C. Calder

Observation of photon recycling in strain-balanced quantum well solar cells

Photon recycling in strain-balanced quantum well solar cells grown on distributed Bragg reflectors has been observed as a suppression of the dark current and a change in electroluminescence spectra. Comparing devices grown with and without distributed Bragg reflectors we have demonstrated up to a 33% reduction in the ideality n=1 reverse saturation current. Furthermore, to validate the observations we demonstrate how both the measured dark currents and electroluminescence spectra fit very well to a photon recycling model. Verifying our observations with the model then allows us to calculate optimized device designs.

Optimisation of Photon Recycling Effects in Strain-Balanced Quantum Well Solar Cells

Photon self-absorption or photon recycling (PR) can be used in strain balanced quantum well solar cells (SB-QWSC) to achieve higher device efficiencies. By growing SB-QWSCs on distributed Bragg reflectors (DBR) suppression of the device dark-current and changes in electro-luminescence (EL) spectra have been observed when compared to devices grown on GaAs substrates. The dark-current is suppressed in the region with ideality factor n=1, which corresponds to ideal Shockley and radiative recombination, while there is no difference in dark-currents in the region where the ideality factor n is ~2 and non-radiative Shockley-Read-Hall recombination is dominant. DBR SB-QWSCs in which the dark-current is dominated by radiative recombination benefit not only from enhancement of short-circuit current (Jsc) due to increased absorption of normal incident photons but also from enhancement of open-circuit voltage (Voc) from PR. Experimental and modeling results of SB-QWSC devices will be presented in the paper. These results show that with optimal device design both the Jsc and Voc can be increased significantly at the same time. Most importantly, to date PR has only been observed in dark-current measurements on fully metalised photodiode structures; but we will show current-voltage characteristics in devices under high concentration which reveal a suppression of the radiative dark-current in DBR devices compared to those without DBRs. The results presented here convincingly demonstrate, for the first time, the PR effect in SB-QWSCs under high concentration

Hot carriers in strain balanced quantum well solar cells

The incorporation of strain-balanced quantum wells into a GaAs solar cell extends the spectral response resulting in a photocurrent increase that can exceed the reduction in voltage performance, leading to higher overall efficiencies [1]. At concentrator current levels the main carrier loss mechanism is radiative recombination from the quantum wells. We have recently reported on evidence of hot carrier effects in the quantum well regions of GaAs based strain-balanced cells incorporating InGaAs quantum wells and GaAsP barriers [2]. This paper extends this work to a greater range of samples, and reports on a bias-dependent broadening in exciton luminescence observed in all samples at high biases. We present two possible interpretations of the data using different parts of the generalised Planck equation.

Modelling the Confined States in Multi Quantum Well Solar Cells

The strain balanced quantum well solar cell (SB-QWSC) provides a novel method of engineering the bandgap of a p-i-n junction to better match the terrestrial solar spectrum. The ability to accurately model the spectral response of a SB-QWSC is vital in optimising cell design. We present a study into two major components of such a model. Firstly, we provide an overview of the model for the confined energy levels within SB-QWSCs and outline the method used to determine a vital parameter for such calculations; the conduction band offset. Secondly, a critical evaluation of the hole in-plane effective masses (required to model the device absorption) is given. A multi-band k.p model is used to calculate the in-plane valence band dispersion and values for the effective masses are extracted from fits to the resulting E-k curves. The results of these investigations are then used to model the spectral response of a typical SB-QWSC. Good agreement is observed between absolute prediction and measured quantum efficiencies

Strain Balanced Quantum Well Monolithic Tandem Solar Cells

The effect of incorporating strain balanced multi-quantum well structures in InGaP/GaAs monolithic tandem solar cells is investigated. At present the majority of InGaP/GaAs tandem cells are current limited by the bottom GaAs junction. Incorporation of multi-quantum well structures in the GaAs bottom junction extends the cell absorption to longer wavelengths. This allows current matched dual junction tandem cells to achieve higher efficiencies. InGaP/GaAs tandem cells have been studied by overgrowing different top cells on two similar quantum well structures and compared to a InGaP/GaAs control cell. A current matched top cell is presented and efficiency enhancement of a tandem by a quantum well cell demonstrated experimentally for the first time

Dark-current suppression due to photon recycling in distributed Bragg reflector strain-balanced quantum well solar cells

Self-absorption or photon recycling (PR) has been observed experimentally as a reduction in the ideality n=1 reverse saturation current of SB-QWSCs. We believe this is the first example of PR effects on solar cell dark-currents. PR is observed at high bias when the primary recombination mechanism in SB-QWSCs is radiative recombination in the quantum wells corresponding to ideality n=1 dark-currents. Results are presented here on the effect of PR on the dark-current and electroluminescence (EL) spectrum; PR has resulted in DBR SB-QWSC dark-current and EL suppression of up to 35% both experimentally and theoretically. Comparison is made to control cells without a DBR. Modelling results will allow us to make predictions for expected efficiency enhancement in optimised SB-QWSC devices at high concentration levels. These results convincingly reveal how PR can increase the efficiency of SB-QWSCs.