An analytical model for organic bulk heterojunction solar cells based on Saha equation for exciton dissociation.

The power conversion efficiencies of organic solar cells (OSCs) have been greatly improved in recent years. However, latest experimental data of high efficiency OSCs, the sublinear relationship between the short circuit current density (Jsc) and light intensity (Pin), and the effects of energetic disorder in bulk heterojunction organic solar cells have not been understood. An analytical model for high-efficiency OSCs is proposed, which takes most physical factors into account that have been ignored in most previous models, including practical solar spectra and absorption spectra, degeneracy effect, exciton effect, space charge limited current, and unified mobility expression dependent on temperature, electric field and density, etc. Three analytical iterative methods are proposed to solve the strong non-linear Poisson equation and the drift-diffusion equations. The method for the drift-diffusion equations involves introducing two constant coefficients and determining their values self-consistently by demanding the space averages of approximate drift and diffusion currents equal to the averages of accurate ones. The theoretical results for five high-efficiency OSCs are in good agreement with experimental data, including current-voltage curves, light intensity-dependent Jsc and open-circuit voltage (Voc) curves. The effects of energetic disorder in bulk heterojunction organic solar cells, and the sublinear relationship Jsc ∝ Pαin (α < 1) can be well explained. The Saha equation for exciton dissociation and the space-charge-limited-current (SCLC) effect are important for modelling high-efficiency OSCs. The Voc ∼ Pin relationship can be influenced by many factors. But, the Jsc ∼ Pin relationship can be mainly and slightly influenced by the exciton effect and energetic disorder, respectively. When aiming to realize higher performance OSCs, one should decrease six material parameters, including the energetic disorder, exciton mass, deep level impurity concentration, the ratios of electron and hole mobilities, densities of states for electrons and holes, and potential barriers at the anode and cathode. The performance parameters of 15 triad compounds are predicted by using ab initio Eg and absorption spectra from the literature along with other input parameters taken from previous optimized values, and the efficiency of two compounds was found to exceed 35%.

[Studies on the non-fatigue specificity of the fatigue-related sEMG signal parameters].

OBJECTIVE: To investigate the non-fatigue specificity of the changes of the fatigue-related parameters of sEMG signal during muscle static contractions. METHOD: The sEMG signals of biceps brachii (BB) and triceps brachii (TB) were recorded in 10 subjects during 1 min sustained right elbow isometric flexion. Changes of a series of fatigue-related indices such as MPF, MF, ZCR, AEMG and Lempel-ziv complexity were then analyzed. MVCs of the extensor (TB) were measured before and immediately after fatiguing flexion. RESULT: MPF, MF, ZCR and Lempel-ziv complexity of BB all decreased regularly and linearly with continuation of contraction. Whereas AEMG of BB increased linearly. The MVCs of TB were (14.0 +/- 3.2)kg and (14.6 +/- 2.6)kg respectively before and after BB fatiguing contraction. But the slopes of decrease of MPF, MF, ZCR and Lempel-ziv complexity of TB were significantly steeper than those of BB. The AEMG of TB was almost unchanged. CONCLUSION: The changes of time- and frequency-domain indices and Lempel-ziv complexity of sEMG signal were not as a result of muscle fatigue specifically. Fatigue might not be the only factor that caused the changes of the indices mentioned above.