Printing of Low‐Melting‐Point Alloy as Top Electrode for Organic Solar Cells

Abstract

coating and aerosol printing. [3][4][5][6][7][8] Various printed transparent bottom electrodes, such as conducting polymers, [9,10] metal nanowires, [11,12] carbon nanotubes, [13] and graphene, [14] have also been demonstrated with performances approaching those made with the magnetron-sputtered ITO electrodes. Researchers also have attempted to produce top electrodes by conductive inks including solutions of conductive polymers, [15][16][17] metal nanowires [18][19][20] or nanoparticles, [21,22] carbon nanotubes, [23] and graphene. [24,25] However, these inks may damage the underneath functional layers due to the organic solvents or the high-temperature sintering step. Therefore, devices with record PCEs still use the silver (Ag) or aluminum (Al) electrodes thermally evaporated in a high degree of vacuum. Such a process is costly, time-consuming, and incompatible with R2R production. The limitations of thermally evaporated top electrodes have become the major obstacle toward fully printed OSCs, and organic-solvent-free top electrode fabricated by printing methods is highly desirable.Requirements for top-electrode materials of OSCs include matching work functions with active materials, high electrical conductivity, printability at moderate temperatures, and low costs. Low-melting-point alloy (LMPA), a class of eutectic alloys with melting points below ≈100 °C, possesses fascinating properties that meet those requirements, making LMPA a promising candidate for vacuum-free electrodes of OSCs. LMPA (including liquid alloys at room temperature) has attracted extensive research interests ranging from stretchable circuitry, strain sensors, and soft robotics, to neural function repairment. [26,27] Recently, LMPAs have also been employed as electrodes in optoelectronic devices, such as quantum-dot lightemitting diodes, [28][29][30] perovskite light-emitting diode, [31] and OSCs. [32][33][34][35][36][37][38][39][40] EGaIn (75% Ga, 25% In) with a melting point of 15.7 °C was pneumatically sprayed as the top electrode of the flexible OSC device with a PCE of 11.2%. [32] By dripping the EGaIn on the functional layers of OSCs, devices with the highest PCE of 14.6% were obtained. [33] However, because EGaIn is molten at room temperature, the unencapsulated devices are vulnerable. LMPAs with a melting point above room temperature were also explored as OSC top electrodes. By using Wood's metal (50% Bi, 25% Pb, 12.5% Sn, 12.5% Cd, melting point 70 °C) as the top electrode, OSC devices based on the P3HT: PCBM reached the highest PCE of 1.8%. [34] An LMPA of The top electrodes of organic solar cells (OSCs) are usually deposited by thermal evaporation in vacuum, which has become the major obstacle toward fully-printed OSCs. The vacuum-free printed top electrodes are highly desirable for the continuous production of OSCs. Here, the printing of lowmelting-point alloy (LMPA) as cathodes in OSCs based on the D18:Y6 system is investigated. The Field's metal (FM) is focused with a melting point of 62 °C, which could be printed under moder...

Current Status and Outlook of Low‐Melting‐Point Metals in Biomedical Applications

“…Yu et al investigated low-melting-point Field's metal printing for organic solar cell cathodes using direct ink writing (Figure 12f,g). [233] Field's metal can be printed at moderate temperatures that do not harm the active layer and cure at room temperature. Field's metal remains solid under solar radiation in actual scenarios.…”

“…Reproduced with permission. [233] Copyright 2022, John Wiley and Sons. h) Differential scanning calorimetry analysis of the FM alloy, FM nanoparticles (NPs), and FM foam presenting low melting points.…”

Blade Coating of Alloy as Top Electrodes for Efficient All‐Printed Organic Photovoltaics

“…More importantly, LMPA cathodes could be fabricated by inexpensive and scalable printing technologies. [30] LMPAs could be printed in the liquid phase and solidify to form solid films below the melting point. The phase transition from liquid to solid is only determined by the temperature change, and it does not involve any solvents.…”

“…The Field's metal (FM) (32.5% Bi, 51% In, 16.5% Sn) with the melting point 62 °C has been deposited on fullerene-based active layers by Maldonado et al [42][43][44][45][46][47][48], which showed the feasibility of using FM as the top electrode. Recently, we printed the FM as top electrodes of OPVs with non-fullerene acceptors via direct-inkwriting process, [30] reaching a PCE of 16.44%.…”