A polymer acceptor with an optimal LUMO energy level for all-polymer solar cells

A new polymer acceptor based on the BNBP unit with an optimal LUMO energy level has been developed. The resulting all-polymer solar cells show high PCEs, remarkably high Voc values and small photon energy losses.


Introduction
All-polymer solar cells (all-PSCs), which utilize polymers as both the electron donor and electron acceptor, have attracted much attention recently because of their great advantages over conventional polymer/fullerene PSCs. 1 These advantages include enhanced light absorption of polymer acceptors, low cost, and improved mechanical/thermal stability. Great progress in all-PSCs has been made by using absorption-complementary polymer donor/acceptors, optimizing the blend morphologies, or developing new polymer acceptors. 2 However, the further development of all-PSCs is severely limited by the lack of excellent polymer acceptors. 3 To date, only several specic polymer acceptors based on the naphthalene diimide (NDI) unit, perylenediimide (PDI) unit and B)N bridged thienylthiazole (BNTT) unit can work as polymer acceptors for efficient all-PSCs with power conversion efficiencies (PCEs) exceeding 4%. 4,5 A key parameter for polymer acceptors is the lowest unoccupied molecular orbital (LUMO) energy level (E LUMO ). In all-PSCs, the E LUMO difference between the acceptor and donor (DE LUMO ) is regarded as the driving force for the charge separation. 6 The difference between the E LUMO of the acceptor and the highest occupied molecular orbital (HOMO) energy level (E HOMO ) of the donor is related to the open-circuit voltage (V oc ) of all-PSCs. 6 Therefore, to get a large DE LUMO for effective charge separation and to maximize V oc , the E LUMO of the polymer acceptor must be carefully optimized. The state-of-the-art polymer acceptors are the NDI-based conjugated polymers. 4 Unfortunately, the E LUMO of these polymers are xed at ca. À3.85 eV and cannot be effectively tuned, leading to a low V oc of the resulting all-PSCs. According to a study by Takimiya et al.,7 the xed E LUMO of NDI-based polymers are due to the localized LUMOs on the NDI units. The E LUMO of the NDI-based conjugated polymers are determined by the NDI unit and are not affected by the copolymerization units. Thus, it is important but challenging to develop polymer acceptors with tunable E LUMO .
Following our strategy to develop polymer acceptors using the B)N unit, 5 we have reported a new electron-decient building block based on the B)N unit, double B)N bridged bipyridine (BNBP), to develop a polymer acceptor. 8 In this manuscript, we report that BNBP-based polymer acceptors show tunable E LUMO because of their delocalized LUMOs over the polymer backbones. The E LUMO of the copolymer of the BNBP unit and selenophene unit (P-BNBP-Se) is lower by 0.16 eV than that of the copolymer of the BNBP unit and thiophene unit (P-BNBP-T) (Fig. 1). As a result, the energy levels of P-BNBP-Se match well with the widely-used polymer donor, poly[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7-Th). 9 While the all-PSC device based on the PTB7-Th:P-BNBP-T blend shows a moderate PCE of 2.27%, the corresponding device with P-BNBP-Se as the acceptor exhibits a PCE as high as 4.26% with a remarkably high V oc of 1.03 V. These results indicate that BNBP-based polymer acceptors have different electronic structures from those of the classical NDIbased polymer acceptors and that they can give all-PSCs with remarkably high V oc values and high PCEs.

Results and discussion
Scheme 1 shows the synthetic route of P-BNBP-Se and P-BNBP-T. The three monomers were prepared following literature methods and the two polymers were synthesized in Stille-polymerization conditions. 8 Their chemical structures are conrmed by 1 H NMR and elemental analysis. According to gel permeation chromatography (GPC), with 1,2,4-trichlorobenzene as the eluent at 150 C, the number-average molecular weight (M n ) and polydispersity (PDI) are 26.3 kDa and 1.93 for P-BNBP-Se and 46.2 kDa and 1.81 for P-BNBP-T, respectively. According to the thermogravimetric analysis (TGA), P-BNBP-T and P-BNBP-Se show a good thermal stability with thermal decomposition temperatures (T d ) of over 350 C (ESI †). In addition, the two polymers show a good solubility in common organic solvents, including chlorobenzene (CB), chloroform (CHCl 3 ) and odichlorobenzene (o-DCB).
To elucidate the molecular orbitals of the two polymers, density functional theory (DFT) calculations at the B3LYP/6-31G* level of theory were performed with the model compounds containing six repeating units with the long alkyl chains replaced by methyl groups. 10 For comparison, we also show the DFT calculation result of the state-of-the-art polymer acceptor, (poly((N,N 0bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl)alt-5,5 0 -(2,2-bithiophene))) (N2200 or P(NDI2ODT2)) ( Fig. 1a). 11 As shown in Fig. 2, the calculated LUMO of the model compound of N2200 is localized on the NDI units, indicating that its E LUMO is determined by the NDI unit and cannot be effectively tuned by changing the co-monomer units. This is consistent with the DFT calculation and experimental results of NDI-based conjugated polymers in the literature. 4,11 In contrast, the calculated LUMOs of the model compounds of P-BNBP-Se/P-BNBP-T are delocalized over the BNBP units and the selenophene/thiophene units. Therefore, the LUMO levels of BNBPbased polymers are determined by both the BNBP unit and the co-monomer unit. The LUMO levels of BNBP-based polymers should be effectively tuned by changing the co-monomer units.
Cyclic voltammetry was employed to estimate the LUMO/ HOMO energy levels of the two polymers (ESI †). 12 As shown in Fig. 3a, P-BNBP-Se exhibits irreversible reduction and oxidation waves with onset potentials of E red onset ¼ À1.14 V and E ox onset ¼ +1.04 V, respectively. Accordingly, the E LUMO/HOMO of P-BNBP-Se are estimated to be À3.66 eV/À5.84 eV (Table 1). Similarly, the E LUMO/HOMO of P-BNBP-T are estimated to be À3.50 eV/À5.77 eV ( Table 1). As reported previously, the model compound of the BNBP unit itself has an E LUMO of À3.19 eV. The E LUMO of the two BNBP-based polymers are much lower than that of the BNBP unit. Moreover, the E LUMO of P-BNBP-Se is lower than that of P-BNBP-T by 0.16 eV. These results conrm that the LUMO levels of BNBP-based polymers can be effectively tuned by changing Scheme 1 Synthetic route of P-BNBP-Se and P-BNBP-T. the co-monomer units. This is consistent with the delocalized LUMOs in the DFT calculation results. The lower-lying E LUMO of P-BNBP-Se is attributed to the lower electronegativity of the Se atom (2.4) than the S atom (2.5) and the empty orbital of the Se atom. 13 Fig. 3b shows the absorption spectra of P-BNBP-Se and P-BNBP-T in dilute o-DCB solutions and in thin lms. Both of the two polymers in solutions show broad absorption bands around l ¼ 580 nm. The absorption spectrum is slightly redshied for P-BNBP-Se compared to P-BNBP-T. In thin lm, P-BNBP-Se exhibits a maximum absorption at 635 nm, while P-BNBP-T shows the absorption peak at 622 nm. Both of the two lms show high absorption coefficients (3), suggesting their intense light absorption. According to the onset absorption wavelength in thin lms, the optical band gaps (E g ) of P-BNBP-Se and P-BNBP-T are estimated to be 1.87 eV and 1.92 eV, respectively. The electron mobilities (m e ) of P-BNBP-Se and P-BNBP-T were estimated using the space-charge-limited current (SCLC) method with the current density-voltage curves of the electrononly devices (device structure: ITO/PEIE/polymer/Ca/Al). 14 The electron mobility of P-BNBP-Se (m e ¼ 2.07 Â 10 À4 cm 2 V À1 s À1 ) is higher than that of P-BNBP-T (m e ¼ 7.16 Â 10 À5 cm 2 V À1 s À1 ) (ESI †). The higher electron mobility of P-BNBP-Se is due to the stronger intermolecular interactions in Se-containing polymers because of the larger and more polarizable radii of the selenium atom than the sulfur atom. This is conrmed by the smaller p- The electron mobility of P-BNBP-Se is comparable to the hole mobilities of typical polymer electron donors, which is very favourable for its application as a polymer electron acceptor in all-PSCs.
To investigate the application of P-BNBP-Se and P-BNBP-T as electron acceptors in all-PSCs, we select a widely-used polymer donor, PTB7-Th. All-PSC devices were fabricated with a conguration of ITO/PEDOT:PSS/PTB7-Th:P-BNBP-Se or P-BNBP-T/ Ca/Al (ESI †). The active layer was spin-coated from the blend in o-DCB solution without any additives. Fig. 4 shows the current density-voltage (J-V) curves under AM 1.5G illumination (100 mW cm À2 ) and the external quantum efficiency (EQE) spectra of the optimal devices. The photovoltaic parameters are summarized in Table 2. The PTB7-Th : P-BNBP-T (3 : 1, w:w) device shows a PCE of 2.27% with a V oc of 1.12 V, a short-circuit current density (J sc ) of 5.24 mA cm À2 and a ll factor (FF) of 0.39. The device based on the PTB7-Th : P-BNBP-Se (2 : 1, w:w) blend exhibits a PCE of 4.26% with a V oc of 1.03 V, a J sc of 10.02 mA cm À2 and a FF of 0.42. This PCE value is comparable to that of the reference all-PSC device based on the PTB7-Th : N2200 (1 : 1, w:w) blend from the chloroform solution (PCE ¼ 4.57%), indicating that P-BNBP-Se is an excellent polymer acceptor. Compared with the device of P-BNBP-T, the device of P-BNBP-Se shows a slightly decreased V oc and much increased J sc . The slightly decreased V oc is attributed to the lower E LUMO of P-BNBP-Se than that of P-BNBP-T. On the other hand, the V oc of the P-BNBP-Se device is higher than that of the N2200 device by 0.22 V (Table 2) because the E LUMO of P-BNBP-Se is higher than   that of N2200. The much increased J sc of the P-BNBP-Se device than that of the P-BNBP-T device is in accordance with their EQE values (EQE max ¼ 0.47 for P-BNBP-Se and EQE max ¼ 0.25 for P-BNBP-T) (Fig. 4b). The J sc calculated from the integration of the EQE spectra agrees well with the J sc values obtained from the J-V scans within an error of 5%. The charge carrier mobilities of the two blends were investigated using the SCLC method with the electron-only and holeonly devices (ESI †). 14 The electron mobility and hole mobility (m h ) of the PTB7-Th:P-BNBP-Se blend are estimated to be 3.34 Â 10 À5 cm 2 V À1 s À1 and 2.38 Â 10 À4 cm 2 V À1 s À1 , respectively. In comparison, the PTB7-Th:P-BNBP-T blend exhibits a m e ¼ 5.96 Â 10 À6 cm 2 V À1 s À1 and m h ¼ 7.28 Â 10 À4 cm 2 V À1 s À1 , respectively. The higher electron mobility and the balanced electron/hole mobilities of the PTB7-Th:P-BNBP-Se blend are due to the enhanced electron mobility of P-BNBP-Se. We also investigated the bimolecular charge recombination in the all-PSC devices using the light-intensity dependence of the J-V curves (Fig. 5). The J sc follows a power-law dependence on the illumination intensity (J sc f P light a ), where P light is light intensity and a is the calculated power-law exponent. The a values are 0.93 for the PTB7-Th:P-BNBP-Se device and 0.94 for the PTB7-Th:P-BNBP-T device, which are close to unity, suggesting that the bimolecular charge recombination is weak in the two devices at a short circuit condition. 15 Both the weak bimolecular recombination and the high and balanced electron/hole mobilities of PTB7-Th:P-BNBP-Se can explain its excellent device performance.
The morphologies of the PTB7-Th:P-BNBP-Se and PTB7-Th:P-BNBP-T blends were characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM). As shown in Fig. 6, the TEM images exhibit similar nano/microstructures without large-size aggregation. The AFM images of the two blends similarly reveal smooth surface morphologies with the same root-mean-square (RMS) roughness of 1.47 nm and domain sizes of around 20-40 nm. The phase separation morphologies of the two blends are benecial for good all-PSC devices.
In organic photovoltaics (OPVs), the DE LUMO of the donor and acceptor is regarded as the driving force for charge separation. The DE LUMO should be larger than a specic value for efficient charge separation. If DE LUMO is too large, there is a lot    of energy loss in the charge separation process, leading to a low V oc because the V oc of OPVs is related to the difference between the E HOMO of the donor and E LUMO of the acceptor. 6 In our previous report, an all-PSC device based on the PTB7:P-BNBP-T blend (DE LUMO ¼ 0.19 eV) showed a good PCE of 3.38%. 8 As shown in Fig. 1b, the DE LUMO is only 0.06 eV for PTB7-Th:P-BNBP-T, and thus the all-PSC device shows a high V oc but produces a low PCE due to the insufficient charge separation. 4g As the E LUMO of P-BNBP-Se is lower than that of P-BNBP-T, the DE LUMO for PTB7-Th:P-BNBP-Se is increased to 0.22 eV and ensures an efficient charge separation, resulting in higher J sc and PCE values. Moreover, due to the suitable E LUMO of P-BNBP-Se, the PTB7-Th:P-BNBP-Se device produces a high V oc of 1.03 V, which is higher than that of the PTB7-Th:N2200 device by 0.22 V. These results indicate that the suitable E LUMO of P-BNBP-Se plays an important role in enhancing the all-PSCs device performance.
It is worthy to note the remarkably low photon energy losses (E loss ) of the all-PSCs based on P-BNBP-Se and P-BNBP-T. E loss is dened as the difference between the lowest optical bandgap of the donor/acceptor and the eV oc of the organic photovoltaic (OPV) device (E loss ¼ E g À eV oc ). 16 Typically, OPVs have large E loss values of 0.7-1.0 eV. It has been proposed that the lowest attainable E loss of OPVs is 0.6 eV, despite several exceptional examples. 17 As listed in Table 2, the E loss for the device of PTB7-Th:P-BNBP-Se and PTB7-Th:P-BNBP-T is 0.56 eV and 0.47 eV, respectively. To our best knowledge, the E loss of 0.47 eV is the lowest one for OPVs reported so far. A small E loss is always observed for all-PSCs with BNBP-based polymers as electron acceptors and the exact reason is as yet unknown. We speculate that the small E loss is related to the high-lying LUMO levels of the BNBP-based polymers.

Conclusions
In summary, we have developed a polymer acceptor based on the BNBP unit and selenophene unit with an optimal E LUMO to simultaneously enable charge separation and maximize V oc . BNBP-based polymers have delocalized LUMOs over the polymer backbones, so their E LUMO can be tuned by changing the comonomer unit. The E LUMO of P-BNBP-Se is lower by 0.16 eV than that of P-BNBP-T and consequently matches well with that of the polymer donor, PTB7-Th. While the all-PSC device based on PTB7-Th:P-BNBP-T shows a moderate PCE of 2.27%, the corresponding device with P-BNBP-Se as the acceptor exhibits a PCE as high as 4.26%. Moreover, the device of P-BNBP-Se shows a V oc of up to 1.03 V and E loss as small as 0.56 eV. These results indicate that BNBP-based polymer acceptors have different electronic structures from those of classical NDI-based polymer acceptors and that they can give all-PSCs with remarkably high V oc values and high PCEs.