Intrinsic zinc oxide (ZnO) is widely used as an electron extraction layer (EEL) for inverted polymer solar cells. Despite the excellent device performance, a major drawback for large area production is its low conductivity. Using microscopic simulations, we derived a technically reasonable threshold value of 10−3 S cm−1 for the conductivity required to overcome transport limitations. For conductivity values typical for ZnO we observed the interface layer thickness restriction at only a few tens of nanometers, either as a fill factor drop due to serial resistance, eventually accompanied by a second diode behavior, or by the need for light soaking. Higher conductive aluminum-doped zinc oxide (AZO), which was introduced earlier, meets the desired conductivity threshold, however, at the cost of high temperature processing. High annealing temperatures (>150 °C) significantly improve the electrical properties of ZnO, but prohibit processing on plastic substrates or organic active layers. Here we report on AZO layers from a sol–gel precursor, which has been already reported to give sufficiently high conductivities at lower processing temperatures (<150 °C). We investigate the influence of different precursor compositions on the electrical properties of the thin films and their performance in inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells. Low temperature AZO layers with thicknesses up to 680 nm maintained comparable performance to devices with thin AZO layers.

In this article, we demonstrate a route to solve one of the big challenges in the large scale printing process of organic solar cells, which is the reliable deposition of very thin layers. Especially materials for electron (EIL) and hole injection layers (HIL) (except poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) (PEDOT:PSS)) have a low conductivity and therefore require thin films with only a few tens of nanometers thickness to keep the serial resistance under control. To overcome this limitation, we investigated inverted polymer solar cells with an active layer comprising a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) with solution processed aluminum-doped zinc oxide (AZO) EIL. Devices with AZO and intrinsic zinc oxide (i-ZnO) EIL show comparable efficiency at low layer thicknesses of around 30 nm. The conductivity of the doped zinc oxide is found to be three orders of magnitude higher than for the i-ZnO reference. Therefore the buffer layer thickness can be enhanced significantly to more than 100 nm without hampering the solar cell performance, while devices with 100 nm i-ZnO films already suffer from increased series resistance and reduced efficiency.Graphical abstractHighlights► We made solution-processed polymer solar cells in the inverted structure. ► We compared doped and intrinsic zinc oxide (ZnO) as electron injection layer (EIL). ► Al doped ZnO shows three orders of magnitude higher conductivity than intrinsic ZnO. ► The impact of these EILs with increased thickness on the efficiency was determined. ► We demonstrate efficient solar cells with over 100 nm thick Al doped ZnO layers.

Intrinsic zinc oxide (ZnO) is widely used as an electron extraction layer (EEL) for inverted polymer solar cells. Despite the excellent device performance, a major drawback for large area production is its low conductivity. Using microscopic simulations, we derived a technically reasonable threshold value of 10−3 S cm−1 for the conductivity required to overcome transport limitations. For conductivity values typical for ZnO we observed the interface layer thickness restriction at only a few tens of nanometers, either as a fill factor drop due to serial resistance, eventually accompanied by a second diode behavior, or by the need for light soaking. Higher conductive aluminum-doped zinc oxide (AZO), which was introduced earlier, meets the desired conductivity threshold, however, at the cost of high temperature processing. High annealing temperatures (>150 °C) significantly improve the electrical properties of ZnO, but prohibit processing on plastic substrates or organic active layers. Here we report on AZO layers from a sol–gel precursor, which has been already reported to give sufficiently high conductivities at lower processing temperatures (<150 °C). We investigate the influence of different precursor compositions on the electrical properties of the thin films and their performance in inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells. Low temperature AZO layers with thicknesses up to 680 nm maintained comparable performance to devices with thin AZO layers.