**9. Conclusions**

27

**Figure 22:** a) PL peak position at 77 K as a function of the cube root of excitation power for 7 ML and 8 ML InAs QDs with optimized cladding scheme, shown in the inset, and b) time dependent PL decay traces for 8 ML InAs SAQDs at different detection wavelengths. The dashed line is the fitting curve for

**Figure 22.** (a) PL peak position at 77 K as a function of the cube root of excitation power for 7 ML and 8 ML InAs QDs with optimized cladding scheme, shown in the inset, and b) time dependent PL decay traces for 8 ML InAs SAQDs at

different detection wavelengths. The dashed line is the fitting curve for the decay trace at peak wavelength.

**b)**

272 Solar Cells - New Approaches and Reviews

the decay trace at peak wavelength.

While multi-junction III-V devices have achieved record-breaking efficiency under select operating conditions, inherent sensitivity to changes in the spectral conditions and high manufacturing costs preclude their wide-spread usage in energy harvesting applications. On the other hand, thin-film single-junction III-V devices can offer more robust performance at lower costs. Moreover, single-junction III-V cells can potentially match or even exceed the peak efficiency performance levels of present day multi-junction devices by incorporating advanced structures that leverage light trapping, optical up-conversion and/or hot carrier effects.

In this chapter, the theoretical performance of optically-thin solar cells has been described using a generalized detailed balance model, specifically adapted for nano-enhanced absorbers. This model has been employed to assess the impact of both absorber thickness and effective optical path length on the performance of III-V photovoltaic devices, using data from GaAsbased structures to validate the approach. In later sections, recent experimental work focused on reducing the diode dark current (and hence increasing the operating voltage) and boosting the current output of nano-enhanced III-V solar cells has been summarized. In particular, the combination of a thin optical absorber and advanced light trapping structures was shown to provide a means to increase the voltage of operation while maintaining current output in photovoltaic devices. However, if the absorber thickness is reduced too far, two-dimensional carrier confinement will effectively enhance radiative recombination and negate the voltage benefits of thin-absorber cells. In addition, optical losses in the high-doped contact layers and surface regions can limit some of the enhanced voltage benefits of light-trapping in thin absorber structures. There are, however, several other mechanisms for reducing radiative emissions in photovoltaic devices. For example, radiative emissions can be minimized and the voltage increased by embedding thin-absorbers in lower refractive index material or employ‐ ing step-graded structures to harness hot carrier effects. Finally, infrared up-conversion has been demonstrated as a pathway to enhance current output in intermediate band solar cells.
