Novel strategies to improve the efficiency and stability of binary-based organic photovoltaic devices
- Autores: Enas Moustafa Mohamed Abd Elghafar
- Directores de la Tesis: Luis Francisco Marsal Garví (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2023
- Idioma: inglés
- Número de páginas: 270
- Tribunal Calificador de la Tesis: Josep Pallarès Marzal (presid.), Mónica Lira Cantú (secret.), Jenny Nelson (voc.)
- Programa de doctorado: Programa de Doctorado en Tecnologías para Nanosistemas, Bioingeniería y Energía por la Universidad Rovira i Virgili
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Abstract: The power conversion efficiency (PCE) of organic solar cells (OPVs) has been promptly improved once emerging the recently developed non-fullerene small-molecules acceptors (NFAs), approaching PCE of 20%. This remarkable increase in the power conversion efficiencies was due to the significant enhancement in the light absorption along with diminishing the energy losses, particularly upon minimizing the trade-off behavior between voltage loss and charge generation in nonfullerene organic solar cells (NF–OPVs). Despite the efficiency, long-term operational stability, is considered as a major challenging issue that must be confronted for the commercialization of OPVs. Several strategies have been investigated to understand the intrinsic photo- degradation mechanism in order to overcome this recent demanding subject. Some of these avenues concern the stability of the bulk-heterojunction photoactive blend microstructure through additives modifications to tune the properties of the photo-active layer. Some other approaches determined the critical role of the interface materials stability and their compatibility with the contacted photo-active layer and electrodes, reflecting their importance for long-term stable OPVs. In this thesis, we combined the interfacial engineering, morphology control, and third component strategies to improve the OPV devices performance and stability. First, we conducted an intermittent spray pyrolysis approach to deposit the ZnO interfacial layers in fullerene based inverted OPVs. It significantly enhanced the interface morphology, resulting in remarkable stability behaviour of the sprayed devices. Then, we focused on optimizing the blend morphology based on the NF-OPV devices through additives and thermal treatment. Furthermore, we presented an extremely efficient PDINO based cathode interlayer for iNF-OSCs, achieving excellent photostability behavior. It has been attributed to the avoidance of the photo-induced shunts and the photocatalytic behavior, which are inevitably in the ZnO based i-OSCs. Then, the employed pre-thermal treatment approach exhibited a significant enhancement in the JSC of the treated devices along with their PCE. Then we worked on the BGJ and LBL deposition strategies along with combining the CsPbI3 nano-crystal materials as a third component to the NF-OPV system. It significantly enhances the FF and VOC, revealing the fine-tuned morphology. Finally, QDs-third component with LBL processing method is an effective strategy to improve the morphology of the active layer and prepare high-performance OPVs.
Conclusion: In Chapter 3, the importance of the surface roughness of the ZnO film (ETL) in identifying the performance of the iF-OPVs was detected by using PTB7-Th: PC70BM as the photoactive layer. We demonstrated that the carefully tuning of the microstructure features -morphology- and the properties of sprayed ZnO film via high and low concentrations of ZnO precursor solution depend sensitively on the chosen number of spraying running cycles using the intermittent spray pyrolysis approach. Then we conducted a comparison between the ZnO-SP based devices and the ZnO-SC ones to reveal the devices performance and stability behavior. For the ZnO-SP based devices with high ZnO concentration precursor solutions, the results demonstrated that the C-7R-SP based devices showed higher JSC than the lab-scale ZnO-SC based ones and fairly similar performance, achieving devices with same VOC (0.79 V) as well efficiency of 10 %. It was noticeable that the main difference between the ZnO-SC and C-7R-SP based devices is the interface roughness effect between the ITO and the ZnO deposited film. The bright side of being textured surface that enhances the light trapping inside the solar cell which increases the absorbance of the incident light and generates higher JSC. Furthermore, this proper interface contact with the active layer enhances the overall stability for the ZnO- SP based devices, where the rate of degradation of the ZnO-SP based devices was slower than the ZnO-SC one. However, the deficient side was regarding the effect of the interface between the ZnO sprayed layer and the ITO that increases the RS. Thus, these investigated results in part I point out that the deposition techniques have a vital role that affects the film formation as well as the performance and stability of the devices. Taking these facts into account leads to the next step of improving the interface between the ZnO layer and the ITO to perform lower RS which might enhance the device stability and performance. Accordingly, we further optimized the morphology of the sprayed ZnO in part II with lowest roughness and full surface coverage that is achieved through 25R with 0.5:9.5 ration of ZnO precursor solution concentration, yielding the benchmark performance of 10 % PCE along with enhanced average VOC (0.80 V) and FF (0.70) of III based devices. Furthermore, we tested their stability behavior that demonstrated a pioneer record with respect to device ZnO-7R-SP and ZnO-SC based devices, maintaining 85% of their starting efficiency even after 16.7 months of storage inside a nitrogen glove box without encapsulation. The difference in the devices performance and stability appears to originate from the different obtained ZnO surface morphologies that control the presence of defects at the surface and their subsequent adjacent organic active layer blend. As a sequence, the surface roughness determines the effective interfacial region between the active layer and the ZnO layer and ITO, thus the density of trap sites at the interface that was investigated by the IS measurements for the fresh and degraded iF-OPVs. Therefore, the proposed electrical equivalent circuit module accounted to fit the experimental data of the IS, allowed us to recognize the impact of each interlayer on the device performance and the correlated stability behavior. Accordingly, we obtained that the remarkable stability enhancement behavior of the III based devices correlates with the marginally interface density of states values for this sample among the others. These models presented a simple way to diagnose the loss mechanisms and investigate degradation mechanisms and stability issues in the fabricated devices. Finally, it is worthy mentioning that the obtained high efficiency and excellent stability of the fabricated inverted OPVs using intermittent spray pyrolysis approach could facilitate their scaling up to the industrial production perspective.
In Chapter 4, fine-tuning of blend morphology is a key factor that limits the performance of the bulk-heterojunction organic photovoltaics (BHJ-OPVs). Here, morphological control of the binary PM6:Y7 blends was conducted through 1- chloronaphthalene (CN, 2%) solvent additives and thermal annealing treatment (TA, 100 • ) with respect to their influence on the photovoltaic performance. Moreover, a distinct study was accomplished on the optical and electric properties of the treated and non-treated based devices by external quantum efficiency measurements and impedance spectroscopy. The results indicated that the 2 % CN solvent addition showed pronounced increment of the JSC by 27 % and FF by 12.5 %. Furthermore, the TA treatment provides a higher PCE for the iNF-OPV mainly due to the increment of the VOC. These performance enhancement was mainly upon suppressing the carrier recombination and assist the excitons in the photoactive layer blend to reach the donor/ acceptor interface, and in turn easily dissociated into free carriers214 which typically contributed to enhance the JSC and FF of the based devices. Moreover, the low RMS as well as more enhanced film crystallization were exhibited for the champion D based devices that conducted the dual modifications of 2 % CN solvent additives along with the TA treatments. That likely facilitated the charge transport providing the closest nid value to 1, highest Pdiss, lowest leakage current, highest JSC, FF, and in turn the best PCEmax Accordingly, the tunning of both CN additives and TA treatments are crucial for achieving the balance of exciton dissociation and charge collection, prompting better film morphology and increase in the PCE as confirmed by the case of D based devices.
In Chapter 5, we presented an extremely efficient PDINO based cathode interlayer for iNF-OSCs, achieving excellent photostability behavior. The iNF-OSCs incorporating the PDINO interlayer (C-PDINO based devices) maintained 80 % of the initial efficiency after continuous illumination (AM 1.5G, 100 mW cm-2) for 520 min, while the ZnO-based control iNF-OSCs (A- ZnO based devices) remained only for 160 min under same conditions. The remarkable photostability of the C-PDINO based devices were mainly attributed to the avoidance of the photo-induced shunts and the photocatalytic behavior, which are inevitably in the ZnO based i-OSCs. Moreover, C-PDINO based devices refrained from the burn-in photo-degradation phenomena, which cause a significant reduction in the performance of the exposed devices by providing negative effects on energy transfer, exciton dissociation and charge recombination processes which may be more related to the bulk of the active layer as appeared for the ZnO based iNF-OSCs. Hence, this piece of work clearly demonstrates that PDINO is a promising cathode interlayer for tremendously photostable i-OSCs, particularly for large-scale production. As it fits the requirements of low-temperature fabrication, can be used in very fine-tuned thicknesses, showing great resistance behavior toward light, and does not react with the contacted active layers and electrodes.
In Chapter 6, aiming to enhance the photovoltaic performance, we explicitly conducted a systematic investigation regarding the influence of the Pre-TT approach on the performance of the NF-OPVs. But first, optimization process was conducted to the devices through sequential thicknesses variation of PM6:Y7, PEDOT:PSS, and PDINO layers to investigate the impact of each layer on the overall device behavior. The obtained results revealed that the key limiting effect was related to the PM6:Y7 layer where the PCE % enhanced by 15% upon reducing its thickness from 150 nm to 100 nm. Surprisingly, another important factor was corelated to the PDINO film which increases the JSC by 7% owing to increment the concentration of the PDINO precursor solution from 1 mg/ml to 1.5 mg/ml in methanol. Then, conducting the Pre-TT step critically affects the PEDOT:PSS/PM6:Y7 blend morphologies and thus the overall performance of the devices. Interestingly, this chapter puts the spotlight on the extraordinary JSC values obtained by the Pre-TT devices, where the optimized treated devices of D8-Pre-TT exhibited a pioneer enhanced JSC value of 32.65 mA/cm2 along with improved PCE of 17.92. We observed that the treated based films exhibited smoother surface roughness, revealing their lower RS, and lower leakage current, which leads to high carrier transport and suppressed charge carrier recombination in the resulting NF-OPVs based devices. Indispensably, a critical challenge for the current NFA-based OPVs is to avoid the strong phase separation upon blending and deposition, which applied through the Pre- TT approach, leading to balanced hole and electron mobility along with low non-geminate recombination that might be the key parameter leading to the remarkable enhancement observed in the performance of the Pre-TT NF-OPVs. Moreover, the enhanced JSC values noticed for the Pre-TT devices were mainly due to their high EQE values and high Pdiss than the corresponding pristine NF-OPVs, correlating their remarkable PL quench of the Pre-TT films, reflecting the efficient charge transfer at the PEDOT:PSS/PM6:Y7 interfaces Furthermore, the recombination study using Plight vs the JSC showed the presence of recombination, but it did not distinguish between each device as it showed all devices have equal contribution of the bimolecular recombination. While the IS characteristics were conducted through investigating the Nyquist plot, Cƒ -DOS calculations and the C-V characteristics, providing an efficient insight study that accurately detected the recombination contribution that was more obvious for the pristine devices which disclose their lower performance. Accordingly, the Pre-TT approach diminishes the traps and enhances the interface charge transfer through enhancing the carrier dynamics of the treated devices, triggering a promising approach for boosting the JSC along with the photovoltaic performance of the treated devices.
In Chapter 7, we comparatively studied the binary and ternary OPVs based on LBL structures as well as BHJ structures for binary based ones with D18 and Y6 as the host system and CsPbI3 as the third component. The devices prepared by LBL processing method, whether based on binary system or ternary system, form a vertical distribution structure facilitating the charge transmission and charge collection, which brings about a higher JSC and FF than BHJ OSCs with the same systems. In addition, It is known that QDs suffer from surface traps behaviour, which might be enhanced regarding the Y6 as a passivation layer on the top of the QDs that reduced the surface trap of the QDs. Then, it could be suggested that, around 15-20 nm QDs film enhanced the charge transfer and lessened interfacial recombination of photo carriers, leading to device performance improvement.. Finally, ternary OPVs with LBL processing method achieve the best PCE of 16.56%. In brief, On the basis of the LBL method, selecting an appropriate ternary system is an effective strategy to improve the morphology of the active layer and prepare high-performance OPVs.
