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New photovoltaic materials: going beyond silicon

Nathanaelle Schneider
Nathanaëlle Schneider
CNRS researcher at Institut Photovoltaïque d'Ile-de-France (IPVF)

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.

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.

Interview by Isabelle Dumé
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

Contributors

Nathanaelle Schneider

Nathanaëlle Schneider

CNRS researcher at Institut Photovoltaïque d'Ile-de-France (IPVF)

Nathanaëlle Schneider works on the synthesis of new materials by ALD (Atomic Layer Deposition) for photovoltaic (solar panel) applications. Using coordination chemistry, in particular, her research has led to new methodological approaches for the design of ALD precursors, material solutions for certain photovoltaic devices and new functional materials for which the UMR-IPVF is now a recognised world pioneer. She was awarded the CNRS bronze medal in 2020.