Comparison of Hybrid Blends for Solar Cell Application

In the area of hybrid and organic solar cells, dye-sensitized solar cells (DSSC), like the ‘Grätzel-cell’, show the highest power conversion efficiency. In such ‘Grätzel-cells’, I−/I3− dissolved in acetonitrile is commonly used as electrolyte. However, cells based on this volatile and corrosive electrolyte present problems in their sealing, which is a major drawback in terms of long term stability. As alternatives to the liquid electrolyte in DSSCs, various solid state and dye sensitized solid state hybrid solar cells have been investigated. A variety of inorganic/organic combinations have been found to be suitable as functional hybrid materials for solar cells. Semiconducting metalchalcogenides and oxides like CdSe, CdTe, PbS, PbSe, ZnO, TiO2 in combination with hole conducting organic materials like MEH-PPV (poly(2-methoxy-5(2′-ethyl)hexoxy-phenylenevinylene)), P3HT (poly(3-hexylthiophene)), Spiro-OMeTAD (2,20,7,70-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,90- spiro-bifluorene), PTPA (poly(triphenylamine)) were found to be promising for photovoltaic applications [. There are several ways of designing an interface between the inorganic electron conducting and organic hole conducting part. In this study we focus on blended systems of different organic hole conductor and TiO2 rods. In such blended systems, phase separation leads to formation of interconnected pathways. Interconnected pathways are desirable because the charge has to travel from the interface towards the electrodes.

Blending the active components also offers additional advantages. Components can be optimized separately, which enables calcination of the inorganic semiconductor at elevated temperatures. This process yields inorganic particles with the desired crystallographic modification. Furthermore, the organic materials can be tuned to improve the miscibility of the organic and inorganic phase in a blend or to selectively dissolve the inorganic part in one phase.

To date, hybrid solar cell blends of CdSe and CdTe combined with poly (thiophenes) show highest efficiencies. However, we concentrate on TiO2 phases in combination with P3HT or PTPA. We believe that because of mild environmental impact and its low cost TiO2 seems to be more promising component than Cd-based blends. Additionally, TiO2/spiro-MeOTAD cells show up to 4% efficiency, depending on the kind of hole conductor.

Comparison of solar cells assembled by different research groups is difficult: the overall power conversion efficiency is strongly influenced by the combination of hole and electron conductor and on the added dye, in case of dye sensitized cells. Also device properties like, e.g., layer thickness, measurement techniques (e.g., measuring at 1 sun) and device preparation conditions (annealing, use of barrier and protecting layers, etc.) have an impact on the solar cell efficiency. To alleviate this problem, two different hole-conducting polymers are used in this study in combination with TiO2. For a direct comparison of these materials, device preparation and measurement were identical.

Read the full article here in PDF format.

Citation: Lechmann, M.C.;Koll, D.;Kessler, D.;Theato, P.;Tremel, W.;Gutmann, J.S. Comparison of Hybrid Blends for Solar Cell Application. Energies 2010, 3, 301-312.

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