Aqueous dispersions of silver nanowires state an environmentally friendly avenue for highly conductive, yet transparent top electrodes for semi-transparent perovskite solar cells. However, for the well-known chemical instability of halide perovskites upon exposure to water, there are no reports of successful aqueous processing on top of perovskite devices. Here, we show that electron extraction layers of AZO/SnOx [1,2], with the SnOx grown by low temperature atomic layer deposition, provide outstanding protection layers, which even afford the spray coating of AgNW electrodes (sheet resistance Rsh =15 Ohm/sq and a transmittance of 90%) from water-based dispersions without damage to the perovskite.
The layer sequence of the inverted cells is ITO/PTAA/perovskite/PCBM/AZO/SnOx/top-electrode. In devices without the ALD SnOx, aqueous spray processing decomposes the perovskite layers. Interestingly, the direct interface of Ag-NW/SnOx comprises a Schottky barrier, with characteristics strongly dependent on the charge carrier density of the SnOx. For a carrier density below 10^18 cm^-3, S-shaped J-V characteristics are found, that successively vanish upon UV-light soaking. For our low-T SnOx with 10^16 cm^-3, the insertion of a thin interfacial layer with a high charge carrier density (10^20 cm^-3), e.g. 10nm of ITO, is found to afford high performance semitransparent PSCs with an efficiency of 15%. Most importantly, compared to ITO electrodes Ag-NW based electrodes provide a key to achieve a higher transmittance in the IR, which is desirable for tandem Si/PSCs.
[1] K. Brinkmann et al., Nat. Commun. 8, 13938 (2017).
[2] L. Hoffmann et al. ACS Applied Mater. & Interfaces 10, 6006 (2018).
Perovskite solar cells (PSCs) suffer from decomposition of the active material in the presence of moisture or heat. In addition, the corrosion of metal electrodes due to halide species needs to be overcome.[1,2]
Here, we introduce ALD-grown tin oxide (SnOx) as impermeable electron extraction layer (EEL), which affords air resilient and temperature stable MAPbI3 PSCs. Being conductive, SnOx is positioned between the metal electrode and the perovskite. Its outstanding permeation barrier properties protect the perovskite against ingress of moisture or migrating metal atoms, while simultaneously the metal electrode is protected against leaking halide compounds.[2] Therefore, SnOx is also excellently suited to sandwich and protect ultra-thin metal layers (Ag or Cu) as cost efficient Indium-free semitransparent electrodes (SnOx/metal/SnOx) in PSCs. Using photoelectron spectroscopy, we unravel the formation of a PbI2 interfacial layer between a SnOx EEL and the perovskite. The resulting interface dipole between SnOx and the PbI2 depends on the choice of oxidant for ALD (water, ozone, oxygen plasma). SnOx grown by using ozone affords hysteresis-free devices with a stable efficiency of 16.3% and a remarkably high open circuit voltage of 1.17 V.[3] Finally, we fabricated semitransparent PSCs with efficiency >11% (Tvis = 17%) and an astonishing stability > 4500h under ambient conditions (>50% RH) or elevated temperatures (60°C).[4]
[1] Y. Kato et al., Adv. Mater. Interf. 2015, 2, 150019
[2] K. Brinkmann et al., Nat. Commun. 8, 13938
[3] T. Hu et al. Adv. Mat. (submitted)
[4] J. Zhao et al. Adv. Energ. Mat. (in press)
Gas diffusion barriers (GDB) are inevitable to protect sensitive organic materials or devices against ambient gases. Typically, thin-film gas diffusion barriers are insulators, e.g. Al2O3 or multilayers of Al2O3/ZrO2, etc.. A wide range of applications would require GDB which are at the same time transparent and electrically conductive. They could serve as electrode and moisture barrier simultaneously, thereby simplifying production. As of yet, work on transparent conductive GDB (TCGDBs) is very limited. TCGDBs based on ZnO prepared by atomic layer deposition (ALD) have been reported. Due to the chemical instability of ZnO, it turns out that their electrical conductivity severely deteriorates by orders of magnitude upon exposure to damp heat conditions after very short time. We will show that these issues can be overcome by the use of tin oxide (SnO2). Conductivities of up to 300 S/cm and extremely low water vapor transmission rates (WVTR) on the order of 10-6 g/(m2 day) can been achieved in SnOx layers prepared by ALD at low temperatures (<150°C). A sandwich of SnOx/Ag/SnOx is shown to provide an average transmittance of 82% and a low sheet resistance of 9 Ohm/sq. At the same time the resulting electrodes are extremely robust. E.g., while unprotected Cu and Ag electrodes degrade within a few minutes at 85°C/85%rH (e.g. Cu lost 7 orders of magnitude in electrical conductivity), sandwich structures of SnOx/(Cu or Ag)/SnOx remain virtually unchanged even after 100 h. The SnOx in this work will also provide corrosion protection for the metal in case of harsh processing steps on top these electodes (e.g. acidic). We demonstrate the application of these TCGDBs as electrodes for organic solar cells and OLEDs.
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