Fabrication of Semiconducting Biofilms for Solar Cell Application

Since common energy resources such as oil and gas become rare and rare, the importance of renewable and sustainable energy resources rises continuously. Considering sustainability, nature provides a vast field for future power supply reaching from wind and water to solar energy. Further the production of biomass and the formation of biogas (methane) by bacteria should also be considered. The iGEM Team of the University Erlangen-Nuremberg aims to integrate current research in the preparation of conducting and semiconducting biofilms [1] to fabricate a series of novel highly absorbing layers in a bottom-up approach from the initial salts. We focus on the control of electron transport through the biofilms as it is crucial for optimization of the system. The result of our research can pave the way to a novel class of solar cells mainly fabricated by living cells. In a first set of experiments, we will combine chemically and biologically synthesized zinc oxide (ZnO) as a wide band gap semiconductor with organic dye molecules and fluorescing proteins to inspect the charge transfer through a biofilm matrix. The emphasis lies on the layer thickness and the morphology of the biofilm, as well as the size of the ZnO colloids, which will affect the electron transport. In the second part, we will address the integration of chemically synthesized, strongly absorbing nanoscale semiconductors into biofilms to analyze the film morphology and the affinity of the different types of nanoparticles to the biofilms. In the third stage we plan to replace the chemical route by biological fabrication of the nanoparticles. Both biofilms (ZnO/Absorber) will be deposited on a substrate, yielding the basic structure of a solar cell, which can be expanded to a complete device using physical or chemical deposition techniques. This primary device will be characterized regarding its photoinduced electron transfer and efficiency.

State of the Art

The rising field of nanoparticular systems is attributed to their unique properties in terms of reactivity and spectroscopic behavior.[2, 3] Thus, biologic routes towards different types of nanoparticles are of current interest, as their “biosynthesis” avoids toxic stabilizing ligands and solvents, which is a crucial aspect of sustainable synthesis.[4] The formation of metal clusters exploits the natural defense mechanism of microorganisms against high metal concentrations.[5] With this strategy, a broad variety of colloids have already been realized, from metallic [6] to semiconducting nanoparticles, also called quantum dots.[4] In the field of semiconducting nanoparticles, metal sulfides are promising absorber materials, being easy to fabricate using bacteria and achieving good yields and morphology.[7] Additionally, Zinc oxide, which can serve as transparent top layer for solar cells, can also be fabricated by bacteria that can tolerate high amounts of zinc in solution.[8]

Biofilms are aggregates of microorganisms frequently embedded within an extracellular matrix promoting adherence of cells to each other and to surfaces.[9] Biofilms are dynamic structures, selforganizing and highly adaptive to their environment. As materials, biofilms and similar multicellular consortia such as bone or shells have various characteristics that are of interest for technological applications.[10] Recently, semiconducting nanoparticles such as CdTe, CdS and ZnS have been successfully integrated into biofilms.[1] This was accomplished by conjugating the nanoparticles with antibodies or appropriate peptide tags, which in turn were linked to amyloid fibrils (curli) produced by E. coli cells. This attempt was further expanded by integrating gold nanoparticles into the film to address electric conductivity in biofilms. A direct preparation of silver nanoparticles within the biofilm matrix has also been shown to facilitate conductivity.[11] We intend to pick up this approach to program bacteria to build a scaffold for a solar cell, utilizing the unique ability of living materials to self-organize into nanoscale structures.

Nanoscale semiconductors provide a powerful platform for the development of numerous classes of solution-processed optoelectronic devices such as photovoltaic cells, photodetectors and lightemission devices. In addition to enabling solution processing, a key advantage of quantum dots is the size confinement effect: optical and electrical properties can be adjusted by varying the size and shape of the nanoparticles. In photovoltaic devices, quantum dot films are combined with a metal (Schottky junction cells) or with another semiconductor (TiO2/ZnO) to form a functional device.[12, 13] Against the background of nanoparticle solar cells, thin film solar cells provide a powerful tool to reduce the amount of material required for fabrication of solar cells. Different highly absorbing metal sulfide and metal selenide materials present the opportunity to reduce the layer thickness of the absorption layer to less than 5 µm. [14] A crucial limitation in the field of thin film solar cells is the Shockley-Queisser limit [15], describing the compromise between charge separation and absorption quality of the film; charge separation accelerates with decreasing layer thickness, while light absorption rises with increasing layer thickness.

References

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