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New photovoltaic materials: going beyond silicon

Isabelle Dumé, Science journalist
On February 16th, 2022 |
4 mins reading time
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New photovoltaic materials: going beyond silicon
Nathanaelle Schneider
Nathanaëlle Schneider
CNRS Researcher at Institut Photovoltaïque d'Ile-de-France (IPVF)
Key takeaways
  • The production capacity of photovoltaic (PV) panels is on the rise. In 2008, it was about 10 GW worldwide, while it is now over 600 GW – enough to power a country like Brazil.
  • This rapid growth in recent years is all the more surprising because it has occurred without any real fundamental change in PV technology.
  • Advances have been made using silicon. In the 1950s, solar cells converted only 5% of solar radiation into energy. Whereas, today, this is closer to 25%.
  • But the technology is so powerful and so inexpensive that its room for improvement is now limited.
  • Researchers are therefore studying new molecules such as perovskites or new production methods such as “atomic layer deposition” to continue to improve PV technology.

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.

Rapid growth

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.

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The search is on for alternative materials

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.

Perovskites, tandem and multi-junction solar cells

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.

Atomic layer deposition

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.

1https://​www​.iea​.org/​r​e​p​o​r​t​s​/​s​o​l​ar-pv
2https://​itr​pv​.vdma​.org/
3https://​www​.sci​ence​.org/​d​o​i​/​1​0​.​1​1​2​6​/​s​c​i​e​n​c​e​.​a​b​d4016
4https://​aip​.sci​ta​tion​.org/​d​o​i​/​1​0​.​1​0​6​3​/​5​.​0​0​48653
5https://​pubs​.rsc​.org/​e​n​/​c​o​n​t​e​n​t​/​a​r​t​i​c​l​e​l​a​n​d​i​n​g​/​2​0​1​7​/​m​h​/​c​6​m​h​0​0​5​2​1​g​/​u​nauth