GEL battery also known as colloidal sealed lead-acid batteries , is a VRLA battery ( a total of two kinds ) , the use of colloidal electrolyte , ie GEL technology to Germany sunshine behalf of the company. Another VRLA batteries using AGM separators, namely AGM technology , with some Japanese companies represented. Both have advantages and disadvantages , communication , development of AGM-VRLA batteries power the rapid development of the last century , high market share is relatively low GEL-VRLA batteries.
Colloidal sealed lead-acid batteries ( GEL batteries ) , which is composed of silica sol and sulfuric acid electrolyte is dubbed , the concentration of sulfuric acid solution is lower than AGM batteries , typically 1.26--1.28g / cm3. Amount of electrolyte AGM batteries to more than 20% , with quite a flooded battery . This state exists colloidal electrolyte filled in the separator between the positive and negative electrode gel surrounded by a sulfuric acid electrolyte , the battery does not flow out . Because of this battery is flooded Noncompact assembly structure , positive grid low antimony alloy material can also be used tubular battery cathode plate . Meanwhile, in order to improve the battery capacity without reducing battery life, the plates can be made thinner . The internal space of the battery compartment can also broaden few .
Its advantages are as follows : GEL type gel batteries is no free electrolyte gel electro-hydraulic rear chances leak acid battery is much smaller than the former ; its perfusion more than 10 - 15 % sulfuric acid , water loss and less so gel batteries will not cause failure due to dehydration ; colloidal poured increase in the strength of the separator to protect the plate , make up the clapboard case of acid contraction defect , the assembling pressure reduction is not obvious reason for having extended battery life one ; colloid filled voids between the plates and separators , reducing the internal resistance of the battery , the charge acceptance may thus be improved . So gel battery overdischarge , Shaanxi multiplexing capability and low temperature charge-discharge performance is superior than AGM batteries; gel battery consistency than similar AGM type battery much better.
Domestic production has been colloid following four : gas gum , silica sol , mixed sol , silicone polymer gel .
Twin pair recombination in polymer donor-acceptor mixing.
Another focus of discussion is the latest progress in experimental research and theoretical analysis of organic solar GEL Battery.The following detailed discussion of basic issues can be found in document [ 47 ]:
The exciton nature of optical excitation in conjugated polymers:
exciton binding energy;
Exciton energy relaxation;
Site - selective spectroscopy.
As the first author of this chapter, professor arkhipov of IMEC, Belgium's inter-school microelectronics research center, died in the process of compiling this book, some errors in the text and pictures in this chapter are difficult to correct, thus causing inconvenience to readers, and the translator is very sorry.
Mechanism of carrier photogeneration
Among unsaturated compounds, there are three or more parallel P orbitals forming a large 7: bond. This system is called conjugated system polymer, which is a high molecular weight compound linked by one or more structural units through covalent bonds, also called polymer compound.Conjugated polymers refer to polymers containing alternating conjugated systems of single and double bonds on the carbon chain skeleton. Due to the inclusion of conjugated systems, the K - bond energy level spacing becomes smaller and the conductivity and light absorption characteristics will change. Therefore, they have their own characteristics in solar cell materials, fluorescent materials, conductive materials and other application fields.
In conjugate polymers, the main optical excitation is neutral singlet exciton or triplet exciton.In singlet excitons.The excited electrons and the electrons left in the ground state have anti-symmetric spin directions and zero spin superposition, and only singlet excitons are generated under the sun's illumination, which has a large energy and can generate carriers.However, the recombination lifetime of singlet excitons is short, on the order of ns, then recombination occurs and fluorescence is emitted.In the triplet exciton, the excited electrons and the electrons left in the ground state have symmetrical spin directions, and there are three ways, the spin superposition is not zero, the energy is low, and carriers cannot be generated, but the triplet exciton has a long recombination life and emits phosphorescence on the order of ms.
Spectral measurements show that the distance between singlet and triplet states is about 0.7 eV, which can directly prove that conjugated polymers mainly excite singlet excitons or triplet excitons [ 812 ].Because this spectrum measures the intensity of the interaction between electrons and holes in excitons, it proves that excitons are tightly bound.According to Stark spectral data and quantum chemistry calculations, the average square root of electron-hole separation in a singlet state is about 1 nm [ 13 - 15 ].Stark spectrum uses an external electrostatic field to shift or split the spectrum of atoms or molecules, thus revealing the physical or physiological and chemical properties of the sample.The electron-hole pair spacing of about 1 nm indicates the energy required for singlet excitons to be separated at room temperature " ht, and the binding energy of excitons is e 0.5 ev [ ls \ however, most excitons' binding energy EB is easily affected by spectral detection except for crystalline polydiacetylene.
Since the exciton binding energy is about 0.5 eV or more, we expect the photoconduction to start with photon energy such as = E ( SJ + ￣ B, where E is the energy of the singlet exciton.The spectrum of action is the spectrum of biological or chemical light response to wavelength or irradiation energy produced by the number of human photons, which is often used to absorb biological systems or solids when the species are unknown.In the spectrum of continuous wave conductance, the 0 - 0 transition of energy greater than the absorption edge is obviously increased for samples of different co-working polymer films such as substituted polystyrene [ 17 ], ladder-type poly - p - benzene ( 1 ) _ [ 18 ] or polythiophene [ 19 ], which proves that intrinsic photogeneration begins to occur. However, the intrinsic photogeneration in this co-polymer film is similar to W occurring in traditional molecular crystals, which is believed to be due to the self-ionization characteristic of the non-relaxed high-energy Frank - Condon state, which leads to coulomb force binding.Similar phenomena occur in conjugated polymers.If additional energy is needed to form coulomb force unbound electron-hole pairs relative to relaxed singlet excitons, it can also be achieved through a two-quantum process, such as two-photon absorption, i.e., continuous photoionization excited by relaxation S, or two-molecule collision of two excitons [ 21 - 23 ].
Relaxation singlet excitons are singlet excitons after all excess vibrational energy has been dissipated. The only way to separate relaxation singlet excitons in undoped systems is to apply a high electric field.The high electric field can compensate for the Coulomb bound force loss of the extended electron-hole pair.In anthracene type molecular crystals, such a high electric field requirement has exceeded the available electric field strength range because exciton binding energy ￠ b is too large.In a 100 nm thick conjugated polymer film, the typical electric field strength of 106 V / cm lower than the breakdown electric field cannot cause exciton separation.The characteristic of electric field-assisted separation of $ excitons is the quenching of burning light at high electric field intensity [ 24,25 ] 0. In a broad sense, fluorescence quenching refers to any effect that can reduce the fluorescence intensity of a given fluorescent substance, or any effect that can make the fluorescence intensity not have a linear relationship with the concentration of the fluorescent substance.In a narrow sense, fluorescence quenching refers to the interaction between fluorescent substance molecules and solvent molecules or other solute molecules, resulting in a decrease in fluorescence intensity.There are many forms and mechanisms of fluorescence quenching, and the main types are collision quenching, generation compound quenching, energy transfer quenching and oxygen quenching.Fluorescence quenching phenomenon is well known in inorganic semiconductors such as GaAs, but due to lower exciton binding energy, fluorescence quenching will occur in the electric field intensity range of 10kV / cm.
Electric field-assisted singlet exciton separation can be directly observed by transient absorption ( TA ).TA measurement method is to use FS - level tunable laser pulse as pump detection to study ultra-fast electron transport reaction.Several reports in the literature have confirmed that by the excitation of 100 fs laser pulses, the polymer film will generate a ta signal close to 2ev, which is much lower than the optical band gap [ 26' 27 ] of the film due to the generation of free radicals of cations and anions, known as polarons.More specifically, the carriers in the solid generate an electric field to polarize the surrounding medium, which is accompanied by carrier movement. The carrier plus the complex of induced polarization in the solid is called polaron.According to the changing signals from time to time, we can draw some conclusions [ 27 ]:
Separation occurs during the whole intrinsic lifetime of vibrational relaxation S, excitons;There will be no strictly defined reaction rate, but dispersion kinetics will be followed.
These characteristics are attributed to energy stochastic systems with non-uniform broadened absorption spectra [ 28' 4 ].The condition for exciton separation is that the energy dependent on the electric field strength of the nearby charge acceptor state is equivalent to the exciton energy.Obviously, the energy disorder state will definitely shift to the reaction distance distribution and the accompanying reaction rate distribution.
The reaction product is a metastable coulomb force bound electron-hole pair, after which complete separation or twin pair recombination will occur. Many experimental results can prove this phenomenon:
If the excitation electric field intensity is turned off, the sample will emit delayed spikes.The delayed intensity is an exponential form of center ( "" ), while 1 [' reflects the twin-to-single molecular recombination that causes fluorescence S: state [ 3 ].
In the delayed electric field strength collection experiment, the sample is excited at a specific electric field strength, and the photoconductive response is measured after a delay time after the instantaneous fluorescence decay.The measurement results show that exciton separation produces metastable electron-hole pairs and attenuates exponentially.
After several hours of photoexcitation at low temperature, thermal excitation luminescence emitted by conjugated polymers can be observed, which is due to the long life of twin pairs and the twin pair life depends on temperature [ 31 ].
According to the conclusion in section 8.2.1, the intrinsic photogeneration of carriers will not occur in the single-component conjugated polymer with medium and low electric field strength, because the higher coulomb bound energy will hinder the separation of singlet excitons.However, at medium electric field strength, photoconduction does occur near the absorption edge [ 17 - 19 ], but quantum yield is far less than 1 %, and depends strongly on electric field strength and weakly on temperature.Quantum yield refers to the amount of reactants consumed or products produced by the system absorbing each photon.The semiconductor energy band model can no longer explain this phenomenon.In the action spectrum, a straight part of the photon energy range of about Lev appears in the curve.Obviously, some S, excitations tend to separate later, while temperature-related effects cannot reach the energy needed to overcome Coulomb attraction.
In Lee's pioneering work [ 32 ], the viewpoint of sensitizing photogeneration was put forward for CM - doped MeH - PPV ( poly ( 2 - Methoxy, 5 - ( 2' - Ethylhexxy ) - 1,4 - Phenylenyenetene ), that is, dopant as sensitizer will cause exciton separation.Controlled doping is an experimental method to confirm this view. Representative data will be presented below.The copolymer used was high-purity PHP PV produced by CovionOrganicsemicondtorsgmbh, Germany, as the matrix material, and the dopant was strong electron acceptor TNF or PDL?The optical density ( 0d ) is the human light intensity j.The logarithm of the ratio to the transmitted light intensity j:
Where " is the absorption coefficient w of the material is the optical path length;If the light intensity ratio + > 10 %, the optical density ol k1.Actual. 1 describes the optical density and photogenerated current of undoped PHP PV intrinsic photogeneration and 1 % TNF content doped PHP PV sensitized photogeneration [ 33 ].The hollow square in the figure shows the absorption spectrum of pure PHP PV. OD0 means that the presence of TNF doping does not change the absorption spectrum of the doped film because TNF is absorbed in the ultraviolet band, while the fluorescence quenching of the matrix material is very strong.If PHP PV is doped, it will also be accompanied by S and shorten the excitation lifetime.Time - resolved fluorescence measurements show that if solid MTHF ( 5,10 - methoyltetrahedron ) is used for isolation and MeH - PPV is used as matrix, S!The lifetime is about 700ps, while the lifetime of undoped films is only 350 ps.The change of twins' life span is attributed to exciton quenching caused by unknown impurity concentration of 0.04 %.The phenomenon that fluorescence depends on doping concentration can be explained by diffusion to incoherent exciton dopants [ 29 ].Since there is no spectral superposition between polymer emission and dopant absorption, fluorescence quenching is believed to be caused by charge transfer centers, more specifically, the transport of electrons from the stimulated PHP PV portion to nearby dopant molecules.
The photoconductive spectrum of a symmetrical Al / PHP PV / A1 diode was measured at rest by irradiating human light through a translucent aluminum electrode.When the photon energy reaches about 3.5 eV, the quantum yield becomes almost a constant.It is worth mentioning that in the spectral range covering the S + SO absorption band, the light response increased by 1 % dopant does not exceed 2 times, while the host fluorescence decreases by more than 10 times.It seems that this result is somewhat weird, but this phenomenon has been reasonably explained.Even in undoped PHP PV films, about 50 % of the $ excitation will be quenched by 0.04 % of unknown impurities, so the photoresponse cannot be increased more than 2 times.
Since the dependence of photogenerated current on electric field strength and temperature is almost the same in undoped and doped films, we can further conclude that Si - S at medium electric field strength.The photogenerated current measured in the transition spectral range is mainly sensitized by electron acceptor impurities.The evidence for the existence of high electron affinity impurities is polymer oxidation products or dissolved oxygen, which is inferred from the study of carrier mobility of conjugated polymers with low ion potential energy without electronic signals.Because the mobility of holes and electrons in polymers without defect matrix isolation is equivalent [ 34 ], this proves that the electron movement of bulk films is limited by traps.Note that the lowest unoccupied orbital LUMO of P - type materials is usually much higher than conventional impurities, so impurities such as polymer chain oxidation products will become deep electron traps rather than hole traps.
Actual. 3 shows the dependence of photogenerated current on electric field strength in undoped intrinsic photogenerated PHP PV film and 1 % TNF doped sensitized photogenerated PHP PV film.Pulse excitation is used to record these data, because under such conditions, the experiment can be extended to higher electric field strength while avoiding the risk of spurious pre-breakdown effect caused by space charge accumulation [ 35 ].The transient and continuous wave data are self-consistent within the range of superimposed electric field intensity.The quantum yield is calculated by integrating the transient photogenerated current into the number of absorbed photons.In the calculation, bimolecular electron-hole recombination was ignored because the photogenerated current became strictly linear with respect to the intensity of human light.Sensitized light productivity is strongly dependent on electric field strength.If we use the three-dimensional model of OnSager' s Theory to analyze the data of twin electron-hole recombination.We will get the initial pair spacing of electron-hole pairs of undoped and doped films of ꆡꆡ and ꆡꆡ nm, respectively.Onzag's theory can be used to calculate the dielectric constant of polar molecular materials, and the local electric field in the molecule can be calculated in the molecular size spherical cavity of the medium by using the Laplace's equation.
A common phenomenon in polymers is the weak dependence of intrinsic photogenerated and sensitized photogenerated photogenerated currents on temperature, which can be confirmed from PHP PV data.Although actual. 4 is about undoped samples, the experimental results of doped thin films are not much different.According to the high temperature slope of Arrhenius curve at medium electric field intensity, the maximum activation energy?70meV。It is noted that in a medium with a dielectric constant of 3.5, the initial pair spacing of electron-hole pairs is 1.4 nm, and the Coulomb binding force of electron-hole pairs is 0.33 eV.One extreme case is the ladder-type poly ( p - phenylene ) LPPP. Under the electric field strength of 105 V / cm, the quantum yield of photo-generated carriers is only - 1 ( T4, and does not depend on the temperature [ 8 ].
If the dopant TNF is replaced by PDL with medium concentration, the phenomenon of sensitized photogeneration will not be changed, but PDL will act as both electron acceptor and singlet exciton acceptor of polymer matrix [ 36 ].The straight photogenerated current spectrum shows that the change of spectral energy does not completely quench excitons, and sensitizing photogeneration must involve PDL molecules.Since quantum yield and electric field strength are both dependent on TNF doping system, it can be considered that the charge transfer process of PDL sensitized photogenerating system is also similar to TNF system, then the precursor entity must be a coulomb force binding pair of PHP PV cation and PHI Ming ion, and further separation needs to take place with the help of electric field strength.If the dopant concentration exceeds 10 %, the dependence of quantum yield on the electric field strength will shift to a lower electric field strength range, while the quantum yield will saturate in the medium electric field strength range and no longer depend on the electric field strength.Actual. 5 indicates that the measured quantum yield will increase significantly in the range of 1 % 10 % doping concentration at a medium electric field strength of 5x105 v / cm.Obviously, the probability of twins separating in these specific systems will increase by two orders of magnitude.Because these phenomena are of great significance to organic solar GEL Battery ( see section 8.2.3 ), it is very worthwhile to measure the dependence of sensitizing light productivity on sensitizer concentration in C60 doping system [ 32 ], requiring a larger sensitizer concentration range to determine the conditions for the relevant threshold effect.
Monte Carlo simulation
Sensitized photogenic experimental results in doped conjugated polymers cannot be simply explained by Braun' Samandent of Onzag's theory [ 37 ] ( see 8.3.1 ), although Onzag's theory is about photogenic twin electron-hole pairs [ 38 ].The key problem is that the dependence of quantum yield on temperature is very weak, which is inconsistent with the strong dependence on electric field.A direct guess is the disorder characteristic associated with random systems.It has been confirmed that charge " jump" transport occurs in an energy dispersive system with Gaussian state density distribution, and the carrier mobility has a temperature dependence of Arrhenius type.The " jump" mode of transport exists in disordered semiconductor systems. Because the energy bands of disordered semiconductors are discontinuous, carriers can only jump in them.The reason for this phenomenon is the local carrier thermal activation transition from the state density tail state to the energy state near the so-called " transport energy" [ 39' 4 ].Because the state density energy decreases with the decrease of temperature under quasi-equilibrium conditions, the activation energy itself is proportional to 1 / to, which is not only limited to the movement of carriers, but also related to the initial generation stage of carriers.
One proven example is the injection of random electrolyte carriers from metal electrodes [ 4143 ].Since the transport energy decreases with the decrease of temperature, and assuming that the carriers are negatively charged, the energy required for injection also decreases with the decrease of temperature.As a result, if there is no disorder, the temperature-dependent injection current will be weaker than expected.By analogy, we also expect a carrier released from a twin electron-hole pair to inject energy closer to the state density to consume less energy, thus relying less on temperature.
The above conjecture has been confirmed by Monte Carlo simulation [?］。In a jump system with Gaussian state density, variance ( variance, at 0.05 - 0.15 eV, dielectric constant E is 3.5 ) produces electron-hole pairs with a certain distance composed of static positive charges and moving negative charges, and assumes the jump probability to be Miller - Abraham probability.Taking the probability of final recombination as the probability of jumping to a lower energy level, the escape probability of carriers leaving twin electron-hole pairs ( escape probability, po actual. 6 ) is mostly a function of 103 k / t, indicating that disordered jumping system has a great influence on the separation of electron-hole pairs.In Monte Carlo simulation, it is assumed that the initial pair spacing Ra - 2.4 nm. Gaussian state density variance Cr = 0.1 eV, dielectric constant ￠ 3.5, and electric field strength F = 105 V / m of electron-hole pairs.In fact, Dash - Dotted Line predicted the 3 - D Ansag theory of twin pair separation.In the high temperature region, the quantum yield 7 is close to the expected Arrhenius curve, while the activation energy is 0.168 eV, and the dependence of quantum yield on temperature 7 becomes flat and even almost constant in the low temperature region.Even if the initial pair spacing of electron-hole pairs is reduced from 2.4 nm to 1.2 nm, such a flat curve in the low temperature region will still appear.Illustration shows that at temperature t = 250 k, escape probability will increase with Gaussian state density variance.The data points with Gaussian state density variance < 7 = 0 are calculated by Onzag's theory.The variation of Gaussian state density variance < 7 given in the illustration shows that the quantum yield increases exponentially with the degree of disorder relative to the expected value of a fully ordered system.
Actual. 7 gives the dependence of escape probability # on electric field strength f.In Monte Carlo simulation, the initial electron-hole pair spacing R0 = 2.4 nm, Gaussian state density variance A = 0.1 eV, and temperature T = 300 K are assumed.The solid and dashed lines represent the electron-hole pair spacing r, = 2.4 nm and ″, respectively.= 3.1 nm 3 - D Ansag theoretical prediction.The premise of calculation is that all carriers move in the direction of electric field intensity from opposite charges.The quantum yield 7, which depends on the electric field strength f, can be fitted by the three-dimensional ansag theory, only assuming that the electron-hole pair spacing is 3.1 nm instead of 2.4 nm.Here, it is worth emphasizing once again that the dependence of stable photogenerated current on electric field strength and temperature depends only on the generation of carriers rather than transport, because all carriers must eventually reach the electrodes unless bimolecular recombination occurs or they are trapped by " infinite depth" trap states, which is a key issue to pay attention to when considering the conversion efficiency of organic solar GEL Battery.Bimolecular recombination refers to the recombination of carriers between two molecules, that is, the recombination of electrons of one molecule with holes of another molecule.
If we consider the disordered system of conjugated polymers doped with medium and low concentration sensitizers, we can at least quantitatively give the dependence of the sensitized photogenerated electric field intensity on temperature [ 45 ].However, it is still a question whether the disordered effect alone can satisfactorily explain the experimental results.At least for Mel - substituted lad - type poly ( p - phenylene ) films, disordered systems are not sufficient to explain the dependence of quantum yield on temperature.Because of the hard polymer backbone, MEL PPP is the system with the lowest degree of disorder.If the temperature is only reduced from 295 K to 120 K, the photogenerated current measured in the S, * - SO transition region will be reduced by a factor of 2. Other mechanisms that can explain this result are:
The triplet exciton causes the optical release of the moving carriers ( holes ) of twin pairs.
Consider the zero vibration of moving carriers in polymer chains [ 47 ] ( see section 8.3.1 ).8.2.3 Carrier photogeneration at donor-acceptor interface In the previous section, we discussed sensitizing photogeneration in random organic systems, especially conjugated polymers with uniformly doped negative sensitizers.It has been confirmed that even at a moderate dopant concentration ( 0.1 % 1 % by weight ), almost all absorbed photons are converted into twin electron-hole pairs through energy transfer from the excited donor to the acceptor and subsequent charge transfer [ 33 ].However, the carrier part that escapes from the twin pairs is strongly dependent on the strength of the collected electric field.At a medium electric field strength of 10SV / cm, the typical quantum yield is only 10 - 3 [ 3 ], which is not conducive to the application of organic solar GEL Battery.
In a specific system, PPV copolymer doped with PDL was observed, but the quantum yield was close to the PDR concentration of the received electrons?20 % and increased significantly [ 36 ].On the other hand, it was found that almost all optical excitations have been harvested by dopants with lower doping levels.Therefore, we can conclude that this increase is only due to the improved separation of twin electron-hole pairs.The evidence to prove this viewpoint comes from the dependence of photon - generated carrier quantum yield 7 on electric field strength f at different doping concentrations.When the critical concentration exceeds 30 %, the quantum yield is in?Saturation occurs at 3x105 v / cm.The device structure is ITO / PHP PV: PDI / Al, the temperature is 293 K, the excitation wavelength is 550 nm ( 2.25 eV ), and the PHI dopant concentration is 1 % 40 %.The saturation electric field intensity depending on the electric field intensity shifts, which indicates that at higher acceptor concentration, the initially generated electron-hole pairs increase.This is very important for the operation of organic solar GEL Battery.This effect and other related aspects of organic solar GEL Battery will be discussed below:
Distinguish the role of energy transfer center or charge transfer center of dopant;* Generation of free carriers;
The recombination of free carriers.
According to energetics, the key requirement for photogenerated electron-hole pairs at the donor-acceptor interface is that the energy of the electron-hole pairs is lower than that of the lowest excited singlet state of the donor or acceptor.Otherwise, the energy transfer will focus on the lowest energy singlet and then radiation attenuation will occur.This is also the premise of effective electroluminescence of light emitting diodes ( LED ).Although such a concept is relatively simple, it is difficult to judge the material system that is most suitable for manufacturing LED or organic solar GEL Battery.The most difficult aspect is that it is difficult to know the exact energy level distribution of electron-hole pairs.Although it will be reduced by the coulomb bound energy of electron-hole pairs, the most important energy level distribution is the difference between the following two energy levels:
Highest occupied orbit ( HOMO ): donor cation group determined by ionization energy;Lowest unoccupied orbit ( LOWENCOCUPIEDMOLOLARRORBITAL, LUMO ): The acceptor anion group determined by electron affinity.
In fact, some people often make the estimated energy of electron-hole pairs depend solely on the measured HOMO level oxidation potential and LUMO level reduction potential, ignoring the Coulomb bound energy of electron-hole pairs.More unfortunately, the oxidation potential or reduction potential is often obtained by cyclic voltammetry, and then the other is given by the s so0 - 0 optical transition.The so-called cyclic voltammetry is to make the potentiostat select a certain initial potential in a controlled potential state, then make the electrode potential change linearly with time according to the specified direction and speed, and automatically return to the initial potential at the same speed after reaching a certain specified potential, while recording the relationship between polarization current and polarization potential.The neglected excitation singlet electron-hole pair coulomb bound energy tends to be > 0.5 ev.On the other hand, even if people can simultaneously measure the oxidation potential and the reduction potential by cyclic voltammetry, they will encounter systematic errors due to the charge transfer from these data measuring electrodes to molecules / polymers.However, the cationic / anionic dissolution energy in the polarized solution is higher than that in the solid electrolyte, because the so-called dissolution layer of the solid electrolyte without the solvent layer O is a chemical layer with solvent function around the outside of the solute substance, and the dissolution layer of the aqueous solution is the hydration layer 0. The conclusive test to determine whether energy transfer or charge transfer is dominant in the donor-acceptor system requires the use of spectra.Only by simultaneously measuring the quenching of donor fluorescence and the initial phase of acceptor fluorescence or the transient absorption of cationic / anionic groups can energy transfer or charge transfer be strictly determined.If the dopant has a high electronegativity, for example, C6.Or TNF, whose LUMO is at least 1eV lower than that of common donors such as PPV or polyfluorene systems, then there is no uncertainty about energy transfer and charge transfer.
When the energy level difference between donor and acceptor is less than 1eV, an interesting case of excimer formation will occur.The exciplex is an exciton located close to the interface and is still attached to a molecule in fact.Affected by the surface, the exciplex experiences a different environment from excitons in vivo, so photoluminescence moves slightly toward the red band, and the surface state makes the exciplex more stable and has a longer lifetime than excitons in vivo.The intermolecular properties of exciplex show that the radiation has a smaller oscillator intensity, while the radiation spectrum is relatively wide and has no obvious characteristics.The fluorescence decay time is usually > 10 ns, which is significantly longer than the fluorescence lifetime on the time scale of < ns for singlet excitation of uncoupled chromophores.The exciplex should not be confused with the charge transfer state or the twin electron-hole pair that completes the charge transfer.It is still unclear whether the transition process of exciplex to tightly bound electron-hole pairs is exothermic or endothermic, because both exothermic and endothermic phenomena exist.On the one hand, the spatial expansion of electron-hole pairs formed by exciplex will reduce Coulomb bound energy.On the other hand, when only part of the state of charge transfer is converted to the state of complete charge separation, the electron polarization energy in the embedded polarized organic solid will increase.
The importance of exciplex in donor-acceptor solar GEL Battery has been widely recognized [ 49 ].The spectroscopy study conducted by the Cambridge University team in England described this phenomenon in more quantitative detail' using PFB or a mixture of TFB and F8BT [ 5' 51 ].
Electron donor PFB: Poly ( 9,9 - Dioctyllforene - Co - Bis - N, N - ( 4 - butyl pheny ) - Bis - N, N _ pheny - 1,4 - phenylene Diameter;
Electron donor TFB: Poly ( 9,9 - Dioctyllforene - Co - N - ( 4 - Butyl Phenyl ) Diphenyl Amine;Electron acceptor f8bt: poly ( 9,9 - dioctyllforene - co - benzohiadiazole ) o through spin coating of mixed solvents, the usual delamination phenomenon of the two polymers is reduced under the condition of rapid drying, while in the film formed, there is a large interfacial area between the two polymers.The experimental results show that the PFB: F8BT hybrid system exhibits higher charge separation yield ( 4 % external quantum efficiency of photogenerated current ) and lower electroluminescent efficiency ( < 0.64 lm / w ).However, TFB: F8BT has higher electroluminescent efficiency ( 19.4 lm / w ) and weaker photovoltaic response.Fluorescence studies show that interface excimer is formed in both systems, and the evidence is that the broad radiation spectrum of excimer shifts to red light compared with the common f8bt superposition radiation spectrum.Different characteristics show the importance of fine tuning of the energy levels of the two polymer components.In PFB: F8BT system, the burning light of the monomer near 530 nm is weak, and the related activation can reach 200 50mev.In TFB: F8BT system, the radiation spectrum of monomer and excimer is similar, and the activation energy of monomer radiation is only 100 Shi 30 MeV.Since the twin pair life of the exciplex reaches 40 5 ns, the photoluminescence intensity IPL of PFB: F8BT hybrid device and the photoluminescence intensity FFL ( - △ PL, solid line ) due to the application of IOV reverse bias are reduced at 340 K, only the spectra ( hollow circle ) of F8BT device are compared, PL, - APL and PLR AIR are plotted on the same scale to reflect their relative intensities;- 4PL spectrum ( reverse bias 10V ), same as ( D ) but different in temperature;At 340 K, the JPL ( solid circle ) of TFB: F8BT hybrid device and the - ( solid line ) spectrum of 15V reverse bias only have the PLFBOT spectrum of F8BT device ( hollow circle > as comparison;APL spectra ( reverse bias 15V ) are the same as those of the same device, but different temperatures can be used as excitation storage with a longer twin pair life, thus regenerating the singlet state of F8BT or completely separating it.If more excitons are regenerated by temperature activation, the system is suitable for electroluminescence, otherwise the system is suitable for use as an organic solar cell.
Other mixed systems forming excimer complexes have also been studied and confirmed by spectroscopy [ 52 ].
Electron donor MDMO - PPV: Poly ( 2 - Methoxy - 5 - ( 3,7 - Dimyloxyloxy ) - 1,4 - Phenylenylene );Electron acceptor PCNEPV: Poly ( OXA - 1,4 - Phenylene - ( 1 - Cyano - 1,2 - Vinylene ) - ( 2 - Methoxy - 5 - ( 3,5 - Dimyloxyloxy ) - 1,4 - Phenylene ) - 1,1 - ( 2 - Cyanonylene ) - 1,4 - Phenylene ) O Actual 10 shows that the absorption spectrum of the mixed system is the superposition of the absorption spectra of PCNEPV and MDMO - PPV, while the peak value is at 1.85 eV.Unexpectedly, the initial lifetime of the hybrid system is only 1.6 ns, which is an intermediate value between MDMO - PPV fluorescence lifetime of 0.36 ns and PCNEPV fluorescence lifetime of 14 ns.In fact, the fluorescence emission intensity of the exciplex of the mixed system in 10 was amplified by 34 times compared with PCNEPV. Such a low radiation yield indicates that recombination is dominant.Some people may guess that this is the separation of coulomb bound electron-hole pairs, that is, the real charge transfer state, and some of the loss of coulomb bound energy is overcompensated by the increase in electrical stable energy.However, this is not consistent with the observed decrease in emission yield of the applied electric field excited complex.
Actual. 11 shows that the transition from the exciplex to the charge transfer state is a weak endothermic process rather than an exothermic process.
S: singlet state;
T: triplet state;
CS: Charge Separation;
ET: Energy transfer;
ISC: The phenomenon that the spin states of atoms or molecules in excited states change nonradiative inactivation does occur in pure materials, but they are quenched in mixed systems.
Up to now, people are still not clear about the reasons for the lower yield of excimer and the lower conversion efficiency of organic solar GEL Battery.According to these results, we can think that the exciplex formed at the interface of the donor-acceptor hybrid system is relatively unfavorable to the conversion efficiency of organic solar GEL Battery, especially when the exciplex energy exceeds the donor or acceptor triplet energy.If the energy level of the exciplex is close to the charge transfer state, frequent charge exchange will occur, and accordingly, the exciplex will effectively generate a triplet state.The exciplex will immediately decay into a low-energy triplet state of one component, so that the exciplex will lose 220.127.116.11 quantum yield through photo-generated carrier generation or electroluminescence.
An important condition for realizing high conversion efficiency organic solar GEL Battery is that as many absorbed photons as possible can generate electron-hole pairs and can be completely separated at last.The donor-acceptor system of two or more components can satisfy such conditions, while the single component system has a larger exciton binding energy, which is not conducive to separation.Since the excited state initially generated is a singlet exciton of the donor phase or the acceptor phase, the exciton needs to diffuse to the charge transfer center.Although the exciton diffusion length in the conjugate polymer reaches 10 - 20nm [ 29 - 53 ], it is still not suitable for a 100 nm thick bilayer combination and requires exciton separation of the hybrid system to occur in the entire sample.
If both components are polymers, lower entropy after mixing will cause phase separation and excitons will diffuse to the internal interface between the two components.The ratio of PFB to F8BT in the mixed system of PFB: F8BT was changed from 155 to 5: 1 [ 54 ]. Fluorescence quenching studies showed that all excitons reached the internal interface regardless of the composition ratio.The conversion efficiency of organic solar GEL Battery depends on whether twin electron-hole pairs are separated or not.If excitons have diffused to the charge transfer center, or light absorption and carrier photogeneration happen just near the charge transfer center, charge transfer is a very fast process.By monitoring the transient absorption TA, it has been confirmed that electrons move from the MEH - PPV chain to nearby C6.Molecular transfer costs only 45 ￠ S15.
In a two-component donor-acceptor system, the conversion efficiency of organic solar GEL Battery measured by photo-generated current method depends on the acceptor concentration and the morphology of the sample.The conversion efficiency depends on the separation yield of electron-hole pairs generated by photons, and the separation of electron-hole pairs is carried out at the charge transfer center under the action of an external electric field or a built-in electric field in the diode.On this phenomenon, several important conclusions can be drawn:
* Increasing the acceptor concentration of PHP PV: PDL system from 0.1 % to > 30 % indicates that the separation yield increases by more than 2 orders of magnitude with the increase of PDI concentration.This is consistent with the dependence of the quantum yield on the electric field strength f in the actual .8 ..
The improvement of the phase structure of the hybrid system has a positive effect on the conversion efficiency.The methods to improve the phase structure include thermal annealing in MDM 0: PCNEPV system or composition adjustment in PFB: F8BT system.
The external photogenic quantum yield of organic solar GEL Battery was studied. The organic absorption layer of the sample was OQQQO - PPV ( poly ( 2 - Methoxy - 5 - ( 3 - 7 - DIMYL - OCTYL OXY ) - 1,4 - Phenylenylene ) ) and soluble fullerene derivative PCBM ( 1 - ( 3 - Methoxy Carbonyl ) Propyl - 1 - Phenylel - ( 6,6 ) - C61 ), and the organic absorption layer was sandwiched by PEDOT ( 3,4 - Ethylethenene ) in a sandwich manner.The dependence of quantum yield on human light intensity, temperature and applied electric field was measured.If the human light intensity changes within the range of 0.02100 MW / cm2, the quantum yield is almost a constant.In the temperature range of 120325 K, the quantum yield is weakly dependent on the temperature and the activation energy is 50mev.
The temporal exponential dependence given by the transient optical study of charge recombination dynamics in the MDMO - PPV polymer and PCBM hybrid system extends to the MS range.
There are three characteristics that may explain the phenomenon that the quantum yield greatly increases with the donor phase concentration exceeding 20 %.
The final monomolecular recombination of the donor cationic group and the acceptor anionic group;Electron - hole pairs are more likely to escape from Coulomb potential later, because charge transfer is easier when the transport penetration limit is reached.Coulomb attraction is completely irrelevant, and the limited quantum yield is caused by carrier recombination in trap state and low carrier mobility caused by imperfect morphology of active layer.
Carrier mobility must have an impact on the performance of organic solar GEL Battery.When the concentration of electron acceptor is low, the generated electrons will be trapped by deep traps.Finally, the space charge will accumulate and generate feedback to the charge to enhance the recombination of the bimolecular charges.Opening the permeation path for minority carrier movement can alleviate this problem.On the other hand, even if the penetration threshold is exceeded, the movement of electrons and holes will be asymmetric.This depends on the energy levels of the constituent intrinsic HOMO and LUMO because they determine whether impurities will become traps.For example, in the case of fullerenes and PPV derivatives ( OC!G。- PPV [ 56 ], the mobility of electrons is greater than that of holes because C6.Extremely low LUMO prevents any possible impurities from trapping electrons.For F8BT such electronegativity is less than ( 36.The situation will not be the same.However, it is also believed that in the PFB: F8BT system, the effective collection of charge by the electrode requires that the conduction path exist in the constituent phase with a small proportion of components.In fact, we found that the charge collection efficiency is proportional to the surface area of the interface between the medium-sized PFB - rich phase and the F8BT - rich phase formed by phase separation.This requirement can be met when charge generation occurs near the interface between the two separated phases.It has also been suggested that medium-scale phase separation on the order of film thickness can optimize the morphology of these polymer hybrid devices.This will maximize the interfacial area of the separated phase and establish a network for the charge to move toward the collecting electrode.
It has been confirmed by experiments that the condition under which the maximum charge-light productivity of these systems can be achieved is usually that the acceptor / donor ratio is as high as 3: 1 [ 57 ], which is significantly higher than the threshold of electron penetration and phase separation.Moreover, the 3: 1 polymer / C6 with the highest conversion efficiency has recently been confirmed.The mixing of derivatives does not show complete phase separation on the polymer side: such a mixing system contains pure g.Derivative acceptor phase and about 1: 1 polymer 8: 6.The mixture acts as a donor phase [ 58 ].Although it is not clear whether such a polymer / C6.The mixture has reached uniformity at the 25 nm scale, but the high concentration of strong electron acceptor will definitely greatly reduce the twin pair lifetime and the accompanying exciton diffusion length in any case.
By thermal annealing or adjusting the composition of the hybrid system, the improved morphology can also affect the initial generation of twin electron-hole pairs, and even become a key factor e % for improving the conversion efficiency of organic solar GEL Battery.This view shows that the initial generation of individuals is coulomb bound rather than completely free electron-hole pairs.Twin electron-hole pair recombination is a strict single molecule process.Coulomb bound hypothesis can be supported by the following facts:
In organic systems, the dielectric constant is in the range of 34, and coulomb force interaction is more important.The conversion efficiency of organic solar GEL Battery depends linearly on the incident light intensity, which indicates that carrier recombination is a first-order reaction rather than a second-order reaction in dynamics.
The so-called first-order reaction means that the rate constant depends only on the concentration of one reactant, while the presence of other reactants has nothing to do with the reaction rate constant:
In PHP PV: PDI system, it can be observed that the dependence of quantum yield of photo-generated carriers on electric field intensity moves from higher PDL concentration to lower electric field intensity, which indicates that the main charge transfer process will change at higher acceptor concentration.This dependence of electric field strength is controlled by the diffusion and escape of electron-hole pairs from Coulomb potential.If it is corrected by disorder effect, it needs to be described by Onzag's theory ( see 18.104.22.168 section ).The dependence on the electric field strength, especially when the quantum yield is saturated, depends on the initial pair spacing of electron-hole pairs.The fact that the saturated electric field intensity moves to a lower value ( see actual. 8 ) shows that the electron-hole binding degree is looser at high PDL concentration than at low acceptor concentration.Since the separation of electron-hole pairs with a spacing of 2.4 nm saturates at an electric field strength of close to 10sv / cm, the 3x105 v / cm saturation electric field strength observed at PD concentration > 30 % indicates that the initial pair spacing of electron-hole pairs reaches?7nm。In practice. 13, the saturation electric field intensity is converted into a function of the initial pair spacing of electron-hole pairs with respect to PDL concentration.In a system with a dielectric constant of 3.5, if the electron-hole pair spacing is 7 nm, the coulomb bound energy is 60 mev, which can be compared with oc &.- PPV: The activation energy of PCBM mixture is compared with M ..
Although it seems unreasonable to conclude the above conclusions, at least in PHP PV: PDL system, the limiting factor of conversion efficiency is obviously the thermalization length of the so-called electron-hole pair in a system higher than the critical acceptor concentration.The mechanism of this phenomenon remains to be studied.The development of conceptual theoretical research will be discussed in section 8.3.2, while the dynamics of twin pair recombination will be discussed in the next section.
Twin electron-hole pair
The latest major research progress on the recombination and separation of twin electron-hole pairs at the donor-acceptor interface is the key assumption in the simulation work of Offr Mans et al. that optical excitation produces the nearest adjacent electron-hole pairs, while the electrons and holes are in the acceptor phase and the donor phase, respectively.The dielectric constants of the donor phase and the acceptor phase are both 4 and are both characterized by a cubic lattice with a lattice constant of 1RUN.Each jumping point forms a Gaussian state density distribution, which is characterized by different variances cre = 70 mev and ah = 120 mev.The changing electric field intensity acts vertically on the interface and jumps to follow Miller - Abraham probability.Unlike earlier simulations by the team at Philippe University in Marburg, Germany, 443, the simulations by Offr Mans et al assume that the nonradiative recombination rate of electron-hole pairs is 1 ns - 1, while the jump rate between the front factor points is 10ps _ 1.In the simulation, it is necessary to independently monitor the ratio of holes and electrons near the interface, the distance between holes and electrons and the interface at lower or higher electric field strength, and the average energy in state density as a function of time.For the electric field strength f > 2x1 o 5v / cm, the separation yield is greatly increased and saturation occurs when the electric field strength f = 2x106 v / cm.If the original temperature-dependent data in document [ 60 ] are redrawn according to Arrhenius curve, a dependency similar to the actual .6 will be obtained.In the high temperature range, the quantum yield is close to the activation characteristic and has an activation energy of 50mev, while in the low temperature range, the dependence of temperature is weak.
The simulation results can be interpreted as random movement of particles in disordered lattice.Initially, carriers were bound by coulomb force and localized at the neighboring donor-acceptor sites.Carriers can then leave another carrier with an opposite charge in the exciton, provided that the loss of coulomb bound energy is compensated for by jumping to a lower energy point.In a system with a dielectric constant of ￠ 4 and a lattice constant of 1 nm, the compensation energy is as high as 0.18 eV.It is relatively simple for holes to reach such compensation energy, and only 1.5 ( JH ) for hole variance ( 7H = 120 MeV, 0.18 eV ).However, such compensation energy is difficult for electrons with variance = 70 MeV, and the probability of finding a transition point at a deep level below 0.18 eV in the center of state density is very low.In order to make such a process more efficient, holes need to jump away from the interface, so the coulomb attraction of electrons will be reduced.However, such simulation cannot reach the experimental observation result of PHP PV: PDL system, that is, when the acceptor concentration is > 10 %, the carrier yield is saturated at a lower electric field strength.
Such reasoning can form a step-by-step model of twin pair separation at the interface:
The hole jumps to a lower energy point and leaves the interface.
The probability of recombination of electrons and holes is greatly reduced. The expanded electron-hole pair is believed to be in metastable state and can be separated by the diffusion motion of electrons, which is a potential well formed by coulomb attraction exerted by holes at a longer distance.
At the same time, cavities may jump back to the interface, promoting rapid twin pair recombination._ _ This simulation reflects the performance of complex systems under set assumptions.Correspondingly, the performance deviation of real samples can be used to counter - embroider the real enemies in the county.Acid.Tepeng, Kurn Sheena's paste samples were compared' the samples are ITO: pedot / MDMO: ppvpcbm / lif / al organic solar GEL Battery studied by dyakonov [ 59 ].The simulation results can also be compared with 70 % PHP PV: PDL system optical conductivity data.The comparison results show that the photoconductive yield has been saturated under the lower electric field in both cases.In the first case, the internal electric field strength?2x105 v / cm, assuming the built-in voltage of a 10 nm thick sample is 2v, then the collection efficiency of each absorbed photon?50 %, which is similar to the quantum yield reported in actual. 5.We hypothesized that the main difference between the real sample and the simulated sample is the increased electron-hole pair spacing at the internal interface.We believe that this effect is due to the ultra-fast movement of holes inside the conjugated polymer chain.However, only when slower carriers ( electrons ) can quickly leave the interface can such effects be beneficially utilized.This requires the establishment of a permeation network, thus requiring the sample structure to be suitable for charge transfer.
Transient absorption TA provides a method to study the photogenerated carrier dynamics at the donor-acceptor interface.As mentioned in the 22.214.171.124 section, the formation of twin electron-hole pairs at the interface is an ultra-fast process that occurs at a time scale of < 100 fs.Any delay in this process must be due to the diffusion of the main excitation to the interface.TA recorded at a longer time scale can monitor subsequent charge decay.From an operational point of view, it is not important to regard carrier absorption as a measure of recombination kinetics according to time. Usually, the monochromatic hole consumption of the cation group occurs in the S SO0 - 0 transition of the neutral donor phase, and the transition shifts from the original spectral position due to the charge.Unfortunately, these spectral characteristics do not allow people to determine whether there is a Coulomb bound between the charge and the other carrier of the exciton, because the Coulomb electric field strength will only cause a slight spectral shift.These spectral characteristics are intramolecular characteristics related to larger non-uniform broadening.This is a serious problem in analyzing experimental data.For example, 1011 bit 11 analyzed the time decay of positively charged MDMO - PPV polaron ( cationic group ) in MDMO - PPV: PCBM system with [ 61' 62 ], studied local charge recombination, and ignored their coulomb interaction.Their time decay follows the algebraic law of factory 1 / 2 and is suitable for hole recombination of tail relaxation to state density distribution.In the same MDMO - PPV: PCBM hybrid system, OFFER MANS et al [ 63 ] measured the decay of monochromatic hole consumption in the Si * - SO0 - 0 transition tail state.The excitation photon energy is 2.76 eV, the pump energy density is 2.0 mJ / cm2, the experimental temperature is 80 K, the acquisition delay time is 10 ns - 10 ms, and the gate pulse width is 2300 ns.
The time decay of monochromatic hole consumption is also similar to the broad 1 / 2 algebraic law, which is consistent with the reference [ 61 ]. Carrying useful information about local hole spectral relaxation shows that the decay of reactants follows a similar law, while the reaction rate more or less follows the plant 1 law [ 3 ].Later studies also confirmed the plant 1 rule of reaction rate, especially the delayed burning attenuation m emitted by the single-component stepped poly ( p - phenylene ) lppp.In this case, it was concluded that the delayed fluorescence was actually confirmed by the experiment under a certain electric field intensity, because the twin pairs of electron-hole pairs were recombined, rather than the double annihilation of two triplet excitons, while the electric field intensity only acted on the charge and did not act on the neutral exciton.
The organic solar cell with high conversion efficiency obviously requires all carriers escaping from the initial coulomb potential well to reach the electrode so as to be completely collected without any further loss.This set a standard for the recombination of bimolecular non-twin pairs, indicating that the transition time of carriers must be less than the recombination time of bimolecular [ 65' 66 ].Assuming the simplest case, the mobility of electrons and holes is equal, what = = and the concentration of electrons and holes is the same, called =, the generation and loss of carriers satisfy the continuity equation:
From inequality ( 8.16 ) and inequality ( 8.18 ), we know that carrier mobility needs to be sufficiently high and electron mobility and hole mobility need to be close.Otherwise, carriers with lower mobility will control the bimolecular recombination of escaping twins to complex carriers.Redox gel battery
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