摘要:近年來,有機無機鈣鈦礦材料興起並被廣泛運用在太陽能電池、發光二極體、以及雷射等光電元件中。其中,採用有機無機鈣鈦礦做為吸光層的光伏元件更被認為是極具潛力的未來綠能來源。原因無他,因為有機無機鈣鈦礦材料屬於直接能隙材料,光吸收效率高、載子傳輸距離長、激子束縛能低,且只需利用簡單快速且低溫的溶液製程即可製備出具有優秀光電特性的有機無機鈣鈦礦薄膜。目前學術界已開發出高於20%光電轉換效率的有機無機鈣鈦礦太陽能電池。也就是說,有機無機太陽能電池提供了一個低成本、高效率的再生能源選擇。然而,即便許多研究團隊致力於研究有機無機鈣鈦礦材料和元件,目前仍有一些未解的問題存在。量測電流電壓特性曲線時觀察到的磁滯現象是其中一個。所謂磁滯現象意指在量測電流電壓特性曲線時,電壓的掃描方向與掃描速度會大大的影響元件效率。這個現象使得有機無機鈣鈦礦太陽能電池元件效率的鑑定需要更加謹慎。因此,為了更進一步提升有機無機鈣鈦礦太陽能電池的元件效率,了解磁滯現象的成因即成為相當重要的課題。目前磁滯現象被猜測起因於不對等的載子傳輸能力、高電容特性、高極化率特性、缺陷、或是有機離子在材料中的遷移所造成。有些報告發現磁滯現象的嚴重程度與晶粒大小與溫度有關。這些發現將磁滯現象的成因指向有機無機鈣鈦礦的材料本身。有趣的是,磁滯現象的嚴重程度也會隨著元件結構的改變而不同。採用平面化二氧化鈦結構製備的鈣鈦礦元件容易觀察到嚴重的磁滯現象,然而,採用多孔隙結構的二氧化鈦結構或者平面化的PEDOT結構卻能夠得到較輕微、甚至是沒有磁滯現象的光伏元件。因此,造成有機無機鈣鈦礦元件產生磁滯現象的因素仍然是未解之謎。必須藉由更多深入的化學以及物理實驗才能夠更加釐清磁滯現象的成因。
這項子計畫希望能夠從材料的觀點出發,藉由謹慎的實驗設計與分析了解有機無機鈣鈦礦的化學、物理特性,希望能夠解釋不同元件結構對元件效率產生的影響。並且,在完成材料特性與元件結構的探討之後,還可以製備串疊型元件,將有機無機鈣鈦礦光伏元件與已商業化的矽基太陽能電池或硒化銅銦鎵太陽能電池相結合,串疊出更高效率的元件。理論預測指出,由於吸收光譜不同,將光電轉換效率約17%的有機無機鈣鈦礦元件與24%的矽基元件結合,能夠得到光電轉換效率將近30%的串疊元件。因此這項子計畫有機會能提升市售的矽基太陽能電池元件效率,並提供增進元件效率的一個研究方向。
Abstract: Organic-inorganic perovskite materials have emerged as a new class of materials for high efficiency optoelectronic devices such as solar cells, light emitting diodes, laser, etc. Perovskite solar cells have become one of the potential candidates to provide low cost renewable solar energy due to their unique advantage of compatibility with solution processing and outstanding optoelectronic properties such as the direct band gap, high absorption coefficient, long range carrier transport length, and low exciton binding energy. Efficiency up to 20% has been certified by NREL. However, in some academic reports, perovskite devices suffered greatly from significant hysteretic behavior. That is, the obtained efficiency can be dramatically different by altering the I-V scan direction and scan rate. The finding of such hysteretic behavior suggests that many previous reports of the device efficiency are likely overestimated. Thus, in order to achieve higher efficiency, in-depth understanding of the origin of this hysteretic phenomenon becomes critical. Hysteretic phenomenon is thought to stem from the unbalanced carrier transport ability, high capacitive character, high polarizability, defects, or the mobile nature of organic cations. Some studies have revealed the grain size and temperature dependent hysteretic phenomenon. According to those results, the hysteretic phenomenon has been attributed to the large dielectric constant and large polarizability of the organic-inorganic perovskite itself. That is, the intrinsic properties of the material may be the cause for this undesirable behavior. Interestingly, however, the prominent hysteretic phenomenon can be significantly minimized or totally eliminated by changing the device structure from the so-called “planar TiO2 structure” to a “mesoporous TiO2 structure” or an “invert PEDOT structure”. Therefore, whether the hysteretic phenomenon is related to the intrinsic materials nature of the perovskite remains an open question. More detailed studies on the chemical and physical properties of materials and the structures of devices are needed to elucidate the origin of the hysteretic phenomenon.
This subproject proposes to carry out a systematic investigation of the fundamental chemistry and physics of the organic-inorganic perovskite materials and their relationship to the device structure and performance. Upon in-depth understanding of the chemistry and physics of the material and device properties, perovskite solar cells may be integrated with Si solar cells or thin film solar cells into a tandem device structure to achieve higher efficiency. A preliminary estimate predicts an improved performance of combined perovskite-Si solar cells to 29.6% efficiency from the 17% efficiency of the perovskite solar cells and the 24% efficiency of the Si solar cells. Therefore, this proposed study has the potential of promoting the performance of existing commercial Si solar cells and even developing a new research direction for developing better solar cells.