新型共軛高分子/富勒烯衍生物異質接面型高分子太陽能電池之光伏特性與熱穩定性探討

Abstract

目前在異質接面型高分子太陽能電池中，主動層的組成通常是以共軛高分子作為電子予體(donor)、富勒烯衍生物作為電子受體(acceptor)，其中又以聚(3-己烷基噻吩)(poly(3-hexylthiophene), P3HT):[6,6]-苯基-C61-丁酸甲酯([6,6]-phenyl-C61-butyric acid methyl ester, PC61BM)的系統最被廣泛地研究。本論文分為五個研究主題，內容含主動層donor與acceptor材料的研究開發與提升電池效率與長時間熱穩定性的元件結構設計。 在第一部份的研究主題研究為富勒烯吡咯烷衍生物(fulleropyrrolidines)在吡咯啶(pyrrolidine)氮上接有不同長度的烷基鏈段，混摻後的形態對於電池的光伏特性有很大的影響，NMC、NHC、NMPC分別與P3HT 混摻所製備成的順式結構元件發現皆有S-kinks的光伏特性產生，經載子傳遞速率的量測，發現三元件之電洞遷移率皆比電子遷移率慢了許多，造成遷移率嚴重不平衡產生S-shape的光伏特性，研究亦藉由混摻後薄膜的TEM、UV及PL圖譜影像，發現P3HT在主動層中仍有足夠的空間自組裝排列堆疊形成結晶，為了找出電洞遷移率過慢的主因，元件設計使用陽極緩衝層與添加劑應用於此章的實驗系統，皆能有效地改善元件S-shape的光伏特性，證實電洞遷移率過慢的原因為主動層垂直方向碳六十衍生物沉降行為使得組成分布不均所致，為了再進一步了解產生沉降的原因，亦合成了另外兩個碳六十衍生物NDPC與NPC，分別帶有強推電子基與電中性官能基，同樣與P3HT混摻組成元件後發現NPC沒有S-shape I-V curve產生，而NDPC元件依然產生了S-shape的光伏特性，而造成碳六十衍生物沉降的原因可能與富勒烯吡咯烷衍生物結構中吡咯啶的三級胺鹼值與PEDOT:PSS強酸特性的PSS (pH < 2)產生作用，促使富勒烯吡咯烷衍生物沉降形成一薄層於PEDOT:PSS表面，造成電洞傳遞路徑受阻，結果使電洞遷移率相對慢了許多，研究亦使用V2O5取代原先使用的電洞傳輸層PEDOT:PSS，使得S-kinks現象全部消失也順著碳六十衍生物沉降的本質行為，製備反式結構太陽能電池，不僅解決了S-shape的光伏特性，更使元件能量轉換效率(PCE)明顯提升。這個研究結果提供一個完整明確且有意義的的研究資訊於設計開發碳六十衍生物與高分子太陽能電池元件。 在第二部分的研究主題，主要探討二苯基環丙烷富勒烯衍生物(diphenylmethanofullerenes, DPMs)結構具推拉電子基對於高分子太陽能電池之開環電壓(Voc)的影響。由於最高可獲得的Voc係取決於donor的HOMO能階與acceptor的LUMO能階差值，為了使元件獲得較大Voc，acceptor需具有較高的LUMO能階。有鑑於此，成功地合成了三個碳六十衍生物，其結構內接有不同推拉電子性質的官能基，分別將其縮寫命名為DPM-O、DPM-OCO及DPM-COO，相當令人覺得有趣的是發現CV測得三者的LUMO能階皆與PC61BM相同，但與P3HT混摻組成的太陽能電池，其Voc卻有相當明顯的不同，實驗結果顯示Voc大小與固態薄膜DPMs分子間through space電荷轉移(charge transfer)行為有密切的關係，其中P3HT/DPM-O的Voc達0.69 V比P3HT/PC61BM高了0.1 V，這個發現，提供了一個嶄新的策略應用於開發高能量轉換效率(PCE)高分子太陽能電池的研究。 在第三部分的研究主題為深入探討當今極具代表性的低能隙的共軛高分子PTB7-Th與PC61BM混摻形成異質接面形態的不穩定性。實驗結果發現PTB7-Th:PC61BM主動層於100 ℃經熱退火處理900分鐘後，主動層會出現大規模的PC61BM聚集，使D/A之間接觸面積大幅地減少，亦降低電洞遷移率，因而使元件PCE由最初未經熱退火處理時候的6.65% 大幅衰減至2.83%，降幅近六成，顯示其熱穩定性不佳。研究提出一個簡單有效的方法，在主動層裡添加少量非晶的bis-PC61BM作為相分離抑制劑，發現不僅可以有效地抑制PC61BM於長時間熱處理後所產生的聚集行為，維持異質接面形態的樣貌，同樣在100 ℃熱退火處理900分鐘後，元件PCE仍可維持於初始狀態之90%，電流值(Jsc)更可穩定相似於初始態。此外更藉由GIWAXS/GISAXS來分析觀察形態的微結構變化，藉由散射圖譜說明PTB7-Th:PC61BM主動層隨著熱退火處理的時間增加，會誘使純PC61BM持續聚集並與中型尺度(meso-scale)的PC61BM-rich區域連結形成大型尺度(macro-scale)的相分離，影響了PTB7-Th規整的排列與結晶生長；倘若以部分bis-PC61BM取代PC61BM作為相分離抑制劑，可以有效地防止純PC61BM擴散到PC61BM-rich區域，藉此維持主動層形態，進而能夠使元件穩定。此研究對高效能高分子太陽能電池於未來商業化提供了一相當值得參考的報告。 在第四部份的研究為開發一系列噻吩作為共軛側鏈之benzodithiophene (BDT)分子分別與thieno[3,4-c]pyrrole-4,6-dione (TPD)、dithienyl thieno[3,4-c]pyrrole-4,6-dione (DTTPD)、dithieno-[3’,2’:3,4;2”,3”:5,6]benzo[1,2-c][1,2,5]thiadiazole (fDTBT) 和 dithienyl-2,1,3-benzothiadiazole (DTBT)，以Stille polycondensation進行共聚，合成一系列新穎的D-A二維共軛高分子，依序分別將其命名為NAP01、NAP02、NAP03及NAP04。BDT單元為一良好的電子予體(donor)，其具有良好之結構平面性、高電洞遷移率與較低的HOMO能階，四種電子受體(acceptor)為TPD、BT、π共軛單元及併環(fused)所組成。BDT-based copolymers的設計可藉由拉電子單元的強弱調整光學吸收性質與HOMO能階。再之藉由XRD實驗分析，NAP02結構含π共軛單元之設計能夠有效地使高分子主鏈不受到龐大側鏈影響，使之能於主鏈及側鏈皆能夠形成結晶，進一步地將其與PC61BM混摻(10:8 w/w)製備組成的元件，最佳PCEmax= 5.54% (PCEave= 5.17%)，Voc高達938 mV，Jsc 為8.28 mA/cm2，FF達66.6%，頗具潛力。此研究替donor材料的開發設計提供了另一全新概念，續往高效率太陽能電池的目標邁進。 於第五部分的研究，使用了一種較低能隙(low bandgap)的新型二維共軛高分子KBP07，以拉電子行為之2,1,3-Benzothiadiazole連結三噻吩共軛側鏈平行於高分子主鏈，使光學吸收能夠更為紅移且寬廣。由實驗結果得知，當主鏈含有龐大的側鏈基團，主鏈結構含π共軛單元spacer，對於高分子堆疊排列越有幫助，結晶行為與電洞遷移率之表現亦越佳；另外於UV-Vis觀察發現KBP07保有二維共軛高分子的特性即側鏈與主鏈擁有各自的吸收特徵峰，吸收範圍寬廣(300 nm-800 nm)，HOMO能階比一維線性高分子P3HT來得低，將之混摻製備組成高分子太陽能電池元件，於最佳化的實驗條件KBP07:PC71BM(1:2 w/w)元件PCEmax達4.90% (PCEave= 4.60%)。上述實驗結果，吾認為還有空間將此類型之共軛高分子做更進一步地優化，使PCE提升。這項研究對於二維共軛高分子的開發與元件性能的優化製程參數有相當助益的實驗報告，藉由主鏈與側鏈雙向搭配，使二維共軛高分子的應用更為彈性與多樣化。

In bulk-heterojunction (BHJ) polymer solar cells (PSCs), conjugated polymer and fullerene derivative are commonly utilized as an electron donor and an electron acceptor, respectively, to form the active layer. Searching potential polymer donor and fullerene acceptor plays an important role in advancing the power conversion efficiency (PCE) and operational lifetime of such solar devices. In the first part, a series of N-substituted fulleropyrrolidines were employed as electron acceptors blending with P3HT as electron donor to fabricate polymer solar cells. It was found that the type of substituent significantly influences the photovoltaic behavior of the solar devices. As the substituent is an alkyl group, such as N-methyl, N-hexyl, N-ethylhexyl, abnormal S-shape current-voltage (I-V) curves are resulted. The analysis of the P3HT/N-alkyl fulleropyrrolidine blends by TEM, UV-vis, and PL shows no obvious difference between these films. This abnormal I-V curve can be ascribed to the formation of a N-alkyl fulleropyrrolidine interlayer at the bottom of the photoactive film in the course of spin-dying that blocks the transport of holes to the anode. To verify this speculation, the para position of phenyl substituent in fulleropyrrolidine was functionalized with an electron-donating group. (NN-dimethyl, methoxyl), or hydrogen atom. As expected, the cell using N,N-dimethylphenyl or methoxylphenyl fulleropyrrolidine as acceptor has an S-shaped I-V curve but the one based on phenyl fulleropyrrolidine behaves a normal photovoltaic performance. Interestingly, as an inverted cell structure is adopted to reverse the transport routes of carriers inside the photoactive blend, the problem of S-shape kink associated with all fulleropyrrolidine are totally solved, suggesting these new fullerene acceptors can be applied as effective acceptor in inverted PSCs. In the second part, we investigated the effect of electron donating and withdrawing group of C60 derivatives on the open-circuit voltage (Voc) of polymer solar cell. Herein, we functionalized C60 with DPM-O, DPM-OCO or DPM-COO groups, which have various electron donating and withdrawing ability. Very interestingly, the cyclic voltamograms showed the LUMO energy levels of these three fullerene derivatives are comparable with that of PCBM, but combination of AC-2 measurements and UV-Vis absorption spectra demonstrated that the LUMO of DPM-O and DPM-OCO films is 0.04-0.08 eV higher than that of PCBM film. This is probably because the electron donated moieties and neighbor C60 cores form charge transfer complexes in the solid film. Their Voc decreased in the trend, DPM-O > DPM-OCO > DPM-COO. This result opens a new approach to develop high-Voc polymer solar cells. In the third part, we observed that thermally annealing the PTB7-Th:PC61BM blends at 100℃ for 900 minutes leads to large-scale PC61BM aggregation with PTB7-Th matrix that decreases the interface area between the donor and the accepter as well as hole mobility, significantly lowering the PCE from 6.65% to 2.83%. The multilength-scale evolution of the morphology of PTB7-Th/PC61BM film from the scattering profiles of grazing incidence small-angle and wide-angle X-ray scattering indicates the PC61BM molecules spatially confine the self-organization of polymer chains into large domains during cast drying and upon thermal activation. However, the addition of bis-PC61BM into the PTB7-Th:PC61BM blends as phase separation inhibitors effectively inhibits the formation of PC61BM clusters during high-temperature aging. As a result, the PCE of the device with bis-PC61BM retains ~90% of its initial value after 900 minutes annealing at 100 oC. In the fourth part, we explored a new class of conjugated polymers, namely NAP01, NAP02, NAP03 AND NAP04 which were synthesized by Stille polycondensation. These polymers have good solubility in common organic solvents due to the presence of 2,3-didecylthiopheneg groups. Their crystallization behavior and optical, electrochemical and electronic properties were measured and discussed. The BHJ PSCs based on the polymer with thiophene spacer show much better performance than the polymers without spacers. These findings indicate that the optoelectronical properties of polymers can be easily control by inserting thiophene spacers and fused rings into the polymer backbone. This study provides an important rout for designing new materials to obtain higher Voc, short-circuit current (Jsc), fill factors (FF) and PCE of BHJ solar devices. In last part, a novel two-dimensional (2-D) conjugated polymer with tertrthiophene-vinylene (TTV) as conjugated side chain, KBP07, containing of 2,1,3-benzothiadiazole (BTD) was synesized by Stille polymerization. The optical and electrochemical properties of KBP07 shows not only a broader absorption wavelength from 300 nm up to 800 nm (λonset~780 nm；Egopt = 1.59 eV) but low-lying HOMO energy level. In addition, as shown as the X-ray diffraction spectrum, the high crystallinity can be observed. Under AM1.5G illumination at 100 mWcm-2, the BHJ PSC fabricated from KBP07/PC71BM exhibits the best PCE of 4.90% (PCEave= 4.60%). These findings indicate that introducing an electron-withdrawing acceptor to build a D-A 2-D conjugated polymer is a simple but effective strategy to broaden the absorption and achieve high photovoltaic performances.