Efficient ternary polymer solar cells by doping fullerene derivatives
Introduction
Bulk heterojunction (BHJ) polymer solar cells (PSCs) have attracted considerable attention due to the potential to realize low-cost, flexible, semi-transparent and lightweight organic photovoltaics devices [1], [2], [3]. During the past decades, the BHJ PSCs have made a significant breakthrough with power conversion efficiencies (PCEs) exceeding 10% [4], [5]. Further increment of PCE and improvement of stability is important to realize industrial production. Therefore, extensive efforts have been focused on molecular structure designing [6], [7], morphology control [8], [9] and fabrication of innovative device architectures [10]. Especially, inverted structure of PSCs (i-PSCs) had been an important approach to achieve stable device without encapsulation [11], [12].
The photo-active layers of PSCs are usually composed of binary blends, conjugated polymer donors and fullerene acceptors with appropriate energy levels alignment forming interpenetrating networks of donor/acceptor (D/A) enriched phases for effective charge transportation. But, thin film of active layer, about 100 nm thickness, and low charge-carrier mobility of D/A binary semiconducting materials has limited the light absorption. To obtain increasing light harvesting and better energy level alignment of the photoactive layers, ternary structures were previously used as an effective method with an increase of PCE [13], [14], [15], [16], [17]. But ternary systems of active layer introduce even more variables and higher level of complexity in comparison with binary solar cells. The fundamental operating principles as well as efficient design of high performance of ternary PSCs still need to be carefully studied. Majority of previous ternary systems contained two donors and one acceptor [18], [19], [20], [21] to compensate the light loss of active layer materials, yet the design of a donor and two acceptors of ternary blends is relatively seldom reported.
[6,6]-Phenyl-C61-butyric acid-methyl-ester (PC61BM) and [6,6]-phenyl-C71-butyric acid-methyl-ester (PC71BM) were the most common electron acceptors, due to their favorable energy alignment, high electron mobility and good affinity with electron donor etc. [22], [23] However, their low lowest unoccupied molecular orbital (LUMO) energy level restricted the improvement of open-circuit voltage (Voc) and increased the energy loss of charge transfer process in devices [24], [25], [26], sometimes leading to a mediocre PCE.
With the unremitting efforts and increasing sophisticated technology, more and more state-of-the-art polymer donors have been designed and synthesized. The popular donor material poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) has been reported with a broad and strong absorption band from visible to near infrared region [27]. And the single junction PSCs based on PTB7-Th:PC71BM has reached a PCE of surpassing 10% [28], [29], [30]. Besides, it also has been reported previously that indene-C60 bisadduct (ICBA) could extensively act as a new fullerene acceptor in PSCs with a notable Voc and PCE due to the excellent electron mobility endowed from fullerene core, as well as its higher LUMO compared to PC61BM and PC71BM [22], [23].
Herein, we doped a cascade acceptor (ICBA) into PTB7-Th:PC71BM as active layer to fabricate the ternary i-PSCs. It was surprising to observe the obviously strong fluorescence quench with the increase of ICBA content though a seemingly inappropriate energy level between the donor and acceptor, suggesting ICBA played an efficient bridging role for charge transport between PTB7-Th and PC71BM. Meanwhile, the film of ternary blends with ~ 10 wt% ICBA exhibited a reduced photoluminescence (PL) lifetime indicating an improved exciton dissociation rates. Additionally, when around 10 wt% ICBA was doped into PTB7-Th:PC71BM, the film displayed the smoother and denser nanostructure maintaining a suitable phase separation, illustrating the favorable compatibility among PTB7-Th, ICBA (10 wt%) and PC71BM. As a result, the ternary device with 10 wt% ICBA demonstrated a notable PCE of 9.2%, which was higher than the average PCE (8%) of binary device based on PTB7-Th:PC71BM. And the Voc reasonably boosted with the addition of ICBA attributing to the higher LUMO relative to PC71BM. Surprisingly, a record Voc of 1.0 V was obtained as the weight percentage of ICBA as fullerene derivatives acceptor increased to 100 wt%.
Section snippets
Devices fabrication
All the chemicals and solvents were obtained commercially and used without any purification. The solvents, 1,8-diiodooctane (DIO), chloroform, chlorobenzene (CB) and methonal, were obtained from J&K chemical Inc. Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) (Mw = 123 kDa, PDI = 1.75), indene-C60 bisadduct (ICBA) and PC71BM were purchased from American Dye Inc. and used without any purification.
I-PSCs were
Device performances of the PSCs
All i-PSCs were fabricated with structure of ITO/ZnO/PTB7-Th:PC71BM:ICBA/MoO3/Ag (Fig. 1 (a)), and the corresponding energy levels of PTB7-Th, ICBA and PC71BM were displayed in Fig. 1 (b). The weight ration of PTB7-Th:fullerene derivatives acceptor was 1:1.5, of which the weight percentage of ICBA in this fullerene derivatives acceptors varied from 0 wt% to 100 wt%. As presented in diagram, the LUMO value of ICBA was suitably intermediate between PTB7-Th and PC71BM, it was suspected the Voc could
Conclusion
A highly efficient ternary architecture i-PSCs is presented by doping ICBA as additional fullerene derivatives acceptor into PTB7-Th:PC71BM binary blend. The results of optical measurements indicated the ternary device with ~ 10 wt% ICBA as fullerene derivatives acceptor possessed good light absorption in full-scale visible region and stronger ultrafast photo induced electron transfer from polymer donor to fullerene derivatives acceptor, beneficial for a better Jsc, in comparison to binary device
Acknowledgements
The author would like to thank Hangzhou Dianzi University for the funding ZX150204307002/016 to support this research.
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