Advanced Materials and Concepts for Full Solar Spectrum Energy Harvesting Kin Man Yu

Collaborators: Wladek Walukiewicz (LBNL) Z. Liliental-Weber, Oscar Dubon (UCB), S. V. Novikov and S. T. Foxon (Nottingham), W. Sarney and S. Svensson (ARL), R. Martin (Strathclyde), C. W. Tu (UCSD), Y. Okada (Tokyo U.), T. Tanaka (Saga U.)

Solar Cell Losses

1. Thermalization loss 2. Transparency loss 3. Recombination loss 4. Junction loss 5. Contact loss

Our research focuses on new materials and concepts to overcome these losses

InGaN: Full spectrum PV The direct energy gap of In1-xGaxN

covers most of the solar spectrum Advantages of using InGaN for solar

cells: – Flexibility in choosing the number

and the bandgaps of junctions to optimize the solar cell performance.

– Fabrication process could be greatly simplified

– Superior radiation resistance – solar cells operated in outer space.

No tunnel junction needed Built on well-established Si solar cell technology

Highly Mismatched Alloys Highly mismatched alloys (HMAs)-novel alloys formed by the substitution

of isoelectronic elements with very different electronegativities/size. Developed a band anticrossing (BAC) model to describe the electronic

structures of HMAs (1000 citations) Utilized the unique properties of HMAs to develop new concepts and

materials for energy conversion devices

Oxides and Interfaces

Intermediate band solar cell using GaNAs HMA

An intermediate band solar cell using dilute GaNAs (~2% N) HMA demonstrates an optical activity of three energy bands that absorb and convert into electrical current the crucial part of the solar spectrum.

The narrow intermediate band acts only as a “stepping stone” enabling efficient use of low energy solar photons.

Highly Mismatched Oxides

InGaN-Si hybrid tandem cell

Electronic structures of ZnOS, ZnSeO and ZnTeO are consistent with valence band anticrossing of ZnO and S, Se and Te localized states in ZnO ZnOS in the whole composition range can be synthesized with bandgap tunable from 3.7eV down to

~2.55eV ZnO1-xTex with x ≈ 0.1 is expected to have a bandgap of ~1.7eV, ideal as a top cell for tandem on a Si

PV The conduction band edge of ZnO lies near the valence band edge of Si.

⇒ A natural tunnel junction between n-ZnO1-xTex and p-Si ZnO1-xTex with x ≈ 0.05 will have a bandgap close to ~2 eV that is optimal for PEC operation..

CdO as High Mobility Transparent Conductor

Ideal uncompensated materials with extremely high mobilities of 300 cm2/Vs with electron concentration of 4x1020/cm3 have been achieved with Ga doping with a maximum conductivity of 20,000 S/cm (or ρ=5x10-5Ω-cm).

CdO can be used for Si and CZTS instead of metal grids Transmittance of CdO:In compatible

with MJ PV

Structurally Mismatched CdO-ZnO

Two distinct phase regimes: RS phase with high electron mobility Optical gap of WZ phase decreases from 3.3 eV (ZnO) to 1.9 eV (x=0.67) Near band gap edge emission decreases from 3.3 eV (ZnO) to 1.7 eV

(x=0.69)

Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231

Single alloy system with band gap tunable from 0.8-3.4 eV High absorption coefficient Amorphous structure: grown on low cost substrate, e.g. glass

Uniform amorphous thin film on sapphire and glass with no nano-crystalline phase for 0.10<x<0.75

ZnO1-xSx ZnO1-xSex