High Efficiency Thin Film Solar Cells

dti

HIGH EFFICIENCY THIN FILM SOLAR CELLS

GROWN BY MOLECULAR BEAM EPITAXY (HEFTY)

CONTRACT NUMBER: S/P2/00369REP

URN NUMBER: 06/668 dti

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11 HIGH EFFICIENCY THIN FILM SOLAFt CELLS

GROWN BY MOLECULAR BEAM EPITAXY (HEFTY)

S/P2/00369/REP

Contractor

BP Solar Limited

Prepared by

N B Mason (BP Solar) KWJ Barnham, IM Ballard & J Zhang (Imperial College)

The work described in this report was carried out under contract as part of the DTI Technology programme: New and Renewable Energy, which is managed by Future Energy Solutions. The views and judgements expressed in this report are those of the contractor and do not necessarily reflect those of the DTI or Future Energy Solutions.

First Published 2006 © Crown Copyright

in EXECUTIVE SUMMARY

Background

The demand for Solar Electricity (photovoltaic power) is growing rapidly. Sales of PV products increased by 40% in 2004 and 25-30% growth rates are anticipated over the next 10 years. A major new industry is being created. The UK position in manufacturing photovoltaic modules is relatively weak compared to our main competitors in Japan, Germany and USA.

More than 90% of the PV modules currently manufactured are based on crystalline silicon wafer technology. These wafers are typically 200 to 300 m thick and are cut from solid silicon crystals (cast multicrystalline blocks or Czochralski gown single crystals). The sawing (wafering) process is highly inefficient, typically wasting up to 200 m of silicon as kerf loss for each wafer produced. Consequently, the cost of the semiconductor grade silicon in a PV module represents up to half the total module cost. In addition, with the rapid growth of the PV industry the worldwide PV demand for semiconductor grade silicon is now comparable to the requirements of the semiconductor electronics industry leading to supply shortage and price increases until new feedstock production capacity becomes available.

Thin films of high quality silicon deposited on low-cost substrates offer a potential solution to significant cost reduction and utilisation of silicon. Silicon films of only 10 m thickness can be engineered to produce high efficiency cells that require only 1 /40th of the silicon used in conventional PV cells. However, this field of thin film crystalline silicon solar cells is relatively new but could be a primary PV technology in the long term. Work in this field could give the UK an opportunity to establish a technology leadership which should attract manufacturing industry.

The motivation for this project was to establish the technology for high quality crystalline silicon film deposition suitable for high efficiency cells and to evaluate techniques for fabricating these devices on low-cost glass substrates.

Objectives

The aim of the project was to demonstrate a world leading result in the field of thin film crystalline solar cells and to establish the UK as a major contributor to this field. The cell design was to be consistent with the large-scale manufacturing and provide a basis for the process to be exploited by industry.

The main targets were: - 1. Demonstrate a 10 pm thick cell grown by deposition of silicon onto a monocrystalline substrate by molecular beam epitaxy (MBE) at temperatures below 700°C achieving a sunlight-to-electricity energy conversion efficiency of 18%. Cell area was to be at least 1 cm2.

iv 2. Demonstration of thin film devices with the same technical characteristics as target 1 deposited on a glass substrate below 700°C by a non-MBE process. The target energy conversion efficiency of these cells to be 8%.

Summary of the work

The work programme was divided into four main tasks; (A) MBE growth, (B) Characterisation of MBE films and solar cells, (C) Development of a high efficiency solar cell for MBE thin silicon films and (D) Transfer of MBE processed cell to a low cost (glass) substrate. The first two tasks were largely the responsibility of Imperial College whilst the latter two were to be undertaken by BP Solar.

It was anticipated that the growth of films for this project by MBE would not represent a significant problem. However, it was discovered though that the thickness of material to be grown, which is many times thicker than the usual 1 m samples, did in fact present problems with early samples suffering from very poor surface morphology. This substantially slowed progress as numerous samples were grown to optimise the growth conditions required for of p+- and p-doped epilayers. Initially, samples were grown using the MBE system in Ultra Low Pressure Chemical Vapour Deposition (ULP-CVD) mode, this was later changed to Gas Source Molecular Beam Epitaxy (GS-MBE) mode as this produced far superior material although at a lower growth rate.

During the course of this project a major re-structuring of BP Solar was undertaken that severely impacted the ability of the contractor to complete the project as originally envisaged. As part of the restructuring process, BP Solar closed-down its two thin film pilot production facilities that used glass superstates as the basis for semiconductor deposition. In addition, the European Technology Centre in Sunbury (UK) was closed and the technology functions were transferred to operations in Spain and the USA. In order to continue to support the project, BP Solar arranged for cell fabrication and cell development to be undertaken at the Fraunhofer Institute for Solar Energy (ISE) in Freiburg, Germany. This institute is a leading solar cell research organisation in Europe with all the skills necessary to convert the silicon film fabricated at Imperial College into solar cells. With this arrangement the project was able to complete Task C but unable to undertake any material work on Task D.

Due to these unforeseen circumstances, it was only possible to complete the first of the two project objectives defined above and little progress was made on the second objective. As a consequence of this, project expenditure was only half of that originally anticipated.

Summary of the main results

The first silicon films on this project were deposited by Ultra Low Pressure Chemical Vapour Deposition (ULP-CVD) but the best PV cell energy conversion efficiencies were very low at around 4%. When the silicon film growth method was changed to

v Gas Source Molecular Beam Epitaxy (GS=MBE) the resulting cell efficiency jumped to over 10%

ULP-CVD would have better material quality at high temperatures but these temperatures were incompatible with growth on glass which limited the upper temperature to 700°C. It was found that emitter structures grown epitaxially had significantly poorer performance and also required exceptionally low growth rates due to the arsenic doping. Arsenic modified the film nucleation behaviour and led to a poor surface morphology at the low growth temperature requirements. Cells with a diffused emitter achieved an efficiency of 10% whereas those with an emitter diffused into the structure during processing achieved 12.7%, which represents an efficiency improvement of 27%. The use of a textured front surface improves the efficiency by two means, first a reduction of reflectivity and second, an increase in path length. Sample thickness was increased to 15 pm to allow for a 7 pm texture depth. Samples without texture achieved 13.3% efficiency, which was improved to 15% efficiency with a front surface texture despite having less material available for absorption. The volume of Si in the textured samples is equivalent to ~10pm thick planar sample.

The best cells produced by the project with optimised epitaxial layer doping and the front finger metallization had a energy conversion efficiency of 16%, independently measured at the calibration facility of the Fraunhofer ISE. The electrical characteristics of the best cells fabricated by the project are listed below. isc 29.6 mA/cm2 Voc 655 mV FF 82.8% Efficiency 16.03% Area4.02 cm2

The deposited silicon material quality and the cell design and fabrication process was been significantly improved through the project. This is demonstrated by the improvement in efficiency from 4.2% to 16.0% from the first to the last samples processed by project, representing an improvement of 380%. This was been achieved by optimisation of the growth conditions, the switch from ULP-CVD mode to GS-MBE mode and diffusing the emitter structures into the epilayers as part of the post growth processing, combined with front surface texturing.

The final result of this project, a cell of 4 cm2 area fabricated on MBE deposited silicon film of 15 m thickness measured at 16% efficiency represents a significant achievement. It represents over 70% of the ideal limit as calculated for this cell thickness by Greenet al. [1]. With thin silicon films grown at a low temperature compatible deposition on glass we have demonstrated a PV cell with energy conversion efficiency within 70% of the ideal limit. The only previous cells to achieve similar results were thicker than 30 m. Clearly, the project has achieved a significant result. MBE silicon films grown at Imperial College are amongst the highest quality for PV application.

vi Conclusions

• Thin silicon films grown at low temperature (suitable for deposition on glass substrates) by Gas Phase Molecular Beam Epitaxy (GS-MBE) has been shown to give superior performance PV cells to Ultra Low Pressure Chemical Vapour Deposition (ULP-CVD). • Best cell performance, independently measured by the calibration laboratory at the Fraunhofer ISE, realised a sunlight-to-electricity energy conversion efficiency of 16% at standard test conditions on a cell of area 4 cm2 with a silicon film of 15 m thickness. • The project target 18% cell efficiency on a cell of area 1 cm2 was not realised. However, the achievement above represents 70% of the ideal limit for a cell on a silicon film of 15 m thickness. • The project has demonstrated one of the highest cell conversion efficiencies for a 15 m thin silicon film. Comparable world leading results have been achieved only of films of thickness greater than 30 m.

Recommendation

There is an obvious need for low cost, stable high efficiency solar cells and modules. Of all the prospective thin film technologies, crystalline thin film silicon is seen as having the most advantages given that the raw materials needed for production are abundant, non-toxic with little in the way of environmental issues in the product life cycle and can produce stable high efficiency solar cells. Its compatibility with current silicon wafer based technology is an asset and there is a marketing pull for successful product.

As noted in the original project proposal, it is unlikely that any immediate commercial product will be the outcome of this project. However, the project is an important building block in the long term development of high efficiency, low cost thin film crystalline silicon solar cells. We have demonstrated a benchmark performance for a cell at a thickness which would be appropriate for a significantly cheaper thin-film silicon cell, and growth at a temperature consistent with glass substrates. Both are important for commercial exploitation of this technology. Further investment is required in the development of silicon deposition growth and device fabrication to build on the excellent progress achieved by Imperial College on high efficiency cells thin silicon cells.

vii LIST OF ABBREVIATIONS

AM Air Mass BSF Back Surface Field CVD Chemical Vapour Deposition E-CV Electrochemical Capacitance Voltage EQE External Quantum Efficiency GS Gas Source I sc Short circuit current LPCVD Low Pressure CVD MBE Molecular Beam Epitaxy MQW Multi Quantum Well PV Photovoltaic QE Quantum Efficiency SIMS Secondary Ion Mass Spectroscopy SR Spectral Response ULP Ultra Low Pressure Voc Open circuit voltage XRD X-Ray Diffraction

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Task

1.1 1.2 1 Section 2.4 2.3.2.3 2.3.2.1 2.3.2.2 2.3.1 2.3.2 2.3 2.2.2.3 2.2.2.4 2.2.2.5 2.2.2.2 2.2.2 2.2.2.1 2.2 2.2.1 2.1.2.3 2.1.2 2.1.2.1 2.1.2.2 2.1 2.1.1 2 8 3 4 in co 1. BACKGROUND, AIMS AND SCOPE OF THE PROJECT

1.1 Background

The demand for Solar Electricity (photovoltaic power) is growing rapidly. Global sales of photovoltaic (PV) modules are believed to have exceeded 1 GWp in 2004 through a year-on-year growth rate of over 60%. Growth rates of 25-35% per annum are anticipated over the next 10 years. A major new industry is being created. However, the UK position in manufacturing photovoltaic modules is relatively weak compared to our main competitors in Japan, Germany and USA.

More than 90% of the PV modules currently manufactured are based on crystalline silicon wafer technology and this technology is expected to dominate the market for at least the next 10 years. However, these wafer-based silicon technologies use high quantities of high quality polysilicon feedstock and it has been argued that widespread adoption of PV technology may be limited by the high utilisation (high cost) of the silicon feedstock. Thin-film PV technologies (amorphous silicon, cadmium telluride, copper indium diselenide) that use substantially less semiconductor material have been developed but have not penetrated the market due to various factors including high investment costs, low performance, poor environmental stability or use of undesirable toxic materials. Future technologies incorporating organic compounds, nano-composite or dye sensitized inorganic materials are not generally expected to be a substantial part of the market until 2020 or beyond.

One proposed solution is to use thin crystalline silicon films. This approach builds on the considerable knowledge and expertise of the existing successful silicon PV technology but such films are 10 to 20 times thinner than current wafer-based silicon technologies and offer the prospect of significant cost reduction. However, this will only be realised if high efficiency cells can be fabricated on thin silicon films grown on low-cost substrates (e.g. glass). Accordingly, this project was to develop and demonstrate high efficiency PV cells on thin crystalline silicon films grown at low temperatures suitable for deposition on glass substrates.

1.2 Aims and Scope of the Project

The aim of the project was to demonstrate a world leading result in the emerging field of thin film crystalline silicon solar cells and to establish the UK as a major contributor in this important topic which many observers believe will be a significant solar electric technology in the long term. The solar cell technology developed was to be consistent with large scale manufacturing and should provide the basis of a cost effective industrial process to be exploited. Molecular Beam Epitaxy (MBE) was used to determine the exact parameters for the technical characteristics of a high efficiency thin film silicon solar cell and the first steps undertaken to realise these properties in a silicon film deposited onto a low cost substrate, such as glass, by a commercial silicon deposition method. The main targets to be achieved by the project were: 1. The demonstration of a thin film crystalline silicon solar cell of 10 microns thickness deposited onto a mono-crystalline silicon substrate at a temperature below 700 °C with an efficiency of 18%. Cell area to be at least 1 cm2 and the efficiency to be verified at an internationally accredited test laboratory. 2. The demonstration of a crystalline silicon film based device, of technical characteristics identical to those of the films used to achieve target 1, deposited onto a glass substrate below 700°C by a non-MBE process. The efficiency of the resultant devices to be at least 8% on a cell area of at least 1 cm2.

Unfortunately, during the course of this project the industrial partner, BP Solar, underwent a major organisational restructuring that severely impacted the ability to undertake work on objective 2 above. As part of the restructuring process, BP Solar closed its two thin film pilot production facilities that used glass superstates as the basis for semiconductor deposition. In addition, the European Technology Centre in Sunbury (UK) was closed and the technology functions were transferred to operations in Madrid, Spain and Frederick, USA.

In order to continue the excellent work started on the project, and in particular to continue to support the MBE silicon deposition technology development at Imperial College, the project continued to complete tasks A to C. Unfortunately it was not possible to carry out task D.

The major technical challenge to realise a successful thin film silicon solar cell is: • either develop a low cost substrate compatible with silicon epitaxy above 1000 °C or • develop a silicon epitaxy process which gives good electronic properties when the film is deposited around 600°C onto a soda-lime glass substrate or other low cost substrate.

It is the second approach upon which this project is focused.

Gas-source MBE routinely gives high quality films at growth temperatures between 600 and 700°C. The key issue for this part of the project is controlling the doping profiles, during MBE film growth, at 700°C, to achieve a graded p+/p film structure with a minority carrier diffusion length greater than 10 microns. Although MBE is not suited to a commodity manufacturing scenario itself, it does offer the most controllable method for the growth of high quality thin films. Nevertheless, these films are an excellent basis on which to demonstrate the future potential of this approach and to model the optoelectronic properties as well as structural and chemical properties.

2. EXPERIMENTAL WORK AND RESULTS

The work undertaken was divided into four tasks. In the remainder of this document these tasks are described together with the main results.

2 2.1 TASK A - Gas-source MBE Growth of thin Crystalline-Si Cells

2.1.1 Introduction

The aim of this task was to grow high-quality p+/p epitaxial Si layers for high- efficiency thin-film solar cells.

The main problem to be tackled was how to maintain the excellent minority carrier properties of epitaxial Si while growing at a sufficiently low substrate temperature that the cells will serve as suitable bench-marks for comparison with lower temperature growth on glass. The epitaxial growth of Si from hydride precursors is largely limited at low temperatures (<600°C) by the rate of desorption of molecular hydrogen, a product of the pyrolysis reaction. In order to achieve the necessary layer thickness (~10 pm) an elevated substrate temperature (~700°C) was employed together with disilane as precursor. The growth system was operated in the ULPCVD mode, recently implemented at Imperial College, to obtain the necessary growth rate for the proposed structures. The investigation focused on the determination of the lowest possible growth temperature that maintains good minority carrier lifetimes while remaining compatible with the subsequent growth on glass substrates.

Diborane and arsine were used as dopant precursors. The p-type doping is generally well behaved and the proposed p+lp structure did not present problems. However, /7-type doping from hydrides generally leads to a significant reduction in growth rates. One important objective of the project was to investigate appropriate conditions for MBE growth of rt emitters. The proposed emitters are relatively thin and therefore their epitaxial growth should not present a fundamental problem.

The work was undertaken in the Physics Department and the Centre for Electronic Materials and Devices at Imperial College. Epitaxial growth at Imperial College was performed on a VG Semiconductors V90 gas source MBE system shown in Figure 1.

Figure 1 VG Semiconductors V90 gas source MBE system. DEP A and DEP B are two separate growth chambers both accessed via the loading stage located in the centre of the picture.

3 2.1.2 Results

2.1.2.1 Determination of growth conditions for n+ emitters (M.A1)

The initial stage of p+/p growth took considerably longer than anticipated and this task was moved to later in the project than originally anticipated. N+ emitters grown by MBE were investigated in June 03 with samples sent for processing along with control wafers without epitaxial n+ regions, where the emitter would be diffused. As was noted in the proposal the n-type doping of hydrides substantially reduced the growth rate, with the rate falling by a factor of 12. The best results for cells grown with epitaxial emitters achieved 10% efficiency, this compares to 12.7% of samples grown at the same time but with emitters diffused during processing. The morphology and material quality of the epitaxially grown emitters was substantially poorer than diffused emitter. In addition the growth of layers thick enough to allow subsequent texturing would not have been practicable.

2.1.2.2 Assessment of minority carrier Dependence on low temperature substrates (M.A2)

This task was combined with M.C2. Lifetimes were obtained by modelling the external quantum efficiency (EQE) of fully processed cells. This technique ensured that that the effects of the deliberate changes in the growth parameters, such as temperature, could be separated from the variations introduced during processing. The best lifetimes and hence cell performance was achieved with the highest growth temperature of 700°C defined by the project. Higher temperatures were not considered due to the requirement that growth conditions were compatible with growth on glass substrates.

2.1.2.3 Details of optimised growth conditions of a 10 m MBE film with ideal p+/p structure (D.1)

Samples were grown on a VG Semiconductors V90 gas source MBE system. Substrates were (001) crystal plane orientated and 0.01 Q cm p-type single crystal silicon from Okmetric. Wafers were prepared prior to epitaxial growth by wet chemical clean consisting of cycles of HF/Nitric etch followed by SC1 and a final HF dip. Growth conditions were optimised by varying the incident precursor flux and substrate temperature via a standard computer controlled interface system. The optimum growth conditions were found to be a gas source pressure of 1 Torr disilane and a substrate temperature of 700°C to boron doped material at 5x1 016 cm-3 grown in MBE mode. This produced a growth rate of 1 pm an hour. Sample thickness was increased to 15 pm to allow for the texture etch, which removed material to a depth of 7 pm. The emitter diffused into structure post growth at Fraunhofer ISE.

4 2.2 TASK B - Characterisation of the MBE films and solar cells and device modelling

2.2.1 Introduction

This task was aimed at the characterisation and optimisation of electronic and optical performance of devices. Routine material characterisation by electro-chemical capacitance-voltage measurements (E-CV), secondary ion mass spectroscopy (SIMS) to ascertain the level of doping of grown epi-layers. Layer thickness was accessed using both SIMS and X-ray diffraction. It is not easy to determine the minority carrier diffusion lengths in layers thinner than the film thickness. Measurement and modelling of spectral response (SR) and dark and comparison with the predictions of the model PC-1 D for devices with different thickness and levels of doping, provided a reasonable determination of the minority diffusion lengths and a quantitative estimate of material quality.

This task made use of the facilities and expertise available in the Quantum Photovoltaic Group and Centre for Electronic Materials and Devices.

2.2.2 Results

2.2.2.1 Establishment of procedures for assessment of material quality of epilayers (M.B1) Grown wafers were characterised using the following techniques:- • Electro-chemical capacitance-voltage measurements (E-CV) were made using a Bio-rad E-CV profiler. This provided doping and depth information for calibration grown samples. • Secondary Ion Mass Spectroscopy (SIMS) - this provided very accurate doping and thickness information for calibration samples. • X-ray Diffraction (XRD) using a commercial Philips X-ray diffractometer. This allowed the accurate calibration of epilayer thicknesses and hence the accurate determination of growth rates. • By visual inspection of surface morphology throughout the project. Examples are shown in figure 2

5 Figure 2 Surface image of BC130 typical of the early samples showing the poor morphology characteristic of these samples! Left X10) and improved surface sample BC222 (right X 2.5). Processed samples were characterised by the following techniques:- • External quantum efficiency was measured using a scanning Bentham monochromator and computer controlled Stanford Research Systems SR510 lock-in amplifier to record the photocurrent as a function of wavelength. This is then compared with a calibrated silicon detector to provide absolute quantum efficiency. • Measurements of the IV characteristics in the dark were performed with a Keithley 238 source measure unit which applies a set voltage and measures the current simultaneously. The cells were mounted on a temperature controlled stage. • Light IV measurements were conducted at the Fraunhofer ISE under AM1.5G illumination to measure the efficiency, fill factor, short circuit current, and open circuit voltage.

2.2.2.2 Modification of SR and dark-current programmes for thin-film silicon devices (M.B2)

The use of a commercial software package PC-1D made it unnecessary to modify the programmes at Imperial College which had been optimised for lll-V materials. Fitting of the best material from the project modelled by PC-1 D is shown in D2.

2.2.2.3 Completion of development of light trapping programme (M.B3)

Commercial light trapping software SUNRAYS was used to model the light trapping. This was more practical than developing the programmes originally designed for MOW lll-V devices. SUNRAYS turned out to be the most appropriate software for modelling the textures and light trapping schemes available from Fraunhofer ISE. Figure 3 show results from SUNRAYS for a planar sample on an inactive silicon substrate, a sample with a front surface texture and a mirror on the rear of the active region, and a sample with textured from surface but on an inactive silicon substrate. This clearly show that the texture dramatically improves the QE but also that light trapping using a rear mirror is required for efficient solar spectrum utilisation.

6 1

0.9 * ' /

0.8 / / f* Xx/Av c& \ a> 0.7 "o / \ t \ \ HI 0.6 E / \ \ 3 c 0.5 (0 ;V \ \ 3 4-* a 0.4 X------\ ...... pi anar devi:e on Si \ 0.3 suusircue I \ \ HI ------Tetxured device on 0.2 Si substrate T-x 0.1 ------Textured device with \ ♦ rear Au mirror 0 ______300 400 500 600 700 800 900 1000 1100 Wavelength (nm)

Figure 3 Example structures modelled with SUNRAYS. One device has a texture on the front surface and a gold mirror on the rear of the active device, an equivalent sample with a planar front surface on a silicon substrate, and also a textured device on an inactive substrate.

2.2.2.4 Assessment of thin film cells grown on glass substrates (M.B4)

As discussed in section 1, due to restructuring changes at BP Solar this work was not undertaken.

2.2.2.5 Details of modelled 18% efficient solar cells structure (D.2)

To achieve the required 18% efficient cell would have require an increase in photocurrent of our best cell by 20%. The material quality of our 15% efficient cell was good enough as our modelling suggest that the 20% increase could have been achieved by reducing substrate thickness to 2 pm and adding a rear mirror. Such thin substrates are not technically feasible at Fraunhofer ISE who use mechanical grinding, minimum thickness after thinning being 50-100 pm. Figure 4 shows the modelling results from PC-1 D on the best quality material processed to date.

7 1.00E+00

------PC-1d Model

1.00E-01 -- ■ Experimental Data ---- PC-1 D External 1.00E-02 Quantum Efficiency S 30 ■ Experimental 1.00E-03 Data

1.00E-04 400 600 800 1000 Wavelength (nm) 1.00E-05

Figure 4 Modelled dark current and QE of best quality material processed as planar devices. In this case the diffusion length obtained by modelling was 50|jm - over three times the active device thickness.

2.3 TASK C - Development of a high efficiency solar cell in an MBE grown silicon layer including emitter formation, texturing of substrate for light trapping and advanced metallisation

2.3.1 Introduction

As a consequence of the restructuring of BP Solar and in order to continue to support the project, BP Solar arranged for cell fabrication to be undertaken at the Fraunhofer Institute for Solar Energy (ISE) in Freiburg, Germany. This institute is a leading solar cell research organisation in Europe with all the skills necessary to convert the silicon film fabricated at Imperial College into solar cells. BP Solar has a long-standing relationship with Fraunhofer ISE and has funded collaborative work this group over a number of years. Through this relationship we were able to arrange for the fabrication of cells from the silicon films deposited by Imperial College. The solar cell fabrication sequence agreed with Fraunhofer ISE for these cells comprised: •

• An RCA clean of the as-received films (a standard semiconductor surface clean for silicon) • Oxidation of the silicon surface to a depth of 105 nm to passivated the surface and form an anti reflection layer to reduce light losses • Photlithographic process sequence to open vias in the oxide film for the front grid contact • Metallisation of the front grid by vacuum deposition of sequential Ti / Pd/ Ag layers • Aluminium contact formation on the rear cell surface • Forming gas anneal • Silver electroplating to "build-up" the contact thickness and reduce resistive power losses

8 In later process sequences the surface was pre-treated to texture the surface to reduce reflection losses. In addition, cells were mechanically thinned to optimise the performance gain from light trapping in the cell.

Throughout this period BP Solar supported the analysis of he deposited films through the characterisation by measurement of minority carrier lifetimes. Cell performance measurements were also undertaken.

2.3.2 Results

2.3.2.1 Identification of the best structure for thin film silicon on silicon substrates (M.C1)

The best structure was found to be n+ on p with a p+ substrate. The material quality was sufficiently good that a more complex design was unnecessary. The diffusion lengths modelled from PC-1 D being much longer than the actual devices.

2.3.2.2 Demonstration of a workable solar cell using the structure in M.C1 on MBE deposited Si films (M.C2)

Initial growth characterisation was done to establish the best growth conditions in terms of growth rate and temperature consistent with high material quality. Initial samples were grown in ULP-CVD mode as this produces higher growth rates. These samples are considerably thicker than the normal epitaxial layers grown on this equipment. Initial samples showed deterioration in surface morphology beyond a thickness of a few microns. The optimum growth conditions were found to be a gas pressure of 1 Torr disilane and a substrate temperature of 700°C to boron doped material at 5x1 016 cm-3. A comparison of material quality for both GS-MBE and ULP- CVD was undertaken by growing nominally identical structures in both growth modes. Although GS-MBE mode was slower it still provided a reasonable growth rate of 1 pm/hour and also produced better quality material. GS-MBE growth was subsequently used for all further samples.

Table II shows the results obtained for PV cells grown on ULP-CVD and GS-MBE silicon films.

Table I Comparison of GS-MBE and ULP-CVD Wafer G rowth Voc I sc Efficiency Thickness mode (mV) (mA/ (%) (pm) cm2) BC139 ULP-CVD 510 11.4 4.1 14 BC142 ULP-CVD 506 10.2 3.9 14 BC154 GS-MBE 610 21.2 10.6 5 BC163 GS-MBE 610 21.2 10.5 10

9 The switch from ULP-CVD to GS-MBE growth increased the efficiency from 4.1% to 10.6%. ULP-CVD would have better material quality at high temperatures but these temperatures were incompatible with growth on glass which limited the upper temperature to 700°C. It was found that emitter structures grown epitaxially had significantly poorer performance and also required exceptionally low growth rates due to the arsenic doping. Arsenic modified the film nucleation behaviour and led to a poor surface morphology at the low growth temperature requirements. Cells with a diffused emitter achieved an efficiency of 10% whereas those with an emitter diffused into the structure during processing achieved 12.7%, which represents an efficiency improvement of 27%. The use of a textured front surface improves the efficiency by two means, first a reduction of reflectivity and second, an increase in path length. Sample thickness was increased to 15 pm to allow for a 7 pm texture depth. Table II compares the results of cells fabricated with both flat (planar) and textured surfaces. Samples without texture achieved 13.3% efficiency, which was improved to 15% efficiency with a front surface texture despite having less material available for absorption. The volume of Si in the textured samples is equivalent to ~10 pm thick planar sample.

Table II Measurements on textured samples Sample I sc Voc Efficiency (mA/cm2) (mV) (%) Flat 25.2 638.5 13.3 Textured 28.5 638.7 15.0 Textured and thinned 26.7 626.3 11.9 Textured and thinned 26.8 634.0 13.9

2.3.2.3 Production of a 1 cm2 solar cell based on a silicon film deposited onto a silicon substrate of 18% efficiency independently verified at an international accredited test centre (D.C)

The best cells achieved prior to the project end date had an energy conversion efficiency of 15.0% on cells of area 3.85 cm2 and independently measured at the Fraunhofer ISE. This result was reported at the international 19th European Photovoltaic Solar Energy Conference and Exhibition, Paris in June 2004. Light confinement was not successful in these samples due to a technical problem in thinning the substrate to add a back mirror during processing.

Following the official end of the funded project, new samples were grown and processed into cells incorporating optimised doping and front surface metallization. The resulting cells had an efficiency of 16.0% with an area of 4.02 cm2. Unfortunately light trapping again failed because of technical problems during the thinning process. The electrical characteristics of the best cells, independently measured by the Fraunhofer ISE in Germany, are shown in table III. These latest results will be submitted for publication in an international journal in 2005. Figure 5 shows a fully processed cell complete with front surface texturing.

10 Table III. Electrical characteristics of best cells (area 4.02 cm2) measured at the calibration lab of Fraunhofer ISE Sample Description /sc Voc FF Efficiency (mA/cm2) (mV) (%) (%) Optimised sample doped to 1e17 cm"3 incorporating texture 29.63 654.6 82.8 16.03 and optimized front metallization

The material quality of the deposited silicon films significantly improved through the project. This is demonstrated by the improvement in efficiency from 4.2% to 16.0% from the first samples processed to the end of the project, representing an improvement of 380%. This was achieved by optimisation of the growth conditions, the switch from ULP-CVD mode to GS-MBE mode and diffusing the emitter structures into the epilayers as part of the post growth processing, combined with front surface texturing. The chronology and rapid learning cycles of the cell fabrication are summarised in the bullets below.

• First processed cells achieved peak energy conversion efficiency of 10.6% for GS-MBE and 4.2% for ULP-CVD material (May 02) • Second processed cells achieved a peak conversion efficiency of 12.7% (Sept 03) • Third processed cells achieved a peak conversion efficiency of 15.0% (May 04) • Final processed cells achieved a peak conversion efficiency of 16.0% (Feb 05)

Figure 5 3.85cm cell with front surface texture

2.4 TASK D - Transfer of MBE process to commercially cost effective deposition systems and solar cell fabrication

Major re-structuring of BP Solar during the course of this project severely impacted our ability to undertake task D and as a consequence no significant progress was made in this field. As part of the restructuring process, BP Solar closed-down its thin film pilot production facilities that used glass superstrates as the basis for semiconductor deposition. In addition, the European Technology Centre in Sunbury

11 (UK) was closed and the technology functions were transferred to operations in Spain and the USA.

3. Discussion

The final result of this project, a cell of 4 cm2 area fabricated on MBE deposited silicon film of 15 m thickness measured at 16% efficiency represents a significant achievement. Figure 6 shows the position of this result relative to those of world leading research groups in this field. With a cell of only 15 m thickness we have achieved efficiency above 70% of ideal at a growth temperature compatible with growth on glass. The only previous cells to achieve similar results were thicker than 30 m. Clearly, the project has achieved a significant result despite the fact that the original (and highly ambitious) 18% efficiency target was not realised. MBE silicon films grown at Imperial College London are amongst the highest quality for PV application.

-----Limit with no light trapping ------70% of limit A Hefty Target ▲ HEFTY Final result □ Max Planck[2] ■ Max Planck[3] O ANU[3] ♦ ANU[3] • UNSW[4] O UNSW[5,6] X Fraunhofer[7] X Fraunhofer[9] - Beiijing[8]

Thickness (pm)

Figure 6 Comparison of Results from Imperial College with world leading results[2-9] As mentioned in section 1.2, during the course of this project a major re-structuring of BP Solar was undertaken that severely impacted the ability of the contractor to complete the project as originally envisaged. As part of the restructuring process, BP Solar closed-down its two thin film pilot production facilities that used glass superstrates as the basis for semiconductor deposition. In addition, the European Technology Centre in Sunbury (UK) was closed. In order to continue to support the project, BP Solar arranged for cell fabrication and cell developmentto be undertaken at the Fraunhofer Institute for Solar Energy (ISE) in Freiburg, Germany. This institute is a leading solar cell research organisation in Europe with all the skills necessary to convert the silicon film fabricated at Imperial College into solar cells. However, it was not possible to make any progress on the development of thin film crystalline silicon cells on glass substrates (task D) and hence no progress was made on the

12 second of the two originally defined objectives. The commercial application and manufacturing cost were not studied on the project.

4. Conclusions

• Thin silicon films grown at low temperature (suitable for deposition on glass substrates) by Gas Phase Molecular Beam Epitaxy (GS-MBE) has been shown to give superior performance PV cells to Ultra Low Pressure Chemical Vapour Deposition (ULP-CVD). • Best cell performance, independently measured by the calibration laboratory at the Fraunhofer ISE, realised a sunlight-to-electricity energy conversion efficiency of 16% at standard test conditions on a cell of area 4 cm2 with a silicon film of 15 m thickness. • The project target 18% cell efficiency on a cell of area 1 cm2 was not realised. However, the achievement above represents 70% of the ideal limit for a cell on a silicon film of 15 m thickness. • The project has demonstrated one of the highest cell conversion efficiencies for a 15 mm thin silicon film. Comparable world leading results have been achieved only of films of thickness greater than 30 m.

5. Recommendation

13 6. References 1. Green, M.A., A. Wang, J. Zhao, S.R. Wenham, P. Campbell, and D. Thorp. Enhanced Light-Trapping In 21.5% Efficient Thin Silicon Solar Ce/is. in Proceedings of the Fourteenth European Photovoltaic Solar Energy Conference. 1997. Barcelona, Spain: H.S. Stephens & associates. 2. Blakers, A.W., J.H. Werner, E. Bauser, and H.J. Queisser, Silicon Epitaxial Solar-Cell with 663-Mv Open-Circuit Voltage. Applied Physics Letters, 1992. 60(22): p. 2752-2754. 3. Blakers, A.W., K.J. Weber, M.G. Struckings, S. Armand, G. Matlakowski , M.J. Stocks, and A. Cuevas. 18% Efficient ThinSilicon Solar Cell by Liquid Phase Epitaxy. i n 13th European Photovoltaic Solar Energy Conference. 1995. Ni ce, France. 4. Zheng, G.F., W. Zhang, Z. Shi , M. Gross, A.B. Sproul, S.R. Wenham, and M.A. G reen, 16.4% efficient, thin active layer silicon solar cell grown by llquidphase epitaxy. Solar Energy Mater i als and Solar Cells, 1996. 40(3): p. 231-238. 5. Zheng, G.F., S.R. Wenham, and M.A. Green, 17.6%o efficient multilayer thin- fiim silicon solar cells deposited on heavily doped silicon substrates. Prog ress i n Photovolta i cs, 1996. 4(5): p. 369-373. 6. Zheng, G.F., A.B. Sproul, S.R. Wenham, and M.A. Green, High-efficiency CVD multi-layer thin-fiim siliconsolar cells, i n Conference Record of the Twenty Fifth IEEE Photovoltaic Specialists Conference. 1996. p. 465-468. 7. Faller, F.R., V. Henn i nger, A. Hurrle, and N. Schi lli nger. Optimization of the CVD Processfor Low-cost crystalline-silicon thin-fiim solar cells, i n Proceedings of the 2nd World PhotoitaicConference and Exhibition on Photovoltaic Solar Energy Conversion. 1998. V i enna, Austr i a. 8. Wang, W., Y. Zhao, X. Xu, X. Luo, M. Yu, and Y. Yu. The Polycrystalline Silicon Thin Film Solar Cells Deposited on Si02 and S i3n4 by RTCVD. i n 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion. 1998. V i enna, Austri a. 9. Hebli ng, C., S.W. Glunz, J.O. Schumacher, and J. Knobloch. High-efficiency (19.2%) silicon thin-fiim solar cells with interdigitated emitter and base front- contacts. i n 14th European Photovoltaic Solar Energy Conference. 1997. Barcelona, Spa i n.

7. Pub!i cati ons ari s ing from th i s project

IM Ballard, KWJ Barnham, J Zhang, T Bruton, S Gledhi ll and S Glunz, 19 h European PVSolar Energy Conference, Pans 2004, p 1229-1232

In preparati on - 16% efficient thin film silicon solar cells - IM Ballard, KWJ Barnham, J Zhang, T Bruton, S Gledhi ll and S Glunz

14 8. Acknowledgements

The authors would l ike to acknowledge the follow ing i nd i v i duals and organ i sat i ons for the i r ass i stance w i th thi s project.

• The Department of Trade and Industry for prov i s i on of fund i ng under i ts New and Renewable Energy Programme • R Chakraborty, Future Energy Solutions • S Glunz and co-workers at the Fraunhofer Inst itute of Solar Energy for dev i ce process i ng • T Bruton, S Gledh i ll, K Heasman and 0 Hartley (formerly BP Solar) for the i r contr i buti on to the project.