Pushing the limits of concentrated photovoltaic solar cell tunnel junctions in novel high-efficiency GaAs phototransducers based on a vertical epitaxial heterostructure architecture

Abstract

A monolithic compound semiconductor phototransducer optimized for narrow-band light sources was designed for achieving conversion efficiencies exceeding 50%. The III-V heterostructure was grown by metal-organic chemical vapor deposition, based on the vertical stacking of 5 partially absorbing GaAs n/p junctions connected in series with tunnel junctions. The thicknesses of the p-type base layers of the diodes were engineered for optimal absorption and current matching for an optical input with wavelengths centered near 830 nm. Devices with active areas of ∼3.4 mm2 were fabricated and tested with different emitter gridline spacings. The open circuit voltage (Voc) of the electrical output is five times or more than that of a single GaAs n/p junction under similar illumination. The device architecture allows for improved Voc generation in the individual base segments because of efficient carrier extraction while simultaneously maintaining a complete absorption of the input photons with no needs for complicated fabrication processes or reflecting layers. With illumination powers in the range of a few 100 mW, the measured fill factor (FF) varied between 88 and 89%, and the Voc reached over 5.75 V. The data also demonstrated that a proper combination of highly doped emitter and window layers without gridlines is adequate for sustaining such FF values for optical input powers of several hundred milliwatts. As the optical input power is further increased and approaches 2 W (intensities ∼58 W/cm2), the multiple tunnel junctions sequentially exceed their peak current densities in the case for which typical (n++)GaInP/ (p++)AlGaAs concentrated photovoltaic tunnel junctions are used. Lower bandgap tunnel junctions designed with improved peak current densities result in phototransducer devices having high FF and conversion efficiencies for up to 5 W optical input powers (intensities ∼144 W/cm2). Measurements at different temperatures revealed a Voc reduction of -6 mV/°C at ∼59 W/cm2.