nanoHUB PhotoVoltaics Reference Zone
Need information on the science of photovoltaics and solar cell technology? Find it here!
The nanoHUB PhotoVoltaics Reference Zone is the right destination for finding general information about photovoltaic solar cell science and technology, as well as for viewing news articles and getting access to some great PV resource links. Sponsored by the nanoHUB and the Center of Integrated Nanomechanical Systems (COINS) - a joint collaboration for nanoscience education and outreach among UC Berkeley, UC Merced, Stanford University, and CalTech - this page is a continuously-updated resource for researchers and educators involved in the rapidly growing world of solar energy research. Check back monthly for new additions!
The goal of COINS is to develop and integrate cutting-edge nanotechnologies into a versatile platform with various ultra-sensitive, ultra-selective, self-powering, mobile, wirelessly communicating detection applications. The success of this mission requires new advances in nano-electro-mechanical devices, from fundamental building blocks to enabling technologies to full device integration. During the past 3.5 years, we have set our Center on a path towards achieving this goal by developing four major research programs, in the areas of Energy, Sensing, Mobility, and Electronics/Wireless. Each of these programs encompasses research projects spanning the full spectrum of basic through to the applied level, and each program has a set of criteria that has been established for use as a means of determining which projects to support, in order to assure optimal project alignment.
The projects sponsored by COINS are efforts to meet the following goals for our society by means of nano-scale science and technological development: (1) It will lead to such major enhancements in environmental monitoring technology that completely new possibilities will emerge due to better spatial and temporal resolution, and (2) It will fundamentally change the way we are able to respond to proliferation events or serious natural catastrophic events by providing much more accurate information on conditions, allowing for substantially better countermeasures and security.
- New Record - 42% Efficient Solar Cell Using Dichroic Optics - Science 3, August 2007
- New Record - 40% Efficient Solar Cell - Appl. Phys. Lett. 90, 4 May 2007
- New Record - 6% Efficient Organic PV - Appl. Phys. Lett. 85, 6 December 2004
- PVCDROM at University of Delaware
The University of Delaware "Photovoltaics CDROM", written by Christiana Honsberg and Stuart Bowden, is an incredibly thorough and practical introduction to the science and applications of basic photovoltaic solar cells. This project is funded by the National Science Foundation.
- University of Oregon Solar Radiation Monitoring Laboratory
The UO SRML is a regional solar radiation data center, whose goal is to provide sound solar resource data for planning, design, deployment, and operation of solar electric facilities in the Pacific Northwest. Creating the long-term solar radiation data base necessary to achieve this goal requires persistence, maintenance of high standards, and an effort to educate people on the importance of a solar radiation database and how to use the database.
- National Renewable Energy Laboratory Photovoltaics
The National Renewable Energy Laboratory's (NREL) Photovoltaic (PV) research is focused on decreasing the nation's reliance on fossil-fuel generated electricity by lowering the cost of delivered electricity and improving the efficiency of PV modules and systems. NREL's PV research contributes to these goals through fundamental research, advanced materials and devices, and technology development.
- Standard Solar Spectra via NREL
The National Renewable Energy Laboratory's Electricity, Resources & Building Systems Integration Center provides the solar spectra in HTML, text and/or MS Excel spreadsheet format.
- SMARTS Solar Irradiance Calculator at NREL
The Simple Model of the Atmospheric Radiative Transfer of Sunshine, or SMARTS, predicts clear-sky spectral irradiances. Earth's atmosphere is a continuously changing filter that modifies the sunlight that travels through it. SMARTS computes how changes in the atmosphere affect the distribution of solar power or photon energy for each wavelength of light.
- Theoretical Semiconductor Bandgaps - Yong Xu and Martin A.A. Schoonen; American Mineralogist, Volume 85, pages 543-556, 2000
The absolute energy positions of conduction and valence band edges were compiled for about 50 each semiconducting metal oxide and metal sulfide minerals. The relationships between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a catalyst or photocatalyst in aqueous redox reactions are reviewed. The compilation of band edge energies is based on experimental flatband potential data and complementary empirical calculations from electronegativities of constituent elements. Whereas most metal oxide semiconductors have valence band edges 1 to 3 eV below the H2O oxidation potential (relative to absolute vacuum scale), energies for conduction band edges are close to, or lower than, the H2O reduction potential. These oxide minerals are strong photo-oxidation catalysts in aqueous solutions, but are limited in their reducing power. Non-transition metal sulfides generally have higher conduction and valence band edge energies than metal oxides; therefore, valence band holes in non-transition metal sulfides are less oxidizing, but conduction band electrons are exceedingly reducing. Most transition-metal sulfides, however, are characterized by small band gaps (<1 eV) and band edges situated within or close to the H2O stability potentials. Hence, both the oxidizing power of the valence band holes and the reducing power of the conduction band electrons are lower than those of non-transition metal sulfides.
- Staebler-Wronski Effect - Strahm, 28 Oct. 2005
This presentation, given by Benjamin Strahm at the Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Federale de Lausanne, discusses the Staebler-Wronski effect in silicon-based PV cells - an effect in which light-induced defects can degrade performance efficiency.
- Unexplained Aspects of the Staebler-Wronski Effect - Bolko von Roedern; Mat. Res. Soc. Symp. Proc. Vol. 808, 2004
This contribution reviews the compatibility of Staebler-Wronski models with experimental data and observations. The review will show that neither the "bond-breaking models" (originally proposed by Dersch and Stutzmann) nor the "defect conversion" models (originally proposed by Adler) can explain all observations on films and/or solar cells. It has been well accepted for some time that experimental stress and recovery phenomena, both on films and devices, always identify both "slow" and "fast" degradation and recovery mechanisms. It is argued that the quintessential understanding of the Staebler-Wronski mechanisms will come from identifying a fundamental physical process that provides a quantitative understanding of the "coupling" between the slow and fast mechanisms.
- "Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene" - Sariciftci, Smilowitz, Heeger, and Wuld; Science, New Series, 27 Nov. 1992
Evidence for photoinduced electron transfer from the excited state of a conducting polymer onto buchminsterfullerene, C60, is reported. After photo-excitation of the conjugated polymer with light of energy greater than the pi-pi* gap, an electron transfer to the C60 molecule is initiated. Photoinduced optical absorption studies demonstrate a different excitation spectrum for the composite as compared to the separate components, consistent with photo-excited charge transfer. A photoinduced electron spin resonance signal exhibits signatures of both the conducting polymer cation and the C60 anion. Because the photoluminescence in the conducting polymer is quenched by interaction with C60, the data imply that charge transfer from the excited state occurs on a picosecond time scale. The charge-separated state in composite films is metastable at low temperatures.
- "Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions" - G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger; Science, New Series, 15 Dec. 1995
The carrier collection efficiency and energy conversion efficiency of polymer photovoltaic cells were improved by blending of the semiconducting polymer with C60 or its functionalized derivatives. Composite films of poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and fullerenes exhibit a carrier collection efficiency of about 29 percent of electrons per photon and an energy conversion efficiency of about 2.9 percent, efficiencies that are better by more than two orders of magnitude than those that have been achieved with devices made with pure MEG-PPV. The efficient charge separation results from photoinduced electron transfer from the MEH-PPV (as donor) to C60 (as acceptor); the high collection efficiency results from a bicontinuous network of internal donor-acceptor heterojunctions.
- "Photoinduced Absorption in P3HT Review" - X.M. Jian, R. Oesterbacka, O. Korovyanko, C.P. An, B. Horovitz, R.A.J. Jannsen, and Z.V. Vardeny; Adv. Funct. Mater., 9 Sep. 2002
Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly-3-alkyl thiophene with regioregular order, which forms self-assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain-like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long-lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long-lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano-type interference mechanism, using the amplitude mode model.
- "High open-circuit voltage photovoltaic devices from carbon-nanotube-polymer composites" - E. Kymakis, I. Alexandrou, and G.A.J. Amaratunga; Journal of Appl. Phys., 12 Nov. 2002
Organic photovoltaic devices based on the bulk heterojunction concept, containing a blend of single-wall carbon nanotubes (SWNTs) and soluble polythiophene (P3OT) were studied. The open circuit voltage V_oc of the devices was found to be 0.75 V, which is larger than the theoretical limit calculated by the metal-insulator-metal (MIM) model. In order to investigate the origin of this unusually high V_oc , we have prepared P3OT-SWNT based devices with different metal negative electrodes. The V_oc measured is only very weakly dependent on the work function of the metal, suggesting that the MIM model does not apply in this case. From the analysis of the current-voltage characteristics and electron microscopy imaging of the composite structure, it is proposed that the photovoltaic response of these devices is based on the introduction of internal polymer/nanotube junctions within the polymer matrix, which due to a photoinduced electron transfer from the polymer to the nanotube contribute to enhanced charge separation and collection. The data suggest that the negligible influence of the negative electrode work function on V_oc can be explained by the metal negative electrode forming ohmic contacts to the nanotube percolation paths.
- "Influence of Single-Walled Carbon Nanotubes Induced Crystallinity Enhancement and Morphology Change on Polymer Photovoltaic Devices" - G. Geng and T. Zeng; JACS Articles, 6 Dec. 2006
Single-walled carbon nanotubes (SWNTs) were determined to have significant interaction with poly(3-hexylthiophene) (P3HT), which is helpful to form continuous active film with interpenetrating structure and improve the crystallinity of the resultant film for SWNTs/P3HT composite. Photovoltaic devices based on an active film with relatively higher crystallinity display much enhanced performance. The work function of carbon nanotubes modulated by electron transferring from P3HT to SWNTs is proposed to explain the high open-circuit voltage (V_oc) obtained from the photovoltaic devices based on the SWNTs/P3HT system.
Dye Sensitized Solar Cells
- "Dye Sensitized Solar Cell Review" - M. Graetzel; Journal of Photochemistry and Photobiology, 1 July 2003
The dye-sensitized solar cells (DSC) provides a technically and economically credible alternative concept to present day p-n junction photovoltaic devices. In contrast to the conventional systems where the semiconductor assume both the task of light absorption and charge carrier transport the two functions are separated here. Light is absorbed by a sensitizer, which is anchored to the surface of a wide band semiconductor. Charge separation takes place at the interface via photo-induced electron injection from the dye into the conduction band of the solid. Carriers are transported in the conduction band of the semiconductor to the charge collector. The use of sensitizers having a broad absorption band in conjunction with oxide films of nanocrstalline morphology permits to harvest a large fraction of sunlight. Nearly quantitative conversion of incident photon into electric current is achieved over a large spectral range extending from the UV to the near IR region. Overall solar (standard AM 1.5) to current conversion efficiencies (IPCE) over 10% have been reached. There are good prospects to produce these cells at lower cost than conventional devices. Here we present the current state of the field, discuss new concepts of the dye-sensitized nanocrystalline solar cell (DSC) including heterojunction variants and analyze the perspectives for the future development of the technology.
- "Detailed Balance Limit of Efficiency of p-n Junction Solar Cells" - W. Shockley and H.J. Queisser; Journal Of Appl. Phys., March 1961
in order to find an upper theoretical limit for the efficiency of p-n junction solar energy converters, a limiting efficiency, called the detailed balance limit of efficiency, has been calculated for an ideal case in which the only recombination mechanism of hole-electron pairs is radiative as required by the principle of detailed balance. The efficiency is also calculated for the case in which radiative recombination is only a fixed fraction f_e of the total recombination, the rest being nonradiative. Efficiencies at the matched loads have been calculated with band gap and f_e as parameters, the sun and cell being assumed to be blackbodies with temperatures of 6000K and 300K, respectively. The maximum efficiency is found to be 30% for an energy gap of 1.1 eV and f_e=1. Actual junctions do not obey the predicted current-voltage relationship, and reasons for the difference and its relevance to efficiency are discussed.
- "Modelling Organic Solar Cells" - A. Moliton and J.M. Nunzi; Polym. Int., 29 March 2006
In this paper we discuss the optimization of various parameters which govern the behaviour of polymer based and organic photovoltaic cells. General mechanisms leading to the generation of charge carriers and the related loss factors are detailed. Theoretical electrical parameters for bilayer and interpenetrating networks of donors and acceptors (open circuit voltages) are established along with current versus voltage characteristics. An equivalent circuit to a solar cell, considering the effects of shunt resistance across the whole layer, is elaborated. After modelling optical interference and its effects on the photocurrent spectrum, orders of magnitude of the required parameters are established for an efficient solar cell. Deviations from optimal values and their effects on the current-voltage characteristics are discussed. Ageing and degradation effects, and calculations demonstrating the necessary photophysical requirements to achieve long-term stable devices are presented.
- AM0, AM1.5D and AM1.5G Standard Solar Spectra
This is a table of data obtained from the National Renewable Energy Laboratory's Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS) enumerating the terrestrial solar radiation spectrum from wavelengths 280nm to 4000nm.
- Solar Energy Industry Association Roadmap
The U.S. solar power industry has developed "Our Solar Power Future" as a roadmap through 2030 and beyond. In this document, the state of solar power today is briefly discussed. Next, the industry looks ahead over several decades and sets targets for the kind of growth and application of solar power that is thought both possible and vital.
Researchers should cite this work as follows: