awn

The ration design of interface materials for inverted perovskite solar cell is demonstrated. This new designed NiO _x /carbon heterostructure interlayer can enable more favorable gradient energy level alignment, improve the interfacial charge transfer dynamics, and minimize open-circuit voltage loss for high-performance P-I-N device.

Abstract

An efficient hole transport layer (HTL) with desirable charge separation and hole extraction efficiency is crucial for inverted perovskite solar cells. However, the interfacial trap recombination loss and mismatched band alignment limit the actual performance of device, especially the open-circuit voltage (V _OC). To address this issue, a unique NiO _x /carbon heterostructure is designed as efficient anode interlayer for optimizing the interfacial charge transport dynamics between HTL and perovskite. Such a buffer interlayer can significantly contribute to the improved hole conductivity and hole extraction efficiency at HTL/perovskite interface. Moreover, the more favorable gradient energy level alignment can be formed to increase the interfacial electric field, inhibit the nonradiative recombination, and minimize the V _OC loss. Therefore, the champion device achieves 19.51% efficiency with high V _OC of 1.13 V, close to the highest power conversion efficiencies of MAPbI₃ device. This work suggests that interface design can be an alternative approach to fabricate efficient inverted NiO-based devices.

Varying thickness of Iridium(Ir) complexes (e.g., bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic)) is used as electron extraction layers (EELs) based on non-fullerene organic photovoltaics (OPVs). The optimal thickness of FIrpic rationally enhances the corresponding short current density (J _sc) and open circuit voltage (V _oc) and finally improves the photovoltaic performance of OPVs. Moreover, Ir complexes show universality in combining with other materials serving as EELs.

Abstract

Ultrathin iridium (Ir) complexes (e.g., bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic)) are first introduced as electron extraction layers (EELs) in non-fullerene organic photovoltaics (OPVs), and a set of devices based on non-fullerene materials is fabricated. It is demonstrated that this approach can rationally enhance the corresponding short current density and open circuit voltage and finally improve the photovoltaic performance of OPVs. Furthermore, optimized bilayer EELs using combined FIrpic and 8-hydroxyquinoline lithium (Liq) are investigated. The resulting device presents excellent power conversion efficiency (PCE) of 15.85%, which is much higher than that of control device without FIrpic layer (14.6%). Moreover, Ir complexes show great universality when combined with other materials serving as EELs. This may provide an extra direction for further enhancement of OPVs performance.

CsPbBr_1.5I_1.5 quantum dots encapsulated in ZrO₂ sol is introduced to improve the stability of perovskite solar cells. The champion performance of the device based on CsPbBr_1.5I_1.5/ZrO₂ nanocomposites achieves 21.23% and maintains 81.8% of initial power conversion efficiency after 500 h.

Abstract

In the last several years, perovskite solar cells (PSCs) are faced with big problems in the stability issues, which also occur in the Interface modification materials. Herein, CsPbBr_1.5I_1.5 quantum dots (QDs) encapsulated in ZrO₂ sol is introduced to improve device stability to some extent. The encapsulated QDs can improve the morphology of the perovskite layer and effectively passivate the perovskite film. The device fabricated by CsPbBr_1.5I_1.5 QDs encapsulated achieves a power conversion efficiency (PCE) of 21.23%, corresponding to 18.86% for the unmodified control devices. The performance of the device based on CsPbBr_1.5I_1.5/ZrO₂ nanocomposites modified remains 81.8% of initial PCE after 500 h.

Publication date: November 2021

Source: Nano Energy, Volume 89, Part B

Author(s): Aleksandra Furasova, Pavel Voroshilov, Mikhail Baranov, Pavel Tonkaev, Anna Nikolaeva, Kirill Voronin, Luigi Vesce, Sergey Makarov, Aldo Di Carlo

Publication date: December 2021

Source: Nano Energy, Volume 90, Part A

Author(s): Tahmineh Mahmoudi, Won-Yeop Rho, Mohammadhosein Kohan, Yeon Ho Im, Sanjay Mathur, Yoon-Bong Hahn

Publication date: December 2021

Source: Nano Energy, Volume 90, Part A

Author(s): Ling Liu, Chuantian Zuo, Liming Ding

Anhydrous SnCl₄ is substituted with SnCl₄·5H₂O to prepare a Sn⁴⁺ precursor solution in air and two solutions containing Sn⁴⁺ and Sn²⁺ are mixed as the final precursor solution, rendering a feasible scheme to obtain denser films and avoid film cracking. When the ratio of Sn⁴⁺ to Sn²⁺ is 1:1, the best efficiency of 11.1% for CZTSSe solar cells is obtained.

The use of different Sn valence states (such as Sn⁴⁺ and Sn²⁺) in the Cu₂ZnSn(S,Se)₄ (CZTSSe) precursor solution is especially important for the quality of the subsequent growth of the CZTSSe films. The latest study has found that replacing SnCl₂·2H₂O with anhydrous SnCl₄ can remarkably improve the performance of CZTSSe solar cells, but it needs to be operated in the glovebox. Herein, for the precursor solution, SnCl₄·5H₂O powder is used instead of anhydrous SnCl₄ in air environment, and the proportion of Sn⁴⁺ and Sn²⁺ precursor solutions is further systematically studied. When the ratio of Sn⁴⁺ to Sn²⁺ is 1:1, a uniform, compact, and noncracking CZTSSe thin film is obtained, effectively alleviating the interface recombination and reducing the concentration of deep-level defects. In particular, the concentration of Cu_Zn antisite defects is decreased by an order of magnitude, and the carrier recombination and band tail effect are alleviated. When J _SC is maintained, V _OC and FF are considerably improved. Finally, CZTSSe thin-film solar cells are fabricated with an efficiency of over 11%. Herein, the feasibility of controlling the ratio of Sn⁴⁺ to Sn²⁺ in the CZTSSe precursor solution for higher efficiency of CZTSSe thin-film solar cells is demonstrated.

An amorphous small molecule with high molecular weight is first designed and used as a solid additive to regulate the phase separation and molecular packing of the PM6:Y6 blends for efficient and stable nonfullerene organic solar cells.

It is incredibly feasible and effective to adopt a solid additive strategy to optimize the active layer blend films morphology for nonfullerene organic solar cells (OSCs) to achieve high efficiency and stable performance. Herein, a novel amorphous small molecule SJ-IC-M with a comparatively high molecular weight is first designed and used as an efficient solid additive in the OSCs based on PM6:Y6 to enhance the power conversion efficiency (PCE) and long-term stability of the device. After the addition of 0.5 wt% SJ-IC-M into the active layer blends, the PCE can be increased to 16.2% compared with that of the reference device without additive displaying an inferior PCE of 15.0%. Moreover, the device containing the SJ-IC-M additive delivers more excellent long-term stability. The PCE can remain over 90% of its initial value when the unencapsulated device is preserved in a N₂-filled glovebox for a month. Systematic analysis reveals that the introduction of the relatively high molecular weight amorphous SJ-IC-M additive can optimize the crystallinity of Y6. As a result, an improved charge transport, stabilized blend morphology, and enhanced device performance are achieved. Moreover, the current research provides a new strategy which can replace the commonly used solvent additive to fabricate efficient and stable nonfullerene OSCs.

Perovskite Solar Cells

In article number 2100243 Do–Hyung Kim, Hyo Jung Kim, and co-workers systematically investigate Li:Cu doped NiO_x films and their characteristics as hole transporting layers for inverted perovskite solar cells (PSCs). The co-doped NiO_x films exhibit enhanced optical/electrical properties and show favorable interfacial properties forming high-quality perovskite films which results in high-efficiency inverted PSCs.

A novel series of 1D/2A PBTPBD terpolymers is developed for high-efficiency and long-lived polymer solar cells (PSCs). The PBTPBD-50:IT-4F PSC, processed with a nonhalogenated solvent, maintains 82% of the initial power conversion efficiency even after 204 days at 85 °C, which is the highest thermal stability achieved among PSCs processed with nonhalogenated solvents.

Donor–acceptor (D–A) copolymer-based polymer solar cells (PSCs) processed with nonhalogenated solvents exhibit relatively low power conversion efficiencies (PCE) due to undesirable morphological properties, including high aggregation and unfavorable orientation. Moreover, they show very poor long-term stability owing to excessive molecular aggregation and unfavorable phase separation. Thus, novel p-type polymers are required for high-efficiency and long-lived PSCs that can be processed in ecofriendly nonhalogenated solvents. Herein, a novel series of 1D/2A terpolymers (PBTPBD) composed of 4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-F), 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD), and 1,3-bis-(4-hexylthiophen-2-yl)-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (HT-TPD) is synthesized and characterized for high-efficiency and long-lived PSCs. A PBTPBD-50:IT-4F blended film exhibits a favorable face-on orientation and superior hole and electron mobility. Therefore, the corresponding PBTPBD-50:IT-4F PSC, processed with a nonhalogenated solvent, exhibits a high PCE of 13.64%, which is 13% higher than that of the related nonhalogenated solvent-processed PSCs. Furthermore, the PBTPBD-50:IT-4F PSC maintains 82% of the initial PCE even after 204 days at 85 °C, which is the highest thermal stability achieved among PSCs processed with nonhalogenated solvents. The high-efficiency and superior long-term thermal stability of the PBTPBD-50:IT-4F PSC are attributed to the excellent miscibility of PBTPBD-50 and IT-4F and the suppression of the morphological changes in the photoactive layer.

A novel process for atmospheric pressure dry etching of polysilicon layers is developed and integrated in industrial TOPCon solar cell process sequence. The process leads to very high etch rates (>3 μm min⁻¹) of parasitic polysilicon layer in a highly single-sided process. The fabricated large area TOPCon solar cells feature high parallel resistance and excellent reverse bias property.

Single-sided etching (SSE) of a-Si/poly-Si is typically considered a challenge for realizing a cost-efficient TOPCon production sequence, as there is a certain degree of unwanted wrap-around for poly-Si deposition technologies such as low pressure chemical vapor deposition, plasma-enhanced chemical vapor deposition, and atmospheric pressure chemical vapor deposition. To date, alkaline or acidic wet-chemical solutions in either inline or batch configurations are used for this purpose. Herein, an alternative SSE process is proposed using an inline dry etching tool, which applies molecular fluorine as the etching gas under atmospheric pressure conditions. The developed etching process performs complete etching of both as-deposited amorphous silicon and annealed polycrystalline silicon layers, either intrinsic or doped, and with measured etch rates of >3 μm min⁻¹ at 10% F₂ concentration allows etching of a typical layer thickness of 200 nm in just a few seconds. The etching process is also configured to perform excellent edge isolation while maintaining a low wrap-around etching (d _rear < 500 μm) at the opposing-side. The etching process is successfully transferred to the industrial TOPCon solar cell architecture, yielding high parallel resistances (S _shunt,avg. > 1500 kΩ cm²), low reverse current density (J _rev,avg < 0.8 mA cm⁻²) measured at a bias voltage of −12 V, and independently certified conversion efficiencies of up to 23.3%.

The effects of Cu (II) poly(styrene sulfonate) (Cu:PSS) on the electronic band structure and characteristics of organic solar cells are investigated. Easily reduced Cu²⁺ ions balance the negative charges on the PSS backbone, supporting p-doping at the interface with PTB7. Photoelectron spectroscopy confirms a band-bending effect, consistent with the observed hole extracting effects of Cu:PSS in solar cells.

In organic solar cells (OSCs), interfacial properties between the donor phase and hole transport layers (HTLs) are critical factors which govern charge extraction efficiency. Many ionic and polar materials are known to function as effective interfacial layers; however, an understanding of how ionic moieties affect the electronic band structure and characteristics of OSCs is lacking. Herein, a new, pH-neutral polyelectrolyte is introduced that resolves several problems which are encountered with the commonly used HTL, poly(3,4-ethylenedioxythiopene):polystyrenesulfonate (PEDOT:PSS). An effective p-type polyelectrolyte dopant is designed, comprising an anionically charged PSS backbone with easily reduced Cu²⁺ counterions (Cu:PSS), and interfacial properties for HTL/donor interfaces by photoelectron spectroscopy are analyzed. The effects of the polyelectrolyte on interfacial energy levels and charge extraction efficiency between the active layer and HTL are quantified. Using optimized processing conditions, the efficiency can be improved from 8.31% to 9.28% in conventional OSCs compared with a standard PEDOT:PSS HTL. The energy-level alignment at the HTLs/donor interface determined by UV photoelectron spectroscopy measurements reveals the origin of distinct differences in device performances. The reduced ionization potential (IP) and hole injections barrier (Φ_h) at the HTL/donor interface play a crucial role in efficient charge extraction in conventional OSCs.

Phosphorus-doped poly-SiN _x as a new material, featuring significant nitrogen doping and low crystalline degree, is introduced to passivating contact, leading to excellent passivation quality with a champion iV _oc of 745 mV and enabling the efficiency of the proof-of-concept tunnel oxide passivated contact solar cell to 23.88%.

A P-doped polycrystalline silicon-nitride (n-poly-SiN _x ) as the electron selective collection layer in a tunnel oxide passivated contact (TOPCon) solar cell is reported. The nitrogen content is controlled by the active gas ratio of R = NH₃/(SiH₄ + NH₃) during the plasma-enhanced chemical vapor deposition (PECVD) process. The effects of R ratio on the material's composition, crystallinity, surface passivation, and contact resistivity are investigated. The poly-SiN _x contact exhibits improved surface passivation in comparison with the reference poly-Si without N incorporation. The best double-sided passivated n-type alkaline-polished crystalline silicon wafer with the n-poly-SiN _x /SiO _x manifests the highest implied open-circuit voltage (iV _oc) of ≈745 mV, with the corresponding single-sided saturated current density of 1.7 fA cm⁻² and the effective lifetime (τ _eff) of 10 ms at the injection level of ≈1 × 10¹⁵cm⁻³. In contrast, the controlled sample with an n-poly-Si/SiO _x passivation contact has a maximal iV _oc of 738 mV. However, the primary drawback of the N doping is to raise the contact resistivity, but which is still in an acceptable range and shows little effect on the performance of solar cell with full-area contact. The proof-of-concept TOPCon solar cell using the n-poly-SiN _x /SiO _x passivating contact has achieved an efficiency of 23.88%, indicating the potential of the n-poly-SiN _x for high-efficiency TOPCon solar cells.

The authors reviewed the influence of the third component on structure–morphology–performance relationships of ternary organic solar cells (TOSCs). This review adopts a bottom-up approach, originating from the influence on intermolecular levels to global morphology, and finally to electronic properties. Critical insights related to global-morphological influence, open-circuit voltage (V _oc) tuning, and dielectric screening are postulated, establishing new perspectives for TOSC research.

The addition of a third component to a binary bulk heterojunction configuration in organic solar cells is called ternary organic solar cells (TOSCs), which is trending as a solar cell configuration that strikes balance between tandem and binary bulk heterojunction organic solar cells by increasing spectral absorption without laborious processing methods. Apart from increasing the spectral range, the third component is also able to tune the morphology and electronic properties for enhanced performance. Existing reviews on TOSCs thus far mainly focus on reporting related research progresses and their respective performances. Instead, a thorough review on understanding the influence of the third component in terms of morphology and electronic properties via a computational chemistry perspective (structure–morphology–performance) is presented here. This bottom-up approach reviews the influence of the third component originating from intermolecular interactions to crystallization and phase properties, and finally to electronic properties such as charge transfer and energy transfer. Herein, new theoretical insights opening up potential research opportunities to propel organic solar cells to one day surpass 20% power conversion efficiency are revealed.

Hole-transport-layer (HTL) free tin perovskite solar cells would solve the stability issue caused by the unstable organic HTL. Formamidinium tin iodide doped with heterogeneous ammonium salts can form an upward band-bending structure to selectively extract the hole in the HTL-free cells. An efficiency of over 10% with reliable light-soaking and thermal stability can be achieved for the cells.

Abstract

Lead-free tin perovskite solar cells (PSCs) have emerged as a promising candidate toward high-performance and eco-friendly photovoltaic technology with great potential for future application. However, tin PSCs with over 10% efficiency usually feature an organic hole transport layer (HTL) at the illumination side that may induce device degradation during long-term operation. Removing the unstable organic HTL is an important way to solve these stability issues, but the efficiency of HTL-free tin PSCs is still much lower than that of the completed cells. Herein, it is demonstrated that formamidinium tin iodide doped with heterogeneous ammonium salts can form an upward band-bending structure to selectively extract the hole in the HTL-free devices. By using this band-bending structure, a promising efficiency of over 10% is first achieved for the lead-free PSCs with a HTL-free structure. More importantly, the optimized cell is highly stable, keeping 95% and 90% of the initial efficiency after continuous light soaking for 40 days and 80 °C annealing for 300 h, respectively. This work paves a route toward the development of efficient, eco-friendly, and highly stable perovskite photovoltaics.

Research on the stability of organic solar cells based on fused-ring electron acceptors (FREAs) is now becoming more urgent. This perspective focuses on discussing the possible degradation mechanisms of FREAs and effective strategies of enhancing their stability reported recently. Also, a conclusion and outlook for the future design of highly efficient and stable FREAs are presented.

Abstract

The power conversion efficiency of organic solar cells (OSCs) has made exceptionally rapid progress in the past five years owing to the emergence of fused-ring electron acceptors (FREAs). To achieve the commercialization, it is urgent to resolve the stability issues of OCSs from materials to devices. In particular, the state-of-the-art FREAs, often synthesized by Knoevenagel condensation, generally contain two exocyclic vinyl groups (CC bond) as the conjugated bridges, which inevitably exhibit an obvious electron-deficient characteristic due to the strong push-pull electronic effect. As a result, these vinyl bridges are vulnerable to nucleophile attacking and/or photooxidation, leading to poor chemical and photochemical stabilities of FREAs that easily cause the degradation of device performance. In this perspective, an in-depth understanding of the degradation mechanism of FREAs is provided, and then effective strategies reported recently are reviewed for improving the chemical and photochemical stabilities of FREAs from interfacial engineering to molecular engineering to additive engineering. Finally, a conclusion and outlook for the future design of highly efficient and stable FREAs are also presented.

To restructure nonuniform CH₃NH₃PbI₃ perovskite crystal surfaces, an effective surface engineering strategy is successfully demonstrated. By thermally evaporating energetic CsI on single-crystal surfaces, a unique nanoisland structure is formed through a cation interdiffusion process. This morphology induces a gradient band bending, which increases the charge carrier mobility from 56 to 93 cm² V⁻¹ s⁻¹.

Abstract

Organometal perovskite single crystals have been recognized as a promising platform for high-performance optoelectronic devices, featuring high crystallinity and stability. However, a high trap density and structural nonuniformity at the surface have been major barriers to the progress of single crystal-based optoelectronic devices. Here, the formation of a unique nanoisland structure is reported at the surface of the facet-controlled cuboid MAPbI₃ (MA = CH₃NH₃ ⁺) single crystals through a cation interdiffusion process enabled by energetically vaporized CsI. The interdiffusion of mobile ions between the bulk and the surface is triggered by thermally activated CsI vapor, which reconstructs the surface that is rich in MA and CsI with reduced dangling bonds. Simultaneously, an array of Cs-Pb-rich nanoislands is constructed on the surface of the MAPbI₃ single crystals. This newly reconstructed nanoisland surface enhances the light absorbance over 50% and increases the charge carrier mobility from 56 to 93 cm² V⁻¹ s⁻¹. As confirmed by Kelvin probe force microscopy, the nanoislands form a gradient band bending that prevents recombination of excess carriers, and thus, enhances lateral carrier transport properties. This unique engineering of the single crystal surface provides a pathway towards developing high-quality perovskite single-crystal surface for optoelectronic applications.

Perovskite Solar Cells

In article number 2101590, Hongqiang Wang and co-workers report stable perovskite solar cells with champion efficiency over 24% and moisture (75%) stability over 10 000 hours. Referencing the Chinese story “Nezha Conquers the Dragon King”, the tower of perovskite is strengthened by the interfacial embedding of laser-manufactured fluorinated gold clusters.

A solar cell device based on (hk1)-oriented Sb₂(S,Se)S₃ grown on top of hexagonal CdS is successfully prepared. The enhanced device performance is attributed not only to the faster charge transport along the vertically oriented (Sb₄X₆) _n ribbons, but also to the efficient charge extraction across the heterojunction owing to the formation of favorable covalent bond at the junction.

Abstract

Antimony sulfoselenide (Sb₂(S,Se)₃) is a promising photoabsorber for stable and high efficiency thin film photovoltaics (PV). The unique quasi-1D (Q1D) crystal structure gives Sb₂(S,Se)₃ intriguing anisotropic optoelectronic properties, which intrinsically require the optimization of crystal growth orientation, especially for electronic devices with vertical charge transport such as solar cells. Although the efficiency of Sb₂(S,Se)₃ solar cells has been improved greatly through optimizing the material quality, the fundamental issue of crystal orientation control in polycrystalline films remains unsolved, resulting in charge carrier recombination losses in the device. Herein, the epitaxial growth of vertically-oriented Sb₂(S,Se)₃ film on hexagonal CdS is successfully realized via a solution-based synergistic crystal growth process. The crystallographic orientation relationship between Sb₂(S,Se)₃ light absorber and the CdS substrate has been rigorously investigated. The best performing Sb₂(S,Se)₃ solar cell shows a high power conversion efficiency of 9.2% owing to the faster charge transport in the bulk and the efficient charge extraction across the heterojunction. This study points to a new direction to control the crystal growth of mixed-anion Sb₂(S,Se)₃, which is crucial to achieve high efficiency solar cells based on antimony chalcogenides with low dimensionality.

A hybrid planar/bulk heterojunction is constructed by introducing a p-type polymer (PTO3) and an n-type naphthalene imide (NDI-i8) on both sides of a mixed donor:acceptor active layer. The tailored hybrid heterojunction presents a decreased energy loss and improved efficiency. As a result, an outstanding PCE of 18.5% is achieved, which is among the top values in the field of organic solar cells.

Abstract

The donor:acceptor heterojunction has proved as the most successful approach to split strongly bound excitons in organic solar cells (OSCs). Establishing an ideal architecture with selective carrier transport and suppressed recombination is of great importance to improve the photovoltaic efficiency while remains a challenge. Herein, via tailoring a hybrid planar/bulk structure, highly efficient OSCs with reduced energy losses (E _losss) are fabricated. A p-type benzodithiophene-thiophene alternating polymer and an n-type naphthalene imide are inserted on both sides of a mixed donor:acceptor active layer to construct the hybrid heterojunction, respectively. The tailored structure with the donor near the anode and the acceptor near the cathode is beneficial for obtaining enhanced charge transport, extraction, and suppressed charge recombination. As a result, the photovoltaic characterizations suggest a reduced nonradiative E _loss by 25 meV, and the best OSC records a high efficiency of 18.5% (certified as 18.2%). This study highlights that precisely regulating the structure of donor:acceptor heterojunction has the potential to further improve the efficiencies of OSCs.