Highly Efficient Thermally Co-evaporated Perovskite Solar Cells and Mini-modules

The rapid improvement in the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has prompted interest in bringing the technology toward commercialization. Capitalizing on existing industrial processes facilitates the transition from laboratory to production lines. In this work, we prove the scalability of thermally co-evaporated MAPbI3 layers in PSCs and mini-modules. With a combined strategy of active layer engineering, interfacial optimization, surface treatments, and light management, we demonstrate PSCs (0.16 cm2 active area) and mini-modules (21 cm2 active area) achieving record PCEs of 20.28% and 18.13%, respectively. Un-encapsulated PSCs retained ∼90% of their initial PCE under continuous illumination at 1 sun, without sample cooling, for more than 100 h. Looking toward tandem and building integrated photovoltaic applications, we have demonstrated semi-transparent mini-modules and colored PSCs with consistent PCEs of ∼16% for a set of visible colors. Our work demonstrates the compatibility of perovskite technology with industrial processes and its potential for next-generation photovoltaics

Structure and functional properties of epitaxial PBZRxTI1-xO3 films

The work described in this thesis is focused on the characterization and understanding of epitaxial, clamped, dense PbZrxTi1-xO3 (PZT) films. A thermodynamic model is developed, which is used to simulate properties of clamped PZT films throughout this work. The free energy equations for single- and poly-domain films are given and allow us to simulate the material properties such as the piezoelectric coefficient, stress, strain, polarization and crystal phase of the PZT films. Experimental work done on tetragonal poly domain PbZr40Ti60O3 films is described. X-ray diffraction (XRD) measurements done with an applied field are used to measure the intrinsic piezoelectric coefficients of individual domains. The intrinsic piezoelectric coefficient of the domains in the PZT film is negative while the average piezoelectric coefficient of the film is positive, which is predicted by the model. An alternative explanation for the origin of high piezoelectric coefficients using the new model is discussed. The model is used to explain the high piezoelectric characteristics of both film and bulk PZT found at the morphotropic phase boundary (MPB) using the characteristics of the rhombohedral and the tetragonal crystal phase or the phase change between them. The model also predicts that in principle arbitrarily high piezoelectric coefficients can be obtained in defect free films. The value of the coefficient is highly dependent on the misfit strain, which can be tuned using different substrates. The structure of the domains and domain walls (DWs) in tetragonal PZT is explored. Data obtained using transmission electron microscopy shows that 2D films exist out of a c/a, c’/a’ domain structure. XRD and piezo force microscopy on 3D films show a complex c/a, c’/a’, c/b, c’/b’ domain structure which reconstructs when the out-of-plane polarization is switched. The complex domain structure can be explained as a mixed micro- and macro-domain structure that describes a domain structure which is required by our model

Process induced poling and plasma induced damage of thin films PZT

This paper treats processing sequence induced changes on PZT. Two kinds of metal-PZT-metal capacitors are compared. The top surface and sidewall of PZT in one kind of capacitor is directly bombarded by energetic particles during ion milling process, whereas PZT in the other kind of capacitor is not. The polarity of plasma charging may depend on the ion milling parameters and influence the self-poling of virgin PZT capacitors. Direct ion bombardment induces a significant decrease of PZT permittivity. The PZT reliability (both RVS and TDDB) at positive voltage worsens because of bombardments of energetic particles; whereas the PZT reliability at negative voltage is not influenced. It indicates that the process induced positively charged defects present in the upper part of the capacitor structure initiate the dielectric breakdown

Controlling the growth of Bi(110) and Bi(111) films on an insulating substrate

We demonstrate the controlled growth of Bi(110) and Bi(111) films on an α-Al2O3(0001) substrate by surface x-ray diffraction and x-ray reflectivity using synchrotron radiation. At temperatures as low as 40 K, unanticipated pseudo-cubic Bi(110) films are grown with thicknesses ranging from a few to tens of nanometers. The roughness at the film–vacuum as well as the film–substrate interface, can be reduced by mild heating, where a crystallographic orientation transition of Bi(110) towards Bi(111) is observed at 400 K. From 450 K onwards high quality ultrasmooth Bi(111) films form. Growth around the transition temperature results in the growth of competing Bi(110) and Bi(111) domains

Controlling the growth of Bi(110) and Bi(111) films on an insulating substrate

We demonstrate the controlled growth of Bi(110) and Bi(111) films on an α-Al2O3(0001) substrate by surface x-ray diffraction and x-ray reflectivity using synchrotron radiation. At temperatures as low as 40 K, unanticipated pseudo-cubic Bi(110) films are grown with thicknesses ranging from a few to tens of nanometers. The roughness at the film–vacuum as well as the film–substrate interface, can be reduced by mild heating, where a crystallographic orientation transition of Bi(110) towards Bi(111) is observed at 400 K. From 450 K onwards high quality ultrasmooth Bi(111) films form. Growth around the transition temperature results in the growth of competing Bi(110) and Bi(111) domains