Metal halide perovskites are promising candidates for many classes of different optoelectronic devices. Apart from being a semiconductor, they additionally show ionic conductivity. It expresses ...itself in slow response times, reversible degradation, and hysteresis in the current–voltage characteristics of solar cells. This Perspective gives a condensed overview about experiments and theory on ion migration in metal halide perovskites focusing on its effects in solar cells. Apart from being a potential stability concern for photovoltaics, ion migration paired with the excellent optoelectronic properties of this material offers opportunities for novel devices such as optically controlled memristors and switchable diodes.
All‐inorganic perovskites are considered to be one of the most appealing research hotspots in the field of perovskite photovoltaics in the past 3 years due to their superior thermal stability ...compared to their organic–inorganic hybrid counterparts. The power‐conversion efficiency has reached 17.06% and the number of important publications is ever increasing. Here, the progress of inorganic perovskites is systematically highlighted, covering materials design, preparation of high‐quality perovskite films, and avoidance of phase instabilities. Inorganic perovskites, nanocrystals, quantum dots, and lead‐free compounds are discussed and the corresponding device performances are reviewed, which have been realized on both rigid and flexible substrates. Methods for stabilization of the cubic phase of low‐bandgap inorganic perovskites are emphasized, which is a prerequisite for highly efficient and stable solar cells. In addition, energy loss mechanisms both in the bulk of the perovskite and at the interfaces of perovskite and charge selective layers are unraveled. Reported approaches to reduce these charge‐carrier recombination losses are summarized and complemented by methods proposed from our side. Finally, the potential of inorganic perovskites as stable absorbers is assessed, which opens up new perspectives toward the commercialization of inorganic perovskite solar cells.
Recent progress of inorganic perovskite materials and photovoltaic solar cells is summarized, including materials design, methods for preparing high‐quality perovskite films, phase instabilities, nanocrystals, quantum dots, lead‐free perovskites, device process, and upscaling. In addition, the energy loss mechanisms within the device are discussed and relevant methods are proposed accordingly.
Inorganic perovskite based solar cells (PSCs) have been receiving unprecedented attention worldwide in the past several years due to their higher intrinsic stability towards high temperatures and ...high theoretical power conversion efficiencies. Since a photovoltaic performance of 20.37% has been achieved for inorganic PSCs recently, the operational stability of these devices has become the major bottleneck which impedes their commercialization. The high thermal stability associated with inorganic perovskites comes along with poorer phase stability compared to their hybrid counterparts and therefore needs thorough understanding. Lattice strain and vacancies within the perovskite crystals are found to be the origin of these phase instability issues. This review summarizes the progress in stability research on inorganic perovskites. Specifically, the degradation mechanisms of inorganic perovskites towards temperature, moisture and oxygen are summarized and discussed. Solutions for tackling these stability issues are reviewed and an outlook on further strategies is provided.
The composition, light, moisture and oxygen all affect the stability of metal halide inorganic perovskites, whose degradation mechanisms are significantly different from those of hybrid perovskites.
Owing to their superior thermal stability, metal halide inorganic perovskite materials continue to attract interest for photovoltaics applications. The highest reported power conversion efficiency ...(PCE) for solar cells based on inorganic perovskites is over 20 %. As this PCE corresponds to 73 % of the theoretical limit, there remains more room for further improving the device PCEs than for improving organic–inorganic hybrid perovskite solar cells (PSCs). The main loss is in the photovoltage, which is limited by interfaces in terms of non‐radiative recombination caused by traps and energy‐level mismatch. Furthermore, inefficient charge extraction at interfacial contacts reduces the photocurrent and fill factor. This Minireview summarizes the recent developments in the fundamental understanding of how the interfaces and interfacial layers influence the performance of solar cells based on inorganic perovskite absorbers. An outlook for the development of highly efficient and stable inorganic PSCs from the interface point of view is also given.
This Minireview summarizes the recent developments on interfaces in inorganic perovskite solar cells, with special focus on the fundamental understanding of how interfaces influence the performance of devices. Directions for developing highly efficient and stable inorganic perovskite solar cells by interface engineering are also provided.
Metal halide perovskite absorber materials are about to emerge as a high-efficiency photovoltaic technology. At the same time, they are suitable for high-throughput manufacturing characterized by a ...low energy input and abundant low-cost materials. However, a further optimization of their efficiency, stability and reliability demands a more detailed optoelectronic characterization and understanding of losses including their evolution with time. In this work, we analyze perovskite solar cells with different architectures (planar, mesoporous, HTL-free), employing temperature dependent measurements (current–voltage, light intensity, electroluminescence) of the ideality factor to identify dominating recombination processes that limit the open-circuit voltage ( V oc ). We find that in thoroughly-optimized, high- V oc (≈1.2 V) devices recombination prevails through defects in the perovskite. On the other hand, irreversible degradation at elevated temperature is caused by the introduction of broad tail states originating from an external source ( e.g. metal electrode). Light-soaking is another effect decreasing performance, though reversibly. Based on FTPS measurements, this degradation is attributed to the generation of surface defects becoming a new source of non-radiative recombination. We conclude that improving long-term stability needs to focus on adjacent layers, whereas a further optimization of efficiency of top-performing devices requires understanding of the defect physics of the nanocrystalline perovskite absorber. Finally, our work provides guidelines for the design of further dedicated studies to correctly interpret the diode ideality factor and decrease recombination losses.
So-called negative capacitance seems to remain an obscure feature in the analysis of the frequency-dependent impedance of perovskite solar cells. It belongs to one of the puzzling peculiarities ...arising from the mixed ionic-electronic conductivity of this class of semiconductor. Here we show that apparently high capacitances in general (positive and negative) are not related to any capacitive feature in the sense of a corresponding charge accumulation. Instead, they are a natural consequence of slow transients mainly in forward current of the diode upon ion displacement when changing voltage. The transient current leads to a positive or negative 'capacitance' dependent on the sign of its gradient. The 'capacitance' appears so large because the associated resistance, when thinking of a resistor-capacitor element, results from another physical process, namely modified electronic charge injection and transport. Observable for a variety of devices, it is a rather universal phenomenon related to the hysteresis in the current-voltage curve.
Inorganic‐organic lead‐halide perovskite solar cells have reached efficiencies above 22% within a few years of research. Achieved photovoltages of >1.2 V are outstanding for a material with a bandgap ...of 1.6 eV – in particular considering that it is solution processed. Such values demand for low non‐radiative recombination rates and come along with high luminescence yields when the solar cell is operated as a light emitting diode. This progress report summarizes the developments on material composition and device architecture, which allowed for such high photovoltages. It critically assesses the term “lifetime”, the theories and experiments behind it, and the different recombination mechanisms present. It attempts to condense reported explanations for the extraordinary optoelectronic properties of the material. Amongst those are an outstanding defect tolerance due to antibonding valence states and the capability of bandgap tuning, which might make the dream of low‐cost highly efficient solution‐processed thin film solar cells come true. Beyond that, the presence of photon recycling will open new opportunities for photonic device design.
Perovskite solar cells show exceptionally high photovoltages. This progress report discusses the current understanding of the main material properties that are responsible for the high electronic quality of the metal‐halide perovskites. Amongst them is a pronounced defect tolerance, which facilitates low non‐radiative recombination rates and high luminescence yields.
The effect of injection and extraction barriers on flat heterojunction (FHJ) and bulk heterojunction (BHJ) organic solar cells is analyzed. The barriers are realized by a combination of p‐type ...materials with HOMOs varying between –5.0 and –5.6 eV as hole‐transport layer (HTL) and as donor in vacuum‐evaporated multilayer p‐i‐metal small‐molecule solar cells. The HTL/donor interface can be seen as a model for the influence of contacts in organic solar cells in general. Using drift‐diffusion simulations we are well able to reproduce and explain the experimental I–V curves qualitatively. In FHJ solar cells the open‐circuit voltage (Voc) is determined by the donor and is independent of the HTL. In BHJ solar cells, however, Voc decreases if injection barriers are present. This different behavior is caused by a blocking of the charge carriers at a spatially localized donor/acceptor heterojunction, which is only present in the FHJ solar cells. The forward current is dominated by the choice of HTL. An energy mismatch in the HOMOs leads to kinks in the I–V curves in the cases for which Voc is independent of the HTL.
The influence of energy barriers (ΔE) for hole transport on the J–V curve is analyzed in experiments and simulations. The investigated system consists of different combinations of organic hole‐transport layers (HTLs) and donors. Depending on the kind of barrier and the configuration of the donor/acceptor heterojunction changes in the open‐circuit voltage (Voc) and the formation of S‐kinks are observed and explained.
Moving a perovskite into the blackThe bandgap of the black α-phase FAPbI3 (where FA is formamidinium) is nearly ideal for solar cells, but it is unstable with respect to the photoinactive yellow ...δ-phase. Lu et al. found that a film of the yellow phase was converted to a highly crystalline black phase by vapor exposure to methylammonium thiocyanate at 100°C, and it retained this structure after 500 hours at 85°C. Solar cells fabricated with this material had a power conversion efficiency of more than 23%. After 500 hours under maximum power tracking and a period of dark recovery, 94% of the original efficiency was retained.Science, this issue p. eabb8985INTRODUCTIONMetal halide perovskite solar cells (PSCs) have reached a power-conversion efficiency (PCE) of 25.2%, thus exceeding other thin-film solar cells. FAPbI3 (where FA is formamidinium) has been shown to be an ideal candidate for efficient, stable PSCs. Obtaining highly crystalline, stable, and pure α-phase FAPbI3 films has been of vital importance. However, FAPbI3 undergoes a phase transition from the black α-phase to the photoinactive δ-phase below 150°C. Previous approaches to overcoming this problem include mixing it with MA, Cs or Br ions. Here, we report a deposition method using methylammonium thiocyanate (MASCN) vapor treatment to convert δ-FAPbI3 to the desired pure α-phase below the thermodynamic phase-transition temperature. Molecular dynamics (MD) simulations show that the SCN– anions promote the formation and stabilization of α-FAPbI3. These vapor-treated FAPbI3 PSCs exhibit outstanding photovoltaic and electroluminescent performance.RATIONALEAlthough the phase transition from δ- to α-phase FAPbI3 requires a high temperature, the treatment of δ-phase FAPbI3 films with MASCN vapor allows the conversion to occur at temperatures below 150°C. MD simulations show that SCN– ions preferentially adsorb on the surface of δ-FAPbI3 to replace iodide ions that are bound to Pb2+. This process disintegrates the top layer of face-sharing octahedra and induces the transition to the corner-sharing architecture of α-FAPbI3. Once the corner-sharing α-form is formed on the top surface, this layer templates the progression of the phase transition from δ- to α-FAPbI3 toward the bulk. Once the pure α-FAPbI3 is formed, its back conversion to the δ-phase is prevented by a high energy barrier.RESULTSWe show a complete conversion from δ- to α-FAPbI3 at 100°C using the MASCN vapor treatment method. This phase transition can also be achieved using FASCN vapor. The vapor-treated FAPbI3 film remained in its pure black phase even after 500 hours of annealing at 85°C, whereas the reference FAPbI3 film formed mainly PbI2 during the heat exposure. X-ray diffraction data showed an improved crystallinity and preferred orientation of the FAPbI3 films after vapor treatment. One- and two-dimensional NMR experiments were used to probe changes in symmetry and quantify the incorporation of MA into the perovskite framework. Time-of-flight secondary ion mass spectrometry measurements confirmed that the MASCN content was mostly located near the surface region of the FAPbI3 films. We used these low-defect-density α-FAPbI3 films to make PSCs with >23% PCE, long-term operational stability, low (330 mV) open-circuit voltage (Voc) loss, and low (0.75 V) turn-on voltage of electroluminescence.CONCLUSIONSCN– anions play a key role in promoting the formation and stabilization of α-FAPbI3. Vapor-treated FAPbI3 films showed long-term thermal stability. MD simulations showed that the pure α-FAPbI3 remained kinetically stable. These findings are important for developing stable and pure black-phase FAPbI3-based PSCs. Our vapor-treated FAPbI3 PSCs showed high efficiency and good long-term stability under maximum power point tracking conditions. Because of its high Voc and high external quantum efficiency electroluminescence yield, pure α-FAPbI3 will be useful for other applications such as light-emitting diodes and photodetectors.Mixtures of cations or halides with FAPbI3 (where FA is formamidinium) lead to high efficiency in perovskite solar cells (PSCs) but also to blue-shifted absorption and long-term stability issues caused by loss of volatile methylammonium (MA) and phase segregation. We report a deposition method using MA thiocyanate (MASCN) or FASCN vapor treatment to convert yellow δ-FAPbI3 perovskite films to the desired pure α-phase. NMR quantifies MA incorporation into the framework. Molecular dynamics simulations show that SCN– anions promote the formation and stabilization of α-FAPbI3 below the thermodynamic phase-transition temperature. We used these low-defect-density α-FAPbI3 films to make PSCs with >23% power-conversion efficiency and long-term operational and thermal stability, as well as a low (330 millivolts) open-circuit voltage loss and a low (0.75 volt) turn-on voltage of electroluminescence.
Promises and challenges of perovskite solar cells Correa-Baena, Juan-Pablo; Saliba, Michael; Buonassisi, Tonio ...
Science (American Association for the Advancement of Science),
11/2017, Letnik:
358, Številka:
6364
Journal Article
Recenzirano
Odprti dostop
The efficiencies of perovskite solar cells have gone from single digits to a certified 22.1% in a few years’ time. At this stage of their development, the key issues concern how to achieve further ...improvements in efficiency and long-term stability. We review recent developments in the quest to improve the current state of the art. Because photocurrents are near the theoretical maximum, our focus is on efforts to increase open-circuit voltage by means of improving charge-selective contacts and charge carrier lifetimes in perovskites via processes such as ion tailoring. The challenges associated with long-term perovskite solar cell device stability include the role of testing protocols, ionic movement affecting performance metrics over extended periods of time, and determination of the best ways to counteract degradation mechanisms.
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