Novel D-pi-A organic dyes for DSSCs based on dibenzo[b,h] [1,6] naphthyridine as a pi-bridge

A new series of D-pi-A organic dyes bearing fused dibenzo[b,h] [1,6]naphthyridine as the conjugated pi-bridge, a cyanoacrylic acid moiety as the electron acceptor/anchoring group and different electron donor groups such as trimethoxy (5a), methoxy (5b) and dimethylamino (5c), were designed and synthesized for dye-sensitized solar cells (DSSCs), for the first time. The effect of donor groups on the performance of the DSSCs was systematically investigated using optical, electrochemical, theoretical and photovoltaic methods. Compared to dyes 5a and 5b, 5c showed a broader and more red-shifted absorption spectrum due to stronger electron donating ability of dimethylamino moiety, which is beneficial for absorbing more photons and thus generating high photocurrent, As a result, the DSSC fabricated with dye 5c has the highest power conversion efficiency of 5.02% (similar to 60% relative to N719-based standard cell), which is greater than those obtained by 5a and 5b-sensitized solar cells (3.22% and 2.13%, respectively) under standard air mass 1.5 global (AM 1.5 G) conditions. The improved photovoltaic performance mainly came from better photocurrent, as also confirmed by IPCE measurements. These results suggest that dibenzo[b,h] [1,6]naphthyridine pi-bridge bearing a strong electron donor group is a promising candidate to construct effective D-pi-A organic dyes for DSSCs.