Sodium Gadolinium Fluoride Nanophosphor-Based Solar Cells: Toward Subbandgap Light Harvesting and Efficient Charge Transfer

In this paper, we have synthesized erbium and ytterbium codoped sodium gadolinium fluoride (NaGdF
₄
:Yb/Er) nanophosphors (NPs), aiming to extend the solar light harvest of PTB7 from visible into near-infrared. Evidence shows that Yb concentration plays an important role in upconversion, because it can inhibit the back energy transfer process and absorb considerable low-energy photons. Subsequently, NaGdF
₄
:Yb/Er NPs have been incorporated into the photocatalytic titania (TiO
₂
) nanoparticle layer to probe into electron transfer dynamics, and the photovoltaic performance of the assembled solar cells has been explored. The results show that NaGdF
₄
:Yb/Er NPs excited at 976 nm present green and red emissions. After interfacing with bare or NP-doped electron donor TiO
₂
, the lifetime of the emerged electron transfer has been shortened from 840 to 466 ps, and correspondingly, the electron transfer rate outstrips that of the bare TiO
₂
by a factor of 2.6. Consequently, an efficiency enhancement has been obtained with power conversion efficiency increasing to 3.61% from 2.81% of pure TiO
₂
/PTB7. This work provides an efficient and facile approach to enhance the device performance by the codoping of robust rare-earth ions to widen the harvesting range of solar spectrum, boost electron transfer rate, and eventually strengthen the device performance.