Studies On The Stability Of Poly(3-hexylthiophene): Carbon Nanomaterials Based Solar Cells

Recent advances have led to power conversion efficiencies?PCEs?of PSCs over 11%,demonstrating their excellent application prospects.In addition to the necessary high PCE,long term stability has become the next key challenge before the commercialization of PSCs can be realized.In this thesis,the performance stability of inverted P3HT:PC₆₁BM solar cells was studied systematically,and some key factors that affect the performance and stability of PSCs were summarized.Based on this,a very effective way to improve device stability was developed.Furthurmore,based on the understanding of degradation of PSCs,carbon quantum dots?CQDs?with advantages of environmentally friendly,easily processable,stable and low cost were used as acceptor and electron transport layer?ETLs?of PSCs,to improve the stability of PSCs.This research work is very helpful in developing high performance and high stability PSCs.The main results are summarized as follows:?1?Inverted-structure P3HT:PC₆₁BM solar cells were selected as model devices,and detailed investigations on the degradation behavior of solar cells operated under different load conditions were conducted.The external load-dependent “burn-in” degradation behavior of P3HT:PC₆₁BM cells was clearly demonstrated.The formation of PC₆₁ BM dimers result in the devices performance decay during aging.A higher exciton concentration within the photoactive layer under higher external load condition speeds up the dimerization of PC₆₁ BM molecules and consequently accelerates the performance decay,which is considered to be the reason for the external load dependence.Blending the photoactive layer with 3% piperazine,which can block the formation of PC₆₁ BM dimers through electron transfer,successfully suppress the external load-dependent “burn-in” degradation.In that,the stability of PSCs effectively improved.?2?The degradation of prototype inverted P3HT:PC₆₁BM solar cells operated at different temperature were detailed investigated in this work.Under the low temperature aging condition?50-70°C?,devices show a fast exponential decay in the initial period and then slowly linear decay.Surprisingly,under high temperature condition?80-90°C?,the inverted P3HT:PC₆₁BM solar cells aged rapidly in the initial period and then almost recovered to the original PCE.The morphology of the active layer during annealing was investigated and results demonstrate that high temperature annealing can induce a PC₆₁ BM aggregates,which is ascribed to be the main reason for performance changing of devices.The results indicate that the active layers of PSCs prepared by traditional method were metastable,and long time high temperature annealing can change the active layer structure from metastable to stable,thus improving the stability of PSCs.?3?A chemical vapor deposition method was developed to synthesize the CQDs.These CQDs have less hydrophilic terminal groups,which offers good solubility in common organic solvents.The lowest unoccupied molecular orbital?LUMO?of CQDs was determined of-3.84 eV,which is close to that of PC₆₁ BM.PSCs based on P3HT:C-CQDs were then fabricated and tested.The performance of devices using C-CQDs as acceptor showed a 2.6 times power conversion efficiency?PCE?enhancement compared with that of device without CQD acceptor.In addition,the C-CQDs was used as additive in P3HT:PC₆₁BM,and results showed that the P3HT:PC₆₁BM:C-CQDs showed a 11% enhancement compared with that of device without using C-CQD.In summary,C-CQDs are of the great potential to act as a new active layer materials in PCSs.?4?Due to good organic solvent dispersion and excellent charge transport properties,the feasibility of using C-CQDs as ETLs in PCSs was explored.PSCs with a device structure of ITO/PEDOT:PSS/active layer/C-CQDs/Al were fabricated and tested,where the blended layer of P3HT:PC₆₁BM,PTB7:PC₆₁BM,PTB7-Th:PC71BM functions as the photoactive layer.Optimized PCE of P3HT:PC₆₁BM,PTB7:PC₆₁BM and PTB7-Th:PC71BM devices using C-CQD ETLs reached 3.11%,6.85% and 8.23% respectively,which are comparable with the LiF based devices.In addition,a wide tolerance of C-CQD-ETLs layer thickness was demonstrated.Furthermore,the long-term thermal stability of devices with C-CQDs as ETLs was improved significantly.In summary,C-CQDs could be used as as a high performance ETLs material in PCSs as well.