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太阳能电池其本质是一个将太阳光转换为电子空穴对的二极管,当太阳光照着在太阳能电池上时,相当于是在P-N结上添加了正向电压,此时太阳能电池的短路电流密度可由下式表示:
$ {J_{{\rm{SC}}}} = {J_0}{\rm{exp}}\left( {\frac{{q{V_{{\rm{OC}}}}}}{{kT}}} \right) $
(1) 式中, JSC是短路电流密度,J0是反向饱和电流密度,VOC是太阳能电池开了电压,k为常数,T为电池温度[18]。
另外, 太阳能电池的转换效率可利用电池的最大输出功率和入射光功率相除而得,由下式所示:
$ \mathit{\eta } = \frac{{{P_{{\rm{max}}}}}}{{{P_{{\rm{in}}}}}} = \frac{{{V_{{\rm{OC}}}}{J_{{\rm{SC}}}}f}}{{\int_0^\infty {{b_{{\rm{in}}}}(\mathit{\lambda })\frac{{hc}}{\mathit{\lambda }}{\rm{d}}\mathit{\lambda }} }} $
(2) 式中,Pmax是电池最大输出功率,它是由电池开路电压VOC、电池短路电流密度和填充因子f决定; Pin是电池的外部入射功率,它是由太阳光入射光子流密度bin(λ)计算而得[12],λ是入射光波长。
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图 2所示的是传统单晶硅TFSC的JSC(15.3mA/cm2)和η(18.7%)。图中得到的JSC和η是在hs=0.4μm, hm=0.08μm, ht=0.02μm的条件下先利用FDTD计算出传统单晶硅TFSC的η,随后将η代入(2)式计算得到JSC。之所以传统单晶硅TFSC的JSC和η相对较低是因为TFSC的吸收层仅有几百纳米,但是若想将长波长完全吸收,则要求吸收层厚度达到几十微米甚至几千微米,因此长波长的光将会透过吸收层而不被吸收,并且即使在薄膜表面由ITO之类的减反层[4],仍然有反射将太阳光反射回空气当中。
为了能够提高传统单晶硅TFSC的短路电流密度和转换效率,本文中提出在传统单晶TFSC的上表面制备硅介质光栅,并且在下表面制备相同周期的Al金属光栅,结构如图 1b所示。Al金属光栅主要作用是利用其表面等离子体效应增加TFSC的背面反射,而硅介质光栅的作用是减少TFSC的表面反射,并且能够将从Al金属光栅反射回来的光又全反射进单晶硅吸收层内。
图 3是当hs=0.4μm, hm=0.08μm, ht=0.02μm, h=0.04μm, F1=1时,表面硅介质光栅参量对光栅单晶硅TFSC短路电流密度的影响。从图 3中可以得到, 当介质光栅占空比F=0.8、光栅周期P=0.632μm、光栅厚度hg=0.42μm时,单晶硅TFSC的JSC可以达到29.13mA/cm2,相比于传统单晶硅TFSC的JSC提高了90.3%。
图 4是hs=0.4μm, ht=0.02μm, h=0.04μm, F=0.8, P=0.632μm, hg=0.42μm时,金属光栅参量对光栅单晶硅TFSC的短路电流密度的影响。通过图 4可以发现, 当F1=0.9,hm=0.005μm时,光栅单晶硅TFSC的短路电流密度可以达到35.15mA/cm2。虽然金属光栅(F1=0.9)厚度hm仅有0.005μm,但是在该参量下其实现的JSC比hm=0.08μm的金属(F1=1)高,之所以出现这种现象是因为太阳入射光波与Al金属光栅之间共振耦合形成表面等离子极化,使Al金属光栅与单晶硅交界面附近的电磁强度由于共振作用明显增强[19],从而使Al金属光栅对太阳光的反射高于厚度更高的金属层,最终提高了光栅单晶硅TFSC的JSC。
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提高光子在吸收层的光程是提高TFSC转换效率的关键问题之一,根据YABLONOVITCH计算得出,理论上最大光子光程[20]可由下式所示:
$ {w_{{\rm{opt}}}}(\mathit{\lambda }) = L{h_{\rm{s}}} = 4{n^2}{h_{\rm{s}}} $
(3) 式中, wopt(λ)是光子光程,L是光程路径提高因子,n是吸收层单晶硅的折射率。
根据参考文献[21]可知,TFSC的反射率与光子在吸收层的光程和吸收层的吸收系数有关,如下式所示:
$ r(\lambda ) = \exp \left[ { - \alpha (\mathit{\lambda }){w_{{\rm{opt}}}}(\mathit{\lambda })} \right] $
(4) 式中, α(λ)为吸收层吸收系数,r(λ)为TFSC的反射率。
根据(3)式和(4)式可知,光程路径提高因子可由下式表示:
$ {L_{\rm{c}}}(\lambda ) = - {\rm{ln}}\left[ {\frac{{{r_{\rm{c}}}(\lambda )}}{{{h_{\rm{s}}}\alpha (\lambda )}}} \right] $
(5) $ {L_{\rm{g}}}(\lambda ) = - {\rm{ln}}\left[ {\frac{{{r_{\rm{g}}}(\lambda )}}{{{h_{\rm{s}}}\alpha (\lambda )}}} \right] $
(6) 式中,Lc(λ)和Lg(λ)分别表示传统单晶硅TFSC路径提高因子和光栅单晶硅路径提高因子,rc(λ)和rg(λ)分别表示传统单晶硅TFSC的反射率和光栅单晶硅TFSC的反射率。
$ {L_r} = \frac{{{L_{\rm{g}}}(\mathit{\lambda })}}{{{L_{\rm{c}}}(\mathit{\lambda })}} = \frac{{{\rm{ln}}\left[ {{r_{\rm{g}}}(\lambda )} \right]}}{{{\rm{ln}}\left[ {{r_{\rm{c}}}(\lambda )} \right]}} $
(7) (7) 式是光栅单晶硅TFSC与传统单晶硅TFSC光程路径提高因子的比值Lr,当Lr>1, 说明相同波长的光子在光栅单晶硅TFSC吸收层的光程路径相对于传统单晶硅TFSC有提高,当Lr < 1, 说明相同波长的光子在光栅单晶硅TFSC吸收层的光程路径相对于传统单晶硅TFSC有减小。
图 5是不同波长对应的Lr值。从图中可以看见,当入射光波长从0.3μm增加到4μm的过程中,Lr几乎都是大于1,而且最大值可以达到40,仅有一小部分波段对应的Lr < 1。由此可知,通过优化后光栅单晶硅TFSC的光程路径有了显著的提高,因此才会使其JSC达到35.15mA/cm2。
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衡量光栅单晶硅TFSC吸收效率相比于传统单晶硅TFSC是提高还是下降,本文中用吸收率增强因子Ae来表述,Ae的表达式如下式所示:
$ {A_{\rm{e}}} = \frac{{{A_{\rm{g}}}(\lambda ) - {A_{\rm{c}}}\left( \lambda \right)}}{{{A_{\rm{c}}}(\lambda )}} $
(8) 式中, Ag(λ)和Ac(λ)分别是最优光栅单晶硅TFSC和传统单晶硅TFSC对应不同波长的吸收效率。当Ae>0,说明光栅单晶硅TFSC的吸收效率比传统单晶硅TFSC的高;而当Ae < 0,则说明光栅单晶硅TFSC的吸收效率比传统单晶硅TFSC的低。
图 6是不同波长对应的吸收率增强因子Ae。从图中可以看到, 大部分的Ae均是大于0,并且最大值可以达到接近700%,只有在一部分波段内是Ae小于0,由此可以得出最优光栅单晶硅TFSC的吸收效率有了显著的提高。
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图 7是由(1)式和(2)式计算得到的最优光栅单晶硅TFSC的短路电流密度和转换效率, 并将其与传统单晶硅的TFSC进行对比。从图中可以发现, 最优光栅单晶硅TFSC的JSC和η分别达到35.15mA/cm2和43.35%,相比传统的有了明显的提高,在转换效率上, 最优单晶硅TFSC相对于传统单晶硅TFSC提高了126.4%。
双光栅结构薄膜太阳能电池的优化
Optimization of thin film solar cells with double-grating structure
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摘要: 为了提高单晶硅薄膜太阳能电池短路电流密度和转换效率, 采用在单晶硅薄膜太阳能电池正背面分别集成硅介质光栅和铝金属光栅的方法, 并利用有限时域差分法软件仿真研究了两种光栅的周期、厚度、占空比对单晶硅薄膜太阳能电池短路电流密度和光转换效率的影响。结果表明, 通过优化可得当正背面光栅都处于最优值时(介质光栅占空比F=0.8、介质光栅周期P=0.632μm、介质光栅厚度hg=0.42μm; 金属光栅占空比F1=0.9、金属光栅周期P=0.632μm、金属光栅厚度hm=0.005μm), 短路电流密度可达35.15mA/cm2, 转换效率为43.35%;将最优光栅单晶硅薄膜太阳能电池与传统单晶硅薄膜太阳能电池对比, 无论是光程路径还是吸收效率, 光栅单晶硅薄膜太阳能电池都有显著的提高。这为以后制备高性能薄膜太阳能电池提供了理论指导。Abstract: In order to improve the short circuit current density and conversion efficiency of crystalline silicon thin film solar cells, a silicon dielectric grating and an aluminium metal grating were integrated on the front and back of single crystal silicon thin film solar cells respectively. The effect of the period, thickness and duty cycle of both the gratings on the short-circuit current density and optical conversion efficiency of single crystal silicon thin film solar cells were simulated with finite difference time-domain software. The results show that, the short-circuit current density can reach 35.15mA/cm2 and the conversion efficiency is 43.35% when both the front and back gratings are at the optimum value (for the dielectric grating, duty cycle F=0.8, period P=0.632μm, thickness hg=0.42μm; for the metal grating, duty cycle F1=0.9, period P=0.632μm and thickness hm=0.005μm). After comparing the optimal grating monocrystalline silicon thin film solar cells with traditional monocrystalline silicon thin film solar cells, the grating monocrystalline silicon thin film solar cells have a significant improvement in both optical path and absorption efficiency. This study provides theoretical guidance for the preparation of high performance thin film solar cells in the future.
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Key words:
- gratings /
- crystalline silicon thin film /
- solar cell /
- conversion efficiency
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