Bulk heterojunction solar cells made from carbazole copolymer and fullerene derivative with an inserted layer of discotic material with improved efficiency

ABSTRACT

Bulk heterojunction photovoltaic cells based on composites of copolymer poly [N-90-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] and the fullerene derivative [6,6]-phenyl C71-butyric acid methyl ester with an inserted layer of discotic liquid crystalline material (2, 3, 6, 7, 10, 11-hexabutyloxytriphenylene) between the interface of active layer and hole transporting layer has been reported. Different hole transporting layers deposited on indium tin oxide substrates such as poly (3,4-ethylenedioxythiophene)-poly (styrenesulphonate) or molybdenum trioxide has been used in these devices. All the devices with inserted discotic liquid crystal layer showed better performance than the reference cells. Power conversion efficiency of 5.14% was achieved for these photovoltaic solar cells containing self-organised discotic liquid crystal layer of 30 nm thickness under one sun condition which is substantial jump as compared to earlier reports. The mobility of holes in the discotic liquid crystal inserted devices was found to be of the order of 10^–6 cm² V^–1 s^–1 due to which high values of current density was achieved. The influence of varying the thickness of liquid crystal layer and annealing on the photovoltaic parameters of these devices was also studied.

GRAPHICAL ABSTRACT

KEYWORDS:

• Bulk heterojunction
• solar cell
• liquid crystals
• mobility
• efficiency

Acknowledgments

This work is financially supported by Department of Electronics and Information Technology (DEITY), New Delhi under a research project no. 12(6)/2011-EMCD. One of us (NY) thanks DEITY for fellowship under the project. MB thanks UGC, New Delhi for D.S. Kothari fellowship under grant number [F no 4-2/2006 (BSR)/13-998 (BSR)]. Indo-Ireland international cooperation through DST, New Delhi is also acknowledged.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work is financially supported by Department of Electronics and Information Technology (DEITY), New Delhi under a research project no. 12(6)/2011-EMCD. One of us (NY) thanks DEITY for fellowship under the project. MB thanks UGC, New Delhi for D.S. Kothari fellowship under grant number [F no 4-2/2006 (BSR)/13-998 (BSR)].