in organic photovoltaic cells
L. Barkat1, M. Hssein2,3, Z. El Jouad3,4, L. Cattin2, G. Louarn2, N. Stephant2, A. Khelil1, M. Ghamnia5, M. Addou3, M. Morsli6, and J. C. Bernede*,4
1Universite d’Oran 1Ahmed Ben Bella, LPCM2E, BP1524 ELM Naouer 31000, Oran, Algeria
2Universite de Nantes, Institut des Materiaux Jean Rouxel (IMN), CNRS, UMR 6502, 2 rue de la Houssiniere, BP 32229, 44322 Nantes cedex 3, France
3Laboratoire Optoelectronique et Physico-chimie des Materiaux, Universite Ibn Tofail, Faculte des Sciences, BP 133, Kenitra 14000, Morocco
4Universite de Nantes, MOLTECH-Anjou, CNRS, UMR 6200, 2 rue de la Houssiniere, BP 92208, 44000 Nantes, France
5Universite d’Oran 1–Ahmed Ben Bella, Laboratoire des Sciences de la Matiere Condensee (LSMC), Oran, Algeria
6Universite de Nantes, Faculte des Sciences et des Techniques, 2 rue de la Houssiniere, BP 92208, 44000 Nantes, France Received 19 August 2016, revised 23 August 2016, accepted 23 August 2016
Published online 12 September 2016
Keywordsbuffer layers, CuI, deposition, MoO3, organic solar cells
*Corresponding author: e-mailjean-christian.bernede@univ-nantes.fr, Phone:þ33 251 125530, Fax:þ33 2 51 12 55 28
In order to improve hole collection at the interface anode/electron donor in organic photovoltaic cells, it is necessary to insert a hole- transporting layer. CuI was shown to be a very efficient hole- transporting layer. However, its tendency to be quite rough tends to induce leakage currents and it is necessary to use a very slow deposition rate for CuI to avoid such negative effect. Herein, we show that the co-deposition of MoO3and CuI avoids this difficulty and allows deposition of a homogeneous efficient hole-collecting
layer at an acceptable deposition rate. Via an XPS study, we show that blending MoO3:CuI improves the hole collection efficiency through an increase of the gap state density. This increase is due to the formation of Mo5þ following interaction between MoO3
and CuI. Not only does the co-evaporation process allow for decreasing significantly the deposition time of the hole- transporting layer, but also it increases the efficiency of the device based on the planar heterojunction, CuPc/C60.
ß2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction The regular improvement of the efficiency of the organic photovoltaic cells (OPVCs) demonstrates that OPVCs are a possible route to low weight andflexible next-generation solar cells, even if they are still in need of power-conversion efficiency [1–4]. One possibility to increase the power-conversion efficiency (PCE) of the OPVCs is to introduce a thin anode buffer layer (ABL) between the anode and the organic electron donor (ED) to improve the hole extraction. This ABL provides an improvement of the band matching between the highest occupied molecular orbital (HOMO) of ED and the work function (WF) of the anode. It must also be selective, i.e., it must facilitate the passage of holes and block the electrons. Moreover, its surface must be smooth in order to avoid leakage current path formation [5].
The first ABL used was the polystyrene sulfonic acid (PEDOT:PSS), which is a conductive polymer. It was
deposited by spin coating onto the ITOfilm before organic layer deposition. This ABL was very efficient since it allowed achieving good adjustment of the work function (WF), passivation of surface defects, and smoothing of the ITO surface. However, PEDOT:PSS is problematic since it degrades under UV illumination; it introduces water into the active layer and it is slightly acidic [6]. Therefore, it needs either improvement in the properties of PEDOT, which was successfully done [7, 8], or use of a different material. In the present paper, we have chosen the second solution, using inorganic materials. For this purpose, different inorganic layers were proposed such as ultrathin goldfilm [9] or transition-metal oxide [5, 10]. Among the transition-metal oxides, MoO3 was shown to be a very suitable ABL [11–15]. Actually, MoO3is very efficient in optimizing the hole extraction due to its high work function
that allows a good agreement with the HOMO value of the ED. Nevertheless, the band matching at the interface is not the only criteria for high OPVC performances. The properties of the organic semiconductors themselves are also important. p-conjugated molecules usually possess highly anisotropic properties, such as light absorption and carrier transport, which strongly depend on the molecular orientation of the organic layers [16]. Therefore, perfor- mance enhancements were also achieved through the management of the molecular orientation. For instance, it was shown that CuI can be used to control the molecular orientation of MPc in organic cells using CuPc [17] and more recently ZnPc [18] or PbPc [19] as an absorbing layer. Due to the templating effect of CuI, very promising performance, an increase of 70–85% of the PCE, was achieved. Not only does the CuI control the MPc molecules orientation, but also, due to its high work function (WFCuI¼5.3 eV [20]), it allows a good band matching at the interface anode/ED. Then, it was shown that CuI is also efficient with small molecules other than MPc [21] and with semiconducting conjugated polymers [22]. Usually, either it increases the organic layer absorption or it improves the electrical conductivity of this layer. It has also been used in organic light-emitting diodes [23]. However, it was shown that the morphology of the CuI layers depends strongly on thefilm-deposition conditions [22], mainly on the deposition rate [24]. For instance, the surface roughness of the CuI layer deposited under vacuum was often very high. If we did not control precisely the conditions of deposition of the layers, they were rough and inhomogeneous, which introduces leakage current paths in the devices and, therefore, poor performance reproducibil- ity. We have shown that, in order to prevent the formation of inhomogeneities during the deposition of CuI, it is necessary to use a very slow deposition rate, i.e., 0.005 nm s1, which is very detrimental to the achievement of cells. On the other hand, we have shown that a CuI layer of thickness 1.5 nm allows achieving performing OPVCs if this CuI layer is deposited onto a layer of MoO3 with a thickness of 3 nm [24].
Therefore, in the present work, we proceed to co- evaporation of an hybrid ABL MoO3:CuI. The co-evaporation process avoids CuI crystallization that prevents rough ABL formation, even for a deposition rate of 0.01 nm s1for each compound. These MoO3:CuI ABL were probed in planar heterojunction OPVCs based on the couple copper phthalocyanine/fullerene (CuPc/C60). By comparison with OPVCs using a reference ABL (MoO3, CuI, or MoO3/CuI) there is a systematic improvement of their PCE and this for a CuI deposition rate two times greater than in the case where CuI is deposited alone.
2 Experimental
2.1 Realization and characterization of the OPVCs The ITO was provided by SOLEMS (France), and the chemical products were provided by CODEX (France). The OPVCs were deposited onto ITO-coated glass
substrates with a sheet resistance of about 25Ω/square. The standard substrate dimensions were 25 mm by 25 mm. Since ITO covered the whole glass substrates, some ITO must be removed to obtain the under electrode. After masking a broad band of 25 mm by 20 mm, the ITO was etched by using Zn powderþHCl as etchant [24]. After scrubbing with soap, the ITO-coated substrates were rinsed in running deionized water. Then the substrates were dried with an air flow and then loaded into a vacuum chamber (104Pa). The deposition rate andfilm thickness were measuredin situby a quartz monitor, after calibration for each material. The planar heterojunctions were based on the couple CuPc/C60
sandwiched between two electrodes, the ITO-coated glass substrate as anode and an Alfilm as cathode. Buffer layers were inserted at the interfaces between the electrodes/
organic materials. Between the Al cathode and the fullerene layer, the exciton blocking layer (EBL) was Alq3[12].
The ABL was the co-evaporated MoO3:CuI hybrid layer, while, as references, MoO3, CuI, and the bilayer MoO3/CuI were also probed. After the ABL, CuPc, C60, Alq3 were successively sublimated under vacuum and finally, the aluminum anode was evaporated on top of the device giving the following OPVC (see Fig. 1):
ITO=ABL=CuPc 35nmð Þ=C60ð40nmÞ=
Alq3ð9nmÞ=Al 120nmð Þ:
The top electrode was deposited through a mask with 28 mm2active area. All these thicknesses were optimized in previous publication [5, 12]. About the ABL, the thickness of the MoO3and CuI layers were 3 and 1.5 nm, respectively. In the case of double or hybrid ABL, the ratio CuI/MoO3was either 0.5 or 1, these ratios corresponding to the optimum results obtained previously with superposed MoO3/CuI ABL [21, 24]. The deposition rate of the MoO3
Figure 1 J–Vcharacteristics of OPVCs with different ABL, in the dark (full symbols) and under AM1.5 irradiation (open symbols).
Inset: scheme of the device structure.
stopped 5 s before that of CuI in order to ensure greater CuI atom density at the surface of the ABL.
Electrical characterizations were performed with an automatedI–Vtester, in the dark and under sun global AM 1.5 simulated solar illumination. The performances of photovoltaic cells were measured using a calibrated solar simulator (Oriel 300W) at 100 mW cm2 light intensity adjusted with a PV reference cell (0.5 cm2CIGS solar cell, calibrated at NREL, USA). Measurements were performed in ambient atmosphere. All devices were illuminated through TCO electrodes.
For comparison, OPVCs with MoO3, CuI, and MoO3/ CuI ABL were realized during the same deposition processes. Successive (3–4) runs were done for each configuration, which corresponds to 9–12 OPVCs, in order to check the reproducibility of the results.
2.2 Characterization of the ABL The morphology of the different structures used as anode was observed through scanning electron microscopy (SEM) with a JEOL 7600F. The SEM operating voltage was 5 kV. Images were obtained using a secondary electron detector.
AFM images of different parts of thefilms were takenex situ at atmospheric pressure and room temperature.
Measurements of surface morphology of the films were carried out using an atomic force microscope (AFM) instrument (dimension Edge-Bruker) in tapping mode. AFM images were acquired in ambient air and digitized into 256256 and 512512 pixels using antimony (n) doped Si tip. The cantilever is characterized by a force constant 80 N m1 and a resonance frequency of 338 kHz. The images were treated using wsxm software [25].
The thin-film structures were analyzed by X-ray diffraction (XRD) by a Siemens D5000 diffractometer using Ka radiation from Cu (lKa¼0.15406 nm). Optical transmission spectra were recorded on a Cary spectrophotometer.
Concerning XPS measurements, an Axis Nova instru- ment from Kratos Analytical spectrometer with the Al Ka line (1486.6 eV) as the excitation source has been used. The core-level spectra were acquired with an energy step of 0.1 eV and using a constant pass energy mode of 20 eV, (energy resolution of 0.48 eV). Concerning the calibration, the binding energy for the C1s hydrocarbon peak was set at 284.8 eV. Then data were analyzed with the CasaXPS software.
3 Results and discussion First, we present the photovoltaic properties of the different OPVCs and then we investigate the surface properties of the different ABLs and the structural properties of the CuPcfilms deposited on these different ABLs.
3.1 J–Vcharacteristics of the different OPVCs It can be seen in Fig. 1 that the best result is obtained with the hybrid MoO3:CuI layer, more precisely, Table 1
summarizes the results obtained with the different ABL under illumination.
When CuI is deposited at 0.01 nm s1, the anode ITO/
CuI gives the worst results, while, as expected, the ABLs containing MoO3and CuI give better results. Furthermore, the best result is obtained with the co-evaporated MoO3:CuI ABL for the ratio 3/1.5, with an open-circuit voltage Voc¼0.48 V, short-circuit currentJsc¼8.48 mA cm2, fill factor FF¼52% and efficiencyh¼2.11%. We can compare these results with those already published in Ref. [22].
These results are briefly summarized in Table 2. It can be seen that, not only are the PCE obtained with the optimum MoO3:CuI ABL better than those obtained with the superposed MoO3/CuI layers deposited at a deposition rate of 0.01 nm s1 (Table 1), but they are also slightly better than those obtained with MoO3/CuI deposited under the optimum conditions, i.e., 0.01 and 0.005 for MoO3and CuI, respectively (Table 2). It can also be seen that, if the anode ITO/CuI gives the worst results in Table 1, this result is in good agreement with that presented in Table 2 for CuI deposited at the same rate, i.e., 0.01 nm s1. Unfortunately, when the deposition rate of CuI is greater than 0.01 nm s1, the OPVC performances, though still good, are smaller than those obtained at that deposition rate (Table 1). On the other hand, when we increase the MoO3/CuI ratio from 1 to 1.5, we improve the performance of the OPVCs (Table 1).
Beyond this report, XRD measurements show that we lose the orientation effect of CuPc molecules by CuI. Thus, we lose the improvements due to this orientation in volume,
ABL thickness
(nm)
Voc
(V) Jsc
(mA cm2) FF (%)
h (%)
Rs
(Ω) Rsh
(Ω)
CuI 1.5 0.39 4.31 47 0.79 9 500
CuI 3 0.42 4.02 49 0.83 3 533
MoO3 3 0.47 5.47 59 1.52 3.7 1660
MoO3/CuI 3/1.5 0.45 6.65 50 1.46 6 510 MoO3/CuI 3/3 0.43 6.13 50 1.30 7.5 620 MoO3:CuI 3:1.5 0.48 8.48 52 2.11 3.7 760
MoO3:CuI 3:3 0.47 7.2 48 1.60 5 765
MoO3:CuI 3:1.5 0.46 7.34 49 1.65 3.8 505
CuI deposition rate: 0.02 nm s1.
Table 2 Optimum results obtained in a preceding study with different ABL configurations [23].
ABL deposition rate (nm s1)
Voc
(V) Jsc
(mA cm2) FF (%)
h (%)
CuI 0.01 0.35 4.9 42 0.73
CuI 0.005 0.50 6.76 53 1.75
MoO3 0.01 0.48 5.43 60 1.58
MoO3/CuI 0.01/0.005 0.50 6.71 57 2.01
even if we cannot exclude some specific arrangement of the first organic layer(s) at the substrate surface [26].
At first glance, all these results confirm that, when deposited alone, CuI must be deposited at a very slow deposition rate, while, when it is co-evaporated with MoO3
it can be deposited at a higher rate without deteriorating OPVC performance.
Therefore, we investigated the properties of the different ABL to put in evidence the modification induced by the co-evaporation process.
3.2 Characterization of the different ABLs In order to relate the photovoltaic performance of the OPVCs to the morphology of the ABL, SEM and AFM topographic studies were performed. Figure 2 presents the surface morphology of the different ABLs. Atfirst glance, it can be seen that the MoO3:CuI ABL is the most homogeneous ABL, while that with MoO/CuI and CuI alone are far less homogeneous. Thisfirst visual impression is confirmed by the AFM study. The variations in morphology and surface roughness of the ITO/ABL structures were examined by AFM and are shown in Fig. 3. The AFM scans were made on 5mm5mm areas for ITO/MoO3/CuI and ITO/MoO3:CuI structures, thefilms being deposited at a deposition rate of 0.01 nm s1. The root mean square thickness (RMS) deduced from these images was 4.7 and 0.38 nm for ITO/
MoO3/CuI and ITO/MoO3:CuI, respectively. Moreover, the maximum peak to peak value was 37.5 nm in the case of superposed films, while it is only 7 nm in the case of co-evaporated layers.
These results help us in explaining the shape of theJ–V characteristics. The slopes at the short-circuit point and at the open-circuit voltage are the inverse values of the shunt resistance (Rsh) and the series resistance (Rs) of the equivalent circuit scheme of a solar cell, respectively. It
can be seen in Table 1 that the smaller shunt resistancesRsh
are obtained with CuI alone, while those obtained with MoO3:CuI are higher than those obtained with MoO3/CuI, which is in good agreement with the SEM and AFM studies, smooth ABLs prevent leakage currents.
Concerning the optical properties of the different ABL, the insert of Fig. 4 shows their transmission spectra. It can be seen that all the curves are similar with a maximum transmission of 96%.
After showing that the MoO3:CuI ABL is smoother than the MoO3/CuI ABL deposited at the same rate of 0.01 nm s1, we checked the structural and optical proper- ties of CuPcfilms deposited onto this ABL, since it is known that CuI modifies the X-ray diffraction spectrum and the absorption properties of CuPc thin films. Therefore, we proceeded to XRD and optical density measurements on ITO/ABL/CuPc structures.
3.3 Properties of the CuPcfilms deposited onto different ABLs If the MoO3:CuI ABL exhibits the most homogeneous structure, it is important to check if the codeposited MoO3:CuIfilms retain the templating proper- ties of the CuI films. Figures 5 and 6 show the surface morphology and the XRD spectra of CuPcfilms deposited onto the different ABLs. One can see in Fig. 5 that the CuPc films deposited onto MoO3 are more homogeneous than those deposited onto CuI. In Fig. 6, the image of the CuPc film deposited onto MoO3/CuI is similar to that of Fig. 5b, i.e., when it is deposited onto CuI. The grains are well faceted, but some pinholes are randomly distributed in the films. In the case of MoO3:CuI ABL, the CuPcfilm, though it shows a similar general appearance to that obtained with CuI, exhibits a smaller grain size, while the CuPcfilm is more homogeneous, without holes similar to those present in the layers deposited on CuI alone.
Figure 2 Surface visualization (SEM) of different structures glass/ITO/ABL, with different ABLs: (a) MoO3, (b) CuI, (d) MoO3/CuI, and (c) MoO3:CuI.
As it appears that the presence of CuI, whatever the ABL configuration, controls the morphology of the CuPc films, it is necessary to check the orientation of the CuPc molecules. As expected, the XRD spectra of CuPc thinfilms deposited onto MoO3(Fig. 5a) or CuI (Figs. 5b and 6a) are quite different. A peak centered at 2u¼6.98is visible in the case of ITO/MoO3substrates. When thefilms are deposited onto CuI, a peak centered at 2u¼27.88is visible, while that situated at 6.98has completely disappeared. It is known that the peak situated at 2u¼6.98can be attributed to the (200) direction of the a-CuPc, while that situated at 27.88cor- responds to an interlayer separation of 0.32 nm, which corresponds to CuPc molecules that lie parallel to the plane of the substrate, while in the other case they are perpendicular to the substrate [17]. Interestingly the same diffraction peak is obtained for CuPc deposited onto MoO3: CuI. Therefore, the conclusion of the XRD study is that,
400 600 800 1000
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
400 600 800 1000 0
20 40 60 80
T (%)
λ (nm) ITO ITO/MoO3 ITO/CuI ITO/MoO3/CuI ITO/MoO3:CuI
O. D. (a. u.)
λ (nm)
Figure 4 Optical absorbance of CuPc films deposited onto different ABL: ̶̶̶ ̶ ̶ITO/MoO3/CuPc, ITO/CuI/CuPc, ITO/MoO3/CuI, and ITO/MoO3:CuI. Insert: Optical transmission of the structures glass/ITOABL, with different ABLs.
Figure 5 Surface visualization and X-ray diffraction spectra of (a) MoO3/CuPc and (b) CuI/CuPc.
Figure 3 AFM images of the topography of (a) ITO/MoO3:CuI and (b) ITO/MoO3/CuI.
when deposited onto CuI, MoO3/CuI or MoO3:CuI, the stacking orientation of the MPc molecules is changed, i.e., the CuPc molecules are parallel to the plane of the substrate.
As is visible in the SEM images, the grain size depends on the ABL. Actually, the full width at half-maximum (FWHM) of the CuPc diffraction peak depends on the ABL, it is 0.2603, 0.2593, and 0.35438for CuI, MoO3/CuI, and MoO3:CuI, respectively. It is known that the grain size increases when the FWHM decreases. The averaged grain size can be estimated using the Scherrer formulae:
D¼Kl=bsinu;
withDthe average grain size in nm,lthe X-ray wavelength, 1.541 Å, K a constant, when b is measured at FWHM, K¼0.9 anduthe diffraction angle [27].
The grain sizes deduced from the XRD spectra are around 200 nm when CuPc is deposited onto CuI or MoO3/CuI and 110 nm when it is deposited onto MoO3:CuI.
The fact that the grain size of the CuPc films deposited onto MoO3:CuI is smaller than that offilms deposited onto
CuI corroborates the conclusion deduced from the SEM study.
Nevertheless, in Fig. 4 one can see that the optical absorption of the CuPcfilms deposited onto MoO3:CuI is similar to that offilms deposited onto CuI or MoO3/CuI. As expected, the optical density of the CuPc films depends strongly on the anode configuration. In the visible region, the absorption band corresponding to the Q band (p–p transition) shows two peaks located at 621 and 695 nm, which is expected fora-CuPcfilms [28]. When deposited onto CuI ABL, there is a broadening of the second peak (704 nm), the Q band surface increases significantly, which increases the absorption and, therefore, the probability of free-carrier formation in OPVCs.
Beyond CuPc, the HOMO value of the electron donors is, in absolute terms, higher than 5 eV, while the work function of ITO is around 4.7 eV. Therefore, the work function of MoO3 and CuI used in the present work is 5.1–5.2 eV [21], the hybrid MoO3:CuI buffer layer appears to improve the band matching at the interface between the anode and the electron donor. However, we cannot exclude the presence of a dipole at the interface. Actually, it was already shown that dipoles can form between a high work function anode and phthalocyanine, which may result in an almost work-function independent charge-carrier extraction barrier [16]. On the other hand, we have shown in Ref. [21]
that the surface energy of MoO3and that of CuI are quite different, which justifies the different properties of the interfaces MoO3/electron donor and CuI/electron donor. In the present work, in order to preserve the influence of the CuI surface properties, we stop the MoO3deposition some seconds (5) before stopping CuI deposition. Since the CuPc molecule orientation depends on the buffer layer, MoO3or CuI, it is easy to check, through simple XRD measurements, the effect of the hybrid buffer layer, MoO3:CuI, on the organic layer properties and it is why we use CuPc as the electron donor.
It was already shown that the molecule orientation of phthalocyanine depends on the substrate roughness [28], smooth underlayer being more favorable to an orientation of the molecules parallel to the substrate plane. Therefore, the ITO/MoO3:CuI hybrid buffer layer being smoother than the ITO/MoO3/CuI buffer layer is more favorable to the orientation via CuI. All this means that, not only does this molecule orientation improve the light absorption, but it also improves the interface properties and the carrier mobility [29, 30].
3.4 XPS study and discussion Therefore, we have shown that the coevaporated MoO3:CuI ABL is as efficient as the CuI or MoO3/CuI ABL with regard to the structural and optical properties of the CuPc films. In Fig. 1 and Table 1, we can see that, not only is the efficiency of the OPVCs using the MoO3:CuI ABL higher than that obtained with MoO3 or CuI, but that it is also higher than that obtained with the double MoO3/CuI ABL. With CuI deposited at 0.01 nm s1, the homogeneity of the CuIfilm Figure 6 Surface visualization and X-ray diffraction spectra of
(a) MoO3/CuI/CuPc and (b) MoO3:CuI/CuPc.
co-evaporatedfilm, which justifies this improvement [24].
When comparing Tables 1 and 2, there is also some efficiency improvement of the cells using MoO3:CuI coevaporated ABL by comparison with MoO3/CuI depos- ited under optimum conditions, i.e., CuI deposited at 0.001 nm s1.
In order to elucidate the influence of the intermixing of MoO3with CuI on the properties of the ABL and how it can induce this OPVCs performance improvement, we proceed to XPS characterization of the different ABLs. The C1s peak of the surface contaminant carbon is situated at 284.8 eV. The I3d and Cu2p photoelectron peaks are shown in Fig. 7, while the Mo3d is shown in Fig. 8. In the case of the binding energy of Cu2p3/2, the peak appears at 932.3 eV (Fig. 7a). This corresponds to the expected value for Cuþof CuI [31, 32]. For copper, the shape of the copper oxide peak is typical [33] and very different from that in Fig. 7a, moreover, each Cu 2p peak could befitted with a single peak, indicating that only one type of copper is present, whatever the ABL is. It can be concluded that Cu is bonded to I only. The case of I is not so simple. It can be seen in Fig. 7b that, here also, the I peaks are symmetric.
However, it is also visible that the binding energy of the peak I3d5/2 varies slightly from 619.75 eV for CuI to 619.54 for MoO3:CuI. While the value measured for CuI corresponds to that given in the literature [31, 33], the small shift of the iodine binding energy means that, in presence of MoO3, there is some modification of the electrical charge of iodine. This hypothesis is strengthened by the modification of the shape of the Mo3d doublet of MoO3in the presence of CuI (Fig. 8). As a matter of fact, the Mo3d signal corresponds to two doublets, thefirst one, situated at
920 930 940 950 960
14000 16000 18000 20000 22000 24000 26000
I (cps)
Binding Energy MoO3:CuI
CuI MoO3/CuI
a-Cu2p HTL
615 620 625 630 635
15000 20000 25000 30000 35000
618 619 620 621
I3d5/2
I (cps)
Binding Energy (eV) MoO3:CuI CuI MoO3/CuI
b-I3d
HTL
Figure 7 XPS spectra of Cu2p (a) and I3d (b).
230 235 240
2000 4000 6000 8000 10000
I (cps)
Binding energy (eV)
Mo 6
Mo5+ MoO3
230,0 232,5 235,0 237,5 240,0 3000
4000 5000 6000 7000
I (cps)
Binding energy
Mo6+
Mo5+ b - Mo3d MoO3/CuI
230,0 232,5 235,0 237,5 240,0 1500
2000 2500 3000 3500 4000
I (cps)
Binding Energy (eV)
c - Mo3d MoO3:CuI
Mo5+
Mo6+
Figure 8 XPS spectra of Mo3d and peaks decomposition for different ABLs: (a) MoO3, (b) MoO3/CuI, and (c) MoO3:CuI.
Mo3d5/2¼232.9 eV and Mo3d3/2¼236.1 eV corresponds to Mo6þ, the second situated at 231.8 and 235.1 eV can be attributed to Mo5þ(Fig. 8) [34, 35]. It is well known that the MoO3 thin films deposited by sublimation of MoO3
powder are deficient in oxygen, which justifies the presence of Mo5þ. However, it can be seen in Fig. 8 that this Mo5þ contribution increases in the presence of CuI, mainly in the case of MoO3:CuI, it increases from 24% for MoO3alone to 45% MoO3:CuI. This is probably due to the fact that, as shown by the shift of the iodine peak, the mixing of MoO3
with CuI improves the probability of interaction between them. It must be noted that such increases of Mo5þdensity due to MoO3:CuI interaction has already been described in a recent paper [22].
Therefore, the XPS study can help us to understand the improvement of the cell efficiency when the MoO3:CuI ABL is used. It is known that the existence of Mo5þin the films induces the presence of states in the bandgap of the oxide. It is now well accepted that these gap states open up a second path for hole collection. Therefore, the presence of these states improves the OPVCs efficiency [35, 36]. Here, it is found that the contact between MoO3and CuI increases the density of these states, as shown by the decompositions in Fig. 8, which justifies a better hole collection and, therefore, a higher short-circuit current in the OPVCs (Fig. 1).
4 Conclusions While it is known that it is difficult to obtain a smooth CuI layer, we show that using a codeposition process of MoO3:CuI allows a smooth anode buffer layer to be achieved. The homogeneous MoO3:CuI ABL are deposited at a deposition rate of 0.01 nm s1, and this deposition rate is twice that necessary to obtain acceptable homogenous CuI films, when it is deposited alone. Moreover, we show that the MoO3:CuI HTL has the same influence as CuI or MoO3/CuI ABL on the structural and optical properties of the CuPc thinfilms, i.e., the CuPc molecules stand parallel to the substrate, which increases the absorption. When introduced in OPVCs, this ABL is more homogeneous than the other ABLs containing CuI, it prevents leakage currents. The efficiency achieved by the OPVCs with the coevaporated MoO3:CuI HTL is higher than that of the OPVCs using different ABLs. The mixing, through the coevaporation, of MoO3with CuI results in an increase of Mo5þ density and, therefore, of gap states.
This increased gap states density improves the hole- collection efficiency, which results in an improvement of the short-circuit current and, therefore, of the OPVCs efficiency.
Acknowledgements The authors would like to thanks CNRST (PPR/2015/9 Ministere, Morocco) for their help.
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