Study On Photovoltaic Performance Optimization By Ternary Strategy And Mechanism Of Organic Solar Cells

Organic solar cells(OSCs)have promising potential in future applications due to their unique advantages of lightweight,solution-processable,low cost and capability to be fabricated into flexible or semitransparent devices.With the development of the photovoltaic materials,device structure design and optimization of processing methods,OSCs have achieved great progress recently.Particularly,the breakthrough in materials design and synthesis of nonfullerene electron acceptors have largely elevated the power conversion efficiencies(PCEs)of OSCs.PCE is still the first-line factor to study before the consideration of commercial applications(another two factors are cost and stability).To boost the PCEs of OSCs,besides the innovation on materials design,research on device engineering is also necessary and important.Normally,for binary systems,there will exist some limits,including the limited absorption spectra,trade-off between photocurrent and open-circuit voltage,large energy loss and morphology problem.To solve above issues,constructing ternary organic solar cells(TOSCs)by introducing a third component to the binary system is an effective strategy to realize efficiency breakthroughs.In addition,the complicated morphology and mechanism of OSCs are still not well understood and worthy studying.In this thesis,we demonstrate high efficiency TOSCs with synergetic enhancement in short-circuit current density(Jsc),open-circuit voltage(Voc)and fill factor(FF)by extending the absorption spectra,modulating the energy levels,mitigating the energy loss and optimizing the active layer morphology.There works are separated into three chapters,each of them study the different roles of the third component acting in the ternary devices and the effect on the device performance and work mechanism of TOSCs.In the first part of work,to a binary system based on a polymer donor PBDB-T and nonfullerene acceptor HF-PCIC with absorption edge around 700 nm and 800 nm respectively,we introduce a low energy loss and near-infrared acceptor material IEICO-4F with absorption edge close to 1000 nm.firstly,since the complementary absorption of HF-PCIC and IEICO-4F,the PL spectrum of HF-PCIC is thus overlapped with the absorption spectrum of IEICO-4F,allowing the realization of energy transfer and the better utilization of absorbed photons.secondly,the third component extends the blend absorption to near-infrared range,benefiting for the capture of more photons.Due to the above reasons,TOSCs show an extremely high Jsc of 23.46 mA cm-2,corresponding to a dramatic Jsc enhancement of 44%compared with the binary devices,which is one of the highest Jsc enhancement reported in TOSCs at that time.Furthermore,we find that the energy loss of TOSCs is decided by IEICO-4F,leading to a reduced energy loss from 0.80 eV for binary OSCs to 0.59 eV for TOSCs.As a result,PCE is dramatically increased from 8.82%for binary OSCs to 11.20%for TOSCs.In the second part of work,we choose PBDB-TF with a deep the highest occupied molecular orbital(HOMO)level as the polymer donor to pair with the unfused-core electron acceptors,HF-PCIC and HC-PCIC,so as to achieve a high Voc.However,this will lead to minimized HOMOD-A and LUMOD-A offsets,thus influencing the quantum efficiency and photocurrent response.Therefore,TOSCs with cascade energy level alignment are constructed by adding the third component of PC71BM which possesses deeper energy levels.Due to the cascade energy level alignment,charge transfer between HF-PCICI/HC-PCIC and PC71BM also exists,thus promoting the exciton separation and charge transport.Finally,obvious quantum efficiency enhancements from around 70%to over 80%in the longer wavelength ranges are observed,leading to high PCEs of 11.55%for PBDB-TF:HF-PCIC:PC71BM-based TOSCs and 12.36%for PBDB-TF:HC-PCIC:PC71BM-based TOSCs.In addition,both ternary OSCs show good thermal stability with～80%initial efficiencies remaining after thermal treatment at 130℃ for 12 h.In the last part of work,we realize the synergetic improvement in three device parameters for TOSCs by combining fused-ring and unfused-core electron acceptors to tune the blend morphology.To a binary system based on PBDB-TF:HC-PCIC,a fused-ring electron acceptor IT-M is introduced as the third component.HC-PCIC has the feature of higher crystallinity,benefiting for electron mobility,but large domain sizes may exist to increase the monomolecular recombination.IT-M has weaker crystallinity,benefiting for small domain sizes,but the lower electron mobility will increase the bimolecular recombination.Through combining the advantages of unfused-core and fused-ring acceptors,the morphology and domain sizes of TOSCs can be balanced,thus maintaining the charge transport and reducing the charge recombination.Finally,synergetic improvement in Jsc,Voc and FF are observed in TOSCs.Relative to the PCE of 11.10%and the FF of 71%for binary OSCs,TOSCs can show the best PCE of 12.51%and the best FF of 75%.More importantly,among the third component ratio of 2.5%-50%,high efficiencies over 11%can be maintained for TOSCs.Such broad composition tolerance is rarely reported in TOSCs.Overall,this thesis presents the construction of high-performance TOSC by introducing a third component to the effi cient binary systems.Thr ough the strategies of extending absorption spectra,reducing energy loss,enlargin g the driving force and optimizing domain sizes and blend morphology,obvious enhancement in efficiencies are realized for TOSCs.Besides,via the study of work mechanism about energy transfer or charge transfer,we disclose that,1)applying near-infrared materials with low energy loss as the third component is possible to simultaneously achieve large photocurrent enhancement and high open-circuit voltage,2)for binary systems with small driving forces,constructing cascade energy level alignment to induce charge transfer is effective to promote the charge separation and increase the quantum efficiency,3)by combining the advantages of different types of acceptor materials to tune the blend morphology and domain sizes,a balance between charge transport and charge recombination can be realized,thus synergistically improving the Jsc and FF,and finally resulting in the improvement of PCEs.