A Study On Efficiency Improvement Of Multicrystalline Solar Cells By Annealing At Low Temperature

Multicrystalline solar cells play a major role in current photovoltaic market. Thekey to promote PV industry technology is to improve cells’ conversion efficiency andlower production cost. Due to its relatively high internal crystal defect and impurities,multicrystalline is sensitive to thermal processing, which bring huge optimizationspace for multicrystalline solar cells by annealing. During fabrication of silicon solarcells, there is no enough time to fully thermally optimize material for the requirementof production efficiency. A systematic study on solar cells efficiency improvement bythermal processing at low temperature was done in this paper.The effect of Ar, N2-10%H2and air on several properties of p-typemulticrystalline silicon solar cells such as conversion efficiency, minority carrierlifetime, reflectivity and cross-section morphology was studied. The results show that:the performance improves at some degree when argon and220℃～400℃is used,and annealing at250℃for15min, the reflectivity reduce, which increase the lightabsorption. When solar cell annealing launch for15min among220℃～400℃inN2-H2atmosphere, both its efficiency and minority carrier lifetime increase. Among300℃～400℃, its efficiency increase obviously and increase1.37%absolutely at350℃. However, it’s bad for efficiency if the annealing temperature is above400℃.And among300℃～600℃, the reflection of light between600nm and1000nmdecreases. Among220℃～300℃, the efficiency of samples annealed in airatmosphere increase differing in degree, but its minority carrier lifetime had notraised. Above350℃, its efficiency drop sharply while minority carrier lifetimeincrease substantially.Then the effect of thermal processing on deep energy level trap center wasstudied by Deep Level Transient Spectroscopy. The results show that: when sampleswere annealed at400℃for15min in air atmosphere, oxide impurity deep energytrap center were introduced and the original Zn impurity hole trap may be translatedto Zn-O complex deep energy trap center. The activating energy of the hole trapchanged from0.47eV to0.11eV and the capture cross section of hole turned from 6.1E-15cm2to3.3E-15cm2. While Fe deep energy trap hadn’t changed. Whensamples annealed in N2-H2atmosphere at300℃for15min, Ti-H2,3complex willbe formed combining Ti and H atom, which act as a deep energy electron trap. Whilesamples annealed in Ar atmosphere at280℃for15min, its hole trap appearedinstead of original electron trap.