Abstract
Although the fundamental limits have been established for the single junction solar cells, tandem configurations are one of the promising approaches to surpass these limits. One of the candidates for the top cell absorber is CdTe, as the CdTe photovoltaic technology has significant advantages: stability, high performance, and relatively inexpensive. In addition, it is possible to tune the CdTe bandgap by introducing, for example, Zn into the composition, forming Cd1−xZnxTe alloys, which can fulfill the Shockley–Queisser limit design criteria for tandem devices. The interdigitated back contact (IBC) silicon solar cells presented record high efficiencies recently, making them an attractive candidate for the rear cell. In this work, we present a combined optical and electrical optimization of Cd1−xZnxTe/IBC Si tandem configurations. Optical and electrical loss mechanisms are addressed, and individual layers are optimized. Alternative electron transport layers and transparent conductive electrodes are discussed for maximizing the top cell and tandem efficiency.
Highlights
Record high efficiencies have recently been announced for many photovoltaic (PV) technologies [1,2]
The optical coefficients (n and k) of C1−x Zx T [12], Mgx Zn1−x O (MZO) [13], Cdx Zn1−x O (CZO) [14], cadmium sulfide (CdS) [15], ethylene/vinyl acetate (EVA) [16], silicon [17], glass [18], and indium-tin-oxide (ITO) [19] used in our calculations are taken from the literature
The gap between the SQ limit and the electrical device simulations (Figure 8b) is seen to be dominated by electrical and optical losses in the top cell as well as losses due to the rear interdigitated back contact (IBC) Si cell, which was not optimized for the IR response
Summary
Record high efficiencies have recently been announced for many photovoltaic (PV) technologies (i.e., silicon, CdTe, CIGS, perovskite, organic, etc.) [1,2]. Approaching their thermodynamic limits, referred to as the Shockley–Queisser (SQ) limit, performance improvements of various types of solar cells have started to slow down. The SQ limit reaches a maximum for the two-junction tandem solar cells having a rear cell of silicon with the bandgap of 1.12 eV when a top cell with a bandgap of 1.81 eV is used [3]. Tandem cells with a two-terminal (2T) connection require continuous device fabrication and processing, a tunneling junction, and current matching of the subcells, all of which present challenges. The current matching requirement necessitates fine-tuning of the subcell parameters, which causes sacrifices in cell properties
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