RISKS AND PERSPECTIVE

The analysis of major thin films tends to underestimate technical risks (despite Table 11.11) and subsequent comments. Risks are pervasive in thin film development, and major setbacks have already occurred. Perhaps the most universal cause is a lack of science base. Because thin films are almost always different from mainstream electronics materials (as opposed to x-Si, which shares much with the mainstream), thin film development is not much supported by scientific understanding outside of PV. Problems that might otherwise be trivial are magnified. Serious problems such as the Staebler-Wronski Effect in a-Si, multielement stoichiometry and uniformity in CIS, and defects and their interactions in CdTe and its contacts are even harder to overcome. Any efforts to follow through on the development of thin films for major energy production should allocate some support to improving their science base if only to reduce the risks associated with explosive growth.

Indeed, the risks associated with explosive growth are paralleled by those of getting started. The existence of one good solar cell (say 10-15 % efficiency at 1 cm2 size) is a needed proof – of-concept; but it is still a factor of 109 away from the size of the annual output (in square meters of module area) needed to make a successful technology at 25 MWp/yr. Newcomers to thin films sometimes miss this developmental challenge, to their detriment. It implies both high technical and financial risk, making the period of scale up often the most challenging. Producing TWs creates a second major scale up challenge – but only another factor of about 105 to get to about 4000 GWp/yr.

Further, for PV to be actually used for TWs of energy, PV electricity storage and PV synthesized fuels (like water splitting or a reverse methanol fuel cell) will be needed. In addition to these technical and energy systems challenges, it will be favorable if PV costs could drop below even those outlined here. For that, further aggressive research work could be highly beneficial.

The point here is that due to substantial, ongoing financial and technical challenges (and the potential for great rewards), thin films need long term, financial support from the private and public sectors to allow them to reach theirpotential. As this chapter should make clear, achieving that potential would be well worth the investment in terms of meeting the TW Challenge.

Thin Film Solar Cells Edited by J. Poortmans and V. Arkhipov © 2006 John Wiley & Sons, Ltd

[2] In the remainder of the chapter the shorter term “epitaxial Si solar cells”, will be used, although this is not the only thin-film crystalline technology in which an epitaxial Si layer is being deposited during the formation of the active layer.

[3] The large investment when building thin-film solar cell production lines is often mentioned as a major barrier.

[4] Si as such is not a rare material and the reduction of sand to metallurgical grade is consistent with a cost of 1-2Euro/kg.

[5] The chemical efficiency describes the ratio between the Si-containing species incorporated in the growing solid Si film and the amount of Si supplied.

[6] The difference in lattice constant between Si and Ge is about 4 %. The lattice constant of the SiGe alloy varies

practically in a linear fashion between the lattice constant of Si and Ge (Vegard’s law).

[8] Chemical efficiency is defined as the ratio of the Si which ends up in the active layer versus the amount of Si introduced in the gaseous or solid phase.

[9] Pursuing a similar concept AstroPower Inc. (now taken over by GE Energy) calculated with 9 $/m2 for substrate costs [17].

[10] For details on this technique see, e. g. [18], p. 41ff.

[11] Now H. C. Starck Ceramics, Germany.

[12] This process to produce RBSiC was developed forPV by H. C. Starck Ceramics, Germany.

[13] Former Pacific Solar Ltd.

[14] Calculated from the data given in Ref. [40].

[15] Mixture of HF (48 %) and K2Cr2O7 (0.15 molar solution) in the ratio 2:1 [68].

[16] Reflection values for liquid silicon calculated from refractive index and extinction coefficient at к = 0.6 pm given in Ref. [85]. For solid silicon and wavelengths >0.6 pm the extinction coefficient is negligible [40]. A more detailed analysis additionally has to take into account the corresponding emissivity values. They differ from the former ones since temperatures of silicon film and lamp filament are not the same [86].

[17] The nomenclature for this growth morphology is not consistent. Some authors distinguish between cellular and faceted growth (e. g. see Ref. [80]). Here we follow the naming in Ref. [60], where cellular is the main category divided into the subcategories of rounded and faceted cellular growth.

[18] In the following also called “bare ZMR layer”.

[19] CP 133: Chemical Polish consisting of HF (50 %), HNO3, CH3COOH in the ratio 1:3:3.

This is of course only true if a chemical equilibrium is reached at some stage in the deposition process.

[21] Labeled “DLAR” for “double layer anti reflection coating” in the figure.

[22] Increase of the conversion efficiency of a-Si:H solar cells. Based on fundamental considera­tions, major performance improvement is expected in the near future from an increase in the current of thin film silicon solar cells [23]. This increase has to result from improved light management schemes such as light trapping and reduction of light absorption losses. For solar cells deposited on glass plates, also called superstrate type cells, the development of a TCO front electrode material with an optimal surface morphology that results in improved light scattering properties is essential. Essential for solar cells deposited on (flexible) opaque carriers, often denoted as substrate type solar cells, is improvement of the texturing and reflectivity of the back contact. Ongoing attention has to be paid to further improvement of the optoelectronic quality of a-Si:H and a-SiGe:H absorbers, the doped layers and the interfaces between the doped layers and intrinsic absorbers.

[23] sccm: standard cubic centimetre per minute. It is a measure for gas flow. One standard cubic centimetre refers to the amount of gas under standard conditions (1 bar, 0 °C) in one cm3.

[24] Aluminium foil is cleaned, heated to around 500 °C while subsequently a TCO layer of SnO2:F of 700 nm is deposited on it by means of an APCVD process at a high rate (order 0.1 qm s-1) and with a natural texture enhancing light trapping within the solar cell.

[25] Deposition of the thin film silicon active layer by means of rf PECVD using a device structure equal to those used on a glass//TCO superstrate. Until now the development has focused on single p-i-n a-Si:H devices.

[26] Deposition of a reflective back contact.

[27] Lamination of a commodity polymer carrier foil, e. g. polyethylene terephthalate (PET).

[28] Removal of the temporary superstrate by means of wet etching.

[29] Monolithic series integration by patterning steps between the preceding processes.

[30] Application of contacts and protective encapsulation layers.

In order to demonstrate manufacturability with sufficient yield and uptime, a roll-to-roll pilot line is being constructed for a 35 cm wide web. The pilot line consists of an APCVD machine for the TCO deposition, a PECVD machine with several rf plasma zones for deposition of amorphous silicon layers, a sputter coater for ZnO:Al, Ag and Al, a compression and curing device for lamination, an etching machine with wet etching, neutralization and rinsing baths,

[31] Grain growth occurs when the initial grains are small (of the order of 1 |rm) but not when the initial grains are large (> 1 |rm). As CdTe produced with CSS, has fairly large grains as-deposited, usually no further grain growth is observed upon CdCl2 treatment, though it has been reported occasionally [64].

[32] The internal crystallographic structure of the grains improves in all cases (disappearance of subgrain structure).

[33] Intermixing between CdS and CdTe at the interface can be promoted. This effect of the CdCl2 treatment has been documented extensively, e. g. [65-68].

[34] The p type doping is established (type conversion) or improved; the minority carrier lifetime Tn is improved.

[35] The density of deep electronic states in the bulk or at the interface can be reduced [64], but other deep states can be introduced [69, 70]. The mechanisms of introducing or annealing out deep states are related with the creation of Cd vacancies during treatment, and interaction with chlorine and oxygen.

[36] Overtreatment can cause a Voc loss due to the introduction of deep recombination centers [69, 71]. It can also result in loss of adhesion [2].

[37] The energy barrier depends on illumination in an indirect way [90]: long wavelength illumination changes the occupation of deep acceptor states near the contact; this, in turn, changes their charge state, and hence the electrostatic potential profile and the energy

[38] Not only holes, but also minority electrons contribute to the current through the contact [87, 88]. A term describing the electron current has to be added to the second equation of Equations (7.14), and an estimate of the electron density at the edge of the contact space charge layer has to be made. It turns out that this mechanism can only be impor­tant when the diffusion length Ln is large, or the cell is thin, or two-dimensional effects at the contact decrease the effective thickness. The source of a nonnegligible electron concentration at the contact SCL can be direct optical generation (then this model is equiv­alent to (1) or a sufficiently large forward bias voltage over the solar cell junction. The model explains the single point cross over of a set of I-V curves observed under certain conditions [88].

[39] Upon turning off the stimulating electric field, delayed fluorescence is emitted from the sample. It decays in a power law fashion, IDF(t) a t-n with n « 1 [24] and reflects the monomolecular recombination of the geminate pair that leads to a fluorescent S1 state [30].

[40] Varying the concentration of the acceptor in the PhPPV:PdI system between 0.1 % and > «30 % shows that the dissociation yield increases by more than two orders of magni­tude as the PdI concentration increases. This correlates with the observation that the n(F) dependence saturates at lower fields (see Figure 8.8).

[41] Improvement of the phase structure in blend systems has a positive effect on the efficiency. It can be accomplished by thermal annealing, for instance, in the MDMO:PCNEPV system, or by varying the composition as in thePFB:F8BT system (for components see Section 8.2.3.1).

[42] If no intermediate states are involved charge generation at a CT center has to be described as an adiabatic quantum mechanical transition from the initial state of two carriers bound in an on-chain singlet exciton to the final state in which the electron is localized in the LUMO of the nearby acceptor and the hole occupies an on-chain state at a distance larger than the Onsager radius, which is around 20 nm at room temperature. It is unlikely that the respective matrix element is large enough for the typical time of exciton dissociation to be shorter than 100 fs.

[43] Provided that the energetic requirements are fulfilled, the yield of direct exciton disso­ciation into free charge carriers should depend neither upon external electric field nor upon temperature. However, measurements do reveal a strong field dependence of the yield although its temperature dependence turns out to be rather weak, especially at low temperatures.

[44] Since CT has to be an adiabatic process, the excess energy should afterwards be released and dissipated within the acceptor molecule occupied by the transferred electron. Since the energy dissipation time is typically shorter than 100 fs it is fairly unlikely that, over such a short time, the electron could escape from the Coulomb potential well by hopping, even if the concentration of acceptor molecules is close to 50 % or larger. Therefore, full separation of charges is feasible only due to motion of the on-chain hole, but then it is not clear how the excess energy, released upon deep trapping of the electron, can be transferred back to the on-chain hole.

[45] The interface morphology must strongly affect the exciton dissociation yield as experi­mentally verified. Any structural disorder at the interface is counterproductive because it will diminish the energy of zero point oscillations and, therefore, destroy the bottleneck for geminate recombination.

1.0

0.8

0.6

0.4

[47]

8

4

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20

[50]

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