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Open Access Publications from the University of California

PbS and Ge Nanocrystals: A Pathway Towards Third Generation Photovoltaics

  • Author(s): Church, Carena
  • Advisor(s): Carter, Sue A
  • et al.
Abstract

Third-generation photovoltaics offer a way around the strict thermodynamic Shockley- Queissar limit of 33% for the efficiency of a single-junction solar cell by utilizing new physics to overcome the SQ limit while also remaining relatively cheap. This thesis deals with two pathways towards III-generation PV candidates: Pb-chalcogenide and Group IV quantum dots (QDs). QDs are attractive solar materials due to their low cost, solution processability, upscaleability, and tunable optical and electronic properties due to the quantum confinement effect. This tunability leads to bandgap engineering, enabling optimal bandgaps both for increased currents due to larger IR response and multiple exciton generation (MEG), the latter being particularly exciting as it could potentially surpass the SQ limit by creating multiple charge carriers from one incoming photon, enabling efficient collection of hot electrons whose excess energy would otherwise be wasted.

Pb-chalcogenides (PbX, where X= S, Se, Te) are good candidate QD systems to study, given their large exciton Bohr radii. Additionally, their robust syntheses offer fine control over size, and hence electric properties. In this work, we approach performance increases in two ways. First, we explore an alloyed PbSxSe1-x system to examine how the inclusion of small amount of Se (x=0.9) can lead to devices with simultaneously high photocurrents and voltages, leading to PCEs of 4.5% . Additionally, this alloyed ternary system exhibits EQE > 100 %, indicating MEG-like behavior and efficient photocurrent generation. Secondly, we use the PbS system and bandgap gradation techniques from traditional PV to increase the limited photovoltages found in QDSC. This route also serves to show the ease with which potential tandems or multijunction QDSC could be made.

Group IV materials are the traditional PV materials, with a large body of research behind them. They are also less toxic than both II-VI and IV-VI systems, increasing the interest in them as viable QDSC systems. However, the synthetic routes are much more complicated, thus systematic studies are largely nonexistent. Here, we use Ge QDs prepared via a facile, up scalable microwave synthesis that offers relatively good size control and crystallinity. These Ge QDs have been incorporated into both photoconductors and photovoltaic devices, while parameter space has been explored to optimize performance. We present the first all-nanocrystalline Ge solar cell, utilizing a donor/acceptor heterojunction structure with TiO2 as the window layer. After a simple ligand exchange, our Ge QDSCs are photoconductive and require no further anneals or surface treatments, potentially lowering future manufacturing costs. For our best TiO2- Ge QD heterojunction devices, short circuit currents of 450 μA and open circuit voltages of 0.335 V are achieved. Our low currents, compared to PbX chalcogenide QD systems, are explained via analysis of intensity-dependent current-voltage characteristics.

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