Billy

A new molecularly engineered nonfullerene acceptor, 2,2′-(5,5′-(9,9-didecyl-9H-fluorene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl)bis(methanylylidene))bis(3-hexyl-1,4-oxothiazolidine-5,2-diylidene))dimalononitrile (BAF-4CN), with fluorene as the core and arms of dicyano-n-hexylrhodanine terminated benzothiadiazole is synthesized and used as an electron acceptor in bulk heterojunction organic solar cells. BAF-4CN shows a stronger and broader absorption with a high molar extinction coefficient of 7.8 × 10⁴m⁻¹ cm⁻¹ at the peak position (498 nm). In the thin film, the molecule shows a redshift around 17 nm. The photoluminescence experiments confirm the excellent electron accepting nature of BAF-4CN with a Stern–Volmer coefficient (K_sv) of 1.1 × 10⁵m⁻¹. From the electrochemical studies, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of BAF-4CN are estimated to be −5.71 and −3.55 eV, respectively, which is in good synchronization with low bandgap polymer donors. Using BAF-4CN as an electron acceptor in a poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3″′-di(2-octyldodecyl) 2,2′;5′,2″;5″,2″′-quaterthiophen-5,5″′-diyl)] based bulk-heterojunction solar cell, a maximum power conversion efficiency of 8.4% with short-circuit current values of 15.52 mA cm⁻², a fill factor of 70.7%, and external quantum efficiency of about 84% covering a broad range of wavelength is achieved.

The non-fullerene acceptor (NFA) BAF-4CN is synthesized for organic photovoltaic (OPV) application to overcome the drawbacks of fullerene. BAF-4CN shows stronger absorption compared to phenyl C₇₁-butyric acid methyl ester and has an excellent electron accepting nature and high charge carrier mobility. A power conversion efficiency of 8.4% is achieved from PffBT4T-2OD:BAF-4CN based bulk heterojunction solar cells. This may be a promising substitute for fullerene in low-cost solution-processed OPV.

The first highly efficient single junction quaternary blend solar cells that break efficiency above 10% with complementary squaraine small molecules and low band-gap polymer combinations are reported by Nilay Hazari, André D. Taylor and co-workers in article number 1600660. The quaternary design demonstrates several advantages: (i) broader light absorption, (ii) improved surface morphology, (iii) enhanced co-crystallization packing, (iv) multiple energy and charge transfer pathways to reduce recombination, and (v) increased charge mobility.

Unintentionally formed carbazole homocouplings occur frequently in the photovoltaic material PCDTBT when made by Suzuki polycondensation resulting in significantly lower power conversion efficiency of PCDTBT/fullerene solar cells. A likely loss mechanism that leads to reduced short circuit currents is exciton localization next to the homocoupling defect. This is reported by Michael Sommer and co-workers in article number 1601232.

The selectivity of electrodes of solar cells is a critical factor that can limit the overall efficiency. If the selectivity of an electrode is not sufficient both electrons and holes recombine at its surface. In materials with poor transport properties such as in organic solar cells, these surface recombination currents are accompanied by large gradients of the quasi-Fermi energies as the driving force. Experimental results from current–voltage characteristics, advanced photo- and electroluminescence as well as charge extraction of three different photoactive materials are shown and compared to drift-diffusion simulations. It can be concluded that in cases of electrodes with reduced selectivity the decrease of the open-circuit voltage can be divided into two distinct contributions, the reduction of the overall steady-state charge carrier density and the gradients of the quasi-Fermi energies. The results clearly show that for photoactive layers with poor transport properties, the gradient of the quasi-Fermi energy in the vicinity of the contact is the main contribution to the loss in open-circuit voltage. For imbalanced mobilities, this gives rise to the phenomenon that it is more challenging to realize a selective contact for the less mobile charge carrier, i.e., the hole contact in most organic solar cells.

The impact of charge carrier mobility and electrode selectivity on surface recombination is investigated by drift-diffusion modeling, charge extraction, electro- and photoluminescence. It is shown that for organic solar cells the reduced open-circuit voltage is not only due to actual recombination of charge carriers at the contact but mainly a consequence of the required driving force for the majority carriers.

This paper firstly reports the effect of deoxyribonucleic acid (DNA) molecules extracted from chickpea and wheat plants on the injection/recombination of photogenerated electrons and sensitizing ability of dye-sensitized solar cells (DSSCs). These high-yield DNA molecules are applied as both linker bridging unit as well as thin tunneling barrier (TTB) at titanium dioxide (TiO₂ )/dye interface, to build up high-efficient DSSCs. With its favorable energy levels, effective linker bridging role, and double helix structure, bifunctional DNA modifier shows an efficient electron injection, suppressed charge recombination, longer electron lifetime, and higher light harvesting efficiency, which leads to higher photovoltaic performance. In particular, a photoconversion efficiency (PCE) of 9.23% is achieved by the binary chickpea and wheat DNA-modified TiO₂ (CW@TiO₂) photoanode. Furthermore, time-resolved fluorescence spectroscopy measurements confirm a better electron transfer kinetics for DNA-modified TiO₂ photoanodes, implying a higher electron transfer rate (k_ET). This work highlights a great contribution for the photoanodes that are linked with DNA molecule, which act as both bridging unit and TTB to control the charge recombination and injection dynamics, and hence, boost the photovoltaic performance in the DSSCs.

Two types of deoxyribonucleic acid molecules, extracted from fresh leaves of chickpea and wheat plants, employ as thin tunneling barrier at TiO₂/dye interface to minimize the recombination rates as well as linker bridging units for the electrons to move toward the TiO₂, thereby enhancing V_oc and J_sc. This strategy might open up new opportunities for the widespread fabrication and application of dye-sensitized solar cells.

After the first report in 2008, diketopyrrolopyrrole (DPP)-based small-molecule photovoltaic materials have been intensively explored. The power conversion efficiencies (PCEs) for the DPP-based small-molecule donors have been improved up to 8%. Furthermore, through judicious structure modification, DPP-based small molecules can also be converted into electron-acceptor materials, and, recently, some exciting progress has been achieved. The development of DPP-based photovoltaic small molecules is summarized here, and the photovoltaic performance is discussed in relation to structural modifications, such as the variations of donor–acceptor building blocks, alkyl substitutions, and the type of conjugated bridges, as well as end-capped groups. It is expected that the discussion will provide a guideline in the exploration of novel and promising DPP-containing photovoltaic small molecules.

Diketopyrrolopyrrole (DPP)-based small-molecule photovoltaic materials are being intensively explored and can be divided into three types: single-DPP, double-DPP, and multi-DPP respectively. The recent progress regarding DPP-based photovoltaic small molecules is highlighted and the photovoltaic performance in relation to structural modification such as the variations of donor–acceptor building blocks, alkyl substitutions, the type of conjugated bridges, and the type of end-capped groups is discussed.

The use of fullerene as acceptor limits the thermal stability of organic solar cells at high temperatures as their diffusion inside the donor leads to phase separation via Ostwald ripening. Here it is reported that fullerene diffusion is fully suppressed at temperatures up to 140 °C in bulk heterojunctions based on the benzodithiophene-based polymer (the poly[[4,8-bis[(2-ethylhexyl)oxy]-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]-thieno[3,4-b]thiophenediyl]], (PTB7) in combination with the fullerene derivative [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₀BM). The blend stability is found independently of the presence of diiodooctane (DIO) used to optimize nanostructuration and in contrast to PTB7 blends using the smaller fullerene derivative PC₇₀BM. The unprecedented thermal stability of PTB7:PC₇₀BM layers is addressed to local minima in the mixing enthalpy of the blend forming stable phases that inhibit fullerene diffusion. Importantly, although the nanoscale morphology of DIO processed blends is thermally stable, corresponding devices show strong performance losses under thermal stress. Only by the use of a high temperature annealing step removing residual DIO from the device, remarkably stable high efficiency solar cells with performance losses less than 10% after a continuous annealing at 140 °C over 3 days are obtained. These results pave the way toward high temperature stable polymer solar cells using fullerene acceptors.

Thermal stability of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]thieno[3,4-b]thiophenediyl]]-based solar cells is found to depend on fullerene size and solvent additive. While [6,6]-phenyl-C71-butyric acid methyl ester diffusion is suppressed inside blends independently of the solvent additive, only removing residual additive by 140 °C annealing leads to solar cells resisting 140 °C over days with performance losses less than 10%.

Organic solar cells are promising in terms of full-solution-processing which enables low-cost and large-scale fabrication. While single-junction solar cells have seen a boost in power conversion efficiency (PCE), multi-junction solar cells are promising to further enhance the PCE. In all-solution-processed multi-junction solar cells, interfacial losses are often encountered between hole-transporting layer (HTL) and the active layers and therefore greatly limit the application of newly developed high-performance donor and acceptor materials in multi-junction solar cells. Here, the authors report on a systematic study of interface losses in both single-junction and multi-junction solar cells based on representative polymer donors and HTLs using electron spectroscopy and time-of-flight secondary ion mass spectrometry. It is found that a facile mixed HTL containing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and MoO _x nanoparticles successfully overcomes the interfacial losses in both single- and multi-junction solar cells based on various active layers by reducing interface protonation, promoting better energy-level alignment, and forming a dense and smooth layer. Solution-processed single-junction solar cells are demonstrated to reach the same performance as with evaporated MoO _x (over 7%). Multi-junction solar cells with polymers containing nitrogen atoms as the first layer and the mixed PEDOT:PSS and MoO _x nanoparticles as hole extraction layer reach fill factor (FF) of over 60%, and PCE of over 8%, while the identical stack with pristine PEDOT:PSS or MoO _x nanoparticles show FF smaller than 50% and PCE less than 5%.

The interface losses in both solution-processed inverted single- and multi-junction solar cells using representative polymer donors and hole-transporting layers (HTLs) are systematically investigated. A facile mixed HTL containing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate and MoO _x nanoparticles successfully overcomes the interfacial losses by reducing interface protonation, promoting better energy-level alignment, and forming a dense and smooth layer.

Advances in the research of graphene in the development of optoelectronic devices have clearly witnessed a strong increase in the past few years. Graphene, a zero bandgap semiconducting material exhibits exceptional properties such as high conductivity, mechanical robustness, optical transparency, flexibility and much more yet to be discovered. Due to its extraordinary properties, graphene is believed to have the potential to replace many traditional electrode materials that are being used in optoelectronic devices. To achieve a high device performance various deposition techniques have been developed to deposit a thin, transparent, and uniform layer of graphene on different substrates. However, the success of these methods strongly relies on the processing conditions, resulting morphology and the work function of the graphene films. This review summarizes the developments in the synthesis and deposition methods of graphene electrodes in combination with organic solar cells over the past 10 years.

The significant increase in graphene research over the past ten years includes the use of graphene in organic solar cells (OSCs). Current challenges and future research directions for graphene electrodes combined with OSCs are discussed, covering the latest developments in deposition methods of graphene electrodes, with an emphasis on manufacturing commercializable graphene electrode-based OSCs.

The origin of photocurrent losses in the power-generating regime of organic solar cells (OSCs) remains a controversial topic, although recent literature suggests that the competition between bimolecular recombination and charge extraction determines the bias dependence of the photocurrent. Here the steady-state recombination dynamics is studied in bulk-heterojunction OSCs with different hole mobilities from short-circuit to maximum power point. It is shown that in this regime, in contrast to previous transient extracted charge and absorption spectroscopy studies, first-order recombination outweighs bimolecular recombination of photogenerated charge carriers. This study demonstrates that the first-order losses increase with decreasing slower carrier mobility, and attributes them to either mobilization of charges trapped at the donor:acceptor interface through the Poole–Frenkel effect, and/or recombination of photogenerated and injected charges. The dependence of both first-order and higher-order losses on the slower carrier mobility explains why the field dependence of OSC efficiencies has historically been attributed to charge-extraction losses.

Fill Factor losses in organic solar cells are decoupled into recombination with first-order and second-order (bimolecular) kinetics. Under steady-state, the former losses dominate the latter from short-circuit to maximum-power point. The first-order photocurrent losses are attributed to geminate charge-transfer state recombination, or recombination of photogenerated and injected charges. Both loss mechanisms can be minimized by increasing the slower carrier mobility.

Ternary polymer solar cells are fabricated based on one donor PBDB-T and two acceptors (a methyl-modified small-molecular acceptor (IT-M) and a bis-adduct of Bis[70]PCBM). A high power conversion efficiency of 12.2% can be achieved. The photovoltaic performance of the ternary polymer solar cells is not sensitive to the composition of the blend.

A synergetic effect of molecular weight (M_n) and fluorine (F) on the performance of all-polymer solar cells (all-PSCs) is comprehensively investigated by tuning the M_n of the acceptor polymer poly((N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′-(2,2′-bithiophene)) (P(NDI2OD-T2)) and the F content of donor polymer poly(2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-dyl-alt-thiophene-2,5-diyl). Both M_n and F variations strongly influence the charge transport properties and morphology of the blend films, which have a significant impact on the photovoltaic performance of all-PSCs. In particular, the effectiveness of high M_n in increasing power conversion efficiency (PCE) can be greatly improved by the devices based on optimum F content, reaching a PCE of 7.31% from the best all-PSC combination. These findings enable us to further understand the working principles of all-PSCs with a view on achieving even higher power conversion efficiency in the future.

To establish a correlation between the effects of molecular weight (M_n) and fluorine (F) on photovoltaic performance, all-polymer solar cells have been comprehensively investigated based on donor polymers of TQ family with various F content and P(NDI2OD-T2) acceptor polymers with various M_n. An efficiency of 7.31% is demonstrated by blending the optimum F content-containing donor with a high M_n acceptor.

Compositional modification and surface treatments of a TiO₂ film prepared by a low-temperature route are carried out by a new promising method. Inverted polymer solar cells incorporating the post-treated TiO₂:TOPD electron-transport layer achieve the highest efficiency of 10.5%, and more importantly, eliminate the light-soaking problem that is commonly observed in metal-oxide-based inverted polymer solar cells.

Morphology control is one of the key strategies in optimizing the performance of organic photovoltaic materials, particularly for diketopyrrolopyrrole (DPP)-based donor polymers. The design of DPP-based polymers that provide high power conversion efficiency (PCE) presents a significant challenge that requires optimization of both energetics and morphology. Herein, a series of high performance, small band gap DPP-based terpolymers are designed via two-step side chain engineering, namely introducing alternating short and long alkyls for reducing the domain spacing and inserting alkylthio for modulating the energy levels. The new DPP-based terpolymers are compared to delineate how the side chain impacts the mesoscale morphology. By employing the alkylthio-substituted terpolymer PBDPP-TS, the new polymer solar cell (PSC) device realizes a good balance of a high V_oc of 0.77 V and a high J_sc over 15 mA cm⁻², and thus realizes desirable PCE in excess of 8% and 9.5% in single junction and tandem PSC devices, respectively. The study indicates better control of domain purity will greatly improve performance of single junction DPP-based PSCs toward 10% efficiency. More significantly, the utility of this stepwise side chain engineering can be readily expanded to other classes of well-defined copolymers and triggers efficiency breakthroughs in novel terpolymers for photovoltaic and related electronic applications.

A new class of small band gap terpolymers featuring diketopyrrolopyrrole unit are designed and their mesoscale morphology are well-correlated with device characteristics. The alkylthio-substituted terpolymer PBDPP-TS exhibits deep highest occupied molecular orbital energy level and optimal domain characteristic length/domain purity, resulting in over 8% and 9.5% efficiency for single junction and tandem polymer solar cells, respectively.

A bimodal texturing effect of semiconducting polymers is investigated by incorporating conjugated small molecules to significantly improve the charge transport characteristics via formation of 3D transport pathways. Solution blending of the electron-transporting polymer, poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)), with small molecular crystals of tetrathiafulvalene and tetracyanoquinodimethane is used, and the thin film microstructures are studied using a combination of atomic force microscopy, transmission electron microscopy, 2D grazing incidence X-ray diffraction, and surface-sensitive near-edge X-ray absorption fine structure. Blended thin films show edge-on and face-on bimodal texture with long-range order and microstructure packing orientation preferable for electron transport through the channel in organic field-effect transistors, which is confirmed by high electron mobility 1.91 cm² V⁻¹ s⁻¹, small contact resistance, and low energetic disorder according to temperature dependence of the field-effect mobility. Structural changes suggest a 3D network charge transport model via lamella packing and bimodal orientation of the semiconducting polymers.

Polymer morphology and molecular orientation are significantly changed by introducing conjugated planar small molecules into semiconducting polymer films. Face-on and edge-on bimodal texture with better long-range arrangement and ordered lamellar stacking enhances efficient charge transport in the polymer semiconductor of organic field-effect transistors.

Organic solar cells (OSCs) are lightweight, have adaptable colors, and can be produced in low-cost procedures on transparent and flexible surfaces. This makes them attractive for markets in which other technologies cannot compete, for example in architectural and consumer product integration. However, both efficiencies and long term operational stability of OSCs do not yet meet the standards set by their inorganic counterparts. This review compiles the growing knowledge about how nanostructured carbon materials, such as fullerenes and carbon nanotubes, decisively influence the operational stability of organic photovoltaics. Firstly, important degradation pathways are introduced and a differential detection scheme is set up to find the dominant loss channel by means of state-of-the-art characterization methods. Then, fullerenes ability to both stabilize and destabilize the donor polymer against photooxidation via different mechanisms (e.g., inner filter effect or radical scavenging) is examined in detail. The “burn-in” problem, an initial rapid efficiency loss in PC₆₀BM-based OSCs, is shown to derive from light-induced PC₆₀BM dimerization, an effect that can also be positively exploited to reduce thermal degradation. Finally, thermal stabilization via additional approaches involving the fullerene derivative, such as crosslinking or incorporation into block copolymers, is presented.

Carbon nanostructures, particularly fullerene derivatives, possess the ability to act as radical scavengers and undergo photodimerization, consequently influencing the photochemical and morphological degradation of the active layer. These properties can be exploited to produce organic solar cells with longer operational lifetimes without compromising device efficiency and manufacturing costs.

Solution-processed organic solar cells with promising photovoltaic performance and extraordinary high thermal stability are achieved by employing novel fullerene-based acceptors in combination with two state-of-the-art polymer donors. The findings demonstrated in this work underline the necessity and importance of novel acceptor design rules for highly efficient organic solar cells with excellent device stability.

In article number 1600637, Alan J. Heeger, Han Young Woo, Jin Young Kim, and co-workers demonstrate high performance ternary blend polymer solar cells including two donor polymers which share the same polymeric backbone. Real charge carrier behavior of ternary blends is firstly clarified via transient absorption spectroscopy as parallel bulk-heterojunction.

Using small amounts of solvent additives during fabrication of organic solar cells has been formerly shown to render a facile approach for improving the performance by tuning the active layer nano-morphology. However, in certain cases, solvent additives are now found to accelerate device degradation. This is reported by Peter Müller-Buschbaum and co-workers in article number 1600712. Cover image by Christoph Hohmann, Nano-systems Initiative Munich (NIM).

o-Chlorobenzaldehyde (CBA) is a derivative of chlorobenzene (CB). The boiling point of CBA is 212 °C, much lower than 332 °C for 1,8-diiodooctane (DIO). With CBA as solvent additive in CB host solvent, CBA could be fast removed during spin-coating 100, 200, and 300 nm thick thieno[3,4-b]thiophene/benzodithiophene polymer (PTB7):[6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) active layers, achieving power conversion efficiencies of 9.11%, 8.24%, and 7.11%, respectively, much higher than 7.53%, 5.71%, and 4.93% for corresponding DIO control devices with vacuum drying.

The work functions for charge transport layers in perovskite solar cells affect device performance significantly. In this work, the regular poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is modified by adding a polymer electrolyte PSS-Na to improve its HTL function in perovskite solar cells. The modified PEDOT:PSS films (called m-PEDOT:PSS) possess higher work function than the regular one. Its energy level matches the valence band of perovskite very well, leading to enhanced V_oc and PCE (power conversion efficiency). When CH₃NH₃PbI₃ is used as the light absorber, the cell with PEDOT:PSS HTL gives a V_oc of 0.96 V and a PCE of 12.35%, while the cell with m-PEDOT:PSS layer gives a V_oc of 1.11 V and a PCE of 15.56%. Enhanced V_oc and PCE are also achieved when CH₃NH₃PbI₂Br or CH₃NH₃PbBr₃ is used as the light absorber. The m-PEDOT:PSS/CH₃NH₃PbBr₃/PC₆₁BM solar cells demonstrate an outstanding V_oc of 1.52 V.

A modified poly(3,4-ethylenedioxythiophene) (PEDOT) layer is developed and used as the HTL for perovskite solar cells, leading to enhanced performance. Using m-PEDOT:PSS (1:2) as the HTL and CH₃NH₃PbI₃ as the light absorber, a V_oc of 1.11 V and a power conversion efficiency of 15.56% are achieved. A V_oc of 1.52 V is obtained from CH₃NH₃PbBr₃ solar cells, which is the highest V_oc for perovskite/PCBM solar cells.

An electroactive polymer poly[4,7-bis(3,6-dihexyloxy-thieno[3,2-b]thiophen-2-yl)]-benzo[c][1,2,5]thiadiazole is synthesized and applied as a color-changing electrode material in supercapacitors via spray coating method. Using poly(3,4-ethylenedioxythiophene) as the negative electrode, the all-polymer asymmetric supercapacitors provide an energy density of 3.5–6.3 W h kg⁻¹ and power density of 0.6–8.8 kW kg⁻¹.

On page 6876, T.-H. Kwon, D. H. Ryu, and co-workers present indoline based sensitizers with a planar geometry and high molar extinction coefficient. Using a very thin active layer (1.8 μm) with an iodine electrolyte, a power conversion efficiency of 9.1% is achieved. On the cover two types of indoline based sensitizers that have different alkyl chain length are represented.