Rational Design of Conjugated Polymers for d-Limonene Processed All-polymer Solar Cells with Small Energy Loss

In this work, we designed and synthesized a novel naphthalenediimide-based n-type conjugated polymer PNDICI, which bears asymmetric backbone containing a 3-chlorothiophene unit. The asymmetric structure associated with steric effects of the chlorine atom imparts remarkable solubility to PNDICI in various organic solvents, enabling the fabrication of all-polymer solar cells (all-PSCs) by using an environmentally friendly solvent of d-limonene. Combined with a novel pyrrolo[3,4-f]benzotriazole-5,7(6H)-dione based p-type conjugated polymer P2F-Si with deep highest occupied molecular orbital energy level, the resulting d-limonene-processed all-PSCs presents an impressively high open-circuit voltage of approaching 1.0 V, corresponding to a very small energy loss of 0.49 eV. Through further morphology optimization by using γ-valerolactone, we demonstrated an impressive device efficiency of 4.2%, which is among the best photovoltaic performance of devices processed using d-limonene and comparable to that processed by conventional solvent, suggesting the great promise of using greener solvent for fabricating high-performance all-PSCs.


INTRODUCTION
Polymer solar cells (PSCs) have attracted tremendous attention owning to their unique merits of low-cost, solution-production, and mechanical flexibility. [1−9] The state-of-the-art PSCs have realized impressive power conversion efficiencies (PCEs) of over 16%, [10−14] mitigating the gap regarding to other emerging photovoltaic techniques. In addition to developing PSCs toward large-scale roll-to-roll production, [15,16] numerous efforts have been devoted to all-polymer solar cells (all-PSCs), [17−23] which include both p-type and n-type polymers in the light-harvesting layer. [24−32] While most all-PSCs are processed using toxic halogenated solvents, much interest has been devoted to seeking non-toxic, environmentally benign processing solvents toward large-scale clean production. In this regard, a variety of nonhalogenated solvents, such as toluene, 1,2,4-trimethylbenzene, tetrahydrofuran, methoxybenzene, 2-methytetrafuran (MeTHF), and cyclopentyl methyl ether (CPME) were thus employed. [33−39] Among the reported non-aromatic solvents, the ether solvents such as MeTHF or CPME are much appreciated, since they can enable pre-aggregation of polymer chains in solution due to its moderate solubility, and thus lead to highlyaligned polymer films that are favorable for inter-molecular charge transport. [40,41] In comparison to those aforementioned non-halogenated and non-aromatic solvents, d-limonene (LM, Fig. 1a) is much more environmentally friendly, since it is originated from the peel of citrus fruits and can be used as a flavoring agent in food. [42] However, the application of LM for processing conjugated polymers is limited, because most conjugated polymers with rigid backbone can be barely dissolved in LM ( Fig. S1 and Table S1, in the electronic supplementary information, ESI). To fabricate all-PSCs using LM, we herein developed a novel n-type polymer, poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-[3(3′)chloro-2,2′-bithiophen]} (PNDICl, Fig. 1a), which has an asymmetric structure containing a chlorinated thiophene on the main chain. Considering that the chlorine substitution generally leads to down-shifted lowest-unoccupied molecular orbital (LUMO) energy level of the resulting polymers, the resulting devices may attain reduced open-circuit voltage (V OC ). [43] To address this issue, we correspondingly developed a novel p-type polymer, namely P2F-Si (Fig. 1a), which con-tains a pyrrolo[3,4-f]benzotriazole-5,7(6H)-dione (TzBI) skeleton with a siloxane-ended alkyl-side chain. The electron-deficient TzBI unit presented similarly moderate electron-withdrawing capability regarding to the difluorinated benzotriazole unit, yet the imide-group allowed for the incorporation of an additional solubilizing group that can impart excellent solubility of the resulting group. [28] Moreover, the siloxane moiety was incorporated as the end-group of the side chain because it can further enhance the solubility without disturbing the intermolecular stacking regarding to the branched alkyl-side chains. [44] It is also worth noting that the electron-withdrawing difluorophenyl substituents of benzo[1,2b:4,5-b′]dithiophene can result in decreased highest occupied molecular orbital (HOMO) energy level, which can potentially compensate for the possibly decreased V OC derived from the slightly down-shifted LUMO energy level of PNDICl. Of particular importance is that both P2F-Si and PNDICl can be easily dissolved in LM (b.p. ~177 °C, Table S2 in ESI). The resulting all-PSCs presented an impressively high V OC of approaching 1.0 V, associating with a relatively small energy loss of 0.49 eV. Through a morphology optimization strategy by using a non-halogenated solvent additive of γ-valerolactone (VA, Fig. 1a), a decent efficiency of 4.18% is achieved, suggesting the great promise of using greener solvent for fabricating high-performance all-PSCs.

Synthesis and Characterization of Materials
The synthetic routes of the polymers are summarized in Scheme  S1 (in ESI) with relevant structures confirmed by proton nuclear magnetic resonance (Figs. S2−S4 in ESI). Density functional theory simulation (B3LYP/6-31G (d, p)) indicated that the chlorine on thiophene leads to enlarged dihedral angle to 57.29° ( Fig. S5 in ESI), presenting more pronounced twisted skeleton than that of counterpart copolymer N2200. The asymmetric structure of PNDICl enabled remarkable solubility in green solvent LM ( Fig. S1 and Table S1 in ESI). The numberaverage molecular weight of P2F-Si and PNDICl was estimated to be 17.9 and 31.7 kDa, respectively. Differential scanning calorimetry demonstrated that PNDICl exhibited a sharp crystallization transition at 295 °C (

Photoelectrical Properties
The HOMO/LUMO energy levels of PNDICl and P2F-Si were estimated to be −5.98/−3.89 eV and −5.51/−3.13 eV, respectively, by cyclic voltammetry measurement ( Fig. S7a in ESI). While the LUMO energy level of PNDICl was slightly deeper than that of N2200, the HOMO energy level of P2F-Si was much deeper than that of PTzBI-Si (Fig. 1b). The combination of these variations would lead to enhanced V OC of P2F-Si:PNDICl than that of PTzBI-Si:N2200. The UV-Vis absorption spectra for PTzBI-Si, P2F-Si, and PNDICl neat films processed by different solvents are illustrated in Fig. 1(c) and Fig. S8 (in ESI). For all three polymers, LM-cast films show enhanced shoulder peaks regarding to those casted from MeTHF-and CPME-solution ( Fig.  S8 in ESI), which should be attributable to the polymer preaggregation in LM. The optical bandgap (E g opt ) of these LMprocessed polymers was estimated by Tauc plots (Fig. S7b, in ESI), where we note that PNDICl exhibits a slightly larger E g opt (1.48 eV) than N2200 (E g opt = 1.46 eV), which can be ascribed to the decreased intra-molecular charge transfer induced by chloride substitution.

Photovoltaic Performance
To investigate the impact of processing solvent on photovoltaic performance, we fabricated all-PSCs with structure of ITO/PEDOT:PSS/photoactive layer/PFNDI-Br/Ag, where PFNDI-Br (~5 nm) was employed to facilitate the electron extraction (Fig. 1a). Since we focused on the processing solvent, the weight ratio of the photoactive layer consisting of donor:acceptor was settled as 2:1 with a fixed thickness of ~100 nm. We also simplified the post-treatments by only thermal annealing at 100 °C for 10 min. The J-V characteristics for all-PSC devices processed by different solvents are depicted in Fig. 2(a), with the corresponding photovoltaic parameters summarized in Table 1.
The control device based on PTzBI-Si:PNDICl processed by CPME presents a V OC of 0.81 V, a short-circuit current (J SC ) of 10.6 mA·cm -2 , and a fill factor (FF) of 63.5%, corresponding to a PCE of 5.4%. In contrast, the LM-processed device exhibits a much higher FF of 72.7% while exhibits an obviously de- creased J SC of 5.1 mA·cm -2 , giving a moderate efficiency of 3.0%. This observation is different for the P2F-Si:PNDICl system, where CPME-and LM-processed devices present similar photovoltaic performance. By exploiting a low-toxic additive of γ-valerolactone (Table S2 in ESI), the LM-processed device presents an optimal power conversion efficiency of 4.2% (V OC = 0.99 V, J SC = 6.6 mA·cm -2 , FF = 64.0%, Table S4 and Fig. S9 in ESI). Note that an impressive V OC approaching 1 V was achieved in this system, demonstrating the validity of decreasing the HOMO energy level of donor toward high V OC . The energy loss (E loss ) was calculated according to the formula of E loss = E g,min -eV OC , where E g,min is the minimal E g between donor and acceptor (Table S3 in ESI). As listed in Table 1, the LM-processed P2F-Si:PNDICl-device has a fairly small E loss of 0.49 eV, much lower than that for PTzBI-Si-based device (E loss = 0.68 eV). Importantly, compared with the devices processed with CPME, devices processed by LM show an obviously increased FF and slightly decreased J SC in both P2F-Si:PNDICl and PTzBI-Si:PNDICl systems. The external quantum efficiency (EQE, Fig. 2b) and the integrated current density (Table 1) confirm the reliability of J SC obtained from the J-V measurement system. To correlate the J SC and FF difference in these devices, we measured the absorption of blend films processed with different solvents. As shown in Fig. S10 (in ESI), one noted an enhanced shoulder peak at 620 and 605 nm attributed to PTzBI-Si and P2F-Si, respectively, for LM-processed blends relative to the MeTHF-and CPME-cases, implying that the polymer pre-aggregation also existed in blend solution. This may contribute to the formation of highly-aligned blend-films and thus account for the much higher FF of LM-processed all-PSCs. However, no obvious difference was discerned in the near-infrared (NIR) region (> 700 nm) that exclusively correlated to the absorption of PNDICl, which can be attributed to the under-estimation of NIR absorption of PNDICl by transmission method. Considering the reflection (R), we depicted the device absorption (1 − R) in Fig. 2(c), where we note a significantly enhanced absorption in the range of 670−750 nm for P2F-Si:PNDICl-based device, while the PTzBI-Si:PNDICldevice maintains a relatively low NIR-absorption (1 − R) that accounts for its poor J SC .
Photoluminescence spectroscopy (PL) was also used to shed light on the difference of current densities, with the thickness of all films fixed as 95 ± 5 nm. As shown in Fig. 2(d), LM-cast neat-film of PTzBI-Si and P2F-Si exhibits a peak at 713 and 690 nm (excited at 580 nm), respectively. Note that the PL intensity of P2F-Si is much higher than that of PTzBI-Si under the same test condition (slit width = 5 nm). After blended with PNDICl, the PL peak of P2F-Si is effectively quenched. In contrast, the quench of PTzBI-Si emission is much less pronounced for the films processed with both CPME-and LM-solution. These observations demonstrate the weak charge transfer in PTzBI-Si:PNDICl-system, especially for the LM-processed films. Additionally, for both systems, the LM-films show less-efficient PL-quenching than the CPMEcounterparts, indicating quite different phase-separated morphology for these blend films ( Table 1). The emission quenching of acceptor was investigated with all films excited at 710 nm. As shown in Fig. S11 (in ESI), the quenching behavior is very similar to that observed for donors.

Morphology Analysis
Atom force microscopy (AFM) was used to reveal the surface morphology of blend films. For P2F-Si:PDNICl, CPME-processed film exhibits a smooth surface with a low root-mean-square (RMS) roughness value of 1.5 nm (Fig. S12a in ESI), while LMprocessing gives large aggregates and much higher RMS roughness of 5.4 nm (Fig. S12b in ESI). Interestingly, the incorporation of 0.5% volume ratio of γ-valerolactone into LM siginificantly reduces the aggregates and decreases the roughness to 2.5 nm (Fig. S12c in ESI), leading to the improved J SC . Note that PTzBI-Si:PDNICl-films show very thick fibers on the surface, especially in the LM-processed case (Figs. S12d and S12e in ESI). The well-organized morphology might contribute to the measured high FF of the resulting devices.
We further conducted the two-dimensional (2D) grazing incidence X-ray diffraction (GIXD) to understand the role of processing solvent in crystalline structures. From the GIXD patterns of neat films processed by LM, we note an evident π-π stacking (010) signal in the out-of-plane (OOP) direction, together with a corresponding in-plane (IP) lamellar stacking (100) peak for both P2F-Si and PNDICl (Fig. S13 in ESI), suggesting the preferential face-on packing orientation relative to the substrates. The P2F-Si:PNDICl blend film produced from CPME maintains face-on π-π stacking, as evidenced from the intense OOP (010) peak with the peak position at q 1.71 Å −1 and a crystal coherent length (CCL) of 17.1 Å (Fig. 3a and Fig. S14 in ESI). LM-processing gives slightly different scattering textures with the (010) peak shifted to a higher q position at 1.73 Å −1 and the CCL increased to 21.0 Å (Table S5 in ESI), indicating a more compact π-π co-facial packing and an increased crystal grain size. However, the (010) reflection arc also extends from OOP direction towards the in-plane (IP) direction (Fig. S13 in ESI), suggesting that a certain amount of π-π stacking was switched from face-on to random arrangement during the LM processing. The incorporation of γ-valerolactone as the solvent additive did not significantly alter the (010) peak, while the (100), (200), and (300) lamellar stacking peaks were mainly centered to OOP direction, implying the ordered side-chain stacking. The improved crystallites embedded in bulk heterojunction (BHJ) framework can facilitate the charge movement and ultimately enhance the fill factor. [45] Table 1 Photovoltaic parameters for all-PSCs processed with different solvents.
Resonant soft x-ray scattering (RSoXS) was also utilized to correlate the phase separation structure with the solar cell efficiency. The RSoXS circularly integrated profiles at beam energy of 285.2 eV are collected in Fig. 3(b). The CPMEprocessed blend film shows poor compositional contrast with a hump at q ~ 0.0044 Å −1 , corresponding to a statistic inter-domain size of 142 nm. When using LM as the processing solvent, the hump is broadened and shifts to 0.0068 Å −1 (domain size ~ 93 nm) with the scattering intensity significantly increasing over the q vector, while additional 0.5% γ-valerolactone leads to a slightly higher domain size of 110 nm (q = 0.0057 Å −1 ). These results demonstrate that using LM for film deposition can decrease the phase separation and meantime lead to higher relative phase purity. The lower disperse of the polymers induced by the increased crystallization is detrimental to the exciton splitting, [46] thus decreasing the PL quenching efficiency (Fig. 2d and Table 1), which agrees well with the slightly decreased J SC of LMdevices.

CONCLUSIONS
In summary, we designed and synthesized a novel naphthalenediimide based polymeric acceptor (PNDICl) containing a chlorinated thiophene in the backbone, and a novel pyrrolo

Electronic Supplementary Information
Electronic supplementary information (ESI) is available free of charge in the online version of this article at https://doi.org/10.1007/s10118-020-2429-3.