Journal of Alloys and Compounds 585 (2014) 696–702

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Journal of Alloys and Compounds

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Improvement of inverted type organic solar cells performanceby incorporating Mg dopant into hydrothermally grownZnO nanorod arrays

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.10.006

⇑ Corresponding author. Tel.: +60 3 89215933; fax: +60 3 89213777.E-mail address: ccyap@ukm.my (C.C. Yap).

Riski Titian Ginting a, Chi Chin Yap a,⇑, Muhammad Yahaya a, Muhamad Mat Salleh b

a School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysiab Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 August 2013Received in revised form 23 September2013Accepted 1 October 2013Available online 12 October 2013

Keywords:Nanostructured materialsOxide materialsPhotoconductivity and photovoltaicsSurfaces and interfaces

The Mg concentration dependence of the performance of inverted type organic solar cells based on Mg-doped ZnO nanorod arrays and poly(3-hexylthiophene) (P3HT) has been investigated. The Mg dopantswith various concentrations (0, 1, 3 and 5 at.%) were introduced during the hydrothermal growth ofthe ZnO nanorod arrays on fluorine-doped tin oxide (FTO) glass substrate. The P3HT was deposited ontoMg-doped ZnO nanorod arrays by spin coating technique, followed by deposition of Ag as anode usingmagnetron sputtering technique. The length and density of Mg-doped ZnO nanorods increased, whereasthe diameter decreased with the Mg concentration. The short circuit current density (Jsc) and open circuitvoltage (Voc) improved with increasing of Mg concentration up to 3 at.%, which could be attributed toincreased interfacial area for more efficient exciton dissociation and reduced charge recombination asa result of lower number of oxygen interstitials which act as electron traps in ZnO. However, the Jsc

and Voc started to decrease at Mg concentration of 5 at.%, mainly due to poor infiltration of P3HT intothe high-density 5 at.% Mg-doped ZnO nanorod arrays and increase of Mg dopant-related trapping cen-ters. The highest power conversion efficiency of 0.36 ± 0.02% was achieved at Mg doping concentration of3 at.%, an enhancement of 225% as compared to that based on undoped ZnO nanorod arrays.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, inverted type organic solar cells (OSCs) havebeen widely investigated due to their good air-stability and lowmanufacturing cost [1–6]. The ZnO nanorods which can be grownperpendicularly to the substrate using a low-cost and low-temper-ature (<100 �C) hydrothermal growth process have been widelyused as electron acceptor in inverted type OSCs [7,8]. The largeinterfacial area between ZnO nanorods acceptor and polymer do-nor as well as rod-to-rod spacing that is compatible with the shortexciton diffusion length account for the efficient exciton dissocia-tion [9]. Furthermore, the introduction of vertically aligned ZnOnanorods with high electron mobility also provides the direct path-way for the electrons to move toward the corresponding electrodeand thus minimizes the charge recombination with the holes inpolymer donor [10–13].

Despite those advantages, the power conversion efficiency(PCE) of inverted type OSCs based on ZnO nanorod arrays andpoly(3-hexylthiophene) (P3HT) remains low [8,14–16]. Various ap-proaches have been proposed to improve the device performance,

such as preparation of ZnO nanorods with different appearances[17], polymer processing [8], and surface modification of ZnOnanorods [18–20]. As compared to other approaches, the effect ofintroduction of dopant into ZnO nanorods on the performance ofinverted type OSCs has rarely been explored [21]. Ruankhamet al. (2011) reported the incorporation of lithium into ZnO nano-rod arrays enhanced device performance as a result of effectivecharge transfer at the interface of oxygen-enriched ZnO nanorodssurface and P3HT [21]. Previous study revealed that Mg-dopedZnO nanorod arrays can be prepared by using aqueous solutionmethod and more importantly the nanorods size can be controlledby the doping concentration [22]. However, there is still no reporton the photovoltaic performance of inverted type OSCs based onMg-doped ZnO nanorod arrays. Therefore, it seems desirable todetermine the correlation between Mg doping concentration andphotovoltaic performance of ZnO nanorod arrays/P3HT-based in-verted type OSCs.

The present work reports the effect of Mg doping concentrationon the performance of inverted type OSCs with fluorine-doped tinoxide (FTO)/Mg-doped ZnO nanorod arrays/P3HT/Ag structure. Theshort circuit current density (Jsc) of the device improved withincreasing of Mg concentration up to 3 at.% as a result of increasedinterfacial area for more efficient exciton dissociation as indicated

R.T. Ginting et al. / Journal of Alloys and Compounds 585 (2014) 696–702 697

by the PL quenching and shorter exciton decay lifetime. The sametrend was observed for the open circuit voltage (Voc). Both ACimpedance spectroscopy and transient open circuit voltage decay(TOCVD) measurements suggest a reduced charge recombinationdue to lower number of oxygen interstitials which act as electrontraps in ZnO. Further increase in Mg concentration to 5 at.% resultsin smaller Jsc and Voc, mainly due to poor infiltration of P3HT intothe high-density 5 at.% Mg-doped ZnO nanorod arrays and increaseof Mg dopant-related trapping centers, respectively. The 3 at.%Mg-doped device exhibited the highest PCE of 0.36 ± 0.02%, whichincreased by 225% as compared to that based on undoped ZnOnanorod arrays.

2. Experimental details

The FTO substrates were cleaned prior to use by sonication in acetone and2-propanol. The ZnO nanorod arrays were synthesized via two-step process: seed-ing and hydrothermal growth at low temperature [23]. The seed layer was spin-coated at 3000 rpm from equimolar solution of 0.2 M zinc acetate dehydrate(Zn(O2CCH3)2�(H2O)2) and diethanolamine in ethanol onto pre-cleaned FTO sub-strates. This process was repeated three times and the samples were then annealedat 300 �C for 1 h in air. The hydrothermal growth of ZnO nanorod arrays was carriedout by immersing the seeded-substrates in equimolar aqueous solution (40 mM) ofzinc nitrate hexahydrate (Zn(NO3)2�6H2O) and hexamethylenetetramine at 90 �C for45 min in oven. The ZnO nanorod arrays with different Mg doping concentrationswere obtained by controlling the atomic ratio of zinc nitrate hexahydrate to mag-nesium acetate tetrahydrate (Mg (CH3COO)2�4H2O).

A solution of P3HT (Rieke Metals) was prepared by dissolving in chlorobenzenewith concentration of 40 mg ml�1 followed by sonication prior to stirring for over-night at 50 �C. The solution was then spin coated onto undoped and Mg-doped ZnOnanorod arrays at 1500 rpm for 60 s. Finally, without further heat treatment, the Agtop electrode was deposited by using magnetron sputtering through mask to definean active area of 0.07 cm2, where the detailed sputtering parameters are describedelsewhere [24].

The surface morphology and element composition of the samples were charac-terized by field emission scanning electron microscopy (FESEM, Carl Zeiss Supra55VP) and energy dispersive X-ray spectroscopy (EDX), respectively. The micro-structural properties of the samples were studies by X-ray diffraction (XRD, BrukerAXS D8 Advance). The thickness of the nanorod arrays was measured by means ofVeeco M6 surface profiler. The steady state photoluminescence (PL) spectra wereinvestigated by using Perkin Elmer LS55 luminescence spectrometer. The time re-solved photoluminescence (TRPL) spectra were obtained by using EdinburgFLS920 spectrophotometer equipped with a 472.4 nm picosecond laser diode asexcitation source. The J–V characteristics of the devices were measured by usingKeithley 237 SMU under illumination at 100 mW cm�2 from a solar simulator withAM 1.5G filter and in dark condition. The incident photon to current conversionefficiency (IPCE) spectra were recorded using Newport IPCE system at a chopping

Fig. 1. FESEM images of ZnO nanorod arrays with Mg doping concentration of (a) 0 at.%,undoped ZnO nanorod arrays on FTO substrates, scale bars 200 nm (b), (c) and (d) respe

frequency of 10 Hz. The impedance measurement of devices was performed in darkcondition by using Solartron 1260 Impedance/gain-phase analyzer, with frequencyrange of 6 MHz to 0.1 Hz and AC oscillating voltage of 50 mV. The obtained spectrawere fitted by using Scribner ZView 2 software. For the TOCVD measurement, thedevices were characterized at open circuit condition under illumination from thesolar simulator [25]. A small perturbation of Voc was generated by a pulse from agreen collimated light emitting diode (wavelength = 505 nm, repetitionrate = 1 kHz, pulse width = 100 ls) and the decay of photovoltage was recordedby a high impedance digital oscilloscope (Siglent 1103CM). All device preparationand measurements were carried out in ambient air.

3. Results and discussions

Fig. 1 shows the surface morphology of ZnO nanorod arraysgrown on FTO substrates with different Mg doping concentrations.The diameter of the Mg-doped ZnO nanorods gradually decreasedfrom 49 ± 2 to 30 ± 1 nm, whereas the nanorod density increasedfrom 125 to 275 rods lm�2 when the Mg concentration was raisedfrom 0 to 5 at.%. In addition, the thickness (determined by usingsurface profiler) of the Mg-doped ZnO nanorod arrays increasedfrom 210 ± 8 to 280 ± 6 nm with Mg concentration from 0 to5 at.%. The thickness was obtained by performing step-height mea-surement of film consisting of thin ZnO seed layer and ZnO nano-rods. It is agreed that the length of ZnO nanorods is probablygreater than the measured thickness especially when the nanorodsdo not grow perfectly in the vertical direction to the substrate. TheFESEM micrograph of the cross sections of the undoped ZnO nano-rods is shown in inset of Fig. 1a. It was found that the thicknessdetermined by using surface profiler is only slightly smaller thanthe length of nanorods (240 ± 10 nm) estimated from the FESEMmicrograph as expected. The Mg element EDX mapping images(inset of Fig. 1b–d) reveal that Mg atoms were uniformly distrib-uted into ZnO crystals during hydrothermal growth and theconcentration of Mg element increased with the concentration ofmagnesium acetate tetrahydrate.

Fig. 2a shows the XRD patterns of ZnO nanorod arrays grown onFTO substrates with various Mg doping concentrations. The stron-gest diffraction peak corresponding to the hexagonal ZnO (002)plane demonstrates that the preferred growth orientation ofMg-doped ZnO nanorods was along the c-axis. It is also noted thatno Mg characteristic peaks and impurity phases were found in XRDspectra within the detection limit of XRD equipment for all dopingconcentrations of Mg. As seen in Fig. 2b, the intensity ratio of (002)

(b) 1 at.%, (c) 3 at.% and (d) 5 at.%. The inset shows (a) cross section FESEM image ofctive EDX mapping image of Mg content. Scale bars 400 nm.

Fig. 2. (a) XRD patterns of ZnO nanorod arrays with various Mg doping concentrations, (b) The intensity ratio of (002) to (101) peaks and lattice parameter of Mg-doped ZnOnanorods as a function of Mg concentration.

Fig. 3. Plot of (ahv)2 as a function of photon energy (hv) for ZnO nanorod arrayswith various Mg doping concentrations.

698 R.T. Ginting et al. / Journal of Alloys and Compounds 585 (2014) 696–702

to (101) peaks increased significantly with Mg concentration, indi-cating higher growth rate along (002) direction as compared tothat of (100) and (101) directions. This finding is consistent withFESEM result shown above. In contrast to previous studies onMg-doped ZnO nanorods [22,26], increase in Mg doping concentra-tion did not lead to lower growth rate along (002) direction, but toan increment in the present work. It should be noted that the Mgprecursor (magnesium acetate tetrahydrate) used in the presentwork was different from those reported previously. The acetategroup could undergo hydrolysis process and more OH- could be

Fig. 4. (a) Normalized PL spectra of ZnO nanorod arrays with various Mg doping concentra function of Mg concentration.

generated in the growth solution [27]. It has been reported thehigher OH� concentration not only increases the nucleation ratedue to higher supersaturation level, but also induces faster growthrate along (002) direction rather than (100) and (101) directions[28,29]. It is believed that the effect of increase in OH� concentra-tion was large enough to offset the restriction of growth rate along(002) direction caused by Mg ions. Therefore, both the density andthe length of the ZnO nanorods increased with Mg concentration inthe present work. The lattice parameters calculated from the (002)peak of the samples with various concentrations of Mg is shown inFig. 2b. The lattice parameter, c calculated from the (002) peak ofthe sample is consistent with the standard value for hexagonalwurtzite ZnO as reported in JCPDS card (No. 36-1451, a = 3.249 ÅA

0

,c = 5.206 ÅA

0

). The lattice parameter decreased slightly from 5.207to 5.187 Å when the Mg concentration increased from 0 to 3 at.%,probably due to the substitution of larger Zn2+ (0.60 Å) sites bysmaller Mg2+ (0.57 Å) [22,30]. However, further increase in Mgconcentration to 5 at.% results in larger lattice parameter whichsuggests the incorporation of Mg in the interstitial sites of theZnO lattice [21] and increase of the structural disorder/distortionat the Zn sites in the Mg-doped ZnO nanorods [31].

The optical band gaps of the ZnO nanorods with various dopingconcentrations of Mg were estimated from the plot of (ahv)2 as afunction of photon energy as shown in Fig. 3. The optical band gapswere determined to be 3.31, 3.33, 3.34 and 3.38 eV for Mg dopingconcentrations of 0, 1, 3 and 5 at.%, respectively. The increase ofoptical band gap with Mg concentration is in agreement with theprevious report [32].

The normalized PL spectra of ZnO nanorod arrays with differentMg doping concentrations are shown in Fig. 4a. The samples

ations excited at 325 nm, and (b) The ratio of UV emission to visible emission peak as

Fig. 5. J–V characteristics of the devices with different Mg concentrations underAM1.5G illumination.

Fig. 6. IPCE spectra of the devices with different Mg concentrations. The inset showa typical absorption spectrum of ZnO nanorod arrays/P3HT film.

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exhibited ultraviolet (UV) emission and broad yellow–orangeemission. It is known that the UV emission corresponds to thenear-band-edge emission, whereas the broad yellow–orange emis-sion could be attributed to oxygen interstitials and dopants [33].The ratio of UV emission peak (IUV) to yellow–orange visible emis-sion peak (IVIS) as a function of Mg doping concentration is shownin Fig. 4b. It can be clearly seen that the ratio of IUV/IVIS increasedwith Mg concentration until it reached a maximum at Mg concen-tration of 3 at.%. It is believed that the incorporation of Mg into ZnOnanorods could reduce the number of oxygen interstitials. As theMg concentration was increased to 5 at.%, the dopant-related emis-sion might contribute to yellow–orange emission and hencedecrease in IUV/IVIS ratio.

Fig. 5 shows the current density–voltage (J–V) characteristics ofdevices with different Mg concentrations under AM1.5G irradia-tion with illumination intensity of 100 mW cm�2. The series resis-tance (Rs) and shunt resistance (Rsh) were calculated from theinverse slope at Voc and Jsc in the J–V graph, respectively. The Jsc

and Voc gradually increased as the Mg concentration increasedfrom 0 to 3 at.%, followed by a decrease at Mg concentration of5 at.%. The same trend can be observed for the fill factor (FF). Bothreduction of Rs and increment of Rsh contribute to the improve-ment of FF. The device with the optimum Mg concentration of3 at.% exhibited Jsc of 1.57 ± 0.03 mA cm�2, Voc of 0.49 ± 0.03 Vand FF of 0.47 ± 0.03, resulting in PCE of 0.36 ± 0.02%. The PCE ofthe optimum device remarkably improved by approximately two-fold over the device without Mg doping. Table 1 summarizes thephotovoltaic parameters data of a series of four individual devicesfor different concentrations with their respective standarddeviation.

In order to investigate the light harvesting property of thedevices with various Mg concentrations, the optical absorptionand IPCE properties were investigated. The inset of Fig. 6 shows atypical optical absorption spectrum of ZnO nanorod arrays/P3HT.Three absorption peaks at around 510, 550 and 625 nm contrib-uted by the vibronic transition of P3HT were observed. All theblend films exhibited similar optical absorption regardless of Mgdoping concentration (not shown here), indicating light absorption

Table 1The photovoltaic parameters of the devices with different Mg concentrations.

Concentration (at.%) Jsc (mA cm�2) Voc (V) PC

0 1.11 ± 0.02 0.38 ± 0.01 0.1 1.34 ± 0.05 0.43 ± 0.02 0.3 1.57 ± 0.03 0.49 ± 0.03 0.5 1.21 ± 0.04 0.41 ± 0.03 0.

did not contribute to the change in Jsc. Fig. 6 shows the IPCE spectraof devices with various Mg concentrations. As expected, the IPCEresult and Jsc followed a similar trend in the tested Mg concentra-tion range. Interestingly, it was found that all the samples exhib-ited low IPCE at around 510 nm corresponding to the absorptionpeak of P3HT. This observation has been reported previously in in-verted type OSCs with thickened polymer active layer, in which theholes generated by the exciton dissociation at the region closed toFTO/ZnO nanorods could not be efficiently extracted to the Aganode [34].

To understand the reason behind the increase in Jsc upon Mgdoping, the steady-state PL and TRPL measurements of nanorodarrays/P3HT films with various Mg concentrations were carriedout. Fig. 7a shows that the PL intensity of P3HT decreased signifi-cantly with increasing Mg concentration up to 3 at.%. The PLquenching implies more efficient exciton dissociation at the inter-face between Mg-doped ZnO nanorods and P3HT. This is furthersupported by TRPL decay curves of the samples shown in Fig. 7b.The calculated PL decay lifetimes (fitted by using single-exponen-tial decay function) of 1 at.% and 3 at.% Mg-doped samples(�489 ps and �326 ps) were smaller than that of the undopedsample (�641 ps). The decay lifetime of undoped ZnO nanorod ar-rays/P3HT was similar to the reported value in literature [11]. Theformation of high-density, long and small-diameter ZnO nanorodsat higher Mg doping concentration provides more interfacial areasfor exciton dissociation, hence resulting in faster PL decay andhigher Jsc. However, the PL decay lifetime of 5 at.% Mg-doped sam-ple increased to �582 ps. Yun et al. reported that poor infiltrationof polymer into high-density ZnO nanorod arrays led to less inter-facial contact between ZnO and polymer, thereby a reduced exci-ton dissociation rate [9]. Although the infiltration of polymer wasnot studied in the present work, it is reasonable to correlate the in-crease in PL decay lifetime at 5 at.% Mg doping concentration topoor infiltration of P3HT into the interspace between ZnO nano-rods with the highest density.

The J–V curves in dark (Fig. 8) shows that leakage current de-creased while the forward bias current increased with the increas-ing of Mg concentration up to 3 at.%. This finding agrees well with

E (%) FF Rs (O cm2) Rsh (O cm2)

16 ± 0.01 0.39 ± 0.01 154 ± 29 886 ± 7624 ± 0.03 0.42 ± 0.03 114 ± 20 980 ± 15136 ± 0.02 0.47 ± 0.03 87 ± 12 1238 ± 17620 ± 0.01 0.41 ± 0.01 123 ± 10 1036 ± 99

Fig. 7. (a) Steady-state PL spectra (excited at 530 nm) and (b) TRPL decay curves of ZnO nanorod arrays/P3HT films with various Mg concentrations.

Fig. 8. J–V characteristics of the devices with different Mg concentrations in darkcondition.

700 R.T. Ginting et al. / Journal of Alloys and Compounds 585 (2014) 696–702

the trend of Rsh and Rs which increased and decreased with Mgconcentration, respectively. It is well known that leakage currentnormally leads to low Voc. Previously, it has been reported thatthe ZnO seed layer consisting of nanoparticles could not fully pre-vent direct contact between organic active layer and the transpar-ent conducting oxide (TCO) electrode and the hole leakage currentfrom donor polymer to TCO might not be completely avoided [12].The dense ZnO nanorod arrays obtained at higher Mg dopingconcentration could minimize the direct contact between P3HTand FTO electrode, resulting in smaller hole leakage current fromP3HT to FTO and hence larger Voc. As the ZnO nanorods growlonger, it is possible for the electron to move from the ZnO to Agelectrode, as indicated by the increase in leakage current andreduction of Rsh at 5 at.% Mg doping concentration. Meanwhile,the increase in forward bias current provides additional evidencethat the number of oxygen interstitials which act as electron trapsin ZnO could be reduced by Mg dopants [31]. On the other hand,dopant-related trapping centers might also increase with the Mgconcentration. Therefore, the forward bias current increased upto the optimum Mg concentration at 3 at.%, after which it startedto decrease due to larger series resistance. Previously, it has beenreported that the Voc of devices based on Zn1�xMgxO thin film in-creased with Mg concentration up to 25% due the up-shifted con-duction band [35]. However, the energy gap only slightlyincreased from 3.31 to 3.34 eV when the Mg doping concentrationwas increased from 0 to 3 at.%. Therefore, it is considered that theinfluence of energy gap on the Voc of the present devices is negligi-ble due to relatively low doping concentration of Mg.

Besides lower leakage current, the enhancement in Voc can alsobe attributed to the reduction of charge recombination at the ZnO

nanorods and P3HT interfaces. AC Impedance spectroscopy, a non-destructive characterization tool, has been used to analyze inter-face characteristics of polymer active layer and ZnO in the invertedtype OSCs based on ZnO nanostructured [36–39]. In order to fur-ther understand the device physics at the interface between ZnOand P3HT, the AC impedance measurement has been carried outin dark condition. Fig. 9a and b shows the Nyquist plot of a seriesof spectra acquired on undoped ZnO and 3 at.% Mg-doped ZnOdevices, respectively with different applied bias voltages. Eachspectrum was composed of a left semicircle in high frequency re-gion and a right semicircle in low frequency region. The impedancespectra could be fitted by equivalent circuit that consisted of twoparallel resistor–capacitor in series as shown in inset of Fig. 9a[36]. It was found that the two parallel combination of RC modelfitted well with the experimental impedance spectra with fittingerror below 5% for R1–C1 and R2–C2 values. The parallel combina-tion of R1 and C1 in high frequency region represents the geometriccontribution of bulk properties of P3HT layer. The R1 gradually de-creased with increasing applied bias, an indication of reduced bulkresistance of P3HT due to introduction of charge carriers throughelectronic injection at the electrodes [40]. The C1 was found to berelatively independent of applied bias voltage which is consistentwith previously reported for ZnO nanorods/P3HT device [36]. Onthe other hand, the strongly-bias dependent parallel R2–C2 is re-lated to the charge transfer process at the interface between ZnOnanorod and P3HT [37]. The extracted R2 and C2 as a function of ap-plied bias voltage are shown in Fig. 9c and d, respectively. The R2

decreased whereas the C2 increased in response to the increasingapplied bias voltage, which could be contributed by the increasingof charge carriers accumulated at the interface. It can be observedthat the R2 of 3 at.% Mg-doped ZnO nanorods/P3HT device was lar-ger in comparison with that of undoped device throughout the biasvoltage, suggesting a higher charge recombination resistance at theinterface between 3 at.% Mg-doped ZnO nanorods and P3HT. Previ-ously, it has been reported that recombination resistance is anindication of whether the generated charge carriers could be ex-tracted to the electrodes or lose to bimolecular recombination[41]. Besides R2, the C2 also increased with the introduction ofMg dopant which can be expected due to the larger interfacial areaof Mg-doped ZnO/P3HT [36]. This finding is in agreement with theFESEM result (Fig. 1) elaborated above.

To determine the charge recombination rate at the interface be-tween ZnO nanorods and P3HT, TOCVD measurement was carriedout [15,38]. Fig. 10a shows the TOCVD curves of the devices withvarious Mg concentrations. The recombination lifetime was esti-mated by using single-exponential decay function. The 1 at.% and3 at.% Mg-doped devices exhibited longer recombination lifetimesof s � 89 ls and s � 120 ls, respectively, as compared to that ofundoped device with s � 73 ls. This indicates that the possibility

Fig. 9. Impedance spectra obtained at different bias voltages in dark condition (symbol) and corresponding fits (solid lines) of (a) undoped device and (b) 3 at.% Mg-dopeddevice. The inset of (a) shows the schematic diagram of ZnO nanorods/P3HT device structure together with equivalent circuit. (c) Extracted resistance (R2) and (d) capacitance(C2) associated with the second semicircle (low frequency) as a function of bias voltages.

Fig. 10. TOCVD curves of the devices with different Mg concentrations (b) extracted recombination lifetime with different Mg concentration as a function of bias lightintensity.

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of separated electrons being trapped by oxygen interstitials be-came smaller and hence leads to lower recombination rate withholes in P3HT. The increase in recombination lifetime also agreeswell with the AC impedance result where recombination resistancealso increased by Mg doping. The electron and hole concentrationsat ZnO/P3HT interface could be enhanced due to longer recombina-tion lifetime [15]. As a result, the increase in difference betweenthe quasi-Fermi levels of electrons in ZnO and holes in P3HT con-tributes to higher Voc and Jsc. However, when the Mg concentrationwas further increased to 5 at.%, the recombination lifetime beganto decrease. This observation provides additional evidence for theincrease of dopant-related trapping centers which could trap theelectrons. As a result, the electron–hole recombination rateincreased and hence results in lower Voc. Fig. 10b shows the

extracted recombination lifetime of the devices with different Mgconcentrations under various bias light intensities. Herein, therecombination lifetime is strongly dependent on the light intensity.Interestingly, the relation between recombination lifetime andlight intensity was found to be consistent with the previous reportfor P3HT:PCBM device with conventional structure [25]. Based onthe linear fitting of the data, similar values of the slopes have beenobtained which suggests that the recombination mechanism didnot vary upon introducing Mg dopants into ZnO nanorods.

4. Conclusions

The effect of Mg doping concentration on the performance of in-verted type OSCs based on Mg-doped ZnO nanorod arrays/P3HT

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films has been investigated. The formation of high-density, longand small-diameter ZnO nanorods by increasing the Mg dopingconcentration provides more interfacial areas for exciton dissocia-tion, hence leading to a higher Jsc. In addition, increase in Mg dop-ing concentration up to 3 at.% also results in lower number ofoxygen interstitials which act as electron traps in ZnO and subse-quently a higher Voc. However, smaller Jsc and Voc were obtainedas the Mg concentration increased to 5 at.%, which could be attrib-uted to the lower exciton dissociation rate as a result of poor P3HTinfiltration and increase of charge recombination rate due to Mgdopant-related trapping centers, respectively. The PCE of thedevice with the optimum Mg concentration of 3 at.% significantlyenhanced by 225% as compared to that without Mg doping. Theincorporation of Mg dopant in ZnO nanorods with appropriateconcentration has been demonstrated to be one of the valuablemethods to enhance the performance of inverted type OSCs.

Acknowledgements

This work has been carried out with the financial support ofMalaysian Ministry of Science, Technology and Innovation (MOSTI)under Science Fund 03-01-02-SF0725. The authors would like tothank Mr. Idris Zulkifle from School of Applied Physics, UniversitiKebangsaan Malaysia for silver sputtering. The authors would alsolike to acknowledge Mr. Mohamad Hasnul Naim Abd Hamid fromCentre Research and Instrumentation Management (CRIM), Uni-versiti Kebangsaan Malaysia for FESEM characterizations.

References

[1] M. Jørgensen, K. Norrman, F.C. Krebs, Sol. Energy Mater. Sol. Cells 92 (2008)686–714.

[2] S.K. Hau, H.-L. Yip, N.S. Baek, J. Zou, K. O’Malley, A.K.Y. Jen, Appl. Phys. Lett. 92(2008) 253301–253303.

[3] B. Zimmermann, U. Würfel, M. Niggemann, Sol. Energy Mater. Sol. Cells 93(2009) 491–496.

[4] S.R. Ferreira, R.J. Davis, Y.-j. Lee, P. Lu, J.W.P. Hsu, Org. Electron. 12 (2011)1258–1263.

[5] T. Stubhan, H. Oh, L. Pinna, J. Krantz, I. Litzov, C.J. Brabec, Org. Electron. 12(2011) 1539–1543.

[6] H. Xia, T. Zhang, D. Wang, J. Wang, K. Liang, J. Alloys Comp. 575 (2013) 218–222.

[7] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430–4431.[8] D.C. Olson, Y.-J. Lee, M.S. White, N. Kopidakis, S.E. Shaheen, D.S. Ginley, J.A.

Voigt, J.W.P. Hsu, J. Phys. Chem. C 111 (2007) 16640–16645.[9] D. Yun, X. Xia, S. Zhang, Z. Bian, R. Liu, C. Huang, Chem. Phys. Lett. 516 (2011)

92–95.

[10] W.I. Park, J.S. Kim, G.-C. Yi, M.H. Bae, H.J. Lee, Appl. Phys. Lett. 85 (2004) 5052–5054.

[11] Y.-Y. Lin, C.-W. Chen, T.-H. Chu, W.-F. Su, C.-C. Lin, C.-H. Ku, J.-J. Wu, C.-H. Chen,J. Mater. Chem. 17 (2007) 4571–4576.

[12] K. Lee, B. Kumar, H.-J. Park, S.-W. Kim, Nanoscale Res. Lett. 5 (2010) 1908–1912.

[13] X. Ju, W. Feng, K. Varutt, T. Hori, A. Fujii, M. Ozaki, Nanotechnology 19 (2008)435706.

[14] D.C. Olson, J. Piris, R.T. Collins, S.E. Shaheen, D.S. Ginley, Thin Solid Films 496(2006) 26–29.

[15] Y.-Y. Lin, Y.-Y. Lee, L. Chang, J.-J. Wu, C.-W. Chen, Appl. Phys. Lett. 94 (2009)063308.

[16] T.-H. Lee, H.-J. Sue, X. Cheng, Nanotechnology 22 (2011) 285401.[17] D.C. Olson, S.E. Shaheen, R.T. Collins, D.S. Ginley, J. Phys. Chem. C 111 (2007)

16670–16678.[18] E.L. Lim, C.C. Yap, M. Yahaya, M.M. Salleh, Semicond. Sci. Tech. 28 (2013)

045009.[19] P. Ruankham, L. Macaraig, T. Sagawa, H. Nakazumi, S. Yoshikawa, J. Phys.

Chem. C 115 (2011) 23809–23816.[20] P. Ruankham, S. Yoshikawa, T. Sagawa, Phys. Chem. Chem. Phys. 15 (2013)

9516–9522.[21] P. Ruankham, T. Sagawa, H. Sakaguchi, S. Yoshikawa, J. Mater. Chem. 21 (2011)

9710–9715.[22] T.-H. Fang, S.-H. Kang, J. Alloys. Comp. 492 (2010) 536–542.[23] D.S. Boyle, K. Govender, P. O’Brien, Chem. Comm. (2002) 80–81.[24] R.T. Ginting, C.C. Yap, M. Yahaya, M.M. Salleh, Int. J. Photoenergy 2013 (2013)

503715.[25] C.G. Shuttle, B. O’Regan, A.M. Ballantyne, J. Nelson, D.D.C. Bradley, J. de Mello,

J.R. Durrant, Appl. Phys. Lett. 92 (2008) 093311–093313.[26] J. Joo, B.Y. Chow, M. Prakash, E.S. Boyden, J.M. Jacobson, Nat. Mater. 10 (2011)

596–601.[27] X. He, H. Yang, Z. Chen, S.S.Y. Liao, Phys. B 407 (2012) 2895–2899.[28] S. Xu, C. Lao, B. Weintraub, Z.L. Wang, J. Mater. Res. 23 (2008) 2072–2077.[29] N.S. Ridhuan, K.A. Razak, Z. Lockman, A.A. Aziz, PloS one 7 (2012) e50405.[30] L.-W. Ji, C.-M. Lin, T.-H. Fang, T.-T. Chu, H. Jiang, W.-S. Shi, C.-Z. Wu, T.-L. Chang,

T.-H. Meen, J. Zhong, Appl. Surf. Sci. 256 (2010) 2138–2142.[31] S.H. Park, K.B. Kim, S.Y. Seo, S.H. Kim, S.W. Han, J. Electron. Mater. 35 (2006)

1680–1684.[32] S.P. Anthony, J.I. Lee, J.K. Kim, Appl. Phys. Lett. 90 (2007) 103107.[33] Y.H. Leung, A.B. Djurišic, Z.T. Liu, D. Li, M.H. Xie, W.K. Chan, J. Phys. Chem.

Solids 69 (2008) 353–357.[34] T.J.K. Brenner, I. Hwang, N.C. Greenham, C.R. McNeill, J. Appl. Phys. 107 (2010)

114501–114509.[35] D.C. Olson, S.E. Shaheen, M.S. White, W.J. Mitchell, M.F.A.M. van Hest, R.T.

Collins, D.S. Ginley, Adv. Funct. Mater. 17 (2007) 264–269.[36] B. Conings, L. Baeten, H.-G. Boyen, D. Spoltore, J. D’Haen, L. Grieten, P. Wagner,

M.K. Van Bael, J.V. Manca, J. Phys. Chem. C 115 (2011) 16695–16700.[37] B. Conings, L. Baeten, H.-G. Boyen, D. Spoltore, J. D’Haen, M.K. Van Bael, J.V.

Manca, Appl. Phys. Lett. 100 (2012) 203905.[38] P.P. Boix, J. Ajuria, R. Pacios, G. Garcia-Belmonte, J. Appl. Phys. 109 (2011)

074514–074515.[39] T. Kuwabara, Y. Kawahara, T. Yamaguchi, K. Takahashi, ACS Appl. Mater. Inter.

1 (2009) 2107–2110.[40] B.J. Leever, C.A. Bailey, T.J. Marks, M.C. Hersam, M.F. Durstock, Adv. Energy

Mat. 2 (2012) 120–128.[41] G. Garcia-Belmonte, P.P. Boix, J. Bisquert, M. Sessolo, H.J. Bolink, Sol. Energy

Mater. Sol. Cells 94 (2010) 366–375.