The roles of ZnTe buffer layers on CdTe solar cell performance

Abstract

The use of ZnTe buffer layers at the back contact of CdTe solar cells has been credited with contributing to recent improvements in both champion cell efficiency and module stability. To better understand the controlling physical and chemical phenomena, high resolution transmission electron microscopy (HR-TEM) and atom probe tomography (APT) were used to study the evolution of the back contact region during rapid thermal processing (RTP) of this layer. After activation the ZnTe layer, initially nanocrystalline and homogenous, transforms into a bilayer structure consisting of a disordered region in contact with CdTe characterized by significant Cd-Zn interdiffusion, and a nanocrystalline layer that shows evidence of grain growth and twin formation. Copper, co-evaporated uniformly within ZnTe, is found to dramatically segregate and aggregate after RTP, either collecting near the ZnTe|Au interface or forming Cu x Te clusters in the CdTe layer at defects or grain boundaries near the interface. Analysis of TEM images revealed that Zn accumulates at the edge of these clusters, and three-dimensional APT images confirmed that these are core-shell nanostructures consisting of Cu 1.4 Te clusters encased in Zn. These changes in morphology and composition are related to cell performance and stability.

The Development of Nanotechnologies and Advanced Materials Industry in Science and Entrepreneurship: Socioeconomic and Technical Indicators. A Case Study of Latvia (Part Two)

“…The greatest human resource provision in engineering and technological sciences belongs to materials sciences -25 % and to biotechnology -2 %. According to the number of doctoral theses defended in Latvia, the number of specialists in engineering accounts for about 13 % of the total number of young scientists in the period of 2000-2013 [41].…”

“…According to the informative report by the Ministry of Education and Science of Latvia, in 2011 a number of "separate clusters were established, for example, in such fields as electronics, chemistry, pharmaceutics, space technology and logistics; however, their added value was unclear. Latvia has undertaken the first modernisation attempts by establishing nine research centres of national importance, but it seems that they are disproportionately much more focused on academic science" [41]. At the micro or business level, the technical indicator group includes three subgroups: the number and capacity of qualitative laboratories, capacity and power of technology and equipment, sufficiency of resources; qualitative indicators of existing product development and processing: the effect from improvement, nanomaterial characteristics; number of patent applications published in the nano-field, number of patents sold in the nano-field.…”

“…As a consequence, there are an extremely low number of spinoffs, as well as start-ups based on the developments made in the public research sector" [41]. Some manufacturing companies in the nano-field consider that they have "strong internal capacities such as more experience in the use of the high technologies, higher ability to increase the capacities of production and level of the salaries" [31].…”

Numerical Simulation of Copper Migration in Single Crystal CdTe

“…In samples that were exposed to higher annealing temperature (350°C samples) or longer duration of the annealing process (300°C for 20 min sample), abnormally high concentration of Cu (>10 18 cm -3 ) is observed in the first 0.5 μm of CdTe (0.5 < x < 1 μm region). This could be explained by the broadening of ZnTe or by formation of ZnCdTe, caused by Zn diffusion at a similar annealing temperature with shorter annealing time [29], [30]. However, such behavior is beyond the scope of this paper as Cu in CdTe bulk is the primary focus of this paper.…”

“…ε is the defect transition energy level (ionization energy) with respect to the valence band minimum. the broadening of ZnTe or the formation of ZnCdTe near the interface [29], [30]. For the simulations, high concentration (5 × 10 20 cm −3 ) of Cu i was assumed in the 0.5 μm thin ZnTe:Cu source layer.…”