The authors consider physical properties of ultrathin photovoltaics with thickness (≲1μm) smaller than both the depletion width and diffusion length, applicable to the cases of amorphous, polycrystalline, and nanostructured devices. Three phenomena underlie the unique physics of such systems: (1) lateral screening by conducting electrodes, (2) leakiness due to defect assisted tunneling, and (3) gigantic capacitive energy conducive to shunting breakdown. The authors give numerical estimates and discuss practical implications of these phenomena.

We report a summary of our recent study of piezo-and pyroelectricity in CdS-based photovoltaics including CdTe and CuIn͑Ga͒Se 2 absorber devices. The strong pyro-and piezo-effects in CdS significantly affect the electric field distribution and the physics of device operations. We introduce the concept of pyro-photovoltaic coupling where the photovoltaic effect and the CdS polarization strongly depend on each other. Our quantitative modeling shows that the CdS polarization turns out to be beneficial for photovoltaic technology, making it more forgiving. Three experimental setups utilized in our work were aimed at exerting mechanical stresses corresponding to the device squeezing, bending, and flexing; we used the substrate, superstrate, and flexible substrate structures. Our understanding points at previously unexplored venues in thin-film photovoltaic technology.

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We propose a simple physical model of CdS-based thin-film photovoltaic junctions including the major types that utilize the CdTe and Cu(In,Ga)Se2 absorber layers. This model allows for field reversal in the CdS layer. It is solved analytically, verified numerically, and predicts a variety of phenomena, such as the lack of carrier collection from CdS, buffer layer effects, light to dark current-voltage curve crossing and rollover.

DEVICE OPERATIONSWe start with the band diagrams of CdS/CdTe devices shown in Fig.2. We consider the CdS polarizations of both signs leading to either the "gull-wing"-shaped potential (Fig. 2b) and the opposite, "hunch" one ( Fig. 2c), in comparison with the "reference" device structure that does not have a dipole layer and represents the standard p-n junction (Fig. 2a). The question of matter is how the major PV parameters, efficiency, Voc, Jsc, and fill factor FF depend on the polarization electric field in the CdS region.