铁电半导体耦合薄膜电池中的反常载流子传输现象

CdS-CdTe铁电半导体耦合太阳能电池是一种新型太阳能电池, 其工作机理是光伏材料CdTe吸收光子产生的电子空穴对, 在铁电材料CdS 极化形成的内建电场作用下向两极运动, 通过前后电极引出形成电流. 本文利用原子力显微镜(AFM)进行导电AFM扫描, 得到的CdS-CdTe 铁电半导体耦合太阳能电池薄膜表面微观电流分布出现了一些反常的现象, CdTe晶粒边界处存在百纳米级别的小颗粒覆盖晶界, 晶界不导电, 大电流区域沿晶界边缘在晶粒内分布. 作为对比, 同样条件下制得的纯CdTe薄膜晶界却存在明显的导电现象. 在进行导电AFM扫描时, 分别对两组薄膜样品施加方向相反的直流偏压, 发现CdS-CdTe 铁电半导体耦合太阳能电池薄膜晶界处存在明显的压电现象, 证明CdS-CdTe 铁电半导体耦合太阳能电池薄膜中不导电晶界很有可能是具有压电性的富S的CdS_1-xTe_x颗粒. 扫描透射电镜分析也证实了这些小颗粒为六方相富S的CdS_1-xTe_x 合金. 同时, 经过六个月的定期测试, 发现CdS 铁电半导体耦合太阳能电池出现效率增长的异常现象, 最高电池效率已达13.2%, 该效率是目前已知的铁电光伏器件中最高的.

Transport phenomenon of anormalous carriers in ferroelectric-semiconductor coupled solar cell

In recent years, a variety of new-concept solar cells have attracted the attention of many scholars. The CdS-CdTe ferroelectric-semiconductor coupled (FSC) solar cell is a novel concept of photovoltaic device that is designed with ferroelectric nano particles of S-rich CdS_1-xTe_x, which are embedded in the light-absorbing semiconductors of Te-rich CdS_yTe_1-y. In our previous work, we have developed a two-step process to fabricate a nano-dipole photovoltaic device, including a thin film deposition in vacuum and high-temperature phase segregation at elevated temperature in sequence. The X-ray diffraction (XRD) and high-resolution scanning transmission electric microscopy (STEM) results confirm the formation of S-rich CdS_1-xTe_x particles with a wurtzite structure embedded in a Te-rich CdS_yTe_1-y film with a zinc blend structure. The localized ferroelectric hysteretic behavior of these particles is confirmed through piezoelectric force microscopy (PFM). Meanwhile, a set of CdS-CdTe FSC devices with a symmetrical structure of ITO/FSC/ITO is fabricated. We observe not only a reasonable photovoltage output on the order of hundreds of mV but also the hysteretic behavior of photovoltage through external electric field post-fabrication.To search for direct evidence of the working mechanism of the FSC solar cell, we further study the film surface micro current distribution of the FSC thin film solar cell. In this work, we adopt the CAFM method to acquire electron distribution of the FSC thin film surface and STEM, the electron diffraction for element distribution, and crystal structure of FSC thin film. Also, Schottky solar cell of FTO/pure CdTe/metal structure which is fabricated by the same process as the FSC solar cell is used as reference sample in the CAFM analysis.In this work, we fabricate the CdS-CdTe FSC film solar cell through a radio-frequency magnetron sputtering method, whose structure is a glass/FTO/CdSTe/back contact (Cu/Au) configuration. In order to enhance the polarization of nano dipole particles in the device, an electric field bias across the FSC film is applied in the high-temperature phase segregation process. Micro-current distribution in CdS-CdTe FSC solar cell is investigated by CAFM. Grain boundaries of the FSC film are found to be non-conductive with high current corridors adjacent to them. And some small particles with diameter about 100 nm are embedded in grain boundaries (GBs) of CdTe grains. By applying positive and opposite voltage separately between measurement tip and TCO of sample, we find that the non-conductive GBs have a strong piezoelectric response, which are most likely S-rich CdS_1-xTe_x in wurtzite structure. By contrast with pure CdTe film, the possibility that the non-conductive particles are CdCl₂ residuals is excluded. We also find by STEM that many particles with sizes about 100-200 nm are embedded in FSC thin film, mostly at the GBs. The XRD results confirm that the small particles are S-rich CdS_1-xTe_x particles with a wurtzite structure and the big grains are Te-rich CdS_yTe_1-y with a zinc blend structure. We could conclude reasonably that the small particles observed in CAFM image probably are S-rich CdS_1-xTe_x:The apparent correlation between the carrier transport channel and nano-dipole material is also established. An interesting discovery from such devices is that such cells exhibit performance improvement over time in months after storage with encapsulation in ambient environment. A linear relationship between V_oc and the external field strength is observed and the best conversion efficiency is improved from 11.3% to 13.2% further after 6-month storage. We believe that all these microscopic and macroscopic evidences are consistent with the FSC photovoltaic mechanism.