多晶硅薄膜太阳能电池电浆增强光捕获

摘要

硼，磷掺杂硅结晶，缺陷钝化和金属化层沉积制备多晶硅薄膜太阳能电池玻璃上。电浆光捕获上限〜45％的光增强扩散反射硅电池表面形成银纳米粒子的引入。

摘要

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.

The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.

To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 μm thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm², which can be increased up to 17-18 mA/cm² (~25% higher) after application of a simple diffuse back reflector made of a white paint.

To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23°C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF₂, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF₂, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm², up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.

Introduction

Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) ¹ and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) ². For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector ³, silicon plasma texturing ⁴ or high refractive index nanoparticle reflector ⁵ have been suggested.

Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect ⁶. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment ^6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested ⁸. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles ⁹. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance ¹⁰.

The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF₂ or SiO₂, from the rear reflector to avoid mechanical and chemical damage ⁷. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric ⁶. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

研究方案

1。多晶硅太阳能电池的制作（动画3）

1. 硅薄膜沉积
   1. 电子束蒸发工具准备烘烤出来〜100°C过夜，以达到<3E-8托基地的压力。预置样本加热到150℃待机温度。
   2. 使用5x5厘米^2（或10×10厘米^2）基板硼硅玻璃（肖特Borofloat33），1.1或3.3毫米厚，涂〜80纳米的氮化硅（由PECVD准备从N ₂和SiH ₄的混合物）制成的基板。
   3. 基材表面干燥氮气吹，以去除灰尘，并放置到样品架。发泄锁的负载，加载样本，泵负载锁定压力<1E5托，样品传送到主室。启动加热到250°C的设定点约20分钟的泵，当压力达到8E-8 Torr或更低。
   4. 检查的掺杂源和硅源百叶窗关闭。预置的掺杂源温度为待机温度，即磷源温度在700°C和硼离子源温度在1250°C。启动电子枪，电子枪电流缓慢增加，熔体硅坩埚。
   5. 时所需的电流达到（从以前的校准：这个电流的电子枪和硅源条件的不同而不同），蒸发所需浓度的P和B掺杂的硅层：35在1E20厘米纳米发射^-3的P 2〜3在5E15厘米^-3 B的微米吸收100 nm的背表面场（BSF），在4E19厘米^-3 B.精确的掺杂浓度达到某些硅沉积速率匹配，测量石英晶体显示器天平（QCM），具有一定的掺杂源的温度，使用SIMS的校准建立了合作关系。
   6. 蒸发后进行切换关闭加热器，降温〜10分钟的样品。反FER样品负载锁，关闭闸门阀，泄负载锁和卸载的硅薄膜样品。
2. 硅结晶
   如果样品是10×10厘米^2，它可以被切割成四个5x5厘米²细胞块大小之前结晶。放置一个粗糙和氮化硅涂层，肖特Robax玻璃（避免粘）持有人（Si薄膜）玻璃上沉积硅薄膜。负载成氮清除烤箱预热至200-300°C。提高温度高达600°C时，3〜5°C /分钟和30小时退火。打开烤箱加热，让烘箱降温自然〜200℃（2〜3小时）起卸前的样品。样品可以有一个凹的形状，由于硅结晶过程中的收缩。这将在下面的快速热加工扁平化。
3. 掺杂活化和缺陷退火（RTA）
   将样品持有人的结晶膜制成的石墨和LOAD以快速热处理器氩清除。坡道的温度高达600°C时，1°C / S，则高达1000°C在20°C /秒，保持1分钟，然后冷却下来自然〜100°C和卸载。
4. 去除表面氧化
   立即前到氢化硅薄膜表面氧化过程中形成的结晶和RTA必须拆除，以确保裸露的硅薄膜表面接触到氢。直到硅片表面变为疏水退火样品浸入5％HF溶液（30〜100秒）。与去离子水冲洗和干燥氮气枪。
5. 缺陷钝化
   样品装入真空室配备了远程氢等离子体源。抽空<1E-4托，加热样品〜620°，打开氩气/氢气的混合物流量（50:150 SCCM），设置压力50-100毫托，开始在3.5千瓦的等离子源微波功率和继续〜10分钟的过程。关闭加热器电源，而mainta伊宁10-15分钟的血浆，直到温度低于350℃之前，血浆和停止气体流量。卸载样品，当温度低于200°C
6. 细胞金属化
   细胞金属化进行了一系列连续的光刻图案，铝薄膜沉积与蚀刻步骤，详细描述在^11。最终细胞看起来像在最后一张幻灯片的动画3所示。金属化细胞的特写视图如图1所示。
7. 测量EQE的金属化细胞。

2。制作电浆银纳米粒子（动画4）

1. 用干燥的氮气吹金属化的细胞表面，去除灰尘和热蒸发器包含一个W的船装满银颗粒（0.3-0.5克）装入样品。抽空蒸发器室基地的压力2〜3E-5托。方案的QCM对Ag参数：密度10.50Z比0.529。
2. 确保样品快门关闭，关闭的W船加热器和增加电流，足够慢，以避免以上8E-5 Torr的压力上升，直到银颗粒融化（如通过观察视图端口）。压力稳定后的电流设定点对应的A / S（校准）0.1-0.2银沉积速率，并打开快门开始沉积过程。
3. 监测使用的QCM增长的Ag膜厚度和关闭快门时达到14纳米的厚度。允许在W船约15分钟冷却下来，卸载样品。电影应退火形成纳米颗粒沉积后尽快，尽可能地避免银氧化。
4. 新鲜沉积的Ag膜的细胞被置于氮气清除烤箱预热到230 0.1-0.2℃，退火50分钟，然后卸载。注意：由于纳米粒子的表面外观的变化。银纳米粒子的扫描电子显微镜图像是SHOWN 图。 2。
5. 测量EQE的细胞与纳米粒子阵列。

3。制作后反射

后方的反射〜300 nm厚的_氟化镁（1.38注册机）介质熔覆与商业白色的天花板漆（多乐士）的外衣。

1. 编造后反射之前，必须用黑色记号笔墨水保护细胞接触，这使得从下介质由升空过程中暴露的接触。
2. 使用氮气枪吹NP阵列和画接触，清除灰尘的样品。使用温和的氮气压力和运动保健不吹纳米粒子。放入热蒸发器包含一个W船充满氟化镁₂件样品。蒸发器泵2〜3E-5 Torr的压力。 _氟化镁的QCM参数为：密度3.05和Z比0.637。
3. 确保样品shutter被关闭，打开船加热器，慢慢加大电流，以避免过多的压力上升，直到_氟化镁熔体通过视图端口。压力稳定后的电流设定点，对应的_氟化镁的沉积速率为0.3 nm / s的和打开样品快门。
4. 监测使用的QCM沉积厚度和关闭快门时达到300海里。
5. 关闭加热器。允许的W船约15分钟冷却下来，卸载样品。注意：在氟化镁₂熔覆细胞形态的变化。
6. 要取出的细胞接触的油墨面膜浸入丙酮熔覆与介质的细胞。等到开始开裂，抬起上面的油墨介质。保持在丙酮中的细胞，直到所有的墨水与介质和金属接触，充分暴露。从丙酮中取出样品，新鲜的丙酮冲洗和干燥氮气枪。
7. 应用层罚款对整个细胞表面的软刷白色油漆（多乐士单涂层的天花板油漆）小心地避免金属接触。油漆层有足够厚，是完全不透明（〜>0.5毫米），所以没有光可以看到在明亮的光源，通过描绘细胞时。让漆干一天。
8. 用白色油漆后反射测量细胞EQE。

4。代表结果

通过整合全球标准太阳光谱的EQE曲线（气团1.5）太阳能电池的短路电流计算。无论是电池的电流，其增强由于光捕获依赖于细胞吸收层厚度：目前本身是较高较厚的细胞，但更薄的设备，看到EQE各自的数据和动画5，表1是当前加强高曲线。原来的2微米厚的细胞，没有光捕获，HAVE JSC测量步骤1.7。〜15毫安/厘米^2）。后的纳米粒子阵列的制造，的JSC增加了约20毫安/平方^厘米 ，其中32％提高。它是略高于由后方的漫反射，只有25-30％的提升效果更好。后加入_氟化镁后熔覆细胞与电浆纳米阵列的漫反射，进一步提高司法人员叙用委员会 22.3毫安/厘米^2，或45％左右提高。请注意，所有电流为3微米厚的细胞较高，而相对提高到25.7毫安/厘米²略低，42％的光捕获在更薄的设备有较大的影响。

电池厚度：	2微米		3微米
	JSC，毫安/厘米2	阮富仲>％以上	JSC，毫安/厘米2	％以上
原始细胞	15.4		18.1
后漫反射（R）的	20.1	30.5	21.5	18.8
纳米粒子（NP方案）	20.3	31.8	21.9	21.0
NP /氟化镁2 /	22.3	45.3	25.7	42.0

表1。电浆电池短路电流和原始细胞相比，其增强。

图1。特写多晶硅薄膜太阳能电池与金属化网格细胞。

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图2银纳米粒子的电子显微镜图像扫描的硅表面上。

图3。1电浆薄膜晶体硅太阳能电池（不按比例）的示意图。

图4外部量子效率和短路电流的多晶硅薄膜与漫反射和电浆纳米细胞：虚线黑-原来的2微米厚的细胞，没有光捕获，JSC15.36毫安/厘米^2;蓝色-细胞。弥漫油漆反射，JSC 20.08毫安/厘米^2;红-与电浆银纳米粒子的细胞，JSC20.31毫安/厘米^2;绿色-细胞的纳米粒子， _氟化镁，和弥漫性涂料的反射，JSC 22.32毫安/厘米^2。紫色- 3微米厚的细胞（3毫米厚的玻璃），纳米粒子， _氟化镁，和漫反射，JSC 25.7毫安/厘米^2（注意蓝色低因无意在AR层和发射层厚度差异）。坚实的黑色- 2微米厚的等离子体增强化学气相沉积法（3毫米厚的玻璃），JSC26.4毫安/厘米^2，比较所示的纹理单元准备。

动画1。 点击这里查看动画 。

动画2。 点击这里查看动画 。

动画3。 点击这里查看动画。

动画4。 点击这里查看动画 。

动画5。 点击这里查看动画 。

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讨论

蒸发多晶硅太阳能电池及电浆纳米粒子光散射的光捕获的理想合作伙伴。这种细胞是平面的，因此，他们不能依靠光散射表面纹理，也可以很容易地在粗糙的表面形成电浆纳米。细胞有直接接触的硅，这也恰好是最好的最有效的电浆光散射纳米粒子的位置只有一个，后表面。此外，纳米热退火形成的最简单的方法也是最合适的光捕获结果与结晶硅薄膜的光捕获峰之间的700和1000 nm的，最重要的广泛...

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材料

Name	Company	Catalog Number	Comments
试剂名称	公司	目录编号	评论
银颗粒	Sigma-Aldrich公司	303372	99.99％
氟化镁，随机晶体，光学级	Sigma-Aldrich公司	378836	> = 99.99％
多乐士大衣天花板涂料	多乐士		R> 90％ （500-1100纳米）