This Review summarizes recent progress in the development of polymer solar cells. It covers the scientific origins and basic properties of polymer solar cell technology, material requirements and device operation mechanisms, while also providing a synopsis of major achievements in the field over the past few years. Potential future developments and the applications of this technology are also briefly discussed.

Polymer solar cells

Approaches, Derivatives and Applications Vasilios Georgakilas,† Michal Otyepka,‡ Athanasios B. Bourlinos,‡ Vimlesh Chandra, Namdong Kim, K. Christian Kemp, Pavel Hobza,‡,§,⊥ Radek Zboril,*,‡ and Kwang S. Kim* †Institute of Materials Science, NCSR “Demokritos”, Ag. Paraskevi Attikis, 15310 Athens, Greece ‡Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University Olomouc, 17. listopadu 12, 771 46 Olomouc, Czech Republic Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v.v.i., Flemingovo naḿ. 2, 166 10 Prague 6, Czech Republic

Functionalization of Graphene: Covalent and Non-Covalent Approaches, Derivatives and Applications

Chemically derived graphene oxide (GO) is an atomically thin sheet of graphite that has traditionally served as a precursor for graphene, but is increasingly attracting chemists for its own characteristics. It is covalently decorated with oxygen-containing functional groups - either on the basal plane or at the edges - so that it contains a mixture of sp(2)- and sp(3)-hybridized carbon atoms. In particular, manipulation of the size, shape and relative fraction of the sp(2)-hybridized domains of GO by reduction chemistry provides opportunities for tailoring its optoelectronic properties. For example, as-synthesized GO is insulating but controlled deoxidation leads to an electrically and optically active material that is transparent and conducting. Furthermore, in contrast to pure graphene, GO is fluorescent over a broad range of wavelengths, owing to its heterogeneous electronic structure. In this Review, we highlight the recent advances in optical properties of chemically derived GO, as well as new physical and biological applications.

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Graphene oxide as a chemically tunable platform for optical applications

http://staff.ulsu.ru/moliver/ref/polymers/pott11.pdf

Graphene-based polymer nanocomposites

Graphene, a two-dimensional, single-layer sheet of sp(2) hybridized carbon atoms, has attracted tremendous attention and research interest, owing to its exceptional physical properties, such as high electronic conductivity, good thermal stability, and excellent mechanical strength. Other forms of graphene-related materials, including graphene oxide, reduced graphene oxide, and exfoliated graphite, have been reliably produced in large scale. The promising properties together with the ease of processibility and functionalization make graphene-based materials ideal candidates for incorporation into a variety of functional materials. Importantly, graphene and its derivatives have been explored in a wide range of applications, such as electronic and photonic devices, clean energy, and sensors. In this review, after a general introduction to graphene and its derivatives, the synthesis, characterization, properties, and applications of graphene-based materials are discussed.

Graphene-Based Materials: Synthesis, Characterization, Properties, and Applications

Three small molecules named DR3TBDTT, DR3TBDTT-HD, and DR3TBD2T with a benzo[1,2-b:4,5-b']dithiophene (BDT) unit as the central building block have been designed and synthesized for solution-processed bulk-heterojunction solar cells. Power conversion efficiencies (PCEs) of 8.12% (certified 7.61%) and 8.02% under AM 1.5G irradiation (100 mW cm(-2)) have been achieved for DR3TBDTT- and DR3TBDT2T-based organic photovoltaic devices (OPVs) with PC71BM as the acceptor, respectively. The better PCEs were achieved by improving the short-circuit current density without sacrificing the high open-circuit voltage and fill factor through the strategy of incorporating the advantages of both conventional small molecules and polymers for OPVs.

Solution-processed and high-performance organic solar cells using small molecules with a benzodithiophene unit.

Small molecules, namely, DCAO3TBDT and DR3TBDT, with 2-ethylhexoxy substituted BDT as the central building block and octyl cyanoacetate and 3-ethylrhodanine as different terminal units with the same linkage of dioctyltertthiophene, have been designed and synthesized. The photovoltaic properties of these two molecules as donors and fullerene derivatives as the acceptors in bulk heterojunction solar cells are studied. Among them, DR3TBDT shows excellent photovoltaic performance, and power conversion efficiency as high as 7.38% (certified 7.10%) under AM 1.5G irradiation (100 mW cm–2) has been achieved using the simple solution spin-coating fabrication process, which is the highest efficiency reported to date for any small-molecule-based solar cells. The results demonstrate that structure fine turning could cause significant performance difference and with that the performance of solution-processed small-molecule solar cells can indeed be comparable with or even surpass their polymer counterparts.

Small Molecules Based on Benzo[1,2-b:4,5-b′]dithiophene Unit for High-Performance Solution-Processed Organic Solar Cells

In the past few years, great progress has been made in organic photovoltaic (OPV) cells for an alternate of silicon semiconductorbased solar cells. OPV has the advantages of clean, low-cost, flexibility, and the possibility of roll-to-roll production.[1–4] Currently, most of the works have been focused on polymer donor molecules using bulk heterojunction (BHJ) architecture and [6,6]-phenyl-C61–butyric acid methyl ester (PC61BM) as the acceptor.[5,6] Indeed, in addition to the currently better OPV performance than small molecules, polymers have the advantages for such as better film forming quality and so on.[7] However, it cannot be denied that there are disadvantages for polymer-based OPV, such as batch to batch reproducibility, difficulty of purification, and so on. In contrast, small molecules intrinsically do not have such flaws;[8] additionally, their band structures could be tuned easily with much more choices of chemical modification. Furthermore, small molecules generally have higher charge mobility and open voltages.[9,10] However, even with these advantages, small-molecule-based OPV cells have not been investigated as intensively as that of their polymer counterparts because one of the major problems for small molecules is their generally poor film quality when using the simple solution spinning process.[11] This has been hampering their performance, and indeed their power conversion efficiencies (PCEs) (4%–5%)[12–18] are still significantly lower compared with that (>7%)[19–25] from polymers. It is thus expected that better PCE could be achieved when their intrinsic bad film quality and morphology in BHJ architecture could be improved combining with their other advantages. But to achieve this, careful molecule design has to be carried out to address many factors collectively, including their molar absorption, morphology compatibility with the acceptors for a better film quality, and so on.

Solution Processable Rhodanine‐Based Small Molecule Organic Photovoltaic Cells with a Power Conversion Efficiency of 6.1%

A conjugated small molecule containing a benzodithiophene unit shows high performance in bulk heterojunction (BHJ) solar cells. Using the simple solution spinning process, a high PCE is achieved by employing this molecule as the donor in BHJ solar cells.

High-performance solar cells using a solution-processed small molecule containing benzodithiophene unit.

Organic solar cells (OSCs) have attracted signifi cant attention as a clean and competitive renewable energy source due to their attractive features such as low-cost, light weight, solution processability and high mechanical fl exibility. [ 1–4 ] Benchmark power conversion effi ciencies (PCEs) of 10% or higher have been predicted if a suitable low bandgap donor material can be designed and implemented. [ 5 ] More recently, bulk heterojunction (BHJ) OSCs using solution-processed small molecules as the donor have attracted great attention. [ 6–8 ] This long-time but recently increased interest lies in the fact that solutionprocessed small molecule based OSCs have numerous advantages, such as relatively simple synthesis and purifi cation methods, monodispersity and well defi ned structures, high open circuit voltage and charge carrier mobilities, and better batch-to-batch reproducibility. [ 6–8 ] However, solution-processed small molecule OSCs have not met such high expectations as those of their polymeric counterparts due to their limited PCEs. In most cases, small molecule devices using solution processing always seem to have poorer fi lm quality than that of their polymeric counterparts in BHJ OSCs. [ 6 , 7 ] It is thus expected that better PCE could be achieved if the intrinsic poor fi lm quality and morphology in BHJ architecture could be improved. However, in order to achieve this, careful molecule design has to be carried out to address many factors simultaneously, including the material’s solar light absorption associated with its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) positions, mobility, fi lm forming quality, electronic band structure and morphology compatibility with the acceptors, and so on. Indeed, several families of solution processed small molecules, such as oligothiophenes, [ 9 ]

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Jiaoyan Zhou

Papers

Solution-processed and high-performance organic solar cells using small molecules with a benzodithiophene unit.

Small Molecules Based on Benzo[1,2-b:4,5-b′]dithiophene Unit for High-Performance Solution-Processed Organic Solar Cells

Solution Processable Rhodanine‐Based Small Molecule Organic Photovoltaic Cells with a Power Conversion Efficiency of 6.1%

High-performance solar cells using a solution-processed small molecule containing benzodithiophene unit.

Spin-Coated Small Molecules for High Performance Solar Cells