Emission-wavelength-dependent photoluminescence decay lifetime of N-functionalized graphene quantum dot downconverters: Impact on conversion efficiency of Cu(In, Ga)Se solar cells.

Graphene quantum dots (GQDs) have several advantages over inorganic quantum dots owing to their beneficial properties. Recently, GQDs have been used as downconverters in photovoltaic devices. However, the application of GQDs in most emergent thin-film-based Cu(In, Ga)Se (CIGS) photovoltaic cells is limited because of either low photoluminescence (PL) quantum yield (QY) or a small Stokes shift (Δλ). Therefore, GQDs with an ultrahigh QY and large Δλ are essential to realizing the two emergent fields, i.e., the application of GQDs in CIGS photovoltaic solar cells. In this regard, we synthesized nitrogen-functionalized GQDs (NGQDs) with an ultrahigh QY (77-99%) and a large Δλ (95-155 nm) via tailoring of the nitrogen and oxygen moieties. The NGQDs were applied in CIGS solar cells to evaluate their downconversion efficiency. Our study shows that the emission wavelength (λ)-dependent photoluminescence decay lifetime (τ) determines the down-conversion efficiency of the nitrogen-functionalized graphene quantum dots. With the increase in τ at λ > 500 nm, the conversion efficiencies of the NGQDs coated-CIGS solar cells increased by 12.22%. Thus, the increase in τ at λ > 500 nm significantly increased the maximum current output and thus enhanced the solar-cell performance.