The Effect of Light-Harvesting Property of Oxasmaragdyrin and Its Impact as Hole Transporting Material in Perovskite Solar Cell
Mario Leonardus a b c, Chen-Hsiung Hung a
a Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529 Taiwan
b National Chiao Tung University Hsinchu, Taiwan, 1001 University Road, Hsinchu, Hsinchu, Taiwan, Republic of China
c Sustainable Chemical Science and Technology Program, Taiwan International Graduate Program Academia Sinica Nankang, Taipei 11529 Taiwan
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP20)
Tsukuba-shi, Japan, 2020 January 20th - 22nd
Organizers: Michio Kondo and Takurou Murakami
Oral, Mario Leonardus, presentation 001
Publication date: 14th October 2019

Porphyrins materials such as tetraarylporphyrins, subphthalocyanines, phthalocyanines, and oxasmaragdyrins have shown compatibility as a hole transporting material (HTM) in perovskite solar cell (PSC) architecture. The devices with porphyrinic macrocycles as HTM show comparable power conversion efficiency (PCE) to those with Spiro-OMeTAD as the HTM.1-6 Nevertheless, the influence of the light-harvesting properties of porphyrinic macrocycles to the efficiency of hole migration has not been investigated. We discuss herein the overall consequence of light-harvesting of these compounds to the PCE performance using the derivatives of SM09, the best HTM among examined oxasmaragdyrins in our previous report, as model compounds.7 The alkyl chains with different chain lengths are introduced in the core position of SM09 to replace the fluorine atoms and attenuate the intermolecular distance which might alter the rate of energy transfer or self-quench. A series of core modification of SM09, encoded by SM09-B(OMe)2, SM09-B(OEt)2, SM09-B(OBut)2, SM09-B(OHex)2 and SM09-B(OOct)2 will be examined and compared.

The presence of methoxy groups in SM09-B(OMe)2 slightly improved the PCE performance of PSC in comparison with parent SM09, consistent with our previous report about the effects of the methoxy group as the HTM to PSC performance. Additionally, we have found that diminished PCE observed upon increasing the alkyl length from two-carbon SM09-B(OEt)2 to four-carbon SM09-B(OBut)2 likely due to the furtherer molecular distance which affected the hole mobility. Interestingly, we found a unique phenomenon in the J-V curves of the devices with SM09-B(OHex) and SM09-B(OOct) as HTMs, i.e. an unsteady Jsc with current continuously decrease at low voltage region. This phenomenon leads to unreasonable Fill Factor (FF) and an unusual low of short-circuit current density (JSC). After a series of examinations, we believe that this phenomenon is related to the self-quenched of SM09 derivatives. The observation that this current drop at low voltage region can be precluded by increasing the scan rate suggests that it is not a rapid exchange process. We used a filter light to measure the PCE performance and found that the dropping current was not observed while the device exposed with short-wavelength light (<600 nm) but still appear when it exposed by long-wavelength light (>600 nm). The presence of longer alkyl chains generates a larger intermolecular distance between the molecule which has not only decreased the hole mobility but also suppress self-quenched of boryl oxasmaragdyrins.  Thereafter, the excited HTM will inject photoelectrons to the LUMO of perovskite cell, which then either retards the excitation of perovskite layer or increases the rate of charge recombination from electrons at LUMO of perovskite and holes on the HOMO of HTM. Additional photophysical lifetime measurements will be conducted to provide conclusive evidence of this phenomenon. 

Searching of new HTMs as replacement of spiro-OMETAD has been a hot topic in PSC studies. Our finding for the first time reveals the effects upon using photoactive materials as HTM. Under the circumstance that there is no significant alternation on Voc and Jsc upon changing the scan rates during the measurements, the solution to obtain a more accurate PCE can be achieved through increasing scan rates. However, the competition on photon absorptions between HTM and perovskite layer appears to exert negative effects to the overall performance of PSC and should be taken into consideration upon the design of HTMs.

The authors acknowledge the support from the Ministry of Science and Technology (MOST), Taiwan International Graduate Program-Sustainable Chemical Science and Technology (TIGP-SCST) Academia Sinica, Advanced nano/micro-fabrication and characterization lab at Institute of Chemistry-Academia Sinica.

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