Chain Length Dependence of the Photovoltaic Properties of Monodisperse Donor–Acceptor Oligomers as Model Compounds of Polydisperse Low Band Gap Polymers

Well‐defined conjugated oligomers ( Sn ) containing from 1 to 8 units of a tricyclic building block involving a dioctyloxybenzothiadiazole unit with two thienyl side rings ( S1 ) are synthesized by a bottom‐up approach. UV–Vis absorption data of solutions show that chain extension produces a narrowing of the HOMO–LUMO gap (ΔE) to values slightly smaller than that of the parent polymer ( P1 ). Plots of ΔE and of the band gap of films (E _g) versus the reciprocal chain length show that ΔE and E _g converge towards a limit corresponding to an effective conjugation length (ECL) of 7–8 S1 units. UV–Vis absorption and photoluminescence data of solutions and solid films show that chain extension enhances the propensity to inter‐chain aggregation. This conclusion is confirmed by GIXD analyses which reveal that the edge‐on orientation of short‐chain systems evolves toward a face‐on orientation as chain length increases while the π‐stacking distance decreases beyond 7 units. The results obtained on solution‐processed BHJ solar cells show a progressive improvement of power conversion efficiency (PCE) with chain extension; however, the convergence limit of PCE remains inferior to that obtained with the polymer. These results are discussed with regard to the role of mono/polydispersity and chain aggregation.     

Alkoxy‐Functionalized Thienyl‐Vinylene Polymers for Field‐Effect Transistors and All‐Polymer Solar Cells

π‐conjugated polymers based on the electron‐neutral alkoxy‐functionalized thienyl‐vinylene (TVTOEt) building‐block co‐polymerized, with either BDT (benzodithiophene) or T2 (dithiophene) donor blocks, or NDI (naphthalenediimide) as an acceptor block, are synthesized and characterized. The effect of BDT and NDI substituents (alkyl vs alkoxy or linear vs branched) on the polymer performance in organic thin film transistors (OTFTs) and all‐polymer organic photovoltaic (OPV) cells is reported. Co‐monomer selection and backbone functionalization substantially modifies the polymer MO energies, thin film morphology, and charge transport properties, as indicated by electrochemistry, optical spectroscopy, X‐ray diffraction, AFM, DFT calculations, and TFT response. When polymer P7 is used as an OPV acceptor with PTB7 as a donor, the corresponding blend yields TFTs with ambipolar mobilities of μ_e = 5.1 × 10^?3 cm² V^–1 s^–1 and μ_h = 3.9 × 10^?3 cm² V^–1 s^–1 in ambient, among the highest mobilities reported to date for all‐polymer bulk heterojunction TFTs, and all‐polymer solar cells with a power conversion efficiency (PCE) of 1.70%, the highest reported PCE to date for an NDI‐polymer acceptor system. The stable transport characteristics in ambient and promising solar cell performance make NDI‐type materials promising acceptors for all‐polymer solar cell applications.     

Fluorobenzotriazole (FTAZ)‐Based Polymer Donor Enables Organic Solar Cells Exceeding 12% Efficiency

The fluorobenzotriazole (FTAZ)‐based copolymer donors are promising candidates for nonfullerene polymer solar cells (PSCs), but suffer from relatively low photovoltaic performance due to their unsuitable energy levels and unfavorable morphology. Herein, three polymer donors, L24 , L68 , and L810 , based on a chlorinated‐thienyl benzodithiophene (BDT‐2Cl) unit and FTAZ with different branched alkyl side chain, are synthesized. Incorporation of a chlorine (Cl) atom into the BDT unit is found to distinctly optimize the molecular planarity, energy levels, and improve the polymerization activity. Impressively, subtle side chain length of FTAZ realizes a dramatic improvement in all the device parameters, as revealed by the short‐current density (J_sc) improved from 7.41 to 20.76 mA cm^?2, fill‐factor from 36.3 to 73.5%, and even the open‐circuit voltage (V_oc) from 0.495 to 0.790 V. The best power conversion efficiency (PCE) of 12.1% is obtained from the L810‐based device, which is one of the highest values reported for FTAZ‐based PSCs so far. Notably, the corresponding external quantum efficiency curve keeps a very prominent value up to 80% from 500 to 800 nm. The notable performance is discovered from the reduced energy loss, improved molecular face‐on orientation, the down‐shifted energy levels, and optimized absorption coefficient regulated by side‐chain engineering.     

Fibril Network Strategy Enables High‐Performance Semitransparent Organic Solar Cells

The development of semitransparent organic solar cells (ST‐OSCs) represents a significant step toward the commercialization of OSCs. However, the trade‐off between power conversion efficiency (PCE) and average visible transmittance (AVT) restricts further improvements of ST‐OSCs. Herein, it is demonstrated that a fibril network strategy can enable ST‐OSCs with a high PCE and AVT simultaneously. A wide‐bandgap polymer PBT1‐C‐2Cl that can self‐assemble into a fibril nanostructure is used as the donor and a near‐infrared small molecule Y6 is adopted as the acceptor. It is found that a tiny amount of PBT1‐C‐2Cl in the blend can form a high speed pathway for hole transport due to the well distributed fibril nanostructure, which increases the transmittance in the visible region. Meanwhile, the acceptor Y6 guarantees sufficient light absorption. Using this strategy, the optimized ST‐OSCs yield a high PCE of 9.1% with an AVT of over 40% and significant light utilization efficiency of 3.65% at donor/acceptor ratio of 0.25:1. This work demonstrates a simple and effective approach to realizing high PCE and AVT of ST‐OSCs simultaneously.     

A 2.20 eV Bandgap Polymer Donor for Efficient Colorful Semitransparent Organic Solar Cells

Semitransparent organic solar cells (ST-OSCs) have attracted increasing attention due to their promising prospect in building-integrated photovoltaics. Generally, efficient ST-OSCs with good average visible transmittance (AVT) can be realized by developing active layer materials with light absorption far from the visible light range. Herein, the development of ultrawide bandgap polymer donors with near-ultraviolet absorption, paired with near-infrared acceptors, is proposed to achieve high-performance ST-OSCs. The key points for the design of ultrawide bandgap polymers include constructing donor–donor type conjugated skeleton, suppressing the quinoidal resonance effect, and minimizing the twist of conjugated skeleton via noncovalent conformational locks. As a proof of concept, a polymer named PBOF with an optical bandgap of 2.20 eV is synthesized, which exhibited largely reduced overlap with the human eye photopic response spectrum and afforded a power conversion efficiency (PCE) of 16.40% in opaque device. As a result, ST-OSCs with a PCE over 10% and an AVT over 30% are achieved without optical modulation. Moreover, colorful ST-OSCs with visual aesthetics can be achieved by tuning the donor/acceptor weight ratio in active layer benefiting from the ultrawide bandgap nature of PBOF. This study demonstrates the great potential of ultrawide bandgap polymers for efficient colorful ST-OSCs.     

“Solvent Annealing” Effect in Polymer Solar Cells Based on Poly(3‐hexylthiophene) and Methanofullerenes

The self‐organization of the polymer in solar cells based on regioregular poly(3‐hexylthiophene) (RR‐P3HT):[6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) is studied systematically as a function of the spin‐coating time t_s (varied from 20–80 s), which controls the solvent annealing time t_a, the time taken by the solvent to dry after the spin‐coating process. These blend films are characterized by photoluminescence spectroscopy, UV‐vis absorption spectroscopy, atomic force microscopy, and grazing incidence X‐ray diffraction (GIXRD) measurements. The results indicate that the π‐conjugated structure of RR‐P3HT in the films is optimally developed when t_a is greater than 1 min (t_s ～ 50 s). For t _s < 50 s, both the short‐circuit current (J_SC) and the power conversion efficiency (PCE) of the corresponding polymer solar cells show a plateau region, whereas for 50 < t_s < 55 s, the J_SC and PCE values are significantly decreased, suggesting that there is a major change in the ordering of the polymer in this time window. The PCE decreases from 3.6 % for a film with a highly ordered π‐conjugated structure of RR‐P3HT to 1.2 % for a less‐ordered film. GIXRD results confirm the change in the ordering of the polymer. In particular, the incident photon‐to‐electron conversion efficiency spectrum of the less‐ordered solar cell shows a clear loss in both the overall magnitude and the long‐wavelength response. The solvent annealing effect is also studied for devices with different concentrations of PCBM (PCBM concentrations ranging from 25 to 67 wt %). Under “solvent annealing” conditions, the polymer is seen to be ordered even at 67 wt % PCBM loading. The open‐circuit voltage (V_OC) is also affected by the ordering of the polymer and the PCBM loading in the active layer.     

Improved power conversion efficiency by insertion of RGO–TiO2 composite layer as optical spacer in polymer bulk heterojunction solar cells

We report that the power conversion efficiency (PCE) can be enhanced in polymer bulk heterojunction solar cells by inserting an interfacial electron transporting layer consisting of pristine TiO₂ or reduced graphene oxide–TiO₂ (RGO–TiO₂) between the active layer and cathode Al electrode. The enhancement in the PCE has been analyzed through the optical absorption, current–voltage characteristics under illumination and estimation of photo-induced charge carrier generation rate. It was found that either TiO₂ or RGO–TiO₂ interfacial layers improve the light harvesting, as well as the charge extraction efficiency, acting as a blocking layer for holes, and also reducing charge recombination. The combined enhancement in light harvesting property and charge collection efficiency improves the PCE of the organic solar cell up to 4.18% and 5.33% for TiO₂ and RGO–TiO₂ interfacial layer, respectively, as compared to a value of 3.26% for the polymer solar cell without interfacial layer.     

Distinguishing the Importance of Fullerene Phase Separation from Polymer Ordering in the Performance of Low Band Gap Polymer:Bis‐Fullerene Heterojunctions

One way to improve power conversion efficiency (PCE) of polymer based bulk‐heterojunction (BHJ) photovoltaic cells is to increase the open circuit voltage (V _oc). Replacing PCBM with bis‐adduct fullerenes significantly improves V _oc and the PCE in devices based on the conjugated polymer poly(3‐hexyl thiophene) (P3HT). However, for the most promising low band‐gap polymer (LBP) system, replacing PCBM with ICBA results in poor short‐circuit current (J _sc) and PCE although V _oc is significantly improved. The optimization of the morphology of as‐cast LBP/bis‐fullerene BHJ photovoltaics is attempted by adding a co‐solvent to the polymer/fullerene solution prior to film deposition. Varying the solubility of polymer and fullerene in the co‐solvent, bulk heterojunctions are fabricated with no change of polymer ordering, but with changes in fullerene phase separation. The morphologies of the as‐cast samples are characterized by small angle neutron scattering and neutron reflectometry. A homogenous dispersion of ICBA in LBP is found in the samples where the co‐solvent is selective to the polymer, giving poor device performance. Aggregates of ICBA are formed in samples where the co‐solvent is selective to ICBA. The resultant morphology improves PCE by up to 246%. A quantitative analysis of the neutron data shows that the interfacial area between ICBA aggregates and its surrounding matrix is improved, facilitating charge transport and improving the PCE.     

Comparison of Two D−A Type Polymers with Each Being Fluorinated on D and A Unit for High Performance Solar Cells

For the purpose of investigating the effect of fluorination position on D?A type conjugated polymer on photophysical and photovoltaic properties, two types of fluorinated polymere are synthesized, HF with fluorination on electron‐donating unit and FH with fluorination on electron‐accepting unit. Compared to non‐fluorinated polymer, fluorinated polymers exhibit deeper HOMO energy levels without change of bandgap and stronger vibronic shoulder in UV?visible absorption, indicating that fluorination enhances intermolecular interaction. HF with fluorinated D unit exhibits well‐developed fibril network, low bimolecular recombination and high hole mobility, which lead a high PCE of 7.10% in conventional single‐junction solar cells, which is higher than the PCE (6.41%) of FH with fluorinated A unit. Therefore, this result demonstrates that fluorination on electron‐donating unit in D?A polymers could be a promising strategy for achieving high performance polymer solar cells.     

Synthesis of a Modified PC70BM and Its Application as an Electron Acceptor with Poly(3‐hexylthiophene) as an Electron Donor for Efficient Bulk Heterojunction Solar Cells

A simple and effective modification of phenyl‐C₇₀‐butyric acid methyl ester (PC₇₀BM) is carried out in a single step after which the material is used as electron acceptor for bulk heterojunction polymer solar cells (PSCs). The modified PC₇₀BM, namely CN‐PC₇₀BM, showed broader and stronger absorption in the visible region (350–550 nm) of the solar spectrum than PC₇₀BM because of the presence of a cyanovinylene 4‐nitrophenyl segment. The lowest unoccupied molecular energy level (LUMO) of CN‐PC₇₀BM is higher than that of PC₇₀BM by 0.15 eV. The PSC based on the blend (cast from tetrahydrofuran (THF) solution) consists of P3HT as the electron donor and CN‐PC₇₀BM as the electron acceptor and shows a power conversion efficiency (PCE) of 4.88%, which is higher than that of devices based on PC₇₀BM as the electron acceptor (3.23%). The higher PCE of the solar cell based on P3HT:CN‐PC₇₀BM is related to the increase in both the short circuit current (J_sc) and the open circuit voltage (V_oc). The increase in J_sc is related to the stronger light absorption of CN‐PC₇₀BM in the visible region of the solar spectrum as compared to that of PC₇₀BM. In other words, more excitons are generated in the bulk heterojunction (BHJ) active layer. On the other hand, the higher difference between the LUMO of CN‐PC₇₀BM and the HOMO of P3HT causes an enhancement in the V_oc. The addition of 2% (v/v) 1‐chloronapthalene (CN) to the THF solvent during film deposition results in an overall improvement of the PCE up to 5.83%. This improvement in PCE can be attributed to the enhanced crystallinity of the blend (particularly of P3HT) and more balanced charge transport in the device.     

Defect‐Minimized PEDOT:PSS/Planar‐Si Solar Cell with Very High Efficiency

Hybrid solar cells made of a p‐type conducting polymer, poly(3,4‐ethyl thiophene):polystyrenesulfonate (PEDOT:PSS), on Si have gained considerable interest in the fabrication of cost‐effective high‐efficiency devices. However, most of the high power conversion efficiency (PCE) performances have been obtained from solar cells fabricated on surface‐structured Si substrates. High‐performance planar single‐junction solar cells have considerable advantages in terms of processing and cost, because they do not require the complex surface texturing processes. The interface of single‐junction solar cells can critically influence the performance. Here, we demonstrate the effect of adding different surfactants in a co‐solvent‐optimized PEDOT:PSS polymer, which, in addition to acting as a p‐layer and as an anti‐reflective coating, also enhances the device performance of a hybrid planar‐Si solar cell. Using time‐of‐flight secondary ion mass spectrometry, we conduct three‐dimensional chemical imaging of the interface, which enables us to characterize the micropore defects found to limit the PCE. Upon minimizing these micropore defects with the addition of optimized amounts of fluorosurfactant and co‐solvent, we achieve a PEDOT:PSS/planar‐Si cell with a record high PCE of 13.3% for the first time. Our present approach of micropore defect reduction can also be used to improve the performance of other organic electronic devices based on PEDOT:PSS.     

Nanoimprinted Grating‐Embedded Perovskite Solar Cells with Improved Light Management

Organic–inorganic halide perovskite solar cells are one of the most efficient photovoltaic devices, and their power conversion efficiency (PCE) continues to grow rapidly due to the optimization of device architecture and functional materials. The involved optical design has an important impact on performance by affecting the light harvesting capability. Here, demonstrated is a simple and effective strategy to boost device performance from the perspective of light management. A diffraction grating pattern into the TiO₂ scaffold and antireflection layer is simultaneously introduced to construct a double grating design for perovskite solar cells. The grating pattern obtained using a commercial compact disk as the hard mold can modulate light transport by enhancing the light transmittance and, thus, light absorption in perovskite solar cells. The grating pattern‐induced light trapping can significantly increase the short‐circuit current density and PCE by 4.8% and 7.9%, respectively, on average. The present work develops a feasible method by tailoring the morphology‐related optical property to further improve the performance of perovskite solar cells. The double grating design can also generate structural color and can provide an alternative solution for fabricating colorful solar cells.     

Enhanced efficiency in polymer solar cells by incorporation of phenothiazine-based conjugated polymer electrolytes

The power conversion efficiency (PCE) of bulk hetero-junction type polymer solar cells (PSCs) based on poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester can be enhanced by insertion of the thin layer of phenothiazine (PT)-based conjugated polymer electrolytes (CPEs), poly{10-[4-(N,N,N-trimethylammonium)butyl]-10H-phenothiazine} bromide (PHPT) and poly{10-butyl-10H-phenothiazine-alt-10-[4-(N,N,N-trimethylammonium)butyl]-10H-phenothiazine} bromide (PcoPT), at the cathode interface. The PHPT has quaternary ammonium salt on every side chains on PT rings whereas PcoPT has quaternary ammonium salt on the side chain on PT rings alternatively. Interestingly, regardless of high HOMO energy levels of PT-based CPEs (ca 5.0 eV), the thin layer of CPE at the cathode interface enhances the PCE by the formation of interface dipole which reduces the work function and a Schottky barrier at the cathode. It is also shown that the number of accumulated ionic groups of PcoPT on the photoactive active is larger than that of PHPT owing to the arrangement of side chain on the CPE backbone. In a similar way, the thin layer of PcoPT exhibits substantially better wetting ability on the photoactive layer than that of PHPT. Thus, the device with PcoPT exhibits higher short circuit current and fill factor than those of PSC with PHPT.     

Alkylthio substituted thiophene modified benzodithiophene-based highly efficient photovoltaic small molecules

Two new small molecules, namely DRSBDTT-EH and DRSBDTT-BO, with alkylthio thienyl substituted benzodithiophene as the central unit and 3-ethylrhodanine as end groups, were designed and studied as the donors for solution processed organic solar cells. The optimized device based on DRSBDTT-EH bearing shorter alkylthio chain gave a power conversion efficiency (PCE) of 8.78%, which is slightly higher than that of device (8.53%) based on DRSBDTT-BO with longer alkylthio chain.     

Large AuAg Alloy Nanoparticles Synthesized in Organic Media Using a One‐Pot Reaction: Their Applications for High‐Performance Bulk Heterojunction Solar Cells

A one‐pot synthesis of large size and high quality AuAg alloy nanoparticles (NPs) with well controlled compositions via hot organic media is demonstrated. Amid the synthesis, complexation between trioctylphosphine (TOP) and metal precursors is found, which slows down the rate of nucleation and leads to the growth of large‐size AuAg nanoalloys. The wavelength and relative intensities of the resulting plasmon bands are readily fine‐tuned during the synthetic process using different Au/Ag precursors molar ratios. In the polymer solar cells, a key step in achieving high efficiency is the utilization of 1% Au₁₁Ag₈₉ alloy NPs embedded in the active layer to promote the power conversion efficiency (PCE) up to 4.73%, which outperforms the reference device based on the control standard device of poly(3‐hexylthiophene) (P3HT):phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) under identical conditions. Corresponding increases in short‐circuit current density (J_sc), open‐circuit voltage (V_oc), fill factor (FF), and incident photon‐to‐current efficiency (IPCE) enable 31% PCE improvement due to the enhancement of the light‐trapping and the improvement of charge transport in the active layer. The findings advance the fundamental understanding and point to the superiority of Au₁₁Ag₈₉ nanoalloys as a promising metallic additive over Au, Ag, and Au₂₈Ag₇₂ alloy NPs to boost the solar cell performance.     

Imide‐Functionalized Heteroarene‐Based n‐Type Terpolymers Incorporating Intramolecular Noncovalent Sulfur∙∙∙Oxygen Interactions for Additive‐Free All‐Polymer Solar Cells

The aggregation/crystallinity of classic n‐type terpolymers based on naphthalene diimide and perylene diimide is challenging to tune due to their rigid and extended cores, leading to suboptimal film morphology. A new strategy for developing high‐performance n‐type terpolymers by incorporating imide‐functionalized heteroarenes is reported here to balance crystallinity and miscibility without sacrificing charge carrier mobilities. The introduction of thienopyrroledione (TPD) into the copolymer f‐BTI2‐FT results in a series of terpolymers BTI2‐xTPD having distinct TPD content. The irregular backbone reduces crystallinity, yielding improved miscibility with the polymer donor. More importantly, TPD triggers noncovalent S?O interactions, increasing backbone planarity and in‐chain charge transport. Such interactions also promote face‐on polymer packing. As a result, all‐polymer solar cells (all‐PSCs) based on BTI2‐30TPD achieve an optimal power conversion efficiency (PCE) of 8.28% with a small energy loss (0.53 eV). This efficiency is substantially higher than that of TPD (4.4%) or a BTI2‐based copolymer (6.8%) and is also the highest for additive‐free all‐PSCs based on a terpolymer acceptor. Moreover, the BTI2‐30TPD cell exhibits excellent stability with the PCE retaining 90% of its initial value after 400 h of aging. The results demonstrate that random polymerization using imide‐functionalized heteroarenes is a powerful approach to develop terpolymer acceptors toward efficient and stable all‐polymer solar cell PSCs.     

Highly Efficient and Thermally Stable Polymer Solar Cells with Dihydronaphthyl‐Based [70]Fullerene Bisadduct Derivative as the Acceptor

The efficiency of polymer solar cells (PSCs) can be essentially enhanced by improving the performance of electron‐acceptor materials, including by increasing the lowest unoccupied molecular orbital (LUMO) level, improving the optical absorption, and tuning the material solubility. Here, a new soluble C₇₀ derivative, dihydronaphthyl‐based C₇₀ bisadduct (NC₇₀BA), is synthesized and explored as acceptor in PSCs. The NC₇₀BA has high LUMO energy level that is 0.2 eV higher than [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), and displays broad light absorption in the visible region. Consequently, the PSC based on the blend of poly(3‐hexylthiophene) (P3HT) and NC₇₀BA shows a high open‐circuit voltage (V_oc = 0.83 V) and a high power conversion efficiency (PCE = 5.95%), which are much better than those of the P3HT:PCBM‐based device (V_oc = 0.60 V; PCE = 3.74%). Moreover, the amorphous nature of NC₇₀BA effectively suppresses the thermally driven crystallization, leading to high thermal stability of the P3HT:NC₇₀BA‐based solar cell devices. It is observed that the P3HT:NC₇₀BA‐based device retains 80% of its original PCE value against thermal heating at 150 °C over 20 h. The results unambiguously indicate that the NC₇₀BA is a promising acceptor material for practical PSCs.     

Dithieno[3',2':3,4;2",3":5,6]benzo[1,2-c][1,2,5]oxadiazole-based polymer donors with deep HOMO levels

Star nonfullerene acceptors like ITIC[1],IDIC[2],O-IDTBR[3],IT-4F[4],CO,8DFIC[5],Y6[6]etc.continuously emerge and keep pushing the power conversion efficiency(PCE)of organic sol-ar cells forward.These small molecules generally show nar-row bandgaps,excellent visible to NIR light-harvesting capabil-ity,good electron mobility,suitable energy levels and miscibil-ity with the donor materials.PCEs up to 18.56％have been achieved for the state-of-the-art nonfullerene organic solar cells[7].On the other hand,donor materials matching non-fullerene acceptors also received considerable interests[8].Ow-ing to complementary light absorption,high hole-mobility and deep HOMO levels,wide-bandgap(WBG)conjugated co-polymers are ideal donor partners for the low-bandgap non-fullerene acceptors.     

Synthesis and Structure–Property Correlations of Dicyanovinyl‐Substituted Oligoselenophenes and their Application in Organic Solar Cells

The convergent synthesis of a series of acceptor–donor–acceptor (A‐D‐A) type dicaynovinyl (DCV)‐substituted oligoselenophenes DCVnS (n = 3–5) is presented. Trends in thermal and optoelectronic properties are studied, in dependence on the length of the conjugated backbone. Optical measurements reveal red‐shifted absorption spectra and electrochemical investigations show lowering of the lowest unoccupied molecular orbital (LUMO) energy levels for DCVnS compared to the corresponding thiophene analogs DCVnT. As a consequence, a lowering of the bandgap is observed. Single crystal X‐ray structure analysis of tetramer DCV4S provides important insight into the packing features and intermolecular interactions of the molecules, further corroborating the importance of the DCV acceptor groups for the molecular ordering. DCV4S and DCV5S are used as donor materials in planar heterojunction (PHJ) and bulk‐heterojunction (BHJ) organic solar cells. The devices show very high fill factors (FF), a high open circuit voltage, and power conversion efficiencies (PCE) of up to 3.4% in PHJ solar cells and slightly reduced PCEs of up to 2.6% in BHJ solar cells. In PHJ devices, the PCE for DCV4S almost doubles compared to the PCE reported for the oligothiophene analog DCV4T, while DCV5S shows an about 30% higher PCE than DCV5T.     

High‐Performance Colorful Semitransparent Polymer Solar Cells with Ultrathin Hybrid‐Metal Electrodes and Fine‐Tuned Dielectric Mirrors

Polymer solar cells (PSCs) possess the unique features of semitransparency and coloration, which make them potential candidates for applications in aesthetic windows. Here, the authors fabricate inverted semitransparent PSCs with high‐quality hybrid Au/Ag transparent top electrodes and fine‐tuned dielectric mirrors (DMs). It is demonstrated that the device color can be tailored and the light harvesting in the PSCs can be enhanced by matching the bandgap of the polymer donors in the active layer with the specifically designed maximum‐reflection‐center‐wavelengths of the DMs. A detailed chromaticity analysis of the semitransparent PSCs from both bottom and top (mirror) views is also carried out. Furthermore, the inverted semitransparent PSCs based on PTB7‐Th:PC₇₁BM with six pairs of DMs demonstrate a maximum power conversion efficiency (PCE) of 7.0% with an average visible transmittance (AVT) of 12.2%. This efficiency is one of the highest reported for semitransparent PSCs, corresponding to 81.4% of the PCE from opaque counterpart devices. The device design and processing method are also successfully adapted to a flexible substrate, resulting in a device with a competitive PCE of 6.4% with an AVT of 11.5%. To the best of our knowledge, this PCE value is the highest value reported for a flexible semitransparent PSC.