Detecting trap states in planar PbS colloidal quantum dot solar cells

Abstract

The recently developed planar architecture (ITO/ZnO/PbS-TBAI/PbS-EDT/Au) has greatly improved the power conversion efficiency of colloidal quantum dot photovoltaics (QDPVs). However, the performance is still far below the theoretical expectations and trap states in the PbS-TBAI film are believed to be the major origin, characterization and understanding of the traps are highly demanded to develop strategies for continued performance improvement. Here employing impedance spectroscopy we detect trap states in the planar PbS QDPVs. We determined a trap state of about 0.34 eV below the conduction band with a density of around 3.2 × 10 16 cm −3 eV −1 . Temperature dependent open-circuit voltage analysis, temperature dependent diode property analysis and temperature dependent build-in potential analysis consistently denotes an below-bandgap activation energy of about 1.17-1.20 eV.PbS colloidal quantum dots (PbS QDs) are attractive materials for next generation photovoltaic devices due to their facile solution processing, low material cost, long term air stability and possibility of tailoring their optoelectronic properties by tuning size, composition and surface chemistry 1-4 . However, PbS QDs in solution are typically surrounded by long aliphatic ligands and once they form solid film, the long ligands act as barriers for charge transfer and transport between neighboring QDs 5-7 . The ligand-exchange procedure, which is used to remove such long ligands can create many surface traps such as vacancies and dangling bonds 8,9 , these traps assist carrier recombination, and hence seriously limit the device performance 10,11 . Great efforts have been put in developing surface passivation approaches to reduce the traps, and power conversion efficiency (PCE) of PbS quantum dot photovoltaics (QDPVs) has been significantly improved (over 10%) [12][13][14][15] . Nevertheless, the achieved PCE is still far below the expected and the surface traps remains a key limiting factor for PbS QDPVs [16][17][18] .Trap states in the PbS QDPVs has been carefully investigated to understand how they limit the device performance. Here we employ Impedance Spectroscopy (IS) to obtain the information of the trap states in our fabricated device with currently most advanced architecture of ITO/ZnO/PbS-TBAI/PbS-EDT/Au (shown in Fig. 1(a)) 12,13 . In such a device the diode property is from ZnO/PbS-TBAI heterojunction, and the PbS-EDT layer acts as an electron-blocking layer between the PbS-TBAI layer and the Au anode 24 . For the ZnO/PbS-TBAI heterojunction, after suitable illumination (mainly for the UV region), the ZnO doping density is much higher than the PbS-TBAI doping density, thereby can be regarded as a N + P abrupt junction, with the depletion region locates in the PbS-TBAI film 25,26 . In our study we use AM 1.5 illumination (10 min AM 1.5 illumination) to generate a N + P ZnO/PbS-TBAI heterojunction, and then performed various measurements including current-voltage and capacitance-voltage under various temperatures to gain information in...

Europium and Acetate Co‐doping Strategy for Developing Stable and Efficient CsPbI2Br Perovskite Solar Cells

“…In order to understand the increase of V oc , capacitance–voltage (C −2 –V) measurements were performed to further investigate the change of the built‐in electric field of the CsPbI 2 Br PSCs with the Eu(Ac) 3 dopant. Figure S5, Supporting Information, shows the C −2 –V plots for the CsPbI 2 Br PSCs, from which the built‐in potential was obtained based on the Mott–Schottky equation of C −2 = (2( V bi − V ))/( A 2 eεε 0 N A ), where C is the capacitance under applied voltage ( V ), V bi is the built‐in potential, V is the applied voltage, A is the device area, ε is the relative permittivity, ε 0 is the vacuum permittivity, and N A is the carrier concentration . The V bi for the PSC made with 0.5 mol% Eu(Ac) 3 dopant is 1.02 V, which is larger than 0.95 V as obtained for the control device.…”

Revealing Trap States in Lead Sulphide Colloidal Quantum Dots by Photoinduced Absorption Spectroscopy

“…[13,14] For PbS treated with tetrabutylammonium iodide (TBAI) a trap distribution with an activation energy of 0.34 eV was found via temperature-dependent impedance spectroscopy. [15] Using optical pump-probe spectroscopy, the groups of Asbury and Sargent observed a broad photoinduced absorption (PIA) around 0.3 eV with a long lifetime (still visible after 200 µs) for a range of ligands, including EDT and MPA, and attributed this observation to surface trap states. [16,17] Bakulin et al finally reported two distributions, independent of each other, at activation energies of 0.2 and 0.3-0.5 eV for EDT and MPA capped PbS, distinguishable by their lifetimes.…”

Anorganische CsPbX3‐Perowskit‐Solarzellen: Fortschritte und Perspektiven

“…Tatsächlich wurden allein im Jahr 2017 weltweit über 100 GW Solarmodule installiert, wobei der Markt immer noch von kristallinen Silicium-Modulen, den Solarzellen der ersten Generation, beherrscht wird. [3] Sie machten 2017 rund 5%der insgesamt hergestellten Module aus.Z ud en Zellen dritter Generation gehçren eine Reihe von Dünnschichtzellen, darunter farbstoffsensibilisierte, [4] Polymer-, [5] Quantenpunkt- [6,7] und Halogenidperowskit(HaP)-Zellen. Hinzu kommen anwendungsabhängige Nachteile wie die relativ schlechte Leistung bei hohen Te mperaturen und unter schwachen Lichtverhältnissen sowie ihre starre und relativ schwer Bauart.…”

“…So ist beispielsweise die bei RT stabile Phase von CsPbI 3 die gelbe d-Phase,d ie als Photovoltaik-Material relativ inaktiv ist, während die sogenannte schwarze Phase (a-Phase) bei RT nur unter besonderen Bedingungen stabil ist. [25] Senkt man die Temperatur von dem Bereich ab,i nd em die kubische "Hochtemperatur"-Phase stabil ist, verformen sich die PbX 6 der kubischen in die geringer symmetrische tetragonale Form und schließlich in die orthorhombische Form um. [24] Andererseits ist die stabile Form von CsPbBr 3 bei RT die orthorhombische g-Form.…”