Search

Benefiting from good solubility of metal halide perovskites, low-temperature solution processes (including one- and two-step spin-coating) have become the most common approach for perovskite solar cells (PSCs). However, one confusing issue is that what are specifically effective strategies for the one-step process may not be applicable for the two-step process, and vice versa. Herein, the PSCs with the same perovskite composition and device configuration were prepared by one- and two-step processes, respectively. The results indicated that high performance PSCs with comparable power conversion efficiency over 22% were achieved by both methods, while the detailed photovoltaic parameters varied greatly depending on the one- or two-step process. Compared to the one-step counterpart, the two-step processed PSCs exhibit lower open-circuit voltage and fill factor, but superior short-circuit current, which was in-depth interpreted in terms of the crystalline growth mode, optical properties, defect types, and carrier transport mechanisms related to a perovskite film surface (including a top and a bottom surface). Understanding the root cause for such differences would be central toward identifying what is really crucial for further producing high performance PSCs.

Hydrophobic organic fluoropolymers (HOFPs) with excellent hydrophobic, heat-resistant, and sunlight-transparent properties were synthesized by emulsion polymerization. The HOFP layer was inserted between a (FAMA)Pb(IBr)3 active layer and a hole transport layer in perovskite solar cells (PSCs). The performance of the resulting PSC devices depends highly on the thickness of the HOFP layer. Under optimized HOFP thickness, a moderate steady power conversion efficiency (PCE) of 16.9% was achieved. Remarkably, the optimized PSCs without any encapsulation exhibit outstanding shelf stability under ambient conditions, and the PCE could maintain 80% of its initial value after 2400 h (100 days), which was among the ever reported best stability whereas, the reference device without HOFP shows rapid severe degradation after only a few days. The significantly improved stability of PSCs was mainly ascribed to the impermeable barrier properties of the HOFP layer, which protect the perovskite active layer against moisture and oxygen from the ambient atmosphere.

We demonstrate an approach for patterning a quasi-two dimensional electron gas (q-2DEG) at the interface of LaAlO3 (LAO) and SrTiO3 (STO) utilizing a structured Al capping layer. The capping of Al enables the formation of q-2DEG at the interface of 1–3 unit cells (uc) of LAO on STO, which was originally insulating before capping. The properties of the q-2DEG induced by the Al capping layer are essentially the same as those of q-2DEG without Al. Therefore, we can pattern q-2DEG by simply patterning the Al film on LAO (2 or 3 uc)/STO using a one-step liftoff process. Our approach circumvents the difficulty of direct patterning of oxide materials and provides a simple and robust patterning method for future device applications based on complex oxide interfaces.

We have systematically studied the magnetoresistance (MR) of Alq3-based organic spin valves using as-grown La0.67Sr0.33MnO3 (LSMO), annealed LSMO, and La0.67Ca0.33MnO3 as the bottom electrode. A giant enhancement of MR ratio (more than one order of magnitude) is observed when the optimal annealed LSMO is used, and the measured MR can be as high as 2.2% at room temperature. Below ∼100 K, the temperature dependence of the normalized MR is almost identical with these three electrodes despite the strong difference in Curie temperature (from 250 K to 360 K). We attribute this similar MR temperature dependence to the spin relaxation in Alq3.

We present a systematical study on the electronic transport properties of the insulating LaAlO3 (3 unit cells)/SrTiO3 interfaces capping with thin layers of La1−xSrxMnO3, whose formal polarization is continually tuned by Sr doping. When the Sr doping is lower than 2/3, the LaAlO3/SrTiO3 interfaces show metallic behaviors. The carrier mobility is almost independent on the Sr doping for metallic interface, indicating that the capping layer does not change the density of the oxygen vacancies and the interface intermixing. However, the sheet carrier densities monotonically decrease as increasing Sr doping, which is ascribed to the decrease of the La1−xSrxMnO3 formal polarization. These results strongly support the intrinsic mechanism of the polar catastrophe model and provide a new approach to tailor the interface states of complex oxide heterostructures.

Magnetic properties of epitaxially grown ultrathin La2/3Sr1/3MnO3 (LSMO) films down to a thickness of one unit cell (u.c.) have been systematically investigated by studying their magnetic behaviors with Co capping layers. For LSMO thickness below 3 u.c., the Co/LSMO bilayers exhibit strong exchange bias (EB) effects after field cooling, suggesting the existence of antiferromagnetic (AFM) phase at the interfaces in ultrathin LSMO. The presence of exchange bias effect for the bilayer with 1 u.c. thick LSMO further demonstrates that the AFM ordering of the LSMO is C-type AFM ordering structure. For 10 u.c. LSMO, the magnetic properties are clearly not altered by the capping Co film, suggesting that the observed phenomena are caused by the intrinsic properties of LSMO.

The correlation of spin injection efficiency and interfacial resistance is investigated in La0.67Sr0.33MnO3 (LSMO)/Alq3/Co organic spin-valve devices. When a thin layer of copper phthalocyanine (CuPc) is inserted between LSMO and Alq3, the magnetoresistance (MR) of the device decrease to only ∼0.4% at 50 K, in sharp contrast to ∼6% MR ratio at the same bias voltage for the device without CuPc interlayer. Meanwhile, the electrical resistance decreases by one order of magnitude, indicating that the interface barrier height is reduced. These results reflect that a strong correlation between the significant decrease of spin injection efficiency at LSMO/CuPc interface and the reduced interfacial resistance. The findings indicate that the conductivity mismatch problem is applicable to organic materials and the interfacial resistance has important impact on the spin injection efficiency.

We have fabricated assemblies of molecular junctions comprised of superparamagnetic Fe3O4 nanoparticles self-assembled with alkane molecules of different lengths as the spacer. The electrical resistance increases exponentially over nearly two decades as the molecular length varies from 0.7 to 2.5 nm, indicating that electrons tunnel through the molecules that are chemically bonded with Fe3O4 nanoparticles. Up to ∼21% room-temperature magnetoresistance is observed. Remarkably, the tunneling magnetoresistance ratio stays nearly independent of molecular length, which entails room-temperature spin-conserving transport in organic molecules.