Significant influence of doping effect on photovoltaic performance of efficient fullerene-free polymer solar cells

The modification mechanism of the water/alcohol cathode interlayer is one of the most complicated problems in the field of organic photovoltaics,which has not been clearly elucidated yet;this greatly restricts the further enhancement of the PCE for polymer solar cells.Herein,we clarified the different effects of PFN and its derivatives,namely,poly[(9,9-bis(3’-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN-Br) in modifying fullerene-free PSCs.It is found for the first time that doping on IT-4F by the amino group of PFN leads to the unfavorable charge accumulation,and hence,forms a dense layer of electronegative molecule due to the poor electron transport capacity of the non-fullerene acceptor IT-4F.The electronegative molecular layer can block the electron transfer from the active layer to the interlayer and cause serious charge recombination at the active layer/cathode interface.This mechanism could be verified by the ESR measurement and electron-only devices.By replacing PFN with PFN-Br,the excessive doping effect between the cathode interlayer and IT-4F is eliminated,by which the charge transport and collection can be greatly improved.As a result,a high PCE of 13.5%was achieved in the fullerene-free PSCs.

Design of ultranarrow-bandgap acceptors for efficient organic photovoltaic cells and highly sensitive organic photodetectors

The fabrication of multifunctional electronic devices based on the intriguing natures of organic semiconductors is crucial for organic electronics.Ultranarrow-bandgap materials are in urgent demand for fabricating high-performance organic photovoltaic(OPV)cells and highly sensitive near-infrared organic photodetectors(OPDs).By combining alkoxy modification and an asymmetric strategy,three narrowbandgap electronic acceptors(BTP-4F,DO-4F,and QO-4F)were synthesized with finely tuned molecular electrostatic potential(ESP)distributions.Through the careful modulation of electronic configurations,the optical absorption onsets of DO-4F and QO-4F exceeded 1μm.The experimental and theoretical results suggest that the small ESP of QO-4F is beneficial for achieving a low nonradiative voltage loss,while the large ESP of BTP-4F can help obtain high exciton dissociation efficiency.By contrast,the asymmetric acceptor DO-4F with a moderate ESP possesses balanced voltage loss and exciton dissociation,yielding the best power conversion efficiency of 13.6%in the OPV cells.OPDs were also fabricated based on the combination of PBDB-T:DO-4F,and the as-fabricated device outputs a high shot-noise-limited specific detectivity of 3.05×10^(13) Jones at 850 nm,which is a very good result for near-infrared OPDs.This work is anticipated to provide a rational way of designing high-performance ultranarrow-bandgap organic semiconductors by modulating the molecular ESP.

Optimizing polymer aggregation and blend morphology for boosting the photovoltaic performance of polymer solar cells via a random terpolymerization strategy

Compared to regular conjugated polymers,the random conjugated terpolymers are usually not beneficial to achieve highly efficient non-fullerene(NF)-based polymer solar cells(PSCs)due to their disordered chemical structures.In this work,we report two random terpolymer donors(PBNB80 and PBNB50)by tuning the molar ratio of electron-accepting units of 1,3-di(thiophen-2-yl)naphtho[2,3-c]thiophene-4,9-dione(NTD)and 1,3-bis(4-chlorothiophen-2-yl)-4 H,8 H-benzo[1,2-c:4,5-c’]dithiophene-4,8-dione(ClBDD),at the same time,the parent polymers(PBNB100 and PBNB00)are also compared to study.These four polymer donors exhibit similar optical bandgaps and gradually deepen highest occupied molecular orbital levels.Importantly,aggregation and self-organization properties of the random terpolymer donors are optimized,which result in the better morphology and crystal coherence length after blending with NF acceptor of BO-4 Cl.Particularly,a PBNB80:BO-4 Cl blend forms an optimal nanoscale phase-separation morphology,thereby producing an outstanding power conversion efficiency of 16.0%,which is much higher than those(12.8%and 10.7%)of their parent binary polymer donor-based devices.This work demonstrates that rational using terpolymerization strategy to prepare random terpolymer is a very important method to achieve highly efficient NF-PSCs.

Facile solution-processed molybdenum oxide as hole transporting material for efficient organic solar cell

The large energy barrier in hole extraction still remains a great challenge in developing hole transporting layer (HTL) materials for organic solar cells (OSCs).Thus,solution-processed HTL materials with excellent hole collection ability and good compatibility with large-area processing technique are strongly desired for OSCs.Herein,we developed a cost-effective and solution-processed MoO_(3)HTL for efficient OSCs.By adding a small amount of glucose as reducing reagent into the ammonium molybdate precursor solution,a deeply n-doped MoO_(3),namely G:Mo,was prepared through the sol–gel method.Compared to pristine MoO_(3),the conductivity of G:Mo was enhanced by two orders of magnitude,which greatly improved the hole collection ability of the HTL.OSCs with G:Mo can exhibit comparable PCE to the PEDOT:PSS device.Using PBDB-TF:BTP-eC9 as the active layer,a PCE of 17.1%is obtained for the device,which is the highest PCE value for OSC using a solution-processed MoO_(3)HTL.More importantly,G:Mo is well compatible with the blade-coating processing.The OSC using a blade-coated G:Mo showed almost no PCE loss as compared to the device with spin-coated G:Mo HTL.The results from this work indicate that G:Mo is a promising HTL material for the practical production of OSCs.

A Thiazole-Based Polymer Donor for Efficient Organic Solar Cells

The development of new materials plays a critical role in improving the efficiency of organic solar cells(OSCs).At present,the relatively high-lying highest occupied molecular orbital(HOMO)level of the high-efficiency polymer donor is regarded as one of the main reasons for the low open-circuit voltage(V_(OC)).In this work,we introduced the strong electron-withdrawing thiazole unit into the construction of a polymer donor.We designed and prepared an alternating donor-acceptor material,namely PSZ,by copolymerizing 4-methyl thiazole with an electron-donating benzodithiophene unit and studied its application in high-efficiency OSCs.The optical and electrical properties of the new material were characterized by UV-Vis absorption spectroscopy and electrochemical cyclic voltammetry.Results show that PSZ is a typical wide-bandgap material with a high optical bandgap of 2.0 eV and a deep HOMO level of-5.70 eV.When a non-fullerene BTP-eC9 was selected as the acceptor material,V_(OC) reached 0.88 V in the resulting device,and the corresponding power conversion efficiency(PCE)was8.15%.In addition,when PSZ was added as the third component to the binary photoactive combination with PBDB-TF as the donor and BTP-eC9 as the acceptor,V_(OC) of the cell device could be increased,thereby obtaining a high PCE of 17.4%.These results indicated that introducing thiazole units into polymer donors can remarkably reduce the HOMO levels and improve V_(OC) and PCE in OSCs.

A New Nonfullerene Acceptor with Suppressed Energy Disorder for High-Efficiency Organic Solar Cells

Energy disorder is an important factor that affects charge transport,recombination,and energy loss in organic solar cells.Here,we designed a ladder-type nonfullerene acceptor and studied the critical role of energy disorder in photovoltaic performance.Taking a typical seven-member fused ring acceptor IT-4F as an example,we replaced its sp3-hybridized bridging carbon atoms and linked bulky groups with triisopropylbenzene-substituted pyrrole units.The newly synthesized acceptor 2,2′-((2Z,2′Z)-((3,9-bis(2-butyloctyl)-6,12-bis(2,4,6-triisopropylphenyl)-6,12-dihydrothieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[2,3-f]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,8-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile(TBB)has a redshifted absorption spectrum with upshifted energy levels than those of IT-4F.More importantly,TBB shows more suppressed energy disorder,which leads to enhanced charge mobility and improved luminescence efficiency.Consequently,TBB-based devices obtained a power conversion efficiency of 16.2%with a relatively low nonradiative energy loss(0.22 eV),which exceeds that of IT-4F-based devices(11.5%)and is one of the top values among non-Y6 systems.This work demonstrates that rational molecular design is crucial for the suppression of energy disorder.

Manipulating Film Formation Kinetics Enables Organic Photovoltaic Cells with 19.5%Efficiency

Organic photovoltaic(OPV)cells have demonstrated remarkable performance in small,spin-coated areas.Nevertheless significant challenges persist in the form of large efficiency losses due to the fact that the ideal morphology cannot be preserved in the transition of small-area cells to large-scale panels.Herein,the ternary strategy of incorporating the third component FTCC-Br into the active layer of PB2:BTP-eC9 is employed to improve absorption response,optimize morphology,and reduce charge recombination,leading to a power conversion efficiency(PCE)of 19.5%(certified as 19.1%by the National Institute of Metrology,China).Moreover,the addition of FTCC-Br can control the aggregation kinetics of the active layer during the film formation process,transferring the optimal morphology to the blade-coated large-area films.Based on the highly efficient ternary bulk heterojunction,the 50 cm^(2) OPVmodules exhibited a PCE of 15.2%with respect to the active area.Importantly,the ternary OPV cells retain 80%of its initial PCE after 4000 h under continuous illumination.Our work demonstrates that the addition of a third component has the potential to improve the efficiency and stability of large-area organic solar cells.

Manipulating Nanowires in Interconnecting Layer for Efficient Tandem Organic Photovoltaics

Owing to the function of manipulating light absorption distribution,tandem organic solar cells containing multiple sub-cells exhibit high power conversion efficiencies.However,there is a substantial challenge in precisely controlling the inter-subcells carrier migration which determines the balance of charge transport across the entire device.The conductivity of"nanowires"-like conducting channel in interconnecting layer between sub-cells should be improved which calls for fine engineering on the morphology of polyelectrolyte in interconnecting layer.Here,we develop a simple method to effectively manipulating the domains of conductive components in commercially available polyelectrolyte PEDOT:PSs.The use of poor solvent could effectively modify the configuration of polystyrene sulfonic acid and thus the space for conductive components.Based on our strategy,the insulated shells wrapping conductive domains are thinned and the efficiencies of tandem organic solar cells are improved.We believe our method might provide guidance for the manufacture of tandem organic solar cells.

Understanding the Interfacial Energy Structure and Electron Extraction Process in Inverted Organic Solar Cells with Phosphine-Doped Cathode Interlayers

Comprehensive Summary Cathode interlayers(CILs)play an essential role in achieving efficient organic solar cells(OSCs).However,the electronic structure at the electrode/CIL/active layer interfaces and the underlying mechanisms for electron collection remain unclear,which becomes a major obstacle to develop high-performance CILs.Herein,we investigate the relationship of the electron collection abilities of four cross-linked and n-doped CILs(c-NDI:P0,c-NDI:P1,c-NDI:P2,c-NDI:P3)with their electronic structure of space charge region at heterojunction interface.By accurately calculating the depletion region width according to the barrier height,doping density and permittivity,we put forward that the optimal thickness of CIL should be consistent with the depletion region width to realize the minimum energy loss.As a result,the depletion region width is largely reduced from 13 nm to 0.8 nm at the indium tin oxide(ITO)/c-NDI:P0 interface,resulting in a decent PCE of 17.7%for the corresponding inverted OSCs.

The impact of reabsorption effect on composition analysis of organic semiconductors

Reabsorption is one of the most fundamental optical phenomena,but it has rarely been considered in spectroscopy-based composition analysis for organic semiconductors.Here,we take four state-of-the-art organic solar cell(OSC)materials as examples,and systematically investigate the influence of reabsorption on photoluminescence emission and excitation spectra by both experimental studies and optical simulations.We find that the overlap between absorption and emission spectra of these OSC materials is strong enough for them to be affected by the reabsorption effect,and the effect becomes more obvious between different species in the multi-components systems.Moreover,three features of the reabsorption effect and the reabsorption strength are identified,with which we have successfully analyzed the composition in a range of OSC materials in both solution and solid-state films.Our work not only provides an important understanding of the largely overlooked feature of reabsorption in the widely used spectroscopic techniques but also offers an effective toolbox for the composition analysis of organic semiconductors.

“Mutually Doped”Conjugated Polyelectrolyte-Polyoxometalate Complex as Hole Transporting Material for Efficient Organic Solar Cells

Comprehensive Summary Compared to electron transporting layer materials,the species and numbers of hole transporting layer(HTL)materials for organic solar cells(OSCs)are rare.The development of HTL materials with excellent hole collection ability and non-corrosive nature is a long-standing issue in the field of OSCs.Herein,we designed and synthesized a series of conjugated polyelectrolytes(CPEs)with continuously varied energy levels toward HTL materials for efficient OSCs.Through a“mutual doping”treatment,we obtained a CPE composite PCT-F:POM with a WF of 5.48 eV and a conductivity of 1.56х10^(-3)S/m,meaning that a good hole collection ability can be expected for PCT-F:POM.The OSC modified by PCT-F:POM showed a high PCE of 18.0%,which was superior to the reference device with PEDOT:PSS.Moreover,the PCT-F:POM-based OSC could maintain 91%of the initial PCE value after storage of 20 d,meaning that the long-term stability of OSCs is improved by incorporating the PCT-F:POM HTL.In addition,PCT-F:POM possesses good compatibility with large-area processing technique;i.e.,a PCT-F:POM HTL was processed by the blade-coating method for fabricating 1 cm^(2)OSC,and a PCE of 15.1%could be achieved.The results suggest the promising perspective of PCT-F:POM in practical applications.

Naphthalene diimide-based cathode interlayer material enables 20.2%efficiency in organic photovoltaic cells

Cathode interlayer(CIL)materials play an important role in improving the power conversion efficiency(PCE)of organic photovoltaic(OPV)cells.However,the current understanding of the structure-property relationship in CIL materials is limited,and systematic studies in this regard are scarce.Here,two new CIL materials,NDI-PhC4 and NDI-Ph C6 were synthesized by varying the alkylamine chain length on the NDI-Ph core.Our investigation reveals a systematic variation in the physical and chemical properties of these materials with increasing alkylamine chain length.Specifically,we observe a sequential decrease in melting point and self-doping effect,accompanied by an enhancement in crystallinity.Among these CIL materials,NDI-PhC4 has a notable balance across various performance metrics.It also exhibits excellent surface modification capabilities,leading to a low surface roughness.Consequently,OPV cells based on NDI-PhC4 achieve a PCE of 20.2%,which is one of the highest reported efficiencies for OPV cells.In addition,the appropriate melting point of NDI-PhC4 contributes to the excellent stability of OPV cells.

Recent progress in organic solar cells(PartⅠmaterial science)

During past several years,the photovoltaic performances of organic solar cells(OSCs)have achieved rapid progress with power conversion efficiencies(PCEs)over 18%,demonstrating a great practical application prospect.The development of material science including conjugated polymer donors,oligomer-like organic molecule donors,fused and nonfused ring acceptors,polymer acceptors,single-component organic solar cells and water/alcohol soluble interface materials are the key research topics in OSC field.Herein,the recent progress of these aspects is systematically summarized.Meanwhile,the current problems and future development are also discussed.

Realizing over 10% efficiency in polymer solar cell by device optimization

The low band gap polymer based on benzodithiophene(BDT)-thieno[3,4-b]thiophene(TT)backbone,PBDT-TS1,was synthesized following our previous work and the bulk heterojunction(BHJ)material comprising PBDT-TS1/PC71BM was optimized and characterized.By processing the active layer with different additives i.e.1,8-diiodooctane(DIO),1-chloronaphthalene(CN)and 1,8-octanedithiol(ODT)and optimizing the ratio of each additive in the host solvent,a high PCE of 9.98%was obtained under the condition of utilizing 3%DIO as processing additive in CB.The effect of varied additives on photovoltaic performance was illustrated with atomic force microscopy(AFM)and transmission electron microscope(TEM)measurements that explained changes in photovoltaic parameters.These results provide valuable information of solvent additive choice in device optimization of PBDTTT polymers,and the systematic device optimization could be applied in other efficient photovoltaic polymers.Apparently,this work presents a great advance in single junction PSCs,especially in PSCs with conventional architecture.

Recent Advances in Fullerene-free Polymer Solar Cells: Materials and Devices

What is the most favorite and original chemistry developed in your research group?We focus on developing new organic photovoltaic materials and exploring their applications in photovoltaic devices. Based on the new materials, we can figure out the correlations among chemical strictures, optoelectronic properties, and photovoltaic behaviors. Our group originally demonstrated quite a few build blocks for making conjugated polymers for photovoltaic applications, some of them have been broadly used by the researchers in the field.

Critical Role of Molecular Electrostatic Potential on Charge Generation in Organic Solar Cells

Revealing the charge generation is a crucial step to understand the organic photovoltaics. Recent development in non-fullerene organic solar cells （OSCs） indicates efficient charge separation even with negligible energetic offset between the donor and acceptor materials. These new findings trigger a critical question concerning the charge separation mechanism in OSCs, traditionally believed to result from sufficient energetic offset between the polymer donor and fullerene acceptor. We propose a new mechanism, which involves the molecular electrostatic potential, to explain efficient charge separation in non-fullerene OSCs. Together with the new mechanism, we demonstrate a record efficiency of -12% for systems with negligible energetic offset between donor and acceptor materials. Our analysis also rationalizes different requirement of the energetic offset between fullerene-based and non-fullerene OSCs, and paves the way for further design of OSC materials with both high photocurrent and high photovottage at the same time.

Fluorination vs. chlorination： a case study on high performance organic photovoltaic materials

Halogenation is a very efficient chemical modification method to tune the molecular energy levels, absorption spectra and molecular packing of organic semiconductors. Recently, in the field of organic solar cells(OSCs), both fluorine-and chlorinesubstituted photovoltaic materials, including donors and acceptors, demonstrated their great potentials in achieving high power conversion efficiencies(PCEs), raising a question that how to make a decision between fluorination and chlorination when designing materials. Herein, we systemically studied the impact of fluorination and chlorination on the properties of resulting donors(PBDB-T-2 F and PBDB-T-2 Cl) and acceptors(IT-4 F and IT-4 Cl). The results suggest that all the OSCs based on different donor and acceptor combinations can deliver good PCEs around 13%–14%. Chlorination is more effective than fluorination in downshifting the molecular energy levels and broadening the absorption spectra. The influence of chlorination and fluorination on the crystallinity of the resulting materials is dependent on their introduction positions. As chlorination has the advantage of easy synthesis, it is more attractive in designing low-cost photovoltaic materials and therefore may have more potential in largescale applications.

Realizing 11.3% efficiency in fullerene-free polymer solar cells by device optimization

In this work, photovoltaic properties of the PBDB-T:ITIC based-NF-PSCs were fully optimized and characterized by tuning the morphology of the active layers and changing the device architecture. First, donor/acceptor(D/A) weight ratios were scanned,and then further optimization was performed by using different additives, i.e. 1,8-diiodooctane(DIO), diphenyl ether(DPE),1-chloronaphthalene(CN) and N-methyl-2-pyrrolidone(NMP), on the basis of best D/A ratio(1:1, w/w), respectively. Finally,the conventional or inverted device architectures with different buffer layers were employed to fabricate NF-PSC devices, and meanwhile, the morphology of the active layers was further optimized by controlling annealing temperature and time. As a result,a record efficiency of 11.3% was achieved, which is the highest result for NF-PSCs. It's also remarkable that the inverted NF-PSCs exhibited long-term stability, i.e. the best-performing devices maintain 83% of their initial PCEs after over 4000 h storage.

Progress in Organic Solar Cells: Materials, Physics and Device Engineering

Organic solar cells(OSCs)have been developed for few decades since the preparation of the first photovoltaic device,and the record power conversion efficiency(PCE)certified by national renewable energy laboratory(NREL)has exceeded 17%.Looking back the whole history of OSCs,its rapid development is inseparable from multi-disciplinary efforts,including the new materials synthesizing,the device physics,and the device engineering,especially the breakthroughs in these disciplines.In this review,we are aiming at reviewing the history of the development of OSCs and summarizing the representative breakthroughs.

A chlorinated low-bandgap small-molecule acceptor for organic solar cells with 14.1% efficiency and low energy loss

A new acceptor-donor-acceptor(A-D-A) type small-molecule acceptor NCBDT-4 Cl using chlorinated end groups is reported.This new-designed molecule demonstrates wide and efficient absorption ability in the range of 600–900 nm with a narrow optical bandgap of 1.40 eV. The device based on PBDB-T-SF:NCBDT-4 Cl shows a power conversion efficiency(PCE) of 13.1%without any post-treatment, which represents the best result for all as-cast organic solar cells(OSCs) to date. After device optimizations, the PCE was further enhanced to over 14% with a high short-circuit current density(Jsc) of 22.35 m A cm-2 and a fill-factor(FF) of 74.3%. The improved performance was attributed to the more efficient photo-electron conversion process in the optimal device. To our knowledge, this outstanding efficiency of 14.1% with an energy loss as low as 0.55 eV is among the best results for all single-junction OSCs.