Implications of low breakdown voltage of component subcells on external quantum efficiency measurements of multijunction solar cells

The electrical and optical coupling between subcells in a multijunction solar cell affects its external quantum efficiency (EQE) measurement. In this study, we show how a low breakdown voltage of a component subcell impacts the EQE determination of a multijunction solar cell and demands the use of a finely adjusted external voltage bias. The optimum voltage bias for the EQE measurement of a Ge subcell in two different GaInP/GaInAs/Ge triple‐junction solar cells is determined both by sweeping the external voltage bias and by tracing the I–V curve under the same light bias conditions applied during the EQE measurement. It is shown that the I–V curve gives rapid and valuable information about the adequate light and voltage bias needed, and also helps to detect problems associated with non‐ideal I–V curves that might affect the EQE measurement. The results also show that, if a non‐optimum voltage bias is applied, a measurement artifact can result. Only when the problems associated with a non‐ideal I–V curve and/or a low breakdown voltage have been discarded, the measurement artifacts, if any, can be attributed to other effects such as luminescent coupling between subcells. Copyright © 2015 John Wiley & Sons, Ltd.     

Positioning and doping effects on quantum dot multi‐junction solar cell performance

The positioning of InAs quantum dot (QD) layers in a triple‐junction GaInP/Ga(In)As/Ge solar cell is studied using numerical modeling techniques. An effective medium is used to describe the absorption characteristics and carrier dynamics in each QD layer. The effects of incorporating 110 layers in the emitter, base layers, as well as between these regions of the middle sub‐cell are analyzed with current–voltage characteristics and energy band diagrams. The cell with QDs positioned between the emitter and base demonstrated an efficiency of 31% under 1 sun illumination at room temperature. The performance was then increased to 31.3% by optimizing the QD region doping. Copyright © 2014 John Wiley & Sons, Ltd.     

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

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. Copyright © 2015 John Wiley & Sons, Ltd.