The pioneering LOSC, which consists of laterally pre-patterned asymmetric electrodes and a simple solution-deposited bulk-heterojunction active layer, was fabricated using solution-processed organic nanowire blends on a flexible substrate. “The main advantage of lateral-architecture devices over conventional vertical architecture is that they can be fabricated on any substrate and can be mechanically deformed without loss of performance.” says Cho Kilwon, chemical engineering professor at Pohang and one of the leaders of the research team.
Traditionally, substrates, electrodes and photo-active layers are stacked in solar cells, making the device stiff. Cho notes that a vertical structure, even when fabricated on flexible plastic substrates, limits device flexibility due to unavoidable delamination at interfaces and the brittleness of the transparent conducting ITO electrode, which leads to electrical shorting when deformed.
His cell, by contrast, has a novel, horizontal structure that includes semiconductor nanowires. The cell with demonstrated photovoltages of over 3 V has only one layer and can maintain photoelectric efficiency even after being bent many times. “The developed integrated LOSC module devices on a flexible plastic substrate exhibit continuous and stable operation under mechanical stress, such as bending and folding,” Cho reports. “Our devices were highly durable, even after 300 bending cycles with a curvature radius of 5 mm.”
Optimizing for commercialization
The novel device structure comprises laterally pre-patterned asymmetric electrodes and a simple solution-deposited active layer. To optimize power conversion efficiency, the team incorporated self-assembled polymer nanowires into the active layer for a high photocurrent generation and an efficient charge collection. “These structures significantly increase the carrier diffusion/drift length and overall generated photocurrent in the photoactive layer,” Cho explains.
Because of the lateral composition, the sunlight directly reaches the photo-active layer, rendering a light-collecting transparent substrate inessential. Additionally, the expert says the LOSC devices are useful for module integration with parallel or series connections simply by designing interdigitated electrode patterns, whereas conventional vertical architectures require the isolation of individual devices connected in series or in parallel.
One major obstacle for the commercialization of lateral organic solar cells are the relatively low power conversion efficiencies compared to conventional vertical devices. “We are now working on enhancing performances of the lateral organic solar cells by optimizing the processing conditions and designing new conjugated polymers,” says Cho. “Moreover, we are trying to combine the lateral solar cell strategy with various solution-printing techniques to deposit electrodes and organic layers for the realization of flexible and foldable large-area LOSCs. We expect that this preliminary study will provide a foundation for the development of low-cost large-area flexible photovoltaic devices within several years.”