In the rapid innovation of organic solar cells, polymer donor plays a significant role in achieving high power conversion efficiencies (PCEs). The strong intermolecular interactions and deep highest occupied molecular orbitals (HOMOs) of donor polymers will facilitate the favorable phase separation and high open-circuit voltage (V oc), resulting in the dramatic improvement of device performance. Herein, combined chlorination of 4,8-bis(thiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene (T-BDT) and selenophene π-bridges, a new polymer donor, named PBBSe-Cl, is designed and synthesized. Compared to its parent polymer without chlorine substitution and π-bridge (named PBB), PBBSe-Cl exhibits much stronger absorption, better molecular planarity, and improved molecular aggregations. Moreover, PBBSe-Cl shows favorable phase separation and bicontinuous interpenetrating network when blending with acceptor Y6. As a result, the inverted device based on PBBSe-Cl achieves a decent PCE of 14.44%, with synchronously improved short-circuit current density (J sc) of 24.07 mA cm–2 and fill factor (FF) of 73.16%. However, its parent polymers PBB and PBBSe-H only present a relatively low device performance. In addition, a very low energy loss (E loss) of 0.51 eV is realized for PBBSe-Cl-based devices. This investigation proves that introducing chlorine atoms on the conjugated side chains and selenophene π-bridges will stepwise increase the polymer solar cell efficiency due to the simultaneous enhancement of device current density and fill factor. The proper usage of chlorination and selenophene π-bridge is a facile and efficient strategy for high-performance solar conversion materials.
The effect of isomerism in polymer donors is appealing as a means of optimization of molecular configurations in organic solar cells (OSCs) but has not been well explored. Two isomers, PAB-α and PAB-γ, with different orientations of their fused thiophene rings were designed and synthesized to investigate the influence of isomerism in polymer donors on their photovoltaic conversions. It was shown that two polymers with almost identical structures exhibited significant differences in the power conversion efficiency (PCE) of solar devices. The PAB-α-based devices achieve an excellent PCE of 15.05%, while the PAB-γ-based devices only obtain an extremely low PCE of 0.04%. Reasons for such a dramatic performance disparity include first, the absorption spectrum of PAB-γ being markedly blue-shifted and failing to match the absorption spectrum of common high-efficiency acceptors, such as Y6, and second, acceptor Y6 has preferable miscibility with PAB-α for a smaller χ value of 0.067 and smaller root-mean-square value of 0.98 nm. What is more, PAB-α has a closer π–π interaction distance compared to its isomer PAB-γ from grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis, and the order-of-magnitude difference between the hole and electron mobilities of two active layers also made the opposing values of their device efficiencies. Therefore, PAB-α has a superior performance in photovoltaic devices, demonstrating that fine tuning of atomic orientation could bring great changes to the properties of the polymer donors. This provides a new train of thought for the material design and evolution of device performance.
In addition to efficiency, stability is another key factor in developing organic solar cells. The quasiplanar heterojunction (Q-PHJ) structure, combining two pure layers as major and tiny nanoscale bulk heterojunction (BHJ) at interface, demonstrates superior device stability compared with BHJ devices. In this contribution, the polymer donor, PBQx-H-TF, and configurationally defined polymeric acceptor, PBTIC-γ-TSe are synthesized and used to fabricate bilayer devices by orthogonal solvents, the corresponding Q-PHJ all-polymer solar cells (all-PSCs) deliver reliable stability with high efficiency. An encouraging PCE of 15.77% is achieved, which is the highest one among Q-PHJ all-PSCs with real-bilayer structure. There is a major improvement over the 13.91% PCE in the BHJ device, and the carrier transport performance is improved substantially following the reduction of recombination in the Q-PHJ all-PSCs. Benefiting from the bilayer morphology, the stability of Q-PHJ all-PSCs has been greatly enhanced over that of the BHJ devices. The charge recombination process is also more serious in the aging BHJ compared with the aging Q-PHJ all-PSCs. This work inspires the application of Q-PHJ in the preparation of high-efficient all-PSCs, but also provides guidance on the improvement of device stability from a dual approach of material and device engineering aspects.
The molecular weight of polymers plays a major role in their aggregation and miscibility in active layer, which eventually dominate the energy loss and device performance. A series of chlorine-substituted PBD-Cl polymers with controlled molecular weight have been synthesized as templates to discern a relationship between molecular weight and the optical properties, energy levels, morphologies, energy loss and photovoltaic performance. Although it has similar optical and electrochemical properties, when blended with acceptor N3, the low molecular weight polymer PBD-Cl L gives the biggest energy loss value, and a PCE of 12.06%. PBD-Cl H shows a moderate energy loss, but displays the lowest PCE of 9.00% as a result of excessive aggregation. PBD-Cl M with a medium molecular weight gives the smallest energy loss and achieves a PCE of 17.17%, which is among one of the highest values recorded to date for the Cl-substituted polymer solar cells. Moreover, the molecular weight mainly affects the nonradiative energy loss (ΔE 3 ), PBD-Cl M also shows the smallest value of 0.252 eV among three polymer donors. These results show the effect of controlling the molecular weight to achieve a small energy loss and provide guidelines which can lead to an understanding of the real photovoltaic performance of new materials.
The light harvesting and photocurrent generation from acceptors largely determine the photovoltaic performance of organic solar cells (OSCs). We have designed and prepared two medium band gap non-fullerene acceptors (NFAs),...
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