New photovoltaic materials: going beyond silicon

Sol­ar (or photo­vol­ta­ic) cells har­ness energy from the sun and con­vert it into elec­tric cur­rent. Until 2008, these devices had a glob­al gen­er­a­tion capa­city of around just 10 gigawatts (GW), but this fig­ure now stands at roughly 600 GW, which is enough to power a coun­try as big as Brazil. What is more, the power gen­er­ated by sol­ar cells is estim­ated to have increased by 22% in 2019 to 720 TWh (ter­awatt hours), which means that this tech­no­logy now accounts for almost 3% of world­wide elec­tri­city pro­duc­tionLes cel­lules sol­aires (ou photo­voltaïques) captent l’énergie du soleil et la con­ver­tis­sent en éner­gie élec­trique. Jusqu’en 2008, la capa­cité de pro­duc­tion était d’environ 10 gigawatts (GW) au niveau mon­di­al, mais elle est désor­mais supérieure à 600 GW – ce qui serait suf­f­is­ant pour ali­menter un pays grand comme le Brésil. De plus, la puis­sance générée par les cel­lules sol­aires aurait aug­menté de 22 % en 2019 pour atteindre 720 TWh (ter­awatt hours), ce qui sig­ni­fie que cette tech­no­lo­gie représente désor­mais près de 3 % de la pro­duc­tion mon­diale d’élec­tri­cité¹.

These fig­ures will con­tin­ue to increase as energy pro­duc­tion becomes more sus­tain­able and because demand is increas­ing world­wide. It is up to us, research­ers and R&D and tech­no­logy play­ers, to meet this demand with new designs, effi­cient photo­vol­ta­ic mater­i­als and new man­u­fac­tur­ing processes.

Such rap­id growth is all the more aston­ish­ing in that it has happened without any real fun­da­ment­al change to the under­ly­ing photo­vol­ta­ic tech­no­logy. Indeed, today’s sol­ar cells are to all intents and pur­poses sim­il­ar to the one demon­strated at Bell Labor­at­or­ies in the US in 1954. This sol­ar cell was based on a simple junc­tion between n‑type (elec­tron-rich) and p‑type (elec­tron-poor) sil­ic­on and it con­ver­ted sun­light into elec­tri­city with an effi­ciency of 5%.

Over the years, this effi­ciency has gradu­ally increased to over 25% thanks to more advanced cell designs con­tain­ing highly doped sil­ic­on, improved elec­tric­al con­tacts and anti-reflec­tion lay­ers, Sil­ic­on-based devices are now cheap­er too: the aver­age mod­ule price is about $0.21/Wp and the LCOE (lev­el­ised cost of energy) is 2.8–6.8 cents/kWh (AC). Accord­ing to the ITRPV 2020 Report², these fig­ures will improve, with an expec­ted LCOE in 2031 of 2–5 cents/kWh. These impress­ive advances, and the fact that the cells can oper­ate for more than 25 years, are the reas­ons why this tech­no­logy now accounts for about 95% of the glob­al sol­ar mar­ket. New mater­i­als could fur­ther increase this mar­ket share.

For PVs to be used in applic­a­tions such as BI-PV (Build­ing-Integ­rated PV) or to meet the grow­ing demand for pro­duc­tion capa­city, sil­ic­on (Si) alone no longer fits the bill. That said, Si tech­no­logy is so effi­cient and inex­pens­ive that improv­ing it fur­ther is dif­fi­cult. One strategy for over­tak­ing Si is to devel­op PVs with even high­er power con­ver­sion effi­cien­cies, i.e. those that con­vert a lar­ger frac­tion of sun­light into electricity.

To achieve this, research­ers are look­ing for altern­at­ive mater­i­als. Thin films based on com­pounds such as cad­mi­um tel­lur­ide (CdTe) and cop­per indi­um gal­li­um (CuInGa) arsen­ide, for example, are already com­mer­cially avail­able. These mater­i­als only need to be a few microns thick to suf­fi­ciently absorb sol­ar radi­ation. They can get away with hav­ing a lower qual­ity too (unlike tra­di­tion­al Si). Pan­els made from these mater­i­als can even be flexible.

There is a prob­lem, how­ever, in that these photo­vol­ta­ic mater­i­als rely on indi­um and tel­luri­um, ele­ments that are rare and thus expensive.

In the hunt for more Earth-abund­ant absorber mater­i­als, research­ers have turned their atten­tion in recent years to per­ovskites, prom­ising crys­tal­line mater­i­als for thin-film sol­ar cells that can absorb light over a wide range of wavelengths in the sol­ar spec­trum. Even though their sta­bil­ity needs to be improved, their effi­ciency is now above 18%, put­ting them on a par with estab­lished sol­ar cell materials.

Anoth­er pos­sib­il­ity: ‘tan­dem’ devices, which are sol­ar cells con­tain­ing two dif­fer­ent but com­ple­ment­ary pho­to­act­ive semi­con­duct­or mater­i­als. These cells can achieve high­er effi­cien­cies when the two mater­i­als are used togeth­er, com­pared to either mater­i­al on its own (the the­or­et­ic­al max­im­um con­ver­sion effi­ciency increases from 33 to 45%). Com­bin­ing Si with a per­ovskite³, for example, can make the most of the dif­fer­ent wavelengths of sun­light: sil­ic­on effi­ciently con­verts photons in the infrared range and per­ovskites con­vert high­er energy photons.

Fur­ther improve­ments are pos­sible by stack­ing sev­er­al pho­to­act­ive semi­con­duct­or mater­i­als and care­fully choos­ing the com­bin­a­tion that best cap­tures sol­ar radi­ation. These multi-mater­i­al devices are called ‘multi-junc­tion cells⁴ and are mainly based on so-called III‑V mater­i­als (GaAs type).

It is not easy to man­u­fac­ture such devices, how­ever. A sol­ar cell, whatever its archi­tec­ture, con­tains dif­fer­ent lay­ers of mater­i­al. Each lay­er has a cru­cial role to play: in addi­tion to the pho­to­act­ive mater­i­al that absorbs light, we need lay­ers that col­lect elec­trons, lay­ers that are trans­par­ent to photons, and coat­ings res­ist­ant to humid­ity, etc. To make an effi­cient sol­ar cell, each lay­er must meet strict spe­cific­a­tions and be assembled with extreme precision.

This is where our expert­ise lies. We syn­thes­ise thin films of mater­i­als for photo­vol­ta­ic cells using a tech­nique widely used in the micro­elec­tron­ics industry called Atom­ic Lay­er Depos­ition (ALD)⁵. Here, the sur­face of a sub­strate is suc­cess­ively exposed to a com­pound, called a pre­curs­or, which reacts with the sub­strate, attaches to it and forms an atom­ic mono­lay­er. Mater­i­al growth con­tin­ues by expos­ing this lay­er to a com­ple­ment­ary molecule to build the entire struc­ture mono­lay­er by mono­lay­er. This tech­nique allows us to make lay­ers as thin as 2 to 100 nanometres.

Not all molecules are suit­able for ALD though. In our labor­at­ory we are try­ing to under­stand which molecules are best suited to the tech­nique and which are not, how they behave on the sur­face of a sub­strate and what phys­ic­al and elec­tron­ic prop­er­ties they bring to the end material.

Although we are now spoilt for choice for when it comes to the dif­fer­ent photo­vol­ta­ic mater­i­als avail­able and under devel­op­ment, it is dif­fi­cult to pre­dict which ones will win the race. It is pos­sible that sev­er­al sim­il­ar com­ple­ment­ary tech­no­lo­gies will co-exist, with dif­fer­ent mater­i­als find­ing dif­fer­ent applic­a­tions. Sil­ic­on, com­bined with a second pho­to­act­ive mater­i­al, may remain the mater­i­al of choice for rigid pan­els, while thin films will be best for build­ings or objects. Whatever the future holds, we are on the verge of a real ‘mater­i­als revolu­tion’ for sol­ar cell technology.