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.     

Relating Recombination,Density of States,and Device Performance in an Efficient Polymer:Fullerene Organic Solar Cell Blend

We explore the interrelation between density of states, recombination kinetics, and device performance in efficient poly[4,8‐bis‐(2‐ethylhexyloxy)‐benzo[1,2‐b:4,5‐b']dithiophene‐2,6‐diyl‐alt‐4‐(2‐ethylhexyloxy‐1‐one)thieno[3,4‐b]thiophene‐2,6‐diyl]:[6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PBDTTT‐C:PC₇₁BM) bulk‐heterojunction organic solar cells. We modulate the active‐layer density of states by varying the polymer:fullerene composition over a small range around the ratio that leads to the maximum solar cell efficiency (50–67 wt% PC₇₁BM). Using transient and steady‐state techniques, we find that nongeminate recombination limits the device efficiency and, moreover, that increasing the PC₇₁BM content simultaneously increases the carrier lifetime and drift mobility in contrast to the behavior expected for Langevin recombination. Changes in electronic properties with fullerene content are accompanied by a significant change in the magnitude or energetic separation of the density of localized states. Our comprehensive approach to understanding device performance represents significant progress in understanding what limits these high‐efficiency polymer:fullerene systems.     

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.     

Toward High‐Temperature Stability of PTB7‐Based Bulk Heterojunction Solar Cells: Impact of Fullerene Size and Solvent Additive

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.     

Structural Characterization of a Composition Tolerant Bulk Heterojunction Blend

The ratio of the donor and acceptor components in bulk heterojunction (BHJ) organic solar cells is a key parameter for achieving optimal power conversion efficiency (PCE). However, it has been recently found that a few BHJ blends have compositional tolerance and achieve high performance in a wide range of donor to acceptor ratios. For instance, the X2 :PC₆₁BM system, where X2 is a molecular donor of intermediate dimensions, exhibits a PCE of 6.6%. Its PCE is relatively insensitive to the blend ratio over the range from 7:3 to 4:6. The effect of blend ratio of X2 /PC₆₁BM on morphology and device performance is therefore systematically investigated by using the structural characterization techniques of energy‐filtered transmission energy microscopy (EF‐TEM), resonant soft X‐ray scattering (R‐SoXS) and grazing incidence wide angle X‐ray scattering (GIWAXS). Changes in blend ratio do not lead to obvious differences in morphology, as revealed by R‐SoXS and EF‐TEM. Rather, there is a smooth evolution of a connected structure with decreasing domain spacing from 8:2 to 6:4 blend ratios. Domain spacing remains constant from 6:4 to 4:6 blend ratios, which suggests the presence of continuous phases with proper domain size that may provide access for charge carriers to reach their corresponding electrodes.     

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.     

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.     

Synergistic Impact of Solvent and Polymer Additives on the Film Formation of Small Molecule Blend Films for Bulk Heterojunction Solar Cells

The addition of polystyrene (PS), a typical insulator, is empirically shown to increase the power conversion efficiencies (PCEs) of a solution‐deposited bulk heterojunction (BHJ) molecular blend film used in solar cell fabrication: p‐DTS(FBTTh₂)₂/PC₇₁BM. The performance is further improved by small quantities of diiodooctane (DIO), an established solvent additive. In this study, how the addition of PS and DIO affects the film formation of this bulk heterojunction blend film are probed via in situ monitoring of absorbance, thickness, and crystallinity. PS and DIO additives are shown to promote donor crystallite formation on different time scales and through different mechanisms. PS‐containing films retain chlorobenzene solvent, extending evaporation time and promoting phase separation earlier in the casting process. This extended time is insufficient to attain the morphology for optimal PCE results before the film sets. Here is where the presence of DIO comes into play: its low vapor pressure further extends the time scale of film evolution and allows for crystalline rearrangement of the donor phase long after casting, ultimately leading to the best BHJ organization.     

Manipulating Backbone Structure to Enhance Low Band Gap Polymer Photovoltaic Performance

A pair of polymers, PBDTBT and PBDTDTBT , was synthesized for application in polymer solar cells (PSCs). Although these two polymers have similar absorption bands and molecular energy levels, PBDTDTBT exhibits much better photovoltaic performance in polymer solar cell (PSC) devices with power conversion efficiency (PCE) of 7.4%. To understand the differences between PBDTDTBT and PBDTBT , we have investigated the correlations of the molecular structure, morphology, dynamics and efficiency of these two polymers. A theoretical investigation using density functional theory (DFT) and time‐dependent DFT (TDDFT) has been employed to investigate the electron density and electron delocalization extent of the unimers. TEM data showed that PBDTDTBT phase separates from PC₇₁BM, while PBDTBT suffers from having a proper morphology on different processing conditions. Grazing incidence wide angle X‐ray diffraction (GIWAXD) was used to probe the crystal structure of the polymers in thin film. A polymorph crystal structure was observed for PBDTBT . Grazing incidence small angle X‐ray scattering (GISAXS) was used to probe the size scale of phase separation, with an optimized 25 nm feature size observed for PBDTDTBT /PC₇₁BM blends, which agrees well with TEM results. Femtosecond transient absorption (TA) spectroscopy was used to probe the dynamics of the fundamental processes in organic photovoltaic (OPV) materials, such as charge separation and recombination. The enhanced absorption coefficient, good charge separation, optimal phase separation and higher charge mobility all contribute to the high PCE of the PBDTDTBT /PC₇₁BM devices.     

Investigation of Charge Carrier Behavior in High Performance Ternary Blend Polymer Solar Cells

This study demonstrates high‐performance, ternary‐blend polymer solar cells by modifying a binary blend bulk heterojunction (PPDT2FBT:PC₇₁BM) with the addition of a ternary component, PPDT2CNBT. PPDT2CNBT is designed to have complementary absorption and deeper frontier energy levels compared to PPDT2FBT, while being based on the same polymeric backbone. A power conversion efficiency of 9.46% is achieved via improvements in both short‐circuit current density (J_SC) and open‐circuit voltage (V_OC). Interestingly, the V_OC increases with increasing the PPDT2CNBT content in ternary blends. In‐depth studies using ultraviolet photoelectron spectroscopy and transient absorption spectroscopy indicate that the two polymers are not electronically homogeneous and function as discrete light harvesting species. The structural similarity between PPDT2CNBT and PPDT2FBT allows the merits of a ternary system to be fully utilized to enhance both J_SC and V_OC without detriment to fill‐factor via minimized disruption of semi‐crystalline morphology of binary PPDT2FBT:PC₇₁BM blend. Further, by careful analysis, charge carrier transport in this ternary blend is clearly verified to follow parallel‐like behavior.     

Self‐Organization of Polymer Additive,Poly(2‐vinylpyridine) via One‐Step Solution Processing to Enhance the Efficiency and Stability of Polymer Solar Cells

Interfaces between the photoactive layers and electrodes play critical roles in controlling the performance of optoelectronic devices. Herein, a novel nonconjugated polymer additive (nPA), poly(2‐vinylpyridine) (P2VP), is reported for modifying the interfaces between the bulk‐heterojunction (BHJ) and cathode/metal oxide (MO) layers. The P2VP nPA enables remarkably enhanced power conversion efficiencies (PCEs) and ambient stability in different types of polymer solar cells (PSCs). Importantly, interfacial engineering can be achieved during deposition of the P2VP nPA‐containing BHJ active layer via simple, one‐step solution processing. The P2VP nPA has much higher surface energy than the BHJ active components and stronger interaction with the surface of MO, which affords spontaneous vertical phase separation from the BHJ layer on the MO surface by one‐step solution processing. The self‐assembled P2VP layer substantially reduces the work function and surface defect density of MO, thereby minimizing the charge‐extraction barrier and increasing the PCEs of the PSCs significantly, i.e., PTB7‐Th:PC₇₁BM (10.53%→11.14%), PTB7:PC₇₁BM (7.37%→8.67%), and PTB7‐Th:P(NDI2HD‐T) all‐PSCs (5.52%→6.14%). In addition, the lifetimes of the PSCs are greatly improved by the use of the P2VP nPA.     

Spectral Engineering of Semitransparent Polymer Solar Cells for Greenhouse Applications

In this study, a wavelength selective semitransparent polymer solar cell (ST‐PSC) with a proper transmission spectrum for plant growth is proposed for greenhouse applications. A ternary strategy combining a wide bandgap polymer donor with a near‐infrared absorbing nonfullerene acceptor and a high electron mobility fullerene acceptor is introduced to achieve PSCs with power conversion efficiency (PCE) over 10%. The addition of PC₇₁BM into J52:IEICO‐4F binary blend contributes to the suppressed trap‐assisted recombination, enhanced charge extraction, and improved open‐circuit voltage simultaneously. ST‐PSC based on the J52:IEICO‐4F:PC₇₁BM ternary blend shows an optimized performance with PCE of 7.75% and a defined crop growth factor of 24.8%. Such high‐performance ST‐PSC is achieved by carefully engineering the absorption spectrum of the light harvesting materials. As a result, the transmission spectra of the semitransparent devices are well‐matched with the absorption spectra of the photoreceptors, such as chlorophylls, in green plants, which provides adequate lighting conditions for photosynthesis and plant growth, and therefore making it a competitive candidate for photovoltaic greenhouse applications.     

Comparison of the Morphology Development of Polymer–Fullerene and Polymer–Polymer Solar Cells during Solution‐Shearing Blade Coating

In this work, the detailed morphology studies of polymer poly(3‐hexylthiophene‐2,5‐diyl) (P3HT):fullerene(PCBM) and polymer(P3HT):polymer naphthalene diimide thiophene (PNDIT) solar cell are presented to understand the challenge for getting high performance all‐polymer solar cells. The in situ X‐ray scattering and optical interferometry and ex situ hard and soft X‐ray scattering and imaging techniques are used to characterize the bulk heterojunction (BHJ) ink during drying and in dried state. The crystallization of P3HT polymers in P3HT:PCBM bulk heterojunction shows very different behavior compared to that of P3HT:PNDIT BHJ due to different mobilities of P3HT in the donor:acceptor glass. Supplemented by the ex situ grazing incidence X‐ray diffraction and soft X‐ray scattering, PNDIT has a lower tendency to form a mixed phase with P3HT than PCBM, which may be the key to inhibit the donor polymer crystallization process, thus creating preferred small phase separation between the donor and acceptor polymer.     

Crosslinkable Amino‐Functionalized Conjugated Polymer as Cathode Interlayer for Efficient Inverted Polymer Solar Cells

A novel crosslinkable aminoalkyl‐functionalized polymer, poly[9,9‐bis(6‐(N,N‐diethylamino)propyl)fluorene‐alt‐9,9‐bis(hex‐5‐en‐1‐yl)‐fluorene] (PFN‐V), is designed and synthesized. The resulting polymer can be rapidly crosslinked by UV‐curing within 5 s in a nearly quantitative yield based on the “click” chemistry of alkyene end‐groups of the PFN‐V side chains and the addition of 1,8‐octanedithiol. The crosslinked PFN‐V film exhibits excellent solvent resistance property and can act as effective cathode interlayer to modify the indium tin oxide (ITO) electrode, which can thus facilitate the formation of Ohmic contact between cathode and active layer. The surface energy of PFN‐V is quite comparable to that of PC₇₁BM, which is favorable for the formation of vertical phase separation in the bulk heterojunction film that can facilitate extraction of charges as verified by transient photocurrent measurements. Based on the resulting PFN‐V as the cathode interlayer, the fabricated polymer solar cells with inverted device structure show a remarkable enhancement of power conversion efficiency from 3.11% for the control device to 9.18% for PFN‐V based device. These observations indicate that the synthesized PFN‐V can be a promising crosslinked copolymer as the cathode interlayer for high performance polymer solar cells.     

Interplay Between Side Chain Pattern,Polymer Aggregation,and Charge Carrier Dynamics in PBDTTPD:PCBM Bulk‐Heterojunction Solar Cells

Poly(benzo[1,2‐b:4,5‐b′]dithiophene–alt–thieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) polymer donors with linear side‐chains yield bulk‐heterojunction (BHJ) solar cell power conversion efficiencies (PCEs) of about 4% with phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) as the acceptor, while a PBDTTPD polymer with a combination of branched and linear substituents yields a doubling of the PCE to 8%. Using transient optical spectroscopy it is shown that while the exciton dissociation and ultrafast charge generation steps are not strongly affected by the side chain modifications, the polymer with branched side chains exhibits a decreased rate of nongeminate recombination and a lower fraction of sub‐nanosecond geminate recombination. In turn the yield of long‐lived charge carriers increases, resulting in a 33% increase in short circuit current (J _sc). In parallel, the two polymers show distinct grazing incidence X‐ray scattering spectra indicative of the presence of stacks with different orientation patterns in optimized thin‐film BHJ devices. Independent of the packing pattern the spectroscopic data also reveals the existence of polymer aggregates in the pristine polymer films as well as in both blends which trap excitons and hinder their dissociation.     

Charge Carrier Dynamics in a Ternary Bulk Heterojunction System Consisting of P3HT,Fullerene, and a Low Bandgap Polymer

The photoresponse of P3HT:PC₆₁BM based organic solar cells can be enhanced by blending the bulk heterojunction with the low band gap polymer Si‐ PCPDTBT. Organic solar cells containing the resulting ternary blend as the photoactive layer deliver short circuit currents of up to 15.5 mA cm^?2. Morphological studies show modest phase separation without the perturbation of the crystallinity of the P3HT:PC₆₁BM matrix, in accordance with the measured acceptable fill factors. Picosecond time‐resolved pump‐probe spectroscopy reveals that the sensitization of P3HT:PC₆₁BM with Si‐PCPDTBT involves the transfer of photogenerated positive polarons from the low band gap polymer to P3HT within few hundreds of picoseconds. Intensity dependent experiments in combination with global fitting show that the charge transfer from Si‐PCPDTBT to P3HT competes with non‐geminate charge carrier recombination of the holes in the Si‐PCPDTBT phase with electrons in the PC₆₁BM phase, both processes being of diffusive nature. At excitation densities corresponding to steady state conditions under one sun, modelling predicts hole transfer efficiencies exceeding 90%, in accordance with IQE measurements. At higher pump intensities, bimolecular recombination suppresses the hole transfer process effectively.     

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.     

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%.     

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.     

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.