1. Comparative LCA of Nanotechnologies in Thin-Film PV Devices
Hyung Chul Kim and Vasilis Fthenakis
Center for Life Cycle Analysis
Columbia University
November 5, 2009

2. LCA of Nano PV - Framework
Materials
Production
Cell/Module manufacturing
Operation/ Maintenance
Recycling/
Disposal
Commercial Micro PV
Parameters
-Purity/amount
-Energy demand
-Particle characteristics
Material utilization
Process parameters
Energy demand
Conversion efficiency
Durability
Recyclability
Environmental fate
Control/process
This is the framework of our LCA of nano PV.
We do have relatively good knowledge on the life cycle impact of commercial micro PV technologies.
But we don’t have much info on the life cycle of nano-PV technologies because they are not fully mature. And only in R&D stages.
Therefore, we need to estimate the life cycle environmental impact with a limited information. One simple way to estimate reduce the knowledge gap is looking at the parameters of energy, material, and processes.
Materials
Production
Cell/Module manufacturing
Operation/ Maintenance
Recycling/
Disposal
R&D
Nano PV

3. 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).
Thin film CdTe PV use around 100 times less semiconductor materials than Si PV.
CdTe is deposited as single junction using Vapor transport Deposition.
The current production cost is less than $<1 per Wp.
Source:

4. Nano CdTe PV
Rationale
Rod-shape CdTe/CdSe nanocrystals processed in colloidal solution
 Low cost processing, e.g. ink-jet printing
There is interesting R&D Recent studies
(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.

5. Nano CdTe PV - Production
Micro CdTe PV
Vapor
Transport
Deposition
Purification
Melting
Atomization
Contact
/Electrode
Module
Assembly
CdTe
Synthesis
Metallurgical, 98-99% grade
Cd, Te
Nano CdTe PV
Nano CdTe
Rod Growing
Degasification
Spin Casting
Contact
/Electrode
CdO
Phosphorous Compounds
Purification
Solvent (toluene, isopropanol, hexane, pyridine) Te
TOP
Waste Solvent
Waste
Inkjet Printing
Module
Assembly
Trioctylphosphine (TOP) tri-n-octylphosphine oxide (TOPO)

6. 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).

7. Nano CdTe PV – Primary Energy Use

8. 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.

9. 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

10. nc-Si Layer
Rationale
Nanocrystalline-Si (nc-Si) layers barely suffer from sunlight induced degradation  higher efficiency.
Issues
Thick layer needed (1-3 µm)
 increased cost, energy and material requirements
Amorphous Solar Modules - This very cost effective solar cell is made by depositing amorphous silicon (a-Si) on the TCO layer.
Micromorph® Solar Modules - In addition to the a-Si layer, the Micromorph® cell has a tandem structure with an additional microcrystalline absorber. This layer converts the energy of the red and near infrared spectrum, allowing an efficiency increase of approximately 30%. Oerlikon Solar's production solution permits a modular upgrade from the single a-Si cell to the Micromorph® tandem cell.
Micromorph PV design
from Oerlikon Solar/ Applied Materials

11. Multi-junction a-Si PVs with nc-Si Layer
TCO
nc-Si
Back Reflector
Substrate µm
a-Si
1.3-2 µm µm
a-SiGe
0.2 µm
0.3 µm
1-3 µm
1-2.5 µm
a-Si/a-SiGe/a-SiGe
Commercial triple-junction
a-Si/nc-Si
Alternative A
a-Si/a-SiGe/nc-Si
Alternative B
a-Si/nc-Si/nc-Si
Alternative C
United Solar’s Multi-junction a-Si PV designs evaluated in this study

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

13. Plasma Enhanced Chemical Vapor Deposition
By applying radio frequency between two electrode, PECVD machine generate ionized plasma gases.
A higher deposition rate and better results than normal CVD.

14. PECVD Parameters 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
SiH4/(SiH4+H2) (%) 2 1 4
SiH4 Utilization (%) 20 80

15. Primary Energy Use

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

17. 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 nm/s), and at the same time the conversion efficiency reaches 10%, the EPBT could drop by 50% from the currently commercial option.

18. Conclusions
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.