| 摘要: | 太陽能由於其具有的巨大能量、廣泛性以及永久性，因此被視為未來綠色能源中重要的來源之一。近年來，太陽能電池產業迅速的蓬勃發展，其中以矽太陽能電池、銅銦鎵硒太陽能電池以及硫化鎘太陽能電池佔據了主要市場，但是這些太陽能電池也有其短處。例如，使用高成本的原料或是具有毒性的元素。為了克服以上問題，開發地球富含元素的太陽能電池是勢在必行的目標。而最近硫化亞錫太陽能電池引起了大家的關注，因為其具備了以上所需的條件。但是硫化亞錫太陽能電池仍面臨著極大的挑戰，主要原因是它不足以商業化的效率。因此在本研究中，主要分成兩個部分來提升硫化亞錫太陽能電池的光電轉換效率。第一部分會專注於設計一個新穎的硫化亞錫薄膜成長方式，克服傳統方式的成長缺點。第二部分則是專注於發展一個全新的緩衝層，以調節硫化亞錫以及緩衝層介面的能帶位置。
在低維度的晶體結構材料中(尤其是硫化亞錫)，晶體取向對於載子遷移、載子複合具有極高的影響力，而此項特性也會間接地影響電子元件效率。目前，有許多方式可以成長硫化亞錫的薄膜。其中以氣相沈積法最為大宗，因為許多研究發現以氣相沈積法合成硫化亞錫所製作的太陽能電池，其薄膜品質以及光電轉換效率都有所提升。但是，目前大部分的文獻並沒有深入探討晶體取向對於載子遷移以及太陽能電池效率的影響，或是如何控制硫化亞錫的晶體取向。因此，在本研究中，我們結合了多段升溫步驟以及氣相沈積法用來控制硫化亞錫的晶體取向。從實驗結果中可以發現040平面並不利於載子遷移，而且會造成嚴重的體內載子複合以及SnS/CdS介面複合。結果中發現，藉由抑制040平面的成長，可以有效的減少載子複合，而硫化亞錫薄膜太陽能電池的效率可以有效的從0.11%提升到2%
薄膜太陽能電池是由多層結構堆積而成的電子元件。例如，緩衝層、前電極、背電極或是吸收層，每一層都具備其重要性以及各自的功能，而太陽能電池的光電轉換效率就取決於各層之間的交互作用。其中，緩衝層大大的影響太陽能電池效率，因為緩衝層決定PN接面的能帶位置，不合適的能帶位置會造成嚴重的介面複合。因此本研究的第二部份發展了無毒且寬能隙的氧化鋅錫(ZTO)作為緩衝層，藉由調控鋅與錫的比例，可以更進一步微調能帶位置。從實驗結果發現當使用硫化鎘作為緩衝層時，導電帶位移會呈現懸崖式能帶(cliff-type)位置，不過當緩衝層替換為氧化鋅錫時，導電帶位移會呈現凸起式能帶(Spike-type)位置。當導電帶位移為Spike-type時，可以明顯的觀察到介面複合顯著地減少，而太陽能電池的光電轉換效率也從1.67%提升到3%，開路電壓也從0.72 eV提升到0.85 eV。
;Solar energy is one of the essential green energy resources owing to its naturally massive abundance, universal accessibility, and long-term sustainability. The evolution of the solar photovoltaic industry has been remarkable in recent years. The main limitations of current well-developed photovoltaic devices (Silicon technology, CIGS, and CdTe) are the high cost and/or the toxic element. The earth-abundant thin-film solar cells (TFSCs) for approaching both environmentally friendly and cost-effective, therefore, are the ideal solution for harvesting solar energy. Among current earth-abundant materials, tin(II) monosulfide (SnS) is considered a promising cost-effective semiconductor. However, the device performance of SnS-based solar cells remains quite low owing to the lack of understanding of SnS properties. This dissertation is attempting to address the main strategies which might play a key role in boosting the device performance of SnS TFSCs. The works mainly focus on presenting a novel experimental design methodology to overcome the SnS growth strategies and developing an alternative eco-friendly buffer layer to obtain favorable band alignment at SnS/buffer layer interface.
With low-dimensional crystal structural materials (particularly SnS), the crystallographic orientation plays a key role in manipulating the charge transport, carrier recombination, and eventually device characteristics. Many deposition methods have been developed to grow SnS thin-film and recently vapor transport deposition (VTD) has shown a great increase in both SnS quality and device performance. However, up to now, not much report provides direct evidence about the effect of crystallographic orientation on the charged transport and device performance, or how to control the crystallographic orientation in SnS thin-film. Herein, we proposed an effective experimental setup/geometry and a multi-step annealing process during the VTD process to intentionally tailor the crystal orientation. These two approaches have directly modified the growth behavior and tailored the crystallographic orientation. All observed results supported that (040) plane is harmful to charge transport, and caused revere recombination in SnS devices (bulk and/or interface). Therefore, the suppression of the (040) plane in SnS thin films led to a dramatic improvement in PCE from 0.11% to almost 2%.
A photovoltaic device is a stack of multiple layers which have their own importance and effect on the final device performance. Therefore, not only the absorber layer, it is essential to take into account some important layers such as the buffer layer, back contact, front contact, etc. Among them, selecting a buffer layer is substantial because the unfavorable band alignment at p-n heterojunction might cause acute recombination at the interface and degrade device efficiency. Therefore, we developed an eco-friendly and wide bandgap buffer layer Zinc-Tin-Oxide (ZTO) with a tunable band offset. The conduction band offset (CBO) switch from the “cliff-type” (CBO = -0.41 eV with CdS buffer layer) to the “spike-type” (CBO = +0.23 eV with ZTO11 buffer layer) by controlling the Zn-to-Sn ratio. The favorable CBO led to the great suppression of the interfacial recombination which was proved by the increase of activation energy from 0.72 eV to 0.85 eV. As expectable result, the dramatical enhancement in the device performance was attained from PCE = 1.67%, Voc = 0.24 V, and Jsc = 13.57 mA/cm2 to PCE = 3.0%, Voc = 0.34 V and Jsc = 18.7 mA/cm2. |
