Chen Weijie

High‐efficiency stable perovskite/gallium arsenide two‐terminal and four‐terminal tandem cells are demonstrated for the first time. For this purpose, high‐performance photostable wide‐bandgap perovskite photovoltaics (PVs) (1.8–1.9 eV) are developed by a solvent‐controlled process. Tandem architectures are shown to be feasible for thin‐film flexible devices with superior bendability, essential to commercialization. This approach is expected to improve the usability of GaAs PV with enhanced efficiency and lower cost.

Abstract

Gallium arsenide (GaAs) photovoltaic (PV) cells have been widely investigated due to their merits such as thin‐film feasibility, flexibility, and high efficiency. To further increase their performance, a wider bandgap PV structure such as indium gallium phosphide (InGaP) has been integrated in two‐terminal (2T) tandem configuration. However, it increases the overall fabrication cost, complicated tunnel‐junction diode connecting subcells are inevitable, and materials are limited by lattice matching. Here, high‐efficiency and stable wide‐bandgap perovskite PVs having comparable bandgap to InGaP (1.8–1.9 eV) are developed, which can be stable low‐cost add‐on layers to further enhance the performance of GaAs PVs as tandem configurations by showing an efficiency improvement from 21.68% to 24.27% (2T configuration) and 25.19% (4T configuration). This approach is also feasible for thin‐film GaAs PV, essential to reduce its fabrication cost for commercialization, with performance increasing from 21.85% to 24.32% and superior flexibility (1000 times bending) in a tandem configuration. Additionally, potential routes to over 30% stable perovskite/GaAs tandems, comparable to InGaP/GaAs with lower cost, are considered. This work can be an initial step to reach the objective of improving the usability of GaAs PV technology with enhanced performance for applications for which lightness and flexibility are crucial, without a significant additional cost increase.

The aprotic butyldimethylsulfonium‐driven MAPbI₃ perovskite shows a much more pronounced effect on the improvement of moisture stability compared to the protic butylammonium (BA)‐based counterpart. The BA having a potential hydrogen donor, which exists on the surface and/or grain boundaries, is vulnerable to H₂O‐induced degradation initiators, resulting in the faster hydration followed by the irreversible degradation of perovskites.

Abstract

Many organic cations in halide perovskites have been studied for their application in perovskite solar cells (PSCs). Most organic cations in PSCs are based on the protic nitrogen cores, which are susceptible to deprotonation. Here, a new candidate of fully alkylated sulfonium cation (butyldimethylsulfonium; BDMS) is designed and successfully assembled into PSCs with the aim of increasing humidity stability. The BDMS‐based perovskites retain the structural and optical features of pristine perovskite, which results in the comparable photovoltaic performance. However, the fully alkylated aprotic nature of BDMS shows a much more pronounced effect on the increase in humidity stability, which emphasizes a generic electronic difference between protic ammonium and aprotic sulfonium cation. The current results would pave a new way to explore cations for the development of promising PSCs.

An unprecedented anchoring‐based coassembly strategy is proposed to acquire highly scalable and wettable hole‐extraction monolayers (HELs) for p–i–n structured perovskite solar cells. It enables ultrathin HELs with high uniformity, facilitates the fabrication of large‐area perovskite films, and guarantees a high quality of interfacial contact. For the first time, a monolayer HEL‐based 36 cm² module achieves 12.67% efficiency.

Abstract

All organic charge‐transporting layer (CTL)‐featured perovskite solar cells (PSCs) exhibit distinct advantages, but their scaling‐up remains a great challenge because the organic CTLs underneath the perovskite are too thin to achieve large‐area homogeneous layers by spin‐coating, and their hydrophobic nature further hinders the solution‐based fabrication of perovskite layer. Here, an unprecedented anchoring‐based coassembly (ACA) strategy is reported that involves a synergistic coadsorption of a hydrophilic ammonium salt CA‐Br with hole‐transporting triphenylamine derivatives to acquire scalable and wettable organic hole‐extraction monolayers for p–i–n structured PSCs. The ACA route not only enables ultrathin organic CTLs with high uniformity but also eliminates the nonwetting problem to facilitate large‐area perovskite films with 100% coverage. Moreover, incorporation of CA‐Br in the ACA strategy can distinctly guarantee a high quality of electronic connection via the cations' vacancy passivation. Consequently, a high power‐conversion‐efficiency (PCE) of 17.49% is achieved for p–i–n structured PSCs (1.02 cm²), and a module with an aperture area of 36 cm² shows PCE of 12.67%, one of the best scaling‐up results among all‐organic CTL‐based PSCs. This work demonstrates that the ACA strategy can be a promising route to large‐area uniform interfacial layers as well as scaling‐up of perovskite solar cells.

Double‐sided 2D surface passivation of 3D perovskite film contributes to a remarkable device V _OC of 1.2 V, which is one of the highest open‐circuit voltages reported for perovskite cells with an optical bandgap of ≈1.6 eV. Discontinuous 2D perovskite films provide conductive pathways through these resistive layers, allowing for efficient charge transport between the 3D perovskite and charge transport layers.

Abstract

Defect‐mediated carrier recombination at the interfaces between perovskite and neighboring charge transport layers limits the efficiency of most state‐of‐the‐art perovskite solar cells. Passivation of interfacial defects is thus essential for attaining cell efficiencies close to the theoretical limit. In this work, a novel double‐sided passivation of 3D perovskite films is demonstrated with thin surface layers of bulky organic cation–based halide compound forming 2D layered perovskite. Highly efficient (22.77%) mixed‐dimensional perovskite devices with a remarkable open‐circuit voltage of 1.2 V are reported for a perovskite film having an optical bandgap of ≈1.6 eV. Using a combination of experimental and numerical analyses, it is shown that the double‐sided surface layers provide effective defect passivation at both the electron and hole transport layer interfaces, suppressing surface recombination on both sides of the active layer. Despite the semi‐insulating nature of the passivation layers, an increase in the fill factor of optimized cells is observed. The efficient carrier extraction is explained by incomplete surface coverage of the 2D perovskite layer, allowing charge transport through localized unpassivated regions, similar to tunnel‐oxide passivation layers used in silicon photovoltaics. Optimization of the defect passivation properties of these films has the potential to further increase cell efficiencies.

Two electron donor (D)–electron acceptor (A)‐type polymers PBDTT and PBTTT are developed as hole‐transporting materials for perovskite solar cells (PVSCs). Both polymers endow the PVSCs promising device performance. A power conversion efficiency of 20.28% is achieved from the devices with dopant‐free PBDTT. High device stability can be expected by employing these compact and hydrophobic polymeric hole‐transporting layers.

Abstract

The rich molecular design of electron donor (D)–acceptor (A) polymers offers many valuable clues to obtain high‐efficiency hole‐transporting materials (HTMs) for use in perovskite solar cells (PVSCs). The fused aromatic or heteroaromatic units can increase the conjugation of the polymer backbone to facilitate electron delocalization, which increases the rigidity of adjacent units to prevent rotational disorder and lower the reorganization energy, leading to improved carrier mobility and optimized film morphology. In this work, fused‐ring ladder‐type indacenodithiophene and indacenodithieno[3,2‐b]thiophene are used as D units, benzodithiophene‐4,8‐dione as the A unit, and thienothiophene as a π‐bridge to form the D–A polymers PBDTT and PBTTT, respectively. Both polymers exhibit favorable properties as HTMs including suitable energy levels, high hole mobility, and excellent film quality. Both dopant‐free HTMs endow n‐i‐p PVSCs with promising performance and stability. A maximum power conversion efficiency of 20.28% is achieved for PBDTT‐based devices, which is among the highest values reported to date.

The interplay of crystal structure and photophysics in the mixed cation, mixed halide perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15 is probed. It is found that changes in crystal structure, quantified by structural parameters such as lattice constant ratios and bond angles, influence optoelectronic properties in the film—the bandgap, Stokes shift, and charge carrier recombination rates all exhibit phase specificity.

Abstract

Mixed cation perovskites currently achieve very promising efficiency and operational stability when used as the active semiconductor in thin‐film photovoltaic devices. However, an in‐depth understanding of the structural and photophysical properties that drive this enhanced performance is still lacking. Here the prototypical mixed‐cation mixed‐halide perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15 is explored, and temperature‐dependent X‐ray diffraction measurements that are correlated with steady state and time‐resolved photoluminescence data are presented. The measurements indicate that this material adopts a pseudocubic perovskite α phase at room temperature, with a transition to a pseudotetragonal β phase occurring at ≈260 K. It is found that the temperature dependence of the radiative recombination rates correlates with temperature‐dependent changes in the structural configuration, and observed phase transitions also mark changes in the gradient of the optical bandgap. The work illustrates that temperature‐dependent changes in the perovskite crystal structure alter the charge carrier recombination processes and photoluminescence properties within such hybrid organic–inorganic materials. The findings have significant implications for photovoltaic performance at different operating temperatures, as well as providing new insight on the effect of alloying cations and halides on the phase behavior of hybrid perovskite materials.

High‐efficiency stable perovskite/gallium arsenide two‐terminal and four‐terminal tandem cells are demonstrated for the first time. For this purpose, high‐performance photostable wide‐bandgap perovskite photovoltaics (PVs) (1.8–1.9 eV) are developed by a solvent‐controlled process. Tandem architectures are shown to be feasible for thin‐film flexible devices with superior bendability, essential to commercialization. This approach is expected to improve the usability of GaAs PV with enhanced efficiency and lower cost.

Abstract

Gallium arsenide (GaAs) photovoltaic (PV) cells have been widely investigated due to their merits such as thin‐film feasibility, flexibility, and high efficiency. To further increase their performance, a wider bandgap PV structure such as indium gallium phosphide (InGaP) has been integrated in two‐terminal (2T) tandem configuration. However, it increases the overall fabrication cost, complicated tunnel‐junction diode connecting subcells are inevitable, and materials are limited by lattice matching. Here, high‐efficiency and stable wide‐bandgap perovskite PVs having comparable bandgap to InGaP (1.8–1.9 eV) are developed, which can be stable low‐cost add‐on layers to further enhance the performance of GaAs PVs as tandem configurations by showing an efficiency improvement from 21.68% to 24.27% (2T configuration) and 25.19% (4T configuration). This approach is also feasible for thin‐film GaAs PV, essential to reduce its fabrication cost for commercialization, with performance increasing from 21.85% to 24.32% and superior flexibility (1000 times bending) in a tandem configuration. Additionally, potential routes to over 30% stable perovskite/GaAs tandems, comparable to InGaP/GaAs with lower cost, are considered. This work can be an initial step to reach the objective of improving the usability of GaAs PV technology with enhanced performance for applications for which lightness and flexibility are crucial, without a significant additional cost increase.

A crosslinkable organic small molecule, thioctic acid (TA), is introduced into perovskite solar cells as a new bifacial passivation agent. This TA can simultaneously be chemically anchored to the surface of TiO₂ and methylammonium lead iodide through coordination effects and then in situ crosslinked to form a robust continuous polymer (Poly(TA)) network after thermal treatment.

Abstract

Defects, inevitably produced within bulk and at perovskite‐transport layer interfaces (PTLIs), are detrimental to power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). It is demonstrated that a crosslinkable organic small molecule thioctic acid (TA), which can simultaneously be chemically anchored to the surface of TiO₂ and methylammonium lead iodide (MAPbI₃) through coordination effects and then in situ crosslinked to form a robust continuous polymer (Poly(TA)) network after thermal treatment, can be introduced into PSCs as a new bifacial passivation agent for greatly passivating the defects. It is also discovered that Poly(TA) can additionally enhance the charge extraction efficiency and the water‐resisting and light‐resisting abilities of perovskite film. These newly discovered features of Poly(TA) make PSCs herein achieve among the best PCE of 20.4% ever reported for MAPbI₃ with negligible hysteresis, along with much enhanced ultraviolet, air, and operational stabilities. Density functional theory calculations reveal that the passivation of MAPbI₃ bulk and PTLIs by Poly(TA) occurs through the interaction of functional groups (COOH, CS) in Poly(TA) with under‐coordinated Pb²⁺ in MAPbI₃ and Ti⁴⁺ in TiO₂, which is supported by X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.

Metal halide perovskite solar cells (PSCs), with their exceptional properties, hold potential as photoelectric converters. However, defects in the perovskite layer, particularly at the grain boundaries (GBs), seriously restrict the performance and stability of PSCs. Herein, we present a simple post‐treatment procedure by applying 2‐aminoterephthalic acid to the perovskite to produce efficient and stable PSCs. By optimizing the post‐treatment conditions, we created a device that achieved a remarkable power conversion efficiency (PCE) of 21.09% and demonstrated improved stability. This improvement was attributed to the fact that the 2‐aminoterephthalic acid acted as a cross‐linking agent that inhibited the migration of ions and passivated the trap states at GBs. These findings provide a potential strategy for designing efficient and stable PSCs regarding the aspects of defect passivation and crystal growth.

Publication date: March 2020

Source: Nano Energy, Volume 69

Author(s): Ngoc Duy Pham, Jing Shang, Yang Yang, Minh Tam Hoang, Vincent Tiing Tiong, Xiaoxiang Wang, Lijuan Fan, Peng Chen, Liangzhi Kou, Lianzhou Wang, Hongxia Wang

Graphical abstract

Alkaline-earth bis(trifluoromethanesulfonyl)imide additives including Mg-TFSI₂ and Ca-TFSI₂, are used to replace Li-TFSI in synthesis of Spiro-OMeTAD-based hole transport layer for enhanced photovoltaic performance and environmental stability of the subsequent perovskite solar cells.

Real‐time grazing incidence X‐ray scattering measurements show that the additive‐tuned film formation of intercalating polymer:fullerene bulk heterojunction (BHJ) blends subdivides into five periods, which correspond to a multistep contraction of the lamellar stacking of the polymer chains.

Fullerene intercalation between the side chains of conjugated polymers has a detrimental impact on both charge separation and charge transport processes in bulk heterojunction (BHJ) organic photovoltaic cells (OPVs). In situ grazing incidence X‐ray scattering experiments allow to characterize the structure formation, drying kinetics, and intercalation in blends of phenyl‐c61‐butyric acid methyl ester (PC₆₀BM) and poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) named (pBTTT‐C14) from their 1,2‐orthodichlorobenzene (oDCB) solutions with different volume fractions of dodecanoic acid methyl ester (Me12) as a solvent additive. The structure formation process during evaporation of the solvent:additive mixture can be described by five periods, which are correlated to a multistep contraction of the lamellar stacking of the bimolecular crystals. The onset of crystallization is delayed by increasing the additive volume fraction in the coating solution leading to a promoted crystallinity. A conclusive picture of fullerene intercalation and additive‐tuned structural evolution during the drying of thin films of the polymer:fullerene BHJ blends will be presented.

To date, the power conversion efficiencies of single‐junction flexible perovskite solar cells (FPSCs) have surged to 19.51% within less than a decade. This Review discusses recent advances of device structural components of FPSC devices, including perovskite absorber layers, flexible substrates, transparent bottom electrodes, and charge carrier transport layers, as well as interface modification layers.

Flexible solar cells have launched potential applications in diverse areas, such as civil engineering, consumer electronics, electric automobile, and aerospace use. In recent years, perovskite solar cells (PSCs) have been successful with a rocketing power conversion efficiency (PCE) from 3.8% to 25.2% in the last decade. Due to its material flexibility, together with low‐cost and facile fabrication processes, perovskites have certainly been considered as a promising candidate for the next generation of flexible solar cells. So far, the PCE of single‐junction flexible perovskite solar cells (FPSCs) has reached 19.51%, which retains researchers' great enthusiasm for further development and applications of FPSCs. Herein, a brief review of the recent advances in the structural components of FPSCs is presented, including perovskite absorber layers, flexible substrates, transparent bottom electrodes, and charge carrier transport layers. In particular, the focus is on the development of low‐temperature fabrication processes of the aforementioned compositional layer structures. Finally, noticeable achievements and annual milestones are discussed and summarized, and suggestions for further improvement of FPSCs and their ways toward commercialization are presented.

A new nonfullerene acceptor (MPU4), bearing a selenophene–diketopyrrolopyrrole (DPP) core, leads to an efficiency of 8.96% in binary solar cells, 24% higher than that using the analogous thiophene–DPP. Ternary solar cells SM1:PC₇₁BM:MPU4 afford an efficiency of 10.04% and a low energy loss of only 0.49 eV, demonstrating the advantages of using selenophene in organic solar cells.

A new near‐IR absorbing nonfullerene acceptor (NFA), MPU4, is synthesized in three steps from an accessible selenophene–diketopyrrolopyrrole and rhodanine. The high planarity of the molecule and the extended conjugation determine that the new NFA presents a high optical absorption coefficient and a narrow bandgap. In thin films, MPU4 shows broad absorption in the visible and near‐IR regions from 550 to 930 nm. When blended with a phenothiazine‐based small‐molecule (SM) donor, SM1, the resulting additive‐free binary organic solar cell (OSC) exhibits an efficiency of 8.96% with a high open‐circuit voltage (V _oc) of 0.99 V and a short‐circuit current (J _sc) of 14.91 mA cm⁻² with a remarkably low energy loss (E _loss) of 0.42 eV. This efficiency is significantly higher (24%) than that achieved with an analogous device with a thiophene‐containing NFA (MPU1, 7.22%) instead of MPU4. Importantly, ternary solar cells prepared with SM1 as the donor and MPU4 and PC₇₁BM as acceptors afford, after solvent vapor annealing, an efficiency of 10.04% with a V _oc of 0.92 V, J _sc of 16.32 mA cm⁻², fill factor of 0.67, and E _loss of 0.49 eV. These results demonstrate the advantages of using selenophene instead of thiophene in SM OSCs.

Tetraethyl orthosilicate processed silica oligomer is in situ introduced into perovskite films to serve as a passivation agent (PA) for perovskite solar cells (PVSCs). Silica oligomer PA can enlarge perovskite grain sizes, prolong carrier lifetime, enhance charge carrier dynamics, and reduce trap state densities, resulting in highly efficient PVSCs with good humid and thermal stability.

Abstract

Perovskite solar cells (PVSCs) have achieved excellent power conversion efficiency (PCE) but still suffer from instability issues. Defect passivation is an important route to simultaneously increase the efficiency and stability of PVSCs. Here, a strategy of incorporating silica oligomer in perovskite films for surface and grain boundary defect passivation is reported. Silica oligomer passivation agent (PA) is in situ formed through hydrolysis and condensation reaction of tetraethyl orthosilicate additive in perovskite precursor. The passivation mechanism is elucidated by density functional theory calculation, revealing stable chelating interaction and hydrogen bond interaction between PA and perovskite. Spectroscopic and electrical characterizations demonstrate that silica oligomer can enlarge grain sizes, prolong carrier lifetime, enhance charge carrier dynamics, and reduce trap state densities in perovskite films. Planar PVSCs with passivation achieve a highly improved PCE of 19.64% with a stabilized efficiency of 18.81%. More importantly, unencapsulated perovskite devices with passivation retain nearly 90% of original efficiency after 1000 h storage under ambient condition and sustained 87% of initial performance after high‐temperature (120 °C) thermal accelerated aging, showing highly enhanced moisture and thermal stability. Therefore, the present study provides a pathway to the future design and optimization of PVSCs with higher efficiency and greater stability.

The effects of adding different Lewis bases on the composition of quasi‐2D crystal phases are investigated. The introduction of stable intermediate complexes not only enhances spatial uniformity of crystallization but also modulates the orientation. Weak dative bonding and strong hydrogen bonding combine to enhance device performance.

Abstract

Crystallographic orientation has a significant impact on the optoelectronic properties of films of quasi‐2D perovskite quantum wells. Here, oxygen‐bearing Lewis bases are employed as additives to explore their ability to modulate spatial uniformity of crystallization and orientation of crystal phases. Different Lewis bases added into the precursor solutions incorporating the large organic ammonium cation, phenethylammonium (PEA⁺), lead to different crystallization kinetics, which are attributed to the varying stability of intermediate complexes. The microscopic photoluminescence heterogeneity and 2D X‐ray diffraction patterns of the thin films reveal that inclusion of Lewis bases can lead to spatially more uniform crystallization and random orientation, resulting in an enhancement in light‐emitting diode performance. In contrast, quasi‐2D phases formed without Lewis bases show poorer uniformity and preferentially vertical orientation. Comparing the Lewis base properties such as Mayer order unsaturation and polarizability suggests that the ability to weakly coordinate with lead and strongly interact with the large organic ammonium is a key factor in controlling the phase composition favorably toward highly luminescent light‐emitting diodes. This work may be of help to provide insight of what kinds of Lewis bases can be helpful to realize the desired phase composition for high performance of optoelectronic applications.

The present study reveals a strong influence of sputtered NiO _x on the perovskite crystallization and the appearance of residual PbI₂ grains resulting in low photovoltaic device performance. Among different methylammonium (MA⁺) halide additives and vapor treatment (to improve the perovskite crystallization) only MA⁺ halide vapor‐treated perovskite shows suppressed recombination, enhanced carrier lifetime, and device efficiency.

Abstract

Investigating the low efficiency issue of radio frequency‐sputtered nickel oxide (sp‐NiO _x )‐based perovskite solar cells (PSCs) due to a limited understanding of the correlation between perovskite growth and sp‐NiO _x on the optoelectronic properties and photovoltaic device performance is critical. Herein, the crystallization of methylammonium (MA) lead iodide (MAPbI₃) thin film (obtained from stoichiometric precursor ratio) on sp‐NiO _x is shown, resulting in appearance of residual PbI₂ grains. This is in contrast to perovskite growth on solution‐processed NiO _x . The amount of residual PbI₂ is suppressed by 1) adding excess MACl/MAI additives and 2) annealing the perovskite film in MACl/MAI vapor atmosphere. Structural and morphological results reveal significant reduction in the amount of residual PbI₂ and enhanced grain size for all the cases while photophysical measurements reveal mitigation of trap/defect sites (within the bulk and at the interfaces) only for MACl/MAI vapor annealing case. As a result, photovoltaic devices exhibit improved performance only for the vapor annealing case. These results elucidate the critical role of maintaining stoichiometric ratio in perovskite and its crystallization on sp‐NiO _x by eliminating the associated defects (influenced by sp‐NiO _x ) in rendering improved performance, which can be insightful to further enhance the performance of PSCs.