title: Lithography-Free Broadband Ultrathin-Film Absorbers with Gap-Plasmon Resonance for Organic Photovoltaics.
abstract: Strategies to confine electromagnetic field within ultrathin film emerge as essential technologies for applications from thin-film solar cells to imaging and sensing devices. We demonstrate a lithography-free, low-cost, large-scale method to realize broadband ultrathi-film metal-dielectric-metal (MDM) absorbers, by exploiting gap-plasmon resonances for strongly confined electromagnetic field. A two-steps method, first organizing Au nanoparticles via thermal dewetting and then transferring the nanoparticles to a spacer-reflector substrate, is used to achieve broader absorption bandwidth by manipulating geometric shapes of the top metallic layer into hemiellipsoids. A fast-deposited nominal Au film, instead of a conventional slow one, is employed in the Ostwald ripening process to attain hemiellipsoidal nanoparticles. A polymer supported transferring step allows a wider range of dewetting temperature to manipulate the nanoparticles' shape. By incorporating circularity with ImageJ software, the geometries of hemiellipsoidal nanoparticles are quantitatively characterized. Controlling the top geometry of MDM structure from hemisphere to hemiellipsoid increases the average absorption at 500-900 nm from 23.1% to 43.5% in the ultrathin film and full width at half-maximum of 132-324 nm, which is consistently explained by finite-difference time-domain simulation. The structural advantages of our scheme are easily applicable to thin-film photovoltaic devices because metal electrodes can act as metal reflectors and semiconductor layers as dielectric spacers.
authors.affiliation: School of Mechanical Engineering, Yonsei University , 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea.
authors.forename: Minjung
authors.initials: M
authors.lastname: Choi
authors.affiliation: School of Mechanical Engineering, Yonsei University , 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea.
authors.forename: Gumin
authors.initials: G
authors.lastname: Kang
authors.affiliation: School of Mechanical Engineering, Yonsei University , 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea.
authors.forename: Dongheok
authors.initials: D
authors.lastname: Shin
authors.affiliation: Korea Institute of Science and Technology , Hwarangno 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea.
authors.forename: Nilesh
authors.initials: N
authors.lastname: Barange
authors.affiliation: Samsung Advanced Institute of Technology , Suwon-si, Gyeonggi-do 16678, Republic of Korea.
authors.forename: Chang-Won
authors.initials: CW
authors.lastname: Lee
authors.affiliation: Department of Applied Chemistry, Kyung Hee University , Yongin, Gyeonggi 17104, Republic of Korea.
authors.forename: Doo-Hyun
authors.initials: DH
authors.lastname: Ko
authors.affiliation: School of Mechanical Engineering, Yonsei University , 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea.
authors.forename: Kyoungsik
authors.initials: K
authors.lastname: Kim
keywords: broadband absorption; gap surface plasmons; nanoparticles; thermal dewetting; ultrathin-film absorber