Photovoltaic effect

Everyday term

In plain words

Light absorbed in a material frees electrons, and a built-in asymmetry – usually a junction between two different layers – pushes them one way rather than the other, producing a voltage. That is how a solar cell works.

Going deeper

A photovoltaic device needs a built-in asymmetry to send electrons and holes in opposite directions – here the staggered band alignment of two stacked layers. Its quality is read from the illuminated current–voltage curve: short-circuit current, open-circuit voltage and fill factor.

From absorbed light to a voltage

Light absorbed in a creates an . Left alone they recombine; to produce power, something must separate them before that. The built-in field of a p–n junction does it in a silicon cell. In the same job is done by a contact, by a lateral junction defined with split gates, or – most characteristically – by stacking two different layers whose bands are staggered, so that electrons find their lowest energy in one layer and holes in the other.

Once separated, the carriers charge the two terminals until their electrostatic potential difference balances the drive, giving the open-circuit voltage; connected instead through a load, they deliver a current. The product of current and voltage is largest somewhere between those extremes, at the maximum power point.

What 2D layers bring

absorb a large fraction of sunlight for their thickness – per unit thickness roughly an order of magnitude more than GaAs or silicon – so a stack a few nanometres thick already absorbs a few percent of incident light. Absolute absorption stays small, which rules out competing with thick silicon on efficiency, but power per weight can be high, and the stacks are flexible and semi-transparent.

The more distinctive property is tunability. In a gate-controlled heterojunction the itself can be changed electrically, so the same device can be switched between photovoltaic and photoconductive behaviour, and the sign of the photocurrent reversed – something a fixed profile in a silicon cell cannot do.

Reading efficiency claims

Efficiency is the electrical power out divided by the light power in, and both numbers invite mistakes. Dividing by the light falling only on a small , rather than on the whole device area, inflates the result; so does illuminating with a laser tightly focused to an intensity thousands of times that of sunlight. Standard conditions (the AM1.5 spectrum, 1 kW per square metre, 25 °C) exist for this reason.

For thin absorbers, quantum efficiency is often the more telling measure: external quantum efficiency counts electrons collected per arriving, internal per photon absorbed. A monolayer device can have excellent internal efficiency and still convert little of the incident light. Stability under illumination matters as well, since many 2D absorbers degrade in air and under light.

For specialists

Separation of photogenerated carriers by a built-in field at a p–n junction, a Schottky contact or a type-II van der Waals interface. Monolayers absorb roughly an order of magnitude more sunlight per unit thickness than GaAs or Si, so a stack a few nanometres thick reaches useful power per unit mass even though its absolute absorption stays small; in a heterojunction the alignment, and with it the sign and size of the , can be switched electrically.

Where this comes from

Extraordinary sunlight absorption and one nanometer thick photovoltaics using two-dimensional monolayer materials Bernardi et al. · Nano Letters 13, 3664 (2013) cited by 2,059
Photovoltaic effect in an electrically tunable van der Waals heterojunction Furchi et al. · Nano Letters 14, 4785 (2014) cited by 1,084

Electron hole

Usually just called a hole: the empty place an electron leaves behind in an otherwise full band. It behaves like a particle of its own with a positive charge. When a neighbouring electron steps into the gap, the gap moves the other way – like the empty space in a queue of cars, which travels backwards as each car edges forward. Many materials conduct mainly with holes, and chips need both kinds of carrier.

An unoccupied state in an otherwise filled band, usually near the valence-band maximum, treated as a quasiparticle with positive charge and a positive effective mass set by the band curvature. Holes carry the current in p-type semiconductors and give a positive Hall coefficient. In 2D semiconductors good p-type transport is harder to obtain than n-type, largely because metal contacts pin the Fermi level nearer the conduction band.

Full explanation

Band alignment

How the energy levels of two materials line up when they are put together: whether one material’s electrons sit above or below the other’s. Electrons roll ‘downhill’ to the lower level, so the alignment decides which way charge moves across the junction, and so decides what the stack can be used for.

The relative positions of the band edges of two materials in contact, classified as straddling (type I), staggered (type II) or broken (type III). Van der Waals stacking avoids chemical bonding and strain, so alignments start close to the isolated layers’ ionisation potentials and electron affinities, shifted by interface dipoles and by the dielectric environment. A type-II alignment puts electron and hole in different layers, which is what makes interlayer excitons and gate-tunable photovoltaic junctions possible.

Full explanation

Photon

The smallest possible amount of light – one indivisible packet of it. The packets of a given colour all carry the same energy, and blue ones carry more than red: that is why ultraviolet light gives you sunburn and red light, however bright, does not. A semiconductor absorbs only photons with enough energy to cross its band gap and gives light back at about the gap’s energy – which is why LEDs of different colours are made from different semiconductors.

The quantum of the electromagnetic field, with energy E = hν and momentum h/λ but no mass or charge. Visible photons carry about 1.7–3.1 eV, comparable to the band gaps of semiconducting TMDCs, but a momentum far smaller than the crystal momenta across the Brillouin zone, so optical transitions are vertical and an indirect gap needs a phonon to make up the difference. Single-photon emission is established by antibunching in photon-correlation measurements.

Full explanation

Gating

Changing a material by putting a voltage on a nearby electrode – a gate – separated from it by an insulator. The voltage pulls electrons into the material or pushes them out, as in every transistor. Because a 2D material is so thin, the gate reaches all of it, so the number of electrons can be dialled up and down continuously: a single device can be turned from insulator to metal, its magnetism strengthened, or superconductivity switched on. Two gates, above and below, can also apply an electric field across the layer.

Electrostatic control of carrier density and electric field through a gate coupled capacitively across a dielectric: the induced density equals the gate capacitance times the voltage beyond threshold, divided by the electron charge, reaching a few 10¹³ cm⁻² with oxide or hBN gates before breakdown. Dual gating sets density and perpendicular displacement field independently, opening the gap of bilayer graphene and tuning moiré flat bands. Ionic-liquid and solid-electrolyte gating form an electric double layer about a nanometre thick and reach 10¹⁴–10¹⁵ cm⁻², enough to induce superconductivity in MoS₂ or raise the Curie temperature of Fe₃GeTe₂, but can intercalate or react electrochemically and work only while the ions are mobile. Hysteresis from traps, contact effects and quantum capacitance are the usual caveats.

Full explanation