Chen Weijie

A novel tin oxide electron-transport layer for perovskite solar cells is developed via chemical bath deposition. Oxalic acid, a volatile dicarboxylic acid ligand, functions as a chemical linker to facilitate film deposition and can be readily removed by thermal annealing and a mild H₂O₂ treatment. The result is an efficient (24.6%) and stable (T ₈₅ = 1500 h) perovskite solar cell.

Abstract

Chemical bath deposition (CBD) is widely used to deposit tin oxide (SnO _x ) as an electron-transport layer in perovskite solar cells (PSCs). The conventional recipe uses thioglycolic acid (TGA) to facilitate attachments of SnO _x particles onto the substrate. However, nonvolatile TGA is reported to harm the operational stability of PSCs. In this work, a volatile oxalic acid (OA) is introduced as an alternative to TGA. OA, a dicarboxylic acid, functions as a chemical linker for the nucleation and attachment of particles to the substrate in the chemical bath. Moreover, OA can be readily removed through thermal annealing followed by a mild H₂O₂ treatment, as shown by FTIR measurements. Synergistically, the mild H₂O₂ treatment selectively oxidizes the surface of the SnO _x layer, minimizing nonradiative interface carrier recombination. EELS (electron-energy-loss spectroscopy) confirms that the SnO _x surface is dominated by Sn⁴⁺, while the bulk is a mixture of Sn²⁺ and Sn⁴⁺. This rational design of a CBD SnO _x layer leads to devices with T ₈₅ ≈1500 h, a significant improvement over the TGA-based device with T ₈₀ ≈250 h. The champion device reached a power conversion efficiency of 24.6%. This work offers a rationale for optimizing the complex parameter space of CBD SnO _x to achieve efficient and stable PSCs.

A dual-asymmetric Y-series acceptor is developed using unidirectional sidechain engineering. Thanks to the low energy loss and efficient charge transfer properties, the corresponding organic solar cells achieved an efficiency of 19.23 %, which is one of the highest values among annealing-free devices.

Abstract

Achieving both high open-circuit voltage (V _oc) and short-circuit current density (J _sc) to boost power-conversion efficiency (PCE) is a major challenge for organic solar cells (OSCs), wherein high energy loss (E _loss) and inefficient charge transfer usually take place. Here, three new Y-series acceptors of mono-asymmetric asy-YC11 and dual-asymmetric bi-asy-YC9 and bi-asy-YC12 are developed. They share the same asymmetric D₁AD₂ (D₁=thieno[3,2-b]thiophene and D₂=selenopheno[3,2-b]thiophene) fused-core but have different unidirectional sidechain on D₁ side, allowing fine-tuned molecular properties, such as intermolecular interaction, packing pattern, and crystallinity. Among the binary blends, the PM6 : bi-asy-YC12 one has better morphology with appropriate phase separation and higher order packing than the PM6 : asy-YC9 and PM6 : bi-asy-YC11 ones. Therefore, the PM6 : bi-asy-YC12-based OSCs offer a higher PCE of 17.16 % with both high V _oc and J _sc, due to the reduced E _loss and efficient charge transfer properties. Inspired by the high V _oc and strong NIR-absorption, bi-asy-YC12 is introduced into efficient binary PM6 : L8-BO to construct ternary OSCs. Thanks to the broadened absorption, optimized morphology, and furtherly minimized E _loss, the PM6 : L8-BO : bi-asy-YC12-based OSCs achieve a champion PCE of 19.23 %, which is one of the highest efficiencies among these annealing-free devices. Our developed unidirectional sidechain engineering for constructing bi-asymmetric Y-series acceptors provides an approach to boost PCE of OSCs.

A facile and effective strategy, i.e., target therapy of buried interface via incorporating formamidine oxalate in colloidal SnO₂, is developed. Both formamidinium and oxalate ions show longitudinal gradient distribution with the highest concentration on the SnO₂ film surface after annealing, which enables matched energy levels, high-quality upper perovskite films, and minimized defects. The perovskite solar cells deliver a high efficiency of 25.05% with enhanced device stability.

Abstract

The buried interface in perovskite solar cells (PSCs) is pivotal for achieving high efficiency and stability. However, it is challenging to study and optimize the buried interface due to its non-exposed feature. Here, a facile and effective strategy is developed to modify the SnO₂/perovskite buried interface by passivating the buried defects in perovskite and modulating carrier dynamics via incorporating formamidine oxalate (FOA) in SnO₂ nanoparticles. Both formamidinium and oxalate ions show a longitudinal gradient distribution in the SnO₂ layer, mainly accumulating at the SnO₂/perovskite buried interface, which enables high-quality upper perovskite films, minimized defects, superior interface contacts, and matched energy levels between perovskite and SnO₂. Significantly, FOA can simultaneously reduce the oxygen vacancies and tin interstitial defects on the SnO₂ surface and the FA⁺/Pb²⁺ associated defects at the perovskite buried interface. Consequently, the FOA treatment significantly improves the efficiency of the PSCs from 22.40% to 25.05% and their storage- and photo-stability. This method provides an effective target therapy of buried interface in PSCs to achieve very high efficiency and stability.

A novel distannylated electron-deficient monomer with high purity and reactivity was synthesized, which enables access to acceptor-acceptor type polymer acceptor PY5-BSeI with high molecular weight, planar backbone, narrow band gap, and low-lying LUMO/HOMO levels. The polymer shows a high electron mobility and very promising efficiency of 17.77 % in all-polymer solar cells.

Abstract

The shortage of narrow band gap polymer acceptors with high electron mobility is the major bottleneck for developing efficient all-polymer solar cells (all-PSCs). Herein, we synthesize a distannylated electron-deficient biselenophene imide monomer (BSeI-Tin) with high purity/reactivity, affording an excellent chance to access acceptor–acceptor (A–A) type polymer acceptors. Copolymerizing BSeI-Tin with dibrominated monomer Y5-Br, the resulting A–A polymer PY5-BSeI shows a higher molecular weight, narrower band gap, deeper-lying frontier molecular orbital levels and larger electron mobility than the donor–acceptor type counterpart PY5-BSe. Consequently, the PY5-BSeI-based all-PSCs deliver a remarkable efficiency of 17.77 % with a high short-circuit current of 24.93 mA cm⁻² and fill factor of 75.83 %. This efficiency is much higher than that (10.70 %) of the PY5-BSe-based devices. Our study demonstrates that BSeI is a promising building block for constructing high-performance polymer acceptors and stannylation of electron-deficient building blocks offers an excellent approach to developing A–A type polymers for all-PSCs and even beyond.

The native AlO _x grown on a thin-sputtered aluminum layer is used as mask for electroplating copper, e.g., for metallizing silicon heterojunction (SHJ) solar cells. Effects that influence the masking quality for selective electroplating are studied herein by atomic force microscopy and scanning electron microscopy characterizations. The mask quality is demonstrated by metallizing industrial size SHJ solar cells.

The native AlO _x grown on a thin sputtered aluminum layer can be used as mask for electroplating copper, e.g., for metallizing silicon heterojunction (SHJ) solar cells. Effects that influence the masking quality for selective electroplating are studied herein. Atomic force microscopy characterization in PeakForce mode highlights the presence of some insulation defects in the native AlO _x due to local contamination, pinholes, or tunneling currents. A focused ion beam/scanning electron microscopy analysis is further conducted to understand some defects in detail. The AlO _x insulation can be improved by adsorbing a self-assembled monolayer, which is mainly required along the process sequence to adjust the surface wetting of the Al for optimal NaOH_aq printing. The mask quality and complete metallization sequence for solar cells are demonstrated on industrial SHJ precursors. Inkjet- and FlexTrail-printing of NaOH_aq are shown to be suitable to pattern the Al layer. Promising conversion efficiency comparable to screen-printing reference is reached on large area.

This review provides an in-depth analysis of strain sources and their influence on FA-based perovskite solar cells (PSCs). It also discusses the utilization of strain engineering to improve the efficiency and stability of PSCs, as well as the challenges and prospects associated with this technique.

The power conversion efficiency (PCE) of organic–inorganic halide perovskite solar cells (PSCs) has increased rapidly in recent years, with the certified best perovskite single-junction photovoltaics reaching an astounding PCE of 26%. Formamidine (FA)-based perovskites possess excellent photovoltaic properties and superior thermal stability, establishing them as one of the most promising perovskite materials for light absorption. However, the issue of the phase instability of black-phase formamidinium lead iodide (α-FAPbI₃) perovskite has seriously impeded its commercialization process, with the strain found in perovskite films being regarded as a significant factor impacting the stability of PSCs. This article begins by examining the sources of strain and the characterization techniques related to perovskites. Subsequently, it outlines the effects of strain on FA-based perovskites and presents strategies to modify lattice strain. Finally, the potential for strain engineering in the future is discussed. This review aims to clarify the impact of strain on FA-based perovskite, determine potential methods of strain engineering to enhance device performance, and ultimately facilitate the commercialization of these materials.

2-Thiophenylethylamine bearing an amine group and an electron rich sulfur from thiophene effectively modifies the surface of perovskite films, outperforming that of phenethylamine without additional electron donor group in terms of passivation effect. 2-Thiophenylethylamine modification results in a significant increase in V _OC from 1.176 to 1.230 V, yielding a device power conversion efficiency of 21.3% and enhanced moisture stability.

Abstract

Metal halide inorganic perovskites show excellent thermal stability compared to organic-inorganic perovskites. However, the performance of inorganic perovskite solar cells (PSCs) is far from theoretical values, together with unsatisfactory stability, mainly due to the poor interfacial properties. In this work, a facial but effective method is reported to realize high-performance inorganic PSCs by post-modifying the perovskite surface with 2-thiophene ethylamine (TEA). It is found that amine group from TEA can favorably interact with the undercoordinated Pb²⁺ via Lewis acid-based coordination, while thiophene ring with electron-rich sulfur assists such interaction by functioning as an electron donor. The synergetic interaction allows TEA to passivate perovskite film defects more efficiently, as compared to phenethylamine (PEA) with less electron-donating ability. Moreover, perovskite valence band is slightly upward shift to match with hole transport material and facilitate hole transfer. These combinations result in a reduced non-radiative charge recombination and improved charge carrier lifetime. Consequently, PSCs with TEA modification shows a drastic improvement of V _OC by 54 mV, yielding a champion PCE of 21.3%, much higher than the control PSCs (19.3%), along with improved ambient stability. This work demonstrates that surface modifier with an electron-rich moiety is critical for achieving efficient and stable inorganic PSCs.

PM6-PBDBT(55):IPC1CN-BBO-IC2Cl-based large-area (58.50 cm²) modules yield an impressive PCE of 11.28% with a small cell-to-module loss in the fill factor. These results suggest that a combination of the asymmetric molecular design using the IPC1CN group and the terpolymer strategy will pave a new path for fabricating highly efficient and scalable large-area OSCs.

Abstract

It remains challenging to fabricate efficient, scalable large-area organic solar cells (OSCs) owing to the unfavorable morphology of photoactive blend films. To address this challenge, two asymmetric nonfullerene acceptors (NFAs) IPC1CN-BBO-IC2F and IPC1CN-BBO-IC2Cl are synthesized, where 12,13-bis(2-butyloctyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2′“,3′”:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno-[3,2-b]indole (BBO) is the molecular core, and two types of end groups are appended to its ends, namely the 9H-indeno[1,2-b]pyrazine-2,3,8-tricarbonitrile (IPC1CN) end group and one of 2-(5,6-dihalo-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile end groups (IC2F or IC2Cl). These NFAs facilitate effective tuning of light absorption and energy levels, offer high carrier mobilities, and allow for the formation of appropriate morphologies. Note that these benefits apply even to large-area devices, unlike typical Y6-based NFAs. In addition, a random copolymer PM6-PBDBT(55) is synthesized and its energy levels are optimally matched with those of the asymmetric NFAs. The blade-coated 1 cm²-area OSCs based on PM6-PBDBT(55):IPC1CN-BBO-IC2Cl exhibit a PCE of 14.12%, which is higher than that of PM6-PBDBT(55)-IPC1CN-BBO-IC2F-based OSCs. More importantly, the PM6-PBDBT(55):IPC1CN-BBO-IC2Cl-based large-area (58.50 cm²) modules yield an impressive PCE of 11.28% with a small cell-to-module loss in fill factor. These results suggest that a combination of the asymmetric molecular design using the IPC1CN group and the terpolymer strategy will pave a new path for fabricating highly efficient and scalable large-area OSCs.

Two side-chain functionalized polymer hole-transporting materials containing methylthio-based passivation groups are developed to improve the buried interface and crystallinity of quasi-2D perovskite films, achieving impressive power conversion efficiency of 21.41% with open-circuit voltage of 1.23 V for inverted quasi-2D perovskite solar cells.

Abstract

Compared with inverted 3D perovskite solar cell (PSCs), inverted quasi-2D PSCs have advantages in device stability, but the device efficiency is still lagging behind. Constructing polymer hole-transporting materials (HTMs) with passivation functions to improve the buried interface and crystallization properties of perovskite films is one of the effective strategies to improve the performance of inverted quasi-2D PSCs. Herein, two novel side-chain functionalized polymer HTMs containing methylthio-based passivation groups are designed, named PVCz-SMeTPA and PVCz-SMeDAD, for inverted quasi-2D PSCs. Benefited from the non-conjugated flexible backbone bearing functionalized side-chain groups, the polymer HTMs exhibit excellent film-forming properties, well-matched energy levels and improved charge mobility, which facilitates the charge extraction and transport between HTM and quasi-2D perovskite layer. More importantly, by introducing methylthio units, the polymer HTMs can enhance the contact and interactions with quasi-2D perovskite, and further passivating the buried interface defects and assisting the deposition of high-quality perovskite. Due to the suppressed interfacial non-radiative recombination, the inverted quasi-2D PSCs using PVCz-SMeTPA and PVCz-SMeDAD achieve impressive power conversion efficiency (PCE) of 21.41% and 20.63% with open-circuit voltage of 1.23 and 1.22 V, respectively. Furthermore, the PVCz-SMeTPA based inverted quasi-2D PSCs also exhibits negligible hysteresis and considerably improved thermal and long-term stability.

A sustainable approach for recycling non-sustainable indium to construct efficient and stable OSCs and scale-up modules is developed, achieving the impressive PCE of 18.92% and 15.20%, respectively. This interlayer significantly extends the thermal stability of derived OSCs with a T₈₀ lifetime of ≈10 000 h.

The fast degradation of the charge-extraction interface at indium tin oxide (ITO) poses a significant obstacle to achieving long-term stability for organic solar cells (OSCs). Herein, a sustainable approach for recycling non-sustainable indium to construct efficient and stable OSCs and scale-up modules is developed. It is revealed that the recovered indium chloride (InCl₃) from indium oxide waste can be applied as an effective hole-selective interfacial layer for the ITO electrode (noted as InCl₃–ITO anode) through simple aqueous fabrication, facilitating not only energy level alignment to photoactive blends but also mitigating parasitic absorption and charge recombination losses of the corresponding OSCs. As a result, OSCs and modules consisting of InCl₃–ITO anodes achieve remarkable power conversion efficiencies (PCEs) of 18.92% and 15.20% (active area of 18.73 cm²), respectively. More importantly, the InCl₃–ITO anode can significantly extend the thermal stability of derived OSCs, with an extrapolated T₈₀ lifetime of ≈10 000 h.

Publication date: October 2023

Source: Nano Energy, Volume 115

Author(s): Chengwen Huang, Jingyu Tian, Song Yang, Yujun Zhao, Huangzhong Yu

Synergetic molecular packing between non-fullerene acceptors BTP-ThMeCl and L8-BO helps to construct 3D charge transport networks in ternary organic solar cells via the formation of NFA co-crystals at the molecular level, realizing a maximum power conversion efficiency of 19.1% with a superior fill factor of 82.2%, which is the highest FF reported so far for OSCs.

Abstract

Ternary strategy is demonstrated as an efficient approach to achieve high short-circuit current and open-circuit voltage to boost the performance of organic solar cells (OSCs), however, the realization of high fill-factor (FF) in ternary OSCs has been rare. In this study, three thiophene terminated non-fullerene acceptors (NFAs) with methyl or chlorine substitutions on their end-groups are designed and synthesized, and further incorporated into the state-of-the-art PM6:L8-BO system to construct ternary OSCs. Subtle changes in their chemical structures significantly modify the molecular packings of these thiophene terminated NFAs. While BTP-ThMe and BTP-ThCl have limited forms of dimer, versatile molecular dimers, including “Z” shaped D-D, “S” shaped A-A, and “F” shaped A-D packings exist in BTP-ThMeCl, which lead to the formation of compact 3D honey-comb network and this is analogous to the host acceptor L8-BO. This synergetic molecular packing between BTP-ThMeCl and L8-BO contributes to maintain the 3D charge transport network in the ternary system via the formation of NFA co-crystals at the molecular level, and consequently realizing a maximum power conversion efficiency of 19.1% with a superior FF of 82.2%, which is the highest FF reported so far for OSCs.

Potassium citrate (PC) is incorporated into the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hole-transport layer to passivate the buried interface of tin-lead perovskites. PC neutralizes the acidity of PEDOT:PSS and coordinates with Sn/Pb cations, preventing oxidation of Sn²⁺ and enhancing interface properties. This strategy enables power conversion efficiencies of 22.7% and 26.1% for Sn-Pb and all-perovskite tandem solar cells, respectively.

Abstract

Easy-to-form tin vacancies at the buried interface of tin-lead perovskites hinder the performance of low-bandgap perovskite solar cells (PSCs). Here, a synergistic strategy by incorporating potassium citrate (PC) into the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hole-transport layer to passivate the buried interface of Sn-Pb PSCs is reported. PC neutralizes the acidity of PEDOT:PSS and stabilizes the perovskite front surface, enhancing device stability. Citrate moieties coordinate with Sn²⁺ on the buried perovskite surface, preventing Sn²⁺ oxidation and suppressing defect formation. Additionally, potassium cations incorporate into Sn-Pb perovskites, enhancing crystallinity and passivating halide defects. The combined benefits enable efficient low-bandgap Sn-Pb PSCs with a power conversion efficiency of 22.7% and a high open-circuit voltage of 0.894 V. Using this method, 26.1% efficiency for all-perovskite tandem solar cells is demonstrated. These results emphasize the significance of buried interface passivation in developing efficient and stable Sn-Pb PSCs and all-perovskite tandem solar cells.

The side-chain engineering of fluorene-based small-molecule (FL02) hole-transport material (HTM) with twisted triphenylamine groups significantly improves the interfacial charge transfer, that is, promoting hole extraction and hindering charge recombination, which also prevents the diffusion of hygroscopic dopants toward the perovskite surface via strong and oriented interfacial interaction as a barrier, for achieving efficient and highly stable n–i–p perovskite solar cells.

The functionalization of small-molecule hole-transport materials (HTMs) heavily relies on the rational design of molecular geometry, which can optimize both intrinsic HTM properties and interfacial properties for realizing high-performance and stable lead halide perovskite solar cells (LHPSCs). Herein, two fluorene-based donor–π linker–donor HTMs are seen, FL01 and FL02, whose side chains are tailored with planar phenyl-carbazole groups and twisted triphenylamine groups, respectively. Benefiting from the high conformational flexibility of twisted side chains, the strong and oriented interaction via PbO bonding is well coordinated at the perovskite and FL02 interface, which favors the interfacial charge transfer as well as the protection of perovskite layer by effectively blocking or mitigating the diffusion of hygroscopic dopants toward the perovskite surface. Consequently, the performance of FL02 HTM-based n–i–p LHPSCs is significantly enhanced by achieving a power conversion efficiency of 17.8%, which is twice higher than that (8.6%) of FL01 HTM-based ones and comparable with the case (18.8%) of conventional spiro-OMeTAD HTM-based devices. More importantly, the FL02-based devices exhibit impressively high operation and storage stabilities with T80 and TS80 lifetimes of >98 h and ≈270 days, respectively, which are among the longest lifespans for the type of hygroscopically doped LHPSCs.

The impact of Ar, H₂, N_2, and O₂ low-pressure plasmas on the surface of MAPbI₃ perovskite is investigated. The performance of the fabricated photovoltaic devices is rationalized based on the optochemical and morphological modifications and confirmed by density-functional theory calculations. The introduction of different chemical functionalities on the perovskite surface is presented as a smart tool for metal halide perovskite surface engineering.

The effective defect passivation of metal halide perovskite (MHP) surfaces is a key strategy to simultaneously tackle MHP solar cell performances enhancement and their stability under operative conditions. Plasma-based dry processing is an established methodology for the modification of materials surfaces as it does not present the disadvantages often associated with wet treatments. This is becoming a fine tool to reach precise atomic-scale engineering of the MHP surfaces. Herein is reported a comprehensive picture of the interaction between different plasma chemistries and MHP thin films. The impact of Ar, H₂, N₂, and O₂ low-pressure plasmas on MHP optochemical properties and morphology is correlated with the performance of photovoltaic devices and rationalized by density functional theory calculations. Strong morphological modifications and selective removal of the uppermost methylammonium moieties are deemed responsible for nonradiative surface defects suppression and higher solar cell performances. Ellipsometry and X-ray photoelectron spectroscopies shine light on the subtle modifications induced by the different plasma environments, paving the way for the more effective engineering of plasma-based (deposition) processing. Notably, for O₂ plasma treatment, deep-state traps induced by the formation of IO₄ ⁻ species are demonstrated and rationalized, highlighting the challenges in optimizing O₂ plasma-based solutions for MHP-based devices.

This work provides a bulk contact structure to increase the contact area of silver nanowires (AgNWs) and interface buffer layer, realizing high performance with 12.27% and 9.54% for 0.09 cm² small-area device and 10.8 cm² full-solution-processed semitransparent mini-module preparation.

The solution-processed electrode is key to the full-solution-processed organic solar cells (OSCs). Silver nanowires (AgNWs) are considered as an attractive solution-processed electrode due to their low sheet resistance and high transmittance. However, the traditional “line-plane” contact between AgNWs and the interface buffer layer is insufficient, resulting in the low performance of full-solution-processed OSCs. Herein, a bulk contact structure is reported between AgNWs and interface, formed by inserting the interface layer material, such as HMoO _x (hydrogen molybdenum bronze) into the AgNWs networks. The extra HMoO _x can be connected with the bottom interface buffer layer, forming a longitudinal network, and wrapping the AgNWs in the longitudinal interfacial layer. This bulk contact electrode-interface structure increases the area of AgNWs/interface layers, promoting charge transfer and collection. Besides, the top-injected method can enable the formation of a water-based ink film on the top of the organic layer, as well as avoid solvent erosion between AgNWs and the interface layer. Based on the bulk contact electrode-interface structure, this work realized high performance of 12.27% and 9.54% for 0.09 cm² small-area device and 10.8 cm² full-solution-processed semitransparent mini-module. These results provide a new idea for full-solution-processed OSCs preparation.

A hydrofluoric acid and the subsequent UV-ozone treatment are employed to reconstruct the indium tin oxide surface to facilitate the adsorption of high-density self-assembled monolayers for achieving efficient perovskite/silicon tandem solar cells.

Abstract

Self-assembled monolayers (SAMs) are widely used as carrier transport interlayers for enabling high-efficiency perovskite solar cells (PSCs). However, achieving uniform and pinhole-free monolayers on metal oxide (e.g., indium tin oxide, ITO) surfaces is still challenging due to the sensitivity of SAM adsorption to the complex oxide's surface chemistry. Here, the hydrofluoric acid and the subsequent UV–ozone treatment are employed to reconstruct the ITO surface by selectively removing the undesired terminal hydroxyl and hydrolysis product. This can significantly increase the ITO surface activity and area, thus facilitating the adsorption of high-density SAMs. The resultant fluorinated surface can also prevent the direct contact of ITO with the perovskite active layer and passivate the perovskite bottom interface. Benefiting from the synergistically improved perovskite film formation, charge extraction, energy level alignment, and interfacial chemical stability, the corresponding PSC achieves a greatly enhanced power conversion efficiency of 21.3%, along with an enhanced long-term stability as compared to the control counterpart. Furthermore, a semitransparent PSC with a certified efficiency of 19.0% (with a record fill factor of 84.1%) and a four-terminal perovskite/silicon tandem with an efficiency of 28.4% are also demonstrated.

This Minireview discusses developments in oligomerized fused-ring electron acceptors (OFREAs), which combine the advantages of small-molecule acceptors (well-defined structures and batch reproducibility) and polymer acceptors (good film formation, low diffusion coefficient, and excellent stability). OFREAs have a great potential to achieve highly efficient and stable organic solar cells.

Abstract

Organic solar cells (OSCs) have attracted wide research attention in the past decades. Very recently, oligomerized fused-ring electron acceptors (OFREAs) have emerged as a promising alternative to small-molecular/polymeric acceptor-based OSCs due to their unique advantages such as well-defined structures, batch reproducibility, good film formation, low diffusion coefficient, and excellent stability. So far, rapid advances have been made in the development of OFREAs consisting of directly/rigidly/flexibly linked oligomers and fused ones. In this Minireview, we systematically summarized the recent research progress of OFREAs, including structural diversity, synthesis approach, molecular conformation and packing, and long-term stability. Finally, we conclude with future perspectives on the challenges to be addressed and potential research directions. We believe that this Minireview will encourage the development of novel OFREAs for OSC applications.

Two asymmetric acceptors, BTP-Cl and BTP-2Cl, were developed to study the relationship between the energy loss mechanism and molecular structure. The results demonstrate that the decreased surface electrostatic potential difference between asymmetric acceptor and donor reduces the charge transfer state ratio, thereby suppressing the non-radiative recombination energy loss.

Abstract

Reducing non-radiative recombination energy loss (ΔE ₃) is one key to boosting the efficiency of organic solar cells. Although the recent studies have indicated that the Y-series asymmetric acceptors-based devices featured relatively low ΔE ₃, the understanding of the energy loss mechanism derived from molecular structure change is still lagging behind. Herein, two asymmetric acceptors named BTP-Cl and BTP-2Cl with different terminals were synthesized to make a clear comparative study with the symmetric acceptor BTP-0Cl. Our results suggest that asymmetric acceptors exhibit a larger difference of electrostatic potential (ESP) in terminals and semi-molecular dipole moment, which contributes to form a stronger π–π interaction. Besides, the experimental and theoretical studies reveal that a lower ESP-induced intermolecular interaction can reduce the distribution of PM6 near the interface to enhance the built-in potential and decrease the charge transfer state ratio for asymmetric acceptors. Therefore, the devices achieve a higher exciton dissociation efficiency and lower ΔE ₃. This work establishes a structure-performance relationship and provides a new perspective to understand the state-of-the-art asymmetric acceptors.

Synergic electron and defect compensation on Sn perovskites has been realized via alloying heterovalent metal halide salts. This concurrently optimized the interfacial carrier dynamics, compensated the prevailing vacancy defects and minimized the nonradiative recombination loss throughout the devices, thus affording best efficiency up to 14.02 % characterized by a record high photovoltage of 1.013 V and the lowest voltage deficit of 0.38 V.

Abstract

Sn perovskite solar cells have been regarded as one of the most promising alternatives to the Pb-based counterparts due to their low toxicity and excellent optoelectronic properties. However, the Sn perovskites are notorious to feature heavy p-doping characteristics and possess abundant vacancy defects, which result in under-optimized interfacial energy level alignment and severe nonradiative recombination. Here, we reported a synergic “electron and defect compensation” strategy to simultaneously modulate the electronic structures and defect profiles of Sn perovskites via incorporating a traced amount (0.1 mol %) of heterovalent metal halide salts. Consequently, the doping level of modified Sn perovskites was altered from heavy p-type to weak p-type (i.e. up-shifting the Fermi level by ∼0.12 eV) that determinately reducing the barrier of interfacial charge extraction and effectively suppressing the charge recombination loss throughout the bulk perovskite film and at relevant interfaces. Pioneeringly, the resultant device modified with electron and defect compensation realized a champion efficiency of 14.02 %, which is ∼46 % higher than that of control device (9.56 %). Notably, a record-high photovoltage of 1.013 V was attained, corresponding to the lowest voltage deficit of 0.38 eV reported to date, and narrowing the gap with Pb-based analogues (∼0.30 V).

Publication date: November 2023

Source: Journal of Energy Chemistry, Volume 86

Author(s): Wei Wang, Jian Zhang, Kaifeng Lin, Jiaqi Wang, Boyuan Hu, Yayu Dong, Debin Xia, Yulin Yang

Charge-transport layers are crucial for maximizing the efficiency of halide perovskite solar cells. A comparative study of a significant variety of different electrical, optical, and photoemission-based characterization techniques is performed to quantify the impact of charge transport layers on solar cell functionality and critically examine the usefulness and complementarity of the techniques.

Abstract

Selecting suitable charge transport layers and suppressing non-radiative recombination at interfaces with the absorber layer is vital for maximizing the efficiency of halide perovskite solar cells. In this study, high-quality perovskite thin films and devices are fabricated with different fullerene-based electron transport layers and different self-assembled monolayers as hole transport layers. Then, a comparative study of a significant variety of different electrical, optical, and photoemission-based characterization techniques is performed to quantify the properties of the solar cells, individual layers, and, importantly, the interfaces between them. In addition, the limitations and problems of the different measurements, the insights gained by combining different methods, and the different strategies for extracting information from the experimental raw data, are highlighted.