Emission-wavelength-dependent photoluminescence decay lifetime of N-functionalized graphene quantum dot downconverters: Impact on conversion efficiency of Cu(In, Ga)Se2 solar cells

Firoz Khan,Jae Hyun Kim

doi:10.1038/s41598-019-47068-w

Abstract

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)Se2 (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 (λem)-dependent photoluminescence decay lifetime (τem) determines the down-conversion efficiency of the nitrogen-functionalized graphene quantum dots. With the increase in τem at λem > 500 nm, the conversion efficiencies of the NGQDs coated-CIGS solar cells increased by 12.22%. Thus, the increase in τem at λem > 500 nm significantly increased the maximum current output and thus enhanced the solar-cell performance.

Highlights

Carbon-based quantum dots (QDs) and graphene QDs (GQDs) have received considerable attention due to their advantageous characteristics[1]
Various N-functionalized GQDs (NGQDs) were synthesized by controlling the polyethyleneimine (PEI)/graphene oxide (GO) weight ratio, synthesis temperature, and synthesis time
The other NGQDs for which PL quantum yield (QY) < 75% do not have much of an impact on enhancing the performance of CIGS solar cells

Summary

Introduction

Carbon-based quantum dots (QDs) and graphene QDs (GQDs) have received considerable attention due to their advantageous characteristics[1]. GQDs are attractive owing to the unique broadband-tunable photoluminescent (PL) properties, their excellent biocompatibility, high photostability, low toxicity, low cost, and due to the abundance of the raw materials used to create them[2,3] They have promising applications in bioimaging[2,4], biosensing[5,6], environmental monitoring[7], light-emitting diodes[8], and photovoltaic (PV) devices[9,10]. For down-conversion, several LDC materials have been investigated, including GQDs1,3,9–11, inorganic QDs18–21, organic dyes[22,23], and rare-earth ions/complexes[24,25,26] These are not convenient downconverters owing to one or more limitations, e.g., a low PL QY, a narrow absorption band, a small value of Δλ, low absorption coefficients, low stability, and toxicity. The produced NGQDs was used to coat a layer onto the top of a CIGS solar cell, and the ensuing performance enhancement was studied[28]

Methods

Results

Conclusion