利用簡單程序製作多功能之介面層應用有機太陽能電池以達高效率之研究

Studies on multiple functionalities of interfacial layer with simple fabrication applying on organic solar cell for high performance

廖思豪(Sih-Hao Liao)

指導教授 ： 陳壽安

國立清華大學/工學院/化學工程學系/博士(2013年)

摘要

本研究提出一種創新簡便的程序藉由介面層的處理與元件的製程改善以提升高分子太陽能電池元件效能。首先瞭解活性層電子予體與電子受體之間的交互作用，進而開發介面層材料以提升元件效率。本研究分為四部分，第一部分是針對活性層高分子材料的分子設計，第二部分是藉由富勒烯衍生物摻雜金屬氧化鋅當作太陽能電池的電極，第三部分是聚茀系高分子側鏈接枝冠醚基團螯合金屬鉀離子當作太陽能電池之電子傳輸層，第四部分是製作單分子自組裝層修飾藉由富勒烯衍生物摻雜氧化鋅之電極應用於小分子太陽能電池。
第一部份是對於活性層的分子設計，提出改進低能隙高分子(PTB7)主鏈的共平面性，即在其中benzodithiophene (BDT)單元引入thiophene基團作為側鏈，而其thienothiophene (TT)單元則維持不變，而設計出新穎的PTB7-Th，可使吸收波長的範圍延長25 nm提高光吸收係數、較深HOMO 能階，在混摻PC71BM下應用於常態太陽能電池其效率可從6.42%提升至7.2%，所以PTB7-Th是適合作為反式高分子太陽能電池之活性層。另一部份新穎低能隙高分子(PIDTTT-E)是以4-hexyl-phenyl 基團作為Indacenodithiophene (IDT) 單元之側鏈與TT單元做交替式之共聚高分子，為了增進較深HOMO 能階，引入拉電子基團氟原子在TT單元的部分，而其IDT單元維持不變，進而合成出含有氟原子之PIDTTTF-E，可使吸收波長的範圍延長50 nm提高光的吸收係數、較深HOMO 能階，應用於常態之高分子太陽能電池其效率可從3.13% 提升至5.43% 增進了2.3%左右，但由於此效率為5% 低於PTB7-Th系列高分子效率(7.2 %) 約2%左右，效率較低之原因是由於此高分子不具有lamellar構型及良好的排列性(由低銳角入射廣角 X 光散射儀(GIWAXS)量測) 。
第二部分是為了有效的收集電子，提出製作出一個修飾ZnO電極的方法，亦即以含有羥基富勒烯衍生物(PCBE-OH)摻雜ZnO (稱此電極為 ZnO-C60)，經此摻雜ZnO在LUMO以下0.39 eV處產生一個新能階。C60在ZnO-C60電極中分佈為階梯型式，由表面往ITO方向遞減，它提供了雙重增進電子收集的功能。第一在陰極表面富有鍵結的富勒烯提供與活性層中富勒烯之直接接觸，有效提升自由電子之萃取；第二增進表面及縱向的導電度，因此可以減少高分子與ZnO直接接觸造成電子電洞於介面複合之機率，又可有效的收集了來自活性層內PC71BM的電子。將此ZnO-C60電極應用活性層為低能隙高分子(PTB7) 混摻PC71BM之反式高分子太陽能電池，其元件效率自6.65 %提升至8.21 %，而已上述新材料 PTB7-Th混摻PC71BM，其元件效率則從7.64 %提升至9.35 %，此值略高於文獻報導之最高值9.21 %。
第三部分是在常態高分子太陽能電池中，將聚茀系高分子側鏈接枝冠醚基團並進一步螯合金屬鉀離子於其中，當作電子傳輸層，應用於活性層為電子予體聚(3-己基噻吩) (P3HT)混摻電子受體indene-C60 bisadduct (ICBA) (或是indene-PCBM (IPCBM))系統下的高分子太陽能電池，當電子受體為ICBA而陰極為Al時，其PCE有效地從3.87 % 提升至6.88 % (當以IPCBM 為電子受體時從 3.06 % 提升到 to 6.21 %)，當電子受體為ICBA與陰極為Ca/Al的元件中效率從5.78 % 提升至7.50 % (對IPCBM，從5.53 % 提升到 6.63 %)。此效率提升的原因，是由於電子傳輸層提供多重功能性，包含光學阻隔作用使整體光場強度重新分佈，阻隔電洞與陰極的電子再結合，介面偶極作用使金屬陰極真空能階提升，提升介面導電度。此效率值7.5 %目前在電子予體為P3HT系統中是最高的。
第四部份是在小分子太陽能電池中，使用上述以富勒烯摻雜氧化鋅(ZnO-C60)當作陰極，當活性層為p-DTS(FBTTh2)2: PC71BM時，其效率為8.3 %，高於未摻雜ZnO之陰極6.08 %。更進一步的使用具有酚官能基的新穎富勒烯衍生物(NPC70-OH)當作單分子自組裝層於ZnO-C60表面，其效率進而提升至8.75 %。此效率值8.75 %目前在反式小分子太陽能電池系統中是最高的。

關鍵字

有機太陽能電池

並列摘要

The object of this research is to develop convenient processes for enhancing device performance of polymer solar cell (PSC) by interface treatment and device fabrication. We first study the physical properties of electron donor and electron acceptor in the active layer and propose novel materials for interfacial layer. Through a combination of them, PSCs with high performance are obtained. The present research contains four parts, being: (1) molecular design of polymer material for active layer; (2) utilization of fullerene derivatives doped zinc oxide as cathode of polymer solar cells; (3) used polyfluorene grafted with metal ion intercalated crown ether as electron transport layer for polymer solar cells; (4) fabricated self-assembled monolayer on fullerene derivatives doped zinc oxide as cathode of small molecule solar cells.
The first part: We propose the novel low band gap polymer PTB7-Th as donor by combining the advantages of incorporations of 2-ethylhexyl-thienyl group into benzodithiophene (BDT) unit in low band gap polymer PTB7 for improving coplanarity of the main chain (so that the absorption band can be extended to longer wavelength by 25 nm along with promoted absorption coefficient) and of retaining the fluorine-substituted TT unit with 2-ethylhexyl carboxylate group for higher HOMO level. This PTB7-Th along with PC71BM as acceptor are applied to compose of the active layer for conventional polymer solar cells (c-PSCs). The resulting device with the active layer PTB7-Th: PC71BM gives the power conversion efficiency (PCE) 7.2%, higher than that by replacing PTB7-Th with PTB7 6.43%. Another molecular design proposed is novel PIDTTT-E type low band gap alternative copolymer as donor by combining 4-hexyl-phenyl group into indacenodithiophene (IDT) unit and TT unit with 2-ethylhexyl carboxylate group (TT-E). For further improvement by making HOMO level deeper, we introduce electron withdrawing F atom onto the TT-E unit to give the new copolymer (PIDTTTF-E) based on fluoro-substituted TT units (TTF-E) and the same IDT unit, which can extend to longer wavelength by 50 nm along with the promoted absorption coefficient and deeper HOMO level. This novel copolymer (PIDTTTF-E) as donor along with PC71BM as acceptor are used to compose of the active layer for the novel c-PSCs. The resulting device with the active layer PIDTTTF-E: PC71BM gives the PCE 5.47%, higher than that by replacing PIDTTTF-E with PIDTTT-E 3.13%. The PIDTTTF-E has lower PCE (5.47%) than PTB7-Th (7.2%) by 1.73% since it has no lamellar packing and no good molecular packing from as determined GIWAXS measurement.
The second part: For effective collection of electron, we propose the modified ZnO cathode for inverted polymer solar cells (i-PSCs) by doping it with hydroxyl containing fullerene derivative (PCBE-OH) (designated as ZnO-C60). The doping of ZnO by PCBE-OH is evidenced by creating an energy level of ZnO (4.53 eV) below its LUMO level (4.14 eV) by 0.39 eV and an optical absorption in the range 400 to 500 nm as determined by ultraviolet photoelectron and optical absorption spectroscopy. This ZnO-C60 cathode provides two functionalities, providing additional transport pathway for electrons created by the high coverage of the fullerene derivatives on cathode surface and promoting electron conduction on surface and in the bulk. Therefore the chance for electron/hole recombination at the cathode interface is reduced by cutting off the chance of direct contact of polymer with ZnO, facilitating a more effective collection of electrons from the active layer. We use the ZnO-C60 as the cathode with the two active layers proposed above to compose of i-PSCs. The resulting i-PSCs with ZnO-C60 provide remarkably enhanced PSC PCE relative to that with pristine ZnO. For the active layer composed of PTB7-Th:PC71BM, the PCE promotes from 7.64% to 9.35%, and for PTB7:PC71BM from 6.65% to 8.21%. This 9.35% is slightly higher than the reported highest record 9.21%.
The third part: For conventional polymer solar cell, we present the novel electron transport (ET) polymer composed of polyfluorene grafted with K+ intercalated crown ether involving 6 oxygen atoms (PFCn6:K+) for bulk heterojunction PSC with regioregular-poly(3-hexyl-thiophene) (P3HT) as donor and indene-C60 bisadduct (ICBA) (or indene-PCBM (IPCBM)) as acceptor in the active layer and with Al or Ca/Al as cathode. Remarkable improvement in PCE by insertion of this ET layer is observed by promoting its value from 3.87% to 6.88% for PSC with ICBA and Al (or from 3.06% to 6.21% with IPCBM) and from 5.78% to 7.5% for PSC with ICBA and Ca/Al (or from 5.53% to 6.63% with IPCBM). This ET layer provides multiple functionalities including optical interference effect for redistribution of light intensity as an optical spacer, blocking holes from recombination with electrons at the interface with cathode, giving interfacial dipole for promoting vacuum level of cathode metal, and enhancing electron conduction. The 7.5% is the highest among the reported values in PSC systems with the simplest donor polymer P3HT.
The fourth part: In small molecule bulk heterojunction inverted solar cells (i-SMSCs), the ZnO-C60 is again used as the cathode. For the device with the active layer p-DTS(FBTTh2)2: PC71BM, which gives the PCE 8.3% higher than that of ZnO without doping 6.08%. Further incorporation of phenol substituted C70 (NPC70-OH) as self-assembled monolayer (SAM) on the top of ZnO-C60 gives further promoted PCE to 8.75%, which is the highest record in i-SMSCs.

並列關鍵字

organic solar cell

參考文獻

[48] Y. -C. Chen, C. -Y. Yu, Y. -L. Fan, L. -I. Hung, C. -P. Chen, C. Ting, Chem. Commun., 2010, 46, 6503

[26] Yoshihisa Inoue, George W. Gokel,“Cation Binding by Macrocycles”

[79] S.-H. Liao, Y.-L. Li, T.-Z. Jen, Y.-S. Cheng, S.-A. Chen, J. Am. Chem. Soc. 2012, 134, 14271

[88] O. Game, U. Singh, A. A. Gupta, A. Suryawanshi, A. Banpurkarb, S. Ogale, J. Mater. Chem. 2012, 22, 17302

[49] Y. Zhang, S. -C. Chien, K. -S. Chen, H. -L. Yip, Y. Sun, J. A. Davies, F. -C. Chenb, A. K.-Y. Jen, Chem. Commun., 2011, 47, 11026