Showing papers on "Perovskite (structure) published in 2016"
TL;DR: In this paper, the triple cation perovskite photovoltaics with inorganic cesium were shown to be thermally more stable, contain less phase impurities and are less sensitive to processing conditions.
Abstract: Today's best perovskite solar cells use a mixture of formamidinium and methylammonium as the monovalent cations. With the addition of inorganic cesium, the resulting triple cation perovskite compositions are thermally more stable, contain less phase impurities and are less sensitive to processing conditions. This enables more reproducible device performances to reach a stabilized power output of 21.1% and ∼18% after 250 hours under operational conditions. These properties are key for the industrialization of perovskite photovoltaics.
3,470 citations
TL;DR: Thin films of near-single-crystalline quality are produced, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport.
Abstract: Three-dimensional organic-inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties, which have led to power conversion efficiencies exceeding 20 per cent, with the prospect of further improvements towards the Shockley-Queisser limit for a single‐junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions. In contrast to their three-dimensional counterparts, Ruddlesden-Popper phases--layered two-dimensional perovskite films--have shown promising stability, but poor efficiency at only 4.73 per cent. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting inorganic slabs. Here we overcome this issue in layered perovskites by producing thin films of near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. We report a photovoltaic efficiency of 12.52 per cent with no hysteresis, and the devices exhibit greatly improved stability in comparison to their three-dimensional counterparts when subjected to light, humidity and heat stress tests. Unencapsulated two-dimensional perovskite devices retain over 60 per cent of their efficiency for over 2,250 hours under constant, standard (AM1.5G) illumination, and exhibit greater tolerance to 65 per cent relative humidity than do three-dimensional equivalents. When the devices are encapsulated, the layered devices do not show any degradation under constant AM1.5G illumination or humidity. We anticipate that these results will lead to the growth of single-crystalline, solution-processed, layered, hybrid, perovskite thin films, which are essential for high-performance opto-electronic devices with technologically relevant long-term stability.
2,566 citations
TL;DR: It is shown that using cesium ions along with formamidinium cations in lead bromide–iodide cells improved thermal and photostability and lead to high efficiency in single and tandem cells.
Abstract: Metal halide perovskite photovoltaic cells could potentially boost the efficiency of commercial silicon photovoltaic modules from ∼20 toward 30% when used in tandem architectures. An optimum perovskite cell optical band gap of ~1.75 electron volts (eV) can be achieved by varying halide composition, but to date, such materials have had poor photostability and thermal stability. Here we present a highly crystalline and compositionally photostable material, [HC(NH2)2](0.83)Cs(0.17)Pb(I(0.6)Br(0.4))3, with an optical band gap of ~1.74 eV, and we fabricated perovskite cells that reached open-circuit voltages of 1.2 volts and power conversion efficiency of over 17% on small areas and 14.7% on 0.715 cm(2) cells. By combining these perovskite cells with a 19%-efficient silicon cell, we demonstrated the feasibility of achieving >25%-efficient four-terminal tandem cells.
2,412 citations
TL;DR: N nanoscale phase stabilization of CsPbI3 quantum dots (QDs) to low temperatures that can be used as the active component of efficient optoelectronic devices and describe the formation of α-CsP bI3 QD films that are phase-stable for months in ambient air.
Abstract: We show nanoscale phase stabilization of CsPbI 3 quantum dots (QDs) to low temperatures that can be used as the active component of efficient optoelectronic devices. CsPbI 3 is an all-inorganic analog to the hybrid organic cation halide perovskites, but the cubic phase of bulk CsPbI 3 (α-CsPbI 3 )—the variant with desirable band gap—is only stable at high temperatures. We describe the formation of α-CsPbI 3 QD films that are phase-stable for months in ambient air. The films exhibit long-range electronic transport and were used to fabricate colloidal perovskite QD photovoltaic cells with an open-circuit voltage of 1.23 volts and efficiency of 10.77%. These devices also function as light-emitting diodes with low turn-on voltage and tunable emission.
2,103 citations
TL;DR: A solution-processed lead halide perovskite solar cell that has p-type NiO(x) and n-type ZnO nanoparticles as hole and electron transport layers, respectively, and shows improved stability against water and oxygen degradation when compared with devices with organic charge transport layers is reported.
Abstract: Lead halide perovskite solar cells have recently attracted tremendous attention because of their excellent photovoltaic efficiencies. However, the poor stability of both the perovskite material and the charge transport layers has so far prevented the fabrication of devices that can withstand sustained operation under normal conditions. Here, we report a solution-processed lead halide perovskite solar cell that has p-type NiOx and n-type ZnO nanoparticles as hole and electron transport layers, respectively, and shows improved stability against water and oxygen degradation when compared with devices with organic charge transport layers. Our cells have a p–i–n structure (glass/indium tin oxide/NiOx/perovskite/ZnO/Al), in which the ZnO layer isolates the perovskite and Al layers, thus preventing degradation. After 60 days storage in air at room temperature, our all-metal-oxide devices retain about 90% of their original efficiency, unlike control devices made with organic transport layers, which undergo a complete degradation after just 5 days. The initial power conversion efficiency of our devices is 14.6 ± 1.5%, with an uncertified maximum value of 16.1%. Using metal oxides for both the hole- and electron-transport layers in perovskite solar cells significantly improves their stability compared with devices containing organic transport layers.
1,834 citations
TL;DR: A perovskite mixed material comprising a series of differently quantum-size-tuned grains that funnels photoexcitations to the lowest-bandgap light-emitter in the mixture functions as charge carrier concentrators, ensuring that radiative recombination successfully outcompetes trapping and hence non-radiatives recombination.
Abstract: Organometal halide perovskites exhibit large bulk crystal domain sizes, rare traps, excellent mobilities and carriers that are free at room temperature-properties that support their excellent performance in charge-separating devices. In devices that rely on the forward injection of electrons and holes, such as light-emitting diodes (LEDs), excellent mobilities contribute to the efficient capture of non-equilibrium charge carriers by rare non-radiative centres. Moreover, the lack of bound excitons weakens the competition of desired radiative (over undesired non-radiative) recombination. Here we report a perovskite mixed material comprising a series of differently quantum-size-tuned grains that funnels photoexcitations to the lowest-bandgap light-emitter in the mixture. The materials function as charge carrier concentrators, ensuring that radiative recombination successfully outcompetes trapping and hence non-radiative recombination. We use the new material to build devices that exhibit an external quantum efficiency (EQE) of 8.8% and a radiance of 80 W sr-1 m-2. These represent the brightest and most efficient solution-processed near-infrared LEDs to date.
1,756 citations
TL;DR: A new metal halide perovskite photovoltaic cell that exhibits both very high solar-to-electric power-conversion efficiency and intense electroluminescence is reported on.
Abstract: We report on a new metal halide perovskite photovoltaic cell that exhibits both very high solar-to-electric power-conversion efficiency and intense electroluminescence. We produce the perovskite films in a single step from a solution containing a mixture of FAI, PbI2, MABr, and PbBr2 (where FA stands for formamidinium cations and MA stands for methylammonium cations). Using mesoporous TiO2 and Spiro-OMeTAD as electron- and hole-specific contacts, respectively, we fabricate perovskite solar cells that achieve a maximum power-conversion efficiency of 20.8% for a PbI2/FAI molar ratio of 1.05 in the precursor solution. Rietveld analysis of x-ray diffraction data reveals that the excess PbI2 content incorporated into such a film is about 3 weight percent. Time-resolved photoluminescence decay measurements show that the small excess of PbI2 suppresses nonradiative charge carrier recombination. This in turn augments the external electroluminescence quantum efficiency to values of about 0.5%, a record for perovskite photovoltaics approaching that of the best silicon solar cells. Correspondingly, the open-circuit photovoltage reaches 1.18 V under AM 1.5 sunlight.
1,659 citations
TL;DR: In this article, an approach to prepare perovskite films of high electronic quality by using poly(methyl methacrylate) (PMMA) as a template to control nucleation and crystal growth is presented.
Abstract: The past several years have witnessed the rapid emergence of a class of solar cells based on mixed organic–inorganic halide perovskites. Today’s state-of-the-art perovskite solar cells (PSCs) employ various methods to enhance nucleation and improve the smoothness of the perovskite films formed via solution processing. However, the lack of precise control over the crystallization process creates a risk of forming unwanted defects, for example, pinholes and grain boundaries. Here, we introduce an approach to prepare perovskite films of high electronic quality by using poly(methyl methacrylate) (PMMA) as a template to control nucleation and crystal growth. We obtain shiny smooth perovskite films of excellent electronic quality, as manifested by a remarkably long photoluminescence lifetime. We realize stable PSCs with excellent reproducibility showing a power conversion efficiency (PCE) of up to 21.6% and a certified PCE of 21.02% under standard AM 1.5G reporting conditions. Controlling the crystallization process of perovskite films is crucial to obtaining high efficiency in perovskite solar cells. Bi et al. propose the use of poly(methyl methacrylate) as a template for the controlled nucleation and growth of perovskite crystals achieving efficiency of 21.6%.
1,626 citations
TL;DR: In this article, the effect of alloying FA0.85Cs0.15PbI3 with CsPbIsI3 was investigated, and it was shown that the effective tolerance factor can be tuned and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA1-xCsxPbisI3 is enhanced.
Abstract: Goldschmidt tolerance factor (t) is an empirical index for predicting stable crystal structures of perovskite materials. A t value between 0.8 and 1.0 is favorable for cubic perovskite structure, and larger (>1) or smaller (<0.8) values of tolerance factor usually result in nonperovskite structures. CH(NH2)2PbI3 (FAPbI3) can exist in the perovskite α-phase (black phase) with good photovoltaic properties. However, it has a large tolerance factor and is more stable in the hexagonal δH-phase (yellow phase), with δH-to-α phase-transition temperature higher than room temperature. On the other hand, CsPbI3 is stabilized to an orthorhombic structure (δO-phase) at room temperature due to its small tolerance factor. We find that, by alloying FAPbI3 with CsPbI3, the effective tolerance factor can be tuned, and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA1–xCsxPbI3 is enhanced, which is in agreement with our first-principles calculations. Thin films of the FA0.85Cs0.15PbI3 p...
1,483 citations
TL;DR: In this paper, a large scale synthesis, crystal structure, and optical characterization of the 2D (CH3(CH2)3NH3)n−1PbnI3n+1 (n = 1, 2, 3, 4, ∞) perovskites is presented.
Abstract: The hybrid two-dimensional (2D) halide perovskites have recently drawn significant interest because they can serve as excellent photoabsorbers in perovskite solar cells. Here we present the large scale synthesis, crystal structure, and optical characterization of the 2D (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 (n = 1, 2, 3, 4, ∞) perovskites, a family of layered compounds with tunable semiconductor characteristics. These materials consist of well-defined inorganic perovskite layers intercalated with bulky butylammonium cations that act as spacers between these fragments, adopting the crystal structure of the Ruddlesden–Popper type. We find that the perovskite thickness (n) can be synthetically controlled by adjusting the ratio between the spacer cation and the small organic cation, thus allowing the isolation of compounds in pure form and large scale. The orthorhombic crystal structures of (CH3(CH2)3NH3)2(CH3NH3)Pb2I7 (n = 2, Cc2m; a = 8.9470(4), b = 39.347(2) A, c = 8.8589(6)), (CH3(CH2)3NH3)2(CH3NH3)2Pb3I10 (...
1,451 citations
TL;DR: Perovskite quantum wells yield highly efficient LEDs spanning the visible and near-infrared as discussed by the authors. But their performance is not as good as those of traditional LEDs, and their lifetime is shorter.
Abstract: Perovskite quantum wells yield highly efficient LEDs spanning the visible and near-infrared.
TL;DR: In this article, the authors discuss the properties of perovskites that benefit light emission, review recent progress in perov-skite electroluminescent diodes and optically pumped lasers, and examine the remaining challenges in achieving continuous-wave and electrically driven lasing.
Abstract: The prospects for light-emitting diodes and lasers based on perovskite materials are reviewed. The field of solution-processed semiconductors has made great strides; however, it has yet to enable electrically driven lasers. To achieve this goal, improved materials are required that combine efficient (>50% quantum yield) radiative recombination under high injection, large and balanced charge-carrier mobilities in excess of 10 cm2 V−1 s−1, free-carrier densities greater than 1017 cm−3 and gain coefficients exceeding 104 cm−1. Solid-state perovskites are — in addition to galvanizing the field of solar electricity — showing great promise in photonic sources, and may be the answer to realizing solution-cast laser diodes. Here, we discuss the properties of perovskites that benefit light emission, review recent progress in perovskite electroluminescent diodes and optically pumped lasers, and examine the remaining challenges in achieving continuous-wave and electrically driven lasing.
TL;DR: It is found that ligand binding to the NC surface is highly dynamic, and therefore, ligands are easily lost during the isolation and purification procedures, and when a small amount of both oleic acid and oleylamine is added, the NCs can be purified, maintaining optical, colloidal, and material integrity.
Abstract: Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX3, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX3 NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr3 NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. 1H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We fi...
TL;DR: This Account critically review the recent progress in understanding the fundamental science on ion migration in OTP based solar cells and raises some questions that need to be understood and addressed in the future.
Abstract: ConspectusOrganometal trihalide perovskites (OTPs) are emerging as very promising photovoltaic materials because the power conversion efficiency (PCE) of OTP solar cells quickly rises and now rivals with that of single crystal silicon solar cells after only five-years research. Their prospects to replace silicon photovoltaics to reduce the cost of renewable clean energy are boosted by the low-temperature solution processing as well as the very low-cost raw materials and relative insensitivity to defects. The flexibility, semitransparency, and vivid colors of perovskite solar cells are attractive for niche applications such as built-in photovoltaics and portable lightweight chargers. However, the low stability of current hybrid perovskite solar cells remains a serious issue to be solved before their broad application. Among all those factors that affect the stability of perovskite solar cells, ion migration in OTPs may be intrinsic and cannot be taken away by device encapsulation.The presence of ion migrat...
TL;DR: In this paper, the highest reported efficiency cesium lead halide perovskite solar cells are realized by tuning the bandgap and stabilizing the black perovsite phase at lower temperatures.
Abstract: Highest reported efficiency cesium lead halide perovskite solar cells are realized by tuning the bandgap and stabilizing the black perovskite phase at lower temperatures. CsPbI2Br is employed in a planar architecture device resulting in 9.8% power conversion efficiency and over 5% stabilized power output. Offering substantially enhanced thermal stability over their organic based counterparts, these results show that all-inorganic perovskites can represent a promising next step for photovoltaic materials.
TL;DR: Although perovskite light-emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light-EMitting diodes accomplished in two decades.
Abstract: Organic-inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light-emitting diodes, lasers, and light-emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light-emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A(-1) have been achieved. Although perovskite light-emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light-emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower-dimensionality layered and three-dimensional perovskites, nanostructures, charge-transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light-emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light-emitting devices.
TL;DR: In this paper, single-crystal perovskite devices 2-3 mm thick exhibit 16.4% X-ray detection efficiency with sensitivity four times higher than α-Se detectors.
Abstract: Single-crystal perovskite devices 2–3 mm thick exhibit 16.4% X-ray detection efficiency with sensitivity four times higher than α-Se X-ray detectors.
TL;DR: This device architecture and materials set will enable “all-perovskite” thin-film solar cells to reach the highest efficiencies in the long term at the lowest costs.
Abstract: The ready processability of organic-inorganic perovskite materials for solar cells should enable the fabrication of tandem solar cells, in which the top layer is tuned to absorb shorter wavelengths and the lower layer to absorb the remaining longer-wavelength light. The difficulty in making an all-perovskite cell is finding a material that absorbs the red end of the spectrum. Eperon et al. developed an infrared-absorbing mixed tin-lead material that can deliver 14.8% efficiency on its own and 20.3% efficiency in a four-terminal tandem cell.
Science , this issue p. [861][1]
[1]: /lookup/doi/10.1126/science.aaf9717
TL;DR: Reduced-dimensionality (quasi-2D) perovskite films are reported that exhibit improved stability while retaining the high performance of conventional three-dimensionalperovskites, and are achieved by the choice of stoichiometry in materials synthesis.
Abstract: Metal halide perovskites have rapidly advanced thin-film photovoltaic performance; as a result, the materials’ observed instabilities urgently require a solution. Using density functional theory (DFT), we show that a low energy of formation, exacerbated in the presence of humidity, explains the propensity of perovskites to decompose back into their precursors. We find, also using DFT, that intercalation of phenylethylammonium between perovskite layers introduces quantitatively appreciable van der Waals interactions. These drive an increased formation energy and should therefore improve material stability. Here we report reduced-dimensionality (quasi-2D) perovskite films that exhibit improved stability while retaining the high performance of conventional three-dimensional perovskites. Continuous tuning of the dimensionality, as assessed using photophysical studies, is achieved by the choice of stoichiometry in materials synthesis. We achieve the first certified hysteresis-free solar power conversion in a p...
TL;DR: In this paper, the optoelectronic properties of mixed A-cation perovskites and the underlying reasons for their excellent performance and high stability were analyzed using first principle computations.
Abstract: ABX3-type organic lead halide perovskites currently attract broad attention as light harvesters for solar cells due to their high power conversion efficiency (PCE). Mixtures of formamidinium (FA) with methylammonium (MA) as A-cations show currently the best performance. Apart from offering better solar light harvesting in the near IR the addition of methylammonium stabilizes the perovskite phase of FAPbI3 which in pure form at room temperature converts to the yellow photovoltaically inactive δ-phase. We observe a similar phenomenon upon adding Cs+ cations to FAPbI3. CsPbI3 and FAPbI3 both form the undesirable yellow phase under ambient condition while the mixture forms the desired black pervoskite. Solar cells employing the composition Cs0.2FA0.8PbI2.84Br0.16 yield high average PCEs of over 17% exhibiting negligible hysteresis and excellent long term stability in ambient air. We elucidate here this remarkable behavior using first principle computations. These show that the remarkable stabilization of the perovskite phase by mixing the A-cations stems from entropic gains and the small internal energy input required for the formation of their solid solution. By contrast, the energy of formation of the delta-phase containing mixed cations is too large to be compensated by this configurational entropy increase. Our calculations reveal for the first time the optoelectronic properties of such mixed A-cation perovskites and the underlying reasons for their excellent performance and high stability.
TL;DR: The double perovskites Cs2AgBiBr6 and Cs 2AgBiCl6 have been synthesized from both solid state and solution routes, and X-ray diffraction measurements reveal band gaps of 2.19 eV and 2.77 eV as discussed by the authors.
Abstract: The double perovskites Cs2AgBiBr6 and Cs2AgBiCl6 have been synthesized from both solid state and solution routes. X-ray diffraction measurements show that both compounds adopt the cubic double perovskite structure, space group Fm3m, with lattice parameters of 11.2711(1) A (X = Br) and 10.7774(2) A (X = Cl). Diffuse reflectance measurements reveal band gaps of 2.19 eV (X = Br) and 2.77 eV (X = Cl) that are slightly smaller than the band gaps of the analogous lead halide perovskites, 2.26 eV for CH3NH3PbBr3 and 3.00 eV for CH3NH3PbCl3. Band structure calculations indicate that the interaction between the Ag 4d-orbitals and the 3p/4p-orbitals of the halide ion modifies the valence band leading to an indirect band gap. Both compounds are stable when exposed to air, but Cs2AgBiBr6 degrades over a period of weeks when exposed to both ambient air and light. These results show that halide double perovskite semiconductors are potentially an environmentally friendly alternative to the lead halide perovskite semico...
TL;DR: In the last few years, the advent of metal halide perovskite solar cells has revolutionized the prospects of next-generation photovoltaics and research on its environmental impact is becoming increasingly relevant.
Abstract: In the last few years, the advent of metal halide perovskite solar cells has revolutionized the prospects of next-generation photovoltaics. As this technology is maturing at an exceptional rate, research on its environmental impact is becoming increasingly relevant.
TL;DR: It is found that exposing PSCs to a temperature of 70 °C is enough to induce gold migration through the hole-transporting layer (HTL), spiro-MeOTAD, and into the perovskite material, which in turn severely affects the device performance metrics under working conditions.
Abstract: Perovskite solar cells (PSCs) have now achieved efficiencies in excess of 22%, but very little is known about their long-term stability under thermal stress. So far, stability reports have hinted at the importance of substituting the organic components, but little attention has been given to the metal contact. We investigated the stability of state-of-the-art PSCs with efficiencies exceeding 20%. Remarkably, we found that exposing PSCs to a temperature of 70 °C is enough to induce gold migration through the hole-transporting layer (HTL), spiro-MeOTAD, and into the perovskite material, which in turn severely affects the device performance metrics under working conditions. Importantly, we found that the main cause of irreversible degradation is not due to decomposition of the organic and hybrid perovskite layers. By introducing a Cr metal interlayer between the HTL and gold electrode, high-temperature-induced irreversible long-term losses are avoided. This key finding is essential in the quest for achieving...
TL;DR: This work investigates the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, and discovers that scattering from longitudinal optical phonons via the Fröhlich interaction is the dominant source of electron–phonon coupling near room temperature.
Abstract: Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here we investigate the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, HC(NH2)2PbI3, HC(NH2)2PbBr3, CH3NH3PbI3 and CH3NH3PbBr3, and discover that scattering from longitudinal optical phonons via the Frohlich interaction is the dominant source of electron–phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3 meV, and Frohlich coupling constants of ∼40 and 60 meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic band-structure picture for describing charge carriers in hybrid perovskites. Phonon scattering limits charge transport in perovskite solar cells, yet the interactions involved are still poorly understood. Here, Wright et al. show by photoluminescence measurements and first-principles calculations that longitudinal optical phonons dominate the electron-phonon coupling at room temperature.
TL;DR: A two-step ligand-exchange strategy is developed, in which the long-carbon- chain ligands on all-inorganic perovskite quantum dots (QDs) are replaced with halide-ion-pair ligands.
Abstract: A two-step ligand-exchange strategy is developed, in which the long-carbon- chain ligands on all-inorganic perovskite (CsPbX3 , X = Br, Cl) quantum dots (QDs) are replaced with halide-ion-pair ligands. Green and blue light-emitting diodes made from the halide-ion-pair-capped quantum dots exhibit high external quantum efficiencies compared with the untreated QDs.
TL;DR: In this paper, a combination of nanoscopic and macroscopic level measurements was used to show that ion migration in polycrystalline perovskites dominates through grain boundary (GBs).
Abstract: The efficiency of perovskite solar cells is approaching that of single-crystalline silicon solar cells despite the presence of a large grain boundary (GB) area in the polycrystalline thin films. Here, by using a combination of nanoscopic and macroscopic level measurements, we show that ion migration in polycrystalline perovskites dominates through GBs. Atomic force microscopy measurements reveal much stronger hysteresis both for photocurrent and dark-current at the GBs than on the grain interiors, which can be explained by faster ion migration at the GBs. The dramatically enhanced ion migration results in the redistribution of ions along the GBs after electric poling, in contrast to the intact grain area. The perovskite single-crystal devices without GBs show negligible current hysteresis and no ion-migration signal. The discovery of dominating ion migration through GBs in perovskites can lead to broad applications in many types of devices including photovoltaics, memristors, and ion batteries.
TL;DR: The adduct approach proposed in this Account is a very promising methodology to achieve high quality perovskite films with high photovoltaic performance and single crystal growth on the conductive substrate is expected to be possible if the authors kinetically control the elimination of Lewis base in the adduct.
Abstract: ConspectusSince the first report on the long-term durable 9.7% solid-state perovskite solar cell employing methylammonium lead iodide (CH3NH3PbI3), mesoporous TiO2, and 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-MeOTAD) in 2012, following the seed technologies on perovskite-sensitized liquid junction solar cells in 2009 and 2011, a surge of interest has been focused on perovskite solar cells due to superb photovoltaic performance and extremely facile fabrication processes. The power conversion efficiency (PCE) of perovskite solar cells reached 21% in a very short period of time. Such an unprecedentedly high photovoltaic performance is due to the intrinsic optoelectronic property of organolead iodide perovskite material. Moreover, a high dielectric constant, sub-millimeter scale carrier diffusion length, an underlying ferroelectric property, and ion migration behavior can make organolead halide perovskites suitable for multifunctionality. Thus, besides solar cell applicati...
TL;DR: In this paper, the authors reported a drastically improved solar cell efficiency via surface optimization of the TiO2 ETL using a special ionic-liquid (IL) that shows high optical transparency and superior electron mobility.
Abstract: The electron-transport layer (ETL) between the active perovskite material and the cathode plays a critical role in planar perovskite solar cells. Herein, we report a drastically improved solar cell efficiency via surface optimization of the TiO2 ETL using a special ionic-liquid (IL) that shows high optical transparency and superior electron mobility. As a consequence, the efficiency is promoted to as high as 19.62% (the certified efficiency is 19.42%), exceeding the previous highest efficiency recorded for planar CH3NH3PbI3 perovskite solar cells. Surprisingly, the notorious hysteresis is completely eliminated, likely due to the improved ETL quality that has effectively suppressed ion migration in the perovskite layer and charge accumulation at the interfaces, higher electron mobility to balance the hole flux at the anode, and a better growth platform for the high quality perovskite absorber. Both experimental analyses and theoretical calculations reveal that the anion group of the IL bonds to TiO2, leading to a higher electron mobility and a well-matched work function. Meanwhile, the cation group interfaces with adjacent perovskite grains to provide an effective channel for electron transport and a suitable setting to grow low trap-state density perovskite for improved device performance.
TL;DR: The radiative efficiency for hybrid lead halide perovskite films is examined and it is found that if high efficiencies are to be obtained for intermediate charge-carrier densities (n ≈ 10(14) cm(-3)) trap-related recombination lifetimes will have to be enhanced well into the microsecond range, and the radiative efficiencies that may be obtained are evaluated.
Abstract: Photovoltaic (PV) devices that harvest the energy provided by the sun have great potential as renewable energy sources, yet uptake has been hampered by the increased cost of solar electricity compared with fossil fuels. Hybrid metal halide perovskites have recently emerged as low-cost active materials in PV cells with power conversion efficiencies now exceeding 20%. Rapid progress has been achieved over only a few years through improvements in materials processing and device design. In addition, hybrid perovskites appear to be good light emitters under certain conditions, raising the prospect of applications in low-cost light-emitting diodes and lasers. Further optimization of such hybrid perovskite devices now needs to be supported by a better understanding of how light is converted into electrical currents and vice versa. This Account provides an overview of charge-carrier recombination and mobility mechanisms encountered in such materials. Optical-pump-terahertz-probe (OPTP) photoconductivity spectroscopy is an ideal tool here, because it allows the dynamics of mobile charge carriers inside the perovskite to be monitored following excitation with a short laser pulse whose photon energy falls into the range of the solar spectrum. We first review our insights gained from transient OPTP and photoluminescence spectroscopy on the mechanisms dominating charge-carrier recombination in these materials. We discuss that mono-molecular charge-recombination predominantly originates from trapping of charges, with trap depths being relatively shallow (tens of millielectronvolts) for hybrid lead iodide perovskites. Bimolecular recombination arises from direct band-to-band electron-hole recombination and is found to be in significant violation of the simple Langevin model. Auger recombination exhibits links with electronic band structure, in accordance with its requirement for energy and momentum conservation for all charges involved. We further discuss charge-carrier mobility values extracted from OPTP measurements and their dependence on perovskite composition and morphology. The significance of the reviewed charge-carrier recombination and mobility parameters is subsequently evaluated in terms of the charge-carrier diffusion lengths and radiative efficiencies that may be obtained for such hybrid perovskites. We particularly focus on calculating such quantities in the limit of ultra-low trap-related recombination, which has not yet been demonstrated but could be reached through further advances in material processing. We find that for thin films of hybrid lead iodide perovskites with typical charge-carrier mobilities of ∼30cm(2)/(V s), charge-carrier diffusion lengths at solar (AM1.5) irradiation are unlikely to exceed ∼10 μm even if all trap-related recombination is eliminated. We further examine the radiative efficiency for hybrid lead halide perovskite films and show that if high efficiencies are to be obtained for intermediate charge-carrier densities (n ≈ 10(14) cm(-3)) trap-related recombination lifetimes will have to be enhanced well into the microsecond range.
TL;DR: In this paper, the authors demonstrate that the operational stability of electronic and optoelectronic devices that exploit the electron transporting properties of MeNH3PbI3 will be critically dependent upon the use of suitable barrier layers and device configurations to mitigate the oxygen sensitivity of this remarkable material.
Abstract: Here, we demonstrate that light and oxygen-induced degradation is the main reason for the low operational stability of methylammonium lead triiodide (MeNH3PbI3) perovskite solar cells exposed to ambient conditions. When exposed to both light and dry air, unencapsulated MeNH3PbI3 solar cells rapidly degrade on timescales of minutes to a few hours. This rapid degradation is also observed under electrically bias driven current flow in the dark in the presence of O2. In contrast, significantly slower degradation is observed when the MeNH3PbI3 devices are exposed to moisture alone (e.g. 85% relative humidity in N2). We show that this light and oxygen induced degradation can be slowed down by the use of interlayers that are able to remove electrons from the perovskite film before they can react with oxygen to form O2−. These observations demonstrate that the operational stability of electronic and optoelectronic devices that exploit the electron transporting properties of MeNH3PbI3 will be critically dependent upon the use of suitable barrier layers and device configurations to mitigate the oxygen sensitivity of this remarkable material.