在傳統式結構有機太陽能電池中，一般使用PEDOT:PSS作為電洞傳輸層。近年來，三氧化鉬具有良好的傳輸載子能力，且較PEDOT:PSS穩定，可有效提升元件壽命，成為另一受重視的電洞傳輸層材料。然而對於三氧化鉬傳導電洞的機制，學界至今還沒有一全面且清楚的論調，更特別的是薄膜形式的三氧化鉬會因為含有氧空缺而呈現一n type半導體性質，這與我們一般認為陽極緩衝層需使用p type材料之觀念相異，也因此激起學界研究此材料之興趣。以目前已發表的研究來看，提升氧化鉬材料作為電洞傳輸層時的元件表現之重點，在於縮小氧化鉬層之價帶邊緣與主動層施體材料之HOMO間的能階差值。根據文獻上顯示之能帶圖，五氧化二釩能階位置略高於三氧化鉬，因此我們假設在氧化鉬中添入釩可能有助於縮小此能階差值。
吾人利用在前驅液添入釩化合物的方法，製備出氧化鉬(釩)薄膜，鉬釩莫耳比為1:0, 1:0.05, 1:0.2, 1:0.5, 0:1，並應用在有機太陽能電池中作為電洞傳輸層，元件結構為ITO/ Mo(V)Ox/ P3HT:PCBM/ ZnO NP/ Al，工作面積0.16cm2。實驗結果顯示5%添釩氧化鉬薄膜有最好之元件表現，光電轉換效率2.16%，開路電壓0.6V，短路電流密度6.93mA/cm2，填充因子51.9%。在使用更高釩比例之添釩氧化鉬薄膜時，其元件轉換效率較差，但仍高於純氧化鉬之元件表現。在穩定性方面，比起使用PEDOT:PSS的元件，添釩氧化鉬元件則表現出一近似或略佳之穩定性。
在材料分析方面，利用掃描式電子顯微鏡及原子力顯微鏡我們可以觀察各式添釩氧化鉬薄膜之表面形貌，結果顯示氧化鉬(釩)薄膜並沒有明顯改變ITO基板表面形貌。另一方面5%添釩氧化鉬薄膜比起純氧化鉬薄膜而言，具有較低之表面粗糙度。由X光光電子能譜可觀察到純氧化鉬薄膜含有6+及少量5+價數之鉬離子存在，然在添入釩元素後幾乎全為6+價數之鉬離子。另外在釩訊號區域我們發現釩主要以5+價數存在，且隨著添入量增加，其訊號強度有上升趨勢，顯示釩元素確有進入薄膜中。在電阻率實驗中方面，所有氧化鉬(釩)薄膜電阻率都在10-3-10-1ohm-cm之間，添入5%釩氧化鉬薄膜有最低之電阻率，然添入20%及50%釩則使電阻率反升至比純氧化鉬薄膜更高之電阻值。5%添釩氧化鉬薄膜因導電率較高以及表面粗糙度較低，使得載子傳輸較為容易，此也反映在較低之Rs值上。由X光繞射實驗我們發現純氧化鉬薄膜顯現出微弱之三氧化鉬結晶相，且結晶性隨著釩添入量增加而有下降趨勢，此導致添入20%及50%釩之氧化鉬薄膜電阻率上升。
由紫外光光電子能譜吾人可計算各式氧化鉬(釩)薄膜價帶邊緣能階值以及費米能階值，輔以由可見光-紫外光吸收光譜計算之各式氧化鉬(釩)薄膜之光學能隙，我們可繪出元件能帶圖。結果顯示5%添釩氧化鉬價帶邊緣能階與P3HT之HOMO能階，具有最小的能階，此較小之差值有助於電洞由P3HT之HOMO位置傳至添釩氧化鉬層，再傳至陽極輸出電流，使得元件有較佳表現。

In the field of conventional organic photovoltaics (OPV), PEDOT:PSS is generally used as hole transport layer (HTL). Recently, MoO3 has recognized as an important HTL because of its good ability of carrier transportation and better stability comparing to PEDOT:PSS. However, the mechanism of hole transportation in MoO3 is still under debate. Furthermore, thin-film MoO3 shows n type characteristic owing to the existence of oxygen vacancies, which conflicts with the general concept that p type material is suitable for hole transportation. According to the literature, the key point of enhancing the device performance using molybdenum oxide as HTL locates on the decreasing of band offset between HOMO of the donor material and valence band edge of oxide. Published energy diagrams show higher energy level of V2O5 than that of MoO3, therefore, we assume that adding V into MoO3 may decrease the band offset.
We fabricate vanadium-doped molybdenum oxide film (V-doped MoOx film) by adding vanadium compound into the precursor solution. The mole ratio of Mo:V are 1:0, 1:0.05, 1:0.2, 1:0.5, 0:1, and these films are used as HTL in OPV. The device structure is ITO/ Mo(V)Ox/ P3HT:PCBM/ ZnO NP/ Al, and the working area is 0.16 cm2. The result shows that MoV0.05Ox device has the best performance, including power conversion efficiency 2.16%, Voc 0.6V, Jsc 6.93 mA/cm2, and FF 51.9%. The devices with higher V contents show weaker performance, but still better than that of pure MoOx device. In stability test, when comparing with PEDOT:PSS device, Mo(V)Ox devices show similar or slightly better performance.
In material analysis, scanning electron microscopy (SEM) and atomic force microscopy (AFM) are employed to examine the surface morphology of the Mo(V)Ox films. The images show that the Mo(V)Ox films barely change the morphology of ITO substrates. On the other hand, MoV0.05Ox film shows a smaller roughness comparing to pure MoOx film. X-ray photoelectron spectroscopy (XPS) reveals that there are Mo6+ and a minor amount of Mo5+ oxidation states in pure MoOx film. However, after adding V, almost only Mo6+ is observed in the Mo(V)Ox films. For V 2p spectra, the spectra show mainly V5+ oxidation state. As the V ratio increases, the intensity of V signal increases as well. As for electrical resistivity test, all Mo(V)Ox films exhibit the resistivity in the range of 10-3-10-1 Ω-cm and MoV0.05Ox shows the lowest resistivity among all, which is beneficial for hole transporting. On the other hand, the resistivity values of MoV0.2Ox and MoV0.5Ox are higher than that of pure MoOx film. Because the MoV0.05Ox film has highest conductivity and a lower roughness, carrier transportation is easier, which corresponds to a lowest Rs value of the device with MoV0.05Ox HTL. X-ray diffraction reveals that pure MoOx film shows weak diffraction peaks of MoO3 phase, and the peak intensity decreases as V ratio increases. The decrease of crystallinity causes the increase of resistivity in MoV0.2Ox and MoV0.5Ox films.
Using ultraviolet photoelectron spectroscopy (UPS), we can define the energy levels of valence band edge and Fermi level of Mo(V)Ox films. With the assistance of optical energy gaps calculated from UV-visible transmission spectroscopy, we can plot the energy band diagrams of devices. The result shows that device using MoV0.05Ox HTL has the smallest band offset between valence band edge of oxide and HOMO of P3HT, which is helpful for hole transporting from HOMO of P3HT to anode via MoV0.05Ox HTL, which leads to a better efficiency of hole extraction.

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