PbS colloidal quantum dot solar-cell photovoltaic energy conversion mechanism studies for fundamental physical electronic process evaluation and device performance optimization.

Exciton kinetics in colloidal quantum dot (CQD) PbS thin films used for the fabrication of quantum dot solar cells were also studied using PCR [1,2]. Through the reconstruction of de-excitation lifetime spectra over broad ranges of temperatures, these studies resulted in the identification of radiative and non-radiative exciton decay pathways.


Excited state manifold of PbS exciton in the singlet−triplet energy range. Photon emission: sine wave arrows indicate radiative emission processes. Phonon emission: sawtooth arrows indicate nonradiative decay processes in the presence of photocarrier exciton traps [1].

To improve understanding of exciton transport and decay pathways that limit current CQD solar conversion efficiency, a full theoretical model of exciton transport processes across the effective Schottky barrier of PbS CQDs has been developed. Current-voltage curves in a wide range of temperatures (100 – 300K) have been used to determine the operating excitonic and dissociated carrier hopping transport [3] and exciton-interface  interaction mechanisms [4], in order to explain observed non-Shockley current-voltage characteristics of PbS CQD solar cells [5].




(a) CQD solar cell band-energy-equivalent structure. (b) Room temperature and (c) low temperature hole accumulation at the PbS CQD–MoO3 interface, also showing transport across the junction. (d) Solar cell architecture from top to bottom: laser beam, glass/FTO (back contact)/TiO2/PbS CQD/MoO3/Au/Ag (front contact). [4]


(a) Experimental set-up for J–V measurement as well as the two-diode electric circuit model of the TiO2/PbS QD/MoO3/Au solar cell. (b)–(f) Current–voltage characteristic curves of solar cell architecture: glass/FTO/TiO2/PbS QD/MoO3/Au/Ag obtained at 100 K, 150 K, 200 K, 250 K and 300 K. Continuous lines: best fits to data using eqn (35) in Ref. [4].


Equivalent electric circuit of a double-diode model as shown in the Figure above, consisting of a hetero- junction diode between ZnO and PbS-TBAI layers and a Schottky diode between PbS-EDT and Au [5].

References II.2

[1] Wang J, Mandelis A, Melnikov A, Hoogland S, Sargent EH. Exciton lifetime broadening and distribution profiles of PbS colloidal quantum dot thin films using frequency- and temperature-scanned photocarrier radiometry, J. Phys. Chem. C 2013; 117, 23333.

[2]  Wang J, Mandelis A. Variational reconstruction of exciton multipath deexcitation lifetime spectra in coupled PbS colloidal quantum dots, J. Phys. Chem. C 2014; 118 (33), 19484.

[3]  Hu L, Yang Z, Mandelis A, Melnikov A, Lan X, Walters G, Hoogland S, and Sargent E. H., Quantitative Analysis of Trap-State-Mediated Exciton Transport in Perovskite Shelled PbS Quantum Dot Thin films Using Photocarrier Diffusion-Wave Non-Destructive Evaluation and Imaging, J. Phys. Chem. C 120, 14416 – 14427 (June 22, 2016). DOI: 10.1021/acs.jpcc.6b04468

[4]   Mandelis A, Hu L, and Wang J, Quantitative Measurements of Exciton Hopping Transport Properties in Depleted-Heterojunction PbS Colloidal Quantum Dot Solar Cells from Temperature Dependent Current-Voltage Characteristics, Royal Soc. Chem. Advances  6, 93180–93194 (2016).

[5] Hu L, Mandelis A, Lan X, Melnikov A, Hoogland S, and Sargent E. H., Imbalanced Hole Mobility and Schottky Junction Induced Anomalous Current-Voltage Characteristics of Excitonic PbS Colloidal Quantum Dot Solar Cells, Solar Energy Materials Solar Cells 155, 155-165 (2016) http://dx.doi.org/10.1016/j.solmat.2016.06.012