Using cathodoluminescence to investigate the electrical properties of
grain boundaries in CdTe/CdS solar cell devices
AA Taylor1, JD Major2, K Durose2 and BG Mendis1
1. Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK
2. Stephenson Institute for Renewable Energy, Chadwick Building, University of Liverpool, Liverpool, L69 7ZF
email: aidan.a.taylor@gmail.com
Keywords: Grain boundaries, CdTe, cathodoluminescence
Single junction solar cells having the optimum band gap of ~ 1.5 eV may convert up to about
30% of incident solar radiation into electrical power. However, in practice, efficiencies are typically
15-19% for small-scale lab devices and 9-13% in commercially produced modules. Understanding
the causes of the discrepancy between theory and practice is crucial to developing higher efficiency
devices.
The estimate of theoretical maximum efficiency is based on the assumption that all the electronhole pairs generated by the incident light are separated and collected by the device. A major cause
of deviation from the theoretical efficiency is that real devices contain defects, including vacancies,
dislocations and grain boundaries. Such defects introduce additional energy levels in the band gap
in semiconductor materials that act as recombination sites. The increased local recombination rate
leads to a diffusive flux of carriers from the bulk. This flux is characterised by the recombination
velocity; higher recombination velocities for defects lead to reduced minority carrier lifetimes and
hence lower collection efficiencies of devices. For the case of cost-effective polycrystalline solar
cells, the issue of extended defects is significant. Figure 1 shows cross-sectional and plan-view
images of a thin-film CdS/CdTe solar cell of the type used in the present study of recombination
effects. Both low angle twin and high angle grain boundaries are evident. The aim is to study the
exact effect of grain boundaries on recombination, and this is crucial for the development of higher
efficiency devices.
In this work a newly demonstrated means of obtaining the grain boundary reduced recombination
velocity, S, and minority carrier diffusion length, L (the reduced recombination velocity is the ratio of
the recombination velocity divided by the bulk carrier diffusion velocity). [1]. With this technique, the
cathodoluminescence (CL) signal from the semiconductor (e.g. in solar cell devices) is quantitatively
assessed in the region of grain boundaries. By assuming that the electron beam may be modelled
as a plane of carrier generation in a semi-infinite slab and extending the van Roosbroeck model [2]
for a free surface to a grain boundary, the CL intensity profile may be shown to vary according to:
 S  xb
ln[∆I (xb )] = ln
−
 S +1 L
(1)
where x b is the distance from the grain boundary and ∆I(x b ) is the relative change in CL intensity
at x b with respect to the bulk value. A plot of ln[∆I(x b )] vs. x b is therefore a straight line, with the
gradient and intercept yielding L and S respectively. A grain boundary in a vapour-grown CdTe
sample analysed according to this method is shown in figure 2.
In this work several aspects of the CL technique for measurement of S are investigated. Firstly,
Monte Carlo-based methods are used to assess the validity of the planar carrier generation
assumption for different beam energies and boundaries. In addition to this, the reduced
recombination velocity, S, is measured in a CdTe/CdS solar cell device using both CL and the more
established electron beam induced current (EBIC) method [3] in order to compare values
determined from the two techniques. Finally, electron backscatter diffraction (EBSD) is used to
determine the local orientation of grains in as-grown and CdCl 2 -treated CdS/CdTe devices. CL of
the orientation-mapped region is then used to investigate the influence of grain boundary type on
the recombination velocity and diffusion length of CdTe-based solar cells. The consequences of this
for device performance and future device development are discussed [4].
References
[1] BG Mendis, L Bowen, QZ Jiang, Appl. Phy. Lett. 97 (2012), 092112.
[2] W Van Roosbroeck, J. Appl. Phys. 26 (1955), p380.
[3] C Donolato, Mater. Sci. Eng. B 24 (1994), p61
[4] The authors gratefully acknowledge EPSRC funding: EP/1028781 (BG Mendis) and the RCUK
Energy Programme Supergen PV21, EP/F029624/1 (K Durose).
Figure 1: Cross-sectional (left) and plan-view (right) SEM micrographs of a CdS/CdTe solar cell device (planview is of the CdTe layer). Images were acquired using a backscattered electron detector.
Figure 2: Panchromatic CL image across a grain boundary in vapour-grown CdTe (left). The CL intensity is
plotted as a function of distance across the boundary (centre). The analysis according to equation (1) is
plotted (right); the variation is linear as predicted, though deviation occurs close to the boundary where some
of the assumptions of the model break down.