Hysteresis-free Perovskite Solar Cells Research Articles

Transport Layer Engineering by Hydrochloric Acid for Efficient Perovskite Solar Cells with a High Open-Circuit Voltage

A large number of defect states that exist at the interface between a perovskite film and an electron transport layer (ETL) are detrimental to the efficiency and the stability of perovskite solar cells (PSCs). It is still a challenge to simultaneously passivate the defects on both sides by a stable and low-cost ion compound. Herein, we demonstrate a simple and effective versatile strategy by introducing hydrochloric acid into SnO2 precursor solution to passivate the defects in both SnO2 and perovskite layers and simultaneously reduce the interface energy barrier, ultimately achieving a high-performance and hysteresis-free PSCs. Hydrogen ions can neutralize -OH groups on the SnO2 surface, whereas the Cl- can not only combine with Sn4+ in ETL but also suppress the Pb-I antisite defects formed at the buried interface. The reduced nonradiative recombination and the favorable energy level alignment result in a significantly increased efficiency from 20.71 to 22.06% of PSCs due to the enhancement of open-circuit voltage. In addition, the stability of the device can also be improved. This work presents a facile and promising approach for the development of highly efficient PSCs.

(Invited) Novel Pyridine-functionalized Fullerene Derivatives toward the Efficient and Hysteresis-free Perovskite Solar Cells

Fullerene derivatives with strong electron-accepting abilities have been extensively used as electron transport layers (ETLs) of inverted (p–i–n) planar heterojunction perovskite solar cells (iPSCs); however the effect of the alkyl chains and a specific heteroatom site within the fullerene derivative on its electron transport properties has scarcely been studied. Herein,by employing pyridine carboxaldehydes bearing nitrogen sites within the pyridine moiety as a reactant, a series of novel pyridine-functionalized fullerene derivatives with variable nitrogen sites and alkyl chains length were synthesized facilely via a one-step 1, 3-dipolar cycloaddition reaction, and their molecular structures as well as molecular packing were unambiguously determined by X-ray single crystal diffraction. Despite that fullerene derivatives exhibit negligible difference in the molecular energy level and optical absorption, however, their electron transport properties are found to sensitively depend on the length of alkyl chains and nitrogen sites within the pyridine moiety. The C60-3-Py-based device shows the highest power conversion efficiency (PCE) of 17.57%. The discrepancy of the electron transport properties of fullerene derivatives is primarily due to the difference in the coordination interaction between the pyridine moiety and the Pb2+ ion of the MAPbI3 perovskite layer interaction between the pyridine moiety and the Pb2+ ion of the MAPbI3 perovskite layer.

Dual-Functional Additive to Simultaneously Modify the Interface and Grain Boundary for Highly Efficient and Hysteresis-Free Perovskite Solar Cells.

The efficiency loss and stability issues of perovskite devices mainly derive from nonradiative recombination, caused by detrimental defects in the perovskite bulk and at the interface between the perovskite absorber and charge transport layer. Therefore, the passivation of these defects is of great concern in achieving high-performance perovskite devices. Here, we report the incorporation of potassium phenyl trifluoroborate (KC6H5BF3) into perovskite films to realize simultaneous passivation of the grain boundaries and the perovskite/SnO2 interface. Apart from the bulk passivation of K+, the accumulation of C6H5BF3- at the buried interface contributes to the compact contact between the perovskite absorber and SnO2 layer and also the perfect columnar perovskite grains. As a result, the KC6H5BF3-containing perovskite films exhibit low trap density. The distinct enhancements of open-circuit voltage and photoelectric conversion efficiency are obtained together with negligible hysteresis. The open-circuit voltage of the KC6H5BF3-containing device increases from 1.09 to 1.18 V, and the corresponding efficiency increases from 19.69 to 22.33%. The finding in this work shows the superiority of the dual-functional additive for preparing highly efficient perovskite devices.

Hysteresis-free perovskite solar cells with compact and nanoparticle NiO for indoor application

The perovskite solar cell technology is showing rapid progress in delivering high power conversion efficiency. This is not limited to conventional 1 SUN illumination, but state-of-the-art results have also been also demonstrated for a low-light condition such as indoor illumination. In this work, we show that very efficient low-light perovskite solar cells can be realized in the inverted configuration with the use of NiO as a hole transporting material by employing a conventional high-temperature process (300 °C, compact NiO) or a low-temperature process (<100 °C) based on nanoparticle NiO. The power density of 90.2 μW/cm2 at 1000 lux was achieved for compact NiO device, while 71.6 μW/cm2 was obtained with nanoparticle NiO cells under the same illumination. A detailed electrical, optical, and morphological characterization permitted to identify the reasons for the difference between compact and nanoparticle NiO. Beside the difference in power density, nanoparticle NiO permitted to realize the first low temperature inorganic hole transporting layer for low-light p-i-n perovskite solar cell and, as we show, this could be easily scaled up to 1 cm2 without suffering for performance losses moving from small to large area.

Exploring Light Polarization Effects of Photovoltaic Actions in Organic-Inorganic Hybrid Perovskites with Asymmetric and Symmetric Unit Structures.

Hybrid perovskites are known as electrically polarizable semiconductors based on theoretical prediction and experimental information. Here we show the light polarization effects on the photovoltaic actions in perovskite solar cells by optically generating directional and random electronic transition dipoles within asymmetric and symmetric unit structures. In an asymmetric tetragonal unit structure, we observed the stripes with different orientations onto perovskite grains and that the randomly polarized photoexcitation can generate a higher photocurrent (Jsc) by 6.5 ± 0.5% as compared to the linearly polarized photoexcitation at same intensity in the hysteresis-free perovskite solar cells [ITO/PEDOT:PSS/MAPbI3/PC61BM/PEI/Ag]. Clearly, switching the photoexcitation between linear and random polarizations leads to a ΔJsc, which provides an experimental indication that all-directional and one-directional transition dipoles generate higher and lower photocurrents in organic-inorganic hybrid perovskites (MAPbI3) with an asymmetric tetragonal unit structure. This implies that all-directional and one-directional transition dipoles develop stronger and weaker dissociative interactions, consequently giving rise to more and less dissociation toward generating photocurrent. This is confirmed by the experimental observation that the ΔJsc almost disappears when the temperature increases up to 55 °C, where the asymmetric tetragonal structure is changed to a symmetric cubic structure. Furthermore, the ΔJsc is shown to decrease with increasing light intensity. This indicates that the electronic transition dipoles encounter a polarization relaxation caused by mutual interaction. We show that the polarization relaxation time in MAPbI3 is comparable to exciton dissociation time (∼ps). This presents the necessary condition to demonstrate light polarization effects of photovoltaic actions in perovskite solar cells.

P-Doping of a Hole Transport Material via a Poly(ionic liquid) for over 20% Efficiency and Hysteresis-Free Perovskite Solar Cells

An efficient metal-free formulation of a hole transport material (HTM) based on an ionic liquid polymer is developed for n–i–p perovskite solar cells (PSCs), to address reproducibility issues relat...

Elimination of Light-Soaking Effect in Hysteresis-Free Perovskite Solar Cells by Interfacial Modification

The hysteresis and light-soaking effect have been observed in organo–metal halide perovskite solar cells (PSCs) under operating conditions, which inhibit the precise evaluation of power output. The mechanisms leading to these effects are little understood. Here, our studies have demonstrated that the light-soaking effect is related to the electron selective layer/perovskite interface in hysteresis-free PSCs. The introduction of [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) doped with CH3NH3I molecules as the interfacial layer can effectively release or eliminate this effect due to the efficient charge transfer, accompanied with the best stable output efficiency of 19% and an ultrahigh fill factor over 84%. Capacitance–voltage and capacitance–frequency curves derived from electrochemical impedance spectroscopy demonstrate that the light-soaking effect in PSCs mainly originates from the charge accumulation at the PC61BM/perovskite interface. The CH3NH3I doped in PC61BM forestalls the ion movement amon...

Numerical simulation of charge transport layer free perovskite solar cell using metal work function shifted contacts

Perovskite solar cells (PSCs) are one of the fastest emerging photovoltaic (PV) technology at the research level. To achieve higher conversion efficiencies from PSCs, a perovskite absorber layer is stacked between two charge transport layers (CTLs) such as electron and hole transport layers. However, fabrication of defect-free multi-layered PSC is a challenging task, and the presence of CTL and their corresponding interfaces with perovskite enhances the recombination, hysteresis and led to poor stability. Here, in this work, CTL free (i.e., electron and hole transport layer free) PSC is simulated using metal work function shifted contacts. The device presented in this work is free from transport layers and the collection process is with the help of an electric field across the perovskite layer. The electric field is created by using two metals of different work function, i.e., 4.35 eV and 5.25 eV (can be realized using self-assembled monolayers technique) used as cathode and anode respectively. Simulated CTL free PSC exhibits JSC = 17.8 mA cm−2, VOC = 712 mV, FF = 68.5% and PCE = 8.7% with 250 nm thick perovskite absorber layer having bulk defect density of 2.5 × 1013 cm−3. Further, a comprehensive study is done in terms of front electrode work function (FEW), front electrode transparency, perovskite thickness and bulk defect density to understand the impact of these parameters on the performance of the device. To understand the behavior of the device, the energy band diagram profile is examined. Reported results show that higher metal work function difference between front and back electrode, higher transparency, and thick perovskite layer with low defect density results in better PV effect in CTL free PSC. Optimized CTL free PSC device delivers JSC = 19.9 mA cm−2, VOC = 726 mV, FF = 66.8% and PCE = 9.7%. The design simulated in this work opens up a new window for next-generation interface defect and hysteresis-free PSC.

Hysteresis-Free Planar Perovskite Solar Cells with a Breakthrough Efficiency of 22% and Superior Operational Stability over 2000 h.

Understanding the transport loss and the ways to improving optoelectronic properties of the charge transporting layers is critical to fabricate highly efficient, long-term stable, and hysteresis-free perovskite solar cells (PSCs). Herein, we report success in suppressing hysteresis and boosting the performance of operationally stable planar solar cells using a ruthenium (Ru) doped tin oxide (SnO2) electron transport layer (ETL) and Zn-TFSI2 doped spiro-OMeTAD hole transport layer (HTL). Apparently, the incorporation of Ru drastically shifts the Fermi level of SnO2 ETL upward, which provides a facile route to tailor the ETL/perovskite band-offset to improve built-in electric fields of devices for improving VOC and electron extraction simultaneously. Meanwhile, rapid injection of the photogenerated electrons from perovskite into ETL with reduced trap density is also observed when Ru doped SnO2 is employed as ETL. On the other hand, the conception of Zn-TFSI2 incorporation into HTL not only further boosts the photovoltaic performance but also prolongs the photostability of the devices. Consequently, a breakthrough efficiency of 22% (average 21.8%) with a JSC of 24.6 mA cm-2, VOC of 1.15 V, and FF of 0.78 has been obtained in planar-type PSCs with a loss in efficiency of only ∼3% at maximum power point tracking over 2000 h.

Enhanced Interfacial Binding and Electron Extraction Using Boron-Doped TiO2 for Highly Efficient Hysteresis-Free Perovskite Solar Cells.

Perovskite solar cells (PSCs) have witnessed astonishing improvement in power conversion efficiency (PCE), more recently, with advances in long‐term stability and scalable fabrication. However, the presence of an anomalous hysteresis behavior in the current density–voltage characteristic of these devices remains a key obstacle on the road to commercialization. Herein, sol–gel‐processed mesoporous boron‐doped TiO2 (B‐TiO2) is demonstrated as an improved electron transport layer (ETL) for PSCs for the reduction of hysteresis. The incorporation of boron dopant in TiO2 ETL not only reduces the hysteresis behavior but also improves PCE of the perovskite device. The simultaneous improvements are mainly ascribed to the following two reasons. First, the substitution of under‐coordinated titanium atom by boron species effectively passivates oxygen vacancy defects in the TiO2 ETL, leading to increased electron mobility and conductivity, thereby greatly facilitating electron transport. Second, the boron dopant upshifts the conduction band edge of TiO2, resulting in more efficient electron extraction with suppressed charge recombination. Consequently, a methylammonium lead iodide (MAPbI3) photovoltaic device based on B‐TiO2 ETL achieves a higher efficiency of 20.51% than the 19.06% of the pure TiO2 ETL based device, and the hysteresis is reduced from 0.13% to 0.01% with the B‐TiO2 based device showing negligible hysteresis behavior.

On the Current-Voltage Hysteresis in Perovskite Solar Cells: Dependence on Perovskite Composition and Methods to Remove Hysteresis.

Current-density-voltage (J-V) hysteresis in perovskite solar cells (PSCs) is a critical issue because it is related to power conversion efficiency and stability. Although parameters affecting the hysteresis have been already reported and reviewed, little investigation is reported on scan-direction-dependent J-V curves depending on perovskite composition. This review investigates J-V hysteric behaviors depending on perovskite composition in normal mesoscopic and planar structure. In addition, methodologies toward hysteresis-free PSCs are proposed. There is a specific trend in hysteresis in terms of J-V curve shape depending on composition. Ion migration combined with nonradiative recombination near interfaces plays a critical role in generating hysteresis. Interfacial engineering is found to be an effective method to reduce the hysteresis; however, bulk defect engineering is the most promising method to remove the hysteresis. Among the studied methods, KI doping is proved to be a universal approach toward hysteresis-free PSCs regardless of perovskite composition. It is proposed from the current studies that engineering of perovskite film near the electron transporting layer (ETL) and the hole transporting layer (HTL) is of vital importance for achieving hysteresis-free PSCs and extremely high efficiency.

Point defect-reduced colloidal SnO2 electron transport layers for stable and almost hysteresis-free perovskite solar cells.

The commercialization of perovskite solar cells has been investigated, but the instability of their light-absorbing layers remains a problem. We demonstrate that the use of colloidal SnO2 nanoparticles prevents perovskite light absorber decomposition, reduces the hysteresis index to 0.1%, and increases the power conversion efficiency to 19.12%.

High-Performance and Hysteresis-Free Perovskite Solar Cells Based on Rare-Earth-Doped SnO2 Mesoporous Scaffold.

Tin oxide (SnO2), as electron transport material to substitute titanium oxide (TiO2) in perovskite solar cells (PSCs), has aroused wide interests. However, the performance of the PSCs based on SnO2 is still hard to compete with the TiO2-based devices. Herein, a novel strategy is designed to enhance the photovoltaic performance and long-term stability of PSCs by integrating rare-earth ions Ln3+ (Sc3+, Y3+, La3+) with SnO2 nanospheres as mesoporous scaffold. The doping of Ln promotes the formation of dense and large-sized perovskite crystals, which facilitate interfacial contact of electron transport layer/perovskite layer and improve charge transport dynamics. Ln dopant optimizes the energy level of perovskite layer, reduces the charge transport resistance, and mitigates the trap state density. As a result, the optimized mesoporous PSC achieves a champion power conversion efficiency (PCE) of 20.63% without hysteresis, while the undoped PSC obtains an efficiency of 19.01%. The investigation demonstrates that the rare-earth doping is low-cost and effective method to improve the photovoltaic performance of SnO2-based PSCs.

Potassium ions as a kinetic controller in ionic double layers for hysteresis-free perovskite solar cells

This work highlights the clear correlation of ion migration and a fast generation of the double layer with severe hysteresis and long-term instability in perovskite solar cells and the importance of K+ ions in reducing the kinetics affecting the ionic attachment to the contact surface.

Interfacial Engineering of TiO2 by Graphene Nanoplatelets for High-Efficiency Hysteresis-free Perovskite Solar Cells

The role of graphene nanoplatelets (GnPs) in a CH3NH3PbI3-based perovskite device was investigated for the first time. The activated GnPs were introduced in the mesoporous TiO2 by a simple solution process. The morphology and optoelectronic properties of the anatase TiO2 layer was modified with the presence of GnPs. The introduction of GnPs modified the bad-gap properties and passivated the oxygen-based surficial defects, leading to reduced interfacial recombination and efficient charge-carrier transportation. An overall increment in the efficiency of 25% was observed from ∼15 to 19% with the introduction of 2.0 wt % of GnPs. In addition, the device exhibits negligible hysteresis and enhanced stability. This work opens up new applications of GnPs to be utilized in solar cells and can be extended to other types of photovoltaic devices.

Indium-doped ZnO mesoporous nanofibers as efficient electron transporting materials for perovskite solar cells

We introduce a viable electrospinning route for the development of highly porous indium-doped (In-doped) ZnO nanofibers as electron transporting materials (ETMs) in perovskite solar cells (PSCs) for the first time ever. The nanofibers ETMs with optimal thickness leads to highly efficient and hysteresis-free PSCs with an average power conversion efficiency (PCEavg) of 16.03% and a best power conversion efficiency (PCEbest) of 17.18%, thanks to high porosity and high crystallinity of the nanofibers, better infiltration of the absorber and rapid charge transport characteristics due to indium (In) doping. Furthermore, the modification of In-doped ZnO nanofibers ETMs, by coating a thin layer of a polymer polyethyleneimine (PEI) further enhances the PCEavg to 16.86% (PCEbest = 18.69%) by effectively reducing the energy barrier for electron extraction due to a reduced work function. This study reveals that In-doped ZnO nanofibers are the best ETMs for generating the inexpensive and high efficiency PSCs with long-term stability.

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.

Improving the Optoelectronic Properties of Mesoporous TiO2 by Cobalt Doping for High-Performance Hysteresis-free Perovskite Solar Cells.

We for the first time report the incorporation of cobalt into a mesoporous TiO2 electrode for application in perovskite solar cells (PSCs). The Co-doped PSC exhibits excellent optoelectronic properties; we explain the improvements by passivation of electronic trap or sub-band-gap states arising due to the oxygen vacancies in pristine TiO2, enabling faster electron transport and collection. A simple postannealing treatment is used to prepare the cobalt-doped mesoporous electrode; UV-visible spectroscopy, X-ray photoemission spectroscopy, space charge-limited current, photoluminescence, and electrochemical impedance measurements confirm the incorporation of cobalt, enhanced conductivity, and the passivation effect induced in the TiO2. An optimized doping concentration of 0.3 mol % results in the maximum power conversion efficiency of 18.16%, 21.7% higher than that of a similar cell with an undoped TiO2 electrode. Also, the device shows negligible hysteresis and higher stability, retaining 80.54% of the initial efficiency after 200 h.

Hysteresis-free perovskite solar cells made of potassium-doped organometal halide perovskite

Potassium-doped organometal halide perovskite solar cells (PSCs) of more than 20% power conversion efficiency (PCE) without I-V hysteresis were constructed. The crystal lattice of the organometal halide perovskite was expanded with increasing of the potassium ratio, where both absorption and photoluminescence spectra shifted to the longer wavelength, suggesting that the optical band gap decreased. In the case of the perovskite with the 5% K+, the conduction band minimum (CBM) became similar to the CBM level of the TiO2-Li. In this situation, the electron transfer barrier at the interface between TiO2-Li and the perovskite was minimised. In fact, the transient current rise at the maximum power voltages of PSCs with 5% K+ was faster than that without K+. It is concluded that stagnation-less carrier transportation could minimise the I-V hysteresis of PSCs.

Engineering Interface Structure to Improve Efficiency and Stability of Organometal Halide Perovskite Solar Cells.

The rapid rise of power conversion efficiency (PCE) of low cost organometal halide perovskite solar cells suggests that these cells are a promising alternative to conventional photovoltaic technology. However, anomalous hysteresis and unsatisfactory stability hinder the industrialization of perovskite solar cells. Interface engineering is of importance for the fabrication of highly stable and hysteresis free perovskite solar cells. Here we report that a surface modification of the widely used TiO2 compact layer can give insight into interface interaction in perovskite solar cells. A highest PCE of 18.5% is obtained using anatase TiO2, but the device is not stable and degrades rapidly. With an amorphous TiO2 compact layer, the devices show a prolonged lifetime but a lower PCE and more pronounced hysteresis. To achieve a high PCE and long lifetime simultaneously, an insulating polymer interface layer is deposited on top of TiO2. Three polymers, each with a different functional group (hydroxyl, amino, or aromatic group), are investigated to further understand the relation of interface structure and device PCE as well as stability. We show that it is necessary to consider not only the band alignment at the interface, but also interface chemical interactions between the thin interface layer and the perovskite film. The hydroxyl and amino groups interact with CH3NH3PbI3 leading to poor PCEs. In contrast, deposition of a thin layer of polymer consisting of an aromatic group to prevent the direct contact of TiO2 and CH3NH3PbI3 can significantly enhance the device stability, while the same time maintaining a high PCE. The fact that a polymer interface layer on top of TiO2 can enhance device stability, strongly suggests that the interface interaction between TiO2 and CH3NH3PbI3 plays a crucial role. Our work highlights the importance of interface structure and paves the way for further optimization of PCEs and stability of perovskite solar cells.