A Silver‐Free, Reflective Substrate Electrode for Electron Extraction in Top‐Illuminated Organic Photovoltaics

Abstract

The choice of metals suitable as the reflective substrate electrode for top-illuminated organic photovoltaics (OPVs) is extremely limited. Herein, we report a novel substrate electrode for this class of OPV architecture based on an Al | Cu | AlOx triple-layer structure, which offers a reflectivity comparable to that of Al over the wavelength range 400-900 nm, a work function suitable for efficient electron extraction in OPVs and high stability towards oxidation. In addition to demonstrating the advantage of this composite electrode over Al in model top-illuminated OPVs, we also present the results of a photoelectron spectroscopy study, which show that an oxidised 0.8 nm Al layer deposited by thermal evaporation onto an Al | Cu reflective substrate electrode is sufficient to block oxidation of the underlying Cu by air or during deposition of a ZnO1-x electron-transport layer. This is remarkable given that the self-limiting oxide thickness on Al metal is greater than 2 nm.

High-Performance Silver Window Electrodes for Top-Illuminated Organic Photovoltaics Using an Organo-molybdenum Oxide Bronze Interlayer

“…AlCuAl was chosen as the back reflective electrode, since it has recently been shown to be a stable low workfunction reflective substrate electrode for topilluminated OPVs. 37 For both device types the open circuit voltage (V oc ) achieved with MPA incorporated into the H y MoO 3−x layer is consistent with the highest reported in the literature. For devices based on a PTB7:PC 70 BM BHJ there is a marked improvement in the short-circuit current density (J sc ) upon addition of MPA into the H y MoO 3−x layer, from 8.27 (±0.71) to 10.92 mA cm −2 (±0.28), which translates to a commensurate increase in power conversion efficiency (PCE) (Figure 7a).…”

“…Finally, the performance of these electrodes was evaluated in model top-illuminated OPV devices, with the structure: Al|Cu|Al|BHJ|H y MoO 3– x |Ag (8 nm), where the BHJ is PTB7:PC 70 BM or PCDTBT:PC 70 BM. AlCuAl was chosen as the back reflective electrode, since it has recently been shown to be a stable low workfunction reflective substrate electrode for top-illuminated OPVs . For both device types the open circuit voltage ( V oc ) achieved with MPA incorporated into the H y MoO 3– x layer is consistent with the highest reported in the literature.…”

Towards all-solution-processed top-illuminated flexible organic solar cells using ultrathin Ag-modified graphite-coated poly(ethylene terephthalate) substrates

“…Another promising strategy to overcome the aforementioned limitations is to develop FOSCs with a top-illuminated cell architecture using an opaque, flexible substrate and a reversal of the illumination direction [15], using, e.g. stainless steel foils, paper, or textiles, for making flexible solar cells [16][17][18]. In top-illuminated FOSCs, the cathode or the electron extraction layer (EEL) is below the BHJ, resulting in stable device performance [19].…”

An electrode design rule for high performance top-illuminated organic photovoltaics

“…We have recently reported a means of rendering Al films sufficiently stable towards oxidation for practical application as a substrate electrode in top-illuminated OPVs, using a very thin capping layer of Cu and Al. 21 This triple layer metal electrode is wellmatched to the requirements of top-illuminated OPVs because it offers the advantages of high reflectivity, low metal cost, and the rare combination of high stability towards oxidation and a very low work function; 3.25 eV AE 0.08 eV. 21 In the current study we have exploited this new reflective electrode, which presents an opportunity to investigate the energetics at the interface between a low work function electrode and solution processed organic semiconductor junction without the complexity associated with interfacial chemical reaction and uncontrolled oxidation.…”