Generating Free Charges by Carrier Multiplication in Quantum Dots for Highly Efficient Photovoltaics

Abstract

CONSPECTUS: In a conventional photovoltaic device (solar cell or photodiode) photons are absorbed in a bulk semiconductor layer, leading to excitation of an electron from a valence band to a conduction band. Directly after photoexcitation, the hole in the valence band and the electron in the conduction band have excess energy given by the difference between the photon energy and the semiconductor band gap. In a bulk semiconductor, the initially hot charges rapidly lose their excess energy as heat. This heat loss is the main reason that the theoretical efficiency of a conventional solar cell is limited to the Shockley-Queisser limit of ∼33%. The efficiency of a photovoltaic device can be increased if the excess energy is utilized to excite additional electrons across the band gap. A sufficiently hot charge can produce an electron-hole pair by Coulomb scattering on a valence electron. This process of carrier multiplication (CM) leads to formation of two or more electron-hole pairs for the absorption of one photon. In bulk semiconductors such as silicon, the energetic threshold for CM is too high to be of practical use. However, CM in nanometer sized semiconductor quantum dots (QDs) offers prospects for exploitation in photovoltaics. CM leads to formation of two or more electron-hole pairs that are initially in close proximity. For photovoltaic applications, these charges must escape from recombination. This Account outlines our recent progress in the generation of free mobile charges that result from CM in QDs. Studies of charge carrier photogeneration and mobility were carried out using (ultrafast) time-resolved laser techniques with optical or ac conductivity detection. We found that charges can be extracted from photoexcited PbS QDs by bringing them into contact with organic electron and hole accepting materials. However, charge localization on the QD produces a strong Coulomb attraction to its counter charge in the organic material. This limits the production of free charges that can contribute to the photocurrent in a device. We show that free mobile charges can be efficiently produced via CM in solids of strongly coupled PbSe QDs. Strong electronic coupling between the QDs resulted in a charge carrier mobility of the order of 1 cm(2) V(-1) s(-1). This mobility is sufficiently high so that virtually all electron-hole pairs escape from recombination. The impact of temperature on the CM efficiency in PbSe QD solids was also studied. We inferred that temperature has no observable effect on the rate of cooling of hot charges nor on the CM rate. We conclude that exploitation of CM requires that charges have sufficiently high mobility to escape from recombination. The contribution of CM to the efficiency of photovoltaic devices can be further enhanced by an increase of the CM efficiency above the energetic threshold of twice the band gap. For large-scale applications in photovoltaic devices, it is important to develop abundant and nontoxic materials that exhibit efficient CM.

Bandgap Engineering of Indium Phosphide-Based Core/Shell Heterostructures Through Shell Composition and Thickness

“…Semiconductor quantum dots (QDs) are used to study the fundamental photophysics of quantum confined systems, as well as, for numerous applications in photovoltaics, solid state lighting, biomedical imaging, and biosensing (Dennis et al, 2012b ; Chuang et al, 2014 ; Lan et al, 2014 ; Ten Cate et al, 2015 ; Kagan et al, 2016 ; Hong et al, 2017 ; Chandran et al, 2018 ; Kong L. et al, 2018 ; McHugh et al, 2018 ). Quantum confinement is easily observed in simple, single semiconductor nanocrystals (NCs): smaller cores are more quantum confined, resulting in an increase in the energy gap between the conduction and valence bands as observed through higher energy (bluer) photon emission following photoexcitation.…”

Broadband Cooling Spectra of Hot Electrons and Holes in PbSe Quantum Dots

“…1 PbSe QDs receive much attention due to their exceptionally strong quantum confinement properties, and in particular because of the occurrence of carrier multiplication (CM). 2−6 CM is a process in which one sufficiently energetic photon excites two or more electrons across the band gap. In this way the power conversion efficiency of a solar cell can be enhanced above the Shockley–Queisser limit.…”

Carrier multiplication detected through transient photocurrent in device-grade films of lead selenide quantum dots

“…While the conducted studies have indicated a significant promise of engineered nanostructures for obtaining enhanced CM, there are still a number of questions and challenges that need to be addressed to fully realize the potential of CM in practical devices. For example, the majority of quantitative insights into CM has been derived from optical spectroscopic studies of solutions of isolated QDs 29 30 31 , while practical devices use films of electronically coupled particles 20 22 32 33 34 35 . The available studies of QD films, however, do not provide a conclusive answer to the question on the effect of electronic coupling on multiexciton yields.…”