Device Physics Of High Efficiency Organic Solar Cell

Energy is the basis of human subsistence and development.Economic development and population growth are inseparable from energy consumption.Photovoltaic,as a clean energy technology based on the photoelectric effect,is one of the outstanding green and renewable energy technics.Organic solar cells is part of photovoltaic,which possesses several advantages such as multiple source of materials,light weight,solution-processed and flexibility,and has become one of the most promising next-generation solar cells and attracted extensive attention.In recent years,the power conversion efficiency?PCE?of organic solar cells has been significantly improved,mainly owing to the material innovation and device optimization,and the state-of-the-art PCE that is reported exceeded 15.7%.The main innovation of this paper is to adopt different device optimization methods.After obtaining the high-efficiency organic solar cells,electroluminescence?EL?,photoluminescence?PL?,charge carriers dynamics and other research methods were used to investigate the influence of different optimization processes on the internal mechanism of organic solar cells.Research directions include exciton separation,charge transport and collection efficiency;quantificationally calculating the ratio of carrier recombination rate to extraction rate;quantificationally calculating the radiation recombination and non-radiation recombination loss.Finally,the fundamental reason why the parameters of organic solar cells are affected by different systems and optimization methods is revealed.Solvent vapor annealing,thermal annealing and mixed solvent processing are commonly used optimization methods for organic solar cell.After device optimization,fill factor can be greatly improved generally,which were often explained by morphology change and there are few in-depth studies on the physical mechanisms.In the third and fourth chapters of this paper,we achieved power conversion efficiency?PCE?of 10.2%small molecule organic solar cells based on BTR:PC₇₁BM bulk-heterojunction and the sources of FF differences in devices are studied by means of photoluminescence and charge carriers dynamics.The PL spectra showed that the fluorescence quenching efficiency of the device optimized by mixed solvents is higher,which indicated that the device has higher charge transfer and exciton separation efficiency.The ratio of carrier recombination rate to extraction rateis calculated by charge carriers dynamics test.is 2.5×10^-2,1.9×10^-22 and 8.6×10^-3 for the CF,CB and mixed solvent device respectively,which is correlated with FF?with lower?corresponding to a higher FF?.Therefore,in organic solar cells where non-geminate recombination is the dominant photocurrent loss mechanism,a further reduction in the ratio between extraction rate and recombination rate provides a guideline to optimize the device performance.Non-fullerene organic solar cells?NF-OSCs?is gaining increasingly attention for its distinct advantages such as facile synthesis,strong optical absorption and minimized energy loss.Compared with binary solar cells,ternary organic solar cells can broaden the active layer absorption,which is a very effective way to obtain high-performance organic solar cells.In the fifth chapter of this paper,ternary non-fullerene organic solar cells based on J52:i-IEICO-4F:INPIC-4F bulk-heterojunction with power conversion efficiency of 14.14%were obtained?VOC:0.83 V,JSC:24.1 mA cm-2,FF:71.0%?.Compared with binary organic solar cells based on J52:INPIC-4F,the open-circuit voltage,short-circuit current and filling factor are enhanced simultaneously,which is relatively rare in ternary solar cells.To go insight into the origin of energy loss of binary and ternary organic solar cells,we calculated the sub-bandgap external quantum efficiency?EQE?and the eletroluminescence external quantum efficiency?EL?,indicating that the main reason for the open-circuit voltage increase is that the radiation recombination loss?0.31 eV?and non-radiation recombination loss?0.30 eV?of the devices were reduced by adding the third component i-IEICO-4F.The total energy loss is only 0.61 eV,which is one of the reported low energy loss systems.UPS measurements showed that J52,i-IEICO-4F and INPIC-4F had cascaded energy levels.The PL spectrum of i-IEICO-4F:INPIC-4F blend films doped with different proportion of INPIC-4F were studied.With the increase of the doping ratio of INPIC-4F,the PL intensity of i-IEICO-4F:INPIC-4F blend films decreased gradually.Therefore,the main working mechanism between i-IEICO-4F and INPIC-4F is charge transfer.Compared with binary solar cells,ternary organic solar cells possesses lower energy loss,which can provide many effective ways for exciton separation,charge transport and collection,thus improving the open-circuit voltage,short-circuit current and filling factor of organic solar cells.