In this perspective, we present an overview of nanoscience applications in catalysis, energy conversion, and energy conservation technologies. We discuss how novel physical and chemical properties of nanomaterials can be applied and engineered to meet the advanced material requirements in the new generation of chemical and energy conversion devices. We highlight some of the latest advances in these nanotechnologies and provide an outlook at the major challenges for further developments.

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Nanoscale Advances in Catalysis and Energy Applications

The intermediate band (IB) solar cell has been proposed to increase the current of solar cells while at the same time preserving the output voltage in order to produce an efficiency that ideally is above the limit established by Shockley and Queisser in 1961. The concept is described and the present realizations and acquired understanding are explained. Quantum dots are used to make the cells but the efficiencies that have been achieved so far are not yet satisfactory. Possible ways to overcome the issues involved are depicted. Alternatively, and against early predictions, IB alloys have been prepared and cells that undoubtedly display the IB behavior have been fabricated, although their efficiency is still low. Full development of this concept is not trivial but it is expected that once the development of IB solar cells is fully mastered, IB solar cells should be able to operate in tandem in concentrators with very high efficiencies or as thin cells at low cost with efficiencies above the present ones.

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The Intermediate Band Solar Cell: Progress Toward the Realization of an Attractive Concept

Absolute limiting efficiencies for photovoltaic energy conversion

The use of quantum dots can turn the old concept of a luminescent solar collector into a practical concentrator. The quantum efficiency, tunability of absorption threshold, and size of the redshift make quantum dots an ideal replacement for the organic dyes whose performance limited this inexpensive technology. Progress in photovoltaic cells, in particular, the ability of quantum-well cells to tune the band gap, also suggests high efficiency is possible in solar and thermophotovoltaic applications. A thermodynamic model is used to show quantitatively how the separation of absorption and luminescent peaks under global illumination is related to the spread of quantum-dot sizes. Hence, the redshift can be determined during the growth process. The model can be used to optimize concentrator performance and to study the effect of reabsorption, which is important for high concentration even if the quantum efficiency is unity. This model provides a quantitative explanation for the contribution of the spread of sizes to the redshift, which should help in the extraction of the much smaller, single-dot effects.

Quantum-dot concentrator and thermodynamic model for the global redshift

A strain-balance multiquantum well (MQW) approach to enhance the GaAs solar cell efficiency is reported. Using a p-i-n diode structure, the strain-balanced GaAsP/InGaAs MQW is grown on a GaAs substrate and equals a good GaAs cell in terms of power conversion efficiency. The cell design is presented together with measurements of the forward bias dark current density, quantum efficiency, and 3000 K light-IV response. Cell efficiencies under standard air mass (AM) 1.5 and AM 0 illumination are projected from experimental data and the suitability of this cell for enhancing GaInP/GaAs tandem cell efficiencies is discussed.

Strain-balanced GaAsP/InGaAs quantum well solar cells

We have studied the forward bias behavior of AlGaAs/GaAs p‐i‐n multiquantum well (MQW) photodiodes. In samples with low background impurity levels in the intrinsic region the high quantum efficiency observed in reverse bias is maintained into forward bias even for carriers photoexcited in the wells. We compare our MQW devices with structures which are identical apart from having AlGaAs intrinsic regions without quantum wells. The short‐circuit currents in the MQW structures are much higher than in the control samples though the open‐circuit voltages are somewhat smaller. In one case the energy conversion efficiency of the MQW device in white light is 110% higher than the control. We discuss the implications of our results for the development of low‐dimensional structure solar cells.

Short‐circuit current and energy efficiency enhancement in a low‐dimensional structure photovoltaic device

The authors have studied the variation in DC photocurrent with bias and temperature from GaAs-Al/sub x/Ga/sub 1-x/As single quantum wells embedded in p-i-n diodes. They found that the observed temperature response shows Arrhenius behaviour with a field-dependent activation energy close to the hole well depth. This can be accounted for using a model based on the competition between photocarrier escape and recombination. Using reasonable values for the diode's built-in voltage and the quantum-well recombination lifetime, good quantitative agreement between theory and experiment is achieved if it is assumed that the recombination rate is governed by the fastest escaping carriers, which are light holes in the present devices. >

Steady-state carrier escape from single quantum wells

The quantum well solar cell is an alternative to more conventional multiband gap approaches to higher cell efficiency. Preliminary studies have shown that the insertion of a series of quantum wells into the depletion region of a GaAs/AlxGa1−xAs p‐i‐n solar cell can significantly enhance the cell’s short‐circuit current. We present here a model for the spectral response of GaAs and AlxGa1−xAs p‐n and p‐i‐n solar cells, with and without quantum wells, based on a standard solution of the minority‐carrier equations. Particular emphasis is placed on modeling the absorption coefficient of the AlxGa1−xAs and of the quantum wells. We find that our model can accurately predict the spectral response of a wide variety of cells: both conventional p‐n junctions in GaAs and AlxGa1−xAs, and various geometries of quantum well solar cell in AlxGa1−xAs/GaAs (x∼0.3). We discuss the strengths and weaknesses of the model and its underlying assumptions, and conclude by using the model to design p‐i‐n quantum well solar cells w...

Modeling the spectral response of the quantum well solar cell

A new approach to higher efficiency solar cells based on low dimensional structures

We have studied the forward bias behaviour of AlGaAs/GaAs p-i-n Multi-Quantum-Well (MQW) photodiodes. In samples with low background impurity levels in the intrinsic region the high quantum efficiency observed in reverse bias is maintained into forward bias even for carriers photo-excited in the wells. We compare our MQW devices with structures which are identical apart from having AlGaAs intrinsic regions without quantum wells. The short-circuit currents in the MQW structures are much higher than in the control samples though the open-circuit voltages are somewhat smaller. In one case the energy conversion efficiency of the MQW device in white light is 110% higher than the control. We discuss the implications of our results for the development of Low-Dimensional Structure (LDS) solar cells.

M. Paxman

Papers

Short‐circuit current and energy efficiency enhancement in a low‐dimensional structure photovoltaic device

Steady-state carrier escape from single quantum wells

Modeling the spectral response of the quantum well solar cell

A new approach to higher efficiency solar cells based on low dimensional structures

The LDS Solar Cell— A New Approach to Higher Efficiency