| Silicon is the most important material in solar cells,however,its low band gap(around 1.1 eV)makes the effective spectral response range from 500 nm to 1100 nm,since the infrared and near-UV light in solar spectrum cannot be used efficiently to generate electric energy.It is expected that the broadband spectral absorption can be realized by forming the all Si-based tandem type solar cells,whose power conversion efficiency can exceed the Shockley-Queisser limit.As a low-dimensional quantum material,the energy band structures of Si quantum dots(Si QDs)can be well changed by controlling the dot size because of the quantum confinement effect,which provides an effective approach to modulate the bandgap of Si QDs to match the broad solar spectrum efficiently.As a consequence,the study on Si QDs-based advanced high efficiency photovoltaic devices has attracted much attention in the recent years.The main topic of the thesis is to study the fabrication and device performance of Si QDs-based hetero-junction solar cells.First of all,the amorphous Si(a-Si)/SiC multilayers(MLs)with various amorphous silicon thicknesses were deposited in plasma enhanced chemical vapor deposition(PECVD)system and the Si QDs/SiC MLs were obtained after high temperature thermal annealing.The microstructures and optical properties were characterized.The prototype p-i-n structure solar cell devices containing size controllable Si QDs were fabricated and the photovoltaic properties were investigated.In order to further enhance the power conversion efficiency(PCE)of the solar cells,we developed nano-sphere lithography technique and metal-assisted chemical etching technique to get Si sub-wavelength nanostructures with good antireflection characteristic.Light harvesting and enhanced performance of Si QDs-based solar cells were achieved.Furthermore,we proposed a novel hetero-junction cell structure containing graded-sized QDs-based multilayers,which extended the spectral response range and improved the device performance.By combining the graded-sized Si QDs multilayers with light-trapping structure,the corresponding PCE of heterojunction exceeded 10%.Moreover,we prepared all-Si QDs-based p-i-n solar cells on glass substrates with laser induced crystallization technique and studied the photovoltaic properties.The main results and innovations of the thesis are shown below.1.We fabricated Si QDs/SiC MLs by annealing a-Si/SiC MLs at 900℃.The average dot size was around 4.9 nm and the optical band gap of Si QDs/SiC MLs was about 1.48 eV.Subsequently,the p-i-n device structure containing phosphorus-doped a-Si and Si QDs/SiC MLs on p-Si substrate was fabricated.Al electrode was evaporated on the surface and rear side of p-type Si substrates.The prototype solar cell device was founded and the PCE of 6.28%was achieved.Compared with that of the solar cell containing a-Si/SiC MLs,the Si QDs-based solar cell had significantly enhanced external quantum efficiency(EQE)result.It can be attributed to the formation of Si QDs which enhances the absorption of incident photons,especially in the short wavelength range.Meanwhile,it was observed that the spectral response wavelength was red shifted with increasing the size of Si QDs,due to the quantum confinement effect.In order to further investigate the contribution from Si QDs in the hetero-junction solar cells,we used heavily-doped p type(p+)Si substrates instead of general Si substrates to get cell structures.Similarly,the observed EQE was located the spectral range of 300 nm-800 nm and was red shifted with increasing the dot size.2.In order to reduce the reflection and enhance the optical absorption of ultrathin Si QDs/SiC multilayers in a wide spectral range,we developed nano-sphere lithography technique to get Si sub-wavelength nanostructures.The mean surface reflectance weighted by AM 1.5 solar spectrum(300 nm-1200 nm)was as low as 5.9%.Subsequently,Si QDs/SiC multilayers were deposited on sub-wavelength nanostructures to obtain hetero-junction solar cells.The photovoltaic properties were improved compared to the reference flat cell and the corresponding PCE was enhanced to 10.55%.Moreover,we used metal-assisted chemical etching method to achieve vertically aligned Si nanowire(Si NW)arrays with high aspect-ratio,which was on a large area simple and low-cost process.The optimal Si NWs lead to the best cell with the short circuit current density(Jsc)of 29.1 mA/cm2,the open circuit voltage(Voc)of 556 mV,the fill factor(FF)of 70.1%and the highest PCE of 11.35%,which can be attributed to the enhanced light harvesting of Si NWs structures and the improved carrier collection efficiency.3.In order to circumvent the narrow spectral response of Si QDs-based solar cells,a novel hetero-junction cell structure containing graded-sized QDs-based multilayers was proposed.The size of Si QDs varies from 8 nm to 2 nm which corresponds with the bandgap from 1.2 eV to 2.1 eV.The graded-sized Si QDs-based hetero-junction cell exhibits an enhanced spectral response in a wavelength range from 400 nm to 1200 nm,which is obviously improved compared with that of conventional Si QDs-based cells,which can be attributed to the efficiently match the incident solar photons and the optical bandgaps of Si QDs with various dot size.Furthermore,by combining the graded-sized Si QDs multilayers with Si NW arrays,a Si QDs/Si NWs hetero-junction solar cell was fabricated and the corresponding PCE can be as high as 12.80%,due to the significant spectral loss suppression and optical absorption enhancement by forming nano-patterned light trapping structures.4.In order to further investigate the potential applications of Si QDs in new generation solar cells,we prepared the p-i-n structure based on Si QDs/SiC multilayers on the Indium Tin Oxide(ITO)transparent electrode coated glass substrates by using the laser induced crystallization technique to get all-Si QDs-based solar cells.KrF pulsed excimer laser was used to crystallize a-Si layer to form Si QDs,which could avoid damaging the ITO glass substrates during the high-temperature annealing process.The rectification characteristics was observed and the rectification ratio of p-i-n structures reached to 104 at the applied voltage V=±3 V,which indicated that the p-i-n structures are well formed with the laser crystallization method.It was found that EQE was located the spectral range of 300 nm-700 nm.The cell area was about 0.8 cm2.The photovoltaic property was observed and the Voc and the Jsc was 475 mV and 0.28 mA/cm2,respectively. |
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