Quantum dots with built-in charge for enhancing quantum dot solar cells and infrared photodetectors

This chapter focuses on a novel approach for engineering nanostructures with advanced properties due to quantum dots with built-in charge (Q-BIC). By implementing this approach, a 50% increase in the photovoltaic efficiency in n-doped InAs/GaAs quantum dot solar cell (QDoSC) with a built-in-dot charge of 6 electrons per dot have been achieved. In addition, we have also demonstrated 25 times increase in the photoresponse of a quantum dot infrared photodetector (QDIP), with increasing the built-in-dot charge from one to six electrons per dot. Self-assembled quantum dots have emerged as one of the most promising systems for a plethora of applications, including quantum computing, infrared (IR) sensing, solid-state lighting and lasing, and energy harvesting due to their fully quantized electron and hole states [1-7]. While knowledge of the tunable energy levels in QDs, which ultimately govern the electrical and optical properties, has been fully exploited, little efforts have been directed to controlling carrier kinetics within QD structures. For example, the phonon bottleneck concept assumes that the phonon-assisted bound-to-bound transitions in QDs are prohibited, unless the energy between two discrete levels matches the phonon energy [8], but completely ignores the interaction between electrons and corresponding modification of electron states. As a result, experimentally measured phonon-mediated electron relaxation turns out to be much faster than expected in the phonon bottleneck concept [9]. In response to this fundamental issue which impedes the performance of QD devices, an unique approach has been proposed for suppressing photoelectron relaxation and increase the photoelectron lifetime. This approach involves manipulating the interdot kinetics by using specially engineered potential barriers [10, 11]. As it will be shown in the next sections, there are two fundamental effects supporting the radical improvements observed in the QDoSCs and QDIPs as a result of these potential barriers. First, the built-in-dot charge strongly increases electron coupling to IR radiation, because a large electron population in QDs enhances the photoinduced intraband transitions in dots and transitions from QD localized states to conducting states in the matrix. Second, the built-in-dot charge creates potential barriers around dots and these barriers suppress capture processes for photocarriers of the same sign as the built-in-dot charge. As a result, the barriers decrease recombination via QDs and increase the photocarrier lifetime. High scalability of QD nano-blocks (single QDs, QD clusters, chains, etc.) allows for numerous possibilities for nano-engineering specifically three-dimensional (3D) potential barriers which provide unique optoelectronic characteristics. This chapter begins with a brief description of the Stranski-Krastanow growth mode used for the self-assembled growth of InAs QDs. The photoelectron capture from a fundamental point of view with emphasis on controlling electron-hole kinetics via selective doping of the interdot space is then described followed by a detailed discussion on the effect of the potential barriers on the performance of QDoSCs and QDIPs.