中介層PCBM交鏈化與應用在倒置結構太陽能電池上的處理

Abstract

在能源需求大增的時代，太陽能電池成為當前重要的課題；其中高分子太陽能電池不僅生産成本低廉、重量輕，而且能夠橈曲，製成各樣型態的太陽能電池。近年來，在有機太陽能電池的發展下，為了促使主動層中的施體與受體材料接觸面積增加，發展出塊材異質接面(bulk-heterojunction)結構，藉由混合大幅增加施體與受體接觸面積進而提升激子分離機率，然而這種做法雖然成功提升效率，卻存在著無法控制混合的施體與受體在主動層中的分布情形的問題，因此後續研究者為了改善施體與受體在主動層中的垂直分層(morphology)利用了幾種方法，熱處理、慢乾法、在主動層中加添加劑、混和溶劑、加額外電場或是加中介層PCBM在ZnO薄膜與主動層之間，惟獨以溶液製程的中介層PCBM簡單又可大面積製程且不受基板限制，且無須考慮高分子材料是否具有結晶性質，只需在ZnO與主動層之間懸塗上中介層PCBM，就可借由PCBM的表面能量將主動層中的PCBM往其方向聚集，然而這方法雖然簡單卻也存在一大問題，中介層PCBM本身溶於有機溶劑，因此在旋塗以有機溶劑配置的主動層時，會將中介層PCBM清洗掉，使得原先塗佈的中介層PCBM蕩然無存，這使得中介層PCBM變成一無法控制的參數，且未能達到因有的效果。 Amare Benor Belay et al.發表了將PC61BM薄膜放入UV-ozone經由紫外光及臭氧的處理，產生C-O-C的鍵結，使得PCBM產生交鍊，可降低對有機溶劑的溶解度，因此本篇研究將PC61BM交鏈化，使得其不容易溶於有機溶劑，如此一來可有效控制中介層PCBM的參數，再將交鏈化的PC61BM做為中介層，製作以P3HT:PC61BM為主動層的倒置太陽能電池，而透過這個研究卻又有一重要發現，交鏈化後的PCBM雖然確實不被有機溶劑所清蝕，但卻因PCBM表面出現大量的OH鍵，使得電子傳輸時產生了電荷累積的現象，導致在JV curve中出現了S shape的情形，反而使得效率降低。為了降低PCBM表面的OH鍵，我們找出了經由UV-ozone處理使PC61BM交鏈化的最短時間，1分鐘，然而元件依然有S shape產生。因此為了改善這現象，本研究利用獨創的方式，在經由UV-ozone處理完後的中介層PC61BM表面旋塗0.1%的Triton X100，透過水滴接觸角的量測，原PC61BM表面與水接觸角為27度，經由UV-ozone處理1分鐘後變為13度，而在由Triton X100旋塗在其表面後，又恢復成了27度。利用此方法成功的消除S shape的，不但比他種同樣消除OH鍵的方式來的更為簡便並成功的提升了元件開路電壓、短路電流、填充因子至67%和元件效率至3.44%。 而在搭配低能隙高分子材料，多以PC71BM與其混層，因此我們同時也研究將中介層PC71BM交鏈化，並在以PBDTT-DPP: PC71BM為主動層的倒置結構中，加入此交鏈化的PC71BM層，並用Triton X100消除其表面OH鍵，使得元件效率從2.8%提升至3.8%。而為了提升短路電流，在氧化鋅種子層上生長氧化鋅奈米柱，透過不同的氧化鋅種子層濃度0.3M~0.6M控制奈米柱的疏密程度，從而影響主動層滲入柱間的狀況，使短路電流提升至14 mA/cm2且元件效率達到4%以上，而在奈米柱與主動層間旋塗的中介層PC71BM並交鏈化，藉由其改善主動層的垂直分層，高分子層與氧化鋅介面的改善，使得元件短路電流提升至17 mA/cm2，更能預防氧化鋅奈米柱直接與施體材料接觸而降低Voc，整體元件效率提升超過5%。 而普遍長柱元件的填充因子(FF)普遍來的較低~40%，雖然在加中介層後有所提升，但並未有大突破，因此我們嘗試了換不同的金屬氧化層V2O5，以及減少氧化鋅奈米柱生長時間，但都未能有效提升填充因子。而後藉由控制氧化鋅奈米柱在氮氣下退火溫度，在退火溫度為175度、1小時下，成功的使得填充因子提升至~53%，而在此倒置結構與奈米柱結構元件效率提升至6.4%。

Using PC61BM as an interlayer between ZnO and the P3HT/PC61BM layer has been promoted as a step to improve the morphology of the active layer; however, the interlayer PCBM is highly soluble in organic solvents and can easily mix with the organic active layer. This paper reports a UV-ozone treatment process to polymerize PCBM, making PCBM insoluble in organic solvents. PCBM’s thin film can be polymerized in 60 s, thereby controlling the parameters of the interlayer. However, the polymerized PCBM has –OH on the surface, which will affect electronic transmission. Our investigation shows that the longer PCBM is treated with UV-ozone, the more –OH exists on the PCBM surface and the worse the performance of the device is. Thus, we explored a technique to resolve the problem. A spin-coated surfactant —Triton X100 (0.1%)—on the polymerized PCBM layer process removes the surface –OH caused by the polymerization of PCBM. Compared to other methods for eliminating the –OH (e.g., annealing the polymerized PCBM thin film or adding Triton X100 to the solvent of the active layer), the spin-coated Triton X100 shows the best performance. The FF is promoted to 67%, and the PCE is promoted to 3.44%. Therefore, the interlayer PCBM can be controlled and the disadvantage in using polymerized PCBM can be eliminated. In recently the low band gap polymer materials are often mixed with PC71BM. In this study, the interlayer PC71BM is polymerized through UV-ozone treatment, and adds in the inverted structure of the active layer PBDTT-DPP: PC71BM. The device with interlayer PC71BM is spin coated Triton X100 to eliminate the surface OH bonds, making the device efficiency from 2.8% to 3.8%, but short-circuit current is not more than 10 mA/cm2. We have grown ZnO nanorods structure to enhance the short-circuit current of devices, the short-circuit current is effectively increased by more than 12 mA/cm2 ,but the open circuit voltage drops to 0.622 V. In order to protect the donor material in contact with zinc oxide nanorods, the polymerized interlayer PC71BM has become particularly important. The interlayer PC71BM successfully reduced the contact probability of the donor material and zinc oxide nanorods. Therefor, the open circuit voltage increased to 0.729 V, the short-circuit current density increased from 12 mA/cm2 to 15 mA/cm2 and device efficiency enhanced from 4% to 5%. Finally, the device with interlayr PC71BM performance has enhanced due to the annealing of ZnO nanorod for 1hour at 175 degree, thus the device short-circuit current density has reached 17.199 mA/cm2, open-circuit voltage maintained in the 0.72 V, FF increased from 40% to 52%, and device efficiency reached 6.4%.