3rd Generation PV
Posted by Max Dunn Fri, 30 May 2008 03:42:00 GMT
There was an interesting talk at the Woods Energy Seminar at Stanford yesterday by Dr. Gavin Conibeer about 3rd generation photovoltaic (PV) devices.
The 1st generation are the PV cells we have now that cost around $6/watt and are around 20% efficient. The 2nd generation are the thin film cells which cost around $1/watt but are only about 12% efficient. The 3rd generation cells will use quantum dot technology created using thin-film manufacturing methods, so they will be a lot less expensive than 1st generation devices but will also use a variety of techniques to boost efficiencies up to 65% which will drive the cost down to $0.20/watt.
This sounds pretty great! However, the catch is that when asked when these 3rd generation PV cells would start going into production, his answer was “It is still a long ways away.”
Following are the notes I took during his talk:
Approaches to Third Generation Photovoltaics
Dr. Gavin Conibeer, University of New South Wales, GCEP Distinguished Lecturer
Abstract
Amongst these renewable energies, photovoltaics is the fastest growing technology with more than 30% growth per year over the last 10 years and more than 60% growth in 2007; although worldwide installation is still small. This growth in manufacture is currently driven by subsidies, primarily in Europe, but the increase itself leads to a learning effect as the technology matures, which brings down the cost per unit. In order to maintain the leverage this steep learning curve applies to unit price, a transition of technology from the First Generation approaches based on single crystal wafer based solar cells to Second Generation Thin Film, with their much lower energy intensity and material usage, is required. However to project this downward pressure on price onto ever larger production volumes, a further generation change is required to push up efficiencies whilst still maintaining the low cost approaches of Thin Film cells.
The reason that such Third Generation technologies can achieve such a ‘best of both worlds’ result is that the vast majority of current production cells consist of only one absorbing semiconductor material. But such single semiconductor band gap devices have to compromise in their absorption of a the very polychromatic solar spectrum, with a wide range of photon energies. This leads to significant energy losses through two main routes. Firstly solar photons at less than the band gap energy are not absorbed at all and are wasted. Secondly, for photons well above the band gap energy, a large fraction of their energy is lost as heat in the device. Third generation devices use multiple energy levels, often in the form of several different semiconductor materials, to extract energy efficiently from a greater fraction of these photons. Examples of such approaches will be discussed, with specific mention of tandem solar cells which use quantum dot nanostructures based on silicon; devices which can up-convert low energy photons such that they are absorbed; and hot carrier cells which seek to extract the energy gained from high energy photons before it can be lost to the lattice. The status of and prospects for these approaches will be assessed.
Booming Photovoltaics
- PV production increasing dramatically in the last few years
- Market growth at 35% for last 20 years, 60% in 2007
- 1 Million jobs in PV by 2020, 1 million jobs in RE by 2010
- Driven by rebates/tariffs in Japan, Germany, power purchase agreements in US
Learning Curves
- Double of installed capacity leads to 20% reduction in cost – same for Gas turbines, Wind and PV
- After certain point, reduction decreases to 10% per doubling in installed capacity
- 2nd generation thin film is following the same curve but is lower cost even for lower capacity
- First Solar is approaching $1/watt
- Third Generation should have efficiencies from 20 to 65% at a cost of $0.20 to $0.50 per watt using multi-junction technology that utilizes more wavelengths
Efficiency Loss Mechanisms
- Loss 1: Sub bandgap losses – low energy photons passing straight through
- Loss 2: Lattice thermalisation – high energy photons losing energy as heat
- These two losses count for about 50% of the losses
- Also have junction loss, contact loss, recombination
- Limiting efficiencies: Single p-n 31% or multiple threshold: 68.2% (at 1 sun)
Third Generation Options
- Increasing levels that utilizes certain wavelengths increases efficiencies but also the cost
- Impact ionisation: converts high energy photons into multiple electron/hole pairs
- Up-converters: take multiple low energy photon and convert higher energy electrons
- Thermionics: Heat up material that produces illumination to the PV layer
- Hot Carrier:
Silicon Based Tandem Cells
- Silicon is a good bottom cell
- Use quantum dots to engineer a wider bad gap – Si QDs
- Start by depositing using thin-film techniques SiO2 and SRO
- When annealed to 1100 degC get silicon nanocrystals that are confined to their layer
- Trying Tin QD in SiO2 and Ge
Hot Carrier Cell
- Theory is to extract hot carriers before they can thermalise, but need to collect carriers over a narrow range of energies
Production
- Still a long ways from production, but costs should be comparable to current thin-film techniques although efficiency rate is not yet known