高效率異質接面太陽能元件

Toward Highly Efficient Heterojunction Solar Devices: An Electrical and Optical Concurrent Design

王新平(Hsin-Ping Wang)

指導教授 ： 林恭如

共同指導教授 ： 何志浩(Jr-Hau He)

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2016年)

https://doi.org/10.6342/NTU201601008

摘要

為了得到高效率的太陽能電池，同時考慮元件光與電特性是必要的。在本論文中，將探討不同材料之異質接面太陽能元件之光電同步設計。第一、二單元討論矽基異質結構在太陽能元件上的應用。第三、四單元討論磷化铟薄膜異質結構在太陽能元件上的應用。
首先，為了克服高表面缺陷的奈米結構帶來的電性影響，在此研究中，包含奈米及微米的階層結構被用來取代傳統高表面積體積比的奈米結構，能夠達到比傳統奈米結構更佳的抗反射效果。就電性而言，利用適當的化學表面拋光，能大大提升載子生命期，與傳統微米金字塔結構不相上下，又同時保有奈米結構的高載子蒐集效率徑向p-n結。又由於非晶矽對晶矽有非常好的表面鈍化效果，能減少表面載子複合率，我們將此階層奈米結構運用在矽基異質接面太陽能電池上，能夠達到15.14%的太陽能轉換效率，是目前矽基奈米結構第二高的效率。與傳統的微米金字塔結構相比，一天能產出高於44.2%的功率。由於良好的表面鈍化，矽基異質接面能產生非常高的光伏，對太陽能分解水是非常有吸引力的。在第二部分，我們利用矽基異直接面設計產氫及產氧的光電極，同時考慮催化劑的光吸收、載子於電池中傳輸損耗，以及載子注入電解液中的速率，能保有非常高的光電流及光伏，達到目前最高的光陰極產氫效率(13.26%)。
接著，磷化铟一直以來是有潛力的太陽能元件材料，因為它的直接能隙及低載子複合率，但高昂的價格一直打壓它的實際應用。我們研發的薄膜氣－液－固成長機制可以成長橫向大面積磷化铟薄膜，有效克服這個問題，但目前材料特性及光電均勻性都還需要更多的探討。這裡我們發現利用氫電漿能夠有效改善橫向光電均勻性，氫離子能擴散至晶界進行鈍化，使得載子捕捉減少。由於電性的改善，在太陽能電池表現上能有明顯的光伏提升。最後，我們研發同時參雜p,n的成長方式，利用簡單的旋塗式玻璃做為成長的蓋子，在高溫薄膜氣－液－固成長時，允許磷擴散至铟成長磷化铟，載子也同時擴散進材料，達到參雜效果。此方法只需改變旋塗式玻璃的參雜p,n型及濃度，不同參雜濃度之三五族材料可以在同一時間成長，符合經濟效益，也提升了薄膜氣－液－固成長在光電元件上的應用。

關鍵字

抗反射次波長階層結構 ； 異質接面太陽能元件 ； 表面鈍化 ； 晶界鈍化 ； 薄膜氣液固成長

並列摘要

To achieve high efficient solar devices, concurrent engineering design involving electrical and optical perspectives in parallel is necessary. The heterojunction is presently popular design in the photovoltaics due to the low recombination rate, leading to high open-circuit voltages (VOC). Here, we’ll focus on boosting the efficiency of heterojunction solar devices.
First of all, hierarchical structures combining micropyramids and nanowires with appropriate control of surface carrier recombination represent a unique class of architectures for radial p-n junction solar cells that synergizes the advantageous features including excellent broadband, omnidirectional light-harvesting and efficient separation/collection of photoexcited carriers. The heterojunction solar cells fabricated with hierarchical structures exhibit the efficiency of 15.14% using cost-effective as-cut Czochralski n-type Si substrates, which is the second highest reported efficiency among all n-type Si nanostructured solar cells. This is also the first described omnidirectional solar cell that exhibits the daily generated power enhancement of 44.2%, as compared to conventional micropyramid control cells. The concurrent improvement in optical and electrical properties for realizing high-efficiency omnidirectional solar cells using as-cut Czochralski n-type Si substrates demonstrated here makes hierarchical architecture concept promising for large-area and cost-effective mass production.
Amorphous Si (a-Si)/ crystalline Si (c-Si) heterojunction (SHJ) photoelectrochemical cells can serve as highly efficient and stable photoelectrodes for solar fuel generation. Low carrier recombination in the photoelectrodes leads to a high photocurrent and high photovoltage. Both SHJ photoanodes and photocathodes are designed for high efficiency oxygen and hydrogen evolution. The SHJ photoanode with sol-gel NiOx as the catalyst shows the current density of 21.48 mA/cm2 at the equilibrium water oxidation potential. The SHJ photocathode displays excellent hydrogen evolution performance with an onset potential of 0.640 V and a solar to hydrogen conversion efficiency of 13.26%, which is the highest ever reported for Si-based photocathodes.
Then, we moved to the next promising photovoltaic materials: InP. The thin-film vapor-liquid-solid (TF-VLS) growth technique presents a promising route for high quality, scalable and cost-effective InP thin films for optoelectronic devices. Towards this goal, careful optimization of material properties and device performance is of utmost interest. Here, we show that exposure of polycrystalline Zn-doped TF-VLS InP to a hydrogen plasma (in the following referred to as hydrogenation) results in improved optoelectronic quality as well as lateral optoelectronic uniformity. Notably, hydrogenation reduces the relative intra-gap defect density by one order of magnitude. As a metric to monitor lateral optoelectronic uniformity of polycrystalline TF-VLS InP, photoluminescence and electron beam induced current mapping reveal homogenization of the grain versus grain boundary upon hydrogenation. At the device level, we measured more than 260 TF-VLS InP solar cells before and after hydrogenation to verify the improved optoelectronic properties. Hydrogenation increased the average VOC of individual TF-VLS InP solar cells by up to 130 mV, and reduced the variance in VOC for the analyzed devices.
Finally, we develop a growth mode that enables to simultaneously obtain InP in-situ doped with different dopants and different concentrations. The process utilizes templated liquid-phase crystal growth with the spin-on dopant (SOD) as the cap and dopant sources. n-type InP with the doping level from 1.0×1017 to 4.8×1018 cm-3 can be successfully obtained in the same growth run by controlling the dilution of Sn-doped SOD. The doping level of p-type InP could be controlled from 9.0×1016 to 3.0×1018 cm-3. Finally, we perform to simultaneously grow both n-type and p-type InP patterns on the same substrate by defining SOD with the pre-patterning metal templates. This result outlines a promising method to achieve partial in-situ doping of materials for future optoelectronic applications.

並列關鍵字

Subwavelength-antireflective hierarchical structure ； heterjunction solar device ； surface passivation ； grain boundary passivation ； thin-film vapor-liquid-solid (TF-VLS) growth technique

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