  Traditional CdS buffer layer in selenium-free Cu(In,Ga)S2 solar cells leads
to reduced open-circuit voltage because of a negative conduction band offset at
the Cu(In,Ga)S2/CdS interface. Reducing this loss necessitates the substitution
of CdS by an alternative buffer layer. However, the substitute buffer layer may
introduce electrical barriers in the device due to unfavorable band alignment
at the other interfaces such as between buffer/ZnO i-layer. This study aims to
reduce interface recombinations and eliminate electrical barriers in
Cu(In,Ga)S2 solar cells using a combination of Zn1-xMgxO and Al-doped Zn1-xMgxO
buffer and i-layer combination deposited using atomic layer deposition and
magnetron sputtering, respectively. The devices prepared with these layers are
characterized by current-voltage and photoluminescence measurements. Numerical
simulations are performed to comprehend the influence of electrical barriers on
the device characteristics. An optimal composition of Zn1 xMgxO x = 0.27 is
identified for a suitable conduction band alignment with Cu(In,Ga)S2 with a
bandgap of ~1.6 eV, suppressing interface recombination and avoiding barriers.
Optimized buffer composition together with a suitable i-layer led to a device
with 14 % efficiency and an open-circuit voltage of 943 mV. A comparison of
optoelectronic measurements for devices prepared with ZnO and Al:(Zn,Mg)O shows
the necessity to replace the ZnO i-layer with Al:(Zn,Mg)O i-layer for a
high-efficiency device.
