Research Overview nanoelectronics E-skin Programmable Materials

 Solar Energy

       Solar energy represents one of the most abundant and yet least harvested sources of renewable energy.  In recent years, tremendous progress has been made in developing solar devices that can be potentially mass employed with costs approaching and in some cases surpassing grid parity. Future research requires development of new material systems and device concepts that simultaneously offer high efficiencies and low processing costs. In this regard, our group has made a number of major advancements, including:


(i)              Development of a new thin film growth mode for the growth of ultra-large grain III-V semiconductor thin films on amorphous substrates with optoelectronic properties approaching those of epitaxially grown, single-crystal layers (Scientific Reports, 2013; Nature Comm, 2016).

(ii)             Development of high efficiency dopant free asymmetric heterocontact (DASH) silicon solar cells with all fabrication process steps performed near room temperature and cell efficiencies >20% (Nature Energy, 2016).

(iii)     Demonstration of the highest efficiency photocathodes for sunlight to fuel conversion using nanotextured InP (Angew. Chem. Int. Ed., 2012).

1. New growth mechanisms for non-epitaxial III-V photovoltaic devices

III-V semiconductors have exhibited the highest conversion efficiencies for solar cell devices to-date. However, their high costs, mainly associated with (i) the use of expensive single crystalline growth substrates and (ii) inefficient growth techniques with low material utilization yields and expensive precursors have prevented their large-scale use. To address these limitations, our group has recently demonstrated a thin-film vapor-liquid-solid (TF-VLS) growth mode which enables micron thick thin films with average grain sizes exceeding 100 microns (R. Kapadia, Z. Yu et al. Scientific Reports, 2013). The TF-VLS process has been implemented on insulators and metal films alike, and provides excellent materials utilization. The projected efficiencies of these cells, in the absence of parasitic contributions not fully optimized in the lab setting, can surpass 20% in our thin-film geometry with high materials.  The process has also been extended to the direct growth of single cyrstalline patterns on amorphous substrates (K. Chen, R. Kapadia, et al., Nature Comm, 2016).


2. Dopant free carrier selective contacts

In crystalline silicon solar cells, electron and holes are separated by either diffusing dopants into or depositing doped layers onto the two surfaces of the silicon wafer. Even with the well-proven efficacy of this approach, device performance can be limited via optoelectronic losses and technological issues associated with high temperature processing etc. An alternative to this approach is to instead use selective materials such as metal oxides or alkaline metal fluorides which achieve the same functionality without the same losses. These advancements have led to the development of high efficiency dopant free asymmetric heterocontact (DASH) silicon solar cells, the first of their kind to demonstrate competitiveness with conventional processes (J. Bullock, Nature Energy, 2016).

3. High efficiency sun light to fuel conversion

    In addition to the solar cell related activities, our group is actively engaged in developing efficient photoelectrochemical (PEC) cells that convert sun light to fuel (e.g., hydrogen).  Specifically, our group holds the highest efficiency photocathode based on a single absorber material by using nanotextured InP (M.H. Lee, et al, Angew. Chem., 2012) for water splitting.  We have shown that by properly texturing the surface of InP to control the surface wetting, the surface bubble formation during fuel generation is drastically reduced which is often a limiting factor in the performance of PEC cells. In addition, we have shown surface texturing to be essential in a PEC cell for antireflective (AR) properties given that conventional AR coating cannot be used since the semiconductor needs to be in direct contact with the electrolyte. As a result, a drastic improvement in the performance of InP PEC cells is demonstrated by nanotexturing the surface, thereby, enabling our group to demonstrate the highest performing photocathode reported to date. Although not as mature as solar cells, PEC devices present yet another platform for harvesting solar energy and present tremendous research opportunities moving forward by properly engineering materials and devices to enhance the conversion efficiencies of the photoelectrodes. This project presents an excellent example of interfacing chemistry with device physics by using materials innovation as a bridge.


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