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.     

Microcrystalline silicon thin film solar modules on glass

We developed microcrystalline silicon (μc-Si:H) thin film solar modules on textured ZnO-coated glass. The single junction (p–i–n) cell structure was prepared by plasma-enhanced chemical vapour deposition (PECVD) at substrate temperatures below 250 °C. Front ZnO and back contacts were prepared by sputtering. A process for the monolithic series connection of μc-Si:H cells by laser scribing was developed. These microcrystalline p–i–n modules showed aperture area efficiencies up to 8.3% and 7.3% on aperture areas of 64 and 676 cm², respectively. The temperature coefficient of the efficiency was −0.4%/K.     

Determination of front surface recombination velocity of silicon solar cells using the short-wavelength spectral response

A method of determination of recombination velocity S_f of minority carriers at the front surface of an n⁺–p–p⁺(p⁺–n–n⁺) silicon solar cell in which the n⁺(p⁺) front emitter is made by diffusion of dopant impurity in the p(n) region is presented. This method uses the short-wavelength spectral response of the cell to determine S_f and is applicable if the front emitter of the cell has a linearly varying built-in field. It was applied to a p⁺–n–n⁺ solar cell that had a Gaussian distribution of the dopant impurity in the p⁺ front emitter up to a depth of 0.078 μm from the surface. Using the spectral response data of cell in 380 nm<λ< 500 nm range S_f was found to have a nearly constant value 6×10⁵ cm s⁻¹ in 400 nm<λ<460 nm range. Below and above this wavelength range the value of S_f was found to be slightly smaller. For comparison the value of S_f was also determined assuming the p⁺ region to be uniformly doped, and this value was found to be significantly smaller than based on the diffused emitter model. The analysis showed that for a diffused junction cell, the assumption that the front emitter is uniformly doped, ignores the presence of the built-in field in the emitter region and leads to overestimation of minority carrier recombination in the emitter. Consequently for a given contribution of the front emitter region to the spectral response of the cell, this assumption underestimates the front surface recombination and determines a smaller value of S_f. On the other hand, the present method can be expected to determine a realistic value of S_f independent of λ for most diffused junction silicon solar cells using the spectral response data in a suitable short-wavelength range since each such cell indeed has a built-in electric field in the emitter region.     

Experimental and theoretical radiation damage studies on crystalline silicon solar cells

An experimental facility was developed to asses in situ the degradation of crystalline silicon solar cells, fabricated by the Solar Energy Group of the National Atomic Energy Commission (CNEA), by measuring the current–voltage characteristic curve. The cells were irradiated with 10 MeV protons and fluences between 10⁸ and 10¹³ p/cm², using an external beam of the linear tandem accelerator TANDAR, at CAC-CNEA. Furthermore, theoretical simulations were performed to establish the relation between the variation of the electrical parameters and the degradation of the lifetime of minority carriers in the base. The damage constant for 10 MeV proton irradiated silicon solar cells of n⁺–p–p⁺ structure and 1 Ω cm base resistivity was determined. Finally, a proposal of a new model of radiation damage for silicon solar cells is discussed.     

Mechanism for the anomalous degradation of proton- or electron-irradiated silicon solar cells

A mechanism of the anomalous increase of the short-circuit current of n⁺–p–p⁺ silicon space solar cells under high fluence of the high-energy 10 MeV protons or 1 Mev electrons is proposed. In distinction to other models this mechanism takes place as a result of the conversion of conductivity type and increased minority carrier lifetime with respect to that of majority carriers. This mechanism occurs in solar cells with deep centers, whose energy level is close to the middle of the band gap.     

Lifetime analysis of silicon solar cells by microwave reflection

Microwave photoconductive decay (μPCD) has become a standard technique for measuring the carrier lifetime of silicon used in solar cells. Here, we have used μPCD to examine the carrier lifetimes at common doping levels used in the base region of silicon photovoltaic devices. For the conductivity range used in the p-type base of n⁺–p structures, the microwave penetration depth is less than the wafer thickness. In this case, the reflectance–conductivity relationship is very nonlinear. We will show that quasi-steady-state photoconductivity (QSSPC) and resonance-coupled photoconductive decay (RCPCD) lifetime measurements track over a wide range of injection level, and generally agree at higher injection levels. Our μPCD data will be compared with the transient RCPCD data over the same range. The data from the latter agree at low-injection levels, but show serious disagreement at higher injection levels. The conclusion is that μPCD must be limited to low-injection levels in the doping range used for solar cells.     

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.     

Technological process for a new silicon solar cell structure with honeycomb textured front surface

This paper presents a new silicon solar-cell structure improved by texturisation of the front surface using silicon micromachining technologies. A ‘honeycomb’-textured front surface has been obtained through a photolithographical process to generate patterns (disc holes) on the front surface followed by isotropic etching (in HNO₃: HF: CH₃COOH) until the wells joined together.For front-surface loss characterisation, the spectral dependence of the front-surface reflectivity has been investigated by spectrophotometrical measurements. The surface reflectivity was lowered under 10% and this value was a good one compared to the reflectivity of silicon monocrystalline wafer untextured surface. The p–n junction made by phosphorous diffusion at 0.8 μm follows the honeycomb profile. In order to obtain low series resistance, a p+ boron diffusion on the back of the structure was made. The fabrication process was completed with an ohmic contact (Al on top and on the back surface).     

Microcrystalline silicon and the impact on micromorph tandem solar cells

Intrinsic microcrystalline silicon opens up new ways for silicon thin-film multi-junction solar cells, the most promising being the “micromorph” tandem concept. The microstructure of entirely microcrystalline p–i–n solar cells is investigated by transmission electron microscopy. By applying low pressure chemical vapor deposition ZnO as front TCO in p–i–n configurated micromorph tandems, a remarkable reduction of the microcrystalline bottom cell thickness is achieved. Micromorph tandem cells with high open circuit voltages of 1.413 V could be accomplished. A stabilized efficiency of around 11% is estimated for micromorph tandems consisting of 2 μm thick bottom cells. Applying the monolithic series connection, a micromorph module (23.3 cm²) of 9.1% stabilized efficiency could be obtained.     

Influence of carrier recombination in the space charge region on minority carrier lifetime in the base region of solar cells

In this paper the minority carrier lifetime (τ) in the base region of an n⁺/p silicon solar cell is calculated. The open circuit voltage decay method is employed. The influence of carrier recombination in the space charge region is considered through an interface recombination velocity, S_i. An analytical expression for τ is obtained and its value for one particular case is reported.     

Optimization of porous silicon reflectance for silicon photovoltaic cells

The antireflection properties of electrochemically formed porous silicon (PS) layers in the 0.3 μm thick n⁺ emitter of Si p–n⁺ junctions, have been optimized for application to commercial silicon photovoltaic cells. The porosity and thickness of the PS layers are easily adjusted by controlling the electrochemical formation conditions (current density and anodization time). The appropriate PS formation conditions were determined by carrying out a two steps experiment. A first set of samples allowed to determine the optimal porosity and a second one to adjust the thickness of the PS layers, by evaluating the interference features of the reflectance produced by the layers. A PS layer with optimal antireflection coating (ARC) characteristics was obtained in 30% HF in only 3.5 s. The effective reflectance is reduced to 7.3% between 400 and 1150 nm which leads to a gain of up to 33% in the theoretical short circuit current of a p–n⁺ shallow junction compared to a reference junction without a PS layer. The effective reflectance with optimized PS layers is significantly less than that obtained with a classical TiO₂ ARC on a NaOH pretextured multicrystalline surface (11%).     

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.     

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.     

Minority carrier lifetime in plasma-textured silicon wafers for solar cells

In this work a comparison between plasma-induced defects by two different SF₆ texturing techniques, reactive ion etching (RIE) and high-density plasma (HDP) is presented. It is found that without any defect-removal etching (DRE), the minority carrier lifetime is the highest for the HDP technique. After DRE, the minority carrier lifetime rises as high as 750 μs for both RIE- and HDP-textured wafers at an excess carrier density of 1×10¹⁵ cm⁻³. The measured lifetimes correspond to an implied one-sun open-circuit voltage of around 680 mV compared to about 640 mV before DRE for the HDP-textured wafers. FZ silicon 1 0 0 wafers were used in this study. We also noted that in the RIE process, the induced defect density was significantly lower for wafers etched at 300 K than those etched at 173 K.     

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.     

Formation and characterization of porous silicon layers for application in multicrystalline silicon solar cells

The electrochemical formation of porous silicon (PS) layers in the n⁺ emitter of silicon p–n⁺ homojunctions for solar energy conversion has been investigated. During the electrochemical process under constant polarization, a variation of the current density occurs. This effect is explained by considering the doping impurity gradient in the emitter and by TEM characterization of the PS layer structure. Optical transmission measurements indicate that modifications of the refractive index and absorption coefficient of PS are mainly related to the porosity value. Reflectivity measurements, spectral response and I–V characteristics show that PS acts as an efficient antireflection coating layer. However, beyond a critical layer thickness, i.e. when PS reaches the p–n⁺ interface, the junction properties are degraded.     

Development of both-side junction silicon space solar cells

This paper reports the recent results of improving the radiation hardness of silicon solar cells, which is SHARP and NASDA's project since 1998 (Tonomura et al., Second World Conference on Photovoltaic Solar Energy, 1998, pp. 3511–3514). Newly developed 2×2 cm² Si solar cells with ultrathin substrates and both-side junction (BJ) structure showed 72.0 mW (13.3% efficiency) maximum output power at AM0, 28°C after 1 MeV electron irradiation up to 1×10¹⁵ e/cm² and the best cell showed 72.5 mW (13.4%) maximum output power. These solar cells have p–n junctions at both front and rear surfaces and showed less radiation degradation and better remaining factor than previous solar cells.     

Experimental evidence of very high open-circuit voltages of inversion–layer silicon solar cells

In this paper the first experimental evidence of the high V_oc-potential of inversion-layer silicon solar cells is given. Minority-carrier lifetime measurements on inversion-layer emitters have been performed and the diffused p–n contact of PN-IL silicon solar cells has been optimized for high open-circuit voltages. PN-IL silicon solar cells with open-circuit voltages of 693 mV have been fabricated on 0.2 and 0.5-Ω cm FZ p-Silicon wafers. These values are the highest ever reported V_oc's for inversion-layer silicon solar cells on p-Silicon. This demonstrates that inversion-layer silicon solar cells exhibit a similar potential for achieving high open-circuit voltages as silicon solar cells with a diffused p–n junction.     

Flexible amorphous and microcrystalline silicon tandem solar modules in the temporary superstrate concept

Encapsulated and series-connected amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) based thin film silicon solar modules were developed in the superstrate configuration using an aluminum foil as temporary substrate during processing and a commodity polymer as permanent substrate in the finished module. For the development of μc-Si:H single junction modules, aspects regarding TCO conductivity, TCO reduction, deposition uniformity, substrate temperature stability and surface morphology were addressed. It was established that on sharp TCO morphologies where single junction μc-Si:H solar cells fail, tandem structures consisting of an a-Si:H top cell and a μc-Si:H bottom cell can still show a good performance. Initial aperture area efficiencies of 8.2%, 3.9% and 9.4% were obtained for fully encapsulated amorphous silicon (a-Si:H) single junction, microcrystalline silicon (μc-Si:H) single junction and a-Si:H/μc-Si:H tandem junction modules, respectively.     

A numerical model of p–n junctions bordering on surfaces

Many solar cell structures contain regions where the emitter p–n junction borders on the surface. If the surface is not well passivated, a large amount of recombination occurs in such regions. This type of recombination is influenced by the electrostatics of both the p–n junction and the surface, and hence it is different from the commonly described recombination phenomena occurring in the p–n junction within the bulk. We developed a two-dimensional model for the recombination mechanisms occurring in emitter p–n junctions bordering on surfaces. The model is validated by reproducing the experimental I–V curves of specially designed silicon solar cells. It is shown under which circumstances a poor surface passivation, near where the p–n junction borders on the surface, reduces the fill factor and the open-circuit voltage. The model can be applied to many other types of solar cells.