Chen Weijie

Two PM6-based D–A₁–D–A₂ type terpolymers PMT-FT-10 and PMT-CT-10 are synthesized by introducing 10% thiazolothiazole as the second electron-accepting (A₂) units connecting with thiophene π-bridges attaching alkyl substituent toward the A₂ unit (PMT-CT-10) or toward D-unit (PMT-FT-10). The polymer solar cells with PMT-CT-10 as donor and Y6 as acceptor show the highest power conversion efficiency of 18.21%.

Abstract

PM6 is a widely used D–A copolymer donor in the polymer solar cells (PSCs). Incorporating second electron-withdrawing (A₂) units into PM6 backbone by ternary D–A₁–D–A₂ random copolymerization strategy is an effective approach to further improve its photovoltaic performance. Here, the authors synthesize the PM6-based terpolymers by introducing thiazolothiazole as the A₂ units connecting with thiophene π-bridges attaching alkyl substituent towards the A₂ unit (PMT-CT) or towards D-unit (PMT-FT), and study the effect of the alkyl substituent position on the photovoltaic performance of them. Two terpolymers PMT-FT-10 and PMT-CT-10 are obtained by incorporating 10% A₂ units in the terpolymers. The film of PMT-CT-10 shows slightly up-shifted highest occupied molecular orbital (HOMO) energy levels while better co-planar structure than that of PMT-FT-10. Meanwhile, the PMT-CT-10:Y6 blend film exhibits better molecular packing properties, more proper phase separation and more balanced hole and electron mobilities, which are beneficial to more efficient exciton dissociation, efficient charge transport and weaker bimolecular recombination. Consequently, the PMT-CT-10 based PSCs obtain the highest power conversion efficiency of 18.21%. The results indicate that side chain position on the thiophene π-bridges influence the device performance of the terpolymer donors, and PMT-CT-10 is a high efficiency polymer donor for the PSCs.

A thin layer of low-dimensional halide (LDH) is inserted at the perovskite/C₆₀ interface in inverted (positive-intrinsic-negative) perovskite solar cells. Induced by imidazolium-based ionic liquid, the LDH enables a strong electronic coupling at the heterointerface to effectively shift the surface gap states out of the perovskite bandgap. This approach enables >24% efficiency along with excellent thermal and operational stability.

Abstract

The photovoltaic performance of inverted (positive-intrinsic-negative) perovskite solar cells (PSCs) is predominantly limited by interfacial recombination loss. Here, by constructing a low-dimensional halide/perovskite heterostructure, non-radiative recombination pathways at the perovskite/C₆₀ contact are effectively eliminated and a voltage loss of only 370 mV is achieved in inverted PSCs. Through molecular engineering of the organic spacer, a strong electronic coupling is enabled at the heterointerface, which effectively shifts the gap states out of the bandgap and leads to a prolonged carrier lifetime of 4.28 µs. Our strategy enables a power conversion efficiency of 24.09% (certified 23.54%) for inverted PSCs with an open-circuit voltage of 1204 mV, and an efficiency of 21.89% (certified 21.48%) for centimeter-scale cells. The devices retain 92% of the initial efficiency after 85 °C thermal aging for over 1400 h, and 95% of the initial efficiency after 1008 h of maximum power point operation under AM1.5G illumination in air.

The effects of sulfonium cation on the evolution of intermediates in quasi-2D Ruddlesden–Popper perovskites are investigated. It is found that the undesired intermediate can be suppressed with a favorable intermediate promoted after treatment, leading to improved film quality and enhanced device performance with superior photovoltaic efficiency and stability.

Abstract

Quasi-2D Ruddlesden–Popper (RP) perovskites with superior stability are admirable candidates for perovskite solar cells (PSCs) toward commercialization. However, the device performance remains unsatisfactory due to the disordered crystallization of perovskites. In this work, the effects of sulfonium cations on the evolution of intermediates and photovoltaic properties of 2D RP perovskites are investigated. The introduction of sulfonium cations leads to preferred intermediate transformation and improved film quality of perovskites. The resulting devices deliver a champion efficiency of 19.08% at room temperature and 20.52% at 180 K, due to reduced recombination and enhanced charge transport. More importantly, the unencapsulated device maintains 84% of the initial efficiency under maximum power point (MPP) tracking at 40 °C for 1000 h. This work helps to gain a comprehensive understanding of the crystallization process of quasi-2D perovskites and provides a simple strategy to modulate the intermediates of perovskites.

Monolayer inverse opal SnO₂ (IO-SnO₂) is synthesized and used as a scaffold for the growth of perovskite layer. The IO-SnO₂ scaffold provides abundant electron transport channels, shortens the electron transport distance, and enhances the built-in electric field. Consequently. the separation and transfer of photogenerated electrons are effectively facilitated in an IO-SnO₂-based perovskite solar cell.

Abstract

Organic–inorganic halide perovskite solar cells (PSCs) have attracted tremendous attention in the photovoltaic field due to their excellent optical properties and simple fabrication process. However, the recombination of photogenerated electron–hole pairs at the interface severely affects the power conversion efficiency (PCE) of the PSCs. Herein, a monolayer of inverse opal SnO₂ (IO-SnO₂) is synthesized via a template-assisted method and used as a scaffold for perovskite layer (PSK). The porous IO-SnO₂ scaffold increases the contact area and shortens the transport distance between the electron transport layer (ETL) and PSK. Ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy results indicate that the built-in electric field is enhanced with IO-SnO₂ scaffold, strengthening the driving force for charge separation. Femtosecond transient absorption spectroscopy measurements reveal that the IO-SnO₂ scaffold facilitates interfacial electron transfer from PSK to ETL. Based on the above superiorities, the IO-SnO₂-based PSCs exhibit boosted PCE and device stability compared with the pristine PSCs. This work provides insights into the development of novel scaffold layers for high-performance PSCs.

An effective method based on the synergy of Pb(SCN)₂ and 2-thiopheneethylammonium chloride to suppress phase segregation and non-radiative recombination is proposed. Finally, the two-terminal perovskite/organic tandem solar cells exhibit a high V _OC of 2.072 V and a power conversion efficiency of 22.29%, and maintain 81% initial efficiencies after 1000 h maximum power point tracking.

Abstract

Wide bandgap (WBG) perovskites through tuning iodine/bromine ratios are capable of merging with narrow bandgap organic bulk heterojunctions to construct tandem solar cells to overcome the Shockley–Queisser limitation. However, WBG perovskites readily suffer from light-induced halide ion migration, leading to detrimental phase segregation and hence severe open-circuit voltage (V _OC) loss. Here, to solve this issue, lead thiocyanate (Pb(SCN)₂) and 2-thiopheneethylammonium chloride (TEACl) are synergistically employed to passivate and stabilize WBG perovskites with 1.79 eV bandgap. It is demonstrated that the synergetic employment of Pb(SCN)₂ and TEACl suppresses light-induced phase segregation, passivates WBG perovskite defects, and reduces non-radiative recombination, hence alleviating V _OC loss. As a result, optimized WBG perovskite solar cells (PSCs) are obtained with an impressive V _OC of 1.26 V and power conversion efficiency (PCE) over 17.0%. Furthermore, the interconnection layer is optimized to minimize the V _OC loss and construct two-terminal perovskite/organic tandem solar cells with a narrow bandgap organic blend bulk heterojunction of PM6:Y6 and achieve a champion PCE of 22.29% with a high V _OC of 2.072 V. In addition, these tandem solar cells maintain 81% of their initial efficiency after 1000 h continuous tracking at the maximum power point (MPP) under 100 mW cm⁻² white light illumination.

A record power conversion efficiency of 19.02% is achieved for a PM6:BTP-eC9:PC₇₁BM-based ternary organic solar cell (OSC) with self-aggregated light-trapping N,N,N′,N′-tetraphenylmalonamide nanodots, opening a window for the rational design of multifunctional cathode buffer layers for efficient and stable OSCs.

Abstract

The typical thickness of the photoactive layer in organic solar cells (OSCs) is around 100 nm, which limits the absorption efficiency of the incident light and the power conversion efficiency (PCE) of OSCs. Therefore, light-trapping schemes to reduce the optical losses in the thin photoactive layers are critically important for efficient OSCs. Herein, light-trapping and electron-collection dual-functional small organic molecules, N,N,N′,N′-tetraphenyloxalamide (TPEA) and N,N,N′,N′-tetraphenylmalonamide (TPMA), are designed and synthesized by a one-step acylation reaction. Driven by strong intermolecular force, TPEA and TPMA tend to self-aggregate into hemispherical light-trapping nanodots on the photoactive layer, resulting in enhanced light harvesting. Meanwhile, TPEA and TPMA demonstrate high electron mobility and excellent electron-collection ability.  Compared with the device without cathode buffer layer (CBL, PCE = 14.09%), PM6:BTP-eC9 based OSCs with TPEA and TPMA light-trapping CBLs demonstrate greatly enhanced PCE of 16.21% and 17.85%, respectively. Furthermore, a record PCE of 19.02% can be achieved for PM6:BTP-eC9:PC₇₁BM based ternary OSC with TPMA light-trapping CBL. Moreover, TPMA exhibits a low synthesis cost of only 0.61 $ g⁻¹ with high yield. These findings could open a window for the rational design of multifunctional CBLs for efficient and stable OSCs.

Commercialization of all-perovskite tandem solar cells requires both, a thermally stable narrow-bandgap perovskite and a tunnel junction. Here all-FA Pb-Sn perovskite with superior intrinsic thermal stability and metal-oxide-based tunnel junction are deployed synchronously, which resulted in a power conversion efficiency of 26.3% and much improved thermal stability in all-perovskite tandem solar cells.

Abstract

Commercialization of all-perovskite tandem solar cells requires thermally stable narrow-bandgap (NBG) perovskites and tunnel junction. However, the high content of methylammonium (MA) and organic hole transport layer used in NBG perovskite subcell undermine the thermal stability of all-perovskite tandems. Here, thermally stable mixed lead-tin NBG perovskite solar cells (PSCs) are developed by using only formamidinium (FA) for the A-site cation. Solution-processed indium tin oxide nanocrystals (ITO NCs) are deployed further to replace the conventional organic charge transport layer. Meanwhile, the ITO NCs layer simultaneously functions as a recombination layer in the tunnel junction, which simplifies the architecture of all-perovskite tandem devices. The thermally stable all-FA Pb-Sn PSCs achieve a high power conversion efficiency (PCE) of 21.0%. With the thermally stable all-FA NBG perovskite and optimized tunnel junction, a stabilized PCE of 26.3% is further obtained in all-perovskite tandems. The unencapsulated tandem devices maintain >90% of their initial efficiencies after 212 h aging at 85 °C in the N₂ atmosphere. The strategies herein offer a crucial step toward efficient and thermally stable all-perovskite tandem solar cells.

The role of the surface modulator cannot simply be attributed to the passivation effect. Here, it is shown that the strong-interaction surface modulator, 2-thiopheneethylammonium iodide (2-TEAI, is helpful in inverted (p-i-n) perovskite solar cells. Through forming a quasi-2D structure and reconfiguring the electronic energy level of perovskite film, 2-TEAI contributes to the reduced interfacial recombination losses, and enhanced device performance.

Abstract

Successful manipulation of halide perovskite surfaces is typically achieved via the interactions between modulators and perovskites. Herein, it is demonstrated that a strong-interaction surface modulator is beneficial to reduce interfacial recombination losses in inverted (p-i-n) perovskite solar cells (IPSCs). Two organic ammonium salts are investigated, consisting of 4-hydroxyphenethylammonium iodide and 2-thiopheneethylammonium iodide (2-TEAI). Without thermal annealing, these two modulators can recover the photoluminescence quantum yield of the neat perovskite film in contact with fullerene electron transport layer (ETL). Compared to the hydroxyl-functionalized phenethylammonium moiety, the thienylammonium facilitates the formation of a quasi-2D structure onto the perovskite. Density functional theory and quasi-Fermi level splitting calculations reveal that the 2-TEAI has a stronger interaction with the perovskite surface, contributing to more suppressed non-radiative recombination at the perovskite/ETL interface and improved open-circuit voltage (V _OC) of the fabricated IPSCs. As a result, the V _OC increases from 1.11 to 1.20 V (based on a perovskite bandgap of 1.63 eV), yielding a power conversion efficiency (PCE) from ≈20% to 21.9% (stabilized PCE of 21.3%, the highest reported PCEs for IPSCs employing poly[N,N′′-bis(4-butylphenyl)-N,N′′-bis(phenyl)benzidine] as the hole transport layer, alongside the enhanced operational and shelf-life stability for unencapsulated devices.

Publication date: 15 December 2022

Source: Nano Energy, Volume 104, Part A

Author(s): Wei Cao, Jian Zhang, Kaifeng Lin, Lele Qiu, Junzhuo Li, Yayu Dong, Debin Xia, Yulin Yang

Non-stoichiometric formamidinium tin triiodide perovskite solar cells without tin fluoride and with ethylenediamine dihydroiodide addition have increased built-in potential, lowered dark currents, and reduced carrier recombination. Consequently, the SnF₂-free solar cells present lower hysteresis, higher efficiency, and better stability than those with SnF₂ additive, enabling the best-performing cell with an impressive efficiency of 10.14%.

Tin halide perovskite solar cells (TPSCs) have attracted extensive attention because of their low toxicity and high theoretical efficiency. However, rapid crystallization, rich defects, and easy oxidation of tin-based perovskite films seriously restrict solar cell performance. Herein, a composition and additive engineering solution by removing commonly used tin fluoride additive from formamidinium tin triiodide perovskite precursors and replacing it with excess tin (II) iodide and ethylenediamine dihydroiodide (EDAI₂) is adopted. The presence of EDAI₂ and excess SnI₂ and the absence of SnF₂ enable tin-based perovskite films with greatly improved morphology, enhanced crystalline quality, boosted optical properties, and prolonged carrier lifetime. Therefore, the SnF₂-free non-stoichiometric TPSCs employing the EDAI₂ additive and excess SnI₂ have increased built-in potential, lowered dark currents, and reduced carrier recombination. The resulting SnF₂-free and SnI₂-rich TPSCs with EDAI₂ addition realize a steady-state efficiency of 10.0%, which is one of the highest power conversion efficiencies among the SnF₂-free TPSCs, along with considerably improved light stability. This work highlights the crucial roles of additives in TPSCs and provides an effective strategy to fabricate efficient and stable low-toxic lead-free perovskite solar cells.

Polyethylenimine ethoxylated-phenyl acetyl chloride (PEIE-PAC) is obtained by quaternization and esterification of PEIE. And PEIE-PAC is used to passivate oxygen defects in ZnO and enhance the surface energy of ZnO, thus forming dense and uniform ZnO-P films for efficient and stable nonfullerene organic solar cells.

Charge transport is a meaningful process in nonfullerene organic solar cells (NOSCs). However, owing to the inherent defects of its metal oxide, the classic ZnO electron transport layer will severely hinder charge transport, and its high surface energy can cause a mismatch contact with the low surface energy active layer. Therefore, polyethylenimine ethoxylated (PEIE) is treated with quaternization and esterification to obtain polyethylenimine ethoxylated-phenyl acetyl chloride (PEIE-PAC). ZnO:PEIE-PAC (ZnO-P) with fewer defects and lower surface energy film is prepared by blending PEIE-PAC with the precursor solution of ZnO. Compared with ZnO-based control device with a power conversion efficiency (PCE) of 15.30%, the PCE of ZnO-P-based device is increased to 16.47% in PM6:Y6 system due to the optimization of oxygen defects and surface roughness. Meanwhile, owing to the decrease of surface energy in ZnO-P, the contact between ZnO-P and hydrophobic nonfullerene acceptors is closer, and it can isolate water and oxygen in the external environment, thus enhancing the stability in air of NOSCs. The ZnO-P-based PM6:Y6 device maintains long-term storage stability in both N₂ and ambient temperature, and after 1000 h aging, the PCE of ZnO-P-based device is still more than 85% of the initial PCE.

Interfacial buffer layers have a decisive impact on the performance and stability of organic solar cells. This review summarizes representative hole transporting materials, including organics, graphene-based molecules, and transition metal oxides, in an attempt to highlight the role of hole transport layers in regulating charge transport, tuning energy levels, reducing potential barriers, and reducing interfacial recombination losses.

Abstract

Recently, organic solar cells (OSCs) have received rapid boosts in the power conversion efficiency (PCE), due to progresses in materials and device engineering. Several groups have reported champion PCEs over 19% in single-junction, ternary, and tandem OSCs. In addition to the concentrated focus on the new design of OSC active materials, buffer layer materials, used for the interface layer providing the functionalities of interface charge transport and collection in OSCs, are of critical importance for the optimization of PCE. Compared with the electron transport layers (ETL), the hole transport layers (HTLs) have received less attention and are still dominated by the commercial material poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), which has limitations for the efficiency and OSC device stability enhancement. In this Review, the recent progress in HTL materials, including the modifications for PEDOT:PSS, alternative HTL materials based on polymers and inorganic oxide materials, etc, is summarized. This review also provides a summative prospect aiming to help scientists apprehend the current possibilities and challenges in this field.

Extending the donor core in a nonfullerene acceptor (NFA) is applied to suppress trap states in organic solar cells (OSCs). TTPIC-4F with an extended core exhibits higher crystallinity than BTPIC-4F. Thus, D18:TTPIC-4F-based OSCs afford a lower trap density of 4.02 × 10¹⁵ cm⁻³ for higher mobility and reduced ΔE _nr, and achieve an impressive PCE of 17.1%, which is the record for A–D–A-type NFAs.

Abstract

The high trap density (generally 10¹⁶ to 10¹⁸ cm⁻³) in thin films of organic semiconductors is the primary reason for the inferior charge-carrier mobility and large nonradiative recombination energy loss (ΔE _nr) in organic solar cells (OSCs), limiting improvement in power conversion efficiencies (PCEs). In this study, the trap density in OSCs is efficiently reduced via extending the donor core of nonfullerene acceptors (NFAs) from a heptacyclic unit to a nonacyclic unit. TTPIC-4F with a nonacyclic unit has stronger intramolecular and intermolecular interactions, affording higher crystallinity in thin films relative to its counterpart BTPIC-4F. Thus, the D18:TTPIC-4F-based device achieves a lower trap density of 4.02 × 10¹⁵ cm⁻³, comparable to some typical high-performance inorganic/hybrid semiconductors, with higher mobility and inhibited charge-carrier recombination in devices. Therefore, the D18:TTPIC-4F-based OSC exhibits an impressive PCE of 17.1% with a low ΔE _nr of 0.208 eV, which is the best known value for A–D–A-type NFAs. Therefore, extending the donor core of NFAs is an efficient method for suppressing trap states in OSCs for high PCEs.

A new polyfullerene material (PFBS-C12) is developed that can work efficiently as the electron transporting material of p-i-n perovskite solar cells (PSCs). PFBS-C12 retains the figure-of-merits of conventional fullerene molecules, and shows suppressed aggregation and more conformal coverage on perovskites compared to PCBM. As a result, the p-i-n PSCs based on PFBS-C12 realize a high efficiency of 23.2 % with good device stability.

Abstract

Electron transporting materials (ETMs) play vital roles in determining the efficiency and stability of inverted perovskite solar cells. The widely used PCBM is prone to undesirable aggregation and migration in a cell, thus impairing device stability. In this work, we develop a new type of ETMs by polymerizing C60 fullerene with an aromantic linker unit. The resultant polyfullerene (PFBS-C12) not only maintains the good optoelectronic properties of fullerenes, but also can address the aforementioned aggregation problem of PCBM. The polyfullerene-based blade-coated cells exhibit a high efficiency of 23.2 % and good device stability that maintain 96 % of initial efficiency after >1300-hour light soaking. An aperture efficiency of 18.9 % is also achieved on a 53.6-cm² perovskite mini-module. This work provides a new strategy for designing ETMs that retain the key figure-of-merits of conventional fullerene molecules and enable more stable perovskite solar devices simultaneously.

Ionic liquid methylammonium acetate (MAAc) is introduced into the PbI₂ layer to produce a perovskite film via a two-step method. Systematic investigations on the influence of MAAc on the perovskite formation mechanism are performed. The theoretical calculations confirm that Ac⁻ can occupy the iodine vacancy of the perovskite material in the bulk and on the surface. The final device shows improvement of the power conversion efficiency over 23%.

Much of the research efforts of late that are directed toward enhancement of the efficiency of perovskite-based photovoltaics are focused on the application of ionic liquids (ILs) in a one-step approach. On the contrary, the details of the alternative two-step approach, such as the role of the ILs in the perovskite film solidification and its optoelectronic properties, remain poorly understood despite the increasing evidence that this latter method might offer considerable advantages, including better reproducibility and control over crystallization. Herein, the effect of IL methylammonium acetate (MAAc) introduced into the PbI₂ layer by a sequential deposition process on the optoelectronic properties of the perovskite film and the performance of the ensuing photovoltaic devices are studied. The addition of MAAc lowers the MAAc–perovskite formation enthalpy, leading to an accelerated solidification process. Moreover, MAAc suppresses the formation of Pb⁰, thereby facilitating the perovskite formation while lowering the deep defect states caused by Pb⁰. In addition to grain boundary passivation, the acetate ions can diffuse into the bulk of the perovskite material, filling up the halide vacancies with reduced trap state density. As a result, a decent power conversion efficiency of 23.36% is achieved, with noticeably improved durability.

A strong chelate coordination bond is designed to terminate the surface of the perovskite absorber layer. The formation of the chelate bond not only alters the tolerance of the perovskite film to high humidity but also results in weak ion migration, thus enhancing the long-term stability and power conversion efficiency.

Abstract

Solar cell efficiency and stability are two key metrics to determine whether a photovoltaic device is viable for commercial applications. The surface termination of the perovskite layer plays a pivotal role in not only the photoelectric conversion efficiency (PCE) but also the stability of assembled perovskite solar cells (PSCs). Herein, a strong chelate coordination bond is designed to terminate the surface of the perovskite absorber layer. On the one hand, the ligand anions bind with Pb cations via a bidentate chelating bond to restrict the ion migration, and the chelate surface termination changes the surface from hydrophilic to hydrophobic. Both are beneficial to improving the long-term stability. On the other hand, the formation of the chelating bonding effectively eliminates the deep-level defects including Pb_I and Pb clusters on the Pb-I and FA-I terminations, respectively, as confirmed by theoretical simulation and experimental results. Consequently, the PCE is increased to 24.52%, open circuit voltage to 1.19 V, and fill factor to 81.53%; all three are among the highest for hybrid perovskite cells. The present strategy provides a straightforward means to enhance both the PCE and long-term stability of PSCs.

A hybrid block copolymer/perovskite heterointerfaces strategy is developed for the first time by introducing a novel and functional block copolymer PBDB-T-b-PTY6, and consequently, such an architecture enhances light harvesting, promotes charge transfer and prevents moisture invasion, delivering a champion efficiency over 24% together with enhanced stability.

Abstract

Solution processable semiconductors like organics and emerging lead halide perovskites (LHPs) are ideal candidates for photovoltaics combining high performance and flexibility with reduced manufacturing cost. Moreover, the study of hybrid semiconductors would lead to advanced structures and deep understanding that will propel this field even further. Herein, a novel device architecture involving block copolymer/perovskite hybrid bulk heterointerfaces is investigated, such a modification could enhance light absorption, create an energy level cascade, and provides a thin hydrophobic layer, thus enabling enhanced carrier generation, promoting energy transfer and preventing moisture invasion, respectively. The resulting hybrid block copolymer/perovskite solar cell exhibits a champion efficiency of 24.07% for 0.0725 cm²-sized devices and 21.44% for 1 cm²-sized devices, respectively, together with enhanced stability, which is among the highest reports of organic/perovskite hybrid devices. More importantly, this approach has been effectively extended to other LHPs with different chemical compositions like MAPbI₃ and CsPbI₃, which may shed light on the design of highly efficient block copolymer/perovskite hybrid materials and architectures that would overcome current limitations for realistic application exploration.

The nucleation process to form quasi-2D perovskites is thermodynamically controlled using the methylammonium chloride additive to achieve films with a degree of preferential orientation of 94%, capable of withstanding 97% relative humidity for 10 h without degradation. Combining microscopic, macroscopic, and spectrographic observations, the thermodynamics enabling preferential crystal growth are inferred.

Abstract

Incorporating large organic cations to form 2D and mixed 2D/3D structures significantly increases the stability of perovskite solar cells. However, due to their low electron mobility, aligning the organic sheets to ensure unimpeded charge transport is critical to rival the high performances of pure 3D systems. While additives such as methylammonium chloride (MACl) can enable this preferential orientation, so far, no complete description exists explaining how they influence the nucleation process to grow highly aligned crystals. Here, by investigating the initial stages of the crystallization, as well as partially and fully formed perovskites grown using MACl, the origins underlying this favorable alignment are inferred. This mechanism is studied by employing 3-fluorobenzylammonium in quasi-2D perovskite solar cells. Upon assisting the crystallization with MACl, films with a degree of preferential orientation of 94%, capable of withstanding moisture levels of 97% relative humidity for 10 h without significant changes in the crystal structure are achieved. Finally, by combining macroscopic, microscopic, and spectroscopic studies, the nucleation process leading to highly oriented perovskite films is elucidated. Understanding this mechanism will aid in the rational design of future additives to achieve more defect tolerant and stable perovskite optoelectronics.

The influences of solvent type, substrate temperature, and coating speed on FA_0.75Cs_0.25PbI_2.7Br_0.3 perovskite by heat-assisted blade coating are systematically investigated for the first time. The structures and properties of perovskite films are studied in detail. Based on the correct strategy, the resulting solar cells achieve 20.2% efficiency and outstanding stability.

Perfecting large-area perovskite film coating technology remains a key challenge to commercialize perovskite solar cells. Herein, nontoxic dimethyl sulfoxide (DMSO) is recommended as the only solvent to fabricate formamidinium (FA)–cesium lead halide perovskite by heat-assisted blade coating. DMSO effectively promotes the formation of α-phase crystals and improves the crystallinity. In addition, high substrate temperature is found to improve the film compactness, preferred facet orientation, and desired phase transition. Fast coating mode is adopted to inhibit ion migration and reduce the hysteresis of solar cells. Without any additives or surface treatments, the resulting perovskite film with chemical formula FA_0.75Cs_0.25PbI_2.7Br_0.3 shows smooth surface, dense grains, and excellent crystallinity. The corresponding solar cells achieve efficiencies of 20.2% and 17.1% with active areas of 0.1 and 1.0 cm², respectively. The unencapsulated devices maintain ≈100% and 70% of their initial efficiencies after 500 h of storage in air with relative humidity of about 25% and heating at 85 °C, respectively.

A molecular design strategy is introduced for carbazole-derived self-assembled monolayers (SAM) to facilitate dense assembly and tune the indium tinoxide (ITO) workfunction. Through asymmetric or helical π-expansion, CbzPh and CbzNaph are obtained as efficient hole-selective layers (HSL) for inverted perovskite solar cells (PSC). Larger molecular dipoles in CbzPh and CbzNaph can better tune the ITO workfunction, whereas the stronger π-π interactions ensure more ordered and denser SAM assembly.

Abstract

Carbazole-derived self-assembled monolayers (SAMs) are promising hole-selective materials for inverted perovskite solar cells (PSCs). However, they often possess small dipoles which prohibit them from effectively modulating the workfunction of ITO substrate, limiting the PSC photovoltage. Moreover, their properties can be drastically affected by even subtle structural modifications, undermining the final PSC performance. Here, we designed two carbazole-derived SAMs, CbzPh and CbzNaph through asymmetric or helical π-expansion for improved molecular dipole moment and strengthened π-π interaction. The helical π-expanded CbzNaph has the largest dipole, forming densely packed and ordered monolayer, facilitated by the highly ordered assembly observed in its π-scaffold's single crystal. These synergistically modulate the perovskite crystallization atop and tune the ITO workfunction. Consequently, the champion PSC employing CbzNaph showed an excellent 24.1 % efficiency and improved stability.

Publication date: February 2023

Source: Journal of Energy Chemistry, Volume 77

Author(s): Yayu Dong, Shuang Gai, Jian Zhang, Ruiqing Fan, Boyuan Hu, Wei Wang, Wei Cao, Jiaqi Wang, Ke Zhu, Debin Xia, Lin Geng, Yulin Yang

Performance of perovskitesolar cells can be improved by reverse-biasing. Characterizations suggest reverse bias can increasecharge recombination resistance and improve carrier transport. It should beattributed to iodine ion migration, which reduces interfacial iodine vacanciesand passivates oxygen vacancies on tin dioxide. Consequently, an efficiency of 23.48% with the open-circuit voltage of 1.16 V is achieved.

Abstract

Perovskite solar cells (PSCs) are being developed rapidly and exhibit greatly potential commercialization. Herein, it is found that the device performance can be improved by manipulating the migration of iodine ions via reverse-biasing, for example, at −0.4 V for 3 min in dark. Characterizations suggest that reverse bias can increase the charge recombination resistance, improve carrier transport, and enhance built-in electric field. Iodine ions including iodine interstitials in perovskites are confirmed to migrate and accumulate at the SnO₂/perovskite interface under reverse-basing, which fill iodine vacancies at the interface and interact with SnO₂. First-principles calculations suggest that the SnO₂/perovskite interface with less iodine vacancies has a stronger interaction and higher charge transfer, leading to larger built-in electric field and improved charge transport. Iodine ions that may pass through the SnO₂/perovskite interface are also confirmed to be able to interact with Sn⁴⁺ and passivate oxygen vacancies on the surface of SnO₂. Consequently, an efficiency of 23.48% with the open-circuit voltage (V _oc) of 1.16 V is achieved for PSCs with reverse-biasing, as compared with the initial efficiency of 22.13% with a V _oc of 1.10 V. These results are of great significance to reveal the physics mechanism of PSCs under electric field.

This work, for the first time introduces an ALD-deposited ultrathin SnO_x buffer layer to further improve the performance of QJSCs. The authors in-depth investigated the significant influence of interfacial carrier density difference on the J−V hysteresis of QJSCs and almost eliminated this hysteresis by the SnO_x buffer layer. This work paves an effective pathway to the development of high-performance all-quantum-dot photovoltaic devices.

Abstract

Solution-processed solar cells are promising for the cost-effective, high-throughput production of photovoltaic devices. Colloidal quantum dots (CQDs) are attractive candidate materials for efficient, solution-processed solar cells, potentially realizing the broad-spectrum light utilization and multi-exciton generation effect for the future efficiency breakthrough of solar cells. The emerging quantum junction solar cells (QJSCs), constructed by n- and p-type CQDs only, open novel avenue for all-quantum-dot photovoltaics with a simplified device configuration and convenient processing technology. However, the development of high-efficiency QJSCs still faces the challenge of back carrier diffusion induced by the huge carrier density drop at the interface of CQDs and conductive glass substrate. Herein, an ultra-thin atomic layer deposited tin oxide (SnO_x) layer is employed to buffer this carrier density drop, significantly reducing the interfacial recombination and capacitance caused by the back carrier diffusion. The SnO_x-modified QJSC achieves a record-high efficiency of 11.55% and a suppressed hysteresis factor of 0.04 in contrast with reference QJSC with an efficiency of 10.4% and hysteresis factor of 0.48. This work clarifies the critical effect of interfacial issues on the carrier recombination and hysteresis of QJSCs, and provides an effective pathway to design high-performance all-quantum-dot devices.

Conductive passivation strategy is proposed to address one of the key issues of non-ideal open-circuit voltage and fill factor in industry-compatible, fully textured perovskite/silicon tandem solar cells. Benefitting from the better electrical property, favorable passivation ability, improved interface charge transfer, and wide processing window, efficiencies of 28.51% on 0.5036 cm² and 25.13% on 11.879 cm² are achieved.

Abstract

Monolithic perovskite/silicon tandem solar cells on commercially textured silicon with conformal perovskite top cells allow compatibility with standard industrial processes of silicon photovoltaic, as well as maximization of light trapping at the least cost. However, the efficiency is still limited by unsatisfactory open-circuit voltage (V _OC) and fill factor (FF), owing to the challenge of growing high-quality perovskite film on textured substrates and the lack of particularly effective passivation at the inferior interface between perovskite and C₆₀. Different from traditionally electricity-insulating passivator, herein, a conductive organic amine salt is introduced into this interface to suppress nonradiative recombination loss and promote carriers transfer synchronously, significantly increasing the V _OC while preserving high FF, thus improving the efficiency substantially. Finally, a champion efficiency of up to 28.51% is obtained. Furthermore, due to the favorable electrical properties of this molecule, there is a wide processing window for the passivation, which helps to achieve an efficiency of 25.13% when the active area is enlarged to 11.879 cm².