Yann Roussillon

Blocking thin-film nonuniformities: Photovoltaic self-healing

An approach is developed to block the effects of lateral nonuniformities in thin-film semiconductor structures. The nonuniformity modulates the surface photovoltage distribution. When exposed to light and immersed in a proper electrolyte, this distribution will generate laterally nonuniform electrochemical reactions. Such treatments result in a nonuniform interfacial layer that balances the original nonuniformity. This approach has been implemented for CdTe/CdS photovoltaic devices, in which it improved the device efficiency from 1%–3% to 11%–12%.

Back contact and reach-through diode effects in thin-film photovoltaics

The physics of back contact effects in photovoltaic devices is revisited. We show that the back contact Schottky barrier can act in either back-diode or reach-through diode regimes. This understanding predicts that rare local spots with low back barrier hole transparency and/or weak main junctions can shunt the photocurrent thus decreasing the measured open-circuit voltage and device efficiency. We derive several more specific predictions of our model and verify them experimentally for the case of thin-film CdTe photovoltaics. Our concept has practical implications: a simple recipe leading to an efficient (13%) copper-free CdTe solar cell.

Reach-through band bending in semiconductor thin films

We describe a phenomenon of reach-through band bending in thin film semiconductors. It occurs through generation of defects that change the semiconductor work function. This translates the effect of the metal presence through the semiconductor film and induces a Schottky barrier in another semiconductor tangent to the film on the opposite side (reach-through band bending). We have found experimental evidence of this effect in CdTe photovoltaics.

Physical modes of thin-film PV degradation

We discuss physical modes of degradation related to the small thickness and lack of crystallinity in thin-film PV. We discriminate between 1) uniform material degradation through defect generation, light-induced diffusion, and electro-migration; 2) nonuniform degradation through ohmic or non-ohmic shunts; 3) metal contact deterioration. The first can equally apply to bulk and thin-film PV. Two others are specific to thin-film PV.

Reach-through mechanism in CdS/CdTe solar cells

We describe an alternative junction mechanism in CdS/CdTe solar cells. It occurs through generation of defects in the CdS layer that changes its work function. These defects appear either by Cu doping or by modification of the TCO/CdS interface. This translates the presence of the front-contact through the CdS film and induces a Schottky barrier at the CdS/CdTe interface, similar to an MIS structure. We call this reach-though band bending. It allows high open-circuit voltage cells (above 800 mV) without any intentional copper doping.

Back contact effects in thin-film photovoltaics

The Schottky barrier formed at the back contact is shown to either act as the commonly accepted back-diode or as a reach-through diode. Rare local spots with low back barrier transparency and/or weak main junctions shunt the photocurrent, thus decreasing the measured open-circuit voltage and device efficiency. This phenomenon was observed experimentally in the case of thin-film CdTe photovoltaics. Combining a proper main junction and an adequate back-contact results in 13% efficient CdTe solar cell without intentional copper doping.