Novel organic dyes (MK dyes), which have a carbazole derivative as an electron donor and a cyanoacrylic acid moiety (dC(sCtN)COOH) as an electron acceptor and an anchoring group, connected with n-hexyl-substituted oligothiophenes as a π-conjugated system, were designed and synthesized for application in dye-sensitized solar cells (DSSCs), which are one of the promising molecular photovoltaics. The photovoltaic performance of the DSSCs based on MK dyes markedly depends on the molecular structure of the dyes in terms of the number and position of n-hexyl chains and the number of thiophene moieties. Retardation of charge recombination caused by the existence of n-hexyl chains linked to the thiophene groups resulted in an increase in electron lifetime. As a consequence, an improvement of open-circuit photovoltage (V oc ) and hence the solar-to-electric power conversion efficiency (η) of DSSCs was achieved upon addition of n-hexyl chains to the thiophene groups. In addition, the adsorption condition (amount of dye molecules and/or dye aggregate thickness) on the nanoporous TiO 2 electrode, depending on the number of hexyl chains, strongly affected the performance of DSSCs. A larger amount and/or thicker aggregate of dye molecules brought about longer electron lifetime, which resulted in higher V oc , and slower diffusion of I 3 ions in the nanoporous TiO 2 electrode, which led to lower short-circuit photocurrent (J sc ) and fill factor (FF). In the result of thorough investigation on the series of MK dyes, a DSSC based on MK-2 consisting of n-hexyl-substituted quarter-thiophene produced 8.3% of η (J sc ) 15.22 mA cm -2 , V oc ) 0.73 V, and FF ) 0.75) under 100 mW cm -2 simulated AM1.5G solar irradiation.

To engineer the NiO x /perovskite interface and promote interfacial hole transfer, two pyridine-terminated conjugated small organic molecules (PTZ-1 and PTZ-2) are synthesized to link the NiO x and perovskite layers for NiO x -based perovskite solar cells (PSCs). One terminal pyridine group interacts with the NiO x layer, while the other one coordinates with the Pb atoms of the perovskite layer, erecting an interfacial hole transfer bridge between NiO x and perovskite. Surface modification of the NiO x film with the PTZ molecules is able to enhance hole extraction, increase hole mobility and conductivity of NiO x , reduce defect density, and retard interfacial charge recombination. As a consequence, power conversion efficiency is improved from 12.53 to 16.25 and 17.00% upon surface modifications of NiO x with PTZ-1 and PTZ-2, respectively. Furthermore, the modified PSCs exhibit almost no hysteresis and show good stability after storage in air (relative humidity of 30–40%) for 500 h without encapsulation.

We designed and synthesized new substituted carbazole dyes, MK-14 and -16, for dye-sensitized solar cells (DSSCs) employing the I À /I 3 À redox couple. By the addition of a hexyloxyphenyl substituent to previously reported carbazole dyes MK-1 and -2, the electron lifetime and open circuit voltage of the DSSCs employing these dyes were increased, showing comparable values with those using a conventional Ru complex dye. This result was achieved by the retardation of the charge recombination, caused by more effective blocking of the I 3 À ion in the electrolyte than that in the cases of MK-1 and -2. The result shows the importance of the position of alkyl chains attached to the main framework of dye molecules.