Chen Weijie

By using different solvents for spin coating, the active layers of non-fullerene-based organic solar cells, the film morphology, and crystallinity of donor and acceptor are tailored. Among the studied solvents, chlorobenzene offers the best way to balance the aggregation and crystallization kinetics in the active layer and thus gives the best performing solar cells.

Non-fullerene acceptor (NFA)-based organic solar cells have made tremendous progress in recent years. For the neat NFA system PBDB-T:ITIC, the film morphology and crystallinity are tailored by the choice of the solvent used for spin coating the active layers. Three different chlorinated solvents, chlorobenzene (CB), chloroform, and dichlorobenzene, are compared and the obtained active layer morphology is correlated with the optoelectronic properties and the device performance. The small domain sizes in the case of CB are most beneficial for the device performance, whereas the largest number or size of face-on PBDB-T crystallites is not causing the highest power conversion efficiencies (PCEs). In addition, when using CB, the number of edge-on crystallites is highest and the distances between neighboring domains are small. The smoothest blend films are realized with CB, which exhibit correlated roughness with their substrates and no large aggregates have formed in these blend films. Thus, CB offers the best way to balance the aggregation and crystallization kinetics in the active layer and enables the highest PCE values.

Herein, the ultraviolet (UV) stability with the solar intensity of solar cell based on organic photovoltaic is studied. The tracking time up to 2000 h is expanded. Non-fullerene organic photovoltaics improves the UV stability by the device compositions. Ternary systems yield a UV half-lifetime up to 1750 h which is close to 2000 h set by the outdoor lifetime of 1 year.

The ultraviolet (UV) radiation in the solar spectrum causes most of the decay under sunlight for solar cells based on organic photovoltaics (OPV). One-year outdoor lifetime still remains challenging for OPV. The lifetime of 2000 h under continuous laboratory light corresponds to a 1 year outdoor lifetime. Herein, the stability of the OPVs is studied under continuous irradiation by a UV light emitting diode of 365 nm with a long tracking time. The intensity of 50 W m⁻² is the same as the sunlight UV. The compositions of the active layer and cathode interfacial layer are sensitive to UV irradiation. In general, ternary devices have better UV stability than binary devices. In particular, a good stability is achieved for the ternary device based on the high-performance blend with poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PM6) as the donor. The acceptor is 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-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-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6). The fullerene derivative [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) is added as the second acceptor, whereas a polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene))-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione)] (PBDB-T) is added as the second donor. For the PM6:Y6 ternary device, the UV half lifetime is 1750 h, which is close to the 2000 h target set by the outdoor of a 1 year lifetime.

Four different quaternary ammonium chloride salts have been studied for posttreatment passivation strategy. Tetramethylammonium chloride (TC) shows the best passivation effect. Due to alleviation of metallic lead on the perovskite surface, it increases the efficiency of the device from 19.7% to 21.7% by largely promoting the open-circuit voltage from 1.06 to 1.12 V with enhanced stability.

Perovskite solar cells (PSCs) have been booming for more than a decade. Defect passivation strategies play an important role in updating the efficiency record. Herein, a posttreatment strategy is proposed to passivate the defects in PSCs by quaternary ammonium chloride (QAC) salts. The tetramethylammonium chloride (TC) molecule shows the best achievement among those passivation agents. Enhanced crystallinity and reduced defects of TC-treated perovskite result in promotion of the open-circuit voltage of the device from 1.06 to 1.12 V, thus largely improving the conversion efficiency from 19.8% to 21.7% for the FA_0.95MA_0.05PbI_2.85Br_0.15 perovskite component. Moreover, the TC-based device retains 93.2% of its initial power conversion efficiency after storage in drying cupboard (relative humidity 20%≈30%) for more than 30 days. By the verification of various characterization techniques (transient resolved photoluminescence, electrochemical impedance spectroscopy, light intensity dependent voltage, and transient absorption spectroscopy), the enhanced performance results from the less recombination retarded by QAC in the treated device compared with the control device. Additionally, X-ray photoelectronic spectroscopy proves the interaction between TC molecule and Pb in perovskite. This system of selection will definitely provide a novel way to search for effective passivators to promote performance of PSCs.

The effect of A-site doping with guanidinium cation (GA⁺) on ion migration is explored based on the temperature-dependent ion conductivity. The increase of activation energy proves the lift of ion migration barrier. It elaborates that doping of GA⁺ can effectively suppress the ion migration and enhance the operational stability of the device.

Perovskite solar cells (PSCs) develop great potential to make photovoltaic power generation systems more cost-effective due to the high power conversion efficiency (PCE), low material cost, and easy fabrication. Alloyed A-site cations surrounded by PbI₆ octahedra play decisive roles in the crystal structure, bandgap, and phase stability, as well as ion migration. Herein, based on the temperature-dependent ion conductivity measurement, the activation energy for iodide ion migration is systematically studied with different proportions of guanidinium cation (GA⁺) substitution. It is found that partial GA⁺ doping could effectively suppress iodide ion migration. The triple-cation perovskite (MA_0.8FA_0.1GA_0.1PbI₃) PSCs achieve a PCE of 22.17% with superior operational stability maintaining 90% of their initial efficiency after 1200 h under continuous light soaking. Furthermore, it is extended to mini perovskite solar modules, 14 cm² active area, and achieves a PCE of 19.18%.

Highly efficient CsPb_0.7Sn_0.3IBr₂ perovskite solar cells (PSCs) are achieved via introducing zinc oxalate (ZnOX) as additive. ZnOX not only reduces the metal ions related trap states but also modulates film crystallization, resulting in a record device efficiency of 14.1%. In particular, chemically reducing oxalate can effectively suppress the oxidation of Sn²⁺, contributing to devices with much improved long-term air stability.

Abstract

All-inorganic CsPbIBr₂ perovskite solar cells (PSCs) have recently gained growing attention as a promising template to solve the thermal instability of organic–inorganic PSCs. However, the relatively low device efficiency hinders its further development. Herein, highly efficient and stable CsPb_0.7Sn_0.3IBr₂ compositional perovskite-based inorganic PSCs are fabricated by introducing appropriate amount of multifunctional zinc oxalate (ZnOX). In addition to offset Pb and Sn vacancies through Zn²⁺ ions incorporation, the oxalate group can strongly interact with undercoordinated metal ions to regulate film crystallization, delivering perovskite film with low defect density, high crystallinity, and superior electronic properties. Correspondingly, the resulting device delivers a champion efficiency of 14.1%, which presents the highest reported efficiency for bromine-rich inorganic PSCs thus far. More importantly, chemically reducing oxalate group can effectively suppress the notorious oxidation of Sn²⁺, leading to significant enhancement on air stability.

The structural design of amine molecules is effective for functional construction of perovskite light-absorbing layer in perovskite solar cells. High-efficiency and robust perovskite solar cells can be achieved through crystal regulation of formamidinium-based perovskites and improvement of the specific perovskite properties including defect condition, charge transfer, and moisture resistance.

Abstract

Formamidinium (FA) based perovskites are considered as one of the most promising light-absorbing perovskite materials owing to their narrower band gap and better thermal stability compared to conventional methylammonium-based perovskites. Constant improvement by using various additives stimulates the potential application of these perovskites. Amine molecules with different structures have been widely used as typical additives in FA-based perovskite solar cells, and decent performances have been achieved. Thus, a systematic review focusing on structural regulation and functional construction of amines in FA-based perovskites is of significance. Herein, we analyze the construction mechanism of different structural amines on the functional perovskite crystals. The influence of amine molecules on specific perovskite properties including defect conditions, charge transfer, and moisture resistance are evaluated. Finally, we summarize the design rules of amine molecules for the application in high-performance FA-based perovskites and propose directions for the future development of additive molecules.

Publication date: 20 April 2022

Source: Joule, Volume 6, Issue 4

Author(s): Kai Wang, Luyao Zheng, Yuchen Hou, Amin Nozariasbmarz, Bed Poudel, Jungjin Yoon, Tao Ye, Dong Yang, Alexej V. Pogrebnyakov, Venkatraman Gopalan, Shashank Priya

Publication date: 16 February 2022

Source: Joule, Volume 6, Issue 2

Author(s): Wei Meng, Kaicheng Zhang, Andres Osvet, Jiyun Zhang, Wolfgang Gruber, Karen Forberich, Bernd Meyer, Wolfgang Heiss, Tobias Unruh, Ning Li, Christoph J. Brabec

Combined transient absorption and time-resolved photoluminescence studies reveal several parallel charge carrier generation pathways, which determine the performance efficiency of a non-fullerene organic solar cell. The small offset between donor and acceptor highest occupied molecular orbit levels enables high open-circuit voltage, yet opens a thermally activated recombination loss channel by reverse electron transfer from the acceptor to the donor.

Transient absorption and time-resolved fluorescence measurements in a wide temperature range are used to investigate the mechanism of charge carrier generation in efficient organic solar cells based on a PM6:Y6 donor–acceptor blend. The generation mechanisms differ significantly under excitation of a donor or acceptor. The investigations reveal a temperature-dependent interplay between the formation of interfacial charge transfer (CT) states and intra-moiety CT states of the acceptor, their separation into free charge carriers and carrier recombination. The efficient charge carrier generation is ensured by the carrier separation over a small energy barrier, which is easily surmountable at room temperature. However, the overall yield of charge carrier generation at room temperature is reduced by the recombination of charge carriers due to the thermally activated back transfer of electrons from the acceptor to the donor via the highest occupied molecular orbit (HOMO) levels, which is enabled by the small energy offset between HOMO levels of the donor and the acceptor.

Unlike blended-cast doping, interfacial doping is an effective universal and important method for tailoring the interfacial properties between the donor (D) and the acceptor (A) in bilayer organic solar cells. Interfacial doping can be effectively controlled within the D/A interfacial miscibility range without disrupting the π–π stacking of the polymer, thus significantly improving the efficiency and stability of the device.

Molecular doping is an effective means to tune the optoelectronic properties of organic semiconductors. Despite its versatility, its application in bulk heterojunction (BHJ) organic solar cells (OSCs) is still limited, especially blend-cast doping. Difficulties in desolving molecular dopants in weakly polar solvents have led to the formation of undesired structures, with relatively high concentrations of dopants affecting the bicontinuous BHJ morphology. Bilayer OSCs are stacked structures with sequential deposition of donor and acceptor films, which make important contributions to the fine tuning of morphology and balanced charge transport. The sequential deposition of thin films in bilayer devices organically facilitates the sequential doping of pure donor polymers by dopants. The sequential doping of the dopant at the interface between the donor and the acceptor can efficiently improve the carrier mobility and the charge transfer between the donor and the acceptor without damaging the quality of the film, thus improving the device performance efficiently. With three different dopants F4TCNQ, F6TCNNQ, and BCF, sequential interfacial doping in bilayer devices is found to be a versatile and effective method to improve device performance. Making this simple doping strategy promising for high-performance bilayer OSCs, it is evaluated as a promising alternative to BHJ OSCs.

This work proposes a C₃N quantum dots insertion method to effectively adsorb bulky organic cations, and provide nucleation sites to realize gradient distribution of solute in 2D CsPbI₃ film to obtain a bi-directional crystallization process. Importantly, the optimized 2D CsPbI₃ film-based device delivers a high power conversion efficiency of 15.63% with strengthened environmental stability, and also versatility to other Ruddlesden-Popper and Dion-Jacobson-type bulky organic.

Abstract

Two-dimensional (2D) CsPbI₃ is developed to conquer the phase-stability problem of CsPbI₃ by introducing bulky organic cations to produce a steric hindrance effect. However, organic cations also inevitably increase the formation energy and difficulty in crystallization kinetics regulation. Such poor crystallization process modulation of 2D CsPbI₃ leads to disordered phase-arrangement, which impedes the transport of photo-generated carriers and worsens device performance. Herein, a type of C₃N quantum dots (QDs) with ordered carbon and nitrogen atoms to manipulate the crystallization process of 2D CsPbI₃ for improving the crystallization pathway, phase-arrangement and morphology, is introduced. Combination analyses of theoretical simulation, morphology regulation and femtosecond transient absorption (fs-TA) characterization, show that the C₃N QDs induce the formation of electron-rich regions to adsorb bulky organic cations and provide nucleation sites to realize a bi-directional crystallization process. Meanwhile, the quality of 2D CsPbI₃ film is improved with lower trap density, higher surface potential, and compact morphology. As a result, the power conversion efficiency (PCE) of the optimized device (n = 5) boosts to an ultra-high value of 15.63% with strengthened environmental stability. Moreover, the simple C₃N QDs insertion method shows good universality to other bulky organic cations of Ruddlesden-Popper and Dion-Jacobson, providing a good modulation strategy for other optoelectronic devices.

Nonradiative recombination induced by C₆₀ is limiting the performance of pin type perovskite solar cells and remains poorly understood. In this manuscript, the possible recombination pathways are systematically examined and it is discovered that across-interface recombination dominates. A point contact strategy is suggested to circumvent the loss, paving the way to improved pin type perovskite solar cells.

Abstract

Perovskite semiconductors are an attractive option to overcome the limitations of established silicon based photovoltaic (PV) technologies due to their exceptional opto-electronic properties and their successful integration into multijunction cells. However, the performance of single- and multijunction cells is largely limited by significant nonradiative recombination at the perovskite/organic electron transport layer junctions. In this work, the cause of interfacial recombination at the perovskite/C₆₀ interface is revealed via a combination of photoluminescence, photoelectron spectroscopy, and first-principle numerical simulations. It is found that the most significant contribution to the total C₆₀-induced recombination loss occurs within the first monolayer of C₆₀, rather than in the bulk of C₆₀ or at the perovskite surface. The experiments show that the C₆₀ molecules act as deep trap states when in direct contact with the perovskite. It is further demonstrated that by reducing the surface coverage of C₆₀, the radiative efficiency of the bare perovskite layer can be retained. The findings of this work pave the way toward overcoming one of the most critical remaining performance losses in perovskite solar cells.

Herein, monolithical perovskite/silicon tandem solar cells with bottom cells featuring a polycrystalline silicon on oxide front junction and locally diffused aluminium-p⁺ rear contacts, fabricated with a passivated emitter and rear cell compatible technology are reported. Proof-of-concept tandem cells demonstrate power conversion efficiencies (PCE) up to 21.3%. Major performance enhancements and a 29.5% PCE potential using optical simulations are identified.

Combining a perovskite top cell with a conventional passivated emitter and rear cell (PERC) silicon bottom cell in a monolithically integrated tandem device is an economically attractive solution to boost the power conversion efficiency (PCE) of silicon single-junction technology. Proof-of-concept perovskite/silicon tandem solar cells using high-temperature stable bottom cells featuring a polycrystalline silicon on oxide (POLO) front junction and a PERC-type passivated rear side with local aluminum-p⁺ contacts are reported. For this PERC/POLO cell, a process flow that is compatible with industrial, mainstream PERC technology is implemented. Top and bottom cells are connected via a tin-doped indium oxide recombination layer. The recombination layer formation on the POLO front junction of the bottom cell is optimized by postdeposition annealing and mitigation of sputter damage. The perovskite top cell is monolithically integrated in a p−i−n junction device architecture. Proof-of-concept tandem cells demonstrate a PCE of up to 21.3%. Based on the experimental findings and supporting optical simulations, major performance enhancements by process and layer optimization are identified and a PCE potential of 29.5% for these perovskite/silicon tandem solar cells with PERC-like bottom cell technology is estimated.

The eco-friendly and highly efficient single-component organic solar cells (SCOSCs) with power conversion efficiencies of over 12% were developed by utilizing a series of halogen-free processing solvents.

Single-component organic solar cells (SCOSCs), which contain only one photoactive material in their active layers, promise many advantages with respect to the excellent device and morphological stabilities and a simplified film fabrication process. However, most of the high-efficiency SCOSCs are fabricated by halogenated solvents. Herein, the eco-friendly and highly efficient SCOSCs based on a conjugated block copolymer PBDB-T-b-PYT with power conversion efficiencies (PCEs) of over 12% are developed by utilizing a series of halogen-free processing solvents (including tetrahydrofuran, toluene, o-xylene), being higher than those of the devices cast from halogenated solvents (including chloroform, chlorobenzene, 1,2-dichlorobenzene). Impressively, by using o-xylene as processing solvent for the PBDB-T-b-PYT active layer, high and balanced charge transport properties, and well-developed morphological features are observed, leading to a high PCE of 12.60%, which is the highest value among SCOSCs to date. Importantly, the non-halogenated solvents-processed devices exhibited better storage and operational stabilities as compared to the chloroform (CF)-processed SCOSCs. More importantly, higher photovoltaic performance (over 12% PCEs) of the green-solvent-processed devices as compared with the CF-processed devices fabricated by blade coating in ambient conditions are achieved, suggesting the promising prospects of SCOSCs toward industrial production, particularly being relevant for environmentally benign solvents.

Perovskite bulk and heterointerface defects that induce non-radiative recombination and trap-density in inverted perovskite solar cells (PSCs) can be suppressed by long-chain alkylamine ligands passivation. The oleylammonium and phenethylammonium ligands act as orgainc spacers to assist growth of perovskite crystals, and suppress bulk, surface, grain boundaries, and buried interfaces defects. Thus, such strategies enable hysteresis-free and efficient NiO _x -based inverted PSCs.

The passivation of perovskite bulk and heterointerface defects is one of the most significant ways to enhance the efficiency and operational stability of perovskite solar cells (PSCs). So far, ammonium-based alkylamine halides have been considered as effective passivation materials to reduce defects of the perovskite absorber layer. Herein, roles of long-chain alkylamine ligands (LALs) in triple-halide perovskites are systematically studied for achieving efficient NiO _x -based inverted PSCs. Two kinds of LALs oleylammonium (OAm) and phenethylammonium (PEA), as perovskite bulk and interface passivation agents, respectively, are introduced. It is found that both OAm and PEA ligands cannot only assist their crystal growth with vertical orientation, but also suppress triple-halide perovskite bulk and interface defects. As precursor additives, OAm ligands can be used as organic spacers to assist the generation of the 2D@3D perovskite bulk crystals. 2D@3D/2D perovskite heterostructures are further formed when the 2D@3D perovskite bulk film is post-treated by PEA ligands. As a result, such strategies enable hysteresis-free and highly efficient NiO _x -based triple-halide inverted PSCs.

Varying the blend ratio is effective to tune the optical properties of a bulk heterojunction (BHJ) film for future application in semitransparent organic solar cells (OSCs). It is discovered that either type of doping helps overcome the discontinuous charge-transport network-led carrier recombination in dilute BHJ films. This discovery provides a new methodology to reduce the electrical–optical gap of semitransparent OSCs.

Abstract

The semitransparent and colorful properties of organic solar cells (OSCs) attract intensive academic interests due to their potential application in building integrated photovoltaics, wearable electronics, and so forth. The most straightforward and effective method to tune these optical properties is varying the componential ratio in the blend film. However, the increase in device transmittance inevitably sacrifices the photovoltaic performance because of severe carrier recombination that originates from discontinuous charge-transport networks in the blend film. Herein, a strategy is proposed via the molecular-doping strategy to overcome these shortcomings. It is discovered that p-doping is able to release the trapped holes in segregated polymer domains leading to short-circuit current enhancement, while n-doping is more effective to fill the bandgap states producing a higher fill factor. More importantly, either type of doping improves the photovoltaic performance in the semitransparent photovoltaic devices. These discoveries provide a new pathway to breaking the compromise between the photovoltaic performance and optical transmittance in semitransparent OSCs, and hold promise for their future commercialization.