Standard DLTS: Still in Reverse Bias Voltage Pulse;Note DLTS: Forward biased voltage pulse;Optical DLTS: Short Laser Pulse.
After receiving the transient excitation, the Motive Power battery returns to the static state again, and the high frequency capacitance value during the transient excitation is recorded, with the typical frequency as high as 1 MHz.Such DLTS measurements can be made at multiple temperatures.The transient capacitance C corresponding to each measured temperature can be processed by several methods to obtain information about the deep energy level in the Motive Power battery:
Deep level distribution;
Deep level density;
Capture cross section.
For more discussion on DLTS technology for measuring cadmium telluride thin film solar Motive Power battery, please refer to document [ 143147 ].
Experimental detection of physical properties
The following various standard spectrometers were used to detect the physical properties of CdTe layer of CdTe thin film solar Motive Power battery:
X - ray photoelectron spectroscopy ( XPS ): As a quantitative spectroscopy technique, XPS can measure the elemental composition, empirical chemical formula, chemical state and electronic state of materials.In XPS under ultra-high vacuum conditions, a beam of X - rays irradiates the material, and the amount and kinetic energy of runaway electrons at 110 nm from the top of the material can be measured and analyzed simultaneously.
Auger Electron Spectroscopy ( AES ): AES is a conventional analytical technique based on the Auger Effect Oaes where energy is transferred to another electron to escape after the electron transits to the inner orbital of the atom, and is especially suitable for studying the surface properties of materials.
X - ray energy dispersion spectrum ( EDS ): The sample is bombarded with electron beams, and EDS technology detects the X - rays emitted by the sample.When the sample is bombarded by an electron beam, electrons escape from atoms on the surface of the sample.The remaining electron vacancies are filled with electrons of higher energy level, thus emitting X - rays.The energy of X - ray has the characteristics of this element.
Secondary ion mass spectrometry SIMS: SIMS used in material science and surface science can analyze the composition of solid surfaces and thin films and is the most sensitive surface analysis technology.In SIMS, the first focused ion beam is sputtered onto the sample surface.Collect and analyze the outgoing second ion beam.According to the second ion beam, the mass spectrometer can determine the elements, isotopes and molecular components of the surface.
In addition to various spectrometers, scanning electron microscopy ( SEM ) and transmission electron microscopy ( TEM ) are also commonly used in the testing of tin telluride thin film solar Motive Power battery.Scanning electron microscope ( SEM ) is a common electron microscope, which scans the surface of a sample in a raster pattern by a high-energy electron beam and forms an image.Electrons interact with the atoms that make up the sample, and the resulting signals contain information such as the surface morphology, composition and conductivity of the sample.TEM is a common microscope technique. A beam of electrons transmits through a very thin sample and interacts with the thin sample during transmission?The image formed by this interaction is magnified and focused on an imaging device such as a fluorescent screen, photographic film or CCD camera.
Electron beam induced current ( EBIC ) and beam induced current ( OBIC ) are direct measurement methods to obtain the micro-scale response of solar Motive Power battery.EBIC is a semiconductor analysis technology based on scanning electron microscope SEM, which relies on electron-hole pairs generated in the semiconductor by microscopic electron beams and can be used to analyze buried junction regions, defects or minority carrier characteristics of the semiconductor.OBIC is a semiconductor analysis technology that uses a scanning laser beam to induce current in a semiconductor sample. Collecting and analyzing the induced current can produce images representing the characteristics of the sample, so as to be able to detect and locate defects or abnormal structures in the semiconductor sample.
In the EBIC and OBIC, the current of the solar cell is measured by exciting with a focused electron beam or laser beam.The electron beam can focus better than the laser beam, so EBIC has higher spatial resolution than OBIC.EBIC and OBIC have different methods for detecting photogenerated current or induced current:
Direct EBIC or Direct OBIC: Detect photo-generated current or induced current at the same position where a human emits an electron beam or a laser beam;Remote EBIC or Remote OBIC: During scanning, electrodes that keep a fixed distance from the human beam or laser beam detect photo-generated current or induced current.
EBIC technology can also be applied to different parts of solar Motive Power battery:
Flat EBIC: solar cell surface;
Transverse EBIC: Solar Cell Transverse Interface.
The information that EBIC and OBIC can obtain is the lateral uniformity and diffusion length, and the measurement of diffusion length also requires a certain Lang hypothesis.EBIC and OBIC have been widely used in the research of cadmium telluride thin film solar Motive Power battery. Using infrared phase locked thermal imaging technology, the non-uniformity of the output of cadmium telluride thin film solar Motive Power battery can be studied. Near infrared radiation on the cell surface is scanned and converted into temperature image D48 ].In the infrared phase-locked thermal imaging technology, the research sample is periodically excited at high temperature.A highly attenuated and scattered " thermal wave" generated inside the sample is recorded as a temperature image after reaching the near surface area.
Photoluminescence PL is the photon absorbed by the substance and then re - radiated, while cathodoluminescence ( CL ) is the visible light generated by electron beam bombarding the luminescent substance generated by the electron gun.Radiation measurements such as PL and CL can give information about band gaps, exciton structures, deep levels, and recombination properties.The excitation mode here is laser or electron beam, and the output is photons of longer wavelength.For the application of PL and CL in the research of cadmium telluride thin film solar Motive Power battery, see document [ 149153 ].
In ultraviolet photoelectron spectroscopy ( UPS ) and X - ray photoelectron spectroscopy XPS, the excitation mode is radiation energy in the order of 10eV to hundreds of eV, and then the energy of emitted electrons is measured.UPS and XPS are collectively referred to as photoelectron spectroscopy ( PES ).This method can give the information of Fermi level position and valence band top EV on the surface of the sample, and combine with the value of band gap to know the conduction band bottom level of the detected surface and the complete band matching ( band alignment user MI55 ).The position of the bottom K of the conduction band can also be directly detected by PES [ 156 \ How to link the band matching of the surface structure with the band matching of the whole cell is an additional problem to be considered.
The working principle of AFM is to mount the probe at one end of an elastic microcantilever and fix the other end of the microcantilever. When the probe scans the surface of the sample, the repulsive force between the probe and the atoms on the surface of the sample will cause the microcantilever to deform slightly, so that the slight deformation of the microcantilever can be used as a direct measure of the repulsive force between the probe and the sample.A laser beam reflected from the back of the microcantilever to the photodetector can accurately measure the micro deformation of the microcantilever, thus reflecting the surface morphology and other surface structures of the sample by detecting the atomic repulsion force between the sample and the probe.Scanning tunneling microscope ( STM ) is based on the concept of quantum mechanics of tunneling effect.When the conductive tip is close to the detection surface, the bias voltage between the two can allow electrons to tunnel through the vacuum between them.The resulting tunnel current is a function of tip position, bias voltage, and sample local state density?The information obtained by scanning the monitoring needle tip position on the surface can be reflected as an image.STM is a technology with high requirements for operating conditions and sample state, requiring the sample surface to be very clean and stable, the conductive tip to be very sharp, the vibration control of equipment to be excellent, and the electronic instrument to be highly precise.
AFM and STM have also developed some advanced scanning techniques.The surface photogenerated voltage SPV technology is widely used to measure the diffusion length and surface potential of semiconductors, which is helpful to solve the minority carrier transport of P - N junction characteristics.As a non-contact method, SPV is an ideal method for describing compound semiconductor ohmic contacts and special device structures, and is also suitable for studying cadmium telluride thin film solar energy [ 157 ].Kelvin force microscope KPFM is a non-contact form of atomic force microscope AFM, which can observe the work function of the surface at the atomic or molecular scale, thus obtaining information' such as the composition and electronic state of the local structure of the solid surface, and can also be used to measure the work function of cadmium telluride thin film solar Motive Power battery [ 158 ].AFM and STM derivatives can also measure electron current density and surface capacitance at submicron resolution [ 117' U8 ].Advanced photons?The measurement can also be used to accurately determine the absorption coefficient of the sub-band gap region to a [ 27 ].
In the double diode model, in cadmium telluride thin-film solar Motive Power battery, the contact barrier of the back contact structure must form a Schottky diode in the opposite direction to the CdS / CdTe junction.The " double diode model" can describe the characteristics of cadmium telluride thin film solar Motive Power battery in a quantitative way, and can satisfactorily explain several unique effects of cadmium telluride thin film solar Motive Power battery.The Motive Power battery structure in the figure corresponds to the DC model and the AC model.The two-diode model was first proposed by Stollwerck and Sites to explain the volt-ampere characteristics of cadmium telluride thin-film solar Motive Power battery under illumination [ 86 ], and then revised by Ghent University in Belgium [ 87' 88, 9' 159 ], further describing the following aspects:
The influence of minority carrier current on back contact:
Dependence of Back Contact on Voltage and Human Light Intensity.
The DC model shows that there is a certain relationship between photogenerated current density JPH, junction current density ( human ) and contact current density ( human ). The current density through CDS / CdTe junction is human - equal to the current density through back contact Schottky diode.On the other hand, the voltage V across the cadmium telluride thin film solar cell is the sum of junction voltage ( VJ ) and contact voltage ( VE ).
Where, j.The reverse saturation current density is the contact saturation current density.The ideal factor of diode is the emission speed.The contact current density is obtained from thermionic emission.The thermionic emission is to make the charged carriers leave the object surface or exceed the potential energy barrier by thermal induction, because the thermal energy obtained by the carriers has exceeded the binding force.If the first order approximation is considered, the emission speed A can be replaced by the drift velocity ( VD ) at the back contact, which is called the contact voltage VE depending on the doping concentration and the back contact.The emission velocity can also be replaced by effective velocity, which describes the combined effect of thermionic emission, drift and diffusion [ 14' 743 ].The double diode model of formula ( 7.20 ) has been able to explain the " flip" phenomenon of the volt-ampere characteristic curve of the telluride thin film solar cell, and depends on the contact barrier 0,,,The dependence of fill factor ff and conversion efficiency 7 on contact barrier less h [ 86 ] can be obtained from such a dual diode model, and the approximate equation of equation ( 7.6 ) to correct conversion efficiency can also be derived from equation ( 7.20 ).
In order to describe the " cross" phenomenon of the volt-ampere characteristic curve of cadmium telluride thin film solar Motive Power battery, it is necessary to further expand the dual diode model [ 16 ].One empirical method is to assume that several parameters in equation ( 7.20 ) depend on the incident light intensity. It is believed that the reverse saturation current density or the contact saturation current density may be caused by the contact barrier that depends on the incident light intensity, while the series resistance and shunt resistance also depend on the incident light intensity.Although all these empirical assumptions can almost perfectly explain the " cross" phenomenon of the volt-ampere characteristic curve of cadmium telluride thin film solar Motive Power battery, they do not provide a physical explanation of the dependence of various parameters on human light intensity and wavelength, and the empirical assumptions can only be valid for one volt-ampere characteristic curve and cannot satisfactorily explain other measurement results.According to the double diode model, some derived theoretical analysis methods can be used in conjunction with experimental measurements and numerical simulations:
Back - contact diode has photovoltaic effect [ 16 ]: in the actual DC model of. 12, it is necessary to add a photo-generated current source in parallel with the back-contact diode, the direction of this photo-generated current source is opposite to the photo-generated current source JPH of CDS / CdTe junction and only responds to long wavelength incident light of a < 820 nm.
Contribution of minority carrier electrons to people with contact current density: the types of carriers that contribute to people with contact current density are not limited to holes, and minority carrier electrons also contribute [ 87' 883 ].It is necessary to add a term of electron current density to the second equation of equation ( 7.20 ) about the contact current density, so the electron concentration of the space charge layer SCL at the back contact edge should be estimated.If such a mechanism is important, it is required that the diffusion length Ln should be large enough, or the cell should be thin enough, or the two-dimensional effect of the back contact reduces the effective thickness O the back contact SCL has a non-negligible electron concentration, meaning that carriers will be generated directly in this region, then this derived theoretical model is equivalent to the first point and will generate a sufficiently large forward bias voltage in both ends of the solar cell.This derived model explains that a set of volt-ampere characteristic curves observed under certain conditions " cross" [ 88 ] at a single point.
The back contact barrier indirectly depends on the human light intensity: long-wavelength human photons will change the dominant state occupation near the back contact, which will change its charge state, electrostatic potential distribution and the barrier of the device, thus making the majority carrier hole current density through the back contact dependent on the human light intensity.
Several kinds of front contact mechanisms similar to the third point are used to explain the effect of short-wavelength human light on the forward bias [ 161 - 165 ]: in the case of forward bias v > 0 and wavelength a < 520 nm, the apparent quantum effect will even become negative and the absolute value will exceed 1, and the volt-ampere characteristic curve will enter the first quadrant.Although the models in different documents differ in detail, their starting point is the same, i.e. the deep host state of the CDs layer or interface will compensate or overcompensate for the shallow donor doping of CDs.Under illumination, assuming that the ratio of hole trapping cross section ( CRP ) to electron trapping cross section ( CRN ) is < TP / trn > 1 and appropriate, the effective doping concentration and potential distribution of the CDS layer will change without charge.This can have a serious impact on current transport to other parts of the Motive Power battery.In document [ 161,162 ], assuming that excessive compensation occurs and a " hump - like" potential distribution appears in the CDS layer, this thin layer acts as a modulation barrier photodiode ( MBP ).A photodiode, like a common diode, is also a semiconductor device' composed of a p - n junction. it also has unidirectional conductivity, but in the circuit it does not act as a rectifying element, but rather converts an optical signal into an electrical signal?The photoelectric sensing device.The modulation barrier photodiode MBP here means that the incident light intensity will change the barrier and affect the characteristics of the solar cell as a photoelectric sensing device.In document [ 163165 ], complete compensation is more or less assumed, explaining the details of the relatively complex characteristics of the Motive Power battery in the forward bias voltage and short wavelength range, especially the effects of high bias voltage and short wavelength, such as quantum efficiency QE and volt-ampere characteristics, are reviewed in document [ 166,167 ].It should be noted that the model mentioned in the fourth point here has exceeded the analytical theoretical analysis level of the dual diode model and requires a large number of numerical simulations' and only MBP model can perform analytical theoretical analysis under simplified assumptions' a.
The " DC model" of the dual diode model was discussed above for the numerical modeling of cadmium telluride thin film solar Motive Power battery, and now the " AC model" will be discussed again.In the equivalent circuit of the actual. 12 AC model, all the components depend on the DC bias voltage applied to the components.
Strong dependence: junction resistance ( ruler ) and contact electrode resistance ( ruler ).) depends exponentially on the junction voltage % and the contact voltage k, respectively;Weak dependency: junction capacitance ( c' ) and contact capacitance ( cc ) are still dependent on bias voltage in square root form, qyvbi -, ccoyvlc + VC, where, and VB, c are junction built-in voltage and contact built-in voltage, respectively
According to the basic circuit theory, we can calculate the low-frequency capacitance Cu and the high-frequency capacitance CHF: when the applied voltage or bias voltage v is negative or medium positive.The current density J is lower than the contact saturation current density, so the back contact does not limit the current J: ruler. Therefore, CLK CID increases with the applied voltage V and Q also increases with the applied voltage V because the junction voltage % increases.From the analysis of Mott - Schottky curve, that is, L / C2 - V curve, we can deduce the doping concentration of CdTe layer near the CdS / CdTe junction.If the applied voltage V is a higher forward bias, the back contact will limit the current, ruler, GJCE.The contact voltage v also increases with the applied voltage v.but contact voltage v ..The back-to-back contact is the reverse voltage.The results of this double diode model enable us to determine the doping of CdTe in the CdS / CdTe junction and back contact region [ 87,159 ], which is also the standard method for analyzing cadmium telluride thin film solar Motive Power battery with contact barriers.
The numerical simulation of solar Motive Power battery is to use a computer program to solve the " semiconductor equations" of a specific solar cell structure.One - dimensional DC semiconductor equations are composed of Poisson' s equation and continuity equation. Poisson equation links electrostatic potential ( 4 ) with charge density, while continuity equation links electron current density 9 and hole current density with the generation and recombination of carriers.
In the formula, the donor concentration NJ and the acceptor concentration NX come from ionized shallow donors and shallow acceptors, respectively.Charge density is a deep level charge density, which depends on electron concentration n and hole concentration p in a non-linear manner.G is the carrier generation rate under illumination, the carrier recombination rate depends on the wavelength A of human light and the position magic ruler in the cell, and the diffusion term and drift term will be included in JP, which is non-linearly dependent on the carrier concentration and external electron current density.
The semiconductor equations composed of Poisson equation and two continuity equations need to solve three unknown functions, namely electrostatic potential 0 and electron concentration W (;C ) and hole concentration p can solve the semiconductor equations according to the appropriate boundary conditions.In the computer algorithm, the solar cell structure is first divided into many slices, with typical slices of several tens to more than 100 layers per semiconductor layer.In the 5 - layer system, the number of slices n = 500, each differential equation is approximately n non-linear algebraic equations, and the equations system to be solved numerically contains 3n = 1500 equations and 3n = 1500 unknowns, i.e. each slice has 3 unknowns of electrostatic potential electron concentration and hole concentration.The computer program needs to input the composite information of the thickness of the five layers and the semiconductor characteristic parameters, especially the shallow level and the deep level, so the number of input parameters will be as high as 50100.
Some differences between different solar cell simulation programs;
Numerical ability: number of semiconductor layers, number of deep energy levels per semiconductor layer, and calculation speed;Physical mechanism of application: different composite mechanism, tunnel effect;Operation difficulty: user interface;
A summary of the simulation program for thin film solar Motive Power battery can be found in reference [ 168 ].The computer simulation results in the following discussion come from SCAPS program developed by Ghent University in Belgium.
Determine parameter settings
The basic setting of numerical simulation parameters for cadmium telluride thin film solar Motive Power battery can be found in Gloeckeli and Sites reports [ 48 ].If we need to carry out predictive numerical simulation, we must determine the self-consistent parameter settings.If a certain set of parameter settings can describe all the measurement results, this means:
There are no special assumptions about the dependence of certain voltage or parameters on irradiance.Non - physical hypothesis: the " medium performance level" of recombination without charge is an ideal situation that does not correspond to physical reality, and the medium performance level is often included in simulation programs such as SCAPS.The numerical simulation results need to be consistent with the experimental data such as deep level transient spectra DLTS.
Since the determination of trap density ( AO, electron capture cross section and hole capture cross section ) is by no means a simple routine work and will cause fundamental errors, we consider trap density / V and capture cross section as fitting parameters.On the contrary, we use the trap energy level E determined by DLTS in the parameter setting.Determining self-consistent parameter settings is a laborious task.The first set of parameter settings comes from fitting forward bias / short wavelength effects [ 164' 1653.This set of parameter settings also needs to be revised in subsequent stages, including fitting the measurement results of J - V, JM - VOE, C - V and C - / on temperature and quantum efficiency QEG, V ) in the dark and in the light, such work comes from two sets of cadmium telluride thin film solar Motive Power battery in Germany's Antacererenergy:
carrying out CDC L2 activation processing in air;
CDC L2 activation processing is performed in a vacuum.
In our parameter setting, the parameters related to defects are shown in actual .6;The values of other parameters are shown in actual .1.
contact potential barrier
The actual parameter setting of .6 will be used as the basic setting for parameter change.In the numerical simulation, one parameter is changed at a time, while the other parameters still maintain the values in the basic settings [ 174 ].The software used for numerical simulation is SCAPS program developed by Ghent University in Belgium.
A numerical simulation of cadmium telluride thin film solar cell was carried out, and the volt-ampere characteristic curve obtained changed with the contact barrier office * to study the activation treatment in vacuum and air respectively.The results of numerical simulation 1 agree with my understanding.When the contact potential barrier is increased, the volt-ampere characteristic curve obtained by numerical simulation begins to " flip" in the forward bias quadrant and the filling factor FF decreases, and the final volt-ampere characteristic curve develops into an " S" shape.
As the contact barrier increases, the conversion efficiency 7 given by the numerical simulation of cadmium telluride thin film solar Motive Power battery gradually decreases.Due to the overall lower doping concentration, the Motive Power battery conversion efficiency of the activation process in vacuum is lower ( see 184.108.40.206 section ).
The CdTe doping concentration near the back contact will have a great impact on the performance of CdTe thin film solar Motive Power battery.When the back contact doping concentration ( DopingDenSityatAckContact ) changes, while other parameters are still in accordance with the basic settings, the volt-ampere characteristic curve will change with the back contact doping concentration NaE.Actual. 16 Voltammetric Characteristic Curve with Back Contact Doping Concentration Na.The variation of the volt-ampere characteristic curve is very similar to the actual variation of the volt-ampere characteristic curve with the contact barrier, which indicates that the influence of back contact doping will be misinterpreted as the influence of the contact barrier.The local concentration of impurities in the back contact of cadmium telluride thin film solar Motive Power battery will mask the effect of back contact. This conclusion has been reported before [ 9 ], but its influence is overestimated.'
In fact, the trap distribution of other shallow and deep doping in .6 has also been carefully studied.If the in vivo doping concentration / vab of the shallow energy level is too low, the filling factor ff and conversion efficiency 7 of the activated treatment cell in vacuum will deteriorate due to the too low electric field strength of the space charge region [ 159 ].
Conversion efficiency?Not only does it depend on the junction doping concentration TVA, near the CdS / CdTe junction, but several other effects also exist at the same time.In the case of high junction doping concentration NAJ, the higher built-in voltage makes the open circuit voltage higher, but the narrower SCL width makes the long wavelength current response lower.In the case of low junction doping concentration NAI, weak SCL electric field strength will reduce current collection.When the doping concentration in the junction region is equal to the middle value, the short-circuit current density / sc reaches the maximum value, while the maximum value area of the sample activated in air is wider.In the region where the doping concentration in the junction region is higher, naj 58 xlo 15cm - 3, the volt-ampere characteristic curve starts to " flip" and the open circuit voltage ke decreases.These complex characteristics make CdTe doping near the junction region the most critical tuning parameter.Especially for the Motive Power battery activated in vacuum, the junction doping concentration naj can only be taken in a relatively narrow range.
The numerical simulation results obtained here can explain why the CdTe layer sometimes induces the opposite effect of Cu dopant from the back contact.Obviously, Cu easily diffuses through the CdTe layer and accumulates in the CdTe / CdS junction and the CdS layer [ 771.We can prove here that the obtained cell characteristics are very sensitive to the variation of doping distribution near the junction region.Therefore, the fabrication technology of cadmium telluride thin-film solar Motive Power battery not only needs to have higher production rate and stability, but also should be able to better adjust the distribution of Cu - related acceptors.
The influence of other deep energy levels on cadmium telluride thin film solar Motive Power battery has also been reported.Due to compensation for shallow acceptor doping near the back contact CdTe, deep donor near the back contact may cause loss of open circuit voltage [ 89 ].This is consistent with the actual result of. 16, but the loss of fill factor FF is more dominant in the Motive Power battery described in our parameter settings.
From the back contact to the junction region, the distribution of the shallow doping concentration is " high - low - high", which has been used to explain the measurement results of the Motive Power battery obtained by other fabrication processes [ 175 ].The change of " high - low - high" distribution can also be used to explain the aging problem of cadmium telluride thin film solar Motive Power battery [ 176 ].Other shallow doping distributions were also reported, for example: the doping concentration steadily increased from the junction region to the contact electrode [ 89 ].The numerical simulation here shows that all the shallow doping and deep doping distributions will have an impact on the final volt-ampere characteristic curve of the cadmium telluride thin film solar cell.
For the numerical modeling results of cadmium telluride thin film solar Motive Power battery, we also need to mention the early work of Purdue University in the United States [ 177182 ].Other aspects that have been extensively simulated include:
The influence of forward bias and short wavelength on quantum efficiency QE and J - V characteristics ( see 220.127.116.11 Section );and features ⑽.183——185：］；
The study of other different parameters [ 186' 1873.
Pseudo two-dimensional modeling
Polycrystalline thin-film solar Motive Power battery have considerable grain boundaries and non-planar interfaces, and two-dimensional or even three-dimensional numerical modeling should be required.The grain boundary effect of cadmium telluride thin film solar Motive Power battery is more obvious than that of copper indium gallium selenium thin film solar Motive Power battery.Although one-dimensional numerical modeling has effectively averaged grain boundary effect in vivo, the modeling results are in good agreement with the experimental results.On the other hand, developing one-dimensional numerical modeling into two-dimensional or three-dimensional will greatly increase the number of input parameters, many of which we know little about.
As a simplified method, pseudo - two - dimensional modeling can be combined with SCAPS program to deal with one-dimensional numerical simulation and SPICE program to deal with other dimensions with network method.SPICE program is a general analog electronic circuit simulation software with open source code, widely used in integrated circuit design, which can confirm the design scheme of integrated circuit and predict circuit characteristics.A practical application of pseudo two-dimensional modeling is to integrate a thin film solar cell into a module ( see section 7.4.2 ) and design the simulator " process [ 188 ] by the program" module.In the computer program, all " basic unit Motive Power battery" are attributed to diode models in the form of exponential functions, the specific form being specified by the user.All two-dimensional series integration is processed into a network of basic unit Motive Power battery and resistors.Although this method has lost its direct connection with the physical nature of the Motive Power battery, the speed and interaction of numerical simulation for designing series contact have greatly increased.Shvydka and Karpov used similar network methods, combined with the analytic effective medium model, to simulate the effects of non-uniformity and small area defects ⑽ - 191.The effects of non-uniform irradiation and small area contact were also treated in a similar way [ 192 ].
Recently, people have developed a specialized nano-scale feature measurement tool to describe the physical and chemical properties of grain boundaries. We expect such pseudo - 2D modeling or full 2D numerical modeling will be applied to quantitatively describe the influence of grain boundaries on Motive Power battery output performance.
In this chapter, we discussed the material characteristics, fabrication process and theoretical model of cadmium telluride thin film solar Motive Power battery.First Solar's commercial success has enabled cadmium telluride thin-film solar cell technology to realize real industrialization and large-scale $ production.
Although the CdTe material forming the absorption layer has relatively poor crystal quality, this does not affect the overall performance of the cadmium telluride thin film solar cell.Although cadmium telluride thin film solar Motive Power battery can be prepared by several different technologies, no matter what deposition technology is used, " magic formula" CDC L2 activation treatment can give better quality Motive Power battery.This seemingly simple preparation technique masks the complexity of the structure and operation of the Motive Power battery.Excellent scientists and professional engineers have accumulated more than a large amount of preparation experience and knowledge of material properties, and have developed cadmium telluride thin film solar cell technology to a new level' in order to further develop this thin film solar cell technology, we need to raise specific preparation methods to understand the structural mechanism of the cell, not only to know " how to do" but also to know " why to do so".Our overall qualitative understanding of the main working mechanism needs to be raised to a detailed quantitative understanding of all materials and phenomena.Recently, important progress has been made in high-level experimental feature tools and numerical modeling, and the mature and active interdisciplinary research talent group is the cornerstone of the further development of cadmium telluride thin-film solar cell technology.We believe that the development of cadmium telluride thin film solar cell technology will make a greater contribution to the solar cell industry.
Organic solar cell
As described in section 0.3.4, " organic" solar Motive Power battery refer to solar Motive Power battery using organic materials at least in the two photovoltaic conversion steps of photon absorption and carrier generation, while organic materials or inorganic materials can be used for carrier transport and contact electrode collection, so organic solar Motive Power battery can be divided into:
Mixed organic solar Motive Power battery: dye-sensitized solar Motive Power battery, which will be introduced in chapter 9.Complete organic solar Motive Power battery: bulk heterojunction solar Motive Power battery will be introduced in chapter 10.
This chapter will mainly discuss the carrier separation mechanism of a fully organic solar cell, developing the past theoretical model from the experimental results, and paying special attention to the carrier photogeneration and exciton separation of the doped hybrid system and the donor-acceptor interface.
Let's first distinguish between excitons, exciplexes, twins and polarons.In organic solar Motive Power battery, excitons are first generated in the donor material after incident light is absorbed;Excitons diffuse to the donor-acceptor interface and become excimer complexes.The negative charge electrons in the exciplex are attracted by the acceptor material on the other side of the interface and separated from the coulomb potential well of the positive charge hole, the exciplex becomes a twin pair with a more separated state, separating the donor-acceptor interface.The twin pairs are further separated into two independent polarons, and finally the polarons leave the interface and are collected by the electrodes.
Excitons are excited quasi-particles in a solid, forming Coulomb bound electron-hole pairs.Compared with inorganic materials, excitons are more important in organic semiconductors, with lower dielectric constant and longer shielding length.Excitons in organic semiconductors become Frinkel excitons, while excitons with weaker binding energy in inorganic materials are called Valny - Mott excitons?Therefore, the two charges of the organic material Frenkel exciton have strong mutual attraction and are usually in the same molecule.There are also special cases in which the two charges of excitons are located in adjacent molecules of the same type, while twin pairs are located in molecules of different types.The spin state of the two charges is more important.If the sum of the two spin vectors is 0, we get singlet excitons.Due to specific selection rules, human light only produces singlet excitons.The other exciton type, the triplet exciton, has a spin vector other than zero and may have three combinations, so it is called triplet.The interaction after charge injection can also form singlet excitons and triplet excitons, with a theoretical ratio of 1: 3. Only 1 / 4 of the excitons are singlet O. Let's discuss the characteristics of singlet excitons and their influence on organic solar Motive Power battery.The binding energy of the Frenkel singlet exciton is 0.30.5 eV, which is larger than the binding energy of the Valny - Mott exciton in the classical semiconductor.Exciters have a certain lifetime. Typical values in organic semiconductors are on the order of ns, followed by radiation recombination and fluorescence, which is called photoluminescence.triplet excitons generally have lower energy and longer lifetime.For organic solar Motive Power battery, triplet excitons are not important, and only play a role of loss mechanism in some cases because the energy is too low to generate free carriers.The lifetime of triplet excitons is usually several ms, and the radiation recombination that occurs afterwards is spin - forbidden transition, emitting phosphorescence.
The exciplex is an exciton located close to the interface.In fact, it is still attached to a molecule.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.
In short, a polaron is a charge, that is, an electron or a hole, plus a distorted environment around the charge.In crystalline inorganic materials, placing charges in one spot does not change the surrounding environment, because the crystal lattice is solid.This is not the case with unordered organic materials.Placing a charge at a specific molecular point will deform the entire molecule.Moving charge from one molecule to another means that a certain amount of energy is first needed to deform the molecule, which is the polaron binding energy.Therefore, charge transport in organic materials is more difficult and carrier mobility is lower.Such a process can also be understood as self - trapping.It is often difficult to distinguish between polaron self-trapping and Gaussian disordered structures.Because these two phenomena have similar effects on carrier transport characteristics.This similarity is also reflected in calculating the jumping probability of carrier transport.The molecular deformation caused by polaron can be quantified as the above polaron binding energy or phonon cloud.However, in most cases, it does not matter whether polaron is understood as charge.
Twin pairs, also known as polaron pairs, are coulomb bound pairs of positively charged polarons and negatively charged polarons, while twin pairs are located in different molecules.Twin pairs are often intermediate states in which excitons transition to a pair of free polarons, and the distance between the pair of free polarons is far enough not to be affected by mutual attraction.Therefore, understanding twin pairs is also an important issue in understanding the photogeneration of carriers in organic solar Motive Power battery.
Organic solar Motive Power battery are made of low-cost materials and are expected to open up many new photovoltaic market segments, from electronic watches to hand-held calculators to notebook computers. Organic solar Motive Power battery can charge almost any portable electric appliance.The soft and light nature makes organic solar Motive Power battery suitable for being placed in tents to provide internal power or integrated into clothing to charge personal electronic equipment.Although the cost advantages of organic materials and polymers in photovoltaic applications are obvious, these materials also have three major disadvantages:
Organic materials do not directly generate free carriers. Unlike traditional crystalline semiconductors such as Si, Ge, GaAs, the photo-induced excitation of organic materials does not immediately generate free carriers.The most important optical excitation in organic materials with crystal structure or amorphous structure is Frenkel singlet exciton with binding energy of 0. 30. 5 eV.It should be emphasized that the excess photon energy beyond the absorption edge cannot increase the number of photon excitations, because the van der Waals force of molecular solid is weak, the oscillator intensity of direct photoionization is very small, and the excess photon energy beyond the 0 - 0 transition does not have much effect.Van der Waals force is a general term for attraction and repulsion between molecules. It is divided into orientation force formed by dipoles between polar molecules, induction force for changing electron distribution between polar molecules and nonpolar molecules or polar molecules, and dispersion force formed by instantaneous moving dipoles between nonpolar molecules.High - energy photons also produce a highly excited franck - Condon state, dissipating excess electron vibrational energy in a time scale of about 100 fs.The classic saying of Frank - Condon Principle is.When an electronic transition occurs, the position of each nucleus in the molecule and its environment can be considered almost unchanged.The state formed is called Frank - Condon state, and the transition mode belongs to vertical transition.These transitions occur at the point where the nuclear kinetic energy is the smallest, that is, at the limit position of vibration, so there is no obvious change in nuclear spacing or momentum during the transition.Therefore, the first problem to be solved in the development of organic solar Motive Power battery is the specific method of separating excitons into free carriers.
Complete collection of photogenerated carriers: For organic materials, especially disordered organic materials, carrier mobility is orders of magnitude smaller than that of crystalline inorganic semiconductors such as Si.This places strict limits on the maximum thickness of organic solar Motive Power battery.Moreover, it is very difficult to find large mobility for both electrons and holes in the same organic material.If enough carriers cannot be accumulated in the body, the collection of carriers by the contact electrode will be impeded, thus greatly reducing the conversion efficiency of the organic solar cell.
Effective exciton harvesting: due to the large binding energy of organic materials, it is almost impossible for relaxed excitons to separate intrinsically and become free carriers, and excitons need to diffuse to the charge transfer center or donor-acceptor interface to realize the photovoltaic effect of organic materials.Charge transfer centers are also called electron - donors?" Electron - donor - acceptor complex" refers to a form in which two or more molecules or different parts of macromolecules are bound to each other, and they transfer charges to each other. Electrons are transported from electron donors to electron acceptors, while electrostatic attraction makes the molecular complex relatively stable.The typical exciton diffusion length of disordered organic materials is not greater than 1020 nm, which limits the size of the donor phase to only 2040 nm.However, an organic film 40 nm thick will be too thin to provide complete light absorption.The solution to the contradiction between too short exciton diffusion length and too weak light absorption of thin films is planar multilayer or bulk heterojunction O - planar multilayer, which is to vacuum deposit small molecular materials and maintain good thickness and shape control of each layer.The main problem with these devices is still exciton harvesting and contact electrode collecting carriers.
The preparation of polymer-based planar heterojunction devices by spin coating or screen printing will encounter considerable problems because the solvent capable of dissolving the donor polymer will not dissolve the acceptor polymer and the solvent capable of dissolving the acceptor polymer will not dissolve the donor polymer.Heterojunction solar Motive Power battery made of semiconductor polymers can make full use of donor-acceptor mixture phase separation in the range of several nm to tens of nm.In principle, this scale of several nm to tens of nm almost perfectly matches the exciton diffusion length, thus the exciton separation at the donor-acceptor interface creates conditions for effective exciton harvesting.
However, the exciton separation of bulk heterojunction solar Motive Power battery has not yet been fully understood.Moreover, only on the side of the acceptor ( usually based on cm ) has the phase separation proved to be complete, i.e. the molecules are in a dispersed state, but it is not clear whether the pure donor polymer phase forms a dispersed state within the range of about 10 nm or whether the donor polymer and the acceptor are uniformly mixed.The main technical problems faced by bulk heterojunction solar Motive Power battery are:
Full light absorption within a typical 100 nm device thickness;
After the donor polymer is mixed with the strong electron acceptor, the open-circuit voltage' decreases' the electron acceptor is also called an oxidat or oxidizing agent, which is a molecule or atom that is reduced by obtaining electrons, F2, O2, Cl2, Br2, etc. are common strong electron acceptors;_ carrier transport through random interpenetrating networks'
This chapter will focus on discussing the mechanism and process related to photoexcitation of organic solar Motive Power battery, while the specific technical introduction of material-sensitized solar Motive Power battery and bulk heterojunction solar Motive Power battery will be carried out in Chapters 9 and 10 respectively. The photoexcitation materials involved include not only undoped pure polymers but also lightly doped or heavily doped polymers.' We will also explain in detail the following important phenomena:
Twins with exciton quenching and exciton separation into carriers at the charge transfer center are composed of a pair of positively charged molecules and negatively charged molecules surrounded by surrounding solvent molecules, usually resulting from chemical reactions such as charge transfer, bond breaking and group transfer of precursors.Onzag type separation of twin pairs;
Interface exciton separation;Charge control means for motive power battery charger
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