Cyanine tandem and triple-junction solar cells
Ultrathin bilayer heterojunction solar cells using cyanine electron donors and electron acceptor C-60 are used to fabricate monolithically stacked tandem and triple junction devices. Sub-cell stack sequences as well as C-60 layer thicknesses are optimized by optical modeling and maximum efficiency is corroborated experimentally. The highest power conversion efficiency of 4.3% under full sun irradiation is achieved with a tandem cell where heptamethine and trimethine cyanine dyes are used in the front and back cell, respectively. The open circuit voltage matches the sum of the two respective open circuit voltages of the individual single junction solar cells within 3%. Triple junction cells using an additional sub-cell with a pentamethine cyanine suffer from electrical series resistance. At low light irradiation intensity, however, both triple and tandem solar cells reach power conversion efficiencies above 5% in agreement with the performance increase predicted from numerical simulation. (C) 2015 Elsevier B.V. All rights reserved.