II. Exploiting Ternary Blends to Accurately Control the Coloration of Semi-Transparent, Non-
2.1 Research Backgrounds
Over the past several decades, OPVs have gained recognition as a potential next-generation clean energy technology because they possess versatile traits such as low-cost, light weight,83, 84 flexibility,85 easy processing86-89 and semi-transparency.90-92 Semi-transparent organic photovoltaics (STOPVs) use conductive, high-transmittance electrodes with variable active layer materials and have characteristics unavailable to other solar technologies, especially for promising applications such as power generating windows and ceilings for more efficiently using solar light and indoor light, agricultural greenhouse in building integrated photovoltaics (BIPVs) and wearable electronics. A well-known challenge in STOPVs is to overcome the trade-off between PCEs and average visible transmittance (AVT) of semi- transparent devices,93-95 however, for feasible commercialization in the context of semi-transparent applications like BIPVs, optical and aesthetic characteristics of devices such as hue, saturation and transparency are of paramount importance.
Notable progress has been made in the field of STOPVs and with OPVs in general due to the advent of effective NFAs. Acceptors based on buckminsterfullerene derivatives were used almost exclusively until about 2015 and have been able to yield PCEs of up to about 912% in well-optimized devices.16-
18, 96 However, fullerene acceptors suffer from several drawbacks including high production cost, poor stability, weak absorption in the visible-near infrared region and limited tunability of energy levels.97-99 Due to their limitations, STOPVs based on fullerene derivatives consequently have been limited to PCE values of 56 % and ~25% AVT.100-102 NFAs are an alternative to fullerenes comprising planar, conjugated structures which have come into prominence recently with many noteworthy developments and impressive improvements in PCEs in the past five years. Previous research has shown that NFAs have many advantages such as tunable energy levels, high electron mobilities and strong absorption in the visible and NIR regions.103-105 Binary opaque OPVs consequently have achieved record PCEs over 18%.106 Along with the emergence of such high performance materials, STOPVs have seen dramatic improvements as well; PCEs over 13% with AVT ~ 22% have recently been achieved in STOPVs.93, 94
In addition to photovoltaic performance, the aesthetic features and coloration of semi-transparent devices are significant considerations since applications for semi-transparent solar cells revolve around human vision. Nonetheless, research efforts so far have primarily focused on improving photovoltaic performance by utilizing absorption of solar irradiation in the NIR region and transmittance of solar irradiation in visible region to improve PCE and AVT simultaneously using NFAs.107-110 Although some reports have described controlling coloration in STOPVs, most of these works have focused on controlling interference effects within semi-transparent electrodes.111-115 To date, there are very few
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reports which describe tuning color via active layer modulation. Kong et al. demonstrated colorful OPVs by controlling ratios of organic dye molecule (2,4-bis-[(N,N-diisobutylamino)-2,6- dihydroxyphenyl]-4-(4-diphenyliminio) squaraine, ASSQ) as a guest material, provided into PTB7- Th:N2200 binary system (N2200: poly{[N,N-bis(2-octyldodecyl)-naphthalene-1,4,5,8- bis(dicarboximide)-2,6-diyl]-alt-5,5-(2,2-bithiophene)}) and recorded 5.46% PCE in a blue, opaque device.116 In addition, Kido et al demonstrated purple, cyan, brown and light brown colored STOPVs using fullerene based acceptors with unique squaraines based donors, which yielded up to 4.9% PCE and 25% AVT with brown colored devices (D-BDT-SQ:PC70BM, PC70BM: [6,6]-Phenyl-C71-butyric acid methyl ester).117 Meanwhile, Yong Cui et al. reported a small band gap NFA, known as IEICO- 4Cl (2,2((2Z,2Z)(((4,4,9tris(4hexylphenyl)9(4pentylphenyl)4,9dihydrosindaceno[1,2b:5,6 b dithiophene2,7diyl)bis(4((2ethylhexyl)oxy)thiophene5,2diyl))bis(methanylylidene))bis(5,6 dichloro3oxo2,3dihydro1Hindene2,1diylidene))dimalononitrile), and used it to demonstrate purple, blue and cyan STOPVs with different three donor polymers achieving PCEs of up to 8.38%
with AVT of 25.7% in cyan colored devices (PTB7-Th:IEICO-4Cl).118 Despite these interesting initial works, the coloration of devices is limited to a handful of discrete colors and there is currently a paucity of information related to systematic color tuning of active layers.
A great deal of effort and expense are spent to design buildings which are visually appealing. In the context of BIPVs, it is necessary to achieve a high level of control over the color of STOPVs in order for them to match the design of buildings and be realistically considered as potential architectural design elements. In order to achieve the necessary level of color control, a delicate understanding of the composition of photoactive materials, including how the absorption bands and spectra of different components mix together to yield specific colors, and a system for balancing wide/medium and narrow band gap organic semiconductors is essential. In this respect, NFA based OPVs offer many opportunities due to their flexibility in color tuning, stemming from their easily modulated energy bands (from wide to narrow band gap materials) and absorption spectra of donor and acceptor materials;
allowing for the manufacture of devices with color controllability for applications which required an aesthetic appearance and color matching with other elements of the building’s design.
In this work, we used PTB7-Th as narrow-band gap donor (1.58 eV)119 and IEICO-4F as the as the acceptor material (1.25 eV)40 to form the basis for semitransparent devices. Additionally, as wide band gap acceptors (2.07 eV) T2-ORH120, 121 and T2-OEHRH122 were included as ternary components, which were used to obtain a diverse range of colors. Colorized STOPVs based on these ternary blends were fabricated using semi-transparent electrodes consisting of Sb2O3/Ag/Sb2O3 (SAS) stacks123-125 whose compositions were systematically formulated to achieve a range of colors including cyan, aqua, indigo, purple and reddish-purple, with a champion PCE of 6.93% and AVT of 34.03%. To the best our knowledge, this study is the first to take advantage of ternary active layer blends to achieve accurate control over the color of STOPVs. Using a combination of optical characterization and calculation,
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colorized blend films with variable acceptor ratios were thoroughly characterized. Our work demonstrates that color modulation by means of ternary active layer blends is a convenient and effective strategy to achieve STOPV colors that can be tuned over a wide range, for applications where color and aesthetic appearance are primary considerations.
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