Perovskite Tandem Solar Cells

The meteoric rise of perovskite single‐junction solar cells has been accompanied by similar stunning developments in perovskite tandem solar cells. Debuting with efficiencies less than 14% in 2014, silicon–perovskite solar cells are now above 25% and will soon surpass record silicon single‐junction efficiencies. Unconstrained by the Shockley–Quiesser single‐junction limit, perovskite tandems suggest a real possibility of true third‐generation thin‐film photovoltaics; monolithic all‐perovskite tandems have reached 18% efficiency and will likely pass perovskite single‐junction efficiencies within the next 5 years. Inorganic–organic metal–halide perovskites are ideal candidates for inclusion in tandem solar cells due to their high radiative recombination efficiencies, excellent absorption, long‐range charge‐transport, and broad ability to tune the bandgap. In this progress report, the development of perovskite tandem cells is reviewed, with presentation of their key motivations and challenges. In detail, it presents an overview of recombination layer materials, bandgap‐tuneability, transparent contact architectures, and perovskite compounds for use in tandems. Theoretical estimates of efficiency for future tandem and triple‐junction perovskite cells are presented, outlining roadmaps for future focused research.     

Interfaces in Perovskite Solar Cells

Rapid improvement in photoconversion efficiency (PCE) of solution processable organometallic hybrid halide based perovskite solar cells (PSCs) have taken the photovoltaic (PV) community with a surprise and has extended their application in other electronic devices such as light emitting diodes, photo detectors and batteries. Together with efforts to push the PCE of PSCs to record values >22% – now at par with that of crystalline silicon solar cells – origin of their PV action and underlying physical processes are also deeply investigated worldwide in diverse device configurations. A typical PSC consists of a perovskite film sandwiched between an electron and a hole selective contact thereby creating ESC/perovskite and perovskite/HSC interfaces, respectively. The selective contacts and their interfaces determine properties of perovskite layer and also control the performance, origin of PV action, open circuit voltage, device stability, and hysteresis in PSCs. Herein, we define ideal charge selective contacts, and provide an overview on how the choice of interfacing materials impacts charge accumulation, transport, transfer/recombination, band‐alignment, and electrical stability in PSCs. We then discuss device related considerations such as morphology of the selective contacts (planar or mesoporous), energetics and electrical properties (insulating and conducting), and its chemical properties (organic vs inorganic). Finally, the outlook highlights key challenges and future directions for a commercially viable perovskite based PV technology.     

Chemical Approaches for Stabilizing Perovskite Solar Cells

Chemical bonding dictates not only the optoelectronic properties of materials, but also the intrinsic and extrinsic stability of materials. Here, the causes of intrinsic and extrinsic instability of perovskite materials are reviewed considering their correlation with the unique chemical‐bonding nature of perovskite materials. There are a number of key standardized stability tests established by the International Electrotechnical Commission for commercialized photovoltaic modules. Based on these procedures, the possible causes and related mechanisms of the material degradation that can arise during the test procedures are identified, which are discussed in terms of their chemical bonds. Based on the understanding of the critical causes, promising strategies for mitigating the causes to enhance the stability of perovskite solar cells are summarized. The stability of the state‐of‐the‐art perovskite solar cells implies a need for the development of improved stability‐testing protocols to move onto the next stage toward commercialization.     

Molecular Interlayers in Hybrid Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells (PSC) are promising third‐generation solar cells. They exhibit good power conversion efficiencies and in principle they can be fabricated with lower energy consumption than many more established technologies. To improve the efficiency and long‐term stability of PSC, organic molecules are frequently used as “interlayers.” Interlayers are thin layers or monolayers of organic molecules that modify a specific interface in the solar cell. Here, the latest progress in the use of interlayers to optimize the performance of PSC is reviewed. Where appropriate interesting examples from the field of organic photovoltaics (OPV) are also presented as there are many similarities in the types of interlayers that are used in PSC and OPV. The review is organized into three parts. The first part focuses on why organic molecule interlayers improve the performance of the solar cells. The second section discusses commonly used molecular interlayers. In the last part, different approaches to make thin and uniform interlayers are discussed.     

Efficient Perovskite Solar Cells Fabricated by Co Partially Substituted Hybrid Perovskite

In the past years, hybrid perovskite materials have attracted great attention due to their superior optoelectronic properties. In this study, the authors report the utilization of cobalt (Co²⁺) to partially substitute lead (Pb²⁺) for developing novel hybrid perovskite materials, CH₃NH₃Pb_1‐xCo_xI₃ (where x is nominal ratio, x = 0, 0.1, 0.2 and 0.4). It is found that the novel perovskite thin films possess a cubic crystal structure with superior thin film morphology and larger grain size, which is significantly different from pristine thin film, which possesses the tetragonal crystal structure, with smaller grain size. Moreover, it is found that the 3d orbital of Co²⁺ ensures higher electron mobilities and electrical conductivities of the CH₃NH₃Pb_1‐xCo_xI₃ thin films than those of pristine CH₃NH₃Pb₄ thin film. As a result, a power conversion efficiency of 21.43% is observed from perovskite solar cells fabricated by the CH₃NH₃Pb_0.9Co_0.1I₃ thin film. Thus, the utilization of Co, partially substituting for Pb to tune physical properties of hybrid perovskite materials provides a facile way to boost device performance of perovskite solar cells.     

Upscaling of Perovskite Solar Cells: Fully Ambient Roll Processing of Flexible Perovskite Solar Cells with Printed Back Electrodes

A scaling effort on perovskite solar cells is presented where the device manufacture is progressed onto flexible substrates using scalable techniques such as slot‐die roll coating under ambient conditions. The printing of the back electrode using both carbon and silver is essential to the scaling effort. Both normal and inverted device geometries are explored and it is found that the formation of the correct morphology for the perovskite layer depends heavily on the surface upon which it is coated and this has significant implications for manufacture. The time it takes to form the desired layer morphology falls in the range of 5–45 min depending on the perovskite precursor, where the former timescale is compatible with mass production and the latter is best suited for laboratory work. A significant loss in solar cell performance of around 50% is found when progressing to using a fully scalable fabrication process, which is comparable to what is observed for other printable solar cell technologies such as polymer solar cells. The power conversion efficiency (PCE) for devices processed using spin coating on indium tin oxide (ITO)‐glass with evaporated back electrode yields a PCE of 9.4%. The same device type and active area realized using slot‐die coating on flexible ITO‐polyethyleneterphthalate (PET) with a printed back electrode gives a PCE of 4.9%.     

Reverse Bias Behavior of Halide Perovskite Solar Cells

The future commercialization of halide perovskite solar cells relies on improving their stability. There are several studies focused on understanding degradation under operating conditions in light, but little is known about the stability of these solar cells under reverse bias conditions. Reverse bias stability is important because shaded cells in a module are put into reverse bias by the illuminated cells. In this paper, a phenomenological study is presented of the reverse bias behavior of halide perovskite solar cells and it is shown that reverse bias can lead to a partially recoverable loss in efficiency, primarily caused by a decrease in V _OC. A general mechanism is proposed, supported by drift–diffusion simulations, to explain how these cells breakdown via tunneling caused by accumulated ionic defects and suggests that the reversible loss in efficiency may be due to an electrochemical reaction of these defects. Finally, the implications of these phenomena are discussed and how they can possibly be addressed is also discussed.     

Spatially Resolved Performance Analysis for Perovskite Solar Cells

This review explores the current state of the art in spatially resolved characterization of mixed‐halide perovskite solar cells. As the size of perovskite cells and modules continues to grow, quantification of the spatial distribution of key cell parameters will become increasingly valuable in predicting ultimate cell‐level performance and tracking process homogeneity. Here, both high resolution microscopic approaches using scanning techniques and camera‐based methods for full‐area cell and/or module analysis are discussed. The value of this local data in predicting performance losses at the cell level is particularly emphasized. Measurable physical parameters sensitive to losses of voltage, current, fill factor, and efficiency are discussed together with selected experimental results. It is demonstrated that a combination of spatially resolved cell parameter mapping/imaging can be used to quantitatively discriminate various loss contributions at high resolution. The impact and control of inhomogeneities become particularly important when upscaling from small devices to large formats compatible with industrial mass production.     

A Review on Additives for Halide Perovskite Solar Cells

Additives are widely adopted for efficient, stable, and hysteresis‐free perovskite solar cells and play an important role in various breakthroughs of perovskite solar cells (PSCs). Herein the various additives adopted for PSCs are reviewed and their functioning mechanism and influence on device performance is described. The main roles of additives, modulating morphology of perovskite films, stabilizing phase of formamidinium (FA) and cesium (Cs)‐based perovskites, adjusting energy level alignment in PSCs, suppressing nonradiative recombination in perovskites, eliminating hysteresis, enhancing operational stability of PSCs, are summarized.     

Emerging 2D Layered Materials for Perovskite Solar Cells

Perovskite solar cells (PSCs) are now at the forefront of the state‐of‐the‐art photovoltaic technologies due to their high efficiency and low fabrication costs. To further realize the potential of this fascinating class of solar cells, nanostructured functional materials have been playing important roles. 2D layered materials have attracted a great deal of interest due to their fascinating properties and unique structure. Recently, the exploration of a wide range of novel 2D materials for use in PSCs has seen considerable progress, but still a lot remains to be done in this field. In this progress report, the advancements that have recently been made in the application of these emerging 2D materials, beyond graphene, for PSCs are presented. Both the advantages and challenges of these 2D materials for PSCs are highlighted. Finally, important directions for the future advancements toward efficient, low‐cost, and stable PSCs are outlined.     

Fundamental Understanding of Photocurrent Hysteresis in Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells (PSCs) have become a promising candidate in the photovoltaic field due to their high power conversion efficiency and low material cost. However, the development of PSCs is limited by their poor stability under practical conditions in the presence of oxygen, moisture, sunlight, heat, and the current–voltage (I–V) hysteresis. In particular, the hysteretic I–V issue casts doubt on the validity of the photovoltaic performance results that are achieved, making it difficult to evaluate the authentic performance of PSCs. This review article focuses on understanding the I–V hysteresis behavior in PSCs and on exploring the possible reasons leading to this hysteresis phenomenon. The various strategies attempted to suppress the I–V hysteresis in PSCs are summarized, and a brief future recommendation is provided.     

Imaging and Mapping Characterization Tools for Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted much attention as efficiencies have gone beyond 24%. To achieve these impressive numbers, the PSC scientific community is working to improve the perovskite optoelectronic properties. Imaging and mapping characterization techniques have been widely used to understand the fundamental properties that allow lead halide perovskites to achieve high performance. In this review, these techniques are evaluated, from simple tools, such as electron microscopy, to more complex systems that include atomic force microscopy, synchrotron‐based X‐ray mapping, and ultrafast and photoluminescence mapping. These tools have helped understand lead halide perovskites and their impressive optoelectronic properties, which make them outstanding materials for solar cell applications.     

The Role of Grain Boundaries in Perovskite Solar Cells

Grain boundaries (GBs) play an important role in most polycrystalline solar cells. In perovskite solar cells, the research community is just starting to understand their effects on performance and long‐term durability. In this essay, three important questions are explored: Do GBs affect: 1) recombination and thus open‐circuit voltage? Not dramatically, if at all; 2) current–voltage hysteresis? Most studies show that hysteresis is dominated by defects at GBs; and 3) long‐term durability? Yes, GBs definitely help increase the rate of perovskite degradation. In this essay, the latest reports are summarized and the authors' perspective on this very important subject is given.     

Additive Engineering for Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) have reached a certified 25.2% efficiency in 2019 due to their high absorption coefficient, high carrier mobility, long diffusion length, and tunable direct bandgap. However, due to the nature of solution processing and rapid crystal growth of perovskite thin films, a variety of defects can form as a result of the precursor compositions and processing conditions. The use of additives can affect perovskite crystallization and film formation, defect passivation in the bulk and/or at the surface, as well as influence the interface tuning of structure and energetics. Here, recent progress in additive engineering during perovskite film formation is discussed according to the following common categories: Lewis acid (e.g., metal cations, fullerene derivatives), Lewis base based on the donor type (e.g., O‐donor, S‐donor, and N‐donor), ammonium salts, low‐dimensional perovskites, and ionic liquid. Various additive‐assisted strategies for interface optimization are then summarized; additives include modifiers to improve electron‐ and hole‐transport layers as well as those to modify perovskite surface properties. Finally, an outlook is provided on research trends with respect to additive engineering in PSC development.