Study On Morphology Control And Properties Of Carrier Transport For Polymer Solar Cells

Polymer solar cells（PSCs）have attracted much attention due to their low-cost,lightweight,flexibility,semitransparency and large-area fabrication.Recently,with the continuous emergence of high-performance narrow-bandgap non-fullerene acceptors and wide-bandgap polymer donors,the power conversion efficiency（PCE）of PSCs has exceeded 18%.Exciton dissociation,charge transport and charge collection in the devices deeply depend on the morphology of the active layer,so ideal morphology of the active layer is the key to obtaining high-performance PSCs.In this work,we control the morphology of the active layer by optimizing molecular structure,film component and film fabrication process to prepare highly efficient PSCs.The relationship between morphology of the active layer and performance of the devices is deeply clarified by analyzing the energy level,molecular stacking,crystallinity and planarity of donor and acceptor materials,as well as the photovoltaic performance,micromorphology,and carrier transport of the devices.The specific research results of this paper are as follows:（1）A series of copolymers donor PBO-X（X=Cl,H,F）were prepared by changing the elements on acceptor unit.PBO-H has a S…O single-side locking,while PBO-Cl has a weakened S…O locking due to the Cl-H steric repulsion and PBO-F has S…O and F…H double-side locking.Density functional theory（DFT）calculations and experimental results show that non-covalent conformation locks can effectively tune the planarity and hole mobility of polymer donors.Then,PBO-X:Y6 PSCs were fabricated.Compared with PBO-Cl:Y6 and PBO-H:Y6,PBO-F:Y6 PSCs have continuous interpenetrating network with better phase separation and crystallinity,which can effectively promote exciton dissociation,charge transport and collection,and reduced charge recombination.As a result,PBO-F:Y6 PSCs exhibit a high PCE of 16.23%.（2）Polymer L3 is synthesized by shifting the position of the alkyl side chain branching point on the thiophene bridges of the polymer donor L2.The main purpose is to study the effect of the position of near-backbone alkyl side chain branching point on the performance of PSCs.DFT calculations and experimental results show that the shift of the alkyl side chain branching point fromβtoγposition can effectively adjust the molecular packing,bandgap,energy levels and hole mobility of the polymer.Then,L2:N3 and L3:N3 PSCs were prepared.L3:N3 PSCs show a higher efficiency of17.23%compare to L2:N3 PSCs（16.69%）.Subsequently,L3:N3:PC₆₁BM ternary PSCs were fabricated by introducing a third component fullerene PC₆₁BM,which can effectively improve electron mobility and balance charge transport of devices.Consequently,L3:N3:PC₆₁BM PSCs exhibit a high PCE of 17.81%（certified PCE17.1%,NIM）.L3 is one of the few polymer donors with a certified efficiency of over17%to date.（3）High-performance D18:Y6:PC₆₁BM ternary PSCs were prepared by introducing the third component fullerene PC₆₁BM into D18:Y6 binary PSCs based on work（2）.After optimization,D18:Y6:PC₆₁BM ternary PSCs with an active layer thickness of 110nm show a high PCE of 17.89%（certified PCE 17.4%,NIM）.Subsequently,thick-film PSCs were prepared by increasing thickness of active layer.It shows that the introduction of PC₆₁BM can effectively improve electron mobility,balance charge transport and suppress charge recombination by studying the carrier transport properties in D18:Y6 and D18:Y6:PC₆₁BM PSCs.When thickness of active layer increases to 300nm and 350 nm,PCE of D18:Y6:PC₆₁BM PSCs can still be maintained at 16.32%and16.19%,respectively.It is the first time report that PCE of 300-nm-thick OSCs has exceeded 16%.（4）The polymer donor D18-Cl was prepared by replacing the fluorine element on benzo[1,2-b:4,5-b＇]-dithiophene（BDT）unit of the polymer donor D18 in work（3）with chlorine element.Compared with D18,D18-Cl has a deeper HOMO energy level,which is beneficial to obtain a higher open-circuit voltage(V_oc).D18-Cl:Y6 and D18-Cl:N3 PSCs were prepared by matching D18-Cl with Y6 and N3,respectively.Then,the D/A ratio and film thickness were optimized,and the solvent additive diphenyl ether（DPE）was introduced to optimize the morphology of the active layer.Compared with D18-Cl:Y6,D18-Cl:N3 PSCs have higher exciton dissociation and charge collection efficiency,faster and more balanced charge transport,and suppressed charge recombination.Consequently,D18-Cl:N3 PSCs show a high PCE of 18.13%（certified PCE 17.6%,the National Institute of Metrology（NIM）（Beijing））,which is the first report that the PCE of organic solar cells based on chlorinated donors has exceeded 18%.（5）On the basis of work（4）,we fabricated high-performance PSCs based on D18-Cl:N3 system by combining sequential deposition（SD）method and volatile solid additive 1,4-diiodobenzene（DIB）to control morphology of active layer.The morphology of active layer and the carrier transport properties in PSCs were systematically analyzed.Volatile solid additive-assisted SD method delivers a bicontinuous interpenetrating network with excellent vertical phase separation and molecular crystallinity,which can promote exciton dissociation,charge transport and collection,and reduce charge recombination.Consequently,D18-Cl/N3（DIB）PSCs deliver a high PCE of 18.42%,which is one of the highest efficiency for reported SD-based binary OSCs to date.