Barrier to Trap Filling CuIn1-xGaxSe2: Preprint Page: 3 of 8
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A Barrier to Trap Filling in CuIni.xGaSe2
David L. Young, Kannan Ramanathan, Miguel Contreras, Jehad AbuShama, Richard S. Crandall
National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401
ABSTRACT
Voltage pulses of variable length were applied to CuIni.XGaSe2/CdS (0 < x < 1) junction
solar cells. The resulting transient capacitance emission signal was recorded for several minutes.
The amplitude of the capacitance emission signal increased linearly with the log of pulse time.
These data do not follow the standard model for trap capture and emission of carriers. Instead
they follow a simple electrostatic model based on electrostatic charging of traps.
INTRODUCTION
The CuIni.XGaXSe2 material system is one of the leading candidates for low-cost
absorbers for thin-film solar cells achieving efficiencies over 19% [1]. Device performance
correlates with the density [2,3] and energy [4] of defect levels detected by capacitance
techniques with a maximum efficiency occurring at x ~ 0.3. Theoretical studies have assigned
defect transition energy levels in the bandgap to specific crystalline defects[5]. However,
metastabilities in the material system complicate the assignments of experimental and theoretical
defect levels [6-10].
The nature of electronic traps is commonly studied using deep-level transient
spectroscopy (DLTS) methods. DLTS approaches can determine defect energy levels relative to
band edges (majority or minority trap), as well their trapping cross-section (o) [11,12]. The
charge emission-rate and oare important quantities for determining whether a defect functions
as a recombination center or a shallow trap. This knowledge is especially important for
photovoltaic (PV) materials because recombination limits the available photocurrent of the solar
cell.
The most reliable method to determine -is to measure the density of trapped charge as a
function of trap-filling pulse time (tp) in a junction device structure. The density of filled traps
(Nf) should increase linearly with time at short tp and finally saturate when charge is being
emitted as fast as it is being captured. This process should obey the following equation [11]:
Nf(t)= No 1- e z, where No is the saturated defect density and z, is the characteristic time
containing o.
This paper outlines a failed attempt to determine -in CuIni.XGaxSe2 using the above
equation. Instead we find that NS(t) obeys the expression: NS(t)= A ln 1+ P2, where Ao is a
constant and to is a characteristic time. Similar behavior was observed in a-Si [13], GaAs [14],
and Ge0 3Si0 7/Si [15]. A model [13] explaining these results is based on the realization that the
traps are clustered, rather than uniformly distributed. Trap-filling charges these regions thus
producing a potential barrier to further filling.1
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Young, D. L.; Ramanathan, K.; Contreras, M.; Abushama, J. & Crandall, R. S. Barrier to Trap Filling CuIn1-xGaxSe2: Preprint, article, April 1, 2003; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc1413255/m1/3/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.