Study Of The Photovoltaic Properties Of Ferroelectric Lead Zirconate Titanate Thin Films

The photovoltaic effect of nonpolar crystals had been discovered in the 19 th century. Nowadays, solar cells based on crystalline silicon, as the most popular photovoltaic drives in the world, whose whole capacity has been over 25 GW. However, the development of crystalline silicon solar cells is limited by its maximum theoretical efficiency so that many scientific researchers are turning their attention to the field of novel photovoltaic materials. Ferroelectrics can usually produce an open-circuit voltage(Voc) much larger than the band gap due to the anomalous photovoltaic effect. Meanwhile, the cost of ferroelectric films is usually very low. These advantages make it a hot research topic recently.Because of the inefficient generation of e-h pairs in ferroelectric materials due to the large band gap(about 3.5e V) and poor conductivity, the short circuit current density(Jsc) in the most metal-ferroelectric-metal(MFM) structure is very small and the energy conversion efficiency is too low to satisfy factual demand. On the other hand, the band gaps of classic ferroelectric materials are usually large(about 3.5e V), the visible light can be hardly absorbed here, which limits the energy conversion efficiency seriously. Therefore, in this paper, we attempt to find a new method to improve the photoelectric transformation efficiency of solar cells based on the ferroelectric films. First of all, we prepared Pb(Zr0.20Ti0.80)O3 films by a sol-gel method on the substrates with a thin coating of ITO(In0.1Sn0.9O2). Then a series of metallic dot electrodes(Ag、Cu、Au、Pt) are fabricated on the same PZT films by sputtering at room temperature. Finally, surface Plasmon(SP) structures are introduced successfully into the inarticial surface of the PZT films. An enhanced EQE and an improved photocurrent output appear at the visible band. We suggest that the photocurrents observed in the visible range are contributed from the light-induced electron-holes excited by the enhanced near fields at the PZT surface. We adopt numeric simulations to verify our idea. We find that the surface roughness of the polycrystalline PZT film here which formed during the anneal process can couple the visible incident light with the localized SPs of Ag and Au electrodes, and enhance the visible light absorption of PZT film. Though the conventional charge-transfer mechanism cannot explain the SP-induced photocurrent in the PZT film, our model can be used to explain well the enhancement of photocurrent at the visible band at the ITO/PZT/Ag and ITO/PZT/Au structures.As we known, some semiconductors can be very active to absorb photons and generate e-h pairs. Therefore, we consider to design a metal/semiconductor/ferroelectric/metal(MSFM) heterostructure, which could take advantages of both the semiconductor for a large Jsc and the ferroelectric for a high Voc, to improve the energy conversion efficiency of ferroelectric photovoltaic devices. Following the thought, we introduce Zn O and Cu2 O layers into the ITO/PZT/Pt structure by sputtering, obtain ITO/PZT/Zn O/Pt and ITO/PZT/Zn O/Cu2O/Pt(MSFM) heterostructure. On the other hand, a reasonable arrangement of band gap is considered in these heterostructures, which could promote the movement of charges effectively. Then, we study the effect of the thickness of Zn O layer and the conditions of Cu2 O layer, respectively. In ITO/PZT/Zn O/Cu2O/Pt structure, we obtain the best structure that the short-circuit current density is sixty times than that of ITO/PZT/Pt structure, which proves that the heterostructure designed in our experiment is practical to improve the photovoltaic properties of ferroelectric thin film devices.