Origin of depressed fill factor in organic solar cells due to S-shape current-voltage characteristics

Often, a depressed fill factor is observed in organic solar cells (OSCs), which is usually known as S-shaped current-voltage (J-V) characteristics. To investigate the origin of the depressed fill factor further, a poly[N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)]:[6,6]-phenyl C71 butyric acid methyl ester based OSC has been characterized through impedance spectroscopy. In addition, the photocurrent vs the applied effective bias plot reveals that the S-shaped J-V characteristics primarily reduce the diffusion current of the device. The present study shows that the diffusion current dominated section of photocurrent reduces with a slope of 2 in the depressed fill factor section given that the slope may further increase as per the impact of the S-shape. The reduction in the diffusion constant (D-n) in the S-shaped region supports the decrease in the diffusion current, which is observed through the photocurrent analysis. It is observed that, in the S-shaped section of the current-voltage characteristics, the transport time for free carriers increases up to 443 mu s. The larger Urbach energy for the active layer of an S-shaped device demonstrates higher subbandgap disorder. Therefore, it is concluded that the accumulation of the charge carriers within the device and disorder in the active layer leads to the S-shaped current-voltage characteristics as well as poor carrier extraction

Non-approximated series resistance evaluation by considering high ideality factor in organic solar cell

The quality performance analysis of any solar cell can only be predicted through the parameters extracted from its current-voltage characteristics. Herein, we demonstrate an efficient analytical method to calculate the series resistance of organic solar cells without any pre approximation. It has been shown that the inaccuracy in series resistance takes place due to the high ideality factor of the organic based solar cells, usually ranging from 2-5. Using a systemic approach, we solved the single diode solar cell model to determine the series resistance expression. The dependence of series resistance on ideality factor of the device and the reverse saturation current calculation through the illuminated current-voltage characteristics has been presented here for the first time. The entire method has been programmatically realized in MAT-LAB. The extracted parameters values have been compared to the other methods for the proof of validation and it shows good agreement where as some outputs are better resolved. Additionally, the effectiveness of the present method is also matched with NREL certified silicon solar cell (reference cell model 60623) parameter extracted from K24XX cell software provided with PET cell tester model, CT200AAA solar simulators. The present proposed method is not restricted to the organic solar cell only but it can also be applied for the other type of solar cell as well. Therefore, the proposed method shows the relevance in the present scenario of solar cell research and development

Ultrafast carrier and phonon dynamics in thin films of bismuth telluride on a flexible substrate

International audienceThin films of topological insulators (TIs) possess exotic nonlinear optical properties such as strong light-matter interaction, broadband spectral sensitivity, thickness-dependent tunable bandgap, higher harmonic generation, etc. Due to the presence of metallic surface states with Dirac fermions and an insulating bulk band in TI, they are projected to be a viable material for studying novel physics, resulting in exciting new properties and technologies. The peculiar electron-phonon interactions at the surface have been linked to various unexpected physical features of topological insulators. Although electron behaviour in topological insulators has been extensively investigated on non-flexible substrates, electron-phonon interactions at TI bulk and surfaces states are less well known on a flexible substrate. Because of its potential uses in wearable devices, communications, sensors, and other fields, there is a significant need for the manufacture of high performance flexible optoelectronic responses employing novel exotic materials. In this paper, we preformed ultrafast pump-probe method to explore TI (Bi2Te3) thin films on a flexible PET (polyethylene terephthalate) substrate. We studied the dynamics of Bi2Te3 thin films' hot carrier relaxation progression and coherent phonon behaviour using transient absorbance measurements. Thickness-dependent low-frequency coherent acoustical phonon oscillations are observed in 10 nm thick films, and the changes vanish for 25 nm thick films, and high-frequency optical phonon oscillations are absent in our work. Subpicosecond range of time constant for the photon excitation, diffusion, carrier thermalization, and relaxation are reported. Longer characteristics time was observed for 25 nm film as compared to 10 nm film, and its variation has been discussed here. The thickness-dependent coherent acoustic phonon oscillating in the terahertz frequency range has been experimentally calculated. The film's terahertz frequency response varies with its thickness, allowing it to be employed in future terahertz applications based on flexible topological insulator thin films

Non-approximated series resistance evaluation by considering high ideality factor in organic solar cell

The quality performance analysis of any solar cell can only be predicted through the parameters extracted from its current-voltage characteristics. Herein, we demonstrate an efficient analytical method to calculate the series resistance of organic solar cells without any pre approximation. It has been shown that the inaccuracy in series resistance takes place due to the high ideality factor of the organic based solar cells, usually ranging from 2-5. Using a systemic approach, we solved the single diode solar cell model to determine the series resistance expression. The dependence of series resistance on ideality factor of the device and the reverse saturation current calculation through the illuminated current-voltage characteristics has been presented here for the first time. The entire method has been programmatically realized in MAT-LAB. The extracted parameters values have been compared to the other methods for the proof of validation and it shows good agreement where as some outputs are better resolved. Additionally, the effectiveness of the present method is also matched with NREL certified silicon solar cell (reference cell model 60623) parameter extracted from K24XX cell software provided with PET cell tester model, CT200AAA solar simulators. The present proposed method is not restricted to the organic solar cell only but it can also be applied for the other type of solar cell as well. Therefore, the proposed method shows the relevance in the present scenario of solar cell research and development