Semi-transparent solar cells are emerging as promising devices for building integrated photovoltaics. However, regardless of the technology considered, the optical transmission of semi-transparent solar cells is fixed during the fabrication process, and hence, cannot adjust to variations in daylight or weather conditions. This becomes an issue when applications such as photovoltaic windows are envisioned. Previously, we have reported that the use of photochromic naphthopyran dyes in dye-sensitized solar cells (DSSC) allows for the fabrication of semi-transparent devices capable of modulating their light transmission and energy production depending on light intensity. Herein, we report a series of naphthopyran dyes with an identical pi-conjugated backbone and varying alkyl substituents. Using this molecular engineering strategy, we demonstrate control of the discoloration kinetics of the photochromic dyes and the reduction of undesirable recombination processes when utilised in solar cells. We establish clear photochromic-photovoltaic structure-property relationships for these new photosensitizers and demonstrate improved photovoltaic performances and photochromic responses in DSSC with notably faster discoloration than previously reported. We also explore, for the first time with photochromic molecules, the co-sensitization of the electrodes and report the highest power conversion efficiency for a photochromic DSSC, reaching up to 4.34% under standard conditions.