Browsing by Author "Yu, Shu-Cheng"

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10% Efficiency Hybrid GaAs/PEDOT:PSS Solar Cells with Monolayer Graphene
(2014-01-01)
Cheng, Kai-Yuan
;
Pan, Huai-Te
;
Yu, Shu-Cheng
;
Weng, Wei-Sheng
;
Lai, Yi-Chun
The development of solution-processed hybrid organic-inorganic heterojunction solar cells have provoked increasing research effort due to potential to significantly reduce the fabrication cost of semiconductor solar cells while maintaining the high power conversion efficiency (PCE). With a recent record of GaAs solar cells achieving 28.8% via a wafer lift-off technique, hybrid organic/GaAs solar cells can offer an alternative route to provide more affordable solar power than current semiconductor technologies. In this work, we demonstrate the hybrid devices based on a conductive polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) directly spun-cast on planar gallium arsenide materials. While still lacking for optimized antireflection, the PCE of the best performing device can achieve nearly 10% by introducing the monolayer graphene as transparent electrode. The current progress presents 2-fold enhancement in comparison to a previous record at 5% based on GaAs nanowire templates.
Doping of Monolayer Graphene for Silicon based Schottky Junction Solar Cells
(2013)
Lai, Yi-Chun
;
Wu, Bing-Shu
;
Yu, Shu-Cheng
;
Yu, Peichen
;
Chi, Gou-Chung
Silicon-based hybrid solar cells have garnered extensive attentions in the photovoltaics industry due to easy processing attributes and high optical absorption and outstanding carrier mobility of silicon. Among all, indium tin oxide (ITO)/silicon solar cells have achieved a power conversion efficiency of 13% due to excellent conductivity, transmittance and applicable surface potential of ITO. However, the cost of ITO has risen significantly recently due to the deficiency of indium. Therefore, graphene has been an inexpensive alternative to ITO. For solar cell applications, graphene plays an important role as transparent electrodes (TE) with tunable work functions for efficient carrier collection. Therefore, graphene-based Schottky junction solar cells (SJSC) on crystalline silicon thin films hold great promises for low-cost photovoltaics owing to potentials for high efficiency and rapid production on flexible substrates. According to previous reports, the key factors to achieve a highly efficient SJSC include excellent transparence and conductance, as well as tunable work functions. Herein, we demonstrate a single layer graphene/n-Si Schottky junction solar cell that exhibits a power conversion efficiency (PCE) of 1.2 % under one-sun AM 1.5G illumination, and an integrated short-circuit photocurrent of 18.3 mA/cm(2) from the external quantum efficiency measurement. The transmittance of the monolayer graphene in this device is over 97 % and the sheet resistance is around 800 to 1200 Omega/square. Furthermore, we investigate a doping method involving bis(trifluoromethanesulfonyl)-amid (TFSA) for the monolayer graphene to improve the separation and collection of photogenerated carriers in the SJSC. The preliminary data show that the sheet resistance is decreased rapidly from 1200 to 300 Omega/square. and the surface potential is also adjusted by the chemical doping. Currently, device fabrication with doped monolayer graphene is still in process and complete characterization data will be presented.
Hybrid Multi-Layer Graphene/Si Schottky Junction Solar Cells
(2013)
Wu, Bing-Shu
;
Lai, Yi-Chun
;
Cheng, Yuan-Hung
;
Yu, Shu-Cheng
;
Yu, Peichen
Hybrid Graphene/Si Schottky junction solar cells are inexpensive alternatives for photovoltaics due to easy and rapid fabrication processes. However, one of the main challenges for graphene-based Schottky junction solar cells is the large sheet resistance of single-layer graphene, leading to low fill factor. Here, we demonstrate multi-layer graphene-based Schottky junction solar cell, which are prepared by stacking monolayer graphene via a chemical vapor deposition (CVD) method on copper foils. The objectives are to improve charge transport and decrease the series resistance of the hybrid solar cell. We present the Raman spectrum results of monolayer graphene and stacked multilayer graphene films, and observe no bonding interaction between the stacked layers. For the optical and electrical properties, the sheet resistance of stacked multilayer graphene is reduced from 3000 Omega/sq to 1500 Omega/sq and 750 Omega/sq with two and three layers, respectively. The reduction of the sheet resistance approaches the properties of highly-oriented pyrolytic graphite (HOPG) when the layer number is more than five. Moreover, from the optical transmittance measurement, the absorption of a monolayer graphene is about 3% at the 555 nm wavelength, giving rise to a transmittance of 90% for triple-layer graphene. The hybrid silicon solar cell with a single-layer graphene achieves a power conversion efficiency of 0.9% with a short-circuit current of 8.75 mA/cm(2), open-circuit voltage of 0.25V, and fill factor of 39% The external quantum efficiency measurement reveals an integrated photocurrent of 18.3 mA/cm(2), which further confirms the potential of the hybrid photovoltaic approach. Device fabrication with multi-layer graphene is still in process, and more data will be presented. Other characterization techniques such as capacitance-voltage and capacitance-frequency measurements will be taken to analyze the performance of the single- and multi-layer graphene solar cells.