On the Effect of Prevalent Carbazole Homocoupling Defects on the Photovoltaic Performance of PCDTBT:PC71BM Solar Cells

The photophysical properties and solar cell performance of the classical donor–acceptor copolymer PCDTBT (poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt ‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole))) in relation to unintentionally formed main chain defects are investigated. Carbazole–carbazole homocouplings (Cbz hc) are found to significant extent in PCDTBT made with a variety of Suzuki polycondensation conditions. Cbz hc vary between 0 and 8 mol% depending on the synthetic protocol used, and are quantified by detailed nuclear magnetic resonance spectroscopy including model compounds, which allows to establish a calibration curve from optical spectroscopy. The results are corroborated by extended time‐dependent density functional theory investigations on the structural, electronic, and optical properties of regularly alternating and homocoupled chains. The photovoltaic properties of PCDTBT:fullerene blend solar cells significantly depend on the Cbz hc content for constant molecular weight, whereby an increasing amount of Cbz hc leads to strongly decreased short circuit currents J_SC. With increasing Cbz hc content, J_SC decreases more strongly than the intensity of the low energy absorption band, suggesting that small losses in absorption cannot explain the decrease in J_SC alone, rather than combined effects of a more localized LUMO level on the TBT unit and lower hole mobilities found in highly defective samples. Homocoupling‐free PCDTBT with optimized molecular weight yields the highest efficiency up to 7.2% without extensive optimization.     

Impact of UV‐Visible Light on the Morphological and Photochemical Behavior of a Low‐Bandgap Poly(2,7‐Carbazole) Derivative for Use in High‐Performance Solar Cells

This paper reports on the photochemical behavior upon exposure to UV‐visible light of a poly(2,7‐carbazole) derivative for use in high‐performance solar cells. Poly[N‐9′‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) is one of a relatively large class of push‐pull carbazole‐based copolymers that have been synthesized to better harvest the solar spectrum. The 2,7‐carbazole building block of PCDTBT is also used with different electron‐accepting units in a large variety of low‐band‐gap polymers. The photochemical and morphological behavior of PCDTBT thin films is investigated from the molecular scale to the nanomechanical properties. The photo‐oxidation mechanism is shown to be governed by chain‐scission and cross‐linking reactions. It results in dramatic evolution of the morphology, roughness and stiffness of thin PCDTBT films. Based on the identification of several photoproducts formed along the macromolecular chains or released into the gas phase, the main pathways of PCDTBT photochemical evolution are discussed. These processes first involve the scission of the C–N bond between the carbazole group and the tertiary carbon atom bearing the alkyl side‐chain. Modifications of the chemical structure of PCDTBT, the evolution of its UV‐visible absorbance, and its nanomechanical properties initiated by light irradiation are shown to be closely related.     

How Photoinduced Crosslinking Under Operating Conditions Can Reduce PCDTBT‐Based Solar Cell Efficiency and then Stabilize It

Bulk heterojunction (BHJ) photovoltaic devices made of PCDTBT (poly[N‐9′‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]) and PC₇₀BM ([6,6]‐phenyl‐C₇₀‐butyric acid methyl ester) are among the most efficient and stable devices studied so far. However, during a short regime called “burn‐in”, a significant decrease of power conversion efficiency was observed. A study of the photochemical mechanisms involved in the PCDTBT:PCBM active layer exposed to light in encapsulated systems is presented. It is found that the photochemical reactions resulting from the absorption of light by PCDTBT involve crosslinking between the 2,7 carbazole unit of PCDTBT and the fullerene unit of PCBM. Those reactions stabilize the BHJ by avoiding the formation of microsized PCBM crystals known to cause failure of BHJ solar cells. Using classical electron paramagnetic resonance spectroscopy (EPR) (without illumination), paramagnetic defects along the polymer chains have been detected. The kinetics of defects intensity show a burn‐in trend. The evolution of their relaxation times upon aging is in good agreement with a structural change (crosslinking) of the BHJ observed from the nanomechanical properties. Finally, light‐induced electron paramagnetic resonance (LEPR) measurements performed on aged samples revealed that electron transfer is not significantly affected upon aging, confirming thus the stabilization of the BHJ in solar cell operating conditions.     

Balanced Carrier Mobilities: Not a Necessary Condition for High‐Efficiency Thin Organic Solar Cells as Determined by MIS‐CELIV

A novel technique based upon injection‐charge extraction by linearly increasing voltage (i‐CELIV) in a metal‐insulator‐semiconductor (MIS) diode structure is described for studying charge transport in organic semiconductors. The technique (MIS‐CELIV) allows selective measurement of both electron and hole mobilities of organic solar cells with active layers thicknesses representative of operational devices. The method is used to study the model high efficiency bulk heterojunction combination poly[N‐9′′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) and [6,6]‐phenyl C70‐butyric acid methyl ester (PC70BM) at various blend ratios. The absence of bipolar transport in PCDTBT‐and‐PC70BM‐only diodes is shown and strongly imbalanced carrier mobility is found in the most efficient “optimized” blend ratios. The mobility measurements are correlated with overall device performance and it is found that balanced and high charge carrier mobility are not necessarily required for high efficiencies in thin film organic solar cells.     

Morphology‐Dependent Trap Formation in High Performance Polymer Bulk Heterojunction Solar Cells

Bulk heterojunction solar cells (BHJs) based on poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) can have internal quantum efficiencies approaching 100% but require active layers that are too thin to absorb more than ～70% of the above band gap light. When the active layer thickness is increased so that the cell absorbs more light, the fill factor and open circuit voltage decrease rapidly, so that the overall power conversion efficiency decreases. We find that hole‐traps in the polymer, which we characterize using space‐charge limited current measurements, play an important role in the performance of PCDTBT‐based BHJs and may limit the active layer thickness. Recombination due to carrier trapping is not often considered in BHJs because it is not believed to be a dominant loss mechanism in the “fruit‐fly” P3HT system. Furthermore, we show that in contrast to P3HT, PCDTBT has only weak short‐range molecular order, and that annealing at temperatures above the glass transition decreases the order in the π–π stacking. The decrease in structural order is matched by the movement of hole‐traps deeper into the band gap, so that thermal annealing worsens hole transport in the polymer and reduces the efficiency of PCDTBT‐based BHJs. These findings suggest that P3HT is not prototypical of the new class of high efficiency polymers, and that further improvement of BHJ efficiencies will necessitate the study of high efficiency polymers with low structural order.     

Inverted Semi‐transparent Polymer Solar Cells with Transparency Color Rendering Indices approaching 100

Window‐ or building‐integrated semi‐transparent solar cells are particularly interesting applications for organic photovoltaic devices. In this work, we present an easy‐to‐process inverted device architecture comprising fully solution processable poly(3,4‐ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) bilayer top‐electrodes for efficient semi‐transparent organic solar cells. By incorporating dyes with a complementary absorption to the light harvesting polymer poly[[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl] (PCDTBT) into the PEDOT:PSS electrode, we achieve fully color neutral transparency perception and a color rendering index approaching 100. This makes the devices suitable for applications such as window shadowing or the integration into overhead glazing.     

Investigating the Influence of Interfacial Contact Properties on Open Circuit Voltages in Organic Photovoltaic Performance: Work Function Versus Selectivity

The role of work function and thermodynamic selectivity of hole collecting contacts on the origin of open circuit voltage (V_OC) in bulk heterojunction organic photovoltaics is examined for poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and [6,6]‐phenyl‐C₇₁ butyric acid methyl ester (PC₇₁BM) solar cells. In the absence of a charge selective, electron blocking contact, systematic variation of the work function of the contact directly dictates the V_OC, as defined by the energetic separation between the relative Fermi levels for holes and electrons, with little change in the observed dark saturation current, J₀. Improving the charge selectivity of the contact through an increased barrier to electron injection from the fullerene in the blend into the hole contact results in a decreased reverse saturation current (decreased J₀ and increased shunt resistance, R_SH) and improved V_OC. Based on these observations, we provide a set of contact design criteria for tuning the V_OC in bulk heterojunction organic photovoltaics.     

Batch‐to‐Batch Variation of Polymeric Photovoltaic Materials: its Origin and Impacts on Charge Carrier Transport and Device Performances

A detailed investigation of the impact of molecular weight distribution of a photoactive polymer, poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT), on photovoltaic device performance and carrier transport properties is reported. It is found that different batches of as‐received polymers have substantial differences in their molecular weight distribution. As revealed by gel permeation chromatography (GPC), two peaks can generally be observed. One of the peaks corresponds to a high molecular weight component and the other peak corresponds to a low molecular weight component. Photovoltaic devices fabricated with a higher proportion of low molecular weight component have power conversion efficiencies (PCEs) reduced from 5.7% to 2.5%. The corresponding charge carrier mobility at the short‐circuit region is also significantly reduced from 2.7 × 10^?5 to 1.6 × 10^?8 cm² V^?1 s^?1. The carrier transport properties of the polymers at various temperatures are further analyzed by the Gaussian disorder model (GDM). All polymers have similar energetic disorders. However, they appear to have significant differences in carrier hopping distances. This result provides insight into the origin of the molecular weight effect on carrier transport in polymeric semiconducting materials.     

Plasmonic Backscattering Effect in High‐Efficient Organic Photovoltaic Devices

A universal strategy for efficient light trapping through the incorporation of gold nanorods on the electron transport layer (rear) of organic photovoltaic devices is demonstrated. Utilizing the photons that are transmitted through the active layer of a bulk heterojunction photovoltaic device and would otherwise be lost, a significant enhancement in power conversion efficiency (PCE) of poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]:phenyl‐C₇₁‐butyric acid methyl ester (PCDTBT:PC₇₁BM) and 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):PC₇₁BM by ≈13% and ≈8%, respectively. PCEs over 8% are reported for devices based on the PTB7:PC₇₁BM blend. A comprehensive optical and electrical characterization of our devices to clarify the influence of gold nanorods on exciton generation, dissociation, charge recombination, and transport inside the thin film devices is performed. By correlating the experimental data with detailed numerical simulations, the near‐field and far‐field scattering effects are separated of gold nanorods (Au NRs), and confidently attribute part of the performance enhancement to the enhanced absorption caused by backscattering. While, a secondary contribution from the Au NRs that partially protrude inside the active layer and exhibit strong near‐fields due to localized surface plasmon resonance effects is also observed but is minor in magnitude. Furthermore, another important contribution to the enhanced performance is electrical in nature and comes from the increased charge collection probability.     

Light‐Induced Degradation of Polymer:Fullerene Photovoltaic Devices: An Intrinsic or Material‐Dependent Failure Mechanism?

Although degradation mechanisms in organic photovoltaic devices continue to receive increased attention, it is only recently that the initial light‐induced failure, or so‐called burn‐in effect, has been considered. Both prototypical polythiophene:fullerene and polycarbazole:fullerene systems exhibit an exponential performance loss of ≈40% upon 150 h of continuous solar illumination. While the decrease in both the short‐circuit current (J_SC) and open‐circuit voltage (V_OC) is the origin of performance loss in poly(3‐hexylthiophene):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PC₆₀BM), in poly(N‐9′‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)):[6,6]‐phenyl‐C71‐butyric acid methyl ester (PCDTBT:PC₇₀BM) the decline of the fill factor dominates. By systematic variation of the interface layers, active layer thickness, and acceptor in polythiophene:fullerene cells, the loss in J_SC is ascribed to a degradation in the bulk of the P3HT:PC₆₀BM, while the drop in V_OC is reversible and arises from charge trapping at the contact interfaces. By replacing the C₆₀ fullerene derivative with a C₇₀ derivative, or by modifying the electron transport layer, the J_SC or V_OC, respectively, are stabilized. These insights prove that the burn‐in process stems from multiple concurrent failure mechanisms. Comparing the ageing and recovery processes in P3HT and PCDTBT blends results in the conclusion that their interface failures differ in nature and that burn‐in is a material dependent, rather than an intrinsic, failure mechanism.     

Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

The use of fullerenes with two or more adducts as acceptors has been recently shown to enhance the performance of bulk‐heterojunction solar cells using poly(3‐hexylthiophene) (P3HT) as the donor. The enhancement is caused by a substantial increase in the open‐circuit voltage due to a rise in the fullerene lowest unoccupied molecular orbital (LUMO) level when going from monoadducts to multiadducts. While the increase in the open‐circuit voltage is obtained with many different polymers, most polymers other than P3HT show a substantially reduced photocurrent when blended with fullerene multiadducts like bis‐PCBM (bis adduct of Phenyl‐C₆₁‐butyric acid methyl ester) or the indene C₆₀ bis‐adduct ICBA. Here we investigate the reasons for this decrease in photocurrent. We find that it can be attributed partly to a loss in charge generation efficiency that may be related to the LUMO‐LUMO and HOMO‐HOMO (highest occupied molecular orbital) offsets at the donor‐acceptor heterojunction, and partly to reduced charge carrier collection efficiencies. We show that the P3HT exhibits efficient collection due to high hole and electron mobilities with mono‐ and multiadduct fullerenes. In contrast the less crystalline polymer Poly[[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl (PCDTBT) shows inefficient charge carrier collection, assigned to low hole mobility in the polymer and low electron mobility when blended with multiadduct fullerenes.     

Electronic Structure of Fullerene Heterodimer in Bulk‐Heterojunction Blends

To increase performance of organic solar cells, the optimization of the electron‐accepting fullerenes has received less attention. Here, an electronic structure study of a novel covalently linked C₆₀‐C₇₀‐heterodimer in blend with the polymer PCDTBT (poly[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl) is presented. Upon optical excitation of polymer:heterodimer solid films, the unpaired electron is shared between both C₆₀ and C₇₀ cages. In contrast, in the solution the electron is localized on one half of the dimer. Electronic structure calculations reveal that for the C₆₀‐C₇₀‐heterodimer two nearly isoenergetic minima exist, essentially the cis and trans conformers, which are separated by a thermodynamically accessible rotational barrier. In the cis conformation, the edge‐to‐edge distance between the two cages is ca. 4 Å and an unpaired electron is shared between two dimer halves, while in the trans conformation the separation between the fullerene cages is larger and favors electron localization on one half of the heterodimer. By comparison with the experimental data, it is concluded that the cis conformation is preferable in films, and the trans conformation in solution. Modification of the linking molecular bridge opens the possibility to influence the electronic properties of fullerene dimers, which in turn may have an impact on the charge carrier generation efficiency in solar cells.     

Morphology‐Limited Free Carrier Generation in Donor/Acceptor Polymer Blend Solar Cells Composed of Poly(3‐hexylthiophene) and Fluorene‐Based Copolymer

The charge generation and recombination dynamics in polymer/polymer blend solar cells composed of poly(3‐hexylthiophene) (P3HT, electron donor) and poly[2,7‐(9,9‐didodecylfluorene)‐alt‐5,5‐(4′,7′‐bis(2‐thienyl)‐2′,1′,3′‐benzothiadiazole)] (PF12TBT, electron acceptor) are studied by transient absorption measurements. In the unannealed blend film, charge carriers are efficiently generated from polymer excitons, but some of them recombine geminately. In the blend film annealed at 160 °C, on the other hand, the geminate recombination loss is suppressed and hence free carrier generation efficiency increases up to 74%. These findings suggest that P3HT and PF12TBT are intermixed within a few nanometers, resulting in impure PF12TBT and disordered P3HT domains. The geminate recombination is likely due to charge carriers generated on isolated polymer chains in the matrix of the other polymer and at the domain interface with disordered P3HT. The undesired charge loss by geminate recombination is reduced by both the purification of the PF12TBT‐rich domain and crystallization of the P3HT chains. These results show that efficient free carrier generation is not inherent to the polymer/fullerene domain interface, but is possible with polymer/polymer systems composed of crystalline donor and amorphous acceptor polymers, opening up a new potential method for the improvement of solar cell materials.     

The Influence of MoOx Anode Stoicheometry on the Performance of Bulk Heterojunction Polymer Solar Cells

Bulk heterojunction solar cells containing molybdenum oxide hole extracting anode contacts have been fabricated with varying stoicheometry using radio frequency reactive sputtering from a Molybdenum metal target. A blend of the newly synthesised conjugated polymer poly[9‐(heptadecan‐9‐yl)‐9H‐carbazole‐2,7‐diyl‐alt‐(5,6‐bis(octyloxy)‐4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole)‐5,5‐diyl] (PCDTBT‐8) and fullerene [6,6]‐Phenyl‐C71‐butyric acid methyl ester (PC70BM) was used as the photoactive layer and device results show that anodes with greater than 98% Molybdenum (VI) oxide result in peak power conversion efficiencies of 3.7%.The presence of up to 28% of Mo (V) results in no significant reduction in efficiency, however the presence of metallic Mo (IV) and lower oxidation states lead to severe reductions in device performance due to a combination of a large hole extraction energy barrier of approximately 0.9eV and reduced device stability.     

Influence of Thermal Annealing on PCDTBT:PCBM Composition Profiles

A variety of measurement techniques including photothermal deflection spectroscopy (PDS), auger electron spectroscopy (AES), (sub–bandgap) external quantum efficiency (EQE), and impedance spectroscopy are applied to poly[N‐900‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(40,70‐di‐2‐thienyl‐20,10,30‐benzothiadiazole (PCDTBT)/[6,6]‐phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) films and devices to probe the stability under thermal annealing. Upon annealing, solar cell performance is drastically decreased for temperatures higher than 140 °C. Detailed investigation indicate changes in polymer:fullerene interactions resulting in the formation of a polymer wetting layer upon annealing at temperatures higher than 140 °C. Upon device completion this wetting layer is located close to the metal electrode and therefore leads to an increase in recombination and a decrease in charge carrier extraction, providing an explanation for the reduced fill factor (FF) and power conversion efficiency (PCE).     

Ternary Organic Solar Cells With 12.8% Efficiency Using Two Nonfullerene Acceptors With Complementary Absorptions

A new small‐molecule acceptor (2,9‐bis(2‐methylene‐(3(1,1‐dicyanomethylene)benz[f]indanone))7,12‐dihydro‐(4,4,10,10‐tetrakis(4‐hexylphenyl)‐5,11‐diocthylthieno[3′,2′:4,5]cyclopenta[1,2‐b]thieno[2″,3″:3′,4′]cyclopenta[1′,2′:4,5]thieno[2,3‐f][1]benzothiophene) (NNBDT) based on naphthyl‐fused indanone ending units is reported. This molecule shows a narrow optical bandgap of 1.43 eV and effective absorption in the range of 700–870 nm. The devices based on poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PBDB‐T):NNBDT yield a power conversion efficiency of 11.7% with a low energy loss of 0.55 eV and a high fill factor (FF) of 71.7%. Another acceptor (2,9‐bis(2‐methylene‐(3(1,1‐dicyanomethylene)benz[f]indanone))7,12‐dihydro‐4,4,7,7,12,12‐hexaoctyl‐4H‐cyclopenta[2″,1″:5,6;3″,4″:5′,6′]diindeno[1,2‐b:1′,2′‐b′]dithiophene (FDNCTF) is introduced as the third component to fabricate ternary devices. The two acceptors (NNBDT and FDNCTF) possess complementary absorption, same molecular orientation, and well‐miscible behavior. It is found that there exists a nonradiative energy transfer process from FDNCTF to NNBDT. The fullerene‐free ternary cells based on PBDB‐T:NNBDT:FDNCTF achieve a high efficiency of 12.8% with an improved short circuit current near 20 mA cm^?2 in contrast to the binary devices. The result represents the best performance for fullerene‐free ternary solar cells reported to date and highlights the potential of ternary solar cells.     

Enhanced Efficiency in Plastic Solar Cells via Energy Matched Solution Processed NiOx Interlayers

We show enhanced efficiency and stability of a high performance organic solar cell (OPV) when the work‐function of the hole collecting indium‐tin oxide (ITO) contact, modified with a solution‐processed nickel oxide (NiO_x) hole‐transport layer (HTL), is matched to the ionization potential of the donor material in a bulk‐heterojunction solar cell. Addition of the NiO_x HTL to the hole collecting contact results in a power conversion efficiency (PCE) of 6.7%, which is a 17.3% net increase in performance over the 5.7% PCE achieved with a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL on ITO. The impact of these NiO_x films is evaluated through optical and electronic measurements as well as device modeling. The valence and conduction band energies for the NiO_x HTL are characterized in detail through photoelectron spectroscopy studies while spectroscopic ellipsometry is used to characterize the optical properties. Oxygen plasma treatment of the NiO_x HTL is shown to provide superior contact properties by increasing the ITO/NiO_x contact work‐function by 500 meV. Enhancement of device performance is attributed to reduction of the band edge energy offset at the ITO/NiO_x interface with the poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothidiazole) (PCDTBT):[6,6]‐phenyl‐C61 butyric acid methyl ester PCBM and [6,6]‐phenyl‐C71 butyric acid methyl ester (PC₇₀BM) active layer. A high work‐function hole collecting contact is therefore the appropriate choice for high ionization potential donor materials in order to maximize OPV performance.     

Germanium‐ and Silicon‐Substituted Donor–Acceptor Type Copolymers: Effect of the Bridging Heteroatom on Molecular Packing and Photovoltaic Device Performance

The effects of heteroatom substitution from a silicon atom to a germanium atom in donor‐acceptor type low band gap copolymers, poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (PSiBTBT) and poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]germole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (PGeBTBT), are studied. The optoelectronic and charge transport properties of these polymers are investigated with a particular focus on their use for organic photovoltaic (OPV) devices in blends with phenyl‐C₇₀‐butyric acid methyl ester (PC₇₀BM). It is found that the longer C‐Ge bond length, in comparison to C‐Si, modifies the molecular conformation and leads to a more planar chain conformation in PGeBTBT than PSiBTBT. This increase in molecular planarity leads to enhanced crystallinity and an increased preference for a face‐on backbone orientation, thus leading to higher charge carrier mobility in the diode configuration. These results provide important insight into the impact of the heavy atom substitution on the molecular packing and device performance of polymers based on the poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole) (PCPDTBT) backbone.     

Designing 1,5‐Naphthyridine‐2,6‐dione‐Based Conjugated Polymers for Higher Crystallinity and Enhanced Light Absorption to Achieve 9.63% Efficiency Polymer Solar Cells

Highly crystalline conjugated polymers represent a key material for producing high‐performance thick‐active‐layer polymer solar cells (PSCs). However, despite their potential, a limited number of crystalline polymers are used in PSCs because of the lack of highly coplanar acceptor building blocks and insufficient light absorptivity (α < 10⁵) of most donor (D)–acceptor (A)‐type polymers. This study reports a series of novel 3,7‐di(thiophen‐2‐yl)‐1,5‐naphthyridine‐2,6‐dione (NTDT) acceptor‐based conjugated polymers, PNTDT‐2T, PNTDT‐TT, and PNTDT‐2F2T, synthesized with 2,2′‐bithiophene (2T), thieno[3,2‐b]thiophene (TT), and 3,3′‐difluoro‐2,2′‐bithiophene (2F2T) donor units, respectively. PNTDT‐2F2T exhibits superior polymer crystallinity and a much higher absorption coefficient than those of PNTDT‐2T or PNTDT‐TT because of adequate matching between highly coplanar A (NTDT) and D (2F2T) building blocks. A bulk heterojunction solar cell based on PNTDT‐2F2T exhibits a power conversion efficiency of up to 9.63%, with a high short circuit current of 18.80 mA cm^?2 and fill factor of 0.70, when a thick active layer (>200 nm) is used, without postfabrication hot processing. The findings demonstrate that the polymer crystallinity and absorption coefficient can be effectively controlled by selecting appropriate D and A building blocks, and that NTDT is a novel and versatile A building block for highly efficient thick‐active‐layer PSCs.     

Computational study on the structural and optoelectronic properties of a carbazole-benzothiadiazole based conjugated oligomer with various alkyl side-chain lengths

The effect of the alkyl side-chain length on the structural and optoelectronic properties of poly[N-9′-heptadecanyl-27-carbazole-alt-55-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) conjugated oligomers have been studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The study was carried out by varying the length of alkyl side-chain attached to the nitrogen atom of the carbazole unit of the PCDTBT oligomers. The structural properties of the optimised oligomers were then studied by determining the bond-length alternation and dihedral angles (Φ) for various side-chain lengths. Total energy calculations for the determination of HOMO energy (E_HOMO), LUMO energy (E_LUMO), and fundamental energy gap (E_Gap) were performed using DFT at the B3LYP/6-31G(d), while the first singlet excitation energies (E_Opt) were calculated by TD-DFT also at the same level of theory. It was observed that there are no significant structural changes occurring as the alkyl chain lengths are varied. For the electronic properties, very small differences (i.e. ~0.01 eV) were observed for E_Gap and E_Opt while the exciton binding energies (E_B) were virtually the same. The results suggest that using shorter alkyl side-chains do not significantly affect the structural and optoelectronic properties of the carbazole-benzothiadiazole based polymer. The observations can aid future computational design studies of analogous systems by reducing large structures thus decreasing computational costs.