Optimum design of ordered bulk heterojunction organic photovoltaics
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An efficiency of 8.62% is observed from inverted organic photovoltaics (OPVs) composed of a bulk heterojunction (BHJ) active layer with a thickness of 280 nm. Remarkably, an efficiency of 7.24% can be obtained using OPVs with a BHJ thickness of 1000 nm. Such high efficiencies from thick BHJ composite films are attributed to the high hole mobility and ordered molecular structure of the electron-donor polymer.
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The dependence of active-layer thickness on the power conversion efficiency (PCE) of inverted organic photovoltaics (OPVs) based on poly(3-hexylthiphene) and [6,6]-phenyl-C61-butyric acid methyl ester was investigated. When PCEs were measured immediately after device fabrication, the optimum thickness was ~100 nm. It was, however, found that thick OPVs exhibit higher PCEs a few months later, whereas thin OPVs simply degraded with time. Consequently, the optimum thickness changed with time. Considering this fact, we discuss the relationship between the active-layer thickness and PCE.
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Comprehensive Summary Organic solar cells (OSCs) present a promising renewable energy technology due to their cost‐effectiveness, adaptability, and lightweight nature. The advent of non‐fullerene acceptors has further boosted their significance, allowing for power conversion efficiencies surpassing 19% even with an active layer thickness of about 100 nm. However, in order to achieve large scale production, it is necessary to fabricate OSCs with thicker active layers exceeding 300 nm that are compatible with large‐area printing techniques. Nevertheless, OSCs with thick active layers have inferior performance compared to those with thin active layers. To expedite the transition of OSCs from laboratory to industrial high‐throughput manufacturing, considerable efforts have been made to comprehend the performance limitations of thick active‐layer OSCs, develop novel photoactive materials that are high‐performance and tolerant towards the thickness of the active layer, and optimize the morphology of the photoactive layer and device structure. This review aims to provide a comprehensive summary of the mechanisms that lead to efficiency loss in thick active‐layer OSCs, the representative works on molecular design, and the optimization strategies for high‐performance thick active‐layer OSCs, and the remaining challenges that must be addressed.
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In organic solar cells (OSCs), the morphology of the active layer plays a significant role in exciton dissociation into charge carriers and their subsequent transportation and extraction. The morphology of the active layer depends upon the thickness, concentration, and pre-post treatments. In this study, we investigated the effect of thickness on the morphology of the active layer and the resulting photovoltaic performance of semitransparent (ST) bulk heterojunction OSCs. We used a blend of PBDB-T (polymer donor) and a well-known nonfullerene small-molecule acceptor ITIC as the active layer for fabricating opaque (OP) and ST OSCs. This study is performed under ambient conditions. To obtain active layer films with thicknesses ranging from 350 to 100 nm, the rotations per minute (rpm) of the spin coating was varied from 1000 to 4000. Through a comprehensive analysis, we identified the optimal rpm for achieving an efficient active layer morphology and improved device performance for both OP and ST devices. We have achieved overall power conversion efficiencies of 10.39 and 7.26% for optimized OP and ST OSCs, respectively. The effect of varying the rpm of the active layer on nonradiative voltage loss is also studied. The correlation between the rpm, active layer morphology, and the overall performance of the ST OSCs is elucidated by our findings, providing valuable insights into optimizing devices. The results of this study could potentially aid in the development of more effective photovoltaic devices.
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This chapter contains sections titled: Introduction to Organic Photovoltaics Performance of Organic Photovoltaics Charge Transport in Organic Semiconductors Energetic Disorder in Organic Semiconductors Morphology of Organic Materials Considerations for Photovoltaics Simulation Methods of Organic Photovoltaics Considerations When Modelling Organic Photovoltaics Acknowledgements
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A sequentially deposited (SD) active layer with bulk-heterojunction (BHJ) like morphology is developed by utilizing a naphthalenediimide-based polymer acceptor PTzNDI-T with a strong interchain interaction and low solubility and a well-soluble polymer donor J52-Cl. The SD active layer is prepared by first depositing PTzNDI-T solution and then depositing J52-Cl solution without any post-treatments, and a traditional blend-cast (BC) active layer is cast from the blend solution of J52-Cl:PTzNDI-T. Both the conventional and inverted all-polymer solar cells (all-PSCs) with the BC active layer present nearly no photovoltaic performance. In contrast, based on the SD active layer, not only do the inverted all-PSCs show a dramatically increased PCE of 6.08% but the conventional all-PSCs with the same deposition sequence also exhibit a similarly high PCE of 6.29%. Notably, the SD active layer shows BHJ-like morphology with well-distributed donor and acceptor phases and thus offers a similarly high photovoltaic performance in conventional and inverted all-PSCs with the same deposition sequence of polymer acceptor and donor, which is the first report of SD all-PSCs. These results provide different insight to the SD active layer for high-performance all-PSCs.
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Morphology, the spatial distribution of traps, interdomain connectivity, and phase separation of the active layer play a critical role in the performance of the bulk heterojunction (BHJ) organic solar cells (OSCs). In this work, we utilize the hopping transport model to simulate the effect of morphological and structural parameters on the diffusion coefficient and efficiency of the polymer-fullerene BHJ solar cells. In BHJ solar cells there are two distinct phases as electron transport material (acceptor) and hole transport material (donor). Here we try to create an almost realistic network containing P3HT polymer chains and PCBM clusters for simulating the charge transport in the active layer. The blend ratio of P3HT:PCBM polymers and alignment of these bicontinuous networks of active layer are considered here as the morphological parameter affecting the cell performance. The dependency of the charge transport on such morphological parameters is obtained in this study by using Monte Carlo continues time random walk simulations.
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At present, bulk heterojunction polymer solar cells are typically fabricated with an active layer thickness of between 80 and 100nm. This active layer thickness has traditionally been chosen based on convenience and empirical results. However, a detailed study of the effects that active layer thickness has on the short circuit current and efficiency has not been performed for bulk heterojunction polymer solar cells so far. We demonstrate that the performance of these devices is highly dependent on the active layer thickness and, using a well established model for optical interference, we show that such effects are responsible for the variations in performance as a function of active layer thickness. We show that the ideal composition ratio of the donor and acceptor materials is not static, but depends on the active layer thickness in a predictable manner. A comparison is made between solar cells comprised of the donor materials regioregular poly(3-hexylthiophene) and poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene) with the acceptor [6, 6]-phenyl C61-butyric acid methyl ester to show that our results are not material specific and that high efficiency solar cells can be fabricated with active layer thickness greater than 100nm for both material mixtures. Finally, a device with an active layer thickness of 225nm is fabricated with a power efficiency of 3.7% under AM1.5 illumination at an intensity of 100mW∕cm2.
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Abstract Developing efficient organic solar cells (OSCs) with relatively thick active layer compatible with the roll to roll large area printing process is an inevitable requirement for the commercialization of this field. However, typical laboratory OSCs generally exhibit active layers with optimized thickness around 100 nm and very low thickness tolerance, which cannot be suitable for roll to roll process. In this work, high performance of thick‐film organic solar cells employing a nonfullerene acceptor F–2Cl and a polymer donor PM6 is demonstrated. High power conversion efficiencies (PCEs) of 13.80% in the inverted structure device and 12.83% in the conventional structure device are achieved under optimized conditions. PCE of 9.03% is obtained for the inverted device with active layer thickness of 500 nm. It is worth noting that the conventional structure device still maintains the PCE of over 10% when the film thickness of the active layer is 600 nm, which is the highest value for the NF‐OSCs with such a large active layer thickness. It is found that the performance difference between the thick active layer films based conventional and inverted devices is attributed to their different vertical phase separation in the active layers.
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Morphology, the spatial distribution of traps, interdomain connectivity, and phase separation of the active layer play a critical role in the performance of the bulk heterojunction (BHJ) organic solar cells (OSCs). In this work, we utilize the hopping transport model to simulate the effect of morphological and structural parameters on the diffusion coefficient and efficiency of the polymer-fullerene BHJ solar cells. In BHJ solar cells there are two distinct phases as electron transport material (acceptor) and hole transport material (donor). Here we try to create an almost realistic network containing P3HT polymer chains and PCBM clusters for simulating the charge transport in the active layer. The blend ratio of P3HT:PCBM polymers and alignment of these bicontinuous networks of active layer are considered here as the morphological parameter affecting the cell performance. The dependency of the charge transport on such morphological parameters is obtained in this study by using Monte Carlo continues time random walk simulations.
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