Role of Ionic Functional Groups on Ion Transport at Perovskite Interfaces

Hybrid organic/inorganic perovskite solar cells are invigorating the photovoltaic community due to their remarkable properties and efficiency. However, many perovskite solar cells show an undesirable current–voltage (I–V) hysteresis in their forward and reverse voltage scans, working to the detriment of device characterization and performance. This hysteresis likely arises from slow ion migration in the bulk perovskite active layer to interfaces which may induce charge trapping. It is shown that interfacial chemistry between the perovskite and charge transport layer plays a critical role in ion transport and I–V hysteresis in perovskite‐based devices. Specifically, phenylene vinylene polymers containing cationic, zwitterionic, or anionic pendent groups are utilized to fabricate charge transport layers with specific interfacial ionic functionalities. The interfacial‐adsorbing boundary induced by the zwitterionic polymer in contact with the perovskite increases the local ion concentration, which is responsible for the observed I–V hysteresis. Moreover, the ion adsorbing properties of this interface are exploited for perovskite‐based memristors. This fundamental study of I–V hysteresis in perovskite‐based devices introduces a new mechanism for inducing memristor behavior by interfacial ion adsorption.     

Device Physics of Hybrid Perovskite Solar cells: Theory and Experiment

Perovskite solar cells (PSCs) exhibit a series of distinctive features in their optoelectronic response which have a crucial influence on the performance, particularly for long‐time response. Here, a survey of recent advances both in device simulation and optoelectronic and photovoltaic responses is provided, with the aim of comprehensively covering recent advances. Device simulations are included with clarifying discussions about the implications of classical drift–diffusion modeling and the inclusion of ionic charged layers near the outer carrier selective contacts. The outcomes of several transient techniques are summarized, along with the discussion of impedance and capacitive responses upon variation of bias voltage and irradiance level. In relation to the capacitive response, a discussion on the J–V curve hysteresis is also included. Although alternative models and explanations are included in the discussion, the review relies upon a key mechanism able to yield most of the rich experimental responses. Particularly for state‐of‐the‐art solar cells exhibiting efficiencies around or exceeding 20%, outer interfaces play a determining role on the PSC's performance. The ionic and electronic kinetics in the vicinity of the interfaces, coupled to surface recombination and carrier extraction mechanisms, should be carefully explored to progress further in performance enhancement.     

Lessons Learnt from Spatially Resolved Electro‐ and Photoluminescence Imaging: Interfacial Delamination in CH3NH3PbI3 Planar Perovskite Solar Cells upon Illumination

The influence of illumination on the long‐term performance of planar structured perovskite solar cells (PSCs) is investigated using fast and spatially resolved luminescence imaging. The authors analyze the effect of illuminated current density–voltage (J–V) and light‐soaking measurements on pristine PSCs by providing visual evidence for the spatial inhomogeneous evolution of device performance. Regions that are exposed to light initially produce stronger electroluminescence signals than surrounding unilluminated regions, mainly due to a lower contact resistance and, possibly, higher charge collection efficiency. Over a period of several days, however, these initially illuminated regions appear to degrade more quickly despite the device being stored in a dark, moisture‐ and oxygen‐free environment. Using transmission electron microscopy, this accelerated degradation is attributed to delamination between the perovskite and the titanium dioxide (TiO₂) layer. An ion migration mechanism is proposed for this delamination process, which is in accordance with previous current–voltage hysteresis observations. These results provide evidence for the intrinsic instability of CH₃NH₃PbI₃‐based devices under illumination and have major implications for the design of PSCs from the standpoint of long‐term performance and stability.     

2‐Thiopheneformamidinium‐Based 2D Ruddlesden–Popper Perovskite Solar Cells with Efficiency of 16.72% and Negligible Hysteresis

Formamidinium (FA)‐based 3D perovskite solar cells (PSCs) have been widely studied and they show reduced bandgap, enhanced stability, and improved efficiency compared to MAPbI₃‐based devices. Nevertheless, the FA‐based spacers have rarely been studied for 2D Ruddlesden–Popper (RP) perovskites, which have drawn wide attention due to their enormous potential for fabricating efficient and stable photovoltaic devices. Here, for the first time, FA‐based derivative, 2‐thiopheneformamidinium (ThFA), is successfully synthesized and employed as an organic spacer for 2D RP PSCs. A precursor organic salts‐assisted crystal growth technique is further developed to prepare high quality 2D (ThFA)₂(MA)_n?1Pb_nI_3n+1 (nominal n = 3) perovskite films, which shows preferential vertical growth orientations, high charge carrier mobilities, and reduced trap density. As a result, the 2D RP PSCs with an inverted planar p‐i‐n structure exhibit a dramatically improved power conversion efficiency (PCE) from 7.23% to 16.72% with negligible hysteresis, which is among the highest PCE in 2D RP PSCs with low nominal n‐value of 3. Importantly, the optimized 2D PSCs exhibit a dramatically improved stability with less than 1% degradation after storage in N₂ for 3000 h without encapsulation. These findings provide an effective strategy for developing FA‐based organic spacers toward highly efficient and stable 2D PSCs.     

Effect of Alkylsilyl Side‐Chain Structure on Photovoltaic Properties of Conjugated Polymer Donors

Side‐chain engineering is an important strategy for optimizing photovoltaic properties of organic photovoltaic materials. In this work, the effect of alkylsilyl side‐chain structure on the photovoltaic properties of medium bandgap conjugated polymer donors is studied by synthesizing four new polymers J70 , J72 , J73 , and J74 on the basis of highly efficient polymer donor J71 by changing alkyl substituents of the alkylsilyl side chains of the polymers. And the photovoltaic properties of the five polymers are studied by fabricating polymer solar cells (PSCs) with the polymers as donor and an n‐type organic semiconductor (n‐OS) m‐ITIC as acceptor. It is found that the shorter and linear alkylsilyl side chain could afford ordered molecular packing, stronger absorption coefficient, higher charge carrier mobility, thus results in higher J_sc and fill factor values in the corresponding PSCs. While the polymers with longer or branched alkyl substituents in the trialkylsilyl group show lower‐lying highest occupied molecular orbital energy levels which leads to higher V_oc of the PSCs. The PSCs based on J70 :m‐ITIC and J71 :m‐ITIC achieve power conversion efficiency (PCE) of 11.62 and 12.05%, respectively, which are among the top values of the PSCs reported in the literatures so far.     

Outstanding Performance of Hole‐Blocking Layer‐Free Perovskite Solar Cell Using Hierarchically Porous Fluorine‐Doped Tin Oxide Substrate

Perovskite solar cells (PSCs) are of great interest in current photovoltaic research due to their extraordinary power conversion efficiency of ≈20% and boundless potentialities. The high efficiency has been mostly obtained from TiO₂‐based PSCs, where TiO₂ is utilized as a hole‐blocking, mesoporous layer. However, trapped charges and the light‐induced photocatalytic effect of TiO₂ seriously degrade the perovskite and preclude PSCs from being immediately commercialized. Herein, a simplified PSC is successfully fabricated by eliminating the problematic TiO₂ layers, using instead a fluorine‐doped tin oxide (FTO)/perovskite/hole–conductor/Au design. Simultaneously, the sluggish charge extraction at the FTO/perovskite interface is overcome by modifying the surface of the FTO to a porous structure using electrochemical etching. This surface engineering enables a substantial increase in the photocurrent density and mitigation of the hysteretic behavior of the pristine FTO‐based PSC; a remarkable 19.22% efficiency with a low level of hysteresis is obtained. This performance is closely approaching that of conventional PSCs and may facilitate their commercialization due to improved convenience, lower cost, greater stability, and potentially more efficient mass production.     

Stability and Dark Hysteresis Correlate in NiO‐Based Perovskite Solar Cells

In perovskite solar cells (PSCs), the interfaces are a weak link with respect to degradation. Electrochemical reactivity of the perovskite's halides has been reported for both molecular and polymeric hole selective layers (HSLs), and here it is shown that also NiO brings about this decomposition mechanism. Employing NiO as an HSL in p–i–n PSCs with power conversion efficiency (PCE) of 16.8%, noncapacitive hysteresis is found in the dark, which is attributable to the bias‐induced degradation of perovskite/NiO interface. The possibility of electrochemically decoupling NiO from the perovskite via the introduction of a buffer layer is explored. Employing a hybrid magnesium‐organic interlayer, the noncapacitive hysteresis is entirely suppressed and the device's electrical stability is improved. At the same time, the PCE is improved up to 18% thanks to reduced interfacial charge recombination, which enables more efficient hole collection resulting in higher V_oc and FF.     

Coordination Engineering of Single‐Crystal Precursor for Phase Control in Ruddlesden–Popper Perovskite Solar Cells

2D Ruddlesden–Popper perovskites (RPPs) have recently drawn significant attention because of their structural variability that can be used to tailor optoelectronic properties and improve the stability of derived photovoltaic devices. However, charge separation and transport in 2D perovskite solar cells (PSCs) suffer from quantum well barriers formed during the processing of perovskites. It is extremely difficult to manage phase distributions in 2D perovskites made from the stoichiometric mixtures of precursor solutions. Herein, a generally applicable guideline is demonstrated for precisely controlling phase purity and arrangement in RPP films. By visually presenting the critical colloidal formation of the single‐crystal precursor solution, coordination engineering is conducted with a rationally selected cosolvent to tune the colloidal properties. In nonpolar cosolvent media, the derived colloidal template enables RPP crystals to preferentially grow along the vertically ordered alignment with a narrow phase variation around a target value, resulting in efficient charge transport and extraction. As a result, a record‐high power conversion efficiency (PCE) of 14.68% is demonstrated for a (TEA)₂(MA)₂Pb₃I₁₀ (n = 3) photovoltaic device with negligible hysteresis. Remarkably, superior stability is achieved with 93% retainment of the initial efficiency after 500 h of unencapsulated operation in ambient air conditions.     

Imidazolium‐Substituted Polythiophenes as Efficient Electron Transport Materials Improving Photovoltaic Performance

In the field of polymer solar cells, improving photovoltaic performance has been the main driver over the past decade. To achieve high power conversion efficiencies, a plethora of new photoactive donor polymers and fullerene derivatives have been developed and blended together in bulk heterojunction active layers. Simultaneously, further optimization of the device architecture is also of major importance. In this respect, we report on the use of specific types of electron transport layers to boost the inherent I–V properties of polymer solar cell devices, resulting in a considerable gain in overall photovoltaic output. Imidazolium‐substituted polythiophenes are introduced as appealing electron transport materials, outperforming the currently available analogous conjugated polyelectrolytes, mainly by an increase in short‐circuit current. The molecular weight of the ionic polythiophenes has been identified as a crucial parameter influencing performance.     

Improvement of Photovoltaic Performance of Polymer Solar Cells by Rational Molecular Optimization of Organic Molecule Acceptors

Two n‐type organic semiconductor (n‐OS) small molecules m‐ITIC‐2F and m‐ITIC‐4F with fluorinated 2‐(2,3‐dihydro‐3‐oxo‐1H‐inden‐1‐ylidene)propanedinitrile (IC) terminal moieties are prepared, for the application as an acceptor in polymer solar cells (PSCs), to further improve the photovoltaic performance of the n‐OS acceptor 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene) indanone) ‐5,5,11,11‐tetrakis(3‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐sindaceno[1,2‐b:5,6‐b′]‐dithiophene (m‐ITIC). Compared to m‐ITIC, these two new acceptors show redshifted absorption, higher molecular packing order, and improved electron mobilities. The power conversion efficiencies (PCE) of the as‐cast PSCs with m‐ITIC‐2F or m‐ITIC‐4F as an acceptor and a low‐cost donor–acceptor (D–A) copolymer PTQ10 as a donor reach 11.57% and 11.64%, respectively, which are among the highest efficiency for the as‐cast PSCs so far. Furthermore, after thermal annealing treatment, improved molecular packing and enhanced phase separation are observed, and the higher PCE of 12.53% is achieved for both PSCs based on the two acceptors. The respective and unique advantage with the intrinsic high degree of order, molecular packing, and electron mobilities of these two acceptors will be suitable to match with different p‐type organic semiconductor donors for higher PCE values, which provide a great potential for the PSCs commercialization in the near future. These results indicate that rational molecular structure optimization is of great importance to further improve photovoltaic properties of the photovoltaic materials.     

Extrinsic Movable Ions in MAPbI3 Modulate Energy Band Alignment in Perovskite Solar Cells

Ionic movement is considered awful in perovskite solar cells (PSCs) for relating with the hysteresis and long‐term instability. However, the positive role of ions to enhance the energy band bending for high performance PSC is always overlooked, let alone reducing the hysteresis. In this work, LiI is doped in CH₃NH₃PbI₃. It is observed that the aggregation of Li⁺/I^? tunes the energy level of the perovskite and induces n/p doping in CH₃NH₃PbI₃, which makes charge extraction quite efficient from perovskite to both NiO and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) layer. Therefore, in NiO/LiI doped perovskite/PCBM solar cells, Li⁺ and I^? modulate the interface energy band alignment to facilitate the electron/hole transport and reduce the interface energy loss. On the other hand, n/p doping enlarges Fermi energy level splitting of the PSCs to improve the photovoltage. The performance of LiI doped PSCs is much higher with reduced hysteresis compared to the undoped solar cells. This work highlights the positive effect of selective ionic doping, which is promisingly important to design the stable and efficient PSCs.     

Energetics and Energy Loss in 2D Ruddlesden–Popper Perovskite Solar Cells

2D Ruddlesden–Popper perovskites (RPPs) are emerging as potential challengers to their 3D counterpart due to superior stability and competitive efficiency. However, the fundamental questions on energetics of the 2D RPPs are not well understood. Here, the energetics at (PEA)₂(MA)_n?1Pb_nI_3n+1/[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) interfaces with varying n values of 1, 3, 5, 40, and ∞ are systematically investigated. It is found that n–n junctions form at the 2D RPP interfaces (n = 3, 5, and 40), instead of p–n junctions in the pure 2D and 3D scenarios (n = 1 and ∞). The potential gradient across phenethylammonium iodide ligands that significantly decreases surface work function, promotes separation of the photogenerated charge carriers with electron transferring from perovskite crystal to ligand at the interface, reducing charge recombination, which contributes to the smallest energy loss and the highest open‐circuit voltage (V_oc) in the perovskite solar cells (PSCs) based on the 2D RPP (n = 5)/PCBM. The mechanism is further verified by inserting a thin 2D RPP capping layer between pure 3D perovskite and PCBM in PSCs, causing the V_oc to evidently increase by 94 mV. Capacitance–voltage measurements with Mott–Schottky analysis demonstrate that such V_oc improvement is attributed to the enhanced potential at the interface.     

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

With 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.     

Tuning Energy Levels without Negatively Affecting Morphology: A Promising Approach to Achieving Optimal Energetic Match and Efficient Nonfullerene Polymer Solar Cells

One advantage of nonfullerene polymer solar cells (PSCs) is that they can yield high open‐circuit voltage (V_OC) despite their relatively low optical bandgaps. To maximize the V_OC of PSCs, it is important to fine‐tune the energy level offset between the donor and acceptor materials, but in a way not negatively affecting the morphology of the donor:acceptor (D:A) blends. Here, an effective material design rationale based on a family of D–A1–D–A2 terthiophene (T3) donor polymers is reported, which allows for the effective tuning of energy levels but without any negative impacts on the morphology of the blend. Based on a T3 donor unit combined with difluorobenzothiadiazole (ffBT) and difluorobenzoxadiazole (ffBX) acceptor units, three donor polymers are developed with highly similar morphological properties. This is particularly surprising considering that the corresponding quaterthiophene polymers based on ffBT and ffBX exhibit dramatic differences in their solubility and morphological properties. With the fine‐tuning of energy levels, the T3 polymers yield nonfullerene PSCs with a high efficiency of 9.0% for one case and with a remarkably low energy loss (0.53 V) for another polymer. This work will facilitate the development of efficient nonfullerene PSCs with optimal energy levels and favorable morphology properties.     

Temperature Dependence of Ideality Factors in Organic Solar Cells and the Relation to Radiative Efficiency

The value and temperature dependence of the ideality factor provides essential information about the dominant recombination route in solar cells. This study presents experimental results of accurate ideality factor determination for representative organic photovoltaic cells (OPV) evaluated at different temperatures over a large current density regime. It is noted that standard dark I–V curves strongly deviate from those obtained by evaluations based on short circuit current density (J _SC)–open circuit voltage (V _OC) pairs. This is attributed to the applied external voltage in a dark I–V measurement not being representative of internal chemical potential, particularly at lower temperatures. Complementary electroluminescence measurements attest that the current density dependence of the ability of the solar cell to emit light is better correlated to the series resistance free ideality factor. For the studied set of OPV devices it is observed that the ideality factors are quite low, and with very weak temperature dependence. The J _SC–V _OC method to determine ideality factors further allows good estimates of activation energies as well as recombination current prefactors J ₀₀. The findings imply that the principal OPV non‐radiative recombination mechanism is not recombination of free carriers with trapped carriers in an exponential density of tail states as previously reported.     

Carrier Interfacial Engineering by Bismuth Modification for Efficient and Thermoresistant Perovskite Solar Cells

Organic–inorganic halide perovskite solar cells (PSCs) have emerged as attractive alternatives to conventional solar cells. It is still a challenge to obtain PSCs with good thermal stability and high permanence, especially at extreme outdoor temperatures. This work systematically studies the effects of Bi³⁺ modification on structural, electrical, and optical properties of perovskite films (FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃) and the performance of corresponding PSCs. The results indicate that Bi³⁺ modified PSCs can achieve better thermal stability, photovoltaic response, and reproducibility compared with control cells due to the decreased grain boundaries, enhanced crystallization, and improved electron extraction from perovskite film. As a result, the modified PSC exhibits an optimized power conversion efficiency (PCE) of 19.4% compared with 18.3% for the optimized control device, accompanied by better thermoresistant ability under 100–180 °C and enhanced long‐term stability. The degradation rate of the modified device is reduced by an order of magnitude due to effective structural defect modification in perovskite photoactive layer. It could maintain more than two months at 60 °C. These results shed light on the origin of crystallization and thermal stability of perovskite films, and provide an approach to solve thermal stability issue of PSCs.     

Extending the Continuous Operating Lifetime of Perovskite Solar Cells with a Molybdenum Disulfide Hole Extraction Interlayer

Solution‐processed organic–inorganic lead halide perovskite solar cells (PSCs) are considered as one of the most promising photovoltaic technologies thanks to both high performance and low manufacturing cost. However, a key challenge of this technology is the lack of ambient stability over prolonged solar irradiation under continuous operating conditions. In fact, only a few studies (carried out in inert atmosphere) already approach the industrial standards. Here, it is shown how the introduction of MoS₂ flakes as a hole transport interlayer in inverted planar PSCs results in a power conversion efficiency (PCE) of ≈17%, overcoming the one of the standard reference devices. Furthermore, this approach allows the realization of ultrastable PSCs, stressed in ambient conditions and working at continuous maximum power point. In particular, the photovoltaic performances of the proposed PSCs represent the current state‐of‐the‐art in terms of lifetime, retaining 80% of their initial performance after 568 h of continuous stress test, thus approaching the industrial stability standards. Moreover, it is further demonstrated the feasibility of this approach by fabricating large‐area PSCs (0.5 cm² active area) with MoS₂ as the interlayer. These large‐area PSCs show improved performance (i.e., PCE = 13.17%) when compared with the standard devices (PCE = 10.64%).     

SrTiO3/Al2O3‐Graphene Electron Transport Layer for Highly Stable and Efficient Composites‐Based Perovskite Solar Cells with 20.6% Efficiency

For practical use of perovskite solar cells (PSCs) the instability issues of devices, attributed to degradation of perovskite molecules by moisture, ions migration, and thermal‐ and light‐instability, have to be solved. Herein, highly efficient and stable PSCs based on perovskite/Ag‐reduced graphene oxide (Ag‐rGO) and mesoporous Al₂O₃/graphene (mp‐AG) composites are reported. The mp‐AG composite is conductive with one‐order of magnitude higher mobility than mp‐TiO₂ and used for electron transport layer (ETL). Compared to the mp‐TiO₂ ETL based cells, the champion device based on perovskite/Ag‐rGO and SrTiO₃/mp‐AG composites shows overall a best performance (i.e., V_OC = 1.057 V, J_SC = 25.75 mA cm^?2, fill factor (FF) = 75.63%, and power conversion efficiency (PCE) = 20.58%). More importantly, the champion device without encapsulation exhibits not only remarkable thermal‐ and photostability but also long‐term stability, retaining 97–99% of the initial values of photovoltaic parameters and sustaining ≈93% of initial PCE over 300 d under ambient conditions.     

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

Lead 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.     

Efficiency Exceeding 11% in Tandem Polymer Solar Cells Employing High Open‐Circuit Voltage Wide‐Bandgap π‐Conjugated Polymers

The emerging field of tandem polymer solar cells (TPSCs) enables a feasible approach to deal with the fundamental losses associated with single‐junction polymer solar cells (PSCs) and provides the opportunity to propel their overall performance. Additionally, the rational selection of appropriate subcell photoactive polymers with complementary absorption profiles and optimal thicknesses to achieve balanced photocurrent generation are very important issues which must be addressed in order to realize paramount device performance. Here, two side chain fluorinated wide‐bandgap π‐conjugated polymers P1 (2F) and P2 (4F) in TPSCs have been used. These π‐conjugated polymers have high absorption coefficients and deep highest occupied molecular orbitals which lead to high open‐circuit voltages (V_oc) of 0.91 and 1.00 V, respectively. Using these π‐conjugated polymers, TPSCs have been successfully fabricated by combining P1 or P2 as front cells with PTB7‐Th as back cells. The optimized TPSCs deliver outstanding power conversion efficiencies of 11.42 and 10.05%, with high V_oc's of 1.64 and 1.72 V, respectively. These results are analyzed by balance of charge mobilities, and optical and electrical modeling is combined to demonstrate simultaneous improvement in all photovoltaic parameters in TPSCs.