Light-splitting photovoltaic system utilizing two dual-junction solar cells

Light-splitting photovoltaic system utilizing two dual-junction solar cells

Available online at www.sciencedirect.com Solar Energy 84 (2010) 1975–1978 www.elsevier.com/locate/solener Light-splitting photovoltaic system utili...

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Available online at www.sciencedirect.com

Solar Energy 84 (2010) 1975–1978 www.elsevier.com/locate/solener

Light-splitting photovoltaic system utilizing two dual-junction solar cells Kanglin Xiong a,b, Shulong Lu a, Jianrong Dong a, Taofei Zhou a, Desheng Jiang b, Rongxin Wang a, Hui Yang a,b,⇑ a

Suzhou Institute of Nano-tech and Nano-bionics, CAS, Ruoshui Road 398, Suzhou 215125, PR China b Institute of Semiconductors, CAS, No. A35, Qing Hua East Road, Beijing 100083, PR China Received 1 July 2010; received in revised form 15 October 2010; accepted 19 October 2010 Available online 9 November 2010 Communicated by: Associate Editor Igor Tyukhov

Abstract There are many difficulties limiting the further development of monolithic multi-junction solar cells, such as the growth of lattice-mismatched material and the current matching constraint. As an alternative approach, the light-splitting photovoltaic system is investigated intensively in different aspects, including the energy loss mechanism and the choice of energy bandgaps of solar cells. Based on the investigation, a two-dual junction system has been implemented employing lattice-matched GaInP/GaAs and InGaAsP/InGaAs cells grown epitaxially on GaAs and InP substrates, respectively. Ó 2010 Elsevier Ltd. All rights reserved. Keywords: Light splitting; GaInP/GaAs; GaInAsP/InGaAs; Dual-junction

1. Introduction Solar cells can convert sunlight into electricity directly (Rappoport, 1959). To make the conversion more efficiently, cells with different energy bandgaps should be adopted in combination to reduce the thermalization loss of hot carriers. According to detailed balance calculation, high efficiency can be achieved by monolithic staked multi-junction solar cells with each junction utilizing corresponding part of the solar spectrum (Yamaguchi et al., 2005). Because the solar irradiance mainly lies in the energy range from 0.3 to 4.5 eV, the most suitable materials for the multi-junction structures are III–V semiconductors (Bett et al., 1999). Following these concepts, III–V based monolithic triple-junction solar cells grown epitaxially on Ge or GaAs substrate have demonstrated ultra high efficiency under concentrated light in recent years. Fig. 1 shows three ⇑ Corresponding author at: Suzhou Institute of Nano-tech and Nanobionics, CAS, Ruoshui Road 398, Suzhou 215125, PR China. E-mail address: [email protected] (H. Yang).

0038-092X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2010.10.011

typical solar cell structures with the arrows indicating their growth order. They are inverted Ga0.5In0.5P/GaAs/ In0.3Ga0.7As solar cell (Inv) with efficiency up to 38.9% at 81 suns under AM1.5d (Geisz et al., 2007), lattice-matched Ga0.5In0.5P/Ga0.99In0.01As/Ge solar cell (LM) with efficiency of 40.1% at 135 suns under AM1.5d low aerosol optical depth spectrum (King et al., 2007) and metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge cell (MM) with efficiency of 41.1% at 454 suns under AM1.5d (Guter et al., 2009). In spite of high efficiencies achieved, there are inherent difficulties restricting the further development of monolithic staked multi-junction solar cells, such as the growth of lattice-mismatched materials (Yamaguchi et al., 2008) and the current-matching restriction (Pehrz et al., 2009) under a specific spectrum. Because the mentioned monolithic triple-junction solar cells have been optimized in many aspects, such as bandgap combination and material quality, there is little room to improve. To obtain a higher efficiency, four-junction monolithic solar cells have been developed with optimized bandgap combination (King

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Fig. 1. Bandgap vs. lattice constant (bottom axis) for AlGaInAsP material system together with AM1.5 g spectrum (top axis, shadow area). Four special solar cell structures are also shown. The lattice-matched Ga0.5In0.5P/Ga0.99In0.01As/Ge (LM), metamorphic GaInP/GaInAs/Ge (1.8, 1.29, 0.66 eV for MM), an inverted GaInP/GaInAs/GaInAs (1.83, 1.40, 1.00 eV for Inv) and lattice-matched InGaAs/InGaAsP on InP (LS). The arrows indicate the growth orders.

et al., 2006; Stan et al., 2010). However, limitations in film quality or current-matching will have to be overcome for further efficiency improvement. An alternative multi-junction approach which is promising is the spectrum-splitting photovoltaic system (Kintisch, 2007). Demonstrated as early as in the 1970s (Moon et al., 1978), the spectrum-splitting method has gained renewed attention recently due to the potential of reaching an efficiency of 50% (Barnett et al., 2007). By splitting the solar spectrum into several parts through beam splitters, each part can be converted to electricity independently by a single junction or monolithic multi-junction solar cell specially designed for the spectrum. So the thermalization loss is reduced and great freedom is offered in the material selection and the design of devices. Furthermore, there are other advantages over monolithic staked solar cells, for example, the system is less sensitive to series resistances and spectrum variations. In this article, the energy loss and the bandgap selection of light-splitting approach will be investigated. Also, a four-junction system will be constructed with one beam splitter and two dual-junction solar cells.

be divided into three parts, i.e. reflection band, transmission band, and a transit region in-between. All the three parts will contribute to energy loss. To investigate the loss introduced by transit region, detailed-balance efficiency under AM1.5 g is calculated for a system schematically shown in Fig. 2. The system employs a dichroic mirror as beam splitter and lattice-matched GaInP/GaAs and InGaAsP/InGaAs cells as receivers. Assuming a linear dependence of reflectivity on wavelength in transit region as well as ideal reflection and transmission bands for the beam splitter, the efficiencies are obtained and shown in Fig. 3 for different widths and center point positions of the transit region. It is found that a wider transit region results in a slight lower efficiency. For a width of 100 nm, the maximum efficiency will be lowered by 3% relatively compared to the 10 nm-width case. What’s more, the performance of dichroic mirror designed for a specific light source and system configuration will degrade if the incident angle and angular distribution of light source changed. Considering two beams with incident angle of h1 and h2, as shown in Fig. 2, the following relationship based on effective medium thickness can be employed as an approximation.   cosðh2 Þ Rðk; h1 Þ ¼ R k ; h2 ð1Þ cosðh1 Þ where k is wavelength of incident light, R is the reflectivity of the dichroic mirror. With R at initial incident angle h1 known, the reflectivity at incident angle h2 can be calculated using Eq. (1). Then the related energy losses caused by variation of incident angle can be estimated. For instance, with h1 = 45°and h2 = 40°, the spectral transit region width will increase from 100 nm to108 nm while the transit center will shift from 870 nm to 942 nm. According to Fig. 3, the system efficiency will be lowered by about 4% relatively.

2. Losses induced by non-ideal splitter It is noted that the light-splitting will introduce energy loss. The absorption and diffuse reflection of the beam splitter will cause optical losses, which can be directly evaluated (Green and Ho-Baillie, 2010). Additionally, in the non-ideal spectrum split, high energy photons will be blended with low energy photons. The higher energy photons in the lower energy beam results in thermalization loss while the lower energy photons in the higher energy beam will be wasted. For a real dichoric mirror, the spectrum can

Fig. 2. Schematic diagram of a light-splitting system. h1 and h2 indicate incident angles of different beams.

K. Xiong et al. / Solar Energy 84 (2010) 1975–1978

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Fig. 3. The dependence of detailed-balance efficiency on the transit region’s center position of a beam splitters’ spectrum characteristic, under AM1.5 g ASTM G173–3 at 298 K, assuming a linear transit characteristic and ideal reflection and transmission outside the transit region. The transit region width varies from 10 nm to 200 nm.

Fig. 4. Detailed-balance efficiency contour plot of a monolithic dualjunction solar cell at 298 K. The long wavelength part of the AM1.5 g ASTM G173–3 spectrum with photon energy lower than 1.42 eV is used as light source. The black dot represents a lattice-matched GaInAsP/InGaAs structure on InP substrate.

3. Consideration on bandgap selection

izontal Aixtron 200/4 Reactor equipped with a rotating disc and an IR-heating system. An InGaAsP tunnel junction is used in the InGaAsP/InGaAs solar cell. The front electrode patterns are identical for both cells with a comb-like structure. The designed illumination area is 2.5 mm  2.5 mm. Only one bus with width of 150 lm is adopted. The width of electrode finger is 10 lm while the spacing between adjacent fingers is 200 lm. The dichroic mirror shown in Fig. 2 is designed to reflect high energy photons to GaInP/GaAs cell and transmit low energy photons to InGaAsP/InGaAs cell with an incident angle of 45°. It consists of SiO2 and TiO2 layers deposited on both sides of a glass substrate. Then it is sliced into chips with area of 1 cm  1 cm. The width of transit region is measured to be 110 nm. The reflectivity in wavelength range of 400–500 nm is affected by the absorption of TiO2. To evaluate the system performance, two different current density–voltage (J–V) characteristics for each cell are measured, i.e., one for “ideal” spectrum splitting, another for “real” splitting under configuration in Fig. 2. During the indoor measurement, a solar simulator with the AM1.5 g spectrum is applied as light source; thermal couple and heat dissipator are used to control the temperature. For the GaInP/GaAs cell, one J–V is measured with the cell under the illumination of solar simulator; it is considered as the J–V under ideal spectrum splitting. The other J–V is measured under light reflected by actual dichroic mirror, as shown in Fig. 2. For the InGaAsP/InGaAs, one J–V is measured under the illumination of solar simulator after the higher energy part of the spectrum has been filtered away by a thick GaAs plate. And the light intensity is adjusted so that the short circuit current is equal to the value as calculated from the external quantum efficiency under an ideal spectrum splitting. The other J–V is measured under light filtered by the dichroic mirror, as shown in Fig. 2.

In view of the light-splitting losses discussed above, the solar spectrum should not be divided into too many parts. During the material selection, a compromise should be made between the thermalization loss and the light-splitting loss. Considering the fact that lattice-matched InGaP/GaAs solar cell preferred as the receiver for higher energy photons, solar spectrum is divided where photon energy equals to the bandgap of GaAs, i.e. 1.42 eV. Though the lower energy photons can be exploited by a silicon cell together with a GaSb or Ge cell with another spectrum splitting (Groß et al., 2009) or by a monolithic dualjunction cell, to reduce optical losses, the latter one should be adopted. For the monolithic dual-junction cell, the dependence of detailed-balance efficiency on the two bandgaps, i.e. Egbottom and Egtop, under the lower energy part of the AM1.5 g spectrum obtained by ideal beam splitting has been calculated, and the result is shown in Fig. 4. The indentations at Egbottom = 0.62 eV and 0.85 eV result from the optical absorption of atmosphere. According to the contour plot, it is found that the lattice-matched InGaAsP/InGaAs cell shown in Fig. 1 is quite close to the optimum bandgap selection. Then, the bandgaps of the selected four junctions are 1.88, 1.42, 1.05 and 0.73 eV respectively. It is interesting to note that this result is also an optimized bandgap combination for monolithic quadruple-junction solar cell (King et al., 2006). Meanwhile, the implementation difficulties of monolithic stacked method are avoided. 4. Experiments Based on the above analyses, a photovoltaic system composed of a dichroic mirror, a GaInP/GaAs cell and an InGaAsP/InGaAs cell, shown in Fig. 2, is constructed. The solar cell structures are grown by MOCVD in the hor-

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dual-junction solar cells have been investigated for the purpose of high-efficiency photovoltaic conversion. A real beam splitters will introduce losses, so a compromise should be made between the thermalization loss and light-splitting loss. A quadruple-junction system is constructed employing two monolithic dual-junction solar cells. The total device efficiency is 31.8% and the system efficiency is realized to be 29.2%. Acknowledgements

Fig. 5. Current density–voltage characteristics of lattice-matched GaInP/ GaAs (H) and InGaAsP/InGaAs (L) solar cells under ideal (I) and real (R) spectrum split. The measurements were carried out at one sun under AM1.5 g spectrum, 25 °C. Open circuit voltage (Voc), fill factor (FF), short circuit current density (Jsc) and efficiency (Eff.) are shown in the table.

5. Results and discussion In Fig. 5, the J–V characteristics with the dichroic mirror as beam splitter (labeled as LR for InGaAsP/InGaAs and HR for GaInP/GaAs) are presented. The J–V curves under the same spectrum with “ideal” spectrum split (labeled as LI for InGaAsP/InGaAs and HI for GaInP/ GaAs) are shown as well. The efficiency of the system (also referred as system efficiency) shown in Fig. 2 is 29.2%, calculated from LR and HR, while the total device efficiency is 31.8%, calculated from LI and HI. According to Fig. 5, the values of short circuit current density (Jsc) are reduced remarkably by using the dichroic mirror. To investigate the degradation, an average photon reflectivity weighted by AM1.5 g spectrum is calculated for the dichroic mirror in four wavelength ranges corresponding to the bandgaps of the four junctions. The obtained results are 94% for 400– 660 nm, 97% for 660–873 nm, 15% for 873–1180 nm and 4% for 1180–1700 nm. Under the ideal spectrum splitting condition, both of the dual-junction solar cells are current-matched. So the decreases of Jsc of GaInP/GaAs and InGaAsP/InGaAs are mainly caused by the absorption of TiO2 in the range of 400–500 nm and the reflection in the range of 873–1180 nm, respectively. In order to further reduce the losses and improve the system efficiency, the dichroic mirror should be made of materials with lower absorption in the short wavelength range. The solar cell system should be further optimized as a whole, too. 6. Conclusion In summary, the energy losses and the material selection of the spectrum-splitting photovoltaic system utilizing two

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