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

Abstract

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 CdS1-xTex, which are embedded in the light-absorbing semiconductors of Te-rich CdSyTe1-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 CdS1-xTex particles with a wurtzite structure embedded in a Te-rich CdSyTe1-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 CdS1-xTex 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 CdS1-xTex particles with a wurtzite structure and the big grains are Te-rich CdSyTe1-y with a zinc blend structure. We could conclude reasonably that the small particles observed in CAFM image probably are S-rich CdS1-xTex: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 Voc 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.