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本文中选用的CdTe太阳电池基本结构为glass/SnO2∶F/CdS/CdTe/ZnTe∶Cu/Au,由四川大学太阳能材料与器件研究所研制。实验中采用在同一玻璃衬底上制作的4组不同效率的电池进行研究分析,其瞬态I-V及全面积QE如图 2所示。图 2b中的纵坐标EQE表示外量子效率(external quantum efficiency)。电池性能如表 1所示。表 1中,VOC为开路电压;JSC为短路电流密度;FF(fill factor)为填充因子;Rs为单位面积的串联电阻;Rsh为单位面积的旁路电阻。
图 2 不同效率的太阳电池光I-V特性及全面积QE特性
Figure 2. I-V characteristics and full-area QE characteristics of solar cells with different efficiencies
表 1 电池性能参数
Table 1. Battery performance parameters
No. VOC/mV JSC/(mA·cm-2) FF/% efficiency/% Rs/(Ω·cm2) Rsh/(Ω·cm2) 1 751.6 23.98 69.06 12.45 5 1173 2 771.5 24.20 67.12 12.53 6 1134 3 741.8 23.23 62.78 10.82 8 614 4 694.1 7.34 52.22 5.31 26 447 综合图 2和表 1中的结果可以看出,电池效率的下降,与电池的串联电阻Rs和旁路电阻Rsh关系密切,结合图 1e中太阳电池的双二极管模型,其I-V曲线方程如下[13]:
$ \begin{gathered} I=I_{\mathrm{ph}}-I_{\mathrm{D}_1}-I_{\mathrm{D}_2}-I_{\mathrm{sh}}= \\ I_{\mathrm{ph}}-I_{01}\left\{\exp \left[\frac{q\left(V+I R_{\mathrm{s}}\right)}{n_1 k_{\mathrm{B}} T}\right]-1\right\}- \\ I_{02}\left\{\exp \left[\frac{q\left(V+I R_{\mathrm{s}}\right)}{n_2 k_{\mathrm{B}} T}\right]-1\right\}-\frac{V+I R_{\mathrm{s}}}{R_{\mathrm{sh}}} \end{gathered} $
(1) 式中,I为输出电流,Iph为光生电流,ID1和ID2为二极管暗电流,I01和I02为相应的暗饱和电流,n1和n2为相应的二极管理想因子,Ish为短路冲击电流,T为温度,q为电荷量,V为输出电压,kB为玻尔兹曼常数。4号电池大的串联电阻(26 Ω·cm2)和小的旁路电阻(447 Ω·cm2),导致短路电流(7.34 mA·cm-2)下降非常明显,对应着很低的光电转换效率(5.31%)。如图 1c所示,电池的串联电阻和旁路电阻与电池组成材料及结构关系密切。而对于全尺寸的电池,由于前后电极的导电性良好,导致载流子的收集存在平均性,图 2中的测试结果,并不能反映电池结构中各个微区的特性。采用激光逐点测试的方式,能够获得电池全面积上不同区域的光诱导电流分布情况,结果如图 3所示。该电流与电池对应区域的特性以及所使用的单色光波长密切相关。微区量子效率由公式获得:
$ E_{\mathrm{EQE}}(\lambda)=\frac{I_{\mathrm{SC}} h c}{e \lambda P(\lambda)} $
(2) 式中, ISC为短路电路的电流,h为普朗克常数,P(λ)为激光照射强度,c为光速,e为元电荷,λ为波长。由于激光的照射面积只占整个电池面积很小的一部分,收集到的电流由方程给出:
$ I_{\mathrm{ph}}=I_{\mathrm{ph}, 0}-C_{\mathrm{rec}} I_{\mathrm{d}}^3 $
(3) 式中, Iph,0为该区域的光生电流,CrecId3为电流复合损失[14], Crec为与载流子复合相关的常数,Id为暗电流。
CdTe太阳电池的电流大小受空间电荷区(space charge region,SCR)和准中性区(quasi neutral region,QNR)的影响。在SCR区中,电流大小与前电极掺杂氟的SnO2透明导电玻璃(fluorine doped tin oxide,FTO)/窗口层CdS的势垒高度,CdS/CdTe耗尽层宽度及界面态有关。在QNR区,电流主要与背接触势垒有关。由于本文中所用的电池均有背接触层ZnTe∶Cu,能够有效地降低QNR区的势垒[15],因此电流大小主要与SCR区中CdS/CdTe结特性有关。对于第4号样品,由于载流子在内建电场下漂移电流的影响,以及CdS和CdTe材料中缺陷形成的复合中心会减小可迁移的载流子数目,使得该处的LBIC信号变小。理想情况下,在少子扩散长度内的光生载流子都有可能通过扩散到达结区边界从而对LBIC信号产生贡献,测量得到的LBIC电流信号根据激光照射处离开结区边界的距离d呈指数规律衰减:
$ I=k \mathrm{e}^{-d / L} $
(4) 式中,k为比例常数,L为扩散长度。因此,电池电流信号的下降,也就意味着激光与P-N结边界作用距离增加,载流子的扩散长度变小[16]。而对于CdS/CdTe太阳电池,其能带结构如图 1d所示。根据半导体材料本征吸收定理,在852 nm波长的激光作用下,光子能够依次透过衬底玻璃,透明导电薄膜SnO2∶F以及硫化镉窗口层,到达CdS/CdTe结区,并产生诱导电流。然而当P-N结特性较差时,往往耗尽层宽度增大,界面缺陷增多,导致载流子的产生、分离及输运发生变化。结合(1)式可知,在光生电流减小的同时,如果串联电阻变大而旁路电阻变小,将直接导致输出电流变小。从测试结果来看,1~4号样品的平均光诱导电流分别为:0.044315 mA,0.043794 mA,0.037668 mA以及0.013295 mA。本文中同时计算了电流的几何分布情况,样品光诱导电流的标准偏差分别为:0.003635,0.003318,0.00309以及0.001091,如图 4所示。选用的电池最高光电转换效率为12.53%,其串联电阻为6 Ω·cm2,旁路电阻为1134 Ω·cm2,与该种电池的实验最高转换效率差距较大[17],电池的激光诱导电流均有一定的起伏,说明本文中选中的4个电池,均匀性有待提高,CdS/CdTe异质结的制作工艺还有待优化,特别是碲化镉的制备及后期的扩散退火工艺需要严格控制[18]。4号电池的激光诱导电流变化幅度相对较小,这可能是由于电池的串联电阻较大而旁路电阻较小,整个器件结特性变差,载流子产生、分离及输运趋于平均所致。另外根据图 1e中的电池等效电路图,在开路电压较小(小于0.7 V)时,流经P-N结的电流将被限制在较小的范围。
CdTe太阳电池激光诱导微区光谱响应研究
Research on laser-induced local spectral response of CdTe solar cell
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摘要: 为了反映器件几何尺度上各个区域的性能分布, 采用波长为852 nm且可调聚焦面积的激光束作用在CdTe薄膜太阳电池的P-N结微区表面, 产生定域诱导光致电流响应, 并通过设置样品台的步进方式, 得到了所测器件在几何面积范围内的微区光谱响应分布图, 获得更直观的器件电流分布均匀性和P-N结特性。结果表明, 这种测试方式能够简化且低成本地建立与CdS/CdTe异质结制作技术密切相关的沉积与后处理工艺参数和材料特性的联系, 进而获得异质结界面分布均匀性与太阳电池电流-电压(I-V)特性参数均匀性的对应关系。该研究可为提高太阳电池的性能提供实验测试依据。
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关键词:
- 激光技术 /
- 微区光谱响应 /
- 激光诱导 /
- CdTe薄膜太阳电池
Abstract: In order to reflect the distribution of the performance of each region on the geometric scale of the device, a laser beam with a wavelength of 852 nm and an adjustable focusing area was used to act on the surface of the P-N junction microregion of the CdTe thin-film solar cell to generate a localized induced photocurrent. By setting the step-by-step method of the sample stage, the micro-region spectral response distribution map of the measured device within the geometric area range was obtained, and a more intuitive device current distribution uniformity and P-N junction characteristics were obtained. The results show that the relationship between the deposition and post-processing parameters and material properties that are closely related to the CdS/CdTe heterojunction fabrication technology can be simplified and cost-effectively established by using this test method, and then the heterojunction interface distribution uniformity and solar cells can be obtained. The corresponding relationship of the uniformity of I-V characteristic parameters provides experimental test basis for improving the performance of solar cells.-
Key words:
- laser technique /
- local spectral response /
- laser-induced /
- CdTe thin-film solar cell
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表 1 电池性能参数
Table 1. Battery performance parameters
No. VOC/mV JSC/(mA·cm-2) FF/% efficiency/% Rs/(Ω·cm2) Rsh/(Ω·cm2) 1 751.6 23.98 69.06 12.45 5 1173 2 771.5 24.20 67.12 12.53 6 1134 3 741.8 23.23 62.78 10.82 8 614 4 694.1 7.34 52.22 5.31 26 447 -
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