Unusual photocapacitance properties of a mono-crystalline silicon solar cell for optoelectronic applications

The current-voltage characteristics of mono-crystalline solar cell device under dark and illumination of 100 mW/m² (AM1.5) were measured. The efficiency of the studied device under AM1.5 was found to be 14.22%±0.2 compared with the company standards. The capacitance properties of mono-crystalline silicon solar cell device were investigated under dark and illumination conditions. The studied mono-crystalline silicon solar cell exhibits an unusual photocapacitance ranging from 50.4 to 4585 nF under dark and 100 mW/m² (AM1.5) of white light, respectively. The drastic increase in the capacitance of the solar cell is due to the space charge polarization induced by the increasing number of photogenerated carriers. The photocapacitance mechanism of the solar cell was interpreted by modified Goswami and Goswami (MGG) model. The relative capacitance C_ph/C_d (the ratio between the capacitance under illumination to the capacitance under dark) and the relative resistance R_ph/R_d (the ratio between the resistance under illumination to the resistance under dark) as a function of the applied frequency at different illuminations were interpreted. The values of the interface state density N_ss and interface capacitance C_ss are increased with the increasing illumination intensities. The prepared mono-silicon solar cell device is a good candidate for photocapacitive and photoresistive sensors in modern electronic and optoelectronic devices.     

UV light protection through TiO2 blocking layers for inverted organic solar cells

Fully organic solar cells (OSCs) based on polymers and fullerenes have attracted remarkable interest during the last decade and high power conversion efficiencies (PCEs) beyond 8% have been realized. However, air stability of these cells remains poor. The conventional geometry of OSCs utilizes strongly oxidizing metal top contacts like Al or Ca. These metals are easily oxidized in air resulting in rapid decrease of PCE if cells are not perfectly encapsulated. Using a thin electron-selective hole-blocking bottom layer like TiO₂ enables fabrication of solar cells in a so-called inverted geometry. In this geometry, noble metals like Ag or Au can be used as top contacts, which are less sensitive to ambient oxygen. Thus, air-stability of these inverted solar cells is significantly improved. In this study we investigate inverted polythiophene-methanofullerene solar cells. We find significant influence of the TiO₂ layer thickness on light absorption and illumination stability of the solar cells, as well as the trap filling by photoinduced carriers. Even though TiO₂ layers as thick as 500 nm seem not to be detrimental for charge transport, light intensity losses limit the device performance. In turn, illumination stability is better for thicker TiO₂ layers, which can serve as UV filters and protect the photoactive materials from degradation, when compared to thin TiO₂ layers. Considering these different effects we state that a thickness of 100 nm is the optimization of the TiO₂ layer.     

Doping effects for organic photovoltaic cells based on small-molecular-weight semiconductors

We have studied highly efficient organic photovoltaic (OPV) cells based on small-molecular-weight semiconductors: zinc phthalocyanine (ZnPc) and C60. To improve the efficiency furthermore, open-circuit voltage (V_oc) has to be increased. We reported that 5,6,11,12-tetraphenylnaphthacene (rubrene) produces the highest V_oc of 0.91 V by p–n heterojunction OPV cells with C60. In this paper, we report rubrene doping effects for OPV cells based on ZnPc to obtain higher V_oc.     

Experimental analysis of upconversion with both coherent monochromatic irradiation and broad spectrum illumination

Upconversion of sub-band-gap photons promises to increase solar cell efficiencies by making these photons useful. In this paper, we investigate the application of β-NaYF₄:20% Er³⁺ to silicon solar cells. We determine the external quantum efficiency of an upconverter silicon solar cell, both under monochromatic excitation and, for the first time in the context of silicon solar cells, under broad spectrum illumination as it is relevant for the application to harvest solar energy. The investigated upconverter silicon solar cell responds under broad spectrum illumination with an average upconversion efficiency of 1.07±0.13% in the spectral range from 1460 to 1600 nm. The resulting efficiency increase for the used solar cell with an overall efficiency of 16.7% is calculated to be 0.014% relative.     

A comparison of fill factor and recombination losses in amorphous silicon solar cells on ZnO and SnO2

Effects of ZnO and SnO₂ TCO (Transparent Conductive Oxide) substrate materials on hydrogenated amorphous silicon (a-Si:H) p-i-n solar cell performances and recombination kinetics have been investigated. DC and Frequency-resolved photocurrent measurements in a-Si:H p-i-n solar cells of 6 have been carried out experimentally. In particular, the I–V characteristics in the dark and light, the quantum efficiency spectra, the intensity-, bias voltage- and frequency-dependence of photocurrent were obtained. Fill factor (FF) values were determined from I–V characteristics for both types of substrate cells under various illumination levels. The exponent v in the power–law relationship, I_ph α G^v, between generating flux density and photocurrent were determined at different bias voltages (DC) and modulation frequencies. High values of V_oc (open-circuit voltage), FF, and DC exponent v for the a-Si:H p-i-n solar cell with SnO₂ were obtained, but the integrated QE (quantum efficiency), the modulated exponent v were found to be low compared to cells prepared on ZnO substrates. Our results show that these parameters are sensitive to the ZnO and SnO₂ substrate materials which act as a window layer allowing most of the incident light to pass into the i-layer of p-i-n cells.     

8 MeV electron irradiation studies on electrical characteristics of Cu(In,Ga)Se2 solar cells

Cu(In,Ga)Se₂ (CIGS) solar cells are gaining considerable interest due to their high optical absorption coefficient and adjustable band gap, which enables them to achieve high conversion efficiency and also present many promising applications in space power systems. In this paper we report the results of the effect of temperature and 8 MeV electron irradiation on the electrical characteristics of ZnO/CdS/Cu(In,Ga)Se₂/Mo polycrystalline thin-film solar cells under forward and reverse bias studied in the temperature range 270-315 K. The solar cells were subjected to 8 MeV electron irradiation from the Microtron accelerator and were exposed to graded doses of electrons up to 75 kGy. I-V characteristics of the cells under dark and AM 1.5 illumination condition were studied before and after the irradiation. Capacitance measurements were also carried out at various frequencies before and after irradiation. In the measured temperature range, the dark current contribution is due to the generation-recombination of the minority carriers in the depletion region. The ideality factor is found to decrease with increase in temperature. It seems that electron irradiation has not altered the dark current conduction mechanism significantly. The effect of electron irradiation on the solar cell parameters such as fill factor (FF), conversion efficiency (η), saturation current (I_o), short circuit current (I_sc), open circuit voltage (V_oc), and ideality factor (n) was studied. They were found to be stable up to 75 kGy of electron dose as only small changes were observed in the solar cell parameters.     

Enhancing methanol oxidation with a TiO2-modified semiconductor as a photo-catalyst

The direct methanol fuel cell (DMFC) is currently one of the most promising alternative power sources because of its high energy, simple design and operation. However, the DMFC still faces several problems, such as sluggish methanol oxidation and oxygen reduction, as well as a high methanol crossover. In other areas of study, it is well-known that methanol can be photocatalytically oxidized by wide band gap semiconductors under solar illumination. Methanol has been used in photocatalytic water splitting to enhance the performance of photo-electrochemical cells (PECs). Therefore, by combining photocatalytic and electro-catalytic mechanisms, methanol is expected to promote a new type of photo-assisted DMFC. In this work, the semiconductor TiO₂ was used as a photo-catalyst in a PEC using a methanol solution. A TiO₂ P25 suspension was cast onto carbon paper and then dried at 80 °C for 60 min. The photo-electrochemical measurements were carried out in a 3-arm electrochemical cell, using Pt wire as the counter electrode and Ag/AgCl as the reference electrode. Linear scan voltammetry (LSV) was carried out using a 20 V/400 mA potentiostat. The current densities of the electrode were monitored with and without simulated solar illumination. From the investigation, the current density of the TiO₂ electrode under solar illumination was higher than it was without solar illumination. However, the value is low due to the low activity of TiO₂ under visible light illumination. Further studies were carried out by combining TiO₂ with a carbon material and a noble metal alloy to maximize the current density. This modified photo-catalyst can be utilized in a new photo-assisted DMFC to produce higher electricity.     

Influence of n-type chemical doping layer on the performance of organic photovoltaic solar cells

We report the performance improvement of organic solar cell by addition of an n-type chemical doping layer in organic bulk heterojunction device. The power conversion efficiency (PCE) of P3HT and PCBM-71 based polymer solar cells increases by adding a mixture of TCNQ (7,7,8,8-tetracyanoquinodimethane) and LCV (Leucocrystal violet) between active layer and cathode electrode. The PCE of the cell increases by 14% compared to the control cell with Al-only cathode electrode. The device with an organic n-doped layer shows the J_SC of 8.88 mA/cm², V_OC of 0.51 V, FF of 60.1%, and thus the PCE of 2.72% under AM1.5 illumination of 100 mW/cm².     

Study of CdS/Cu(In,Ga)Se2 interface by using n values extracted analytically from experimental data

This paper presents that an analytical method based on Lambert W-function can be applied to estimate the value of the diode ideality factor n, of a ZnO/CdS/Cu(In,Ga)Se₂ (CIGS) solar cell by using its dark current-voltage characteristics. The method is tested at different temperatures in the dark and found that the resulting n(T) values are in good agreement with those estimated experimentally from the slopes of the straight-line regions of Log I-V plots. The suggested values of n(T) under illumination are also determined using the exact explicit analytic solutions for the current-voltage relation expressed in terms of Lambert W-functions and experimentally estimated parasitic series and shunt resistances (R_s, R_sh), diode saturation current (I_o), open circuit voltage (V_oc) and short circuit current (I_sc) values at various temperatures. Temperature dependence of the diode ideality factor revealed that after illumination still tunnelling enhanced interface recombination mechanism dominates the current transport with relatively low tunnelling energy as compared to the dark case.     

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.     

Inverted and transparent polymer solar cells prepared with vacuum-free processing

Inverted transparent polymer solar cells were fabricated by sequentially depositing several organic layers from fluids, on ITO/glass substrates. ITO was used as a cathode to collect electrons. The photovoltage of these diodes can be increased by up to 400 mV by inserting a buffer layer of polyethylene oxide between ITO and the active layers, which results in 4-fold enhancement of power conversion efficiency under the illumination of 100 mW/cm² simulated AM1.5 solar light. The enhancement of V_oc is consistent with the work function change between ITO and ITO/PEO measured by photoelectron spectroscopy. Solar cell production without vacuum processing may lower production costs.     

Efficiency improved by acid texturization for multi-crystalline silicon solar cells

In this paper, we will show that efficiency of multi-crystalline silicon (mc-Si) solar cells may be improved by acid texturization. In order to enhance overall efficiency of mc-Si for solar-cell applications, the surface treatment of texturization with wet etching using appropriate solutions can improve incident light into the cell. Alkali etchant cannot produce uniformly textured surface to generate enough open circuit voltage (V_OC) and high efficiency of the mc-Si due to the unavoidable grain randomly oriented with higher steps formed during etching process. Optimized acid etching conditions can be obtained by decreasing the reflectance (R) for mc-Si substrate below levels generated by alkali etching. Short-circuit current (I_SC) measurements on acid textured cells reveal that current gain can be significantly enhanced by reducing reflection. The optimal acid etching ratio HF:HNO₃:H₂O = 15:1:2.5 with etching time of 60 s and lowering 42.7% of the R value can improve 112.4% of the conversion efficiency (η) of the developed solar cell. In order to obtain more detailed information of different defect region, high-resolution light beam induced current (LBIC) is applied to measure the internal quantum efficiency (IQE) and the lifetime of minority carriers. Thus, the acid texturing approach is instrumental to achieve high efficiency in mass production using relatively low-cost mc-Si as starting material with proper optimization of the fabrication steps.     

Performance of low series-resistance interconnections on the polycrystalline solar cells

The performance of four ribbons of lead SnAgPb (SAP) and lead-free SnAgCu (SAC) photovoltaic (PV) ribbons in patterned (pat-) and high-conductivity (con-) types soldered on silicon solar cells are investigated by electron characteristics and microstructure analyses. The electrical performances of the solar strings were characterized before and after the soldering process by a solar flasher, and strings with commercial ribbons of lead and lead-free are compared. In general, lead ribbon solar strings show lower power loss value than lead-free counterparts. And the commercial lead and lead-free ribbon strings show highest power loss around 6.66%-10.65% of all the solar strings. Furthermore, the power losses for patterned lead (pat-SAP) and lead-free (pat-SAC) solar string are 3.93% and 5.51%, respectively, implying that a patterned structure should improve the soldering process and result in lower power loss. Yet, significant low output power loss values are detected in the solar strings which are soldered with high-conductivity ribbons. It is estimated that the power loss for high-conductivity lead (con-SAP) and lead-free (con-SAC) solar strings are 3.89% and 4.43%, respectively. And a similar condition was revealed when the solder materials were practically interconnected the solar cells in PV modules. According to the microstructure analyses, the low power loss of con-SAP solar string should be due to the good electrical performance in low series resistance (R_s) and high shunt resistance (R_sh), thus resulting in a high fill factor of the solar device.     

Intensity modulated short circuit current spectroscopy for solar cells

Understanding charge separation and transport is momentously important for the rectification of solar cell performance. To probe photo-generated carrier dynamics, we implemented intensity modulated short circuit current spectroscopy (IMSCCS) on porous Si and Cu(In_x,Ga_1−x)Se₂ solar cells. In this experiment, the solar cells were lightened with sinusoidally modulated monochromatic light. The photocurrent response of the solar cell as a function of modulation frequency is measured as the optoelectronic transfer function of the system. The optoelectronic transfer function introduces the connection between the modulated light intensity and measured AC current of the solar cell. In this study, interaction of free carriers with the density of states of the porous Si and Cu(In_x, Ga_1−x)Se₂ solar cells was studied on the basis of charge transport time by IMSCCS data.     

Organic solar cells incorporating buffer layers from indium doped zinc oxide nanoparticles

Monodisperse, indium doped zinc oxide (IZO) nanoparticles were prepared via the polyol-mediated synthesis and incorporated into regular and inverted poly-(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C₆₁-butyric acid methyl ester organic photovoltaic devices as buffer layers between the active layer and the cathode. Efficient hole blocking at the particle buffer layers leads to an enhanced open-circuit voltage of the solar cells. This effect is even more pronounced for inverted device architectures. Device degradation studies revealed a solar cell performance reduction upon sample exposition to ambient atmosphere. However, this degradation is fully reversible under UV illumination. In addition, the n-doped IZO particles form suitable charge carrier transport layers for an efficient recombination in an intermediate recombination zone in tandem solar cells. Accordingly we have fabricated fully solution-processed tandem solar cells and investigated their optoelectronic properties.     

Efficiency enhancement in large-area organic photovoltaic module using theoretical power loss model

For efficiency enhancement of a large-area monolithic organic photovoltaic (OPV) module, we studied the influence of the OPV cell geometry parameters using theoretical and experimental methods. For this work, a unit OPV cell as a reference device and four types of monolithic OPV module with different active cell lengths were fabricated together on a glass substrate. The characteristics of the fabricated unit OPV cell were measured and the voltage (V_mp) and current density (J_mp) at the maximum power point were extracted. The parasitic power losses were calculated from the extracted parameters and the material parameters using a theoretical power loss model, taking into consideration the series resistance, contact resistance, and shading (or dead area) losses at the calculated maximum power of the monolithic OPV module. To analyze the influence of OPV cell layout on efficiency of the large-area monolithic OPV module, the power conversion efficiency of the four type monolithic OPV modules with different active cell lengths was measured and compared with the calculated power conversion efficiency. The calculated PCE ratio of the monolithic OPV module with three cells was approximately 78%, and the measured PCE ratio of the fabricated monolithic OPV module with three cells was also approximately 78%. The measured PCE ratio of fabricated monolithic OPV modules with two, four, and five cells also exhibited this tendency for the calculated PCE ratio. Thus, a large-area monolithic OPV module with optimum electrical power loss and an appropriate number of OPV cells can be designed by extracting the parameters of the unit OPV cell and calculating the electrical power loss using the proposed theoretical power loss model.     

Over 30% transparency large area inverted organic solar array by spray

We report the fabrication and characterization of large scale inverted organic solar array fabricated by all-spray process. The inverted polymer solar cell geometry consists of four layers, in the order of ITO-Cs₂CO₃-(P3HT:PCBM)-modified PEDPT:PSS, on a glass substrate. With semitransparent PEDPT:PSS as anode, the encapsulated solar array shows more than 30% transmission in the visible-near IR range. Optimization of device is done by thermal annealing, and the optimal annealing conditions are shown to be different in single-cell test device and the multiple-cell array. Solar illumination has been demonstrated to improve solar array efficiency up to 250%. Device efficiency of 1.80% was observed with the array under AM1.5 irradiance. Our preliminary data have shown that the performance enhancement under illumination only happens with sprayed devices, not devices made by spin coating. This means that solar cells made with our spray-on technique performs better under sunlight, which is beneficial for solar energy application.     

Recent progress in metal-doped TiO2, non-metal doped/codoped TiO2 and TiO2 nanostructured hybrids for enhanced photocatalysis

Titanium dioxide remains a benchmark photocatalyst with high stability, low cost, and less toxicity, but it is active only under UV light; thus, in practical applications using visible light, its catalytic reactions are stalled. To enhance its catalytic activity under visible light, non-metal/codoped TiO₂ structures are being studied. These structures improve the photocatalytic activity of TiO₂ in visible light by reducing its energy bandgap. This might be useful in wastewater treatment for the photocatalytic degradation of organic contaminants under visible and UV light irradiation. In this intensive review, we describe recent developments in TiO₂ nanostructured materials for visible-light driven photocatalysis, such as (i) mechanistic studies on photo-induced charge separation to understand the photocatalytic activity and (ii) synthesis of non-metal doped/codoped TiO₂ and TiO₂ nanostructured hybrid photocatalysts. Furthermore, the effects of various parameters on their photocatalytic efficiency, photodegradation of various organic contaminants present in wastewater, and photocatalytic disinfection are delineated.     

Oxide/polymer interfaces for hybrid and organic solar cells: Anatase vs. Rutile TiO2

In this work, we study the effect of the transparent conducting oxide (TCO) and the polymer applied (MEH-PPV or P3HT) on the photovoltaic properties of TCO/TiO₂/polymer/Ag bi-layer solar cells. The solar cells were analyzed under inert atmosphere conditions resembling an encapsulated or sealed device. We demonstrate that the substrate applied, ITO or FTO, modifies the crystalline structure of the TiO₂: on an ITO substrate, TiO₂ is present in its anatase phase, on an FTO, the rutile phase predominates. Devices fabricated on an FTO, where the rutile phase is present, show better stability under inert atmospheres than devices fabricated on an ITO, anatase phase. With respect to the polymer, devices based on MEH-PPV show higher Voc (as high as 1 V), while the application of P3HT results in lower Voc, but higher J_sc and longer device stability. These observations have been associated to (a), the crystalline structure of TiO₂ and (b) to the form the polymer is bonded to the TiO₂ surface. In-situ IPCE analyses of P3HT-based solar cells show a red shift on the peak corresponding to TiO₂, which is not present on the MEH-PPV-based solar cells. The latter suggest that P3HT can be linked to the TiO₂ though the S-end atom, which results in devices with lower Voc. All these observations are also valid for devices, where the bare TiO₂ is replaced by an Nb-TiO₂. The application of an Nb-TiO₂ with rutile structure in these polymer/oxide solar cells is the reason for their higher stability under inert atmospheres. We conclude that the application of TiO₂ in its rutile phase is beneficial for long-term stability devices. Moreover there is an interplay between low Voc and J_sc in devices applying P3HT, since power conversion efficiency can be partially canceled by their lower Voc in comparison with MEH-PPV. These findings are important for polymer/oxide solar cells, but also for organic solar cells, where a layer of semiconductor oxides are in direct contact with a polymer, like in an inverted or tandem organic solar cells.