Chalcogen Atoms Regulate the Organic Solar Cell Performance of B-N-Based Polymer Donors

Donor polymers play a key role in the development of organic solar cells (OSCs). B-N-based polymer donors, as new types of materials, have attracted a lot of attention due to their special characteristics, such as high E(T1), small ΔEST, and easy synthesis, and they can be processed with real green solvents. However, the relationship between the chemical structure and device performance has not been systematically studied. Herein, chalcogen atoms that regulate the OSCs performance of B-N-based polymer donors were systematically studied. Fortunately, the substitution of a halogen atom did not affect the high E(T1) and small ΔEST character of the B-N-based polymer. The absorption and energy levels of the polymer were systematically regulated by O, S, and Se atom substitution. The PBNT-TAZ:Y6-BO-based OSCs device demonstrated a high power conversion efficiency of 15.36%. Moreover, the layer-by-layer method was applied to further optimize the device performance, and the PBNT-TAZ/Y6-BO-based OSCs device yielded a PCE of 16.34%. Consequently, we have systematically demonstrated how chalcogen atoms modulated the electronic properties of B-N-based polymers. Detailed and systematic structure-performance relationships are important for the development of next-generation B-N-based materials.

Electrostatic Potential Design of Solid Additives for Enhanced Molecular Order of Polymer Donor in Efficient Organic Solar Cells

Polymeric semiconducting materials struggle to achieve fast charge mobility due to low structural order. In this work, five 1H-indene-1,3(2H)dione-benzene structured halogenated solid additives namely INB-5F, INB-3F, INB-1F, INB-1Cl, and INB-1Br with gradually varied electrostatic potential are designed and utilized to regulate the structural order of polymer donor PM6. Molecular dynamics simulations demonstrate that although the dione unit of these additives tends to adsorb on the backbone of PM6, the reduced electrostatic potential of the halogen-substituted benzene can shift the benzene interacting site from alkyl side chains to the conjugated backbone of PM6, not only leading to enhanced π-π stacking in out-of-plane but also arising new π-π stacking in in-plane together with the appearance of multiple backbone stacking in out-of-plane, consequent to the co-existence of face-on and edge-on molecular orientations. This molecular packing transformation further translates to enhanced charge transport and suppressed carrier recombination in their photovoltaics, with a maximum power conversion efficiency of 19.4% received in PM6/L8-BO layer-by-layer deposited organic solar cells.

Isomerization of Benzothiadiazole Yields a Promising Polymer Donor and Organic Solar Cells with Efficiency of 19.0

The exploration of high-performance and low-cost wide-bandgap polymer donors remains critical to achieve high-efficiency nonfullerene organic solar cells (OSCs) beyond current thresholds. Herein, the 1,2,3-benzothiadiazole (iBT), which is an isomer of 2,1,3-benzothiadiazole (BT), is used to design wide-bandgap polymer donor PiBT. The PiBT-based solar cells reach efficiency of 19.0%, which is one of the highest efficiencies in binary OSCs. Systemic studies show that isomerization of BT to iBT can finely regulate the polymers' photoelectric properties including i) increasing the extinction coefficient and photon harvest, ii) downshifting the highest occupied molecular orbital energy levels, iii) improving the coplanarity of polymer backbones, iv) offering good thermodynamic miscibility with acceptors. Consequently, the PiBT:Y6 bulk heterojunction (BHJ) device simultaneously reaches advantageous nanoscale morphology, efficient exciton generation and dissociation, fast charge transportation, and suppressed charge recombination, leading to larger VOC of 0.87 V, higher JSC of 28.2 mA cm-2 , greater fill factor of 77.3%, and thus higher efficiency of 19.0%, while the analog-PBT-based OSCs reach efficiency of only 12.9%. Moreover, the key intermediate iBT can be easily afforded from industry chemicals via two-step procedure. Overall, this contribution highlights that iBT is a promising motif for designing high-performance polymer donors.

Biaxially Conjugated Materials for Organic Solar Cells

Organic solar cells (OSCs) represent one of the most important emerging photovoltaic technologies that can implement solar energy conversion efficiently. The chemical structure of organic semiconductors deployed in the active layer of OSCs plays a critical role in the photovoltaic performance and chemical/physical stability of relevant devices. With the structure innovation of organic semiconductors, especially nonfullerene acceptors (NFAs), the performance of OSCs have been promoted rapidly in recent years, with state-of-the-art power conversion efficiencies (PCEs) exceeding 19.5%. Compared with other photovoltaics like perovskite, the shortcoming of OSCs mainly lies in the high nonradiative recombination loss. However, the photocurrent density is superior in OSCs owing to the easy modulation of the NFA band gap toward the near-infrared region. In these regards, the effort to further boost the PCE of OSCs to achieve a milestone >21% should be devoted to reducing the nonradiative loss while further broadening the absorption band. Developing organic semiconductors with biaxially extended conjugated structures has provided a potential solution to achieve these goals. Herein, we summarize the design rules and performance progress of biaxially extended conjugated materials for OSCs. The descriptions are divided into two major categories, i.e., polymers and NFAs. For p-type polymers, we focus on the biaxial conjugation on some representative building blocks, e.g., polythiophene, triphenylamine, and quinoxaline. Whereas for n-type polymers, some structures with large conjugated planes in the normal direction are presented. We also elaborate on the biaxial conjugation strategies in NFAs with modification site at either the π-core or side-group. The general structure-property relationships are further retrieved within these materials, with focus on the short-wavelength absorption and nonradiative energy loss. Finally, we provide an outlook for the further structure modification strategies of biaxially conjugated materials toward highly efficient, stable, and industry-compatible OSCs.

Manipulating the Alkyl Chains of Naphthodithiophene Imide-Based Polymers to Concurrently Boost the Efficiency and Stability of Organic Solar Cells

Morphology instability holds the major responsibility for efficiency degradation of organic solar cells (OSCs). However, how to develop polymer donors simultaneously with high efficiency and excellent morphology stability remains challenging. Herein, we reported naphtho[2,1-b:3,4-b']dithiophene-5,6-imide (NDTI)-based new polymers PNDT1 and PNDT2. The alkyl chain engineering leads to high crystallinity, high hole mobility (>10-3 cm2 V-1 S-1), and nanofibrous film morphology, which enable PNDT2 to exhibit an efficiency of 18.13% and a remarkable FF value of 0.80. Moreover, the NDTIs have short π-π stacking and abundant short interactions, and their polymers exhibit superior morphological stability. Therefore, the PNDT2-based OSCs exhibit much better device stability than that of PNDT1, PAB-α, and benchmark polymers PM6 and D18. This work suggests the great importance of the large conjugated backbone of the monomer and alkyl chain engineering to develop high-performance and morphology-stable polymers for OSCs.

Facile Strategy for Third Component Optimization in Wide-Band-Gap π-Conjugated Polymer Donor-Based Efficient Ternary All-Polymer Solar Cells

Emerging organic solar cells based on a ternary strategy is one of the most effective methods for improving the blend film morphology, absorption ability, and device performances. On the other hand, this strategy has had very limited success in all-polymer solar cells (all-PSCs) because of the scarcity of new polymers and the challenges faced during third component optimization. Herein, highly efficient ternary all-PSCs were developed from siloxane-functionalized side chains with a wide-band-gap (E_g) polymer, Si-BDT, which is blended with a medium and ultra-narrow E_g polymer donor and acceptor, PTB7-Th, and DCNBT-TPIC. An impressive power conversion efficiency (PCE) of 13.45% was achieved in the ternary all-PSCs [PTB7-Th(0.6):Si-BDT(0.4):DCNBT-TPIC(0.6)] with the addition of 0.4 wt equivalent Si-BDT into binary all-PSCs [PTB7-Th(1):DCNBT-TPIC(0.6) PCE of 10.11%]. In contrast, the binary all-PSCs with a Si-BDT(1):DCNBT-TPIC(0.6) active layer only exhibited a good PCE of 9.92%. More importantly, the siloxane-functionalized side chains increase the light-absorption ability, carrier mobility, blend miscibility, and film morphology in ternary devices compared to those of the binary devices. Hence, exciton dissociation, charge carrier transport, and suppressed recombination properties were facilitated. In the presence of Si-BDT, both binary and ternary all-PSCs PCEs are significantly improved. Indeed, 13.45% PCE is one of the best values reported for all-PSCs except for those based on polymerized small molecule acceptors. In addition, the ternary all-PSCs showed excellent environmental and thermal stabilities with 95 and 84% of the initial PCE retained after 900 and 500 h, respectively. These results offer effective device engineering, providing a new avenue for improving the device performance in ternary all-PSCs.

Influence of Fluorine Substitution on the Photovoltaic Performance of Wide Band Gap Polymer Donors for Polymer Solar Cells

The design and development of wide band gap (WBG) polymer donors are critical for achieving high power conversion efficiencies (PCEs) in polymer solar cells. In this work, four WBG polymer donors, Q4, Q5, Q6, and Q7, with different numbers and positions of fluorine substitution (n = 0, 2, 2, and 4, respectively) were prepared, and the effect of fluorination on their photovoltaic performance was systematically investigated. When blended with a small-molecule electron acceptor MeIC, the devices based on Q4, Q5, Q6, and Q7 showed PCEs of 10.34, 11.06, 5.26, and 0.48%, respectively. When coupled with a low band gap polymer acceptor PYIT to fabricate all-polymer solar cells (all-PSCs), while the other three polymers (Q5-Q7) exhibited much lower PCEs in the range of 0.12-6.71%, the Q4 polymer-based all-PSCs showed the highest PCE of 15.06%, comparable to that of the devices fabricated with the star polymer donor PM6 (PCE = 15.00%). Detailed physicochemical and morphological studies revealed that an over-substitution of F in Q7 results in undesired low-lying HOMO levels and phase separation with the acceptors, thus resulting in its inferior PCEs. Moreover, the less F-substitution and controlling of the positions of F-substitution position in Q4 and Q5 can improve the HOMO energy level matching as well as morphologies between these two polymers with the acceptors, which in turn gives rise to higher performances. Clearly, our results indicate that Q4 is a promising donor candidate for high-performance all-PSCs, and the fine-tuning of both the number and positions of F-substitution in the polymer backbone is essential in developing high-performance WBG polymer donors.

Electron-deficient TVT unit-based D-A polymer donor for high-efficiency thick-film OSCs

As the power conversion efficiencies of organic solar cells (OSCs) have been improved continuously in recent years, more attention will be paid to the industrial production and practical application of OSCs. However, there are still many problems to be solved in the process of large-scale production. Among them, reducing the costs of the materials and enhancing the film-thickness tolerance of the active layer are the two key points. Therefore, it is urgent to develop organic semiconductor materials which are easy to synthesize and suitable for the construction of high-efficiency, thick-film OSCs. In this work, we have focused on the (E)-2-[2-(thiophen-2-yl)vinyl]thiophene (TVT) unit because of its unique coplanar structure. And we noticed that TVT was mostly used as an electron-donating unit in the previous reports. However, we have modified TVT into electron-withdrawing unit by the introduction of fluorine atoms/ester groups. And two new donor-acceptor (D-A) copolymers have been obtained by combining the electron-withdrawing TVT unit with benzo[2,1-b:4,5-b']dithiophene (BDT) unit. Among them, the polymer based on the ester modified TVT unit presents excellent photovoltaic performance by virtue of its good solubility and preferable molecular stacking mode, and the corresponding devices also show extraordinarily high-thickness tolerance. The emergence of this new electron-withdrawing TVT unit will undoubtedly further promote the development of low-cost, high-efficiency, thick-film OSCs.

Alkylsilyl Fused Ring-Based Polymer Donor for Non-Fullerene Solar Cells with Record Open Circuit Voltage and Energy Loss

The energy loss (E_loss ), especially the nonradiative recombination loss and energetic disorder, needs to be minimized to improve the device performance with a small voltage (V_OC ) loss. Urbach energy (E_U ) of organic photovoltaic materials is related to energetic disorder, which can predict the E_loss of the corresponding device. Herein, a polymer donor (PBDS-TCl) with Si and Cl functional atoms for organic solar cells (OSCs) is synthesized. It can be found that the V_OC and E_loss can be well manipulated by regulation of the energy level of the polymer donor and E_U , which is dominated by the morphology. A low energetic disorder with an E_U of 23.7 meV, a low driving force of 0.08 eV, and a low E_loss of 0.41 eV are achieved for the PBDS-TCl:Y6-based OSCs. Consequently, an impressive open circuit voltage (V_OC ) of 0.92 V is obtained. To the best of knowledge, the V_OC value and E_loss are both the record values for the Y6-based device. These results demonstrate that fine-tuning the polymer donor by functional atom modification on the side chain is a promising way to reduce E_U and energy loss, as well as obtain small driving force and high V_OC for highly efficient OSCs.

A Wide Bandgap Polymer Donor Composed of Benzodithiophene and Oxime-Substituted Thiophene for High-Performance Organic Solar Cells

Oxime-substituted thiophene (TO) is used as an acceptor (A) unit to copolymerize with the benzodithiophene (BDT) donor (D) unit to form a novel D-A polymer donor, PBDTTO, which has a low-lying highest occupied molecular orbital energy level (E_HOMO) of -5.60 eV and a wide bandgap of 2.03 eV, forming complementary absorption and matching energy levels with the narrow bandgap nonfullerene acceptors. Organic solar cells using PBDTTO and Y6 as the donor and acceptor, respectively, exhibited a J_SC of 27.03 mA cm^-2, a V_OC of 0.83 V, and a fill factor of 0.59, reaching a high power conversion efficiency of 13.29%. The unencapsulated devices show good long-term stability in ambient air. Compared with the acceptor monomers used in other high-performance BDT-based D-A polymer donors, which are synthesized tediously in low yields, the TO acceptor monomer can be conveniently synthesized in only two steps with a high overall yield of 70%. These results demonstrate that TO unit can be used as a promising acceptor unit for developing BDT-based D-A polymer donors at low cost while maintaining high photovoltaic performance.

Improved Stability of All-Polymer Solar Cells Using Crosslinkable Donor and Acceptor Polymers Bearing Vinyl Moieties in the Side-Chains

Crosslinkable polymers have attracted tremendous attention in various fields of science and technology, owing to their potential utilization in applications requiring dimensional and morphological stability under thermal and mechanical stress. In this study, random terpolymers were successfully synthesized by introducing thiophene-based monomers bearing vinyl functional groups in the side-chain of the polymer donor (PBDBT-BV₂₀) and polymer acceptor (N2200-TV₁₀) structures. The physical properties of the blend films of PBDBT-BV₂₀ and N2200-TV₁₀ before and after thermal crosslinking were extensively investigated and compared to those of the homogeneous individual polymer films. The results revealed that a network polymer with donor and acceptor polymer chains, which can lock the internal morphology, could be achieved by inducing crosslinking between the vinyl groups in the mixed state of PBDBT-BV₂₀ and N2200-TV₁₀. In addition, the power conversion efficiency (PCE) of the polymer solar cells (PSCs) containing the blend films that were crosslinked by a two-step thermal annealing process was improved. The enhanced PCE could be attributed to the individual crystallization of PBDBT-BV₂₀ and N2200-TV₁₀ in the blend phase at 120 °C and then thermal crosslinking at 140 °C. In addition, the PSCs with the crosslinked blend film exhibited an excellent shelf-life of over 1200 h and a thermally stable PCE. Furthermore, the crosslinked blend film exhibited excellent mechanical stability under bending stress in flexible PSCs using plastic substrates.

Reduced Nonradiative Recombination Energy Loss Enabled Efficient Polymer Solar Cells via Tuning Alkyl Chain Positions on Pendent Benzene Units of Polymers

Nonradiative recombination energy loss (ΔE₃) plays a key role in enhancing device efficiencies for polymer solar cells (PSCs). Until now, there is no clear resolution for reducing ΔE₃ via molecular design. Herein, we report two conjugated polymers, PBDB-P-p and PBDB-P-m, which are integrated from benzo[1,2-b:4,5-b']dithiophene with alkylthio chain substituted at para- or meta-position on pendent benzene and benzo[1,2-c:4,5-c']dithiophene-4,8-dione. Both the polymers have different temperature-dependent aggregation properties but similar molecular energy levels. When BO-4Cl was used as an acceptor to fabricate PSCs, the device of PBDB-P-p:BO-4Cl displayed a maximal power conversion efficiency (PCE) of 13.83%, while the best device of PBDB-P-m:BO-4Cl exhibited a higher PCE of 14.12%. The close J_SCs and fill factors in both PSCs are attributed to their formation of effective nanoscale phase separation as confirmed by atomic force microscopy measurements. We find that the PBDB-P-m-based device has 1 order of magnitude higher electroluminescence quantum efficiency (EQE_EL) than in the PBDB-P-p-based one, which could arise from the relatively weak aggregation in the PBDB-P-m-based film. Thus, the PBDB-P-m-based device has a remarkably enhanced V_OC of 0.86 V in contrast to 0.80 V in the PBDB-P-p-based device. This study offers a feasible structural optimization way on the alkylthio side chain substitute position on the conjugated polymer to enhance V_OC by reducing nonradiative recombination energy loss in the resulting PSCs.

Random Polymer Donor for High-Performance Polymer Solar Cells with Efficiency over 14

Constructing random copolymers has been regarded as an easy and effective approach to design polymer donors for state-of-the-art polymer solar cells (PSCs). In this work, we develop a naphtho[2,3-c]thiophene-4,9-dione-based copolymer PBN-Cl as a donor material for PSCs, and a moderate power conversion efficiency (PCE) of 11.21% is achieved with a relatively low fill factor (FF) of 0.615. We then incorporate a similar acceptor unit benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) into the polymeric backbone of PBN-Cl to tune its photovoltaic performance, and a significantly higher PCE of 14.05% is achieved from the random polymer PBN-Cl-B80 containing 80% BDD unit. The enhanced PCE of the PBN-Cl-B80-based device mainly relies on the higher FF value, resulting from the improved charge mobility properties, reduced bimolecular and trap-assisted recombination, and more appropriate phase separation. The results demonstrate a feasible strategy to tune the photovoltaic performance of polymer donors by constructing a random polymer with a compatible component.

Benzodithiophene-Dithienylbenzothiadiazole Copolymers for Efficient Polymer Solar Cells: Side-Chain Effect on Photovoltaic Performance

A new series of low band gap D-A alternating polymers based on 4,5-bis((2-ethylhexyl)oxy)benzo[2,1- b:3,4- b']dithiophene (BDT) and 5-fluoro-4,7-bis(4-alkylthien-2-yl)benzo[ c][1,2,5]thiadiazole bearing different size of lateral alkyl substituents, namely, PfBB- n, n = 8, 10, 12, 14, and 16, was designed and synthesized for high-performance bulk heterojunction (BHJ) polymer solar cells (PSCs). PfBB- n-bearing linear alkyl side chains exhibited strong and controllable aggregation in both solution and solid states, which gives rise to a significant bathochromic shift of the absorption cut-off down to ∼780 nm in thin film. In addition, the strong and wide absorption (350-800 nm) of PfBB- n polymers can compensate for the relatively weak absorption of PC₇₁BM, particularly in the 300-400 range nm to enhance light harvesting of such an active blend. BHJ solar cells based on PfBB- n:PC71BM blends as an active layer showed power conversion efficiency (PCE) in the range 7.8-9.7%. Because of the strong stacking interchain interactions, PfBB-12-based PSC exhibited aggregation-induced spectral broadening, superior structural order, higher exciton dissociation, higher and more balanced charge carrier mobilities, as well as reduced recombination losses. As a result, PfBB-12-based device afforded the best PCE of 9.7%, with the highest short-circuit current density ( J_sc) of 16.6 mA cm^-2 and open-circuit voltage ( V_oc) of 0.92 V among devices fabricated. These results demonstrate that the alkyl side chain of the polymer significantly affects the absorption, morphology, and electronic properties of the active blend of PfBB- n/PC71BM, which would provide an alternative useful tool to fine-tune the device performance. Our results also highlight that the electron-rich benzo[2,1- b:3,4- b']dithiophene building block, BDT, is highly useful for the construction of low band gap D-A polymer for highly efficient PSCs.

High-Performance Non-Fullerene Organic Solar Cells Based on a Selenium-Containing Polymer Donor and a Twisted Perylene Bisimide Acceptor

A novel polymer donor (PBDTS-Se) is designed to match with a non-fullerene acceptor (SdiPBI-S). The corresponding solar cells show a high efficiency of 8.22%, which result from synergetic improvements of light harvesting, charge carrier transport and collection, and morphology. The results indicate that rational design of novel donor materials is important for non-fullerene organic solar cells.