Introduction

Light absorption in CdTe devices

Where do the carriers go?

The decay slope and device properties

Interface and bulk recombination on TR

Impact of Interface Recombination on Time Resolved Photoluminescen

(TRPL) in CdTe solar cells (Numerical Simulation Analysis)

Ana Kanevce, Darius Kuciauskas, Timothy A. Gessert, Dean H. Levi and David S. A

National Renewable Energy Laboratory, Golden, CO, USA

Presented at the 38th IEEE Photovoltaic Specialists Confere

This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08GO28308 with the National Renewable Energy Laboratory. 

The information contained in this poster is subject to a government license.

Abstract 

Using Sentaurus Device Software, we analyze how bulk and 

interface recombination affect time-resolved photoluminescence (TRPL) decays in CdTe solar cells. This modeling analysis could improve the interpretation of TRPL data and increase the possibility of rapid defect characterization in thin-film solar cells. By illuminating the samples with photons of two different wavelengths, we try to deduce the spatial origin of the dominant recombination 

loss. Shorter-wavelength photons are more affected by the interface recombination and drift compared to the longer ones. Using the two-wavelength TRPL characterization method, it may be possible to determine whether a specific change in deposition process has affected the properties of interface or the bulk of the absorber. 

Conclusions

Carrier decays after a short-light pulse illumination have been s

interpretation of TRPL measurements. The TRPL decays in thin-film dthe carrier dynamics, including drift, diffusion, interface and bulk recothat at 635 nm illumination, the decay is affected more by the interfacwith the 808 nm decay. We have also calculated the n*p product at dthe decay to show that the interface recombination affects the second

By comparing the decays at different wavelengths, it may be possible dominant recombination is located at the interface or in the bulk. 

Contou

• TRPL is a contactless and quick method to determine the carrier lifetime in CdTe absorbers.• The decay shape is affected by complex carrier dynamics, including drift, diffusion, and recombination.• The bi-exponential nature of the decay is pronounced for  = 635 nm.

• For  = 635 nm, more carriers are generated close to the junction where the electric field is the strongest, and many of them are separated quickly. • For  808 nm, the difference between 

1m and 2m is much smaller, and the decay 

appears more single-exponential. 

• For typical doping levels of CdTe cells, 1013 cm-3

to 1015 cm-3, most of the light is absorbed within the 

depletion region. • The junction field plays a significant role in the decay.• The 635 nm illumination generates 3.3 times more carriers next to the CdS/CdTe interface. • The decay generated by 635 nm illumination is expected to be more influenced by drift and interface recombination.

6

1014

246

1015

24

n [cm-3]

0.6 0.8 1.0 1.2 1.4 1.6

Position [m]

 = 635 nm; S = 100 cm/s

 0 0.01 0.03 0.05 0.1 0.2

 0.5 

6

1014

246

1015

24

p [cm-3]

0.6 0.8 1.0 1.2 1.4 1.6

Position [m]

 = 635 nm; S =100 cm/s

CdS/CdTeinterface

6

1014

246

1015

24

n [cm-3]

0.6 0.8 1.0 1.2 1.4 1.6

Position [m]

 = 635 nm; S = 105 cm/s

 0.5 0.6 0.8 1 2 3

6

1014

246

1015

24

p [cm-3]

0.6 0.8 1.0 1.2 1.4 1.6

Position [m]

 = 635 nm; S =105 cm/s

Experimental and numerically 

simulated TRPL decay for two 

excitation wavelengths. The decay 

is measured at 840 nm with 

excitation through junction.

The simulations match the 

experimental data well.

Simulated carrier decays 

= 1.25 ns, 2.5 ns, and 6.

for  = 635 nm and  = 80

-6-4-2

0

Energy [eV]

0 1 2 3 4

Position [m]

3020100

Generation rate a.u. CdTe 635 nm808 nm

SnO2 CdS

• Both interface and bulk recombination accelerate the carrier decay. • In a case of 635 nm illumination, the decays are affected by interface rethe longer wavelengths.• If S < 104 cm/s, its impact on the TRPL decay becomes significant for 2

• When S is low, 2m for 635 nm excitation is higher than 2m for 808 nm e

the 2m(808nm) > 2m(635nm).

• The Voc can be predicted from the decay slope. 

Contour plots of simulated decay times (2m) as a function of 

input carrier lifetime and interface recombination velocity 

102103104105106

S [cm/s]

2 4 6 8 10

i [ns]

 6.5 

 6 

 5 5.5 4.5 

 4 

 3.5 3 

 2.5 

 1.5 2 

 1 2m [ns] 635 nm

102103104105106

S [cm/s]

2 4 6 8 10

i [ns]

 5 

 4 4.5 

 1.5 2 2.5 3 3.5 

 1 

2m [ns] 808 nm

• The slower part of the biexponential decay, 2m, is more 

affected by the recombination in CdTe.• The variation in 2m for both 

wavelengths 635 nm and 808 nm is comparable, because the defect density is assumed to be uniform throughout the absorber. • In a case of 635 nm illumination, more carriers are generated next to the heterointerface, and the decay is significantly more affected by the recombination at the interface. • Interface recombination has a stronger influence on the slower part of the decay, where 2m is 

deduced. 

0.01

246

0.1

246

1

Normalized PL intensity

0.0 1.0 2.0 

Time [ns]

635 nm

 6.25 n 2.5 ns 1.25 n

0.01

246

0.1

246

1

Normalized PL intensity

0.0 1.0 2.0 

Time [ns]

808 nm

 6.25 ns 2.5 ns 1.25 ns

0.01

246

0.1

246

1

Normalized PL Intensiy

0 5 10 15 20n(s)

Time [ns]

 experiment simulation

635 nm

808 nm

2m

1m Simulated equilibrium band diagram and 

generation profiles for  = 635 nm and 808 nm

The different curves correspond to times: 0, 10 ps, 30 ps, 50 ps, 0.1 ns, 0.2 ns, 0.5 ns, 0.6 ns, 0.8 ns, 1 ns, 2 ns, and 3 ns.

Calculated n*p product 

 = 635 nm

• The maximum electron density shifts away from the interface in the first 0.5 ns, and then slowly toward the interface.• For low S, the maximum hole density remains close to the interface.• The recombination at the interface reduces hole density during the slower part of the decay.• The electron and hole densities after 808 nm excitation are more widely distributed. 

Calculated n*p product 

 = 808 nm

Calculated hole density for  = 635 

nm, S = 100 cm/s and 105 cm/s 

Calculated electron density for  = 

635 nm, S = 100 cm/s and 105 cm/s 

-2.0-1.5-1.0-0.5

0.00.51.0

Energy [eV]

0 1 2 3 4

Position [m]

The electric field 

is pushing us one way, the concentration gradient the other, there are so many traps to recombine

through ... 

Where to go?

?

102103104105106

S [cm/s]

1027102810291030

n*p [cm-6]

0.6 0.8 1.0 1.2 1.4 1.6

Position [m]

t

 = 635 nm S = 100 cm/s

1027102810291030

n*p [cm-6] 

0.6 0.8 1.0 1.2 1.4 1.6

Position [m]

 = 808 nmS =100 cm/s

1027102810291030

n*p [cm-6]

0.6 0.8 1.0 1.2 1.4 1.6

Position [m]

 = 635 nm S = 105 cm/s

1027102810291030

n*p [cm-6]

0.6 0.8 1.0 1.2 1.4 1.6

Position [m]

 = 808 nmS = 105 cm/s