Professor Chi-Kuang Sun

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Graphene is often deposited on substrates for practical purposes. The graphene-substrate interactions have been the focus of many discussions because they induce property modifications and result in considerable energy transfer at the interface. However, the proposed mechanisms were restricted to the 2D plane and interface, while the energy conduction in the third dimension is hardly considered. Herein, we applied nano-ultrasonic spectroscopy with a piezoelectric nano-layer embedded in the substrate to study the energy transfer perpendicular to the interface of the combined system of the 2D graphene and the 3D base. The result revealed energy dissipation of optically excited graphene via emitting out-of-plane longitudinal acoustic phonon into the substrate. We found that, under photo-excitation by a femtosecond laser pulse, graphene can emit longitudinal coherent acoustic phonons (CAPs) with frequency bandwidth over 1 THz into the substrate. In addition, the waveform of the CAP pulse infers that the photo-carriers and sudden lattice heating in graphene caused modification of graphene-substrate bond and consequently generated longitudinal acoustic phonons in the substrate. The direct observation of this unexplored graphene to substrate vertical energy transfer channel brings new insights into the understanding of the energy dissipation and limited transport properties of supported graphene.

Reference: I-Ju Chen, Pierre-Adrien Mante, Cheng-Kai Chang, Szu-Chi Yang, Hui-Yuan Chen, Yu-Ru Huang, Li-Chyong Chen, Kuei-Hsien Chen, Vitalyi Gusev, and Chi-Kuang Sun, “Graphene-to-substrate energy transfer through out-of-plane longitudinal acoustic phonons,” Nano Letters, 1317−1323, 14, 2014

Professor Gong-Ru Lin's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Si quantum dots (Si-QDs) and SiC quantum dots (SiC-QDs) co-doped Si-rich silicon carbide (Si_xC_1-x) based p-i-n junction light-emitting diodes (LEDs) with blue-green light emission is demonstrated. By growing the Si_xC_1-x with plasma-enhanced chemical vapor deposition (PECVD) at relatively low temperature, the turn-on voltage of Si-rich Si_xC_1-x LEDs can be reduced to 6.1 V when thinning i-Si_xC_1-x layer to 25 nm because of higher tunneling probability and lower series resistance. The electroluminescent (EL) power increases to 136 nW, however, which inversely attenuates due to the reduced SiC-QDs if the i-Si_xC_1-x thickness further shrinks to 25 nm. The principle EL peak at 500 nm with narrower shape and blue-green emission pattern is attributed to the self-trapped excitons at surface states among SiC-QDs. The external quantum efficiency (EQE) of the Si-rich Si_xC_1-x LEDs with i-Si_xC_1-x thickness of 50 nm is up to 1.58×10^-1% with enhanced carrier tunneling probability. The carrier injection efficiency is enhanced to 46% by increasing the doping concentration to 10¹⁶ cm^-3, leading to almost one order of magnitude improvement on the EQE of Si-rich Si_xC_1-x LEDs.

Reference: Hung-Yu Tai, Chih-Hsien Cheng, and Gong-Ru Lin, “Blue-Green Light Emission from Si and SiC quantum dots co-doped Si-rich SiC p-i-n Junction Diode,” IEEE J. Sel. Top. Quantum Electron., Vol. 20, Issue 4, 8200507, Jul.-Aug. 2014.

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Open-circuit voltage is one of the key parameters affecting the performance of organic photovoltaic devices. Several electronic mechanisms originating from energy band alignment within the path of charge transport and carrier extraction are the major factors that limit the V_oc. We demonstrated that the influence of energetic disorder due to introduced cathode buffer layers between active layer and metal cathode can contribute to energetic favorable charge transport and extraction, leading to V_oc enhancement. To explore the dependence of V_oc on the energetic properties of the contacting materials prior to cathode deposition, we proposed a comprehensive model for buffer layers based on molecular materials commonly employed in OLED and OPV cells.

Energy band alignment of various cathode buffer layers in P3HT/PCBM based bulk-heterojunction (BHJ) solar cell are checked via in situ ultra-violet photoemission spectroscopy (UPS) and the results are organized as follows: First, we clarified the correlation between charge transport property and carrier extraction ability at buffer layers with energetic favorable band alignment. This includes a presentation of qualitative models for the dependence of HOMO and LUMO offset between active layer and buffer layer on the shunt resistance and series resistance, and short-circuit current density, or saturated value of V_oc based on Shockley’s theory for p-n junctions. Second, the dependence of V_oc and OPV operating parameters on the cathode contacting materials with inappropriate energy offset or transport ability, such as narrow band gap and hole transport materials, were obtained. We investigated not only the LUMO offset related to electron transport, but also the influence of HOMO offset on carrier extraction at cathode contact. Experimental details and device performance are provided in our recent publication. As a result, the performance of P3HT/PCBM based BHJ solar cells with various buffer materials can be classified into three class according to the energy offset. Class I contained large HOMO and LUMO offset so the better hole blocking and electron transport ability lead to a saturated value, 0.6 V, of V_oc in P3HT/PCBM system. Class II has either large HOMO offset or LUMO offset making a small decrease in V_oc and a poor V_oc for Class III due to inappropriate energy offset of buffer materials. Finally, we summarize the key findings to propose and verify a new model which gives predictions for the maximum V_oc in BHJ solar cells consisting of buffer materials.