GIPO newsletter

High Efficiency Organic – Inorganic Hybrid Solar Cells by employing flexible film-Silicon with Hierarchical Surface

Professor Ching-Fuh Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Crystalline silicon (c-Si) photovoltaics (PVs) have dominated PV industry for several decades. However, current crystalline-silicon solar cells based on thick wafers have little flexibility, so limiting their applications on curved or bendable surfaces. Highly flexible and light-weight solar energy devices have received significant attention owing to their potential inspiration provided in various modern applications such as robotics, portable electronics, flexible displays, building windows, curved building structures and other systems. In recent years, fabulous progresses have been made in developing flexible and thin-film solar cells that can be potentially massively deployed.

On the other hand, conventional silicon solar cells have thickness in the range of 180-200 μm, which limits their flexibility. Therefore, exploring flexible single-crystalline-film-silicon solar cells without exploiting highly expensive and complicated methods are very attractive. We demonstrate a high-efficiency organic – inorganic hybrid solar cell Si/PEDOT:PSS with hierarchical surface composed of silicon nanoholes and micro-desert textures, showing a power conversion efficiency of 12%. Such Si/PEDOT:PSS hybrid solar cells exhibit high J_sc of 36.81mA/cm², V_oc of 0.492V, FF of 66.3%, and thus PCE of 12.00%. The hierarchical surface provides an excellent light absorption over 97% for the spectral range of 300 nm to 1100 nm with the thickness of 60µm due to internal multiple reflections caused by subwavelength features of high density silicon nanoholes and micro-desert textures. At an angle of incidence up to 65^º, the reflectance maintains less than 0.7% and at the large angle of 75^º the value is still less than 1%. In addition, we made the back surface of flexible-Si as a highly doped n-type layer to reduce the surface recombination rate and decrease the contact resistance.


Fig. 1 shows that optical measurement of wide range of AOI and wavelength for the thin wafer with silicon nanohole.	Fig. 2 shows that high efficiency hybrid solar cells are demonstrated by employing flexible thin silicon with Hierarchical surface composed of high-density nanoholes on micro-desert textures.

Investigation of focused ion beam milling process

Professor Chieh-Hsiung Kuan

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Focused ion beam (FIB) technique enables nanomachining of materials directly and site-specifically. Some kinds of surface structures which require complex conventional processes to fabricate can be facilely obtained with FIB. During a FIB machining process, the desired patterns are milled with a scan of adjacent ion beam spots. In this study, we investigated the surface topography evolution of Si(001) milled with a single Ga⁺ beam spot of different dwell time.

Fig. 1 shows typical AFM images of single-spot-milled Si(001) surfaces. For the samples of short dwell time, the surface shows a protruding topography which comes from the amorphization of Si after a low dose of ion bombardment (swelling effect). As the dwell time exceeds 1 ms, the milling process begins to dominate. The depth at each radial distance of the pit varies almost linearly with dwell time in the range of 1.5-84 ms (Fig. 2). In addition, the slopes of the depth-dwell time graph can be approximately described by a Gaussian function of radial distance (Fig. 3). Thus the surface profiles of the milled pits can be effectively expressed as an equation of radial distance and dwell time. Fig. 4 shows the calculated profiles as well as the AFM data. The difference between measured and calculated values is within only 3 nm. Based on these results, 3D surface structures can be accurately realized for further device applications.

Fig. 1. AFM images of Si(001) surfaces after the process of single spot milling with a dwell time of (a) 88 μs, (b) 445 μs, (c) 667 μs, (d) 1.5 ms, (e) 3.3 ms, (f) 7.4 ms, (g) 16.7 ms, and (h) 38 ms.

Fig. 2. Change of depth with dwell time at each radial distance of the pit. The slopes decrease outward from the center.

Fig. 3. Change of the slope of depth-dwell time graph (Fig. 2) with radial distance. The red solid line represents a Gaussian fitting curve.

Fig. 4. Calculated depth profiles (solid lines) as well as the AFM data (hollow dots). The difference between measured and calculated values is within 3 nm.

Collinear and non-collinear multi-wavelength optical parametric generation in 1D PPLT

Professor Lung-Han Peng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We demonstrated multiple wavelengths optical parametric generation (OPG) from a single grating, one dimensional periodically poled lithium tantalate crystal. Such OPG spectra are found in continuous spatial positions symmetrical to the collinear direction, with decreasing signal and increasing idler wavelengths, associated with decreasing powers when the far-field angle increases. These observations were ascribed to the simultaneous collinear and non-collinear optical parametric generation (OPG) processes due to the pump intensity and the resultant wavelength-dependence on the angular bandwidth of the parametric gain.

This work was collaborated with Prof. Boudrioua of U. Paris 13, France and Prof. Simohamed of´ Éole Militaire Polytechnique (Algeria)

Fig. 1. Spatial dispersion of the generated spectrum

Fig. 2. Generated OPG spectra recorded at different symmetrical spatial positions

Fig. 3. Calculated single-pass gain spectra and measured signal wave intensities

Ref: M. Lazoul, A. Boudrioua, L. M. Simohamed, A. Fisher, and L.-H. Peng, “Simultaneous collinear and non-collinear parametric generation in 1D single grating periodically poled lithium tantalate,” Appl. Phys. B 110, 459 (2013).

Spectroscopic measurement of absorptive thin films by spectral-domain optical coherence tomography

Professor Sheng-Lung Huang's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A non-invasive method for measuring the refractive index, extinction coefficient and film thickness of absorptive thin films using spectral-domain optical coherent tomography is proposed, analyzed and experimentally demonstrated. Such an optical system employing a normal-incident beam of light exhibits a high spatial resolution. There are no mechanical moving parts involved for the measurement except the transversal scanning module for the measurement at various transversal locations. The method was experimentally demonstrated on two absorptive thin-film samples coated on transparent glass substrates. The refractive index and extinction coefficient spectra from 510 to 580 nm wavelength range and film thickness were simultaneously measured. More info can be found at Optics Express, 22, No. 5, pp. 5675–5683, 2014.

Figure: The dispersions of index refraction and extinction coeff. of a yellow (a, b) and a green (c,d) color filters.