Combining Energy Transfer and Optimized Morphology for Highly Efficient Ternary Polymer Solar Cells

Abstract

photovoltaic performance, as they not only maintain the simple device configuration and low-cost processing of single-junction polymer solar cells (PSCs), but also inherit the major benefit of incorporating multiple polymers in tandem cells. [1][2][3] In general, the third component mainly plays a role in energy transfer or electron transfer on the condition of proper cascaded energy levels, which lead to an increase in shortcircuit current density (J sc ) by extended absorption and more efficient charge generation. [2,[4][5][6] Besides, the third component with relatively high crystallinity may also act as a template to facilitate supramolecular assembly for achieving ideal morphology of active layer, [2,7,8,4] providing numerous D/A area for exciton dissociation and affording optimized phase separation to facilitate the charge transport. [9][10][11][12][13] Therefore, compared with the corresponding binary systems, TSCs have the potential to achieve higher light-harvesting and/or improved charge transport, leading to increased J sc and/or FF. [14][15][16] However, it should be noted that the emission of the third component and the absorption of the donor or acceptor must overlap in order to get efficient energy transfer. In addition, the morphology tuning highly depends on the crystallinity and miscibility properties of the materials involved. Unfortunately for ternary systems, Aimed at achieving ideal morphology, illuminating morphology-performance relationship, and further improving the power conversion efficiency (PCE) of ternary polymer solar cells (TSCs), a ternary system is designed based on PTB7-Th:PffBT4T-2OD:PC 71 BM in this work. The PffBT4T-2OD owns large absorption cross section, proper energy levels, and good crystallinity, which enhances exciton generation, charge dissociation and transport and suppresses charge recombination, thus remarkably increasing the short-circuit current density (J sc ) and fill factor (FF). Finally, a notable PCE of 10.72% is obtained for the TSCs with 15% weight ratio of PffBT4T-2OD. As for the working mechanism, it confirmed the energy transfer from PffBT4T-2OD to PTB7-Th, which contributes to the improved exciton generation. And morphology characterization indicates that the devices with 15% PffBT4T-2OD possess both appropriate domain size (25 nm) and enhanced domain purity. Under this condition, it affords numerous D/A interface for exciton dissociation and good bicontinuous nanostructure for charge transport simultaneously. As a result, the device with 15% PffBT4T-2OD exhibits improved exciton generation, enhanced charge dissociation possibility, elevated hole mobility and inhibited charge recombination, leading to elevated J sc (19.02 mA cm −2 ) and FF (72.62%) simultaneously. This work indicates that morphology optimization as well as energy transfer plays a significant role in improving TSC performance.

Miscibility Tuning for Optimizing Phase Separation and Vertical Distribution toward Highly Efficient Organic Solar Cells

“…[25] Although both ternary OSCs and tandem OSCs could improve the power conversion efficiency (PCE) value to about 15%, the ternary strategy possessing simplicity of processing active layer exhibits great potential in R2R technology. [29] Especially, many researchers have realized the importance of miscibility in ternary OSCs, [28,[35][36][37] such as the appearance of Förster resonance energy transfer, [38] distribution of third additive on the interfaces of donor and acceptor, [39] formation of alloy with donor or acceptor, [36,37,40] etc. [6,[32][33][34] Unfortunately, most of the research work about how to select appropriate third additive is based on the trial and error strategy, which is a process of labor and time consumption.…”

“…Nevertheless, the majority of ternary components show their highest performance at the appropriate blend films thickness (about 100 nm), further increasing the film thickness eventually would bring about the sharp reduction in PCE values. [38] Furthermore, as the blend film thickness increased, the effect of surface energy of substrates on the vertical distribution of donor and acceptor lessened, leading to the unfavorable enrichment of donors or acceptors at cathode and anode, which is harmful to the charge collection in the respective electrodes. When the crystallization of donor or acceptor is disturbed by the incorporation of third additive, [6,43] the decreased domain purity may lead to the depression of charge mobility and the enhanced charge recombination, causing the severe reduction of PCE and fill factor (FF).…”

The Critical Impact of Material and Process Compatibility on the Active Layer Morphology and Performance of Organic Ternary Solar Cells

“…[64,65] Concurrently, tuning composition of the ternary blend was found to give rise to a change in the energy of the charge transfer states, which is in turn directly related to V oc . [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] This subtle optimization point is also evident in the series of TSC devices studied here. On the other hand, V oc is also highly limited by the intrinsic nonradiative recombination losses, and this issue has recently been rigorously investigated for binary blend systems.…”

“…In recent years, organic solar cells (OSCs) have shown enormous progress primarily due to an emergence of new materials featuring proper optoelectronic properties, [1][2][3][4][5][6][7][8][9][10][11][12][13] and the Successful TSC designs incorporate combinations of two conjugated polymers and a fullerene electron acceptor molecule [18][19][20][21][22] or a polymer electron donor and two small molecules. [23][24][25][26][27][28][29][30][31] Those TSCs have to overcome material incompatibility and processing problems, which create a major bottleneck in obtaining the desirable ternary blend nanomorphologies.…”

“…[16] The second polymer is poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7diyl)-alt-(3,3′″-di-(2nonyltridecyl)2,2′;5′,2″;5″,2′″-quaterthiophen-5,5′″-diyl)] (PffBT4T-C 9 C 13 ) with a quaterthiophene donor unit and a difluorobenzothiadiazole acceptor unit and it gives high efficiency in binary BHJ solar cells. [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] This intriguing morphological changes are highly connected to the electrical properties and device FF. The LBG polymer has enabled the extension of absorption to 900 nm.…”