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◆ Overview : Organic Photovoltaics

  Harvesting energy directly from sunlight using photovoltaic(solar cell) technology is a way to address growing global energy needs with a renewable resource while minimizing detrimental effects on the environment by reducing atmospheric emissions. Recent advances in solar power conversion efficiencies have propelled organic-based photovoltaics (OPVs) out of the realm of strictly fundamental research at the university level and into the industrial laboratory setting. Fabricated from organic materials—polymers and small molecules—these devices are potentially easier to manufacture than current technologies based on silicon or other materials. Some key advantages for OPVs are that organic small-molecule and polymer materials are inherently inexpensive; they can have very high optical absorption coefficients that permit the use of films with thicknesses of only several hundred nanometers; they are compatible with plastic substrates; and they can be fabricated using high-throughput, low temperature approaches that employ one of a variety of well-established printing techniques in a roll-to-roll process.

These are the list of recent works of our research group
 I. High Performance Organic Photovoltaics via Interface Engineering
 II. High Performance Organic Photovoltaics via Morphology Control
 III. Ordered Bulk hetrojunction Photovoltaic Cell

  A key issue faced by OPVs in general is relatively low power conversion efficiency. First and foremost, higher power conversion efficiencies must be demonstrated. Several groups are now reporting values in the 5% regime. These are very encouraging results; however, efficiencies of laboratory-scale devices must be pushed higher before large scale manufacturing can really be considered. The main focus of our laboratory in OPVs research is to demonstrate high efficiency OPVs via Device Interface Engineering, Morphology Control of Photoactive layers and Fabrication of ordered bulk hetero structures. OPVs have several interfaces can be modified. By inserting effective buffer layers, such as self-assembled monolayers and metal nanoparticles, we can control the carrier transport and enhance the incoming light intensity. Ordered bulk heterostructure reduces exciton recombination and provide straight carrier pathways to the electrodes. We fabricated well-aligned hexagonal array of p-type P3HT Nano-Rods oriented perpendicularly to the ITO glass by using AAO template. The optimized morphology of photoactive layers having high and balanced electron/hole mobility has also been accomplished by changing end function group of conducting polymers or applying solvent vapors to the active films.


I. High Performance OPV via Interface Engineering
 1.1. Control of electrode work function & morphology with self assembled monolayer

  We have correlated the changes in electrode work function and active layer structure due to the surface energy change produced by treating ITO with various SAMs with the electrical properties of OPVs. SAMs with electronwithdrawing groups increase the work function of the ITO/active layer interface more than SAMs with electrondonating groups. However, the phase separation of the active layer is a more important factor than the match between the injection barrier and the active layer’s HOMO level. The best power conversion efficiency value we obtained was 3.15% for a CF3 SAM treated surface after annealing, in the absence of a PEDOT:PSS layer. By carrying out further optimization of this device, we expect to increase the performance of devices obtained by SAM treatment of ITO substrates.

 1.2. Spectral surface plasmon excitation of OPV by anisotropic gold nanoparticles

  We fabricated interfacial gold nanostructured surfaces by simple solution process to investigate the spectral response of LSPR. Excited-state plasmon creates propagating polariton wave, resulting exciton-plasmon interaction in conjugated polymer. Exciton-plasmon interaction can accelerate the hot exciton without finding their trap sites for the enhanced efficiency of bulk heterojunction solar cells.

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II. High Performance OPV via Morphology Control
 2.1. Morphology Control by using End Group Modified Poly(3-hexylthiophene)

  We controlled the domain size from miscibility control between PCBM and end functional group modified P3HT. The miscibility was caused by surface energy matching of modified P3HT. The surface energy matching between PCBM and CF3 terminated P3HT was most favorable, and it causes the well mixed film morphology of the blend film. Well mixed and advantageous nano-morphology which can help the exciton diffusion and charge separation makes the series resistance of the active layer decrease; therefore FF of the device was maximized. This FF causes the efficiency increase.

 2.2. Effect of Solvent Annealing on Performance of P3HT/PCBM Solar Cells

  The solubility and volatility of annealing solvent have a substantial effect on the degree of nanoscale phase separation of photoactive organic films. By controlling solubility and vapor pressure of annealing solvent, we can optimize morphology and molecular ordering of solar cells.

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III. Ordered bulk heterojunction photovoltaic cell

  Ordered bulk heterostructure PV cells are more difficult to fabricate than disordered blends, but there are several good reasons to do so. First, the dimensions of both phases can be controlled to ensure that every spot in a film is within an exciton diffusion length of an interface between the two semiconductors. Second, there are no dead ends in the structure. After excitons are dissociated by electron transfer, the electrons and holes have straight pathways to the electrodes. This geometry ensures that the carriers escape the device as quickly as possible, which minimizes recombination. Third, in an ordered structure it is possible to align conjugated polymer chains, which increases the mobility of their charge carriers. Another advantage, which is particularly important during the current stage of organic photovoltaic cell research, is that ordered structures are much easier to model and understand.

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Written by OPV Team ( )
Edited by Kwangwoo Cho ( 2019.11.05 )


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