Turn defects into strengths

The power conversion efficiency of solar cells sensitized with colloidal quantum dots is believed to be limited by surface defects. Research now finds that photocarriers trapped at shallow defect states can actually be recovered and ultimately contribute to device efficiency

How Do PerovskiteSolar Cells Work?

Since the first publication of all-solid perovskite solar cells (PSCs) in 2012, this technology has become probably the hottest topic in photovoltaics. Proof of this is the number of published papers and the citations that they are receiving—greater than 3,200 and 110,000, respectively— in just the last year (2017). However, despite this intensive effort, the working principles of these kind of devices are not yet fully understood. The manuscript of Ravishankar et al. will contribute significantly to this debate, as the authors have shown that the work function of the electron selecting layer plays a minor role on the final open circuit voltage, Voc

Interaction between Colloidal Quantum Dots and Halide Perovskites: Looking for Constructive Synergies

Colloidal quantum dots (QDs) have received extensive attention during the last few decades because of their amazing properties emerging from quantum confinement. In parallel, halide perovskites have attracted attention because of the demonstration of very high performance, especially in solar cells, light-emitting diodes (LEDs), and other optoelectronic devices. Both families of materials can be prepared in a relatively simple way, facilitating their integration. There are several examples of their interaction enhancing the properties of the final nanocomposite. Perovskites can effectively passivate QDs or act as efficient charge transporters. QDs can be used to modify the selective contacts in perovskite devices or can be used as efficient light emitters or absorbers for enhanced LEDs and photodetectors, respectively. Moreover, QDs can seed the perovskite crystal growth, improving the morphology and ultimately the solar cell performance. In addition, new advanced devices can emerge as a result of the constructive synergy between both families of materials

High-throughput analysis of the ideality factor to evaluate the outdoor performance of perovskite solar minimodules

Halide perovskite solar cells exhibit a unique combination of properties, including ion migration, low non-radiative recombination and low performance dependence on temperature. Because of these idiosyncrasies, it is debatable whether standard procedures for assessing photovoltaic technologies are sufficient to appropriately evaluate this technology. Here, we show a low dependence of the open-circuit voltage on the temperature of a MAPbI3 minimodule that allows a high-throughput outdoor analysis based on the ideality factor (nID). Accordingly, three representative power loss tendencies obtained from I–V curves measured with standard procedures are compared with their corresponding nID patterns under outdoor conditions. Therefore, based on the linear relationship between T80 and the time to reach nID = 2 (TnID2), we demonstrate that nID analysis could offer important complementary information with important implications for outdoor development of this technology, providing physical insights into the recombination mechanism dominating performance, thus improving the understanding of degradation processes and device characterization

Welcoming the First Decade of Perovskite Solar Cells

The swift emergence of perovskite solar cells (PSCs) is a “miracle” development in the history of photovoltaics. Since Miyasaka and co‐workers (Toin University of Yokohama, Japan) reported the first use of halide perovskites (HPs) in solar cells in 2009, the past ten years have witnessed a skyrocketing increase in power conversion efficiency (PCE) to 24.2% for single‐junction PSCs and 28.0% for Si‐perovskite tandem solar cells

Optical characterization of lead-free Cs2SnI6 double Perovskite Fabricated from degraded and reconstructed CsSnI3 films

Halide perovskites have experienced a huge development in the past years, but they still have two major challenges for their massive implantation: the long-term stability and the use of lead. One of the most obvious lead-free candidates to replace these perovskites is CsSnI3, but due to its poor environmental stability, it has been discarded for the fabrication of stable devices. Nevertheless, ambient degradation of CsSnI3 and ulterior reconstruction produce a relatively stable lead-free Cs2SnI6 double perovskite with interesting optical properties that have not been deeply characterized previously. In this work, the potential use for the optical properties of Cs2SnI6 is studied and compared with that of the most common halide perovskite, CH3NH3PbI3 (MAPbI3). The Cs2SnI6 films stayed in a standard atmosphere for a week without showing any signs of degradation. They also demonstrated better reflective behavior than MAPbI3 and higher absorption in the 650 and 730 nm spectral range, making this material interesting for the development of photodetectors in this region. This study demonstrates that Cs2SnI6 is a promising material for photodevices, as it highlights its main characteristics and optical parameters, giving an original view on the use of the double perovskite, but at the same time emphasizing the need to improve the electrical properties for the development of efficient optoelectronic devices.E.L.-F. wants to express his gratitude to the Ministerio de Educación y Formación Profesional for his doctoral grant (FPU research fellowship FPU17/00612) and his research stay grant (EST18/00399). This work was partially supported by the European Research Council (ERC) via Consolidator Grant (724424-No-LIMIT) and the European Commission via FET Open Grant (862656 - DROP-IT). We acknowledge SCIC from Jaume I University (UJI) for help with XRD and SEM-EDS characterization

Diffusion-Recombination Impedance Model for Solar Cells with Disorder and Nonlinear Recombination

The diffusion–recombination model is a key tool in understanding the photovoltaic operation of solar cells. Dye-sensitized solar cells, organic solar cells, and inorganic semiconductor solar cells are systems affected by disorder that are often characterized with impedance spectroscopy. In this paper, we extend the previous theory of diffusion–recombination impedance including traps and nonlinear recombination. We show the transmission line equivalent circuit representation, and we describe the physical meaning of a number of model parameters that can be obtained: the chemical capacitance, ; the recombination resistance, ; the transport resistance, ; the electron lifetime, ; the electron conductivity, ; the chemical diffusion coefficient of electrons, ; and the diffusion length, . At most, three of these parameters are independent, but if the diffusion length is short, the impedance model collapses to a function that has one degree of freedom less, known as the Gerischer impedance. We show the connection of the two parameters that remain to the diffusion length and the lifetime

Evaluation of multiple cation/anion perovskite solar cells through life cycle assessment

After the great initiation of perovskite as a photovoltaic material, laboratory efficiencies similar to other photovoltaic technologies already commercialised have been reached. Consequently, recent research interests on perovskite solar cells try to address the stability improvement as well as make its industrialisation possible. Record efficiencies in perovskite solar cells (PSCs) have been achieved using as active material a multiple cation/anion perovskite by combining methylammonium (MA) and formamidinium (FA), but also Cs cation and I and Br as anions, materials that also have demonstrated a superior stability. Herein, the environmental performance of the production of such perovskite films was evaluated via life cycle assessment. Our study points out that multiple cation/anion perovskite films show major detrimental environmental impacts for all categories assessed, except for abiotic depletion potential, when they are compared with a canonical perovskite MAPbI3. In addition, a closer analysis of the materials utilised for the synthesis of the different multiple cation perovskites compositions revealed that lead halide reagents and chlorobenzene were the most adverse compounds in terms of impact. However, the former is used in all the perovskite compositions and the later can be avoided by the use of alternative fabrication methods to anti-solvent. To this extent, FAI, with the current synthesis procedures, is the most determining compound as it increases significantly the impacts and the cost in comparison with MAI. A further economic analysis, exposed that multiple cation perovskites need a significantly higher photoconversion efficiency to produce the same payback times than canonical perovskite

Progress in halide-perovskite nanocrystals with near-unity photoluminescence quantum yield

Colloidal halide perovskite nanocrystals (PNCs) are an outstanding case study due to their remarkable optical features, such as a high photoluminescence (PL) quantum yield (PLQY), tunable band gap, and narrow emission. Despite the impressive first reports of PLQYs beyond 70%, it has been observed that PLQY is limited by structural defects arising from labile interactions between the organic capping ligand and the inorganic core. Structural defects acting as trap states are key factors limiting both PNC PLQY and stability. In this review, we present the most studied, common, and alternative protocols to fully compensate for surface defects (e.g., halide vacancies, loss of protective capping ligands) as well as how to increase their stability and PLQY to unity (i.e., 100% when PLQY is expressed as a percentage)

Stabilization of Black Perovskite Phase in FAPbI3 and CsPbI3

Although halide perovskites allow a great versatility, the application on single-absorber solar cells restricts significantly the number of available materials. In this context, CsPbI3 and FAPbI3 (FA, formamidinium) present a huge potential for the inorganic approach with enhanced stability and narrow bandgap, respectively. However, for these materials, Cs+ and FA+ are relatively too small and too big to stabilize the perovskite black phase at room temperature, both presenting a nonphotoactive yellow phase as the most stable phase. This fact limits dramatically their application and also helps in the understanding of the main research lines in the halide perovskite photovoltaic field in the quest for the stabilization of FAPbI3. In this Perspective, we present an overview of different strategies for the stabilization of the perovskite black phase of these two materials. We evaluate the stability approaches envisioning efficient and stable materials, with a particular focus on the positive and limiting aspects of the exploitation of low dimensionality and chemi-structural mechanisms