Solar power is the conversion of energy from sunlight into electricity, either directly using photovoltaics (PV), indirectly using concentrated solar power, or a combination. Concentrated solar power systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam.
Actual. 1 Band Gap and Lattice Constant of Different Chalcopyrite Compounds [ 1 ] As early as 1974, single crystal chalcopyrite materials were used in solar cell research [ 2 ].The absorption layer of the first chalcopyrite solar cell was Cui NSE2, which became the most advanced small band gap technology at that time.At present, the laboratory conversion efficiency record of Cu - In - Ga - Se thin film solar POWER is close to 20 % [ 3 ], which can be compared with polysilicon solar POWER.Many problems faced by mass production have been properly solved, test lines have been run in many countries, and commercial components have been sold in the solar energy market.As of 2005, the market share of Cu - In - Ga - Se thin film solar POWER is not large, but its successful commercialization is just around the corner.
Advantages of Cu - Ga - Se Thin Film Solar Cells
The advantages of Cu - In - Ga - Se thin film solar POWER are summarized as follows:
high conversion efficiency;
Production cost or manufacturing cost is low;There will be no shortage of raw materials;
The energy recovery period is short;
There are many researchers around the world.
We will briefly introduce these advantages and explain them in further detail in the rest of this chapter.
Among all types of thin film solar POWER, copper indium gallium selenium thin film solar POWER have obvious advantages of high conversion efficiency.The laboratory conversion efficiency record of the copper indium gallium hit thin film solar cell is close to 20 %, and this small-sized cell only uses a layer of antireflection film and standard metal grid lines, and does not use any complicated concept like the crystalline silicon solar cell with laboratory conversion efficiency record.The conversion efficiency of subassemblies is close to 17 % w, and assemblies with square feet or more have a conversion efficiency of 12 % 14 %.It can be imagined that the highest conversion efficiency of copper indium gallium selenium thin film solar POWER will be further improved in the future.The development of multi-junction POWER can even make their conversion efficiency exceed the theoretical conversion efficiency limit O of crystalline silicon solar POWER. Compared with amorphous silicon thin-film solar POWER, copper indium gallium selenium thin-film solar POWER do not decay under light and have higher stability.The excellent performance of Cu - In - Ga - Se thin film solar POWER is not only limited to standard test conditions, but also has excellent test data under outdoor installation conditions.Outdoor tests show that its service life can be compared with that of crystalline silicon solar POWER.
The low cost advantage of copper indium gallium selenium thin film solar cell is similar to other thin film solar cell technologies, thanks to cheap substrate, effective utilization of raw materials, large production rate, large area deposition at low temperature and monolithic integration.
If the thickness of the substrate is not taken into account, the total thickness of all thin films in the Cu - In - Ga - Se thin film solar cell is 24 mm, which means that the use of semiconductor materials is much less than that of crystalline silicon solar POWER.There will be no shortage of raw materials for mass production.
When considering the contribution of photovoltaic technology to future energy supply, the energy recovery period is obviously an important parameter.Thin film technology has lower process temperature and shorter process time, which will result in shorter energy recovery period EPBT.
As we will introduce in this chapter, different chalcopyrite materials and different process methods can be selected to prepare copper indium gallium selenium thin film solar POWER.Therefore, various application products of solar POWER can be designed according to different application requirements, weighing conversion efficiency and cost.The power demand may be MW or MW, and the human light intensity may be indoor or outdoor or concentrated.The copper indium gallium hit thin film solar cell can be grown on a rigid substrate or a flexible substrate.Even in the harsh environment, the Cu - In - Ga thin-film solar cell still performs well, not only has high mechanical stability, but also can operate at extreme temperatures and is suitable for space applications with high radiation.
Copper, indium, gallium and selenium thin film solar POWER have been extensively studied all over the world.Researchers from relevant universities, research institutions and high-tech companies have considerable experience and will continue to invest in them for a long time.This ensures that any problems encountered in industrialization can be solved through sufficient background knowledge, and the technology being developed can be developed into future products.The huge financial risks brought about by large-scale production to a certain extent limit the commercialization of copper indium gallium smashed thin film solar POWER.However, more and more companies have begun to participate in the research and development and production of copper indium gallium selenium thin film solar POWER, which will strongly promote the commercialization process.
Process for Cu - Ga - Se thin film solar cell
Preparation process of copper indium gallium selenium thin film solar cell
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Type c ds is used as a buffer layer to form a heterojunction.Finally - Layer 4. VSHHWFLL is N - type heavily doped ZnO with wide band gap as transparent front
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Electron microscope, scanning the surface of the sample with raster pattern by high-energy electron beam and forming an image.The electron interacts with the atoms of the copper, indium, gallium and selenium thin film given by SEM, and the signals generated include information such as the shape, composition and conductivity of the cross-sectional surface of the solar cell on the sample surface.
Films suitable for photovoltaic applications can be prepared by various processes.In the test line, the growth mode of the absorption layer can be multi-source evaporation or process technology, i.e. reaction annealing of the metal film.The process flow for preparing cuins 2 assembly.In the following sections, we will introduce the common processes for preparing each layer of film, and these advanced processes are the foundation of the production line.Section 6.6 will discuss the process that is still in the development stage, which can greatly reduce the cost in the future.
The most advanced absorbing layer material for Cu - In - Ga - Se thin film solar POWER is based on CUI NSE 20, which has a small band gap of only 1.0 eV and can increase Ga, thus obtaining Cu ( In, Ga ) Se2 and achieving a standard band gap of 1.15 eV.The process can also add S to obtain Cu ( In, Ga ) ( Se, S ) 2.CUI NSE2 or Cu ( In, Ga ) ( Se, S ) 2 absorption layers all need to reach an optimized Na concentration in order to obtain better film properties ( see 188.8.131.52 Section ).
The other technical route starts with cuins 2.In contrast, the growth of cuins 2 requires excess Cu ( Cu - excess ) to have a high band gap ( eg = l 5ev ), so it is unnecessary to add elements such as na or ga.
The most common absorption layer preparation methods are multi-source evaporation and process technology.Evaporation can control film formation more directly, determine film thickness distribution, realize band gap design, and improve the utilization of Cu ( S, Se ) phase.This may be the reason why evaporation has reached the laboratory conversion efficiency record.As long as enough Ga is included, the film obtained by evaporation can achieve a standard band gap of 1.15 eV.
The process is different from evaporation, which will cause Ga to gather near the back contact and lower Ga content in the main area of the absorption layer.so that the band gap is lower than the optimum value.Adding H & " to the annealing atmosphere can overcome this problem to some extent, but it will also increase the overall complexity of the process and equipment.
Multi - source evaporation
Multi - source evaporation or co-evaporation is a more direct method of film growth. The required film can be formed by co-evaporating the constituent elements onto a heated substrate.The stoichiometry of the concentration ratio of group vi elements to metal elements requires that the group v elements remain at high pressure until the initial stage of cooling of the substrate begins.On the other hand, the molecular state of the concentration ratio of group I metal elements to group di metal elements is adjusted by strictly controlling the temperature of the metal source.It is necessary to use a single crystal substrate with appropriate lattice constant and surface characteristics to realize epitaxial growth of thin films.The stoichiometry refers to the molar ratio relationship between reactants and products in the chemical reaction. Group VI element Se reacts with metal elements Cu, In and Ga. The generated Cu in and Gai - XSe2 need to reach the required element ratio, which is also the ratio of oxidant Group VI element and reducing agent metal element in the reaction process, namely stoichiometry.The molecular state refers to the proportion of different molecular forms in the chemical reaction. the chemical reaction not only generates cuinigahses. Cu - rich, but also generates 11 additional Cu - vi binary phases, i.e. the proportion of group I metal elements and group m gold j elements, i.e. the molecular state.
The morphology and other properties of the | film are very dependent on the sub - state of SiO - 4 - ▲ sub - state.The Cu - rich film presents larger grain size, which is a mixture of chalcopyrite crystals CIGS Se and binary phases Cu and Se, which are close to the ideal composition, while binary phases Cu and Se usually appear on the surface after the sample is cooled.Compared with Cu, it is only 0.480.490.500.510.520.53.
The Cu - rich chalcopyrite film of the material grown with Ga / ( Cu + Ga ) remaining before etching actually has a lateral conductivity of .4 Cug ASE 2.The low density of growth @ trap under Cu - rich conditions also reduces the appearance of films that compensate for JFE length than those grown under Cu - poor conditions.
High effective doping.Before the test, these phenomena were removed by chemical etching, indicating that the binary phase Cu xse is very important for the growth mechanism Curse local segregation, ensuring the accuracy of the measurement, and also has a great influence on the mixing of group vi elements. if the molecular state of the film is close to 1, excessive binary phase Cu. se will show local segregation, and the film will be uneven within the range of number p ( m ).Segregation refers to the phenomenon of uneven distribution of each constituent element in the alloy during crystallization.
Although the grain size is small, this simple co-evaporation method can produce Cu ( In, Ga ) Se2 thin films slightly depleted in Cu ( Cu - Pool ), and has achieved a conversion efficiency of 14 %.On the other hand, several schemes have been developed to fully play the role of Cu - rich film growth.
In the double-layer method [ 1 ], Mr. Wang grew a Cu - rich coarse grain seed layer.Then, the proportion of Cu is reduced so that the excess amount of Cu in the seed layer is gradually consumed in the second stage, resulting in the formation of a thin film slightly depleted in Cu.The purpose of this is to achieve excellent properties of Cu - rich films while reducing binary phase Cui Se in Cu - poor films.If the binary phases Cu and Se are completely consumed, this growth mechanism will not be maintained and the interrupted crystal growth will cause new nucleation.The advantage and potential of the double-layer method is that it can be converted into a commercial on-line process so that the substrate passes through the Cu source first, then through the in source and the ga source.
Other schemes are based on Cu - poor films, even films without Cu, but adding more Cu in the second stage will become an inverted double-layer method o if enough Cu is added in this stage, the recrystallization of Cu - rich films will be formed.In this case, it is necessary to reduce the Cu content again in the third stage to form a single-phase material. Such a scheme is called a three-stage process [ 11 ].The three-stage process has created a record of laboratory conversion efficiency ( close to 20 % ) to date.In principle, the problem of double-layer process still exists in the three-stage process, and the growth mechanism may not be maintained after excess Cu is consumed in the third stage.However, the three-stage process can achieve more precise control. By observing the temperature change of the substrate, the critical point [ 12 ] of the film entering and leaving the Cu - rich state can be determined.In - situ process control can monitor the power transferred from the substrate to the heater in real time to judge the temperature change and Cu - rich state of the substrate.Another process control is the laser light scattering method, i.e. the three-stage process of monitoring the reflected light intensity of the substrate off the base axis u can obtain a certain film thickness distribution and an accurate ga / ( in + ga ) ratio, and achieve good SOLAR POWER performance through band gap design.
The two-step process is another method for preparing the absorbent layer [ 14' 15 ].In the first step of the process, the metal precursor is deposited by sputtering.Because pure Ga has a low melting point, it is not suitable for sputtering, and Cu / Ga alloy is often used as sputtering target.In the second step of the process, the precursor is exposed to a high-temperature atmosphere containing sulfur groups and selenized or sulfided to form a chalcopyrite structure.
The process is especially suitable for large-scale production, and the commercial process can use off - the _ shelf equipment for direct current sputtering.The process is characterized by good reproducibility and high thickness uniformity of large-area films.The important Cu / M group element ratio can be accurately controlled in the first sputtering step.The problem of equipment degradation caused by long-term high-temperature corrosive environment is limited to the second step.Since the second step of controlling the sulfur ideal ratio can largely self - adjust, the equipment corrosion problem of the process is not important.
Selenization or vulcanization as a second step can be carried out by a tube furnace or rapid thermal treatment RTP.The tube furnace is filled with a mixture of inert carrier gas and reactive component 144 or H2Se for annealing treatment.In this slow batch process, several substrates can be processed simultaneously, and the size of the substrates is limited by the maximum diameter of the tube furnace.RTP is a relatively new process 17 ], one of which is to evaporate Se onto the element stacking layer formed by the metal precursor and then carry out high temperature annealing in H2S atmosphere through RTP.In a two-step process, the chalcopyrite crystallization process largely depends on the thermodynamics of the material system and the kinetics of crystallization.According to the detailed study of these basic characteristics, the process [ 18' ] 9 can be optimized.
In addition to preparing Cu ( In, Ga ) ( S, Se ) 2 absorption layer with small band gap, the process is also particularly suitable for preparing Cuins: ( EG = 1.5 eV ) [ 17 ], simply placing massive or powdery S next to the substrate in RTP cavity, thus avoiding the separate chalcogenide evaporation process and the use of toxic gas.Moreover, the annealing time is very short and only takes a few minutes at the highest temperature.Cu - rich precursors satisfying Cu / in = l 21.8 can generate Cu and s, which is helpful for crystal growth.The films obtained under these conditions exhibit high P - type conductivity, so sodium doping is not important ( see 184.108.40.206 section ) [ 2 ] and diffusion barrier and sodium precursor films are not required.As mentioned earlier, it is also unnecessary to introduce other elements to adjust the band gap.
Recognizing that Na has an important impact on device performance is a major breakthrough in the research of Cu - In - Ga - Se thin film solar POWER.Na is beneficial to the growth mechanism and obtains good morphology, and more obviously forms high-efficiency P - type doping.In the POWER dominated by space charge region recombination, the increase of open circuit voltage is observed to be consistent with the increase of net doping [ 21 \ na diffusion onto the prepared thin film can also improve device performance, which further shows that the optimization of electrical properties by sodium doping is more obvious than the improvement of morphology [ 22 ].
Especially in the slow evaporation process, Na diffused from the soda-lime glass substrate to the absorption layer through back contact can produce the required Na concentration of the absorption layer film.If the substrate does not contain Na, the sodium salt can be co - evaporated or deposited on the back contact as a precursor film before the absorption layer is deposited.In a faster process, this method is often used with soda-lime substrate in combination with diffusion barrier under back contact to achieve more accurate Na concentration control.
Cu ( In, Ga ) Se2, which is poor in Cu, needs sodium doping, but CuIns is often excessive in Cu. Culn thin films have the characteristics of large grain size and high conductivity and do not need sodium doping.
The most common back contact of copper indium gallium selenium thin film solar cell is Mo thin film prepared by sputtering.By controlling the pressure of the working gas in the sputtering process, the internal stress in the mo film can be adjusted to yang 3 in a wide range.The optimized film can be well attached to glass or other substrates, while laser scribing or photolithography can directly prepare the desired pattern on the Mo back contact.One alternative to sputtering is electron gun evaporation of EGE, which is still limited to laboratory-scale preparation, but may have cost advantages in large-scale production.
During the preparation of the absorption layer, a more complex Mo - chalcogenide intermediate layer may be formed.At low temperatures, kV at forward bias ), the curve often deviates from the ideal diode model.Such curved bend - over is believed to be caused by the barrier that the SOLAR POWER's back contact has a blocking effect.However, with the relevant instruments, it is impossible to detect the voltage drop M0 on the back contact and other models unrelated to the barrier barrier to explain the bending of the KV curve [ 25 ].At present, people are studying other back contact materials instead of Mo to achieve greater light reflection and novel device structure ( see Section 6.6 ).
As mentioned earlier, it is not absolutely necessary to deposit a diffusion barrier layer of SiO 2 or Si 3N 4 between the Mo back electrode and the glass substrate, but doping can be controlled more accurately.On the other hand, foil substrates often require the preparation of an additional thin film under the Mo back electrode to block impurities and achieve substrate planarization and substrate isolation [ 26 ].
CdS buffer layer is very thin, usually only 50nm. The solution of [ 27 ] OCBD prepared by chemical water bath deposition ( CBD ) uses CD salt and S source thiourea as solute and ammonia as solvent.The substrate on which the absorption layer was deposited was immersed in a cold solution and then heated to 6080 c.After thiourea hydrolysis, S ions and CD ions form the required CDs.The film is grown directly on the substrate or nanoparticles are formed in solution and then deposited on the substrate.Depending on the deposition conditions and solution environment, the film may contain a large amount of O and H ..CDS CBD has good reproducibility and good SOLAR POWER performance for any chalcopyrite absorbing layer.
The window layer of the Cu - In - Se thin film solar cell, like ZnO as transparent conductive oxide TCO, is prepared by sputtering or metal organic chemical vapor deposition ( MOCVD ).ZnO thin films are required to have high lateral conductivity to reduce ohmic losses.Therefore, the Zn window layer needs to be heavily doped with A1 or Ga ( sputtering ) or B ..However, depositing a thin film with low lateral resistance directly on the buffer layer will increase pinhole defects [ 28 ] and affect the characteristics of absorption layers such as band gaps [ 29 ].In order to avoid such problems, a 100nm thin layer of undoped ZnO with low conductivity can be deposited first, such as sputtering with undoped targets or adding 02 to the working gas.
The cost of ZnO window layer is high, which has a great influence on the component cost of Cu - In - Ga - Se thin film solar cell.Therefore' lower resistivity is required to reduce the thickness of the ZnO window layer.In fact, the resistivity of large-area ZnO thin films is on the order of 5x10 - 4 FLCM, and there is little room for improvement, because higher doping will reduce electron mobility and carrier absorption will affect carrier transport.The performance of ZnO thin films is considered to be very close to the physical limit [ 3 < in the long run, ZnO needs to be replaced by other materials with higher mobility ( lower electron mass ).Methods to reduce the cost of ZnO preparation include using ceramic targets or using metal targets for accelerated aging test required by OIEC 61646 certification, including wet heat test 0. The characteristics of ZnO window layer in wet heat test are very important to the stability of components.The lateral resistance tends to increase, causing loss of fill factor.Therefore, it is necessary to optimize the preparation of ZnO, taking into account not only the performance of the resulting components, but also the degradation under humid and hot conditions.
In addition to the above cell preparation process, the production of copper indium gallium selenium thin film solar cell module also requires monolithic integration, or monolithic interconnection, i.e. splitting the module into interconnected regions by laser scribing, photolithography or mechanical scribing. After depositing transparent conductive oxide TCO, Mo back contact and TCO form series connection.The front surface may be metallized, but such a process is generally not used.The monolithic integration method is divided into three graphic characterization steps:
First patterning 1, PL: Mo back electrode is scribed to form isolation;Second patterning 2, P2: scribe the absorption layer to form TCO - filled pores;Third patterning 3, P3: scribe the interconnect region to the mo back electrode to form the isolation of the interconnect region and the series connection between the interconnect regions.
The suggested P1 pattern scribing tool is a pulsed Nd - YAG laser, but lithography is also feasible.After laser scribing, the substrate needs to be cleaned again with a rotating brush to remove dust scattered after scribing.P2 graphics and P3 graphics are mechanically scored.The scribing of P2 pattern can be performed before or after the undoped ZnO layer is deposited, and the latter method can obtain a better contact electrode.Optical microscope is the type of microscope that uses visible light and lens system to display large and small sample images and observe them directly with naked eyes. It is also one of the oldest microscopes, and its design can be traced back to about 1600 years.
The monolithic integration divides the interconnection area, but reduces the effective absorption area of the SOLAR POWER and also reduces the photogenerated current.In principle, the score line needs to be as thin as possible, the distance between PL, P2 and P3 needs to be as small as possible, and the distance between interconnected areas needs to be as large as possible.In practice, scribe lines of a certain width should maintain sufficient isolation ( PL, P3 ) and contact resistance.Therefore, the total width of the score line is 0.5
mm。In addition, the allowable distance between interconnect regions is limited by the lateral resistance of ZnO thin films.If the ZnO thin film is thicker, the distance between the interconnected regions can be larger.However, thicker films not only have higher cost, but also lower transparency will reduce photogenerated current.Therefore.The typical distance between interconnected areas is 510 mm, limiting the loss of effective absorption area to about 10 %.The absorption layer of the wide band gap has a smaller current density, which can make the design of components more flexible.Under standard test conditions, CISE with band gap of 1.5 eV has a typical photogenerated current density of 42 mA / cm2, while CIS with band gap of 1 eV has a typical photogenerated current density of 22 mA / cm2.Computer simulation can give an optimized component characterization pattern [ 32 ] based on a series of determined film properties.
Single Chip Integration for Accelerated Aging Test ( Damp Heat Test )
The degradation of the components of the has a considerable impact.A preliminary model of contact electrode degradation can be obtained by scanning the specially prepared test structure with X - ray emission spectrum ( the scribe line distance increases and ZnO thickness decreases ).Before the wet heat test, a large S signal can be observed at the P3 score line, indicating that the Mo surface is vulcanized during the formation of the absorption layer.Due to the attenuation of the signal by ZnO thin film, the S signal observed at P2 scribe line is smaller.After the damp heat test, the S signal at the P2 score line increased greatly.This spectrum indicates the formation of ZnSO _ 4, i.e. the humid and hot environment causes a chemical reaction between MOS _ 2 and ZnO, which degrades the contact electrode at the P2 score line.
because that graphic score affects the on-line vacuum proces,
There is still room for improvement in the standard process flow described here.Graphic characterization is also critical to production rate or production cycle.Finally, the lamination package of the copper indium gallium selenium thin film solar cell module is the same as that of the crystalline silicon solar cell, and the glass / glass lamination can be completed by using a standard laminating machine for EVA film.The quality of the laminated package is critical to passing the accelerated aging test.
Characteristics of Cu - Ga - Se Thin Film Solar Cells
Copper - indium - gallium - selenium thin film solar cell is a rather complicated device, which includes several layers of polycrystalline compound semiconductor thin films and several heterogeneous interfaces.However, the test methods and theoretical models for studying various characteristics of devices have developed to a certain extent, and now there are several novel measurement methods for various main characteristic parameters.For example, KelvinProForcemicroscopy ( KPFM ) can measure the work function, which has a sub-micron lateral resolution.Reflective electron emission spectroscopy ( IPES ) can measure the conduction band structure on the same straight line.The X - ray emission spectrum' can analyze the interface in the material body.KPFM is a non-contact form of atomic force microscope AFM, which can observe the work function of the surface on the atomic or molecular scale, thus obtaining information such as the composition and electronic state of the local structure on the solid surface.As a surface science and technology, IPES is used to study the unoccupied electronic structures of surfaces, films and adsorbates.The calibrated electric f beam has an accurately limited energy ( < 20ev ). after the electrons are irradiated on the surface of the sample, they are coupled with the unoccupied electron state of the high energy level, attenuated to the unoccupied electron state of the low energy level, and the transition is accompanied by emission radiation.In IPES technology, photons emitted by the decay process are detected to form an energy spectrum of the number of photons with respect to the energy of the incident electron.IPES is a surface-sensitive technology because of the low energy of human-emitted electrons and their penetration depth is only a few layers of atoms thick.
Generally speaking, if you want to get the quantitative parameters of devices and materials from the measurement results, you need a certain theoretical model to describe the correlation between measurement characteristics and physical parameters.Because polycrystalline compound semiconductor thin films are complex, only theoretical models under certain approximate conditions can be used, and the obtained physical parameters are also effective parameters under specific theoretical models?These effective parameters are useful for quantitatively comparing different samples, but some errors may occur in the calculation of different situations.Numerical simulation has become a useful tool for understanding device characteristics.Due to different approximate conditions, theoretical models useful for other types of solar POWER may not be applicable to copper indium gallium selenium thin film solar POWER.To sum up, the material science of chalcopyrite crystal structure and the copper indium gallium selenium thin film solar cell are an extensive and active research field.In the following, we can only discuss several issues. Readers can look up relevant literature and materials, and a deeper understanding can begin with an overview [ 33 ].
As a direct band gap semiconductor, the absorption coefficient of chalcopyrite crystal structure is very high, so the absorption layer can be thinner in the design of copper indium gallium impact thin film solar cell.However, this also means that the part absorbing human light is very close to the surface of the cell, and photo-generated carriers are prone to surface recombination.Even if the doping of chalcopyrite crystal can be well controlled and most of the photo-generated carriers generated between the surface and the P - N junction will still be lost due to more surface recombination after surface passivation, so the cell design of homogeneous junction is not easy to achieve higher conversion efficiency.In order to avoid such problems, it is necessary to design a copper indium gallium selenium thin film solar cell as a heterojunction with a window layer / absorption layer.Due to the wide band gap of the window layer, the heterojunction transfers light absorption from the surface to the interface in the body.Even if the surface passivation is not used and the surface recombination speed is high, the window layer does not absorb much human light.The carrier loss of the heterojunction will not be large.
In an ideal solar cell, dark current ( several * ) is used to describe the volt-ampere characteristics conforming to the superposition principle.However, in Cu - In - Ga - Se thin film solar POWER, dark current is often replaced by shielding current, because the shielding current under illumination is different from dark current due to more complex compound machines.The current of copper indium gallium selenium thin film solar cell is photogenerated current minus shielding current.
Although the heterogeneous interface in the body will also cause certain interface recombination, resulting in loss of photogenerated current, certain shielding current and lower open-circuit voltage v ..., the calculation results show that the interface recombination has little influence on the SOLAR POWER performance and depends on:
The composite center density of the interface;
doping the absorption layer and the window layer;
The density of state at the interface and the type of charge carried;The conduction band structure.
Although we hope to reduce the density of interface states as much as possible, the actual large-area crystal growth technology is not easy to reach the absolute clean environment required for defect-free epitaxial growth.On the other hand, reducing the electron concentration or hole concentration at the interface can also reduce recombination. This more feasible method requires special design of doping, band structure and interface charge distribution.
For shielding current ( open circuit voltage VOC ), the effect of reducing the electron concentration or hole concentration at the interface is similar.Considering the photogenerated carriers, the electrons to be collected from the absorption layer are the majority carriers of the interface, forming the N - type CIGSE inversion interface O of the N + type window layer / P - type absorption layer heterojunction. Therefore, the Fermi energy level of the interface is close to the bottom of the conduction band ￢The Fermi level near the interface of the absorption layer is in the center of the band gap, and the interface charge is positive charge ( donor impurity ), while the P - N homojunction of CIGSE is deep inside the CIGSE, far away from the interface with more defects, reducing the interface recombination speed.
The conduction band of the absorption layer should be lower than the conduction band of the window layer, forming a spike to ensure that electrons are the majority carriers.If it is not a spike but an opposite cliff, the conduction band of the absorption layer will be far away from the Fermi level, reducing the electron concentration, and the electrons from the interface on one side of the window layer will recombine with holes from the interface on the other side of the absorption layer, lowering the barrier of the heterojunction.
In a real copper indium gallium hit thin film solar cell, the buffer layer and its preparation process can meet such design requirements.Chalcopyrite crystal structure has certain surface and interface characteristics [ 37 ], but the measurement methods of energy band structure and Fermi level are not direct and the results vary greatly [ 38 - 41 ].The energy band diagram described in .11 is a reasonable approximation and can form an inverted interface that meets the above requirements.Theoretical and test results show that the band gap increases along the front surface direction [ 39' 42' 43 ], and the increased band gap lowers the valence band top, reduces the hole concentration, and further reduces the surface recombination.Obviously, only when the defect density of the increased interface band gap is small enough can the buffer layer play a role in improving device performance, and lattice matching between CDS and CIGSE is achieved through the CDSM layer [ 44 ].
In Cu - In - Ga - Se thin film solar POWER, the ordered vacancy compound ( OVC ) on the surface of CIGSE thin film plays an important role.The Cu - depleted ordered vacancy compound OVC is a polymer semiconductor material with the molecular formula Cu in 3Se 5 or Cui N2 Se 3.5, and when Ga is doped, ( ( 11!After the ordered vacancy compound OVC is formed in the CIGSE thin film by 1 -, 03, 々5 or CUDN GAJZSEA. S, the heterojunction between CIGSE and CDS becomes a series of two heterojunctions: one is an inverse heterojunction composed of P - CIGSE and N - OVC;The other F is the P _ N junction of the homotype heterojunction heterojunction composed of N - OVC and N - CDS, which is prepared in the narrow band gap absorbing layer CIGSE, greatly reducing the defects at the interface.At the same time, a transition is formed between the CIGSE and CDS band gaps, reducing the band gap step between them, thus reducing lattice mismatch, reducing the interface state density, and improving the performance of heterojunction and Cu - In - Ga - Se thin film solar POWER.
Gradient band gap is considered to improve the performance of Cu - In - Ga - Se thin film solar POWER.The band gap of the absorption layer is no longer uniform, but is larger near the back contact and buffer / absorption layer heterostructure, which is called V - profile.The gradient band gap near the back contact drives photogenerated carriers away from the back contact, similar to the back surface field BSF of crystalline silicon solar POWER.However, the gradient band gap near the buffer layer / absorber layer heterogeneous interface can effectively reduce interface recombination.Because the larger the Ga 8 in + Ga ratio is, the larger the band gap of Cu ( In, Ga ) Se2 is, increasing the Ga 8 in + Ga ratio on both sides of the absorption layer can realize V - shaped distribution.The advantage of co-evaporation is that the ratio of Ga 8 in + Ga ) can be controlled to form a gradient band gap.6.4.2 The majority carrier concentration ( hole ) of chalcopyrite film is 10141017 CNT 3, which is an intrinsic P - type semiconductor, if there is no additional doping.Donor O and acceptor Na are the most common impurities that affect carrier concentration.The donor concentration is close to the acceptor concentration. Compensation or self-compensation 0 compensation means that the donor impurity and the acceptor impurity cancel each other out, while self-compensation means that the impurity acts as both the donor and the acceptor.The effective mobility of polycrystalline thin films at room temperature is 1100 cm2 / vs.The dependence of conductivity on temperature and Hall effect data in polycrystalline or single crystal samples can only be obtained from CuG ASE 2 of excess Cu, which has been summarized in the literature [ 16 ].
Charges captured by grain boundaries expand the space charge region into the grain, forming band bends and barriers.Hall effect measurements show that the barrier height is 50 - 150 MeV and can be explained by the charge density of 1012 cm - 2 at the CGSE grain boundary [ 47 ].Kelvin force microscope KPFM under high vacuum is a novel tool for detecting characteristics, which can further describe the grain boundary model for trapping charges.KPFM has a high lateral resolution and can measure the work function of specific grain boundaries.According to KPFM measurements, the average work function difference of 110 MeV at grain boundaries is consistent with Hall effect measurements [ 48 ].The electric field near the grain boundary will cause minority carriers to drift into the grain boundary.However, even in thin films with fine grains, higher photogenerated current can be observed, which contradicts the excessive grain boundary recombination in the grain boundary model of trapped charge.It has been proposed that the grain boundary model based on energy band structure calculation can compare the effects of various grain boundaries on device performance by two-dimensional numerical calculation [ 5D ].In KPFM measurement, the work function or surface photogenerated voltage SPV varies locally with the intensity of human light, which indicates that the trapped charge model and the energy band structure model can describe the characteristics of grain boundaries from different aspects respectively [ 51 ].
According to the steady-state photoluminescence and photoluminescence luminescence decay phenomenon, many studies have come up with the way and life of radiation recombination.At low temperature ( 8.5K ), radiation recombination is the main recombination mechanism of high quality polycrystalline thin films [ 52 ].According to the measurement, the lifetime of free carriers is on the order of several ns.The lifetime associated with defects may be higher due to carrier trapping.Recent photoluminescence studies [ 53 ] have found that Cu ( In, Ga ) Se2 compounds have one shallow donor level and two shallow acceptor levels ( Al, A2 ).It was found that the depth of shallow donor level and shallow acceptor level deepened from ( D = 10 MeV, A1 = 40 MeV, A2 = 60 MeV ) of CUI NSE 2 to ( D = 13 MeV, A1 = 60 MeV, A2 = 100 MeV ) O of CUG ASE 2, and the concentration of shallow donor level and shallow acceptor level was related to QV ( In + Ga ) during film preparation.For Cug ASE2, shallow level defects can explain the measurement data of photoluminescence and Hall effect [ 54 ].It can be considered that these shallow energy level defects are point defects of the ternary system, and their formation energy and defect depth [ 55 ] have been obtained by theoretical calculation, but such a theory cannot explain more complex defects and impurities, which are very important.
At room temperature, the lifetime of photo-induced luminescence decay caused by non-radiative recombination is tens of ns.The parameters describing minority carrier transport are minority carrier diffusion length, which can be obtained from electron beam induced current EB ICT 5 and quantum efficiency 5 ], and the minority carrier diffusion length of high quality films is typically 12 mm ..This means that the minority carrier diffusion length is close to the space charge region width, and it is not easy to get two parameters from the same measurement result.In order to solve this problem, measurements need to be made with the applied bias voltage changing [ 58 ].Wind and solar power systems: design, analysis, and operation
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