Hamidreza Esmaielpour

Suppression of phonon‐mediated hot carrier relaxation in type‐II InAs/AlAsxSb1 − x quantum wells: a practical route to hot carrier solar cells

InAs/AlAs

Hot-carrier effects in type II heterostructures

_{x}

Enhanced hot electron lifetimes in quantum wells with inhibited phonon coupling

Sb

The effect of an InP cap layer on the photoluminescence of an InxGa1–xAs1–yPy/InzAl1– zAs quantum well heterostructure

_{1-x}

Evidence of suppressed hot carrier relaxation in type-II InAs/AlAs1-xSbx quantum wells

quantum wells are investigated for their potential as hot carrier solar cells. Continuous wave power and temperature dependent photoluminescence indicate a transition in the dominant hot carrier relaxation process from conventional phonon-mediated carrier relaxation below 90 K to a regime where inhibited radiative recombination dominates the hot carrier relaxation at elevated temperatures. At temperatures below 90 K photoluminescence measurements are consistent with type-I quantum wells that exhibit hole localization associated with alloy/interface fluctuations. At elevated temperatures hole delocalization reveals the true type-II band alignment; where it is observed that inhibited radiative recombination due to the spatial separation of the charge carriers dominates hot carrier relaxation. This decoupling of phonon-mediated relaxation results in robust hot carriers at higher temperatures even at lower excitation powers. These results indicate type-II quantum wells offer potential as practical hot carrier systems.

Evidence of hot carriers at elevated temperatures in InAs/AlAs0.84Sb0.16 quantum wells

Hot-carrier solar cells have high theoretical limiting efficiency however, absorber materials with slow carrier thermalization remain a development barrier for these devices. In previous studies, charge separation in core-shell colloidal quantum dots has been shown to result in slow carrier relaxation. Charge separation also occurs in III-V heterostructures with type-II band alignments. We characterize hot-carrier effects in InAlAs/InP and InAlAs/InGaAsP quantum well structures, with type-II and quasi-type-II band alignments respectively. InGaAsP is identified as a promising hot-carrier absorber candidate, with thermalization coefficient 1.77±0.12 W.K-1.cm-2, corresponding to limiting solar conversion efficiency >42%, under 2000X.

Effect of occupation of the excited states and phonon broadening on the determination of the hot carrier temperature from continuous wave photoluminescence in InGaAsP quantum well absorbers

Hot electrons established by the absorption of high-energy photons typically thermalize on a picosecond time scale in a semiconductor, dissipating energy via various phonon-mediated relaxation pathways. Here it is shown that a strong hot carrier distribution can be produced using a type-II quantum well structure. In such systems it is shown that the dominant hot carrier thermalization process is limited by the radiative recombination lifetime of electrons with reduced wavefunction overlap with holes. It is proposed that the subsequent reabsorption of acoustic and optical phonons is facilitated by a mismatch in phonon dispersions at the InAs-AlAsSb interface and serves to further stabilize hot electrons in this system. This lengthens the time scale for thermalization to nanoseconds and results in a hot electron distribution with a temperature of 490 K for a quantum well structure under steady-state illumination at room temperature.

Hot-carrier dynamics in type-II semiconductor quantum wells (Conference Presentation)

The effect of an InP cap on the photoluminescence (PL) spectrum of an InGaAsP/InAlAs quantum well (QW) is investigated using excitation power and temperature dependent PL. An as-grown sample with the InP cap layer shows an inverted interface created between InP and InAlAs that has a transition energy very close to the transition energy of the QW; consequently, there is an overlap between them. On the other hand, the QW sample with the cap layer etched away does not have a feature due to the inverted interface; even at very low power, the only observed feature is due to the QW transition.

Valence band states in an InAs/AlAsSb multi-quantum well hot carrier absorber

Hot carrier solar cells (HCSCs) have been proposed as devices, which can increase the conversion efficiency of a single junction solar cell above the Shockley-Queisser limit. For practical implementation of such systems, solar cells operating with efficient hot carrier extraction must circumvent two fundamental challenges: 1. Find an absorber material in which hot carriers are sustained either via inhibiting or circumventing phonon relaxation pathways; 2. Implement energy selective contacts in which only a narrow range of energy within the hot carrier distribution is extracted; thereby, reducing cooling losses in the contacts. Here, type-II InAs/AlAs0.16Sb0.84 quantum-wells are investigated as a candidate system for hot carrier absorbers. Continuous wave power and temperature dependent photoluminescence measurements are presented that indicate: a transition in the dominant hot carrier relaxation process from conventional phonon-mediated carrier relaxation − below 90 K − to a regime where inhibited radiative recombination dominates the hot carrier relaxation − at higher temperatures1. The reduction in the PL efficiency is strongly coupled to an increase in the hot carrier temperature extracted from the measurements. This behavior is attributed to a build-up of electrons in the QWs, which appears to inhibit electron-phonon relaxation2.

The effect of excited state occupation and phonon broadening in the determination of the non-equilibrium hot-carrier temperature in InGaAsP quantum-well absorbers (Conference Presentation)

InAs/AlAs0.84Sb0.14 quantum wells (QWs) are investigated as a potential system for applications in hot carrier solar cells. Temperature and power dependent photoluminescence (PL) measurements show evidence of carrier localization. Evidence of for the presence of hot carriers is provided through the broadening of the high-energy tail in PL with increasing excitation power. Moreover, with increasing temperature, the stability of the hot carriers appears to improve despite the increased contribution of phonons at elevated temperatures. This is attributed to the reduced radiative recombination rate driven by the type-II band offset inherent in this system; which is suggested to result in inhibited hot carrier relaxation through electron pile-up in the conduction band