Joseph Boisvert

III-V multijunction solar cells for concentrating photovoltaics

Concerns about the changing environment and fossil fuel depletion have prompted much controversy and scrutiny. One way to address these issues is to use concentrating photovoltaics (CPV) as an alternate source for energy production. Multijunction solar cells built from III–V semiconductors are being evaluated globally in CPV systems designed to supplement electricity generation for utility companies. The high efficiency of III–V multijunction concentrator cells, with demonstrated efficiency over 40% since 2006, strongly reduces the cost of CPV systems, and makes III–V multijunction cells the technology of choice for most concentrator systems today. In designing multijunction cells, consideration must be given to the epitaxial growth of structures so that the lattice parameter between material systems is compatible for enhancing device performance. Low resistance metal contacts are crucial for attaining high performance. Optimization of the front metal grid pattern is required to maximize light absorption and minimize I2R losses in the gridlines and the semiconductor sheet. Understanding how a multijunction device works is important for the design of next-generation high efficiency solar cells, which need to operate in the 45%–50% range for a CPV system to make better economical sense. However, the survivability of solar cells in the field is of chief concern, and accelerated tests must be conducted to assess the reliability of devices during operation in CPV systems. These topics are the focus of this review.

Direct Semiconductor Bonded 5J Cell for Space and Terrestrial Applications

Spectrolab has demonstrated a 2.2/1.7/1.4/1.05/0.73 eV 5J cell with an efficiency of 37.8% under 1 sun AM1.5G spectrum and 35.1% efficiency for 1 sun AM0. The top three junctions and bottom two junctions were grown on GaAs and InP substrates, respectively, by metal organic vapor phase epitaxy. The GaAs- and InP-based cells were then direct bonded to create a low-resistance, high-transmissive interface. Both the space and terrestrial cells have high 1 sun Voc between 4.75 and 4.78 V. Initial tests of the terrestrial cells at concentration are promising with efficiencies increasing up to 10× concentration to a maximum value close to 41%.

Origin of dark counts in In0.53Ga0.47As∕In0.52Al0.48As avalanche photodiodes operated in Geiger mode

A dark count rate in InP-based single photon counting avalanche photodiodes is a limiting factor to their efficacy. The temperature dependence of the dark count rate was studied to understand its origin in In0.53Ga0.47As∕In0.52Al0.48As separate-absorption-charge-multiplication avalanche photodiodes. The dark count rate was observed to be a very weak function of temperature in the range from 77Kto300K. Various mechanisms for dark count generation were considered. Simulations of band-to-band tunneling in the In0.52Al0.48As multiplication layer were found to agree well with the experimental temperature dependence of dark count rate at various excess biases. To reduce tunneling-induced dark counts, a suitable design change to the detector structure is proposed.

Development of advanced space solar cells at Spectrolab

High efficiency multi-junction solar cells utilizing inverted metamorphic1,2 and semiconductor bonding technology3 are being developed at Spectrolab for use in one-sun space and near-space applications. Recently that effort has been extended to include low-concentration space applications. This paper will review the present state-of-the-art cell technologies at Spectrolab, with an emphasis on performance characterization data at both 1-sun and low-concentration operating conditions that these cells will experience in flight‥ A cell coupon utilizing IMM solar cells has been assembled and subjected to thermal cycling. Pre-and post thermal cycling data have been collected and there is no performance degradation or mechanical issues after the test.

35.8% space and 38.8% terrestrial 5J direct bonded cells

Spectrolab has fabricated a direct semiconductor bonded space solar cell with an efficiency of 35.8% under the AM0 space spectrum. Using a similar technology, Spectrolab has achieved a 5-junction (5J) direct bonded terrestrial cell with a record efficiency of 38.8% under the one-sun AM1.5G terrestrial spectrum. Efforts to further improve the 5J cell efficiency have focused on development of the top 3 junctions (T3J) grown on GaAs. Experiments with top 3J isotype cells have yielded an improvement of 1% in current and 100 mV in voltage for the T3J. Spectrolab has also made significant improvements in its direct bonding process. The improved process has increased bond strengths by more than a factor of 5 and eliminated issues with large voids.

Semiconductor-bonded III–V multijunction space solar cells

Boeing-Spectrolab recently demonstrated monolithic 5-junction space solar cells using direct semiconductor-bonding technique. The direct-bonded 5-junction cells consist of (Al)GaInP, AlGa(In)As, Ga(In)As, GaInPAs, and GaIn(P)As subcells deposited on GaAs or Ge and InP substrates. Large-area, high-mechanical strength, and low-electrical resistance direct-bonded interface was achieved to support the high-efficiency solar cell structure. Preliminary 1-sun AM0 testing of the 5-junction cells showed encouraging results. One of the direct-bonded solar cell achieved an open-circuit-voltage of 4.7V, a short-circuit current-density of 11.7 mA/cm2, a fill factor of 0.79, and an efficiency of 31.7%. Spectral response measurement of the five-junction cell revealed excellent external quantum efficiency performance for each subcell and across the direct-bonded interface. Improvements in crystal growth and current density allocation among subcells can further raise the 1-sun, AM0 conversion efficiency of the direct-bonded 5-junction cell to 35 – 40%.

Single photon counting Geiger mode InGaAs(P)/InP avalanche photodiode arrays for 3D imaging

We have designed, fabricated and characterized InGaAs/InP Geiger-mode avalanche photodiode (APD) 32 x 32 arrays optimized for operation at both 1.06 and 1.55 μm wavelengths Single element devices with a thick multiplication layer thickness showed dark count rate as low as 60 kHz at a 3 V overbias, while photon detection efficiencies at a wavelength of 1.55 μm exceed 30% at 2 V overbias. Back illuminated 32 x 32 detector arrays exhibited breakdown uniformity of greater than 97% and excellent dark current uniformity. Detector arrays were integrated with low-noise read-out integrated circuits for an imaging demonstration. 3D imaging was demonstrated using 1.06 micron detector arrays.

32 x 32 Geiger-mode ladar camera

For the wide applications of LAser Detection and Ranging (LADAR) imaging with large format Geiger-mode (GM) avalanche photodiode (APD) arrays, it is critical and challenging to develop a LADAR camera suitable to volume production with enough component tolerance and stable performance. Recently Spectrolab and Black Forest Engineering developed a new 32x32 Read-Out Integrated Circuit (ROIC) for LADAR applications. With a specially designed high voltage input protection circuit, the ROIC can work properly even with more than 1 % of pixels shorted in the APD array; this feature will greatly improve the camera long-term stability and manufacturing throughput. The Non-uniform Bias circuit provides bias voltage tunability over a 2.5 V range individually for each pixel and greatly reduces the impact of the non-uniformity of an APD array. A SMIA high speed serial digital interface streamlines data download and supports frame rates up to 30 kHz. The ROIC can operate with a 0.5 ns time resolution without vernier bits; 14 bits of dynamic range provides 8 μs of range gate width. At the meeting we will demonstrate more performance of this newly developed 32x32 Geiger-mode LADAR camera.

Direct Semiconductor Bonding Technology (SBT) for high efficiency III-V multi-junction solar cells

In recent years, the concept of wafer bonding has attracted significant attention since this approach allows direct integration of lattice-mismatched hetero-structure devices grown on different substrates by eliminating the major limitations of lattice matching requirements. The wafer bonding approach is particularly attractive for III-V based device structures since elimination of the lattice matching constraints allows novel device designs with otherwise impossible band-gap combinations, such as in multi-junction photo-voltaic devices with subcell band gaps tailored for optimum absorption of the solar spectrum. Several other optoelectronic applications, such as photonic crystals, resonant cavity photo-detectors and surface emitting lasers, have also extensively investigated the wafer bonding approach [1]. For III-V based photo-voltaic devices in particular, which have continued to yield the highest conversion efficiency amongst all photo-voltaic technologies [2], the possibility of integration of devices grown lattice matched to GaAs and InP with optimum band-gap combinations provides a way to break the barrier posed by lattice matching constraints and achieve even higher conversion efficiencies. Recently, fabrication of a simple 2-junction solar cell has been reported by Tanabe et al [3] via direct bonding of GaAs and InP wafers.

Geiger-mode ladar cameras

The performance of Geiger-mode LAser Detection and Ranging (LADAR) cameras is primarily defined by individual pixel attributes, such as dark count rate (DCR), photon detection efficiency (PDE), jitter, and crosstalk. However, for the expanding LADAR imaging applications, other factors, such as image uniformity, component tolerance, manufacturability, reliability, and operational features, have to be considered. Recently we have developed new 32×32 and 32×128 Read-Out Integrated Circuits (ROIC) for LADAR applications. With multiple filter and absorber structures, the 50-μm-pitch arrays demonstrate pixel crosstalk less than 100 ppm level, while maintaining a PDE greater than 40% at 4 V overbias. Besides the improved epitaxial and process uniformity of the APD arrays, the new ROICs implement a Non-uniform Bias (NUB) circuit providing 4-bit bias voltage tunability over a 2.5 V range to individually bias each pixel. All these features greatly increase the performance uniformity of the LADAR camera. Cameras based on these ROICs were integrated with a data acquisition system developed by Boeing DES. The 32×32 version has a range gate of up to 7 μs and can cover a range window of about 1 km with 14-bit and 0.5 ns timing resolution. The 32×128 camera can be operated at a frame rate of up to 20 kHz with 0.3 ns and 14-bit time resolution through a full CameraLink. The performance of the 32×32 LADAR camera has been demonstrated in a series of field tests on various vehicles.