No abstract

Photovoltaic technologies have shown efficiencies of over 40%, however, manufacturing costs have prevented a more significant energy market penetration. To bridge the gap between the high efficiency technology and low cost manufacturing, a research and development tool and process was built and tested. This fully automated single vacuum photovoltaic manufacturing tool utilizes multiple inline close space sublimation (CSS) sources with automated substrate control. This maintains the proven scalability of the CSS technology and CSS source design but with the added versatility of independent substrate motion. This combination of a scalable deposition technology with increased cell fabrication flexibility has allowed for high efficiency cells to be manufactured and studied. The single vacuum system is capable of fabricating a 3.1 × 3.6 in. substrate every 45 min with a cell efficiency of 12% with a standard deviation of 0.6% as measured over 36 months. The substrate is generally scribed into 25 small area devices allowing for over 250 small area devices to be fabricated each day. The system can operate uninterrupted for maintenance for over 21 days.

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.

We report a strong reversible piezo-effect in CdTe∕CdS photovoltaics consistent with the piezo parameters of CdS. Our finding suggests a different understanding of CdS-based solar cells including CdTe- and CuIn(Ga)Se-based devices. Because the CdS film is put into compression in the device, the piezo coupling generates surface charges and the electric field opposing that of the absorber layer. The corresponding potential barrier makes CdS insulating and the device operate in a metal-insulator-semiconductor mode. Our understanding introduces the concept of piezo-photovoltaics and suggests specific practical implications.

We report on the development of Mo/CdTe/CdS/indium-tin-oxide, thin-film solar cells grown by radio-frequency magnetron sputtering. This is an inverted configuration compared to the conventional glass/tin-oxide/CdS/CdTe/metal cells. Molybdenum was chosen as a substrate because its thermal expansion coefficient and the work function are close to those of CdTe. We have achieved AM1.5 conversion efficiencies of 7.8 percent on 0.05 cm2 area devices. Our best cells had a nitrogen-doped ZnTe layer between the molybdenum and the CdTe for a somewhat improved back contact. However, we observe a significant rollover in the IV curve in forward current that indicates a back-diode effect. This implies the need for improvement of the electronic properties of the molybdenum - CdTe and possibly CdS - ITO interfaces.