Back-Surface Passivation of CdTe Solar Cells Using Solution-Processed Oxidized Aluminum

Abstract

Although back-surface passivation plays an important role in high-efficiency photovoltaics, it has not yet been definitively demonstrated for CdTe. Here, we present a solutionbased process, which achieves passivation and improved electrical performance when very small amounts of oxidized Al 3+ species are deposited at the back surface of CdTe devices. The open circuit voltage (V oc ) is increased and the fill factor (FF) and photoconversion efficiency (PCE) are optimized when the total amount added corresponds to ∼1 monolayer, suggesting that the passivation is surface specific. Addition of further Al 3+ species, present in a sparse alumina-like layer, causes the FF and PCE to drop as the interface layer becomes blocking to current flow. The optimized deposit increases the average baseline PCE for both Cu-free devices and devices where Cu is present as a dopant. The greatest improvement is found when the Al 3+ species are deposited prior to the CdCl 2 activation step and Cu is employed. In this case, the best-cell efficiency was improved from 12.6 to 14.4%. Time-resolved photoluminescence measurements at the back surface and quantum efficiency measurements performed at the maximum power point indicate that the performance enhancement is due to a reduction in the interface recombination current at the back surface.

Why Increased CdSeTe Charge Carrier Lifetimes and Radiative Efficiencies did not Result in Voltage Boost for CdTe Solar Cells

“…[ 19 ] Third, effective front interface passivation has been achieved with MgZnO (MZO), [ 20 ] Al 2 O 3 , [ 21 ] TeO 2 , [ 22 ] and other materials. [ 23 ] Yet, device voltages stayed approximately the same. This was attributed to reduced bandgap E g ( E g ≈ 1.4 eV for CdSeTe compositions used in solar cells versus 1.5 eV for CdTe).…”

“…[ 25 ] According to this model bulk properties (implied voltage or quasi‐Fermi level splitting) are sufficient for ≈ 1 V solar cells, but incorrect interface band offsets impede charge carrier collection. However, devices with a broad range of interface layers and interface band offsets (front–MZO, [ 20 ] SnO 2 , [ 19 ] graded Cd(O,S,Se,Te), [ 26 ] and back–TeO 2 , [ 22 ] ZnTe, [ 19,27 ] doped Al 2 O 3 [ 23 ] ) have voltages equal to or below 900 mV, making interface selectivity unlikely to dominate V OC ‐losses for such a large range of front and back contact architectures.…”

Design optimization and efficiency enhancement of axial junction nanowire solar cells utilizing a forward scattering mechanism

“…55,56 Al 2 O 3 and SiO 2 are two of the most investigated materials used for the passivation of these cells due to their superior properties and the technological feasibility of using them for passivation using several processes including chemical bath deposition, gas phase epitaxy and sputtering. 53,[55][56][57][58][59][60][61][62][63][64][65][66] For nanowire solar cells, the enhanced surface area that improves utilization of solar spectrum also presents the challenge of enhanced surface recombination resulting in deterioration of external quantum efficiency of the solar cell due to lost charge carriers at the surface. 67 Surface passivation is needed for the alleviation of surface recombination which can signicantly improve the carrier collection efficiency.…”

Hierarchical Transparent Back Contacts for Bifacial CdTe PV

“…Solution-processed AlGaO x hole transport layers have a tunable band gap based on the ratios of aluminum and gallium precursors. Evidence for the passivating quality of an aluminum oxide thin film has been previously reported for CdTe surfaces . Akkuly et al report an undoped CdS/CdTe AlGaO x passivated device with an opaque 45 nm Au contact .…”

“…Evidence for the passivating quality of an aluminum oxide thin film has been previously reported for CdTe surfaces. 11 Akkuly et al report an undoped CdS/CdTe AlGaO x passivated device with an opaque 45 nm Au contact. 12 However, the role of AlGaO x as a surface passivant has not been comprehensively explored or confirmed, particularly in doped devices.…”