Different Bandgaps in Cu$_2$ ZnSnSe$_4$: A High Temperature Coevaporation Study

Abstract

Abstract-We present a high-temperature Cu 2 ZnSnSe 4 coevaporation study, where solar cells with a power conversion efficiency of 7.1% have been achieved. The process is monitored with laser light scattering in order to follow the incorporation of the Sn into the film. We observe the segregation of ZnSe at the Mo/CZTSe interface. Optical analysis has been carried out with photoluminescence and spectrophotometry. We observe strong band tailing and a bandgap, which is significantly lower than in other reported efficient CZTSe absorbers. The photoluminescence at room temperature is lower than the bandgap due to the existence of a large quantity of tail states. Finally, we present effects of low-temperature postannealing of the absorbers on ordering of the Cu/Zn atoms in CZTSe and solar cell parameters. We observe strong changes in all solar cell parameters upon annealing. The efficiency of the annealed devices is significantly reduced, although ordering is improved compared with ones made from nonannealed absorbers.

Is the Cu/Zn Disorder the Main Culprit for the Voltage Deficit in Kesterite Solar Cells?

“…These results are in accordance with the ones reported recently for pure Se co-evaporated CZTSe devices. [ 88 ] In order to verify that these changes in V oc and E g-IQE originate genuinely from the difference in crystalline order and not from altered front or back junctions, we have evaluated the optical absorption of PO and PD fi lms lifted-off from their Mo/ glass substrates. A second set of bandgap values, noted E g -abs , was thus derived from these optical measurements.…”

Highly conductive ZnO films with high near infrared transparency

“…An efficiency of 8.4 % has been achieved based on a rather high open circuit voltage of 378 mV and a short circuit current of nearly 39 mA cm -2 . Both of these values are bet-ter than those obtained for our previous cells [27,46]. We attribute the high V OC to the quality of the absorber and the high J SC (IV) to the elevated transparency of the b-ZnO layer and to the anti-reflective coating.…”

“…The substrate temperature was controlled with a pyrometer monitor. In addition, the laser light scattering technique was used for end point detection in the case of Cu(In,Ga)Se 2 deposition [26], and for determination of Sn incorporation in the case of Cu 2 ZnSnSe 4 [27].…”

“…Afterwards, this secondary phase is removed by an etching step using potassium cyanide (10 wt% KCN during 5 minutes at room temperature). Cu 2 ZnSnSe 4 absorbers were grown via multiplestage coevaporation as described in reference [27]. The composition of the latter samples was chosen to be Cu-poor (Cu/(Zn+Sn) <1) and slightly Zn-rich (Zn/Sn > 1).…”

Erratum: “Multiple phases of Cu2ZnSnSe4 detected by room temperature photoluminescence” [J. Appl. Phys. 116, 073509 (2014)]

“…6-10, or why other kesterite samples do not show these interference peaks in the PL spectra. 11,12 For the interference to occur in the PL spectra, two conditions have to be fulfilled: (i) the PL light must be weakly absorbed and (ii) the surface must be smooth. Condition (i) is often fulfilled in kesterite films, since the PL originates mostly from the tail states, well below the band gap of the extended states, 13 where absorption is already weak.…”