To date, antisolvent treatment has become one of the most important means to fabricate high efficiency perovskite solar cells (PSCs); however, the few reported antisolvents have not been analyzed on a uniform platform, and there is hitherto no clear reasoning in the choice of antisolvents toward high performance PSCs. Here, we study the role of the antisolvents in the nucleation kinetics of perovskite solutions and their residual influence on perovskite crystal growth, film formation, and device performance. Through X-ray diffraction analysis on the complicated double mixed perovskite, we qualitatively evaluate the impact of thermal annealing and antisolvent treatment (A.S.T.) on the phase composition and microstructure of the films. By using miscible antisolvents with high boiling point instead of immiscible low boiling point solvents, we obtain homogeneous and almost pinhole-free perovskite films. When using trifluorotoluene (TFT) to replace toluene and chlorobenzene as a novel antisolvent, we achieve a power conversion efficiency (PCE) of 20.3% under optimized device fabrication conditions with a composite perovskite as active layer. The conclusions from this study should assist in establishing reproducible fabrication processes and finding better antisolvent candidates for perovskite solar cells.
We report a 2µm ultrafast solid-state Tm:Lu 2 O 3 laser, mode-locked by single-layer graphene, generating transform-limited∼410fs pulses, with a spectral width∼11.1nm at 2067nm. The maximum average output power is 270mW, at a pulse repetition frequency of 110MHz. This is a convenient high-power transformlimited laser at 2µm for various applications, such as laser surgery and material processing.Ultrafast lasers operating at∼2µm are of great interest due to their potential in various applications, e.g. telecoms 1 , medicine 2,3 , material processing 3,4 and environment monitoring 5 . They can be used for light detection and ranging measurements 5 and free-space optical communications 5 , due to the 2-2.5µm atmospheric transparency window 5 . Because water (main constituent of human tissue) absorbs more strongly at∼2µm (∼100/cm) 3 than at other conventional laser wavelengths (e.g.∼10/cm at∼1.5µm, and∼1/cm at∼1µm) 3 , sources working at∼2µm are promising for medical diagnostic 3 and laser surgery 3 . Currently, the dominant technique for ultrafast pulse generation at 2µm relies on semiconductor saturable absorber mirrors (SESAMs) 6,7 . In-GaAsSb quantum-well-based SESAMs have been used to mode-lock Tm,Ho:NaY(WO 4 ) 2 8 and Tm:Sc 2 O 3 9 lasers, generating 258fs pulses with 155mW output power at 2µm 8 , and 246fs pulses with 325mW output at 2.1µm 9 . However, SESAMs require complex growth techniques (e.g. molecular beam epitaxy 6 ), often combined with ion implantation 8,9 to reduce recovery time 6,7 .Nanotubes and graphene have emerged as promising saturable absorbers (SA), due to their low saturation intensity 10-14 , low-cost 10 and easy fabrication 12,14,15 . With nanotubes, broadband operation can be achieved by using a distribution of tube diameters 10,16 . With graphene, this is intrinsic, due to the gapless linear dispersion of Dirac electrons 12,14 . Ultrafast pulse generation at 0.8 17 , 1 18 , 1.3 19 and 1.5µm [10][11][12]14,[20][21][22][23] was demonstrated with graphene-based SAs (GSAs). Ref.25 reported a 1.94µm Tm-doped fiber laser mode-locked by a polymer composite with graphene produced by liquid phase exfoliation of graphite 14,24 . Compared to solidstate lasers, fiber lasers have some advantages, such as compact geometry and alignment-free operation. However, their output power is typically very low (∼mW 26 ) and their output spectrum generally has side-bands 26 . Solid-state lasers have the advantage, compared to fibre lasers, of sustaining ultrafast pulses with higher output power (typically≥100mW) 6,7 and better pulse quality (e.g. transform-limited with sideband-free profile in
We report a versatile and cost-effective way of controlling the unsaturated loss, modulation depth and saturation fluence of graphene-based saturable absorbers (GSAs), by changing the thickness of a spacer between SLG and a high-reflection mirror. This allows us to modulate the electric field intensity enhancement at the GSA from 0 up to 400%, due to the interference of incident and reflected light at the mirror. The unsaturated loss of the SLG-mirror-assembly can be reduced to∼0. We use this to mode-lock a VECSEL from 935 to 981nm. This approach can be applied to integrate SLG into various optical components, such as output coupler mirrors, dispersive mirrors, dielectric coatings on gain materials. Conversely, it can also be used to increase absorption (up to 10%) in various graphene based photonics and optoelectronics devices, such as photodetectors.Ultrafast mode-locked lasers play an increasingly important role in numerous applications, ranging from optical communications[1] to medical diagnostics[2] and industrial material processing [3]. In particular, ultrafast vertical-external-cavity surface-emitting lasers (VECSELs), also referred to as semiconductor disk lasers (SDLs) [4] or optically pumped semiconductor lasers (OPSLs) [1,2,4], are excellent pulsed sources for various applications, such as multi-photon microscopy [5], optical data communications [4], supercontinuum generation [6] and ultra-compact stabilized frequency combs [2,4]. In such lasers, light propagates perpendicular to the semiconductor gain layers [4]. In contrast to vertical-cavity surface-emitting lasers (VCSELs) [7], a VECSEL consists of an external cavity, formed by high-reflection mirrors, and an output coupler, with typical cavity lengths of a few mm up to tens cm [1,2]. The gain chip generally contains a highly reflective bottom section to reflect the laser and pump light, an active semiconductor gain section in the middle, and an anti-reflective top layer [1,2,4] [17,18]. Furthermore, large-area (compared to a typical laser spot), high quality, single layer graphene (SLG) can be easily grown [19] and integrated in a variety of lasers [16,20]. Graphene has emerged as a promising saturable absorber (SA) for ultrafast pulse generation because of its simple, low-cost fabrication and assembly [16,21,22], ultrafast carrier lifetime [17,18] and broadband absorption [16,23,24]. The unsaturated loss (i.e. the loss of a device at low incident power) of a typical intracavity transmission device based on single layer graphene (SLG) is typically ∼2×2.3% (the factor 2 accounting for the double-pass per round-trip) for the most common linear cavities [25,26]. While this allows to use SLG as SA (GSA) to mode-lock a variety of lasers, such as fiber [21, 22], solid-state[16, 26] and waveguide [27], it poses serious limitations for VECSELs [2]. These typically require a SA mirror with losses<3%[28] because the small-signal gain (i.e. the optical gain for a low-intensity signal where no saturation occurs during amplification) of VECSELs suitable for...
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