Physics Department, University of Pavia

http://fisica.unipv.it http://fisica.unipv.it/nanophotonics

Lucio Claudio Andreani

Dip.to di Fisica, Università di Pavia, 13-09-2018

Semiconductors and photonics for energy

OUTLINE

1. Introduction: energy & climate (in a bad mood)

2. Solar cells: towards the efficiency limits

3. Silicon photonics & optical communication

4. Conclusions

Global temperature increase

Earth’s surface temperature has increased by ≈1 °C over pre-industrial levels.This is correlated with cumulated CO2 emission, mainly due to the use of fossile fuels.

Conclusions from Intergovernmental Panel on Climate Change, http://www.ipcc.ch/

www.climatecentral.org

Global primary energy demand & related CO2 emissions by scenario

450 scenario = limit global temperature increase in 2100 to 2 °C above pre-industrial levels, by keeping CO2 concentration in atmosphere below 450 ppm

International Energy Agency (IEA) – World Energy Outlook, 2016

Global energy-related CO2 emissions & additional CO2 abatement in the 450 scenario

450 scenario requires strong increase in energy efficiency & use of renewables

IEA – World Energy Outlook, 2016

Grid vs solar electricity: grid parity…?

… yes, reached in many countries! But depending on solar irradiation & on grid market

EU-JRC PV Status Report 2017

Solar cells: towards the efficiency limits

Marie Curie action FP7-PEOPLE-2010-ITN 264687 “PROPHET”

Fondazione CARIPLO "Nanophotonics for thin-film photovoltaics"

ENI SpA "Fotonica per sistemi fotovoltaici basati su concentratori fluorescenti"

RSE SpA "Nanostructured antireflectioncoatings for multijunction solar cells"

Participants: M. Liscidini, M. Galli, M. Patrini, A. Bozzola, S. Flores, P. Kowalckzewski, M. Passoni, S. Rafizadeh, G. Timò, E. Achilli, L. Lavit…

Detailed balance limit to the efficiency of single-junction photovoltaic cells

Shockley and Queisser, JAP 32, 510 (1961); Tiedje, Yablonovitch et al (1984)

• Thermodynamic efficiency limit is ~33%. Si and GaAs have optimal band gaps.• Beyond Shockley-Queisser limit? 3rd-generation concepts (very hard),

multijunction structures, concentration

Main photovoltaic technologiesSilicon (wafer-based)

photovoltaicsConcentration photovoltaics

Organic photovoltaics

Mono- and poly c-Si: ∼94% of global

photovoltaic market

Efficiency record: 26.7%

Multijunctions of III-V semiconductors:

satellites, solar fields

Efficiency record: 46% (4J, ~500 suns)

Dye-sensitized solar cells, polymeric (BHJ),

fluorescent concentrators

Low efficiencies (<10%) building-integrated PV

Light trapping in silicon photovoltaic cells

Goals: - to study the effects of light trapping in c-Si photovoltaic cells - to determine the ultimate efficiency limits and the optimal design

Light harvesting…

Light harvesting + electronic transport

• Optical calculation yields spatial profile of photogenerated carriers

• Electronic transport is treated by solving drift-diffusion equations with generation and recombination terms

• Generation rate profile for a Lambertian scatterer is calculated analytically and it is isotropic ⇒ 1D electronic simulations

Efficiency limits and optimal thickness of Si solar cells: effect of impurity recombination

1 10 1002021222324252627282930

0.2 mm

0.5 mm

1 mm

4 mm2 mm

Effic

ienc

y η

(%)

Thickness (µm)

Ldiff=infty

Maximum efficiency is ∼29%. The optimal thickness is reduced from ∼80 to 20 µm when the efficiency decreases from 29% to <23%

Theoretical treatment of transport in PV cells

Drift-diffusion equations:hh

ee

RGtp

RGtn

−=⋅∇+∂∂

−=⋅∇+∂∂

J

J

Currents (number and charge): hhhhh

eeeee,,

JjEJJjEJ

epDpenDn

+=∇−=−=∇−−=

µµ

Einstein relations: TkeD

TkeD

BBh

he

e , == µµ

Poisson equation: φε

φ −∇=+−−−=∇ E),(2ad NNpne

Boundary condition at surface for minority carriers:

)()(

0h0|h

0e0|eppSJnnSJ

xx

−=−=

=

= J

0 x

Photogeneration rate: calculated from

Maxwell equations

Beyond SQ limit: perovskite-silicon tandems

P. Kowalczewski et al., J. Opt. 18, 054001 (2016)

Perovskite (ABX3) structure with Methylammonium lead iodide: CH3NH3PbI3

Tandem solar cells on silicon are a promising approach to overcome the Shockley-Quessier limit for single-junction solar cells

Nanostructured antireflection coatings for multijunction solar cells

In collaboration with RSE SpA

1 µm

Moth-Eye Antireflection Coating:ACS Appl. Mat. Interf 2014, 6, 5827

⇒ reduce reflection losses, increase the efficiency by developing broadband, omnidirectional antireflection coatings.

Final goal: improve efficiency record for MJ solar cells (now 46% 50%...)

Thermophotovoltaics: exploiting thermal radiation for cogeneration of heat and electricity

A fuel (prophane or methane) is burned in a furnace,heating a ceramic element, which emits intense infraredradiation. A photovoltaic array surrounding this emitterconverts this IR energy into electric power.

Lewis M. Fraas, Low-Cost Solar Electric Power (Springer, 2014)

Combines physics of thermal radiation and heat exchange with photovoltaics (photonics + semiconductors) ⇒ prospects for micro-cogeneration technology

Silicon photonics and optical communication

Participants: D. Gerace, M. Passoni, L. Carroll, A. Bozzola…

EU FP7 STREP project 2011-2015 FABULOUS - "FDMA Access By Using Low-cost Optical Network Units in Silicon Photonics"

EU Horizon 2020 STREP project 2016-2018 COSMICC - " CmOs Solutions for Mid-board Integrated transceivers with breakthrough Connectivity at ultra-low Cost".

Information & Communication Technologies (ICT)

Short distance: treatment of information by electrical signals on a silicon chip integrated circuit

Long distance: transmission of information by optical signals propagating in silica fibers optical fiber

Integrating electrical with optical functions on a Si chip Silicon photonics

Main component: Si strip waveguide on SiO2

Si

SiO2

Light propagation by total internal reflection

Compatible with CMOS technology electrical and optical integration

on the same chip

Si photonics chip

Image from IBM Zürich

The FABULOUS project

Project goal: to realize an optical modem (optical network unit, ONU) integrated on a silicon chip

http://www.fabulous-project.eu/

UNIPV: Physics + DIII (Electrical Engineering)

Our goal: to design a grating coupler for coupling light from an optical fiber to a silicon waveguide, using photonic crystal concepts

The COSMICC project

Project goal: to realize an optical transceiver integrated on a silicon chip

http://www.h2020-cosmicc.com/

UNIPV: Physics + DIII (Electrical Engineering)

Our goal: to develop a silicon waveguide with slow light in order to reduce energy dissipation in the Mach-Zehnder modulator

Optical modulation by electrical signal (phase shifter)

Photonic band gap and band-edge slow light in silicon grating waveguides

Periodic variation of dielectric profile along the waveguide (1D photonic crystal) photonic band gap group velocity vg=dω/dk0 at the band edge

higher interaction in phase shifter reduced dissipation in MZ modulator

⇒ band-edge slow light

M. Passoni et al., Optics Express 26, 8470 (2018)

A major application of silicon photonics: energy consumption in data centers

More than 3% of electrical energy consumption in the world (and 2% of CO2 emission) is due to data centers. This number grows rapidly. Replacing optical fiber cables with interconnections on a silicon chip would lead to major energy savings.

1 google search = 0.3 Wh = a 60 W light bulb x 17 s 0.2 g of CO2 emission

PublicationsSolar cells: A. Bozzola et al., Opt. Expr. 20, A224 (2012); Prog. Photov. 22, 1237 (2014)P. Kowalczewski et al., Opt. Lett. 37, 4868 (2012); Opt. Expr. 21, A808 (2013)A. Bozzola et al. J. Appl. Phys. 115, 094501 (2014); 117, 026102 (2015)P. Kowalczewski et al., J. Appl. Phys. 115, 194504 (2014)S. Flores Daorta et al., Appl. Phys. Lett. 104, 153901 (2014)L.C. Andreani et al., Solar Energy Mat. & Solar Cells 135, 78 (2015) P. Kowalczewski et al., Solar Energy Mat. & Solar Cells 143, 260 (2015) A. Bozzola et al., Advanced Optical Materials 4, 147-155 (2016)P. Kowalczewski et al., J. Opt. 18, 054001 (2016)C Mennucci et al., Nanotechnology 29, 355301 (2018) Two patents with with ENI: MI2013A001062 (2013), MI2015A000091 (2015)

Silicon photonics: L. Carroll et al, Optics Express 21, 21556 (2013); 22, 14769 (2014)A. Bozzola et al., Optics Express 23, 16289 (2015)M. Passoni et al., APL 110, 041107 (2017); Opt. Express 26, 8470 (2018).

Summary

The energy transition to a sustainable society will require strong effortsto boost energy efficiency and renewables (solar PV and wind+ storage, electrification, grid infrastructure, … ). Very hard job.

Solar PV market is dominated by silicon solar cells, which are reachingthe conversion efficiency limits (∼29%). Further improving the efficiencywill require developing tandem/multijunction structures.

Silicon photonics may provide a breakthrough in optical communicationvia high-speed, low-cost, low-consumption devices.

Applied research is strongly linked to basic physics research on semiconductors and photonics: radiation-matter interaction, transport, nanophotonics, photonic crystals…