Flexible perovskite solar cells based on indium-free AZO/Ag/AZO multilayer transparent electrodes

Abstract

<p indent=0mm>Perovskite solar cells (PSCs) fabricated on transparent polymer substrates are considered to be one of the most disruptive competitors in the solar cell industry for their favorable traits, including high efficiency, flexibility, light weight, portability, and compatibility with curved surfaces. More importantly, PSCs based on transparent polymer substrates could dramatically lower down the processing cost, by introducing high-throughput roll-to-roll manufacturing and low-temperature energy-saving technological processes. However, due to the low glass transition temperatures of common transparent polymer substrates, it is difficult to prepare the traditional transparent electrodes such as indium tin oxide (ITO) and fluorine doped SnO₂ (FTO) and the electron transport layer with efficient charge transport to meet the specifications of flexible PSCs as on rigid glass substrate, which has become a major bottleneck to improve the device performance of flexible PSCs based on transparent polymer substrates. In this paper, multilayer films of AZO/Ag/AZO are deposited onto heat-sensitive polyethylene terephthalate (PET) substrate by radio frequency magnetron sputtering technique at room temperature to develop indium-free flexible transparent conductive films (TCFs), which are utilized as transparent electrode to prepare flexible PSCs with planar regular structure. The effects of Ag mid-layer thickness on microstructure and photoelectric properties of the AZO/Ag/AZO multilayer films are investigated by scanning electron microscopy, UV-VIS transmission spectra and four-point probe resistance measurements. It is found that when Ag layer thickness is less than <sc>5 nm,</sc> Ag layer has the island structure and the electrical connection is very poor, so that the samples have a higher sheet resistance. As the thickness of Ag layer is increased, the Ag layer changes gradually from discontinuous structure to continuous structure, the sheet resistances of the samples decrease drastically. However, when the thickness of Ag layer is beyond <sc>9 nm,</sc> the sheet resistances of the samples decrease slowly with further increasing the thickness of Ag layer. The analysis of optical transmission spectra indicates that with increasing the Ag layer thickness, the average transmittance in the visible light range of the AZO/Ag/AZO multilayer films increases first and then decreases. The Haacke figure of merit has been calculated for the AZO/Ag/AZO multilayer films. The best Haacke figure of merit of 2.3×10^–2Ω^–1 is obtained for the 11-nm-thick Ag mid-layer sample, which has an average transmittance of 83.3% in the spectral range of <sc>400–800 nm</sc> and a sheet resistance of 7.0 Ω/sq. In addition, the bending test indicates that the developed AZO/Ag/AZO multilayer TCFs also have excellent mechanical flexibility, and the relative change rate of sheet resistance is less than 8% after 1000 bending cycles with a <sc>5-mm</sc> bending radius. The flexible PSCs are fabricated with a configuration of Ag/MoO₃/spiro-OMeTAD/perovskite/SnO₂/AZO/Ag/AZO on PET substrates. The mixed cation and mixed halide perovskite films are prepared by a single step inverse solvent crystallization method. Considering that SnO₂ layer presents advantages of low-temperature processability and modulating the band level mismatch at the interface between the AZO/Ag/AZO and the perovskite layer, SnO₂ electron transport layer is fabricated by spin-coating the aqueous soluble SnO₂ nanoparticles and annealing at a low temperature of 120°C. The flexible PSC based on the optimized AZO/Ag/AZO transparent electrode achieves power conversion efficiency (PCE) of 11.9%. At a bending radius of <sc>10 mm,</sc> flexible PSC based on AZO/Ag/AZO multilayer electrodes maintains 95% of their initial PCE even after 100 bending cycles.