Improved Performance and Stability of Inverted Organic Solar Cells with Sol–Gel Processed, Amorphous Mixed Metal Oxide Electron Extraction Layers Comprising Alkaline Earth Metals

Abstract

Stability of organic photovoltaic devices (OPVs) is a limiting factor for their commercialization and still remains a major challenge whilst power conversion efficiencies are now approaching minimum requirements. The inverted organic solar cell (iOSC) architecture shows promising potential for improving significantly the cell's working lifetime. However, when solution processed ZnO is used as electron extraction layer, an undesirable light‐soaking step is commonly required before the device reaches a non‐permanent maximum performance. This work investigates the use of Sr and Ba doped ZnO films, ZnSrO and ZnBaO, formed by sol‐gel deposition using molecular alkoxide precursor solutions, as electron extraction layers in a model iOSCs with poly [3‐hexylthiophene] (P3HT): [6, 6]‐phenyl C60 butyl acid methyl ester (PCBM) as the active layer. We show that using these ternary oxides the light‐soaking step can be circumvented by preventing a dipole forming between the oxide and the active organic layer as supported by electroabsorption spectroscopy measurements of the device built‐in field. It is suggested that Sr or Ba doping results in suppression/reduction of the oxygen adsorption at mobile oxygen vacancy sites on the metal oxide surface. Like in thin film transistor (TFT) applications, where materials like InGaZnO are rapidly becoming an important technology, the use of amorphous, mixed metal oxides allows improving the performance and stability of interfacial charge extraction layers for organic solar cells.

Overcoming Electrode‐Induced Losses in Organic Solar Cells by Tailoring a Quasi‐Ohmic Contact to Fullerenes via Solution‐Processed Alkali Hydroxide Layers

“…Again, we want to confi rm that these fi ndings are in agreement with the formation of a O A bond, reducing the otherwise oxygen and humidity susceptible dangling bonds. [ 38,41 ] Therefore, no obvious "light-soaking" is observed in devices with alkali hydroxide/metal oxides ETLs. We anticipate that the fi nal OSC product will not rely on UV light treatment due to the interface treatment by solution-processed alkali hydroxides.…”

ZnO cathode buffer layers for inverted polymer solar cells

“…Besides aluminium and the group III elements, the group-II elements (alkaline earth), such as magnesium (Mg), strontium (Sr) and barium (Ba) doped ZnO ( named as MZO, ZnSrO and ZnBaO, respectively) layers also have been introduced into inverted PSCs. 48,202 Besides the metals, the fullerene derivatives have been used to dope ZnO for forming an efficient CBL in inverted PSCs. 49,114 The photovoltaic performance of inverted devices based on these doped ZnO CBLs in literature is summarized in Table 2.…”

“…48,202 Pachoumi et al reported an improved performance and stability of inverted PSCs by employing the sol-gel processed amorphous Sr and Ba doped ZnO CBLs prepared at relatively low temperature. 202 The inverted PSCs based on Sr and Ba doped ZnO CBLs exhibited higher performance and better stability than the devices with pure ZnO CBLs. In addition, the commonly requirement of light soaking for reaching a maximum photovoltaic performance of inverted device based on i-ZnO CBLs was successfully circumvented.…”

Amorphous oxide alloys as interfacial layers with broadly tunable electronic structures for organic photovoltaic cells

“…Additionally, the localized O 2p based valence bands, which are typically deep and poorly dispersed, should block hole injection from typical organic donor HOMOs (43). Although compositional investigations on amorphous oxides have been reported in Zn 1-x Ba x O, Zn 1-x Sr x O, and Ga 1-x Zn x O systems (44,45), systematic band alignment tuning via doping such oxides has not been demonstrated.…”