No abstract

Interface recombination in a complex multilayered thin‐film solar structure causes a disparity between the internal open‐circuit voltage (VOC,in), measured by photoluminescence, and the external open‐circuit voltage (VOC,ex), that is, a VOC deficit. Aspirations to reach higher VOC,ex values require a comprehensive knowledge of the connection between VOC deficit and interface recombination. Here, a near‐surface defect model is developed for copper indium di‐selenide solar cells grown under Cu‐excess conditions. These cell show the typical signatures of interface recombination: a strong disparity between VOC,in and VOC,ex, and extrapolation of the temperature dependent q·VOC,ex to a value below the bandgap energy. Yet, these cells do not suffer from reduced interface bandgap or from Fermi‐level pinning. The model presented is based on experimental analysis of admittance and deep‐level transient spectroscopy, which show the signature of an acceptor defect. Numerical simulations using the near‐surface defects model show the signatures of interface recombination without the need for a reduced interface bandgap or Fermi‐level pinning. These findings demonstrate that the VOC,in measurements alone can be inconclusive and might conceal the information on interface recombination pathways, establishing the need for complementary techniques like temperature dependent current–voltage measurements to identify the cause of interface recombination in the devices.

No abstract

No abstract

No abstract

No abstract

Metastabilities in Cu(In,Ga)Se2 based solar cells were investigated. Capacitance and conductance transients were measured in order to analyze carrier trapping and emission processes related to the creation and relaxation of metastable states. Our experimental findings support the theoretical predictions of Lany and Zunger [Lany and Zunger, J. Appl. Phys. 100, 113725 (2006)] for a (VSe-VCu) complex, a defect with negative-U energy that can exist in both the donor and acceptor configurations. We show that two different defect reactions induced by either voltage bias or illumination lead to the same acceptor configuration of the defect. The relaxation process is the same for light- and bias-induced metastabilities in devices and thin films. Time constants and activation energies for all investigated processes have been obtained. The results agree very well with the values calculated for (VSe-VCu) divacancy.

The impact of sodium on the electrical properties of Cu(In,Ga)Se2 (CIGS) thin films and corresponding solar cells was investigated by preparing nearly alkali-free CIGS layers and doping them with different Na amounts via NaF post-deposition treatment (PDT) at temperatures between 110 and 400 °C. The mean Na concentrations in the CIGS layers ranged from 0.1 to 400 ppm. Sodium was found also in the grain interior even for the lowest PDT temperature. All samples were subjected to extensive electrical characterization: current–voltage, capacitance profiling, conductivity, steady-state, and transient capacitance spectroscopy. A continuous increase in open-circuit voltage VOC and fill factor FF, an accompanying increase in hole density and mobility, and a decrease in secondary barriers responsible for the distortion of current–voltage characteristics were observed with increasing sodium content. An abrupt change in defect spectra and a dominant transport mechanism was found for PDT temperatures T(PDT) of ≥150 °C. We attribute a further improvement in VOC observed above 150 °C PDT temperature to the reduced concentration of recombination centers with increased sodium content. An explanation of both gradual evolution and the abrupt change is proposed based on passivation of grain boundaries and interfaces by sodium.