Carrier Multiplication in InAs Nanocrystal Quantum Dots with an Onset Defined by the Energy Conservation Limit

Abstract

Carrier multiplication (CM) is a process in which absorption of a single photon produces not just one but multiple electron−hole pairs (excitons). This effect is a potential enabler of next-generation, high-efficiency photovoltaic and photocatalytic systems. On the basis of energy conservation, the minimal photon energy required to activate CM is two energy gaps (2E g ). Here, we analyze CM onsets for nanocrystal quantum dots (NQDs) based upon combined requirements imposed by optical selection rules and energy conservation and conclude that materials with a significant difference between electron and hole effective masses such as III−V semiconductors should exhibit a CM threshold near the apparent 2E g limit. Further, we discuss the possibility of achieving sub-2E g CM thresholds through strong exciton−exciton attraction, which is feasible in NQDs. We report experimental studies of exciton dynamics (Auger recombination, intraband relaxation, radiative recombination, multiexciton generation, and biexciton shift) in InAs NQDs and show that they exhibit a CM threshold near 2E g .Carrier Multiplication Thresholds in Bulk and Nanocrystalline Semiconductors. It has recently been established that carrier multiplication (CM), the production of multiple excitons following absorption of a single photon of sufficient energy, is efficient within the range of solar photon energies in semiconductor nanocrystal quantum dots (NQDs). [1][2][3][4][5][6][7][8][9] This finding is important because CM can potentially provide increased power conversion efficiency in low-cost, singlejunction photovoltaics via an enhanced photocurrent. 10,11 To obtain optimum photovoltaic power conversion efficiency using a single semiconductor material with energy gap E g , it is desirable that the CM onset energy is minimal and that the CM efficiency above the onset is as large as is allowed by energy conservation.According to energy conservation, the apparent minimal photon energy that is required to activate CM (pω CM ) is simply 2E g . However, the CM thresholds observed in bulk materials are significantly larger than this value. 12,13 In bulk semiconductors, the generally accepted mechanism for multiexciton generation is impact ionization in which a highenergy electron (or a hole) transfers its energy to a valenceband electron that is excited across the energy gap. 14 The CM threshold in this case is determined not only by conservation of energy but also by conservation of translational momentum, which increases the threshold above 2E g . For example, using a free-particle approximation 15,16 and applying both energy and momentum conservation laws, one can obtain that the CM threshold is approximately 4E g (Figure 1a).In NQDs, translational momentum conservation is relaxed. Therefore, one might expect that the CM threshold could reach the 2E g limit as defined by energy conservation. However, available experimental data indicate that pω CM is still greater than twice the energy gap in such materials. For example, in PbSe and PbS NQDs, it ...

Assessment of carrier-multiplication efficiency in bulk PbSe and PbS

“…Hence, a direct comparison between carrier multiplication in bulk and quantum dots requires assessment of bulk carriermultiplication efficiencies on a similar, ultrafast, timescale (see Supplementary Information). Whereas time-resolved optical and infrared spectroscopies are ideally suited to probe carrier populations in colloidal quantum dots 2,5,7,8,10,11,14,15,[17][18][19][22][23][24] , light of terahertz frequencies interacts strongly with free carriers and allows for the direct characterization of carrier density and mobility [25][26][27] . Here, we quantify carrier multiplication in bulk PbSe and PbS on ultrafast timescales using terahertz time-domain spectroscopy 26 (THz-TDS).…”

Exciton Dynamics in InSb Colloidal Quantum Dots

“…In particular, the measured biexciton recombination times, 4 attributed to Auger processes, are one order of magnitude faster in InSb CQDs than observed [5][6][7] and predicted 8 in materials such as CdSe, whereas the electron cooling times, also attributed to Auger decay, are in line with those commonly found in nanostructures. 5,6,[8][9][10][11][12] These observations raise the question of whether, unlike in the case of other materials, different (intra-band and inter-band) non-radiative decay processes may be governed by different (i.e., Auger and non-Auger) mechanisms in InSb CQDs. (The unexpectedly small interlevel energy spacings within the conduction band of InSb CQDs resulting from very recent scanning tunneling spectroscopy and atomic force microscopy measurements 13 also remain unexplained.…”

Distribution of Multiexciton Generation Rates in CdSe and InAs Nanocrystals

“…band-gap independent [4,5,11,12]. However, several recent studies have questioned the efficiency of CM in NCs, in particular for CdSe [13] and InAs [14].…”