Back Interface Recombination Velocity Research Articles

Is a passivated back contact always beneficial for Cu (In,Ga)Se2solar cells?

AbstractUltrathin CIGSe solar cells on TCOs (transparent conductive oxides) have many promising potential applications due to their transparency. However, their performance is strongly constrained due to the back contact, often referred to acting as a Schottky junction. In this work, we show that the current limitation due to a back Schottky junction can be responsible for the apparent photocurrent loss and suppression of the forward current, which deteriorates cell performance and leads to the S‐shapedJ‐Vcharacteristics, respectively. However, due to the non‐negligible photocurrent in the back Schottky diode and the current amplification effect of a transistor, cells with a reduced absorber thickness overall exhibit a relieved current suppression, resulting in a lightJ‐Vcurve closer to the one with an Ohmic back contact. Notably, a higher back interface recombination velocity is capable of increasing the forward current of the Schottky diode by recombination, which mitigates the limitation of current flow through the back Schottky junction and thus improves the cell performance. The results show strong implications that introducing high density defects at the back interface is favorable for CIGSe solar cells on TCOs, rather than employing point‐contact structures for interface passivation. This suggests a different path to improve ultrathin CIGSe solar cells on TCOs compared to the devices on Mo with an Ohmic back contact.

Characteristic research of uniform-doping and exponential-doping Ga1-xAlxAs/GaAs photocathode with femtosecond laser illumination

The properties and characteristics of graded Al component Ga1-xAlxAs/GaAs photocathode are investigated in this paper, especially the transient reflectivity of Ga0.37Al0.63As under the condition of femtosecond laser illumination. Using software CASTEP, we calculate and analyze the optoelectronic property of varied Al composition Ga1-xAlxAs photocathode based on first principle theory. Ga0.37Al0.63As is emphasized studied because its lattice constant is very close to GaAs, reducing the probability of lattice mismatch and the back interface recombination velocity on the interface of substrate and emission layer. In this work, we design two reflection mode GaAs photocathode samples with different doping method: uniform-doping and exponential-doping used in emission layer grown by MBE. The impacts of different doping method on photocathode are analyzed by comparing some typical performance parameters. Transient reflectivity rates of different doping method Ga1-xAlxAs/GaAs photocathode are investigated by the device of pump-probe transient reflectivity system in the paper and we found that the way of exponential-doping is better for improving the performance of Ga1-xAlxAs /GaAs photocathode. The potential value of transient property of photocathode is enormous in future.

High-performance GaAs nanowire cathode for photon-enhanced thermionic emission solar converters

GaAs nanowire cathodes with exponential doping and graded Al composition structures are proposed for photon-enhanced thermionic emission (PETE) devices. The conversion efficiency models with these two nanowire cathodes are deduced on the basis of one-dimensional continuity equations. The conversion efficiency as a function of wire length, wire width, Al composition distribution, cathode temperature, emissive surface and back interface recombination velocity are also simulated, respectively. Results show that exponential doping and graded Al composition cathode structures can obviously improve the conversion efficiency of devices through introducing a built-in electric field along the growth direction of nanowire. Besides, the optimum wire length and wire width range are 300–340 nm and 5.9–6.4 μm, respectively. Moreover, wider Al composition range is beneficial to achieve higher conversion efficiency. These simulations provide an interesting attempt to explore the working mechanism of GaAs nano-based PETE devices and are expected to be verified by the experimental results in the future.

Comparative research on reflection-mode GaAs photocathode with graded AlxGa1−xAs buffer layer

The graded Al compositional AlxGa1−xAs buffer layer can not only form continuous internal electric field from buffer layer to active layer but also optimize the interface properties by decreasing the misfit dislocations and stacking faults arising from lattice mismatch. By measuring the spectral response current (SRC) for two reflection-mode (r-mode) designed samples of graded and stationary Al compositional structure, we can find the special phenomenon that the graded structure had quite influence at the middle wavelength band from 550nm to 850nm, but not the short wavelength band from 400nm to 550nm, though the buffer layer can only absorb photon energy at the short wavelength band. Through the comparative research for designed samples through SPV before Cs–O activation and SRC after Cs–O activation, the graded structure can well optimize the key parameters such as LD, Ln, Sv and P. For the photon absorption lengths are relative little at the short wavelength band and relative long at the middle wavelength band, so the optimizations of key parameters have little influence on photo-excited electrons at the short wavelength band which are mainly excited from the region in active layer near surface barriers. The optimizations of key parameter, mainly the back interface recombination velocity (Sv), can have quite impact on photo-excited electrons at the middle short wavelength band which are mainly excited from the internal active layer near the back interface. This comparative research can help to well study the photo-emission theory and structure design on graded Al compositional design for r-mode GaAs photocathodes in the future research.

Comparative research for GaAs photocathodes with graded compositional AlxGa1−xAs buffer layer

The graded Al compositional structure design in AlxGa1−xAs buffer layer can well increase the quantum efficiency than the stationary Al compositional structure design, especially at short-wave band. For deep study, the samples of two structures were prepared by metal organic chemical vapor deposition for carrying out the comparative research between surface photovoltage before Cs–O activation and spectral response current after Cs–O activation. Through fitting calculations and analyzes, the mixed structure of graded Al composition in AlxGa1−xAs buffer layer and exponential doping concentration in GaAs active layer cannot only form continuous internal electric field throughout window layer and active layer, but also decrease the misfit dislocations and stacking faults arising from lattice mismatch at the AlxGa1−xAs/GaAs interface. The better interface properties can further decrease the back interface recombination velocity than the standard structure design. Furthermore the continuous internal electric field can well increase the electron diffusion and drift length for active layer and surface escape probability for surface barriers and can also drive more photo-excited electrons at short-wave band in buffer layer to cathode surface.

Quantum efficiency study of the sensitive to blue–green light transmission-mode GaAlAs photocathode

The quantum efficiency of the blue–green transmission-mode GaAlAs photocathode has been studied. Two transmission-mode GaAlAs photocathodes with different structures are prepared, and the quantum efficiency curves are measured. We use the quantum efficiency formula to fit the experimental curves, and obtain the performance parameters of photocathodes such as the electron diffusion length, the back interface recombination velocity, and the surface electron escape probability. The effects of the Al compositions, the thickness of emission layer, and the electron diffusion length on quantum efficiency are investigated. The results show that both of transmission-mode GaAlAs photocathodes are sensitive to the blue–green light. The peak quantum efficiency of one photocathode appears at about 565nm, while that of another photocathode appears at about 535nm. The Al composition of emission layer plays a major role on the peak position of quantum efficiency of transmission-mode GaAlAs photocathode. Besides, the thickness of emission layer and the Al composition of window layer also have a large influence on the quantum efficiency.

Research on quantum efficiency of transmission-mode InGaAs photocathode

To investigate the quantum efficiency of high performance transmission-mode InGaAs photocathode, the quantum efficiency curve is fitted by using the multilayer optical thin film theory and the first principle calculation. A series of the performance parameters are obtained with the relative error less than 5%. It indicates that the thickness of the emission layer is 0.7–0.9μm, the thickness of the window layer is 0.1–0.3μm. Meanwhile the results from the first principle calculation are proved to be reliable as well. In addition, the long-wave response will increase and the highest integral sensitivity will be obtained when the thickness of the emission layer is the optimum value. For the InGaAs photocathode, the back interface recombination velocity mainly leads to the low integral sensitivity, which is caused by the material lattice mismatch.

Evaluation of chemical cleaning for Ga1−xAlxAs photocathode by spectral response

The spectral response has been used to evaluate the chemical cleaning for Ga1−xAlxAs photocathode by an on-line spectral response measurement system. The spectral response curves of Ga1−xAlxAs photocathodes treated by different chemical cleaning methods are measured and analyzed in detail. We use the quantum efficiency formulas to fit the experimental curves transforming from the spectral response curves, and obtain the related performance parameters such as the surface electron escape probability, the back-interface recombination velocity, the electron diffusion length, and the thickness of the etching GaAs layer. The results show that the GaAs photocathode cleaned by the HF solution could obtain a good photoemission effect, while the Ga0.37Al0.63As photocathode could be well cleaned by the solution of sulfuric acid and hydrogen peroxide.

Comparison between exponential-doping reflection-mode GaAlAs and GaAs photocathodes

A reflection-mode GaAlAs photocathode and a reflection-mode GaAs photocathode using exponential-doping technique are prepared by metal organic chemical vapor deposition, and the Al content of GaAlAs emission layer is 0.63. The two photocathodes are activated in an ultra-high vacuum system, and the spectral response curves are measured after activation. The quantum efficiency formula for exponential-doping reflection-mode photocathode is used to fit the experimental curves of the two photocathodes respectively, and the effects of some performance parameters on photoemission are analyzed, such as electron diffusion and drift length, back-interface recombination velocity, surface electron escape probability, etc. The results show that the Al content of the GaAlAs photocathode plays a bad role in the photoemission compared with that the GaAs photocathode, but it solves the problem that the GaAs photocathode cannot be well used in the area of detecting the narrow wavelength light due to the broad spectral response. The reflection-mode GaAlAs photocathode prepared is responsive to the blue and green light.

The optimal thickness of a transmission-mode GaN photocathode

A 150-nm-thick GaN photocathode with a Mg doping concentration of 1.6 × 1017 cm−3 is activated by Cs/O in an ultrahigh vacuum chamber, and a quantum efficiency (QE) curve of the negative electron affinity transmission-mode (t-mode) of the GaN photocathode is obtained. The maximum QE reaches 13.0% at 290 nm. According to the t-mode QE equation solved from the diffusion equation, the QE curve is fitted. From the fitting results, the electron escape probability is 0.32, the back-interface recombination velocity is 5 × 104 cm·s−1, and the electron diffusion length is 116 nm. Based on these parameters, the influence of GaN thickness on t-mode QE is simulated. The simulation shows that the optimal thickness of GaN is 90 nm, which is better than the 150-nm GaN.

Comparison between transmission-mode extended blue GaAs photocathodes in optical structure

One transmission-mode extended blue GaAs photocathode is prepared by MOCVD , whose integral sensitivity is 1980 A/lm. Its spectral curve is compared with the spectral curve of ITT photocathode for analyzing optical structure. The comparison indicates that the differences lie in the thickness and the Al mole value of the Ga1-xAlxAs window layer, electron diffusion length, and back-interface recombination velocity, which make the photocathode in this experiment inferior to that of ITT in extended blue performance. However our surface electron-escape probability and the thickness of the GaAs active layer are in accordance with those of ITT, which leads their difference in the long waveband part to be less than in the short one. In addition, our absorptivity in the whole response waveband is smaller than that of ITT photocathode, which leads the spectral response and integral sensitivity of the domestic transmission-mode extended blue GaAs photocathode to be inferior to the exotic one.

Effect of different epitaxial structures on GaAs photoemission

The quantum efficiency equations of two different structure reflection-mode GaAs photocathodes with back interface recombination velocity have been solved from the diffusion equations. One structure consists of GaAs substrate and an epitaxial GaAs active layer (GaAs-GaAs) and another structure consists of GaAs substrate, an epitaxial AlGaAs buffer layer, and a GaAs active layer (AlGaAs-GaAs). The experimental results show that the quantum efficiency of long-wavelength photons and the integral sensitivities for GaAs-GaAs cathodes both increase with the increase in the active layer thickness, which is due to the increase of electron diffusion length. The quantum efficiency of long-wavelength photons and the integral sensitivity of AlGaAs-GaAs cathodes are greater than those of GaAs-GaAs cathodes with an identical active layer thickness, which is attributed to the AlGaAs buffer layer. The buffer layer can reflect electrons and improve the quality of the GaAs active layer. Through the theoretical simulation, we found the active layer thickness for AlGaAs-GaAs cathodes has an optimum value at which the cathodes achieve the maximum sensitivity.

Theoretical Revision and Experimental Comparison of Quantum Yield for Transmission-Mode GaAlAs/GaAs Photocathodes

The quantum yield formula for uniform-doping GaAlAs/GaAs transmission-mode photocathodes is revised by taking into account the light absorption in the window layer. By using the revised quantum yield formula, the domestic and ITT's experimental quantum yield curves are fitted and the fitted curves match well with the experimental curves. In addition, the fit results show that the integral sensitivity and quantum yield of domestic image intensifier tube has achieved 2130μA/lm and 45%, nearly reaching ITT's third generation level in 2002, whereas the discrepancy in cathode performance is mainly embodied in the electron diffusion length and back interface recombination velocity.

Study of spectral response for transmission-modeNEA GaN photocathodes

Transmission-mode GaN photocathodes with the emission layer thickness of 150 nm and the doping concentration of 1.6×1017 cm-3 were grown by metal-organic chemical vapor deposition (MOCVD) and were activated in ultra-high vacuum system. The result was tested by Multi-information test system. The shape of transmission-mode NEA GaN photocathode quantum yield curves looks like the Chinese charocter 门 for "door", the photocathode had flat and high response between 255 and 355 nm, the highest quantum yield of 13% appeared at 290 nm. When the wavelength was less than 255 nm the quantum yield was decreased because of the high absorption coefficient of AlN buffer layer at short wavelengths. The quantum yield was also decreased beyond 355 nm and fell to 3.5% at the threshold of 365 nm, the quantum yield at 385 nm was reduced to 0.1% and the cut-off character of long wave was well shown. The quantum yield formula of transmission-mode GaN photocathode has been solved from diffusion equations, and the main factors affecting the quantum yield mostly, including electron diffusion length, electron escape probability, active-layer thickness and the back-interface recombination velocity, were analysed and discussed. The future work is optimizing the structure of the photocathodes.

发射层厚度对透射式GaAs光电阴极表面光电压谱的影响

Equations for surface photovoltage spectroscopy were deducted,by solving the on-dimensional diffusion equation for equilibrium minority carriers of transmission-mode GaAs phtotocathode.Through measuring the surface photovoltage curves for GaAs photocathodes with the active layer thichness of 1.6 and 2.0μm,doping concentration of 1×1019 cm-3,experiments and fitting curves fit very well.By leading the formulas for integral sensitivity of surface photovoltage spectroscopy,the inflection of active layer thickness for integral sensitivity was analyzed under certain body parameters through emulations.It was found that GaAs photocathodes have a optimal active layer thickness and the back-interface recombination velocity inflects more on optimal thickness than electron diffusion length.Furthermore,GaAlAs window layer could help to well reduce the back-interface recombination velocity for active layer thickness.

Theoretical calculation of quantum yield for exponential-doping GaAs photocathodes

Exponential-doping photocathodes, in which from the GaAs bulk to the surface the doping concentration is distributed exponentially from high to low, can form a stable built-in electric field in the active layer, and the electric field facilitates the excited photoelectron emission. The quantum yield formulas of both reflection-mode and transmission-mode of exponential-doped photocathodes have been solved from the 1-diemnsion continuity equations, in which the build-in electric field is considered. According to these formulas, we calculate the theoretical quantum yield of the exponential-doped photocathodes. The calculated results show that the exponential-doping structure can increase the quantum yield of photocathodes significantly. To compare with the uniformly-doped photocathodes, the integral sensitivity of the reflection-mode exponential-doped photocathodes increases by nearly 20%, and for transmission-mode photocathodes the increase is more than 30%. The performance improvements of exponential-doping photocathodes are mainly attributed to the built-in electric field, the photoelectrons driven by the field move towards the cathode surface by way of diffusion and drift, accordingly, decrease the influence of the back-interface recombination velocity on photoemission and increase the equivalent electron diffusion length of cathodes.

Effects of fluorine in silicon solar cells with polysilicon contacts

The effects of fluorine on the performance of solar cells with polysilicon contacts at the front surface and at both: front and back surfaces, are reported. It is shown that a fluorine implant applied to the front surface of the device has two different effects: it improves the passivation of the free oxidised surface leading to improvements in open circuit voltage and short circuit current, and it reduces the series resitance. The back polysilicon contact improves the back interface recombination velocity. Specific series resistance values from 0.16 to 0.5 Ω cm 2 have been measured. The effects of the polysilicon contact processing conditions on the short-circuit current, open-circuit voltage and spectral response are also discussed.

Correlation of implantation defects with interface recombination velocities and carrier lifetimes in annealed SIMOX SOI materials by using a contactless optical modulation technique

A contactless dual-beam optical modulation (DBOM) technique [Y. S. Chang and S. S. Li, Solid-State Electron., 1995, 38, 297–304, Y. S. Chang, S. S. Li and P. C. Yang, Solid-State Electron., 1995, 38, 1359–1366] was employed to determine the interface recombination velocities and substrate carrier lifetimes in the SIMOX SOI wafers processed under different implant energies (155–185 keV), temperatures (540–640°C), beam currents (45–65 mA), and with channeling and non-channeling implants. The results revealed that using an implant energy of 185 keV, implant temperature of 640°C, beam current of 45 mA, and with channeling implant yielded a better front-interface (Si film/buried oxide layer (BOX) interface) quality. In contrast, when an implant energy of 155 keV, a beam current of 65 mA, and non-channeling implant were used, a better back interface ( BOX Si substrate interface) was obtained. Similar to the back interface recombination velocity study, the DBOM measurements also showed that using an implant energy of 155 keV, implant temperature of 540°C, beam current of 45 mA, and with non-channeling implant yielded a higher substrate carrier lifetimes. An etch-pit method was used to correlate the Si-overlayer defect density with the front-interface recombination velocity determined by the DBOM technique. Good correlation was obtained between the measured interface recombination velocities and the Si-overlayer defect densities.

Properties of high sensitivity GaP/InxGa1−xP/GaAs : (Cs–O) transmission photocathodes

Negative electron affinity (NEA) GaAs photocathodes were vapor grown on GaP/InxGa1−xP substrates with 0.4<x<0.65. The dependence of cathode sensitivity, electron diffusion length, cathode back-interface recombination velocity, and surface-escape probability on buffer-layer composition was determined. The optimal InxGa1−xP composition for these samples was in the range of 52–55% InP. Indium unintentionally introduced into the GaAs causes a shift in the InxGa1−xP composition for good lattice match. SIMS and AES measurements were made to obtain the chemical and electronic structure of the epitaxial layers. The photosensitivity dependence on both Zn doping concentration and cathode thickness are presented. The highest transmission mode photosensitivity measured was 740 μA/lm.

The effect of lattice parameter mismatch in NEA GaAs photocathodes grown on GaP/InxGa1−xP substrates

Transmission-mode negative electron affinity (NEA) GaAs photocathodes were investigated. In order to assess quantitatively the effect of lattice-parameter mismatch between the cathode and substrate, several 1-μm-thick GaAs photocathode layers were vapor deposited onto GaP/InxGa1−xP composite substrates with x ranging from O to about 0.5; the corresponding lattice mismatch varied from 0.2 Å to approximately zero. Photoluminescence and ir free-carrier absorption measurements of the Zn doping concentration in the 1-μm-thick GaAs layers revealed that the doping concentration was about 3–6 times greater than that obtained on 15-μm-thick GaAs layers grown under identical Zn partial pressure conditions. Photoemission measurements showed that the transmission-mode quantum efficiency at a wavelength of 0.7 μm increased from about 0.045 electron/photon for GaAs on GaP to 0.13 electron/photon for GaAs on In0.50Ga0.50P as the lattice-parameter mismatch decreased to approximately zero. Correspondingly, transmission-mode white-light sensitivity values increased from 195 to 390 μA/lm. The photoemission spectral response curves were analyzed to determine values for back interface recombination velocity s and electron diffusion length L. In general, several combinations of these parameters gave a good fit to the data. However, for samples near the lattice match point, s was restricted to relatively low values and, for a given value of s, the corresponding value of L was significantly higher than that for any sample with an appreciable amount of lattice mismatch.