Studies On The Property Of Organic Tandem Solar Cells

Organic solar cells(OSCs),as a new photovoltaic technology,have attracted a wide spread attention in recent years owing to their low cost,light weight,and potential for making large-area and flexible devices through roll-to-roll processing.With the development of new materials and device engineering,the power conversion efficiency(PCE)of over 17%for OSCs has been achieved.However,for the commercial application,there is still a long way to go.Tandem OSCs composed of two or more subcells with complementary absorption stacked together can utilize the solar radiation more effectively by reducing transmission and thermalization losses.Currently,it is still challenging to fabricate tandem OSCs due to the more complicated device structure compared with single-junction devices.In this thesis,the following studies have been performed to make highly efficient and stable tandem OSCs:(1)The morphology of active layers is very important for highly efficient OSCs,especially for tandem OSCs.We studied the relationship between the morphology of active layer and the photovoltaic performance of single-junction device.Through thermal annealing treatment,the OSCs based on PTZ1:IDIC exhibit a high PCE of up to 11.5%with enhanced short-circuit current density(Jsc)of 16.4 mA cm-2 and fill factor(FF)of 76%.Systematic research shows that thermal annealing treatment contributes to the enhanced absorption,efficient exciton dissociation,suppressed trap-assisted recombination,enhanced crystallinity,preferable molecular orientation and purer domains,leading to the high performance of the nonfullerene OSCs.These results indicate that PTZ1 is a promising polymer donor material for highly efficient fullerene-free OSCs and large-scale devices fabrication.(2)The influence of tandem device performance by the active layers has been studied.By choosing active layer materials with different absorption spectra in the subcells,the influence of Jsc of tandem device by the active layers was obtained.Based on proper active layer materials and optical simulations,tandem OSCs with 14.2%efficiency were achieved by employing PBD1:PC71BM in the front cell and PTB7-Th:IEICO-4F in the rear cell.(3)The relationship between the interconnecting layer and photovoltaic performance of tandem OSCs has been studied.Inverted tandem devices were fabricated with HTL Solar:SPS and AI 4083:NPA as the hole transport layer in the interconnecting layer.To obtain good photovoltaic performance of the tandem OSCs,the conductivity of PEDOT:PSS layer in the interconnecting layer should be lower than 0.08 S cm-1.The work function of PEDOT:PSS layer should be near to the highest occupied molecular orbital(HOMO)energy level of the donor material in the front cell.(4)To replace the evaporated metal electrode in the device,PEDOT:PSS:silver nanowire blend(HYE)was used as the transparent top electrode and PMMA reflector was used to make full use of the sunlight.Upon a variation of the foamed film’s thicknesses,the ratio of reflected/transmitted light can be easily tuned.The enhancement effect of the PMMA reflectors equals the optical performance of metallic ones in opaque OSCs and the process of PMMA reflectors is cost-effective.Owing to the optical field tuning effect,the performance enhancement for tandem devices with PMMA reflector is higher than single-junction devices.