Double-Mesoscopic Hole-Transport-Material-Free Perovskite Solar Cells: Overcoming Charge-Transport Limitation by Sputtered Ultrathin Al₂O₃ Isolating Layer

Abstract

The electrically insulating space layer takes a fundamental role in monolithic carbon-graphite based perovskite solar cells (PSCs) and it has been established to prevent the charge recombination of electrons at the mp-TiO 2 /carbon-graphite (CG) interface. Thick 1 μm printed layers are commonly used for this purpose in the established triple-mesoscopic structures to avoid ohmic shunts and to achieve a high open circuit voltage. In this work, we have developed a reproducible large-area procedure to replace this thick space layer with an ultra-thin dense 40 nm sputtered Al 2 O 3 which acts as a highly electrically insulating layer preventing ohmic shunts. Herewith, transport limitations related so far to the hole diffusion path length inside the thick mesoporous space layer have been omitted by concept. This will pave the way toward the development of next generation double-mesoscopic carbon-graphite-based PSCs with highest efficiencies. Scanning electron microscope, energy dispersive X-ray analysis, and atomic force microscopy measurements show the presence of a fully oxidized sputtered Al 2 O 3 layer forming a pseudo-porous covering of the underlying mesoporous layer. The thickness has been finely tuned to achieve both electrical isolation and optimal infiltration of the perovskite solution allowing full percolation and crystallization. Photo voltage decay, light-dependent, and time-dependent photoluminescence measurements showed that the optimal 40 nm thick Al 2 O 3 not only prevents ohmic shunts but also efficiently reduces the charge recombination at the mp-TiO 2 /CG interface and, at the same time, allows efficient hole diffusion through the perovskite crystals embedded in its pseudo-pores. Thus, a stable V OC of 1 V using CH 3 NH 3 PbI 3 perovskite has been achieved under full sun AM 1.5 G with a stabilized device performance of 12.1%.

Triple-Mesoscopic Carbon Perovskite Solar Cells: Materials, Processing and Applications

“…However, it should be noted that optimal spacer thickness can vary when alternative perovskite formulations are used. For Thinner, sputtered Al 2 O 3 spacer layers in the order of hundreds of nm for AVA-MAPbI 3 devices have also recently been reported, proving that that mCPSCs can work without very thick spacer layers as long as the carbon layer is properly insulated [60]. Here, Al 2 O 3 at thicknesses of 10, 20, 40 or 100 nm was sputtered onto screen-printed mesoporous TiO 2 ( Figure 6).…”

“…The statistical PV parameters for each group of devices is shown in (a-d). Adopted with permission from reference [60]. Further permissions related to the material excerpted should be directed to the ACS (https://pubs.acs.org/ doi/10.1021/acsanm.9b02563).…”

Low-temperature carbon-based electrodes in perovskite solar cells

“…31,33 Sputtered layers of Al 2 O 3 as thin as 10 nm, which can also electrically isolate electrons in the conduction band of m-TiO 2 from reaching the back-contact and the respective electrodes against pin-holes to the TCO layer have been demonstrated by us earlier. 135 The latter advantage is particularly useful for large-area device fabrication, to avoid the shunted regions in the modules. Because of the nature of the sputtering process, the inner surface (i.e.…”

“…To date most of the high-efficient HTCB-cells have open circuit voltages around 1 V, 135,137,170,171 despite tremendous efforts to boost this value closer to the cells with HSL and the metallic electrode. In contrast, the versatility of implementation of the LTCB-layer is the most attractive advantage of this approach, as the layer can be simply used to replace the costly metal-based electrodes on top of the HSL to improve cell stability at low cost.…”

Electrochemical Impedance Spectroscopy Analysis of Hole Transporting Material Free Mesoporous and Planar Perovskite Solar Cells

“…Significant research on carbon-based HTM-free PSCs has been dedicated to mesoporous configurations. The multiple mesoporous layers in this type of architecture prevent the recombination of electrons and holes and also avoids pure ohmic shunts [52]. Bhatt et al [46] fabricated an HTM-free mesoscopic carbon-based PSC in ambient air, where polyaniline on FTO was used as a current collector electrode.…”