Flexible Crystalline-Silicon Photovoltaics: Light Management with Surface Structures

ConspectusFlexible solar cells have been intensively studied in recent years for their applicability on curved or uneven surfaces, which augments their versatility toward various applications. Although emerging materials such as organics/polymers, perovskite, amorphous silicon, and copper indium gallium selenide have been used as light absorption materials for flexible solar cells, the commercialization of these materials is limited owing to their efficiency degradation, usage of toxic materials, short lifespan, or scarcity. On the contrary, crystalline silicon (c-Si) solar cells have been commercialized because of their low manufacturing cost, long lifespan of over 20 years, and high power-conversion efficiency (PCE) of ≤26.7%. However, the development of flexible solar cells using c-Si substrate poses an intrinsic problem resulting from its rigid material characteristics. In recent years, flexible solar cells using thin c-Si wafers have become more attractive with archiving a higher PCE than that of the emerging flexible solar cells. In addition, the mechanical flexibility can be realized using a thin c-Si film with a thickness of ≤50 μm, which is a quarter of the substrate thickness of conventional c-Si solar cells. Nonetheless, thin c-Si-based flexible solar cells face critical challenges because of severe light absorption loss in the entire wavelength region (300–1100 nm) because of the low absorption coefficient and surface reflection of c-Si. The development of the c-Si flexible solar cells should focus on improving the light absorption of thin c-Si films as well as maintaining the mechanical flexibility and stability of the thin c-Si solar cells. Thus, in this Account, we introduce high-aspect-ratio c-Si microwires and a random inverted-pyramidal-transparent optical film as promising surface structures for the efficient trapping of incident light in thin c-Si films. Moreover, the principles regarding the improvement in light absorption of these surface structures are discussed along with the implementable strategies for maximizing PCE of the c-Si flexible solar cells. Lastly, perspectives on further improvement of the PCE and stability of the flexible c-Si solar cells are presented

Dopant-Free All-Back-Contact Si Nanohole Solar Cells Using MoO_<i>x</i> and LiF Films

We demonstrate novel all-back-contact Si nanohole solar cells via the simple direct deposition of molybdenum oxide (MoO_<i>x</i>) and lithium fluoride (LiF) thin films as dopant-free and selective carrier contacts (SCCs). This approach is in contrast to conventionally used high-temperature thermal doping processes, which require multistep patterning processes to produce diffusion masks. Both MoO_<i>x</i> and LiF thin films are inserted between the Si absorber and Al electrodes interdigitatedly at the rear cell surfaces, facilitating effective carrier collection at the MoO_<i>x</i>/Si interface and suppressed recombination at the Si and LiF/Al electrode interface. With optimized MoO_<i>x</i> and LiF film thickness as well as the all-back-contact design, our 1 cm² Si nanohole solar cells exhibit a power conversion efficiency of up to 15.4%, with an open-circuit voltage of 561 mV and a fill factor of 74.6%. In particular, because of the significant reduction in Auger/surface recombination as well as the excellent Si-nanohole light absorption, our solar cells exhibit an external quantum efficiency of 83.4% for short-wavelength light (∼400 nm), resulting in a dramatic improvement (54.6%) in the short-circuit current density (36.8 mA/cm²) compared to that of a planar cell (23.8 mA/cm²). Hence, our all-back-contact design using MoO_<i>x</i> and LiF films formed by a simple deposition process presents a unique opportunity to develop highly efficient and low-cost nanostructured Si solar cells

Improved Interfacial Crystallization by Synergic Effects of Precursor Solution Stoichiometry and Conjugated Polyelectrolyte Interlayer for High Open-Circuit Voltage of Perovskite Photovoltaic Diodes

The open-circuit voltage (Voc) of perovskite photovoltaic diodes depends largely on the selection of charge transport layers (CTLs) and surface passivation, which makes it important to understand the physical processes occurring at the interface between the perovskite and a CTL. We provide a direct correlation between Voc and the interfacial characteristics of perovskites tuned through stoichiometry engineering of precursor solutions and surface modification of the underlying poly­(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) layer. Poor quality interfacial perovskite crystals were observed on top of the PEDOT:PSS layer, resulting in strong interfacial recombination and a low Voc. In contrast, the growth of the interfacial perovskite crystals was significantly improved by the synergic effects of varying the precursor solution composition and covering the surface with a pH-neutral conjugated polyelectrolyte, poly­[2,6-(4,4-bis­(potassium butanylsulfonate)-4H-cyclopenta­[2,1-b;3,4-b′]­dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (CPE-K), which possesses potassium ions as counter ions. The influence of the energy-level alignment at the interface on Voc was also discussed. Our findings highlight that improved perovskite crystallization at the interface can facilitate bulk growth of perovskite grains in the vertical direction and effectively suppress nonradiative surface charge recombination, thus enhancing the short-circuit current and Voc

Large Electroabsorption Susceptibility Mediated by Internal Photoconductive Gain in Ge Nanowires

Large spectral modulation in the photon-to-electron conversion near the absorption band-edge of a semiconductor by an applied electrical field can be a basis for efficient electro-optical modulators. This electro-absorption effect in Group IV semiconductors is, however, inherently weak, and this poses the technological challenges for their electro-photonic integration. Here we report unprecedentedly large electro-absorption susceptibility at the direct band-edge of intrinsic Ge nanowire (NW) photodetectors, which is strongly diameter-dependent. We provide evidence that the large spectral shift at the 1.55 μm wavelength, enhanced up to 20 times larger than Ge bulk crystals, is attributed to the internal Franz–Keldysh effect across the NW surface field of ∼10⁵ V/cm, mediated by the strong photoconductive gain. This classical size-effect operating at the nanometer scale is universal, regardless of the choice of materials, and thus suggests general implications for the monolithic integration of Group IV photonic circuits

Two GPSes in a Ball: Deciphering the Endosomal Tug-of-War Using Plasmonic Dark-Field STORM

Live video recording of intracellular material transport is a promising means of deciphering the fascinating underlying mechanisms driving life at the molecular level. Such technology holds the key to realizing real-time observation at appropriate resolutions in three-dimensional (3D) space within living cells. Here, we report an optical microscopic method for probing endosomal dynamics with proper spatiotemporal resolution within 3D space in live cells: plasmonic dark-field STORM (pdf-STORM). We first confirmed that pdf-STORM has a spatial resolution comparable to that of scanning electron microscopy. Additionally, by observing two optical probes within a single organelle, we were able to track rotational movements and demonstrate the feasibility of using pdf-STORM to observe the angular displacements of an endosome during a “tug-of-war” over an extended period. Finally, we show various biophysical parameters of the hitherto unelucidated dynamics of endosomesangular displacement is discontinuous and y-axis movement predominates and follows a long-tail distribution

Impact of Hydroxyl Groups Boosting Heterogeneous Nucleation on Perovskite Grains and Photovoltaic Performances

Surface energy is a key factor in controlling the kinetics of nucleation and growth of perovskite, which are crucial for the formation of high quality films and the photovoltaic efficiency of solar cells. It has been reported that substrate wettability and perovskite grain size are to be compromised with necessity, as promoted heterogeneous nucleation that occurs on a hydrophilic surface reduces the grain size for a two-step deposition method. Herein, the increase in grain size on hydrophilic surfaces in the presence of hydroxyl groups and the direct correlation between the perovskite grain formation and photovoltaic performance are investigated. The surface energy of the hole transport layer in planar p–i–n type perovskite solar cells is modulated by the introduction of polymer surfactant additive, poly­(ethylene glycol) tridecyl ether (PTE). Perovskite films deposited on a hydrophilic surface by a two-step method contain small grain size, leading to a reduction in photovoltaic performance. In contrast, surface hydroxyl groups were found to induce the preferential (110) orientation and large grain size in the perovskite films deposited by means of a one-step method. Nucleation and growth mechanisms are proposed to explain those different behaviors of the dependence of grain size on surface energy. The enlarged perovskite grains on hydrophilic surfaces lead to an efficiency improvement owing to an increase in the short-circuit current and fill factor. Our study highlights that the grain size increase and high crystallinity can be achieved even with accelerated heterogeneous nucleation on a hydrophilic substrate surface

Two GPSes in a Ball: Deciphering the Endosomal Tug-of-War Using Plasmonic Dark-Field STORM

Live video recording of intracellular material transport is a promising means of deciphering the fascinating underlying mechanisms driving life at the molecular level. Such technology holds the key to realizing real-time observation at appropriate resolutions in three-dimensional (3D) space within living cells. Here, we report an optical microscopic method for probing endosomal dynamics with proper spatiotemporal resolution within 3D space in live cells: plasmonic dark-field STORM (pdf-STORM). We first confirmed that pdf-STORM has a spatial resolution comparable to that of scanning electron microscopy. Additionally, by observing two optical probes within a single organelle, we were able to track rotational movements and demonstrate the feasibility of using pdf-STORM to observe the angular displacements of an endosome during a “tug-of-war” over an extended period. Finally, we show various biophysical parameters of the hitherto unelucidated dynamics of endosomesangular displacement is discontinuous and y-axis movement predominates and follows a long-tail distribution

Two GPSes in a Ball: Deciphering the Endosomal Tug-of-War Using Plasmonic Dark-Field STORM

Live video recording of intracellular material transport is a promising means of deciphering the fascinating underlying mechanisms driving life at the molecular level. Such technology holds the key to realizing real-time observation at appropriate resolutions in three-dimensional (3D) space within living cells. Here, we report an optical microscopic method for probing endosomal dynamics with proper spatiotemporal resolution within 3D space in live cells: plasmonic dark-field STORM (pdf-STORM). We first confirmed that pdf-STORM has a spatial resolution comparable to that of scanning electron microscopy. Additionally, by observing two optical probes within a single organelle, we were able to track rotational movements and demonstrate the feasibility of using pdf-STORM to observe the angular displacements of an endosome during a “tug-of-war” over an extended period. Finally, we show various biophysical parameters of the hitherto unelucidated dynamics of endosomesangular displacement is discontinuous and y-axis movement predominates and follows a long-tail distribution

Photophysics of Delocalized Excitons in Carbazole Dendrimers

The photophysical properties in solution of three generations of carbazole-based dendrons and dendrimers with fluorenyl surface groups were studied using steady-state, time-resolved femtosecond transient absorption and anisotropy, and coherent two-dimensional ultraviolet spectroscopy. It was found that increasing the generation caused a switch in the nature of the emissive state between the first-generation compounds and the second- and third-generation dendrimers. Time-resolved anisotropy measurements revealed low initial anisotropies that decreased with increasing dendrimer generation consistent with increasing intradendrimer interchromophore coupling. Two-dimensional UV spectroscopy showed that the signal from the second- and third-generation dendrimers is the product of multiple chromophores interacting. The maximum number of interacting chromophores is reached by the second generation

Two GPSes in a Ball: Deciphering the Endosomal Tug-of-War Using Plasmonic Dark-Field STORM

Live video recording of intracellular material transport is a promising means of deciphering the fascinating underlying mechanisms driving life at the molecular level. Such technology holds the key to realizing real-time observation at appropriate resolutions in three-dimensional (3D) space within living cells. Here, we report an optical microscopic method for probing endosomal dynamics with proper spatiotemporal resolution within 3D space in live cells: plasmonic dark-field STORM (pdf-STORM). We first confirmed that pdf-STORM has a spatial resolution comparable to that of scanning electron microscopy. Additionally, by observing two optical probes within a single organelle, we were able to track rotational movements and demonstrate the feasibility of using pdf-STORM to observe the angular displacements of an endosome during a “tug-of-war” over an extended period. Finally, we show various biophysical parameters of the hitherto unelucidated dynamics of endosomesangular displacement is discontinuous and y-axis movement predominates and follows a long-tail distribution