New generations of photovoltaics

The photovoltaic effect was first observed by Becquerel in 1839: the visible radiation produces a voltage on certain materials (namely, semiconductors) while striking them. Electrical current can therefore be extracted through an external load, giving power.

The first solar cell was produced by Chapin, Fuller and Pearson (Bell Labs, New Jersey [1]) in 1954, with a p-n junction of doped crystalline silicon (c-Si).

In 1960, 15% cell efficiency had already been achieved. Silicon solar cells dominated the scientific research up to the 1970s, when both thin-film devices (amorphous silicon, a-Si; CdTe and Copper-Indium-Selenide, CIS) and high efficiency III-V compound cells (GaAs, GaInAs, GaInP) became object of investigation (Fig. 1).

Fig. 1 Timeline of PV devices: record efficiency solar cells as a function of time, from 1954 to 2010 [2].

Nowadays, solar cells have shown great improvements in the energy conversion efficiency. With values around 26-27% at 1000 Wm-2, (also referred to as "1X"), both c-Si and GaAs solar cells are very close to the theoretical efficiency limit of 31% for single junction PV cells. This theoretical limit is based on the principles of thermodynamics [3].

More junctions, more efficiency

c-Si and GaAs solar cells are examples of single-junction PV cells. Several single junction devices have been studied that may absorb a wide range of wavelengh bands, but typically in each single junction device the radiation at smaller wavelenghs is converted to electricity less efficiently, mainly due to thermal losses.

It is possible to (either mechanically stacking or epitaxially growing) add several junctions to form a multi-junction device. Increasing the number of junctions in the PV cell helps to increase the thermodynamic efficiency limit (Fig. 2).

Fig. 2 Theoretical limit of efficiency (thermodynamic limit) as a function of the number of junctions, at 1X global irradiance and under direct irradiance at maximum possible concentration. Record efficiencies for single-, double- and triple-junction devices are shown.

Ideally, a solar cell with infinite number of junctions has a thermodynamic limit of efficiency above 80% under concentrated light.

Practically, 3-junction cells above 40% efficiency under concentrated light have already been experimentally demonstrated. GaInP/GaInAs/Ge cells with energy efficiencies around 37% (at 500X) are also already on the market and 40% efficiency products are on the roadmap.

Where do we are? Where should we go?

More than half a century of research has helped to reach almost the thermodynamic limit for single-junction PV cells. On the other hand, multi-junction solar cells are still far from their theoretically possible best performance.

From a cost-of-energy point of view, Figure 3 shows the efficiency-cost chart of PV devices. Blue area is the so-called "Generation-1" PV (mainly c-Si and poly-Si), with price-per-watt between 1 and 2 $/W and above 150 $/m2. The chart shows also a selection of commercialized modules of various technologies (full blue dots: c-Si; empty blue: poly-Si; full green: a-Si compound; empty green: CIS, CIGS, CdTe, CSG; red: concentrators): data are based on the efficiency stated by the company and by spot prices in Germany in January 2010.

Fig. 3 Efficiency-cost chart of a selection of PV products (spot prices, January 2010) and definition of 1st, 2nd and 3rd generation PV [4].

Generation-2 PV are thin-film modules, with typical low-efficiency and low-price trend, leading to 0.50-1 $/W, but hardly exceeding 15-20% efficiency.

Cost per watt may be further decreased with Generation-3 PV, where the high cost of high-efficiency (above 30%) solar cells may be greatly reduced by concentrating light and sun-tracking systems. Research and development of Gen-3 PV is still on going and lots of research centres and universities are investigating over this topic worldwide.

A comlementary way under study to reduce cost-per-watt in new generation PV is the use of novel, potentially inexpensive materials as organic materials, dye-sensitized solar cells [5] or luminescent solar concentrators (LSC, [6-7]).

List of publications

All publications are available in electronic format by request to the author

photovoltaics:

M. Pravettoni et al., “Characterization of a Pulsed Solar Simulator for Concentrator Photovoltaic Cell Calibration”, Meas. Sci. Technol. 21 (2010), 115901 (8pp)

M. Pravettoni et al., “Electrical Characterisation of Concentrating Photovoltaic Cells: a Comparison between Outdoor Testing under Direct Solar Radiation and Indoor Measurements on a High Intensity Solar Simulator”, Proceedings of the Thirty-fifth IEEE Photovoltaic Specialists Conference, Honolulu (2010), to be published

M. Pravettoni et al., “Characterization of High Efficiency CPV Cells”, Progr. Photovolt: Res. App., to be published

M. Pravettoni et al., “From an Existing Large Area Solar Simulator to a High Intensity Pulsed Solar Simulator: Characterization, Standard Classification and First Results at ESTI”, Proceedings of the Thirty-fifth IEEE Photovoltaic Specialists Conference, Honolulu (2010), to be published

M. Pravettoni et al., “Electrical Characterization of Multi-junction Thin-film Photovoltaic Modules: Spectral Mismatch Correction to Standard Conditions and Comparison with Outdoor Measurements”, in Amorphous and Polycrystalline Thin-Film Silicon Science and Technology 2010, edited by Q. Wang, B. Yan, S. Higashi, C.C. Tsai, A. Flewitt (Mater. Res. Soc. Symp. Proc. Volume 1245, Warrendale, PA, 2010), A13-06

M. Nikolaeva-Dimitrova et al., “Energy Yield on Seasonal Variations of a-Si, Hybrid and Crystalline Si PV Modules”, Prog. Photovolt: Res. Appl. 18(5), 311-320 (2010)

M. Pravettoni, "Un Sistema a Concentrazione Complementare. Il Concentratore Solare a Luminescenza", PV Technology, 3, 42-45 (2009), in Italian

M. Pravettoni et al., “External Quantum Efficiency Measurements of Luminescent Solar Concentrators: a Study of the Impact of Backside Reflector Size and Shape”, Proceedings of the Twenty-fourth European Photovoltaic Solar Energy Conference, Hamburg (2009), 332-335

M. Pravettoni et al., “Outdoor Characterisation of High Efficiency Luminescent Solar Concentrators”, Mater. Res. Soc. Symp. Proc: Three-Dimensional Architectures for Energy Generation and Storage (2009), 1168-P02-06

M. Pravettoni et al., “Outdoor Characterization of Luminescent Solar Concentrators and Their Possible Architectural Integration on a Historically Relevant Site in Milan (Italy)”, Proceedings of the Thirty-fourth IEEE Photovoltaic Specialists Conference,
Philadelphia (2009), 187-192

M. Pravettoni et al., “Indoor-Outdoor Characterisation of Luminescent
Solar Concentrators at the European Solar Test Installation”, Proceedings of the Twenty-third European Photovoltaic Solar Energy Conference, Valencia (2008), 744-747

M. Pravettoni et al., “Classical Behaviour of Output Light Emitted by the Edge of a Luminescent Solar Concentrator”, Proceedings of the Thirty-third IEEE Photovoltaic Specialists Conference, San Diego (2008), DOI: 10.1109/ PVSC.2008.4922806

L. H. Slooff et al., “A Luminescent Solar Concentrator with 7.1% Power Conversion Efficiency”, Phys. Stat. Sol. (RRL) 2(6) (2008), 257-259

else:

M. Pravettoni, "Risorse, fabbisogno e loro evoluzione", MSc in Engineering and Plasma Physics, University of Padua (2005), unpublished

M. Romé et al., "Three Dimensional PIC Simulation of Electron Plasmas", Proceedings of the 12th International Conrgress on Plasma Physics, Nice (2004)

M. Pravettoni, "Equilibrio e Stabilità di un Fascio di Elettroni di Bassa Energia in una Macchina di Malmberg-Penning", MSc Thesis, University of Milan (2003), in italian

Fig. 4 Timeline of PV: two centuries of research. In red, developments in PV standards. * D. Trivich, Ohio J. Sci. 53(5), 300-314 (1953). ** "Terrestrial Photovoltaic Measurement Procedures"; NASA Tech. Report TM 73702, June 1977.

References:

[1] D. M. Chapin, C. S. Fuller and G. L. Pearson, J. Appl. Phys. 25(5), 676 (1954)

[2] M. Green, K. Emery, Y. Hishikawa and W. Warta, Prog. Photovolt: Res. Appl. 18(5), 346-352 (2010)

[3] A. Martí and G. Araújo, Sol. En. Mat. Solar Cells 43, 203-222 (1996)

[4] M. Green, Third Generation Photovoltaics, Springer-Verlag, 2003.

[5] B. O'Regan and M. Graetzel, Nature 353, 737-740 (1991)

[6] W. H. Weber and J. Lambe, Appl. Opt. 15(10), 2299-2300 (1976)

[7] M. Pravettoni, PV Technology, 3, 42-45 (2009), in Italian