Lucideon Examines Layer Growth and Doping Uniformity in Solar Photovoltaics

The Centre for Solar Energy Research (CSER) carries out renewable energy R & D with a focus on novel photovoltaic materials and devices research.

One of the key priority areas identified in Solar Photovoltaic (PV) technology is a better understanding of the role and functions of layers and junctions in thin film PV devices. Surface analysis techniques, particularly Secondary Ion Mass Spectrometry (SIMS) therefore play a key role in furthering this aim.

Cadmium telluride (CdTe) thin films are currently receiving considerable attention in PV technology since CdTe has the advantage of a direct bandgap which matches well with the solar spectrum. CdTe has a high absorption coefficient, is capable of receiving n-type and p-type dopa nts and can be integrated into low cost, large scale manufacturing processes.

The Challenge

One of the main challenges for PV fabrication is the scaling up from small laboratory Metal-Organic Chemical Vapour Deposition (MOCVD) chamber reactors to large scale manufacturing platforms. The uniformity of dopant distribution across the entire cell area is paramount for optimising product yield and performance.

In a study commissioned by the CSER, the uniformity of As and Cl doping in CdTe solar cell devices was investigated by SIMS depth profiling at various points across a target structure comprising of CdS (240nm)/CdTe:As (2um)/CdTe:As+ (250nm).

What We Delivered

Comparison of the arsenic and chlorine dopant levels between two samples at upstream (Position 1) and downstream (Position 2) points in the MOCVD chamber indicated significant variation in their respective levels, e.g. there is an ~2.5 fold difference in As concentration between the two positions.

Figure 1 - Variation of the dopant concentrations from upstream (Position 1) and downstream (Position 2) areas of the solar cell

Figure 2 - SIMS depth profile of a CdTe solar cell device

The SIMS depth profile of a sample is shown in Figure 1; the upper CdTe layer is approximately 1.5µm thick, i.e. significantly below the target thickness. The arsenic profile shows evidence for out-diffusion to the CdTe surface, whereas chlorine indicates the opposite behaviour.

Value to the Client

From the data described above, CSER was able to feedback the target layer thickness and doping variations to adjust MOCVD chamber programming conditions. This will lead to an optimising of layer growth and doping uniformity with consequent yield improvement.

Professor Stuart Irvine, Director of CSER commented: "Being able to monitor the impurity variations in PV materials is vital for the development of new materials for solar energy modules. This study shows the potential to make accurate analysis of samples from different parts of the deposition system so that improvements and optimization can be realised."

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