Performance optimization of P3HT:PCBM solar cells ...

31st European Photovoltaic Solar Energy Conference and Exhibition

Performance optimization of P3HT:PCBM solar cells by controlling active layer thickness Burak Kadem, Aseel Hassan and Wayne Cranton Material and Engineering research institute, Sheffield Hallam University, UK Tel. +441142253500 Fax. +441142253501 [email protected]

ABSTRACT: Optimization of organic solar cells (OSCs) active layer thickness for improved performance has been investigated for P3HT:PCBM hybrid bulk heterojunction OSCs. Active layers in the range 65-266 nm were produced using a conventional device structure (ITO/PEDOT:PSS/P3HT:PCBM/Al). UV-visible absorption spectra revealed typical P3HT:PCBM absorption features for all obtained thicknesses. The dark J-V characteristics were employed to determine the charge carrier mobility using space charge limited conduction theory (SCLC) and series resistance of investigated devices were also derived. Series resistance was found to decrease with decreasing active layer thickness reaching lowest value of 33.9 for film thickness of 95nm. Furthermore, charge carrier mobility was found to increase with decreasing thickness of the active layer, with a maximum mobility value of 1.37 ×10 -5 cm2V-1s-1 obtained for the 95nm thick films. Measurements of the PV characteristics of the investigated devices have revealed optimum performance when active layer thickness was 95 nm. Power conversion efficiency (PCE) as high as 3.86%, fill factor (FF) of 50%, short circuit current (Jsc) of 12.6 mAcm-2 were achieved for optimum active layer thickness of 95nm. On the other hand open circuit voltage (V oc) remained almost unchanged in range of 0.61-0.62V for all investigated devices. Moreover, device stability was shown to be largely improved for optimum active layer thickness with constancy for more than three months. Keywords: Active layer thickness, Charge carrier mobility, SCLC, Series resistance.

1

INTRODUCTION

within the solar cell were examined. Fig. 1 shows a schematic representation of the effect of the active layer thickness on charge carriers generation and collection.

Nowadays, improving the organic solar cells (OSCs) performance is considered as one of the attractive research themes due to their promising characteristics such as low cost, large scale manufacturing, solution processability and light weight [1]. Poly (3hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are considered as the most investigated materials considering the light harvesting properties [2]. P3HT:PCBM bulk heterojunction requires nanomorphology structure blend which controls the charge separation and transportation within the blend [3].Power conversion efficiencies (PCE) more than 5% have been achieved for polymer solar cells [4,5], however, 10% or larger is projected for future OSCs [6]. This makes this type of solar cells a viable low-cost replacement of the well-established but more expensive silicon SCs. Absorption of light by the polymer generates excitons, which diffuses to the donor/acceptor interface where they will be separated to electrons and holes by the internal electric field. These generated carries will then be transported by the donor and acceptor components of the blend and selectively collected by the electrodes. It could be argued that several factors play an important role in optimizing OSCs performance; among those is the active layer thickness which has the potential to influence the device performance by improving charge collection rate and reducing the recombination rate [7]. Although, a thicker film is favourable for absorbing more light compared to thinner films; thicker film however could also increase the series resistance (Rs) and reduce the charge carriers mobility [8], whereas thinner films are capable of showing nearly 100% conversion of absorbed photons into collected carriers [9]. In this study, active layer thickness in the range of 65-266 nm has been investigated in relation to its role in improving OSCs efficiency. Furthermore, the film's thickness effects on the contact resistance and charge transport mechanism

Fig. 1 The P3HT:PCBM organic solar cell outline. Role of the active layer thickness 2 EXPERIMENTAL METHODS 2.1 Sample preparation ITO coated glass substrates with sheet resistance of 8-10/□ were cleaned using isopropanol, acetone and DI water in ultrasonic bath for 10 min each, respectively. For the deposition of hole collection electrode, PEDOT:PSS solution was spin coated on ITO coated glass substrates with 2000 rpm for 30 sec and annealed on hot plate at 150 ºC for 10 min. P3HT:PCBM (1:1) with solution concentration 14mg/ml were dissolved in chlorobenzene:chloroform (CB:CF) co-solvents with the ratio (1:1). These P3HT:PCBM active layers were spin coated onto the PEDOT:PSS layers inside a nitrogenfilled glove box at different spin speeds in the range 5002500 rpm for 30 sec. The obtained layers were annealed on a hot plate at 120 ºC for 10 min inside the glove box. Aluminium top contacts with the thickness of ~100 nm were evaporated by vacuum thermal deposition and monitored by quartz crystal microbalance with a deposition rate of 0.1-0.2 nm/sec. The final devices were heat treated at 120 ºC for further 10 min inside the glove box.

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2.2 Measurement techniques UV-visible spectrophotometer (Varian 50-scan UVvisible) in the range of 190-1100 nm was used to study the absorption spectra of the active layers spun onto cleaned glass substrates. The active layer and PEDOT:PSS layer thicknesses were determined using M2000 (J.A. Woollam Co., Inc.) spectroscopic ellipsometer operating in the wavelength range 370-1000 nm. DC electrical measurements in the form of current density-voltage (J(V)) dependence were carried out using 4200 Keithley semiconductor characterization system and the photo current was generated under AM 1.5 solar simulator source of 100mW.cm-2. The fill factor (FF) and the overall light to-electrical energy conversion efficiency (PCE) of the solar cell were calculated according to the following equations [10]:

FF =

Jmax Vmax

Jmax Vmax

Fig. 3 shows the current density-voltage (J-V) characteristics plotted on a log-log scale of P3HT:PCBM solar cells with different active layer thicknesses under dark condition. In general four regions of J-V dependence were observed; at lower applied voltage an ohmic behaviour dominates and is represented by the following equation [14]: J = pqμ

(1)

Pin

(2)

Jsc Voc

where Jsc is the short-circuit current density (mA.cm-2), Voc is the open-circuit voltage (V), Pin is the incident light power and Jmax (mA.cm-2) and Vmax (V) are the current density and voltage at the point of maximum power output in the J-V curves, respectively. . 3 RESULTS 3. 1. UV-Vis absorption spectra:

Absorption

0.4 0.3

𝑑3

(4)

N

E

Nt

kT

(5)

where k is the Boltzmann’s constant, Nv the effective density of states in the valance band and Nt is the traps concentration at energy level Et and T is the absolute temperature. 3

2.2

TF-SCLC

2

2.0

266 nm 254 nm 233 nm 215 nm 182 nm 169 nm 137 nm 111 nm 95 nm 80 nm 65 nm

1.8

1 1.6

log (J)

0.02

0.03

0.04

0.05

0.06

0.07

TSCLC

0.08

log (V)

SCLC


Ohm's law

-1 -2 -3 -4 -5 -1.4

0.2

-1.2

-1.0

-0.8

-0.6 log (V)

-0.4

-0.2

0.0

0.2

Fig. 3 the log-log scale for the P3HT:PCBM solar cell with different active layer thickness under dark condition

0.1 0.0 300

𝑉2

8

θ = ( v) exp ( t )


0.5

9

where ϵ is the dielectric constant of the polymer which is determined from the C-V measurements (data not included) and was found to be 3.09 [17], ϵo (=8.854 ×1012 F/m) is the permittivity of free space and θ is the traplimiting factor, which represents the ratio of free to trapped charges and is given by [14]:

0

0.6

(3)

d

J = ϵϵ𝑜 θμ

It is well known that active layers of large thickness absorb more light than thinner ones [8], however, organic semiconductors have fairly strong absorption coefficients (105 cm-1), resulting in high light absorption in even lower than 100 nm film thickness [11]. Fig. 2 shows the absorption spectra of P3HT:PCBM active layers prepared with different thicknesses in the range of 65-266 nm. P3HT:PCBM films show an absorption band in the range 400-650 nm with main absorption peak of different intensities around 510 nm which are attributed to the π– π* transition within the P3HT main polymer [12], while two shoulders appeared around 550 and 610 nm which are ascribed to the extended conjugated P3HTand the inter-chain stacking of P3HT, respectively [7]. On the other hand, the absorption peaks around 330 nm were related to the PCBM molecules [13]. Furthermore, the increase in absorption intensity is consistent with increased film thickness. 0.7

V

where p is the holes concentration, q is the elementary charge, µ is the mobility, V is the applied voltage and d is the active layer thickness. At applied voltages above 0.15V the curves demonstrate power law dependence of the form Jα Vm where m takes different values over different voltage regimes [15]. After the ohmic region and when the gradient m takes a value of ≈2 at the second region the J-V curves can be associated with charge transport mechanism ascribed to trap-controlled space charge limited conduction (SCLC) with traps located at a single energy level inside the band gap; this mechanism can be represented by the following equation [16]:

log (J)

PCE (%) =

3. 2. Charge carrier mobility using SCLC method:

400

500

600

700

When m takes a value larger than 2 (in the current study m≈8) this can be associated with trap filling limit beyond which trap-filled SCLC mechanism is observed. This is evident by the onset of a trap-free space-charge

Wavelength (nm)

Fig. 2 UV-visible absorption spectra of P3HT:PCBM with different active layer thicknesses

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limited conductivity (TFCLC) with m taking on a value of ≈2 according to Child's law [16]: 9

V2

8

d3

J = ϵϵo μ

where IL is the light current, Io is the saturation current, n is the ideality factor, kT/q is the thermal energy. Under dark conditions, the generated photocurrent is zero and it could be assumed that the shunt resistance (RSH) is as high as enough to eliminate the last term in Eq. 7 [21]. Hence, the dark I-V relation becomes:

(6)

This occurs when the injected carriers from the electrodes exceed the generated carries and there are no traps in the SCLC region or all traps are filled; the current density will increase quadratically with increasing applied voltage [15]. The upper region of logJ-logV dependence given in Fig. 3 (the curves in this region are included within a circle) represents the TFSCLC, satisfying the square law dependence. Using the data in this region, the mobility of charge carriers in the active layer are determined by plotting J vs. V2/d3 (see eqn. 6) and the variation of mobility with film thickness is presented in Fig. 4.

I = Io [eq(V−IRS )⁄nkT − 1]

(8)

From eqn. (8) the applied voltage could be expressed as a function of current as follows: V=

nkT q

I

ln ( + 1) + IR S Io

(9)

At higher applied voltage when I>>Io, Eq. 9 becomes: I

100

dV dI

= IR S +

nkT q

(10)

14

RS could therefore be easily extracted from the slope of I(dV⁄dI) versus I plotted at the high current region. The P3HT:PCBM solar cells with different active layer thickness have revealed a decrease in RS with decreasing the active layer thickness. The highest RS was obtained for the thicker film (266 nm) while the optimum thickness of 95 nm was associated with the lower RS. Further increase in RS values was observed on decreasing active layer thickness below 95 nm. This could be attributed to the carrier diffusion length which will be affected by thicker layer leading to increased possibility of recombination and therefore higher RS [ 18].

80

-6

70

10

60 8

50

6

RS (Ohm)

2

-1

-1

Mobility x10 (cm . V . S )

90 12

40 30

4 50

100

150

200

250

20 300

Active layer thickness (nm)

Fig. 4 The mobility and series resistance as a function of active layer thickness for P3HT:PCBM solar cells Usually charge carrier mobility in organic thin films is very low; this is mainly ascribed to structural disorder in the films [18]. The charge carrier mobility was found to increase with decreasing active layer thickness; the maximum recorded mobility of 1.37 ×10-5 cm2.V-1.s-1 was found for film thickness of 95nm. The mobility was found to be smaller for film thickness below 95 nm. Furthermore the mobility was found to decrease when film thickness increases above 95nm. Therefore, it could be inferred that the organic film thickness is an essential factor in determining the charge carrier mobility. Increasing film thickness is found to result in decreased charge carrier mobility due to increasing bulk resistivity of the active layer [19], as well as decreasing electric field (at constant applied voltage) leading to reduced separation rate of excitons, hence decreasing charge carrier mobility [20].

Fig. 5 Equivalent circuit model of solar cells 4

PHOTOVOLTAIC PERFORMANCE: In order to examine the photovoltaic performance as a function of different active layer thickness, J-V characteristics were measured under 1.5AM illumination and the results are shown in Fig.6. The solar cells performance is mainly characterized by their short circuit current density (Jsc), fill factor (FF) and the open circuit voltage (Voc). Therefore, to achieve higher power conversion efficiency (PCE), higher values of these parameters are required [22] and those have been examined as a function of active layer thickness. The latter may directly affect the Jsc and FF of the solar cell, while Voc is mainly determined by the energy level difference between the HOMO of the donor and the LUMO of the acceptor, as depicted by the following equation [23]:

3. 3. Extracting series resistance (RS) from the dark J(V) According to solar cell equivalent circuit shown in Fig. 5, two effective parameters should be taken into account during solar cell fabrication; these are series resistance RS and shunt resistance RSH where film thickness can play a significant role [21]. Generally, the I-V characteristics of the solar cell could be represented by the following equation: I = IL − Io [eq(V−IRS )⁄nkT − 1] −

V−IRS RSH

Voc =

1 q

(LUMOAcceptor − HOMODonor ) − 0.3

(11)

where q is the elementary charge, LUMOacceptor is the energy of the lowest unoccupied molecular orbital of the acceptor and HOMOdonor is the energy of the highest occupied molecular orbital of the donor.

(7)

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0 0.0 -2

0.1

0.2

0.3

0.4

0.5

0.6

0.7

2

-2

JSC (mA.cm )

Moreover, increasing the series resistance could be the main reason for reducing FF in thicker films and the devices with thickness below 100 nm. [25].

Voltage (V)

-4

-10 -12

-2

-8

JSC (mA.cm )

-6

Voltage (V)

0 0.0 -2


-14

0.1

0.2

0.3

0.7

-6 -8 First day 5 days 21 days 28 days 98 days

-14 0.7

4

14

50 48 3

10

44

-2

8

(12)

2 42

PCE %

0.6

46

JSC (mA.cm )

VOC (volt)

12

40 1 38 36

0.5

6 First day

where µ is the carrier mobility, τ is the carrier recombination lifetime and E is the electrical field.

5 days

21 days

28 days

Stability time

98 days

0 First day

5 days

21 days

28 days

98 days

Staibility time

Fig. 8. Stability of P3HT:PCBM device with the thickness 95 nm.

5

(a)

50

Fig. 7 (a) shows the PCE and FF for OSC devices as a function of P3HT:PCBM active layer thickness. Current density was found to decrease when the photoactive layer thickness increased, while Voc remained almost unchanged at about 0.61-0.63V as revealed in Fig. 7 (b). The best examined device with the highest PCE has demonstrated good stability with time when tested over a period of more than three months corresponding to a decrease in PCE to 1.9%, FF to 37% and JSC 8.3 mA.cm-2 however, Voc remained unchanged again. It is expected that further stability could be achieved as a result of device encapsulation. Fig. 8 demonstrates device stability over a period of three months

4

46

3

PCE %

48

FF %

0.6

-12

The improvement in the device performance could only be occurred once the photo-generated carriers are extracted without recombination loss; hence the fill factor of the solar cell is limited by the carrier diffusion length (Ld) [24]:

44 2 42 50

100

150

200

250

1 300


0.70

14

(b)

0.68 0.66

12

0.64 10

0.60 0.58

8

Jsc (mA.cm )

0.62

Voc (V)

0.5

-10

Fig. 6. J-V curves of P3HT:PCBM with different active layer thickness

Ld = μτE

0.4

-4

FF%

2

Conclusion: P3HT:PCBM solar cells were fabricated with different active layer thickness in the range 65 - 266nm. UV-Vis absorption spectra show typical P3HT:PCBM behavior with thickness dependence. The OSC device performance was examined in dark and under light illumination. The charge carrier mobility was determined from the SCLC theory, where the mobility was found to decrease with increasing film thickness. Furthermore series resistance was found to decrease with decreased film thickness where active layer thickness of 95nm has exhibited the lowest series resistance as well as the highest mobility. The photovoltaic performance was evaluated for these devices as a function of active layer thickness and the maximum PCE was recorded for the device with 95 nm active layer thickness with 3.86% and FF 50%, Jsc 12.6 mA.cm-2 whereas Voc remains constant at 0.61-0.63V. The best examined device has shown a good stability with time without using any encapsulation or protection layers for a period of over three months.

-2

0.56 0.54

6

0.52 0.50 50

100

150

200

250

4 300


Fig. 7 The photovoltaic parameters as a function of active layer thickness (a) FF and PCE (b) Jsc and Voc Therefore, to prevent charge recombination Ld must be higher than the active layer thickness; higher mobility or thinner active layer is beneficial for charge carrier extraction. PCE and other performance parameters were found to increase with decreasing the active layer thickness until reaching an optimum thickness of 95 nm; this thickness was shown to exhibit a PCE of 3.86% with Jsc 12.6 mA.cm-2 and FF 50%. The Voc on the other hand was found to be constant within the range 0.61-0.63V. Devices with active layer thickness larger than 95 nm have demonstrated a deteriorated performance compared to those with the optimum film thickness. In general, PCE of organic solar cells are limited by two factors; low carrier mobility and short exciton diffusion lengths [19]

Acknowledgements Burak Kadem wishes to acknowledge the PhD

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