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The unique morphology of 2D van der Waals materials enables them to withstand large deformations and significant nonuniform strain, potentially inducing a strong flexoelectric effect. Despite the size‐dependent flexoelectric effect showing potential for modulating the optoelectronic performance of 2D van der Waals materials, it is far from being fully exploited owing to various challenges. Herein, the use of nanowires with different diameters as the bending media to fabricate 2D α‐In2Se3/β‐InSe heterojunctions with large curvatures of 0.1–1 µm−1 is proposed. The significant band alignment modulation in α‐In2Se3 resulting from the bending‐induced flexoelectric effect is verified through Kelvin probe force microscopy. The strain‐induced piezoelectric effect can be negated because of the weak vdW forces at the interface. The flexoelectric polarization in β‐InSe is screened via the accumulated electrons in the unilateral depleted heterojunction. Compared to the flat heterojunction, the curved heterojunction with an average curvature of 0.9 µm−1 shows 2.48‐fold and 7.62‐fold increases in open‐circuit voltage and zero‐biased responsivity, respectively. This study demonstrates the first successful modulation of photodetection in 2D heterojunctions by exploiting the flexoelectric effect, providing a new perspective for high‐performance 2D vdW optoelectronic devices. [ABSTRACT FROM AUTHOR]

The external radiative efficiency of a photovoltaic device is a useful metric to compare different photovoltaic technologies and material systems. However, under low‐intensity indoor lighting, a particular open‐circuit voltage dependence arises that cannot be described using the assumption of a constant external radiative efficiency, as often assumed at higher illumination intensities. As a result, the external radiative efficiency at 1 sun conditions no longer becomes a useful comparison metric for devices at indoor lighting levels. By use of absolute electroluminescence measurements of gallium arsenide photovoltaic (PV) cells, the current density dependence of the external radiative efficiency is explored and it is shown that the functional form of this dependence dictates the open‐circuit voltage losses in the low‐intensity limit. This dependence is quantified with a mathematical treatment that takes advantage of a widely accepted recombination current model in III–V photovoltaic devices, and this model is used to study the dependence of PV device open‐circuit voltage at low intensity with varying non‐radiative recombination rates. [ABSTRACT FROM AUTHOR]

The power conversion efficiency (PCE) of organic solar cells (OSCs) is improved dramatically in recent years and now approaches >19% for single‐junction cells and >20% for tandem cells. Therefore, the practical use of OSCs is becoming a reality. This perspective summarizes the state of the art of OSC characteristics and discusses the challenges that remain in further improving PCE. The short‐circuit current density (J SC) of the state‐of‐the‐art OSCs almost approaches 30 mA cm−2. As the internal quantum efficiencies of these devices exceed 90%, for further improvement in J SC, it is necessary to suppress reflection at interfaces and increase the active layer thickness to absorb as many photons as possible. Further suppression of the nonradiative voltage loss is imperative, as there is still large room for improvement compared to inorganic and perovskite counterparts. Hence, increasing the exciton lifetime and photoluminescence quantum yield of nonfullerene acceptors are pivotal to improving the open‐circuit voltage of OSCs. The fill factors of the latest OSCs approach 80%; however, because the optimized active layer thickness remains ≈100 nm, further suppression of bimolecular charge recombination is needed. Finally, this perspective discusses to what extent PCE can be improved and what can be done to achieve this.

An overshoot of the open‐circuit voltage (VOC) after switching off the illumination is observed for perovskite solar cells, while the simultaneously measured photoluminescence (PL) intensity decreases continuously. Similarly, a dip in the photovoltage transient is detected at the beginning of a light pulse added to a continuous bias light, while the PL increases. This decoupling of external and implied VOC (as derived from the PL data) originates from a strong gradient of the majority charge carrier quasi‐Fermi level in the vicinity of a nonideal contact. This gradient reduces the external voltage much more than the implied voltage. The VOC overshoot is observed whenever the gradient decreases faster than when the separation of the quasi‐Fermi levels is reduced by charge carrier recombination. As shown in previous work, in perovskite solar cells, the magnitude of the gradient is strongly influenced by mobile ionic species and it decreases upon light soaking. This is why a fully light‐soaked device does not show this kind of VOC overshoot. [ABSTRACT FROM AUTHOR]

A bandgap gradient at the front surface of solar absorbers can effectively suppress surface recombination while not affecting photocurrent. Herein, it is demonstrated that a front‐surface gradient can be formed in an inverted perovskite cell by introducing perovskite quantum dots (QDs) between the hole‐transporting layer (HTL) and the remaining absorber. Ultraviolet photoelectron spectroscopy reveals that, with the addition of CsPbBr3 QDs onto the HTL substrate, the subsequently deposited MAPbI3 is converted from mild p‐type to n‐type, and the resultant band alignment can effectively reduce the electron concentration at the front surface without significantly affecting hole extraction. Multiple independent characterizations further confirm the reduction of surface recombination. As a result, the inverted MAPbI3 cells exhibit an open‐circuit voltage of 1.154 V, which translates to a nonradiative recombination loss of 0.15 V and a power conversion efficiency of 20.51%. [ABSTRACT FROM AUTHOR]

The performance of thermally deposited Sb2Se3 solar cells are severely limited by various bulk and interfacial recombination, instigating a large open‐circuit voltage (VOC) deficit. Ternary Sb2(S,Se)3 is considered as a remedy, however, it is also subjected to a dilemma that improvement in VOC will be escorted by JSC loss due to the shrinkage of light harvest. Thus, a gradient of S/Se across the film is a prerequisite to avoid this detrimental compromise. Herein, the incorporation of S in the Sb2Se3 absorber layer evaporated from a CdS buffer layer during vapor transport deposition (VTD) process, and its further self‐activated diffusion at the interface upon ambient storage is explored. For the gradient indium tin oxide (ITO)/CdS/Sb2(S,Se)3/Sb2Se3/Au solar cell, the large bandgap Sb2(S,Se)3 at the heterojunction side contributes to high VOC, while the narrow bandgap Sb2Se3 at the top side confirms high JSC. Sulfur diffusion at the CdS/Sb2Se3 interface also improves the junction quality with an enlarged Vbi, reduced interfacial defects and recombination loss, thus improving VOC from 393 to 430 mV. Such VOC represents the highest value for that of thermally deposited Sb2Se3 solar cells. The champion device also delivers an interesting efficiency of 7.49%. This research provides substantial guidance in exploring efficient approaches to improve the performance of Sb2Se3 solar cells. [ABSTRACT FROM AUTHOR]

An asymmetric wide‐bandgap (WBG) nonfullerene acceptor (C6‐IDTT‐T) is developed by shearing one alkyl side‐chain from a symmetrically alkyl‐substituted indacenodithieno[3,2‐b]thiophene (IDTT) core of the fused‐ring electron acceptor 2C6‐IDTT‐T. These two acceptors both exhibit wide optical bandgaps over 1.8 eV. Investigations on the optical, electrochemical, and active layer morphology are conducted to understand the effect of asymmetric side chains on the electrical and photovoltaic properties. Compared with symmetric 2C6‐IDTT‐T, asymmetric C6‐IDTT‐T is found to exhibit redshifted absorption and higher electron mobility. As a result, the C6‐IDTT‐T blend with a thienothiophene‐benzodithiophene copolymer (PTB7‐Th) presents higher electron mobility and more balanced charge carrier transport, which leads to an enhanced power conversion efficiency of 8.51% for C6‐IDTT‐T‐based device with a high open‐circuit voltage of 1.052 V and a low energy loss of 0.60 eV. [ABSTRACT FROM AUTHOR]

Abstract: One of the most important factors that limits the efficiencies of bulk‐heterojunction organic solar cells (OSCs) is the modest open‐circuit voltage (Voc) due to their large voltage loss (Vloss) caused by significant nonradiative recombination loss. To boost the performance of OSCs toward their theoretical limit, developing high‐performance donor: acceptor systems featuring low Vloss with suppressed nonradiative recombination losses (<0.30 V) is desired. Herein, high performance OSCs based on a polymer donor benzodithiophene‐difluorobenzoxadiazole‐2‐decyltetradecyl (BDT‐ffBX‐DT) and perylenediimide‐based acceptors (PDI dimer with spirofluorene linker (SFPDI), PDI4, and PDI6) are reported which offer a high power conversion efficiency (PCE) of 7.5%, 56% external quantum efficiency associated with very high Voc (>1.10 V) and low Vloss (<0.60 V). A high Voc up to 1.23 V is achieved, which is among the highest values reported for OSCs with a PCE beyond 6%, to date. These attractive results are benefit from the suppressed nonradiative recombination voltage loss, which is as low as 0.20 V. This value is the lowest value for OSCs so far and is comparable to high performance crystalline silicon and perovskite solar cells. These results show that OSCs have the potential to achieve comparable Voc and voltage loss as inorganic photovoltaic technologies. [ABSTRACT FROM AUTHOR]

Triboelectric nanogenerators (TENGs) based on solution-processed zinc-oxide (ZnO) nanoripples acting as a triboelectric layer were fabricated to enhance their output voltage. Scanning electron microscopy images showed that ZnO nanoripples were uniformly formed on glass substrates coated with indium-tin oxide. The TENG containing ZnO nanoripples and operating in the vertical contact-separation mode exhibited an output voltage of 68 V, which was almost 3 times larger than that of the TENGs with ZnO nanorods. The open-circuit output voltages of TENGs were found to depend significantly on the condition of the ZnO nanoripple precursors. The friction force on the ZnO nanoripple matrix increased with increasing density of ZnO nanoripples due to an increase in the contact area between the ZnO nanoripples and the PI layer, resulting in an enhancement of the output voltage of the TENGs based on nanoripples. [ABSTRACT FROM AUTHOR]

All-oxide solar cells are presently attracting extensive research interest due to their excellent stability, low-cost and non-toxicity. However, the band gap of metal oxides is lack of effective optimization and results in poor photovoltaic performance, thus hindering their practical applications. In this work, Co 3 O 4 was investigated for application as a photo-absorber in all-oxide solar cells, and its band gap was optimized by introducing Li dopant into the spinel structure. Li x Co 3−x O 4 nanoparticles, prepared via the hydrothermal method, were homogenously coated onto TiO 2 mesoporous films, which were then used to fabricate planar heterojunction TiO 2 /Li x Co 3−x O 4 solar cells (SCs). The effects of Li-doping on the heterojunction solar cell performance were further investigated. The findings revealed that the incorporation of Li ions into Co 3 O 4 led to a significant enhancement in short-circuit current density (J sc ). Remarkably, a high open-circuit voltage (V oc ) of 0.70 V was also achieved. Besides, reasons for the enhanced cell performance are the narrower band gap, reduced photogenerated carrier recombination and the more favorable energy band structure as compared with SCs assembled from pure Co 3 O 4 . [ABSTRACT FROM AUTHOR]

The development of small molecule-based (SMs) organic photovoltaic (PV) materials is under intense attention by the researchers for organic solar cells (OSCs) to constantly revise their power conversion efficiencies (PCEs). Therefore, we have designed a series of Z-shaped non-fullerene acceptors (NFAs) (SNAA1-SNAA6), and characterized them theoretically for the possible use of in efficient OSCs devices. We have designed this Z-shaped series after employing several end-capped modifications over reference molecule (COi5DFIC (R)) to enhance their photovoltaic parameters further. We have extensively characterized these Z-shaped NFAs and explored their optoelectronic, photovoltaic and structure-property relationship with density functional theory (DFT) and time-dependent (TD-DFT). As a result, this Z-shaped NFA series (SNAA1-SNAA6) showed attractive narrower bandgaps (E g) compared to parent molecule R, and all are extremely red-shifted in the UV-Visible absorption spectrum. Moreover, we have also performed a complex study of donor:acceptor (D:A) blend (SNAA1:PTB7-Th) to show their compatibility in fabricating highly efficient OSCs devices. • Z-Shaped Non-fullerene acceptors (SNAA1-SNAA6) are designed for organic solar cells. • Highly red-shifted absorption, and an attractive narrow band-gaps are observed. • The non-fullerene acceptors showed an outstanding electron/hole reorganizational energy. • The designed materials (SNAA1-SNAA6) exhibited better optoelectronic properties as compared with reference R. [ABSTRACT FROM AUTHOR]

The theoretical mechanism of open-circuit voltages (VOC) in OSCs based on various small molecule organic materials is studied. The structure under investigation is simple planar heterojunction (PHJ) by thermal vacuum evaporation deposition. The various wide band gaps of small molecule organic materials are used to enhance the power conversion efficiency (PCE). The donor materials used in the device include: Alpha-sexithiophene (α-6T), Copper(II) phthalocyanine (CuPc), boron subnaphthalocyanine chloride (SubNc) and boron Subphthalocyanine chloride (SubPc). It is combined with fullerene or SubPc acceptor material to obtain a comprehensive understanding of the charge transport behavior. It is found that the VOC of the device is largely limited by charge transport. This was associated with the space charge effects and hole accumulation. These results are attributed to the improvement of surface roughness and work function after molybdenum trioxide (MoO3) is inserted as an anode buffer layer. [ABSTRACT FROM AUTHOR]

We present an experimental study combined with computer simulations on the effects of wide band-gap absorber and window layers on the open-circuit voltage ( Voc) in single junction thin film silicon solar cells. The quantity Δ Ep, taking as the difference between the band gap and the activation energy in ⟨p⟩ layer, is treated as a measure of the p-layer properties and shows a linear relation with Voc over a range of 100 mV with a positive slope of around 430 mV/eV. Two limiting mechanisms of Voc are identified: the built-in potential at lower Δ Ep and the band gap of the absorber layer at higher Δ Ep. The results of the experimental findings are confirmed by computer simulations. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A new semiconducting polymer based on terthiophene-thienopyrrolodione alternating building blocks with a deep HOMO energy level (5.66 eV) is presented. The polymer is prepared by a direct heteroarylation polycondensation reaction, which is a low-cost and green alternative to the standard Stille coupling reaction and thus avoids any use of toxic stannyl derivatives. Integrating the polymer into bulk heterojunction solar cells with [6,6]-phenyl C71-butyric acid methyl ester ([70]PCBM) leads to a PCE of over 6% and a high open-circuit voltage of up to 0.94 V. To obtain these results a unique processing additive, 1-chloronaphthalene, is used, and a relatively low concentration of [70]PCBM is used in the blend solution. [ABSTRACT FROM AUTHOR]

The key strategy to enhance the intra-molecular push-pull effects by broadening the optial absorption of small molecule based organic photovoltaic (SM-OPV) materials is considered an effective approach to enhance the power conversion efficiencies (PCEs) of SM-OPV devices. However, in case of acceptor materials, the highly desirable molecular modelling strategy of halogenation generally effects in downward-shifting of molecular energy levels, resulting decrease in open-circuit voltages (V oc) in the devices. Herein, we investigate a fluorinated, chlorinated and cyanide (CN) based end-capped acceptor materials, which shows a broader optical absorption phenomenon and exhibited a good voltages than it chlorinated counterparts. These new molecularly engineered SM-OPV were characterize theoretically by density functional theory (DFT) and time-dependent (TD-DFT) approaches. The estimation of electron/hole mobility, and V oc was done by calculating the geometric parameters, electronic structures, frontier molecular orbitals (FMOs), charge transfer rates, and exciton binding energies of the designed OPV materials. The outcomes of these investigations revealed that all newly engineered SM-OPV acceptor materials displays an enhanced exciton dissociation and absorption efficiency and underneath LUMO levels which might be responsible to improve the V oc , reorganization energies, and photo-current density parametrs, resulting enhancement in the PCEs of the organic solar cells (OSC) devices. [Display omitted] • Non-fullerene acceptor molecules (SiOA1-SiOA5) are studied for organic solar cells applications. • Significant lowering of energy gap with concomitant red shifting of the absorption spectra is achieved. • All acceptor molecules have remarkable optoelectronic properties compared to R. [ABSTRACT FROM AUTHOR]

The effect of mutual coupling in an adaptive antenna array for the nulling of interferences is investigated. The concept of mutual impedance is re-defined to take into account of the scattering effect due to the other antenna elements in the array. The re-defined mutual impedances are used to reduce the mutual coupling effect by calculating the open-circuit voltages from the measured voltages on the antenna terminals. Results show that by using the re-defined mutual impedances, substantial improvements in term of the depths and the accuracy of the nulls can be obtained over a previous method [ABSTRACT FROM PUBLISHER]

A trade‐off between open‐circuit voltage (VOC) and high short‐circuit (JSC) becomes one of the most vital problems limiting further improvement in polymer solar cells' (PSCs) efficiency. In this work, two asymmetric polymer donors PBDT‐F‐2TC and PBDT‐SF‐2TC are designed and synthesized. When blended with a state‐of‐the‐art acceptor IT‐4F with low lowest‐unoccupied molecular orbital level, simultaneously high VOC (up to 0.94 V) and JSC (up to 20.73 mA cm−2) are obtained for both copolymers. Note that the VOC value of 0.94 V is the highest value of PSCs based on IT‐4F reported so far. The simultaneously improved VOC and JSC in resulting devices are discovered from the deep highest‐occupied molecular orbital levels (−5.5 to −5.7 eV) and the hyperchromic effect of the polymers, the small driving force, and the small energy loss during the charge transfer, due to the synergistic effect of asymmetric carboxylate unit and fluorine/sulfur atoms. More importantly, thanks to the asymmetric 2TC, both PBDT‐F‐2TC‐ and PBDT‐SF‐2TC‐based PSCs can be successfully processed by non‐halogenated solvent 1,2,4‐trimethylbenzene (TMB) to yield device efficiencies of 10.29% and 10.39%, respectively, which are the maximum values for non‐fullerene PSCs fabricated using the eco‐friendly solvent TMB. [ABSTRACT FROM AUTHOR]

In this work, we present a novel small molecule based on dithienylthienothiadiazole units (named SM1) acting as an efficient component in ternary blend organic solar cells to modify the hole extraction at the interface. Our findings show that the SM1 suppresses the surface recombination and enhances the open-circuit voltage ( V oc ). By introducing SM1 in a host system composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl- C 61 -butyric acid methyl ester (PCBM), we obtained V oc values of up to 0.75 V and fill factors larger than 70% for the ternary blends. As a consequence, the power conversion efficiency is improved by about 30% compared to P3HT:PCBM binary devices. Interestingly, external quantum efficiency and absorption spectra in the near-infrared region do not show any contribution of SM1 in dried films. Instead, the addition of the small molecule improves the V oc by reducing the surface recombination losses. To shed light on the recombination processes, we carried out Fourier-transform photocurrent spectroscopy and impedance spectroscopy measurements. This work shows that the ternary concept can also have functionalities other than photosensitization and can even act as a morphology-directing agent or an interface modifier.

We present here a series of wide-band-gap ( E g : >1.8 eV) polymer donors by incorporating thiophene-flanked phenylene as an electron-donating unit and quinoxaline as an electron-accepting co-unit to attain large open-circuit voltages ( V oc s) and short-circuit currents ( J sc s) in nonfullerene organic solar cells (OSCs). Fluorination was utilized to fine-tailor the energetics of polymer frontier molecular orbitals (FMOs) by replacing a variable number of H atoms on the phenylene moiety with F. It was found that fluorination can effectively modulate the polymer backbone planarity through intramolecular noncovalent S···F and/or H···F interactions. Polymers (P2-P4) show an improved molecular packing with a favorable face-on orientation compared to their nonfluorinated analogue (P1), which is critical to charge carrier transport and collection. When mixed with IDIC, a nonfullerene acceptor, P3 with two F atoms, achieves a remarkable V oc of 1.00 V and a large J sc of 15.99 mA/cm 2 , simultaneously, yielding a power-conversion efficiency (PCE) of 9.7%. Notably, the 1.00 V V oc is among the largest values in the IDIC-based OSCs, leading to a small energy loss ( E loss : 0.62 eV) while maintaining a large PCE. The P3:IDIC blend shows an efficient exciton dissociation through hole transfer even under a small energy offset of 0.16 eV. Further fluorination leads to the polymer P4 with increased chain-twisting and mismatched FMO levels with IDIC, showing the lowest PCE of 2.93%. The results demonstrate that quinoxaline-based copolymers are promising donors for efficient OSCs and the fluorination needs to be fine-adjusted to optimize the interchain packing and physicochemical properties of polymers. Additionally, the structure-property correlations from this work provide useful insights for developing wide-band-gap polymers with low-lying highest occupied molecular orbitals to minimize E loss and maximize V oc in nonfullerene OSCs for efficient power conversion.

Herein, the open-circuit voltage (V OC ) loss in both polymer solar cells and perovskite solar cells is quantitatively analyzed by measuring the temperature dependence of V OC to discuss the difference in the primary loss mechanism of V OC between them. As a result, the photon energy loss for polymer solar cells is in the range of about 0.7-1.4 eV, which is ascribed to temperature-independent and -dependent loss mechanisms, while that for perovskite solar cells is as small as about 0.5 eV, which is ascribed to a temperature-dependent loss mechanism. This difference is attributed to the different charge generation and recombination mechanisms between the two devices. The potential strategies for the improvement of V OC in both solar cells are further discussed on the basis of the experimental data.

The theoretical mechanism of open-circuit voltages (V OC ) in OSCs based on various small molecule organic materials is studied. The structure under investigation is simple planar heterojunction (PHJ) by thermal vacuum evaporation deposition. The various wide band gaps of small molecule organic materials are used to enhance the power conversion efficiency (PCE). The donor materials used in the device include: Alpha-sexithiophene (α-6T), Copper(II) phthalocyanine (CuPc), boron subnaphthalocyanine chloride (SubNc) and boron Subphthalocyanine chloride (SubPc). It is combined with fullerene or SubPc acceptor material to obtain a comprehensive understanding of the charge transport behavior. It is found that the V OC of the device is largely limited by charge transport. This was associated with the space charge effects and hole accumulation. These results are attributed to the improvement of surface roughness and work function after molybdenum trioxide (MoO₃) is inserted as an anode buffer layer.