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Enhancement the Power Conversion Efficiency of Organic Solar Cells via Unveiling the Appropriate Rational Design Strategy in Indacenodithiophene-alt-Quinoxaline #-Conjugated Polymers Christos Chochos, Ranbir Singh, Vasilis Gregoriou, Min Kim, Athanasios Katsouras, Efthymis Serpetzoglou, Ioannis Konidakis, Emmanuel Stratakis, Kilwon Cho, and Apostolos Avgeropoulos ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18381 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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ACS Applied Materials & Interfaces

Enhancement the Power Conversion Efficiency of Organic Solar Cells via Unveiling the Appropriate Rational Design Strategy in Indacenodithiophenealt-Quinoxaline π-Conjugated Polymers Christos L. Chochos,*[a,b] Ranbir Singh,*[c] Vasilis G. Gregoriou,[b,d] Min Kim,[c] Athanasios Katsouras,[a] Efthymis Serpetzoglou,[e] Ioannis Konidakis,[e] Emmanuel Stratakis,[e] Kilwon Cho,[c] Apostolos Avgeropoulos*[a] [a]

Department of Materials Science Engineering, University of Ioannina, Ioannina 45110, Greece

[b]

Advent Technologies SA, Patras Science Park, Stadiou Street, Platani-Rio, 26504, Patra, Greece [c]

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790–784, Korea

[d]

National Hellenic Research Foundation (NHRF), 48 Vassileos Constantinou Avenue, Athens 11635, Greece

[e]

Institute of Electronic Structure and Laser, Foundation for Research and Technology – Hellas, P. O. Box 1527, Heraklion, Crete, Greece

E-mail address: [email protected]; [email protected]; [email protected]; [email protected] KEYWORDS. organic photovoltaics, polymeric relationships, indacenodithiophene, quinoxaline

semiconductors,

structure-property

ACS Paragon Plus Environment

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ABSTRACT. We report on the photovoltaic parameters, photophysical properties, optoelectronic properties, self-assembly and morphology variation in a series of high performance donoracceptor (D-A) π-conjugated polymers based on indacenodithiophene and quinoxaline moieties തതതതn ), the nature of aryl substituents and as a function of the number-average molecular weight (M the enlargement of the polymer backbone. One of the most important outcome is that from the three optimization approaches followed to tune the chemical structure towards enhanced photovoltaic performance in bulk heterojunction (BHJ) solar cell devices with the fullerene derivative [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor, the choice of the aryl substituent is the most efficient rational design strategy. Incorporation of thienyl rings as substituents versus phenyl rings accelerates the electron-hole extraction process to the respective electrode, despite the slightly lower recombination lifetime and thus, improves the electrical performance of the device. Single junction solar cells based on ThIDT-TQxT feature a maximum power conversion efficiency (PCE) of 7.26%. This study provides significant insights towards understanding of the structure-properties-performance relationship for D-A πconjugated polymers in solid state, which provide helpful inputs for the design of next generation polymeric semiconductors for organic solar cells with enhanced performance.


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1. INTRODUCTION One of the most challenging task in the scientific field of “donor-acceptor” (D-A) π-conjugated polymers as electron donating materials in organic solar cells (OSCs) is the systematic optimization of their chemical structure for which precise guidelines are yet to be determined.1-8 Complete understanding of the polymer’s molecular characteristics in (i) the optoelectronic properties and the corresponding device performance and (ii) the aggregation properties of the polymer chains both in solution and in solid state, as well as the resulting thin film morphology is of outmost importance. Accomplishing these correlations will lead to π-conjugated polymers that will represent desirable organic semiconductors for highly efficient OSCs. Up to now, a variety of structural and molecular parameters have been identified as essential components9-40 for obtaining D-A π-conjugated polymers that instantaneously demonstrate high power conversion efficiencies (PCEs) and high solubility in non-toxic organic solvents in OSCs. Therefore, significant efforts have been currently devoted to correlate the optical, electrochemical and electrical properties of the polymers, as well as the resulting efficiency when applied in various optoelectronic applications with the rational design concept of π-conjugated polymer’s chemical structure. Indacenodithiophene (IDT) and quinoxaline (Qx) monomers represent two of the most favorable building blocks that have been extensively utilized for the development and synthesis of a plethora of organic electronic materials (small molecules and conjugated polymers) function either as p-type (electron donors) or n-type (electron acceptors) in various optoelectronic applications. 41-45 In particular, PhIDT-TQxT (inset of Figure 1) has been emerged as a highly promising electron donor polymeric material that exhibits PCEs > 7% in bulk heterojunction (BHJ) solar cells with the fullerene derivative [6,6]-phenyl-C71-butyric acid methyl ester


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(PC71BM) as the electron acceptor.46-48 Up to now, our group along with the research groups of Hou et al. and Wang et al. systematically studied the influence of the donor/acceptor ratio,46 the para- and meta- hexyl side chains positions anchored onto the phenyl substituents of the IDT47 and the length of the repeat unit on the photovoltaic performance.48 In order to further exploit the potential of this type of D-A π-conjugated polymers, our efforts have been focused to identify how the influence of the average molecular weight, the arylated substituents and the further enlargement of the polymer backbone in the PhIDT-TQxT are affecting the photovoltaic performance, which remains unexplored up to now. In particular, it is not well understood how exactly the aforementioned optimization chemical approaches are associated with the: (i) optical, electrochemical and electrical properties, (ii) self-assembly, (iii) thin film morphology and (iv) efficiency when applied into various optoelectronic applications. For shading light to these unanswered questions, a specific type of D-A conjugated polymers, effectively produced by varying the average molecular weights, replacing the phenyl substituent with thienyl and finally the further increament of the repeat unit of the polymer chain by the use of an indacenodithienothiophene (IDTT) instead of IDT (Figure 1) were extensively examined. The synthesized π-conjugated polymers allowed us to accomplish a detailed correlation between structure-property relationship, unknown up to now, on the optical, electrochemical and electrical properties, device performance, self-assembly, thin film morphology, hole and electron mobilities and photophysical properties. The examined systems are either pristine conjugated polymers or blends thereof with the PC71BM. The obtained results reveal that the replacement of the phenyl substituents with thienyl in this series of D-A polymers is the appropriate approach to further enhance the photovoltaic performance in PC71BM based- BHJ solar cells.


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Figure 1. Chemical structures of the studied polymers along with the rational design strategies followed to further enhance the photovoltaic performance of the PhIDT-TQxT(LMW):PC71BM system.

2. RESULTS AND DISCUSSION 2.1 Synthesis For the correlation between structure-optoelectronic properties-OSC efficiency as regards the IDT’s side group substitution, impact of molecular weight and enlargement of the polymer backbone by the extension of the repeat unit, the 5,8-bis(5-bromothien-2-yl)-2,3-bis(3(octyloxy)phenyl)quinoxaline (M1) is combined with the bis(trimethylstannyl)tetrahexylphenylIDT (M2) or the bis(trimethylstannyl)tetrahexylthienyl-IDT (M3) to provide PhIDTTQxT(LMW)/PhIDT-TQxT(HMW) or ThIDT-TQxT, respectively and finally with the


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bis(trimethylstannyl)tetrahexylphenyl-IDTT (M4) to afford the PhIDTT-TQxT (Scheme S1 in Supporting Information (SI)). Stille aromatic cross-coupling polymerization conditions49 were employed for the synthesis of the polymers using as catalyst 0.02 equivalent of tris(dibenzylideneacetone)dipalladium(0)

[Pd2(dba3)]

and

0.08

equivalent

of

tri(o-

tolyl)phosphine [P(o-tol)3] in a solution of toluene. To obtain the lower average molecular തതതതn ), PhIDT-TQxT(LMW), the polymerization reaction was carried out in weight per number (M dry toluene at 120 oC for 6 h. Increasing the reaction time to 48 h, the molecular weight is തതതതn PhIDT-TQxT(HMW) (see experimental methods in SI for enhanced, providing the higher M details).

After

purification,

using

Soxhlet

extraction,

PhIDT-TQxT(LMW)/PhIDT-

TQxT(HMW), ThIDT-TQxT and PhIDTT-TQxT are received from the chloroform (CF) batch. തതതതn , the average molecular weight per weight (M തതതത The M w ) and dispersity (Đ) of the polymers as estimated by gel permeation chromatography (GPC) based on polystyrene standards of narrow dispersity at elevated temperature (150 oC) and employing ortho-dichlorobenzene (o-DCB) as solvent are presented in Figure S1 (in (SI)) and are depicted in Table S1 (in SI). It is revealed the monomodal GPC profiles of all polymers with absence of any unreacted starting compound or തതതതn = 30700 g/mol), all polymers exhibit oligomer fractions. Except PhIDT-TQxT(LMW) (M

തതതn of 125900 g/mol (PhIDT-TQxT(HMW)), 85700 g/mol (ThIDT-TQxT) and extremely high തM 232100 g/mol (PhIDTT-TQxT).

2.2 Structural characterization and optical properties Grazing incidence wide angle X-ray scattering (GIWAXS) was used to understand the effect of chemical structure on the crystal packing and structural conformation properties of the neat thin films. The out-of-plane and in-plane profiles were extracted from 2D GIWAXS images


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(Figure 2) and summarized in Table 1. The in-plane diffraction profile of the neat polymer films showed several diffraction peaks. The diffraction peaks at ~0.29 Å-1 is related with the lamellar packing of the polymer chains. The lamellar packing distance can be calculated to be 21.7 Å. Furthermore, while the three polymers, PhIDT-TQxT (LMW-, HMW-) and ThIDT-TQxT, showed the (001) diffraction peak at 0.56 Å-1 (d-spacing, 11.2 Å), the PhIDTT-TQxT polymer film exhibited the (001) diffraction peak at 0.48 Å-1 (d-spacing, 13.1 Å). These diffraction peaks are correlated with the backbone unit length. The unit length of the extended polymer backbone of PhIDTT-TQxT is 26.7 Å, which is nearly double of the (001) d-spacing (13.1 Å). The other three polymers present an identical backbone structure with different side groups has the backbone unit length of 22.7 Å, which is also double of the (001) d-pacing (11.2 Å). These XRD results successfully reflected the molecular structures.

Figure 2. GIWAXS scans of qxy (in-plane) and qz (out-of-plane) for the neat polymers in solid state.

The absorption spectra of the polymers both in solution (chloroform) and as thin films are presented in Figure 3, and the optical and electrochemical properties are depicted in Table 1. It is shown that the absorption spectra of the polymers are qualitatively the same. In solution, two


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1 2 3 absorption peak maxima are spotted (Figure 3a), a common characteristic of the D-A π4 5 conjugated polymers. The low-wavelength absorption maxima has been assigned to a π-π* 6 7 electronic excitation, whereas the high-wavelength absorption maxima has been attributed to an 8 9 10 intramolecular D-A charge transfer.50 Overall, it is revealed that the high- and low- maximum 11 12 absorption peaks of the ThIDT-TQxT, consisting of the thienyl side groups on the IDT, are blue 13 14 തതതതn (Table shifted versus the PhIDT-TQxT (phenyl side group) on the IDT, independent of the M 15 16 17 1). Furthermore, comparing the phenyl substituted PhIDT-TQxT and PhIDTT-TQxT it is shown 18 19 that the low- wavelength peak of PhIDTT-TQxT is ~10 nm red shifted whereas the high20 21 wavelength peak of PhIDTT-TQxT is slightly blue shifted versus PhIDT-TQxT in agreement 22 23 24 with recently reported results.48 In addition, comparing the two molecular weight fractions of 25 26 PhIDT-TQxT, the PhIDT-TQxT(HMW) reveals slightly higher absorption peak maxima than the 27 28 PhIDT-TQxT(LMW) (Table 1).19 Finally, the molar coefficient (ε) of the polymers was 29 30 estimated (Figure 3a) showing that ThIDT-TQxT exhibits the highest ε then PhIDTT-TQxT and 31 32 33 തതതn fractions of PhIDT-TQxT, the lower M തതതതn last the PhIDT-TQxT. Among the two different തM 34 35 presents the higher ε. 36 37 38 39 40 Table 1. Optical and structural properties of the synthesized polymers. 41 42 43 Polymer λmaxsol εsol Egopt unit length (001) diffraction λmaxfilm 44 [Å Å] peak [Å Å-1] -1 -1 45 [nm] [cm L mol ] [nm] [eV] 46 47 445, 573 44250[573 nm] 447, 587 1.75 22.7 0.56 PhIDT-TQxT(LMW) 48 49 30930[576 nm] 452, 600 1.75 22.7 0.56 PhIDT-TQxT(HMW) 447, 576 50 51 442, 570 60560[570 nm] 447, 583 1.78 22.7 0.56 ThIDT-TQxT 52 53 456, 570 48780[570 nm] 461, 587 1.75 26.7 0.48 PhIDTT-TQxT 54 55 56 57 58 59 ACS Paragon Plus Environment 60

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d-spacing [Å Å] 11.2 11.2 11.2 13.1

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70000

(a)

PhIDT-TQxT(LMW) PhIDT-TQxT(HMW) ThIDT-TQxT PhIDTT-TQxT

60000

å (cm-1 L mol-1)

50000 40000 30000 20000 10000 0 350

400

450

500

550

600

650

700

750

800

850

Wavelength (nm)

Normalized absorbance

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


1.0

0.8

(b)


0.6

0.4

0.2

0.0

400 450 500 550 600 650 700 750 800 850

Wavelength (nm) Figure 3. Polymers (a) molar coefficient (ε) in chloroform solution and (b) absorption spectra as pristine films.

Passing from solution to the solid state (Figure 3b), it is presented a slightly (2-5 nm) red shift for the low- wavelength absorption peak of the polymers, while for the high- wavelength absorption peak of the polymers a more than 10 nm red shift is observed (Table 1). This supports efficient π-π stacking of the polymer chains passing from solution to the solid state. It is


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തതതതn PhIDT-TQxT(HMW) and PhIDTTinteresting to note that comparison between the two high M TQxT demonstrates that increment of the length among the electron donating (D) and electron deficient (A) units results to a red-shift of the low-wavelength peak, whereas a blue shift of the high-wavelength is observed in agreement with recently reported findings.12,51 Moreover, the optical energy gaps, as extrapolated by the UV-Vis absorption onset as thin films, all the polymers exhibit optical bandgaps (Egopt) of 1.75 eV, except ThIDT-TQxT (1.78 eV). This points തതതതn for a specific polymer or replacement of IDT with IDTT is not out that variation on the M affecting the Egopt only the absorption peak maxima. The electrochemical properties of the polymers were investigated by cyclic voltammetry. The redox Fc/Fc+ couple was utilized as standard (in these experiments the oxidation potential was +0.44 V). The voltagram graphs presenting the reduction potentials of PhIDT-TQxT(LMW), PhIDT-TQxT(HMW), ThIDT-TQxT and PhIDTT-TQxT as thin films are shown in Figure S6. It should be noted that no reliable oxidation potentials were recorded, hence the EHOMO (HOMO: highest occupied molecular orbital) levels of the polymers were not estimated. All the polymers exhibit irreversible reduction peaks. The reduction onsets vs. Ag/AgCl were calculated at -0.89, 0.80, -0.70 and -0.80 V for PhIDT-TQxT(LMW), PhIDT-TQxT(HMW), ThIDT-TQxT and PhIDTT-TQxT, respectively. The resulting ELUMO (LUMO: lowest unoccupied molecular orbital) levels of PhIDT-TQxT(LMW), PhIDT-TQxT(HMW), ThIDT-TQxT and PhIDTT-TQxT are 3.47, -3.56, -3.66 and -3.56 eV, respectively. The resulting deeper ELUMO of ThIDT-TQxT vs PhIDT-TQxT (both LMW and HMW) is elucidated by the σ-inductive effect of thienyl substituents in accordance with recently published findings.52,53 On the contrary, replacement of the IDT with IDTT in the polymer backbone (PhIDT-TQxT (both LMW and HMW) vs PhIDTTTQxT) doesn’t affect the ELUMO levels of the polymers..


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2.3 Photovoltaic characterization and photophysical properties OSC devices were fabricated based on the bulk heterojunction (BHJ) concept to assess the solar cell characteristics of the polymer:PC71BM systems. Different optimization approaches were performed, such as the weight ratios of the electron donor and acceptor (Table S2 in SI) and additive [1-chloronaphthalene (CN) in various volume fractions] (Table S3 in SI). The PCE depends significantly on the rational design strategy followed to optimize the chemical structure of the polymers both in the blends with no additive treatment (Figure S7 and Table S2 in SI) and in the addition of CN (Figure 4 and Table 2). In the former case (Figure S7 and Table S2 in SI), ThIDT-TQxT:PC71BM presents a PCE of 6.12%, higher than PhIDT-TQxT(LMW):PC71BM (5.14%), PhIDT-TQxT(HMW):PC71BM (4.62%) and PhIDTT-TQxT:PC71BM (3.92%). For better understanding the increased PCE of the ThIDT-TQxT:PC71BM, morphological and charge carrier mobility studies were achieved by atomic force microscopy (AFM), transmission electron microscopy (TEM) and space charge limited current (SCLC) model, respectively. The nanoscale morphology of the as-spun ThIDT-TQxT:PC71BM blend film as pictured in the TEM and AFM images (Figures S8, S9 in SI) show a reduced nanophase separation leading to a better mixing between the donor and acceptor components as compared to the other three systems. In addition, the ThIDT-TQxT:PC71BM as spun film presents higher hole mobility (1.43 × 10-4 cm2/Vs) as determined by utilizing the SCLC model (Figure S10 in SI) versus the other systems (Table S4 in SI). This result in combination with the reduced nanophase separation supports the higher PCE of the ThIDT-TQxT:PC71BM. The hole mobilities of the other systems as spun films are 1.23 × 10-4 cm2/Vs (PhIDT-TQxT(HMW):PC71BM), 1.22 × 10-4 cm2/Vs (PhIDTT-TQxT:PC71BM) and 1.06 × 10-4 cm2/Vs (PhIDT-TQxT(LMW):PC71BM).


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The PCE of the blend films treated with CN is higher than the untreated ones with ThIDTTQxT:PC71BM showing the highest PCE (7.26%) versus to PhIDT-TQxT(LMW):PC71BM (6.31%), PhIDT-TQxT(HMW):PC71BM (5.51%) and PhIDTT-TQxT:PC71BM (4.47%) (Figure 4). The photovoltaic parameters of the devices are analytically presented in Table 2. The enhanced PCE of the CN-aided ThIDT-TQxT:PC71BM as compared to the other systems is mainly attributed to the increased short circuit current (Jscs), fill factor (FFs) and external quantum efficiency (EQE) (Table 2). However, the physical origin of such a correlation is not straightforward to identify. Previous reports suggested that differences in Jsc, FF and EQE might attributed to thin film morphological variations, differences in hole/electron mobilities and/or photophysical pathways, or in various proportions of charge generation. To clarify among these parameters, a detailed examination of hole/electron mobilities, thin film morphology variation and photo-physical effects was performed.

Table 2. Solar cell characteristics under standard AM1.5G at 1 sun illumination (100 mW/cm2) and charge transporting properties based on SCLC model of the studied blends.

Polymer:PC71BM

Jsc (mA/cm2)

JscEQE

Voc

FF

(mA/cm2)

(V)

(%)

PCEaver PCEmax (%)

(%)

µh

µe

(cm2/Vs)

(cm2/Vs)

PhIDT-TQxT(LMW)

13.16±0.23

12.92

0.82±0.006 56.4±0.73 6.09±0.22 6.31 1.31×10-4±2.04×10-5 6.81×10-6±3.26×10-7

PhIDT-TQxT(HMW)

12.76±0.14

12.71

0.84±0.004 50.8±0.89 5.41±0.09 5.50 1.46×10-4±1.24×10-5 5.72×10-6±3.74×10-7

ThIDT-TQxT

14.38±0.16

14.05

0.84±0.003 58.2±0.92 7.03±0.23 7.26 1.47×10-4±3.24×10-5 9.91×10-6±4.14×10-7

PhIDTT-TQxT

11.21±0.17

10.69

0.82±0.002 46.7±0.67 4.29±0.18 4.47 1.37×10-4±1.24×10-5 9.29×10-7±2.67×10-8


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(a)

-2

Current density (mA cm )

10


5

0

-5

-10

-15 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Voltage (V)

100

(b)


80

EQE (%)

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


60

40

20

0 300

350

400

450

500

550

600

650 700

750

800

Wavelength (nm) Figure 4. (a) Current density-voltage characteristics under standard AM1.5G solar simulator illumination (100 mW/cm2) and (b) external quantum efficiency (EQE) graphs of the CN-aided polymer:PC71BM inverted organic solar cells.


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Our studies were initiated by examining the hole/electron mobilities of the CN-aided thin film systems employing the SCLC model. For every polymer:PC71BM blend, unipolar devices were built and their dark J–V graphs were prepared. The dark J-V graphs are shown in Figure S10 in SI. Table 2 presents the hole/electron mobility data of the systems. The recorded charge transporting characteristics are in the order of 10-4 cm2/Vs according to SCLC model are analogous to those reported for usual high-PCE BHJ films.54,55 Slightly higher hole mobilities are observed for the devices containing the ThIDT-TQxT (1.47 × 10-4 cm2/Vs) and PhIDTTQxT(HMW) (1.46 × 10-4 cm2/Vs) versus PhIDT-TQxT(LMW) (1.47 × 10-4 cm2/Vs) and PhIDTT-TQxT (1.46 × 10-4 cm2/Vs), but not in such extent to justifies the variation on the Jscs or FF (especially taking account the recorded electron mobilities as well). Then, AFM and TEM studies of the CN-aided thin films were achieved. Figure 5 presents the AFM and Figure S11 in SI the TEM images of the polymer:PC71BM blend films. The AFM and TEM images of the CN-aided polymer:PC71BM systems (Figures 5 and S11 in SI) present improved mixing of the two components (smaller domain size) as compared to their as-spun ones (Figures S8, S9 in SI). Moreover, the root mean square (RMS) roughness of the blend films from the AFM images decreases from 2.2-13.9 nm (for the as-spun, Figure S9 in SI) to 0.5-0.6 nm (for the CN-aided, Figure 5). This might be an indication for the higher PCE of the CN-aided polymer:PC71BM systems versus the as-spun ones, but based on the obtained morphological features (Figures 5, S9 in SI) a conclusive clarification for the differences in the OSC performance amongst the CN-aided studied blends and most importantly the significantly increased PCE of the ThIDT-TQxT versus the PhIDT-TQxT(LMW), PhIDT-TQxT(HMW) and PhIDTT-TQxT.


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Figure 5. Height (top) and phase (bottom) AFM morphologies of CN-treated polymer:PC71BM films. PhIDT-TQxT(LMW) (a,e), PhIDT-TQxT(HMW) (b,f), ThIDT-TQxT (c,g) and PhIDTTTQxT (d,h).

According to the GIWAXS spectra of Figure 6, the optimized polymer:PC71BM systems reveal similar results as those of the neat polymer films. In particular, in the PhIDTT-TQxT:PC71BM blend film, the PhIDTT-TQxT also showed the longer (001) d-spacing of 13.1 Å compared to the PhIDT-TQxT(LMW)-, PhIDT-TQxT(HMW)- and ThIDT-TQxT:PC71BM blend films. The lamellar diffraction peaks of the polymers in the polymer:PC71BM blend films are not very marked in the out-of-plane profile, which means that in all the CN-aided blend films the polymer chains are self-assemble to a preferential face-on orientation that explains the similar hole mobilities of the polymer:PC71BM systems as estimated by the SCLC model.


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Figure 6. GIWAXS scans of qxy (in-plane) and qz (out-of-plane) for the CN-aided blend systems.

Since that a vibrant conclusion cannot be safely obtained for the PCE variation of the polymer:PC71BM systems by the results deduced from the charge transporting properties, the morphological behavior and the microstructure organization, we studied the photophysical properties of the devices by transient absorption spectroscopy (TAS) in order to elucidate the photophysical properties of the devices and to answer the open question why from the three

തതതതn , replacement of phenyl substituent with thienyl and further different approaches (increase of M enlargement of the polymer backbone by the use of IDTT instead of IDT) the most efficient one is the replacement of phenyl substituent with thienyl. TAS studies of all samples were accomplished under inert atmosphere within a thoroughly sealed sample holder that contained highly purified nitrogen gas. Figure 7a displays typical profiles of the relative optical density (∆OD) against wavelength at several time delays for the PhIDTT-TQxT:PC71BM device, after photo-excitation with an IR laser operating at 1026 nm. For all TAS measurements a pump fluence of 1.5 mJ cm-2 was employed. The dominant ∆OD feature at ca. 606 nm corresponds to the transient photo-induced bleaching of the band edge transition. In addition, inspection of


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Figure 7a reveals a transient photo-induced absorption profile in the range of 660-800 nm that is also found to significantly diminish as time progresses.

Figure 7. (a) Typical transient absorption spectroscopy ∆OD vs. wavelength profiles at several time delays for sample PhIDTT-TQxT (see text for photo-excitation details). (b) Transient band edge bleach kinetics (empty symbols) and corresponding exponential fits (solid lines) for the polymer:PC71BM devices with PhIDT-TQxT(LMW), PhIDT-TQxT(HMW), ThIDT-TQxT and PhIDTT-TQxT as the polymers.

The transient photo-induced bleaching relaxation dynamics, as well as, the corresponding decay kinetic parameters were determined by exponential fittings, as depicted in Figure 7b for all the devices. For this aim, a triple-exponential equation of the form y = y0 + A1exp(-x/τ1) + A2exp(x/τ2) + A3exp(-x/τ3) was used in order to fit the time-resolved progression of the excited state decay kinetics, i.e. ∆OD against time. Notably, for all fitting procedures, the adjusted r squared (R2) values were exceeding 0.997, i.e. indicating fittings of good quality. Table 3 summarizes the obtained kinetic fit parameters (time components) following this analysis.


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Table 3. Kinetic fit parameters of the polymer:PC71BM systems in the devices. Polymer

λmax (nm)

τ1 (ps)

τ2 (ps)

τ3 (ps)

PhIDT-TQxT(LMW)

606

0.8

25

336

PhIDT-TQxT(HMW)

606

1.0

33

377

ThIDT-TQxT

606

0.9

17

404

PhIDTT-TQxT

606

1.6

46

422

The photophysical processes occurring when a photon is absorbed within the active layer of an organic solar cell,56-62 along with the physical meaning of the corresponding time decay components, have been described explicitly in one of our previous papers.51 Inspection of Table 3 reveals that the first time component (τ1) varies faintly from 0.8 to 1.6 ps for all the examined device architectures. This time component (τ1), represents the time required for an exciton dissociation,51,63,64 while its small variation reveals that such process is an ultrafast process for all samples in question, and consequently, has an insignificant effect on the photovoltaic performance of the devices (Table 2). More importantly, the second time component (τ2) is attributed to the electron-hole transfer from the active layer to the hole transport layer (HTL) of the device.51,63-67 On the contrary to the exciton dissociation, the electron-hole transfer process is acknowledged to be a key element for the device electrical performance and characteristics.51,6367

As reported in Table 3, PhIDT-TQxT(LMW) exhibits a τ2 time component of 25 ps.

Remarkably, the corresponding τ2 for the most efficient device polymer ThIDT-TQxT is found smaller, i.e. 17 ps, suggesting that the incorporation of thienyl rings as substituents versus phenyl rings accelerates the electron-hole transfer process, and thus, improves the electrical performance of the device (Table 2). On the contrary, as depicted from Table 3, both PhIDT-TQxT(HMW) and PhIDTT-TQxT exhibit slower (higher) τ2 time components, i.e. 33 and 46 ps respectively, as


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compared to both ThIDT-TQxT and PhIDT-TQxT(LMW). These findings imply that by increasing the polymer molecular weight (in the case of PhIDT-TQxT(HMW)) or further enlargement of the polymer backbone (PhIDTT-TQxT) the electron-hole transfer process becomes considerably slower, and thus, suppressing the electrical performance of both devices (Table 2). These findings are in agreement with an earlier report by Guo et al.67 in which faster electron-hole injection rates were found for the photovoltaic devices with advanced electrical characteristics. Moreover, Table 3 includes the longer life time decay component (τ3) which is indicative of the electron-hole recombination speed upon excitation.51,68-70 In principle, longer exciton lifetimes are meant to be beneficial for the electrical efficiency of the devices as there is more time available for the so-formed free charge carriers, i.e. holes and electrons, to diffuse towards the transport layers. Inspection of Table 3 reveals significantly longer τ3 decay component for ThIDT-TQxT (404 ps) versus that of PhIDT-TQxT(LMW) (336 ps) or PhIDT-TQxT(HMW) (377 ps). Thus, the introduction of the thienyl rings instead of phenyl rings in this type of polymers, not only accelerates the electron-hole transfer process as depicted from τ2 component, but it provides also excitons with longer lifetimes, a combination that boosts the average PCE from 6.09% to 7.03%, i.e. an improvement of ~13%. Finally, Table 3 data reveals longer exciton lifetimes for samples PhIDT-TQxT(HMW) and PhIDTT-TQxT as compared to PhIDTTQxT(LMW), suggesting that increasing the polymer molecular weight and the donor-acceptor distance helps to generate excitons of better lifetimes. However, as revealed by the corresponding τ2 component of these samples and especially if they are also associated to ThIDT-TQxT, both approaches resulted to considerably slower electron-hole transfer processes and poorer electric performances (Table 2). Such findings provide additional evidence that in this


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type of device architectures the dominant factor for achieving advanced PCEs, among others, is the efficient electron-hole transfer process. 3. CONCLUSION Three D-A polymeric semiconductors based on IDT and Qx were effectively developed by altering the average molecular weights, the aryl substituent and the polymer backbone length. From the extensive structure-property relationship study on the optical, electrochemical and electronic properties, efficiency when integrated into various optoelectronic applications, self assembly, thin film morphological features, charge transporting characteristics and photophysical properties, it is demonstrated that even though the studied π-conjugated polymers present similar Egopt, preferential self-assembly, hole mobilities and morphological features in the optimized solar cell devices, the choice of the aryl substituent is the most efficient rational design strategy to further increase the photovoltaic performance in this series of polymers due to the quicker extraction process of the charges to the respective electrode vis-a-vis the slightly lower recombination lifetime. We believe the obtained results will provide to organic (polymer) chemists and materials (synthetic) researchers helpful inputs for the design of next generation polymeric semiconductors with predetermined optical, electrochemical and electronic characteristics for the fabrication of highly efficient OSCs. Further studies including the replacement of the PC71BM with non-fullerene acceptors (NFA) and the stability of the devices are in progress for the further enhancement of the PCE and the understanding of the chemical structure optimization on the stability of the OSC devices.

ASSOCIATED CONTENT


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Supporting Information. Experimental section, synthetic procedures, gel permeation chromatography and theoretical calculation of the polymers. Construction and characterization of OSCs, hole/electron mobilities of the devices. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of the as-spun and CN-aided systems.

AUTHOR INFORMATION Corresponding Author *Christos

L.

Chochos,

*Ranbir

Singh,

*Apostolos

Avgeropoulos.

E-mail

address:

[email protected]; [email protected]; [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡Christos L. Chochos, Ranbir Singh, Apostolos Avgeropoulos contributed equally.

ACKNOWLEDGMENT This project has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under the Grant Agreements n° 607585 project OSNIRO and no 604603 project MatHero.

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67. Guo, J.; Liang, Y.; Szarko, J.; Lee, B.; Son, H. J.; Rolczynski, B. S.; Yu, L.; Chen, L. X. Structure, Dynamics, and Power Conversion Efficiency Correlations in a New Low Bandgap Polymer:PCBM Solar Cell. J. Phys. Chem. B 2010, 114, 742-748. 68. Tautz, R.; Como, E.; Limmer, T.; Feldmann, J.; Egelhaaf, H. J.; Hauff, E.; Lemaur, V.; Beljonne, D.; Yilmaz, S.; Dumsch, I.; Allard, S.; Scherf, U. Structural Correlations in the Generation of Polaron Pairs in Low-Bandgap Polymers for Photovoltaics. Nat. Commun. 2012, 3, 970. 69. Loslein, H.; Ameri, T.; Matt, G. J.; Koppe, M.; Egelhaaf, H. J.; Troeger, A.; Sgobba, V.; Guldi, D. M.; Brabec, C. J. Transient Absorption Spectroscopy Studies on Polythiophene– Fullerene Bulk Heterojunction Organic Blend Films Sensitized with a Low-Bandgap Polymer. Macromol. Rapid. Commun. 2013, 34, 1090-1097. 70. Chow, P. C. Y.; Albert-Seifried, S.; Gelinas, S.; Friend, R. H. Nanosecond Intersystem Crossing Times in Fullerene Acceptors: Implications for Organic Photovoltaic Diodes. Adv. Mater. 2014, 26, 4851-4854.

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Enhancement of the Power-Conversion Efficiency ... - ACS Publications

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