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2.1. Quantum materials for energy

2.1.2. Photovoltaics

Research for renewable energies at the HZB focusses on the direct conversion of light into electricity, i.e. photovoltaics (PV), and on the direct conversion of abundant resources such as water and CO2 into hydrogen or hydrocarbons using sunlight, i.e. solar fuels (see section 2.4.2).

 Silicon and compound semiconductor based thin-film PV represent the technological base of the Solar Energy Research activities at HZB. A self-consistent scientific approach enables research and development on the full chain from the very fundamentals of the materials ending at scalable prototype devices that demonstrate industrial relevance. The requirement of short-term support of industrial partners is facilitated by the Competence Centre for Thin Film and Nanotechnology for Photovoltaics Berlin (PVcomB). The aim is to provide the scientific and technological base to advance present and develop next generation thin film photovoltaics.

The long-term goal of HZB’s PV research is to develop even more cost effective and more efficient devices. Here, new materials and concepts for future photovoltaic devices are explored. Solar cell concepts based on, e.g. nano-particles will provide extended chemical flexibility and include quantum-size and optical coherence effects. The research strategy behind this activity is to generate the scientific knowledge needed to create photovoltaic devices beyond the present cost and efficiency limitations.

The most prominent techniques used at BESSY II for this at the current stage are photoemission spectroscopy, x-ray absorption and emission spectroscopy, photoemission electron microscopy and energy-dispersive x-ray diffraction and fluorescence. While most of the methods make full use of and require a tunable soft x-ray source of high brilliance, BESSY II is also able to provide x-rays at much higher energies as needed for diffraction and fluorescence experiments. A crucial extension of the experimental capabilities using X-rays at BESSY II will be realized by the Energy Materials In-situ Laboratory Berlin, EMIL.

In recent years, thin film solar cell devices have impressed by their very successful development and fast progress. Today’s thin film photovoltaic systems are hence on the verge to commercialization. In addition to the continuous need for higher efficiencies, respective solar cell devices have thus to face new challenges in particular in terms of stability, cost, and scale-up potential. At this point, the next crucial level of performance can only be reached by a knowledge-based optimization, extending current trial-and-error device improvement. Thus, in order to lead the thin film solar cell technology to a breakthrough, a detailed knowledge about potential inherent limitations is necessary in order to identify new routes to overcome these obstacles. Since each layer in the photovoltaic device thin film layer stack has a different chemical (lattice constant, crystal structure, thermal expansion or diffusion coefficient, mechanical adhesion, chemical affinity) and electronic structure (work function, electron mobility, doping level, defect concentration, electronic band position, conductivity), the interfaces between those layers can cause stresses and are often the place of an increased density of defect states which can act as recombination centers when not designed carefully. In addition, interdiffusion processes can take place at those interfaces leading to significant interfacial intermixing, which induce changes in the optoelectronic properties of the whole device. Furthermore, the interfaces not only influence, but in most cases determine the local electric fields necessary for efficient charge carrier separation.

The key to more efficient PV systems is the exact knowledge of the interface properties and the dynamics of charge carrier excitation, generation, transfer, and recombination. BESSY-VSR  will be essential in elulcidating these. Usually charge carrier lifetimes in today’s PV devices are in the ms – ns regime. Thus, recombination mechanisms occurring in these solar cells are already accessible by BESSY II today. However, e.g., the formation of free charge carriers after optical excitation, the processes involved in photon up- and/or down-conversion systems, and/or the charge carrier lifetimes in new PV materials/concepts take places on time scales a few ps. In particular the use of novel concepts (e.g. excitonic) in solar cells depends crucially on understanding generation and recombination. In order to study these key mechanisms in next-generation PV materials and solar cell devices BESSY-VSR with the option to use short X-ray pulses at high repetition rates will be essential. It will be in particular the flexibility of BESSY-VSR with radiation from the Terahertz to the hard x-ray range with flexible time-structure and filling patterns that will enable unprecedented insight into the interface properties and the charge carrier dynamics of PV systems and solar cells.