National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Characterizing Recombination 

in CdTe Solar Cells with TimeResolved Photoluminescence 

Preprint 

W.K. Metzger, M.J. Romero, P. Dippo, and 

M. Young 

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39926 

May 2006 
NOTICE

The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a 

contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US 

Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes. 

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or 

usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not 

infringe privately owned rights. Reference herein to any specific commercial product, process, or service by 

trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

government or any agency thereof. 

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy 

and its contractors, in paper, from: 

U.S. Department of Energy 

Office of Scientific and Technical Information 

P.O. Box 62 

Oak Ridge, TN 37831-0062 

phone: 865.576.8401 

fax: 865.576.5728 

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from: 

U.S. Department of Commerce 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

phone: 800.553.6847 

fax: 703.605.6900 

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
CHARACTERIZING RECOMBINATION IN CdTe SOLAR CELLS WITH 

TIME-RESOLVED PHOTOLUMINESCENCE* 

W.K. Metzger, M.J. Romero, P. Dippo, and M. Young 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 USA 

ABSTRACT 

Time-resolved photoluminescence (TRPL) computer 

simulations demonstrate that under certain experimental 

conditions it is possible to assess recombination in CdTe 

solar cells in spite of the junction. This is supported by 

experimental findings that open-circuit voltage (Voc) is 

dependent on lifetime in a manner consistent with device 

theory. Measurements on inverted structures show that 

the CdCl2 treatment significantly reduces recombination in 

the CdTe layer without S diffusion. However, S diffusion is 

required for lifetimes comparable to those observed in 

high-efficiency solar cells. The results indicate that 

substrate solar cells can be fabricated with recombination 

lifetimes similar to superstrate cells. 

INTRODUCTION 

Solar cell technology requires the ability to 

understand and manipulate transport and recombination 

processes in microstructures. A standard method to 

characterize recombination in semiconductor materials is 

to inject excess carriers into confinement devices such as 

double heterostructures. Optical excitation is produced 

with a fast laser pulse, and one observes the subsequent 

decay in photoluminescence (PL), photoconductivity, or 

any other physical property related to the excess carrier 

density. The confinement limits excess carriers to the 

material under investigation, simplifies analysis, and can 

help control surface recombination. 

However, polycrystalline CdTe cannot be 

incorporated into a confining double heterostructure 

suitable for recombination studies, and completed devices 

may have recombination properties due to interface, 

material, and growth issues that cannot be observed in a 

test structure. Recombination measurements in the 

presence of the CdTe/CdS junction are needed, but 

interpretation is complicated by the carrier dynamics 

introduced. The electric field can quickly alter the spatial 

profiles of the excess electrons and holes, thereby 

distorting recombination rates and making analysis 

complex. 

In this paper, we briefly describe three separate 

TRPL studies which together help characterize 

recombination in CdTe solar cells. First, we use 

computational modeling to examine if and under what 

experimental conditions TRPL on CdTe solar cells can 

give meaningful values of the recombination rate in the 

CdTe layer. We then compare experimental TRPL results 

on CdTe solar cells with device performance to see if 

measured lifetime values are consistent with basic theory 

and computer models. Finally, we examine TRPL on 

samples before and after the CdTe/CdS junction is 

formed. 

CdTe solar cells are usually grown by depositing 

CdTe on CdS/SnO2/glass substrates. In this superstrate 

configuration, the CdTe layer cannot be measured without 

the influence of the CdTe/CdS junction. Furthermore, 

when CdS is present, the CdCl2 treatment required for 

reasonably efficient CdTe solar cells induces S diffusion 

from the CdS layer into the CdTe. So the recombination 

effects of the CdCl2 treatment on bulk CdTe cannot be 

separated out from interfacial S diffusion. To observe the 

role of CdCl2 treatment on CdTe, without the junction or S 

diffusion, CdTe was deposited on glass/SnO2 substrates 

and PL lifetimes were measured on samples with and 

without a CdCl2 treatment. To observe the effects of the 

junction and S diffusion, CdS was subsequently deposited 

and half of the samples were CdCl2 treated. 

SIMULATION AND EXPERIMENT 

In the TRPL simulations, a 500-fs laser pulse injects 

carriers into a structure consisting of 500-nm SnO2, 80-nm 

CdS, and 8-μm CdTe. The light has a wavelength of 600 

nm, penetrates the SnO2 and the CdS layers, and 

generates carriers primarily within the first 500 nm of the 

CdTe/CdS metallurgical junction. The computer tracks the 

generation, drift, diffusion, and recombination of excess 

carriers as a function of time. From this, we are able to 

generate simulated PL decay curves as described in detail 

in reference 1. The material parameters are taken from 

conventional models and experimental estimates [2], 

however, the electron and hole lifetimes in the CdTe 

region are fixed at 20 ns. By extracting the PL decay time 

from the simulated curves, we can determine if TRPL 

measurements on a completed solar cell can properly 

assess the recombination lifetime assigned to the CdTe 

layer. The impact of the junction on PL decay is easily 

visualized by comparing the PL decay curves generated 

on a complete solar cell structure with PL decay curves 

generated from a solitary CdTe layer. 

Real TRPL measurements were performed using the 

photoexcitation generated by a mode-locked, cavitydumped dye laser operating at a repetition rate of 1 MHz 

with a wavelength of 630 nm and a pulse width of 5 ps. 

* This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy. The 

United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, 

irrevocable, worldwide license to publish or reproduce the published form of this work or allow others to do so, for United States Government purposes.

1
The laser spot size was about 1 mm in diameter, and the 

average power was 10 mW. PL was passed through a 

spectrometer to a photomultiplier tube. The overall 

system resolution was about 20 ps using deconvolution 

and standard time-correlated single-photon counting [3]. 

All measurements were performed at room temperature. 

Reference 4 gives a detailed description of TRPL 

measurements and analysis on CdTe solar cells. 

To grow CdTe films from 2.3 to 8.2 μm thick directly 

on SnO2/Corning 7059 glass substrates, a lowtemperature nucleation thermal profile during the closespaced sublimation (CSS) process was used [5]. CdS 

films about 80 nm thick were grown using the chemicalbath deposition (CBD) technique. CdCl2 treatments were 

performed by suspending either CdTe/SnO2/glass or 

CdS/CdTe/SnO2/glass samples directly over a CdCl2-

coated source plate in a CSS configuration; the anneal 

temperature was 400°C and the ambient consisted of 100 

torr oxygen and 400 torr helium. 

A total of eight samples were grown and are labeled 

A through H. The CdTe layers in samples A, B, and C 

were 2.3, 3.2, and 4.4 μm thick, respectively; the CdTe 

layer in samples D through H were all about 8 μm thick. 

After depositing the CdTe, each sample was divided into 

two pieces: one piece was CdCl2 treated, and the other 

was not. After performing lifetime measurements, CdS 

was deposited on samples A through G (but not H). 

These samples were again divided into two pieces, and 

only one piece was CdCl2 treated. As a result, there are 

six measurement categories. One letter, Y or N, means 

the measurement was performed on a sample that did not 

yet have a CdS layer, and either did (Y) or did not (N) 

have a CdCl2 treatment. Two letters, YY, YN, NY, and 

NN, means the measurement was performed on a sample 

that did have a CdS layer. The first of the two letters 

refers to whether the sample did (Y) or did not (N) have a 

CdCl2 treatment prior to the CdS deposition, and the 

second letter refers to whether the sample did (Y) or did 

not (N) have a CdCl2 treatment after the CdS deposition. 

Sample E (YN and NN) received a heat treatment 

equivalent to the CdCl2 treatment, except that CdCl2 was 

absent. 

RESULTS AND DISCUSSION 

The TRPL computer simulations provide snapshots 

of the electron and hole concentrations at discrete times 

before and after a laser pulse on a complete CdTe solar 

cell with an 8-μm CdTe layer. For clarity, Fig. 1 focuses on 

the CdTe region beginning at the CdTe/CdS interface 

(positioned at 0 μm in the graph) and ending 3 μm into the 

CdTe region. In the experiments and simulations, PL is 

collected only from the CdTe region. For easy reference, 

we will define P0 as the injection level that corresponds to 

8.8x1011 photons/cm2

 incident on the CdTe layer. 

 Fig. 1a shows the equilibrium and excess carrier 

distributions for an injection level of 0.01 P0. The laser 

pulse generates excess electrons and holes in a Beer’s 

law distribution. Just 1 ns later, long before bulk 

recombination is prevalent, the electrons and holes have 

been separated out by the electric field. 

 

Fig. 1. Excess electron and hole concentrations at 

given times after a laser pulse for an injection level of 

(a) 0.01 P0 and (b) 10 P0. The solid lines represent 

the electron concentration; the dashed line or circles 

represent the hole concentration. 

For an injection level of 10P0 (Fig. 1b), the dynamics 

are very different. Photoinjected carriers overwhelm the 

equilibrium carrier concentrations and screen the 

equilibrium electric field. As a result, electrons and holes 

diffuse throughout the CdTe layer together, and there is 

very little charge separation. This is identical to the 

behavior one would expect in an ideal CdTe layer without 

any junction. 

So the degree of charge separation, and hence its 

effect on TRPL measurements, is a function of injection 

level. Figure 2 illustrates this by comparing simulated 

TRPL curves from an ideal CdTe layer (no junction) with 

TRPL curves from a CdTe solar cell (with junction) at 4 

different injection levels. The radiative recombination rate 

per unit volume, Rrad, is given by 

 Rrad (t) = B p(r,t)n(r,t) − p0(r)n0 [ (r)], (1) 

where B is the radiative coefficient, p and n are the total 

hole and electron concentrations, and p0 and n0 are the 

equilibrium hole and electron concentrations, respectively. 

The PL intensity depends on the spatial overlap of excess 

holes and electrons. At the lowest injection level shown, 

0.01 P0, the junction quickly separates electrons and holes 

and the PL intensity decays at a rate determined by 

charge separation, not recombination. In high injection 

conditions, the field is screened and the PL intensity 

initially decays at a rate determined primarily by 

recombination. After enough excess carriers recombine, 

the equilibrium field eventually recovers and charge 

separation again emerges (not shown). However, in high 

2
injection, this occurs beyond the dynamic range of most 

TRPL measurements and recombination will dominate 

most of the PL decay signal. Consequently, as the 

injection level is increased (see Fig. 2), the PL curves from 

a completed CdTe solar cell overlap those from an ideal 

CdTe layer, until it is as though the junction is not there. 

Fitting these high injection PL decay curves reproduces 

the lifetime that was assigned to the CdTe layer. 

Actual TRPL measurements cannot explore the 

same parameter space as the simulations. CdTe has poor 

radiative efficiency and very low doping. So in practice, the 

TRPL experiment is not sufficiently sensitive to acquire a 

signal when the injection levels are low enough that 

charge separation is dominant. The experimental cutoff is 

typically around 0.5 Po. 

In high-injection conditions, the modeling predictions 

are consistent with experimental results. In general, the 

charge separation time should be negatively correlated 

with device performance. For example, increased doping 

and mobility lead to faster charge separation, but superior 

device performance. On the other hand, the 

recombination lifetime should be positively correlated with 

device performance. For injection levels corresponding to 

about 1 to 10 P0, room-temperature TRPL measurements 

on more than 80 completed CdTe solar cell devices 

fabricated under diverse conditions show a strong 

correlation with Voc (see Fig. 3). A detailed account of 

sample preparation, measurements, and analysis are 

provided in reference 4. 

Fig. 2. PL decay curves for injection levels of 0.01, 

0.1, 1, and 10 Po, respectively, on log-linear and loglog scales. 

Device theory gives specific predictions for how Voc

and other device parameters should vary with 

recombination lifetime. Current-voltage characteristics 

under AM1.5 illumination were modeled using the same 

device structure and material parameters as the TRPL 

simulations. However, here the lifetimes were adjusted by 

inserting a midgap trap with a density of 2x1014 cm-3 and 

altering the electron and hole capture cross-sections, 

which were set equal to one another. Figure 3 shows that 

the lifetime values are consistent with CdTe device 

models, experimental observations, and standard device 

physics. In spite of the junction, TRPL is a technique 

capable of quantitatively assessing recombination in the 

interface and depletion region, which is driving device 

performance.

 

Fig. 3. Voc vs. PL lifetime data compiled from seven 

separate studies that used different growth conditions [4]. 

By growing glass/SnO2/CdTe and 

glass/SnO2/CdTe/CdS structures, we are able to 

distinguish and quantify to some degree the roles of 

junction formation, S diffusion, and the CdCl2 treatment on 

recombination. Reference 4 gives a detailed description of 

TRPL measurements and analysis on completed CdTe 

solar cells. In this study, a similar injection level (about 5 

P0) was used, and the fits were again done by fitting a 

single exponential through the initial section of the decay 

for direct comparison. When necessary, the system 

response was deconvoluted from the measured PL decay 

[3]. The monochromator was tuned to the bandgap 

luminescence, and all the lifetime data were measured at 

room temperature. So the contribution of recombination 

from sub-bandgap features frequently observed in very 

low-temperature PL, such as donor-acceptor pairs and 

excitons, is negligible. Overall, the gross recombination 

characteristics of the inverted structures with or without 

the CdS layer are very similar to those observed in the 

superstrate devices. The radiative efficiency is about the 

same; the lifetimes range from 50 ps to 1 ns; and the 

lifetimes do not vary appreciably with injection. These 

3
observations are inconsistent with strong radiative 

recombination or Auger recombination. The dominant 

recombination mechanisms are nonradiative and 

associated with Shockley-Read-Hall and surface 

recombination. The initial decay may reflect a mixture of 

interface recombination, grain-boundary recombination, 

bulk recombination, and transients caused by carrier 

kinetics. There is not enough information to accurately 

quantify the individual contributions, but device 

performance will be affected by the aggregate 

recombination rate. 

Figure 4 illustrates the PL lifetimes for samples A-H 

with and without CdCl2 treatments before and after CdS 

deposition. The PL is excited mostly within 1 μm of the 

incident surface. First, we look at measurements where 

the laser light is incident through the glass. Modeling 

indicates that for the applied injection levels, charge 

separation at the SnO2/CdTe interface or grain boundaries 

does not significantly affect the measured lifetime. Asreceived samples (N) have lifetimes less than 100 ps. 

When this layer is CdCl2 treated (Y), the treatment 

penetrates the entire CdTe layer and the lifetime increases 

to values between 350 and 600 ps. The addition of CdS 

(NN, YN, NY, and YY) predictably has no effect on the 

lifetime at the bottom of the CdTe layer. The impact of the 

CdCl2 treatment is the same with or without the CdS layer, 

so S diffusion does not affect recombination throughout 

the entire CdTe region. 

The trends are different when the laser light is 

incident on top of the cell. As-received layers have 

lifetimes that vary from 100 to 300 ps and are correlated 

with thickness. The CdCl2 treatment preferentially 

improves the lifetimes of thin cells, perhaps by 

preferentially affecting the small grains associated with 

thinner cells [6]. The addition of CdS without any further 

CdCl2 treatment (NN and YN) also increases the lifetime. 

This cannot be explained by charge separation, and 

suggests that the CdS layer may passivate the free CdTe 

surface. Secondary-ion mass spectrometry measurements 

indicate that when a CdS layer is present, the CdCl2

treatment causes S diffusion [5]. The longest lifetimes, 

which are commensurate with those measured in highquality CdTe solar cells, are only achieved when there is 

significant S diffusion. 

 

CONCLUSIONS 

Computational modeling shows that TRPL 

measurements can assess recombination in CdTe solar 

cells in high-injection conditions. This is supported by 

measurements on more than 80 samples, which indicate 

that the open-circuit voltage is dependent on lifetime in a 

manner consistent with device theory. Measurements on 

inverted structures indicate that the CdCl2 treatment, 

without any S present, significantly reduces 

recombination. However, S diffusion is required for 

lifetimes comparable to those observed in high-efficiency 

solar cells. The results indicate that substrate solar cells 

can be fabricated with recombination characteristics 

similar to those of superstrate cells. 

ACKNOWLEDGEMENTS 

This work is supported under DOE Contract No. DEAC36-99GO10337. We would also like to acknowledge D. 

Albin. 

REFERENCES 

[1] W.K. Metzger, R.K. Ahrenkiel, J. Dashdorj, and D. 

Friedman, Phys. Rev. B 71, 035301 (2005). 

[2] M. Gloeckler, A.L. Fahrenbruch, and J.R.Sites, in 

Proceedings of 3rd World Conference on Photovoltaic 

Energy Conversion (2003), p. 491. 

Fig. 4. Lifetimes for samples A-H after different 

processing and treatment steps. 

[3] D.V. O’Connor and D. Phillips, Time Correlated Single 

Photon Counting, Academic Press:San Diego, 1984. 

[4] W.K. Metzger, D.Albin, D. Levi, P. Sheldon, X. Li, B.M. 

Keyes, and R.K. Ahrenkiel, J. Appl. Phys. 94, 3549 (2003). 

[5] W.K. Metzger, D. Albin, M.J. Romero, P. Dippo, and M. 

Young, J. Appl. Phys. 99, in press (2006). 

[6] H.R. Moutinho, R.G. Dhere, M.M. Al-Jassim, D.H. Levi, 

and L.L. Kazmerski, J. Vac. Sci. Technol. A 17,1793 

(1999). 

4