Anita Ho-Baillie

Benefit of Grain Boundaries in Organic–Inorganic Halide Planar Perovskite Solar Cells

The past 2 years have seen the uniquely rapid emergence of a new class of solar cell based on mixed organic-inorganic halide perovskite. Grain boundaries are present in polycrystalline thin film solar cell, and they play an important role that could be benign or detrimental to solar-cell performance. Here we present efficient charge separation and collection at the grain boundaries measured by KPFM and c-AFM in CH3NH3PbI3 film in a CH3NH3PbI3/TiO2/FTO/glass heterojunction structure. We observe the presence of a potential barrier along the grain boundaries under dark conditions and higher photovoltage along the grain boundaries compare to grain interior under the illumination. Also, c-AFM measurement presents higher short-circuit current collection near grain boundaries, confirming the beneficial roles grain boundaries play in collecting carriers efficiently.

Optical Properties of Photovoltaic Organic-Inorganic Lead Halide Perovskites.

Over the last several years, organic-inorganic lead halide perovskites have rapidly emerged as a new photovoltaic contender. Although energy conversion efficiency above 20% has now been certified, improved understanding of the material properties contributing to these high performance levels may allow the progression to even higher efficiency, stable cells. The optical properties of these new materials are important not only to device design but also because of the insight they provide into less directly accessible properties, including energy-band structures, binding energies, and likely impact of excitons, as well as into absorption and inverse radiative recombination processes.

Defect trapping states and charge carrier recombination in organic–inorganic halide perovskites

Organic–inorganic perovskite solar cells have attracted huge research interest due to rapid improvement in device performance showing great potential to be the next generation flexible solar cells. Unique defect properties in perovskite have been considered as the possible mechanism for the superior performance, and closely relevant to the effects of hysteresis and light soaking. To date, the quantitative correlation and in-depth understanding of defects in organic–inorganic perovskite are still lacking although extensive investigation have been undertaken. Here we study defect trapping states and carrier recombination dynamics in organic–inorganic halide perovskites. At low excitation the photoluminescence (PL) intensity exhibits a super-linear increase with increasing excitation, due to the slow depopulation rate of the defect states. The steady state and time-resolved photoluminescence (PL) carried out in this work reveal that the carrier recombination dynamics is ultimately correlated with both the defect density and the relaxation rate of the carriers in defects. A model is established for the relationship between the properties of the defect trapping state and steady state PL intensity. Two key parameters, (i) the ratio of the trap-state density to the depopulation rate of trapped states and (ii) ratio of the maximum density of covalence band electrons to the trapping rate, can be extracted from the model based on the excitation dependent steady state PL. This work demonstrates that the properties of defect trapping states are closely related to the fabrication technique, and suggests that the organic–inorganic halide perovskite is partly defect-tolerant.

Morphology and Carrier Extraction Study of Organic–Inorganic Metal Halide Perovskite by One- and Two-Photon Fluorescence Microscopy

The past two years have seen the uniquely rapid emergence of a new class of solar-cell-based on mixed organic-inorganic halide perovskite. In this work, we demonstrate a promising technique for studying the morphology of perovskite and its impact on carrier extraction by carrier transport layer using one-photon and two-photon fluorescence imaging in conjunction with time-resolved photoluminescence. This technique is not only effective in separating surface and bulk effects but it also allows the determination of lifetimes in localized regions and local carrier extraction efficiency. The difference in sensitivities of transport materials to grain boundaries and film uniformity is highlighted in this study. It is shown that the PCBM fabricated in this work is more sensitive to film nonuniformity, whereas spiro-OMeTAD is more sensitive to grain boundaries in terms of effective carrier extraction.

Input Parameters for the Simulation of Silicon Solar Cells in 2014

Within the silicon photovoltaics (PV) community, there are many approaches, tools, and input parameters for simulating solar cells, making it difficult for newcomers to establish a complete and representative starting point and imposing high requirements on experts to tediously state all assumptions and inputs for replication. In this review, we address these problems by providing complete and representative input parameter sets to simulate six major types of crystalline silicon solar cells. Where possible, the inputs are justified and up-to-date for the respective cell types, and they produce representative measurable cell characteristics. Details of the modeling approaches that can replicate the simulations are presented as well. The input parameters listed here provide a sensible and consistent reference point for researchers on which to base their refinements and extensions.

Polaronic exciton binding energy in iodide and bromide organic-inorganic lead halide perovskites

The last 4 years have seen the rapid emergence of a new solar cell technology based on organic-inorganic lead halide perovskites, primarily CH3NH3PbI3 and related halides involving Cl and Br. Debate continues on the role of excitons and free carriers in these materials. Recent studies report values of exciton binding energy for the iodide ranging from 0.7 meV to 200 meV, with vastly different implications for device operation and design. In the present work, previously neglected polarons are shown likely to have a major impact in determining excitonic properties. Polaronic exciton binding energies calculated using effective longitudinal optical phonon energies, deduced from permittivity measurements, are shown consistent with experimental energies for good quality samples of CH3NH3PbI3 and CH3NH3PbBr3, as determined over a large temperature range from optical absorption data. Bandgaps determined simultaneously show a discontinuity at the orthorhombic to tetragonal phase transition for the iodide, but not ...

Optical analysis of perovskite/silicon tandem solar cells

Semi-transparent perovskite solar cells have significant potential as the top cell in a tandem structure with silicon (Si) or copper indium gallium selenide (CIGS) as the bottom cell. The efficiency of such tandem cells depends on careful light management and optical design of the device. Here, using the transfer matrix method, we report a detailed optical analysis of the absorption distribution, parasitic absorption and reflection losses in various semi-transparent perovskite solar cell structures and their impact on tandem cell efficiencies. The optical properties of some key layers of a semi perovskite cell are measured and modelled to facilitate the optical analysis. It is found that the encapsulation glass and the absorption in the transparent conductive electrode have a huge impact on the tandem efficiency. The tandem efficiency limit is also calculated to present the design of a two-terminal current-matched tandem device required to achieve 30% conversion efficiency. This work provides directions for further improving the power conversion efficiency of perovskite/Si tandem cells.

Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites

The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA+/MA+/Cs+, X=I−/Br−) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials.

Four-Terminal Tandem Solar Cells Using CH3NH3PbBr3 by Spectrum Splitting.

In this work, the use of a high bandgap perovskite solar cell in a spectrum splitting system is demonstrated. A remarkable energy conversion efficiency of 23.4% is achieved when a CH3NH3PbBr3 solar cell is coupled with a 22.7% efficient silicon passivated emitter rear locally diffused solar cell. Relative enhancements of >10% are demonstrated by CH3NH3PbBr3/CH3NH3PbI3 and CH3NH3PbBr3/multicrystalline-screen-printed-Si spectral splitting systems with tandem efficiencies of 13.4% and 18.8%, respectively. The former is the first demonstration of an all perovskite split spectrum system. The CH3NH3PbBr3 cell on a mesoporous structure was fabricated by the vapor-assisted method while the planar CH3NH3PbI3 cell was fabricated by the gas-assisted method. This work demonstrates the advantage of the higher voltage output from the high bandgap CH3NH3PbBr3 cell and its suitability in a tandem system.

Ultimate efficiency limit of single-junction perovskite and dual-junction perovskite/silicon two-terminal devices

Theoretical calculation based on detailed balance and incorporating different realistic optical and electrical losses predicts conversion efficiency beyond 22% for single-junction perovskite devices. In dual-junction perovskite/silicon devices, theoretical conversion efficiency around 40% is been determined. However, dramatic drop in the conversion efficiency is shown to be due to the glass reflection and FTO parasitic absorption losses. Additionally, practical conversion efficiency limits of dual-junction two-terminal perovskite/silicon tandem solar cell of 30% are achievable as reported in this work using state-of-the-art demonstrated devices. Additionally, various crystalline silicon (industry and laboratory demonstrated) technologies are used as the bottom cell for the current matched tandem cell stacks with higher relative improvements when using commercial c-Si solar cells. Moreover, the effect of eliminating the parasitic resistances and enhancing the external radiative efficiency (ERE) in the perovskite junction on tandem performance are also investigated enhancing the stack efficiencies.