Proceedings of the European Summer School of Photovoltaics 4 â 7 ...

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Fig. 4. Current-voltage curve of single junction InGaAs cell Fig. 2. Photolithographic mask for front electrode Results of electro-optical measurements All three separately made structures of p-n junctions were measured in terms of their electrical parameters and spectral efficiency. Current-voltage characteristics were measured in a solar simulator PhotoEmission ss300B at Warsaw University of Technology. Epitaxial structures were not cut out because that could create shunts on edges. In long term, the front electrode will be created on the whole surface of the wafer. Fig. 3 shows light current-voltage curves of the PV cell with the area of 1 cm 2 under Standard Test Conditions (irradiation 1000 W/m 2 , spectrum AM1.5, cell temperature 25°C) for different pressures in the reactor during epitaxial growth. With pressure increasing up to 400 mbar, the surface quality was enhanced and the surface roughness decreased, which has influence on obtaining better electrical parameters. Open circuit voltage increased up to 230 mV and the efficiency of a single Ge junction was 5,5%. The middle InGaAs junction on the GaAs substrate has open circuit voltage equal to 0.78 V and the efficiency up to 9.9%. GaAs substrates have good surface quality , and there are no problems with epitaxial growth of layers with low roughness. Fig. 4 shows current-voltage and power characteristics of the cell. An evident influence of shunt resistance, the value of which is about 50 Ω, is observed. Such an increase would be possible after etching of edges, which will be done for the purpose of future works. Fig. 5. External quantum efficiency InGaP and InGaAs cell The external quantum efficiency of InGaP and InGaAs is showed in Fig. 5. The InGaAs cell absorbs photons in the range between 400 and 900 nm, although after stacking junctions one on the top of the other, spectrum in the range between 400 and 650 nm will be absorbed by the top InGaP junction. For an efficient operation of the cell it is important to match current flows through all three junctions, so that absorption coefficients, junction thickness, energy bandgaps are matched. It will cause uniform current flow and will reduce recombination loses. Top junction made of InGaP, as an independent structure has the efficiency of 11.1% and open circuit voltage 0.92 V. Summary Epitaxial structures made from 3 different materials act as photovoltaic solar cells in different spectrum ranges. Their electrical parameters are still improved by optimization of technological processes. We also plan to interconnect the separate junction with the stacked triple junction cell by tunnel junctions. This work was financed within the project POIG.01.03.01-00- 015/09 „Advanced materials and Technologies of their production. Development of technology of photovoltaics materials production.” References Fig. 3. Improvement of I-V curve caused by increase of pressure in reactor from 100 to 400 mbar [1] Market Report 2011, European Photovoltaic Industry Association, http://www.epia.org/publications/publications.html [2] Martin A. Green, Keith Emery, Yoshihiro Hishikawa and Wilhelm Warta, PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICA- TIONS Prog. Photovolt: Res. Appl. 2011; 19:84–92 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.1088. [3] G.M.M.W. Bissels*, M.A.H. Asselbergs, E.J. Haverkamp, N.J. Smeenk and J.J. Schermer, A new circular contact grid pattern, designed for solar cells in a mechanical stack. 108 Elektronika 6/2012
GaAsN as a photovoltaic material – photoelectrical characterization Paulina Kamyczek 1) , Ewa Placzek-Popko 1) , Piotr Biegański 1) , Eunika Zielony 1) , Beata Sciana 2) , Marek TłaczaŁa 2) 1) Institute of Physics, Wroclaw University of Technology 2) Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology GaAsN and InGaAsN materials have attracted considerable attention due to their unique physical properties and wide range of their possible application in optoelectronics, especially in infrared laser diodes for 1.3 and 1.55 mm [1, 2] and high efficiency multi-junction (MJ) solar sells [3, 4], where these low-band-gap materials can, in principle, be used to efficiently collect the lowphoton-energy portion of the solar spectrum [4]. In this paper the results of studies on the layers of GaAs 1-x N x grown on (100)-oriented Si-doped n-type GaAs substrates by atmospheric pressure metal organic vapour phase epitaxy APMO- VPE are presented. In the first step the layers of GaAs 1-x N x were characterized with the use of optical methods, then Schottky diodes were realized and the rectifying properties of the diodes were studied with electrical methods. The diodes exhibit light-energy conversion effect. Basic parameters of the solar cells ( short-circuit current I sc , open circuit-voltage, V oc , and fill factor, FF) were determined. Obtained results confirm that the diodes are promising as efficient solar cells. Samples The investigated structures were grown on (100)-oriented Sidoped n-type GaAs substrates by APMOVPE with AIX200 R&D AIXTRON horizontal reactor. Trimethylgallium (TMGa), tertiarybutylhydrazine (TBHy) and arsine (AsH3) were used as the growth precursors. High purity hydrogen with the total flow rate of 9.6 l/min was employed as a carrier gas. The hydrogen flow rate through the saturator was changed during runs with TBHy - VH2/ TBHy = 1500...2500 ml/min. Stable parameters were as follows: the growth temperature was Tg = 566°C , the arsine flow rate VAsH3 = 50 ml/min (for GaAsN) and 300 ml/min (for GaAs), the organic source temperatures: TTMGa = −10°C, TTBHy = 30°C. The thickness of Si-doped n-type GaAs substrates was set to 350 μm. Subsequently GaAs buffer layer of the thickness of 450 nm was grown. The layer of 200...300 nm thick GaAs 1−x N x was grown on top of the buffer layer. Gold Schottky contacts of 0.5 mm 2 area were prepared by electrolitography technique on the front side of the GaAs 1−x N x layer. An AuGe served as the ohmic contact to the n-type GaAs substrates (cf. Fig. 1). In this paper two kinds of samples labeled as N42N, N48N with various nitrogen content were investigated. Nitrogen content was determined from the spectral characteristics of transmittance (T) , reflectance (R) and photocurrent (PC). Experimental The optical properties were analysed using transmittance, reflectance and photocurrent spectral room-temperature (RT) measurements. The transmitted and reflected light was dispersed by BENTHAM spectrometer and detected by the Ge and Si detectors with a lock-in amplifier. The same system was used to perform photocurrent measurements. The latter were realized on Schottky Au–GaAs 1−x N x /GaAs diodes. The solar cell figures of merit (Isc, Voc and FF) were determined from current-voltage characteristics carried out in darkness and after illumination with halogen lamp. The dark and illuminated current-voltage (I-V) characteristics were measured by using Keithley 2601 current source meter. From transmittance or reflectance spectra the excitonic band gap can be obtained. The first derivative of the absorption coefficient has a maximum in the vicinity of the band gap [5] whereas reflectance spectrum exhibits a “dip” at the same wavelength. The value of the band gap (GaAs 1-x N x layers) can be also determined from the photocurrent spectrum more specifically from the midpoint of the PC drop. Once the energy of the excitonic band gap is determined, the GaAs 1-x N x nitrogen content x can be extracted from the equation: 2/3 ∆ E = 3.91x (1) where ΔE g = E g (GaAs) – E g (GaAs 1x N x ) [5, 6] (assuming RT E g (GaAs) = 1.42 eV). From I-V characteristics basic parameters characterizing solar cell can determined: open circuit voltage V OC and short circuit current I SC and a point of maximum power P max . Using this parameters a fill factor FF can be calculated: (2) Results and discussion g Optical properties of the studied heterostructures were verified by the room temperature (RT) transmittance (T), reflectance (R) and photocurrent (PC) spectral measurements. A comparison of the reflectance spectra with the derivative of transmission for N42N, N48 are given in Fig. 2a and 2b. The arrows correspond to the excitonic transition of GaAs 1-x N x and the dotted lines indicate GaAs band gap shifted with respect to the expected value of 1.42 eV due to Urbach tail. Figure 3 shows PC spectra of the structures with different nitrogen content in the GaAs 1−x N x epilayers (N42N and N48N). The photocurrent generated in GaAs 1−x N x can be seen above the absorption edge of the GaAs. In Table 1 the nitrogen content x obtained for the studied samples (N42N, N48N) from transmittance and reflectance as well as from PC measurements are collected. Tabl. 1. Nitrogen content obtained for the studied samples Sample x (%) T x(%)PC N42 1.08 1.2 N48 1.88 1.9 Fig. 1. A diagram of a GaAs 1−x N x /GaAs heterostructure The dark and illuminated I-V curves of the two structures are shown in Fig. 4, while the basic solar cell parameters of the diodes obtained from the curves are reported in Table 2. Elektronika 6/2012 109
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