Germanium photovoltaic cells with MoOx hole selective contacts
Year: 2019

Very thin, thermally evaporated MoOx (x 3) layer has been used as transparent hole selective contact on an ntype Germanium substrate to effectively demonstrate PV conversion capability. The fabricated MoOx/Ge heterojunction PV cell shows a photocurrent density of 44.8 mA/cm under AM1.5G illumination, which is comparable to that of conventional Ge PV cells. However, a low open circuit voltage of 138mV is obtained, which might be explained by the presence of tunnelling mechanisms through the MoOx/Ge interface. To our knowledge, this is the first demonstration of a hole selective contact made of transition metal oxide on an n type semiconductor different from c Si. Thus, this work may have important implications toward the development of new device architectures, such as novel low cost Ge PV cells with possible applications in multijunction solar cells and thermophotovoltaics. Historically, the driving force for the use of Ge in photovoltaic (PV) applications has been as a substrate for GaAs space solar cells (Miller and Harris, 1980) the main reason being the higher thermal conductivity and the possibility of manufacturing thinner and lighter wafers with Ge than with GaAs. Later on, Ge/GaAs tandem solar cells were pursued to enhance the conversion efficiency (Chand et al. 1986) by using the Ge bottom cell to convert the infrared part of the solar spectrum. This progress eventually derived in the development of the current standard technology for space solar cells that consists of triple junction Ge/GaAs/GaInP structures, with AM0 conversion efficiencies in the range of 28 30% These cells have been also used in terrestrial applications within concentrated PV (CPV) systems, where they reached AM1.5D conversion efficiencies of 41.6% King et al. 2009) just below the current world record for solar to electricity conversion efficiency of 46.0% (Dimroth et al. 2016) Apart from solar applications, Ge PV cells have been considered as a low cost replacement for low band gap III V semiconductors in thermophotovoltaic (TPV) converters, in which thermal radiation is directly converted into electricity by infrared sensitive PV devices (Bauer, 2011; Chubb, 2007) In this context, Ge TPV cells could be used in a broad range of applications such as waste heat recovery (Bauer et al. 2003) solar thermal power (Ungaro et al. 2015; Lenert et al. 2014; Datas and Algora, 2013) space power (Datas and Marti, 2017) and energy storage (Datas et al. 2016) among many others. Current state of the art of Ge PV cells consist of p n junctions created by diffusion of dopants at high temperatures (Bitnar, 2003) For instance, p n junctions in p Ge have been created by diffusion of Vgroup atoms (typically P and As) during the first growing step of GaInP or GaAs nucleation layers within a Metal Organic CVD (MOCVD) reactor at temperatures of ~650 degC (Fernandez et al. 2008; Fernandez, 2010;  2014) Other groups have used the diffusion of Zn in n Ge substrates within a LPE reactor (Khvostikov et al. 2002) In an effort to reduce manufacturing costs of standalone Ge PV cells, IMEC reported devices with p n junctions created by spin on diffusion of P on p Ge by rapid thermal annealing at different temperatures (450 700 degC) (Posthuma et al. 2007; van der Heide, 2009; van der Heide et al. 2009) leading to the best reported 1 sun AM1.5G conversion efficiency for stand alone Ge PV cells of 7.9% (van der Heide et al. 2009) Surface passivation has been accomplished by forming different kinds of heterojunctions on Ge surface, such as Ge/GaAs (Khvostikov et al. 2002) or Ge/GaInP (Fernandez et al. 2008; Fernandez, 2010;  2014) by MOCVD or LPE (Khvostikov et al. 2002) or Ge/a Si (Posthuma et al. 2007; van der Heide, 2009; van der Heide et al. 2009; Posthuma et al. 2005) Ge/ SiNx (Nagashima et al. 2007) and Ge/a SixC1 x (Fernandez et al. 2008; Fernandez, 2010; Weiss et al. 2018) by PECVD. In order to further reduce the fabrication cost of Ge PV cells, it is desirable to eliminate the high temperature diffusion, and complex MOCVD or PECVD processes. In this regard, a particularly appealing option consists of substituting the doping step by carrier selective coatings with surface passivation properties that could be deposited at low temperatures. For this purpose, high electron affinity transition https:/doi.org/10.1016/j.solener.2019.02.009 Received 29 August 2018; Received in revised form 21 January 2019;  February 2019 Corresponding authors. Solar Energy 181 (2019) 357 360 0038 092X/ (c) 2019 International Solar Energy Society.  T metal oxides (TMOs) such as MoO3, WO3, and V2O5, are very interesting candidates that have already been found effective to produce hole selective contacts on both n type and p type c Si (Gerling et al. 2015; Battaglia et al. 2014; Bullock et al. 2014; Vijayan et al. 2018) In this letter we report a Ge PV cell formed by a thin sub stoichiometric MoOx (x 3) layer on top of an n type crystalline Ge (c Ge) substrate, which behaves as a hole selective contact. To our knowledge, this is the first demonstration of a hole selective contact made with a TMO on an n type semiconductor different than c Si. Thus, it might open the door to new device architectures, not only for PV applications, but also in photonics and CMOS electronics, where the integration of TMOs is being investigated (Sanchez et al. 2016) along with the use of different semiconductors having higher carrier mobilities and extended spectral response than c Si, such as Ge (Reboud et al. 2017; Toriumi and Nishimura, 2017) The PV cell structure was fabricated on (1 0 0) oriented, Czochralski, n type Ge substrates (r=0.37O*cm, 350 mm thick) The substrate was cleaned by HCl: H2O (33% immediately prior to rear side passivation by PECVD of (i/n) a SiCx:H (4/15 nm, x~ 0.2) and a SiC (80 nm) stack deposited at ~300 degC. Next, the rear contact was created by laser firing of the a SiC stack to produce an array of ~60 mm diameter local diffusion points, separated by 600 mm pitch. This distance was chosen to introduce reasonable series resistance of ~0.1Ocm (Fischer, 2003) Laser firing was accomplished by means of a ~1200mW, l=1064 nm Nd/YAG laser system at a frequency of 4 kHz with 6 pulses per spot, following a similar approach developed for c Si devices (Lopez et al. 2018) The rear contact was finalized by means of an e beam evaporated Ti/Pd/Ag metal stack that provides lateral interconnection between fired points. The hole selective contact was formed at the front side of the device by means of very thin (nominally 20 nm) MoOx layer thermally evaporated from powdered MoO3 sources at ~8*10 mbar and a deposition rate of ~0.2 A/s (Gerling et al. 2015) A 75 nm thick ITO layer was subsequently deposited by RFSputtering on top of the MoOx layer to increase lateral electrical conductivity and minimize optical reflectivity. A sketch of the full PV cell structure and the TEM image of the MoOx/ITO interface are shown in Fig. 1, where a pronounced inter diffusivity between the layers is clearly observed. The 1x1 cm active area of the PV cells was defined by conventional lithographic techniques followed by mesa etching of the MoOx/ITO layers. Finally, the front Ag grid electrode (2 mm thick) was evaporated through a shadow mask for a 4% contacted area. One sun I V characterization of the PV cell was performed at 25 degC using a ORIEL 94021A (Newport) solar simulator, where the distance between the cell and the lamp was varied until the photo generated current density coincides with the one calculated through the integration of the cell' experimental external quantum efficiency (EQE) and the AM1.5G spectrum with 100mW/cm. The corresponding current density voltage (J V) curve under 1 sun illumination is shown in Fig. 2. The short circuit current density (JSC= 44.8mA/cm) outperforms that of the best performing state of the art Ge PV cells (43.2mA/cm) (van der Heide et al. 2009) On the other hand, a much lower open circuit voltage (138mV) is measured, compared to those reported in (Fernandez, 2010; van der Heide et al. 2009) (up to 265mV) which ultimately results in a lower FF (40.9% partially due to a non optimized metal grid that introduces a series resistance of 0.65 Ocm. As a result, an AM1.5G conversion efficiency of 2.53% is obtained. External quantum efficiency (EQE) of the PV cell is shown in Fig. 3 at short circuit conditions along with the EQE of Ge PV cells reported in (van der Heide, 2009) for a direct comparison. The improved EQE for wavelengths shorter than 600 nm might be explained by the reduction of the recombination close to the front surface compared to the one existing in the highly doped emitters (10 10 cm) used in (van der Heide, 2009) Such a low recombination does not necessary indicate a good chemical surface passivation, i.e. strong reduction of interface state density, but it could be related to a strong electric field that unbalances carrier densities, i.e. field effect passivation. The electric field located at the surface creates an electrostatic potential which, under thermal equilibrium, is the potential barrier built at the junction (Vbi) Then, the higher Vbi, the stronger the electric field and, thus, the better the surface passivation. In order to measure Vbi, capacitance voltage measurements of the cell in reverse bias were performed at 10 kHz using HP4294A impedance analyser with a signal amplitude of 30mV following the same approach than in (Almora et al. 2017) where similar structures on c Si substrates are characterized. This data can be Fig. 1. Sketch of the fabricated solar cell.  TEM image of the MoOx interlayer of the final device.  Current density voltage curve of the Ge PV cell manufactured in this work under AM1.5 G illumination conditions. A. Alcaniz, et al. Solar Energy 181 (2019) 357 360