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Efficient photovoltaic energy harvesting requires device structures capable of absorbing a wide spectrum of incident radiation and extracting the photogenerated carriers at high voltages. In this paper, we review the impact of active layer thickness on the voltage performance of GaAs-based photovoltaic device structures. We observe that thin absorber structures can be leveraged to increase the operating voltage of energy harvesting devices. Thin absorbers in combination with advanced light trapping structures provide an exciting pathway for enhancing the performance of flexible, lightweight photovoltaic modules suitable for mobile and portable power applications.

Optoelectronic materials for advanced IR sensing should combine wide strong electron coupling to the IR radiation, spectral tunability, adjustable dynamic range, manageable trade-off parameters, such as the noise characteristics and the operating time. Modern nanomaterials based on quantum dots and quantum wells provide wide possibilities to manage photoelectron processes via tuning the charge of quantum dots and quantum wells by the electric field and/or optical pumping. Variations in charge built in dots and wells change spectral characteristics, photocarrier lifetimes, and noise processes. These effects are especially strong in nanomaterials with strong selective doping of dots and wells. Manageable built-in charge provides wide possibilities to control the spectra, detector responsivity, and recombination processes.

Most of investigations of quantum dot photovoltaic devices are aimed at the development of the intermediate band solar cell. To form the intermediate band by quantum dot electron levels, the dots should be placed close to one to another. This leads to strain accumulation and defects, which increase the photocarrier recombination, and recombination losses. To avoid the nanostructuring-induced recombination, we proposed and studied an alternative approach, which is based on the separation of quantum dots (QDs) or QD clusters from the conducting channels by potential barriers created by quantum dots with built-in charge (Q-BIC). Charging of QDs improves the performance of QD solar cells due to the following factors: Negative dot charging increases electron coupling to sub-bandgap photons and provides effective harvesting of IR energy. Because of the strong difference in effective masses of electrons and holes, an electron level spacing in QDs substantially exceeds a level spacing for holes. Therefore, QDs act as deep traps for electrons, but they are shallow traps for holes. Thus, the holes trapped in QDs may be excited by thermal phonons, while excitation of localized QDs electrons requires IR radiation or the interaction with hot electrons. Therefore, n-doping of QD structures is strongly preferable for photovoltaic applications. Charging of QDs is also an effective tool for managing the potential profile at micro- and nanoscales. Filling QDs predominantly from dopants in the QD medium allows one to maintain the macroscale profile analogous to that in the best conventional single-junction solar cells.

We investigated the effects of doping on the photovoltaic efficiency in a GaAs reference cell, and in undoped, n-doped, and p-doped InAs/GaAs quantum-dot (QD) solar cells. We found that the photovoltaic efficiency of the undoped QD solar cell is almost the same as that of the reference cell. However, the efficiency improves monotonically with increasing inter-dot ndoping, while p-doping deteriorates the photovoltaic conversion. We observed a 50 % increase in photovoltaic efficiency in the device n-doped to provide approximately six electrons per dot as compared with the undoped QD cell. In this QD solar cell, the short circuit current density increases to 24.30 mA/cm 2 compared with 15.07 mA/cm 2 in the undoped QD solar cell without deterioration of the open circuit voltage. To identify the physical mechanisms that provide this improvement, we investigated the spectral characteristics of the photovoltaic response and photoluminescence of our QD solar cells. We found that the electron capture into QDs is substantially faster than the hole capture, which leads to an accumulation of electrons in QDs. The electrons trapped in dots enhance IR transitions. The built-in-dot electron charge together with charged dopants outside the dots creates potential barriers, which suppress the fast electron capture processes and at the larger scale form a potential profile which precludes degradation of the open circuit voltage. All of these factors lead to the enhanced harvesting of IR energy and a radical improvement of the QD solar cell efficiency. Higher efficiencies are anticipated with further increase of doping level and at higher radiation intensity. This makes the QD solar cells promising candidates for use with concentrators of solar radiation.