The growing interest in sustainable renewable energy sources has led to significant attention towards solar power generation. This shift has been driven not only by the lower operating costs of solar power as compared to traditional fossil fuel generation but also due to the wide range of potential applications. Organic-based solar cells, distinguished by their excellent flexibility, lightweight nature, and ability to manifest diverse colors or transparent solar panels via specific material selection, are at the forefront of this evolution. Continuous research into these cells has unveiled the potential for groundbreaking applications, including incorporation into curved building facades, vehicle windows, and greenhouse roofs.

 Despite these promising attributes, a notable hurdle persists within organic solar cell technology. The conventional bottom electrode material, Indium Tin Oxide (ITO), while widely used, exhibits low mechanical stability, failing to withstand severe mechanical deformation¹. This characteristic makes ITO unsuitable for flexible components, limiting its application within the versatile, dynamic environments organic solar cells aim to service.

 In pursuit of improved mechanical stability and enhanced transparency, we have developed highly flexible, transparent electrodes, namely the metal thin film electrode² and the Dielectric/Metal/Dielectric (DMD) electrode^3,4,5. To supplant ITO, an ultrathin Silver (Ag) film electrode was crafted via the thermal evaporation of metal Ag in ultra-thin layers. This approach maintains the flexibility of the cell while ensuring the stability of the electrode. Additionally, to achieve optimum transparency performance, we utilized a DMD electrode. This innovative solution mitigates the reflection typically caused by metal components, providing an efficient balance between transparency and mechanical stability. These advancements led to the successful fabrication of a semi-transparent Organic Photovoltaics (OPV) based on PTB7-Th: IEICO-4F. The resultant solar cell demonstrated a commendable power conversion efficiency of 6.93% and a transmittance exceeding 30% in the visible light range (Fig.1).

Fig. 1. a, Schematic of device structure and b, photograph of ultra-flexible ST-OPV. c, Chemical structures of donor polymer, PTB7-Th, and non-fullerene acceptor, IEICO-4F. d, Absorbance spectra of PTB7-Th : IEICO-4F film (purple) and transmittance spectra of glass/parylene/SU-8 (black), PEI/Ag/ZnO (red), PTB7-Th:IEICIO-4F/MoO3/Ag (8 nm)/ (green).

 For effective performance in the visible light spectrum, alongside similar conductivity to the traditional ITO electrode, we meticulously fine-tuned the layer thicknesses of each electrode. Ultimately, this led to the fabrication of an 8 nm Ag thin film and a DMD electrode with layers of 5 nm, 8 nm, and 30 nm. Post-optimization of the ultrathin Ag film and DMD structure, we conducted comprehensive tests to assess the compression strain durability of each electrode and the ultrathin semi-transparent OPV. Remarkably, after enduring repetitive cycles of compression-release, neither the ultra-thin Ag nor DMD electrodes exhibited significant performance degradation for up to 1,000 cycles. In stark contrast, the ITO electrode demonstrated its limitations, failing after merely 20 cycles. (Fig.2).

Fig. 2. a, Comparison of R_sheet according to Ag layer thickness in PEI/Ag/ZnO (5/x/50 nm) and DMD (5/x/30 nm) electrodes. e, Comparison of AVTs according to Ag layer thickness in PEI/Ag/ZnO (5/x/50 nm) and DMD (5/x/30 nm). b, Comparison of AVTs according to Ag layer thickness in PEI/Ag/ZnO and DMD electrodes.c, Images of ultrathin PEI/Ag electrodes laminated into polymeric elastomer from pre-stretched condition (tensile strain of 200%) to compressed condition (tensile strain of 0%) in tensile strain steps of 50%. Change in resistance in the ITO, PEI/Ag, and DMD electrodes d, under a series of compression and e, after repetitive cycles of compression–release.

 Following the durability tests, we analyzed the photovoltaic parameters of the semi-transparent OPV constructed with the ultrathin Ag film and DMD electrodes (Fig. 3). The compression strain tested ranged from 200% to 0%, mirroring the conditions applied during the electrode compression test. Encouragingly, the results indicated that the nanoscale morphology and charge extraction capabilities of the devices were maintained under severe mechanical stress. Finally, we evaluated the mechanical stability of the ultra-flexible semi-transparent organic photovoltaic (ST-OPV) under 1,000 cycles of 200% compressive strain and release. Throughout the compressive strain, the photovoltaic parameters, including the power conversion efficiency (PCE), remained intact. Impressively, the PCE value persevered at over 96% and 73% of the initial values up to 100 and 1,000 cycles, respectively. This indicates that the development of these electrodes during this research holds significant importance due to its achievement of both transparency and flexibility, distinguishing it from conventional electrodes previously used.

Fig. 3. Changes in photovoltaic parameters of a, V_OC, b, I_SC,0 c, FF, and d, PCE under forward and backward compression. e, Change in PCE of ST-OPVs under repetitive cycles of compression–release.

 In conclusion, our research has heralded a significant shift in the field of organic solar technology by successfully integrating flexibility as a core characteristic in transparent organic solar devices. Its exceptional flexibility outperforms previous technological milestones, underscoring the substantial progress made in this domain. This advancement holds vast potential for expanding the usage of organic solar cells in an array of diverse fields, from integrating with building exteriors and greenhouse walls to being embedded in wearable devices. While the journey towards fully optimized organic solar cells continues, these advancements represent a significant step forward. By addressing the mechanical stability and transparency limitations of traditional electrode materials, this innovative approach opens a new chapter for organic solar cells, accelerating their potential in the realm of sustainable renewable energy.

 For more information, please refer to our article published in npj Flexible Electronics :

Lee, H., Jeong, S., Kim, JH. et al. Ultra-flexible semitransparent organic photovoltaics. npj Flex Electron 7, 27 (2023). (https://doi.org/10.1038/s41528-023-00260-5)

References

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