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