交叉型共軛的低能隙高分子合成及其在有機太陽能電池之應用

Abstract

有機高分子太陽能電池因製程簡單、成本低廉、具有透光性、可大面積製造以及可與可撓式基板相結合等優點，使得近年來受到國內外矚目。至今，大部分高效率的有機太陽能電池元件均使用混摻異質接面型(BHJ)結構，其由p-型的施體(共軛高分子)材料及n-型的受體(富勒烯衍生物)材料混摻而成，進而創造出最大接觸面積使激子有效的分離。本研究目的在於設計及合成一系列新的p-型共軛高分子，以達到高效率的混摻異質接面型太陽能電池。 本研究發表了六個新穎的交叉型共軛低能隙高分子的合成及性質。合成上利用鈀金屬為催化劑進行Stille coupling的反應，成功結合了三部分的電子施體與電子受體，其化學結構可以(thiophene donor)m-(thiophene acceptor)n表示，而電子施體除了以thiophene為主體的D1結構外，另一個電子施體結構是以1,4-Dibromo-2,5-{bis(4-[N,N-(dioctylamino)styryl])}-benzene) D2為聚合單體，之後再分別配合2,1,3-benzothiadiazole A1及diketopyrrolopyrrole A2二種電子受體結構聚合出二系列(PCSTBT及PCSTDPP)的共聚高分子。在性質上，六個共聚高分子皆是熱穩定性高的材料，且可溶於一般有機溶劑。此外，因高分子鏈上電子施體及電子受體間具有強大的分子間電荷傳遞(ICT)作用，使其吸收範圍可擴大至近紅外光區，且光學能隙在1.36到1.75 eV。 本實驗所採用的元件結構為ITO/PEDOT:PSS/Copolymer:PCBM(1:2, w/w)/Ca/Al。在元件表現上，共聚高分子PCSTBT系列以PCSTBT75表現最佳，其Voc值為0.59 V，Isc值為1.96 mA/cm2，FF值為29 %，PCE值為0.44 %；共聚高分子PCSTDPP系列以PCSTDPP75表現最佳，其Voc值為0.5 V，Isc值為3.71 mA/cm2，FF值為33.4 %，PCE值為0.62 %，而PCSTDPP75也是六個共聚高分子中表現最佳的材料，在於其能隙最低，達到1.36 eV。 雖然目前所得的光電轉換效率都不算高，但在元件上進行調整與改善(例如加入添加物或者採用退火處理等等)，使元件性質達到最佳化，是有可能讓光電轉換效率大大突破的。

In recent years, polymer solar cells (PSCs) have been attracting considerable attention for many advantages, such as low cost, light weight, easy fabrication and their potential application in large area flexible devices. Since the discovery of the photovoltaic effect in bulk heterojunction (BHJ) devices, the considerable publications in PSCs have been reported. PSCs based on the concept of bulk heterojunction (BHJ) configuration where an active layer comprises of a p-type donor (conjugated polymer) and an n-type acceptor (fullerene derivative) materials, represents the most useful strategy to maximize the internal donor-acceptor interface area allowing for efficient charge separation. The goal of this research is to design and synthesize a series of new p-type conjugated polymers to achieve highly efficient BHJ solar cells. Six new cross-conjugated and low bad-gap copolymers have been synthesized and characterized. These of three-component donor-acceptor random copolymers are symbolized as (thiophene donor)m-(thiophene acceptor)n. The PCSTBT series are prepared by Stille coupling polymerization of 2,5-bis(trimethylstannyl)thiophene D1 with 1,4-dibromo-2,5-{bis(4-[N,N-(dioctylamino)styryl])}-benzene) D2 and4,7-dibromo-1,2,3-benzothiadiazole A1, while PCSTDPP series are prepared by Stille coupling polymerization of 2,5-bis(trimethylstannyl)thiophene D1 with1,4-dibromo-2,5-{bis(4-[N,N-(dioctylamino)styryl])}-benzene) D2 and3,6-di(2-bromothien-5-yl)-2,5-dioctylpyrrolo[3,4-c]pyrrole-1,4-dione A2. Thesynthesized copolymers are soluble in common organic solvents and possess good thermal stability. The UV-vis absorption spectra of these copolymers contain an intramolecular charge transfer (ICT) transition band, which lead to an absorption extending into near-infrared region and optical band gaps ranging from 1.36 eV to 1.75 eV. Polymer solar cells of a BHJ were fabricated with the structure of ITO/PEDOT:PSS/Copolymer:PCBM(1:2, w/w)/Ca/Al. The PCE were 0.10 % (PCSTBT25), 0.18 % (PCSTBT50), 0.32 % (PCSTBT75), 0.37 % (PCSTDPP25), 0.54 % (PCSTDPP50), 0.62 % (PCSTDPP75). The higher PCE for PCSTDPP75 copolymer solar cell is attributed to the low band gap of this copolymer compared to others, which increases the numbers of photogenerated excitons and corresponding photocurrent of device. Although their PCE is still relatively low, further improvement on device performance can be achieved through morphology control by thermal annealing and chemical annealing, and carefully device engineering.