Flexible Crystalline-Silicon Photovoltaics: Light Management with Surface Structures

Abstract

Conspectus Flexible 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.

Tunable Photovoltaics: Adapting Solar Cell Technologies to Versatile Applications

“…However, by thinning down c-Si below 50 µm, a mechanical bending occurs and the bendability and the flexibility increases as the thickness of the c-Si is further reduced. [58,191,192] Typically, the flexibility and the bendability of the functional materials and the SC devices are characterized and quantified by relevant mechanical parameters such as bending curvature radius, Young's modulus and elongation yield strength and elastic or total elongation. [26][27][28]83,193] For stretchable SC, fiber shaped or intrinsically stretchable structures are employed due to their multidirectional stretching ability, their resistance against tensile fracture and their accommodation to strains.…”

“…[481] Such thickness level can promote the application of ultrathin lightweight flexible c-Si SCs in indoor applications, albeit the non-optimal bandgap. [58,191,192] Challenges and Prospects: The main critical difficulties the field of IPV is facing, are the distance from the light source, manufacturing costs to enable competitively-commercialized technologies, the scalability to generate sufficient power from weak-intensity sources, the mechanical flexibility to comply with different applied surfaces, and the environmental reliability to extend the lifetime of PV cells and minimize their harmful ecotoxicity impact. [38,40,487,488,496] Once such obstacles are well tackled on the research and development level and successfully transferred to the industrial level, IPV can be efficiently utilized and widely commercialized in internet of things (IoT) applications including wireless sensors and other standalone electronic devices which operate at low illuminance and consume very low power.…”

“…However, by thinning down c‐Si below 50 µm, a mechanical bending occurs and the bendability and the flexibility increases as the thickness of the c‐Si is further reduced. [ 58,191,192 ]…”

“…[ 481 ] Such thickness level can promote the application of ultrathin lightweight flexible c‐Si SCs in indoor applications, albeit the non‐optimal bandgap. [ 58,191,192 ]…”

Mg$_2$Si is the new black: introducing a black silicide with $>$95% average absorption at 200-1800 nm wavelengths

“…Simultaneously, a gradual thinning of Si wafers for reduction of the panel cost takes place. This search for a thinner light absorbing layer is additionally motivated by the much wider flexible SC applicability [5][6][7] . Therefore, modern Si SC industry has been narrowed down to the motto "thinner & cheaper" resulted in 10-fold decrease in thickness (20 µ m) 8 representing ultrathin SC niche and extremely thin ( < 3 µ m) freestanding SC beating 12% efficiency 9 .…”