Non-halogenated Solvent Processed All-Polymer Solar Cells over 7.4

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Energy, Environmental, and Catalysis Applications

Non-halogenated Solvent Processed All-Polymer Solar Cells over 7.4% Efficiency from Quinoxaline Based Polymers Liuyang Zhou, Xuan He, Tsz-Ki Lau, Beibei Qiu, Tao Wang, Xinhui Lu, Beata Luszczynska, Jacek Ulanski, Shutao Xu, Guohui Chen, Jun Yuan, Zhi-Guo Zhang, Yongfang Li, and Yingping Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13949 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018

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ACS Applied Materials & Interfaces

Non-halogenated Solvent Processed All-Polymer Solar Cells over 7.4% Efficiency from Quinoxaline Based Polymers Liuyang Zhoua†, Xuan He b†, Tsz-Ki Lauc, Beibei Qiud, Tao Wanga,e, Xinhui Luc, Beata Luszczynskaf, Jacek Ulanskif, Shutao Xua, Guohui Chena, Jun Yuana, Zhi-Guo Zhangd, Yongfang Lid, Yingping Zou a * a

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083,

China. * E-mail: [email protected](Y.Zou) b

Institute of Inorganic and Analytical Chemistry, University of Münster, Corrensstraße 28/30, 48149

Münster, Germany cDepartment

of Physics, The Chinese University of Hong Kong, New Territories, Hong Kong, P. R.

China. dBeijing

National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of

Sciences, Beijing 100190, China. eState

Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082,

China fDepartment

of Molecular Physics, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz,

Poland † These authors contributed equally to this work

ABSTRACT Two conjugated polymers, with different side chains of alkoxy substituted difluorobenzene and alkyl substituted difluorobenzene based on quinoxaline (Qx) as the

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electron acceptor unit and the benzodithiophene (BDT) as the electron donor unit, named HFQx-T and HFAQx-T, were used as electron donor polymers to fabricate all-polymer solar cells (all-PSCs) with a naphthalenediimide-bithiophene n-type semiconducting polymer (N2200). Usually halogenated solvents are harmful to the natural environment and human beings, and solvent additive were disadvantageous in the process of roll-to-roll technology. The Qx based polymers are successfully used to fabricate high performance all-PSCs, which processed with the non-halogenated solvent tetrahydrofuran (THF) at room temperature. With THF as processing solvent, the active layer showed higher absorption coefficient, better phase separation, exciton dissociation and charge carrier mobilities than that processed with CHCl3. Moreover, the photovoltaic properties have been dramatically improved with THF. The optimized device of HFAQx-T:N2200 processed with THF delivered an efficient power conversion efficiency (PCE) of 7.45％, which is the highest PCE for all-PSCs from Qx based polymers processed by a non-halogenated solvent. KEYWORDES: all-PSCs, efficient photovoltaics, non-halogenated solvent, side chains, additive free

INTRODUCTION Over the past decades, organic solar cells (OSCs) have attracted more and more attention by reason of their unique advantages for simple device structure, inexpensive and solution-processing for flexible and large area printing techniques1-5. Although OSCs


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based on fullerene derivatives exhibited an encouraging power conversion efficiency (PCE) over 10%6-7, their inherent defects of fullerene derivatives hindered further development, such as low light absorption efficiency, expensive preparation and purification procedures, difficult adjustment of energy levels and their instability8 . Recently, OSCs based on nonfullerene acceptors (NFA) achieved inspiring PCEs over 13％ 9 ， which

overcame the mentioned issues, such as easy synthesis, easy adjustment of

optical and electronic properties. Simultaneously, what is noteworthy is that, a more ideal type of OSCs, all-polymer solar cells (all-PSCs) have been attracting great interest, since the blend present superior photochemical, thermal and morphological stability10-13, unique mechanical strength14 compared to the fullerene based OSCs. During the past few years, all-PSCs have made enormous progress with the efficiencies increasing from approximately 5% to over 9%15-19. Recently, Li and co-works obtained a great PCE of 8.27%, with the blend of a medium bandgap polymer donor of J51 and a commercial polymer acceptor N220020. Very recently, the highest PCE up to 10%21 has been reported by using a new wide bandgap polymer of PTzBI paired with polymer acceptor N2200. Moreover, new polymers have also been optimized towards higher photovoltaic performance, for example, random polymer PNDI-T10 was used with PTB7-Th to fabricate all-PSCs, which delivered a high PCE of 7.6%22, this efficiency is 2-fold increase with a pair of PTB7-Th and N2200. Although of much less attention to all-PSCs, the fast development within a short span of time indicates that all-PSCs are promising to approach performances similar or superior to those of fullerene



based PSCs. However, the overall performance of all-PSCs is still behind the outstanding efficiency obtained from the other type devices which contain small molecule NFA or fullerene derivative acceptors. Thus, it remains high importance to develop all-PSCs with high PCEs by rational molecular design and device fabrication23. As well-known, chlorinated solvents are harmful to the environment and human health10,

24-27,

furthermore, green development is an important index to the society,

however, halogenated products are the most successful and widely used solvents in the device fabrications. That means chlorinated solvents are useful for spin-coating under laboratory conditions but not applicable for large-area production in the future28. In consequence, it is extremely urgent to find some eco-friendly solvents, like tetrahydrofuran (THF), toluene and 1, 2, 4-trimethylbenzene and so forth, for reducing contaminants. For example, Hou et al employed THF as processing solvent to achieve higher PCE by using layer-by-layer approach29. For another reason, the solvents of high boiling point (b.p.) were commonly used as solvent additives. Frequently-used solvent additives are halogenated, which remained much contaminant. Some investigations showed that solvent additives are difficult to be removed with adverse effects for practical roll to roll printing techniques due to reduced stability30-33. Hence, new polymer donors should be explored for matching well with typical polymer acceptors, like N2200. N2200 has dual absorption, therefore, the medium polymers34 were required for complementary absorption with N220035. Recently, our group has reported a new quinoxaline (Qx) based copolymer, named TTFQx-T1, which


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exhibited a high PCE of 10.52% while blending with ITIC2. Qx is an universal and promising building block due to the advantages of easily modifiable structure, electron-deficient and planarity36-44, and so on. In this study, we employed two electron-donating conjugated polymers with similar structures named HFAQx-T45 and HFQx-T46, and a typical n-type polymer, N220047-48 to construct all-PSCs (the structures are shown in Figure 1b). The difference of two polymers is located in Qx side chains, where alkoxy substituted difluorobenzene and alkyl substituted difluorobenzene corresponding to HFQx-T and HFAQx-T, respectively. Side chain has played an important part in photovoltaic performance. For example, HFAQx-T has lower the highest occupied molecular orbital (HOMO) energy level resulting in a higher open circuit voltage (Voc) than that of HFQx-T. In addition, molecular weight plays an important role in all-PSCs49-50, HFAQx-T, HFQx-T and N2200 possess good solubility, suitable energy levels although of comparable molecular weight (shown in Table 1). The optimized device based on HFAQx-T:N2200 (2:1, w/w) exhibited a high PCE of 7.45% with a high Voc of 0.92 V, a short circuit current density ( Jsc) of 12.47 mA cm-2 and a fill factor (FF) of 65% with THF and thermal annealing (TA) treatment. Our work indicates that Qx based polymers have great potential for developing high-efficiency all-PSCs. Through modified structures and optimized device of Qx based polymers, higher photovoltaic performance can be anticipated.



Figure 1. (a) Device architecture of the all-PSCs; (b) Molecular structures of HFAQx-T, HFQx-T and N2200.

RESULTS AND DISCUSSION Optical and Electrochemical Properties The ultraviolet-visible (UV-Vis) absorption spectra of the neat polymers and blend films were characterized (shown in Figure S3a-b). As depicted in Figure S3a, HFAQx-T and HFQx-T processed by THF and CHCl3 present similar absorption spectra in film state (300-800 nm) and broad absorption plateau between 600 nm and 650 nm. The absorption edge (λonset) of HFAQx-T is located at 730nm with the optical bandgap (Eopt g ) of 1.70 eV, which was calculated by the formula of Eopt g =1240/onset. In the same way, the λ onset of HFQx-T is located at 700nm with the Eopt of 1.77 eV (shown in Table 1). It is worth g mentioning that N2200 film shows dual absorption band located at 300-450 nm and 530-900 nm. The λ onset for N2200 is at 850 nm, corresponding to the Eopt of 1.46 eV. g HFAQx-T or HFQx-T and N2200 exhibited a good complementary absorption toward better spectral coverage of the sunlight and benefitted higher Jsc. As shown in Figure S3b,


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the active layer of polymer:N2200 demonstrated broad and improved light absorbance. The neat films of HFAQx-T and HFQx-T and blend films show similar absorption spectra with THF and CHCl3 solvents, therefore, we measured the absorption coefficients for comparing light harvesting abilities. The absorption coefficients of HFAQx-T processed with THF and CHCl3 are 4.8×104 cm-1 and 4.6×104 cm-1, respectively (shown in Figure 2a and S4a). HFQx-T processed with THF shows decreased absorption coefficient of 4.3 × 104 cm-1 compared to that of its alkyl counterpart HFAQx-T. Nevertheless, N2200 processed with THF possesses a little higher absorption coefficient and

more redshifted absorption compared to those with CHCl3. To further explore the

light harvesting capacity, the absorption coefficients of polymer:N2200 processed with different solvents were measured and there are some differences of HFAQx-T:N2200 processed with THF and CHCl3 , followed by TA at 130 oC (shown in Figure 2b and S4b), corresponding to the absorption coefficients of 4.05×104 cm-1 and 3.90×104 cm-1, respectively. As shown in Figure S9d-f, it is noteworthy that the blend film of HFAQx-T:N2200 processed with THF exhibited a notably enhanced intensities of (010) π-π stacking. Moreover, HFAQx-T:N2200 blend with TA treatment indicates slightly higher absorption than those films of as-cast.



Figure 2. Absorption coefficients of: (a) neat films of polymers donor and N2200; (b) blend films processed with THF and CHCl3. The HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels of polymers donor and N2200 were carried out by electrochemical cyclic voltammetry (CV) (Figure 3b), to explore the effects of side chains on the electrochemical energy levels. As shown in Figure 3a, the energy level alignments of the materials are demonstrated. The HOMO and LUMO energy levels (EHOMO/ELUMO) can be calculated from the onset oxidation potential (EOx) and the onset reduction potential (ERed) from CV curves, according to the equations of EHOMO/LUMO = - (EOx/Red + 4.8-EFc/Fc+ ) (eV), and the EFc/Fc+ is the redox potential which was carried out to be 0.44 eV with Ag/AgCl as reference electrode. Finally, the calculation equations were modified as EHOMO/LUMO = - (Eox/red + 4.36 ) (eV).


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Figure 3. (a) Energy level diagram of related materials in devices; (b) CV curves of HFAQx-T, HFQx-T and N2200. The EHOMO of HFAQx-T, HFQx-T and N2200 could be calculated to be -5.53 eV, -5.51 eV and -5.87 eV from the onset oxidation potential (EOx(onset)) (shown in Figure S11), corresponding ELUMO were estimated to be -3.55 eV , -3.52 eV and -3.85 eV from the onset reduction potential (ERed(onset)) (listed in Table 1). It should be noted that the EHOMO of HFAQx-T is lowered by 0.02 eV compared to that of HFQx-T which results in a slightly higher Voc of 0.92 V than that of HFQx-T of 0.90 V, on the other hand, EHOMO/LUMO of donor ought to be higher than the corresponding EHOMO/LUMO of acceptor to overcome the binding force of excitons at the BHJ interface. In this work, the difference of HOMO and LUMO energy levels (△EHOMO and △ELUMO) were suitable. The △ELUMO of polymer donors (HFAQx-T and HFQx-T) and acceptor (N2200) is 0.30 eV and 0.33 eV, respectively, suggesting enough driving force for efficient exciton dissociation51. Table 1. Molecular weights, optical data and energy levels of polymer donors and



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acceptor Polymer

Mn(kDa)

Mw(kDa)

PDI

λonset(nm)

Eg(eV)

EHOMO(eV)

ELUMO(eV)

HFAQx-T HFQx-T N2200

23.8 18.0 42.0

44.0 43.6 78.0

1.85 2.40 1.86

730 700 850

1.70 1.77 1.46

-5.53 -5.51 -5.87

-3.55 -3.52 -3.85

Photovoltaic Properties

As shown in Figure 1a, the conventional device structure was employed to carry out the

photovoltaic

properties

of

all-PSCs,

which

containing

ITO/PEDOT:PSS/polymer:N2200 (2:1, w/w) /PDINO52/Al, PDINO was dissolved in methanol. The current density–voltage (J-V) curves were measured and corresponding photovoltaic parameters of devices are summarized in Figure 4a and Table 2, respectively. Different weight ratios of HFAQx-T (or HFQx-T) and N2200 for 1.5:1, 2:1 and 2.5:1 were tried and the optimized weight ratio of polymer donor and N2200 was 2:1. Furthermore, further optimization was performed by changing processing solvents, additives and the thermal annealing temperatures for all-PSCs. As a result, the processing solvent of the optimized devices is THF, and the devices processed with THF showed a great PCE improvement than those of CHCl3 based devices. The solvent additives made negative effects on PCE, on the other hand, TA has notably increased FF with slightly increased Jsc and Voc. Using THF as processing solvent, as cast device efficiency based on HFAQx-T:N2200 (2:1, w/w) was 6.51% with a moderate FF of 59%. A higher PCE of 7.45％ was achieved processed with THF followed by TA at 130 oC, The photoactive layer processed with CHCl3 exhibited a relatively lower PCE of 6.08% than those from


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the devices processed with THF under the similar conditions due to lower Jsc. For comparison, HFQx-T:N2200 (2:1, w/w) processed by THF and annealing at 130 ℃ showed a moderate PCE of 6.20% mainly from the decreased FF and Jsc (Figure S1a-b and Table S1). The detailed photovoltaic performances are shown in Figure S1a-b and Table S1. Table 2. Photovoltaic data of blend films processed with different conditions Blend films

Ratio

HFAQx-T:N2200(THF) HFAQx-T:N2200(CHCl3)

2:1

at

130 oC

PCE(Avg) (％)b

12.47±0.19

65±1

7.45

7.40±0.05

0.91±0.02

12.18±0.28

59±2

6.51

6.44±0.11

130

0.92±0.01

11.80±0.14

62±1

6.71

6.63±0.08

130

0.90±0.02

11.69±0.21

59±2

6.20

6.14±0.09

130

0.92±0.01

None

HFQx-T:N2200(THF) aAnnealing

PCE(Max)(％)

Voc(V)

HFAQx-T:N2200(THF)

for 10min.

Jsc(mA

FF(％)

Annealing(℃)a

bAverage

cm-2)

value of PCEs obtained from 10 devices.

As depicted in Figure 4b, the external quantum efficiency (EQE) spectra was measured to confirm accuracy of the Jsc measurements and estimate the spectral responses based on devices. All devices demonstrate broad photo responses. The optimized device showed the broad EQE plateau about 350nm-750 nm. Comparing with the EQE curves, other devices had a slightly lower spectral responses than those optimized devices. The values of Jsc were obtained from the EQE curves were 11.86 mA cm-2 and 11.79 mA cm-2 for HFAQx-T:N2200 of optimized and as cast devices using THF as processing solvent, respectively. The Jsc obtained from the integral EQE curves were in accordance with those from the J-V measurements within 5% mismatch.



Figure 4. (a) The J-V curves; (b) EQE curves of all-PSCs processed with different conditions. To better understand the synergistic effects of solvents, thermal annealing and structures on charge carrier mobility of the active layers, the mobilities were carried out using the space charge limited current (SCLC) method. The J1/2-V curves and corresponding data of the hole mobilities (μh) and electron mobilities (μe) are shown in Figure S5 and Table S3. For the as-cast blend film of HFAQx-T:N2200, μh and μe are calculated for 3.54×10-4 cm2V-1s-1 and 4.20×10-4 cm2 V-1 s-1, respectively, with μe/μh of 1.19. After TA, the device of HFAQx-T:N2200 exhibited more balanced charge carrier mobilities with values of μh and μe are 5.52 ×10-4 cm2V-1s-1 and 5.81×10-4 cm2V-1s-1 (1.05 of μe/μh), respectively, which may be one of the factors to explain notably increased FF and a bit higher Jsc to obtain better photovoltaic performance. The mobilities of THF processed devices were obviously higher than those devices processed with CHCl3 (μh /μe values of 9.56×10-5/3.22×10-4). The device based on HFQx-T:N2200 under optimization conditions showed lower μh (1.52×10-4 cm2 V-1 s-1) and μe (3.13×10-4 cm2V-1s-1) values compared to


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their alkyl substituted counterpart-HFAQx-T, which can partially explain lower Jsc and FF. The charge recombination behavior in the devices was considered to have significant influence on the FF and Jsc. Generally speaking, Jsc and Plight complied with a power-law, expressed as Jsc∝Plightα, where Plight is light intensity and α is a power-law component. Experimentally, the values of α tend to be unity when the bimolecular recombination is negligible53. In this study, we carried out the Jsc∝Plight curves (shown in Figure 5a and S6a). α values are 0.95, and 0.92, corresponding to the devices of optimized and as cast, respectively. The values of α indicated the weaker bimolecular recombination, which conforms to the higher charge carrier mobilities and better photovoltaic performance of the device mentioned above.

Figure 5. (a) Jsc dependence upon different Plight of the corresponding devices based on polymer donor:N2200. (b) Jph-Veff curves of all-PSCs based on devices. To further study the effects of the devices processed with different conditions and side chains on high-performance all-PSCs, the photocurrent (Jph) versus the effective voltage



(Veff) of devices were carried out. In general, Jph is complied with the formula of Jph = JL-JD, where JL and JD are the current densities in illumination and darkness conditions. Besides, Veff follows the formula of Veff =V0-Vbias, where the voltage of V0 is defined when Jph = 0 and Vbias is external voltage. In this study, the Jph is saturated (Jsat) when Veff is about 2 V (Figure 5b and S6b), which indicates nearly all of the photo-generated free charge in the active layers could be gathered by the Cathode and anode. Besides, the exciton dissociation probabilities (Pdiss) follows the formula of Pdiss = Jph / Jsat , and all values of Jsat could be obtained from the original data of Jph - Veff curves. Accordingly, the values of Pdiss are 95% and 93%, corresponding to the devices of HFAQx-T:N2200 (optimized), HFAQx-T:N2200 (as cast), respectively, which were obtained under the short circuit condition. Finally, we could come to a conclusion based on the Pdiss values that all devices of mentioned above exhibited a high exciton generation and dissociation efficiency.

Morphology and GIWAXS analysis The film surface topography plays a vital role for high-performance all-PSCs. Hence, we measured the surface morphology of active layers through atomic force microscopy (AFM) (Figure 6a-d) and transmission electron microscopy (TEM) (Figure 6e-h). The active layer of HFAQx-T:N2200 processed with CHCl3 exhibited a relatively strong aggregation with root-mean-square (RMS) value about 1.41 nm, which may be adverse to exciton dissociation, and lead to unbalanced hole and electron mobilities, on the other


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hand, the active layer based on HFAQx-T:N2200 processed with THF demonstrated a proper phase separation which may facilitate efficient exciton dissociation and charge carrier mobilities, and RMS values were obtained were 1.06 nm and 1.36 nm, corresponding to the HFAQx-T:N2200 devices with TA and without TA processed by THF solvent, respectively. As AFM images shown that the annealed and as cast blend films exhibited a big difference of RMS and surface topographies, therefore, as cast device demonstrated a slightly lower Voc and Jsc and

much lower FF resulting in lower

PCE. The active layer without TA may have slight aggregations and bring about geminate recombination54. The morphology of active layer based on HFQx-T:N2200 processed by THF and TA

showed a larger RMS of 1.88 nm, exhibited relatively rough

surface and poor phase separation, which may limit exciton transport as well as result in lower charge carrier mobilities. TEM were measured to explore inner phase separation of active layers. The blend films based on HFAQx-T:N2200 processed with THF and TA exhibited a homogeneous fibrills with better interpenetrating network which improved the exciton dissociation and charge carrier mobilities; the devices of as cast or processed with CHCl3 and TA demonstrated slight aggregation and thick, fibrous textures, which result in lower photovoltaic performance. On the other hand, the devices of HFQx-T:N2200 processed with THF and TA exhibited weak phase separation but lumpish morphology which may affect exciton dissociation and give rise to lower PCE.



Figure 6. AFM height images (5 × 5 μ m2) and TEM photos for HFAQx-T:N2200 processed by THF and TA (a and e), as cast (b and f), CHCl3 and TA (c and g); and HFQx-T:N2200 processed by THF and TA (d and h) based all-PSCs.

To further illustrate the effect of different structures, processing solvents and TA on morphological structure based on the devices of all-PSCs, we carried out the grazing-incidence wide angle X-ray scattering (GIWAXS)55 and all patterns and line cuts shown in Figure S9. The films of neat and blend polymer were fabricated under identical conditions. As observed from the patterns of neat films, a distinct (010) π-π stacking and (100) lamellar stacking are observed from out-of -plane (OOP) and in-plane (IP) direction56, respectively. The mirrored reflection of (010) and (100) implies that the neat polymer films of donor and acceptor adopt a face-on orientation toward the substrate. The neat film of HFAQx-T processed with THF exhibited a notably strong intensities of (010) π-π stacking but HFQx-T has shown hardly any intensities of (010) π-π stacking, which


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means the neat HFAQx-T adopt a more favorable face-on orientation. The blend films exhibited similar peak location with preferential face-on packing for lamellar (100) stacking and π - π (010) stacking. The blend film of HFAQx-T:N2200 processed with THF and TA exhibited a enhanced intensities of (010) π-π stacking than as cast blend film and those processed with CHCl3 and TA. Generally, efficient charge carrier mobilities benefited by tight π-π stacking of blend films, which can help improve the JSC and FF for high-performance all-PSCs.

CONCLUSIONS Ultimately, two medium bandgap polymers and commercial narrow bandgap N2200 were employed to fabricate all-PSCs. The optimized device delivered the highest PCE of 7.45% processed with THF followed by TA post-treatment. In this work, we proposed that TA could reduce the geminate recombination which is beneficial to efficient charge carrier transport, moreover, THF can optimize phase separation and improve the charge mobility. The synergistic effects of TA and processing solvents in all-PSCs, combining relatively higher absorption coefficients of HFAQx-T based blend, made the relatively higher efficiency with outstanding FF, high Voc and good Jsc. Our work demonstrates that non-halogenated solvent, TA and the optimized structure of Qx based polymers will have great potentials toward high-performance solar cells.

Supporting Information



The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental sections including materials, device fabrication and characterization; Photovoltaic data of all-PSCs processed with different conditions (Figure S1-S2 and Table S1-S2); Optical properties are shown in Figure S3-S4; Charge mobilities of the polymer:N2200 processed with different conditions (Figure S5 and Table S3); AFM height images, XRD patterns, 2D GIWAXS images, DSC thermograms, CV curves and the solubility of N2200 are shown in Figure S7-S12, respectively.

Acknowledgements This work has been financially supported by National Key Research & Development Projects of China (2017YFA0206600), Science Fund for Distinguished Young Scholars of Hunan Province (2017JJ1029), the Natural Science Foundation of Hunan Province(2017JJ2325), the National Natural Science Foundation of China (21875286).

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Non-halogenated Solvent Processed All-Polymer Solar Cells over 7.4

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