Yingzhi Jin

In article number 2003771, Rujun Ma and co‐workers report a highly efficient double‐unit solid‐state refrigeration device based on two‐layer modified electrocaloric polymer stacks actuated by electrostatic forces. The availability and the high‐efficiency of the device for cooling the central processing unit provides a unique option for thermal management of microcircuits in the future.

In article number 2003576, Kung‐Hwa Wei and co‐workers demonstrate that semi‐transparent organic photovoltaics with a sequential deposited (SD) active layer—individually deposited polymer donor layer and a small‐molecule acceptor layer—forming pseudo p–i–n structures have larger power conversion efficiency and transmittance values than those for the devices with bulk heterojunction (BHJ) structures, and the enhancements for SD versus BHJ devices increase with the decreasing active layer thickness.

The significant advances in efficient photoabsorbent materials have been instrumental in the performance enhancement of semitransparent organic solar cells (ST‐OSCs) from <7% to 12–14% (with good visible transmittance) only in the last 3 years. This study reviews the progress of photoabsorbent materials for ST‐OSCs, and discusses the structure–property relationships and future perspectives for the development of multifunctional ST‐OSCs.

Abstract

Semi‐transparent organic solar cells (ST‐OSCs) have revolutionized the field of photovoltaics (PVs) due to their unique abilities, such as transparency and color tunability, and have transformed normal power‐harvesting OSC devices into multifunctional devices, such as building‐integrated photovoltaics, agrivoltaics, floating photovoltaics, and wearable electronics. Very recently, ST‐OSCs have seen remarkable progress, with a rapid increase in power conversion efficiency from below 7% to 12–14%, with an average visible transparency of 9–25%, especially due to the use of low bandgap semiconductors including polymer donors and non‐fullerene acceptors that exhibit absorption in the near‐infrared region as photoabsorbent materials. From this perspective, the latest developments in ST‐OSCs stemming from the innovations in photovoltaic materials that delivered multifunctional ST‐OSCs with top‐of‐the‐line power conversion efficiencies are discussed to shed light on the structure‐property relationship between molecular design and current challenges in this cutting‐edge research field. Finally, personal perspectives, including research directions for the future use of ST‐OSCs in multifunctional applications, are also proposed.

In poly(3,4‐ethylenedioxythiophene) (PEDOT) thin films with a highly face‐on orientation, the charge transport between chains within a crystallite becomes a rate‐limiting factor, which is highly sensitive to the π–π stacking distance. Engineering the π–π stacking distance in PEDOT films grown by water‐assisted oxidative chemical vapor deposition (oCVD) yields a record high electrical conductivity of 7520 ± 240 S cm⁻¹.

Abstract

Engineering the texture and nanostructure to improve the electrical conductivity of semicrystalline conjugated polymers must address the rate‐limiting step for charge carrier transport. In highly face‐on orientation, the charge transport between chains within a crystallite becomes rate‐limiting, which is highly sensitive to the π–π stacking distance and interchain charge transfer integral. Here, face‐on oriented semicrystalline poly(3,4‐ethylenedioxythiophene) (PEDOT) thin films are grown via water‐assisted (W‐A) oxidative chemical vapor deposition (oCVD). Combining W‐A with the volatile oxidant, antimony pentachloride, yields an optimized electrical conductivity of 7520 ± 240 S cm⁻¹, a record for PEDOT thin films. Systematic control of π–π stacking distance from 3.50 Å down to 3.43 Å yields an electrical conductivity enhancement of ≈1140%. The highest electrical conductivity also corresponds to minimum in Urbach energy of 205 meV, indicating superior morphological order. The figure of merit for transparent conductors, σ_dc/σ_op, reaches a maximum value of 94, which is 1.9× and 6.7× higher than oCVD PEDOT grown without W‐A and utilizing vanadium oxytrichloride and iron chloride oxidizing agents, respectively. The W‐A oCVD is single‐step all‐dry process and provides conformal coverage, allowing direct growth on mechanical flexible, rough, and structured surfaces without the need for complex and costly transfer steps.

In this study, the importance of terminal group match in the design of polymer donor and small-molecule acceptor for optimal blend morphology, reduced voltage loss, and high device performances are demonstrated.

Abstract

As a variety of non-fullerene small molecule acceptors (SMAs) have been developed to improve power conversion efficiency (PCE) of organic solar cells (OSCs), the pairing of the SMAs with optimal polymer donors (P _Ds) is an important issue. Herein, a systematic investigation is conducted with the development of the SMA series, named C6OB-H, C6OB-Me, and C6OB-F, which contain distinctive terminal substituents –H, –CH₃, and –F, respectively. These SMAs are paired with two P _Ds, PBDT-H and PBDT-F. Interestingly, the P _D/SMA pairs with similar terminal groups yield enhanced molecular compatibility and energetic interactions, which suppress voltage loss while improving blend morphology to enhance simultaneously the open–circuit voltage, short–circuit current, and fill factor of the OSCs. In particular, the OSC based on the PBDT-F:C6OB-F blend sharing fluorine terminal groups achieves the highest PCE of 15.2%, which outperforms those of PBDT-H:C6OB-F (10.1%) and PBDB-F:C6OB-H OSCs (11.2%). Furthermore, the PBDT-F:C6OB-F OSC maintains high PCEs with active layer thicknesses between 85 and 310 nm. In contrast, the PCE of PBDT-H:C6OB-F-based OSC already drops by 80% from 10.1% to 2.1% when the active layer thickness increases from 100 to 200 nm. This study establishes an important P _D/SMA pairing rule in terms of terminal functional groups for achieving high-performance OSC.

A new series of conjugated molecules with spatially 2D sidechains are designed and utilized as the non-fullerene acceptors in all-small-molecule organic solar cells. The multi-dimensional lamellar packing induced by the orthogonal sidechains is able to tune the morphology as effective as the stacking of conjugated backbones, thus providing an impressive power conversion efficiency of 15.67%.

Abstract

Organic semiconductors consist of a conjugated backbone and flexible sidechains. Compared to the meticulous design of backbones, less attention has been paid to the investigation of sidechains, in particular their spatial orientation. Herein, three non-fullerene acceptors, anti-PDFC, syn-PDFC, and PDFC-Ph, are applied in all-small-molecule organic solar cells (ASM-OSCs) to reveal the varied effects of sidechains on morphology and device performance. With spatially orthogonal alkyl chains, anti-PDFC and syn-PDFC show unique bimodal lamellar packing and moderate crystallinity. When blending with an efficient binary BTR-Cl/Y6 system, anti-PDFC as well as syn-PDFC not only form their own crystal phase but also improve the packing order of BTR-Cl, consequently enhancing the power conversion efficiency (PCE) of ternary ASM-OSC to be 14.56%. However, although PDFC-Ph has an identical backbone with anti-PDFC, the alternated sidechains make it relatively amorphous, which is prone to damage the original packing of the host donor/acceptor, and thus deteriorating the device performance. When PC₇₁BM is added to optimize the morphology further, the triple-acceptor device involving anti-PDFC realizes a PCE of 15.67%, which is among the best efficiencies in ASM-OSCs. This study demonstrates that a multi-dimensional sidechain can optimize the morphology of a bulk heterojunction as effective as a conjugated backbone.

Reducing and mitigating non‐radiative recombination defects in perovskite materials are still crucial prerequisites for achieving high performance in light‐emitting applications. Ethoxylated trimethylolpropane triacrylate is introduced in antisolvent to passivate surface and bulk defects during the spinning process, and external quantum efficiency of quasi‐2D perovskite light‐emitting diodes as high as 22.49% is demonstrated.

Abstract

Perovskite light‐emitting diodes (PeLEDs) are considered as particularly attractive candidates for high‐quality lighting and displays, due to possessing the features of wide gamut and real color expression. However, most PeLEDs are made from polycrystalline perovskite films that contain a high concentration of defects, including point and extended imperfections. Reducing and mitigating non‐radiative recombination defects in perovskite materials are still crucial prerequisites for achieving high performance in light‐emitting applications. Here, ethoxylated trimethylolpropane triacrylate (ETPTA) is introduced as a functional additive dissolved in antisolvent to passivate surface and bulk defects during the spinning process. The ETPTA can effectively decrease the charge trapping states by passivation and/or suppression of defects. Eventually, the perovskite films that are sufficiently passivated by ETPTA make the devices achieve a maximum external quantum efficiency (EQE) of 22.49%. To our knowledge, these are the most efficient green PeLEDs up to now. In addition, a threefold increase in the T ₅₀ operational time of the devices was observed, compared to control samples. These findings provide a simple and effective strategy to make highly efficient perovskite polycrystalline films and their optoelectronics devices.