Performance Characterization of Thin-Film In GaAs Solar Cells with Double-Hetero-Structure and InP Window-Layers of Various Thicknesses

III-V based semiconductors are ideally suited to high efficiency thin film solar cells due to their direct bandgap, high absorption coefficient, and high carrier mobility. The absorbance cutoff wavelength of In0.53Ga0.47 is approximately 1650 nm at room temperature, which provides a wide spectral range and small bandgap of 0.75 eV. Solar cells with a small bandgap tend to provide high short circuit-current density (J(SC)), but low open circuit voltage (V-oc). Improving the performance of small bandgap solar cells depends largely on increasing the V-oc. This paper presents an In GaAs solar cell, comprising a 0.5 mu m-thick n(+)-InP buffer layer, a 2.5 mu m-thick un-doped i-In0.53Ga0.47 As layer, and 1 mu m-thick un-doped n(+)-InP window layer grown by metal organic chemical vapor deposition (MOCVD) on an n(+)-InP substrate with the aim of achieving high J(SC) as well as high V-oc. Under optimal annealing conditions, we achieved specific contact resistivity of 2.71 x 10(-6) Omega-cm(2) from AuBe/Ni/Au on p-InP and 9.85 x 10(-7) Omega-cm(2) from Ni/Ge/Au/Ni/Au on n-InP. With the aim of enhancing the J(SC), V-oc and conversion efficiency (eta), we fabricated thin-film In GaAs cells with window layers of various thicknesses and compared them according to optical reflectance, dark current voltage (I-V), photovoltaic I-V, and external quantum efficiency (EQE). Under AM 1.5G illumination, we achieved J(SC) = 43.313 rnA/cm(2), V-oc = 0.385 V, P-max = 1.5 mW, and eta = 10.35% from a cell with a 0.2 mu m window layer. This represents an impressive 57.14% improvement in J(SC) (from 27.56 to 43.31 mA/cm(2)), an 18.46% improvement in V-oc (0.325 to 0.385 V) and a 71.55% increase in eta (from 6.06% to 10.35%), compared to cells with a 1.0 mu m window layer.