Ultrathin high band gap solar cells with improved efficiencies from the world's oldest photovoltaic material

Abstract

Selenium was used in the first solid state solar cell in 1883 and gave early insights into the photoelectric effect that inspired Einstein's Nobel Prize work; however, the latest efficiency milestone of 5.0% was more than 30 years ago. The recent surge of interest towards high-band gap absorbers for tandem applications led us to reconsider this attractive 1.95 eV material. Here, we show completely redesigned selenium devices with improved back and front interfaces optimized through combinatorial studies and demonstrate record open-circuit voltage (V _OC) of 970 mV and efficiency of 6.5% under 1 Sun. In addition, Se devices are air-stable, non-toxic, and extremely simple to fabricate. The absorber layer is only 100 nm thick, and can be processed at 200 ˚C, allowing temperature compatibility with most bottom substrates or sub-cells. We analyze device limitations and find significant potential for further improvement making selenium an attractive high-band-gap absorber for multi-junction device applications.Wide band gap semiconductors are important for the development of tandem photovoltaics. By introducing buffer layers at the front and rear side of solar cells based on selenium; Todorov et al., reduce interface recombination losses to achieve photoconversion efficiencies of 6.5%.

Fig. 1

a Cross sectional scanning electron microscopy of the optimized device structure which consists of glass/SnO₂:F/n-type buffer (ZnMgO or TiO₂)/Se/MoO_x/Au. Only 100-nm-thick Se absorber is used. b A schematic of combinatorial sputtering setup for accelerated studies on ZnMgO n-type buffer layers. A color map indicates the band gap (E _g) of ZnMgO thin films measured by optical absorption spectra. The measured film thickness values in the map indicate a gradient of deposition rate

Fig. 2

a As-measured current–voltage (J–V) characteristics of Se solar cell devices under dark (dashed lines) and illuminated (solid lines) condition, showing the effect of buffer and hole-transport layer. ZnMgO/Se/MoO_x/Au, TiO₂/Se/MoO_x/Au, and TiO₂/Se/Au devices are shown in red, black, and purple, respectively. b Illuminated J–V plots of ZnMgO/Se/MoO_x/Au devices with as-deposited Se, annealed at 200 ˚C, 5-day aged after annealing, and 5-month aged after annealing Se are shown in purple, black, orange and red. c Raman spectra of as-deposited Se (purple) and annealed Se at 200 °C (red). The annealed Se shows convoluted peaks at 235 and 237 cm⁻¹. d External quantum efficiency spectrum of a ZnMgO/Se/MoO_x/Au device

Fig. 3

Effects of band gap (E _g) of ZnMgO buffer layer on the photovoltaic characteristics. a Power conversion efficiency, b Open-circuit voltage (V _OC), c Sort circuit current (J _SC), and d fill factor of the ZnMgO/Se/MoO_x/Au devices prepared by the combinatorial deposition process are plotted against measured E _g of ZnMgO layer. Square symbols and error bars indicate average values and standard deviations, respectively

Fig. 4

a Femtosecond ultraviolet photoelectron spectroscopy (fs-UPS) spectra of Se (red), TiO₂ (green), and ZnMgO (blue) under flat-band conditions. b Valence band and conduction band lineups extracted from the fs-UPS and optical absorption spectra

Fig. 5

Temperature-dependent J–V characteristics of the SnO₂:F/ZnMgO/Se/MoO_x/Au device. a Current–voltage (J–V) plots under 1-Sun illuminated (solid line) and dark (dashed line) conditions measured at the device temperature of 300 °K (black), 240 °K (purple), and 170 °K (blue). b Efficiency (black) and pseudo-efficiency (red) plots. c Fill factor (black) and pseudo-fill-factor (red). The pseudo-efficiency and pseudo-fill-factor were extracted from the short circuit current (J _SC)—open-circuit voltage (V _OC) measurements under various illumination intensities. d J _SC (black) and V _OC (red). e Series resistance (R _S) extracted from dark J–V measurements. f Shunt conductance (G _sh) extracted from the illuminated (red) and dark (black) J–V measurements