Solar Cells: Current status and Future prospects Naveen Khandelwal 2008PH10628

History “ Photovoltaic effect” was first recognized in 1839 by French physicist A. E. Becquerel. First photovoltaic cell was built, by Charles Fritts, semiconductor selenium coated with an extremely thin layer of gold ;1% efficient.1883 Modern photovoltaic cell was developed at Bell Laboratories, 1954, using a diffused silicon p-n junction. Space satellite use only. (crystalline Si, solar cells with efficiency of 6–10%) cost >$250per watt. Cost reduction only due to semiconductor industry growth. $100 per watt in 1971.

Cont. SPC, New Jersey,$20 per watt, Elliot Berman.1973 o “reject” Si from Existing manufacturers,very low cost. 1970s-80s, solar powered Navigation Buoy as Natural market. o Automatic Power (Battery manufacturer) buys solar cell design rights and suppressed to save battery market. o Tideland Signal o Increase in offshore oil platform, o “1973 oil crisis", Major oil giants (ARCO, EXXON, SHELL) came into solar market. o Technology companies like GE,IBM,Motorola,Tyco etc.

Some terms Price per watt: Total capital cost by the amount of peak power can be produced. Grid parity:is the point at which alternative means of generating electricity is at least as cheap as conventional grid power Balance of system(BOS):all components of a photovoltaic system other than the photovoltaic panels like wiring, support rack, switches, inverter and batteries.

Current Capacity In 2010, Installed PV capacity was 40GW,producing 50TWh of electricity every year. Growth rate of PV market is around 40%. EU leads the way with almost 30 GW as of Japan (3.6GW),the USA(2.5GW). India has merely 10MW PV capacity. c-Si and pc-Si wafers are the main materials.80% Module prices are in the range of $3.0–4.5/Wp, and the system prices are in the range of $5–7/Wp, depending on technology and size.

Global Top 10 Solar Cell Manufacturer(2011) CompanyCapacity (MW)Country Suntech2,400China JA Solar2,100China Trina1,900China Yingli1,700China Motech1,500Taiwan Gintech1,500Taiwan Canadian Solar1,300China Neo Solar Power1,300Taiwan Hanwha Solar One1,100China JinkoSolar1,100China

Top 10 thin-film Solar cell manufacturers(2011) Thin films represented 16.8% of total global production MW First Solar MW Suntech solar 94.0 MW Sharp 60.0 MW HELIOSPHERA 60.0 MW Sungen Solar 50.0 MW Trony 43.0 MW Solar Frontier 42.0 MW Mitsubishi 40.0 MW Kaneka 40.0 MW vtech solar

Thermodynamic Efficiency Limits Classic thermodynamic efficiency for a single homo junction cell is ~31%,limited by transmission loss for photons below band gap and thermal relaxation loss for above band gap.1961(Shockley and Queisser model) Quantum efficiency >1 in SW range of a-Si solar cell explained by Auger mechanism. increase the power conversion efficiency of solar cells, 1.tandem cells, (68.5 % theoretical) 2.impurity-band and intermediate-band devices,(as high as 63.2%) 3.hot-electron extraction, (66%) 4.carrier multiplication, (44.7% to 85.9%) the so-called “third generation” PV.

Silicon solar cells C-Si solar cell: boron-doped single-crystal wafers (around 400µm thick) grown by the Czochralski (CZ) process. residual impurities such as oxygen, carbon, and transition-metal ions. Oxygen reacts with the boron to form an electronically active defect. Si grown by the float-zone(FZ) process,has lowest recombination losses. Si cell efficiencies close to 25% have been achieved. improved contact and surface passivation of the cell, along the front and rear surfaces, as well as an improved understanding of the significant role of light-trapping in Si devices Commercial c-Si cells have efficiencies in the range of 15–22%.

P-Si Solar cell : reactive-ion etching, this process allows about a 40% relative increase in absorption. Bulk hydrogenation and nitride passivation of the cell surface have produced good results. Commercial pc-Si cells have efficiencies of 12–15%.

Thin Film Solar Cell Thin a-Si:H Films : ◦ Hydrogen passivation of dangling bonds. ◦ collection of current by having very thin n- and p-layers, with an intrinsic intermediate layer. ◦ Intrinsic layer degrade under illumination, because Si-H bonds are destroyed under visible light. ◦ 8–10% of the worldwide PV production uses a-Si technology. ◦ Deposition technique: Silane (SiH 4 ) based PECVD and glow discharge CVD ◦ around 10% dilution with hydrogen,deposition rate 3A 0 /s. ◦ Efficiency,6% for single-junction a-Si up to 9% to 10% for commercial high-performance micromorph modules

a-Si:H Configuration

commercial thin-film silicon PV modules by developer

CdTe thin film modules Direct band gap, ~1.45ev bandgap quite favorable. In 2009, commercial CdTe module sales surpassed 1,000 MW. production costs of $0.76/Wp. Starting with a TCO-coated glass(SnO x :F) the (CdS) and CdTe layers are sequentially added via a vapor- transport deposition or closed-space sublimation process The CdTe active layer is then treated at 400 °C to 450 °C in a CdCl 2 atmosphere. After which a carbon-based paste containing Cu is added and annealed before the final backside Al electrode is added.

Commercial CdTe PV module Developer

CIGS solar modules

20% (the highest value for thin-film cells) on a CIGS cell was achieved at NREL. Commercial efficiency ~10%. band gap of CIGS can be varied continuously between 1.04 and 1.68 eV. Different techniques for growth of active layer o Co-evaporation of Cu,In,Ga in Se environment o RF sputtering then selenization around( C) o Reactive sputtering o Electrochemical deposition

Commercial (CIGS) PV modules by developer

Organic photovoltaics (OPV) and dye-sensitized solar cell (DSSC) newest entrants to the commercial PV product market. DSSCs based on liquid electrolytes have reached efficiencies as high as 10% under AM 1.5 OPV conversion efficiency ~1%.

Schematic of OPV and DSSC

Terrestrial Solar cell efficiency atAM1.5 at 25 o C ClassificationEfficiencyFill factor Si (Crystalline)25.0+/ Poly-Si20.4+/ Thin film-Si16.7+/ Thin film-GaAs27.6+/ CIGS19.6+/ CdTe12.5+/ a-Si10.1+/ DSSC10.4+/ Organic (Polymer)8.3+/ GaInP/GaAs/Ge32.5+/ a-Si/µcSi11.9+/ Organic (2 tandem)8.3+/

References “Solar photovoltaic electricity: Current status and future prospects” :T.M. Razykov, C.S. Ferekides, D. Morel, E. Stefanakos b, H.S. Ullal,H.M. Upadhyaya, 2010 “Commercial status of thin-film photovoltaic devices and materials” :Johanna Schmidtke, EPIA :Global market outlook for phtovoltaics,2010 “Solar cell efficiency tables” :Martin A. Green, Keith Emery, Yoshihiro Hishikawa and Wilhelm Warta, Oct, 2010