Morphology Related Photodegradation of Low‐Band‐Gap Polymer Blends

The morphology related photodegradation of low band‐gap polymer blends is investigated using optical microscopy and scanning probe microscopy. Poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7(2,1,3‐benzothiadiazole)] (C‐PCPDTBT):[6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) blend films without and with ODT, as well as poly[(4,40‐bis(2‐ethylhexyl)dithieno[3,2‐b:20,30‐d]silole)‐2,6‐diylalt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (Si‐PCPDTBT):PCBM blend films exposed to a focused 632.8 nm laser under ambient condition with and without inert gas protection are studied. The photodegradation of the polymer starts in the vicinity of the PCBM molecules (first sphere degradation), which effectively blocks the electron transfer processes. Stern‐Volmer type kinetics are observed in the C‐PCPDTBT:PCBM blend with ODT, which indicates that only a small number of photo‐oxidized monomer units act as quenchers of the C‐PCPDTBT polymer luminescence. Furthermore, in addition to the permanent damage of the polymer molecules, as witnessed from their Raman intensity decrease, the polymer photoluminescence demonstrates partial reversible recovery when inert gas protection is resumed, indicating the involvement of temporary polymer/O₂‐charge transfer complexes in the photodegradation process.     

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%.     

Charge Transport and Recombination in Low‐Bandgap Bulk Heterojunction Solar Cell using Bis‐adduct Fullerene

Charge transport and recombination are studied for organic solar cells fabricated using blends of polymer poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole)‐5,5′‐diyl] (Si‐PCPDTBT) with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (mono‐PCBM) and the bis‐adduct analogue of mono‐PCBM (bis‐PCBM). The photocurrent of Si‐PCPDTBT:bis‐PCBM devices shows a strong square root dependence on the effective applied voltage. From the relationship between the photocurrent and the light intensity, we found that the square‐root dependence of the photocurrent is governed by the mobility‐lifetime (μτ) product of charge carriers while space‐charge field effects are insignificant. The fill factor (FF) and short circuit current density (J_sc) of bis‐PCBM solar cells show a considerable increase with temperature as compared to mono‐PCBM solar cells. SCLC analysis of single carrier devices proofs that the mobility of both electrons and holes is significantly lowered when replacing mono‐PCBM with bis‐PCBM. The increased recombination in Si‐PCPDTBT:bis‐PCBM solar cells is therefore attributed to the low carrier mobilities, as the transient photovoltage measurements show that the carrier lifetime of devices are not significantly altered by using bis‐PCBM instead of mono‐PCBM.     

High Fill Factor Polymer Solar Cells Incorporating a Low Temperature Solution Processed WO3 Hole Extraction Layer

We demonstrate solution‐processed tungsten trioxide (WO₃) incorporated as hole extraction layer (HEL) in polymer solar cells (PSCs) with active layers comprising either poly(3‐hexylthiophene) (P3HT) or poly[(4,4'‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole)‐5,50‐diyl] (Si‐PCPDTBT) mixed with a fullerene derivative. The WO₃ layers are deposited from an alcohol‐based, surfactant‐free nanoparticle solution. A short, low‐temperature (80 °C) annealing is sufficient to result in fully functional films without the need for an oxygen‐plasma treatment. This allows the application of the WO₃ buffer layer in normal as well as inverted architecture solar cells. Normal architecture devices based on WO₃ HELs show comparable performance to the PEDOT:PSS reference devices with slightly better fill factors and open circuit voltages. Very high shunt resistances (over 1 MΩ cm²) and excellent diode rectification underline the charge selectivity of the solution‐processed WO₃ layers.     

Morphological Characterization of a Low‐Bandgap Crystalline Polymer:PCBM Bulk Heterojunction Solar Cells

Understanding the morphology of polymer‐based bulk heterojunction (BHJ) solar cells is necessary to improve device efficiencies. Blends of a low‐bandgap silole‐containing conjugated polymer, poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b;2′,3′‐d]silole)‐2,6‐diyl‐alt‐(4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole)‐5,5′‐diyl] (PSBTBT) with [6,6]phenyl‐C61‐butyric acid methyl ester (PCBM) were investigated under different processing conditions. The surface morphologies and vertical segregation of the “As‐Spun”, “Pre‐Annealed”, and “Post‐Annealed” films were studied by scanning force microscopy, contact angle measurements, X‐ray photoelectron spectroscopy, near‐edge X‐ray absorption fine structure spectroscopy, dynamic secondary ion mass spectrometry, and neutron reflectivity. The results showed that PSBTBT was enriched at the cathode interface in the “As‐Spun” films and thermal annealing increased the segregation of PSBTBT to the free surface, while thermal annealing after deposition of the cathode increased the PCBM concentration at the cathode interface. Grazing‐incidence X‐ray diffraction and small‐angle neutron scattering showed that the crystallization of PSBTBT and segregation of PCBM occurred during spin coating, and thermal annealing increased the ordering of PSBTBT and enhanced the segregation of the PCBM, forming domains ～10 nm in size, leading to an improvement in photovoltaic performance.     

The Role of Photon Energy in Free Charge Generation in Bulk Heterojunction Solar Cells

To determine the role of photon energy on charge generation in bulk heterojunction solar cells, the bias voltage dependence of photocurrent for excitation with photon energies below and above the optical band gap is investigated in two structurally related polymer solar cells. Charges generated in (poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothia­diazole)] (C‐PCPDTBT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) solar cells via excitation of the low‐energy charge transfer (CT) state, situated below the optical band gap, need more voltage to be extracted than charges generated with excitation above the optical band gap. This indicates a lower effective binding energy of the photogenerated electrons and holes when the excitation is above the optical band gap than when excitation is to the bottom of the CT state. In blends of PCBM with the silicon‐analogue, 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] (Si‐PCPDTBT), there is no effect of the photon energy on the electric field dependence of the dissociation efficiency of the CT state. C‐PCPDTBT and Si‐PCPDTBT have very similar electronic properties, but their blends with PCBM differ in the nanoscale phase separation. The morphology is coarser and more crystalline in Si‐PCPDTBT:PCBM blends. The results demonstrate that the nanomorphological properties of the bulk heterojunction are important for determining the effective binding energy in the generation of free charges at the heterojunction.     

Domain Compositions and Fullerene Aggregation Govern Charge Photogeneration in Polymer/Fullerene Solar Cells

The complex microstructure of organic semiconductor mixtures continues to obscure the connection between the active layer morphology and photovoltaic device performance. For example, the ubiquitous presence of mixed phases in the active layer of polymer/fullerene solar cells creates multiple morphologically distinct interfaces which are capable of exciton dissociation or charge recombination. Here, it is shown that domain compositions and fullerene aggregation can strongly modulate charge photogeneration at ultrafast timescales through studies of a model system, mixtures of a low band‐gap polymer, poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]germole)‐2,6‐diyl‐alt‐(2,1,3‐benzothia‐diazole)‐4,7‐diyl], and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester. Structural characterization using energy‐filtered transmission electron microscopy (EFTEM) and resonant soft X‐ray scattering shows similar microstructures even with changes in the overall film composition. Composition maps generated from EFTEM, however, demonstrate that compositions of mixed domains vary significantly with overall film composition. Furthermore, the amount of polymer in the mixed domains is inversely correlated with device performance. Photoinduced absorption studies using ultrafast infrared spectroscopy demonstrate that polaron concentrations are highest when mixed domains contain the least polymer. Grazing‐incidence X‐ray scattering results show that larger fullerene coherence lengths are correlated to higher polaron yields. Thus, the purity of the mixed domains is critical for efficient charge photogeneration because purity modulates fullerene aggregation and electron delocalization.     

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.     

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.     

Photo‐Carrier Recombination in Polymer Solar Cells Based on P3HT and Silole‐Based Copolymer

Photo‐current loss in donor‐acceptor (DA) polymer‐fullerene bulk heterojunction solar cells was studied via carrier transport and recombination measurements. Focusing on the DA polymer poly((4,4‐dioctyldithieno (3,2‐b:2',3'‐d) silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl) (DTS‐BTD) we found that the carrier transport is well‐balanced and attribute the loss mechanism in DTS‐BTD solar cells to carrier recombination. Using carrier extraction with linear increasing voltage (photo‐CELIV) and transient photo‐voltage (TPV), we show that carrier recombination plays an important role in photo‐current extraction at open circuit conditions due to increase in photo‐excited carrier concentration. Delay time dependent photo‐CELIV and temperature dependent transport studies suggest that the recombination rate is related to the degree of energetic disorder in the polymer: fullerene blends.     

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.     

Real‐Time Photoluminescence Studies of Structure Evolution in Organic Solar Cells

A method to study the structural evolution of organic bulk heterojunctions via real‐time, in‐situ, steady‐state photoluminescence (PL) is presented. In‐situ PL, in combination with real‐time transmission and reflection measurements, allows us to quantitatively describe the progression of intimate mixing during blade coating of two OPV systems: the common model system poly(3‐hexylthiophene‐2,5‐diyl)/phenyl‐C61‐butyric‐acid‐methyl ester (P3HT/PCBM), and the higher power conversion efficiency system 7,7′‐(4,4bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(6‐fluoro‐5‐(5′‐hexyl‐[2,2′‐bithiophen]‐5‐yl)benzo[c][1,2,5]thiadiazaole), p‐DTS(FBTTh₂)₂/[70]PCBM. Evaluating the time dependence of the PL intensity during drying using a 3D‐random‐walk diffusion model allows for the quantitative determination of the ratio of characteristic domain size to exciton diffusion length during solidification in the presence of the processing additives 1‐chloronaphtalene (CN), 1,8‐octanedithiol (ODT), and 1,8‐diiodooctane (DIO). In both cases, the obtained results are in good agreement with the typically observed fibril widths and grain sizes, for P3HT and p‐DTS(FBTTh₂)₂, respectively.     

Significance of Average Domain Purity and Mixed Domains on the Photovoltaic Performance of High‐Efficiency Solution‐Processed Small‐Molecule BHJ Solar Cells

Whereas the role of molecularly mixed domains in organic photovoltaic devices for charge generation is extensively discussed in the literature, the impact on charge recombination and thus fill factor is largely unexplored. Here, a combination of soft X‐ray techniques enables the quantification of phases at multiple length scales to reveal their role regarding charge recombination in a highly efficient solution processed small molecule system 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) . A quantitative (linear) relationship between the average composition variations and the device fill‐factor is observed. The results establish the complex interrelationship between average phase purity, domain size, and structural order and highlight the requirement of achieving sufficient phase purities to diminish bimolecular and geminate recombination in solution processed small molecule solar cells.     

Dopamine Semiquinone Radical Doped PEDOT:PSS: Enhanced Conductivity,Work Function and Performance in Organic Solar Cells

Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been is applied as hole transport material in organic electronic devices for more than 20 years. However, the redundant sulfonic acid group of PEDOT:PSS has often been overlooked. Herein, PEDOT:PSS‐DA is prepared via a facile doping of PEDOT:PSS with dopamine hydrochloride (DA·HCl) which reacts with the redundant sulfonic acid of PSS. The PEDOT:PSS‐DA film exhibits enhanced work function and conductivity compared to those of PEDOT:PSS. PEDOT:PSS‐DA‐based devices show a power conversion efficiency of 16.55% which is the highest in organic solar cells (OSCs) with (poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)‐4‐fluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithio‐phene))‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)‐benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione))] (PM6):(2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐ethylhexyl)‐3,9‐diundecyl‐12,13‐dihydro‐[1,2,5]thiadiazolo[3,4‐e]thieno[2′′,3′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2‐g]thieno[2′,3′:4,5]thieno[3,2‐b]indole‐2,10‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile) (Y6) as the active layer. Furthermore, PEDOT:PSS‐DA also exhibits enhanced performance in three other donor/acceptor systems, exhibiting high compatibility in OSCs. This work demonstrates that doping PEDOT:PSS with various amino derivatives is a potentially efficient strategy to enhance the performance of PEDOT:PSS in organic electronic devices.     

Photoactive Blend Morphology Engineering through Systematically Tuning Aggregation in All‐Polymer Solar Cells

Polymer aggregation plays a critical role in the miscibility of materials and the performance of all‐polymer solar cells (APSCs). However, many aspects of how polymer texturing and aggregation affect photoactive blend film microstructure and photovoltaic performance are poorly understood. Here the effects of aggregation in donor–acceptor blends are studied, in which the number‐average molecular weights (M_ns) of both an amorphous donor polymer, poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b′]dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl)] ( PBDTT‐FTTE ) and a semicrystalline acceptor polymer, poly{[N,N′‐bis(2‐octyldodecyl)naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} ( P(NDI2OD‐T2) ) are systematically varied. The photovoltaic performance is correlated with active layer microstructural and optoelectronic data acquired by in‐depth transmission electron microscopy, grazing incidence wide‐angle X‐ray scattering, thermal analysis, and optical spectroscopic measurements. Coarse‐grained modeling provides insight into the effects of polymer aggregation on the blend morphology. Notably, the computed average distance between the donor and the acceptor polymers correlates well with solar cell photovoltaic metrics such as short‐circuit current density (J_sc) and represents a useful index for understanding/predicting active layer blend material intermixing trends. Importantly, these results demonstrate that for polymers with different texturing tendencies (amorphous/semicrystalline), the key for optimal APSC performance, photovoltaic blend morphology can be controlled via both donor and acceptor polymer aggregation.     

Stability of Nonfullerene Organic Solar Cells: from Built‐in Potential and Interfacial Passivation Perspectives

Remarkable progress has been made in the development of high‐efficiency solution‐processable nonfullerene organic solar cells (OSCs). However, the effect of the vertical stratification of bulk heterojunction (BHJ) on the efficiency and stability of nonfullerene OSCs is not fully understood yet. In this work, we report our effort to understand the stability of nonfullerene OSCs, made with the binary blend 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):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 (ITIC) system. It shows that a continuous vertical phase separation process occurs, forming a PBDB‐T‐rich top surface and an ITIC‐rich bottom surface in PBDB‐T:ITIC BHJ during the aging period. A gradual decrease in the built‐in potential (V₀) in the regular configuration PBDB‐T:ITIC OSCs, due to the interfacial reaction between the poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer and ITIC acceptor, is one of the reasons responsible for the performance deterioration. The reduction in V₀, caused by an inevitable reaction at the ITIC/PEDOT:PSS interface in the OSCs, can be suppressed by introducing a MoO₃ interfacial passivation layer. Retaining a stable and high V₀ across the BHJ through interfacial modification and device engineering, e.g., as seen in the inverted PBDB‐T:ITIC OSCs, is a prerequisite for efficient and stable operation of nonfullerene OSCs.     

Multi‐Length‐Scale Morphologies in PCPDTBT/PCBM Bulk‐Heterojunction Solar Cells

Additives are known to improve the performance of organic photovoltaic devices based on mixtures of a low bandgap polymer, 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) and [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM). The evolution of the morphology during the evaporation of the mixed solvent, which comprises additive and chlorobenzene (CB), is investigated by in‐situ grazing incidence X‐ray scattering, providing insight into the key role the additive plays in developing a multi‐length‐scale morphology. Provided the additive has a higher vapor pressure and a selective solubility for PCBM, as the host solvent (CB) evaporates, the mixture of the primary solvent and additive becomes less favorable for the PCPDTBT, while completely solubilizing the PCBM. During this process, the PCPDTBT first crystallizes into fibrils and then the PCBM, along with the remaining PCPDTBT, is deposited, forming a phase‐separated morphology comprising domains of pure, crystalline PCPDTBT fibrils and another domain that is a PCBM‐rich mixture with amorphous PCPDTBT. X‐ray/neutron scattering and diffraction methods, in combination with UV–vis absorption spectroscopy and transmission electron microscopy, are used to determine the crystallinity and phase separation of the resultant PCPDTBT/PCBM thin films processed with or without additives. Additional thermal annealing is carried out and found to change the packing of the PCPDTBT. The two factors, degree of crystallinity and degree of phase separation, control the multi‐length‐scale morphology of the thin films and significantly influence device performance.     

Highly Efficient Polymer Tandem Cells and Semitransparent Cells for Solar Energy

Highly efficient tandem and semitransparent (ST) polymer solar cells utilizing the same donor polymer blended with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) as active layers are demonstrated. A high power conversion efficiency (PCE) of 8.5% and a record high open‐circuit voltage of 1.71 V are achieved for a tandem cell based on a medium bandgap polymer poly(indacenodithiophene‐co‐phananthrene‐quinoxaline) (PIDT‐phanQ). In addition, this approach can also be applied to a low bandgap polymer poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7‐(5‐fluoro‐2,1,3‐benzothia‐diazole)] (PCPDTFBT), and PCEs up to 7.9% are achieved. Due to the very thin total active layer thickness, a highly efficient ST tandem cell based on PIDT‐phanQ exhibits a high PCE of 7.4%, which is the highest value reported to date for a ST solar cell. The ST device also possesses a desirable average visible transmittance (≈40%) and an excellent color rendering index (≈100), permitting its use in power‐generating window applications.     

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.     

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.