Kentaroh Watanabe

A quantum-well superlattice solar cell for enhanced current output and minimized drop in open-circuit voltage under sunlight concentration

Insertion of quantum wells (QWs) extends the absorption edge to a longer wavelength than the value of a p-i-n cell without the QWs, which is preferable for the improved current matching of a InGaP/GaAs/Ge multijunction cell. The QWs, however, reduce the open-circuit voltage (Voc) and degrade the fill factor; the latter is significant for a large number of QWs that are mandatory for sufficient light absorption. As a structure to minimize these drawbacks, a QW superlattice, a strain-balanced In0.13Ga0.86As (4.7 nm)/GaAs0.57P0.43 (3.1 nm) stack, was implemented by metalorganic vapour-phase epitaxy. It brought about an enhancement in short-circuit current density (3.0 mA cm−2) with a minimal drop in Voc(0.03 V) compared with a p-i-n cell without the superlattice. The collection efficiency of photocarriers from the wells to an external circuit was evaluated: the efficiency was above 0.95 for the superlattice, while it was below 0.8 at a large forward bias for a conventional QW cell with thicker barriers. With the fast electron–hole separation in the superlattice owing to tunnelling transport, the superlattice cell exhibited a steeper increase in Voc as a function of the sunlight concentration ratio than the conventional QW cell: at the concentration ratio of 50, the value of Voc for the superlattice cell was almost equivalent to the value of the GaAs p-i-n cell without QWs. As a possible mechanism behind such an enhancement in Voc, photocurrent generation by two-step photon absorption was observed, using the electron ground state of the superlattice as an intermediate state.

Evaluation of Carrier Collection Efficiency in Multiple Quantum Well Solar Cells

The carrier collection efficiency (CCE) is proposed as an effective parameter for indicating the efficiency of carrier transport in quantum nanostructured solar cells. The CCE can be estimated by normalizing the illumination-induced current enhancement to its saturation value at reverse bias. The derivation procedure for the CCE is experimentally validated by examining the bias-dependence of light absorption, and by investigating the balance between the absorbed photons and collected carriers at reverse bias. The effect of AM1.5 bias-illumination for CCE characterization was also investigated and found to be significant for more accurate evaluation of carrier dynamics during actual device operation under sunlight. Furthermore, simulation of the resistance effects showed that a large series resistance leads to underestimation of the CCE at forward bias whereas a shunt resistance has only a slight influence on the CCE calculation.

Strain-compensation measurement and simulation of InGaAs/GaAsP multiple quantum wells by metal organic vapor phase epitaxy using wafer-curvature

Precise strain compensation for lattice-mismatched quantum wells is crucial for obtaining high performance devices such as quantum well solar cells. High-accuracy in situ curvature monitoring is a more efficient tool to adjust growth conditions for perfect strain balancing, and we have achieved curvature measurement during growth of InGaAs/GaAsP multiple quantum wells by metal organic vapor phase epitaxy. We have also developed the curvature calculation model taking into account of thermal expansion and lattice relaxation effects based on Stoney’s equation. The measured periodical curvature behavior corresponds to the growth of compressive InGaAs well layers and tensile GaAsP barrier layers and fits perfectly with a theoretical curve assuming the structural parameters (thicknesses and atomic contents) obtained by x-ray diffraction analysis, confirming correctness of the developed calculation method. Considering the proper thermal expansion coefficients for InGaAs and GaAsP, we have obtained much accurate ...

Compensation doping in InGaAs / GaAsP multiple quantum well solar cells for efficient carrier transport and improved cell performance

A major challenge for multiple quantum well (MQW) solar cells is to extract sufficient photo-excited carriers to an external circuit through the MQW region under forward bias. The present study reports the effectiveness of compensation doping in the i-region, which includes MQWs, for more efficient transport of both electrons and holes. Unintentional p-type background doping occurs in GaAs by inevitable carbon incorporation during metal-organic vapor phase epitaxy, causing undesirable bending of the band lineup in the i-region of p-on-n devices. By cancelling this out by sulfur compensation doping to obtain a uniform electric field distribution, we achieved much a high carrier collection efficiency (CCE) >90% at the operating bias voltage regardless of the excitation wavelength, compared to < 50% without compensation doping. Consequently, cell performance was greatly improved, in particular showing an enhancement of the fill factor from 0.54 to 0.77, and degradation-free quantum efficiency within the GaAs...

High-Aspect Ratio Structures for Efficient Light Absorption and Carrier Transport in InGaAs/GaAsP Multiple Quantum-Well Solar Cells

The high aspect ratio (HAR) quantum well was proposed as a general design principle to overcome the tradeoff problem between light absorption and carrier collection in multiple quantum-well (MQW) solar cells. An HAR-MQW structure consists of thin wells and barriers, and its fundamental strategies are 1) thinner wells to enhance the light absorption for 1HH transition and make it possible to absorb the same amount of light with a thinner MQW region; 2) thinner barriers to allow the photogenerated carriers to be extracted by means of tunneling transport; and 3) deeper wells to obtain the same effective bandgap as thicker wells because of the stronger confinement. The enhanced absorption coefficient for HAR-MQW was proved by the measurement of both photoabsorption and the quantum efficiency at a sufficiently large reverse bias. Stronger photon absorption via 1HH transition was achieved with a smaller total thickness of the wells area. In the HAR-MQW cell, although the transport of the heavy holes was found to still be dominated by thermionic processes due to its large effective mass, tunneling of the electrons was clearly observed, and the extraction efficiency of photoexcited electrons remained much higher than that of a normal MQW cell at forward biases.

Photocurrent generation by two-step photon absorption with quantum-well superlattice cell

We have assessed a possibility of using multiple quantum wells as an absorber for an intermediate-band solar cell. Photocurrent due to two-step photon absorption was observed using an InGaAs/GaAsP strain-balanced quantum-well superlattice cell, the barrier thickness of which was 3 nm. Upon infrared irradiation with a filtered AM 1.5 light source (λ>1.4 μm), quantum efficiency was increased by 0.8% at the wavelength range corresponding to the absorption by the quantum wells. Such enhancement in quantum efficiency was not observed either for a conventional quantum-well solar cell with thick (11 nm) barriers or for a GaAs pin cell, suggesting that efficient separation between photo-generated electrons and holes in the superlattice is essential for this achievement of two-step photon absorption. The quantum efficiency of such two-step photocurrent generation can be enhanced by elaborate design of a superlattice layer structure, by which quantum-well superlattice will be a possible active region for an intermediate-band solar cell.

A Superlattice Solar Cell With Enhanced Short-Circuit Current and Minimized Drop in Open-Circuit Voltage

A quantum-well (QW) solar cell including InGaAs wells is a promising candidate for the purpose of current matching in InGaP/GaAs/Ge tandem solar cells by extending the edge of quantum efficiency to longer wavelengths. Even though QWs increase short-circuit current by the extended effective band edge, they tend to obstruct carrier transport and degrade the efficiency of a cell. Therefore, a superlattice (SL) structure has been proposed to prevent the recombination of carriers inside of the wells and, more importantly, to enable carriers to tunnel to a neighboring well, leading to an efficient carrier transportation in such a photovoltaic device. In this paper, a SL solar cell was implemented with a strain-balancing technique. It exhibited excellent performance: Enhanced photocurrent (3.0 mA/cm

Carrier Escape Time and Temperature-Dependent Carrier Collection Efficiency of Tunneling-Enhanced Multiple Quantum Well Solar Cells

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Effect of Quantum Well on the Efficiency of Carrier Collection in InGaAs/GaAsP Multiple Quantum Well Solar Cells

) with minimized drop (0.03 V) in open-circuit voltage. Behind these achievements, substantial contribution of tunneling transport has been confirmed for the SL cell by external quantum efficiency measurement at 77 K.

InGaAs/GaAsP superlattice solar cells with reduced carbon impurity grown by low-temperature metal-organic vapor phase epitaxy using triethylgallium

Tunneling enhancement of cell performance in InGaAs/GaAsP multiple quantum well (MQW) solar cells has been studied to investigate the potential in overcoming the carrier collection problem, which hinders the maximum performance of quantum structure solar cells. To accurately investigate the effects of the tunneling effect, the study was carried out in samples with different GaAsP barrier thickness, controlled absorption edge, and constant built-in field. The tunneling effect has been confirmed by evaluating carrier escape times using the time-resolved photoluminescence technique and measuring carrier collection efficiency at various temperatures. The collection efficiencies at low temperature are found to be remarkably improved when barrier thickness was below 3 nm, which can be regarded as the critical thickness for efficiently facilitating tunneling enhancement. It can also be concluded that the carrier transport model based on thermal and tunneling processes is practical enough to describe most of the carrier sweep-out dynamics in MQW solar cells.