For decades, solar-cell efficiencies have remained below the thermodynamic limits. However, new approaches to light management that systematically minimize thermodynamic losses will enable ultrahigh efficiencies previously considered impossible.
View full text
At a glance
: Solar-cell characteristics.
a, Energy diagram of a single-junction solar cell. Light at an energy (red arrow) creates an excitation from the valence (V) to the conduction (C) band of a semiconductor. After thermalization in the conduction band an electron–hole pair is formed across the bandgap with energy ħ ω E g. Light with an energy below the bandgap (purple arrow) is not absorbed. b, Typical current–voltage ( I– V) characteristics of a solar cell. The short-circuit current I sc is a direct measure of the conversion efficiency from incident photons to electrical current. The open-circuit voltage V oc is determined by the factors described in the main text; it is significantly lower than E g due to entropic reasons. The maximum-power operating point of the solar cell is indicated by the dashed lines.
: Light-management architectures for reaching ultrahigh efficiency.
a, Three-dimensional parabolic light reflectors direct spontaneous emission back to the disk of the Sun. b, Planar metamaterial light-director structures. c, Mie-scattering surface nanostructure for light trapping. d, Metal–dielectric–metal waveguide or semiconductor–dielectric–semiconductor slot waveguide with enhanced optical density of states to increase the spontaneous emission rate.
: Multi-junction solar cells.
a, Multi-junction energy diagram. Semiconductors with different bandgaps convert different portions of the solar spectrum to reduce thermalization losses. The quasi-Fermi levels defining the open-circuit voltage are indicated by the horizontal blue dashed lines. The yellow dots represent the electrons. b, Parallel-connected architecture that can be realized using epitaxial liftoff and printing techniques of the semiconductor layers, followed by printing of a micro- or nanophotonic spectrum splitting layer. Each semiconductor layer can be combined with one of the structures in Fig. 2 to reduce entropy losses and these structures can be separately optimized for each semiconductor.
: Scalable inexpensive large-area layer transfer and nanofabrication techniques.
a, Fabrication of ultrathin silicon wafers: hydrogen-ion implantation into a silicon wafer followed by annealing leads to formation of hydrogen bubbles at a well defined depth; the surface silicon layer can subsequently be peeled off and the remaining wafer is polished for re-use. b, Fabrication of ultrathin GaAs layers: AlAs (blue) and GaAs (orange) layers are epitaxially grown onto a GaAs substrate by chemical vapour deposition (CVD). Selective chemical etching removes the AlAs layer, subsequently the GaAs layer is lifted-off. In the lift-off processes in a and b, the thin layer is often laminated to a flexible substrate before it is peeled off. c, Soft-imprint lithography: a patterned rubber stamp (pink) is printed into a sol–gel layer (green) that is spin-coated onto the substrate. On drying of the sol–gel the stamp is removed and the pattern is transferred into the wafer by reactive-ion etching and the mask is removed. Using soft imprinting a spatial resolution of 10 nm is routinely achieved over a full 6” wafer.
: Thermodynamic losses in solar-energy conversion.
The maximum efficiency realized for a conventional single-junction solar cell is 28.3% (indicated in green). Dark blue bars indicate entropy-related losses and light blue bars indicate energy-related losses . The main energy loss is due to thermalization and lack of absorption. The solutions to reducing the entropy- and energy-loss problems are listed in the right-hand column.