Effect of spatial variation of incident radiation on spectral response of a large area silicon solar cell and the cell parameters determined from it

Effect of spatial variation of incident monochromatic light on spectral response of an n⁺–p–p⁺ silicon solar cell and determination of diffusion length of minority carriers (L_b) in the base region and the thickness of the apparent dead layer (x_d) in the n⁺ emitter from the spectral response have been investigated. Spectral response of a few 10 cm diameter and 10×10 cm² pseudo-square silicon solar cells was measured with the help of a standard silicon solar cell of 2×2 cm² area in 400–1100 nm wavelength range. Different areas (4, 9, 16, 25 and total area 78.6 or 96 cm²) were exposed. The effect of the radial variation of incident radiation was determined quantitatively by defining a parameter f₁ as the ratio of the average intensity falling on the reference cell to that on the exposed area of the test cell. The value of f₁ varied between 1 and 1.15 (1.25) as the exposed area of the cell varied from 4 cm² to 78.6 (96) cm² indicating that the spatial inhomogeneity of intensity increased with the increase in the exposed cell area. Short-circuit current densities, J_sc, computed from spectral response data for AM1.5 spectrum were less compared to the directly measured values by a factor which was nearly equal to f₁. However, radial variation of intensity does not affect the determination of diffusion length of minority carriers in the base region (by the long wavelength spectral response, LWSR method using the measured spectral response data in 0.85<λ<1.05 μm range) and the thickness of the dead layer (by the method of Singh et al. using the data of 0.45<λ<0.65 μm range) significantly.     

Study of silicon solar cell at different intensities of illumination and wavelengths using impedance spectroscopy

The electrical properties of an n⁺–p–p⁺ structure-based single-crystalline silicon solar cell were studied by impedance spectroscopy, I–V and spectral response. The impedance spectrum is measured in dark, under different intensities (14, 43, 57, 71, 86, 100 mW/cm²) of illumination and wavelengths (400–1050 nm) of light. Under dark and at low intensities of illumination (<50 mW/cm²) the impedance spectra show perfect semicircles but at high intensities the semicircles are distorted at low frequencies. It is found that illumination provides an additional virtual R₁C₁ network parallel to the initial bulk R_pC_p network observed under dark conditions. The value of virtual resistance R₁ depends on the illumination wavelength and shows an inverse relationship with the spectral response of the device.     

The use of the self-calibration method for accurate determination of silicon solar cell internal quantum efficiency

The application of the self-calibration method as a means of increasing the accuracy of spectral response, SR, and internal quantum efficiency, Q(λ), measurements is discussed. The principle of the method is the precise calculation of Q(λ_m) of a test cell for light at λ_m≈0.8 μm, where the response is weakly dependent on the emitter and base parameters. The ratio of the calculated and measured Q(λ_m) values gives the corresponding factor for shifting the experimental spectral response curve. The sequence of calculations is described, and an algorithm of the necessary operations for a computer is developed. Several examples of the use of the self-calibration method for correction of SR measurements of solar cells with low shunt resistance demonstrate its very high effectiveness. The corrected Q(λ) values follow the respective actual data with very high accuracy even when the measured SR is decreased by factor 2–3 due to low shunt resistance of the solar cell.     

Current–voltage characteristics of electrovoltaic and electrophotovoltaic cells

Electrovoltaic (EV) effect provides a way of generating voltage across an unbiased junction under dark. Electrovoltaic (EV) cell in its simplest form is a device based on n⁺–p–n⁺ (or p⁺–n–p⁺) like structure in which if one p–n junction is subjected to an external forward bias, then, a voltage is developed across the other p–n junction such that the n-side gets a negative polarity with respect to the p-side. Connecting to a load across one of the n⁺–p junctions a bipolar transistor can be operated as a three-terminal EV cell. A new device henceforth known as electrophotovoltaic (EPV) cell wherein EV and PV effects could be expected to work cooperatively was also realized. It is based on a structure which is a combination of n⁺–p–n⁺ EV and n⁺–p–p⁺ photovoltaic (PV) cell structures having a common n⁺–p junction and is able to operate in EV, PV and EPV modes. We have developed one-dimensional physical models of EV and EPV cells and have applied them to explain the observed I–V characteristics of an n–p–n silicon bipolar transistor 2N3055 in EV mode and the EPV cell in EV, PV and EPV modes. While the photovoltaic efficiency η_PV decreases slowly with d/L, where d is the thickness and L is the diffusion length of minority carriers in the base region, the electrovoltaic efficiency η_EV has a strong dependence on d/L and decreases sharply with increase in d/L. Transistor 2N3055 with d/L=0.7 demonstrated η_EV>60%, whereas, our EPV cell with d/L>2.7 had η_EV<3%. However, in the EPV cell, the PV and the EV effects were indeed found to work cooperatively and the output power was enhanced in the EPV mode over the PV mode value although the efficiency η_EPV was less than 4.5%. To achieve substantially high values of efficiencies in EV and EPV modes the EPV cell should be designed to have d/L1.     

The influence of porous silicon coating on silicon solar cells with different emitter thicknesses

The influence of the emitter thickness on the photovoltaic properties of monocrystalline silicon solar cells with porous silicon was investigated. The measurements were carried out on n⁺p silicon junction whose emitter depth was varied between 0.5 and 2.2 μm. A thin porous silicon layer (PSL), less than 100 nm, was formed on the n⁺ emitter. The electrical properties of the samples with PS were improved with decrease of the n⁺p junction depth. Our results demonstrate short-circuit current values of about 35–37 mA/cm² using n⁺ region with 0.5 μm depth. The observed increase of the short-circuit current for samples with PS and thin emitter could be explained not only by the reduction of the reflection loss and surface recombination but also by the additional photogenerated carriers within the PSL. This assumption was confirmed by numerical modeling. The spectral response measurements were performed at a wavelength range of 0.4–1.1 μm. The relative spectral response showed a significant increase in the quantum efficiency of shorter wavelengths of 400–500 nm as a result of the PS coating. The obtained results point out that it would be possible to prepare a solar cell with 19–20% efficiency by the proposed simple technology.     

Simplified edge isolation of buried contact solar cells

The aim of this work is to implement simple edge isolation techniques in buried contact solar cell (BCSC) process by preserving the active cell area. Here we present results of two simplified edge isolation techniques for BCSC and they are compared with the standard process incorporating mechanical edge isolation using a dicing saw. The first technique is chemical wet etching of the solar cell's rear side in an inline system recently developed by University of Konstanz and Rena. The second technique is edge removal carried out in a fluoride/oxide radicals environment of a Asyntis plasma etcher. While the shunt resistance R_sh obtained with wet etching is between 1500 and 7000 Ω cm², the standard process shows R_sh values ranging from 2100–6300 Ω cm². The R_sh after plasma processing is between 1000 and 3600 Ω cm². These cell results show that both wet and plasma etching achieve results close to mechanical edge isolation.However, a slight reduction of short circuit current is observed for the cells undergone standard as well as plasma processing. This is due to the presence of floating volume shunts formed at the rear n–p⁺ junction, which are not removed by either the standard or plasma process. These shunts do not influence the IV-curve of the solar cells and are nearly invisible with conventional thermography, as they are not connected to the front side emitter grid. Hence, light-modulated lock-in thermography measurements were carried out to analyse these shunts.     

Calculation of the photocurrent-potential characteristic for regenerative, sensitized semiconductor electrodes

The steady state anodic photocurrent for sensitized semiconductor electrodes, where the sensitizer is regenerated by a redox electrolyte, has been modeled taking into account the rate of light absorption by the sensitizer S, the rate of electron injection from the excited state S^* of the sensitizer to the conduction band of the semiconductor, the rate of decay of S^* (radiative and non-radiative) and the rate of reductive regeneration of the sensitizer by the redox electrolyte. In this model the rate of recombination between the conduction band electron and the oxidized sensitizer, S⁺, and the reactions between S^* and the redox couple have been assumed to be negligible. The rate constant for injection, k_inj, the injection efficiency, φ_inj, the photocurrent density, J_P, and the steady state concentrations of S^* and S⁺, have been calculated as a function of light intensity, Helmholtz potential, λ_max and halfwidth, ΔE_0.5, of the sensitizer absorption spectrum, and the semiconductor band gap and electron affinity both for monochromatic light and for AM1.5 sunlight simulated by radiation from a 5600K black body. For the calculation of k_inj as a function of Helmholtz potential, the Gurney -Gerischer-Marcus (GGM) model has been used. Allowance for the distribution of electrode potential between the semiconductor and the electrolyte has been introduced in principle. The steady state concentrations of S^* and S⁺ were used in the Nernst equation to calculate the S^*/S⁺ quasi-reversible potential. It is shown that the short-circuit current density of the cell is a maximum for a sensitizer with a λ_max of about 1550 nm. Inter-relationships between variables involving the sensitizer have been used to show that only four sensitizer parameters are needed when considering the effects of changing the sensitizer. These parameters are the reorganization energies and the standard reduction potentials for the couples S⁺/S^* and S⁺/S. For a related series of sensitizers, such as obtained by changing a substituent group, only the two standard reduction potentials are required to predict the effects of changing the sensitizer.     

Photoelectrical properties of doped cadmium sulphide powders

Photosensitive powders of CdS were prepared with different concentrations of dopants. Doses of donors (Cl⁻) and acceptors (Cu²⁺, Ag⁺) varied from 0 to 41 and from 0 to 9 mg/g CdS, respectively. Reflectance, absorption coefficient and resistance dependence on illumination intensity and voltage at the wavelength of about λ=630 nm and photovoltage spectra in the range 450–900 nm were measured on layers prepared from the powders. The value of absorption coefficient grew with the increasing dopant concentrations; acceptors appeared more efficient than donors. Reflectance decreased with growing acceptor dose. Using the values of reflectance, absorption coefficient and resistance the corrected photovoltage, as the measure of the concentration photogenerated charge carriers, was calculated. The ratio (σ_IGB) of the corrected photovoltage and photocurrent was used as the criterion of intergrain barrier conductivity. All doped samples exhibited similar value of σ_IGB which was about three orders of magnitude lower than that of undoped sample.     

Photovoltaic and electronic properties of quercetin/p-InP solar cells

In this work, quercetin/p-InP heterojunction solar cell has been fabricated via solution-processing method and characterized by current–voltage and capacitance–voltage measurements at room temperature. A barrier height and an ideality factor value of 0.86 eV and 3.20 for this structure in dark have been obtained from the forward bias current–voltage characteristics. From the capacitance–voltage measurement, the barrier height and free carrier concentration values for the quercetin/p-InP device have been calculated as 1.63 eV and 3.8×10¹⁷ cm⁻³, respectively. Also, series resistance calculation has been performed by using Cheung theory. The device exhibits a strong photovoltaic behavior with a maximum open circuit voltage V_oc of 0.36 V and short-circuit current I_sc of 35.3 nA under 120 lx light intensity only.     

Self-calibration as a tool for high-accuracy determination of silicon solar cell internal quantum efficiency

Light nonuniformity, uncertainty in the illuminated photoactive area, and relative, but not absolute radiometric data for the reference detector, can be the reasons for the inaccuracy or impossibility of solar cell spectral response and quantum efficiency determination. The use of a self-calibration principle permits minimization of the errors caused by the above factors. This principle consists of quite precise calculation of the internal quantum efficiency Q(λ_m) of the test cell at λ_m≈0.8 μm, where the cell response is weakly dependent on emitter and base parameters. Experimentally determined short- and long-wavelength internal quantum efficiencies, Q(0.4) and Q(0.95), respectively, based on relative radiometric data for a reference detector, are used as starting data for the Q(λ_m) calculation. The ratio of the calculated to measured Q(λ_m) values gives the correction factor for shifting the experimental quantum efficiency curve. Computer modeling supports the assumption that uniform deviation of measured Q(λ) can be precisely corrected by calculation. Analysis of the accuracy of the self-calibration method demonstrates very small uncertainties in the corrections of quantum efficiency measurements, attainable for many practical situations. Confirmation of correctness of the proposed method is shown by analysis of the results of spectral response measurements of several solar cells.     

Analysis of thin silicon solar cells for high efficiency

A comprehensive theoretical analysis taking into account the contribution from both the emitter and base regions having finite surface recombination velocity has been developed for computing short-circuit current, open-circuit voltage, and efficiency of thin AR coated thin silicon solar cells with textured front surface. The dependence of efficiency on the front surface and back surface recombination velocities and on the cell parameters have been investigated in details for varying cell thickness considering the effects of bandgap narrowing and Auger recombination in the material. It is shown that efficiency exceeding 24% can be attained with silicon solar cells having thickness as low as 25 μm provided both front and back surfaces are well passivated (S < 10³cm/s) and the doping concentration in the base and emitter are in the range of 5 × 10¹⁶ to 10¹⁷cm⁻³ and 10¹⁸ to 5 × 10¹⁸cm⁻³, respectively. It is also shown that an efficiency of about 23% can be obtained for thin cells of 25 μm thickness with a much inferior quality materials having diffusion length of about 40 μm.     

Investigation of various surface passivation schemes for silicon solar cells

In this work, we have investigated three different surface passivation technologies: classical thermal oxidation (CTO), rapid thermal oxidation (RTO) and silicon nitride by plasma enhanced chemical vapor deposition (PECVD). Eight different passivation properties including SiO₂/SiN_x stacks on phosphorus diffused (100 and 40 Ω/Sq) and non-diffused 1 Ω cm FZ silicon were compared. Both types of SiO₂ layers, CTO and RTO, yield a higher effective lifetime on the emitter surface than on the non-diffused surface. For the SiN_x layers the situation is reverted. On the other hand, with SiO₂/SiN_x stacks high lifetimes are obtained not only non-diffused surface but also on the diffused surface. Thus, we have chosen the RTO/SiN_x stack layers as front and rear surface passivation in solar cells, which passivate relatively good on the surface and has very low-weighted reflection. On planar cells passivated with RTO/SiN_x a very high V_oc of 675.6 mV and a J_sc of 35.1 mA/cm² was achieved. Compared to a planar cell using CTO the efficiency of RTO/SiN_x cell is 0.8% higher (4.5% relative). It can be concluded that the RTO/SiN_x layers are the optimal passivation for the front and rear surface. On the other hand, for textured cells, the J_sc and FF of RTO/SiN_x cells are lower than those of CTO cells. The main reasons of these J_sc and FF losses were also discussed systematically.     

La0.84Sr0.16MnO3−δ cathodes impregnated with Bi1.4Er0.6O3 for intermediate-temperature solid oxide fuel cells

La_0.84Sr_0.16MnO_3−δ–Bi_1.4Er_0.6O₃ (LSM–ESB) composite cathodes are fabricated by impregnating LSM electronic conducting matrix with the ion-conducting ESB for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The performance of LSM–ESB cathodes is investigated at temperatures below 750 °C by AC impedance spectroscopy. The ion-impregnation of ESB significantly enhances the electrocatalytic activity of the LSM electrodes for the oxygen reduction reactions, and the ion-impregnated LSM–ESB composite cathodes show excellent performance. At 750 °C, the value of the cathode polarization resistance (R_p) is only 0.11 Ω cm² for an ion-impregnated LSM–ESB cathode, which also shows high stability during a period of 200 h. For the performance testing of single cells, the maximum power density is 0.74 W cm⁻² at 700 °C for a cell with the LSM–ESB cathode. The results demonstrate the ion-impregnated LSM–ESB is one of the promising cathode materials for intermediate-temperature solid oxide fuel cells.     

Promotion of electrochromism in spray-deposited molybdenum oxide-doped iridium oxide thin films

Electrochromic molybdenum oxide-doped iridium oxide thin films were prepared by using a pneumatic spray pyrolysis technique onto fluorine-doped tin oxide (FTO) coated conducting glass substrates. An aqueous solution of 0.01 M ammonium molybdate was mixed with 0.01 M iridium trichloride in different volume proportions and the resultant solution was used as a precursor for spraying. An aqueous electrolyte (0.5 N H₂SO₄) was used to study electrochromic properties of thin films using cyclic voltammetry (CV), chronoamperometry (CA) and spectrophotometry. During the potential scan the iridium oxide electrode switches between coloured and bleached state due to Ir⁺⁴–Ir⁺³ intervalency charge transfers. The optical density difference (ΔOD)_λ=630 nm and colouration efficiency was maximum for 2% molybdenum oxide-doped sample. Moreover, loss in charge density during extended cycling is less than undoped and other doped (>2%) samples.     

Optimized spectral transmittance of sun protection glasses

A spectral optical transmission function τ(λ) for sun protection insulating glasses is proposed in order to reduce the solar energy load of a building’s interior and thus reducing overheating. τ(λ) is based on standard functions such as spectral distribution of solar radiation S_λ(λ), spectral photopic luminous efficiency V(λ), standard illuminant D₆₅(λ), and the CIE color-matching functions . In the framework of the present approach an optimized spectral transmittance τ_min(λ) with a minimal normalized solar energy load (i.e., solar direct transmittance normalized to light transmittance, τ_e/τ_v) has been obtained on the condition of color neutrality of the transmitted light. A comparison with the performance of actual commercial sun protection glasses on the market shows that the present model for an optimized spectral transmittance could reduce the solar energy load by roughly one third for equal light transmittance τ_v.     

A new model of very high efficiency buried emitter silicon solar cell

In this work, we report a new model of symmetrical silicon solar structure where the emitter is buried, thus, creating many depletion regions in series inside the cell. The photocurrent in this model is computed for AM0 solar spectrum and is compared to the classical p-n junction. The first results of the calculation show that the buried emitter solar cell (BESC) has 10–35% more short-circuit current than the classical cell depending on the surface recombination velocity S. The biggest difference is obtained for S = 10⁵cm/s. The ratio of the photocurrent in the BESC to the classical photocurrent as a function of the absorption coefficient a goes from 1.9 for small α to 6.5 times for α = 10⁷cm⁻¹ with a minimum of 1.1 for α around 1800 cm⁻¹ for S = 10⁵cm/s.     

Silicon thin-film solar cells on insulating intermediate layers

An interdigitated front grid structure for both the emitter and base was simulated and realized. This contact design is suitable for thin-film solar cells on insulating substrates or insulating intermediate layers. Confirmed efficiencies of up to 18.2% were achieved on a 46 μm thick epitaxial silicon layer which was grown on a SIMOX wafer with an implanted compact SiO₂ intermediate layer.Samples with and without a highly doped back surface field were prepared to study the influence of the back-side recombination velocity. L_eff values of 250 and 52 μm, corresponding to S_back values of 800 and 10⁵ cm/s, respectively, were measured, thus, underlining the importance of a low back-side recombination velocity. The optical confinement properties of the SiO₂ intermediate layers were calculated depending on the angle of the incident rays. An angle from the plane normal which is larger than 23° is necessary in order to achieve the condition of total internal reflection.Future work will focus on recrystallized Si layers on foreign substrates [1]. Since the surface of the silicon layer is fairly rough after the recrystallisation process, another set of masks was designed which is more tolerant to aligning accuracy. This is mainly relevant for the area where the base contacts are located between the locally diffused emitter. The technology for CVD Si-layer deposition, zone melting recrystallization (ZMR), as well as for a simplified solar cell process is under investigation.     

Application of 1/f current noise for quality and age monitoring of electrochromic devices

This is a continuation of an earlier study on 1/f noise in electrochromic (EC) devices undergoing discharge via a resistor. The EC devices comprised films of W oxide and Ni–V oxide joined by a polymer electrolyte, and with this three-layer stack positioned between transparent conducting In₂O₃:Sn films backed by polyester foils. We also investigated “symmetrical” devices with two identical films of W oxide or Ni–V oxide. The power spectral density S_i at fixed frequency scaled with current (I) as S_iI². Color/bleach cycling for about 2500 times degraded the optical properties and homogeneity of the EC devices and increased the 1/f noise intensity by a factor of four, which confirms the earlier assumption that 1/f noise has a good potential to serve as quality and aging assessment for EC devices. Studies of “symmetrical” devices proved that the noise was mainly associated with the Ni oxide, and measurements on individual parts of an EC device indicated that the 1/f noise originated from localized areas.     

Photoelectrochemical studies of chemically deposited nanocrystalline p-type HgS thin films

Nanocrystalline mercury sulfide (HgS) thin films were deposited by chemical bath deposition (CBD) method onto the glass and fluorine doped tin oxide (FTO) coated glass substrate from an aqueous alkaline bath (pH  8) at room temperature (300 K). Mercuric acetate and thiourea were used as Hg²⁺ and S²⁻ ion sources, respectively. The photoelectrochemical (PEC) studies of HgS films were carried out, and the nanocrystalline films were found to be photoactive in polyiodide solution. The PEC cell configuration was p-HgS/0.1 M (KOH–KI–I₂)/C. From the current–voltage (I–V) characteristics, it is concluded that the HgS films are of p-type electrical conductivity. The photovoltaic output characteristics were used to calculate the fill factor (ff) and solar conversion efficiency (η). The low value of η may be due to the high value of series resistance (R_s) and interface states in the cell, which are responsible for the recombination mechanism.     

Self-aligned locally diffused emitter (SALDE) silicon solar cell

This paper presents, for the first time, a low-cost, high-throughput manufacturing approach for fabricating n-base dendritic web silicon solar cells with selectively doped emitters and self-aligned aluminum contacts using rapid thermal processing (RTP) and screen printing. The self-aligned locally diffused emitter (SALDE) structure is p⁺ nn⁺⁺ where aluminum is screen-printed on a boron-doped emitter and fired in a belt furnace to form a deep self-doped p⁺-layer and a self-aligned positive contact to the emitter according to the well-known aluminum-silicon (Al---Si) alloying process. The SALDE structure preserves the shallow emitter (20.2 μm) everywhere except directly beneath the emitter contact. There the junction depth is greater than 5 μm, as desired, in order to shield carriers in the bulk silicon from that part of the silicon surface covered by metal where the recombination rate is high. This structure is realized by using n-base (rather than p-base) substrates and by utilizing screen-printed aluminum (rather than silver) emitter contacts. Prototype dendritic web silicon (web) cells (25 cm² area) with efficiencies up to 13.2% have been produced.