Improved Efficiency of Polymer Solar Cells by Modifying the Side

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Improved Efficiency of Polymer Solar Cells by Modifying the Side Chain of Wide-Band Gap Conjugated Polymers Containing Pyrrolo[3,4‑f ]benzotriazole-5,7(6H)‑dione Moiety Peng Zhu,† Baobing Fan,† Xiaoyan Du,‡ Xiaofeng Tang,‡ Ning Li,‡ Feng Liu,§ Lei Ying,*,† Zhenye Li,† Wenkai Zhong,† Christoph J. Brabec,‡,∥ Fei Huang,*,† and Yong Cao†

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†

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China ‡ Institute of Materials for Electronics and Energy Technology (i-MEET), FAU Erlangen-Nürnberg, 91058 Erlangen, Germany § Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ∥ Bavarian Center for Applied Energy Research (ZAE Bayern), Immerwahrstraße 2, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: Two novel wide-band gap donor−acceptortype conjugated copolymers, PTzBI-S and PTzBI-Ph, are designed and synthesized, based on alkylthio-thienyl- or alkylphenyl-substituted benzodithiophene (BDT) derivatives as the electron-donating unit and pyrrolo[3,4-f ]benzotriazole5,7(6H)-dione as the electron-withdrawing unit. The asgenerated copolymers show the comparable optical and electrochemical properties. The alkylthio-thienyl-substituted BDT unit facilities a benign decrease of the highest occupied molecular orbital (HOMO) levels. This consequently enhances open-circuit voltages (VOC) over 0.9 V in relevant solar cells with the fullerene acceptor ([6, 6]-phenyl-C71-butyric acid methyl ester, PC71BM) or the nonfullerene acceptor (3,9bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene, ITIC). The combination studies of Fourier transform photocurrent spectroscopy and electroluminescence further rationalize the VOC difference between solar cells with fullerene and nonfullerene acceptors. An impressively high power conversion efficiency of 10.19% is obtained for the device based on PTzBI-Ph:ITIC, outperforming the 8.84% achieved by the PC71BM-based device. Our results demonstrate that the modification of substituents of BDT units can effectively decrease the HOMO level and consequently improve VOC, ultimately allowing the attainment of high-efficiency polymer solar cells. KEYWORDS: polymer solar cells, pyrrolo[3,4-f ]benzotriazole-5,7(6H)-dione, wide-band gap copolymers, side-chain modification

1. INTRODUCTION Polymer solar cells (PSCs) have drawn much attention because of their promising properties such as mechanical flexibility, economic competitiveness, ease of integration into existing infrastructures, and short-term energy payback.1−3 Previous efforts in the development of new light-harvesting species, optimization of the nanostructures of bulk-heterojunction films, and rational device engineering have significantly boosted the power conversion efficiency (PCE) of PSCs.4−7 In general, the PCE is determined by the open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF).8−10 To achieve highly efficient devices with long-term operational stability, the electron-donating polymers and the acceptor need to be well paired in terms of absorption spectra, energy levels, charge carrier mobility, and miscibility.11−14 Because VOC is correlated to the discrepancy between the highest occupied molecular orbital (HOMO) energy level of © XXXX American Chemical Society

the electron-donating material and the lowest unoccupied molecular orbital (LUMO) of the electron-accepting material,15−17 electron donors with relatively deep HOMO energy levels are highly desirable.18−21 To maximize VOC of the resulting devices, much effort has focused on developing conjugated polymers with wide band gaps (WBGs) that possess relatively deep HOMO energy levels.22−24 One of the most effective classes of electrondonating WBG conjugated polymers is based on a donor− acceptor-type backbone, which can be achieved by combining weak electron-rich moieties with weak electron-deficient moieties.25,26 Although WBG conjugated polymers can achieve the deep HOMO energy levels necessary for high VOC,27 the Received: April 9, 2018 Accepted: June 13, 2018

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DOI: 10.1021/acsami.8b05700 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthetic Routes of PTzBI-S and PTzBI-Ph

Table 1. Molecular Weight and Electrochemical and Optical Properties of Polymers polymer

Mn (kDa)

PDI

HOMOa (eV)

LUMOc (eV)

λmax (nm)

λonset (nm)

b Eopt (eV) g

PTzBI-S PTzBI-Ph

21.6 19.0

1.81 2.85

−5.38 −5.43

−3.54 −3.58

573 573

675 670

1.84 1.85

Obtained from the CV measurements. bCalculated from the onset of UV−vis absorption in thin films. cObtained by adding Eopt g to the obtained HOMO energy levels.

a

resulting devices typically exhibit relatively low JSC owing to the limited light-harvesting capability, as their absorption profile is not well matched with the solar spectrum. This tradeoff between VOC and JSC can be resolved by pairing the electron-donating conjugated polymers with narrow-band gap nonfullerene acceptors (NFAs), such as 3,9-bis(2-methylene(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC).28−30 The relatively narrow absorption band of ITIC and its derivatives can compensate for the poor absorption of WBG polymers in the long-wavelength region while maintaining a preferable LUMO energy level to supply the driving force for charge separation.10,31 Here, we design and synthesize two novel WBG conjugated copolymers, PTzBI-S and PTzBI-Ph, which consist of an imide-functionalized benzotriazole (TzBI) electron-withdrawing unit and an electron-donating alkylthio-thienyl- or alkylphenyl-substituted benzodithiophene (BDT) unit (with structures shown in Scheme 1). The modification of the side chains of the BDT unit leads to reduced electron density and thus decreased HOMO energy levels32,33 relative to the previously reported copolymer PTzBI that was based on a 2ethylhexylthienyl-substituted BDT unit.19,34 Although the strategy of copolymerizing alkylthio-thienyl- or alkylphenylBDT unit with an electron-deficient unit has been reported, the WBG conjugated polymers integrated with BDT derivatives and the TzBI unit that presented enhanced VOC significantly lagged behind. Considering that most of recently developed nonfullerene electron acceptors are developed based on the trivial modification of molecular structures, it is highly

required to develop WBG conjugated polymers with carefully tailored molecular structures to pair with those NFAs. Here, we fabricated devices based on the developed copolymers PTzBI-S and PTzBI-Ph as donors and PC71BM (or ITIC) as the acceptor, presenting improved VOC of about 0.92 V. These observations indicate that developing TzBI-based WBG copolymers through the rational design of side chains is a promising strategy for the fabrication of high-performance PSCs with high VOC values.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization. Scheme 1 shows the synthetic routes of the target copolymers PTzBI-S and PTzBI-Ph. By treating the compounds 4,8-bis(5-((2ethylhexyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b′]-dithiophene (BDT-S) and 4,8-bis(4-(2-ethylhexyl)phenyl)benzo[1,2-b:4,5b′]dithiophene (BDT-Ph) with n-butyllithium (n-BuLi), followed by adding trimethyltin chloride, the target monomers, bis-stannylated (4,8-bis(5-((2-ethylhexyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane) (M1) and (4,8-bis(4-(2-ethylhexyl)phenyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane) (M2), were obtained in good yields of 93 and 89%, respectively. The palladium-catalyzed Stille polymerization of M1 and M2 with the dibromo-monomer M3 gave the target copolymers PTzBI-S and PTzBI-Ph, respectively, in good yields of over 90%. These polymers can be easily dissolved in common solvents such as chloroform or chlorobenzene at room temperature. The number-average molecular weights (Mn) and polydispersity indices (PDIs) of B


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ACS Applied Materials & Interfaces

Figure 1. Normalized UV−vis absorption spectra of PTzBI-S and PTzBI-Ph in chlorobenzene solution (a) and in thin films (b). The weight ratio of donor:acceptor is denoted in the parenthesis (b). The energy level alignment of the relevant components in the photoactive layer (c).

Figure 2. J−V characteristics (a) and EQE spectra (b) for devices with the device architecture of ITO/PEDOT:PSS/active layer/PFNDI-Br/Al.

measured to be 0.95 and 1.00 V for PTzBI-S and PTzBI-Ph, respectively. The half-wave potential of the reference ferrocene/ferrocenium redox couple (Fc/Fc+), denoted as EFc/Fc+, was measured to be 0.37 V under the same conditions. The HOMO energy levels for PTzBI-S and PTzBI-Ph were calculated to be −5.38 and −5.43 eV, respectively (Table 1), according to the equation: HOMO = −e[Eox + (4.80 − EFc/Fc+)] eV. The LUMO energy levels of these copolymers are estimated by adding the optical band gap (Eopt g ) to the obtained HOMO energy levels, which are estimated to be −3.54 and −3.58 eV for PTzBI-S and PTzBI-Ph, respectively. The detailed alignments of frontier molecular orbital energy levels are depicted in Figure 1c. 2.3. Photovoltaic Properties. To evaluate the photovoltaic performances of the two electron-donating copolymers, we prepared PSCs with the conventional architecture of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/active layer/poly[(9,9bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-5,5′-bis(2,2′-thiophene)-2,6-naphthalene-1,4,5,8-tetracarboxylic-N,N′-di(2-ethylhexyl)imide]di-bromide (PFNDIBr)/Al, where the active layer comprised PTzBI-S and PTzBIPh as the electron donor and PC71BM or ITIC as the electron acceptor. Here, a thin layer of PFNDI-Br35 was used as the cathode interlayer because it can facilitate charge extraction from the active layer. The current density−voltage (J−V) characteristics of these devices were measured under simulated AM 1.5G, 100 mW cm−2 illumination; and the corresponding photovoltaic characteristics are shown in Figure 2a, and the relevant photovoltaic parameters are summarized in Table 2.

PTzBI-S and PTzBI-Ph were estimated to be 21.6 kDa (PDI = 1.81) and 19.0 kDa (PDI = 2.85), respectively (Table 1). These values were estimated by high-temperature gel permeation chromatography at 140 °C using 1,2,4-trichlorobenzene as the eluent and linear polystyrene as the reference. It is also worth noting that these copolymers with a higher molecular weight exhibited poor solubility, which is unfavorable for the fabrication of high-quality films by a solutionprocessing procedure. 2.2. Optical and Electrochemical Properties. PTzBI-S and PTzBI-Ph exhibited similar UV−vis absorption both in chlorobenzene solution and in thin films (Figure 1a,b), presenting relatively strong absorption in the range of 300− 680 nm. PTzBI-S and PTzBI-Ph exhibited nearly identical absorption maxima at 573 nm, with a relatively weak shoulder peak at about 615 and 608 nm, respectively, because of aggregation in the film state. It is also worth noting that the absorption onsets of PTzBI-S and PTzBI-Ph were located at 670 and 675 nm, respectively, corresponding to the optical band gaps (Eopt g ) of 1.84 and 1.85 eV, respectively (Table 1). Figure 1b shows the absorption profiles of the two copolymers blended with the electron acceptors PC71BM and ITIC. Note that the polymer:ITIC blended films exhibited much broader absorption profiles (from 300 to 780 nm) and much stronger absorption coefficients than those of the polymer:PC71BM blended films, implying the greater light-harvesting ability of the former, which was supposed to present increased JSC of the corresponding PSCs. The HOMO levels of the resulting copolymers were evaluated by cyclic voltammetry (CV, see Figure S1 in the Supporting Information). The onsets of oxidation (Eox) were C


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slightly higher VOC values than the device based on the electron-donating polymer PTzBI, which bears a BDT unit with 2-ethylhexylthienyl substituents, indicating that the modification of the side groups is an effective strategy to improve the VOC of the resulting PSCs. The external quantum efficiency (EQE) spectra of the fabricated devices are illustrated in Figure 2b. It is clear that the ITIC-based devices show much broader photoresponse, from 300 to about 780 nm, than those of the PC71BM-based devices, which indicates that ITIC contributes greatly to sunlight harvesting and thus potentially leads to much higher JSC values than those of the PC71BM-based counterparts. The current densities, calculated by integrating the EQE spectra, are 12.29 and 11.75 mA cm−2 for the PTzBI-S:PC71BM- and TzBIPh:PC71BM-based devices, respectively, and 15.61 and 15.48 mA cm−2 for those based on PTzBI-S:ITIC and TzBI-Ph:ITIC, respectively, all of which are consistent with the corresponding J−V characteristics. We note that the obtained EQE spectra (Figure 2b) are quite different from those observed for UV−vis spectra based on the same donor:acceptor blend films (Figure 1b). This can be understood as the EQE values refer to the overall efficiency of a range of factors for PSCs, including light absorption, exciton diffusion to the donor−acceptor interfaces, charge separation, and charge carrier collection.36 2.4. Charge Separation and Recombination Dynamics. To probe the charge carrier generation and charge collection behavior, we plotted the photocurrent density (Jph) versus the effective voltage (Veff). Here, Jph is defined as JL − JD, where JL and JD are the current densities under illumination and in the dark, respectively. Veff equals Vbi − Vappl, where Vbi is the built-in voltage (the voltage when Jph = 0) and Vappl is the applied voltage.37−40 Note that Jph reaches saturation (Jsat) in the high-voltage region (≥2 V, Figure 3a,b). Given that such a

Table 2. Photovoltaic Parameters of Devices Measured under AM 1.5G, 100 mW cm−2 donor:acceptor

ratio (w/w)

VOC (V)

JSC (mA cm−2) FF (%)

PTzBI-S:PC71BM

1:1

0.923

13.10

70.95

PTzBI-Ph:PC71BM

1:1

0.943

12.70

73.80

PTzBI-S:ITIC

1:1.5

0.915

16.62

60.01

PTzBI-Ph:ITIC

1:1

0.918

16.39

67.72

PCE (avg)a (%) 8.58 (8.42) 8.84 (8.72) 9.12 (9.03) 10.19 (10.05)

a

The average PCE of eight individual devices. Device structure: ITO/ PEDOT:PSS/active layer/PFNDI-Br/Al.

Initially, the processing conditions of the PSCs were optimized by using PC71BM as the electron acceptor. The polymer:PC71BM (1:1, w/w) blend films were cast from the 1,2-dichlorobenzene (DCB) solution, followed by thermal annealing at 120 °C for 10 min. The device based on PTzBIS:PC71BM presented an impressive PCE of 8.58%, with a VOC of 0.92 V, a JSC of 13.10 mA cm−2, and an FF of 70.95%. The device based on PTzBI-Ph:PC71BM exhibited a slightly higher PCE of 8.84% (VOC = 0.94 V, JSC = 12.70 mA cm−2, FF = 73.80%). In addition, we fabricated nonfullerene PSCs and optimized the processing conditions by screening the weight ratio of donor:ITIC blends, processing solvents, and solvent additives (see Figures S2−S6 and Tables S1−S5, Supporting Information). The optimized nonfullerene devices exhibited higher PCEs of 9.12% (VOC = 0.92 V, JSC = 16.62 mA cm−2, FF = 60.01%) and 10.19% (VOC = 0.92 V, JSC = 16.39 mA cm−2, FF = 67.72%) for PTzBI-S and PTzBI-Ph as donors, respectively (Table 1). Interestingly, these devices showed

Figure 3. Jph−Veff characteristics (a,b) and JSC dependence upon light intensity (c,d) for PC71BM- (a,c) and ITIC-based (b,d) devices, respectively. D


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Figure 4. FTPS (EQEFTPS), the directly measured EQE, the EQE determined from the emitted photon flux (ϕEL), and the room-temperature blackbody photon flux (ϕbb) to determine the radiative VOC limit (VOC,rad) for devices based on PTzBI-S:PC71BM (a), PTzBI-Ph:PC71BM (b), PTzBI-S:ITIC (c), and PTzBI-Ph:ITIC (d).

strong internal field is enough to transport all generated carriers to the electrodes, Jsat is limited by the number of absorbed photons (or generated carriers). Here, Jsat equals qLGmax, where q and L are the elementary charge and the thickness of the active layers, respectively, and Gmax is the maximum carrier generation rate. The Gmax values for the devices based on PTzBI-S:PC71BM and PTzBI-Ph:PC71BM were calculated to be 1.01 × 1028 and 0.98 × 1028 m−3 s−1 (see Table S6, Supporting Information), respectively, which are lower than the corresponding ITIC-based devices PTzBIS:ITIC (Gmax = 1.28 × 1028 m−3 s−1) and PTzBI-Ph:ITIC (Gmax = 1.15 × 1028 m−3 s−1). These findings indicate that the polymer:ITIC blends achieve greater photogeneration of charge carriers than the PC71BM blends. The charge collection probability (PC) can be evaluated as PC = Jph/Jsat, from which the PC values under short-circuit conditions for the PTzBIS:PC71BM, PTzBI-Ph:PC71BM, PTzBI-S:ITIC, and PTzBIPh:ITIC devices were calculated to be 93.9%, 94.8%, 88.8%, and 92.2%, respectively. Note that the PTzBI-Ph-based devices show slightly higher PC values than the PTzBI-S-based devices, implying the better charge collection ability of the former. Considering that the PTzBI-S-based devices are more efficient at generating charge carriers than the PTzBI-Ph-based devices, but less efficient at actually delivering charge to the electrode, it stands to reason that the two congeneric devices exhibit comparable JSC values. To assess the charge recombination behavior of these devices, we measured the variation of JSC as a function of illumination intensity (Plight) from 5 to 100 mW cm−2. The power law dependence of JSC upon Plight is expressed as JSC ∝ (Plight)S, where S is close to unity as a result of weak bimolecular recombination during sweep-out.41 From the slopes of the fitted lines of the JSC−Plight characteristics (Figure 3c,d), one can obtain the S values of 0.983, 0.987, 0.935, and 0.964 for the devices based on PTzBI-S:PC71BM, PTzBIPh:PC71BM, PTzBI-S:ITIC, and PTzBI-Ph:ITIC, respectively.

The PC71BM-based devices show slightly higher S values (i.e., closer to unity) than the ITIC-based devices, demonstrating the weaker bimolecular recombination of the former. These findings agree with the higher FF values of the PC71BM-based devices than those of the ITIC-based devices (Table 2). To investigate the effect of the side chain on the charge transport ability, we measured the hole and electron mobility of these blend films using the space-charge-limited current model, and the hole- and electron-only devices had the structures of ITO/PEDOT:PSS/active layer/MoO3/Al and ITO/ZnO/active layer/PFNDI-Br/Al, respectively. The hole/ electron mobilities of PTzBI-S:ITIC and PTzBI-Ph:ITIC were calculated to be 2.39 × 10−5/2.04 × 10−4 cm2 V−1 s−1 and 3.55 × 10−5/4.53 × 10−4 cm2 V−1 s−1, respectively (see Figure S7 and Table S6, Supporting Information). This indicates that the side-chain alkylthiol-thienyl or alkylphenyl substituents attached to the BDT unit have a negligible effect on charge carrier mobility in the polymer:ITIC blend films. 2.5. VOC Loss Analysis. It is noted that even though devices based on polymer:PC71BM blend films exhibited obviously different absorption onsets from the polymer:ITIC blend films (Figure 1b), the obtained VOC values of such two sets of devices are quite similar (0.91−0.95 V, see Table 2). Therefore, in order to compare the VOC losses with respect to the band gap (Eg), we determined Eg using the edge of EQE spectra, which is the intersection of the extrapolated absorption edge and the EQE isoline passing through the local EQE maximum at the edge of the spectrum (Figure S8, Supporting Information) according to the method proposed by Nikolis et al.42 Note that this method gives higher but more accurate band gap values than those determined using the onset of UV−vis absorption spectra and can be used to specify the energy loss according to the detailed balance limit and the reciprocity relation.43,44 The band gaps (Eg’s) of the PTzBI-S and PBTzBI-Ph-based fullerene cells were determined to be 1.95 and 1.98 eV, respectively, and those for the ITIC systems E


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Figure 5. AFM height images (1.5 × 1.5 μm2) and TEM images for devices based on PTzBI-S:PC71BM (a,e), PTzBI-Ph:PC71BM (b,f), PTzBIS:ITIC (c,g), and PTzBI-Ph:ITIC (d,h).

Figure 6. Two-dimensional GIWAXS patterns (a−d) and scattering profiles of IP and OOP directions for blend films (e).

The VOC losses with respect to the CT states energy can be further divided into radiative and nonradiative losses.47 According to the reciprocity theory, the highest VOC can be achieved when the recombination is only radiative.44 The limit of the radiative VOC (VOC,rad) is therefore calculated by combining the FTPS and EL data as shown in Figure 4. The nonradiative VOC loss (ΔVOC,nr) is then the difference between VOC,rad and the VOC experimentally measured under AM 1.5G solar irradiation (VOC,meas).48−50 The VOC,rad values for PTzBIS and PTzBI-Ph-based fullerene cells were both 1.32 V, and those for the ITIC systems were 1.30 and 1.28 V, respectively. The systems based on fullerenes have slightly higher ΔVOC,nr compared to their counterparts based on ITIC, which is consistent with the decreased CT state energy when the energy gap law is considered.51 It is worthy of note that modification of the substituents of the BDT unit did not significantly change the VOC losses (Table S7, Supporting Information). 2.6. Film Morphology. To correlate the blend film morphology with the photovoltaic performance of our devices, we investigated the active layer morphology by tapping-mode atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM images of all of the blend films exhibit rough morphology with root-mean-square roughness ranging from 1.4 to about 3.0 nm, wherein one can clearly

were both 1.70 eV. The calculated results imply that the overall VOC loss with respect to Eg/q was approximately 1.03 and 0.78 V for the fullerene and ITIC cells, respectively. The charge-transfer (CT) state energy (E CT ) was determined by fitting the high-energy tail of the CT emission [N(E)] in electroluminescence (EL) spectra using eq 1 and low-energy tail of CT absorption [A(E)] in Fourier transform photocurrent spectroscopy (FTPS) using eq 2.45 ÅÄÅ ÑÉ Å (E − λ − E) ÑÑÑ ÑÑ N (E) ∝ E expÅÅÅÅ− CT ÑÑ ÅÅÇ 4λkBT (1) ÑÖ ÄÅ É ÅÅ (ECT + λ − E) ÑÑÑ 1 Å Å ÑÑÑ A(E) ∝ expÅÅ− ÑÑ ÅÅÇ E 4λkBT ÑÖ

(2)

Here, kB is the Boltzmann constant, T is the temperature, and λ represents the reorganization energy, which is related to nuclear deformations within the donor−acceptor complex and surroundings.46 As shown in Figure S8, the ECT of the fullerene-based cells is 1.49 eV and those for the ITIC systems are both around 1.55 eV. In addition, the reorganization energy of ITIC systems (∼0.17 eV) is significantly lower than that of the fullerene-based systems (∼0.27 eV). F


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ACS Applied Materials & Interfaces discern large agglomerations for the PTzBI-S:PC71BM and PTzBI-Ph:PC71BM blend films (Figure 5a,b). In contrast, the polymer:ITIC blend films exhibit more uniform film morphology associated with less pronounced aggregation (Figure 5c,d). Moreover, in the PC71BM-based devices, one can clearly observe a filamentous fibril texture, localized in the bright spots that are dotted throughout the blend films (Figure 5e,f). However, in the polymer:ITIC blend films, fibrous structures are distributed across the entire film, without large aggregates (Figure 5g,h). Grazing incidence wide-angle X-ray scattering (GIWAXS) was used to study the molecular orientation and crystallinity of the blend films (Figure 6). The blend films of PTzBIS:PC71BM and PTzBI-Ph:PC71BM exhibited distinct (010) π−π stacking peaks with q values of 1.66 Å−1 (d = 3.78 Å) in the out-of-plane (OOP) direction (Figure 6a,b). This observation, together with the (100) in-plane (IP) stacking direction, demonstrated the preferential “face-on” orientation of polymer chains in the blend films with regard to the substrate. The blend films based on PTzBI-S:ITIC and PTzBIPh:ITIC also exhibited distinct π−π stacking peaks (q = 1.66 Å−1) in the OOP direction (Figure 6c,d), again demonstrating the face-on orientation. Note that this orientation is favorable for charge transport between the two electrodes.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.Y.). *E-mail: [email protected] (F.H.). ORCID

Xiaoyan Du: 0000-0002-7614-1673 Ning Li: 0000-0003-1208-4638 Lei Ying: 0000-0003-1137-2355 Fei Huang: 0000-0001-9665-6642 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology of China (no. 2014CB643501) and the National Natural Science Foundation of China (nos. 91633301, 51673069, 51521002, and 21520102006) and the Science and Technology Program of Guangzhou, China (nos. 201710010021, 201707020019, and 2017A050503002). N.L. gratefully acknowledges the financial support from the DFG research grant: BR 4031/13-1 and the Bavarian Ministry of Economic Affairs and Media, Energy and Technology by funding the HI-ERN (IEK11) of FZ Jülich. C.J.B. gratefully acknowledges the financial support through the “Aufbruch Bayern” initiative of the state of Bavaria (EnCN and “Solar Factory of the Future”), the Bavarian Initiative “Solar Technologies go Hybrid” (SolTech), and the SFB 953 (DFG).

3. CONCLUSIONS In summary, we developed two novel donor−acceptor-type WBG conjugated copolymers based on an electron-deficient imide-functionalized benzotriazole unit and an alkylthiothienyl or alkylphenyl moiety as the substituent of the electron-donating BDT unit. The modification of the substituents of the resulting copolymers did not significantly change the optical properties, frontier molecular orbitals, or loss of open-circuit voltage, whereas the resulting copolymer PTzBI-Ph based on the alkylphenyl substituent exhibited enhanced hole mobility and higher crystallinity than the counterpart copolymer PTzBI-S. Benefitting from the relatively deep HOMO energy levels of about −5.4 eV, the resulting PSCs exhibited relatively high VOC of about 0.9 V. An impressively high PCE of 10.19% was observed for the devices based on PTzBI-Ph. These observations indicate that the development of TzBI-based WBG copolymers via side-chain modification is a promising strategy for the fabrication of highperformance PSCs.

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photovoltaic parameters of CB-processed devices based on PTzBI-Ph:ITIC incorporating different solvent additives, hole- and electron-only characteristics of devices, relevant parameters obtained from Jph−Veff and J1/2−V (SCLC) characteristics of devices, real EQE used to calibrate FTPS, FTPS and EL spectra with their corresponding fit to determine the energy of CT states and reorganization energy for blend films, and calculated parameters for VOC losses and ΔVOC (PDF)

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REFERENCES

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05700. Fabrication of PSCs, instruments and characterization, synthesis of monomers and polymers, cyclic voltammograms of PTzBI-S and PTzBI-Ph, J−V characteristics and photovoltaic parameters of devices based on PTzBIS:ITIC with different donor:acceptor ratios, J−V characteristics and photovoltaic parameters of devices based on PTzBI-S:ITIC devices based on PTzBI-S:ITIC processed by different solvents, J−V characteristics of CB-processed devices and photovoltaic parameters of devices based on PTzBI-S:ITIC incorporating different contents of DBE, J−V characteristics and photovoltaic parameters of devices based on PTzBI-Ph:ITIC processed by different solvents, J−V characteristics and G


Research Article

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Improved Efficiency of Polymer Solar Cells by Modifying the Side

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