J. Furlan

Analysis of TCO/p(a-Si:C:H) heterojunction and its influence on p-i-n a-Si:H solar cell performance

The ASPIN computer simulator, which enables analysis of transparent conducting oxide (TCO)/a-Si:C:H/a-Si:H/TCO heterostructures, was used to examine the influence of different front TCO/p(a-Si:C:H) heterojunctions on TCO/p-i-n/TCO/metal a-Si:H solar cell performance. Separate analysis of TCO/p(a-Si:C:H) structure for both SnO2 and ZnO indicates that the mismatch between the high contact potential and the measured potential barrier at the p-layer surface can be resolved by a large density of interface defect states, causing a steep potential decrease in the interface. Analysis of the detrimental effects of a-Si:C:H chemical oxidation in SnO2/p(a-Si:C:H), which were simulated by the increased surface state density in the a-Si:C:H, showed that the potential barrier in a p-layer with oxidized surface is increased. The impact of both TCO/p(a-Si:C:H) interface states and a-Si:C:H surface states on the photoelectric properties of p-i-n a-Si:H solar cells is discussed, and a possible improvement of Voc is envisaged.

Examination of blocking current-voltage behaviour through defect chalcopyrite layer in ZnO/CdS/Cu(In,Ga)Se2/Mo solar cell

Abstract Blocking current-voltage behaviour of ZnO/CdS/Cu(In,Ga)Se2/Mo solar cells, which is either temperature- or light-conditioned, is examined using a comprehensive numerical device simulator. Effects of defect states in the defect-chalcopyrite layer and at the CdS/defect-chalcopyrite interface are investigated. Acceptor-like defect states either in a defect-chalcopyrite layer or at the CdS/defect-chalcopyrite interface cause different trapping under red light or white light. This results in different potential profiles throughout the structure, which determine the changeable I−V behaviour under forward bias. Simulation results show that these acceptor-like defect states can also control the temperature-conditioned blocking I−V behaviour.

Effects of abrupt and graded a‐Si:C:H/a‐Si:H interface on internal properties and external characteristics of p‐i‐n a‐Si:H solar cells

Using a computer simulation, the effects of abrupt and graded a‐Si:C:H/a‐Si:H interfaces on the performance of a‐Si:H p‐i‐n solar cells are discussed. It is shown that structures with graded heterojunction transitions possess much lower recombination near the junction and a higher accelerating built‐in electric field in the i layer, both of which increase the open‐circuit voltage and improve the solar cell fill factor.

Stacked a-SiC:H/a-Si:H heterostructures for bias-controlled three-colour detectors

Abstract A family of three-terminal three-colour detectors based on stacked a-SiC:H/a-Si:H heterostructures is presented. The detectors have the assembly TCO/PIN/TCO/PINIP/metal and TCO/PINIP/TCO/PIN/metal and are sensitive for the fundamental chromatic components. These two structures are mutually compared with regard to the calculated spectral responsivity using our simulation program. They both exhibit a linear photocurrent/generation-rate relationship with high rejection ratios for blue, green and red colours at peak wavelengths 430, 530, 630 nm, applying ± 1 V or more. Based on the optimisation analysis of their geometrical dimensions TCO/PIN/TCO/PINIP/metal structures were fabricated. In the fabricated assembly, due to the stringent thickness condition, the top PIN diode does not yet provide satisfactory results, but the PINIP structure renders an excellent spectral separation for green and red colour.

Tunnelling-assisted generation-recombination in pn a-Si junctions

Abstract A theoretical model is developed describing the trap-assisted tunnelling capture and emission mechanisms in amorphous silicon pn junctions. Adding these transitions to a pure thermal capture–emission mechanism gives expressions for the nonequilibrium occupancy function. Integrating over all states in the gap of an amorphous semiconductor, the equations for the generation-recombination rate are obtained, taking into account not only the thermal capture–emission process, but also the tunnelling transport of charge carriers and the enhanced carrier transport due to the Poole–Frenkel effect.

Numerical modelling of trap-assisted tunnelling mechanism in a-Si:H and μc-Si n/p structures and tandem solar cells

The trap-assisted tunnelling theory was developed to describe the tunnelling of charge carriers via bandgap energy levels in structures based on hydrogenated amorphous silicon and microcrystalline silicon. Its implementation into ASPIN numerical simulator is explained. Models that were verified on n/p single junctions were applied in the tunnel recombination junction area of a tandem solar cell. Thus, it is possible to study a multi-layer solar cell without separately simulating any of its components.

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.

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.