Hysteresis In Perovskite Solar Cells Research Articles

Surface modification via self-assembling large cations for improved performance and modulated hysteresis of perovskite solar cells

Novel surface modification of self-assembling large cations enables the achievement of high quality perovskite films for hysteresis-free and stable solar cells with an optimized efficiency over 20%.

Tailoring Crystal Structure of FA0.83Cs0.17PbI3 Perovskite Through Guanidinium Doping for Enhanced Performance and Tunable Hysteresis of Planar Perovskite Solar Cells

AbstractCurrent–voltage hysteresis of perovskite solar cells (PSCs) has raised the concern of accurate performance measurement in practice. Although various theories have been proposed to elucidate this phenomenon, the origin of hysteresis is still an open question. Herein, the use of guanidinium cation (Gu+)‐dopant is demonstrated to tailor the crystal structure of mixed‐cation formamidinium‐cesium lead triiodide (FA0.83Cs0.17PbI3) perovskite, resulting in an improved energy conversion efficiency and tunable current–voltage hysteresis characteristic in planar solar cells. Particularly, when the concentration of Gu‐dopant for the perovskite film increases, the normal hysteresis initially observed in the pristine PSC is first suppressed with 2%‐Gu‐dopant, then changed to inverted hysteresis with a higher Gu‐dopant. The hysteresis tunability behavior is attributed to the interplay of charge/ion accumulation and recombination at interfaces in the PSC. Furthermore, compared to the cell without Gu+‐dopant, the optimal content of 2% Gu+‐dopant also increases the device efficiency by 14%, reaching over 17% under one sun illumination.

Hysteresis Index: A Figure without Merit for Quantifying Hysteresis in Perovskite Solar Cells

Hysteresis Index: A Figure without Merit for Quantifying Hysteresis in Perovskite Solar Cells

Resolving Hysteresis in Perovskite Solar Cells with Rapid Flame‐Processed Cobalt‐Doped TiO2

AbstractTo further increase the open‐circuit voltage (V oc) of perovskite solar cells (PSCs), many efforts have been devoted to doping the TiO2 electron transport/selective layers by using metal dopants with higher electronegativity than Ti. However, those dopants can introduce undesired charge traps that hinder charge transport through TiO2, so the improvement in the V oc is often accompanied by an undesired photocurrent density–voltage (J–V) hysteresis problem. Herein, it is demonstrated that the use of a rapid flame doping process (40 s) to introduce cobalt dopant into TiO2 not only solves the J–V hysteresis problem but also increases the V oc and power conversion efficiency of both mesoscopic and planar PSCs. The reasons for the simultaneous improvements are two fold. First, the flame‐doped Co‐TiO2 film forms Co‐Ov (cobalt dopant‐oxygen vacancy) pairs and hence reduces the number density of Ti3+ trap states. Second, Co doping upshifts the band structure of TiO2, facilitating efficient charge extraction. As a result, for planar PSCs, the flame doping of Co increases the efficiency from 17.1% to 18.0% while reducing the hysteresis from 16.0% to 1.7%. Similarly, for mesoscopic PSCs, the flame doping of Co increases the efficiency from 18.5% to 20.0% while reducing the hysteresis from 7.0% to 0.1%.

Bias-Dependent Normal and Inverted J- V Hysteresis in Perovskite Solar Cells.

Perovskite solar cells (PSCs) typically exhibit hysteresis in current density-voltage ( J- V) measurements. The most common type of J- V hysteresis in PSCs is normal hysteresis, in which the performance in the reverse scan is better than that in the forward scan. However, inverted hysteresis also exists, in which the reverse scan performance is worse than in the forward scan; this hysteresis, however, is significantly less well studied. In this work, we show that the hysteresis decreases when the sweep rate is decreased only in cases involving a small bias range, and it does not decrease with a large bias range. Under large forward bias and slowing sweep rate, we observe enhanced normal hysteresis or inverted hysteresis in PSCs. Moreover, the degree of normal and inverted hysteresis can be adjusted by varying the bias. Here, we hypothesize that the tunable hysteresis is derived from the different distribution of ionic defects (VI and VMA) at the electron (hole) transport layer/perovskite interface due to ionic movement in the perovskite layer under the different bias scanning conditions. This conclusion is confirmed using Kelvin probe force microscopy with different bias voltages and scanning rates, which shows surface potential hysteresis based on ionic-migration-related Fermi level shifting in perovskite films and agrees with the tunable J- V hysteresis hypothesis. Moreover, the increased time response in the milliseconds region in open-circuit voltage decay after J- V scanning further corroborates the mechanism of ionic migration under bias. Our work provides new insights into the ionic movement hypothesis for the J- V hysteresis in PSCs.

Ion-Migration Inhibition by the Cation-π Interaction in Perovskite Materials for Efficient and Stable Perovskite Solar Cells.

Migration of ions can lead to photoinduced phase separation, degradation, and current-voltage hysteresis in perovskite solar cells (PSCs), and has become a serious drawback for the organic-inorganic hybrid perovskite materials (OIPs). Here, the inhibition of ion migration is realized by the supramolecular cation-π interaction between aromatic rubrene and organic cations in OIPs. The energy of the cation-π interaction between rubrene and perovskite is found to be as strong as 1.5 eV, which is enough to immobilize the organic cations in OIPs; this will thus will lead to the obvious reduction of defects in perovskite films and outstanding stability in devices. By employing the cation-immobilized OIPs to fabricate perovskite solar cells (PSCs), a champion efficiency of 20.86% and certified efficiency of 20.80% with negligible hysteresis are acquired. In addition, the long-term stability of cation-immobilized PSCs is improved definitely (98% of the initial efficiency after 720 h operation), which is assigned to the inhibition of ionic diffusions in cation-immobilized OIPs. This cation-π interaction between cations and the supramolecular π system enhances the stability and the performance of PSCs efficiently and would be a potential universal approach to get the more stable perovskite devices.

Rationally Induced Interfacial Dipole in Planar Heterojunction Perovskite Solar Cells for Reduced J–V Hysteresis

AbstractWith the rapid progress in developing hybrid perovskite solar cells, the allure of current density–voltage ( J–V) hysteresis has attracted quite a lot of interest in the research community. It requires feasible approaches that further deepen the fundamental understanding of device physics in specific device architecture in order to solve this problem eventually. Here, perovskite solar cells configured with different counter electrodes are systematically investigated with the focus on charge accumulation within the devices responsible for J–V hysteresis. The results indicate that J–V hysteresis is affected by charge accumulation which can be modulated by carrier extraction efficiency of the electrodes. Through a rationally induced interfacial dipole, the devices have shown improvement in carrier extraction, which thus reduces J–V hysteresis significantly. It provides solid evidence for the proposition that interface charge plays an important role in J–V hysteresis, and demonstrates an applicable strategy that effectively alleviates J–V hysteresis in perovskite solar cells.

Switching Off Hysteresis in Perovskite Solar Cells by Fine‐Tuning Energy Levels of Extraction Layers

AbstractLead halide perovskites often suffer from a strong hysteretic behavior on their j–V response in photovoltaic devices that has been correlated with slow ion migration. The electron extraction layer has frequently been pointed to as the main culprit for the observed hysteretic behavior. In this work three hole transport layers are studied with well‐defined highest occupied molecular orbital (HOMO) levels and interestingly the hysteretic behavior is markedly different. Here it is shown that an adequate energy level alignment between the HOMO level of the extraction layer and the valence band of the perovskite, not only suppresses the hysteresis, avoiding charge accumulation at the interfaces, but also degradation of the hole transport layer is reduced. Numerical simulation suggests that formation of an injection barrier at the organic/perovskite heterointerface could be one mechanism causing hysteresis. The suppression of such barriers may require novel design rules for interface materials. Overall, this work highlights that both external contacts need to be carefully optimized in order to obtain hysteresis‐free perovskite devices.

Eliminating J-V hysteresis in perovskite solar cells via defect controlling

Organometal halide perovskite solar cell enjoys a rapid development in the last decade. However, the severe J-V hysteresis existed in perovskite solar cells leads to an over- or under-estimation of device efficiency. Herein, we introduce an ammonium salt as a new dopant of perovskite. As a result, the J-V hysteresis is almost eliminated, and a higher average efficiency is obtained. Besides, the degradation under constant illumination in ambient atmosphere observed in control devices is greatly improved. Thin film characterizations reveal that the addition of ammonium salt leads to smoother, more homogenous and better crystalline perovskite films, which exhibit more intense photoluminescence and a little longer fluorescent lifetime. Such improvements are owing to the dual-functional sulfoxide (SO) in the dopant that can assist in perovskite crystallization and passivate the surface defects, thus giving rise to the eliminated J-V hysteresis. Our results provide an easy way to eliminate the J-V hysteresis in perovskite solar cell.

Influence of Fermi Level Alignment with Tin Oxide on the Hysteresis of Perovskite Solar Cells.

We tune the Fermi level alignment between the SnO x electron transport layer (ETL) and Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 and highlight that this parameter is interlinked with current-voltage hysteresis in perovskite solar cells (PSCs). Furthermore, thermally stimulated current measurements reveal that the depth of trap states in the ETL or at the ETL-perovskite interface correlates with Fermi level positions, ultimately linking it to the energy difference between the Fermi level and conduction band minimum. In the presence of deep trap states, charge accumulation and recombination at the interface are promoted, affecting the charge collection efficiency adversely, which increases the hysteresis of PSCs.

Influence of Charge Transport Layers on Open-Circuit Voltage and Hysteresis in Perovskite Solar Cells

Summary Perovskite materials have experienced an impressive improvement in photovoltaic performance due to their unique combination of optoelectronic properties. Their remarkable progression, facilitated by the use of different device architectures, compositional engineering, and processing methodologies, contrasts with the lack of understanding of the materials properties and interface phenomena. Here we directly target the interplay between the charge-transporting layers (CTLs) and open-circuit potential ( V OC ) in the operation mechanism of the state-of-the-art CH 3 NH 3 PbI 3 solar cells. Our results suggest that the V OC is controlled by the splitting of quasi-Fermi levels and recombination inside the perovskite, rather than being governed by any internal electric field established by the difference in the CTL work functions. In addition, we provide novel insights into the hysteretic origin in perovskite solar cells, identifying the nature of the contacts as a critical factor in defining the charge accumulation at its interface, leading to either ionic, electronic, or mixed ionic-electronic accumulation.

Post-treatment of perovskite film with phenylalkylammonium iodide for hysteresis-less perovskite solar cells

Superb solar-to-electricity conversion property of organic-inorganic halide perovskite has recently received considerable attention. Although the power conversion efficiency (PCE) reaches over 22%, severe current-voltage hysteresis in perovskite solar cells has been issues. To solve the hysteresis, we report here on interfacial engineering via post-treatment of perovskite films with phenylalkylammonium iodide. The 145 °C-annealed FA.9Cs.1PbI2.9Br.1 perovskite films are post-treated with series of phenylalkylammonium iodides with chemical formula of (C6H5)CnH2n(NH3I), where effects of concentration and solvent on photovoltaic performance and hysteresis are systematically investigated. For n = 2 phenylethylammonium iodide (PEAI), hysteresis is reduced by PEAI treatment but there is optimal concentration to minimize the hysteresis. Average PCE, estimated from reverse and forward scanned data, of 15.94% before PEAI treatment is improved to 17.62% after 4 mM PEAI treatment mainly due to increase in average FF from 0.696 to 0.751. At higher concentration of 10 mM, average PCE is declined again to 15.60% due to the large difference in FF between reverse and forward scan. Post-treatment with PEAI increases carrier life time from 500 ns (pristine) to 795 ns, which is responsible for the reduced hysteresis. Longer alkyl chain such as PEAI (n = 2) is found to be better to reduce the hysteresis than shorter one like PAI (n = 0) because of longer carrier life time, which indicates that dipole moment of phenylalkylammonium iodide is involved in time-dependent charge extraction.

Amide-Catalyzed Phase-Selective Crystallization Reduces Defect Density in Wide-Bandgap Perovskites.

Wide-bandgap (WBG) formamidinium-cesium (FA-Cs) lead iodide-bromide mixed perovskites are promising materials for front cells well-matched with crystalline silicon to form tandem solar cells. They offer avenues to augment the performance of widely deployed commercial solar cells. However, phase instability, high open-circuit voltage (Voc ) deficit, and large hysteresis limit this otherwise promising technology. Here, by controlling the crystallization of FA-Cs WBG perovskite with the aid of a formamide cosolvent, light-induced phase segregation and hysteresis in perovskite solar cells are suppressed. The highly polar solvent additive formamide induces direct formation of the black perovskite phase, bypassing the yellow phases, thereby reducing the density of defects in films. As a result, the optimized WBG perovskite solar cells (PSCs) (Eg ≈ 1.75 eV) exhibit a high Voc of 1.23 V, reduced hysteresis, and a power conversion efficiency (PCE) of 17.8%. A PCE of 15.2% on 1.1 cm2 solar cells, the highest among the reported efficiencies for large-area PSCs having this bandgap is also demonstrated. These perovskites show excellent phase stability and thermal stability, as well as long-term air stability. They maintain ≈95% of their initial PCE after 1300 h of storage in dry air without encapsulation.

Preventing Hysteresis in Perovskite Solar Cells by Undoped Charge Blocking Layers

Preventing hysteresis in lead halide perovskite solar cells remains one of the key challenges hindering their integration into industrial applications. Herein, we numerically study a model solar cell system that is based on a mixed electron–ion conducting perovskite active layer and vary the configuration of undoped charge-blocking layers within the device. We find that the use of undoped blocking layers significantly reduces the potential drop across the perovskite active layer. This redistribution of voltage across the device suppresses the ion accumulation, or deficiency, which would otherwise develop at the two ends of the active layer. The fill factor is not compromised, provided that the blocking layers’ mobility value is not lower than 0.01 cm2 V–1 s–1, which results in devices with power-conversion efficiencies surpassing 20% and minimal hysteresis. We believe that this method not only can suppress hysteresis effectively but could also contribute to the long-term stability of such cells that have ...

Evidence of Tailoring the Interfacial Chemical Composition in Normal Structure Hybrid Organohalide Perovskites by a Self-Assembled Monolayer.

Current-voltage hysteresis is a major issue for normal architecture organo-halide perovskite solar cells. In this manuscript we reveal a several-angstrom thick methylammonium iodide-rich interface between the perovskite and the metal oxide. Surface functionalization via self-assembled monolayers allowed us to control the composition of the interface monolayer from Pb poor to Pb rich, which, in parallel, suppresses hysteresis in perovskite solar cells. The bulk of the perovskite films is not affected by the interface engineering and remains highly crystalline in the surface-normal direction over the whole film thickness. The subnanometer structural modifications of the buried interface were revealed by X-ray reflectivity, which is most sensitive to monitor changes in the mass density of only several-angstrom thin interfacial layers as a function of substrate functionalization. From Kelvin probe force microscopy study on a solar cell cross section, we further demonstrate local variations of the potential on different electron-transporting layers within a solar cell. On the basis of these findings, we present a unifying model explaining hysteresis in perovskite solar cells, giving an insight into one crucial aspect of hysteresis for the first time and paving way for new strategies in the field of perovskite-based opto-electronic devices.

Universal Approach toward Hysteresis-Free Perovskite Solar Cell via Defect Engineering.

Organic-inorganic halide perovskite is believed to be a potential candidate for high efficiency solar cells because power conversion efficiency (PCE) was certified to be more than 22%. Nevertheless, mismatch of PCE due to current density (J)-voltage (V) hysteresis in perovskite solar cells is an obstacle to overcome. There has been much lively debate on the origin of J-V hysteresis; however, effective methodology to solve the hysteric problem has not been developed. Here we report a universal approach for hysteresis-free perovskite solar cells via defect engineering. A severe hysteresis observed from the normal mesoscopic structure employing TiO2 and spiro-MeOTAD is almost removed or does not exist upon doping the pure perovskites, CH3NH3PbI3 and HC(NH2)2PbI3, and the mixed cation/anion perovskites, FA0.85MA0.15PbI2.55Br0.45 and FA0.85MA0.1Cs0.05PbI2.7Br0.3, with potassium iodide. Substantial reductions in low-frequency capacitance and bulk trap density are measured from the KI-doped perovskite, which is indicative of trap-hysteresis correlation. A series of experiments with alkali metal iodides of LiI, NaI, KI, RbI and CsI reveals that potassium ion is the right element for hysteresis-free perovskite. Theoretical studies suggest that the atomistic origin of the hysteresis of perovskite solar cells is not the migration of iodide vacancy but results from the formation of iodide Frenkel defect. Potassium ion is able to prevent the formation of Frenkel defect since K+ energetically prefers the interstitial site. A complete removal of hysteresis is more pronounced at mixed perovskite system as compared to pure perovskites, which is explained by lower formation energy of K interstitial (-0.65 V for CH3NH3PbI3 vs -1.17 V for mixed perovskite). The developed KI doping methodology is universally adapted for hysteresis-free perovskite regardless of perovskite composition and device structure.

Hysteresis in Hole-free Perovskite Solar Cells with Carbon Counter Electrode Doped Reduced Graphene Oxide (RGO)

Perovskite solar cells achieved high efficiencies but they suffer from the hysteresis of (J-V) curve that depends on the scan direction. The architecture introduced in this paper is hole –free cell with Reduced Graphene Oxide (RGO) as a dopant. The good electrical and optical properties of (RGO) make it acts as a hole transfer layer (HTL) beside its basic role as a dopant of carbon counter electrode. For different areas, the hysteresis decreases by decreasing the cell area. The TiO2/CH3NH3PbI3-xClx interface is truly responsible for the hysteresis in the Perovskite solar cells due to the dependence of the porosity from area to other.

How the formation of interfacial charge causes hysteresis in perovskite solar cells

In this study, we use time-resolved Kelvin probe force microscopy to investigate current–voltage hysteresis in a hybrid lead-halide perovskite solar cell.

Restrained light-soaking and reduced hysteresis in perovskite solar cells employing a helical perylene diimide interfacial layer

An n-type helical molecule perylene diimide (PDI2) has been explored as an efficient interfacial layer between TiO2 and perovskite, which can restrain the light-soaking instability and reduce hysteresis in perovskite solar cells. A champion device PCE of 19.84% has been achieved after incorporating the PDI2 interlayer.

The Influence of the Relaxation Time on the Dynamic Hysteresis in Perovskite Solar Cells

We investigate the dynamic behavior of perovskite solar cells by focusing on the relaxation time τ, which corresponds to the slow de-polarization process from an initial bias pre-poled state. The dynamic electrical model (DEM) is employed for simulating the J-V characteristics for different bias scan rates and pre-poling conditions. Depending on the sign of the initial polarization normal or inverted hysteresis may be induced. For fixed pre-poling conditions, the relaxation time, in relation to the bias scan rate, governs the magnitude of the dynamic hysteresis. In the limit of large τ the hysteretic effects are vanishing for the typical range of bias scan rates considered, while for very small τ the hysteresis is significant only when it is comparable with the measurement time interval.