Hybrid organo-metal halide perovskites are an exciting new class of solar absorber materials and have exhibited a rapid increase in solar cell efficiencies throughout the past two years to over 17% in both meso-structured and thin-film device architectures. We observe slow transient effects causing hysteresis in the current–voltage characterization of these devices that can lead to an over- or underestimation of the solar cell device efficiency. We find that the current–voltage (IV) measurement scan direction, measurement delay time, and light and voltage bias conditions prior to measurement can all have a significant impact upon the shape of the measured IV light curves and the apparent device efficiency. We observe that hysteresis-free light IV curves can be obtained at both extremely fast and slow voltage scan rates but only in the latter case are quasi-steady-state conditions achieved for a valid power conversion efficiency measurement. Hysteretic effects are also observed in devices utilizing alternative selective contacts but differ in magnitude and time scale, suggesting that the contact interfaces have a big effect on transients in perovskite-absorber devices. The transient processes giving rise to hysteresis are consistent with a polarization response of the perovskite absorber that results in changes in the photocurrent extraction efficiency of the device. The strong dependence of the hysteresis on light and voltage biasing conditions in thin film devices for a period of time prior to the measurement suggests that photo-induced ion migration may additionally play an important role in device hysteresis. Based on these observations, we provide recommendations for correct measurement and reporting of IV curves for perovskite solar cell devices.

https://web.stanford.edu/group/mcgehee/publications/EES2014b.pdf

Hysteresis and transient behavior in current–voltage measurements of hybrid-perovskite absorber solar cells

Thin-film electronics in its myriad forms has underpinned much of the technological innovation in the fields of displays, sensors, and energy conversion over the past four decades. This technology also forms the basis of flexible electronics. Here we review the current status of flexible electronics and attempt to predict the future promise of these pervading technologies in healthcare, environmental monitoring, displays and human-machine interactivity, energy conversion, management and storage, and communication and wireless networks.

Flexible Electronics: The Next Ubiquitous Platform

This paper describes the use, within p–i–n- and n–i–p-type solar cells, of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) thin films (layers), both deposited at low temperatures (200°C) by plasma-assisted chemical vapour deposition (PECVD), from a mixture of silane and hydrogen. Optical and electrical properties of the i-layers are described. These properties are linked to the microstructure and hence to the i-layer deposition rate, that in turn, affects throughput in production. The importance of contact and reflection layers in achieving low electrical and optical losses is explained, particularly for the superstrate case. Especially the required properties for the transparent conductive oxide (TCO) need to be well balanced in order to provide, at the same time, for high electrical conductivity (preferably by high electron mobility), low optical absorption and surface texture (for low optical losses and pronounced light trapping). Single-junction amorphous and microcrystalline p–i–n-type solar cells, as fabricated so far, are compared in their key parameters (Jsc, FF, Voc) with the [theoretical] limiting values. Tandem and multijunction cells are introduced; the μc-Si: H/a-Si: H or [micromorph] tandem solar cell concept is explained in detail, and recent results obtained here are listed and commented. Factors governing the mass-production of thin-film silicon modules are determined both by inherent technical reasons, described in detail, and by economic considerations. The cumulative effect of these factors results in distinct efficiency reductions from values of record laboratory cells to statistical averages of production modules. Finally, applications of thin-film silicon PV modules, especially in building-integrated PV (BIPV) are shown. In this context, the energy yields of thin-film silicon modules emerge as a valuable gauge for module performance, and compare very favourably with those of other PV technologies. Copyright © 2004 John Wiley & Sons, Ltd.

Thin‐film silicon solar cell technology

Organo-lead halide perovskite solar cells have gained enormous significance and have now achieved power conversion efficiencies of ∼20% However, the potential toxicity of lead in these systems raises environmental concerns for widespread deployment Here we investigate solvent effects on the crystallization of the lead-free methylammonium tin triiodide (CH3NH3SnI3) perovskite films in a solution growth process Highly uniform, pinhole-free perovskite films are obtained from a dimethyl sulfoxide (DMSO) solution via a transitional SnI2·3DMSO intermediate phase This high-quality perovskite film enables the realization of heterojunction depleted solar cells based on mesoporous TiO2 layer but in the absence of any hole-transporting material with an unprecedented photocurrent up to 21 mA cm–2 Charge extraction and transient photovoltage decay measurements reveal high carrier densities in the CH3NH3SnI3 perovskite device which are one order of magnitude larger than CH3NH3PbI3-based devices but with comparable

Solvent-Mediated Crystallization of CH3NH3SnI3 Films for Heterojunction Depleted Perovskite Solar Cells

Physics of Amorphous Silicon–Carbon Alloys

Long exposure to light decreases the photoconductivity and dark conductivity of some samples of hydrogenated amorphous silicon (a‐Si : H). Annealing above ∼150 °C reverses the process. The effect occurs in the bulk of the films, and is associated with changes in density or occupation of deep gap states. High concentrations of P, B, or As quench the effect. Possible models involving hydrogen bond reorientation at a localized defect or electron‐charge transfer between defects are discussed. An example is shown where these conductivity changes do not affect the efficiency of an a‐Si : H solar cell.

Optically induced conductivity changes in discharge‐produced hydrogenated amorphous silicon

Unencapsulated, amorphous silicon indium tin oxide/n‐i‐p/stainless‐steel solar cells were tested for stability. All cells have excellent shelf life. Changes occur during exposure to light, but can be controlled by the deposition conditions of the amorphous silicon. The changes are due to trapping and recombination of optically generated carriers in the i layer, and are reversibly annealed out above 175 °C. Preliminary life tests on two relatively stable cells showed a small initial drop to 5%, followed by a weak logarithmic decay that predicts only ∼20% further decrease in efficiency after 20 years in sunlight. Work is continuing on improving the efficiency and stability of these cells.

Stability of n‐i‐p amorphous silicon solar cells

Amorphous silicon films prepared by dc glow discharge of silane on room‐temperature substrates darken when exposed to a focused laser beam. The process is primarily thermal, has an activation energy of ∼1 eV, and is accompanied by the desorption of hydrogen. It also affects the etch rate in basic solutions. A bleaching effect, not reported before, is seen at low exposures. All coloration changes are permanent. Possible applications for optical storage of information are discussed.

D.L. Staebler

Papers

Optically induced conductivity changes in discharge‐produced hydrogenated amorphous silicon

Stability of n‐i‐p amorphous silicon solar cells

Laser‐beam annealing of discharge‐produced amorphous silicon

Properties of amorphus silicon and a-si solar cells.

Properties of the defect causing solar cell degradation