Electron -reflector strategy for CdTe thin-film solar cells
Electron -reflector strategy for CdTe thin-film solar cells
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CdTe薄膜太阳能电池的电子反射策略
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发表时间：
2010
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通讯作者：
K. Hsiao
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文献类型：
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作者：
K. Hsiao
CdTe薄膜太阳能电池具有较大的吸收系数和较高的理论效率。此外，可以经济地制造大面积光伏面板。这些特性使CdTe薄膜太阳能电池成为领先的替代能源。然而，记录的CdTe效率（16.5%）远低于其理论最大效率（29%），主要是因为开路电压（0.845 V）远低于其带隙（1.5 eV）的预期。引入电子反射器是提高太阳能电池开路电压的一种策略，因此很有可能提高CdTe薄膜太阳能电池的效率。电子反射器是太阳能电池背表面处的导带能量势垒，其可以减少由于电子流到背表面而引起的复合。本文介绍了几种不同的电子反射器的制备方法：（1）带隙扩展法，即带隙扩展层或体带隙减小法;（2）通过反向背势垒或重掺杂背表面改变能带弯曲。研究表明，扩展带隙层是最有效和实用的电子反射器的机制，任何两种机制的组合不会产生额外的改善。iv为了从电子反射器策略中获得最佳效果，需要合理的CdTe寿命（1 ns或以上）和CdTe层的完全耗尽，以确保高载流子收集。此外，p型CdTe层和电子反射层之间的良好质量的反射界面是必不可少的。初步实验证据表明，具有ZnTe背层的CdTe电池确实具有稍高的开路电压。电子反射器对于具有完全耗尽的CdTe吸收层的薄（小于2 μm）CdTe电池应该特别有益。薄的CdTe电池也可以受益于后表面处的光学反射。为了研究更高效率的可能性，将电子和光学反射数值应用于CdTe记录单元基线模型。然而，对于大于2 μm的CdTe厚度，几乎没有改善。为了从组合的电子和光学反射中获得最佳效果，需要大约一微米厚的单元。即使没有改善的电流质量的CdTe，电池效率超过19%，应该是可以实现的0.2-eV的电子反射器。此外，如果还可以实现大的光学背反射，则高于20%的效率应该是可能的。与此同时，具有竞争力的CdTe…
The CdTe thin-film solar cell has a large absorption coefficient and high theoretical efficiency. Moreover, large-area photovoltaic panels can be economically fabricated. These features potentially make the CdTe thin-film solar cell the leading alternative energy source. However, the record CdTe efficiency (16.5%) is much less than its theoretical maximum efficiency (29%), primarily because the open-circuit voltage (0.845 V) is well below what is expected for its band gap (1.5 eV). The incorporation of an electron reflector is a strategy to improve the open-circuit voltage of solar cells, and thus a strong possibility to improve the efficiency of CdTe thin-film solar cells. An electron reflector is a conduction-band energy barrier at the back surface of the solar cell, which can reduce the recombination due to the electron flow to the back surface. Different methods to create an electron reflector are explained in the thesis: (1) expanded band gap, either an expanded-band-gap layer or a bulk-band-gap reduction, and (2) alteration to the band bending through a reversed back barrier or a heavily-doped back surface. Investigation shows that the expanded-band-gap layer is the most efficient and practical mechanism for an electron reflector, and the combination of any two mechanisms does not yield additional improvement. iv To have the optimal effect from the electron-reflector strategy, reasonable CdTe lifetime (1 ns or above) and full depletion of the CdTe layer are required to ensure high carrier collection. Furthermore, a good-quality reflector interface between the p-type CdTe layer and the electron-reflector layer is essential. Preliminary experimental evidence has shown that CdTe cells with a ZnTe back layer do have a slightly higher open-circuit voltage. An electron reflector should be particularly beneficial for thin (less than 2 µm) CdTe cells which have a fully-depleted CdTe absorber layer. Thin CdTe cells can also benefit from the optical reflection at the back surface. To investigate the possibility of still higher efficiency, both electron and optical reflection were numerically applied to the CdTe record-cell baseline model. However, there is little improvement for CdTe thicknesses greater than 2 µm. To have the optimal effect from combined electron and optical reflection, cells approximately one micron thick are required. Even without the improvement to the current quality of CdTe, cell efficiency above 19% should be achievable with a 0.2-eV electron reflector. Moreover, efficiency above 20% should be possible if one can also achieve large optical back reflection. At the same time, competitive CdTe …