CIS solar cell Essay Example

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CIS Solar Cells

  1. Introduction

    1. Background

The development of clean energy resources to substitute fossil fuels has never been so as aggressively been attended to as it is today[CITATION Ham04 l 1033 ]. For the 21st century science and technology, the task of finding alternatives to fossil fuels to reduce the carbon print as a result of environmental pollution requires new philosophy urgently to entirely change the power generation field[ CITATION Sch11 l 1033 ]. Hamakawa [CITATION Ham04 n t l 1033 ] highlights that the urgent call to find alternatives was strongly motivated by the Kyoto Protocol’s call to stop air pollution as a result of massive fossil fuel consumption and the need to develop and sustain the earth’s biosystems’ ecological cycles. Additionally, the need for alternative sources energy was concluded from influences of the industrial developments especially on the development of energy from James Watt’s construction of the steam engine in the 18th century to evolution of other energy like coal, oil, and liquid petroleum gas all of which are none renewable. However, the most promising renewable energy source for energy technology’s future has been identified to be photovoltaic or PV despite the drawback impending large-scale expansion which is associated to high prices for the solar cells. However, Hamakawa highlights that the production of thin-film solar cell that saves on both materials and energy has offered a solution in the reduction of costs associated to solar power cells. Hamakawa (2004) also reveals that Large Scale Intergrated Cicuit or LSIC is also another solution to the solar cells cost reduction given its production in large scale. However,the utilization of large-scale work requires roof construction PV materials or construction of government subsidized solar cells[CITATION Ham04 l 1033 ]. Accordign to Pistor [ CITATION Pis09 n t l 1033 ], there are three material systems applied as absorbers in thin film PV and these are CdTe, Chalcopyrites or CuInSe2 Cu(InGa)Ses, and thin film Si. In this paper, the discusion focuses on the formation and properties of CIS that makes it the viable material for the future of renewable energy implementation.

    1. Relevant Issues with formation and properties of CIS solar cells

According to Romeo, et al [ CITATION Rom04 n t l 1033 ], the CIS or Copper Indium Diselenide or CuInSe2 is one of the numerous available polycrystalline thin-film solar cells preferred for terrestrial and space applications. When used together with its alloys and Cadmium Telluride or CdTe, CIS appears to be the most promising contestant for use in large application of photovoltaic energy conversion. Birkmire & Eser [ CITATION Bir97 n l 1033 ] reveals that CIS is promising for its laboratory efficiencies of 15%. Other polycrystalline thin-film solar cells include CIGS or Copper Indium Gallium Selenide, all of which are preferred in solar cells for their high coefficient of absorption about 105 cm-1. Additionally, thin-layer polycrystalline solar cells have a thin layer that sufficiently absorbs the valuable part of the spectrum at about 2 micrometers. According to Repins, et al [ CITATION Rep08 n t l 1033 ], the highest record efficiencies for CIS are 17% efficiency [ CITATION Rom04 l 1033 ]. This new record retains the CIS as one of the most preferable polycrystalline thin film solar cells and the efficiency improvement is linked to the improved performance obtained from the Current- voltage analysis and is expected to reduce recombination. In thin-film Photovoltaic or PV segments, three different materials are applied for absorbers [ CITATION Mil05 l 1033 ] and these are: Thin film Si; CdTe, and Chalcopyrites or CuInSe­2 with these three technologies, chalcopyrites or solar cells based on CuInSe2 has efficiencies greater than 10%. Unlike CIGS, flexible thin film CIS solar cells have been fabricated on light-weight polymeric substrates by evaporating CuIn alloy predecessor and selenizing them in an atmosphere of H2Se about 4000 C for absorber layers growth. This low temperature technique is attuned to polymeric substrates for depositing the window layers of CdS and ZnO. Additionally, power generation using light-weight CIS is preferred especially where power needs are high and specific and mechanical flexibility is called for[ CITATION Bas96 l 1033 ].

    1. Problems to be solved

Compared to standard to standard wafer-oriented silicon solar cells technologies, the substrate CIS or Copper Indium Selenium solar cells demonstrate multiple advantages that resolve efficiency issues experienced in the former. According to Khartchenko & Kharchenko [CITATION Kha13 n t l 1033 ], the operation of solar cells depends on factors such as light absorption for electron-hole pairs creation or carriers; carriers diffusion; electrons and holes separation; and carriers collection. This elements are essential given that a solar cell represents a battery that is driven by light and that has an open current voltage of Voc, short circuit current Isc, parallel and series resistance (Rs, Rsh), and voltage (In Vn), maximum power point current and voltage[CITATION Kha13 l 1033 ]. Additionally, an ideal solar cell is expected to be cheap, availability of materials, and simple; cost efficient; and integrated Large scale manufacturability. In the case of CIS, the advantages that gives it an advantage over silicon cells is that it has high conversion efficiency, low costs of fabrication, materials economy hence very low weight GHT per unit power, multiple topologies ranging from very rough to atomically smooth, and small thickness which promotes high absorption, small length of diffusion and high velocity of recombination[CITATION Kha13 l 1033 ]. In addition, Yoosuf [CITATION Yoo12 n t l 1033 ] reveals that CIS solar cells also have various customizable opto-electronic properties like energy gap, electron affinity, graded gap, and work function. As a result, CIS solar cells can either be single junction cells, tandem junction cells, or triple cell which have an efficiency of up to 15%. When rated with other solar cells in terms of spectral response, CIS has a high relative spectral response as indicated below.
CIS solar cell 1

Chopra (2012)
Figure 1: Spectral response of CIS solar cells in relation to other cells. Source:

Despite the proven the proven track record that makes CIS or CuInSe2 very promising material for photovoltaic applications for a band width 1.05 eV, higher band gaps are recommended for higher efficiency levels and this calls for a band gap of about 1.2-1.3 eV. This band gap is considered to be optimal for maximized efficiency conversions. Conversely, the band gap of 1.0eV limits the open circuit voltage in CuInSe2 which falls way below 500mV. In return, the conversion efficiencies are limited to for CuInSe2. However, these limitations in band gap for CuInSe2 can be improved to match the solar spectrum by replacing part of Selenium by sulphur and part of indium by Gallium.

    1. Project aims and objectives

This section summarizes the aims and objectives of this work.

  1. Investigating the epitaxial growth and characterization of CuInSe2 crystallographic polytypes

  1. Methodologies and Analysis

    1. Experimental methods including material preparation and characterization

To investigate CuInSe2, the experimental samples were first prepared by growing them in a custom rotating disk. The preparation involved growing the samples of CuInSe2 in a rotating disk reactor that was designed to implement MEE or Migration-enhanced epitaxy process variant of the molecular beam epitaxy MBE technique. Samples preparation using MEE was advantageous from usign MBE in that it involved sequential and not simulteneous exposure of the substrates to fluxes of cationic or ionic nature; and a fluxless “relaxation” step is used between each exposure. For the CuInSe2 samples, sequential substrates exposure was used for maximization of surface or adaton reaction rates while minimizing anion or cation adatom binocular rates of reaction. Consequently, the sample preparation promoted two-dimensional growth while at the same time inhibiting clustering by intra-adlayers. Further, the fluxless relaxation period is intended at allowing relaxation for sequentially anion- or cation-terminated surface hence facilitation of equilibrium configuration every step of the way and before further deposition. Since the preparation proces may involve flux alternation, the reactor utilizes metal and selenium sources shielding thereby creating four zones. The four zones offer regions for substrates’ cyclic transportation by substrate carrier platen rotation.

The next step is to have each substrate sequentially exposed throughout a entire cycle to a assorted metallic (Cu+In) flux, vacuum ambient background, an Se vapour flux, and vacuum ambient background coupled with radiant heating.

The vacuum reactor used here is ultrahigh nature designed to utilize water-coolled shielding with the confinement of volatile selenium flux within the zone of selenium-deposition being done using liquid-nitrogen to facilitate the attainment of a system-base pressure of about ~10-7 Pa while the background pressure is at 10-5 Pa in the course of the deposition. This substrates are contained within the MEE system’s load lock where polished semi-insulating GaAs substrates are introduced. The preparation of GaAs for use in MEE involves etching GaAs in a solution at room temperature in a 5:1:1 ratio before rinsing it in water and methanol. Each substrate is then heatedin situ at >6000 C in ten minutes and directly exposed to Se. during film growth, the temperature was maintained at 5500 +/­ — 50 0 C.

CIS solar cell 2

Figure 2: A comparison of chalcopyrite and CuAu-Like P4m2 primitive unit cell

With the CuAu-like films, the several characterization techniques were utilized. First, X-Ray diffraction or XRD which was used to acquire data using PW3710 defractometer through copper anode filtered for provision of Kα radiation[ CITATION Sta02 l 1033 ]. Raman characterization was also used to characterize the film where a DILOR XY800 triple monochromator spectrometer was deployed capable of supporting multichannel charge coupled liquid-nitrogen device detector. Further, dynamical transmission electron diffraction simulation was calculated through Bloch-wave method for electrons transmitted using high energy[ CITATION Sta02 l 1033 ].

    1. Analytical models

In this study, description of other people’s work or a survey of published materials is involved particularly those that deal with Copper, Indium, Selenide or CIS photovoltaic solar cells. The paper provides a critical evaluation of each work to achieve intended descriptions and summaries. The literature analyzed includes scholarly journals, dissertations, books and conference proceedings. The choice of the sources was influenced by the topic being examined on the properties and formation of CIS solar cells in order to contribute to the context of gaining a better understanding of the history of CIS solar cells and the progress in adaptation and implementation and provide new interpretations of previous work. Additionally, the choice of literature to include was influenced by the author’s credentials including past work, education and authority in terms of their excellence and relevance. The identified most convincing arguments are that CIS properties are very dependent on the formation process which has to follow prescribed measurement guidelines. Finding literature involved consulting professors and librarians, identifying text in listed texts and using them to locate the sources needed in the research. Since CIS solar cell are promising and their use in solar cells have gained momentum during the 21st century, and relevant literature selection for CIS experimental method and characterization was on basis of its contribution to the growth and characterization of CuInSe2.

    1. Theoretical backgrounds

In the past, MEE has been applied in the growth of epitaxial films of CuInSe2 on (001) GaAs compound. This compound demonstrates distinct coexisting domains of unstable crystallographic structure. The characteristics of that structure are cation ordering CuAu (CA), and a chalcopyrite compound stable structure. However, no characterization has been done which is why there was need to use XRD, Raman Scattering, and transmission electron diffraction so as to offer the evidence on the polytype structure to replace the wrong perception of single-phase CuInSe2 ­structure[CITATION Sta02 p 3598-3600 l 1033 ].

    1. Key techniques in history

According to Schimidt [ CITATION Sch11 n t l 1033 ], the CuInSe2 are p-type semi-conductors that form p-n junction with n-type counterparts. These thin films are layers within the solar cell stack all of that work towards the absorption of majority of the photons that results to solar power coversion into electrical signals. In the solar cells, the layers are known as absorbers.

CuInSe2 uses three types of characterization techniques which are full area measurements, mapping, and imaging[ CITATION Hon14 l 1033 ]. Full area characterization involves measuring the whole solar cell or a large portion of the waferby issuing a single data point per wafer. These measurements are swift enough to make inline characterization on each and every wafer that goes throough the production line. Further, the produced data is used for statistical process control or SPC. IED test is the perfect example of full area measurement and is normally done at the end of solar cell processing, or at the end of lifetime test, or at the endof the measurement of a parameter like reflectivity on a part of the wafer. This technique is however disadvantaged by the fact that it does not reveal problems associated to an area like poor printing of an area on the wafer.

Mapping unlike full area characterization depends upon point by point measurement for wafer surface scanning. Mapping may provide high accuracy levels concerning each of the points on the wafer as well as expose imperfections like grain boundaries. While using the mapping characterization technique,one has to take into consideration the element of time since this method is proven to be time consuming. Consequently, the long times taken on each one wafer makes the mapping technique inappropriate for in-line characterization. The best wafer mapping example is LBIC system that involves scanning a laser over the wafer surface and for each data point, current is measured. When this technique is applied for two-dimenasional mapping, too much time is required and simplification is obtained through the use of line scans.

Imaging characterization involves the use of photograph taking techniques. To take the measurements, a sensor array is utilized to test multiple points concurrently. The main benefits of using imaging characterization is that enormous data point arrays can be taken in very small short time durations. Conversely, imaging characterization technique is disadvantaged by the fact that the sensors utilized are very expensive. However, with the emergence of cheap sillicon CCD camera, the use of imaging technique is gainig momentum since the silicon cameras resemble those in digital cameras.

Thin films of Chalcopyrite period in of CuInSe2 placed by MEE on (001) GaAs substrates extended the findings obtained using conventional MBE technique’s (001) substrate orientation reported in previous reports. When subjected to some conditions, the substrate appeared to contain CA-ordered structure domains. For this resultant structural model, XRD and dynamic electron-diffraction simulations were deployed resulting to type 1 CuAu-regimented cation sublatice and a single CuInSe2 molecular unit characterized by in atom in site 1c, copper atom in site 1a, and at site 2g, two Se atoms in site. There is consistence in this analysis with corresponding characterization data for CA-regimented structure under the assumption that the structure alternating Cu and In cation planes are inclined parallel to the (001) GaAs substrate plane and causing it to be parallel to the Chalcopyrire structure. From previous studies, there was prediction of a possible coexistence of CA and CH-regimented CuInSe2 revealed that only simple translation of alternating pairs of chalcopyrite (001) cation planes linked these structures. This translation transformation as indicated in figure 2 above show consistence between CA-ordered structure axes and the chalcopyrite (010) and (001) axes are in parallel.

X-Ray Diffraction

The X-Ray diffraction 0-20 used for characterization involved scanning some epitaxial CIS films on GaAs and the outcome is an unusually prominent diffraction series peaks at 20 and which are then assigned to respective reflections on the CA structure[ CITATION Sta02 l 1033 ]. During X-ray diffraction to proof the polytype nature of CuInSe2 the 20= 47.190 sufficiently designates the existence of CA-ordered and CuInSe2 since this peak as reported in XRD spectra is way too far from any of the reported others in any ternary compound in the field of Cu-In-Se phase.

TED or transmission electron diffraction and Microscopy

In this characterization, TEM reveals the cross-sectional imager taken using two varying superlattice diffraction spots. This comparison indicates that the two crystallographic structures are different and are distributed in segregated domains in real-time.

CIS solar cell 3

Figure 3: A comparison of dark-field TEM image b and c are from two unique superlatice diffraction sports, a is TED pattern, b taken using spot labelled CA in Image a, and c was taken using spot CH in image a.

  1. Conclusion or recommendation

From the experiment, the epitaxial layers of CuInSe2 were prepared in MEE conditions that allowed coexistence of chalcopyrite CH and CuAu cation-ordered (CA) domains while the CuAu structure was assigned to P4m2 space group. Using characterization, the two structures were discriminated as evident in XRD, TED, and Raman scattering. In XRD characterization, 20-47.190 at peak 003 was outstanding although this may be insufficient if the CA-regimented size of domain or fraction volume is small for a given sample. With TED, the characterization technique is destructive but it also permits identification of CA structure in the 001 spot while CH is identifiable in the 011 spot[ CITATION Sta02 l 1033 ]. However, in small CA ordering TED is a less definitive test since CH-ordered CuInSe2 reveals a non-vanishing 001 peak owing to double diffraction. Since there is sensitive non-destructive probe at 185cm-1, using techniques like Raman scattering spectroscopy could be used to test if there is any CA structure in any domain. During preparation of substrate, flux alternation qualities of MEE choice and process led to extraordinary prevalence of CA ordering as observed in epitaxial CuInSe2 film growth by MEE.

  1. References List

Basol, B. M. et al., 1996. Flexible and light weight copper indium diselenide solar cells on polyimide substrates. Solar Energy Materials and Solar Cells, 43(1), pp. 93-98.

Birkmire, R. W. & Eser, E., 1997. Polycrystalline thin film Solar cells: Present Status and Future Potential. Annu. Rev. Mater. Sci, Volume 27, pp. 625-653.

Chopra, K. L., 2012. Thin film Solar Cells-A Status Review. Available at:
[Online] http://www2.kfupm.edu.sa/cent/img/KFUPM-TFSC-Dec20.pdf [Accessed 5 June 2014].

Hamakawa, Y., 2004. Thin-film solar cells: Next generation photovoltaics and its applications. illustrated ed. Berlin: Springer.

Honsberg, C. & Bowden, S., 2014. the Photovoltaic Education Network. Available at:
[Online] http://www.pveducation.org/pvcdrom/characterisation/introduction [Accessed 5 June 2014].

Khartchenko, N. V. & Kharchenko, V. M., 2013. Advanced Energy Systems. 2nd ed. New York: CRC Press.

Miles, R., Hynes, K. & Forbes, I., 2005. Photovoltaic Solar Cells: An Overview. Progress in Crystal Growth and characterization of Materials Research Bulletin, 51(1-3), pp. 1-42.

Pistor, P., 2009. formation and Electronic Properties of CIGS junctions and relatd thin film solar cells, Berlin: University of Berlin.

Repins, I. et al., 2008. Characterization of 19.9% efficient CIGS absorbers. San Diego, California, National Renewable Energy Laboratory.

Romeo, A. et al., 2004. Development of Thin-film Cu(In, Ga)Se2 and CdTe Solar Cells. Progress in Photovoltaic Research and Applications, Volume 12, pp. 93-111.

Schimidt, S. S., 2011. Microscopic Properties of Grain Boundaries in Cu(In, Ga)Se2 and CuLnS2 thin-Film Solar Cells Studied By Transmission Electron Microscopy, Berlin: Helmholtz Centre Berlin.

Stanbery, B. J. et al., 2002. Epitaxial Growth and Characterization of CuInSe2 Crystallographic Polytypes. Journal of Applied Physics, 91(6), pp. 3598-3604.

Yoosuf, R., 2012. 2 Thin Films for Solar Cell Application. illustrated ed. New York: Shodhganga.