Organic Solar Cells : Problems and Perspectives

For photovoltaic cells to convert solar into electric energy is probably the most interesting research challenge nowadays. A good efficiency of these devices has been obtained by using inorganic semiconductor materials. On the other hand, manufacture processes are very expensive in terms of both materials and techniques. For this reason organic-based photovoltaic (OPV) cells are attracting the general attention because of the possible realization of more economical devices. Organic materials are abundant and easily handling. Unfortunately OPV cells efficiency is significantly lower than that of inorganic-based devices, representing a big point of weakness at the present. This is mainly due to the fact that organic semiconductors have a much higher band gap with respect to inorganic semiconductors. In addition, OPV cells are very susceptible to oxygen and water. In this paper we will describe some of the different approaches to the understanding and improving of organic photovoltaic devices.


Introduction
Semiconducting conjugated polymers are the organic materials used in OPV cells, since they possess the base property required to activate the fundamental mechanisms to transform the radiative energy of light into an electric current.When a donor (D) and an acceptor (A) material are being in contact, the result is the so-called heterojunction and this is the basis for the operation of organic solar cells [1][2][3][4][5][6][7][8][9].As a solar photon is absorbed, an exciton occurs which is a coulombically bound electron-hole pair, and it distributes to the D/A interface.At this stage the excitons are separated into free holes and electrons by the electric field.Two types of architectures are currently used to create a D/A heterojunction, that is, bilayer heterojunction and bulk heterojunction (BHJ) solar cells.The latter system is to date the most investigated way to produce solar devices.This is mainly due to the favorable combination of an easy manipulation in the production process and a higher conversion efficiency because of a markedly larger D/A interface.From a schematic point of view, organic solar cells operate the conversion of the incident solar irradiation to electrical current, through essentially four-steps process.Figure 1 depicts this process.In this view, the donor is termed the holes transporting material and it makes contact with the anode, while the electrons transporting material is the acceptor, which is in contact with the cathode.
The creation of an exciton after the absorption of a photon is the first step.The exciton diffuses inside the material to reach the donor-acceptor interface where it will be separated.All of these four steps are potential targets for researchers to improve performances of the OPV.Absorption efficiency, that is how much light is harvested, mainly depends upon the absorption spectra of the organic materials.However the design of the devices, including their thickness, has not a minor role in capturing as much as incident wavelengths.The exciton diffusion length is the parameter that accounts for the efficiency of the second step.It comes out that longer diffusion length corresponds to a greater probability that the exciton reaches the D/A interface raising the holes-electrons production.Clearly the morphology of the D/A boundary also is of great importance.In fact the acceptor has possibly to be very close to the donor so that the D/A interface is always next to the point where the exciton is created.Indeed the third step is the exciton split-up into free charges and this step depends mainly on the properties of the donor and acceptor but also the overall device architecture plays an important role.The last step, that is the transport of the free charges through the sample and their collection at the electrodes, is the final step and it can be considered as central in a organic photovoltaic device.This crucial function derives from the fact that organic film structures are generally amorphous and disordered and, consequently, charge recombination is strongly favored.In addition, film resistance is large and reduces, thus, the current efficiency.The molecular structures of some of the materials are shown in Figures 2(a

Absorption Efficiency
Conjugated π-systems are extensively used as both donor and acceptor materials (Figures 2(a) and 2(b)) and they are often referred as organic semiconductors.However they are intrinsically different from classic crystalline inorganic semiconductors which, for instance, can absorb a continuous spectrum of light and the three-dimensional rigid lattice provides large carrier mobility and broad conduction and valence bands.Typically conjugated materials exhibit intense and broad absorption, but since the thickness of organic semiconductors has to be tiny, because of short exciton diffusion lengths, the incident light is not captured efficiently.In addition organic semiconductors have well-defined electronic transitions that are typically quite narrows and have a propensity to be very susceptible to the background.Nevertheless when organic semiconductors can reach an ordered solid crystalline structure, their absorption band becomes broadened with evident benefits with respect to the solar cell efficiency.Hence the molecular structures and the eventual propensity to aggregate have a considerable influence on the absorption spectrum.For instance, many papers reported CuPc and SubPc (Figure 3) as materials used in organic solar cells.Phthalocyanines in general have been used as the only donor material within the system or as dye molecule in combination with a different donor [10][11][12][13][14].The absorption spectra of the above mentioned CuPc and SubPc have similar profiles when recorded in very dilute solutions, but they are very different in the solid state [15].Furthermore the absorption spectrum of a thin film of SubPc is comparable to that of SubPc in solution while the CuPc spectrum recorded in solid state is quite different from the spectrum of CuPc in solution.This is mainly due to the fact that films made of CuPc show the presence of the cofacial aggregates of planar phthalocyanine molecules [16][17][18][19] whereas SubPc forms a very smooth and amorphous film [20].
Sometimes although a good combination of donor and acceptor is found, the efficiency of the cell is low unless a dye is introduced into the system to improve its performances.For example, one of the most investigated organic solar cell is based on the bulk heterojunction poly (3hexylthiophene) (P3HT) : [6,6]-phenyl-C(61)butyric acid methyl ester (PCBM) (Figures 2(a) and 2(b)) blend film, where relatively high values of efficiencies (4%-5%, see Table 1) have been reported [21][22][23][24][25].However, the major inconvenience of P3HT is the scarce matching of its absorption spectrum with the solar emission spectrum.The absorption of P3HT is limited to wavelengths below 650 nm, and as a consequence it is only able to collect little more than 20% of the solar photons [19,26].Introducing a dye in this system can improve performances.For example, 9,10diphenylanthracene (DPA) has been used as a conjugated dye with different concentrations into a solar cell P3HT:PCBM system [27] and the photocurrent was improved by a factor 3,7.In addition all of the solar cell performance parameters and power conversion efficiency improved as a result of improvement in the light harvesting and charge carrier transfer taking place between P3HT and PCBM through the conjugated DPA molecules.The efficiency of the photogeneration can be also improved rising the exciton generation rate.We have already mentioned that the low charge mobility (high resistance) of organic semiconductors obliges the device thickness to stay thin.Hence the wavelength of the incident light is longer than film thickness causing optical interference.A standing wave occurs at each interface within the device and the complex refraction indices and the thickness of the different layers govern this wave pattern.The latter interference pattern can be precisely calculated and it corresponds to the exciton generation rate within the whole structure [28][29][30].Therefore the enhancement of photocurrent might be possible, at least for planar heterojunction cells, through the modulation of the thicknesses of the layers [31,32].In the next paragraphs we will resume the critical points of the other three steps of the working sequence of an OPV.Although each step can be viewed and treated separately from the others, the four different phases are strictly interrelated.Improving a single step might not lead to the overall improvement of the device or the enhancement might be less than expected.It is the case, for example, of the bulk heterojunction that greatly enlarged the interface area between donor and acceptor leading to a great production of separated charges, but the random nature of the phase separation did not help the charges to reach the electrodes and be collected.

Exciton Diffusion
Once the exciton is generated, the electron and hole remain localized on a few polymer repetition units or a molecule and they are bound to each other by the electrostatic attraction.This is also called intrachain exciton to indicate that the constituent charges are situated on the same polymer chain.It is believed that intra-chain excitons represent the main species that are formed after photoexcitation in conjugated polymers [33,34].Upon adding a charge to a polymer chain, the chain will deform in order to reduce the energy of the carrier.This charge and deformation together constitute a polaron.The energy levels of a polaron are within the HOMO-LUMO gap.Polarons can drift along the conjugated chain.Once they reach the end of a conjugated segment, a hopping process to another conjugated chain can occur.It is known that in molecular crystals, excitons can diffuse efficiently by energy transfer and that the same mechanism takes place in thin layers of molecular semiconductors [35].However the research efforts are currently not so much devoted to increase diffusion lengths by modeling new molecules, but most of the researchers focus their action.todecreasing the distance between donor and acceptor [36][37][38][39].

Charge Dissociation
BHJ structures were designed for this purpose and they accomplished the requirement of reducing distances between D and A. In fact only when the exciton reaches the D/A boundary they will separate effectively.The energetic driving force for separating the positive and negative charges relies in the difference between the LUMO levels of the donor and acceptor.If this energy is larger than the binding energy of the exciton in the donor, then the dissociative process became more favorable.In this view donor and acceptor should stay very close each other as they do in the BHJ devices.

Charge Transport and Materials
However once separated, charges need to travel through the materials towards electrodes and the random oriented domains of donor can greatly reduce the cell efficiency not providing the necessary conductivity that charges require in order to be collected in the last step of the operating sequence.In this view, ordered structures are supposed to improve conductivity of the system thus cell efficiency.
Unfortunately a real control on the morphology of these systems is not achieved yet.
Trying to accomplish the necessity of an ordered array of D/A, nanorods and nanotubes of donor and acceptor materials have been investigated.In particular organic solar cells were made of Single Walls carbon Nano Tubes (SWNTs) employed as acceptor, and the poly(3-octylthiophene) (P3OT) as donor [40].The open circuit voltage V oc of the device was found to be 0.75 V, quite larger than expected.It was proposed that the improvement in the photovoltaic properties is caused by the internal polymer/nanotube links within the polymer matrix.Once dissociating the exciton, there is a uninterrupted way for electron and hole transport towards electrodes.On the other hand, the photocurrent of such devices is relatively low due to the partial phase separation and scarce quantity of absorbed light.The difference between the optical absorption of the polymer and the solar spectrum causes the low photocurrent.In addition the nanotubes do not supply the photogeneration process.Thus the same authors incorporated a high absorbing organic dye at the polymer/nanotube junctions and the photogeneration process was improved, especially for the UV portion of the solar spectrum [41].The short circuit current was found 5 times larger than the nondye system.However the open circuit voltage was lowered by 0,1 V and under solar illumination the overall performances did not improve significantly (current 1 μAcm −2 ).A blend of two conjugated polymers as the photoactive film was reported in 1995 [42,43], and since then, polymer-polymer solar cells have not attracted so much attention.However, regardless their modest performances (less than 2% efficiency) [44], these kinds of devices might be exploited for a series of advantages they can provide.As for solar absorption, for instance, two conjugated polymers could be tuned as harvesting a wider spectral wavelength.Moreover, the manipulation of polymers in terms of adjusting the donor-acceptor energy levels, is fairly easy.The main obstacle for the technology  based on polymer-polymer solar cells resides in designing conducting acceptors polymers which should have properties similar to those showed by fullerenes.In this view the use of block copolimers for solar cells is an early stage research area.Block copolymers (BCPs) synthesis would be one potential approach because periodic, adjustable nanostructures can be obtained [45].An ideal BCP can be described as a block containing a p-type donor and block containing an n-type acceptor (Figure 4).Nevertheless as the use of block copolymer for solar cell devices was theoretically predicted [46], efficiencies beyond the 0.5% were not achieved [47,48].In the field of materials, when order and mobility are required, it is natural to think about liquid crystals.They are currently considered as the new generation of organic semiconductors.By using conjugated LCs one can, in principle, control order in the bulk and at interfaces, from molecular to macroscopic distances.Because of their liquid-like character they can self-repair structural defects.Large single domains can be obtained by simply thermal annealing [49,50].By easily tuning parameters such as concentration or temperature [51,52] and irradiation with polarized light [53] or surface alignment layers [54,55], one can orient molecules inside these large domains.In addition defect-free chemical structures and a high purity level can be obtained due mainly to the low molecular weight of conjugated LCs.In this frame discotic liquid crystals have been reported as semiconductors [56][57][58][59]48].Disk-like LCs molecules can self-assembly in a columnar array (Figure 5) [58].Columns of discotic mesogens display one-dimensional charge transport and within the columns adjacent disclike molecules experience a large orbital overlap.The band width has been measured as 1.1 eV [60], and high values of the charge carrier mobility (m), that is, 0.2-1.3cm 2 V −1 s −1 , in their liquid crystalline (LC) mesophases were reported [61][62][63][64].Furthermore the measurement of the exciton diffusion length in the same columnar mesophases was about 70 nm [65].Probably the best discotic-based photovoltaic devices would be made by the columnar array of the donor, perpendicularly oriented to the substrate, which is embedded in an acceptor environment.Such a device has been proposed and an external efficiency of 5% with an external quantum efficiency (EQE, photon to current) of 34% at a monochromatic wavelength of 490 nm has been claimed [66].The discotic liquid crystal hexaperi-hexabenzocoronene (Figure 6(a)) as hole conductor was blended with a perylene dye (Figure 6(b)) to create thin films with separated perylene and hexabenzocoronene perpendicularly oriented.
However the ideal morphology showed in Figure 5 that it is quite difficult to be obtained and the overall efficiency of about 0.5% reported is an evidence that the perfect blend discotic-perylene was not achieved.Very likely the high performance for the EQE is quenched by a scarce phase separation and high percentage of recombination phenomena due to the fact that perylene molecules infiltrate within columnar stacks of the discotic liquid crystal.Thus the optimal device would be composed of ordered structures of separate donor and acceptor lamellae as Figure 7(a) shows.
To get this goal the homeotropic alignment of liquid crystals can be a promising way to pursue.In fact uniformly oriented textures of oligomers and polymers each other connected can be obtained by polymerization processes (photo induced, as an example) performed in liquid crystalline systems that have been prepared, in a homeotropic alignment by application of electric fields or by standing surface interaction (Figure 8) [67].In the sketch reported below is represented a possible way to overcome the problems caused by the bulk heterojunction architectures.
The red drawn molecules are polymerizable liquid crystals while the blue ones are not polymerizable liquid crystals.The result of the polymerization of "red molecules" allows getting the textures as presented in Figure 9 [67].
Electro-optical data, performed in our laboratory, confirm the noticeable evidence: the polymer forms structures International Journal of Photoenergy   that can be assumed as small cylinders filled with the not polymerized liquid crystal.The cylinder diameter is less than 1 μm, but it can be controlled through parameters such as temperature and relative concentrations of the components.
The not polymerized component can experience a phase transition to crystalline solid without any effect for the preformed structures.This kind of ordered microscopic structure would represent a great promise for application in organic photovoltaic.Although they cannot be strictly classified as OPVs, DSSC (Dye Sensitizer Solar Cells) photovoltaic cells [68], because of their very good performances (11% conversion efficiency), are worthy to be mentioned.They are made with a dye which is absorbed into nanocrystalline TiO 2 , ruthenium complex sensitizers, electrolytes containing fluid I − /I 3 − redox couple, and Pt-coated counter electrode.The redox couple donates its own electron in order to regenerate the dye and it avoids the reduced form of the dye to take back the injected electron.However, I − /I 3 − redox system can substantially decrease long-term stability and can be incompatible with some metallic components.Furthermore iodine is an oxidizing agent and it can rust metals, particularly if water and oxygen are present.Additionally, photocurrent loss can be induced because of the electrolytes I − /I 3 − absorb visible light (λ ∼ 430 nm).In the view of the mentioned working principles, the titania layer can be considered as the electron-transporting and the I − /I 3 − redox couple as the hole-transporting layer.Thus I − /I 3 − redox couple can be substituted by a p-type semiconducting material as a hole-transporting material (HTM).A first reported alternative to I − /I 3 − was a new couple, cobalt(II)-bis[2, 6-bis(1 -butylbenzimidazol-2 -yl)pyridine] (Figure 10), and it was found to act as a redox couple in DSSC systems [69].
Other copper complexes were examined as substitutes of the I − /I 3 − redox couple [70] and the operative working of the cells was demonstrated.However, low efficiencies (under 2%) were obtained.In addition, the time decay of the original DSSC performances has been attributed to a solvent loss of the electrolyte mixture.Therefore gelled-electrolyte-based DSSC systems were prepared and studied [71], but none of the proposed nonvolatile solvent-based cells has reached the original performance of the liquid electrolyte system.Iodide/iodine-free DSSCs using conductive polymers (Polymeric Hole-Transport Materials pHTM) such as polythiophene derivatives (Figures 11 and 12) were attempted to construct using a spin-coating technique and related dip-coating methods.
The nanoporous structures of the titania layer represent a barrier for polymeric hole transporting material to infiltrate into the nanospace.A more complex way is to synthesize HTM polymers in situ; that is, the HTM is introduced as monomer in the system and then it is polymerized within the nanopore of the dyed nc-TiO 2 electrode.The first reported system is a polypyrrole-based DSSC as a iodide/iodinefree DSSC.In order to plug polypyrrole into the pores of the dye-adsorbed titania layer, in situ photoelectrochemical polymerization (PEP) of pyrrole (Figure 13) [74] was used but a scarce conversion efficiency has been achieved (0.8%).
Among the attempts to reach the perfect structure, it is worth to mention the devices based onto ZnO nanorods [75].Vertically aligned ZnO nanostructures coupled to a polythiophene (P3HT) through a fullerene mediator gave an efficiency of about 3%.In the end we cite another interesting experimental approach which has been carried out by Sicot et al. [76] that verified the improving of the performance of a polythiophene-based photovoltaic cell when a molecular orientation has been induced.The authors reported the so-called molecular rectification through dipole orientation.Layers of polythiophene, dye and small amount of a polar molecule (4-(dibutylamino)-4 -nitroazobenzene-DRPR) embedded in a polymer matrix, were used to make up the photovoltaic cell.By applying a DC-field under resonant illumination of DRPR, a photoinduced reorientation is achieved.Although the measured efficiencies were very low, the authors observed an increase of one order of magnitude of the power efficiency of the oriented as compared to the nonoriented cells.

Conclusion
As a concluding remark of this brief review concerning organic solar cell, we like to stress the fact that this field of research is just at the beginning.Efforts need to be done in order to get bulk-ordered separated structures of p and n organic semiconductors in order to improve contemporaneously both the charge separation processes and the transport of the free charge to the electrodes.In our opinion one of the most promising work directions is to investigate the possibility to use liquid crystal semiconductor molecules and to study phase separation strategies between these base components, in order to obtain fine cell bulk architectures.

Figure 1 :
Figure 1: Scheme of operative sequence of an OPV.

Figure 2 :Figure 3 :
Figure 2: Examples of materials used as donors (a) and acceptors (b).

Figure 4 :
Figure 4: Schematic representation of block copolimer as an OPV device.

Table 1 :
Best performances of different systems.