Charge-density-based analysis of the current-voltage response of polythiophene/fullerene photovoltaic devices.

A key challenge for organic electronics research is to develop device models that correctly account for the structural and energetic disorder typically present in such materials. In this paper we report an approach to analyze the electrical performance of an organic electronic device based upon charge extraction measurements of charge densities and transient optoelectronic measurements of charge carrier dynamics. This approach is applied to a poly(3-hexyl thiophene) (P3HT)/6,6 phenyl C61 butyric acid methyl ester (PCBM) blend photovoltaic device. These measurements are employed to determine the empirical rate law for bimolecular recombination losses, with the energetic disorder present in the materials being accounted for by a charge-density-dependent recombination coefficient. This rate law is then employed to simulate the current/voltage curve. This simulation assumes the only mechanism for the loss of photogenerated charges is bimolecular recombination and employs no fitting parameters. Remarkably the simulation is in good agreement with the experimental current/voltage data over a wide range of operating conditions of the solar cell. We thus demonstrate that the primary determinant of both the open-circuit voltage and fill factor of P3HT:PCBM devices is bimolecular recombination. We go on to discuss the applicability of this analysis approach to other materials systems, and particularly to emphasize the effectiveness of this approach where the presence of disorder complicates the implementation of more conventional, voltage-based analyses such as the Shockley diode equation.

Influence of a TiCl4 post-treatment on nanocrystalline TiO2 films in dye-sensitized solar cells.

In this study, the influence of the TiCl(4) post-treatment on nanocrystalline TiO(2) films as electrodes in dye-sensitized solar cells is investigated and compared to nontreated films. As a result of this post-treatment cell efficiencies are improved, due to higher photocurrents. On a microscopic scale TiO(2) particle growth on the order of 1 nm is observed. Despite a corresponding decrease of BET surface area, more dye is adsorbed onto the oxide surface. Although it seems trivial to match this finding with the improved photocurrent, this performance improvement cannot be attributed to higher dye adsorption only. This follows from comparison between incident photon to current conversion efficiency (IPCE) and light absorption characteristics. Since the charge transport properties of the TiO(2) films are already more than sufficient without treatment, the increase in short circuit current density J(SC) cannot be related to improvements in charge transport either. Transient photocurrent measurements indicate a shift in the conduction band edge of the TiO(2) upon TiCl(4) treatment. It is concluded that the main contribution to enhanced current originates from this shift in conduction band edge, resulting in improved charge injection into the TiO(2).

The effect of Al2O3 barrier layers in TiO2/dye/CuSCN photovoltaic cells explored by recombination and DOS characterization using transient photovoltage measurements.

Solid-state dye-sensitized solar cells of the type TiO(2)/dye/CuSCN have been made with thin Al(2)O(3) barriers between the TiO(2) and the dye. The Al(2)O(3)-treated cells show improved voltages and fill factors but lower short-circuit currents. Transient photovoltage and photocurrent measurements have been used to find the pseudo-first-order recombination rate constant (k(pfo)) and capacitance as a function of potential. Results show that k(pfo) is dependent on V(oc) with the same form as in TiO(2)/dye/electrolyte cells. The added Al(2)O(3) layer acts as a "tunnel barrier", reducing the k(pfo) and thus increasing V(oc). The decrease in k(pfo) also results in an increased fill factor. Capacitance vs voltage plots show the same curvature (approximately 150 mV/decade) as found in TiO(2)/dye/electrolyte cells. The application of one Al(2)O(3) layer does not cause a significant shift in the shape or position of the capacitance curve, indicating that changes in band offset play a lesser role in the observed V(oc) increase. Cells made with P25 TiO(2) have, on average, 2.5 times slower recombination rate constants (longer lifetimes) than those made with colloidal TiO(2). The cells with P25 also show 2.3 times higher trap density (DOS), which results in little change in the V(oc) between the two types of TiO(2). It is further noted that the recombination current in these cells cannot be calculated from the total charge times the first order rate constant.