高分子太陽能電池光電性質之研究

Research on photovoltaic characteristics of polymer based solar cells

蔡欣宏(Shin-Hung Tsai)

指導教授 ： 何志浩

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2011年)

https://doi.org/10.6342/NTU.2011.01496

摘要

有機高分子太陽能電池有著市售太陽能電池所缺乏的性質，成本低廉、製程方便快速、重量輕且可曲，現已廣泛地被研究。
本篇論文著重在討論有機無機混成太陽能電池及倒置結構太陽能電池，加入了無機材料讓載子傳輸方向不同於傳統結構。最重要的是避免了poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS)與Indium Tin Oxide (ITO)電極的接觸及低功函數金屬的使用，大大提升了有機高分子太陽能電池的使用壽命。
首先，利用簡單方法讓有機層薄薄的附在矽奈米線表面上的有機無機混成太陽能電池，載子在有機材料中的傳輸更加有效率，溶液濕蝕刻製程的矽奈米線更具備抗反射的優點。比起有機層將奈米線空隙填滿並形成元件平整表面的電池比起來，此種結構的太陽能電池有61%短路電流的增強，造成效率比其高出約31%。研究中也發現，元件表現受到持續照光的影響而漸趨提高(提高至約140%)，拉曼量測及電子穿隧式顯微鏡結果證實，這是因為高分子材料的排列受到無機材料表面能的影響。
第二，論文中利用一個簡單的摻雜方法來製作氧化鋅鋁薄膜，且證明摻雜後無論在穿透度以及導電率上都表現較氧化鋅薄膜良好。實驗顯示，利用此種薄膜當作倒置結構有機太陽能電池的電子導電層讓元件效率有明顯的提升(約14%)。主要是因為它讓電子的傳輸更容易，同時優化了串聯及並聯電阻。由光譜分析得到，摻雜鋁之後的氧化鋅(Al-doped ZnO, AZO)薄膜費米能階會提高，讓電子從主動層到電子傳輸層的躍遷能力有效提升，進而增加元件的填充因子(fill factor)。從AZO/P3HT的吸收譜，在AZO上的poly(3-hexylthiophene) (P3HT)會有較好的結晶，這可以減少在此接面上的能量耗損。
最後，有鑑於濺鍍的AZO薄膜有跟ITO一樣優越的導電度及穿透度，最重要是還擁有較ITO低的功函數，直接使用摻鋁的氧化鋅薄膜當作電極可以快速達到能階匹配的效果。又由於ITO中In的產量降低及In所含有的毒性，成本不斷上升及使用上的憂慮成了有機元件普及的一大絆腳石，使用氧化鋅鋁是一個未來的趨勢。此外，配合使用簡單的酸液蝕刻方式，讓光在主動層中路徑增加，達到效率改善的結果。降低材料的使用且效率不比含有很多有機主動層材料的電池低，是有機太陽能電池普及化的一大步。

關鍵字

有機無機混成太陽能電池 ； 抗反射 ； 倒置結構有機太陽能電池 ； 氧化鋅鋁 ； 電子傳導層 ； 氧化鋅鋁電極 ； 鹽酸蝕刻

並列摘要

Organic photovoltaics have received an intensive attention due to their promising features, such as low cost, light weight, simple solution-based process, and flexibility.
This thesis focuses on organic-inorganic hybrid solar cells (HSCs) and inverted organic solar cells (OSCs). Employing inorganic materials prevent photovoltaic devices from weak air stability due to the acidic, hygroscopic nature of poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) and oxidation of low work function front electrode.
First of all, a novel strategy employing core-shell nanowire arrays (NWAs) consisting of Si/regioregular poly(3-hexylthiophene) (P3HT) was demonstrated to facilitate efficient light harvesting and exciton dissociation/charge collection for HSCs. Consequently, core-shell HSCs exhibit the 61% improvement of short circuit current density (JSC) for a conversion efficiency (η) enhancement of 31.1 % as compared to the P3HT-infiltrated Si NWA HSCs with layers forming a flat air/polymer cell interface.Surprisingly, the device performance exhibits a dramatic enhancement under air mass 1.5 global (AM 1.5G) illumination. The improvement of crystal quality of P3HT shells due to the formation of ordering structure at Si interfaces after AM 1.5G illumination was confirmed by transmission electron microscopy and Raman spectroscopy. The core-shell geometry with the interfacial improvement by AM 1.5G illumination promotes more efficient exciton dissociation and charge separation, leading to η improvement (~140.6 %) due to the considerable increase in Voc from 257 to 346 mV, Jsc from 11.7 to 18.9 mA/cm2, and FF from 32.2 to 35.2 %, which is not observed in conventional P3HT-infiltrated Si NWA HSCs.
Second, we demonstrate that the power conversion efficiency of OSCs based on ZnO/P3HT:PCBM was improved by incorporating Al into ZnO acting as an electron transport layer (ETL). The Al-doped ZnO (AZO) thin film was deposited via an aqueous solution method. Compared with devices based on pure ZnO, AZO-based solar cells lead to the enhancement in power conversion efficiency by about 114% under simulated AM 1.5G full-sun illumination. It was found that the increase in the Fermi level as introducing Al into ZnO film would assist the interfacial electron transfer from active region to ETL, contributing to the improvement in fill factor. Besides, the enhancement in JSC was attributed to the higher transmission, interfacial area and electrical conductivity of AZO thin films than those of un-doped ZnO. The P3HT crystalline tends to be higher ordered on AZO ETL, which was suggested by normalized absorbance spectra, resulting in a reduction in energy loss due to insufficient donor/acceptor phase segregation. This study provides new insights into the design optimization of the ETL for inverted OSCs.
Finally, an efficient way to achieve high electron transfer efficiency from active layer to cathode has been demonstrated here, replacing ITO with sputtered AZO due to lower work function of AZO. Addressing the problem of diminishing reservation of indium was also found to be an advantage. After introducing a simple HCl etching treatment to form a rough AZO electrode, the distribution of light wave within active layer can be effectively widened, leading to a significant enhancement in EQE and JSC. With the roughened AZO film, even a thin active layer based cells can reach efficiency comparable to that with thick organic layer. This novel method provides an important scheme applicable to OSCs and benefits the practicability of OSCs by increasing the efficiency-cost ratio.

並列關鍵字

organic-inorganic hybrid solar cells ； anti-reflection ； inverted organic solar cells ； Al-doped ZnO ； electron transporting layer ； AZO electrode

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