Novel narrow bandgap polymer donors based on ester-substituted quinoxaline unit for organic photovoltaic application
Lingzhi GuoXuelong HuangYingtong LuoShengjian LiuZhixiong CaoJiale ChenYue LuoQingduan LiJiaji ZhaoYue‐Peng Cai
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Keywords:
Quinoxaline
Active layer
Stille reaction
Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71 butyric acid methyl ester (PCBM) polymer solar cell is studied by using GPVDM simulations and experiments. The research focuses on the effects of active layer thickness on solar cell structures as bulk heterojunction (BHJ) (ITO/P3HT:PCBM/Al) as compared to a bilayer structure (ITO/P3HT/PCBM/Al). The optimal active layer thickness of 200 nm is obtained in the simulation for BHJ solar structure. The results also indicate that bulk heterojunctions exhibit slightly higher efficiency than bilayer solar cell with the same thickness, possibly due to a better and worthier total surface region for charge separation and reduced recombination between the electrons and holes. BHJ solar cell is fabricated in the experiment by using spin coating. The results show that higher spin speeds result in a thinner active layer, and the device coated at 2500 rpm had the highest power conversion efficiency of 0.91 % because of a higher Isc and fill factor, despite a low absorption. The results suggest that bulk resistance, and morphology of the active layer play important roles in the carrier transport in the P3HT:PCBM solar cell.
Active layer
Spin Coating
Hybrid solar cell
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At present, heterojunction polymer solar cells are typically fabricated with an active layer thickness of approximately 80 nm to 100 nm. This active layer thickness has traditionally been chosen based upon convenience and empirical results. However, a detailed mechanistic study of the effects of active layer thickness on the short circuit current and efficiency has never been performed for polymer solar cells. We demonstrate that using the high mobility materials regio regular poly(3-hexylthiophene and [6,6]-phenyl (P3HT) and C61-butyric acid methyl ester (PCBM), that high efficiency solar cells can be fabricated with active layer thickness greater than 100 nm. Devices with an active layer thickness of 200 nm are fabricated with a power efficiency of 4.1% under AM1.5 illumination at and intensity of 80 mW/cm2. In addition, we explain the variation in short circuit current density as a function of thickness using calculations of the distribution of the optical electric field intensity as a function of device thickness.
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Light intensity
Intensity
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The Nanoscale morphology has been shown to be a critical parameter governing charge transport properties of polymer bulk heterojunction (BHJ) solar cells. Recent results on vertical phase separation have intensified the research on 3D morphology control. In this paper, we intend to modify the distribution of donors and acceptors in a classical BHJ polymer solar cell by making the active layer richer in donors and acceptors near the anode and cathode respectively. Here, we chose [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to be the acceptor material to be thermally deposited on top of [poly(3-hexylthiophene)] P3HT: the PCBM active layer to achieve a vertical composition gradient in the BHJ structure. Here we report on a solar cell with enhanced power conversion efficiency of 4.5% which can be directly correlated with the decrease in series resistance of the device.
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Morphology
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PCEs of 15.81% and 15.29% are achieved in LbL and BHJ all-PSCs with polymer donor PM6, polymer acceptor PY-IT and CN as an additive. Over 15% PCE improvement can be obtained in LbL and BHJ all-PSCs with CN in LbL and BHJ active layers.
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Layer by layer
Acceptor
Photoactive layer
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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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Acceptor
Deposition
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The morphology of active layer for polymer solar cells is critical to enhance the performance especially for fill factor of the devices. To investigate the relationship between active layer morphology and performance of polymer solar cells (PSCs), 1,8-diiodooctane (DIO) additive, and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) electron acceptor were used to regulate the aggregation morphology of copolymer poly(thieno[3,4-b]-thiophene/benzodithiophene) (PTB7) electron donor from solution state to solid state. Atom force microscopy (AFM), steady-state absorption (UV-Vis), time-resolved absorption (TA), spectroelectrochemistry (SEC) and current-voltage (J-V) measurements were employed to characterize the morphology, optical and electrical characteristics of active layers and to reveal the relationship among the morphology, photophysical property, and performance of PTB7-based devices. The results show that DIO can refine the aggregation scale of PTB7 during the dissolution process, whereas both the aggregation scale and aggregation behaviors of PTB7 donor are affected by PC71BM acceptor molecules. Furthermore, the bulk heterojunction structure (BHJ) morphology of active layer can be optimized during the DIO evaporation process. TA kinetic data indicate that the population and lifetime of charged species are improved in the DIO-treated BHJ active layer. Moreover, the active layers with DIO treatment have a relative low highest occupied molecular orbital (HOMO) energy level, which makes hole transport more easily in PTB7 donor phase. As a result, the performance of PTB7-based PSCs is enhanced.
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Acceptor
Electron acceptor
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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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Acceptor
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An efficient route for the synthesis of 1-iodo-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione as a key intermediate to build new electron-deficient monomers and related conjugated polymers is reported. Along these lines, two new low bandgap copolymers were synthesized from Stille or Suzuki coupling. These polymers were characterized and their properties compared to those of analogous conjugated polymers.
Stille reaction
Pyrrole
Suzuki reaction
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Nanoscale morphology has been shown to be a critical parameter governing charge transport properties of polymer bulk heterojunction (BHJ) solar cells. Recent results on vertical phase separation have intensified the research on 3D morphology control. In this paper, we intend to modify the distribution of donors and acceptors in a classical BHJ polymer solar cell by making the active layer richer in donors and acceptors near the anode and cathode respectively. Here, we chose [6,6]-phenyl- C(61)-butyric acid methyl ester (PCBM) to be the acceptor material to be thermally deposited on top of [poly(3-hexylthiophene)] P3HT: the PCBM active layer to achieve a vertical composition gradient in the BHJ structure. Here we report on a solar cell with enhanced power conversion efficiency of 4.5% which can be directly correlated with the decrease in series resistance of the device.
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Acceptor
Morphology
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A series of quinoxaline-based copolymers, namely, poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(5′,8′-di-2-thienylquinoxaline)] (P1), poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(5′,8′-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)quinoxaline] (P2), and poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(5′,8′-di-2-thienyl-2,3-bis(4-(3,7-dimethyloctyloxy)phenyl)quinoxaline] (P3), were synthesized and characterized for use in polymer solar cells (PSCs). We describe the effect of modifying the alkyl group of the side chain of the quinoxaline derivatives on the electronic and optoelectronic properties of the polymers. The field-effect hole mobility as well as the electronic energy levels and processability of the materials for PSC applications were investigated. Among the studied quinoxaline-based copolymers, P2 showed the best photovoltaic performance with an open-circuit voltage (VOC) of 0.82 V, a short-circuit current density (JSC) of 9.96 mA/cm2, a fill factor (FF) of 0.49, and a power-conversion efficiency of 4.0% when a P2/PC71BM blend film was used as the active layer under AM 1.5 G irradiation (100 mW/cm2).
Quinoxaline
Carbazole
Open-circuit voltage
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