利用下轉移螢光粉以提升結晶矽太陽能電池轉換效率

Abstract

目前矽基太陽能電池雖具原料豐富與成熟的半導體技術支援等特點,市佔率較其他 種電池為高,但其在紫外與近紫外光下照射下的量子效率,卻仍遠低於其他波段;此乃 因高能量的光子在電池表面被吸收後所產生的電子-電洞對,大部分都無法被外電路有 效地收集利用而只是經由表面缺陷再結合而消耗殆盡。此外,由於矽基材本身折射係數 與空氣不相匹配,當光通過空氣/矽材介面時約有四成因反射而損失,商業生產的太陽 能電池表面常需耗費昂貴成本製作 SiNx 作為抗反射層。 本論文提出並探討兩種下轉移螢光粉之製備、特性鑑定,與其應用於太陽能電池的 可行性。本研究利用 Pechini 溶膠-凝膠法製備次微米級 Ba2SiO4:Eu2+螢光體並將其作為 光譜下轉移層塗佈於鍍有 SiNx 的單晶矽太陽能電池表面。在標準太陽光照度(AM1.5G) 量測下,原試片短路電流與光電轉換效率分別可提升 0.48 mA/cm2 與 0.23%。我們亦藉 由螢光粉的激發光譜、電池試片的反射率與外部量子效率之變化來探討此增益的背後原因。 此外我們為利用尿素輔助共沉澱法及高溫硫化等過程製備高分散性球狀之次微米級 Gd2O2S:Eu3+螢光體,並藉由前驅物粒徑大小分布控制以調變產物的發光強度與尺寸。 我們將所得到的 Gd2O2S:Eu3+分散在聚乙烯吡咯烷酮(Polyvinylpyrrolidone)溶液中,並利 用旋轉塗佈法將其沉積在多晶矽太陽能電池表面(無 SiNx 鍍膜)作為一具抗反射與光譜 下轉移之雙功能塗層。在 AM1.5G 照度下,此一 PVP/Gd2O2S:Eu3+複合層可使原試片的短路電流與光電轉換效率分別提升 6.74 mA/cm2 (增幅 26.2%)與 2.76% (增幅 24.2%),而分析塗佈前後太陽能電池的反射率與外部量子效率之變化及螢光粉的激發光譜顯示此 一增益主要來自於入射紫外光的下轉移機制與試片全波段的反射率下降所致。我們也評 估此設計應用於綠色能源發展之潛力。

Silicon based solar cells have been the workhorse in the market due to its abundance and relatively inexpensive processing. However, under UV and n-UV illumination, most of the electron-hole pairs are generated near the device surface and consumed easily through the recombination with surface defects, which is responsible for inducing rather poor performance. In addition, the high contrast of refractive indices between air and silicon also causes a high Fresnel reflection loss (~40%) at air-silicon interfaces and limits the performance of silicon-based photovoltaic devices. Typically, coating an anti-reflective layer of SiNx is one of the most common methods to suppress Fresnel reflection in industry, yet this layer requires high reaction temperature and fabrication apparatus with high-vacuum capability, which could put the overall cost of solar cells intolerable. In regard to the above-mentioned problems, in this report we proposed two developmental directions for down-shifting phosphors applied to silicon solar cells; the preparation and characterization for these phosphors were also investigated. First, the Ba2SiO4:Eu2+ sub-micron particles have been synthesized by the Pechini sol-gel method and applied as a luminescent down-shifting layer coated on the textured surface of crystalline Si solar cells (with SiNx layer). Under a simulated one-sun illumination, the short-circuit current density of the device with phosphor coated can be increased from 34 to 34.48 mA/cm2, meanwhile the power conversion efficiency is enhanced from 15.13 to 15.36%. The related underlying mechanism for the enhancement has been investigated and discussed by photoluminescence excitation spectrum, reflectance, and external quantum efficiency. Second, we prepared highly-monodispersed submicrometer Gd2O2S:Eu3+ spheres through a simple urea-based homogeneous precipitation and subsequent annealing process. Then the particle size of the Gd2O2S:Eu3+ can be finely tuned by adjusting the conditions of nucleation of the precursor particles. The dual-functional anti-reflection coating of polyvinylpyrrolidone/Gd2O2S:Eu3+ was formed by dispersing the phosphor particles in the polyvinylpyrrolidone aqueous solution and spraying the mixture onto the textured surface of pc-Si solar cell through a spin-coating technique. Under a simulated one-sun illumination, this predesigned device achieved an enhancement of 6.74 mA/cm2 in short-circuit current density and 2.76% in power conversion efficiency relative to those for a pristine cell. Based on the observed results, we can conclude that these improvements are attributed to the luminescent down-shifting capability in the ultraviolet range and additional promotion of light absorption within the measured spectral region.