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. 2021 Apr 15;1(5):612–622. doi: 10.1021/jacsau.1c00064

High-Molecular-Weight Electroactive Polymer Additives for Simultaneous Enhancement of Photovoltaic Efficiency and Mechanical Robustness in High-Performance Polymer Solar Cells

Jin-Woo Lee ‡, Boo Soo Ma §, Hyeong Jun Kim ||, Taek-Soo Kim §,*, Bumjoon J Kim ‡,*
PMCID: PMC8395705  PMID: 34467323

Abstract

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The development of small-molecule acceptors (SMAs) has significantly enhanced the power conversion efficiency (PCE) of polymer solar cells (PSCs); however, the inferior mechanical properties of SMA-based PSCs often limit their long-term stability and application in wearable power generators. Herein, we demonstrate a simple and effective strategy for enhancing the mechanical robustness and PCE of PSCs by incorporating a high-molecular-weight (MW) polymer acceptor (PA, P(NDI2OD-T2)). The addition of 10–20 wt % PA leads to a more than 4-fold increase in the mechanical ductility of the SMA-based PSCs in terms of the crack onset strain (COS). At the same time, the incorporation of PA into the active layer improves the charge transport and recombination properties, increasing the PCE of the PSC from 14.6 to 15.4%. The added PAs act as tie molecules, providing mechanical and electrical bridges between adjacent domains of SMAs. Thus, for the first time, we produce highly efficient and mechanically robust PSCs with a 15% PCE and 10% COS at the same time, thereby demonstrating their great potential as stretchable or wearable power generators. To understand the origin of the dual enhancements realized by PA, we investigate the influence of the PA content on electrical, structural, and morphological properties of the PSCs.

Keywords: polymer solar cell, nonfullerene small-molecule acceptor, high-molecular-weight polymer additive, mechanical robustness, high efficiency

1. Introduction

Polymer solar cells (PSCs) are considered as one of the most attractive candidates for portable and wearable power generators due to their advantages of being lightweight, semitransparent, and mechanically flexible. During the past few years, the power conversion efficiencies (PCEs) of the PSCs have increased rapidly to over 17–18% with the development of various small-molecule acceptors (SMAs).1−5 These SMAs that contain strongly light-absorbing dye molecules enable much greater light harvesting than the conventional fullerene-based acceptors, leading to dramatic improvements of the short-circuit current densities (Jscs) and PCEs.2,3,6−8 However, the SMAs, which contain multiple fused rings in their backbone structures, are highly crystalline and mechanically fragile, limiting the mechanical properties and long-term stabilities of the SMA-based PSCs.9−12 Considering that one of the most attractive features of PSCs is to use them as wearable and portable power generators,13−15 the mechanical properties of SMA-based PSCs should be significantly enhanced without sacrificing any of their high efficiencies.

To this end, recent efforts have been made to improve the mechanical properties of SMA-based PSCs, mostly by polymerizing the SMA building blocks.9,10,16−20 Polymerization of SMAs can enhance the mechanical properties of SMAs by decreasing the crystallinity and bridging different crystalline domains. For example, the use of polymer acceptors (PAs) made from an IDIC-based SMA increased the mechanical ductility of the PSCs with a crack onset strain (COS) of 8.6%, whereas the active layers composed of the same SMA mechanically failed at a strain of less than 2%.9 More recently, we designed new polymerized SMAs which share the same building units with the polymer donors for better molecular compatibility, leading to both efficient (PCE ≈ 11%) and mechanically robust (COS ≈ 15%) PSCs.10 However, although the mechanical properties of PSCs strongly rely on the molecular weights (MWs) of the consisting polymers,12,21−24 polymerized SMAs having high MWs (above the critical MW) have not been achieved thus far. Moreover, the PSCs with polymerized SMAs still show a lower PCE than that of the state-of-the art SMA-based PSCs.5,9,10,19

A more simple and straightforward method to modify the mechanical properties of the SMA-based PSCs is to incorporate ductile ternary additives into the donor:acceptor binary blend.25−32 For example, Chen et al. incorporated poly(dimethylsiloxane-co-methyl phenethylsiloxane) polymer additives into the TQ-F:N2000 solar cell system, resulting in improved toughness and ductility of the active layers.26 Nevertheless, the use of electrically insulating polymeric additives resulted in serious degradation of the PCE of the PSCs.26,33 In that, polymeric acceptors (PAs) having high mechanical and electrical properties at the same time can be exciting candidates for modifying the mechanical properties of SMA-based PSCs.9,12,34−36 In particular, naphthalene diimide (NDI)-based PAs typically have excellent electron mobility and mechanical properties, making them suitable as active components in efficient PSCs.12,37−39 In addition, NDI-based PAs having a high MW above the critical MW—essentially required to achieve high mechanical toughness and ductility—are easily accessible by coupling reactions.12,21,40−43

Here, we report a simple and efficient strategy of using a high-MW poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)] (P(NDI2OD-T2)) PA additive for simultaneously enhancing the PCE and stretchability of the PSCs. The P(NDI2OD-T2) PA additives are designed to have a very high MW (weight-average molecular weight (Mw) = 267 kg mol–1), which is above their critical MW (Mw ≈ 100 kg mol–1).12,24,40 The addition of high-MW PA additives improved the PCE value from 14.62 to 15.44% in the PSCs consisting of poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)] (PM6) donor and 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3′′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-dichloro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y7) SMA. Importantly, the addition of the high-MW PA significantly increased the mechanical ductility of the PM6:Y7 PSCs by more than 4-fold in terms of the COS (i.e., COS = 2% for the pristine PM6:Y7 blend; COS = 10% with the addition of 20 wt % PA). As a result, for the first time, we demonstrate a PSC system satisfying a high efficiency (PCE = 15%) and mechanical stretchability (COS = 10%) at the same time. These simultaneous enhancements by PA additives are mainly attributed to the formation of an entangled network and tie molecules between adjacent acceptor domains, providing both electron hopping pathways and stress dissipation routes. Detailed analyses regarding the electrical, structural, and mechanical properties of the PSCs for various amounts of the PA were performed to elucidate their roles.

2. Results and Discussion

Basic Material Properties

We employed the PM6 donor and Y7 SMA as a representative of a highly efficient PSC system6 and selected the P(NDI2OD-T2) PA as a ductilizing additive (Figure 1a). The PA additive strategy in this work is distinguished from previous additive strategies employing electrically insulating polymers,26,33,44−46 in that the P(NDI2OD-T2) PA has high electron mobility due to its electroactive conjugated backbones and strong assembly into the intermolecular structures. Among the various types of PAs, P(NDI2OD-T2) is one of the most extensively used PA materials producing high-performance PSC devices,47−51 due to its superior electron mobility (μe) and suitable energy level offset with efficient benzodithiophene (BDT)-based polymer donors.13,52,53 These features suggest that the P(NDI2OD-T2) would not deteriorate the electrical properties of the PSC blends when they are embedded as an additive into the PM6:Y7 system. Importantly, synthetic methods to produce very high-MW P(NDI2OD-T2) polymers are easily accessible.24,40 Thus, the P(NDI2OD-T2) having a very high MW is targeted to be synthesized in order to maximize the enhancement of mechanical properties of the PM6:Y7 blend by the addition of P(NDI2OD-T2) PA. It is noted that the Mw = 267 kg mol–1 of the PA in this study is much higher than the critical MW (Mw ≈ 100 kg mol–1), above which polymers start to fold and entangle.24,40 Our hypothesis is that tie molecules and entangled chains of the PA additive can effectively dissipate the external mechanical stresses on the PM6:Y7 blend; otherwise, the blend without the additive suffers from brittle crack propagation through sharp interfaces between the PM6 and Y7 domains (Figure 1d).

Figure 1.

Figure 1

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(a) Molecular structures of the donor (PM6), acceptor (Y7), and PA additive (P(NDI2OD-T2)), (b) normalized UV–vis absorption in film state, (c) energy level alignments for the materials, and (d) schematic illustrations describing different blend morphologies under mechanical stresses w/o (left) and w/ (right) PA additive.

The optical and electrochemical properties of PM6, Y7, and P(NDI2OD-T2) are presented in Figure 1 and Table 1. The maximum absorption wavelengths (λmaxs) of PM6 and Y7 were 620 and 846 nm, respectively, and their ultraviolet–visible (UV–vis) spectra indicated that their light absorption properties are complementary (Figure 1b).54 In particular, the absorption coefficient (εmax) of Y7 at its maximum absorption wavelength was significantly high, over 1.0 × 105 cm–1. The P(NDI2OD-T2) additive had a λmax of 705 nm, located between those of PM6 and Y7, which additionally complements the light absorption of the PM6:Y7 blend.55 The lowest unoccupied and highest occupied molecular orbital (LUMO/HOMO) energy levels of PM6 and Y7 are well-matched with sufficient offset to generate a driving force for efficient charge transfer (Figure 1c). The P(NDI2OD-T2) showed the LUMO/HOMO levels located between those of the PM6 and Y7, further confirming its suitability as an additive.56,57 These optical and electrochemical properties suggest that the electroactive P(NDI2OD-T2) is an excellent additive candidate for the PM6:Y7 blend that does not impair the charge generation process.

Table 1. Optical/Electrochemical Properties and Molecular Weight Information of the Materials.

material λmax (nm)a εmax [× 104 cm–1]b HOMO (eV)c LUMO (eV)c Mw (Đ) (kg mol–1)
PM6 620 7.1 –5.45 –3.65 84 (3.1)
Y7 846 12.4 –5.68 –4.12 –
P(NDI2OD-T2) 705 4.3 –5.52 –4.02 267 (2.3)
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a

Obtained from the UV–vis absorbance in film state spin-coated from the solution in chlorobenzene with 10 mg mL–1 concentration.

b

Absorption coefficients at λmaxs from film absorption spectra.

c

Measured from the cyclic voltammetry.

Photovoltaic Properties

We investigated the photovoltaic performances of PM6:Y7 systems loaded with different amounts of P(NDI2OD-T2) PA additives. The normalized photovoltaic parameters of the systems are plotted as a function of PA content in Figure 2a. The detailed results are shown in Table 2. The current density–voltage (J–V) curves under simulated 1 sun illumination are plotted in Figure 2b. The PCE distribution histograms of the respective devices in Figure 2c demonstrated a near-Gaussian distribution. We also included the photovoltaic performances of the PM6:P(NDI2OD-T2) blend in Figure S1 and Table S1. To investigate the effect of the PA content on the photovoltaic properties, the ratio of donor to acceptor in the PSC blends was fixed to 1:1. Thus, the percentage of the PA in a PSC blend sample refers to the weight percentage of the PA in the total acceptor content (Y7 + PA) of the PSC. For example, a blend containing 30 wt % of the PA has a PM6:Y7:PA weight ratio of 1:0.7:0.3. First, the PM6:Y7 blend without PA content showed a maximum PCE of 14.62%. Notably, the PCE of the PSCs increased to 15.44% with incorporation of 10 wt % of the PA additive, and it remained at 15.01% with the 20 wt % PA additive (Table 2). The PCE slightly decreased to 14.08% at the 30 wt % addition. The Jscs of the PSCs showed a similar trend to their PCEs, i.e., the Jscs initially increased with increasing PA content up to 20 wt %; however, the open-circuit voltages (Vocs) remained almost constant across the range of contents. However, the PCE value started to decrease rapidly when the PA amounts were higher than 30 wt %, which was mainly caused by the decreases of fill factor (FF) and Jsc. The external quantum efficiency (EQE) spectra of the solar cells loaded with different amounts of PA are presented in Figure 2d. The calculated Jscs are matched with the actual Jscs within a 4% error (Table 2). The EQE responses of the PSCs in the Y7 absorption range (800–950 nm) decreased, but the EQE values in the PM6 absorption range (450–600 nm) increased with increasing PA content (0–30 wt %). Also, the EQE values in the absorption range between 650 and 800 nm increased for the higher PA content.

Figure 2.

Figure 2

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(a) Normalized PCE, Jsc, Voc, and FF as a function of PA content, (b) J–V curves, (c) PCE distribution histograms, and (d) EQE response spectra of the PSCs with different PA contents.

Table 2. Photovoltaic Parameters of the PSCs Depending on Their PA Contents.

PA content (%) Voc (V)a Jsc (mA cm–2)a cal. Jsc (mA cm–2) FFa PCEavg(max) (%)a
0 0.86 ± 0.01 24.27 ± 0.19 23.95 0.69 ± 0.01 14.43 ± 0.16 (14.62)
10 0.87 ± 0.01 25.31 ± 0.20 24.81 0.69 ± 0.02 15.19 ± 0.21 (15.44)
20 0.87 ± 0.01 25.74 ± 0.18 25.22 0.67 ± 0.01 14.86 ± 0.17 (15.01)
30 0.87 ± 0.01 24.41 ± 0.27 23.52 0.64 ± 0.02 13.68 ± 0.28 (14.08)
40 0.87 ± 0.00 23.52 ± 0.20 22.95 0.58 ± 0.01 11.35 ± 0.21 (11.91)
50 0.88 ± 0.01 21.73 ± 0.36 21.01 0.44 ± 0.03 8.36 ± 0.33 (8.70)
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a

Average values from more than 15 PSCs.

Charge Transport and Recombination Properties

To elucidate the trends observed in the Jsc and FF values of the PSCs, we analyzed their charge generation, transport, and recombination properties. Space-charge-limited current (SCLC) mobilities of the pristine constituent materials and blend films were analyzed (Tables S2 and S3).58 The devices for the SCLC measurements were fabricated as described in the previous literature.37 In Table S3, the blends containing 0–20 wt % of the PA demonstrated high μes (exceeding 10–4 cm2 V–1 s–1). However, the μes of the blend films decreased at above 20 wt % PA addition. Since hole mobilities (μhs) of the blends remained fairly constant regardless of the PA content, the near-unity μh/μe value of the PM6:Y7 blend was well-retained by the blends up to a PA content of 20 wt %. However, the μh/μe values of the blend films increased as the PA content increases above 20 wt %, resulting in unbalanced charge transport. For instance, the μh/μe value of the blend containing 50 wt % of the PA is 21.8 (Table S3). As the deviated μh/μe value from unity causes charge recombination and lowers the Jsc and FF of the PSC devices,59,60 this SCLC mobility trend supports the changes of the Jsc and FF values in terms of PA loadings.

To analyze the charge dissociation behaviors of the PSCs at their donor–acceptor interfaces, their photocurrent densities (Jphs) were measured under effective voltage (Veff) (Figure S2).61 The exciton dissociation probabilities (P(E,T)s) were estimated by obtaining the ratios of Jscs (Veff = Voc, dashed line in Figure S2) relative to the saturated Jphs (Veff = 4 V) (Table S3).61 The PM6:Y7 blends with 0 to 10 wt % PA addition displayed a high P(E,T) of over 90%, but a steep decrease of the P(E,T) values was observed for a high PA loading of more than 20–30 wt % (Figure S2). For example, the P(E,T) values decreased from 94.4% (0 wt % PA) to 87.7% (30 wt % PA) and to 77.3% (50 wt % PA). This result implies that the addition of the PA could reduce the donor–acceptor interfaces where exciton dissociation occurs.62

In addition, the Jsc and Voc of the PSCs were measured under different light intensities (P) from 10 to 100 mW cm–2, to elucidate the charge recombination characteristics (Figure 3). Jsc is known to follow an equation of Jsc ∝ Pα, and the slope (α) of the power-law relationship becomes closer to unity when bimolecular recombination is suppressed.63 In Figure 3a, the α values in all of the blends were almost the same, around α = 0.94, indicating all the blends did not suffer from severe bimolecular recombinations in their PSC operations. Under the open-circuit condition, Voc is proportional to ln(P) with a unit of kTq–1 (k = Boltzmann constant, T = temperature, and q = elementary charge), and the slope (S) of the linear regression approaches unity when all charge carriers reach to electrodes without monomolecular or trap-assisted recombination.64 A clear difference in the S values of the PSC blends depending on the contents of the PA was observed (Figure 3b). The PM6:Y7 blend showed an S value of 1.13 kTq–1; however, those associated with the PSCs containing 10 and 20 wt % of the PA were lower, i.e., 1.06 and 1.09 kTq–1, respectively. Thereby, the monomolecular or trap-assisted recombinations became suppressed by an appropriate amount of PA additive. However, the S values increased to 1.23 and 1.33 kTq–1 for the 40 and 50 wt % PA loadings, respectively. It suggests that excessive amounts of the PA cause charge recombination. As a result, the increases of Jsc and PCE with the 10 and 20 wt % PA additions compared to the blend without additive can be explained by the suppressed monomolecular/trap-assisted recombinations.

Figure 3.

Figure 3

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Light-intensity dependences of the (a) Jsc and (b) Voc of PSCs containing different amounts of the PA.

Mechanical Properties of the Thin Films

To demonstrate the importance of PA additive in enhancing the mechanical properties of the PM6:Y7 blend film, the tensile properties of both pristine and PM6:Y7:PA blend thin films were investigated using a pseudo-free-standing tensile test.65−67 Each thin film was transferred from its glass substrate onto a water surface to measure its intrinsic tensile properties in a pseudo-free-standing state. Figure S3 and Table S4 show the stress–strain (S–S) curves, COSs, calculated toughnesses, and elastic moduli (Es) of the pristine constituent materials. While the PM6 polymer exhibited a moderate ductility (COS = 14.66%) and toughness (3.84 MJ m–3), the tensile properties of Y7 could not be measured due to extreme brittleness of the thin films. This brittleness of the Y7 thin film was attributed to its highly rigid ladder-type backbone structure, which is consistent with the reported characteristic brittleness of SMA materials.10,12 In stark contrast, the P(NDI2OD-T2) PA had high ductility with a COS and toughness of 39.20% and 12.0 MJ m–3, respectively. The superior ductility of P(NDI2OD-T2) was attributed to its high MW (Mw = 267 kg mol–1) exceeding its critical MW, which affords the formation of tie molecules and entangled chains in the thin film.12,24,40

Figure 4a displays the S–S curves of the blend thin films, and Table S5 lists their COS, toughness, and E values. Without the P(NDI2OD-T2) PA content, the PM6:Y7 binary blend showed brittle tensile properties, showing only 2.21% of COS and 0.31 MJ m–3 of toughness. This was ascribed to the failure of stress dissipations due to the brittleness of the Y7 SMA.10,12,68 The addition of the PA significantly increased the COS and toughness values of the blends. For example, the addition of 30 and 50 wt % of the PA increased the COS values to 11.93 and 19.97%, respectively, and the toughness values to 3.76 and 5.15 MJ m–3, respectively (Figure 4b). The crack formation behavior of blend thin films with and without PA contents (see the images in Figure 4c) was distinctly different. The pristine PM6:Y7 thin film developed a brittle break (upper image), whereas the blend thin film containing the PA experienced ductile plastic deformation, resulting in significant wrinkling (lower image). This notable enhancement in the mechanical ductility of the blends was attributed to tie molecules and chain entanglements formulated by the long chains of high-MW PA, dramatically improving the dissipation of external stress.10,68 Based on the aforementioned results and the photovoltaic parameters presented in Figure 2 and Table 2, the PSC incorporating 20 wt % of the PA demonstrates optimal photovoltaic performance (PCE exceeding 15%) and mechanical robustness (COS exceeding 10%). The PCEs and COS values (obtained by the pseudo-free-standing tensile test) of the PSCs developed in this study were compared with those of previously reported PSCs (Figure 4d and Table S6).

Figure 4.

Figure 4

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(a) Stress–strain curves, (b) COS and toughness values of the blend films depending on the PA content, (c) images of blends without PA content (upper) and with PA content (lower) after the tensile test, and (d) a comparison of the PCE and COS values of previously reported PSCs and those of the PSCs developed in this work.

Morphological Properties

To better understand the variances in the photovoltaic/mechanical properties of the PSCs with different PA contents, their blend morphologies including the domain sizes and purities were investigated using resonant soft X-ray scattering (RSoXS) (Figure 5).69 The scatterings were acquired in the incidence beam energy of 284.2 eV, which can maximize the material contrasts in the blends.69 The RSoXS profiles of all the blends reflect clear peaks at q = 0.0081 Å–1 (domain spacing = 77 nm). Notably, the scattering peaks got more distinct and sharper with increasing PA content, suggesting gradational development of independent phases of the blend components.70 For a more quantitative analysis, we estimated the relative domain purity of each blend, which is proportional to the square root of the integrated scattering intensity (√ISI) (Table S7).71 To note, the addition of 10 wt % of the PA increased the relative domain purity to 0.20. This suggests that even small amounts of the high-MW PA can formulate its own crystalline domains, owing to the highly semicrystalline and self-aggregating properties of the long PA chains.40 As the PA content of the blend increased, the relative domain purities gradually increased. For example, the addition of 20 and 40 wt % of the PA increased the relative domain purity of the blend to 0.49 and 1.00, respectively. These results indicate that high PA content promoted the formation of domains with high purities.

Figure 5.

Figure 5

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Lorentz-corrected RSoXS profiles in the (a) low- and (b) high-q regions of blends with different PA contents (incident-beam energy = 284.2 eV).

The structural properties of the pristine constituent materials and the blend films were estimated using grazing incidence X-ray scattering (GIXS) (Figures S4 and S5 and 6). The GIXS line-cut profiles of both the pristine and blend films from GIXS are represented in Figures S4 and 6, respectively. In Figure S4, all of the PM6, Y7, and P(NDI2OD-T2) PA showed the face-on dominant packing orientations with in-plane (IP) (100) peaks and out-of-plane (OOP) (010) peaks, which is beneficial to the vertical charge transport properties.72,73 Especially, both Y7 and PA showed distinct (001) scatterings in the IP direction with d spacings of 15.9 and 13.8 Å, respectively.74 The scattering profiles in Figure 6 indicate that the blend films also had the face-on dominant packing orientations, consistent with those of the pristine constituent materials. Interestingly, the IP (001) peaks of Y7 and PA showed different patterns depending on the PA contents. The PM6:Y7 blend without the PA did not show any peak around q = 0.40–0.46 Å–1 in the IP direction, while the blends with 10 and 20 wt % PA showed detectable peaks in the same regime (Figure 6a). Meanwhile, in the profiles of the blends containing 40 and 50 wt % of the PA, two separate peaks were observed. In a quantitative analysis of the GIXS results, the d spacing and coherence length (Lc) values of the blends associated with the IP (001) peaks were calculated (Table S8). The Lc values were estimated by a Scherrer equation,75 and the procedure is detailed in the Experimental Section. It is noted that absolute value of Lc would be varied from the actual size of crystals, since the solution-processed blends consisting of semicrystalline polymers and small molecules can be fairly disordered. The Lc values of the (001) scattering of P(NDI2OD-T2) (qxy = 0.46 Å–1) increased significantly with increasing PA content, e.g., from 12.9 to 16.4 nm as the PA content increased from 30 to 50 wt %. The Lc values associated with the (001) scattering of Y7 (qxy = 0.40 Å–1) also increased with increasing PA content, e.g., from 10.7 to 14.8 nm as the PA content increased from 30 to 50 wt %. In addition, distinct (200) peaks in the OOP direction associated with P(NDI2OD-T2) (qz = 0.58 Å–1) were observed in the profiles of the blends containing 40 and 50 wt % of the PA, while the profiles of the blends containing less than 40 wt % of the PA did not show any peaks. The PA forms its own crystal networks in the blends even at low PA contents (10–20 wt %), and independent crystalline domains of Y7 and PA are evident in blends with high PA contents (40–50 wt %). These results are consistent with the observations from the RSoXS analyses.

Figure 6.

Figure 6

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GIXS line-cut plots for the blend films in the (a) IP and (b) OOP directions.

From the electrical, mechanical, and morphological analyses of the blends, the different blend morphologies are described in Figure 7 in terms of PA content. The PM6:Y7 blend (without PA content) includes locally isolated Y7 domains (Regime I). In this regime, some of the electrons become trapped in isolated Y7 domains. Moreover, mechanical stresses concentrate along sharp and fragile PM6–Y7 interfaces, avoiding the hard Y7 crystallites; thereby, cracks propagate easily under small strains (i.e., a COS of 2%).12,76,77 In blends with optimal PA contents (10–20 wt %), the three components formulate properly intermixed domains with moderate domain purities (Regime II). Tie molecules formed by long PA chains bridge and interconnect the isolated Y7 domains. This facilitates the effective hopping of electrons from one acceptor domain to the adjacent ones, suppressing monomolecular or trap-assisted recombinations in comparison to the blend without the PA. Also, the entangled chains and tie molecules of the PA effectively dissipate external mechanical stress, making the blend films more resistant to the crack formation and mechanical failure. In comparison, the blends with excessive PA contents (over 30 wt %) have highly segregated and pure domains of the three components with comparatively smaller donor–acceptor interfacial areas (Regime III). In this regime, the PA does not percolate well to the PM6 and Y7 domains, owing to their high purities and severe segregation. In conclusion, the blends containing 10–20 wt % of the PA exhibit optimal photovoltaic and mechanical properties, demonstrating 15% of PCE and 10% of COS simultaneously.

Figure 7.

Figure 7

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Schematic descriptions of the different blend morphologies in terms of PA content.

3. Conclusions

We developed a PSC system that demonstrates both a high PCE and mechanical robustness, by incorporating a high-MW PA additive into the SMA-based PSC blend. This additive approach is highly effective in enhancing the mechanical properties of the characteristically brittle SMA-based PSCs without compromising any photovoltaic performances. The addition of an appropriate amount of the PA (≤20 wt %) increased the PCE of the PM6:Y7 PSC, while excessive amounts of the PA decreased its PCE. For example, the PSCs incorporating 10 and 20 wt % of the PA demonstrated higher PCEs (15.44 and 15.01%, respectively) than the PSC featuring the pristine blend (PCE = 14.62%). We revealed that the PA content of a blend determines the extent of its acceptor-phase separation and, consequently, its electrical and photovoltaic performances in the PSC by affecting its charge transport and recombination behaviors. Importantly, the mechanical ductility of the blend film dramatically increased with increasing PA content, e.g., its COS increased from 2 to over 10% as the PA content increases from 0 to 20 wt %. Based on these results, the blend containing 20% of the PA produced optimal PSC devices that simultaneously demonstrate high photovoltaic performance (PCE > 15%) and mechanical robustness (COS > 10%). Thus, we developed an effective strategy for achieving PSCs with high PCEs potentially suitable for stretchable or wearable applications.

4. Experimental Section

Polymerization of P(NDI2OD-T2) PA

The PA was prepared by a Stille coupling reaction by following a method from the previous literature.40 Particularly, we performed the polymerization during a long time (i.e., 2 days) to provide sufficient time for the PA growth and at high temperature (100 °C) to prevent gelation (yield: 80%, Mw = 267 kg mol–1, Đ = 2.3).

Materials

The 2,9-bis(3-((3-(dimethylamino)propyl)amino)propyl)anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10(2H,9H)-tetraone (PDINN) material was synthesized by following a method described in the literature.78 PM6 PD and Y7 SMA were purchased from Derthon. PEDOT:PSS (AI4083) was purchased from Heraeus. The other chemical compounds were purchased from Sigma-Aldrich.

Characterizations

A Shimadzu Scientific UV-1800 instrument was used to measure the UV–vis spectra. Size exclusion chromatography (Agilent GPC 1200) was applied to obtain the MW information on the polymers relative to polystyrene references (eluent: ortho-dichlorobenzene). GIXS data were collected at beamline 3C in Pohang Accelerator Laboratory in South Korea (incidence angle = 0.12–0.14°). Lcs from GIXS scatterings were estimated by Scherrer equation

graphic file with name au1c00064_m001.jpg

where K = shape factor (0.9), and Δq = the full width half-maximum of the peaks.

PSC Fabrication

The PSC devices have a structure of ITO/PEDOT:PSS/photoactive layer/PDINN/Ag. First, ITO substrates were washed by sequential ultrasonications with acetone and isopropyl alcohol. After drying, they were plasma-treated for 10 min. Then, PEDOT:PSS was spin-coated onto the ITO substrates (3000 rpm, 30 s). The PEDOT:PSS-coated substrates were thermally treated (165 °C, 20 min). Next, the substrates were moved into a N2-filled glovebox. The active layer films were prepared onto the substrate by spin-coating the blend solution (i.e., concentration in chlorobenzene = 20 mg mL–1, blend ratio = 1:1, and 1 vol % of 1-chloronaphthalene). Next, the active layer films were thermally annealed at 100 °C for 10 min in a high-vacuum condition. Then, the PDINN was dissolved into methanol with a 1 mg mL–1 concentration. The solution was spin-casted on the substrates (3000 rpm, 30 s), and Ag (120 nm) was thermally evaporated on the PDINN layer.

PSC Measurement

A Keithley 2400 SMU and McScience K201 LAB55 solar simulator were used to measure the photovoltaic characteristics of the PSCs under AM1.5G 100 mW cm–2. A McScience K3100 IQX was applied to obtain EQE spectra of the PSCs. Solar intensity was calibrated using a standard cell (K801SK302, McScience). The photoactive area used for the efficiency measurement was 0.164 cm2.

Thin-Film Tensile Measurement

For the sample preparation, poly(styrenesulfonate) solution (10 mg mL–1 in water) was casted on a glass substrate for a sacrificing layer. Then, pristine and blend solutions were spin-coated on the substrates. The resulting films on the substrate were cut into a dog-bone shape using a femtosecond laser. The samples were slowly floated onto a water surface to yield free-standing films. Then, strains were applied under ambient conditions (strain rate = 0.8 × 10–3 s–1), and tensile loads were obtained using a LTS-10GA (KYOWA) instrument.

Acknowledgments

This work is supported by a National Research Foundation of Korea (NRF) Grant of the Korean Government (2019R1A2B5B03101123 and 2020R1A4A1018516). This work is also supported by a Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant (No. 20183010014470) of the Korean Government. The experiment at the Advanced Light Source is supported by a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.1c00064.

  • Pristine SCLC mobility; Jph vs Veff curves; additional GIXS analysis; additional mechanical properties (PDF)

Author Contributions

† J.-W.L. and B.S.M. equally contributed to this work.

The authors declare no competing financial interest.

Supplementary Material

au1c00064_si_001.pdf (600.1KB, pdf)

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