Silicon Wafer Solar Cells

Transcription

1 Silicon Wafer Solar Cells Armin Aberle Solar Energy Research Institute of Singapore (SERIS) National University of Singapore (NUS) April

2 1. PV Some background Photovoltaics (PV): Direct conversion of solar energy to electrical energy via solar cells Advantages: Clean energy Uses an inexhaustible renewable energy source Modular (from mw to TW) Very low safety risks Reliable; low maintenance cost Also suited for developing countries 2

3 The PV market Annual PV production [GW] The PV market is booming (> 30 %/a since 1999) Market share of silicon in 2007: Approx 97% (!) Si wafers ~93%, Si thin-films ~4% Year

4 Evolution of the global PV market 1 GW p / a Mono-Si Multi-Si Thin film (a-si) Ribbon Si 1990 Graph: G. Willeke,

5 Why is silicon so dominant in PV? Si dominates the semiconductor industry (microelectronics, displays) Large variety of machines for industrial production exists already. Almost ideal bandgap for PV (efficiency i limit it = 29% at 1 sun). Excellent PV efficiency already realised in industry (> 22%). Good electronic and mechanical properties. Abundant and non-toxic material. PV modules are long-term stable (> 20 years). Si can be made as a wafer or as a ribbon or as a thin-film on rigid or flexible substrates.... 5

6 Future growth of the silicon PV industry Aim: To lower the $/W cost of PV modules, via higher PV efficiencies i i and/or lower manufacturing costs ($/m 2 ). Two strategies: Larger & thinner & cheaper Si wafers Si thin-film technologies 6

7 2. Silicon wafer solar cells PV module with Si wafers: Module assembly 0.92 US$/W Si wafer (mc-si) 1.37 US$/W Solar cell process 0.72 US$/W Cost = 3-6 US$/W Cost distribution of a PV module with mc-si wafers (13%, 3 US$/W) Fantastic technology, but: Need further cost reductions ($/W) Major R&D efforts required 7

8 Structure of a simple Si wafer solar cell Front contact Light beam Antireflection coating Emitter (n-type) - + Base (p-type) Rear contact

9 Photogeneration of electron-hole pairs in a semiconductor E - - Conduction band hf red hf green Bandgap energy + + Vl Valence band

10 Main loss mechanisms in single-bandgap solar cells (Example: Silicon) 1600 [W m -2 µm -1 ] Powe er density #1: Poor usage of energy of short-wavelength photons Available ab energy e for PV conversion o using a c-si solar cell #2: Non-absorbed photons Wavelength [nm]

11 Single-bandgap p-n junction solar cells under one-sun illumination Theoretical limit for PV efficiency of such cells: ~31% at 25ºC (W. Shockley and H.-J. Queisser, 1960/61, calculated using thermodynamic principles) Best such solar cell realised as yet: 26.1% (GaAs, 2009, Radboud University Nijmegen) Best such silicon solar cell realised as yet: 25.0% (1999, UNSW)

12 Bulk recombination a major problem in standard industrial Si wafer cells (~250 µm thick) - + PV efficiency: 15-16% 16% - + +

13 Step 1 towards improved PV efficiency: Use of a thinner wafer (~150 µm) PV efficiency: 15% (Ouch!)

14 Step 2 towards improved PV efficiency: Thin wafer with optimised rear surface (and narrower front fingers) Narrower front finger PV efficiency: ~20% (nice!) Reduced Optical Passivating contact area mirror film

15 Highly efficient laboratory solar cells using thin monocrystalline Si wafers Wafer thickness 42 µm PV efficiency 20.2% Cell area 1.0 cm 2 (Fraunhofer ISE)

16 Laser-grooved buried-contact cells Invented in 1980s at UNSW Many excellent properties for cost-effective ti high-eff h PV Lab cells up to 21%, factory cells up to 18% (BP Solar) Narrow copper contact SiO 2 coating lightly diffused emitter p-type locally diffused contact BSF metal back contact 16

17 RISE cell (Rear Interdigitated Single Evaporation) Source: ISFH, 2006 Invented at ISFH in 2005 Only 1 diffusion process, only 1 metallisation process All-back-contact cell Holes are laser drilled through the Si wafer to connect the front junction with the corresponding electrode 17

18 3. Summary Silicon Wafer PV Si wafer PV is booming (> 25% p.a.) Its market share is approx 90% 3 technologies: mono, multi, ribbon Modules are long-term stable Good price/performance ratio Wafers are getting thinner Trend towards high-eff structures Cost of modules are falling ($/W p ) Si wafer PV is the benchmark PV technology and a moving target for any competing technology 18