A fundamental analysis of the impact of Ge on the synthesis of Cu2ZnSnSe4:Ge by a sequential process is presented, reporting the consequences on the absorber morphology and solar cell devices performance.

We have derived an analytical formula for the temperature dependence of the photoluminescence (PL) intensity when the “negative” thermal quenching phenomenon is observed. The formula was examined using available experimental data for GaAs and ZnS, and the agreement of the results was quite good. It was confirmed quantitatively that the principal mechanism of negative thermal quenching is the thermal excitation of electrons to the initial state of the PL transition from the eigenstates with smaller energy eigenvalues. In addition, it was revealed that the real activation energy for the nonradiative recombination processes is given by ε+E ′ when the negative thermal quenching phenomenon is observed, where ε is the activation energy estimated simply from the apparent slope in the experimental data, and E ′ is the activation energy for the negative thermal quenching phenomenon.

We demonstrate the improved efficiency of a Cu2Zn(Sn1− x Ge x )Se4 (CZTGSe) thin-film solar cell with a conversion efficiency of 12.3%; this cell exhibits a greatly improved open-circuit voltage (V OC) deficit of 0.583 V and a fill factor (FF) of 0.73 compared with previously reported CZTGSe cells. The V OC deficit was found to be improved through a reduced band tailing via the control of the Ge/(Sn + Se) ratio. In addition, the high FF was mainly induced by a reduced carrier recombination at the absorber/buffer interface and/or in the space charge region, whereas parasitic resistive effects on FF were very small.

The systematic variations in the structural, optical, and electrical properties of polycrystalline Cu͑In, Ga͒Se 2 ͑CIGS͒ thin films with Na doping level were investigated. Precise control of the Na concentration in CIGS films was demonstrated using alkali-silicate glass thin layers of various thicknesses deposited on substrates prior to CIGS growth. The CIGS grain size was observed to decrease with increasing Na concentration, although the surface morphology became smoother and exhibited a stronger ͑112͒ texture, which has been demonstrated consequence of larger grain size. The Ga composition gradient in the CIGS films was found to become large due to the presence of Na during growth, which in turn led to a decrease in the nominal band gap energy. Variations in the photoluminescence spectra and electrical properties suggested that the formation of an acceptor energy state, which may originate from O Se point defects, was enhanced in the presence of Na. This result suggests that not only Na, but also the presence of O in combination with Na contributes to the compensation of point defects and enhances p-type conductivity in CIGS films.

The emerging technological demands for high‐efficiency solar cells and flexible optoelectronic devices have stimulated research on transparent conducting oxide (TCO) electrodes. High‐mobility TCOs are needed to achieve high conductivity with improved visible and near‐infrared transparency; however, the fabrication of TCO films on heat‐sensitive layers or substrates is constrained by the trade‐off between fabrication temperatures and TCO properties. Historically, Sn‐doped indium oxide and amorphous In–Zn–O have been used as standard TCOs to achieve high mobility using low fabrication temperatures. However, two polycrystalline In2O3 films with significantly higher mobilities have recently been reported: i) polycrystalline (poly‐) In2O3 films doped with metal (Ti, Zr, Mo, or W) impurities instead of Sn exhibit mobilities greater than ≈80 cm2 V−1 s−1 even when grown at low temperatures and ii) solid‐phase crystallized (spc‐) H‐doped In2O3 (In2O3:H) and In2O3:Ce,H films exhibit mobilities greater than 100 cm2 V−1 s−1 when processed at low temperatures of 150–200 °C. Here, poly‐In2O3, In2O3:W, and In2O3:Ce films and spc‐In2O3:H, In2O3:W,H, and In2O3:Ce,H films are fabricated. Comparative studies of these films reveal the effect of the i) metal dopant species; ii) metal and hydrogen codoping; and iii) solid‐phase crystallization process on the resultant transport properties.