Hybrid organic–inorganic semiconducting perovskite photovoltaic cells are usually coupled with organic hole conductors. Here, we report planar, inverse CH3NH3PbI3–x Cl x -based cells with inorganic ...hole conductors. Using electrodeposited NiO as hole conductor, we have achieved a power conversion efficiency of 7.3%. The maximum V OC obtained was 935 mV with an average V OC value being 785 mV. Preliminary results for similar cells using electrodeposited CuSCN as hole conductor resulted in devices up to 3.8% in efficiency. The ability to obtain promising cells using NiO and CuSCN expands the presently rather limited range of available hole conductors for perovskite cells.
Highly reproducible and reversible thermochromic nature of dihydrated methylammonium lead iodide is found. A wide bandgap variation of the material (∼2 eV) is detected between room temperature and 60 ...°C under ambient condition as a result of phase transition caused by moisture absorption and desorption. In situ X-ray diffraction and Fourier transform infrared spectroscopy studies are performed to understand the mechanistic behavior during the phase transition. This thermochromic property is further explored as absorber material in mesostructured solar cells. Temperature-dependent reversible power conversion efficiency greater than 1% under standard test conditions is demonstrated; revealing its potential applicability in building integrated photovoltaics.
Recent reports on temporal photoluminescence (PL) intensity fluctuations (blinking) within localized domains of organo-metal lead halide (hybrid) perovskite microcrystals have invoked considerable ...interest to understand their origins. Using PL microscopy, we have investigated the effect of atmospheric constituents and photoillumination on spatially extended intensity fluctuations in methylammonium lead bromide (MAPbBr3) perovskite materials, explicitly for micrometer (ca. 1–2 μm)-sized crystals. Increase in the relative humidity of the ambience results in progressive reduction in the PL intensity, and beyond a threshold value, individual microcrystalline grains exhibit multistate PL intermittency (flickering), which is characteristically different from quasi two-state blinking observed in nanocrystals. Such flickering disappears upon removal of moisture, accompanied by considerable enhancement of the overall PL efficiency. We hypothesize that initiation of moisture-induced degradation marked by the lowering of PL intensity correlates with the appearance of PL flickering, and such processes further accelerate in the presence of oxygen as opposed to an inert (nitrogen) environment. We find that the intrinsic defects not only increase the threshold level of ambient moisture needed to initiate flickering but also modulate the nature of PL intermittency. Our results therefore establish a strong correlation between initiation of material degradation and PL flickering of hybrid perovskite microcrystals, induced by transient defects formed via interaction with the ambience.
Abrupt fluorescence intermittency or blinking is long recognized to be characteristic of single nano‐emitters. Extended quantum‐confined nanostructures also undergo spatially heterogeneous blinking; ...however, there is no such precedent in dimensionally unconfined (bulk) materials. Herein, we report multi‐level blinking of entire individual organo–lead bromide perovskite microcrystals (volume=0.1–3 μm3) under ambient conditions. Extremely high spatiotemporal correlation (>0.9) in intracrystal emission intensity fluctuations signifies effective communication amongst photogenerated carriers at distal locations (up to ca. 4 μm) within each crystal. Fused polycrystalline grains also exhibit this intriguing phenomenon, which is rationalized by correlated and efficient migration of carriers to a few transient nonradiative traps, the nature and population of which determine blinking propensity. Observation of spatiotemporally correlated emission intermittency in bulk semiconductor crystals opens the possibility of designing novel devices involving long‐range (mesoscopic) electronic communication.
Blink In‐Sync: A novel phenomenon of spatiotemporally correlated fluorescence intermittency is reported for entire individual organo–metal halide perovskite microcrystals, a bulk polycrystalline material. This observation unambiguously points out to extremely long‐range (>μm) communication between photogenerated carriers and dispels the long‐standing notion that quantum nanoconfinement is essential to exhibit photoluminescence blinking.
The attractive optoelectronic properties of MAPbI3 (MA = CH3NH3), one of the most common halide perovskites, can be complicated by its tetragonal → cubic structural phase transition just above room ...temperature. We show that decreasing the ambient pressure can move that phase transition by ∼40 °C (at ∼10–3 mbar). Our report also includes control experiments, which show that desorption of water or oxygen can be excluded as possible causes for the change in phase transition temperature. On the basis of diffraction data, we postulate that an optimum volume is required to initiate a T → C phase transition. The pressure-induced phase change in effect stabilizes the tetragonal phase for work around room temperature, even if some natural heating occurs.
For the increasing demand for renewable energy to reduce carbon dioxide emissions, a high-efficiency, low-cost perovskite solar cell (PSC) is one of the most promising technologies. To further ...improve the power conversion efficiency, a tandem device comprising a bottom cell with low band gap perovskite is a potential candidate. Tin–lead alloyed perovskite is suitable for this purpose due to its low band gap of about ∼1.2–1.3 eV. However, the stability of Sn(II) in the alloyed perovskite films poses a significant challenge to the progress of the technology. Here, we identify the origin of tin instability in tin–lead perovskite and analyze the promising stability improvement strategies available in the literature. In addition, we offer a perspective for further development of strategies to enhance the stability of the material and the device’s performance.
Attempts to dope halide perovskites (HaPs) extrinsically have been mostly unsuccessful. Still, oxygen (O2) is an efficient p‐dopant for polycrystalline HaP films. To an extent, this doping is ...reversible, i.e., the films can be de‐doped by decreasing the O2 partial pressure. Here results are reported, aimed at understanding the mechanism of such reversible doping, as it has been argued that doping involves interaction of oxygen with defects inside bulk HaP. These experimental results clearly point out that O2‐surface interactions suffice to dope the bulk of the films. Such behavior fits what is known for other polycrystalline semiconductors, where surface charge transfer‐adducts can form and be removed. Thus, controlling the O2 partial pressure to which the HaP film is exposed, can, after proper encapsulation, achieve the desired bulk doping of the film.
The challenge of doping Pb‐halide perovskites, HaPs can be met by surface charge transfer of oxygen (O2) and is shown to allow p‐doping in HaP thin films. Therefore, by controlling the O2 surface concentration and grain size of the film, with proper encapsulation, these films can be doped controllably for consistent performance.
Pseudohalide thiocyanate anion (SCN–) has been used as a dopant in a methylammonium lead tri-iodide (MAPbI3) framework, aiming for its use as an absorber layer for photovoltaic applications. The ...substitution of SCN– pseudohalide anion, as verified using Fourier transform infrared (FT-IR) spectroscopy, results in a comprehensive effect on the optical properties of the original material. Photoluminescence measurements at room temperature reveal a significant enhancement in the emission quantum yield of MAPbI3–x (SCN) x as compared to MAPbI3, suggestive of suppression of nonradiative channels. This increased intensity is attributed to a highly edge specific emission from MAPbI3–x (SCN) x microcrystals as revealed by photoluminescence microscopy. Fluoresence lifetime imaging measurements further established contrasting carrier recombination dynamics for grain boundaries and the bulk of the doped material. Spatially resolved emission spectroscopy on individual microcrystals of MAPbI3–x (SCN) x reveals that the optical bandgap and density of states at various (local) nanodomains are also nonuniform. Surprisingly, several (local) emissive regions within MAPbI3–x (SCN) x microcrystals are found to be optically unstable under photoirradiation, and display unambiguous temporal intermittency in emission (blinking), which is extremely unusual and intriguing. We find diverse blinking behaviors for the undoped MAPbI3 crystals as well, which leads us to speculate that blinking may be a common phenomenon for most hybrid perovskite materials.
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