混合型有機-矽奈米柱太陽能電池之形貌與界面特性控制

Abstract

由於混合型有機矽異質接面太陽能電池具有低製程成本的優勢，有潛力成為下世代的光伏元件。然而，目前其光電轉換效率仍無法與傳統矽基太陽能電池相比，其主要原因是表面反射以及接面復合的損耗。在本論文中，我們研究藉由小球微影術及金屬輔助蝕刻而形成之矽奈米柱結構的側壁形貌與界面，以及應用於混合型太陽能電池上的特性。 矽奈米柱的側壁形貌的控制對於有機材料在結構上的覆蓋率、以及薄矽基板的光吸收特性很重要，而界面缺陷更是光伏元件開路電壓提升的重大關鍵。首先，利用金作催化層蝕刻的結構較為光滑平整，但由於在蝕刻過程中，金離子不容易被完全去除，進而在結構表面產生載子復合中心，這樣缺陷的存在使得元件開路電壓受到嚴重影響，在各蝕刻深度下皆不超過450mV，使光電轉換效率受限，儘管透過缺陷移除蝕刻能夠將開路電壓提升至569mV，使元件效率提升至13.52%，但由於缺陷移除蝕刻是等向性的蝕刻，其對週期性奈米柱陣列會造成相當程度的毀壞，失去抗反射結構的優勢。再者，利用銀催化層蝕刻的結構較為粗糙，但也因此有較低的反射率，經優化深度實驗後，發現深寬比1.5的元件結構有最高的效率，其開路電壓550mV，藉由低反射率及較大的PN接面面積，短路電流有超過30mA的表現，使元件效率高達12.35%。最後，透過原子沉積法在結構上沉積一層氧化鋁薄膜，在薄膜厚度3Å的條件下，既可造成優異的表面鈍化效果，又不會對載子傳輸造成影響，使其開路電壓從548mV提升至573mV，光電轉換效率則高達13.33%。 在能穩定控制矽奈米柱結構的條件下，我們可以藉由調變有機材料的參數，使其在結構上的覆蓋率產生變化，能更有效的探討混合型太陽能電池的界面特性，我們期望當矽奈米柱結構能被PEDOT:PSS完整包覆時，因接面面積的增加，該元件開路電壓的表現會再提升。

Hybrid organic-silicon solar cells are promising candidates for next-generation photovoltaics due to their low fabrication cost. However, their power conversion efficiency still cannot compete with that of traditional silicon solar cells owing to surface reflection and interface recombination losses. The control of nanorod sidewall profile is important to organic surface coverage, as well as optical absorption for thin-silicon substrates, whereas the control of interface properties is important to boost the open-circuit voltage (Voc) of hybrid devices. In this study, we investigate the sidewall profile and interface properties of silicon nanorod templates, fabricated by means of polystyrene lithography, followed by metal-assisted chemical etch (MACE) for hybrid photovoltaics. The optical and electrical characteristics of hybrid organic- silicon nanorod devices made with gold (Au) and silver (Ag) catalysts are systematically compared. First, the nanorod textures with gold catalyst exhibit smoother surface than that with silver. However, the residue gold particles result in carrier recombination centers and hence limit the Voc. In all the etch conditions, the Voc cannot exceed 450mV, leading to a low power conversion efficiency (PCE). With the help of damage removal etching (DRE), a hybrid solar cell achieves a Voc of 569mV and a PCE of 13.52%. However the anisotropic etch of DRE nearly remove the nanorod textures, losing the advantages for antireflection and large junction area. Moreover, the nanorods etched by silver catalyst exhibit a rough surface and low reflectance. The hybrid solar cells with an aspect ratio of 1.5 device show best performance with a Voc of 550mV, and a high short-circuit current (Jsc) over 30mA, and a high PCE of 12.35%. By passivating the nanorod surface with a thin Al2O3 layer of 3Å, , the Voc is further enhanced from 548mV to 573mV, and the PCE achieves 13.33%. By stably controlling the nanorod structures, we can investigate the dependence of organic surface coverage on photovoltaic characteristics of hybrid solar cells. We expect that the device performance can be further enhanced when the silicon nanorods are fully covered by PEDOT:PSS due to superior optical absorption and carrier collection benefited from the enlarged junction area.