Rotaxane Structure Optimizes the Photovoltaic Performance of Polymer Solar Cells★

doi:10.6023/A23070353

Abstract

The degree of polymerization often affects the blending microstructure of the active layer in polymer solar cells (PSC), resulting in a large difference in device performance. Generally, when the degree of polymerization is low, the power conversion efficiency (PCE) of PSC is usually significantly reduced due to the impact of charge transport. To address this issue, we synthesized a series of polymer donors with rotaxane structure by introducing crown ether into the polymers, and four molecules PM6-C1, PM6-C2, PM6-C3 and PM6-C4 with different contents of crown ether were obtained. The rotaxane structure is composed of cyclic compounds and polymers through non-covalent bonds, which can affect intermolecular stacking by non-covalent interaction while retaining the excellent photoelectric properties of the polymer. The crown ether was randomly inserted into the backbone of polymer in the process of Stille coupling. By limiting the reaction time, the number-average molecular weight was guaranteed to be lower than 50 kDa. The devices and blend films based on the resultant polymers were characterized by means of external quantum efficiency (EQE), light intensity dependence, mobility, atomic force microscopy (AFM), transmission electron microscopy (TEM), grazing incidence wide-angle X-ray scattering (GIWAXS), etc. In order to study the differences in device performance caused by the different introduction methods, the crown ether was directly added as an additive, and the open-circuit voltage (V_oc) and PCE of the device were significantly reduced. However, the device performance was different when the crown ether and the polymer backbone formed a rotaxane structure. Compared with PM6-L, the active layer based on PM6-C2 and Y6 exhibited an optimal fiber-like network structure and better phase separation scale, which contributed to better charge extraction efficiency and reduced trap/bimolecular recombination. Ultimately, more excellent short-circuit current density (J_sc) and fill factor (FF) were obtained because of high charge transport and collection efficiency. Therefore, the PM6-C2-based device achieved a PCE of 16.23%, exceeding that of the PM6-L-based device (15.33%). This work indicates that rotaxane structure is beneficial to improve the active layer morphology and show great potential in developing PSC materials.

Key words: polymer solar cells, non-covalent bond, rotaxane structure, crown ether, phase separation scale

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Dongxu Li, Xiang Xu, Jiage Song, Songting Liang, Yuang Fu, Xinhui Lu, Yingping Zou. Rotaxane Structure Optimizes the Photovoltaic Performance of Polymer Solar Cells^★[J]. Acta Chimica Sinica, 2023, 81(11): 1500-1507.

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Fig. & Tab. 7

Figure 1. Synthetic route of the PM6-C series polymers

Figure 2. (a) The normalized absorption spectra of polymer donors and acceptor in thin film state; (b) energy level diagram of polymer donors in this work

Polymer	λ_max^sol/nm	λ_max^film/nm	λ_onset^film/nm	E_g^opt/eV	HOMO/eV	LUMO/eV	E_g^cv/eV
PM6-L	609	614	684	1.81	–5.55	–3.38	2.17
PM6-C1	611	619	690	1.80	–5.54	–3.37	2.17
PM6-C2	613	620	688	1.80	–5.54	–3.37	2.17
PM6-C3	612	622	686	1.81	–5.53	–3.39	2.14
PM6-C4	612	620	685	1.81	–5.52	–3.40	2.12

Table 1. Optical and electrochemical properties of the polymers

Figure 4. AFM height images and the corresponding phase images of (a, f) PM6-L:Y6, (b, g) PM6-C1:Y6, (c, h) PM6-C2:Y6, (d, i) PM6-C3:Y6, and (e, j) PM6-C4:Y6 blend films (2 μm×2 μm)

Figure 4. (a) The 2D GIWAXS patterns, and (b) corresponding in-plane (red) and out-of-plane (black) 1D integral curves of PM6-L:Y6, PM6-C1:Y6, PM6-C2:Y6, PM6-C3:Y6, and PM6-C4:Y6 blend films

Active layers	V_oc/V	J_sc/(mA•cm^-2)	J_sc-EQE/(mA•cm^-2)	FF/%	PCE/%
PM6-L:Y6	0.86 (0.85±0.01)	24.84 (24.53±0.50)	24.79	72.10 (70.75±1.50)	15.33 (15.10±0.23)
PM6-C1:Y6	0.84 (0.84±0.01)	24.81 (24.63±0.30)	24.20	75.47 (74.66±1.61)	15.82 (15.55±0.27)
PM6-C2:Y6	0.85 (0.84±0.01)	25.80 (25.56±0.33)	25.58	73.46 (72.66±1.22)	16.23 (16.02±0.21)
PM6-C3:Y6	0.85 (0.84±0.01)	25.13 (24.88±0.40)	24.99	75.63 (73.56±2.00)	16.08 (15.88±0.20)
PM6-C4:Y6	0.85 (0.84±0.01)	25.58 (25.28±0.36)	25.28	69.94 (68.30±1.70)	15.21 (14.99±0.22)

Table 2. Photovoltaic parameters of PSC devices based on PM6-L and PM6-C series donors

Figure 5. (a) The J-V curve, (b) the external quantum efficiency (EQE) response curve, (c) the plot of photocurrent against effective voltage characteristic curve, (d) the plots of open circuit voltage against incidence light intensity characteristic curve, (e) the short circuit current density against incidence light intensity characteristic curve, (f) the hole transport mobility (μh) of the blend films based on the PM6-L and PM6-C series polymers:Y6 blends

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