Fig. 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^[18].

Fig. 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 (N_total, photon number in AM 1.5 G spectrum with energy larger than E_ind) as a function of ΔE and E_ind. The blue-dashed line indicates a boundary of the 85%. A₁ and A₂ of dashed line 1 are 10⁵ and 10⁴ cm^–1; A₁ and A₂ of dashed line 2 are 4 × 10⁴ and 10⁴ cm^–1; A₁ and A₂ of dashed line 3 are 2 × 10⁴ and 10⁴ cm^–1; A₁ and A₂ of dashed line 4 are 2 × 10⁴ and 10³ cm^–1. (c) The ratio of electrons in indirect band (N_ind) to the total number of electrons (N_total) as a function of ΔE and temperature.

Fig. 3. (Color online) Point defects in Sb₂Se₃ taking into account, it contains two kinds of Sb and three kinds of Se. Reprinted from Chen et al.^[27] with permission.

Fig. 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 et al. with permission^[29].

Fig. 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^[61].

Fig. 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^[62].

Table 1. Summary of low-dimensional material characteristics and champion solar cell PCE.