GIPO newsletter

Enhancing performance of CH₃NH₃PbI₃ perovskite solar cell with low-pressure control via Sandwich Evaporation Technique

Professor Ching-Fuh Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A sandwich evaporation technology (SET) process was established to modify vapor assisted solution process. The MAI dissolved in DMF was coated as a seed layer. Then PbI₂-MAI was evaporated at the condition of thermal evaporation and SET machine, respectively. The concept of MAI-PbI₂-MAI is originated from double mutual diffusion. It significantly promoted the crystallization of perovskite films. In order to further improve the quality, the low-vacuum controlling was applied to SET process to slow down the reaction rate. In this way, we have produced perovskite solar cells with efficiency up to 16%. This is of great significance to the industrial production of perovskite solar cells.

Under low-vacuum condition, the unreacted perovskite in the evaporation process can continue to finish completely, and the sandwich structure in MAI-PBI₂-MAI further promoted the full progress of the reaction. Finally, the highly quality perovskite solar cells were formed. Fig.2. shows that the fully reflected perovskite solar cell exhibits superior performance. The photoelectric conversion efficiency can reach 16%.

Fig.1 The schematic structure of the device

Fig.2 J-V curve of with and w/o low-pressure control for perovskite solar cells

Generation of multi-wavelength yellow-orange lasers using chi⁽²⁾ chirped nonlinear photonic crystals

Professor Lung-Han Peng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Strong optical absorption in the yellow spectral range due to chromophore in Melanin, Hemoglobin (Hb) and Oxyhemoglobin (HbO₂) make clinic benefits by using yellow lasers for medical treatment to ophthalmology and dermatology. Interest has been invoked on yellow lasers as a depletion beam in a STED microscopy to achieve super-resolution bio-images when incorporated with green-yellow fluorescent beads.

Yellow lasers can be achieved via diode pump rare-earth doped crystals such as Dy:YAG (emission wavelength 580nm). However, this approach typically operates the laser crystal with a slope efficiency 10-15%. In comparison, yellow lasers can also be prepared via a nonlinear optical process. It combines an NIR pump laser with a second harmonic generation (SHG) crystal to facilitate the frequency doubling provided the laws of momentum and energy conservation can be maintained. Therefore the SHG efficiency would hinge up the NIR beam intensity, the chi⁽²⁾ nonlinearity of the SHG crystal and the acceptance bandwidth (angle, temperature, wavelength) of the crystal.

In order to engage an efficient scheme of nonlinear green to yellow wavelength conversion, we reported an intra-cavity operation of cascaded optical parametric oscillation-sum frequency generation (OPO-SFG) monolithic chi⁽²⁾ nonlinear photonic crystal (NPC). The chi⁽²⁾ NPC was prepared by incorporating three 15mm-long quasi-phase-matching (QPM-OPO) structures with periodicity of 7.67, 7.68, 7.69 µm in parallel followed by a 5mm-long chirped QPM-SFG section with periodicity varied from 9.85 to 10.34 µm on periodically poled lithium tantalate (PPLT). Such intra-cavity operation of cascaded OPO-SFG nonlinear optics processes enables yellow-orange wavelength conversion from a green 532nm pump laser. A single-side slope efficiency > 17.8% can be easily achieved when placed inside a confocal laser cavity. This work was collaborated with Prof. Azzedine BOUDRIOUA’s group (U. Paris 13, France), Prof. Hiroyuki YOKOYAM’s group (Tohoku Univ., Japan), and Dr. Katrin PASCHKE’s group (Ferdinand Braun Inst. Germany), and received outstanding achievement award from Laser Display and Lighting Conference 2019. The technical abstract can be found in the following website.

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11145/2540345/8th-Laser-Display-and-Lighting-Conference-Abstract-Book/10.1117/12.2540345.full?SSO=1

Corneal Tomograms Using Sub-micron Resolution Optical Coherence Tomography

Professor Sheng-Lung Huang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Accurate diagnosis of corneal pathology and morphological identification of different corneal layers require clear delineation of corneal three-dimensional structures and en face or cross-sectional imaging of palisade of Vogt (POV), neovascularization (NV) or corneal nerves. Here we report a prototype of full-field optical coherence tomography (FF-OCT) system with isotropic sub-micron spatial resolution in the en face and cross-sectional views. It can also provide three-dimensional reconstructed images and a large field of view (FOV) by stitching tomograms side by side. We validated the imaging power of this prototype in in vivo rat and rabbit eyes, and quantified anatomical characteristics such as corneal layer thickness, endothelial cell density and the intensity profile of different layers. This FF-OCT delineated the ridge-like structure of POV, corneal nerve bundles, and conjunctival vessels in rat eyes. It also clearly identified the vessel walls and red blood cells in rabbit model of corneal NV. The findings provided by this FF-OCT are expected to facilitate corneal disease diagnosis and treatment.

With 0.2-µm interval of voxel depth, Fig. 1 shows the sequential results of en face imaging on in vivo rat corneas after a moving average filter of every five frames along the z (depth) direction. Images of superficial squamous epithelial cells, wing epithelial cells and basal epithelial cells were taken at depths of 0–20 μm, 20–40 μm, and 40–53 μm, respectively, from the corneal surface. The interface between Bowman’s membrane (BM) and the epithelial layer was highly-reflective (red arrows in Fig. 1), possibly due to the different refractive indices. As seen from the yellow arrows in Fig. 1, the posterior side of BM (from 53- to 57-μm depth in Fig. 1) merges with the anterior stromal layer.

Reference:

Y. T. Chen, C. Y. Tsai, Y. K. Chiu, T. W. Hsu, W. L. Chen, and S. L. Huang, “En Face and cross-sectional corneal tomograms using sub-micron spatial resolution optical coherence tomography,” Scientific Reports, 8, 14349, 2018.

Fig. 1. FF-OCT en face image sequence of in vivo normal rat cornea from the epithelial layer to anterior stroma. The following layers can be identified at the corresponding corneal depths: superficial squamous epithelial layer at 0–20 μm , superficial-to-wing transitional region at around 20 μm , wing layer at 20–40 μm , wing-to-basal transitional region at around 40μm , and basal layer at 40–53 μm . The red arrows mark the interface between BM and epithelium. The yellow arrows mark the anterior stroma. (Scale bar=10 μm ; value within each image corresponds to the depth in relation to corneal surface.)