| Ga As based thin film solar cells possess tremendous applicable value in military and aerospace fields.America has mastered the proven technologies and manufactured his relevant products of Ga As based thin film solar cells,however our corresponding technologies are still in developing stages and exist considerable gaps with the international advanced levels.In this work we designed and fabricated a type of thin film In Ga P/Ga As double junction solar cells on a metal substrate and research the characteristics comprehensively,aiming at the special requirements in the fields of satellite,space station,unmanned aerial vehicle and mobile military equipment in the future.An In Ga P etching stop layer was sandwiched between the epilayers and Ga As grown substrate based on the conventional epitaxial structure of conventional Ga As based solar cells.The epilayers were grown uprightly by a MOCVD system,with the key grown technologies of thickness and doping concentration of each layer,diffusion of dopant source,heterojunction,tunnel junction,and uniformity of wafer solved.The upper surface processes,consisting of the preparation of top grid electrodes,the isolation trench etch of multilayers and the design and optimization of antireflection layers,which are analogous to the conventional processes of Ga As based solar cells preparation,were developed firstly.A substrate transferred process,based on Ga As substrate removal and electroplating filmy metal,was then adopted to accomplish the fabrication of thin film solar cells.The Ga As substrate was removed through wet etching assisted by the high selective ratio with In Ga P layer,which resulted in a more pure and smooth back surface with less chemical damage and residues on the exposed epilayer,improving the metal-semiconductor contact characteristics of bottom electrodes,compared with epilayers lift-off process.The metal handle was prepared by electroplating,and the thickness uniformity,flat and smooth property,and stress of metal film were improved by adjusting electroplating process parameters comprehensively.Electroplating is an electrochemical deposition process in solutions without demanding requirement for the cleanliness of operating environment.What is more,electroplating avoids the mechanical damage to epilayers caused by residual particles or wafer bowing as the wafer do not suffer from the high temperature and high pressure processing like bonding.Electroplating allows the metal films with adjustable thickness,making the thin film devices great flexibility,especially competitive in the fabrication of large area flexible thin film solar cells.This substrate transferred process is compatible flexibly to the upright or inverted epitaxial structure,and can be extended to the fabrication of other thin film photo-electronic devices such as light emitting diodes,photoelectric detectors and semiconductor lasers.The thin film In Ga P/Ga As double junction solar cells on a metal substrate showed an open circuit voltage of 2.4 V,a short circuit current density of 13.67 m A/cm2,a fill factor of 88.7%,an efficiency of 29.09% under 1 sun AM 1.5 illumination,which are approaching the international advanced levels.The total thickness of thin film solar cells was about 30 ?m,exhibiting super high power to mass ratio and power to volume ratio with ultra light and thin property.The excellent flexibility permits Ga As based thin film solar cells attached onto uneven surfaces.We researched the performance variations when bending thin film solar cells into different strain and after bending the thin film solar cells for different recycles.The efficiency showed a stable maintainment when the epilayers strain was less than 0.06%,but an increasing degradation with the strain further increased.The thin film solar cells were resistant to mechanical damage,showing no obvious cracks or falling caused by bending recycles.Nearly no performance degradation was observed after up to 100 bending recycles,verifying the thin film solar cells with mechanical reliability.The double junction solar cells showed strong photoluminescence(PL)and electroluminescence(EL)characteristics,and particularly the luminescence under simulated solar light can be taken as a convenient means of checking the materials growth and device fabrication.The PL peaks of In Ga P top cell and Ga As bottom cell were at 648 nm and 874 nm respectively excited by 532 nm laser,which were consistent with the EL peaks.The peaks and full widths at half maximum showed no changes after transferring the substrate,which meant no obvious train attached into the epilayers.The thin film solar cells showed an enhanced PL intensity compared with that before transferring Ga As substrate,but a 7 nm red shift of PL peaks occurred when the epilayers were bent into intense strain.We have developed some our own technologies for the fabrication process and accumulated some relevant data of Ga As based thin film solar cells for the moment,but still lag behind the international groups.The epilayers structure design,materials growth and device fabrication still need to be further optimized and developed to realize our own Ga As based thin film solar cells industrialization. |
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