COMPARATIVE LCA OF NANOTECHNOLOGIES IN THIN-FILM PV DEVICES

Hyung Chul Kim and Vasilis FthenakisCenter for Life Cycle AnalysisColumbia University CLCA

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November 5, 2009

LCA of Nano PV - Framework

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Cell/Module manufacturing

Commercial Micro PV

R&DNano PV

MaterialsProduction

Operation/ Maintenance

Recycling/Disposal

Cell/Module manufacturing

MaterialsProduction

Operation/ Maintenance

Recycling/Disposal

Parameters

-Purity/amount-Energy demand-Particle characteristics

-Material utilization -Process parameters-Energy demand

-Conversion efficiency-Durability

-Recyclability-Environmental fate-Control/process

Thin Film CdTe PV

Direct bandgap semiconductor - About100 times less material needed per watt than indirect bandgap semiconductor, e.g. Si.

Simple device structures and manufacturing processes - Low cost production (<$1/Wp).

Source: www.firstsolar.com

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Nano CdTe PV

(A) CdSe and (B) CdTe NCs. Scale bar, 40 nm. (C) An energy diagram of valence and conduction band.(D) Spin-cast film of colloidal NCs. Scale bar, 1 mm.Source: Gur et al 2005, Science.

RationaleRod-shape CdTe/CdSe nanocrystals processed in colloidal solution Low cost processing, e.g. ink-jet printing

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Micro CdTe PV

Vapor Transport Deposition

PurificationMeltingAtomization

Contact/Electrode

Module Assembly

CdTe Synthesis

Metallurgical, 98-99% gradeCd, Te

Nano CdTe PV

Nano CdTe Rod Growing

Degasification

Spin Casting

Contact/Electrode

CdOPhosphorous Compounds

Purification

Solvent (toluene, isopropanol, hexane, pyridine)

TeTOP

Waste Solvent

Waste

Inkjet Printing

Module Assembly

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Nano CdTe PV - Production

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10

100

1000

10000

100000

1000000

Cell materials Solvents Phosphorus material

g/m2

Micro-commercial

Nano - Lab level

Nano - Projected Commercial

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Nano CdTe PV – Material Use

-Laboratory to pilot plant scale-up ratios between 10 to 100 for solution grown processes (Bisio and Kabel 1985; Slater and Savelski 2007). - Material utilization: spin casting ~1%; Ink jet printing > 98% (Bharathan and Yang 1998).

1

10

100

1000

10000

100000

Cell Materials Solvents + Phosphorus

materials

Semiconductor Deposition

Total

MJ p/m

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Micro- commercial

Nano – Lab Level

Nano – Projected Commercial

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Nano CdTe PV – Primary Energy Use

Findings on Nano CdTe PV

The current laboratory-based mass utilization numbers have to be scaled-up to sustainable production scales. Spin-casting is inefficient and will have to be replaced by ink-jet printing

Under the laboratory condition, the solvents and phosphorus compounds used in growing and purifying the nanoparticles dominate the primary energy demands, accounting for 99% of the total of 41,000 MJp/m2.

The primary energy demand of future commercial line to manufacture nanoparticle-based CdTe could potentially drop to ~50 MJp/m2 excluding encapsulation based on more efficient solvent and material usages projected.

Solvents used in nano-crystalline CdTe synthesis have to be selected in consideration of recyclability, human- and eco- toxicity, and environmental impacts.

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a-Si PV

Pros- Perfect for building integrated applications - Roll-to-roll deposition- Aesthetic attractiveness- Easy alloying with Ge to adjust the bandgap - suitable for multi-junction cells

Cons - “Staebler-Wronski Effect (SWE)” significantly (20-30%) degrades the efficiency of the cell upon initial 1000 hours of exposure to sunlight –low conversion efficiency. i.e. 6.7% for commercial module triple junction a-Si/a-SiGe/a-SiGe

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nc-Si Layer

Rationale Nanocrystalline-Si (nc-Si) layers barely suffer from sunlight induced degradation higher efficiency.

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Micromorph PV design from Oerlikon Solar/ Applied Materials

Issues Thick layer needed (1-3 µm) increased cost, energy and material requirements

Multi-junction a-Si PVs with nc-Si Layer

TCO

nc-Si

Back Reflector

Substrate

0.2-0.3 µma-Si

1.3-2 µm

nc-Si 1.7-2.5 µm

TCO

Back Reflector

Substrate

a-SiGe

0.2 µm

0.3 µm

0.3 µma-SiGe

a-Si

TCO

nc-Si

Back Reflector

Substrate

0.2-0.3 µma-Si

1-3 µm 1-2.5 µm

TCO

Back ReflectorSubstrate

a-SiGe

0.2 µm

0.3 µma-Si

nc-Si

a-Si/a-SiGe/a-SiGe Commercial triple-

junction

a-Si/nc-SiAlternative A

a-Si/a-SiGe/nc-SiAlternative B

a-Si/nc-Si/nc-SiAlternative C

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United Solar’s Multi-junction a-Si PV designs evaluated in this study

Breakdown of Primary Energy Use for a-Si/a-SiGe/a-SiGe

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Plasma Enhanced Chemical Vapor Deposition

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Parameter a-Si nc-Si, current nc-Si, future

Deposition rate (Å/s) 3 5-8 20-30

RF Frequency (MHz) 13.56 40-70 40-70

SiH4/(SiH4+H2) (%) 2 1 4

SiH4 Utilization (%) 20 20 80

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PECVD Parameters

Primary Energy Use15

Energy Payback Time16

Based on the average US insolation of 1800 kWh/m2/yr and a performance ratio of 0.75.

Findings on Nano Si

Depositing nc-Si layer use significantly more energy than depositing a-Si layer

The conversion efficiency of nc-Si designs is not yet higher than that of current triple triple junction a-Si design.

Consequently, the new designs have a 20-30% longer energy payback time (EPBT) than the currently commercial option.

If nc-Si film is deposited at a higher rate, (i.e. 2-3 nm/s from 0.5-0.8 nm/s), and at the same time the conversion efficiency reaches 10%, the EPBT could drop by 50% from the currently commercial option.

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Conclusions18

Despite the publicity of nano technologies in high tech industries including the photovoltaic (PV) sector, their life cycle environmental impacts are understood to a limited degree as they remain in R&D stage.

The benefits will be paramount if the potential environmental risks of a nanotechnology are properly assessed and addressed before it fully matures.

The energy and environmental performance of nano-based PV technologies can be estimated based on parametric analysis of mature, micro-based PV designs.

A timely update of such analyses will be critical.