Alexei Gruverman

Is Cu a stable electrode material in hybrid perovskite solar cells for a 30-year lifetime?

One grand challenge for long-lived perovskite solar cells is that the common electrode materials in solar cells, such as silver and aluminum or even gold, strongly react with hybrid perovskites. Here we report the evaluation of the potential of copper (Cu) as the electrode material in perovskite solar cells for long-term stability. In encapsulated devices which limit exposure to oxygen and moisture, Cu in direct contact with CH3NH3PbI3 showed no reaction at laboratory time scales, and is predicted to be stable for almost 170 years at room temperature and over 22 years at the nominal operating cell temperature of 40 °C. No diffusion of Cu into CH3NH3PbI3 has been observed after thermal annealing for over 100 hours at 80 °C, nor does Cu cause charge trap states in direct contact with CH3NH3PbI3 after long-term thermal annealing or illumination. High performance devices with efficiency above 20% with a Cu electrode retain 98% of the initial efficiency after 816 hours storage in an ambient environment without encapsulation. The results indicate Cu is a promising low-cost electrode material for perovskite solar cells for long-term operation.

Giant switchable photovoltaic effect in organometal trihalide perovskite devices

Organolead trihalide perovskite (OTP) materials are emerging as naturally abundant materials for low-cost, solution-processed and highly efficient solar cells. Here, we show that, in OTP-based photovoltaic devices with vertical and lateral cell configurations, the photocurrent direction can be switched repeatedly by applying a small electric field of <1 V μm(-1). The switchable photocurrent, generally observed in devices based on ferroelectric materials, reached 20.1 mA cm(-2) under one sun illumination in OTP devices with a vertical architecture, which is four orders of magnitude larger than that measured in other ferroelectric photovoltaic devices. This field-switchable photovoltaic effect can be explained by the formation of reversible p-i-n structures induced by ion drift in the perovskite layer. The demonstration of switchable OTP photovoltaics and electric-field-manipulated doping paves the way for innovative solar cell designs and for the exploitation of OTP materials in electrically and optically readable memristors and circuits.

Efficiency enhancement in organic solar cells with ferroelectric polymers

The recombination of electrons and holes in semiconducting polymer-fullerene blends has been identified as a main cause of energy loss in organic photovoltaic devices. Generally, an external bias voltage is required to efficiently separate the electrons and holes and thus prevent their recombination. Here we show that a large, permanent, internal electric field can be ensured by incorporating a ferroelectric polymer layer into the device, which eliminates the need for an external bias. The electric field, of the order of 50 V μm(-1), potentially induced by the ferroelectric layer is tens of times larger than that achievable by the use of electrodes with different work functions. We show that ferroelectric polymer layers enhanced the efficiency of several types of organic photovoltaic device from 1-2% without layers to 4-5% with layers. These enhanced efficiencies are 10-20% higher than those achieved by other methods, such as morphology and electrode work-function optimization. The devices show the unique characteristics of ferroelectric photovoltaic devices with switchable diode polarity and tunable efficiency.

Nanoscale ferroelectrics : processing, characterization and future trends

This review paper summarizes recent advances in the quickly developing field of nanoscale ferroelectrics, analyses its current status and considers potential future developments. The paper presents a brief survey of the fabrication methods of ferroelectric nanostructures and investigation of the size effects by means of scanning probe microscopy. One of the focuses of the review will be the study of kinetics of nanoscale ferroelectric switching in inhomogeneous electrical and elastic fields. Another emphasis will be made on tailoring the electrical and mechanical properties of ferroelectrics with a viewpoint of fabrication of nanoscale domain structures.

Tunneling electroresistance effect in ferroelectric tunnel junctions at the nanoscale.

Using a set of scanning probe microscopy techniques, we demonstrate the reproducible tunneling electroresistance effect on nanometer-thick epitaxial BaTiO(3) single-crystalline thin films on SrRuO(3) bottom electrodes. Correlation between ferroelectric and electronic transport properties is established by direct nanoscale visualization and control of polarization and tunneling current. The obtained results show a change in resistance by about 2 orders of magnitude upon polarization reversal on a lateral scale of 20 nm at room temperature. These results are promising for employing ferroelectric tunnel junctions in nonvolatile memory and logic devices.

Mechanical Writing of Ferroelectric Polarization

Changing Polarization with Applied Stress The direction of electric polarization in ferroelectric materials can be switched with an applied field, but mechanical stresses can also couple to the polarization, forming the basis for piezoelectric effects. In principle, it should be possible to change the polarization of a ferroelectric material mechanically through stress gradients. Lu et al. (p. 59; see the Perspective by Gregg) demonstrate such switching for nanoscale-sized regions created by the stress induced with an atomic force microscope. The substrates are single-crystalline barium titanate films that have a vertically aligned dipole moment created by compressive stresses in the film. This approach may lead to memory devices in which bits are written mechanically but read electrically. The stress gradient created with the tip of an atomic force microscope can locally change the polarization of a barium titanate film. Ferroelectric materials are characterized by a permanent electric dipole that can be reversed through the application of an external voltage, but a strong intrinsic coupling between polarization and deformation also causes all ferroelectrics to be piezoelectric, leading to applications in sensors and high-displacement actuators. A less explored property is flexoelectricity, the coupling between polarization and a strain gradient. We demonstrate that the stress gradient generated by the tip of an atomic force microscope can mechanically switch the polarization in the nanoscale volume of a ferroelectric film. Pure mechanical force can therefore be used as a dynamic tool for polarization control and may enable applications in which memory bits are written mechanically and read electrically.

Nanoscale investigation of fatigue effects in Pb(Zr,Ti)O3 films

Scanning force microscopy has been used to perform a comparative nanoscale study of domain structures and switching behavior of Pb(ZrxTi1−x)O3 (PZT) thin films integrated into heterostructures with different electrodes. The study revealed a significant difference between polarization state of as‐deposited PZT films on RuO2 and Pt electrodes. The PZT/RuO2 films exhibit polydomain crystallites and show almost symmetric switching behavior, while the PZT/Pt films are mainly in a single polarity state and exhibit highly asymmetric piezoelectric hysteresis loops. Formation of unswitchable polarization within the grains of submicron size as a result of fatigue process was directly observed.

Grain boundary dominated ion migration in polycrystalline organic–inorganic halide perovskite films

The efficiency of perovskite solar cells is approaching that of single-crystalline silicon solar cells despite the presence of a large grain boundary (GB) area in the polycrystalline thin films. Here, by using a combination of nanoscopic and macroscopic level measurements, we show that ion migration in polycrystalline perovskites dominates through GBs. Atomic force microscopy measurements reveal much stronger hysteresis both for photocurrent and dark-current at the GBs than on the grain interiors, which can be explained by faster ion migration at the GBs. The dramatically enhanced ion migration results in the redistribution of ions along the GBs after electric poling, in contrast to the intact grain area. The perovskite single-crystal devices without GBs show negligible current hysteresis and no ion-migration signal. The discovery of dominating ion migration through GBs in perovskites can lead to broad applications in many types of devices including photovoltaics, memristors, and ion batteries.

Nanoscale imaging of domain dynamics and retention in ferroelectric thin films

We report results on the direct observation of the microscopic origins of backswitching in ferroelectric thin films. The piezoelectric response generated in the film by a biased atomic force microscope tip was used to obtain static and dynamic piezoelectric images of individual grains in a polycrystalline material. We demonstrate that polarization reversal occurs under no external field (i.e., loss of remanent polarization) via a dispersive continuous-time random walk process, identified by a stretched exponential decay of the remanent polarization.

Scanning force microscopy for the study of domain structure in ferroelectric thin films

A piezoresponse technique based on scanning force microscopy (SFM) has been used for studying domain structure in ferroelectric thin films. Studies were performed on Pb(Zrx,Ti1−x)O3(PZT) thin films produced by a sol–gel method. The piezoresponse images of the PZT films were taken before and after inducing polarization in the films by applying a direct current voltage between the bottom electrode and the SFM tip. Polarization induced patterns were written with 20 V pulses and subsequently imaged by the SFM piezoresponse technique. The effect of the film structure on the imaging resolution of domains is discussed.