High Performance Polymer Solar Cell with Single Active Material of

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High Performance Polymer Solar Cell with Single Active Material of Fully Conjugated Block Copolymer Composed of Wide-Bandgap Donor and Narrow-Bandgap Acceptor Blocks Ji Hyung Lee, Chang Geun Park, Aesun KIm, Hyung Jong Kim, Youngseo Kim, Sungnam Park, Min Ju Cho, and Dong Hoon Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03580 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Applied Materials & Interfaces

High Performance Polymer Solar Cell with Single Active Material of Fully Conjugated Block Copolymer Composed of Wide-Bandgap Donor and Narrow-Bandgap Acceptor Blocks Ji Hyung Lee, Chang Geun Park, Aesun Kim, Hyung Jong Kim, Youngseo Kim, Sungnam Park, Min Ju Cho* and Dong Hoon Choi*

Department of Chemistry, Research Institute for Natural Sciences, Korea University, 145 Anam-Ro, Sungbuk-gu, Seoul 136-701, Korea. * Corresponding authors: E-mail: [email protected]; [email protected].

Keywords: conjugated block copolymer, single active material, charge transfer, polymer solar cells, stability

ABSTRACT We synthesized a novel fully conjugated block copolymer (CBC), P3, in which a widebandgap donor block (P1) was connected to a narrow-bandgap acceptor block (P2). As P3 contains P1 block with a wide bandgap and P2 block with a narrow bandgap, it exhibits very wide complementary absorption. Transient photoluminescence (PL) measurement using P3 dilute solution demonstrated intramolecular charge transfer between the P1 block and P2 block, which was not observed in a P1/P2 blend solution. A P3 thin film showed complete PL quenching because the photoinduced inter/intramolecular charge transfer states were effectively formed. This phenomenon can play an important role in the photovoltaic properties of P3-based polymer solar cells. A single active material polymer solar cell (SAMPSC) fabricated from P3 alone exhibited a high power conversion efficiency (PCE) of 3.87 % with a high open-circuit voltage of 0.93 V and a short-circuit current of 8.26 mA/cm2, demonstrating much better performance than a binary P1/P2-based PSC (PCE = 1.14%). This 1

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result facilitates the possible improvement of the photovoltaic performance of SAMPSCs by inducing favorable nano-phase segregation between the p- and n-blocks. In addition, owing to the high morphological stability of the block copolymer, excellent shelf-life was observed in P3-based SAMPSC compared with a P1/P2-based PSC.

1. INTRODUCTION Bulk heterojunction (BHJ)-type polymer solar cells (PSCs) have been extensively employed in binary composite films consisting of p-type donors and n-type acceptors as active layers. Many studies have been conducted on conventional BHJ PSCs fabricated using polymer donors and fullerene derivatives or non-fullerene small molecules or polymer acceptors.1-6 Recently, intensive research on BHJ PSCs fabricated with a non-fullerene acceptor has been undertaken, and BHJ PSCs with power conversion efficiencies (PCEs) up to 13% were achieved using solvent additives and special treatment of the active layer and fabrication methods.7 During the study of small molecule acceptors, studies on all-polymer solar cells (all-PSCs) fabricated using blend films consisting of electron-donor and -acceptor polymers as active layers were also conducted.8-11 All-PSCs have attracted considerable attention owing to their advantages such as their film-forming properties and the mechanical stability of their active layers.12-15 However, the major disadvantages of polymer/polymer blend films include poor miscibility, difficulty in controlling crystallinity, non-uniform internal phase composition, and large-scale phase segregation,16-18 all of which adversely affect the performance of the final PSC device. Although the miscibility of the two polymers is very good at room temperature, it is very difficult to control the phase separation, which may be caused by the variation in 2


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external environmental conditions. Nonetheless, BHJ all-PSCs with the maximum efficiency of more than 9% have been achieved through the efforts of some researchers.19 Additionally, much effort is being expended to develop a single-component (SC) active layer in PSCs rather than a binary blend active layer. This can be accomplished by the development of materials that include both donors and acceptors, which will promote light absorption, exciton dissociation, and charge transport. The donor and acceptor materials often used in the active layer of BHJ PSCs are usually chemically bonded to produce the dual functional materials required for the SC active layer in PSCs.20 Such single active material PSCs (SAMPSCs) exhibit several key advantages such as a considerably simplified device fabrication and an active internal morphology more stable than those of binary blend materials. In addition, the light absorption, exciton diffusion, dissociation, and charge separation occurring in the same polymer matrix overcome the problems observed in BHJ PSCs, which are related to the short exciton diffusion length in organic semiconductors. Although few studies on SAM organic solar cells (SAMOSCs) have been reported, a study on SAMOSCs fabricated using organic materials containing a conjugated molecular structure produced with p-type donors and n-type acceptors has been published. Structures in which n-type acceptors are chemically bonded to conjugated polymers or to small molecules displaying mainly electron-donating properties have been reported. SAMOSCs have been fabricated using such structures and their properties have been reported.21-32 Although a significant amount of research has been focused on synthesizing conjugated block copolymers (CBCs) with donor and acceptor blocks, most studies have been limited to the 3



investigation of CBCs synthesized with donor units consisting of poly(3-alkylthiophene) blocks. Apart from the poly(3-alkylthiophene) blocks, which can couple with various acceptor blocks in CBCs, the introduction of other types of donor blocks has been limited because of the difficulties in CBC synthesis. Despite such difficulties, however, research on SAMPSCs has been carried out recently by synthesizing CBCs containing other types of donor and acceptor blocks.33-37 For example, Verduzo et al. reported that P3HT-b-PFTBT block copolymers were utilized as the active layer for PSCs. A PCE of 2.3–3.0% was achieved without the additional use of a fullerene or non-fullerene acceptor.38 Wang et al. synthesized donor–acceptor block copolymers (i.e., P3HT-b-PBIT2) via Stille coupling polycondensation. They fabricated all-PSCs with a simple SC active layer consisting of P3HT-BPIT2, which achieved a PCE of 1.0% when the active layer was thermally annealed at 150 °C.39 In fact, owing to the complex synthesis, tedious purification, and challenging structural analysis of donor moieties, few have been developed that could be included in the aforementioned block copolymer structure employed in PSC applications. Herein, we demonstrated a fully CBC, P3, containing a wide-bandgap p-type benzodithiophene-thiophenecarboxylate-based copolymer (P1) and a narrow-bandgap n-type naphthalenedicarboximide-selenophene-based copolymer (P2). P3 was synthesized via Stille coupling polycondensation using P1 and P2, which were synthesized and purified separately. After several purification steps via the Soxhlet method, the increased molecular weight of P3 was also measured using gel permeation chromatography (GPC). The structural, optical, and electrochemical properties and the PSC characteristics of P3 were studied in detail and were compared with those of blend samples consisting of the p-type oligomeric donor P1 and the n-type oligomeric acceptor P2 used to form P3. As P1 has a wide bandgap (Eg = 2.0 eV) and P2 has a much narrower bandgap (Eg = 1.72 eV), the resultant P3 can provide very broad 4


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complementary absorption, as confirmed using absorption spectroscopy. A P3 thin film showed complete photoluminescence (PL) quenching because the photoinduced charge transfer (CT) states were effectively formed. Transient PL (TRPL) measurement demonstrated efficient photoinduced CT property in the P3 film in terms of fast decaying lifetime (~110 ps). Finally, an inverted-type PSC was fabricated to investigate the photovoltaic property observed in an SC active layer fabricated with P3. The SAMPSC based on P3 exhibited a PCE of 3.52% and a PCE higher than 3.84% when a small amount of solvent additive was introduced into the active layer. The performance of the PSC was unique when it was fabricated using an SC active layer and was much better than that of the PSC fabricated using the P1/P2 blend film (PCE: 0.91–1.14%). In addition, the SAMPSC showed an excellent shelf-life of over 2000 h under atmospheric conditions, much better than that of the PSC fabricated using the P1/P2 blend film. These SAMPSC results are mainly due to the excellent internal morphology stability of the P3 film.

2. RESULTS AND DISCUSSION 2.1. Synthesis and characterization We synthesized a CBC, P3, in which P1 and P2 were covalently bonded. The structure of P3 is shown in Figure 1. In order to prepare P3, the two oligomeric monomers (P1 and P2) were synthesized (Scheme S1) and purified following the method described in previous reports.4043

P3 was thereafter synthesized via a Stille coupling reaction of P1 and P2, which contain trimethylstannyl and bromo end groups, respectively. After the reaction, the crude product was purified via Soxhlet extraction with acetone, hexane, methylene chloride, and chloroform 5



in succession. Relatively low-molecular-weight species, unreacted oligomers, and catalyst residues were removed during the extraction.

(b)

(a)

1.2 1.0

P1

Normallized Intensity

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

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P2

P1 P2 P3

0.8 0.6 0.4 0.2 0.0 12

P3

14

16

18

Elution Volume (mL)

Figure 1. (a) Chemical structures of the oligomeric monomers (P1 and P2) and the conjugated block copolymer (P3). (b) GPC profiles of P1, P2, and P3.

The molecular weights and polydispersity indices (PDIs) of the two oligomeric blocks (P1 and P2) and the resultant P3 were measured using GPC with o-dichlorobenzene as an eluent at 80 °C and were calibrated against polystyrene standards, as shown in Figure 1b and Table 1. The number average molecular weights (Mn) of P1, P2, and P3 were 6.84, 10.7, and 18.1 kg mol−1, respectively, and their PDIs were 1.53, 2.16, and 2.31, respectively. P1, P2, and P3 were readily soluble in common chlorinated organic solvents such as chloroform, chlorobenzene, and o-dichlorobenzene at room temperature. To assess the structural characteristics of the block copolymer and to determine the weight 6


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ratios of P1 and P2 in the block copolymer, 1H nuclear magnetic resonance (NMR) was carried out, and the results were confirmed using UV–vis absorption spectroscopy. As shown in Figure 2, the presence of both P1 and P2 blocks in P3 was confirmed by integrating the peaks for P1 (i.e., δ = 8.01–8.04, 7.68, 7.31–7.34, and 6.93 ppm for the aromatic ring of methyl thiophene-3-carboxylate and 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene; δ = 3.84, 2.88–2.89, 1.71, 1.35, and 0.91–0.96 ppm for the methyl group of methyl thiophene-3-carboxylate and 2-ethylhexyl) (Figure S1) and for P2 (i.e., δ = 8.78–9.00 and 7.57–7.65 ppm for 1,4,5,8-naphthalenetetracarboxylic diimide and selenophene; δ = 4.11–4.16, 2.02, 1.24–1.37, and 0.81–0.97 ppm for 2-hexyldexyl (Figure S2). In addition, the end group on one side of P1 was identified from the methyl protons of trimethyltin (i.e., δ = 0.41 ppm), as shown in Figure S1.

Figure 2. 1H-NMR spectra of P3 (top) and of the blend of oligomeric monomers, P1 and P2 (bottom) 7



For comparison, the two oligomeric monomers (P1 and P2) and a physical mixture of P1 and P2 were characterized. Blend solutions of various compositions of P1 and P2 were prepared, and NMR spectra were obtained to determine the composition of the synthesized P3 structure. We prepared a 2:1 wt. ratio P1/P2 blend and observed that the integration areas of the characteristic well-isolated chemical shifts at 3.84 and 4.16 ppm in the spectra for the P3 and the P1/P2 polymer blends were almost identical. The weight ratios of P1 and P2 in P3 were determined by comparing the integrated area of the branched alkyl chain of the 2-hexyldecyl peak (δ = 4.16 ppm) for the naphthalene monomer and the alkyl chain peak (δ = 3.84 ppm) for the methyl thiophene-3-carboxylate. Even if a tiny amount of the low-molecular-weight oligomer remained, the main product was shown to be a copolymer composed of p- and nblocks.

2.2. Optical and electrochemical properties The UV–visible absorption spectra of the synthesized P3, oligomeric monomers (P1 and P2), and the physical blend of P1 and P2 (2:1 wt. ratio) in chloroform solutions and thin films are shown in Figures 3 and S4a, respectively, and the corresponding spectroscopic data are summarized in Table 1. In a dilute solution, P3 exhibited the characteristic absorption features of both P1 and P2, and its spectrum was almost identical to the superimposed spectra of P1 and P2, which is not observed in a binary random copolymer. The compositions of P1 and P2 were estimated using the NMR spectra shown in Figure 2. In order to confirm the results, the absorption spectra were also measured by varying the compositions in dilute solutions of P1 and P2. The absorption spectra of the synthesized P3 and the dilute solution (conc. 10-5 M) prepared with a 2:1 wt. ratio of P1 and P2 were almost identical, as shown in 8


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Figure 3a. The absorption spectra of thin films applied to an actual device were measured and are shown in Figure S4a. The absorption spectra of the P1, P2, and P3 thin films appeared more redshifted than their corresponding solution spectra, suggesting that strong intermolecular interactions exist between the polymer chains. The optical bandgaps of P1, P2, and P3 calculated from the onset of the film absorption spectra are in the range 1.70–2.0 eV.

Figure 3. (a) UV–vis absorption spectra recorded for solutions. (b) PL spectra corresponding to the solutions. The excitation wavelength was 481 nm. (c) TRPL decaying behaviors of P1, P3, and P1/P2 blend. Samples: toluene solutions. (conc. ~10-7 M). (d) Schematic representation of photoinduced charge transfer between a donor and an acceptor in dilute solutions.

Furthermore, the PL spectra of the P1, P3, and P1/P2 blend (2:1 wt. ratio) solutions (conc.

9



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~10-7 M) are shown in Figure 3b. The spectrum of the P1 solution shows an emission band at 560 nm when the solution was excited at 481 nm. In the spectra of the P1/P2 blend and P3 solutions, the maximum emission intensity was observed at 560 nm corresponding to P1. To investigate the possible molecular CT in the P3 solutions, TRPL measurements were performed using toluene solutions of P1, P3, and the 2:1 wt. ratio P1/P2 blend (Figure 3c). To exclude the possibility of intermolecular interactions as much as possible, considerably low concentrations of all the solutions were used (10-7 M). In Figure 3c, three PL decay curves were suitable for the exponential decay function. In particular, the double exponential decay function fitted the curve data well. The solution samples were excited at 520 nm, and the PL decay was observed by probing at 560 nm, where the PL intensity was measured to a large extent.

Table 1. Molecular weights and optical and electrochemical properties of polymers.

Absorption Mn (kDa)

PDI

λ

a peak

λ

b peak

Emission

λem

a@481

(nm)

(nm)

(nm)

Eg c (eV)

Eg d (eV)

Eox d (V)

Ered d (V)

HOMO (eV)

LUMO (eV)

P1

6.84

1.53

481

532

560

2.00

2.05

1.15

-0.90

-5.59d

-3.59e

P2

10.7

2.16

555

616

-

1.72

-

-

-0.48

-5.68e

-3.96d

P3

18.1

2.31

490

528

560

1.80

1.74

1.22

-0.52

-5.66d

-3.86e

Blend (P1+P2)

-

-

486

535

560

1.76

1.72

1.23

-0.49

-5.67d

-3.91e

a

Maximum absorption peak in dilute chloroform solution. b As-cast film from chloroform solution. c The optical bandgaps were obtained from the absorption spectra of the film samples. d Calculated by using cyclic voltammetry (sample: film). e Eg (eV) = LUMO – HOMO.

The PL decay curve of the P1 solution showed an average lifetime, τavg, of 690 ps, which is 10


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the typical time scale for the fluorescence decay of conjugated polymers, identical to that of the P1/P2 blend solution. Therefore, for very dilute solutions (10-7 M), we can neglect the formation of intermolecular CT states owing to the very low concentration. The red curve, representing the PL decay of the P3 solution, showed a τavg of 630 ps, which is shorter than the lifetime of other samples and is attributed to the existence of intramolecular CT between the p-type block and the n-type block within the P3 structure. The small difference in lifetimes measured above is different from the results of the donor– acceptor molecular dyad generally reported in the literature.44 The PL lifetime of the molecular dyad with a long alkylene spacer between the donor and acceptor was reported to be very small in solution state compared with that of the donor. This was explained by the folded conformation, which makes the electron transfer in the solution efficient in the molecule.45 This unique phenomenon is due to the rigid conformation of the fully conjugated structure of P3. In a diluted solution of P3, the electron transport in the polymer chain might occur only in a very small localized region at the junction area between the donor and acceptor blocks, and thus, the difference in the PL lifetimes of the two solutions is observed to be small. The possible CT state formation and occurrence of PL in the visible wavelength range are depicted schematically in Figure 3d. Thus, these results support the formation of the block copolymer with p- and n-blocks. The TRPL measurements were also carried out for the P1, P3, and P1/P2 blend films to understand the exciton dynamics in the films (Figure S5). It was expected that more delocalized CT excitons in the films would lead to fast PL decay. The curve for the P1 film showed the typical fluorescence decay lifetime of approximately 730 ps (τavg), whereas the average lifetime (τavg) for the P3 and P1/P2 blend films was measured to be identical at 110

11



ps. The much faster TRPL decay lifetimes of the P3 and P1/P2 blend films can be explained by the photoinduced intermolecular CT states between the p- and n-type blocks. The electrochemical properties of the P3 film, 2:1 wt. ratio P1/P2 physical blend film, and oligomeric monomer (P1 and P2) films were studied using cyclic voltammetry. The polymer films were prepared by dispensing a polymer chloroform solution onto the working electrode at room temperature. Cyclic voltammograms of all the films are illustrated in Figure S6, and the corresponding data are summarized in Table 1. The highest occupied molecular orbital (HOMO) levels of the P3 and P1/P2 blend films were calculated from the onset of the oxidation curves, whereas the lowest unoccupied molecular orbital (LUMO) levels were calculated from the difference between the HOMO and the optical bandgap. Interestingly, as shown in the energy diagram of Figure S6, the LUMO levels of the P3 (−3.86 eV) and P1/P2 blend films (−3.91 eV) are between those of the P1 and P2 films.

2.3. Photovoltaic properties Consequently, we measured the photovoltaic properties of a diode device for SAMPSCs fabricated with P3. Thus, a series of PSCs was prepared with different active layers in an inverted device configuration: ITO/ZnO/P1/P2 blend or P3/MoO3/Ag as shown in Figure 4. The active layers were fabricated by spin-coating the P3 or P1/P2 blend dissolved in either chlorobenzene or a mixture of chlorobenzene and 1-chloronaphthalene (CN) (99:1 v/v). As shown in Figure 5a and Table 2, the PSCs fabricated with the P3 and P1/P2 blend films exhibited drastically different performances. The PSC fabricated with the 2:1 wt. ratio P1/P2 blend film had a maximum PCE (PCEmax) of 1.14%, short-circuit current density (Jsc) of 3.05 12


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mA/cm2, Voc of 0.86 V, and fill factor (FF) of 43.2%. It is evident that the device fabricated with the P1/P2 blend dissolved in chlorobenzene with 1.0 vol.% CN showed poorer performance (Voc = 0.83 V, Jsc = 2.71 mA/cm2, FF = 40.3%, and PCE = 0.91%) than the devices fabricated with the P1/P2 blend dissolved in chlorobenzene without any CN additive. (Figure S8 and Table 2)

Figure 4. Schematic device configuration of inverted type PSC (a) and energy level diagram of inverted type PSC based on P1/P2 blend (b) and P3 (c).

Interestingly, the devices containing the P3 active layers displayed peculiar photovoltaic performances. In particular, the P3 device fabricated without any added CN showed a significantly better performance with a PCEmax of 3.52% (Voc = 0.94 V, Jsc = 7.82 mA/cm2, and FF = 47.89%) than the PSC fabricated with the P1/P2 blend film. The PSC fabricated with an active layer prepared using 1.0 vol.% CN additive in the P3 solution showed the 13



highest efficiency of 3.87% with Voc = 0.93 V and Jsc = 8.26 mA/cm2 among all the PCEs fabricated in this study. Compared with the device with the blend of P1 and P2, the enhancement in Jsc for the device with P3 was confirmed via EQE measurements (Figure 5b). Regardless of the use of the CN additive, the PSCs fabricated using P3 showed relatively high EQEs over 40% for wavelengths in the range 420–600 nm. In contrast, the PSCs fabricated using the P1/P2 blend only showed EQE in the range 15–25%, indicating that the Jsc of the device fabricated using P3 is higher than that of the device fabricated using the P1/P2 blend.

(b) 60

9 2

P3 Blend (P1+P2)

50

0

40

EQE (%)

-2 -4

6

30 20

3

-6

10 -8

0 -0.2

0.0

0.2

0.4

0.6

0.8

0 400

1.0

(d) 2

2

1.0

P(E,T)

0.8 0.6 0.4 0.2 0.0 0.01

0.1

1

log Veff (V)

0.1

600

700

800

9

Current Density (mA/cm )

10

1

500

Wavelength (nm)

Voltage (V)

(c)

0.1

Integrated Jsc (mA/cm )

2

2


(a)

log Jph (mA/cm )

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

Page 14 of 31

1

log Veff (V)

8

P3 (α = 0.999) Blend (P1+P2) (α = 0.730)

7 6 5 4 3 2 1 0.2

0.4

0.6

0.8 -2

1.0 2

Light intensity (x 10 mW/cm )

Figure 5. a) J–V and b) external quantum efficiency (EQE) curves of inverted devices processed with P3 and P1/P2 blend under optimized conditions with 1.0 vol.% CN. Integrated Jsc curves were included in this plot; c) photocurrent density (Jph) and exciton dissociation probability [P(E,T)] versus effective bias (Veff); d) dependence of short-circuit current density on light intensity. 14


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Table 2. Summary of photovoltaic properties Polymer

Additive (CN)

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Jsc a (mA/cm2)

P3

-

0.94 (0.93 ± 0.005)

7.82 (7.83 ± 0.097)

47.89 (44.82 ± 2.06)

3.52 (3.27 ± 0.14)

7.26

P3

1.0 vol%

0.93 (0.92 ± 0.01)

8.26 (8.30 ± 0.082)

50.33 (49.97 ± 0.28)

3.87 (3.84 ± 0.021)

7.59

Blend (P1+P2)

-

0.86 (0.84 ± 0.02)

3.05 (2.95± 0.10)

43.2 (40.9 ± 1.77)

1.14 (0.91 ± 0.23)

2.92

Blend (P1+P2)

1.0 vol%

0.83 (0.80 ± 0.03)

2.71 (2.57 ± 0.14)

40.3 (39.3 ± 2.97)

0.91 (0.81 ± 0.10)

2.60

a

Closed circuit current (Jsc) values were obtained from EQE measurements. Average values and standard deviations are in parentheses and obtained from more than 5 devices.

To improve our understanding of the PSC performance, the maximum exciton generation rates (Gmax) of the devices characterized herein were calculated by plotting the photocurrent density (Jph) as a function of the effective voltage (Veff), as shown in Figure 5c (Jph = JL − JD, where JL and JD are the current densities measured under illumination and dark conditions, respectively, and Veff = V0 − Va, where V0 is the voltage when Jph = 0, and Va is the applied bias).46 For PSCs, the Gmax of the device can be calculated using the equation Jsat = qLGmax, where Jsat is the saturated photocurrent density, q is the electron charge, and L is the thickness of the active layer in the PSC. The Gmax and Jsat of the P3-based device fabricated without any CN additive (6.47 × 1027 m−1 s−1 and 82.87 A/m2, respectively) and those of the P3-based device fabricated with 1.0 vol.% CN (6.70 × 1027 m−1 s−1 and 85.90 A/m2, respectively) were approximately three times higher than those of the corresponding P1/P2 blend-based devices (Figures 4c, S8c, and Table S2). As a portion of the photoinduced excitons dissociate into holes and electrons in the BHJ 15



structure, the exciton dissociation probability, P(E,T), can be obtained using the estimated Gmax.46 Using the Gmax value, the photocurrent density could be calculated using the equation Jph = qGmaxP(E,T)L. The normalized photocurrent density (Jph/Jsat) is plotted against Veff to obtain the P(E,T) of the PSC device, as shown in the inset of Figures 5c and S8c. Under a short-circuit condition (Jph = Jsc at Va = 0 V), the value of P[(E,T)] for the P3-based devices was estimated to be much higher—94% without CN additive and 96% with 1.0 vol% CN— than those (e.g., 91% without CN additive and 88% with 1.0 vol% CN) of the blend (P1 and P2)-based devices under an applied bias. This result attests to the superior exciton dissociation efficiency and charge carrier generation capability of P3 and thus its higher Jsc than that of the P1/P2 blend. Further, the result suggests that a large portion of the excitons recombine in the P1/P2 blend before the charge separation. The light-intensity-dependent Jsc was also measured to investigate the nongeminate bimolecular recombination losses of the devices fabricated with and without the CN additive (Figures 5d and S8d). The relationship between Jsc and the incident light intensity can be described as Jsc ∝ Plightα, where α is 1.0 if all the dissociated free carriers are collected at the corresponding electrodes without any charge recombination, whereas α < 1.0 indicates the presence of bimolecular recombination to some extent.47 Both P3-based devices showed a linear dependence of current density on the light intensity when plotted in logarithmic coordinates. The slopes of the graphs for the P3-based PSC devices fabricated without any CN additive and with 1.0 vol.% CN were 0.992 and 0.999, respectively. However, the graphs for the P1/P2 blend-based PSC devices fabricated without any CN additive and with 1.0 vol.% CN exhibited relatively low α values of 0.752 and 0.730, respectively, which are significantly lower than those obtained from the graphs of the corresponding P3-based devices. In 16


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SAMPSC, the charge carriers separated owing to exciton dissociation are efficiently transferred to the electrode sides, demonstrating negligible bimolecular recombination compared with the PSC fabricated with the polymer blend.48

Figure 6. AFM images (scale bar = 1 µm) of the active layer in PSC devices fabricated with 1.0 vol.% CN: topography of a) P3, d) P1/P2 blend films and phase images of films fabricated with b) P3, e) P1/P2 blend. TEM images (scale bar = 200 nm) of the active layer in PSC devices fabricated with 1.0 vol.% CN: (c) P3, (f) P1/P2 blend films.

The surface morphologies of the P3 and P1/P2 blend films were studied using atomic force microscopy (AFM). The AFM images of the films are presented in Figures 6 and S9. The ascast P1/P2 blend films showed larger surface domains than the P3 films. In particular, the surface domains were significantly larger on the P1/P2 blend film fabricated with the 1.0 vol.% CN additive. In contrast, the P3 film surface displayed a uniform, smooth morphology—even 17



in the case of the P3 film fabricated with the additive—because of the nanophase separation between the p- and n-blocks in the polymer chain, indicating that the P3 film fabricated with 1.0 vol.% CN exhibited surface morphology characteristics typical of high-performance CBCs. Furthermore, transmission electron microscopy (TEM) was used to investigate the internal morphologies of the films in two different systems (P3 film-based PSCs and P1/P2 blend film-based PSC), as shown in Figures 6 and S9. The P3 films showed well-defined nanophase segregation and a much more uniform internal morphology than the P1/P2 blend films. Moreover, the P3 films showed a uniform, homogeneous morphology, which is favorable for exciton diffusion and charge transport owing to the formation of continuous charge migration channels. These results indicate that the SAMPSCs fabricated with the P3 films would show higher Jsc values than those fabricated with the P1/P2 blend films.

2.4. PSC shelf-life The PSC device stability was evaluated by measuring the J–V characteristics for unencapsulated PSC devices stored under ambient conditions and AM 1.5 G at room temperature for over 1000 h, according to the ISOS-D-1 (shelf) protocol.49 To investigate the device stability, PSC devices were fabricated with the P3 and P1/P2 blend films. In Figures 7 and S11, the J–V characteristics and the corresponding photovoltaic parameters (i.e., PCE, Jsc, Voc, and FF) of the devices are plotted as functions of storage time. Figures 7a and 7b show the effects of aging on the photovoltaic parameters of the PSC during long-term storage. The P3-based device fabricated with 1.0 vol.% CN retained ~80% of its initial PCE for over 1000 h. The P3-based device fabricated without any CN additive 18


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showed a reduction of approximately 23% in its stability under similar conditions, as shown in Figure S11.

(b) Normalized parameters

1.0

0.8

Voc Jsc FF

0.6

0.4

PCE 0

200

400

600

800

(d)

2

-2 -4

0h 180 h 346 h 538 h 730 h 922 h 1018 h

-6 -8 -0.2

0.0

0.2

0.4

0.6

0.8

Voc Jsc FF

0.6

0.4

PCE

1.0

200

400

600

800

1000

Time (h) 1

2

2

0

0.8

0

Time (h)

(c)

1.0

1000


Normalized parameters

(a)


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


0

-1

0h 180 h 346 h 538 h 730 h 922 h 1018 h

-2

-3 -0.2

0.0

Voltage (V)

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 7. Performance stability of PSCs stored under ambient conditions up to 1018 h. a) P3based device fabricated with 1.0 vol.% CN and b) P1/P2 blend-based device fabricated with 1.0 vol.% CN. The average photovoltaic parameters were obtained from four PSCs. Variation in J–V characteristics of c) P3-based device fabricated with 1.0 vol.% CN and d) P1/P2 blend-based device fabricated with 1.0 vol.% CN.

In comparison with the devices composed of the P1/P2 blend film without CN additive, it was observed that the devices fabricated with 1.0 vol% CN exhibited poorer stability after storage for over 1000 h (Figure S11). The performance degradation of the PSC devices is mainly due to the decrease in Jsc associated with the internal morphology of the active layer. This phenomenon can be explained by analyzing the EQE spectra for the devices aged for 19



1018 h. The maximum EQE of the 2:1 wt. ratio P1/P2 blend-based device was significantly lower than that of the P3-based device over the entire range of wavelength measurement (Figure S12). In order to study the effect of morphological change on shelf-life, AFM was used to analyze the topography of the film surface during the measurement period, as shown in Figures S13 and S14. The PSC device fabricated with the P1/P2-blend film prepared without any CN additive and stored for 1018 h showed many micro-aggregates. The corresponding PSC device fabricated with the P1/P2-blend film prepared using 1.0 vol.% CN showed a PCE reduction of approximately 42% compared with the initial value. In contrast, the PSC devices fabricated with the P3 films and stored for 1018 h under ambient conditions maintained a homogeneous morphology and showed hardly any micro-aggregates. Owing to the stable surface morphology of the P3-based films, the SAMPSC exhibited excellent shelf-life compared with the PSC produced by mixing two donor/acceptor elements (i.e., the P1/P2 blend films). In particular, after the photovoltaic parameters of the P3-based device were measured, the device was maintained under ambient conditions for more 1000 hours, and the PV parameters were measured again when the device had been stored for 2016 h. The device exhibited a remarkably persistent PCE, maintaining 72% of its initial efficiency, as shown in Figure S15. Thus, the long-term stability of the device in air at room temperature will be a significant advantage for practical PSC applications.

3. CONCLUSION We introduced a very intriguing CBC (P3) containing a p-type oligomeric unit (P1) and an ntype oligomeric unit (P2). P3 showed very broad complementary absorption owing to the 20


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wide bandgap of the P1 block and the narrow bandgap of the P2 block. Complete PL quenching was observed in the P3 film because the photoinduced inter/intramolecular charge transfer states were effectively formed. A P3-based SAMPSC exhibited an unusually high PCE of 3.87%, high Voc of 0.93 V, and Jsc of 8.26 mA/cm2, demonstrating significantly better performance than the P1/P2 blend film-based PSC. Moreover, owing to the highly stable surface morphology of the P3 film, the SAMPSCs fabricated with P3 exhibited an excellent shelf-life of over 2000 h under ambient conditions compared with the PSC fabricated with the P1/P2 blend film. To the best of our knowledge, the PCE of the CBC-based SAMPSC demonstrated in this study is one of the highest ever achieved for an SAMPSC, and this is the first study to report the excellent shelf-life of up to 2000 h for a CBC-based SAMPSC under ambient conditions.

4. EXPERIMENTAL 4.1. Transient Photoluminescence Spectroscopy The transient photoluminescence (TRPL) signals, I(t) of the samples were collected over a series of wavelengths, using a time-correlated single-photon counting (TCSPC) technique. The samples were excited by a 520-nm pulse (LDH-P-C-520, PicoQuant). The emitted PL of the solution and film samples was measured at 560 and 656 nm, respectively, using a photomultiplier tube (PMA 182, PicoQuant). The TRPL signals were well fitted by a biexponential function, I(t)/I(0) = A1 exp(-t/τ1) + A2 exp(-t/τ2). The average lifetime was determined by τavg= (A1/(A1+A2))) τ1 + (A2/(A1+A2)) τ2.

21



4.2. Fabrication of inverted polymer solar cell The device was fabricated using a solution process and had an inverted device configuration of glass/ITO/ZnO/active layer/MoO3/Ag. An ITO (150 nm)-coated glass was cleaned by ultrasonication in deionized water and isopropyl alcohol for 10 min, respectively. The dried ITO glass was subjected to UV-ozone treatment for 20 min. A ZnO (40 nm) as electron transport layer was spin-coated on the top of ITO glass using ZnO precursor solution at 3000 rpm for 40 s. After drying at 170 °C for 1 h, it was transferred to a glove box filled with nitrogen gas for use. P3 or P1, and P2 were dissolved over 12 h in anhydrous chlorobenzene with or without 1 vol% of 1-chloronaphthalene (CN). The prepared solution was then spin-coated onto the ZnO layer. The resulting active layer was 80–100 nm thick. Finally, a MoO3 (10 nm) layer and a Ag (100 nm) layer were deposited on the active layer using a thermal evaporator to form a 4 mm2 active region through a shadow mask. The J–V of the devices were measured with a Keithley 2400 source meter under simulated AM 1.5G illumination (100 mW/cm2). The EQE spectra were recorded using a certified EQE instrument (McScience Inc., EQX 3100).

ASSOCIATED CONTENT Supporting Information Synthetic procedure, NMR spectra, UV-Vis absorption spectra, photoluminescnece(PL) spectra, transient PL decay spectra, cyclic voltammograms, GIWAXD patterns, J-V characteristics, EQE spectra, AFM images, performance stability. This material is available free of charge via the Internet at http://pubs.acs.org. 22


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

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This research was supported by the (NRF2015R1A2A1A05001876)

and

by

National Research Foundation of Korea the

Key

Research

Institute

Program

(NRF20100020209). We are grateful to Pohang Accelerator Laboratory (Pohang, Korea) for allowing us to conduct the grazing incidence wide angle X-ray scattering (GIWAXS) measurements.

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High Performance Polymer Solar Cell with Single Active Material of

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