Probing the Origin of the Open Circuit Voltage in Perovskite Quantum Dot Photovoltaics

Abstract

Perovskite quantum dots (PQDs) have many properties that make them attractive for optoelectronic applications, including expanded compositional tunability and crystallographic stabilization. While they have not achieved the same photovoltaic (PV) efficiencies of top-performing perovskite thin films, they do reproducibly show high open circuit voltage (V OC) in comparison. Further understanding of the V OC attainable in PQDs as a function of surface passivation, contact layers, and PQD composition will further progress the field and may lend useful lessons for non-QD perovskite solar cells. Here, we use photoluminescence-based spectroscopic techniques to understand and identify the governing physics of the V OC in CsPbI3 PQDs. In particular, we probe the effect of the ligand exchange and contact interfaces on the V OC and free charge carrier concentration. The free charge carrier concentration is orders of magnitude higher than in typical perovskite thin films and could be tunable through ligand chemistry. Tuning the PQD A-site cation composition via replacement of Cs+ with FA+ maintains the background carrier concentration but reduces the trap density by up to a factor of 40, reducing the V OC deficit. These results dictate how to improve PQD optoelectronic properties and PV device performance and explain the reduced interfacial recombination observed by coupling PQDs with thin-film perovskites for a hybrid absorber layer.

Inhibiting lattice distortion of CsPbI3 perovskite quantum dots for solar cells with efficiency over 16.6%

“…Compared to the bulk CsPbI 3 perovskite, PQDs could generate an order of magnitude higher concentration of free carriers by tailoring their surface ligand chemistry, allowing higher open-circuit voltage ( V OC ) in solar cells. 6 Meanwhile, PQDs with larger surface-to-volume ratios over bulk counterparts can maintain their better black phase (α-, β-, or γ-phase) stability by leveraging their surface energy. 7 Furthermore, PQDs could be crystallized in large batches during the colloidal synthesis, eliminating limitations of the preferential crystallization process that is essential for bulk perovskites.…”

Nanoscale Photoexcited Carrier Dynamics in Perovskites

“…Also shown in Figure e are time-resolved microwave conductivity (TRMC) , and time-resolved photoluminescence (TRPL) , traces taken from sibling samples (see Supporting Information for experimental details). The comparable lifetimes of TRMC and MIM are expected, as both probe the microwave conductivity and are sensitive to a linear superposition of the electron and hole conductivities σ tot = σ h + + σ e – , though they probe different regions of the sample.…”

Ion-Assisted Ligand Exchange for Efficient and Stable Inverted FAPbI3 Quantum Dot Solar Cells

“…As compared to the pristine film, the PL intensities of the ligand-exchanged films drop by over six times (Figure 3a), which agrees with previous reports on the PL quantum yield of QDs during each step of QDSC fabrication. 18 While the MeOAc and GAOAc films show similar PL intensities, the PEAOAc film shows slightly higher PL intensity, possibly due to the passivation effect of the surface-anchored PEA + . The PL intensity of the FAOAc film is much higher than those of other ligand-exchanged QD films, indicating a better suppression of nonradiative recombination.…”

“…14,15 Recent works have demonstrated perovskite QD solar cells (QDSCs) based on phase-stable α-CsPbI 3 QDs and α-FAPbI 3 QDs. 16,17 Compared to their bulk counterparts, perovskite QDSCs usually possess a low opencircuit voltage (V OC ) deficit but large short-circuit current (J SC ) loss, 18 which highlights the importance of charge transport improvement and defect reduction in the research of perovskite QDSCs. Perovskite QD consists of a perovskite nanocrystalline core and an organic ligand shell, usually composed of both oleate (OA − ) and oleylammonium (OAm + ).…”