Abstract
1. Introduction
Sunlight provides the most abundant sustainable energy to our world. Flexible thin-film photovoltaics (TF-PV) are important technologies in the PV community due to the reduced pay-back time[
The utilization of nanostructures for advanced light management is a realistic path to minimize optical losses of TF-PV. Implementing these nano/microstructures, for instance, nano/micro-pyramid[
It is highly desirable to have a nanostructured film with broadband anti-reflection and self-cleaning capacity on the top surface of solar panels for real application. As the incident angle of sunlight varies during the day, the angular dependence performance determines a solar cell's daily electrical energy output[
The active layer itself can also be nanostructured. Taking one-dimensional (1D) NW, for example, photovoltaics based on high crystallinity (even monocrystalline) NW provide remarkable improvement in PCE. One contributing factor is the sufficient photogenerated carrier generation and collection in optoelectronic nanodevice[
This review provides a comprehensive review of the recent developments in various light management nanostructures for photovoltaics. In particular, we focus on nanostructures on top as a broadband anti-reflection layer with self-cleaning capacity and survey micro-structured back-reflectors to reduce transmission via multiple reflections. We also summarize the recent progress in developing flexible photovoltaics based on NW with improved bendability, longevity, and PCE. The distinct merits and challenges of these strategies are discussed.
2. Nanostructures at the front surface
It is known that there are two categories of absorption losses: reflection and transmission. The transparent dielectrics or metal oxide layer with a high refractive index produce an unfavored reflection. One practical solution to minimize reflection is the nanostructures implemented on the top surface of solar panels. Nanostructures offer an efficient pathway for the photons flux and reduce solar radiation power loss due to the antireflection effect from the geometry and gradual refractive index gradient provided by nanostructures[
Figure 1.(Color online) Three-dimensional (3D) nanostructured silicon solar cells and their corresponding absorption spectra. (a1, a2) Double-sided nanostructure. (b1, b2) Top-only nanostructure. (c1, c2) Bottom-only nanostructure. (d1, d2) Flat film. Red curves stand for the Yablonovitch limit, green curves are the single-pass absorption spectra, and black curves represent spectra for corresponding structures. Reproduced with permission[
Apart from the light-trapping effect, wavelength-scale nanospheres can diffractively couple photons and assist confined resonant modes. Moreover, owning to whispering gallery resonances within the spheres, the light coupling between the spheres is witnessed in the highly periodic array of dielectric nanospheres[
Tsui et al. reported a cost-effective method for flexible plastic with three-dimensional (3D) light-trapping nanocone (NC) arrays[
Figure 2.The scanning electron microscope (SEM) of inverse nanocone (NC) template (a) and NC arrays (b). (c) The external quantum efficiency (EQE) spectra of CdTe solar cells with and without NC film. The inset of (c) is the schematic structure of the device. (a–c) Reproduced with permission[
The energy output of solar cells can be interfered with or even cut down by the dust on the top of solar modules when it comes to outdoor conditions, especially in the solar farm located in the desert. The dust stuck on solar panels will block the solar radiation and lead to performance degradation. Therefore, there is a need for manpower or machine to maintain the surface cleaning of solar panels. Apart from the constant maintenance cost, an abundant amount of water is needed to clean the solar system that is precious in the desert area. The nanostructured film with the function of self-cleaning capacity becomes a practical solution to this issue[
Tavakoli et al. fabricated flexible perovskites solar cells on ultrathin willow glass substrates with polydimethylsiloxane (PDMS) NC array films on top as a light-trapping and self-cleaning layer[
Similarly, through nanoimprinting lithography, Zhang et al. realized highly ordered metal oxide nanotextures on polyimide (PI) substrate for the highly flexible amorphous silicon (a-Si:H) solar cells[
3. Nanostructures at the back surface
As mentioned above, antireflection coatings affect the optical loss caused by reflection. In contrast, light-trapping schemes address the loss by transmission. Especially, photons at the longer wavelength are less absorbed in the single path because of decreased absorption with increasing the wavelength towards the bandgap[
The loss of light absorption in the red region is the cause of undesired reddish-brown color for conventional semi-transparent perovskite solar cells (ST-PSCs) based on the continuous TF[
Figure 3.(Color online) (a) Complete compound moth eyes and a moth-eye-inspired structure (MEIS) device structure diagram. (b) Reflectance spectra of MEIS and human luminosity curve, inset is the photo of MEIS (scale bars, 3 cm). (c)
Similarly, to address the insufficient light-harvesting, Zheng et al. proposed a strategy of TiO2 nanobowl (NB) array with controlled morphology and fabricated carbon cathode-based perovskite solar cells[
Xiao et al. systematically investigated the performance of a-Si:H solar cells based on the different thicknesses of oxide spacer layers[
4. 3D nanostructured device
4.1. 3D nanostructured for light management & boosted carrier collection
The most crucial part of photovoltaics is the adequate harvesting of photons, exciting electrons to the conductive band, and leaving holes behind. Nanomaterials provide the opportunity to minimize loss of each step, for instance, absorption, carrier generation, separation, and collection. Besides, the nano-scale geometry offers unique advantages, including suppressed reflection, light trapping, facile strain relaxation, new charge separation mechanisms, better defect tolerance, etc.[
In Figs. 4(a) and 4(b), Fan et al. pioneered a nanopillar-array CdTe/CdS photovoltaics with a 3D geometric configuration[
Figure 4.(Color online) (a) Cross-sectional schematic diagram of a 3D solar nanopillar cell, demonstrating improved carrier separation and collection. (b) SEM images of d a CdS nanopillar array. The experimental (c) and simulated (d) absorption spectra of the nanowire (NW) plotted as a function of diameter and pitch. (c, d) Reproduced with permission[
The carrier separation and collection advantages of the radial geometry are more noticeable. As the single crystallinity of nanopillar (Figs. 4(e) and 4(f)) is beneficial to form guiding channels for carriers, the direct charge transport pathway makes electron mobility in NWs several orders of magnitude higher than in the polycrystallinity TF counterpart[
To achieve the optimum performance cell, the detailed optimization of the optical and electronic properties is required, which are strongly determined by the geometry of the nanopillar[
Multilayered photovoltaic absorbers, such as BiI3, two-dimensional (2D) perovskites, and transition metal dichalcogenides have gained enormous attention because of their unique properties. Shown in Fig. 4(h), BiI3 is a layered 2D material constructed by the repeating unit of the I-Bi-I layer. Carriers are mobile in the layer and immobile across planes[
4.2. 3D nanostructured devices for better flexibility
High-performance flexible electronics increasingly gained attention during recent decades, owing to the promising potential in building-integrated photovoltaics, portable and wearable power supplies, etc.[
For flexible photovoltaics, the bending and stretching of the device should not have a notable impact on PCE[
In Fig. 5, Leung et al. fabricated flexible, nanospike arrays of Al substrate for single-junction a-Si:H solar cells[
Figure 5.(Color online) (a) Schematic diagram of the 3D nanospike. (b) Angular and wavelength-dependent absorption of a nanospike solar cell and a planar reference. (c) Normalized PCE of the nanospike device under different bending angles, inset is the schematic of a flexible nanospike solar cell. (a–c) Reproduced with permission[
Tavakoli et al. reported efficient, flexible, and mechanically robust organometallic perovskite solar cells on plastic substrates with inverted NC structures[
In Fig. 5(d), Lin et al. present a cost-effective approach towards periodical NC arrays of polyimide (PI), which possesses excellent mechanical flexibility and unique optical management[
5. Summary and outlook
Nanostructures and nanomaterials possess promising potential to improve the light-harvesting capability of solar cells. Nanostructures on the top surface offer broadband anti-reflection and self-cleaning capacities for solar cells. More importantly, the device's overall performance has been remarkably upgraded via implementing these nanostructures. Nevertheless, it is still far from the terminal objectives of the entire solar spectrum coverage for solar cells. Future work still requires optimizing geometry design and fabrication process to draw out the full potency of these light-trapping strategies.
The nanostructured light absorbers offer photovoltaics unique optoelectronic and mechanical properties. The 3D geometric configuration leads to sufficient orthogonalization light-harvesting and carrier collection achieved in nanodevice. Also, nanodevices possess excellent mechanical highly flexibility. Despite the promising potential, surface recombination is the core hindrance for a high-performance nanodevice. Thus, the investigation of the surface property of material really matters. In this perspective, efforts are required to understand better the carrier dynamics at the surface, such as charge transfer, surface recombination, minority carrier diffusion, dopant density, surface state, and conductivity measurements. It is noteworthy that the PCE relies on efficient photogenerated carrier collection. The nanodevices' performance can be significantly improved through interface engineering for better surface quality and interfacial band alignment. Therefore, to explore the potentiality of nanodevice, efforts should be devoted to nanodevice geometry design, material choice, surface passivation/treatments, and energy-level alignment engineering.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Project No. 51672231), the Science and Technology Plan of Shenzhen (Project Nos. JCYJ20170818114107730, JCYJ20180306174923335), the General Research Fund (Project Nos. 16309018, 16214619) from the Hong Kong Research Grant Council. Guangdong-Hong Kong-Macao Intelligent Micro-Nano Optoelectronic Technology Joint Laboratory (Project No. 2020B1212030010), HKUST Fund of Nanhai (Grant No. FSNH-18FYTRI01). The authors also acknowledge the support from the Center for 1D/2D Quantum Materials and the State Key Laboratory of Advanced Displays and Optoelectronics Technologies at HKUST and Foshan Innovative and Entrepreneurial Research Team Program (2018IT100031).
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