Correlation between location of defects in electrodeposited ZnOand performance for the corresponding hybrid solar cells

Xin Ren • Jiao Cao • Shuai Yuan • Liyi Shi

Received: 26 March 2014 / Accepted: 17 April 2014 / Published online: 30 April 2014

� Springer Science+Business Media New York 2014

Abstract ZnO nanorod array and dense grain film on ITO

glass was grown by one-step electrodeposition at 85 and

55 �C, respectively. Hybrid solar cells composed of poly-

mer blends and the electrodeposited ZnO were produced.

We demonstrated the correlation between the location of

the dominant defects in the electrodeposited ZnO and the

performance of the corresponding hybrid solar cells by

correlating the morphologies of the hybrid solar cells, the

shift phenomena of the photoluminescent visible emission

bands of the electrodeposited ZnO, and the photovoltaic

behaviors of the solar cells. The defects located at the root

of the ZnO rods can cause serious current leakage for the

hybrid solar cells. The elimination of the current leakage

can be achieved by cutting off the direct contact between

the polymer blend and the root of the nanorods by either

growing denser rods or annealing treatment.

1 Introduction

Organic-based photovoltaics have attracted great interest

due to their potential for the realization of a low cost, easily

processed and flexible renewable energy source [1–4].

Polymer–fullerene solar cells based on composites of an

electron-donating conjugated polymer and an electron-

accepting fullerene has proven to be one of the most suc-

cessful of them so far [5–7]. An n-type inorganic material

is usually incorporated to act as a hole blocking layer

[8–10], and more importantly, as a scaffold to structure the

blend and to favor the alignment of the chains of the

organic material to optimize the mobility of the charge

carriers [11, 12]. ZnO nanorod array has been one of the

best candidates for inorganic nanomaterials in polymer-

inorganic hybrid solar cells owing to the characteristics of

low work function, [9] high electron mobility [12–14] and

easiness of control of nanostructures. [15, 16] Preparation

of ZnO nanorods by electrodeposition process from aque-

ous solutions attracts extensive research because the

method is relatively simple, can easily control the mor-

phology of nanostructures, [17–22] and has the potential

for scale-up production. However, due to the low growth

temperature (\100 �C), the crystalline quality of such

samples is often lower than those fabricated by physical

methods, which will influence the performance of the solar

cells. How the defects effect on the cell performance and

how to eliminate or to minimize the detrimental effects is a

key factor in improving the efficiency of the photovoltaic

devices. Seldom reports on this were published so far

despite its importance.

In this work, we produced two different hybrid solar cell

architectures with ZnO nanorod arrays and dense grain

films. The ZnO nanorod arrays and the dense grain films on

ITO glass substrates were prepared by a simple one-step

electrodeposition method. No additives, seeded layers, or

template was added so as to minimize all the other possible

interferential factors. We demonstrate the relationship

between the location of the defects in the deposited ZnO

and the device performance for the hybrid solar cells by

comparing the XRD patterns and the photoluminescence

spectra of the electrodeposited ZnO nanorod arrays and

dense grain films, and the morphologies and the photo-

voltaic behaviors of the solar cells with the corresponding

ZnO samples.

X. Ren (&) � J. Cao � S. Yuan � L. Shi

Research Center for Nano-Science and Technology, Shanghai

University, Shanghai 200444, People’s Republic of China

e-mail: renxin108@shu.edu.cn

123

J Mater Sci: Mater Electron (2014) 25:2923–2928

DOI 10.1007/s10854-014-1960-9

2 Experimental

Electrodeposition was performed using a Keithley 2400

sourceMeter under constant current density of 0.15 mA cm-2

at 85 and 55 �C for 10–60 min, respectively. The ITO sub-

strate (sheet resistance *10 X/sq) was connected to the

cathode, and a 2.25 cm2 platinum foil was employed as the

anode. This electrode was immersed in a 0.01 M Zn(NO3)2

solution, parallel to the ITO substrate at a distance of*2 cm.

After electrodeposition, several samples were placed in a

furnace and annealed at 300 �C in air for 6 h. Spin coating was

carried out in the globe box at nitrogen atmosphere. Before

coating, the as-grown ZnO samples were kept at 150 �C for

20 min to decompose Zn(OH)2 and to eliminate the H2O

molecules absorbed on the samples. The solution composed of

20 mg/ml P3HT and 20 mg/ml PCBM, using chlorobenzene

as the solvent, was prepared, stirred for about 8 h and spin

coated on the top of the ZnO nanorod arrays at 1,000 rpm for

60 s. Then, the samples were annealed at 120 �C for 30 min to

further crystallize the blend. Silver electrodes (100 nm thick)

were deposited on the blend at a pressure of\10-6 Torr in

thermal evaporator. After evaporation, the samples were

exposed to air in the dark for 4 days to help oxidize the ZnO

surface which was kind of deoxidized when in vacuum [23].

A JEOL 6340F Field Emission Scanning Electron

Microscope (FE-SEM) was employed to study the mor-

phology of the electrodeposited ZnO and the hybrid solar

cells. PL measurements were performed at room temperature

with an ACCENT RPM 2000 compound semiconductor PL

system equipped with a Nd:YAG laser of wavelength

266 nm. Current density–voltage (J-V) characteristics of the

fabricated devices were measured using a Keithley 2400

sourcemeter in dark, and under 100 mW/cm2 white light

illumination (Oriel 91160 300 W solar simulator equipped

with an AM 1.5 G filter) through the ITO/glass side.

3 Results and discussion

Figure 1a shows a large area ZnO nanorod array electrode-

posited at 85 �C (denoted as ZnO_85) on an ITO substrate.

The diameter of the rods is 150–200 nm, and the average

distance between the rods is around 550 nm. Figure 1b

shows a ZnO film electrodeposited at 55 �C (denoted as

ZnO_55) on an ITO substrate. The film is composed of dense

grains with average size of 350 nm without interspaces.

Figure 1c shows the XRD patterns of the electrodeposited

ZnO on ITO substrates without and with annealing treatment

at 300 �C. The peaks associated with In2O3 from ITO sub-

strates are labeled for clarity. The patterns associated with

ZnO reveal a hexagonal wurtzite structure (JCPDS No.

36-1451). The intensities of the (002) diffraction are much

higher than those of the other ZnO peaks in all samples,

indicating that the ZnO nanorods have a preferential orien-

tation along the c-axis. The intensities of the peaks of the as-

grown ZnO_85 are higher than those of the as-grown

ZnO_55 due to the better crystallization at higher growth

temperature. After annealing treatment at 300 �C, all the

intensities of the ZnO peaks became higher. Since our ZnO

Fig. 1 SEM images of ZnO nanorod array and dense grain film

electrodeposited on ITO glass at a 85 �C and at b 55 �C for 30 min,

respectively. c XRD patterns of the as-grown and the annealed ZnO

on ITO glass. To distinguish, the ZnO nanorod array deposited at

85 �C was denoted as ZnO_85, and the ZnO dense grain film

deposited at 55 �C was denoted as ZnO_55

2924 J Mater Sci: Mater Electron (2014) 25:2923–2928

123

rods were prepared from aqueous solution, the as-grown

ZnO should contain some Zn(OH)2 [24], which can be

detected in the XRD pattern of the as-grown ZnO_55. The

increase of the intensities of ZnO patterns after annealing

should attribute to the new formed ZnO by the decomposi-

tion of Zn(OH)2.

Figure 2 shows the PL spectra from the ZnO/ITO glass

excited by laser with a wavelength of 266 nm. The spectra

are normalized to the peak value of the UV emission band in

order to compare the variations of the ratio of the visible

emission intensity to the UV band-edge emission intensity.

Both PL spectra for the as-grown and the annealed ZnO_85

consist of a strong UV peak and a weak broad visible emis-

sion band. The UV peak centered at 378 nm is due to the

exciton recombination [25, 26]. The visible emission band

centered at 580 nm may be related to the oxygen vacancy

(Vo). Vo, which is the one of the most mentioned defects in

the literatures about ZnO has the lowest formation energy

among the defects that behave as donors. Several groups

have suggested that Vo was the source of green luminescence

[27–30]. Considering the relatively low temperature of the

electrodeposition of ZnO, Vo is the most possible defect

appearing in our ZnO rod array. After annealing, the UV

emission remains at the same position, while the center of the

visible emission band shifts from *580 to *520 nm. For

the as-grown ZnO_55, the visible emission is much higher

than the UV emission due to the inadequate decomposition

of Zn(OH)2 at low temperature [31]. After annealing, the

intensity of the visible emission decreases fleetly, indicating

the effective decomposition of Zn(OH)2. The yellow and

green emission bands from the as-grown and the annealed

ZnO_55 appear at the same wavelength ranges as the iden-

tically treated ZnO_85, suggesting that the ZnO deposited at

55 and 85 �C have the same types of defects. The shift of the

visible emission band center cannot be contributed to the

change of defect type since the 300 �C annealing is not

powerful enough to afford the formation energy for a new

type of defects in ZnO. Figure 2c shows the PL spectra of the

ZnO nanorod arrays deposited at 85 �C for 10, 30 and

60 min, and then annealed at 300 �C. It can be seen that

increasing deposition time can reduce the relative intensity

of the visible emission. This implies that the dominant

defects responsible for the visible emission may originate

from the ZnO rod root rather than the inner. Since the crystal

lattices between the as-grown ZnO and the ITO are quite

different, a transition region that contains a great deal of

defects should locate at the ZnO rod root.

We supposed that the blueshift of the visible emission

bands in Fig. 2a, b resulted from the migration of the

dominant defects from the root to the inner of the rods. As

Fig. 3 illustrated, in the as-grown ZnO rod array, most Vo

defects located at the transition region of ZnO rod root.

Since ZnO and ITO have different work functions UZ and

UI, respectively, their contact leads the line up of the Fermi

levels, which in turn results in the upwards band bending of

ZnO. Since the Vo is a deep level defect of localized state,

the band bending effect has little influence in altering its

energetic position as compared to those shallow donor/

acceptors [32]. Hence, the electronic transition from the

deep level state to the valence band maximum occurred at a

lower energy (*2.14 eV), giving rise to a long wavelength

yellow emission band (*580 nm). After the 300 �C

annealing treatment, part exposed Vo on the ZnO surface

were vanished by the oxygen at ambient atmosphere. The

unexposed defects at the root of the ZnO rods diffused into

Fig. 2 PL spectra of the unannealed (square) and annealed (circle)

a ZnO_85 and b ZnO_55 deposited for 30 min on ITO glass,

respectively; c nanorod arrays deposited for 10 min (square), 30 min

(circle) and 60 min (triangle), and then annealed for 6 h

J Mater Sci: Mater Electron (2014) 25:2923–2928 2925

123

the rod stem under the drive of the concentration gradient.

As a result, most electron transitions occurred in the flat

band region far from the ZnO/ITO interface, and the visible

emission band shifted from yellow to green centered at

*520 nm. It is worth noting that an annealing treatment to

the ZnO with Vo concentrating on the utmost surface could

also result in a visible band shift [33]. Whereas, the band

redshifted to a lower energy level in such a case. This

conversely indicated that the dominant defects Vo con-

centrated at the roots rather than on the surface of the as-

grown ZnO in our experiment.

Figure 4 presents the top and cross-sectional views of

the Ag/P3HT:PCBM blend/ZnO/ITO composite films with

ZnO_85 and ZnO_55, respectively. Figure 4a shows that

the blend effectively intercalates into the ZnO_85 nanorod

array. The morphology of the blend still partly keeps the

original morphology of the ZnO nanorod array. Compared

with the morphologies of the nanorod based bulk hetero-

junction structures reported before where the polymers

usually filled and even overflowed the nanorods, [12, 23,

34] this conformal morphology will shorten the path for the

hole transmission and increase the interface area between

the blend and the metal electrode layer. Different to the

morphology of the ZnO_85 cell, we can clearly distinguish

the layers of Ag, blend, ZnO and ITO in Fig. 4b. In this

structure, the blend in the ZnO_55 cell only coats the top of

the ZnO dense grain film, and cannot touch the bottom

region of the ZnO where the defects concentrate.

Figure 5 shows the photovoltaic performance for the ITO/

ZnO/P3HT:PCBM blend/Ag devices with the corresponding

ZnO_85 and ZnO_55 in dark and under 100 mW cm-2 AM

1.5 simulated illumination. In Fig. 5a, the dark J-V curve of

the ZnO_85 cell shows the cell without annealing suffers a

severe current leakage. The Voc of the as-grown ZnO_85 cell

under illumination is only 0.048 V. After the annealing

treatment to the ZnO_85, the Voc of the cell improved to

0.42 V, which is much higher than that of the unannealed

cell. The improvement of Voc accords well with the general

expression Voc = nVth ln ((Jsc/Jdark) ? 1), where n is the

diode ideality factor, Vth is the thermal voltage, Jsc is the

short circuit current density and Jdark is the dark current

density. Thus, the increase of Voc should be due to the

effective suppression of the current leakage by the 300 �C

annealing. In Fig. 5b, it can be seen that the dark current

density is much lower than that of the ZnO_85 cell. The Voc

of the unannealed ZnO_55 cell is 0.38 V, which is much

higher than that of the unannealed ZnO_85 cell. Since the

ZnO_55 was deposited at lower temperature than the

ZnO_85, it had more intrinsic defects. However, the

Fig. 3 Schematic diagram of upwards band bending of ZnO with Vo

defects

Fig. 4 Top view (top row) and

cross-sectional view (bottom

row) SEM images of Ag/blend/

ZnO/ITO composite films with

a ZnO_85 b ZnO_55,

respectively

2926 J Mater Sci: Mater Electron (2014) 25:2923–2928

123

performance of the ZnO_55 cell behaves much better than

that of the ZnO_85 cell. By comparing the morphologies

between the ZnO_85 and the ZnO_55 cells, we can deduce

that the current leakage of the ZnO_85 cell happened at the

interface between the blend and the root of the ZnO rods,

where the dominant defects Vo concentrated and caused the

visible emission with center at *580 nm. In the ZnO_55

cell, the dominant defects Vo also existed at the bottom of the

ZnO. The ZnO dense grain film prevented the blend infil-

trating into the bottom, avoiding a serious current leakage. In

the annealed ZnO_85 cell, the annealing treatment to the

ZnO resulted in the migration of the dominant defects from

the root to the inner, as we supposed in Fig. 3. The polymer

could not touch the defect concentrated region, then the

current leakage got suppressed. The efficiency of the

annealed ZnO_85 cell is 1.19 %, which is 18 % higher than

that of the identically treated ZnO_55 cell. This mainly

results from the difference between the Jsc of the ZnO_85 and

the ZnO_55 cells. Compared to the ZnO_55 cell, the nano-

rods in the ZnO_85 cell provides larger interface, shorter

route for the holes to transfer from the interface to the Ag

electrode, and better scaffold to fix the blend and to favor the

alignment of the chains of the organic material to optimize

the mobility of the charge carriers. These lead to the

improvement of the photocurrent.

From the above cases, we deduce that the current

leakage can be suppressed by cutting off the direct contact

between the polymer blend and the root of the ZnO

nanorods. To verify our deduction, a ZnO film with denser

and slantwise rods (Fig. 6a) was deposited under constant

voltage of 2 V (denoted as ZnO_2 V), with all other

parameters identical with the ZnO_85. The slantwise rods

could prevent the blend infiltrating into the bottom of the

ZnO array, thus avoided the direct contact between the

blend and the dominant defects region. The J-V plots of the

solar cells with the as-grown and the annealed ZnO_2 V

rods are shown in Fig. 6b. With regard to the cell with the

as-grown ZnO_2 V, the Jsc is 10.15 mA, which is higher

than those of the former cells due to the higher polymer

load on the denser rod structure. The Voc is 0.33 V, which

is much higher than that of the as-grown ZnO_85 cell but

lower than that of the as-grown ZnO_55 cell, due to a

Fig. 5 J-V plots of the a ZnO_85 cell and b ZnO_55 cell, where the

as-grown and annealed ZnO cells in dark is denoted as square and

circle, respectively, and under 100 mW cm-2 AM1.5 simulated

illumination is denoted as triangle and star, respectively

Fig. 6 a Top view SEM image of the ZnO nanorod array deposited

under constant voltage of 2 V at 85 �C for 30 min (denoted as

ZnO_2 V). b J-V plots of the cells with the as-grown and annealed

ZnO rods in dark (square and circle, respectively), and under

100 mW cm-2 AM 1.5 simulated illumination (triangle and star,

respectively)

J Mater Sci: Mater Electron (2014) 25:2923–2928 2927

123

somewhat contact between the rod roots and the polymer.

After annealing to the ZnO_2 V, the Jsc and fill factor got

kind of fall. The Voc increased to 0.44 V due to the sup-

pression of the Vo defects by the oxygen in the ambient

atmosphere. The efficiency of the annealed ZnO_2 V cell

is 1.41 %, which is better than all the former solar cells.

Moreover, this efficiency is better than the efficiencies

(1.02–1.28 %) of the hybrid solar cells using the ZnO

nanorods of similar dimensions [35, 36], although the latter

cost more polymer blends. This can be contributed to the

conformal morphologies of our solar cells, which shortens

the carrier transport route from the interface of the ZnO/

polymer blend to the Ag electrode.

4 Conclusions

In summary, ZnO nanorod arrays and dense grain films

have been prepared by one-step electrodeposition methods

with identical parameters except deposition temperature.

We proved that the shift of the centers of the PL visible

emission bands was caused by the migration of the domi-

nant defects from the root to the inner of the ZnO nanorods.

We demonstrated that the dominant defects located at the

root of the ZnO nanorods were the keys of the current

leakage of the hybrid solar cells. The cutoff of the contact

between the polymer blend and the root of the nanorods by

either growing denser rods or annealing treatment can

suppress the leakage effectively. The solar cells with the

conformal morphologies showed a higher efficiency than

the reported conventional bulk solar cells using nanorods

of similar dimensions. This can be contributed to the

shorter carrier transport route from the interface of the

ZnO/polymer blend to the Ag electrode.

Acknowledgments The authors are very Grateful to the financial

support by the National Natural Science Foundation of China (Grant

No. 51202140, 51202138, 51311130128), Natural Science Founda-

tion of Shanghai (12ZR1410500), Funding Project for Young Uni-

versity Faculty of Shanghai, Shanghai University Innovation Fund

(2012-120417).

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