P. Popovic

Numerical analysis of the transient response in amorphous silicon

Using our program, TRansient Amorphous DEvice Simulator (TRADES), for the simulation of transient phenomena in amorphous silicon devices, the transient response after turning the light off is numerically calculated. Parameters for the numerical analysis are obtained by fitting the two families of the measured steady-state characteristics: light-intensity and temperature dependence of the secondary photocurrent. Using these parameters, both temperature and light-intensity dependence of the transients are calculated. It is shown that the transient is faster at higher temperatures and at higher illumination levels. Results are compared with measured transients at different temperatures.

Charge carrier transport in n–i–p and p–i–n a-Si/c-Si heterojunction solar cells

Abstract The regional approximation method, developed recently for the analysis of a p–i–n a-Si/c-Si heterojunction solar cell structure, is applied to simulate the internal operation and external characteristics of a n–i–p a-Si/c-Si cell. The derived closed-form solutions have equal basic forms. However, in as much as material parameter values in these two structures differ, also the calculated plots of output characteristics of p–i–n and n–i–p a-Si/c-Si cells are different. The dominant effects which influence the charge-carrier transport in both cells are mutually compared and discussed.

Internal and external characteristics in transient forward bias simulations of a-Si:H devices

Abstract A numerical program for transient modeling of multilayer a-Si:H devices is described. Using a computer simulation, transient currents in a-Si:H PIN and NIN devices using different magnitudes of step voltage excitations were studied. The results of calculations show first a rapid drop in electric current, followed by an intermediate interval of current rise in both devices. During longer times after step excitation, the transient current in a NIN device decays monotonically towards the new steady-state value, while the current in a PIN device shows a sudden rise to its final value. Such behaviour is explained on the basis of calculated results for different internal quantities such as free and trapped carrier concentrations, space charge density, electric field, potential, net recombination rates and others. It is shown that initially the current in both devices is space-charge limited. The presence of both types of carrier in a PIN diode enables space-charge neutralization at the end of the transient, causing the final abrupt current increase and transition to recombination-limited current flow. The influence of varying the density of localized states (DOS) on simulated transient currents is analyzed and discussed.

Analytical model of a-Si/c-Si HIT solar cell

Using suitable simplifying approximations inside the particular regions of a p-i a-Si/n c-Si heterojunction solar cell, the analytical expressions for the solar cell current-voltage characteristics are derived showing clearly the dominating first-order effects on solar cell performance. The derived closed form solutions indicate that in the useful forward voltage range the largest dark current component of this cell is the interface recombination current and that the main contribution to the photocurrent comes from the light generated holes in the c-Si substrate layer. The transfer of holes across the intrinsic layer and over the {Delta}E{sub {nu}} barrier is strongly suppressed resulting in an attenuation of solar cell dark and photocurrent.

Small‐signal capacitance and conductance of biased a‐Si structures

Small‐signal capacitance and conductance of experimental samples of a‐Si n‐i‐n structures were measured in a wide frequency range under various bias conditions. The measured capacitance at low frequencies greatly exceeds the expected value derived from the ΔQ/ΔV ratio, where ΔQ is a change of the trapped charge corresponding to a change ΔV of the applied voltage. This capacitance increases with the steady‐state bias and decreases with the frequency of the measuring signal. The measured low‐frequency small‐signal conductance equals the differential conductance obtained from the steady‐state current–voltage characteristics, but it increases with the rising frequency of the measuring signal. A small‐signal analytical model of an a‐Si n‐i‐n structure is developed which agrees well with the experimental results. With this model, the high capacitive effect of the n‐i‐n device at low frequencies is explained on the basis of a phase shift which arises from the delayed capture–emission mechanism of carriers in the...

Analysis of silicon solar cells incorporating an extra defect rich layer

Abstract Recently, it has been reported that the efficiency of a crystalline silicon solar cell had been strongly increased by implanting a thin defect layer absorbing light at higher wavelengths, thus causing additional light generations. In this work the effects of a such layer on the electric properties of a c-Si and an a-Si solar cell are theoretically explored. In the examination of the most favorable position of the defect layer in the cell and its effect on solar cell characteristics, a piecewise analytical approach was used dividing the solar cell structure into segments and maintaining the continuity of electric properties at interfaces between these segments. Using this approach it was deduced that a slight increase of short-circuit current — accompanied by a greatly increased dark current lowering the efficiency — can be expected in a c-Si cell, whereas in case of an a-Si cell the added defect layer results in a degradation of all relevant solar cell parameters.

Temperature dependence of p-i-n HIT solar cell characteristics

The regional approximation method is used for calculating temperature dependent p-i-n a-Si/c-Si HIT (heterojunction with thin intrinsic layer) solar cell characteristics. The emphasis in the analysis is given to the mechanisms which dominantly govern the temperature dependence of HIT cell conversion efficiency. The current transport in a p-i-n HIT cell is suppressed by drift-diffusion limitations in the intrinsic layer and by large valence-band offset at the a-Si/c-Si heterojunction. With increasing temperature, the onset of transport limitations is shifted toward higher forward voltages, causing an enhanced transfer of photogenerated holes and resulting in a lower temperature dependence of HIT cell conversion efficiency.

Response time distribution. A way to describe the photocurrent transient

The authors studied the secondary photocurrent decay in thin layers of intrinsic amorphous silicon solar cells. The decay of the secondary photocurrent in the coplanar arrangement was measured and simulated in the temperature range from 140 K to room temperature. Data were analysed to obtain response times using different definitions. Instead of observing the measured and modelled transients in the time domain, they introduce an additional processing of the data leading to response time distributions. Different types of response time spectra are compared and the physical origin for the distribution of response times is discussed for Si solar cells.

Investigation of a-Si:H p-i-n Solar Cell Degradation

Light-induced degradation in amorphous silicon (a-Si:H)--also known as the Staebler-Wronski effect--is among the most important issues facing improvements in quality of a-Si:H solar cells and other device applications. Based on measured characteristics of degraded p-i-n structures, simulations of degraded structures were performed using the numerical simulator ASPIN in order to fit and explain pronounced hump-shaped voltage-dependent internal collection efficiency (ICE) characteristics under weak short-wavelength illumination. Agreement with measured hump-shaped ICE characteristics was obtained only if in addition to the introduction of light-induced dangling bond defect states, their capture cross-sections were also increased, in particular capture cross-section for the charged defect states. This causes a change in occupancy of defect states at the p-i interface and front part of the i-layer under forward bias. Consequently, it increases the electric field in the front part of the cell, which results in recovery of the ICE.

Internal opto-electric properties of p-i-n a-Si:H solar cell on grooved TCO texture

Effects of textured TCO layer on a-Si:H solar cell electric properties are investigated by computer modelling of thermal equilibrium profiles of charge carriers, electric field and potential distribution. 2D analysis of p-i-n a-Si:H cell on TCO layer, simulated by V-grooved morphology, was performed using the commercial program MEDICI, for which input data were modified in order to account for continuous distribution of states in the gap of a-Si:H. Results of calculations show strongly increased trapped hole concentration at the p-i interface around the peaks of the illuminated side of V-shaped grooves. At these locations, the built-in electric field is lowered so that the collection of light-generated carriers in these regions is reduced. The paper also demonstrates the dependence of the light generation profile on the wavelength of incident light and on V-groove tilt angle.