Light absorption broadening and exciton diffusion length enhancement for organic solar cells

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Light absorption broadening and exciton diffusion length enhancement for organic solar cells


Author: Wang, Jingchuan
Title: Light absorption broadening and exciton diffusion length enhancement for organic solar cells
Degree: Ph.D.
Year: 2013
Subject: Solar cells -- Materials.
Photovoltaic cells -- Materials.
Solar cells -- Design and construction.
Thin film devices -- Design and construction.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: ii, xiii, 150 leaves : ill. ; 30 cm.
Language: English
InnoPac Record:
Abstract: Although the main advantages of organic photovoltaics (OPVs) include low-cost, lightweight, large area available and compatible with flexible and transparent substrates, the power conversion efficiency (PCE) is still low for real application. The key problems with OPVs have not been solved. Some of the primary culprits such as narrow absorption band and short exciton diffusion length in organic semiconductors are still hindering the solar radiation utility of organic solar cells. In the thesis, a multijunction structure was designed for broadening light absorption and a novel thin film deposition method was adopted to generate a crystallized thin film with a long-range exciton diffusion length. Both of them might provide alternative solutions for the inefficient light absorption and exciton employment in organic solar cells. At first, the mechanism of S-shape characteristic current-voltage (I-V) curve in solar cell characterization was investigated by experiments and numerical simulation. The finding indicates that the inefficient charge extraction can to a large extent induce the kink in I-V curve, especially in the low mobility organic semiconductor photovoltaic devices. The charge accumulation within the device can cause a serious recombination problem, which will modulate the normal exponential I-V curve into S-shape. This part of research provided a guideline for avoiding S-shape when designing OPVs device especially for the high performance tandem devices which easily formed charge accumulation with the inefficient intermediate layers involving. With the secondary dye added, a multijunction structure was designed to broaden light absorption spectra which mimicked the photosynthesis in natural green leaves, where the pigments (primary chlorophylls and carotenoids) complementarily absorb light energy and then transfer into the reaction center. Comparing with tandem structure, which was conventionally used for complementary light absorption, the recombination centers can be eliminated in our design which will reduce the complexity of fabrication processing. As a result, the short circuit current was improved around 50% with maintaining the value of open circuit voltage in our devices. Finally, to overcome the limitation brought forward by short exciton diffusion length in organic semiconductor thin films, a crystallized boron subphthalocyanine chloride (SubPc) thin film was grown on negative electrode using solution method. In this novel thin film growth technique, the surface charges were suggested to promote the nucleation of SubPc molecules on the substrate. The crystallized thin film structure was characterized by GIXRD and TEM techniques and results indicated a turbostratic-graphite-like structure. With the preferred molecule packing, the exciton diffusion length was measured up to five-fold increase comparing with that in the thermal evaporated amorphous thin film.

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