Abstract
ABSTRACTThe performance improvement of conventional CdTe solar cells is mainly limited by doping concentration and minority carrier life time. Alloying CdTe with an isovalent element changes its properties, for example its band gap and behaviour of dopants, which has a significant impact on its performance as a solar cell absorber. In this work, the structural, optical, and electronic properties of CdTe1-xSex films are examined for different Se concentrations. The band gap of this compound changes with composition with a minimum of 1.40 eV for x = 0.3. We show that with increasing x, the lattice constant of CdTe1-xSex decreases, which can influence the solubility of dopants. We find that alloying CdTe with Se changes the effect of Cu doping on the p-type conductivity in CdTe1-xSex, reducing the achievable charge carrier concentration with increasing x. Using a front surface CdTe1-xSex layer, compositional, structural and electronic grading is introduced to solar cells. The efficiency is increased, mostly due to an increase in the short-circuit current density caused by a combination of lower band gap and a better interface between the absorber and window layer, despite a loss in the open-circuit voltage caused by the lower band gap and reduced charge carrier concentration.
Highlights
Recent improvements in CdTe solar cell efficiency were mainly reached by increasing the short-circuit current density (JSC) in state-of-the-art superstrate solar cells
We find that alloying CdTe with Se changes the effect of Cu doping on the p-type conductivity in CdTe1-xSex, reducing the achievable charge carrier concentration with increasing x
We investigate the properties of the CdTe1-xSex alloy with a focus on its influence on Cu doping
Summary
Recent improvements in CdTe solar cell efficiency were mainly reached by increasing the short-circuit current density (JSC) in state-of-the-art superstrate solar cells. The majority of Cu in solar cells is segregated at grain boundaries [5] This can influence the minority carrier life time which is limited by recombination at bulk defects and at grain boundaries in the CdTe layer [9]. Swanson et al [10] studied solar cells with a CdTe1-xSex layer between a Mg0.25Zn0.75O window layer and the CdTe absorber layer They reported an increase in JSC with a reduction in band gap. Based on the results from optical and electrical measurements presented in this paper, we introduce the concept of a substrate configuration CdTe solar cell containing an absorber with compositional grading, which corresponds to grading of structural and electronic properties. This strategy has been applied in perovskite solar cells [17]
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