In article https://doi.org/10.1002/smll.2019014661901466, Bo Chi, Gregory J. Wilson, and co‐workers investigate superior electronic properties of nanostructured tin (IV) oxide (SnO2) as an ideal inorganic electron transport layer (ETL) in n–i–p perovskite solar cells. The bilayer ETL architecture attaining impressive power conversion efficiency (PCE) greater than 20% is depicted.
Methylamine lead bromide (MAPbBr3, MA: CH3NH3 +) perovskite nanocrystals (NCs) are decorated on few layer MXene (Ti3C2T x ) nanosheets to form heterostructures. An optimal mixed solvent ratio is essential to realize the growth of perovskite NCs on Ti3C2T x nanosheets. Photoluminescence lifetime of MAPbBr3 NCs is greatly shortened in the heterojunctions, indicating the efficient interfacial energy transfer behavior of MAPbBr3/Ti3C2T x heterostructures.
The performance of perovskite nanocrystals (NCs) in optoelectronics and photocatalysis is severely limited by the presence of large amounts of crystal boundaries in NCs film that greatly restricts energy transfer. Creating heterostructures based on perovskite NCs and 2D materials is a common approach to improve the energy transport at the perovskite/2D materials interface. Herein, methylamine lead bromide (MAPbBr3, MA: CH3NH3 +) perovskite NCs are homogeneously deposited on highly conductive few‐layer MXene (Ti3C2T x ) nanosheets to form heterostructures through an in situ solution growth method. An optimal mixed solvent ratio is essential to realize the growth of perovskite NCs on Ti3C2T x nanosheets. Time‐resolved photoluminescence spectroscopy, transient absorption spectroscopy, and the photoresponse of electron‐ and hole‐only photoelectric conversion devices reveal the interfacial energy transfer behavior within MAPbBr3/Ti3C2T x heterostructures. The present investigation may provide a useful guide toward use of halide perovskite/2D material heterostructures in applications such as photocatalysis as well as optoelectronics.
By finely optimizing the alkyl chains, the nonfullerene acceptor named BTP‐eC9 is synthesized and a maximum power conversion efficiency of 17.8% in organic photovoltaic cells is recorded. This work demonstrates that the optimization of alkyl chains to get suitable solubility and enhanced intermolecular packing has a great potential in further improving photovoltaic performance.
Optimizing the molecular structures of organic photovoltaic (OPV) materials is one of the most effective methods to boost power conversion efficiencies (PCEs). For an excellent molecular system with a certain conjugated skeleton, fine tuning the alky chains is of considerable significance to fully explore its photovoltaic potential. In this work, the optimization of alkyl chains is performed on a chlorinated nonfullerene acceptor (NFA) named BTP‐4Cl‐BO (a Y6 derivative) and very impressive photovoltaic parameters in OPV cells are obtained. To get more ordered intermolecular packing, the n ‐undecyl is shortened at the edge of BTP‐eC11 to n ‐nonyl and n ‐heptyl. As a result, the NFAs of BTP‐eC9 and BTP‐eC7 are synthesized. The BTP‐eC7 shows relatively poor solubility and thus limits its application in device fabrication. Fortunately, the BTP‐eC9 possesses good solubility and, at the same time, enhanced electron transport property than BTP‐eC11. Significantly, due to the simultaneously enhanced short‐circuit current density and fill factor, the BTP‐eC9‐based single‐junction OPV cells record a maximum PCE of 17.8% and get a certified value of 17.3%. These results demonstrate that minimizing the alkyl chains to get suitable solubility and enhanced intermolecular packing has a great potential in further improving its photovoltaic performance.
In article number https://doi.org/10.1002/adfm.2019059511905951, Jong H. Kim, Sang Hyuk Im, O‐Pil Kwon, and co‐workers report a series of electron transporting homochiral and heterochiral stereoisomers of naphthalene diimide crystalline materials and show simultaneous achievement of low‐temperature solution processability, high device performance, and long‐term temporal device stability.
A fullerene derivative (indene‐C60 bisadduct) is introduced into organic solar cells via blade coating, which can eliminate the efficiency loss caused by different printing methods. This ternary strategy overcomes the morphology issues, and based on this strategy, the blade‐coating device (1.05 cm2) achieves an efficiency of 13.70%.
Regarded as a critical step in commercial applications, scalable printing technology has become a research frontier in the field of organic solar cells. However, inevitable efficiency loss always occurs in the lab‐to‐manufacturing translation due to the different fabrication processes. In fact, the decline of photovoltaic performance is mainly related to voltage loss, which is mainly affected by the diversity of phase separation morphology and the chemical structures of photoactive materials. Fullerene derivative indene‐C60 bisadduct (ICBA) is introduced into a PBDB‐T‐2F:IT‐4F system to control the active layer morphology during blade‐coating process. Accordingly, as a symmetrical fullerene derivative, ICBA can regulate the crystallization tendency and molecular packing orientation and suppress charge carrier recombination. This ternary strategy overcomes the morphology issues caused by weaker shear impulse in blade‐coating process. Benefiting from the reduced nonradiative recombination loss, 1.05 cm2 devices are fabricated by blade coating with a power conversion efficiency of 13.70%. This approach provides an effective support for recovering the voltage loss during scalable printing approaches.
The structure and formation mechanism of spin‐coated films of lead‐free Sn‐based Ruddlesden–Popper (Sn‐RDP) perovskites are unveiled by combining results from in situ grazing incidence wide‐angle X‐ray scattering measurements and other extensive ex situ characterization methods. The formation of films with oriented Sn‐RDP crystallites is the result of bulk crystallization suppression induced by the presence of the 2D component (PEA+) during the drying process.
Knowledge of the mechanism of formation, orientation, and location of phases inside thin perovskite films is essential to optimize their optoelectronic properties. Among the most promising, low toxicity, lead‐free perovskites, the tin‐based ones are receiving much attention. Here, an extensive in situ and ex situ structural study is performed on the mechanism of crystallization from solution of 3D formamidinium tin iodide (FASnI3), 2D phenylethylammonium tin iodide (PEA2SnI4), and hybrid PEA2FA n −1Sn n I3 n +1 Ruddlesden–Popper perovskites. Addition of small amounts of low‐dimensional component promotes oriented 3D‐like crystallite growth in the top part of the film, together with an aligned quasi‐2D bottom‐rich phase. The sporadic bulk nucleation occurring in the pure 3D system is negligible in the pure 2D and in the hybrid systems with sufficiently high PEA content, where only surface crystallization occurs. Moreover, tin‐based perovskites form through a direct conversion of a disordered precursor phase without forming ordered solvated intermediates and thus without the need of thermal annealing steps. The findings are used to explain the device performances over a wide range of composition and shed light onto the mechanism of the formation of one of the most promising Sn‐based perovskites, providing opportunities to further improve the performances of these interesting Pb‐free materials.
By applying Al‐doped TiO2 as an electron extraction layer of organic and perovskite solar cells, power conversion efficiency is improved by 5–10%. In order to suggest the cause of the improved efficiency, the physical characteristics (recombination and density of states analysis) and electrical characteristics (impedance, conductive‐atomic force microscope) of the solar cell device are conducted.
Fabricating an electron extraction layer (EEL) with a metal doping transition metal oxide (TMO) in inorganic–organic hybrid perovskite solar cells (PeSCs) and organic solar cells (OSCs) is a simple and efficient process for enhancing photovoltaic properties. Here, the universal benefits and common factors that influence both PeSCs and OSCs as a result of changes in Al‐doped TiO2 properties are investigated. These common factors are identified in two separate mechanisms. The first involves surface smoothing of TiO2 films, which affects the formation of a high crystalline active layer and reduces recombination between the electron extraction and active layers. The second involves bandgap widening of TiO2, which reduces the activation energy and enhances the quenching efficiency of devices. These factors are demonstrated in various measurements. The results will help in understanding the fundamental benefits of Al‐doped TiO2 in solution‐processed thin‐film solar cells.
Herein, polyethylene glycol (PEG) is used as an additive for the morphology control of lead‐reduced perovskite films. The power conversion efficiency of lead‐reduced perovskite solar cells with PEG additive improves from 13.7% to 16.1% without J –V hysteresis due to pinhole elimination of the perovskite film.
The organic–inorganic halide perovskite solar cells (PSCs) are rapidly developed in just a few years due to its high power conversion efficiency. However, it still faces some critical issues, one of which is the presence of toxic lead (Pb2+). Recent researches show that barium (Ba2+) can partially replace the Pb2+ in perovskite structure and achieve a promising device performance because of its adequate ionic radius. However, the optimal replacement amount of Ba2+ in perovskite is still limited. Herein, the methylammonium (MA)/formamidinium (FA) mixed‐cation perovskite is used as the active layer in PSCs and Pb2+ is partially substituted with Ba2+. Compared with the pure MA system, the best device efficiency can be achieved using higher Ba2+ replacement ratio. In addition, while introducing the appropriate polymer additive, the replacement ratio can be further increased without compromise of device efficiency. Using polyethylene glycol as polymer additive, 10.0 mol% Ba‐doped MA/FA mixed‐cation PSC with an efficiency of 16.1% can be realized. It is believed that this report provides an effective strategy to fabricate high‐performance lead‐reduced PSCs.
Polystyrene is added into PBDB‐T:ITIC active layers of organic solar cells, leading to a power conversion efficiency enhancement of up to 16% relative to the control device. Other insulating polymers can also improve the performance of the organic solar cells for different levels dependent on the polymer‐side chain size. This work provides a guideline for the selection of polymer additives in organic solar cells.
A series of insulating polymers are used as additives in nonfullerene organic solar cells (OSCs) for the first time. A significant relative power conversion efficiency (PCE) enhancement of up to 16% is observed with an introduction of polystyrene for only 5.0 wt% into the active layer of OSCs. Other insulating polymers possessing linear nonconjugated backbones with different side chains are also incorporated into OSCs and the resultant PCE enhancement decreases with the decrease in the side chain size. Another important issue that is noted is the glass transition temperature of the polymer additive. When the glass transition temperature is higher than the thermal annealing temperature of the active layer, the polymer additive plays a negative effect on the device performance and the device efficiency decreases monotonically with the increase in addition amount. So the effect of the insulating polymer additives in nonfullerene OSCs can be attributed to the reconstruction of the active layer films, which increases the crystallinity, carrier mobility, and carrier lifetime of the organic semiconductors in the bulk heterojunction of the devices. This work provides a guideline for the selection of polymer additives in OSCs apart from the consideration on the optoelectronic property of the additives.
The introduction of F5PEA+ to partially replace PEA+ as the 2D perovskite passivation agent, with a strong noncovalent interaction between the two bulky cations and enhanced charge transport, is reported to improve the performance (from 19.58% to 21.10%) and stability of the corresponding wide‐bandgap perovskite solar cells.
The replacement of a small amount of organic cations with bulkier organic spacer cations in the perovskite precursor solution to form a 2D perovskite passivation agent (2D‐PPA) in 3D perovskite thin films has recently become a promising strategy for developing perovskite solar cells (PSCs) with long‐term stability and high efficiency. However, the long, bulky organic cations often form a barrier, hindering charge transport. Herein, for the first time, 2D‐PPA engineering based on wide‐bandgap (≈1.68 eV) perovskites are reported. Pentafluorophenethylammonium (F5PEA+) is introduced to partially replace phenylethylammonium (PEA+) as the 2D‐PPA, forming a strong noncovalent interaction between the two bulky cations. The charge transport across and within the planes of pure 2D perovskites, based on mixed ammoniums, increases by a factor of five and three compared with that of mono‐cation 2D perovskites, respectively. The perovskite films based on mixed‐ammonium (F5PEA+‐PEA+) 2D‐PPA exhibit similar surface morphology and crystal structure, but longer carrier lifetime, lower exciton binding energy, less trap density and higher conductivity, in comparison with those using mono‐cation (PEA+) 2D‐PPA. The performance of PSCs based on mixed‐cation 2D‐PPA is enhanced from 19.58% to 21.10% along with improved stability, which is the highest performance for reported wide‐bandgap PSCs.
The driving force for charge collection in perovskite solar cells is clarified by means of theoretical device simulations and phase‐sensitive photocurrent measurements. While a drift field is not required for charge collection, a built‐in potential is crucial to ensure efficient charge extraction through the charge transport layers and avoid the formation of reverse electric fields inside the active layer.
Perovskite semiconductors as the active materials in efficient solar cells exhibit free carrier diffusion lengths on the order of microns at low illumination fluxes and many hundreds of nanometers under 1 sun conditions. These lengthscales are significantly larger than typical junction thicknesses, and thus the carrier transport and charge collection should be expected to be diffusion controlled. A consensus along these lines is emerging in the field. However, the question as to whether the built‐in potential plays any role is still of matter of some conjecture. This important question using phase‐sensitive photocurrent measurements and theoretical device simulations based upon the drift‐diffusion framework is addressed. In particular, the role of the built‐in electric field and charge‐selective transport layers in state‐of‐the‐art p–i–n perovskite solar cells comparing experimental findings and simulation predictions is probed. It is found that while charge collection in the junction does not require a drift field per se, a built‐in potential is still needed to avoid the formation of reverse electric fields inside the active layer, and to ensure efficient extraction through the charge transport layers.
A power conversion efficiency of 16.88% (certified as 16.4%) is achieved based on PM6:Y6 by morphology optimization, which is the most efficient for organic solar cells. Through the study of single structure and film morphology, a well‐ordered 2D crystal is found, which helps to enhance ultrafast hole and electron transfer, thus improving performance.
Single‐layered organic solar cells (OSCs) using nonfullerene acceptors have reached 16% efficiency. Such a breakthrough has inspired new sparks for the development of the next generation of OSC materials. In addition to the optimization of electronic structure, it is important to investigate the essential solid‐state structure that guides the high efficiency of bulk heterojunction blends, which provides insight in understanding how to pair an efficient donor–acceptor mixture and refine film morphology. In this study, a thorough analysis is executed to reveal morphology details, and the results demonstrate that Y6 can form a unique 2D packing with a polymer‐like conjugated backbone oriented normal to the substrate, controlled by the processing solvent and thermal annealing conditions. Such morphology provides improved carrier transport and ultrafast hole and electron transfer, leading to improved device performance, and the best optimized device shows a power conversion efficiency of 16.88% (16.4% certified). This work reveals the importance of film morphology and the mechanism by which it affects device performance. A full set of analytical methods and processing conditions are executed to achieve high efficiency solar cells from materials design to device optimization, which will be useful in future OSC technology development.
A chemically orthogonal hole transport layer for lead sulfide colloidal quantum dot (CQD) solar cells is introduced. By substituting the 1,2‐ethanedithiol‐treated CQDs with malonic‐acid‐treated CQDs, the surface chemistry of the active layer is preserved. This increases the charge diffusion length by 1.4×, enabling near‐unity charge extraction efficiency at the back electrode, achieving 13.0% efficiency.
Colloidal quantum dots (CQDs) are of interest in light of their solution‐processing and bandgap tuning. Advances in the performance of CQD optoelectronic devices require fine control over the properties of each layer in the device materials stack. This is particularly challenging in the present best CQD solar cells, since these employ a p‐type hole‐transport layer (HTL) implemented using 1,2‐ethanedithiol (EDT) ligand exchange on top of the CQD active layer. It is established that the high reactivity of EDT causes a severe chemical modification to the active layer that deteriorates charge extraction. By combining elemental mapping with the spatial charge collection efficiency in CQD solar cells, the key materials interface dominating the subpar performance of prior CQD PV devices is demonstrated. This motivates to develop a chemically orthogonal HTL that consists of malonic‐acid‐crosslinked CQDs. The new crosslinking strategy preserves the surface chemistry of the active layer beneath, and at the same time provides the needed efficient charge extraction. The new HTL enables a 1.4× increase in charge carrier diffusion length in the active layer; and as a result leads to an improvement in power conversion efficiency to 13.0% compared to EDT standard cells (12.2%).
We report the profiling of spatial and energetic distributions of trap states in metal halide perovskite single-crystalline and polycrystalline solar cells. The trap densities in single crystals varied by five orders of magnitude, with a lowest value of 2 x 1011 per cubic centimeter and most of the deep traps located at crystal surfaces. The charge trap densities of all depths of the interfaces of the polycrystalline films were one to two orders of magnitude greater than that of the film interior, and the trap density at the film interior was still two to three orders of magnitude greater than that in high-quality single crystals. Suprisingly, after surface passivation, most deep traps were detected near the interface of perovskites and hole transport layers, where a large density of nanocrystals were embedded, limiting the efficiency of solar cells.
Publication date: 15 April 2020
Source: Joule, Volume 4, Issue 4
Author(s): Jason A. Röhr, Jason Lipton, Jaemin Kong, Stephen A. Maclean, André D. Taylor
Ligands around inorganic perovskite nanocrystals (PNCs) play a critical role in improving the PNCs‐based solar cell performance. Herein, a facile hexane/ethyl acetate solvent treatment method to manage the ligand amount around PNCs is reported. Finally, a power conversion efficiency of 12.2%, which is the highest performance reported for mixed‐halide CsPbX3 NCs solar cells, is achieved.
CsPbX3 (X = Cl, Br, I) inorganic perovskite nanocrystals (PNCs) not only maintain the excellent optical and electronic properties of bulk material but also possess the features of nano‐materials, such as tunable bandgap and easily processable colloidal ink, and enable them to be suitable for incorporation into various electronic devices and compatible with printing techniques. In contrast to the traditional II‐VI and III‐V semiconductor nanocrystals, the unique defect‐tolerance effect makes the CsPbX3 PNCs promising materials for optoelectronic applications. The ligands around the PNCs play a critical role in the optoelectronic devices performance. Herein, through a facile hexane/ethyl acetate (MeOAc) solvent treatment method to control the ligand amount around CsPbBrI2 PNCs, the impact of ligand amount on the performance of solar cell is systematically demonstrated and the ligand amount is quantified precisely via the nuclear magnetic resonance internal standard method. Through controlling the ligand amount, the film quality, charge transfer, and transport properties are largely improved. In addition, a simple annealing process is applied to improve the interface properties by partial crystal fusion. As a consequence, the photovoltaic power conversion efficiency of 12.2% is achieved, which is the highest performance reported for mixed‐halide CsPbX3 PNCs solar cells.
Polycrystalline FAPbI3 and monocrystalline MAPbBr3 are synthesized from low‐grade purity commercial products (FAI, PbI2, MABr, and PbBr2). The crystal powder‐derived precursor (CP) and commercial products‐derived typical precursor (TP) are used to fabricate planar inverted (FAPbI3)0.85(MAPbBr3)0.15 perovskite solar cells. CP devices yield a champion power conversion efficiency of 20.5%, which is higher than TP of 16.7%.
Solution‐processed perovskite precursors, especially for MAPbBr3‐assisted FAPbI3 crystallization, has been noted to achieve high power conversion efficiency (PCE) for perovskite solar cells (PSCs). However, this low‐temperature processed (FAPbI3) x (MAPbBr3)1−x typical precursor derived from commercial products (FAI, PbI2, MABr, and PbBr2) suffers from environmental sensitivity, poor film crystallinity and less than ideal device reproducibility. Herein, (FAPbI3) x (MAPbBr3)1–x (0.80 ≤ x ≤ 0.90)‐based planar inverted PSCs are fabricated, employing grinded monocrystalline MAPbBr3 and powdered polycrystalline FAPbI3 as precursors. The champion device with optimal molar ratio x = 0.85 comprising highly crystalline larger‐grained perovskite film with enhanced carrier transport kinetics and reduced trap‐state density exhibits boosted efficiency reaching 20.50%, which shows a 22.90% improvement over typical precursors with a PCE of 16.68%. In addition, the crystal powder precursor yields obvious film stability under ambient conditions (23 °C, 65–85% humidity) for 150 days and improved device storage stability in the glove box within two months. This protocol using stock crystal powders for perovskite precursor formulation provides a relatively facile and reproducible device fabrication route for the commercialization of PSCs.
Light transmission is largely decoupled from harvesting by optically tailoring an organic cell architecture with 50% average visible transmission. In an outdoor measurement of vertically positioned devices, a 9.80% sunlight energy conversion into electricity during 1 day is demonstrated.
For many years, it has been recognized that potential organic photovoltaic cells must be integrated into elements requiring high transparency. In most of such elements, sunlight is likely to be incident at large angles. Here it is demonstrated that light transmission can be largely decoupled from harvesting by optically tailoring an infrared shifted nonfullerene acceptor based organic cell architecture. A 9.67% power conversion efficiency at 50° incidence is achieved together with an average visual transmission above 50% at normal incidence. The deconstruction of a 1D nanophotonic structure is implemented to conclude that just two λ/4 thick layers are essential to reach, for a wide incidence angle range, a higher than 50% efficiency increase relative to the standard configuration reference. In an outdoor measurement of vertically positioned 50% visible transparent cells, it is demonstrated that 9.80% of sunlight energy can be converted into electricity during the course of 1 day.
A 2D/3D layered heterostructure with 2D perovskites self‐assembled atop 3D MAPbI3 via a one‐step printing process is reported. The 2D perovskite capping layer significantly suppresses nonradiative recombination of the devices, leading to a remarkably high open‐circuit voltage of 1.2 V. Moreover, notable enhancement in light, thermal, and moisture stability is obtained as a result of the protective barrier of 2D perovskites.
As perovskite solar cells (PSCs) are highly efficient, demonstration of high‐performance printed devices becomes important. 2D/3D heterostructures have recently emerged as an attractive way to relieving the film inhomogeneity and instability in perovskite devices. In this work, a 2D/3D ensemble with 2D perovskites self‐assembled atop 3D methylammonium lead triiodide (MAPbI3) via a one‐step printing process is shown. A clean and flat interface is observed in the 2D/3D bilayer heterostructure for the first time. The 2D perovskite capping layer significantly suppresses nonradiative charge recombination, resulting in a marked increase in open‐circuit voltage (V OC) of the devices by up to 100 mV. An ultrahigh V OC of 1.20 V is achieved for MAPbI3 PSCs, corresponding to 91% of the Shockley–Queisser limit. Moreover, notable enhancement in light, thermal, and moisture stability is obtained as a result of the protective barrier of the 2D perovskites. These results suggest a viable approach for scalable fabrication of highly efficient perovskite solar cells with enhanced environmental stability.
