Search

Highly performing mixed Sn/Pb-metal halide perovskite solar cells (PSCs) are among the most promising options to reduce Pb content in perovskite devices and enable, owing to their reduced bandgap, the fabrication of all-perovskite tandem solar cells. Whereas pure-Pb perovskite devices exhibit efficiency up to 25.5%, alongside a high open-circuit voltage (≈1.2 V), Sn-Pb PSCs still show lower performances (22.2%) due to higher open-circuit voltage losses. Here, we introduced 2,3,4,5,6-pentafluorophenethylammonium cations in a perovskite active layer of composition (FASnI3)0.5(MAPbI3)0.5 to obtain highly oriented films with improved thermal stability. The treated films exhibit merged grains with no evidence of 2D structures, which could help to reduce the trap state density at the surface and grain boundaries. Solar cells fabricated with the fluorinated cation added to the active layer displayed reduced trap-assisted recombination losses and lower background carrier density, which leads to enhanced open-circuit voltages with respect to the reference samples and the active layers incorporating unfluorinated phenethylammonium cations. The best device reached an efficiency of 19.13%, with an open-circuit voltage of 0.84 V, which is substantially improved with respect to the reference sample showing 17.47% efficiency and 0.77 V open-circuit voltage. More importantly, the fluorinated cations' addition is instrumental to improve the device's thermal stability; 90.3% of the solar cell initial efficiency is maintained after 90 min of thermal stress at 85 °C in a nitrogen atmosphere. [ABSTRACT FROM AUTHOR]

This study broadens the family of 2D layered perovskites by demonstrating that it is possible to self-assemble organic charge-transfer complexes in their organic layer. Organic charge-transfer complexes, formed by combining charge-donating and charge-accepting molecules, are a diverse class of materials that can possess exceptional optical and electronic properties. [ABSTRACT FROM AUTHOR]

Organic photovoltaics (OPV) have attracted great interest as a solar cell technology with appealing mechanical, aesthetical, and economies-of-scale features. To drive OPV toward economic viability, low-cost, large-scale module production has to be realized in combination with increased top-quality material availability and minimal batch-to-batch variation. To this extent, continuous flow chemistry can serve as a powerful tool. In this contribution, a flow protocol is optimized for the high performance benzodithiophene-thienopyrroledione copolymer PBDTTPD and the material quality is probed through systematic solar-cell evaluation. A stepwise approach is adopted to turn the batch process into a reproducible and scalable continuous flow procedure. Solar cell devices fabricated using the obtained polymer batches deliver an average power conversion efficiency of 7.2 %. Upon incorporation of an ionic polythiophene-based cathodic interlayer, the photovoltaic performance could be enhanced to a maximum efficiency of 9.1 %. [ABSTRACT FROM AUTHOR]

Organic photovoltaics (OPV) represent a thin-film PV technology that offers attractive prospects for low-cost and aesthetically appealing (colored, flexible, uniform, semitransparent) solar cells that are printable on large surfaces. In bulk heterojunction (BHJ) OPV devices, organic electron donor and acceptor molecules are intimately mixed within the photoactive layer. Since 2005, the power conversion efficiency of said devices has increased substantially due to insights in the underlying physical processes, device optimization, and chemical engineering of a vast number of novel light-harvesting organic materials, either small molecules or conjugated polymers. As Nature itself has developed porphyrin chromophores for solar light to energy conversion, it seems reasonable to pursue artificial systems based on the same types of molecules. Porphyrins and their analogues have already been successfully implemented in certain device types, notably in dye-sensitized solar cells, but they have remained largely unexplored in BHJ organic solar cells. Very recent successes do show, however, the strong (latent) prospects of porphyrinoid semiconductors as light-harvesting and charge transporting materials in such devices. Here, an overview on the state-of-the-art of porphyrin-based solution-processed BHJ OPV is provided and insights are given into the pathways to follow and hurdles to overcome toward further improvements of porphyrinic materials and devices. [ABSTRACT FROM AUTHOR]

Recently, the efficiency of p‐i‐n perovskite solar cells drastically increased, a pivotal factor being the incorporation of self‐assembled monolayers (SAMs) as a hole‐transporting layer (HTL). SAMs offer many advantages over conventional HTLs, including minimal material requirements, low cost, and facile processing. Current research is mainly focused on the development of carbazole‐derived SAMs. However, the versatility of organic chemistry allows for the design of SAMs with alternative organic cores that may possess specific benefits. In this study, three novel SAMs are incorporated in p‐i‐n perovskite solar cells, each based on an aromatic core commonly used in organic semiconductors. The novel SAMs vary in their energy level alignment with the perovskite active layer. Optimal alignment is achieved with a pyrene‐based SAM (4PAPyr), resulting in solar cells that outperform the commercially available 2PACz. Moreover, due to improved surface coverage, the use of 4PAPyr leads to a significantly higher number of working solar cell devices when compared to 2PACz, which is of particular interest with regard to upscaling. After device optimization, a power conversion efficiency of 22.2% is achieved with 4PAPyr. This research underlines the importance of diversifying SAMs to unlock further advancements in perovskite solar cell efficiency and scalability. [ABSTRACT FROM AUTHOR]

A significant number of organic electronic devices rely on blends of electron-donating and electron-accepting molecules. In bulk heterojunction organic photovoltaics, the nanoscopic phase behavior of the two individual components within the photoactive layer has a major impact on the charge separation and charge transport properties. For polymer:fullerene solar cells, it has been hypothesized that an increased accessibility of the electron-deficient monomer unit in push-pull type low bandgap polymers allows for fullerene 'docking'. The close proximity of electron donor and acceptor molecules enables more efficient charge transfer, which is beneficial for the device efficiency. With this in mind, we synthesized a series of PBDTTPD [poly(benzodithiophene-thienopyrroledione)] low bandgap copolymers with varying side chains. Solar cells were fabricated for all polymers and the device characteristics were compared. The combination of proton wideline solid-state NMR (ssNMR) relaxometry and sensitive external quantum efficiency (sEQE) measurements was shown to provide essential information on donor-acceptor interactions and phase separation in bulk heterojunction organic photovoltaics. The reduced charge transfer state absorption and the observed phase separation of crystalline PC 71 BM domains for the polymers containing the most accessible methyl-TPD unit indicate a diminished contact between donor and acceptor, leading to a loss in performance. Image 1 • PBDTTPD low bandgap copolymers are synthesized. • The polymers are applied in bulk heterojunction organic solar cells. • Steric control of the fullerene approach is realized through side chain variation. • Efficiencies go down when introducing short methyl (TPD) side chains. • ssNMR relaxometry and sEQE give detailed insights on the nanoscale organization. [ABSTRACT FROM AUTHOR]

Abstract Fullerene-free organic photovoltaics have recently reached impressive power conversion efficiencies above 14% for single junctions, increasing their competitiveness with respect to alternative thin-film technologies. In most record devices, electron-donating conjugated polymers are combined with novel generation small molecule acceptors. All-polymer organic solar cells, on the other hand, still lag behind in efficiency, although they have specific advantages in terms of ink formulation and long-term operational stability. Another point of attention is the synthetic complexity of the active layer materials, notably on the side of the new acceptor molecules. Therefore, the present study focuses on the implementation of the stable and cost-effective perylene diimide structure as the key component of high-performance electron-accepting polymers. The synthesis, structural and optoelectronic characterization of four push-pull type copolymers containing the electron-deficient bis(perylene diimide) (bis-PDI) unit is reported, as well as the photovoltaic analysis of these acceptor materials in combination with a well-known donor polymer (PTB7-Th). The acceptor polymers differ in the electron-rich part of the alternating push-pull structure and their solar cell power conversion efficiencies range from 3.2 to 4.7%. The maximum efficiency - the best performance achieved with a bis-PDI polymer so far - is obtained for the structurally most simple polymer, containing merely thiophene as the electron-rich building block. Controlled degradation under blue light in air is monitored by the bleaching of the relevant UV–Vis absorption bands, demonstrating high stability for the bis-PDI-thiophene containing polymers as compared to some prototype small molecule acceptors (FBR and ITIC). Graphical abstract Image 1 Highlights • A series of electron acceptor polymers based on bis(perylene diimide) is prepared. • All-polymer solar cells are fabricated in combination with PTB7-Th. • The best efficiency (4.7%) is obtained for the structurally most simple polymer. • The acceptor polymers have a low synthetic complexity and high photostability. [ABSTRACT FROM AUTHOR]

Porphyrinoid small molecules can be used as electron donor or acceptor components in bulk heterojunction organic solar cells, which has resulted in steadily improving power conversion efficiencies. However, the effect of material purity is often neglected. In this work, a series of D1-A-π-D2-π-A-D1 conjugated chromophores based on the A 2 B 2 - meso- ethynylporphyrin core is synthesized. Different electron-donating (D1) end groups are chosen to investigate their influence on the material properties and optoelectronic characteristics. The porphyrin small molecules are tested as electron donor materials in bulk heterojunction organic solar cells in combination with PC 71 BM, affording an efficiency up to 4.6% under standard illumination. Furthermore, it is shown that (Glaser type) homocoupled side products are generated during the final Sonogashira cross-coupling reactions, which can be very challenging to remove. Their presence reduces solar cell performance by affecting the open-circuit voltage and fill factor. Image 1 • Efforts to increase our understanding of the effect of homocoupling on photovoltaic performance are presented • D1-A-π-D2-π-A-D1 conjugated chromophores based on an A 2 B 2 - meso- ethynylporphyrin core are synthesized • Power conversion efficiencies up to 4.6% are achieved • The presence of (Glaser type) homocoupled side products mainly reduces the open-circuit voltage and fill factor [ABSTRACT FROM AUTHOR]

Extensive research on organic photovoltaics has granted impressive power conversion efficiencies, nowadays exceeding 13% for state-of-the-art photoactive material combinations. Nevertheless, different strategies can be adopted to further enhance the efficiency and the competitiveness with alternative photovoltaic technologies. Conjugated polyelectrolytes have been applied as anode or cathode interlayers to optimize ohmic contacts and lower the contact resistance, thereby improving the ultimate device efficiency. Here, we present an interlayer material belonging to the emerging class of narrow bandgap conjugated polyelectrolytes, based on an imidazolium functionalized 4 H -cyclopenta[2,1- b :3,4- b ']dithiophene (CPDT) as the electron-rich polymer building block and 4 H -thieno[3,4- c ]pyrrole-4,6(5 H )-dione (TPD) as the electron-deficient subunit. The ionic polymer is applied as cathode interlayer for PBDTTPD:[70]PCBM (poly[bis(2′-ethylhexyloxy)benzo[1,2- b :4,5- b ′]dithiophene- alt - N -octylthieno[3,4- c ]pyrrole-4,6-dione]:[6,6]-phenyl-C 71 -butyric acid methyl ester) bulk heterojunction polymer solar cells, improving the overall device performance from 6.9 to 7.8%. [ABSTRACT FROM AUTHOR]

The incorporation of conjugated polyelectrolytes as cathode interlayers in organic photovoltaics has been proven to be an effective way to boost the device efficiency. Nevertheless, more detailed investigations of the structure–property relationships of these interlayer materials, in particular related to the film deposition behavior, can provide further insights into their mode of action. With this aim, a series of ionic (co)polythiophenes is successfully synthesized via Kumada catalyst-transfer condensation polymerization and subsequent introduction of ionic moieties on the polymer side chains. Both the topology (i.e. homopolymers, random and block copolymers) and the amount of ionic groups are systematically varied. The polymers are fully characterized and then applied as cathode interlayers in polymer solar cells based on PCDTBT:PC 71 BM, affording an average efficiency increase of ∼15%. The structural screening on one hand indicates that the efficiency gain is a rather general phenomenon for this material class. On the other hand, the best photovoltaic responses are observed for the conjugated polyelectrolytes with a higher triethylene glycol side chain ratio and the block copolymer structure performs slightly better as compared to the random copolymer with the same (50/50) monomer ratio. Based on these findings, the field can move on to a more rational development of novel interfacial materials and thereby push the device efficiency even further. [ABSTRACT FROM AUTHOR]

Ternary organic photovoltaic devices were prepared through the addition of small amounts of metalloporphyin-terthiophenes to two established low bandgap polymer:fullerene blends. The methodology afforded a PCDTBT:PC 71 BM-based device that displayed an initial power conversion efficiency of 5.1%, an increase of 16% when compared to the binary blend. Among the range of metalloporphyrins considered in this study, the Cu(II)-porphyrin derivatives were found to result in the most significant efficiency improvements. [ABSTRACT FROM AUTHOR]

Theperformance of polymer solar cells is strongly dependent onthe morphology of the photoactive layer, which can be optimized bytuning the polymer side chain pattern. Whereas most studies focuson length and bulkiness, the side chain density receives much lessattention. In this work, the effect of the number of side chains onPCPDTQx(2F) low bandgap copolymers on material properties and solarcell characteristics is investigated. The active layer morphologyis strongly affected, affording more favorable finely intermixed blendswhen decreasing the side chain density. As a result, the efficiencyincreases to a maximum of 5.63% for the device based on the copolymerwith intermediate side chain density. Moreover, removal of the sidechains also has a positive effect on device stability under prolongedthermal stress. A single structural parameteralkyl side chaindensityis hence used for simultaneous enhancement of bothsolar cell efficiency and lifetime. [ABSTRACT FROM AUTHOR]

The molecular weight of poly(3-hexylthiophene) is an important factor influencing the photovoltaic properties of bulk heterojunction organic solar cells based on this material. However, since different synthetic processes or repetitive soxhlet extractions – generally applied to obtain the different molecular weight batches under study – result in samples with simultaneously varying regioregularity ( RR ) and polydispersity index ( PDI ), it has not been possible yet to find an unambiguous correlation between the molecular weight and the photovoltaic performance. In the present work preparative gel permeation chromatography is introduced as a versatile technique to fractionate the donor polymer and thereby obtain a systematic variation of the number average molecular weight ( M n = 11–91 kg mol −1 ) with an almost constant PDI and RR . Polymer crystallinity and conjugation length are evaluated by UV–Vis spectroscopy, rapid heat-cool calorimetry and selected area electron diffraction, and are found to be deeply affected by M n . This in turn influences the behavior of the charge transfer state energy, measured via Fourier transform photocurrent spectroscopy, and therefore the open-circuit voltage. The short-circuit current is also affected by M n , but mainly due to a change in absorption coefficient. The apparent recombination order is shown to be linked to the morphology of the polymer:fullerene blend and is determined using transient photovoltage and photocurrent techniques. Finally, a correlation between recombination and fill factor is also suggested. [ABSTRACT FROM AUTHOR]

Three distinct low bandgap copolymers are synthesized by the combination of N -(2′-propylpentanoyl)dithieno[3,2- b :2′,3′- d ]pyrrole (DTP) and (fluorinated) 2,3-bis[5′-(2”-ethylhexyl)thiophen-2′-yl]quinoxaline (Qx) and these PDTPQx derivatives are investigated as electron donor materials in bulk heterojunction polymer solar cells. Due to the DTP N-acylation and the introduction of the Qx units, both the open-circuit voltage (Voc) and the short-circuit current density (Jsc) increase compared to previous devices based on DTP-type donor polymers. Organic solar cells with an average Voc of 0.67 V, a Jsc of 12.57 mA/cm² and a fill factor of 0.54 are obtained, affording a power conversion efficiency of 4.53% (4.81% for the top-performing device), a record value for (N-acyl-)DTP-based polymer solar cells devoid of special interlayer materials. Despite further enhancement of the Voc, the solar cell efficiency declines for the fluorinated PDTPQx copolymers because of the inability to achieve a finely intermixed bulk heterojunction blend nanomorphology. [ABSTRACT FROM AUTHOR]

Abstract: Detailed optimization of the device architecture of bulk heterojunction organic solar cells is of crucial importance when eventually targeting commercial applications. It has been shown before that the efficiency of such devices can be increased significantly upon incorporation of an imidazolium-substituted polythiophene interlayer. The cause of this increase in efficiency was, however, still unclear and is investigated here by a combination of PeakForce Quantitative Nanomechanical Mapping, PeakForce TUNA and Kelvin Probe Force Microscopy. A link between the local morphological and electrical properties is established. We show that the conjugated polyelectrolyte interlayer acts as a hole blocking layer. Additionally, illuminated Kelvin Probe Force Microscopy indicates that the interlayer creates an extra built-in electric field promoting charge transfer from the bulk heterojunction active layer into the interlayer. [Copyright &y& Elsevier]

Highlights: [•] Functionalized random copolythiophenes are synthesized via an optimized Rieke zinc protocol. [•] Solar cell efficiencies are similar to P3HT:PC61BM for low functionalization degrees (5–15%). [•] The intrinsic stability of the bulk heterojunction active layer blends enhances significantly. [•] Side chain functionalization is a viable approach toward OPV devices with longer lifetimes. [ABSTRACT FROM AUTHOR]

Abstract: An optimized synthetic protocol toward highly reactive Rieke zinc is applied for the preparation of a series of ω-(2,5-dibromothiophene-3-yl)alkyl acetate monomers and statistical copolythiophenes derived thereof, with 10% of ester-functionalized side chains. The obtained conjugated polymers are attractive electron donor materials for organic solar cells, notably to increase the thermodynamic stability of the bulk heterojunction polymer:fullerene active layer blend, and are fully characterized by a combination of spectroscopic, thermal analysis and electrochemical techniques. [Copyright &y& Elsevier]

Abstract: P3HT:PCBM blends applied as active layers for bulk heterojunction organic solar cells generally show unstable morphologies upon prolonged thermal annealing, severely limiting the lifetime of the devices. As such, the thermodynamic instability of the blend is a limiting factor in the overall performance of organic photovoltaics, and a strong disadvantage in the fierce competition with other photovoltaic technologies. This paper shows whether different blend preparation conditions and intrinsic structural changes in the side chains of poly(3-alkylthiophene) (P3AT) derivatives can influence the thermal stability of the resulting solar cells. A combination of Bright Field Transmission Electron Microscopy (BFTEM) and the analysis of Selected Area Electron Diffraction (SAED) patterns revealed that the investigated preparation conditions do not really affect the thermal stability, whereas the introduction of a small ratio (10%) of specific functional moieties in the side chains of random P3AT copolymers does improve the thermal stability significantly. It was demonstrated that demixing of the blend components upon prolonged thermal annealing is strongly delayed in the functionalized P3AT:PCBM blends. The enhanced thermal stability was confirmed by in-situ monitoring of the short circuit current of organic solar cells based on the respective active layers. The introduction of functionalized side chains hence represents an attractive approach to increase the operational stability of organic photovoltaics based on the bulk heterojunction concept. [Copyright &y& Elsevier]

Abstract: The introduction of functional moieties in the donor polymer (side chains) offers a potential pathway toward selective modification of the nanomorphology of conjugated polymer:fullerene active layer blends applied in bulk heterojunction organic photovoltaics, pursuing morphology control and solar cell stability. For this purpose, two types of poly(3-alkylthiophene) random copolymers, incorporating different amounts (10/30/50%) of ester-functionalized side chains, were efficiently synthesized using the Rieke method. The solar cell performance of the functionalized copolymers was evaluated and compared to the pristine P3HT:PCBM system. It was observed that the physicochemical and opto-electronic characteristics of the polythiophene donor material can be modified to a certain extent via copolymerization without (too much) jeopardizing the OPV efficiency, as far as the functionalized side chains are introduced in a moderate ratio (<30%) and that preference is given to side chains with a small molar volume. A range of complementary techniques – UV–Vis spectroscopy, (modulated temperature) differential scanning calorimetry, transmission electron microscopy and X-ray diffraction analysis – indicated that variations in polymer crystallinity, while maintaining a high level of regioregularity, are probably the main factor responsible for the observed differences. [Copyright &y& Elsevier]

Abstract: The influence of octanedithiol (ODT) on the film morphology of PCPDTBT:PCBM (1:2) blends processed from solutions in chlorobenzene was studied by chemical shift selective carbon-13 solid-state NMR relaxation experiments. A close contact between PCBM and PCPDTBT is demonstrated for PCPDTBT:PCBM films processed with as well as without ODT, i.e. the dimensions of the substructures in the PCBM rich and polymer rich domains are smaller than 10nm. The PCBM organization is shown to be reduced towards very small and weakly organized clusters. The main difference of processing the active film in the presence of ODT can be found in the ability of the polymer to improve its chain packing towards a state that closely resembles this of pure PCPDTBT. This difference in physical interaction between the polymer chains can explain the improved power conversion efficiency and the EQE red-shift of solar cells processed with ODT. [Copyright &y& Elsevier]

Abstract: Within the field of organic bulk heterojunction solar cells, the morphology of the active layer has a key role in obtaining high power conversion efficiencies. P3HT nanofibers, obtained in highly concentrated solutions, are able to give controlled morphologies directly upon deposition. Since the solar cell efficiency of fiber solar cells depends on the fiber content of the casting solution, it is important to control this parameter. Here, we demonstrate an easy way to control the fiber content in the casting solution, i.e. changing the solution temperature. By using solution heating, the overall molecular weight of the polymer in the blend is kept constant, fiber isolation is not needed and the use of solvent mixtures is avoided. The obtained optimal power conversion efficiency is shown to be linked to the morphology of the active layer, which is studied with Transmission Electron Microscopy (TEM). [Copyright &y& Elsevier]

Abstract: We report on the comparison of photophysical and photovoltaic properties of three different soluble alkyl derivatives of the low bandgap poly(2,5-thienylene vinylene) (PTV), synthesized using the dithiocarbamate precursor route. The solubility of the precursor material in dichlorobenzene is enhanced by the addition of hexyl, dihexyl and dodecyl sidegroups to the polymer chain. The materials were characterized in solid state by means of absorption, ellipsometry and atomic force microscopy of films made from both the pristine alkyl-PTVs and alkyl-PTVs:([6,6]-phenyl C61-butyric acid methyl ester) (PCBM) mixtures in a 1:1 ratio. The materials showed an optical bandgap below 1.7eV, derived from the absorption spectrum of the polymers. Field-effect transistors made of these materials showed hole mobilities in the range of 10−7 to 10−6 cm2/Vs. Bulk heterojunction solar cells made with the polymer:PCBM blend reached efficiencies above 0.6%. [Copyright &y& Elsevier]

Abstract: In high performance polymer:fullerene bulk heterojunction solar cells the nanoscale morphology of interpenetrating acceptor:donor materials is optimized through appropriate preparation conditions such as annealing and choice of solvent, but this initial state-of-the-art morphology will not remain stable during long-term operation. We report the effects of prolonged storage at elevated temperatures on both the morphology and the photovoltaic performance for the model systems MDMO-PPV:PCBM and P3HT:PCBM as compared to ‘High T g PPV’:PCBM based solar cells, where the ‘High T g PPV’ is characterized by its high glass transition temperature (138°C). In situ monitoring of the photocurrent–voltage characteristics at elevated temperatures, in combination with a systematic transmission electron microscopy (TEM) study and complementary optical spectroscopy, reveals distinct degradation kinetics and morphological changes that indicate the occurrence of different underlying physico-chemical mechanisms. [Copyright &y& Elsevier]

Abstract: A new approach is presented in order to improve the thermal stability of polymer: [6-6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction solar cells. The central idea in this approach is the use of a polymer with high glass transition temperature (T g), well above the normal operating temperatures of the devices. In this paper, a PPV-derivative with a T g of 150°C was used as an electron donor and the thermal stability of the obtained solar cells was compared with solar cells based on the reference material poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) with a T g of 45°C. The use of the material with higher glass transition temperature resulted in a significant improvement of the thermal stability of the photovoltaic parameters. Furthermore, a systematic transmission electron microscope (TEM) study demonstrates that the better thermal stability of performance coincides with a more stable active layer morphology. Both improvements are attributed to the reduced free movement of the electron donor material (PCBM) within the active layer of the solar cell. [Copyright &y& Elsevier]

Abstract: Photophysical studies and photovoltaic devices on a low bandgap, high-charge carrier mobility poly(thienylene vinylene) (PTV), prepared from a soluble precursor polymer synthesised via the “dithiocarbamate route”, are reported. In composites with an electron acceptor ([6,6]-phenyl C61-butyric acid methyl ester (PCBM), a soluble fullerene derivative), photoinduced absorption characteristic for charged excitations together with photoluminescence quenching are observed indicating photoinduced electron transfer. The “bulk heterojunction” photovoltaic devices using PTV and PCBM composites show short-circuit currents up to 4mA/cm2 under AM 1.5 white-light illumination. The photocurrent spectrum of the photovoltaic device shows an onset about 1.65eV (750nm), which corresponds to the absorption spectrum of the polymer. [Copyright &y& Elsevier]

An analytical model has been developed to estimate the short-circuit current density of conjugated polymer/fullerene bulk heterojunction solar cells. The model takes into account the solvent-dependent molecular morphology of the donor/acceptor blend, which was revealed by transmission electron microscopy. Field-effect transistors based on single and composite organic layers were fabricated to determine values for the charge carrier mobilities of such films. These values served as input parameters of the model. It is shown that the difference in short-circuit current density that was measured between toluene-cast and chlorobenzene-cast conjugated polymer/fullerene photovoltaic cells (Appl. Phys. Lett. 78 (2001) 841) could be very well simulated with the model. Moreover, the calculations illustrate how increasing the hole and electron mobilities in the photoactive blend can improve the overall short-circuit current density of the solar cell. [Copyright &y& Elsevier]

The rapid advancement in the development of non-fullerene acceptors has led to single-junction polymer solar cells with efficiencies over 18%. Even with these novel acceptor materials, the choice of the donor polymer remains important. Tuning of the donor and acceptor compatibility in terms of absorption, frontier orbital energy levels, mixing enthalpy and charge carrier mobility is routinely performed by side chain variation. Fluorination presents an additional powerful approach to optimize these parameters. Although significantly less studied, chlorination can give rise to similar effects, while donor-acceptor phase separation due to fluorophobic interactions is less of an issue. Moreover, from a material synthesis point of view, the introduction of chlorine groups is in many cases much more straightforward. In this work, we present a series of push-pull type benzo[1,2- b :4,5- b ']dithiophene- alt -quinoxaline donor polymers and compare the behavior of the non-halogenated, fluorinated and chlorinated derivatives in polymer solar cells when combined with small molecule and polymer type non-fullerene acceptors. The solar cell efficiencies vary from 2.4 to 8.4%, elucidating the large impact of these small structural variations. Best results are achieved for the chlorinated donor polymer, affording a high open-circuit voltage, balanced charge carrier mobilities and favorable donor-acceptor interactions. Combined with the easier synthesis of chlorinated materials, this suggests that more emphasis should be put on chlorination as a valuable approach to tune the properties of organic semiconductors for solar cell blends (and other optoelectronic applications). Image 1 • Benzo[1,2- b :4,5- b ']dithiophene- alt -quinoxaline push-pull copolymers are synthesized. • The effect of backbone halogenation (fluorination vs chlorination) is studied. • Organic solar cells are prepared from the donor polymers and non-fullerene acceptors. • Best results are achieved for the chlorinated polymer (affording 8.4% efficiency). • Chlorination is an undervalued approach to tune organic semiconductor properties. [ABSTRACT FROM AUTHOR]