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Review slides of a few papers on CZTS journal articles
Citation preview
CZTS paper review
Mitzi 1: Loss mechanisms in Hydrazine processed Cu2Zn(Se,S)4 solar cells - Mitzi
APL 97, 233506 (2010)
4 Characterization techniques to identify main loss mechanisms limiting device efficiency
1. Light-dark J-V (and pseudo J-V) Magnitude of FF gives series resistance Pseudo J-V is what J-V would be without any Rs
2. EQE, EQE(V) Which wavelengths have low response
3. Voc vs. T Identify main recomb as interface or bulk
4. Time resolved PL lifetime
Main observations
Voc is low wrt to Eg
Voc is usually (Eg/q 0.5) for CIGS. CZTS > 0.6
Low FF -> high Rs
CZTSSe-A
CZTSSe-B
CIGS-A
Efficiency
8.73 9.50 13.8
FF 57.8 64.3 72.4
Voc (mV)
395 499.3 578.4
Jsc (mA/cm2)
38.24 29.55 33.06
EG (eV) 1.05 1.21 1.14
EQE
EQE response is low in long lambda (ZB 900nm)
EQE(V)
EQE carrier collection efficiency
EQE(V) ratio: EQE(-1)/EQE(0)
CZTS increases at long wavelengths poor min carrier collection deep in the abs and a V dep current collection
Temp dependent Voc
Voc vs. T
A, J_oo and J_L are assumed to be temp independent
Really there is T dep
Mitzi says rearranging 1st eqn
ln ooAocL
JE AkTV
q q J
exp
exp
o s L
Ao oo
qJ J V R J J
AkT
EJ J
AkT
ln lnAo ooE
A J A JkT
Temp dependent Voc
Voc vs. T
Plot should be straight line and T = 0K should yield activation energy
If intercept yields band gap
Bulk recombination (limited)
If intercept < Egap
Interface recombination
ln ooAocL
JE AkTV
q q J
Temp dependent Voc
So, CZTS is interface limited. Why?
Buffer-absorber layer has electrical defects
Cliff-type band alignment -> absorber has smaller EA than buffer
CIGS shows bulk recombination due to grain boundaries of poly xtal
Can this work for amorph materials?
TR-PL
Solution for n(t) n0 excess carrier density; C1/C2 constants
Intially fast rise due to radiative recomb and HLI that redistributes excess carriers. long tail is LLI and time constant -> minority carrier lifetime
CZTS 1.2 ns
Series resistance
Low FF due to high Rs (sheet series resistance)
T dep of the efficiency and dark series R
So, plot of dV/dJ vs 1/(J-GsdV) gives y-intercept as Rsd
, ,s d s ddV AkT
R J G VdJ q
Series resistance
Efficiency increases with lower T and plateaus for CIGS
CZTS efficiency drops at low T
This is due to Rs inc at low T
Why does this happen?
Presence of blocking contact at abs/Mo interface
Model back contact as Sch diode
Series resistance
When solar cell is fwd biased; back contact diode is in rev bias and conduction is limited by the rev. sat. J which diminishes at low T, increasing Rs
Can now estimate barrier height of this contact
Ro is contact grid resistance and resistance from TCO can be ignored at low T
0 *exp bs
kR R
qA T kT
Equivalent cct
When solar cell is fwd biased; back contact diode is in rev bias and conduction is limited by the rev. sat. J which diminishes at low T, increasing Rs
Series resistance
By plotting ln(Rs*T) vs 1/T get barrier heights as 5.9 for CIGS and 99 ,115 for CZTS
Large barrier leads to large Rs
Also, cross-over of J-V curves indicates Schottky contact
Summary
Key loss mechanisms
Low Voc limited by buffer/abs interface recomb
Voc vs T indicates this; low min. carrier lifetime limits Voc, leads to limited long lambda EQE which limits Jsc
Large series resistance could be because of Schottky contact at the CZTSSe/Mo interface limits FF