Research Of InP-based Narrow Band Gap Solar Cell

Most III-V compound semiconductor materials have direct band gap structure,and solar cell fabricated with this material exibit excellent spectral response,has been widely used in various spacecraft due to its highest conversion efficiency,strong radiation resistance and good high temperature characteristic.III-V compound solar cell has leading the development of solar cell with the highest conversion efficiency for decades.The rapid development of epitaxial growth technology and device process has made the conversion efficiency of single-junction,double-junction and triple-junction solar cells approach their theoretical limits.Further improvement needs to further divide the solar spectrum to improve the utilization of photon energy base on spectrum matching principle.Therefore,it is necessary to continuously enrich the optoelectronic conversion material system,optimize the device structure,and improve the quality of the epitaxial material to meet the requirements of the high-efficiency solar cell for various band gap III-V materials.This paper focuses on solar cells with four and more junction structure,conduct research on epitaxial growth and characterization of In P-based long-wavelength(>900nm)optoelectronic response materials,structure design and optimization of longwave response solar cell,and the performance improvement of multi-junction cell,based on the insufficient energy utilization of long-wavelength solar spectrum.In epitaxial growth,the quality of the long-wavelength optoelectronic response material is improved by optimizing the growth temperature,lattice matching and interface switching and other epitaxial growth parameters based on metal organic chemical vapor deposition(MOCVD)technology;High Resolution X-ray Diffraction,photoluminescence,time-resolved photoluminescence,electrochemical ECV and other methods are used to fully characterize bandgap adjustment,lattice matching,fluorescence lifetime and doping profile of long-wavelength response materials,providing experimental basis for structure optimization;In terms of solar cell devices,based on material quality characterization and improvement results,optimizing cell structure design by improving bulk material quality and interface quality in back surface field/ base region,leading to the development of high open circuit voltage sub-cells.The main content of this paper can be summarized as the following aspects:1.The quality of In P-based long-wavelength response material is improved.The difficulty of epitaxial growth of long-wavelength response material is that the small growth window of double-V group quaternary compound semiconductor InGaAs P,the lattice mismatch induced by the P-type Zn doping of the In-containing compound,and the large lattice mismatch buffer layer growth required for the narrower band gap InGaAs.Based on the above growth difficulties,from the perspectives of source material cracking efficiency,growth competition mechanism,and epitaxial growth parameters,this paper analyzes the quality of the MOCVD grown materials using highresolution X-ray diffraction,time-resolved photoluminescence,and electrochemical ECV characterization techniques,quickly obtained the effect of material quality improvement,successfully obtained InGaAs and InGaAs P materials with precise controllable composition,lattice matching,and clear doping characteristics.In addition,this paper designed large lattice mismatch buffer layers with step composition,the defect induced by lattice mismtch is suppressed at the interface,leading to the successful preparation of InGaAs material with a wavelength extending above 2.5 ?m.2.Optimization of 1.0eV InGaAs P sub-cell.The growth window of the InGaAs P material is very small,and the quality of the heterojunction interface formed with other layers also has an important impact on the device performance.To evaluate the quality of InGaAs P,this paper designed the In P / InGaAs P / In P double heterojunction structure,simulating the base/BSF interface in the cell structure,then measure its fluorescence lifetime by TRPL,which can be used to predict the theoretical lifetime of InGaAs P material based on the radiation recombination theory.The results show that the theoretical value is close to the experimental value,demonstrated that radiation recombination is dominant,resulting high quanlity material,which is suitable for the preparation of cell devices.On this basis,the InGaAs P sub-cell optimization work was carried out from the aspects of lattice matching,device growth temperature,and interface processing.Finally,the open circuit voltage of the sub-cell is raised from the initial 633 m V to 693 m V,and the band gap compensation difference(Woc)is as low as 325 m V.The result is close to the value calculated based on the radiation recombination theory Woc of 323 m V,reaching the best level ever public reported.Then 1.13 e V and 0.85 eV InGaAs P sub-cells are fabricated with the optimized parameters.The Woc are close to the calculated values based on the radiation recombination theory,and a highperformance In P-based InGaAs P sub-cell library is obtained,laid solid foundation for the fabrication of four-junction and five-junction solar cells.3.Optimization of 0.75 eV InGaAs sub-cell and multijunction solar cell.InGaAs material is the first choice for In P-based long-wavelength response materials with bandgap extending below 0.8 e V,and the bandgap of the lattice-matched InGaAs is 0.75 e V.In this paper,the BSF of the lattice-matched InGaAs sub-cell is optimized based on the III-V solar cell structure design.The experimental results show that the use of In Al As: Zn BSF instead of In P: Zn BSF in conventional cell leading to significantly increase in the Voc of the sub-cell,and the short-circuit current density also increased to a certain extent.Based on the optimized InGaAs sub-cell,combined with the best structure of 1.0eV InGaAs P sub-cell,this paper develops InGaAs P(1.0e V)/ InGaAs(0.75 e V)double junction cell that can be applied to the four-junction solar cell.The use of a new BSF structure instead of In P BSF in the InGaAs bottom cell of the double junction cell can effectively reduce the Voc loss.The experimental results show In Al As BSF can optimize the reflection barrier of the minority carriers(electrons)of p-InGaAs to increase the lifetime.However,neither of the BSF can inhibit the dopant diffusion of highly doped layers,and as the thermal annealing time prolongs,the minority carrier lifetime continues to decline.Analysis shows that the increase of interface recombination caused by the thermionic emission between base and BSF is always the dominant mechanism.Based on these conclusions,this paper proposes a new In Al As / In P superlattice BSF structure,apply to double-junction solar cell,the Voc is increased from 967 m V to 997 m V compare with that use In P BSF,resulting 30 m V reduce in Woc,reaching the highest level of this type of solar cell ever public report,meaning that the new BSF can effectively reduce the Voc loss In In P based double-junction solar cell,which is conducive to the improvement of the overall Voc of 4-or more-junction solar cells.The dual-junction cell was applied to bonded fourjunction solar cell,under AM0 illumination,the open circuit voltage was 3423.1m V,the short circuit current density was 15.6m A/cm~2,and the fill factor was 0.873.The final conversion efficiency of the four-junction cell was 34.47%,approaching the leading domestic level.4.Optimization of 0.5eV InGaAs sub-cell.In this paper,a large lattice mismatch buffer layer structure is used to extend the bandgap of In P-based long-wavelength response material to 0.486 e V.The lattice relaxation of the MOCVD grown buffer layer is calculated to be over 99% using the reciprocal space mapping.The lattice mismatching dislocations are well suppressed at the interface through TEM picture,thereby low dislocation large lattice mismatch epitaxial layer is well deceloped.On this basis,this paper fabricates InGaAs sub-cells with bandgap extend to less than 0.5e V.Because this narrow bandgap cell is more suitable for thermal photovoltaic applications,this paper uses black-body radiant heat source as the energy source instead of solar spectrum,to characterize and analyze the performance of lattice mismatch sub-cell.Under the irradiation of 1200 K heat source,the Voc of the sub-cell reached 92.6m V and the short-circuit current density reached 94.0m A/cm~2.