以電漿輔助式化學氣相沉積法開發微晶矽鍺吸收膜應用於多接面矽基薄膜太陽能電池

Abstract

在本研究著重於開發氫化微晶矽鍺(µc-Si1-xGex:H)薄膜應用於矽機薄膜型太陽電池吸收層。薄膜鍺含量主要受到反應氣體中鍺甲烷(GeH4)濃度及電漿功率影響，於本研究中較佳品質的吸收膜鍺含量約12 at.%;同時氫流率增加以及低製程壓力可分別對薄膜沉積表面進行氫蝕刻效應和提升反應前驅物在表面的擴散長度，對薄膜的結晶化有所助益。藉由適當控制製程參數得到合適的薄膜結晶率約50-60%，並減少鍺誘發缺陷及晶界缺陷得到高品質氫化微晶矽鍺吸收膜，擁有大約10-5 S/cm的高光電導。 進一步氫化微晶矽鍺吸收膜應用在單接面電池應用開發上增加太陽頻譜紅外光能擷取，在0.9 μm厚的吸收膜厚度下有較佳太陽能電池表現，可達6.53%轉換效率。電池開發上藉由整合以下五個主要的研究方案來改善電池效能：(1) 在氫化微晶矽鍺吸收層初期沉積過程中，藉由調變反應氣體矽甲烷(SiH4)和鍺甲烷(GeH4)流量控制氫氣氣體稀釋比例，提升整體吸收膜結晶均勻度，減少非晶相孵化層厚度改善利於光生載子於吸收膜傳輸。(2) 引入低溫硼摻雜窗層(window layer)改善電池載子傳輸及提升電流密度貢獻。(3) 以硼摻雜之非晶矽/微晶矽窗層減少製程中氫電漿效應在前電極透明導電層(TCO)的錫還原反應及表面傷害，改善入光量與TCO/p層介面品質。(4) 引入一層氫化微晶矽層以增加吸收層在i/n介面附近電場強度，提升長波段光生電洞之傳輸。(5)開發氫化微晶矽氧(μc-SiOx:H)，整合p型摻雜膜(μc-SiOx:H(p))於的窗層及n型摻雜膜(μc-SiOx:H(n))於背反射層，藉由其本身寬能矽及低折射率特性，減少吸收膜以外的入射光損耗同時提升電池內部光侷限能力，增加吸收層中光獲取量。 此外在串疊型太陽電應用上，開發了適合的穿隧複合介面(TRJ)/中間反射層(IRL)。在0.24 μm氫化非晶矽/0.9 μm氫化微晶矽鍺串疊型電池結構上以引入 a-Si:H(n)/μc-SiOx:H(n)/μc-Si:H(n)/μc-SiOx:H(p)多層膜結構，連結上電池吸收膜及下電池吸收膜來構成，可得到10.02%的高轉換效率：VOC= 1.38 V、JSC = 10.06 mA/cm2、FF= 72.1%、以及上下電池總電流密度貢獻JSC,Sum = 22.77 mA/cm2。對於外部量子效率(EQE)在無偏光量測之結果顯示，電池效能仍可藉由TRJ/ IRL改善減少電池間漏電來提升。基於減少非晶矽基薄膜光誘發劣化效應(Staebler–Wronski Effect)影響電多接面池穩定性，0.12 μm氫化非晶矽/0.9 μm氫化微晶矽鍺/1.5 μm氫化微晶矽鍺在本研究中成功開發並且可達9.54 %轉換效率：VOC= 1.79 V、JSC = 7.55 mA/cm2、FF= 70.6%、以及上中下電池總電流密度貢JSC,Sum = 25.10 mA/cm2。在EQE結果上顯示整體電流輸出限制在1.5 μm氫化微晶矽鍺吸收層，但是受到載子傳輸在更厚的吸收膜中變差的因素考量，未來改善電池效率預期可藉由光侷限技術應用達成: (1) 研究開發高紅外光(IR)穿透率的前電極TCO玻璃基板，減少TCO前電極IR光耗損。(2) 開發同時具有小及大絨面(texture)結構的前電極TCO表面，以因應短波段及長波段太陽光，能在電池中保有較長的光學路徑以利吸收。

In this thesis, the hydrogenated microcrystalline silicon germanium alloy (µc-Si1-xGex:H) deposited by Plasma-Enhanced Chemical Vapor Deposition (PECVD) has been investigated. Germance concentration (RGeH¬4¬) and RF power were effective in variation of SiGe composition. Unfortunately, Ge incorporation induced defects in the film. The film Ge content of 12 at.% was acceptable for µc-Si1-xGex:H absorber to enhance IR absorption while minimizing film-quality degradation. Hydrogen played an important role in surface reaction which promoted the film crystallization from H-etching effect. In addition, lower growth pressure shifted to high-electron temperature in plasma which led to a transition from amorphous to crystalline growth. With the careful parameter controlling, the solar grade μc-Si1-xGex:H absorber with a moderate crystalline volume fraction (XC) of 50-60%, less Ge-induced defects, and grain-boundary defects provided a high photo-conductivity of ~10-5 S/cm. Employment of 0.9 μm-thick μc-Si1-xGex:H alloy as an IR absorber, the efficiency of single-junction solar cells can be achieved 6.53% by integrating with the several approaches: (i) a proper gas profiling for improving microstructural homogeneity in μc-Si1-xGex:H absorber growth; (ii) a low-temperature deposition for p-type window layer; (ⅲ) an a-Si:H/μc-Si:H p-type window layer for alleviating tin reduction to improve TCO/p interface and TCO transparency; (ⅳ) a introduction of μc-Si:H field-enhanced layer (FEL) for enhancing electric field at i/n interface and improving hole collection; (ⅴ) the use of wide-bandgap Si-rich microcrystalline SiOx:H doped layer for improving light confinement in cells. As for 0.24μm a-Si:H/ 0.9 μm μc-Si1-xGex:H tandem solar cell, the development of TRJ/IRL structure was investigated. With use of the IRL TRJ/IRL structure of a-Si:H(n)/μc-SiOx:H(n)/μc-Si:H(n)/μc-SiOx:H(p) multilayers, a high efficiency of 10.02% was exhibited with VOC= 1.38 V, JSC= 10.06 mA/cm2, FF= 72.1%, and 22.77 mA/cm2 for sum contribution in JSC. The departure of EQE result which measured under without light bias suggested that the cell performance should be further improved by modification of TRJ/IRL for reducing leakage path in tandem cell. Regarding the triple-junction cells, the low-refractive-index μc-SiOx:H(n) IRL was appied to develop the 0.1 μm a-Si:H/0.3 μm a-Si1-xGex:H/1.5 μm µc-Si1-xGex:H cell, which, corresponded to a 8.59% efficiency with VOC= 1.88 V, JSC= 6.93 mA/cm2, FF= 65.8%. For avoiding light-induced degradation in amorphous Si-based absorbers, the 0.12 μm a-Si:H/ 0.9 μm μc-Si1-xGex:H/ 1.5 μm μc-Si1-xGex:H triple-junction solar cell has been developed. The 9.54 % efficiency was obtained with with VOC= 1.79 V, JSC= 7.55 mA/cm2, FF= 70.6%, and the sum contribution in JSC can achieve 25.1 mA/cm2. μc-Si1-xGex:H middle absorber with high IR absorption provided a good modulation of light harvesting in the solar cells, which achieved a relatively high JSC. The further improvement of cell performance in triple-junction cells could be expected by introducing a proper textured TCO-coated substrate and a low-refractive-index IRL for enhancing the light management.