Study On Fabrication And Performance Of Silicon Based Heterojunction Solar Cells With Metal Oxides As Carrier Selective Layers

The mainstream dopant-diffused crystalline silicon（c-Si）solar cells（SCs）are currently the most efficient,technically mature and widely applied photovoltaic（PV）technologies among the various SCs.However,recombination losses at the directly-metalized and heavily-doped regions have a significant,and often limiting effect on device performance.The wisdom on addressing the losses is to fabricate SCs with metal oxides as carrier selective layers,which avoids Auger recombination loss caused by doping and the direct contact between c-Si and metal and simplify cell architectures and fabrication procedures.However,carrier selective materials are greatly influenced by the environment and fabrication process,which seriously affects device efficiency.Based on these,this paper focused on carrier selective layers,explored the influential factors and optimized interfacial effects,aiming to improve interfacial properties and fabricate high efficiency SCs.The main contents are summarized as follows:（1）MoO_x film was deposited using electron-beam-evaporation process.After optimization high passivation quality,a low contact resistance and a strong surface inversion layer were achieved.An additional indium tin oxide（ITO）layer upon the MoO_x film was also required to improve the collection of carriers,yielding a high efficiency of 12.8%and an open circuit voltage(V_oc)of 580 mV without passivation layers on both sides,which demonstrates the possibility of using evaporation-processed metal oxides as an effective hole-selective passivating contact.The thickness of MoO_x and ITO films were optimized to reduce the optical reflection loss and thus enhance the short-circuit current density(J_sc)of the related SCs.Besides,under the protection of ITO,V_oc of this type of SC keeps high stability for 20 days.Moreover,heterojunction SCs with MoO₂ film as the hole-selective contact were fabricated using electron-beam-evaporated to verify the influence of O/Mo atomic ratio.After comparison,we found that SCs with low stoichiometric MoO₂ film could also achieve a high efficiency and a high V_oc of 570 mV,because the higher conductivity MoO₂ film could also induce a strong band bending.（2）Based on the above mentioned process,the electron-beam-evaporated molybdenum oxide（MoO_x）and magnesium oxide（MgO_x）were directly deposited upon the front and rear surface of c-Si substrates to form carrier selective contacts with asymmetric band offset for holes and electrons,respectively.Considering a good carrier-selectivity at the rear side,the optimum thickness in terms of contact resistivity and electrical parameters was obtained under 1.5 nm-thick MgO_x film.External quantum efficiency spectra（EQE）,capacitance-voltage（C-V）,scanning kelvin probe microscopy（SKPM）and X-ray photoelectron spectroscopy（XPS）measurements confirmed the positive roles of MgO_x in reducing the barrier height and suppressing the carrier recombination at Si/Al interface.Finally,planar heterojunction SCs with a core structure of MoO_x/n-Si/MgO_x were fabricated with the efficiencies up to 14.2%via a dopant-free and low-temperature fabrication procedure,demonstrating new possibilities in designing and fabricating low-cost optoelectronic devices.（3）As to the issue that carrier-selective materials,especially for those metal oxides deposited via a low temperature evaporation,lack of or even without chemical passivation to c-Si wafers irrespective of their successful effect on reduction of contact resistivity.Here,we demonstrated that electron beam evaporated magnesium oxide（MgO_x）thin film can work as a promising electron-selective passivating contact for n-type c-Si SCs after a post-annealing treatment and an alumina-initiated atomic hydrogenation.Comprehensive characterizations revealed the formation of Si-O-Mg bonds and the activition of atomic hydrogen passivation were the main reasons for such high-level passivation.10 nm MgO_x provided excellent surface passivation for the n-Si wafer,manifested by a low surface recombination velocity of 14.9 cm/s.1 nm MgO_x/n-Si showed a low contact resistivity of 14 mΩ·cm².A PERC-like dopant-free rear contact was formed by using the 1 nm MgO_x as electron-collector and the 10 nm MgO_x as passivating layer.The SCs featuring PERC-like MgO_x contact with optimum area-ratio achieved 27%increment in efficiency and 51 mV increase in V_oc in comparison with reference devices（n-Si/Al contact）.The ways of improving the passivation quality of MgO_x and novel design of contact structure obviously open up the possibility of using evaporation-processed metal oxides as an effective and low-cost carrier-selective passivating contact for c-Si PV devices.