Sequential Processing for Organic Photovoltaics: Design Rules for Morphology Control by Tailored Semi‐Orthogonal Solvent Blends

Design rules are presented for significantly expanding sequential processing (SqP) into previously inaccessible polymer:fullerene systems by tailoring binary solvent blends for fullerene deposition. Starting with a base solvent that has high fullerene solubility, 2‐chlorophenol (2‐CP), ellipsometry‐based swelling experiments are used to investigate different co‐solvents for the fullerene‐casting solution. By tuning the Flory‐Huggins χ parameter of the 2‐CP/co‐solvent blend, it is possible to optimally swell the polymer of interest for fullerene interdiffusion without dissolution of the polymer underlayer. In this way solar cell power conversion efficiencies are obtained for the PTB7 (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)]) and PC₆₁BM (phenyl‐C₆₁‐butyric acid methyl ester) materials combination that match those of blend‐cast films. Both semicrystalline (e.g., P3HT (poly(3‐hexylthiophene‐2,5‐diyl)) and entirely amorphous (e.g., PSDTTT (poly[(4,8‐di(2‐butyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)‐alt‐(2,5‐bis(4,4′‐bis(2‐octyl)dithieno[3,2‐b:2′3′‐d]silole‐2,6‐diyl)thiazolo[5,4‐d]thiazole)]) conjugated polymers can be processed into highly efficient photovoltaic devices using the solvent‐blend SqP design rules. Grazing‐incidence wide‐angle x‐ray diffraction experiments confirm that proper choice of the fullerene casting co‐solvent yields well‐ordered interdispersed bulk heterojunction (BHJ) morphologies without the need for subsequent thermal annealing or the use of trace solvent additives (e.g., diiodooctane). The results open SqP to polymer/fullerene systems that are currently incompatible with traditional methods of device fabrication, and make BHJ morphology control a more tractable problem.     

Efficient Semi‐Transparent Organic Solar Cells with Good Transparency Color Perception and Rendering Properties

Window integrated photovoltaics for automotive and building applications are a promising market segment for organic solar modules. Besides semi‐transparency, window integrated applications require a reasonable transparency perception and good color rendering properties in order to be suitable for realistic scene illumination. Here, the transmitted light through semi‐transparent organic solar cells comprising the polymer/fullerene blend 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]: [6,6]‐phenyl C₇₁‐butric acid methyl ester (PSBTBT:PC₇₀BM) as active layer and a sputtered aluminum doped zinc oxide cathode were found to exhibit a color neutral perception for the human eye and very good color rendering properties. Moreover, the electrical cell properties allow for efficient energy harvesting with an overall power conversion efficiency η ≈ 3%.     

Fabricating High Performance,Donor–Acceptor Copolymer Solar Cells by Spray‐Coating in Air

We report the fabrication of high performance organic solar cells by spray‐coating the photoactive layer in air. The photovoltaic blends consist of a blend of carbazole and benzothiadiazole based donor–acceptor copolymers and the fullerene derivative PC₇₀BM. Here, we formulate a number of photovoltaic inks using a range of solvent systems that we show can all be deposited by spray casting. We use a range of techniques to characterize the structure of such films, and show that spray‐cast films have comparable surface roughness to spin‐cast films and that vertical stratification that occurs during film drying reduces the concentration of PCBM towards the underlying PEDOT:PSS interface. We also show that the active layer thickness and the drying kinetics can be tuned through control of the substrate temperature. High power conversion efficiencies of 4.3%, 4.5% and 4.6% were obtained for solar cells made from a blend of PC₇₀BM with the carbazole‐based co‐polymers PCDTBT, P2 and P1. By applying a low temperature anneal after the deposition of the cathode, the efficiency of spray‐cast solar‐cells based on a P2:PC₇₀BM blend is increased to 5.0%. Spray coating holds significant promise as a technique capable of fabricating large‐area, high performance organic solar cells in air.     

Revealing Hidden UV Instabilities in Organic Solar Cells by Correlating Device and Material Stability

With state‐of‐the‐art organic solar cells (OSCs) surpassing 16% efficiency, stability becomes critical for commercialization. In this work, the power of using photoluminescence (PL) measurements on plain films is demonstrated, as well as high‐performance liquid chromatography analysis to reveal the origin of UV instabilities in OSCs based on the most commonly used acceptors PC₇₀BM ([6,6]‐phenyl‐C71‐butyric acid methyl ester), ITIC (3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene), and o‐IDTBR (indacenodithiophene‐based non‐fullerene acceptor). The UV dependent stability tests reveal instabilities in solar cells based on PC₇₀BM and ITIC while devices based on o‐IDTBR are highly stable even under UV illumination. The analysis of solar cell devices based on charge extraction and sub‐bandgap external quantum efficiency only shows the UV‐dependent emergence of traps, while PL spectra of plain films on glass allows the disentanglement and identification of individual instabilities in multi‐component bulk‐heterojunction devices. In particular, the PL analysis demonstrates UV instabilities of PC₇₀BM and ITIC toward the processing additive 1,8 diiodooctane (DIO). The chemical analysis reveals the in‐depth mechanism, by providing direct proof of photochemical reactions of PC₇₀BM and ITIC with UV‐induced radicals of DIO. Based on this scientific understanding, it is shown how to stabilize PBQ‐QF:PC₇₀BM devices.     

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.     

Unraveling the Morphology of High Efficiency Polymer Solar Cells Based on the Donor Polymer PBDTTT‐EFT

The microstructure of the polymer PBDTTT‐EFT and blends with the fullerene derivative PC₇₁BM that achieve solar conversion efficiencies of over 9% is comprehensively investigated. A combination of synchrotron techniques are employed including surface‐sensitive near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy and bulk‐sensitive grazing‐incidence wide angle X‐ray scattering (GIWAXS). A preferential “face‐on” orientation of PBDTTT‐EFT is observed in the bulk of both pristine and blend thin films, with π–π stacking largely normal to the substrate, which is thought to be beneficial for charge transport. At the surface of the blend, a slight “edge‐on” structure of the polymer is observed with side‐chains aligned normal to the substrate. The effect of the solvent additive 1,8‐diiodooctane (DIO) on solar cell efficiency and film microstructure is also investigated, where the addition of 3 vol% DIO results in an efficiency increase from ≈6.4% to ≈9.5%. GIWAXS studies indicate that the addition of DIO improves the crystallization of the polymer. Furthermore, atomic force microscopy and transmission electron microscopy are employed to image surface and bulk morphology revealing that DIO suppresses the formation of large PC₇₁BM aggregates.     

Triplet Excitons in Highly Efficient Solar Cells Based on the Soluble Small Molecule p‐DTS(FBTTh2)2

Triplet exciton formation in neat 7,7‐(4,4‐bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b′] dithiophene‐2,6‐diyl)bis(6‐fluoro‐4‐(5′‐hexyl‐[2,2′‐bithiophen]‐5‐yl)benzo[c][1,2,5]thiadiazole) (p‐DTS(FBTTh₂)₂) and blends with [6,6]‐Phenyl C₇₀ butyric acid methyl ester (PC₇₀BM), with and without the selective solvent additive 1,8‐diiodooctane, is investigated by means of spin sensitive photoluminescence measurements. For all three material systems, a significant amount of long living triplet excitons is detected, situated on the p‐DTS(FBTTh₂)₂ molecules. The characteristic zero‐field splitting parameters for this state are identified to be D = 42 mT (1177 MHz) and E = 5 mT (140 MHz). However, no triplet excitons located on PC₇₀BM are detectable. Using electrically detected spin resonance, the presence of these triplet excitons is confirmed even at room temperature, highlighting that triplet excitons form during solar cell operation and influence the photocurrent and photovoltage. Surprisingly, the superior performing blend is found to have the largest triplet population. It is concluded, that the formation of triplet excitons from charge transfer states via electron back transfer has no crucial impact on device performance in p‐DTS(FBTTh₂)₂:PC₇₀BM based solar cells.     

Influence of Fullerene Acceptor on the Performance,Microstructure, and Photophysics of Low Bandgap Polymer Solar Cells

The morphology, photophysics, and device performance of solar cells based on the low bandgap polymer poly[[2,6′‐4,8‐di(5‐ethylhexylthienyl)benzo[1,2‐b;3,3‐b]dithiophene]3‐fluoro‐2[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl (PBDTTT‐EFT) (also known as PTB7‐Th) blended with different fullerene acceptors: Phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM), phenyl‐C₇₁ ‐butyric acid methyl ester (PC₇₁BM), or indene‐C₆₀ bisadduct (ICBA) are correlated. Compared to PC₇₁ BM‐based cells – which achieve a power conversion efficiency (PCE) of 9.4% – cells using ICBA achieve a higher open‐circuit voltage (V_OC) of 1.0 V albeit with a lower PCE of 7.1%. To understand the origin of this lower PCE, the morphology and photophysics have been thoroughly characterized. Hard and soft X‐ray scattering measurements reveal that the PBDTTT‐EFT:ICBA blend has a lower crystallinity, lower domain purity, and smaller domain size compared to the PBDTTT‐EFT:PC₇₁BM blend. Incomplete photoluminescence quenching is also found in the ICBA blend with transient absorption measurements showing faster recombination dynamics at short timescales. Transient photovoltage measurements highlight further differences in recombination at longer timeframes due to the more intermixed morphology of the ICBA blend. Interestingly, a mild thermal treatment improves the performance of PBDTTT‐EFT:ICBA cells which is exploited in the fabrication of a homo PBDTTT‐EFT:ICBA tandem solar cell with PCE of 9.0% and V_OC of 1.93 V.     

Electron Collection as a Limit to Polymer:PCBM Solar Cell Efficiency: Effect of Blend Microstructure on Carrier Mobility and Device Performance in PTB7:PCBM

The poor photovoltaic performance of state‐of‐the‐art blends 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] (PTB7) and [6,6]‐phenyl‐C61‐butyric acid (PCBM) at large active layer thicknesses is studied using space‐charge‐limited current mobility and photovoltaic device measurements. The poor performance is found to result from relatively low electron mobility. This is attributed to the low tendency of PTB7 to aggregate, which reduces the ability of the fullerene to form a connected network. Increasing the PCBM content 60–80 wt% increases electron mobility and accordingly improves performance for thicker devices, resulting in a fill factor (FF) close to 0.6 at 300 nm. The result confirms that by improving only the connectivity of the fullerene phase, efficient electron and hole collection is possible for 300 nm‐thick PTB7:PCBM devices. Furthermore, it is shown that solvent additive 1,8‐diiodooctane (DIO), used in the highest efficiency PTB7:PCBM devices, does not improve the thickness dependence and, accordingly, does not lead to an increase in either hole or electron mobility or in the carrier lifetime. A key challenge for researchers is therefore to develop new methods to ensure connectivity in the fullerene phase in blends without relying on either a large excess of fullerene or strong aggregation of the polymer.     

8.9% Single‐Stack Inverted Polymer Solar Cells with Electron‐Rich Polymer Nanolayer‐Modified Inorganic Electron‐Collecting Buffer Layers

Enhanced power conversion efficiency (PCE) is reported in inverted polymer solar cells when an electron‐rich polymer nanolayer (poly(ethyleneimine) (PEI)) is placed on the surface of an electron‐collecting buffer layer (ZnO). The active layer is made with bulk heterojunction films 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]] (PTB7) and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM). The thickness of the PEI nanolayer is controlled to be 2 nm to minimize its insulating effect, which is confirmed by X‐ray photoelectron spectroscopy and optical absorption measurements. The Kelvin probe and ultraviolet photoelectron spectroscopy measurements demonstrate that the enhanced PCE by introducing the PEI nanolayer is attributed to the lowered conduction band energy of the ZnO layer via the formation of an interfacial dipole layer at the interfaces between the ZnO layer and the PEI nanolayer. The PEI nanolayer also improves the surface roughness of the ZnO layer so that the device series resistance can be noticeably decreased. As a result, all solar cell parameters including short circuit current density, open circuit voltage, fill factor, and shunt resistance are improved, leading to the PCE increase up to ≈8.9%, which is close to the best PCE reported using conjugated polymer electrolyte films.     

A High‐Performance D–A Copolymer Based on Dithieno[3,2‐b:2′,3′‐d]Pyridin‐5(4H)‐One Unit Compatible with Fullerene and Nonfullerene Acceptors in Solar Cells

Development of high‐performance donor–acceptor (D–A) copolymers is vital in the research of polymer solar cells (PSCs). In this work, a low‐bandgap D–A copolymer based on dithieno[3,2‐b:2′,3′‐d]pyridin‐5(4H)‐one unit (DTP), PDTP4TFBT, is developed and used as the donor material for PSCs with PC₇₁BM or ITIC as the acceptor. PDTP4TFBT:PC₇₁BM and PDTP4TFBT:ITIC solar cells give power conversion efficiencies (PCEs) up to 8.75% and 7.58%, respectively. 1,8‐Diiodooctane affects film morphology and device performance for fullerene and nonfullerene solar cells. It inhibits the active materials from forming large domains and improves PCE for PDTP4TFBT:PC₇₁BM cells, while it promotes the aggregation and deteriorates performance for PDTP4TFBT:ITIC cells. The ternary‐blend cells based on PDTP4TFBT:PC₇₁BM:ITIC (1:1.2:0.3) give a decent PCE of 9.20%.     

The Effect of Fluorination in Manipulating the Nanomorphology in PTB7:PC71BM Bulk Heterojunction Systems

The best performing low bandgap copolymers PTB series to date which is based on thieno[3,4‐b]thiophene‐alt‐benzodithiophene units blended with [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM), have been the focus of polymer‐based solar cells. Here, novel fluorinated polymers PTB7‐Fx (fluorine units coupled with submonomer thieno[3,4‐b]thiophene) with varied degree of fluorination are used as electron donor materials. The PTB7‐Fx:PC₇₁BM bulk heterojunction (BHJ) films spin‐coated from the host solvent chlorobenzene without and with solvent additive 1,8‐diiodooctane (DIO) and the corresponding solar cell devices are systematically investigated to address the morphology‐efficiency relationship. Self‐assembled BHJ morphology is already observed for as‐spun blend films. After adding the solvent additive DIO, the pronounced ordered structures are suppressed and better intermixed films with much smaller domain sizes result. Full fluorination of the third C‐atom of thienothiophene gives rise to the highest power conversion efficiency. As the absorption properties, film morphology and crystallinity remain similar for different degrees of fluorination, the main influence of the photovoltaic performance is ascribed to the different lowest unoccupied molecular orbital (LUMO) of each polymer instead of the film morphology. Thus the device performance can be efficiently improved by tuning the energy level of the polymer without necessarily changing either the film nanomorphology or crystallinity dramatically.     

The Crucial Influence of Fullerene Phases on Photogeneration in Organic Bulk Heterojunction Solar Cells

The conjugated polymer, poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (pBTTT‐C₁₆), allows a systematic tuning of the blend morphology by varying the acceptor type and fraction, making it a well‐suited structural model for studying the fundamental processes in organic bulk heterojunction solar cells. To analyze the role of intercalated and pure fullerene domains on charge carrier photogeneration, time delayed collection field (TDCF) measurements and Fourier‐transform photocurrent spectroscopy (FTPS) are performed on pBTTT‐C₁₆:[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) solar cells with various stoichiometries. A weak influence of excess photon energy on photogeneration along with a photogeneration having a weaker field dependence at increasing fullerene loading is found. The findings are assigned to a dissociation via thermalized charge transfer (CT) states supported by an enhanced electron delocalization along spatially extended PC₆₁BM nanophases that form in addition to a bimolecular crystal (BMC) for PC₆₁BM rich blends. The highly efficient transfer of charge carriers from the BMC into the pure domains are studied further by TDCF measurements performed on non‐intercalated pBTTT‐C₁₆:bisPC₆₁BM blends. They reveal a field dependent charge generation similar to the 1:4 PC₆₁BM blend, demonstrating that the presence of pure acceptor phases is the major driving force for an efficient, field independent CT dissociation.     

The Effect of PCBM Dimerization on the Performance of Bulk Heterojunction Solar Cells

Increasing the lifetime of polymer based organic solar cells is still a major challenge. Here, the photostability of bulk heterojunction solar cells based on the polymer poly[4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐[2,5‐bis(3‐tetradecylthiophen‐2‐yl)thiazole[5,4‐d]thiazole)‐1,8‐diyl] (PDTSTzTz) and the fullerene [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₀BM) under inert atmosphere is investigated. Correlation of electrical measurements on complete devices and UV‐vis absorption measurements as well as high‐performance liquid chromatography (HPLC) analysis on the active materials reveals that photodimerization of PC₆₀BM is responsible for the observed degradation. Simulation of the electrical device parameters shows that this dimerization results in a significant reduction of the charge carrier mobility. Both the dimerization and the associated device performance loss turn out to be reversible upon annealing. BisPC₆₀BM, the bis‐substituted analog of PC₆₀BM, is shown to be resistant towards light exposure, which in turn enables the manufacture of photostable PDTSTzTz:bisPC₆₀BM solar cells.     

Interplay of Optical,Morphological, and Electronic Effects of ZnO Optical Spacers in Highly Efficient Polymer Solar Cells

Optical spacers based on metal oxide layers have been intensively studied in poly(3‐hexylthiophene) (P3HT) based polymer solar cells for optimizing light distribution inside the device, but to date, the potential of such a metal oxide spacer to improve the electronic performance of the polymer solar cells simultaneously has not yet be investigated. Here, a detailed study of performance improvement in high efficient polymer solar cells by insertion of solution‐processed ZnO optical spacer using ethanolamine surface modification is reported. Insertion of the modified ZnO optical spacer strongly improves the performance of polymer solar cells even in the absence of an increase in light absorption. The electric improvements of the device are related to improved electron extraction, reduced contact barrier, and reduced recombination at the cathode. Importantly, it is shown for the first time that the morphology of optical spacer layer is a crucial parameter to obtain highly efficient solar cells in normal device structures. By optimizing optical spacer effects, contact resistance, and morphology of ZnO optical spacers, poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6diyl] [3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl] thieno[3,4‐b]thiophenediyl]] (PTB7):[6,6]‐phenyl‐C71‐butyric acid (PC₇₀BM) bulk heterojunction solar cells with conversion efficiency of 7.6% are obtained in normal device structures with all‐solution‐processed interlayers.     

Layer‐by‐Layer Solution‐Processed Low‐Bandgap Polymer‐PC61BM Solar Cells with High Efficiency

Polymer solar cells (PSCs) are fabricated without solvent additives using a low‐bandgap polymer, PBDTTT‐C‐T, as the donor and [6,6]‐phenyl‐C61‐butyric‐acid‐methyl‐ester (PC₆₁BM) as the acceptor. Donor‐acceptor blend and layer‐by‐layer (LL) solution process are used to form active layers. Relative to the blend devices, the LL devices exhibit stronger absorption, better vertical phase separation, higher hole and electron mobilities, and better charge extraction at correct electrodes. As a result, after thermal annealing the LL devices exhibit an average power conversion efficiency (PCE) of 6.86%, which is much higher than that of the blend devices (4.31%). The best PCE of the LL devices is 7.13%, which is the highest reported for LL processed PSCs and among the highest reported for PC₆₁BM‐based single‐junction PSCs.     

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.     

A Thieno[3,2‐c]Isoquinolin‐5(4H)‐One Building Block for Efficient Thick‐Film Solar Cells

An aromatic lactam acceptor unit, thieno[3,2‐c]isoquinolin‐5(4H)‐one (TIQ), is developed. Compared with its analogues, dithieno[3,2‐b:2′,3′‐d]pyridin‐5(4H)‐one (DTP) and phenanthridin‐6(5H)‐one (PN), TIQ shows its advantage in constructing donor–acceptor (D–A) copolymers for efficient solar cells. TIQ‐based D–A copolymer PTIQ4TFBT delivers a power conversion efficiency (PCE) of 10.16% in polymer:fullerene solar cells, while those based on DTP and PN copolymers, PDTP4TFBT and PPN4TFBT, afford PCEs around 8.5%. The higher performance of PTIQ4TFBT:PC₇₁BM solar cells originates from enhanced short‐circuit current density (J_sc) and fill factor (FF), because of favorable morphology, less bimolecular recombination, and balanced charge transport in the active layer. Moreover, the performance for PTIQ4TFBT:PC₇₁BM solar cells is less sensitive to active layer thickness than PDTP4TFBT:PC₇₁BM and PPN4TFBT:PC₇₁BM solar cells. Over 8% PCEs can be obtained from PTIQ4TFBT:PC₇₁BM solar cells when the active layer thickness is over 500 nm.     

Effect of Polymer–Fullerene Interaction on the Dielectric Properties of the Blend

It is commonly believed that large dielectric constants are required for efficient charge separation in polymer photovoltaic devices. However, many polymers used in high‐performance solar cells do not possess high dielectric constants. In this work, the effect of polymer–fullerene interactions on the dielectric environment of the active layer blend and the device performance for several donor–acceptor conjugated polymer systems is investigated. It is found that, while none of the high‐performing polymers studied has a dielectric constant value larger than 3, all polymer–fullerene blends have a significantly larger dielectric constant compared to their pristine constituents. Additionally, it is found that the blend dielectric constant reaches a maximum value in fully optimized devices. Using PTB7:PC₇₁BM blends as an example, it is showed that, in addition to a small increase in the dielectric constant, devices fabricated using the optimum processing additive concentration exhibit almost 3X larger excited state polarizability. This large increase in excited state polarizability results in a substantial difference in short‐circuit current and ultimately device performance. The results show that the excited state polarizability critically depends on polymer–fullerene interactions, and can be a leading indicator of device performance for a given material system.     

Impact of Fullerene Molecular Weight on P3HT:PCBM Microstructure Studied Using Organic Thin‐Film Transistors

The clustering and diffusion of C₇₁‐butyric acid methyl ester (PC₇₁BM) in poly(3‐hexylthiophene) (P3HT) has been studied using single layer blend and bilayer organic field‐effect transistors (OFETs) and by atomic force microscopy (AFM). P3HT:PC₇₁BM blend based OFETs were found to undergo phase‐segregation upon annealing, which was detectable as a fall in electron mobility with increasing annealing temperature. By employing carefully designed bilayer P3HT:PC₇₁BM OFETs, the diffusion‐properties of PC₇₁BM in P3HT could additionally be inferred from electron mobility measurements. It was found that the prerequisite annealing temperatures for detectable PC₇₁BM clustering and diffusion in P3HT was approximately 20 °C higher than for PC₆₁BM. The diffusion coefficient of PC₆₁BM in P3HT was found to be several times higher that that of PC₇₁BM. The present work provides unique insights into the diffusion process of fullerenes in conjugated polymers and could prove highly valuable for future materials development and device optimization.