Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SOLACYLIN (A preparative approach to geometric effects in innovative solar cell types based on a nanocylindrical structure)

Teaser

Two necessary requirements of solar cells are the efficient absorption of light and the subsequent efficient collection of the charge carriers generated. In planar devices, which constitute the vast majority of current commercial cells, these two physical phenomena impose...

Summary

Two necessary requirements of solar cells are the efficient absorption of light and the subsequent efficient collection of the charge carriers generated. In planar devices, which constitute the vast majority of current commercial cells, these two physical phenomena impose contradicting constraints on the only one experimentally variable geometric parameter, the thickness of the semiconductor layer(s). Thus, planar systems necessitate that materials parameters be exploited in the photovoltaic optimization, as well. In c-Si cells, light absorption is maximized by a large layer thickness (>100 µm), which is deleterious to the collection of charge carriers. To compensate for this, a material is used (crystalline Si 99.9999%) of extreme purity, very large carrier lifetimes and diffusion distances. In thin film cells, most prominently of the chalcopyrite family (\'CIGS\'), carrier collection is maximized by a small layer thickness (<100 nm), deleterious to the absorption of photons. To com­pen­sate for this, a material is used of extraordinarily large light absorption coefficient and short light penetration depth.

The success of each of these two types of photovoltaic devices crucially relies on the outstanding properties of a very specific material. Correspondingly, material costs represent a significant fraction of the overall device costs. This and their rather long energy payback times put a hurdle on the widespread adoption of photovoltaics as a primary energy source. Unexpensive materials containing exclusively earth-abundant elements and processed without high-vacuum techniques cannot compete with c-Si and CIGS in terms of electrical and optical quality. Therefore, the optimization of photovoltaic devices based on them must exploit an additional experimentally adjustable parameter: Conceptually, this is, in many types of \'third-generation\' cells, the lateral size of structures elongated perpendicularly to the plane of the bulk device. In principle, the length L of such structures can be adjusted to the material\'s optical density and their thicknesses D to the diffusion lengths of the carriers. In reality, systems prepared on these principles to date (dye-sensitized, quantum dot, perov­skite, and some organic cells) are usually disordered.

Such photovoltaic structures based on a nano­crys­talline semiconductor prepared with colloi­dal approaches have provided viable alter­natives to c-Si and CIGS based on in­expensive materials combined with nano­struc­tu­ring. However, these systems have not reached the efficiencies of planar types of cells. One difficulty in their optimi­zation resides in the lack of control over the structure of the interface. The disordered geometry may put limitations on the charge collection efficiency,8 it also prevents systematic investi­ga­tions of the device physics, given that no well-defined geometric parameters can be addressed. Such a systematic investigation would be possible if the semiconductor junction were organized in cylindrical nanostructures of tunable length and diameter. To date, however, a direct, systematic and exhaustive investigation of the geometric effects on photovoltaic properties in solar cells of this nanorod structure has been missing. This may be related to the difficulties of preparing large parallel arrays of cylinders combining two semi­con­duc­tors in a geometry that is well defined, tunable, and homogeneous over each sample. Approaches to the preparation of photovoltaic solar cells based on elongated nano­structures have been published recently, however none combines all aspects of geometric control necessary to directly highlight expe­ri­mentally the relationships between geometry and photovoltaic performance using materials of low cost and low quality.

Thus, experimental work in this research area to date has been limited in terms of materials and of ability to tune the nanostructures’ geometry. In SOLACYLIN, we develop novel preparative met

Work performed

\"SOLACYLIN research so far has focused on the development of several distinct aspects of sample preparation which will be needed for generating solar cells in the novel \'SOLACYLIN geometry\'. This geometry consists of parallel nanocylinders in ordered arrays, each of which combines three semiconductors in a coaxial manner.

One piece of work has focused on the nanoporous matrices to be used as templates for the cylinders. We use \"\"anodization\"\", an electrochemical procedure used industrially for the surface treatment of some metals. In SOLACYLIN, we have explored simple methods for generating anodized arrays of pores that display a high degree of order and a homogeneous geometry from the very beginning. This contrasts with existing methods, in which either the pores start out disordered and only become ordered later, or expensive manufacturing techniques (lithography) are used to define the order preliminarily.

In another line of research, we have developed methods allowing for the coating of such elongated pores based on surface chemical reactions, a strategy called \"\"atomic layer deposition\"\" (ALD). We have optimized methods for depositing thin silica films by ALD, into which we mix in small amounts of either aluminum or antimony. Upon thermal reduction using lithium vapor, the material is converted to amorphous silicon with either p or n doping as needed in photovoltaics. Further ALD work has been pursued towards controlling the surface reactions from \'precursors\' dissolved in liquids, instead of bringing them from the gas phase in a vacuum chamber. This evolution shall render novel types of ALD chemistry possible, and thereby, allow for the ALD coatings of materials which to date have not been accessible, in particular the so-called \'perovskites\' that represent the most modern development in photovoltaics.

Finally, we have worked on making full perovskite solar cell prototypes in various geometries -- planar films, colloidal nanoparticles, and coaxial nanocylinders. We have also started investigating materials that could replace the lead-based perovskites used currently. We hope to be able to circumvent two major impediments of the lead-based perovskites, namely their toxicity and their instability in air.

In the coming months, we will combine all these novel methods and assemble nanocylindrical solar cells of three modern types: the \"\"extremely thin absorber\"\" model, the amorphous silicon type, and the perovskite-based one. investigating how their performance depends on variations in their geometric parameters will allow us to uncover strategies to improve their efficiencies based on readily available materials.\"

Final results

The scientific advances achieved in SOLACYLIN so far have been mostly of technical nature and engineered towards the final project goal. However, several of the advances will certainly become relevant much beyond the project.

Firstly, the ability to generate parallel arrays of nanopores that are ordered from the beginning will enable their use beyond bulk materials towards films deposited on various substrates, including flexible ones. This feature will be of interest for the development of various embedded devices -- not limited to photovoltaics but also including batteries and sensors, among others.

Secondly, the novel ALD chemistry that we are advancing should render the coating method relevant to a range of novel applications, from protective organic coatings on displays to inorganic and hybrid semiconductors. It will become of particular interest for the development of future embedded devices based on printed electronics.

In essence, SOLACYLIN shall provide original material alternatives and corresponding preparative methods to render the production of energy converting and other high-end devices less expensive and more sustainable.