The intermediate-band solar cell is designed to provide a large photogenerated current while maintaining a high output voltage. Nanostructured materials and certain alloys have been employed in the practical implementation of these devices. This Progress Article reviews the range of different approaches and discusses how to resolve the remaining technical issues.

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Understanding intermediate-band solar cells

Extensive literature and publications on intermediate band solar cells (IBSCs) are reviewed. A detailed discussion is given on the thermodynamics of solar energy conversion in IBSCs, the device physics, and the carrier dynamics processes with a particular emphasis on the two-step inter-subband absorption/recombination processes that are of paramount importance in a successful implementation high-efficiency IBSC. The experimental solar cell performance is further discussed, which has been recently demonstrated by using highly mismatched alloys and high-density quantum dot arrays and superlattice. IBSCs having widely different structures, materials, and spectral responses are also covered, as is the optimization of device parameters to achieve maximum performance.

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Intermediate band solar cells: Recent progress and future directions

Ten-layer InAs/GaAs quantum dot (QD) solar cells exhibiting enhanced short circuit current (Jsc) and open circuit voltage (Voc) comparable to a control GaAs p-i-n solar cell are reported. 1 sun Jsc is enhanced by 3.5% compared to that of the GaAs control, while the Voc is maintained at 994 mV. Results were achieved using optimized InAs QD coverage and a modified strain balancing technique, resulting in a high QD density (3.6×1010 cm−2), uniform QD size (4×16 nm2), and low residual strain (103 ppm). This enhanced Voc is a promising result for the future of InAs QD-enhanced GaAs solar cells.

Near 1 V open circuit voltage InAs/GaAs quantum dot solar cells

We report a 50% increase in the power conversion efficiency of InAs/GaAs quantum dot solar cells due to n-doping of the interdot space. The n-doped device was compared with GaAs reference cell, undoped, and p-doped devices. We found that the quantum dots with built-in charge (Q-BIC) enhance electron intersubband quantum dot transitions, suppress fast electron capture processes, and preclude deterioration of the open circuit voltage in the n-doped structures. These factors lead to enhanced harvesting and efficient conversion of IR energy in the Q-BIC solar cells.

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Strong Enhancement of Solar Cell Efficiency Due to Quantum Dots with Built-In Charge

Ten-, 20-, and 40-layer InAs/GaAs quantum-dot (QD)-embedded superlattice solar cells were compared with a baseline GaAs p-i-n solar cell. Proper strain balancing and a reduction of InAs coverage value in the superlattice region of the QD embedded devices enabled the systematic increase in short-circuit current density with QD layers (0.02-mA/cm $^2$ /QD layer) with minimal open-circuit voltage loss (∼50 mV). The improvement in voltage was found to be due to a reduced nonradiative recombination resulting from a reduced density of larger defective QDs and effective strain management. The 40-layer device exceeded the baseline GaAs cell by 0.5% absolute efficiency improving efficiency relative to the baseline by 3.6%.

http://bargh121.persiangig.com/Maghalat/06179500_(Open-Circuit%20Voltage%20Improvement%20of%20InAsGaAs%20Quantum-Dot%20Solar%20Cells%20Using%20Reduced%20InAs%20Coverage_2012).pdf

Open-Circuit Voltage Improvement of InAs/GaAs Quantum-Dot Solar Cells Using Reduced InAs Coverage

We report the fabrication of InAs/GaAs quantum dot solar cells (QDSCs) with enhanced photocurrent and no degradation in open circuit voltage (VOC) compared to a solar cell grown without QDs and composed solely of wetting layers. Notably, the achievement of such high VOC does not require electronic coupling. We report QDSCs with a light absorption range extended up to 1.3 μm and evidence a trade-off between VOC and QD ground-state energy. These results are of major significance to the design of high efficiency QDSCs.

Fabrication of InAs/GaAs quantum dot solar cells with enhanced photocurrent and without degradation of open circuit voltage

Quantum dot solar cells can potentially realize ultrahigh efficiencies in single p-n junction structures utilizing intermediate-level energy bands. However, so far most fabricated quantum dot solar cells have suffered from severe reduction of open-circuit voltage by incorporation of quantum dots resulting in significantly lower efficiencies than those without quantum dot. Here we fabricate a high-efficiency InAs/GaAs quantum dot solar cell. Our cell contains five layers of high-density (4 × 1010 cm−2 per layer) self-assembled InAs quantum dots grown by metalorganic chemical vapor deposition suppressing open-circuit-voltage degradation. We develop a dual-layer anti-reflection coating of optimum thicknesses. The resulting cell exhibits efficiencies of 18.7% under AM1.5G, 1 sun and 19.4% for 2 suns, the highest reported thus far, for any kind of quantum dot cell. Our high-efficiency demonstration in a cell grown by MOCVD is a strong encouragement towards the commercialization of quantum dot solar cells.

High-efficiency InAs/GaAs quantum dot solar cells by MOCVD

Quantum dot (QD) solar cells have been proposed as a means to exceed the Shockley and Queisser efficiency limit of 31%, via the absorption of sub-band-gap photons, conventionally lost in a single junction solar cell (SC). Previous reports on fabricated InAs/GaAs QDSCs showed a slight increase in photoresponse, due to below-band gap absorption. However, they all showed a severe degradation of open circuit voltage (V OC ). Here, we report the fabrication of solar cells, containing InAs QDs. These QD solar cells show enhanced photoresponse in the below GaAs band gap region, and above all, yield almost no degradation of V OC .

Ryo Morihara

Papers

Fabrication of InAs/GaAs quantum dot solar cells with enhanced photocurrent and without degradation of open circuit voltage

High-efficiency InAs/GaAs quantum dot solar cells by MOCVD

InAs/Sb∶GaAs quantum dot solar cells grown by metal organic chemical vapor deposition