tandem qw solar cells

7/30/2019 Tandem QW Solar Cells

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TANDEM QUANTUM WELL SOLAR CELLS

Ben Browne*1,

Andreas Ioannides1, James Connolly

1, Keith Barnham

1, John Roberts

2, Robert Airey

2, Geoffrey Hill

2,

Guy Smekens3

and Jose Van Begin3

1Experimental Solid State Physics, Blackett Laboratory, Imperial College London, SW7 2BW, UK

2EPSRC National Centre for III-V Technologies, Sheffield S1 3JD, UK.3Energies Nouvelles et Environnement, B-1150 Brussels, Belgium

*Corresponding author: [email protected]

ABSTRACT

Quantum wells offer advantages in conventional bulktandem solar cells since they allow the independenttailoring of the absorption edge of either cell with no latticemismatch and subsequent relaxation. We describe

progress in the band gap engineering of InGaP/GaAssolar cells using strain balanced quantum wells andpresent a tandem quantum well structure which hasachieved 30.6% efficiency under 54 suns AM1.5g. This isa record for photovoltaic nanostructured devices. Wepredict realistic efficiencies of 34% under 600suns,

AM1.5d low AOD for optimized devices. Finally, thepossibility and potential gains of introducing quantum wellsinto both cells of an InGaP/GaAs device are discussed.

INTRODUCTION

The energy density of sunlight is relatively weakcompared to fossil fuels. Concentrator systems have the

potential to reduce the cost of photovoltaics by collectingthe direct component of the dilute terrestrial insolation withinexpensive lenses or mirrors. The economics of thisapproach demands high efficiency solar cells and thispaper discusses the application of a nano-structuredtechnology to such an approach

GaAs provides the highest efficiency single-junctionsolar cells under all concentrations. Its bandgap, (1.42eV)however is higher than the 1.1eV required for optimalefficiency at both one sun and high concentration [1].Strain balanced InGaAs quantum wells (QWs) in theintrinsic region of GaAs single junction solar cells extendthe absorption edge without the dislocations inherent invirtual substrate devices [2, 3]. The resulting increased

short circuit current prevails over the accompanying dropin open circuit voltage [4] increasing efficiency overcomparable conventional cells [5].

A similar band gap optimum exists in the case of twojunction cells. The record efficiency for a two junctiontandem solar cell under AM1.5d is 30.2% for an

InGaP/InGaAs device at 300 concentration. However, asFigure 1 shows, the 1.8eV/1.42eV bandgap combination

of an InGaP/GaAs cell is significantly higher than theoptimum combination under both direct and globalspectra. Approaches being actively pursued to lower thebandgaps of tandem cells include lattice mismatchedInGaP/InGaAs grown on a virtual substrate and dilutenitrides [6]. Both of these introduce dislocations thuslowering the voltage of the cell though the virtual substrateapproach has recently achieved a record (40.72.4)%under 240 suns in a low AOD spectrum [7].

Strain balanced QWs in tandem solar cells [8] allowthe absorption edge of either cell to be independentlyadjusted with no lattice mismatch and subsequentrelaxation. Provided there is sufficient absorption in thewells these devices can climb the contours in Figure 1 andbe tailored to any spectrum.

Figure 1. Ideal efficiency contour plot for a tandem cell

under 500 low AOD. Dark lines show the band gaps of

GaInP and GaAs. The red cross shows the position of theQW tandem described in this paper and the blue star, aproposed structure with QWs in both junctions.

In this paper we present recent progress onInGaP/GaAs tandem structures with QWs incorporatedinto the GaAs cell, showing measured and predictedefficiencies, whilst outlining the optimisation possibilities.


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STRAIN BALANCING

We are able to grow up to 65 QWs in the intrinsicregion of a p-i-n solar cell without dislocations [9] bybalancing the compressive stress exerted by the low(relative to GaAs) lattice spacing of InxGa1-xAs QWs withGaAsyP1-y barrier material of higher lattice spacing [10].The strain balancing method and a resultant band diagram

are shown in Figure 2.

Figure 2. The process of strain balancing (top) and theband structure of three strain balanced QWs in a GaAscell at short circuit current conditions (bottom).

DEVICE SUMMARY

All the devices described in this paper were fabricatedusing Metal-Organic Vapour Phase Epitaxy. We havegrown two tandem GaAs/InGaP solar cells with QWs inthe intrinsic region of the bottom cell. The second cell(TS151) was grown with higher top cell emitter doping andlower QW Indium content than the initial device(QT1821AD). In both cases the bottom junction was grown

at the EPSRC National Centre for III-V Technologies,Sheffield and the top cell at ENE, (Energies Nouvelles etEnvironnement) in Belgium. The TS151 top cellovergrowth was performed alongside a conventional GaAscell on a passive Ge substrate to create a control device(ENE1864). The top cell of the QT1821AD controlperformed poorly due to an insufficiently thick p-n top celland is not discussed here. A schematic of the tandemstructure is shown below.

Figure 3. A cross section of the QW tandem structure ofTS151.

All devices were processed at the facilities in theCentre for Integrated Photonics (CIP) in Ipswich.Photodiodes for quantum efficiency measurements, fullymetallised devices for dark current measurements, andconcentrator devices were prepared from each wafer.

RESULTS AND MODELLING

The quantum efficiency of all samples wascharacterised as described in [11] by optically biasing onecell by as little as possible to ensure that the other iscurrent limiting then measuring the response of the limitingcell. The results are shown in Figure 4. The lower InGaAsindium content of the TS151 QWs raises their band gap.Thus the exciton absorption of TS151 peaks at 922nmcompared to 932nm for QT1821AD.


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Figure 4. The external quantum efficiencies of the controland QW tandem devices.

By integrating the product of the spectral responseand the spectrum of interest we calculate an expectedshort circuit current for each junction. The Shockleyinjection and Shockley-Read-Hall dark currents of eachdevice are simulated using a 1D drift diffusion model [12].The radiative component of the dark current is calculatedwith no free parameters from the generalised Planckformula.

QT1821AD was independently measured at TheFraunhofer Institute for Solar Energy Systems who foundan efficiency of (22.10.7)% under 1 sun, AM1.5d lowaerosol optical depth (AOD). We have constructed thelight current curve of each cell by subtracting the modelleddark current of each junction from the Fraunhofer shortcircuit current measurement. The current mismatch

between the top and bottom cells was calculated from inhouse quantum efficiency measurements. This methodaccurately reproduces the experimental light current curveand efficiency to within the quoted error.

Figure 5. Tandem dark current modelling results relative tothe light current curve measured at Fraunhofer.

Due to a low top cell emitter doping, the efficiency ofQT1821AD peaked at 22 suns AM1.5d with a value of

27.2%. Extrapolating our model for the dark current tohigher concentrations we predict that the same cell with noseries resistance losses would have achieved 29.8%under 200 suns AM1.5d low AOD.

TS151 was grown with a higher top cell emitter dopingto overcome the series resistance limitations of

QT1821AD. The QW band gap was increased to counterthe slight over production of current in the QW cell.

Concentrator measurements under a Xenon lightsource have been performed on TS151 at ENE. Theresults in Table I show the maximum recorded efficiencieswhich occurred at 54 suns for both devices.

Device Fill Factor (%) Efficiency (%)

TS151 81.4 30.6

ENE1864 81.9 31.7

Table I. The measured performance of the QW and controldevices under concentration.

The superior performance of the control cell in Table Imay be explained by considering the xenon spectrumbelow and the resultant short circuit currents in Figure 7:In the control device, the current of the limiting bottom cellis enhanced under the xenon spectrum relative to AM1.5dlow AOD, current matching the control. Under theseconditions TS151 is top cell limited and the controloutperforms the QW device because its top cell spectralresponse extends to longer wavelengths.

Figure 6. The xenon spectrum used to characterise TS151

alongside a low AOD concentrator spectrum.


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Figure 7. Short circuit currents under a xenon lampspectrum (green) alongside those calculated from spectralresponse curves (blue and red). Values are for an intensityof 541000W/m

Figure 7 shows that the QW cell should outperformthe control under a concentrator spectrum and could

perform better still beneath a top cell with a larger spectralresponse such as that of the control. The control cell wasgrown on a 6 Germanium substrate whereas the QW cellused a 3 GaAs substrate. Growth of InGaP at 6 is knownto give rise to a higher degree of order in the arrangementof indium and gallium atoms which lowers the bandgap[13]. This seems the likely cause of the discrepancybetween the top cells and a promising avenue foroptimisation of the tandem QW solar cell design.

To investigate the relative performance of the QW andcontrol cells we have combined experimental quantumefficiencies and dark currents under the assumption ofadditivity and an AM1.5d low AOD spectrum to produceFigure 8. The red dots represent a proposed tandem

structure in which the control top cell with high disorder isgrown on the QW bottom cell. Such a cell should achievean energy conversion efficiency of 34%.

Figure 8. Efficiency predictions under the assumptionsdescribed in the text above.

FUTURE WORK

QW solar cells have the potential to achieve improvedbandgap optimisation relative to lattice mismatched cells ifthe top cell band gap can be lowered in unison with that ofthe bottom cell. This can be achieved by introducingGaInP QWs into an intrinsic region in the top cell. In thisscenario a tandem cell would posses the bandgap

combination denoted by Dual SB-QWSC in Figure 1. Theproposed GaAs cell in Figure 1 would present littleproblem in fabrication since we have alreadydemonstrated high quality material out to and beyond theabsorption edge required at 980nm [14]. We are currentlydeveloping a top cell to complete such a structure.

Once grown, a dual QW tandem cell would have theadvantage over the record cell in [7] of containing nodislocations implying less recombination and a longerlifetime. In addition we have shown previously that GaAsQWSCs are radiatively dominated [15]. A distributedBragg reflector can therefore boost efficiency byincreasing absorption and decreasing recombination in thebottom cell [16]. Alternatively, the dual QW tandem couldbe grown on an active Ge substrate in a similar manner tothe present multijunction record cell [7].

CONCLUSION

It has been demonstrated that QWs can tailor bandedges which provides the flexibility to current matchtandem solar cells under any predefined spectrum.

We have achieved 30.6% efficiency under an AM1.5gspectrum at 54 suns. This is a record for a nanostructuredsolar cell.

Strain balanced quantum well solar cells withoptimised conventional top cells should reach efficienciesof 34% and above.

We have proposed a new high efficiency tandemconcept consisting of QWs in both the GaAs and InGaPcells. This has the potential to achieve efficienciescomparable to the current lattice mismatched, recordmultijunction cells.


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References

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