CdTe‐Based Solar Cells with Variations in Mg Concentration in the MgZnO Emitter

Abstract

The optimal fraction of Mg incorporation in sputter‐deposited MgXZn1−XO (MZO) emitters for thin‐film CdTe‐based solar cells is evaluated by varying it over a range of x from 0 to 0.35. This range allows a variation in the conduction band offset from −0.1 eV (cliff like) to +0.32 eV (spike like). A maximum efficiency of 18.5% for cells with the bilayer CdSeTe/CdTe absorber occurs at x = 0.15, which corresponds to a spike‐like band offset near 0.2 eV, as confirmed by X‐ray photoelectron spectroscopy. In addition, good cell performance is seen over a fairly broad range of x extending from 0.1 to 0.25. The MZO optical bandgap increases with the Mg fraction, consistent with an increasing conduction band offset. Temperature‐dependent current−voltage measurements and time‐resolved photoluminescence show improvement in the emitter/absorber interface with the incorporation of Mg. Capacitance−voltage measurements show that the depletion region extends further into the absorber with more Mg, and X‐ray diffraction confirms a change from a hexagonal‐dominant crystal structure toward zinc blende at x = 0.35.

Charge Carrier Lifetime Determination in Graded Absorber Solar Cells Using Time‐Resolved Photoluminescence Simulations and Measurements

“…This indicates that CBO = +0.2 eV is superior for reduced interface recombination, in agreement with V OC and efficiency simulation and experimental results in the literature. [ 39,40 ]…”

“…It reduces interface recombination, in agreement with V OC and efficiency simulation and experimental results in the literature. [ 39,40 ] These results can be utilized to direct research focus between bulk and interface defect density reduction for a more targeted approach to photovoltaic efficiency increases.…”

“…Additional input parameters are given in Table 1 with values held fixed unless otherwise specified. [ 6,12,22,39 ]…”

“…The front-side conduction band offset (CBO) between the n-type buffer and p-type absorber (MgZnO and CdSeTe, respectively, in the structures presented here) significantly impacts carrier recombination and open-circuit voltage (V OC ), where V OC is maximized with CBO = þ0.2 eV for MgZnO/CdSeTe device structures. [39,40] Here, we present the impact that CBO has on interface recombination. 4)) for CBO = 0.0 eV (Figure 7a,c) and þ0.2 eV (Figure 7b,d) with variation in S front and S back .…”

“…This indicates that CBO = þ0.2 eV is superior for reduced interface recombination, in agreement with V OC and efficiency simulation and experimental results in the literature. [39,40] Finally, the CBO has little impact on late decay times, which is discussed in greater detail in the supplementary material section. Therefore, to investigate CBO with TRPL, early decay times should be utilized.…”