| Silicon heterojunction(SHJ)solar cells are p-n junction devices,which are formed by depositing amorphous silicon films on crystalline silicon substrates.SHJ solar cells with high conversion efficiency,low temperature coefficient,no light induced degradation(LID)and potential induced degradation(PID)effects can generate energy from both sides.SHJ solar cells will play an important role in the development of high efficiency solar cells.However,for a long time,the research and development of SHJ solar cells in China are lagged behind,so the research on SHJ solar cells is of great practical significance and strategic significance.Based on the transport mechanism of photo-generated carrier,SHJ solar cells were studied through solving the Poisson equation,the continuity equations and transport equations of electrons and holes.Interface passivation effect,spectral response and carrier transport characteristics were studied.Finally,SHJ solar cells with total area conversion efficiency of 22.4%were prepared.Preparation process which can be transferred to the production line has been summarized,which greatly promoted the development in industrialization of SHJ solar cells in China.Major works and some significant research progresses are as follows:(1)Based on the theoretical study,it is pointed out that low interface defect density and optimized band structure are the key requirements for high efficiency SHJ solar cells.Effects of material properties and structural parameters on the performance of SHJ solar cells were studied,showing it is an effective way to improve the conversion efficiency of solar cell through decreasing the thickness of amorphous silicon layers under the premise of ensuring good interface passivation and carrier transport characteristics.(2)For a large area(238.95 cm~2)n-type monocrystalline(100)silicon substrate,the key to obtaining high quality interface passivation has been summarized,and the passivation mechanism is clarified,the open-circuit voltage of SHJ solar cells exceeds730 mV(close to the highest reported value).The surface morphology and roughness of silicon substrate will seriously affect the quality of interface passivation,our study shows that controlling surface roughness and smoothing pyramid texture are essential for high efficiency SHJ solar cells.Two-step method of preparing amorphous silicon layer has been proposed,the interface passivation quality is greatly improved,since the plasma damage is avoided.Hydrogen with high chemical activity plays an important role in saturating dangling bonds,increasing hydrogen content can effectively improve the interface quality and the degradation in passivation after the deposition of p-type amorphous silicon film is inhibited.Through process optimization,the minority carrier lifetimes of the crystal wafers reach 5000μs and 3500μs after the deposition of intrinsic amorphous silicon films and doped amorphous silicon films respectively.(3)Rear-junction structure is proposed,through adopting n-type amorphous silicon film as window layer,excellent spectral response and carrier transport characteristics as well as good filed passivation are achieved,the short-circuit current density reaches 39 mA/cm~2,and the fill factor exceeds 78%which are nearly 95%of the highest reported values.Energy conversion efficiency of SHJ solar cells reaches22.4%by introducing new TCO films.The study shows that the introduction of amorphous silicon oxide films effectively improves the short wavelength spectral response.The free carrier concentration could be regulated by controlling the oxygen vacancy concentration,which improves the long wavelength spectral response.The studies also show that hydrogen can also passivate defect states of TCO films,weaken scattering effect of defects on the carriers.It is a new method to improve carrier mobility of TCO films,and the development of hydrogen,titanium co-doped indium oxide film greatly improves the transverse transport of carriers. |
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