Gradated Mixed Hole Transport Layer in a Perovskite Solar Cell: Improving Moisture Stability and Efficiency

We demonstrate a simple and facile way to improve the efficiency and moisture stability of perovskite solar cells using commercially available hole transport materials, 2,2′,7,7′-tetrakis-(<i>N</i>,<i>N</i>-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) and poly­(3-hexylthiophene) (P3HT). The hole transport layer (HTL) composed of mixed spiro-OMeTAD and P3HT exhibited favorable vertical phase separation. The hydrophobic P3HT was more distributed near the surface (the air atmosphere), whereas the hydrophilic spiro-OMeTAD was more distributed near the perovskite layer. This vertical separation resulted in improved moisture stability by effectively blocking moisture in air. In addition, the optimized composition of spiro-OMeTAD and P3HT improved the efficiency of the solar cells by enabling fast intramolecular charge transport. In addition, a suitable energy level alignment facilitated charge transfer. A device fabricated using the mixed HTL exhibited enhanced performance, demonstrating 18.9% power conversion efficiency and improved moisture stability

A Benzodithiophene-Based Novel Electron Transport Layer for a Highly Efficient Polymer Solar Cell

We designed and synthesized a novel conjugated polyelectrolyte (CPE), poly­{3-[2-[4,8-bis­(2-ethyl-hexyloxy)-6-methyl-1,5-dithia-<i>s</i>-indacen-2-yl]-9-(3-dimethylamino-propyl)-7-methyl-9H-fluoren-9-yl]-propyl}-dimethyl-amine (PBN). We employed PBN as an electron-transporting layer on a ZnO layer and constructed a highly efficient, inverted structure device consisting of a mixture of poly­({4,8-bis­[(2-ethylhexyl)­oxy]­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene-2,6-diyl}­{3-fluoro-2-[(2-ethylhexyl)­carbonyl]­thieno­[3,4-<i>b</i>]­thiophenediyl}) (PTB7) and PC₇₀BM, achieving a high power conversion of up to 8.6%, constituting a 21.1% improvement over the control device performance (7.1%) prepared without a PBN layer. This result was ascribed to the reduced interfacial resistance and the improved charge transport and collection through the PBN electron transport layer

Switchable Photovoltaic Effects in Hexagonal Manganite Thin Films Having Narrow Band Gaps

Ferroelectric photovoltaics (FPVs) are being extensively studied owing to their anomalously high photovoltages, coupled with reversibly switchable photocurrents. However, FPVs suffer from their extremely low photocurrents, which is primarily due to their wide band gaps. Herein, we present a new class of FPV by demonstrating (i) a nearly optimum band gap of ∼1.55 eV and (ii) the ferroelectric polarization switching in the epitaxial hexagonal manganite thin films, <i>h</i>-RMnO₃, where R = Lu and Y. According to the thickness-dependent photovoltaic measurements, the ITO/<i>h</i>-LuMnO₃/Pt solar cell shows a power conversion efficiency of ∼0.11% when the thickness of the <i>h</i>-LuMnO₃ layer is ∼150 nm. We have shown that the PCE is 1–3 orders higher than those of classical FPVs such as undoped Pb­(Zr,Ti)­O₃ and BiFeO₃ under the standard AM 1.5G illumination. We have further elucidated that the switchable photovoltaic effect dominates over the nonferroelectric internal field effect

Green-Solvent-Processable, Dopant-Free Hole-Transporting Materials for Robust and Efficient Perovskite Solar Cells

In addition to having proper energy levels and high hole mobility (μ_h) without the use of dopants, hole-transporting materials (HTMs) used in n-i-p-type perovskite solar cells (PSCs) should be processed using green solvents to enable environmentally friendly device fabrication. Although many HTMs have been assessed, due to the limited solubility of HTMs in green solvents, no green-solvent-processable HTM has been reported to date. Here, we report on a green-solvent-processable HTM, an asymmetric D–A polymer (asy-PBTBDT) that exhibits superior solubility even in the green solvent, 2-methylanisole, which is a known food additive. The new HTM is well matched with perovskites in terms of energy levels and attains a high μ_h (1.13 × 10^–3 cm²/(V s)) even without the use of dopants. Using the HTM, we produced robust PSCs with 18.3% efficiency (91% retention after 30 days without encapsulation under 50%–75% relative humidity) without dopants; with dopants (bis­(trifluoromethanesulfonyl) imide and <i>tert</i>-butylpyridine, a 20.0% efficiency was achieved. Therefore, it is a first report for a green-solvent-processable hole-transporting polymer, exhibiting the highest efficiencies reported so far for n-i-p devices with and without the dopants