開發寬能隙氫化非晶及微晶矽氧材料應用於多接面矽基薄膜太陽能電池

在本研究中，我們開發及優化氫化非晶矽氧及氫化微晶矽氧合金薄膜並將其應用於單接面太陽能電池及多接面太陽能電池中，藉由結合以下兩種不同的研究方案，可有效提升轉換效率。首先，開發寬能隙之N型氫化非晶矽氧薄膜及N型氫化微晶矽氧薄膜，並結合成為雙N層結構於電池中來提升光利用率。第二個方案為開發高品質之非摻雜型(undoped)氫化非晶矽氧薄膜作為p/i介面的緩衝層，緩衝層之目的是透過漸變能隙製程來提升載子傳輸。本研究之矽基薄膜太陽能電池皆是透過具27.12 MHz之射頻電漿輔助化學氣相沉積系統(PECVD)來製作完成。在第一部分，我們開發N型氫化非晶矽氧薄膜及N型氫化微晶矽氧薄膜，並應用於矽薄膜太陽能電池中之N層，中間反射層及背反射層，來提升光利用率。在開發氫化矽氧薄膜中，透過調變射頻功率、磷化氫濃度、二氧化碳濃度及氫氣濃度來優化氫化矽氧薄膜的光學及電學特性。經優化後，適當氧含量摻入薄膜中，可使N型氫化非晶及微晶矽氧薄膜提升能隙並維持足夠的導電性。在氫化非晶矽單接面太陽能電池中，我們將N型氫化非晶矽氧薄膜(氧含量為3.6 at.%)取代傳統之N型氫化非晶矽薄膜作為N層，使元件效率提升7.2%，這主要是因為降低寄生吸收損失於N層中。此外，我們也發現透過全電漿輔助式化學氣相沉積製程所製作完成之N型氫化微晶矽氧薄膜可取代背面氧化銦錫層做為背反射層，並提升轉換效率達9.4%，使用N型氫化微晶矽氧做為背反射層的優點在於可簡化製程並降低成本，以及有更佳之介面特性。在氫化非晶矽/氫化非晶矽鍺串疊型太陽能電池方面，將N型氫化微晶矽氧薄膜作為中間反射層能有效提升上層電池的輸出電流密度，使電流匹配。此外，將N型氫化非晶矽氧薄膜取代傳統之N型氫化非晶矽薄膜於上層電池中，可有效提升下層電池之電流密度，其主要是因為降低寄生吸收損失於上層電池之N層中。結合上述優化成果，氫化非晶矽/氫化非晶矽鍺串疊型太陽能電池之轉換效率可達10.5%，及VOC =1.58 V，JSC = 9.68 mA/cm2，FF = 68.4%。於第二部分，我們開發具可調變能隙之高品質非摻雜型氫化非晶矽氧薄膜，作為矽薄膜太陽能電池中之p/i緩衝層，以降低載子於介面之復合損失。在薄膜開發方面，透過調變氫氣濃度於製程中，可得到高品質的非摻雜型氫化非晶矽氧薄膜。經優化後，當本質氫化非晶矽氧薄膜之氧含量於4 at.%至7 at.%之間時，薄膜之光響應可超過105。將單一能隙之非晶矽氧薄膜作為p/i緩衝層於氫化非晶矽單接面太陽能電池中，元件之開路電壓可從0.85提升至0.90 V 及提升短波長從400 nm至550nm之光響應，其主要是因為減小了p/i介面的能帶偏移(band offset)。我們進一步開發具四層漸變能隙之氫化非晶矽氧薄膜作為p/i緩衝層於氫化非晶矽單接面太陽能電池中，元件於可見光範圍從350至660 nm皆有明顯的提升光譜響應，並減少元件中之反向飽和電流，其主要是因為提升了載子傳輸及減少載子複合機率，而使元件效率提升11.2%.。將其具有漸變能隙之氫化非晶矽氧p/i緩衝層之氫化非晶矽電池結合低能隙之氫化微晶矽鍺下層電池，使串疊型電池之加總電流相對於無緩衝層之電池可從22.06 提升至 23.20 mA/cm2。經優化過後，具漸變能隙氫化非晶矽氧緩衝層之氫化非晶矽/氫化微晶矽鍺串疊型太陽能電池可將轉換效率由9.91%大幅提升至11.04%，相對提升轉換效率11.4%，優化後之串疊型太陽能電池VOC =1.33 V，JSC = 11.6 mA/cm2，FF = 71.6%。

關鍵字

氫化非晶矽氧；氫化微晶矽氧；背反射層；漸變能隙氫化非晶矽氧緩衝層；氫化微晶矽鍺；多接面太陽能電池

並列摘要

In this thesis, we have developed and optimized the hydrogenated amorphous silicon-oxide (a-SiOx:H) and hydrogenated microcrystalline silicon-oxide (μc-SiOy:H) films for single-junction and multi-junction solar cell applications. The high efficiency silicon thin-film solar cell were achieved by integrating two approaches, as following. The first approach is developing the wide-bandgap n-type a-SiOx:H and n-type μc-SiOy:H films to form a double n-layer structure in cell for improving the light management. The second approach is developing the high-quality undoped hydrogenated amorphous silicon-oxide (a-SiOx:H(i)) film as graded p/i buffer layer in cell for enhancing carrier transport. All silicon thin films and solar cells were prepared by radio-frequency plasma-enhanced chemical vapor deposition (RF PECVD) with 27.12 MHz. In the first part, we developed a-SiOx:H(n) and μc-SiOy:H(n) films as n-type layer, intermediate reflecting layer (IRL), and back-reflecting layer (BRL) to improve the light management in silicon thin-film solar cells. In the development of a-SiOx:H(n) and μc-SiOy:H(n) films, we have optimized the optical and electrical properties by properly adjusting RF power, PH3 concentration, CO2 concentration and H2 concentration. After optimization, the optical bandgap of a-SiOx:H(n) and μc-SiOy:H(n) can be increased while maintaining sufficient conductivity. In a-Si:H single-junction cells, employing n-type a-SiOx:H (oxygen content of 3.6 at.%) as the replacement for a-Si:H(n) resulted in a relative efficiency enhancement of 7.2% due to the reduced parasitic absorption loss. Besides, we have also found that μc-SiOy:H(n) can replace back ITO layer as BRL by all in-situ PECVD process, resulting in obtaining the high efficiency of 9.4% and simplifying the fabrication process. For a-Si:H/a-Si1-zGez:H tandem cell, employing μc-SiOy:H(n) as IRL increased the current density of top cell, leading to the current matching. In addition, employing a-SiOx:H(n) as a replacement of a-Si:H(n) in the top cell increased the current density of bottom cell due to the reduction of absorption loss. Combining all the improvements, the a-Si:H/a-Si1-zGez:H tandem cell with efficiency of 10.5%, VOC of 1.58 V, JSC of 9.68 mA/cm2, and FF of 68.4% was obtained. In the second part, we have developed the high-quality a-SiOx:H(i) films with variable bandgap as p/i buffer layer in silicon thin-film solar cells for reducing the carrier recombination loss at interface. In development of a-SiOx:H(i) films, the high-quality a-SiOx:H(i) films was achieved by adjusting the H2 concentration and CO2 concentration. After optimization, the a-SiOx:H(i) films with oxygen content from 4 to 7 at.% exhibited high photo-response of over 105. By employing a single-bandgap a-SiOx:H(i) as p/i buffer layer in the a-Si:H single-junction cell, the improved VOC from 0.85 to 0.90 V and the increased short-wavelength from 400 to 550 nm response were attributed to the improved p/i band offset. Further employing the graded-bandgap a-SiOx:H buffer layer with 4 graded-steps in a-Si:H single-junction cell, the enhanced visible-wavelength from 350 to 660 nm response and the reduced reverse saturation current density were due to the improved carrier transport and the reduced carrier recombination, which resulted in a relative efficiency enhancement of 11.2%. By combining the a-Si:H top cell having the graded-bandgap a-SiOx:H buffer layer together with the hydrogenated microcrystalline silicon-germanium (μc-Si1-aGea:H) bottom cell, the total current density significantly increased from 22.06 to 23.20 mA/cm2 compared to the tandem cell without buffer layer. As a result, compared to the cell without buffer layer, the efficiency of a-Si:H/μc-Si1-aGea:H tandem cell with graded-bandgap a-SiOx:H buffer layer increased from 9.91% to 11.04%, which had a relative enhancement of 11.44%, Furthermore, the tandem cell performance of VOC of 1.33 V, JSC of 11.6 mA/cm2 and FF of 71.6% were obtained.

並列關鍵字

hydrogenated amorphous silicon-oxide ； hydrogenated microcrystalline silicon-oxide ； back-reflecting layer ； graded-bandgap a-SiOx:H buffer layer ； hydrogenated microcrystalline silicon-germanium ； multi-junction solar cell

參考文獻

[1] United Nations, “United Nations Framework Convention on Climate Change,” 1992. Website: http://unfccc.int/essential_background/convention/items/6036.php

Google Scholar

[2] United Nations, “Kyoto Protocol to the United Nations Framework Convention on Climate Change”, Website: http://unfccc.int/kyoto_protocol/items/2830.php