Operando Direct Observation of Charge Accumulation and the

Subscriber access provided by Kaohsiung Medical University

Organic Electronic Devices

Operando Direct Observation of Charge Accumulation and the Correlation with Performance Deterioration in PTB7 Polymer Solar Cells Takaya Kubodera, Masaki Yabusaki, Vanadian Astari Suci Atina Rachmat, Yujin Cho, Toshihiro Yamanari, Yuji Yoshida, Nobuhiko Kobayashi, and Kazuhiro Marumoto ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06211 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 15, 2018

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

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

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

ACS Applied Materials & Interfaces

Operando Direct Observation of Charge Accumulation and the Correlation with Performance Deterioration in PTB7 Polymer Solar Cells

Takaya Kubodera†, Masaki Yabusaki†, Vanadian Astari Suci Atina Rachmat†, Yujin Cho‡, Toshihiro Yamanari§, Yuji Yoshida§, Nobuhiko Kobayashi⁋, and Kazuhiro Marumoto†ǁ*

†

Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan

‡

Semiconductor Device Materials Group, Nano Materials Field, International Center for

Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan §

Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial

Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 3058565, Japan ⁋

Division of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan

ǁ

Tsukuba Research Center for Energy Materials Science (TREMS), University of Tsukuba,

Tsukuba, Ibaraki 305-8571, Japan

*Correspondence should be addressed to [email protected]

1 Environment ACS Paragon Plus

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

ABSTRACT: Polymer solar cells are one of the promising energy sources due to the easy solutionprocessable production with large area at a low cost without toxicity. Among the polymer materials, a donor-acceptor conjugated copolymer PTB7 has been extensively studied because of the typical high-performance polymer solar cells.

Here, we show operando direct

observation of charge accumulation in PTB7:PC71BM blend solar cells from a microscopic viewpoint using electron spin resonance (ESR) spectroscopy. The accumulation of ambipolar charges in the PTB7-based cells is directly observed for the first time, which shows a clear correlation with the performance deterioration during device operation. The sites of the ambipolar charge accumulation are elucidated at the molecular level, whose information would be useful for improving the cell durability in addition to the performance improvement. KEYWORDS: polymer solar cells, PTB7, electron spin resonance spectroscopy, charge accumulation, performance deterioration

1. INTRODUCTION Among solution processable polymer solar cells with large-area production at a low cost without toxicity,1-3 a donor-acceptor (D-A) conjugated copolymer poly({4,8-bis[(2ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl}{3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) has been widely studied as a typical


Page 2 of 40

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


p-type organic semiconductor because it has a lower bandgap compared to that of a typical ptype organic semiconducting homopolymer poly(3-hexylthiophene) (P3HT) and the light absorbance in almost the whole visible light region has been obtained by the combination of PTB7 and an n-type semiconductor soluble fullerene [6,6]-phenyl C71-butyric acid methyl ester (PC71BM).4-7 Also, a suitable nano structure has been demonstrated by adding 1,8diiodooctane (DIO) due to the control of the phase separation of PTB7 and PC71BM.7-9 By using PC71BM and DIO, a high power conversion efficiency of 9.2% for PTB7:PC71BM blend solar cells has been achieved even for single junction cells.10,11 The effects of the DIO addition on the device performance and the durability have been studied for the PTB7:PC71BM blend cells.12

In addition to the efficiency improvement, the durability improvement is another important issue to the commercialization of polymer solar cells.

Extrinsic and intrinsic

factors for the performance deterioration of organic solar cells have been discussed.13-24 Organic semiconductors irreversibly deteriorate by extrinsic deterioration factors such as oxygen and moisture.13-15 The extrinsic irreversible deteriorations can be suppressed by applying a device sealing.16-22

However, the intrinsic internal deteriorations cannot be

prevented by the device sealing.

For one of the intrinsic deterioration factors, charge

accumulation in solar cells during device operation has been studied using macroscopic



investigations such as thermally stimulated current (TSC) and current density (J)−voltage (V) measurements.23,24 In addition, the effects of fullerene dimerization on the performance of polymer solar cells have been reported, where the change in the chemical structure of the materials under light illumination has been discussed.25,26 Such chemical change may form the trapping sites for photogenerated charges, which may cause the charge accumulation.

Electron spin resonance (ESR) spectroscopy is one of the most effective methods to directly observe the charge accumulation in the devices operando from a microscopic viewpoint.27-30 The ESR method can nondestructively investigate materials and devices with high sensitivity at the molecular level.27-38 The ESR studies of organic solar-cell materials such as polymers and fullerenes have been reported by several groups.31-35 However, the ESR study of organic solar cells has not yet been reported except for our studies.28-30,36,37 With respect to the initial device performance, the ESR studies of organic solar cells have clarified the internal degradation mechanisms due to the charge formation in the cells during device fabrication.36,37 Also, the ESR study has directly observed the charge accumulation during device operation for P3HT polymer solar cells with a soluble fullerene [6,6]-phenyl C61butyric acid methyl ester (PC61BM) or indene-C60 bisadduct (ICBA).28-30 However, direct evidence for ambipolar charge accumulation in polymer:fullerene blend solar cells has not yet


Page 4 of 40

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


been reported. Moreover, D-A polymer solar cells have not yet been studied by the ESR method from a microscopic viewpoint.

Here, we report operando direct observation of ambipolar charge accumulation in PTB7:PC71BM blend solar cells using a light-induced ESR technique.

The charge

accumulation clearly correlates with the performance deterioration of the cells. The charge accumulation is found to occur in both PTB7 and PC71BM.

In particular, the hole

accumulation nearby PTB7 chain ends with residual bromines is found to have a correlation with the deterioration of the device performance.

Therefore, the present microscopic

characterization using ESR would be useful for studying the intrinsic deterioration mechanism.

2. MATERIALS AND DEVICE FABRICATION Commercially available PTB7 (1-Material, OS007), PC71BM (Solenne BV, purity >99 %), poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) (Clevios P AI4083), and DIO (Sigma-Aldrich) were used to fabricate the solar cells.

The device

structure of indium tin oxide (ITO)/PEDOT:PSS (≈25 nm)/PTB7:PC71BM (≈100 nm)/LiF (0.1 nm)/Al (100 nm) was fabricated by spin coating PEDOT:PSS and PTB7:PC71BM solution on an ITO substrate, followed by the vacuum deposition of LiF and Al on the organic



layers to form an electron collecting electrode. The obtained power conversion efficiency (PCE) of the fabricated solar cells is ≈7.8%. For the PTB7-based solar cells with an inverted device structure, the PCE of 9.2% has been reported.11 For those with a conventional device structure, the PCEs of 7.4-8.4% have been reported.7,10,12 Thus, the PCE of our cells is reasonable because a conventional device structure is used in this study. To attain high signal-to-noise (S/N) ratio of the light-induced ESR signal by increasing the active area of the device, we utilized a rectangular solar-cell structure (3 mm × 20 mm) with an asymmetric active area of 2 mm ×10 mm in an ESR sample tube with an inner diameter of 3.5 mm.30,36 The fabricated device was sealed in an ESR sample tube with helium gas at 100 Torr after evacuating the tube below 4.0×10−5 Pa. Further details are described in Experimental Section.

3. RESULTS AND DISCUSSION The charge accumulation in the cells can be directly investigated by observing the light-induced ESR signal. In our experiments, we simultaneously observed the light-induced ESR signal and the device performance during device operation, where a continuous-wave method with a modulation frequency of 100 kHz for the external magnetic field H, that is, lock-in detection, was used in the ESR experiments.28-30 Thus, the photogenerated charge carriers with a lifetime of 10 µs in solar cells, namely, accumulated (or deeply trapped) photogenerated carriers.28-30

A

lifetime of 10 µs is calculated from one cycle of the H modulation frequency of 100 kHz, that is, 1/100×103 (1/s−1). A charge with a lifetime to response the H modulation over at least one cycle can be detected. Figure 1a,b present typical data for the dependence of the lightinduced ESR spectra of a cell of ITO/PEDOT:PSS/PTB7:PC71BM/LiF/Al on the duration of simulated solar irradiation (AM 1.5G, 100 mW cm−2) under (a) short-circuit condition and (b) open-circuit condition, respectively. Under short-circuit condition, the ESR signals and the short-circuit current were measured simultaneously using the same device under applied bias voltage of Vbias = 0 V. Under open-circuit condition, the ESR signals and the open-circuit voltage were measured simultaneously using the same device under applied bias current of Jbias = 0 mA cm−2. The light-induced ESR spectrum was obtained by subtracting the ESR spectrum under dark conditions from that under simulated solar irradiation. The vertical axis is plotted using a unit of the peak-to-peak ESR intensity of a standard Mn2+ marker sample, IMn. As revealed in Figure 1a,b, the gradual increases in the light-induced ESR signals due to the charge accumulation were clearly observed under both operation conditions, respectively. The observed ESR signals gradually decreased after turning off the light irradiation, which supported that the origins of the ESR signals were due to the charge accumulation, not due to



molecular degradations such as oxidation.23,24,28-30

Similar results for the transient ESR

responses have been obtained from six fabricated cells. Two signal components with narrow or broad ESR linewidth were observed. The observation directly demonstrates the charge accumulation with a lifetime of >10 µs in the D-A polymer solar cells for the first time. Hereinafter, these signal components are defined as the narrow and broad components, respectively. The reason why the light-induced ESR signals due to the charge accumulation under the short-circuit condition (Figure 1a) is larger than that under the open-circuit condition (Figure 1b) may be ascribed to the increase in a possibility of encountering the trapping sites for photogenerated charge carriers because of their migration due to an internal electric field existed in the active layer in the cell under the short-circuit condition.

To analyze the light-induced ESR spectra in detail, we performed a fitting analysis using a least-squares method as shown in Figure 1c,d. The ESR spectra of charges in semiconductors are typically described by Lorentzian or Gaussian functions. In this study, the Lorentzian and Gaussian function describe mobile and trapped charges, respectively. Several formulas were examined for the fitting analysis, and the sum of Lorentzian and Gaussian functions has been found to obtain the best result. The fitting formula for the ESR spectrum, I(H), using a Lorentzian function IL(H) and a Gaussian function IG(H), which is used for the best fitting shown in Figure 1c,d, is described as follows:


Page 8 of 40

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


I (H ) = I L (H ) + I G (H ) I L (H ) = −

(1)

2hL (H − H 0,L )   H − H 0,L  2  2   wL 1 +    wL  

I G (H ) = −2

hG (H − H 0,G ) wG

2

2

  H − H 0,G  2    exp −    wG    

(2)

(3)

Here, the fitting parameters of IL(H) and IG(H) are the resonance magnetic field (H0,L, H0,G), half width at half maximum (wL, wG), and spectrum height (hL, hG) for the Lorentzian and Gaussian functions, respectively. Here, the g factor was evaluated from the H0 where the ESR spectrum with a first derivative form has a value of zero, and the peak-to-peak ESR linewidth (∆Hpp) value was evaluated as the difference between the two magnetic fields at a peak and a valley in the ESR spectrum, which are defined using the half width at half

(

maximum as ∆H pp = 2

)

3 wL for the Lorentzian function and as ∆H pp = 2 ln 2 wG for the

Gaussian function, respectively. The value of the g factor is calculated using the resonance magnetic field H0 and the magnetic resonance condition of hν = gµBH0, where h is the Planck constant, ν is the frequency of the microwave used in this study, and µB is the Bohr magneton. The analysis using equations (1), (2), and (3) has been adopted for organic solar cells for the first time. The green dotted lines show the calculated narrow component with a Lorentzian lineshape and ESR parameters g = 2.0027 ± 0.0001 and ∆Hpp = 0.20 ± 0.02 mT. The orange dotted lines show the calculated broad component with the Gaussian lineshape and ESR



parameters g = 2.0036 ± 0.0002 and ∆Hpp = 1.38 ± 0.02 mT. The red solid lines represent the sum of the green and orange dotted lines, which explains the experimental results (blue open circles). The g factor is determined by the g shift from a free electron’s value of 2.0023 due to spin-orbital interactions. The g shift of the broad component has been obtained as 0.0013, which is more than three times larger than that of the narrow component of 0.004. This larger g shift is ascribed to the effect of bromine heavy atoms due to the large spin-orbital interactions, as discussed later.

It is interesting to compare the variation of these ESR signals with that of the device performance during device operation. To present the ESR intensity, we use the number of spin (Nspin) of the cell derived from the charge accumulation. The Nspin was obtained by integrating the light-induced ESR spectrum twice and comparing the value with that of the standard Mn2+ marker sample. The ESR spectrum has a differential form of the magnetic resonance absorption as a function of the H due to the lock-in detection. Thus, the double integral of the ESR spectrum is needed to evaluate the total absorption, which is proportional to the Nspin. Figure 1e,f present the dependences of the Nspin of both narrow and broad components and the solar-cell parameters short-circuit current density (Jsc) and open-circuit voltage (Voc) of the same cell on the duration of the simulated solar irradiation, respectively. The Nspin due to the narrow or broad component is proportional to the double integral of the


Page 10 of 40

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


IL(H) (πhLwL) or that of the IL(H) (√hGwG), respectively. The total Nspin is proportional to the sum of πhLwL and √hGwG, and can be evaluated using the standard Mn2+ marker sample as mentioned above. From these relations, each Nspin due to the narrow or broad component can be obtained by calculating each ratio to the total Nspin. For the solar-cell parameters, normalized values of Jsc(t)/Jsc(0) and Voc(t)/Voc(0) with respect to the initial states are used under short-circuit and open-circuit conditions, respectively.

The Nspin of the broad

component increased monotonically and the solar-cell parameters decreased concomitantly under both device-operation conditions. The increase in the Nspin of the broad component correlates with the decreases in the solar-cell parameters. This result demonstrates that the charge accumulation has a correlation with the deterioration of the D-A polymer solar cells for the first time. The Nspin of the broad component is one order of magnitude larger than that of the narrow component, and has a large contribution to the total Nspin. The number of trapped charges (total Nspin) for 1 h after simulated solar irradiation on has been measured as 5.3×1012 (see Figure 1e). The number of photoinduced charges in the cell for 1 h is calculated as 6.6×1019 using the average of the Jsc (≈15 mA/cm2) at 1 h after the light irradiation on and the elementary charge of 1.6×10−19. From these values, ≈0.08ppm of photoinduced charges is found to be presented as trapped charges at 1 h after the light irradiation on. We comment the reason why the Nspin under the short-circuit condition (Figure 1e) is larger than that under the open-circuit condition (Figure 1f). This may be ascribed to the migration of photogenerated



charge carriers. Under the short-circuit condition, an internal electric field exists in the active layer in the cell, which may further enhance the chance to encounter the trapping sites for photogenerated charges carriers due to their migration. Similar behavior has been observed for the P3HT:PC61BM solar cells.28 Several different types of traps have been discussed for the active layer of P3HT:PCBM polymer solar cells, such as hole trap, electron trap, electron trap in isolated PCBM, molecular oxygen, etc.39 The trapped charges are referred to as the accumulated charges in this study where hole traps in PTB7 and electron traps in PC71BM are discussed. The possibility of electron traps in PTB7 is excluded by considering the energy instability and the density functional theory calculation of a radical anion on PTB7, as mentioned later. The electron traps in molecular oxygen cannot be observed by the present ESR study because of the rapid spin relaxation at room temperature.40

The elucidation of the origin of the charge accumulation is important for discussing the performance deterioration mechanism. To clarify the origin, we studied polymer:fullerene blend films for the assignment of the signals. The film structures were quartz/PTB7:PC61BM and quartz/PTB7:PC71BM, which were studied similarly with the light-induced ESR technique. Figure S1a shows the light-induced ESR spectrum of the PTB7:PC61BM film (Supporting Information). A main signal with a Lorentzian lineshape and ESR parameters g = 2.0024 ± 0.0001 and ∆Hpp = 0.21 ± 0.01 mT was observed. Since the ESR signal due to


Page 12 of 40

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


radical anions (electrons) on PC61BM cannot be detected at room temperature (RT),28,30,34 we assign the origin of the observed signal to photogenerated radical cations (holes or positive polarons) on PTB7. However, the PTB7:PC71BM film exhibited a different light-induced ESR spectrum with a Lorentzian lineshape and ESR parameter g = 2.0027 ± 0.0001 and ∆Hpp = 0.20 ± 0.01 mT (Figure S2a, Supporting Information). This signal is consistent with that of the narrow component of the cells (see Figure 1c,d).

The obtained g value of the

PTB7:PC71BM film (g = 2.0027) is different from that of the PTB7:PC61BM film (g = 2.0024). Thus, a contribution of radical anions (electrons) on PC71BM is found because the PC61BM signal is undetectable at RT.28,30,34 To evaluate this contribution, we subtracted the lightinduced ESR spectrum of the PTB7:PC61BM film from that of the PTB7:PC71BM film. As a result, a differential ESR spectrum with g = 2.0030 ± 0.0003 and ∆Hpp = 0.21 ± 0.02 mT was obtained (Figure S2b, Supporting Information), which is assigned to the signal of PC71BM radical anions. This assignment is further supported by an additional ESR experiment for thermally annealed quartz/PC71BM/LiF/Al layered films (see the S2 section, Supporting Information). Therefore, it is demonstrated that the narrow component of the cells is ascribed to both PTB7’s holes and PC71BM’s electrons. Such electron accumulation has not been observed for the P3HT:PC61BM or P3HT:ICBA blend cells.28-30 No ESR anisotropy was observed for the H direction to the substrate plane, which may indicate that the amorphous nature in the PTB7:PC71BM films relates to the charge accumulation.7,8



Page 14 of 40

The origin of the broad component of the cells is an important issue because the Nspin of the broad component correlates with the performance deterioration.

The fullerene

dimerization in polymer solar cells under light illumination has been reported to have the correlation with the performance deterioration.25,26

However, the origin of the broad

component cannot be ascribed to the charge accumulation in dimeric or monomeric PC71BM or in pure PTB7 because the ESR parameters of the broad component are larger than those of dimeric or monomeric PC71BM or of pure PTB7 as mentioned above, respectively. Thus, from now on, for a possible origin of the broad component, we discuss the hole accumulation nearby PTB7 chain ends with residual bromines, as reported for the P3HT cases.29,30,41 The P3HT chain ends with bromine atoms have been reported to act as hole trapping sites from the study of P3HT:PCBM blend cells and P3HT field-effect transistors.41 Also, the ionization potential of the thiophene ring attached with a bromine atom is smaller than that without a bromine atom, which supports the hole trappings at the thiophene rings with residual bromines.42,43 The ESR studies of polymer solar cells with P3HT:PCBM or P3HT:ICBA blend film have reported that the hole accumulation nearby P3HT chain ends with residual bromines is the origin of the broad component.29,30 For the PTB7 case, a thienothiophene acceptor unit attached with bromine atoms is used for the synthesis.6 It has been reported that although the chain-end treatment of synthesized polymers is performed, the treatment is


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


imperfect and bromines remain at the chain ends.44-46 Actually, we have confirmed the existence of residual bromines in the present PTB7 material from energy-dispersive x-ray spectroscopy (EDS) measurements (Figure S3 and Table S1, Supporting Information). The bromine atom has a nuclear spin 3/2 with a 100 % natural abundance, which may result in a large ESR linewidth, as discussed later.47 In addition, the g value increases owing to the large spin-orbit interactions due to bromine heavy atoms.48

To examine the effects of residual bromines at PTB7 chain ends, we performed density functional theory (DFT) calculation using Gaussian09.49 As a model molecule of PTB7, we utilized oligomers shown in Figure 2a,b. The dimer attached with hydrogen atoms at both ends of thienothiophene (T) and benzodithiophene (B) is defined as H-2TB-H (Figure 2a), and the dimer attached with a bromine atom at the left end of T is defined as Br-2TB-H (Figure 2b). All side chains are replaced with hydrogen atoms. The DFT calculations for the radical cation states of H-2TB-H and Br-2TB-H were performed with UB3LYP functional and 6-31G(d,p) basis set using an anti-PTB7 structure under structure-optimization conditions according to the literature.50 The calculated spin density distributions of these states are shown in Figure 2c,d, respectively. Table 1 shows the calculated spin density (σ) and the tensor components of the hyperfine coupling (hfc) constants for bromines and neighbour atoms in H-2TB-H and Br-2TB-H. As shown in Figure 2c,d and Table 1, the Br-2TB-H has



larger σ and hfc constants at the bromine atom compared to those at the hydrogen atom in H2TB-H. Large spin density on an atom gives rise to a large hyperfine interaction.47 The hyperfine interaction with a bromine atom splits one resonance line into four, because the natural abundant

79

Br and

81

Br nuclei have a nuclear spin I = 3/2. This could make ESR

linewidth further broader. These effects rationally explain the broader ESR linewidth (∆Hpp = 1.38 mT) than that of the narrow component (∆Hpp = 0.20 mT) because the ESR linewidth arises from the hyperfine interactions between the π-electron’s spin and the nuclear spins.47 Note that a similar broad component was observed in the PTB7:PC61BM film (Figure S1b, Supporting Information). In addition, larger averaged g values (gave) are calculated for Br2TB-H and for a TB trimer attached with a bromine atom (Br-3TB-H) (Tables S2 and S3, Supporting Information), which also support the experimental results. Therefore, the broad component is ascribed to the hole accumulation nearby PTB7 chain ends with residual bromines. No observation of the ESR anisotropy for the cells and the blend films indicates that the hole-accumulation sites are amorphous. The powder pattern simulation of the ESR spectrum with taking into account the anisotropy in the hfc constants is an interesting issue. However, this simulation is generally difficult because the simulation needs to take into account the principal axes of the anisotropy in the hfc constants and their direction cosines for the H at each atom in the calculated molecules, which needs a theoretical technique and seems to be beyond the scope of this study. The simulation will be reported in a separate paper.


Page 16 of 40

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


Also, there is a possibility that the broad component is from the reduced polymer species (radical anion), since the B unit can be an electron accepting group. This possibility may be excluded by the energy instability of the anion formation in PTB7 in PTB7:PC71BM thin-film system compared with the energy stability of the cation formation in PTB7. To examine the possibility further, we have performed the DFT calculation for the radical anion species in the same model oligomers using UB3LYP functional and 6-31G+(d,p) basis set. As a result, the obtained g factor of the radical anions species shows smaller or almost the same values compared to those of the radical cations. Thus, the formation of the radical cations in PTB7 is excluded from the possibility of the broad component.

The previous light-induced ESR study of PTB7:fullerene blend films have reported the principal values of the g tensor of positive polarons on PTB7 as 2.0023, 2.0031, and 2.0045.35 The smaller value of g = 2.0023 may be related to that of the present narrow component due to photogenerated radical cations (holes or positive polarons) on PTB7 (g = 2.0024). The larger values of g = 2.0031 and 2.0045 calculate an averaged value of g = 2.0038, which may be related to the present hole accumulation nearby PTB7 chain ends with residual bromines (g = 2.0036).



Judging from the reported correlation between the fullerene dimerization in polymer solar cells and the performance deterioration under light illumination,25,26 the change in the chemical structure of the materials such as fullerene dimerization under illumination may lead to the formation of trapping sites. The effect of forming more trapping sites is the hole accumulation. Thus, the origin of the performance deterioration may be the change in the chemical structure nearby PTB7 chain ends with residual bromines. The residual bromines in the present PTB7 material have been confirmed by the EDS measurements, as mentioned above (Figure S3 and Table 1, Supporting Information). Such hole accumulation leads to lower performance due to the changes of charge extraction/recombination kinetics, as discussed for the P3HT:PCBM or P3HT:ICBA blend cells.28,29,37,51 It has been reported that the absorption spectra of PTB7-Th based thin films greatly decrease after light irradiation where the effect of DIO on radical formation in the PTB7-Th side chain with a thiophene ring is discussed.52 If the dissociation of bromine and the formation of radical occurs in PTB7, leading to crosslinking or scission at the chain ends, bromine does not remain in PTB7. This possibility may be excluded by the explanation of the hole accumulation at PTB7 chain ends because the larger g factor cannot be explained without the existence of bromine. Thus, bromine at the PTB7 chain ends seems to remain. In the present study, the absorption spectra of the samples containing PTB7 based thin films did not show a large decrease after light irradiation, which indicates that DIO seems to have no significant effect on the degradation of


Page 18 of 40

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


the samples. The present PTB7 based solar cells contain DIO while the above-mentioned P3HT based solar cells do not contain DIO which shows the hole accumulation at the P3HT chain ends with residual bromines.28-30 Thus, the charge accumulation at the polymer chain ends with residual bromines in the cells seems to occur regardless of the existence of DIO.

The energy-level shift in the highest occupied molecular orbital (HOMO) of PTB7 at the interfaces between PTB7 and PEDOT:PSS was studied using organic thin films. In the PTB7:PC71BM solar cells, this energy-level shift should occur because of the energy difference between the HOMO of PTB7 (−5.15 eV) and the work function of PEDOT:PSS (−5.3 eV).11,28,36 This energy-level shift is related to the interfacial electric dipole layer, which can be caused by an electron transfer from PTB7 to PEDOT, forming additional holes in PTB7 under dark conditions, as discussed in previous ESR studies.28,36 This hole formation has been confirmed by measuring ESR signals of layered films of quartz/PTB7:PC61BM and ITO/PEDOT:PSS/PTB7:PC61BM under dark conditions. Figure S4 shows these ESR signals, where the ESR signal of ITO/PEDOT:PSS/PTB7:PC61BM is obtained by subtracting the contributions of the ESR signals of PEDOT:PSS and PTB7:PC61BM under dark conditions. The data of ITO/PEDOT:PSS/PTB7:PC61BM shows an ESR signal with g = 2.0034±0.0001 and ∆Hpp = 0.33±0.01 mT. This result demonstrates the additional hole formation in PTB7



Page 20 of 40

due to the electron transfer from PTB7 to PEDOT:PSS under dark conditions, which indicates the energy-level shift in the HOMO of PTB7 at the PEDOT:PSS/PTB7 interfaces.

It has been reported that the residual bromines at polymer chain ends decrease the initial solar-cell performance of polymer:fullerene blend cells,53 which may indicate the another effect of the residual bromines for the device performance. The charge accumulation is considered to depend on trapping densities, which may be due to the change in the chemical structure in polymer solar cells. The ESR method can directly measure the number of trapped charges in solar cells. With an assumption of the volume where trapped charges exist, the trap site density within the solar cells can be evaluated from the ratio of the number to the volume. Thus, it would be an interesting subject to perform ESR experiments on polymer solar cells with different chemical structures and bromine content.

Such cells may be

fabricated using different fullerene and polymer structures, different polymer-molecular weights, and polymers with different end-capping or prepared without the bromine. By using our research method, the charge-accumulation sites can be identified at a molecular level. For example, an isolated domain of fullerene could be a place for electron charge trap, which should provide a clear ESR signal.39 This electron trap may be the origin of the observed ESR signal due to radical anions in PC71BM in this study. Thus, the information for chargeaccumulation sites is important because the optimization of molecular and device structures


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


for the performance improvement can be easily performed on the basis of the information obtained from a microscopic viewpoint.

4. CONCLUSION Our results directly show the ambipolar charge accumulation in the PTB7:PC71BM solar cells operando using light-induced ESR spectroscopy. Two ESR signals of the narrow and broad components were observed for the cells. The simultaneous measurements of ESR and device performance demonstrate the clear correlation between the increase in the Nspin due to the broad component and the decreases in the solar-cell parameters Jsc and Voc during device operation. The origins of the narrow and broad components have been elucidated from the fitting analyses of the light-induced ESR spectra and the DFT calculations. The narrow component is ascribed to the hole accumulation in pure PTB7 and the electron accumulation in PC71BM. The broad component is ascribed to the hole accumulation nearby PTB7 chain ends with residual bromines. Although accessible time-scales of our presented experiments are limited to above 10 µs due to the lock-in detection of 100 kHz, our method has higher sensitivity for detecting accumulated photogenerated carriers compared to current-detected ESR experiments because the current-detected ESR experiments are not suitable for detecting accumulated carriers. Our method is general and should be applicable to the elucidation of the performance deterioration mechanism in other solar cells. One of the origins of the



Page 22 of 40

performance deterioration may be the charge accumulation due to the change in the chemical structure nearby polymer chain ends with residual bromines. This chemical structure change may be further clarified with other methods such as infrared (IR) spectroscopy.

EXPERIMENTAL SECTION Device Fabrication. A polymer:fullerene blend solution with 8 mg of PTB7 (1-Material, OS007) and 12 mg of PC71BM (Solenne BV, purity >99%) (1:1.5 w/w) dissolved in 1 mL of CB/DIO (97/3, v/v) solvent was fabricated by vibrational mixing at RT. The ITO-coated (3 mm × 15 mm) rectangular quartz substrate with a size of 3 mm × 20 mm was cleaned by O2 plasma treatment for 30 min. The PEDOT:PSS (Clevios P AI4083) layer of ≈25 nm thickness was fabricated by a spin-coating method on the ITO substrate. The first and second rotation conditions were 500 rpm for 5 sec and 3000 rpm for 180 sec under air atmosphere, respectively. Then the film was annealed at 135 °C for 10 min under ambient air.

The

PTB7:PC71BM blend film of ≈100 nm thickness was fabricated on the PEDOT:PSS layer by the spin-coating method in an N2-filled glove box (O2 < 0.2 ppm, H2O < 0.5 ppm). The rotation condition was 900 rpm for 60 sec. After the spin coating, the layered film was dried in the glove box for 30 min. The LiF and Al layers were deposited onto the PTB7:PC71BM film to form an electron-collecting electrode using a conventional vacuum-sublimation technique under vacuum conditions below 4.0×10−5 Pa. The LiF layer of 0.1 nm thickness


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


was formed at a deposition rate of 0.2 Å/s. The Al layer of 100 nm thickness was formed at a deposition rate of 2.0 Å/s. After forming the LiF/Al layer, the cell was transferred into the glove box and was wired by silver paste. After wiring, the cell was sealed in an ESR sample tube with helium gas at 100 Torr after evacuating the tube below 4.0×10−5 Pa.

Thin-Film Fabrication.

For quartz/PTB7:PC71BM blend films, the above-mentioned

PTB7:PC71BM solution was used for the film fabrication. For quartz/PTB7:PC61BM blend films, a polymer:fullerene blend solution with 8 mg of PTB7 and 12 mg of PC61BM (Frontier Carbon, nanom spectra E100, purity: 99.2%) (1:1.5 w/w) dissolved in 1 mL of CB/DIO (97/3, v/v) solvent was fabricated by vibrational mixing at RT and was used for the film fabrication. The rectangular quartz substrate with a size of 3 mm × 20 mm was cleaned by O2 plasma treatment for 30 min. The PTB7:PC71BM or PTB7:PC61BM blend film of 100 nm thickness was fabricated on the quartz substrate using the spin-coating method in the glove box. The rotation condition was 900 rpm for 60 sec. After the spin coating, the layered film was dried in the glove box for 30 min. For quartz/PC71BM/LiF/Al layered films, a PC71BM solution with 12 mg of PC71BM dissolved in 600 µL of CB/DIO (97/3, v/v) solvent was fabricated by vibrational mixing at RT and was used for the film fabrication. The PC71BM film was deposited on the quartz substrate using a drop-casting method in the glove box. After the drop-casting, the film was dried in the glove box for 30 min. The LiF and Al layers were



Page 24 of 40

deposited onto the PC71BM film using the above-mentioned vacuum-sublimation technique and fabrication conditions. After the deposition of the LiF/Al layer, the layered film was annealed at 160 °C in the glove box. All fabricated films were sealed in the ESR sample tubes with helium gas at 100 Torr after evacuating the tube below 4.0×10−5 Pa.

Characterization.

ESR measurements were performed using an X-band spectrometer

(JEOL RESONANCE, JES-FA200). The number of spins, g factor, and linewidth of the ESR signal were calibrated using a standard Mn2+ marker sample. The absolute value of the number of spins of the Mn2+ marker sample was calculated using a solution (220 µL) of 4hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) as a standard. The calibration of the g factor was performed by using a software program of the ESR system considering high second-order correction of the effective resonance field. Its correctness was also confirmed by using 2,2-diphenyl-1-picrylhydrazyl (DPPH) as another standard sample.

Device

characteristic measurements were carried out using a source meter (Keithley, 2612A) under AM 1.5G 100 mW cm−2 simulated solar irradiation.

A solar simulator (Bunkoukeiki,

OTENTOSUN-150BXM) was used as the light source. The light irradiation was performed through a fully opened optical window in the wall of the ESR cavity resonator. The light power was calibrated using a standard silicon photodetector in the ESR cavity resonator. Chemical composition analyses were carried out using a field emission scanning electron


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


microscopy (FE-SEM) (HITACHI, SU-8000) equipped with an EDS spectrometer (Bruker, QUANTAX) under an accelerating voltage at 5 kV and current of 10 µA. All measurements were performed at RT.

Device Performance.

The fabricated solar cells had the following parameters: a short-

circuit current density (Jsc) of ≈15 mA cm-2, an open-circuit voltage (Voc) of ≈0.77 V, a fill factor (FF) of ≈0.68, and a power conversion efficiency (PCE) of ≈7.8% (Figure S5, Supporting Information).



Figure

1.

(a,b)

Light-induced

ESR

spectra

Page 26 of 40

of

a

cell

of

ITO/PEDOT:PSS/PTB7:PC71BM/LiF/Al for various exposure times to simulated solar light (AM 1.5G, 100 mW cm−2) at room temperature (RT) under short-circuit conditions (a) and open-circuit conditions (b), respectively. The external magnetic field H is parallel to the substrate plane. The data were obtained by averaging light-induced ESR spectra measured under irradiation during 1 h. (c,d) Fitting analyses using a least-squares method for the lightinduced ESR spectra of the cells after 20 h irradiation under short-circuit conditions (c) and open-circuit conditions (d), respectively. (e,f) Dependences of the Nspin (red circle) and the Jsc (blue solid line) (e) or Voc (blue solid line) (f) of the cells on the duration of simulated solar irradiation at RT. The Nspin is obtained from the averaged light-induced ESR spectra under irradiation for 1 h, and is plotted at each averaged time over 1 h. The green squares and the orange triangles show contributions of the narrow and broad components obtained in Fig. 1c,d, respectively.


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


Figure 2. (a,b) The chemical structures of: a) H-2TB-H, and b) Br-2TB-H used as a model molecule of PTB7 in the DFT calculation. (c,d) The spin density distribution of: c) H-2TB-H, and d) Br-2TB-H for a cationic state obtained from the DFT calculation.



Page 28 of 40

Table 1. The spin density (σ) and the tensor components of hyperfine coupling (hfc) constants for bromines and neighbour atoms in model molecules of PTB7 (H-2TB-H and Br-2TB-H) obtained from the DFT calculation. The atomic number is defined in Figure 2a,b.

Chemical structure

No.

Atom

σ

16

S

-0.000086

15

C

0.068287

13

C

-0.019154

64

H

-0.003182

16

S

-0.002747

15

C

0.064376

13

C

-0.018303

64

Br

0.009275

H-2TB-H

Br-2TB-H


hfc (mT) -0.020694 -0.005294 0.029306 0.117126 0.124126 0.666626 -0.208881 -0.106481 -0.092881 -0.274800 -0.186900 -0.073800 -0.014940 0.001760 0.004660 0.161376 0.169176 0.735476 -0.200307 -0.106607 -0.093007 -0.380409 -0.187309 0.814791

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


ASSOCIATED CONTENT Supporting Information ESR study of PTB7:PC61BM blend film, ESR study of PTB7:PC71BM blend film, Energy-dispersive X-ray spectroscopy, Density functional theory (DFT) calculation, ESR study of the energy-level shift in PTB7 at the interfaces, Device performance

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interests.

Acknowledgements This work was partially supported by JSPS KAKENHI Grant Numbers JP24560004 and JP15K13329, by JST PRESTO, by SEI Group CSR Foundation, and by JST ALCA Grant Number JPMJAL1603, Japan.



REFERENCES (1) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer-Based Organic Solar Cells. Chem. Rev. 2007, 107, 1324-1338. (2) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736-6767. (3) Søndergaard, R.; Hösel, M.; Angmo, D.; Larsen-Olsen, T. T.; Krebs, F. C. Roll-to-Roll Fabrication of Polymer Solar Cells. Mater. Today 2012, 15, 36-49. (4) Lu, L.; Yu, L. Understanding Low Bandgap Polymer PTB7 and Optimizing Polymer Solar Cells Based on It. Adv. Mater. 2014, 26, 4413-4430. (5) Liang, Y.; Wu, Y.; Feng, D.; Tsai, S.-T.; Son, H.-J.; Li, G.; Yu, L. Development of New Semiconducting Polymers for High Performance Solar Cells. J. Am. Chem. Soc. 2009, 131, 56-57. (6) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. Highly Efficient Solar Cell Polymers Developed via Fine-Tuning of Structural and Electronic Properties. J. Am. Chem. Soc. 2009, 131, 7792-7799. (7) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright Future—Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%. Adv. Mater. 2010, 22, E135-E138.


Page 30 of 40

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


(8) Lou, S. J.; Szarko, J. M.; Xu, T.; Yu, L.; Marks, T. J.; Chen, L. X. Effects of Additives on the Morphology of Solution Phase Aggregates Formed by Active Layer Components of High-Efficiency Organic Solar Cells. J. Am. Chem. Soc. 2011, 133, 20661-20663. (9) Collins, B. A.; Li, Z.; Tumbleston, J. R.; Gann, E.; McNeill, C. R.; Ade, H. Absolute Measurement of Domain Composition and Nanoscale Size Distribution Explains Performance in PTB7:PC71BM Solar Cells. Adv. Energy Mater.2013, 3, 65-74. (10) He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells. Adv. Mater. 2011, 23, 4636-4643. (11) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-Conversion Efficiency in Polymer Solar Cells using an Inverted Device Structure. Nat. Photon. 2012, 6, 591-595. (12) Kim, W.; Kim, J. K.; Kim, E.; Ahn, T. K.; Wang, D. H.; Park, J. H. Conflicted Effects of a Solvent Additive on PTB7:PC71BM Bulk Heterojunction Solar Cells. J. Phys. Chem. C 2015, 119, 5954-5961. (13) Seemann, A.; Sauermann, T.; Lungenschmied, C.; Armbruster, O.; Bauer, S.; Egelhaaf, H.-J.; Hauch, J. Reversible and Irreversible Degradation of Organic Solar Cell Performance by Oxygen. Sol. Energy 2011, 85, 1238-1249.



Page 32 of 40

(14) Jørgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.; Andreasen, B.; Krebs, F. C. Stability of Polymer Solar Cells. Adv. Mater. 2012, 24, 580-612. (15) Soon, Y. W.; Cho, H.; Low, J.; Bronstein, H.; McCulloch, I.; Durrant, J. R. Correlating Triplet

Yield,

Singlet

Oxygen

Generation

and

Photochemical

Stability

in

Polymer/Fullerene Blend Films. Chem. Commun. 2013, 49, 1291-1293. (16) Chang, C.-Y.; Chou, C.-T.; Lee, Y.-J.; Chen, M.-J.; Tsai, F.-Y. Thin-Film Encapsulation of Polymer-Based Bulk-Heterojunction Photovoltaic Cells by Atomic Layer Deposition. Org. Electron. 2009, 10, 1300-1306. (17) Sarkar, S.; Clup, J. H.; Whyland, J. T.; Garvan, M.; Misra, V. Encapsulation of Organic Solar Cells with Ultrathin Barrier Layers Deposited by Ozone-Based Atomic Layer Deposition. Org. Electron. 2010, 11, 1896-1900. (18) Clark, M. D.; Jespersen, M. L.; Patel, R.J.; Leever, B. J. Ultra-Thin Alumina Layer Encapsulation of Bulk Heterojunction Organic Photovoltaics for Enhanced Device Lifetime. Org. Electron. 2014. 15, 1-8. (19) Hermenau, M.; Schubert, S.; Klumbies, H.; Fahlteich, J.; M.-Meslamp, L.; Leo, K.; Riede, M. The Effect of Barrier Performance on the Lifetime of Small-Molecule Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2012, 97, 102-108.


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


(20) Lee, H.-J.; Kim, H.-P.; Kim, H.-M.; Youn, J.-H.; Nam, D.-H.; Lee, Y.-G.; Lee, J.-G.; Yusoff, A. R. B. M.; Jang, J. Solution Processed Encapsulation for Organic Photovoltaics. Sol. Energy Mater. Sol. Cells 2013, 111, 97-101. (21) Abdel-Fattah, T. M.; Younes, E. M.; Namkoong, G.; El-Maghraby, E. M.; Elsayed, A. H.; Elazm, A. H. A. Stability Study of Low and High Band Gap Polymer and Air Stability of PTB7:PC71BM Bulk Heterojunction Organic Photovoltaic Cells with Encapsulation Technique. Synth. Met. 2015, 209, 348-354. (22) Yang, L.; Xu, H.; Tian, H.; Yin, S.; Zhang, F. Effect of Cathode Buffer Layer on the Stability of Polymer Bulk Heterojunction Solar Cells. Sol. Energy Mater. Sol. Cells 2010, 94, 1831-1834. (23) Kawano, K; Adachi, C. Evaluating Carrier Accumulation in Degraded Bulk Heterojunction Organic Solar Cells by a Thermally Stimulated Current Technique. Adv. Funct. Mater. 2009, 19, 3934-3940. (24) Yamanari, T.; Ogo, H.; Taima, T.; Sakai, J.; Tsukamoto, J.; Yoshida, Y. PhotoDegradation and Its Recovery by Thermal Annealing in Polymer-Based Organic Solar Cells. in 2010 35th IEEE Photovoltaic Specialists Conf. (PVSC), IEEE, 2010, 001628001631.



(25) Distler, A.; Sauermann, T.; Egelhaaf, H.-J.; Rodman, S.; Waller, D.; Cheon, K.-S.; Lee, M.; Guldi, D. M. The Effect of PCBM Dimerization on the Performance of Bulk Heterojunction Solar Cells. Adv. Energy Mater. 2014, 4, 1300693-1-6. (26) Heumueller, T.; Mateker, W. R.; Distler, A.; Fritze, U. F.; Cheacharoen, R.; Nguyen, W. H.; Biele, M.; Salvador, M.; von Delius, M.; Egelhaaf, H.-J.; McGehee, M. D.; Brabec, C. J. Morphological and Electrical Control of Fullerene Dimerization Determines Organic Photovoltaic Stability. Energy Environ. Sci. 2016, 9, 247-256. (27) Marumoto, K.; Kuroda, S.; Takenobu, T.; Iwasa, Y. Spatial Extent of Wave Functions of Gate-Induced Hole Carriers in Pentacene Field-Effect Devices as Investigated by Electron Spin Resonance. Phys. Rev. Lett. 2006, 97, 256603-1-4. (28) Nagamori, T.; Marumoto, K. Direct Observation of Hole Accumulation in Polymer Solar Cells During Device Operation using Light-Induced Electron Spin Resonance. Adv. Mater. 2013, 25, 2362-2367. (29) Tamai, Y.; Ohkita, H.; Namatame, M.; Marumoto, K.; Shimomura, S.; Yamanari, T.; Ito, S. Light-Induced Degradation Mechanism in Poly(3-hexylthiophene)/Fullerene Blend Solar Cells. Adv. Energy Mater. 2016, 6, 1600171-1-7. (30) Son, D.; Kuwabara, T.; Takahashi, K.; Marumoto, K. Direct Observation of UV-Induced Charge Accumulation in Inverted-Type Polymer Solar Cells with a TiOx Layer:


Page 34 of 40

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


Microscopic elucidation of the light-soaking phenomenon. Appl. Phys. Lett. 2016, 109, 133301-1-5. (31) De Ceuster, J.; Goovaerts, E.; Bouwen, A.; Hummelen, J. C.; Dyakonov, V. HighFrequency (95 GHz) Electron Paramagnetic Resonance Study of the Photoinduced Charge Transfer in Conjugated Polymer-Fullerene Composites. Phys. Rev. B 2001, 64, 195206-1-6. (32) Schultz, N. A.; Scharber, M. C.; Brabec, C. J.; Sariciftci, N. S. Low-Temperature Recombination Kinetics of Photoexcited Persistent Charge Carriers in Conjugated Polymer/Fullerene Composite Films. Phys. Rev. B 2001, 64, 245210-1-7. (33) Poluektov, O. G.; Filippone, S.; Martín, N.; Sperlich, A.; Deibel, C.; Dyakonov, V. Spin Signatures of Photogenerated Radical Anions in Polymer-[70]Fullerene Bulk Heterojunctions: High Frequency Pulsed EPR Spectroscopy. J. Phys. Chem. B 2010, 114, 14426-14429. (34) Aguirre, A.; Meskers, S. C. J.; Janssen, R. A. J.; Egelhaaf, H.-J. Formation of Metastable Charges as a First Step in Photoinduced Degradation in π-Conjugated Polymer:Fullerene Blends for Photovoltaic Applications. Org. Electron. 2011, 12, 1657-1662. (35) Niklas, J.; Mardis, K. L.; Banks, B. P.; Grooms, G. M.; Sperlich, A.; Dyakonov, V.; Beaupré, S.; Leclerc, M.; Xu, T.; Yu, L.; Poluektov, O. G. Highly-Efficient Charge



Separation and Polaron Delocalization in Polymer–Fullerene Bulk-Heterojunctions: a Comparative Multi-Frequency EPR and DFT Study. Phys. Chem. Chem. Phys. 2013, 15, 9562-9574. (36) Marumoto, K.; Fujimori, T.; Ito, M.; Mori, T. Charge Formation in Pentacene Layers During Solar-Cell Fabrication: Direct Observation by Electron Spin Resonance. Adv. Energy Mater. 2012, 2, 591-597. (37) Liu, D.; Nagamori, T.; Yabusaki, M.; Yasuda, T.; Han, L.; Marumoto, K. Dramatic Enhancement of Fullerene Anion Formation in Polymer Solar Cells by Thermal Annealing: Direct Observation by Electron Spin Resonance. Appl. Phys. Lett. 2014, 104, 243903-1-5. (38) Namatame, M.; Yabusaki, M.; Watanabe, T.; Ogomi, Y.; Hayase, S.; Marumoto, K. Direct Observation of Dramatically Enhanced Hole Formation in a Perovskite-SolarCell Material Spiro-OMeTAD by Li-TFSI Doping. Appl. Phys. Lett. 2017, 110, 1239041-5. (39) Yu, C.-Y.; Jen, T.-H.; Chen S.-A. Traps in Regioregular Poly(3-hexylthiophene) and its Blend with [6,6]-Phenyl‑C61-Butyric Acid Methyl Ester for Polymer Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 4086−4092. (40) Kusumi, T.; Kuwabara, T.; Fujimori, K.; Minami, T.; Yamaguchi, T.; Taima, T.; Takahashi, K.; Murakami, T.; Rachmat, V. A. S. A.; Marumoto, K. Mechanism of


Page 36 of 40

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


Light-Soaking Effect in Inverted Polymer Solar Cells with Open-Circuit Voltage Increase. ACS Omega 2017, 2, 1617−1624. (41) Kim, Y.; Cook, S.; Kirkpatrick, J.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; Heeney, M.; Hamilton, R.; McCulloch, I. Effect of the End Group of Regioregular Poly(3-hexylthiophene) Polymers on the Performance of Polymer/Fullerene Solar Cells. J. Phys. Chem. C 2007, 111, 8137-8141. (42) Rabalais, J. W.; Werme, L. O.; Bergmark, T.; Karlsson, L.; Siegbahn, K. The High Resolution Electron Spectra of Thiophene, 2-Bromothiophene and 3-Bromothiophene. Int. J. Mass Spectrom. Ion Phys. 1972, 9, 185-196. (43) Butler, J. J.; Baer, T. Thermochemistry and Dissociation Dynamics of State-Selected C4H4X Ions. 1. Thiophene. J. Am. Chem. Soc. 1980, 102, 6764-6769. (44) McCullough, R. D.; Tristram-Nagle, S.; Williams, S. P.; Lowe, R. D.; Jayaraman, M. Self-orienting Head-to-Tail Poly(3-alkylthiophenes): New Insights on StructureProperty Relationships in Conducting Polymers. J. Am. Chem. Soc. 1993, 115, 49104911. (45) Chen, T.-A.; Wu, X.; Rieke, R. D. Regiocontrolled Synthesis of Poly(3-alkylthiophenes) Mediated by Rieke Zinc: Their Characterization and Solid-state Properties. J. Am. Chem. Soc. 1995, 117, 233-244.



Page 38 of 40

(46) Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D. A Simple Method to Prepare Head-to-Tail

Coupled,

Regioregular

Poly(3-alkylthiophenes)

Using

Grignard

Metathesis. Adv. Mater. 1999, 11, 250-253. (47) Son, D.; Marumoto, K.; Kizuka, T.; Shimoi, Y. Electron Spin Resonance of Thin Films of Organic Light-Emitting Material Tris(8-hydroxyquinoline) Aluminum Doped by Magnesium. Synth. Met. 2012, 162, 2451-2454. (48) Son, D.; Shimoi, Y.; Marumoto, K. Study on N-Type Doped Electron Transporting Layers in OLEDs by Electron Spin Resonance. Mol. Cryst. Liq. Cryst. 2014, 599, 153156. (49) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.;


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


Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT, 2013. (50) Khoshkho, M. J.; Marsusi, F.; Abolhassani, M. R. Density Functional Theory Investigation

of

Opto-Electronic

Properties

of

Thieno[3,4-b]thiophene

and

Benzodithiophene Polymer and Derivatives and their Applications in Solar Cell. Spectrochim. Acta A: Mol. Biomol. Spectrosc. 2015, 136, 373-380. (51) Marumoto, K.; Nagamori, T. Correlation between Hole Accumulation and Deterioration of Device Performance in Polymer Solar Cells as Investigated by Light-Induced Electron Spin Resonance. Mol. Cryst. Liq. Cryst. 2014, 597, 29-32. (52) Tremolet de Villers, B. J.; O’Hara, K. A.; Ostrowski, D. P.; Biddle, P. H.; Shaheen, S. E.; Chabinyc, M. L.; Olson, D. C.; Kopidakis, N. Removal of Residual Diiodooctane Improves Photostability of High-Performance Organic Solar Cell Polymers. Chem. Mater. 2016, 28, 876−884. (53) Kuwabara, J.; Yasuda, T.; Choi, S. J.; Lu, W.; Yamazaki, K.; Kagaya, S.; Han, L.; Kanbara, T. Direct Arylation Polycondensation: A Promising Method for the Synthesis of Highly Pure, High-MolecularWeight Conjugated Polymers Needed for Improving the Performance of Organic Photovoltaics. Adv. Funct. Mater. 2014, 24, 3226-3233.



Table Of Contents (TOC) graphic


Page 40 of 40

Operando Direct Observation of Charge Accumulation and the

Recommend Documents