2, California Institute of Technology, Pasadena, California, United States
3, California Institute of Technology, Pasadena, California, United States
The strong absorption and visible spectrum energy bandgaps for the transition metal dichalcogenides (TMDCs) of molybdenum and tungsten render them as attractive candidates for photovoltaics (PV) and optoelectronics. Further, the atomically thin nature is favorable for efficient separation and collection of photo-excited charge carriers. Therefore, if the three major optoelectronic criteria, i.e., i) sunlight absorption, ii) carrier collection and iii) operating voltage can be addressed for TMDC materials, they may be candidates for high efficiency photovoltaics. We have recently demonstrated near-unity broad-band absorption of above band-gap photons for < 15 nm TMDC layers.1 This has been further extended to concurrently achieve high external quantum efficiency in < 10 nm thick active layer photovoltaic devices in a p-n junction of WSe2/MoS2 with graphene contacts2, thereby addressing the first two criteria. However, achievement of high open-circuit voltage (Voc) remains an outstanding challenge towards achieving high photovoltaic efficiency.
Here, we experimentally demonstrate high open-circuit voltages (Voc) in TMDC absorbers based photovoltaic devices. We achieve this by tailoring the conduction and valence band-alignments between a single TMDC absorber layer and carrier-selective contact layers for electron collection (titanium oxide) and hole collection (nickel oxide), respectively. We further verify the band alignments using X-ray photoelectron spectroscopy to demonstrate the asymmetric and selective nature of the metal oxide carrier-selective contacts. We observe open circuit voltage values exceeding 700 mV under AM 1.5 illumination at 1 Sun, a record for TMDC-based photovoltaic devices. In contrast, the current maximum Voc in similar class of devices is limited to 300-400 mV. We will also present detailed electrical characterization of these devices, combined with coupled optoelectronic simulations combining device physics modeling with full wave electromagnetic simulations of optical absorption. Devices form using both sulfides and selenides of molybdenum and tungsten will be presented, in a comparative analysis of performance, material quality, doping levels and band alignments. Our results presented here will serve as guiding design principles towards overcoming the final major hurdle in achieving high power conversion efficiency in devices with ultrathin van der Waals absorber layers.3
1. Jariwala, D.; Davoyan, A. R.; Tagliabue, G.; Sherrott, M. C.; Wong, J.; Atwater, H. A. Nano Lett. 2016, 16, (9), 5482-5487.
2. Wong, J.; Jariwala, D.; Tagliabue, G.; Tat, K.; Davoyan, A. R.; Sherrott, M. C.; Atwater, H. A. ACS Nano 2017, 11, 7230–7240.
3. Jariwala, D.; Wong, J.; Davoyan, A. R.; Atwater, H. A. ACS Photonics 2017, doi: 10.1021/acsphotonics.7b01103.