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Abstract Fast reaction between organic salt and lead iodide always leads to small perovskite crystallites and concentrated defects. Here, polyacrylic acid is blended with organic salt, so as to regulate the crystallization in a two‐step growth method. It is observed that addition of polyacrylic acid retards aggregation and crystallization behavior of the organic salt, and slows down the reaction rate between organic salt and PbI 2 , by which “slow‐release effect” is defined. Such effect improves crystallization of perovskite. X‐ray diffraction study shows that, after addition of 2 m m polyacrylic acid, average crystallite size of perovskite increases from ≈40 to ≈90 nm, meanwhile, grain size increases. Thermal admittance spectroscopy study shows that trap density is reduced by nearly one order (especially for deep energy levels). Due to the improved crystallization and reduced trap density, charge recombination is obviously reduced, while lifetime of charge carriers in perovskite film and devices are prolonged, according to time‐resolved photoluminescence and transient photo‐voltage decay curve tests, respectively. Accordingly, power conversion efficiency of the device is promoted from 19.96 (±0.41)% to 21.84 (±0.25)% (with a champion efficiency of 22.31%), and further elevated to 24.19% after surface modification by octylammonium iodide. 

Crystallization of perovskite is monitored in carbon-electrode based, low-temperature, mesoscopic perovskite solar cells. Crystallographic and morphological properties of the perovskite are examined through changes in the film thickness of carbon-electrode or the volume of perovskite precursor. It is observed that, when a relatively thin carbon-electrode or large volume of perovskite precursor is used, perovskite crystallites mainly form on the device surface, leaving the bottom part of the device un-wetted. However, if a thicker carbon-electrode or less perovskite precursor is used, crystallization could be seen in the whole porous skeleton, and relative uniform distribution of perovskite crystallites is achieved. As such, uneven crystallization is observed. Such behavior is due to solvent evaporation on the surface, which facilitates nucleation processes on the surface, while retards crystallization on the bottom due to the Ostwald ripening effect. Charge transfer/recombination processes and photo-to-electric power conversion properties are studied. As expected, uneven crystallization results in retarded charge transfer and increased risk of recombination, and poor power conversion efficiency, for example, ∼3%. In contrast, uniform crystallization accelerates charge transfer and reduces recombination risk, and increases the efficiency to higher than 11% (AM1.5G, 100 mW/cm2). 

Hydrophobic and long-chain molecule oleylamine is used to modify the spiro-OMeTAD matrix, which is then adopted for the hole-transport layer in perovskite solar cells. It is observed that after moderate doping, the power conversion efficiency of the devices increases from 17.82 (±1.47)% to 20.68 (±0.77)%, with the optimized efficiency of 21.57% (AM 1.5G, 100 mW/cm2). The improved efficiency is ascribed to the favored charge extraction and retarded charge recombination, as reflected by transient photovoltage/photocurrent curves and impedance spectroscopy measurement. In addition, the grazing incidence photoluminescence spectrum reveals that oleylamine doping causes a blue shift of the luminescence peak of the surface layer of the halide perovskite film, while the Mott−Schottky study observes 100 mV increment in the built-in potential, both of which indicate possible defect passivation behavior on the perovskite. Moreover, an accelerated damp test observes that moisture resistance of the device is also upgraded, which is due to the improved hydrophobicity of the spiro-OMeTAD matrix. 

SnO2 modified mesoporous ZrO2 is used to replace the mesoporous TiO2 layer and serves as a kind of mesoporous electron-transport layer during the low-temperature fabrication of mesoscopic perovskite solar cells that are based on carbon electrode. X-ray/ultraviolet photoelectron spectroscopy studies and electrical test observe that SnO2 modification brought down the work function while increasing the conductivity of the mesoporous ZrO2. Transient photovoltage/photocurrent decay curves, impedance spectroscopy, and photoluminescence mapping show that after the bottom layer of ZrO2 is modified by SnO2, the charge extraction process is accelerated while recombination is retarded. This modification helps to increase the power conversion efficiency from 4.70 (±0.85)% to 10.15 (±0.35)%, along with the optimized efficiency at 13.37% (AM1.5G, 100 mW/cm2) for the low-temperature devices. In addition, the effects of modification layers of SnO2 on the power conversion properties are carefully studied. This study shows that SnO2 modified mesoporous ZrO2 could serve as an efficient electron-transport layer for the low-temperature mesoscopic devices. 

Capsaicin is used to modify SnO 2 quantum dots and then used as an electron-transfer material for perovskite solar cells. After capsaicin modification, the power conversion efficiency of the devices increases from 19.90 (± 0.47)% to 21.87 (± 0.28)% with a champion device of 22.24% (AM 1.5G, 100 mW/cm 2 ). Transient photovoltage and photocurrent decay show that, after the capsaicin doping, the lifetime increases from 21.55 (± 1.54) to 27.63 (± 1.45)  μs, while the charge extraction time reduces from 1.90 (± 0.09) to 1.67 (± 0.06)  μs. Time-resolved photoluminescence and impedance spectrum studies show similar results. The accelerated charge transfer and retarded recombination are due to defect passivation. Space charge limited current study shows that, after modification, the trap density of devices is reduced from 2.24 × 10 15 to 1.28 × 10 15  cm −3 . X-ray photoelectron spectroscopy and theoretical calculation indicate that the reduced trap density is due to the chemical interaction between carbonyl group (from capsaicin) and Sn atom, and that between carbonyl group and Pb atom. 

Abstract “Perovskite/carbon” interface is a bottle‐neck for hole‐conductor‐free, carbon‐electrode basing perovskite solar cells due to the energy mismatch and concentrated defects. In this article, in‐situ healing strategy is proposed by doping octylammonium iodide into carbon paste that used to prepare carbon‐electrode on perovskite layer. This strategy is found to strengthen interfacial contact and reduce interfacial defects on one hand, and slightly elevate the work function of the carbon‐electrode on other hand. Due to this effect, charge extraction is accelerated, while recombination is obviously reduced. Accordingly, power conversion efficiency of the hole‐conductor‐free, planar perovskite solar cells is upgraded by ≈50%, or from 11.65 (± 1.59) % to 17.97 (± 0.32) % (AM1.5G, 100 mW cm⁻²). The optimized device shows efficiency of 19.42% and open‐circuit voltage of 1.11 V. Meanwhile, moisture‐stability is tested by keeping the unsealed devices in closed chamber with relative humidity of 85%. The “in‐situ healing” strategy helps to obtain T₈₀time of >450 h for the carbon‐electrode basing devices, which is four times of the reference ones. Thus, a kind of “internal encapsulation effect” has also been reached. The “in situ healing” strategy facilitates the fabrication of efficient and stable hole‐conductor‐free devices basing on carbon‐electrode. 

Improving efficiency and stability has become an urgent issue in the application of perovskite solar cells (PSCs). Herein, a kind of long‐chain polymer or polymethylmethacrylate (PMMA) is added into the spiro‐OMeTAD matrix to improve the film formation process and hence the device performance. It is observed that, after modification, the spiro‐OMeTAD‐based hole‐transporting layer becomes uniform, continuous, and condensed. Meanwhile, the power conversion efficiency of the devices is upgraded. Compared with the control device, open‐circuit voltage of the modified one (with moderate doping) increases from 1.06 (±0.03) to 1.10 (±0.02) V, fill factor increases from 72.20 (±3.44)% to 75.59 (±3.35)%, and the power conversion efficiency increases from 18.82 (±1.06)% to 20.51 (±0.82)% (highest at 21.78%) under standard test condition (AM 1.5G, 100 mW cm⁻²). Transient photocurrent/photovoltage decay curves, time‐resolved photoluminance, and impedance spectroscopy studies show that the modification could accelerate charge transfer and retard interfacial recombination. In addition, the modification improves device stability. Due to the strengthened barrier against penetration of “H₂O/O₂/Ag,” the efficiency of the unsealed device could retain 91.49% (by average) of the initial one after 100 days storage in the dark [relative humidity = 30(±5)%]. This work shows that long‐chain polymer doping could simultaneously improve efficiency and stability of spiro‐OMeTAD‐based PSCs.