Hybrid organic–inorganic inks flatten the energy landscape in colloidal quantum dot solids

Abstract

Bandtail states in disordered semiconductor materials result in losses in open-circuit voltage (V) and inhibit carrier transport in photovoltaics. For colloidal quantum dot (CQD) films that promise low-cost, large-area, air-stable photovoltaics, bandtails are determined by CQD synthetic polydispersity and inhomogeneous aggregation during the ligand-exchange process. Here we introduce a new method for the synthesis of solution-phase ligand-exchanged CQD inks that enable a flat energy landscape and an advantageously high packing density. In the solid state, these materials exhibit a sharper bandtail and reduced energy funnelling compared with the previous best CQD thin films for photovoltaics. Consequently, we demonstrate solar cells with higher V and more efficient charge injection into the electron acceptor, allowing the use of a closer-to-optimum bandgap to absorb more light. These enable the fabrication of CQD solar cells made via a solution-phase ligand exchange, with a certified power conversion efficiency of 11.28%. The devices are stable when stored in air, unencapsulated, for over 1,000 h.

Multiple exciton generation in quantum dot-based solar cells

“…Furthermore, the reducing character of molecules such as hydrazine has been shown to increase the n-type doping density of IV-VI QD films significantly [96]. Most efficient QD-based PV devices employ a heterojunction analogous to a p-n junction at an interface between an n-type MO and a depleted, p-type QD layer [82,103,104]. It follows that the significant, ligand-associated changes in QD energetics and doping level to promote MEG in a device environment (e.g.…”

“…There is general consensus that improved QD trap state passivation played a vital role in this improvement particularly in improving the typically poor V oc of QD devices [97,104]. The most effective method of surface passivation of QDs has been demonstrated to be a latticematched, wide band-gap shell material, grown on the particle core [85,87,88,105].…”

“…That is due to recently emerging concepts based on rigorous light-trapping techniques [140], solar photon concentration [141] and highly luminescent semiconductors [132,142], which are already being introduced in present day's PV market [143]. Similarly, techniques to reduce photovoltage losses in QD-based solar cells are put in the research focus of an increasing number of groups [95,104]. Interestingly, the resulting highly efficient solar cells do not necessarily rely on a fully depleted QD film for charge extraction but function similar to conventional bulk semiconductor devices where carrier diffusion is responsible for efficient charge transport to the electrodes.…”

“…Interestingly, the resulting highly efficient solar cells do not necessarily rely on a fully depleted QD film for charge extraction but function similar to conventional bulk semiconductor devices where carrier diffusion is responsible for efficient charge transport to the electrodes. It is worth noting that these improvements have been the result of refined QD material synthesis and deposition techniques, but increasingly due to adjustment in the device architecture [104,144].…”

PbSe Nanorod Field‐Effect Transistors: Room‐ and Low‐Temperature Performance

“…Most recently, the use of halide ions (Cl − , Br − , I − ) have led to rapid improvements in PbS/PbSe QD solar cell power conversion efficiencies (up to 11.3%) and long-term stability in air. [1,26] The proposed …”