High-Performance Polymer Solar Cells with Minimal Energy Loss

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High-Performance Polymer Solar Cells with Minimal Energy Loss Enabled by a Main-Chain Twisting Nonfullerene Acceptor Yan Zou, Yingying Dong, Chenkai Sun, Yue Wu, Hang Yang, Chaohua Cui, and Yongfang Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01175 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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Chemistry of Materials

High-Performance Polymer Solar Cells with Minimal Energy Loss Enabled by a Main-Chain Twisting Nonfullerene Acceptor Yan Zou,† Yingying Dong,† Chenkai Sun,‡ Yue Wu,† Hang Yang,† Chaohua Cui,*,† and Yongfang Li†,‡ †Laboratory

of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China. ‡Beijing

National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ABSTRACT: We design and synthesize a new main-chain twisting fused-ring narrow-bandgap n-type organic semiconductor acceptor (namely IE4F-S) with terminate group on the 4-position of 3-alkylthio substituted thiophene. The acceptor IE4F-S shows zero ionization potential (IP) offset 9=&6< with wide bandgap polymer donor PTQ10. Notably, the polymer solar cell (PSC) device operating at such a negligible =EHOMO can still yield an outstanding power conversion efficiency (PCE) of 12.20%, with a high open-circuit voltage (Voc) of 0.996 V and a remarkably low energy loss of 0.47 eV. The energy loss of 0.47 eV should be the lowest value for the reported PSCs with PCE over 12%. In addition, by using PBDB-T as donor to blend with IE4F-S, the PSC device also demonstrates a promising PCE of 13.72%, which is one of the top efficiencies for the PBDB-T based devices. The results demonstrate that the appropriate modulation of the main-chain planarity is an effective approach to construct high-performance fused-ring based acceptors for PSCs.

INTRODUCTION Solution-processed polymer solar cells (PSCs) have been regarded as a promising renewable energy technology due to their unique advantages of easy fabrication, light weight, and capability to be fabricated into flexible and semitransparent devices.1-5 Benefited from their merits of broad and strong absorption, easy tunable energy levels, and favorable molecular packing properties over fullerene acceptors, the emerging of fused-ring n-type organic semiconductor (n-OS) acceptors significantly boosts the efficiency of PSC up to 13~15% in recent years.6-11 In spite of the huge progress, developing photovoltaic materials to realize high power conversion efficiency (PCE) of PSCs is still the major priority in the research field. With respect to PSCs, one of the main factors limiting the efficiency is the significant amounts of energy loss (Eloss) which results in relatively low open-circuit voltage (Voc) of the PSCs.12-14 In high-performance inorganic solar cells and perovskite solar cells, the Eloss values are around 0.4-0.5 eV,15-18 while most of the state-of-the-art PSCs exhibit higher Eloss around 0.6-0.7 eV. The Eloss in PSCs is defined as Eloss = Eg eVoc, where Eg is the lowest optical bandgap of donor or acceptor evaluated from the absorption edge in its active layer,19 commonly it is the Eg value of the narrow bandgap acceptor in the nonfullerene PSCs. Typically, decreasing the HOMO (highest occupied molecular orbital) energy offset 9=EHOMO) between the donor and acceptor is an effective strategy to reduce the Eloss of PSC. In particular, several cases have been demonstrated that only a small driving force (ca. 0.1 eV, even negligible in some cases)

can satisfy for efficient charge separation in the n-OS acceptor based PSCs.20-23 For instance, Yan et al. developed a polymer donor PvBDTTAZ which only exhibited a slightly higher HOMO than n-OS acceptor OIDTBR 9=EHOMO = 0.04 eV), and the PvBDTTAZ:O-IDTBR based device can afford a high PCE of 11.6%, with a high Voc of 1.08 V and a low Eloss of 0.55 eV.21 In the case for the PffBT2T-TT:O-IDTBR based device, a promising PCE of 10.4% can be realized, with a notable Voc of 1.08 V and a low Eloss of 0.55 eV, in spite of the both very small =EHOMO and LUMO (lowest unoccupied molecular orbital) offset 9=ELUMO) 9=EHOMO = 0.09 eV and =ELUMO = 0.03 eV).22 Regarding the PSCs based on fullerene derivative acceptor, a low Eloss of 0.59 eV with a high PCE reached 10.28% was reported very recently.24 Although much progress has been achieved in reducing Eloss for PSCs, there is still much room for minimizing the Eloss of PSC devices. Figure S1 in Supporting Information (SI) summarized the reported state-of-the-art n-OS based PSCs with PCE 9% and Eloss 0.7 eV, that the Eloss values of the PSCs are mainly located in the 0.55 0.70 eV region, seldom PSCs can afford PCE 12% with Eloss below 0.5 eV. Typically, realizing high main-chain planar configuration is the main priority in photovoltaic materials design in terms of expanding absorption profile and enhancing efficient charge transportation. On the other side, appropriate twisted configuration in some extent is required for the photovoltaic materials to prevent the over aggregation which would lead to the

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formation of large crystalline domains and phase separation in blend film. Regarding the fused-ring based n-OS acceptors with high planar main-chain, the most common used molecular design strategy to address this issue is the manipulation of molecular steric hindrance via out-of-plane side chains engineering on the fused aromatic rings.25-28 Besides, tuning the main-chain twisting should be an effective approach which is widely used in perylene diimide (PDI) based molecules for reducing aggregation and thus realizing high efficiency,2934 while it is seldom utilized in the fused-ring n-OS acceptors design so far. For instance, the I8 " " of thiophene unit are the most frequently used to connect in the conjugated backbones as J8 0 However, moving the end-capped groups of IEICO-4F from 5-position of 3alkoxy substituted thiophene to its 4-positon can significantly give rise to its overall photovoltaic performance, in spite of the resulting twisted mainchain.35 Based on the above discussion, herein we designed and synthesized a new main-chain twisting fused-ring acceptor with terminate group on the 4-position of 3alkylthio substituted thiophene J8 " (namely IE4F-S, Figure 1a). Due to the empty 3d-orbital of sulfur atom, the alkylthio side-chain on the IE4F-S can potentially redshift the absorption spectrum, down-shift the HOMO level, and enhance the intramolecular charge transfer (ICT) effect within the molecule.36-39 On the other hand, density functional theory (DFT) calculation results revealed that attaching the terminate groups on K8 position of 3-alkylthio increased the degree of twisting (from planarity) in the backbone compared to the counterpart IEICS-4F end-capped on the I8 " of 3alkylthio, resulting in a blue-shifted absorption spectrum and up-shifted LUMO level. The up-shifted LUMO level of acceptor material is favorable for obtaining higher Voc of PSCs. Encouragingly, the PSC device using IE4F-S as acceptor and PBDB-T40 (Figure 1a) as donor demonstrate an outstanding PCE of 13.72%, which is one of the top PCEs reported for the PBDB-T based PSCs so far. In particular, even if the ionization potential (IP) offset 9=&6< is zero for the PTQ1041:IE4F-S blend (Figure 2a. It should be pointed out that the =&6 value is calculated from the individual IP of donor and acceptor components and do not take into account the specific effects occurring at the donor-acceptor interface), a promising PCE of 12.20% can still be realized in the PTQ10:IE4F-S based device, with a high Voc of 0.996 V and a minimal Eloss of 0.47 eV. It is worth noting that the Eloss of 0.47 eV should be the lowest value for the reported PSCs with PCE over 12%. The results clearly demonstrate that modulating main-chain twisting is a promising strategy to design state-of-the-art fused-ring acceptors, and IE4F-S is an outstanding acceptor for highly efficient PSCs. In particular, the results also demonstrate that high efficiency can be realized for the PSCs based on the donor and acceptor materials even with negligible =&60 RESULTS AND DISCUSSION

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Material Synthesis and Optoelectronic Properties. The n-OS acceptor IE4F-S was synthesized by formylation, Stille coupling and Knoevenagel condensation reactions, as shown in Figure 1b. The detailed synthetic procedures are described in the Experimental Section in SI. IE4F-S can be readily dissolved in common organic solvents, such as chloroform, chlorobenzene, and o-dichlorobenzene. (a)

C4H9 NC

CN

C2H5

S

S

S

F

O

O O

n

NC

PBDB-T

F

F

n

PTQ10

CN

C4H9

N

S

F S

N S

S

S

C2H5 C6H13

C6H13

C2H5

IE4F-S

C4H9

C4H9

C4H9

C2H5

C2H5 S

OHC

S

1. THF, -78 oC, LDA Br

S

C4H9

S

S S

(b)

C6H13

C2H5 S

O

S O

F

C2H5 C4H9

S

S F

C8H17

C4H9

C6H13

C6H13 C2H5

2. N-Formylpiperidine

Br

S

2

1 C6H13

C4H9

C6H13

C4H9 S

S

OHC

+ S

Sn

Sn

S

S

S

C6H13

C6H13

3

F

S

Br

2

O F

S

Pd(PPh3)4 Toluene, 110 oC

S

C6H13

C6H13 C2H5

S

OHC

C2H5

C6H13

C2H5 C6H13

CN NC

IE4F-S

CHO CHCl3, pyridine

C4H9

4

Figure 1. (a) Chemical structures of IE4F-S, PBDB-T, and PTQ10. (b) Synthetic routes of IE4F-S. The absorption spectra of IE4F-S in chloroform and solid thin film are presented in Figure S2 in SI. The absorption spectrum of IE4F-S in solution covers the wavelength range of 600-760 nm with a high absorption coefficient ( ) of 1.54 × 105 M-1 cm-1. Notably, significantly broader and red-shifted absorption spectrum was observed in solid thin film with its absorption edge extending from 774 in solution to 843 nm in the solid thin film, corresponding to an Eg of 1.47 eV (extracting by the onset of the absorption spectrum). The intense absorptions of IE4F-S at the wavelength of 700-850 nm are complemented with the wide bandgap polymer donors PBDB-T and PTQ10 in the visible-near infrared region (Figure 2a). The main absorption peak at 761 nm in solid thin film should be ascribed to interchain J8J transition, resulting from the J8J stacking of the conjugated backbone; while the identifiable peak at 695 nm should be attributed to J8J transition along the molecular conjugated backbone.42,43 Moreover, such strong main absorption peak at 761 nm (corresponding to J-aggregates) with weak shoulder peak at 695 (corresponding to H-aggregates) of IE4F-S film suggests that J-aggregates (end-to-end arrangement) dominate the molecular packing (as depicted in Figure S3), which is in favor of hopping transport of electrons for PSCs. The IP and electron affinity (EA) of IE4F-S were measured by electrochemical cyclic voltammetry,44 as shown in Figure S4. The p-doping onset potential ( ox) and n-doping onset potential ( red) of IE4F-S film were measured to be 0.80 V and -0.85 V vs. Ag/Ag+, and thus IP and EA are calculated to be -5.54 and -3.89 eV, respectively (Figure 2b and Figure S4 in SI), according to the equation of EIP/EA = - e( ox/red + 4.74) (eV). In comparison with the counterpart with 3-alkoxyl


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Chemistry of Materials


Chemistry of Materials Table 1 The optimal photovoltaic performance of the PSCs with a device architecture of glass/ITO/PEDOT:PSS/active layer/PDINO/Al under the illumination of AM 1.5 G at 100 mW cm-2. Active layer PBDB-T:IE4F-Sa PTQ10:IE4F-Sb

Voc [V]

Jsc [mA cm-2]

FF [%]

PCE [%]

0.868

22.88

69.1

13.72

[0.867±0.002]

[22.52±0.35]

[69.0±0.21]

[13.48±0.19]

0.996

19.67

62.3

12.20

[0.990±0.003]

[19.59±0.37]

[61.8±0.85]

[11.98±0.14]

Eloss [eV] 0.60 0.47

aWith

TA at 140°C for 10 min. bWith TA at 160°C for 10 min. The statistical values in square bracket are the average PCE obtained from 12 devices. 100

(a)

5

-2

90

(b)

80 0

70

-5

EQE (%)

Current density (mA cm )

10

PBDB-T:IE4F-S PTQ10:IE4F-S

-10

60 50 40 PBDB-T:IE4F-S PTQ10:IE4F-S

30

-15

20

-20

10

-25 -0.2

0.0

0.2

0.4 0.6 Voltage (V)

0.8

0 300

1.0

(c)

400

500 600 700 Wavelength (nm)

800

900

-2

)

(d) Current Density (mA cm

-2

Jph (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

10 PBDB-T:IE4F-S PTQ10:IE4F-S

1 0.01

Veff (V)

1

PBDB-T:IE4F-S PTQ10: IE4F-S Fitting curve

= 0.954 = 0.956

1 10

-2

Light Intensity (mW cm

)

100

Figure 3. (a) J-V characteristics under AM 1.5G illumination at 100 mW cm-2 and (b) EQE spectra of the optimized PSCs based on PBDB-T:IE4F-S and PTQ10:IE4F-S. (c) Plots of Jph versus Veff and (d) light dependence of Jsc of the PSCs based on PBDB-T:IE4F-S and PTQ10:IE4F-S. EQE values indicate the efficient charge transfer can occur in the two blends, despite the small =&6 for PBDBT:IE4F-S and zero =&6 for PTQ10:IE4F-S blends. The photoluminescence (PL) quenching efficiencies were measured to investigate the charge transfer in the two blends. As shown in Figure S7 in SI, the PBDB-T:IE4F-S (1:1, w/w) (excited at either 629 nm or 761 nm) and PTQ10:IE4F-S (1:1.5, w/w) (excited at either 550 nm or 761 nm) blend films both exhibited similar emission spectral shape with pristine IE4F-S film, which implied the complete quenching of donor material emission and some residual fluorescence of acceptor material. The emissions were highly quenched for the PBDB-T:IE4F-S blend (the quenching efficiencies were 93% (excited at 629 nm) and 89% (excited at 761 nm), suggesting its efficient exciton dissociation. Interestingly, although the =IP of PTQ10 and IE4F-S is zero, high quenching efficiencies of 80% (excited at 550 nm) and 72% (excited at 761 nm) were observed for the PTQ10:IE4F-S blend. The higher PL

quenching efficiency of PBDB-T:IE4F-S than that of PTQ10:IE4F-S indicates the more effective photo-induced exciton dissociation and charge transport, which is agreement with the higher Jsc and EQE values of the devices. Relative to the device based on PBDB-T:IE4F-S, the lower FF of the PTQ10:IE4F-S based device should be resulted from the lower PL quenching efficiency and more unbalanced hole and electron mobility ratio. We then further investigated the exciton dissociation probability (Pdiss) of the two devices.50 Figure 3c plots the curves of Jph (Jph = JL JD, where JL and JD represent the light and dark current densities, respectively) versus effective voltage (Veff) of the devices. It is supposed that all of the photogenerated excitons were dissociated into free charges and collected by electrodes when Jph reached saturation (Jsat) at large reverse bias (Veff = 2 V in this case), and the Pdiss is defined as Jph/Jsat.50 The PSCs based on PBDB-T:IE4F-S and PTQ10:IE4F-S exhibited high Pdiss values of 98.3% and 97.2%, respectively, indicating the efficient exciton


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Chemistry of Materials dissociation and charge collection in the devices. In addition, the charge recombination of the two devices was evaluated by plotting the light intensity (Plight) dependence of the Jsc. The correlation of the Plight and Jsc can be described as Jsc PlightI, where I is an exponential factor that should be equal to 1 if all charge carriers are swept out before recombination.51 As shown in Figure 3d, the I values of the PBDB-T:IE4F-S and PTQ10:IE4F-S based devices were fitted to be 0.954 and 0.956, respectively, suggesting the weak bimolecular recombination of the two devices. A nanoscale phase separated and bicontinuous D/A interpenetrating network in the blend film is critical for efficient charge separation and transportation in PSCs. The surface and bulk morphologies of the PBDB-T:IE4F-S (1:1, w/w) and PTQ10:IE4F-S (1:1.5, w/w) blends were studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM). As shown in Figure 4a and c, the PBDB-T:IE4F-S (1:1, w/w) and PTQ10:IE4F-S (1:1.5, w/w) blends shows a smooth and uniform surface with a root-mean-square surface roughness (Rq) of 1.86 and 1.07 nm, respectively. In addition, clear and defined nanoscale phase separation with well-developed fibrillar structure was observed both in the PBDB-T:IE4F-S (1:1, w/w) and PTQ10:IE4F-S (1:1.5, w/w) blends (Figure 4b and d), which should favor efficient dissociation and charge transport. Based on the AFM and TEM results, we infer that the main-chain twisting acceptor IE4F-S has appropriate aggregation which match well with polymer donor PBDB-T and PTQ10, enabling the excellent photovoltaic performance of the devices.

(a)

Rq = 1.86 nm

In summary, we designed and synthesized a main-chain twisting fused-ring based acceptor IE4F-S by modulating the end-capped positions on the alkylthio substituted thiophene units. IE4F-S shows complementary absorption spectrum and well-matched morphology properties with polymer donors PBDB-T and PTQ10. The PSC device using IE4F-S as acceptor and PBDB-T as donor demonstrated an outstanding PCE of 13.72%, which is one of the top PCEs reported for the PBDB-T based PSCs so far. In particular, a promising PCE of 12.20% can still be realized in the the PSCs based on PTQ10:IE4F-S with zero =&6 showing a notable Voc of 0.996 V and a minimal Eloss of 0.47 eV. The PCE of 12.20% should be the highest one reported so far for the PSC devices with Eloss as low as 0.47 eV. These results demonstrate that the appropriate modulation of the main-chain planarity is a successful strategy to construct high-performance fused-ring based acceptors for PSCs. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental details for synthesis of IE4F-S, material characterization, device fabrication and characterization, and mobility measurement. Summarized the reported state-of-the-art n-OS based PSCs. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Author Contributions All authors have given approval to the final version of the manuscript.

(b)

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51873140, 51603136, and 91633301).

(c)

Rq = 1.07 nm

REFERENCES

(d)

Figure 4. (a) AFM 3D topography and (b) TEM image of the PBDB-T:IE4F-S (1:1, w/w) blend. (c) AFM 3D topography and (d) TEM image of the PTQ10:IE4F-S (1:1.5, w/w) blend. CONCLUSIONS

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High-Performance Polymer Solar Cells with Minimal Energy Loss

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