Microscopic theory and numerical simulation of quantum well solar cells

Abstract

Quantum well solar cells have been introduced as high efficiency photovoltaic energy conversion devices nearly twenty years ago. Since then, their capability to outperform bulk devices under concentrated illumination was established and efficiencies close to the single gap Shockley-Queisser limit for the corresponding bulk materials were reached. In order to further increase the efficiency, the mechanisms behind the extraordinary performance need to be analyzed and understood. To this end, novel theoretical approaches to the simulation of quantum optoelectronic devices are required, since the device behavior depends on complex processes between localized and extended states, such as carrier capture and escape into and from quantum wells, which cannot be consistently described by the combination of macroscopic semiconductor transport equations with detailed balance rate models conventionally used in photovoltaics. This paper presents a suitable theoretical framework based on the nonequilibrium Green's function formalism that allows the unification of quantum optics and dissipative quantum transport on the quantum kinetic level and is thus able to provide insight into the microscopic mechanisms of quantum photovoltaic device operation.

Design and Simulation of Multiquantum-Well AlGaAs/GaAs Single Junction Solar Cell with Back Surface Reflector

“…The single junction structure proposed consists of an AlGaAs/GaAs MQW top cell and an AlAs/GaAs DBR reflector at the back. Incorporating GaAs quantum wells in the intrinsic region of a p-i-n Al x Ga 1−x As solar cell can increase the efficiency of the single bandgap baseline cell as it is reported by several investigators [1][2][3][4].…”

Photovoltaics at the mesoscale: insights from quantum-kinetic simulation

“…7(a) for the case of a 5 nm wide AlGaAs-GaAs SQW with band offsets ∆E C = 200 meV and ∆E V = 150 meV (see Ref. [86] for a complete list of parameters), excitation at the absorption edge of the QW (E γ = 1.45 eV) leads to a spectral density with pronounced phonon satellites, while excitation at higher energies generates carriers also above the ground state, which results in a broadened spectral excess carrier density. The associated spectral current flow displayed in Fig.…”

“…The associated spectral current flow displayed in Fig. 7(b) reflects the photogenerated carrier density and reveals a transition in the photocarrier escape mechanism from phonon-assisted tunneling to direct escape [86,120]. For further assessment of the photocarrier escape in SQW, the carrier extraction efficiency is evaluated as the ratio η ext = J sc /J abs of short-circuit current (J sc ) and generation current (J abs ) at low carrier lifetime in 5 nm wide GaAs-InGaAs SQW for different values of the built-in field, the band offsets ∆E C,V and photon energy E γ .…”

Quantum-kinetic perspective on photovoltaic device operation in nanostructure-based solar cells

“…In quantum well solar cells, the absorber contains regions of lower dimensional electronic states that are partially confined in transport direction owing to the variation in band gap and electron affinity of the constituent bulk materials. In conventional multi-quantum-well architectures, the quantum well states are not directly coupled to the contacts, and carriers generated in such localized states have to be transferred to extended continuum states prior to extraction [46,47]. The escape of carriers proceeds via thermionic emission, direct or phonon assisted tunneling, depending on the temperature, injection level and strength of the built-in field.…”