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Through a charge-transfer doping strategy to tune the carrier-type, P/N homojunction CsPbI₃ perovskite QD solar cells are demonstrated; the P/N homojunction significantly improves the carrier dynamic process and outputs record high efficiency in a QD active layer with thickness over 1 µm, demonstrating great potential for the future printing manufacturing of QD PVs.

Abstract

The solution-processed solar cells based on colloidal quantum dots (QDs) reported so far generally suffer from poor thickness tolerance and it is difficult for them to be compatible with large-scale solution printing technology. However, the recently emerged perovskite QDs, with unique high defect tolerance, are particularly well-suited for efficient photovoltaics. Herein, efficient CsPbI₃ perovskite QD solar cells are demonstrated first with over 1 µm-thick active layer by developing an internal P/N homojunction. Specifically, an organic dopant 2,2′-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6TCNNQ) is introduced into CsPbI₃ QD arrays to prepare different carrier-type QD arrays. The detailed characterizations reveal successful charge-transfer doping of QDs and carrier-type transformation from n-type to p-type. Subsequently, the P/N homojunction perovskite QD solar cell is assembled using different carrier-type QDs, delivering an enhanced power conversion efficiency of 15.29%. Most importantly, this P/N homojunction strategy realizes remarkable thickness tolerance of QD solar cells, showing a record high efficiency of 12.28% for a 1.2 µm-thick QD active-layer and demonstrating great potential for the future printing manufacturing of QDs solar cells.

Terpolymer: Y6 system promotes significant breakthroughs in photovoltaic performance for both traditional opaque organic solar cells and semitransparent organic solar cells (ST-OSCs) based on all narrow bandgap (all-NBG) semiconductors. The terpolymer PCE10-BDT2F-0.8:Y6-based ST-OSCs achieve power conversion efficiencies (PCEs) of 12.00% and 10.85% with average visible transmittance (AVT) of 30.98% and 41.08%, respectively.

Abstract

Semitransparent organic solar cells (ST-OSCs) based on all narrow bandgap (all-NBG) semiconductors are attractive for building integration. Unfortunately, advanced NBG Y-series acceptors cannot well match with the NBG donors, resulting from their mismatched energy levels and poor compatibility. Herein, a facile terpolymer design strategy is adopted to improve the matching of Y6 with efficient NBG polymer donor PCE10. F or Cl atom functionalized benzodithiophene (BDT) are introduced into the PCE10 matrix to afford two series of terpolymers, namely PCE10-BDT2F and PCE10-BDT2Cl. Compared with PCE10, all terpolymers show deeper energy levels, higher extinction coefficients, enhanced face-on orientation, and better compatibility with Y6. Consequently, significant breakthroughs are achieved for both opaque and semitransparent devices. Particularly, a record power conversion efficiency (PCE) of 13.80% is achieved by PCE10-BDT2F:Y6-based device, nearly 40% higher than PCE10:Y6-based device. ST-OSCs also achieve impressive PCEs of 12.00% and 10.85% with average visible transmittance (AVT) of 30.98% and 41.08%, respectively, and both PCEs are the highest values with AVT over 30% and 40%. An outstanding light utilization efficiency (LUE) of 4.46% further demonstrates the successful balance of PCE and AVT. These results demonstrate that the design of NBG terpolymers is a facile and highly encouraging strategy for promoting breakthroughs in ST-OSCs.

4-Hydrazinobenzoic acid is introduced to enhance the efficiency and stability of Sn–Pb mixed-perovskite solar cells. Moreover, a vertical compositional gradient with a continuous change of the Pb/Sn ratio is found in the Sn–Pb mixed-perovskite films, leading to graded heterojunctions that are beneficial for photocarrier separation. The resultant devices show a record efficiency up to 22%.

Abstract

Sn–Pb mixed perovskites with bandgaps in the range of 1.1–1.4 eV are ideal candidates for single-junction solar cells to approach the Shockley–Queisser limit. However, the efficiency and stability of Sn–Pb mixed-perovskite solar cells (PSCs) still lag far behind those of Pb-based counterparts due to the easy oxidation of Sn²⁺. Here, a reducing agent 4-hydrazinobenzoic acid is introduced as an additive along with SnF₂ to suppress the oxidation of Sn²⁺. Meanwhile, a vertical Pb/Sn compositional gradient is formed spontaneously after an antisolvent treatment due to different solubility and crystallization kinetics of Sn- and Pb-based perovskites and it can be finely tuned by controlling the antisolvent temperature. Because the band structure of a perovskite is dependent on its composition, graded vertical heterojunctions are constructed in the perovskite films with a compositional gradient, which can enhance photocarrier separation and suppress carrier recombination in the resultant PSCs. Under optimal fabrication conditions, the Sn–Pb mixed PSCs show power conversion efficiency up to 22% along with excellent stability during light soaking.

Light intensity analysis of photovoltaic parameters is introduced as a simple method, allowing understanding of the dominating mechanisms limiting the device performance in perovskite solar cells. The method is based on the drift-diffusion model and is aimed at helping in the explanation of parasitic losses from the trap-assisted recombination or ohmic losses in devices.

Abstract

The number of publications on perovskite solar cells (PSCs) continues to grow exponentially. Although the efficiency of PSCs has exceeded 25.5%, not every research laboratory can reproduce this result or even pass the border of 20%. Unfortunately, it is not always clear which dominating mechanism is responsible for the performance drop. Here, a simple method of light intensity analysis of the JV parameters is developed, allowing an understanding of what the mechanisms are that appear in the solar cell and limit device performance. The developed method is supported by the drift-diffusion model and is aimed at helping in the explanation of parasitic losses from the interface or bulk recombination, series resistance, or shunt resistance in the perovskite solar cell. This method can help not only point toward the dominating of bulk or interface recombination in the devices but also determine which interface is more defective. A detailed and stepwise guidance for such a type of light intensity analysis of JV parameters is provided. The proposed method and the conclusions of this study are supported by a series of case studies, showing the effectiveness of the proposed method on real examples.

A systematic understanding of 2D perovskite's carrier transport mechanism is critical for the development of high-performance 2D perovskite solar cells (PSCs). The recent advances on the carrier behavior of 2D Ruddlesden–Popper PSCs from the view of crystal structure, grain orientation, quantum-well width distribution, and device structure are summarized, and guidelines for successfully elevating carrier transports are provided.

Abstract

In recent years, 2D Ruddlesden–Popper (2DRP) perovskite materials have been explored as emerging semiconductor materials in solar cells owing to their excellent stability and structural diversity. Although 2DRP perovskites have achieved photovoltaic efficiencies exceeding 19%, their widespread use is hindered by their inferior charge-carrier transport properties in the presence of diverse organic spacer cations, compared to that of traditional 3D perovskites. Hence, a systematic understanding of the carrier transport mechanism in 2D perovskites is critical for the development of high-performance 2D perovskite solar cells (PSCs). Here, the recent advances in the carrier behavior of 2DRP PSCs are summarized, and guidelines for successfully enhancing carrier transport are provided. First, the composition and crystal structure of 2DRP perovskite materials that affect carrier transport are discussed. Then, the features of 2DRP perovskite films (phase separation, grain orientation, crystallinity kinetics, etc.), which are closely related to carrier transport, are evaluated. Next, the principal direction of carrier transport guiding the selection of the transport layer is revealed. Finally, an outlook is proposed and strategies for enhancing carrier transport in high-performance PSCs are rationalized.

Single-photon emitters are universally observed from all-inorganic perovskite films at the cryogenic temperature, owing to local thickness variations of the composing CsPbBr₃ microcrystals and the resulting low potential-energy regions. The discovery of such novel emitting species in a perovskite film, with the enriched structure–property relationship, will impart significant influences on the advancement of relevant optoelectronic devices and quantum-light sources.

Abstract

All-inorganic halide perovskites have drawn a lot of research attention very recently owing to their potential solution to the instability issue currently faced by the organic–inorganic counterparts. Meanwhile, the halide perovskites in a solid film are manifested as microscale morphologies whose functionalities are unavoidably affected by the interior or exterior presence of various nanoscale entities. Here all-inorganic solid films are fabricated with varying densities of single CsPbBr₃ microcrystals, showing that very sharp photoluminescence peaks can be universally observed at 4 K with the linewidths being as narrow as hundreds of μeV. The single-photon emission nature is confirmed for such a photoluminescence peak, whose intensity is completely quenched above ≈30 K to suggest its possible origin from a low potential-energy region of the single microcrystal. The discovery of such a novel emitting species in halide perovskites, with the enriched structure–property relationship, will surely impart significant influences on the advancement of relevant optoelectronic devices and quantum-light sources.

Application of photoluminescence-based contactless series resistance imaging using nonuniform illumination is demonstrated on perovskite solar cells. The operating point of particular regions is controlled spatially via photoexcitation pattern manipulation across the device. The capability of this proposed contactless method to identify features with high and low absolute effective series resistance is validated qualitatively by comparison with other luminescence-based imaging techniques.

A contactless effective series resistance imaging method for large-area perovskite solar cells that is based on photoluminescence imaging with nonuniform illumination is introduced and demonstrated experimentally. The proposed technique is applicable to partially and fully processed perovskite solar cells if laterally conductive layers are present. The capability of the proposed contactless method to detect features with high effective series resistance is validated by comparison with various contacted mode luminescence imaging techniques. The method can reliably provide information regarding the severeness of the detected series resistance through photoexcitation pattern manipulation. Application of the method to subcells in monolithic tandem devices, without the need for electrical contacting the terminals, appears feasible.

The design strategy of thermally stable all-perovskite tandem solar cells is presented, where metal oxides are used for the charge transport layers and tunnel junction. The tandem devices retained 85% of their initial efficiency after thermal stressing at 85 °C for 2500 h. Achieving such remarkable thermal stability represents a crucial step toward commercial viability of all-perovskite tandem solar cells.

All-perovskite tandem solar cells offer a promising avenue to go beyond the efficiency limit of single-junction devices. Their efficiencies have been increasing rapidly in the past few years; however, their commercial viability is hindered by the instability under thermal stressing. Herein, comprehensive device design strategies are proposed to achieve thermally stable all-perovskite tandem solar cells while retaining the advantages of solution processing. Metal oxides, i.e., NiO _x and SnO₂, are used for the hole and electron transport layers in both wide bandgap and narrow subcells. The metal-based recombination layer is replaced with a stable and conductive indium tin oxide nanocrystals film to fabricate an all metal-oxide-based tunnel junction. Based on those design strategies, the encapsulated all-perovskite tandem solar cells retained 85% of their initial efficiency after stressing at 85 °C for 2500 h and maintained >80% of their initial performance after 900 h operation at the maximum power point and operating temperature of ≈65 °C. Achieving such thermal stability represents a crucial step toward commercial viability of all-perovskite tandem solar cells.

Herein, a promising power conversion efficiency of a simple additive-treated single-cell inverted perovskite solar cell fabricated under moderate humidity is further used in a novel mini-module architecture. This work opens possibilities of the next steps towards commercialization of the perovskite solar cells.

An additive to perovskite material can play a crucial role in the fabrication of perovskite solar cells (PSCs); the additive treatment investigated herein produces promising overall power conversion efficiency (PCE) enhancements. Currently, the fabrication of PSCs is mainly carried out under the dry air of a glove box to avoid moisture adsorption by methylammonium lead iodide perovskites; these tight restrictions in how PSCs are fabricated hinder the possibility of their commercialization. In this study, a technique that uses C₁₂F₄N₄ as an additive to the MAPbI₃ for the one-step deposition of perovskite layers under moderate humidity is presented; this approach is able to achieve high-quality perovskite films despite the adverse condition. The added C₁₂F₄N₄ bridges the gaps between the MAPbI₃ grain boundaries and significantly enhances all the photovoltaic parameters compared to using pristine methylammonium lead iodide; this method even eventually enhances the overall PCE. Both, single-cell and optimized five-cell integrated mini-module devices in moderately humid conditions are fabricated. This facile method presents new possibilities for scaling up PSCs production in humid environments.

The baseplate temperature-dependent blend morphology of sequential deposition-processed organic solar cells at vertical and lateral scales is systematically studied, and the temperature-microstructure-performance relationship is established.

Abstract

Numerous previous reports on the sequential deposition (SD) technique have demonstrated that this approach can achieve a p-i-n active layer architecture with an ideal vertical composition gradient, which is one of the critical factors that can influence the physical processes that determine the photovoltaic performance of organic solar cells. Herein, a commonly used photovoltaic system comprised of PM6 as a donor and Y6 as an acceptor is investigated with respect to sequential blade-processing deposition to comprehensively explore the morphology characteristics as a function of baseplate temperature. A systematic study of the temperature-dependent blend morphology elucidates the SD-processed configuration merits and device physics behind temperature-controlled degree of vertical composition gradient, and constructs the temperature-microstructure-property relationship for the corresponding photovoltaic parameters. The result shows, as the temperature increases, the morphology of the active layer has undergone a distinct evolution from the pseudo-bulk heterojunction to a pseudo-planar heterojunction and then to a pseudo-planar bilayer, leading to a non-monotonic correlation between baseplate temperature and device performance. This investigation not only reveals the importance of precisely controlling baseplate temperature for gaining vertical morphology control, but also provides a path toward rational optimization of device performance in the lab-to-fab transition.

Three nonfused ring electron acceptors (NFREAs) with different π-conjugation length are designed and synthesized. The π-conjugation length can significantly influence the molar extinction coefficient and the electron mobility. The first 3D network packing is observed for 2BTh-2F. 2BTh-2F derivative based organic solar cells give a high power conversion efficiency of 15.44%, which is the highest value reported based on NFREAs.

Abstract

Three nonfused ring electron acceptors (NFREAs; 2Th-2F, BTh-Th-2F, and 2BTh-2F) with thieno[3,2-b]thiophene bearing two bis(4-butylphenyl)amino substituents as the core, 3-octylthiophene or 3-octylthieno[3,2-b]thiophene as the spacer, and 3-(1,1-dicyanomethylene)-5,6-difluoro-1-indanone as the terminal group are designed and synthesized. The molar extinction coefficient of acceptors and the electron mobility of blend films gradually increase with increasing π-conjugation length. Moreover, 2BTh-2F displays a planar molecular conformation assisted by S···N and S···O intramolecular interactions. More importantly, the molecular stacking changes from 2D packing for the 2Th-2F analog to 3D network packing for 2BTh-2F. Due to these comprehensive merits, 2BTh-2F:PBDB-T-based organic solar cells give a high power conversion efficiency of 14.53%. More impressively, when D18 is used as the donor polymer, the power conversion efficiency is further enhanced to 15.44%, which is the highest value reported for solar cells based on NFREAs.

A series of small molecule acceptor-polymerized acceptors (P _As) with 2D control of regioregularity and ii) donating units are developed to investigate a direct relationship between the electron mobility of P _A and the performance of all-polymer solar cells (all-PSCs). The Y5-Se-In-based all-PSC showed the most well-developed crystalline properties, the electron mobility and, thus, the best efficiency of 13.4%. The authors report, for the first time, the opposite effects of regioisomers on the chain conformation, electron mobility of P _As, and the all-PSC performance depending on the backbone building unit.

Abstract

The charge transport ability of polymer acceptors (P _As) is crucial for achieving high power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs). However, the electron mobilities (μ_es) of most P _As are inferior to those of their small molecule acceptor (SMA) counterparts. Herein, the authors design a new series of the polymerized SMA-based P _As (Y5-A-B), where the donating moiety (A = selenophene (Se)/biselenophene (BiSe)) and the backbone regioregularity (B = In/Mix/Out) are 2D controlled, for enhancing both the μ_e and PCEs. Interestingly, the effects of regioisomers on the μ_e and all-PSC performance are the opposite depending on the donating unit. For the Y5-Se-based P _As, the PCEs increase in order of Out (7.52%) < Mix (9.33%) < In (13.38%). In contrast, for the Y5-BiSe-based P _As, the PCEs decrease in order of Out (10.67%) > Mix (9.58%) > In (8.52%). These opposite trends in each series originate from the different planarity and intermolecular assembly of P _As depending on the regioregularity. Thus, the Y5-Se-In blend exhibits the highest μ_e and achieves the highest PCE (13.38%) among the all-PSCs in this study. Therefore, the authors report the importance of simultaneous engineering of the backbone building unit and regioregularity to realize high-mobility P _A and highly efficient all-PSCs.

Device engineering is an effective way to improve the photovoltaic performance of organic solar cells. Herein, PM6:BO-4F system is selected to prepare planar heterojunction (PHJ) devices. The PHJ devices are successfully fabricated by using green orthogonal solvents of o-xylene (O-XY) and tetrahydrofuran (THF), achieving an excellent power conversion efficiency (PCE) of 16%, which is the highest efficiency of the PHJ structure.

Abstract

Device engineering is an effective way to improve the photovoltaic performance of organic solar cells (OSCs). Currently, the widely used bulk heterojunction (BHJ) structure has problems such as material solubility limitations and the emerging pseudoplanar heterojunction (PPHJ) structure is also troubled by printing technology requirements. However, these issues can be solved by the reasonable application of traditional planar heterojunction (PHJ) structure. Herein, PM6:BO-4F system is selected to prepare PHJ devices by combining sequential spin-coating and orthogonal solvent strategy. In view of the good solubility of PM6 and BO-4F in commonly used high-boiling solvent chlorobenzene (CB) and green solvent tetrahydrofuran (THF), respectively, the PHJ devices are successfully prepared by using these two orthogonal solvents, achieving a power conversion efficiency (PCE) of 15.6%. On this basis, green nonhalogen reagent o-xylene (O-XY) is further used to process PM6. Due to the large polarity difference between O-XY and THF, all-green solvent-processed PHJ devices are successfully fabricated and obtain an astonishing PCE of 16%. As far as it is known, it is the highest efficiency for PHJ OSCs. The results prove the huge research potential of PHJ structure and point out new direction for solving OSC materials compatibility, long-term stability, and future commercial applications.

A surface reconstruction strategy is proposed to optimize the surface defect states and crystallization dynamics in an all-inorganic perovskite/organic two-terminal tandem solar cell, leading to efficient hole transport and charge recombination in the interconnecting layer. Finally, a power conversion efficiency of 21.04% and robust operational stability are obtained.

Abstract

The construction of monolithic two-terminal tandem solar cells (2T TSCs) offers the possibility of pursuing high power conversion efficiency (PCE) by overcoming the single-junction Shockley–Queisser limit in photovoltaics. However, little attention is paid to simultaneously improve the stability by utilizing the complementary properties of various photoactive layers. Here, beyond the stacked photoactive layers featuring complementary absorption, all-inorganic perovskite (CsPbI_1.8Br_1.2) is chosen as the photoactive layer of the front wide-bandgap subcell for its intrinsic high thermal stability and ultraviolet (UV)-filtering function to address the burn-in and UV degradation of organic rear subcells. To realize their monolithic integration, the charge recombination efficiency in the interconnecting layer (ICL) between the two types of subcells is tentatively improved by surface reconstruction of all-inorganic perovskite using trimethylammonium chloride. The repaired CsPbI_1.8Br_1.2 surface enables effective suppression of nonradiative recombination and facilitates hole transport, providing efficient charge recombination in the ICL in the 2T TSC. As a result, the all-inorganic perovskite/organic 2T TSC delivers a promising PCE of 21.04%, accompanied by an ultrahigh open-circuit voltage (V _oc) of 2.05 V, which is nearly equal to the superposition of the respective V _oc values of the subcells. More importantly, the 2T TSC simultaneously shows outstanding operational and UV stabilities.

J- and H-aggregates constitute archetype functional materials due to unique properties originating from exciton coupling. Here, a broader perspective on the origin of red-shifted absorption bands including intermolecular charge transfer-mediated J-coupling is provided. Further, important classes of dye aggregates with regard to the prevailing intermolecular couplings and their relevance for organic solar cells and photodetectors are analyzed.

Abstract

Dye–dye interactions affect the optical and electronic properties in organic semiconductor films of light harvesting and detecting optoelectronic applications. This review elaborates how to tailor these properties of organic semiconductors for organic solar cells (OSCs) and organic photodiodes (OPDs). While these devices rely on similar materials, the demands for their optical properties are rather different, the former requiring a broad absorption spectrum spanning from the UV over visible up to the near-infrared region and the latter an ultra-narrow absorption spectrum at a specific, targeted wavelength. In order to design organic semiconductors satisfying these demands, fundamental insights on the relationship of optical properties are provided depending on molecular packing arrangement and the resultant electronic coupling thereof. Based on recent advancements in the theoretical understanding of intermolecular interactions between slip-stacked dyes, distinguishing classical J-aggregates with predominant long-range Coulomb coupling from charge transfer (CT)-mediated or -coupled J-aggregates, whose red-shifts are primarily governed by short-range orbital interactions, is suggested. Within this framework, the relationship between aggregate structure and functional properties of representative classes of dye aggregates is analyzed for the most advanced OSCs and wavelength-selective OPDs, providing important insights into the rational design of thin-film optoelectronic materials.

Solar cells incorporating 2D perovskites show tradeoffs between efficiency and stability. The challenges these solar cells face are identified and select works the community has undertaken to overcome them are highlighted in this review. Several recommendations are proposed on how to further improve perovskite solar cells so their performance and stability can be commensurate with application requirements.

Abstract

Perovskite solar cells (PSCs) have rapidly emerged as one of the hottest topics in the photovoltaics community owing to their high power-conversion efficiencies (PCE), and the promise to be produced at low cost. Among various PSCs, typical 3D perovskite-based solar cells deliver high PCE but they suffer from severe instability, which restricts their practical applications. In contrast to 3D perovskites, 2D perovskites that incorporate larger, less volatile, and generally more hydrophobic organic cations exhibit much improved thermal, chemical, and environmental stability. 2D perovskites can have different roles within a solar cell, either as the primary light absorber (2D PSCs), or as a capping layer atop a 3D perovskite absorbing layer (2D/3D PSCs). Tradeoffs between PCE and stability exist in both types of PSCs—2D PSCs are more stable but exhibit lower efficiency while 2D/3D PSCs deliver exciting efficiency but show relatively poor stability. To address this PCE/stability tradeoff, the challenges both the 2D and 2D/3D PSCs face are identified and select works the community has undertaken to overcome them are highlighted in this review. It is ended with several recommendations on how to further improve PSCs so their performance and stability can be commensurate with application requirements.

Nature Energy, Published online: 18 October 2021; doi:10.1038/s41560-021-00912-8

An extended π-conjugated organic spacer, namely TTDMAI, is successfully developed as spacers for 2D Dion–Jacobson perovskites. A champion efficiency of 18.82% is demonstrated due to the improved film quality and preferred crystal vertical orientation thanks to the templated grain growth by the large crystal nuclei size in the precursor solution.

Abstract

2D Dion–Jacobson (DJ) perovskites have become an emerging photovoltaic material with excellent structure and environmental stability due to their lacking van der Waals gaps relative to 2D Ruddlesden–Popper perovskites. Here, a fused-thiophene-based spacer, namely TTDMAI, is successfully developed for 2D DJ perovskite solar cells. It is found that the DJ perovskite using TTDMA spacer with extended π-conjugation length exhibits high film quality, large crystal size and preferred crystal vertical orientation induced by the large crystal nuclei in precursor solution, resulting in lower trap density, reduced exciton binding energy and oriented charge transport. As a result, the optimized 2D DJ perovskite device based on TTDMA (nominal n = 4) delivers a champion PCE up to 18.82%. Importantly, the unencapsulated device based on TTDMA can sustain average 99% of their original efficiency after being stored in N₂ for 4400 h (over 6 months). Moreover, light, thermal, environmental and operational stabilities are also significantly improved in comparison with their 3D counterparts.