Abstract
Molecularly engineered Ir(III) complexes can transfer energy from short-wavelength photons (λ < 450 nm) to photons of longer wavelength (λ > 500 nm), which can enhance the otherwise low internal quantum efficiency (IQE) of crystalline Si (c-Si) nanowire solar cells (NWSCs) in the short-wavelength region. Herein, we demonstrate a phosphorescent energy downshifting system using Ir(III) complexes at short wavelengths (300–450 nm) to diminish the severe surface recombination that occurs in c-Si NWSCs. The developed Ir(III) complexes can be considered promising energy converters because they exhibit superior intrinsic properties such as a high quantum yield, a large Stokes shift, a long exciton diffusion length in crystalline film, and a reproducible synthetic procedure. Using the developed Ir(III) complexes, highly crystalline energy downshifting layers were fabricated by ultrasonic spray deposition to enhance the photoluminescence efficiency by increasing the radiative decay. With the optimized energy downshifting layer, our 1 cm2 c-Si NWSCs with Ir(III) complexes exhibited a higher IQE value for short-wavelength light (300–450 nm) compared with that of bare Si NWSCs without Ir(III) complexes, resulting in a notable increase in the short-circuit current density (from 34.4 mA·cm−2 to 36.5 mA·cm−2).
Highlights
Emit long-wavelength photons, is one way to diminish surface recombination
To control the absorption and emission characteristics of the Ir(III) complexes, four different hydrophobic primary ligands (Fig. 1b) were used: 1-phenylisoquinoline (1pq), 2-phenylquinoline (2pq), phenylpyridine, and difluorophenylpyridine (F2ppy). These ligands have been shown to afford high-quantum yield (QY) materials when incorporated with iridium[26,30,31], which emit light of different colors ranging from blue (Ir-Blue), green (Ir-Green), and orange (Ir-Orange) to red (Ir-Red) with increasing conjugation length (Fig. 1b)
The Ir(III) complex-based thin films were employed as energy downshifting layers in crystalline Si (c-Si) nanowire solar cells (NWSCs), which can convert UV light to visible light
Summary
Emit long-wavelength photons, is one way to diminish surface recombination. The energy downshifting phenomenon has been observed in rare-earth materials and some II−VI semiconductor nanoparticles, which are usually costly and toxic[20]. We designed four different iridium-based phosphorescent energy downshifting materials for application in c-Si NWSCs; c-Si NWSCs generally have a low EQE in the short-wavelength region (300–450 nm), which matches well with the absorption range of Ir(III) complexes. Compared to conventional quantum-dot-based energy downshifting systems[20,21,22,23], the triplet-singlet relaxation based phosphorescent system has several advantages, such as tremendous molecular engineering strategies, large Stokes shift preventing self-quenching, a long exciton diffusion length in crystalline film, and reproducible synthetic processes[24,25,26,27,28]. We expect this work to open up a new approach for overcoming the short-wavelength photon induced poor carrier collection of c-Si NWSCs via unique phosphorescent energy downshifting systems
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