Spatial correlation of structure / composition trends in Cu2ZnSnSe4-based solar cells with sub-nanometer resolution
2015 (English)In: 2015 Materials Research Society Fall Meeting, 2015Conference paper, Abstract (Refereed)
Thin film photovoltaic absorbers based on the kesterite crystalline structure have generated intense interest in recent years as a possible replacement material for Cu(In,Ga)Se2 (CIGSe) and CdTe. In particular, the Cu2ZnSn(S,Se)4 (CZTSSe) phase as well as the Cu2ZnSnSe4-CZTSe and Cu2ZnSnS4-CZTS quaternary phases are considered especially promising for industrial applications due to the relative abundance and low price for all of the critical elements. While CZTSSe absorber layers have recently demonstrated efficiencies in excess of 12%, a considerable gap still exists between this system and the more well-established CIGSe and CdTe technologies. The primary challenge can be attributed to the large voltage deficit of the kesterites. Critically, the origins of this deficit are not currently understood, but are largely considered to arise from nanoscale features such as bulk defects, grain boundary impurities, local chemical/potential/band-gap fluctuations, and interfacial features that contribute to a short minority carrier life-time. Accordingly, an increase in performance of the CZTSSe system is intimately linked to an improved understanding of its nanoscale landscape and correlation to its macroscopic properties.
We recently reported a CZTSe-based solar cell with a record performance of 10.1% through the introduction of a superficial Ge-nanolayer. The absorber was synthesized using a sequential process based on DC-magnetron sputtering deposition of Cu/Sn/Cu/Zn metallic stacks, followed by a thermal evaporation of a 10 nm thick Ge layer, and finally by a reactive annealing under Se+Sn atmosphere. In this work, we present a detailed analytical investigation into the nanoscale makeup of this full photovoltaic device as well as other high performance CZTSe-based photovoltaic devices. We combine traditional Transmission Electron Microscopy (TEM) methods with Electron Energy Loss Spectroscopy (EELS) hyperspectral imaging techniques to enable the spatial mapping of structure / composition trends in this system over micron-sized regions with sub-nanometer spatial resolution. We levy this information to target structural features of significance, including grain boundaries, allowing us to classify their nature in terms of chemical, structural, and even electronic configuration over the entire absorber layer thickness. By using this advanced methodology, we observe two types of grain boundaries referred as “dirty” and “clean”. The “dirty” grain boundaries are in general parallel to the surface and connecting pores, while the “clean” ones are mainly perpendicular and with a more marked Cu-metallic character. Additionally, we identify for the first time nanoscale Cu and Zn compositional fluctuations that correlate with possible band-gap fluctuations. Finally, we will present the predictable impact of these features on the overall performance of the devices, giving insights about the major challenges of CZTSSe solar cells at the nano-scale level.
Place, publisher, year, edition, pages
CZTSe, kesterites, solar cells, photovoltaic, transmission electron microscopy, TEM, Electron Energy Loss Spectroscopy, EELS, hyperspectral imaging, thin films
Energy Systems Nano Technology Condensed Matter Physics
Research subject Engineering Science with specialization in Materials Analysis; Engineering Science with specialization in Materials Science; Engineering Science with specialization in Nanotechnology and Functional Materials; Engineering Science with specialization in Solid State Physics
IdentifiersURN: urn:nbn:se:uu:diva-268455OAI: oai:DiVA.org:uu-268455DiVA: diva2:876773
2015 Materials Research Society Fall Meeting