Title:Graphene based quantum dot organic solar cells ResearchersProf. Louis Brus Hyesung Park , Sehoon Chang , Joel Jean , Jayce J. Cheng , Paulo T. Araujo , Mingsheng Wang , Moungi G. Bawendi , Mildred S. Dresselhaus , Vladimir Bulovi , Jing Kong , and Silvija Gradeak Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, United States Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, United States Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, United StatesAbstractCurrent research in organic photovoltaic (OPV) is largely focused on the development of low cost OPV materials such as semiconductor quantum dots (QDs). Graphene quantum dots (GQDs) are a fascinating class of QDs having size below 10 nm. They have emerged as an alternative to semiconductor QDs in photovoltaics due to their size-dependent photoluminescence (PL) and tunable band gap properties. They are expected to be a versatile candidate due to their low cost and biocompatibility. Recently, it has been shown that they are promising for efficient light harvesting in solar cells. Keeping this in view, we present a comprehensive review of the progress made so far for the application of GQDs in organic solar cells.

IntroductionQuantum dots*Quantum dots are tiny man-made crystals. They are so small that you cant see them with a typical microscope. In fact, theyre 10,000 times narrower than a human hair. Thats incredibly small but dont let their size fool you. Quantum dots are actually very powerful devices and its their size that gives them a unique ability: to convert light into nearly any color in the visible spectrum with very high efficiency.Each quantum dot is actually a tiny semiconductor -- which means it can convert incoming energy. The electronic characteristics of quantum dots are determined by their size and shape. This means we can control the color of light given off by a quantum dot just by changing its size. Bigger dots emit longer wavelengths like red, while smaller dots emit shorter wavelengths like green. Think of a guitar string. When a guitar string is shortened, it produces a higher pitch and when it is lengthened, it creates a lower pitch. The tune of a quantum dot the wavelength of the light it emits behaves in a similar way.Each quantum dot is actually a tiny semiconductor -- which means it can convert incoming energy. The electronic characteristics of quantum dots are determined by their size and shape. This means we can control the color of light given off by a quantum dot just by changing its size. Bigger dots emit longer wavelengths like red, while smaller dots emit shorterwavelengths like greenGraphene**Graphene is the only form of carbon (or solid material) in which every atom is available for chemical reaction from two sides (due to the 2D structure). Atoms at the edges of a graphene sheet have special chemical reactivity. Graphene has the highest ratio of edge atoms of any allotrope. Defects within a sheet increase its chemical reactivityIt is the thinnest compound known to man at one atom thick, the lightest material known (with 1 square meter coming in at around 0.77 milligrams), the strongest compound discovered (between 100-300 times stronger than steel and with a tensile stiffness of 150,000,000 psi), the best conductor of heat at room temperature (at (4.840.44) 10^3 to (5.300.48) 10^3 Wm1K1) and also the best conductor of electricity known (studies have shown electron mobility at values of more than 15,000 cm2V1s1).

DescriptionGraphene-based quantum dot-sensitizedsolar cellsIn GQDSCs, light absorbing materials in QuantumDot. The absorption spectrum of semiconductor QDs, probing the particle size is an efficient way to harvest the entire range of the solar spectrum.In addition, owing to the unique electronic band structure, QDs can overcome the ShockleyQueisser limit of energy conversion efficiency.The ability of QDs to harvest hot electrons and to generate multiple carriers makes them a viable candidate for light-harvesting sensitizersin solar cells. Several semiconductor materials (CdS, CdSe, PbS, etc.) have been used as light sensitizers on wide band gap mesoporous metal oxide layers (TiO2 and ZnO) due to their low cost and simple sensitization processing. The device function of QDSCs is analogous to DSSCs, where the dye molecules are replaced with semiconductor QDs. Besides, the theoretical efficiency of QDSCs is as high as 44%, the practical performance still lags behind that of DSSCs at present exclusively compared the critical factors of QDSCs to answer the question, why it is inferior to DSSCs. The main issues in QDSCs are (i) fast electron recombination at surface states of the TiO2/QD junction ascribed to a slower electron injection rate from QDs to TiO2 and (ii) hole trapping between QDs. To achieve competitive photoconversion efficiency with DSSCs, the above factors have to be bottlenecked in QDSCs. In the context of promoting QDSC performance, different approaches were demonstrateddoping anode, shell layer,hierarchical photoanodes etc. However, graphene has attracted growing interest as a potential candidate for improving QDSC performance. Guo et al. first demonstrated the feasibility of QD-decorated graphene as photoelectrode, which showed best incident photon to converted electron (IPCE) compared to QD-decorated CNT scaffold. This confirms graphene as a good candidate for the collection and transport of photogenerated charges, and thus, the layered nanofilm provides a new and promising direction toward developing high-performance light-harvesting devices for the next generation solar cells. Following that another interesting approach has been laid, where QD-decoratedHYBRID Type Graphene-based quantum dot solar cells or C-Si

Graphene quantum dots (GQDs) possess extraordinary optical and electrical properties and show great potential in energy applications. Here, with combing of crystalline silicon (c-Si) and GQDs, a new type of solar cells based on the c-Si/GQDs heterojunction was developed. Thanks to the unique band structure of GQDs, photogenerated electronhole pairs could be effectively separated at the junction interface. The GQDs also served as an electron blocking layer to further prevent the carrier recombination at the anode. These characteristics endow the heterojunction solar cells with much enhanced photovoltaic performance compared to the device counterparts without GQDs or with graphene oxide sheets.

Conclusion(i) Eventually, an optimum power conversion efficiency of 6.63% was obtained by tuning the GQDs size and layer thickness. Our results demonstrate the great potential of the c-Si/GQDs heterojunctions in future low-cost and high-efficiency solar cells.(ii) Several processes have been developed for graphene synthesis, and its utilization in different components of next generation solar cells were demonstrated. To date, graphene is applied in solid-state solar cells, DSSCs, QDSCs, and OPVs, owing to its versatile multifunctional properties such as optical, electrical, electrocatalytic, and mechanical propertiesREFERENCES*/**1. A.A. Balandin: Thermal properties of graphene and nanostructuredcarbon materials. Nat. Mater. 10, 569581 (2011).2. A.K. Geim and K.S. Novoselov: The rise of graphene. Nat. Mater.6, 183191 (2007).3. S. Das and W. Choi: Graphene synthesis. In Graphene: Synthesisand Applications, Vol. 3, 1st ed.; W. Choi and J-W. Lee eds.;Taylor & Francis Group: Boca Raton, FL, 2011; pp. 2763.4. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang,S.V. Dubonos, I.V. Grigorieva, and A.A. Firsov: Electric fieldeffect in atomically thin carbon films. Science 306, 666669 (2004).5. L.M. Viculis, J.J. Mack, and R.B. Kaner: A chemical route tocarbon nanoscrolls. Science 299, 1361 (2003).