High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion

Abstract

We demonstrate for the first time that impact ionization (II) [the inverse of Auger recombination (AR)] occurs with very high efficiency in semiconductor nanocrystals (NCs). Interband optical excitation of PbSe NCs at low pump intensities, for which less than one exciton is initially generated per NC on average, results in the formation of two or more excitons (carrier multiplication) when pump photon energies are more than three times the NC band gap energy. Generation of multiexcitons from a single photon absorption event is observed to take place on an ultrafast (picosecond) timescale and occurs with up to 100% efficiency depending upon the excess energy of the absorbed photon. Efficient II in NCs can be used to considerably increase the power conversion efficiency of NC-based solar cells. [2,6]. II is an Auger-type process whereby a high-energy exciton, created in a semiconductor by absorbing a photon of energy ≥2E g , relaxes to the band edge via energy transfer of at least 1E g to a valence band electron, which is excited above the energy gap ( Fig. 1(a)). The result of this energy transfer process is that two excitons are formed for one absorbed photon. Thus, this process converts more of the high photon energy portion of the solar spectrum into usable energy.Here, for the first time, we demonstrate carrier multiplication via II in NCs. By directly monitoring exciton conversion to biexcitons in the time domain, we show that II in PbSe NCs is highly efficient, extremely fast, and occurs in a wavelength range that has potential to provide significantly increased solar cell power conversion efficiency.2 Schaller et al.Auger recombination (AR), the opposite of II, is a process in which an exciton recombines via energy transfer to an electron (or hole) that is excited to a higher energy state within or outside a NC (Fig. 1(b)). Because of restrictions imposed by energy and momentum conservation, AR is inefficient in bulk materials. However, AR becomes efficient in NCs due to enhanced Coulomb interactions and relaxation of momentum conservation [7][8][9][10]. Because of symmetry of the matrix elements that describe both II and AR, the former can also be very efficient in quantum-confined systems.Here, we use transient absorption (TA) to monitor carrier population dynamics in high quality, oleic acid-passivated, colloidal PbSe NC samples [11] (size dispersity was ~5-10%, studied NC diameters were ~4 to 6 nm). Pump pulses (50 fs) from an amplified Ti-sapphire laser (pump photon energies, ћω = 1.55 or 3.10 eV) or from a tunable optical parametric amplifier (OPA) excited NCs dissolved in hexane. The absorption change, ∆α, within the photo-excited spot is probed with 100 fs pulses that are tuned via another OPA to the band-edge (A 1 ) absorption maximum. As a measure of excitation density, we use an average number of photo-generated e-h pairs per NC, N eh , produced by the pump pulse that we can accurately calculate and experimentally verify [9]. One important problem in experimental studies of II is to rel...

Optical Investigation of Quantum Confinement in PbSe Nanocrystals at Different Points in the Brillouin Zone

“…[7][8][9][10] The mechanism for MEG, however, is still under debate. Two theoretical papers used impact ionization (II) to explain the ultrafast carrier multiplication.…”

“…[3][4][5][6] For example, PbSe QDs hold great promise in the field of photovoltaics because of the recently observed multiple exciton generation (MEG) in these nanocrystals. [7][8][9][10] Here we report on detailed investigations of the optical absorption spectra of PbSe QDs. First, the assignment of the multiple features (up to 11) in the absorption spectrum of highly monodisperse PbSe QDs is discussed and compared to calculated spectra of PbSe QDs that were recently published.…”

Schottky Solar Cells Based on Colloidal Nanocrystal Films

“…Demonstrating an EQE above 100% at hυ ) 3E g to 4E g in a PbSe NC PV cell would provide unequivocal proof that multiple exciton generation 11 (MEG), observed in many colloidal semiconductor NCs [12][13][14][15][16][17][18][19] and in NC films, 20 can be observed and exploited in devices. However, because our cells show a maximum EQE of only ∼65%, we must rely on a determination of the IQE or a comparison of the shape of the EQE and optical absorption spectra to identify any anomalously large photocurrents that may be attributable to MEG.…”

Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots

“…12 The magnitude of the photoinduced absorption change at the band edge is proportional to the number of electron-hole pairs created in the sample. The transients are detected by probing either with a band edge (energy gap or HOMO-LUMO transition energy ≡ E g ) probe pulse, or with a mid-IR probe pulse that monitors intraband transitions in the newly created excitons.…”

“…The biexciton effect does not influence the observed MEG efficiency results, which are based on an analysis of scaled dynamics and not absolute magnitudes. 12 Utilizing an infrared probe near the intraband 1S e -1P e (or 1S h -1P h ) transition energy, we have conducted preliminary measurements of the rise time for pump photon energies producing multiple excitons. In these data, we observe a relatively slow (∼2 ps) and featureless rise time, and no sign of biexciton effect.…”