Monograinmaterialsforsolarcells

Monograinmaterialsforsolarcells

Monograin materials for solar cells

E. Mellikov , D. Meissner, T. Varema, M. Altosaar, M. Kauk, O. Volobujeva, J. Raudoja, K. Timmo, M. Danilson

Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, Estonia article i n f o

Article history: Received 3 September 2007 Received in revised form 31 March 2008

Keywords: Powders Solar cells CuInSe Cu ZnSnSe abstract

This paper reviews results of studies on different materials and technologies for monograin layer (MGL) solar cells conducted at Tallinn University of Technology. The MGL consists of monograin powder crystals embedded into an organic resin. The MGL combines the superior photoelectrical parameters of single crystals with the advantages of polycrystalline materials, such as the low cost and simple technology of materials and layers preparation and the possibility of making devices of practically unlimited area. A main technological advantage is the separation between absorber and cell formations.

The developments in the field of monograin materials of CuInSe2,C u2ZnSnS4 and Cu2ZnSnSe4 and technical parameters of MGL solar cells are summarized.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

The principal technologies used today for manufacturing solar cells are the planar and the thin-film technologies [1]. The planar technology is based on the use of very expensive large 3D single crystals. These large single crystals possessing a high degree of chemical purity and physical perfection are then sawn into thin 2D wafers and then are subjected to sophisticated methods of oxidation, diffusion and photoetching in order to form localized regions of different types of conductivity. The loss of very expensive monocrystalline material in this sawing process exceeds 50% [2]. This method of growing large crystals and then cutting them into ultra-thin wafers is therefore certainly not the best way around of producing materials, which must be largescale mass products of as low as possible price. As an alternative, thin-film technologies can be applied, but here the electronic parameters of the such obtained polycrystalline thin-film solidstate solar cells are in general much worse than those of monocrystalline solar cells [3]. On the other hand, thin-film technologies are, as a rule, much cheaper when compared to monocrystalline material-based technologies.

During the last years there has been a rising interest for the socalled spheral technologies of producing solar cells [4–6]. Spheral technologies are using powder materials [4] or powder-like materials [5,6] to form absorber layers. However, the biggest advantage of spheral technologies lies in the the simplicity of making large-area and still single-crystalline layers. In general, powder technologies are the cheapest technologies for producing materials.

It was shown by us [7] that isothermal recrystallization of initial powders in different molten fluxes appears to be a relatively simple, inexpensive and a convenient method to produce powder materials with an improved crystal structure and reduced concentration of inherent defects that are perquisite for solar cell use. The developed powder materials consist of small single-crystalline grains and it is possible to prepare the powder in such a way that the crystals are each physically perfect [8].I n many cases the chemical composition and the size of the powder grains can also be well controlled. Additional advantages of the developed powder materials besides their single-crystalline structure of every grain are their uniform distribution of doping impurities and a rather narrow granulometric composition. The main feature of monograin layer (MGL) technology is that fabrication of absorber/junction formation and cell/module formation is separated, which leads to the several benefits in both stages of MGL production. High temperatures are allowed in adsorber material production, and the possibility of using cheap, flexible, low temperature substrates allows production of cheap flexible solar cells.

2. Experimental

Monograin powder materials with different compositions were synthesized from metal binaries (Cu–In and elemental Se for

CuInSe2, CuSe, ZnSe(S), SnSe and elemental Se for Cu2ZnSnSe(S)4)

Contents lists available at ScienceDirect journal homepa ge: w.elsevier .com/locate/solmat Solar Energy Materials & Solar Cells in a molten flux in an isothermal recrystallization process. The grounded precursors were thermally annealed in evacuated quartz ampoules. The crystal size of the materials was controlled by the temperature and duration of the recrystallization process and by the chemical nature of the flux. The details of the monograin growth technology can be found elsewhere [9].

Surface morphology, phase structure and composition of the powder crystals were analyzed by high resolution SEM, XRD and EDS, respectively. The evolution of crystal shape and morphology of the monograin powders was analyzed by electron imaging using a high-resolution scanning electron microscope (SEM) Zeiss ULTRA 5 with the compositional contrast detector EbS. The chemical composition and the distribution of components in powder crystals were determined using an energy dispersive X- ray analysis (EDX) system (Rontex). XRD patterns were recorded using a Bruker AXS D5005 diffractometer with the monochro- matic Cu Ka-radiation in the 2p-interval of 12–901 using steps of 0.041 and a counting time of 2s/step. Room temperature (RT) micro-Raman spectra were recorded by using a Horiba’s LabRam HR high-resolution spectrometer equipped with a multichannel detection system in backscattering configuration. The incident laser light with a wavelength of 532nm was focused on samples within a spot of 1mm in diameter and the spectral resolution of the spectrometer was about 0.5cm 1.

Narrow granulometric fractions of grown powders (20–50mm) were used for the formation of the absorber layer in the MGL solar cell structure: graphite/CIS/CdS/ZnO [8,10,1]. Powder crystals were covered with chemically deposited CdS buffer layers. For

MGL formation a monolayer of CuInSe2 (Cu2ZnSnSe4) powder crystals was glued together by a thin layer of epoxy. The polymer film thickness was adjusted to 20mm on top of a 10mm glue film before inserting the monograins. Since the grains sink into the polymer and reach the underneath glue layer, after washing off the glue completely, the lower part of each grain sticks out of the polymer film. After polymerization of this epoxy, i-ZnO and conductive ZnO:Al were deposited by RF-sputtering onto the open (i.e. not covered by epoxy) surface of the layer. Solar cell structures were completed by vacuum evaporation of 1–2-m thick In grid contacts onto the ZnO window layer. Following this, the layer was glued onto glass substrates. The opening of back contact areas of crystals that were originally inside the epoxy was done by etching of the epoxy by H2SO4 and by additional abrasive treatment. This finalized the preparation of the solar cell structure (Fig. 1).

Graphite paste was used for the back contacts.

Photovoltaic properties of graphite/CuInSe2(Cu2ZnSnSe4)/CdS/ ZnO structures were characterized by I–V measurements under

100mW/cm2 tungsten halogen illumination. Spectral response measurements were performed by the help of a computercontrolled SPM-2 monochromator and a 100-W tungsten halogen lamp. Halogen lamp was calibrated using a power meter.

3. Results and discussion 3.1. Modification of the composition of monograin powders

It is known that the structural, morphological, electrical and optical properties of CuInSe2 materials depend strongly on the composition and the method of preparation. In most cases, after the removal of the used flux material, the as-grown monograin powders needed a post-treatment in different gaseous ambients to improve their properties for use as absorber materials. For this purpose, annealing in dynamic vacuum (0.01Torr) (continuous pumping) and in Se or in S vapor (0.1Torr) at different temperatures and durations was applied and studied. Heattreatments were performed for materials with a ratio of Cu/ In ¼ components of 0.92. A short-time vacuum annealing at low temperatures, where the loss of selenium only from absorber material prevails, resulted in a decrease of the Voc. At higher temperatures and for longer durations, when an In loss was also detected, the values of Voc increased somewhat, but still remained relatively low. The dependences of the open circuit voltage (Voc)o f MGL solar cells that were made from powders annealed in different ambients at 803K, as a function of the annealing time, are presented in Fig. 2. Annealing CuInSe2 powder in Se vapor

(0.1Torr for 40h) led to an increase of the open circuit voltage Voc of solar cell by up to 150mV in comparison to cells prepared from materials that were annealed in vacuum. At the same time, the values of the fil factor (F) remained low. The efficiency of MGL solar cells improved remarkably if the CuInSe2 absorber was treated in a sulfur-containing atmosphere. Fig. 3 shows the dependence of the output parameters on annealing temperatures in sulfur vapor. We found that the optimal annealing temperature for sulfurization was 803K. Fig. 4 shows the correlation of the Voc of such completed solar cell structures on the sulfurization time. An increase in the annealing

Fig. 1. Monograin layer: schematic structure.

V oc (mV)

1 PS = 0.1Torr

PSe = 0.1Torr vacuum

Fig. 2. Maximum values of V as a function of post-treatment time in different ambient atmosphere.

E. Mellikov et al. / Solar Energy Materials & Solar Cells 93 (2009) 65–6866

time resulted in a marginal increase in the Voc values. However, sulfur incorporation consistently resulted in decreased values of short-circuit currents (Isc).

After sulfurization, the efficiency of the active area of these cells improved dramatically up to 9.5% with a maximum values of

Voc ¼ 530mV, Jsc ¼ 25mA/cm2 and F ¼ 0.63. As can be seen in

Fig. 4, value of Voc improved remarkably, while the value of Jsc decreased somewhat, both of which are probably attributable to an increase in the band gap of the absorber due to the formation of a wide bandgap CuIn(Se,S)2 layer on the surface (Fig. 5). The improvement in Voc is assumed to be due to the passivation of deep trap states, resulting in a decrease of space charge recombination events [12], which also improved the F. The values of open circuit voltages of MGL solar cells with

Cu2ZnSnSe4 (0oxo2) synthesized in molten KI were dependent on the Zn/Sn concentration ratio in materials and on post-growth heat-treatment parameters. Current–voltage dependences (one of them is presented in Fig. 6)o fC u2ZnSnSe4 MGL solar cells show

Voc values of over 420mV, FFs of up to 4% and short circuit currents of up to 15.5mA/cm2, resulting in efficiencies of the active area of these solar cells of up to 2.16%.

4. Conclusions

CuInSe2 andCu2ZnSnSe4 solar cells were preparedin a monograin layer design. The influence of annealing treatment for the CuInSe2

V oc (mV)

I sc (mA)

Fig. 3. Solar Cell output parameters as a function of annealing temperatures (p ¼ 0. 1Torr, t ¼ 18h).

V oc (mV)

I sc (mA)

Fig. 4. Output parameters versus annealing time in sulfur vapor at 803K (p ¼ 0.1Torr).

x (m)

C In Cu

Se S

In C

Intensity (count)

Fig. 5. EDS scan over a sulfur-treated CuInSe monograin powder crystal.

ab) Cu2ZnSnSe4a) CuInSe2 Voc = 422mV j = 15.5mA/cm2

F = 4%

Voc = 479mV2 j = 36mA/cm2

F = 58% η = 9.5%

Current (mA/cm η = 2.16%

Fig. 6. I/V curves of different monograin layer solar cells.

E. Mellikov et al. / Solar Energy Materials & Solar Cells 93 (2009) 65–68 67 absorber material on the output parameters of monograin layer solar cells was investigated.The results show a pronounced improvement in solar cell parameters. However, additional studies especially of

CuInSe2–CdS interface modifications are needed to optimize the parameters of the newly developed solar cell structures.

Acknowledgment

Financial support of the European Union in the Energy Program and of the Estonian Science Foundation under contract Nos. 6160 and 6179 is gratefully acknowledged.

References

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