Poly(benzo[2,1-b:3,4-b′]dithiophene-alt-isoindigo): a low bandgap polymer showing a high open circuit voltage in polymer solar cells

Abstract

A novel optical band gap of 1.57 eV isoindigo (IID)-containing polymer of PBDP-IID was synthesized and the HOMO of the polymer descends to −5.44 eV due to strong electron-deficient isoindigo unit. Bulk heterojunction solar cells made from PBDP-IID exhibited a high V_oc of 0.95 V, which shows the highest value among the PSCs based on the narrow bandgap polymer donors with E_g < 1.6 eV.

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Low band gap (E_g) conjugated polymer photovoltaic donor materials have been a hot research subject in the field of polymer solar cells (PSCs) in recent years, owing to their distinguished optical and optoelectronic response to visible and NIR light.¹ Compared to poly(3-hexylthiophene) (P3HT) which is the most representative polymer donor material, more photons can be absorbed from the low band gap polymers and transferred into effective currents. Thus developing low band gap polymers has become an effective strategy to realize highly efficient PSCs.² The low bandgap conjugated polymers can be obtained by copolymerization of an electron-donating unit (D) and an electron-accepting unit (A),^3,4 attaching electron-rich or electron-deficient functional substituents, or using quinoid resonance structure.² However, lowering E_g often results in a high-lying HOMO energy level of the conjugated polymers,⁵ which will make the polymer unstable against oxidation and lead to a lower open circuit voltage (V_oc) of the PSCs based on the polymer as donor. Therefore, in the molecular design of the photovoltaic donor polymers, decreasing the HOMO and LUMO energy levels whilst keeping E_g low should be considered.

Isoindigo, known since antiquity, is industrially used as dyestuff,⁶ and now it is also found important photovoltaic effect.⁷ As shown in Scheme 1, isoindigo bears two symmetrical oxindole motif and can be regarded as a compound with trans stilbene hydrocarbon backbone linked by two lactame rings. The two lactame rings affiliated to adjacent aryl rings in isoindigo ensure its planar molecular structure. Recently, it has been reported that isoindigo is obviously a strong electron-accepting unit.⁷ Therefore, isoindigo could be a promising acceptor unit in D–A copolymers with low bandgap and lower LUMO energy level.^7d–j In the other hand, benzo[2,1-b:3,4-b′]dithiophene (BDP) as a derivative of benzo[1,2-b:4,5-b′]dithiophene (BDT) is modified by a benzo-annellation. From molecular simulations and experiments results in some papers, the HOMO levels of homopolymers of BDP and BDT are −5.70 eV and −5.16 eV, respectively.⁸ Obviously, the former exhibits a lower HOMO level than that of the former, which suggests that the a new polymer containing BDP unit can show a lower-lying HOMO than those similar structure based BDT polymers. Meanwhile, BDP containing polymers have showed high photovoltaic performances.^8–10

Scheme 1 Molecular model of isoindigo (IID) unit.

Based on these considerations, in this work we synthesized a new low band gap D–A copolymer of poly(benzo[2,1-b:3,4-b′]dithiophene-alt-isoindigo) (PBDP-IID) with BDP as the donor unit and isoindigo as a acceptor unit. PBDP-IID possesses a low E_g of 1.57 eV and a lower HOMO energy level of −5.44 eV. The PSCs based PBDP-IID as donor demonstrated a power conversion efficiency of 5.07% with a high open circuit voltage (V_oc) of 0.95 V. To our knowledge, the V_oc of 0.95 V is the highest value among the PSCs based on the narrow bandgap polymer donors with E_g < 1.6 eV.

The synthetic route of PBDP-IID is shown in Scheme 2. In acidic condition, 6-bromoisatin (1) and 6-bromooxindole (2) was condensed to form 1,1′-dihydro-6,6′-dibromoisoindigo (3) as brown solid power with 90% yield. Then alkyl side chains was employed to increase the solubility of isoindigo. The monomer of isoindigo was copolymerized with the monomer of BDP unit by typical Stille coupling reaction to obtain deep dark green solid power PBDP-IID with a 64% yield.

Scheme 2 Synthetic route of PBDP-IID: (i) acetic acid, conc. hydrochloric acid, reflux, 9 h; (ii) 2-ethylhexyl bromide, 12 h, DMF, K₂CO₃ 100 °C, 10 h, argon; (iii) Pd(PPh₃)₄, toluene, reflux, 20 h.

The polymer of PBDP-IID is readily dissolved in common solvents such as chloroform and THF. The corresponding weight average molecular weight (M_w) of PBDP-IID is 37.6 K, with a polydispersity index (PDI) of 2.3. Thermal stability of the polymer was investigated with thermogravimetric analysis (TGA) (see ESI†). The TGA result reveals that, in the air, the onset temperature with 5% weight-loss (T_d) of PBDP-IID is 379 °C. This indicates that it has good thermal stability against oxygen, which is very important in device fabrication process and other kinds of applications.

Fig. 1 shows the absorption spectra of PBDP-IID in dilute chloroform solutions (s) and solid thin films (f). The absorption spectrum in dilute chloroform shows two absorption bands, which are common characters for D–A type low band gap polymers. The absorption band located at 300–500 nm should be attributed to the π–π* transition of the conjugated polymer main chains and the longer wavelength absorption band located at 500–800 nm are caused by intramolecular charge transfer (ICT) interaction between electron-rich BDP and electron-deficient IID units. The main absorption peak and onset of PBDP-IID appeared at 678 nm and 769 nm, respectively. In solid film state, the absorption spectrum of PBDP-IID shows red shift in some degree. The main absorption peak is red shifted to 691 nm and absorption edge extended to 792 nm due to more aggregated configurations formed in film than in solution. The optical bandgaps (E^opt_g) calculated from the absorption edge of the PBDP-IID film is 1.57 eV, which confirms that the low band gap conjugated polymer is indeed realized by incorporating new isoindigo acceptor unit in the D–A copolymers.

Fig. 1 Normalized UV-Vis absorption spectra of PBDP-IID in dilute CHCl₃ solutions (s) and in solid films (f).

Electrochemical cyclic voltammetry has been widely employed to investigate the redox behavior and to estimate the HOMO and LUMO energy levels of a conjugated polymer. Fig. 2 shows the cyclic voltammogram of PBDP-IID. Clearly, PBDP-IID shows reversible p-doping/dedoping and n-doping/dedoping processes in the positive and negative scanning potential range. The onset oxidation potential (E_ox) is 0.73 V vs. Ag/Ag⁺ and the onset reduction potential (E_ox) is −0.94 V vs. Ag/Ag⁺, respectively. The corresponding HOMO and LUMO values are calculated according to the equations LUMO = −e(E_red + 4.71) (eV) and HOMO = −e(E_ox + 4.71) (eV)¹¹ (where the units of E_ox and E_red are V vs. Ag/Ag⁺). The corresponding HOMO and LUMO values are −5.44 eV and −3.77 eV, respectively. The electrochemical band gap (E^ec_g), taken as the difference between the onsets of oxidation and reduction potentials, is equal to 1.67 eV for PBDP-IID. The deeper HOMO level of PBDP-IID is favorable for realizing high open-circuit voltage (V_oc) of the PSCs based on the polymer as donor.

Fig. 2 Cyclic voltammograms of the PBDP-IID film on a platinum electrode in acetonitrile solution containing 0.1 mol L⁻¹ Bu₄NPF₆ at a scan rate of 20 mV s⁻¹.

To evaluate the effect of the strong electron-deficient IID unit in the copolymer with BDP unit on the photovoltaic properties, polymer solar cells (PSCs) were fabricated with a structure of ITO/PEDOT:PSS/PBDP-IID/PC₇₀BM/Ca/Al. The active layers were spin-coated from o-dichlorobenzene solutions with different PBDP-IID/PC₇₀BM weight ratios of 1 : 1, 1 : 2 and 1 : 3, respectively. The devices were completed by evaporating Ca/Al metal electrodes with an area of 4 mm² as defined by masks. Fig. 3 shows J–V curves of the devices under illumination of AM 1.5G (100 mW cm⁻²) and the device performance parameters are listed in Table 1. The photovoltaic performance of PBDP-IID exhibited a remarkable high V_oc of above 0.9 V for such low band gap of 1.57 eV. The J_sc increases from 6.77 mA to 9.21 mA and to 7.81 mA when the PBDP-IID/PC₇₀BM weight ratio changes from 1 : 1 to 1 : 2 and to 1 : 3, but FF changed little at about 55%. A higher power conversion efficiency (PCE) of 5.07% was obtained under the condition of 1 : 2 weight ratio. The results indicate more acceptor content in the active layer could lead to better balance between electrons and holes transfer process. Additional, the J–V curves of with different incident light intensity (from 100 to 1 mW cm⁻²) are measured and it was found J_sc is linear with illuminated light intensity and there is no obvious change for V_oc in the devices. The FF appears maximum at 70 mW cm⁻² but ultimately the optimized PCE reaches its maximum of 5.07% at 100 mW cm⁻². Meanwhile, Fig. 4 shows the EQE curves of the PSC based on a blend of PBDP-IID/PCBM with a optimized weight ratio, which matches well the absorption range of the active layer blend.

Fig. 3 J–V curves of the polymer solar cells based on PBDP-IID/PC₇₀BM under illumination of AM 1.5, 100 mW cm⁻².

Table 1 Photovoltaic properties of the PSCs based on the PBDP-IID:PC₇₀BM

Weight ratio	Thickness (nm)	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)
1 : 1	95	0.91	6.77	54	3.33
1 : 2	100	0.95	9.21	58	5.07
1 : 3	92	0.97	7.81	56	4.24

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Fig. 4 EQE curves of the PSCs with a structure of ITO/PEDOT:PSS/PBDP-IID:PC₇₀BM (1 : 2, w/w)/Ca/Al.

It is very interesting to note that an impressive high V_oc of 0.95 V was obtained for the PSC based on this rather low band gap (<1.6 eV) polymer donor. The high V_oc origination could be discussed from the energy level chart shown in Fig. 5. Considering a charge separation drive force of about 0.2–0.3 eV for the exciton dissociation from the LUMO offset of PBDP-IID and PC₇₀BM, there is only 1.44 eV left between the HOMO of PBDP-IID and the LUMO of PC₇₀BM. According to the theory equation: V_oc = |E^donor HOMO| − |E^acceptor LUMO| − 0.3 V,¹² little energy loss during the charge transfer and lead to a very high V_oc of 0.95 V. Obviously, the high V_oc originates from the low-lying HOMO of the PBDP-IID donor material.

Fig. 5 Energy level chart of PBDP-IID and PC₇₀BM.

Morphology study

Surface morphology of the blend films of polymers/PC₇₀BM with different weight ratios were investigated by the atomic force microscope (AFM). As shown in Fig. 6(a) and (c), there is very large domain sizes between D/A interfacial areas, which makes the surface morphology rather roughness. In Fig. 6(b), on the contrary, the domain sizes becomes smaller and surface becomes smoother than that in Fig. 6(a) and (c). From our measurement results, root-meansquare (RMS) of these films are 6.6 nm, 2.1 nm and 3.1 nm for AFM height images with different weight ratio of 1 : 1, 1 : 2 and 1 : 3, respectively, which coincides above conclusions. These very different morphologies imply that the different weight ratio for polymer/PCBM blends affect the interactions between D/A interfacial areas and at a ratio of 1 : 2 condition the morphology has been better optimized. Therefore, it is reasonable to get the highest short circuit current and PCE from a blend of 1 : 2 weight ratio for PBDP-IID based devices.

Fig. 6 AFM height images (2 × 2 μm²) of the active layers from polymer:PC₇₀BM with different weight ratio of (a) 1 : 1; (b) 1 : 2; (c) 1 : 3.

In summary, a new low band gap D–A copolymer of PBDP-IID containing BDP donor unit and IID acceptor unit was synthesized for the application as donor materials in PSCs. The polymer showed a low optical band gap of 1.57 eV and possess a low-lying HOMO energy level of −5.44 eV. The PSCs made from blends of this polymer and PC₇₀BM exhibited an efficient PCE of 5.07% a high V_oc of 0.95 V, which is the highest value reported so far for the PSCs based on the polymer donors with a band gap below 1.6 eV. The low-lying HOMO energy level of PBDP-IID plays a main role in determining this high V_oc. The results indicate that the isoindigo-based D–A copolymer could possess a narrow bandgap as well as low-lying HOMO level. Therefore it shows a promising potential for the PSCs with tandem structures.

Acknowledgements

This work was supported by National Natural Science Foundation of China (NSFC) (no. 51273203 and 51261160496).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12171f

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