Polycrystalline thin-film solar cells – A review

Solar Energy Materials and Solar Cells 55 (1998) 75—81

Polycrystalline thin-film solar cells — A review Allen M. Hermann* Department of Physics, Superconductivity Laboratories, University of Colorado, Boulder, CO 80309-0390, USA

Abstract This paper reviews recent progress in polycrystalline thin-film solar cell research and development. Results from both small area cells and larger area modules/submodules are discussed. Emphasis is given to results from deposition techniques with potential for fabrication of large-area cells of the type CdS/CdTe and CdS/Cu(In,Ga)Se . Small area high-efficiency cell 2 results are discussed in terms of manufacturability issues for large-area cells. A discussion of recent successes in electrodeposition of high-quality absorbing layers of Cu(In,Ga)Se is given. 2 Hybrid approaches involving final adjustment of local and overall stoichiometries using post-deposition vacuum evaporation of selected substituent elements and thermal anneals are also discussed. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Polycrystalline; Thin films; Solar cells

1. Introduction According to one National renewable energy laboratory study [1], 15% solar-to electric conversion efficiency is required to reach a competitive goal of $0.06/kWh, the cost of electric power in many US locations today. Current photovoltaic systems costs based on crystalline silicon wafers have been estimated between $0.25 and $0.40/kWh. These high costs are traceable largely to the high costs of crystalline silicon photovoltaic device manufacture in spite of reasonably high efficiencies [2]. There are currently several low-cost thin-film solar cell options which have potential for high efficiency. Amorphous silicon (a-Si) enjoys a well-established successful large-area deposition technology (plasma-enhanced chemical vapor deposition). The

* E-mail: [email protected] 0927-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 7 - 0 2 4 8 ( 9 8 ) 0 0 0 4 8 - 8

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A.M. Hermann/Solar Energy Materials and Solar Cells 55 (1998) 75—81

degradation of a-Si solar cell efficiency in sunlight (Staebler—Wronski instability), however, is still a major difficulty requiring much research. One approach to solving this instability problem involves the use of the hot-wire technique (thermal CVD) to limit the number of defects during deposition. CdTe-based thin-film cells are being developed by low-cost non-vacuum techniques [2—4] in spite of difficulties with contacts and stability problems. High efficiencies for small-area cells fabricated by vacuum technology have also been reported for CdTe [2]. It has also been shown recently [5—7] in research laboratory tests that cells of type CdS/Cu(In,Ga)Se (CdS/ 2 CIGS) films deposited onto inexpensive soda lime glass substrates have solar-toelectric conversion efficiencies approaching 18%. We survey in this paper recent results on small-area cells and larger modules/submodules based on CdTe and Cu(In,Ga)Se absorber films, the latter system being discussed first. 2 2. Recent results 2.1. CdS/CuIn Ga Se 1~x x 2 Table 1 gives a compilation of recent thin-film results reported for cells of the type CdS/CuIn Ga Se . Some authors report these materials systems as indium tin x 1~x 2 oxide (ITO)/CdS/CuIn Ga Se cells with CdS denoted as a buffer layer, but the 1~x x 2 more common nomenclature is as we specify here with ITO denoted as a window layer often not mentioned in the cell designation. It is worth noting from Table 1 that one entry, that of Tokyo Institute of Technology, has In—Se as a buffer layer, however, with no Cd present. Also the NREL entries are worthy of special note. The record efficiency for a small laboratory cell, approaching 18%, is a remarkable result. This 13.6% efficiency for the NREL electrodeposited CIGS precursor is also important since electrodeposition holds great promise for large-area deposition. This particular entry corresponds to a hybrid process in which In,Ga and Se were subsequently added by a vacuum deposition process to the electrodeposited precursor. An entry corresponding to a 15 cm2 small area multi-cell module (Nordic Solar) has also been included in Table 1. A wide variety of deposition techniques were used for cell fabrication corresponding to the entries in Table 1. Some cells have uniform Ga content, others do not. Some of these features will be discussed in Section 3. Table 2 is an updated version of that reported in Ref. [15] and gives recent reported results for modules/submodules ranging in area from 40 to 3830 cm2. Of special note is the 11.2% aperture area efficiency of the 3830 Siemens Solar module which delivered 43.1 W. 2.2. CdS/Cd¹e Table 3 gives a compilation of recent thin-film results reported for cells of the type CdS/CdTe. Again very high efficiencies for small-area laboratory cells whose CdTe absorber layers and whose CdS layers were deposited by close-spaced sublimation are noteworthy (University of South Florida). CSS—CdTe and CdS deposited by

&0.2

0.39

NREL#

NREL#

0.633

0.592

—

—

0.689

34.54

31.55

27.6

34.0

31.5

32.8 29.7 30.4

75.6

69.6

71.6

77.2

—

73.1 78.6 72.0

J (mA/cm2) FF (%) 4#

!Institute for Energy Conversion, University of Delaware. "Institu¨t fur Physikalische Elektronik, Universita¨t Stuttgart. #National Renewable Energy Laboratory.

Energy Photovoltaics Solarex

2.9

—

0.674

0.623 0.661 0.595

0.3 — &0.2

IEC! IPE", Stuttgart Tokyo Institute of Tech. Nordic Solar

» 0#(V)

x

Group

15.5

13.9

13.6

17.7

12.4

14.9 15.4 13.0

g(%)

0.413

0.422

0.419

0.41

15.0

0.4 — 0.179

area (cm2)

[15]

[14]

[13]

[12]

[11]

[8] [9] [10]

Ref.

Electro-deposited CIGS precursor followed by elemental vacuum evaporation Cu sputtered, selenides and Se evaporated Sputtering/evaporation of binaries and elements with subsequent co-evaporation

moving source elemental vacuum evaporation elemental vacuum evaporation

elemental vacuum evaporation elemental vacuum evaporation elemental vacuum evaporation

Absorber deposition process

Table 1 Small area cell results for cells of the type CdS/CuIn Ga Se at AM 1.5 illumination, 25°C x 1~x 2

—

—

Record efficiency for all non-single-crystal thin films —

Uniform Ga content Uniform Ga content ZnO/In-Se/CIGS, more Ga at Mo contact Small area module

Comments

A.M. Hermann/Solar Energy Materials and Solar Cells 55 (1998) 75—81 77

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A.M. Hermann/Solar Energy Materials and Solar Cells 55 (1998) 75—81

Table 2 Module results for CdS/CuIn Ga Se structures at AM 1.5, 25°C [16] x 1~x 2 Group

Efficiency (%)

Area (cm2)

Power (W)

Siemens Solar EPVs! ISET" Solarex

11.2 6.9 6.9 13.0

3830 3746 845 40.4

43.1 25.7 5.8 0.525

!Energy Photovoltaics. "International Solar Electric Technology.

a chemical bath technique (CBD) produced a greater than 15% efficient cell (Matsushita Battery). Again it is clear from Table 3 that a wide variety of deposition techniques (vacuum and non-vacuum) have led to quite respectable efficiencies. Table 4 presents recent module results for CdS/CdTe structures of areas ranging from 706 to 3716 cm2 in area. The 9.1% efficient 6728 cm2 module of Solar Cells, Inc. is especially noteworthy.

3. Manufacturability issues 3.1. CIGS Quite a lot of information in manufacturability can be gleaned from laboratory-scale small-area experiments. The potential manufacturer should of course be encouraged by the wide variety of manufacturing techniques (vacuum and non-vacuum) which can produce high cell efficiencies. The relative insensitivity of cell parameters to a 10% variation of Ga content [5] has further encouraged R&D directed towards large-area devices. The lowering of Ga control requirements should enhance manufacturability. The achievement of small-cell high efficiency for both Ga-graded and non-graded cells is another indicator of the ease of manufacturability. Some issues, however, need to be addressed. The lowest sheet resistance of the ZnO window layer is about &10 )/square for relatively transparent layers. This puts restrictions on module development due to scribing for interconnections, and makes higher voltage devices desirable [24]. A solution is to develop higher content Ga cells with correspondingly higher bandgap, but this has not been successfully done to date [8]. Other issues currently being addressed include the necessity of Na diffusion from the soda-lime glass through the Mo back contact and corresponding optimization of the Mo thickness. The cost and availability of In and Ga for very large scale deployment may also be a relevant issue.

» 0# (7)

0.853 0.853 0.78 0.827 0.824 0.806 0.793 0.824

Group

University of South Florida University of South Florida Colorado School of Mines NREL Matsushita Battery University of Toledo Golden Photon Golden Photon

25.1 21.7 23.0 — 25.4 19.7 24.18 25.4

J (mA/cm2) 4# 74.5 76.5 72.0 — 71.9 73.3 63.7 70.2

FF (%) 15.8 14.2 12.9 12.0 15.05 11.6 12.2 14.7

g (%)

Table 3 Small-Area cell results for cells of the type CdS/CdTe at AM 1.5 illumination, 25°C

1.05 — — — 1.0 — 0.30 0.34

Area [17] [18] [19] [20] [21] [22] [16] [23]

Reference

CSS CdTe and CBD Cds CSS CdS and CdTe Electro-deposited CdTe, CBD CdS CSS CdTe, CBD CdS CSS CdTe, CVD CdS all-sputtered films, PLD CdCl 2 — —

Deposition process

A.M. Hermann/Solar Energy Materials and Solar Cells 55 (1998) 75—81 79

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A.M. Hermann/Solar Energy Materials and Solar Cells 55 (1998) 75—81

Table 4 Module results for CdS/CdTe structures at AM 1.5 illumination 25°C [21] Group

Efficiency (%)

Area (cm2)

Power (W)

BP Solar Matsushita Battery Solar Cells, Inc. Golden Photon

10.1 8.7 9.1 8.8

706 1200 6728 3716

7.1 10.0 61.3 29.3

3.2. CdTe One expects that a binary system reduces complexity in a manufacturing arena. Again one is encouraged by the large number of processes (vacuum and non-vacuum) producing high-efficiency cells. There are, however, problems to be overcome in developing large-area cells/modules. A major issue in the CdS/CdTe system relates to the difficulty in making an ohmic contact to p-CdTe and the process-related CdCl /O treatment required for the CdTe 2 2 surface [24]. The use of the heavy metal Cd is a problem common to CdS/ Cu(In,Ga)Se and CdS/CdTe structures from the n-type CdS, but it is exacerbated in 2 the CdTe cell with Cd present also in the absorber layer. Another problem common to both systems relates to environmental issues in the chemical bath deposition of CdS. The chemical utilization is poor and there are waste disposal problems.

4. Conclusions Significant progress in the development of polycrystalline thin-film photovoltaic devices and modules thereof has been made. The future looks promising, but significant problems, many of which we have identified above, must be solved prior to large scale deployment of these devices.

References [1] [2] [3] [4]

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