The batteries made by Sunbright include backup batteries applied in telecommunications, Power Plant, UPS battery, fire alarm system, emergency lighting and efficient energy storage batteries used in solar energy, wind energy and, as well as motive power batteries for electric vehicles, golf carts, electric forklift, electric traction trucks and other fields. All products are CE certificated, UL certificated, and TLC, ROSH certificated.
Recently, the dependence of photo-generated carrier mobility and recombination on time in organic SOLAR POWER BATTERY has been further studied [ 68 ].The active layer is prepared by spin coating, with MDMO - PPV as the hole acceptor and PCBM as the electron acceptor. The research method is the linearly increasing voltage charge extraction ( CELIV ) technique.At first, Celiv technology was developed to measure carrier transport in the dark. Carriers can be extracted by applying a linearly increasing voltage [ 69 ].In document [ 68 ], 3 ns laser pulses that can be highly absorbed can inject people to generate carriers.The temperature ranges from 120 K to 300 K ..In order to prevent injection from the electrode, reverse bias is applied after adjusting the delay time ( ZDD ) in the range of 1m11s.Within the delay time, carriers that have escaped twin pair recombination and have avoided bimolecular recombination have been detected.The results show that the dependence of carrier mobility on time follows the law of power function algebra, which is a characteristic of the tail relaxation of carrier energy distribution to Gaussian state density in an energy disordered organic jump system.The bimolecular recombination coefficient / 3 is no longer a constant, but depends on time and also follows the law of power function algebra because ( the gate is controlled by the diffusion of carriers.The experimental results show that the cold ⑴ / > ( 0 ratio does not depend on time and is close to Langevin's theoretical formula ( 8.15 ).
However, RR - P3HT ( 3 - hex yl - thiophene ) and PCBM with a mixing ratio of 1: 2 have completely different characteristics with respect to photogenerated carrier bimolecular recombination 11661.The sample thickness is 80100 nm, the product w of the absorption coefficient and the optical path length is 4.4, and the optical density CD is 1.9 according to formula ( 8.1 ).The transient photo-generated carrier response was measured. The laser pulse excited the sample was 3 ns and the maximum energy was 0.3 mJ / pulse. The measurement condition was that the resistance-capacitance time constant was much larger than the carrier transition time.At the highest human intensity, the normalized ratio of extracted charge to capacitance charge is close to 30. According to the extraction time, the bimolecular recombination coefficient cold = 2x10 - 13cm3 / s is calculated.Combined with the independently measured maximum carrier mobility " = 4x1 ( T3cm2 / VS, the sample is the same organic solar cell, and the test method is time-of-flight TOF technology.TOF measures the time it takes for particles, sound waves, electromagnetic waves, etc. to move a certain distance in a certain medium. Moving objects can be detected directly by ion mass spectrometry or indirectly by light scattering by laser Doppler velocimetry ( LDV ).The results show that bimolecular recombination coefficient and mobility can easily calculate the ratio = 5XIO " 11 VCM, which is about 4 orders of magnitude smaller than the ratio of 9 / given by Langevin's theoretical formula ( 8.15 ).This result well illustrates the great influence of donor-acceptor mixed morphology.Obviously, the self-alignment of the region regular arrangement instead of applying polythiophene chains will greatly reduce bimolecular recombination, as this will form a continuous interpenetrating network, thus establishing independent paths for electrons and holes.This will lead to a guess as to whether such a phenomenon is related to the decrease in the recombination rate of twin pairs that initially produce electron-hole pairs in the appropriate mixed composition.In any case, this confirms the important role of mesoscopic order in the mixed structure of organic SOLAR POWER BATTERY.
Theoretical Model of Exciton Separation
Before we begin to discuss the theoretical model of organic SOLAR POWER BATTERY, we need to summarize the experimental results.The exciton theory of conjugated polymer photon physics explains free carrier photogeneration in this way.By absorbing the incident light in the energy band, the conjugated polymer is excited to form a Frenkel singlet exciton?The binding energy EB is about 0.5 eV.The Frank - Condon state produced by photon absorption can be electron vibration relaxation, or human photon energy exceeds absorption edge and " heats up".Through the vibration process, the hot Frank - Condon state relaxes, the excess photon energy is transferred to the vibration excitation part, and excitons occupy the hot part.The condition for fast separation of these excitons into free electron-hole pairs is a medium external electric field strength [ 7173 ], while the electric field strength required for intrinsic separation of relaxed excitons is in the MV / cm range.However, the relaxed singlet exciton may be quenched and separated into twin electron-hole pairs at the charge transfer center.Typical charge transfer centers in conjugated polymers contain conjugated moieties and adjacent electron scavengers, i.e. deep electron traps.If the electron affinity of the electron scavenger is large enough to compensate for exciton binding energy, exciton separation at the charge transfer center may be achieved.
However, exciton quenching at the energy transfer center does not generate free carriers.On the contrary, twin electron-hole pairs with strong coulomb binding will be produced.It is estimated that the coulomb bound energy is about 0.5 eV, the distance between the conjugated moiety and the electron scavenger is about 1 nm, and the dielectric constant is about 3.This estimate shows that the binding energy of twin pairs is stronger than that of vibrational relaxation singlet excitons.This explains the troublesome important difference between photoluminescence quenching efficiency and photogenerated carrier generation rate in doped conjugated polymers [ 33' 36 ].When the photoluminescence intensity drops to about 0 when the quencher concentration is 1 %, effective charge generation requires a doping concentration greater than 10 % [ 31 ], and the probability of free carrier photogeneration in donor-acceptor mixing is much higher than that of uniformly doped polymer.
Ansag - Braun model
It seems to be possible to directly explain the photogeneration of carriers in donor-acceptor organic SOLAR POWER BATTERY by using the Onsager - Braun model, which is the twin pair correction of Onsager's theory.In 1934, Lars - Onzag proposed a theoretical model for infinite-life ion separation for weak electrolytes [ 74 ].In 1938, Onzag explained the problem that radiation-induced charge pairs recombine as soon as they collide with each other, that is, infinite sinking approximation [ 38 ].In 1984, Charles Braun considered the limited life of twin pairs [ 37 ], indicating that multistage drift of twin pairs can lead to complete escape from each other's Coulomb potential, or final recombination.This is related to hybrid organic SOLAR POWER BATTERY because the premise of effective photogeneration is that the donor-acceptor interface becomes the lowest energy level of the twin pair system and the twin pair has a limited lifetime.Prolongation of the lifetime of multistage drift twin pairs in any exciton - excimer - charge transfer state cycle will definitely increase the complete separation yield of twin pairs 7 ( 1, t ) depending on the electric field strength f and temperature t:
OQC with composition ratio of 20: 80.- PPV mixed with PCBM.The parameters used are: the spatial average of the relative dielectric constant < e > = 3.4, the initial pair spacing a = 1.3 nm, the limited twin pair life 1, and the dependence of the electric field strength in the experiment has repeatability.According to the temperature dependence measured under very low applied voltage ( 0.01 V ) and built-in voltage, the calculated activation energy is 94.1 MeV and 35.4 MeV, respectively, and then converted into electron-hole pair spacing of 4.511111 and 1211111, respectively, which is much larger than the average value of 1.5 nm based on the initial pair spacing " = = 1.311111".This difference is a common phenomenon in random organic systems and can be attributed to the intrinsic disorder structure ( see 18.104.22.168 Section ).Another problem is about the selected value of twin electron-hole pair or excimer precursor lifetime, which should be controlled by the nonradiative symmetry of twin pair and molecular vibration.The twin pairs of charge transfer centers have a life longer than that of excimer complexes, typically reaching 150 ns, which is still controversial and needs further experimental demonstration.
Although the disorder effect described by the Onzag - Braun model is self-consistent in parameterization and explains the photogeneration of carriers in bulk heterojunction organic SOLAR POWER BATTERY, people are not fully convinced by this theoretical model.At a critical dopant concentration that exceeds the photon harvesting and the full generation limit of the main electron-hole pair, the photogenerated carrier generation rate will suddenly increase, a phenomenon that cannot be explained by the Onzag - Braun model.The Onzag - Braun model can not explain the quantum yield increase in the morphology optimization system.In the following, we will further develop the exciton separation theory model to explain the exciton separation phenomenon that these Onzag - Braun models do not apply.
Zero Vibration Energy Model of Doped Polymer
At high dopant concentrations, the photo-generated carrier yield increases sharply, which can be attributed to two effects:
The formation of a random interpenetrating network is conducive to the leaping transport of electrons and holes.The donor phase and the acceptor phase start to separate partially or completely.so that better exciton separation will occur at the improved donor-acceptor interface.
Bipolar transport is obviously needed for complete carrier separation, but if exciton separation at the beginning of light harvesting is not effective enough, the quantum yield of organic SOLAR POWER BATTERY can hardly be improved.The complete exciton harvesting at the interface is the basic condition for effective carrier photogeneration, which requires the exciton diffusion length to be greater than or at least close to the maximum distance from the exciton to the nearest interface.By controlling the deposition of each layer, the optimized planar multilayer organic solar cell can meet such requirements.It is more difficult to control the morphology of donor-acceptor system of bulk heterojunction.
From the viewpoint of energy conservation, it is feasible in principle to separate relaxed excitons into free holes and localized electrons at the charge transfer center, provided that the energy level difference between the matrix and the lowest unoccupied orbital LUMO is greater than the exciton binding energy.However, this overall conclusion is not yet able to answer the questions raised by the relevant experimental results:
If the intermediate state is not included, the charge generation in the charge transfer center needs to be described as an adiabatic quantum mechanical transition. In the initial state, two carriers are bound in a singlet exciton on one chain, and in the final state, electrons are localized in the nearby LUMO. Holes occupy an on-chain state, and the distance between electrons and holes is further than the Onzag radius.About 20 nNi at room temperature.For exciton separation with a typical time of less than 100 fs, the corresponding matrix element is unlikely to be large enough.
If the energy requirements are met, the yield of excitons directly separated into free carriers should not depend on the external electric field strength or temperature.However, the measurement results show that the quantum yield is strongly dependent on the electric field strength, while the dependence on temperature is relatively weak, especially at low temperatures.
Because the charge transfer requirement is an adiabatic process, excess energy needs to be released directly and dissipated in the acceptor molecules occupied by the transferred electrons.Because the typical energy dissipation time is less than 100 fs, electrons are unlikely to escape from the coulomb potential well by jumping in such a short time, even when the acceptor molecule concentration is close to 50 % or more.Therefore, complete charge separation is possible due to the movement of the on - chain hole alone, but the value of the effective mass of the holes released in the deep-electric conjugate polymer is still unclear.Quantum chemical calculations performed in several polymers [ 14 ] show that we are about ( 0.030.08 ), where me is the free electron mass.Using such WEFF value in Equation ( 8.27 ) can get twin pair binding energy = - E0 close to zero, which means that twin pairs will be separated into free carriers immediately even in the absence of an applied electric field.However, the experimental study of dopant - assisted exciton separation reveals that quantum yield is strongly dependent on electric field strength [ 33,35.36 ].Another evidence that the effective mass of holes in the chain should take a larger value comes from the study of microwave conductivity of conjugated polymers?.The experimental results show that the mobility on the chain of the relaxed polaron is still not higher than 0.11 cm2 / vs, indicating that the effective mass of carriers is zne, while the mobility of the carriers just generated is as high as 1500 cm2 / vs.
We can explain the obvious contradiction between the calculation of the zero-point vibrational energy model and the experimental data, and we need to consider the difference in time scale between exciton separation and carrier transport in the chain.Exciton separation is a quasi-instantaneous process, and the state immediately after exciton separation is a molecular environment in which holes in the chain have not been structurally and electrically relaxed.In this case, the hole effective mass ZZM can actually reach the small value M given by the theoretical model of zero vibration energy.It is worth noting that the electric reflection experiment of single crystal polydiacetylene revealed the effective mass history of carriers in the chain.It is much smaller than the free electron mass 7 and is close to the theoretical calculation value of zero vibration energy [' 7 ].Since the time scale of polaron and vibration relaxation in the co-working polymer is less than 100 fs, we expect that the conductivity measurement at a longer time scale will only reveal the contribution of relaxation polaron.
Based on these facts, we can summarize the general process of exciton separation in charge transfer centers.Immediately after the negative charge is transferred from the polymer chain to the adjacent electron scavenger, holes in the non-relaxation chain with lighter effective mass occupy the ground state or an excited state of the potential well formed by the negative charge of the trapped electrons.Since the effective mass of the hole is still small before the polaron relaxes, the zero vibration energy must be relatively large, so the excess energy released in the deep electron trap is more inclined to the hole in the chain.In the fast polaron and vibration relaxation process of < 100 fs, the effective mass of holes in the chain increases to zn6, accompanied by the decrease of the zero vibration energy AE.According to equation ( 8.27 ), the reduction of zero vibration energy is:
Where " Shu" and " Shu" are respectively the transient effective mass and relaxation effective mass of holes.If = 0.03 ME, = 2ME, A = 0.5 nm, equation ( 8.29 ) gives AE 1.2 eV.Such excess energy will dissipate into the local vibration system, making this part of the system " hot" for the time being.As mentioned in the previous thermal exciton separation model, 72 ], holes in the chain will use this excess energy to escape the Coulomb potential well formed by the corresponding electrons and follow the Boltzmann thermal excitation release process.Since the initially occupied conjugated polymer fraction is " hot", the escape probability is only weakly dependent on the ambient temperature [ 723.However, the dependence of quantum yield on electric field strength remains strong because the applied electric field determines the height of the barrier, and holes need to exceed this height to be free.
It is worth mentioning that the above estimation of excess energy released by hole polaron relaxation in the chain can be consistent with other data, especially the typical excess photon energy required to start intrinsic thermal exciton separation [ 18' 78 ], as well as the electron affinity energy to provide effective exciton separation acceptor materials in organic donor-acceptor mixing.If the energy obtained by electron transfer is not large enough to provide enough energy for the zero vibration of temporarily lighter holes, we believe that the charge transfer process will be slower and directly generate holes or excimer complexes on the relaxation chain, both of which are not conducive to the final complete charge separation.In principle, such a zero-point vibration energy model viewpoint can explain the lower yield of excimer in MDMO - PPV / PCNEPV mixing ( see 22.214.171.124 Section ).8.3.3 Bipolar layer model of donor-acceptor interface As mentioned earlier, if the conjugated polymer is highly doped by electron acceptor ( polymer donor-electron acceptor mixture ), the photo-generated yield of intrinsic carriers can be increased to almost 100 %, provided that the morphology requirements are met [ 36' 57 ].In addition, such an effective exciton separation can be achieved even at moderate electric field strength and low temperature.This means a truly effective function to prevent twin pairs from recombination at the polymer donor-electron acceptor interface.The obvious difference between exciton separation in weakly doped polymer and strongly doped polymer is supported by some facts. Electron - hole pairs can be detected at low acceptor concentration. In the solution reaction of polymer, a large number of solvent molecules surround the reactant molecules, like a cage surrounding the reactant in the middle.The reactant molecules are alone in one solvent cage, and it is possible to extrude the solvent cage to another solvent cage after many collisions, and it is possible to encounter another reactant molecule.On the other hand, when the two reactant molecules are in the same solvent cage, repeated collisions will occur.Although the probability of the two reactants meeting becomes low, once they meet, they have a high collision frequency, which is not lower than the collision frequency in the gas phase reaction as a whole, so there are more chances of reaction. This phenomenon is called cage effect.
Energy level difference e' as a function of carrier effective mass, dark charge transfer parameter " take different values.As shown in actual. 17.It is worth mentioning that when the value of the effective mass of carriers is small enough and the energy level difference e - e is positive, the repulsive barrier separates the negatively charged electron acceptor and the hole occupying part of the polymer chain next closest to the interface, which can prevent the twin pairs from recombination and stabilize the interface twin pairs, provided that the effective mass zne ( < 0.2zne ).However, even if holes on the chain are transferred from the nearest interface polymer chain to the next nearest interface polymer chain, the effective mass and total energy of holes on the chain increase, which is weaker than without cage effect.According to Onzag's theory of twin pair recombination.Even if the attraction potential energy is reduced slightly, especially in the case of short distance, the separation probability will be greatly increased.This can explain the obvious increase in electron-hole pair separation when the acceptor concentration threshold is exceeded in PHP PV: PDL system.Therefore, when the effective mass of carriers is large, the cage effect should also be able to greatly increase the separation of twin opposite free carriers.
Because the bipolar model requires that polymer chains be completely parallel to the interface, at least the two polymer molecular layers closest to the interface, and also requires that the arrangement of electron acceptor molecules be ordered, the bipolar model has explained why the degree of order of the molecular arrangement near the internal donor-acceptor interface and the high concentration of electron acceptor are very important for organic SOLAR POWER BATTERY to achieve high conversion efficiency.The importance of zero vibration on the chain also gives the reason why polymer organic SOLAR POWER BATTERY have higher conversion efficiency than small molecular organic SOLAR POWER BATTERY.
Bipolar layer model explains the increase of photoproductivity caused by the increase of twins' separation rate constant under the condition of high acceptor concentration.Alternatively, we can also think that the higher acceptor concentration increases the electron permeation in the acceptor molecular array.However, if the twin pair separation rate constant does not increase, any change in carrier diffusion will not affect the twin pair separation and recombination trade - off.The bipolar model can also give several practical conclusions:
The interface morphology must have a strong influence on exciton separation yield, which has been confirmed by experimental results.The disordered structure of the interface is not conducive to exciton separation because it will reduce the zero vibration energy and thus damage the bottleneck of twin pair recombination.
The existence of a bipolar layer at the donor-acceptor interface facilitates exciton separation.Although it has been confirmed by the actual. 17 data that the degree of dark charge transfer has relatively little influence on exciton separation yield, the separation yield will increase with the increase of dark charge transfer parameter.Especially in the case of weak electric field strength.
Under the condition of medium electric field strength, effective exciton separation is also possible, provided that the potential barrier separates carriers in electron-hole pairs to prevent twin pairs from recombination.Such a model is not applicable to the so-called " double cable" polymer because the acceptor portion is directly attached to the main donor polymer chain [ 83 ].Therefore, the conversion efficiency of the dual-cable polymer organic solar cell predicted by the existing interface separation model is relatively low.
In order to make organic SOLAR POWER BATTERY have higher conversion efficiency, certain requirements need to be met in several aspects:
All major optical excitations reach the charge transfer center and produce metastable twin electron-hole pairs.The generated twin electron-hole pairs need to easily escape each other's Coulomb potential to avoid twin pair recombination.The released carriers are collected by the built-in electric field to avoid bimolecular recombination or deep energy level capture.
As the exciton binding energy of single-component organic solid is large, only the two-component system can satisfy the above first condition, so the energy of charge transfer state is lower than that of donor or acceptor excited singlet state.Effective light harvesting also requires that the exciton diffusion length be at least comparable to the length of the donor or acceptor phase.Because the exciton diffusion length is less than 10 nm, this sets an upper limit for the structure size of the hybrid system where phase separation occurs.In fact, a properly organized interpenetrating network can meet these requirements.
A problem still to be demonstrated is that twin electron-hole pairs are generated at the internal donor-acceptor interface and escape the detailed mechanism of Coulomb potential.One possible explanation for the experimental results is to use the Ansag - Braun model and think that the separation of electron-hole pairs has a limited lifetime.However, in order to explain the troublesome weak temperature dependence of quantum yield, it is necessary to assume that the final twin pair life is LMS, or to ensure that the rate constants of twin pair recombination and separation can be compared, as shown in Equation ( 8.19 ).In addition, the separation rate of twin pairs will be strictly proportional to the separation rate constant of twin pairs, and the separation rate constant of twin pairs will be determined by the coulomb bound energy of electron-hole pairs.This will mean unrealistic electron-hole pair spacing, but the disorder effect can alleviate this problem to some extent.
However, there is an urgent need for experimental studies on the dynamics of photogenic twin pairs to demonstrate whether the longevity of long twin pairs is a function of electron acceptor concentration.The Onzag - Braun model contains only two adjustable parameters: twin pair lifetime and electron-hole pair spacing, not acceptor concentration.On the other hand, we also know that quantum yield is strongly dependent on acceptor concentration.For a specific system such as PHP PV: PDL, an increase of more than 2 orders of magnitude in photo-generated carrier yield has indeed been observed.The cause of this phenomenon has not yet been satisfactorily explained. Whether this indicates that donor-acceptor mixing is beneficial to the main charge transfer, or whether it simply opens the electron transport path, thus reducing the recombination and space charge effects.
Another problem also needs to be clarified by further experiments. Whether the excess energy dissipated in the separation of excitons from twin electron-hole pairs is also conducive to the generation of electron-hole pairs with larger spacing.This seems to explain why the hybrid system should contain C6 … because the lowest unoccupied track LUMO is very low, C6.It is the strongest electron acceptor and is suitable for preparing high conversion efficiency organic SOLAR POWER BATTERY.On the other hand, it is observed that the photogenic quantum efficiency of PHP PV: PDI system can reach 20 people. It is worth mentioning that the spectral yield of this process is constant in the absorption spectrum of the whole hybrid system, while the optical band gap of the hybrid system is determined by the absorption of PDL.Obviously, there is not much difference between PDL excitation through fluorescence resonance energy transfer ( F6 RSEFRESONENTERYTRANSFER, FRET ) or through direct excitation.This proves that PDI excited singlet excitons are separated into PHP PV / PDL twins, regardless of how they are excited.Since the energy of PDI singlet excitons is only 2.2 eV, the excess energy must be small relative to the energy of electron-hole pairs.The same discussion applies to PFB: F8BT systems ( see 126.96.36.199 section ).Obviously, the order of magnitude of excess energy is a key parameter in designing organic SOLAR POWER BATTERY with high conversion efficiency.However, a systematic study of the possible effects of excess energy will help to understand the mechanism of electron-hole pair formation.
In the fluorescence resonance energy transfer FRET, the fluorescence spectrum of one fluorescent molecule ( called donor molecule ) overlaps the excitation spectrum of another fluorescent molecule ( also called acceptor molecule ), and the excitation of donor fluorescent molecule can induce the acceptor molecule to emit fluorescence while the fluorescence intensity of donor fluorescent molecule itself is attenuated.The intensity of FRET is closely related to the spatial distance between the donor molecule and the acceptor molecule, and FRET can occur at 710 nm.With the distance increasing, FRET decreased significantly.The intensity of FRET also depends on the degree of overlap between the donor emission spectrum and the acceptor excitation spectrum, as well as the relative orientation of the dipoles for energy transfer between the donor and the acceptor.
Recently, the theoretical model for explaining effective photogeneration in donor-acceptor mixing has been further developed, and at least one component of donor-acceptor mixing here is conjugate polymer ( 7t * - conjugate polymer ).The basic idea is that the carrier mobility is determined by the strong coupling between the electrical chains in the first time carriers are generated on the polymer chain, the mobility can reach a value of 103 ￠ 2 / $ #, and the effective mass is about 0.05 / 7 1.Due to the imperfect polymer chain, molecular vibration and carrier scattering caused by phonons, carriers will relax quickly in the sub - PS time scale, but this ultra-fast movement can ensure that the initial pair spacing of electron-hole pairs reaches several times the length of polymer chain repeating units.Such twin pairs will certainly suffer from twin pair recombination unless the coulomb potential is shielded.The bipolar layer established at the internal interface or potential energy change of the donor-acceptor heterojunction can act as a Coulomb potential shield.This will have a concomitant effect on the recombination of non-twin pairs, because the recombination of non-twin pairs also depends on the attraction of Coulomb potential.This shows the key role of morphology and control of organic SOLAR POWER BATTERY.
Dye-sensitized solar cell
At present, the mainstream of thin-film SOLAR POWER BATTERY is amorphous silicon thin-film SOLAR POWER BATTERY, copper indium gallium smashed thin-film SOLAR POWER BATTERY and cadmium telluride thin-film SOLAR POWER BATTERY, which are made using the classic P - N structure and benefit from the rich experience of processing equipment and mature raw material supply chain in the semiconductor industry.However, more and more people in the industry recognize the potential advantages of mesoscopic inorganic or organic semiconductor devices.Due to the interconnected three-dimensional structure, these device structures are also called internal junctions, and the most representative of which is the dye-sensitized solar cell first invented by the author at Lausanne Institute of Technology in Switzerland [ 1 ].Dye - sensitized SOLAR POWER BATTERY are composed of nanocrystalline inorganic oxides, ionic liquids and conductive polymers.The biggest advantage of dye-sensitized SOLAR POWER BATTERY is that the potential preparation cost is very low, and expensive high temperature, high vacuum and high energy consumption processes are not needed.Another advantage is that it is compatible with flexible substrates and is widely used in the market and suitable for integration of indoor devices or architectural decoration.Unlike the classic solid-state SOLAR POWER BATTERY, dye-sensitized SOLAR POWER BATTERY are based on interpenetrating network junctions, with mesoscopic morphology forming a large area interface, giving the system interesting photoelectric properties.Unexpectedly, dye-sensitized SOLAR POWER BATTERY based on semiconductor interpenetrating networks have surprisingly high conversion efficiency and can even compete with traditional crystalline silicon SOLAR POWER BATTERY.Dye - sensitized SOLAR POWER BATTERY combine sensitizers as light-absorbing materials with wide-band gap semiconductor materials with mesoporous or nanocrystalline morphologies to separate light absorption and carrier separation processes [ ]' 2 ].At present, dye-sensitized SOLAR POWER BATTERY have a conversion efficiency of > 11 %, and the device contains liquid electrolyte, and the measurement is carried out under standard test conditions: solar spectrum am 1.5, irradiance 1000 w / m2, and temperature 298 k.Dye - sensitized SOLAR POWER BATTERY using solid electrolyte as organic hole conductors also achieved a conversion efficiency w of more than 4 %, while nanocomposite films containing inorganic materials TiO 2 and cuins 2 achieved a conversion efficiency l5' 6 of 5 % 6 %.The newly developed dyes show a higher optical cross section and can absorb human light of longer wavelength.Similarly, TiO _ 2 mesoscopic films that collect electrons also benefit from the latest progress in nanomaterials' performance is significantly improved.Taking advantage of the high transparency of sensitized nanocrystalline oxide film, the dye-sensitized solar cell and the copper indium gallium selenium film solar cell have achieved a conversion efficiency of > 15 % respectively as stacked cells of the top cell and the bottom cell.
Long - term stability at 8085' c is a major challenge for dye-sensitized SOLAR POWER BATTERY, but significant progress has been made recently [ 8 ].People have also introduced solvent-free electrolytes such as ionic liquids or solid polymers as hole conductors to solve practical problems.By combining hydrophobic sensitizer with hydrophilic lipophilic co - adsorbent, the self-assembly of hydrophobic sensitizer has a better effect of stabilizing the interface.The running stability m has been confirmed by 85 c extended thermal stress test and 60 c AMI. 5 photoaging test.These dye-sensitized SOLAR POWER BATTERY can maintain an initial conversion efficiency of 98 % after 1000 h of high temperature aging.Long - term accelerated testing shows that dye-sensitized SOLAR POWER BATTERY can work in a stable state for more than 20 years, provided that internal interface design issues are properly addressed.This chapter will comprehensively describe the latest progress of dye-sensitized SOLAR POWER BATTERY in academic and industrial fields, and focus on the author's research in the laboratory of Lausanne Institute of Technology in Switzerland.
Band Structure and Operating Mechanism
Actual. 1 illustrates the energy band structure of dye-sensitized SOLAR POWER BATTERY, which can explain its operation principle.Unlike crystalline silicon SOLAR POWER BATTERY or other P - N junction thin film SOLAR POWER BATTERY, dye-sensitized SOLAR POWER BATTERY separate the light absorption process from the carrier transport process.Solar energy is harvested by the sensitizer, which adheres to the surface of the large band gap semiconductor oxide film, usually TiO 2.Photoexcitation of the dye injects electrons into the conduction band of the oxide.The electrolyte gives electrons to regenerate the dye. The electrolyte is usually an organic hole conductor or an ionic liquid and generally contains a wide / 17 pair as redox system O, which intercepts the oxidation state S + of the sensitizer to recapture conduction band electrons.Then, through the reduction of 17 at the counter electrode, I was regenerated, and the electrons completed the loop through the migration of the external load.The voltage generated by illumination corresponds to the energy level difference between the quasi - Fermi level of electrons in the semiconductor oxide and the redox potential in the electrolyte, i.e. the work function of the hole conductor.In this way, dye-sensitized SOLAR POWER BATTERY are regenerated, generating electricity from sunlight, and only temporary chemical changes occur.
Actual. 2 illustrates two coupled redox cycles included in the energy conversion mechanism of dye-sensitized SOLAR POWER BATTERY.Similar to photosynthesis in nature, human light flows from the conduction band of a semiconductor oxide to an external circuit as a charge excited by an electron chestnut.After the dye is regenerated by the electrons given by R, R diffuses to the counter electrode, and the electrons injected into the external circuit by the sensitizer reduce IR to R, thus completing two redox cycles in the energy conversion process.Under sufficient sunlight, the turnover frequency of the sensitizer is 0.155 s _ 1 ( see section 9.7.1 ), so 20 years of outdoor use requires the sensitizer to be able to carry out 100 million turnover times.
The nanocrystalline morphology of semiconductor oxide films is the basic guarantee for the effective operation of dye-sensitized SOLAR POWER BATTERY.On a flat surface, a single layer of dye absorbs up to several percentage points of human light because it occupies an area hundreds of times larger than the optical cross section.The use of multilayer sensitizers also does not solve this problem.Only molecules in direct contact with the oxide surface have photosensitivity, and the rest can only filter incident light.In addition to the unsatisfactory light harvesting, compact semiconductor films also need to be doped with N - type semiconductors to conduct electrons.In this case, the electrons in the semiconductor will cause the energy transfer quenching of the excited sensitizer, which will surely reduce the conversion efficiency.
TiO _ 2 has anatase structure and SEM images of mesoscopic TiO _ 2 layer.The average diameter of the particles is 20nm and they have a biconical shape. The exposed facets are mainly crystal planes, corresponding to anatase crystal planes with the lowest surface energy.Covering a single layer of sensitizer on the surface of oxide nanocrystals as a light harvesting unit can overcome the troublesome problem of low conversion efficiency, which has plagued dye-sensitized SOLAR POWER BATTERY for a long time.
Light harvesting efficiency
Assume that the mesoscopic semiconductor oxide film is 10 mm thick, in which the particle diameter is 20 nm and the real surface area of the particle is 1000 times larger than the projected area.Since the particle size is very small, the semiconductor oxide film exhibits high transparency and light scattering can be ignored.Beer - Lambert's law should be applied to describe the relationship between light harvesting efficiency ( LHE ) and absorption coefficient A ..The light harvesting efficiency is the percentage of human photons absorbed by the sensitizer.
Where ex is the optical cross section of the sensitizer that absorbs human light;C is the concentration of sensitizer in mesoporous film.The value of the optical cross section ER can be obtained from the extinction coefficient ZC of the decimal sensitizer:
N3 dye CIS - RUL 22 ( L = 2,2 - bipyridyl - 4,4' dicarboxylate ) is a widely used sensitizer, and the molecular structure of N3 dye.The absorption of the sensitizer at 530 nm reached the maximum, its optical cross section was 1.4x107 cm2 / mol, and the typical concentration of the sensitizer in the single-layer nanocrystalline film was 2x1 ( t4mol / cm3 ).Therefore, the absorption coefficient of 530 nm is a - 2.8x103 cm - 1, and the absorption length l / a is 3.6 x01.The relationship between light harvesting efficiency and absorption coefficient A is:
Where is the thickness of the nanocrystalline film.The light harvesting efficiency LHE = 99.8 % was obtained by using the formula of = 10 FXM and A = 2.8x103 cm - 1 ( 9.3 ).This explains why the nanocrystalline TiO _ 2 layer can be deeply dyed by covering only a single layer of sensitizer.
In the sample prepared by block copolymer template method, the semiconductor oxide film shows an ordered mesoporous structure, and its internal surface area is much higher than that of randomly bound nanoparticles.Because the same film thickness absorbs more sensitizers, the absorption length is reduced by 1 / 2, and the conversion efficiency of dye-sensitized SOLAR POWER BATTERY is improved.
The light trapping effect can further improve the light harvesting on the surface of the absorbing sensitizer.For example, the incorporation of 100 400 nm size anatase particles can significantly improve the absorption of red and infrared photons by the film.SEM images of related particles.The light trapping technique uses scattering effect and photon band gap effect ell - 13 to localize human light in the mesoporous structure, significantly increasing the optical path length, far exceeding the film thickness, and effectively increasing photon harvesting in the spectral band where the optical cross section of the sensitizer is very small.The benefits of using light trapping technology are very clear in practice. 6. The light scattering layer will improve the photogenerated current response of dye-sensitized SOLAR POWER BATTERY in infrared and visible solar radiation bands, and the light trapping structure can increase the short-circuit current density by 30 %.
The photoelectric conversion efficiency ( IPCE ) is sometimes called external quantum efficiency EQE, which refers to the number of electrons measured by the photogenerated current of the external circuit divided by the monochromatic photon flux reaching the surface of the solar cell.Photoelectric conversion efficiency JPCE is a key parameter of dye-sensitized SOLAR POWER BATTERY, and its expression is:
Where lhe is the light harvesting efficiency for a photon of wavelength a;AndIt is the electron injection quantum yield from the stimulated sensitizer to the semiconductor oxide conduction band.Know | is the efficiency of electronic collection.We have already analyzed the relationship between the light harvesting efficiency LHE and the absorption coefficient A of the dye film above, and will now discuss the other two parameters.
The electron injected into the quantum yield sum.Indicates the percentage of photons absorbed by the dye that can be converted to conduction band electrons.Excited sensitizers inject people into the charge of semiconductor oxides to compete with other radiation inactivation or non-radiation inactivation effects.
In the formula, TNI is the electron injection rate constant, and Yamato is the deactivation rate constant, which combines all the effects of radiation inactivation and non-radiation inactivation.
A typical deactivation rate constant I has a value of 103101.S - 1, and in order to make the electron injection quantum yield as close as possible!, requiring the electron injection rate constant ( .in ) to be in the PS - 1 range.Now commonly used sensitizers can meet such requirements.These dyes bind certain functional groups, such as COOH -, OH - PO3 - 14, to fix sensitizers on the surface of semiconductor oxides.By establishing strong coordination bonds with Ti surface ions, these functional groups increase the electron co knocking between the lowest unoccupied track LUMO of the sensitizer and the conduction band of the semiconductor oxide.The lowest energy electron transition of the pyridopyridine complex such as N3 dye is characterized by metal-to-ligand charge transfer ( MLCT ).
When excitation occurs, electron density is transferred from Ru metal to surface attached ligand, and then rapid electron injection from ligand to semiconductor oxide occurs.Through these sensitizers designed by the molecules, the injection time is in the PS or FS range, and the quantum yield of charge injection is generally over 90 %.In fact, for several sensitizers, the electron transport to the semiconductor oxide conduction band is so fast that this process is caused by the vibrational thermal excited state [ 162 ].
N - 719 dye absorbed on the surface of nanocrystalline TiO _ 2, i.e. cis - rul 22, l = 2,2 - bipyridyl - 4,4' dicarboxylate, the transient absorption signal [ 21 ] of the sample after fs laser excitation will punish the strong tip and return the wave to the surface to run away.0fs, thus the corresponding rate constant Bi, > 5x1013s - 1.Considering the electron coupling of TT sensitizers LUMO and Z2G conducting multiple wave functions Ti and the large acceptor density in semiconductors, such a high rate is reasonable.Since the movement of the molecules and their nuclei in the environment occurs at a time scale of at least 20fs, the observed charge injection dynamics must exceed the vibrational electron transfer model ⑽' for example.Therefore, the rate of the process is likely to be limited only by the phase shift of electrons in the solid.Interestingly, it was observed that the slower injection kinetics was extended to PS time range, while the sensitizer was present in the aggregated form on the surface of TiO 2 film [ 21 ].
As the next mechanism of converting human light into current, complete charge separation must be achieved.According to thermodynamics, electron injection into the conduction band of TiO _ 2 film is a desired process, and the reverse reaction between electron and oxidized sensitizer is undesirable because the reverse reaction does not generate current but only heat.In order to describe the recombination rate, an important parameter, namely the inverse reaction rate constant ( b ) o, needs to be as high as possible, while the unintelligent value is as low as possible in the sensitizer system we want to develop.
For N3 sensitizer, forward injection of human is a fast process, taking place in FS time range, and the reverse reaction between electrons and oxidized Ru complex takes place longer?ms ) time range.One reason for this surprising characteristic is that for reverse electron transfer, the electron coupling element of the reverse reaction is 1 to 2 orders of magnitude lower.The process of recapturing electrons includes the D orbit of Ru metal, the overlap between electrons and the conduction band of TiO 2 is very small, and the space contraction of wave function caused by Ru to Ru oxidation is further reduced.
Another important reason for the kinetic delay of charge recombination is that the driving force of the reverse reaction is larger and the reconstruction energy is smaller. The corresponding values of N - 719 are 1.5 eV and 0.3 eV, respectively.This makes electron recapture clearly exist in the reverse Marcus region, reducing its rate constant by several orders of magnitude.For the same reason, the interfacial redox process is almost independent of temperature and is not even sensitive to the ambient temperature of the contact film [ 24 ].
Charge recombination will be further suppressed by the electric field strength existing on the surface of TiO _ 2 film.Due to the smaller particle size and lower doping concentration, the electric field strength of the depletion layer in the semiconductor oxide can be ignored, positive ions are transferred from hydroxyl acid groups of Ru complexes to the oxide surface, and dipoles emerge on the surface.In a positive ion inert medium, Li + or Mg2 * is a potential determinant ion of TiO 2 and will positively charge the surface.The local potential gradient from the negatively charged sensitizer to the positively charged semiconductor oxide drives the injector in the desired direction and inhibits electrons from leaving the solid again.
Finally, the capture of conduction band in mesoscopic thin films will strongly affect the inverse reaction kinetics.If the diffusion of trapped electrons to the particle surface determines the rate, the time dependence of the inverse reaction is an exponential function of expansion [ 26 ].If the interfacial reverse electron transfer is slow and determines the rate, then the reverse reaction is the first order reaction of kinetics.
The problem of carrier penetration in mesoscopic particle networks has now attracted many people's attention.This important process leads to a certain amount of electron collection from the sensitizer.These large band gap semiconductor oxide films are insulated in the dark, but single electrons injected into 20nm - sized particles will produce an electron concentration of 2.4x1017 cm - 3.If the electron diffusion coefficient ( dn ) is io - 4cm2 / s, this corresponds to the specific conductivity 71o of 1.6x10 - 4s cm 1. in fact, this situation is more complicated because the transport of carriers in these films involves trapping of traps, unless the fermi level of electrons is so close to the conduction band that all traps are f - filled, so that electrons can move freely.Therefore, the trap depth involved in the electron movement will affect the value of the diffusion coefficient' which explains the phenomenon that the diffusion coefficient increases with the incident light intensity [ 28.29 ].Recently, Monte Carlo simulation has given a good description of the complex relationship of electron transport in this mesoscopic F - conducting thin film [ 3 ].The fact that it is of great significance to the operation of dye-sensitized SOLAR POWER BATTERY is that the charges injected with nanoparticles can be shielded at mesoscale by the surrounding electrolyte, which greatly facilitates the penetration of electrons into UL.The electron charge is shielded by the cation of the electrolyte, which limits the internal electric field, so there is no drift term in the transport equation.Actual. 9 illustrates this local shielding effect.
In fact, the equivalent circuit given in the lower part of. 9 treats each particle as a resistive element coupled to the electrolyte through the interface.The resistance element is expressed as a parallel structure of an electrical double-layer capacitor CD and an interface electron transfer resistor RT.
Actual. 9 shows that the movement of electrons in the conduction band of mesoscopic thin films must be accompanied by charge compensated cation diffusion near the electrolyte layer on the surface of nanoparticles.The Coulomb potential of cation shielded electrons avoids the formation of uncompensated local space charges.Otherwise, local space charges will damage the movement of electrons in the film.This adjusts the bipolar diffusion length, i.e. the effective diffusion length, so that the bipolar diffusion length includes the contribution of both electrons and charge compensation cations [ 33' 343 ], thus more accurately describing the charge transport in this mesoscopic interpenetrating network dye-sensitized solar cell.Electrochemical impedance spectroscopy provides useful clues to explain the complex phenomena of these films under the irradiation of human light in the band gap band [ 35 ].
Actual. 10 describes the process of electron injection and recombination.
Quantum dot sensitizer
Quantum dot structures prepared from semiconductor materials can replace dyes as light harvesting units of dye-sensitized SOLAR POWER BATTERY [ 38' 39 ].In quantum dots, absorbed human light produces excitons or electron-hole pairs.The electrons are then injected into the semiconductor oxide, and the holes are transferred to the electrolyte as hole conductors, which is present in the pores of the nanocrystalline oxide film.Effective and rapid hole injection can be achieved from PBS quantum point to triarylamine hole conductor, with the photoelectric conversion efficiency ZPCE exceeding 50 %, and no measures have been taken to optimize the collection structure or slow down the recombination of electrons and holes [ 38 ].The optical cross section of quantum dots is much larger than that of molecular sensitizers and also depends on particle size.However, quantum dots also occupy a larger area on the surface of the mesoporous electrode, thus reducing the concentration of quantum dots in the film.Therefore, the absorption length of quantum dots is similar to that of dye-loaded films.
Recent studies have given surprising results. By the impact of quantum dots on ions, absorption of a single photon can produce multiple excitons, provided that the photon energy is more than three times the band gap [ 39.4 ].The challenge now is to find?To find an effective way to collect exciton before recombination.Because recombination occurs at FS time scale, using mesoporous oxide collection electrodes to remove electrons is a promising strategy, which creates a possibility for the photoelectric conversion efficiency FPCE to exceed 100 %.
Characteristics of dye-sensitized SOLAR POWER BATTERY
We have already introduced the operation mechanism and nanostructure of dye-sensitized SOLAR POWER BATTERY, and we will discuss the latest characteristic data of this new type of solar cell.The structure of dye-sensitized SOLAR POWER BATTERY used in these experiments can be explained by the cross-section cabinet'
As shown in actual. 11.The working electrode as the front contact and the counter electrode as the back contact are both composed of soda-lime float glass covered with transparent conductive oxide TCO.The material of TCO is S NO2: F, namely fluorine doped tin dioxide ( FTO ), which has a square resistance of 10 - 15FL / □ and a visible light transmittance of 80 % 90 % of human reflection loss.The back contact is plated with a small amount of Pt to catalyze the transfer of electrons from the FTO electrode to the interface with a loading of 50mg / m2.Nanocrystalline TiO _ 2 film was deposited on FTO glass by screen printing as a front electrode, followed by rapid sintering at 450 C in air to remove organic impurities and enhance the interconnection between nanoparticles.By self - assembly, nanoparticles are absorbed from the solution and a sensitizer monolayer is formed.Dye - sensitized SOLAR POWER BATTERY use U.S. DuPont's Bailey as hot melt adhesive.The redox electrolyte can be injected through small holes in back contact.
monochromatic incident photon to electron conversion efficiency
The standard preparation method of mesoscopic TiO 2 film is hydrothermal method, which can form a film containing 1520 nm size anatase crystals.Hydrothermal method is to heat the aqueous solution with reactants at high pressure and low temperature in a pressure pot to form crystals on the substrate.The mesoscopic morphology formed has a great influence on the characteristics of dye-sensitized SOLAR POWER BATTERY.Actual. 12 The photoelectric conversion efficiency JPCE of the two samples was compared. The collecting electrode was anatase single crystal and anatase nanocrystalline film, which were sensitized with standard Ru dye N - 719, i.e. CIS - RuL2Z, L = 2,2 - bipyridyl - 4,4' dicarboxylate.Photoelectric conversion efficiency 7PC ￠ or external quantum efficiency EQE is plotted as a curve with respect to wavelength.The photoelectric conversion efficiency JPCE obtained at 530 nm on the single crystal electrode is only 0.13 %, 530 nm is the wavelength at which the N - 719 sensitizer absorbs the maximum, while the photoelectric conversion efficiency IPCE at 530 nm on the nanocrystalline electrode reaches 88 %.Under illumination, the photogenerated current of the nanocrystalline electrode is more than 1000 times higher than that of the single crystal electrode.Such a large improvement is far beyond expectations, because it was thought that defects on the disordered surface would increase the recombination of photogenerated carriers, and such a large area of semiconductor junction is not considered conducive to achieving a high photoelectric conversion efficiency IPCE.
If the light loss in front of FTO glass is taken into account, the photoelectric conversion efficiency has an equivalent value in the range of 500600 nm where the sensitizer absorption reaches the maximum value.From equation ( 9.4 ), it can be seen that the light harvesting efficiency AHE, the electron injection quantum yield INJ and the electron collection efficiency % II need to be close to 1 in order to have such a result.Electrochemical impedance spectroscopy shows that the typical diffusion length of conduction band electrons in dye-sensitized SOLAR POWER BATTERY is 20100 mm.This exceeds the thickness of the nanocrystalline TiO _ 2 film, which explains why all photo-generated carriers are collected.
Laboratory conversion efficiency records
The conversion efficiency of dye-sensitized SOLAR POWER BATTERY is also determined by short-circuit current density, open-circuit voltage Voe, filling factor FF and human light intensity of 8:
If the most advanced Ru sensitizer is used, under the standard test condition STC, the human light intensity of AM 1.5 is 8 = 1000 W / cm2, and the short-circuit current density is 9 at 16 —
Ma / cm2, open circuit voltage 1 is in the range of 0. 70. 86 v and fill factor ff is in the range of 0. 650. 8.The laboratory conversion efficiency record of the dye-sensitized solar cell certified in 2001 was 10.2005 and the updated laboratory conversion efficiency record was 11.18 %. The volt-ampere characteristic curve of this dye-sensitized solar cell is shown in the actual .13.Dye N - 719 is absorbed by double-layer nanocrystalline TiC ) 2.Apply light scattering centers ( e.g. actual .6
As shown ), the electrolyte of the R / IF redox system uses a mixed solvent of acetonitrile and valeronitrile.
We have confirmed that additives such as guanidine ions can suppress dark current in TiO 2 - electrolyte junctions.Although this effect cannot be fully explained, these ions seem to contribute to the self-assembly of dye molecules on the TiO surface, making TiO 2 more impenetrable, thus reducing the dark current of dye-sensitized SOLAR POWER BATTERY.In addition, it has also been found that guanidinium butyric acid can inhibit the number of surface states as complex centers [ improvement of gas 9.5 formula ]
Although the characteristics of dye-sensitized SOLAR POWER BATTERY have been greatly improved recently, it is difficult to achieve higher conversion efficiency using standard N - 719 sensitizers unless the formula is improved to change the matching of redox systems.Because the redox potential of the N - 719 sensitizer does not match the R / IR pair very well, the regeneration reaction of the sensitizer will consume a very large part of the absorbed photon energy. This result is obvious from the actual .1.Now, people are developing some alternative redox systems to make their redox systems and N719 have a more matching Nernst potential, that is, the equilibrium voltage of redox pairs on the electrodes [ 43 ], which will make the open circuit voltage VOC of dye-sensitized SOLAR POWER BATTERY exceed 1V.
Of course, the improved formula can also maintain the existing R / I " redox system and introduce a full-color sensitizer or dye mixture, thus further improving the conversion efficiency.In order to make the conversion efficiency of dye-sensitized SOLAR POWER BATTERY reach 15 %, the improved design needs to give a short-circuit current density of at least 24mA / cm2 under full solar illumination, while the filling factor and open-circuit voltage need to be maintained at the existing values.In order to realize such photogenerated current, it is necessary to greatly increase the light harvest in the 650900 nm range.
Actual. 14 illustrates the structure of Ru mixed complex labeled K - 19.Due to the extended K system in one ligand, K - 19 has an increased absorption coefficient.The K - 19 dye showed good light conversion characteristics and SOLAR POWER BATTERY stability.Similar to K - 19, the bipyridyl group with alkyl side chains is Z - 907, and also exhibits good light conversion characteristics and stability M.These Z series dyes have proved to be very useful for solid-state dye-sensitized SOLAR POWER BATTERY, and their hydrophobic properties are indeed a useful element.Therefore, K - 19 or Z - 907 can improve the characteristic W of the conductor system containing ion electrolyte and hole.
Solid - state dye-sensitized solar cell
Solid - state dye-sensitized SOLAR POWER BATTERY use hydrophobic Z - 907 dye and hole conductor Spiro - Omet AD, combined with Li2N, T - Butyl Pyrridine additive, N 3 SBCL 6 as a P - type dopant, achieving a conversion efficiency of > 4 % under AMI. 5 illumination.The self-assembly of dye molecules in the dense layer on the surface of TiO _ 2 once again plays an important role. A COOH functional group plays a role in fixing and the hydrophobic chain forms a barrier between the hole conductor and TiO _ 2.
Stability of dye-sensitized SOLAR POWER BATTERY
The long-term accelerated photoaging test has confirmed the intrinsic stability of dye-sensitized SOLAR POWER BATTERY [ 45 ], and has recently achieved stable operation under 8085 C high temperature stress, using careful sensitizer molecular design and combining a solid electrolyte that does not volatilize.
Mechanism of stability
Actual. 16 illustrates that the mechanism of the catalytic cycle o of sensitizer limiting the stability of dye-sensitized SOLAR POWER BATTERY during the operation of the cells is a side reaction caused by the excited state s * or the oxidized state s +.The side reaction of excited state S * will compete with the electron donor from the excited dye to the mesoscopic oxide conduction band, while the side reaction of oxidized state S + will compete with the regeneration of sensitizer.These side reactions that are not conducive to the stability of dye-sensitized SOLAR POWER BATTERY are considered to be kinetic first-order reactions or pseudo-first-order reactions:
In the formula, and are respectively the excited state side reaction branching ratio and the oxidized state side reaction branching ratio affecting stability;Bi He is.X is an excited state side reaction rate constant and an oxidized state side reaction rate constant, respectively.々 * and are respectively the electron injection rate constant and the regeneration rate constant of the sensitizer;The excited state side reaction rate constant * ex and the electron injection rate constant center jointly determine the excited state side reaction branching ratio, while the oxidation state side reaction rate constant Bi and the regeneration rate constant jointly determine the oxidation state side reaction branching ratio P … The portion of the loss of sensitizer molecules through a catalytic cycle can be described by the " and oxidation state side reaction branching ratio" through the excited state side reaction branching ratio.If dye-sensitized SOLAR POWER BATTERY are required to operate normally for 20 years, the excited state side reaction branching ratio / or the oxidized state side reaction branching ratio p.The maximum limit of is 1x1 ( t \ dye turnover frequency should reach 0.155f1 according to the average season and all-day time.
As mentioned earlier, for most conventional sensitizers, the electron injection rate constant from the excited state to the conduction band of TiO _ 2 particles is constant.It's all in the FS range.Assuming the electron injection rate constant Yao n = 1x1013 factory 1, if the excited state side reaction rate constant x < 105 s -' o of s " is required to exceed this range, the excited state side reaction branching ratio / will exceed the maximum limit ix 1.018, and the side reaction will seriously affect the stability of the dye-sensitized solar cell.N3 Ru sensitizer has met such conditions.The dye-sensitized solar cell prepared by N3 can withstand photo-induced loss or [ SCN IT ligand exchange, [ SCN ] - the rate constant of ligand exchange is far lower than the IO5S 1 limit of excited state side reactions.At present, there is still a dispute as to whether these side reactions do affect stability, because the products of the side reactions are still sensitizers with charge transfer effect.In ethanol solution, prolonged photolysis and photooxidation of N3 dye will cause S loss and formation of NC ( ) - Ru complex.However, no such reaction was observed in dyes absorbed on the oxide surface.Battery energy storage for enabling integration of distributed solar power generation
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