5Advanced Amorphous Silicon Solar Cell Technologies
Miro Zeman
Delft University of Technology
5.1INTRODUCTION
The?rst amorphous silicon layers were reported in1965as?lms of‘silicon from silane’deposited in a radio frequency glow discharge[1].Ten years later,Walter Spear and Pe-ter LeComber from Dundee University reported that amorphous silicon had semiconducting properties.They demonstrated that the conductivity of amorphous silicon can be manipulated by several orders of magnitude by adding some phosphine or diborane gas to the glow discharge gas mixture[2].This was a far reaching discovery since until that time,it had generally been thought that amorphous silicon could not be made n type or p type by substitutional doping. It was not recognized immediately that hydrogen plays an important role in the newly made amorphous silicon doped?lms.In fact,amorphous silicon suitable for electronic applications, requiring doping,is an alloy of silicon and hydrogen.Electronic grade amorphous silicon is therefore called hydrogenated amorphous silicon(a-Si:H).
The successful doping of amorphous silicon created tremendous interest in this material for two reasons.First,the material had several interesting properties that opened up many opportunities for semiconductor device applications.For example,due to the high absorption coef?cient of a-Si:H in the visible range of the solar spectrum,a1micrometer(μm)thick a-Si:H layer is suf?cient to absorb90%of the usable solar energy.Second,the glow discharge depo-sition technique,also referred to as plasma enhanced chemical vapour deposition(PECVD), enabled the production of a-Si:H?lms over a large area(larger than1m2)and at a low tem-perature(100to400?C).The low processing temperature allows the use of a wide range of low cost substrates such as glass sheet,metal or polymer foil.The a-Si:H is simply doped and alloyed by adding the appropriate gases to a source gas,usually silane.These features have made a-Si:H a promising candidate for low cost thin?lm solar cells.At present,besides solar cells,this material is used for thin?lm transistors in?at panel displays and photoconductive layers in electrophotography.
Since the?rst a-Si:H solar cell Carlson and Wronski made in1976,which had an energy conversion ef?ciency of2.4%[3],a-Si:H solar cell technology has improved considerably, and today,it is capable of producing solar cells with initial ef?ciencies exceeding15%[4]. Today,amorphous silicon solar cell technology is a mature thin?lm solar cell technology that in2003already delivered modules with a total output power of25.8MW p[5].
Thin Film Solar Cells: Fabrication, Characterization and Applications Edited by J. Poortmans and V. Arkhipov C 2006 John Wiley & Sons, Ltd. ISBN: 0-470-09126-6
174THIN FILM SOLAR CELLS
5.2OVERVIEW OF AMORPHOUS SILICON SOLAR CELL
TECHNOLOGY DEVELOPMENT AND CURRENT ISSUES
5.2.11970s
Carlson and Wronski announced that they had made the?rst experimental a-Si:H solar cell at the RCA Laboratory in1976[3].This single junction p-i-n a-Si:H solar cell deposited on a glass substrate coated with transparent conductive oxide(TCO)and aluminium back contact exhibited a2.4%conversion ef?ciency.In order to increase the output voltage of a-Si:H solar cells,the concept of a stacked(also called multi-junction)solar cell structure was introduced [6].A key step to industrial production was the development of a monolithically integrated type of a-Si:H solar cell[7].Using the monolithic series integration of a-Si:H solar subcells,a desired output voltage from a single substrate can easily be achieved.In1980,the integrated type a-Si:H solar cells were commercialized by Sanyo and Fuji Electric and applied in consumer electronics such as calculators and watches.
5.2.21980s
Much research in the?eld of a-Si:H solar cells was devoted to developing and optimising a-Si:H based alloys in the1980s.A p type hydrogenated amorphous silicon carbide(a-SiC:H) was incorporated in solar cells as a low absorbing layer,usually denoted as a window layer[8]. Hydrogenated amorphous silicon germanium(a-SiGe:H)became an attractive low bandgap material for stacked solar cells[9].Surface textured substrates were introduced to enhance optical absorption[10].The laboratory cells reached an initial ef?ciency in the range of11to 12%.The next generation of a-Si:H modules came on the market in the second half of the 1980s and was aimed at off grid power generation.These modules were single junction p-i-n a-Si:H solar cells,produced mainly in a single chamber batch process.The typical area of the modules ranged from0.1to0.3m2and they were aimed to deliver a power of around14W (stabilized ef?ciency up to5%).However,this promising technology suffered from some setbacks that gave it a bad reputation:pronounced initial degradation due to illumination, insuf?cient protection and framing of these modules against moisture,which resulted in the corrosion of contacting electrodes.
5.2.31990s
In the1990s,the main research and manufacturing efforts were directed towards achieving 10%stabilized module ef?ciency and a high throughput process.Several companies optimized and implemented an a-SiGe:H alloy in tandem(BP Solar[11],Sanyo[12],Fuji Electric[13]) and triple junction(United Solar[14])solar cell structures.The main characteristics of a-Si:H modules developed in the1990s were a multijunction solar cell structure,improved encapsu-lation and framing.Lightweight frames from organic materials that provided better protection against corrosion substituted the aluminium frames.The module area reached1m2and the total area stabilized module ef?ciency was increased to6–7%.The improved environmental protection of the modules enabled the producers to guarantee more than20years of power
ADV ANCED AMORPHOUS SILICON SOLAR CELL TECHNOLOGIES175 output.At the end of the20th century,the annual total production capacity for amorphous sil-icon single and multijunction modules reached around30MW p.The focus on the application of the modules shifted from off grid to building integrated applications.
Hydrogenated microcrystalline silicon deposited by the low temperature PECVD technique emerged in this period as a new candidate for the low bandgap material in multijunction a-Si:H based solar cells.The University of Neuch?a tel introduced a micromorph tandem solar cell in 1994,which comprized an a-Si:H top cell and aμc-Si:H bottom cell[15].The promising poten-tial of the micromorph cell concept was soon demonstrated by the fabrication of micromorph tandem and triple solar cells with stabilized ef?ciencies in the range of11to12%[16,17],and Kaneka Corporation started the development of micromorph module production technology [17].The introduction and implementation ofμc-Si:H in thin?lm silicon solar cells shifted attention to increasing the deposition rate.Several new deposition techniques[18]started to be investigated and developed for fabricating absorber layers at high deposition rates(10to 20?A/s),such as very high frequency and microwave PECVD,hot wire CVD,and expanding thermal plasma CVD.
5.2.4After2000
Research has concentrated on understanding and improving light trapping techniques,where surface textures as well as new TCO materials play a crucial role.This activity has resulted in the commercialization of novel deposition techniques for ZnO as an alternative TCO material for SnO2[19,20].Several deposition machine manufacturers have started developing commercial production machines for the fabrication of thin?lm silicon solar cells[21,22].Today the most advanced a-Si:H production lines are characterized by fully automated facilities and large area deposition over more than1m2,with an annual production capacity in the range of 10MW p to30MW p(Mitsubishi Heavy Ind.10MW p,Kaneka Corporation20MW p,United Solar30MW p).
5.2.5Current technology issues
In order to increase the competitiveness of a-Si:H modules on the market,several cost-to-performance aspects of the a-Si:H solar cell technology are of importance,which can be pided into the following performance and production related issues:
