• Energy Research

Experimental study of the band structure at the heterojunction interfaces of GaInP solar cells was performed by admittance spectroscopy. Admittance measurements were analyzed using numerical simulations. A good agreement between simulation and experiment was obtained. A potential barrier of about 0.6 eV at the p-GaAs/p-AlInP interface formed due to the high valence band offset was observed by the experiment. This high barrier creates fundamental limitation for the usage of this interface in p–n GaInP solar cells. A way to reduce the effective barrier height to 0.257 0.02 eV using a double layer p-AlGaAs/p-AlGaInP window avoiding deterioration of I–V curves was demonstrated.

A three-dimensional computer simulation of flexible double-junction solar cells (SC) consisting of Si wires and p-i-n a-Si:H structures was carried out. The performance dependence on geometrical and electrical parameters was calculated. With an increase in the height of the Si wires, the open-circuit voltage ( V OC ) decreases monotonically for both the bottom Si and the top p-i-n a-Si:H junctions. The short-circuit current density ( J SC ) for the top p-i-n a-Si:H junction increases sharply with Si wire height, and then, it goes into saturation at a wire height of more than 10-15 μm. The absolute value of J SC increases (from 10.2 to 12.7 mA/cm2) with a decrease in the wire diameter (from 2 to 0.5 μm). For the bottom junction based on Si wires, the dependence of J SC on the wire height is determined by the charge carrier lifetime, doping level, and diameter, which can be associated with the effect of complete inversion of the Si wire conductivity type. For tandem SCs, the optimal wire height is 10 μm, at which efficiency of 14% can be achieved for structures based on Si wires with a diameter of 0.5 μm and a charge carrier lifetime of 10 μs. The practical implication of the developed design was experimentally demonstrated.

Low-temperature plasma enhanced atomic layer deposition (PE-ALD) was successfully used to grow silicon (Si) doped amorphous and microcrystalline gallium phosphide (GaP) layers onto p-type Si wafers for the fabrication of n-GaP/p-Si heterojunction solar cells. PE-ALD was realized at 380 °C with continuous H2 plasma discharge and the alternate use of phosphine and trimethylgallium as sources of P and Ga atoms, respectively. The layers were doped with silicon thanks to silane (SiH4) diluted in H2 that was introduced as a separated step. High SiH4 dilution in H2 (0.1%) allows us to deposit stoichiometric GaP layers. Hall measurements performed on the GaP:Si/p-Si structures reveal the presence of an n-type layer with a sheet electron density of 6–10 × 1013 cm−2 and an electron mobility of 13–25 cm2 V−1 s−1 at 300 K. This is associated with the formation of a strong inversion layer in the p-Si substrate due to strong band bending at the GaP/Si interface. GaP:Si/p-Si heterostructures exhibit a clear photovoltaic effect, with the performance being currently limited by the poor quality of the p-Si wafers and reflection losses at the GaP surface. This opens interesting perspectives for Si doped GaP deposited by PE-ALD for the fabrication of p-Si based heterojunction solar cells.

The GaP/Si heterojunctions fabricated by molecular beam epitaxy (MBE) and plasma enhanced atomic layer deposition (PE-ALD) were studied. The degradation of charge carrier lifetime in Si was observed during the growth of single-crystalline GaP layer on Si substrates by MBE at 500-600 °C. The study performed by PL and DLTS has demonstrated the presence of the defective layer in Si, which is located within ~30 nm near to the GaP/Si interface. This defective layer leads to significant reduction of solar cell performance for anisotype n-GaP/p-Si heterojunction due to strong recombination in the space charge region. The GaP/Si heterostructure with Si n-p homojunction exhibits better performance compared to the anisotype n-GaP/p-Si heterojunction because the defective layer is located in the n-Si emitter formed by intentional P diffusion. On the contrary, the deposition of amorphous GaP layer by PE-ALD at T < 380 °C does not lead to the degradation of Si wafer charge carrier lifetime.

AbstractThe n-GaInP/n-p Ge heterostructure sloar cells grown by OMVPE were studied by new technique based on the diffusion capacitance measurements under illumination at open circuit voltage conditions. The value of the effective minority carrier lifetime in the p-Ge base τeff=1.5×10−6s was determined from the frequency dependence of the diffusion capacitance, C(f). The numerical simulation model was developed, which is in a good agreement with the experimental C(f), capacitance-voltage and dark current-voltage measurements. The value of the bulk minority lifetime equal to τb=5×10−6s was obtained by the simulation fit of the experimental data. The simulations have also demonstrated that the diffusion capacitance is very sensitive to the recombination velocity at the back contact, Sbc, for Sbc¡105 cm/s providing a way to characterize the back contact quality. Thus the proposed technique could be very useful for the back surface passivation development in GaInP/Ge solar cells.

Boron phosphide (BP) thin films are potential selective contact layers for photovoltaic (PV) devices. However, lower deposition temperatures are preferred in the fabrication of many PV devices, such as solar cells, to reduce their cost. Here, boron phosphide layers were grown on silicon (100) substrates using the standard plasma-enhanced chemical vapor deposition (PECVD) and time-modulated PECVD methods with trimethylboron [B(CH3)3, TMB] and phosphine precursors. The effect of plasma power and Ar addition on structural properties and chemical composition is investigated, while material properties are analyzed by transmission electron microscopy. Chemical characterization by the electron diffraction x-ray spectroscopy method showed high carbon content in the BP layer. Electron energy loss spectroscopy demonstrated almost stoichiometric B and P (1:1) content. Raman spectroscopy of annealed samples showed an increase in carbon-related peaks, therefore indicating that annealing does not lead to the crystallization of boron phosphide. Thus, using TMB as a precursor of boron leads to carbon contamination in both standard and time-modulated modes. Optical emission spectroscopy showed that the low-temperature growth of BP without plasma (Ar or phosphine) assistance using TMB is impossible. We conclude that there is a need to investigate other boron precursors for boron phosphide low-temperature growth to avoid carbon contamination in BP films.

The paper presents the results of the study of the effect of plasma etching of n-type silicon spin-coated with polystyrene balls on bulk silicon properties using careful surface chemical treatments and passivation. Plasma etching was carried out under different substrate temperatures (up to -140 °C). Photoluminescence decay time imaging was used to evaluate effective carrier lifetime in silicon samples after dry etching and a-Si:H passivation. According to the results obtained, plasma etching of n-Si(100) spin-coated with polystyrene balls within the temperature up to -140 °C in gas mixture of SF6/O2, RF and ICP plasma power of 30 W and 1000 W, respectively leads to minority carrier lifetime decrease. The difference in the rate of carrier lifetime degradation with etching time was found.