Study On The Nature Of Solar Cells Based On Ferroelectric

The ferroelectric thin film materials with excellent ferroelectric, piezoelectric, pyroelectric, electro-optic and nonlinear optical properties are widely applied in electronic devices such as storage, pressure sensor, brake and so on. And the ferroelectric thin films with simple preparation technology and low cost are popular. As a result, ferroelectric thin films as a new kind of materials are one of the hot researches in recent years in the world. Recently, the photovoltaic mechanism of ferroelectrics, such as Pb(Zr,Ti)O3(PZT),(Bi,Nd)4Ti3O12(BNT), Ba Ti O3 and Bi Fe O3(BFO) have attracted a great deal of attention. Many groups have put their efforts into studying the ferroelectric PV effect and enhancing the PV output of ferroelectric-based solar cells. The maximum open-circuit voltage(VOC, above 15 V) was reported in BFO films, however, the short-circuit current(JSC) of BFO film was very low(about dozens of μA/cm2). So far, the PZT film shows the highest photoelectric conversion efficiency(η) among the ferroelectric-based solar cells, due to its large remnant polarization and high Schottky barrier at the film–electrode interface. However, the reported maximum η of pure PZT film(about 0.01%) is still far less than that of a crystalline silicon solar cell, which is mainly attributed to the poor conductivity and large band gap of the ferroelectrics.The main contents in this aticle include the following two aspects:(1) To improve the spectrum response at the visible light band for a typical ferroelectric film with a large band gap, it should be effective that a semiconductor material with a narrow band gap is combined with the ferroelectric fillm if their energy levels can match each other. The specific method is as follows:Pb(Zr,Ti)O3(PZT) film with a band gap of 3.6 e V was prepared on In2O3:Sn(ITO) coated glass, and then an amorphous silicon(a-Si) film with a narrow band gap of 1.8 e V was deposited on the PZT/ITO/glass. The short-circuit current of Ag/a-Si/PZT/ITO/glass sample is 2.56 m A/cm2, about 50 times greater than that of Pt/PZT/ITO/glass sample. The energy level of the a-Si film is well matched with that of PZT film, which is beneficial to the transport of UV-visible light-induced charges in the Ag/a-Si/PZT/ITO/glass sample. The photovoltaic output can be tuned by changing the dopant type of the a-Si film, and the maximum photoelectric conversion efficiency is measured to be up to 1.25% in the PZT film with an n-type a-Si film(phosphorus doped).(2) Ion doping technique is thought as another effective method to improve the remnant polarization and lower the crystallization temperature of ferroelectric thin film.We studied the effects on microstructure, ferroelectric and photovoltaic property of different Cr3+ dopants concentrations of Pb(Zr0.2Ti0.8)O3(PCZT) ferroelectric films. Our experimental results show that Cr3+ ions, as the accepters substituting for Ti4+ ions, decrease the remnant polarization(2Pr) when Cr3+ concentration is less than about 1%(molar ratio). However, Cr3+ ions were the donors substituting for Pb2+ ions and 2Pr is improved greatly when Cr3+ concentration is more than about 1%. The ferroelectric and photovoltaic outputs of PCZT films are optimized. When the concentration of Cr3+ ions is about 3%, the value of(2Pr) reaches 124.44 μC/cm2, which is about 2.8 times compared with that of PZT film(no Cr3+ dopants, 45.15 μC/cm2). Accordingly, the photovoltaic output of PCZT film(Cr3+ dopants 3%) is about 5 times compared with that of PZT film.