Improved performance and stability in quantum dot solar cells through band alignment engineering

Abstract

Solution processing is a promising route for the realization of low-cost, large-area, flexible and lightweight photovoltaic devices with short energy payback time and high specific power. However, solar cells based on solution-processed organic, inorganic and hybrid materials reported thus far generally suffer from poor air stability, require an inert-atmosphere processing environment or necessitate high-temperature processing, all of which increase manufacturing complexities and costs. Simultaneously fulfilling the goals of high efficiency, low-temperature fabrication conditions and good atmospheric stability remains a major technical challenge, which may be addressed, as we demonstrate here, with the development of room-temperature solution-processed ZnO/PbS quantum dot solar cells. By engineering the band alignment of the quantum dot layers through the use of different ligand treatments, a certified efficiency of 8.55% has been reached. Furthermore, the performance of unencapsulated devices remains unchanged for over 150 days of storage in air. This material system introduces a new approach towards the goal of high-performance air-stable solar cells compatible with simple solution processes and deposition on flexible substrates.

Figure 1. Photovoltaic device architectures and performance

a, Device architectures. b, Representative J-V characteristics of devices with Au anodes under simulated AM1.5G irradiation (100 mW/cm²). The PbS-TBAI device consists of 12 layers of PbS-TBAI and the PbS-TBAI/PbS-EDT device consists of 10 layers of PbS-TBAI and 2 layers of PbS-EDT. c, EQE spectra.

Figure 2. Energy level diagrams of PbS QDs and photovoltaic devices containing the QDs

a, Energy levels with respect to vacuum for PbS-TBAI, PbS-EDT, and PbS-TBAI films covered with different thicknesses of PbS-EDT layers. The Fermi levels (E_F, dashed line) and valence band edges (E_V, blue lines) were determined by UPS. The conduction band edges (E_C, red lines) were calculated by adding the optical bandgap energy of 1.33 eV, as determined from the first exciton absorption peak in the QD thin films, to E_V. b, Schematic energy level alignment at PbS-TBAI and PbS-EDT interfaces deduced from UPS. E_VAC: vacuum level. c, Schematic illustration of band bending in ZnO/PbS-TBAI and ZnO/PbS-TBAI/PbS-EDT devices at short-circuit conditions.

Figure 3. Evolution of photovoltaic parameters with air storage time in devices with Au and MoO₃/Au anodes

a, V_OC. b, J_SC. c, FF. d, power conversion efficiency (PCE). Measurements were performed in a nitrogen-filled glovebox. Day 0 denotes measurements performed after anode evaporation in vacuum. Between each measurement, the unencapsulated devices were stored in air without any humidity control. The average (symbols) and standard deviation (error bars) were calculated based on a sample of between 6 and 9 devices on the same substrate.

Figure 4. Long-term stability assessment of unencapsulated devices with Au anodes

a, Evolution of photovoltaic parameters of PbS-TBAI (black) and PbS-TBAI/PbS-EDT(red) devices. Open symbols represent the average values and solid symbols represent the values for the best-performing device. b, Device performance of a PbS-TBAI/PbS-EDT device certified by an accredited laboratory after 37 days of air-storage.