ANALYSIS OF FILL FACTOR LOSSES IN THIN FILM CdS/CdTe PHOTOVOLTAIC DEVICES

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1 ANALYSIS OF FILL FACTOR LOSSES IN THIN FILM CdS/CdTe PHOTOVOLTAIC DEVICES T. Ptlg, N. Spalatu, V. Cibanu,. Hiie *, A. Mere *, V. Mikli *, V. Valdna * Department Physics, Mldva State University, 60, A. Mateevici str., MD-2009, Chisinau, Republic Mldva * Department Materials Science, Tallinn University Technlgy, 5, Ehitajate tee, 19086, Tallinn, Estnia (Received 6 Octber 2010) Abstract In this paper, CdS/CdTe slar cells with cnversin eiciency values η = 5.1, 3.6, 3.5, and 2.9% are analyzed and the eects the device parameters, such as the dide ideality actr (n), the saturatin current-density ( ), and the series resistance (R s,) n the ill actr, bth in the dark and under illuminatin are investigated. The slar cells with eiciency η = 5.1, 3.6, and 2.9% were grwn rm 3N surce material. A slar cell with 3.5% eiciency was made a 6N CdTe surce. The temperature current density-vltage (-U-T) and capacitance-vltage characteristics (C-U-T) CdS/CdTe slar cells were measured in a temperature range K. Fr all studied cells, the dide ideality actr in the dark is much higher than 2 and the saturatin current density increases under illuminatin, while bth the series and shunt resistances decrease under illuminatin and temperature. 1. Intrductin Thin-ilm CdS/CdTe devices have been studied extensively, but sme basic underlying prperties are nt well understd, and prgress twards higher cell perrmance has nt been rapid. Shrt circuit current-density ( sc ) lsses are attributed t relectin, glass absrptin, TCO absrptin, CdS absrptin, and deep-penetratin lsses. In general, the pen circuit vltage (V c ) is limited by the dminant current transprt mechanisms. Recmbinatin in the depletin regin can reduce the ill actr (FF) thrugh the increase in n-dide ideality actr and the decrease in V c. Series resistance (R s ) and shunt resistance (R sh ) will als reduce the ill actr, and any vltage dependent current cllectin, (U), can additinally aect FF. T analyze FF lsses, we used empirical expressins that relate FF t the pen-circuit vltage V c, the dide ideality actr n, the series resistance R s, and the shunt resistance R sh [1-3]. In this paper, CdS/CdTe slar cells with cnversin eiciency values η = 5.1, 3.6, 3.5, and 2.9% are analyzed and the eects the device parameters (n,, R s,) n the ill actr, bth in the dark and under illuminatin are investigated. An attempt t understand the lw ill actr values r these cells is made. The temperature current density-vltage (-U-T) characteristics CdS/CdTe slar cells are measured in a temperature range K. 2. Experimental Thin ilm CdS/CdTe slar cells were grwn by clse space sublimatin [4]. The CdTe thin ilms were grwn rm 3N and 6N surce material. The CdTe thickness was abut 8 µm. CdS thickness was 380 nm. Ater the CdTe layers were depsited, the structures were held in 363

2 Mldavian urnal the Physical Sciences, Vl.9, N3-4, 2010 CdCl 2 :H 2 O saturated slutins r 3-4 h. Ater that, the structures were annealed in the air at 390±5 C r 30 min and etched in K 2 Cr 2 O 7 :H 2 SO 4 :H 2 O. All cells were cmpleted with a Ni cntact thermally depsited in vacuum. SnO 2 served a transparent rnt cntact t CdS. The standard current vltage characteristics were measured at rm temperature at 100 mw/cm 2 illuminatin and in the dark. The light measurements were calibrated t the shrt circuit current density ( sc ) a Si reerence slar cell. At the base the measure principle the capacity the semicnductr junctins, a generatr resnance with circuit LC stands. At the beginning the experiment, we measured the requency the generatr withut sample that it is studied als withut the capacitr reerence. Then, in parallel with the capacitr rm circuit LC, the reerence calibrated capacitr is cnnected and the requency the generatr is measured again. Ater this, the capacitr reerence is switched and the studied sample is plugged in. The characteristics capacitancevltage are recrded using a prgram cmmand and cntrl. 3. Analysis and discussin 0-0,6-0,4-0,2 0,0 0,2 0,4 0,6 0,8 1,0-5 Figures 1 and 2 illustrate the current-vltage characteristics r a set the slar cells at the rm temperature under illuminatin 100 mw/cm 2 thrugh the wide gap cmpnent CdS and in the dark, respectively. The devices with η = 5.1, 3.6, and 2.9% dented as (), (C2), and (C3), respectively, were grwn rm 3N surce material and its current-vltage characteristic behavirs in the dark (Fig. 2) are similar. The device with η = 3.5% is dented as C4 and is grwn rm a 6N CdTe surce. The best phtvltaic parameters are achieved r the device. As ne can see rm the table, the value the pen circuit vltage (U c ) and current density ( sc ) r the device achieves 0.62 V and 19.8 ma/cm 2, respectively. Phtvltaic parameters CdS/CdTe phtvltaic devices under 100 mw/cm 2 illuminatin, T = 300 K. Cell C2 C4 C3 sc, ma/cm U c, V F, % η, % R sd Ω cm R sl Ω cm R sh D,Ω cm R shl, Ω cm n D n L D, ma/cm L, ma/cm 2 2,4E-3 0.5E-6 3.4E-4 2.5E-4 R sd, R shd are the series and shunt resistances in the dark; R si, R shi are the series and shunt resistances under illuminatin; n D is the dide ideality actr in the dark; n I is the dide ideality actr under illuminatin. D is the saturated current density in the dark; and L is the saturated current density under illuminatin., ma/cm C3 C4 C2 Fig. 1. Current-vltage characteristics the CdS/CdTe phtvltaic devices under 100 mw/cm 2 illuminatin: η = () 5.1%, (C2) 3.6%, (C3) 2.9%, and (C4) 3.5%. 364

3 Fill actr (FF) is lw in general. Accrding t the thery, the ill actr is determined by the series resistance, saturated dark current density ( ), and dide quality actr (n). As ne can see rm the table, the value series resistance is high r all devices and is prbably due t the act that the cells used wet CdCl 2 treatment that may cntain xide n the surace CdTe. The ill actr depends n bth R s and R sh in a cmplex way. As ne can see rm the table, bth R s and R sh change under illuminatin. The variatin R sh under illuminatin is the mst dramatic. We may cnclude that the light-dependent R sh negatively inluences the eiciency ur cells. The btaining high eiciency slar cells requires the understanding the junctin current mechanism transprt. Therere, the temperature current-vltage and capacitancevltage characteristics thin ilm slar cell layers are investigated in a temperature range K. The saturatin current and the ideality dide actr are reprted, they were btained by itting the experimental data t the standard dide equatin: = exp ( eu nkt )-1, where is the utput current density, is the saturatin current-density, and n is the dide ideality actr. C3 C4 C2, ma/cm Fig. 2. Dark current-vltage characteristics the CdS/CdTe slar cells with dierent eiciencies. n , dark, light T, K Fig. 3. The ideality actr as a unctin temperature; under 100 mw/cm 2 and in the dark r cell. n T,K C4, dark C4, light Fig. 4. The ideality actr as a unctin temperature; under 100 mw/cm 2 and in the dark r cell C4. ln , dark, light T -1,K -1 Fig. 5. Plt ln 0 vs 1/T r cell under 100 mw/cm2 and in the dark. 365

4 Mldavian urnal the Physical Sciences, Vl.9, N3-4, 2010 Fr example, phtvltaic device exhibits a temperature-independent n value ~3 under 100 mw/cm 2 and a temperature-dependent n in the dark (see Fig. 3). Thus, in these cells, generatin-recmbinatin current appears t be dminant in the dark. The dminant current lw mechanism under 100 mw/cm 2 illuminatins is tunneling. Phtvltaic device C4 shws a temperature-independent ideality actr value ~13 and higher in the dark and ~5 under 100 mw/cm 2 illuminatin (see Fig. 4). These data indicate that the tunneling carriers is an imprtant junctin transprt mechanism in this cell. The saturatin current-density, which was btained by extraplating the rward current curves ln =(U) t zer vltage r the device, is und t vary expnentially with 1 T in the K temperature interval as shwn in Fig. 5 accrding t the relatin ( ) exp( ) 0 T Δ EA kt, where ΔE A is the activatin energy the charge carriers in the rward bias. The activatin energy the cell calculated rm the slpe the ln 0 vs 1 T plt is und t be 0.63 ev clse t ne-hal the band gap CdTe. This suggests that the generatin-recmbinatin carriers in the depletin regin determines the dark current. ln T -1, K -1 C4, dark C4, light Fig. 6. Plt ln 0 vs 1 T r cell C4 under 100 mw/cm 2 and in the dark K, C4 313 K, C4 333 K, C K, C4 373 K, C4 293 K, K, 333 K, 353 K, K, W, μm Fig. 7. Depletin layer width (W) as a unctin vltage, cells and C4. It is und that the plt ln 0 vs 1 T r cell C4 under 100 mw/cm 2 and in the dark des nt vary expnentially with reverse temperature interval as shwn in Fig. 6. Capacitance-vltage characteristics at dierent measurement temperatures r cells, C2, and C3 are similar. The curves are almst independent vltage until rward biases near diusin ptential when the depletin regin rapidly cllapses leading t an increase in capacitance. Depletin regin width versus vltage with temperature variatin is nt changed r these cells, while r the C4 cell, a decrease in the depletin layer width at higher rm temperature is bserved (see Fig. 7). This indicates that thermal excitatin carriers between the traps and the energy band in CdTe plays an imprtant rle in the behavir junctin in the C4 cell. We bserve rm Fig. 8 that, r the cell, at all measurement temperatures, the value the eective carrier cncentratin in CdTe is clse t cm -3 near the surace, which is abut 3.8 μm away rm the CdTe/CdS junctin. The value eective carrier cncentratin is much lwer, ~10 14 cm -3, in the CdTe regin which is μm away rm the CdTe/CdS junctin. Thus, the CdTe layer appears t be p-type near the surace and p-type (lw dped) between the surace and the CdTe/CdS junctin. Als, C2 and C3 shwed a similar CdTe carrier cncentratin being relatively high (~10 17 cm -3 ) near the surace and lw (~10 14 cm -3 ) between the surace and the CdTe/CdS junctin. 366

5 p, cm K, 313 K, 333 K, 353 K, 373 K, 293 K, C4 313 K, C4 333 K, C4 353 K, C4 373 K, C4 FF, % W, μm Fig. 8. Dependence hle density as a unctin distance, cells and C x x x x x10-3 / 0 Fig. 9. Variatin ill actr with reverse saturatin current, cell. Current-vltage measurements revealed that, under 100 mw/cm 2 illuminatin, tunneling was the dminant current lw mechanism in these cells and a generatin-recmbinatin type current lw prcess in the dark. Tunneling was als the dminant current lw mechanism in the dark and under illuminatin r the C4 cell. The experimental analysis the ill actr by pltting FF vs is a useul technique r evaluating the phtvltaic behaviur slar cells [5]. The curve rm Fig. 9 shws the variatin ill actr with reverse saturatin current, taking int accunt the perating current dependence series resistance (each pint is rm a -U curve at ne illuminatin) r cell. A decrease in FF r high values indicates a lss due t series resistance; a decrease in FF r lw values indicates a parallel resistance lss. Cnclusins The highest ill actr lsses are due t the changes the saturatin current density and ideality actr under temperature and illuminatin. The FF is a unctin series/shunt resistance. Bth resistances vary with light level and temperature. The eect shunt resistance n the ill actr may be neglected in the dark due t high value in all cells. The eect the series resistance n the ill actr is higher in the dark than under illuminatin. Acknwledgements This wrk was supprted by EU 7 th FP prject FLEXSOLCELL GA Reerences [1] S.M. Sze, Physics Semicnductr Devices, 2 nd Editin, Wiley, New Yrk, 422, [2] D. Bnnet, Mater. Res. Sc. Symp. Prc., 1012, 249, (2007). [3] K. Mitchell, A.L. Farenbruch, and R. Bube,. Appl. Phys., 48, 4365, (1977). [4] T. Ptlg, L. Ghimpu, P. Gashin, A. Pudv, T. Nagle, and. Sites, Sl. Energy Mat. & Sl. Cells, 77, 327, (2003). [5] S. Hegedus and B. McCandless, Sl. Energy Mat. & Sl. Cells, 88, 75, (2005). 367