以昇陳

In tandem organic photovoltaics, most ultraviolet–visible photons are absorbed by the front sub‐cell, so in the rear sub‐cell, excitons generated on large‐bandgap donors will be reduced significantly. This reduces the conductivity and limits the hole‐transporting property of the rear sub‐cell. An infrared‐absorbing polymer donor is introduced, which provides a second hole‐generation/transporting mechanism to minimize the aforementioned detrimental effects.

Abstract

In tandem organic photovoltaics, the front subcell is based on large‐bandgap materials, whereas the case of the rear subcell is more complicated. The rear subcell is generally composed of a narrow‐bandgap acceptor for infrared absorption but a large‐bandgap donor to realize a high open‐circuit voltage. Unfortunately, most of the ultraviolet–visible part of the photons are absorbed by the front subcell; as a result, in the rear subcell, the number of excitons generated on large‐bandgap donors will be reduced significantly. This reduces the (photo) conductivity and finally limits the hole‐transporting property of the rear subcell. In this work, a simple and effective way is proposed to resolve this critical issue. To ensure sufficient photogenerated holes in the rear subcell, a small amount of an infrared‐absorbing polymer donor as a third component is introduced, which provides a second hole‐generation and transporting mechanism to minimize the aforementioned detrimental effects. Finally, the short‐circuit current density of the two‐terminal tandem organic photovoltaic is significantly enhanced from 10.3 to 11.7 mA cm⁻² (while retaining the open‐circuit voltage and fill factor) to result in an enhanced power conversion efficiency of 15.1%.

Nature Energy, Published online: 05 October 2020; doi:10.1038/s41560-020-00705-5

A narrow‐bandgap polymer acceptor L14 with an acceptor–acceptor (A–A) backbone is synthesized, showing lower‐lying frontier molecular orbitals, higher electron mobility, and larger absorption coefficient without sacrificing photovoltage compared to its donor–acceptor (D–A) analog polymer, L11. When applied in all‐polymer solar cells, L14 yields an outstanding efficiency of 14.3%.

Abstract

Narrow‐bandgap polymer semiconductors are essential for advancing the development of organic solar cells. Here, a new narrow‐bandgap polymer acceptor L14, featuring an acceptor–acceptor (A–A) type backbone, is synthesized by copolymerizing a dibrominated fused‐ring electron acceptor (FREA) with distannylated bithiophene imide. Combining the advantages of both the FREA and the A–A polymer, L14 not only shows a narrow bandgap and high absorption coefficient, but also low‐lying frontier molecular orbital (FMO) levels. Such FMO levels yield improved electron transfer character, but unexpectedly, without sacrificing open‐circuit voltage (V _oc), which is attributed to a small nonradiative recombination loss (E _loss,nr) of 0.22 eV. Benefiting from the improved photocurrent along with the high fill factor and V _oc, an excellent efficiency of 14.3% is achieved, which is among the highest values for all‐polymer solar cells (all‐PSCs). The results demonstrate the superiority of narrow‐bandgap A–A type polymers for improving all‐PSC performance and pave a way toward developing high‐performance polymer acceptors for all‐PSCs.

High‐efficiency solution‐processed hybrid tandem photovoltaic devices, employing inorganic perovskite and organic bulk‐heterojunction as the photoactive layers, are demonstrated. A PCE of 18.04% in the hybrid tandem device is achieved, which is significantly higher than the comparable single‐junction devices, owing to a near‐optimal absorption spectral match.

Abstract

Although the power conversion efficiency (PCE) of inorganic perovskite‐based solar cells (PSCs) is considerably less than that of organic‐inorganic hybrid PSCs due to their wider bandgap, inorganic perovskites are great candidates for the front cell in tandem devices. Herein, the low‐temperature solution‐processed two‐terminal hybrid tandem solar cell devices based on spectrally matched inorganic perovskite and organic bulk heterojunction (BHJ) are demonstrated. By matching optical properties of front and back cells using CsPbI₂Br and PTB7‐Th:IEICO‐4F BHJ as the active materials, a remarkably enhanced stabilized PCE (18.04%) in the hybrid tandem device as compared to that of the single‐junction device (9.20% for CsPbI₂Br and 10.45% for PTB7‐Th:IEICO‐4F) is achieved. Notably, the PCE of the hybrid tandem device is thus far the highest PCE among the reported tandem devices based on perovskite and organic material. Moreover, the long‐term stability of inorganic perovskite devices under humid conditions is improved in the hybrid tandem device due to the hydrophobicity of the PTB7‐Th:IEICO‐4F back cell. In addition, the potential promise of this type of hybrid tandem device is calculated, where a PCE of as much as ≈28% is possible by improving the external quantum efficiency and reducing energy loss in the sub‐cells.

A novel BDT‐based random polymeric hole‐transporting layer (asy‐ranPBTBDT) is developed with irregularity from asymmetric substitution and random copolymerization. The resulting low crystallinity from the irregularity leads to superior solubility capacity and suppressed charge recombination and morphological changes. Therefore, the colloidal quantum dot solar cells using asy‐ranPBTBDT‐based device show highly efficient power conversion efficiency of 13.2% with superior operational stability.

Abstract

Next‐generation solution‐processed solar cells will hopefully be processed using green solvents, and will unite high performance with operating stability. Colloidal quantum dot/polymer hybrid solar cells are of interest for their harvest of the visible as well as the near infrared; however, today's best polymer hole‐transporting layers (HTLs) rely on processing using hazardous solvents such as chlorobenzene. This stems from the strong polymer–polymer attraction in polymeric p‐type materials, which accounts for their limited solubility. Here, a new random polymeric HTL (asy‐ranPBTBDT) is reported that is soluble in green solvents such as 2‐methylanisole without compromising ultimate device power conversion efficiency. The new polymer structure induces a strong π–π stacking face‐on orientation and less lateral grain growth compared to control asy‐PBTBDT, leading to reduced charge recombination and improved device stability. The resulting device exhibits a power conversion efficiency (PCE) of 13.2% and retains 89% of its initial efficiency after 120 h of continuous device operation at the maximum power point, compared to a PCE of 11.4% and 71% degradation for control devices.

To generate cost‐efficient and high‐performanced polymers, a simple chemical steric effect (SE) is introduced to benzothiophene (BDT)‐based side chains. The polymeric crystallinity and miscibility are rebalanced and a power conversion efficiency (PCE) of 14.53% is achieved. Thus, the SE applied in crystalline polymer pave an easier and cheaper route to realize the coordination of low‐cost fabrication and high‐performance.

Abstract

Low synthetic cost and high performance are becoming a new challenge in designing polymer donors for large‐scaled polymer solar cells (PSCs) fabrication; however, complicated synthetic routes and high material costs hamper the widespread commercial application of OPVs. Here, a simple and low‐cost chemical steric effect (SE) is introduced to BDT‐based side chains. Through adjusting alkyl side chains, the polymeric crystallinity and miscibility are rebalanced and subsequently the photovoltaic device based on the meta‐positioned alkyl polymer outperforms its para‐positioned counterpart. The champion device based on the polymer with the meta‐positioned side chains affords a PCE of 14.53% without sacrificing its high fill factor of 0.77, which could be attributed to a more balanced charge‐carrier transport ability and optimized morphology. This is the highest PCE value reported in BTZ based polymer donors to date. Thus, it shows that the SE applied in high crystalline polymer could pave an easier and cheaper chemical route to realize the coordination of low‐cost fabrication and high‐performance.

Systematic photoelectron spectroscopy measurements are performed on perovksite solar cells in classical and inverted architecture in the dark and under illumination. The photovoltage is identified at the n‐MAPI (methylammonium‐lead‐iodide)/p‐HEL (hole extraction layer) interfaces for both architectures. From this, energy band diagrams for the full devices are derived for the dark and the illuminated case.

Abstract

High power conversion efficiency (PCE) perovskite solar cells (PSCs) rely on optimal alignment of the energy bands between the perovskite absorber and the adjacent charge extraction layers. However, since most of the materials and devices of high performance are prepared by solution‐based techniques, a deposition of films with thicknesses of a few nanometers and therefore a detailed analysis of surface and interface properties remains difficult. To identify the respective photoactive interfaces, photoelectron spectroscopy measurements are performed on device stacks of methylammonium‐lead‐iodide (MAPI)‐based PSCs in classical and inverted architectures in the dark and under illumination at open‐circuit conditions. The analysis shows that vacuum‐deposited MAPI perovskite absorber layers are n‐type, independent of the architecture and of the charge transport layer that it is deposited on (n‐type SnO₂ or p‐type NiO _x ). It is found that the majority of the photovoltage is formed at the n‐MAPI/p‐HEL (hole extraction layer) junction for both architectures, highlighting the importance of this interface for further improvement of the photovoltage and therefore also the PCE. Finally, an experimentally derived band diagram of the completed devices for the dark and the illuminated case is presented.

A proof‐of‐concept of high‐performance wearable thermocell is made through systematic investigations into wearable thermocells with regard to the development of n‐type gel electrolytes, the integration of gel electrolytes into 3D porous electrode, and advanced device design. The demonstration of wearable thermocells harvesting body heat, charging supercapacitors and illuminating LED lights, shows the potential for commercial applications.

Abstract

Thermoelectrochemical cells (thermocells) designed for harvesting human body heat can provide constant power output for wearable electronics, supplementing state‐of‐the‐art flexible power storage and conversion solutions. However, a systematic investigation into the optimization of wearable thermocells is lacking, especially with regard to device design, n‐type electrolytes, and electrode/electrolyte integration. Here, a n‐type gel electrolyte: polyvinyl alcohol‐FeCl_2/3 with outstanding flexibility and elasticity and exceptional electrolyte/electrode integration into a 3D porous poly(3,4‐ethylenedioxythiophene)/polystyrenesulfonate (PEDOT/PSS) electrode, is produced via an in situ chemical crosslinking method. The integrated n‐type cell shows excellent seebeck coefficients (0.85 mV K⁻¹) and output current density (1.74 A m⁻² K⁻¹) that are comparable with an optimized p‐type cell consisting of a carboxymethylcellulose‐K_3/4Fe(CN)₆ electrolyte with a 3D PEDOT/PSS‐edge functionalized graphene/carbon nanotube electrode (−1.22 mV K⁻¹ and 1.85 A m⁻² K⁻¹). The equivalent performance of the n‐type and p‐type cells enables the effective series connection of up to 18 pairs of p–n cells that combines to give an output voltage of 0.34 V (∆T = 10 K). This in‐series device is fabricated into a proof‐of‐concept watch strap, which can harvest body heat, charge supercapacitor (up to 470 mF) as well as illuminate a green light emitting diode, demonstrating the practical applications.

Organic photovoltaics (OPVs) demostrate certified cell efficiencies of over 17% and are expected to contribute to versatile applications powered by solar energy. By taking into consideration different critical and “soft” key performance indicators, this work demonstrates material strategies to accelerate the development of OPV technology toward a GW era.

Abstract

With the rise of the solar power century, photovoltaic applications and installations will go beyond the traditional green field power plants and enter any aspect of daily life. Organic photovoltaics (OPVs) demonstrate certified cell efficiencies of over 17% and are expected to contribute to versatile applications powered by solar energy, for instance, applications rely on flexibility, transparency, color management, or integrability. In this work, the progress of OPV technology is briefly reviewed and the material strategies to accelerate OPV technology toward a GW era are analyzed. In addition to the exciting efficiency values achieved for small area devices, there are many important criteria deciding the success of OPV technology. By taking into consideration the synthetic complexity of OPV materials and the operational stability of OPV devices, the industrial figure of merit (i‐FoM) is proposed as a fast and reliable method to verify the true potential of a novel material. Furthermore, “soft” key performance indicators are introduced, such as toxicity, flexibility, transparency, processing, which require different development strategies to reflect the potential of OPV technology for specific applications.

Three dithienylphenylene‐bridged diketopyrrolopyrrole dimers, linked in ortho (o‐DPP), meta (m‐DPP), and para (p‐DPP) positions, are characterized in the context of intramolecular singlet fission (i‐SF). In the cases of o‐DPP and m‐DPP, SF is mediated by a charge transfer state, while population of a symmetry‐breaking charge‐separated state dominates p‐DPP and, in turn, hampers SF.

Abstract

Three diketopyrrolopyrrole (DPP) dimers, linked via different dithienylphenylene spacers, ortho‐DPP (o‐DPP), meta‐DPP (m‐DPP), and para‐DPP (p‐DPP), are synthesized, characterized, and probed in light of intramolecular singlet fission (i‐SF). Importantly, the corresponding DPP reference (DPP‐Ref) singlet and triplet excited state energies of 2.22 and 1.04 eV, respectively, suggest that i‐SF is thermodynamically feasible. The investigations focus on the impact of the relative positioning of the DPPs, and give compelling evidence that solvent polarity and/or spatial overlap govern i‐SF dynamics and efficiencies. Polar solvents make the involvement of an intermediate charge transfer (CT) state possible, followed by the population of ¹(T₁T₁) and subsequently (T₁ + T₁), while spatial overlap drives the mutual interactions between the DPPs. In o‐DPP, the correct balance between polar solvents and spatial overlap leads to the highest triplet quantum yield (TQY) of 40%. Notable is the superimposition of CT and triplet excited states, preventing an accurate TQY determination. For m‐DPP, poorer spatial overlap correlates with weaker CT character and manifests in a TQY of 11%. Strong CT character acts as a trap and prevents i‐SF, as found with p‐DPP. The DPP separation is decisive, enabling a symmetry‐breaking charge‐separated state rather than CT formation, shutting down the formation ¹(T₁T₁).

Charge transfer state parameterization based upon external quantum efficiency and electroluminescent is influenced by cavity effects yielding arbitrary results. This is shown to be due to the thickness dependent cavity in(out)put coupling. A practical approach is presented to overcome this issue. The effect is demonstrated in several organic solar cells including non‐fullerene‐based devices.

Abstract

Free carrier photogeneration in bulk‐heterojunction solar cells composed of blends of acceptor and donor organic semiconductors proceeds via intermolecular charge transfer (CT) states. Non‐adiabatic Marcus theory has proven valid to explain the absorption and emission of these sub‐gap states which have extremely weak emission probabilities and absorption cross sections making them difficult to probe directly using optical spectroscopy. Therefore, the CT state parameters involved in the Marcus model are often extracted from fittings on the photovoltaic external quantum efficiency (EQE _PV) and electroluminescence. These two spectra are (ideally) interrelated via the so‐called reciprocity principle. In this paper, the limitations of such an approach are demonstrated, in particular the impact of simple low finesse cavity interference effects acting as an uneven spectral filter for emission and absorption. This can produce almost spurious CT state parameterization with, for example, relative errors as large as 90% in absorption coefficients obtained from EQE _PV. It is shown how these limitations can be partially lifted using an iterative transfer matrix approach applied to the EQE _PV.

Nature Photonics, Published online: 23 September 2020; doi:10.1038/s41566-020-0696-8