**1. Introduction**

[49] Gratzel M, Park NG. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%, Science Rep. 2012;2591. [50] SnaithHJ, LeeMM. Efficient hybrid solar cells based on meso-superstructuredorgano‐

[51] MalinkiewiczO, YellaA, LeeY H, EspallargasG M, GraetzelM, NazeeruddinM K,Bo‐ linkH. Perovskite solar cells employing organic charge-transport layers, J. Nat Pho‐

[52] Stranks S, Eperon G, Grancini G, Menelaou C, Alcocer M, Leijtens T, Herz L, Petroz‐ zaA, Snaith H. Electron-hole diffusion lengths exceeding 1 micrometer in an organo‐

metal halide perovskites, Science 2012; 338(6107)643-7.

metaltrihalideperovskite absorber, Science 2013; 342(6156)341-4.

[53] Gregg B A. Excitonic solar cells, J. Phys. Chem. B 2003; 107(20) 4688–4698.

tonics2014; 8, 128‐132.

246 Solar Cells - New Approaches and Reviews

Optically-thin absorber structures represent an interesting class of photovoltaic devices, both in terms of their performance characteristics and the economic advantages of employing thinner semiconductor material layers. This chapter reviews the underlying physics of highefficiency optically-thin solar cells employing thin-film III-V materials. By combining thin III-V absorber structures with advanced light-trapping structures, single-junction devices can deliver high efficiency performance over a wide range of operating conditions at a fraction of the cost of multi-junction structures. Moreover, by leveraging hot carrier and/or optical upconversion mechanisms to extend infrared absorption, the power conversion efficiencies in single-junction nano-enhanced solar cells can potentially exceed the Shockley-Queisser limit and outperform multi-junction devices. Experimentally, suppressed radiative recombination and high voltage operation have been observed in step-graded InGaAs quantum well struc‐ tures. In addition, recent results from a novel InAs/AlAsSb quantum dot structure prove the validity of the intermediate band solar cell approach for infrared up-conversion, and underline the potential of thin-film III-V materials for realizing cost-effective, high-efficiency solar cells.

By minimizing semiconductor material content, optically-thin absorber structures provide a pathway to lower the manufacturing cost of high-performance photovoltaic (PV) devices. Thin-film solar cells are also an attractive source of portable and mobile power, as they can be integrated into flexible, lightweight photovoltaic modules that can operate in both terrestrial and space environments. Several different emerging technologies can be employed to fabricate flexible thin-film PV cells [1-2]. Although the deposition of copper indium gallium diselenide (CIGS) directly onto flexible substrates offers some advantages in terms of ease of manufac‐ turing, the epitaxial lift-off (ELO) of III-V devices can provide much higher efficiency per‐ formance.

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

By stacking multiple p-n junctions of different III-V semiconductor materials into one twoterminal device, multi-junction solar cells have achieved record-high efficiency at converting solar power into electrical power. Under air mass zero (AM0) spectral conditions found in space, InGaP/GaAs/InGaAs inverted metamorphic (IMM) cells have been demonstrated with efficiencies in excess of 32% [3-4]. Even higher efficiencies exceeding 35% have been achieved under an air mass spectrum (AM1.5) typically used to characterize terrestrial performance [5]. However, changes in the solar spectrum can dramatically degrade the performance of multijunction devices – changes that occur naturally throughout the day, from season to season, and from location to location as sunlight passes through the earth's atmosphere. As illustrated in Figure 1, the efficiency of a multi-junction device will decrease by more than a factor of two under higher air mass spectra. This reduction in efficiency is due to a decrease in the current output of the series-connected multi-junction device, which is limited by the subcell generating the least amount of photocurrent. Series-connected multi-junction cells can also degrade more rapidly than single-junction III-V cells upon irradiation, particularly as the current output of the limiting subcell fails. Moreover, multi-junction III-V cells require thick, complex epitaxial layers and are therefore inherently expensive to manufacture. Thus the inconsistent perform‐ ance under changing environmental conditions and high manufacturing costs of multijunction III-V cells severely hamper the application of this established high-efficiency technology.

**Figure 1.** Projected un-concentrated efficiency versus air mass spectrum for several different types of solar cell struc‐ tures, including a high-performance IMM triple-junction III-V structure, a single-junction CIGS cell, and a single-junc‐ tion GaAs solar cell. Also shown is the theoretical performance of a thin GaAs-based device incorporating upconverting and light-trapping structures to harness a notable fraction of the available low energy photons. The calculations assume a Bird – Riordan model of the air mass spectrums and realistic spectral response and dark diode characteristics, as detailed in reference [6].

Thin-film single-junction III-V cells can potentially address the performance and cost limita‐ tions of multi-junction devices. By avoiding current matching constraints, single-junction structures can offer a more robust performance than multi-junction devices [6]. The efficiency of established single-junction CIGS and GaAs cell technologies is more stable to changes in the incident spectrum, and can actually outperform III-V multi-junction structures under higher air mass spectrums, as depicted in Figure 1. Moreover, the efficiency of single-junction III-V cells can be dramatically increased by employing additional structures that leverage optical up-conversion and/or hot carrier effects. Theoretically, both up-conversion and hot carrier mechanisms have been projected to increase the limiting one-sun efficiency of single-junction photovoltaic devices to over 50% [7-8]. By combining thin III-V absorber structures with advanced light-trapping structures, nano-enhanced III-V single-junction devices can poten‐ tially deliver high efficiency performance in a flexible format at a fraction of the cost of multijunction structures. As illustrated in Figure 1, efficiencies of more than 40% over a wide range of spectrums are projected for an optically-thin GaAs-based device that can harness 85% of the infrared photons falling within 500 meV of the GaAs band edge [6].

By stacking multiple p-n junctions of different III-V semiconductor materials into one twoterminal device, multi-junction solar cells have achieved record-high efficiency at converting solar power into electrical power. Under air mass zero (AM0) spectral conditions found in space, InGaP/GaAs/InGaAs inverted metamorphic (IMM) cells have been demonstrated with efficiencies in excess of 32% [3-4]. Even higher efficiencies exceeding 35% have been achieved under an air mass spectrum (AM1.5) typically used to characterize terrestrial performance [5]. However, changes in the solar spectrum can dramatically degrade the performance of multijunction devices – changes that occur naturally throughout the day, from season to season, and from location to location as sunlight passes through the earth's atmosphere. As illustrated in Figure 1, the efficiency of a multi-junction device will decrease by more than a factor of two under higher air mass spectra. This reduction in efficiency is due to a decrease in the current output of the series-connected multi-junction device, which is limited by the subcell generating the least amount of photocurrent. Series-connected multi-junction cells can also degrade more rapidly than single-junction III-V cells upon irradiation, particularly as the current output of the limiting subcell fails. Moreover, multi-junction III-V cells require thick, complex epitaxial layers and are therefore inherently expensive to manufacture. Thus the inconsistent perform‐ ance under changing environmental conditions and high manufacturing costs of multijunction III-V cells severely hamper the application of this established high-efficiency

**0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0**

**Thin GaAs with Up-conversion (500 meV @ 85%)**

**Air Mass Number**

**Figure 1.** Projected un-concentrated efficiency versus air mass spectrum for several different types of solar cell struc‐ tures, including a high-performance IMM triple-junction III-V structure, a single-junction CIGS cell, and a single-junc‐ tion GaAs solar cell. Also shown is the theoretical performance of a thin GaAs-based device incorporating upconverting and light-trapping structures to harness a notable fraction of the available low energy photons. The calculations assume a Bird – Riordan model of the air mass spectrums and realistic spectral response and dark diode

technology.

248 Solar Cells - New Approaches and Reviews

characteristics, as detailed in reference [6].

**CIGS**

**IMM-3J**

**GaAs**

**One Sun (Unconcentrated) Efficiency (%)**

In this chapter, the theoretical performance of optically-thin solar cells is first described using a generalized detailed balance model, specifically adapted for nano-enhanced absorbers. This model is then employed to describe the impact of both absorber thickness and effective optical path length on the performance of III-V photovoltaic devices, using data from GaAs-based 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 is summarized. In particular, the combination of a thin optical absorber and advanced light trapping structures is 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 effects will in essence enhance radiative recombination and negate the voltage benefits of thinabsorber cells. In addition, optical losses in the high-doped contact layers and surface regions can limit some of the benefits of light-trapping on voltage 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 voltage enhanced by embed‐ ding thin-absorbers in lower refractive index material or employing step-graded structures to harness hot carrier effects. Finally, infrared up-conversion provides a pathway to enhance current output and thus increase efficiency.
