Diode Quality factor

A major degradation mechanism of modules is the decrease in fill factor. This is caused by an increase in the diode quality factor of the cells making up the module and by an increase in series resistance. The former is related more to the absorber and heterojunction properties and less to the ZnO properties. The series resistance increases because the conductivity of the ZnO drops and because the interconnects are deteriorating. Wennerberg et al. have assessed the individual contributions to increased series resistance [50]. Klaer et al. [52,53] have described a transmission-line test structure that allows to separate the contributions of contact and sheet resistance, respectively. The test structure is prepared by the same scribing techniques as those used in module manufacturing. 

An experimental quantity which can provide valuable informations on steps on layer crystal surfaces as well as on recombination or trapping processes is the diode quality factor. The dependence of the photopotential on the light intensity follows frequently a logarithmic law (Up = photopotential, IL = light intensity)... 

Performance and Diode Parameters. For finding the optimum absorber composition, organic solar cells of the type A with various [CuPc] [C60] ratios were prepared at a non-optimised substrate temperature. The Eff behaviour with the absorber composition (Fig. la) is dominated by the behaviour of the Jso and FF [4], In the compositional range investigated, 0.2 < [CuPc]/([CuPc] + [C60]) < 0.8, the devices V00 is almost constant taking values of about 400 mV (not shown). An efficiency of 1.6% is achieved in Fig. la at a [CuPc] [C60] composition of about 1 1 (by weight) [4], The diode quality factor, n. of the devices decreases from 2.6-2.7 at the either minimum of CuPc or C6o content to 1.8 at 1 1 absorber composition. At the same time, the OSCs series resistance decreases almost symmetrically from 0.34-0.38 Q x cm2 at a content of 17% of either CuPc or C to -0.2 Q x cm2 at an identical content of CuPc and C6o-... 

Fig. 2. (a) Diode quality factor and (b) series resistance of the ITO/PEDOT PSS/ CuPc C60/ Mg/Ag OSCs as a function of preparation temperature. The doted lines are guides to the eye. 

The diode quality factor n of type A devices decreases as a function of substrate temperature in Figure 2a from 2.4-2.6 at Tsubstrate = 131°C or Tsubstrate = 187°C to 1.5 at Tsubstrate = 151°C. The behavior of the series resistance (Rs) with temperature is shown in Fig. 2b. The minimum Rs = 0.27 Q x cm2 is achieved for type B OSCs at Tsubstrate = 148°C. The enhancement of the devices PV and diode parameters with the temperature up to 150°C can be explained by (i) an improved separation of the CuPc and Cm donor and acceptor materials in an interpenetrated absorber network, (ii) enhanced crystalline perfection of the CuPc domains [4] and therefore improved transport properties, i.e., better collection efficiency of photogenerated carriers at the respective electrode. Alteration of the photoelectrical parameters at higher temperatures can be attributed to the potential degradation of the PEDOT buffer layer. 

In 1983 the application of doped tin oxide films to silicon solar cells has been reported [5]. lida and coworkers realized a setup with the following characteristics 7sc = 14 mAcm", Kic = 800 mV, efficiency = 7.5 % and fill factor = 0.67. Vishwakarma et al. [166, 167] prepared arsenic-doped tin oxide films for silicon solar cells and investigated the diode properties of Sn02 As/Si02/n-Si and Sn02 As/n-Si cells. The barrier height 0 was 0.78-0.89 eV and 0.68-0.69 eV, respectively, and the reverse saturation current density 7u was 2-45 pAcm and 0.07-9.2 pAcm", respectively, with diode quality factors of 2.2-2.9 and 1.7-1.9. The optimized results for solar cell applications are given below... 

The diode quality factor n, is in fact a function of the applied bias voltage V, for all interfacial oxide thicknesses. However, for many devices in which the oxide is thin the change of n with Vp over the bias range of interest is small and n may be treated as a constant. From equation 13 it is clear that as the interfacial oxide thickness increases and the potential V, which appears across the oxide, increases, so n increases. A quantitative analysis of the relationship between n and oxide thickness 6 is given in Section 2.3. 

Effect of the MIS Potential Distribution upon the Diode Quality Factor n... 

The effect of the interfacial layer upon the diode quality factor n has been investigated by Card and Rhoderick on the assumptions that ... 

We observe that the diode quality factor relates to the recombination exponent as m = up [17]. The balance of currents shown at (18) is indicated in Fig. 2a. 

Therefore acmal measurements of carrier density via chemical capacitance or any related stepped technique will provide the total carrier density n, but since actual measurements are often performed as function of voltage, the free carrier density (and not total carrier density) is a useful index of the voltage Fp. This is the advantage of expressing -recombination model in terms of free carriers as in (15) [14], as then the exponent immediately translates into the diode quality factor of (19). Since one can convert from free to total carrier density by the expression at (39), recombinatiOTi of excess carriers can be phenomenologically modeled as a power-law dependence of the total carrier density ... 

Pandey et al. [983] described In/CP Schottky devices fabricated via thermal evaporation of In on chemically synthesized P(ANi), poly(o-anisidine) and poly(aniline-co-o-anisidine). In the case of die last copolymer, the rectification ratio, ideality factor and barrier height were found to be 300, 4.41 and 0.497 V respectively, while they were 60, 5.5 and 0.510 V for the P(ANi) device. Bantikassegn and Inganas described [984] a Schottky contact made from poly(3-(4-octylphenyl)-2,2 -bithiophene) (P(TOPT)) in its neutral and PF -doped states and Al metal as the sandwich structure ITO/P(TOPT)/Al. Rectification ratios for the neutral and doped CP were observed to be ca. 5 and 3 orders of magnitude respectively, with diode quality factors (n) being 1.2 and 4.2 respectively. Liu et al. [122] fabricated Schottky diodes from L-B films of poly(3-alkyl-thiophenes) doped with an... 

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