Browsing by Author "Cheng, Kai-Yuan"

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10% Efficiency Hybrid GaAs/PEDOT:PSS Solar Cells with Monolayer Graphene
(2014-01-01)
Cheng, Kai-Yuan
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Pan, Huai-Te
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Yu, Shu-Cheng
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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.
Conductive Polymer/GaAs Hybrid Heterojunction Photovoltaic Devices
(2013)
Pan, Huai-Te
;
Huang, Yang-Yue
;
Weng, Wei-Sheng
;
Chen, Po-Han
;
Cheng, Kai-Yuan
Hybrid solar cells combining organic polymers and inorganic semiconductors are extensively investigated recently due to relatively inexpensive cost and simple fabrication processes. In this work, we demonstrate organic/inorganic hybrid heterojunction solar cells based on gallium arsenide (GaAs) substrate and conjugated polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS). First we performed a one-dimensional device simulation based on a self-consistent Poisson and drift-diffusion solver to survey the band alignment between the conductive polymer and GaAs materials and achieve a practical device design. Second, for device fabrication, we prepare a cleaned one-side-polished planar GaAs wafer, followed by thermal evaporation of back-side metal using either Aluminum or titanium/gold as the cathode. Next, PEDOT:PSS is spun-cast onto the wafer and annealed at 115 degrees C for 10 minutes. To improve carrier conduction, we use a self-assembled polystyrene (PS) nanosphere lithography technique to form the sacrificial mask layer, and perform anisotropic metal-assisted chemical etching on GaAs substrates. Various nanostructures such as nanowires or nanorods allow the conformal p-n heterojunction formation at the interface of organic/inorganic semiconductors, which can be beneficial for both light absorption and carrier collection. The optical and electrical characteristics such as reflectance, current voltage, and external quantum efficiency are measured. Currently, we achieve a 3.2% power conversion efficiency with an open-circuit voltage of 0.565 V, short-circuit current of 8.95 mA/cm(2), and a fill factor of 63.29% under a simulated AM1.5G illumination for planar substrates. Device fabrication with GaAs nanowires is still in process and more data will be presented.
Low-Temperature and Ultrafast Synthesis of Patternable Few-Layer Transition Metal Dichacogenides with Controllable Stacking Alignment by a Microwave-Assisted Selenization Process
(2016-02-23)
Chen, Yu-Ze
;
Medina, Henry
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Wang, Sheng-Wen
;
Su, Teng-Yu
;
Li, Jian-Guang
To date, the chemical vapor deposition (CVD) process is the most popular approach because of its high yield and quality. Nevertheless, the need for a high temperature and the relatively long process time within each cycle hinder the commercial development in terms of production cost. In this work, we demonstrate a fast (<3 min) and feasible approach to synthesizing a few-layers of WSe2 and MoSe2 on arbitrary substrates by a microwave-assisted selenization process. The transition metal dichalcogenides (TMDs) can be patterned by standard photolithography. Furthermore, controllable layered growth from horizontal to vertical alignment can be achieved, leading to an enhanced chemical catalytic activity caused by large edge sites (exposed areas). As a proof, a highly sensitive NO gas sensor based on vertical WSe2 was fabricated. Moreover, our microwave assisted selenization process can be further extended to achieve other two-dimensional TMD materials because of the simplicity of the process.
Projected Efficiency of Organic/Inorganic Hybrid Tandem Solar Cells
(2013)
Chen, Po-Han
;
Huang, Yang-Yue
;
Pen, Huai-Te
;
Lai, Yi-Chun
;
Tsai, Chia-Ying
We propose a series-connected hybrid tandem solar cell which consists of an organic solar cell (P3HT/PCBM) as the top cell and an organic/crystalline silicon hybrid solar cell (PEDOT:PSS/c-Si nanowires) as the bottom cell. Based on the device structure, the organic materials can be directly spun-cast onto the inorganic silicon substrate with thermally evaporated metal contacts, making solution-based processes possible for rapid and low-cost production. With a proper design, the hybrid device architecture can achieve a high open-circuit voltage and junction-matched photocurrent, offering a promising approach for next-generation high-efficiency photovoltaics. In this work, we established a device model to investigate the photovoltaic characteristics of the proposed hybrid tandem solar cells by combining the organic and hybrid silicon solar cells with a hypothetic recombination layer (RL). First, the model of single junction solar cells is fitted to the current-voltage curve of fabricated devices. Next, we investigate the properties of the RL between the sub-cells and observe strong correlations with the photovoltaic performance of tandem cells. In our preliminary model, we have realized a cell with an open-circuit voltage (Voc), short-circuit current (Jsc), fill-factor (FF) and power conversion efficiency (PCE) of 1.093 V, 9.715 mAkm2, 43.725 % and 4.644 %, respectively. We will further tailor the properties of the RL, the active-layer thickness of sub-cells, as well as the band alignment, in order to achieve practical device designs. Currently, the characteristics of real hybrid tandem solar cells remain significantly lower than the simulation result. The reason of such limited cell performance is the poor interfacial contact, which makes it difficult to provide efficient recombination and transport for electrons and holes generated from sub-cells. A number of challenging issues, including interface physics and device design will be discussed.