The uncertain dopant location in the bulk heterojunction (BHJ) film hinders the wide application of molecular doping in polymer solar cells (PSCs) as is in other organic devices. It is known that the interaction between the dopant and component governs the dopant distribution in the BHJ film and thus largely controls the effectiveness of molecular doping. After excluding the strong dopant/component interaction by forming the charge-transfer complex in the solution, we estimate the dopant/component miscibility by calculating the difference of Hansen’s total solubility parameters (△δi‑Hansen) and prove its correctness by contact angle measurements, and two model systems of poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophe-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))] (PBDB-T)/poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (N2200) and poly[4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl]] (PCE10)/N2200 are selected to reveal the miscibility–photovoltaic performance relations. Only the material combination with large △δi‑Hansen between the n-dopant (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) and the donor polymer achieves enhanced photovoltaic performance. After that, we examine the doped morphology of polymer blends. Since the polymers’ crystallizations are negatively affected by N-DMBI addition, we ensure the significance of n-doping on the enhanced device performance. Besides the dopant/polymer interaction, the solvent/polymer and solvent/dopant interactions are also considered to evaluate the kinetic effect on N-DMBI distribution by drawing the ternary phase diagram. We conclude that the kinetic morphological evolution does not change the miscibility-governed N-DMBI distribution in the BHJ film. Finally, we provide a direct relationship between the N-DMBI position and the device property by fabricating the bilayer devices. The enhancement of photovoltaic performances is observed in both material systems only if the N-DMBI distributes in N2200. Our work outlines a basis for using the dopant/component interaction and ternary phase diagram to predict the dopant distribution before extensive experiments. It significantly reduces the trial-and-error work and increases the reliability of molecularly doped PSCs.
The electron donor/acceptor (D/A) heterojunction is the core for photocharge generation and recombination in organic photovoltaics (OPVs). Developing practical methods for the D/A heterojunction modification remains challenging and is rarely discussed in OPV research. Herein, the roles of molecular doping at the D/A heterojunction in the charge-transfer exciton dissociation and detailed energy loss are investigated, and new insights are gained into the functions of doping on the OPV performance. Heterojunction doping simultaneously enhances all three OPV parameters, especially the short-circuit current (J sc ). It is shown that the J sc improvement is due to the combined effects of strengthened electric field and reduced activation energy, which is regulated via an entropy-related mechanism. The performance enhancement is further demonstrated in homojunction devices showing the great potential of interfacial doping to overcome the intrinsic limitation between high J sc and open-circuit voltage (V oc ) in OPVs.
To achieve efficient doping in polymer solar cells (PSCs), the dopant needs to be selectively located in the binary components of a bulk heterojunction (BHJ) film according to its polarity. The rarely studied n-type dopant is thoroughly examined in a simplified planar heterojunction (PHJ) device to address its favored location in the active layer. Results show that the n-dopant distribution in the acceptor layer or at the donor/acceptor interface produces enhanced device performance, whereas it harms the device when located in the donor layer. Based on the results, the benefit of n-type doping is then transferred to the highly efficient BHJ devices via a sequential coating procedure. The performance improvement is closely linked to the variations in the dopant’s location in the BHJ film, which is carefully examined by the synchrotron techniques with delicate chemical sensitivity. More interestingly, the sequential coating procedure can be easily extended to the p-doped device only by changing the dopant’s polarity in the middle layer. These findings pave the way for ambipolar doping in PSCs and enable performance improvement by molecular doping within the expectations.
Overfitting is a common and critical challenge for neural networks trained with limited dataset. The conventional solution is software-based regularization algorithms such as Gaussian noise injection. Semiconductor noise, such as 1/f noise, in artificial neuron/synapse devices, which is often regarded as undesirable disturbance to the hardware neural networks (HNNs), could also play a useful role in suppressing overfitting, but that is as yet unexplored. In this work, we proposed the idea of using 1/f noise injection to suppress overfitting in different neural networks, and demonstrated that: (i) 1/f noise could suppress the overfitting in Multilayer Perceptron (MLP) and long short-term memory (LSTM); (ii) 1/f noise and Gaussian noise performs similarly for the MLP but differently for the LSTM; (iii) The superior performance of 1/f noise on LSTM can be attributed to its intrinsic long range dependence. This work reveals that 1/f noise, which is common in semiconductor devices, can be a useful solution to suppress the overfitting in HNNs, and more importantly, further evidents that the imperfectness of semiconductor devices is a rich mine of solutions to boost the development of brain-inspired hardware technologies in the AI era.
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