Quantum Dot Solar Cell   

Surface Trap Density Reduction of Quantum Dot

Quantum Dot Ink by Phase-Transfer-Exchange

 Quantum dots Solar Cells is promising to be next generation solar cells due to low cost and easy material synthesis. Quantum dots are nanometer-sized particles of semiconductor PbS, PbSe, CdS, etc, which can absorb the light and convert into electricity. Our goals are developing and optimizing PbS quantum dots solar cells, to get transparent, flexible, and high efficiency.

 Dye - Sensitized Solar Cell  



 Our research is mainly focused on developing highly efficient, stable, solid-state DSSCs. In order to achieve this goal, we are actively investigating four major components for DSSCs; dye, photoelectrode, electrolyte, and counter electrode.




Anode:    hv + Dye --> S*   Light Absorption
              S* --> S+ + e(TiO2 Electron Injection

  S+ + 3I- --> S + I3-   Dye Regeneration

Cathode I3- + 2e- (Pt--> 3I-    Electrolyte Reduction



 Principal of DSSC



 Polymer based Solar Cells (PSC) 



Converting energy directly from sunlight using photovoltaic technology is a way to approach growing global energy needs with a renewable resource. While silicon-based solar cells have dominated the commercial market, Organic photovoltaics (OPVs) have been considered as a promising candidate for the next generation solar cells, which can open new niches, including ultralow-cost disposable devices and flexible renewable energy devices.


  Establishment of soluble spary process

High-throughput Continuous Process



Novel External Post Treatment




 We are also interested in developing a practical post-treatment methods which may not rely on the deposition techniques to manipulate the nanomorphology of polymer active layers. Our novel solvent-assisted treatment technique successfully develop the polymer crystals which is comparable to be obtained by conventional state-the-art post treatment techniques.







 Organic Field Effect Transistors (OFETs)  


 Our spray techniques used for PSC fabrication can be easily extended for the fabrication of other organic electronic devices. OFETs are successfully fabricated using our spray technique and the microcrystal domains of the polymer active layers are very effectively manipulated using the post-treatment techniques. The device performance was high and comparable to the devices fabricated by other conventional methods.




 CNT & Graphene Chemistry  


 Carbon nanotubes (CNT) and graphene are nano-sized carbon materials possessing various unique properties which can be applied for a range of applications such as electronic devices, energy devices, catalysts, and sensors. The main obstacle for the utilization of these materials are limited processability. We develop the chemistry for the solution processable CNTs and Graphene.

Preparation of Graphene
 We prepare graphene via chemical synthetic method. Chemical synthetic methods of graphene has potentials for large scale preparation which has industrial feasibility. The decent exfoliation of graphene is confirmed by TEM and AFM which indicate the large portion of single layer graphene is prepared. In the state of single layer or a few layer graphene, high surface area, catalytic effect, and charge transporting properties are observed. We are currently applying the chemically prepared graphene to the electrodes for energy devices and active materials for transistors. 


  Organic and aqueous soluble SWCNTs


 Aqueous soluble SWCNT was prepared by hybridizing SWNCT with a conjugated block copolymer (PEDOT-b-PEO). This block copolymer functions as a surfactant to disperse the aggregated SWCNT; the conjugated PEDOT block interact with the basal plane of SWCNTs while the water soluble PEO block to offer solubility. The CNT/polymer nano-hybrids has solubility in various organic and aqueous systems. When the solvents were dried, the nano-hybrids had a morphology where the polymers are wrapped SWCNTs which are well-exfoiliated one another.





 The molecular level interaction was confirmed by the PL emission quenching upon increase of the SWCNT contents. Since the conjugated PEDOT moieties are known to have a good electrical properties, the nano-hybrids displayed advanced properties as the transparent conducting electrodes.


  Water soluble MWCNTs


 Water soluble MWCNTs were synthesized by grafting PSSNa on the MWCNTs. The synthetic strategy yielded a polyelectrolyte brushed MWCNTs which exhibits high solubility in water. This materials was used for the cathode of DSSC and the fabrication of transparent conducting electrode.


  Metal nanoparticles decorated CNTs 


 Metal nanoparticle (NP) decorated CNTs are synthesized via a novel sonochemical method in the ionic liquid media. Uniform dispersion of metal-NPs on the CNTs is achieved, and they exhibited enhanced solubility in organic solvents. Transparent conducting electrodes are successfully fabricated vacuum-assisted filtration method. The electrical conductivity of the metal-NP decorated CNTs  was > 2 times higher than that of their pristine CNT counterparts.




 Molecular Thermoelectrics




 Study on revealing for thermoelectric properties of organic molecules are in very early stage. Thermopower in metal-molecule-metal junctions (MMMJ) are first reported at 2007 (by S-Y. Jang et al.), and there are many important issues to be addressed. The goal of this research is to understand the relationship between the molecular structures and thermoelectric properties, and to develop organic materials possessing improved thermopower. Single molecule or single molecular layers which contains the functional groups to contact metal electrodes has to be prepared, and nanoscale measurements of electronic properties are performed. We synthesize various conjugated molecules which have different conjugation length, chemical structure, and end-functional groups, and characterize the thermoelectric properties.



 The thermopower effect and charge transport in MMMJs are determined by the relative location between energy levels of molecules and metal Fermi levels. The thermopower of the junctions is obtained by the voltage built by the given temperature difference. The higher voltage was generated, the higher thermopower is. Depending on the sign of the thermopower, the major carrier for charge transport can be determined (hole or electron). The charge transport within molecules is governed by the energy barrier from the metal Fermi level to the closest molecular orbital (typically HOMO or LUMO). The locations of energy levels of molecules are often dependant on the chemical structure and the metal-molecular contacts.






 Our recent results (JACS 2011) showed that the conjugation length and contacting functional groups influenced the resulting thermopower. As the conjugation length become longer the thermopower increased due to lowering of HOMO level. The thermopower was relatively insensitive upon change in contacting end-groups, however the sign of thermopower could be changed by changing from –SH group to –NC group.



Energy & Nanoelectronic Materials Group, Department of Chemistry, Kookmin University,
861-1, Jeongneung-Dong, Seongbuk-Gu, Seoul 136-720, Republic of Korea, Tel:82-2-910-5583

All Rights Reserved 2011. Energy & Nanoelectronic Materials Group