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Timestamp: 2019-04-19 12:20:40+00:00

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c and metallo-organic) solar cells necessitates to at least partly master fundamental solid state physics of molecular materials as well as the chemistry of dyes and conjugated compounds. This multidisciplinary approach allows to envisage practical applications. This publication is divided in two parts depending on the type of molecular materials used: easily doped insulators or intrinsic molecular semiconductors. It is important to distinguish between these two types of materials since the electrical properties of easily doped insulators are as dependent of the concentration and type of the impurities present as of the chemical nature of the molecular compound itself.
intrinsic semiconductor". The density of intrinsic charge carriers must indeed be, as stated, related to the number of -conjugated electrons. However, it does also influence the facility with which oxidizing or reducing impurities can act as "dopants". In other words, the density of extrinsic charge carriers is related as well to the number of conjugated double bonds since it influences the redox properties of the molecular unit. As early as 1953 , D.D. Eley et al. by studying the d.c. electrical properties of crystalline powders (previously sublimed in vacuo) proposed that "...the conductivity is associated with the -electrons of the aromatic molecules. The problem was separated into two factors: (a) the energy levels of the -electrons in the isolated molecule and (b) the broadening of these levels into bands in the crystal, due to interactions between orbitals of adjacent atoms, but situated in two different molecules". The same approach has been formulated from a symmetry point of view under the name "supramolecular orbitals" (see page 212 of ). D.D. Eley et al. (page 84 of ) also outlined that "...we adhere to the view that the crystals are intrinsic semiconductors, although it is possible that in phthalocyanine below...310°C, impurities may play some role...". In a subsequent paper , it was mentioned that the "...promoted electron might ... tunnel through the barrier to the next molecule... if this theory is correct we should expect a zero or very small energy gap for a solid free radical". It is implicitly postulated that the electron transfer occurs from a single level. Many other factors have been demonstrated since that time to be important in the charge carrier process: vibrational relaxation, self trapping polarization, charge induced lattice distortion... This led the authors to propose the use of :-diphenyl -picryl hydrazyl  to lead to organic semiconductors a.c. (105-106 Hz) electrical measurements under vacuum (10-5 mm Hg) on freshly recrystallized DPPH powders (compression 0.5 Kg/cm2) gave a thermal activation energy of conduction = 0.263 eV in 0exp (/2kT), with no transition in the temperature range 84-312°C. The experimental curves may be extrapolated to give the room temperature conductivity: (27°C) = 3.5 10-9 -1cm-1 . The d.c. conductance of DPPH under light compression gave = 1.47 eV. A degradation with time of the DPPH sample was noticed characterized by a change of the melting point from 147°C for freshly recrystallized compounds to 126°C. Correlatively, decreases to 0.86 eV (d.c. value). In the same work , the conductivity of PcH2 powders were determined. The a.c. (50 kHz-50 MHz) electrical measurements in the range 150°-350°C lead to a value = 1.458 eV. This is indicative of a "doped" sample (the main references are indicated pp. 104-112 of ). Studies on single crystals of -PcH2 in the range 273-600°K (10-7 Torr) unambiguously showed that the intrinsic value of the thermal activation energy is 2.00 eV [9,10]) and it is written: "Above about 410°K (137°C), there is evidence that the conduction is intrinsic" . There is therefore some doubt that one can obtain the intrinsic properties of any molecular compounds by studying powders. In particular the previously mentioned results on powders of DPPH are influenced by the presence of impurities.
This has been confirmed latter on by experiments carried out on single crystals, powders and thin films of DPPH . Another organic radical (galvinoxyl, see chemical fomula figure 1) was studied in the same time. The thin films were obtained by solvent evaporation because vacuum sublimation caused some decomposition of DPPH (Table 1).
Figure 1: Chemical formula of :-diphenyl -picryl hydrazyl (abbreviated as DPPH) and galvinoxyl.
Neff is therefore of the order of 1021 molecules/ cm3 leading to approximately 107 carriers per cm3. The concentration of electroactive impurities must therefore be lower than 10-8 ppm to be negligible in the conduction process at room temperature. Galvinoxyl single crystals show very close electrical properties ( = 1.36 eV; log100 = -2.69) . The fact that an extrinsic conductivity is observed in all the experiments is confirmed by the fact that a relationship has been observed between 0 and  : log100 = a + b a,b : constants The preexponential factor 0 can be related to the density of effective states and to the mobility of charge carriers. The more reliable upper value for the mobility in single crystals is 1 cm2/V.s (see  and references therein). For ultrapurified organic crystals , the mobility has been found to increase by decreasing the temperature to reach 60 cm2/V.s at 30°K in the case of perylene. The time-of-flight method used to determine the mobilities is associated with a photogeneration of charge carriers instead of a thermal one for the previous studies. The quantity of charge generated depends on the power of irradiation. Whenever the density of photogenerated carriers is higher than the density of traps, these latter do not anymore influence the mobility measurements. Very early , the bandwith, which is related to the intermolecular overlap, has been the object of calculations (tight binding approximation) for anthracene. A bandwidth of the order of 0.56 kT ( 0.01 eV) was found. In 1962 , transient photocurrent measurements on pyrene crystals indicated a hole mobility of 0.35 cm2/V.s. At 155°C, above the melting point (149°C), the mobility decreases to 3. 10-4 cm2/V.s. d.c. experiments on anthracene type crystals indicate in most cases that holes are considerably more mobile than electrons (see references mentioned in ). This can be hardly correlated with a better - of the LUMO than with the HOMO. On the other hand, drift mobilities measured by a pulsed photoconductivity technique indicate that electrons and holes have about the same mobility. This discrepancy is due to the different processes of production of the charge carriers in the two cases. Irradiation can lead to a fairly high concentration of carriers contrarily to the thermal excitation. Preferential trapping of electrons by impurities (the ambient can almost always furnishes oxidant doping agents) would readily rationalize the difference in the results. The molecular radical lutetium bisphthalocyanine (Pc2Lu) has been shown to be an intrinsic molecular semiconductor with a room temperature conductivity in the range 10-5-10-6-1cm-1 for thin films as well as single crystals [17-19]. Rare earth bisphthalocyanines were first synthesized in 1965  and their electrochromic properties were latter studied . The conductivity of a bisphthalocyanine complex, formulated as Pc2NdH, was reported in 1967  to be 4.3 10-2 -1cm-1 ( = 0.122 eV) around 25°C. This does not correspond to the intrinsic conductivity as will be shown latter. Part of the molecular units is very probably oxidized leading to an enhancement of the positive charge carriers. The authors I.S. Kirin and P. N. Moskalev  compared their results to those of A. T.
2, the use of intrinsic molecular semiconductor (mainly Pc2Lu) is treated. The major differences unavoidably arising when using the two classes of materials are detailed.
Silicon single crystals have an intrinsic conductivity of 3.8 10-6-1cm-1 (see page 58 of ). The effective density of states in the conduction band is 2.8 1019/ cm3 to be compared to the number of silicon atoms per cm3 (5. 1022 atoms/cm3) deduced from the density (2.328 g/ cm3) and the atomic mass (28.08) of silicon. Only the levels situated close to the bottom of the conduction band contribute to the conductivity. In a molecular crystal with narrow or negligible bandwidths, the density of density levels should be at most equal to the number of molecular units per cm3 ( 1021 molecules/ cm3). The energy gap of silicon is 1.12 eV (300°K) to be compared with the thermal activation of conductivity found in single crystals of Pc2Lu.CH2Cl2 (Eact = 0.64 eV ). The drift mobilities are 1500 cm2/V.s. for electrons and 600 cm2/V.s. for holes. In molecular materials depending on the purity and the type of condensed phases considered mobilities varying from 10-4 to 1 cm2/V.s. are found. It is worth outlining that the value of the mobility strongly depends on the method and conditions of measurements, especially whether or not the generated carriers can fill all the traps or not (for an early reference see ). A high-efficiency (6%) solar cell using a silicon P-N junction was first described in 1954 . These devices were produced in a large scale in the 1960's mainly for uses in the American space program. A schematical representation of a P-N junction is given in figure 2.
Figure 2: Schematic representation of a conventional P-N junction derived from silicon type device.
Table 2: Diffusion length of holes as a function of a dopant concentration in Ndoped silicon . 1.E+15 stands for 1015 .
The contribution of minority carriers to the efficiency of the solar cells is hardly possible in the case of organic or molecular materials with low mobilities. Instead, the absorption of a photon generates an excited state with a strong interaction between the hole remaining in the Highest Occupied Molecular Orbital (HOMO) and the electron promoted to the Lowest Unoccupied Molecular Orbital (LUMO). The essential difference between the inorganic and organic devices may be also viewed by considering silicon under the form of nanocrystals of different sizes . The exciton exchange splitting between the singlet and the triplet states decreases from 16.5 meV to 8.4 meV when the diameter of the silicon nanocrystal increases from 2 to 5.5 nm. As a matter of comparison, the singlet/triplet splitting in PcZn is 0.68 eV (for a review of photophysical results see ). The molecular excited state can migrate from molecular unit to molecular unit without dissociation. In the case of a coherent transport, the energy transfer can be properly called an "exciton". The difference between mere incoherent energy migration and real excitons is however rarely made. Even under the influence of strong electrical fields, the excited state is not expected to significantly yield free charge carriers and a Stark effect is more often observed (see  for a comparison of Stark effects in PcCu and Pc2Lu). It has been seen that in standard solar cells, the amount of doping is of the order of 1016 atoms per cm3. A crystal of silicon contains approximately 5 1022 atoms/cm3, the mean distance between impurities is therefore of the order of 50 nm. The electrostatic repulsion between ionised impurities is consequently about 0.002 eV, a small value relative to kT at room temperature (Si: relative dielectric constant: 11.8). This will be rarely the case in molecular insulators because of the lower dielectric constant and of the high density of impurities contained in molecular materials which are never, or very exceptionally, purified to the same extent as silicon crystals. From the previous considerations, various very different mechanisms may be effective to demonstrate a photovoltaic effect depending on the type of single component materials used as previously defined: inorganic semiconductors (silicon type), molecular semiconductors (PcLi, Pc2Lu), molecular insulators (all closed shell molecular compounds polymeric or not), doped insulators. In the last case, it must be outlined that most experimentally studied conjugated organic compounds are said to be of P-type because they are polluted with air related oxidizing impurities.
coefficient of anthracene at 365 nm, the light is almost totally absorbed within the first 0.5µm. (ii) an oxidation-reduction (redox) process must take place at the solution-crystal interface, reduction occurring (to yield probably H2) at the illuminated face. Violanthrene instead of phthalocyanine has been used to study the photogeneration of carriers in the presence of dopants, such as o-chloranil, iodine, tetracyanoquinodimethane, deposited on the surface of the molecular material thin film . At the interface, electron transfer from the donor to the acceptor layer occurs giving rise to positive and negative ions on either side of the interface. The relative dielectric constant of molecular material is low (r = 3-4), the difference of redox potentials between the donor and the acceptor hardly reaches 1V and the density of dopants is fairly high. For these reasons, the width of the charged region must be of the order of one or two molecular units  for an abrupt interface. The diffusion of the donor and acceptor layers in each other can lead to significantly different results. The intensity dependence of photoconductivity of PcH2 layers of different thicknesses irradiated from the front or the back side allowed to have an idea of the width of the photoactive area which includes the doping region but also the layer which can be reached by energy migration. A value of 100 nm is found in this way . In conventional silicon based solar cells, there is a well established relationship between the rectification behaviour and the performances of the solar cells. The rectification phenomena have been studied for polycrystalline samples of various phthalocyanines . It is worth noting that the phthalocyanine used (PcH 2, PcCu, PcNi...) were of good purities with reported conductivities of the order of 10-15 -1cm-1. For cells such as Ag/PcM/M1, a silver paste electrode was used whereas small pieces of metal foils (Ag,Al,Sn,Pt,Cu) were contacted by a slight pressure. The presence of moisture had a pronounced effect on the rectification behaviour. All samples which initially showed rectification lost this characteristic when dried for several days in a dessicator containing concentrated sulphuric acid. An "ionic" space charge in the proximity of the least noble electrode was envisaged. A small amount of various acids significantly increased the rectification ratio. Electrolytic rectification due to an asymmetric growth of oxide on metal electrodes was already known . The importance of ionic currents was unambiguously demonstrated years after [50,51] by studying sandwich cells ITO/PPV:PEO:lithium triflate (1:1:0.2)/Al (PPV: PolyPhenyleneVinylidene, PEO: PolyEthyleneOxide). An electrochemical doping at the interfaces is thought to occur resulting in the formation of oxidized and/or reduced PPV, the molecular charge is compensated by the added counterions. The photocurrent and photovoltage of tetracene single crystals (20µm) with aqueous electrodes were studied  as a function of the excitation wavelength (220560 nm). The results are interpreted by involving the formation of neutral "exciton" diffusing over approximately 200 nm to reach the interface where an electron is transferred to the aqueous medium (to form presumably H2 as in reference ) while the remaining positive charge remains within the crystal. The absorption of the photon occurs around 2.4 eV. The lowest conducting band is thought to lie at 3.0 eV.
Q=CV Q in C V in V C = 0 r S/e It can be readily calculated with 0 = 8.85 10-14 F/cm, r = 4, S = 1cm2 and the PcM thickness e = 0.1 µm (10-5 cm) that for V = 1 V, there are approximately 2.2 1011 charges per cm2 on the electrode. The size of a molecular unit is of the order of 1 nm by considering 1021 molecules per cm3. In consequence there is approximately 1 charge for 4.5 102 interfacial molecular units. The space charge limited current for injected charges in insulators is classically given by  : ISCLC= 10-13 (µ) r V2 / e3 : proportion of free carriers (for the numerical equation see for instance page 53 of ). By taking a photovoltage V = 1V, e =10-5 cm, a photocurrent of 1 mA/cm2 necessitates a value of (µ)= 2.5 10-6 cm2/V.s. (This calculation has been made following J.-M. Nunzi's suggestion). Other ways may be taken to estimate the effective mobility of carriers necessary to have decent efficiencies of solar cells. The interfacial areas within the molecular material may be considered to be charged and "screen" the external voltage. Practically no electrical field is present in the bulk. Charges must then be transported in a purely diffusive way. With these assumptions, a mobility of the order of 0.1-0.5 cm2/V.s. is needed. Molecular units in direct contact with the metallic electrode can undergo a redox process (case B). The charge transfer occurring enhances the conductivity in the interfacial region until most of the molecular units are reduced (or oxidized). In the real cases, impurities, which may or may not influence the electrical properties, and doping agents, which does, are always present within the molecular layer (case C). The incorporation of O2 or some other compounds present in the ambient (O3, NO2 ,CO2, H2O...) is unavoidable. They can react with the charged metallic electrode and /or undergo a reduction process leading to a large variety of ionized impurities (O2-, OH-, NO3-, HCO3-...) near the electrode. In the example detailed in Table 4 , the large open circuit photovoltage observed in the case of the samarium electrode is very instructive as to the mechanisms involved in the photovoltaic effect. When the work function of the active metallic electrode is lowered (enhancing its electron donating ability), the consequence can be an increase in the density of ionised electron acceptor dopant IA-. This should result in a decrease of the "majority carriers", PcM+. The "excitons" ionise at defects or chemical impurities. Irradiation will thus tends to form ion pairs PcM+, IA-. However, Sm is sufficiently reactive to reduce the PcM unit itself. This reaction will occur when all the electron acceptor impurities IA around the Sm electrode will be reduced. The excited state PcM* can then hardly increase the density of IA- since the impurities are already reduced. The formation of the excited state of the anion (PcM-)* may be alternatively postulated.
Both the presence of impurities and chemical diffusion or reaction at the interface may be postulated (case D). It has been shown  by X-ray photoemission and ultraviolet photoemission spectroscopies that aluminium deposited on a thin layer of F16PcCu lead to a penetration of the metal deep within the molecular material and that some chemical reaction occurs between the two constituents. On the contrary, little, if any, inter-diffusion takes place when an organic material is deposited on a metal surface, the metal/molecule interaction is in this case limited to the first layer. A non-destructive fabrication of metal over molecular material layers has been described . Other types of dyes were used for making devices  yielding efficiencies of the order of 0.02% under white light irradiation (AM0; 135 mW/cm2). In these studies, preliminary results indicate a significant efficiency increase by bromine doping.
Figure 3: The dye used in reference . High efficiency organic solar cells based on merocyanines [69,70] have been described: $O$O2O3/dye/Ag.
which can occur from the excited state or the radiative decays commonly arise from the lowest excited state. It seems not to be the case in this example. Electromodulated absorption spectra of anthracene crystals , indicate that for photons of energy lower than 3.4 eV, the "neutral transitions" dominate whereas at higher energies, polar charge transfer transitions become important. This could rationalize the increase of quantum efficiency observed around 400 nm for the merocyanine based device. Charge transfer may however also occur from the dye to some defect or impurity. Dark and photoconductivity of nematic, smectic, and cholesteric liquid crystals have been measured . Smaller photoresponses occur in the mesomorphic states as compared to the corresponding properties at room temperature. A short description of the photovoltaic properties of cells ITO/APAPA (nematic liquid crystal)/ITO partially coated with gold has been reported .
Figure 5: Anisylidene-p-aminophenyl acetate (APAPA) mesogen . It is concluded in this publication that the key point to obtain a photovoltaic effect is related to the boundary organic material/electrode; the illuminated electrode is always found to be negative. A symmetrical glass coated ITO cell filled with a liquid crystalline porphyrin derivative has also been described . However, the results obtained under irradiation concerns only the molecular solid phase, the liquid crystalline properties are used only to prepare an "ordered solid" after cooling down to room temperature.
In all previous examples, the generation of charges involved the excited state of a dye and a dye/metallic electrode interface. A different approach has been taken in which two different dyes are used. The "active interface" is then constituted of two organic materials [75,76]. The device prepared has the following structure: JG/In2O3/PcCu (30 nm)/Perylene derivative (50 nm)/Ag The following characteristics were obtained under AM2 (75 mW/cm2) illumination: VOC = 0.45 V, ISC = 2.3 mA/cm2, FF = 0.65, = 0.95 % (without corrections for reflection or electrode absorption losses).
Figure 6: Copper phthalocyanine (PcCu) and the perylene derivative abbreviated ImPTC or (more commonly) PTCBI.
Figure 7: The methyl substituted perylene derivative used by M. Hiramoto et al. in ref. . Different abbreviations are employed: Me-PTC and in recent publications, MPCI or MePTCDI.
Irradiation was carried out from both the gold and ITO sides. A "masking effect" was noticed by studying the wavelength dependence of the photo-response: when illumination is carried out from the ITO/Me-PTC side, the PcH2 layer contributes predominantly to the photocurrent. On the contrary, irradiation through Au/PcH2 shows a major contribution from the Me-PTC part (Table 5) . Although the devices are very close to the one described in the very first publication , the masking effect was not mentioned in this latter. Doping of the Me-PTC layer with H2 or NH3 , both considered as electron donors, increases the performances of the solar cells  (Table 5).
Illumination is carried out through ITO. In the presence of the connecting Au layer between the two cells, the photo-voltage increases by a factor of approximately two as compared to the tandem cell without the connecting layer. However, the best photovoltages are comparable to the performances of a single cell: VOC = 0.67 V instead of 0.66 V. In the same time, the short circuit photocurrent of the tandem cell is about one third of the one of a single cell . Triple hetero-junction devices of overall efficiencies of the order of 1% (AM 1.5; 60 mW/cm2) have been described . M. Hiramoto et al. proposed some other related devices . Single solar cells G/ITO/Me-PTC/PcZn/Au reported by another group  confirmed the main results previously described. Sandwiches PcTiO(50nm)/Me-PTC(50nm) have also been used  for making solar cells. Exposure of the PcTiO thin film to ethanol causes the appearance of a strong new band around 830 nm which is active to generate a photocurrent. The Langmuir-Blodgett (LB) method has been proposed to make hetero-junctions .
Figure 8: Some of the organic compounds used in Ref. .
were fabricated and studied . No more than three or four monolayers of each type were used to make the devices. A rectifying effect RR (± 3V) = 3 was detectable. An irradiation from Al side leads to currents of the order of 10-10 A/cm2 . Interestingly, for some dyes (MX; X = S,Se) a photoresponse is attributed to Jaggregates. An extended review has been published  which details the mechanisms occurring in single or multiple hetero-junction devices. It is mentioned that "a direct contact between the deposited electrode and the active organics leads to quenching of excitons". This point has already been previously mentioned  (see also ). To avoid this difficulty, it is proposed to intercalate a bathocuproine thin film (BCP) between the perylene derivative and the metallic electrode.
Cells G/ITO/PcCu/PTCBI/BCP/Ag were studied and the thicknesses of both PcCu and PTCBI (3,4,9,10-perylenetetracarboxylic bis-benzimidazole) were varied to optimize the photovoltaic performances. Energy transfer from the two excited states PcCu* or PTCBI* to bathocuproin cannot occur due to the relative positions of the HOMO and LUMO levels. The molecular excited state therefore cannot come in close vicinity to the metallic electrode. The LUMO of bathocuproin is situated at a higher energy that the LUMO of PTCBI. The transfer of electron from PTCBI- to BCP is however necessary to deliver a photovoltaic response. This creates a barrier and this is in consequence associated to the introduction of an additional series resistance. The BCP layer is also said to avoid the damage of the organic layers when the metallic contacts are finally deposited. This point has been already discussed . The device ITO/PcCu(15nm)/PTCBI(15nm)/BCP(15nm)/Ag(80nm) leads to an external power conversion efficiency of 1.1% from 0.1 to 10 suns [90-92] (AM 1.5; for a precise definition of Air Mass 1.5 see ). In related devices , the BCP layer is cosublimed with 10% (w:w) of PTCBI to "prevent film recrystallisation rapid at high illumination intensities". The introduction of PTCBI in BCP (called "exciton blocking layer") must concomitantly influence the electrical properties of the thin film. The action spectra of the PcCu/PTCBI solar cells closely follows the corresponding absorption spectra of the constitutive parts as in the first publications on the subject [75,76].
Attempts have been made to improve the quality of the contact between ITO and the photoactive molecular material. ITO is thus recovered with a thin film of PEDOT:PSS deposited by spin coating.
been fabricated. Under standard conditions (100mW/cm2; AM 1.5), the bilayer cell leads to a conversion efficiency of 1.71% whereas the cosublimed thin film annealed at 150°C leads to a 2.14% yield. The effect of annealing is significant since the yield is only 1.49% before heat treatment. Mixed polymer/mesomolecule photoactive layers have been deposited by spin coating .
Figure 11: The products and the abbreviations (P3BT, PR 3072) used in Ref. .
The dependence of VOC and ISC with the percentage of the dye PR 3072 in the substituted polythiophene P3BT shows an abrupt change when the concentration of the dye reaches 16%: VOC increases from 30 to 160 mV and ISC from 5 to 35 µA. This phenomenon has been attributed to the establishment of "percolated paths" of PR 3072 at the concentration of the transition providing a continuous conduction channel for electrons. Compounds which can show mesomorphic phases in some temperature ranges have been used to form bilayer type devices with a perylene derivative . Devices G/ITO/RS-PcH2/Ethyl-Phenyl-Per./Al have been fabricated. The phthalocyanine layer is deposited by spin coating, heated up to the isotropic phase (292°C) under nitrogen and slowly cooled down to room temperature. Such a procedure is thought to favour a perpendicular alignment of the columns. The columnar structure is indeed a characteristic of discotic mesophases in which segregation between the rigid central cores and the flexible chains spontaneously occurs (see for instance Ref. ).
Figure 12: The mesomorphic compound used in Ref. : RS-PcH2 (the nature of R is not indicated in the publication; the isotropic liquid forms at 292°C) and the perylene derivative (Ethyl-Phenyl-Per.) employed to make the corresponding bilayer devices. At room temperature the material is probably polycrystalline and important structural changes can have occurred. Photoelectric studies have been carried out on this cell; the assumed "alignment effect" has not been clearly evidenced. Another couple of compounds has been used for closely related purposes . The hexa-substituted coronene derivative HBC-PhC12 forms a discotic liquid crystalline phase at room temperature whereas the substituted perylene is polycrystalline. Photodiodes based on a blend HBC-PhC12: Et-But.-Per. (40:60) with ITO and Al electrodes have been irradiated at 490 nm (0.47 mW/cm2) yielding ISC= 33.5 µA/cm2, VOC= 0.69 V, FF = 40%. The corresponding conversion efficiency is 1.95% for monochromatic light. The bilayer type device based on the same compounds but where the perylene layer is evaporated on top of a HBC-PhC12 thin film made by solvent evaporation leads to lower performances . A N,N'-diheptyl perylene product (C7-PTCD) forms two different liquid crystalline phases in the temperature ranges 214-387°C and 387-403°C. At a higher temperature, an isotropic phase is found. Bilayer type solar cells G/ITO/PcZn(25nm)/ C7-PTCD(25nm)/Ga:In have been made . The devices are made by sequentially evaporating the two organic compounds. The currentvoltage characteristics under irradiation (> 400 nm; 100mW/cm2) are presented at the temperature of 23°C, outside the mesomorphic domain of the perylene derivative: ISC= 1.58 mA/cm2, VOC=0.6 V, FF = 0.4, = 0.38% .
Figure 13: The coronene derivative (HBC-PhC12) and the substituted perylene derivative (Et.-But.-Per.) used in Ref. .
Two-organic layer solar cells: polymeric compounds Conjugated polymers can be used alternatively to mesomolecules for making electronics components. They are already massively used as conductors after heavily doping. Numerous techniques available to make thin films, such as ink jet printing, which do not necessitate physical masks , make the use of polymeric compounds attractive. However, the fabrication of printing inks with mesomolecular dyes is also well established . On the other hand, these materials suitable for printing compulsory contain large quantities of impurities and traps and the sublimation procedure under vacuum which is generally used to make thin films eliminate most of the chemically undesired products. The non-substituted polymeric materials are not soluble nor sublimable and cannot be extensively purified. Large amounts of impurities (several per cent or more) cannot be removed and may act as traps. A high concentration of carriers is necessary to fill these levels and to yield a conductive state. The purity expected for "Organic Electronic Materials" may be estimated from the description of commercial products . The dyes of molar mass in the range 300-1000 are available with purity comprised between 80% and 99.9% depending they are sublimed or not. The high molar mass conjugated polymers have probably less controlled and higher impurity content. In this last case, the compounds are made soluble by chain grafting and can then be purified by standard methods such as High Pressure Liquid Chromatography (HPLC). Even in this case the concentration of impurities is many orders of magnitude higher than the density of intrinsic carriers and the electrical properties of these materials is still governed by external sources of dopants.
The couple MEH-PPV(100nm)/C60(100nm) has been used in 1993 to make solar cells .
contrarily to usual cases. The rectification ratio (± 3.5 V) of G/ITO/MEH-PPV:CNPPV/Al cell is 103 . Modest performances under irradiation have been reported. It has been proposed to use self organization of block co-polymers for "photonic" applications . Related studies have been reviewed [121,122]. Block copolymers where electron donor sequences alternate with electron acceptor moieties have been proposed . Composite films with MEH-PPV and a substituted fullerene derivative (PCBM) have been studied . The two constituents are reported to form a "bi-continuous network" following a process close to the one described for two high molar mass polymers. The quasi-three dimensional interface thus formed is said to be a "bulk hetero-junction" .
Figure 15: Chemical formula of the two fullerene derivatives used to make "bulk hetero-junctions"(PCBM: 5,6 and 6,6 adducts) . Solar cells G/ITO/MEH-PPV:C60 (blend: 100-200 nm)/Ca or Al have been tested (surface area from 0.1 cm2 to 15 cm2 yield similar performances). The morphology of the network is strongly dependent upon the solvent (xylene or 1,2dichlorobenzene) used to cast the film. Short circuit currents of the order of 0.5 mA/cm2 (monochromatic light: 430 nm; 20mW/cm2) were observed . Studies on glass or polyester/ITO/MDMO-PPV:PCBM (+ 0-90% polystyrene)/M  irradiated at 488 nm (10 mW/cm2) leads to energy conversion efficiencies of 1.2%. In this publication, it is stressed that the combination of light and oxygen induces a very fast degradation of the conjugated polymer. The exclusion of O2 (in fact more probably ambient air) permits to extend the stability to 8 hours.
. Three different devices have been fabricated  to further elucidate the influence of the contacts: ITO/MEH-PPV:PCBM/Al ITO/PEDOT-PSS/MEH-PPV:PCBM/Al ITO/PEDOT-PSS/MEH-PPV:PCBM/LiF/Al The open circuit photo-voltage, the short circuit current and the the fill factor are all increased by intercalating the two layers of PEDOT:PSS and LiF. For a MEHPPV:PCBM ratio in the blend of 1.5, the following results under standard irradiation (100mW/cm2; AM 1.5) are obtained: VOC= 0.79 V, ISC= 6.1 mA/cm2, = 1.49%. As mentioned previously, the chemical or physicochemical differences brought by LiF and PEDOT-PSS are complex. Additionally, some ionic current may be involved in the electrical properties. Various well known electrochemical effects (double layer, Helmholtz and Gouy capacitie for a general reference see ) are probably interfering in the mechanisms. The stability of: G/ITO/BCP or perylene derivative/MEH-PPV:PCBM/PcCu/Au without encapsulation  has been determined. A decrease of the performances of the order of 30% is observed after two weeks under ambient air. Sealed cells have been also reported ( see S. Schuller et al. in Ref. ). Fullerene can be reacted with various reagents via charged nucleophile addition, amine addition, cycloaddition.... Chemically connected donor and acceptor moieties have been synthesized with these chemical tools: phthalocyanine-C60 , oligophenylenevinylene-C60 [134,135], oligophenyleneethynylene-C60 ,t-butyl substituted phthalocyanine-C60 . These molecular units are potentially highly polarizable, this effect can intervene in the photo-generation of separated charge carriers. The corresponding solar cells present until now fairly poor performances: = 10-2-10-3% (400 nm; 1 mW/cm2) , = 2 10-2% (simulated solar illumination, 80 mW/cm2) . A supramolecular engineering approach could allow to form linked nano- or micro-domains for transporting electrons and holes in different channels as already explored in the case of polymers. The compounds described may also play a role in devices based on other physico-chemical properties such as second or third harmonic generation (see for instance ). Molecular Field Effect Transistor (MFET or OFET, O standing for Organic) based on oligophenyleneethynylene-C60 compounds have been also reported .
notions from most scientific fields, from Chemistry to Physics for so-called basic Sciences, from materials to commercial applications for applied aspects. From the data reported in 1958, the energy conversion efficiencies can be hardly estimated but the main processes involved were already correctly outlined. In the period of time 1958-1985, mostly molecular material/metal interfaces were studied. They were called "Schottky devices" by analogy with the conventional inorganic based systems. This terminology is misleading since the basic phenomena and the associated relationships which can be derived are entirely different in the two types of devices. The corresponding energy conversion efficiencies of the organic solar cells were in the range 10-4-10-2% under standard conditions (white light, 100 mW/cm2; see page 148 of Ref. ). The irradiation with photons on a large wavelength spectrum is important to characterize the performances of the solar cells but it has also deep consequences on the choice of the dyes and on the basic physicochemical processes involved. The same remark could be made concerning the intensity of the irradiation. In an isolated case, a "Schottky" type organic solar cell was reported to show a conversion efficiency as high as 0.7% in 1978 . In a patent disclosed in 1977  followed nine years latter by a publication , interfaces between two dyes differing by their electrochemical properties (the respective location of the HOMO and LUMO) have been proposed. This interface is used to ionize the molecular excited state formed under irradiation instead of the molecule/metal contact. Photo-induced electron transfers have been very extensively studied in solution. The same fundamental mechanisms govern the various processes in thin films. Some additional factors must however be taken into account whenever the distances between the photo-chemical events induces interactions between them. The "bilayer" structure with two different dyes permitted to increase the solar cell efficiencies by several orders of magnitude (0.95% in Ref. ). This result was confirmed by latter independent studies. Subsequently, most of the efforts focussed on the bilayer type of solar cells with rather minor variations on the type of dyes employed. A bilayer PcCu/PTCBI demonstrated a 1.1% efficiency in 2000 . The efficiency raised to 3.6% for a PcCu/C60 bilayer in 2001 . This report was confirmed by the same group in 2003 . The structure of the contact area between the donor and the acceptor moieties is an essential parameter for the performances of the solar cells. Quasi threedimensional junctions have been proposed in 1995. Segregation of the donors and acceptors into differently spaced domains ensures continuous pathways to collect the negative and positive charges once formed after photo-ionization [119,124]. In 2001 , a conjugated polymer and a fullerene derivative led to a 2.5% efficiency. A composite cell , MEH-PPV:PCBM, yielded 2.9% in 2004. A comparison has been made between a PcCu/C60 bilayer and a corresponding device where PcCu is co-sublimed with C60 . The efficiencies measured in both cases are in the same range: 1.71% for the bilayer compared to 2.14% for the composite.
In 2005 , for a cell ITO/PEDOT:PSS(40nm)/P3HT:PCBM(1:0.8 from chlorobenzene)/Al a 5% yield was reported. In this case, the whole device was annealed at 150°C in a glove box (nitrogen atmosphere). Before heat treatment, = 0.82%. It is stressed in the publication that no LiF is used to improve the contacts. The annealing probably favours the reaction of the superficial part of the aluminium electrode with some O2 related impurities which are efficient traps for electrons. "Tandem cells" constituted by the superimposition of two single cells have been demonstrated to be able to yield energy conversion efficiencies higher than 5% : G/ITO/PcCu(7.5nm)/PcCu:C60(12.5nm)/C60(8nm)/PTCBI(5nm)//Ag nanoclusters(0.5nm)inMTDATA(5nm)//PcCu(6nm)/PcCu:C60(13nm)/C60(16nm)/BC P(7.5nm)/Ag(100nm).
sublimation have been used to sandwich PcZn:C60 blends [143-144]. The reported yields are in the range 1-2% for single cells and 2.4% for a tandem cell. The use of "collector mirrors" permitting the passage of the incident light several times through the photo-active area has been proposed . The multiple absorption can increase the effectiveness of light collection but the parabolic mirrors employed need to be properly oriented relatively to the incident light. This implies the use of a costly solar tracking system. A comparison is often made in the recent publications between the yields obtained with amorphous silicon and the ones demonstrated with organic compounds. Although the overall efficiencies seem to be comparable, it is highly probable that the degree of technological achievement of organic solar cells is still far from the one obtained with amorphous silicon. It must be also remarked that amorphous silicon is only one of the condensed phase used to make solar cells or more generally devices. Thirty years ago, the relative advantages of the various forms of silicon for industrial applications were already compared. One report among many others may be found in Ref. . The title of this article is: "Silicon-The Perpetual Material for Micro-, Power-, and Solar Electronics". After several decades, single crystal silicon is the only material massively used in the fabrication of electronics circuits. On the other hand, the technology is not yet stabilized for solar cell applications mainly because mass production started more recently, the silicon used to make solar cells is a side product of the electronics industry. Once heavy investments have been made in an industrial way, many examples in the past demonstrate that alternatives may hardly emerge even if significant advantages could be found. In present days even a promise of financial benefit is sufficient to overcome all other considerations. The originality and the quality of the researches are beyond the preoccupation of most deciders. It is a mere observation, which cannot be contested by the facts, that the major breakthroughs, even for pure industrial applications, were the results of the unconstrained talent of scientists or technicians rather than the accomplishment of a large group of obedient people. Creative scientific efforts do not need to be justified by short, middle or even long term applications: it is probably the best way to never find any. As an example, the liquid crystals discovered by chance in 1888 found large industrial applications only one century latter. However, the "fluidity" of the scientific community must be sufficient to allow the passage from basic to applied preoccupations without purely administrative or ideological barriers. Another point can be hardly disputed. The domain of "solar energy" has suffered constantly from a total dereliction from political and industrial leaders in the past thirty years. It was easier to use oil or nuclear energy not for technical or social reasons but only because they can accompany military efforts and is a way to have the "power", i.e. the political and social controls of population. It is true that solar energy cannot be used as a source of energy for tanks or submarines. On the contrary, it is well adapted to deliver energy. These aspects cannot be ignored even in a purely scientific publication. This long world wide neglect, which seems hopefully to end, is not justified from scientific, technological, industrial, environmental, social points of view.
twisting (see Ref.  and related references). Molecular solar cells could be advantageously used to make artificial retina, for instance. Silicon based solar cells have been studied towards that goal . Inorganic semiconductors, mainly silicon, have been also deposited on flexible substrates [155-157]. Amorphous silicon thin films deposited on "plastic foils" have been made by plasma enhanced chemical vapour deposition; the temperature of the substrate is limited to 150°C to avoid thermal degradations. A Field Effect Transistor (FET) presenting mobilities around 0.5 cm2/V.s.  was thus fabricated. Polycrystalline silicon thin layers have been obtained by spin coating and ink jet printing . Cyclic Si5H10 is irradiated (405 nm) in solution partially yielding high molar mass (MW 2600) polysilanes. The irradiated solution was used for spin coating. The sample was baked at 540°C to yield amorphous silicon. This material leads to low mobility (10-3-10-4 cm2/V.s.) FET. The amorphous material is finally transformed into a polycrystalline film under high energy irradiation (308 nm; XeCl excimer laser). The polycrystalline thin films of silicon yield high mobility FET (74-108 cm2/V.s.) as calculated from the transconductance in the saturation region . This point has been detailed to try to show the difficulty to use an approach with certainty when a given application is targeted. The decision is related to the results rather than to the method. (iii) the use of molecular materials to make functional devices is a way to connect the macroscopic world almost entirely driven by electrical currents to the biological one where electrons are never the active species transporting information or energy. They are replaced by a large variety of inorganic and molecular anions or cations giving rise to ionic currents (for a short illustration of the importance of this difference see Ref. ). (iv) it is now generally accepted that molecular diodes, transistors, photodetectors, solar cells...constitute a few elements of an immense puzzle that Scientists can constitute more and more precisely. The proposition of "Molecular Electronics" made years ago  is still of actuality. This approach is not anymore devoted to replace the present computers by molecular systems. The challenge is to succeed to make "molecular complex systems" close to natural neural networks (see ) as to the way the information is processed. The term "ionoelectronics"  has been proposed to designate that field of research.
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