CdTe solar cells: growth phenomena and device performance

Abstract

A systematic study is presented on the control of CdTe and CdS layers during their growth,
with the understanding gained being implemented in the production of solar cells with
enhanced performance. In particular the growth mechanisms for close space sublimation (CSS)
— grown CdTe were evaluated as a function of processing gas (N2, 02 and H2) and nitrogen
pressure. Films were shown to form via the Volmer-Weber growth mode with films deposited
under nitrogen showing well defined crystal facets. Inclusion of oxygen in the deposition
ambient produced islands of a rounded morphology, reduced size and increased number density,
whilst hydrogen was shown to increase the island number density and the level of substrate
coverage. Growth mechanisms were deduced from the morphologies observed at different
stages of growth by ex-situ AFM and SEM and by comparison with growth literature,
especially the work of P. Barna. Nucleation density, step flow and impurity incorporation are
all invoked in the discussion.Factors influencing the cell performance were evaluated with the
aid of a optical beam induced current (OBIC) and external quantum efficiency (EQE) system
built as part of this work and having the capacity to measure EQE or OBIC maps with a
resolution of 12.5pm. The system was used to evaluate the photovoltaic response of CdTe/CdS
devices as a function of wavelength with the impact of the nitric-phosphoric acid (NP) etch on
the back surface, the uniformity of CdTe/CdS devices deposited by different methods and the
effect of absorber layer thickness of PV uniformity being assessed.
The performance of CdTe/CdS devices was evaluated as a function of variables that could be
influenced by growth of the CdTe and CdS layers. The use of lower substrate temperature and
the incorporation oxygen in CdS increased V„ from 0.51 to 0.65V is discussed. Oxygen in the
CdTe was also shown to influence the junction position and hence efficiency, while oxygen in
the CdS layer was also shown to be vital for the formation of hetero-junctions. The CdTe grain
size was shown to be significantly increased for deposition under higher nitrogen pressures
(Grain diameter = [0.027P + 0.9]gm, where P is the pressure in Torr), with the average grain
diameters being 0.94pm at 2Torr and 5.63pm at 200Torr. Device performance was improved as
a result with the peak device efficiency being increased from 2.1% at 2Torr to 14.1% at
100Torr. The series resistance was shown to be minimised for larger grain size, owing to the
reduced contribution of grain boundaries. Suggestions for the fabrication of high efficiency
solar cells are given with reference to the efficiency limiting factor.