SUPHANNEEPONGKITWITOON
    
Photovoltaic Device
Presented by Suphannee Pongkitwitoon
Content
 
  1. Photovoltaic                                                                                                                  2

1.1  Organic Photovoltaic Materials                                                                   3

1.2  Types of Photovoltaic                                                                                 4

1.3  Principles of Operation                                                                               4

 

  1. Mixture of Conjugated Polymer with Nano-Particles of C60 used as Photovoltaic          6

2.1  Limitations by Problems of Electron Extraction                                            8

2.2  Improvement by Using Nano-Rods instead of Spherical Nano-Crystals       9

2.3  Synthesis of Central Core Structures                                                           12

 

  1. Previous and Present Works of Photovoltaic                                                                 13

3.1  Present Developments                                                                                13

3.2  Breakthroughs in Cell Performance                                                 14

 

  1. Future Trends                                                                                                               16

4.1  Light Harvesting                                                                                          17

4.2  Improving Charge Transport                                                                       17

4.3  Control of Morphology                                                                               18

4.4  Performance and Production Issues                                                 19

4.5  Understanding Function and Experimental Techniques                                 19

 

  1. References                                                                                                                   21
  

1. Photovoltaic

 

From Nelson, 2002, an unprecedented growth of interest for future application in solar cells made from organic electronic materials has seen within the last two years.  Although results for single layer organic materials have been disappointing, high photocurrent quantum efficiencies can be achieved in composite system including both electro donating and electron accepting components.  This is might be partly because of the rapid growth of the photovoltaic device [Power, 2001], which has stimulated research into longer term, more innovative photovoltaic technologies, and partly due to the development of organic electronic materials for display applications.

 

Organic materials are attractive for photovoltaic primarily through the prospect of high throughput manufacture using reel-to-reel or spray deposition.  Additional attractive features are the possibilities for ultra thin flexible devices which maybe integrated into appliances or building materials and tuning of color through chemical structure [Wallace, et. al., 2000].  The revolution in optoelectronic molecular materials has introduced range of potential new photovoltaic materials as well as an improved understanding of the capacities of such materials and confidence in their application [Heeger, 2001].

 

Generally, photovoltaic devices were fabricated as shown in Figure 1.1 (Figure 2 from Baoquan, et. al., 2003).  A blend of poly (3,4-ethylenedioxythiophene) with poly (styrene sulfonate) or PEDOT / PSS was used as the anode and was spin coated with a thickness of 70 nm onto an oxygenplasma-treated indium-tin oxide (ITO) substrate, followed by baking at 150 oC for 2 h under nitrogen.  The tetra-pod solution (300 mL) and 125 mL of 10 mg/mL OC1C10-PPV in 1,2-dichlorobenzene were thoroughly mixed and spin coated onto the surface of the PEDOT/PSS.  Typical film thicknesses were 160-180 nm.  The films were baked at 150 oC for 30 min under nitrogen to remove residual solvent, and aluminum cathodes were then deposited by thermal evaporation.   Devices were encapsulated using epoxy resin and were measured under ambient conditions.

 

1.1  Organic Photovoltaic Materials

 

Organic electronic materials are conjugated solid where both optical absorption and charge transport are dominated by partly delocalized p and p* orbitals.  Candidates for photovoltaic applications include crystalline or polycrystalline films of ‘small molecules’ (molecules of molecular weight of a few 100), amorphous films of small molecules prepared by vacuum deposition or solution processing, films of conjugated polymers or oligomers processed from solution, and combinations of any of these either with other organic solids or with inorganic materials.

 

Organic photovoltaic materials differ from inorganic semiconductors in the following aspects.

 

1.      Dissociation requires an input of energy of ~ 100 meV compared to a few meV for a crystalline semi-conductor.  This is because photogenerated excitations, or ‘excitons’, are strongly bound and do not spontaneously dissociate into charge pairs.  Therefore, the carrier generation does not necessarily result from the absorption of light.

2.      Charge transport proceeds by hopping between localized states, rather than transport within a band, and mobilities are low.

3.      The spectral range of optical absorption is relatively narrow compared to the solar spectrum.

4.      Absorption coefficients are high (~ 10 cm) so that high optical densities can be achieved, at peak wavelength, with films less than 100 nm thick.

5.      Many materials are susceptible to degradation in the presence of oxygen or water.

6.      As one-dimensional semiconductors, their electronic and optical properties can be highly anisotropic.  This is potentially useful for device design.

 

The first two features are due to the fact that the intermolecular van der Waals forces in organic solids are weak compared to bonds in inorganic crystals and much weaker than the intra-molecular bonds.  As a consequence all electronic states are localized on single molecules and do not form bands.  Low mobility is ‘made worse’ by the high degree of disorder present in many organic solids.  The optical excitations accessible to visible photons are usually p to p* transitions.  Most conjugated solids absorb in the blue or green; absorption in the red or infrared is harder to achieve.  However, the absorption bandwidth depends on the degree of conjugation and wider spectral sensitivity can be achieved in highly conjugated dye molecules.

 

These properties impose some constraints on organic photovoltaic devices.

 

1.      A strong driving force such as an electric field should be present to break up the photogenerated excitons.

2.      Low charge carrier mobilities limit the useful thickness of devices.

3.      Limited light absorption across the solar spectrum limits the photocurrent.

4.      Very thin devices mean interference effects can be important.

5.      Photocurrent is sensitive to temperature through hopping transport.

 

 

 

 

1.2  Types of Photovoltaic

 

There are two general types of photovoltaic, which are based on the polymer blends [Halls, et. al., 1995] and the other on mixtures of conjugated polymers with nano-particles or nano-crystals of inorganic semiconductors [Yu, et. al, 1995 and Shaheen, et. al., 2001].

 

Photovoltaic devices based on solution processable conjugated polymers are attractive for the production of low cost solar cells [Nelson, 2002].  To obtain high efficiencies, it is necessary to have a hole-accepting component within devices [Baoquan, et. al., 2003].  This can be achieved using polymer blends or mixtures of conjugated polymer with nano-particles such as C60 or nano-crystal of inorganic semiconductors such as CdSe also act as good electron acceptors from conjugated polymers [Yu, et. al, 1995 and Shaheen, et. al., 2001].  However, the efficiency of photovoltaic devices made with spherical nano-crystal is limited by the problem of electron extraction through the nano-crystal network [Greenham, et. al., 1996 and Ginger, et. al., 1999].

 

 

1.3  Principles of Operations

 

1.3.1. Homo-junctions

 

The simplest device structure is a layer of organic material sandwiched between two different conducting contacts, typically indium tin oxide, or ITO, and a low work function metal such as Al, Ca or Mg in Figure 1.3.1.1 (Figure 1) [Nelson, 2002].

 

The difference in work function provides an electric field which drives separated charge carriers towards the respective contacts, in rough analogy to a p–i–n junction in amorphous silicon.  This electric field is seldom sufficient to break up the photo-generated exciton.  Instead, the exciton diffuses within the organic layer until it reaches a contact, where it may be broken up to supply separate charges, or recombine.  Since exciton diffusion lengths are short, typically 1–10 nm, exciton diffusion limits charge carrier generation in such a device.  Photo-carrier generation is therefore a function not only of bulk optical absorption, but also of available mechanisms for exciton dissociation.  Other loss factors are non-radiative recombination at the interfaces and non-geminate recombination at impurities or trapped charges. 

 

Single layer solar cells of this type deliver quantum efficiencies (QE) of less than 1% and power conversion efficiencies of less than 0.1%.  The QE is the ratio of electrons delivered to the external circuit per incident photon of a given wavelength, and is the figure of merit in organic photovoltaics.  High QE is a necessary, though not sufficient, condition for high photovoltaic efficiency.  In organic devices the value is still far from the values of 80–90% typical in inorganic solar cells.

 

 

 

 

1.3.2. Hetero-junctions and Dispersed Hetero-junctions

 

Most of the developments that have improved performance of organic photovoltaic devices are based on donor–acceptor hetero-junctions.  At the interface between two different materials, electrostatic forces result from the differences in electron affinity and ionization potential.  If both electron affinity and ionization potential are greater in one material or at the electron acceptor than the other or at the electron donor then the interfacial electric field drives charge separation in Figure 1.3.2.1 (Figure 2).

These local electric fields are strong and may break up photo-generated excitons provided that the differences in potential energy are larger than the exciton binding energy.  In a planar hetero-junction, or ‘bi-layer’ device, the organic donor–acceptor interface separates excitons much more efficiently than the organic-metal interfaces in a single layer device and with very high purity materials, efficient photovoltaic devices may be made.

 

 

2. Mixtures of Conjugated Polymers with Nano-Particles C60 used as Photovoltaic

 

Yu, et. al., 1995, reported the improvement of the carrier collection efficiency or hc and the energy conversion efficiency or he of polymer photovoltaic cells by blending the semi-conducting polymer with C60 or its functionalized derivatives.  They used composite films of poly (2-methoxy-5-(2’-ethy-hexyloxy)-1,4-phenylene vinylene) or MEH-PPV and fullerenes exhibit hc of about 29 percent of electrons per photon and he 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 MEH-PPV.  The efficient charge separation results from photo-induced electron transfer from the MEH-PPV as a donor to C60 as an acceptor.  The high collection efficiency results from a bi-continuous network of internal donor-acceptor hetero-junctions.  The efficient photovoltaic cells made with MEH-PPV : C60 composites thin film of Yu, et. al., 1995, was shown in following Figure 2.1 (Figure 1).

 

 

Sean, et. al., 2000, showed that the power conversion efficiency of organic photovoltaic devices based on a conjugated polymer/methanofullerene blend is dramatically affected by molecular morphology.  By structuring the blend to be a more intimate mixture that contains less phase segregation of methanofullerenes, and simultaneously increasing the degree of interactions between conjugated polymer chains.  They have fabricated a device with a power conversion efficiency of 2.5 percent under AM 1.5 illumination, which is a nearly threefold enhancement over previously reported the power conversion efficiency values for such a device and also it approaches what is needed for the practical use of these device for harvesting energy from sunlight.  Their photovoltaic devices high performance were fabricated in an individual manner except for spin coating the active layer of MDMO-PPV : PCBM, 1:4 by weight.  The device layers of spin-coated blend film consist of an indium tin oxide / PEDOT or poly (3,4-ethylenedioxythiophene)-poly(styrenesulfonate) / MDMO – PPV or poly(2-methyl 1,5-dimethyloctyloxy-p-phenylene vinylene) : PCBM or 6,6-phenyl C61 butylric acid methyl ester / LIF / Al layered structure which is described as Figure 2.2 (Figure 2).

 

 

 

2.1 Limitations by problems of electron extraction

 

Greenham, et. al., 1996, studied the processes of charge separation and transport in composite materials formed by mixing cadmium selenide or cadmium sulfide nano-crystals with the conjugated polymer poly (2-methoxy, 5-(2’-ethyl)-hexyloxy-p-phenylenevinylene) or MEH-PPV as Figure 2.1.1 (Figure 1).

 

When the surface of the nano-crystals is treated so as to remove the surface ligand, they found that the polymer photoluminescence is quenched, consistent with rapid charge separation at the polymer/nano-crystal interface.  Thin-film photovoltaic devices using the composite materials show quantum efficiencies that are significantly improved over those for pure polymer devices, consistent with improved charge separation.  At high concentrations of nano-crystals, where both the nano-crystal and polymer components provide continuous pathways to the electrodes, they found quantum efficiencies of up to 12%.  They described a simple model to explain the recombination in these devices, and show how the absorption, charge separation, and transport properties of the composites can be controlled by changing the size, material, and surface ligands of the nano-crystals.

 

 

Ginger, et. al., 1998, studied photo-induced electron transfer from derivatives of poly~p-phenylenevinylene or PPV and nano-crystals of cadmium selenide via photoluminescence or PL quenching and photo-induced absorption or PIA spectroscopy.  They presented a systematic investigation of charge transfer in these polymer/quantum-dot composites and the observed peaks showed that their frequency and temperature dependence are consistent with the formation of long-lived positive polarons on MEH-PPV following electron transfer to the nano-crystals.  They explained the insensitivity of the electron transfer process to nano-crystal size in the context of the relevant polymer and nano-crystal energy levels and discussed the influence of the polymer side chains on the charge-transfer process.

 

 

2.2 Improvement by Using Nano-Rods instead of Spherical Nano-Crystals

 

Using nano-rods structures instead of spherical nano-crystals helps improve the electron transferring from nano-particles of inorganic electron donor to conjugated polymer electron acceptor.  This is mainly because of the smaller the number of inter-particles locates inside, the easier the electron will leave as seen in works of Wendy, et. al., 1999.

 

Wendy, et. al., 1999 used CdSe nano-crystal rods / Poly (3-hexylthiophene) composite photovoltaic devices.  The solar cells comprised of dye-sensitized, nano-crystalline TiO2 have achieved a considerable overall power conversion efficiency of 10%, which is comparable to current polycrystalline silicon devices as shown in following Figure 2.2.1 (Figure 3).

 

By improving microstructures, the limitations of a tendency of electron lied in plane, which is limits the electron extraction, will be decreased [Baoquan, et. al., 2003] such as the work of Wendy, et. al., 2003.

 

In Wendy, et. al., 2003 work, they controlled the morphology of nanocrystal-polymer composites for solar cell.  The combining nano-rods with a good hole-transporting polymer, such as poly (3-hexylthiophene) or P3HT in Figure 2.1.2 (Figure 1a from Wendy, et. al., 2003), enables the good transport characteristics of inorganic semi-conductors to be exploited, without sacrificing the solution processibility of organic materials.

 

 

 

A red shift in the absorption bands of nano-crystals has been observed in superlattices relative to independent particles as a consequence of the coherent interaction of nano-crystals in close proximity in the absence of surface ligands.  Moreover, it is possible for nano-crystals to be sintered together at relatively low temperatures compared to the bulk melting point t produce larger crystals.  Consequently, heat treatment is seen to aid both in the removal of interfacial pyridine and in bringing nano-rods closer together as shown in Figure 2.1.3 a (Figure 8a).  This aggregation of neighboring nano-rods is likely to improve electron transport between nano-rods, which occurs through thermally activated hopping.  Furthermore, the removal of interfacial pyridine can also have the effect of enhancing charge transfer between CdSe and P3HT by bringing these two materials into closer electronic contact as seen in Figure 2.1.3 b (Figure 8b).  These two effects most likely resulted in the overall photocurrent enhancement by a factor of about 2.5 across all absorbed wavelengths.

 

 

 

2.3 Synthesis of Central Core Structures

 

Manna, et. al., 2000, reported that the formation of extremely high aspect ratio CdSe nano-rods (30:1), as well as arrow-, teardrop-, tetra-pod-, and branched tetra-pod-shaped nano-crystals of CdSe, has been achieved by growth of the nano-particles in a mixture of hexylphosphonic acid and trioctylphosphine oxide.  The most influential factors in shape control are the ratio of surfactants, injection volume, and time-dependent monomer concentration.

 

In their paper, they demonstrated systematic variation of the shape of colloidal semiconductor nano-crystals, using thermal decomposition of organometallic precursors in a hot ( ~ 300 oC ) mixture of trioctylphosphine oxide and hexylphosphonic acid.  As in the growth of spherical CdSe nano-crystals in hot trioctylphosphine oxide, the surfactants dynamically adsorb to the growing crystallites, allowing atoms to add and subtract for high crystallinity.  This enables the growing crystallites to anneal, resulting in good crystallinity, while suppressing particle aggregation.  It is important to note that the growth mode of the nano-crystals depends strongly upon monomer concentration.  At low monomer concentration, Ostwald ripening occurs, and small nano-crystals dissolve at the expense of larger ones.  Such slow growth conditions favor the formation of a spherical particle shape or least surface area.  On the other hand, at high monomer concentration, relative differences between the growth rates of different faces can lead to anisotropic shapes.  The relative growth rates of the different faces can be controlled by suitable variation of the ratio of trioctylphosphine oxide and hexylphosphonic acid.  Using a combination of these parameters, they demonstrated the controlled formation of CdSe nano-crystals with rod, arrow, teardrop, and tetra-pod shapes.  This wide variation of unusual shapes provides important information about the growth of the nano-crystals.

 

The formation of arrows unequivocally points to uni-directional growth.  The hexagonal CdSe nano-crystals do not have inversion symmetry, meaning the top and bottom faces of the crystals are intrinsically different.  As can been seen in Figure 2.3.1 (Figure 8), Cd atoms on the {001} face have one dangling bond, while Cd atoms on the {001’} face have three dangling bonds.  It appears that, in the presence of HPA, the relative growth rate of the {001’} face is much greater than that of the others.

 

 

 

The tetra-pods are formed when a CdSe nano-crystal nucleates in the zinc blend structure instead of the Wurtzite structure.  Then Wurtzite arms grow out of the four {111} equivalent faces of the tetrahedral zinc blend core as seen in following Figure 2.3.2 (Figure 9).

 

 

 

3. Previous and Present Works of Photovoltaic

 

The last 2 years have seen developments in the synthesis of new photovoltaic materials; the combination of materials into new device architectures; studies of the effect of processing conditions and other factors on morphology and performance; and manipulation of materials at a molecular level, exploiting molecular self assembly and modification

 

3.1 Present Developments

 

A revolutionary development in organic photovoltaics and photodetectors came in the mid 1990s with the introduction of a dispersed hetero-junction, where an electron accepting and an electron donating material are blended together.  If the length scale of the blend is similar to the exciton diffusion length, then wherever an exciton is photo-generated in either material, it is likely to diffuse to an interface and break up.  If continuous paths exist in each material from the interface to the respective electrodes, then the separated charge carriers may travel to the contacts and deliver current to the external circuit in Figure 3.1 (Figure 3 from Nelson, 2002).

 

 

This effect was reported independently by several groups [Yu, et. al., 1995, Halls, et. al., 1995 and Yoshino, et. al., 1997] for a blend of two conjugated polymers.  The blend improved QE to around 6–8% from less than 1% for either polymer alone.  Around the same time, Yu and coworkers reported a QE of 29% for a blend of the hole transporter, PPV, with a derivative of C60 [Yu, et. al., 1995], where the C60 acts as the electron transporting component.  This was followed by observations of enhanced QE in hetero-junctions made from conjugated polymers with inorganic nano-crystals [Salafsky, et. al., 1999 and Greenham, et. al., 1996] and organic dye crystals [Petritsch, et. al., 2000].  The demonstration of improved QE with dispersed hetero-junctions represents a departure from the device physics of conventional solar cells and has led to new device and materials designs.  Dye sensitized solar cells, however, function on similar principles [Hagfeldt, 2000].

 

 

3.2 Breakthroughs in Cell Performance

 

Power conversion efficiencies of over 2% have now been achieved in the following four classes of device.

 

3.2.1 A Single Layer and Hetero-Junction Solar Cells using Doped Pentacene Single Crystals [Schon, et. al., 2000 and 2001]

 

Researchers at Bell Labs have made single layer and hetero-junction solar cells using doped pentacene single crystals [Schon, et. al., 2000 and 2001].  These organic crystals have high anisotropic minority carrier (electron) mobilities which allows them to be used in Schottky barrier and p–n structure, where collection is aided by the built-in electric field of a depletion region as in inorganic solar cells. In the single layer structure a vacuum deposited pentacene crystal forms a Schottky barrier with an aluminium back contact and an Ohmic contact with an ITO window layer.  In the bi-layer case the pentacene forms a hetero-junction with a ZnO:Al window and an Ohmic contact with platinum.  The location of the field bearing region closer to the window should assist collection in this structure.  Doping with iodine or bromine is essential for successful function.  The dopants are responsible for exciton dissociation, increasing the QE, as well as improving absorption in the red and reducing series resistance.

 

3.2.2 Planar Hetero-Junction Devices made by Vacuum Deposition of Thin Films of Small Molecules

 

Planar heterojunction devices made by vacuum deposition of thin films of small molecules have been studied by several groups, for application to LEDs as well as solar cells. An impressive result has been achieved at Princeton with a four layer hetero-junction, containing wide band gap hole transporting and electron transporting ‘window’ layers [Peumans, et. al., 2000 and 2001].  These buffer layers function to block excitons from lossy metal contacts and to enhance optical field strength in photoactive layers via interference effects Figure 3.2.2.1 (Figure 4, Granstrom, 1998).

 

 

3.2.3 Blends of Poly (phenylene vinylene) derivatives and Methanofullerenes

 

Blends of polyphenylenevinylene derivatives and methanofullerenes are a well studied combination and are under intense development at Linz [Brabec, et. al., 2001].  Photon absorption in the polymer is followed by electron transfer to the fullerene on a sub ps time scale.  Current collection depends on charge percolation through the fullerene network and is therefore critically dependent upon the blend ratio and the degree of phase separation. MDMO-PPV and PCBM appear to be a promising materials combination.  A recent breakthrough was achieved by using chlorobenzene as a solvent in place of toluene, leading to QE of over 50% and power conversion efficiency of 2.5% [Shaheen, et. al., 2001].  The much improved performance is attributed to improved phase separation with chlorobenzene.

 

3.2.4 Solid State Dye Sensitized Solar Cells, DSSC

 

Solid state dye sensitized solar cells or DSSC are the most promising amongst organic–inorganic composite devices to date.  In the DSSC three active materials are used: an organic dye as light absorber, a nano-crystalline metal oxide film as electron transporter and liquid or organic hole transporting material (HTM) [Hagfeldt, et. al. 2000].  The original design used a redox active liquid electrolyte for hole transport, but a non-volatile HTM is desirable for commercialization.  The ideal material should regenerate the photo-oxidized dye quickly and transport holes with high mobility.  Candidates for solid state HTMs include doped arylamine based small molecules (OMeTAD) [Bach, et. al., 1998] and polythiophenes [Spiekermann, et. al., 2001].  A power conversion efficiency of 2.5% using OMeTAD was recently reported [Kruger, et. al., 2001] by the EPFL group, greatly improving on earlier studies of that material.  The improvement is attributed to the suppression of electron-hole recombination at the metal oxide surface using adsorbed pyridine.  A number of other device types have achieved QE comparable with the devices, though lower power conversion efficiencies.  These include donor–acceptor polymer blend devices with well controlled morphology [Granstorm, et. al., 1998], three layer donor–sensitizer acceptor structures [Takahashi, et. al., 2000], inorganic–organic hetero-junctions [Arango, et. al., 2000]; and liquid crystal-crystalline dye devices [Schmidt-Mende, et. al., 2001].

 

 

4. Future Trends

 

The predicted trends for photovoltaic are mostly to achieve the current challenges based on blends of nano-particles or nano-crystals of inorganic semiconductor with conjugated polymers to improve the efficiency of electron extraction through tetra-pods and hoe transportation in polymers.

 

The whilst organic solar cells produce quite respectable, which should be reasonably high, open-circuit voltages, the short circuit photocurrent and fill factor are much lower than those available from inorganic devices.  The lower photocurrent is due to poorer light absorption as well as photocurrent generation and transport; the fill factor is due to poor transport and recombination. Most of current research is therefore focused on the following goals:

 

4.1 Improving light harvesting.

4.2 Improving photocurrent generation.

4.3 Improving charge transport.

4.4 Addressing manufacturing issues and improving stability.

4.5 Understanding device function and limits to performance.

 

 

4.1 Light Harvesting

 

A preferred strategy is to replace conducting polymers in devices with others which absorb further into the red.  In the polymer–fullerene cell, possible lower band gap replacements for the PPV include polythiophene derivatives [Brabec, et. al., 2001] polypyrrole / thiazadole copolymers [Dhanabalan, et. al., 2001] and thiophene /naphthene copolymers [Shaheen, et. al., 2001].  Synthesis of new materials with red absorbing moities is underway.  An alternative is to replace the electron transporting polymer in a blend with conjugated crystalline dyes, such as anthracene or perylene, with wider absorption bands [Dittmer, et. al., 2000].

 

Dye sensitization is a different strategy where a mono-layer of a third material, usually an organic dye, is introduced between donor and acceptor to function as light absorber. Since light absorption and charge transport are carried out by different materials, the light absorber does not need to be a good bulk transporter of charge.  Efforts to improve light harvesting in DSSCs include development of alternative dyes and combinations of dyes.  A similar concept, of an all-organic donor–absorber–acceptor structure was proposed by Yoshino, et. al. 1997.

 

A different approach is to improve the utilization of absorbed photons with light trapping structures. This improves the capture of photons where absorption is weak and allows thinner photovoltaic films to be used, which has advantages for transport. The possibilities for light trap- ping for organic solar cells are discussed by Inganas, et. al., 2001.  Some of these have been investigated using embossed polymer layers for light trapping [Roman, et. al., 2000] and exploiting interference effects inside a cavity made from photoactive and transmitting organic layers [Peumans, et. a., 2000 and Rostalski, et. al., 2000]. Embossed polymer light trapping structures are already used in thin film silicon solar cells. Interference effects need to be included in the calculation of light absorption in organic thin films, and theoretical tools have been developed by for planar structures [Petterson, et. al., 1999 and Wan, et. al., 2000].

 

 

4.2 Improving Charge Transport

 

Charge transport is limited by the low intrinsic mobilities of organic solids, which are typically 10 E-7 cm2 V-1 s-1 up to 10 E-3 cm2 V-1 s-1 for hole transporting materials, smaller for electron transporters) and by the charge trapping effects of impurities and defects.  In several recent studies, higher mobility polymers such as fluorine-triarylamine and thiophene copolymers have been used to replace PPV in blend devices [Arias, et. al., 2001 and Halls, et. al., 2000].  Materials with ordered phases offer high, though anisotropic, mobilities and in this respect liquid crystals [Schmidt-Mende, et. al., 2001 and Struijk, et. al., 2000] and polymers with ordered phases, such as polythiophenes [Videlot, et. al., 2000], are interesting.  Since organic electron mobilities are generally very poor, an inorganic electron transporting component may be preferred.  Thin film [Arango, et. al., 2000 and Fan, et. al., 2001] and dispersed nano-crystalline [Salafsky, et. al., 1999] TiO2 has been used in several approaches.  Efficient charge transfer to TiO2 from various polymers has been reported (PPV [Salafsky, et. al., 1999 and Savenije, et. al., 1998], phenyl-amino-PPV [Arango, et. al., 2000], polythiophene [Spiekermann, et. al., 2001]). TiO2 is the most widely studied material in such structures, but the sensitization of other oxides such as SnO2 with polymers has been demonstrated [Anderson, et. al., 2003].  Elongated crystalline components are attractive as electron transporters if crystal size and orientation can be controlled. Blend devices using crystalline dyes [Dittmer, et. al., 2000], CdSe nano-rods [Huynh, et. al., 1999] and carbon nano-tubes [Lee et. al., 2001 and Ago, et. al., 1999] have been studied.  Meanwhile, organic electron transporters with improved mobility and stability are being developed [Katz, et. al., 2000 and Murata, et. al., 2001].

 

 

4.3 Control of Morphology

 

In a dispersed hetero-junction device, both photocurrent generation and charge transport are functions of morphology.  Photocurrent generation requires uniform blending on the scale of the exciton diffusion length while transport requires continuous paths from interface to contacts.  In polymer / nano-crystal or polymer / fullerene blends, the concentration of the particulate component should be sufficient for charge percolation [Greenham, et. al., 1996 and Brabec, et. al., 1999].  One attractive hypothetical configuration is a set of intersecting electron and hole transporting channels, directed perpendicular to the contacts.  This is one motivation for the study of rod-like nano-crystals.  Another is a compositionally graded blend with an excess of donor type material on one side and acceptor type on the other. This concept has been demonstrated by Yoshino using vacuum deposited layers [Yoshino, et. al., 1997], by Granstrom for a laminated assembly [Granstrom, 1998] (see Figure 4).  The concept was taken further by Takahashi and co-workers who reported QE of 49% for a three layer structure where electron and hole transporting layers are separated by a hetero-dimer light absorbing layer [Takahashi, et. al., 2000].  The absorbing layer is polarized upon light absorption to drive the charges towards the appropriate transport layers.  This is essentially an all-organic version of the dye sensitized solar cell.

 

In practice many materials tend to segregate when blended, and much attention has focused on ways of controlling the morphology of blends.  Routes include:

 

4.3.1 Control of blend morphology through processing conditions.

 

Choice of solvent, atmosphere and substrate temperature strongly influence the morphology of polymer blends [Arias, et. al., 2001 and Halls, et. al., 2000].  Choice of solvent appears to influence segregation of fullerenes in PPV [Shaheen, et. al., 2001].

 

4.3.2 Self organization.

 

Self assembly by discotic liquid crystals [Schmidt-Mende, et. al., 2001], and by ionically or electrostatically interacting monolayers [Schroeder, et. al., 2001 and Baur, et. al., 2001] have been used to construct structured hetero-junctions.  Self assembled mono-layers can also be used to modify substrate surfaces to control the segregation of blend components.

 

4.3.3 Synthesis of donor (D) – acceptor (A) copolymers

 

Synthesis of donor (D) – acceptor (A) copolymers is shown such as polymer with pendant fullerene groups [Ramos, et. al., 2001] and block copolymers.  Positioning D and A groups on the same polymer backbone can ensure effective photo-induced D→A electron transfer under all conditions and avoids the problems of phase segregation.  D–A copolymers may be designed to absorb longer wave length photons than single polymers, so improving light harvesting, but charge extraction may be more difficult.

 

4.3.4 Use of porous organic or inorganic films as templates [de Boer, et. al., 2001].

 

4.3.5 Co-sublimation of small molecules

 

Co-sublimation of small molecules to form graded D–A hetero-structures [Pfeiffer, et. al., 2000 and Murgia, 2001].

 

 

4.4 Performance and Production Issues

 

Several studies of performance and stability of organic solar cells have been carried out on polymer–fullerene devices.  These devices exhibit improved performance with increasing temperature, an advantage over inorganic devices which is attributed to temperature dependent mobility, [Katz, et. al., 2001] but poor stability [Padinger, et. al., 2001].  Stability is a common problem with conjugated polymers and will need to be addressed by encapsulating cells and by using more stable materials such as organic dyes, liquid crystals, and metal oxide nano-crystals and siloles are promising in this respect.  The studies of the effects of increasing cell area [Padinger, et. al., 2000] and deposition by screen printing [Shaheen, et. al., 2001] show that polymer–fullerene cells can be scaled up without large losses in performance.  In the case of doped pentacene solar cells, replacing the single crystal with a thin film of pentacene degrades efficiencies slightly to 2% indicating that this technology could be used with flexible substrates [Schon, et. al., 2001].

 

One frequent problem with new device designs is a degradation of QE with increasing light intensity so that cells perform well only under low illumination.  Such behavior is a signature of recombination mechanisms which may be due to impurities, and must be eliminated for the cells to be useful in solar conditions.

 

The search for efficient ways to generate interpenetrating networks, discussed above, is also a manufacturing issue.  For large scale production, complicated procedures such as lamination are not feasible, while self assembly from a single solution is attractive.

 

 

4.5 Understanding Function and Experimental Techniques

 

Understanding of the device physics of organic solar cells is still at a primitive stage, compared to inorganic solar cells.  Important differences are that light absorption is not equivalent to photo-carrier generation, and that fundamentally different recombination mechanisms may dominate in the light and the dark.  The subject is complicated by uncertainties in material parameters, the effects of hetero-junctions and interfaces on materials, and the effects of light, bias and ageing.  Further understanding and the development of appropriate models requires fundamental studies on model systems.

 

Here we mention some of the characterization techniques relevant to hetero-junction solar cells.  Techniques for characterizing the optical, electrochemical and transport properties of single polymers are amply described else-where in the literature.

 

Photo-induced absorption is useful for studying the kinetics of electron transfer, i.e. of exciton dissociation and polaron recombination.  Ultrafast spectroscopy has been used to study charge separation in polymer–fullerene structures [Barbec, et. al., 2001], polymer nano-crystal blends [Ginger, et. al., 1999] and in donor–acceptor copolymers.  Nanosecond–microsecond spectroscopy has proved useful for monitoring the rate of charge recombination in dye sensitized devices [Nogueira, et. al., 2001] and may be applied elsewhere.  Electron spin resonance can be used to detect the unpaired spin of polaron states and is useful for monitoring the formation and lifetime of charge separated states [Dyakonov, et. al., 2001].

 

Photocurrent generation in planar structures is reasonably well understood and can be interpreted in terms of the filter effect [Harrison, et. al., 1997] although interference must be accounted for [Petterson, et. al., 1999].  In blends, photocurrent generation is a function of morphology, as has been discussed above.  Novel characterization techniques have been developed to study blend morphology, including spatially resolved fluorescence [Halls, et. al., 2000] and confocal Raman spectroscopy [Stevenson, et. al., 2001] as well as AFM and SEM imaging techniques.

 

The influences on photovoltage are much less well understood. In single layer devices voltage appears to be limited by the difference in work functions of the electrode materials.  In blends, however, the photovoltage can be larger than that difference and appears instead to be related to the difference in electron affinity of the donor and ionization potential of the acceptor [Brabec, et. al., 2001]. This indicates that photocurrent collection does not require a macroscopic electric field, a situation which is largely agreed to apply to dye sensitized solar cells [Cahen, et. al., 2000].  Electroabsorption is useful for measuring electric fields inside organic devices [Lane, et. al., 2000] and may assist in answering such questions.  Ac admittance measurements relating device performance to charge accumulation and Fermi level profile [Dakonov, et. al., 2001] and optical measurements of recombination kinetics may be equally useful.  These questions remain of active interest.

 

In conclusions, the progress with organic photovoltaic materials and devices in the last 2 years has been impressive.  Power conversion efficiencies over 2% have been achieved in four different device structures, varying from high quality, vacuum deposited multilayer molecular films to dispersed hetero-junctions in spin cast soluble polymers.  All are based on the concept of a donor–acceptor system where photo-generated excitons are split by forces at the donor– acceptor interface.  Higher efficiency requires improvement experimental in absorption of red light, in charge transport and in material stability.

 

Recent research focuses on the synthesis and testing of new photovoltaic materials in established device structures, and the development of new structures where morphology is controlled through self assembly and processing conditions.  Experience with commercial scale devices is still limited, as is the theoretical understanding of device function.  Based on current trends, efficiencies of 5% appear to be within reach, although stability remains an obstacle.

 

 

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