Interface Engineering of Chalcogenide Semiconductors in Thin Film Solar Cells: CdTe as an Example

Modelling Interfaces in Thin-Film Photovoltaic Devices

Developing effective device architectures for energy technologies-such as solar cells, rechargeable batteries or fuel cells-does not only depend on the performance of a single material, but on the performance of multiple materials working together. A key part of this is understanding the behaviour at the interfaces between these materials. In the context of a solar cell, efficient charge transport across the interface is a pre-requisite for devices with high conversion efficiencies. There are several methods that can be used to simulate interfaces, each with an in-built set of approximations, limitations and length-scales. These methods range from those that consider only composition (e.g. data-driven approaches) to continuum device models (e.g. drift-diffusion models using the Poisson equation) and ab-initio atomistic models (developed using e.g. density functional theory). Here we present an introduction to interface models at various levels of theory, highlighting the capabilities and limitations of each. In addition, we discuss several of the various physical and chemical processes at a heterojunction interface, highlighting the complex nature of the problem and the challenges it presents for theory and simulation.

Electronic Structures of Ge2Sb2Te5/Co2FeX (X: Al, Si) Interfaces for Phase Change Spintronics

Phase change materials (PCMs), such as Ge₂Sb₂Te₅, are highly attractive in modern electronics and photonics. However, their spintronic applications remain largely unexplored. Here, we propose a tentative modality of phase change spintronic devices based on the ferromagnet/PCM/ferromagnet structure. The electrically tunable properties of a PCM interlayer give rise to new possibilities of manipulating spin transport through phase change, adding new functionalities and modes of operation to the spintronic devices. As the first step toward realizing such phase change spintronic devices, we calculate the electronic structures of the interfaces of c-Ge₂Sb₂Te₅ and half-metallic ferromagnetic Co₂FeX (X: Al, Si). The interfaces are found not to be genuine half-metallic, indicating room for improvement. The band alignments are largely determined by the termination of c-Ge₂Sb₂Te₅. Two types of band alignments are found for c-Ge₂Sb₂Te₅/Co₂FeX interfaces. Considering c-Ge₂Sb₂Te₅ as heavily p-type-doped, interfaces with Te termination are generally suitable such that they offer low contact resistance for hole injection from Co₂FeX to c-Ge₂Sb₂Te₅ in the majority spin channel; at the same time, they naturally form tunneling barriers, alleviating the degradation of spin injection efficiency because of occasional hole injection in the minority spin channel. This work provides important insights into this proposed phase change spintronic framework.

Energy band alignment in chalcogenide thin film solar cells from photoelectron spectroscopy

Energy band alignment plays an important role in thin film solar cells. This article presents an overview of the energy band alignment in chalcogenide thin film solar cells with a particular focus on the commercially available material systems CdTe and Cu(In,Ga)Se2. Experimental results from two decades of photoelectron spectroscopy experiments are compared with density functional theory calculations taken from literature. It is found that the experimentally determined energy band alignment is in good agreement with theoretical predictions for many interfaces. These alignments, in particular the theoretically predicted alignments, can therefore be considered as the intrinsic or natural alignments for a given material combination. The good agreement between experiment and theory enables a detailed discussion of the interfacial composition of Cu(In,Ga)Se2/CdS interfaces in terms of the contribution of ordered vacancy compounds to the alignment of the energy bands. It is furthermore shown that the most important interfaces in chalcogenide thin film solar cells, those between Cu(In,Ga)Se2 and CdS and between CdS and CdTe are quite insensitive to the processing of the layers. There are plenty of examples where a significant deviation between experimentally-determined band alignment and theoretical predictions are evident. In such cases a variation of band alignment of sometimes more than 1 eV depending on interface preparation can be obtained. This variation can lead to a significant deterioration of device properties. It is suggested that these modifications are related to the presence of high defect concentrations in the materials forming the contact. The particular defect chemistry of chalcogenide semiconductors, which is related to the ionicity of the chemical bond in these materials and which can be beneficial for material and device properties, can therefore cause significant device limitations, as e.g. in the case of the CuInS2 thin film solar cells or for new chalcogenide absorber materials.

Interfaces in CdTe Solar Cells: From Idealized Concepts to Technology

In thin film solar cells interfaces between lattice mismatched or dissimilar materials are used for the front and the back contact. A p-i-n device structure should be possible as most simple but ideally suited thin film solar cell. In contrast the interfaces in CdTe solar cells are found to be much more complex containing interdiffused phase boundaries at the front as well as at the back contact. By comparison to non-interdiffused interfaces using contact phases of adapted work functions it can be shown that the contact potentials of the front contact but also of the back contact are dominated by Fermi level pinning. The pinning states are evidently due to dislocation defects at the boundary of CdTe to the contact phases. Based on these results it is concluded that interdiffused phase boundaries or appropriate passivation layers are a precondition for efficient solar cells whenever strongly lattice mismatched or dissimilar materials are combined.