Jože Furlan

Band‐gap engineering in CdS/Cu(In,Ga)Se2 solar cells

Using a numerical device simulation program, the band‐gap engineering in CdS/Cu(In,Ga)Se2 solar cells is examined. The device physics of different design concepts is analysed. Normal band‐gap grading improves performance, especially due to the additional quasi‐electric field, and the analysis showed that the best results are achieved if the grading extends from the highest band‐gap value at the back up to the space charge region. The double grading concept does not yield further improvement, because the front grading—even if located in the space charge region—repels the minority carriers (electrons) away from the CdS interface, and consequently, the fill factor drops significantly. Notch structures in the base also exhibit lower performance than the uniform band‐gap base due to the lower open‐circuit voltage and poorer fill factor. Therefore, the best results are achieved by a normal grading in a Cu(In,Ga)Se2 base from the edge of the space charge region to the back contact.

Approximation of the carrier generation rate in illuminated silicon

Abstract In order to shorten the calculation time for solar cell properties, the light generation rate usually expressed as a sum of individual contributions over the whole solar spectrum is replaced by a curve-fitted approximation. This approximation is represented by a series of three to five exponential terms resulting in an analytical solution of the continuity equation which has the same form as for the actual generation rate. Using the proposed approximation the calculated contribution of the base region to the short-circuit current fits closely the result obtained with the actual generation rate.

Amorphous silicon solar cell computer model incorporating the effects of TCO/a-Si:C:H junction

Abstract A simulation model of amorphous silicon solar cells ASPIN has been extended to incorporate the material properties of the TCO/a-Si:C:H interface region, which plays an important role in p-i-n a-Si:H solar cells and can strongly influence their photoelectrical characteristics. The analysis includes the impact of band bending at the front a-Si:C:H surface due to the difference between the work functions of TCO and a-Si:C:H and the influence of an increased surface density of states at the TCO/a-Si:C:H interface on internal and external characteristics.

Tunnelling generation–recombination currents in a-Si junctions

Abstract In describing high-field effects on electric conductivity and on carrier transport in pn junctions, different approaches and approximations have been used as the basis for the theoretical representation of high-field effects. In this review the basic principles of various models describing high-field effects are presented. The main emphasis is given to carrier generation–recombination in amorphous silicon pn junctions in which quantum tunnelling contributes a great deal to carrier transitions between band states and the traps, resulting in strongly enhanced generation–recombination rates. A model describing trap-assisted tunnelling carrier capture and emission in amorphous silicon is given a thorough presentation. Carrier capture at traps is regarded as a thermally assisted tunnelling transition combined by Poole–Frenkel barrier-lowering. The incident carriers which take part in the tunnelling capture process are majority carriers in the low field regions at the edges of the pn junction depletion region. Carriers, penetrating or crossing the barrier, are captured by thermalising to the ground level of traps. In the inverse process of carrier emission, the carriers are thermally lifted from traps, tunnelling subsequently to band states. Summing up tunnelling carrier transitions and pure thermal carrier capture–emission transitions gives expressions for the non-equilibrium occupancy function of traps. The generation–recombination rate within a-Si pn junctions is then expressed by virtue of the occupancy function and the continuous distribution of states in the gap of amorphous silicon.

Stacked a-Si:H-based three-colour detectors

Abstract A new a-Si:H based three colour-detector with the assembly transparent conducting oxide ( tco )/PINIP/ tco /PIN/metal is theoretically and experimentally investigated. This detector assembly overcomes the problems with a thin PIN diode for the detection of the blue colour in the tco /PIN/ tco /PINIP/metal assembly. The theoretical results demonstrate the advantages of the new detector type. The three colours could be detected with full width half magnitude

The wave nature of light in computer analysis of solar cells

Abstract In this work, the accurate light-generation model which accounts for the wave nature of light is compared to the classical model which assumes the simple exponential decay of absorbed light and considers only the two most important reflections. Both models are used to calculate both internal and external chectrical characteristics of the illuminated amorphous silicon sola cell. The results are compared to find out when the classical formula can be used, and when accounting for light interference is necessary.

New Bias-Controlled Three-Color Detectors using Stacked a-SiC:H/a-Si:H Heterostructures

A novel family of three-terminal three-color (blue, green, red) detectors based on stacked a-SiC:H/a-Si:H heterostructures is presented: TCO/PIN/TCO/PINIP/TCO/metal and TCO/PINIP/TCO/PIN/TCO/metal structure. The analysis of stacked photodetectors and the optimization of their geometrical dimensions is performed using the adopted ASPIN simulation program. Both structures are mutually compared with regard to calculated current-voltage characteristics and spectral responsivity. They both exhibit linear photocurrent/generation-rate relationship for all three colors at peak wavelengths 430, 530, 630 nm, applying ±1 V or more. This linearity allows that all three colors can be detected with high rejection ratio using simple system electronics.

Transient properties of PINIP structures in three-terminal a-Si:H based three-color detectors

Abstract a-Si:H-based three-terminal three-color detectors are experimentally investigated. Transient properties of photocurrent after bias switching are studied in PINIP structures for the detection of either blue and green light (PI 1 NI 2 P) or green and red light (PI 2 NI 3 P) under different illumination intensities and under different voltages before and after bias switching. A reciprocal relationship between the delay times and the illumination intensity is established for both structures over a wide range of illumination intensity. Only for the larger intensities is a deviation from the reciprocal relation observed. The delay time, t d NP , due to switching from negative to positive bias, determines the speed of the device. It is for both structures only moderately wavelength dependent, but it strongly depends on the switching voltage levels, V N(egative) and V P(ositive) . Optimized selection of V 1N and V 1P reduces the delay time of the PI 1 NI 2 P structure, speeding it up to the generally faster PI 2 NI 3 P structure.

Numerical analysis of a thin microcrystalline p layer in p-i-n a-Si:H solar cells

We have used our numerical simulator to examine the function of a thin microcrystalline p(μc-Si:H) layer in a p-i-n hydrogenated amorphous silicon (a-Si:H) based solar cell. In the analysis of a p(μc-Si:H)-i-n structure, three parameter sets were chosen for the purpose of verifying the validity of the reported measured p(μc-Si:H) material parameters. The results revealed that the correct setting of the p(μc-Si:H) work function is of prime importance. Analysis of a double window layer with an inserted p(a-SiC:H) layer and an accompanying i(a-SiC:H) buffer layer in the p(μc-Si:H)-i-n structure showed that the i(a-SiC:H) buffer layer beneficially affects the JSC and fill factor and furthermore, the insertion of a few nanometers thick p(a-SiC:H) layer between the p(μc-Si:H) layer and i(a-SiC:H) buffer layer additionally improves the solar cell performance.

Effects of High Electric Fields on Tunneling-Assisted Optical Electron Transitions in a-Si

A theoretical model for the optical absorption of amorphous silicon (a-Si) at high electric field aimed for ease of use is developed. Optical absorption is proportional to the number of possible transitions from valence band to conduction band. When an electron is excited by a photon, the atom it was bound to is ionized and bends all the energy levels to form a Coulombic potential funnel. A high electric field tilts all the energy levels. The Coulombic funnel and the tilted conduction band form a potential barrier through which an electron can tunnel, giving rise to additional available density of conduction band states, thus increasing the optical absorption. The model was used to calculate the optical absorption at zero electric field, 100 kV/cm, and 1 MV/cm. Results show that, although optical absorption change is barely perceptible at electric fields up to 100 kV/cm, the change is much more pronounced at an electric field strength of 1 MV/cm.