Abstract
1. Introduction
Silicon based solar cells are currently dominate with over 90% share in the photovoltaic (PV) market[
Materials with a highly symmetrical crystal structure and direct band gap are preferred in solar cells, because it results in high absorption coefficient and isotropic carrier transport. This is especially important for polycrystalline thin films where an absorber of less than few micrometers is desirable to reduce material demand and to ensure effective carrier collection. Mainstream polycrystalline solar cell technologies such as Cu(In,Ga)Se2, CdTe and CH3NH3PbI3 all have a highly symmetrical crystal structure and have demonstrated over 22% PCE[
There is a group of compounds with binary and ternary compositions as well as PV-relevant characteristics, which have not been studied extensively because of their reduced structural dimensionality. Low-dimensional materials (namely layered (2D) and chained (1D), see Fig. 1) might be considered to be unsuitable for PV application but have been shown to have potential, both theoretically and experimentally[
Figure 1.(Color online) The evolution of crystal structure and morphology of the grain as a function of structural dimensionality. CdTe, MoS2 and Sb2Se3 structures were selected as representative materials in each case. Grain morphology was calculated using Bravais−Friedel−Donnay−Harker (BFDH) theory[
There are some excellent reviews on low-dimensional nanomaterials for catalysis, electronics and photonic applications[
Throughout this review we probe material properties that are critical for PV application and correlate them with unique features of low-dimensional materials. In principal, the key condition for good PV material is that diffusion length of photo-generated carriers should be larger than the light penetration depth and can be expressed as
2. Low-dimensional materials for photovoltaic application
2.1. Absorption and band gap
Direct band gap semiconductors are highly favored in many optoelectronic devices and would also be the first prerequisite for an absorber in solar cells. However, providing the difference between direct and indirect gaps, ΔE, is small, semiconductor can be characterized as quasi-indirect type, showing both direct- and indirect-like properties. These characteristics are quite common in low-dimensional materials (Table 1) and, as we will show, are highly suitable for PV devices.
Indirect transition requires the participation of phonon(s) which leads to a lower absorption event probability and therefore low absorption coefficient. However, when the energy of photons is equal or greater than direct gap (Fig. 2(a)), absorption increases abruptly. Therefore, assuming that the ratio of proportional constants between direct and indirect absorption is one tenth, we have calculated the number of photons absorbed in indirect semiconductor with a thickness of 2 μm as a function of ΔE using AM1.5G solar spectrum (dashed line-I in Fig. 2(b), Fig. S1 and Fig. S2). If the indirect band gap of absorber is lower than 1.35 eV, then 85% of photons will be absorbed when ΔE is in the range of 0.125–0.175 eV. For larger band gap absorbers (> 1.35 eV), the conditions are stricter requiring ΔE to be lower than 0.125 eV because of lower photon flux density in the shorter wavelength region of solar spectrum. Evidently, on a theoretical basis, a large part of the solar radiation can still be absorbed by an indirect band gap semiconductor providing that ΔE does not exceed 0.175 eV. Although we have used typical values of absorption parameters (A1 and A2, defined as the proportional constants between absorption coefficient and (hv – Eg)1/2 and (hv – Eg)2, respectively), requirement for ΔE can range (decrease or increase) depending on the material’s absorption capabilities. For instance, for materials with low absorption, ΔE is required to be below 0.1 eV to absorb the same number of photons (Fig. 2(b), dashed line-III, IV).
Figure 2.(Color online) (a) Photon absorption and carrier dynamics for a quasi-indirect band gap semiconductor. Photons are first absorbed via direct band gap (I) or indirect band gap (II), they then thermodynamically relax to the indirect band gap (III) and eventually recombine (IV). (b) The ratio of photons absorbed to the total number of photons (
2.2. Carrier recombination and lifetime
Besides the light absorption, another key parameter to evaluate the potential of PV material is the carrier lifetime. There is a general consensus that an absorber with > 1 ns of carrier lifetime is necessary to achieve PCE of solar cell over 10% [
When ΔE is in the 0.045–0.175 eV range (for highly absorbing materials) semiconductor can be described as quasi-indirect gap type because of direct-like absorption and indirect-like recombination characteristics. These features are highly desirable in PV materials because they lead to a sufficiently high absorption and long carrier lifetime without sacrificing one or the other. Recently, the quasi-indirect character has been observed in methylammonium lead iodide (CH3NH3PbI3) perovskite (ΔE = 0.04–0.07 eV)[
We acknowledge that quasi-indirect band gap is not a unique feature of low-dimensional materials, but due to low crystal symmetry they possess an indirect type band gap (Table 1). Therefore, materials with low-symmetry structure offer many ways to fully exploit quasi-indirect characteristics, which is highly desirable in PV applications.
2.3. Defects and doping
Understanding the formation of defects and impurities in the semiconductors allows the control of its electrical properties and is important for optimization of functional optoelectronic devices[
Figure 3.(Color online) Point defects in Sb2Se3 taking into account, it contains two kinds of Sb and three kinds of Se. Reprinted from Chen
2.4. Anisotropy and mobility
Low structural dimensionality leads to the restricted electronic dimensionality and therefore to highly anisotropic carrier movement[
Figure 4.(Color online) (a) Carrier movement in Sb2Se3 along [120] (red dashed arrows) and [221] (solid red arrow) directions. (b) Atomistic view of Sb2Se3 grain boundary oriented [001] direction perpendicular to substrate. All of the atoms at the edge of these ribbons are saturated (highlighted as red spheres) and introduce no recombination loss at the GBs. Reprinted from Tang
Mobility is one of the parameters describing the transport of photo-generated carriers is the mobility. Assuming zero-optical phonon scattering model carrier mobility depends on effective mass because μ– m*–5/2[
2.5. Surface properties
Another aspect of low-dimensional materials is the large difference in the surface energies of specific facets. This originates from the fact that surfaces terminated by vdW bonds will not produce dangling bonds and will result in low surface energy[
Figure 5.(Color online) Layered crystal structure of SnS with Pnma space group and calculated morphology of the grain based on the surface energy. Surface energy (SE), EA and IP of various SnS facets. Printed with permission[
2.6. Growth kinetics
The growth principals of low-dimensional materials will differ from 3D because of severe bonding anisotropy. For instance, the classical island growth regime in 3D crystals occurs when atoms are bonded stronger with each other than with substrate, leading to an island-like morphology (Fig. 6(a)). In the case of low-dimensional materials under the same growth conditions, atoms will prefer to bond with each other. However because of large difference in bond strength, adatoms will be preferably attached along the strong bond axis (Fig. 6(b), red arrow). This will lead to the growth of nanowires or sheets parallel to the substrate, which is more characteristic to the layer-by-layer growth regime than the island regime. In 3D materials, a strong interaction of adatoms and substrate atoms will result in layer-by-layer growth regime because the most energetically favored position will be at the terrace forming strong bond with substrate and growing layer (Fig. 6(c)). For low-dimensional materials, a strong interaction with the substrate will lead to nuclei orientation perpendicular to the surface (in terms of strong bond direction), and therefore positions on top of growing nuclei and on substrate will be equally favored leading to growth of nanowires or sheets normal to the substrate (akin to the island-like growth regime), Fig. 6(d). This hypothetical model shows the fundamental difference in growth dynamics of 3D and low-dimensional materials and highlights the importance of the substrate to control film orientation.
Figure 6.(Color online) Schematics of growth process of 3D and 1D materials on (a, b) inert and (c, d) strongly interacting substrates. (a) represents an island-like growth mode, whereas (c) layer-by-layer[
In thin film solar cells, polycrystalline substrates are commonly employed and therefore will lead to the growth of films with more dispersed orientations than presented above. Commonly, on polycrystalline substrates, the orientation of 3D materials adopts close-packed plane parallel to the surface, i.e. (111) for cubic, (112) for tetragonal or (0001) for hexagonal crystal structures. In contrast, for low-dimensional materials the substrate will play significant role for film orientation, according to the nature of interaction discussed above. For instance, vdW planes in thermally evaporated Bi2S3, Sb2Se3, Sb2S3, SnS and Bi2Se3 films were predominantly oriented parallel to the surface when deposited on inert substrates such as glass or atomically smooth Si[
The properties of the substrate will dictate the orientation of low-dimensional thin films, however, kinetically limited growth conditions can also be used to steer growth dynamics. During the growth of SnS, increased deposition flux reduced the adatom’s surface diffusion length. Consequently instead of thermodynamically favored growth of sparse platelet-like grains, a compact cube-shaped grain morphology was obtained[
2.7. Current research status of solar cells based on low-dimensional materials
Many low-dimensional materials have been explored as potential PV absorbers, such as the one-dimensional Bi2S3, Sb2Se3, Sb2S3 and their alloys, two-dimensional CuSbSe2, CuSbS2, GeSe, SnS, and recently Se and BiOI gained an interest for wide bandgap PV application (Table 1). Among them, the record PCE has been achieved in solar cells with Sb2(Se,S)3 absorber[
3. Conclusion
We discussed and showed that low-dimensional materials could be applied to device level. This broadens the scope of potential application from nanomaterials to the macro-scale devices such as solar cells, photodetectors, light emitting diodes and thermoelectrics. We probed various PV-related material properties and growth mechanism for low-dimensional materials with an aim to provide new insights and design options for functional devices. Low-dimensional materials offer a bright future for various electronic, optic and photonic applications and fabrication of high-quality films with controlled properties will be the next step for their successful implementation.
Acknowledgments
The authors thank the Analytical and Testing Center of HUST and the facility support of the Center for Nanoscale Characterization and Devices (CNCD), WNLO-HUST. This work was supported by the National Natural Science Foundation of China (61725401, 61904058, 61904058), the National Key R&D Program of China (2016YFA0204000), China Postdoctoral Science Foundation Project (2019M662623) and the National Postdoctoral Program for Innovative Talent (BX20190127).
Appendix A. Supplementary materials
Supplementary materials to this article can be found online at https://doi.org/1674-4926/42/3/031701.
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