Selective Control of Novel TiO2 Nanorods: Excellent Building Blocks for the Electron Transport Layer of Mesoscopic Perovskite Solar Cells.

Novel TiO2 nanorods (NRs) with various lengths of 70-200 nm and uniform widths of 46-48 nm are selectively synthesized by a solvothermal reaction under a basic environment. The length of TiO2 NRs is reproducibly tuned by varying the concentration of tetramethylammonium hydroxide (TMAH), while the NRs in the pure anatase phase are grown in the [001] direction, caused by the preferential binding affinity of TMAH to the TiO2 (101) facet. TiO2 NRs of various lengths are then applied to form the electron transporting layer (ETL) of mesoscopic perovskite solar cells (PSCs). We found that PSC devices with NRs exhibit superior photovoltaic (PV) performance to those with conventional 46 nm-sized TiO2 nanoparticles (NP46). Particularly, the PSC with TiO2 NRs of 110 nm length (NR110) exhibits the optimum PV conversion efficiency (PCE): the average PCE is 22.64% with a VOC of 1.137 V, a JSC of 24.60 mA·cm-2, and a FF of 80.96%, while the champion PCE is 23.18%. In addition, the PSC with NR110 (PSC-NR110) reveals significantly improved long-term stability in air with a relative humidity of 40-50%. In 1000 h, its PCE is reduced by only 9% whereas that of PSC with NP46 decreases by 25%. The PSC properties analyzed by impedance spectroscopy and J-V curve measurements under dark conditions and at various light intensities provide evidence that PSC-NR110 has fewer defects and shows significantly reduced charge recombination. We discuss the advantages of NR structures in preparing the ETL of PSC devices and also explain why the charge recombination is suppressed.

Future Research Directions in Perovskite Solar Cells: Exquisite Photon Management and Thermodynamic Phase Stability.

As power conversion efficiency (PCE) of perovskite solar cells (PSCs) has rapidly increased up to 25.7% in 2022, a curiosity about the achievable limit of the PCE has prevailed and demands understanding about the underlying fundamentals to step forward. Meanwhile, outstanding long-term stability of PSCs over 1000 h has been reported at operating conditions or under damp heat test with 85 °C/85% relative humidity. Herein comes the question as to whether the phase stability issue of perovskite crystal is completely resolved in the most recent state-of-the-art perovskite film or if it deceives everyone into believing so by significantly slowing the kinetics. On the one hand, the fundamental origins of a discrepancy between reported values and the theoretical limit are thoroughly examined, where the importance of light management is greatly emphasized with the introduction of external luminescence as a key parameter to narrow the gap. On the other hand, the phase stability of a perovskite film is understood from thermodynamic point of view to address viable approaches to lower the Gibbs free energy, distinguishing the kinetically trapped condition from the thermodynamically stable phase.

Oriented Crystal Growth during Perovskite Surface Reconstruction.

Surface passivation has become a key strategy for an improvement in power conversion efficiency (PCE) of perovskite solar cells (PSCs) since PSCs experienced a steep increase in PCE and reached a comparably matured point. Recently, surface passivation using a mixed salt of fluorinated alkyl ammonium iodide and formamidinium bromide demonstrated a remarkable improvement in both performance and stability, which can be tuned by the length of the alkyl chain. Nevertheless, the role of the alkyl chain in manipulating surface-limited crystal growth was not fully understood, preventing a further progress in interface control. In this study, we found that the length of the fluorine-substituted alkyl chain governed the crystal formation dynamics by manipulating surface tensions of different crystal orientations. The overall enhancement of the (001) plane, being the most favored, commonly resulted from the surface reformation of the perovskite film regardless of the chain length, while the highly oriented (001) over (111) was monitored with a particular chain length. The enhanced crystal orientation during surface recrystallization was responsible for the low trap density and thus effectively suppressed charge recombination at the interface, resulting in a considerable increase in open-circuit voltage and fill factor.

Enhanced Phase Stability of Compressive Strain-Induced Perovskite Crystals.

Control of strain in perovskite crystals has been considered as an effective strategy to ensure the phase stability of perovskite films where a compressive strain is particularly preferred over a tensile strain due to a lowered Gibbs free energy by the unit cell contraction effect. Here we adapt the strategy of strain control into perovskite solar cells in which the compressive strain is applied by utilizing a thermal expansion difference between the perovskite film and an adjacent layer. Poly(4-butylphenyldiphenylamine), with a higher thermal expansion coefficient compared to that of perovskite, is employed as a substrate for perovskite crystal growth at 100 °C, followed by cooling to room temperature. The applied compressive strain at the interface, as a result of a greater contraction of the polymer compared to the perovskite film, is confirmed by grazing incidence X-ray diffraction showing a red peak shift with increasing secondary angle. The compressive strain-induced perovskite film shows relatively constant absorbance spectra as a function of time. In the meantime, the absorbance spectra of a film without strain control exhibit a gradual decay with developing an Urbach tail. Importantly, the effect of strain engineering is remarkably prominent in the long-term photovoltaic performance. The photocurrent drops by 41% over 911 h without controlling strain, which is significantly improved by employing compressive strain, showing only a 6% drop in photocurrent from a shelf-stability test without encapsulation. It is also noted that an S-shaped kink appears in the current-voltage curves since 579-h-long storage for the device without strain control, leading to unreliable and overestimated fill factor and conversion efficiency. On the other hand, a 16% increase in fill factor with a stable performance is derived over 911 h from the compressive strain-induced device.

Challenges for Thermally Stable Spiro-MeOTAD toward the Market Entry of Highly Efficient Perovskite Solar Cells.

Perovskite solar cells (PSCs) have drawn great attention because they have seen a dramatic increase in power conversion efficiency (PCE) over only a decade and reached 25.5% of certified PCE in 2021. The efficiency competitiveness with a low production cost puts up PSCs as a candidate for next-generation photovoltaics, encouraging the stability assessment. Research on PSCs, however, still struggles with the stability issue, particularly at elevated temperature, which is mainly ascribed to the use of spiro-MeOTAD as a hole transport material (HTM). Though many attempts have been made to explore a new HTM to replace spiro-MeOTAD, the improved stability is mostly obtained at the expense of losing efficiency. Likewise, the question of the effectiveness of alternatives for spiro-MeOTAD consistently remains. In this perspective, the morphological stability of spiro-MeOTAD at elevated temperatures is discussed to determine the underlying origins of the thermal stability issue and find feasible strategies to resolve it.

3D/2D Bilayerd Perovskite Solar Cells with an Enhanced Stability and Performance.

The formation of a thin 2D perovskite layer on the surface of 3D perovskite films has become a popular strategy for obtaining a high-efficiency perovskite solar cell (PSC) with an ensured device stability. In this review paper, various experimental methods used for growth of the 2D layer are introduced with the resulting film properties. Furthermore, a variety of organic cation sources for the 2D layer, ranging from alkyl to phenyl ammonium, are explored to investigate their impact on the device stability and photovoltaic performance.

Morphological and compositional progress in halide perovskite solar cells.

Perovskite solar cells (PSCs) reached a certified 23.7% efficiency in 2018 by boosting their surprisingly high open-circuit voltage (VOC) and photocurrent. The suppressed recombination in PSCs significantly cut down the voltage loss between the bandgap energy and VOC, which encouraged the VOC to reach closer to the bandgap. In addition, the photocurrent is considerably closer to the theoretical value at a given bandgap, leaving almost no room for further improvement. This remarkable development in the performance of PSCs is mainly ascribed to high-quality perovskite material being consistently tailored in the progress of technology. At the beginning of the progress, the morphology of the perovskite was a major target for improvement to enhance the crystal quality. The need for compositional engineering of the perovskite was raised in later stages of the progress by considering the benefits from different compositions of perovskites and their structural stability. Here we review the overall progress in perovskite materials from two perspectives: morphological progress and compositional progress.

Correction: Wafer-scale reliable switching memory based on 2-dimensional layered organic-inorganic halide perovskite.

Correction for 'Wafer-scale reliable switching memory based on 2-dimensional layered organic-inorganic halide perovskite' by Ja-Young Seo, et al., Nanoscale, 2017, DOI: 10.1039/c7nr05582j.

Wafer-scale reliable switching memory based on 2-dimensional layered organic-inorganic halide perovskite.

Recently, organic-inorganic halide perovskite (OHP) has been suggested as an alternative to oxides or chalcogenides in resistive switching memory devices due to low operating voltage, high ON/OFF ratio, and flexibility. The most studied OHP is 3-dimensional (3D) MAPbI3. However, MAPbI3 often exhibits less reliable switching behavior probably due to the uncontrollable random formation of conducting filaments. Here, we report the resistive switching property of 2-dimensional (2D) OHP and compare switching characteristics depending on structural dimensionality. The dimensionality is controlled by changing the composition of BA2MAn-1PbnI3n+1 (BA = butylammonium, MA = methylammonium), where 2D is formed from n = 1, and 3D is formed from n = ∞. Quasi 2D compositions with n = 2 and 3 are also compared. Transition from a high resistance state (HRS) to a low resistance state (LRS) occurs at 0.25 × 106 V m-1 for 2D BA2PbI4 film, which is lower than those for quasi 2D and 3D. Upon reducing the dimensionality from 3D to 2D, the ON/OFF ratio significantly increases from 102 to 107, which is mainly due to the decreased HRS current. A higher Schottky barrier and thermal activation energy are responsible for the low HRS current. We demonstrate for the first time reliable resistive switching from 4 inch wafer-scale BA2PbI4 thin film working at both room temperature and a high temperature of 87 °C, which strongly suggests that 2D OHP is a promising candidate for resistive switching memory.

Effect of Selective Contacts on the Thermal Stability of Perovskite Solar Cells.

Thermal stability of CH3NH3PbI3 (MAPbI3)-based perovskite solar cells was investigated for normal structure including the mesoporous TiO2 layer and spiro-MeOTAD and the inverted structure with PCBM and NiO. MAPbI3 was found to be intrinsically stable from 85 °C to 120 °C in the absence of moisture. However, fast degradation was observed for the encapsulated device including spiro-MeOTAD upon thermal stress at 85 °C. Photoluminescence (PL) intensity and the time constant for charge separation increased with thermal exposure time, which is indicative of inhibition of charge separation from MAPbI3 into spiro-MeOTAD. A full recovery of photovoltaic performance was observed for the 85 °C-aged device after renewal with fresh spiro-MeOTAD, which clearly indicates that thermal instability of the normal structured device is mainly due to spiro-MeOTAD, and MAPbI3 is proved to be thermally stable. Spiro-MeOTAD with additives was crystallized at 85 °C due to a low glass transition temperature, and hole mobility was significantly deteriorated, which was responsible for the thermal instability. Thermal stability was significantly improved for the inverted structure with the NiO hole transporting layer, where the power conversion efficiency (PCE) was maintained at 74% of its initial PCE of 14.71% after the 80th thermal cycle (one cycle: heating at 85 °C for 2 h and cooling at 25 °C for 2 h). This work implies that the thermal stability of perovskite solar cells depends on selective contacts.

Effect of Cs-Incorporated NiO x on the Performance of Perovskite Solar Cells.

The effect of Cs-incorporated NiO x on perovskite solar cells with an inverted structure was investigated, where NiO x and PCBM were used as selective contacts for holes and electrons, respectively. It was found that the generation of an Ni phase in an NiO x layer was significantly suppressed by employing cesium. Furthermore, Cs-incorporated NiO x enabled holes to be efficiently separated at the interface, showing the improved photoluminescent quenching and thus generating higher short-circuit current. The effect of Cs incorporation was also prominent in the inhibition of recombination. The recombination resistance of Cs-incorporated NiO x was noticeably increased by more than three-fold near the maximum power point, leading to a higher fill factor of 0.78 and consequently a higher power conversion efficiency of 17.2% for the device employing Cs-incorporated NiO x .

Material and Device Stability in Perovskite Solar Cells.

Organic-inorganic halide perovskite solar cells have attracted great attention because of their superb efficiency reaching 22 % and low-cost, facile fabrication processing. Nevertheless, stability issues in perovskite solar cells seem to block further advancements toward commercialization. Thus, device stability is one of the important topics in perovskite solar cell research. In the beginning, the poor moisture resistivity of the perovskite layer was considered as a main problem that hindered further development of perovskite solar cells, which encouraged engineering of the perovskite or protection of the perovskite by a buffer layer. Soon after, other parameters affecting long-term stability were sequentially found and various attempts have been made to enhance intrinsic and extrinsic stability. Here we review the recent progresses addressing stability issues in perovskite solar cells. In this report, we investigated factors affecting stability from material and device points of view. To gain a better understanding of the stability of the bulk perovskite material, decomposition mechanisms were investigated in relation to moisture, photons, and heat. Stability of full device should also be carefully examined because its stability is dependent not only on bulk perovskite but also on the interfaces and selective contacts. In addition, ion migration and current-voltage hysteresis were found to be closely related to stability.

Lewis Acid-Base Adduct Approach for High Efficiency Perovskite Solar Cells.

Since 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 applications, perovskite material has recently been applied to a variety fields of materials science such as photodetectors, light emitting diodes, lasing, X-ray imaging, resistive memory, and water splitting. Regardless of application areas, the growth of a well-defined perovskite layer with high crystallinity is essential for effective utilization of its excellent physicochemical properties. Therefore, an effective methodology for preparation of high quality perovskite layers is required. In this Account, an effective methodology for production of high quality perovskite layers is described, which is the Lewis acid-base adduct approach. In the solution process to form the perovskite layer, the key chemicals of CH3NH3I (or HC(NH2)2I) and PbI2 are used by dissolving them in polar aprotic solvents. Since polar aprotic solvents bear oxygen, sulfur, or nitrogen, they can act as a Lewis base. In addition, the main group compound PbI2 is known to be a Lewis acid. Thus, PbI2 has a chance to form an adduct by reacting with the Lewis base. Crystal growth and morphology of perovskite can be controlled by taking advantage of the weak chemical interaction in the adduct. We have successfully fabricated highly reproducible CH3NH3PbI3 perovskite solar cells with PCE as high as 19.7% via adducts of PbI2 with oxygen-donor N,N'-dimethyl sulfoxide. This adduct approach has been found to be generally adopted, where formamidinium lead iodide perovskite, HC(NH2)2PbI3 (FAPbI3), with large grain, high crystallinity, and long-lived carrier lifetime was successfully fabricated via an adduct of PbI2 with sulfur-donor thiourea as Lewis base. The adduct approach proposed in this Account is a very promising methodology to achieve high quality perovskite films with high photovoltaic performance. Furthermore, single crystal growth on the conductive substrate is expected to be possible if we kinetically control the elimination of Lewis base in the adduct.

Mesoscopic perovskite solar cells with an admixture of nanocrystalline TiO2 and Al2O3: role of interconnectivity of TiO2 in charge collection.

Perovskite solar cells with high power conversion efficiency usually employ mesoporous TiO2, however the role of the TiO2 layer has not been clearly resolved. Here we prepared MAPbI3 (MA = CH3NH3) perovskite solar cells with an admixture of nanocrystalline TiO2 and Al2O3 to investigate the role of the mesoporous TiO2 layer. The Al2O3 content was varied from 0% (pure TiO2) to 100% (pure Al2O3) with nominal composition of (1 - x)TiO2 + xAl2O3 (x = 0, 0.25, 0.5, 0.75 and 1). The photocurrent density and fill factor decreased as Al2O3 content increased, whereas the open-circuit voltage was hardly changed. Steady-state photoluminescence (PL) was less quenched as the Al2O3 content increased due to its non-electron-injecting characteristics, where a decrease in PL intensity with increasing TiO2 content was correlated to an increase in photocurrent. Electron injection to TiO2 was also evidenced by time-resolved PL and time-limited photocurrent measurements, where interconnection of TiO2 particles played an important role in charge collection. The slight change in voltage with Al2O3 content was explained by balancing the Fermi position due to a trade-off between charge recombination and the Fermi level. The results observed from the admixture mesoporous layer comprising electron-injecting and electron-non-injecting oxides suggest that electron-injection characteristics play an important role in determining photovoltaic parameters.

Real-Space Imaging of the Atomic Structure of Organic-Inorganic Perovskite.

Organic-inorganic perovskite is a promising class of materials for photovoltaic applications and light emitting diodes. However, so far commercialization is still impeded by several drawbacks. Atomic-scale effects have been suggested to be possible causes, but an unequivocal experimental view at the atomic level is missing. Here, we present a low-temperature scanning tunneling microscopy study of single crystal methylammonium lead bromide CH3NH3PbBr3. Topographic images of the in situ cleaved perovskite surface reveal the real-space atomic structure. Compared to the bulk we observe modified arrangements of atoms and molecules on the surface. With the support of density functional theory we explain these by surface reconstruction and a substantial interplay of the orientation of the polar organic cations (CH3NH3)(+) with the position of the hosting anions. This leads to structurally and electronically distinct domains with ferroelectric and antiferroelectric character. We further demonstrate local probing of defects, which may also impact device performance.

Control of I-V hysteresis in CH3NH3PbI3 perovskite solar cell.

Mismatch of current (I)-voltage (V) curves with respect to the scan direction, so-called I-V hysteresis, raises critical issue in MAPbI3 (MA = CH3NH3) perovskite solar cell. Although ferroelectric and ion migration have been proposed as a basis for the hysteresis, origin of hysteresis has not been apparently unraveled. We report here on the origin of I-V hysteresis of perovskite solar cell that was systematically evaluated by the interface-dependent electrode polarizations. Frequency (f)-dependent capacitance (C) revealed that the normal planar structure with the TiO2/MAPbI3/spiro-MeOTAD configuration showed most significant I-V hysteresis along with highest capacitance (10(-2) F/cm(2)) among the studied cell configurations. Substantial reduction in capacitance to 10(-3) F/cm(2) was observed upon replacing TiO2 with PCBM, indicative of the TiO2 layer being mainly responsible for the hysteresis. The capacitance was intensively reduced to 10(-5) F/cm(2) and C-f feature shifted to higher frequency for the hysteresis-free planar structures with combination of PEDOT: PSS, NiO, and PCBM, which underlines the spiro-MeOTAD in part contributes to the hysteresis. This work is expected to provide a key to the solution of the problem on I-V hysteresis in perovskite solar cell.

Ferroelectric Polarization in CH3NH3PbI3 Perovskite.

We report on ferroelectric polarization behavior in CH3NH3PbI3 perovskite in the dark and under illumination. Perovskite crystals with three different sizes of 700, 400, and 100 nm were prepared for piezoresponse force microscopy (PFM) measurements. PFM results confirmed the formation of spontaneous polarization in CH3NH3PbI3 in the absence of electric field, where the size dependency to polarization was not significant. Whereas the photoinduced stimulation was not significant without an external electric field, the stimulated polarization by poling was further enhanced under illumination. The retention of ferroelectric polarization was also observed after removal of the electric field, in which larger crystals showed longer retention behavior compared to the smaller sized one. Additionally, we suggest the effect of perovskite crystal size (morphology) on charge collection at the interface of the ferroelectric material even though insignificant size dependency in electric polarization was observed.

Evaluation of limiting factors affecting photovoltaic performance of low-temperature-processed TiO2 films in dye-sensitized solar cells.

Limiting factors affecting photovoltaic performance of dye-sensitized solar cell employing low-temperature-processed TiO2 films were investigated. TiO2 films were prepared at a low temperature of 200 °C using the normal alcohol-containing binder-free TiO2 paste (LT200). Their photovoltaic performance was compared to a high-temperature (550 °C) annealed TiO2 film prepared using a polymer binder containing TiO2 paste (HT550). Compared to the proportional increase in conversion efficiency with TiO2 film thickness upto 14 µm for HT550, the increase in efficiency was terminated at relatively smaller thickness of about 8 µm for LT200 mainly due to unaugmented photocurrent. From the transient photocurrent-voltage studies, the electron transport rate was found to be almost identical, while charge recombination was one order of magnitude faster for LT200. Consequently, the electron diffusion length was more than 2-3 times shorter for LT200 than for HT550. Electron diffusion length and electron life time obtained from electrochemical impedance analysis were well consistent with those observed from transient measurement. Density of states (DOS) was evaluated to be shallow and narrow in LT200, which was responsible for limiting photovoltaic performance in the low-temperature processed TiO2 film.

Parameters Affecting I-V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer.

Current-voltage (I-V) characteristics of CH3NH3PbI3 perovskite solar cells are studied using a time-dependent current response with stepwise sweeping of the bias voltage. Compared with the crystalline Si solar cell showing time-independent current at a given bias voltage, the perovskite solar cells exhibit time-dependent current response. The current increases with time and becomes steady at forward scan from short-circuit to open-circuit, whereas it is decayed and saturated with time at reverse scan from open-circuit to short-circuit. Time-dependent current response eventually leads to I-V hysteresis depending on the scan direction and the scan rate. Crystal size of CH3NH3PbI3 and the mesoporous TiO2 (mp-TiO2) film are found to influence I-V hysteresis, where the I-V hysteresis is alleviated as crystal size increases and in the presence of mp-TiO2. The capacitance observed at low frequency (0.1 to 1 Hz), associated with dipole polarization, tends to diminish as size of perovskite and mp-TiO2 layer thickness increases, which suggests that the origin of hysteresis correlates to the capacitive characteristic of CH3NH3PbI3 and the degree of hysteresis depends strongly on perovskite crystal size and mesoporous TiO2 layer.