Flexible Solar Cells : fabrication and charge conduction analysis Vishal Sharma1,2*, Vinamrita Singh1, Swati Arora3 and R. P. Tandon1 1Department
of Physics & Astrophysics, University of Delhi, Delhi-110007, India 2Department of Electronics, Govt. degree College Udhampur, J&K, India. 3Cluster Innovation Center, University of Delhi, Delhi-110007, India.
INTRODUCTION In this study, we have fabricated flexible organic solar cells (OSC) composed of blended films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) on ITO coated PET flexible substrate. In BHJ organic solar cells, the donor (P3HT) and acceptor (PC71BM) materials are blend together in an organic solvent and are phase separated to form large interfacial area for efficient exciton dissociation, which leads to high quantum efficiency of these solar cells. The current density-voltage (J-V) (Fig.3) characteristics are measured which gives the values of short circuit current density (Jsc), open circuit voltage (Voc), series resistance (Rs), shunt resistance (Rsh), fill factor (FF) and efficiency (Ρ). To understand the charge transport behavior, theoretical analysis of J-V characteristics was performed which shows that the thickness of interfacial layer (δ) plays an important role in the current conduction phenomenon in flexible solar cells.
EXPERIMENTAL DETAILS Solar cells were fabricated on 10 Ί/sq. ITO coated glass substrates with the ITO patterned in two 1.5 mm strips. The substrates were cleaned with soap solution in ultrasonic bath (15 min) followed by rinsing in DI water. Subsequently, the substrates were then boiled in acetone, trichloroethylene and isopropanol, followed by an oxygen plasma treatment prior to film deposition. The vertical layer configuration was ITO/PEDOT- PSS/P3HT:PCBM/Aluminum. PEDOT-PSS layer was spin coated at 2000rpm for 1 minute to get a film thickness of 50nm and then annealed at a temperature of 120oC for 20 minutes. The active layer of P3HT:PCBM (20mg/ml) at the blend ratio of 1:1 was then spin coated at 1000rpm to get a film thickness of 150nm. Aluminum electrodes (100nm) are finally thermally evaporated (5-8 Å/s) to form electrodes for the solar cells. Solar Simulator SS50AA at 100mWcm-2 and Keithley-2400 source meter is used for taking current- voltage measurements. All the chemicals are procured from Sigma Aldrich chemicals.
Fig1. Schematic Diagram of P3HT:PCBM Organic Solar Cell
Fig.2 Photograph of the real device
Fig3. J-V Characteristics of P3HT:PCBM Organic Solar Cell
THEORETICAL ANALYSIS
Total dark Current JD,total = JD+JT đ?‘ąđ?‘Ť =
đ?’’đ?&#x;? đ?‘Ťđ?’? đ?‘ľđ?‘Ş
đ?? đ??Ł =
đ?‘˝đ?’ƒđ?’Š −đ?‘˝ đ?’†đ?’™đ?’‘
đ?’…đ?’?đ?’Œđ?‘ť đ?&#x;?−đ?’†đ?’™đ?’‘ đ?›…đ??Ł đ?›†đ??Šđ??Ł đ?›†đ??˘đ??Ł đ??? đ?›…đ??Ł đ?&#x;?+ đ???
đ?›†đ??Šđ??Ł đ?›†đ??˘đ??Ł
−đ??‹ đ?’’đ?‘˝ đ?’†đ?’™đ?’‘ đ?’Œđ?‘ť đ?’?đ?’Œđ?‘ť −đ?’’(đ?‘˝đ?’ƒđ?’Š −đ?‘˝) đ?’?đ?’Œđ?‘ť
−đ?&#x;?
đ?‘ąđ?‘ť =
and đ?&#x;? đ?’‹=đ?&#x;? đ?’’đ?‘Ťđ?’”đ?’‹ đ??ˆđ?’”đ?’• đ?‘˝đ?’Ž
Total Illuminated Current JI,total= JD,total-JL(V) đ?&#x;? đ?&#x;?
đ?’ˆđ?’‹ đ?‘Źđ?’” ∆đ?‘Źđ?’” đ?’†đ?’™đ?’‘ −đ?’ƒđ?œżđ?’”đ?’‹ đ?œšđ?’‹
đ?’‡đ?’”đ?’‹ − đ?’‡đ?’”đ?&#x;Žđ?’‹ đ?’†đ?’™đ?’‘ −
đ?‘Šđ?’‹ đ?’’đ?‘˝ đ?’Œđ?‘ť
and đ??‰đ??‹ đ??• =
(đ??•đ??›đ??˘ −đ??•) |đ??‰đ??’đ??‚ |đ?›?đ?›• đ???đ?&#x;?
JD-diode current, JT-tunneling current, Ds-Density of surface states, ĎƒSt = 10-14 m2 is the capture cross-section for surface states, gj(ES) is density of states at energy E = Es around a small energy range ΔEs , b = 0.6 is tunneling constant for surface states, Îşsj is the barrier height for surface states, δj is thickness of the interface layer, fsoj is occupation probability of surface states and fsj is final occupation probability, Îľpj and Îľij are the dielectric constants of the polymer and interface under consideration, respectively. Figure 4. (a) Dark experimental and theoretical J-V characteristics (b) Illuminated experimental and theoretical J-V Characteristics
Hole mobility (cm2V-1s-1) Electron mobility (cm2V1s-1)
Dark
Illuminated
0.9Ă—10-4
1.01Ă—10-4
1Ă—10-4
1.1Ă—10-4
RESULTS AND DISCUSSIONS ďƒ˜ It was found from theoretical analysis of the dark and illuminated J-V characteristics that the interface state density and thickness of the interfacial layers play an important role in the current conduction phenomenon in flexible solar cells. Hence the current through the interfaces contributes significantly in the conduction phenomenon of flexible solar cells, for this reason organic/electrode interfaces should be studied in more detail to fabricate high efficiency and more stable flexible organic solar cells