Perovskite-based Photovoltaics Research Articles

Manipulated Crystallization and Passivated Defects for Efficient Perovskite Solar Cells via Addition of Ammonium Iodide

Organic-inorganic metal halide perovskite materials have been widely studied as the light absorber for efficient photovoltaics. However, perovskite layers with defective nature are typically prepared with an uncontrollable crystallization process, intrinsically limiting further advance in device performance, and thus require delicate manipulation of crystallization processes and defect density. Here, we demonstrate an ammonium-assisted crystallization of perovskite absorbers during a two-step deposition to fabricate efficient solar cells. Addition of ammonium iodide (NH4I) is devised to manipulate the nucleation and crystal growth of perovskite, wherein the formation and transition of intermediate x[NH4+]•[PbI3]x- enables high-quality perovskite layers with an enlarged grain and reduced defect density. As a result, the perovskite solar cells (PSCs) achieve an average efficiency of 21.36% with a champion efficiency of 22.15% and improved environmental stability over 30 days in ambient conditions with varied relative humidity. These results with addition of NH4I provide an available and ingenious way to construct high-quality perovskite layers for efficient solar cells and will advance the commercial application of perovskite-based photovoltaics.

Discovery of temperature-induced stability reversal in perovskites using high-throughput robotic learning

Stability of perovskite-based photovoltaics remains a topic requiring further attention. Cation engineering influences perovskite stability, with the present-day understanding of the impact of cations based on accelerated ageing tests at higher-than-operating temperatures (e.g. 140°C). By coupling high-throughput experimentation with machine learning, we discover a weak correlation between high/low-temperature stability with a stability-reversal behavior. At high ageing temperatures, increasing organic cation (e.g. methylammonium) or decreasing inorganic cation (e.g. cesium) in multi-cation perovskites has detrimental impact on photo/thermal-stability; but below 100°C, the impact is reversed. The underlying mechanism is revealed by calculating the kinetic activation energy in perovskite decomposition. We further identify that incorporating at least 10 mol.% MA and up to 5 mol.% Cs/Rb to maximize the device stability at device-operating temperature (<100°C). We close by demonstrating the methylammonium-containing perovskite solar cells showing negligible efficiency loss compared to its initial efficiency after 1800 hours of working under illumination at 30°C.

Wide-Bandgap Halide Perovskites for Indoor Photovoltaics.

Indoor photovoltaics (IPVs) are receiving great research attention recently due to their projected application in the huge technology field of Internet of Things (IoT). Among the various existing photovoltaic technologies such as silicon, Cadmium Telluride (CdTe), Copper Indium Gallium Selenide (CIGS), organic photovoltaics, and halide perovskites, the latter are identified as the most promising for indoor light harvesting. This suitability is mainly due to its composition tuning adaptability to engineer the bandgap to match the indoor light spectrum and exceptional optoelectronic properties. Here, in this review, we are summarizing the state-of-the-art research efforts on halide perovskite-based indoor photovoltaics, the effect of composition tuning, and the selection of various functional layer and device architecture onto their power conversion efficiency. We also highlight some of the challenges to be addressed before these halide perovskite IPVs are commercialized.

Transparent Electrode Techniques for Semitransparent and Tandem Perovskite Solar Cells

Inorganic–organic halide perovskite solar cells have attracted significant attention to the photovoltaic community considering their high-efficiency, tunable bandgap, low-cost, and easy fabrication. Perovskite solar cells are especially an attractive top cell partner for tandem applications with silicon bottom cells and other solar cell types with lower bandgap absorbers. For such tandem applications, semitransparent perovskite solar cell techniques with high near-infrared transparency become important, since the incident light needs to efficiently reach the bottom cell. This review article will summarize the status and progress of perovskite-based tandem solar cells development, while focusing on the transparent electrode approaches and techniques. Future directions and challenges of transparent electrodes for semitransparent and tandem perovskite-based photovoltaics will also be discussed.

Advent of alkali metal doping: a roadmap for the evolution of perovskite solar cells.

Metal-halide hybrid perovskites have prompted the prosperity of the sustainable energy field and simultaneously demonstrated their great potential in meeting both the growing consumption of energy and the increasing social development requirements. Their inimitable features such as strong absorption ability, direct photogeneration of free carriers, long carrier diffusion lengths, ease of fabrication, and low production cost triggered the development of perovskite solar cells (PSCs) at an incredible rate, which soon reached power conversion efficiencies up to the commercialized level. During their evolution process, it has been witnessed that alkali metal cations play a pivotal role in the crystal structure as well as intrinsic properties of hybrid perovskites, thus enabling the unique positioning of the correlated doping strategy in the development history of PSCs in the past decade. Herein, we summarize the growth and progress of the state-of-the-art alkali metal cation (Cs+, Rb+, K+, Na+, Li+) doping in the field of hybrid perovskite-based photovoltaics. To start with, the accurate identification of different alkali metal-occupied locations in the perovskite crystal lattice are discussed in detail with highlighted advanced characterization methods. Beyond that, the location-dependent functions induced by alkali metal doping are intensely focused upon and comprehensively assessed, indicating their versatile and special effects on perovskites in terms of bottleneck issues such as crystallinity modulation, crystal structure stabilization, defect passivation, and ion-migration inhibition. Thereafter, we are committed to analyze their responsible working mechanisms so as to unveil the relationship between occupied locations and crucial roles for each doped cation. The systematical overview and in-depth understanding of the superiorities of such strategies together with their future challenges and prospects would further boost the advancement of perovskite-related fields.

Materials Chemistry Approach for Efficient Lead-Free Tin Halide Perovskite Solar Cells

The commercial development of perovskite-based photovoltaics is hindered by the toxicity of lead perovskite. Although tin perovskite is a promising alternative, the power conversion efficiency of t...

Local ferroelectric polarization in antiferroelectric chalcogenide perovskite BaZrS3 thin films

Bulk chalcogenide perovskite BaZrS3 (BZS), with a direct band gap in visible region, is an important photovoltaic material, albeit with limited applicability owing to its antiferroelectric (AF) nature. Presently, ferroelectric (FE) perovskite-based photovoltaics are attracting enormous attention for environmental stability and better energy conversion efficiency through enhanced charge separation, owing to loss of center of inversion symmetry. We report on antiferroelectric-ferroelectric (AF-FE) phases of BZS thin film, grown with chemical vapor deposition (CVD), using temperature-dependent Raman investigations and first-principles calculations. The origin of FE phases is established from anomalous behavior of A7g ~ 300 cm-1 and B1g5 ~ 420 cm-1 modes, which involves the vibration of atoms at apical site of ZrS6 octahedra. Additionally, below 60 K, B1g1 and B2g2 ( ~ 85 cm-1) modes appear whereas B12g (~ 60 cm-1) disappears to stabilize the Pnma structure against ferroelectricity by local distortion. Here, B2g2 and B1g2 involve vibrations of Ba atoms in AF manner while B1g1 involves, in addition, the rotation of octahedra as well. Our first-principles calculations confirm that FE appears as a result of loss of center of inversion symmetry in ZrS6 octahedra due to existence of oxygen (O) impurities placed locally at apical sites of sulfur (S) atom.

Molecular-Level Insight into Correlation between Surface Defects and Stability of Methylammonium Lead Halide Perovskite Under Controlled Humidity.

Perovskite-based photovoltaics (PVs) have garnered tremendous interest, enabling power conversion efficiencies exceeding 25%. Although much of this success is credited to the exploration of new compositions, defects passivation and process optimization, environmental stability remains an important bottleneck to be solved. The underlying mechanisms of thermal and humidity-induced degradation are still far from a clear understanding, which poses a severe limitation to overcome the stability issues. Herein, in situ X-ray diffraction (XRD), in operando liquid-cell transmission electron microscopy (TEM) and ex situ solid-state (ss)NMR spectroscopy are combined with time-resolved spectroscopies to reveal new insights about the degradation mechanisms of methylammonium lead halide (MAPbI3 ) under 85% relative humidity (RH) at different length scales. Liquid-cell TEM enables the live visualizations from meso-to-nanoscale transformation between the perovskite particles and water molecules, which are corroborated by the changes in local structures at sub-nanometer distances by ssNMR and longer range by XRD. This work clarifies the role of surface defects and the significance of their passivation to prevent hydration and decomposition reactions.

Minimized surface deficiency on wide-bandgap perovskite for efficient indoor photovoltaics

Abstract Wide-bandgap perovskite solar cells (PVSCs) possess significant potential in providing reliable power sources for applications in the Internet of Things (IoT) ecosystem under indoor light illumination. However, the wide-bandgap PVSCs usually suffer from photo-induced phase segregation and non-radiative energy loss caused by the Shockley-Read-Hall (SRH) type trap-assisted recombination at the interfaces in the devices. To address these issues, a simple strategy by applying phenethylammonium halides to reduce the energy loss and suppress the phase segregation of wide-bandgap PVSCs is developed. The devices incorporated with phenethylammonium chloride (PEACl) is revealed to achieve a high open-circuit voltage (VOC) of 1.26 V, leading to a merit power conversion efficiency (PCE) of 18.3%, which is the best performance among the inverted wide-bandgap PVSCs (~1.75 eV) under one sun illumination. Meanwhile, the photovoltaic performance of the device is also significantly enhanced, especially under a white light-emitting diode (LED) with an illumination of 1000 lux, showing a PCE of 35.6% with a high VOC of 1.08 V. Impressively, the device delivers a minimum energy loss of 670 meV, which is among the smallest value reported for perovskite-based indoor photovoltaics.

Inorganic narrow bandgap CsPb0.4Sn0.6I2.4Br0.6 perovskite solar cells with exceptional efficiency

The instability of organic/inorganic hybrid perovskite solar cells (PSCs) has motivated the development of the inorganic halide PSCs. However, the representative inorganic CsPbI 3 still suffers from phase instability in ambient air and an unfavorable wide bandgap (1.75 eV), thereby limiting its efficiency. In this study, a binary metal composition of Pb:Sn = 4:6 in CsPb 1-x Sn x I 3 is found to stabilize the cubic CsPbI 3 phase and reduce its bandgap. Based on the parental CsPb 0.4 Sn 0.6 I 3 , compositional engineering is further conducted for CsPb 0.4 Sn 0.6 I 3-y Br y perovskites. After introducing a suitable Br content (y = 0.6), there are remarkable improvements in the crystalline quality and a denser morphology in the perovskite films. Furthermore, in the novel inorganic CsPb 0.4 Sn 0.6 I 2.4 Br 0.6 perovskite, trap-assisted recombination is effectively suppressed, with a desirable narrow bandgap of 1.35 eV. As a result, the corresponding PSC delivered an unprecedented efficiency of 12.34%, which is the highest among the inorganic Sn-rich (Sn > 50%) PSCs reported to date. Additionally, the unencapsulated PSC demonstrates impressive long-term air stability, which exceeds the performance of 100% Pb-based inorganic CsPbI 3 and/or CsPbI 2 Br PSCs reported. This near-infrared absorbing (~930 nm) inorganic PSC with exceptional efficiency, durability and Pb-reduction generates a promising route for further progress of perovskite-based photovoltaics. • All-inorganic CsPbI 3 suffers from phase instability, wide bandgap, and toxic-Pb. • The partial Pb-replacement by Sn (x = 0.6) stabilizes the cubic CsPbI 3 and reduces bandgap. • The suitable Br addition suppresses trap-assisted recombination of CsPb 0.4 Sn 0.6 I 3 with a narrow bandgap of 1.35 eV. • A record efficiency of 12.34% for inorganic Sn-rich perovskite solar cell is demonstrated based on CsPb 0.4 Sn 0.6 I 2.4 Br 0.6 .

Technique offers improved resolution and characterization of perovskite-based photovoltaics

Researchers have developed a method for directly measuring the electronic transport at interfaces within a perovskite-based solar cell.

Sn(IV)-free tin perovskite films realized by in situ Sn(0) nanoparticle treatment of the precursor solution

The toxicity of lead perovskite hampers the commercialization of perovskite-based photovoltaics. While tin perovskite is a promising alternative, the facile oxidation of tin(II) to tin(IV) causes a high density of defects, resulting in lower solar cell efficiencies. Here, we show that tin(0) nanoparticles in the precursor solution can scavenge tin(IV) impurities, and demonstrate that this treatment leads to effectively tin(IV)-free perovskite films with strong photoluminescence and prolonged decay lifetimes. These nanoparticles are generated by the selective reaction of a dihydropyrazine derivative with the tin(II) fluoride additive already present in the precursor solution. Using this nanoparticle treatment, the power conversion efficiency of tin-based solar cells reaches 11.5%, with an open-circuit voltage of 0.76 V. Our nanoparticle treatment is a simple and broadly effective method that improves the purity and electrical performance of tin perovskite films.

A Preferential Analysis of Physical Properties in Graphene Oxide Doped PEDOT:PSS Hole Transport Layers

Flexible and semitransparent hole transport layers in perovskite-based photovoltaics have been attracting increasing attention in recent years. Among the benefits they provide, such as being easily reproducible and affordable, they pose their own problems with device degradation, stability, and surface wettability. We probed the graphene oxide doped PEDOT:PSS (GO)/poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) composite for the application in 2D carbonaceous hole transport layers to improve perovskite solar cells. Advantages and problems are pointed out as well as new perspectives to further modify and retract the drawbacks of conducting polymers. We discussed the advancement of the hole transport layer and described the effects of graphene oxide (dopant) on device transmission, surface wettability, and film quality. Basic principles and mechanisms are covered in the development of polymeric hole transport layers and interfacial properties. Findings included the decrease in device transmission, lessened oxidation of the perovskite, and superior surface wettability. This work highlights the importance and relevance of thin, flexible and powerful photovoltaic systems.

(Invited) Carbon Nanotubes As an Effective and Stable p-Type Contact for Perovskite Solar Cells

The enormous strides researchers have made in advancing perovskite-based photovoltaics in the past few years are fueling the dream of a rapid entry of this highly promising technology into the global energy market. This seems like serendipitous timing since solar power has gained an enormous momentum over the past decade exceeding all projections. This expansion in solar power is driven by an increasing understanding that climate change requires immediate action, in particular with regards to power generation; and simultaneously, the price for photovoltaic modules has remarkably quickly fallen to make solar power generation competitive vis-à-vis conventional technologies.Compositional tuning of the perovskite absorber has led to immense improvements in stability and efficiency of perovskite solar cells leading to a certified efficiency of over 25% for a single-junction laboratory device. The successful transition of a photovoltaic technology from the lab into the field requires, however, two additional important parameters aside from efficiency which are essential for real-world deployment: stability and scalability.We propose using single-walled carbon nanotubes (SWCNTs) as an alternative p-type layer for future application in perovskite solar cells.Carbon nanotubes combine several highly attractive characteristics such as chemical inertness and mechanical resilience with intrinsically high charge-carrier mobilities endowing them with a unique potential for a much more stable dopant-free charge-selective contact. Furthermore, the SWCNT deposition techniques for large-scale use, such as spray coating, are already well-established opening clear avenues for direct implementation in a real-world industrial setting.We demonstrate that a cascade-like transfer extraction through SWCNTs can minimize photovoltage losses by suppressing non-radiative recombination. By further improving the absorber quality, the voltage losses in solar cells can be reduced to around 340 mV which is comparable to conventional III-V semiconductor-based photovoltaics.By further enhancing the selective charge extraction at the perovskite-SWCNT interface through interfacial charge transfer doping, we achieve steady-state efficiencies approaching 22% for alloyed narrow bandgap perovskites, illustrating the versatility and excellent performance of SWCNTs as contact material.

Nanostructured front electrodes for perovskite/c-Si tandem photovoltaics.

The rise in the power conversion efficiency (PCE) of perovskite solar cells has triggered enormous interest in perovskite-based tandem photovoltaics. One key challenge is to achieve high transmission of low energy photons into the bottom cell. Here, nanostructured front electrodes for 4-terminal perovskite/crystalline-silicon (perovskite/c-Si) tandem solar cells are developed by conformal deposition of indium tin oxide (ITO) on self-assembled polystyrene nanopillars. The nanostructured ITO is optimized for reduced reflection and increased transmission with a tradeoff in increased sheet resistance. In the optimum case, the nanostructured ITO electrodes enhance the transmittance by ∼7% (relative) compared to planar references. Perovskite/c-Si tandem devices with nanostructured ITO exhibit enhanced short-circuit current density (2.9 mA/cm2 absolute) and PCE (1.7% absolute) in the bottom c-Si solar cell compared to the reference. The improved light in-coupling is more pronounced for elevated angle of incidence. Energy yield enhancement up to ∼10% (relative) is achieved for perovskite/c-Si tandem architecture with the nanostructured ITO electrodes. It is also shown that these nanostructured ITO electrodes are also compatible with various other perovskite-based tandem architectures and bear the potential to improve the PCE up to 27.0%.

Ligand-assisted cation-exchange engineering for high-efficiency colloidal Cs1−xFAxPbI3 quantum dot solar cells with reduced phase segregation

The mixed caesium and formamidinium lead triiodide perovskite system (Cs1−xFAxPbI3) in the form of quantum dots (QDs) offers a pathway towards stable perovskite-based photovoltaics and optoelectronics. However, it remains challenging to synthesize such multinary QDs with desirable properties for high-performance QD solar cells (QDSCs). Here we report an effective oleic acid (OA) ligand-assisted cation-exchange strategy that allows controllable synthesis of Cs1−xFAxPbI3 QDs across the whole composition range (x = 0–1), which is inaccessible in large-grain polycrystalline thin films. In an OA-rich environment, the cross-exchange of cations is facilitated, enabling rapid formation of Cs1−xFAxPbI3 QDs with reduced defect density. The hero Cs0.5FA0.5PbI3 QDSC achieves a certified record power conversion efficiency (PCE) of 16.6% with negligible hysteresis. We further demonstrate that the QD devices exhibit substantially enhanced photostability compared with their thin-film counterparts because of suppressed phase segregation, and they retain 94% of the original PCE under continuous 1-sun illumination for 600 h.

Theoretical study of building-integrated photovoltaics based on perovskite single junction and perovskite/silicon tandem solar cells

Considering the perovskite-based building-integrated photovoltaics, we use the single box model of 4 × 4 × 3 m3 to illustrate a promising solution for the zero-energy residential building and the distributed power generators in the Wuhan urban area in China (N30° E114°), where we have the high ratio of façades on the tall buildings. According to our estimation, the energy gains from the solar cells on the façades could be competitive with the ones on the rooftops. In comparison with the perovskite/silicon tandem solar cells, the perovskite-based single-junction solar cells could boost the annual energy gains more than 1.5 times on the façades. It is because of the perovskite-based single-junction solar cells being less angle-dependent, even if their efficiencies under the vertical illumination are lower than that of the perovskite/silicon tandem solar cells. Within the same single box, the energy demands caused by the air-conditioning system are simulated by the program THERB with 100 and 65% solar irradiation; the previous one is the standard case, and the later one assumes that 35% of solar irradiation is converted to the electricity. To control the indoor temperature above 18°C in winter and below 26°C in summer, the single box could achieve zero-energy demand except for January and December. And the annual surplus photovoltaic energy gains of 3071 kW h could be used as the distributed generator for the resident downstairs.

Efficient All-Evaporated pin-Perovskite Solar Cells: A Promising Approach Toward Industrial Large-Scale Fabrication

Vacuum-based deposition techniques are a common route for the fabrication of high-quality optoelectronic devices on an industrialized scale at low cost and high yield. In the field of perovskite-based photovoltaics, however, vacuum deposition methods are less researched in the community today. Even though the fundamental concept of thermal evaporation of perovskite-based solar cells has been demonstrated, the number of reports about efficient upscalable all-evaporated approaches employing inexpensive raw materials is still limited. In this contribution, a novel architecture for efficient all-evaporated perovskite solar cells in pin -architecture based on a co-evaporated CH3NH3PbI3 absorber deposited on top of an electron-beam evaporated NiOx hole transport layer is reported. Stabilized power conversion efficiencies as high as 16.1% are achieved, resulting in the most efficient thermally evaporated perovskite solar cells employing a pin -architecture. Moreover, it is the first time in the literature that a co-evaporated perovskite absorber deposited directly on top of a metal oxide exceeeds a stable power conversion efficiency above 15%. Next to efficient devices, a remarkable stability against temperature variations up to 80 °C is demonstrated, highlighting the promising thermal stability of the employed charge extracting layers. Replacing the expensive gold rear electrode by copper reduces the material costs of the approach significantly while maintaining a good device performance and stability. The homogeneity and ease of upscaling of the all-evaporated approach toward industrial relevant areas is demonstrated by light-beam induced current mapping. Finally, a homogeneous deposition of the functional layers of the approach on top of a textured silicon wafer is shown.

Beneficial impact of materials with reduced dimensionality on the stability of perovskite-based photovoltaics

Organometallic lead-halide solar cells exhibited immense potential over the past years and reached the transition point from lab to industry-scale fabrication. However, bridging this gap and establishing perovskites as a viable competitor to conventional Si-based photovoltaics, hinges on the success of cost-effective upscaling process. The key factor impeding this transition is operational stability of solar cells under realistic photoconversion conditions. To this extent, reducing the dimensionality of cell constituents appears as a promising and very attractive approach to tackle this issue. The beneficial influence of such materials on device stability, which is explicitly tied to the engineered interface quality with underlying layers, comes as a result of complex interplay between energy alignment, strain-induced interactions and barrier-like properties of 2D components. The aim of this perspective is to briefly outline key challenges regarding the exploitation of 2D materials within the framework of perovskite photovoltaics, as well as to suggest further development directions.

Carbon nanotubes embedded poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hybrid hole collector for inverted planar perovskite solar cells

This study presents a hybrid hole collector that consists of metallic single-walled carbon nanotubes (CNTs) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and is applicable in inverted planar perovskite solar cells. The drop-casted CNTs exhibit superior optical transmission and hole extraction properties compared to conventional PEDOT:PSS; however, the inherent random networks of CNTs result in many vacancies between nanotubes, causing recombination losses when employed solely as a hole transport layer in the planar architecture of solar cells. Thus, the proposed hybrid hole collector is designed by blending CNTs with various mixture ratios (10–50%) of PEDOT:PSS to enhance the electron-blocking properties. The preferred CNT (70%)/PEDOT:PSS (30%) composition shows a dense, pinhole-free surface and better photoluminescence quenching properties than pristine PEDOT:PSS. After device fabrication, we demonstrate that this hybrid hole collector impressively enhanced average power conversion efficiency from 13.2% to 15.6% (up to 16.0% for best-performing cell) with negligible hysteresis. Time-correlated single-photon counting and conductive atomic force microscopy analyses elucidate the performance progress for the CNT/PEDOT:PSS composite in terms of better hole collection and highly conductive characteristics. This approach supports simple solution-processing techniques at low temperatures, which can construct promising routes for the development of inverted planar perovskite-based photovoltaics with reduced hygroscopic and acidic PEDOT:PSS content.