The dependence of open-circuit voltage on illumination level in p-n junction solar cells

Simple analytical dependencies of solar cell open-circuit voltage on illumination level, valid for high injection, are derived. The developments are guided and verified by exact computer-aided numerical simulations of silicon cells. The results are related to an easily measured device parameter, the uncompensated photocurrent, through the use of the principle of superposition. An advantage of p⁺-n over n⁺-p cells with respect to open-circuit voltage at high levels of illumination is predicted.     

A novel high open-circuit voltage p-n InP solar cell design

The performance of InP solar cells has been limited by low open-circuit voltages. While the reported short-circuit current densities are approaching the theoretical limit, the open-circuit voltages have yet to obtain what is expected from a semiconductor with a direct band gap of 1.35 eV. This work investigates the factors that determine the open-circuit voltage and presents the design and fabrication of a novel high open-circuit voltage p-n InP solar cell. the key aspect of the novel design is a complete analysis of the top contact metallization effects on the reverse saturation current density and the open-circuit voltage. the features of the design are not specific to InP solar cells but are applicable to other advanced material solar cells that require a thin emitter for an optimal design (those materials with a high absorption coefficient). By minimizing the reverse saturation current density, a high open-circuit voltage and high efficiency may be obtained. In addition, a complete analysis of the solar cell modelling is provided, with comparisons to other published InP solar cell models and device results to juxtapose the key material and design parameter effects.     

An analytical model for high-low-emitter (HLE) solar cells in concentrated sunlight

A current-voltage characteristic is derived for the high-low emitter (HLE) solar cell in concentrated sunlight. For high-level injection, the ambipolar approach is used to yield the complete information of the low emitter concentration region, including the ohmic drop, the Dember voltage, the minority carrier current density, the minority-carrier distribution and the electric field distribution. High doping effects including Auger recombination and bandgap narrowing are considered. The dependences of short-circuit current, open-circuit voltage, fill factor and conversion efficiency on the variations of the geometrical dimensions and material parameters are discussed in detail for silicon single crystal materials. It is shown that the maximum conversion efficiency of 22% at 100 suns AMO can be obtained for silicon high-low emitter solar cell.     

The limiting efficiency of silicon solar cells under concentrated sunlight

The intrinsic limits on the energy conversion efficiency of silicon solar cells when used under concentrated sunlight are calculated. It is shown that Auger recombination processes are even more important under concentrated sunlight than nonconcentrated sunlight. However, light trapping can be far more effective under concentrated light due to the better defined direction of incident light. As a result of these effects, the limiting efficiency lies in tile 36-37-percent range regardless of concentration ratio compared to the limiting value of 29.8 percent for a nonconcentrating cell with isotropic response.     

Influence of the junction area to edge area ratio on the open-circuit voltage of silicon solar cells

This work shows experimentally the decrease in open-circuit voltage produced by edge recombination in silicon solar cells. The effect is related to the edge area to junction area ratio.     

Saturation of photovoltage and photocurrent in p-n junction solar cells

Expressions for the photovoltage and photocurrent of a strongly illuminated p-n junction solar cell are derived by solving the ambipolar diffusion equation. A complete boundary condition is derived for the junction, which is valid for all levels of injection. In the open-circuit case, results are in agreement with those given by earlier theories, while in the short-circuit case, the current is found to saturate at the ratio of the diffusion potential to the internal resistance. Results are used to explain the experimental results of earlier workers.     

Effects on the open-circuit voltage of grain boundaries within the junction space-charge region of polycrystalline solar cells

A physical explanation is given for the observed dependence of open-circuit voltage on grain size in polycrystalline solar cells when no such dependence is seen for short-circuit current. This explanation identifies carrier recombination through grain-boundary surface states within the junction space-charge region as a primary mechanism underlying these dependencies. Experimental data that support this explanation are discussed, and possible ways of improving the conversion efficiency of polycrystalline solar cells are indicated.     

Limiting efficiency of silicon solar cells under highly concentrated sunlight

Recent work has shown that the upper bound on the energy conversion efficiency of silicon cells under concentrated sunlight lies in the 36-37-percent range regardless of the concentration ratio. These bounds are reassessed at very high cencentration levels where loss of conductivity modulation and loss in carrier collection efficiency due to Auger effects become important. Previous work is shown to overestimate the efficiency bound at such levels as well as the cell thickness required to attain this bound.     

Efficiency calculations for thin-film polycrystalline semiconductor p-n junction solar cells

Numerical calculations have been made of the effect of grain size on the short-circuit current and the AM1 efficiency of polycrystalline thin-film GaAs and InP (2 µm thick) and silicon (25 µm thick) p-n junction solar cells. Junction solar cells are seen to be more efficient than Schottky-barrier cells, due to the higher dark current associated with Schottky diodes. GaAs shows the highest efficiency and both GaAs and InP attain 90 percent of their maximum efficiencies at a grain size of 10 µm, while silicon requires grain sizes of 200 µm to attain 90 percent of maximum efficiency. However, the deleterious effect of poor lifetimes and mobilities is less for silicon polycrystalline cells than for the direct-bandgap devices.     

Three-dimensional modeling and simulation of p-n junction spherical silicon solar cells

A three-dimensional numerical model is presented to simulate spherical p-n junction silicon solar cells, which is a promising new technology for photovoltaic (PV) energy conversion for terrestrial applications. Material properties imposed by the sphere formation method, geometry of the device, and the specific device structure stemming from the fabrication technology are taken into account in the optical and electrical models of the device. The spherical device is numerically simulated based on these models using finite-difference method in a spherical system of coordinates, generating the internal quantum efficiency and current-voltage (I-V) characteristics of the device. It has been shown that the efficiency of a spherical solar cell is slightly lower than a conventional device; however, the slightly inferior performance does not outweigh the cost advantage. It has been also found that subsurface diffusion length from effective impurity segregation and the depth of the denuded zone in spherical devices are parameters that mainly affect the device efficiency. Based on the simulation and analysis, design guidelines have been presented for spherical PV devices.     

Prediction of the open-circuit voltage of solar cells from the steady-state photoconductance

The steady-state photoconductance is a relevant parameter for solar cells. Minority carrier lifetimes, surface recombination velocities, diffused region recombination and open-circuit voltages can be determined from an analysis of photoconductance data. Computer simulations of archetypical cases are used here to demonstrate the usefulness of such an analysis and, in particular, the possibility of predicting device voltages from contactless photoconductance measurements. Simple theoretical models are given to provide physical insight and guide the analysis. A systematic approach is described that permits diagnosis and identification of the dominant recombination mechanisms for a given device structure.© 1997 John Wiley & Sons, Ltd.     

Measurement of minority carrier lifetime in solar cells from photo-induced open-circuit voltage decay

We present an experimental technique for determining the excess minority carrier lifetime within the base region of p-n junction solar cells. The procedure is to forward-bias the solar cell with a flash from a stroboscope and then to monitor the decay of the open-circuit voltage. Results are given for conventional horizontal-junction devices, as well as for vertical single- and multijunction solar cells. Lifetimes obtained with this technique are compared with those obtained from a method based on open-circuit voltage decay following the abrupt termination of a forward current, and with results obtained from a traveling light spot measurement of base minority carrier diffusion length in vertical-junction solar cells, from which the lifetime can be inferred. It is found that the forward current method does not yield a reliable lifetime estimate.     

Surface damage caused by electron-beam metallization of high open-circuit voltage solar cells

Metallization of high open-circuit voltage metal-insulator n-type-p-type(MINP)solar cells using an electron-beam deposition apparatus has been observed to cause degradation of device performance. Investigations using metal-insulator-semiconductor (MIS) diodes as test devices showed that the damage is consistent with a massive increase in surface-state density, and may be due to both radiation and the impact of charged particles. The damage can be eliminated by resistive evaporation of 100 nm of metal prior to use of the electron beam. It is likely that use of an electron beam to deposit grid lines and antireflection coatings on any high open-circuit voltage solar cell will lead to irreversible degradation in performance.     

Experimental determination of series resistance of p-n junction diodes and solar cells

Various methods for determining the series resistance of p-n junction diodes and solar cells are described and compared. New methods involving the measurement of the ac admittance are shown to have certain advantages over methods proposed earlier.     

Performance of n+-p silicon solar cells in concentrated sunlight

Performance data for n⁺-p silicon solar cells operating at illuminations up to 90 suns (9 W/cm²) and temperatures up to 100°C are presented. Experimental results for 2-cm²cells with different base resistivities are compared to performances predicted by a numerical device analysis computer code. Excellent agreement between numerical simulation and experiment is observed. For the illumination levels considered, an optimum base resistivity of approximately 0.3 Ω. cm is predicted by the numerical analyses and verified experimentally. The 0.3-Ω. cm cells exhibit conversion efficiencies above 11.8 percent up to 90 suns with a peak efficiency of 14 percent at approximately 30 suns. Preliminary results for a large-area (15.2 cm²) circular cell design are also presented for illuminations up to 60 suns. A peak conversion efficiency of 13.5 percent is measured for this cell at ∼25 suns.     

Influence of the density of states on the open-circuit voltage in small-molecule solar cells

Open-circuit voltages are strongly dependent on the density-of-states in solar cells based on disordered semiconductors. In this work, organic solar cells based on tetraphenyldibenzoperiflanthene and fullerene C₇₀ with a bilayer structure were fabricated to investigate the variation in the density-of-states with the substrate temperature during deposition of the donor. The maximum open circuit voltage was reached at a substrate temperature of 60 °C. Organic thin-film transistors were also fabricated to study their electrical properties, such as the mobility and the density-of-states. Finally, an organic solar cell with p–i–n structure was fabricated at the optimized substrate temperature, and a power conversion efficiency of almost 4% was obtained.     

Validated front contact grid simulation for GaAs solar cells under concentrated sunlight

In a common approach, the electric behavior of a solar cell is modeled by dividing it into smaller sub‐circuits and solving the resulting network by a circuit simulator. In this paper detailed network simulations are presented for a GaAs single‐junction solar cell. All resistive losses and losses influencing the diode saturation currents, such as recombination in the depletion region or at the perimeter are taken into account. With this model the maximum power point of a solar cell can be calculated for one‐sun and for higher illumination intensities. The results were validated experimentally using suitable test structures. This includes solar cell devices with varying dimensions, grid finger spacing and lengths. An excellent agreement between theoretical and experimental results was obtained. The network simulation model allows determining the optimum size and concentration ratio at which a solar cell operates at its maximum efficiency. In the case of a GaAs single‐junction solar cell this global efficiency maximum was found for an area of 1 mm² and at a concentration ratio of 450 suns. Under these conditions the largest loss mechanisms are the finger shading with 36.1% and the emitter resistance losses with 21.5% of the total power losses. Copyright © 2010 John Wiley & Sons, Ltd.     

Efficient electron-blocking layer-free planar heterojunction perovskite solar cells with a high open-circuit voltage

Perovskite solar cells (PSCs) with a simple device structure are particularly attractive due to their low cost and convenient fabrication process. Herein, highly efficient, electron-blocking layer (EBL)-free planar heterojunction (PHJ) PSCs with a structure of ITO/CH₃NH₃PbI₃/PCBM/Al were fabricated via low-temperature, solution-processed method. The power conversion efficiency (PCE) of over 11% was achieved in EBL-free PHJ-PSCs, which is closed to the value of PSC devices with the PEDOT:PSS as the EBL. It is impressed that the open-circuit voltage (V_oc) up to 1.06 V, an average value of 1.0 V for 43 devices, was obtained in EBL-free PHJ-PSCs. The electrochemical impedance spectroscopy (EIS) results suggested that the high PCE and V_oc are attributed to the relatively large recombination resistance and low contact resistance in EBL-free PHJ-PSCs. The solution-processed, EBL-free PHJ structure paves a boulevard for fabricating high-efficiency and low-cost PSCs.