Chen Weijie

An investigation on the role of nonfullerene acceptor trace impurities on the optoelectronic characteristics and lifetime of resulting organic photovoltaics devices is presented. Interestingly, the less pure materials lead to longer device lifetimes along with an enhancement in accumulative power generation by a factor 3, compared to the purest ITIC-based devices as increased crystallization alters the favorable morphology.

The introduction of nonfullerene acceptors (NFAs) has pushed the power conversion efficiency and organic photovoltaics (OPV) device stability to new standards. In this aspect, removal of trace impurities from one purification stage to the next is frequently stressed throughout the synthesis of photoactive OPV materials and NFAs to obtain the highest-purity material. However, detailed studies of the effect of purification on device performance are less reported. Herein, the role of NFA trace impurities on the optoelectronic characteristics and lifetime of resulting OPV devices is studied. The optimization of PBDB-T:ITIC-X devices, with various ITIC purity levels (X), has been thoroughly studied via a combination of photophysical, chemical, morphological, electrical, and optical characterization techniques, to shine light on the role of these impurities on device performance and lifetime. The findings suggest that, even in materials with larger concentrations of trace impurities, careful tuning can produce high efficiencies. Interestingly, the less-pure materials lead to longer device lifetimes along with an enhancement in accumulative power generation by a factor 3, compared to the purest ITIC-based devices. This demonstrates that selecting a material with the highest purity may not always be the best option for NFA OPV and that any positive effects of NFA purification must be carefully considered in light of both the device efficiency and stability.

Introduction of the donor–π–acceptor organic blue dye between the perovskite and the spiro-hole transport material leads to robust perovskite solar cells presenting enhanced interfacial hole extraction, suppressed nonradiative carrier recombination, and power conversion efficiency values reaching 20.90%. Machine learning suggests the relative importance of photovoltaic parameters and online prediction model permits the estimation of power conversion efficiencies with high accuracy.

To prevent the degradation of perovskite solar cells (PSCs) and optimize the solar energy conversion process, a donor–π–acceptor (D–π–A) organic blue dye as a passivation layer and as a hole-transporting layer is introduced. The terminal chains of D–π–A dye confer the ultrahydrophobic character (contact angle > 100°) of the interface layer, protecting the perovskite from ambient moisture while mitigating ionic diffusion in the device. The dye interlayer primarily improves the perovskite by reducing grain boundary defects. The perovskite/D–π–A architecture enhances the interfacial hole extraction, suppressing nonradiative carrier recombination and enabling power conversion efficiency (PCE) reaching 20.90%, outperforming by 2.05% the PCE of control cells. Unsealed PSCs retain 84% and 62% of their efficiency after photovoltaic operation for 1000 and 3000 h, respectively. Statistical correlation of bivariant and multivariant analyses of photovoltaic parameters is performed and Pearson's correlation identifies underlying patterns in experimental data collections. Machine learning (ML) of regression algorithms is used to predict the minimum errors and the coefficient of determination, which confirm the analysis quality. The linear regression ML model suggests the importance of photovoltaic parameters (R _s > V _mpp > J _sc > V _oc> fill factor > J _mpp > R _sh) toward higher PCE. An efficient online prediction model is also developed to support the estimation of PCEs with high accuracy.

Two judiciously engineered passivators for use in perovskite solar cells based on the unique benzothiophene moiety involving the primary and secondary ammonium terminals, BTMA-1 and BTMA-2, respectively, are developed for the first time. Interestingly, the champion efficiency of the BTMA-2-treated device without the formation of low-dimensional perovskite increases to 23.10% from ≈20%, along with great stabilities and negligible hysteresis.

Aromatic ammonium salts have been regarded as the promising passivators in perovskite solar cell (PSC) fabrications. However, the complicated passivation procedure and inevitable formation of undesirable low-dimensional (LD) perovskite layers limit further development. Furthermore, how the steric and electronic properties of different ammonium cations would influence the passivation is not well understood. Herein, two carefully engineered passivators based on the unique benzothiophene moiety involving the primary and secondary ammonium terminals, BTMA-1 and BTMA-2, respectively, are developed. It is shown that defects and, thus, nonradiative recombination reactions are effectively suppressed by simple posttreatments without the formation of LD perovskite. Interestingly, the champion efficiency of the BTMA-2-treated device increases to 23.10% from ≈20%, along with great stabilities and negligible hysteresis. An in-depth understanding of the passivation effect influenced by steric and electronic properties is explored. The extra electron-donating methyl on the ammonium nitrogen (BTMA-2) increases the electron density on the N atom and the N–H⁺ ionic bond is, thus, boosted, which helps the positive terminal to anchor more tightly to the [PbI₆]⁴⁻ structure of the perovskite resulting in improved passivation effects. This novel and promising design strategy for ammonium passivators can promote PSCs to achieve further breakthroughs in both efficiency and stabilities.

Structural fine-tuning of coumarin-based donor by varying the central acceptor units resulted in optimal energy level modulation showing an excellent efficiency of 14.84% in a binary organic solar cell.

The choice of molecular material plays a crucial role in improving the performance of organic solar cells. Herein, two coumarin-based small molecules are designed and synthesized in which the π-system is extended by the introduction of double bonds. The molecules are differed by the attachment of central acceptor units. The replacement of the carbonyl group in C3 by the dicyanomethylene unit in C3–CN red-shift the absorption, reduced the energy gap, and stabilized the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. When blended with a complementary absorbing all-fused acceptor F13, the binary devices using C3 donor delivered a power conversion efficiency (PCE) of 11.09%. In contrast, C3–CN:F13-based binary device displayed an impressive PCE of 14.84% due to the obvious increase in all open circuit voltage, short circuit current density, and fill factor values. The inclusion of CN group to the molecular backbone induces a tighter intermolecular packing, which is beneficial for the charge transport properties. The study demonstrates that the subtle modification in conjugation length and tuning of the energy level by the proper choice of acceptor could be a promising way to improve the device's performance.

Transparent photovoltaics utilizing a layer-by-layer (LBL) approach demonstrate a power conversion efficiency of 8.8%, average visible transmittance of 40.9%, and light utilization efficiency of 3.6%. The impact of the acceptor polymer PTB7-Th thickness on optical and electronic device performance is thoroughly evaluated, revealing the tunability of the LBL device architecture and exceptional excitonic and electronic properties of these nonfullerene acceptors.

Transparent and semitransparent photovoltaics offer an exciting opportunity to integrate existing infrastructure with renewable energy. Organic photovoltaics (OPVs) are key enablers for wavelength-selective transparent photovoltaics (TPVs) because of their selective absorption in the near-infrared (NIR) that enables simultaneously high power conversion efficiency (PCE) and average visible transmittance (AVT). The recent rise of OPVs and TPVs has been accelerated in large part by the development of nonfullerene acceptors (NFAs) as highly adaptable deep NIR harvesting materials. Herein, sequential layer-by-layer (LBL) deposition of a selectively NIR absorbing nontraditional acceptor polymer is paired with a NIR absorbing donor IEICO-4F that is typically considered an NFA via solvent orthogonality. With detailed optimization of the active layers and top electrode, semi-transparent photovoltaics with a PCE of 8.8%, AVT of 40.9%, and a light utilization efficiency of 3.6% are demonstrated. The LBL approach enables explicit optical modeling of the device structure to extract exciton diffusion lengths >100 nm for both the polymer and IEICO-4F with a transition in charge collection length regimes dependent on the acceptor thickness. Furthermore, the LBL deposition technique enables an investigation of the full range of polymer thickness and its impact on power generation and optical performance.

An octylammonium tetrafluoroborate (OABF₄) additive is reported to enhance the efficiency and photostability for Sn-Pb PSCs. OABF₄ suppresses the iodide defect density in Sn-Pb perovskite films, boosting the efficiency to 23.7%. The OABF₄-based perovskite solar cells show less interstitial-iodides and I₂ generation under light soaking, leading to greatly enhanced photostability, retaining 88% of the initial power conversion efficiency for 1000 h under 1-sun operation.

Abstract

High-performance tin-lead perovskite solar cells (PSCs) are needed for all-perovskite-tandem solar cells. However, iodide related fast photodegradation severely limits the operational stability of Sn-Pb perovskites despite the demonstrated high efficiency and thermal stability. Herein, this work employs an alkylammonium pseudo-halogen additive to enhance the power conversion efficiency (PCE) and photostability of methylammonium (MA)-free, Sn-Pb PSCs. Density functional theory (DFT) calculations reveal that the pseudo-halogen tetrafluoroborate (BF₄ ⁻) has strong binding capacity with metal ions (Sn²⁺/Pb²⁺) in the Sn-Pb perovskite lattice, which lowers iodine vacancy formation. Upon combining BF₄ ⁻ with an octylammonium (OA⁺) cation, the PCE of the device with a built-in light-scattering layer is boosted to 23.7%, which represents a new record for Sn-Pb PSCs. The improved efficiency benefits from the suppressed defect density. Under continuous 1 sun illumination, the OABF₄ embodied PSCs show slower generation of interstitial iodides and iodine, which greatly improves the device photostability under open-circuit condition. Moreover, the device based on OABF₄ retains 88% of the initial PCE for 1000 h under the maximum-power-point tracking (MPPT) without cooling.

Kesterite solar cells on transparent electrodes are fabricated via direct phase transformation grain growth from an Sn⁴⁺-based solution. An efficiency of 7.02% is achieved on bare fluorine-doped tin oxide. Through the use of a MoO₃ back contract interfacial layer to improve absorber morphology, Na-doping to compensate alkali ions, and Ag alloying to mitigate band tailing, kesterite solar cells with a certified efficiency of 11.43% are demonstrated.

Abstract

Thin film solar cells on semitransparent substrates are attracting much attention due to new application scenarios including building-integrated photovoltaics (BIPV). Environmentally-benign element constituted and highly stable kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film solar cells are ideal candidates for such applications. However, the efficiency of kesterite solar cells on semitransparent substrates is far behind that on opaque Mo-based substrates. Here, fabrication of CZTSSe solar cells on fluorine-doped tin oxide (FTO) substrates from molecular solution and how step-by-step absorber engineering improves device performance is reported. A power conversion efficiency of 7.02% is obtained when the absorber is fabricated on bare FTO, which is improved to 9.56% after adding a MoO₃ interfacial layer. Investigations show the enhancement originates from the transformation of MoO₃ to MoSe₂ during film selenization which initiates crystallization at the back contact and at the same time prevents oversize grains at the absorber surface. Na-doping and Ag alloying further facilitate grain growth and mitigate band tailing, resulting in a certified effective area efficiency of 11.43% with all device parameters comparable to that on an Mo-substrate. This is the first time highly efficient kesterite solar cells are demonstrated on transparent electrodes, which opens up new opportunities for these earth-abundant elements composed of thin film photovoltaics.

Herein, an asymmetric modification strategy is developed by incorporating a thiazole derivative, 1,3-thiazole-2,4-diammonium (TDA), into a perovskite/SnO₂ interface for high performance FACsPbI₃ perovskite solar cells (PSCs). N3 cures the free OH defects on the SnO₂ surface, while N1 passivates the Pb²⁺/I⁻ defects from the perovskite-buried interface. Consequently, the TDA-modified device achieves nearly 25% efficiency with a high V _oc of 1.20 V.

Abstract

Surmounting complicated defects at the electron transport layer (ETL) and perovskite interface plays a non-trivial role in improving efficiency and stability of perovskite solar cells (PSCs). Herein, an asymmetric interface modification strategy (AIMS) is developed to passivate the defects from both a SnO₂ ETL and the perovskite buried surface via incorporating 1,3-thiazole-2,4-diammonium (TDA) into the SnO₂/perovskite interface. Detailed experimental and calculated results demonstrate that N3 (the nitrogen atom bonding to the imine) in the TDA preferentially cures the free hydroxyl (OH), oxygen vacancy (V _O), and the Sn-related defects on the SnO₂ surface, while N1 (the nitrogen atom bonding to the vinyl) is more inclined to passivate the Pb²⁺ and I⁻ related defects at the perovskite buried surface. As a result, the TDA-modified FACsPbI₃ PSC yields a champion power conversion efficiency (PCE) of 24.96% with a gratifying open-circuit voltage (V _oc) of 1.20 V. In addition, the optimized PSCs exhibit charming air-operational stability with the unencapsulated device sustaining 97.04% of its initial PCE after storage in air conditions for 1400 h. The encapsulated device maintains 90.21% of its initial PCE after maximum power point tracking for 500 h.

Polyacrylic acid (PAA) retards aggregation and crystallization behavior of the organic salt, and slows down the reaction rate between organic salt and PbI₂, by which “slow-release effect” is defined. Such effect improves crystallization and reduces trap density, and helps to obtain a power conversion efficiency from 19.96 (±0.41)% to 21.84 (±0.25)%, and further elevated to 24.19% after surface modification.

Abstract

Fast reaction between organic salt and lead iodide always leads to small perovskite crystallites and concentrated defects. Here, polyacrylic acid is blended with organic salt, so as to regulate the crystallization in a two-step growth method. It is observed that addition of polyacrylic acid retards aggregation and crystallization behavior of the organic salt, and slows down the reaction rate between organic salt and PbI₂, by which “slow-release effect” is defined. Such effect improves crystallization of perovskite. X-ray diffraction study shows that, after addition of 2 mm polyacrylic acid, average crystallite size of perovskite increases from ≈40 to ≈90 nm, meanwhile, grain size increases. Thermal admittance spectroscopy study shows that trap density is reduced by nearly one order (especially for deep energy levels). Due to the improved crystallization and reduced trap density, charge recombination is obviously reduced, while lifetime of charge carriers in perovskite film and devices are prolonged, according to time-resolved photoluminescence and transient photo-voltage decay curve tests, respectively. Accordingly, power conversion efficiency of the device is promoted from 19.96 (±0.41)% to 21.84 (±0.25)% (with a champion efficiency of 22.31%), and further elevated to 24.19% after surface modification by octylammonium iodide.

In this study, a double-side passivation strategy is proposed to reduce the defects on both the top and buried surfaces of perovskite layer. To passivate both positive and negative defects, a multifunctional passivating molecule with both electron-rich and electron-poor domains is developed. As a result, these synergistic virtuous effects are used here to demonstrate flexible perovskite solar cells (f-PSCs) with unprecedented, simultaneous improvements in power conversion efficiency (PCE) (a high PCE of 24.40% and 22.04% for rigid and flexible PSCs, respectively), operational stability (>1000 h) and bending durability (>10 000 cycles), where refer to retention of 90% and 80%, respectively, of the initial PCE.

Abstract

Although much progress is made toward enhancing the efficiency of perovskite solar cells (PSCs), their operational reliability, particularly their mechanical stability, which is a crucial factor for flexible PSCs (f-PCSs), has not attracted sufficient attention. The defects in the perovskite layer, especially on the top and the buried surface of the perovskite layer, can induce perovskite fracture, highly limiting the performance of f-PSCs. Herein, a novel multifunctional organic salt, metformin hydrochloride, which can passivate cationic and anionic defects, is incorporated on both the top and buried surfaces of perovskite layer to suppress defects. As a result, a power conversion efficiency (PCE) of 24.40% for rigid PSCs and a PCE of 22.04% for f-PSCs are achieved. Simultaneously, the device can retain 90% and 80% of the initial efficiency after 1000 h of light illumination and 10 000 bending cycles, respectively, showing excellent operational stability. This study may provide a global way to design a passivation strategy and fabricate flexible perovskite solar cells with high efficiency and stability.

Antisolvent-assisted in situ cation exchange of perovskite quantum dots (PQDs) is demonstrated. The vacancy-mediated cation diffusion can efficiently passivate the surface defects of PQDs without affecting the size distribution and surface ligand chemistry of PQDs. Meanwhile, using the antisolvent-assisted cation exchange method, the PQD solid with a flattened energy landscape, improved stacking orientation, and favorable charge carrier transport is realized.

Abstract

Cesium-formamidinium lead iodide perovskite quantum dots (FA _x Cs_{1− x} PbI₃ PQDs) show high potential for next-generation photovoltaics due to their outstanding optoelectronic properties. However, achieving composition-tunable hybrid PQDs with desirable charge transport remains a significant challenge. Herein, by leveraging an antisolvent-assisted in situ cation exchange of PQDs, homogeneous FA _x Cs_{1− x} PbI₃ PQDs with controllable stoichiometries and surface ligand chemistry are realized. Meanwhile, the crystallographic stability of PQDs is substantially improved by substituting the cations of the PQDs mediated by surface vacancies. Consequently, PQD solar cell delivers an efficiency of 17.29%, the highest value among the homostructured PQD solar cells. The high photovoltaic performance is attributed to the broadened light harvesting spectra, flattened energy landscape, and rationalized energy levels of highly oriented PQD solids, leading to efficient charge carrier extraction. This work provides a feasible approach for the stoichiometry regulation of PQDs to finely tailor the optoelectronic properties and tolerance factors of PQDs toward high-performing photovoltaics.

A hole transport material (HTM), TPASF, is designed, exhibiting high hole mobility, superior interfacial properties, and good growth template, benefiting from the self-assembled network via synergistic F⋅⋅⋅S dipole-dipole intra- and intermolecular interactions. The dopant-free TPASF HTM-based inverted perovskite solar cells achieve a high power conversion efficiency of 21.01% and a long T ₈₀ lifetime of ≈632 h under the operational conditions.

Designing dopant-free small-molecule hole transport materials (HTMs) with self-assembly behavior via noncovalent interactions is considered as one effective strategy to achieve high-performance inverted perovskite solar cells (PSCs). Herein, two donor-π bridge-donor (D–π–D) HTMs are presented, TPASF and TPAOF, containing 3,6-dimethoxythieno[3,2-b]thiophene as a core part with 3-fluoro-N,N-bis(4-(methylthio)phenyl)aniline and 3-fluoro-N,N-bis(4-methoxyphenyl)aniline as side group. The synergistic F⋅⋅⋅S dipole–dipole intra- and intermolecular interactions in TPASF drive the self-assembly of this molecule into a supramolecular nanofibrillar network, leading to high hole mobility, superior interfacial properties, and providing a good growth template for the perovskite layer atop. The corresponding dopant-free TPASF-based inverted devices exhibit a promising power conversion efficiency of 21.01% with a long T ₈₀ lifetime of ≈632 h under operational conditions. This work paves the way for the further development of new dopant-free self-assembled HTM designs for highly efficient and stable inverted PSCs.

A comprehensive passivation strategy based on 4-(chloromethyl) benzonitrile is proposed for defect treatments of both hole and electron transport layers. This additive can improve wettability of poly(triarylamine) and reduce agglomeration of [6,6]-phenyl-C61-butyric acid methylester particles. It also demonstrates improvement of interfacial contact between the charge transport layers and perovskites for fabrication of efficient and stable inverted perovskite solar cells.

Abstract

Poor carrier transport capacity and numerous surface defects of charge transporting layers (CTLs), coupled with misalignment of energy levels between perovskites and CTLs, impact photoelectric conversion efficiency (PCE) of inverted perovskite solar cells (PSCs) profoundly. Herein, a collaborative passivation strategy is proposed based on 4-(chloromethyl) benzonitrile (CBN) as a solution additive for fabrication of both [6,6]-phenyl-C61-butyric acid methylester (PCBM) and poly(triarylamine) (PTAA) CTLs. This additive can improve wettability of PTAA and reduce the agglomeration of PCBM particles, which enhance the PCE and device stability of the PSCs. As a result, a PCE exceeding 20% with a remarkable short circuit current of 23.9 mA cm⁻², and an improved fill factor of 81% is obtained for the CBN- modified inverted PSCs. Devices maintain 80% and 70% of the initial PCE after storage under 30% and 85% humidity ambient conditions for 1000 h without encapsulation, as well as negligible light state PCE loss. This strategy demonstrates feasibility of the additive engineering to improve interfacial contact between the CTLs and perovskites for fabrication of efficient and stable inverted PSCs.

The bidirectional anions gathering strategy (BAG) is reported by using Methylammonium acetate (MAAc) and Methylammonium thiocyanate (MASCN) as mix PbSn perovskite bulk additives, which Ac^- escapes from the perovskite film top surface while SCN^- gathers at the perovskite film bottom in the crystallization process. The novel strategy enables a controllable crystallization process, thus getting a champion efficiency of 22.14%.

Abstract

Mixed lead-tin (PbSn) perovskite solar cells (PSCs) possess low toxicity and adjustable bandgap for both single-junction and all-perovskite tandem solar cells. However, the performance of mixed PbSn PSCs still lags behind the theoretical efficiency. The uncontrollable crystallization and the resulting structural defect are important reasons. Here, the bidirectional anions gathering strategy (BAG) is reported by using Methylammonium acetate (MAAc) and Methylammonium thiocyanate (MASCN) as perovskite bulk additives, which Ac⁻ escapes from the perovskite film top surface while SCN⁻ gathers at the perovskite film bottom in the crystallization process. After the optoelectronic techniques, the bidirectional anions movement caused by the top-down gradient crystallization is demonstrated. The layer-by-layer crystallization can collect anions in the next layer and gather at the broader, enabling a controllable crystallization process, thus getting a high-quality perovskite film with better phase crystallinity and lower defect concentration. As a result, PSCs treated by the BAG strategy exhibit outstanding photovoltaic and electroluminescent performance with a champion efficiency of 22.14%. Additionally, it demonstrates excellent long-term stability, which retains ≈92.8% of its initial efficiency after 4000 h aging test in the N₂ glove box.

A multifunctional Lewis acid (p-toluenesulfonic acid, TsOH) is used to significantly reduce the energy loss in single-wall carbon nanotube/silicon (SWCNT/Si) solar cells. Owing to the charge transfer doping effect of TsOH, the conductivity and work function of SWCNT films are optimized and tuned. More importantly, a chemical bridge is constructed at the interface of SWCNT/Si heterojunction, which effectively suppresses the recombination of photogenerated carriers. As a result, a champion power conversion efficiency of 17.7% is achieved.

Abstract

Single-wall carbon nanotube/silicon (SWCNT/Si) heterojunction shows appealing potential for use in photovoltaic devices. However, the relatively low conductivity of SWCNT network and interfacial recombination of carriers have limited their photovoltaic performance. Herein, a multifunctional Lewis acid (p-toluenesulfonic acid, TsOH) is used to significantly reduce the energy loss in SWCNT/Si solar cells. Owing to the charge transfer doping effect of TsOH, the conductivity and work function of SWCNT films are optimized and tuned. More importantly, a chemical bridge is constructed at the interface of SWCNT/Si heterojunction. Experimental studies indicate that the phenyl group of TsOH can interact with SWCNTs through π–π interaction, meanwhile, the oxygen in the sulfonic functional group of the TsOH molecule can graft on the dangling bonds of the Si surface. The chemical bridge structure effectively suppresses the recombination of photogenerated carriers. The TsOH coating also works as an antireflection layer, leading to a 19% increment of the photocurrent. As a result, a champion power conversion efficiency of 17.7% is achieved for the TsOH-SWCNT/Si device, and it also exhibits an excellent stability, retaining more than 96% of the initial efficiency in the ambient air after 1 month.

3D polydentate complexing agents are explored to synchronously passivate defects of perovskite absorber directly in multiple spatial directions. The strong electron-donating groups (H₂PO₄) in the phytic acid can passivate non-coordinated Pb²⁺ at the ground boundaries/surface and modulate perovskite crystallization. Especially, phytic acid dipotassium (PAD) containing additional (K→PO) push–pull structure passivates the amphoteric defects. Therefore, the PAD-passivated perovskite solar cells deliver a champion photoelectric conversion efficiency of 23.18%.

Abstract

The grain boundaries (GBs)/surface defects within perovskite film directly impede the further improvement of photoelectric conversion efficiency (PCE) and stability of planar perovskite solar cells (PSCs). Herein, 3D phytic acid (PA) and phytic acid dipotassium (PAD) with polydentate are explored to synchronously passivate the defects of perovskite absorber directly in multiple spatial directions. The strong electron-donating groups (H₂PO₄) in the PA molecule afford six anchor sites to bind firmly with uncoordinated Pb²⁺ at the GBs/surface and modulate perovskite crystallization, thus enhancing the quality of perovskite film. Particularly, PAD containing an additional (K→PO) push–pull structure promotes the dominant coordination of phosphate group (PO) with Pb²⁺ and passivates halide anion defects due to the complexation of potassium ions (K⁺) with iodide ions (I^-). Consequently, the PAD-complexed PSCs deliver a champion PCE of 23.18%, which is remarkably higher than that of the control device (19.94%). Meanwhile, PAD-complexed PSCs exhibit superior moisture and thermal stability, remaining 79% of their initial PCE after 1000 h under continuous illumination, while the control device remain only 48% of their PCE after 1000 h. This work provides important insights into designing multifunctional 3D passivators for the purpose of simultaneously enhancing the efficiency and stability of devices.

Quinoxaline-based X-shaped molecules (1-4) are designed and synthesized as p-type self-assembled monolayer (SAM) for tin perovskite solar cells (TPSC). SAM; TQxD (4) exhibits excellent device performance of 8.3%, and shows great enduring stability for the performance retaining ≈90% of their original values for shelf storage over 1600 h. This is the current best result for SAM-based TPSC ever reported.

Abstract

Four X-shaped quinoxaline-based organic dyes, PQx (1), TQx, (2), PQxD (3), and TQxD (4) (D = dye sensitizers) are developed and served as p-type self-assemble monolayer (SAM) for tin perovskite solar cells (TPSC). Thermal, optical, and electrochemical properties of these SAMs are thoroughly investigated and characterized. Tin perovskite layers are successfully deposited on these four SAM surfaces according to a two-step approach and the devices exhibit power conversion efficiency in the order of TQxD (8.3%) > TQx (8.0%) > PQxD (7.1%) > PQx (6.1%). With thiophene π-extended conjugation unit in SAM structure, TQxD (4) exhibits the highest hole extraction rates, greatest hole mobilities, and slowest charge recombination to achieve great device performance of 8.3%, which is the current best result for SAM-based TPSC ever reported. Furthermore, all devices except PQx shows great enduring stability for the performance retaining ≈90% of their original values for shelf storage over 1600 h.

A series of small-molecule and polymeric acceptors with different amounts and types of halogenations are synthesized and investigated to reveal the effect of fluorination and chlorination between small-molecule and polymeric acceptors. It is found that chlorinated small-molecule acceptors lead to longer exciton diffusion length and better performance, while fluorinated polymers achieve a denser packing mode and better performance.

Abstract

Tuning the properties of non-fullerene acceptors (NFAs) through halogenation, including fluorination and chlorination, represents one of the most promising strategies to boost the performance of organic solar cells (OSCs). However, it remains unclear how the F and Cl choice influences the molecular packing and performance between small-molecule and polymeric acceptors. Here, a series of small-molecule and polymeric acceptors with different amounts and types of halogenation is synthesized, and the effects of fluorination and chlorination between small-molecule and polymeric acceptors are investigated. It is found that chlorinated small-molecule acceptors lead to longer exciton diffusion length and better performance compared to the corresponding fluorinated ones, which attributes to their stronger intermolecular packing mode. For polymer acceptors, in contrast, the fluorinated polymers achieve a denser packing mode and better performance, because chlorinated polymers exhibit reduced intrachain conjugation between end group moieties and linker units. This study demonstrates different halogenation effects on the packing modes and performances for small-molecule and polymeric acceptors, which provides important guidance for the molecule design of high-performance acceptors for OSCs.

A simple yet scalable interfacial layer consisting of self-assembled molecule and conjugated polymer facilitates resolving bilateral surface issues between charge-selective metal oxide and perovskite, successfully leading to high-performance inverted perovskite solar cells and modules.

Abstract

Challenges remain hindering the performance and stability of inverted perovskite solar cells (PSCs), particularly for the nonstable interface between lead halide perovskite and charge extraction metal oxide layer. Herein, a simple yet scalable interfacial strategy to facilitate the assemble of high-performance inverted PSCs and scale-up modules is reported. The hybrid interfacial layer containing self-assembly triphenylamine and conjugated poly(arylamine) simultaneously improves the chemical stability, charge extraction, and energy level alignment of hole-selective interface, meanwhile promoting perovskite crystallization. Consequently, the correspondent inverted PSCs and modules achieve remarkable power conversion efficiencies (PCEs) of 24.5% and 20.7% (aperture area of 19.4 cm²), respectively. The PSCs maintain over 80% of its initial efficiency under one-sun equivalent illumination of 1200 h. This strategy is also effective to perovskite with various bandgaps, demonstrating the highest PCE of 19.6% for the 1.76-eV bandgap PSCs. Overall, this work provides a simple yet scalable interfacial strategy for obtaining state-of-the-art inverted PSCs and modules.

SnO₂–MXene (0.2 wt%) nanocomposites serve as the electron transport layer of perovskite solar cell (PSC). It not only inhibits SnO₂ agglomeration and induces the vertical growth of CsPbIBr₂, but also passivates the defects of the SnO₂/CsPbIBr₂ interface. Finally, a carbon-based CsPbIBr₂ PSC with an efficiency of 11.21% is obtained.

Due to its unique photoelectric properties, MXene has a wide range of potential applications. Herein, different MXene contents (0, 0.10, 0.15, 0.20, 0.25, 0.4 wt%) of SnO₂–MXene nanocomposites are used as the electron transport layer. The introduction of MXene not only inhibits SnO₂ agglomeration and induces the vertical growth of CsPbIBr₂, but also passivates defects of the SnO₂/CsPbIBr₂ interface. It effectively increases the power conversion efficiency from 8.98% to 11.21%. In addition, even after 1000 h of storage with a relative humidity of more than 50% in the ambient air, the unsealed device still maintains about 92% of its initial efficiency. This strategy provides a promising way to improve the efficiency of solar cells.

Herein, an alternative electron-selective strontium oxide (SrO _x ) contact for silicon solar cells is developed. By the implementation of the SrO _x -based rear contact, the champion power conversion efficiency of 20.0% is achieved featuring a high fill factor of 82.8% and a high thermal stability up to 500 °C. These results exhibit an intriguing path to fabricate competitive efficiency and thermal stability devices.

Extensive efforts have been made to develop wide-bandgap metal compound-based carrier-selective contacts to improve the performance of crystalline silicon (c-Si) solar cells, by mitigating the deleterious effects of metal–Si contact directly. Herein, thermally evaporated wide-bandgap strontium oxide (SrO _x ) is exploited as an electron-selective contact for c-Si solar cells. Benefiting from a lower work function (3.1 eV) of SrO _x , a strong downward band-bending is achieved at the n-type c-Si/SrO _x interface, enabling the electron-selective transport characteristic. Thin SrO _x films simultaneously provide moderate surface passivation after annealing and enable a low contact resistivity on c-Si surfaces. By the implementation of a single-dielectric-layer SrO _x -based rear contact, a champion power conversion efficiency of 20.0% is realized on the n-type c-Si solar cell featuring an intriguing fill factor of 82.8%. Moreover, electron-selective SrO _x contact is demonstrated to show high thermal stability up to 500 °C. The SrO _x layer formed by a facile thermal evaporation process presents a unique opportunity to develop highly efficient and low-cost c-Si solar cells.

Nature Energy, Published online: 23 February 2023; doi:10.1038/s41560-023-01204-z