Morphology Control And Photovoltaic Performance Of Polymer Solar Cells With Multi-components Active Layers

Polymer solar cells?PSCs?have been demonstrated to be an important means of generating renewable energy due to advantages including their low-cost,lightweight nature,and large scale fabrication from solution,thus exhibiting promising prospects for applications such as wearble flexible electronics.However,compared to traditional inorganic photovoltaics,the exciton diffusion lengths of PSCs are shorter and the carrier mobilities are lower,which makes the nanoscale morphology of the photoactive layer critical in approaching high performance PSCs.In order to extensively and comprehensively clarify the correlation between the nature of the nanoscale morphology of the active layer with device performance and further develop the power conversion efficiency?PCE?of PSCs,we fabricated a series of binary and ternary polymer:fullerene and polymer:nonfullerene bulk heterojunction solar cells,and studied the impact of compontents,chemical structures and processing methods of the active layers on their molecular order,condensed state and photovoltaic property of the PSC devices.The research contents are as follows:?1?Contrasting effects of two crystallizable polymers,namely,PffBT4T-2OD and PDPP2TBT,in determining the efficiency improvements in PTB7-Th:PC₇₁BM host blends are demonstrated.A notable power conversion efficiency of 11%can be obtained by introducing 10%PffBT4T-2OD?relative to PTB7-Th?,while the efficiency of PDPP2TBT incorporated ternary devices decreases dramatically despite an enhancement in light absorption.Blend morphology studies suggest that both PffBT4T-2OD and PDPP2TBT are well dissolved within the host PTB7-Th phase and facilitate an increased degree of phase separation between polymer and fullerene domains,result in improved hole mobility.While negligible charge transfer is determined in binary blends of each polymer mixture,effective energy transfer is identified from PffBT4T-2OD to PTB7-Th that contributes to an improvement in ternary blend device efficiency.In contrast,energy transfer from PTB7-Th to PDPP2TBT worsens the efficiency of the ternary device due to inefficient exciton dissociation between PDPP2TBT and PC₇₁BM.?2?High efficiency TSCs with the maximum PCE over 11%were achieved by introducing a medium band gap conjugated polymer PCDTBT8 into the PffBT4T-2OD:PC₇₁BM binary photovoltaic system.Morphological investigation shows that the third component PCDTBT8 locates at the interface between PffBT4T-2OD and PC₇₁BM so as to maintain decent charge mobility,and loosens the fullerene aggregation networks to facilitate exciton dissociation.The efficient F?rster energy transfer from PCDTBT8 to PffBT4T-2OD enables the ternary devices to retain a high short-circuit current density despite the slightly decreased light absorption.Device physics studies suggest that the addition of PCDTBT8 can enhance the built-in voltage,prolong the carrier lifetime,reduce the defect density and suppress the trap-assisted charge recombination,leading to an improved FF and V_OCC to enhance the efficiency of ternary devices.?3?The impact of the molecular organization time during film casting process on the film nanoscale morphology as well as device photovoltaic in PBDB-T:INPIC-4F system has been studied,and the crucial role of retarding the crystallization of INPIC-4F in achieving high performance is demonstrated.When PBDB-T:INPIC-4F is cast with the presence of solvent vapor to prolong the organization time,INPIC-4F molecules form spherulites,resulting in large phase separation and device efficiency below 10%.On the contrary,casting the film on a hot substrate is effective in suppressing the formation of the polycrystalline structure,and encourages face-on?-?stacking of INPIC-4F.This molecular transformation of INPIC-4F significantly enhances the absorption ability of INPIC-4F at long wavelengths and facilitates a fine phase separation to support efficient exciton dissociation and balanced charge transport,leading to the achievement of a maximum PCE of 13.1%.A rational guide was provided for optimizing nonfullerene polymer solar cells consisting of highly crystallizable small molecular electron acceptors.?4?Adjusting molecular ordering,orientation and nanoscale morphology within the photoactive layer of polymer:non-fullerene organic solar cells is crucial to achieve high power conversation efficiency?PCE?.The molecular ordering and orientation of n-type small molecule acceptor COi8DFIC were tuned from flat-and edge-on lamellar crystalline to H-and J-type?-?stacking during the solution-casting process,resulting in broadened photon absorption and fine phase separation with the electron donor PTB7-Th.This favorable morphology with face-on?-?stacked electron donors and acceptors promotes efficient exciton dissociation at the donor/acceptor interface,together with enhanced and balanced carrier mobility.The enhanced short-circuit current density and fill factor lead to the achievement of a maximum PCE of 13.8%in binary,single-junction PTB7-Th:COi8DFIC non-fullerene polymer solar cells,whilst also exhibiting superior stability.?5?A series of ladder-type electron acceptors having different carbon-oxygen-bridged electron-donating cores were systematically investigated.The results revealed the effects of core structure and film casting condition on molecular ordering and optoelectronic properties in pure acceptor films as well as in their photovoltaic blends with the electron donor PTB7-Th.We found that NFAs containing the thieno[3,2-b]thiophene centered,6 or 8 fused-rings?i.e.COi6DFIC,COi8DFIC?exhibit much narrower optical band gaps than NFAs containing the benzene centered,5 or 7 fused-rings?i.e.COi5DFIC,COi7DFIC?.The amount of fused-rings in the carbon-oxygen-bridged core structure also influences the molecular ordering of NFAs,with COi5DFIC and COi6DFIC exhibits edge-on orientation,which is in contract to the face-on orientation of electron donor PTB7-Th,therefore resulting in poor device performance.COi7DFIC and COi8DFIC which have longer cores,exhibits pronounced flat-on molecular orientation induced by its strong backbone stacking.Therefore,casting film on hot substrate is needed to suppress the backbone stacking of COi7DFIC and COi8DFIC,and rendering high performance in solar cells.