Tuning optical and electronic properties of star-shaped conjugated molecules with enlarged π-delocalization for organic solar cell application

Abstract

Three structurally related conjugated molecules (BTT-BTD-0, BTT-BTD-1 and BTT-BTD-2) in star shape have been designed and synthesized as donor materials for small molecule based bulk heterojunction (BHJ) solar cells. The structural features of these molecules include a planarized benzo[1,2-b:3,4-b 0 :5,6-b 00 ]trithiophene (BTT) with a C 3h symmetry as the central core and three conjugated arms incorporating electron deficient benzo[2,1,3]thiadiazole (BTD) units, with arms being linked to the core via different number of thiophene connecting units (e.g., 0, 1, 2 corresponding to BTT-BTD-0, BTT-BTD-1 and BTT-BTD-2, respectively). Comparative analyses of optical and electronic properties indicate that the molecules bearing more thiophene units between the BTT core and the BTD arms possess higher-lying HOMO levels while their LUMO levels remain almost unchanged. The improvement of BHJ device performance, with [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) as the acceptor, is observed with increasing number of thiophene units between the BTT core and BTD arms, from BTT-BTD-0 to BTT-BTD-1 and BTT-BTD-2. The BTT-BTD-2:PC 61 BM based BHJ devices show the highest power conversion efficiency (PCE) of 0.74%, with an open-circuit voltage (V oc ) of 0.69 V, a short-circuit current density (J sc ) of 2.93 mA cm À2 , and a fill factor (FF) of 0.37 under 1 sun (100 mW cm À2 ) AM 1.5G simulated solar illumination. The PV performance of BTT-BTD-2 is further improved when [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) is used as the electron acceptor, yielding the best device performance with J sc of 4.13 mA cm À2 , V oc of 0.72 V, FF at 0.46 and PCE of 1.36%. The effect of the different number of thiophenes linking the BTT core and the conjugated BTD arms has been clearly demonstrated on regulating optical and electrochemical properties of the three molecules and their BHJ device performances.

Advances in Star‐Shaped π‐Conjugated Systems: Properties and Applications

“…These studies have revealed that introducing more than two thiophene units has marginal effect on the HOMO level, as well as on the bandgap. The reported results have, however, shown that the above significantly influences the operating parameters of a device based on the above systems, as its efficiency increased from 0.16% for n = 0 up to 1.36% for n = 2 . This indicates that the effect of structure on the performance of photovoltaic devices is both significant and not limited to only electronic properties.…”

The role of structural and electronic factors in shaping the ambipolar properties of donor–acceptor polymers of thiophene and benzothiadiazole

“…An electron-deficient BT unit has been attached to electron rich molecules such as thiophene derivatives resulting in unique optical and electrochemical properties [23][24][25][26][27]. When BT is incorporated with electron donor units, the extension of the conjugated length appears, due to the occurrence of intarmolecular charge transfer (ICT) in D-A structure [28,29].…”

Rational design of multifunctional star-shaped molecules with a 1,3,5-triazine core and different arms for application in organic light-emitting diodes and organic solar cells

“…The increased steric hindrance associated with star-shaped conjugated molecules can prevent the formation of ordered, long-range, and coplanar π-π stacking, which can improve the charge-transport properties of these molecules. At the same time, π-π stacking of the conjugated arms is promoted by their high structural symmetry and planarity [21]. Moreover, the absorption and emission spectra of these materials can be tuned by adjusting the core and arm units, the π-conjugated bridge units, or the length of the molecule [22].…”