Chen Weijie

An HBr-assisted slow cooling method is developed for the growth of centimeter-sized Cs₄PbBr₆ crystals. The obtained crystals show strong green photoluminescence with absolute photoluminescence quantum yields up to 97%. More importantly, the evolution process and structural characterizations support that the nonstoichiometry of initial Cs₄PbBr₆ crystals induce the formation of nanosized CsPbBr₃ nanocrystals in crystalline Cs₄PbBr₆ matrices. Furthermore, high efficiency and wide color gamut prototype white light-emitting diode devices are also demonstrated by combining the highly luminescent Cs₄PbBr₆ crystals as green emitters and commercial K₂SiF₆:Mn⁴⁺ phosphor as red emitters with blue emitting GaN chips. The optimized devices generate high-quality white light with luminous efficiency of ≈151 lm W⁻¹ and color gamut of 90.6% Rec. 2020 at 20 mA, which is much better than that based on conventional perovskite nanocrystals. The combination of improved efficiency and better stability with comparable color quality provides an alternative choice for liquid crystal display backlights.

Centimeter-sized Cs₄PbBr₆ crystals with embedded CsPbBr₃ NCs are fabricated through an HBr-assisted slow cooling technique, achieving a photoluminescence quantum yield about 97%. The formation can be explained by the nonstoichiometry induced partial transformation from defective Cs₄PbBr₆ crystals. Furthermore, a luminous efficiency up to 151 lm W⁻¹ with a color gamut of 90.6% of Rec. 2020 is achieved, showing the bright future for display backlights.

Two new wide-bandgap D–A–π copolymer donor materials, PBDT-2TC and PBDT-S-2TC, based on benzodithiophene and asymmetric bithiophene with one carboxylate (2TC) substituent are synthesized by a facile approach for fullerene-free organic solar cells (OSCs). The combination of one carboxylate-substituted thiophene with one thiophene bridge in the backbone substantially reduces the steric hindrance, thereby favoring a planar geometry for efficient charge transport and molecular packing. A reasonable highest-occupied-molecular-orbital energy level in relation to that of the acceptor and balanced hole and electron transport are observed for both polymers. This asymmetric structure unit is flexible and versatile, allowing the absorption, energy levels, and morphology of the blend films to be tailored. Fullerene-free OSCs based on PBDT-S-2TC:ITIC achieve a high power conversion efficiency of 10.12%. More impressively, a successful nonhalogen solvent-processed solar cell with 9.55% efficiency is also achieved, which is one of the highest values for a fullerene-free OSC processed using an ecofriendly solvent.

New wide-bandgap D–A–π copolymers based on an asymmetric bithiophene with one carboxylate substituent were synthesized. The asymmetric structure unit is flexible and versatile, which allows the absorption, energy levels and morphology of the blend films to be adjusted easily. D-A-p copolymers produced a high power conversion efficiency of 10.0% for halogen solvent-processed OSCs and 9.55% for non-halogen solvent-processed devices.

Kesterite is an attractive material for absorber layers in thin film photovoltaics. Solar cells based on kesterite have shown a substantial progress over the last decade; nevertheless, further improvements in device efficiency are pending due to the open-circuit voltage (V_oc) deficit (i.e., difference between the maximum V _oc that can be achieved according to Shockley–Queisser limit and actual V _oc from the device). In this study, the optoelectronic properties of the author's internal record Cu₂ZnSnSe₄ solar cell, which shows a power conversion efficiency of 11.4%, are presented. The device measurements reveal a V_oc deficit of 337 mV, which is one of the lowest V _oc deficits in the literature. Moreover, an unusual behavior for kesterite is observed: (i) photon energy of the photoluminescence emission and (ii) the extrapolated V _oc for 0 K are both matching the band gap region of the absorber. These results indicate a significant improvement in the recombination characteristics and absorber quality in comparison to other kesterite devices in literature.

Optoelectronic properties of an 11.4% Cu₂ZnSnSe₄ solar cell are presented. The device shows an unusual behavior for kesterite: (i) photon energy of the photoluminescence emission and (ii) the extrapolated V_oc for 0 K are both matching the band gap region of the absorber. These properties lead to one of the lowest V_oc deficits for kesterite in the literature.

In this report, highly efficient and humidity-resistant perovskite solar cells (PSCs) using two new small molecule hole transporting materials (HTM) made from a cost-effective precursor anthanthrone (ANT) dye, namely, 4,10-bis(1,2-dihydroacenaphthylen-5-yl)-6,12-bis(octyloxy)-6,12-dihydronaphtho[7,8,1,2,3-nopqr]tetraphene (ACE-ANT-ACE) and 4,4′-(6,12-bis(octyloxy)-6,12-dihydronaphtho[7,8,1,2,3-nopqr]tetraphene-4,10-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (TPA-ANT-TPA) are presented. The newly developed HTMs are systematically compared with the conventional 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamino)-9,9′-spirbiuorene (Spiro-OMeTAD). ACE-ANT-ACE and TPA-ANT-TPA are used as a dopant-free HTM in mesoscopic TiO₂/CH₃NH₃PbI₃/HTM solid-state PSCs, and the performance as well as stability are compared with Spiro-OMeTAD-based PSCs. After extensive optimization of the metal oxide scaffold and device processing conditions, dopant-free novel TPA-ANT-TPA HTM-based PSC devices achieve a maximum power conversion efficiency (PCE) of 17.5% with negligible hysteresis. An impressive current of 21 mA cm⁻² is also confirmed from photocurrent density with a higher fill factor of 0.79. The obtained PCE of 17.5% utilizing TPA-ANT-TPA is higher performance than the devices prepared using doped Spiro-OMeTAD (16.8%) as hole transport layer at 1 sun condition. It is found that doping of LiTFSI salt increases hygroscopic characteristics in Spiro-OMeTAD; this leads to the fast degradation of solar cells. While, solar cells prepared using undoped TPA-ANT-TPA show dewetting and improved stability. Additionally, the new HTMs form a fully homogeneous and completely covering thin film on the surface of the active light absorbing perovskite layers that acts as a protective coating for underlying perovskite films. This breakthrough paves the way for development of new inexpensive, more stable, and highly efficient ANT core based lower cost HTMs for cost-effective, conventional, and printable PSCs.

First time low-cost anthanthrone dye based hole transporting materials (HTMs) 4,10-bis(1,2-dihydroacenaphthylen-5-yl)-6,12-bis(octyloxy)-6,12-dihydronaphtho[7,8,1,2,3-nopqr]tetraphene (ACE-ANT-ACE) and 4,4′-(6,12-bis(octyloxy)-6,12-dihydronaphtho[7,8,1,2,3-nopqr]tetraphene-4,10-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (TPA-ANT-TPA) end capped with dihydroacenaphthylene and triphenyleamine groups are designed and synthesized, respectively. Among both, dopant-free TPA-ANT-TPA cut-rate HTM ($67 g⁻¹) exhibits higher performance with 17.5% efficiency and retains respectable performance after 50 h in 58% relative humidity than conventional expensive 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamino)-9,9′-spirbiuorene.

This study proposes a novel strategy of controllable deamination of Co–NH₃ complexes in a system containing Ni(OH)₂ to synthesize ultrasmall ternary oxide nanoparticles (NPs), NiCo₂O₄. Through this approach, ultrasmall (5 nm on average) and well-dispersed NiCo₂O₄ NPs without exotic ligands are obtained, which enables the formation of uniform and pin-hole free films. The tightly covered NiCo₂O₄ films also facilitate the formation of large perovskite grains and thus reduce film defects. The results show that with the NiCo₂O₄ NPs as the hole transport layer (HTL), the perovskite solar cells reach a high power conversion efficiency (PCE) of 18.23% and a promising stability (maintained ≈90% PCE after 500 h light soaking). To the best of the author's knowledge, it is the first time that spinel NiCo₂O₄ NPs have been applied as hole transport layer in perovskite solar cells successfully. This work not only demonstrates the potential applications of ternary oxide NiCo₂O₄ as HTLs in hybrid perovskite solar cells but also provides an insight into the design and synthesis of ultrasmall and ligand-free NPs HTLs to enable cost-effective photovoltaic devices.

An ultrasmall and well-dispersed ternary oxide of NiCo₂O₄ nanoparticles, achieved by a new strategy of controllable deamination of the Co–NH₃ complexes in a system containing Ni(OH)₂ , facilitate the formation of large perovskite grains, which is first applied as hole transport layer for highly efficient perovskite solar cells.

Realizing over 10% efficiency in printed organic solar cells via scalable materials and less toxic solvents remains a grand challenge. In article number 1705485, Harald Ade and co-workers report chlorine-free, in-air blade-coating of a new photoactive combination, FTAZ:IT-M, which is able to yield an efficiency of nearly 11%, despite a high humidity of ≈50%.

In article 1705875, Zhao-Kui Wang, Peng-Fei Fang, Liang-Sheng Liao, and co-workers incorporate graphitic carbon nitride (g-C₃N₄) into the perovskite layer of perovskite solar cells. The additive retards the crystallization rate, improving crystalline quality and reducing the intrinsic defect density of the perovskite film. Increasing the fill factor from 0.65 to 0.74 yields a stable device with a power conversion efficiency of 19.49%.

In article 1703398, Jong-Hong Lu, Chih-Ping Chen and, co-workers demonstrate a Ag/ITO/Ag based microcavity (MC) structure for colorful organic photovoltaics applications. OPVs with an ultra-wide vivid color-gamut (blue, green, yellow-green, yellow, orange, and red), with PCEs as high as 8.2% for the yellow-green [CIE 1931: (0.364, 0.542)] device with a highest transmittance of 17.3% at 561 nm are demonstrated.

2D halide perovskites have recently been recognized as a promising avenue in perovskite solar cells (PSCs) in terms of encouraging stability and defect passivation effect. However, the efficiency (less than 15%) of ultrastable 2D Ruddlesden–Popper PSCs still lag far behind their traditional 3D perovskite counterparts. Here, a rationally designed 2D-3D perovskite stacking-layered architecture by in situ growing 2D PEA₂PbI₄ capping layers on top of 3D perovskite film, which drastically improves the stability of PSCs without compromising their high performance, is reported. Such a 2D perovskite capping layer induces larger Fermi-level splitting in the 2D-3D perovskite film under light illumination, resulting in an enhanced open-circuit voltage (V_oc) and thus a higher efficiency of 18.51% in the 2D-3D PSCs. Time-resolved photoluminescence decay measurements indicate the facilitated hole extraction in the 2D-3D stacking-layered perovskite films, which is ascribed to the optimized energy band alignment and reduced nonradiative recombination at the subgap states. Benefiting from the high moisture resistivity as well as suppressed ion migration of the 2D perovskite, the 2D-3D PSCs show significantly improved long-term stability, retaining nearly 90% of the initial power conversion efficiency after 1000 h exposure in the ambient conditions with a high relative humidity level of 60 ± 10%.

2D perovskite capping layers are grown in situ on top of the 3D perovskite film, leading to an enhanced efficiency of 18.5% in the stacking-layered 2D-3D perovskite solar cells (PSCs). Moreover, the unencapsulated 2D-3D PSCs show drastically improved long-term stability, retaining nearly 90% of the original efficiency after 1000 h exposure in a highly humid environment.

Hole transport layer (HTL) plays a critical role for achieving high performance solution-processed optoelectronics including organic electronics. For organic solar cells (OSCs), the inverted structure has been widely adopted to achieve prolonged stability. However, there are limited studies of p-type effective HTL on top of the organic active layer (hereafter named as top HTL) for inverted OSCs. Currently, p-type top HTLs are mainly 2D materials, which have an intrinsic vertical conduction limitation and are too thin to function as practical HTL for large area optoelectronic applications. In the present study, a novel self-assembled quasi-3D nanocomposite is demonstrated as a p-type top HTL. Remarkably, the novel HTL achieves ≈15 times enhanced conductivity and ≈16 times extended thickness compared to the 2D counterpart. By applying this novel HTL in inverted OSCs covering fullerene and non-fullerene systems, device performance is significantly improved. The champion power conversion efficiency reaches 12.13%, which is the highest reported performance of solution processed HTL based inverted OSCs. Furthermore, the stability of OSCs is dramatically enhanced compared with conventional devices. The work contributes to not only evolving the highly stable and large scale OSCs for practical applications but also diversifying the strategies to improve device performance.

A novel self-assembled quasi-3D nanocomposite is demonstrated to be an effective top hole transport layer (HTL) for both fullerene and non-fullerene inverted organic solar cells. Due to the better conductivity of this nanocomposite HTL, the thickness sensitivity issue of graphene oxide is addressed. Surface recombination is suppressed and the highest power conversion efficiency can reach 12.13%.

A novel wide-bandgap electron-donating copolymer containing an electron-deficient, difluorobenzotriazole building block with a siloxane-terminated side chain is developed. The resulting polymer, poly{(4,8-bis(4,5-dihexylthiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-4,7-di(thiophen-2-yl)-5,6-difluoro-2-(6-(1,1,1,3,5,5,5-heptamethyltri-siloxan-3-yl)hexyl)-2H-benzo[d][1,2,3]triazole} (PBTA-Si), is used to successfully fabricate high-performance, ternary, all-polymer solar cells (all-PSCs) insensitive to the active layer thickness. An impressively high fill factor of ≈76% is achieved with various ternary-blending ratios. The optimized all-PSCs attain a power conversion efficiency (PCE) of 9.17% with an active layer thickness of 350 nm and maintain a PCE over 8% for thicknesses over 400 nm, which is the highest reported efficiency for thick all-PSCs. These results can be attributed to efficient charge transfer, additional energy transfer, high and balanced charge transport, and weak recombination behavior in the photoactive layer. Moreover, the photoactive layers of the ternary all-PSCs are processed in a nonhalogenated solvent, 2-methyltetrahydrofuran, which greatly improves their compatibility with large-scale manufacturing.

A novel electron-donating copolymer, PBTA-Si, containing a benzotriazole building block with a siloxane-functionalized side chain, is developed and used to fabricate thick-film all-polymer solar cells (all-PSC). By means of ternary blending, the all-PSCs attain a power conversion efficiency of 9.17% with a 350 nm thick active layer and 8.34% with a thickness of 420 nm.

A new naphthalene diimide (NDI)-based polymer with strong electron withdrawing dicyanothiophene (P(NDI2DT-TTCN)) is developed as the electron transport layer (ETL) in place of the fullerene-based ETL in inverted perovskite solar cells (Pero-SCs). A combination of characterization techniques, including atomic force microscopy, scanning electron microscopy, grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, space-charge-limited current, electrochemical impedance spectroscopy, photoluminescence (PL), and time-resolved PL decay, is used to demonstrate the interface phenomena between perovskite and P(NDI2DT-TTCN) or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). It is found that P(NDI2DT-TTCN) not only improves the electron extraction ability but also prevents ambient condition interference by forming a hydrophobic ETL surface. In addition, P(NDI2DT-TTCN) has excellent mechanical stability compared to PCBM in flexible Pero-SCs. With these improved functionalities, the performance of devices based on P(NDI2DT-TTCN) significantly outperform those based on PCBM from 14.3 to 17.0%, which is the highest photovoltaic performance with negligible hysteresis in the field of polymeric ETLs.

A novel naphthalene diimide (NDI)-based polymer (P(NDI2DT-TTCN)) is used as the electron transport layer in inverted flexible perovskite solar cells. Photovoltaic performances of the P(NDI2DT-TTCN)-based device show a significant improvement up to 17.0%, whereas the control device for [6,6]-phenyl-C61-butyric acid methyl ester based device only shows power conversion efficiency of 14.3%. In addition, P(NDI2DT-TTCN) improves not only the light-induced and long-term stability but also mechanical stability.

In article number 1701683, In Hwan Jung, Sung-Yeon Jang, and co-workers report the synthesis of ‘high-efficiency low-temperature processed perovskite solar cells’ by the combination of two techniques; the modification of solution-processed ZnO using a self-assembled monolayer, and the fabrication of perovskite layers by a sequential deposition method. This technique may pave the way to efficiently reducing the processing temperature of perovskite solar cells.

In article number 1702235, Taiho Park and co-workers demonstrate that the efficiency of a planar-type perovskite solar cell is increased by the addition of CuI ionic salt to the TiO₂ electron transport layer. Due to the characteristics of CuI (Visualized on the cover as yellow and red spheres), the electrons from perovskite layer (pink octahedra) show fast extraction to TiO₂ layer (blue and purple spheres).

Wide-bandgap (WBG) formamidinium–cesium (FA-Cs) lead iodide–bromide mixed perovskites are promising materials for front cells well-matched with crystalline silicon to form tandem solar cells. They offer avenues to augment the performance of widely deployed commercial solar cells. However, phase instability, high open-circuit voltage (V_oc) deficit, and large hysteresis limit this otherwise promising technology. Here, by controlling the crystallization of FA-Cs WBG perovskite with the aid of a formamide cosolvent, light-induced phase segregation and hysteresis in perovskite solar cells are suppressed. The highly polar solvent additive formamide induces direct formation of the black perovskite phase, bypassing the yellow phases, thereby reducing the density of defects in films. As a result, the optimized WBG perovskite solar cells (PSCs) (E_g ≈ 1.75 eV) exhibit a high V_oc of 1.23 V, reduced hysteresis, and a power conversion efficiency (PCE) of 17.8%. A PCE of 15.2% on 1.1 cm² solar cells, the highest among the reported efficiencies for large-area PSCs having this bandgap is also demonstrated. These perovskites show excellent phase stability and thermal stability, as well as long-term air stability. They maintain ≈95% of their initial PCE after 1300 h of storage in dry air without encapsulation.

The highly polar solvent additive, formamide, enables phase-selective crystallization in wide-bandgap (WBG) perovskites. By suppressing the formation of non-perovskite phases, the WBG perovskites (E_g ≈ 1.75 eV) exhibited excellent light-induced phase-, thermal, and air stability, as well as device performance with a high V_oc of 1.23 V and reduced hysteresis, with a power conversion efficiency (PCE) of 17.8%.