Impact of the Siloxane-Terminated Side Chain on Photovoltaic

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Impact of Siloxane-Terminated Side Chain on Photovoltaic Performances of Dithienylbenzodithiophene-Difluorobenzotriazole Based Wide Bandgap Polymer Donor in Non-fullerene Polymer Solar Cells Haiying Jiang, Feilong Pan, Lianjie Zhang, Xiaobo Zhou, Zhen Wang, Yaowen Nian, Cang Liu, Wei Tang, Qiao Ma, Zhenyu Ni, Mingjun Chen, Wei Ma, Yong Cao, and Junwu Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08722 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 24, 2019

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Impact of Siloxane-Terminated Side Chain on Photovoltaic Performances of Dithienylbenzodithiophene-Difluorobenzotriazole Based Wide Bandgap Polymer Donor in Non-fullerene Polymer Solar Cells Haiying Jiang,†,‖ Feilong Pan,†,‖ Lianjie Zhang,*,† Xiaobo Zhou,‡ Zhen Wang,† Yaowen Nian,† Cang Liu,† Wei Tang,† Qiao Ma,† Zhenyu Ni,† Mingjun Chen,† Wei Ma,*,‡ Yong Cao,† and Junwu Chen*,† †

Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent

Materials & Devices, South China University of Technology, Guangzhou 510640, P. R. China ‡

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an

710049, P.R. China ABSTRACT: To thoroughly disclose the role of siloxane-terminated side chain with different substituent positions, three difluorobenzotriazole-dithienylbenzodithiophene (FTAZ-BDTT) based polymers PBZ-1Si, PBZ-2Si and PBZ-3Si with the siloxane-terminated side chain on the FTAZ unit (PBZ-1Si), on the BDTT unit (PBZ-2Si), and both on BDTT and FTAZ unit (PBZ-3Si), respectively, were synthesized. The different side chain substitutions have slight influences on absorption behavior, thermal stability, and frontier molecular orbitals, but showing great effect on the aggregation of the polymers. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements reveal that, relative to PBZ-1Si with branched alkyl on the BDTT unit, polymers PBZ-2Si and PBZ-3Si, bearing the siloxane-terminated side chains on the BDTT unit, exhibit smaller π-π stacking distances and larger crystal coherence lengths (CL), suggesting that adopting the siloxane-terminated side chain on the BDTT unit can promote the interchain π-π interaction and the ordering of molecular packing. With ITM as the non-fullerene acceptor, among the three polymers, the PBZ-2Si based active layer possesses 1


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the highest ordered crystals for both polymer and IT-M as well as the purest domain, which affords efficient exciton dissociation, the most balanced hole-electron transport, and reduced recombination, leading to the highest short-circuit current density (Jsc) and fill factor (FF) and then the highest power conversion efficiency (PCE) of 11.14%. In contrast, PBZ-1Si and PBZ-3Si based devices show lower PCEs of 8.98% and 9.92%, respectively. Moreover, PBZ-2Si:IT-M also exhibits good thickness tolerance and its thick active layer of 240 nm shows the most limited decreasing of efficiency after a storage for 77 days, supplying good potential for mass farbication. Our work suggests that the fine pairing of a siloxane-terminated side chain and an alkyl side chain is benifical for the opitmizing of a conjugated polymer donor towards high performance non-fullerene polymer solar cells. Keywords: polymer solar cell; wide band gap polymer; side chain engineering; siloxane-terminated side chain; molecular packing

1. Introduction Bulk heterojunction (BHJ) polymer solar cells (PSCs) have been considered as the promising candidate for the green energy technology due to their unique advantages of lightweight, flexibility, short payback time and capability of roll-to-roll high speed printing technique.1-5 The active layer of PSCs usually comprises a p-type conjugated polymer as the electron donor and an n-type organic semiconductor as the electron acceptor. In the past few years, acceptor-donor-acceptor (A-D-A) type small molecular non-fullerene acceptors (NFAs) have emerged as a significant progress owing to their excellent absorption properties, tunable molecular properties, morphology stability and low cost.6-8 The power conversion efficiencies (PCEs) of non-fullerene based PSCs have exceeded 15% for single 2



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junction devices9-12 and 17% for tandem devices13. Specifically, A-D-A type NFAs with narrow-band gaps (NBG, Eg1.8 eV) polymer with complementary absorption can be employed as the donor to compose the active layer, which is beneficial for the improvement of short circuit current density (Jsc).26-38 Recently, WBG (D-A) copolymers consisting of the two-dimensional (2D) dithienylbenzodithiophene (BDTT) as the D unit and difluorobenzotriazole (FTAZ) as the A unit have been demonstrated huge potential to construct high-efficiency non-fullerene PSCs due to their excellent charge transport ability and strong absorption in the short wavelength region.39-41 Li and co-workers reported a series of FTAZ-BDTT based polymers (J-series polymers).42-48 They modified the polymers via side chain engineering with mainly focus on the BDTT unit, and obtained high efficiency beyond 12%. For example, the introduction of linear alkylthio side chain on the BDTT unit yielded J61 polymer with an improved efficiency compared to J52 with alkyl side chain on the BDTT unit. Thus, the side-chain of the polymer should be carefully designed to achieve the optimum photovoltaic performance. Most of the FTAZ-BDTT based polymer adopt second-position branched aliphatic side chain as the solubilizing group to guarantee the polymer solubility for solution process. However, the second-position branched aliphatic side chain may impede the further intermolecular π-π interaction between the polymer backbones due to steric hindrance effect.31, 49 So far, attaching far-branched side chains on the FTAZ-BDTT based polymers to enhance the backbone π-π interaction have not been reported. As a matter of fact, siloxane-terminated side chain is a unique solubilizing side chain, which 3


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supplies a far-branched point away from conjugated backbone and the bulky siloxane terminated group with steric hindrance to provide the polymer with enough solubility. Bao and co-workers introduced the siloxane-terminated side chain to the isoindigo-based polymer, which decreased π-π stacking distance and hence improved the lateral mobility.50 In our previous reports, a series of PFBT-4T and N2200 analogues bearing different content of siloxane-terminated side chains were synthesized as well.51-52 The content of the siloxane-terminated side chains has great impact on the properties of polymer donor and polymer acceptor, including the energy level, solubility, crystallinity and crystal orientation. Fan et al. introduced the siloxane-terminated side chain to the imide-functionalized benzotriazole based polymer yielding PTzBI-Si and a PCE of over 10% for the all-polymer solar cells can be achieved.53-54 These reports demonstrate that the far-branched siloxane-terminated side chain has great potential to improve the photovoltaic performance of a conjugated polymer in PSCs. Si O Si O Si

Si O Si O Si

S N

N

S

N S

F

F

PBZ-1Si

N

S

S S

Si O Si O Si

Si O Si O Si

S

N S

S F

N

S

S n

S

N

F

N

N

S

S n

S

S

S F

F

n S

PBZ-3Si

PBZ-2Si Si O Si O Si

Si O Si O Si

Chart 1. The chemical structures of polymers PBZ-1Si, PBZ-2Si and PBZ-3Si comprising one, two, and three siloxane-terminated side chains, respectively. Herein, we systematically attached siloxane-terminated side chains on the FTAZ-BDTT based polymers so as to investigate the impact of siloxane-terminated side chains on the photovoltaic performance. As shown in Chart 1, polymer PBZ-1Si comprises one siloxane-terminated side chain 4



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on the FTAZ unit while PBZ-2Si comprises two siloxane-terminated side chains on the BDTT unit. For PBZ-3Si, all the alkyl side chains on the FTAZ and BDTT units have been replaced by three siloxane-terminated side chains. It was found that the siloxane-terminated side chain could significantly influence the polymer aggregation in solution state, polymer packing in solid state, and the interaction between polymer donor and non-fullerene acceptor in the BHJ blend film. When IT-M was employed as the acceptor, PBZ-2Si with the siloxane-terminated side chain on BDTT unit showed the highest PCE of 11.14% owing to the balanced hole and electron mobility, efficient exciton dissociation, and the favorable morphology. Relatively, the PBZ-1Si and PBZ-3Si based devices displayed lower PCEs of 8.98% and 9.92%, respectively. Moreover, it is worth noting that the PSCs based on the PBZ-2Si:IT-M active layer also exhibited good thickness tolerance and excellent shelf stability. Our study suggests that judicious introducing of siloxane-terminated side chains on the polymer backbone is crucial, which has strong impact on its photovoltaic performance.

5


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R1

R2

R2

S

S

S

(a) i)

S

S

S

ii)

R1

S

iii)

S

R1

N

Br

Br F

H 2N

v)

NH2

S

R2

R2

Br

F

F

4

HN

Br

N

viii)

N

Br

F

N

N

vii) Br

F R2 N

N

ix)

N

S

Br

R1=

F

7 N

x) Br

F

9

8

N

F

S F

F

R1 N

Br

F

6

Br F

N

Br

5 R2 N

BDT-Th-C6Si-Sn

3

vi)

R2 N

Si O Si O Si

R2=

N

S

S F

Sn

S

S

2

1 S

Sn

S S

N

S

iv)

F

Br

FTAZ-C6Si-Br

C8H17

(b)

C8H17 C6H13 N

Br

N

S

Sn S

F

S

N

F

(c)

C6H13

S

BDT-Th-HD-Sn

FTAZ-HD-Br

+

BDT-Th-C6Si-Sn

FTAZ-C6Si-Br

+

BDT-Th-C6Si-Sn

S

C6H13 C8H17

FTAZ-HD-Br

+

xi)

PBZ-1Si

Sn

S

Br

FTAZ-C6Si-Br

xi) xi)

PBZ-2Si

PBZ-3Si

BDT-Th-HD-Sn

Scheme 1. (a) Synthesis routes of monomers BDT-Th-C6Si-Sn and FTAZ-C6Si-Br. (b) The chemical structures of monomers FTAZ-HD-Br and BDT-Th-HD-Sn. (c) The polymerizations for polymers PBZ-1Si, PBZ-2Si, and PBZ-3Si. Conditions: i) n-BuLi, 6-bromo-1-hexene, THF, -78 ºC; ii) n-BuLi, benzo[1,2-b:4,5-b’]dithiophene-4,8-dione, SnCl2∙2H2O, HCl, 0 ºC; iii) 1,1,1,3,5,5,5-heptamethyltrisiloxane, Karstedt catalyst, THF, 50 ºC; iv) n-BuLi, SnMe3Cl, THF, −78 ºC; v) NaBH4, EtOH, 0 ºC; vi) NaNO2, HAc, 0 ºC; vii) DIAD, 5-hexen-1-ol, THF, 0 ºC; viii) 1,1,1,3,5,5,5-heptamethyltrisiloxane, Karstedt catalyst, THF, 50 ºC; ix) 2-(tributylstannyl)thiophene, Pd(PPh3)4, o-xylene, 120 ºC; x) NBS, CF, DMF, 0 ºC; xi) Pd2(dba)3, P(o-tol)3, o-xylene, 120 ºC.

2.Results and Discussion 2.1 Synthesis and Polymer characterization The

detailed

synthesis

routes

of

4,8-bis(5-(6-(1,1,1,3,5,5,5-heptamethyltrisiloxan-36



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yl)hexyl)thiophen-2-yl) benzo [1,2-b:4,5-b'] dithiophene (BDT-Th-C6Si-Sn) and 4,7-bis(5bromothiophen-2-yl)-5,6-difluoro-2-(6-(1,1,1,3,5,5,5-heptamethyltri-siloxan-3-yl)hexyl)benzo[d][1,2,3]triazole (FTAZ-C6Si-Br), two key monomers with the siloxane-terminated side chains, are shown in Scheme 1a. The BDT-Th-C6Si-Sn was obtained from thiophene in four steps. Compound 1 was synthesized via the lithiation of thiophene and then alkylation with the 6-bromohex-1-ene. Compound 2 was obtained by lithiation of 1, coupled with benzo[1,2-b:4,5-b']dithiophene-4,8-dione, and finally reduction with the SnCl2. After which, treating compound 2 with 1,1,1,3,5,5,5heptamethyltrisiloxane in the presence of the Karstedt catalyst, intermediate 3 with the siloxaneterminated alkyl side chain was obtained. Finally, compound 3 was transferred to the target distannyl monomer BDT-ThC6Si-Sn by lithiation and the subsequent reaction with chlorotrimethyltin. Another monomer FTAZ-C6Si-Br with the siloxane-terminated side-chain was synthesized with 4,7-dibromo5,6-difluoro[2,1,3]benzothiadiazole 4 as the starting reagent, which included converting to diamine compound 5 with NaBH4, cyclization to compound 6 with NaNO2, alkylation to 7 using 5-hexen-1-ol, transformation to 8 with the siloxane-terminated side in the presence of Karstedt catalyst, generation of compound 9 by Stille-coupling, and eventually the bromination reaction. The chemical structures of FTAZ-HD-Br and BDT-Th-HD-Sn, two monomers with the 2-hexyldodecyl (HD) side chains, are shown in Scheme 1b. The polymers PBZ-1Si, PBZ-2Si, and PBZ-3Si were prepared via Stille copolymerization using Pd2(dba)3 as catalyst and P(o-tol)3 as ligand (Scheme 1c). The crude polymers were purified by sequential Soxhlet extraction with methanol, acetone, hexane, ethyl acetate, dichloromethane, and chloroform (CF). Finally, the CF fraction was precipitated in methanol and collected. 7


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Molecular weights of the polymers were measured by high temperature gel permeation chromatography (GPC) at 150 C using 1,2,4-trichlorobenezene as eluent. The number-average molecular weights (Mn) of PBZ-1Si, PBZ-2Si, and PBZ-3Si are 105.2, 87.9, and 65.1 kg/mol with corresponding polydispersity index (PDI) of 1.82, 2.94, and 2.71, respectively. The degrees of polymerization for PBZ-1Si, PBZ-2Si, and PBZ-3Si are about 74, 58, and 41, respectively, calculated from the Mn and molecular weight of repeated unit for each polymer. All the polymers show good solubility in common solvents, including CF, chlorobenzene (CB) and o-dichlorobenzene (o-DCB). Additionally, the Mn of PBZ-1Si, PBZ-2Si, and PBZ-3Si are much higher than that (6.9757.5 kg/mol) of previous FTAZ-BDTT polymer with common alkyl side chains45, suggesting that using siloxaneterminated side chain can simultaneously maintain good solubility and high molecular weight. In general, higher Mn polymers are desired to construct a more effective pathway for charge transports and then obtain better device performance, mainly ascribed to the enhanced aggregation of polymer chains, the increased crystallization, and the improved face-on orientation of polymer crystallites.49, 55 The thermal stability of the polymers was investigated by thermogravimetric analysis (TGA). The TGA plots are shown in Figure S1. Polymers PBZ-1Si, PBZ-2Si, and PBZ-3Si all exhibited good thermal stability with high decomposition temperature (Td) for 5% weight loss of 421, 424, and 425 C, respectively. The thermal stabilities of the polymers are high enough for their application in PSCs.

8



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Figure 1. (a) Normalized UV-vis absorption spectra of PBZ-1Si, PBZ-2Si, PBZ-3Si, and IT-M pristine films. (b) Energy level diagrams of PBZ-1Si, PBZ-2Si, PBZ-3Si, and IT-M. 2.2 Photophysical Property, Energy Level, and Polymer Crystallinity The UV-vis absorption spectra of thin films of the three polymers and IT-M are shown in Figure 1a. The polymer films exhibit almost comparable absorption 0-0 and 0-1 peaks at 595 and 550 nm, respectively, with small red-shifts of ca.10 nm if compared with the solutions (Figure S2). The results suggest that the polymers possess fairly strong aggregations in the solution and the solid state. The absorption edges of the polymers are at 640 nm, corresponding to an optical bandgap of 1.94 eV. Thus the attachments of the different siloxane-terminated side chains on the polymers have little effect on the absorption property. Relatively, the IT-M acceptor shows a broad absorption in the longer wavelengths, from which complementary absorptions till the near IR region can be established between the polymers and IT-M, as reflected by their blend films (Figure S3). Cyclic voltammetry (CV) measurements were performed to evaluate the highest occupied molecular orbital (HOMO) level of the polymers with an Ag/Ag+ electrode as the reference electrode and ferrocene as the internal standard (Figure S4). The HOMO levels of PBZ-1Si, PBZ-2Si, and PBZ3Si are of 5.29, 5.26, and 5.21 eV, respectively. The lowest unoccupied molecular orbital (LUMO) 9


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level was calculated from the HOMO level and the corresponding optical bandgap. The LUMO levels of PBZ-1Si, PBZ-2Si and PBZ-3Si are 3.35, 3.32, and 3.27 eV, respectively (Figure 1b). Therefore, as increasing number of the siloxane-terminated side chain in the repeat unit, the HOMO and LUMO levels of the polymers were elevated in the meantime. Similar results have been reported before.52 The HOMO and LUMO levels are of 5.58, and 3.98 eV, respectively, which can match those of the PBZ polymers with sufficient driving forces for the charge dissociations.

Figure 2. Temperature-dependent UV-vis absorption spectra of (a) PBZ-1Si, (b) PBZ-2Si, and (c) PBZ-3Si in chlorobenzene (CB). Temperature dependence absorption spectra of the dilute solutions of the three polymers were performed to evaluate their de-aggregation ability upon heating from 25 to 105 C (Figure 2). As increasing of the temperature, the positions of the 0-0 and 0-1 peaks of the polymers show very little blue-shifts and the intensities of the 0-0 peaks decrease more obviously if compared with the corresponding 0-1 peaks. At the 105 C, the 0-0 peaks for PBZ-1Si and PBZ-3Si can be higher than the 0-1 peaks with 0-0/0-1 intensity ratios of 1.06 and 1.02, respectively, but that for PBZ-2Si can be slightly lower than the 0-1 peak with 0-0/0-1 intensity ratio of 0.99. Therefore, the three polymers exhibit the de-aggregation ability in an order of PBZ-2Si  PBZ-3Si  PBZ-1Si, demonstrating that 10



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using the siloxane terminated side chain to replace the HD alkyl side chain shows some complexities in the contribution to the de-aggregation ability upon the heating. In a previous report, higher content of the siloxane terminated side chain relative to the 2-dodecyltetradecyl (DT) side chain results in slower de-aggregation of the polymer.52

Figure 3. (a) 2D-GIWAXS patterns of neat films for PBZ-1Si, PBZ-2Si, and PBZ-3Si. The white line from top to down of the pattern is related to the existed gap of different detectors used in the measurement. (b) Line-cut profiles along in-plane and out-of-plane direction for corresponding films. (c) Lamellar diastance and π-π distance of polymers. 11


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Further insight into the polymer aggregation in solid state, grazing-incident wide angle X-ray scattering (GIWAXS) was performed. The 2D patterns are shown in Figure 3a and the corresponding line-cut profiles are depicted in Figure 3b. All polymers exhibit distinct (010) diffraction peaks in the out-of-plane (OOP) direction and (100) diffraction peaks in the in-plane (IP) direction, indicating faceon orientations in the pristine polymer films. Based on the (100) and (010) peak positions, the lamellar d-spacing and π-π distances of the polymers can be caculated (Table S1). The results are also shown in Figure 3c for a better comparision. The higher content of siloxane terminated side chain in the PBZ polymer gives the longer lamellar d-spacing (from 27.13 to 29.22 and then 30.04 Å) but shorter π-π distance (from 3.68 to 3.62 and then 3.61 Å). As such, both of the substitutions of the siloxaneterminated side chain to the HD side chain on the FTAZ and BDTT units can weaken the lamellar interactions but enhance the π-π interactions of the polymers in the meantime. Generally, the increased π-π interaction is benefical for the carrier transport, however, it may greatly deteriorate the solubility. In a previous report, Yan et al. found that the varying of branching from 1- to 2- and then 3- position for an alkyl side chain could significantly enhance the π-π interaction, and the alkyl side chain with 3position branching gave an insoluble FBT-4T polymer.49 Delightly, the weakened the lamellar interaction from siloxane-terminated side chain can supply a chance to compensate the solubility, giving enough solubility for the PBZ-1Si, PBZ-2Si, and PBZ-3Si. It should be noted that decreasing of Mn was found for our polymers with more siloxane-terminated side chains, also demonstrating the decreased solubility tendency with more siloxane-terminated side chains. This may a reason for the complexities of the de-aggregation ability upon the heating process as shown in Figure 2. On the other hand, according to the Scherrer equation, the crystalline coherence length (CL) of 12



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(100) peak in the IP direction and 010 peak in the OOP direction are also listed in Table S1. The complete line-cut profile and the Gaussian peak fitting of the (100) and (010) peaks are shown in Figure S5. The CL values of (100) peaks for PBZ-1Si, PBZ-2Si, and PBZ-3Si are of 105.5, 126.6, and 153.7 Å, respectively, while the related CL values of (010) peak of 21.6, 23.9, and 24.5 Å, respectively. Overall, higher content of the siloxane-terminated side chain can help the ordering of the polymer’s face-on packing. 2.3 Device Characteristics To investigate the impact of the siloxane-terminated side chain on the photovoltaic performance of the PBZ polymers, PSCs with an inverted device structure of ITO/ZnO/PFN-Br/active layer (100 nm)/MoO3/Al were fabricated, and the corresponding fabrication process and the full name of materials used can be found in Supporting Information. The measurements of the PSCs were carried out in air under AM1.5G illumination at 100 mW/cm2. We compared the effect of blend ratios of PBZ polymer:IT-M (D:A) on the photovoltaic performances of as-cast blend films, which showed a D:A ratio of 1:1 was suitable for PBZ-1Si and PBZ-3Si while that of 1:1.3 was optimal for PBZ-2Si (Table S2). The best PCEs of 6.79%, 10.40%, and 9.11% were found for PBZ-1Si, PBZ-2Si, and PBZ-3Si based as-cast devices, respectively, demonstrating that PBZ-2Si was more powerful relative to PBZ-1Si or PBZ-3Si. Solvent additives are often applied to optimize the photovoltaic performances of PSCs. With 0.5% diphenyl ether (DPE) as solvent additive, the PCEs of PBZ-1Si and PBZ-3Si could be elevated to 8.98% and 9.92% (Table 1), respectively. The PCE of 8.98% for PBZ-1Si is based on open-circuit voltage (Voc) of 0.94 V, Jsc of 14.41 mA/cm2, and fill factor (FF) of 65.93%. The PBZ-3Si based device shows slightly lower Voc of 13


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0.92 V, but with obviously higher Jsc and FF of 15.86 mA/cm2 and FF of 68.00%, respectively. Generally, the optimized efficiencies for PBZ-1Si and PBZ-3Si are still lower than the PBZ-2Si based as-cast device. Then we paid more efforts to optimize the photovoltaic performance of PBZ-2Si (Table S3). The optimization of DPE contents from 0.5% to 2% demonstrated that only a 0.5% content could show a limited improved PCE to 10.67% and higher DPE contents of 1% and 2% gave the adverse effect. Relative to the as-cast device, small amounts of DPE as solvent additive can contribute the active layers with slightly higher Jsc and FF. With N-methyl pyrrolidone (NMP) as the solvent additive, lower device performances relative to the as-cast device were found. However, the PBZ-2Si based active layer with 0.5% NMP could contribute the device with a modified Jsc of 17.10 mA/cm2, showing some potential of NMP as the solvent additive. Therefore, we tried to use a binary solvent additive based on the DPE and NMP (Table S3). With 0.25% DPE plus 0.25% NMP, the PSC gave a comparable PCE of 10.36% if compared with the as-cast device. Delightedly, a higher content of 0.5% DPE plus 0.5% NMP could boost the efficiency to 11.14%, based on Voc of 0.88 V, Jsc of 17.69 mA/cm2, and FF of 71.55% (Table 1). The averaged PCE based on over 10 devices is 10.88%. It should be noted that reports of enhanced photovoltaic performances by binary processing additives are extremely rare in non-fullerene PSCs56-57. The current density-voltage (J-V) curves for the three optimized PSCs are shown in Figure 4a. The variations of the side chains for the three PBZ polymers can result in quite different photovoltaic performances. The highest PCE achieved by the PBZ-2Si with two siloxaneterminated side chains suggests that the PBZ-2Si:IT-M active layer may possess the best morphology for exciton dissociation and carrier transports. Table 1. The optimized photovoltaic parameters of the PSCs for the three PBZ polymers 14



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Polymer

Conditions

Voc (V)

Jsc (mA/cm2)

FF (%)

PCEa) (%)

PBZ-1Si

0.5% DPE

0.94

14.41 (13.53±0.62)

65.93 (62.15±2.39)

8.98 (7.94±0.64)

PBZ-2Si

0.5% DPE+0.5% NMP

0.88

17.69 (17.50±0.20)

71.55 (71.01±0.63)

11.14 (10.88±0.22)

PBZ-3Si

0.5% DPE

0.92

15.86 (15.56±0.51)

68.00 (67.69±1.93)

9.92 (9.70±0.12)

a)

Average values and the standard deviation are in the parentheses obtained from over ten devices.

Figure 4. (a) J-V curves for the optimized devices. (b) EQE spectra of the optimized PBZ-1Si:IT-M, PBZ-2Si:IT-M and PBZ-3Si:IT-M devices. The external quantum efficiency (EQE) spectra of the optimized devices are presented in Figure 4b. All devices cover almost identical photo-response ranges from 300 to 800 nm, which match well with corresponding absorption spectra. The maximum EQE values of 71%, 81%, and 75% were obtained for the PBZ-1Si, PBZ-2Si and PBZ-3Si based devices, respectively. The PBZ-2Si:IT-M active layer shows the highest EQE response, contributing to the highest Jsc. The integrated current densities are of 14.35, 16.81 and 15.72 mA/cm2, respectively, which all agree well with those values obtained from the J-V curves within 5% mismatch.

15


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Figure 5. (a) Jph versus Veff curves of the optimized devices. (b) Jsc versus the light intensity. (c) SCLC hole and electron mobility of the blend films. 2.4 Exciton Dissociation and Charge Recombination Study To gain deep insight into the exciton dissociation and charge collection porperties, the relationship between the effective photocurrent (Jph) and the effective applied voltage (Veff) were conducted, as 16



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shown in Figure 5a. The exciton dissociation probability (P(E,T)) can be estimated from the value of Jph/Jsat (here, Jph is the current under the short-circuit condition). Jph is defined by JL – JD (JL and JD are light and dark current densities); Jsat is where the Jph reaches its saturation at high revese voltage, which means all the excitons are dissociated to free charge carries and collected by the electrodes. Veff can be gained by subtracting the applied voltage from the voltage where Jph is zero. Under the short circuit condition, the P(E,T) values of 81.6%, 94.2%, and 92.6% can be obtained for PBZ-1Si, PBZ2Si, and PBZ-3Si based devices, respectively. The result suggests the highest exciton dissociation and charge collection efficiency in PBZ-2Si-based devices, which agrees well with its highest Jsc among the three BHJ blends. The J-V curves under different light intensity were measured to investigate the charge recombination mechanism of the devices. The Jsc versus light intensity curves for the optimal devices are described in Figure 5b. In general, the relation between the light intensity (Plight) and Jsc agrees with the formula of Jsc  PlightS. The exponential factor (S) of PBZ-1Si, PBZ-2Si and PBZ-3Si based devices are 0.95, 0.95, and 0.93, respectively. Therefore, relative to PBZ-3Si, the bimolecular recombination can be suppressed more effectively in PBZ-1Si or PBZ-2Si based device. Subsequently, the

space

charge-limited

current

(SCLC)

measurements

with

the

hole-only

devices

(ITO/PEDOT:PSS/Active layer/MoO3/Al) and electron-only devices (ITO/ZnO/Active layer/PFN/Al) were conducted to investigate the charge transport properties of the PSCs. The J1/2-V characteristics of hole-only and electron-only devices are shown in Figure S6. The hole mobility (μh) and electron mobility (μe) values of the blend films are depicted in Figure 5c. The PBZ-1Si:IT-M blend film possesses μh and μe of 2.33 × 104 and 4.05 × 105 cm2 V1 s1, respectively, leading to a very high μh 17


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/μe ratio of 5.75. Relatively, more balanced hole and electron transports could be achieved with PBZ2Si and PBZ-3Si based blend films. The slightly lower μh of 1.28 × 104 and the highest μe 8.62 × 105 cm2 V1 s1 for the PBZ-2Si:IT-M blend can afford the most balanced carrier transports (μh /μe = 1.48). The PBZ-3Si:IT-M blend film displays the lowest μh and μe of 1.04 × 104 and 2.36×105 cm2 V1 s1, giving a μh /μe ratio of 4.41. Consequently, the balanced hole and electron transports and the suppressed bimolecular recombination can be achieved with the PBZ-2Si:IT-M active layer, resulting in the superior FF of 71.55%.

2.5 Morphology Moreover, atomic force microscopy (AFM) was performed to investigate the surface morphology of the optimized blend films. As shown in Figure S7, the three blend films showed similar surface morphology. The root-mean-square (RMS) roughness values were obtained from the height images, corresponding to 1.07, 0.96, and 1.31 nm for the PBZ-1Si:IT-M, PBZ-2Si:IT-M and PBZ-3Si:IT-M blends, respectively. The three blend films all possess smooth surface morphology and similar phase seperation size. Thus the surface morphology may not be a curical factor that leads to the different device performances of the three blends.

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Figure 6. (a) 2D GIWAXS patterns for PBZ-1Si:IT-M PBZ-2Si:IT-M and PBZ-3Si:IT-M blend film under optimized condition. The white line from top to down of the pattern is related to the existed gap of different detectors used in the measurement. (b) Line-cut profiles along in-plane and out-of-plane direction for corresponding films. (c) The relation between conherence length and PCE. (d) R-SoXS scattering profile of the polymer:IT-M blend films. GIWAXS was further employed to investigate the molecular packing of the optimized blend films. Their GIWAXS patterns are shown in Figure 6a while the GIWAXS pattern for the IT-M pristine film is shown in Figure S8. The corresponding line-cut profiles are listed in Figure 6b, with crystal 19


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parameters listed in Table S1. and the complete line-cut profile and the Gussian peak fitting can be found in Figure S9. The distinct (010) diffraction peaks at qz ≈ 1.8 Å1 in the OOP direction were observed in all blends, indicating predominant face-on oritentation that is beneficial for vertical charge transport. Meanwhile, two pronounced (100) peaks at 0.21 Å−1 and 0.34 Å−1 can be observed in the IP direction, which belong to the (100) peaks of donor polymer and IT-M, respectively. The observations indicate that both the PBZ polymer and IT-M can retain their own crystallization properties in the blend films, which afford well-seperated and pure domains. The IT-M displays stronger crystallinity and tighter lamellar packing when blended with the three polymers, evidenced by more pronounced peak (Figure 6b) and larger CL as well as higher qxy (100) in the three blend films compared to its neat film (Table S1). Notably, for PBZ-2Si:IT-M blend, the IT-M (100) peak in the IP direction has the highest intensity among three blends, indicating the highest crystalline degree of IT-M in PBZ-2Si:ITM blend films, which is in well agreement with its highest electon mobility in SCLC measurment. Meanwhile, PBZ-2Si:IT-M blend exhibits the highest CL of 27.2 Å for the (010) peak in OOP direction among the three blends (Figure 6c), which contributes to its highest electron transport and balanced charge transports and then the highest Jsc and FF. Generally, the crystal sizes based on the (010) peak in OOP direction for the three blend films can correlate their efficiencies in PSCs. Resonant soft X-ray scattering (R-SoXS) was also performed, so as to determine the domain size and relative domain purity (Figure 6d). A photon energy of 284.8 eV was selected to probe the blends owing to the highest polymer:IT-M contrast at this energy. R-SoXS data reveal that the domain sizes of the opitimal blend films of PBZ-1Si:IT-M, PBZ-2Si:IT-M and PBZ-3Si:IT-M are 30, 38, and 41 nm, respectively. Under the assumption of a globally isotropic 3D morphology, the root-mean-square 20



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composition variation, which relates to the average domain purity, is proportional to the square root of the normalized integrated scattering intensity. The R-SoXS profiles reveal that PBZ-2Si:IT-M has the highest average domain purity while the relative domain purities of PBZ-1Si:IT-M and PBZ-3Si:ITM are of 85% and 90%, respectively. The appropriate domain size and the purest domain of PBZ2Si:IT-M blend contribute to its highest Jsc and FF and finally the highest PCE.

Figure 7. (a) The photovoltaic performance for PBZ-2Si:IT-M based devices with different thickness of active layer; (b) Shelf stability of PBZ-2Si:IT-M based devices with different thickness of active layer in N2 fill grove box. 2.6 Thick-Film Devices and Stability For the practical application of PSCs, the active materials should have good thickness tolerance for compatibility with high-throughput roll-to-roll printing as well as good stability. Thus, we fabricated PBZ-2Si:IT-M devices with various active layer thickness from 100 to 240 nm (Figure 7a). The detailed photovoltaic parameters are listed in Table S4. As the thickness increases, the devices show continuously increased Jsc and decreased FF. We also observed the slightly decreasing of Voc. Similar results had been reported before.58-60 Finally, a decent PCE of 9.48% can be achieved with the active layer thickness of 240 nm. Additionally, we performed shelf-life test of PBZ-2Si:IT-M devices in the 21


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N2 filled glove box (Figure 7b). The detailed variations of the photovoltaic parameters are shown in Figure S10. Despite the quite different initial efficiencies of the 100240 nm thick active layers, all the devices exhibited averaged PCEs around 8.3% after 77 days, suggesting that the thick active layer of 240 nm shows the most limited decreasing of efficiency. The degradation of the devices mainly originated from the decline of Jsc and FF which can be ascribed to the morphology transition. These observations indicate that PBZ-2Si is capable of being the promising donor material for large area fabrication by roll-to-roll printing and future application of PCSs.

3. Conclusion In summary, we designed and synthesized three FTAZ-BDTT polymers comprising different siloxaneterminated side chain, namely, PBZ-1Si, PBZ-2Si, and PBZ-3Si, to study the impact of siloxaneterminated side chain substitutions on the photovoltaic performances. The different substituent positions have slight influences on the polymer absorption and energy levels, but showing significant impact on the molecular packing. Relative to PBZ-1Si, polymers PBZ-2Si and PBZ-3Si bearing two siloxane-terminated side chains on the BDTT unit, show smaller π-π stacking distances but with larger lamellar distances. Using IT-M as an acceptor to construct the active layer, a PCE of 11.14% was obtained from PBZ-2Si:IT-M based devices. The PBZ-2Si:IT-M blend can exhibit stronger π-π stacking, more ordered packing, a purer domain, as well as the enhanced crystallinity of IT-M, so that the most balanced charge transport, efficient exciton dissociation, and suppressed charge recombination were achieved, which contributed to the highest Jsc and FF in PBZ-2Si:IT-M based devices. Noteworthy, the PBZ-2Si:IT-M exhibits good thickness tolerance and its thick active layer of 240 nm shows the most limited decreasing of efficiency after a storage for 77 days, supplying good 22



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potential for mass farbication. Our work suggests that the fine pairing of a siloxane-terminated side chain and an alkyl side chain is benefical for the optimizing of photovoltaic performance of a conjugated polymer donor. ASSOCIATED CONTENT Supporting Information Synthesis and measurement details, TGA, UV-vis absorption, and CV measurement for the polymers, CLs as well as the complete line-cut profiles for the pristine polymer and relevant blend films, the optimization of the photovoltaic devices, J1/2-V plot, AFM images and the stability test are available. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (L. Zhang) * E-mail: [email protected] (W. Ma) * E-mail:; [email protected] (J. W. Chen) ORCID Lianjie Zhang: 0000-0003-3555-4372 Junwu Chen: 0000-0003-0190-782X Author Contributions ‖

These authors contributed equally.

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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support of National Natural Science Foundation of China (51673070, 51521002, 91633301, U1401244, and 21225418) and Natural Science Foundation of Guangdong Province (2017A030313326). This work was also supported by the Fundamental Research Funds for the Central Universities (2180040). Parts of this research were conducted at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory, which was sustained by the DOE, Office of Science, and Office of Basic Energy Sciences. Chenhui Zhu and Cheng Wang are acknowledged for beamline support.

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Impact of the Siloxane-Terminated Side Chain on Photovoltaic

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