雙功能型交聯添加劑對有機光伏太陽能電池之效率與熱穩定性探討

Abstract

本篇論文設計合成以二酮吡咯並吡咯(Diketopyrrolopyrrole, DPP)中心且末端具有疊氮官能基的交聯小分子材料，並將其添加入以高光電轉換效率的導電高分子Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th)與碳球衍生物[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)為主動層的有機太陽能電池中。藉由小分子適當的能階與填充主動層材料之可見光吸收範圍提升元件效率，也透過加熱產生化學交聯反應以提供穩定性，達到雙功能性的效果。合成出的三種交聯小分子以DPP為中心，雙噻吩、苯環與三聯苯為臂鏈，分別命名為DPPBTDA、DPPPDA與DPPTPTA，透過加入不同化學結構的交聯小分子探討其對有機太陽能電池效率提升與熱穩定性的影響。以高效率的導電高分子PTB7-Th為施體與碳球衍生物PC61BM為受體 (1:1.5, w/w) 混摻不同比例小分子材料以製備成太陽能電池之主動層，元件採反式結構。在AM 1.5照度下發現添加適當比例之交聯小分子能夠有效提升其光電轉換效率，混摻5wt % DPPBTDA元件的光伏參數:開路電壓為 0.70 V，短路電流為14.77 mAcm-2，填充因子為65.5%，轉換效率為6.75 %；導入5wt % DPPPDA之開路電壓為 0.76 V，短路電流為14.63 mAcm-2，填充因子為66.0 %，轉換效率為7.35 %。而混摻3wt % DPPTPTA之開路電壓為 0.76 V，短路電流為15.90 mAcm-2，填充因子為66.8 %，轉換效率更能達到8.11 %。另一方面，在高溫環境下維持穩定光伏轉換效率，主動層表面形貌之維持尤為重要。由傅里葉轉換紅外光譜分析儀 (FT-IR)與紫外光可見光光譜儀(UV-vis)鑑定交聯小分子之化學性交聯反應生成，經過高溫作用下，疊氮官能基會與碳球衍生物PC61BM進行3+2環化反應並生成共價鍵結，藉此穩定主動層形貌防止聚集，達到高效熱穩定性。利用光學顯微鏡觀察PTB7-Th與PC61BM組成之主動層，添加適量交聯小分子，於長時間150 oC高溫環境下主動層形貌之相分離程度減緩，結晶數量明顯減少，其中又以DPPTPTA效果最佳。混摻5wt % DPPTPTA的系統在18小時的加熱後仍能夠維持4.02 %的轉換效率。為了探討交聯小分子對太陽能電之效率提升的機制，我們透過螢光光譜(PL)證實DPPPDA與DPPTPTA能透過螢光共振能量傳遞(FRET)的方式將能量傳遞至PTB7-Th。也經由EQE的測量發現添加DPPPDA與DPPTPTA能夠形成緊密的階梯式能階幫助載子傳遞。最後也以原子力顯微鏡(AFM)觀察添加合成出的小分子對於主動層材料的影響。至於熱穩定性提升機制的方面，我們分別透過AFM與光學顯微鏡(OM)觀察主動層形貌的變化，由結果顯示添加是當比例的交聯添加劑確實能夠抑制相分離的產生並增強元件熱穩定性。經由元件效率的測量以及機制的探討，證實添加適量的DPPPDA與DPPTPTA確實能夠達到提升元件效率並增強熱穩定性的雙功能性效果。

A novel type of crosslinkers based on diketopyrrolopyrrole (DPP) as core, azide groups on the terminal of alkyl chains along with bithienyl, phenyl or terphenyl as conjugated arms (DPPBTDA, DPPPDA and DPPTPTA, accordingly) were developed. In order to investigate the enhancement of efficiency and thermal stability caused by crosslinkers, DPPBTDA, DPPPDA and DPPTPTA were respectively blend into the active layer which consist of a high performance conducting polymer (PTH7-Th) and a fullerene derivative (PC61BM). Inverted organic solar cells (OSCs) were fabricated by spin-coating the blends of PTB7-Th as donor, PC61BM as acceptor and different amounts of DPPBTDA, DPPPDA or DPPTPTA. In terms of photovoltaic performance, the OSC with 5% DPPBTDA showed a power conversion efficiency of 6.75 % whereas the OSC with 5% DPPPDA showed a power conversion efficiency of 7.35 %. Furthermore, a power conversion efficiency of 8.11 % was observed for the sample with 3 % DPPTPTA. On the other hand, the morphology of active layer has to be carefully controlled to offer optimum photovoltaic performances. According to FT-IR and UV-vis analysis, the crosslinking reaction did occur between the additives and PC61BM. The optical microscope (OM) result also indicates that only few fullerene crystals can be observed in the active layers with the crosslinkers upon heating the samples at 150˚C for 18 hours. Especially, the device with 5% DPPTPTA remained a power conversion efficiency of 4.02 % after heated at 150˚C for 18 h. The photoluminescence (PL) result showed that energy transfer was present between PTB7-Th, and two individual crosslinkers, DPPPDA and DPPTPTA. The external quantum efficiency (EQE) measurement also showed that the respective addition of DPPPDA and DPPTPTA increased the power conversion by enhancing the charge transfer. These two measurements revealed the mechanisms of the enhancement of power conversion efficiency. Apart from that, the result of atomic force microscope (AFM) and OM showed that the addition of crosslinkers inhibited macrophase separation in the active layers. Based on the above, two of the crosslinkers (DPPPDA and DPPTPTA) were able to increase the power conversion efficiency with fluorescence resonance energy transfer (FRET) and ladder-like energy levels and all of them could bring about stable morphology and further enhance the thermal stability via crosslinking reactions.