Photovoltaic & Optoelectronic Device Group

Abstract:

Successful chemical doping of metal halide perovskites with small amounts of heterovalent metals has attracted recent research attention because of its potential to improve long-term material stability and tune absorption spectra. However, some additives have been observed to impact negatively on optoelectronic properties, highlighting the importance of understanding charge-carrier behavior in doped metal halide perovskites. Here, we present an investigation of charge-carrier trapping and conduction in films of MAPbBr3 perovskite chemically doped with bismuth. We find that the addition of bismuth has no effect on either the band gap or exciton binding energy of the MAPbBr3 host. However, we observe a substantial enhancement of electron-trapping defects upon bismuth doping, which results in an ultrafast charge-carrier decay component, enhanced infrared emission, and a notable decrease of charge-carrier mobility. We propose that such defects arise from the current approach to Bi-doping through addition of BiBr3, which may enhance the presence of bromide interstitials.

Abstract:

Metal‐halide perovskites (MHP) are highly promising semiconductors for light‐emitting and photovoltaic applications. The colloidal synthesis of nanocrystals (NCs) is an effective approach for obtaining nearly defect‐free MHP that can be processed into inks for low‐cost, high‐performance device fabrication. However, disentangling the effects of surface ligands, morphology, and boundaries on charge‐carrier transport in thin films fabricated with these high‐quality NCs is inherently difficult. To overcome this fundamental challenge, terahertz (THz) spectroscopy is employed to optically probe the photoconductivity of CsPbBr3 NC films. The vibrational and optoelectronic properties of the NCs are compared with those of the corresponding bulk polycrystalline perovskite and significant deviations are found. Charge‐carrier mobilities and recombination rates are demonstrated to vary significantly with the NC size. Such dependences derive from the localized nature of charge carriers within NCs, with local mobilities dominating over interparticle transport. It is further shown that the colloidally synthesized NCs have distinct vibrational properties with respect to the bulk perovskite, exhibiting blue‐shifted optical phonon modes with enhanced THz absorption strength that also manifest as strong modulations in the THz photoconductivity spectra. Such fundamental insights into NC versus bulk properties will guide the optimization of nanocrystalline perovskite thin films for optoelectronic applications.

Abstract:

Abstract Controlling the transport and minimizing charge carrier trapping at interfaces is crucial for the performance of various optoelectronic devices. Here, how electronic properties of stable, abundant, and easy‐to‐synthesized carbon dots (CDs) are controlled via the surface chemistry through a chosen ratio of their precursors citric acid and ethylenediamine are demonstrated. This allows to adjust the work function of indium tin oxide (ITO) films over the broad range of 1.57 eV, through deposition of thin CD layers. CD modifiers with abundant amine groups reduce the ITO work function from 4.64 to 3.42 eV, while those with abundant carboxyl groups increase it to 4.99 eV. Using CDs to modify interfaces between metal oxide (SnO 2 and ZnO) films and active layers of solar cells and light‐emitting diodes (LEDs) allows to significantly improve their performance. Power conversion efficiency of CH 3 NH 3 PbI 3 perovskite solar cells increases from 17.3% to 19.5%; the external quantum efficiency of CsPbI 3 perovskite quantum dot LEDs increases from 4.8% to 10.3%; and that of CdSe/ZnS quantum dot LEDs increases from 8.1% to 21.9%. As CD films are easily fabricated in air by solution processing, the approach paves the way to a simplified manufacturing of large‐area and low‐cost optoelectronic devices.