Toward indium-free optoelectronic devices: Dielectric/metal/dielectric alternative transparent conductive electrode in organic photovoltaic cells
L. Cattin*,1, J. C. Berne`de2, and M. Morsli3
1Universite´ de Nantes, Institut des Mate´riaux Jean Rouxel (IMN), CNRS, UMR 6502, 2 rue de la Houssinie`re, BP 32229, 44322 Nantes Cedex 3, France
2Universite´ de Nantes, LUNAM Universite´, MOLTECH-Anjou, CNRS, UMR 6200, 2 rue de la Houssinie`re, BP 92208, Nantes, 44000 France
3Faculte´ des Sciences et des Techniques, Universite´ de Nantes, LUNAM Universite´, 2 rue de la Houssinie`re, BP 92208, Nantes, 44000 France
Received 3 February 2013, revised 13 February 2013, accepted 22 February 2013 Published online 27 March 2013
Keywordsdielectric materials, metal electrodes, organic solar cells, transparent conductive oxides
*Corresponding author: e-mailjean-christian.bernede@univ-nantes.fr, Phone: 33 251 125 530, Fax: 33 251 125 528
Depending on their resistivity and their transmittance, the thin films of transparent conductive oxide (TCO) are widely used in optoelectronic devices. In2O3:Sn (ITO) is the most widely used TCO in optoelectronic devices. As indium is scarce and ITO is limited in flexibility due to its ceramic structure, many studies have been dedicated to new transparent conductive electrodes.
This review article presents the state-of-the-art concerning the dielectric/metal/dielectric structures and their application as transparent electrodes in organic photovoltaic cells (OPVCs).
First, TCO/Ag/TCO structures were created to achieve higher conductivity than ITO films. Then others dielectrics have been used such as transition-metal oxides (WO3, MoO3, V2O5, etc.), ZnS, etc. Such structures exhibit excellent flexibility, high conductivity, and good transparency. They can be deposited onto substrates at room temperature by simple evaporation
under vacuum. Moreover, it is possible to manage the anode work function through the choice of the dielectric, which can allow them to be used as cathodes or anodes and as intermediate electrodes in tandem solar cells. The properties of the dielectric/
metal/dielectric (D/M/D) structures depend on the thickness of the different layers. The threshold thickness value of the metal film is usually around 10 nm, where the structures change from an insulating state to a highly conductive state. This is attributed to the percolation of conducting metal paths. The transmittance of the films increases when the metal thickness increases up to the percolation thickness, while further increase induces a decrease in transmittance. Finally, the nature and the thickness of the dielectric layers can be chosen as a function of the device properties requested, which is illustrated through different examples of OPVCs.
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1 Introduction Thin films with high optical transmit- tance and electrical conductivity have a multitude of applications in contemporary technologies [1, 2]. Nowadays, besides the domain of buildings and windows for energy savings [1], the main market for transparent electrodes consists in displays and photovoltaic. Actually, there is an increasing demand for transparent conductive electrodes (TCE) for application in large-area optoelectronic devices:
flat-panel displays, photovoltaic cells, touch screens and also, light-emitting devices, and for all kinds of organic
optoelectronic devices. Today, more than 80% of the total area of the TCE used every year consists in indium tin oxide (ITO) thin films. Usually ITO thin films are deposited by magnetron sputtering. Often the ITO films properties are optimized by annealing at 2508C or higher temperatures.
ITO is the most often used as TCE because it presents many advantages such as excellent optical properties (large transmittance in the visible region and large bandgap), good conductivity, and high work function. Therefore, it is the most efficient transparent conductive oxide (TCO) in
applications and materials science
Feature Article
optoelectronic devices and it is nearly systematically used in the case of organic devices. However, it has also some disadvantages such as indium scarcity, aggressive tech- niques of deposits for organic materials and brittleness.
Concerning indium scarcity, it should be noted that the ITO film are very thin, between 100 and 300 nm and, at this moment, there is enough indium on earth to satisfy the demand. However, the cost of indium is very high because it is extracted as a byproduct of Zn mining at very low concentration. Moreover, the price pressure will increase due to the breakthrough of the market of CIGS (CuInGaSe2) photovoltaic panels [3].
As noted above, ITO films are deposited by magnetron sputtering. This technique induces a plasma, which interacts with the substrate and may damage it, if, for instance, the substrate is covered with an organic material, which is the case of organic semitransparent solar cells.
Moreover, usually, organic material does not support high temperatures. To date, there is an increasing demand for flexible optoelectronic devices, achieved on plastic and the
brittleness of ITO makes it incompatible with flexible plastic substrates. Therefore, there is a rising demand for alternative TCEs. This TCE should fulfill some specific requirements such as high optical transmittance in the visible and high conductivity. Its surface work function should be easy to manage. Its constituents should be abundant and have good environmental neutrality. The properties of the electrode should be stable under room atmosphere. The techniques used for its deposition should be as soft as possible and at room temperature. It should be scalable for industrial application, for example, it should be compatible with roll- to-roll deposition technique. Its flexibility and adhesion should be compatible with plastic substrate. Its surface roughness should be small [4]. In fact, in a first approach, it can be thought that an alternative approach to afford a large interfacial area in organic solar cells is the use of a rough layer underelectrode that extends into the active organic layer by using for instance an underelectrode rough layer.
However, it is known that under electrode protrusions can reduce parallel resistance, increasing the leakage current of the current–voltage (I–V) diode characteristics, which results in low short-circuit current and fill factors (FFs) [5, 6].
More specifically, the sheet resistance of the alternative TCE should be around 5Vsq1, its transmittance in the visible should be higher than 80%, its work function should be tunable from around 4 eV to more than 5 eV, its roughness should be of the order of 1–2 nm, or less. The TCE should support a radius of curvature of the order of 1.5 cm without degradation of the performances beyond 10% and of the adhesion to the substrate or to the lower layer.
Numerous alternative TCEs have been explored in order to avoid the use of ITO.
The idea that comes first to mind to realize electrodes without indium is to replace the ITO by another TCO. Zinc oxide (ZnO) and tin oxides (SnO2) are well known. They have a broad bandgap (>3.2 eV) and they are therefore transparent in the visible. After doping, ZnO with Al (AZO) or Ga (GZO), SnO2with F (FTO), they are conductive. After a specific treatment they can be used as electrodes in the organic devices. For these materials several reviews was already published [7–12].
They allow making a fast current situation (inventory of fixtures) according to the wished application [11]. If standard deposition procedures for these films allow TCE electrical and optical requirements to be achieved routinely, they meet some specific criteria of organic material (deposition should be as soft as possible and at room temperature) with more difficulty.
Beside other TCO, conducting polymers, metal grid embedded in polymer, carbon nanotubes, graphene, metal nanowires, semitransparent metal electrodes, and multi- layers structures have been proposed [12]. If, up to now, these alternatives do not have all the potential of ITO, some of them have already the potential to answer the prerequisites for specific applications. Moreover, any limited detrimental effects due to the use of a TCO alternative could be tolerated if the overall cost for manufacturer of the devices were Jean Christian Berne`deis a special-
ist in thin films at the MOLTECH- Anjou Laboratory at L’UNAM (Nantes University, France). He is a solid-state physicist. In 1983, he obtained the ‘‘Doctorat d’Etat’’
degree for research on threshold and memory switching in chalcogenide thin films. Then he oriented his research on renewable energy, especially solar energy.
First, he began work on transition-metal chalcogenides.
Then he investigated CIGS-based cells via buffer layers, working on the replacement of CdS with In2S3. He showed that it was possible to modulate the bandgap of this buffer layer from 2 to 2.9 eV by introducing oxygen.
He is now involved in the field of organic photovoltaic cells. Through organic solar cell investigations he is also involved in the research of surface passivation of transparent conductive oxides films, and more recently he addressed the scope of ITO-free transparent conductive electrodes. He is strongly involved in international collaborations with European and emerging countries of South America and Africa. He is a board member of Mediterranean Scientific Societies.
Linda Cattinis an associate profes- sor at Science Faculty of Nantes University and a member of Institut des Mate´riaux Jean Rouxel (IMN) of Nantes. She prepared her PhD thesis (1989) at Ecole Centrale de Paris in the field of optoelectronic devices.
Her current research focuses on ITO- free organic solar cells.
significantly reduced. For instance, in the case of the growing market of touch screens, the requirements for resistive touch screens are a sheet resistance in the range 300–1000 ohm sq1 a transmission of at least 85% and a good robustness under flexing. Even though ITO easily satisfies the first two conditions, this is not the case of the third, while the poly(3,4-ethylenedioxythiophene):poly- (styrenesulfonate) (PEDOT:PSS) satisfies correctly these three conditions [13]. In fact, PEDOT:PSS electrodes exhibit good flexibility in several types of flexibility tests, including outer/inner bending, twisting, and stretching [14], which made them compatible with roll-to-roll process [15, 16]. The resistivity of PEDOT:PSS is a little too high, different attempts have been dedicated to improve the PEDOT conductivity [17–19], while alternative deposition techniques are probed to avoid spin coating, which is not adequate for industrial processes [20]. Even if PEDOT is the most investigated conductive polymer, some alternatives are also under studies, such as polyaniline [21]. Besides the conductive polymers, other carbon-based electrodes can be used, among which carbon nanotube and also graphene attract interest that is quickly growing. A recent review, by Po et al. [22], gives the state-of-the-art of carbon-based electrodes. In the case of carbon nanotubes one of the difficulties consists in finding the optimum thickness compromise in order to achieve simultaneously high optical transmittance and electrical conductivity. One of the possibilities is to add to the layer of carbon nanotubes, a nanostructured metal grid [23]. Another difficulty encoun- tered is the high surface roughness of the electrodes induced by the carbon nanotubes, even if specific processes such as self-lamination, allow this problem to be limited [24]. An alternative carbon-based electrode for flexible substrates consists in the graphene films, graphene being highly flexible [25, 26]. Different ways can be used to achieve these electrodes but they are not trivial, and, moreover, up to now, the experimental electrical conductivity of the graphene is far from the theoretical one [27]. Since the optical transmittance is very high, one possibility to circumvent the small conductivity is the use of multilayers. For instance, it is possible to optimize the optoelectonic device perform- ance by using layer-by-layer stacked graphene anodes [28].
Recently, it has been shown that inkjet-printed graphene films allow high-performance thin-film transistor (TFT) to be achieved. It shows that inkjet printing of liquid-phase exfoliated graphene is a promising technique for flexible optoelectronic devices realization [29]. However, large-area graphene electrodes are still in need of investigation.
Another possibility is the use of metal, Ag or Cu, nanowires. Actually, solution-processed transparent electro- des consisting of random meshes of metal nanowires that exhibit an optical transparency equivalent to or better than that of metal–oxide thin films for the same sheet resistance [30–33]. Here also, nanowires imply a high surface rough- ness, which can be the source of components dysfunction. In order to prevent such a limitation, it has been shown that metal-wire electrodes fabricated by nanoimprint lithography
can be used. The nanopatterned metal electrodes showed high optical transmittance in the visible range as well as high electrical conductivity [34]. Finally, a metal gird embedded in a conducting polymer allows transparent conductive and smooth electrodes to be achieved [35].
Another classical alternative solution is the use of semitransparent thin metal films. These layers have received renewed interest because of their higher flexibility than that of the TCO, what makes them potential candidates for deposition on plastic substrates. Moreover, the new concept of bilayer thin metal films is also being investigated due to their higher performances than those of equivalent single- layer structures.
The idea of multilayer structures is carried at its peak by the realization and the study of the structures oxide/metal/
oxide. Initially, the idea was to increase the conductivity of the ITO films. Actually, if these layers allow very successful devices in the case of small surface area, in the case of large surface area to be realized, the limit of the ITO conductivity induces the appearance of a series resistance, which reduces the device performances. Therefore, it was proposed to increase the ITO conductivity by introducing a thin metal film sandwiched between two thin ITO films. Then, in order to use ITO free structures, it was proposed to use the other TCO. Finally, in order to avoid the sputtering deposition technique, which is nearly universally used for TCO deposition, and to facilitate the tuning of the electrode work function, oxide/metal/oxide (O/M/O0) and even dielectric/
metal/dielectric (D/M/D0) multilayers structures are pro- posed.
This brief discussion shows that several alternatives in the electrodes of ITO are in the course of investigation.
However, in the present review, only alternative TCE issued from the multilayer structure technology are presented. Their performances are illustrated through their use as transparent electrodes in organic photovoltaic cells (OPVCs). This limitation of the domain of study is due to the fact that it already represents a wide spectrum of possibilities. Further- more, the multilayers structures are achieved by thermal deposition under vacuum, which allows electrodes with reproducible properties to be obtained with high purity.
In addition, vacuum deposition allows facile fabrication of multilayered devices and facile control of the layers thickness.
2 Generality on the contact transparent conductive electrode/organic material Since, in the present review, the performances of the new TCE based on multilayer structures are illustrated through their use as transparent electrode in OPVCs, first we will briefly describe the behavior of the interface electrode/organic material and the solution used when the electrode is a TCO. Two technological routes are mainly used to grow OPV cells, the deposition of polymers and nanoparticles from solution by a spin-coating technique and the vacuum sublimation of small molecules. The former route has given the concept of bulk heterojunction solar cells (BHJ), while the latter gives
the planar heterojunction (PHJ) solar cells family. Although impressive progress was made during recent years [36], the OPVCs efficiency is still in need of improvement. A general problem in organic electrical devices is the transport of charge carriers at the electrode/organic material interface.
Many studies have experimentally demonstrated a strong correlation between the metal work functionFM and the barrier height for hole exchange at anode/organic electron donor (ED) interfaces [37, 38], however, it is now thought that the Schottky–Mott model does not exactly describe the band scheme at this interface. The real energy-level alignment should consider the vacuum-level discontinuity associated with an interface dipole,D, resulting from charge rearrangement upon interface formation [39]. Interfacial phenomena represent a challenge and important area in organic devices science and technology. A variety of interfacial treatments have been applied to both the cathode/organic and the anode/organic interfaces, various, more or less thin, buffer layers have been placed at these interfaces, resulting in varying degrees of devices improve- ment in terms of charge exchange. In fact, theFMvalue is very important. Actually, if the transparent electrode is used as a cathode in optoelectronics its work function must be small. If it is used as an anode it must be high. Therefore, it is necessary to be able to tune readily the work functionFMof the TCE used as electrode [39, 40]. A typical contact electrode/organic material, in an optoelectronic device, is schematized in Fig. 1. The organic material in contact with the anode is an ED in the case of organic solar cells (OPVs) or a hole-transporting layer in the case of organic light-emitting diodes (OLEDs). Indeed if, as shown in Fig. 1, there is a large difference between theFMvalue of the TCE and that of the highest occupied molecular orbital (HOMO) of the organic material, a barrier forms at the TCE/ED interface after contact. This barrier will decrease the efficiency of the hole injection, in the case of OLEDs and of the hole collection in the case of OPVs. Therefore, it is necessary to improve the band matching between the TCO and the ED.
Among the different techniques that can be used to improve the band matching at the contact, it has been shown that the introduction of an anode buffer layer (ABL) at the interface is very efficient. This ABL must be quite transparent and its band structure has to allow a good adaptation of the band structure. This means that it should have a high work function value.
For instance, the PEDOT:PSS, i.e. (poly(3,4-ethylene- dioxythiophene) doped with poly(4-styrenesulfonate);
Fig. 2) is a conducting polymer that has a high FM (>5 eV). It is well known to be as efficient as ABL in optoelectronic devices [41]. Also, transition-metal oxides such as MoO3[42–48], WO3[49, 50] V2O5[51–53] WO3– V2O5 [54], and NiO [55–59] are very efficient. A more original ABL has been used, an ultrathin Au film (0.5 nm).
Gold is used because it has a high work function (5.1 eV) [60]. Generally, these ABL are used with ITO because better results are achieved. However, recently, it has been shown that they are also effective than the other TCO. Successful attempts have been done to replace ITO by FTO [61–63].
The other well-known TCO, zinc oxide, has also been used as electrode in OLEDs, OPVs, and TFT [64, 65]. As in the case of FTO, in order to achieve high-performance devices, it is necessary to introduce an ABL between the electrode and the organic material, when AZO is used as anode. As in the case of FTO, an ultrathin Au film (0.5 nm) allows highly performing multilayer OPVs to be achieved based on the junction CuPc/C60 [66]. As can be seen in Fig. 3, under AM1.5 illumination similarI–Vcharacteristics are obtained, whatever the TCO used.
Up to now, we have presented studies in which the TCO is used as the anode. However, it can also be interesting to use it as cathode. For example, while usually, in forward OPVs structures, the bottom TCE is used as the anode, in inverted OPVs structures, this TCE is used as the cathode (Fig. 4). The interest of such inverted structure is that the OPV cells using this configuration have longer lifetime. In fact, the top electrode is the anode that must have a high work function, such as gold. Therefore, it is not so easy oxidized as a cathode, which must have a low work function such as aluminum (Fig. 5a) [67]. In the inverted configuration (Fig. 5b), if ITO is used as TCO, its work function must be decreased by introducing a thin cathode buffer layer (CBL), Anode–Organic material
h+
ED HTL TCO
PEDOT :PSS
Au
Injection (OLEDs) ΦM
HOMO LUMO
Figure 1 Band structure, before contact, at the anode/organic
electron donor interface. Figure 2 PEDOT:PSS (poly(3,4-ethylenedioxythiophene).
that is to say a low work function layer (Fig. 5b). In that case, ZnO [68], TiO2[69], CsCO3[70] are often used.
When it is a question of substituting a new electrode to ITO, the description above of the properties of the interface electrode/organic material and of the various processes allowing to improve its properties, will serve as a reference to guide the choices of materials constituting this new electrode.
3 Semitransparent metal electrodes Another possibility as an alternative transparent electrode is the use of a semitransparent metal electrode. It is an ‘‘old new’’ idea that was widely investigated some years ago. However, there
is a renewed interest in this topic due to the flexibility of these semitransparent films and to the emergence of the idea of multilayers electrodes. With such semitransparent metal films it is necessary to find the best compromise between the transmittance and the conductivity of the film. Such a compromise can be managed through the film morphology, as shown by a work dedicated to the optimization of organic solar cells with thin film Au as the anode [71]. Au, with its highFMis used as the anode of a BHJ cell. The gold films, at least 10 nm thick, have different morphology. It has been shown by an atomic force microscopy (AFM) study that evaporated gold films exhibit a surface containing peaks with sharp profile. If the peaks induce the presence of small thickness domains, which can improve the transmittance of the film from 38% up to 66%, they can also act as leakage paths for the current and reduce the shunt resistance and hence FF and h. Therefore, it should be possible to manage the morphology of the gold film to optimize the cell performance. The highest efficiency (1.83%) is achieved when the Au film transmittance and sheet resistance are 54% and 9.07Vsq1, respectively (Table 1). However, the surface roughness of the Au films is rather random and difficult to control. The same team has shown that a flexible Ag electrode can be used advantageously as electrode onto plastic substrate [72]. Ag films, 19–23 nm thick have been deposited onto planarized PEN (polyethylene naphthalate) film that was 125mm thick. They have a surface roughness of 2–5 nm, a very small shunt resistanceRsof 4Vsq1and an acceptable transmission, Tmax (380 nm) of 57%. Their response to the curvature test is improved with regard to ITO.
When PEN/anode/PEDOT:PSS/P3HT:PCBM/Al OPV cells are fabricated using Ag/PEN and ITO/PET as anode, the efficiency (1.38%) of the cells with Ag as the anode is higher than that with ITO anode (0.92%; Table 1). Moreover, after submitting these cells to bending test, by rolling into a cylinder of curvature 0.7 mm1, the efficiency decrease is far higher (40–45%) in the case of ITO than in the case of silver.
0,6 0,4
0,2 -100,0
-5 0 5 10
J (mA/cm2 )
V (V)
ITO/Au AZO/Au
ZnO
Figure 3 TypicalJ–Vcharacteristics of TCO/Au(0.5 nm)/CuPc/
C60/Alq3/Al cells, with TCO¼AZO () and ITO (~), in the dark (full symbols) and under illumination of AM1.5 solar simulation (100 mW cm2) (open symbols).
Substrate
Transparent conductive Cathode
Anode buffer layer
Electron donor Electron Acceptor
Cathode buffer layer
Anode
b)
Substrate
Transparent conductive anode
Anode buffer layer
Electron donor Electron Acceptor
Exciton blocking layer
Cathode
a)
Figure 4 Classical (a) and inverted (b) multiheterojunction OPV cell.
Anode
cathode ED
EA C B L ABL
a)
b)
Cathode
Anode ED
EA
ABL CBL
Figure 5 Band scheme before contact: (a) classical and (b) inverted multiheterojunction OPV cell.
Another possibility of optimization is to use bilayer structures such as Cu/Ni [73]. The metal films are deposited by DC sputtering. The threshold thickness of the Cu films to change from insulating film to highly conductive films is
6 nm. The optimum thickness is 7 nm. This Cu film has a high conductivity but, because of its high coefficient of diffusion, it is not stable. The Cu films properties can be stabilized by covering it with an ultrathin Ni film (1 nm). Moreover, the Table 1 Synthesis of the influence of different anodes on the performances of OPV cells.
anode electrical and optical properties
anode OPV cells: BHJ (P3HT :PCBM) or
PHJ (CuPc/C60) performances
T(%) Rsq(Vsq1) substrate Voc(V) Jsc(mA/cm2) FF (%) h(%) ref.
semitransparent metal electrodes
54a 9.07 glass Au/BHJ 0.55 6.75 49 1.83 [71]
55a 4 PEN Ag/PEDOT:PSS/BHJ 0.60 8.8 43 1.38 [72]
80a 12 PEN ITO/PEDOT:PSS/BHJ 0.92 [72]
57a 16 PET Cu/Ni/BHJ 0.50 8.26 59 2.51 [73]
86a 21 PET ITO/BHJ 0.52 9.62 59 3.31 [73]
ITO/M/ITO
87b 4.15 glass ITO/Ag/ITO/PHJ 0.54 8.13 69 3.05 [77]
81b 5.49 glass ITO/Au/ITO/PHJ 0.56 7.04 68 2.66 [77]
89b 15 glass ITO/Ag/ITOPHJ 0.47 2.89 40 0.57 [84]
15 glass ITO/PEDOT :PSS/PHJ 0.50 10.2 42 2.1 [125]
ADO/M/ADO (D¼Al or Ga)
94b 30 glass GZO/PEDOT:PSS/BHJ 0.50 8.00 39 1.57 [86]
87b 6 glass GZO/Ag/GZO/PEDOT:PSS 0.54 9.86 53 2.84 [86]
93b 74 glass AZO/PEDOT:PSS/BHJ 0.48 9.20 31 1.36 [86]
82b 7 glass AZO/Ag/AZO/PEDOT:PSS 0.50 9.41 46 2.14 [86]
86b 4 Glass ZTO/Ag/ZTO/ZnO/BHJ 0.55 7.95 59 2.55 [92]
glass ITO/ZnO/BHJ 0.55 7.79 58 2.46 [92]
82a 8.8 ZTO/Ag/ZTO/MoO3/BHJ 0.55 7.9 61 2.6 [93]
ITO/MoO3/BHJ 0.56 7.8 54 2.3 [93]
TCO/M/O
87b 7 glass SnOx/Ag/SnOx/PEDOT:PSS 0.62 8.11 54 2.7 [98]
glass ITO/PEDOT:PSS/BHJ 0.63 8.37 58 3.1 [98]
85a 3 glass ITO/Ag/MoO3/PHJ 0.45 6 51 1.38 [104]
85a 20 glass ITO/PHJ 0.45 3.9 38.5 0.68 [104]
O/M/O
93a 20 glass ITO/PHJ 0.44 5.3 30 0.68 [110]
85b 8 glass ITO/MoO3/PHJ 0.42 5.3 54 1.3 [110]
85b 8 glass MoO3/Ag/MoO3/PHJ 0.43 4.9 52 1.15 [110]
85b 8 glass MoO3/Ag/MoO3/CuI/PHJ 0.50 6.0 52.5 1.56 [113]
93a 20 glass ITO/CuI/PHJ 0.51 6.9 48.5 1.73 [113]
glass WO3/Ag/WO3/BHJ (70 nm) 0.59 5.86 57.9 2.00 [115]
glass ITO/BHJ (70 nm) 0.59 4.62 57.7 1.57 [115]
glass WO3/Ag/WO3/BHJ(150 nm) 0.59 4.83 57.9 1.65 [115]
glass ITO/BHJ (150 nm) 0.59 6.06 57.6 2.06 [115]
glass MoO3/Ag/MoO3/BHJ 0.59 5.5 61 2.0 [116]
glass MoO3/Au/MoO3/BHJ 0.59 6.7 63 2.5 [116]
glass ITO/BHJ 0.62 8.1 61 3.1 [116]
PET MoO3/Au/MoO3/BHJ 0.60 6.5 62 2.4 [116]
D/M/O
85b 14.5 ZnS/Ag/WO3/BHJ 0.61 6.83 70 2.92 [118]
ITO/PEDOT:PSS/BHJ 0.61 8.12 65 3.22 [118]
13 glass ZnS/Cu/WO3/BHJ 0.60 8.1 70 3.4 [119]
67 glass Cu/WO3/BHJ 0.59 5.3 67 2.1 [119]
12 glass ITO/PEDOT:PSS/BHJ 0.62 9 69 3.9 [119]
a400 nm<l<700 nm;bl¼550 nm.
work function varies from 4.65 to 5.06 eV. A mild annealing at 1808C in room air induces the formation of an ultrathin NiO film at the surface of the structure and therefore it increasesFMup to 5.4 eV. These structures have been used as the anode in OLEDs [74] and organic solar cells such as: Cu(7)/Ni(1)/NiO/P3HT:PCBM/LiCoO2/Al. These struc- tures have a good flexibility and their efficiency is around 2.51%, while that achieved with ITO is 3.31% (Table 1). The difference is mainly due to the higher transmittance of ITO (51% for Cu (7 nm)/Ni (1 nm), 86% for ITO). However, it is difficult to increase the transmittance further without degrading the conductivity and the new concept of multi- layer TCO/M/TCO should be easier to manage.
Actually, in the case of semitransparent metal film if high FF is obtained due to the high conductivity of the electrode, small Jsc values are achieved due to its small transmittance. Therefore, the oxide/metal/oxide structures could allow the high conductivity of the semitransparent metal films to be maintained, while the oxide films will decrease its reflectivity, as we will to see below.
4 Oxide–metal–oxide multilayer structures As shown in Fig. 6, the oxide–metal–oxide multilayer structures (OMO) correspond to very thin metal film sandwiched between two oxide layers. First, this new concept was proposed for increasing of the ITO conductivity [75]. Indeed, if ITO is very efficient as a TCE in the case of small surface area optoelectronic devices, in the case of large-area devices it induces a series resistance that decreases the device efficiency. Therefore, it was suggested to grow ITO/M/ITO structures in order to increase the ITO conductivity. Then, in the search for In-free electrodes, TCO/M/TCO multilayer structures have been investigated. Finally, in order to avoid the use of sputtering deposition, which is the technique used for the TCO thin-film realization, it was proposed to use other oxides that can be produced by soft deposition techniques such as thermal evaporation. Moreover, by broadening the field of the oxide that can be used, it may be easier to manage the work function of the structure through the choice of the oxide.
4.1 Multilayer structures ITO/M/ITO, M¼Ag, Au, Cu As noted above the first studies were dedicated to structures using ITO as TCO. A recent critical review written by Guille´n and Herrero [76] has been dedicated to the, TCO/M/TCO multilayer structures. It is shown in Table 2 of this review that the metal that is most
often used is silver, while the TCO is most commonly ITO. The other metals used are Au and Cu, this is due to the fact that, with Ag, these metals exhibit the smallest resistivity: rAg¼1.6mVcm, rAu¼1.7mVcm, and rCu¼2.4mVcm at 208C. Indeed, if we look at the electric circuit equivalent to the multilayer structure, we have (Fig. 6): 1/R¼1/RMþ2/RO. Therefore, since the resistivity of these metals is more than one order of magnitude smaller than that of TCOs, the resistance of the OMO structures is mainly related to that of the metal film.
In order to discuss more easily the main works dedicated to the structures O/M/O we will first present the universal features encountered in these multilayer structures.
Regarding the electrical behavior of these structures, the typical shape of the variation of the sheet resistance with metal thickness is visualized in Fig. 7 in the case of a MoO3/Ag/MoO3structure. It can be seen that for a threshold thickness the sheet resistance changes from a quite high value to a very small value.
The threshold thickness value corresponds to the percolation of the metal nanostructures. Below this thickness the films are discontinuous, above this thickness they are continuous. In a similar manner, the optical transmittance variation with the metal thickness is schematically described in Fig. 8. It can be seen that, of course, it varies with the metal film thickness. However, it is interesting to note that the transmittance increases with the metal thickness up to a critical value. Then, as expected, it decreases with the increase of the metal film thickness. The initial transmittance increase has been attributed to the evolution of the morphology of the metal film before it becomes continuous. The fairly low transmittance for thin Ag layers was seen to be due to the absorption of the aggregated Ag islands. An increase in Ag thickness leads to an improvement of transmittance because a continuous Ag layer has low absorption. At a Ag thickness of 10–11 nm, it shows the highest transmittance. However, a further increase of Ag thickness resulted in a decrease of transmittance.
Therefore, the optimum Ag thickness in TCO/M/TCO multilayer is 10–11 nm.
RM
Substrate
RO
RO
TCO M
TCO
Figure 6 Equivalent scheme of the O/M/O structures.
11 10 9 8 7 6 5 0,00 2,50x105 5,00x105 7,50x105 1,00x106
11,0 10,5 10,0 0,00 9,5 2,50x10-5 5,00x10-5 7,50x10-5 1,00x10-4 1,25x10-4 1,50x10-4 1,75x10-4 2,00x10-4
Ag (nm)
Resistivity (cm-1 )
Ag film thickness (nm)
Figure 7 Typical variation of the resistivity of a structure O/M/O with the metal thickness (here the structure is MoO3(20 nm)/Ag (xnm)/MoO3(35 nm)).
It should be noted that the metal film thickness, which gives the highest transmittance (Fig. 8), corresponds to the threshold thickness of the sheet resistance curve (Fig. 7).
Since in the majority of the studies devoted to TCO/M/
TCO the TCO is ITO, first, we will present some typical results by means of structures containing In2O3.
For instance, in their work, Jeong et al. [77] compare the performances of structures using gold or silver as metal. The transparent oxide IZO (InZnO) is deposited by dc sputtering from a target of In2O3with 10% of ZnO, while the metals are deposited by evaporation. They show that the metal thickness necessary to achieve the commutation from the resistive to the conductive state is higher when gold is used, which means that gold layers are continuous for thickness above that of silver. After changing to the high conductivity state, the multilayer structure resistivity is the same whatever the metal. This resistivity, r105Vcm, is more than one order of magnitude smaller than that of typical ITO (r¼2104Vcm), which shows that the introduction of a thin metal film is very efficient to increase the electrode conductivity. Regarding the optical properties of these structures, the wavelength of maximum transmittance, lTmax, depends of the metal, lTmax¼450 nm with Ag and lTmax¼523 nm with Au. This wavelength difference is due to the dependence of the plasmon frequency with the metal [78]. Another important difference is that the maximum transmittance is l550 nm¼87.7% with Ag and only l650 nm¼82.5% with Au.
If we use the figure of merit that has been defined by Haacke [79] as follow FTC¼T10/Rsh, with Rsh sheet resistance andT maximum transmittance the difference in transmittance induces a better figure of merit for the structure with Ag: FTC¼60103V1 for 14 nm of silver and FTC¼20103V1with Au. Therefore, the efficiency of the BHJ-OPV cells probed is higher with silver (3.05%) than with gold (2.66%; Table 1). It should be noted that there is no metal diffusion into the oxide, whatever the metal used, Au or Ag. Such a result is reproducible whatever the TCO used [77].
In another study [80] where the same IZO blend is used as TCO and Ag as metal it is shown that the vacuum deposition techniques are compatible with a continuous
roll-to-roll technique, which is very important for industrial utilization. It is also shown that, due to the ductile Ag layer, the response of the IZO/Ag/IZO structure to the curvature test is improved with regards to IZO. It should be noted that in both studies the IZO thickness of all the IZO films was 40 nm. Regarding IZO/Ag/IZO structures deposited on flexible substrate, a systematic study of the critical strain in bending tests shows that it is improved compared to IZO films [81]. It has also been shown that, when a very thin Ag film (5 nm) is introduced between the TCOs films, it is possible to decrease the sheet resistance of the ITO/Ag/ITO and IZO/Ag/IZO structures by electron-beam irradiation.
It decreases from 66 to 20.4Vsq1and 17.8 to 3.5Vsq1, respectively [82]. An original structure, ITO/Ni/ITO struc- ture was also realized by RF magnetron sputtering [83].
However, it is difficult to achieve a high figure of merit (maximum value 2103V1), for 5 nm of Ni, T¼76%
but the sheet resistance is 32Vsq1, while for 10 nm of Ni, the sheet resistance decreases to 22Vsq1, but T is only 54%.
Also, a ITO/Ag/ITO trilayer anode, were deposited by an ion-beam sputter deposition method (IBS). The IBS technique allows separation of the plasma from the deposition chamber, which avoids organic material degra- dation when the film is used as a top or intermediate electrode (Fig. 9). Actually, sputter deposition is accompanied by a plasma, leading to excited atoms and ions, which can cause chemical degradation of the organic molecules. The structures obtained with this technique were used as the anode in OPV cells based on CuPc/C60 heterojunction.
The PCE obtained, even though better than that obtained with an ITO anode deposited by IBS (Table 1), is still far from that usually obtained with such an OPV cell using commercial ITO.
The last study mentioned among the structures containing In, concerns a three-oxide blend, InZnSnOx. If the electrical and optical properties of the structures InZnSnOx/Ag/InZnSnOx(IAI) follow the same behavior as described above, it is interesting to note that it appears that the work function of these IAI multilayer structures is 5.12 eV, measured by UV photoelectron spectroscopy.
Such a highFMvalue allows these structures to be used as efficient anodes in flexible OLEDs [85].
Therefore, not only can the optical and electrical properties of the three-layer electrode be managed, but its work function can also be managed through the oxide composition.
4.2 Multilayer structures TCO/M/TCO, M¼Ag, Au, Cu With the objective to look for In-free TCEs, ZnO can be used as TCO. It is well known that ZnO can be doped with Al and Ga. Therefore, the performances of ZnO/Ag/
ZnO multilayers structures using either AZO or GZO have been compared [86]. All the films are deposited by sputtering. It is shown by X-ray diffraction study that all the films, ZnO and Ag, are crystallized, with a better crystallinity in the case of GZO thin films. The optical and
800 700 600 500 400 0300 10 20 30 40 50 60 70 80 90 100
Transmittance (%)
λ (nm) 8 nm 10 nm 11 nm Ag thickness
Figure 8 Typical variation of the transmittance of a O/M/O struc- ture (here the structure is MoO3(20 nm)/Ag (xnm)/MoO3(35 nm)).
electrical properties of structures using GZO are better than those of AZO films, which leads to a higher figure of merit for the structure using GZO (40103V1 for GZO, 22103V1for AZO). When these structures are used as the anode in P3HT:PBCM BHJ cells, they are covered with PEDOT:PSS since ZnO is a low work function TCO.
The results show that, as expected the best results are achieved with GZO. However, it should be noted that, when the GZO (or AZO) is in direct contact with the organic absorbing layer, the efficiency is not very high due to the small value ofVoc. Another interesting result is that the multilayer structures allows cell efficiencies far higher than that achieved with a single TCO film to be achieved, (2.84 and 1.57%, respectively) which shows the relevance of the TCO/M/TCO concept. Here also, the TCO thickness is 40 nm, while the optimum Ag thickness is 12 nm. Thermal stability of the compositional, optical and electrical properties of AZO/Ag/AZO structures has been investigated up to 4008C [87]. It was shown that these properties are stable and that AZO is an excellent barrier to Ag diffusion.
Copper has also been used as the metal layer. The films were deposited by magnetron sputtering onto PEN substrates (polyethylene naphthalate). While the TCO film thickness was 30 nm, the Cu thickness was varied from 3 to 8 nm. Electrical and optical curves with shapes classical for OMO multilayer structures, with an optimum resistivity of 6.9105Vcm and a maximum transmittance of 88%, are obtained [88]. An interesting result is that the bandgap increases with increasing thickness of the copper layer. The increase in carrier concentration with increased Cu thickness leads to filling of the small density of states of ZnO near the conduction-band minimum, which induces a shift in the absorption edge to higher energies due to the well-known Moss–Burstein effect [89, 90]. Another study gives electrical and optical curves with, as expected, the same shape [91].
However, there is a noticeable difference, the threshold copper thickness value for achieving conducting structures is far higher, since 18 nm of Cu are necessary against 6 nm previously.
Blends of zinc and tin oxides can also be used [92].
ZnSnO3 (ZTO) amorphous films were deposited by RF magnetron sputtering, while Ag was deposited by DC magnetron sputtering. First, ZTO (35 nm)/Ag (xnm)/ZTO
(35 nm) were studied with 0<x<18 nm. The variation of the resistivity of the structures has a metallic like behavior for a silver thickness of 12 nm. After optimization of the silver thickness, the influence of the top oxide blend was studied. The asymmetric Ag/ZTO multilayer structures without the top ZTO layer exhibited a much lower optical transmittance than did the ZTO/Ag/ZTO structures due to the absence of the antireflection effect in bilayer structures.
An XPS study shows that there is neither diffusion of Ag into ZTO nor any interfacial reaction between the Ag and the ZTO layers. After optimization, these multilayer structures have been used as the cathode in ZTO/Ag/ZTO/ZnO/
P3HT:PCBM/PEDOT:PSS/Ag inverted OPV cells. They are covered with ITO/ZnO since they are used as the cathode.
These OPV cells showed power-conversion efficiency (2.55%) comparable to that using ITO/ZnO structures as the cathode. This study shows that these indium-free TCEs are a promising substitute for conventional ITO electrode.
In another study of ZTO/Ag/ZTO multilayer top electrodes, it was shown that the top oxide capping layer prevented diffusion of silver [93], which improves strongly the lifetime of the structures, because it prevents breaking of the silver continuous path. The structures that are used as top electrode in semitransparent OPV cells are deposited by sputtering and therefore it is necessary to protect the organic under layer. In fact, it is shown by transmission electron microscopy that there is an intermixing between ZTO and MoO3 the buffer layer deposited by evaporation above the organic layer. This MoO3layer not only protects the organic film from plasma damage but also optimizes the band matching between the multilayer anode and the ED of the OPV cells. The semitransparent reverse OPV cells, ITO/TiOx/PCBM:P3HT/MoO3/anode, with anode¼ZTO/Al/ZTO or ITO have been illuminated from both sides of the cells. The results show that, for small-area OPV cells, the ZTO/Al/ZTO structures give an efficiency equal to that obtained with ITO, whatever the illumination side of the cell. Moreover, for large-area cells (>2 cm2), it becomes higher for ZTO/Al/ZTO structure. This is due to the higher conductivity of ZTO/Al/ZTO structures.
This result is a nice demonstration of the potential of these ZTO/Al/ZTO multilayer structures.
A study of annealing effects on the properties of TCE shows that the factor of merit of AZO/M/AZO structures, with M¼Au or Cu, is stable up to 3008C, while that of the AZO layer alone degrades continuously [94]. In another study of the effect of annealing on ZnO/Au/ZnO properties it is shown that not only are they stable up to 3008C, but also that their work function increases up to 4.1 eV [95].
By using more sophisticated structures, that is to say ZnO (42 nm)/Ag (15 nm)/ZnO (87 nm)/Ag (15 nm)/ZnO (42 nm) transparent and very conductive electrode (T¼75%, Rsq¼1.6Vsq1) can be achieved [96].
A structure with a factor of merit of 4.5103V1and a work function of 4.55 eV has also been obtained with ZnO-based multilayer structure using the alloy CuSn (90:10 wt%) as metal [97].
Substrate
holder Target
Ion beam
Vacuum chamber
Figure 9 Scheme of an ion-beam sputtering (IBS) system.
Tin oxide (TO) has also been used in multilayer transparent conducting electrodes [98]. A SnOx(40 nm)/Ag (11 nm)/SnOx(40 nm) multilayer structure allows maximum optical transmittance of 87.3% at 550 nm and an electrical resistivity of 6.5105Vcm, corresponding to a figure of merit of 36103V1to be achieved. In order to use these structures as the anode in OPV cells, they are covered with a PEDOT:PSS film. We can see in Table 1 that PCE comparable with that obtained with the commercial ITO reference is achieved. Moreover, it is shown that, while AZO/Ag/AZO structures are corroded after exposition to PEDOT:PSS, in the case of SnOx/Ag/SnOx structures no noticeable change or damage is observed after dropping PEDOT:PSS onto the surface [98].
An original structure, a SiON/Ag/SiON multilayer, was recently described [99]. The authors studied the effect of the oxygen flow rate on the electrical properties of these transparent structures deposited by dc sputtering. They showed that for low oxygen flow rates (below 0.6 sccm) the structures are conductive with a low resistivityr¼2104, while they are insulating for high oxygen flow. This opposing electrical behavior is explained, as expected, to the transition of the inserted Ag layer from a continuous layer to a layer of randomly disconnected islands with increasing flow rates.
In all the multilayer structures reported above, the TCO is n-type, but some p-type oxides are also available. An example of a p-type oxide/M/p-type oxide structure has been recently reported [100]. The p-type oxide is CuAlO2, it was deposited by magnetron sputtering [101]. The variation of the properties of the CuAlO2 (40 nm)/Ag (xnm)/CuAlO2 (40 nm), with the Ag film thickness, showed that for Ag¼18 nmr¼2.8105Vcm, Ag 8 nmT¼89%. How- ever, it should be noted that if these maximum values are promising, they are given for a different optimal Ag thickness.
In all the work dedicated to TCO/M/TCO multilayer structures presented above, the TCO films have been deposited by sputtering. Therefore, if we want to use these structures as top or intermediate electrodes, it will be necessary to protect the substrate covered by organic layers. Sometimes, it is not easy, and even not possible, to introduce a protective layer without degrading the device performances. Therefore, it could be interesting to use a deposition technique less damaging than sputtering. For instance, some attempts have been made to substitute electron-beam deposition for sputtering. The investigation of conductive and transparent Al-doped ZnO/Ag/Al-doped ZnO multilayer coatings deposited by electron-beam evaporation shows that the optimal film thicknesses are 25, 11, and 23 nm, respectively, the figure of merit being used as the criterion of choice (FTC¼2.87102V1). An annealing at Ta¼4008C allows FTC to be increased up to 7102V1, when Ta>5008C, Ag diffuses and Rs increases [102]. However, such an annealing temperature is incompatible with organic thin films. Actually, if we compare the sheet resistance of the multilayer structures to
that of the corresponding silver thin film alone, the values are nearly the same [76], which shows that the multilayer structures resistivity depends only slightly on the oxide resistance (Fig. 6). Therefore, it is possible to use other oxide films than TCO, without degrading the conductivity of the structures. This extension of the list of oxides that can be introduced in the multilayer structures allows, not only to choose oxides that can be simply deposited, but also that have a work function compatible with the future role as an electrode: anode or cathode.
4.3 Multilayer structures O/M/O, M¼Ag, Au, Cu Initially, the idea was to use oxides that can be deposited by simple thermal evaporation. Transition-metal oxides such as WO3, MoO3 can be easily deposited by this technique and therefore have been used as the oxide in OMO structures. First, ITO/Ag/WO3was used successfully as electrode in OLEDs [103]. Similarly ITO(45 nm)/
Ag(10 nm)/MoO3(45 nm) was used as the anode in OPV cells. MoO3being an excellent ABL the PCE of the OPV cell using this anode is double that using ITO alone (Table 1) [104]. Then, WoO3/Ag/WoO3 structures have been used in transparent organic light-emitting diodes (TOLED) using a multilayer oxide as a low resistance transparent cathode [105]. After optimization, the structure WO3 (40 nm)/Ag (14 nm)/WO3(40 nm)/Ag (1 nm), (WAW(Ag)), deposited by evaporation have a maximum transmission Tmax¼80% and a sheet resistance Rs¼12V/sq. This structure deposited by evaporation was used as a top cathode of the TOLED ITO/NPB/Bphen:Li/(Ag)WAW, which justifies the thin top Ag film, WO3 being a well-known ABL. The Ag film is very efficient as a CBL and the TOLED emits light efficiently from both sides. In the same way, a MoO3/Ag/MoO3multilayer structure has been used in an OPV cell. Here, this multilayer structure has been used as a top anode, the OPV cell being an inverted semitransparent cell such as ITO/nc-TiO2/P3HT:PCBM/MoO3/Ag/MoO3. Since MoO3is a well-known ABL, the first MoO3film thick of 1 nm allows the band matching between the anode and the organic layer to be improved. The thickness of the second layer, the silver layer, is optimized to 10 nm, in order to achieve a conductive top electrode. The top MoO3capping layer is used to enhance the light coupling. First, the authors compared the efficiency of OPV cells with either a three- layer structure [106], with a MoO3capping layer thick of 20 nm, or a bilayer structure as the top electrode. When submitted to light illumination better efficiencies are achieved when the top electrode is the three-layer structure, mainly in the case of illumination of the top electrode. This is due to the high reflectance, and therefore the poor transmittance of the bilayer structure. Then, they showed that by increasing further the MoO3capping layer thickness,his enhanced when the cell is illuminated from the ITO side, but it is decreased when it is illuminated from the MAM side, which means that it is possible to manage the semitransparent cell behavior through the thickness of the top MoO3capping layer. The same team has obtained similar
results using MoO3 (1 nm)/Ag (10 nm)/WO3 (0–80 nm) structures, V2O5 (10 nm)/Ag (10 nm)/V2O5 (20–80 nm), and MoO3 (1 nm)/Ag 10 nm)/V2O5 (0–100 nm) structures [107–109], which shows that these results can be extended to other oxides, at least transition-metal oxides.
MoO3/Ag/MoO3 multilayer structures have also been used in classical OPV cells [110]. The cells are based on the junction CuPc/C60such as: anode/CuPc/C60/Alq3/Al. In Fig. 10, theI–Vcharacteristics of OPV cells using different anodes are presented. It can be seen that, if the efficiency of the cell using ITO/MoO3as the anode is slightly higher than that of the cells using the MoO3/Ag/MoO3 multilayer structure, that of cells using bare ITO anode is far smaller (Table 1). Moreover, the shape of the curve shows that, in the case of bare ITO there is a barrier at the electrode/organic material interface (S-shaped curve), while that of the other curves shows that the contact of the other anodes is ohmic [111] which is due to the MoO3. More recently it was shown that, following the recent works on the CuI ABL [112], when the MoO3/Ag/MoO3 multilayer structure are covered by a thin (3 nm) CuI layer the PCE of the OPV cells is significantly improved (Table 1) [113]. As shown in Table 1, even if the performances of the OPV cells are improved in the presence of CuI, the highest efficiency is still achieved with ITO/CuI.
Although many studies have been dedicated to the optimization of the metal layer, not many have been dedicated to the study of the influence of the oxide layers.
There is a red shift of the absorption domain when the MoO3
thickness increases. Mainly in the case of the top capping MoO3 layer, by varying the thickness we move the transmittance spectrum [114]. Therefore, it is possible to tailor the domain of transmission to the maximum absorption range of the organic material (Fig. 11).
Another possibility to manage the transmittance spec- trum of the multilayer structure is to change the metal film.
Actually, the plasmon frequency depends on the metal. For instance it can be seen in Fig. 12 that the wavelength of
maximum transmission in the case of Ag is 450 nm, while it is 600 nm in the case of a Cu metal layer. This means that both structures can be used in the case of CuPc absorbing layer, while only MoO3/Ag/MoO3can be used with P3HT.
Similar structures, where WO3was substituted by MoO3 were also probed as anodes in OPV cells but based on BHJ. It was shown that the PCE achieved depends on the BHJ thickness. When it is only 70 nm thick, it is better than that obtained with ITO, while it is smaller when the BHJ thickness is 150 nm [115].
Inverted solar cells using MoO3as oxide and Au or Ag as metal were realized on glass and flexible substrates. It is shown that both Au and Ag can be used as intermediate metal without affecting the overall device performance, which is within 20% of that of the similar device using an ITO electrode [116]. When deposited onto plastic substrate, the PCE was only reduced by 6% from its original performance after 500 bending cycles with a bending radius of 1.3 cm.
Up to now all the structures described are TCE, which can be used as the anode. It is also possible to achieve cathodes. TiO2/Ag/TiO2structures have been grown by rf sputtering, they exhibit a transparency of 84% at 550 nm and they are stable after annealing at 2008C [69].
If the transition-metal oxides allow good band matching to be achieved when the multilayer structures are used as the anode, a dielectric (D) with higher refractive index can more
400 200
0 -6-200
-4 -2 0 2 4 6
J (mA/cm2 )
V (mV) ITO ITO/MoO3 MAM
Figure 10 TypicalJ–Vcharacteristics of anode/CuPc/C60/Alq3/Al cells, with anode¼ITO (&), (~) ITO/MoO3, and (!) MoO3/Ag/
MoO3 under illumination of AM1.5 solar simulation (100 mW cm2).
35 30 25 20 15 10 5 320 340 360 380 400 420 440 460 480 500
λ (nm) transmittance maximum.
x: Thickness (nm) MoO3 top layer Glass/MoO3 (20 nm)/Ag (10 nm)/MoO3 ( x nm)
Figure 11 Variation of the maximum transmittance wavelength with the thickness of the top MoO3layer.
1400 1200 1000 800 600 0 400
20 40 60 80 100
Transmittance (%)
λ (nm) MoO3/Ag/MoO3
MoO3/Al/Cu/Al/MoO3
Figure 12 Transmittance of typical MoO3(20 nm)/Ag (10 nm)/
MoO3(35 nm) and MoO3/Al/Cu/Al/MoO3.
easily fulfill a zero reflection condition. For instance, while the refractive index of WO3 is 1.95 that of ZnS is 2.35.
Therefore, it can be advantageous to choose other dielectrics than oxides for specific uses. For example, in the case of classical light-emitting diodes, that is, to say when the light is emitted through the glass substrate, it is necessary to minimize the reflection of the transparent bottom layer. If a multilayer structure such as ZnS/Ag/WO3 (ZAW), is introduced in an OLED such as: PET/ZAW/NPB/Alq3/ LiF/Al, the WO3layer allows a good band matching to be achieved between the organic NPB layer and the anode ZAW. The Ag layer, which becomes continuous for a thickness of 10 nm, allows a highly conductive and flexible structure to be achieved. This is due to the high conductivity and the ductility of Ag. The third layer, ZnS, due to its highn value, produces an approach to a zero-reflection effect, as shown by the luminance characteristics of the OLED [117].
The same idea has been used in organic solar cells, ZnS/Ag/
WO3/P3HT:PCBM/Ca/Al, deposited onto flexible substrate [118]. Here also WO3 (30 nm) is an efficient buffer, Ag (12 nm) allows conductive and flexible structures to be obtained and the third ZnS (30 nm) layer, which has no direct role in electrical properties, allows the reflection to be managed. It should be noted that these structures have been submitted to bending tests. After bending, the degradation of the performances of the cells using a ZnS/Ag/WO3multi- layer structure is smaller than that of cells using ITO.
Therefore, a ZAW multilayer structure can lead to ITO-free as well as highly flexible OPV cells, and it can be used as a top electrode as well as a bottom electrode.
The same kind of study has been realized by replacing Ag by Cu [119]. The OPVs performances using these anodes have been compared to that obtained with an ITO/
PEDOT:PSS classical anode, 3.4% has been obtained against 3.9% for the reference (Table 1). It should be noted that the ZnS layer has a beneficial influence on the Cu layer morphology. It allows growth of a homogeneous thin (12 nm) Cu layer.
Finally, oxides are not necessary in the multilayer structures, D/M/D structures such as ZnS/Ag/ZnS structures have been proved to have good electrical and optical properties [120, 121]. The ZnS (40 nm)/Ag (0–20 nm)/ZnS (40 nm) multilayer films have been deposited by ion-beam- assisted deposition (IBAD) [120]. After deposition, the ZnS films are crystallized in the cubic zincblende structure, Ag films are also crystallized. The Ag films become continuous for an Ag film thickness of 6 nm and above. The roughness of the structures decreases when the Ag film thickness increases. For an Ag film thickness of 14 nm the sheet resistance is 77.1Vsq1. After percolation of the Ag islands, the carrier concentration and mobility increase with the Ag thickness. The improvement of the crystallinity and surface smoothness with the increase of Ag thickness is attributed to the bombardment of the Ar ion beam, which decreases the formation possibilities of Ag islands. The average transmit- tance of 80 nm pure ZnS is 80%, and it increases up to 90%
when the Ag thickness increases up to 12 nm, for greater
thicknesses it decreases. Such ZnS/Ag/ZnS structures, when deposited by IBAD onto PET were shown to be highly flexible [122]. The optimum structure exhibits a sheet resistance of 10Vsq1and a transmittance of 92.1% with an Ag thickness of 10 nm. When the silver layer is 12 nm thick the ZnS/Ag/ZnS structures have improved resistance stabilities when submitted to bending, which is attributed to the ductile Ag metal layer.
Therefore, these multilayer structures have the proper- ties required for a good TCE.
To end this review, we present some applications that show the impressive ease of adaptation of the D/M/D0 multilayer structures to the organic devices such as organic tandem solar cells (Fig. 13).
In the first example [123], the intermediate multilayer structure MoO3(10 nm)/Al (1 nm)/ZnO (30 nm) was used as an intermediate electrode in high-efficiency polymer tandem cells utilizing the inverted configuration (Fig. 4). The cell being in series, one side of the electrode (MoO3) permits an ohmic contact with the ED of the bottom cell. At the other side of the electrode, the ZnO allows a good band matching with the electron acceptor of the bottom cell. The Al film serves as a wetting layer for ZnO deposition as well as providing additional recombination sites. The tandem cells efficiency is 5.1% withVoc¼1.20 V andJsc¼7.8 mA cm2. TheVocof the tandem cell is equal to the sum of the two subcells, moreoverJscis slightly higher than that of a cell alone, which can be due to the higher transmittance of the multilayer structure compared to that of the PEDOT:PSS used in subcells. Therefore, the multilayer structure works as an effective interlayer electrode since it possesses high transmittance, good electrical contact for both subcells and presents the ability to protect against solvents.
In another example [124], the system is more or less the same. The intermediate electrode is as follow: MoO3/Ag/Al/
Ca. The CBL is Ca and not ZnO, in order to grow the whole intermediate electrode by deposition under vacuum. Here, Ag is introduced to increase the conductivity of the electrode.
Therefore, these examples show the impressive ease of adaptation of the DMD multilayer structures to the organic optoelectronic devices.
Globally, we can see in Table 1 that the trilayer structures allow PCE of the same order of magnitude as that obtained with ITO to be achieved. More precisely, often,Jscis better
D/M/D’ structure Top cell
Bottom cell Buffer layer 2
Buffer layer 1
Electrode 1 Electrode 2
Substrate
Figure 13 Tandem organic photovoltaic solar cell.
