No abstract

Electrical transport properties of poly (3-hexylthiophene) (P3HT) (6,6)-phenyl C61-butyric acid methyl ester (PCBM) solar cells, with and without encapsulation, have been investigated and analyzed using admittance spectroscopy and capacitance voltage measurements at different temperatures. The admittance spectroscopy clearly reveals two defect states with activation energies of 53 and 100 meV, and a concentration ten times higher in the unencapsulated sample. These defects seem to have a strong effect on the charge transport and the solar cell performance when they are present with a high concentration, since they lead to a decrease of the mobility and also the short-circuit current and the efficiency. The origin of these defects has been assigned to reaction of the blend with O2 which is also known to induce p-type doping in pure P3HT. In an attempt to understand the effect of these defects on the organic solar cell performance, modeling and simulation were carried out using the effective medium layer model and gave good agreement with the measurements results.

No abstract

The non‐geminate recombination of charge carriers in polymer‐fullerene solar cells has been modeled in the last few years with a trap‐assisted recombination model, which states that the apparent recombination order depends on the concentration of trapped charges tailing into the band gap. Higher concentrations of trapped charges lead to higher apparent recombination orders. In this work, the mass fraction f of highly crystalline nanofibrillar P3HT to the total P3HT content in P3HT:PCBM solar cells is consistently varied, controlling the temperature of a nanofibers‐P3HT casting dispersion. A systematic study of the apparent recombination order, measured with a transient photovoltage technique, as a function of f is presented. A correlation is shown between the apparent recombination order, the P3HT crystallinity, and the trap concentration in the band gap measured with an admittance spectroscopy technique.

The removal of secondary phases from the surface of the kesterite crystals is one of the major challenges to improve the performances of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. In this contribution, the KCN/KOH chemical etching approach, originally developed for the removal of CuxSe phases in Cu(In,Ga)(S,Se)2 thin films, is applied to CZTSe absorbers exhibiting various chemical compositions. Two distinct electrical behaviors were observed on CZTSe/CdS solar cells after treatment: (i) the improvement of the fill factor (FF) after 30 s of etching for the CZTSe absorbers showing initially a distortion of the electrical characteristic; (ii) the progressive degradation of the FF after long treatment time for all Cu-poor CZTSe solar cell samples. The first effect can be attributed to the action of KCN on the absorber, that is found to clean the absorber free surface from most of the secondary phases surrounding the kesterite grains (e.g., Se0, CuxSe, SnSex, SnO2, Cu2SnSe3 phases, excepting the ZnSe-based phases). The second observation was identified as a consequence of the preferential etching of Se, Sn, and Zn from the CZTSe surface by the KOH solution, combined with the modification of the alkali content of the absorber. The formation of a Cu-rich shell at the absorber/buffer layer interface, leading to the increase of the recombination rate at the interface, and the increase in the doping of the absorber layer after etching are found to be at the origin of the deterioration of the FF of the solar cells.

No abstract

The voltage dependence of the derivative of the capacitance to (the logarithm of) the measurement frequency is investigated. Relations describing this dependence are derived for the influence of carrier freeze out, of a defect distribution, and of a back contact barrier. The validity of these relations is investigated with numerical simulations. Considering the extraction of the defect density from capacitance-frequency measurements, the extension of existing formulas to different bias voltages leads to an improved accuracy and the possibility to investigate spatial non-uniformities while preserving a direct link between the defect level energy and the apparent defect density. This is illustrated with voltage dependent admittance measurements of thin film Cu(In,Ga)Se 2-based solar cell devices. V

No abstract