Electron. Mater. Lett., Vol. 10, No. 2 (2014), pp. 433-437

Temperature-Dependent Photovoltaic Characterization of a CdTe/CdSe Nanocrystal’s Solar Cell

Huichao Zhang,1 Zhengyang Li,

1 Jun Qian,

2 Qiumei Guan,

1 Xiaowei Du,

1 Yiping Cui,

1 and Jiayu Zhang

1,*

1Advanced Photonics Center, School of Electronic Science and Engineering, Southeast University,

Nanjing 210096, China2Electrical Information Engineering Institute, Jiangsu University, Zhenjiang 212013, China

(received date: 13 April 2013 / accepted date: 18 July 2013 / published date: 10 March 2014)

An all-inorganic thin film solar cell was fabricated with colloidal CdTe and CdSe nanocrystals. Its temperature-dependent photovoltaic characterization was measured, and an open circuit voltage decay technique was usedto determine the electron lifetime. The photovoltaic parameters, such as the exact carrier lifetime, were remark-ably different between under low-temperature region and under temperature above 267 K. It is suggestedthat the temperature-dependent carrier conductivity results from a thermally-activated hopping process andthis nanocrystals device is a kind of donor-acceptor solar cell.

Keywords: semiconductor nanocrystals, photovoltaic effect, temperature-dependent, CdSe, CdTe

1. INTRODUCTION

Colloidal semiconductor nanocrystals (NCs) have some

practical properties of organic photovoltaic materials, such

as the solution processibility and the controllable synthesis,

while they still retain the photovoltaic features of traditional

inorganic semiconductors such as the broadband absorption

and superior transport properties,[1] therefore they have the

potential applications in the field of solar cell. Recently,

various colloidal nanocrystals, such as cadmium compound

nanocrystals (dots,[2,3] nanorods,[1,4,5] tetrapods[6]), type-II

nanocrystals,[7] and lead compound nanocrystals,[8-10] have

been integrated in different types of solar cells. At the

present stage, the power conversion efficiencies of 0.16% -

6% in the colloidal NCs based solar cells are still lower than

those of the traditional Silicon (Si) based solar cells.[1-10] It is

necessary to study the mechanism, such as the carrier’s

extraction and transfer, involved in this kind of solar cells to

improve the power efficiency. For example, the result of a

CdTe/CdSe nanorods solar cell indicated that carrier extraction

was primarily driven by directed diffusion, similar to that in

the type II heterojunction. By analyzing capacitance-voltage

and current-voltage characteristics, Olson et al.[5] demon-

strated that the performance of the CdTe nanorod solar cell

was dominated by the formation of a p-CdTe/Al Schottky

junction. Results from the SnO2:F/TiO2/PbS/Au solar cell

suggested that carrier extraction was driven by a built-in

field created from a depletion region. Therefore, we are

investigating temperature effects to understand the incon-

sistency.

Temperature is an important factors that affect the carrier’s

transport in semiconductor materials. The temperature-

dependent photovoltaic characterization helps to understand

the mechanism in photovoltage generation, charge carrier

transport, and in particular, thermally activated charge carrier

mobility.[11] The temperature-dependent open-circuit voltage

decay (OCVD) measurement demonstrated that the carrier

lifetimes at low (50 - 170 K) and high (190 - 330 K) tem-

perature were remarkably different in porous Si based solar

cells because the interface effect were not stable with

temperature.[12] There are few reports about temperature-

dependent study on colloidal NCs involved solar cells.

In this paper, the temperature-dependent effect on

photovoltage and OCVD were investigated on colloidal

CdTe/CdSe NCs based solar cells. The electron lifetime was

0.369 µs in low temperature region (87 - 267 K) and

6.25 × 10−3µs in high temperature region (267 - 307 K),

respectively. The photovoltaic mechanism was discussed

with a donor-acceptor charge transfer model.

2. EXPERIMENTAL PROCEDURE

High-quality CdTe and CdSe NCs were prepared and

purified using following method,[13,14] and then dissolved in

toluene. The concentrations of NCs solution, estimated with

empirical function,[15] was 20 mg/mL for CdTe NCs solution

and 34 mg/mL for CdSe NCs one, respectively. Figure 1

illustrate the ITO/CdTe/CdSe/Al structure of the solar cell.

The CdTe layer and CdSe layer were obtained by spin-

DOI: 10.1007/s13391-013-3106-2

*Corresponding author: jyzhang@seu.edu.cn©KIM and Springer

434 H. Zhang et al.

Electron. Mater. Lett. Vol. 10, No. 2 (2014)

coating the corresponding NC’s solution on the ITO glass.

The film was baking at 60°C for 15 minutes. Both of the two

layers were 100 nm thick. As both of CdTe and CdSe NCs

could be dissolved in toluene, the lower CdTe layer might be

destroyed with the subsequent spin-coating process of the

upper CdSe layer. In order to avoid this situation, an

additional anneal treatment (250°C, 15 minutes) inside a

vacuum chamber (~1 × 10−4 Pa) was done after the spin

coating of the CdTe layer.[16] Finally, an Al film (100 nm

thick) was deposited as the cathode by an evaporation

method.

Absorption spectra were recorded by an UV-3600

Shimazu spectrometer. A JEOL 100CX transmission

electron microscope (TEM) was used to get the NCs’ TEM

images. X-ray diffraction (XRD) spectra were obtained

using a Rigaku RU-200 B spectrometry. A Veeco DI Ilia

Atomic Force Microscope (AFM) was used to take the

morphologic images of the NC’s film. The current-voltage

characteristics were obtained with a Keithley 2400 multi-

meter, in which AM1.5G solar light was simulated by a 1000

W/m2 xenon lamp. The OCVD measurement was done by

using a pulse laser beam (pulse width of 8 ns, frequency of

10 Hz) as the excitation source. For the temperature-dependent

measurement, the device was placed into an Oxford cryostat.

3. RESULTS AND DISCUSSION

Figure 1 illustrates the absorption spectra of pure CdSe

and CdTe NCs. The first exciton absorption peak is ~608 nm

for CdSe NCs and ~703 nm for CdTe NCs. Therefore the

combination of their absorption covers most of the visible

spectrum.

Figure 2 illustrates the TEM images of the synthesized

CdTe and CdSe NCs. The average size of CdSe NCs is

5.0 nm and that of CdTe NCs is 6.4 nm. The size distribution

is 7% and 16% for CdSe and CdTe NCs, respectively.

Figure 3 shows the XRD patterns of the CdTe NC’s films.

Both the as-deposited film and the annealed one are of

wurtzite structure. The annealing process did not induce any

shift in the XRD peak position but slight narrowed the peak

width from 5.7 nm for the as-deposited film to 5.9 nm after

annealing, calculated using Scherrer formula. The slight

increase of NC’s size means that CdTe NCs do not fuse

thermally during the annealing process. It should be noted

that the “real” value of the CdTe NCs’ average size measured

from the TEM image is 6.4 nm, which is larger than what is

calculated with the XRD spectrum. Defects in nanocrystals

could broaden the x-ray diffraction peaks,[17] which make the

calculated value with Scherrer formula smaller than the

“real” value. The narrowing of XRD peaks after annealing

may result from the reduction of defects in CdTe NCs.

Figure 4 shows the AFM images of the CdTe NC’s films.

The surface of the CdTe film becomes rough after the

annealing process, which may be due to the thermal decom-

position and/or desorption of organic ligands attached to the

CdTe NCs. However, the annealing process enhances the

Fig. 1. Absorption spectra of colloidal CdTe (dot line) and CdSe(solid line) NCs in toluene. The inset shows the schematic diagram ofthe solar cell.

Fig. 2. TEM images of CdTe NCs (left) and CdSe NCs (right).

Fig. 3. XRD spectra of the CdTe NCs’ films. The vertical lines aredrawn accordingly to the XRD pattern of wurtzite CdTe (JCPDSNO.19-0193 standard).

H. Zhang et al. 435

Electron. Mater. Lett. Vol. 10, No. 2 (2014)

effective contact between CdTe and CdSe NCs. Drndic et al.

suggested that thermal annealing could improve the

conductivity of CdSe NCs’ films because of the enhancement

of inter-dot tunneling caused by the decreased separation

between NCs and the chemical changes in their organic cap,

whose length was proposed to be 1.1 ± 0.1 nm.[18] As

indicated in Fig. 3 and Fig. 4, the annealed CdTe film

exhibits the similar characteristics.

Figure 5 shows the typical temperature-dependent current-

voltage curves. At room temperature, the short-circuit current

(ISC) is 4.0 mA/cm2, the open-circuit voltage (VOC) is 0.67 V

and the fill factor (FF) is 0.36, so the power conversion

efficiency is ~1%. The current-voltage characteristics shown

in Fig. 5 are remarkably temperature-dependent. For example,

at 107 K, ISC is 0.14 mA/cm2 and VOC is 0.076 V, and when

temperature goes up to 207 K, ISC becomes 0.91 mA/cm2 and

VOC becomes 0.153 V. The energy band structure of the

device is sketched in Fig. 5 also. A model based on organic

donor-acceptor (D-A) charge transfer has been proposed to

explain the photovoltaic conversion in these undoped active

materials. Carrier extraction is primarily driven by directed

diffusion, similar to that in the type II heterojunction.[1] The

well-accepted metal-insulator-metal (MIM) model, in which

a field across the dielectric active materials is formed due to

the work function difference of the two electrodes, seems to

provide an additional driving force for carrier extraction. It is

suggested that the maximum value of VOC is determined by

the difference between work functions of the two electrodes

in the MIM model[19] but the value of VOC at 297 K (0.67 V)

is larger than the difference in work functions between ITO

and aluminum (0.6 V). This may result from the modification

of the work functions of the electrodes. The diffusive metal

Al may be able to penetrate into the columns existing in the

CdSe film, opening up enough surface states to pin the

metal’s work function at the level of the CdSe’s conduction

band[20] and the electric parameters of the ITO substrate may

be changed by surface treatments.[21] Moreover, experimental

results with some polymer-based photovoltaic cells have

indicated that the VOC value is independent of the work

functions of the electrodes but determined by the difference

between the HOMO level of the electron donor and the

LUMO level of the electron acceptor.[22]

The traditional Si-based solar cells have been quanti-

ficationally analyzed with an equivalent circuit model,[23]

and Cheknane et al.[24] used an equivalent circuit based on a

single-diode model, as shown in the inset of Fig. 6, to

analyze organic D-A solar cells (their structure is ITO/

PEDOT: PSS/Active layer/LiF/Al and the active layer refers

to the blend of MEH-PPV with PCBM). The values of series

resistance (RS) and shunt resistance (RSh) for our device,

which are calculated according to this model, are shown in

Fig. 6. The RS value reflects the mobility of specific charge

carrier in the CdTe and CdSe NCs’ layers, where the mobility

is affected by space charges and traps or other barriers

(hopping), because the two NCs’ layers exhibit larger

resistivity than the two electrodes. With the increasing of

temperature, the RS value is decreased from 1024 Ω to 98 Ωin this cell, which means that carrier mobility is increased

Fig. 4. The AFM images of the NCs’ CdTe films before annealing (a)and after annealing (b).

Fig. 5. Current-voltage curves at temperatures of 107 K (), 207 K(), 297 K (). The inset is the diagram of the band structure of thedevice.

Fig. 6. The temperature dependence of RS () and RSh (). The insetshows the equivalent circuit model. The values of RS and RSh are cal-culated from the current-voltage curves according to the model.

436 H. Zhang et al.

Electron. Mater. Lett. Vol. 10, No. 2 (2014)

with temperature. Generally, RSh is mainly determined by

leakage due to recombination of charge carriers.[23] RSh is

increased from 350 Ω to 763 Ω with temperature, indicating

that the leakage is reduced. The improvement of the carrier

mobility will reduce the possibility of the carrier’s recom-

bination before carriers are collected by electrodes, and this

will increase the RSh value.

Figure 7 shows the temperature dependence of VOC and ISC.

At 87 K, ISC is 0.11 mA/cm2, and it rises slightly towards

0.84 mA/cm2 at 247 K, then it rises remarkably towards

6.2 mA/cm2 at 307 K. Ginger et al.[25] suggested a thermally-

activated hopping transport model to describe the transport

of carriers in NC’s films, and the photocurrent ISC can be

written as:

(1)

where k is Boltzmann constant, T is temperature and EA is

the activation energy for charge transport, which may be

associated with the distribution of traps within the NC’s thin

films. The EA value of 0.427 eV is yielded by fitting the

experimental data with Eq. (1), and this EA value is in the

same level as that of Ginger’s report. On the other hand, the

results from the organic donor-acceptor solar cells have

suggested that the temperature dependence of VOC can be

described as the following equation:[11]

(2)

where q is quantity of electric charge, n is the ideal factor

and I0 is the reverse saturation current. The above equation

indicates that VOC is significantly affected by ISC so it is

remarkably increased with ISC for temperature above 247 K.

The inset of Fig. 8 shows the typical VOC response curve

measured using OCVD technique.[26] The voltage decay

curve can be fitted well with a single exponential equation:

(3)

where V0 is a constant voltage and τ is a time constant.[27,28]

The OCVD time constant τ, a function of temperature (T),

are plotted in Fig. 8. Kavasoglu et al. suggested that the exact

carrier lifetime (τp) could be extracted with the OCVD time

constant τ according the following equation:

(4)

where ∆E is the barrier energy difference.[12] The exact

carrier lifetime is influenced by several factors such as trap

density and the capture cross section of the trap state.[12] As

shown in Fig. 8, a plot of lnτ against 1/T gives good

correlation efficiency in two distinct regions (low-temperature

region 87 - 267 K and high-temperature region 267 - 307 K).

The τp value is estimated by fitting with Eq. (4). The τp value

is 0.369 µs in the low-temperature region and it exhibits

significant change (6.25 × 10−3µs) when the temperature

increases above the threshold temperature (267 K). Such

abrupt change, which has been observed in porous-Si based

solar cells also, possibly stems from critical discrete trap

excitation energy.[12] In other words, there are some thermally

activated trap state in the photovoltaic device. With the

increase of temperature, these trap state are activated,

resulting in the abrupt change of the carrier lifetime. In the

NCs-polymer hybrid solar cells, the temperature-dependent

measurements of the photovoltaic properties revealed that

the kind of thermally activated traps play a significant role in

the photovoltaic process of NCs, similar to the case of the

NC’s photoluminescence.[19,28-30]

ISC T( ) exp −EA

kT------∝

VOC

nkT

q---------⎝ ⎠⎛ ⎞ ln

ISCI0------ 1+⎝ ⎠⎛ ⎞V=

V t( ) V0e−t/τ

=

lnτ T( ) lnτp

E∆kT-------+=

Fig. 7. The temperature dependence of VOC () and ISC (). Thesolid line is the fitting curve of ISC according to Eq. (1). The values ofVOC and ISC are calculated from the current-voltage curves accordingto the equivalent circuit model.

Fig. 8. Dependence of lnτ on the inverse temperature in the range of87 - 307 K. The inset is the photovoltage response of solar cell at307 K.

H. Zhang et al. 437

Electron. Mater. Lett. Vol. 10, No. 2 (2014)

4. CONCLUSIONS

An inorganic CdSd/CdTe NCs based solar cell was

fabricated, and its temperature-dependent photovoltaic

characteristics were investigated. With the rising tem-

perature, the series resistance becomes smaller due to the

increase of carrier mobility, and the shunt resistance is

increased because of the reduction of leakage current. The

exact carrier lifetime is obtained with the open-circuit voltage

decay method, which is 0.369 µs at low temperature region

(87 - 267 K) and 6.25 × 10−3µs at high temperature region

(267 - 307 K), respectively. The photovoltaic mechanism

is discussed with the donor-acceptor solar cell model.

ACKNOWLEDGEMENTS

This work was supported by funds provided by the

National Basic Research Program of China (973 Program,

2012CB921801) and the National Natural Science Foundation

of China (60778041).

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