Sensitivity of Thin Film CdTe Device Operation to Processing Chemistry

1
Sensitivity of Thin Film CdTe Device Operation
to Processing Chemistry
Brian McCandless, Steve Hegedus, Kevin Dobson,
Darshini Desai, Michael Angelo, Meijun
Lu Institute of Energy Conversion University of
Delaware Newark, Delaware 19716 NREL
sub-contract ADJ-1-30630-12
2
Problem

• Understanding role of materials and processing
  chemistry on polycrystalline CdTe device
  operation with respect to high throughput
  deposition and post-deposition treatments

Approach
Fabricate cells using baseline approach, analyze
materials properties and devices using vapor
transport and evaporated CdTe films
3
Topics
CdTe deposition Thickness and growth rate O2
ambient Post-deposition treatment CdTe film
oxidation with varying humidity Separating
effects of CdCl2, Cu, O2 Vapor ZnCl2 versus
CdCl2
4
Vapor Transport (VT) Deposition
Exit slit located 1.5 cm above substrate surface
Deposition rate from 1 to 85 mm/min at 550ºC
5
VT CdTe Deposition
Baseline Conditions Tsubstrate 550ºC
99.9998 Alfa CdTe crystals He/O2 Ambient 3
cm/min translation GR 8 mm/min 5-7 mm
thick Thickness uniformity (10 x 10 cm) 5
direction of travel 10 along each edge
Film Properties Single phase CdTe Sharp band
edge, Eg 1.50 eV T/(1-R) gt 60
6
VT CdTe Film Morphology
AFM 20 x 20 mm
570ºC 5 mm, 8 mm/min
Substrate temp
500ºC 5 mm, 8 mm/min
Faceted morphology, grain size increases with
substrate temperature
Film thickness
550ºC 23 mm, 8 mm/min
550ºC 1 mm, 8 mm/min
Grain size increases proportional with CdTe
thickness
Growth rate
550ºC 6 mm, 8 mm/min
550ºC 6 mm, 81 mm/min
Grain size inversely proportional to CdTe growth
rate
Without O2 during growth pinholes and
discontinuous morphology ? shunted cells
7
VT CdTe Devices TEC15/30 nm Ga2O3/80 nm CdS
Effect of CdTe thickness and growth rate on cell
performance
Vapor CdCl2 415ºC, 25 min Cu/Ni
Vapor CdCl2 400ºC, 20 min CuI/Ni
8
VT CdTe Devices TEC15/30 nm Ga2O3/80 nm CdS
Effect of CdTe thickness on cell performance
single piece
Final CdS thickness estimates 60 nm for 1.45 mm
CdTe 45 nm for 0.75 mm CdTe
2X thickness reduction ? 10 reduction in Voc,
Jsc, FF
CdTe band edge shifted 5 nm to lower energy for
0.75 mm CdTe
Thinner CdTe, smaller grain size, enhanced CdS
consumption
9
Post-Deposition Treatment Interface Diffusion
GIXRD with in situ heating, 400ºC, W 5º
scan acquisition 5 min each
CdTe0.95S0.05 and CdS0.95Te0.05 CdTe0.96S0.04
and mixed CdSTe CdTe and CdS
10
CdTe Film Oxidation with Varying Humidity
Do oxidation/humidity in post-deposition
processing significantly enahnce CdTe film
oxidation? Background CdTe surfaces can be
diffusion sources (e.g., Cu, Cl) CdTe thin
films oxidize in room air CdTeO3, CdTe2O5
CdTe crystals oxidize via TeO2 and CdO formation
PVD CdTe Films VT CdTe Films RH 0,
20, 40 RH 40 T 400ºC, 500ºC, 550ºC T
500ºC Bottled dry air RH 0 Bubbler RH
20 Room Air RH 40 PVD data shown 500ºC,
RH 0 and 40
11
CdTe Film Oxidation
GIXRD with in situ heating, W 1º
ambient sparged onto film surface scan
acquisition 10 min each

• Humid ambient initially TeO2 then CdTeO3
• CdTe intensity ? as oxide intensity ?
• Oxidation rate enhanced

• Dry ambient a-CdTeO3
• CdTe intensity ? as oxide intensity ?

12
Estimated Oxide Thickness vs Time 500ºC
Calculated from attenuation of CdTe (111) GIXRD
peak intensity
13
Oxidized CdTe Films
PVD Films, 500ºC, 100 min XPS PHI 5000 Al Ka 200
W, 15 kV
RH 0
RH 40
Oxidized
Oxidized
Ar etched 2 kV, 25 mA P 0.02 Pa 1 nm/min
Etched 180 min
Etched 260 min
14
Oxidation vs Depth
PVD Films, 500ºC XPS PHI 5000, Al Ka
RH 40
RH 0
Etch Time (min)
0 180
Binding Energy (eV)

• Surface oxide and penetrating oxide
• Transition from Te-O to Te-Cd bonding with
  sputter depth
• Oxidation enhanced in humid ambient

15
CdTe Film Oxidation VT CdTe Devices
Pieces cut from adjacent locations on VT 161 6
mm CdTe, Cu/Ni contact

• Exposure to humidity prior to CdCl2 HT
  detrimental to device operation
• Suspect stoichiometric change preferential Te
  removal from CdTe lattice
• CdCl2 treatment doesnt heal film

16
Separating CdCl2, Cu, O2
LDX light-dark crossover on JV plot Roll
rollover in JV characteristic at V gt Voc Hyst
hysteresis between forward and reverse JV sweeps

• Without CdCl2 HT, little Cu benefit eliminates
  rollover
• CdCl2 with no O2 improves current collection, Cu
  improves Voc
• Cu and O independently influence Voc and FF

17
Separating Effects halide vapor, Cu

• Cu in contact can significantly influence Voc and
  FF
• ZnCl2 treatment with Cu/Ni contact similar to
  CdCl2 treatment with Ni only

18
Conclusions
VT CdTe grain size increases with substrate
temperature, proportional to film thickness,
inversely proportional to growth rate Thin CdTe
smaller grains, enhanced CdS-CdTe diffusion,
promising results with modified CdCl2, etching
and contact CdTe film storage prior to CdCl2
HT can contribute to variability in
batch-processed pieces suspect stoichiometric
changes Grain boundaries likely decorated by
oxides CdCl2 primary effect on minority carrier
collection Cu, O primary effect on junction
(mixed results for Cu buffer role?) ZnCl2
shows promise as alternative to CdCl2