Wide Bandgap Molecular Acceptors with a Truxene Core for Efficient Nonfullerene Polymer Solar Cells: Linkage Position on Molecular Configuration and Photovoltaic Properties

Two wide bandgap star‐shaped small molecular acceptors, para‐TrBRCN and meta‐TrBRCN , are synthesized for efficient nonfullerene polymer solar cells (PSCs). The tiny structural variation by just changing the linkage positions affects largely the inherent properties of the resulting molecules. Both molecules have a nonplanar 3D structure, which can prevent the excessively aggregation to realize the optimized morphology and ideal domain size in their active blends. Compared to para‐TrBRCN , meta‐TrBRCN exhibits the smaller distortions between the truxene skeleton and the benzothiadiazole units, which would also lead to the enhanced π–π stacking and charge transfer. When blending with PTB7‐Th, high power conversion efficiencies (PCEs) of 10.15% and 8.28% are obtained for meta‐TrBRCN and para‐TrBRCN devices, respectively. To make up the weak absorption of above binary active blend in the longer wavelength region and increase the whole device performance further, low bandgap 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone)‐5,5,11,11‐tetrakis(4‐hexylthienyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]‐dithiophene (ITIC‐Th) is added as the second acceptor material to fabricate ternary blend PSCs. After adding 20 wt% of ITIC‐Th, the resulting devices exhibit the well‐balanced optical absorption and fine‐tuned morphology, giving rise to the significantly improved PCE of 11.40% with much higher J _sc of 18.25 mA cm^?2 and fill factor of 70.2%.     

A Terminally Tetrafluorinated Nonfullerene Acceptor for Well‐Performing Alloy Ternary Solar Cells

Fabricating ternary solar cells is a pivotal strategy to improve the power conversion efficiencies (PCEs) of organic photovoltaic devices. However, it is still a challenge to simultaneously improve the performance parameters of ternary devices. Therefore, the third ingredient in ternary blends should be precisely designed or selected. Herein, a new medium‐bandgap small‐molecule acceptor, namely, 3,9‐bis(2‐methylene‐(3‐(1‐(3,5‐dimethylphenyl)‐1cyanomethylene)indanone))‐5,5,11,11‐tetrakis‐(4‐hexylphenyl)dithieno[2,3‐d:2′,3′‐d′]‐sindaceno[1,2‐b:5,6‐b′]dithiophene (ITIF), is synthesized by end‐capping with a new fluorinated, asymmetric terminal group, (Z)‐2‐(3,5‐difluorophenyl)‐2‐(3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene) acetonitrile. Replacing the CN substituent with the asymmetric 3,5‐difluorophenyl substituent obviously up‐shifts the lowest unoccupied molecular orbital (LUMO) level of ITIF to ?3.78 eV, enlarges the bandgap to 1.82 eV, and improves the absorption coefficient to ≈50% higher than that of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)indanone))‐5,5,11,11‐tetrakis‐(4‐hexylphenyl)dithieno[2,3‐d:2′,3′‐d′]‐sindaceno[1,2‐b:5,6‐b′]dithiophene (ITIC). Due to the similar structures, ITIF and ITIC can combine as an alloyed acceptor, which makes it convenient to tune the morphology and optical and electrochemical properties of ternary blends. The enhanced absorption coefficient of ITIF and the rapid fluorescence resonance energy transfer from ITIF to ITIC remarkably improve the absorption of the ternary blend film, hence compensating for the external quantum efficiency (EQE) curves. When ITIF is introduced into ternary solar cells 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):ITIF:ITIC blends, the PCEs of the ternary devices are increased from 9.2% to 10.5%, and the short‐circuit currents, open‐circuit voltages, and fill factors are simultaneously improved.     

An Electron Acceptor with Broad Visible–NIR Absorption and Unique Solid State Packing for As‐Cast High Performance Binary Organic Solar Cells

A novel acceptor–donor–acceptor (A–D–A) type electron acceptor 6TIC‐4F with terthieno[3,2‐b]thiophene (6T) as the core unit is rationally designed and synthesized, which exhibits an extraordinarily narrow bandgap (≈1.24 eV) and strong absorption between 650 and 1000 nm. X‐ray crystallographic analysis reveals that it has unique intermolecular π–π stacking. The solar cells based on the as‐cast 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): 6TIC‐4F binary blends exhibit an excellent power conversion efficiency (PCE) of 11.14% with a high J_SC of 23.00 mA cm^?2, and a high fill factor of 0.67, which represents one of the best PCE values for low bandgap (E_g < 1.3 eV)–based organic solar cells.     

Simultaneously Achieved High Open‐Circuit Voltage and Efficient Charge Generation by Fine‐Tuning Charge‐Transfer Driving Force in Nonfullerene Polymer Solar Cells

To maximize the short‐circuit current density (J_SC) and the open circuit voltage (V_OC) simultaneously is a highly important but challenging issue in organic solar cells (OSCs). In this study, a benzotriazole‐based p‐type polymer (J61) and three benzotriazole‐based nonfullerene small molecule acceptors (BTA1‐3) are chosen to investigate the energetic driving force for the efficient charge transfer. The lowest unoccupied molecular orbital (LUMO) energy levels of small molecule acceptors can be fine‐tuned by modifying the end‐capping units, leading to high V_OC (1.15–1.30 V) of OSCs. Particularly, the LUMO energy level of BTA3 satisfies the criteria for efficient charge generation, which results in a high V_OC of 1.15 V, nearly 65% external quantum efficiency, and a high power conversion efficiency (PCE) of 8.25%. This is one of the highest V_OC in the high‐performance OSCs reported to date. The results imply that it is promising to achieve both high J_SC and V_OC to realize high PCE with the carefully designed nonfullerene acceptors.     

Metal Ion/Dendrimer Complexes with Tunable Work Functions in a Wide Range and Their Application as Electron‐ and Hole‐Transport Materials of Non‐Fullerene Organic Solar Cells

The work functions of electrodes can be modified by adding charge transport layers to have good energy level matching with the active materials for organic solar cells (OSCs). Usually, a certain material gives rise to one definite work function of an electrode. In this work it is demonstrated that complexes of poly(amido amine) (generation 3) (PAMAM) with Cu²⁺ can continuously tune the work function of indium tin oxide (ITO) in a range of 4–5 eV by controlling the ratio of Cu²⁺ to PAMAM. PAMAM can lower the work function of ITO from 4.60 to 4.07 eV, while Cu²⁺‐PAMAM can increase the work function. The work function increase depends on the Cu²⁺‐to‐PAMAM molar ratio, and the work function can be up to 4.96 eV. The Cu²⁺ effect is ascribed the Cu²⁺‐caused change in the dipole moment of PAMAM. Moreover, this method can be used to continuously modify the work function of other materials, including Ag, Au, poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate, and reduced graphene oxide. In addition, PAMAM and Cu²⁺‐PAMAM are investigated as the charge collection buffer materials of non‐fullerene OSCs of 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))]: 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). The power conversion efficiency can reach 9.2%, which is comparable to that using conventional charge transport materials.     

Wide Bandgap Copolymers Based on Quinoxalino[6,5‐f].quinoxaline for Highly Efficient Nonfullerene Polymer Solar Cells

Two novel wide bandgap copolymers based on quinoxalino[6,5‐f]quinoxaline (NQx) acceptor block, PBDT–NQx and PBDTS–NQx, are successfully synthesized for efficient nonfullerene polymer solar cells (PSCs). The attached conjugated side chains on both benzodithiophene (BDT) and NQx endow the resulting copolymers with low‐lying highest occupied molecular orbital (HOMO) levels. The sulfur atom insertion further reduces the HOMO level of PBDTS–NQx to ?5.31 eV, contributing to a high open‐circuit voltage, V _oc, of 0.91 V. Conjugated n ‐octylthienyl side chains attached on the NQx skeletons also significantly improve the π–π* transitions and optical absorptions of the copolymers in the region of short wavelengths, which induce a good complementary absorption when blending with the low bandgap small molecular acceptor of 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. The wide absorption range makes the active blends absorb more photons, giving rise to a high short‐circuit current density, J _sc, value of 15.62 mA cm^?2. The sulfur atom insertion also enhances the crystallinity of PBDTS–NQx and presents its blend film with a favorable nanophase separation, resulting in improved J _sc and fill factor (FF) values with a high power conversion efficiency of 11.47%. This work not only provides a new fused ring acceptor block (NQx) for constructing high‐performance wide bandgap copolymers but also provides the NQx‐based copolymers for achieving highly efficient nonfullerene PSCs.     

Complementary Absorbing Star‐Shaped Small Molecules for the Preparation of Ternary Cascade Energy Structures in Organic Photovoltaic Cells

Two anthracene‐based star‐shaped conjugated small molecules, 5′,5″‐(9,10‐bis((4‐hexylphenyl)ethynyl)anthracene‐2,6‐diyl)bis(5‐hexyl‐2,2′‐bithiophene), HBantHBT, and 5′,5″‐(9,10‐bis(phenylethynyl)anthracene‐2,6‐diyl)bis(5‐hexyl‐2,2′‐bithiophene), BantHBT, are used as electron‐cascade donor materials by incorporating them into organic photovoltaic cells prepared using a poly((5,5‐E‐alpha‐((2‐thienyl)methylene)‐2‐thiopheneacetonitrile)‐alt‐2,6‐[(1,5‐didecyloxy)naphthalene])) (PBTADN):[6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) blend. The small molecules penetrate the PBTADN:PC₇₁BM blend layer to yield complementary absorption spectra through appropriate energy level alignment and optimal domain sizes for charge carrier transfer. A high short‐circuit current (J_SC) and fill factor (FF) are obtained using solar cells prepared with the ternary blend. The highest photovoltaic performance of the PBTADN: BantHBT :PC₇₁BM blend solar cells is characterized by a J_SC of 11.0 mA cm^?2, an open circuit voltage (V_OC) of 0.91 V, a FF of 56.4%, and a power conversion efficiency (PCE) of 5.6% under AM1.5G illumination (with a high intensity of 100 mW^?2). The effects of the small molecules on the ternary blend are investigated by comparison with the traditional poly(3‐hexylthiophene) (P3HT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PC₆₁BM) system.     

Subtle Polymer Donor and Molecular Acceptor Design Enable Efficient Polymer Solar Cells with a Very Small Energy Loss

A new wide bandgap polymer donor, PNDT‐ST, based on naphtho[2,3‐b:6,7‐b′]dithiophene (NDT) and 1,3‐bis(thiophen‐2‐yl)‐5,7‐bis(2‐ ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) is developed for efficient nonfullerene polymer solar cells. To better match the energy levels, a new near infrared small molecule of Y6‐T is also developed. The extended π‐conjugation and less twist of PNDT‐ST provides it with higher crystallinity and stronger aggregation than the PBDT‐ST counterpart. The higher lowest occupied molecular orbital level of Y6‐T than Y6 favors the better energy level match with these polymers, resulting in improved open circuit voltage (V_oc) and power conversion efficiency (PCE). The high crystallinity and strong aggregation of PNDT‐ST also induces large phase separation with poorer morphology, leading to lower fill factor and reduced PCE than PBDT‐ST. To mediate the crystallinity and optimize the morphology, PNDT‐ST and PBDT‐ST are blended together with Y6‐T, forming the ternary blend devices. As expected, the two compatible polymers allow continual optimization of the morphology by varying the blend ratio. The optimized ternary blend devices deliver a champion PCE as high as 16.57% with a very small energy loss (E_loss) of 0.521 eV. Such small E_loss is the best record for polymer solar cells with PCEs over 16% to date.     

Impact of Nonfullerene Molecular Architecture on Charge Generation,Transport, and Morphology in PTB7‐Th‐Based Organic Solar Cells

Despite the rapid development of nonfullerene acceptors (NFAs), the fundamental understanding on the relationship between NFA molecular architecture, morphology, and device performance is still lacking. Herein, poly[[4,8‐bis[5‐(2‐ethylhexyl)thiophene‐2‐yl]benzo[1,2‐b:4,5‐b0]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]‐thieno[3,4‐b]thiophenediyl]] (PTB7‐Th) is used as the donor polymer to compare an NFA with a 3D architecture (SF‐PDI4) to a well‐studied NFA with a linear acceptor–donor–acceptor (A–D–A) architecture (ITIC). The data suggest that the NFA ITIC with a linear molecular structure shows a better device performance due to an increase in short‐circuit current ( J_sc) and fill factor (FF) compared to the 3D SF‐PDI4. The charge generation dynamics measured by femtosecond transient absorption spectroscopy (TAS) reveals that the exciton dissociation process in the PTB7‐Th:ITIC films is highly efficient. In addition, the PTB7‐Th:ITIC blend shows a higher electron mobility and lower energetic disorder compared to the PTB7‐Th:SF‐PDI4 blend, leading to higher values of J_sc and FF. The compositional sensitive resonant soft X‐ray scattering (R‐SoXS) results indicate that ITIC molecules form more pure domains with reduced domain spacing, resulting in more efficient charge transport compared with the SF‐PDI4 blend. It is proposed that both the molecular structure and the corresponding morphology of ITIC play a vital role for the good solar cell device performance.     

The Impact of Sequential Fluorination of π‐Conjugated Polymers on Charge Generation in All‐Polymer Solar Cells

The performance of all‐polymer solar cells (all‐PSCs) is often limited by the poor exciton dissociation process. Here, the design of a series of polymer donors ( P1 – P3 ) with different numbers of fluorine atoms on their backbone is presented and the influence of fluorination on charge generation in all‐PSCs is investigated. Sequential fluorination of the polymer backbones increases the dipole moment difference between the ground and excited states (Δµ_ge) from P1 (18.40 D) to P2 (25.11 D) and to P3 (28.47 D). The large Δµ_ge of P3 leads to efficient exciton dissociation with greatly suppressed charge recombination in P3 ‐based all‐PSCs. Additionally, the fluorination lowers the highest occupied molecular orbital energy level of P3 and P2 , leading to higher open‐circuit voltage (V_OC). The power conversion efficiency of the P3 ‐based all‐PSCs (6.42%) outperforms those of the P2 and P1 (5.00% and 2.65%)‐based devices. The reduced charge recombination and the enhanced polymer exciton lifetime in P3 ‐based all‐PSCs are confirmed by the measurements of light‐intensity dependent short‐circuit current density (J_SC) and V_OC, and time‐resolved photoluminescence. The results provide reciprocal understanding of the charge generation process associated with Δµ_ge in all‐PSCs and suggest an effective strategy for designing π‐conjugated polymers for high performance all‐PSCs.     

Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero‐interface band structures while maintaining surface passivation

Interdigitated back contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high open circuit voltage (V_OC) due to the surface passivation and heterojunction contacts, and high short circuit current density (J_SC) due to all back contact design. Intrinsic amorphous silicon (a‐Si:H) buffer layer at the rear surface improve the surface passivation hence V_OC and J_SC, but degrade fill factor (FF) from an “S” shape J–V curve. Two‐dimensional (2D) simulation using “Sentaurus device” demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero‐interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness, (2) increased conductivity, and/or (3) reduced band gap. Experimental IBC‐SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF, consistent with simulation. However, this has only marginal effect on efficiency since J_SC and V_OC also decrease due to poor surface passivation. A narrow band gap a‐Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero‐interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation, and 26% with light trapping. This work provides criterion to design IBC‐SHJ solar cell structures and optimize cell performance. Copyright © 2010 John Wiley & Sons, Ltd.     

Thiocyanate‐Free Ru(II) Sensitizers with a 4,4′‐Dicarboxyvinyl‐2,2′‐bipyridine Anchor for Dye‐Sensitized Solar Cells

A new class of thiocyanate‐free Ru(II) sensitizers with 4,4′‐dicarboxyvinyl‐2,2′‐bipyridine anchor and two trans‐oriented pyrid‐2‐yl pyrazolate (or triazolate) functional chromophores is synthesized, characterized, and evaluated in dye‐sensitized solar cells (DSCs). Despite their enhanced red response and absorptivity when compared to the parent sensitizer TFRS‐2 that possesses standard 4,4′‐dicarboxyl‐2,2′‐bipyridine anchor and shows the best conversion efficiency of η = 9.82%, the newly synthesized carboxyvinyl‐pyrazolate sensitizers, TFRS‐11 – TFRS‐13 , exhibit inferior performance characteristics in terms of short‐circuit current density (J_SC), open‐circuit voltage (V_OC), and power conversion efficiency (η), the latter being recorded to be in the range 5.60–7.62%. The reduction in device efficiencies is attributed to a combination of poor packing of these sensitizers on the TiO₂ surface and less positive ground‐state oxidation potentials, which, respectively, increase charge recombination with I₃^? in electrolytes and impede the regeneration of sensitizers by I^? anions. The latter obstacle can be circumvented in part by the replacement of the pyrazolates with triazolates, forming the TFRS‐14 sensitizer, which exhibits an improved J_SC, V_OC, and η of 16.4 mAcm^?2, 0.77 V, and 9.02%, respectively.     

Understanding Temperature‐Dependent Charge Extraction and Trapping in Perovskite Solar Cells

Understanding the factors that limit the performance of perovskite solar cells (PSCs) can be enriched by detailed temperature (T)‐dependent studies. Based on p‐i‐n type PSCs with prototype methylammonium lead triiodide (MAPbI₃) perovskite absorbers, T‐dependent photovoltaic properties are explored and negative T‐coefficients for the three device parameters (V_OC, J_SC, and FF) are observed within a wide low T‐range, leading to a maximum power conversion efficiency (PCE) of 21.4% with an impressive fill factor (FF) approaching 82% at 220 K. These T‐behaviors are explained by the enhanced interfacial charge transfer, reduced charge trapping with suppressed nonradiative recombination and narrowed optical bandgap at lower T. By comparing the T‐dependent device behaviors based on MAPbI₃ devices containing a PASP passivation layer, enhanced PCE at room temperature is observed but different tendencies showing attenuating T‐dependencies of J_SC and FF, which eventually leads to nearly T‐invariable PCEs. These results indicate that charge extraction with the utilized all‐organic charge transporting layers is not a limiting factor for low‐T device operation, meanwhile the trap passivation layer of choice can play a role in the T‐dependent photovoltaic properties and thus needs to be considered for PSCs operating in a temperature‐variable environment.     

Efficient Polymer Solar Cells with a High Open Circuit Voltage of 1 Volt

A series of polymers containing benzo[1,2‐b:4,5‐b′]dithiophene and N‐alkylthieno[3,4‐c]pyrrole‐4,6‐dione are designed. By incorporating different alkylthienyl side chains, the fill factor (FF) and open circuit voltage (V_oc) of the copolymers are further improved. The experimental results and theoretical calculations show that the size and topology of the side chains can influence the polymer solubility, energy levels, and intermolecular packing by altering the molecular coplanarity. As a result of improved morphology and fine‐tuned energy levels, an increased FF and a high V_oc of 1.00 V are achieved, as well as a power conversion efficiency of 6.17%, which is the highest efficiency ever reported for polymer solar cells with a V_oc over 1 V.     

Effects of Nonradiative Losses at Charge Transfer States and Energetic Disorder on the Open‐Circuit Voltage in Nonfullerene Organic Solar Cells

The considerable improvement on the power conversion efficiency (PCE) for emerging nonfullerene polymer solar cells is still limited by considerable voltage losses that have become one of the most significant obstacles in further boosting desired photovoltaic performance. Here, a comprehensive study is reported to understand the impacts of charge transport, energetic disorder, and charge transfer states (CTS) on the losses in open‐circuit voltage (V_oc) based on three high performing bulk heterojunction solar cells with the best PCE exceeding 11%. It is found that the champion poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene)‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)‐benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione))] (PBDB‐T):IT‐M solar cell (PCE = 11.5%) is associated with the least disorder. The determined energetic disorder in part reconciles the difference in V_oc between the solar cells. A reduction is observed in the nonradiative losses (ΔV_nonrad) coupled with the increase of energy of CTS for the PBDB‐T:IT‐M device, which may be related to the improved balance in carrier mobilities, and partially can explain the gain in V_oc. The determined radiative limit for V_oc combined with the ΔV_nonrad generates an excellent agreement for the V_oc with the experimental values. The results suggest that minimizing the energetic disorder related to transport and CTS is critical for the mitigation of V_oc losses and improvements on the device performance.     

Bithiazole-based copolymer with deep HOMO level and noncovalent conformational lock for organic photovoltaics

To match with the state-the-art-of non-fullerene acceptor for polymer solar cells (PSCs), it is urgent to develop high performance wide-bandgap (WBG) copolymer donors with deep highest occupied molecular orbital (HOMO) level and highly planar backbone to further promote the device efficiency. A new WBG copolymer PBDTBTz, poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithi ophene))-alt-(5,5’-(-4,4’-dinonyl-2,2’-bithiazole))], which is based on benzodithiophene (BDT) as donor unit and 4,4’-dinonyl-2,2’-bithiazole (BTz) as acceptor unit is prepared for non-fullerene PSCs. Due to the strong electron-withdrawing effect, BTz unit can dramatically lower the HOMO level of PBDTBTz to −5.60 eV. Notably, double noncovalent conformational locks (N⋯S) of BTz are formed in the backbone to reduce the steric hindrance and favor a highly planar geometry for efficient charge transport and molecular packing. As a result, the device based on PBDTBTz as donor and 3,9-bis(2-methylene-((3-(1,1-dic-yanomethylene)-6,7-difluoro)-indanone))-5,5,11, 11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d']-s-indaceno[1,2-b:5,6-b']dithiophen e(IT-4F) as acceptor afforded a high open circuit voltage (V_oc) of 0.92 V, which is the highest value for IT-4F-based PSCs reported so far. Furthermore, the device operated well with a negligible driving force (ΔE_HOMO) of as small as 0.06 eV. These results revealed that combination of electron-withdrawing bithiazole and double noncovalent conformational lock of N⋯S is a promising molecular design concept of polymer donor with deep and planar structure for high performance PSCs.     

Near‐Infrared Small Molecule Acceptor Enabled High‐Performance Nonfullerene Polymer Solar Cells with Over 13% Efficiency

One of the most promising approaches to achieve high‐performance polymer solar cells (PSCs) is to develop nonfullerene small molecule acceptors (SMAs) with an absorption extending to the near‐infrared (NIR) region. In this work, two novel SMAs, namely, BTTIC and BTOIC, are designed and synthesized, with optical bandgaps (E_g^opt) of 1.47 and 1.39 eV, respectively. Desipte the narrow E_g^opt, the PBDB‐T:BTTIC‐ and PBDB‐T:BTOIC‐based PSCs can maintain high V_OCs of over 0.90 and 0.86 V, respectively, with low energy losses (E_loss) < 0.6 eV. Meanwhile, due to the favorable morphology of the PBDB‐T:BTTIC blend, balanced carrier mobilities are achieved. The high external quantum efficiencies enable a high power conversion efficiency (PCE) up to 13.18% for the PBDB‐T:BTTIC‐based PSCs. In comparison, BTOIC shows an excessive crystallization propensity owing to its oxyalkyl side groups, which eventually leads to a relatively low PCE for the PBDB‐T:BTOIC‐based PSCs. Overall, this work provides insights into the design of novel NIR‐absorbing SMAs for nonfullerene PSCs.     

Enhanced Light‐Harvesting Capability of a Panchromatic Ru(II) Sensitizer Based on π‐Extended Terpyridine with a 4‐Methylstylryl Group for Dye‐Sensitized Solar Cells

A novel Ru π‐expanded terpyridyl sensitizer, referred to as HIS‐2, is prepared based on the molecular design strategy of substitution with a moderately electron‐donating 4‐methylstyryl group onto the terpyridyl ligand. The HIS‐2 dye exhibits a slightly increased metal‐to‐ligand charge transfer (MLCT) absorption at around 600 nm and an intense π–π* absorption in the UV region compared with a black dye. Density functional theory calculations reveal that the lowest unoccupied molecular orbital (LUMO) is distributed over the terpyridine and 4‐methylstyryl moieties, which enhances the light‐harvesting capability and is appropriate for smooth electron injection from the dye to the TiO₂ conduction band. The incident photon‐to‐electricity conversion efficiency spectrum of HIS‐2 exhibits better photoresponse compared with black dye over the whole spectral region as a result of the extended π‐conjugation. A DSC device based on black dye gives a short‐circuit current (J_SC) of 21.28 mA cm^?2, open‐circuit voltage (V_OC) of 0.69 V, and fill factor (FF) of 0.72, in an overall conversion efficiency (η) of 10.5%. In contrast, an HIS‐2 based cell gives a higher J_SC value of 23.07 mA cm^?2 with V_OC of 0.68 V, and FF of 0.71, and owing to the higher J_SC value of HIS‐2, an improved η value of 11.1% is achieved.     

Efficient Infrared Solar Cells Employing Quantum Dot Solids with Strong Inter-Dot Coupling and Efficient Passivation

Lead chalcogenide quantum dot (QD) infrared (IR) solar cells are promising devices for breaking through the theoretical efficiency limit of single-junction solar cells by harvesting the low-energy IR photons that cannot be utilized by common devices. However, the device performance of QD IR photovoltaic is limited by the restrictive relation between open-circuit voltages (V_OC) and short circuit current densities (J_SC), caused by the contradiction between surface passivation and electronic coupling of QD solids. Here, a strategy is developed to decouple this restriction via epitaxially coating a thin PbS shell over the PbSe QDs (PbSe/PbS QDs) combined with in situ halide passivation. The strong electronic coupling from the PbSe core gives rise to significant carrier delocalization, which guarantees effective carrier transport. Benefited from the protection of PbS shell and in situ halide passivation, excellent trap-state control of QDs is eventually achieved after the ligand exchange. By a fine control of the PbS shell thickness, outstanding IR J_SC of 6.38 mA cm⁻² and IR V_OC of 0.347 V are simultaneously achieved under the 1100 nm-filtered solar illumination, providing a new route to unfreeze the trade-off between V_OC and J_SC limited by the photoactive layer with a given bandgap.     

Morphology optimization of organic solar cells enabled by interface engineering of zinc oxide layer with a conjugated organic material

A diphenylphosphine-oxide-based conjugated organic molecule, ((1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T), was doped into ZnO to improve the characteristics of the electron transport layer (ETL) in inverted organic solar cells (OSCs). A series of characterization techniques were carried out to demonstrate the function of PO-T2T in film aspect, including transmittance, atomic force microscopy (AFM), transmission electron microscopy (TEM), water contact angle and grazing incidence wide angle X-ray scattering (GIWAXS). Light dependent, space-charge-limited current, exciton dissociation possibility were aimed to explore the influence of PO-T2T for internal carrier behaviors based on PTB7-Th: PC₇₁BM system. It's found that the PO-T2T doped ETLs played a role in morphology optimization of ETL and undermined the trap-assistant recombination through filling the defects ZnO itself had, simultaneously. Besides, the electron mobility was also improved. With the optimized functionalities, the OSCs' efficiency based on fullerene system Poly[4,8- bis(5-(2-Ethylhexyl)thiophen-2-yl) benzo [1,2-b:4,5-b′] dithiophene-co-3-fluorothieno [3,4-b] thiophene-2-carboxylate] (PTB7-Th): [6,6]-Phenyl C71 butyric acid methyl ester (PC₇₁BM) was improved from 9.03% to 9.84%. Finally, when this strategy was applied into another hot-topic system, poly((2,6-(4,8-bis(5-(2-ethylhexyl-3- fluoro)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-TF):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), a high PCE of 16.34% was obtained. These results demonstrated that the PO-T2T had a positive role in OSC performance improvement.