N‑Functionalized Graphene Quantum Dots with Ultrahigh Quantum Yield and Large Stokes Shift: Efficient Downconverters for CIGS Solar Cells Firoz Khan and Jae Hyun Kim* Smart Textile Convergence Research Group, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 333 Techno Jungang-Daero, Hyeonpung-Myeon, Dalseong-Gun, Daegu-42988, Republic of Korea * S Supporting Information ABSTRACT: Copper indium gallium selenide (CIGS) is the most promising thin film solar cell technology. However, its high performance is hampered by its poor short-wavelength response. The short-wavelength response can be enhanced via photon downconversion using quantum dots. Unfortunately, most graphene quantum dots (GQDs) are not suitable as downconverters in CIGS cells owing to their low photoluminescence quantum yield (PL QY) and/ or low Stokes shift. Herein, an ultrahigh PL QY (99%) and a large Stokes shift (98 nm) are achieved for N-doped GQDs via a novel method. The performance of a CIGS solar cell is enhanced via photon downconversion and the light- trapping effect using the NGQDs. The effectiveness of the NQGDs is manifested in a conversion efficiency (η) of 15.31%. In addition, improvements in the short-circuit current density from 30.69 mA/cm 2 to 31.77 mA/cm 2 and fill factor from 71.25% to 73.09% are observed. The n and J 0 values are decreased by insertion of NGQDs, indicating a reduction in the recombination losses. KEYWORDS: N-doped graphene quantum dots, downconverters, quantum yield, N moieties, CIGS solar cells C u(In,Ga)Se 2 (CIGS)-based solar cells, the most promis- ing thin film technology, are receiving worldwide attention for solar power generation. 1 Owing to their high absorption coefficient, only 1-2 μm thickness is sufficient to absorb more than 95% of the useful solar radiation (in the wavelength range of 300 to 1300 nm). 2,3 Unfortunately, the high performance of CIGS solar cells is hampered by parasitic absorption losses in the ZnO window and CdS buffer layers. 4 ZnO strongly absorbs photons of λ < 390 nm, while CdS absorbs photons of λ < 500 nm. Only a fraction of photons absorbed by CdS are utilized for current generation, whereas the photons absorbed by ZnO are completely lost. 5 Thus, CIGS solar cells typically exhibit a poor short-wavelength (λ) response, which reduces their current output. The maximum losses in current density corresponding to absorbed photons in ZnO and CdS are ∼1.1 and 7.4 mA/cm 2 , respectively. These losses can be reduced either by improving the electronic properties of the solar cell or by photon manage- ment via downconversion. 6-8 The electronic properties of CIGS solar cells can be improved by creating a very narrow junction or employing low doping levels or very thin window/ buffer layers. 6 However, it is difficult to implement these steps, as they increase the production cost. Additionally, this approach may have detrimental effects on the overall solar cell performance. 6 On other hand, in the photon management approach, a luminescent down-shifting (LDS) material is used. The LDS material absorbs short-λ photons and re-emits them at a more favorable λ, at which the solar cells exhibit a significantly better response. Since the LDS layer is applied on top of the solar cell, no change occurs in the junction and interfaces of the cell. Therefore, it is a passive approach to enhancing the short-λ response. Moreover, a theoretical efficiency of 33% can be achieved via photon management. 9 The most suitable LDS material for CIGS solar cells should exhibit (i) a high photoluminescence quantum yield (PL QY); (ii) a wide absorption band for wavelengths <500 nm; (iii) a high absorption coefficient; (iv) a narrow emission band for wavelengths >500 nm; and (v) good separation between the absorption and emission bands to minimize losses due to reabsorption. In addition, they should be inexpensive. A large number of luminescent materials have been investigated for LDS and can be classified into three main categories: inorganic quantum dots (QDs), 10 organic dyes, 11 and rare-earth ions/complexes. 12 Inorganic QDs are nanosized semiconducting crystals, whose absorption and emission bands can be tuned according to their size. They exhibit a wide absorption band, high emission intensity, and relatively good photostability. 11 On the other hand, they result in high reabsorption losses due to the large overlap of absorption and emission bands, exhibit relatively poor PL QYs, and are Received: August 12, 2018 Published: September 24, 2018 Article pubs.acs.org/journal/apchd5 Cite This: ACS Photonics 2018, 5, 4637-4643 © 2018 American Chemical Society 4637 DOI: 10.1021/acsphotonics.8b01125 ACS Photonics 2018, 5, 4637-4643 Downloaded via DAEGU GYEONGBUK INST SCI & TECHLGY on November 30, 2018 at 00:11:06 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.