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 mm 2 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/cm 2 ), 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/cm 2 ). Measurements at different temperatures revealed a Voc reduction of À6 mV/°C at~59 W/cm 2 .

Measurement of strong photon recycling in ultra‐thin GaAs n/p junctions monolithically integrated in high‐photovoltage vertical epitaxial heterostructure architectures with conversion efficiencies exceeding 60%

“…Also as shown here, because the VEHSA devices do not display the series resistance and the shunting behavior, they can readily convert in excess of 3 W of output power with an efficiency over 60%. The behavior of the VEHSA devices away from the peak of their spectral response can be studied by using a low input power EQE measurement or by scanning a tunable laser across the wavelength range of interest. Figure shows the I – V curves obtained with a PT6 device while probing with a ∼1.4 W tunable laser input.…”

“…Furthermore, high photovoltages and conversion efficiencies have been reported with GaAs side by side planar or vertical arrangements, in particular at high optical intensities with light management or in other material systems . Moreover, record‐high photovoltages and unprecedented monochromatic conversion efficiencies have now been demonstrated for phototransducer applications with the vertical epitaxial heterostructure architecture (VEHSA) design which is engineered with multiple thin, partially absorbing, subcells of the same material . The monolithic epitaxial design of the VEHSA devices naturally allows to construct n/p junction devices with exceptionally low shunting and series resistance issues.…”

“…An output power of 5.87 W is reported and an open circuit voltage enhancement of 92 meV per n/p junction is measured compared to a device with a side by side planar architecture. engineered with multiple thin, partially absorbing, subcells of the same material [27][28][29][30][31][32]. The monolithic epitaxial design of the VEHSA devices naturally allows to construct n/p junction devices with exceptionally low shunting and series resistance issues.…”

High-photovoltage GaAs vertical epitaxial monolithic heterostructures with 20 thin p/n junctions and a conversion efficiency of 60%

“…Here, we present results obtained with 3.7 mm  3.7 mm devices equipped with typical antireflection coatings and gridlines. As reported previously, 14 the structure uses a 1 lm InGaP window with a doping of n տ 1E18 cm 3 and here using AlGaAs nþþ/pþþ tunnel junctions with an Al alloy <30%. We have shown previously that the non-uniform and partial illumination of the active area of VEHSA devices typically does not degrade significantly the conversion efficiency.…”

“…These multi-subcell photovoltaic devices are featuring record conversion efficiencies. [12][13][14][15][16][17][18][19][20][21][22] However, while the upper limit of the p/n junction thickness has been well characterized in the past for GaAs or other III-V semiconductor alloys, the lower limit of the base thickness is still unknown despite the great progress in the field recently. [23][24][25][26][27][28] In this work, we therefore explore the thinnest p/n junctions achievable in practice in GaAs VEHSA structures.…”