Optimization of front surface texturing processes for high-efficiency silicon solar cells

This paper presents the results of an experimental study regarding the increase in the efficiency of the silicon solar cells by texturing the front surface. Designing, patterning and surface etching processes led to refined structures with very low losses of the incident optical radiation. Photolithography has been used to generate patterns (disc hole) through the silicon dioxide layer grown at the beginning on silicon wafers. The holes (4 μm in diameter) have been uniformly distributed on the entire surface (2×2) cm² and the distance between the hole centres was determined to be 20 μm. Semispherical walls have been defined in holes by isotropic etching up to join together of the wells.     

Double porous silicon layer on multi-crystalline Si for photovoltaic application

Double porous silicon (d-PS) layers formed by acid chemical etching on a top surface of n⁺/p multi-crystalline silicon solar cells were investigated with the aim to improve the performance of standard screen-printed silicon solar cells. First a macro-porous layer is formed on mc-Si. The role of this layer is texturization of surface. Next, the cells have been manufactured using standard technology based on screen-printing metallization. Finally, a second mezo-porous layer in n⁺ emitter of cell has been produced. The role of this PS layer is to serve as an antireflection coating. In this way, we have obtained d-PS layers on these solar cells. The paper present observation of d-PS microstructure with SEM as well as measurements of its effective reflectance at the level of 2.5% in the 400–1000 nm length wave range. The efficiency of the solar cells with this structure is about 12%.     

Heterogeneous growth of microcrystalline silicon germanium

Microcrystalline silicon germanium films showing excellent opto-electronic properties have been prepared at a substrate temperature of 195°C by radio frequency plasma enhanced chemical vapor deposition at 13.56 MHz. A white light (AM 1.5) photoconductivity of 5×10⁻⁵/Ω cm and ambipolar diffusion length of 114 nm (from SSPG) established the device quality. Films are intrinsic (Fermi level near midgap; activation energy E_a (0.49 eV) is approximately half the band gap (1.01 eV)). Performance of preliminary n–i–p solar cells (with μc-SiGe:H i-layer) on stainless steel and molybdenum substrates justify their photosensitivities. A current density of 9.44 mA/cm² has been generated in an i-layer of only 150 nm thick without any back-reflector. A deposition rate of 0.75 Å/s for such a thin layer gives this material much advantage than a μc-Si cell, where a thickness of >2 μm is needed. A high V_oc of 0.43 eV has been achieved for such a low mobility gap cell (Ge fraction 60%).     

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (σ_ph), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p–i–n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100 mw/cm²) at room temperature.     

Reactive ion etching (RIE) technique for application in crystalline silicon solar cells

Saw damage removal (SDR) and texturing by conventional wet chemical processes with alkali solution etch about 20 micron of silicon wafer on both sides, resulting in thin wafers with which solar cell processing is difficult. Reactive ion etching (RIE) for silicon surface texturing is very effective in reducing surface reflectance of thin crystalline silicon wafers by trapping the light of longer wavelength. High efficiency solar cells were fabricated during this study using optimized RIE. Saw damage removal (SDR) with acidic mixture followed by RIE-texturing showed the decrease in silicon loss by ∼67% and ∼70% compared to conventional SDR and texturing by alkaline solution. Also, the crystalline silicon solar cells fabricated by using RIE-texturing showed conversion efficiency as high as 16.7% and 16.1% compared with 16.2%, which was obtained in the case of the cell fabricated with SDR and texturing with NaOH solution.     

Minority carrier lifetime of silicon solar cells from quasi-steady-state photoluminescence

Minority carrier lifetime is the most crucial material parameter for the performance of a silicon solar cell. While numerous methods exist to determine carrier lifetime on solar cell precursors prior to metallization, only very few techniques are capable of implicitly extracting effective minority carrier lifetimes from metallized silicon samples. In this paper, a measurement technique for effective minority carrier lifetime on silicon solar cells and metallized cell precursors via quasi-steady-state photoluminescence is presented. The setup requirements for this measurement technique are elaborated, experimental evidence of the reliability of such measurements down to carrier lifetimes in the range of a microsecond is provided, and a lifetime calibration of spatially resolved photoluminescence images of solar cells via the presented measurement technique is sketched. Finally, the very good agreement between the obtained effective carrier lifetime and the corresponding open circuit voltage of a solar cell is demonstrated.     

Classification of shunting mechanisms in crystalline silicon solar cells

The efficiency of a solar cell is given by its average electrical parameters. On inhomogeneous materials and especially on large-area solar cells the inhomogeneity of the short circuit current, the open circuit voltage and the fill factor are important factors to reach high and stable efficiencies and may limit the overall performance of the device.A locally increased dark forward current (shunt) reduces the fill factor and the open circuit voltage of the whole cell. The inhomogeneity of the forward current in a solar cell can be measured using lock-in thermography. The quantitative and voltage-dependent evaluation of these thermographic investigations of various solar cell types on mono- or multi-crystalline silicon enables the classification of the different shunting mechanisms found. By further microscopic investigations the physical reasons for the increased dark forward currents can be determined.It turns out that a high density of crystallographic defects like dislocation tangles or microdefects can be responsible for an increased dark forward current. Unexpectedly, grain boundaries in solar cells on multicrystalline silicon do not show any measurable influence on the local dark forward current. In most cases shunts caused by process-induced defects are dominating the current–voltage characteristic at the maximum power point of the solar cell. In commercial solar cells shunts at the edges are most important, followed by shunts beyond the grid lines.     

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.     

Forming openings to semiconductor layers of silicon solar cells by inkjet printing

An inkjet printing method for forming openings to buried semiconductor layers of silicon solar cells is described. The method uses an overlying resist as a sacrificial layer onto which a plasticiser for the resist polymer is deposited in a programmed pattern using inkjet printing. At the locations where the plasticiser is printed, the resist becomes permeable to aqueous etching solutions, enabling openings to be created in underlying dielectric or silicon layer(s). The formed openings can be used to create metal contacts to the buried silicon layers of the solar cell. The permeability of the resist to aqueous etchants can be reversed, thus enabling a single resist layer to be used to create more than one set of openings in the underlying layers. The proposed method may also be applied more generally to the formation of patterns of openings in layers of semiconductor or microelectromechanical devices.     

Measurements of effective optical reflectivity using a conventional flatbed scanner—Fast assessment of optical layer properties

A conventional flatbed scanner equipped with an additional diffusor is used for rapid measurements of an important figure of merit for optical surfaces, the effective reflectivity of devices such as solar cells. The application of the technique to multicrystalline silicon wafers with light-trapping structures obtained by electrochemical etching is shown. The use of this method for rapid quality control in production environments as well as in the lab is envisaged: even with a non-optimized diffusor, a spatial resolution of 0.1×0.1 mm² can be achieved, with an accuracy of the reflectivity measurements of 1% and data-acquisition times around 10 s per wafer.     

External bias as the factor of efficiency increase of silicon MIS/IL solar cells

A new method of improvement of efficiency of silicon solar cells (SC) with structure of Al/tunneling thin SiO_x/p-Si with induced inversion layer (MIS/IL) during their operation in solar modules structure has been presented. The method proposes modification of joining of the SC into modules as well as their photovoltaic properties improvement by means of external electrical bias. A mechanism of the reverse bias influence on structural parameters of the MIS/IL SC was studied theoretically. Relations expressing functional dependence of the MIS/IL structure parameters, and output electrical characteristics of SC on its base as function of the bias voltage value were obtained. Results of numerical calculations demonstrating efficiency of use of reverse electrical bias in the range from 0 to 0.6 V to increase the efficiency of the MIS/IL SC are presented. The method of external impact proposed has been compared with methods using technological aspects of silicon MIS/IL structures improvement.     

A simple fabrication process for 20% efficient silicon solar cells

Recently, a substantially simplified PERC silicon solar cell has been developed at ISFH with independently confirmed 1-sun efficiencies of up to 20.0%. This paper describes the details of the relatively simple cell fabrication process and experimentally characterizes the new cells. The simplified design involves reflection control by means of random pyramids, the direct evaporation of the front metal grid onto the random pyramids, elimination of the need for nontextured areas underneath the contact grid, and the use of a single phosphorous diffusion (1-step emitter).     

Direct patterned etching of silicon dioxide and silicon nitride dielectric layers by inkjet printing

An inkjet method for the direct patterned etching of silicon dioxide and silicon nitride dielectric is described. The method involves fewer steps, lower chemical usage and generates less hazardous chemical waste than existing resist-based patterning methods (e.g., photolithography), which employ immersion etching. Holes of diameter 40–50 μm and grooves 50–60 μm wide were etched in 300 nm silicon dioxide layers. Grooves were also etched in 75 nm silicon nitride layers formed on textured silicon surfaces. The resulting patterned dielectric layers were used to facilitate masked etching, local diffusions and metal contacting of underlying silicon for solar cell applications.     

Rapid thermal technologies for high-efficiency silicon solar cells

This paper shows that rapidly formed emitters in less than 6 min in the hot zone of a conveyor belt furnace or in 3 min in an rapid thermal processing (RTP) system, in conjunction with a screen-printed (SP) RTP Al-BSF and passivating oxide formed simultaneously in 2 min can produce very simple high-efficiency n⁺-p-p⁺ cells with no surface texturing, point contacts, or selective emitter. It is shown for the first time that an 80 Ω/□ emitter and SP Al-back surface field (BSF) formed in a high throughput belt furnace produced 19% FZ cells and greater than 17% CZ cells with photolithography (PL) contacts. Using PL contacts, we also achieved 19% efficient cells on FZ, >18% on MCZ, and 17% boron-doped CZ by emitter and SP Al-BSF formation in <10 min in a single wafer RTP system. Finally, manufacturable cells with 45 Ω/□ emitter and SP Al-BSF and Ag contacts formed in the conveyor belt furnace gave 17% efficient cells on FZ silicon. Compared to the PL cells, the SP cell gave 2% lower efficiency along with a decrease in J_sc and fill factor. This loss in performance is attributed to a combination of the poor blue response, higher series resistance and higher contact shading in the SP devices     

Full process for integrating silicon nanowire arrays into solar cells

A novel process was developed for integrating silicon nanowire arrays into solar cells. n-Type silicon nanowires were grown by chemical-vapour deposition via the gold-catalysed vapour-liquid-solid method, on a p-type silicon substrate. After the growth, the nanowire array was planarized, by embedding the nanowires in a spin-on glass matrix and subsequent chemical-mechanical polishing of the front surface. This planarization step allows to deposit a continuous and uniform conductive film on top of the nanowire array, and thus to form a high-quality front electrical contact. For an illumination intensity of 100 mW/cm², our devices exhibit an energy conversion efficiency of 1.9%. The main performance limiting factor is a high pn junction reverse current, due to contamination by the growth catalyst or to a lack of passivation of surface electronic defects.     

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.     

Fundamental understanding and implementation of Al-enhanced PECVD SiNx hydrogenation in silicon ribbons

A low-cost, manufacturable defect gettering and passivation treatment, involving simultaneous anneal of a PECVD SiN_x film and a screen-printed Al layer, is found to improve the lifetime in Si ribbon materials from 1–10 μs to over 20 μs. Our results indicate that the optimum anneal temperature for SiN_x-induced hydrogenation is 700°C for EFG and increases to 825°C when Al is present on the back of the sample. This not only improves the degree of hydrogenation, but also forms an effective back surface field. We propose a three-step physical model, based on our results, in which defect passivation is governed by the release of hydrogen from the SiN_x film due to annealing, the generation of vacancies during Al–Si alloying, and the retention of hydrogen at defect sites due to rapid cooling. Controlled rapid cooling was implemented after the hydrogenation anneal to improve the retention of hydrogen at defect sites by incorporating an RTP contact firing scheme. RTP contact firing improved the performance of ribbon solar cells by 1.3–1.5% absolute when compared to slow, belt furnace contact firing. This enhancement was due to improved back surface recombination velocity, fill factor, and bulk lifetime. Enhanced hydrogenation and rapid heating and cooling resulted in screen-printed Si ribbon cell efficiencies approaching 15%.     

Cost effective process for high-efficiency solar cells

A new method for patterning the rear passivation layers of high-efficiency solar cells with a mechanical scriber has been developed and successfully adapted to fabricate high-efficiency passivated emitter and rear cell (PERC). Three types of the rear contact patterns: dot patterns with a photolithography process, line and dashed line patterns with a mechanical scriber process have been processed in order to optimize the rear contact structure. An efficiency of 19.42% has been achieved on the mechanical-scribed (MS)-PERC solar cell on 0.5 Ω cm p-type FZ-Si wafer and is comparable to that of conventional PERC solar cells fabricated by using photolithography process. The mechanical scriber process shows great potential for commercial applications by achieving high efficiency above 20% and by significantly reducing the fabrication costs without an expensive photolithography process. Low-cost Ni/Cu metal contact has been formed by using a low-cost electroless and electroplating. Nickel silicide formation at the interface enhances stability and reduces the contact resistance resulting in an energy conversion efficiency of 20.2% on 0.5 Ω cm FZ wafer.