Defect Densities, Diffusion Lengths and Device Physics in Nanocrystalline Si:H Solar Cells

2004 ◽  
Vol 808 ◽  
Author(s):  
Vikram L. Dalal ◽  
Puneet Sharma ◽  
David Staab ◽  
Max Noack ◽  
Keqin Han
Keyword(s):  
Solar Cells ◽  
Quantum Efficiency ◽  
Diffusion Length ◽  
Deep Level ◽  
Plasma Deposition ◽  
Solar Spectrum ◽  
Base Layer ◽  
Degree Of Crystallinity ◽  
High Hydrogen ◽  
Ecr Plasma

ABSTRACTWe report on the properties of nanocrystalline Si:H solar cells. The solar cells were of the p+nn+ type, with the n+ layer deposited first on a stainless steel substrates. The solar cells were prepared under high hydrogen dilution conditions using either ECR plasma deposition, or VHF diode plasma deposition processes. The deposition pressures were kept low, 5 mTorr in the ECR reactor and 50 mTorr in the VHF reactor. All the solar cells reported showed a high Raman ratio of crystalline to amorphous peaks. Properties such as dark current, deep level defects and shallow doping densities, and hole diffusion lengths were measured in these cells. It was found that the base layer was always n type, but that its doping could be changed by adding ppm levels of B during growth. A sufficient B doping even type converted the base layer to p type. It was found that there was a good one-to-one correlation between the shallow doping and deep level defects, suggesting that the same element, probably oxygen, is responsible for generating both shallow dopants and deep levels. The diffusion length of holes was measured in these cells using quantum efficiency vs. voltage techniques, and it was found that the diffusion length data could be explained very well by invoking trap-controlled recombination statistics. The dark I(V) curves could be represented by a standard diode model for highly crystalline materials, but as the degree of crystallinity was reduced, the diode factor increased. Voltage could be improved by reducing the crystallinity of the layer, but doing so resulted in a decrease in quantum efficiency in the infrared regions of the solar spectrum.

Microcrystalline (Si,Ge):H Solar Cells

2003 ◽  
Vol 762 ◽  
Author(s):  
Jianhua Zhu ◽  
Vikram L. Dalal
Keyword(s):  
Solar Cells ◽  
Buffer Layer ◽  
Quantum Efficiency ◽  
Fill Factor ◽  
Open Circuit Voltage ◽  
Defect Density ◽  
Cell Structure ◽  
Open Circuit ◽  
Base Layer ◽  
Ecr Plasma

AbstractWe report on the growth and properties of microcrystalline Si:H and (Si,Ge):H solar cells on stainless steel substrates. The solar cells were grown using a remote, low pressure ECR plasma system. In order to crystallize (Si,Ge), much higher hydrogen dilution (∼40:1) had to be used compared to the case for mc-Si:H, where a dilution of 10:1 was adequate for crystallization. The solar cell structure was of the p+nn+ type, with light entering the p+ layer. It was found that it was advantageous to use a thin a-Si:H buffer layer at the back of the cells in order to reduce shunt density and improve the performance of the cells. A graded gap buffer layer was used at the p+n interface so as to improve the open-circuit voltage and fill factor. The open circuit voltage and fill factor decreased as the Ge content increased. Quantum efficiency measurements indicated that the device was indeed microcrystalline and followed the absorption characteristics of crystalline ( Si,Ge). As the Ge content increased, quantum efficiency in the infrared increased. X-ray measurements of films indicated grain sizes of ∼ 10nm. EDAX measurements were used to measure the Ge content in the films and devices. Capacitance measurements at low frequencies ( ~100 Hz and 1 kHz) indicated that the base layer was indeed behaving as a crystalline material, with classical C(V) curves. The defect density varied between 1x1016 to 2x1017/cm3, with higher defects indicated as the Ge concentration increased.

Nanocrystalline Germanium and Germanium Carbide Films and Devices

2005 ◽  
Vol 862 ◽  
Author(s):  
Xuejun Niu ◽  
Jeremy Booher ◽  
Vikram L. Dalal
Keyword(s):  
Grain Size ◽  
Plasma Deposition ◽  
Open Circuit ◽  
Image Sensors ◽  
High Hydrogen ◽  
Strong Function ◽  
Germanium Carbide ◽  
Ecr Plasma ◽  
Hydrogen Dilution ◽  
Steel Substrates

AbstractNanocrystalline Ge and its alloys with C are potentially useful materials for solar cells, thin film transistors and image sensors. In this paper, we discuss the growth and properties of these materials using remote, low pressure ECR plasma deposition. The materials and devices were grown from mixtures of germane, methane and hydrogen. It was found that high hydrogen dilutions (>40:1) were needed to crystallize the films. Studies of x-ray spectra revealed that the grains were primarily <220> oriented. The grain size was a strong function of hydrogen dilution and growth temperature. Higher growth temperatures resulted in larger grain size. High hydrogen dilution tended to reduce grain size. These results can be explained by recognizing that excessive amounts of bonded H can inhibit the growth of <220> grain, which is the thermodynamically favorable direction for grain growth. Grain sizes as large as 80 nm were obtained in nc-Ge. Addition of C reduced the crystallinity. Mobility and carrier concentrations in nc-Ge were measured using Hall effect. Mobility values of ˜5cm2/V-sand carrier concentrations of ˜1x1016/cm3were obtained in larger grains. p+nn+ devices were fabricated on stainless steel substrates and compared with similar devices deposited in nc-Si:H. It was found that the voltage decreased and current increased in nc-Ge devices, in comparison with devices in nc-Si:H. Addition of C to Ge devices increased the open circuit voltage and shifted the quantum efficiency to larger photon energies, as expected.

Properties of Substrate-Type a-Sihh Devices Prepared Using ECR Conditions

1996 ◽  
Vol 420 ◽  
Author(s):  
Vikram L. Dalal ◽  
Sanjeev Kaushal ◽  
Robert Girvan ◽  
Levent Sipahi ◽  
Swati Hariasra
Keyword(s):  
Stainless Steel ◽  
Quantum Efficiency ◽  
High Temperatures ◽  
Plasma Deposition ◽  
Type A ◽  
Substrate Type ◽  
Ecr Plasma ◽  
Hydrogen Dilution ◽  
Steel Substrates ◽  
Deposition Techniques

AbstractWe report on the preparation and properties of a-Si:H devices prepared at high temperatures on stainless steel substrates using low pressure ECR plasma deposition techniques. The devices were prepared using either Hydrogen or Helium as the plasma diluent gas. The use of He as the plasma gas led to films having significantly lower H concentration(4–5%) and a lower bandgap than comparable films made using hydrogen dilution. We find that we can make excellent devices, with good fill factors(over 70%) and voltages(over 0.86 V) using either H or He dilution. The use of He led to devices having smaller bandgap, by about 35–40 meV compared to devices made using H dilution. Detailed quantum efficiency measurements show that the hole collection in both types of devices is excellent, and that Urbach energies of tail states in each case is in the range of 43–45 meV.

Properties of Nanocrystalline Germanium-Carbon Films and Devices

2003 ◽  
Vol 762 ◽  
Author(s):  
X.J. Niu ◽  
Vikram L. Dalal ◽  
Max Noack
Keyword(s):  
Crystal Structure ◽  
Plasma Deposition ◽  
Amorphous Material ◽  
Carbon Films ◽  
Open Circuit ◽  
Base Layer ◽  
Photovoltaic Properties ◽  
Ecr Plasma ◽  
Hydrogen Dilution ◽  
N Doping

AbstractNanocrystalline Germanium-Carbon alloys, denoted by nc- (Ge,C):H, are a potentially useful new electronic material whose bandgap can be varied by changing the Ge:C ratios. We have shown previously that nanocrystalline (Ge,C):H films can be grown using remote ECR plasma deposition. In this paper, we report on the crystal structure, electron mobility and some device related properties of these materials. The materials were grown using mixtures of either Germane and methane, with significant hydrogen dilution, or from mixtures of ethylene and germane, also with significant hydrogen dilution. X-ray diffraction measurements indicated a predominantly <111> crystal structure. The grain size was in the range of 10 nm. Raman measurements clearly show the 300 cm-1 Ge peak in the films. Electron Hall mobilities were measured in these films and were found to be in the range of 2.5-3 cm2/V-sec. Proof-of-concept p+nn+ junction devices were fabricated and showed distinct photovoltaic properties. The open circuit voltage was found to be a strong function of the itnerfaces between n+ and n layers, and between p+ and n layers. The use of an amorphous n+/n interface at the back of the improved the device performance significantly. Capacitance measurements indicated that the device behaved according to standard p/n junction theory, with the n- doping in the base layer being in the 1017/cm3 range. Thus, the base layer was not intrinsic, but rather n type, as may be expected for a crystalline as opposed to amorphous material. Quantum efficiency data indicated that as C was added to the material, the edge of the QE curve shifted to higher photon energies, indicating larger bandgaps.

Oxygenated Protocrystalline Silicon Thin Films for Wide Bandgap Solar Cells

2010 ◽  
Vol 1245 ◽  
Author(s):  
Ruud E.I. Schropp ◽  
Jan Willem Schüttauf ◽  
Karine van der Werf
Keyword(s):  
Solar Cells ◽  
Temperature Coefficient ◽  
Solar Spectrum ◽  
Wide Bandgap ◽  
Energy Yield ◽  
High Hydrogen ◽  
Silicon Thin Films ◽  
Hydrogen Dilution ◽  
Multijunction Solar Cells ◽  
Reduced Temperature

AbstractProtocrystalline silicon, which is a material that has enhanced medium range order (MRO), can be prepared by using high hydrogen dilution in PECVD, or, alternatively, using high atomic H production from pure silane in HWCVD. We show that this material can accommodate percentage-level concentrations of oxygen without deleterious effects. The advantage of protocrystalline SiO:H for application in multijunction solar cells is not only that it has an increased band gap, providing a better match with the solar spectrum, but also that the solar cells incorporating this material have a reduced temperature coefficient. Further, protocrystalline materials have a reduced susceptibility to light-induced defect creation. We present the unique result in the PV field that these oxygenated protocrystalline silicon solar cells have an efficiency temperature coefficient (TCE) that is virtually zero (TCE is between -0.08%/°C and 0.0/°C). It is thus beneficial to make this cell the current limiting cell in multibandgap cells, which will lead to improved annual energy yield.

Microcrystalline Si:H solar cells fabricated using ECR plasma deposition

2003 ◽  
Vol 150 (4) ◽  
pp. 316 ◽  
Author(s):  
V.L. Dalal ◽  
J.H. Zhu ◽  
M. Welsh ◽  
M. Noack
Keyword(s):  
Solar Cells ◽  
Plasma Deposition ◽  
Ecr Plasma

Determining the exciton diffusion length of copper phthalocyanine in operating planar-heterojunction organic solar cells

2020 ◽  
Vol 89 (3) ◽  
pp. 30201 ◽  
Author(s):  
Xi Guan ◽  
Shiyu Wang ◽  
Wenxing Liu ◽  
Dashan Qin ◽  
Dayan Ban
Keyword(s):  
Solar Cells ◽  
Organic Solar Cells ◽  
Diffusion Length ◽  
Copper Phthalocyanine ◽  
Short Circuit ◽  
Thick Layer ◽  
Short Circuit Current ◽  
Short Circuit Current Density ◽  
Exciton Diffusion ◽  
Exciton Diffusion Length

Organic solar cells based on planar copper phthalocyanine (CuPc)/C60 heterojunction have been characterized, in which a 2 nm-thick layer of bathocuproine (BCP) is inserted into the CuPc layer. The thin layer of BCP allows hole current to tunnel it through but blocks the exciton diffusion, thereby altering the steady-state exciton profile in the CuPc zone (zone 1) sandwiched between BCP and C60. The short-circuit current density (JSC) of device is limited by the hole-exciton scattering effect at the BCP/CuPc (zone 1) interface. Based on the variation of JSC with the width of zone 1, the exciton diffusion length of CuPc is deduced to be 12.5–15 nm. The current research provides an easy and helpful method to determine the exciton diffusion lengths of organic electron donors.

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