CdTe薄膜太阳电池器件中的关键科学问题研究

Compared to the traditional crystalline Si solar cell, CdTe thin film solar cell has its own specific characteristics regarding both to the material and the cell device. CdTe solar cell is fabricated based on the CdS/CdTe hetero-junction diode and polycrystalline CdTe absorber film. In a CdTe thin film solar cell there are several heterojunctions. Therefore the control of defect types and density at the heterojunctions, and the micro-shunting/shorting channels at the CdS/CdTe junction diode on carrier generation, recombination and transport are the most important problems for the study of CdTe thin film solar cell. In this research proposal we propose to carry out the following studies. 1) An energy barrier for electron transport is to be introduced at the front electrode, this would lead to carrier type inversion in CdTe near the CdS/CdTe diode junction. Then admittance spectroscopy technique can be employed to study the effect of defects on minority electron carrier recombination and transport near the CdS/CdTe junction diode. 2) To introduce a highly resistive non-continuous and continuous oxide layer between CdS and transparent conductive oxide (TCO) electrode. The energy band of the inserted oxide layer is fully matched to the front TCO electrode. In this way the formation of space charge limited current (SCLC) can be controlled by the oxide layer covering thickness. The controlled SCLC formation density can be employed to qualitatively and/or quantitatively study the effect of the current shunting induced by SCLC on the cell open-circuit voltage. 3) To study the microstructure and material/composite inhomogeneity at the CdS/CdTe junction by employing nano-metal enhanced plasmonic Raman scattering.

CdTe薄膜太阳电池和传统晶体硅电池相比，有着较为独特的材料和器件结构特点。CdTe电池是基于CdS/CdTe异质结和多晶CdTe薄膜光吸收层结构的电池，电池结构中存在多个异质结构界面，因此界面附近缺陷的控制，以及CdS/CdTe异质结处的微观漏电通道对光生载流子的产生、复合和输运，是CdTe电池研究面临的重要科学问题。本申请提出如下研究方案，1）在电池前电极处引入导带势垒，在靠近CdS/CdTe结处的CdTe薄膜内，实现载流子种类的翻转，即电子浓度高于空穴浓度，进而利用导纳谱研究CdS/CdTe结处的缺陷能级及载流子复合；2）引入与透明导电电极能带完全匹配的SnO2高电阻层，调控空间电荷限制电流（SCLC）微观漏电通道形成密度，定性/定量研究SCLC漏电对电池开路电压的影响；3）利用纳米金属等离激元增强的拉曼散射，在微米/亚微米尺度上研究CdS/CdTe界面处的结构及成分的不均匀性。

硅基太阳电池以其成本较低、较高的光电转化效率，占据了全球太阳电池市场90 %以上的份额。在光伏市场上成功实现大规模产业化的薄膜电池，在效率和成本上唯一能和晶硅电池进行竞争的是CdTe薄膜太阳电池。CdTe电池的产业化，在中国面临较大的挑战。.在本基金的资助下，开展的研究工作和取得的研究成果如下。1）研究了碲化镉薄膜太阳电池中的空间电荷限制电流，电池中的FTO/CdTe/metal结构会导致空间电荷限制电流增强，CdTe电池的漏电电流增大，填充因子降低，效率下降。2）制备了以MoOx、CuI、WO3纳米片等作为背接触缓冲层的高转换效率CdTe太阳电池，提高了电池的热稳定性。3）制备了MgZnO、TiO2、Zn(S,O)等薄膜作为窗口层的新型结构CdTe太阳电池，通过对窗口层的热处理提高载流子浓度，电池效率明显提升。4）研究了金属氧化物对碲化镉表面的钝化，发现3 nm V2O5和Al2O3中存在一定量的固定负电荷，该固定负电荷的能够在CdTe中诱导产生电场，而电场能够起到加速电子空穴分离降低复合的作用。5）研究了MgZnO/CdTe太阳电池中复合引起的光生电流收集损失机制，并与CdS/CdTe电池进行了对比研究，发现MgZnO/CdTe太阳能电池则以p-n结界面复合为主，CdS/CdTe太阳能电池中的复合主要是CdTe体材中的Shockely-Read-Hall（SRH）复合。6）利用金属纳米等离激元增强效应，定量地研究了纳米厚度Cu薄膜的Cu原子在CdTe薄膜中的扩散。.通过本基金项目的实施，可以发现在CdTe薄膜太阳电池中，限制电池转换效率提高的因素，既有电池本身结构的内在因素，例如空间电荷限制电流的存在，这是基于电池基本结构导致的，又有外在因素，例如碲化镉表面缺陷的钝化。