Quaternary polymer solar cells with over 13% efficiency enabled by improving film-morphologies via binary mixed fullerene additive

Abstract

In this article, we report two new cases of quaternary polymer solar cells with over 13% efficiency. By introducing bis-PC₇₁BM:PC₆₁BM or bis-PC₇₁BM:IC₆₀BA into a known nonfullerene system – PBDB-T:IT-M, the quaternary solar cells significantly outperform the nonfullerene binary and ternary (PBDB-T:IT-M:fullerene) cells with a significant increase in short-circuit current-density (18.2 vs. 16.5 and 16.8–17.4 mA cm⁻², respectively) and fill-factor (0.73 vs. 0.67 and 0.705–0.726, respectively) and hence large power conversion efficiencies (13.3% for the quaternary vs. 11% for the binary and 12% for the ternary solar cells). Compared with the clear phase-separated domains in the bis-PC₇₁BM based ternary blend, the PC₆₁BM based film showed the finest morphologies and the IC₆₀BA based film had the largest domains. In sequence, the bis-PC₇₁BM:PC₆₁BM-based quaternary blend showed finer morphology than the bis-PC₇₁BM:IC₆₀BA based blend, while both the quaternary blends showed clear phase-separated bright and dark domains. One-dimensional grazing incidence X-ray diffraction (1D GIXRD) data indicated that the addition of PC₆₁BM and IC₆₀BA helped more IT-M molecules to form compacted π–π stacks at the out-of-plane directions, which resulted in the increase in electron mobilities. Our results indicate that the use of the fourth fullerene component provides more choices and more mechanisms than the ternary systems for tuning the photon-to-electron conversion; therefore, this study sheds light on the realization of high-efficiency polymer solar cells by designing multi-acceptor component energy levels.

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Chuanlang Zhan

Chuanlang Zhan received his PhD degree from the Institute of Photographic Chemistry, Chinese Academy of Sciences (CAS) under the supervision of Prof. Duoyuan Wang. Then, he joined the Institute of Chemistry, CAS, as a Postdoctoral fellow at the Laboratory of Organic Solids, where he worked with Prof. Daoben Zhu and Prof. Deqing Zhang. Later, he was promoted as an associate professor at this institute. He is now a professor at ICCAS and working with Prof. Jiannian Yao, a Professor at College of Chemistry and Environmental Science, Hebei University. His research activities focus on the synthesis and self-assembly of functional molecular materials, solar energy conversion and solar cells.

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Introduction

Bulk-heterojunction (BHJ) polymer solar cells (PSCs) have several merits such as semi-transparency, light-weight, low-cost, and flexibility at the same time, which endow PSCs with potentials to meet the future demands of sustainable and green energy sources. The power conversion efficiencies (PCEs) in PSCs are however largely restricted by the intrinsic trade-off between the open-circuit voltage (V_oc), the short-circuit current-density (J_sc), and the fill factor (FF). The ternary blended bulk-heterojunction (T-BHJ)^1–5 and tandem devices^6–9 are two materials that release the trade-off, and the former is of relative simplicity in both device structure and fabrication in keeping with the single junction device structure. A T-BHJ allows for the use of one more donor (D) or one more acceptor (A) to resolve the narrow absorption of organic blend films, giving rise to larger J_sc.^10–13 Moreover, as the third component forms an alloy with the parent D/A component,^14–17 the resulting ternary system behaves like a pseudo-binary system. The mixing becomes as easy as the fabrication of a binary system and the single-junction device structure is maintained. More importantly, the formation of the electronic alloy state allows for (1) the tuning of the interfacial charge transfer (CT) state energy level between the parent two-components' extreme values, which associates with the adjustment of the V_oc between the two binary devices' extremes,^15,16 and (2) the tuning of the film-morphology for the out-of-plane (OOP) charge transport and for the reduction of the recombination losses,¹⁷ leading to a higher FF. Collectively, a larger V_oc·J_sc·FF product is available with the T-BHJ than with the binary blend by judiciously designing the ternary blended photo-active systems.

Nevertheless, the ternary approach has several issues. First, success in the mixing of two or more polymers is still limited to rare cases because of the structure similarity requirement¹⁸ and also due to the weak intermolecular interactions and lack of entropic force between polymers.¹⁹ Second, the mixing between a polymer and a small molecule donor^17,20 or between two small molecule donors^21,22 is still highly challenging because achieving more efficient devices is largely restricted by the complexity in obtaining the ideal film morphologies and donor/acceptor phase crystallinity.^23,24 Distinctly, the structure similarity, the small energy difference (<0.3 eV), the spherical shape, and the entropic force²⁵ benefits the easy mixing between two fullerene acceptors, which largely relaxes the complexity in obtaining the ideal film-morphologies.^26–29 A big issue in this form is the limitation of the number of fullerene derivatives.

Relative to fullerene, the frontier molecular orbitals and the optical band gap, E^opt_g, of fused-ring-based nonfullerene acceptors can be tuned in a wide range with structural modifications.^30–32 Thus far, reports have indicated that the highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) energy levels can be tuned between −5.3 to −6.0 eV and between −3.6 to −4.2 eV, respectively, and the E^opt_g can be modulated between 2.0 and 1.2 eV through modifications on either the fused-ring electron-donating (D)/the electron-accepting (A) units and/or the π-bridges, by which over 13% PCEs have been reported.^33–43 Such a wide range of tuning in both the optical and electrochemical properties provides possibilities for fabricating ternary blended organic solar cells. In the cases of two nonfullerene acceptors, the judicious selection of two structurally miscible nonfullerenes is very important to obtain improved device performance.^44–51 Relatively, the mixing of one nonfullerene with one fullerene may integrate the merits from both the nonfullerene and fullerene acceptors. Particularly, the excellent film-forming ability of fullerene helps to obtain ideal film-morphologies.^52–60

Recently, we reported the first case of a quaternary solar cell, in which the binary blended PCBM:ICBA was introduced into PBDB-T:ITIC, a known fullerene-free system, leading to the formation of the so-called quaternary BHJ (Q-BHJ).⁶¹ The Q-BHJ integrates the merits of both fused-ring based nonfullerene and fullerene acceptors and also those of the mixture of two fullerene acceptors for adjusting the film-morphologies, device electric properties and solar cell V_oc values. In this case, we found that the minor difference in the structures of the fullerene acceptors was able to lead to a very different device performance and the structure-matched IC₆₀BA:PC₆₁BM combination gave rise to the best device function. In this article, we report two new cases of quaternary polymer solar cells by introducing bis-PC₇₁BM:PC₆₁BM or bis-PC₇₁BM:IC₆₀BA into another known nonfullerene system – PBDB-T:IT-M. Here, PBDB-T is poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione)]⁶² and IT-M is 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone-methyl))-5,5,11,11-tetrakis(4-n-hexylphenyl)-dithieno[2,3d:2′,3′d′]-s-indaceno[1,2b:5,6b′]dithiophene (Fig. 1).⁶³ Compared to the bis-PC₇₁BM-based ternary device, the PC₆₁BM- and IC₆₀BA-based devices had larger J_sc but lower FF. As the mixed bis-PC₇₁BM:PC₆₁BM or bis-PC₇₁BM:IC₆₀BA was used as the additive, both the quaternary devices showed improved J_sc and FF, and the IC₆₀BA-based device exhibited slightly larger V_oc than the PC₆₁BM-based device. Finally, the quaternary solar cells showed over 13.3% PCEs, while the ternary and fullerene-free binary devices had PCEs of 12% and 11%, respectively.

Fig. 1 Chemical structures of polymer donor as well as nonfullerene and fullerene small-molecule acceptors.

Results and discussion

Fig. 2a shows the film absorption spectra of the polymer donor and the nonfullerene and fullerene acceptors. IC₆₀BA and PC₆₁BM show absorption in the visible region typically below 600 nm and bis-PC₇₁BM has relatively stronger absorption in the visible region typically below 750 nm. Their LUMO energy levels are close to each other and match well with that of IT-M (Fig. 2b). The energy difference between IT-M and fullerene component is 0–0.19 eV, offering the possibility for the formation of alloyed energy states.⁶⁴

Fig. 2 Film absorption spectra (a) and diagram of energy levels (b) of the donor and acceptors.

The binary, ternary and quaternary cells were fabricated with a traditional device structure of ITO/PEDOT:PSS⁶⁵/active layer/ETL/Al. Here, ITO is indium tin oxide, PEDOT:PSS is poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate), and the electron transporting layer (ETL) is the binary N719:PrC₆₀MAI (1 : 1, w/w)-based blend, which has been recently reported by us⁵³ with dual functions.^66–71 N719 is di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II)) and PrC₆₀MAI is C₆₀,N,N,N-trimethyl-1-(2,3,4-tris(2-(2-methoxyethoxy)ethoxy)phenyl)dimethanaminium monoadduct iodide salt.⁷² Optimizations towards the best-performing solar cells are given in Tables S1–S4 (ESI†). Table 1 lists the photovoltaic data of the optimal devices, and their current-density–voltage (J–V) curves are shown in Fig. 3a. The integrated J_sc values from their external quantum efficiencies (EQEs, Fig. 3b) agreed with the values from the J–V measurements (Table 1).

Table 1 The photovoltaic data of the ternary and quaternary devices. The donor material is PBDB-T for all cells and all the data were obtained under illumination of AM 1.5G, 100 mW cm⁻² light source

PBDB-T:IT-M:bis-PC70BM:fullerene	V oc [V]	V BI [V]	J sc [mA cm^−2]	J sc [mA cm^−2]	FF^a	PCEave^a [%]	PCEmax [%]
a Average values from 20–35 devices. b Estimated from the dark J–V curves (Fig. S1, ESI). c Integrated from the EQE spectra from 360 to 900 nm.
1 : 1 : 0 : 0	0.952 ± 0.007	1.04	16.59 ± 0.32	15.85	0.675 ± 0.013	10.66 ± 0.39	11.15
1 : 1 : 0.2 : 0	0.959 ± 0.008	1.09	16.79 ± 0.30	16.10	0.726 ± 0.012	11.69 ± 0.32	12.04
1 : 1 : 0 : 0.2 (IC60BA)	0.956 ± 0.006	1.07	17.21 ± 0.35	16.49	0.705 ± 0.011	11.60 ± 0.35	12.06
1 : 1 : 0 : 0.2 (PC61BM)	0.948 ± 0.005	1.04	17.38 ± 0.36	16.54	0.709 ± 0.010	11.68 ± 0.34	12.09
1 : 1 : 0.2 : 0.2 (IC60BA)	0.968 ± 0.006	1.11	18.21 ± 0.33	17.29	0.731 ± 0.011	12.89 ± 0.37	13.31
1 : 1 : 0.2 : 0.2 (PC61BM)	0.957 ± 0.007	1.08	18.38 ± 0.31	17.40	0.731 ± 0.010	12.86 ± 0.35	13.28

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Fig. 3 Light J–V curves (a) and EQE spectra (b) of the ternary and quaternary devices.

Comparable PCEs (12%) were obtained from the bis-PC₇₁BM, IC₆₀BA and PC₆₁BM-based ternary devices. The bis-PC₇₁BM ternary cells showed higher V_oc and FF, while the IC₆₀BA- and PC₆₁BM-based ternary devices exhibited higher J_sc. Therefore, we introduced IC₆₀BA and PC₆₁BM as the forth component into the bis-PC₇₁BM-based ternary system to fabricate the quaternary devices. Interestingly, compared with the parent ternary systems – the bis-PC₇₁BM-, IC₆₀BA-, and PC₆₁BM-based systems – large V_oc, J_sc and FF were simultaneously obtained for the two quaternary systems, which finally gave rise to the high PCEs (13.3%). The V_oc values were 0.95–0.97 V, obtained under illumination for the PBDB-T:IT-M based optimal binary, ternary and quaternary devices. In the dark, the turn-on voltage was estimated to be 1.04–1.11 V (Fig. S1, ESI†), which indicated that these devices exhibited high built-in voltage (V_BI).

The electron and hole mobilities (μ_e/μ_h) were measured by the space-charge-limited current (SCLC) method (Fig. S2, ESI†). The μ_e/μ_h values were estimated to be 0.95/0.13, 1.86/0.21, and 2.07/0.21 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively, for the bis-PC₇₁BM-, IC₆₀BA- and PC₆₁BM-based ternary solar cell blend films. The IC₆₀BA- and PC₆₁BM-based blended films showed larger electron and hole mobilities, which was consistent with the observation of larger J_sc values. For the quaternary devices, even larger μ_e/μ_h values of 2.57/0.36 and 2.47/0.39 × 10⁻⁴ cm² V⁻¹ s⁻¹ were obtained for the IC₆₀BA- and PC₆₁BM-based devices, respectively. The larger mobilities contributed to the larger J_sc values for the two quaternary solar cells.

Fig. 4 shows the transmission electron microscopy (TEM) pictures of the blended ternary and quaternary solar cell films. All the five blends showed nanoscaled film morphologies. Among the three blends, the largest bright and dark domains are observed for the IC₆₀BA-based ternary blend, while the finest morphology is seen for the PC₆₁BM-based blend. Very clear phase-separated bright and dark domains are seen for the bis-PC₇₁BM-based ternary film and also in the two quaternary films, which agree with their much higher FF values compared with those of the IC₆₀BA- and PC₆₁BM-based ternary devices. The atomic force microscopy (AFM) height images (Fig. 4) showed similar morphologies for the three blends. The root-mean-square roughness (rms) values were 2.15, 4.06, and 1.99 nm for the IC₆₀BA-, bis-PC₇₁BM-, and PC₆₁BM-based ternary blends, respectively. For the quaternary blend, larger bright and dark domains are seen for the IC₆₀BA-based quaternary blend than the PC₆₁BM-based quaternary blend. Again, both the IC₆₀BA- and PC₆₁BM-based quaternary blends showed longer fibril morphologies than the three ternary films, which might help to give rise to the larger J_sc, as observed from the J–V characterizations. The rms values were 3.16 and 3.61 nm for the IC₆₀BA- and PC₆₁BM-based quaternary blends, respectively.

Fig. 4 TEM (upper) and AFM height (lower) images of the ternary (a–c; f–h) and quaternary (d and e; i and j) blend films.

Fig. 5 shows the (010) regions of one-dimensional graze-incidence X-ray diffraction (1D GIXRD) data of the host fullerene-free binary, the three ternary, and the two quaternary solar cell blends. The 1D GIXRD data were directly measured from the samples at their q_z and q_xy directions.⁷³ First, the diffraction patterns of the six samples were quite similar, indicating that the IT-M π–π stacking was well maintained after the addition of the fullerene additives. All the six samples showed two peaks at 1.70–1.73 and 1.83–1.86 Å⁻¹ (Fig. S3, ESI†), which corresponded to a π–π stacking distance of 3.69–3.63 and 3.43–3.39 Å, respectively. These two diffractions can be assigned as those originated from out-of-plane (OOP) π–π stacks of the acceptor IT-M. The right side diffraction tail can be contributed from the compacted π–π stacks of IT-M.⁷⁴ Second, there are small differences in the intensity of the two (010) diffractions. For the host binary blend, the 1.85 Å⁻¹ peak is much weaker than the 1.72 Å⁻¹ diffraction peak, while for the ternary blend, we can see that the intensity of 1.83–1.86 Å⁻¹ peak increased relatively to that of the 1.70–1.73 Å⁻¹ peak, particularly with the use of bis-PC₇₁BM or PC₆₁BM as the third additive (Fig. S3, ESI†). This increase suggests that more IT-M molecules form compact π–π stacks upon the addition of bis-PC₇₁BM, IC₆₀BA and PC₆₁BM. For the quaternary blend, the use of bis-PC₇₁BM:IC₆₀BA led to the shift in band position to a larger Q vector value, indicating the formation of more compacted π–π stacks at the OOP direction than with the use of a single fullerene component. Similar result was obtained using bis-PC₇₁BM:PC₆₁BM (Fig. S3, ESI†). Again, compared with the host binary blend, the larger Q-vector peak also increased, indicating that more IT-M molecules form compacted π–π stacks with the use of binary mixing fullerene, particularly with the use of bis-PC₇₁BM:PC₆₁BM. The compact packing of IT-M molecules along the OOP direction benefits the OOP electron transport, which is consistent with the large electron mobilities in the IC₆₀BA- and PC₆₁BM-based ternary and quaternary solar cell blends.

Fig. 5 The (010) regions of the GIXRD 1D profiles of the host binary, ternary and quaternary solar cell blends at the out-of-plane (solid lines) and in-plane (dot lines), respectively.

Conclusions

Two new quaternary PSCs were fabricated by introducing bis-PC₇₁BM:PC₆₁BM or bis-PC₇₁BM:IC₆₀BA into the fullerene-free PBDB-T:IT-M system, and bis-PC₇₁BM-, PC₆₁BM-, and IC₆₀BA-based ternary PSCs were also fabricated for comparisons. The PC₆₁BM- and IC₆₀BA-based ternary devices had larger J_sc than the bis-PC₇₁BM-based ternary device, while the latter showed higher FF and hence, all the three ternary devices showed 12% PCEs. On using mixed bis-PC₇₁BM:PC₆₁BM or bis-PC₇₁BM:IC₆₀BA as the additive, the quaternary solar cells showed improved V_oc, J_sc, and FF, and finally increased PCEs, reaching 13.3%. TEM images indicated clear phase-separated bright and dark domains in the bis-PC₇₁BM-based ternary and the two quaternary blends, while the IC₆₀BA-based ternary and quaternary blends had larger domains and the PC₆₁BM-based ternary and quaternary blends had finer film-morphologies. 1D GIXRD data indicated that the addition of IC₆₀BA and PC₆₁BM helped more IT-M molecules to form compact π–π stacks at the out-of-plane direction, which is consistent with their larger electron mobilities. These results again demonstrated the large outstanding performance of the quaternary blend devices compared with the ternary blend devices, which opens windows for the further improvement of PSCs device performance as the four-component-based active layer provides much more choices to improve the optical and electrical functions of the photoactive layer.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (NSFC, No. 91433202, 21773262, 21327805, 91227112, and 21521062) and Chinese Academy of Sciences (CAS, No. XDB12010200). The authors gratefully acknowledge Beijing Synchrotron Radiation Facility (BSRF) for supports of GIXRD measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qm00571k

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