which they ascribed to pinning to the HOMO of spiro-OMeTAD.56 They reported the vertical concentration gradient of Li dopants and facilitation of oxidative reactions by stabilizing the reduced oxygen in the form of, for example, Li2O. Other groups have also reported the same findingsthat is, that oxidation of spiro-OMeTAD by an Li dopant in air substantially increases its conductivity and reduces series resistance.22,57 In the absence of a dopant, the O2 in air is insufficient for high doping, and thus, much more time is needed for the increase of η0, ηsat, and k. The power factor β

Figure 3. Kinetic analysis of hole transfer yield. (a) ϕΣμ transients of MAPbI3 with doped or nondoped HTLs (PTAA as red or FT55 as blue) normalized by those of MAPbI3 without HTL (indicated by “No HTL”). (b) Semilogarithmic plot of hole transfer yield ηHT and time in s. The green mesh region represents η0 or ηsat. (c) Semilogarithmic plot of normalized ηHT vs time in μs. The black solid lines are leastmean-square fits of the stretched exponential function, exp(−(kt)β). Time evolutions of (d) η0; (e) ηsat; and (f) k at the minute time scale after exposure to air. The doped HTLs and nondoped HTLs are shown by the closed symbol with solid lines and open symbols with dotted lines, respectively. The polymer is distinguished by color (red: PTAA, purple: P3HT, blue: FT55, yellow: FT37).

Table 2. Data Analysis To Find Correlation between Kinetic Factor of Doped HTL and Device Output r (rLasso)b

explanatory variable synonym x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 x11 x12 x13 x14 x15

a

entity

Jsc

Voc

FF

PCE

ln(η0k) η0 η0 + ηsat η0k ln((η0 + ηsat)k) (η0 + ηsat)k η0 + ηsat + lnk ln(ηsatk) ηsat k η0 + lnk ηsat ηsat + lnk lnk k β

0.96 (1.40) 0.95 (1.33) 0.94 (1.02) 0.93 (0) 0.87 (0) 0.85 (0) 0.83 (0) 0.80 (0) 0.80 (0) 0.80 (0) 0.76 (0) 0.76 (0) 0.71 (0) 0.66 (0) 0.19 (0)

0.76 (0) 0.72 (0) 0.66 (0) 0.76 (1.81) 0.70 (0) 0.70 (0) 0.67 (0) 0.67 (0) 0.57 (0) 0.72 (0) 0.71 (0) 0.66 (0) 0.63 (0) 0.60 (0) 0.09 (0)

0.88 (0.63) 0.83 (0) 0.74 (0) 0.87 (0) 0.77 (0) 0.77 (0) 0.72 (0) 0.73 (0) 0.57 (0) 0.78 (0) 0.77 (0) 0.72 (0) 0.70 (0) 0.70 (0) 0.31 (0)

0.93 (2.37) 0.88 (0.18) 0.89 (0.39) 0.92 (0) 0.84 (0) 0.83 (0) 0.84 (0) 0.78 (0) 0.78 (0) 0.82 (0) 0.61 (0) 0.77 (0) 0.74 (0) 0.74 (0) 0.20 (0)

a

Sorted in descending order of r of Jsc. br: correlation coefficient (−1−1). rLasso: regression coefficient (arb. unit) of LASSO analysis. The highest r (rLasso), namely the most correlated variable, is indicated by bold for each column. D


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Figure 4. Statistics and LASSO analyses on the correlation between the hole transfer process (doped HTL) and device output. (a) Correlation coefficient (r, blue bar) and regression coefficient of LASSO (rLASSO, red bar) for Jsc. Correlation of Jsc with (b) x1 (= ln(η0k)); (c) x2 (= η0); and (d) x3 (= η0 + ηsat). (e) r and rLASSO for PCE. Correlation of PCE with (f) x1; (g) x2; and (h) x3. The explanatory variables xn (n = 1−15) were calculated from TRMC measurements of doped HTLs. See Table 2 for each xn. The solid gray lines are least-squares-mean fits of A × xnB. The colorized symbols represent the polymers as shown in the inset of (a) or (e). The closed black circles are molecular HTLs and the open circle (mostly overlapped) is spiroOMeTAD taken from ref 38.

the PCEs of solar cells in a straightforward manner, as expressed by ηHT × ηHTP × ηHC. Therefore, each factor must be as close to 1 as possible. Thus, it is important to elucidate how the ηHT and device outputs are influenced by the energetics and kinetics of HTLs with and without dopants. Exploration of Explanatory Variables for Doped HTL. On the basis of the η0, ηsat, k, and β values obtained by TRMC measurements, we explored the explanatory variables that account for the objective variables (device outputs) using conventional statistics and least absolute shrinkage and selection operator (LASSO) analysis.58 The latter is a regression analysis method allowing enhanced prediction accuracy. As the explanatory variables xn (n = 1−15), we employed not only sole variables (e.g., η0) but also their combinations, such as summation (e.g., η0 + ηsat), product (e.g., η0k), and logarithmic (e.g., ln(η0k)), as listed in Table 2. η0k signifies that the delayed hole transfer quantified by k may

of the stretched exponential function ranges from 0.8 to 1.0 over the whole air exposure time (Supporting Figure S11), except for nondoped FT55 and FT37, which possess deep HOMO levels of −5.48 and −5.36 eV, respectively. They exhibit continuous decreases of β from 1.0 at 1 min to 0.4−0.5 at 30 min. This may be due to their small offset against the VB of MAPbI3 (ΔHOMO‑VB = 0.02−0.14 eV), which causes backtransfer of holes from the HTL to MAPbI3 and resultant hole retention at the HTL/MAPbI3 interface. Given that much worse PCEs are commonly observed for nondoped HTLs, we can conclude that oxygen-mediated doping improves the perovskite-to-HTL hole transfer yield, reduces series resistance, and facilitates Ohmic contact with the metal anode.22 In addition to ηHT, the yield of hole transport in HTL (ηHTP) associated with series resistance and long-range mobility, and that of hole collection at the HTL/metal interface (ηHC) associated with Ohmic contact, are presumed to impact E


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Table 3. Summary of the Most Accurate Descriptor of Device Parameter doping of HTL doped

b

nondoped

device parameter −2

Jsc/mA cm Voc/V FF PCE/% Jsc/mA cm−2 Voc/V FF PCE/%

descriptor

scaling factor: Aa

power factor: Ba

correlation coefficient: r

ln(η0k) η0k ln(η0k) ln(η0k) ln(η0k + ηsatk) η0 + ln(k) η0 + ηsat η0 + ηsat

0.294 0.626 6.69 × 10−3 1.57 × 10−3 3.33 × 10−2 0.95 0.615 11.3

1.59 0.03 1.72 3.38 2.35 0.09 0.46 0.84

0.96 0.76 0.88 0.93 0.90 0.90 0.98 0.95

The data were fitted by a least-squares-mean method using A × (descriptor)B. bDoped with LiTFSI/TBP for polymer HTL and CoTFSI/LiTFSI/ TBP for molecular HTL. a

performing molecular HTLs (i.e., spiro-OMeTAD and oxygenbridged triarylamine connected to azulene molecules) with CoTFSI, LiTFSI, or TBP dopants38 (Supporting Figure S9 and Table S1). Remarkably, the plots exhibit the same trend, indicating that hole transfer evaluation by TRMC is a versatile screening method for polymer and molecular HTLs. It should be emphasized that the data for azulene molecules include different CoTFSI concentrations (0.05−0.20 equiv) affording Jsc values from 16 to 21 mA cm−2. Explanatory variables for Voc, FF, and PCE were surveyed in the same fashion (Supporting Figures S13−S15). The LASSO analysis of Voc and FF revealed η0k and ln(η0k) to be the best descriptors, respectively (Table 3 and Supporting Figures S16). Consequently, ln(η0k) allowed us to interpolate PCE with the high r of 0.93, supported by decisive prediction by LASSO, as shown in Figure 4e. The correlation plots of PCE as a function of the best three descriptors are presented in Figure 4f−h. Here, we reiterate that the PCEs ranging from 1 to 17% for these HTLs are governed by the hole transfer process at the MAPbI3/HTL interface (i.e., ηHT correlates with ln(η0k) rather than hole transport (ηHTP) and collection (ηHC)). Exploration of Explanatory Variables for Nondoped HTLs. Correlation analysis was also performed on the same explanatory variables (x1−x15) for the nondoped HTLs (Supporting Table S2). Evaluation without dopant could offer advantages over that with dopants owing to facile film preparation and direct comparison with neutral HOMO levels. As displayed in Figure 5a, the parameter screening and LASSO analysis reveals (η0 + ηsat) as the most accurate descriptor of PCE (Supporting Figures S17−S20 and Table S3). Conversely, the correlation with ln(η0k) of nondoped HTLs is much worse than that observed for doped HTLs (Figure 5b). The resultant r of (η0 + ηsat) vs PCE is 0.95, which is even higher than the r for ln(η0k) exhibited by the doped HTLs (0.93). The improved accuracy is mainly ascribed to the sublinear correlation of (η0 + ηsat) with FF (Supporting Figure S21). The hole transport process in nondoped HTLs is largely based on ηsat, because the slow hole transfer occurs mostly in the delayed time domain. Thus, ηsat is likely more preferable than k for representing the delayed hole transfer process in nondoped HTLs. In addition, η0 still contains the information on the prompt hole transfer process, leading to a correlated (η0 + ηsat) with PCE. This is in contrast to the doped HTLs, where the initial hole transfer yield (η0) and rate (k) are significant, leading to a good correlation of ln(η0k) with device output. Importantly, some of the shallow-HOMO polymers (P3HT, PTAA, and FT37) exhibit large values of both ηsat (0.86−0.99) and k (5.7−9.2 × 106 s−1), suggesting that a dopant is intrinsically unnecessary to achieve high hole transfer yields.

contribute to the photocurrent of a solar cell as well as the fast transfer quantified by η0. This is also inferred from the good correlation of ϕΣμ maximum × (lifetime of TRMC decay) with Jsc × FF of a bulk heterojunction polymer−fullerene solar cell, where a large number of long-lived charge carriers cause a large photocurrent.59,60 The explanatory variable was further formulated to A × xnB, where A is the scaling factor and B is the power factor. B (>0) was introduced in order to treat not only simple linear correlations but also sublinear and superlinear correlations. The objective variables (e.g., Jsc and PCE) were fitted by the least-squares-mean method with A × x n B , and the maximized correlation coefficient r was subsequently calculated. Figure 4a shows the bar plot of r analyzed on the basis of the TRMC parameters of a doped HTL, and J sc of the corresponding devices (the fitting results are provided in Supporting Figure S12). r is an indicator of degree of correlation (−1 ≤ r ≤ 1), where −1 and 1 represent perfect negative and positive correlation, respectively. A high r of 0.93− 0.96 was found for the four explanatory variables (x1−x4), all of which comprise η0 (Table 2), underscoring the key role of fast hole transfer (0.85 cm2 V−1 s−1 at 300 K. Indeed, this is close to values reported for wet-processed MAPbI3 films as determined by photoluminescence lifetime spectroscopy.61−63 The delayed hole transfer rate k (105−107 s−1) leads to mobilities of 10−3−10−1 cm2 V−1 s−1 by assuming reciprocal k as a measure of time. The doped PTAA exhibits a large k of 8.7 × 106 s−1, inversely affording a mobility of 0.3 cm2 V−1 s−1. As a consequence, the remaining holes still have the opportunity to diffuse toward the HTL layer, even though their mobilities are reduced to one-third by, for example, shallow traps or relaxation to density of state.38,42 The efficient hole diffusion on the delayed time scale is in line with the eventual ηsat of 0.99 found for PTAA (Figure 3a). Therefore, the observed two-order variation of k is due to the hole transfer process at the MAPbI3/HTL interface, followed by hole dissipation into the bulk of the HTL. These findings are in agreement with the good correlation between ln(η0k) and Jsc. Figure 4b−d show the resulting correlation plots of Jsc against the best three descriptors, i.e., ln(η0k), η0, and η0 + ηsat. The data analysis was performed for not only polymer HTLs with LiTFSI/TBP dopants, but also for reportedly highF


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10% drop in Voc (0.906 V) accompanying a 4−6% decrease in Jsc and FF. This is rationalized by the trade-off between the increasing hole transfer yield relating to the energetic driving force and the decrease in voltage gain. More specifically, the hole transfer yield is likely to have a significant impact on Voc. For instance, TQ1 and poly[2- methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) exhibit moderate HOMO levels (−5.18 and −5.21 eV, respectively) close to the optimal value; however, their low ln(η0k)s appear to impede Voc by 0.06−0.11 V. We also evaluated the HOMO levels of doped conjugated polymers as listed in Table 1, and we observed relatively large shifts of +0.2 to −0.4 eV, in almost linear relationship with the neutral HOMO levels (Supporting Figure S23). Subsequent correlation between HOMO (doped) and Voc becomes less definite (Figure 5d). Regarding the chemical structures, triphenylamine (TPA)-based polymers indicated by the triangle symbols in Figure 5c exhibit high Voc and ln(η0k). Although TQ1 containing nitrogen atoms has the same HOMO levels as PTAA, Voc and ln(η0k) are lowered. This suggests that the TPA unit enhances hole transfer yield and increases Voc via electronic interaction with the MAPbI3 layer. Sun et al. have reported improved photoluminescence quenching using an alkylaminofunctionalized fluorene-naphthalene diimide copolymer as the ETL in a perovskite solar cell, which outperformed PCBMbased devices.68 They postulated that electron-rich nitrogen can passivate the trap sites on the perovskite surface arising from I− vacancies. With respect to HTLs, the high-performing spiroOMeTAD and its analogues contain a TPA unit, implying that such an electron-rich unit is essential for enhancing hole transfer yield. Lewis bases like thiophene or pyridine also work as passivation agents.69 Conversely, conjugated polymers containing thiophene (e.g., P3HT) and quinoxaline (e.g., TQ1) seem to have a large distance over which to facilitate the interaction of sulfur or nitrogen atoms with the undercoordinated Pb of perovskite owing to the steric hindrance or the presence of side-alkyl chains that prefer edge-on orientation on the underlying perovskite. Thus, we suggest that the conjugated polymer should be designed to have a > 0.14 eV higher HOMO than VB maxima of the perovskite and have sterically accessible electron-rich atoms (S or N) on the backbone. Furthermore, the present method is expected to be applicable to the facile and fast screening of hole transport materials, as well as electron transport materials.

Figure 5. Correlation of PCE or Voc with hole transfer to nondoped HTLs. Correlation of PCE with (a) x3 (= η0 + ηsat) and (b) x1 (= ln(η0k)). The solid gray lines are least-squares-mean fits of A × xnB. (c) Plot of Voc with the HOMO of neutral HTLs. The dotted black line is an eye-guide. The colorized symbols represent the data of nondoped polymers as shown in the inset of (a). (d) Plot of Voc with the HOMO of HTLs with dopant.

However, the use of these HTLs without dopants results in poor device efficiencies (7.9, 4.4, and 5.9% for P3HT, PTAA, and FT37, respectively, Supporting Figure S22 and Table S4). Therefore, the inefficient downstream factors of ηHTP and/or ηHC following the hole transfer process degrade the device output, in particular FF (0.4−0.6) and Jsc (17−19 mA cm−2). This is consistent with the literature on conductivity-varied PEDOT:PSS.64 Meanwhile, substantial drops in PCE (0.2− 2.4%) and ηsat (0.13−0.23) are observed for nondoped FT55, F8T2, and FT73, for which their neutral HOMO levels (−5.48 to −5.59 eV) are close to the VB of MAPbI3 (−5.50 eV). From these observations, we suggest that dopant-free HTL polymers should be designed to have a HOMO − VB offset (ΔHOMO‑VB) larger than 0.14 eV (shallower than −5.36 eV) alongside a moderate hole mobility and secured Ohmic contact with a metal anode. It should be mentioned that the ΔHOMO‑VB is similar to the reorganization energy of conjugated materials.65 Recently, Shi et al. reported enhancement of hole mobility of single-crystal spiroOMeTAD by 3 orders of magnitude (∼1.3 × 10−3 cm2 V−1s−1) compared to an amorphous film.66 Therefore, mesoscale ordering of HTL is also expected as a key to achieving high ηHTP even in the same hole transport material. Determinant of Voc. Correlating the HOMO of the HTL with Voc and the overall PCE of the perovskite solar cell is a matter of great concern. In this regard, Polander et al. have surveyed molecular HTLs comprising spiro-OMeTAD analogues and highlighted the delicate energetic balance between the driving force for hole-extraction and maximizing the photovoltage.67 Figure 5c shows the plot of the Voc of MAPbI3 solar cells using doped HTL against the HOMO levels of neutral (nondoped) conjugated polymers. It presents a roughly bell-shape trend with the maximum HOMO at ca. −5.2 eV corresponding to PTAA (−5.17 eV and Voc = 1.01 V). A linear dependence of Voc on HOMO level is observed in the deeper HOMO regions, while P3HT, for which the HOMO level (−4.92 eV) is shallower than the maximum, exhibits a ca.

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CONCLUSION We revealed that local hole and electron mobilities of MAPbI3 prepared by an updated one-step method with poor-solvent treatment were improved and more balanced (33 and 40 cm2 V−1 s−1, respectively) than those in materials prepared by conventional procedures. Using this high-electronic-quality perovskite, the hole transfer process from MAPbI3 on mpTiO2 to different HTLs was investigated for eight kinds of conjugated polymers. The hole transfer yield (ηHT) at nanosecond to microsecond time scales was directly evaluated by kinetic analysis using TRMC transients, where the initial, delayed, and saturated yields were quantified as η0, k, and ηsat, respectively. Through statistical and LASSO analysis, we showed that ln(η0k) and (η0 + ηsat), derived from the doped and nondoped HTLs, respectively, correlate well with device PCE, indicating that both initial and delayed hole transfer processes contribute to the overall device performance. The energy offset between the HOMO of a neutral HTL and the G


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voltage curves were measured using a source-meter unit (ADCMT Corp., 6241A) under AM 1.5 G solar illumination at 100 mW cm−2 (1 sun, monitored by a calibrated standard cell, Bunko Keiki SM-250 KD) from a 300 W solar simulator (SAN-EI Corp., XES-301S). The size of the active area was defined by a black metal mask with a square hole (2 × 2 mm2). Time-Resolved Microwave Conductivity (TRMC). MAPbI3 with/without HTL (doped or nondoped) or PCBM (purchased from Frontier Carbon Inc.) films were prepared in the same manner as the solar cells, without the compact TiO2 and Au electrode steps, and with FTO/glass being replaced by a quartz plate. The sample was set in a resonant cavity and probed by continuous microwaves at ca. 9.1 GHz. The excitation laser from an optical parametric oscillator (OPO, Continuum Inc., Panther) seeded by the third harmonic generation of a Nd:YAG laser (Continuum Inc., Surelite II, 5−8 ns pulse duration, 10 Hz) was set at 500 nm except for P3HT (630 nm) and MDMO-PPV (550 nm) at I0 = 1.3−1.8 × 1011 photons cm−2 pulse−1. The latter two exceptions are due to their large absorptions at 500 nm (Supporting Figure S3). The laser pulse was exposed from the HTL side (not from mpTiO2 side) and the excitation wavelength was chosen to minimize laser absorption by the HTL. The I0 incident to a MAPbI3 layer was tuned to the same intensity regardless of the presence of HTL by taking account of the HTL absorption. The low I0 allows for precise evaluation of ηHT, because ϕΣμmax lies in the flat region, unaffected by a small change of I0 (Supporting Figure S4). The photoconductivity transient Δσ is converted to the product of the quantum efficiency (ϕ) and the sum of charge carrier mobilities, Σμ (= μ+ + μ−) by ϕΣμ = Δσ (eI0Flight)−1, where e and FLight are the unit charge of a single electron and a correction (or filling) factor, respectively. Note that both ϕΣμ(t) and ϕΣμHTL(t) were derived for the same film, so as to compensate the film-to-film variation of ϕΣμ(t) (change in the TRMC transients of pristine perovskite was confirmed negligible within a few hours). All experiments were conducted at room temperature in air. The LASSO analysis was performed using R language free software with glmnet package.71

VB maximum of the perovskite should be larger than 0.14 eV so as to ensure a high ηHT. Notably, such an HTL exhibited high ηsat (0.86−0.99), even without dopant, suggesting that doping is intrinsically unnecessary for hole transfer processes (ηHTP and ηHC are regarded as other factors). Exposure of the HTLs to air was found to impact ηHT, with the doped HTLs showing a rapid increase of ηHT up to an air-exposure time of 5 min, while nondoped HTLs underwent prolonged change up to an exposure time of 30 min. We illustrated how ηHT is influenced by HOMO levels, chemical structure, doping, and air exposure, highlighting the versatility of our direct approach to the evaluation of ηHT, and indicating that our method may be applicable to a wide range of polymeric and molecular HTLs.

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EXPERIMENTAL SECTION General Measurement. Steady-state photoabsorption spectroscopy was performed using a Jasco V-730 UV−vis spectrophotometer. The molecular weights of the polymers were determined using gel permeation chromatography (GPC) with polystyrene standards (Supporting Table S5). GPC analysis was performed with chloroform as an eluent at a flow rate of 1 cm3 min−1 at 40 °C, on a Shimadzu LC-20AT, CBM-20A, CTO-20A chromatography instrument connected to a Shimadzu SPD-M20A UV−vis detector. Photoelectron yield spectroscopy (PYS) experiments were carried out using a Bunko Keiki BIP-KV202GD. AFM and SEM observations were performed using a Bruker Innova and a Hitachi Hi-Tech S5500, respectively. Device Fabrication. A F-doped SnO2 (FTO) layer on a glass substrate was etched with 6 N HClaq and Zn using masking tape. After cleaning with detergent, acetone, isopropyl alcohol, and deionized water, a compact TiO2 layer was deposited onto the FTO/glass by spray pyrolysis using a solution of titanium diisopropoxide bis(acetylacetonate) (Tokyo Chemical Industry Co. Ltd.) in ethanol (1:40 v/v) at 450 °C. The substrate was immersed in a 50 mM aqueous solution of TiCl4 (Wako Chemical Industries Ltd.) for 30 min, followed by rinsing with deionized water and sintering at 500 °C for 20 min. A 200 nm-thick mpTiO2 layer (average particle size: 20 nm, anatase) was deposited onto the compact TiO2 layer by spin-coating (slope 5 s, 5,000 rpm 30 s, slope 5 s) of a diluted TiO2 paste (PST−18NR, JGC Catalysts and Chemicals Ltd.) in ethanol (paste/ethanol = 1:7 w/w), followed by sintering at 500 °C for 20 min. A 1 M DMSO (Wako) solution of PbI2 (TCI, solar cell grade) and CH3NH3I (TCI, solar cell grade) at 1:1 stoichiometry was prepared in a N2-filled glovebox. Subsequently, a MAPbI3 precursor layer was formed by spin-coating the DMSO solution (slope 1 s, 1,000 rpm 40 s, slope 1 s, 0 rpm 20 s, slope 5 s, 4,000 rpm 20 s, slope 5 s). After 80 s, poor-solvent treatment was applied by slowly dropping 0.5 mL of toluene onto the rotating substrate. The resultant transparent film was annealed at 100 °C for 10 min, affording a 300 nm-thick MAPbI3 layer. Conjugated polymers of P3HT, MDMO-PPV, F8T2, and PTAA were purchased from Aldrich, while TQ1 was from Luminescence Technology Corp. The same batch of FT37, FT55, and FT73 from our previous report was used.70 A 50 nm-thick HTL of conjugated polymer with/ without LiTFSI (TCI) and TBP (TCI) dopants (polymer:LiTFSI = 1:0.15) was spin-coated from a toluene solution (ca. polymer: 10 mg mL−1, TBP: 15 μL mL−1) at an appropriate rotation speed depending on the viscosity of each polymer solution. Subsequently, a 100 nm-thick stripe-shaped gold anode was thermally deposited in a vacuum chamber. Current−

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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/acsphotonics.6b00331. Supplemental data as mentioned in the text (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.S.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Kakenhi (Grant-in-aid for Scientific Research A: JP16H02285, Scientific Research B: JP252880840, and Innovative Areas “Element block” JP15H00747) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan, and the PRESTO program (16021140600) from the Japan Science and Technology Agency (JST). The authors acknowledge Dr. Masayuki Karasuyama at the Nagoya Institute of Technology H


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for his advice on LASSO analysis. We thank Dr. Hiroki Yamamoto and Prof. Takahiro Kozawa at The Institute of Scientific and Industrial Research, Osaka University for the support of SEM measurements.

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