First publ. in: Solar Energy Materials and Solar Cells ; 95 (2011), 12. - pp. 3450-3454
http://dx.doi.org/10.1016/j.solmat.2011.08.004
UV light protection through Ti0 2 blocking layers for inverted
organic solar cells
Haiyan Sun, Jonas W eickert, Ho\ger Christian Hesse, Lukas Schmidt-Mende *
Ludwig-Muxil1lili,l/Is-Ulliversity Munich. Dep"rtmellt oj Physics ,mel Center Jor NunuSciellce (CeNS). Allwliellstr. 54.80799 MUllic/l. Cerl1lullY
AB ST RACT
Keywords :
TiO, layer
Orga nic solar cell
Trap Alling
UV Alter effect
Stabi lity
Fu ll y organic so la r cell s (OSCs) based on po lymers and fullerenes have attracted remarkable interest
during the last decade and high power conversion efficie ncies (PCEs) beyond 8% have been realized .
However. air stabi li ty of these ce ll s rem ains poor. The co nvent ional geometry of OSCs utili zes strongly
oxidizing metal top contacts like AI or Ca. These m etals are eas ily oxid ized in air resu lting in rapid
decrease of PCE if cell s are not perfectly encapsu lated . Using a thin elect ron-se lective hole- bl ocki ng
bottom layer like TiO, enabl es fabricat ion of so lar ce ll s in a so-ca ll ed inve rted geometry. In this
geo metry. nob le metal s like Ag or Au can be used as top contacts. which are less se nsitive to ambie nt
oxygen . Thus. air-s tabi li ty of these inverted solar ce ll s is signifi ca ntly improved. In this study we
inves tiga te inverted polythiophene-methanofu ll erene solar ce ll s. W e find sign ifi ca nt influ ence of the
TiO, layer thickn ess on li ght abso rption and illumination stability of the solar cells. as w ell as the trap
filling by photoindu ced carr ie rs. Even thou gh TiO, layers as thick as 500 nm seem not to be detrimental
for cha rge transport, light intensity losses limit th e dev ice pe rforman ce. In turn. illumination s tability is
better for thicker TiO, layers. which can se rve as UV filt ers and protect the photoactive material s from
degradation. wh en compared to thi n TiO, layers. Considering these different effects we state that a
thi ckn ess of 100 nm is the optimization of the TiO, layer.
1. Introduction
Polymer so lar cells (PSCs) have been significantly improved
during t he last few years. and power co nv ers ion efficiencies
(PCEs) as high as 8.3% have been re ported [l-5J . Even though
PSCs hold the promise of low m ass produ ction costs when
compared to conventional Si- based photovoltaics. mu ch work
ha s to be don e to invest igate th e stab ility [6-8 1 and to enhance
the li fetime of PSCs [91. Th e most commonly used geo m etry of
PSCs uses non - nobe l meta ls like AI a nd Ca as top contacts. which
qui ckly oxidize in air. As a co nseque nce. PCEs d ec rease ra pidly if
cells are operated in ambi e nt air without e laborate encapsu lati on.
PSCs with a so-ca ll e d inverted structure with a Ti0 2 layer as hol e
blocking material s [10]. a ll ow the use of noble metal s as top
e lectrodes. thu s in creasing air stabi li ty [11\. As both a photoactive and an inte rfa cial layer (1). Ti0 2 layers play an important
role on the performance of PSCs.
In this work. we inv es ti gate the influence of Ti0 2 blocking
layer thickn ess on the power conversion effi ci e ncy and illumination stability of PSCs. Ti0 2 layers are fabricated by s pray pyrolysis
• Correspondi ng author.
E-l1lail address: L.Schmidt-Mend [email protected] chen.de
(L. Schmidt-Mend e).
method. which a ll ows control over layer thickness. Solar ce lls
with a thicker layer of Ti0 2 s how lower short circuit current
density Usc ) and PCE, but hig her s tability. W e prove that th e
d e pe nd e nce of performance o n Ti0 2 thickness is ca used by a filter
effect due to the trap s tates in our Ti0 2 • rather than its e lectron
tran sport abi lity. Th ese findin gs are important 'not only for bl e nd I
oxide so lar cell s. but a lso for hybrid organic solar ce ll s. which
have a layer of semiconductor oxides.
2. Experimental
2.1. Device fabri cation
Solar cells are produced on ITO coated glass substrates (Kintec,
100./ 0 ). ITOs are clea ne d consecutively by ultraso nication for
30 min in acetone and 30 min in 2-pro panol. dri e d in a nitroge n
s tream and treated for 7 min in a plasma clea ne r. Ti0 2 is
de pos ited by spray pyro lysis as described by other g roup earlier
11 2 J. Briefly we sp rayed a 1 : 10 so lution of di - isop ropoxytitanium
bis (acetylacetonate) in ethanol (Sigma Aldrich) at 450 "C on our
ITO s ubstrates and sa mpl es a re cured at this temperature for
15 min . The heating ancl coo lin g rates are 30 and 2.5 "C/min .
res pecti ve ly. Ti0 2 laye rs are ultra so ni cated for 5 min in acetone.
rin sed with e thanol and dried in a nitroge n stream.
Konstanzer Online-Publikations-System (KOPS)
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-179798
3451
Poly-3- hexyl- thioph e ne (P3 HT, Me rck) and [6,6] - ph e nyl -C6 1butyric acid methyl es te r (PCBM, Nano-C) solutions are pre pared
in chlorobe nzene (Sigma Aldri ch) at 24 and 20 mg jml, res pective ly, mixed at a 1: 1 volum e ratio and s pin coated at 600 rpm for
1 min, th e n slow-dried for 20 min cove red with a Pe tri di s h [13].
A thin laye r of PEDOT:PSS was de posited on top of the active laye r
from a 1: 10 dilution in 2- propanol by s pray-spin coating as
described elsewh ere [14]. Sampl es w e re pre-anneal ed at 10S "C
for S min in ambi e nt air before 100 nm thi ck sil ver top contacts
are DC s puttered by KS7SX (Emitech ) sputter coater onto th e
organic material through a s hadow mas k. During sputte rin g th e
va cuum in th e chambe r is 6 x 10- 3 mbar, and tile whol e process
takes 30 min . After top contact de position solar cells are ann eal ed
a t 140 "C for 10 min.
Cell fabrication and all electrical and s pectral m easure m e nts
a re carri ed out in ambi e nt a ir at a humidity of approximately 30%
and room tempera ture (betwee n 22 and 2S ·> C).
N -2
E
~
.s.c -4
'00
c
Q)
'0
C -6
~
~
-
u
50nm
-
100nm
200nm
---T- 500nm
-8
---A-
0.1
0.0
0.2
0.3
0.4
0.6
0.5
Bias (V)
2.2, Device characterization
Curre nt de nsity- voltage UV) chara cte ri s tics are acquired with
a Keithl ey 2400 Source-Me ter usin g a self- mad e LabView program . Curves a re measured in dark or under illumination with an
AM l.5G solar simulator. Lig ht intensity is calibrated with a
Fraunhofe r Ins titute certified silicon refe re nce cell with a KG S
filte r and adjusted to 100 mWj cm 2 Solar cells are illuminated
throug h a shadow mask, yielding an active area of 0.12S cm 2 .
Shunt and series res istan ce (Rsh and Rs) have been estimated
usin g a fit to the slope of th e lV-curves at hi gh reverse and
fOlward bias, respectively, as s hown previously [lS]. For external
qu a ntum efficienci es (EQEs ) alSO W Xe lamp is focu sed onto a
monochromator. Curves are record ed us ing a Keithley 2400
Source-M eter. Absorption curves are acquired with an Agilent
84S3 UV- vis spectrome ter in transmi ssion mode. Ti0 2 layer
thicknesses are probed with a De ktak profilomete r. Carrie r
de nsiti es are meas ured by Hall effect using an Ecopia syste m of
HMS3 000.
Fig. 1. Current-voltage cha racte ristics und er illuminati on w ith simula ted AM
1.5G solar light (100 mW/cm 2 ) . Data is s hown for devices with 4 d iffere nt
thicknesses of TiO, . (blac k squares : 50 nm ; red circles: 100 nm ; green up
tria ngles : 200 nm ; blue down tri angles : 500 nm ) Devi ces with thin TiO, exhibi t
highe r PCE and Jsc. (For in te rpreta ti on of the refe rences to color in thi s fi gure
legend. the reader is refe rred to the we b ve rsion of th is article.)
Table 1
Photovoltai c param eters of a representative solar cell frolll current- voltage data
acquired und er simu\(ltcd solar illuminati on. Valu es arc given for solar ce lls witl1
different thi cknesses of TiO, .
Thi ckn esses
(nm )
PCE
(%)
Jsc
(mA/cm' )
FF
(%)
Rsh
R,
(V)
(o./ cm')
(o./ CI\1 , )
50
100
200
500
2.4 1
2.06
1.84
1.19
0.59
0.58
0.58
0.57
7.44
6.38
5.74
3.86
55
55.4
55.5
54.6
5546
296 1
1967
1622
2. 18
2.27
2.01
6.62
voc
photoactive organi c laye r. To inves ti ga te this effect w e measure
th e absorption of the different Ti0 2 layers.
3. Results and discussion
3.2. Absorption
3. 1. Optimized Ti0 2 layer thickn ess
In this work, w e fabri cate sola r cells of th e type ITO jTi0 2 j
P3HT:PCBMjPEDOT:PSSjAg. W e used th e same thickness of ble nd
layer ( l S0nm ), Ag top contact (100nm) and PEDOT: PSS (about
10 nm ). Variations mad e on th e solar ce lls are on th e thickn ess of
th e Ti0 2 oxid e laye r. Four diffe rent thi ckness ofTi0 2 were s tudi ed:
SO, 100, 200 a nd SOO nm . Current- volta ge curves acquired und e r
simulated illumination of AM ·1.5G solar lig ht (100 mWjcm 2 ) for
diffe rent thi ckn esses of Ti0 2 layer are shown in Fig. 1.
Data are s hown in Tabl e 1 and re prese nted in Fig. 1 for devi ces
with diffe re nt thi cknesses ofTi0 2 laye r (black squares : SO nm; red
circl es: 100 nm; green up tri a ngles : 200 nm; blue down triangles :
SOO nm ). Among th e four types of solar ce ll s, bes t performa nce is
shown on th e one with th e thinn es t Ti0 2 layer of only SO nm .
Whil e th e thickn ess ofTi0 2 in creases, bo th efficiency and s hortjsc
a re lin early redu ced, but ope n circui t voltage (Vocl and fill factor
(FF ) kee p almos t th e same. Th e sampl e with SO nm Ti0 2 gives
2.4% effici e ncy, whil e th e SOO nm one gives only 1.1 %. Thi s 60%
loss in effi cie ncy is m a inly cau sed by redu ced 15c,
Since th e FF and se ries res istan ce, as s hown in Table 1, are
barely change d depending on Ti0 2 thickn ess , we can infe r that th e
l sc 01' th e solar cells s hould not be limited by th e res is tance of th e
Ti0 2 laye r. A poss ibl e rea son for reduce of l sc mi g ht be li ght
absorption in the Ti0 2 reducin g th e li g ht inte ns ity insid e of' th e
Ti0 2 lay e rs s how a li ght ye ll ow to brown color with in creasing
thickn ess, s ugges ting their a bility to ab sorb li ght in th e vi sibl e
reg ion. As s hown in Fig. 2, th e absorption peak of Ti0 2 laye r at
33 S nm in creases with its thickn ess. A sli ght vi sible li ght absorption s ugges ts that th e re are sub-band gap states in th e Ti0 2 ,
probably du e to impurities [161. Compared to pure Ti0 2 , whi ch
has tran sitions from th e val e nce band to th e condu ction ba nd,
Ti0 2 introdu ced with dopants and traps exhibits additional
ex trin sic el ectronic levels, whi ch can be located in its en ergy
ba nd gap. As our Ti0 2 laye rs a re fabri cated by s pray pyrolysis
m e thod in air w e ass um e that th e precursor is directly decomposed at an elevated te mperature of 4S0 'C. Although great ca re
has been take n, it is expec ted that at such a di spersion and slow
appli ca tion of the solution prec ursor it is impossibl e to avoid
impurities prese nt in th e Ti0 2 . Th e refore some trap s tates due to
impuriti es will form in th e Ti0 2 laye rs [17].
3.3. Photoindu ced tmp filling of Ti0 2
During the lV m eas urements for all th e samples with diffe re nt
thi ckn esses of Ti0 2 laye rs, w e find a trap filling effect in photoinduced Ti0 2 , depending on th e tim e of illumination. Fig. 3
s umm a ri zes the dependence of PCE and FF on illumination time.
Upon firs t m eas urem e nt und e r solar illumination, solar ce lls s how
3452
obvious S-s ha pe on th e JV curves, with quite low FF a nd peE. As
being illuminated longe r, the pe rforman ces of all solar cell s are
improved and reach th eir maximum poin ts afte r a ce rta in tim e.
This illumina tin g tim e is proportiona l to th e thickn ess of Ti0 2
laye rs. Devi ces with thi cke r Ti0 2 take longe r tim e to get th eir bes t
performan ces. For all th e param ete rs of solar cell durin g th e
improve m e nt, the FF and effi ciency increase significantly, howeve r th e Jsc and Voc s tay almost cons tant.
As Weidmann reported, defects in Ti0 2 laye rs can redu ce th e
el ec tron drift mobi lity [181. Th e refore w e propose that trap s limit
electron tra ns port in Ti0 2 . The absorption s hift of Ti0 2 to the
visibl e li g ht, cau sed by th e tra p states in Ti0 2 laye r, is clearly
s hown in Fig. 2, so th e trap fillin g e ffec t of solar cells co uld be
ex plained as follows.
Photo-excited elec trons and hol es in Ti0 2 are firstly obtained to
fill th e traps close to th e conduction ba nds [191. a nd the tra p
becomes ina ctivated for furth er el ectron capture [201. This causes
the low FF and effi ciency, whe n a firs t current- voltage cha racte risti c is recorded without illumination prior to the m easure me nt.
Eve n a lower Jsc of SCs can be observed with the thick es t Ti0 2
layer, whi ch probably contains the high es t number of traps that
limited th e charge trans port. As light induced fillin g of electron
traps can cau se a n increase of th e electron drift mobility, ass ist
electron trans port through Ti0 2 [211 , and improve the qua ntum
yi eld of carri e rs a t higher light intensities [22]. The pe rforman ces of
SCs are improving with cons tant illumination in solar spectrum.
- ._ - ....-,,-
3
0'
Q.
50nm
100nm
200nm
500nm
2
c
.Q
i5.
0
If)
.n
<{
3.4. Filter effect
We fabri ca te a gro up of se mitrans pare nt sol a r cells wi t h
differe nt Ti0 2 layers, by s putte rin g a 20 nm thin Ag top contact.
Thus, devi ces can be illuminated from th e backs id e with a li g ht
pa th throug h Ag/PEDOT:PSS/P3HT:PCBM/Ti0 2 , Without the Ti0 2
absorption of both UV and partly vi sibl e lig ht. w e can confirm that
th e li ght inte nsity insid e th e active layer is th e sam e for all th e SC,
thu s th e excited charge carri es are supposed to b e ge ne rated
ind e pe nd e ntly of th e thi ckn ess of Ti0 2 as w ell.
As s how n in Fig. 4, wh en illuminated from th e front sid e, i. e.
solar li ght is ab sorbed by th e Ti0 2 laye r first, the EQE results of
SCs a re obvi o usly in inv e rse pro por tion to Ti0 2 thi c kn ess (addi tional plots s hown in Fi g. Sl supportin g information). With
thi ckn ess of Ti0 2 increasing, EQE of SCs decreases, followin g th e
sam e tre nd that was found for th e PCE, Whil e ill uminated from
th e bac ksid e, both PCE a nd EQE a ppea r ind epe nde nt on Ti0 2 layer.
During thi s investigation of reverted illumination , w e cou ld
confirm that th e power conve rsion of solar cells d e pe nding on
th eir Ti0 2 thi c kn ess is cau sed by a filte r effec t of th e Ti0 2 laye r.
Li ght intensity is reduced wh e n illuminated throu g h Ti0 2 laye r.
As apparent from Fig. 2, th e a bsorpti on band of Ti0 2 , will be
obse rved be tween 340 nm and 450 nm, and thicke r Ti0 2 layers
absorb s tron ge r at thi s wavele ngth. Th e n th e light inte nsity is
redu ced reaching the active laye r, thus th e Jsc and peE of SCs with
thi cker Ti0 2 layer a re redu ced. Thicke r Ti0 2 laye rs ab sorb and
filte r more sola r li ght and lower li ght redu ces th e ligh t inte nsity at
th e active laye rs, whi ch contributes to charge ge ne ration. Accordin gly, less light participates in power conve rsion in SCs.
Hall effect mea s urements on our Ti0 2 laye rs s how a charge
carrie r de nsity in the ran ge of 2 x 106 for all diffe re nt thi ckn esses .
Thi s is well in accordan ce with th e se ri es res is tan ces for diffe re nt
solar cell types , whi ch was found to be relatively consta nt among
the diffe re nt Ti0 2 thi ckn esses. Since th e PCE of solar ce lls are
ind e pe nd ent of Ti0 2 thi ckn ess wh e n illumina ted from bac ksid e,
w e ass ume that th e Ti0 2 laye r does not limit th e charge tran sport
in orga nic so la r cell s eve n with a thickn ess of 500 nm .
3.5. Illumination stability
o
300
SCs with thi cker Ti0 2 laye rs contain more tra ps, a nd th us th ey take
longer tim e to be fill ed and to reach the bes t charge tra ns port state.
400
500
Wavel e ngth (nm)
600
700
Fig. 2. Absorpti on s pectra or dirre re nt th ickn esses o r Ti O,. (For inte rpre tatio n or
the rere re nces to color in this fi gure legend . the rea der is rci'e rred to the we b
ve rsion or th is a rticle.)
We inves ti ga ted th e illumina tion stability of th e sola r ce ll s
with di fferent Ti0 2 thi ckn ess. In a mbi e nt condition within
100 min solar illumina tion, th e 50 nm Ti0 2 laye r solar cell has a
33.5% deg rada tion of PC E, whil e th e thi cke r on es are mu ch more
s tabl e, 0.13% for 100 nm Ti0 2 , 0.10% a nd 0.1 6% fo r 200 nm and
500 nm Ti0 2 , res pec tively.
60
50
~
LL
LL
40
30
~
- - 50nm
--+- 100nm
- A - 200nm
~ 500nm
20
2
3
Ill umination Time (min)
4
A~~'--'--"
~.
~~
w
u
a..
E
o
- m- 50nm
--+- 100nm
-A- 200nm
..
~ 500nm
3
Illumination Time (min)
2
Fig. 3. Le rt: FF, ri ght: peE illumina tion time trap fillin g erfect.
4
z
3453
1.75 - , - - - - - - - - - - - - - - - - - - - ,
~ Back illuminated
2.7
-+-- Front illuminated
1.50
__ 50nm
__ 100nm
--4- 200nm
-"f'- 500nm
2.4
2. 1
~
o~
1.25
0~
1.8
()
w
w
a.. 1.5
()
a.. 1.00
1.2
0.9
0.75
0.6
0.50
-t---,----.--~--r-~-----,-~-_.__~-___.__----l
o
100
200
30 0
400
20
500
Thickness (nm)
- - -- - --
~'
40
I
I" I
I
I
I
500nm Back
500nm Front
50nm Back
50nm Front
"
30
"\
"
\
0~
w
aw
20
10
____ -_01,,,. ..,_,, ...------- _______ _
"
"
"
,--
0
300
400
500
600
700
Wavelength (nm)
Fig. 4. peE (up) and EQE (down) depend ence of the TiO, thi cknesses in front
(soli d) and back (dash) illuminated dev ices.
40
60
Time (min)
80
100
Fig. S. Ill umination stabi li ty of different thicknes s ofTiO,.
4. Conclusion
The trap states in TiOz layers fabricated by sta nd ard spray
py rolys is technique caused s li ght absorpt ion in t he visibl e. Thi s
redu ces j sc and PCE of so lar ce lls with thicker TiO z laye rs, because
li ght inte ns ity is reduced when so lar cell s are illuminate d throug h
TiO z. Howeve r, according to series res is ta nce a nalys is, EQE of
backs id e illuminated dev ices and Hall effect meas ure m e nts,
cha rge trans port seems not to be li mited even for 500 nm thick
Ti0 2 . The trap s tates in TiO z layers fabricated by sp ray pyrolysis,
not only cause the filte r effe ct, but also demand for extended
photodoping tim es. During jV m eas ure m e nts und e r so la r illumi natio n, so lar ce ll pe rforman ces initia ll y imp rove, due to fillin g of
traps in TiO z. However, upon lo ng-term illumination so lar ce ll
pe rformance decreases, because of the degradation of orga n ic
mate rial s caused by UV il lum in ation. The illumination stability is
in creased with thickn ess of TiO z layer, whil e the performance
d ec reases. Co ns id e rin g both competing effects, 100 nm is the
optimal thickness of TiOz for our inve rted so lar ce lls.
Aclmowledgment
Th e degradation of po lymer so lar ce lls has m a ny reaso ns,
a nd the most commo n one is t hat oxygen is readily activated by
UV illumination o n th e inte rface be tween titanium oxid e and the
organ ics. As the Ti0 2 laye rs are fabr ica ted on ITO subst rates a nd
coated with blend, we think that air, including m o isture, w ill
affect on th e ble nd laye r rath e r than the Ti0 2 layer. In thi s case
the in nu ence of oxygen on so lar ce ll s s hould be ind e pend e nt on
th e thi ckn ess of Ti0 2 layers . Thi s leaves us with the poss ibl e
ex planation t hat, if th e TiO z layer is not thick e nough to fi lter out
a ll the UV light, th e s uper-ox ide or hydroge n peroxide form ed, is
lil<e ly to agg ress ive ly attack the organic compound s [6 ].
Howeve r, fi ltering of UV li g ht res ults in mo re stable so lar ce ll s
making devices wit h thicker TiOz more durabl e to so lar irradiation. TiOz layers s how a s ig ni fica nt absorption in th e UV-spect ra l
reg ion. However, as already discussed, their fi lter effect reduces
PCE of SCs. As s hown in Fig. 2, t he th inn est TiO z layers und e r
investiga tion (5 0 nm ) abso rb on ly abo u t 90% of hi gh ene rgetic
photons (). $; 300 nm ). As s uch, t he d egradat ion process ca used by
UV- li g ht in cide nt to th e orga ni c material s cannot be e ntire ly rul ed
out fo r the thinn est TiOz layers. This competiti o n betw ee n
efficiency a nd stab ility d e pe ndin g on the t hi ck ness of TiO z laye rs
gets in balan ce at 100 nm, wh ich has a reaso nable s low degradat io n a nd hi g h effic ie ncy (Fig. 5).
H.S. t han ks th e Ch inese Scholarship Counc il (C RC) for fun d ing.
Th is work has bee n supported by the German Exce ll ence Ini tiative
of t he De utsc he Forschu ngsge m e insc haft (DFG) via th e " Na nosyste ms Ini tiat ive Munich (N IM)" and th e Ge rm a n Resea rch Fo und atio n (DFG) in th e program "SPP 1355: e le m entary processes of
orga ni c photovoltaics".
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