Effect of Fullerene Passivation on the Charging and Discharging

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Effect of Fullerene Passivation on Charging and Discharging Behavior of Perovskite Solar Cells: Reduction of Bound Charges and Ion Accumulation Yen-Chen Shih, Leeyih Wang, Hsiao-Chi Hsieh, and King-Fu Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03116 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Applied Materials & Interfaces

Effect of Fullerene Passivation on Charging and Discharging Behavior of Perovskite Solar Cells: Reduction of Bound Charges and Ion Accumulation Yen-Chen Shih, †, ‡ Leeyih Wang,*, ‡ Hsiao-Chi Hsieh, †, ‡ King-Fu Lin*,† †

Department of Materials Science and Engineering and ‡ Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan.

ABSTRACT: Ion accumulation of organometal halide perovskite (OHP) induced by electrode polarization of perovskite solar cells (PSCs) under illumination has been intensely studied and associated with widely observed current-voltage hysteresis behavior. This work dedicates to the investigation of charged species’ behavior at the compact TiO2/OHP interface with respect to electrode polarization in PSC devices. By providing a comprehensive discussion of open-circuit voltage (VOC) buildup and VOC decay under illumination and in the dark for the PSCs modified with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) at the TiO2/OHP interface and their corresponding electrochemical impedance spectroscopies (EIS), a justified mechanism is proposed attempting to elucidate the dynamics of interfacial species with respect to the time and frequency domains. Our results demonstrate that the retarded VOC buildup and decay observed in PSC devices are related to the formation of bound charges in TiO2, which is essential to

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neutralize the oppositely charged ions accumulating at the OHP side. Besides, inserting thicker PCBM at TiO2/OHP interface as a passivation layer can alleviate the electrode polarization more efficiently as verified by the less dielectric constant measured from EIS.

Moreover,

photoluminescence measurements indicate that PCBM at TiO2/OHP interface is capable of passivating trap state and improving charge transfer. However, with respect to the timescale investigated in this work, the reduction of hysteresis behavior on milliseconds scale is more likely due to less bound charge formation at the interface rather than shallow trap state passivation by PCBM.

After all, this work comprehensively demonstrates the interfacial

properties of PSCs associated with PCBM passivation and helps to further understand its impact on charging/discharging as well as device performance.

KEYWORDS: ion accumulation; electrode polarization; bound charge; fullerene; passivation effect; organometal halide perovskite; perovskite solar cell.

1. INTRODUCTION Organometal halide perovskites (OHPs) for the emerging perovskite solar cells (PSCs)1–3 have drawn significant attention over the past few years on account of their high absorption coefficient almost covering the entire visible region4,5 and balanced bipolar transportation with long carrier diffusion length.5,6 Notably, some interesting properties of these promising materials have been recently reported such as ferroelectricity7–9 and ion migration.10–12 The later has also been pointed out to cause current-voltage hysteresis in devices that may detrimentally affect the reliability of performance evaluation especially for the photovoltaics constructed with TiO2 as electron transporting material (ETM).13–15


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Perovskite oxides such as La1-xSrxMnO3 and yttria-stabilized ZrO2 are well known as the oxygen-ion conductors, which can be applied for solid oxide fuel cells.16,17

Similar ionic

conducting properties have also been observed for inorganic halide perovskite (CsPbCI3, CsPbBr3) as well as for OHPs.12,18 Unlike perovskite oxides where the ionic conducting channel is through oxygen vacancy, the origin of ionic conductivity in OHP such as methylammonium lead triiodide (CH3NH3PbI3) likely comes from ion-vacancies of iodide anion and methyl amine cation.11,19,20 These charged vacancies can drift in OHP film by applying an electric field and consequently cause band bending due to the accumulated ions near interfaces, and hence change the feature of electric potential and the current response.10,21–23 In addition, such phenomenon is switchable by altering the direction of applied electric field, indicating that the accumulated ions are not fastened to their redistributed loci and the composition of OHP film is reversible.24 A recent research shows that the origin of current-voltage hysteresis is likely due to large capacitive current originated from electrode polarization through accumulated ions, rather than the ion migrating process under electric field poling.25 Notably, a few reported experimental evidence, that inserting a layer of [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) or other fullerene derivatives in the TiO2/OHP interface is capable of reducing capacitive effect as well as hysteresis behavior,26,27 also supported the above statements. On the other hand, a recent work showed that “inverted structure” PSCs composed of PCBM have less surface trap and are almost free of hysteresis.28 OHPs are known to suffer from a significant and broad density of sub gap trap states, which makes the trapped electrons slowly recombine with free holes on microsecond time scale.29,30 From this aspect, trap passivation by fullerene may be responsible for the reduction of hysteresis behavior. Despite intensive report on alleviation of hysteresis in PSCs through the modification of TiO2/OHP interface, the role of


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PCBM still remained for debate. Very recently, open-circuit voltage decay (OCVD) technique has been carried out to investigate the interfacial recombination in PSCs,31,32 showing that the ion migration and traps of electron-selective contact may affect recombination rate of photogenerated carriers, and thus the photovoltage relaxation is expected to change depending on the condition at the interface. However, to the best of our knowledge the features of electrode polarization and its impact on open-circuit voltage (VOC) in PCBM modified devices have not been well investigated. Herein, to further investigate how the drifting charges/ions influence the potential in devices and how it is affected by an inserting layer of PCBM, we measured the open-circuit voltage buildup (OCVB) of PSCs in planar architecture with and without modification of PCBM to see the rise of potential with time under illumination. OCVD technique was also subsequently conducted to see the decline of built-up potential just after turning off the light. Besides, electrochemical impedance spectroscopy (EIS) was carried out to determine the time scale of the relaxation response of charges/ions driven by alternative electric field with varied frequencies, which can be correlated with the OCVB and OCVD data. Thereafter, a justified mechanism is proposed attempting to elucidate the findings obtained from the above results with respect to the time and frequency domains.

2. RESULTS

CH3NH3PbI3 films as OHP were deposited on top of compact TiO2 (cTiO2) and PCBM/cTiO2 by using solvent-assisted casting method,33 respectively and a layer of 2,2',7,7'-tetrakis-(N,N-di-4methoxyphenylamino)-9,9'-spirobifluorene (spiro-MeOTAD) was then applied on top of


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CH3NH3PbI3 film as hole transporting material (HTM) (see Supporting Information for details). The architecture of PSCs is schematically illustrated in Figure S3. Before OCVB measurement, the device was open-circuited and kept in dark to eliminate the remaining charges until no voltage offset was detected. Upon illuminating by a 530 nm LED, the VOC of non-modified PSC rises immediately to around 0.35 V (Figure 1a) but starts to climb slowly before reaching to a plateau. Here, three different power densities (20.65, 10.55 and 2.11 mW cm-2) were employed and brought in deviation from the immediate raise of voltage after stage I but quite parallel in the latter stage of stage II. Besides, we also observed the higher illumination power density leads to higher steady VOC. OCVD is subsequently carried out after one minute of constantly illuminating. We observe VOC declines rapidly to around 0.8 V (Figure 1b) just after turning off the LED light. Interestingly, all three curves seem to drop down to the same voltage level in the initial decay (defined as stage i), then gradually decay over tens of seconds to zero but split with respect to the different power densities at the moment around the sixth second in stage ii (see Figure 1b). On the other hand, the VOC of PCBM-modified PSC rises faster in OCVB (Figure 1c), reaches to higher value compared to that of non-modified device, decays faster in OCVD (Figure 1d), and also splits earlier after light off. Such abnormal delay for voltage buildup and decay has been reported previously and was attributed to the population and depopulation of trap states.34

However, very recently there are other

interpretations for the delayed OCVD with regard to the light-induced self-poling effect contributed by drift of ions in OHP layer35 and light-soaking stimulation of slow motion of mobile ions in the double layer of selective contact.36


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Before giving a comprehensive explanation behind the time delay of OCVB and OCVD in PSCs, we would like to firstly elucidate where the voltage comes from. It is known that the VOC of photovoltaic responses under illumination is the result of quasi Fermi level splitting, which is

Figure 1. (a, c) OCVB and (b, d) OCVD of PSCs modified with and without PCBM (20 mg mL1

in chlorobenzene, 2000 rpm). In OCVB measurement, devices are open-circuited and

constantly illuminated with a 530 nm LED (at three indicated power density conditions) turned on at “1 s” in the presented time scale. The VOC of devices rises rapidly in stage I, increases slowly in stage II, and then reaches a steady value after tens of seconds. After one minute illuminating, OCVD is conducted by turning off the LED at “1 s” as indicated. The VOC of devices declines in dark rapidly in stage i, and then decays slowly in stages ii and iii.

related to the excited charge density n in the device as37


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1 ≅

Figure 2. OCVD and charge density decay of the indicated PSCs in the dark after one minute illumination with 530 nm LED (20.65 mW cm-2). Charge densities (denoted as circle symbols) of PSCs modified with and without PCBM (20 mg mL-1 in chlorobenzene, 2000 rpm) are extracted by short-circuiting the device at corresponding delay time after turning off the illumination.

where kB, T and q are the Boltzmann constant, temperature and elementary charge, respectively, and n0 and p0 are intrinsic electron and hole concentrations. To be more precise, we provide an additional remark here that eqn 1 represents a steady-state relationship between VOC and charge concentration in an equilibrium state with respect to the concurrent charge generation and recombination. Notably, the measured charge density extracted from bare cTiO2 device after different delay times since light off approximately matches with the OCVD data as shown in Figure 2, indicating that photovoltage and stored charge extracted from device follow the relation of VOC ∝ ln(n). Therefore, the decay of VOC in the dark is attributed to the decrease of charge


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density resulting from the charge recombination process. Notably, the charge density at the initial value of 165.10 µC cm-2 drops rapidly to 87.17 µC cm-2 during 20 ms just after light off and then starts to decay in extremely slow rate. At this moment, based on the 300 nm thickness of OHP layer, the concentration of stored charge being extracting out was estimated to be 1.81 ⨯ 1019 cm-3, which is similar to the value observed by Deng et al.35 and is far higher than the reported concentration of electron/hole trap states in OHP which is within the order of 1013-1016 cm-3.34,38 In addition, because of the time scale of trapping process being in the order of 10-5 s,39 the dielectric relaxation40 and slow redistribution process of accumulated ions seem more accountable for our observation of time delay in the scale of tens of seconds. Interestingly, the charge density of PCBM-modified PSC extracted after the same delay time of 20 ms is 68.23 µC cm-2, which is around 20% lower than that of bare cTiO2 device, and decreases with a steeper slope thereafter. Moreover, the stored charge extracted from PCBM-modified device has some deviation from the OCVD curve (see Figure 2), implying that the inserted PCBM layer tends to capture part of piled-up charges that barely contribute to the VOC of device. To sum up the findings so far, in stage ii large amount of charges remained in devices sluggishly decayed via recombination process because of extremely slow redistribution of ions


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Figure 3. (a) OCVB and (b) OCVD of the PSCs modified with the indicated thickness of PCBM and without PCBM. The devices are illuminated by a 530 nm LED with power density of 20.65 mW cm-2. and charges piled up at the interface of OHP side. To further investigate the role of PCBM in PSC, we increased the concentration of PCBM from 20 to 30 mg mL-1 in chlorobenzene to ensure the full coverage of PCBM on cTiO2/FTO (Figure S4). We also changed the spin-coating rate to obtain different thickness of PCBM layer, the thickness was estimated by cross-section scanning electron microscope images (Figure S5). Figure 3 shows the OCVB and OCVD of PSCs modified with PCBM in different thickness, the measurement of which was performed in the same way as mentioned previously. The results in full time duration under light on and then light off are shown in Figure S6. Obviously, from OCVB measurement, we can find that the thicker PCBM leads to higher VOC in stage I upon illumination, then the VOC starts to build up tardily in stage II for all devices, and successively reach to higher steady value in an order according to the PCBM thickness (Figure 3a). After light off, the PCBM-modified devices show rapid decay of VOC in milliseconds to around 0.2 V (Figure 3b), such value is remarkably lower than that without PCBM. Besides, VOC of the thickest PCBM (60 nm) declines in the rapidest rate just after turning off the illumination, but the subsequent decay is in the most sluggish rate. In contrast with the OCVB and OCVD studies, hysteresis behavior of PSCs were investigated by biasing the devices under backward scan (BS, forward bias to short circuit) and forward scan (FS, short circuit to forward bias) with different delayed time for each voltage step (0.01 V) as shown in Figure S7, and the parameters are listed in Tables S1~S4. Unlike bare TiO2 device that suffers from significantly delay-time-dependent J-V hysteresis on timescale of subseconds as observed in our results and in the literatures,13,14 PCBM modified devises show


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apparently suppressed hysteresis behavior. Moreover, in the same scanning condition, thicker PCBM further reduces the difference of fill factors (FFs) between FS and BS (see Table 1), which is hereafter defined as the hysteresis factor (HF) for convenience by Table 1. Average photovoltaic parameters of PSCs measured by backward scan (BS) and forward scan (FS) with 50 ms delay time for each 0.01 V voltage step under AM 1.5 illumination (100 mW cm−2), statistics from 6 devices. RS (Ω cm2)a

Device

JSC (mA cm−2)

VOC (V)

FF (%)

PCE (%)

Bare cTiO2, BS

20.42±0.36

1.06±0.01

65.93 ±0.57

14.28±0.26

5.13±0.13

Bare cTiO2, FS

19.92±0.23

1.04±0.01

52.47±0.71

10.84±0.28

10.70±0.36

PCBM 40 nm, BS

19.39±0.04

1.05±0.01

71.80±1.38

14.63±0.34

4.69±0.47

PCBM 40 nm, FS

19.07±0.01

1.06±0.01

68.16±1.46

13.78±0.31

5.10±0.66

PCBM 50 nm, BS

17.93±0.53

1.01±0.01

71.83±0.82

13.04±0.27

5.03±0.16

PCBM 50 nm, FS

17.54±0.50

1.02±0.01

68.56±1.19

12.31±0.27

5.15±0.06

PCBM 60 nm, BS

17.35±0.10

1.00±0.01

71.11±1.49

12.33±0.21

5.64±0.27

PCBM 60 nm, FS

16.97±0.02

1.02±0.01

68.59±0.74

11.81±0.11

5.69±0.26

HF (%)b

20.42

5.07

4.55

3.54 a b

The series resistance (RS) of devises were calculated from J-V curve by using the equation of RS = dV/dJ (J = 0). Hysteresis factor is defined as HF = ( | FFBS – FFFS |/ FFBS) ⨯ 100%, where FFBS and FFFS are the FF measured under BS direction and FS direction respectively.

HF = ( | FFBS – FFFS |/ FFBS) ⨯ 100%, where FFBS and FFFS are the FF measured under BS direction and FS direction respectively. Take 50 ms delayed time for example, the HF of bare TiO2 device is 20.42%, which decline to 5.07%, 4.55%, and 3.54%, when the thickness of PCBM is 40, 50, and 60 nm respectively. However, it should be noted that decrease of shortcircuit current density (JSC) and rise of series resistance (RS) resulting lower power conversion efficiency (PCE) were also observed for the thicker PCBM modified device.


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Considering that charge trapping and detrapping processes from trap states may be the reason behind the hysteresis behavior, we conducted the photoluminescence (PL) measurements to explore this issue. Generally, radiative recombination from trap states would result in a redshifted emission compared with the case of trap state being passivated.38 We used an excitation light of 532 nm, whose penetration depth is smaller than the thickness of perovskite layer, to investigate the steady-state PL response from air side and glass side. As shown in Figure S8, all samples excited from air side show the same peak position at 762 nm, implying that the excited OHP films near top surface have same band transition energy whether the cTiO2 was modified by PCBM or not. When bare cTiO2 sample was excited from glass side, a red-shifted PL peak from 762 to 764 nm indicates the presence of trapping states at the TiO2/OHP interface. For the case of the TiO2/OHP interface modified with 40 nm PCBM, the PL peak excited from glass side is at 763.5 nm, exhibiting a slightly suppressed red-shift compared to the case of bare cTiO2. When PCBM thickness was increased to 50 and 60 nm, both PL peaks shifted to 762.5 nm, implying that more trapping states were passivated. Time-resolved PL (TRPL) of OHP films coated on different substrate was also measured as shown in Figure S9, where a 485 nm laser was employed to illuminate the samples from glass side. The PCBM modified samples exhibit faster PL decay compared to that of bare cTiO2. Here, three components of exponential decay to estimate the carrier lifetime are employed to fit the PL decay curve, and the results are listed in Table S5. Surprisingly, the lifetimes extracted from the fitting results does not largely change, ~2 ns for τ1, ~10 ns for τ2, and ~50 ns for τ3, but the fractional intensities (F) of each component show huge difference as the interface is modified with PCBM. The fractional intensities of the longest lifetime component (τ3) decrease from over 85% to around 50% for all PCBM samples owing to the increase of fractional intensities of short and median lifetime components (τ1 and τ2


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respectively) indicating that PCBM tends to attract the excited charges in nanoseconds. However, the timescale of nanoseconds investigated here cannot explain the slow VOC build-up and decay observed on the timescale of milliseconds to seconds (Figure 3), neither the delaytime-dependent J-V hysteresis observed on sub-seconds (Figure S7). Nevertheless, the results obtained from PL and TRPL manifest the faster excited charge transfer at the interface when PCBM was applied, which corresponds to the faster VOC build-up upon illumination (stage I) and faster decay after light off (stage i). Therefore, based on our PL study, the shallow trap state passivation by PCBM at the grain boundaries of OHP near PCBM/OHP interface as indicated in the literature26,38,41 should not be the primary reason for the reduction of hysteresis behavior. Instead, the motion of the species such as electrons/holes and ions in the interface on the time scale of milliseconds to seconds should be an essential issue for the observation of slow VOC buildup and decay.

Figure 4. Real part of dielectric constant as a function of frequency. Devices are measured at zero bias in the dark and under illumination by using a 530 nm LED with power density of 20.65 mW cm-2.


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EIS technique is a useful tool to characterize the kinetics of charges in PSCs under illumination and in the dark with regard to the variation in impedance associated with the different interfaces of the device.42,43 By plotting the real part of dielectric constant ε’ as a function of frequency (calculated from the raw data shown in Figure S10, see Supporting Information for detail), we can obtain a typical Bode plot with some interesting results as shown in Figure 4. Firstly, by comparing the devices measured under illumination with that in the dark, we can observe the ε’ response has a slight increase at intermediate frequencies but a huge increase at lower frequencies. Notably, such drastic rise of ε’ response in low frequency regime is reduced for the device modified with PCBM, and is further reduced with thicker PCBM. However, there is no obvious change in the dark condition of all devices. Secondly, inserting a layer of PCBM leads to the shift of onset point to lower frequency for the rising of ε’ from the plateau in the intermediate regime as shown in Figure 4.

These observations illustrate a

significant effect of PCBM on the dynamic motion of interfacial species and also lay the basis for the following discussion with respect to the role of PCBM in PSCs.

3. DISCUSSION For the frequency-dependent dielectric constant in PSCs, it has been suggested that the plateau at intermediate frequencies corresponds to “dipolar polarization”, whereas the ascending ε’ by decreasing the frequency from the intermediate regime is related to “electrode polarization” caused by interfacial ion accumulation that induces hefty increase of capacitance.44 Accordingly, the results shown in Figure 4 indicate that illumination not only induces the dipolar polarizations such as rotation of methylammonium dipoles (orientation polarization) and ion redistribution of bulk OHP, but also significantly induces the ion accumulation at interface. Moreover, the


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interfacial modification with PCBM has shifted the onset point of ascending ε’ response from 5 ⨯ 103 to 2 ⨯ 103 Hz. Since frequency domain is related to the time domain by τ=1/2πf, we may intuitively deduce that PCBM has retarded the electrode polarization, although it has been overwhelmingly associated with the trap state passivation and/or the formation of PCBM–halide radicals at the PCBM/OHP interface.45,46 Here, we simply consider the device as a plate capacitor that two electrodes sandwich a layer of OHP with dielectric constant of εr, whose geometrical capacitance Cg can be expressed by the following equation: =

= 2

where C0 is the geometrical capacitance in vacuum, ε and ε0 are the absolute dielectric constant and vacuum dielectric constant respectively. Then the built-up potential V, which has been neutralized by dielectric to deviate from V0 in vacuum, can be related to electrical charge Q and geometrical capacitance as follows:47 =

/ = = 3

from which one can obtain that only a fraction of “free” charges Q/εr contribute to the potential; or we can say, part of the charges is neutralized by polarization and can be regarded as the “bound” charges being attracted at the interface. Therefore, the observed high ε’ response in low frequency regime is related to the bound charges that cannot be freely extracted from ETM to external circuit. Compared to the bare cTiO2 device, PCBM-modified devices display smaller ε’ at low frequency regime, implying that the piled up charges at the interface (interfacial capacitance) is mitigated probably due to the enhanced charge separation48 by means of ultrafast electron


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delocalization in aggregated fullerene domain.49,50 Under illumination, 60 nm thick PCBM (ε’=1.7 ⨯ 105) exhibits about two-fold decrease of ε’ compared to bare cTiO2 (ε’=3.3 ⨯ 105) at the low frequency region (~10-1 Hz), the time domain of which unequivocally corresponds to the time scale to reach steady VOC. Therefore, less retardation for the VOC buildup after immediately raised VOC for PCBM-modified PSCs observed in Figure 3 can be explained as that less charges are needed to counteract with the dipolar polarization and the upcoming oppositely charged ions driven by photo-induced electric field. The retardation of VOC buildup thereafter can be deduced as a dynamic competition between electrode charging up and the bound charge formation (electrode polarization). To further deliberate the observed experimental results, we provide schematic mechanisms as shown in Figure 5 to illustrate the charging stage of PSCs with and without modification of PCBM under exposure of light, where only electrons are highlighted in order to emphasize the feature at TiO2/OHP interface. Upon illumination (stage I), free carriers, particularly high concentration of electrons near illuminated side with an exponential distribution following Beer– Lambert law, transport swiftly to the interface and promptly transfer to TiO2 within a range of sub-picoseconds,51 but that cannot be monitored by our equipment due to the detection limit. However, it is detectable that those injected electrons survived from charge recombination starts to fill up the electronic states of TiO2 and rises up the VOC of device. Nonetheless, part of them become bound charges induced by polarization in OHP layer in milliseconds, which leads to the retardation of VOC buildup at the end of stage I. The curvature appears at the beginning of stage II is likely influenced by the dipolar polarization near the interface induced by the electric field and further dominated by a large quantity of ions migrating in opposite direction toward electrodes that lead to much slower VOC buildup until reaching a balance among charging up


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electrons, free charge recombination, and bound charge formation. However, on the other hand, PCBM-modified PSCs exhibit more efficient VOC charging up in stage I. Thick film of PCBM with larger domains for electrons to delocalize efficiently diminishes the bound charges that mitigate the polarization electric field (Ep) in the direction opposite to the charging electric field (E0), leading to a shorter period of time to reach steady VOC in stage II. After VOC is built-up steadily, a conceivable encounter among charging up electrons, polarized dipoles, accumulated positive charged ions, and the bound charges piled up at interface is schematically illustrated as stage i in Figure 6. When the illumination is just turned off, high concentration of charges in electrodes tends to diffuse back to OHP layer for recombination with holes. According to the statement proposed by Baumann et al., the fast VOC decay in early submilliseconds is attributed to the recombination of free electrons and free holes.52

The

observation that all three OCVD curves firstly decayed down to almost the same voltage level regardless the

Figure 5. Schematic illustration of the device charging, charge recombination, dipolar polarization, ion migration, and bound charge formation of PSCs under constant illumination


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based on the OCVB results. The second row of the cartoons depicts the role of PCBM as a passivation layer to retard and mitigate the bound charge formation. The presence of PCBM facilitates the buildup of “effective electric field” E, which is the result of the competition between the charging electric field (E0) and the polarization electric field (Ep).

illumination power densities at the end of stage i (Figure 1b, d) probably comes from the already saturated electrode polarization, where the excess charging under higher illumination intensity all turns into free charges that can be swiftly recombined with free holes in OHP layer. After several milliseconds for the relaxation of dipolar units in early period of stage ii, the slow depolarization dominated by back diffusion of accumulated species takes several seconds to unleash the bound charges and leads to very slow VOC decay due to insufficient free charges for recombination. It is worth noting that the degree of electrode polarization is dependent on the illumination intensity.53


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Figure 6. Schematic illustration of the charge recombination, dipolar depolarization, redistribution of piled-up ions, and relaxation of bound charges of PSCs in the dark based on the OCVD results. The second row of the cartoons depicts the role of PCBM as a passivation layer to alleviate the impact of accumulated species in OHP.

The prolonged depolarization process in the stage ii (Figure 1b) can be thus regarded as more strongly bonded charges owing to more accumulated ions at the interface under stronger illumination. At the end of depolarization, the VOC decay becomes dominated by remaining bound charges that relax to free charges in electrodes transporting back to OHP layer for recombination and thus further lowers the VOC to nil as displayed in stage iii. In general, charge recombination in the photovoltaics is a result of several mechanisms associated with the recombination order α, such as Shockley-Read-Hall recombination via sub-bandgap of traps/defects (α=1), free electron-hole recombination (α=2), and Auger recombination that involves three species process (α=3). Notably, Auger-like recombination has been observed from fluorescence intermittency of OHPs, which was attributed to the ionized surrounding of grain boundaries that leads to non-radiative recombination.54 In our case, when illumination is turned off, the split quasi Fermi level in photovoltaic starts to relax back into the original position. The decrease of excited charge concentration associated with the charge recombination can be represented by the following equation,55 −

"

= $ 4 "#


18

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where k is the kinetic constant and α is the recombination order. By combining eqns 1 and 4 (see Supporting Information for detailed derivation), we obtain the relationship between the excited charge density n and VOC of device as

& ' ≅ 5

2 and &

1 +) − 1, '≅ # − * -

. 6 ) − 1 −) − 1

from which we can relate the OCVD result to the dynamics of charge decay as shown in Figure 7.

Figure 7. Charge recombination dynamics of the indicated PSCs after illuminating with light intensity of 20.65 mW cm-2. The data were converted from the OCVD results based on eqns 5 and 6. Table 2. Charge recombination order α at different fitting regimes of the indicated PSCsa Device

α1

α2

α3

α4

Bare cTiO2

1.96 (0-0.01s)b

1.68 (0.015-0.045s)

2.94 (0.14-1.84s)

1.10 (34.855-46.875s)

PCBM 40 nm

1.44 (0-0.01s)

1.10 (0.03-0.04s)

1.70 (0.07-0.78)

1.37 (1.025-3.51s)

PCBM 60 nm

1.45 (0-0.01s)

1.13 (0.025-0.045s)

1.32 0.09-0.165s)

1.59 (5.545-17.125s)


19


a

b

Page 20 of 33

The α values obtained from the linear fitting results of Figure 7 based on eqns 5 and 6 with the R-squared value above 0.99. Time range for the fitting regime.

By fitting the linear regimes of the curves, α values of the devices at different specific decay regimes are obtained as listed in Table 2. For bare cTiO2 device, its recombination order is initially 1.96, which is close to the value obtained by Baumann et al. and can be associated with free carrier recombination.52 Subsequently, α turns into a lower value of 1.68 in several milliseconds, and then changes to 2.94 in the regime of delayed time from sub-seconds to seconds.

Such high recombination order is considered to associate with the Auger-like

recombination, probably related to the high concentration of charged species in the interface, including relaxed bound charges (or reclaimed free charges) and accumulated ions. After tens of seconds, it declines to 1.1 that can be related to trap-assisted recombination dominated by the remaining charges in TiO2. Such long period of time for the transition of recombination order from 2.94 to 1.1 is related to the hypothesis of slow redistribution of ions in stage ii. As for the case of devices modified with PCBM (40 nm), the initial α value of 1.44 decreased to 1.1 in milliseconds, both of which are lower than those with bare cTiO2 (1.96 and 1.68 respectively), indicating that the free carrier recombination became controlled by trapping process. Most surprising is that the Auger-like recombination was suppressed.

The

corresponding α value is only 1.70 and it is hard to distinguish between stages ii and iii of OCVD plots. Moreover, thicker PCBM layer (60 nm) exhibits further reduced α value to 1.32, suggesting that the interaction between the charges in TiO2 and the accumulated species in OHP in the interfacial region is further mitigated resulting in rapid VOC decay. Obviously, it is the thicker PCBM causes less bound charges that induce rapid VOC decay, the result of which is consistent with the decrease of ε’ response observed in EIS measurement and clarifies the puzzle


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why thicker PCBM can reduce current-voltage hysteresis more as reported in the literature.56 Besides, as the depolarization process is almost complete, the α value was slightly increased from 1.10 (bare cTiO2) to 1.37 (PCBM 40 nm) and further to 1.59 (PCBM 60 nm). Although the exact mechanism is still unclear, the observation of slower VOC decay with thicker PCBM device could be rationalized with longer transport distance/time for the remaining charges redistribute back to OHP for charge recombination, which would experience higher charge transfer resistance as indicated in the Nyquist plots of EIS measurements (Figure S10a) and also relate to the increase of RS of devices (see Table 1). Notably, the OCVD of the devices composed of mesoporous TiO2 reported in the literature56 is much faster than that with planar structure observed in this study. Since the device with mesoporous heterojunction is expected to have much larger interfacial area with more trapping sites at the TiO2/OHP interface, its faster OCVD decay time should not be caused by interfacial trap states. Rather, it is in agreement with the lower ε’ response obtained for the mesoporous devices53 and could reasonably explain their alleviation of hysteresis as reported in the literature.58,59

4. CONCLUSION In summary, we employed OCVB and OCVD techniques to measure the VOC responses of PSCs under illumination and in the dark. The EIS results that recorded dielectric response in the similar frequency range agree with the slow VOC build up and imply that the anomalous long time decay of VOC in the dark are mainly originated from the dipolar polarization and the accumulated ions of OHP at interface. We propose that the bound charges piling up at electrode side are related to the polarized dipoles and the oppositely charged species accumulated at OHP


21


Page 22 of 33

side. The formation of bound charges can be retarded and mitigated by inserting a layer of PCBM as verified by the less ε’ response in low frequency regime of the EIS plots. Despite thicker PCBM can reduce the bound charge formation and efficiently alleviate hysteresis behavior, it inevitably rises the charge transfer resistance between OHP and the electrode, which augments RS and decreases the PCE of devices. In all, this work opens up another window to investigate the interfacial issue of PSCs and the passivation effect of fullerene, and helps to further understand the impact of ion migration on bound charge formation as well as photovoltaic performance.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: Experimental section, equation derivation, scheme of device architecture, SEM images for PCBM coating, full view of OCVB and OCVD results, and Nyquist plots of PSCs measured under illumination and in the dark (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Professor King-Fu Lin: Tel: +886-2-3366-1315; Fax: +886-2-2363-4562; E-mail: [email protected]; * Professor Leeyih Wang: E-mail: [email protected].


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Mr. Chang-Chi Yuan and Mr. Hsin-Hsiang Huang for PL measurements and the set-up of J-V measurements. This work was supported by the Ministry of Science and Technology of the Republic of China through grant MOST 103-2221-E-002-280-MY3. L. Wang acknowledges the funding supported by Ministry of Science and Technology (Grant No. MOST 105-2119-M-002-030-MY3), Academia Sinica in Taiwan (Grant No. 2394-104-0500; AS-106SS-A02), and National Taiwan University.

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TABLE OF CONTENTS


33

Effect of Fullerene Passivation on the Charging and Discharging

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