Carbazoles on same main chain for polymer solar cells

Abstract

Planar conjugated 2,7-linked carbazole blocks: Q1(2,7-dibromo-9-octyl-9H-carbazole); Q2(7,7 0 -dibromo-9,9 0 -dioctyl-9H,9 0 H-2,2 0 -bicarbazole); Q3 (7-bromo-7 0 -(7-bromo-9-octyl-9H-carbazol-2-yl)-9,9 0 -dioctyl-9H,9 0 H-2,2 0 -bicarbazole) were coupled with same acceptor (4,7-di[2,5-thiophene]-5,6-dioctyloxy-2,1,3-Benzothiadiazole) to prepare polymers HXS-1, 2, 3 (Scheme 1). Bulkheterojunction polymer solar cells (BHJ PSCs) with these polymers were made. Power conversion efficiency of HXS-1 was proved to be over 5.4%. It declined dramatically to 0.43% and 0.23% for HXS-2 and 3 respectively. Their absorption and X-ray diffraction pattern show the torsion angle in main chain increased when more carbazole units were added. More carbazoles will make polycondensation reaction more difficult to get high molecular weight polymers. The torsion angle was calculated using a semiempirical molecular orbital method. All the results pointed out that the coplanarity in the conjugated backbone was destroyed. Electron delocalization was disturbed because p-orbital overlapping only occurs effectively in the parallel orbit so charge cannot move a longer distance. This study offers a useful and important insight to designing polymers for high performance PSCs.

Dithienocarbazole- and benzothiadiazole-based donor-acceptor conjugated polymers for bulk heterojunction polymer solar cells

“…This device performance is largely comparable to that of HXS-1 [26] but lower than that of the polymer based on C2 and IID [44]. We expect further improvement upon optimizing device-fabrication conditions [26][27][28]30]. As shown in the external quantum efficiency (EQE) profiles (Figure 6(b)), the devices based on both polymers showed a photoresponse range of 300-720 nm, which largely matched the film-absorption spectra of polymer:PC 71 BM blends; the highest EQE value reached 65%.…”

“…As shown in Figure 7, both blend films were very smooth and uniform, and possessed a nanoscale phase separation although no additive was used. This behavior is different from HXS-1:PC 71 BM, for which the additive 1,8-diiodooctane was used in order to attain appropriate film morphology [26][27][28]. Compared to the film of P(BT-C2): PC 71 BM, the film of P(BT-C1):PC 71 BM had a more obvious phase separation and exhibited a fiber-like morphology.…”

“…Due to their high mobilities, appropriate HOMO energy levels, and narrow bandgaps, which are required properties for the fabrication of high-performance photovoltaic devices [3-6], donoracceptor (D-A)-conjugated polymers have become the most successful donor materials. Of those, D-A conjugated polymers based on carbazole and 4,7-bis(thiophen-2-yl)-benzo[c][1,2,5]thiadiazole derivatives, such as poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and poly(2-(5-(5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)-9-octyl-9H-carbazole) (HXS-1), have attracted intense attention [22][23][24][25][26][27][28]; PCE has surpassed 7% after the device fabrication condition was optimized [29,30]. Because polyaromatics generally have rigid and planar configurations, incorporating a polyaromatic unit into the backbones of conjugated polymers can make the -electron delocalization more effective.…”

Fine-tuning of polymer photovoltaic properties by the length of alkyl side chains

“…Maximizing the short-circuit current in a low-band gap planar polymer is one of the critical factors towards enabling highefficiency solar cells. Coplanarity in the conjugated backbone is crucial for electron delocalization, which requires a minimal torsional bond angle because p-orbital overlapping occurs effectively in the planar structure [23] . We optimized all these factors resulting in high performance of the device.…”

Synthesis of two D‐π‐A polymers π‐bridged by different blocks and investigation of their photovoltaic property

“…However, unmatched conjugated units will result in notable large dihedral angle along the main chain. Large torsion angle will destroy the electronics delocalization of the polymer backbone . This will narrow the light absorption range of the active layer.…”