Chen Weijie

25 cm² organic solar module based on PM6:BTP-eC9 processed from the high boiling point solvent (chlorobenzene) is fabricated with an efficiency of over 14%. The large side chain on the pyrrole ring of Y6-based nonfullerene can inhibit the excessive aggregation and obtain the wide processing window for doctor blading.

Abstract

It is still challenging to fabricate efficient large-area organic solar modules by solution processing. Processing window is important to obtain optimal aggregation of an active layer in large area for high efficiency. The star active layer of PM6:Y6 is processed from chloroform (for high efficiency) that has a narrow processing window due to the low boiling point of the solvent. In this work, the correlation between chemical structure (side chains) and processing solvents is investigated to obtain high efficiency and long processing windows. It is found that large side chains on the pyrrole ring are the key factor influencing the aggregation of active layer films. Short side chain (in Y6 and Y6-1O) will cause excess aggregation when processed from high-boiling-point solvent (chlorobenzene, CB), while long side-chain (in BTP-BO-4F, BTP-BO-4Cl, and BTP-eC9) can inhibit such aggregation and maintain high photovoltaic performance when processed from CB with wide processing window. In the end, over 25 cm² organic solar module via doctor blading based on PM6:BTP-eC9 active layer has been fabricated with a PCE of 14.07%.

This article presents a light-trapping radiative cooler (LTRC)-integrated multijunction SC (MJSC) for simultaneously enhancing light-trapping and radiative cooling effects. Theoretical opto-electro-thermal analyses show that the MJSC is the most effective cell when the radiative cooler is applied. Field tests confirm that the LTRC can contribute to a temperature drop of 6 °C and both maximization of open-circuit voltage and short-circuit current.

Abstract

The power-conversion efficiency of solar cells (SCs) is reduced at high temperatures. A radiative cooling process can be implemented to overcome this issue. The radiative cooler (RC) presents considerable potential in the design of an ideal broadband emitter, which emits heat through the entire atmospheric transmittance window for devices with operating temperatures that significantly exceed the ambient temperature. However, the performance of these devices varies based on the type of SCs. This study aims to determine the dependency of the radiative cooling power for various types of SCs and proposes the multi-junction SC (MJSC), which is the SC that benefits the most from RCs. The integrated cooler is designed with a micro-grating which can enhance the emissivity within entire atmospheric transmittance window and can also lead to the light-trapping aspect in the solar spectrum. Outdoor field tests demonstrate both the enhanced cooling performance and the power conversion efficiency of the proposed MJSC when compared to a conventional glass-mounted MJSC under direct sunlight of ≈900 Wm⁻² including a temperature drop of ≈6 °C and minimization of the variation of the open-circuit voltage to ≈6%. Future research is expected to develop a theoretical bridge between the field of SCs and radiative cooling.

The multistrategy of ThMAAc addition and BTCIC-4Cl modification to prepare CsPbI₂Br perovskite solar cells based on dopant-free poly(3-hexylthiophene) (P3HT) is applied. The multi-optimization can shrink the crystal lattice, release stress, passivate defects, and promote carrier transport, thereby obtaining a smaller open-circuit voltage deficit, a champion power conversion efficiency of 16.3% and excellent thermal stability.

All-inorganic perovskites have attracted substantial interest due to their outstanding thermal stability. However, the device performance is still inferior to the typical organic–inorganic counterparts because of the unsatisfying phase stability and defects of the inorganic perovskite films. Herein, a multistrategy to optimize CsPbI₂Br perovskite solar cells (PSCs) based on dopant-free poly(3-hexylthiophene) (P3HT) by applying thienylmethylamine acetate additive to enhance the α phase stability and passivate the bulk defects of CsPbI₂Br perovskite is successfully demonstrated, followed by implementing BTCIC-4Cl interlayer at CsPbI₂Br/P3HT interface, which can coordinate with both perovskite and P3HT to suppress the surface defects and promote the hole transport. Benefitting from these, a champion power conversion efficiency (PCE) of 16.3% is achieved, and the unencapsulated optimized device can retain 97% of the initial PCE after aging under N₂ atmosphere at 85 °C for 530 h. This work opens up a new era of multistrategy for improving performance and stability of CsPbI₂Br PSCs based on dopant-free hole transport layer.

A series of regioregular polymer acceptors with different molecular weights are developed and the relationship between batch factors and properties is studied without isomer interference. PA-5 has unique absorption and better crystallization, which produces state-of-the-art binary all-polymer solar cells with a high efficiency of 16.11%. PA-6-L/M presents negligible batch difference in performance (close to 15%), while PA-6-H displays inferior efficiency.

Abstract

Random conjugated polymers, such as typical polymerized small molecular acceptors (PSMAs), concurrently suffer from the dual batch factors of molecular weights (MWs) and regioregularity, which seriously interfere with the study of the relationship between batch factors and polymer properties. Here, four isomer-free PSMAs, PA-5 and three members of a PA-6 series with low (L), medium (M), and high (H) MWs, in which 5 and 6 define linkage position throughout conjugated backbone, are designed and synthesized to clearly investigate polymer batch effects. These studies reveal that PA-6-L and PA-6-M have ignorable batch differences within deviations, which deliver comparable maximum efficiencies of 14.81% and 14.99%, respectively. The PA-6-H based cell is processed from chlorobenzene with its high boiling point, due to the limited solubility in other common solvents, leading to large-size phase separation during prolonged film drying process, and thereby inferior performance. In contrast, PA-5 possesses diverse absorption characteristics, and ordered crystallization, which prompts higher short-circuit current density and fill factor in the cell. As a result, the corresponding device realizes a photovoltaic performance of 16.11%, which is one of the best binary all-polymer solar cells in the reported literature to date. This study provides a new insight into complicated batch effects of PSMAs on device performance while avoiding cross-talk between them.

A novel solution-printing strategy based on meniscus-assisted coating for all-polymer solar cells (all-PSCs) is presented. The champion device based on PM6:PY-IT shows a power conversion efficiency (PCE) of 15.53%, which is attributable to the enhanced crystallinity and nanofiber network morphology. It is worth mentioning that 15.53% is the highest PCE reported for solution-printing-based all-PSCs.

Abstract

Morphology control is the key to engineering highly efficient solution-processed solar cells. Focusing on the most promising application-oriented photovoltaic all-polymer solar cells (all-PSCs), herein a facile and effective meniscus-assisted-coating (MAC) strategy is reported for preparing high-quality blend films with enhanced crystallinity and an interpenetrating nanofiber network morphology. The all-PSCs based on MAC exhibit excellent optoelectronic properties with efficiencies exceeding 15%, which is the best performance of solution-printing-based all-PSCs, as well as better stability. The crystallization kinetics of the polymer blend film is investigated by in situ UV–vis absorption spectroscopy, and the result explains the linear relationship between the meniscus advance speed and the crystallinity (crystallization rate) of the polymer. To verify the compatibility and universality of this strategy, the MAC strategy is applied to the other three binary systems. By precisely controlling the meniscus advancing speed, 1 cm² all-PSC devices whose efficiencies exceed 12% are fabricated. Such progress demonstrates that the application of the MAC strategy is a promising approach for the fabrication of high-efficiency all-PSCs.

A functionalized polyurethane is used as an additive to self-heal a CsPbIBr₂ film by a disulfide-exchange reaction upon heat treatment. The inorganic CsPbIBr₂ perovskite solar cell achieves a champion efficiency of 10.61 % with efficiency recovery after heat treatment.

Abstract

One great challenge for perovskite solar cells (PSCs) lies in their poor operational stability under harsh stimuli by humidity, heat, light, etc. Herein, a thermal-triggered self-healing polyurethane (PU) is tailored to simultaneously improve the efficiency and stability of inorganic CsPbIBr₂ PSCs. The dynamic covalent disulfide bonds between adjacent molecule chains in PU at high temperatures self-heal the in-service formed defects within the CsPbIBr₂ perovskite film. Finally, the best device free of encapsulation achieves a champion efficiency up to 10.61 % and an excellent long-term stability in an air atmosphere over 80 days and persistent heat attack (85 °C) over 35 days. Moreover, the photovoltaic performances are recovered by a simple heat treatment.

The modulation effect of light in- and outcoupling on the photoluminescence from layered structures of widely employed organic photovoltaics materials, used for the assessment of the exciton diffusion length L _D, is investigated. The effect is shown to depend on the optical constants of the layers and the sample architecture. A possible correction method is introduced.

The correct determination of the exciton diffusion length (L _D) in novel organic photovoltaics (OPV) materials is an important, albeit challenging, task required to understand these systems. Herein, a high-throughput approach to probe L _D in nonfullerene acceptors (NFAs) is reported, that builds upon the conventional photoluminescence (PL) surface quenching method using NFA layers with a graded thickness variation in combination with spectroscopic PL mapping. The method is explored for two archetypal NFAs, namely, ITIC and IT-4F, using PEDOT:PSS and the donor polymer PM6 as two distinct and practically relevant quencher materials. Interestingly, conventional analysis of quenching efficiency as a function of acceptor layer thickness results in a threefold difference in L _D values depending on the specific quencher. This discrepancy can be reconciled by accounting for the differences in light in- and outcoupling efficiency for different multilayer architectures. In particular, it is shown that the analysis of glass/acceptor/PM6 structures results in a major overestimation of L _D, whereas glass/acceptor/PEDOT:PSS structures give no significant contribution to outcoupling, yielding L _D values of 6−12 and 8−18 nm for ITIC and IT-4F, respectively. Hence, practical guidelines for quencher choice, sample geometries, and analysis approach for the accurate assessment of L _D are provided.

Ionic motion is known to impact perovskite solar cells' performance and leads to the appearance of hysteresis in the current–voltage characteristics. However, there is no simple method to quantify their impact on the stabilized performance. Herein, a new method is presented to measure the ion-free efficiency and determine the loss due to the ions.

Perovskite semiconductors differ from most inorganic and organic semiconductors due to the presence of mobile ions in the material. Although the phenomenon is intensively investigated, important questions such as the exact impact of the mobile ions on the steady-state power conversion efficiency (PCE) and stability remain. Herein, a simple method is proposed to estimate the efficiency loss due to mobile ions via “fast-hysteresis” measurements by preventing the perturbation of mobile ions out of their equilibrium position at fast scan speeds ( ≈ 1000 V s⁻¹). The “ion-free” PCE is between 1% and 3% higher than the steady-state PCE, demonstrating the importance of ion-induced losses, even in cells with low levels of hysteresis at typical scan speeds ( ≈ 100 mV s⁻¹). The hysteresis over many orders of magnitude in scan speed provides important information on the effective ion diffusion constant from the peak hysteresis position. The fast-hysteresis measurements are corroborated by transient charge extraction and capacitance measurements and numerical simulations, which confirm the experimental findings and provide important insights into the charge carrier dynamics. The proposed method to quantify PCE losses due to field screening induced by mobile ions clarifies several important experimental observations and opens up a large range of future experiments.

This review introduces various optical coupling structures for semitransparent organic and perovskite solar cells. Furthermore, different optimization strategies for materials of transparent electrodes (metal nanowires, carbon-based materials, and conductive polymers) are summarized in detail. This review article aims to give readers a better understanding of light management methods and transparent electrode optimization strategies for semitransparent organic and perovskite solar cells.

Comparing with inorganic solar cells, organic solar cells and perovskite solar cells have attracted considerable attention owing to their unique properties such as the color tunability of their photoactive layers, low cost, catering to solution processing, and flexibility, which have been considered as the most promising technologies in wearable energy resources and show huge potential in fabricating transparent devices. In recent years, numerous light coupling constructions and transparent electrode optimization strategies have been applied to further enhance device photoelectric performance. In this review, common strategies focusing on light modulation and for semitransparent organic solar cells and semitransparent perovskite solar cells, which are crucial to increase J _sc, achieving adjustable chromaticity coordinates, high average visible transmittance and color rendering index have been summarized in detail.

Herein, one dimensional (1D) trimethylsulfonium lead triiodide (Me₃SPbI₃) nanoarrays are synthesized in aqueous condition with excellent water resistivity and environmental stability. Moreover, an efficient and stable 1D/3D device with power conversion efficiency (PCE) of 22.06% is demonstrated, which also maintains 97% of its initial value after 1000 h storage under ambient condition (RH ≈50%) without encapsulation.

Heterojunctions constructed upon multidimensional perovskites (1D/3D or 2D/3D) has emerged as an effective approach to improve the photovoltaic performance and stability of perovskite solar cells (PSCs). Herein, 1D trimethyl sulfonium lead triiodide (Me₃SPbI₃) 1D Me₃SPbI₃ nanoarrays are successfully synthesized via a two-step method in aqueous condition, which reflects excellent water resistivity and environmental stability. By incorporating this 1D Me₃SPbI₃ into lead halide 3D perovskites, heterostructural 1D/3D perovskite photoactive layer with improved morphology, crystallinity, enhanced photoluminescence lifetime, and reduced carrier recombination in comparison to its 3D counterpart is obtained. Moreover, an efficient and stable 1D/3D PSCs with power conversion efficiency (PCE) of 22.06% by using this 1D/3D perovskite are demonstrated. It noticeably maintained 97% of their initial efficiency after 1000 h storage under ambient condition (RH≈50%) without encapsulation. Our study opens up the design protocol for the development of next-generation highly efficient and stable perovskite solar cells.

Herein, a self-formed multifunctional grain boundary (GB) passivation strategy is reported, where an ultrathin GB passivation layer is in situ constructed via incorporating K₂SO₄ into perovskite precursor solution. The efficiency is increased from 20.4% to 22.4% after K₂SO₄ modification along with improved stability.

The deep-level defects at grain boundary (GB) result in serious trap-assisted non-radiative recombination. Moreover, the degradation of perovskite films is preferentially triggered by the attack of GBs by water and/or oxygen. Therefore, it is urgently needed to develop a multifunctional GB tailoring strategy to address the abovementioned issues. Herein, a self-formed multifunctional GB passivation strategy is reported, where an ultrathin GB passivation layer is in situ constructed via incorporating K₂SO₄ into perovskite precursor solution. The self-formed GB passivation layer plays multiple functions, including crystallization improvement, defect passivation, and moisture resistance. The GB manipulation strategy endows perovskite films reduced defect density, boosted carrier lifetime, and thus suppressed non-radiative recombination, which contributes to efficiency enhancement from 20.39% to 22.40%. The GB tailoring approach makes the unencapsulated target device exhibit no degradation while the control device degrades to 93% of its initial power conversion efficiency after 1200 h ambient exposure with a relative humidity of 10–20%. The modified device maintains 98% of its original efficiency after aging at 60 °C for 1200 h, whereas only 89% for the control device. Herein, the importance of developing an in situ GB modification strategy in enhancing performance of perovskite photovoltaics is highlighted.

Heating colloidal tin oxide solutions changes the particle size distribution from bimodal to unimodal. This mitigates agglomerates, resulting in uniform, compact, and gap-free SnO₂ electron transport layers. Processing perovskite layers on top results in films with reduced grain boundaries and fewer defects, along with improved perovskite/SnO₂ interfaces. This facile process results in significantly enhanced perovskite solar cell performance.

Tin dioxide is a frequently reported electron transporting material for perovskite solar cells (PSCs) that yields high-performance devices and can be solution processed from aqueous colloidal solutions. While being very simple to process, electron transport layers deposited in this manner often lead to nonuniform film morphology, significantly affecting the morphology of the subsequent perovskite layer, lowering the overall device performance. Herein, it is shown that heating the SnO₂ colloidal solution (70 °C) results in compact SnO₂ films with increased surface coverage and fewer gaps in the SnO₂ film. Such films possess threefold higher lateral electrical conductivity than those obtained from room-temperature solutions. The narrow gaps in the SnO₂ film also reduce the chances of direct contact between the indium tin oxide electrode and the perovskite layer, yielding better contact with less voltage loss. The improved SnO₂ surface coverage induces larger perovskite grains (≈565 nm) than those prepared from the room-temperature solution (≈273 nm). Finally, using these compact SnO₂ layers, efficient and stable PSCs that retain ≈85% of the initial power conversion efficiency of 20.67% after 100 h of maximum power point tracking are demonstrated.

Novel Ullazine derivatives bearing thiophene units along with various functional group are reported, and their use as hole-transporting materials (HTMs) in perovskite solar cells (PSCs) is investigated. The high potential of Ullazine-based HTMs for the fabrication of efficient and stable PSCs is substantiated for the first time.

Organic hole-transporting materials (HTMs) based on the Ullazine core yield so far only moderate power conversion efficiencies of up to 13.08% in perovskite solar cells (PSCs). Aiming to fabricate efficient and stable PSCs, novel Ullazine derivatives bearing thiophene units were designed and synthesized, allowing modulation of the electronic states of the HTMs and further providing defect passivation of the perovskite surface. Experimental and theoretical analysis show that thiophene units with -N(p-MeOC₆H₄)₂ groups improve the conductivity of Ullazine HTMs, boosting the efficiency of PSCs to 20.21%. This value is the highest reported to date for Ullazine-based HTMs, and is close to the performance of Spiro-OMeTAD. In addition, unencapsulated PSCs based on the champion Ullazine exhibit superior stability with respect to Spiro-OMeTAD, retaining nearly 90% of the initial efficiency following 1000 h aging, which is ascribed to a combination of higher water repellency and passivation of defects on the perovskite surface. This work demonstrates the high potential of HTMs based on Ullazine core as substitutes to Spiro-OMeTAD.

The results of the modeling study are summarized in the figure. The figure shows the performance parameters of the solar cell device configurations containing a single perovskite CsSn_0.5Ge_0.5I₃, a double perovskite Cs₄CuSb₂Cl₁₂, and a ternary perovskite Cs₃Bi₂I₉ as light-capturing layers, respectively, after the optimization of various input parameters of the absorber layer and the device operating conditions.

A comprehensive device performance optimization of perovskite solar cells (PSCs) having the device configurations FTO/IGZO/CsSn_0.5Ge_0.5I₃/CuO/Au, FTO/IGZO/Cs₄CuSb₂Cl₁₂/CuO/Au, and FTO/IGZO/Cs₃Bi₂I₉/CuO/Au containing the single, double, and ternary perovskites CsSn_0.5Ge_0.5I₃, Cs₄CuSb₂Cl₁₂, and Cs₃Bi₂I₉, respectively, is done utilizing the SCAPS 1D tool. The bandgap tuning of the perovskite layers and the work function optimization of the rear-contact metals for the chosen device designs provide an enhanced power conversion efficiency of 23.15%, 17.39%, and 9.75% for the solar cell architecture with the light-captivating layers CsSn_0.5Ge_0.5I₃, Cs₄CuSb₂Cl₁₂, and Cs₃Bi₂I₉, respectively. Herein, it is identified the variation of electron affinity of the perovskite layer for the aforementioned device configurations does not produce any significant enhancement in the output parameters of the simulated configurations.

Herein, vacuum-deposited p–i–n perovskite solar cells employing very thin films of intrinsic organic semiconductors as the hole transport layers is studied. When no dopants nor high work function interlayers are used, the devices show a dynamic electrical behavior before reaching efficient steady state operation. Devices with small molecules are not thermally stable, as opposite to polymer transport layers in the same configuration.

Thin polymeric and small-molecular-weight organic semiconductors are widely employed as hole transport layers (HTLs) in perovskite solar cells. To ensure ohmic contact with the electrodes, the use of doping or additional high work function (WF) interlayer is common. In some cases, however, intrinsic organic semiconductors can be used without any additive or buffer layers, although their thickness must be tuned to ensure selective and ohmic hole transport. Herein, the characteristics of thin HTLs in vacuum-deposited perovskite solar cells are studied, and it is found that only very thin (<5 nm) HTLs readily result in high-performing devices, as the HTL acts as a WF enhancer while still ensuring selective hole transfer, as suggested by ultraviolet photoemission spectroscopy and Kelvin probe measurements. For thicker films (≥5 nm), a dynamic behavior for consecutive electrical measurements is observed, a phenomenon which is also common to other widely used HTLs. Finally, it is found that despite their glass transition temperature, small-molecule HTLs lead to thermally unstable solar cells, as opposed to polymeric materials. The origin of the degradation is still not clear, but might be related to chemical reactions/diffusion at the HTL/perovskite interface, in detriment of the device stability.

Herein, two spirobifluorene-based dyes are deposited from aqueous solutions as efficient anode interlayers (AILs) as a step towards all-solution-processed environmentally friendly organic solar cells. The nonacidic nature of these water-soluble small molecules solves the electrode corrosion issue associated with the commonly employed poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) interlayers while producing higher efficiencies than the photovoltaic devices prepared with PEDOT:PSS AILs.

Solution-processed inverted organic solar cells (OSCs) generally use poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate (PEDOT:PSS) as hole selective anode interlayer (AIL). However, the acidic nature of PEDOT:PSS considerably accelerates the degradation dynamics of OSCs, which shortens the durability of these low-cost photovoltaic devices. Small organic molecules are attracting growing interest as alternative AIL materials, but their solubility limited to toxic organic solvents hinders the production of environmentally friendly OSCs. Herein, the first inverted OSCs employing non-PEDOT:PSS solution-processed top small organic molecule AILs deposited from aqueous solution are reported. The investigated water-soluble spirobifluorene (SBF) derivatives 1 and 2 show hole mobility (≈4 × 10⁻³ cm² V⁻¹ S⁻¹) higher than PEDOT:PSS. Because of their nonacidic nature, the interlayers formed with derivatives 1 or 2 considerably delay the degradation of the top metal electrode compared to OSCs employing PEDOT:PSS interlayers. The PEDOT:PSS-free OSC devices with inverted configuration with the water-soluble SBF derivatives as AIL produce power conversion efficiencies above 5% with PTB7-Th:ITIC active layers and above 8% with PBDB-T-2Cl:Y6 active layers, respectively, with an enhancement up to 28% compared to OSCs employing PEDOT:PSS. These results correspond to the highest reported values for PEDOT:PSS-free small-molecule inverted OSCs deposited from an aqueous solution.

Publication date: March 2022

Source: Nano Energy, Volume 93

Author(s): Zhen Li, Xin Wu, Shengfan Wu, Danpeng Gao, Hua Dong, Fuzhi Huang, Xiaotian Hu, Alex K.-Y. Jen, Zonglong Zhu

The authors demonstrate that designing functional fullerenes with roles beyond defect passivation is essential for perovskite solar cells (PSCs). By taking the advantages of fullerene, porphyrin, and pentafluorophenyl, a novel fullerene-porphyrin dyad is intentionally synthesized to stitch grain boundaries and trap lead ions in the perovskite film by forming chemical interactions in-situ. This robust strategy yields high-performance and eco-friendly PSCs.

Abstract

Designing functional fullerenes with roles beyond defect passivation and electron-transporting for perovskite solar cells (PSCs) is essential to the development of fullerenes and PSCs. Here, the authors design and synthesize a functional fullerene, FPD, composed of a C₆₀ cage, a porphyrin ring, and three pentafluorophenyl groups. The structure features of FPD enable it can form chemical interactions with the perovskite lattices. These interactions enhance the defect passivation effect and prevent the decomposition of perovskite under irradiation. As a result, the FPD-based device yields an improved power conversion efficiency of 23% with substantially enhanced operational stability (T ₈₀ > 1500 h). Furthermore, once got damaged, the FPD can prevent lead leakage by forming a stable and water-insoluble complex (FPD-Pb). Their findings provide a novel strategy to achieve high-performance and eco-friendly PSCs with functional fullerene materials.

To reveal the charge-carrier dynamics in single-component organic solar cells based on a double-cable polymer, investigations across seven orders of magnitude in time scale are demonstrated. Thermal post-treatment demonstrates a positive effect on charge generation in parallel to suppressed recombination.

Abstract

Single-component organic solar cells (SCOSCs) have witnessed great improvement during the last few years with the champion efficiency jumping from the previous 2–3% to currently 6–11% for the representative material classes. However, the photophysics in many of these materials has not been sufficiently investigated, lacking essential information regarding charge-carrier dynamics as a function of microstructure, which is highly demanded for a better understanding and potential guidance for further improvements. In this work, for the first time, the charge-carrier dynamics of a representative double-cable polymer, which achieves efficiencies of over 6% as an active layer in SCOSCs, is investigated across seven orders of magnitude in time scale, from fs–ps charge generation to ns–µs charge recombination processes. Specific emphasis is placed on understanding the impact of thermal post-treatment on the charge dissociation and recombination dynamics. Annealing the photoactive layer at 230 °C results in the highest photovoltaic performance because of efficient charge generation in parallel to suppressed recombination. This work intends to present a complete picture of the charge-carrier dynamics in SCOSCs using the representative double-cable polymer PBDBPBICl.