Mingguang Li

Donor/Acceptor Molecular Orientation-Dependent Photovoltaic Performance in All-Polymer Solar Cells

The correlated donor/acceptor (D/A) molecular orientation plays a crucial role in solution-processed all-polymer solar cells in term of photovoltaic performance. For the conjugated polymers PTB7-th and P(NDI2OD-T2), the preferential molecular orientation of neat PTB7-th films kept face-on regardless of the properties of processing solvents. However, an increasing content of face-on molecular orientation in the neat P(NDI2OD-T2) films could be found by changing processing solvents from chloronaphthalene (CN) and o-dichlorobenzene (oDCB) to chlorobenzene (CB). Besides, the neat P(NDI2OD-T2) films also exhibited a transformation of preferential molecular orientation from face-on to edge-on when extending film drying time by casting in the same solution. Consequently, a distribution diagram of molecular orientation for P(NDI2OD-T2) films was depicted and the same trend could be observed for the PTB7-th/P(NDI2OD-T2) blend films. By manufacture of photovoltaic devices with blend films, the relationship between the correlated D/A molecular orientation and device performance was established. The short-circuit current (Jsc) of devices processed by CN, oDCB, and CB enhanced gradually from 1.24 to 8.86 mA/cm(2) with the correlated D/A molecular orientation changing from face-on/edge-on to face-on/face-on, which could be attributed to facile exciton dissociation at D/A interface with the same molecular orientation. Therefore, the power conversion efficiency (PCE) of devices processed by CN, oDCB, and CB improved from 0.53% to 3.52% ultimately.

Balancing the H- and J-aggregation in DTS(PTTh2)2/PC70BM to yield a high photovoltaic efficiency

The content and ratio of two stacking styles, namely H-aggregation and J-aggregation, of 5,5′-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene, DTS(PTTh2)2, had a profound influence on the performance of solar cells based on DTS(PTTh2)2/[6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) (7/3, w/w) blend films. It was found that the H/J ratio could be tuned from 0.30 to 1.40 by controlling the boiling point of the main solvent, the content of additives and the selective dissolution by additives (such as 1,8-diiodooctane (DIO), 1,6-diiodohexane (DIH) and 1,4-diiodobutane (DIB)) of the DTS(PTTh2)2 side chains. The power conversion efficiency (PCE) of DTS(PTTh2)2/PC70BM was firstly improved, then decreased with increasing H/J ratio. The best PCE of 6.51% was achieved by adding 0.20 vol% 1,6-diiodohexane (DIO) into solutions in thiophene (Th), which improved it by 203% compared to the reference without additives, when the H/J ratio was 1.01. Balance between the two stacking styles is needed for the device to obtain best performance. On one hand, J-aggregation was favorable for forming more excitons because a lower energy was needed to excite the J-aggregation. On the other hand, the H-aggregation favored exciton dissociation for two reasons: providing excitons with a longer lifetime and stronger driving force for exciton dissociation. In addition, the complementary light absorption of the two stacking styles is advantageous for maximal light absorption.

A bi-continuous network structure of p-DTS(FBTTh2)2/EP-PDI via selective solvent vapor annealing

A critical requirement of small molecule non-fullerene acceptor-based solar cells for efficient charge separation and collection is the formation of interconnected phase-separated domains of 10–20 nm. The phase-separation behavior of small molecule donor 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) and acceptor N,N′-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI) was regulated by donor selective solvent vapor annealing (D-SVA), poor donor solvent vapor annealing (P-SVA) and thermal annealing (TA). It was found that the power conversion efficiency (PCE) was significantly improved from less than 0.2% up to 3.0% after D-SVA. In contrast, a limited improvement of PCE was obtained for P-SVA (1.9%) and TA (1.8%). The difference in the improvement of PCE values was attributed to the formation of a phase-separated structure and the regulation of crystallite sizes under different post treatments. The fibrous crystals of p-DTS(FBTTh2)2 were formed during D-SVA treatment while the EP-PDI component was in the form of small microcrystals, thus leading to the required interconnected phase-separated structure. As a consequence, the moderate phase-separated morphology accompanied by a pure crystalline phase with a medium size could maximize the carrier transport process without compromising exciton separation efficiency, and thus contributes to the final optimal PCE value of up to 3.0% under D-SVA. For P-SVA or TA treatment, we only observed a significant enhancement of the crystallinity of both p-DTS(FBTTh2)2 and EP-PDI components, leading to remarkable film coarsening, or even undesired large phase separation, which is detrimental to the exciton diffusion as well as exciton separation processes.

Morphological transformation of pyrazine-based acene-type molecules after blending with semiconducting polymers: from fibers to quadrilateral crystals

Micron-sized fibers of n-type pyrazine-based acene-type molecule (PBA) spin-coated films were formed after solvent vapor annealing (SVA). PBA fibers dramatically transformed into quadrilateral crystals after blending with semiconducting polymers under identical post-treatment procedures. For polymer blended systems, a much longer post-treatment time was needed to achieve the nucleation of PBA molecules. Furthermore, the orientation of PBA molecules in the quadrilateral crystals became much more ordered. There are two prerequisites for choosing the blended polymers: good compatibility with PBA molecules and comparatively strong crystallinity. Even though the blended polymers suppress the PBA nucleation process because of interpenetrating polymer networks, the self-organization of semiconducting polymers may control the diffusion rate of PBA molecules in the mixture and finally facilitate PBA self-assembly into perfect ordered aggregates.