There are two approaches for tapping the sun's energy: photovoltaic and solar-thermal. The photovoltaic approach absorbs photons streaming from the sun and reacts by giving off a flow of electrons (electricity).Traditionally, inorganic substances – silicon based have been used for solar cells. In today's inorganic solar cells, roughly 20-30 of every 100 bumped-off electrons make it out in the real world. This means an efficiency of 20-30%. Inorganic solar cells represent ~ 95% of today’s photovoltaic market. [1]
With the advent of nanotechnology, better and cheaper engineered materials hold prospect. One such option is Organic materials. Examples of such materials are "buckeyball" mixtures on plastic substrates. Advantages of solar cells made from organic materials, are [4]:
The potential of semi-conducting organic materials to transport electric current and to absorb light in the UV-visible part of the solar spectrum is due to the delocalized π electrons of the sp2 hybridized carbon atoms. Organic solar cells suffer from the following problems:
The Organic Photovoltaic process can be summarized as:
The electric current delivered by an organic solar cell corresponds to the number of created charges that are collected at the electrode. This number depends on:
Hence, the overall efficiency (ηj) is defined as,
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Single layer or MIM (Metal Insulator Metal) Structures – These were the first generation of solar cells based on single organic layers sandwiched between two metal electrodes of different work functions. [2] The rectifying behaviour was achieved by the asymmetry in the electron and hole injection into the molecular π and π* orbitals and the formation of the schottky barrier between the p-type hole conducting organic layer and the metal with lower work function. The power conversion efficiency was generally poor (< 1%). The following diagram explains the rectifying behavior of the MIM assembly. Metal ‘A’ is the high work function metal and metal ‘B’ is the low work function metal. Figure 1(a) is referred as short circuit condition, where there is no current flowing in the dark. However under illumination the electrons move to the electrode B and holes to A. Figure 1(b) is the Flat band condition with zero current and VOC corresponding to the difference in the metal work function values. Figure 1(c) is the reverse bias mode, when the device acts as a photo detector under illumination. Figure 1(d) is the Forward bias mode, when the contacts can inject charges into the semiconductor and on radiative recombination the device works as an LED.
Bilayer Heterojunction – This architecture is based on the concept of Photo Induced Charge Transfer (PICT), Figure 2. [3] As per the figure, the photo-induced electron transfer occurs when it is energetically favorable for the electron in the excited state of the polymer for example PPV, to be transferred to the more electronegative C60. This results in the effective quenching of the excitonic photoluminescence of the polymer. Because the electron is transferred from a p-type hole conducting polymer onto an n-type electron conducting C60 molecule, the notation of Donor (D) and Acceptor (A) with respect to electron transfer has been introduced. The donor and acceptor material are stacked together with a planar interface. The charge separation occurs, mediated by a large potential drop between the donor and acceptor. The bilayer assembly is sandwiched between the electrodes matching the donor HOMO (Highest Occupied Molecular Orbital) and the acceptor LUMO (Lowest Unoccupied Molecular Orbital) for efficient extraction of electrons and holes. Power Conversion efficiencies of ~3-4% have been demonstrated using Copper phthalocyanine and C60
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Bulk Heterojunction – In this approach, the donor and acceptor components of the solar cell are intimately mixed in a bulk volume so that each donor-acceptor interface is within a distance less than the exciton diffusion length of each absorbing site. This architecture has a larger interfacial area where charge separation occurs and higher efficiency can be achieved.
Diffuse Bilayer Heterojunction – This approach is hybrid between bilayer heterojunction and bulk heterojunction. [5] It buys the advantages of both approaches viz. large donor-acceptor interface and an uninterrupted pathway for the opposite charge carriers to their corresponding electrode.
The fabrication of solar cell requires processing of thin-films. Generally two approaches are used depending on the material:
Evaporation-The thin films are processed in < 10-5 mbar pressure. The mean free path of the evaporated molecule is greater than the distance between the evaporation source and the sample holder. Ultra high vaccum can eliminate contaminants like oxygen and water. Evaporation is used to make thin films of Aluminum, Indium Tin Oxide (ITO) and other short polymers and oligomers in solar cells.
Wet Processing–This approach mandates the solvation of organic material in an appropriate solvent like water or any other polar or non-polar organic solvent. This can be achieved by spin coating, doctor blading, screen printing or inkjet printing. Also less soluble molecules like C60 can be made soluble when modified by attaching soluble groups (e.g. PCBM)[5]. A modern organic solar cell with wide gap p-doped hole (MeO-TPD) and n-doped electron (C60) transport layers provide good contacts with Aluminium and ITO electrodes.
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The overall aim is to increase the power conversion efficiency of the solar cells. This can be achieved by:
Device/Material Engineering - Modification of the photoactive layer & Improvement of contacts by introduction of transport/blocking layers.
Modulation of Absorption Range: One of the main reasons for low efficiency of solar cells is the spectral mismatch of the organic material absorbers to the solar spectrum. Most of the organic semiconductors investigated today absorb in the visible range, while the sun has its maximum photon density at around 700nm. The only way to increase the solar photon harvesting is to introduce low band gap materials. The introduction of inorganic/hybrid nanorods / nanocrystals [6] is another way to increase the visible absorption. One such example is to use Copper Indium disulphide (CIS) nanoparticles. The nanomorphology of these new materials requires further research.
© 2007 Gaurav Thareja. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Another dawn for Solar Power," Business Week, Sep 6, 2004.
[2] Hoppe et al., "Organic solar cells: An overview," J. Mater. Res. 19, No. 7, Jul 2004.
[3] Yoshino et al., "Novel Photovoltaic Devices Based on Donor-Acceptor Molecular and Conducting Polymer Systems," IEEE Trans. On Elec. Dev. 44, No. 8, Aug 1997.
[4] Brabec et al., "Organic photovoltaics: technology and market," Solar Energy Materials and Solar Cells 83, 273 (2004).
[5] Peumans et al., "Small molecular weight organic thin-film photo-detectors and solar cells," J. Appl. Phys. 93, 3693 (2003).
[6] Huynh et al., "Hybrid Nanorod-Polymer Solar Cells," Science 295, 2425 (2002).