探討與改善氮化銦鎵/氮化鎵太陽能電池效率之研究

Abstract

III-V 族太陽電池屬於與化合物半導體（Compound semiconductor）太陽電池，由週期表中 III 族與 V 族元素所構成。其中氮化銦鎵材料由於是直接能隙和寬能隙材料，已廣泛地被應用在發光二極體(LED)和雷射(laser)。加上其具有高吸收係數、優良的抗輻射能力…等特性，近年來在太陽能應用上和學術發表上漸漸受到重視。氮化銦鎵材料多使用分子束磊晶(MBE) 或者有機金屬化學氣相沉積(MOCVD)機台成長，由於氮化銦鎵與基板材料（藍寶石或碳化矽）間的晶格不匹配，故其成長時，會產生的應力（stress）作用與應變（strain）效應，造成極化(polarization)現象。有研究指出極化電荷會使氮化銦鎵材料堆疊造成能帶結構產生彎曲，進而影響太陽能電池效率降低。

在本研究中，首先固定i層氮化銦鎵的銦含量，模擬不同銦含量的p型氮化銦鎵取代傳統p型氮化鎵材料，探討極化匹配效應對氮化銦鎵雙接面異質結構太陽能電池效率的影響。使用有限元素差分法的方法模擬氮化銦鎵太陽能電池的電特性，模擬結果可以發現隨著p型氮化銦鎵中的銦含量增加，電池效率也隨之增加，主要的原因是隨著增加p型氮化銦鎵中的銦含量會減少p層和i層之間的極化電荷，減少極化不匹配，減緩p型氮化銦鎵之間的能帶彎曲現象。當p層氮化銦鎵中的銦含量與i層氮化銦鎵中的銦含量相同時，達到極化匹配，這個氮化銦鎵太陽能電池達到此組效率最佳的效率。接著探討p層氮化銦鎵的銦含量從0%~25%和i層氮化銦鎵的銦含量從10%~25%，從匹配圖(mapping)的結果顯示當極化匹配的情況下，隨著銦含量增加，效率增加，最佳的效率在20%的時候，之後效率減少。這是因為n層和i層之間的能帶不連續造成電子和電洞在介面處累積造成複合，致使載子無法有效被電極蒐集的結果。最後探討隨著壓電極化的減少，p層氮化銦鎵的銦含量從0%~25%和i層氮化銦鎵的銦含量從10%~25%的匹配圖亦有相同的趨勢。

在第二個部分，由於氮化銦鎵太陽能電池成長在不同方向的藍寶石基板上，會產生不同比例的極化電荷。我們先探討不同比例的極化電荷對氮化銦鎵雙接面異質結構太陽能電池效率的影響，從模擬結果發現氮化銦鎵太陽能電池成長於非極性基板上，由於層和層之間的介面處不會產生極化電荷，造成能帶彎曲和傾斜，所以有最佳的效率。接著探討主動區中的氮化銦鎵的銦含量5~45%，模擬結果顯示隨著銦含量增加，效率增加，最佳的效率在主動區的銦含量為37%，之後隨之減少，顯示銦含量大於37%後，p-i和n-i介面的能帶不連續造成電子和電洞載子的累積，電子和電洞在i層中大量複合，造成載子無法有效被電極蒐集。為了改善i層中大量複合的行為，插入漸變層，在主動區的銦含量為45%為最佳化的結果。

第三部分，代入磊晶結構中應用能帶工程化的概念，探討改變量子井(well)和阻擋層(Barrier) 不同結構的設計對14對多重量子井(MQW) 結構氮化銦鎵太陽能電池的能帶、內建電場、載子複合率和效率的影響。從模擬結果顯示量子井(well)和阻擋層(Barrier)的結構，兩側漸變的結構結構有利於電子、電洞蒐集。 從穿透式電子顯微鏡(TEM)所量測實作14對多重量子井(MQW) 結構氮化銦鎵太陽能電池的銦含量成分的結果顯示，量子井結構因為銦含量攜出，能帶圖呈現類高斯函數 (guassian)分布。我們將穿透式電子顯微鏡(TEM)所量測出實作14對多重量子井(MQW) 結構氮化銦鎵太陽能電池的銦含量成分代入模擬中，比較銦攜出對多重量子井氮化銦鎵太陽能電池的能帶、內建電場、載子複合率和效率的影響。

接著製作真實氮化銦鎵/氮化鎵太陽能電池的元件，並探討製作仿生結構提升太陽能電池效率。在第四部份使用聚苯乙烯小球當做遮罩(mask)，使用ICP-RIE轟擊，在p-GaN的表面製作彈球型(nipple)的仿生結構，增加入光量和光散射，達到提高24%的效率。並模擬最佳化彈球型的結構。最後，由於使用ICP-RIE轟擊P-GaN，會造成P-GaN表面缺陷，造成載子大量複合，而減少光電流的萃取。因此相同的使用聚苯乙烯小球當做遮罩，而將彈球型的仿生結構製作於透明導電層上，達到增加入光量和光散射且避免表面缺陷的目的。

Ⅲ-Ⅴ compound semiconductors are promising materials for highly reliable and high output power optoelectronic devices, including light-emitting diodes (LED) and laser diodes (LD) and solar cells application. InGaN materials recently have more attention for solar cell application caused by their tunable band gap, from InN (0.7 eV) to GaN (3.4 eV), which can provide wide range absorption of the entire solar spectrum. For InGaN alloys, previous studies also showed the advantages of direct bandgap transition, high carrier mobility and drift velocity, high radiation resistance and high optical absorption and good thermal stabilities. These advantages are promising that InGaN/GaN solar cells have potential to the highly efficient solar cells under concentrated sunlight. The InGaN/GaN-based solar cells are normally grown on c-facet sapphire and GaN substrate by Metalorganic vapour phase epitaxy (MOCVD) and/or Molecular beam epitaxy (MBE). The growth of InGaN/GaN hetero-junction suffers from both spontaneous and strain-induced piezoelectric polarization field. The polarization mismatch effect gives rise to strong band bending, and the effect of band bending between inhomogeneous InGaN/GaN layer may hinder photogenerated carrier transport and collection.

In this study, we assume that the implementation of a p-InGaN layer is achievable. To relieve the polarization effects using a p-doped InGaN layer, the current density-voltage curves of InGaN heterojunction solar cells with an intrinsic In0.1Ga0.9N layer are first simulated for p-layers with different indium compositions, ranging from 0% to 10%. The conversion efficiency of the p-In0.1Ga0.1N/i-In0.1Ga0.1N/n-GaN solar cell is significantly enhanced by 4.14 times over the reference cell, which the enhancement is due to polarization mismatch. Therefore we propose the use of a p-type InGaN layer to reduce the polarization field, and we calculate the efficiency map of ternary p-InxGa1-xN/i-InyGa1-yN/n-GaN solar cells with various indium compositions for both p-type and intrinsic layers for x= 0.00 ~ 0.25 and y= 0.10 ~ 0.25.

In the second part, we first calculate the theoretical efficiencies of InxGa1-xN solar cells in polar and non-polar orientations. And then we evaluate the abrupt and graded junction design of InxGa1-xN/GaN double heterojunction structure solar cell on non-polar orientation. The discontinuity at GaN/InGaN interface is too large to collect hole and electron carriers when indium content goes above 37%. Finally, the optimal structure of non-polar graded InGaN solar cell can be demonstrated by removing the band discontinuity at interface which improves the minority carrier collection.

In the third part, we design the barriers and the wells structure to substitute for the effect of indium fluctuation and/or structural disorder in active region by using finite element analytical simulation. The simulation results show that the efficiencies can be enhanced by a factor of indium composition fluctuation of barrier and well, due to uniform electric field for each QW and reducing the total recombination. And then we demonstrated the 14 pairs multiple quantum wells In0.15Ga0.85N/GaN solar cell fabricated structure of TEM image into our simulation. The efficiency of inhomogenous QWs structure displays better performance than homogenous cell, which the improvements can be attributed to the markedly increased Jsc by indium composition fluctuation.

Other than material issues, how to let more sunlight into cell is another important issue. The sub-wavelength structure (SWS), which acts like a gradient refractive index varying layer, possesses broadband and omni-directional anti-reflective properties and thus becomes a suitable candidate of solar cell’s top surface coating. We demonstrate the fabrication and characteristic of double-heterojunction GaN/In0.11Ga0.89N solar cell grown on the pattern sapphire substrate and biomimetic surface anti-reflection process. This biomimetic structure is proved to be beneficial to the performance of the solar cell, and we utilized a rigorous coupled-wave analysis simulation to find out the optimized size of texture. The p-GaN surface will generate defects by using ICP-RIE to form p-GaN biomimetic surface, and we also demonstrate this texture on transparent conductive film to solve this problem. We focus on the enhancement of the electrical and structure characteristic of GaN/In0.25Ga0.75N multiple quantum well solar cells.