Interfacial Strain Engineering in Wide-Bandgap GeS Thin Films for Photovoltaics.

Wide-bandgap semiconductors exhibiting a bandgap of ∼1.7-1.9 eV have generated great interest recently due to their important applications in tandem solar cells as top cells and emerging indoor photovoltaics. However, concerns about the stability and toxicity especially in indoor application limit the choice of these materials. Here we report a new member of this family, germanium monosulfide (GeS); this material displays a wide bandgap of 1.7 eV, nontoxic and earth-abundant constituents, and high stability. We find that the little success of GeS solar cells to date is primarily attributed to the challenge in fabricating high-quality polycrystalline GeS films, wherein the high thermal expansion coefficient (α = 3.1 × 10-5 K-1) combined with high crystallization temperature (375 °C) of GeS induces large tensile strain in the GeS film that peels off GeS from the substrate. By introducing a high-α buffer layer between GeS and substrate, we achieve a high-quality polycrystalline GeS thin film that compactly adheres to substrate with no voids. Solar cells fabricated by these GeS films show a power conversion efficiency of 1.36% under AM 1.5G illumination (100 mW cm-2). The unencapsulated devices are stable when stored in ambient atmosphere for 1500 h. Their efficiencies further increase to 3.6% under indoor illumination of 1000 lux.

Strain-engineering the in-plane electrical anisotropy of GeSe monolayers.

As a representative in-plane anisotropic two-dimensional (2D) material, germanium monoselenide (GeSe) has attracted considerable attention recently due to its various in-plane anisotropic material properties originating from the low symmetry of a puckered honeycomb structure. Although there have been plenty of reports on the in-plane anisotropic vibrational, electrical and optical properties of GeSe, the strain effect on those appealing anisotropies is still under exploration. Here we report a systematic first-principles computational investigation of strain-engineering of the anisotropic electronic properties of GeSe monolayers. We found that the anisotropic ratio of the effective mass and mobility of charge carriers (electrons and holes) of GeSe along two principle axes can be controlled by using simple strain conditions. Notably, the preferred conducting direction of GeSe can be even rotated by 90° under an appropriate uniaxial strain (>5%). Such effective strain modulation of the electronic anisotropy of GeSe monolayers provides them abundant opportunities for future mechanical-electronic devices.

Investigation of Oxygen Passivation for High-Performance All-Inorganic Perovskite Solar Cells.

Defect passivation using oxygen has been identified as an efficient and convenient approach to suppress nonradiative recombination and improve the photovoltaic performance of hybrid organic-inorganic halide perovskites (HHPs). However, oxygen can seriously undermine the chemical stability of HHPs due to the reaction of superoxide with protonated organic cations such as CH3NH3+ and [(NH2)2CH]+, thus hindering the deep understanding of how oxygen affects their defect properties. Here we substitute free-proton inorganic Cs+ for organic moiety to avoid the negative effect of oxygen and then systematically investigate the oxygen passivation mechanism in all-inorganic halide perovskites (IHPs) from theory to experiment. We find that, in contrast to conventional oxygen molecule passivation just through physisorption on the surface of perovskites, the oxygen atom can provide a better passivation effect due to its stronger interaction with perovskites. The key point to achieve O-passivated perovskites rather than O2 is the dry-air processing condition, which can dissociate the O2 into O during the annealing process. O-passivated IHP solar cells exhibit enhanced power conversion efficiency (PCE) and better air stability than O2-passivated cells. These results not only provide deep insights into the passivation effect of oxygen on perovskites but also demonstrate the great potential of IHPs for high photovoltaic performance with simplified ambient processing.

Guiding Uniform Li Plating/Stripping through Lithium-Aluminum Alloying Medium for Long-Life Li Metal Batteries.

The uncontrolled growth of Li dendrites upon cycling might result in low coulombic efficiency and severe safety hazards. Herein, a lithiophilic binary lithium-aluminum alloy layer, which was generated through an in situ electrochemical process, was utilized to guide the uniform metallic Li nucleation and growth, free from the formation of dendrites. Moreover, the formed LiAl alloy layer can function as a Li reservoir to compensate the irreversible Li loss, enabling long-term stability. The protected Li electrode shows superior cycling over 1700 h in a Li|Li symmetric cell.

Air-Stable In-Plane Anisotropic GeSe2 for Highly Polarization-Sensitive Photodetection in Short Wave Region.

In-plane anisotropic layered materials such as black phosphorus (BP) have emerged as an important class of two-dimensional (2D) materials that bring a new dimension to the properties of 2D materials, hence providing a wide range of opportunities for developing conceptually new device applications. However, all of recently reported anisotropic 2D materials are relatively narrow-bandgap semiconductors (<2 eV), and there has been no report about this type of materials with wide bandgap, restricting the relevant applications such as polarization-sensitive photodetection in short wave region. Here we present a new member of the family, germanium diselenide (GeSe2) with a wide bandgap of 2.74 eV, and systematically investigate the in-plane anisotropic structural, vibrational, electrical, and optical properties from theory to experiment. Photodetectors based on GeSe2 exhibit a highly polarization-sensitive photoresponse in short wave region due to the optical absorption anisotropy induced by in-plane anisotropy in crystal structure. Furthermore, exfoliated GeSe2 flakes show an outstanding stability in ambient air which originates from the high activation energy of oxygen chemisorption on GeSe2 (2.12 eV) through our theoretical calculations, about three times higher than that of BP (0.71 eV). Such unique in-plane anisotropy and wide bandgap, together with high air stability, make GeSe2 a promising candidate for future 2D optoelectronic applications in short wave region.

GeSe Thin-Film Solar Cells Fabricated by Self-Regulated Rapid Thermal Sublimation.

GeSe has recently emerged as a promising photovoltaic absorber material due to its attractive optical and electrical properties as well as earth-abundant and low-toxic constituent elements. However, no photovoltaic device has been reported based on this material so far, which could be attributed to the inevitable coexistence of phase impurities Ge and GeSe2, leading to detrimental recombination-center defects and seriously degrading the device performance. Here we overcome this issue by introducing a simple and fast (4.8 µm min-1) rapid thermal sublimation (RTS) process designed according to the sublimation feature of the layered structured GeSe. This new method offers a compelling combination of assisting raw material purification to suppress deleterious phase impurities and preventing the formation of detrimental point defects through congruent sublimation of GeSe, thus providing an in situ self-regulated process to fabricate high quality polycrystalline GeSe films. Solar cells fabricated following this process show a power conversion efficiency of 1.48% with good stability. This preliminary efficiency and high stability, combined with the self-regulated RTS process (also extended to the fabrication of other binary IV-VI chalcogenide films, i.e., GeS), demonstrates the great potential of GeSe for thin-film photovoltaic applications.

[Lidong needling therapy combined with functional exercise in treatment of upper limb lymphedema after breast cancer surgery: a randomized controlled trial].

OBJECTIVE: To observe the clinical efficacy of lidong needling therapy (acupuncture technique combined with therapeutic movement of the body) on upper limb lymphedema after breast cancer surgery in combination with functional exercise. METHODS: A total of 73 patients with postoperative lymphedema of breast cancer in the upper limbs were randomized into an observation group (36 cases) and a control group (37 cases). The routine nursing care and functional exercise were given in the control group, twice a day, for about 10-15 min each time, lasting 8 weeks. On the basis of the treatment as the control group, lidong needling therapy was applied to the acupionts on the affected upper limb, i.e. Jianyu (LI 15), Waiguan (TE 5), Hegu (LI 4) and ashi points (the most obvious swelling sites), as well as to bilateral Yinlingquan (SP 9) and Zusanli (ST 36), etc. The needles were retained for 30 min. While the needles retained, the patients were asked to move the affected shoulder to 90° by the sagittal anteflexion and keep it elevated. Simultaneously, the hand on the affected side was clenched and opened slowly and coordinately. Lidong needling therapy was delivered once every two days, three times weekly for 8 weeks. Before and after treatment, the difference of the circumference between the affected and healthy limbs, the score of visual analogue scale (VAS) for swelling and the score of disability of arm, shoulder and hand (DASH) were compared in the patients of the two groups. The clinical efficacy was evaluated. RESULTS: After 2, 4, 6 and 8 weeks of treatment, except for the circumference of the area 10 cm below the cubitel crease in the control group, the differences in the circumferences of the rest parts between the affected and healthy limbs were reduced in comparison with those before treatment in the two groups (P<0.01, P<0.05). After 6 weeks of treatment, in the observation group, for the circumference at the level of hand between the thumb and the index finger and that of the wrist, the differences between the affected and healthy limbs was smaller compared with those in the control group (P<0.05). After 8 weeks of treatment, except for the areas 5 cm below and above the cubitel crease, the differences of circumferences between the affected and healthy limbs in the observation group were smaller than those in the control group in the rest parts (P<0.01, P<0.05). After 8 weeks of treatment, the swelling VAS scores were reduced when compared with those before treatment in the two groups (P<0.05), and the score in the observation group was lower than that in the control group (P<0.01). After 4 and 8 weeks of treatment, DASH scores were reduced in comparison with those before treatment in the two groups (P<0.01). The total effective rate of the observation group was 83.3% (30/36), which was higher than that of the control group (35.1%, 13/37, P<0.01). CONCLUSION: Lidong needling therapy combined with the functional exercise obtains the satisfactory clinical effect on the upper limb lymphedema after breast cancer surgery. This treatment effectively relieves swelling and improves the upper limb function.

In-plane anisotropic 2D Ge-based binary materials for optoelectronic applications.

In-plane anisotropic two-dimensional (2D) materials possess unique in-plane anisotropic physical properties arising from their low crystal lattice symmetry. Among these low-symmetry 2D materials, anisotropic Ge-based binary materials have the advantages of simple binary and earth-abundant compositions, good stability, highly anisotropic physical properties along two principle axes, and wide coverage of bandgaps, enabling use in broadband photodetection from the infrared to ultraviolet region. Here, we review recent progress in in-plane anisotropic 2D Ge-based binary materials, focusing on their anisotropic structural, electrical and optical properties. We then discuss demonstrations of optoelectronic applications related to those anisotropic properties including polarization-sensitive photodetection and polarization-based all-optical switches. Finally, we provide further possible opportunities for this relatively new, but quickly expanding family of materials.

Strain in perovskite solar cells: origins, impacts and regulation.

Metal halide perovskite solar cells (PSCs) have seen an extremely rapid rise in power conversion efficiencies in the past few years. However, the commercialization of this class of emerging materials still faces serious challenges, one of which is the instability against external stimuli such as moisture, heat and irradiation. Much focus has deservedly been placed on understanding the different origins of intrinsic instability and thereby enhancing their stability. Among these, tensile strain in perovskite films is an important source of instability that cannot be overcome using conventionally extrinsic stabilization approaches such as encapsulation. Here we review recent progress in the understanding of the origin of strain in perovskites as well as its corresponding characterization methods, and their impacts on the physical properties of perovskites and the performance of PSCs including efficiency and stability. We then summarize the latest advances in strain-regulation strategies that improve the intrinsic stability of perovskites and photovoltaic devices. Finally, we provide a perspective on how to make further progress in stable and high-efficiency PSCs via strain engineering.

An antibonding valence band maximum enables defect-tolerant and stable GeSe photovoltaics.

In lead-halide perovskites, antibonding states at the valence band maximum (VBM)-the result of Pb 6s-I 5p coupling-enable defect-tolerant properties; however, questions surrounding stability, and a reliance on lead, remain challenges for perovskite solar cells. Here, we report that binary GeSe has a perovskite-like antibonding VBM arising from Ge 4s-Se 4p coupling; and that it exhibits similarly shallow bulk defects combined with high stability. We find that the deep defect density in bulk GeSe is ~1012 cm-3. We devise therefore a surface passivation strategy, and find that the resulting GeSe solar cells achieve a certified power conversion efficiency of 5.2%, 3.7 times higher than the best previously-reported GeSe photovoltaics. Unencapsulated devices show no efficiency loss after 12 months of storage in ambient conditions; 1100 hours under maximum power point tracking; a total ultraviolet irradiation dosage of 15 kWh m-2; and 60 thermal cycles from -40 to 85 °C.

Regulating strain in perovskite thin films through charge-transport layers.

Thermally-induced tensile strain that remains in perovskite films following annealing results in increased ion migration and is a known factor in the instability of these materials. Previously-reported strain regulation methods for perovskite solar cells (PSCs) have utilized substrates with high thermal expansion coefficients that limits the processing temperature of perovskites and compromises power conversion efficiency. Here we compensate residual tensile strain by introducing an external compressive strain from the hole-transport layer. By using a hole-transport layer with high thermal expansion coefficient, we compensate the tensile strain in PSCs by elevating the processing temperature of hole-transport layer. We find that compressive strain increases the activation energy for ion migration, improving the stability of perovskite films. We achieve an efficiency of 16.4% for compressively-strained PSCs; and these retain 96% of their initial efficiencies after heating at 85 °C for 1000 hours-the most stable wide-bandgap perovskites (above 1.75 eV) reported so far.

Weak Interlayer Interaction in 2D Anisotropic GeSe2.

Germanium diselenide (GeSe2) has recently emerged as a new member of in-plane anisotropic 2D materials, notable for its wide bandgap of 2.74 eV, excellent air stability, and high performance in polarization-sensitive photodetection. However, the interlayer interaction in GeSe2 has never been reported, which usually plays an important role in layer-number-dependent physical properties. Here, the interlayer coupling in GeSe2 is systematically investigated from theory to experiment. Unexpectedly, all of density functional theory (DFT) calculations about layer-dependent band structures, cleavage energy, binding energy, translation energy, and interlayer differential charge density demonstrate the much weaker interlayer interaction in GeSe2 when compared with black phosphorus (BP). Furthermore, both thickness-dependent and temperature-dependent Raman spectra of GeSe2 flakes, which exhibit no detectable changes of Raman peaks with the increase in thickness and a small first-order temperature coefficient of -0.0095 cm-1 K-1, respectively, experimentally confirm the weakly coupled layers in GeSe2. The results establish GeSe2 as an unusual member of in-plane anisotropic 2D materials with weak interlayer interaction.