Study Of Semi-transparent Organic Solar Cells Based On Active Layer Manipulation

Organic solar cells(OSCs)have gained much attention in past decades due to their low-cost,solution processability,flexibility,excellent mechanical properties and semitransparency.The maximum power conversion efficiency(PCE)of OSCs has exceeded 18%with the continuous development of donor and acceptor materials and advances in device engineering.Semi-transparent OSCs(ST-OSCs)can convert sunlight into electricity while allowing a portion of light to pass through,and have potential applications in building-integrated photovoltaics and portable electronic devices.Ideal ST-OSCs should selectively absorb the infrared and ultraviolet light more efficiently for achieving both high PCE and high average visible transmittance(AVT).However,most active layer systems for ST-OSCs have strong absorption in the visible region,and the overall absorption of the active layer is limited in order to balance the transmission of light in the visible region.The trade-off between PCE and AVT hinders the further improvement of the performance of ST-OSCs.In this thesis,the problem of the strong absorption of the active layer in the visible region will be addressed.The absorption of the active layer in the visible region will be reduced by selecting the active layer materials,manipulating the active layer structure and the ratio of donor to acceptor in order to enhance simultaneously the AVT and PCE of ST-OSCs.The main conclusions are as follows.1.The wide bandgap small molecule donor TAPC combined with the non-fullerene small molecule acceptor Y6 was chosen to fabricate ST-OSCs.TAPC has a wide bandgap(Eg=3.54 eV)and almost no absorption in the visible region,while Y6 has strong absorption in the near-infrared region and could absorb sunlight without seriously affecting AVT.The combination of TAPC and Y6 can minimize the absorption of the active layer in the visible region.TAPC:Y6(1:1,w:w)blend films are subjected to solvent vapor annealing(SVA)with chloroform.It is found that SVA could improve the charge collection efficiency,reduce bimolecular recombination and obtain a better morphology,thus improving the PCE of the prepared ST-OSCs.Finally,the SVAtreated TAPC:Y6 based ST-OSCs realize a PCE of 3.01%and a high AVT of 47.99%.The CIE 1931 coordinates of the ST-OSCs are(0.292,0.313),close to the D65 standard light source.The color rendering index(CRI)is 94.5,indicating their good color rendering ability.2.The ternary ST-OSCs via sequential depositing(SD)based on classical polymer donor PTB7-Th,PCDTBT and narrow bandgap small molecule acceptor IEICO-4C1 were prepared and studied.First of all,SD-type and bulk heterojunctions(BHJ)type ST-OSCs based on PTB7-Th:IEICO-4Cl were compared and it is found that SD-type binary ST-OSCs can maintain comparable PCE with corresponding BHJ-type ST-OSCs at different active layer thicknesses but have higher AVT,and therefore have higher light utilization efficiencies(LUE).The SD method is more suitable for decreasing the absorption of the donor material in the visible light region.A second wide-bandgap donor PCDTBT was doped into the PTB7-Th donor layer in SD-type device to further optimize the performance of ST-OSCs.The introduction of PCDTBT not only helps to resist chloroform spin-rinsing but also improves charge generation and charge collection.Finally,the transparency of the ST-OSCs is further enhanced by introducing a thin light out-coupling layer of MgF2 atop the semi-transparent Au electrode.The optimized ST-OSC achieves a PCE of 6.30%and an AVT of 43.93%,leading to a LUE of 2.77%.3.The PM6:L8-BO combination was selected to prepare high-efficiency ST-OSCs through decreasing the donor content to reduce the absorption of the active layer in the visible region.The influence of decreasing the weight ratio of the donor:acceptor blend on the absorption,charge transport and morphology of the active layer was investigated.It is found that with the decrease of the PM6 content,the photon capture capacity and charge extraction capacity of the blend films are decreased.The charge recombination is increased,while the PCE decreases slowly in a certain interval.A series of ST-OSCs with different PM6:L8-BO weight ratios were fabricated with 15 nm semi-transparent Au as the top electrode.The PCE of ST-OSCs is declined with the decrease of PM6 content,but the higher AVT leads to the improvement of overall performance,such as the LUE.Finally,the ST-OSCs based on PM6:L8-BO weight ratio of 1:3 exhibit a PCE of 12.35%and an AVT of 21.38%,resulting in an LUE of 2.64%,higher than the LUE of 2.10%for ST-OSCs based on PM6:L8-BO(1:1.2)blend.It indicates the effectiveness of the strategy of reducing the donor ratio to achieve high-performance ST-OSCs in the PM6:L8-BO system.