Final Report Summary - EPOS CRYSTALLI (Epitaxial thin-film organic semiconductor crystals and devices)
Organic semiconductors (OSCs) radically differ from other semiconductors because of their molecular nature that confers them tunability, processability and intriguing opto-electronic properties. The strong disorder in thin films of OSCs is, however, severely limiting device performance. This disorder is not an intrinsic molecular property. Indeed, single-crystal OSCs are known, and display exciting characteristics and high performance. In Epos Crystalli, we wanted to develop scalable fabrication techniques to harness the potential of organic single crystals into high performance opto-electronic devices. To this end, we worked in four distinct technological platforms.
The first platform relies on the vacuum evaporation of small molecular OSCs through stencil masks with micron-sized apertures. By controlling crystallite nucleation, this method produces arrays of single organic crystals on inert substrates. Through careful mask alignment, metal electrodes were then patterned on top of these crystal arrays, leading to micron-sized, single-crystal electrical devices. This included single crystalline thin film transistors with a charge carrier mobility beyond 12 cm2/Vs. This investigation increased our understanding of organic crystal growth and pinpoints the potential and challenges of aggressively downscaled electrical devices based on single-crystalline OSC films.
A second platform focused on the formation of large area single crystalline thin films of OSCs on inert substrates. We developed methods to grow, pattern and integrate such films for circuit applications. For thin film growth, we propose a combination of a slow solution coating method that forms a crystalline template with cm-long crystallites, followed with an homoepitaxial regrowth by vacuum evaporation of the same molecule. This yields films with a charge carrier mobility in excess of 10 cm2/Vs before integration. During device downscaling, contact resistance represents a major hurdle for these high mobility materials. We have investigated contact doping and ripening methods to overcome this issue. As a result, we successfully fabricated 5 µm channel length integrated circuits such as ring oscillators based on single-crystalline thin films of OSCs.
The third platform aimed at the fabrication of highly crystalline solar cells. We developed a thermal treatment for complete recrystallization of an evaporated amorphous organic thin film followed by a regrowth of a thick homoepitaxial film on top of this template. Thanks to their long exciton diffusion lengths (> 200 nm), solar cells based on these multicrystalline films exhibit increased efficiency up to very high crystalline film thicknesses. We also explored the use of a crystalline donor layer to template the growth of organic acceptor layers, inducing crystallinity throughout the whole cell. This was key to the success of novel fullerene-free planar heterojunctions based on a unique tri-layer architecture. In this cell concept, three complementary absorbers contribute to generating the current, resulting in an 8.4% efficient organic solar cell with high impact in the field.
In the fourth platform, we have developed the overlapping-gate organic light emitting transistor based on a novel dual-stacked gate architecture. The device utilizes patterned crystalline films of high mobility OSCs to independently channel electron and holes towards the light emissive recombination zone. This novel device topology exhibits good charge transport balance, leading to bright light emission and a record external quantum efficiency of 14.2%. By demonstrating efficient and intense light emission 100 µm away from injecting metal contacts, this light emitting transistor represents an important step towards the achievement of high brightness thin film light sources.
In conclusion, the Epos Crystalli project has generated a broad multi-disciplinary effort to advance technologies based on highly crystalline organic semiconductors. This work spanned a broad research spectrum from fundamental material research to applied device engineering. The project impact was immediate in the scientific community and is now diversifying to other communities.
The first platform relies on the vacuum evaporation of small molecular OSCs through stencil masks with micron-sized apertures. By controlling crystallite nucleation, this method produces arrays of single organic crystals on inert substrates. Through careful mask alignment, metal electrodes were then patterned on top of these crystal arrays, leading to micron-sized, single-crystal electrical devices. This included single crystalline thin film transistors with a charge carrier mobility beyond 12 cm2/Vs. This investigation increased our understanding of organic crystal growth and pinpoints the potential and challenges of aggressively downscaled electrical devices based on single-crystalline OSC films.
A second platform focused on the formation of large area single crystalline thin films of OSCs on inert substrates. We developed methods to grow, pattern and integrate such films for circuit applications. For thin film growth, we propose a combination of a slow solution coating method that forms a crystalline template with cm-long crystallites, followed with an homoepitaxial regrowth by vacuum evaporation of the same molecule. This yields films with a charge carrier mobility in excess of 10 cm2/Vs before integration. During device downscaling, contact resistance represents a major hurdle for these high mobility materials. We have investigated contact doping and ripening methods to overcome this issue. As a result, we successfully fabricated 5 µm channel length integrated circuits such as ring oscillators based on single-crystalline thin films of OSCs.
The third platform aimed at the fabrication of highly crystalline solar cells. We developed a thermal treatment for complete recrystallization of an evaporated amorphous organic thin film followed by a regrowth of a thick homoepitaxial film on top of this template. Thanks to their long exciton diffusion lengths (> 200 nm), solar cells based on these multicrystalline films exhibit increased efficiency up to very high crystalline film thicknesses. We also explored the use of a crystalline donor layer to template the growth of organic acceptor layers, inducing crystallinity throughout the whole cell. This was key to the success of novel fullerene-free planar heterojunctions based on a unique tri-layer architecture. In this cell concept, three complementary absorbers contribute to generating the current, resulting in an 8.4% efficient organic solar cell with high impact in the field.
In the fourth platform, we have developed the overlapping-gate organic light emitting transistor based on a novel dual-stacked gate architecture. The device utilizes patterned crystalline films of high mobility OSCs to independently channel electron and holes towards the light emissive recombination zone. This novel device topology exhibits good charge transport balance, leading to bright light emission and a record external quantum efficiency of 14.2%. By demonstrating efficient and intense light emission 100 µm away from injecting metal contacts, this light emitting transistor represents an important step towards the achievement of high brightness thin film light sources.
In conclusion, the Epos Crystalli project has generated a broad multi-disciplinary effort to advance technologies based on highly crystalline organic semiconductors. This work spanned a broad research spectrum from fundamental material research to applied device engineering. The project impact was immediate in the scientific community and is now diversifying to other communities.