Polymer solar cell
||It has been suggested that this article be merged with Organic solar cell. (Discuss) Proposed since August 2010.|
A polymer solar cell is a type of flexible solar cell made with polymers, large molecules with repeating structural units, that produce electricity from sunlight by the photovoltaic effect. Polymer solar cells include organic solar cells (also called "plastic solar cells"). They are one type of thin film solar cell, others include the currently more stable amorphous silicon solar cell. Polymer solar cell technology is relatively new and is currently being very actively researched by universities, national laboratories, and companies around the world.
Currently, most commercial solar cells are made from a refined, highly purified silicon crystal, similar to the material used in the manufacture of integrated circuits and computer chips (wafer silicon). The high cost of these silicon solar cells and their complex production process has generated interest in developing alternative photovoltaic technologies.
Compared to silicon-based devices, polymer solar cells are lightweight (which is important for small autonomous sensors), potentially disposable and inexpensive to fabricate (sometimes using printed electronics), flexible, and customizable on the molecular level, and they have lower potential for negative environmental impact. An example device is shown in Fig. 1. The disadvantages of polymer solar cells are also serious: they offer about 1/3 of the efficiency of hard materials, and they are relatively unstable toward photochemical degradation. For these reasons, despite continuing advances in semiconducting polymers, the vast majority of solar cells rely on inorganic materials.
Polymer solar cells currently suffer from a lack of enough efficiency for large scale applications and stability problems but their promise of extremely cheap production and eventually high efficiency values has led them to be one of the most popular fields in solar cell research. It is worth mentioning that state-of-the-art devices produced in academic labs – with the record currently held by Yang Yang’s group in UCLA – have reached certified efficiencies above 8% while devices produced which have remained unpublished – probably to maintain secrecy for industrial applications – are known to have already gone above 10%.
Polymer solar cells usually consist of an electron- or hole-blocking layer on top of an indium tin oxide (ITO) conductive glass followed by electron donor and an electron acceptor (in the case of bulk heterojunction solar cells), a hole or electron blocking layer, and metal electrode on top. The nature and order of the blocking layers – as well as the nature of the metal electrode – depends on whether the cell follows a regular or an inverted device architecture.
In bulk heterojunction polymer solar cells, light generates excitons with subsequent separation of charges in the interface between an electron donor and acceptor blend within the device’s active layer. These charges then transport to the device’s electrodes where these charges flow outside the cell, perform work and then re-enter the device on the opposite side. The cell's efficiency is limited by several factors especially non-geminate recombination. Hole mobility leads to faster conduction across the active layer.
Organic photovoltaics are made of electron donor and electron acceptor materials rather than semiconductor p-n junctions. The molecules forming the electron donor region of organic PV cells, where exciton electron-hole pairs are generated, are generally conjugated polymers possessing delocalized π electrons that result from carbon p orbital hybridization. These π electrons can be excited by light in or near the visible part of the spectrum from the molecule's highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), denoted by a π -π* transition. The energy bandgap between these orbitals determines which wavelength of light can be absorbed.
Unlike in an inorganic crystalline PV cell material, with its band structure and delocalized electrons, excitons in organic photovoltaics are strongly bound with an energy between 0.1 and 1.4 eV. This strong binding occurs because electronic wavefunctions in organic molecules are more localized, and electrostatic attraction can thus keep the electron and hole together as an exciton. The electron and hole can be dissociated by providing an interface across which the chemical potential of electrons decreases. The material that absorbs the photon is the donor, and the material acquiring the electron is called the acceptor. In Fig. 2, the polymer chain is the donor and the fullerene is the acceptor. After dissociation, the electron and hole may still be joined as a "geminate pair", and an electric field is then required to separate them.
After exciton dissociation, the electron and hole must be collected at contacts. If charge carrier mobility is insufficient, the carriers will not reach the contacts, and will instead recombine at trap sites or remain in the device as undesirable space charges that oppose the drift of new carriers. The latter problem can occur if electron and hole mobilities are not matched. In that case, space-charge limited photocurrent (SCLP) hampers device performance.
Organic photovoltaics can be fabricated with an active polymer and a fullerene-based electron acceptor. Illumination of this system by visible light leads to electron transfer from the polymer to a fullerene molecule. As a result, the formation of a photoinduced quasiparticle, or polaron (P+), occurs on the polymer chain and the fullerene becomes an radical anion (C60-). Polarons are highly mobile and can diffuse away.
As described in Mayer's review, the simplest organic PV device features a planar heterojunction (figure 1). A film of active polymer (donor) and a film of electron acceptor is sandwiched between contacts. Excitons created in the donor region may diffuse to the junction and separate, with the hole remaining behind and the electron passing into the acceptor. Because charge carriers have diffusion lengths of just 3–10 nm in typical organic semiconductors, planar cells must be thin, but the thin cells absorb light less well. Bulk heterojunctions (BHJs) address this shortcoming. In a BHJ, a blend of electron donor and acceptor materials is cast as a mixture, which then phase-separates. Regions of each material in the device are separated by only several nanometers, a distance suited for carrier diffusion. BHJs require sensitive control over materials morphology on the nanoscale. A number of variables, are important including choice of materials, solvents, and the donor-acceptor weight ratio.
The next logical step beyond BHJs are ordered nanomaterials for solar cells, or ordered heterojunctions (OHJs). OHJs minimize the variability associated with BHJs. OHJs are generally hybrids of ordered inorganic materials and organic active regions. For example, a photovoltaic polymer can be deposited into pores in a ceramic such as TiO2. Since holes still must diffuse the length of the pore through the polymer to a contact, OHJs do suffer thickness limitations. Mitigating the hole mobility bottleneck is key to further enhancing device performance of OHJ's.
Active layer deposition and annealing process
Since its active layer largely determines device efficiency, the morphology of this component has received much attention.
If one material is more soluble in the solvent than the other then it will deposit first on top of the substrate, causing a gradient in concentration along the film. This has been demonstrated to be the case for poly-3-hexyl thiophene (P3HT), phenyl-C61-butyric acid methyl ester (PCBM) devices where the PCBM tends to accumulate towards the bottom of the device upon spin coating from ODCB solutions. This effect is seen because the more soluble component tends to migrate towards the “solvent rich” phase during the spin coating procedure, generating an accumulation of the more soluble component towards the bottom part of the film which is where the solvent dries last. It is also worth noting that the thickness of the generated film also affects the phases segregation because the dynamics of crystallization and precipitation are different for more concentrated solutions or faster evaporation rates (either one is needed to build thicker devices). Enrichment of crystalline P3HT closer to the hole collecting electrode can only be achieved for relatively thin (100 nm) P3HT/PCBM layers.
The gradients in the initial morphology are then mainly generated by the solvent evaporation rate and the differences in solubility between the donor and acceptor inside the blend. This dependence on solubility has been clearly demonstrated using fullerene derivatives and P3HT by Troshin’s group. When using solvents which evaporate at a slower rate (as chlorobenzene (CB) or dichlorobenzene (DCB)) you can get larger degrees of vertical separation or aggregation while with solvents that evaporate quicker you can get a much less effective vertical separation. In a similar manner larger solubility gradients should lead to more effective vertical separation while smaller gradients should lead to more homogeneous films. These two effects have been studied extensively and have been verified on P3HT:PCBM solar cells.
The evaporation speed of the solvent as well as posterior solvent vapor or thermal annealing procedures have also been the subject of additional studies. Some blends like P3HT:PCBM seem to greatly benefit from thermal annealing procedures while other blends like PTB7:PCBM seem to show no benefit from the application of such a procedure. In the case of P3HT the benefit seems to come from an increase of crystallinity of the P3HT phase which is generated through an expulsion of PCBM molecules from within these domains. This has been demonstrated through studies of PCBM miscibility in P3HT as well as the changes in compositions of domains as a function of annealing times.
The above hypothesis based on miscibility does not fully explain the efficiency of the devices as solely pure amorphous phases of either donor or acceptor materials never exist within bulk heterojunction devices. A 2010 paper suggests that current models which assume pure phases and discrete interfaces might run into problems as pure amorphous regions never exist within the devices. Since current models assume phase separation at interfaces without any consideration for phase purity the models might need to be changed to account for these important aspects inherent to real devices.
The thermal annealing procedure is also different depending on precisely when it is applied. Since the vertical migration of species is determined in part by the surface tension between the active layer and either air or another layer, it is different to anneal cells before or after the deposition of additional layers (most often the metal cathode). In the case of P3HT:PCBM solar cells there is a clear difference in vertical migration when cells are annealed before or after the deposition of the metal cathode with better results attained for the post-annealing treatment.
Donor or acceptor accumulation next to the adjacent layers might be beneficial as these accumulations can lead to hole or electron blocking effects which might benefit device performance. As Schwartz et al. showed in 2009, the difference in vertical distribution on P3HT:PCBM solar cells can cause problems with electron mobility which ends up with the yielding of very poor device efficiencies. In their work, they effectively demonstrate that simple changes to device architecture – spin coating a thin layer of PCBM on top of the P3HT – can greatly enhance cell reproducibility by providing reproducible vertical separation between the device components. Since higher contact between the PCBM and the cathode is required for better efficiencies, this largely increases device reproducibility.
According to neutron scattering analysis, P3HT:PCBM blends have been described as “rivers" (P3HT" interrupted by “streams” (PCBM regions).
Conditions for spin coating and evaporation affect device efficiency. Solvent and additives influence the donor-acceptor morphology. Additives slow down evaporation, leading to more crystalline polymers and thus improved hole conductivities and efficiencies. Typical additives include 1,8-octanedithiol, ortho-dichlorobenzene, 1,8-diiodooctane (DIO), and nitrobenzene. The effect of the DIO was attributed to the selective solubilization of the PCBM component. Additives can also lead to big increases in efficiency for polymers. For HXS-1/PCBM solar cells, the effect was also correlated with charge generation, transport and shelf-stability. Other polymers such as PTTBO also benefit significantly from DIO, achieving PCE values of more than 5% from around 3.7% without the additive.
Small differences in polymer structure can also lead to significant changes in crystal packing which inevitably affect device morphology. In the case of PCPDTBT Vs PSBTBT, there is a significant difference caused by the difference in bridging atom between the two polymers (C vs. Si) which implies that better morphologies are achievable with the PCPDTBT:PCBM solar cells containing additives as opposed to the Si system which achieves good morphologies without any help from additional substances.
Cells by self-assembly
Work has examined using supramolecular chemistry, using donor and acceptor molecules that assemble upon spin casting and heating. Most supramolecular assemblies employ small molecules. Donor and acceptor domains in a tubular structure appear ideal for organic solar cells.
Diblock polymers containing fullerene yield stable organic solar cells upon thermal annealing. Solar cells with pre-designed morphologies have been built when appropriate supramolecular interactions are introduced.
Device efficiency based on co-polymer approaches have yet to cross the 2% barrier, whereas bulk-heterojunction devices exhibit efficiencies >7% in single junction configurations.
Fullerene-grafted rod-coil block copolymers have been used to study the domain organization. The active layer morphology of polymer organic solar cells affects device performance. Device efficiency correlates with the distribution of donor and acceptors. Electron and hole mobility within the active layer usually correlates with efficiencies. Vertical segregation within the active layer is important to the device performance. Solvent evaporation rate, donor and acceptor miscibilities and additives also influence device characteristics. Overall, the influence of donor and acceptor interactions on morphology is poorly understood. Supramolecular approaches to organic solar cells, while in its infancy, provides understanding about the macromolecular forces that drive domain separation.
Infrared polymer cells
Infrared cells preferentially absorb light in the infrared range rather than the visible wavelength range. As of 2012, such cells can be made nearly 70% transparent to the latter. The cells allegedly can be made in high volume at low cost using solution processing. The cells employ silver nanowire/titanium dioxide composite films as the top transparent electrode, replacing conventional opaque metal electrodes. With this combination, 4% power-conversion efficiency has been achieved.
At the moment, an open question is to what degree polymer solar cells can commercially compete with silicon solar cells and the other thin-film cells. The silicon solar cell industry has the important industrial advantage of being able to leverage the infrastructure developed for the computer industry. Besides, the present efficiency of polymer solar cells lies near 10%, much below the value for silicon cells. Polymer solar cells also suffer from environmental degradation owing the lack of effective protective coatings.
Further improvements in performance are needed to promote charge carrier diffusion; transport must be enhanced through control of order and morphology; and interface engineering must be applied to the problem of charge transfer across interfaces. Novel molecular chemistries and materials offer hope for revolutionary, rather than evolutionary, breakthroughs in future device efficiencies.
Polymer solar cells are not widely produced commercially. Starting in 2008, Konarka Technologies started production of polymer-fullerene solar cells. The initial cells from the factory were 3–5% efficient, and only last a couple years. Further improvements are planned in both efficiency and durability.
Other third-generation solar cells
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