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

• Main Authors: Liao, Sih-Hao, 廖思豪
• Other Authors: Chen, Show-An
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/43202637510821750520

博士 === 國立清華大學 === 化學工程學系 === 102 === 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.