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The choice of metals suitable as the reflective substrate electrode for top-illuminated OPVs is extremely limited. In this chapter the development of a novel substrate electrode for this class of OPV architecture based on an Al (>60 nm)  Cu (8 nm)  AlOx

(~1 nm) triple layer is described. This new electrode offers a reflectivity comparable to that of Al over the wavelength range λ= 400-900 nm and a work function suitable for efficient electron-extraction in OPVs along with high stability towards oxidation. In addition to reporting the advantage of this electrode over Al in model top-illuminated OPVs, the results of a photoelectron spectroscopy study show that the oxidised ~1 nm AlOx layer, formed by evaporation then oxidation of Al on a thermally evaporated Al 

Cu reflective substrate electrode, is sufficient to block oxidation of the underlying Cu. This is remarkable given that the self-limiting oxide thickness of bulk Al metal is greater than 2 nm.

Chapter 3.

74

3.2 Introduction

In conventional OPV architectures electrons are extracted to the external circuit by the reflective metal electrode which is deposited onto a photoactive organic semiconductor layer supported on a transparent electrode, most commonly ITO coated glass.146 For

efficient electron-extraction the reflective metal electrode is preferably a low Φ metal such as Ca or Mg, although parasitic optical absorption by these metals is known to reduce the photocurrent in OPVs, so the highly reflective metals Al and Ag are most often used.72,81 The Φ of both Al; 4.2-4.4 eV;147 and Ag; 4.3-4.7 eV;148 is too high for efficient electron-extraction from the LUMO of most electron acceptor molecules used in OPVs (e.g. PC71BM = 3.9 eV71, C60= 4.2 eV149 and perylene = 3.8 eV150) (Figure

3.1) and so these metals must be used in conjunction with a charge extraction layer to ensure good alignment between the LUMO of the electron acceptor and the Ef of the

metal electrode.151

Figure 3.1: Schematic energy diagram showing the LUMO energy of three widely used

electron acceptor molecules and the energy of the Ef (with respect to the vacuum level)

for both Al and Ag.

M PC 71 BM 60C V P er y e n e A En er gy 4 .3 eV Ag 4 .5 e V 3 . eV 4.2 eV 3 .9 eV H M

75

The two most widely used electron-extracting layers are the wide band-gap partially reduced oxides ZnOx152,153 and TiOx154,155which serve to block the unwanted

extraction of holes by the electrode - a process that competes with the photovoltaic effect in BHJ OPVs because both donor and acceptor phases can contact both electrodes. These interface layers are ordinarily processed by deposition of an organometallic precursor from solution onto the supporting electrode followed by heating in air to form the oxide, a process that is incompatible with metal substrate electrodes that are susceptible to oxidation, such as Al.

Record laboratory efficiencies for top-illuminated OPVs are now approaching those of conventional architecture OPVs.104,129 However, these devices invariably use an

optically thick ( 50 nm) Ag or Al reflective substrate electrode. Unfortunately, Al is extremely susceptible to oxidation and the oxide formed at its surface is self-passivating at a thickness of 2-4 nm.156 This thickness is too great to be transparent to the flow of

electrons, as has been demonstrated for other insulating materials used in OPVs and organic light emitting diodes, such as LiF.157,158 Oxidation of the Al electrode is also

known to be a major degradation pathway in conventional architecture OPVs159,160 and

so, in general, Al is poorly suited as the substrate electrode in top-illuminated OPVs. Whilst Ag is much more stable towards oxidation than Al, its use erodes the cost advantage of OPVs over other types of thin film PVs due to its high cost.161 The pallet

of potential alternatives to Ag for top-illuminated OPVs in which the reflective substrate electrode also serves as the electron-extracting electrode is extremely limited, since most low Φ metals either exhibit unacceptable optical absorption losses72 _{or are too}

easily oxidised. Petoukhoff et al.72 have shown that Cu, which is 1% of the cost of

Ag,161 is a potential alternative to Ag in OPVs. However, Cu is susceptible to oxidation

Chapter 3.

76 Φ of Cu (4.5-4.6 eV)162 is also too high to be suitable as an electron-extracting electrode. It is therefore evident that for top-illuminated OPVs to achieve their full potential there is a need for a new reflective substrate electrode with high stability towards oxidation based on low cost earth abundant materials.

In this chapter a new substrate electrode for top-illuminated OPVs is described based on an Al  Cu  AlOx triple layer structure which offers a reflectivity comparable

to that of Al over the spectral range  = 400-900 nm and low Φ comparable to that of Mg163_{, combined with high stability towards oxidation. The latter is achieved using a}

strategy recently proposed by the Hutter et al. for the passivation of optically thin Cu films on glass; namely passivation of Cu with a 0.8 nm AlOx capping layer.164 In

addition to demonstrating the advantage over Al as the reflective substrate electrode in model top-illuminated OPVs, the results of a photoelectron spectroscopy study of the surface of this composite electrode provide new insight into the nature of the ultra-thin surface passivation layer.